Development of Visible Light-Responsive Photocatalysts

International Journal of Photoenergy

Development of Visible Light-Responsive PhotocatalystsGuest Editors: Jinlong Zhang, Masaya Matsuoka, Jae Sung Lee, and Shifu Chen

Development of Visible Light-ResponsivePhotocatalysts

International Journal of Photoenergy

Development of Visible Light-ResponsivePhotocatalysts

Guest Editors: Jinlong Zhang, Masaya Matsuoka,Jae Sung Lee, and Shifu Chen

Copyright © 2012 Hindawi Publishing Corporation. All rights reserved.

This is a special issue published in “International Journal of Photoenergy.” All articles are open access articles distributed under theCreative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided theoriginal work is properly cited.

Editorial Board

M. Sabry Abdel-Mottaleb, EgyptMuhammad Abdul Rauf, UAENihal Ahmad, USANicolas Alonso-Vante, FranceWayne A. Anderson, USAVincenzo Augugliaro, ItalyDetlef W. Bahnemann, GermanyIgnazio Renato Bellobono, ItalyRaghu N. Bhattacharya, USAGion Calzaferri, SwitzerlandVikram L. Dalal, USAD. Demetriou Dionysiou, USAAbderrazek Douhal, SpainSamy El-Shall, USABeverley Glass, AustraliaShahed Khan, USA

Cooper H. Langford, CanadaYuexiang Li, ChinaGianluca Li Puma, UKPanagiotis Lianos, GreeceStefan Lis, PolandGilles Mailhot, FranceUgo Mazzucato, ItalyJacek Miller, PolandFranca Morazzoni, ItalyLeonardo Palmisano, ItalyDavid Lee Phillips, Hong KongXie Quan, ChinaTijana Rajh, USASaffa Riffat, UKPeter Robertson, UKJ.-Louis Scartezzini, Switzerland

P. Joseph Sebastian, MexicoPanagiotis Smirniotis, USASam-Shajing Sun, USAMasanori Tachiya, JapanGopal N. Tiwari, IndiaVeronica Vaida, USARoel van De Krol, The NetherlandsMark van Der Auweraer, BelgiumJohannes Vos, IrelandDavid Worrall, UKF. Yakuphanoglu, TurkeyJimmy C. Yu, Hong KongKlaas Zachariasse, GermanyJincai Zhao, China

Contents

Development of Visible Light-Responsive Photocatalysts, Jinlong Zhang, Masaya Matsuoka, Jae Sung Lee,and Shifu ChenVolume 2012, Article ID 280297, 4 pages

Photosensitized Oxidation of 9,10-Dimethylanthracene on Dye-Doped Silica Composites, Elim Albiter,Salvador Alfaro, and Miguel A. ValenzuelaVolume 2012, Article ID 987606, 8 pages

Photoresponse of Visible Light Active CM-n-TiO2, HM-n-TiO2, CM-n-Fe2O3, and CM-p-WO3 towardsWater Splitting Reaction, Yasser A. Shaban and Shahed U. M. KhanVolume 2012, Article ID 749135, 20 pages

Photocatalytic Degradation of Pesticides in Natural Water: Effect of Hydrogen Peroxide,Natividad Miguel, Marıa P. Ormad, Rosa Mosteo, and Jose L. OvelleiroVolume 2012, Article ID 371714, 11 pages

Photocatalytic Activity and Characterization of Carbon-Modified Titania for Visible-Light-ActivePhotodegradation of Nitrogen Oxides, Chun-Hung Huang, Yu-Ming Lin, I-Kai Wang, and Chun-Mei LuVolume 2012, Article ID 548647, 13 pages

Sm2FeTaO7 Photocatalyst for Degradation of Indigo Carmine Dye under Solar Light Irradiation,Leticia M. Torres-Martınez, Miguel A. Ruiz-Gomez, M. Z. Figueroa-Torres, Isaıas Juarez-Ramırez,and Edgar MoctezumaVolume 2012, Article ID 939608, 7 pages

Photocatalytic Oxidation of Gaseous Isopropanol Using Visible-Light Active Silver Vanadates/SBA-15Composite, Ting-Chung Pan, Hung-Chang Chen, Guan-Ting Pan, and Chao-Ming HuangVolume 2012, Article ID 314361, 8 pages

Application of Pt/CdS for the Photocatalytic Flue Gas Desulfurization, Xiulan Song, Weifeng Yao,Bo Zhang, and Yiping WuVolume 2012, Article ID 684735, 5 pages

An Enthusiastic Glance in to the Visible Responsive Photocatalysts for Energy Production and PollutantRemoval, with Special Emphasis on Titania, Padikkaparambil Silija, Zahira Yaakob, Viswanathan Suraja,Njarakkattuvalappil Narayanan Binitha, and Zubair Shamsul AkmalVolume 2012, Article ID 503839, 19 pages

Highly Active Rare-Earth-Metal La-Doped Photocatalysts: Fabrication, Characterization,and Their Photocatalytic Activity, S. Anandan, Y. Ikuma, and V. MurugesanVolume 2012, Article ID 921412, 10 pages

Hierarchical CuO/ZnO Membranes for Environmental Applications under the Irradiation of VisibleLight, Zhaoyang Liu, Hongwei Bai, and Darren Delai SunVolume 2012, Article ID 804840, 11 pages

Synthesis, Characterization, and Evaluation of Boron-Doped Iron Oxides for the PhotocatalyticDegradation of Atrazine under Visible Light, Shan Hu, Guanglong Liu, Duanwei Zhu, Chao Chen,and Shuijiao LiaoVolume 2012, Article ID 598713, 4 pages

Silver Orthophosphate Immobilized on Flaky Layered Double Hydroxides as the Visible-Light-DrivenPhotocatalysts, Xianlu Cui, Yaogang Li, Qinghong Zhang, and Hongzhi WangVolume 2012, Article ID 263254, 6 pages

Thickness Dependent on Photocatalytic Activity of Hematite Thin Films, Yen-Hua Chen and Kuo-Jui TuVolume 2012, Article ID 980595, 6 pages

Synthesis, Property Characterization, and Photocatalytic Activity of Novel Visible Light-ResponsivePhotocatalyst Fe2BiSbO7, Jingfei Luan and Zhitian HuVolume 2012, Article ID 301954, 11 pages

One-Step Cohydrothermal Synthesis of Nitrogen-Doped Titanium Oxide Nanotubes with EnhancedVisible Light Photocatalytic Activity, Cheng-Ching Hu, Tzu-Chien Hsu, and Li-Heng KaoVolume 2012, Article ID 391958, 9 pages

Photodegradation of Malachite Green by Nanostructured Bi2WO6 Visible Light-Induced Photocatalyst,Yijie Chen, Yaqin Zhang, Chen Liu, Aimin Lu, and Weihua ZhangVolume 2012, Article ID 510158, 6 pages

Preparation of Porous F-WO3/TiO2 Films with Visible-Light Photocatalytic Activity by MicroarcOxidation, Chung-Wei Yeh, Kee-Rong Wu, Chung-Hsuang Hung, Hao-Cheng Chang, and Chuan-Jen HsuVolume 2012, Article ID 285129, 9 pages

Preparation of TiO2-Fullerene Composites and Their Photocatalytic Activity under Visible Light,Ken-ichi Katsumata, Nobuhiro Matsushita, and Kiyoshi OkadaVolume 2012, Article ID 256096, 9 pages

Photocatalytic Ethanol Oxidative Dehydrogenation over Pt/TiO2: Effect of the Addition of BluePhosphors, J. J. Murcia, M. C. Hidalgo, J. A. Navıo, V. Vaiano, P. Ciambelli, and D. SanninoVolume 2012, Article ID 687262, 9 pages

Development of Visible Light-Responsive Sensitized Photocatalysts, Donghua Pei and Jingfei LuanVolume 2012, Article ID 262831, 13 pages

Sol-Gel-Hydrothermal Synthesis of the Heterostructured TiO2/N-Bi2WO6 Composite withHigh-Visible-Light- and Ultraviolet-Light-Induced Photocatalytic Performances, Jiang Zhang,Zheng-Hong Huang, Yong Xu, and Feiyu KangVolume 2012, Article ID 469178, 12 pages

Preparation, Characterization, and Photocatalytic Property of Cu2O-TiO2 Nanocomposites,Longfeng Li and Maolin ZhangVolume 2012, Article ID 292103, 4 pages

Effect of Electronegativity and Charge Balance on the Visible-Light-Responsive Photocatalytic Activityof Nonmetal Doped Anatase TiO2, Jibao Lu, Hao Jin, Ying Dai, Kesong Yang, and Baibiao HuangVolume 2012, Article ID 928503, 8 pages

CO2 Reforming Characteristics under Visible Light Response of Cr- or Ag-Doped TiO2 Prepared bySol-Gel and Dip-Coating Process, Akira Nishimura, Go Mitsui, Katsuya Nakamura, Masafumi Hirota,and Eric HuVolume 2012, Article ID 184169, 12 pages

Visible-Light Photodegradation of Dye on Co-Doped Titania Nanotubes Prepared by HydrothermalSynthesis, Jung-Pin Wang, Hsi-Chi Yang, and Chien-Te HsiehVolume 2012, Article ID 206534, 10 pages

Photocatalytical Properties and Theoretical Analysis of N, Cd-Codoped TiO2 Synthesized by ThermalDecomposition Method, Hongtao Gao, Bing Lu, Fangfang Liu, Yuanyuan Liu, and Xian ZhaoVolume 2012, Article ID 453018, 9 pages

Nitrogen-Doped TiO2 Photocatalyst Prepared by Mechanochemical Method: Doping Mechanisms andVisible Photoactivity of Pollutant Degradation, Yu-Chao Tang, Xian-Huai Huang, Han-Qing Yu,and Li-Hua TangVolume 2012, Article ID 960726, 10 pages

AgBr-Coupled TiO2: A Visible Heterostructured Photocatalyst for Degrading Dye Pollutants,Jianjun Liu, Yingchun Yu, Zhixin Liu, Shengli Zuo, and Baoshan LiVolume 2012, Article ID 254201, 7 pages

Photocatalytic Degradation of Phenolics by N-Doped Mesoporous Titania under Solar Radiation,Priti A. Mangrulkar, Sanjay P. Kamble, Meenal M. Joshi, Jyotsna S. Meshram, Nitin K. Labhsetwar,and Sadhana S. RayaluVolume 2012, Article ID 780562, 10 pages

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2012, Article ID 280297, 4 pagesdoi:10.1155/2012/280297

Editorial

Development of Visible Light-Responsive Photocatalysts

Jinlong Zhang,1 Masaya Matsuoka,2 Jae Sung Lee,3 and Shifu Chen4

1 Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road,Shanghai 200237, China

2 Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 599-8531 Osaka, Japan3 Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang University of Science andTechnology (POSTECH), Pohang 790784, Republic of Korea

4 Department of Chemistry, Huaibei Coal Normal College, Anhui, Huaibei 235000, China

Correspondence should be addressed to Jinlong Zhang, [email protected]

Received 20 December 2011; Accepted 20 December 2011

Copyright © 2012 Jinlong Zhang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Photocatalysis has received considerable attention because ofits promising applications such as in photocatalytic degra-dation of organic pollutants, photocatalytic dissociation ofwater, solar energy conversion, and disinfection. As animportant semiconductor, photocatalytic materials, titaniahas been attracting the worldwide attention due to its goodchemical stability, insolubility in water, and nontoxic, lowcost, and readily available raw materials. It has become a hottopic in photocatalysis scopes that how to expand the spectralresponse range and improve the photocatalysis quantumefficiency of photocatalysts.

Of course, the selected topics and papers are notan exhaustive representation of the area of visible light-responsive photocatalysts. Nonetheless, they represent therich and many-faceted knowledge, that we have the pleasureof sharing with the readers. We would like to thank theauthors for their excellent contributions and patience inassisting us. Finally, the fundamental work of all reviewerson these papers is also very warmly acknowledged.

This special issue contains twenty nine papers, wheretwo reviews are related to development of visible light-responsive sensitized photocatalysts. Nine papers are dealingwith the doping or codoping TiO2 photocatalysts. Six papersare regarding the composite photocatalysts. Two papersare related to modified photocatalysts by deposition ofnoble metal. Three papers are about the preparation ofphotocatalytic film. Six papers belong to the non-TiO2

photocatalysts. Finally, one paper addresses photocatalyticdegradation of pesticides in natural water.

In the paper entitled “Development of visible light-responsive sensitized photocatalysts,” D. Pei and J. Luan

present a review of studies about the visible-light-promotedphotodegradation of the contaminants and energy conver-sion with sensitized photocatalysts. Herein authors studymechanism, physical properties, synergism effect of thesensitized photocatalysts, and the method for enhancing thephotosensitized effect.

In the paper entitled “An enthusiastic glance in to thevisible responsive photocatalysts for energy production andpollutant removal with special emphasis on titania,” Z. Yaakobet al. present some of the recently published papers onvisible responsive photocatalysts. The influence of variousmetal oxides and their sulfides on energy production andpollutant removal are presented with special emphasis ontitania photocatalysts. A keen look into the photoactivitytitania for various pollutant degradation, modified titaniasystems, physical and chemical characteristics are employedat this juncture.

In the paper entitled “Photocatalytic degradation of phe-nolics by N-doped mesoporous titania under Solar radiation,”S. S. Rayalu et al. present preparation of nitrogen dopedmesoporous titania by templating method using chitosan.This biopolymer chitosan plays the dual role of acting as atemplate (which imparts mesoporosity) and precursor fornitrogen. The doping of nitrogen into TiO2 lattice and itsstate is substantiated and measured by XPS. The photocat-alytic activity of the prepared N-doped mesoporous titaniafor phenol and o-chlorophenol degradation is investigatedunder solar and artificial radiation.

In the paper entitled “Visible-light photodegradation ofdye on co-doped titania nanotubes prepared by hydrothermalsynthesis,” Chien-Te Hsieh et al. deal with preparation of

2 International Journal of Photoenergy

highly porous codoped TiO2 nanotubes from a hydrothermaltreatment and investigate their photocatalytic activity tophotodecompose methylene blue (MB) in liquid phase undervisible light irradiation.

The research of A. Nishimura et al. entitled “CO2

reforming characteristics under visible light response of Cr- orAg-doped TiO2 prepared by sol-gel and dip-coating process”presents the preparation of Cr- or Ag-doped TiO2 film by sol-gel and dip-coating process and study their photocatalyticactivity for CO2 reforming under the visible light.

In the paper entitled “Photocatalytical properties andtheoretical analysis of N, Cd-codoped TiO2 synthesized bythermal decomposition method,” X. Zhao et al. present thepreparation of N, Cd-codoped TiO2 by thermal decompo-sition method. The products represented good performancein photocatalytic degradation of methyl orange. The effectof the incorporation of N and Cd on electronic structureand optical properties of TiO2 are studied by first-principlecalculations based on density functional theory (DFT).

There is also the paper by Tang et al. “Nitrogen-dopedTiO2 photocatalyst prepared by mechanochemical method:doping mechanisms and visible photoactivity of pollutantdegradation.” Nitrogen-doped TiO2 (N/TiO2) photocata-lysts are prepared using a mechanochemica1 method withraw amorphous TiO2 as precursors and various nitroge-nous compounds doses (NH4F, NH4HCO3, NH3·H2O,NH4COOCH3, and CH4N2O). Their photocatalytical activ-ities were evaluated with the degradation of p-nitrophenoland methyl orange under UV or sunlight irradiation. Thecatalysts have a strong visible light absorption which iscorresponding to doped nitrogen and consequent oxygendeficient.

In the paper entitled “Effect of electronegativity and chargebalance on the visible-light-responsive photocatalytic activity ofnonmetal doped anatase TiO2,” Y. Dai et al. investigate theorigin of visible light absorption and photocatalytic activityof nonmetal doped anatase TiO2 in details in this workbased on density functional theory calculations. Their resultsindicate that the electronegativity is of great significance inthe band structures, which determines the relative positionsof impurity states induced by the doping species, andfurther influences the optical absorption and photocatalyticactivities of doped TiO2. The effect of charge balance on theelectronic structure is also discussed, and it is found that thecharge-balance structures may be more efficient for visible-light photocatalytic activities.

In the paper entitled “Photocatalytic activity and char-acterization of carbon-modified titania for visible-light-activephotodegradation of nitrogen oxides,” C.-H. Huang et al.present the preparation of carbon-modified titania powders,which are prepared by impregnation method using a com-mercial available titania powder, Hombikat UV100, as matrixmaterial while a range of alcohols from propanol to hexanolare used as precursors of carbon sources. Rising the carbonnumber of alcoholic precursor molecule, the modified titaniashows increasing visible activities of NOx photodegradation.

In the paper entitled “One-step cohydrothermal synthesisof nitrogen-doped titanium oxide nanotubes with enhancedvisible light photocatalytic activity,” T.-C. Hsu et al. present

a kind of nitrogen-doped TiO2 nanotubes synthesized usingcommercial titania P25 as raw material by a facile P25/ureacohydrothermal method, which shows enhanced visible lightphotocatalytic activity. The nitrogen content and surfacearea, rather than the crystallinity is found to be the crucialfactors in affecting the photocatalytic efficiency of thenitrogen-doped TiO2 nanotubes.

In the paper entitled “Highly active rare-earth metalLa-doped photocatalysts: fabrication, characterization, andtheir photocatalytic activity,” S. Anandan et al. present theinvestigation of highly active La-doped TiO2 nanoparticleswith different proportion of La content for the degradationof MCP in aqueous solution. It is observed that the rateof degradation of MCP over La-doped TiO2 increases withincreasing La loading. 1.0 wt% La-doped TiO2 is found tobe the most active among all the catalysts, which shows highrelative photonic efficiencies. The effects of electron trappingby lanthanum metal ions, particle size, surface area, andsurface roughness of the photocatalysts are supposed to bethe reason for the enhanced activity.

In the paper entitled “Preparation, characterization, andphotocatalytic property of Cu2O-TiO2 nanocomposite,” M.Zhang and L. Li present a serial of Cu2O-TiO2 nanocom-posits with high visible light photocatalytic activity for thedegradation of methyl orange, which is prepared by thehomogeneous hydrolysation, followed by the solvothermalcrystallization and ethylene glycol-thermal reduction pro-cess, respectively. The prepared Cu2O-TiO2 nanocompositesexhibit higher photocatalytic activities for the decompositionof MO than the pure Cu2O and the commercial Degussa P25under visible light irradiation.

In the paper entitled “AgBr coupled TiO2: a visible het-erostructured photocatalyst for degrading dye pollutants,” Liuet al. present a series of AgBr/TiO2 photocatalysts with het-erojunction structure and high-visible-light photocatalyticactivity, which is synthesized using Ti(OC4H9)4, KBr andAgNO3 as precursors. It is found that the coupled AgBr/TiO2

shows a stable and enhanced photodegradation rate ofmethylene blue under visible light irradiation, comparedwith the noncoupled photocatalysts of AgBr, AgBr/P25 andP25. The synergetic effect of heterostructured AgBr/TiO2 isresponsible for the strongest absorption in whole UV-vislight region.

The research of Z. Liu et al. entitled “HierarchicalCuO/ZnO membranes for environmental applications underthe irradiation of visible light” studies a new kind of highactive hierarchical CuO/ZnO nanomaterial prepared usinga facile process, which has a great potential in environ-mental applications with solar visible light. This novelCuO/ZnO membrane shows improved photodegradation ofcontaminants and antibacterial activity under the irradiationof visible light. It is found that the special hierarchicalnanostructure of CuO/ZnO is in favour of enhancing lightutilization rate, enlarging specific surface and reducing therecombination of electrons and holes at the interfacialbetween CuO and ZnO, which is the reason for the highphotocatalytic activity.

In the paper entitled “Photocatalytic oxidation of gaseousisopropanol using visible-light active silver vanadates/SBA-15

International Journal of Photoenergy 3

composite,” C.-M. Huang et al. present an environ-mentally friendly visible-light driven photocatalyst, silvervanadates/SBA-15, which is prepared through an incipi-ent wetness impregnation procedure with silver vanadates(SVO). All the composites loaded with various amountof SVO inherit the higher adsorption capacity and largermineralization yield than those of P25 and pure SVO, whichis resulting from a favorable crystalline phase combined withthe high intensities of Brønsted and Lewis acids.

In the paper entitled “Sol-gel-hydrothermal synthesis ofthe heterostructured TiO2/N-Bi2WO6 composite with high-visible-light and ultraviolet-light-induced photocatalytic per-formances,” Z.-H. Huang et al. present a heterostructuredTiO2/N-Bi2WO6 composite prepared by a facile sol-gel-hydrothermal method, which has a high UV and visible lightphotocatalytic performance. The TiO2/N-Bi2WO6 compos-ites exhibit much higher photocatalytic performances thanTiO2 as well as Bi2WO6, owing to the effective electron-holeseparations at the interfaces of the two semiconductors.

In the paper entitled “Preparation of TiO2-fullerenecomposites and their photocatalytic activity under visiblelight,” Ken-ichi Katsumata et al. present the preparationand characterization of TiO2-fullerene composites which areprepared by a solution process. It is found that the rutile-C60 exhibit higher activity than the rutile under visible light,resulting from the transfer of photogenerated electrons fromthe C60 to the rutile under visible light irradiation.

The research of J. J. Murcia et al. entitled “Photocatalyticethanol oxidative dehydrogenation over Pt/TiO2: effect of theaddition of blue phosphors” investigates the effect of bluephosphors on the ethanol oxidative dehydrogenation overPt/TiO2 photocatalyst. It is found that the blue phosphorsproduced an increase in the level of ethanol conversionover the Pt/TiO2 catalyst keeping at the same time the highselectivity to acetaldehyde.

In the paper entitled “Application of Pt/CdS for thephotocatalytic flue gas desulfurization,” W. Yao et al. design aphotocatalytic flue gas desulfurization technology to controlemissions of SO2 from the combustion of fossil fuels. CdSloaded with Pt are selected as the model photocatalyst forthe photocatalytic flue gas desulfurization, and the factorsinfluencing the rate of hydrogen production and ammoniasulfite solution oxidation are detected in this paper.

The research of Y.-H. Chen and K.-J. Tu entitled“Thickness dependent on photocatalytic activity of hematitethin films” obtains a result that the photocatalytic activity ofhematite films increases with the increasing film thickness,which is because the hematite film with a thicker thicknesshas a rougher surface, providing more reaction sites forphotocatalysis. In addition, the lower band gap of a hematitefilm would generate more electron-hole pairs under visible-light illumination to enhance the photocatalytic efficiency.

In the paper entitled “Photoresponse of visible light activeCM-n-TiO2, HM-n-TiO2, CM-n-Fe2O3, and CM-p-WO3

towards water splitting reaction,” Y. A. Shaban and S. U. M.Khan summarize their studies on thin film photoelectrodesof Visible light active carbon modified titanium oxides (CM-n-TiO2); Visible light active hydrogen modified n-type tita-nium oxide (HM-n-TiO2) thin films; carbon modified iron

oxides (CM-n-Fe2O3) thin films; visible light active carbonmodified p-type tungsten oxides (CM-p-WO3) thin film.

In the paper entitled “Preparation of porous F-WO3/TiO2

films with visible-light photocatalytic activity by microarcoxidation,” K.-R. Wu et al. present a kind of porous F-WO3/TiO2 (mTiO2) films prepared on titanium sheet sub-strates using microarc oxidation (MAO) technique, whichshows an enhanced photocatalytic degradation of dye underUV and visible light irradiation, owing to its high specificsurface area from the porous microstructure.

In the paper entitled “Silver orthophosphate immobi-lized on flaky layered double hydroxides as the visible-light-driven photocatalysts,” Q. Zhang et al. present a visible-light driven photocatalyst FLDH/Ag3PO4 fabricated by thecoprecipitation method. It is found that the photocatalyticactivities of Ag3PO4 immobilized on the surface of FLDHare significantly enhanced for the degradation of acid red Gunder visible light irradiation compared to bare Ag3PO4.

In the paper entitled “Photodegradation of malachite greenby nanostructured Bi2WO6 visible light induced photocatalyst,”Zhang et al. study the photodegradation of malachite greenby the Bi2WO6 photocatalyst for the first time. The effects ofthe concentration of malachite green, the pH value, and theconcentration of Bi2WO6 on the photocatalytic efficiency areinvestigated in this paper.

In the paper entitled “Synthesis, property characterization,and photocatalytic activity of novel visible light-responsivephotocatalyst Fe2BiSbO7,” J. Luan and Z. Hu present thepreparation and characterization of visible light inducedphotocatalyst Fe2BiSbO7, which is synthesized by a solid-state reaction method for the first time. It is found that theFe2BiSbO7 possesses higher photocatalytic degradation ofMB under visible light irradiation, compared with Bi2InTaO7

or pure TiO2 or N-doped TiO2. The possible photocatalyticdegradation pathway of MB over Fe2BiSbO7 is obtained inthis paper.

In the paper entitled “Sm2FeTaO7 photocatalyst for degra-dation of indigo carmine dye under solar light irradiation,” L.M. Torres-Martınez et al. study the degradation of indigocarmine dye over Sm2FeTaO7 pyrochlore-type compound,which is synthesized by using conventional solid statereaction and sol-gel method. It is found that the solar-light-induced degradation active of sol-gel photocatalyst is 8 timesto the active of solid state. When Sm2FeTaO7 is impregnatedwith CuO as cocatalyst the photocatalytic activity is increasedbecause CuO acts as electron trap decreasing electron-holepair recombination rates.

In the paper entitled “Photosensitized oxidation of 9,10-dimethylanthracene on dye-doped silica composites,” M. A.Valenzuela et al. present a series of cationic dyes, methyleneblue (MB), safranin O (SF), toluidine blue (TB), andneutral red (NR) successfully incorporated into a silicamatrix by using ultrasound irradiation during the Stoberprocess. Among these four different types of dye-doped silicacomposites, the SiO2-SF composite shows the most efficientdelivery of singlet oxygen. This result is explained in terms ofa higher dispersion of the SF on the silica matrix.

In the paper entitled “Synthesis, characterization, andevaluation of boron-doped iron oxides for the photocatalytic


degradation of atrazine under visible light,” D. Zhu et al.investigate the photocatalytic degradation of atrazine byboron-doped iron oxides under visible light irradiation.Boron-doped goethite and hematite are successfully preparedby sol-gel method. It is found that the B-doped ironoxides shows higher atrazine degradation rate than that ofpristine iron oxides, and the B-doped goethite exhibits betterphotocatalytic activity than B-doped hematite. The bettercrystal structure, larger BET surface area, enhanced lightabsorption ability, and narrowed band-gap energy inducedby the B-doping are responsible for the high photocatalyticactivity.

In the paper entitled “Photocatalytic degradation of pesti-cides in natural water: effect of hydrogen peroxide,” N. Miguelet al. evaluate the effectiveness of photocatalytic treatmentwith titanium dioxide in the degradation of 44 organic pesti-cides analyzed systematically in the Ebro river basin (Spain).The effect of the addition of hydrogen peroxide in thistreatment is studied in this paper. It is found that the additionof hydrogen peroxide could increase the average degrada-tion of pesticides. The pesticides which are best degradedare parathion methyl, chlorpyrifos, α-endosulphan, 3,4-dichloroaniline, 4-isopropylaniline, and dicofol, while theworst degraded are HCHs, endosulphan-sulphate, hep-tachlors epoxide, and 4,4′-dichlorobenzophenone.

Jinlong Zhang

Masaya Matsuoka

Jae Sung Lee

Shifu Chen


Research Article

Photosensitized Oxidation of 9,10-Dimethylanthracene onDye-Doped Silica Composites

Elim Albiter, Salvador Alfaro, and Miguel A. Valenzuela

Laboratorio Catalisis y Materiales, ESIQIE Instituto Politecnico Nacional, Zacatenco, 07738 Mexico City, DF, Mexico

Correspondence should be addressed to Miguel A. Valenzuela, [email protected]

Received 15 July 2011; Accepted 17 November 2011

Academic Editor: Shifu Chen

Copyright © 2012 Elim Albiter et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A series of cationic dyes, methylene blue (MB), safranin O (SF), toluidine blue (TB), and neutral red (NR), were successfullyincorporated into a silica matrix by using ultrasound irradiation during the Stober process. Several analyses were performed,including scanning dynamic light scattering (DLS), electron microscopy (SEM), nitrogen physisorption, FTIR spectroscopy, UV-vis, and diffuse reflectance spectroscopy. The entrapped dyes on silica were evaluated in singlet oxygen (1O2) generation undervisible light irradiation, by means of the photosensitized oxidation of 9,10-dimethylanthracene (DMA). According to the results,the photocatalytic performance of the silica composites was improved, and the leakage of the dye from the particles was suppressed.Among these four different types of dye-doped silica composites, the SiO2-SF composite showed the most efficient delivery of 1O2.

1. Introduction

A photosensitized reaction is defined as the process leadingto photochemical or photophysical changes in a substrate bymeans of the absorption of radiation by other entity calleda photosensitizer [1]. Since the 1960s, several fundamentalworks have been published related to photosensitization inpresence of oxygen and have distinguished two competingmechanisms referred as Type I and Type II [2–5]. Type I pho-tooxygenation involves the formation of a sensitizer tripletstate (3S∗) which interacts with a substrate (XH) giving riseto a pair of free radicals by electron-transfer or hydrogen-transfer mechanisms [1]. These produced radicals react withoxygen to regenerate the sensitizer and to form peroxyor superoxide radicals. Type II mechanism implicates thedirect interaction of the sensitizer excited state with oxygen,generating upon energy transfer singlet oxygen [1O2, ( 1Δg)]which reacts directly with numerous organic substrates [6].

In spite of many efforts and successful results in thestudy of photosensitized reactions in homogeneous media,there are some disadvantages by this route mainly due to thesolubility of the sensitizers in the reaction solvent and theirremoval from the reaction mixture [7]. These problems canbe overcome by the immobilization of the sensitizer in ap-propriated solid carriers. For instance, the advantages of

using a dye dispersed on a solid carrier are the oligomeriza-tion of the dye can be prevented, a higher photostability ofthe dye is obtained, a higher purity of the final products, andthe reusability of the dye [8]. Important applications, forexample, fine chemical synthesis, wastewater treatment, andphotodynamic processes, among others, have been found forphotosensitized singlet oxygen production [7–14].

Although many solid carriers such as polymers, zeolites,semiconductors, glasses, silica gel, among others, have beenused to support a variety of sensitizers [15], we are interestedto develop a hybrid organic-inorganic systems made of dye-doped silica particles active under visible-light irradiation forfine chemical synthesis. Silica is a very attractive materialfor many industrial and medical applications because it isinexpensive, chemically inert, thermally stable, and biocom-patible. In fact, silica particles have captured much attentionover the past two decades for their application in catalysis,separation, biosensors, and adsorption [16]. Silica particlescan be tuned from 50 to 300 nm containing pore diametersbetween 2 and 10 nm allowing for different dyes loadings.Also, they have a high surface area (>700 m2/g) and largepore volume (>0.9 mL/g) allowing high loading of chemicals[16]. The doping of organic dyes into a silica matrix is notan easy task due to the weak interaction between the or-ganic-inorganic hybrid compound [17]. Usually, the main


NH

N

N

S

N

S

N

N

I II

IV

H3C H3C

H3C

H2N H2N

H3C

H2N

Cl−

Cl−Cl−

Cl−

CH3

CH3CH3

CH3

CH3

CH3

CH3

CH3

N+N+

N+N+

NH2

III

Figure 1: Molecular structure of the dyes: (I) neutral red (NR), (II) methylene blue (MB), (III) safranin O (SF), and (IV) toluidine blue O(TB).

problem to solve is the instability of the composites, anddye molecules with small size are easy to leak out fromthe matrix [17]. Several approaches have been developed toincorporate organic dyes into a silica matrix, for instance,covalent coupling [18], reverse microemulsion [19], andelectrostatic interaction (Stober method) [20]. However,there is still no effective method to control the dispersion ofthe dye molecules, since they are quickly and spontaneouslyaccumulated into the silica particles [21].

Herein, we reported a simple and modified Stober meth-od to synthesize SiO2-dyes composites by employing ultra-sound irradiation during the hydrolysis and condensationsteps of the silica source and dye addition. It was found thatthe as-synthesized composites were well dispersed into thesilica particles with high stability during the photooxidationof 9,10-dimethylanthracene under visible light.

2. Experimental

2.1. Synthesis of SiO2-Dye Composites. Tetraethyl orthosili-cate (TEOS), methylene blue (MB), safranin O (SF), tolui-dine blue (TB), neutral red (NR), and SiO2 nanopowder werepurchased from Sigma-Aldrich and used without furtherpurification. Ethanol (HPLC grade) was used as solvent; dou-ble distilled water and ammonium hydroxide (NH4OH) werealso used during the synthesis. The molecular structure of thedyes can be seen in Figure 1.

The composites were synthesized by using a modifiedStober method where the dye was incorporated since theformation of the SiO2 particles. TEOS was used as silicasource and NH4OH as catalyst, and the composites silica-dyewere obtained in one-step process. Briefly, it was preparedtwo solutions, one containing TEOS and ethanol (solution 1)and another with NH4OH, water, and dye (solution 2). Solu-tion 1 was poured drop by drop into solution 2, ultrasonicirradiation was applied during 10 min, and the sampleswere aged for 12 h under constant agitation. The obtainedpowders were completely dried under vacuum at 318 K.The molar ratios used in the preparation of the compositeswere 1/20/0.1/30 for TEOS/water/NH4OH/ethanol, and the

nominal concentration of dye was 1×10−5 or 1×10−6 molesof dye/g of SiO2.

2.2. Characterization. The dynamic light scattering (DLS)measurements were determined by a Zetasizer Nano instru-ment (Malvern), and ethanol was used as dispersant. AQuanta 3D FEG (Fei) scanning electron microscope (SEM)was used; the dry samples were deposited onto a carbon tapebefore analysis. The SiO2 and SiO2-MB samples were an-alyzed in an Asap 2405 (Micromeritics) automated gas sorp-tion system to obtain the nitrogen adsorption isothermsat 77.4 K. The specific surface area was estimated with theBrunauer-Emmett-Teller (BET) model, and the pore sizedistribution was evaluated with the BJH model. All sampleswere outgassed at 373 K for 1 h prior to analysis. FTIR spectrain the region 4000–400 cm−1 were obtained with a Nexus 470Spectrometer (Nicolet). The powders were mixed with KBrto form a pellet. UV-vis spectra were obtained on a Cintra 20Spectrometer (GBC). The diffuse reflectance measurementswere done using the lab sphere RSA-PE-20 accessory usingBaSO4 as reference. In all cases the spectra were recorded inthe 400–800 nm region.

2.3. Photocatalytic Evaluation. The photooxygenation of 9,10-dimethylanthracene (DMA) was carried out in acetoni-trile (HPLC grade) using a Newport solar simulator (Model67005) equipped with a 150 W Xe lamp with a maximumemission around 460 nm. It was used a 10 mL batch reactorfor the evaluation of the SiO2-MB composites and an 80 mLbatch reactor for the evaluation of the different SiO2-dyecomposites. The temperature was kept constant at 298 K, andthe incident light was filtered in order to cut out light below400 nm and eliminate any photochemical reaction of DMA.The initial concentration of DMA was 16 mg/L, and the com-posite loading was 1 g/L or 0.5 g/L. The reaction samples wereanalyzed using HPLC (GBC model 1120) equipped with aUV detector (λ = 254 nm), using a 70/30 acetonitrile/watermobile phase (1 mL/min) and a 15 cm column (Grace PrevailC18 5 μ). The samples were also analyzed using a GC-MS


1 μm

Figure 2: SEM image of SiO2-MB composite; [MB] = 1×10−5 mol/g SiO2.

(Perkin-Elmer model TurboMass) using a 30 m capillary col-umn (Alltech EC5).

3. Results and Discussion

3.1. Characterization Results of SiO2-MB Composites. The in-fluence of the dye over the final particle size of the compositewas determined by DLS. During the polymerization of TEOSthrough the formation of Si–O–Si bonds, the cationic partof MB molecule can interact with part of Si–O groups byelectrostatic forces, increasing the stability of the hybrid sys-tem [17] and leading to an amount of the average size from110 nm (as-prepared SiO2) to 141 and 185 nm by using 1 ×10−6 and 1× 105 mol/g SiO2 MB concentration, respectively.The stability of the SiO2-MB composites was compared ina blank experiment, where a solution of MB in ethanol wasmixed under stirring with the as-prepared SiO2 particlesduring 12 h and then dried under vacuum and 318 K. Theresulting material was rinsed with ethanol, and the MBwas completely washed out, causing a total decolorization.However, the SiO2-MB composite prepared by the modifiedStober method never exhibited this behavior. The remainingethanol used in the SiO2-MB composite washes was analyzedby UV-vis, and the characteristic absorption bands of MBwere not observed, confirming the high stability of thecomposite.

A SEM image of the SiO2-MB composite shows themorphology of the spheres in Figure 2. It can be seenspheres ranging from 100 to 200 nm in size, in some caseswell dispersed but also forming particle agglomerates. Themorphology of the silica spheres (not shown here) wassimilar than that of the composite.

Figure 3 shows the nitrogen adsorption-desorption iso-therm of the SiO2 and the SiO2-MB composite. The shapecorresponds to a type IV isotherm according to the IUPACclassification [22]. The hysteresis loop (H3/H4 types) canbe attributed to the presence of slit-shaped pores and openpores where no condensation was evident. According tothese results, the pore size varies between 10 and 100 nmdia-meter (see Figure 3(a)). The corresponding surface

Table 1: Characteristic bands of different MB species [26].

Species Wavelength (nm) Assignment

(AM+)n 570–590 Trimer and higher aggregates

(AM+)2 600–620 Dimer

AM+ 650–675 Monomer

area was 91 m2/g for SiO2 and 23 m2/g for the SiO2-MBcomposite. Note that slight changes in the composite adsorp-tion-desorption isotherm (Figure 3(b)) caused a dramaticdecrease in specific surface area and a bimodal behavior ofpore size distribution with an important amount of macro-pores. These results could be indicative that MB was in-corporated, probably of irregular form, into the silica matrixmodifying the shape and size of the pores.

FT-IR spectra of MB, SiO2, and SiO2-MB composite([MB] = 1 × 10−5 mol/g SiO2) are presented in Figure 4.The main absorption bands of MB at 1610, 1505, 1405, and1355 cm−1 are assigned to the = N+ cation, the heterocyclicskeleton, and to the −CH3 symmetric and asymmetricbending vibrations, respectively [23, 24] (Figure 4(a)). Theobtained spectrum of SiO2 (Figure 4(b)) reproduces thegeneral features often reported for this compound [24, 25]. Itis worth noting that bands at 1200, 1100, 800, and 460 cm−1

are attributed to Si–O–Si vibrations and band at 960 cm−1

corresponds to the Si–OH vibration. The spectrum also pre-sented a broad band located between 3750 and 3000 cm−1;this can be generated by the hydration of the solid (bandslocated at 3350 and 1630 [24]) or by the presence of SiO–Hvibrations [25]. The FT-IR spectrum of the SiO2-MB com-posite was quite similar than that of SiO2, which can beattributed to the low concentration of MB in the material, sothat the main absorption bands of MB overlap with SiO2

bands (Figure 4(c)). Note that a slight deformation ofSi–O–Si bands at 1200 cm−1 and 3000–3750 cm−1 is detectedin the composite which can be indicative of a bonding inter-action between the organic dye and SiO2 [17].

The visible light absorption spectra of SiO2-MB compos-ites and a mechanical mixture (MB + commercial SiO2) arecompared in Figure 5. In the mechanical mixture sample, astrong band appears at 670 nm and one shoulder at 610 nm(Figure 5(b)). The main band is associated to free moleculesof MB (monomer, see Table 1), and the shoulder is assignedto the formation of the so-called H-aggregates (dimers)[26, 27]. In the diffuse reflectance spectrum of SiO2-MBcomposite [MB] = 1× 10−6 mol/g SiO2 (Figure 5(a)), a wideband appears with a maximum at around 610 nm indicat-ing that the predominant species was MB dimers. If theconcentration of MB is increased to 1 × 10−5 mol/g SiO2,the maximum of absorption is shifted to around 590 nm,which is interpreted as the formation of trimers and higheraggregates [26].

The visible light absorption spectra of the different SiO2-dye composites prepared by the modified Stober methodand the mechanical mixture preparations are shown inFigure 6. In general, each kind of preparations presenteddifferent optical properties. For instance, the compositesprepared by the modified Stober method showed a better


0 0.2 0.4 0.6 0.8 10

50

100

150

200

250

Relative pressure

Vol

um

e ab

sorb

ed (

cm3/g

)

AdsorptionDesorption

0

0.5

1

1 10 100Pore diameter (nm)

Pore

vol

um

e (c

m3/g

)

(a)

0 0.2 0.4 0.6 0.8 10

50

100

150

200

250

Relative pressure

Vol

um

e ab

sorb

ed (

cm3/g

)

0

0.3

0.6

1 10 100Pore diameter (nm)

Pore

vol

um

e (c

m3/g

)

Adsorption

Desorption

(b)

Figure 3: Nitrogen adsorption isotherms: (a) SiO2 material as-synthesized; (b) SiO2-MB composite; [MB] = 1× 10−5 mol/g SiO2.

16101355

14051505

1610

33501200

800

960

1100 460

5001000150020002500300035004000

Tran

smit

tan

ce (

a.u

.) (

%)

Wavenumber (cm−1)

(a)

(b)

(c)

1630

Figure 4: FT-IR spectra of (a) MB powder, (b) as-prepared SiO2,(c) SiO2-MB, [MB] = 1× 10−5 mol/g SiO2.

band definition compared with the wide and irregularprofile of the mechanical mixture bands. Note that a broadabsorption band with a maximum at 490 nm was observedfor commercial SiO2 + SF (Figure 6(d)); in comparisonwith the SiO2-SF composite, a sharper absorption bandappeared at 515 nm (Figure 6(a)). These results indicatedthat, in the first case, dye aggregates were the predominantspecies meanwhile; in the second case there were moremonomeric dye species incorporated into the silica matrix[28, 29]. The SiO2-NR composite and its mechanical mixtureare shown in Figures 6(b) and 6(e). In the mechanicalmixture, a broad absorption band from 400 to 650 nm, with amaximum centered at 505 nm, was previously assigned to theabsorption of H-aggregates (dimers) of the dye [29]. Whenthe dye was incorporated since the formation of the SiO2

matrix, a solvatochromical shift in the absorption maximum

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Abs

orba

nce

(a.

u.)

400 500 600 700 800Wavelength (nm)

[MB] = 1.1 × 10−6 mol/g

610

590

[MB] = 1.1 × 10−5 mol/g400 500 600 700 800

Wavelength (nm)

0

0.3

0.6610

670

Abs

orba

nce

(a.

u.)(a) (b)

Figure 5: Visible light absorption spectra of (a) SiO2-dye nanocom-posites and (b) mechanical mixture of MB + commercial SiO2

nanoparticles.

to 535 nm was absorbed, and a slight shoulder at 480 nmwas also observed. The absorption band associated to thefree molecules of NR is located around 535 nm [29–31], andthe absorption associated with the uncharged form of NRis located around 450–460 nm in methanol [30, 32]. Hence,the predominant species in the SiO2-NR composite was thefree molecules of the dye although the neutral form (pKa =6.8 [30]) could be present, due to basic media used in thepreparation of the materials, causing the observed shoulder.In the case of TB, the mechanical mixture with commercialSiO2 (Figure 6(f)), the absorption spectrum presented an ill-defined band with a maximum centered at 585 nm, producedby the presence of TB dimers [33, 34]. When TB was


0

0.2

0.4

0.6

400 500 600 700 800

515

Abs

orba

nce

(a.

u.)

Wavelength (nm)

(a)

0

0.2

0.4

0.6

400 500 600 700 800

Abs

orba

nce

(a.

u.)

480 535

Wavelength (nm)

(b)

0

0.2

0.4

0.6

Abs

orba

nce

(a.

u.)

615

Wavelength (nm)

400 450 500 550 600 650 700 750 800

(c)

400 500 600 700 800A

bsor

ban

ce (

a.u

.)

0

0.2

0.4

0.6

0.8

1

Wavelength (nm)

(d)

400 500 600 700 800

Abs

orba

nce

(a.

u.)

0

0.2

0.4

0.6

0.8

Wavelength (nm)

(e)

Abs

orba

nce

(a.

u.)

0

0.2

0.4

0.6

0.8

1

Wavelength (nm)

400 450 500 550 600 650 700 750 800

(f)

Figure 6: Visible light absorption spectra of SiO2-dye composites prepared by modified Stober method: (a) SF, (b) NR, (c) TB, and preparedby mechanical mixture with commercial SiO2: (d) SF, (e) NR, and (f) TB.

incorporated into the SiO2 composites (Figure 6(c)), a well-defined band appeared in the absorption spectrum centeredat 615 nm and a slight shoulder also appeared at 495 nm. Theabsorption at 615 nm was produced by the presence of TBmonomers in the composite.

3.2. Photosensitized Oxidation of DMA . In photosensitizedreactions, the photosensitizer molecule absorbs the energyof a photon (hν) of ultraviolet or visible radiation to be-come an excited singlet state which rapidly converts into anexcited triple state. The lifetime of the triplet is longer (mi-croseconds) than that of the singlet (nanoseconds) so thatenergy transfer from the triplet to dissolved oxygen moleculeto form singlet oxygen (1O2) is possible. The amount ofsinglet oxygen generated by a photosensitizer is determinedby the rate of absorption of photons, the triplet quantumyield, and the efficiency of the energy transfer process [35].After singlet oxygen is generated, it either reacts with a

substrate or losses its excitation energy as heat or lightemission (phosphorescence). Scheme 1 shows a reactionscheme considering the visible light irradiation of a SiO2-dye composite in presence of oxygen, ethanol, and the mainsubstrate DMA.

Scheme 1 (reaction scheme of the photosensitized oxidationof DMA on dye-doped silica composites).

SiO2-dye + hν −→ SiO2-dye∗ I , (1)

SiO2-dye∗ + 3O2 −→ SiO2-dye + 1O2 k, (2)

DMA + 1O2 −→ DMA endoperoxide kc, (3)

1O2 −→ 3O2 kd, (4)

where I is the light intensity, k is a rate constant for thequenching of excited SiO2-dye by triplet oxygen to produce


0 20 40 60 80 100 120

Time (min)

0

0.2

0.4

0.6

0.8

1

[DM

A] t

/[D

MA

] 0

0 20 40 60 80 100 120

Time (min)

ln([

DM

A] 0

/[D

MA

] t)

0

2

4(a) (b)

Figure 7: (a) Relative concentration of DMA versus time for SiO2-MB composites (�) [MB] = 1 × 10−6 mol/g SiO2 (�) [MB] = 1 ×10−5 mol/g SiO2. (b) First-order kinetics data fitting.

singlet oxygen, kc is the rate constant of chemical quenchingof singlet oxygen in presence of DMA; however, singlet ox-ygen also decays to the ground state by energy transfer to thesolvent or with other species in solution with a rate constantkd [36]. A rough estimation of the rate constant in (3) canbe obtained by considering that an excess of singlet oxygen isproduced compared with the initial concentration of DMAand then a pseudofirst-order reaction is proposed (5)–(8):

−d[DMA]dt

= kc[DMA][

1O2

]= k′[DMA], (5)

where

k′ = kc[

1O2

]= ΦI

kc

kd, (6)

[DMA]t = [DMA]0e−k′t, (7)

ln[DMA]0

[DMA]t= k′t. (8)

The concentration profiles of DMA, during the photooxida-tion in presence of SiO2-MB, are shown in Figure 7. Notethat raw data fit well to a first-order reaction at two concen-trations of MB, with k′ values of 0.0762 and 0.0154 min−1 for1× 10−5 and 1× 10−6 moles of dye/g of SiO2, respectively.

As shown in Figure 8, the conversion of DMA followed asimilar behavior (first order kinetics) with the different SiO2-dye composites. However, the rate constants had differentvalues, and it is clearly seen that SiO2-MB is not a goodphotocatalyst for this reaction. Note that the k′ values hadthe following increasing order depending of the dye used inthe photooxidation of DMA (Figure 9): SF > TB > NR >MB. These results revealed that the singlet oxygen generationdepended on the type of composite used. Therefore, in theparticular case of SiO2-SF composite instead of presentingalmost the same absorption properties than MB, it presenteda rate constant one order of magnitude higher compared

0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

[DM

A] t

/[D

MA

] 0

Time (min)

(a)

0 20 40 60 80 100 120

Time (min)

ln[D

MA

] 0/[

DM

A] t

0

1

2

3

4

5

(b)

Figure 8: (a) Relative concentration of DMA versus time fordifferent SiO2-composites: (�) MB; (•) NR; (�) TB; (�) SF. (b)First-order kinetics data fitting.

0

5

10

15

20

25

30

35

40

SiO2-MB SiO2-NR SiO2-TB SiO2-SF

k×

10−3

(min

−1)

Figure 9: Rate constant values (k ′) of the photooxidation of DMAwith different SiO2-dye composites.


O O

Product V

Product VICH3

CH3 CH3

CH3

+ 1O2

Figure 10: Reaction path scheme of DMA endoperoxide formationas well as the unidentified V and VI products detected by HPLC andGC-MS (see Supplementary Information available online at doi:10.1155/2012/987606).

with MB which should be interpreted as SF dye was homo-genously dispersed on the silica matrix, as indicated by theUV-vis spectra results (Figures 5 and 6), where the SiO2-SFcomposite presented a sharper absorption band at 515 nm,mainly as a consequence of the presence of monomeric spe-cies of SF in the material. Further studies are in progress inour lab in order to know in detail the properties and be-havior of the SiO2-SF composites for fine chemical synthesisunder visible light irradiation.

Finally, a reaction path scheme showing the formationof DMA endoperoxide and two unidentified byproducts (Vand VI), which were found with all series of dye-doped silicaparticles, is presented in Figure 10. As mentioned before, themain product was the DMA endoperoxide; however, the twounidentified byproducts were detected by HPLC and GC-MS (supplementary information). As is well known, the en-doperoxides are highly instable compounds so that easilydecompose to other oxygenated compounds following aparallel path forming stable products as the unidentified Vand VI byproducts.

4. Conclusion

In this study, a novel procedure (modified Stober method)to obtain stable and active SiO2-dye composites for thephotosensitized oxidation of 9,10-dimethylanthracene wasdeveloped. We found important differences in the mechan-ical mixtures and Stober’s method preparations of our fourdye-doped silica composites than can be attributed to thehigher electrostatic interaction and dispersion of the dyeinto the silica matrix by the second procedure. According tothe results, all dyes had a larger affinity to the matrix andcan be easily incorporated. These findings agreed with thephotocatalytic behavior, and the SiO2-SF composite showedthe most efficient delivery of 1O2. Two byproducts werealso detected during the photooxidation of DMA which areprobably assigned to the decomposition of the endoperoxide.

Acknowledgments

This work was financially supported by the National Councilof Science and Technology (Conacyt) Projects: 106891 and153356. The authors also acknowledge the support ofCOFAA-IPN. E. Albiter thanks Conacyt for the scholarshipsupport.

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[36] W. Tang, H. Xu, R. Kopelman, and M. A. Philbert, “Photody-namic characterization and in vitro application of methyleneblue-containing nanoparticle platforms,” Photochemistry andPhotobiology, vol. 81, no. 2, pp. 242–249, 2005.


Research Article

Photoresponse of Visible Light Active CM-n-TiO2, HM-n-TiO2,CM-n-Fe2O3, and CM-p-WO3 towards Water Splitting Reaction

Yasser A. Shaban1 and Shahed U. M. Khan2

1 Department of Marine Chemistry, Faculty of Marine Sciences, King Abdulaziz University, P.O. Box 80207,Jeddah 21589, Saudi Arabia

2 Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, PA 15282, USA

Correspondence should be addressed to Shahed U. M. Khan, [email protected]

Received 14 August 2011; Accepted 25 September 2011


Copyright © 2012 Y. A. Shaban and S. U. M. Khan. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Photoresponses of visible light active carbon modified titanium oxide (CM-n-TiO2), hydrogen modified titanium oxide (HM-n-TiO2), carbon modified iron oxide (CM-n-Fe2O3), carbon modified tungsten oxide (CM-p-WO3) towards water splitting reactionare reported in this article. Carbon and hydrogen in titanium oxide were found to be responsible for red shift from UV regionto visible region which in turn enhanced the photoconversion efficiency by an order of magnitude for water splitting reaction.Photocurrent densities and photoconversion efficiencies of regular n-TiO2 and CM-n-TiO2 towards water splitting reaction undermonochromatic light illumination from a xenon lamp and sunlight were compared and found in reasonable agreement. Theseoxides were characterized by photocurrent measurements, UV-Vis spectra, scanning electron microscopy (SEM), energy dispersivex-ray spectroscopy (EDS) and x-ray diffraction (XRD) studies and these results are also reported in this article.

1. Introduction

Sunlight is the unlimited source of clean and renewableenergy if it could be efficiently utilized to split waterto hydrogen and oxygen. Numerous studies focused onphotoelectrochemical water splitting reaction [1–36]. Theenhancement of efficiency of a solar hydrogen productionprocess is of considerable importance because high efficiencytranslates into lower costs. It is, therefore, important tosearch for a stable and low-cost material for efficiently har-vesting the solar energy. To use semiconductors as possiblephotoelectrodes, their viability depends on their ability toabsorb enough sunlight as well as their stability againstphotocorrosion. The electronic structure of a semiconductorplays a key role in its photoactivity. Unlike a conductor,a semiconductor consists of a valence band (VB) and theconduction band CB). Energy difference between these twolevels is said to be the bandgap (Eg) [37].

For efficient H2 production using a visible-light-drivensemiconductor (Figure 1), the bandgap should be less than3.0 eV, but larger than 1.23 eV [38–41]. Moreover, theconduction band (CB) and valence band (VB) levels should

satisfy the energy requirements set by the reduction andoxidation potentials for H2O, respectively. For hydrogenproduction, the CB level should be more negative thanhydrogen production level EH2O/H2 while the VB shouldbe more positive than water oxidation level EH2O/O2 forefficient oxygen production from water by photocatalysis(Figure 1). Figure 2 shows the bandgap energies; CB andVB positions at pH 2 for different oxide materials thatcan be used as photoelectrodes. It shows that the CBlevels of SrTiO3 and KTaO3 are above H+/H2 but theVB levels of these oxides are below H2O/O2 redox energylevel. This means that only the SrTiO3 and KTaO3 canphotosplit water without external bias. The TiO2 needs atleast 0.3 volt of external bias to photosplit water. How-ever, most of these oxides have very high bandgap energy(>3.1 eV) and hence cannot absorb the visible light of solarspectrum.

1.1. Mechanism of Hydrogen Production from Water. Pho-toelectrolysis of water in a photoelectrochemical cell (PEC)involves several processes within photoelectrodes and at thephotoelectrode/electrolyte interface, including the following.


0

+1

+2

1.23 V

O2

OH−

H+

H2e−

h+

Conduction band

Valence band

V/NHE

hA ≥EH2O/H2

EH2O/O2

Eg

Eg

Figure 1: Basic principle of overall water splitting on a cocatalyst-loaded semiconductor.

Autoionization of Water Molecules. In an electrolyte solution,water molecules dissociates into H+ and OH−:

4H2O −→ 4H+ + 4OH−. (1)

Light-Induced Excitation. When a semiconductor electrodeis illuminated by a light source or directly by solar radiationthat can provide the electrode with photonic energy (hν)greater than its bandgap energy (Eg), then an electron fromthe valence band will be excited into the conduction band,and correspondently, a positively charged hole will be left inthe valence band:

photoanode + sunlight −→ 4h+ + 4e−. (2)

Reduction of Hydrogen Ions. The electrons generated as aresult of (2) are transferred over the external circuit tocathode, resulting in the reduction of hydrogen (H+) ions togaseous hydrogen:

4H+ + 4e− −→ 2H2(gas) (3)

at cathode.

Oxidation of Hydroxyl Ions. The light-induced holes from(2), at the photoanode/electrolyte interface, oxidize thehydroxyl (OH−) ions to gaseous oxygen as

4OH− + 4h+ −→ O2(gas) + 2H2O (4)

at photoanode.The overall reaction will be

photoanode + sunlight + H2O −→ 12

O2(gas) + H2(gas).

(5)

1.2. Development of Photoelectrodes for Water Splitting.Among the photoelectrode materials, titanium dioxide (n-TiO2) was found to be the most promising because of itslow cost, chemical inertness, nontoxicity, and photostability.However, its wide bandgap (3.0–3.2 eV) limits its photore-sponse in the ultraviolet region which is only a small fraction(∼5%) of the sun energy compared to visible light from

400 nm to 750 nm (∼49%). Hence, any shift in the opticalresponse of n-TiO2 from the UV to the visible spectral rangewill have a profound positive effect on the photocatalyticefficiency of the material. Several attempts were made tolower the bandgap of n-type titanium oxide (n-TiO2) bytransition metal dopants [42, 43] but no noticeable changein bandgap energy of n-TiO2 was observed. The visiblelight absorption by the transition metal-doped n-TiO2 wasfound to be mainly due to d-d transition of electrons in thetransition metal dopants but not due to bandgap lowering.The transition metal dopants acted adversely on photocat-alytic activity of n-TiO2 because they acted as recombinationcenters for the photogenerated electron-hole pairs.

However, recent studies involved doping of n-TiO2

by carbon [1, 15–24, 44–49], nitrogen [50, 51], sulphur[52], also fabricating the carbon-modified (CM)-n-TiO2 inmesoporous [53, 54], and nanostructure [55, 56] forms.Photocatalytic activity of sulphur-doped n-TiO2 was foundto diminish under UV and visible light illuminations due tocatalytic poisoning induced by oxidation of sulfur to SO2 andSO2−

4 . Photocatalytic activity of nitrogen-doped n-TiO2 wasreported to be much lower than carbon-doped n-TiO2 [44].

It was found that carbon modification of n-TiO2 photo-catalyst synthesised by thermal oxidation of Ti metal sheetin a natural gas flame lowered the bandgap energy of n-TiO2 to 2.32 eV and exhibited water splitting to hydrogen andoxygen with a photoconversion efficiency of 8.35% [1] underartificial light illumination from a Xenon lamp. This progressstimulated further investigation into carbon-modified n-TiO2 (CM-n-TiO2) as visible light active photocatalysts [43–49] and also as photoelectrodes [14–24, 55–59] for water-splitting reaction with enhanced photoconversion efficiency.Xu and Khan [16] also showed enhanced rate of watersplitting at spray pyrolytically synthesised visible light activecarbon-doped n-TiO2 thin films. Enhanced photoresponsefor water splitting at nanocrystalline carbon-doped n-TiO2

thin films was also reported by Xu et al. [17]. Argonwas found to be the most effective calcining ambient toenhance the carbon doping of n-TiO2 films. The resultantcarbon-modified n-TiO2 thin film calcined in Ar at theoptimum temperature of 700◦C contained 3.8 at % carbonand showed higher photoresponse in both UV and visibleregions compared to those calcined in air at the optimumtemperature of 500◦C that contained 2.3 at % carbon.Carbon content in these samples calcined in presence ofoxygen in air reduced due to removal of some carbon in theforms of CO and CO2. This wet process synthesis allowedmolecular scale mixing of the carbon source with precursorof TiO2 for improved doping homogeneity and was capableof tuning the carbon content in the film.

Importantly, Mohaputra et al. [18] synthesised CM-n-TiO2 thin films by flame oxidation of Ti metal sheet andreported photoconversion efficiency of 8.5% for water split-ting which is slightly higher than earlier results [1]. Carbondoping was considered to be responsible for the lowering ofthe bandgap of n-TiO2 and consequent high photocatalyticactivity under visible light illumination. Noworyta andAugustynski [19] observed shift in the spectral response (upto 425 nm) during water-splitting reaction for carbon-doped


En

ergy

(re

lati

ve to

vac

uu

m le

vel)

(eV

)

pH = 2

En

ergy

(re

lati

ve to

NH

E)

(eV

)

O2/H2O

−1

−4

−5

−6

−7

−8

−9

0

1

2

3

4

H+/H2

TiO

2,E

g=

3.2

eV

PbO

,Eg=

2.8

eV

WO

3,E

g=

2.7

eV

SrT

iO3,E

g=

3.7

eV

BaT

iO3,E

g=

3.3

eV

KTa

O3,E

g=

3.5

eV

Bi 2

O3,E

g=

2.9

eV

FeT

iO3,E

g=

2.9

eV

Mn

TiO

3,E

g=3

.1 e

V

SnO

2,E

g=

3.5

eV

Fe2O

3,E

g=

2.3

eV

Figure 2: Diagram showing bandgap energy of different oxide materials and relative energies in terms of vacuum level and normal hydrogenelectrode level in electrolyte of pH = 2 [69].

n-TiO2 film electrodes formed in the flame of a burner fedwith various gas mixtures. Shankar et al. [57] investigatedthe effect of flame annealing on the spectral photoresponseof titania nanotubes and demonstrated the enhancementof the visible light absorption and the photocurrent forwater splitting due to annealing in the flame. However, intheir study, the flame annealing was carried out at 1020◦Cwhich is much higher than the optimum temperature of850◦C observed earlier [1]. Such high temperature generallyremoves considerable amounts of carbon from the sampleand thus reduces its photoresponse. Given the sensitivity ofCM-n-TiO2 to flame temperature, Shankar et al. [57] and Xuet al. [59] prepared highly ordered n-TiO2 nanotube arraysby electrochemical anodization of Ti metal sheet followedby calcinations in an electric oven and further oxidation innatural gas flame at optimum temperature to incorporatecarbon in it. Significant enhancement of the photoresponseof these carbon-modified (CM) n-TiO2 nanotube films forwater splitting was also reported [58, 59]. Hahn et al.[60] prepared carbon-doped self-organized TiO2 nanotubeslayers formed by electrochemical anodisation of Ti in anHF/Na2HPO4 electrolyte. The tubes were treated at 500◦Cunder a mixed flux of N2 and acetylene (C2H2). Nakanoet al. [61] prepared TiO2:C films by oxidative annealingof sputtered TiC films. They reported three bands havingenergies 0.86 eV, 1.30 eV, and 2.34 eV below the conductionband of carbon-doped n-TiO2. They attributed the 0.86 eVlevel to the intrinsic nature of TiO2, whereas the 1.30 eV andthe 2.34 eV levels were newly introduced by C-doping andbehaved as deep-level bands. In particular, the pronounced2.34 eV band contributed to bandgap narrowing by mixingC 2p with the O 2p valence bands. Therefore, the 2.34 eVlevel played a significant role for the visible-light sensitivityin TiO2:C. Ren et al. [62] synthesised a visible light activeTiO2 photocatalyst by carbon doping using glucose as carbonsource. The preparation was performed by a hydrothermal

method at temperature as low as 160◦C. The carbon-dopedn-TiO2 showed absorption in the 400–450 nm range witha red shift in the bandgap transition. Sun et al. [63]reported that carbon-doping-made In2O3 red shifted fromUV to visible range up to 500 nm and 40% contributionfrom visible light was observed for the photoelectrochemicalsplitting of water to hydrogen. Cardenas et al. [64] reportedmarked decrease in resistivity of carbon-doped antimonysulfide (Sb2S3) thin films to 102Ω cm compared to 108Ωcm for undoped samples, and electrical resistivity was alsopossible to tune by controlling the carbon content (wt %)in the sample. The bandgap energy the carbon-doped Sb2S3

thin films was found to decrease to 1.7 eV from 2.57 eV forundoped sample. This result indicates that carbon doping ofSb2S3 is not only capable of bandgap lowering but can alsoenhance the conductivity of the sample.

In an important theoretical study, Nie and Sohlberg [65]reported the lowering of the bandgap of n-TiO2 to 2.32 eV(535 nm) due to carbon incorporation and predicted thatthe bandgap value of 1.58 eV may be possible to achieveby some complex carbon incorporation. Interestingly, itwas found experimentally that enhanced carbon dopinglowered the bandgap of n-TiO2 up to 1.45 eV [16]. Wangand Lewis [66, 67] addressed theoretically the effects ofcarbon dopants concentration on the photoresponse ofn-TiO2 in the visible-light region. They found that thesubstitutional and interstitial carbon dopants incorporatedinto TiO2 drastically affected the electronic structure of thematerial, thus improving its photoactivity. They predictedthe low bandgap of 2.35 eV for carbon-doped TiO2 as itwas experimentally observed earlier [1]. The theoreticalfindings of Di Valentin et al. [68] revealed the presenceof substitutional and interstitial carbon in carbon-modified(CM)-n-TiO2 (Figure 3) which were found to be responsiblefor the lowering of its main bandgap as well as generating amid-gap band.


CS–O

(a)

CS–Ti

(b)

CI

(c)

CI + VO

(d)

Figure 3: Partial geometry of the models for (a) one substitutionalC atom to O (CS−O), (b) one substitutional C atom to Ti (CS−Ti), (c)one interstitial C atom (CI), and (d) one interstitial C atom nearbyan oxygen vacancy (CI+VO) in the rutile supercell. The yellow spheresrepresent O atoms, the small brown spheres represent Ti atoms, andthe black sphere presents the carbon impurity [68].

Among various oxide semiconductor photocatalysts, iron(III) oxide (Fe2O3) is a low-cost semiconductor with highstability. Iron (III) oxide has a bandgap of 2.0–2.3 eV;therefore, it can absorb solar radiation up to 620 nm, whichcovers ∼37% of the photons of the solar spectrum of globalAM 1.5 (1 sun) [70]. However, challenges are set by itsquite low photoresponse due to its high resistivity andconsequent recombination of photogenerated carriers. Tominimise these limitations, it has been reported that dopants,either n-type [4, 7, 26, 27] or p-type [28–30], improvedthe photoresponse of iron (III) oxide (Fe2O3) towards watersplitting to hydrogen. Thin films of n-Fe2O3 were extensivelystudied [4, 7, 25–36]. For example, n-Fe2O3 films weresynthesised by a sputtering method [30], by pressing powdersof Fe2O3 [32–34], and by spray pyrolysis method [4, 7, 28–30, 32]. Silicon-doped Fe2O3 films synthesised by ultrasonicspray pyrolysis showed 1.0 mA cm−2 at +0.1 V versus AgCl[36] under solar-simulated light illumination at 1 sun, AM1.5 (100 mW cm−2).

Tungsten trioxide (WO3) is also of great interest and hasbeen investigated extensively due to its promising physicaland chemical properties [70–73] The WO3 films are nontoxicand have relatively low price. Semiconducting tungsten oxide(WO3) is an interesting candidate for photocataylsis becauseof its relatively low bandgap energy (Eg = 2.6 eV), [74–76] resulting in possible utilization of 12% of the solarspectrum [77]. Spectral response studies were carried outfor n-type WO3 by different workers [78, 79]. For example,n-WO3 films were synthesized by conventional evaporationtechniques [80, 81] and, more recently, with the sol-gelprocess [82, 83]. It is important to note that regular undopedtungsten oxide is an n-type semiconductor, but when it

is carbon-doped or carbon-modified, it was found to bea p-type tungsten oxide. Carbon-modified p-type tungstenoxide (CM-p-WO3) films were synthesized by thermal flameoxidation method at several flame temperatures for differentlengths of time [84].

In this paper, we summarize our studies on thin-filmphotoelectrodes of the following

(i) visible light active carbon-modified titanium oxides(CM-n-TiO2);

(ii) visible light active hydrogen modified n-type tita-nium oxide (HM-n-TiO2) thin films;

(iii) carbon-modified iron oxides (CM-n-Fe2O3) thinfilms;

(iv) visible light active carbon-modified p-type tungstenoxides (CM-p-WO3) thin film.

The results of photocurrent density measurements onthese photoelectrodes for water splitting to H2 and O2

and the maximum photoconversion efficiencies under bothXenon lamp and natural sunlight of global AM 1.5 illu-minations are also provided. Synthesized thin-film photo-electrodes are characterized in terms of bandgap energies,X-ray diffraction pattern, scanning electron micrograms,and carbon contents to correlate with their photoresponsetowards water-splitting reaction.

2. Experimental Details and Results

2.1. Visible Light Active Carbon-Modified Titanium Oxides(CM-n-TiO2) Thin Films. Shaban and Khan [23] synthe-sized visible light active carbon-modified n-type titaniumoxide (CM-n-TiO2) thin films by flame oxidation of Ti metalsheets both flat and grooved and also by using combinationof spray pyrolysis and flame oxidation methods. Also un-doped reference n-TiO2 samples were synthesized in an elec-tric oven for comparison. The flame was custom designedby Knight Co. The flame temperature was controlled byadjusting the flow rates of oxygen and natural gas.

2.1.1. Dependence of Photocurrent Density on Measured Elec-trode Potential for Titanium Oxides. Plots of photocurrentdensity ( jp, mA cm−2) as a function of measured potential,Emeas (V/SCE) for oven-made n-TiO2 synthesized at 825◦Cfor 16 min (sample 1); CM-n-TiO2 synthesized by onlythermal flame oxidation of Ti at 825◦C for 16 min (sample2); grooved CM-n-TiO2 synthesized by only thermal flameoxidation of Ti (0.005 inch groove depth) at 820◦C for 18 min(sample 3); CM-n-TiO2 synthesized by spray pyrolysis of0.175 M TiCl4 in ethanol at substrate temperature of 460◦Cfor 20 sec at carrier gas (oxygen) pressure of 20 psi followedby thermal flame oxidation at 825◦C for 16 min (sample4) are shown in Figure 4. Sample 1 shows the lowest pho-tocurrent density of 1.6 mA cm−2 measured at –0.4 V/SCE.The observed low photocurrent density for sample 1 wasdue to absence of carbon as well as lower thickness insuch electric oven-made samples. Sample 4 generated thehighest photoresponse compared to samples, 1, 2, and 3.


0 0.20

20

18

16

14

12

10

2

4

6

8

(4)

(3)

(2)

(1)

14.68 mA cm−2

11.44 mA cm−2

1.6 mA cm−2

−1 −0.8 −0.6 −0.4 −0.2

Emeas (V/SCE)

9.17 mA cm−2

Ph

otoc

urr

ent

den

sity

,J p

(mA

cm−2

)

Figure 4: Photocurrent density, jp (measured under light intensityof 100 mW cm−2 from a 150 Watt Xenon lamp) as a function ofmeasured potential, Emeas (V/SCE), for oven-made n-TiO2 synthe-sized at 825◦C for 16 min (sample 1); CM-n-TiO2 synthesized byonly thermal flame oxidation of Ti at 825◦C for 16 min (sample 2);grooved CM-n-TiO2 synthesized by only thermal flame oxidationof Ti (0.005 in groove depth) at 820◦C for 18 min (sample 3); CM-n-TiO2 synthesized by spray pyrolysis of 0.175 M TiCl4 in ethanolat substrate temperature of 460◦C for 20 sec at carrier gas (oxygen)pressure of 20 psi followed by thermal flame oxidation at 825◦C for16 min (sample 4); electrolyte solution of 5 M KOH was used. Theelectrode potentials at open circuit condition under illumination,Eaoc, were found to be −0.977 V/SCE (sample 1), −0.940 V/SCE(sample 2), −0.942 V/SCE (sample 3), and −0.915 V/SCE (sample4) [21–23].

Photocurrent density, measured at –0.7 V/SCE, increased sig-nificantly from 9.17 mA cm−2 for sample 2 to 11.44 mA cm−2

for sample 3. The surface grooving of Ti metal increasedthe effective surface area of oxide layer in sample 3 andconsequently generated higher photocurrent density. A sharpincrease in photocurrent density to 14.68 mA cm−2 measuredat –0.7 V/SCE for sample 4 can be attributed to increasedthickness of the oxide layer synthesized by combinationof spray pyrolysis and flame oxidation methods. This isbecause increased thickness allows absorption of more lightto generate more electron-hole pairs (excitons).

2.1.2. Dependence of Photoconversion Efficiency on AppliedPotential for CM-n-TiO2 Films. The calculation of totalpercent photoconversion efficiency (%εphoto(total)) of lightenergy under white light illumination to chemical energyin the presence of an external applied potential (Eapp) wascarried out using the equation given earlier [1, 4] as

%εphoto(total) =jp[Eo

rev −∣∣∣Eapp

∣∣∣]Po

× 100, (6)

where jp is the photocurrent density (mA cm−2), Eorev is

the standard reversible potential (which is 1.23 V for watersplitting reaction), Po is the power density of incident light

0 0.2 0.4 0.6 0.8 1 1.20

2

4

6

8

10

12

14

16

(4)

(3)

(2)

(1)1.2%

14.04%

11.31%

9.08%

−0.2

Ph

otoc

onve

rsio

n effi

cien

cy (

%ε p

hot

o(t

otal

))

Eapp (V versus Eaoc)

Figure 5: Dependence of photoconversion efficiency,%εphoto(total), on applied potential Eapp (V versus Eaoc), for samples1, 2, 3, and 4 [21–23].

(mW cm−2), |Eapp| is the absolute value of the appliedpotential which is obtained as [1]

Eapp = (Emeas − Eaoc), (7)

where Emeas (versus SCE) is the potential at which thephotocurrent density was measured, Eaoc (versus SCE) is theelectrode potential at open circuit condition in the sameelectrolyte solution under the same illumination intensity atwhich the photocurrent density was measured. The valuesof Eaoc (versus SCE) were found to be −0.847 V, −0.940 V,−0.942 V, −0.915 V for samples 1, 2, 3, and 4, respectively.

The corresponding maximum photoconversion efficien-cies (see Figure 5) were found to be 1.2% at 0.447 V bias,9.08% at 0.240 V bias, 11.31% at 0.242 V bias, and 14.04%at 0.215 V bias for samples 1, 2, 3, and 4, respectively, underillumination intensity of 100 mW cm−2cm−2 from a 150 wattXenon lamp.

2.1.3. Monochromatic Photocurrent Density-WavelengthDependence for CM-n-TiO2 Films. The plots of monochro-matic photocurrent density, jp(λ), as a function ofwavelength of light, λ, for samples 1, 2, 3, and 4 areshown in Figure 6. The monochromatic photocurrentdensity, jp(λ), was measured at measured potentialscorresponding to applied potential at which maximum%εphoto(total) was observed (see Figure 5) under whitelight illumination. The values of Eapp used were 0.477 V,0.240 V 0.242 V, and 0.215 V for samples 1, 2, 3, and 4,respectively. Performing integration under the curve, totalphotocurrent densities of 1.25 mA cm−2, 8.95 mA cm−2,11.30 mA cm−2, and 13.58 mA cm−2 were found for samples1, 2, 3, and 4, respectively. These values are a little lower thanthe corresponding photocurrent density values under whitelight illumination from the Xenon lamp because of loss ofsome light passing through the monochromator.

Note that the observed photocurrent in the visible regionfor the CM-TiO2 samples cannot be due to artifact of


300 400 500 600 700 800

4

3

2

1

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

(1) Jp flat by oven = 1.25 mA cm−2

(2) Jp flat by flame = 8.95 mA cm−2

Wavelength, λ (nm)

(3) Jp grooved by flame = 11.3 mA cm−2

(4) Jp flat by spray and flame = 13.58 mA cm−2

Ph

otoc

urr

ent

den

sity

(J p

(λ),

(m

A c

m−2

)

Figure 6: The monochromatic photocurrent density, jp(λ), as afunction of wavelength of light, λ, for samples 1, 2, 3, and 4,respectively [21–23].

a monochromator or due to second harmonic generationbecause if that would be the case the regular n-TiO2

would have shown photocurrent in the visible region.Figure 6 clearly shows no photocurrent for regular n-TiO2

in the wavelength region between 500 nm 750 nm. Recentcombined sputtering and XPS studies showed that carbon

incorporation was limited to few angstroms (15 Á to 20 Á)deep. Hence, the amounts of visible light absorption were lowas demonstrated by low photocurrent in the visible regionbetween 500 nm to 750 nm (see Figure 6).

2.1.4. Photoconversion Efficiency from Monochromatic Pho-tocurrent Density for CM-n-TiO2 Films. Alternatively, thepercent total photoconversion efficiency (%εphoto(total)) oflight energy to chemical energy can be obtained by using thefollowing equation [20, 21, 68, 70]:

%εphoto(total) =∫ λg

λminjp(λ)

[Eo

rev −∣∣∣Eapp(λ)

∣∣∣]dλ∫∞λmin

Po(λ)dλ× 100,

(8)

where jp(λ) is wavelength-dependent photocurrent densityunder monochromatic light illumination (mA cm−2 nm−1)and Po(λ) is the power density of incident wavelength-dependent monochromatic light (mW cm−2 nm−1). Notethat the applied potential, Eapp(λ), was also found to bewavelength-dependent and can be expressed as

Eapp(λ) = Emeas − Eaoc(λ), (9)

where Eaoc(λ) is the electrode potential in volts at open-circuit conditions under monochromatic light illumination.However, λmin = 250 nm for light from Xenon lamp and

300 400 500 600 700 8000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

1

34

2

(1) %εphoto (total) flat by oven = 0.94%(2) %εphoto (total) flat by flame = 8.86%(3) %εphoto (total) grooved by flame = 11.16%(4) %εphoto (total) flat by spray and flame = 13.79%

Ph

otoc

onve

rsio

n effi

cien

cy (

%ε p

hot

o)

Wavelength, λ (nm)

Figure 7: The photoconversion efficiency, %εphoto(λ), as a functionof wavelength of light, λ, for samples 1, 2, 3, and 4 [21–23].

300 nm for sunlight were used, and λg = 1239.85/Eg is thethreshold wavelength corresponding to bandgap energy, Eg ,of the sample in electron volt unit.

Note that wavelength-dependent percent photoconver-sion efficiency, %εphoto(λ) can be expressed as [20, 21],

%εphoto(λ) =jp(λ)

[Eo


∣∣∣]∫ λmaxλmin Po(λ)dλ

× 100. (10)

The plot of %εphoto(λ) versus wavelength, λ, using (10) isgiven in Figure 7 where total intensity in the denominator of(10) was taken as 100 mW cm−2. Performing the integration(as given in (8)) from λmin = 250 nm (for Xenon lamp whitelight) to λg nm or determining the area under the curvein Figure 7 obtained using (10), the total photoconversionefficiencies of 0.94%, 8.86%, 11.16%, and 13.79% wereobtained for samples 1, 2, 3, and 4, respectively. Notethat λmax represents the maximum wavelength available insunlight which is 4200 nm. These values are little lowerbut comparable to 1.2%, 9.08%, 11.31%, and 14.04%(when calculated using (6) under white light illuminationof intensity 100 mW cm−2 from a 150 W Xenon lamp) forsamples 1, 2, 3, and 4, respectively. These lower values can beattributed to loss of some light through the monochromatoras well as the use of average value of Eapp(λ) instead of usingits experimentally determined value at each wavelength, λ.

2.1.5. Bandgap Energy Determination for n-TiO2 and CM-n-TiO2 Films. The bandgap energy, Eg , can be determinedfrom the frequency of light, ν, dependent quantum efficiency,η(ν), using the following equation [4, 5, 7]:

η(ν)hν = A(hν− Eg

)n, (11)


0

0.1

0.2

0.3

0.4

0.5

0.6

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4

Flat by ovenFlat by flame

Grooved by flameFlat by spray and flame

hA (eV)

(ηhA)1/

2

Figure 8: The respective plots of [η(λ)hν]1/2 versus hν to determinethe bandgap for samples 1, 2, 3, and 4 [21–23].

where A is a constant, n equals either 0.5 for allowed directtransition or 2 for allowed indirect transitions, hν is thephoton energy of frequency ν. The wavelength, λ, dependentquantum efficiency, η(λ), or incident photon conversionefficiency, IPCE(λ), can be expressed as [20, 21]

η(λ) = IPCE(λ) = jp(λ)

[eIo(λ)]

=[

1239.85(Vnm) jp(λ)(mA cm−2nm−1

)][λ(nm)Po(λ)(mW cm−2nm−1)]

,

(12)

where e is the electronic charge and the incident photonflux, and the wavelength-dependent photon flux, Io(λ),(cm−2 nm−1 s−1) was expressed as [20, 21]

Io(λ) = Po(λ)λ

hc

Io(λ) =[Po(λ)

(mW cm−2nm−1

)λ(nm)

][1239.85(V nm)e]

,

(13)

where h is the Planck’s constant and c is the velocity of light.Figure 8 shows [η(λ)hν]1/2 versus hν plot for samples 1,

2, 3, and 4, respectively. From the intercepts of the straightlines the bandgaps were obtained. It shows two indirectbandgap energies for all CM-n-TiO2 photoelectrodes. Thefirst bandgap energy of 2.65 eV corresponds to lowering oforiginal bandgap (samples 2, 3, and 4) and second bandgapwith values of 1.6 eV and 1.4 eV can be attributed to mid-gap band generated by carbon doping as shown in theschematic diagram in Figure 9. Theoretical analysis [65, 66,70] and experimental findings [20, 21] also showed multiplebandgaps in CM-n-TiO2. Oven-made n-TiO2 showed onlyone bandgap of 2.9 eV, and as a result, poor photoresponsewas obtained. Note that the first bandgap energy valuesfor all CM-n-TiO2 samples were found equal. The slopesof bandgap plots for sample 4 were found to be the

2.65 eV

h+h+

h+

e−

e− e−

1.23 eV

H2O/H2

O2/H2O

Ev

Ec

E f

CM-n-TiO2

Semiconductor electrolyte interface

1.4 eV

Mid-gap band

hA

5 M KOH

Figure 9: A schematic diagram of valence, conduction, and mid-gap bands to show the bandgaps in CM-n-TiO2 films synthesizedby thermal flame oxidation of Ti metal sheet [21–23].

highest, which is in accordance with the highest observedphotoresponse generated by this sample.

2.1.6. Spectral Distribution of Light for Global AM 1.5 NaturalSunlight (1 Sun) and Xenon Lamp Light (100 mW cm−2). Itis important to compare the spectral distribution of lightfrom both Xenon lamp and sunlight. Figure 10 compares thespectral distribution of these two lights. The integrated valueof the intensity from 300 up to 800 nm for global sunlightAM 1.5 (1 sun = 100 mW cm−2 measured by NREL [85])was found to be 59.00 mW cm−2 clearly indicating that thecontribution from remaining light from 800 nm to 4100 nmpresent in global sunlight will be 41.00 mW cm−2 [= (100–59) mW cm−2]. Hence, total Xenon lamp light comparable tosunlight (1sun) will be 98 mW cm−2 [= (57 + 41) mW cm−2]where 57.00 mW cm−2 is the integrated value of intensityfrom 200 nm to 800 nm for Xenon lamp light.

Note that the spectral distribution of Xenon lamp lightbegins below 250 nm compared to global AM 1.5 sunlightthat begins at 300 nm. Furthermore, the spectral distributionof Xenon lamp light is higher in the UV region, butin the visible region, it is lower than that of sunlight.Hence, one will expect comparable photocurrent densityand photoconversion efficiencies for CM-n-TiO2 samplesfor water splitting under actual global sunlight illuminationof AM 1.5 (1 sun) and the Xenon lamp illumination ofintensity of 100 mW cm−2. However, relatively higher valuesfor photocurrent density and photoconversion efficiencyunder Xenon lamp illumination will be expected comparedto those under sunlight for CM-TiO2 due to higher spectraldistribution of the former in the UV region than in the visibleregion (see Figure 10) and that the absorption coefficient oflight for CM-TiO2 in UV region is much higher comparedto that in the visible region. Also, only 1.5 to 2.0 nm deepcarbon modified region in CM-n-TiO2 could not absorbenough light in the visible region.

2.1.7. Photocurrent Density and Photoconversion Efficiency forTitanium Oxides from IPCE(λ) Using Illumination Intensity,Io(λ) of Natural Global AM 1.5 Sunlight. Utilizing the valuesof the wavelength-dependent incident photon conversion


250 300 350 400 450 500 550 600 650 700 750 8000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

(2)

(1)

(1) Global AM 1.5(2) Xenon lamp

Wavelength, λ (nm)

Inte

nsi

ty (

mW

cm−2

nm−1

)

Figure 10: The wavelength-dependent intensity of light from a150 W Xenon lamp at intensity of 100 mW cm−2, measured byusing silicon photon detector (UDT Sensors. Inc., Model 10DP/SB),compared to the wavelength-dependent intensity of natural globalAM 1.5 sunlight in the wavelength range from 300 to 800 nmobtained from NREL website [85].

efficiency IPCE (λ) or quantum efficiency η(λ) of thesesamples, it is possible to determine the photocurrent density,

jsunlightp (λ), under natural global AM 1.5 sunlight illumina-

tion as [20, 21]

jsunlightp (λ) = IPCE(λ)eI

sunlighto (λ), (14)

where Isunlighto (λ) is the wavelength-dependent photon flux of

incident global AM 1.5 sunlight which can be obtained using

(13) in which photon power density of sunlight Psunlighto (λ)

was used from the tabulated data given in National Renew-able Energy Laboratory (NREL) website [85].

The plots of jsunlightp (λ) [data obtained from (14)] versus

the wavelength of light, λ, for samples 1–4 are given inFigure 11. The integrated values of total current densityunder sunlight illumination were found to be 0.85 mAcm−2, 5.89 mA cm−2, 7.71 mA cm−2, and 12.27 mA cm−2 forsample 1, 2, 3, and 4, respectively.

The total photocurrent density, jsunlightp (total), under

sunlight illumination can be expressed as [20, 21]

jsunlightp (total) = e

∫ λg

λminIPCE(λ)Io(λ)dλ. (15)

The total photoconversion efficiency under natural sunlightillumination can be expressed as [20, 21]

%εsunlightsolar (total) =

∫ λg

λminj

sunlightp (λ)

[Eo


∣∣∣]dλ∫ λmaxλmin

Psunlighto (λ)dλ

× 100,(16)

300 400 500 600 700 8000

2

4

6

8

10

12

14

16

18

43

2

1

Wavelength, λ (nm)

×10−2

Jligh

tp

(λ)=

(IP

CE

(λ)×e×Ilig

ht

o(λ

))

(1) Jsunlightp flat by oven = 0.85 mA cm−2

(2) Jsunlightp flat by flame = 5.89 mA cm−2

(3) Jsunlightp grooved by flame = 7.71 mA cm−2

(4) Jsunlightp flat by spray and flame = 12.27 mA cm−2

Figure 11: Wavelength-dependent photocurrent density under

global sunlight illumination, jsunlightp (λ) = [IPCE(λ) × e ×

global sunlight intensity, Isunlighto (λ)], versus wavelength, λ of inci-

dent sunlight for samples 1, 2, 3, and 4 (14) [23].

where the denominator was taken as 100 mW cm−2 for actualnatural global AM 1.5 sunlight (1 sun).

Hence, using (15) and the denominator in (16) as100 mW cm−2 for global AM 1.5 sunlight, one can expressthe simplified form of (16) as

%εsunlightsolar (λ)

=j

sunlightp (λ) mA cm−2

[1.23−

∣∣∣Eapp

∣∣∣average

volt]

100 mW cm−2

× 100,(17)

%εsunlightsolar (total)

=j

sunlightp (total) mA cm−2

[1.23−

∣∣∣Eapp

∣∣∣average

volt]

100 mW cm−2

× 100,(18)

where Eorev = 1.23 volt was used for water-splitting reaction

and an average value of the absolute value of appliedpotential, |Eapp|average, was used for all wavelength, λ values.

Photoconversion efficiencies under sunlight illuminationwere obtained using (16) or (18). Figure 12 shows thecalculated wavelength-dependent percent photoconversion

efficiencies under sunlight illumination, %εsunlightsolar (λ), as

a function of wavelength of incident light λ using (17).


300 400 500 600 700 8000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

43

2

1Ph

otoc

onve

rsio

n effi

cien

cy (

%ε p

hot

o(λ

))

Wavelength, λ (nm)

(1) εsunlight flat by oven = 0.67%(2) εsunlight flat by flame = 5.63%(3) εsunlight grooved by flame = 7.62%(4) εsunlight flat by spray and flame = 12.26%

Figure 12: Wavelength-dependent photoconversion efficiency

under sunlight illumination, %εsunlightsolar (λ), versus wavelength, λ of

incident light for samples 1, 2, 3, and 4 (17) [21–23].

Performing the integration under the curve from 300 nm toλg, photoconversion efficiencies of 0.67%, 5.63%, 7.62, and12.26% were found for samples 1, 2, 3, and 4, respectively.These results indicate that CM-n-TiO2 thin films synthe-sized by combination of spray pyrolysis and natural gasflame oxidation (sample 4) can be expected to split waterwith ≥10% even under actual sunlight illumination. Thephotoconversion efficiency values for these 4 samples underdifferent illumination conditions are summarized in Table 1.

2.1.8. UV-Vis Spectra of n-TiO2 and CM-n-TiO2 Films. TheUV-Vis absorbance of n-TiO2 and CM-n-TiO2 photoelec-trodes is shown in Figure 13. The absorption spectra ofCM-n-TiO2 samples 1, 2, 3, and 4 demonstrate a wideabsorption in the UV and visible regions with a tail extendingto near infrared region up to 800 nm. This indicates lowbandgap energy values for these samples. The referenceoven-made n-TiO2 (sample 1) shows absorption only upto 415 nm which corresponds to bandgap energy of 3.0 eV.The lower bandgaps of CM-n-TiO2 samples may be dueto carbon incorporation during the flame oxidation of Timetal sheet. Note that carbon modification also generatesoxygen vacancy by partial reduction of TiO2 and this internmay be partially responsible for visible light activity ofcarbon-modified titanium oxide (CM-n-TiO2). The UV-Visspectroscopic results show higher absorption in UV as wellas in the visible regions for CM-n-TiO2 films synthesized bycombination of spray pyrolysis and flame oxidation (sample3) compared to sample 2. Also, these UV-Vis spectra areconsistent with the higher photocurrent densities at CM-n-TiO2 thin-film electrodes prepared by the combinationof two methods. Importantly, this observation conforms to

Abs

orba

nce

(a.

u.)

350 400 450 500 550 600 650 700 750 800

(1)

(2)

(3)(4)

Wavelength, λ (nm)

(1) Flat by oven (εphoto (total) = 1.2%)(2) Flat by flame (εphoto (total) = 9.08%)(3) Grooved by flame (εphoto (total) = 11.31%)(4) Flat by spray and flame (εphoto (total) = 14.04%)

Figure 13: UV-Vis Spectra of titanium oxides (samples 1–4) [21–23].

the important hypothesis that the spray pyrolysis increasedthe thickness of the visible light active CM-n-TiO2 films.Consequently, these films absorbed more light.

However, the UV-Vis spectra (see Figure 13) do notmatch well with the monochromatic photocurrent density-wavelength dependence (see Figure 6). The UV-Vis spectrarepresent the excitation of electron from lower energy state(to generate holes in this state) to higher energy state byabsorption of light. However, to have observed photocurrent,photogenerated holes will have to (1) transport to theinterface and (2) undergo transition across the interface toavailable (thermally distributed) hole acceptor states (e.g.,OH−) in solution. These two extra processes are responsiblenot having UV-Vis spectra to match with the monochro-matic photocurrent density-wavelength-dependent plots.

The electrons that are excited to conduction band fromthe shallow mid-gap states in CM-n-TiO2 (see Figure 9) bylight of around 500 nm reasonable amount of absorptionwere observed in the UV-Vis spectra but the holes generatedin the mid gap band (see Figure 9) were lost due to very lowavailability of acceptor states of holes in solution (e.g., OH−

ion) and consequently generated negligible photocurrentdensity (see Figure 6). However, absorption in UV-Vis spec-tra around 500 nm is observed since such absorption doesnot need availability of acceptor state of holes in solution.Furthermore, the light around 500 nm (2.48 eV) havingsub-bandgap energy cannot be absorbed by CM-n-TiO2 togenerate electron-hole pairs in its valence band. However,the light around 600 nm to 700 nm can excite electronsfrom the valence band to mid-gap band states of CM-n-TiO2 (see Figure 9) and these electrons move to the counterelectrode. The corresponding holes generated in the valenceband undergo transition to available hole acceptor states insolution. Consequently, some photocurrents are observed


Table 1: Maximum photoconversion efficiencies (%) under Xenon lamp light and actual natural sunlight illumination conditions forreference electric oven-made (n-TiO2), flame-made, and spray-flame-made CM-n-TiO2 samples and their bandgap and mid-gap Energies[21–23].

TitaniumOxide,TiO2

samples

%εphoto(total) Bandgap energies

Under white lightillumination fromXenon lamp light

Under monochromatic lightilluminations from Xenon lamp

Under monochromatic actualnatural sunlight illuminations(integrated from 300 nmpresent in sunlight)

Bandgapenergy (eV)

Mid-gapenergy (eV)

Integrated from250 nm

Integrated from300 nm

Oven-made(sample 1)

1.2% 0.94% 0.84% 0.67% 2.9 None

Flame made(sample 2)

9.08% 8.86% 7.62% 5.63% 2.65 1.6

Flame made ongrooved Ti surface(sample 3)

11.31% 11.16% 8.41% 7.62% 2.65 1.6

Spray pyrolysisand Flame made(sample 4)

14.04% 13.79% 12.89% 12.26% 2.65 1.4

1 μm

Figure 14: SEM images for a CM-n-TiO2 (sample 2) [21–23].

in this wavelength region (see Figure 6). Furthermore, itshould be noted that the carbon modification or dopingwas confined to surface region (2.0 nm deep) in CM-n-TiO2, the absorption of light in the visible region was foundlow.

2.1.9. Scanning Electron Microscopy (SEM) of CM-n-TiO2

Films. The surface characterization of a CM-n-TiO2 film(sample 2) was performed by employing a scanning electronmicroscope (SEM) and the image is shown in Figure 14.The surface looks extremely rough and nanocrystallinenanowalls hence provided much higher surface area toabsorb incident light, multiple absorption, and reflectionto absorb more light and consequently generated enhancedphotoresponse. Generation of multiple electron-hole pairs(excitons) by absorption of single high-energy photon inUV-region in nanocrystals (Ncs) was observed and alsoexplained theoretically in terms of impact ionization [86–92]. Furthermore, the microscopic rough surface couldtemporarily trap microscopic oxygen gas bubbles on roughsurface that may have acted as tiny lens and spherical mirrorsthat helped to concentrate the incident light [93] on theelectrode surface and hence augmented the photoresponsefor water splitting by an order of magnitude.

RRR

R R

R

R

R

R

R

A R

RR

R

R

RR

R

30 40 50 60 70

(S2)

(S3)

Inte

nsi

ty (

a.u

.)

(S1) RR

A R

Angle (2θ)

Figure 15: X-ray diffraction (XRD) patterns for samples 1, 2 and 3,and identified as S1, S2, and S3, respectively [21–23].

2.1.10. X-Ray Diffraction (XRD) for Titanium Oxides. X-ray diffraction (XRD) measurements were carried out tocharacterize the structure of n-TiO2 and CM-n-TiO2 sam-ples. Figure 15 shows the XRD patterns for oven-made n-TiO2 (S1); CM-n-TiO2 synthesized by only thermal flameoxidation (S2) and the CM-n-TiO2 synthesized by spraypyrolysis and thermal flame oxidation (S3).

The peaks of these XRD spectra of both CM-n-TiO2

photoelectrodes (S2 and S3) are consistent with the rutilestructure. The XRD results demonstrate that the crystalstructure of oven-made n-TiO2 (S1) is of mixture of anataseand rutile forms. It is important to note that some peaks ofrutile structure are missing in both CM-n-TiO2 samples (S2and S3) compared to those in oven-made n-TiO2 (S1). Thisindicates the marked influence of carbon modification on thestructure of n-TiO2.

2.1.11. Energy-Dispersive Spectroscopy (EDS) for CM-n-TiO2

Films. Figure 16 shows the pattern of energy-dispersivespectroscopic (EDS) data for samples 1, 2, 3, and 4.


Ti

Ti

O

(keV)

Sample 1

0 5

(a)

Ti

Ti

O

C

Sample 2

(keV)

0 5

(b)

Sample 3

(keV)

0 5 10

Ti

Ti

O

C

(c)

Ti

TiO

C

Sample 4

(keV)

0 5

(d)

Figure 16: Pattern of energy-dispersive spectroscopy (EDS) of samples 1, 2, 3, and 4 [21–23].

The carbon contents increased significantly from 17.60atom % for CM-n-TiO2 synthesized by only thermal flameoxidation to 23.23 atom % for CM-n-TiO2 synthesized bycombination of spray pyrolysis and thermal flame oxidation(see Table 2 for detail).

2.2. Visible Light Active Hydrogen-Modified n-Type TitaniumOxide (HM-n-TiO2) Thin Films. Frites and Khan [94]reported a new method for the synthesis of hydrogen-modified (HM)-n-TiO2 thin films by thermal oxidation ofTi metal sheet (Alfa Co. 0.25 mm thick) to TiO2 in anelectric oven followed by incorporation of hydrogen in itby electrochemical generation of hydrogen gas on it undercathodic polarization at −1.6 V versus Pt in an alkalinemedium at room temperature and also provided its photo-electrochemical behavior during water-splitting reactions.

2.2.1. Photocurrent Density-Potential Dependence for HM-n-TiO2. The photoresponse of the HM-n- TiO2 was evaluatedby measuring the rate of water-splitting reaction to hydrogenand oxygen in terms of photocurrent density, Jp. Theoptimized electric oven-made n-TiO2 and HM-n-TiO2 pho-toelectrodes showed photocurrent densities of 0.2 mA cm−2

and 1.60 mA cm−2, respectively, at a measured potential of−0.4 V versus Pt at illumination intensity of 100 mWcm−2

from a 150 W Xenon lamp (Figure 17). This indicated aneightfold increase in photocurrent density for HM-n-TiO2

compared to oven-made n-TiO2 at the same measuredelectrode potential. The band-gap energy of HM-n-TiO2

was found to be 2.7 eV compared to 2.82 eV for electricoven-made n-TiO2 and a mid-gap band at 1.67 eV abovethe valence band was also observed. The HM-n-TiO2 thin-film photoelectrodes were characterized using photocurrentdensity under monochromatic light illumination and UV-Visspectral measurements.

2.2.2. Monochromatic Photocurrent-Wavelength Dependencefor HM-n-TiO2. The monochromatic photocurrent den-sity, Jp(λ), was measured under wavelength, λ, dependent

(2)

(1)

−0.9 −0.7 −0.5 −0.3 −0.1 1 3

2.5

2

1

0.5

0

1.5

Emeas (V)/Pt

J p(m

A/c

m2)

Figure 17: Photocurrent density, Jp, as a function of measuredpotential, Emeas (V versus Pt) at light intensity of 100 mW/cm2 froma 150 W Xenon lamp for (1) oven-made n-TiO2 at 835◦C for 5 minand (2) HM-n-TiO2 samples [94].

monochromatic light illumination from a Xenon lamp byusing a Spectra Physics monochromator (Model 77250) at ameasured potential of −0.1 V versus Pt (see Figure 18). Per-forming integration under the curve in the wavelength rangebetween 250 nm and 775 nm in Figure 18, a total photocur-rent density of 2.2 mA/cm2 was observed at HM-n-TiO2

electrode. This value is little higher within the experimentalerror than the corresponding photocurrent density values of2.1 mA/cm2 for HM-n-TiO2 samples under white light illu-mination, at the same measured potential of −0.1 V versusPt (see Figure 17). The monochromatic photocurrent densityversus wavelength plots shows a clear shift of the photosen-sitivity of the HM-n-TiO2 to the visible region with a secondpeak around 630 nm compared to oven-made n-TiO2 whichshows only one pick in the UV region only (see Figure 18).

2.2.3. Bandgap Energy Determination for HM-n-TiO2. Thebandgap of HM-n-TiO2 becomes narrower from 3.0 eV for


Table 2: Atomic percent of elements Ti, O, and C in n-TiO2 and CM-n-TiO2 films from EDS data.

ElementAtomic %

Flat n-TiO2 by flame CM-n-TiO2 by flame Grooved CM-n-TiO2 by flame CM-n-TiO2 by spray and flame

Ti 33.33 15.73 13.96 10.10

O 66.66 66.67 66.67 66.67

C 0 17.60 19.38 23.23

2.5

2

1.5

1

0.5

0300 400 500 600 700

Wavelength (nm)

Wavelength (nm)

500 550 600 650 700 750

0.001

0.0008

0.0006

0.0004

0.0002

0

HM-n-TiO2

n-TiO2

J p(m

A/c

m2)

J p(m

A/c

m2)

×10−2

Figure 18: The monochromatic photocurrent density, Jp(λ), asa function of wavelength of light, λ (nm), for HM-n-TiO2, andunreduced n-TiO2 under monochromatic light illumination froma Xenon lamp; using a Spectra Physics monochromator (Model77250) with a 0.75 mm slit width. The applied potential was keptat −0.1 V/Pt for both electrodes [94].

regular rutile TiO2 to 2.7 eV (Figure 19). Also, a mid-gapband was created within the bandgap of n-TiO2 due tohydrogen reduction; which is located 1.67 eV above thevalence band for the hydrogen-modified titanium oxide.

2.3. Carbon-Modified Iron Oxides (CM-n-Fe2O3) Thin Films.Shaban and Khan [23] synthesized nanocrystalline carbon-modified iron (III) oxide (CM-n-Fe2O3) thin films by flameoxidation of Fe metal sheet at different temperatures (700–850◦C) and oxidation time (3–15 min), using a custom-designed large area flame (Knight, model RN. 3.5 xa wc)under controlled oxygen and natural gas flows.

2.3.1. Photocurrent Density-Potential Dependence for IronOxides Photoelectrodes. Figure 20 shows the dependence ofphotocurrent density (Jp, mA cm−2) for water splittingon measured electrode potentials (Emeas, V/SCE) at lightintensity of 100 mW cm−2, for oven-made-n-Fe2O3 (sampleA) and flame-made CM-n-Fe2O3 (sample B). Under illumi-nation of light intensity of 100 mW cm−2, and at measured

1

0.8

0.6

0.4

0.2

01 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4

HM-n-TiO2

HM-n-TiO2n-TiO2

(hA)(ηhA)0.

5

Figure 19: [η(λ)hν]1/2 versus hν plot for electric oven-made n-TiO2

and hydrogen-modified HM-n-TiO2 films [94].

0 0.2 0.4 0.6 0.8

0

2

4

6

8

10

12

14 B

B

A

A

Photocurrent

−0.6 −0.4 −0.2

2.8 mA cm−2

7.83 mA cm−2

Emeas (V/SCE)

Dark current

Ph

otoc

urr

ent

den

sity

,Jp

(mA

cm

−2)

Figure 20: Photocurrent density ( jp, mAcm−2) versus measuredpotential (Emeas, V/SCE) for oven-made n-Fe2O3 (sample A) andflame-made CM-n-Fe2O3 (sample B) under light intensity of100 mW cm−2. Both samples were prepared at the same optimumconditions of 850◦C for 15 minutes. 5.0 M KOH was used as anelectrolyte solution [23].

potential of −0.1 V/SCE, photocurrent density increasedfrom 2.8 mA cm−2 for oven-made n-Fe2O3 to 7.83 mA cm−2

for carbon-modified n-CM-n-Fe2O3 synthesized by thermalflame oxidation. The electrode potentials at open circuitcondition under illumination of light, Eaoc, were found tobe –0.498 V/SCE and −0.510 V/SCE for samples A and B,respectively.


0 0.2 0.4 0.6 0.8 1

(A)

(B)

1.20

1

2

3

4

5

6

7 6.5%

3.61%

Oven-made n-Fe2O3

Flame-made CM-n-Fe2O3


Ph

otoc

onve

rsio

n effi

cien

cy(%

ε Ph

oto(t

otal

))

Figure 21: Photoconversion efficiency, %εphoto(total), at lightintensity of 100 mWcm−2 versus applied potential, Eapp (V versusEmeas) for oven-made n-Fe2O3 (samples A) and flame-made CM-n-Fe2O3 (sample B) [23].

250 300 350 400 450 500 550 6000

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Ph

otoc

urr

ent

den

sity

(J p

(λ),

mA

cm

−2)

Wavelength, λ (nm)

Jp = 7.2 mA cm−2

Figure 22: The monochromatic photocurrent density, jp(λ), as afunction of wavelength of light, λ, for CM-Fe2O3 [23].

2.3.2. Photoconversion Efficiency-Potential Dependence forIron Oxides Photoelectrodes. Figure 21 shows the photocon-version efficiency, %εphoto(total), as a function of appliedpotential, Eapp (V versus Emeas), for samples A and B. Underillumination intensity of 100 m W cm−2, the maximumphotoconversion efficiency for water splitting improvedsignificantly from 3.61% (at a minimal applied potential of+0.5 V versus Eaoc) for sample A 6.5% (at a minimal appliedpotential of +0.402 V versus Eaoc) for sample B.

2.3.3. Monochromatic Photocurrent-Wavelength Dependencefor Iron Oxides. The dependence of monochromatic pho-tocurrent density, jp(λ), versus the wavelength, λ, of lightfor CM-n-Fe2O3 (sample B) is shown in Figure 22. Themonochromatic photocurrent density, jp(λ), at measured

Ph

otoc

onve

rsio

n effi

cien

cy (

%ε p

hot

o(λ

))

%εphoto = 5.9%

250 300 350 400 450 500 550 6000

0.01

0.02

0.03

0.04

0.05

0.06

Wavelength, λ (nm)

Figure 23: The photoconversion efficiency, %εphoto(λ), as a functionof wavelength of light, λ, for CM-Fe2O3 [23].

0

0.1

0.2

0.3

3.2

0.4

0.5


Oven-made n-Fe2O3

1.8 2 2.2 2.4 2.6 2.8 3

(ηhA)1/

2

hA (eV)

Figure 24: The respective plot of [η(hν)]1/2 versus hν to determinethe bandgap energies of both oven-made n-Fe2O3 and flame-madeCM-n-Fe2O3 films [23].

potential of 0.0 V/SCE was measured under monochromaticlight illumination from a 150 watt Xenon lamp by using aspectra physics monochromator (Model 77250). Performingintegration under the curve, total photocurrent densities of7.2 mA cm−2 were found for CM-n-Fe2O3. This value is lessthan that under white light illumination (7.83 mA cm−2).This is due to loss of light through the monochromator. Itis also due to steady-state measurements of photocurrentsunder monochromatic light illumination. Note that thephotocurrent densities under white light illumination weremeasured with a scan rate of 50 mV/sec.

2.3.4. Photoconversion Efficiency from Monochromatic Pho-tocurrent Densities at Iron Oxide Electrodes. The plot of%εphoto(λ) versus wavelength, λ, using (10) for CM-n-Fe2O3

is given in Figure 23 where total intensity in the denominatorof (10) was used as 100 mW cm−2. Performing integration


200 300 400 500 600 700

Abs

orba

nce

(a.

u.)


Oven-made n-Fe2O3

Wavelength, λ (nm)

Figure 25: UV-visible spectra for iron oxides [23].

20 30 40 50 60 70

d

a

aa

d

aa

a a ab

bc c

b

bcb

b

e fff fh fg hh b

c

eb

Inte

nsi

ty (

a.u

.)

Angle (2θ)

Figure 26: X-ray diffraction (XRD) patterns for flame-made CM-n-Fe2O3 (top plot) and the reference oven-made n-Fe2O3(bottomplot) films. The detected probable formulas to comply with peaksare a = Fe2O3; b = Fe3O4; c = FeO; d = Fe; e = FeC; f = Fe2C; g =Fe3C; h = FeCO3 [23].

Table 3: Atomic percent of elements Fe, O, and C in n-Fe2O3 andCM-n- Fe2O3 films from EDS data.

ElementAtomic %

Oven-made n-Fe2O3 Flame-made CM-n-Fe2O3

Fe 43.7 36.2

O 56.3 55.4

C 0.0 8.4

in (8) from λmin = 250 nm to λg = 1240/Eg or determiningthe area under the curve in Figure 23 obtained using (10), atotal photoconversion efficiency of 5.9% is also lower thanthat under white light illumination (6.5%) for the reasonsmentioned in the above conditions.

2.3.5. Bandgap Energy Determination for Iron Oxides.Figure 24 shows [η(hν)]1/2 versus hν for samples A and B,respectively. From the intercept of these plots, the bandgaps

were found to be 2.3 eV and 1.95 eV for samples A and B,respectively. These bandgap values are in agreement withthose obtained from UV-Vis spectra (see Figure 25). Notethat η(λ) is the wavelength, λ, dependent quantum efficiencyas expressed in (12).

2.3.6. UV-Vis Spectra for Iron Oxides. Figure 25 compares theUV-Vis absorbance of flame-made CM-n-Fe2O3 and oven-made n-Fe2O3 photoelectrodes having maximum photocon-version efficiencies of 6.5% and 3.61%, respectively. Bothelectrodes were synthesized at the same optimum conditionsof 850◦C for 15 min. The UV-Vis spectroscopic results showa significant increase in the absorbance of light in the UVas well as in the visible regions for flame-made CM-n-Fe2O3 film compared to that of the oven-made n-Fe2O3. TheUV-Vis spectra are consistent with the higher photocurrentdensities of flame-made CM-n-Fe2O3 thin-film electrodes.The absorption spectra demonstrate a wide absorption in theUV and visible regions with a tail extending up to 600 nmwhich corresponds to a bandgap energy of 1.95 eV for flame-made CM-n-Fe2O3 and up to 545 nm which correspondsto bandgap energy of 2.3 eV for oven-made n-Fe2O3. Thelow bandgap energy value for flame-made CM-n-Fe2O3 canbe attributed to carbon incorporation during the flameoxidation process of Fe metal sheet.

2.3.7. X-Ray Diffraction (XRD) for Iron Oxides. XRD pat-terns (Figure 26) show that in addition to Fe2O3, Fe3O4 andFeO are also present in both oven-made and flame-made thinfilms. Analysis of XRD confirms carbon incorporation inflame-made iron (III) oxide film (sample B) by the presencethe peaks for iron carbide compounds in the forms of FeC,Fe2C, Fe3C, and FeCO3. None of these peaks for iron carbidecompounds were observed in oven-made iron (III) oxide(sample A). It can be concluded that the flame oxidationprocess is responsible for carbon incorporation onto iron(III) oxide thin film.

2.3.8. Energy-Dispersive Spectroscopy (EDS) for Iron Oxides.Table 3 shows the EDS results of oven-made n-Fe2O3 (sampleA) and flame-made CM-n-Fe2O3 (sample B) photoelec-trodes. The presence of carbon in CM-n-Fe2O3 photo-electrode and its absence in the oven-made n-Fe2O3 wereobserved. The corresponding EDS spectra of oven-made n-Fe2O3 and flame-made CM-n-Fe2O3 samples are shown inFigures 27(a) and 27(b), respectively. Note that the regularoven-made n-Fe2O3 did not show any carbon peak.

2.3.9. Scanning Electron Microscopic (SEM) Images of IronOxides. Morphological differences were observed from thescanning electron microscopic (SEM) images (Figure 28)between (a) oven-made n-Fe2O3 film and (b) flame-madeCM-n-Fe2O3 film. The surface of oven-made n-Fe2O3 showsnanowires but CM-n-Fe2O3 surface of sample B does notshow nanowires even at higher magnification (200 nmcompared to 1 micro meter used for oven-made sample a).The nanowires on oven-made n-Fe2O3 films (Figure 28(a))


Fe

(keV)

Fe

O

0 5 10

(a)

Fe

Fe

O

C

(keV)0 5 10

(b)

Figure 27: Pattern of energy-dispersive spectroscopy (EDS) of samples (a) and (b) [23].

1 μm ZEISSZEISS Gemini FE-SEM EHT = 3 kV

WD = 6.6 mmDate: 27 Oct 2008

Signal A Inlens=Reference mag 300 mm dispay=

File name duquesne Fe2O3 03.tif=Mag 26.24 kX=

(a)

ZEISSZEISS Gemini FE-SEM200 nm EHT = 1.5 kV

WD = 6.7 mmDate: 27 Oct 2008File name duquesne Fe2O3 17.tif

Signal A Inlens=Reference mag 300 mm dispay=

=Mag 100 kX=

(b)

Figure 28: SEM images of (a) oven-made-n Fe2O3 [23] and (b) flame-made CM-n-Fe2O3 films.

helped to minimize the recombination rate of photogen-erated carriers and consequently generated much higherphotoresponse compared to its nanocrystalline thin filmssynthesized by spray pyrolysis methods earlier (4). The CM-n-Fe2O3 films show only the nanocrystals (Figure 28(b)).

2.4. Visible Light Active Carbon-Modified p-Type TungstenOxides (CM)-p-WO3 Thin Film. Shaban and Khan [84]synthesized carbon-modified p-type tungsten oxide (CM-p-WO3) photoelectrodes by flame oxidation of tungsten metalsheets (Alfa Co. 0.25 mm thick) at several temperatures fordifferent lengths of time.

2.4.1. Photocurrent Density-Potential Dependence for TungstenOxides Photoelectrodes. The photocatalytic activity of CM-p-WO3 was evaluated by measuring the rate of water-splittingreaction to hydrogen and oxygen, which is proportional tophotocurrent density, jp. Carbon-modified tungsten oxidephotoelectrodes exhibited p-type photoresponse in acidic

medium of 0.5 M H2SO4. The highest photocurrent densityof 2.08 mA cm−2 was observed under illumination intensityof 80 mW cm−2 from a 150 Watt (Kratos Model LH 150/1)Xenon arc lamp at an optimal thermal oxidation temperatureof 900◦C for 15 min at applied potential of−0.4 V versus Eaoc

(Figure 29).

2.4.2. Photoconversion Efficiency-Potential Dependence forTungsten Oxides Photoelectrodes. The optimized CM-p-WO3

photoelectrodes were found to generate the highest photo-conversion efficiency of 2.16% for photosplitting of water(Figure 30).

2.4.3. Monochromatic Photocurrent Density-Wavelength De-pendence for CM-p-WO3 Electrodes. The dependence ofmonochromatic photocurrent density, jp(λ), versus thewavelength, λ, of light for CM-p-WO3 thin-film electrodes,having photoconversion efficiency of 2.16% (synthesized bythermal oxidation of W metal at oxidation temperature of900◦C for 15 minutes), is shown in Figure 31.


0 0.1 0.20

0.5

1

1.5

2

−0.2 −0.1

800◦C

950◦C

900◦C

Emeas (V versus SCE)

Ph

otoc

urr

ent

den

sity

,Jp

(mA

cm

−2)

Figure 29: Photocurrent density ( jp, mAcm−2) versus measuredpotential (Emeas, V/SCE) for CM-p-WO3 synthesized by thermaloxidation of tungsten metal sheets at different flame temperatures850, 900, and 950◦C. All these samples were synthesized at the sameflame oxidation time of 15 minutes [84].

00

0.5

1

1.5

2

2.5

−0.4 −0.3 −0.2 −0.1

800◦C

950◦C

900◦C

Ph

otoc

onve

rsio

n effi

cien

cy, (

%ε p

hot

o(t

otal

))


Figure 30: Photoconversion efficiency (%εphoto(total)) versus mea-sured potential (Emeas, V/SCE) for CM-p-WO3 synthesized bythermal oxidation of tungsten metal sheets at different flame tem-peratures 850, 900, and 950◦C. All these samples were synthesizedat the same flame oxidation time of 15 minutes [84].

Significant visible light absorption activity by carbon-modified p-WO3 can be noticed, where a maximummonochromatic photocurrent density value of 3.25 μA cm−2

at 425 nm was observed.

2.4.4. Dependence of Percent Incident Photon ConversionEfficiency (IPCE %) on Wavelength of Light, λ. MaximumIPCE (%) of 42.7% at wavelength of 260 nm can be observed(Figure 32). However, in the visible region, the CM-p-WO3

shows incident photon conversion efficiency percent (IPCE%) in the range between 0.9% at 500 nm and 9.9% at 400 nm.These activities in the visible region can be attributed to

250 300 350 400 450 500 5500

0.5

1

1.5

2

2.5

3

3.5

Wavelength (nm)

Ph

otoc

urr

ent

den

sity

,Jp

(μA

cm

−2)

Figure 31: Monochromatic photocurrent density, jp(λ), versusthe wavelength, λ, of light for CM-p-WO3 thin-film electrodessynthesized by thermal oxidation of W metal at temperature of900◦C for 15 minutes; electrolyte solution of 0.5 M H2SO4; undermonochromatic light illumination of Xenon lamp; using a SpectraPhysics monochrometer (Model 77250) with a 0.75 mm slit width,and Silicon detector (UDT Sensors. Inc. Model 10DP/SB) [84].

250 300 350 400 450 500 5500

10

20

30

40

50

Wavelength (nm)

IPC

E (

%)

Figure 32: Incident photon conversion efficiency percent (IPEC%) as a function of wavelength, λ, of light for CM-p-WO3

synthesized by thermal oxidation of W metal at temperature of900◦C for 15 minutes; electrolyte solution of 0.5 M H2SO4; undermonochromatic light illumination of Xenon lamp; using a SpectraPhysics monochrometer (Model 77250) with a 0.75 mm slit width,and Silicon detector (UDT Sensors. Inc. Model 10DP/SB) [84].

carbon modification of WO3 during the flame oxidationprocess of tungsten metal.

2.4.5. Bandgap Energy. Figure 33 shows the plot for[η(hν)]1/2 versus hν for thermally prepared CM-p-WO3 atoxidation temperature of 900◦C for 15 min. From the inter-cept of the straight line, the bandgap is obtained. It can be


2.35 2.4 2.45 2.5 2.55 2.6 2.650

0.05

0.1

0.15

0.2

0.25

0.3

CM-p-WO3

n-WO3

hA (eV)

(η(λ

)hA

(1/2

)

Figure 33: Plot of [η(hν)]1/2 versus hν to determine the bandgap ofCM-p-WO3 thin-film electrodes synthesized by thermal oxidationof W metal at temperature of 900◦C for 15 minutes; electrolytesolution of 0.5 M H2SO4; under monochromatic light illuminationof Xenon lamp; using a Spectra Physics monochrometer (Model77250) with a 0.75 mm slit width, and Silicon detector (UDTSensors. Inc., Model 10DP/SB) [84].

250 300 350 400 450 500 550

Abs

orba

nce

(a.

u.)

Wavelength (nm)

Oven-made-n-WO3

Flame-made-CM-p-WO3

Figure 34: UV-Visible spectra of flame-made-CM-p-WO3 andoven-made n-WO3 photoelectrodes synthesized by heating attemperature of 900◦C for 15 minutes [84].

observed that the bandgap energy became lower from 2.6 eVfor regular n-WO3 (to 2.42 eV for of CM-p-WO3). This lowbandgap energy can be attributed to carbon incorporationduring the flame oxidation process of tungsten metal.

2.4.6. UV-Vis Spectra on CM-p-WO3. The UV-Vis absorp-tion spectra show (Figure 34) a significant improvementin the absorbance of light in UV and visible regions forflame-made CM-p-WO3 films compared to oven-made n-WO3. The absorption spectra of flame-made p-WO3 showhigher absorption in the visible region with a tail extendingto 512 nm which corresponds to bandgap energy value of2.42 eV.

The threshold of absorption for oven-made-n-WO3 isat 475 nm (2.6 eV) which is in agreement with a bandgapvalue of 2.6 eV for n-WO3 [74, 75]. The low bandgap energyvalue of 2.42 eV for CM-p-WO3 films can be attributed tothe carbon incorporation during the flame oxidation processof tungsten metal sheet. This low bandgap energy valuesfor CM-p-WO3 determined by UV-Vis spectra (Figure 34)agree well with the values obtained from the monochromaticphotocurrent densities and the corresponding quantumefficiencies (Figure 33).

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Research Article

Photocatalytic Degradation of Pesticides in Natural Water:Effect of Hydrogen Peroxide

Natividad Miguel, Marıa P. Ormad, Rosa Mosteo, and Jose L. Ovelleiro

Department of Chemical Engineering and Environmental Technologies, University of Zaragoza, Pedro Cerbuna 12,50009 Zaragoza, Spain

Correspondence should be addressed to Natividad Miguel, [email protected]

Received 12 August 2011; Revised 20 November 2011; Accepted 1 December 2011

Academic Editor: Jae Sung Lee

Copyright © 2012 Natividad Miguel et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The aim of this paper is to evaluate the effectiveness of photocatalytic treatment with titanium dioxide in the degradation of44 organic pesticides analyzed systematically in the Ebro river basin (Spain). The effect of the addition of hydrogen peroxidein this treatment is studied, and a monitoring of effectiveness of photocatalytic processes is carried out by measurements ofphysical-chemical parameters of water. The application of photocatalytic treatment with 1 g L−1 of TiO2 during 30 minutesachieves an average degradation of the studied pesticides of 48%. Chlorine demand, toxicity, and dissolved organic carbon (DOC)concentration of water are reduced. If hydrogen peroxide is added with a concentration of 10 mM, the average degradation ofpesticides increases up to 57%, although chlorine demand and toxicity of water increase while DOC concentration remainsunchanged with this treatment. The application of either photocatalytic treatments does not produce variations in the physical-chemical parameters of water, such as pH, conductivity, colour, dissolved oxygen, and hardness. The pesticides which are bestdegraded by photocatalytic treatments are parathion methyl, chlorpyrifos, α-endosulphan, 3,4-dichloroaniline, 4-isopropylaniline,and dicofol while the worst degraded are HCHs, endosulphan-sulphate, heptachlors epoxide, and 4,4′-dichlorobenzophenone.

1. Introduction

During recent years, numerous organic substances consid-ered to be hazardous have been detected in waters of Ebroriver basin (Spain). These substances have been detected insurface and ground waters and they can be considered haz-ardous substances according to the current legislation be-cause they are toxic, persistent, and bioaccumulative sub-stances.

Pesticides, artificially synthesized substances used to fightpests and improve agricultural production, are especiallyproblematic. These are monitored by the Pesticides ControlNetwork (Ebro river basin), which systematically analyzes 44organic pesticides in surface waters. These pesticides wereselected because of their appearance in lists of hazardoussubstances and/or their high level of use in Spanish agri-culture. The 44 pesticides analyzed in the Ebro riverbasin are alachlor, aldrin, ametryn, atrazine, chlorfenvin-fos, chlorpyrifos, pp′-DDD, op′-DDE, op′-DDT, pp′-DDT,

desethylatrazine, 3,4-dichloroaniline, 4,4′-dichlorobenzo-phenone, dicofol, dieldrin, dimethoate, diuron, α-endos-ulphan, endosulphan-sulphate, endrin, α-HCH, β-HCH,γ-HCH, δ-HCH, heptachlor, heptachlor epoxide A, hep-tachlor epoxide B, hexachlorobenzene, isodrin, 4-isopropyl-aniline, isoproturon, metholachlor, methoxychlor, molinate,parathion methyl, parathion ethyl, prometon, prometryn,propazine, simazine, terbuthylazine, terbutryn, tetradifon,and trifluralin.

Due to the presence of hazardous substances, both natu-ral water used to supply the population and water which isto be reused have to be treated in water treatment plants toensure that they comply with the minimum criteria estab-lished in the current legislation [1–3].

Water treatments consist of several operations whichoften do not achieve the removal of hazardous substances orentail other problems deriving from the treatment.

In the case of drinking water production, an impor-tant successful development is the possible formation of


organic-halogenated compounds, potentially carcinogenicand chlorinated deriving from methane, as reaction productsbetween chlorine (common disinfectant product) and theorganic matter in water. The most abundant of these aretrihalomethanes (THMs) whose concentration is limited bySpanish Royal Decree 140/2003 at to 100 μg L−1.

The drawbacks in the use of compounds with chlorine,among others, have derived in the research of other alterna-tive treatments for removing pollutants in water treatments.Among these, advanced oxidation processes (AOPs) arebased on the generation of reactive oxygen species, whichare highly reactive, nonselective, and do not generate toxicbyproducts [4, 5]. These species are capable of degrading asubstantial number of pollutants by radicalary mechanisms.The generation of these species can be carried out byprocesses with ozone, hydrogen peroxide, metallic catalysts,UV radiation, and so forth.

One of the most frequently investigated AOPs in recentyears is the photocatalytic process. Photocatalysis entail thecombination of radiation and catalyst. Both are necessaryin order to achieve or accelerate a chemical reaction, andtherefore photocatalysis can be defined as the “accelerationof a photoreaction by the presence of a catalyst.”

Catalysts used in these processes are semiconductormetallic oxides materials. The surface of semiconductormetallic oxides provides a place where oxidation-reductionreactions can be started by radiation. Semiconductors havebands associated with energy levels spaced between them.Photoexcitation with energy equal to or greater than that ofthe gap between the valence and conduction bands movesan electron from the valence band to the conduction band,generating a hole deficient in electrons. The oxidationof an adsorbed molecule can be produced in holes andsimultaneously the reduction of another molecule can beproduced in the opposite part of the catalyst.

Numerous semiconductor substances have been tried ascatalyst in photocatalytic processes. Generally, it is knownthat titanium dioxide is one of the most effective. Moreover,it has a high stability and photoactivity, low cost, nontoxicity,and solubility. The activation of titanium dioxide is producedwith radiation at λ < 387 nm.

As a result of the application of photocatalytic processesbased on titanium dioxide, the number of references in theliterature in recent decades related to the degradation oftoxic and hazardous substances in water can be counted intheir thousands. As regards the group of pesticides understudy, photocatalytic processes have been applied in severalways and there are many references to the degradation ofsuch pesticides such as triazines [6, 7], organic-phosphoratedpesticides [8–10], alachlor [11, 12], diuron [7, 13], andparathion methyl [14].

The addition of hydrogen peroxide to photocatalytictreatment with titanium dioxide can be used to increase theeffectiveness of the latter process because, as hydrogenperoxide is a more powerful oxidant than oxygen, it generatesa larger collection of electrons and this avoids the recombi-nation of electron-hole pairs formed in the photocatalyticprocess. Hydrogen peroxide is an electron acceptor and it

can react with electrons in the conduction band and generateradicals.

Similarly, there are several references concerning theremoval of pesticides by photocatalytic treatments withhydrogen peroxide, such as the degradation of atrazine,isoproturon, alachlor and diuron [7], organic-phosphoratedpesticides [9, 10], and other triazines [15].

However, the results show a big disparity with respect tothe effect of hydrogen peroxide. It is sometimes beneficialwhile at other times it is damaging to the effectivenessof the process. This phenomenon, the effect of hydrogenperoxide in photocatalytic treatments, produces additionaldrawbacks when complex mixtures of pollutants are used.The damaging effect in the degradation rate is produced bya modification of the catalyst surface by hydrogen peroxideadsorption [16] and the inhibition of generated holes in thevalence band and reaction with hydroxyl radicals [17]. Whenthe effect of hydrogen peroxide is beneficial, a substantialconsumption of hydrogen peroxide is sometimes necessaryin order to obtain only a small increment of the degradationrate.

The aim of this research work is to evaluate the degrada-tion of the 44 pesticides systematically analyzed in the Ebroriver by photocatalytic treatment with titanium dioxide andto study the effect of hydrogen peroxide in this treatment.Moreover, a monitoring of the effectiveness of the photo-catalytic processes is carried out by measuring the physical-chemical parameters of water.

2. Materials and Methods

2.1. Samples. Samples of natural water from the Ebro riverupstream from Zaragoza (Spain) were prepared by fortifica-tion with 44 pesticides in concentrations of 500 ng L−1. These44 organic pesticides, listed in the introduction section, aresystematically analysed in surface waters by the Network ofControl of Pesticides in the Ebro river basin.

2.2. Sample Characterization. The characterization of thesamples was carried out using the parameters shown inTable 1. The equipment used and standard methods appliedare detailed below.

The analysis of the chromatographic conditions of thepesticides is shown in Table 2 and the results of the validationof this analysis in Table 3.

Previously to the analysis of pesticides by GC/MS a solid-liquid extraction was carried out. This extraction consistedof the retention of organic compounds in a solid phase andsubsequent elution with an organic solvent.

The extraction was carried out using an AUTOTRACEWorkStation automatic extractor (Zymark). Before extrac-tion, 100 ng L−1 of surrogate compounds used to control theextraction process (simazine-D5, atrazine-D5, and prome-tryn-D6) were added to the water sample. During solid-liquid extraction, 900 mL of the sample was passed throughcartridges containing a solid ENV+ filter (polystyrene divinylbenzene copolymer) (ISOLUTE cartridges, 200 mg 6 mL−1).The pesticides contained in the sample were retained in


Table 1: Parameters, equipment, and standardized methods used for the characterization of samples.

Parameter Equipment Standard method Reference

pH and temperature pH-meter CRISON GLP21 SM 4500-HB [18]

Conductivity Conductivity-meter CRISON Basic30UNE-EN

27888 : 1994[19]

Dissolved organic carbon (DOC)concentration

Analyzer Shimadzu TOC-VCSH SM 5310B [18]

Chlorine Demand — SM 5710B [18]

Turbidity Turbidity-meter Hanna LP2000 ISO 7027: 1999 [20]

Color Multiparametric Photometer Hanna C99 SM 2120B [18]

Dissolved oxygen concentration Multiparametric Photometer Hanna C99 SM 4500-O C [18]

Hydrogen peroxide concentration — — [21]

Hardness Multiparametric Photometer Hanna C99 SM 2340B [18]

Suspended solids concentrationMultiparametric Photometer Hach Lange

DR2800SM 2540D [18]

Phosporous concentration Multiparametric Photometer Hanna C99 SM 4500-P C [18]

Ammonia concentration Multiparametric Photometer Hanna C99 SM 4500-NH3 C [18]

Cyanide concentration Multiparametric Photometer Hanna C99 SM 4500-CN E [18]

Fluoride, chloride, nitrate, phosphate,and sulphate concentration

Ionic Chromatographer DIONEXICS-1000

SM 4110B [18]

Toxicity Toxicity-meter LUMIStox 300 ISO 11348: 1999 [22]

PesticidesGas chromatographer TRACE2000 and

mass spectrometer POLARISEPA 525.2 [23]

Table 2: Conditions of pesticide analysis.

Gas chromatographer TRACE GC 2000 (Thermo Finnigan)

Column DB5-MS (J&W, 30 m, 0.25 mm, 0.25 μm)

Program of temperatures 90◦C (1 min), 20◦C min−1, 180◦C (1 min), 2◦C min−1, 240◦C (1 min), 20◦C min−1, 310◦C (10 min)

Injector temperature 250◦C

Injection volume 1 μL, splitless 0.8 min

Carrier gas He (N55), 1 mL min−1

Mass spectrometer POLARIS (Thermo Finnigan)

Ionization energy 70 eV

Acquisition mode Full scan

Mass interval 50–450 amu

Screen speed 1 scan s−1

Acquisition time 32.5 min

the solid phase and dried under N2 for 10 minutes. Theywere then eluted by passing 10 mL of ethyl acetate (SDS forpesticide analysis) through the cartridge, thus facilitating thepassage of these compounds from the water phase to anorganic phase. The extracts thus obtained were concentratedunder a N2 flow until an approximate volume of 1 mL wasobtained, after which 3 mL of isooctane was added (SDSfor pesticide analysis) in order to carry out a change ofsolvent. The extract was then concentrated until obtaining anapproximate volume of 0.5 mL. Anthracene deuterate (D10,SUPELCO) was added to each extract as an internal standardfor subsequent quantification of the pesticides present in thesamples. These extracts were analysed by GC/MS.

2.3. Experimental Procedure in Photocatalytic Treatments.The titanium dioxide used in this work was Degussa P25.The tests were carried out with a dose of 1 g L−1, the same asthat used in previous studies and determined as optimal inthe stages of disinfection [24–26]. The TiO2 was added to 1 Lof sample to be treated and shaken at 300 rpm to promotecontact between the sample and the catalyst and provideoxygen to the reaction medium. The reaction time was 30minutes.

For tests with hydrogen peroxide, the compound wasadded to the sample H2O2 30% v/v (Carlo Erba) with a con-centration of 10 mM. This same dose has been used anddetermined as optimal in many research works evaluating


Table 3: Results of the methodology validation of the pesticide analysis.

PesticideQuantification limit (μg L−1) Calibration

interval (μg L−1)Validity

interval (μg L−1)Recovery interval (%)

Instrumental step Full method Instrumental step Full method

Isoproturon 20 0.030 20–500 0.030–300 75–130 63–110

Diuron 20 0.030 20–500 0.030–300 82–128 70–123

3,4-Dichloroaniline 20 0.030 20–500 0.030–300 88–130 47–106

4-Isopropylaniline 20 0.030 20–500 0.030–300 80–130 60–125

Desethylatrazine 20 0.030 20–500 0.030–300 76–130 80–129

Trifluralin 20 0.015 20–500 0.030–300 70–130 70–127

Dimethoate 20 0.030 50–500 0.030–300 66–124 54–137

Simazine 50 0.030 20–500 0.030–600 75–135 64–127

Prometon 20 0.030 20–500 0.030–300 76–124 0–125

Atrazine 200 0.100 200–5000 0.100–300 78–130 75–127

Propazine 20 0.015 20–500 0.015–300 86–130 73–127

Terbuthylazine 20 0.015 20–500 0.015–300 79–130 83–128

Parathion methyl 50 0.030 50–500 0.030–300 78–139 72–130

Parathion ethyl 20 0.030 20–500 0.030–300 74–122 64–128

Alachlor 20 0.015 20–500 0.015–300 75–125 70–124

Ametryn 20 0.030 20–500 0.030–300 78–130 0–116

Prometryn 20 0.030 20–500 0.030–300 80–120 17–116

Terbutryn 20 0.030 20–500 0.030–300 80–120 13–114

Chlorpyrifos 20 0.015 20–500 0.015–300 75–120 73–116

Chlorfenvinfos 20 0.015 20–500 0.015–300 76–130 70–126

HCHs 20 0.015 20–500 0.015–300 84–124 70–120

Hexachlorobenzene 20 0.030 20–500 0.030–300 70–130 74–136

Heptachlor 20 0.015 20–500 0.015–300 75–130 58–113

Heptachlor epoxide A 20 0.015 20–500 0.015–300 85–125 62–112

Heptachlor epoxide B 20 0.015 20–500 0.015–300 84–130 58–113

Aldrin 20 0.015 20–500 0.015–300 85–125 64–126

4,4′-Dichlorobenzophenone 20 0.015 20–500 0.015–300 75–120 68–126

Isodrin 20 0.015 20–500 0.015–300 85–125 66–120

α-Endosulphan 20 0.015 20–500 0.015–300 70–125 70–93

pp′-DDE 20 0.015 20–500 0.015–300 89–122 64–107

Dieldrin 20 0.015 20–500 0.015–300 70–125 62–120

Endrin 20 0.015 20–500 0.015–300 80–125 74–122

pp′-DDD + op′-DDT 40 0.030 40–1000 0.030–600 79–125 66–139

Endosulphan-sulphate 20 0.015 20–500 0.015–300 83–125 73–126

pp′-DDT 20 0.030 20–500 0.030–300 76–130 50–120

Dicofol 50 0.030 50–500 0.030–300 80–148 63–136

Methoxychlor 20 0.015 20–500 0.015–300 77–126 75–130

Metholachlor 20 0.015 20–500 0.015–300 76–115 73–128

Molinate 20 0.015 20–500 0.015–300 91–130 75–113

Tetradifon 20 0.015 20–500 0.015–300 85–130 70–116

the effectiveness of photocatalytic treatment with hydrogenperoxide on the degradation of different compounds, oreven on the generation of ROS: degradation of dyes [27–30], pesticides [31, 32], antibiotics [33], and ROS production[34]. In these tests, hydrogen peroxide was removed after thetreatment.

The UV/VIS radiation source was provided using anATLAS SUNTEST CPS+/XLS+ solar chamber. This chamberis an instrument equipped with a xenon lamp used forlighting and the ageing of materials. It can be used assimulator of natural sunlight. The chamber also includes anagitation system, a quartz filter, UV radiation, visible light,


temperature control. It is equipped with a programmablesystem for measuring and for regulating the irradiationintensity. The irradiation range of the equipment is from250 to 785 W m−2 with a wavelength of 300 to 800 nm.The samples were subjected to a radiation intensity from500 W m−2 which corresponds to 50% of the intensity ofnatural solar radiation at midday [35].

All the tests were performed in duplicate, using glassbeakers of 1 L and with shaking. Moreover, blank tests werecarried out.


The results of the two photocatalytic treatments (TiO2/radi-ation and TiO2/H2O2/radiation) on the solutions of naturalwater fortified with the 44 pesticides under study relating tothe sample characterization and the removal of pesticides arepresented below.

3.1. Sample Characterization. The characterization of theinitial sample and the samples treated with the two photo-catalytic processes is shown in Table 4.

The results show the following.

(i) The application of photocatalytic treatments doesnot produce significant variations with respect to theinitial values of pH, conductivity, color, dissolvedoxygen, hardness, ammonium concentration, andconcentration of ions in solution.

(ii) The turbidity of the sample, in both cases, declinesslightly, and solids in suspension make it significantlythrough the chemical oxidation of organic matterproduced in the treatment.

(iii) Chlorine demand in the TiO2/radiation-treated sam-ple is reduced significantly, up to a value of of1 mg L−1. However, the application of TiO2/H2O2/ra-diation treatment produces an enormous increase inthis parameter. This is possibly due to the variousbyproducts formed after the application.

(iv) Regarding the toxicity of treated sample, TiO2/radia-tion treatment reduces the initial toxicity of thesample to approximately half its initial value. On theother hand, the application of TiO2/H2O2/radiationproduces an increase in the initial toxicity, due tothe formation of byproducts more toxic than theoriginal. This has occurred in other research worksconcerning the application of these photocatalytictreatments [14, 36–38].

(v) The COD undergoes no variation through the TiO2/H2O2/radiation treatment while it is reduced by 36%after the application of TiO2/radiation. This suggeststhe mineralization of organic matter present in theprocess with hydrogen peroxide [39].

(vi) Suspended solids are removed by both photocatalytictreatments. This is due to the organic matter presentin the water by this advanced oxidation process.

(vii) Hydrogen peroxide appears after the TiO2/radiationtreatment formed as a by-product in photocatalyticreactions. In the case of the TiO2/H2O2/radiationtreatment, where it is used initially as a reagent,almost its total consumption is observed.

3.2. Pesticide Removal. Photocatalytic experiments were car-ried out in duplicate. Table 5 shows the average concen-tration of each pesticide studied in the initial sample andthe final concentration after the photocatalytic treatments.It is worth noting that average concentrations are shownbecause the variations obtained in these analyses were verylow, always below 2%.

In addition to the photocatalytic treatments, blank testswere carried out. These blank tests were solution of pesticideswith titanium dioxide without radiation and with radiationwithout catalyst. The results obtained are shown in Table 6.Only for some pesticides were degradations different to zeroachieved.

As can be observed, some of the studied pesticides can bedegraded by TiO2 without radiation and by radiation with-out TiO2. The degradation of these pesticides by TiO2 onlyis due to their adsorption in the catalyst. The degradationof these pesticides by radiation only is due to the advancedoxidation process with UV by which hydroxyl radicals aregenerated.

Table 7 shows the average removal percentages of thepesticides.

The photocatalytic treatments achieved an average degra-dation of 48% by photocatalysis (TiO2/radiation) and 57%by photocatalysis with hydrogen peroxide (TiO2/H2O2/radi-ation).

The photocatalytic treatments were very effective in theremoval of parathion methyl, chlorpyrifos, α-endosulphan,3,4-dichloroaniline, 4-isopropylaniline, and dicofol. Thetreatments were less effective in the degradation of HCHs,endosulphan-sulphate, heptachlors epoxide, and 4,4′-di-chlorobenzophenone.

As can be seen in Tables 5 and 7, the addition of hydro-gen peroxide to the process slightly improves the averagepercentage of pesticide removal. However, this improvementdoes not occur for all the pesticides studied. In fact, someof them present the same removal percentages by both pho-tocatalytic treatments. The increase in the average percentageof degradation is due to an improvement in some of thestudied pesticides to add hydrogen peroxide. These aresome triazines, α-endosulphan, molinate, trifluralin, and an-ilides, for which removal is improved by 5–20% using hy-drogen peroxide; isodrin, aldrin, DDTs, and 4,4′-dichlo-robenzophenone, for which removal is improved by 25–50%using hydrogen peroxide.

The degradation percentages of the pesticides in theirindividual group are as follows.

Triazines. The rates of degradation of triazines obtainedby the photocatalytic treatments are between 35 and 60%.By photocatalysis, these percentages are from 35 to 55%.Degradation of these pesticides is very similar, 40–60%, when


Table 4: Characterization of samples of surface water fortified with pesticides in photocatalytic treatments.

Initial sample

Physical-chemical parameters

pH 8.0 Colour (PCU) 0

Conductivity (μS cm−1) 508 Dissolved oxygen (mg L−1) 9.9

Turbidity (NTU) 69 Hardness (mg CaCO3 L−1) 108

Suspended solids (mg L−1) 27 NH4+ concentration (mg L−1) 0.15

Phosphorous (mg L−1) 0.8 CN− concentration (mg L−1) 0.002

DOC (mg C L−1) 22 Toxicity (% inhibition) 33.6

Chlorine demand (mg Cl2 L−1) 6.0

Anion concentrations in solution (mg L−1)

Fluorides 0.1 Nitrates 10.8

Chlorides 63.8 Phosphates <0.2

Sulphates 98

Sample after treatment TiO2/radiation








H2O2 concentration (mg L−1) 5 Chlorine demand (mg Cl2 L−1) 1.0


Fluorides <0.1 Nitrates 11.6


Sulphates 101.0

Sample after treatment TiO2/H2O2/radiation








H2O2 concentration (mg L−1) 0.5 Chlorine demand (mg Cl2 L−1) 55.6


Fluorides <0.1 Nitrates 11.4


Sulphates 99.2

hydrogen peroxide is added to the treatment. Therefore,the addition of this reagent is not compensated for by theincrease of the effectiveness in the degradation of triazinesby photocatalytic treatment. Among the triazines studied areatrazine and desethylatrazine, the most difficult to degradeby photocatalytic treatments.

Organic Phosphorated. The five organic-phosphorated pesti-cides studied are degraded between 40 and 90% by photo-catalytic treatments. The degradation percentages obtainedfor these pesticides are similar whether or not hydrogen

peroxide is added. The degradation of dimethoate and chlor-fenvinfos only slightly improves when hydrogen peroxide isadded. Methyl parathion and chlorpyrifos are the pesticidesthat degrade best by photocatalysis (degradation of 80–90%) while chlorfenvinfos is the most difficult organic-phosphorated pesticide to degrade (maximum degradationof 55%).

HCHs and HCB. With regard to HCHs, the degradationpercentages obtained by the photocatalytic treatments arethe lowest. In all cases, adding hydrogen peroxide makes


Table 5: Concentration of pesticides in samples in photocatalytic treatments.

PesticideConcentration (ng L−1)

Initial After TiO2/radiation After TiO2/H2O2/radiation

Alachlor 505 253 177

Aldrin 512 230 26

Ametryn 501 225 225

Atrazine 551 358 331

Chlorfenvinfos 492 295 221

Chlorpyrifos 520 104 104

pp′-DDD 510 332 102

op′-DDE 480 288 144

op′-DDT 482 313 96

pp′-DDT 482 386 145

Desethylatrazine 593 385 356

3,4-Dichloroaniline 658 0 0

4,4′-Dichlorobenzophenone 519 493 363

Dicofol 568 57 57

Dieldrin 508 356 356

Dimethoate 608 274 243

Diuron 501 125 125

α-Endosulphan 475 48 0

Endosulphan-sulphate 483 459 435

Endrin 486 243 243

α-HCH 511 109 409

β-HCH 519 441 441

γ-HCH 521 443 417

δ-HCH 504 428 428

Heptachlor 491 246 246

Heptachlor epoxide A 495 347 347

Heptachlor epoxide B 487 341 341

Hexachlorobenzene 503 327 327

Isodrin 516 206 0

4-Isopropylaniline 512 0 0

Isoproturon 521 78 78

Metholachlor 524 262 210

Methoxychlor 519 234 130

Molinate 551 248 165

Parathion ethyl 507 228 228

Parathion methyl 508 51 51

Prometon 492 271 271

Prometryn 489 220 220

Propazine 508 330 305

Simazine 554 305 277

Terbuthylazine 524 262 262

Terbutryn 514 231 206

Tetradifon 493 296 296

Trifluralin 566 255 170

no difference to the degradation percentages obtained soits use is redundant. The HCB is degraded 35% by thephotocatalyisis, and the addition of hydrogen peroxide doesnot produce any improvement.

Heptachlors. Degradation rates obtained for the heptachlorsare 50% while the heptachlors epoxides are more difficultto degrade (30%). The addition of hydrogen peroxide tothe process does not improve the degradation percentages


Table 6: Degradation of pesticides in blank experiments.

Pesticide Removal by TiO2 Removal by radiation

Trifluralin 0 28

Heptachlor 14 45

Aldrin 20 50

Isodrin 24 55

pp′-DDE 33 46

pp′-DDD + op′-DDT 21 13

pp′-DDT 25 28

of these pesticides. In case of heptachlors, the blanks exper-iments show a degradation of 45% by radiation. The pho-tocatalytic treatment does not therefore improve its degrada-tion with respect to radiation only.

Endosulphans. Under photocatalysis, α-endosulphan showsalmost total degradation while endosulphan-sulphate ispractically undegraded by photocatalytic treatments. Bothhave the same behavior after the addition of hydrogen per-oxide, showing an improvement of 5%, although again thissmall improvement does not compensate for its use.

Drins. With regard to these pesticides, it can be seen bythese treatments that aldrin and isodrin (isomers betweenthem) are more easily degradable that endrin and dieldrin(isomers between them), especially in the case of usinghydrogen peroxide. The rates of degradation of dieldrinand endrin, 50% and 30% respectively, do not improvewhen adding hydrogen peroxide. In the case of aldrin andisodrin, blank experiments show an important degradationby radiation. Thus radiation and not the photocatalytictreatment is responsible for their degradation. However, thedegradation of aldrin and isodrin is noticeably more effectivewhen using hydrogen peroxide treatment, achieving theircomplete degradation.

DDTs. The DDTs studied degrade between 20% and 40%under the photocatalytic treatments. However, similar degra-dations are achieved in the blank experiments. The degra-dation could thus occur by the effect of titanium dioxideor radiation only, not by photocatalytic treatment. Thesedegradation rates increase significantly by adding hydrogenperoxide, reaching 70–80%. Therefore, for the DDTs thepresence of hydrogen peroxide notably favours their degra-dation.

Anilines. The two studied anilines, 3,4-dichloroaniline and4-isopropylaniline, are completely degraded by the photocat-alytic treatments. Therefore, the use of hydrogen peroxideis not necessary for increasing the effectiveness of thetreatment.

Ureas. Isoproturon and diuron, the studied pesticides thatare derivatives of urea, present high degradation ratesunder the photocatalytic treatments, being 85% and 75%,

respectively. The same percentages are obtained when usinghydrogen peroxide, so that its use does not improve theprocess.

Anilides. The three anilides studied also show the samebehaviour with photocatalytic treatments, and the addi-tion of hydrogen peroxide produces a slight improvement,from 5% to 20%. Under photocatalysis, the degradationpercentages are 50–55%. These percentages increase slightlywhen hydrogen peroxide is added to the treatment system,achieving rates of 60–75% degradation in this case.

Other Pesticides. For the rest of the pesticides under study,molinate, trifluralin, tetradiphon, dicofol, and 4,4′-dichlo-robenzophenone, very different degradations are achieved.Molinate and trifluralin are degraded 55% by the photo-catalysis and this percentage increases to 70% when hydro-gen peroxide is added. Therefore, the use of this reagentrepresents a significant improvement in the degradation ofthese two pesticides. In the case of trifluralin, the blankexperiment with radiation produced 30% degradation butits removal is greater by photocatalysis. Degradation ofdicofol is very effective by the photocatalytic treatment, being90%. The presence of hydrogen peroxide does not favourdegradation in this case. The same applies to tetradiphon,which degrades to a lesser extent (40%), but hydrogenperoxide does not increase the effectiveness of the process.The 4,4′-dichlorobenzophenone is barely degraded at all byphotocatalysis but its degradation increases up to 30% whenadding hydrogen peroxide to the treatment.

4. Conclusions

After the completion of this study, the following conclusionscan be drawn.

(i) The pesticides most effectively removed by the photo-catalytic treatments, reaching yields higher than 80%,are parathion methyl, α-endosulphan, chlorpyrifos,3,4-dichloroaniline, 4-isopropylaniline, and dicofol.The least degraded, below 30%, are HCHs, endos-ulphan-sulphate, heptachlors epoxides, and 4,4′-di-chlorobenzophenone.

(ii) Photocatalytic treatment, TiO2/radiation, achieves apartial removal of the studied pesticides of 48%.When hydrogen peroxide is added the average per-centage increases to 57%. The addition of hydrogenperoxide improves the degradation of some of thepesticides studied, mainly isodrin, aldrin, DDTs, 4,4′-dichlorobenzophenone, some triazines, molinate, α-endosulphan, trifluralin, and anilides.

(iii) TiO2/radiation treatment produces a reduction of36% of the initial COD, 43% of the initial toxicity,and chlorine demand is reduced to 1 mg L−1. Thesame treatment with hydrogen peroxide produces alarge increase in toxicity and chlorine demand of thetreated sample and there is no variation in the CODdue to the generation of intermediate compounds


Table 7: % Removal of pesticides by photocatalytic treatments.

Group Pesticide Removal after TiO2/radiation (%) Removal after TiO2/H2O2/radiation (%)

Triazines

Simazine 45 50

Atrazine 35 40

Propazine 35 40

Terbuthylazine 50 50

Prometon 45 45

Ametryn 55 55

Prometryn 55 55

Terbutryn 55 60

Desethylatrazine 35 40

Organic phosphorated

Parathion methyl 90 90

Parathion ethyl 55 55

Chlorpyrifos 80 80

Chlorfenvinfos 40 55

Dimethoate 55 60

HCHs

α-HCH 20 20

β-HCH 15 15

χ-HCH 15 20

δ-HCH 15 15

HCB Hexachlorobenzene 35 35

HeptachlorsHeptachlor 50 50

Heptachlor epoxide A 30 30

Heptachlor epoxide B 30 30

Endosulphansα-Endosulphan 90 100

Endosulphan-sulphate 5 10

Drins

Endrin 50 50

Dieldrin 30 30

Isodrin 60 100

Aldrin 55 95

DDTspp′-DDE 40 70

pp′-DDD + op′-DDT 35 80

pp′-DDT 20 70

Anilines3,4-Dichloroaniline 100 100

4-Isopropylaniline 100 100

UreasIsoproturon 85 85

Diuron 75 75

Carbamate Molinate 55 70

Nitroderivate Trifluralin 55 70

AnilidesAlachlor 50 65

Metholachlor 50 60

Methoxychlor 55 75

ChlorophenolsTetradiphon 40 40

Dicofol 90 90

Chlorinated diphenyl 4,4′-Dichlrobenzophenone 5 30

Average 48 57

(no mineralization) that can be more toxic than theoriginal.

(iv) Both the studied photocatalytic processes produce areduction of turbidity and of the solids in suspensionin the treated samples.

Acknowledgments

The authors wish to thank “MICINN-FEDER” for financingthis paper through the project “Regeneracion de aguas depu-radas mediante procesos de oxidacion avanzada (CTM2008-01876/TECNO).”


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Research Article

Photocatalytic Activity and Characterization ofCarbon-Modified Titania for Visible-Light-ActivePhotodegradation of Nitrogen Oxides

Chun-Hung Huang,1 Yu-Ming Lin,2 I-Kai Wang,3 and Chun-Mei Lu4

1 Product Development Division, Daxin Material Co., No. 15, Keyuan 1st Road, Central Taiwan Science Park, Taichung 40763, Taiwan2 ITRI South, Industrial Technology Research Institute, Room 602, Building 3, 31 Gongye 2nd Road, Annan District,Tainan 70955, Taiwan

3 Department of Chemical Engineering, National Tsing-Hua University, Hsinchu 30043, Taiwan4 Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, Taichung 41101, Taiwan

Correspondence should be addressed to Yu-Ming Lin, [email protected] and I-Kai Wang, [email protected]

Received 15 July 2011; Revised 24 October 2011; Accepted 24 October 2011

Academic Editor: Jinlong Zhang

Copyright © 2012 Chun-Hung Huang et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

A variety of carbon-modified titania powders were prepared by impregnation method using a commercial available titania powder,Hombikat UV100, as matrix material while a range of alcohols from propanol to hexanol were used as precursors of carbonsources. Rising the carbon number of alcoholic precursor molecule, the modified titania showed increasing visible activities ofNOx photodegradation. The catalyst modified with cyclohexanol exhibited the best activities of 62%, 62%, 59%, and 54% for thetotal NOx removal under UV, blue, green, and red light irradiation, respectively. The high activity with long wavelength irradiationsuggested a good capability of photocatalysis in full visible light spectrum. Analysis of UV-visible spectrum indicated that carbonmodification promoted visible light absorption and red shift in band gap. XPS spectroscopic analysis identified the existence ofcarbonate species (C=O), which increased with the increasing carbon number of precursor molecule. Photoluminescence spectrademonstrated that the carbonate species suppressed the recombination rate of electron-hole pair. As a result, a mechanism ofvisible-light-active photocatalyst was proposed according to the formation of carbonate species on carbon-modified TiO2.

1. Introduction

In the field of environmental protection, photocatalysis isbelieved to have an extraordinary potential to treatmentof contaminating compounds in water purification and airpollutants control, such as degradation of nitrogen oxides(NOx), sulfur oxides (SOx), and volatile organic compounds(VOCs) [1–5]. Among the semiconductors, titania has beenregarded as the most promising material for photocatalysisdue to its several advantages, including complete mineraliza-tion without other derivative chemicals, solar-light-inducedoperation at room temperature, non-toxicity, and low cost[6, 7]. Generally, titania may exist in three different formsof crystalline phases: rutile, brookite, and anatase phase[8]. Rutile is thermodynamically stable and formed aftercalcination over 600◦C with dense structure, resulting in lowphotocatalytic efficiency [9, 10]. Brookite and anatase are

structures of metastable states, whereas the former is notstable enough to exist alone. Anatase is recognized as themost suitable phase for photocatalysis [5]. However, withthe band gap of 3.2 eV, anatase phase TiO2 can be photo-activated under UV light irradiation (wavelength shorterthan 388 nm), which only accounts for 4% of the solarspectrum. To extend the utilization over the main part (45%)of visible region in solar spectrum, several attempts havebeen made to reduce the band gap.

One strategy is preparing metal-modified titania bydoping or loading transitional metals, such as V, Mn, Fe,Cr, Cu, Ni, W, and others [11–15]. The noble metals werereported to produce the highest Schottky barrier and tofacilitate electron capture [16]. Therefore, Ag, Ru, and Pt[17–21] were employed to modify TiO2 for enhancing UVphotocatalytic efficiency and/or visible light activities.


Nonmetal doping is the other approach to achieve thegoal of TiO2 modification. Sato [22] discovered the red-shift phenomena of N-promoted TiO2. Afterward Asahi et al.[23] reported that a small amount of substitutional Natoms into the TiO2 structure by the sputtering method willinduce the Ti3+ sites on the TiO2 surface and significantlynarrow the band gap. Nakamura et al. [24] found that TiO2

treated with hydrogen plasma can generate oxygen vacanciesand create subband gap to promote visible light response,despite the expensive appliance. Organic dyes as the lightsensitization agents were also considered to be added onthe TiO2 surface to improve the light efficiency [25, 26].However, the consumption of dye molecules during thereaction process will limit the application. Recently, carbonwas widely used for visible modification of TiO2 [27–31].It was reported that carbon modification is more efficient,even higher than nitrogen modification [28]. The carbondoped TiO2 could be prepared by simple method such asmechanochemical operation by grinding TiO2 with ethanolin air [32]. On a comprehensive survey, carbon is one ofthe practicable materials, for the consideration of economicand practical mass production. Therefore, in this study, wetried to use carbon precursors to modify TiO2 powder byimpregnation method, enabling the significant visible pho-toactivity. Moreover, a possible photocatalytic mechanismwas proposed to speculate the role of carbonaceous speciesfunctioning with TiO2, based on the evidences observedfrom various characterization techniques.

2. Experimental

2.1. Preparation of Carbon-Modified TiO2 Photocatalysts.All chemicals in this study are reagent-grade without fur-ther purification. A commercially available TiO2 powder(Hombikat UV100, Sachtleben Chemie, 100% anatase),symbolized as UV100, was used as the matrix materialfor modification. Seven kinds of alcohols were adopted asthe carbon sources: 2-propanol, n-propanol, 2-butanol, i-butanol, 4-methyl-2-pentanol, n-hexanol, and cyclohexanol,symbolized as 2-C3, C3, 2-C4, iC4, 2-C6, C6, and cC6,respectively, according to the carbon number of moleculeand the isomer structure. Impregnation method was used toprepare carbon-modified TiO2. The synthesis procedures aredepicted in Figure 1. Firstly, 5 g UV100 powder was addedinto an alcoholic solution containing 30 mL alcohol and70 mL D.I. water. After ultrasonic bath for several minutes,the slurry was then magnetically stirred for 30 min withsufficient mixing. Subsequently, the modified sample wasevaporated in an oven at 110◦C for 4 h and then milledto powder, followed by calcination at a heating rate of 1◦C/min up to 200◦C for 5 h in a furnace. Finally, seven kindsof carbon-modified TiO2 were obtained and symbolizedas 2-C3/UV100, C3/UV100, 2-C4/UV100, iC4/UV100, 2-C6/UV100, C6/UV100, and cC6/UV100, ascribed to theiralcoholic precursors.

2.2. Experimental Apparatus and Procedure. Photocatalyticdegradation of nitrogen oxides (De-NOx) was carried out at

D.I. water

Mixing

30 mL alcohol

Stirred for 30 min

Pulverized

Calcined at 200◦C for 5 h

Carbon-modifiedTiO2 powder

Synthesis process

5 g UV100added

Dried at 110◦C

Figure 1: Schematic procedure for synthesis of carbon-modifiedTiO2 powder by impregnation method.

room temperature by the setup consisting of a continuousflow reactor (a round-shaped Pyrex glass vessel) connectedto the gas suppliers and the analytic system, as shown inFigure 2. To simulate atmospheric environment and reachthe optimal value, the relative humidity was maintained at50% by adjusting a humidified air stream with a dry airstream [4]. Then, a nitric oxide (NO) gas stream, providedby a cylinder containing 100 ppmv NO (San Fu ChemicalCo., N2 balance), was added to the air stream to reach adesired NO concentration of 1 ppmv and a total volumetricflow rate of 1 L/min as the feedstock. The photocatalyst withamount of 0.5 g was dispersed into an appropriate amountof D.I. water contained in a 50 mL beaker, followed by wellstirring. Then the mixed slurry was uniformly spread ona disk-like glass plate (diameter = 12 cm). After drying at110◦C for 30 min, dehydrated powder attached to the platewas obtained. Subsequently, the glass plate coated with thephotocatalyst was set in the center of reactor for use. AUV light lamp over the reactor delivered the ultravioletillumination in the wavelength range of 330–420 nm. For vis-ible light reaction, three light-emitting diodes (LEDs) wereemployed, including blue LEDs (BLED, 430–530 nm), greenLEDs (GLED, 470–570 nm), and red LEDs (RLED, 590–680 nm). The related photon energy distribution profiles areshown in the inset of Figure 2. The intensity of the lightsource was controlled at 1 mW/cm2 and was measured by aspectrophotometer (Ocean Optics, USB2000).

In a typical experiment, the feedstock was introducedto the reactor with sample plate for several minutes inthe dark. Light was turned on as the system reached theadsorption equilibrium (i.e., NO concentration recoveringto 1 ppmv). Meanwhile, the effluent concentrations of NOand nitrogen dioxide (NO2) from reactor were measured


Aircylinder

Humidifier

MFC

MFC

MFC

Inte

nsi

ty

Mixer

UV or LED light

300

UVBLED

GLED RLED

350 400

Wavelength (nm)

450 500 550 600 650 700

Vent

Pyrex glass reactor

Glass plate coated with photocatalyst

NO/N2

cylinder(100 ppmv)

NOx

analyzer

Figure 2: Schematic diagram of De-NOx system. The inset shows photon energy distribution of UV lamp and LEDs.

by an on-line chemiluminescent NOx analyzer (Eco Physics,CLD 700AL). In addition, blank test was performed underthe same condition except photocatalyst existence. The resultcertainly guaranteed all the materials of the system inactiveon photocatalytic De-NOx.

NO is oxidized in a serial steps (NO→NO2 →NO3−)

over the photocatalyst under illumination [4]. The finalproduct, NO3

− ion, was adsorbed on the photocatalystsurface and could be easily washed out by water forphotocatalyst in reuse [17]. In data analysis, the totalNOx concentration ([NOx]), NO conversion (NOC), NO2

selectivity (NO2S), and total NOx removal (NOxR) weredefined as follows:

[NOx] = [NO] + [NO2], (1)

NO conversion = [NO]in − [NO]out

[NO]in, (2)

NO2 selectivity = [NO2]out

[NO]in − [NO]out, (3)

Total NOx removal = [NO]in − [NOx]out

[NO]in. (4)

The photocatalyst performance could be evaluated by thetotal NOx removal, or the NO conversion conjunctive withthe NO2 selectivity.

2.3. Characterization and Analysis. The particle sizes andthe morphology of photocatalysts were examined by field-emission scanning electron microscopy (FE-SEM, HitachiS4800-1) and transmission electron microscopy (TEM,Philips Tecnai 20). Each powder sample was dispersed inanhydrous alcohol and then transferred to a copper grid(with carbon support film, 300-mesh). After drying at 50◦Cin an oven, the pretreatment was completed for use.

The specific surface areas (SSAs) were measured bynitrogen adsorption at 77 K using Micrometrics equipment(Model ASAP 2000). Samples were outgassed to a vacuumlower than 10−4 Torr at 200◦C for 2 h to remove adsorbedimpurities prior to each measurement. Specific surface areaswere evaluated by Barrett-Emmet-Teller (BET) method [33].

The crystal structures were analyzed by an X-ray powderdiffractometry (XRD, Rigaku RU-H3R) in the reflectionmode with Cu Kα radiation. The angular domain was 20–80◦ (2θ). The surface structures were identified by a Micro-Raman spectrometer (Renishaw 1000B), conducted with alow-power green laser (100 mW) at 532 nm. Weak laser of1% to 10% power was used to examine the surface structureof the samples.

The UV-visible absorption spectra of all samples wereperformed by a powder UV-visible spectrophotometer (UV-VIS, Shimadzu UV-2450) equipped with an integratingsphere accessory for diffuse reflectance spectra over a rangeof 200–800 nm by using BaSO4 as the reference. The obtainedreflectance data (R) was converted to the absorbance value,F(R), based on the Kubelka-Munk theory, as follows:

F(R) = (1− R)2

2R. (5)

The surface composition and the chemical states ofcatalysts were investigated by an X-ray photoelectron spec-troscopy (XPS, Physical Electronics ESCA PHI 1600) withMg Kα radiation. The charging effects were corrected byadjusting the C 1s peak to a position of 284.5 eV.

The decomposition behavior of products was determinedby thermogravimetric analysis (TGA) using a thermo-gravimeter mass spectrometer (Seiko instruments, TG/DT6200) at a heating rate of 10◦C/min between 200 to 800◦C inflowing nitrogen. Samples (about 3 mg) were set and heatedto 200◦C for 30 min prior to each experiment.

The photoluminescence (PL) emission spectra weremeasured by using a Xe lamp with wavelength of 325 nmas an excitation light source. The reflectance spectra of thesamples are over a range of 200–800 nm.


0

10

20

30

40

50

60

70

80RLEDGLEDBLEDUV

NO

con

vers

ion

(%

)

a b c d e f g h a b c d e f g h a

(A)

b c d e f g h a b c d e f g h

RLEDGLEDBLEDUV

a b c d e f g h a b c d e f g h a b c d e f g h a b c d e f g h0

10

20

30

40

50

60

70

NO

2se

lect

ivit

y (%

)

(B)

RLEDGLEDBLEDUV

0

10

20

30

40

50

60

70

a b c d e f g h a b c d e f g h a b c d e f g h a b c d e f g h

Tota

l NOx

rem

oval

(%

)

(C)

Figure 3: De-NOx activities including NO conversion (A), NO2 selectivity (B), and total NOx removal (C) on UV100 (a), 2-C3/UV100 (b),C3/UV100 (c), 2-C4/UV100 (d), iC4/UV100 (e), 2-C6/UV100 (f), C6/UV100 (g), and cC6/UV100 (h).


3.1. Photocatalytic Ativity of Carbon-Modified TiO2. TheNO conversion (NOC), NO2 selectivity (NO2S), and totalNOx removal (NOxR) on unmodified UV100 and carbon-modified UV100 under various light irradiations are illus-trated in Figure 3. In a serial oxidation steps (NO→NO2 →NO3

−), NOC, NO2S, and NOxR represent the initialoxidation of NO converted, the undesired byproduct ratio,and the total oxidation ability, respectively. Accordingly, awell-active photocatalyst should basically provide a highNOC as well as a low NO2S, resulting in a high NOxR. Withthe same NO conversion, the one with lower NO2 selectivitycatalyzed the oxidation of NO2 to NO3

− more completely.

In UV light reaction, unmodified UV100 exhibited a highNOC (71%) attributed to its pure anatase phase structureand high specific surface area [34]. However, the high NO2Srevealed (27%) the insufficiency in NO2 suppressive abilityand led to the reduction of NOxR (51%). Compared withUV100, carbon-modified UV100 showed a similar or slight-lower NOC. It was referred that carbonaceous residues onTiO2 surface partly obstructed UV absorption, as evidenced

in UV-VIS analysis (discussed later). Even so, carbon speciesprobably functioned with TiO2 and substantially lowered theNO2S. As a result, all carbon-modified TiO2 displayed higherNOxR than UV100.

In visible light reaction, apparently UV100 providedpoor activities, especially under GLED and RLED irradiationwith the NOxR of 3% and 1%, respectively. In contrast,carbon-modified UV100 showed progressive activity underwhichever visible light irradiation. It suggested that thevisible light response may be offered by the conjugationof carbonaceous residues and TiO2. Additionally, like theperformance in UV reaction, NO2S was also successfullysuppressed on carbon-modified UV100 under visible lightirradiation. Furthermore, the visible light activity increasedwith the increase of carbon number in alcoholic precursors.Visible-photocatalytic activities were in the order of UV100 <2-C3/UV100 < C3/UV100 < 2-C4/UV100 < iC4/UV100 < 2-C6/UV100 < C6/UV100 < cC6/UV100. Among the products,cC6/UV100, prepared by modification of the highest carbonnumber in alcoholic precursor, exhibited the highest visibleactivity of 62%, 59%, and 54% for BLED, GLED, and RLEDreaction, respectively. Compared with its UV activity (62%),


Table 1: Characterization of the band gap, specific surface area, and weight loss in TGA.

Sample Band gapb (eV) Specific surface area (m2/g)Weight lossa (wt.%, between 200 and 800◦C)

Total 200–380◦C 380–600◦C 600–800◦C

UV100 3.26 249 4.23 2.33 1.01 0.90

2-C3 3.24 252 3.53 1.71 0.95 0.87

C3 3.24 272 4.84 2.10 2.18 0.57

2-C4 3.24 279 4.88 2.13 2.18 0.58

iC4 3.23 298 5.73 2.86 2.01 0.89

2-C6 3.18 290 5.71 2.44 2.69 0.58

C6 3.07 294 7.73 3.70 3.18 0.86

cC6 3.04 297 6.82 2.91 3.27 0.64aAll samples were pretreated at 200◦C before test.

bDetermined by the intercept in Tauc Plot.

Table 2: The elemental surface composition and atomic ratio ofO/Ti resulted from XPS analysis.

Sample Ti (%) O (%) N (%) C (%) O/Ti

UV100 15.7 58.9 1.4 24.0 3.8

2-C3 20.6 52.7 — 26.7 2.6

C3 18.8 49.9 — 31.3 2.7

2-C4 16.7 49.3 — 34.0 3.0

iC4 11.4 49.9 — 38.7 4.3

2-C6 16.0 52.6 — 31.4 3.3

C6 12.1 45.2 — 42.7 3.7

cC6 14.0 42.3 — 43.7 3.0

cC6/UV100 was stable enough to extend its photoactivity tovisible region, even under red light irradiation. As can be seenin Tables 1–3, specific surface area, carbon amount, and C–C and C=O ratio obviously increase with increasing carbonnumber of alcoholic precursor. In addition, higher visiblelight absorbance and lower band gap were observed as thecarbon number of alcoholic precursor increased (Figure 7).All these positive factors may contribute to the highestactivity of cC6/UV100 catalyst.

In summary, via impregnation with alcoholic precursorsfollowed by low temperature calcination at 200◦C, car-bonaceous residues acted as sensitizing agent with TiO2 tolower NO2 production efficiently, facilitated more completereaction, and improved the visible photoactivity. Visibleactivities increased with the rise of carbon number inprecursor molecule. And with the same carbon number (e.g.,n = 6), photocatalytic levels were in the order of 2-Cn <Cn < cCn, depending on the isomer structure. Therefore,the carbonaceous species resulted from different molecularstructure of alcoholic precursors may play an important roleon visible light activity.

3.2. Characterization of Carbon-Modified TiO2

3.2.1. Morphology. Figure 4 shows the FE-SEM and TEMimages of various photocatalysts. FE-SEM photographsindicated that the primary particle size was unchanged

(7–14 nm) after carbon-modification, while the secondaryparticle was more agglomerate. From TEM photographs, itcould be found that amorphous carbonaceous species existon the interfaces or surface of TiO2. It may be the reason forhigh agglomeration of carbon-modified TiO2 catalysts.

3.2.2. Specific Surface Area. The specific surface areas ofall samples were in range of 249–298 m2/g (Column 3 inTable 1). All the modified samples showed larger specificsurface areas than UV100 (249 m2/g). The higher the carbonnumber in precursor molecule was, the higher the specificsurface area of the catalyst was obtained. Zhang et al.[35] investigated carbon-modified TiO2 by using glucoseas precursor and mentioned that carbon residue did notmake TiO2 growth. Same thing was observed in this study.Therefore, the carbonaceous species, existing on the TiO2

surface, may create more specific surface area and favor thephotocatalytic efficiency.

3.2.3. Crystal Structure. The XRD patterns are shown inFigure 5. All the crystal structures of samples were almostanatase, attributed to several main peaks (2θ = 25.3 with[101], 37.8 with [004], 48.1 with [200], 54.0 with [105],and 62.7 with [204]). There was no significant differencebetween samples, which meant that carbon modificationdid not obviously change the crystal phase through low-temperature calcination. According to Scherrer’s equation,the crystal size is larger with sharper main peak. All thecrystal sizes of samples were in range of 9–12 nm, consistentwith those observed in TEM graphs.

Raman spectrum is surface-sensitive for surface charac-terization. The Raman spectra of all samples are shown inFigure 6. Firstly, a weak laser power (1% of 100 mW) wasintroduced as the source to detect Raman signals. As shownin Figure 6(a), all samples showed anatase peaks mainly.However, a shoulder peak at 480 cm−1 grew up with theincreasing carbon number. It was attributed to brookite,regarded as a visible-light-responsive phase [36]. When thelaser power was enhanced to 10% of 100 mW, the brookitepeak disappeared (Figure 6(b)) due to its characteristicsof unstable phase. This phenomenon confirmed that theunstable phase was brookite. Moreover, 10% of 100 mW


Table 3: Deconvolution results of C 1s and O 1s spectra from XPS analysis.

SampleDeconvolution of C 1s spectraa Deconvolution of O 1s spectrab

C–C ratio(%)

C=O ratio(%)

Ti–O ratio(%)

O–H ratio(%)

C=O ratio(%)

UV100 22.6 1.4 47.2 10.7 1.0

2-C3 19.9 6.8 44.9 2.9 4.9

C3 22.8 8.5 42.5 2.7 4.7

2-C4 25.4 8.6 40.1 2.7 6.5

iC4 26.1 12.6 38.3 3.2 8.3

2-C6 18.8 12.6 39.3 3.7 9.6

C6 29.2 13.4 33.4 2.6 9.1

cC6 31.4 12.3 28.1 3.1 11.1aPeaks fitted at 284.5–284.7 eV and 287.2–287.6 eV for C–C and C=O bonding.

bPeaks fitted at 529.3–529.9 eV, 530.9–531.5 eV, and 531.9–532.8 eV for Ti–O, O–H, and Ti=O bonding, respectively.

power was employed to treat the same sample for severaltimes. The main crystal phase would transform from anataseto rutile (Figure 6(c)) over repeated treatment because thestrong incident power might cause transformation to theultimately stable phase, that is, rutile. Basically, UV100 wasalmost of anatase phase. After carbon impregnation, smallportion of brookite phase was observed on UV100 surface.And visible light activities might be enhanced with thebrookite phase.

3.2.4. UV-Visible Absorption. The apparent colors are whit-ish for UV100, pale-yellowish for 2-C3/UV100, C3/UV100and 2-C4/UV100, yellowish for iC4/UV100 and 2-C6/UV100, and brownish for C6/UV100 and cC6/UV100. Thecolors shift from white to brown indicated the extent ofvisible light absorption influenced by carbon modification,depending on the carbon number of precursor molecule.The UV-visible absorbance spectra for various TiO2 samplescan be seen in Figure 7(a). Obviously, UV100 displayedstrong absorbance in UV region, corresponding to a sharpabsorption edge near 380 nm. The band gap of UV100was well known as 3.2 eV based on the pure anatase phasestructure. In the presence of carbon residue, the visibleabsorbance increased with the increasing carbon numberof precursor molecule for carbon-modified TiO2. Despitea little expense of UV absorbance, the modified samplesstill provided sufficient UV activities in De-NOx reaction.In addition, the optical absorption edges of carbon-modifiedTiO2 showed obvious red shifts and long absorption tailingover 700 nm in visible region, especially in samples ofC6/UV100 and cC6/UV100. This phenomenon impliedcertain change in band gap structure occurred due to theexistence of carbon residue.

For semiconductor material, the direct band gap can bederived by establishing Tauc Plot of transformed Kubelka-Munk function versus the absorbed light energy [37]. As canbe seen in Figure 7(b), the so-called Tauc optical band gapis obtained at the intercept between the extension line ofslop and the base line. For example, the band gap of UV100is calculated as 3.26 eV from the Tauc Plot, correspondingto the wavelength of 380 nm. The direct band gaps were

thus calculated and listed in Table 1 (Column 2). The bandgap reduced with increasing carbon number of precursormolecule, which evidenced that the alcoholic modificationcertainly made a red shift and thus the absorption of visiblelight. The red shift essentially provided the activity of catalystunder the visible light illumination.

For carbon-modified TiO2, there exist two types ofstructure, that is, oxygen substitution to form Ti-C bondingand carbon residue deposition in the form of carbonatespecies (C=O). In former case, a subband gap may becreated to show both main peak and shoulder peak inUV-Vis spectrum [27, 38]. In the latter case, the carbonresidues are usually deposited on the interfaces or surface ofTiO2, making a red shift in UV-VIS spectrum [39]. In thisstudy, obvious red shift was found for carbon-modified TiO2

samples. Therefore, the formation of carbonate species wasexpected.

3.2.5. XPS Analysis

Analysis of Chemical Elements. XPS analysis revealed theelemental surface composition of the samples, including Ti,O, N, and C, as summarized in Table 2. According to thesynthesis procedure, element N was not detectable in thecarbon-modified TiO2 sample as expected, while trace Nwas detected in UV100 catalyst probably introduced withstray ions in the commercial manufacturing process. Thecomposition of elements Ti and O and its atomic ratioof O/Ti are demonstrated in Table 2. The atomic ratios ofO/Ti were in range of 2.6–4.3, greater than theoretical valueof 2, probably due to the surface phydrophilicity. Carbon-modified TiO2 showed lower O/Ti ratio than that of UV100(3.8), indicating the decrease of hydrophilicity caused bythe existence of carbon residue. The carbon content showedrelatively high values in range of 24.0–43.7%. The carbonmight come from two sources: adventitious element carbonfrom the impurity of equipment chamber [36], and carbonresidues from the impregnation of alcoholic precursor. Thecarbon content detected in UV100 could be regarded asthe baseline amount of adventitious element carbon. Thecontent higher than that baseline could be determined as


40 nm 20 nm

20 nm20 nm

(a) (b)

(c)

(A)

(d)

Amorphous carbon

5 nm

(B)

Figure 4: FE-SEM images (A) of UV100 (a), C3/UV100 (b), C6/UV100 (c), and cC6/UV100 (d) and TEM image of cC6/UV100 (B).

20 30 40 50 60 70 80

cC6/UV100

C6/UV100

2-C6/UV100

iC4/UV100

2-C4/UV100

C3/UV100

2-C3/UV100

A A AA

A

UV100

Inte

nsi

ty

2θ (deg)

Figure 5: XRD patterns of the photocatalysts.

the amount of carbon residues from impregnated alcoholicprecursor. Obviously, the higher the carbon number ofprecursor molecule was, the larger amount of carbon residuewas obtained.

Analysis of Chemical States. Ti 2p, C 1s, and O 1s spectrawere conducted for analysis of chemical states. No differ-ence was found in Ti 2p spectrum (not shown here) forvarious samples, indicating no change of surface Ti bondingoccurred because of the impregnation of carbon precursor.For C 1s spectrum (Figure 8(a)), two peaks at 284.5 eVand 287-288 eV were observed for all the carbon-modifiedsamples, except for the original UV100 sample. The peakat 284.5 eV denoted C–C bonding caused by adventitiouselement carbon. The peak at 287-288 eV denoted C=O

bonding represented by carbonate species. All the carbon-modified samples showed higher intensity of C=O peak thanthat of UV100, indicating the effect of carbon residues. Also,the intensity increased with increasing carbon number ofprecursor molecule. For O 1s spectrum (Figure 8(b)), threekinds of peaks, that is, Ti–O (529-530 eV), O–H (531 eV),and C=O (532 eV), could be observed. Ti–O bonding isthe main peak of TiO2 and can be seen in all the samples.O–H bonding might be introduced by the hydrophilicityof water vapor to form the surface-bound hydroxyl group[1]. The third one was C=O bonding which represented theoccurrence of carbonate species due to the carbon modifica-tion. Therefore, the intensity of C=O bonding increased withincreasing carbon number of precursor molecule, which wasconsistent with the results from C 1s spectrum.

To further investigate the contribution of individualbonding, the ratio of two peaks in C 1s and three peaks in O1s spectrum would be calculated separately. Deconvolutionmethod was thus conducted to calculate the ratio of eachbonding. Taking examples of UV100 and cC6/UV100, thepeaks of C=O and C–C bonding in C 1s spectrum could beseparated and the ratio could be calculated by the integrationarea, as shown in Figure 9(a). The same procedure could beconducted for O 1s spectrum (Figure 9(b)). Using deconvo-lution method, the ratio of each bonding in all samples couldbe calculated and listed in Table 3.

For C 1s analysis, adventitious element carbon (rep-resented by C–C bonding) possessed the ratio of 18.8–31.4% and no obvious trend among the samples wasobserved due to the characteristics of adventive impurityitself. In contrast, carbonate species (represented by C=Obonding) showed obvious trend with the carbon number ofprecursor molecule. In UV100 sample, only trace carbonatespecies (1.4%) were observed. In carbon-modified samples,


1% power

cC6/UV100

C6/UV1002-C6/UV100

iC 4/UV1002-C4/UV100

C3/UV100

2-C3/UV100

UV100

BA

A AIn

ten

sity

(a.

u.)

300 400 500 600 700 800 900 1000

Raman shift (cm−1)

(a)

10% power

cC6/UV100

C6/UV1002-C6/UV100

2-C4/UV100

C3/UV1002-C3/UV100

UV100

AA A

Inte

nsi

ty (

a.u

.)

300 400 500 600 700 800 900 1000


(b)

Inte

nsi

ty (

a.u

.)

300 400 500 600 700 800 900 1000

10% poweriC 4/UV100

RR

A

A

6th test

3rd test2nd test

4th test

5th test

1st test

A


(c)

Figure 6: Raman spectra with 1% (a) and 10% (b) power for all samples; 10% power for iC4/UV100 over 6 times of test (c).

400 500 600 700 800

0

0.5

1

300 400 500 600 700 8000

1

2

3

4

5

6

7

cC6/UV100C6/UV1002-C6/UV100iC4/UV1002-C4/UV100C3/UV1002-C3/UV100UV100

VISUV

UV100

C3/UV1002-C4/UV100

cC6/UV100

iC4/UV100

C6/UV1002-C6/UV100

2-C3/UV100

Wavelength (nm)

F(R

)

(a)

3.63.53.43.33.23.13

0

1

2

3

cC6/UV100C6/UV1002-C6/UV100

2-C4/UV100iC4/UV100

C3/UV1002-C3/UV100UV100

Ebg (eV)

[F(R

)∗E

bg]1/

2(a

.u.)

(b)

Figure 7: UV-visible absorption spectra (a) and the plot of transformed Kubelka-Munk function versus the energy band gap (b).


292 290 288 286 284 282 280 278 276

C 1s

C=O

cC6/UV100C6/UV1002-C6/UV100iC 4/UV100

2-C4/UV100C3/UV1002-C3/UV100UV100

Inte

nsi

ty

Binding energy (eV )

C–C

(a)

Inte

nsi

ty

Binding energy (eV )

536 534 532 530 528 526 524 522

C=O

O 1s

cC6/UV100

C6/UV1002-C6/UV100iC 4/UV100

2-C4/UV100C3/UV100

2-C3/UV100

UV100

Ti–O

O–H

(b)

Figure 8: C 1s (a) and O 1s (b) XPS spectra for all samples.

Raw dataFitting sumPeak 1 (C–C)

Peak 2 (C=O)Baseline

UV100

(5.93%)

Inte

nsi

ty (

cps

)In

ten

sity

(cp

s )

294 292 290 288 286 284 282 280 278

C=O(28.1%)

C–C(71.9%)

Binding energy (e.V)

294 292 290 288 286 284 282 280 278


C=O

C–C94.07%

cC6/UV100

(a)

Inte

nsi

ty (

cps

)In

ten

sity

(cp

s )


538 536 534 532 530 528 526 524


538 536 534 532 530 528 526 524

C=O(26.3%) O–H

(7.2%)

Ti–O(66.5%)

cC6/UV100

C=O(1.7%) O–H

(18.2%)

Ti–O(80.1%)

UV100

Raw dataFitting sumPeak 1 (Ti–O)

Peak 2 (O–H)Peak 3 (C=O)Baseline

(b)

Figure 9: Deconvolution of C 1s (a) and O 1s (b) XPS spectra for UV100 and cC6/UV100.


92

94

96

98

100

cC6/UV100

C6/UV100

2-C6/UV100iC4/UV100

2-C4/UV100

UV100C3/UV100W

eigh

t (%

)

2-C3/UV100

200 300 400 500 600 700 800 900 1000

Temperature (◦C)

(a)

UV100

200 300 400 500 600 700 800 900 1000

cC6/UV100

C6/UV100

2-C6/UV100

iC4/UV100

2-C4/UV100

C3/UV100

2-C3/UV100

Wei

ght

loss

rat

e

Temperature (◦C)

(b)

Figure 10: TGA (a) and DTG (b) curves for all samples obtained in flowing N2.

300 400 500 600

0

1

Wavelength (nm)

0.5

F(R

)

cC6/SiO2

SiO2

cC6/SiO2 110◦C

200◦C

Figure 11: UV-visible absorption spectra for SiO2, cC6/SiO2 110◦C, and cC6/SiO2 200◦C.

the amount of carbonate species clearly increased withincreasing carbon number of precursor molecule.

For O 1s analysis, Ti–O bonding possessed major portionas expected in TiO2 matrix. The ratio of O–H bondingshowed great difference between UV100 and carbon-modified TiO2 samples. High ratio of O–H bonding (10.7%)exhibited high hydrophilicity of UV100. However, low ratioof O–H bonding (2.6–3.7%) implied low hydrophilicityof carbon-modified TiO2, as described in literature [40,41]. Carbonate species (represented by C=O bonding)showed obvious trend with the carbon number of precursormolecule, consistent with the result from C 1s analysis.In UV100 sample, only trace carbonate species (1.0%)were observed. In carbon-modified samples, the amount ofcarbonate species clearly increased with increasing carbonnumber of precursor molecule. In summary, the trend ofcarbonate species is in good correlation with the visiblelight absorption and photoactivity of nitrogen oxides degra-

400 450 500 550 6000

50

100

150

200

250

300

2-C6/UV100

UV100C3/UV100

cC6/UV100C6/UV100

Inte

ns

ity

(a.u

.)

Wavelength (nm)

iC4/UV100

Figure 12: PL spectra for UV100, C3/UV100, iC4/UV100, 2-C6/UV100, C6/UV100, and cC6/UV100. Excitation light source: Xelamp with wavelength of 325 nm.

dation under visible light illumination. Therefore, it canbe concluded that the carbonate species may make majorcontributions for visible-light-active photocatalysis.

3.2.6. Thermogravimetric Analysis (TGA). Figure 10(a)shows TGA spectra of samples over the temperature rangeof 200–800◦C under nitrogen atmosphere. All the sampleswere preheated at 200◦C to remove the physical adsorbedwater [42, 43]. Figure 10(b) shows DTG spectra of samples,resulted from first derivation of TGA curves, to indicatethe rate of weight loss. Three weight loss zones could becategorized as follows: Zone I: 200–380◦C, weight lossfrom chemical adsorbed water and some low-temperaturevolatile carbon residues; Zone II: 380–600◦C, weight lossfrom the high-temperature pyrolytic carbon species, that is,carbonate species mainly; and Zone III: 600–800◦C, weightloss probably accompanied with collapse of interagglomerate


UV

Visible

Carbonaceousspecies

Photocatalyticoxidation

Assistant tooxidation

Assistant tooxidation

O=C

O=C

C=O

VB

CB e−e−e−

h+

h+

TiO2

NO → NO2 → NO3−



O2

O2

OH−

OH−

H+

H+

•OH

•OH

•O2−

•O2−

•O2H

•O2H

Figure 13: Schematic illustration of photocatalytic mechanism on carbon-modified TiO2.

and escape of trace moiety during the phase transformationfrom anatase to rutile [43–45]. The weight loss data wassummarized in Table 1 (Column 4–7). The total weightloss of each sample was in range of 3.53–7.73%. Thedistributions in Zone I and Zone III could be found ineach sample, ascribed to the common properties of allthe samples. However, the weight loss in Zone II showeddifferent trend and obviously increased with increasingcarbon number of precursor molecule. As a result, largeamount of carbon residues were existed on TiO2 samplemodified by precursor with large carbon number. Andcarbonate species should be the main composition accordingto the XPS analysis as discussed earlier.

3.3. Role of Carbonate Species in Mechanism of

Photocatalytic Reaction

3.3.1. Visible Light Absorption of Carbonate Species. In thispaper, we already proved that carbonate species could beformed onto interfaces or surface of TiO2 by impregnat-ing alcoholic precursor and calcining at 200◦C. The pre-pared carbon-modified TiO2 illustrated obvious visible lightabsorption. To verify the absorption property mainly orig-inated from the carbonate species, we prepared the samplewith carbon residue on fumed silica by using cyclohexanol asprecursor. Two calcination temperatures (110◦C and 200◦C)were employed. The sample calcined at 110◦C was denoted ascC6/SiO2 110◦C and the other at 200◦C as cC6/SiO2 200◦C.The UV-Vis spectra of carbon deposited SiO2 are shownin Figure 11. SiO2 sample exhibited no absorbance of light.The light absorbance of carbon deposited SiO2 increasedwith rising calcination temperature, where absorbance washigher at 200◦C than that at 110◦C. The enhancement ofabsorbance may be attributed to the existence of conjugatestructure in the carbon species [30, 34, 46]. Long alkyl

chain or cyclic configuration of alcoholic precursor mightfavor the formation of conjugate structure in carbonatespecies. Therefore, the precursor with high carbon numberof molecule could make better visible light absorption andthen the photocatalytic activity.

3.3.2. Photoluminescence(PL) Emission Spectra. Photolumi-nescence emission occurs when a short wavelength (e.g.,32 nm) photoenergy is absorbed to excite an electron fromvalence band and then a longer wavelength luminescenceis emitted via recombination of electron-hole pair. Thehigher the intensity of luminescence is, the faster is therecombination of electron and hole. Therefore, high intensityof luminescence may imply low photocatalytic activity [47].As shown in Figure 12, UV100 had the highest intensityof luminescence which meant the lowest photocatalyticactivity as discussed in previous paragraph. The inten-sity of modified sample decreased with increasing carbonnumber of precursor molecule. The sample modified withcyclohexanol, cC6/UV100, showed the much lower intensityof luminescence and led to higher photocatalytic activity,especially in visible region (Figure 3). This suppression ofluminescence might be due to the existence of carbonatespecies, which acted as the sensitizing agent of TiO2 andprohibitor of electron-hole recombination.

3.3.3. Photocatalytic Mechanism on Carbon-Modified TiO2.In summary, a mechanism of photocatalytic degradationof nitrogen oxides over carbon-modified TiO2 might beproposed in Figure 13. According to the mechanism, thecarbonate species in carbon residue on TiO2 surface wereregarded as the sensitizer to absorb visible light and inducethe electron-hole pair of TiO2 so that hydroxyl radicals(•OH) or superoxide ions (•O2

−), which may transform to•O2H with H+ in humid environment, could be generated


as active species for oxidation reaction [29]. The existence ofcarbonate species could also suppressed the recombinationrate of electron-hole pair to enhance the photocatalyticactivity. In addition, the carbonate species acted as promoterfor deep oxidation in the degradation of nitrogen oxides andlower the selectivity of NO2. Therefore, highly active visiblelight photocatalyst could be achieved by carbonmodifiedTiO2.

4. Conclusions

An active visible light photocatalyst was prepared by impreg-nation method using a range of alcohols, that is, frompropanol to cyclohexanol, as precursor to modify TiO2

powder. The prepared carbon-modified TiO2 showed betterphotoactivity of nitrogen oxides degradation than that ofunmodified TiO2. The activity increased with increasingcarbon number of alcoholic precursor. The form of car-bonate species on carbon-modified TiO2 was identified byXPS and TGA analysis. A photocatalytic mechanism wasthus proposed for photodegradation of nitrogen oxides.According to the mechanism, the carbonate species incarbon residue onto TiO2 interfaces or surface acted assensitizer for visible light response, prohibitor for electron-hole recombination, and promoter for oxidation of nitrogenoxides. High carbon number of alcoholic precursor wouldmake high amount of carbonate species so that betterperformance of photocatalysis could be achieved.

Acknowledgment

The authors gratefully express thanks for the assistance ofMicro-Raman spectrometer provided by Professor Chia-Liang Cheng (Department of Physics, National Dong HwaUniversity).

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Research Article

Sm2FeTaO7 Photocatalyst for Degradation of Indigo Carmine Dyeunder Solar Light Irradiation

Leticia M. Torres-Martınez,1 Miguel A. Ruiz-Gomez,1, 2 M. Z. Figueroa-Torres,1

Isaıas Juarez-Ramırez,1 and Edgar Moctezuma2

1 Departamento de Ecomateriales y Energıa, Facultad de Ingenierıa Civil, Universidad Autonoma de Nuevo Leon (UANL),Avenida Universidad S/N, Ciudad Universitaria, 66451 San Nicolas de los Garza, NL, Mexico

2 Facultad de Ciencias Quımicas, Universidad Autonoma de San Luis Potosı, Avenida Manuel Nava No. 6, 78290 San Luis Potosı,SLP, Mexico

Correspondence should be addressed to Leticia M. Torres-Martınez, [email protected]



Copyright © 2012 Leticia M. Torres-Martınez et al. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

This paper is focused to study Sm2FeTaO7 pyrochlore-type compound as solar photocatalyst for the degradation of indigocarmine dye in aqueous solution. Sm2FeTaO7 was synthesized by using conventional solid state reaction and sol-gel method. X-raydiffraction results indicated that Sm2FeTaO7 exhibit a monoclinic crystal structure. By scanning electron microscopy analysis, itwas observed that sol-gel material presents particle size of around 150 nm. The specific surface area and energy bandgap valueswere 12 m2 g−1 and 2.0 eV, respectively. The photocatalytic results showed that indigo carmine molecule can be degraded undersolar light irradiation using the synthesized materials, sol-gel photocatalyst was 8 times more active than solid state. On the otherhand, when Sm2FeTaO7 was impregnated with CuO as cocatalyst the photocatalytic activity was increased because CuO acts aselectron trap decreasing electron-hole pair recombination rates.

1. Introduction

Nowadays, most of the investigations on photocatalysts areconducted in order to modify the nanostructure to promotetheir use under visible-light, especially when solar light isused because sun light is an available free energy source [1–5].

Actually, most of the common visible-light-sensitivephotocatalysts are binary compounds such as CdS, CdSe,WO3, TiO2, ZnO, and Fe2O3 that are unstable or havelow activity during the photocatalytic process [6–9]. Whileternary oxides are a promising family of interesting com-pounds that can offer the properties desired for an idealvisible-light photocatalyst [10].

Photodegradation using mixed oxides such as A2BB’O7

pyrochlore-type compounds has attracted considerableattention because those compounds could be acting asphotocatalyst under visible-light irradiation. The slight mod-ification into their crystal structure will cause a variation intheir electronic properties provoking an enhancement in the

photocatalytic activity. Those compounds have the advan-tage that A and B sites can be substituted by several metalions in order to develop visible responsive photocatalysts.

Previous works reported that pyrochlore containingmetal ions like Bi3+, Gd3+, Sm3+, In3+, Fe3+, Ta5+, andSb5+ improve the photocatalytic activity for dye degradationin aqueous solution under visible-light irradiation. This isbecause those metals favor the mobility of photoinducedelectrons and holes to reach easily the reactive sites oncatalyst surface [11–16]. However, to our knowledge, thereare no reported studies concerning the photocatalytic activityof pyrochlore compounds under solar light.

Recently in our group, a novel pyrochlore-type com-pound Sm2FeTaO7 has been synthesized with attractivecharacteristics to be evaluated as potential photocatalyst [17].For that reason in this paper, the attention is focused to studySm2FeTaO7 as photocatalyst for the degradation of indigocarmine dye in aqueous solution under real and varyingsolar light illumination. Samples were taken at differenttimes to monitor the progress of the reaction by UV-vis


10 20 30 40 50 60 70 80 90 100

SS

SG

Inte

nsi

ty (

a.u

.)

2θ (degree)

Figure 1: X-ray diffraction patterns of Sm2FeTaO7 synthesized bysolid state reaction at 1400◦C during 36 hours and by sol gel at800◦C during 6 hours [17].

spectroscopy and total organic carbon analysis. Additionallyit is proposed the relation between preparative methods andthe reaction mechanism of the photocatalytic degradation ofindigo carmine dye.

2. Experimental

2.1. Synthesis by Solid State Reaction. Sm2FeTaO7 wasobtained by solid state reaction using Sm2O3, Fe2O3 andTa2O5 (Aldrich purity >99.9%) as starting materials. Thepowders were dried at 200◦C for 4 hours before the synthesis.Then, stoichiometric amounts of each reactant were perfectlymixed with acetone in an agate mortar. The mixture wasground until complete evaporation of the acetone. Thissolid was placed into a platinum crucible and calcined at1400◦C for 36 hours under air atmosphere with intermediateregrinding to complete the reaction. The furnace was pro-grammed to reach the calcination temperature at a heatingrate of 1◦C/min. This sample was labeled as SS.

2.2. Synthesis by Sol-Gel Method. Sm2FeTaO7 was also syn-thesized by the sol-gel method. In this purpose, a stoichio-metric amount of iron (III) acetylacetonate was dissolved inacetylacetone. The reaction mixture was kept under magneticstirring for 1 hour and refluxed at 70◦C. Then, samarium(III) acetate was dissolved in ethylene glycol and water andrefluxed for 1 hour at 70◦C. Glacial acetic acid was thenadded to obtain a colorless samarium solution. At the sametime, tantalum ethoxide was mixed with ethanol. Both thesamarium and tantalum solutions were slowly added to theiron (III) acetylacetonate solution, and the resulting solutionwas refluxed at 70◦C for 48 hours. After this time, pH wasadjusted to 10 using a solution of ammonium hydroxide.Afterwards, the mixture was kept under the same conditionsfor 48 hours. The final product was dried for 24 hours at100◦C to obtain the fresh sample. The material was heatedup to 800◦C at a heating rate of 1◦C/min and calcined for sixhours under air atmosphere. This sample was labeled as SG.

2.3. Wet Impregnation Method. Sm2FeTaO7 synthesized byboth, solid state and sol-gel, was impregnated with 1% CuO

using the stoichiometric amount of an aqueous solutionof cupric nitrate hydrate. Sm2FeTaO7 powders and coppersolution were mixed and then stirred at 80◦C until completeevaporation of the solvent. Then, the materials were ther-mally treated at 400◦C by 1 hour under an air atmosphereusing a heating rate of 10◦C/min. These samples were labeledas CuO/SS and CuO/SG.

2.4. Characterization. Sm2FeTaO7 materials were character-ized by X-ray powder diffraction (XRD) using a Bruker D8Advance diffractometer and CuKα radiation (λ = 1.5406 A)as the incident X-ray source. XRD data were collected atroom temperature from 10 to 100◦ with a step interval of0.01◦ and a counting time of 1s/step.

Morphology of the samples was observed using a JEOL6490 LV Scanning Electron Microscope (SEM). All sampleswere stuck to graphite tape and then placed on an aluminumsample holder and located in the SEM chamber. The contentof CuO of the impregnated catalysts was determined byenergy dispersive X-ray spectroscopy (EDS) analyzing fiverandom zones.

The optical absorption properties of the samples wereanalyzed in the range of 200–900 nm at room temperaturewith a UV-vis spectrophotometer (Lambda 35 Perkin ElmerCorporation) equipped with an integrating sphere attach-ment. The energy bandgap of Sm2FeTaO7 was determinedby reflection spectra following the equation [13, 18] αhv =A(hv − Eg)n. Here, α, v, A, and Eg are absorption coefficient,light frequency, proportional constant, and bandgap, respec-tively.

The specific surface area (SBET) was determined bynitrogen adsorption isotherms from the BET method usingthe Quantachrome NOVA 2000e equipment. The sampleswere degassed for 3 hours at 300◦C prior to the analysis.

2.5. Photocatalytic Activity. The degradation of indigocarmine dye (Indigo-5,5

′-disulfonic acid disodium salt) was

carried out using a glass container as reactor under solarlight irradiation. Four photodegradation experiments werecarried out with an indigo carmine solution (10 mg/L). Ineach of the experiments, 400 mL of the solution were mixedwith 400 mg of catalyst (SS, SG, CuO/SS, and CuO/SG) inthe glass reactor. The system was allowed to reach adsorptionequilibrium under dark conditions. Then, the reactors wereplaced under solar illumination. In addition, a similarexperiment without using any catalyst was also carried outto monitor pure photochemical degradation reactions.

During the experiment, the reaction mixture was keptunder stirring. The temperature was controlled at 30◦Crunning cold water on the external surface of the glassreactor. Samples for analysis were taken at different times tomonitor the reaction. Each sample was analyzed by UV-visspectrophotometer (Lambda 35 Perkin Elmer Corporation)and TOC apparatus (TOC-VCSH, Shimadzu Corporation).All tests were carried out simultaneously to ensure iden-tical experimental conditions. Solar irradiation data wereobtained from the Environmental Integrated MonitoringSystem (SIMA) at Monterrey, Nuevo Leon, Mexico.


1 μm1 μm

1 μm

SS SG

CuO/SG

TaTa

Ta EDS of CuO/SGO

Sm

Sm

Sm

SmCu

Fe

1 2 3 4 5 6

(KeV)

Full

scal

e, 1

200

cts

Figure 2: SEM images for Sm2FeTaO7 synthesized by solid state and sol gel.


3.1. Characterization of Pyrochlore-Type

Compound Sm2FeTaO7

3.1.1. X-Ray Diffraction Analysis. Sm2FeTaO7 was obtainedas a single phase at 1400◦C by solid state reaction andat 800◦C by sol-gel method; see Figure 1. This compoundcrystallized in a monoclinic system with the space groupC2/c [17]. According to XRD patterns both materials showedthe same crystal structure but with different crystallinitydegree. This result is related to the differences in thesynthesis conditions, 1400◦C/36 h and 800◦C/6 h for solidstate reaction and sol-gel method, respectively.

Crystal size calculation from the broadening of the mainpeak (2θ = 29.1◦) using the Scherrer formula revealed valuesof 77 nm for solid state and 43 nm for sol-gel samples. Thissituation is in agreement with the fact that, by solid statemethod, material is suffering sintering due to higher thermaltreatment than sol-gel, reaching a high crystallization degree.

When CuO/SS and CuO/SG samples were analyzed anyXRD peaks attributed to CuO phase were detected sincethe CuO content in the impregnated catalysts is very low(1 wt.%).

The cell parameters of synthesized compounds areshowed in Table 1.

3.1.2. Morphology and Specific Surface Area. Figure 2 showsthe morphology of Sm2FeTaO7 synthesized by solid stateand sol-gel. It can be seen that morphology and dimensionsof particles were strongly dependent on the synthesis route.The particles prepared by solid state reaction have a smoothsurface and size of 1 to 4 μm due to the high temperature

Table 1: Cell parameters for Sm2FeTaO7[17].

Sm2FeTaO7

Solid state 1400◦CSm2FeTaO7

Sol-gel 800◦Ca (A) 13.1307 (5) 13.0913 (2)b (A) 7.5854 (3) 7.5622 (6)c (A) 11.6425 (4) 11.7358 (6)β (◦) 100.971 (2) 100.933 (4)Z 8 8

Table 2: Physicochemical properties of Sm2FeTaO7 materials.

Sample CuO (wt. %) Surface area (m2 g−1)SS — 1SG — 12CuO/SS 1.2 1CuO/SG 1.1 11

of synthesis. While the particles prepared by sol gel havespherical shape and size of around 100 to 150 nm, themicrographs also show these particles are forming aggregatesof large size.

Also in Figure 2 are showed the EDS analysis of a specificzone and its respective spectra corresponding to CuO/SGmaterial. The results of this analysis are reported in Table 2.

The results of the BET analysis are also reported inTable 2. Sm2FeTaO7 catalysts prepared by sol-gel had aspecific surface area of one order of magnitude higher thanthe corresponding area of materials prepared by solid statereaction. This result corroborates XRD and SEM analysis,and it was observed that sol-gel material has small particlesize. Therefore the highest specific surface area of sol-gelmaterial is due to the soft chemical route used.


CuO/SS

SS

CuO/SG

SG

Abs

orba

nce

(a.

u.)

Wavelength (nm)

400 500 600 700 800 900

Figure 3: UV-vis spectra of Sm2FeTaO7 materials.

SS SG

1.6 1.8 2 2.2 2.4 2.6

Energy (eV)

(αhv)

1/n

Figure 4: Bandgap determination of Sm2FeTaO7.

1

0.8

0.6

0.4

0.2

0

02:00 05:00 08:00 11:00 14:00 17:00 20:00 23:00

Time (h)

Sola

r ir

radi

atio

n (

KW

/m2)

(a)

100

80

60

40

20

0Tran

smit

tan

ce (

%)

Wavelength (nm)

200 300 400 500 600 700 800 900

(b)

Figure 5: (a) Solar light irradiation conditions during test and (b)percentage of transmittance of the glass container.

Wavelength (nm)

Irradiation time

Initial

10 hours

200 300 400 500 600 700 800

Abs

orba

nce

(a.

u.)

Figure 6: UV-vis absorption spectra during photodegradation ofindigo carmine under solar light irradiation using CuO/SG.

1

0.9

0.8

0.7

0.6

C/C

o

Time (h)

Dar

knes

s

Solar light irradiation

SS

CuO/SS

Photolysis

SG

CuO/SG

0 2 4 6 8 10

Figure 7: Photodegradation of indigo carmine under solar lightirradiation using Sm2FeTaO7 and CuO/Sm2FeTaO7 materials.

In the case of CuO/SS and CuO/SG surface analysis, it isobserved that surface area variation after wet impregnationis not significant. Although it is well known that the oxidesupport nature and metal oxide cocatalyst play an importantrole in surface area modification [19, 20], it seems that ourresults are depending on the amount of cocatalyst used.

3.1.3. UV-Vis Analysis. Figure 3 shows the UV-vis spectra forall synthesized samples. It is observed a strong absorptionin the visible light region from 400 nm to 900 nm. CuO/SSand CuO/SG showed an additional absorption band at 600–800 nm, which is indicative of the presence of CuO dispersedat the surface [21]. It could be noted that absorption of thisband is more evident in CuO/SS sample due to its low surfacearea, that is, CuO particles are covering much of the availablesupport surface in comparison with CuO/SG.

The bandgap (Eg) of Sm2FeTaO7 synthesized by solidstate and sol-gel was determined by the Tauc plot [18] of(αhv)1/n versus hv showed in Figure 4. The Eg value obtainedwas 1.99 eV and 2.01 eV for material synthesized by solid


Solid state reaction

Solar light

ReductionReduction

OxidationO

OO

O

OOCuO CuO

O2 O2

Indigo carmine molecule

CB

VB

Recombination

CB

VBh+ h+ h+

e− e− e−

h+ h+

e−e−HO•

H2O

HO•

H2O

Low photoactivity

Sm2FeTaO7

Sm2FeTaO7

High photoactivity!!!

Na+

Na+

−O

NH

HNS

S

CO2 + H2O

Solar light

Sol-gel method

O−O2

•−O2

•−

Figure 8: Schematic mechanism of the photocatalytic degradation of indigo carmine dye. CB conduction band and VB valence band.

state and sol-gel, respectively. According to Eg results, weassumed that the ability of Sm2FeTaO7 to absorb in thevisible light region is directly associated to the presence of Fe.Iron generally acts as electron donor and its 3d electrons areeasily excited by the visible light allowing the shift of bandgapto the visible region [10, 16, 22].

3.2. Photocatalytic Evaluation. The photodegradation testswere realized under the solar light irradiation conditions pre-sented in Figure 5(a). Samples were exposed to irradiationfrom 9 : 00 AM to 7 : 00 PM, where the maximum energy wasreceived. Figure 5(b) shows transmittance spectra of glasscontainer used as photocatalytic reactor. It can be noted thatthe glass container allows the incidence of radiation withwavelength larger 300 nm.

During the photocatalytic experiment, the intensity ofthe blue color of indigo carmine solution decreased withtime. The analysis of the reaction samples by UV-visspectroscopy shows a slight decrement of all absorptionbands (610, 286, 252, and 206 nm). Figure 6 shows theabsorption spectra for indigo carmine photodegradationusing CuO/SG. It is clear that indigo carmine molecule issuffering degradation under solar light irradiation.

Degradation of indigo carmine solution as a function ofirradiation time using the synthesized materials is showed inFigure 7. It can be seen that photolysis reached around 20%due to the small portion of UV light of the solar spectrum,which participate directly in the photochemical reaction,whereas in the presence of the materials synthesized by solidstate reaction only 5–10% of degradation is achieved. Wecan assume that a part of photons are absorbed by materialsfor the subsequent formation of the electron (e−) and hole(h+) pairs limiting the photolysis effect. Besides the pho-togenerated charges are not participating efficiently for theindigo carmine degradation according to the low activityobserved. This fact could be due to several factors such as lowinteraction between material and dye solution, the low sur-face area as well as the high rate of the recombination process.

On the other hand, materials synthesized by sol-gel routeshowed the higher activity for degradation of the aqueous

solution of indigo carmine. In concordance with our results,sol-gel material exhibited an activity 8 times higher thansolid state catalyst. It is known that photoactivity is stronglyinfluenced by the nature of material. In our case, sol-gelmaterials present high surface area and small particle sizeallowing a better interaction between solution and catalystmaterial. The presence of small particles provides more activesites and decreases the migration distance of photogeneratedelectrons and holes promoting the reaction on the catalystsurface [23].

With respect to the use of CuO as cocatalyst promoter,it increases the photocatalytic activity of Sm2FeTaO7. In par-ticular CuO/SG material reached 38% of photodegradation;see Figure 7. In this case CuO acts as an electron trap [24]decreasing the rate of recombination of the photogeneratedelectron-hole pairs and more holes will be available to oxidizethe organic molecules [25]. Additionally, it is known thatthe recombination competes strongly with the photocatalyticprocess being it the major limitation in photocatalysis as itreduces the overall quantum efficiency [26].

Additionally, photocatalytic tests were followed by TOCanalysis. Results of all samples showed that mineralizationis occurring during the reaction. Materials SG and CuO/SGshowed 7% and 11% of TOC reduction, respectively, whilemineralization for solid state materials is less than 5%.

On basis of the above results, the relation betweenpreparative methods and the reaction mechanism of thephotocatalytic degradation of indigo carmine dye is schemat-ically illustrated in Figure 8. It is an attempt to explain whysol gel samples presented higher activity than solid statesamples during the photocatalytic process.

In the first step, both CuO/SS and CuO/SG absorbphotons with energy greater than their bandgap to formelectron-hole pairs. Then, the charge carriers migrate tothe surface where redox reactions will occur; simultaneouslyrecombination process is occurring and competes withthe photocatalytic process. In our case, both processes areaffected by the preparation method. When samples wereprepared by sol gel, particle size is smaller than solid statesamples; consequently the distance that electrons and holes


have to migrate to the surface becomes short decreasing therecombination rate and providing more active sites for thesurface redox reactions. Finally, CuO is acting as electrontrap reducing the electron-hole recombination rate, whereasholes are forming oxidative species like hydroxyl radicals thatreact with indigo carmine dye molecule until forming CO2

and H2O [23–25].According to our results Sm2FeTaO7 can be considered

as photocatalyst for organic compounds degradation underreal and varying solar light illumination as an alternativematerial for the treatment of wastewater from textile anddying industries.

4. Conclusions

The photophysical characteristics of Sm2FeTaO7 allowed itsuse as a visible-light photocatalyst for degradation of indigocarmine dye under solar light irradiation. Its activity wasenhanced 8 times when this material was synthesized by sol-gel route due to the high surface area and small particle sizecompared with solid state material. In addition, the presenceof CuO as cocatalyst increases the photocatalytic activitybecause CuO acts as electron trap avoiding electron-hole pairrecombination rates, which favors the oxidation of the indigocarmine molecule.

Acknowledgments

Authors want to thank CONACYT for the financial supportthrough the CB-98740-2008, CB-84809-2007, CB-103532-2008 and CB-83923-2007, and PAICYT-UANL2010. Also E.Moctezuma would like to thank to the projects UASLPC10-FRC-07-03. M. Z. Figueroa-Torres thanks CONACYTfor financial support through the program A.C.C.I.G.I.-Retencion 2010-1 no. 144226. M. A. Ruiz-Gomez thanksCONACYT for Ph.D. scholarship no. 239336.

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Research Article

Photocatalytic Oxidation of Gaseous Isopropanol UsingVisible-Light Active Silver Vanadates/SBA-15 Composite

Ting-Chung Pan,1 Hung-Chang Chen,2 Guan-Ting Pan,2 and Chao-Ming Huang3

1 Department of Environmental Engineering, Kun Shan University, Tainan 71003, Taiwan2 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan3 Department of Materials Engineering, Kun Shan University, No. 949 Da-Wan Road, Yong-Kang District, Tainan 71003, Taiwan

Correspondence should be addressed to Chao-Ming Huang, [email protected]



Copyright © 2012 Ting-Chung Pan et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

An environmentally friendly visible-light-driven photocatalyst, silver vanadates/SBA-15, was prepared through an incipient wet-ness impregnation procedure with silver vanadates (SVO) synthesized under a hydrothermal condition without a high-temperaturecalcination. The addition of mesoporous SBA-15 improves the formation of nanocrystalline silver vanadates. In situ diffusereflectance infrared Fourier transform spectroscopy (DRIFTS) confirms the presence of Brønsted and Lewis acids on the SVO/SBA-15 composites. The results of photoluminescence spectra indicated that the electron-hole recombination rate have been effectivelyinhibited when SVO was loaded with mesoporous SBA-15. All the composites loaded with various amount of SVO inherit thehigher adsorption capacity and larger mineralization yield than those of P-25 (commercial TiO2) and pure SVO. The sampleloaded with 51% of SVO (51SVO/SBA-15) with mixed phases of Ag4V2O7 and α-Ag3VO4 exhibits the best photocatalytic activity.A favorable crystalline phase combined with high intensities of Brønsted and Lewis acids is considered the main cause of theenhanced adsorption capacity and outstanding photoactivity of the SVO/SBA-15 composites.

1. Introduction

Semiconductor photocatalysts have been studied extensivelyfor degradation of organic compounds or water splitting [1–3]. Among semiconductor photocatalysts, titanium dioxide(TiO2) is the most widely employed. Due to the band gap,around 3.0 eV for rutile and 3.2 eV for anatase, TiO2 exhibitsstrong oxidation activity under ultraviolet (UV) light witha wavelength of 390 nm or less. Since UV light is only 3–5%part of the solar spectrum, the photocatalytic activity of TiO2

is not enough under the visible and/or solar light irradiation.Therefore, numerous attempts have been made to improveTiO2 as a visible-light-active photocatalyst, including aniondoping, cation doping, coupling of other semiconductor, andoxygen deficiency generation [4–6]. In addition, the degrada-tion rate of TiO2 largely depends on the adsorption capacitywhich is associated with the specific surface area; therefore, alot of research effort has been devoted to loading TiO2 spe-cies onto or incorporating them into the mesoporous silica

SBA-15 to be a high-surface-area composite. SBA-15 materialhas extremely high surface area, large pore volume, and tun-able pore size [7]. The motivation of TiO2/SBA-15 compositeis that the dispersion and stabilization of the TiO2 species ona high surface area support leads to the synergistic effect withadsorptive and photocatalytic ability [8–10]. However, thereare some basic restrictions using titania-SBA-15 compositesas the visible-light-driven photocatalysts. First, TiO2 has lowphoto efficiency under visible-light illumination due to itswide band gap. Second, the most general method for syn-thesizing TiO2 is the sol-gel method, which requires high-temperature calcination (673 K or higher) to obtain goodcrystallinity. In recent years, Ag2ZnGeO4 [11], Ag3VO4 [12],and BiVO4 [13] were found to be effective as the visible-lightactive photocatalysts for water splitting and pollutant de-composing under visible-light illumination. These photocat-alysts were prepared by hydrothermal synthesis method sincethis method offers many advantages, such as controllableparticle size, high degree of crystallinity, and high purity


while using milder synthesis temperatures and simpler proc-ess configurations.

The present work demonstrates the preparation of silvervanadate/SBA-15 composites (SVO/SBA-15) via hydrother-mal synthesis using a postsynthesis step without high-tem-perature calcination. Photodecomposition of isopropanol(IPA) was selected as a model reaction to evaluate the photo-catalytic performance of silver vanadate/SBA-15 compositessince IPA is frequently detected in indoor and industrial airanalyses. In situ diffuse reflectance infrared Fourier trans-form spectroscopy (DRIFTS), a very powerful technique toprovide a high sensitivity and to allow the tracking of vari-ations in the composition occurring in seconds, was used toidentify reaction intermediates and to monitor the progressof the photocatalytic processes on the composite surface.

2. Experimental Procedure

2.1. Preparation of Photocatalyst. SBA-15 was synthesizedwith Pluronic P123 (EO20PO70EO20, Mav = 5800; Aldrich)and tetraethylorthosilicate (TEOS) according to a previousreport [14]. Briefly, 4.0 g of P123 was dissolved in 30 g ofdeionized water and 120.0 g of HCl solution (2 M) withstirring at 313 K for 2 h. Then, 8.5 g of TEOS was addedinto the P123 solution, which was stirred for another 22 huntil a white gel precipitated. The gel was transferred to aTeflon bottle and heated at 373 K for 24 h. The precipitatewas filtered, washed several times with deionized water, driedovernight at 373 K, and then calcined at 773 K (heating rateof 1 K/min) for 4 h in air.

The silver-vanadate-loaded SBA-15 (SVO/SBA-15) wasprepared via the incipient wetness impregnation procedure.In the preparation process, 0.204 g AgNO3 was dissolved inurea aqueous solution (120 g H2O, 0.577 g urea) with stirringat room temperature for 0.5 h to obtain solution A. SolutionB was prepared by mixing 0.047 g NH4VO3 with 0.3 g SBA-15in deionized water at 343 K for 1 h under an ultrasonic bath.A suspension formed when solution A was added dropwiseto solution B under vigorous stirring for 1 h. The molarcomposition of the suspension AgNO3/NH4VO3/CO(NH2)2

was 3.0/1.0/12.0. The suspension was titrated to pH 7 usingammonia solution, followed by additional stirring at roomtemperature for 24 h. Finally, the as-obtained suspension wastransferred into a Teflonlined autoclave with hydrothermaltreatment (temperature: 413 K, time: 4 h). After the hydro-thermal procedure, the resulting precipitates were collectedand washed with deionized water three times and then driedat 353 K for 12 h. The samples were named as xSVO/SBA-15, where x was the amount of SVO loading by weight.The sample synthesized under identical conditions withoutthe addition of SBA-15 was denoted SVO. The experimentalprocedure for preparing SVO/SBA-15 composites usingincipient wetness impregnation is schematically shown inScheme 1.

2.2. Sample Characterization. The X-ray diffraction (XRD)patterns of the powders were measured using an X-raydiffractometer (PANalytical X’Pert PRO) with Cu radiation

(λ = 0.15418 nm) in the 2θ range of 20 to 60◦. High-resolu-tion transmission electron microscopy (HRTEM) imagesof the samples were observed on a Philips Tecnai G2 F20microscope equipped with energy-dispersive X-ray spectro-scopy (EDX) operated at an accelerating voltage of 200 kV.Photoluminescence (PL) spectra were recorded by a fluores-cence spectrophotometer (Dongwoo Optron) at 1 atm and25◦C. In situ DRIFTS measurements were performed using aPerkinElmer FTIR spectrometer (Spectrum GX) and a dif-fuse reflectance accessory (Harrick Scientific, DRP-PE9)with a temperature- and atmosphere-controlled high-tem-perature low-pressure reaction cell (Harrick Scientific, HVC-DRP-3). To determine the types of acid site present on thesamples, temperature-programmed desorption of ammonia(NH3) was carried out using DRIFT. Ammonia usually pro-vides the probe molecules in spectroscopic experiments todetermine the type of acid site in heterogeneous catalysis:Brønsted sites or Lewis sites. Prior to the experiments, thesamples were heated in situ from room temperature to 523 Kat 10 K/min in N2 flow (30 mL/min), held at 523 K for30 min, and then cooled down to 303 K. The samples weresaturated at 303 K with a gas mixture of 5% NH3 in N2

(30 mL/min) for 30 min. At the end of the saturation process,the samples were flushed with N2 flow (30 mL/min). Then,the samples were heated again at a heating rate of 10 K/minfrom 30 K to 523 K and held at 523 K for 60 min. The DRIFTspectra with a resolution of 4 cm−1 were collected in the in-terval of 1200–1700 cm−1 for determining surface acidity[12]. The surface area and pore volume of the as-preparedsamples were determined using a volumetric sorption ana-lyzer (Micromeritics ASAP 2020). The samples were degassedat 473 K under vacuum conditions for a period of at least 4 hprior to measurements. The nitrogen adsorption/desorptionisotherms were measured over a relative pressure (P/P0)range of approximately 10−3 to 0.995. The surface areas werecalculated using the Brunauer-Emmett-Teller (BET) methodin the relative pressure range of 0.06–0.2. The pore size dis-tributions were determined from the analysis of the adsorp-tion isotherm using the Barret-Joyner-Halenda (BJH) algo-rithm. The total pore volumes were estimated from the ad-sorbed N2 amount at P/P0 = 0.973.

2.3. Photocatalytic Activity Evaluation with MS and DRIFTS.The photocatalytic oxidation of IPA was performed in situin an IR cell with ZnSe windows. An LED lamp, with a wave-length ranging from 430 to 620 nm with a photon intensity of4 mW/cm2, was used as the visible-light source [13]. A gase-ous IPA/O2 mixture was generated, corresponding to the tar-get IPA concentration of 610 ppmv. Prior to the experiments,the samples were pretreated by heating and flushing with N2

flow (20 mL/min) from room temperature to 523 K, held at523 K for 30 min, and then cooled to 303 K. The procedurefor all PCO experiments was as follows: (1) IPA/O2 flow wasintroduced into the photoreactor at a constant flow rate of10 mL/min; (2) when the photoreactor inlet and outlet IPAconcentrations were approximately equal, the LED lamp wasturned on, and the IPA/O2 flow was stopped; (3) the pho-toreactor was flushed with the O2 flow at 10 mL/min during


xSVO/SBA-15

Solution A Solution B

AgNO3 + CO (NH2)2

+ H2O + H2O

pH = 7

stirring 24 h (R.T.)

Autoclave

Filtered, washed, dried

(413 K 4 h)

(353 K 12 h)

NH4VO3 + SBA-15

Scheme 1: Schematic diagram for the preparation of SVO/SBA-15 composite photocatalysts through the incipient wetness impregnation.

PCO experiment. To verify the gaseous composition (inter-mediate), the samples were monitored online with a quad-rupole mass spectrometer (MS, SRS QMS300) at regularintervals. The spectra of the adsorbed species on the catalystsurface were recorded under both darkness and illumination.The spectra of the catalyst and the reaction intermediatesduring the reaction were expressed in units of Kubelka-Munk(K-M).


3.1. Characterization of Samples. The X-ray powder diffrac-tion patterns of the SVO and three SVO/SBA-15 samples areshown in Figure 1. The SVO/SBA-15 composites have twokinds of XRD patterns, assigned to the pure α-Ag3VO4

(JCPDS 43-0542) for 17SVO/SBA-15 and 34SVO/SBA-15samples and to the mixed phases of Ag4V2O7 (JCPDS 77-0097) and α-Ag3VO4 for 51SVO/SBA-15, respectively. Thebulk SVO had the same crystalline structure with 51SVO/SBA-15. Figure 2 shows the adsorption-desorption isothermsof SBA-15 and SVO/SBA-15 samples. According to theIUPAC classification of adsorption isotherms, the isothermsof SBA-15 and SVO/SBA-15 can be classified as type IV whichis a typical indication of mesoporous materials. With increas-ing the SVO loading, the SVO/SBA-15 sample gives isothermwith similar inflection but with reduced sharpness and a shiftto higher relative pressure P/P0. It is well studied that thelocation of inflection point is related to pore diameter in themesopore range and the sharpness of these slopes displaysthe uniformity of the mesopore size distribution. Comparedto SBA-15, the capillary condensation of SVO/SBA-15 shifted

to higher relative pressure, indicating an increase of the porediameter when SVO was loaded into/onto SBA-15. Thesharpness of the steeps is decreased as the amount of SVOincreases, indicating that the pore size distribution becomesbroad with an increase of SVO amount in the SBA-15 frame-work. The textural properties of SBA-15 support and SVO/SBA-15 samples are listed in Table 1. It can be seen that thespecific surface area and pore volumes of SVO/SBA-15 sam-ples significantly decrease with increasing silver vanadateloading, whereas the pore diameter increases. When theSVO/SBA-15 samples were synthesized using a postsynthesismethod, the silver and vanadate species reacted on the sur-face of SBA-15; thus, the BET surface and pore volumes ofSVO/SBA-15 composites decreased with increasing amountof SVO. The average pore diameter increased for SVO/SBA-15, which might be due to the small pores of SBA-15 beingobstructed by silver vanadate nanoparticles. Direct evidencesfrom TEM images can be used to confirm the formation ofSVO nanocrystals on the SBA-15. Figure 3 shows the TEMimages of SBA-15 and 51SVO/SBA-15, which are viewedalong the <110> orientations. The TEM image of SBA-15(Figure 3(a)) clearly shows well ordered cylindrical channels,indicative of 2D hexagonal p6mm mesostructure. Neverthe-less, the surfaces of 51SVO/SBA-15 (Figure 3(b)) seemed liketo be covered by clouds which were caused by aggregation ofthe SVO particles on the surface of SBA-15. The micrographresult indicates that high loading of nanocrystalline SVO canbe well dispersed inside the mesoporous channels of the SBA-15 using urea as the chelating agent, which exhibits a stronginteraction between urea and metal ions and prevents theprecipitation of sliver and vanadium ions before the forma-tion of silver vanadates-silica composites.


20 30 40 50 60

SVO

Monoclinic Ag3VO4JCPDS NO.

Monoclinic Ag4V2O7JCPDS NO.

17SVO/SBA-15

Inte

nsi

ty (

a.u

.)

2θ (degrees)

34SVO/SBA-15

51SVO/SBA-15

77-0097

43-0542

Figure 1: XRD patterns of SVO and SVO/SBA15 composites.

Table 1: Specific surface area and pore properties of SBA-15, SVO/SBA-15, SVO, and P25 materials.

Sample SBET (m2g−1)Pore volume

(cm3g−1)Pore size (nm)

SBA-15 942 1.04 6.3

17SVO/SBA-15 217 0.66 11.9

34SVO/SBA-15 149 0.46 12.3

51SVO/SBA-15 125 0.41 13.3

SVO 2 0.002 7.3

P25 56 0.25 17.5

3.2. DRIFTS and MS Studies of Adsorbed and PhotocatalyticOxidation of IPA. The DRIFTS was employed to simultane-ously analyze the products of the reaction as well as thecatalytic surface to get a better understanding of the correla-tion between structure and activity of the catalysts. Thespectra of the samples before adsorption have been sub-tracted in order to highlight the features of adsorbed IPA. ForIPA adsorption, three peaks in the range of 1500–1200 cm−1

assigned to the δ(CH) mode of IPA; in the high wavenumberregion, a very intense band at 2978 and a weak band at2888 cm−1 were observed, corresponding to the stretchingν(CH) mode of methyl groups of IPA [15]. Figure 4 showsthe DRIFTS of IPA adsorbed on the surface of sample in darkand after illumination. It was observed that the intensity ofadsorbed IPA bands increased with increasing flushing time,and IPA bands reached stable equilibrium after 30 min (fig-ure not shown here). Figure 4(a) shows the spectrum of ad-sorbed IPA on fresh SVO/SBA-15, SVO, and P25 samples inthe dark for 30 min at 300 K. Since the same amount of eachsample was used for the DRIFTS experiments, SVO/SBA-15sample adsorbed a much higher amount of IPA than did SVOand P25. The photocatalytic oxidation of IPA started afterattaining adsorption equilibrium, and the IR spectra of sam-ples equilibrated with IPA were taken as the initial state un-der illumination. Figure 4(b) shows the IR spectra of thesample surfaces after 1 h of photocatalytic oxidation of IPA.

0 0.2 0.4 0.6 0.8 1

SBA-15100

51SVO/SBA-1534SVO/SBA-1517SVO/SBA-15

Qu

anti

ty a

dsor

bed

(cm

3ST

P/g

) (a

.u.)

Relative pressure (P/P0)

Figure 2: N2 adsorption-desorption isotherms of SBA-15 and SVO/SBA-15 samples.

The IPA bands decreased progressively for all SVO/SBA-15composites; however, a slight reduction of IPA was observedfor P25. Moreover, new features at about 1703, 1571, 1425,1371, and 1241 cm−1 were developed for all samples. Thepeak at 1571 cm−1 is clearly visible, which is associated withthe νas (COO) mode of formate, while new bands at 1703,1425, 1371, and 1241 cm−1 are attributed to acetone. Fromthe spectra distribution, FTIR results have indicated that IPAadsorbed under illumination can be degraded to acetone andto formate, the intermediate species adsorbed on the surfaceof the photocatalysts. The adsorption and photodegradationcurves of gaseous IPA during visible-light irradiation wereshown in Figure 5. The concentration of gaseous IPA, deter-mined using a quadrupole MS, rapidly decreased during theinitial 10 min, which was due to the adsorption of IPA oncatalyst surface. After 30 min, the concentration of gaseousIPA returned to the initial state, indicating that the adsorp-tion of IPA reached equilibrium. In the absence of irradiationthe as-prepared SVO/SBA-15 composites exhibited muchhigher adsorption capabilities of IPA than those of SVO andP-25. The adsorption capability of the samples decreased asthe following order: 51SVO/SBA-15 > 34SVO/SBA-15 >17SVO/SBA-15 > SVO > P25. It is well known that photo-catalytic oxidation of organic pollutants follows Langmuir-Hinshelwood kinetics [16]; the L-H model can be simplifiedto a pseudo-first-order expression: ln(Ce/C) = kt (where Ce

and C are the equilibrium concentration of adsorption andthe concentration of VOC at the exposure time, t, resp., andk is the apparent rate constant). The calculated kapp of IPAdecreased in the order: 17SVO/SBA-15 (0.048 min−1) >34SVO/SBA-15 (0.045 min−1) > 51SVO/SBA-15(0.042 min−1) > SVO (0.035 min−1) > P25 (0.029 min−1).For the initial stage of photocatalytic reaction, the compositematerials (17SVO/SBA-15 and 34SVO/SBA-15) with α-Ag3VO4 crystalline had stronger initial photoactivity thanthat of mixed structures of α-Ag3VO4 and Ag4V2O7 (51SVO/SBA-15). Only the decrease of the IPA concentration in theearly stage is not sufficient for the comparison of photo-catalytic activity; therefore, the amount of CO2, a final


800

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Cou

nts

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SiV

V V

Cu

Cu

Cu

Cu

Ag

AgAg

Ag AgAg

AgAg

Ag AgAg

5 15 25

100 nm

(b)

Figure 3: HRTEM images of (a) SBA-15 and (b) 51SVO/SBA-15.

decomposition product of gaseous IPA, was measured toevaluate the long-term photoactivity of all samples. Themineralization yield of gaseous IPA is defined as [4]

Mineralization (%) = 1/3[CO2]production

[CH3CHOHCH3]original× 100%.

(1)

For a 100% mineralization yield, three moles of carbon diox-ide are formed from each mole of IPA. Figure 6 representsthe CO2 concentrations of various samples; after visible-lightirradiation for 270 min, the CO2 concentrations of SVO/SBA-15 composites are much higher than those of SVO andP25. The mineralization yields of IPA were 60%, 55%,

50%, 46%, and 44% for 51SVO/SBA-15, 34 SVO/SBA-15,17 SVO/SBA-15, SVO, and P25, respectively. When the massratio of silver vanadates to SBA-15 is small (17SVO/SBA-15),each nano silver vanadates particle is surrounded by a largeramount of mesoporous silica, which leads to the increase ofthe average distance from the adsorption sites to the photo-active sites, bringing about a decrease of the mineraliza-tion yield. Generally speaking, the photocatalytic activity isstrongly related to the crystalline phase. Konta et al. [17]reported that α-Ag3VO4 has stronger photocatalytic activitythan those of β-AgVO3 and Ag4V2O7 for oxygen productionfrom water splitting under visible-light irradiation. In thisstudy, it was observed that the 51SVO/SBA-15 sample, withmixed phases of Ag4V2O7 and α-Ag3VO4, exhibits the highest


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17031571

14711425

1383137112521241

0.025

Ku

belk

a-m

un

k

Wavenumber (cm−1)

(b)

Figure 4: DRIFTS spectra of samples (a) after adsorption of IPA for 30 min and (b) after visible-light illumination for 60 min.

0 50 100 150 200 250 3000

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34SVO/SAB-1551SVO/SAB-15

PhotoDark

Time (min)

IPA

(C/C

0)

Figure 5: Concentrations of gaseous IPA during dark and visible-light irradiation.

mineralization yield, whereas 34SVO/SBA-15 has low miner-alization yield even though it has a high crystallinity of α-Ag3VO4.

3.3. Correlation between Photocatalytic Activity and SBA-15Support. From Figures 4 and 5, the adsorption of gaseousIPA on sample surface is highly relevant to the contents ofsilver vanadate in SVO/SBA-15 composites. In the absence ofirradiation, the composite samples exhibit a much higheradsorption capability of gaseous IPA than those of P25 andbulk SVO, which is attributed to their much higher specificsurface area and pore volumes than P25 and bulk SVO.However, the adsorption of gaseous IPA showed differenttrends relative to the specific surface area of composite.

0 50 100 150 200 250 300300

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1000

1100

1200

1300

P25SVO17SVO/SBA-15

34SVO/SBA-1551SVO/SBA-15

Time (min)

CO

2(p

pm)

Figure 6: Time-dependent concentrations of CO2 as final productunder visible-light irradiation.

17SVO/SBA-15 has the highest specific surface area of217 m2g−1 but its adsorption capability for gaseous IPA waslower than those of 34SVO/SBA-15 and 51SVO/SBA-15 sam-ples. This phenomena is quite different with other studies,which have shown that the much higher specific surface areaand pore volume of TiO2-containg mesoporous silica com-posites compared to those of pure titania are beneficial forthe adsorption of organic pollutants [18–20]. Therefore, pa-rameters other than surface area need to be investigated. Itis well known that the adsorption capacity is mainly deter-mined by the specific surface area and nature of the surfaceof the photocatalyst. As shown in Figure 5, 51SVO/SBA-15exhibited the highest adsorption capacity of IPA instead of17SVO/SBA-15. However, 17SVO/SBA-15 had much higher


1700 1600 1500 1400 1300 1200

1425

1425

1425

1605

1605

1605

Ku

belk

a-m

un

k

0.002

14251605SVO

51SVO/SBA-15

17SVO/SBA-15

34SVO/SBA-15

Wavenumber (cm−1)

Figure 7: IR spectra of NH3 adsorbed onto SVO and SVO/SBA-15samples at 523 K.

specific surface area and pore volume than those of 51SVO/SBA-15, implying that a large surface area may not be bene-ficial to the adsorption capability in case of composite mate-rials. Therefore, the IPA adsorption capacity is thus greatlyinfluenced by the nature of the surface of SVO, rather thanthe specific surface area. To further characterize the surfaceattributes that affect IPA adsorption, the surface acidity, theexistence of Brønsted and Lewis acid sites, of the samples wasexamined using DRIFTS to detect ammonia adsorption onthe photocatalysts surface. A literature survey indicates thatIR spectroscopic studies of ammonia adsorbed on solid sur-faces have made it possible to distinguish between Brønstedand Lewis acid sites of a catalyst [21–26]. Before the DRIFTmeasurements, the samples were saturated with NH3/N2,flushed with N2 flow to remove physically adsorbed ammo-nia, and then heated from 303 K to 523 K. The intensity ofchemisorption was determined based on the irreversible ad-sorption of ammonia. Figure 7 shows the IR spectra of am-monia adsorbed onto SVO and SVO/SBA-15 samples whichwere heated at 523 K for 30 min. The existence of NH+

4 ionsadsorbed onto Brønsted acid sites of the silver vanadatesurfaces is supported by the presence of a band at 1425 cm−1

due to the asymmetric deformation mode [21–23]. The bandat 1604 cm−1 is assigned to NH3 coordinately bonded toLewis acid sites [24–26]. It can be concluded that bothBrønsted and Lewis acid sites exist on the surfaces of SVOand SVO/SBA-15. The intensities of Brønsted and Lewis acidsites, detected at 1425 and 1604 cm−1, respectively, follow thesequence: 51SVO/SBA-15 > 34SVO/SBA-15 > 17SVO/SBA-15 ∼ SVO. The trend of Brønsted and Lewis acid sites is inagreement with that of the adsorption capacity. That is, high-er intensities of Brønsted and Lewis acids of silver vanadatesindicate a larger adsorption capacity for IPA.

3.4. Photoluminescence Spectra. In this study, photolumi-nescence spectra were measured to disclose the migration,transfer, and recombination processes of the photogeneratedelectron-hole pairs of all samples. For the PL intensity mea-surements, the same amount of sample was used. As shown

400 425 450 475 500 525 550 575 600

0.1

PL

inte

nsi

ty

Wavelength (nm)

P25SVO17SVO/SBA-15

34SVO/SBA-1551SVO/SBA-15

Figure 8: Normalized PL intensity of the samples measured at300 K.

in Figure 8, the photoluminescence (PL) intensity of thesesamples decreases in the order of P25 > SVO > 17SVO/SBA-15 > 34SVO/SBA-15 > 51SVO/SBA-15. The PL of SVO/SBA-15 composites show obvious decrease in the intensity of PLspectra as compared to SVO, indicating the recombination ofphotoelectrons and holes is efficiently suppressed in the com-posite semiconductors. Moreover, the resulting 51SVO/SBA-15 composite shows much lower intensity of PL spectra thanthose of 17SVO/SBA-15 and 34SVO/SBA-15. However, theactive SVO content of 51SVO/SBA-15 is much higher thanthose of the others. Therefore, the intensity of PL spectra ofcomposite decreased with increasing SVO content. As shownin Figure 3, the TEM micrograph result indicates that highloading of nanocrystalline SVO can be well dispersed insidethe mesoporous channels of the 51SVO/SBA-15. The reduc-tion of PL intensity seems to depend on the amount of nano-sized SVO rather than SBA-15, an insulator. It is suggestedthat the intensity of photoluminescence spectra correspondsto the recombination rates of the holes formed in the O2p

band and the electron in the V3d band. The slower recom-bination process of photogenerated charges (the less the PLintensity) can facilitate the enhancement of photocatalyticactivity of SVO/SBA-15 composite.

4. Conclusion

In this study, the presence of nanoscaled silver vanadates sig-nificantly promoted the adsorption capacity and photocata-lytic activity. XRD and TEM results indicate the nanosized α-Ag3VO4 for 17 and 34SVO/SBA-15 and to the mixed phasesof Ag4V2O7 and α-Ag3VO4 for 51SVO/SBA-15 and that SVOdispersed well in the channels or on the surface of SBA-15without affecting the SBA-15 mesoporous structure, respec-tively. The DRIFT spectra identify the formation of interme-diates, formate and acetone, on the surface of SVO/SBA-15


composites. The composite photocatalyst exhibits highly en-hanced photocatalytic activity under visible-light irradiation,and the highest efficiency is for the 51SVO/SBA-15 compos-ite. A favorable crystalline phase and the reduction of therecombination of photogenerated hole-electron pairs areresponsible for the enhanced adsorption capacity and highmineralization yield of SVO/SBA-15 composites.

Acknowledgment

The authors are grateful to the National Science Council ofTaiwan (Grant no. NSC 99-2221-E-168-027), for supportingthis study.

References

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Research Article

Application of Pt/CdS for the Photocatalytic FlueGas Desulfurization

Xiulan Song, Weifeng Yao, Bo Zhang, and Yiping Wu

College of Energy and Environmental Engineering, Shanghai University of Electric Power, 2103 Pingliang Road,Shanghai 200090, China

Correspondence should be addressed to Weifeng Yao, [email protected]

Received 12 July 2011; Accepted 29 September 2011


Copyright © 2012 Xiulan Song et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A photocatalytic flue gas desulfurization technology was designed to control emissions of SO2 from the combustion of fossil fuels.With the photocatalytic technology, we cannot only achieve the purpose of solving the problem of SO2 emissions but also realize thedesire of hydrogen production from water. CdS loaded with Pt were selected as the model photocatalyst for the photocatalytic fluegas desulfurization. The factors influencing the rate of hydrogen production and ammonia sulfite solution oxidation were detected.

1. Introduction

Sulfur oxide (SO2) is one of the important air pollutants,which mainly originates from the combustion of coal andfuel derived from petroleum [1, 2]. In atmosphere, sulfuroxide could combine with oxygen and water resulting insulfuric acid and may cause serious damage to agricultureand wild life [1]. It is indispensable to find out efficient waysto avoid the SO2 molecules from reaching to the atmosphere.Flue gas desulphurization (FGD) is considered as one of themost effective ways to control emissions of SO2 from thecombustion of fossil fuels [3–6]. Among which, ammonia-based wet flue gas desulfurization has drawn increasingattention because of its lower investment, higher desulfuriza-tion efficiency, and useful byproducts. The reaction processof the ammonia-based flue gas desulfurization is

2NH3 + H2O + SO2 −→ (NH4)2SO3, (1)

(NH4)2SO3 + SO2 + H2O −→ 2NH4HSO3, (2)

NH4HSO3 + NH3 −→ (NH4)2SO3, (3)

2(NH4)2SO3 + O2 −→ 2(NH4)2SO4. (4)

The process of ammonia sulfite oxidation (Function (4))always decides the prospect of applying ammonia-based fluegas desulfurization technology, because high temperatureand special instruments are needed to completely oxidize

ammonia sulfite. It is of interest if we could find new waysto reduce the expense of ammonia sulfite oxidization.

Semiconductor photocatalysis is one of the hopeful waysto solve current environment and energy problem usingthe abundant solar light [7, 8]. It can decompose harmfulorganic and inorganic pollutants present in air and aqueoussolution and can also split water to produce clean and recy-clable hydrogen energy. Up to now, a lot of photocatalysts,such as TiO2 [9], CdS [10], Ag3PO4 [11], AgCl [12, 13],Bi12TiO20 [14], have been prepared and demonstrated to beable to produce hydrogen and decompose pollution underUV or visible light irradiation. CdS is known as one ofthe efficient photocatalysts for hydrogen production undervisible light irradiation with sodium sulfite and sodiumsulfide as the sacrificial materials.

In this paper, a photocatalytic process combined withammonia-water method was developed for flue gas desul-furization. It should point out that with the photocatalytictechnology, we cannot only achieve the purpose of solvingthe problem of SO2 emissions but also realize the desire ofhydrogen production from water. The chemical reaction ofthe photocatalytic process can be written as

(NH4)2SO3 + H2Ophotocatalyst, light−−−−−−−−−−→ (NH4)2SO4 + H2. (5)

The factors influencing the rate of hydrogen production andammonia sulfite solution oxidation were detected.


Ammoniasulfite solution

Methanolaqueous solution

Glacial aceticacid

Light irradiationfor 17 hours

Pt/CdS

Commercial CdS

21 3

H2PtCl6solution

Scheme 1: Schematic procedure of the Pt/CdS catalysts preparation.

2. Experimental

2.1. Preparation of Pt/CdS Catalysts. The CdS powder wasobtained commercially and used without further purifica-tion. Pt was loaded on CdS by using a photoreductionmethod. The procedure of three catalysts’ preparation meth-ods is shown in Scheme 1. CdS was dispersed in (NH4)2SO3

(CH3OH or glacial acetic acid) solution containing differentamount of H2PtCl6. The mixture was then exhausted andirradiated with 350 nm∼800 nm light for 17 h. The resultingPt/CdS powder was collected, washed, and then dried at333 K for 4 h.

2.2. Photocatalytic Reaction. The photocatalytic reactionswere carried out in a Pyrex reaction cell connected to a closedgas circulation and evacuation system. Certain amount ofthe prepared Pt/CdS, for example, 0.05 g, was suspendedin 100 mL of (NH4)2SO3 aqueous solution. The solutionwas then thoroughly degassed and irradiated by a Xe lampequipped with an optical cutoff filter (λ > 420 nm) toeliminate UV light and a water filter to remove infrared light.The amounts of H2 evolved were detected using an online gaschromatography.


3.1. Photocatalytic Oxidation of Ammonia Sulfite. The rate ofH2 production and ammonia sulfite oxidation over Pt/CdSphotocatalysts is very high. Figure 1 shows a typical resultof the photocatalytic H2 production from ammonia sulfitesolution with simulated sunlight (350∼800 nm) irradiation.It is noted that hydrogen was generated continuously fromthe ammonia sulfite solution and the rate of hydrogenproduction has no decreases after an 8-hour reaction. Theamount of H2 generated in the reaction (6.44 mmol) was

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Figure 1: Photocatalytic hydrogen production from aqueousammonia sulfite solution. Catalyst: 0.05 g Pt/CdS; 100 mL 1.25 M(NH4)2SO3; incident light: (350∼800 nm).

much greater than that of CdS (0.35 mmol) used in thereaction, indicating that the reaction of hydrogen productionfrom the ammonia sulfite solution over Pt/CdS photocat-alysts is a photocatalytic process but not a photocorrosionprocess.

Couple with hydrogen production, the ammonia sulfitewas oxidized to ammonia sulfate by the photocatalyticreactions. The production of ammonia sulfate was confirmedby using the ion chromatography (IC), which shows that theamount of sulfate ions in the solution increases but sulfiteions decrease linearly in the process of the photocatalyticreactions. It should point out that the sulfite ions were finallycompletely oxidized to sulfate after a long-term photo-catalytic reaction (about 120 h). This result indicates that


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e of

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Figure 2: Influence of (NH4)2SO3 concentration on photocatalytic activity. (NH4)2SO3 concentration: (B) 0.125 M; (C) 0.25 M; (D) 0.5 M;(E) 0.75 M; (F) 1 M; (G) 1.25 M; (I) 1.5 M; (K) 1.75 M; incident light (420∼800 nm).

the oxidation of ammonia sulfite, which is a key problemof an ammonia-based wet flue gas desulfurization process,could be efficiently operated by using the technology ofphotocatalysis.

3.2. Influence of the Ammonia Sulfite Concentration on Hydro-gen Generation. The influence of the ammonia sulfite con-centration on the rate of hydrogen formation was measuredfor the oxidation reactions of sulfite ions. As shown inFigure 2, hydrogen evolution is very sensitive to the concen-tration of ammonia sulfite. The rate of hydrogen productionfrom a 1.25 M ammonia sulfite solution was almost 4 timeshigher than that from 0.125 M ammonia sulfite solution. Fora photocatalytic reaction, the increase of reactant concentra-tion normally favors a forward reaction, because a high reac-tant concentration could be expected for the diffusion of thereacting species to and from the surface of the photocatalysts[10]. However, as shown in Figure 2, the hydrogen evolutionrate declined when the ammonia sulfite’s concentration ishigher than 1.25 M. A similar loss of activity of hydrogen pro-duction at high Na2SO3 concentrations has been observed byReber and Meier [15] and Aruga et al. [16] when using ZnSand CdS as the photocatalysts.

This phenomenon is attributed to the higher solutionviscosity and lower ionic transport ability of the higher elec-trolyte concentration [17], which may increase the electron-hole recombination [15] and make hydrogen bubbles formedon the surface of photocatalyst more difficult to be released.The optimal electrolyte concentration reported by Reber andMeier (1.0 M) is in agreement with our data [15].

3.3. Influence of Platinum Coating Amounts on Hydrogen Gen-eration. Noble metals such as Pt, Pd, Ru, and Rh function

as efficient H2 evolution promoters for many photocatalysts[18–21]. Among which, Pt is usually the best promoter forH2 evolution, which has a superior performance for theactivation of H2 in the electrochemical system. The influenceof the platinum coating at the surface of the CdS particles onthe rate of hydrogen production and ammonia sulfite oxida-tion at the present experimental condition was detected. Itis noted that noble metal Pt loading plays an important roleon hydrogen evolution from ammonia sulfite solution. Thephotocatalytic activity of pure CdS for hydrogen evolution isvery low, as shown in Figure 3, the rate of hydrogen produc-tion over CdS photocatalysts was only about 7 μmol/h undervisible light irradiation (larger than 420 nm). However, when0.5 wt% Pt was deposited on CdS, the H2 evolution ratequickly increased to 62 μmol/h. The optimum amount of Ptloading appeared to be about 0.7 wt% yielding a maximumrate of hydrogen production, 97.7 μmol/min, under visiblelight irradiation.

For the photocatalytic reactions, especially for the oxida-tion of SO3

2− ions, the Pt fraction at the surface of the pho-tocatalysts has been reported must be relatively high (largerthan 0.5 wt%) to produce an acceptable activity. Whereas, anincreasing amount of platinum loading is believed to resultin an increasing fraction of dissolved CdS [10], which wouldbe harmful for the catalysts to show a high activity. Note thatby increasing the Pt loading to 0.8 wt%, the rate of hydro-gen production dropped somewhat to about 85.3 μmol/h,under visible light irradiation (Figure 3).

3.4. Influence of the Amount of Cadmium Sulfide on HydrogenGeneration. Figure 4 shows the influence of CdS amounton the photocatalytic activity of hydrogen production fromammonia sulfite solution. In this experiment, a 1.25 M and


0 0.5 1 1.5

0

50

100

150

200

Irradiation time (hour)

BCD

EGI

Am

oun

t of

H2

prod

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ion

(μ

mol

)

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0 0.6 0.7 0.8 0.90.50

20

40

60

80

100

Amount of Pt (wt%)

Rat

e of

H2

prod

uct

ion

(μ

mol

/h)

(b)

Figure 3: Influence of platinum coating amounts (Pt versus CdS) on hydrogen generation. (B) Pure phase of CdS; (C) 0.5 wt.%; (D) 0.6 wt%;(E) 0.7 wt%; (G) 0.8 wt%; (I) 0.9 wt%. incident light: 420∼800 nm.

0 0.025 0.05 0.075 0.1

0

20

40

60

80

100

Amount of CdS (g)

Rat

e of

H2

prod

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ion

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mol

/h)

Figure 4: Influence of the amount of cadmium sulfide on hydrogengeneration.

100 mL aqueous ammonium sulfite solution was used asphotolyte and 0.7 wt% of Pt was loaded on CdS for thephotocatalytic hydrogen production. The result shows thatthe optimal amount of catalyst for hydrogen productionwas 0.05 g. Normally, more photocatalysts favor to absorbmore incidents light and would provide more active sitesfor hydrogen evolution. However, when more CdS particleswere suspended in the solution, the light scattering effect isenhanced. This would cause the photonic energy loss anddecrease the photocatalytic activity. As shown in Figure 4,the activity of hydrogen production is declined when thecatalyst’s amount was higher than 0.05 g.

3.5. Influence of Pt Coating Method on Hydrogen Generation.Figure 5 shows the influence of Pt coating method on the

0

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Am

oun

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0 0.5 1 1.5 2 2.5 3 3.5 4

a

b

c

Figure 5: Influence of Pt coating method on hydrogen generation.(a) Pt/CdS (Sample 1) prepared from a methanol solution; (b)Pt/CdS (Sample 2) prepared from an ammonia sulfite solution; (c)Pt/CdS (Sample 3) prepared from a glacial acetic acid.

photocatalytic activity of Pt/CdS for hydrogen productionand ammonia sulfite oxidation. Three differently preparedsamples of Pt/CdS were illuminated in the ammonia sulfitesolution under visible light irradiation. It is apparent thathydrogen is generated in all the cases when the catalystswere illuminated. However, the activity of these catalysts forhydrogen production is strongly dependent on the prepa-ration method of Pt/CdS catalysts. It is noted that Sample1, which was prepared from a methanol solution, showed amuch low activity for hydrogen production from ammoniasulfite solution. Only 70 μmol H2 was produced fromammonia sulfite solution within 3 hours under visible lightirradiation (Line a, Figure 5). In contrast, 330 μmol H2 was


obtained over Sample 2 prepared from ammonia sulfitesolution (Line b, Figure 5) under the same condition. Thehighest rate was observed by illuminating the Sample 3,which was prepared from the glacial acetic acid. It is notedthat 950 μmol H2 was evolved from ammonia sulfite solutionwithin 3 hours under the present working condition (Line c,Figure 5). The high activity of the Pt/CdS samples, preparedfrom the glacial acetic acid, for hydrogen production andammonia sulfite oxidation is not clear. The process oftreatment in acetic acid is thought helpful for the eliminationof CdO fraction, which is lying at the CdS surface andharmful for the photocatalytic reactions [10, 22]. Furtherworks, such as detection of the ingredient, valence state,particle size of Pt over CdS, are still under way to determinethe mechanism of the different activities of the three samplesprepared with different methods.

4. Conclusion

In summary, the oxidation of ammonia sulfite, which is a keyproblem of an ammonia-based wet flue gas desulfurizationprocess, could be efficiently operated by using the technologyof photocatalysis. With the photocatalytic technology, wecannot only achieve the purpose of solving the problem ofSO2 emissions but also realize the desire of hydrogen pro-duction from water. The photocatalytic activity for hydro-gen production and ammonia sulfite oxidation was foundstrongly dependent on (NH4)2SO3 concentration, amount ofPt/CdS catalyst, and Pt coating method.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (21103106), Shanghai Rising-Star Program (10QA1402800), Shanghai Nano SpecialFoundation (1052nm02600), Construction Capacity Project(10160502300) and (09230501400). W. Yao thanks the fundof Specially-Appointed Professors (Oriental Scholars) forShanghai Universities.

References

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[2] A. V. Slack and G. A. Hollinden, Sulfur Dioxide RemoVal fromWaste Gases, Noyes Data Corporation, Park Ridge, NJ, USA,2nd edition, 1975.

[3] B. B. Hansen, S. Kiil, J. E. Johnsson, and K. B. Sønder, “Foam-ing in wet flue gas desulfurization plants: the influence ofparticles, electrolytes, and buffers,” Industrial and EngineeringChemistry Research, vol. 47, no. 9, pp. 3239–3246, 2008.

[4] X. Ma, T. Kaneko, T. Tashimo, T. Yoshida, and K. Kato, “Useof limestone for SO2 removal from flue gas in the semidryFGD process with a powder-particle spouted bed,” ChemicalEngineering Science, vol. 55, no. 20, pp. 4643–4652, 2000.

[5] G. Astarita, D. W. Savage, and A. Bisio, Gas Treating withChemical Solvents, Wiley-Interscience, New York, NY, USA,1983.

[6] H. F. Johnstone, “Recovery of Sulfur Dioxide from WasteGases,” Industrial & Engineering Chemistry Research, vol. 32,no. 8, pp. 1037–1049, 1940.

[7] X. Chen, S. Shen, L. Guo, and S. Mao, “Semiconductor-basedphotocatalytic hydrogen generation,” Chemical Reviews, vol.110, no. 11, pp. 6503–6570, 2010.

[8] S. Shen, J. Shi, P. Guo, and L. Guo, “Visible-light-driven pho-tocatalytic water splitting on nanostructured semiconductingmaterials,” International Journal of Nanotechnology, vol. 8, no.6-7, pp. 523–591, 2011.


[10] N. Buhler, K. Meier, and J. F. Reber, “Photochemical hydrogenproduction with cadmium sulfide suspensions,” Journal ofPhysical Chemistry, vol. 88, no. 15, pp. 3261–3268, 1984.

[11] Z. Yi, J. Ye, N. Kikugawa et al., “An orthophosphate semi-conductor with photooxidation properties under visible-lightirradiation,” Nature Materials, vol. 9, no. 7, pp. 559–564, 2010.

[12] P. Wang, B. Huang, X. Qin et al., “Ag@AgCl: a highlyefficient and stable photocatalyst active under visible light,”Angewandte Chemie, vol. 47, no. 41, pp. 7931–7933, 2008.

[13] P. Wang, B. Huang, Z. Lou et al., “Synthesis of highly efficientAg@AgCl plasmonic photocatalysts with various structures,”Chemistry, vol. 16, no. 2, pp. 538–544, 2010.

[14] W. Yao, H. Wang, X. H. Xu et al., “Photocatalytic property ofbismuth titanate Bi12TiO20 crystals,” Applied Catalysis A, vol.243, no. 1, pp. 185–190, 2003.

[15] J. F. Reber and K. Meier, “Photochemical production ofhydrogen with zinc sulfide suspensions,” Journal of PhysicalChemistry, vol. 88, no. 24, pp. 5903–5913, 1984.

[16] T. Aruga, K. Domen, S. Naito, T. Onishi, and K. Tamaru, “Therole of sulfite anion as a hole scavenger in the photocatalytichydrogen formation from water under illumination of visiblelight,” Chemistry Letters, vol. 7, pp. 1037–1040, 1983.

[17] R. de Souza, J. Padilha, R. Goncalves, and J. Rault-Berthelot,“Dialkylimidazolium ionic liquids as electrolytes for hydrogenproduction from water electrolysis,” Electrochemistry Commu-nications, vol. 8, no. 2, pp. 211–216, 2006.

[18] H. Kato, K. Asakura, and A. Kudo, “Highly efficient watersplitting into H2 and O2 over lanthanum-doped NaTaO3 pho-tocatalysts with high crystallinity and surface nanostructure,”Journal of the American Chemical Society, vol. 125, no. 10, pp.3082–3089, 2003.

[19] X. Zong, H. Yan, G. Wu et al., “Enhancement of photocatalyticH2 evolution on CdS by loading MoS2 as cocatalyst undervisible light irradiation,” Journal of the American ChemicalSociety, vol. 130, no. 23, pp. 7176–7177, 2008.

[20] W. Yao, C. Huang, N. Muradov, and A. T-Raissi, “A novelPd-Cr2O3/CdS photocatalyst for solar hydrogen productionusing a regenerable sacrificial donor,” International Journal ofHydrogen Energy, vol. 36, no. 8, pp. 4710–4715, 2011.

[21] W. Yao, C. Huango, and J. Ye, “Hydrogen production andcharacterization of MLaSrNb2NiO9 (M = Na, Cs, H) basedphotocatalysts,” Chemistry of Materials, vol. 22, no. 3, pp.1107–1113, 2010.

[22] M. Matsumura, S. Furukawa, Y. Saho, and H. Tsubomura,“Cadmium sulfide photocatalyzed hydrogen production fromaqueous solutions of sulfite: effect of crystal structure andpreparation method of the catalyst,” Journal of PhysicalChemistry, vol. 89, no. 8, pp. 1327–1329, 1985.


Review Article

An Enthusiastic Glance in to the Visible ResponsivePhotocatalysts for Energy Production and Pollutant Removal,with Special Emphasis on Titania

Padikkaparambil Silija,1 Zahira Yaakob,1 Viswanathan Suraja,1

Njarakkattuvalappil Narayanan Binitha,2 and Zubair Shamsul Akmal1

1 Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia,Selangor 43600 Bangi, Malaysia

2 Department of Chemistry, Sree Neelakanta Government Sanskrit College, Pattambi, Kerala, Palakkad 679306, Malaysia

Correspondence should be addressed to Zahira Yaakob, [email protected]

Received 15 July 2011; Accepted 19 August 2011


Copyright © 2012 Padikkaparambil Silija et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

As a consequence of the rapid growth of industry, major problems are created related to energy and environment. Sunlight beingone of the most potential alternative source of energy, the development of efficient solar-energy storage systems is an importantsubject in the fields of science and technology. Here we have reviewed and summarized some of the recent reports on visibleresponsive photocatalysts. In this review, the influence of various metal oxide photocatalysts on energy production and pollutantremoval are presented with special emphasis on titania based photocatalysts. The photoactivity of titania for various pollutantdegradation, modified titania (TiO2) systems, their physical and chemical characteristics, and so forth, are described in detailat this juncture. Different methods used to enhance the visible light absorption of TiO2, like doping with metals and nonmetals,coupling with other metal oxides, and so forth, have been discussed. Various applications of photocatalysts including photocatalytictreatment of waste water, pesticide degradation and water splitting to produce hydrogen are summarized. The development ofphotocatalysts that function under visible light for the efficient utilization of sunlight is an area of current interest and thus thedifferent methods of preparation for the visible active photocatalysts are also explored.

1. Introduction

Photochemistry deals with the chemical changes broughtabout by light. Its important aim is to discover or to designstructurally organized and functionally integrated artificialsystems that are capable of harvesting solar energy, toperform useful functions. Energy crisis is the main impetusto the study of photochemical reactions. The photochemicalchanges such as photofading of coloured materials, pho-tosynthesis in plants, and blackening of silver halides, andwere observed and so forth, studied qualitatively from 1817onwards. The quantitative approach to photochemistry wasinitiated by Grotthus and Draper in the beginning of thenineteenth century, and it was formulated that only thelight which is absorbed by a system can cause any chemical

change. The probability or rate of absorption is given by theLambert-Beer Law [1].

Photocatalysis is found to be an eco-friendly cheapmethod for removing various pollutants from gas and liquidstreams and conversion of solar energy to chemical energyby splitting water (H2 generation) and reducing CO2 intolight hydrocarbons. Splitting of water is a process thathas great potential for the conversion of photo energy tochemical energy, in the form of hydrogen [2]. Millions oftonnes of H2S are produced in petroleum refinery plants inevery year and is expected to increase considerably in thefuture [3, 4]. Thus, among various methods of solar energyconversions, much attention has been paid to photocatalyticdecomposition of H2S, for its potential in obtaining cleanand high energy containing H2 from abundant H2S. As the


direct thermal decomposition of H2S, for the productionof hydrogen and elemental sulfur is energy intensive andeconomically unviable, there has been immense emphasison the development of visible light photocatalysts forthe production of hydrogen [5]. The development of aparticulate photocatalyst that catalyzes overall water splittingunder visible light for large-scale production of H2 fromwater and sunlight, has become an important endeavor [6–9]. It is also essential to find an alternative to fossil-fuel-based energy sources, for establishing new energy systems forthe 21st century. Thus, photocatalysis appeared as a “green”technology with promising applications in a wide assortmentof chemical and environmental technologies.

Over the past decades, photocatalytic activity of semicon-ducting inorganic solids have attracted passionate researchinterest for the degradation of organic pollutants and/orthe decomposition of water molecules [10, 11]. Semicon-ductors (such as TiO2, ZnO, Fe2O3, CdS, and ZnS), whichare characterized by a filled valence band and an emptyconduction band [12], are proved to be important materialsdue to the electronic structure of the metal atoms in chemicalcombination.

The major organic compounds that constitute theindustrial waste water include dyes, phenols, chlorophenols,aliphatic alcohols, aromatics, polymers, and carboxylic acids.Although, dyes are creating a colorful world, they are becom-ing a major source of environmental contamination, as theirrelease into the ecosystem is a dramatic source of aestheticpollution, eutrophication, and perturbation in aquatic sys-tems [13]. Colour removal, especially from textile wastewater, has been a big challenge over the last decades [14].The conventional technologies are not capable of reducingthem to the lowest levels demanded by the environmentallaws. The complexity and variety of dyestuffs employed in thedying processes made it difficult to find a unique treatmentprocedure that entirely covers the effective elimination of alltypes of dyes. Principally, biochemical oxidations go throughconsiderable restrictions in view of the fact that the majorityof dyestuffs commercially available have been deliberatelyplanned to resist aerobic microbial degradation and arethus, converted to toxic or carcinogenic compounds. Physicalprocesses, for example, flocculation, reverse osmosis, andadsorption on activated charcoal are nondestructive andsimply transfer the pollutant to another media, consequentlycausing secondary pollution. Among the advanced oxidationprocesses (AOP) prompted for treating both gas and waterpollutants, the heterogeneous photocatalysis is a powerfultool to solve environmental and energy problems [15, 16].Injurious atmospheric pollutants, which are exhausted frominternal combustion engines and furnaces, like nitrogenoxides (NOx) such as nitric oxide (NO), nitrous oxide(N2O), and nitrogen dioxide (NO2) originates acid rain,photochemical smog, and greenhouse effects. Therefore,successful removal of the above pollutants has become avital topic in the meadow of environmental protection, andthere had appeared many winning reports on the catalyticoxidation of NOx under photo irradiation.

Photocatalytic treatment plays an important role in theenvironmental and energy applications, including purifi-

cation and recycling of waste water and degradation ofpesticides, dyes, and other volatile organic compounds. Pro-duction of hydrogen by water splitting becoming an inter-esting area of research. For the efficient utilization of solarenergy, photocatalysts exhibiting longer wavelength absorp-tion are highly desirable. In the present review, we have triedto make an illumination into the visible responsive photo-catalytic world.

2. Effect of Band Gap

Generally, photodissociation driven by a semiconductororiginates from a redox reaction with transient electronsor holes generated by an electronic transition from valenceband (VB) to conduction band (CB), by the absorption oflight with energy equal to or greater than the band gap of thesemiconductor. The separated photo-induced electrons andholes transferred themselves efficiently to the semiconductorsurface. At the surface, electrons react with acceptors (usuallyO2 dissolved in the solution) to produce radical anionssuch as O.

2−. Meanwhile, holes react with donors (H2O,

OH−) to produce oxidant species such as OH·. These highlyactive species, which are produced from electrons and holes,have strong oxidizing and reducing abilities and can directlyoxidize organic compounds into CO2 and H2O. So theefficiency of photocatalysts strongly depends on their bandstructures such as bandgap energy (Eg) and the positions ofVB and CB.

Due to the large band gap, these semiconductors could beactivated only by UV irradiation. This limited the utilizationof sunlight as an irradiation source in photocatalytic reac-tions. It is known that the UV part of the solar spectrumaccounts only for about 5% of the incoming solar energywhile the rest is visible light. The holes and electronsexcited by the UV light can recombine easily, which willreduce the efficiency of photons. Thus suppression of therecombination of hole-electron pairs is a necessity. There-fore, a photocatalyst material having visible light activity isrequired for harvesting solar energy and interior lightingapplications. It is therefore of great significance to adjust theband structure of photocatalyst to improve photocatalysis byefficient utilisation of solar energy. There have been manyattempts such as dye sensitization, coupling of differentsemiconductors, and so forth are performed to optimize theband structure of semiconducting photocatalysts [17, 18].Recently, attempts have been made to modify the band gapenergy simply by substituting metal or oxygen ions in part ofthe oxide lattice [19–21].

3. Visible Light Responsive Photocatalysts

Semiconductor photocatalysts had been widely employedin pollutant removal as well as energy production, afterthe early work on TiO2 photoelectrochemical hydrogenproduction reported by Fujishima and Honda [10]. Nowa-days scientific and engineering interests in semiconductorphotocatalysis have full grown extensively. The major limi-tations of the application of semiconductor photocatalysts,


as mentioned before, are their high band gap and UV lightactivity. Smoothening of the progress of their applicationsnecessitated the use of sunlight or visible light responsivesystems, which can make the process of semiconductor pho-tocatalysis economical. In the preceding sections differentphotocatalysts and the factors leading to their visible lightresponse are sighted (see Figure 1).

3.1. Titania. TiO2 is one of the most talented heterogeneousphotocatalysts because of its first-rate properties such ashigh photocatalytic activity, strong oxidizing power, low cost,chemical and thermal stability, resistance to photo corrosionand nontoxicity, in addition to its favorable optoelectronicproperty [22]. TiO2 is also a very popular material for opticaland protective applications, because of its high transparencyin the visible region [23] and excellent mechanical durability.Anatase, rutile, and brookite are the three phases in whichTiO2 usually exists in nature. Among them anatase andrutile are commonly utilized as photocatalysts. Even thoughanatase is believed to be the more reactive phase of TiO2

than rutile crystalline phase, it has low quantum yield foroxidation steps (∼5%) as a result of rapid recombination ofphotogenerated electron-hole pairs [24]. The high intrinsicband gap energy of pure TiO2 photocatalysts (3.2 eV foranatase phase and 3.0 eV for rutile) made anatase operateeffectively as a photocatalyst only when the wavelengths oflight are shorter than 387 nm. Thus, pure anatase is able touse only around 4% of the terrestrial solar spectrum becauseof its wide band gap [25]. Many attempts have been madeto make it active in the visible range, which include dyesensitization, metal ion doping, nonmetal doping, and soforth [19–21].

The rutile phase of TiO2 has a slightly smaller bandgap (3.0 eV), and, therefore, rutile should be photo-excited more easily [26]. The higher recombination rate ofphoto-generated charge carriers is the major reason forthe poor photocatalytic activity of rutile as compared toanatase [19]. The most feasible methods for humanizingthe photocatalytic performance of TiO2 are doping withmetals and nonmetals. Doped metal atoms could suppressthe recombination of photo-induced electron-hole pairs,where the excited electron migrates from the inside of thephotocatalyst to the surface of the metal so as to increasethe photo quantum efficiency [19, 27] by trapping theelectron from recombination. On the other hand, nonmetaldoping decreases the band gap, and shifts the responseto the visible part of the solar spectrum by incorporatingnonmetal atoms into the lattice structure of TiO2 [21, 28, 29].Based on literature review, the outcome of different attemptsto improve the photoactivity of TiO2 is described in thefollowing sections.

3.1.1. Metal-Doped Visible Light Active TiO2. The secondmost detrimental aspects of the photoactivity of TiO2 isits relatively high electron-hole recombination rate. Pre-vious studies demonstrated that the appropriate amountof metals doped on TiO2 could inhibit the recombinationof photo-induced electron-hole pairs [30, 31]. The change

−O2

•−O2

OH−

H2OH+

+ Valance band

Conduction band

Photocatalyst

hA

+

Pollutantmolecules

band gap

CO2 + H2O

Figure 1: Pictorial representation of semiconductor photocatalystband structure that leads to pollutant degradation upon exposureto light.

in electronic properties of TiO2 by substitution of metalion for Ti4+ sufficiently reduced the energy band gap toabsorb visible light. Abundant studies were reported forthe characteristic behavior of visible-active metal-dopedsemiconductor photocatalysts where the dopants includenoble metals [32], rare earth metals [31], and transitionmetals, and so forth [33]. Silver and gold nanoparticlespossess additional ability to absorb visible light, due tolocalized surface plasmon resonance (LSPR) [34], whichagain contributes to the visible light activity, when the noblemetals are doped over TiO2.

A series of transition metal ions, such as Cr, Mn, andCo, when doped in TiO2 powders by hydrolysis method[35] and showed considerable shift of absorption towardsvisible light region. Li and Li reported Au3+ doped TiO2 forthe photodegradation of methylene blue (MB) under visiblelight [36]. Kim et al. reported the preparation of Pt ion-doped TiO2 for the photodegradation of chlorinated organiccompounds under visible light irradiation [37]. The photo-activity of TiO2 could be enhanced by the incorporation oflanthanide ions into TiO2 matrix. Lanthanide ions have theaptitude to form complexes with a variety of Lewis bases(including organic acids, amines, aldehydes, alcohols, andthiols) by the interaction of functional groups with f-orbitalsof lanthanides. This could provide a way to concentrateorganic pollutant at the semiconductor surface [38]. Thus,TiO2, doped with lanthanide metals, such as La3+, Eu3+,Nd3+, and Ce4+, produces effective photocatalysts that arefound to be visible-light active [39–42]. The vanadium ionsshowed a marked effect in the red shift of the spectralresponse of TiO2 [43]. Vanadium dopant is accounted as oneof the best metal dopants to extend the optical absorptionof TiO2 towards the visible light region [44]. The enhancedabsorption to visible light region and improved quantumefficiency, owing to the improvement in e−–h+ pair separa-tion make TiO2 an efficient photocatalyst in the presence ofvanadium. The V4+ species in the V-doped TiO2 materialscould act as a trapping center for both h+ and e−, favoringcharge separation, while V5+ might have acted as an electronacceptor [45–47]. Xu et al. [48] reported that, the presence ofGd in TiO2 accelerated interfacial electron transfer processand thus exhibited better photocatalytic activity. ThoughDRS results showed more red-shift on higher Gd-doping,


the photoactivity was not consistent with the result. It maybe due to the large amount of Gd which would formrecombination centers where photo-induced carriers couldbe captured, which led to a lower photocatalytic activity.Fe doping is also found to be highly efficient to impartvisible activity to TiO2. Fe2O3-modified TiO2 nanotubearrays were first prepared by Kuang et al. [49] by annealinganodic TiO2 nanotubes preloaded with Fe(OH)3 which wasuniformly clung to the TiO2 nanotubes using sequentialchemical bath deposition (S-CBD). Fe2O3-modified systemsshowed higher photopotential and photocurrent valuesthan those of unmodified TiO2 nanotubes. The maximumphotocurrent was obtained with 0.5 at% Fe content, whichwas 7 times greater than that achieved with unmodified TiO2

nanotubes. The enhanced photoelectrochemical behaviourscan be attributed to the shifting of photoresponse of TiO2

from UV to visible region due to the low band gap of 2.2 eVof Fe2O3. The Fe2O3-modified samples also resulted in anegative shift of the zero-current potential from −0.36 to−0.78 V, which further confirmed that the enhancement inthe separation of the photogenerated e−–h+ [50].

3.1.2. Nonmetal Doping on TiO2. Considerable efforts havebeen made to extend the photoactivity of TiO2-based systemsinto the visible light region for the efficient utilization of solarenergy. Nonmetal doping is an alternative for improvingthe visible light response of TiO2, and extensive researchwork has been done on synthesis of N-doped, S-doped,F-doped, and C doped TiO2 [51–54]. The doping withnonmetal could narrow down the band gap by modifyingthe electronic structure around the conduction band edge ofTiO2, by incorporating anion into the crystal lattice of TiO2

[21]. This might drive better photocatalytic performanceunder visible light [21, 28]. Recently, many researcherspaid much attention to nitrogen-doped TiO2, which canbe produced using different techniques, such as hydrolyticprocess [55, 56], mechanochemical technique [57, 58], andreactive DC magnetron sputtering, and so forth [59, 60].The first study on nitrogen-doped TiO2 including its visiblelight photocatalysis was conducted by Sato in 1986 [61].The mixtures of a commercial titanium hydroxide andammonium chloride calcined at about 400◦C showed higherphotocatalytic activity in the visible lightregion due to thepresence of nitrogen and, thereafter N-doping is consideredas an efficient method to extend the optical absorptionedge of TiO2 into the visible light region [21, 59, 61].Similarly, carbon and sulphur were identified to form newimpurity levels closest to the valence band whilst maintainingthe largest band gap for maximum efficiency. Carbon-doped TiO2 obtained by controlled combustion of Ti metalshowed an impressive performance for photochemical watersplitting under visible light [28]. Khan et al. [28] reporteda chemically modified C-doped TiO2 as the photochemicalwater splitting material under visible light. Chou et al.reported carbon containing nanostructured mixed TiO2

phases with enhanced visible light photoactivity [62]. Wonget al. [63] fabricated carbon-incorporated TiO2 films in ananatase structure. The carbon was present both in the form ofsubstituted Ti–C bonds and free graphitic carbon. The films

exhibited absorption bands in the visible light region, with anabsorption edge red-shifted up to ∼450 nm, correspondingto a band gap ∼2.7 eV, with the gradual increment ofcarbon content. The films possessed outstanding visiblelight-induced photocatalytic properties in the reduction ofsilver ions, degradation of MB, and super-hydrophilicity.Park et al. demonstrated the catalytic efficiency of the C-doped TiO2 nanotubes, for water splitting under visible lightirradiation, which was prepared at an elevated temperaturerange of 500–800◦C, by using carbon monoxide precursor[64]. Visible light responsive, carbon doped TiO2 films weredeveloped by ion-assisted electron-beam evaporation usingrutile powder as source material and two different gases,CO2 and CO, in the ion source as dopant source by Wonget al. [65]. The annealed, carbon-doped, anatase TiO2 film,with a carbon content of 1.25 wt%, gave the best visible lightphotocatalytic activity for super hydrophilicity, degradationof (MB), and reduction of silver ions.

Asahi et al. [21] and other investigators [58, 66] reportedthe shifting of optical absorption edge to the visible regionbecause of the narrowing of the band gap, by mixing theN 2p and O 2p states in N-doped TiO2. It has initiated anew research area to extend the photo absorbance into thevisible light region using nitrogen-doped TiO2. But mostof current researchers supported an alternative proposal inwhich the dopant atom orbitals generate an isolated mid-gaplevel above the valence band [51, 56, 67]. Irie et al. suggestedthat the formation of an isolated N 2p narrow band above theO 2p valence band is responsible for the visible light responsein the nitrogen-doped TiO2 [51]. Doped nitrogen can beincorporated into the TiO2 structure as substituted N and/oras interstitial N. Peng et al. [68] found that the interstitial Nimpurities can enhance the photoactivity of TiO2 in visiblelight and the activity was found to be higher than that ofsubstitutional N-doped TiO2. But, some researches [69, 70]presented that the interstitial N impurities might reducethe direct oxidation ability of sample in the photocatalyticprocess by acting as stronger hole trapping sites.

Burda et al. [71] used triethylamine as the N-dopant toget nitrogen-doped TiO2, with an average grain size of 6–10 nm, which could absorb well in the visible light regionup to 600 nm. Based on theoretical predictions and availableexperimental results, recently Wang and Lewis [72] hadreported that carbon doping of TiO2 gave the best photo-response compared to nitrogen or sulfur doping. A seriesof nitrogen-doped TiO2 catalysts were generated by Qin etal. [73] with N/Ti proportioning of 4, 8, 12, 20, 24, 28,and 32 mol%. Among them N/Ti = 20 mol% showed highestcatalytic activity and, they stated that an optimal content ofTi3+ might be the critical factor leading to the improvementof the photoactivity. This observation is supported by theprevious report, which affirmed that in nitrogen-dopedTiO2 catalysts, the oxygen sites were partially replaced withnitrogen atoms, while TiO2 was simultaneously reduced [74]and it leads to an increase in oxygen vacancy and amountof Ti3+. As the concentration of dopant increased, moreTiO2 was reduced and the amount of oxygen vacanciesincreased. When it is excessively high, the space chargeregion became very narrow and the penetration depth of


light into TiO2 greatly exceeded the space charge layer andthus the recombination of photo-generated electron holepairs became easier [39]. Here, excessive oxygen vacancy andTi3+ acted as a recombination center for holes and electrons[51]. N-doped TiO2 is mainly applied for the removal ofvarious types of organic pollutants from the environmentsuch as acetaldehyde [75], methylcyclohexene [76], benzoicacid [77], and dyes [78]. Wang et al. [79] demonstrateda unique incorporation of carbon into TiO2 films withcarbon-covered (along grain boundaries) columnar grainsof carbon-doped (inside the grains) anatase phase, usingthe method of reactive magnetron sputtering. The charac-terizations revealed that carbon is incorporated both in theform of substituted Ti–C bonds as well as free graphiticnature. The gradual increment of power of the graphitetarget shifted the absorption edge of the TiO2−xCx : Cfilms from ultraviolet to visible region which increased thephotocatalytic performance with retained crystallinity, evenat a high carbon content of 9.3%.

3.1.3. Effect of Codoping on TiO2. The modification of TiO2

by codoping is an effective method apart from doping TiO2

with a single metal or nonmetal. It is highly anticipatedthat doping TiO2 with an appropriate combination of metalsand/or nonmetals would, of course, result in more visiblelight-sensitive photocatalysts for a desired application. Manyof the recent efforts and strategies revealed that codopingof TiO2 with a metal and a nonmetal can result in thedevelopment of highly efficient visible active photocatalysts[80–82]. Zhao et al. [82] reported an improvement in bothspectral response and photocatalytic efficiency through acombined action of a nonmetal and a metal oxide. Theyhad prepared Ni2O3/TiO2−xBx photocatalyst by a simplemodified sol-gel method. They demonstrated that incorpo-ration of B into TiO2 could extend the spectral responseto the visible region, and the photocatalytic activity wasgreatly enhanced when Ni2O3 was further loaded into it.Sakatani et al. [81] reported that La–N–TiO2 photocatalystprepared by means of a Polymerization method whichcould effectively decompose acetaldehyde under visible lightirradiation. Shi et al. [83] reported the cooperative effectsof the two dopants Fe3+ and Ho3+ ions that resulted inimproved photocatalytic activity to the codoped TiO2. Liet al. [84] pointed out high photocatalytic activity for N–F codoped catalyst prepared by pyrolysis method, and thehigh visible light activity was ascribed to a synergetic effectof its unique surface characteristics, doped N atoms, anddoped F atoms. Yuan et al. [85] revealed the cooperativeaction of Zn2+ and Fe3+, over TiO2, in the photocatalyticdegradation of phenol. The mechanisms of modificationusing metal and nonmetal element are completely different.Metal atoms either form individual phases dispersed intoTiO2 or accommodate into the lattices of TiO2, which areprimarily related to metal ion radii [86] while nonmetalatoms can enter into TiO2 lattice [21, 28]. So it is expectedthat the doping with different transition metals, rare earthmetals, nonmetals as well as their combination can be usedto raise the photoactivity of TiO2 synergistically. Balek etal. [87] reported that the nitrogen and fluorine codoped

TiO2 photocatalyst prepared by spray pyrolysis using a mixedsolution of TiCl4 and NH4F showed high photocatalyticactivity in the visible region of spectrum for acetaldehydedecomposition. They demonstrated that the observed highphotocatalytic activity of the samples could be ascribed toa synergetic effect due to the codoping of nitrogen andfluorine. Ling et al. [88] prepared the B and N codopedTiO2 nanopowders using boric acid and ammonium fluorideas the precursors of boron and nitrogen and proved thesynergistic effect of B/N codoping, in its photocatalyticperformance. Ling et al. [88] prepared B/Fe/Ce codopedTiO2, and their characterization results showed that B dopingled to the modification of electronic structure around theconduction band edge of TiO2, and eventually resulted inthe visible light response. In fact, the Ti 3d orbital of TiO2

split into two parts, and the CB was divided into the lowerand upper parts. On doping TiO2 with B, the B 2p states aresomewhat delocalized and thus resulted in the modificationof the electronic structure around the conduction band edgeof TiO2. The mixing of the B 2p states with VB increasedthe width of the VB itself and thus decreases the bandgap energy [89]. By investigating the electronic and opticalproperties of several possible B-doped models, Yang et al.[90] pointed out that the transition of excited electronsfrom the valence band to the empty gap states above theFermi level might be responsible for the red shift of theabsorption edge in substitutional B- to O-doped anatase.Zhao et al. [82] had preformed theoretical calculations andfound that the mixing of p orbital of B with O 2p orbitalwas responsible for narrowing of the band gap by calculatingthe densities of states (DOSs). According to the reports[31, 85] on degrading dichlorophenol, Fe3+ ion trapped anelectron to change into Fe2+, thus demolishing the stable halffilled electronic configuration. Thus for maintaining steadystructure, the trapped electron could rapidly be transferredfrom Fe2+ to the oxygen molecules adsorbed on the surfaceof the photocatalyst and the Fe2+ recurred to the originalhalf-filled state (Fe3+), thus accelerating charge transfer andprohibiting the electron-hole recombination. Additionally,the titanium atoms could enter into the lattice and replacedCe3+ or Ce4+, as the ionic radius of Ti4+ is much smallerthan that of Ce3+ or Ce4+. This substitution led to a chargeimbalance and to keep the equilibrium, more hydroxide ionswere adsorbed onto the surface, thus benefiting the photo-generated electron-hole pairs separation, which eventuallyincreased the photoactivity. Ling et al. [88], therefore,suggested that the cooperative actions of boron, ferrum,and cerium resulted in the increase of the photoactivityfor degradation of DCP. Silija et al. [91] developed anefficient photocatalytic system with unique incorporation ofcarbon and nitrogen into TiO2, using urea as N precursorand ethanol as cosolvent. The systems, calcined at lowtemperature retained the carbon and showed excellentphotoactivity. The coexistence of both the anion dopants,C and N, enhanced the catalytic efficiency and was evidentfrom the activity comparison studies. Surprisingly, thismost photocatalytically active system, which was calcined at300◦C, is found to be amorphous in nature and possesseshighest surface area and lowest band gap among the prepared


samples. The nitrogen was incorporated interstitially andit is found to be one of the best photocatalyst for MBdegradation under visible light irradiation. Codoping ofnanosilver, carbon, and nitrogen was done by Binitha et al.[92] on TiO2 to get efficient degradation in visible lightfor methyl orange degradation. Results showed a synergeticeffect of carbon and nitrogen doping as well as nanosilverloading on the performance of TiO2. A dual-functionalcomposite of nitrogen-doped TiO2 supported on activatedcarbon (N–TiO2/AC) were prepared by Yap et al. [93]. Bothadsorption and photocatalytic degradation effects of N–TiO2/AC were evaluated using bisphenol-A (BPA) as thetarget pollutant, in the aqueous phase. The effect of pHand influence of excitation wavelengths were investigated.Inhibition of BPA adsorption occurred at pH 11.0 thusresulted in a slower kinetics of BPA photodegradation. N–TiO2/AC was found to be photoresponsive under visiblelight (420–630 nm) illumination. Pt- and CdS-codoped TiO2

system CdS/(Pt/TiO2) showed the highest rate of hydrogenproduction under visible light irradiation in comparisonwith CdS/TiO2 or Pt/CdS single component doped systemdue to the cooperation of the electron trapping ability ofnoble metal and decreased band gap energy due to thepresence of CdS [94]. Similarly, the same observation hadfound in the case of Ag- and InVO4-codoped TiO2 compositethin film with 1% Ag doping, where the system exhibitedhigher visible light photocatalytic activity for decompositionof aqueous methyl orange compared with TiO2 or InVO4–TiO2 [95].

Yu et al. [96] reported visible light active catalyst with Ga-and W-codoped TiO2 with Cu2+ modification and found itsquantum yield under visible light irradiation as 13%. Manysystems such as CdS/TiO2 [97, 98]. CdSe/TiO2 [99, 100],Bi2O3/SrTiO3 [101], Bi2S3/TiO2 [98, 102], ZnMn2O4/TiO2

[103], and TiO2/Ti2O3 [104] were formed sensitizer-loadedTiO2, which have shown efficient visible light photoactivity.In most of these catalysts, the addition of sensitizers reducedthe band gap of the material and enhanced the visible lightabsorption. But the photogenerated holes of the sensitizerremain in the valence band (VB), and its accumulation leadsto photocorrosion of the catalyst resulting in low stability ofthe composite photocatalyst [105].

3.2. ZnO-Based Photocatalysts. Nanostructured ZnO is espe-cially important in hi-tech applications owing to its uniquechemical and physical properties [106, 107]. It is a directwide gap and n-type semiconductor with a band-gap of3.37 eV and found to be useful in various opto-electronicapplications such as light emitting diodes and solar cells, andso forth [108, 109]. It is widely employed as a raw material inthe textile, cosmetic, ceramic, and glass industries. ThoughTiO2 is the most extensively used photocatalyst [110], inrecent years ZnO has attracted special attention owing toits low cost [111, 112], efficient synthesis, easy handling,reliability, and its application as a chemisensor and photo-catalyst for the detection of toxic chemicals such as H2, NH3,liquid petroleum gas (LPG), HCHO, ammonium hydroxide,and alcohols, and so forth [113, 114]. Moreover, ZnO hasperformance advantages over TiO2 for the decomposition of

volatile organic compounds, including azo dyes [115]. ZnO-based visible light photocatalyst seems to be an alternative toTiO2 and WO3 [116]. The energy levels for the conductionand valence bands and the electron affinity of zinc oxide aresimilar to those of TiO2, making ZnO a likely candidate asa semiconductor material for photocatalysis. Therefore, itsphotocatalytic ability had attracted much interest. ZnO hasfound to be a suitable alternative to TiO2 in a number ofstudies such as photodegradation of pesticides and herbi-cides [117], photocatalytic oxidation of pulp mill, bleachingwaste water [118], degradation of 2,4,6-trichlorophenol[119] and 4-chlorophenol [120], photodegradation of AcidRed B dye [121] and degradation of cyanide, and so forth[122]. Poulios et al. [123] studied photocatalytic degradationof Auramine O in aqueous suspension using ZnO and TiO2

separately in a batch reactor, and they concluded that therate of degradation of pollutants is faster with ZnO than withTiO2 (Degussa P25). Its low efficiency of photocatalysis andlow stability due to photocorrosion are the two main draw-backs facing by these ZnO photocatalysts compared withthe common photocatalyst, TiO2. The principal advantage ofZnO is that it absorbs over a larger fraction of solar spectrumthan TiO2 [124].

Many efforts have been made to develop ZnO-based visi-ble light photocatalysts, particularly, combination with GaN[125] or Co doping [126] into ZnO, to extend the absorptionspectrum of ZnO to the visible region. Efficient ZnO-basedvisible light photocatalysts, Cu(II)-modified CdxZn1−xO,were developed by adopting a hybrid approach, consistingof band-engineering by formation of a solid solution andmultielectron reduction by the modification of cocatalystsby Anandan et al. [116]. Due to the similar physical char-acteristics of Cd2+ and Zn2+, a substitution of Zn2+ by Cd2+

results in the visible light absorption without the formationof defects. CdxZn1−xO showed high photocatalytic activitiesunder visible light illumination as compared to pure ZnO forthe decomposition of gaseous acetaldehyde. Further, Cu2+

modification ensures the enhancement in the visible lightactivity of CdxZn1−xO by capturing the photoexcited elec-trons [116]. A novel approach to the oxidative degradation oftoluene, salicylic acid, and 4-chlorophenol using a speciallydesigned photoelectrochemical detoxification reactor withZnO electrodes under solar radiation has been proposed byShinde et al. [127]. Pardeshi and Patil [128] had investi-gated the photocatalytic degradation of resorcinol, a potentendocrine disrupting chemical, in aqueous medium usingZnO under sunlight irradiation in a batch photoreactor.Resorcinol solutions of lower concentration were completelymineralized by photocatalytic degradation on the surface ofZnO under irradiation of sunlight, and the degradation wasfound to be favorable in neutral and basic solutions.

The ZnO/Cu2O compound photocatalysts were pre-pared by Xu et al. [129] using “soak-deoxidize-air oxida-tion” method. The prepared ZnO/Cu2O compound showedabsorption in the visible light region, between 400 and610 nm. All the ZnO/Cu2O compounds have higher photo-catalytic activity than pure ZnO, and some compounds arebetter than pure Cu2O. The degradation of MO reached 73%after 180 min of reaction when the ZnO/Cu2O compound


(the mole ratio of Cu2O to ZnO was 0.138) was used asphotocatalyst. Kanade et al. [5] reported a new approach forthe synthesis of lattice-doped copper in the nanocrystallinewurtzite ZnO in different solvents, and their use as aphotocatalyst for the decomposition of H2S to generatehydrogen. The average particle size was found to be in therange of 40–85 nm. The maximum hydrogen productionrate achieved was 1932 μmol h−1 over the as synthesized Cudoped ZnO by H2S decomposition under visible light irra-diation. There, Cu doped ZnO acts as a photocathode, andadsorption of light promoted electrons in the conductanceband (CB) of semiconductor where the potential (−0.88 V)was sufficient to liberate hydrogen. At the same time, holesin the valence band (VB) moved into the bulk to facilitatethe oxidation process, where they were transferred to thereduced species S2−, which acts as a hole-scavenger andprevented the photocorrosion of the photocatalyst. In theirstudy, it was observed that the visible light photocatalyticactivity of Cu-ZnO synthesized in organic media was higherwhen compared to that synthesized in water medium dueto well-crystallined self-aligned particles. Chen et al. [130]modified ZnO photocatalysts with thiourea to enhance theirphotocatalytic activity by nonmetal dopants under visiblelight illumination. The photocatalytic activity of modifiedZnO had also been examined for the degradation of AO7and phenol under UV and visible light illumination. N-, S-,and C-doped ZnO (N,S,C–ZnO) particles were synthesizedfrom thiourea and zinc sulfate via precipitation method.The visible light activity of the N,S,C–ZnO samples wasconfirmed by degradation of phenol. The optimal molarratios of thiourea to zinc sulfate were 1 and 2 for thehighest photocatalytic activities under UV and visible lightillumination, respectively. The 500◦C-calcined N, S and C–ZnO showed strong photoabsorbance in the visible lightregion.

Pardeshi et al. [131] had synthesized ZnO crystallitesby two-steps solution-free mechanochemical method for thecomplete mineralization of resorcinol solutions of 100 ppmconcentration by photocatalytic degradation under sunlightirradiation. Li and Haneda [132] reported the synthesize oftwo series of N-containing MOx–ZnO composite powders(WO3–ZnO and V2O5–ZnO) by spray pyrolysis. Acetalde-hyde photodecomposition was used as a probe reaction toevaluate their photocatalytic activity. The MOx additionenhanced the photocatalytic activity of the N-containingZnO powder under both UV and visible light irradiation.This enhancement under visible light is due to a synergisticeffect of N-doping and MOx–ZnO coupling. The N dopinginduced the formation of an electronic impurity level inthe band structure of ZnO; therefore, the electron transitionfrom valence band to conduction band in a ZnO semicon-ductor could be achieved through two-step transitions evenwith the lower energy of visible light. ZnO has emerged tobe a more efficient catalyst concerning water detoxificationin an efficient way. Also it has more number of surfaceactive sites with high surface reactivity. Due to its low-price, very simple synthesis process, biocompatibility nature,high-stability, high-activity towards photo-induced redoxreactions, various applications in photonics and electronics,

photocatalytic reduction, the eradication of environmentalpollutants, it is predicted that ZnO as well as modified ZnOcould be one of the best photo-catalysts for the degradationof organic dyes and other pollutants.

3.3. Tantalum Oxide, Tantalum Nitride, and Tantalum Oxyni-tride. As a typical semiconductor, tantalum oxide (Ta2O5)has attracted increasing interests due to its superior proper-ties, such as good chemical resistance, high melting point,and photocatalytic activity under UV irradiation [133].Recently, it is identified that the tantalum oxynitride andtantalum nitride are potential photocatalysts that functionunder visible light irradiation [134]. Tantalum (V) nitride,with an optical band gap of 2.08 eV [135], is found tobe an efficient photocatalyst in the visible region of theelectromagnetic spectrum, with a quantum efficiency of ca.10% for overall water splitting and showed good hydrolyticstability [134]. Tantalum nitride with an anosovite struc-ture [136] has been studied as a 600 nm light-absorbingphotocatalyst [134, 137] and as a photoanode for watersplitting reactions [138]. The conduction band of Ta3N5

is composed of vacant Ta 5d states, and the valence bandconsists of occupied N 2p orbitals. The conduction bands oftantalates consisting of a tantalum (Ta 5d) orbital located atmore negative potential than titanates (Ti 3d) and niobates(Nb 4d) [139], Maeda et al. [140] attempted to increase thephotocatalytic activity of tantalum oxynitride (TaON) for H2

evolution under visible light by reducing the particle size,since smaller particle size usually results in a shorter diffusionlength for photogenerated electron-hole pairs in a givenphotocatalyst. Tantalum oxynitride (TaON) and tantalumnitride (Ta3N5) are investigated in detail as visible lightdriven photocatalyst for overall splitting of water. The Ta3N5

is found to be in red (Eg = 2 : 08 eV), whereas TaON isin yellow colour (2.5 eV) [141]. It was found that TaONand Ta3N5 evolved H2 or O2, respectively, in the presenceof a sacrificial electron donor or acceptor, via band gapexcitation. The band gap energies of Ta2O5, TaON, andTa3N5 are to be 3.9, 2.5 and 2.1 eV, respectively. This bandgap narrowing occurs by substituting nitrogen for oxygen inthese oxides since nitrogen has a lower electronegativity thanoxygen and the presence of O2− in the anionic frameworkwill increase the band gap. As the electronic potential ofthe N 2p orbitals is higher than that of O 2p, it is expectedthat the N 2p orbitals will dominate the occupancy of thetop of the valence band, leading to band-gap narrowing[135]. Thus the electronic structural changes in the valence-band states may occur as a result of the incorporationof nitrogen. Density functional theory (DFT) calculationsshowed that the upper part of the valence band is dominatedby N 2p orbitals on account of the higher potential energyof the N 2p orbital compared to the O 2p orbital [135].Luan et al. [142] synthesized new pyrochlore-type structurecompounds Bi2GaTaO7 and Ga2BiTaO7 by solid-state (SS)reaction methods and evaluated photocatalytic properties forthe degradation of MB dye under ultraviolet and visible lightirradiation. A systematic study of the structural, morpholog-ical, optical, and visible light photocatalytic properties of Ta-doped ZnO and pure ZnO samples has been carried out by


Kong et al. [143]. The Ta doping also changes the mor-phology, surface adsorption, specific surface area, crystallinesize and increases the lattice constants and band gap of thenanocrystals. The photocatalytic efficiency of the Ta-dopedZnONPs is much more excellent than the pure ZnO andis attributed to the increase of the active hydrogen-relateddefect sites caused by Ta5+ doping, leading to the enhancedspecific surface area and optical absorption in the visible lightregion [143]. Zhang et al. [144] successfully synthesized anovel, nanosized, Bi3–TaO7 photocatalyst which showed astrong optical absorption in the visible light region (k >400 nm) and a high adsorption ability for the 4 BS dye.They had evaluated the photocatalytic properties of theBi3TaO7 nanopowders, by the degradation of 4 BS aqueoussolution under visible light irradiation. It has also been foundthat niobium and tantalum ions are efficient codopants tomaintain the charge balance when titanate photocatalystsare modified by doping with the low valence ions such asCr3+ [145]. Because of all the above mentioned incentives,in recent years, Ta-based compounds will get more exposureto the field of photocatalysis.

3.4. Fe2O3-Based Photocatalysts. Fe2O3 exists widely innature and is an environmentally friendly n-type semi-conductor (Eg = 2.1 eV). It is an important functionalmaterial because of its applications (e.g., magnetic andcatalytic properties, chemical stability, biocompatibility, lowtoxicity), and so forth. It is widely used in catalysts, pigments,sensors, photoelectrodes for solar energy conversion, clinicaltherapy and diagnosis and as a raw material for synthesizingother compounds [146–149]. Most of the iron oxideshave been revealed as photochemically active [150], andits incorporation with TiO2 not only efficiently inhibitsthe recombination between the photo-generated electronsand holes, but also enhances the absorption of solar light[19, 151–154]. In recent years, it has been found that thecomposites of TiO2 and Fe2O3 can effectively respond tovisible light [88, 155] due to the narrow band-gap of Fe2O3.It has also been reported that when Fe3O4, reacted withTiO2 powders and produced Fe containing TiO2 compounds,such as FeTiO3, the electrons in the valence bands of FeTiO3

are excited to the conduction band of FeTiO3 first, andthen will get promoted to the conduction band of TiO2.This leads to an improved photocatalytic activity due to thedecrease of electron recombination [156]. Iron, as a dopantin TiO2-based systems, has been investigated to enhance thephotocatalytic efficiency under visible light irradiation tocounter the potential negative environmental problems ofusing heavy metal sensitizers [157–161].

Transparent α-Fe2O3 films were synthesized [162] withvarying thickness of film on an SnO2 transparent conductingfilm-coated glass substrate by metal organic deposition. Itshowed high photocatalytic activity for the decompositionof 2-naphthol with visible light irradiation under anodic-biased conditions. The α-Fe2O3 was transformed to inactiveFe(OH)3 as the reaction proceeded, while the activity wasalways maintained in acetonitrile. Methylene blue was cho-sen as a model pollutant for testing the photocatalytic activityof the p-n junction photocatalyst p-CaFe2O4/n-Ag3VO4

prepared by ball milling Ag3VO4 in H2O doped with p-type CaFe2O4 [163]. The visible light irradiation resultsshowed that the photocatalytic activity of the p-CaFe2O4/n-Ag3VO4 was higher than that of Ag3VO4. The 2.0 wt.%p-CaFe2O4 doped p-CaFe2O4/n-Ag3VO4, which was ballmilled for 12 h, showed 85.4% photocatalytic degradation.Since CaFe2O4 is a p-type semiconductor, and Ag3VO4,an n-type semiconductor, the photogenerated electrons andholes separation is efficient, to show better photocatalyticactivity. Highly ordered ZnFe2O4 nanotube arrays weresuccessfully prepared by anodic Al2O3 templates from sol-gel solution, and its photocatalytic capability under visiblelight irradiation was evaluated [164], using 4-chlorophenolas the model contaminant, which constitutes an importantclass of soil and water pollutants arising from their wide useas pesticides, herbicides, and wood preservatives.

Fe2O3–TiO2 composite photocatalyst was synthesized byan ethanol-assisted hydrothermal method, from Fe2(SO4)3

and Ti(SO4)2 [165]. The presence of α-Fe2O3 and anataseTiO2 in the composites was confirmed from the results.This sample led to a photodegradation efficiency of 90%and 40% of auramine, under visible light and solar light,respectively, and it was significantly higher than that ofpure TiO2. The optimal photocatalyst contains 1 : 2 formolar ratio of Ti to Fe, and it exhibited the highestphotocatalytic degradation efficiency of auramine. Withfurther increase in the ratio to 1 : 4, the degradation efficiencyon the organic dye decreased slightly. The Fe2O3–TiO2

synthesized under optimum condition (2 h reaction time,160◦C reaction temperature, 20% for the volume fraction ofethanol) consisted of mesoporous structure with an averagepore size of 4 nm and a surface area of 43 m2/g. SulfatedFe2O3–TiO2 (SFT) was synthesized by Smith et al. [166],and photocatalytic activity was evaluated by the oxidationof 4-chlorophenol (4-CP) in aqueous medium under UVand visible light irradiation. The SFT calcined at 500◦Cdemonstrated the highest photocatalytic activity, and itpossessed a band gap value of 2.73 eV. Despite of the lowsurface area of the SFT samples, (12–17 m2/g) comparedwith the surface area of sulfated TiO2 (275 m2/g), the betterphotocatalytic activity was due to the presence of iron. Theseobservations revealed the significance of the presence ofiron in TiO2 for photocatalytic activity. So in view of theefficient utilization of solar light, the α-Fe2O3 (hematite),with narrow band gap had been explored as photocatalysts,and it has been investigated for its photochemical behavioras a semiconductor electrode.

3.5. A Glance to the Photoactivity of Other Metal Oxides.Tremendous efforts have been dedicated to synthesize mon-oclinic BiVO4 (m-BiVO4) owing to its fascinating structure-related properties after the first report of the preparationof bismuth vanadate crystals in 1925 [167]. The key factorsin determining the photocatalytic activity of m-BiVO4 areits optical absorption properties, which are related to itselectronic structure [168, 169]. The crystal structure of m-BiVO4, with its calculated band gap of 2.2 eV, makes it apromising, nontoxic, visible light responsive photocatalyst[170], for the degradation of harmful pollutants and as a


thermochromic material for indicating temperature [170–172]. BiVO4 is found to be one of the photocatalysts thatexhibits high activity for photocatalytic O2 evolution undervisible light irradiation [173, 174]. Due to the low band gap(2.4–2.5 eV) [175] and reasonable band edge alignment withrespect to the water redox potentials, it has shown particularpromise for water photodecomposition. In addition to highphoton-to-current conversion efficiencies (>40%) [175], thesystem shows both n- and p-type semiconducting properties[176]. It has been reported that the BiVO4 is found tobe a direct band gap semiconductor, despite having bandextrema away from the Brillouin zone center [177]. Thedirect gap is maintained via coupling between V 3d, O 2p,and Bi 6p, which lowered the conduction band minimum.DFT calculations suggested that the hybridization of Bi 6sand O 2p levels resulted in the valence band which favorsthe mobility of photo-excited holes and, thus, enhanced thephotocatalytic oxidation of organic pollutants [168].

SrTiO3 is one of the best host materials for the designof visible light driven photocatalysts, possessing the H2

production ability, by transition metal ion doping. Kudoand Hijii [178] and Tang et al. [179] studied the activity ofa potential, visible light responsive photocatalyst, Bi2WO6,the simplest member in the Aurivillius family, for the firsttime. Their study revealed that Bi2WO6 could perform as anexcellent photocatalyst and solar energy transfer material. Itwas demonstrated that the visible light photocatalytic activityof Bi2WO6, which has a novel octahedron-like hierarchicalstructure, in the degradation of Rhodamine B (RhB), wasbetter when compared to that of the Bi2WO6 synthesizedby solid-state reaction (SSR) [180]. When the pH valueof Bi2WO6/RhB suspension was 7.5, the photodegradationwas apparently enhanced and 95% of the RhB could bedegraded after 6 h, which had shown the pH dependence ofthe prepared catalyst. Recently, a novel series of InMO4 (M= Nb, Ta, V) catalysts was reported to show high activity forwater splitting reaction under visible light [181, 182].

Konta et al. [183] reported that Ag3VO4 showed acompetent activity for the evolution of O2 from an aqueoussilver nitrate solution under visible light irradiation. Sincethen silver vanadate (Ag3VO4) material had attracted muchattention [184–187]. It has been reported that WO3 [188,189], RbPb2Nb3O10 [188], BiVO4 [188, 189], Bi2WO6 [189],chromium/antimony-doped TiO2 [190], AgNbO3 [191], andAg3VO4 [183] are visible light driven photocatalysts forthe evolution of O2 from water, containing a sacrificialreagent. Pt/HPb2Nb3O10 [188] and chromium/antimony-doped SrTiO3 [190] were also found to be active metaloxides. Recently, a simple pristine metal oxide tungsten oxide(WO3) is reported as a visible light driven photocatalystbecause of its small band-gap (2.8 eV) and a deeper VB(+3.1 V versus SHE, pH 0). Though the WO3 is a goodcandidate for visible light active photocatalysts, tungsten isa rare metal with high cost, and WO3 is chemically unstableespecially in alkaline solution. The visible light activity ofWO3 was greatly enhanced via efficient oxygen reductionprocess by the addition of cocatalysts, such as Pt [192],Cu2+ [193], Pd [194], or Tungsten Carbide (WC) [195],where the cocatalysts acted as electron pools to initiate

multielectron reductions. The quantum yield obtained forthe decomposition of gaseous iso-propanol using WO3 was17% [193], which is much higher than that obtained withnitrogen-doped TiO2 [51]. Thus, a metal oxide with lowconduction band, like WO3, can be used as a promisingcandidate for visible light active photocatalysts, by modifyingits surface with co-catalysts like Cu2+ ions that can help inmultielectron reduction. Table 1 also discusses a number ofimportant visible light photocatalysts.

4. Different Means for the Reduction ofBand Gaps

The mixing effect of band gaps of the composite semiconduc-tor leads to large red shift. Cheng et al. [196] stated that whena relatively broad band gap anatase TiO2 was mixed witha narrow band gap zinc ferrite, the band gap of compositesemiconductor would locate between the band gaps of thesetwo semiconductors, that is, it shifts to a lower energy, ascompared with that of TiO2. Due to the interfacial couplingeffect, the zinc ferrite could induce lattice defects on thesurface of TiO6 octahedra, which will serve as the centres ofbound excitons, which is another factor that resulted in largered shift [197].

Hur et al. [198] reported that the incorporation of Pbor Sn ions led to a distinct depression of the Eg valueof ternary metal oxide Sr(In1/2Nb1/2)O3, which is a UV-active semiconductor with a wide bandgap separation of3.6 eV to 3.1 eV for Pb and 3.5 eV for Sn. A similar trendin the variation of bandgap energy was also observed forthe Ba-based compounds upon B-site cation substitution inA(In1/3Nb1/3B1/3)O3 with Pb and Sn. The band gap energy(3.30 eV) of Ba(In1/2Nb1/2)O3 showed a depression of 3and 1.48, respectively, for Sn and Pb. The lead-substitutedA(In1/3Nb1/3Pb1/3)O3 compounds resulted in more effec-tive decomposition of MB, under UV-vis (λ > 300 nm)radiations compared to the pristine A(In1/2Nb1/2)O3. TheBa(In1/3Nb1/3Pb1/3)O3 compound induced an effectivedegradation of 4-chloro phenol under visible light radi-ation (λ > 420 nm), whereas there was no significantchange in 4-chloro phenol concentration when exposed toSr(In1/3Nb1/3Pb1/3)O3. A series of InVO4 and NiO/InVO4

catalysts were prepared by Lin et al. [199] with a lower bandgap of 1.8 eV for InVO4 which is consistent with the resultreported by Ye et al. [181], where they had obtained InVO4

band gap as 1.9 eV. The small band gap of these catalystsindicates its ability to split water into H2 and O2 in visiblelight. Loading of NiO significantly increases the adsorptionof visible light and the 1% NiO loaded InVO4 showeda highest absorbance in visible light region and exhibitedhighest photocatalytic activity for water splitting. kudo et al.[200] developed an efficient visible light driven photocatalystZn1−xCuxS for the evolution of H2 from an aqueous Na2SO3

solution. They had succeeded to get a quantum yield of 3.7%at 420 nm, without a co-catalyst such as Pt. ZnS photocat-alysts, which possess a band gap of 3.7 eV (α phase: 3.8 eV,β phase: 3.6 eV), have an absorption band only in the UVregion, whereas the absorption edge of the Zn0.957Cu0.043Ssolid solution reached to the visible light region around


Table 1: Some visible light responsive photocatalysts.

Catalyst Photocatalytic reaction Reference

Zn1−x CuxS solidsolution

The H2 evolution reaction from an aqueous solution of Na2SO3 were performed and gota quantum yield of 3.7% at 420 nm.

[200]

zinc ferrite dopedTiO2 (ZFDT)

ZFDT powders effectively photodegraded methyl orange under visible light irradiationand achieved maximum photoactivity when the amount of zinc ferrite was 1.5%.

[196]

zeolite-basedcomposite

Showed high efficiency when compared to TiO2 photocatalyst for photoreduction of waterto hydrogen in the visible light range with the combination of Co2+, TiO2 and heteropolyacid.

[222]

N-doped TiO2The colloidal TiO2 nanoparticles treated with triethylamine by a batch system was foundto show particularly efficient photocatalytic activity under visible illumination.

[223]

m-BiVO4 quantumtubes

Photodegradation of RhB reached 98.7% after 15 min irradiation [224]

N-TiO2 The photocatalytic degradation of trichloroethylene was well-reproduced several times. [225]

CoTa2O6H2 is produced from aqueous CH3OH solution using Pt-loaded powder samples and O2

from aqueous AgNO3 solution under visible light irradiation.[226]

InVO4 andNiO/InVO4

The 1% NiO/InVO4 catalyst which was reduced at 500◦C and oxidized at ambientcondition for 48 h gave the highest activity with a rate of 896 μmol h−1 g−1.

[199]

BiVO4Showed good O2 evolution activity over BiVO4 nano-leaves by photocatalytic watersplitting under visible light irradiation

[227]

500 nm and the energy gap of the Zn0.957Cu0.043S solidsolution was estimated to be 2.5 eV. Because of the improperposition of conduction band edge (−0.45 V), WO3 did notshow photocatalytic activity for water splitting. Hwang et al.[201] reported that doping of 5–10% Mg in WO3 showedimproved photocatalytic activity for water splitting, wherethe conduction band level was shifted leading to a bandgap of −2.7 V. ZnFe2O4 is another catalyst with a relativelysmall band gap (1.9 eV), which was used as an effectivephotocatalyst due to its capability of utilizing visible lightand good photochemical stability [202, 203]. So the band gapnarrowing is an efficient method to shift the absorption edgeof a photocatalyst to visible light region of the spectrum.

5. Effect of Preparation Method on the Activityof Visible Light Driven Photocatalysts

Numerous efforts have been postulated for the fabricationand application of efficient semiconductor manufacture.Photoactivity is highly dependent on surface area, crys-tallinity, or crystal sizes, which in turn is influenced bythe synthetic method [204]. As far as such methods areconcerned, hydrothermal, sol-gel, impregnation, chemicalvapour deposition, and coprecipitation are most widely used.

5.1. Sol-Gel Method. Sol-gel technology is a low-temperaturemethod of preparing inorganic materials by chemical routes.Many researchers had paid attention in synthesizing newphotocatalysts. Among the various methods, sol-gel methodhas attracted most attention because of low process cost,easy control of composition, and relatively low calcinationtemperatures. Here we evaluate some promising reports ofvisible active photocatalysts prepared via sol-gel method.

Jung et al. [205] have synthesized Al2O3-TiO2 solidsolution by sol-gel method via adjusting the pH and it is used

for the decomposition of acetaldehyde under visible light.Here, anatase phase was maintained even after calcinationat 800◦C, and the BET surface area for mixed oxides(116.8 m2/g) was much higher than either that of pureTiO2 or Al2O3. They attained a maximum conversion ofacetaldehyde for Ti (70%)–Al (30%) mixed oxide, that is,79% for 2 h reaction. Parida et al. [206] prepared, sulfate-modified TiO2 by sol-gel method, which could reduce thecrystallite size and increase the specific surface area of thecatalysts. They investigated the effects of different parameterssuch as pH of the solution, amount of catalyst, additives, andkinetics. At 2.5 wt% sulfate loading, the average percentage ofdegradation of methyl orange was nearly two times than thatof neat TiO2. This material showed photocatalytic activity of61% of MO degradation under solar radiation against 12%of adsorption in the dark. The effect of vanadium ions onthe photocatalytic activity of TiO2 nanocrystals, prepared bysol-gel method was investigated using MB by Doong et al.[207]. BET-BJH analysis clearly showed that the fabricatedV-doped TiO2 was mesoporous, with high specific surfacearea. The specific surface areas of V-doped mesoporousTiO2 nanoparticles increased from 88 ± 2 to 93 ± 19 m2/gwhen V/Ti ratios increased from 0.5 to 1.0 wt%. A furtherincrease in the V/Ti ratio to 2.0 wt% reduced the specificsurface area of V-doped TiO2 slightly to 86 ± 9 m2/g. Itwas observed that anatase is the main crystalline phase ofV-doped mesoporous TiO2, at a calcination temperature of500◦C. The photodegradation efficiency of MB by V/TiO2

was increased under visible light and was consistent withsurface area analysis [208].

5.2. Hydrothermal Method. Hydrothermal method is oftenthe method of choice because of its various advantages likecost-effectiveness, low energy consumption, mild reactioncondition, and simple equipment requirements. The particle


properties such as morphology and size can be controlledvia the hydrothermal process by adjusting the reactiontemperature, time, and concentration of precursors. It isa low-temperature technology thus can save energy and isenvironmentally benign, because the reactions take placein closed-system conditions using water as the reactionmedium. This method is very versatile for the synthesis ofnanophase materials and has been well established.

Shen et al. [209] presented Cetyltrimethylam-moni-umbromide-(CTAB-) assisted hydrothermally synthesizedZnIn2S4 as an efficient visible light driven photocatalyst forhydrogen production. They studied the effects of hydrother-mal treatment time and influence of surfactant, CTAB, onthe crystal structures, morphologies, and optical propertiesof ZnIn2S4 and the activity was evaluated by photocatalytichydrogen production from water under visible light irra-diation. It was found that the photocatalytic activities ofthese ZnIn2S4 products decreased with the hydrothermaltreatment time period while increased with the amountof CTAB. C-doped Zn3(OH)2V2O7 has been prepared byhydrothermal method, in the presence of polyethyleneglycol and ethylenediamine tetracetic acid, and used forthe degradation of MB. The catalyst exhibited high photo-catalytic performance with 90.3% MB decolorization after30 min, much better than the ZnO photocatalyst (34.6%),and the TOC removal rate of the solution reached 71.9%within 30 min [210]. Zn-doped CdSe prepared by a simplehydrothermal method exhibited high visible light drivenactivity [211]. Many researchers studied the effect of Zn inthe visible light driven photoactivity of catalysts preparedby hydrothermal method. Photocatalytic decomposition ofacetaldehyde was achieved by iron hydroxide (FeOOH)particles, prepared by a hydrothermal method, under visiblelight [212]. Wu et al. studied the effect of the hydrothermaltreatment temperature on the preparation of visible lightactive TiO2 photocatalyst. The photocatalyst, treated at160◦C achieved much more absorption in the visible lightcompared to N-TiO2 prepared at 120◦C and 140◦C [213].Jing et al. [214] prepared decahedral Cu2WS4, by a facilehydrothermal method and employed it as a photocatalystfor photocatalytic hydrogen production for the first time.The hydrothermal method avoided the traditional use of H2Sfor the preparation of such chalcogenide, which guaranteesan environmental friendly process. Cu2WS4 synthesized at200◦C for 72 h showed the highest activity with the apparentquantum yield of 11% at 425 nm. Qi Xiao et al. [215]prepared a novel (C, S, Sm)-tridoped TiO2 photocatalystby one-step hydrothermal method, and the photocatalyticdegradation of potassium ethyl xanthate (KEX) was carriedout under visible light irradiation. It was found that the orderof photocatalytic activity of C–S–xSm–TiO2 (x representsthe mol% of Sm) samples was dependant on the mol%of Sm and followed the order 1.0 > 0.5 > 1.5 > 0 mol%.Ho et al. [216] showed that the S-doped TiO2 prepared bythis hydrothermal approach resulted on photodegradationof 4-chlorophenol under visible light irradiation, with muchhigher photocatalytic activity (86% degradation in 6 h) thanthat obtained by the traditional high temperature (66% ofthe 4-chlorophenol in 6 h) thermal annealing method. The

photocatalytic activity of Bi2WO6 powder prepared by asimple hydrothermal method at 150◦C for 24 h was evaluated[217] using the photocatalytic oxidation of formaldehyde atroom temperature, under visible light irradiation. At 500◦C,Bi2WO6 powder photocatalyst showed the highest visiblelight photocatalytic activity due to the good crystallizationof the samples.

5.3. Chemical Vapour Deposition. Metal organic chemicalvapour deposition (MOCVD) is an interesting and promis-ing method to prepare supported photocatalytic TiO2 films.It is an industrial process applicable to large area deposition.High-quality TiO2 films can be easily and cheaply anchoredon various substrates, even bearing the complicated shapes.This method eliminated steps such as saturation, aging, dry-ing, and reduction of any subtle change which critically affectthe performance of the catalyst in traditional methods forsupported catalysts preparation. This technique has superioradhesion, durability, and uniformity than the correspondingphysical vapour deposition (PVD) counterpart [218]. Yoshi-naga et al. [219] have proposed a novel technique, chemical-vapor-reductive deposition, for the deposition of metallicnickel nanoparticle onto TiO2 thin film, which was used forphotocatalytic hydrogen evolution by the decomposition ofethanol under visible light. For the effective immobilizationof TiO2 particles, and to improve the catalytic activity undervisible light, Zhang and Lei [153] had prepared Fe2O3–TiO2 coatings supported on activated carbon fiber (ACF), inone step by MOCVD. A maximum catalytic efficiency wasreached when the loading of TiO2 was 13.7 wt%, and it couldbe reused without decrease in the catalytic activity. Fe–TiO2

coatings, supported on activated carbon were developedthrough a codeposition process of MOCVD method in onestep, and were used for the effective degradation of methylorange under visible light by Zhang et al. [86]. Akhavanet al. [220] synthesized TiO2/multiwall carbon nanotube(MWNT) heterojunction arrays and immobilized on Si forphotoinactivation of E. coli in visible light irradiation. Thevertically aligned MWNT arrays were grown on ∼5 nm Nithin film deposited over Si by using plasma-enhanced chem-ical vapor deposition at 650◦C. The MWNTs were coated byTiO2 using dip-coating sol-gel method. The order of visiblelight-induced photoinactivation of the bacteria was MWNTs< TiO2 < TiO2/MWNTs. Kafizas et al. [221] describedthe combinatorial atmospheric pressure chemical vapourdeposition (cAPCVD) synthesis of anatase TiO2 thin-filmwith gradating nitrogen dopant. A single film with significantvariations in thickness, phase, and composition has beenachieved by this method. Transition from predominantlyNs-doped, to Ns/Ni mixtures, to purely Ni-doped TiO2 wasobserved by X-ray photoelectron spectroscopy analysis, andthe photocatalytic activity was confirmed using stearic acid.

In addition to the advanced methods described, theconventional methods of impregnation, coprecipitation, andso forth was also widely engaged in photocatalyst prepara-tion. The supplementary advantage of specific preparationmethods is the development of nanosized metal oxides withuniform pore structure and well defined crystal structurewhich has pronounced influence in its visible light activity.


6. Conclusions

The use of visible light responsive photocatalysts are foundto be a suitable method for degrading harmful organicpollutants and also it can provide a way for the produc-tion of energy, and can thus, help to solve many urgentenvironmental issues faced by mankind today. TiO2 is themost widely studied photocatalyst where visible light activityis achieved mainly by nonmetal doping. A combination ofmetal as well as nonmetal incorporation prevents electronhole-pair recombination in addition to the extended visiblelight response. Among the other suitable visible light activephotocatalysts, ZnO based systems are found to be abetter choice than that of modified TiO2 photocatalysts,owing to its low-price, very simple synthesis process, bio-compatibility nature, high stability, high activity towardsphoto-induced redox reactions and photocatalytic reducingpower for the eradication of environmental pollutants.Photoactivity depends on the synthetic method adapted.Among the various methods, sol gel, hydrothermal andchemical vapour deposition had resulted in highly crystallineand porous catalysts with improved surface area and betteractivity.

Acknowledgment

The authors acknowledge the UKM, Grant no. UKM-RF-06-FRGS010-2010, for providing assistance.

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Research Article

Highly Active Rare-Earth-Metal La-Doped Photocatalysts:Fabrication, Characterization, and Their Photocatalytic Activity

S. Anandan,1, 2 Y. Ikuma,2 and V. Murugesan1

1 Department of Chemistry, Anna University, Chennai 600 025, India2 Department of Applied Chemistry, Kanagawa Institute of Technology, 1030 Shimoogino, Atsugi, Kanagawa 243-0292, Japan

Correspondence should be addressed to S. Anandan, [email protected] and Y. Ikuma, [email protected]

Received 15 July 2011; Revised 23 September 2011; Accepted 24 September 2011


Copyright © 2012 S. Anandan et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Efficient La-doped TiO2 photocatalysts were prepared by sol-gel method and extensively characterized by various sophisticatedtechniques. The photocatalytic activity of La-doped TiO2 was evaluated for the degradation of monocrotophos (MCPs) in aqueoussolution. It showed higher rate of degradation than pure TiO2 for the light of wavelength of 254 nm and 365 nm. The rate constantof TiO2 increases with increasing La loading and exhibits maximum rate for 1% La loading. The photocatalytic activities of La-doped TiO2 are compared with La-doped ZnO; the reaction rate of the former is ∼1.8 and 1.1 orders higher than the latter for thelights of wavelength 254 nm and 365 nm, respectively. The relative photonic efficiency of La-doped TiO2 is relatively higher thanLa-doped ZnO and commercial photocatalysts. Overall, La-doped TiO2 is the most active photocatalyst and shows high relativephotonic efficiencies and high photocatalytic activity for the degradation of MCP. The enhanced photocatalytic activity of La-doped TiO2 is mainly due to the electron trapping by lanthanum metal ions, small particle size, large surface area, and high surfaceroughness of the photocatalysts.

1. Introduction

The remarkable progress of scientific technologies in recentyears has made extremely high demands on semicon-ductor material. Photocatalysis using semiconductors hasbeen extensively performed worldwide to find solutions toenergy and environmental problems, since the discovery of“Honda-Fujishima Effect” three decades ago. Among thesemiconductors, outstanding stability and oxidative powermake TiO2 the best semiconductor photocatalyst for envi-ronmental remediation and energy conversion processes [1–3]. However, the application of TiO2 is yet limited by the fastrecombination of electron-hole pairs and their wide bandgap, which corresponds with UV light [4]. Therefore, thestudies of modifying TiO2 to reduce the electron-hole recom-bination and sensitization towards visible light have beenextensively investigated. Metal ion doping has been widelyperformed on semiconductors to minimize electron/holerecombination and enhance their absorption towards visiblelight region [5–8]. For example, Choi et al. found thatTiO2 doping with Fe3+, Mo5+, Ru3+, Os3+, Re5+, V4+, and

Rh3+ increased photocatalytic activity in the liquid-phasephotodegradation of CHCl3. However, (transition metal-)doped TiO2 suffers from a thermal instability or an increasein the carrier recombination centers [9]. Alternately, rareearth-doped metal oxides are potentially attractive materialsfor various optical and electronic applications because suchmaterials exhibit unique physical and chemical propertiessuch as fluorescence [10–12], persistent spectral hole burning[13–15], and ion conductivity [16]. Moreover, rare earth ionshave radii larger than Ti4+, so they are mainly distributed onthe surface when deposited onto TiO2, that keep large surfaceareas of TiO2 when treated at high temperatures. Atribaket al. [17] studied the catalytic oxidation of soot using La-modified TiO2 and concluded that TiO2 doped with 0.2% Laexhibits best photocatalytic activity. In our previous studies,we developed La-doped ZnO photocatalysts and successfullyapplied for the degradation of organic pollutants in aqueoussolution [18]. In the present study, we mainly focus on thesynthesis of La-doped TiO2, since the photocatalytic activityof TiO2 was greater than ZnO. Moreover, the photocatalyticdegradation of MCP using TiO2, ZnO, La-doped ZnO, and


some of the commercial photocatalysts, carried out. Finallythe photocatalytic efficiency of La-doped TiO2, comparedwith above-mentioned photocatalysts. Herein we report thesynthesis, characterization, and application of La-dopedTiO2 particles with series of lanthanum loading. La-dopedTiO2 prepared by sol-gel method and extensively character-ized using various sophisticated techniques. Photocatalyticactivity of TiO2, La-doped TiO2, and other photocatalystshave evaluated by the degradation of MCP in aqueoussolution. Photocatalytic decomposition of MCP is of greatsignificance from the viewpoint of practical applicationsbecause MCP is one of the organophosphorous insecticideswhich are widely used in agriculture and animal husbandry.Also, MCP has been identified as endocrine disruptingchemicals (EDCs) which causes serious adverse effects onhumans. The most serious among them are prematureageing, congenital abnormalities, and impotence [19]. Ithas been observed from the experimental results that La-doped TiO2 showed an excellent photocatalytic activity andrelative photonic efficiency compared to other photocata-lysts. Electron trapping by lanthanum metal ions, smallerparticle size, large surface area, high porosity, and increasein surface roughness may be the reasons for the enhancedphotocatalytic activity.

2. Experimental

2.1. Materials Preparation and Characterization. La-dopedTiO2 was prepared by sol-gel method using titanium tet-raisopropoxide (Analytical grade, Merck Ltd., India) andlanthanum nitrate hexahydrate (La (NO3)3·6 H2O) (Analyt-ical grade, CDH, India) as titanium and lanthanum sources,respectively. The technical grade sample of monocrotophos(MCP) was received from Sree Ramcides Chemicals, India.HPLC grade acetonitrile was purchased from Merck, India.The lanthanum-doped TiO2 samples were prepared by sol-gel method. Required amount of Ti (O-Bu)4 was dissolvedin absolute ethanol and the solution mixed vigorously ina solution containing appropriate amounts of water, aceticacid, and ethanol. Then lanthanum nitrate solution wasadded to the above sol to form a colloidal suspension. Theresultant colloidal suspension was stirred and aged to forma gel. The gel was dried in vacuum and then ground. Theresulting powder was calcined at 500◦C and depending onthe concentration of lanthanum nitrate; the photocatalystswere expressed in terms of weight percentage. The crys-tallinity of pure TiO2 and La-doped TiO2 photocatalysts wasanalyzed by X-ray powder diffraction (Model: PANalytical)with Cu-Kα radiation in the scanning range of 2θ between10 to 80◦. The accelerating voltage and applied current were40 kV and 40 mA, respectively. Data were recorded at a scanrate of 0.02◦ s−1 in the 2θ range of 10◦ to 80◦. The sizeof the crystallite was calculated from X-ray line broadeningfrom the Scherrer equation: D = 0.89λ/β cos θ, where Dis the average crystal size in nm, λ is the Cu-Kα wavelength(0.15406 nm), β is the full width at half maximum, and θis the diffraction angle. Specific surface area, pore volume,and pore diameter of the materials were determined from N2

adsorption-desorption isotherms at 77 K by using a Belsorb

mini II sorption analyzer. Surface morphology of TiO2 andLa-doped TiO2 photocatalysts were investigated by AFM(Digital Instruments, 3100) and FE-SEM-(Hitachi, S-4800).X-ray photoelectron spectroscopy (XPS—Thermo ElectronCorporation Theta Probe) equipped with ultrahigh vacuumchambers were used to evaluate the presence of elements inLa-doped TiO2 photocatalysts. Mg K-alpha X-rays (100 W)was used as the source at a takeoff angle of 5–75◦ and vacuumpressure of 10−6–10−7 Torr. The energy of monochromaticMg K-alpha X-rays used for analysis is 1486.6 eV.

2.2. Photocatalytic Degradation Procedure and AnalyticalMethods. A cylindrical photochemical reactor setup wasused as reported previously [20] for the degradation of MCP.The photocatalytic degradation was carried out by mixing100 mL of aqueous MCP solution and 100 mg of photocata-lyst. The experiments were performed at room temperature,and the pH of the reaction mixture was kept at solutionpH. Before irradiation, the slurry was aerated for 30 min toreach adsorption equilibrium followed by UV irradiation atspecific wavelength, 254 or 365 nm. Aliquots were withdrawnfrom the suspension at specific time intervals and centrifugedimmediately at 1500 rpm. Then it was filtered through a0.2 μm millipore filter paper to remove suspended particles.The filtrate was analyzed by HPLC and TOC to find outthe extent of degradation and mineralization of MCP. Theconcentration of MCP was analyzed by HPLC instrument(Shimadzu, Model: SPD-10A VP) with a UV-Vis detector.In the HPLC analysis, Shim-pack CLC-C8 column (5 μmparticle size, 250 mm length, and 4.6 mm inner diameter)and mobile phase of acetonitrile/water (6 : 4 v/v) were usedwith a flow rate of 1.0 mL min−1. An injection volume of20 μL was used. The total organic carbon was determined bya TOC analyzer (Shimadzu, Model: 5000A) equipped witha single injection autosampler (ASI-5000). The concept ofrelative photonic efficiency (ξr) is very useful to comparecatalyst efficiencies using a given photocatalyst (La-dopedTiO2) material and a given standard photocatalyst (TiO2-Degussa P25) [21]. The relative photonic efficiency (ξr),was obtained by comparing the photonic efficiency of La-doped TiO2 with that of the standard photocatalyst (TiO2-Degussa P25). To evaluate ξr , a solution of MCP (40 mg L−1)with a pH of 5 was irradiated with 100 mg of TiO2 or La-doped TiO2 for 0.5 h. Then, relative photonic efficiency wascalculated based on the above experiments.


3.1. Characterization of Photocatalysts

3.1.1. XRD Patterns and TEM Analysis of TiO2 and La-Doped TiO2. XRD patterns of TiO2 and La-doped TiO2 areshown in Figure 1, in which the peaks marked “A” and “R”correspond with anatase and rutile phase, respectively. XRDanalysis reveals that TiO2 and La-doped TiO2 photocatalystswas comprised of both anatase and rutile phases. The diffrac-tion pattern of La-doped TiO2 photocatalysts was similar tothat of pure TiO2. There are no peaks for the formation ofcomposite metal oxides such as La2O3 in La-doped TiO2. It


Inte

nsi

ty(a

.u.)

(a)

(b)

(c)

(d)

(e)

20 40 60 80

A(1

01)

A(0

04)

A(2

00)

A(2

11)

R(2

11)

R(0

02)

A(1

16)

A(2

20)

A(3

01)

Angle 2θ (◦)

Figure 1: XRD patterns of TiO2 and La-doped TiO2: (a) TiO2, (b)0.5 wt%, (c) 0.7 wt%, (d) 1.0 wt%, and (e) 1.5 wt% La-doped TiO2.

Table 1: Physicochemical characteristic of TiO2 and La-doped TiO2

photocatalysts.

PhotocatalystCrystalsize (D)

nm

Latticeparameter

(a) nm

Latticeparameter

(c) nm

Unit cellvolume(nm3)

TiO2 40.10 0.37728 0.95015 0.1356

0.3 wt%La-TiO2

33.12 0.37772 0.95069 0.1357

0.5 wt%La-TiO2

30.56 0.37726 0.95051 0.1356

0.7 wt%La-TiO2

21.41 0.37722 0.95031 0.1357

1.0 wt%La-TiO2

19.87 0.37735 0.95024 0.1356

1.5 wt%La-TiO2

19.80 0.37754 0.95075 0.1356

Pure ZnO 53.11 0.3248 0.5205 0.0476

1.0% La-ZnO 20.58 0.3267 0.5182 0.0472

was observed that the peak at 2θ = 25.4◦ for La-doped TiO2

photocatalysts were slightly shifted to lower angles whichshows that the presence of the large radius of La3+ (1.15 A)may interstitially substitutes in TiO2 lattice rather substitutefor relatively small radius Ti4+ (0.745 A), and this cause,lattice distortion in La-doped TiO2 [22]. The crystal size wascalculated using Scherer’s formula for the (101) plane of TiO2

and La-doped TiO2 photocatalysts, and the values are givenin Table 1. The average grain size from the broadening of the(101) peak of anatase was 19–40 nm. The crystal size of La-doped TiO2 decreased with increase in La content, and theircrystal size was less than that of pure TiO2. The decreasein crystal size was due to the incorporation of La-ion intoTiO2, which decreases the grains growth [23]. The latticeparameters of La-doped TiO2 (Table 1) are a little smallerthan the standard values of bulk TiO2 (a = 0.37728 nm, c =0.95015 nm). The difference in lattice parameters shows thatLa3+ is successfully incorporated into the TiO2 lattice whereit interstitially substitutes the Ti4+ ion sites as the ionic radius

Table 2: Textural properties of pure and La-doped semiconductors.

Photocatalyst ABET/(m2g−1) Vp/(cm3 g−1) dp/(nm)

TiO2 118.5 0.1071 2.718

1.0 wt% La-TiO2 98.36 0.117 4.267

Pure ZnO 53.11 0.0942 1.131

1.0% La-ZnO 39.56 0.1053 2.378

of La3+ (0.106 nm) is larger than that of Ti4+(0.061 nm)[24, 25]. The shift in the peak position and the changein the lattice parameters show that La3+ ions are replacingthe Ti4+ ions. Representative TEM images of La-dopedTiO2 are shown in Figure 2. Figure 2(a) reveals the uni-form size of the TiO2 nanoparticles with a few aggregations,and the average particle diameter was estimated to be about20 nm. HRTEM image (Figure 2(b)) revealed the presenceof highly crystalline TiO2 nanoparticles in La-doped TiO2

photocatalysts. The electron diffraction pattern of La-dopedTiO2 photocatalysts shown in Figure 2(c) further indicatesthat the samples were composed of highly crystalline TiO2

nanoparticles.

3.1.2. Nitrogen Adsorption Measurements. The nitrogenadsorption-desorption isotherms of pure and La-dopedTiO2 and their textural parameters are shown in Figure 3and Table 2, respectively. TiO2 and La-doped TiO2 samplesexhibit adsorption isotherms similar to Type II [26]. TypeII isotherm is often observed when multilayer adsorptionoccurs on a nonporous solid. The surface area of pureTiO2 was about 118.5 m2/g while La-doped TiO2 exhibiteda surface area of 98.36 m2/g. The decrease in the surfacearea for La-doped TiO2 can be attributed to the partialfilling of the pores in TiO2 by La3+ nanoparticles. The porediameters of pure and 1 wt% La-doped TiO2 were 2.718 nmand 4.267 nm, respectively. The decrease in surface area andcorresponding increase in average pore diameter may bedue to the collapse of considerably narrow pores to formbroad pores in presence of La ions. Similar trend of resultswere previously observed for semiconductors doped withtransition metals [27]. This shows that drastic modificationsof pore structures take place when semiconductor oxidesare doped with transition metals, and further, the effectprobably depends on the individual metal characteristics.However, this trend is not common for all metal ion-doped semiconductors. The large pore of La-doped TiO2 alsoallows an easy diffusion of the pollutant molecules in andaround the semiconductor, thus enhancing the adsorption ofpollutant molecules and its intermediate on the surface of thephotocatalysts. It is interesting to note that the surface areaof TiO2 and 1 wt% of La-doped TiO2 are relatively higherthan that of ZnO (53.11 m2/g) and 1 wt% of La-doped ZnO(39.56 m2/g) [18].

3.1.3. Atomic Force Microscope (AFM) and HR-SEM Analysis.The two-dimensional surface AFM images and surfaceroughness profiles of TiO2 and 1 wt% La-doped TiO2 areshown in Figures 4 and 5. From Figure 4(a), it could be


100 nm

50 nm

(a)

(b)

(c)

Figure 2: TEM images of 1.0 wt% La-doped TiO2 (a) HR-TEM image (b) HR-TEM of the selective area, and (c) TEM image with electrondiffraction pattern.

Relative pressure (p/p0)

0.2 0.4 0.6 0.8 1

0

100

200

300

400

500

1 wt% La-TiO2

TiO2

Am

oun

tad

sorb

ed(c

m3·g

−1)

Figure 3: Nitrogen adsorption-desorption isotherms of TiO2 and1 wt% La-doped TiO2.

observed that the size of TiO2 was not widely distributedand the grain size was big. The average roughness value(Rav) of pure TiO2 was 0.304 nm. However, for La-dopedTiO2 the grains are sharpened and their size decreased toabout 50%. The average roughness value (Rav) increased to0.576 nm. So we safely conclude that the incorporation ofLa3+ into the lattice of TiO2 can control the grain growthand enhance the surface roughness of TiO2 photocatalysts.From the surface roughness values it can be revealed thatLa-doped TiO2 possesses high rough and porous surfacethan pure TiO2. These high rough and porous surfaces ofLa-doped TiO2 are beneficial to enhance the photocatalyticactivity for the degradations of organic pollutants presentin aqueous solution [28]. The high resolution-scanningelectron micrographs (HR-SEM) of TiO2 and that of 1 wt%La-doped TiO2 are presented in Figures 6(a) and 6(b). TiO2

particles in Figure 6(a) seem to be well separated with regularshape. The morphology of TiO2 appears to be retained in

1 wt% La-doped TiO2 (Figure 6(b)) with random particlesize distributions.

3.1.4. X-Ray Photoelectron Spectroscopy (XPS) Analysis. Sur-vey spectrum (Figure 7(a)) of La-doped TiO2 photocatalystexhibits the presence of Ti, O, and La elements. XPS spec-trum of Ti-2p of TiO2 is shown in Figure 7(b). Ti 2p peakappeared as a single, well-defined, spin-split doublet withthe typical interval of 6 eV between its two peaks whichcorresponds to Ti4+ in a tetragonal structure (i.e., Ti 2p1/2

and Ti 2p3/2, shown in Figure 7(b)). The binding energiesof the peaks within the doublet were found to be 464.8 eVfor Ti 2p1/2 and 459.0 eV for Ti 2p3/2 signal, and this was ingood agreement with the binding energies of TiO2 found inthe literature [29] (464.34 eV for Ti 2p1/2 peak and 458.8 eVfor Ti 2p3/2 peak). The spectrum for Ti 2p of 1 wt% ofLa-doped TiO2 is also shown in Figure 7(b). It carries asimilar feature as that of XPS spectrum, Ti 2p of TiO2.The XPS spectrum of O2− of TiO2 is shown in Figure 7(c).An intense signal about 531 eV due to O2− ions of TiO2

was observed. This peak was attributed to the Ti–O inTiO2 and OH groups on the surface of the photocatalysts[30, 31]. The shoulder about 532 eV of the main O1s peakcan be attributed to the presence of loosely bound oxygenof the surface-adsorbed CO3

2−, H2O, or O2 [32]. The XPSspectrum of O 1s of 1.0 wt% La-doped TiO2 is also shownin Figure 7(c). The spectrum of O 1s broadened due tothe incorporation of La3+ into the TiO2 lattice. The XPSspectrum of La 3d is shown in Figure 7(d). La3+ ions thatincorporated into TiO2 lattice make interaction with oxidicsites of TiO2 (Ti–O–La) and appear to provide a broadenedspectrum of La 3d. A Similar broad peak for La 3d spectrumof La3+ was also observed by Liqiang et al. [33] for La-dopedTiO2 photocatalysts. These results conclude that La elementsexisted mainly as +3 valence in La-doped TiO2, Ti elementswere both mainly as +4 valence, and both O elements hadat least two kinds of chemical states, crystal lattice oxygen(1), and adsorbed oxygen (2). The presence of La, Ti, and O


0.5

1

1.5

μm( )

(a)

0 0.5 1 1.5 2−6

−4

−2

0

2

4

μm( )

nm

)

)

(b)

Figure 4: (a) AFM image and (b) surface roughness profile of TiO2.

elements in TiO2 and La-doped TiO2 is also observed fromthe EDX spectra (See Figures 1sa and 1(b) in Supplementarymaterial available on line at doi: 10.1155/2012/921412), andthe results are consistent with XPS analysis.

3.2. Degradation Mechanism of La-Doped TiO2. To evaluatethe photocatalytic activity of TiO2 and La-doped TiO2 aseries of experiments were carried out for MCP degradationin aqueous suspension with the light of wavelengths 254and 365 nm. The photocatalytic degradation of MCP followsa pseudo-first-order reaction. The apparent reaction rateconstants (k) and t1/2 values of TiO2 and La-doped TiO2 arepresented in Tables 3 and 4. For comparison the rate constantand t1/2 values of pure ZnO La-doped ZnO are also presentedin Tables 3 and 4. La-doped TiO2 showed higher rate ofdegradation than TiO2 for the light of wavelength of 254 nmand 365 nm. The rate constant increased with increase inLa loading up to 1.0 wt%, and with further increase inloading, the rate constant decreased. It was found thatthe reaction rate increased with the increase of lanthanumcontent (0.3–1 wt% La) at first and then declined when thecerium ion content (1.5 wt% La) exceeded its optimal value.In general, when increasing the metal ion concentrationthe carrier mobility decreased [34]. In the present study,

1

2

1.5

0.5

μm( )

(a)

1

2

3

0

−1

−21 20

μm( )

nm

)

)

(b)

Figure 5: (a) AFM image and (b) surface roughness profile of1.0 wt% La-doped TiO2.

the higher concentration of lanthanum in La-doped TiO2

photocatalysts decreased its crystallinity and carrier mobility,which led to reduction of the photocatalytic activity of TiO2.The reasons for the enhanced photocatalytic activity of TiO2

by the incorporation of lanthanum can be explained asfollows. The incorporation of La3+ in the lattice of TiO2

decreases crystallite sizes and inhibits the electron-holerecombination on excitation of La-doped TiO2. Moreover itincreased the surface roughness and provided more activesurface area for photocatalytic reaction. The introduction ofLa3+ produces lattice swelling [35] and hence increases thesurface roughness of La3+ (0.576 nm) than Ti4+ (0.304 nm).Thus, the increase in roughness of La3+ in La-doped TiO2

makes the structure loose and the grains activated whichcan be attributed to the high photocatalytic activity of TiO2.Further, an interesting observation was that the rate constantof 1 wt% La-doped TiO2 was higher than that of ZnOand 1 wt% La-doped ZnO [18] (Tables 3 and 4). Smallerparticle size, high porosity, and high surface roughnessare the reasons for the excellent photocatalytic activity of


Table 3: Apparent reaction rate constants and t1/2 values for the degradation of MCP with 254 nm.

PhotocatalystApparent reaction rate constant k

(× 10−2 min−1)t1/2 values (min) Correlation coefficient (R2 value)

TiO2 6.0 11.55 0.985

0.3 wt% La-TiO2 8.0 8.66 0.976

0.5 wt% La-TiO2 9.5 7.29 0.981

0.7 wt% La-TiO2 12.0 5.76 0.990

1.0 wt% La-TiO2 14.0 4.95 0.982

1.5 wt% La-TiO2 11.0 6.3 0.989

Pure ZnO 5.25 13.20 0.976

1.0% La-ZnO 8.01 8.65 0.977

MCP = 40 mg L−1, TiO2 or La-TiO2= 100 mg/100 mL, pH = 5, UV = 8 lamps, λ = 254 nm, adsorption equilibrium time = 30 min, and irradiation time =60 min.

S4800 8 mm SE(U) 25 kV ×25 μmk

(a)

SE(U)8.7 mm 45 kV μm×13 k

(b)

Figure 6: HR-SEM pictures of (a) TiO2 and (b) 1.0 wt% La-dopedTiO2.

La-doped TiO2 than ZnO and La-doped ZnO. The half-life of degradation decreased with increase in La loadingup to 1.0 wt%, and with further increase of La the half-life of degradation gets increase. The half-life of degradationwith 1.0 wt% La-doped TiO2 was almost half the value ofTiO2. These results also reveal the essential role of La3+ inLa-doped TiO2 for the degradation of MCP.

3.3. Photocatalytic Mineralization of MCP. Mineralization ofhazardous pollutants by a cost-effective process is very im-portant for industrial applications. To study the complete

Table 4: Apparent reaction rate constants and t1/2 values for thedegradation of MCP with 365 nm.

Photocatalyst

Apparentreaction rateconstant k (×10−2 min−1)

t1/2 values(min)

Correlationcoefficient(R2 value)

TiO2 5.3 13.08 0.999

0.3 wt% La-TiO2 5.8 11.95 0.995

0.5 wt% La-TiO2 6.7 10.34 0.993

0.7 wt% La-TiO2 8.2 8.45 0.994

1.0 wt% La-TiO2 9.0 7.7 0.985

1.5 wt% La-TiO2 8.6 8.06 0.989

Pure ZnO 3.00 13.20 0.999

1.0% La-ZnO 8.00 8.66 0.985

MCP = 40 mg L−1, TiO2 or La-TiO2= 100 mg/100 mL, pH = 5, UV = 8lamps, λ = 365 nm, adsorption equilibrium time = 30 min, and irradiationtime = 60 min.

mineralization of MCP, the degradation was carried out witheither TiO2 or La-doped TiO2 photocatalysts. The plot oftotal organic carbon value of MCP with time is illustratedin Figures 8 and 9 for the light of wavelength 254 nmand 365 nm, respectively. For comparison the mineralizationresults of pure ZnO and la-doped ZnO are also presentedin Figures 8 and 9. It can be clearly seen from Figures8 and 9 that the decrease of TOC concentration of MCPfor La-doped TiO2 was relatively higher compared to pureTiO2. Decrease of TOC value of MCP showed maximum for1 wt% La-doped TiO2 and achieved complete mineralization(100% TOC removal) of MCP within 3 h, whereas only 30%TOC removal was observed for pure TiO2 over the sameperiod of irradiation time. Rapid mineralization of MCPover La-doped TiO2 can be associated with the suppressionof electron-hole recombination by La3+ in the lattice ofTiO2 and generation of more number of •OH radicals byoxidation of holes. •OH radicals are the primary oxidizingspecies which break down organic pollutants into a varietyof intermediate products on the way to total mineralizationto carbon dioxide and harmless inorganic ions [36]. Theresults demonstrated that 1.0 wt% La-doped TiO2 was found


Binding energy (eV)

02004006008001000

Inte

nsi

ty(c

oun

ts)

0

1e+3

2e+3

3e+3

4e+3

5e+3

6e+3 Survey spectrum

Ti2

pO1s

Ti3

pT

i3s

Ti2

s

OK

LL

Ti L

MM

(a)

Binding energy (eV)

Ti 2p

472 470 468 466 464 460 458

Cou

nts

(a.u

)

(a)

(b)

462

(b)

(a)

(b)

524526528530532534

Binding energy (eV)

O 1s

Cou

nts

(a.u

)

(c)

Binding energy (eV)

Cou

nts

(a.u

)

845 840 835 830

La 3d

(d)

Figure 7: XPS spectra of pure and 1.0 wt% La-doped TiO2: (a) survey spectrum of TiO2 (b) Ti 2p (a = TiO2; b = 1.0 wt% La-TiO2) (c) O 1s(a = TiO2; b = 1.0 wt% La-TiO2), and (d) La 3d.

to be more active than other photocatalysts, namely, pureTiO2, 0.3 wt%, 0.5 wt%, 0.7 wt%, and 1.5 wt% La-dopedTiO2 and those photocatalysts exhibited TOC removal of30%, 28%, 75%, 70%, 75%, 87%, and 90%, respectively.1.0 wt% La-doped TiO2 required shorter irradiation time forthe complete mineralization of MCP compared to previouslyreported photocatalysts (pure ZnO, 1.0 wt% La-doped ZnO),and they disclosed only 28% and 70% TOC removal of MCPwithin 3 h, under the same experimental conditions [20].Small particle size, high surface area, high surface roughnessand porous surface of La-doped TiO2, and the suppressionof electron-hole recombination by La3+ were the reasons forthe high photocatalytic activity of La-doped TiO2 than otherphotocatalysts.

3.4. Relative Photonic Efficiency. The concept of relative pho-tonic efficiency (ξr) affords comparison of catalyst efficien-cies for the photocatalytic degradation of organic pollutants.To evaluate ξr , photocatalytic degradation of MCP wascarried out over TiO2 (Degussa P-25) or La-doped TiO2 orother photocatalysts (ZnO and 1 wt% La-doped ZnO) with

the lights of wavelengths 254 and 365 nm, and the resultsare presented in Table 5. The relative photonic efficienciesof La-doped TiO2 photocatalysts were greater than TiO2

and this revealed the effectiveness of metal-doped systems.The relative photonic efficiencies of light of wavelength254 nm for La-doped TiO2 are greater than light of wave-length 365 nm. The results were in good agreement withdegradation and mineralization studies. Comparing the highefficiency of La-doped TiO2 photocatalysts with standardcatalyst (Degussa P-25), 1.0 wt% La-doped TiO2 is about2.3 and 1.8 times more efficient than Degussa P-25 forthe light of wavelength 254 nm and 365 nm, respectively.While comparing with La-doped ZnO photocatalysts, La-doped TiO2 was about 1.54 and 1.46 times higher for thelight of wavelength 254 nm and 365 nm, respectively. Therelative photonic efficiency of La-doped TiO2 photocatalystswas compared with already reported photocatalysts [21],and the values are shown in Table 5. 1 wt% La-doped TiO2

(254 nm) photocatalyst was about 6.13 times more efficientthan Baker and Adamson whereas 9.32 times higher thanHombikat UV-100. Further, the relative photonic efficiency


Irradiation time (min)

0 20 40 60 80 100 120 140 160 1800

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

TiO2

0.3 wt% La-TiO2

0.5 wt% La-TiO2

0.7 wt% La-TiO2

1 wt% La-TiO2

1.5 wt% La-TiO2

(TO

Ct)

/(T

OC

0)

Figure 8: Comparison of photocatalytic mineralization of MCPusing TiO2 and La-doped TiO2 photocatalysts. (MCP concentra-tion = 40 mg L−1; catalyst amount = 100 mg/100 mL; solution pH =5; λ = 254 nm).

of 1 wt% La-doped TiO2 (254 nm) was relatively higher thanthe values reported for other commercial photocatalysts(Tioxide, Sargent-Weich and Fluka AG). It may be presumedthat the incorporation of La3+ in the lattice of TiO2 greatlyenhance the photocatalytic performance of TiO2 and henceshowed high relative photonic efficiency value than all otherphotocatalysts.

3.5. Photocatalytic Reaction Mechanism. Metal ions doped onthe semiconductors have been shown to improve the pho-tocatalytic electron-transfer processes at the semiconductorinterface [9, 37–39]. For example, the deposition of Au onTiO2 led to improvement in the efficiency of photocatalyticoxidation of thiocyanate ions [40]. Doped metal ions influ-ence the photocatalytic activity of TiO2 by acting as electron(or hole) traps and by altering e−/h+ pair recombination ratethrough the following processes:

Charge pair generation

TiO2 + hν −→ ecb− + hνb

+ (1)

Mn+ + hν −→M(n−1)+ + hνb+ (2)

Mn+ + hν −→M(n+1)+ + ecb−. (3)

Charge trapping

Mn+ + ecb− −→M(n−1)+ electron trap (4)

Mn+ + hνb+ −→M(n+1)+ hole trap. (5)


0 20 40 60 80 100 120 140 160 1800

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

TiO2

0.3 wt% La-TiO2

0.5 wt% La-TiO2

0.7 wt% La-TiO2

1 wt% La-TiO2

1.5 wt% La-TiO2

(TO

Ct)

/(T

OC

0)

Figure 9: Comparison of photocatalytic mineralization of MCPusing TiO2 and La-doped TiO2 photocatalysts. (MCP concentra-tion = 40 mg L−1; catalyst amount = 100 mg/100 mL; solution pH =5; λ = 365 nm).

Recombination

ecb− + hνb

+ −→ TiO2 (6)

M(n−1)+ + hνb+ −→Mn+ (7)

M(n+1)+ + ecb− −→Mn+, (8)

where Mn+ is a metal ion dopant. The energy level forMn+/M(n−1)+ lies below the conduction band edge (Ecb) andthe energy level for Mn+/M(n+1)+ above the valence bandedge (Evb). In the present study, photocatalytic reactionmechanism for the degradation of MCP over La-doped semi-conductor is speculated as follows. Under light illumination,La-doped TiO2 photocatalysts are exposed, and the electronsare excited from the valence band state to conduction bandstate. Liqiang et al. [33] demonstrated that surface oxygenvacancies and defects states by La doping could be favorablein capturing the photoinduced electrons during the processof photocatalytic reactions. Previously Chen et al. [41]reported that the introduction of new impurity states dueto the incorporation of La3+ decreases the recombination ofphotogenerated electrons and holes. Hence, in the presentstudy, we believe that the excited electrons in the conductionband transfer to the La3+ states, and the holes present in thevalence band are available for the photocatalytic oxidationof MCP. The rare earth metal (here, La), which usually actas a reservoir for photogenerated electrons, promotes anefficient charge separation in La-doped TiO2 photocatalysts.In addition, small particle size, high surface area, high surfaceroughness, and porous surface of La-doped TiO2 were alsothe reason for the high photocatalytic activity of La-dopedTiO2.


Table 5: Comparison of relative photonic efficiencies in thephotocatalytic degradation of MCP by pure and La-doped semicon-ductors (TiO2 and ZnO).

PhotocatalystRelative photonic

efficiency (ξr)

TiO2 1.0 ± 0.01 In this study

1.0 wt% La-TiO2

(254 nm)2.33 ± 0.01 In this study

1.0 wt% La-TiO2

(365 nm)1.82 ± 0.01 In this study

Pure ZnO 1.03 ± 0.01 [18]

1.0% La-ZnO (254 nm) 1.51 ± 0.01 [18]

Baker and Adamson 0.38 ± 0.02 [18]

Tioxide 1.9 ± 0.1 [21]

Sargent-Welch 2.1 ± 0.1 [21]

Fluka AG 2.2 ± 0.2 [21]

Hombikat UV-100 0.25 ± 0.02 [21]

MCP = 40 mg L−1, TiO2 or La-TiO2 = 100 mg/100 mL, pH = 5, UV =8 lamps, λ = 254/365 nm, adsorption equilibrium time = 30 min, andirradiation time = 60 min.

4. Conclusion

Highly efficient photocatalyst, La-doped TiO2, was success-fully synthesized and characterized by various sophisticatedtechniques. XRD and TEM analysis revealed the presence ofhighly crystalline TiO2 nanoparticles. AFM results demon-strated that La-doped photocatalysts have rough and highlyporous surface, which is a critical parameter to enhancethe photocatalytic activity. The photocatalytic activity of La-doped TiO2 in the degradation of MCP was studied, andthe results were compared with degradation results of La-doped ZnO photocatalysts. La-doped TiO2 photocatalystswere found to be very active, and its rate constant was1.75 and 1.125 times higher than La-doped ZnO photo-catalysts for the light of wavelength 254 nm and 365 nm,respectively. The relative photonic efficiency of La-dopedTiO2 catalyst was relatively higher than that of previouslyreported and some of the commercial photocatalysts. Smallparticle size, high surface area, high surface roughness, andthe suppression of electron-hole recombination by La3+ werethe reasons for the high photocatalytic activity of La-dopedTiO2 for the removal of MCP than other photocatalysts. Itis concluded that incorporation of rare earth metal into thesemiconductors has been proven to be a promising approachto improve the photocatalytic activity of semiconductors.

Acknowledgment

The authors acknowledge the Ministry of Education, Cul-ture, Sports, Science and Technology (MEXT) for the fund-ing through “High-Tech Research Center” a project for pri-vate universities, 2004–2008.

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Research Article

Hierarchical CuO/ZnO Membranes for EnvironmentalApplications under the Irradiation of Visible Light

Zhaoyang Liu, Hongwei Bai, and Darren Delai Sun

School of Civil & Environmental Engineering, Nanyang Technological University, Singapore 639798

Correspondence should be addressed to Zhaoyang Liu, [email protected] and Darren Delai Sun, [email protected]

Received 19 June 2011; Accepted 14 September 2011


Copyright © 2012 Zhaoyang Liu et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Solar visible light is a source of clean and cheap energy. Herein, a new kind of hierarchical CuO/ZnO nanomaterial wassynthesized using a facile process. Characterized by FESEM, TEM, XRD, XPS, and so forth, this CuO/ZnO naomaterial showsa special hierarchical nanostructure with CuO nanoparticles grown on ZnO nanorods. By assembling the hierarchical CuO/ZnOnanomaterials on a piece of commercial glassfiber membrane, a novel hierarchical CuO/ZnO membrane was fabricated. ThisCuO/ZnO membrane demonstrated excellent environmental applications, such as improved photodegradation of contaminantsand antibacterial activity, under the irradiation of visible light. Compared with pure ZnO nanorod membrane, the improvedphotodegradation and antibacterial capacities of this hierarchical CuO/ZnO membrane result from the special hierarchicalnanostructure of CuO/ZnO nanomaterials, which could enhance light utilization rate, enlarge specific surface area, and retardthe recombination of electrons and holes at the interfacial between CuO and ZnO. This hierarchical CuO/ZnO membrane is alsoeasy to be regenerated by completely mineralizing the adsorbed contaminants under the irradiation of visible light. All the abovecharacteristics of this hierarchical CuO/ZnO membrane indicate its great potential in environmental applications with solar visiblelight.

1. Introduction

Widespread water source contamination and increasing de-mand for consumable water necessitate a novel and sus-tainable approach to the remediation of polluted water [1].Photocatalytic nanomaterials have shown high efficiency incontaminant removal through the degradation of organiccontaminants and the inactivation of bacteria [2–4]. Solarenergy is a clean and renewable resource ideal for environ-mental and sustainable applications. Photocatalytic degra-dation utilizing visible light radiation is an increasinglypopular research topic [5]. ZnO is an extensively studiedsemiconductor employed in the photocatalytic removal ofcontaminants in water [6, 7]. However, pure ZnO nanoma-terials have limited efficiency because of the low light utiliza-tion rates resulting from the wide-band gap (3.2 eV). ZnOnanomaterials absorb only a small portion of light in the UVspectrum and exhibit rapid recombination of electrons andholes reducing their photocatalytic activity [8, 9]. The keyto broadening the applications for ZnO nanomaterials lies in

overcoming these shortcomings. Over several decades, effortshave been made to enhance the photocatalytic activity ofZnO through metal [10] or carbon/nitrogen doping [4, 11].ZnO nanomaterials have also been coupled with narrow-band gap semiconductors like Fe2O3, WO3, CdS, Cu2O,and CuO in attempts to improve photocatalytic activity byencouraging red shift [12–16]. The resulting increase inwavelength produces light in the visible spectrum, thereby,increasing light absorption and retarding electron and holerecombination. ZnO has also received attention for itsantibacterial properties through a mechanism which destroysthe cell membrane affecting bacterial morphology and theefflux of intracellular components [3].

Hierarchically structured nanomaterials with multiscaledorganizations increase light reflection and absorption insidethe material to enhance light utilization rate and enlargespecific surface area [17]. This effectively increases contami-nant attachment and adsorption. These attributes make hier-archical nanomaterials a desirable approach to enhancingphotocatalytic activity [18].


In practical application, the recovery of the photocatalystfrom treated water is an important economic and sustain-ability consideration. The incorporation of the photocat-alyst into a functional membrane is an ideal approach asit facilitates convenient recovery and regeneration whilstdegrading foulants retained in pores or on the membranesurface by simple irradiation using either UV or visiblelight [19, 20]. It is meaningful to fabricate a multifunctionalmembrane for contaminant removal via photodegradationand inactivation of bacterial under the irradiation of visiblelight. The multiple functions of such a membrane comesfrom the functional layer created by a hierarchical structuredCuO/ZnO materials, which could demonstrate high photo-catalytic activity because of the enhanced light utilizationrate, enlarged specific surface area, and the efficient an-tirecombination of electrons and holes. In this case, thephotocatalysts are easy for reuse, and the multiple functionalmembrane is easy to be regenerated after complete miner-alization of the adsorbed contaminants via extending theirradiation time of visible light.

In this study, a new kind of hierarchical CuO/ZnO nano-materials is synthesized using low-temperature hydrother-mal and photodeposition process. A piece of multifunctionalmembrane is fabricated by assembling the hierarchicalCuO/ZnO nanomaterials on top of a piece of commercialglassfiber membrane. Under the irradiation of visible light,this functional membrane demonstrated better photodegra-dation of contaminants and bacterial inactivation over acomparative functional membrane with a pure ZnO layer.This membrane is also characteristically easy to regenerateand has high reusability. All the above characteristics indicateits good potential in environmental purification with theusage of solar energy.

2. Experimental Section

2.1. Materials Synthesis

2.1.1. The Synthesis of ZnO Nanorods. Zinc acetate dihydrate(Zn(CH3COO)2, 2H2O) was purchased from Alfa, hexam-ethylenetetramine (HMTA: C6H12N4) was purchased fromSigma, and copper sulfate (Cu(SO4)2 · 5H2O) was fromAlfa. Other chemicals were purchased from Sigma. Accord-ing to the reports [21–23], a mixed solution of 0.03 MZn(CH3COO)2 and 0.03 M HMT was well prepared, andadjusted the pH of the solution around 7.5. 100 mL-mixedsolution was filled in a high temperature resistance glassbottle (<140◦C) and sealed tightly. After hydrothermallytreated for 6 h–12 h at 95◦C, the precipitation at the bottomof the glass bottle was collected and sufficiently washed forfurther characterization and usage.

2.1.2. The Synthesis of CuO/ZnO Hierarchical Nanomaterials.A given amount of hydrothermally synthesized ZnO nano-rods were dispersed in 100 mL deionized (DI) water. 0.1 gCu(SO4)2, 5H2O, and 0.1 g NaCl were added in the solutionand were continuously stirred for 10 min and ultrasonicatedfor another 10 min to make a mixed solution [24]. A weak

Pressure sensor

Vacuum pump

Membrane

Sunlight simulator

Scheme 1: Schematic setup for membrane assembly and pho-todegradation of contaminants.

black UV light with a predominant wavelength of 365 nmwas suspended at the middle of the solution [25]. Underthe continuous stirring, the mixed solution was illuminatedfor 6–12 h. Collect the precipitations at the bottom andwash them with DI water sufficiently. After being calcinedat 500◦C for 1 h in a furnace under atmospheric condition,the samples were ready for further characterizations andenvironmental application tests.

2.1.3. Assembly of the Hierarchical CuO/ZnO MultifunctionalMembrane. 100 mg of synthesized hierarchical CuO/ZnOnanomaterials were dispersed in 100 mL DI water. Onepiece of commercial glass fiber membrane (Adventc discsØ 47 mm, 0.50 μm) was mounted at the bottom of thevacuum filtration cup (Scheme 1), then slowly poured themixed solution into the filtration cup and switched on thevacuum pump. After certain time, the water was filtratedaway, leaving a uniformly assembled membrane with theCuO/ZnO hierarchical materials. This membrane is namedas CuO/ZnO membrane. As a comparison, a piece of pureZnO membrane with ZnO nanorods was assembled by thesame method; this membrane is named as ZnO membrane.

2.1.4. Photodegradation Test. Three kinds of commonly usedpollutants, including methyl blue (MB), acid orange (AO7),and Rhodamine B (RhB) aqueous solution, were used forthe test. The experiment detail was as follows. Firstly, theaqueous solutions of MB (100 mg/L), AO7 (50 mg/L), andRhB (50 mg/L) were prepared, respectively; then the solution(50 mL) was poured into the filtration cup, at the bottomof which the CuO/ZnO membrane was mounted, thenlet it adsorb for 1 h to reach adsorb equilibrium. Thenswitch on the sunlight simulator (100 mA) to start thephotodegradation experiment. The whole test system is acontinuous process. At a given internal, 3 mL reactant solu-tion was drawn with a syringe, and its UV-visible adsorptionvalue was measured by a UV-visible photospectrometer. As


a comparison, the photodegradation ability of ZnO mem-brane was also tested using the same methods as above.

The concurrent photocatalysis and membrane filtrationexperiment was conducted, and the permeate quality wasmonitored by measuring the UV-visible adsorption value ofthe collected permeate at a given internal.

The photolysis experiment without CuO/ZnO nanoma-terials and ZnO nanorods was also carried out before eachphotodegradation experiment.

2.1.5. Antibacterial Test. Microorganism (E. coli) was cul-tivated in sterilized LB broth culture and then incubatedovernight at 37◦C with a shaking incubator. The concen-tration of E. coli was 107 CFU/mL. Before the antibacterialtest, one new CuO/ZnO membrane was assembled, andthen the whole vacuum filtration setup except the sunlightsimulator was sterilized in a UV-sterilization fume hose.The antibacterial capability of the CuO/ZnO membrane wasinvestigated with/without the irradiation of visible light after40 mL E. coli solution was poured into the filtration cup(without switching on the vacuum pump). At an internalof 5 min, 1 mL reactant solution was drawn with a sterilizedsyringe and uniformly streaked on the well-solidified agarnutrient plate. After overnight’s cultivation at 37◦C, thecolonies were counted.

At the same time, the antibacterial capability of theZnO membrane and the blank control experiment (withoutCuO/ZnO nanomaterials and ZnO nanorods) with/withoutthe irradiation of visible light were also carried out using thesame methods as above.

2.1.6. Characterizations. The structure and the crystal phaseof the synthesized ZnO nanorods, and the CuO/ZnO hierar-chical nanomaterials were analyzed by powder X-ray diffrac-tometer (XRD, Bruker AXS D8 advance) with monochro-mated high-intensity Cu Kσ radiation (λ = 1.5418 A) oper-ated at 40 kV and 30 mA. The obtained XRD spectra werematched with a powder diffraction file (PDF) databasemaintained by the International Centre for Diffraction Data(ICDD). The morphology of the samples was characterizedby field emission scanning electron microscopy (FESEM,JEOL JSM-6360F) and high resolution transmission electronmicroscopy (HRTEM, Jeol JEM-2010). The element contentsof the samples were measured by energy dispersive X-ray spectrometer (EDS) attached to the SEM. The UV-Visdiffuse reflectance spectra of the samples were measured by aThermo Scientific Evolution 300 UV-Vis spectrophotometer(Thermo Fisher Scientific, Mass, USA) equipped with anintegrating sphere assembly and a Xenon lamp source. BaSO4

was used as the reflectance standard. The BET specific sur-face areas of the samples were determined at liquid nitro-gen temperature (77 K) using a Micromeritics ASAP 2040system. Before the measurement, 0.1 g sample was outgassedunder vacuum for 6 h at 250◦C.

XPS analysis was carried out at room temperature in anultra high vacuum (UHV) chamber with a base pressurebelow 2.66 × 10−7 Pa. Photoemission spectra were recordedby a Kratos Axis Ultra spectrometer (Shimadzu Corporation,Kanagawa, Japan) with a monochromatic Al Kα excitation

source (hν = 1486.71 eV). Curve fitting was performedusing a nonlinear least square Gaussian-Lorenzian function.Carbon calibration shift was carried out at 284.6 eV toremove the presence of residual adventitious carbon.

3. Results and Discussions

The morphology of the hydrothermally synthesized ZnOnanorods was observed by FESEM in Figure 1(a). It can beseen that the ZnO nanorod has a length of 10 to 20 μm,and the average diameter is around 1 μm, so the aspect ratioof length to diameter was around 10 to 20, which belongsto a typical rod-like structure. And the synthesized ZnOnanorod has a smooth surface with a hexagonal end. TheHRTEM image in Figure 1(b) obviously confirmed that ZnOgrew along the direction of [0001] plane forming a rod likestructure with a typical lattice of 0.52 nm.

With the illumination of weak black light on the mixedsolution of copper sulfate, sodium chloride, and ZnOnanorods, Cu nanoparticles were uniformly deposited on thesurface of the ZnO nanorods. The CuO/ZnO nanomaterialswere well observed by FESEM as shown in Figures 2(a)–2(c)at different magnifications. These Cu nanoparticles are sup-posed to nucleate from the defect parts of the ZnO nanorodsand grow up. After the illumination, the ZnO nanorods werethoroughly covered by Cu nanoparticles, forming a “corn-like” architecture. The followed calcination at 500◦C for 1 hmakes Cu become CuO without changing the morphologyof the Cu nanoparticles, as shown in Figure 2(a). Thesizes of the CuO nanoparticles were around 100 nm. FromFigure 2(b), it is found that only partial surface of the ZnOnanorod was covered by CuO nanoparticles. The distancebetween individual CuO nanoparticle was in the range ofnanoscale, but the distance between individual CuO/ZnOnanorod was in the range of microscale. Therefore, theseCuO/ZnO nanomaterials form a kind of hierarchical struc-ture, which possesses the advantages of promoting masstransfer, enlarging the specific surface area, and promotingthe charge transfer from ZnO nanorod to CuO nanoparticle.The energy disperse X-ray spectrum of the CuO/ZnOhierarchical material (Cu (10 wt%)) in Figure 2(c) clearlyproved that the elements of Zn, O, and Cu dominate thecomposition of these novel hierarchical materials. Furtherquantitative analysis of the EDS reveals that the atomicratio of O : Zn : Cu is about 10 : 9 : 1, indicating that thestoichiometric [(Zn + Cu)/O = 1 : 1] CuO/ZnO hierarchicalnanomaterial is consisted of ZnO and CuO.

The X-ray diffractometer was employed to measurethe crystallization of the hydrothermally synthesized ZnOnanorods and the CuO/ZnO hierarchical nanomaterials. Asshown in Figure 3, the hydrothermally synthesized ZnOnanorods were crystallized into a standard hexagonal struc-ture (JCPDS 89–1397) [26]. After CuO was formed on theZnO nanorod, all the dominant peaks of the original ZnOnanorods were kept, but the peaks of CuO (002) and CuO(110) at 2θ = 39.2◦ and 2θ = 32.6◦ appears respectively,and the overlapped peaks of CuO (111) and ZnO (101)around 2θ = 36.4◦ also revealed the existence of CuO in thismaterial and further confirmed that the CuO nanoparticles


1 μm

(a)

5 nm

0.52 nm[0

001] [0110]

(b)

Figure 1: (a) FESEM image and (b) HRTEM image of the hydrothermally synthesized ZnO nanorods.

1 μm

(a)

1 μm

(b)

Zn

ZnO

Cu Cu

0 1 2 3 4 5 6 7 8 9 10

10 μm

(keV)

(c)

Figure 2: (a)-(b), FESEM images of the hierarchical CuO/ZnO nanomaterials at different magnifications, (c) the EDS spectrum of thehierarchical CuO/ZnO nanomaterials, insert is an overview SEM image.


Table 1: Binding energies (eV), fwhm of the hierarchical CuO/ZnO nanomaterials.

Position (fwhm) Position (fwhm) Position (fwhm)

O 1s O 1s Zn 2p3/2 Zn 2p1/2 Cu 2p3/2 Cu 2p1/2

CuO/ZnO hierarchicalnanomaterials

530.508(2.009)

532.265(2.643)

1021.45(3.162)

1044.44(3.027)

934.832(3.559)

954.913(3.656)

20 30 40 50 60 70 80

Inte

nsi

ty(a

.u.)

2θ (degrees)

(a)

(b) ZnO nanorod

(CuO)

(002)(CuO)

(a)

(b)

(CuO)

(110)

“corn-like” CuO/ZnO material

Figure 3: XRD patterns of the pure ZnO nanorods and the hier-archical CuO/ZnO nanomaterials.

were uniformly distributed on the surface of this material[27–29].

The chemical composition and electronic structure of theCuO/ZnO hierarchical nanomaterials was investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectra ofthe Cu 2p, Zn 2p, and O 1s were presented in Figure 4 whilethe detailed binding energies (eV) and their correspondingfull width at half maximum (fwhm) were listed in Table 1.The characteristic intense peaks were centered at 1021.45 eV(Zn 2p3/2) and 1044.44 eV (Zn 2p1/2) from XPS spectra ofZn 2p in Figure 4(a), clearly indicating that the oxidationstate of Zn is +2 in the form of ZnO on the surface ofthe CuO/ZnO hierarchical nanomaterials [30]. The char-acteristic peaks were located at 934.832 (Cu 2p3/2) and954.913 (Cu 2p1/2), and their corresponding satellite peaksof the Cu 2p XPS spectra presented in Figure 4(b), obviouslyconfirming that the oxidation state of Cu is +2 in the form ofCuO on the surface of the CuO/ZnO hierarchical nanomate-rials [31–33]. The broad O 1s peak in Figure 4(c) was com-prised of two small peaks, one is located at 530.508 eV andthe other one is located at 532.265 eV. The former is inherentO atoms bound to metals such as Cu and Zn, while the latterpeak resulted from the possible surface contaminations byhydroxyl species and carbonate species [34].

The UV-visible spectra of the CuO/ZnO hierarchical na-nomaterials and ZnO nanorods were presented in Figure 5.Obviously, the CuO/ZnO hierarchical nanomaterials dis-played a stronger UV absorption ability over ZnO nanorods

in both UV and visible ranges, which indicates that the UV-visible absorption spectra has been red shifted to visiblerange. This is because (1) on one hand, the rough surface ofthe CuO/ZnO hierarchical nanomaterials would allow morelight reflection and absorption inside the structure [18],which is different from the smooth surface of ZnO nanorods;(2) on the other hand, CuO has a potential to red-shift thelight absorption spectrum to visible range [16].

The N2 adsorption/desorption isotherm curve of theCuO/ZnO hierarchical nanomaterials was measured asshown in Figure 6. Clearly, the CuO/ZnO hierarchical nano-materials show the characteristic mesoporous materials, [35]which is favorable for the mass transfer, light reflection,and bacterial attachment [36]. As measured, the specificsurface area of the CuO/ZnO hierarchical nanomaterials is11.24 m2/g, which is much higher than that of ZnO nanorods(4.81 m2/g) and that of commercial ZnO powder (3 m2/g)[35].

3.1. Photodegradation of Contaminants by the MultifunctionalCuO/ZnO Membrane. The CuO/ZnO membrane, assembledby depositing CuO/ZnO nanomaterials on a glassfiber mem-brane, was tested for the removal of contaminants by pho-todegradation and filtration. As a blank control, commercialZnO powder and physically mixed CuO/ZnO nanoparticleswere also depositing on glassfiber membranes to investigatetheir photodegradation abilities of contaminants. Firstly, thephotodegradation of industrial dye, such as MB, under theirradiation of visible light on the surface of CuO/ZnO mem-brane was investigated, as shown in Figure 7. MB solutionwith an initial concentration of 100 ppm was completely de-graded after visible light irradiation for 60 min (Figure 7(a)).The CuO/ZnO membrane demonstrated better photodegra-dation ability compared with ZnO membrane and physicallymixed CuO/ZnO membrane under the same conditions(Figure 7(b)) because of the special hierarchical CuO/ZnO“corn-like” nanostructure [37]. After switching on thevacuum pump as shown in Scheme 1, the MB solution wasremoved by the concurrent photodegradation and filtrationthrough the assembled membranes. By measuring the per-meate from CuO/ZnO membrane, MB was found to betotally removed (after 60 min reaction), but there was stillabout 60% MB remaining in the permeate from ZnO mem-brane (Figure 7(b)). Moreover, the CuO/ZnO membrane iseasy to be regenerated after the adsorbed MB was completelydegraded by extending the visible light irradiation time.

The CuO/ZnO membrane and ZnO membrane werealso tested for the removal of AO7 (Figure 8). The AO7solution (initial concentration of 50 mg/L and pH of 7)was completely photodegraded by the CuO/ZnO mem-brane after 90 min irradiation of visible light, but longer


930 935 940 945 950 955 960 965

Inte

nsi

ty(a

.u.)

Binding energy (eV)

Cu 2p3/2

Cu 2p1/2

(a)

1015 1020 1025 1030 1035 1040 1045 1050

Inte

nsi

ty(a

.u.)

Binding energy (eV)

Zn 2p3/2

Zn 2p1/2

(b)

526 528 530 532 534 536 538

Inte

nsi

ty(a

.u.)

Binding energy (eV)

O 1 s

(c)

Figure 4: XPS spectra of the Cu 2p, Zn 2p, and O 1s state on the surface of the hierarchical CuO/ZnO nanomaterials.

300 400 500 600 700 8000

0.2

0.4

0.6

0.8

1

1.2

400350 4500

0.2

0.4

0.6

0.8

1

1.2

ZnO nanorods

Ads

orba

nce

Wavelength (nm)

CuO/ZnO “corn-like” architecture

Figure 5: The UV-visible spectra of the hierarchical CuO/ZnO nanomaterials and ZnO nanorods.


0 0.2 0.4 0.6 0.8 10

2

4

6

8

10

12

14

16

18

20

Vol

um

e(c

c/g)

AdsorptionDesorption

Relative pressure, P/P0

Figure 6: N2 adsorption/desorption isotherm curve of the hierarchical CuO/ZnO nanomaterials.

400 500 600 700 800

0

0.5

1

1.5

2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

0 min20 min

40 min60 min

(a)

0 10 20 30 40 50 60

0

0.2

0.4

0.6

0.8

1

Time (min)

PhotolysisZnO membrane Physically mixed CuO/ZnO

membrane

CuO/ZnO membrane

C/C

0

(b)

Figure 7: (a) The absorption spectra of MB solution under the irradiation of visible light on the CuO/ZnO membrane, and (b) theconcentration change of MB solution under the irradiation of visible light on the CuO/ZnO membrane and ZnO membrane.

time is needed for the complete degradation of AO7by the ZnO membrane. Through measuring the con-centration change of AO7 during the visible light irra-diation process, the CuO/ZnO membrane demonstratedbetter photodegradation ability than that of ZnO mem-brane under the same condition (Figure 8(b)). The con-current membrane filtration and photocatalysis of AO7was also investigated, no AO7 was found in the permeatefrom CuO/ZnO membrane (after 90 min), while there wasstill 63% AO7 found in the permeate from the ZnO mem-brane because of pure ZnO’s low photodegradation ability.At the same time, the CuO/ZnO membrane is easy for regen-eration after complete photodegradation of AO7 adsorbed

on the surface of CuO/ZnO membrane by extending the ir-radiation time of visible light.

The CuO/ZnO membrane and ZnO membrane wasalso tested for the removal of dye contaminant RhB,the concentration of RhB was monitored by measuring theUV-visible adsorption value at 553 nm. The concentrationchange of RhB under the irradiation of visible light on theCuO/ZnO membrane and ZnO membrane was presentedin Figure 9. Obviously, the CuO/ZnO membrane demon-strated better photodegradation ability compared with ZnOmembrane. After 120 min irradiation of visible light, theCuO/ZnO membrane completely degraded the RhB in thesolution, while ZnO membrane only degraded about 45%.


300 400 500 600 700

0

0.2

0.4

0.6

0.8

1

Abs

orba

nce

(a.u

.)

Time (min)

0 min30 min

60 min90 min

(a)

0 20 40 60 80 1000

0.2

0.4

0.6

0.8

1

Time (min)

PhotolysisZnO membrane

CuO/ZnO membrane

C/C

0

(b)

Figure 8: (a) The absorption spectra of AO7 solution under the irradiation of visible light on the CuO/ZnO membrane, and (b) theconcentration change of AO7 solution under the irradiation of visible light on the CuO/ZnO membrane and ZnO membrane.

PhotolysisZnO membrane

CuO/ZnO membrane

0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

Time (min)

C/C

0

Figure 9: The concentration change of RhB under the irradiationof visible light on the CuO/ZnO membrane and ZnO membrane.

The enhanced photodegradation ability of CuO/ZnO mem-brane was attributed to the following reasons. Firstly, theenlarged specific surface area of CuO/ZnO nanomateri-als (11.24 m2/g) over that of ZnO nanorods (4.81 m2/g)is favorable for contaminant adsorption, providing morereaction sites for the photocatalytic reactions. Secondly,the CuO/ZnO nanomaterials improved the light utilizationrate compared with ZnO nanorods. The rough surface ofCuO/ZnO nanomaterials can enhance the light reflectionand absorption inside the CuO/ZnO nanomaterials, whileon the contrast, ZnO nanorods only can be excited by UV

e−e−e−e−e−

e−e−e−

e− ZnO CuO

Visible light

Dye

Dye+

O2

O2•

Scheme 2: Schematic diagram of the electron transfer and theenergy band positions of ZnO and CuO in the hierarchicalCuO/ZnO materials for the photodegradation of dye contaminantsunder the irradiation of visible light.

light which just accounts for a minor part of the solar light[16]. Moreover, the CuO/ZnO hierarchical nanomaterialwith larger specific surface area favors the adsorption of dyecontaminants, the adsorbed dye contaminants is favorablefor the utilization of visible light [38, 39], and the producedelectrons and holes would be transferred to CuO/ZnOnanomaterials [40]. The transferred electrons were capturedby the surface adsorbed O2 to yield O2

•−and HOO• radicals[40]. Then the dye contaminants are photodegraded in situby the produced radicals. At the same time, the band gapdifference between CuO and ZnO facilitates the transferof photogenerated electrons and holes from ZnO to CuO(Scheme 2) [8]; in this way, the recombination of electronsand holes is retarded to a certain extent [41]. Therefore, thephotocatalytic activity is enhanced.

3.2. Antibacterial Capability Test. Besides the photodegra-dation of contaminants, the antibacterial capabilities ofthe CuO/ZnO membrane and ZnO membrane were alsoinvestigated. Figure 10 shows the inactivation of E. coli on


0 5 10 15 20 25 30

0

20

40

60

80

100

Surv

ival

(%)

Time (min)

Blank (dark)Blank (light)ZnO membrane (dark)

ZnO membrane (light)CuO/ZnO membrane (dark)CuO/ZnO membrane (light)

Figure 10: Plot of % E. coli survival rate as the function ofvisible light irradiation time on CuO/ZnO membrane and ZnOmembrane.

the CuO/ZnO membrane and ZnO membrane with/withoutthe irradiation of visible light. The survival of E. coli can becalculated by % survival = B/A∗100 (where A is the numberof surviving microbial colonies in the control, and B is thenumber of surviving microbial colonies in the tested sample).To rule out the synergic effect of CuO/ZnO membrane onthe inactivation of E. coli, both blank control experimentsfor the pure glassfiber membrane and inactivation of E.coli by ZnO membrane with/without the irradiation wereconducted using the same method. It is found that thedirect visible light irradiation has no obvious effect on theinactivation of E. coli on pure glassfiber membrane. As weknow, ZnO [3] and CuO [17] have certain antibacterialgrowth ability; hence, both CuO/ZnO membrane and ZnOmembrane demonstrated antibacterial growth in the dark asexpected. However, under the irradiation of visible light, theantibacterial capability of CuO/ZnO membrane was greatlyenhanced and was significantly higher than that of ZnOmembrane; wherein, CuO/ZnO membrane can completelykill all the E. coli in the solution after 20 min irradiationof visible light, while ZnO membrane can only kill around40% E. coli even after 30 min irradiation of visible light. Thisindicates that the synergic effect of CuO/ZnO nanomaterialsplayed a significant role in the improvement of antibacterialcapability, which is in consistent with previous reports [42].The enhanced antibacterial capability is attributed to (1)the enlarged specific surface area and mesoporous property[36] of CuO/ZnO nanomaterial which is convenient forbacterial attachment and (2) the enhanced photocatalyticactivity by enlarging the light utilization rate and retardingthe recombination of photogenerated electrons and holesbetween CuO and ZnO, as explained in previous section[8, 16].

4. Conclusion

The hierarchical CuO/ZnO nanomaterials have been suc-cessfully synthesized for the first time by the combinationof low-hydrothermal and photodeposition methods. Thishierarchical CuO/ZnO nanomaterial was well characterizedby FESEM, TEM, XRD, XPS, and so forth. The CuO/ZnOmembrane was assembled by depositing the hierarchicalCuO/ZnO nanomaterials on the surface of glassfiber mem-brane with the help of vacuum filtration. The CuO/ZnOmembrane demonstrated high photodegradation ability fordye contaminants, such as MB, AO7, and RhB, under visiblelight irradiation, which results from the enlarged specificsurface area for contaminants adsorption, the improved lightutilization rate, and the retarded recombination of electronsand holes. At the same time, the CuO/ZnO membrane iseasy for regeneration by complete degradation the adsorbeddyes with extending the visible light irradiation time.Furthermore, the CuO/ZnO membrane also demonstratedbetter antibacterial capability than ZnO membrane due tothe synergic effect of CuO/ZnO nanomaterials. All theseadvantages of the CuO/ZnO membrane would bring greatbenefits to water purification applications.

Acknowledgment

The authors are grateful for the financial support receivedfrom the National Research Foundation/NTU Joint R&D(REF. COY-25-24/N370-4).

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Research Article

Synthesis, Characterization, and Evaluation of Boron-DopedIron Oxides for the Photocatalytic Degradation of Atrazine underVisible Light

Shan Hu, Guanglong Liu, Duanwei Zhu, Chao Chen, and Shuijiao Liao

Laboratory of Plant Nutrition and Ecological Environment Research, Centre for Microelement Research of Huazhong AgriculturalUniversity, Key Laboratory of Subtropical Agriculture and Environment, Ministry of Agriculture, Wuhan 430070, China

Correspondence should be addressed to Duanwei Zhu, [email protected]



Copyright © 2012 Shan Hu et al. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Photocatalytic degradation of atrazine by boron-doped iron oxides under visible light irradiation was investigated. In this work,boron-doped goethite and hematite were successfully prepared by sol-gel method with trimethylborate as boron precursor. Thepowders were characterized by XRD, UV-vis diffuse reflectance spectra, and porosimetry analysis. The results showed that borondoping could influence the crystal structure, enlarge the BET surface area, improve light absorption ability, and narrow theirband-gap energy. The photocatalytic activity of B-doped iron oxides was evaluated in the degradation of atrazine under the visiblelight irradiation, and B-doped iron oxides showed higher atrazine degradation rate than that of pristine iron oxides. Particularly,B-doped goethite exhibited better photocatalytic activity than B-doped hematite.

1. Introduction

Atrazine, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-tri-azine, has been widely used in the fields of corn, sorghum,orchard, and forest, controlling broad-leaf and grassy weeds[1]. However, due to the toxicity to aquatic organisms andmammals, high mobility, low-sorption affinity, and slowbiodegradability [2, 3], atrazine has been banned by manyEuropean countries. It is frequently detected in ground waterand surface water [4] and seriously influenced water quality.Therefore, many ways have been found to resolve atrazinecontamination, such as advanced oxidation processes [5],microorganism removal [6], and microwave irradiation [7].

It has been reported that photocatalysis is effective way inthe degradation of organic pollutants. TiO2 is considered tobe the most promising photocatalyst due to its nontoxicity,chemical inertness, and high reactivity. Parra found thatboth suspended and supported TiO2 could destroy atrazinealthough atrazine could not be completely mineralized[8]. However, the widespread technological use of TiO2 isimpaired by its wide-band gap (3.2 eV), which can onlybe activated under UV light. Iron oxides especially goethite

and hematite have been studied as photocatalysts in recentyears because their lower band gap (2.2 eV), and nonmentaldoping could improve reactivity of photocatalysts [9, 10].It is reported that PE films with boron-doped goethite hashigher photo-induced degradation than pure PE films underthe UV irradiation [11]. In this paper, B-doped goethiteand hematite were prepared as photocatalysts, and enhance-ment of photocatalytic activity of atrazine degradation wasobserved under visible light irradiation.

2. Experimental

2.1. Materials. Fe(NO3)3, (CH3O)3B, KOH, methanol weresupplied from Guoyao Chemical Co. (Shanghai, China)and atrazine was supplied from the Laboratories of Dr.Ehrenstorfer (Germany). All chemicals were used withoutfurther purification, and deionized water was used in all theexperiments.

2.2. Preparation of Photocatalysts and Characterization. Theoriginal goethite (G-S-B0%) was prepared according to the


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Figure 1: X-ray diffraction patterns of goethite and B-dopedgoethite.

method of Atkinson et al. [12], and the preparation of B-doped goethite photocatalysts was the same as the methodof Liao et al. [13], while the atomic weight ratio of B toFe was 2% (G-S-B2%). Goethite and B-doped goethite werecalcined at 400◦C for 2 h to obtain the original hematite (H-S-B0%) and B-doped hematite (H-S-B2%), respectively.

The Brunauer-Emmett-Teller surface areas of the pow-der samples were determined by nitrogen adsorption-desorption isotherm measurements on a ST-08 nitrogenadsorption apparatus. The X-ray powder diffraction patternwas obtained with a Brook D8 diffractometer using Fe Kαradiation with an accelerating voltage of 40 kV and currentof 20 mA. The UV-vis diffuse reflectance spectra of differentiron oxides in the 190–900 nm were recorded using anAmerican Lambda35 UV-vis spectrophotometer.

2.3. Photocatalytic Evaluation with Atrazine under VisibleLight. The photocatalytic activities of pure and B-dopediron oxides nanoparticles were evaluated by the degradationof atrazine under visible light irradiation at a constanttemperature (25◦C). 25 mL 10 mg·L−1 atrazine solution wasput in 50 mL of centrifugal test tube with 100 mg differentphotocatalysts, and then all tubes were placed in a constanttemperature shaking incubator at a speed of 190 r·min−1.A 250 W metal halide lamp (λ > 385 nm, JLZ250KN,Shanghai Yaming Co.) was put above all tubes as the visiblelight irradiation with a distance of 80 cm. At differenttime intervals during the irradiation, samples were col-lected, filtered, and finally analyzed by HPLC (Agilent1100).Atrazine was detected at 222 nm and the mobile phasewas methanol/water mixture (80 : 20, v/v) at a flow rate of1.0 mL·min−1 using C18 column (4.6 mm × 150 mm).


3.1. Crystal Structure. XRD was carried out to investigatethe changes of goethite and hematite phase structure after

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Figure 2: X-ray diffraction patterns of hematite and B-dopedhematite.

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Figure 3: UV-visible light reflection spectrum of goethite and B-doped goethite.

boron doping and heat treatment. Figure 1 shows the X-ray diffraction patterns of goethite and B-doped goethite.Compared with G-S-B0%, there is no significant new peakappearing in G-S-B2%, but the intensity of some peaksbecome weaker or stronger. Maybe the content of boron istoo small to make perceptible crystal change of goethite’sstructure by X-ray diffraction. But boron does make aninfluence in the crystal structure of goethite. Figure 2 showsthe crystal form change of common hematite and hematitewith 2% boron doping. It seems that H-S-B0% and H-S-B2% have the same peaks. Perhaps the high calcinationtemperature destroyed the changes of doping.

3.2. UV-vis Diffuse Reflectance Spectra. Figure 3 illus-trates the UV-vis light reflection spectrum of goethite and


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Figure 4: UV-visible light reflection spectrum of hematite and B-doped hematite.

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Figure 5: Atrazine degradation in present of goethite and B-dopedgoethite under visible light.

Table 1: The properties of different B-doped goethite and hematite.

Samples BET(m2/g) Band-gap energy (eV)

G-S-B0% 60.24 2.06

G-S-B2% 91.46 1.97

H-S-B0% 56.26 1.69

H-S-B2% 71.48 1.68

boron-doped goethite. In the UV part, G-S-B0% and G-S-B2% show the same reflection rate, while in the visiblepart, G-S-B2% shows stronger light absorption than G-S-B0%. The UV-vis light reflection spectra of undoped and2% boron-doped hematite were shown in Figure 4. H-S-B2% shows stronger absorption than H-S-B0% during the

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Figure 6: Atrazine degradation in present of hematite and B-dopedhematite under visible light.

whole wavelength range. The band-gap energy of these fouriron oxides were estimated by Eg = 1240/λonset [14] andpresented in Table 1. It is inferred that boron doping maynarrow the band-gap energy of iron oxides and improve theirlight absorption ability.

3.3. BET Surface Area. Table 1 lists the BET surface areas offour iron oxides. G-S-B2% exhibits higher BET surface areathan that of G-S-B0% (34.13%), and the BET surface areaof H-S-B2% is also larger than that of H-S-B0%(21.29%).These results confirm that boron doping can efficientlyinhibit the crystal size growth and increase the surface areaof goethite and hematite.

3.4. Visible Light Photocatalysis of Atrazine. To examine thephotocatalytic activity of B-doped iron oxides, atrazine waschosen as target contaminant. And the degradation rate ofatrazine through the reaction time in present of these fouriron oxides under visible light was displayed in Figures 5and 6. The photocatalytic degradation of atrazine followedthe first-order reaction kinetics under visible light accordingto lnCt = lnC0 − kt, where C0 stands for the initialconcentration of atrazine and Ct is the concentration ofatrazine at t time. All the numbers were collected in Table 2.The results clearly indicated that G-S-B2% and H-S-B2%revealed a substantially enhanced activity for degradationof atrazine, as compared to undoped G-S-B0% and H-S-B0% under visible light irradiation. The first-order kineticsconstants (k) for atrazine degradation by G-S-B0%, G-S-B2%, H-S-B0%, and H-S-B2% were 0.0295 h−1 (R =0.9774), 0.0301 h−1 (R = 0.9857), 0.0199 h−1 (R = 0.9757),and 0.0202 h−1 (R = 0.9932), respectively, and the half lives(t1/2) of atrazine degraded by them were 23.49 h, 23.02 h,34.82 h, and 34.31 h, respectively. It was confirmed thatboron doping could show good optical activity and goethite


Table 2: Degradation kinetic results of atrazine under visible light irradiation with different iron oxides.

Experiments conditions lnCt = lnC0 − kt R k (h−1) t1/2 (h)

G-S-B0% lnCt = 0.8261− 0.0295t 0.9774 0.0295 23.49

G-S-B2% lnCt = 0.7895− 0.0301t 0.9857 0.0301 23.02

H-S-B0% lnCt = 0.8240− 0.0199t 0.9757 0.0199 34.82

H-S-B2% lnCt = 0.7983− 0.0202t 0.9932 0.0202 34.31

had better catalytic activity than hematite under visible lightirradiation.

4. Conclusion

The degradation of atrazine by using visible light-activatedB-doped iron oxide as photocatalyst is demonstrated inthis paper. Goethite, hematite, and B-doped goethite andhematite were successfully synthesized by a novel modifiedsol-gel method. Although there is no significant differentin XRD results between pure iron oxide and B-doped ironoxide, however, the BET surface area and UV-vis spectraindicate that boron doping greatly influenced the propertiesof iron oxide. G-S-B2% and H-S-B2% exhibited enhancedvisible light photocatalytic activity in degradation of atrazinecompared with G-S-B0% and H-S-B0%, which maybe dueto the stronger light adsorption and boron-doped goethiteexhibited better photocatalytic activity than boron-dopedhematite.

Acknowledgments

This work was supported jointly by the National NaturalScience Foundation of China (40973056; 40371064) andSpecialized Research Fund for the Doctoral Program ofHigher Education (SRFDP) of the Ministry of Education ofPRC (20100146110020).

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Research Article

Silver Orthophosphate Immobilized on Flaky Layered DoubleHydroxides as the Visible-Light-Driven Photocatalysts

Xianlu Cui,1 Yaogang Li,2 Qinghong Zhang,2 and Hongzhi Wang1

1 State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China2 Engineering Research Center of Advanced Glasses Manufacturing Technology, MOE, Donghua University, Shanghai 201620, China

Correspondence should be addressed to Qinghong Zhang, [email protected] and Hongzhi Wang, [email protected]



Copyright © 2012 Xianlu Cui et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Flaky layered double hydroxide (FLDH) was prepared by the reconstruction of its oxide in alkali solution. The composites withFLDH/Ag3PO4 mass ratios at 1.6 : 1 and 3 : 1 were fabricated by the coprecipitation method. The powders were characterized byX-ray diffraction, field-emission scanning electron microscopy, transmission electron microscope, and UV-vis diffuse reflectancespectroscopy. The results indicated that the well-distributed Ag3PO4 in a fine crystallite size was formed on the surface of FLDH.The photocatalytic activities of the Ag3PO4 immobilized on FLDH were significantly enhanced for the degradation of acid red Gunder visible light irradiation compared to bare Ag3PO4. The composite with the FLDH/Ag3PO4 mass ratio of 3 : 1 showed a higherphotocatalytic efficiency.

1. Introduction

The treatment of environmental pollutants in waste waterby active semiconductor photocatalysts has recently attractedconsiderable attention for its ability to completely oxidizeorganic contaminants to carbon dioxide, water, and mineralacids [1–5]. Among the semiconductor photocatalysts, TiO2

has been extensively studied because of its outstanding pho-tocatalytic activity, long-term stability, low cost, and non-toxicity [6, 7]. However, TiO2 can only be activated with UVlight with a wavelength of less than 385 nm due to its wideband gap (∼3.2 eV), which limits its utilization of the solarenergy. Therefore, a growing interest is also focused on thedevelopment of the new photocatalysts that can operateeffectively under visible light irradiation. The doping ofTiO2 with foreign elements such as metals and nonmetals toimprove the energy band structure of TiO2 has been exten-sively investigated [8], and nitrogen doping TiO2 has beendemonstrated as a visible light photocatalyst [9]. However,the absorption edge of N-doped TiO2 was just extended tothe wavelength below 450 nm with a lower absorption cons-tant. The non-TiO2-based photocatalysts with a larger absor-ption coefficient, such as Ta3N5 [10], Ga1−x ZnxN1−x Ox

[11, 12], BiVO4 [13, 14], Bi2WO6 [15, 16], and Ag2CrO4

[17] have also been studied during the past decade. Up tonow, the development of novel visible-light-responsive pho-tocatalysts with expanded spectral response range and highphotocatalysis quantum efficiency is still necessary.

Recently, Yi et al. reported the new use of Ag3PO4 semi-conductor as a visible light photocatalyst, which can oxidizewater as well as decompose organic contaminants in aqueoussolution [18]. It was found that it exhibited extremely highphotocatalytic efficiency for organic dye decompositionunder visible light irradiation and the decomposition ofmethylene blue over Ag3PO4 was dozens of times quickerthan that over monoclinic BiVO4 and commercial TiO2−xNx.However, the effect about reducing the crystallite size ofAg3PO4 on photocatalysis is still unknown.

Layered double hydroxide (LDH), also called hydrotal-cite-like compound or anionic clay, is a family of lamellarionic solids that in terms of layer charge are the counterpartof cationic clay minerals [19]. The LDH has a uniqueproperty known as “memory effect,” which refers to thatLDH is decomposed into mixed metal oxide (MMO) whenLDH calcined at 300–600◦C. The calcined LDH is able torecover the original layered structure easily when exposed tothe air or put into anion aqueous solution [4, 20]. In ourprevious work, we have prepared flaky layered double


hydroxides (FLDH) composed of cross-linked nanoflakes bythe reconstruction of their oxides in alkali solution, andthe ZnO immobilized on FLDH generated highly dispersedZnO nanoparticles (∼9.5 nm in diameter) with an enhancedphotocatalytic efficiency [21]. Nevertheless, the band gap ofthis composite (∼3.18 eV) is too wide to absorb sunlightefficiently. In this work, we synthesized the Ag3PO4/FLDHcomposites, and the much finer Ag3PO4 was obtained in thepresence of FLDH and the resulted composites exhibiteda higher visible light photocatalytic efficiency compared tobare Ag3PO4.

2. Experimental

2.1. Synthesis of Materials. The Mg-Al-CO3 LDH was pre-pared by coprecipitation by a high supersaturation method[22]. A typical preparation process was described as fol-lows. Solution A was prepared by dissolving AlCl3·6H2O(62.5 mmoL) and MgCl2·6H2O (125 mmoL) in 87.5 mL ofdeionized water and solution B prepared via dissolvingNaOH (437 mmoL) and Na2CO3 (208 mmoL) in 125 mL ofdeionized water. Solution A was added to solution B ina three-neck round-bottom reaction flask and then stirredat 50◦C, subsequently heated to 90◦C for 4 h. LDH wasobtained after collection of the precipitates by filtration,washing several times with distilled water, and drying at 90◦Covernight.

FLDH was prepared by the reconstruction of its oxide inalkali solution [21]. Firstly, LDH was calcined in air at 500◦Cfor 4 h, with a heating rate of 4◦C/min, to prepare the MMO.Secondly, 3.0 g MMO was dispersed into 200 mL 1.0 MNa2CO3 solution and stirred for 5 min; the suspension wasthen transferred into a temperature-humidity chamber andstirred for 24 h at 6◦C. FLDH was obtained after collectionof the precipitates by filtration, washing several times withdistilled water, and drying at 90◦C overnight.

The Ag3PO4/FLDH composites were prepared as follows.3.0 g FLDH mixed with 4.8 g Na2HPO4 was placed into150 mL of the distilled water under room temperature andstirred for 5 min. A proper amount of AgNO3 was dissolvedin 50 mL of the distilled water and added to the suspensionabove. After stirring vigorously for 20 min at room tem-perature, the suspension was filtered, washed several timeswith distilled water, and dried at 60◦C overnight. By varyingthe amount of AgNO3, the composites with Ag3PO4/FLDHmass ratios at 1 : 1.6 and 1 : 3 (labeled Ag3PO4/FLDH-1.6 andAg3PO4/FLDH-3, resp.) were prepared. Bare Ag3PO4 samplewithout FLDH was also prepared by a similar method.

2.2. Characterization. The X-ray diffraction (XRD) patternsof the powder phase compositions were identified by X-raydiffractometer (Model D/Max-2550, Rigaku Co., Japan) us-ing Cu Kα irradiation (λ = 1.5406 A) at 40 kV and 100 mA.The size and morphology of the samples were determined byfield emission scanning electron microscopy (FE-SEM)(Model S-4800, Hitachi, Japan) and transmission elec-tron microscope (TEM) (Model JEM-2100F, JEOL, Japan).The Brunauer-Emmett-Teller (BET) specific surface area

measurement and the Barrett-Joyner-Halenda (BJH) porevolume were performed using a nitrogen adsorption appara-tus (Model Autosorb-1MP, Quantachrome Instruments Co.,USA). The UV-vis diffuse reflectance spectra of the powdersamples were collected in the form of a dry-pressed disk atroom temperature with a spectrophotometer (Model Lam-bda 950, Perkin-Elmer Co., USA).

2.3. Photocatalytic Tests. Photocatalytic activities of the re-sultant bare Ag3PO4 and Ag3PO4/FLDH composites wereevaluated by the photocatalytic decomposition of acid red G(ARG) under visible light irradiation. A 500 W Xe lamp wasused as the light source, and a ZJB 420 filter glass was usedto cut off light of wavelength <420 nm. In view of the factthat Ag3PO4 was slightly soluble in aqueous solution [23],the amount of catalysts in every experiment was normalizedby the mass of Ag3PO4 really present in each sample andmade sure that the amount of Ag3PO4 in ARG solutionequal to 1 g/L. The experiments were performed as follows.Amount of photocatalyst was added into 100 mL ARG(50 mg/L). The aqueous suspension was stirred at room tem-perature and irradiated with visible light. About 3 mL ofreaction suspension was sucked at a defined time interval,and the solid material was separated by centrifugation. Theremoval rates of ARG were measured on a UV-vis spec-trophotometer (Model Lambda 35, Perkin-Elmer Co., USA)at the wavelength of 530 nm.


3.1. Characterization. Figure 1 shows the XRD patterns ofthe original LDH, MMO, and FLDH. The typical X-ray pat-tern of the original LDH (Figure 1(a)) exhibits the typical re-flections of Mg-Al-CO3-LDH with a series of narrow, sym-metric, and sharp peaks, indicating a high degree of crys-tallinity [24]. Calcination of the LDH at 500◦C resulted inthe formation of a mixed metal oxide phase with an MgO-like structure (Figure 1(b)). After regeneration by soakingthe MMO into a Na2CO3 solution, the calcined LDH havesuccessfully recovered the original layered structure accord-ing to the characteristic reflections corresponding to the ori-ginal LDH (Figure 1(c)), though a loss of some degree ofcrystallinity as reported elsewhere [25].

The XRD patterns of bare Ag3PO4, Ag3PO4/FLDH-1.6,and Ag3PO4/FLDH-3 are shown in Figure 2. For bareAg3PO4 (Figure 2(a)), all of the diffraction peaks are wellindexed as the body-centered cubic Ag3PO4 (JCPDS No. 06-0505). For the Ag3PO4/FLDH composites (Figures 2(b) and2(c)), most of the diffraction peaks could be attributed to thebody-centered cubic Ag3PO4. Meanwhile, small diffractionpeaks of LDH have been detected just as that in Figure 1(c),confirming that the FLDH preserve its original structure afterbeing coated with Ag3PO4.

The FE-SEM images in Figure 3 show the morphologiesof LDH and FLDH. Figure 3(a) shows the plate-like mor-phology typical of LDH [26] with diameters in the range 50–100 nm. After regeneration, as shown in Figure 3(b), FLDH ismade up of flaky sheets coalesced irregularly with each other.


(003

)

(006

)

(009

)(0

12)

(015

)

(018

)

(110

)

(113

)

(a)

(b)

(c)

MgO

2θ (deg)

Inte

nsi

ty (

a.u

.)

10 20 5030 6040 70

Figure 1: XRD patterns of (a) the original LDH, (b) MMO, and (c)FLDH.

(110

)

(a)

(b)

(c)

LDH

2θ (deg)

Inte

nsi

ty (

a.u

.)

10 20 5030 6040 70 80

(200

)(2

10)

(211

)

(220

)

(310

)

(320

)(3

21)

(332

)

(400

)

(411

)(4

20)

(421

)

(222

)

Figure 2: XRD patterns of (a) bare Ag3PO4, (b) Ag3PO4/FLDH-1.6,and (c) Ag3PO4/FLDH-3.

The sizes of the flaky sheets range from 50 to 200 nm, and thethickness is ∼10 nm. It means that these sheets have a largeaspect ratio (defined as platelet diameter/thickness) whichmay lead to an efficient adsorption capability.

The TEM images in Figure 4 show the sizes and mor-phologies of Ag3PO4 and Ag3PO4/FLDH-3. The TEM imageof bare Ag3PO4 shows agglomerated particles with diametersin the range 200–1000 nm (Figure 4(a)). The average diam-eter of the well-distributed Ag3PO4 in the Ag3PO4/FLDH-3composite (∼200 nm) is smaller than that of the bare Ag3PO4

(Figure 4(a)). From these results, it can be concluded thatFLDH plays an important role in hindering the crystalgrowth and agglomeration of Ag3PO4. Figure 4(c) shows theenergy dispersive spectroscopy (EDS) pattern of Ag3PO4/FLDH-3. Except for the Cu and C peaks coming from thecopper grid for the TEM analysis, the Mg and Al peaks are

Table 1: Corresponding specific surface area and pore volume forthe samples.

Sample SaBET (m2/g) V b (cm3/g)

LDH 110.4 0.336

FLDH 146.4 0.769

Ag3PO4 1.6 0.002

Ag3PO4/FLDH-1.6 106.1 0.495

Ag3PO4/FLDH-3 122.3 0.601aBET surface area determined by a multipoint BET method, using theadsorption data in the relative pressure (P/P0) range 0.05–0.3. bBJH porevolume determined by the volume of nitrogen adsorbed at a relative pressure(P/P0) of 0.994.

corresponding to FLDH, and the presence of Ag andP elements indicates the formation of Ag3PO4. This re-sult confirms that Ag3PO4/FLDH composites were success-fully prepared in this study.

The BET specific surface area and pore volume of thesamples are summarized in Table 1. It can be seen that theoriginal LDH has a BET surface area 110.4 m2/g with a rela-tive lower pore volume 0.336 cm3/g. After regeneration, theBET specific surface area and pore volume of FLDH aremuch larger than that of LDH, this may be because the newlyformed FLDH has a coarse surface due to the irregular inter-connected nanosheets and the remaining slit-like mesoporesas shown in Figure 3(b) [21]. This higher surface area and thespecial structure are considered to be a suitable support forAg3PO4 dispersion. The surface area of Ag3PO4/FLDH-1.6,and Ag3PO4/FLDH-3 are slightly lower than that of FLDHand decrease with increasing the Ag3PO4/FLDH mass ratios.This could be attributed to that the presence of Ag3PO4 canclog the pore of FLDH, and this effect is more and more sig-nificant with the amount of Ag3PO4, which is further con-firmed by change in the pore volume. It can also be observedthat the surface area and pore volume of bare Ag3PO4 arevery low, which is expected to be responsible for the sub-sequent poor adsorption capacity of ARG.

The UV-vis diffuse reflectance spectra of the bare Ag3PO4,Ag3PO4/FLDH-1.6 and Ag3PO4/FLDH-3 are shown inFigure 5. It can be clearly seen that the bare Ag3PO4 canabsorb solar energy with a wavelength shorter than ∼530 nmas reported by Yi et al. [18]. As for Ag3PO4/FLDH compos-ites, the absorption edge of each sample shifts to a slightlyshorter wavelength compared with bare Ag3PO4 (∼525 nmfor Ag3PO4/FLDH-1.6 and ∼520 nm for Ag3PO4/FLDH-3), though the absorption intensity slightly decreases. Thedecrease of absorption intensity is dependent on the contentof Ag3PO4 in these composites.

3.2. Degradation of ARG Solution. The photocatalytic activ-ity of Ag3PO4 may be explained by that Ag3PO4 has alarge dispersion of conduction band due to the form of thedelocalized π∗ antibonding states in the conduction bandand the inductive effect of PO4

3−, which helps the separationof electron-hole pairs [27], and a strong oxidation ofphotoexcited holes in the valence band could be responsiblefor the dye degradation [18].


100 nm

(a)

100 nm

(b)

Figure 3: SEM images of (a) LDH and (b) FLDH.

200 nm

(a)

200 nm

(b)

C

O

Mg

Cu

Cu

Cu

AlP

Ag

Ag

(keV)

0 2 3 4 5 6 7 8 9 10 111

(c)

Figure 4: TEM images of Ag3PO4 (a) and Ag3PO4/FLDH-3 (b), and EDS elemental microanalysis of Ag3PO4/FLDH-3 (c).

The photocatalytic activities in decomposing ARG areplotted in Figure 6, where C0 and Ct are the concentration ofaqueous ARG in the starting aqueous (50 mg/L) and at timet, respectively. All of the samples showed efficient photo-catalytic activities under visible light irradiation, indicatingthe potential of Ag3PO4 as a photofunctional material forwaste water cleaning. It can be clearly seen that both of theAg3PO4/FLDH photocatalysts exhibited higher photocat-alytic activities for the ARG degradation reaction than bareAg3PO4, especially Ag3PO4/FLDH-3, the ARG dye can becompletely degraded within 20 min under visible light irrad-iation.

The enhanced photocatalytic properties can be attributedto two main factors. First is the much smaller size and gooddispersion of Ag3PO4 immobilized on the FLDH. It is wellknown that the light-generated charge carriers in small-sizedsemiconductor grains can efficiently transfer to the surface,

which results in the decrease of the opportunities for recom-bination [28]. Meanwhile, for bare Ag3PO4, the by-products,black metallic Ag particles, would appear because of thephotocorrosion during the photocatalytic process and attachthemselves onto the surface of the Ag3PO4 catalyst, whichwould inevitably prevent visible light absorption and de-crease its photocatalytic activity [23]. For the Ag3PO4/FLDHcomposites, the photocorrosion still existed, but the Ag parti-cles resulted from the decomposition of much finer Ag3PO4

in the composite were also much smaller compared to thatfrom the bare Ag3PO4. The finer Ag particles scattered lesslight and improved the light harvesting in the photocatalyticprocess. Moreover, a large part of Ag particles would depositonto the surface of FLDH not coated by Ag3PO4 in thecomposites, which also further reduce the negative influence.Second is a higher adsorption capability for the compositescompared with bare Ag3PO4. The photocatalytic degradation


Ag3PO4

Abs

orba

nce

(a.

u.)

350 400 450 500 550 600 650

Wavelength (nm)

Ag3PO4/FLDH-1.6

Ag3PO4/FLDH-3

Figure 5: UV-vis diffuse reflectance spectra of bare Ag3PO4 andAg3PO4/FLDH composites.

Ct/C

0

Time (min)

(a)

(b)

(c)

0 10 20 30 40 50 60

0.8

0.6

0.4

0.2

0

1

Figure 6: Photocatalytic activities of (a) Ag3PO4, (b) Ag3PO4/FLDH-1.6, and (c) Ag3PO4/FLDH-3 for ARG degradation undervisible light irradiation.

of a pollutant must proceed at the surface of the catalyst afterthe reactant has been adsorbed on the surface, so theadsorption is considered to be the prestep for the consequentphotocatalytic reaction [29]. According to the blank tests car-ried out like the way of photocatalytic degradation of ARGbut without light irradiation, which are shown in Figure 7,it can be clearly seen that the adsorption of ARG on bareAg3PO4 in the dark was negligible after 60 min, however,the percentages of ARG removal with Ag3PO4/FLDH-1.6 andAg3PO4/FLDH-3 were ∼35% and 55%, respectively. Theseresults indicate that the high adsorption capability is verycrucial to the improvement of photocatalytic activation forthe Ag3PO4/FLDH composites. For Ag3PO4/FLDH-3, the ex-cellent efficiency may be explained based on the smallerparticle size, better dispersion of Ag3PO4 on the FLDH, andhigher adsorption capability with the amount of FLDH con-tent.

Ct/C

0

Time (min)

(a)

(b)

(c)

0 10 20 30 40 50 60

0.8

0.6

0.4

1

Figure 7: The adsorption capabilities of ARG on (a) Ag3PO4, (b)Ag3PO4/FLDH-1.6 and (c) Ag3PO4/FLDH-3 in the dark.

4. Conclusions

In summary, we synthesized the Ag3PO4/FLDH compositeswith fine Ag3PO4 crystalline grains through a wet chemi-cal method. The Ag3PO4/FLDH composites, the Ag3PO4/FLDH-3 in particular, exhibited much higher catalytic effi-ciency than bare Ag3PO4 for the degradation of ARG undervisible light irradiation. The enhanced photocatalytic prop-erties can be attributed to the combination of the smaller-sized and well-distributed Ag3PO4 immobilized on theFLDH and the strong adsorption of the dye on the FLDH.

Acknowledgments

This work was supported by National Key Technology R&DProgram (no. 2006BAA04B02-01) and Shanghai LeadingAcademic Discipline Project (B603).

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Research Article

Thickness Dependent on Photocatalytic Activity ofHematite Thin Films

Yen-Hua Chen and Kuo-Jui Tu

Department of Earth Sciences, National Cheng Kung University, No. 1, University Road, Tainan 701, Taiwan

Correspondence should be addressed to Yen-Hua Chen, [email protected]



Copyright © 2012 Y.-H. Chen and K.-J. Tu. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Hematite (Fe2O3) thin films with different thicknesses are fabricated by the rf magnetron sputtering deposition. The effects of filmthicknesses on the photocatalytic activity of hematite films have been investigated. Hematite films possess a polycrystalline hexa-gonal structure, and the band gap decreases with an increase of film thickness. Moreover, all hematite films exhibit good photo-catalytic ability under visible-light irradiation; the photocatalytic activity of hematite films increases with the increasing filmthickness. This is because the hematite film with a thicker thickness has a rougher surface, providing more reaction sites forphotocatalysis. Another reason is a lower band gap of a hematite film would generate more electron-hole pairs under visible-lightillumination to enhance photocatalytic efficiency. Experimental data are well fitted with Langmuir-Hinshelwood kinetic model.The photocatalytic rate constant of hematite films ranges from 0.052 to 0.068 min−1. This suggests that the hematite film is a super-ior photocatalyst under visible-light irradiation.

1. Introduction

Many industrial dyes are toxic and carcinogenic [1, 2]. Theorganic dyes present in industrial wastewater often posesignificant threats against human health and environmentalpollution control. Therefore, it is important to remove or-ganic dyes from wastewater. However, wastewater exhibitsstable behavior under harsh conditions and resists biodegra-dation, making it difficult to remove organic dyes easily.Many wastewater treatment methods have been explored [3,4], and the most commonly reported technique is the adsor-ption technique [5, 6]. However, this method is now becom-ing unpopular because it is expensive, and the adsorbent haslow recyclability [7–9].

Photocatalysis is an environment friendly process thatutilizes irradiation energy for catalytic reactions. Hence, pho-tocatalytic technology has been widely investigated for appli-cations to the decomposition of pollutants [10–12]. The pho-tocatalytic decomposition of wastewater is a process thatcombines heterogeneous catalysis with solar technologies[13]. Researchers are especially interested in developing pho-tocatalysts that can extend the absorption wavelength intothe visible-light region [14–16].

Hematites (Fe2O3) are minerals belonging to the groupof iron-oxide minerals; they have a hexagonal structure andexhibit paramagnetic behavior. Moreover, they show strongcatalytic activity, widely and easily available, and are extre-mely environment friendly. In particular, hematite may be apromising candidate for visible-light photocatalysis—it canabsorb visible light, collect up to 45% of solar-spectrumenergy, and is one of the cheapest semiconductor materialsavailable.

Photocatalysts are usually in the form of powders andthin films. The powders have a better adsorption activity andphotocatalytic efficiency than thin films because of a con-siderably large surface area. However, the drawback of thepowders, especially nanopowders, is that they are not easy toretrieve and may be harmful to human beings. In this study,hematite thin films are employed for the removal of methy-lene blue (MB) dye by photocatalytic decomposition. MBis nonbiodegradable and extensively used in the industry;therefore, it is selected as the model contaminant. The photo-catalytic activity for different thicknesses of hematite thinfilms (1H, 2H, and 4H) will be evaluated, and their thicknessdependence is discussed.


H(1

16)

H(0

24)

H(1

13)

H(0

06)

H(1

10)

Si(1

00)

H(1

04)

30 35 40 45 50 55 60

Inte

nsi

ty (

cps)

2θ (degrees)

1H2H4H

Figure 1: XRD patterns of hematite thin films with different depo-sition times.

2. Experimental

2.1. Preparation and Characterization of Hematite Thin Films.The hematite thin films were grown using the rf magnetronsputtering system on a Si(100) substrate. The Fe2O3 targetwas prepared as the starting material. The deposition wascarried out at 600◦C in the atmosphere having an Ar/O2 ratioof 14/6, at a total pressure of 20 mtorr. Several depositiontimes of the Fe2O3 films were studied: 1.0, 2.0, and 4.0 hr,which were in different thicknesses. The crystal structure ofFe2O3 thin films was characterized by X-ray powder diffrac-tion (XRD). The morphology and the grain size of Fe2O3

films were observed by a field-emission scanning electronmicroscope (FE-SEM). Magnetic properties of Fe2O3 filmswere measured by a superconducting quantum interferencedevice (SQUID) magnetometer at room temperature withina magnetic field up to 1 T. The band gap of the specimens wasexamined via a UV-Vis diffuse reflectance spectrometer.

2.2. Photocatalytic Experiment. In the photocatalytic exper-iment, the degradation of MB (C16H18ClN3S•3H2O)was measured under visible-light irradiation (wavelength>400 nm) in a black box. The power of the visible light was500 W. The visible-light lamp was placed vertically on theclosed reaction vessel at a distance of 60 cm. The reactionsystem was kept at 25◦C with cooling water. The initial con-centration of MB was adjusted to 1.0 ppm (100 mL), andthen, 1 cm × 1 cm of the Fe2O3 thin film (photocatalyst) wasadded. After every measurement, the MB solution was cen-trifuged and filtered, and subsequently, the 2 mL MB solutionwas removed for concentration measurements. The varia-tions in the MB concentration were measured using a UV-Visspectrometer (UV-Vis).

2.3. Photocatalytic Model. The kinetic mechanism is impor-tant because it describes the removal rate of the photocatalystand controls the residual time of the entire process. The

Acc.V10 kV

Spot2

Magn80000 x

DetSE

WD9.8

Exp1

200 nmFESEM1000330

(a)

Acc.V10 kV

Spot2

Magn80000 x

DetSE

WD9.9

Exp1

200 nmFESEM1000330

(b)

Acc.V10 kV

Spot2

Magn80000 x

DetSE

WD9.8

Exp1

200 nmFESEM1000330

(c)

Figure 2: Plane view of FE-SEM images of hematite thin films withvarious deposition times: (a) 1.0 hr, (b) 2.0 hr, and (c) 4.0 hr.

heterogeneous photocatalysis usually follows a pseudo-first-order equation, which can be expressed by the Langmuir-Hinshelwood kinetic model [17, 18]

rLH = −dC

dt= k · KC0

1 + KC0or ln

C

C0= k′t, (1)

where rLH is the photocatalytic reaction rate, C0 is the initialconcentration, C is the concentration at time t, k is the reac-tion rate constant, K is the equilibrium adsorption constant,and k′ is the pseudo-first-order rate constant (photocatalyticrate constant).


172 nm

HV10 kV

WD9.7 mm

Mag40000 x

SigSE

10/7/20098:51:51 AM

1 μmNCKU ESEM

(a)

367 nm

Acc.V10 kV

Spot2

Magn40000 x

DetSE

WD11

Exp1

500 nmFESEM981028

(b)

553 nm

10.1 mmHV

10 kVWD Mag

40000 xSigSE

10/7/20098:53:15 AM

1 μmNCKU ESEM

(c)

Figure 3: Cross-section of FE-SEM images of hematite thin filmswith different deposition times: (a) 1.0 hr, (b) 2.0 hr, and (c) 4.0 hr.


3.1. Characterization of Hematite Thin Films. The XRD pat-terns of the prepared Fe2O3 films are shown in Figure 1. Thediffraction peaks can be identified as hematite with a hexa-gonal structure, which is in agreement with the standard datagiven in its JCPDS card (33-0664). No other impurity peakscan be detected, and the diffraction intensity increases withthe increasing deposition time. Moreover, the strong andsharp diffraction peaks of the films indicate well-crystallizedhematite films that can be easily obtained using the afore-mentioned sputtering parameters.

−10000 −5000 0 5000 10000

−30

−20

−10

0

10

20

30

M(e

mu

/cm

3)

Magnetic field (Os)

1H2H4H

Figure 4: Magnetization plots as a function of magnetic field ofhematite thin films with various film thicknesses.

Table 1: Grain size, magnetic properties, band gap, and relatedparameters of photocatalysis of hematite thin films with variousfilm thicknesses.

Properties 1H 2H 4H

Film thickness (nm) 172 367 553

Particle size (nm) 17 35 77

Band gap (eV) 2.77 2.74 2.71

Maximal magnetization (emu/cm3) 4.7 8.6 22.6

Rate constant (1/min) 0.052 0.055 0.068

The film thickness, morphology, and grain size of thehematite thin films investigated by FE-SEM are shown inFigures 2 and 3. The film thickness of hematite films is 172,367, and 553 nm for the deposition time of 1.0, 2.0, and4.0 hr, respectively. All the films have fine and irregularlyshaped crystal grains, and the grain size gradually increaseswith an increase in the layer thickness (deposition time). Thegrain size of hematite films is around 17, 35, and 77 nm, andwe name these films 1H, 2H, and 4H, as listed in Table 1.

The magnetization plots as a function of magnetic field,at 300 K, are shown in Figure 4. All the three specimensexhibit superparamagnetic behavior however, the bulk mag-netite is paramagnetic. The maximal magnetization increaseswith the increasing film thickness of hematite films, which isin agreement with the previous studies [19, 20]. The resultssuggest that hematite films could be manipulated or recov-ered rapidly by an external magnetic field. Furthermore, thenonmagnetic impurities can be excluded during hematitefilms recovery.

The absorbance of the UV-Vis diffuse reflectance spec-trum for hematite films is shown in Figure 5(a); the absor-bance for the three samples is fairly high and within the


Table 2: The comparison of photocatalytic activity for various photocatalysts.

Photocatalyst Material Rate constant (k′: min−1) Reference

Nanohematite Congo red 0.0026 [21]

Fe2O3/RR (20 mM H2O2) Orange II 0.0029 [22]

Fe2O3 thin film MB dye 30% [23]

TiO2/Fe2O3 thin film MB dye 15% [23]

α-Fe2O3/SnO2 thin film 2-naphthol 8% [24]

Fe2O3 nanorod thin film Rhodamine B 0.3% [25]

Fe2O3 thin film MB dye 0.068/99% This study

400 500 600 7000

0.1

0.2

0.3

0.4

0.5

0.6

Abs

orba

nce

(%)

Wavelength (nm)

1H2H4H

(a)

2 2.5 3 3.5 40

0.2

0.4

(αhA)2

hA (eV)

1H2H4H

(b)

Figure 5: The absorbance of the (a) UV-Vis diffuse reflectance spectrum, (b) Tauc plot for hematite thin films with different film thicknesses.

region of visible-light wavelength. This result indicates thatthe photocatalytic properties of hematite films may existunder visible-light irradiation. By using the absorbance pat-tern and the following equation [21]: αhν = A(hν− Eg)0.5

(where α is the considered absorption coefficient, A is aconstant, h is Planck’s constant, ν is the photon frequency,and Eg is the optical band gap energy), an extrapolation ofthe linear plot of (αhν)2 on the y-axis versus (hν) on the x-axis gives the Eg value. The estimate Eg is to be approximately2.77, 2.74, and 2.71 eV for 1H, 2H, and 4H, respectively(Figure 5(b)). It is observed that the band gap of hematitefilms increases with a decrease of film thickness (grain size).It has been reported by Bao et al. [22] and Miller et al. [23]that the film with a small grain size would have a high poten-tial barrier at the grain boundary; thus, it has a large bandgap. An increase of the band gap with a decrease of the grainsize (film thickness) is also associated with the quantum sizeeffect, which is reported by Lee et al. [24] and Ubale et al.[25].

3.2. Photocatalytic Activity of Hematite Thin Films

3.2.1. Time Effect of Photocatalysis. The changes in the MBconcentration as a function of time in the presence of visible

light for hematite thin films (1H, 2H, and 4H) are shown inFigure 6(a). The degradation efficiency is higher during theearly stages of the reaction and decreases with time until theend of the reaction. It can be found that the photocatalyticability increases with an increase of film thickness. The pho-tocatalytic efficiency at the reaction time of 60 min is 95%,99%, and 99% for the films of 1H, 2H, and 4H, respectively.

3.2.2. Photocatalytic Kinetics. The kinetics of the degradationreaction is investigated by plotting the variations in MB con-centration with time (Figure 6(b)). A linear section is obtain-ed within the reaction time. The obtained values of the rateconstant (k′) of the photocatalytic reactions are summarizedin Table 1. The results reveal that the photocatalytic degra-dation of MB dye obeys the rule of the pseudo-first-orderkinetic reaction. Moreover, the corresponding rate constantsare 0.052, 0.055, and 0.068 per min with the increasing filmthickness. Note that these radicals initially lead to the gene-ration of free electrons and holes in the conduction andvalence bands of the solid. They also lead to the formation ofsurface excited states that subsequently react with the surfacemolecules of the reagents and then accelerate the decompo-sition reaction rate as the film thickness of hematite filmsincreases. The reason for a better photocatalytic ability of


0 10 20 30 40 50 60

0

0.2

0.4

0.6

0.8

1

1H2H4H

Ct/C

0

Time (min)

(a)

1H2H4H

0 10 20 30 40 50 60Time (min)

0

2

4

ln(C

t/C

0)

(b)

Figure 6: (a) Plot of photocatalytic efficiency versus the degradation time for the hematite films; (b) plot of log C/C0 versus irradiation timefor hematite films.

the hematite film with a thicker film thickness is a large sur-face roughness and a small band gap. A large surface rough-ness provides a large number of reaction sites for photocatal-ysis; a small band gap generates more electron-hole pairsunder light irradiation. Consequently, a thicker hematite filmhas a better photocatalytic activity.

Previous studies have shown that the photocatalytic ratesconstant of the nanohematite and Fe2O3/RR (0.02 M H2O2)are 0.0026 and 0.0029 min−1, respectively [26, 27]. Moreover,the photocatalytic efficiency at the reaction time of 60 minfor Fe2O3 thin film, TiO2/Fe2O3 thin film, α-Fe2O3/SnO2

glass thin film, and Fe2O3 nanorod thin film is 30%, 15%,8%, and 0.3% [28–30]. The detailed comparisons of photo-catalysts are listed in Table 2, which indicates that the pho-tocatalytic activity of hematite thin films is competitive withthe reported literatures. Therefore, the hematite thin film canact as a good photocatalyst under visible-light excitations,and it is able to successfully utilize solar energy for perform-ing photocatalysis.

4. Conclusion

In this study, it is observed that hematite thin films withdifferent film thicknesses can be fabricated by the rf magnet-ron sputtering deposition. The FE-SEM images show that thegrain size and the surface roughness increase with an increasein the film thickness. The band gap decreases with the in-creasing grain size (film thickness) because of the potentialbarrier at the grain boundary. The result of the photocatalyticdegradation of MB reveals that all the films have a good pho-tocatalytic ability; a thicker hematite film, with a larger grainsize, has a better photocatalytic efficiency because of a smallerband bap and a rougher surface. We believe that with contin-ued development, this type of thin film will have the potentialto be employed in environmental remediation applications.

Acknowledgment

The authors would like to thank the National Science Councilfor the financial support.

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Research Article

Synthesis, Property Characterization, and Photocatalytic Activityof Novel Visible Light-Responsive Photocatalyst Fe2BiSbO7

Jingfei Luan and Zhitian Hu

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, China

Correspondence should be addressed to Jingfei Luan, [email protected]



Copyright © 2012 J. Luan and Z. Hu. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Fe2BiSbO7 was synthesized by a solid-state reaction method for the first time. The structural and photocatalytic properties ofFe2BiSbO7 have been characterized. The results showed that Fe2BiSbO7 was crystallized with the pyrochlore-type structure,cubic crystal system, and space group Fd3m. The lattice parameter for Fe2BiSbO7 was a = 10.410297 A. The photocatalyticdegradation of methylene blue (MB) was realized under visible light irradiation with Fe2BiSbO7 as catalyst. Fe2BiSbO7 ownedhigher catalytic activity compared with Bi2InTaO7 or pure TiO2 or N-doped TiO2 for photocatalytic degradation of MB. Thephotocatalytic degradation of MB with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 followed the first-order reaction kinetics, andthe first-order rate constant was 0.01189, 0.00275, or 0.00333 min−1. After visible light irradiation for 230 min with Fe2BiSbO7,complete removal and mineralization of MB was observed. The reduction of the total organic carbon, the formation of inorganicproducts, SO4

2− and NO3−, and the evolution of CO2 revealed the continuous mineralization of MB during the photocatalytic

process. The photocatalytic degradation pathway of MB was obtained. Fe2BiSbO7/(visible light) photocatalysis system was foundto be suitable for textile industry wastewater treatment.

1. Introduction

Dye effluents from textile industries and photographicindustries are becoming a serious environmental problembecause of their toxicity, unacceptable color, high chemicaloxygen demand content, and nonbiological degradation [1].Many conventional methods have been proposed to treatindustrial effluents, but each method has its shortcomings[1–7]. In recent years, the photocatalytic degradation pro-cesses have been widely applied as techniques of destructionof organic pollutants in wastewater and effluents, especiallyfor degrading dyes [1, 7–21]. However, among various dyes,methylene blue (MB) dye was difficult to be degraded andwas often utilized as a model dye contaminant to estimate theactivity of a photocatalyst under both ultraviolet light irradi-ation [18, 19, 22] and visible light irradiation [20, 21, 23, 24].There were many reports about the photodegradation of MB.Unfortunately, most of these reports were carried out underUV light irradiation. Up to now, there were only few reportsof MB dye degradation under visible light irradiation such asthe research by Asahi et al. with a reduced TiOx(TiO2−xNx)

as catalyst and the research by Tang et al. and Cui et al. withPt-TiO2 as photocatalyst [21, 24]. Zhang [25] utilized N-doped TiO2 as catalyst to degrade MB under visible lightirradiation and found that the removal ratio of MB wasonly 35% after 180 min. It is known that ultraviolet lightonly occupies 4% of the solar energy. For this reason, manyendeavors should be taken up to develop new visible light-responsive photocatalysts which are capable of utilizing morevisible light, which accounts for about 43% of the solarenergy. Therefore, it is urgent to develop novel visible light-responsive photocatalysts.

With the development of investigation of photocatalysisprocess, investigators also paid much attention to researchingand developing novel photocatalysts [26–30]. Currently,TiO2 was the most common photocatalyst, however, TiO2

could not be utilized in the visible light region and could onlydegrade RhB under ultraviolet light irradiation which wasa restrained factor for photocatalysis technology with TiO2

as catalyst. Therefore, some efficient catalysts which couldgenerate electron-hole pairs under visible light irradiationshould be developed. Fortunately, A2B2O7 compounds were


often considered to own photocatalytic properties undervisible light irradiation. In our previous work [31], we havefound that Bi2InTaO7 was crystallized with the pyrochlore-type structure and acted as a photocatalyst under visible lightirradiation and seemed to have potential for improvementof photocatalytic activity upon modification of its structure.According to the above analysis, we could assume thatsubstitution of Ta5+ by Sb5+, substitution of Bi3+ by Fe3+, andsubstitution of In3+ by Bi3+ in Bi2InTaO7 might increase car-riers concentration. As a result, a change and improvementof the electrical transportation and photophysical propertiescould be found in the novel Fe2BiSbO7 compound whichmight own advanced photocatalytic properties.

Fe2BiSbO7 has never been produced before and thedata about its structural and photophysical properties suchas space group and lattice constants have not been foundpreviously. In addition, the photocatalytic properties ofFe2BiSbO7 have not been studied by other investigators. Themolecular composition of Fe2BiSbO7 was very similar withother A2B2O7 compounds. Thus the resemblance suggestedthat Fe2BiSbO7 might possess photocatalytic propertiesunder visible light irradiation, which was similar with thoseother members in A2B2O7 family. Fe2BiSbO7 also seemedto own potential for improvement of photocatalytic activityupon modification of its structure because it had been provedthat a slight modification of a semiconductor structure willresult in a remarkable change within photocatalytic proper-ties [21]. In this paper, Fe2BiSbO7 was prepared for the firsttime by the solid-state reaction method and the structureand photocatalytic properties of Fe2BiSbO7 were investigatedin detail. The photocatalytic degradation of MB undervisible light irradiation was also performed to evaluate thephotocatalytic activity of Fe2BiSbO7. A comparison amongthe photocatalytic properties of Fe2BiSbO7, Bi2InTaO7, andN-doped TiO2 was achieved in order to elucidate the rela-tionship between the structure and photocatalytic activity ofFe2BiSbO7.

2. Experimental

2.1. Synthesis of Fe2BiSbO7 and N-Doped TiO2. Fe2BiSbO7

powder was first synthesized by the solid-state reactionmethod. Fe2O3, Bi2O3, and Sb2O5 with the purity of 99.99%were utilized as raw materials which were purchased fromSinopharm Group Chemical Reagent Co. (Shanghai, China)and used without further purification. All powders weredried at 200◦C for 4 h before synthesis. In order to synthesizeFe2BiSbO7, the precursors were stoichiometrically mixed ina quartz mortar, subsequently pressed into small columns,and put into an alumina crucible (Shenyang Crucible Co.,Ltd., China). Finally, calcination was carried out at 1020◦Cfor 25 h in an electric furnace (KSL 1700X, Hefei KejingMaterials Technology Co., Ltd., China). Similarly, Bi2InTaO7

was synthesized by calcination at 1050◦C for 46 h. Aftersintering and grounding within a quartz mortar, ultrafineFe2BiSbO7 powder was fabricated. Nitrogen-doped titania(N-doped TiO2) catalyst with tetrabutyl titanate as a tita-nium precursor was prepared via the sol-gel method at

room temperature. The procedure was as follows: 17 mLtetrabutyl titanate and 40 mL absolute ethyl alcohol weremixed as solution a, subsequently solution a was addeddropwise under vigorous stirring into the solution b thatcontained 40 mL absolute ethyl alcohol, 10 mL glacial aceticacid, and 5 mL double distilled water to form transparent col-loidal suspension c. Subsequently aqua ammonia with N/Tiproportion of 8 mol% was added into the resulting transpar-ent colloidal suspension under vigorous stirring conditionand kept stirring for 1 h. Finally, the xerogel was formed afterbeing aged for 2 days. The xerogel was grounded into powderwhich was calcined at 500◦C for 2 h. Finally, above powderwas grounded in agate mortar and screened by shaker toobtain N-doped TiO2 powders.

2.2. Characterization of Fe2BiSbO7. The crystalline phase ofFe2BiSbO7 was analyzed by X-ray diffractometer (D/MAX-RB, Rigaku Corporation, Japan) with CuKα radiation (λ =1.54056). The patterns were collected at 295 K with a step-scan procedure in the range of 2θ = 10− 95◦. The step inter-val was 0.02◦ and the time per step was 1 s. The acceleratingvoltage and applied current were 40 kV and 40 mA, respec-tively. The chemical composition of the compound wasdetermined by scanning electron microscope-X-ray energydispersion spectrum (SEM-EDS, LEO 1530VP, LEO Cor-poration, Germany), X-ray fluorescence spectrometer (XFS,ARL-9800, ARL Corporation, Switzerland), and X-ray pho-toelectron spectroscopy (XPS, ESCALABMK-2, VG ScientificLtd., UK). The particle morphology of Fe2BiSbO7 wasobserved by transmission electron microscope (Tecnal F20S-Twin, FEI Corporation, USA). The Fe3+ content, Bi3+

content, Sb5+ content, and O2− content of Fe2BiSbO7 and thevalence state of elements were also analyzed by X-ray pho-toelectron spectroscopy (XPS). The chemical compositionwithin the depth profile of Fe2BiSbO7 was examined by theargon ion denudation method when X-ray photoelectronspectroscopy was used. UV-visible diffuse reflectance spec-trum of Fe2BiSbO7 was measured with a Shimadzu UV-2550 UV-Visible spectrometer, and BaSO4 was used as thereference material. The surface areas of Fe2BiSbO7 and N-doped TiO2 were determined by the Brunauer-Emmett-Teller (BET) method (MS-21, Quantachrome InstrumentsCorporation, USA) with N2 adsorption at liquid nitrogentemperature. The particle sizes of the photocatalysts weremeasured by Malvern’s mastersize-2000 particle size analyzer(Malvern Instruments Ltd., UK).

2.3. Photocatalytic Activity Tests. The photocatalytic activ-ity of Fe2BiSbO7 was evaluated with methylene blue(C16H18ClN3S) (Tianjin Bodi Chemical Co., Ltd., China) as amodel material. The photoreaction was carried out in a pho-tochemical reaction apparatus (Nanjing Xujiang MachinePlant, China). The internal structure of the reaction appa-ratus is as follows: the lamp is put into a quartz hydrazinewhich is a hollow structure and located in the middle ofthe reactor. The recycling water through the reactor main-tains a near constant reaction temperature (20◦C) and thesolution was continuously stirred and aerated. Twelve holes


which are used to put quartz tubes evenly distribute aroundthe lamp and the distance between the lamp and each holeis equal. Under the condition of magnetic stirring, the photo-catalyst within the MB solution is in the state of suspension.In this paper, the photocatalytic degradation of the MBsolution was performed with 0.3 g Fe2BiSbO7 in 300 mL0.025 mM MB aqueous solution in quartz tubes with 500 WXenon lamp (400 nm < λ < 800 nm) as visible-light source.Prior to visible light irradiation, the suspensions whichcontained the catalyst and MB dye were magnetically stirredin the dark for 45 min to ensure establishment of an adsorp-tion/desorption equilibrium among Fe2BiSbO7, the MB dye,and atmospheric oxygen. During visible light illumination,the suspension was stirred at 500 rpm and the initial pHvalue of the MB solution was 7.0 without pH adjustment inthe reaction process. The above experiments were performedunder oxygen-saturation conditions ([O2]sat = 1.02 ×10−3 M). One of the quartz tubes was taken out from thephotochemical reaction apparatus at various time intervals.The suspension was filtered through 0.22 μm membranefilters. The filtrate was subsequently analyzed by a ShimadzuUV-2450 UV-Visible spectrometer with the detecting wave-length at 665 nm. The experimental error was found to bewithin ±2.2%.

The incident photon flux Io measured by a radiometer(Model FZ-A, Photoelectric Instrument Factory BeijingNormal University, China) was determined to be 4.76×10−6

Einstein L−1 s−1 under visible light irradiation (wavelengthrange of 400–700 nm). The incident photon flux on thephotoreactor was varied by adjusting the distance betweenthe photoreactor and the Xe arc lamp. The pH valueadjustment was not carried out, and the initial pH value was7.0. The inorganic products which were obtained from MBdegradation were analyzed by ion chromatograph (DX-300,Dionex Corporation, USA). The identification of MB and thedegradation intermediate products of MB were performed bygas chromatograph—mass spectrometer (GC-MS, HP 6890Series Gas Chromatograph, AT column, 20.3 m × 0.32 mm,ID of 0.25 μm) which operated at 320◦C and was connectedto HP 5973 mass selective detector and a flame ionizationdetector with H2 as the carried gas. The intermediate prod-ucts of MB were also measured by liquid chromatograph—mass spectrometer (LC-MS, Thermo Quest LCQ Duo, USA,Beta Basic-C18 HPLC column: 150 × 2.1 mm, ID of 5 μm,Finnigan, Thermo, USA). Here, 20 μL of postphotocatalysissolution was injected automatically into the LC-MS system.The fluent contained 60% methanol and 40% water, andthe flow rate was 0.2 mL min−1. MS conditions included anelectrospray ionization interface, a capillary temperature of27◦C with a voltage of 19.00 V, a spray voltage of 5000 V, anda constant sheath gas flow rate. The spectrum was acquired inthe negative ion scan mode and the mz−1 range swept from50 to 600. Evolution of CO2 was analyzed with an intersmatIGC120-MB gas chromatograph equipped with a porapackQ column (3 m in length and an inner diameter of 0.25 in.),which was connected to a catharometer detector. The totalorganic carbon (TOC) concentration was determined witha TOC analyzer (TOC-5000, Shimadzu Corporation, Japan).

The photonic efficiency was calculated according to the fol-lowing equation [32, 33]:

ϕ = R

Io, (1)

where ϕ is the photonic efficiency (%), R is the rate of MBdegradation (Mol L−1 s−1), and Io is the incident photon flux(Einstein L−1s−1).


3.1. Crystal Structure of Fe2BiSbO7. Figure 1 presents TEMimage and the selected area electron diffraction pattern ofFe2BiSbO7. The TEM image of Fe2BiSbO7 showed that themorphology of the Fe2BiSbO7 particle was very similar andregular. It could be seen that the Fe2BiSbO7 particles crystal-lized well and the mean particle diameter of Fe2BiSbO7 wasabout 150 nm. SEM-EDS spectrum of Fe2BiSbO7 revealedthat Fe2BiSbO7 was pure phase without any other impureelements and Fe2BiSbO7 displayed the presence of iron,bismuth, antimony, and oxygen. It could be seen fromFigure 1 that Fe2BiSbO7 was crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m.The lattice parameter for Fe2BiSbO7 was proved to bea = 10.410297 A. According to the calculation results fromFigure 1, the (h k l) value for the main peaks of Fe2BiSbO7

could be found and indexed.Full-profile structure refinements of the collected X-ray

diffraction data of Fe2BiSbO7 were obtained by the RIETAN[34] program, which was based on Pawley analysis. Therefinement results of Fe2BiSbO7 are shown in Figure 2. Theatomic coordinates and structural parameters of Fe2BiSbO7

are listed in Table 1. The results of the final refinementfor Fe2BiSbO7 indicated a good agreement between theobserved and calculated intensities in a pyrochlore-typestructure and cubic crystal system with space group Fd3m.Our XRD results also showed that Fe2BiSbO7 and Bi2InTaO7

were crystallized in the same structure, and 2 theta anglesof each reflection of Fe2BiSbO7 changed with Fe3+ beingreplaced by Bi3+, Bi3+ being replaced by In3+, and Sb5+ beingreplaced by Ta5+. Bi2InTaO7 was also crystallized with a cubicstructure by space group Fd3m and the lattice parameterof Bi2InTaO7 was a = 10.746410 A. The lattice parameterof Fe2BiSbO7 was a = 10.410297 A, which indicated thatthe lattice parameter of Fe2BiSbO7 decreased compared withthe lattice parameter of Bi2InTaO7 because the In3+ ionicradii (0.92 A) or the Bi3+ ionic radii (1.17 A) was larger thanthe Fe3+ ionic radii (0.78 A). The outcome of refinementfor Fe2BiSbO7 generated the unweighted R factor, RP =11.56% with space group Fd3m. Zou et al. [35] refined thecrystal structure of Bi2InNbO7 and obtained a large R factorfor Bi2InNbO7, which was ascribed to a slightly modifiedstructure model for Bi2InNbO7. Based on the high purity ofthe precursors which were used in this study and the EDSresults that did not trace any other elements, it was unlikelythat the observed space groups originated from the presenceof impurities. Therefore, it was suggested that the slightlyhigh R factor for Fe2BiSbO7 was due to a slightly modified


200 nm

(a)

51 nm

(b)

Figure 1: TEM image of Fe2BiSbO7 (a) and the selected area electron diffraction pattern of Fe2BiSbO7 (b).

10 20 30 40 50 60 70 80 90

(311

)

(840

)(622

)(4

44)

(622

)

(331

)

(440

)

(111

)

(400

)

(222

)

ExperimentSimulation

DifferenceObserved reflections

Inte

nsi

ty(a

.u.)

2θ (deg)

Figure 2: Pawley refinements of XRD data for novel photocatalystFe2BiSbO7 prepared by the solid state reaction method at 1020◦C.The solid line represents experimental X-ray diffraction pattern(—). The dot line represents simulation X-ray diffraction pattern(. . .). The tic marks represent reflection positions. A difference(observed-calculated) profile is shown beneath.

Table 1: Atomic coordinates and structural parameters ofFe2BiSbO7 prepared by the solid state reaction method.

Atom x y z Occupation factor

Fe 0.00000 0.00000 0.00000 1.0

Bi 0.50000 0.50000 0.50000 0.5

Sb 0.50000 0.50000 0.50000 0.5

O(1) −0.14538 0.12500 0.12500 1.0

O(2) 0.12500 0.12500 0.12500 1.0

structure model for Fe2BiSbO7. It should be emphasizedthat the defects or the disorder/order of a fraction of theatoms could result in the change of structures, includingdifferent bond-distance distributions, thermal displacementparameters, and/or occupation factors for some of the atoms.

In order to reveal the surface chemical compositions andthe valence states of various elements of Fe2BiSbO7, the X-ray

Table 2: Binding energies (BE) for key elements from Fe2BiSbO7.

CompoundsBi4f7/2

BE (eV)Sb3d5/2

BE (eV)Fe2p3/2

BE (eV)O1s

BE (eV)

Fe2BiSbO7 155.80 530.88 708.10 527.00

photoelectron spectrum of Fe2BiSbO7 for detecting Fe, Bi,Sb, and O was performed. The full XPS spectrum confirmedthat the prepared Fe2BiSbO7 contained elements of Fe, Bi,Sb, and O, which was consistent with the results of SEM-EDS. The different elemental peaks which are correspondingto definite binding energies are given in Table 2. The resultsillustrated that the oxidation states of Fe, Bi, Sb, and Oions from Fe2BiSbO7 were +3, +3, +5, and −2, respectively.Besides, the average atomic ratio of Fe : Bi : Sb : O forFe2BiSbO7 was 2.00 : 0.97 : 1.01 : 6.98 based on our XPS,SEM-EDS and XFS results. Accordingly, it could be deducedthat the resulting material was highly pure under ourpreparation conditions. It was remarkable that there were notany shoulders and widening in the XPS peaks of Fe2BiSbO7,which suggested the absence of any other phases.

3.2. Photocatalytic Properties. Generally, the direct absorp-tion of band-gap photons would result in the generationof electron-hole pairs within Fe2BiSbO7, subsequently; thecharge carriers began to diffuse to the surface of Fe2BiSbO7.As a result, the photocatalytic activity for decomposingorganic compounds with Fe2BiSbO7 might be enhanced.Changes in the UV-Vis spectrum of MB upon exposure tovisible light (λ > 400 nm) irradiation with the presenceof Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 indicated thatFe2BiSbO7, Bi2InTaO7, or N-doped TiO2 could photode-grade MB effectively under visible light irradiation. Figure 3shows the photocatalytic degradation of methylene blueunder visible light irradiation in the presence of Fe2BiSbO7,Bi2InTaO7, pure TiO2, N-doped TiO2, as well as in theabsence of a photocatalyst. The results showed that a reduc-tion in typical MB peaks at 665 nm and 614.5 nm was clearlynoticed and the photodegradation rate of MB was about1.980 × 10−9 mol L−1 s−1 and the photonic efficiency wasestimated to be 0.0416% (λ = 420 nm) with Fe2BiSbO7


as catalyst. Similarly, the photodegradation rate of MB wasabout 1.001 × 10−9 mol L−1s−1and the photonic efficiencywas estimated to be 0.0210% (λ = 420 nm) with N-dopedTiO2 as catalyst. Moreover, the photodegradation rate ofMB was about 0.891 × 10−9 mol L−1 s−1 and the photonicefficiency was estimated to be 0.0187% (λ = 420 nm) withBi2InTaO7 as catalyst. By contrast, the photodegradation rateof MB within 200 min of visible light irradiation was only0.8338 × 10−9 mol L−1 s−1 and the photonic efficiency wasestimated to be 0.0175% (λ = 420 nm) with pure TiO2 as cat-alyst. The photodegradation rate of MB was about 0.6830 ×10−9 mol L−1 s−1 and the photonic efficiency was estimated tobe 0.0143% (λ = 420 nm) in the absence of a photocatalyst.The results showed that the photodegradation rate of MBand the photonic efficiency with Fe2BiSbO7 as catalyst wereboth higher than those with N-doped TiO2 or Bi2InTaO7,or pure TiO2 as catalyst. The photodegradation rate of MBand the photonic efficiency with N-doped TiO2 as catalystwere both higher than those with Bi2InTaO7 or pure TiO2 ascatalyst. The photodegradation rate of MB and the photonicefficiency with Bi2InTaO7 as catalyst were both higher thanthose with pure TiO2 or the absence of a photocatalyst. Thephotodegradation rate of MB and the photonic efficiencywith pure TiO2 as catalyst were both higher than those withthe absence of a photocatalyst. When Fe2BiSbO7, N-dopedTiO2, Bi2InTaO7 or pure TiO2 was used as photocatalyst,the photodegradation conversion rate of MB was 96.59%,48.05%, 42.76%, and 40.02% after visible light irradiation for200 min, respectively. Moreover, the photodegradation con-version rate of MB was 32.78% after visible light irradiationfor 200 min with the absence of a photocatalyst because ofthe MB dye photosensitization effect [36]. After visible lightirradiation for 230 min with Fe2BiSbO7 as catalyst, completeremoval of MB was observed and the complete disappearanceof the absorption peaks which presented the absolute colorchange from deep blue into colorless solution occurred.Based on above results, the photocatalytic degradationactivity of Fe2BiSbO7 was much higher than that of N-doped TiO2, Bi2InTaO7, or pure TiO2. Meanwhile, N-dopedTiO2 showed higher photocatalytic degradation activity forMB photodegradation compared with Bi2InTaO7 or pureTiO2. Bi2InTaO7 showed higher photocatalytic degradationactivity for MB photodegradation compared with pure TiO2.Pure TiO2 was more suitable for MB photodegradationthan the absence of a photocatalyst. The photocatalyticproperty of novel Fe2BiSbO7 under visible light irradiationwas amazing compared with that of N-doped TiO2 or pureTiO2, and the main reason was that the specific surfacearea of Fe2BiSbO7 was much smaller than that of N-doped TiO2 or pure TiO2. BET isotherm measurements ofFe2BiSbO7, N-doped TiO2, and pure TiO2 provided a specificsurface area of 2.78 m2 g−1, 45.53 m2 g−1, and 46.24 m2 g−1,respectively, which indicated that the photocatalytic degra-dation activity of Fe2BiSbO7 could be greatly improved byenhancing the specific surface area of Fe2BiSbO7.

Figure 4 shows the change of TOC during photocatalyticdegradation of MB with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation. TheTOC measurements revealed the disappearance of organic

400 450 500 550 600 650 700 750 800

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Abs

orba

nce

(a.u

.)

Time (min)

0 min30 min60 min90 min

120 min150 min180 min200 min

(a)

0 40 80 120 160 200

0

0.009

0.018

0.027M

Bco

nce

ntr

atio

n(m

mol

L−1

)

Time (min)

Fe2BiSbO7

Absence of catalystN-doped TiO2

Pure TiO2

Bi2InTaO7

(b)

Figure 3: The absorbance pattern of methylene blue photocatalyt-ically degraded by Fe2BiSbO7 (a), and photocatalytic degradationof methylene blue under visible light irradiation in the presence ofFe2BiSbO7, Bi2InTaO7, pure TiO2, N-doped TiO2, as well as in theabsence of a photocatalyst (b).

carbon when the MB solution which contained Fe2BiSbO7,Bi2InTaO7, or N-doped TiO2 was exposed under visiblelight irradiation. The results showed that 89.51%, 46.77%,or 41.71% of TOC decrease was obtained after visible lightirradiation for 200 min when Fe2BiSbO7, or N-doped TiO2

or Bi2InTaO7 was used as photocatalyst. Consequently, aftervisible light irradiation for 230 min with Fe2BiSbO7 as cat-alyst, the entire mineralization of MB was observed becauseof 100% TOC removal. The turnover number which repre-sented the ratio between the total amount of evolved gas anddissipative catalyst was calculated to be more than 0.204 for


0 50 100 150 200

0

0.09

0.18

0.27

0.36

0.45

TO

C(m

mol

L−1)

Time (min)

Fe BiSbON-doped TiO2

72

Bi2InTaO7

Figure 4: Disappearance of total organic carbon (TOC) duringphotocatalytic degradation of methylene blue with Fe2BiSbO7,Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradi-ation.

0 50 100 150 200

0

0.03

0.06

0.09

0.12

CO

2(m

mol

)

Time (min)


72

Bi2InTaO7

Figure 5: CO2 production kinetics during the photocatalyticdegradation of methylene blue with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation.

Fe2BiSbO7 after 200 min of reaction time under visible lightirradiation and this turnover number was evident to provethat this reaction occurred catalytically. Similarly, when thelight was turned off in this experiment, the stop of thisreaction showed the obvious light response.

Figure 5 shows the amount of CO2 which was yield-ed during the photodegradation of MB with Fe2BiSbO7,Bi2InTaO7 or N-doped TiO2 as catalyst under visible light

irradiation. The amount of CO2 increased gradually withincreasing reaction time when MB was photodegraded byFe2BiSbO7, Bi2InTaO7 or N-doped TiO2. At the same time,after 200 min visible light irradiation, the CO2 productionof 0.11063 mmol with Fe2BiSbO7 as catalyst was higherthan the CO2 production of 0.05600 mmol with N-dopedTiO2 as catalyst. Meanwhile, after visible light irradiationfor 200 min, the CO2 production of 0.05600 mmol with N-doped TiO2 as catalyst was higher than the CO2 productionof 0.04934 mmol with Bi2InTaO7 as catalyst.

The first-order nature of the photocatalytic degradationkinetics with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as cat-alyst is clearly demonstrated in Figure 6. The results showeda linear correlation between ln(C/Co) (or ln(TOC/TOCo))and the irradiation time for the photocatalytic degradationof MB under visible light irradiation with the presence ofFe2BiSbO7, Bi2InTaO7, or N-doped TiO2. Here, C repre-sented the MB concentration at time t, Co represented theinitial MB concentration, TOC represented the total organiccarbon concentration at time t, and TOCo represented theinitial total organic carbon concentration. According toFigure 6, the first-order rate constant kC of MB concen-tration was estimated to be 0.01189 min−1 with Fe2BiSbO7

as catalyst, 0.00275 min−1 with Bi2InTaO7 as catalyst, and0.00333 min−1 with N-doped TiO2 as catalyst. The differentvalue of kC indicated that Fe2BiSbO7 was more suitable forthe photocatalytic degradation of MB under visible lightirradiation than N-doped TiO2 or Bi2InTaO7. Meanwhile N-doped TiO2 was more suitable for the photocatalytic degra-dation of MB under visible light irradiation than Bi2InTaO7.Figure 6 also showed that the first-order rate constant KTOC

of TOC was estimated to be 0.01101 min−1 with Fe2BiSbO7

as catalyst, 0.00275 min−1 with N-doped TiO2 as catalyst, and0.00259 min−1 with Bi2InTaO7 as catalyst, which indicatedthat the photodegradation intermediate products of MBprobably appeared during the photocatalytic degradation ofMB under visible light irradiation because of the differentvalue between kC and KTOC. It could also be seen fromFigure 6 that Fe2BiSbO7 showed higher mineralization effi-ciency for MB degradation compared with N-doped TiO2 orBi2InTaO7. At the same time, N-doped TiO2 showed highermineralization efficiency for MB degradation compared withBi2InTaO7.

Some inorganic ions such as NH4+, NO3

−, and SO42−

were formed in parallel as the end products of nitrogen andsulfur atoms which existed in MB. Figures 7 and 8 showedthe concentration variation of SO4

2− and NO3− during pho-

tocatalytic degradation of MB with Fe2BiSbO7, Bi2InTaO7,or N-doped TiO2 as catalyst under visible light irradiation.The results showed that the concentration of NO3

− or SO42−

increased gradually with increasing reaction time when MBwas photodegraded by Fe2BiSbO7, Bi2InTaO7, or N-dopedTiO2. Monitoring the presence of ions in the solutionrevealed that the SO4

2− ion concentration was 0.01849 mM,0.00924 mM, or 0.00757 mM with Fe2BiSbO7, N-dopedTiO2, or Bi2InTaO7 as catalyst after visible light irradiationfor 200 min, indicating that 63.22%, 36.94%, or 30.28%of sulfur from MB was converted into sulfate ions withFe2BiSbO7, N-doped TiO2, or Bi2InTaO7 as catalyst after


−3.5

−3

−2.5

−2

−1.5

−1

−0.5

0

N-doped TiO2, K = 0.00333Fe2BiSbO7, K = 0.01189Bi2InTaO7, K = 0.00275

0 50 100 150 200

Time (min)

Ln( C

/C) 0

(a)

N-doped TiO2, K = 0.00275Fe2BiSbO7, K = 0.01101Bi2InTaO7, K = 0.00259

0 50 100 150 200

Time (min)

−3

−2.5

−2

0

−1.5

−1

−0.5

Ln(T

OC

/TO

C) 0

(b)

Figure 6: Observed first-order kinetic plots for the photocatalyticdegradation of methylene blue with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation.

visible light irradiation for 200 min. It could be seen fromFigure 8 that the NO3

− ion concentration was 0.05258 mM,0.0351 mM, or 0.02232 mM with Fe2BiSbO7, N-doped TiO2,or Bi2InTaO7 as catalyst after visible light irradiation for200 min, which indicated that 70.11%, 46.80%, or 29.76%of nitrogen from MB was converted into nitrate ions withFe2BiSbO7, N-doped TiO2, or Bi2InTaO7 as catalyst aftervisible light irradiation for 200 min. The sulfur was firsthydrolytically removed, and subsequently was oxidized andtransformed into SO4

2− . At the same time, nitrogen atoms

0

0.004

0.008

0.012

0.016

0.02

0 50 100 150 200

Time (min)

SO−2 4

con

cen

trat

ion

(mm

olL−1

)


72

Bi2InTaO7

Figure 7: The concentration variation of SO42− during photocat-

alytic degradation of methylene blue with Fe2BiSbO7, Bi2InTaO7,or N-doped TiO2 as catalyst under visible light irradiation.

0 50 100 150 200

Time (min)


72

Bi2InTaO7

0

0.02

0.04

0.06

NO

− 3co

nce

ntr

atio

n(m

mol

L−1

)

Figure 8: The concentration variation of NO3− during photocat-

alytic degradation of methylene blue with Fe2BiSbO7, Bi2InTaO7,or N-doped TiO2 as catalyst under visible light irradiation.

in the −3 oxidation state produced NH4+ cations that

subsequently were oxidized into NO3− ions. As expected, the

formation kinetics with Fe2BiSbO7 was significantly fasterthan that of N-doped TiO2 or Bi2InTaO7 by using the sameamount of photocatalyst. Moreover, the formation kineticswith N-doped TiO2 was faster than that of Bi2InTaO7 byusing the same amount of photocatalyst. It was noteworthy


(

(

H3C)2N

H3C)2N

(H3C)2N

Cl−

Cl−

Cl−

N(CH3)2

N(CH3)2H3C

S

+

NH2

NH2

NH2NH2

NH2

Cl−

Cl−

+ Cl−

Cl−

NH2

HOC

CH3

SO3H

CO2 H

H

2O N

N

H +

+

S+

4 N

N

N

Cl−H3C

H2N

H

N S+

N

S+

N

S+

N

S+

N

O −3 SO4

2−

Demethylation

Deamination

Ring breaking

SO3H OH

+ Cl−

N

Figure 9: Suggested photocatalytic degradation pathway scheme for methylene blue under visible light irradiation in the presence ofFe2BiSbO7.

that the amount of SO42− ions which was released into the

solution was lower than the amount of SO42− which should

come from stoichiometry. One possible reason could be a lossof sulfur-containing volatile compounds such as SO2. Thesecond possible reason was a partially irreversible adsorptionof some SO4

2− ions on the surface of the photocatalyst whichhad been observed by Lachheb et al. by titanium dioxide[37]. Regardless, whether the sulfate ions were adsorbedirreversibly on the surface or not, it was important to stressthat the evidence for restrained photocatalytic activity wasnot noticed.

The photodegradation intermediate products of MBin our experiment were identified as azure B, azure A,azure C, thionine, phenothiazine, leucomethylene blue, N,N-dimethylp-phenylenediamine, phenol, and aniline. Accord-ing to the intermediate products which were found in thiswork and the observed appearance time of other inter-mediate products, a possible photocatalytic degradation

pathway for MB was proposed. Figure 9 shows the suggestedphotocatalytic degradation pathway scheme for methyleneblue under visible light irradiation in the presence ofFe2BiSbO7. The molecule of MB was converted into smallorganic species, which were subsequently mineralized intoinorganic products such as SO4

2− ions, NO3− ions, CO2, and

water ultimately.

3.3. Photocatalytic Degradation Mechanism. The action spec-tra of MB degradation with Fe2BiSbO7 as catalyst wereobserved under visible light irradiation. A clear photonicefficiency (0.0103% at its maximal point) at wavelengthswhich corresponded to sub-Eg energies of the photocatalysts(λ from 375 to 700 nm) was observed for Fe2BiSbO7. Theexistence of photonic efficiency at this region revealed thatphotons are not absorbed by the photocatalysts. In particular,the correlation between the low-energy action spectrum andthe absorption spectrum of MB clearly demonstrated that


any photodegradation results at wavelengths above 545 nmshould be attributed to photosensitization effect by the dyeMB itself (Scheme I).

Scheme I. Consider

MBadsVisible light−−−−−−→MB∗ads

MB∗ads + Fe2BiSbO7 −→ Fe2BiSbO7(e) + MB+ads

Fe2BiSbO7 (e) + O2 −→ Fe2BiSbO7 + ·O2−

(2)

According to the mechanism which was shown in SchemeI, MB which was adsorbed on Fe2BiSbO7 was excited by vis-ible light irradiation. Subsequently, an electron was injectedfrom the excited MB to the conduction band of Fe2BiSbO7

where the electron was scavenged by molecular oxygen.Scheme I explained the results which were obtained withFe2BiSbO7 as catalyst under visible light irradiation, wherethe photocatalyst Fe2BiSbO7 could serve to reduce recombi-nation of photogenerated electrons and holes by scavengingof electrons [38].

Below the wavelength of 545 nm, the situation was dif-ferent. The results of photonic efficiency correlated well withthe absorption spectra of Fe2BiSbO7. These results evidentlyshowed that the mechanism which was responsible for thephotodegradation of MB went through band gap excitationof Fe2BiSbO7. Despite the detailed experiments about theeffect of oxygen and water were not performed, it waslogical to presume that the mechanism in the first step wassimilar to the observed mechanism for Fe2BiSbO7 undersuprabandgap irradiation, namely Scheme II.

Scheme II. Consider

Fe2BiSbO7Visible light−−−−−−→ h+ + e−

e− + O2 −→ ·O2−

h+ + OH− −→ ·OH

(3)

According to first principles calculations, we deducedthat the conduction band of Fe2BiSbO7 was composed ofFe 3d and Sb 5p orbital component, and the valence bandof Fe2BiSbO7 was composed of O 2p and Bi 6s orbitalcomponent. Fe2BiSbO7 could produce electron-hole pairsby absorption of photons directly, and it indicated thatenough energy which was larger than the band gap energy ofFe2BiSbO7 was necessary for the photocatalytic degradationprocess of MB.

Former luminescent studies had shown that the closer theM–O–M bond angle was 180◦, the more delocalized was theexcited state [39], as a result, the charge carriers could movemore easily in the matrix. The mobility of the photoinducedelectrons and holes influenced the photocatalytic activitybecause high diffusivity indicated the enhancement of prob-ability that the photogenerated electrons and holes wouldreach the reactive sites of the catalyst surface. For Fe2BiSbO7,the bond angle of Bi–O–Sb was 119.76◦, which indicatedthat the bond angle of Bi–O–Sb was close to 180◦. Thus,the photocatalytic activity of Fe2BiSbO7 was consequently

higher. The crystal structure and the electronic structureof Fe2BiSbO7 and N-doped TiO2 were totally different. ForFe2BiSbO7, Fe was 3d-block metal element, and Bi was 6p-block metal element, and Sb was 5p-block metal element. Butfor N-doped TiO2, Ti was 3d-block metal element, indicatingthat the photocatalytic activity might be affected by notonly the crystal structure but also the electronic structureof the photocatalysts, as well. In conclusion, the differentphotodegradation effect of MB between Fe2BiSbO7 and N-doped TiO2 could be mainly attributed to the difference oftheir crystalline structures and electronic structures.

The present results indicated that the Fe2BiSbO7-visiblelight photocatalysis system might be regarded as a practicalmethod for treatment of diluted colored wastewater. Thissystem could be utilized for decolorization, purification, anddetoxification of textile, printing, and dyeing industries inthe long-day countries. Meanwhile, this system did not needhigh pressure of oxygen, heating, or any chemical reagents.Much decolorized and detoxified water were flowed from ournew system for treatment, and the results showed that theFe2BiSbO7-visible light photocatalysis system might providea valuable treatment for purifying and reusing coloredaqueous effluents.

4. Conclusions

Fe2BiSbO7 was prepared by the solid-state reaction methodfor the first time. The structural and photocatalytic proper-ties of Fe2BiSbO7 were investigated. XRD results indicatedthat Fe2BiSbO7 was crystallized with the pyrochlore-typestructure, cubic crystal system, and space group Fd3m.The lattice parameter of Fe2BiSbO7 was found to be a =10.410297 A. Photocatalytic decomposition of aqueous MBwas realized under visible light irradiation in the presence ofFe2BiSbO7, Bi2InTaO7, or N-doped TiO2. The results showedthat Fe2BiSbO7 owned higher catalytic activity comparedwith pure TiO2, Bi2InTaO7, or N-doped TiO2 for photo-catalytic degradation of MB under visible light irradiation.The photocatalytic degradation of MB with Fe2BiSbO7,Bi2InTaO7, or N-doped TiO2 as catalyst followed the first-order reaction kinetics, and the first-order rate constant was0.01189 min−1, 0.00275 min−1, or 0.00333 min−1. Completeremoval and mineralization of MB was observed after visiblelight irradiation for 230 min with Fe2BiSbO7 as catalyst.The reduction of the total organic carbon, the formationof inorganic products such as SO4

2− and NO3−, and the

evolution of CO2 revealed the continuous mineralizationof MB during the photocatalytic process. The possiblephotocatalytic degradation pathway of MB was obtainedunder visible light irradiation. Fe2BiSbO7/(visible light)photocatalysis system was found to be suitable for textileindustry wastewater treatment and could be used to solveother environmental chemical pollution problems.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (no. 20877040), by a grant from the


Technological Supporting Foundation of Jiangsu Province(no. BE2009144), by a grant from China-Israel Joint ResearchProgram in Water Technology and Renewable Energy (no.5), by a grant from New Technology and New Methodologyof Pollution Prevention Program from Enviromental Pro-tection Department of Jiangsu Province of China during2010 and 2012 (no. 201001), by a grant from The FourthTechnological Development Scheming (Industry) Programof Suzhou City of China from 2010 (SYG201006), and by agrant from the Fundamental Research Funds for the CentralUniversities.

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Research Article

One-Step Cohydrothermal Synthesis of Nitrogen-DopedTitanium Oxide Nanotubes with Enhanced Visible LightPhotocatalytic Activity

Cheng-Ching Hu,1 Tzu-Chien Hsu,1 and Li-Heng Kao2

1 Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan2 Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Science, Kaohsiung 80778, Taiwan

Correspondence should be addressed to Tzu-Chien Hsu, [email protected]

Received 15 July 2011; Revised 7 September 2011; Accepted 8 September 2011


Copyright © 2012 Cheng-Ching Hu et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Nitrogen-doped TiO2 nanotubes with enhanced visible light photocatalytic activity have been synthesized using commercialtitania P25 as raw material by a facile P25/urea cohydrothermal method. Morphological and microstructural characteristicswere conducted by transmission electron microscopy, powder X-ray diffraction, and nitrogen adsorption/desorption isotherms;chemical identifications were performed using X-ray photoelectron spectroscopy, and the interstitial nitrogen linkage to theTiO2 nanotubes is identified. The photocatalytic activity of nitrogen-doped TiO2 nanotubes, evaluated by the decompositionof rhodamine B dye solution under visible light using UV-vis absorption spectroscopy, is found to exhibit ∼ four times higherthan that of P25 and undoped titanate nanotubes. Factors affecting the photocatalytic activity are analyzed; it is found that thenitrogen content and surface area, rather than the crystallinity, are more crucial in affecting the photocatalytic efficiency of thenitrogen-doped TiO2 nanotubes.

1. Introduction

Titanium oxide (TiO2) is one of the well-known photocat-alysts with many promising properties such as nontoxicity,excellent chemical stability, high photocatalytic activity, highresistance to photocorrosion, and great photostability. Thegood semiconductor characteristics of TiO2 render itselfwilder applications, especially in the photoelectrochemicaldevices. They have been applied to the decomposition ofsome pollutants under light irradiation, such as nitrogenmonoxide in atmosphere and organic pollutants in water.However, applications of pure titania suffer from limitationto only UV light (λ < 380 nm) because of its wide bandgap value of 3.2 eV. Since UV light accounts for only asmall fraction (4∼5%) of the natural sun light as comparedto visible light (45%), any shift in the optical response ofTiO2 from UV to visible spectral range will have a profoundpositive effect on its photocatalytic efficiency [1–4].

There are two basic strategies to overcome the limitationand to improve the efficiency of photocatalysis. One isenlarging the light absorption range of the photocatalyststo enhance the harvesting efficiency of sun light by dopingmetal or nonmetal ions. The second one is the control of themorphology and size of the particles and their distribution,phase composition, and porosity of the photocatalyst. Earlyattempts on the shifting of TiO2 absorption into visible lightregion mainly focus on the doping with transition metals[5–7]. But it suffers from thermally instability, its tendencyto form charge carrier recombination centers [6, 8], and theexpensive ion implantation facilities; all these make metal-doped TiO2 impractical.

Efforts have been made to modify titanium dioxide withnonmetal elements such as boron, carbon, sulfur, fluorine,and nitrogen [2, 4, 9–11] to efficiently extend the photore-sponse from UV to visible light region. Asahi et al. showed aneffective shift to visible light region by doping with nitrogen


[2]. The reason for the improvement of photocatalyticactivity has been often attributed to the decrease of the bandgap, which is due to either mixing the N 2p states with O 2pstates on the top of valence band, or the creation of an ion-induced mid-gap level.

Beside metal and nonmetal doping method, it is wellknown that the performance of TiO2 depends stronglyon its crystallite phase, size, and morphology, since theygive an important influence on the chemical and physicalproperties of TiO2. Kasuga et al. [12, 13] reported that thethermal treatment of TiO2 particles in NaOH resulted in theformation of TiO2 nanotubes with large surface areas. Onthe other hand, one-dimensional nanostructured TiO2, suchas nanotubes and nanofibers, are of particular significancedue to their superior electronic, magnetic, optical, catalytic,and mechanical properties, and their potential applicationsin environmental purification, gas sensors, and solar cell [14–16].

N-doped TiO2 has been prepared by various methodssuch as mechanochemical reaction [17], sputtering [18],ion implantation [19], chemical vapor deposition [20],and sol-gel processing [21, 22]. Most of the preparationmethods adopted NH3 under higher temperatures as thenitrogen source, which did not meet the ever-increasingenvironmental restrictions and energy-saving requirement;other methods such as ion implanter or complex processwere either complicated or expensive [23, 24]. Therefore, itis a worthwhile effort to find a simple and low-temperatureapproach to synthesize N-doped TiO2 nanotubes withoutdestroying their inherent tubular morphology and keep theirsuperior photocatalytic property to extend the applicationof the N-doped TiO2 photocatalysts. In this study, N-dopedTiO2 nanotubes are synthesized via a facile cohydrothermalprocess. This unique process combines the chemical vapordeposition of urea as the nitrogen source and the alkalinehydrothermal treatment of 10 M NaOH at the same time,combining the nitrogen-doping and TiO2 nanotube synthe-sis in a single process. The effects of calcination temperatureand nitrogen content on the microstructure of the nanotubesare investigated in detail. The photocatalytic activity of N-doped TiO2 nanotubes is evaluated by the photocatalyticdegradation of rhodamine B (RB) dye solution under visiblelight irradiation.

2. Experimental

2.1. Preparation of N-Doped TiO2 Nanotubes. Titaniumdioxide used for synthesis of the nanotubes was a commer-cially available P25 TiO2 powder (P25, Acros), consisting ofabout 30% rutile and 70% anatase with a primary particlesize of about 21 nm; it was used without further treatment.Urea was purchased from Soha Co. Ltd. and used as thenitrogen source. All other reagents were of analytical grade,and all the water used was deionized.

TiO2 nanotubes were synthesized by a hydrothermalmethod analogous to the one proposed by Kasuga et al.[12]. 1.5 g P25 TiO2 powder was dispersed in 50 mL NaOHsolution with a concentration of 10 molL−1 and stirred for

30 min. Then, a certain amount of urea was added and stirredfor another 30 min. The solution was heated at 110◦C for24 h under hydrothermal condition in a PTFE-lined stainlesssteel vessel. It was then filtered and washed with 0.1 N HClsolutions until pH < 7, followed by washing with DI waterand drying at 80◦C in air overnight. Samples with differenturea loadings (0, 1.9 g) are designated as TC-0 and TC-1. Sample TC-1 was calcined under various temperaturesranging from 200◦C to 400◦C.

2.2. Characterizations. X-ray diffraction patterns were col-lected on a Siemens D5000 powder diffraction systemequipped with a position-sensitive detector, using Cu Kα (λ =0.154 nm) radiation under 40 kV working voltage and 30 mAworking current. The patterns were collected in the range of2θ = 5–80◦ with a speed of 1.5◦/min in the continuous scanmode. Nitrogen physisorption was performed at −196◦Cwith a Micromeritics ASAP 2010 apparatus. Prior to themeasurements, the powdered samples were degassed for 24 hat 200◦C in vacuum. Surface area, pore volume, and pore sizedistribution were determined with standard BET and BJHmethods, respectively. Samples ultrasonicated and filteredon holey carbon grids were examined by the transmissionelectron microscopy (TEM, JEOL AEM 3010, Tokyo, Japan)operating at 200 kV. The chemical nature of N in TiO2− x Nx

was studied using X-ray photoelectron spectroscopy (XPS)by a Krato Axis Ultra DLD (delay line detector) with Al Kαradiation as the exciting source. All the binding energy wasreferenced to the C 1s peak at 285 eV of the surface carbonfor calibration.

2.3. Photocatalytic Activity. The photocatalytic activity of theas-prepared samples was evaluated by the photodegradationof rhodamine B dye (RB) under a 500 W xenon lamp througha cutoff filter (UVCUT425, λ > 400 nm, Rocoes). The mix-ture containing the N-doped TiO2 nanotubes (50 mg) andRB (7.5 mg/L, 100 mL) was magnetically stirred in a 250 mLbeaker for 1 h in darkness to reach adsorption equilibriumbefore irradiation. At the given time intervals during theirradiation period, samples were taken from the suspensionand immediately centrifuged for 20 min. After recoveringthe catalyst by centrifugation, the absorbance of the clearsolution was measured by using a UV-vis spectrophotometer(Hitachi U-1500) at 554 nm (λmax for RB). For comparison,the photocatalytic activity of pure P25 and the undoped TiO2

nanotubes was also performed. The UV-vis light absorptionspectra were obtained from a spectrophotometer (Hitachi U-4100) equipped with an integrating sphere assembly, usingthe diffuse reflection method and BaSO4 as a reference.


3.1. Morphological and Microstructural Characterizations ofN-Doped TiO2 Nanotubes. The purpose of adding urea isto provide nitrogen source during the synthesis process ofTiO2 nanotube. Urea is a very weak Brønsted base, highlysoluble in water, and its hydrolyzing rate may be controlledby temperature, its decomposition giving rise to ammonium


carbonate, and when the temperature is above 90◦C, itobviously begins to hydrolyze in a basic solution and can bedescribed by:

CO(NH2)2 + 2H2O −→ 2NH4+ + CO3

2− (1)

TEM image of the as-prepared TiO2 nanotubes shownin Figure 1(a) reveals a large amount of nanotubes with anouter diameter of 8–15 nm and a length of several hundrednanometers. HRTEM image in Figure 2(a) indicates thatthe as-prepared N-doped TiO2 nanotubes possess ratheruniform inner and outer diameters along their length.

A closer observation further suggests that the nanotubesare multilayered and open-ended, in good agreement withthe previous reports [14, 25]. As shown in Figure 1(b), aftercalcination at 400◦C, some of the nanotubes with porousmultilayer structure become dense and aggregated to formnanorods with a diameter of 8–15 nm and a length of severalhundred nanometer, also in agreement with the previousreport [26]; other nanotubes begin to form particles.

HRTEM in Figure 2(b) for calcined TiO2 nanotube TC-1(400) (the number in parenthesis indicates that sample wascalcined at 400◦C) suggests that the crystalline structure ofthe nanorods is of anatase phase. The interplanar spacing ofthe nanorods is measured to be ca. 0.35 nm, correspondingto the (101) crystal plane of anatase. Selected area electrondiffraction pattern (inset, Figure 2(b)) reveals that fiveindiscernible diffraction rings can be identified as (101),(004), (200), (211), and (204); all belong to the anatase phaseof the TiO2 crystal.

Figure 3 shows the XRD patterns of the titanatenanotubes TC-1 calcined at 200◦C–400◦C, along with thestarting material P25 for comparison. For the as-preparedsample, there exists three broad yet weak peaks at 2θ =24.5◦, 28.2◦, and 48.4◦, corresponding to (110), (130),and (200) reflections, respectively. This suggests a ratherlow crystallinity of the as-prepared TC-1. In addition, thebroad peak at 2θ = 9.6∼13◦ can be assigned to the differentinterlayer spacings of the nanotubes having the trititanateconfiguration (H,Na)2Ti3O7 [27]. They can be ascribedto the lepidocrocite-type sodium titanate compounds[27–29]. As reported, this sodium titanate is probably firstformed from original titania powder through hydrothermaltreatment and then changes to hydrogen titanate afterwashing with HCl solution through an ion-exchangemechanism [29]. Several crystal structures of the TiO2-derived titanates have been reported, including H2Ti3O7

(or Na2Ti3O7), NaxH2− x Ti3O7, Na2Ti2O4(OH)2, orNayH2− y TinO2n + 1 · x H2O [30], where x and y dependstrongly on the pH value of the acidic washing solutionduring the postcalcination treatment.

As the calcination temperature further increases, thosepeaks belonging to lepidocrocite-type sodium titanate phasedisappear gradually, and the anatase phase of (101), (004),(200), and (211) reflections becomes the main crystal form.Meanwhile, the peak located at ca. 12◦ (nanotubes) wasweakened moderately, accompanying with the formation ofanatase phase. This indicates a decrease of the interlayerspacing due mainly to the dehydration of the interlayered

OH groups, while the titanate structure remains almostunchanged. It has been reported that for titanate nanotubeswith the postheat treatment above 350◦C, some of thenanotubes began to break into particles of anatase phase, andthe others remained as nanotubes. The remaining nanotubeswas believed to be stabilized by the remnant Na atoms[27]. Our XRD results demonstrate that the crystallinityof the resultant nanotubes increases with increasing heat-treatment temperatures. The diffraction peaks of the samplesTC-1(400) can be indexed as the anatase phase of TiO2.(JCPDS 21-1272). No nitrogen-derived peaks are detected,presumably due to the fact that the content of N dopingis very low and the doped N is uniformly distributed inthe nanotubes. It can be concluded that N doping does notcause the change in crystallite structure of TiO2, which wasconsistent with previous report [31].

From the nitrogen physisorption, the adsorption-desorption isotherm of TC-1 shown in Figure 4 is thetype IV isotherm with H1 hysteresis, according to IUPACclassification [32]. The shapes of hysteresis loops have oftenbeen identified with specific pore structures. Thus, Type H1hysteresis appearing in the multilayer range of physisorptionisotherms is usually associated with capillary condensationin mesopore structures. As expected, the as-prepared titanatenanotubes have a large mesopore volume of 1.01 cm3/g.The BJH method was employed to analyze the pore sizedistributions, and the results are depicted in the inset ofFigure 4. The pore structures of TC-1 samples determinedaccording to the adsorption data are also collected in Table 1.The distributions are relatively narrow within 15 nm. Weinfer that both tips of nanotubes are opened and their innercavities are accessible to N2 gas molecules. Therefore, theenhancement of pore volume can be mainly contributed bythe tubular-type titanate. This type of mesoporous TiO2 iscredited to show an excellent performance in photocatalysisand photovoltaic applications.

As compared to the as-prepared one, the specific surfaceareas of TC-1(400) decreased rapidly from 292 to 73 m2/g,pore volume reduced from 1.01 to 0.57 cm3/g, and the poresize distribution extended from 15.0 to 31.8 nm. All thesecould be inferred to the substantial destruction of interlayerstructure in nanotubes and the formation of particles withsmaller specific surface areas. The results of reduced specificsurface area are in good agreement with those of the XRDand TEM observations from this study.

3.2. Chemical Identification of N-Doped TiO2 Nanotubes.XPS was conducted for chemical identification of the valencestate of the doping nitrogen in detail. Figure 5 shows thatthe XPS survey spectrum of the nitrogen-doped titanatenanotubes TC-1, revealing strong peaks of Ti, O, and Celements. The minor peak at about 400 eV can be assigned tothe binding energy of N 1s. The C element can be ascribedto the adventitious hydrocarbon in carbon tape from XPSinstrument itself. Table 2 summarizes the XPS spectra of TC-0, TC-1, and TC-1(400).

The high-resolution XPS spectra of the N 1s region onthe surface of TC-0, TC-1, and TC-1(400) are displayed


100 nm

(a)

100 nm

(b)

Figure 1: TEM images of (a) as-prepared TC-1 and (b) TC-1(400).

20 nm

(a)

5 nm

0.35 nm

(204)(200)

(101)

(211)(004)

(b)

Figure 2: HRTEM images of (a) as-prepared TC-1 and (b) TC-1(400).

in Figure 6, from which the nitrogen concentrations areestimated to be about 0.50%, 2.69%, and 2.46% for TC-0,TC-1, and TC-1(400), respectively. The low nitrogen contentof the undoped TC-0 might be due to the existence of themolecularly chemisorbed γ-N, which should not be ignored.Sample TC-1 shows a single peak at 400.6 eV for N 1s corelevel. The assignment of the XPS peak of N 1s has stillbeen under disputation, and some controversial hypotheseshave been provided. Some reports suggested that the N 1speak at 399-400 eV is due to the NH3 adsorbed on theTiO2 surface [2, 33]. Other researchers pointed out that thepresence of oxidized nitrogen such as Ti–O–N or Ti–N–Olinkages should appear above 400 eV [34–36]. In most cases,the N 1s peaks at around 400 eV has been assigned to themolecularly chemisorbed γ-N [37]. Although not provenexperimentally, it has been postulated that the bondingenergy of N 1s is higher when the nitrogen atom in achemical linkage shows more positive formal charge [34, 36].Therefore, the N 1s peak at 400 eV in this study is ascribed to

a characteristic peak of interstitial N, which is hosted in aninterstitial position and directly bound to lattice oxygen. Inthis study, urea was used as nitrogen source and containedin cohydrothermal synthesis process, it is highly soluble inwater, and its controlled hydrolysis in aqueous solutions canyield ammonium cyanate or its ionic form (NH4

+, NCO−)[38]. From these viewpoints, the N 1s peak explored in thiswork can be assigned to the anionic N− in Ti–O–N or Ti–N–O linkages. In Figure 6, the N 1s peak of TC-1(400) centeringat 399.9 eV has a lower N concentration. This low N contentmay be due to the replacement of N in the matrix by O duringthe annealing process.

The high-resolution XPS spectra of the Ti 2p and O1s on the surface of TC-1 and TC-1(400) are shown inFigure 7. Both peaks are significantly reduced in intensity bycalcination. It can be seen that peaks of Ti 2p3/2 (459.8 eV),Ti 2p1/2 (465.5 eV), and O 1s (531.1 eV) of sample TC-1(400) shift to higher binding energy, as compared to thoseof sample TC-1. This can be attributed to the formation of


10 20 30 40 50 60 70 80

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004 20

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211

204

116

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(d)

(c)

(b)

(a)

Inte

nsi

ty(a

.u.)

2θ

P25

Anatase

Figure 3: XRD patterns of P25 and sample TC-1 calcined at varioustemperatures; (a) as-prepared, (b) 200◦C, (c) 300◦C, and (d) 400◦C.

0 0.2 0.4 0.6 0.8 1

0

200

400

600

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TC-11 10 100

0

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0.05TC-1TC-1(400)

Pore diameter (nm)

15

3.3 31.8

Vol

um

ead

sorp

ted

(cm

3/g

)

dV/dD

(cm

3/g∗n

m)

Relative pressure P/P0

TC-1(400)

Figure 4: Nitrogen adsorption isotherms for as-prepared TC-1 andcalcined TC-1(400).

1000 800 600 400 200 0

0

15000

30000

45000

60000

Inte

nsi

ty(c

ps)

TC-1

N1s

C1sTi2

pO

1s

Ti2

s

Binding energy (eV)

Figure 5: XPS survey of TC-1.

408 404 400 396 392

Inte

nsi

ty(a

.u.)

Binding energy (eV)

N 1s

400.6

399.9

TC-1

TC-1(400)

N%

=2.

69N

%=

2.46

TC-0

Figure 6: XPS N1s spectra of TC-0, nitrogen doped TC-1 and TC-1(400).

Table 1: Nitrogen physisorption properties of N-doped TC-series.

Sample Urea (g) SBET (m2/g)a DBJH-ads (nm)b VPore (cm3/g)c

TC-0 0.00 207 9.2 0.48

TC-1 1.88 292 15.0 1.01

TC-1(400) 1.88 73 31.8 0.57aBET surface area calculated from the linear part of BET plot.

bEstimated using the adsorption branch of the isotherm by BJH method.cSingle-point total pore volume of pores at P/P0 = 0.97.

Table 2: XPS of TC-series.

SampleNitrogen content

(%)aN 1s(eV)

O 1s(eV)

Ti 2p3/2

(eV)

TC-0 0.50 398.0 528.0 457.0

TC-1 2.69 400.6 530.3 458.8

TC-1(400) 2.46 399.9 531.1 459.8aNitrogen doping percentage calculated according to the curve fitting of the

XPS micrographs for the N 1s region.

hyponitrite (N2O2)2− in sample TC-1(400) after calcination,supporting further the postulate of the interstitial dopingof nitrogen atoms into the lattice in sample TC-1 [31].These XPS results lead to the confirmation that nitrogen issuccessfully incorporated into the titania in this study.

A controversial viewpoint on the nitrogen doping mech-anism has been proposed, and it was postulated that thenitrogen should be doped into the oxide side of the nanotu-bes during calcination. Apparently, this is not what found,particularly from a series of XPS data in this work.

The urea adopted is highly soluble in water, and itshydrolyzing rate may be controlled by temperature, givingrise to ammonium carbonate when the autoclave temper-ature is above 90◦C. Similar urea applications have alsobeen reported. Cong et al. applied urea as nitrogen sourcein the sol-gel hydrothermal reaction to synthesize N-dopedTiO2 successfully [39]. Another study by Peng et al. usedP25 and urea in ethanol via a solvothermal approach by


468 464 460 456 4520

2000

4000

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8000

10000

12000

Inte

nsi

ty(c

ps)

Binding energy (eV)

Ti 2p1/2

458.8

459.8

464.5

465.5

TC-1

TC-1(400)

Ti 2p3/2

Ti 2p

(a)

537 534 531 528 525

4000

8000

12000

16000

20000

Inte

nsi

ty(c

ps)

Binding energy (eV)

O 1s 530.3

TC-1

TC-1(400)

531.1

(b)

Figure 7: High-resolution XPS spectra of TC-1 and TC-1(400), (a) Ti 2p (b) O1s.

microwave; the temperature was set above 135◦C, higherthan the decomposition temperature of urea. They reportedthe as-synthesized yellowish powder of nitrogen-dopedtitanium dioxide possessed the best photocatalytic activity[31]. Although the exact mechanism of urea hydrolysis inautoclave was not clear, it was confirmed by XPS in this studythat the as-prepared sample TC-1 has the stronger N1s peakand the higher nitrogen content up to 2.69%. All of these datastrongly indicate that nitrogen was directly doped during theone-step cohydrothermal reaction.

3.3. Photocatalytic Activity. The UV-vis diffuse reflectancespectra of samples TC-1, TC-1(400), TC-0, and P25 areshown in Figure 8. Compared with P25, samples TC-1,TC-1(400), and TC-0 present a noticeable increase in theabsorbance extending to the visible-light region between400–600 nm, which is the typical absorption feature of thenitrogen-doped TiO2.

The photocatalytic activity has been tested by the pho-todegradation of aqueous solutions of Rhodamine B dye.Figure 9 shows the degradation of RB with time over the N-doped titania nanotube TC-1 under visible light irradiation.Sample TC-1 exhibits a remarkable photocatalytic activitythan P-25. Over 95% of the Rhodamine B dye is degradedafter 1 h irradiation for TC-1, but only 22% for P-25powder. The photocatalytic degradation rate of N-dopedTC-1 is far greater than that of P-25 in the first hour.The commercially available P-25 is believed to be a high-efficiency photocatalyst due to the synergetic contributionby the mixing phase of anatase and rutile [40]. The extrahigh photocatalytic efficiency of TC-1 can be interpreted inthe view of the high nitrogen content, high surface area,and large mesopore volume. It is noteworthy that the as-prepared amorphous TC-1 shows the highest photocatalyticactivity and not the well-crystallized samples TC-1(400).The crystallinity has been reported as the key factor indetermining the photoactivity under visible light [41, 42].However, our results above point out that the crystallinity

300 400 500 600

0

0.2

0.4

0.6

0.8

1

400 500 600A

bsor

ban

ce(a

.u.)

Wavenumber (nm)

TC-1TC-1(400)TC-0P25

Abs

orba

nce

Wavenumber (nm)

Figure 8: The UV-Vis diffuse reflectance spectra of TC-1, TC-1(400), TC-0, and P25.

0 60 120 180 2400

0.2

0.4

0.6

0.8

1

Time (min)

a

b

c

d

Deg

rada

tion

(C/C

0)

Figure 9: Photocatalytic degradation of Rhodamine B under theirradiation of visible light in the presence of (a) TC-0, (b) TC-1(400), (c) P25, and (d) TC-1.


might not be the only factor; the microstructure, nitrogencontent, pore volume, and surface area also play influentialroles in photoactivity.

It is interesting to point out the photocatalytic activity ofP25 under the visible light in Figure 9, since the original P25does not normally absorb visible light. Two possible reasonscan be rationalized. First, according to the manufacture’sspecification, the UV cutoff filter used in this study cannormally screen the irradiation with a wavelength longerthan 400 nm but still has a T50% at 425 nm. This mayaccount for the mild absorbance of P25 from 420 to 425 nmin the early stage of irradiation. Similar result has alsobeen reported by Li et al. in 2010 [43]. Second, the cutofffilter effect has also been checked using a spectrometer(HR4000CG, Ocean Optics). It demonstrates a satisfactoryscreening effect of wavelength less than 400 nm; yet, thereis a strong peak from 400 nm, rising to 450 nm sharply. Webelieve that this is another reason for the activated P25 withinthis wavelength region.

The visible light response in nitrogen N-doped TiO2

titania has been extensively investigated but remains undercontroversy. Chen and Burda reported that the substitutionaltype nitrogen species with the N–O type bonding wasresponsible for the observed increase in photocatalytic activ-ity in the visible region [36]. Diwald et al. indicated that abinding energy of 399.6 eV for N 1s was effective in reducingthe band gap from 3.0 to 2.4 eV, which is a shift of 0.6 eV intothe visible spectral region [44]. In general, an N 1s bindingenergy near 400 eV is regarded as an active dopant althoughassignments of its chemical nature differed. Di Valentinet al. investigated the difference in the dopant states forsubstitutional versus interstitial type impurities in anataseTiO2 by electron paramagnetic resonance spectroscopy andXPS and also by calculations using density functional theory[22]. Very distinct differences in the calculated electronicstructure for substitutional versus interstitial type nitrogenwere reported. Both types of impurities were found to addlocalized states within the band gap. For substitutional typenitrogen, these states were located 0.14 eV above the valenceband, and for interstitial type nitrogen species (referred toas N–O), the localized states were 0.73 eV above the valenceband.

The N1s XPS spectra of TC-1 display a peak at 400.6 eV,which can thus be assigned to the interstitial Ti–O–N orTi–N–O linkages or molecularly chemisorbed γ-N [37]. Allthese findings lead us to infer that NH4

+ has successfullyincorporated as nitrogen doping source during the forma-tion of TiO2 nanotube and that N-doped TiO2− x Nx istransformed into multilayer tubular structure with meso-pores which eventually becomes the nanocrystalline anatase,However, the photoactivity of the N-doped nanotube TC-1decreases, as the calcination temperature increases to 400◦C.This decrease in photoactivity can be explained by the lossof nitrogen content and the destruction of tubular structure,accompanied by a decreased specific area. On the other hand,formation of larger amounts of oxygen vacancies due to highcalcination temperatures should be promoted, which furtherencourages the recombination of photogenerated electronand holes, resulting in a lower photoactivity.

4. Conclusions

Nitrogen-doped TiO2 nanotubes were successfully preparedby a simple urea/P25 cohydrothermal method. Morpholog-ical study by TEM reveals that the nanotubes change fromporous multilayer and tube-shaped structure to dense andaggregated rod-shaped structure when the postannealingtemperature increases. Microstructure study by XRD andBET indicates that the as-prepared N-doped TiO2 nanotubeis a lepidocrocite-type sodium titanate compound havinga surface area of 292 m2/g; it changes to the anatase TiO2

after calcination. Chemical identification by XPS showsincorporation of high nitrogen content and the formationof Ti–O–N or Ti–N–O linkages within the nanotubes. Pho-tocatalytic activity experiments indicate that it exhibits highphotoactivity under visible light, which is about four timesgreater than that of P25. The results suggest that the nitrogencontent and surface area, rather than crystallinity, are morecrucial in determining the photocatalytic activity. We believethat the preparation method of N-doped titanium oxidenanotube derived from this facile cohydrothermal methodis simple, costeffective, and environmentally friendly. Withfurther investigation and improvement, it should providegreat potential applications in photocatalysis.

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Research Article

Photodegradation of Malachite Green by Nanostructured Bi2WO6

Visible Light-Induced Photocatalyst

Yijie Chen, Yaqin Zhang, Chen Liu, Aimin Lu, and Weihua Zhang

College of Science, Nanjing Agricultural University, Nanjing 210095, China

Correspondence should be addressed to Weihua Zhang, [email protected]

Received 14 August 2011; Accepted 6 September 2011


Copyright © 2012 Yijie Chen et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Bi2WO6 photocatalyst was first utilized to degrade malachite green. The effects of the concentration of malachite green, thepH value, and the concentration of Bi2WO6 on the photocatalytic efficiency were investigated. This study presents a strategyto eliminate highly toxic and persistent dyes such as malachite green.

1. Introduction

Malachite green is a kind of triphenylmethane dye whichhas been widely used in the production of ceramics, leather,textile industry, food coloring, cell coloring, and so on. Dueto its high efficiency in disinfection, it has also been usedin aquaculture industry to treat scratch on the fish bodiesand defend against bacterial infections. Since 1990s, however,researchers found that malachite green and its reduced formsare highly toxic, persistent, carcinogenic, and mutagenic [1].It will bring irretrievable damage to the environment ifdischarged into water body.

In recent years, much effort has been devoted to utilizephotocatalysis to eliminate organic dyes in contaminatedwater [2–9]. TiO2-based nanomaterials are typical photocat-alysts which have been extensively investigated, especially inthe photodegradation of dyes [10–12]. TiO2 possesses highactivity in degrading pollutants under UV light irradiationand will not bring secondary pollution. However, TiO2 isonly active under UV light irradiation, which is unfavorablefor practical application [13–15]. Semiconductors of theAurivillius oxides with general formula of Bi2An−1BnO3n+3 (A= Ca, Sr, Ba, Pb, Na, K and B = Ti, Nb, Ta, Mo, W, Fe) havegained much attention due to their unique electronic struc-ture and good stability [16]. Such oxides have become newtypes of highly effective photocatalysts. Bi2WO6, as a typicalAurivillius oxide (n = 1), is attractive due to its narrow bandgap, good stability, and excellent antioxidation ability. It hasbeen reported that Bi2WO6 could be used as a photocatalyst

to decompose water and degrade organic pollutants [17–20].However, the investigated organic pollutants degraded by thephotocatalysis of Bi2WO6 are limited.

In this paper, Bi2WO6 is firstly used as a photocatalystto degrade a new kind of organic pollutant, malachite green.The effects of the concentration of malachite green, the pHvalue, and the concentration of photocatalyst on the degra-dation efficiency were studied. Under optimal condition, thedegradation efficiency of malachite green is as high as∼100%only after 15 min of irradiation with Bi2WO6 photocatalyst.These results reveal that Bi2WO6 photocatalyst is a veryperspective photocatalyst in wastewater remediation.


All the reagents were of analytical purity and were usedas received from Shanghai Chemical Company. In a typ-ical process, aqueous solutions of Bi(NO3)3·5H2O andNa2WO4·2H2O in 2 : 1 molar ratio were mixed together,then the pH value of the final suspension was adjusted toabout 7. The mixture was stirred for several hours at roomtemperature. Then the suspension was added into a 50 mLTeflon-lined autoclave up to 80% of the total volume. Thesuspension in the autoclave was heated at 160◦C for 24 hunder stirring. Subsequently, the autoclave was cooled toroom temperature naturally. The products were collectedby filtration and then were washed by deionized water andabsolute ethanol until no anions were left in the solution astested. The samples were then dried at 80◦C for several hours.


The X-ray diffraction (XRD) patterns of the sampleswere measured on a D/MAX 2250V diffractometer (Rigaku,Japan) using monochromatized Cu Kα (λ = 0.15418 nm)radiation under 40 kV and 100 mA and scanning over therange of 10◦ ≤ 2θ ≤ 80◦. The morphologies and microstruc-tures of as-prepared samples were analyzed by transmissionelectron microscopy (TEM) (JEOL JEM-2100F, acceleratingvoltage 200 kV). The UV-vis diffuse reflectance spectra ofthe samples were recorded with a UV-vis spectrophotometer(Hitachi U-3010) using BaSO4 as reference.

For photocatalysis test, typically aqueous Bi2WO6 sus-pensions were prepared by adding certain amount of Bi2WO6

into a 50 mL solution of malachite green with different con-centration. The pH values of the suspensions were adjustedby adding KOH or HCl solution when necessary. Beforeillumination, the suspensions were magnetically stirred indark to reach the adsorption/desorption equilibrium. A500 W Xe lamp was used as the light source. The distancebetween the lamp and the suspension was kept at 20 cm. Theexperiments were processed at room temperature. At giventime intervals, the suspension was sampled and centrifugedto remove the photocatalyst particles. For the photocatalyticdegradation of malachite green under anoxic condition, N2

bubbles were introduced into the reaction system for severalhours to drive O2 out, and then the reaction vessel wasirradiated by Xe lamp. Malachite green exhibits the highestabsorption at 616 nm, and the concentration of malachitegreen was monitored by the change of the absorption (A) at616 nm by a Shimadzu UV-1700 spectrophotometer in thephotocatalytic reaction process. The degradation efficiency(η) was described by the equation: η = (c0− c)/c0 × 100% =(A0 − A)/A0 × 100% (c0 and c were the concentrations ofmalachite green at the beginning and after the photocatalyticreaction for certain time, while A0 and A were the absorptionintensities at the beginning and after photocatalytic reactionfor certain time).


The phase and composition of the products, which wereprepared at neutral pH value and the temperature of 160◦Cfor 24 hours, were investigated by using XRD measurement.Figure 1 shows the XRD pattern of the Bi2WO6 sample.Obviously, the Bi2WO6 sample exhibits high-intensity andnarrow-diffraction peaks in the XRD pattern, which isdue to the well crystallization. All the diffraction peakscan be indexed to orthorhombic Bi2WO6 according to theJCPDS card no. 39-0256. After refinement, the crystal latticeparameter of Bi2WO6 was calculated to be as follows: a =5.456 A, b = 16.445 A, and c = 5.444 A.

As shown in Figure 2, the diffuse reflectance spectrumof as-prepared Bi2WO6 indicates it absorbs visible lightwith wavelength shorter than 470 nm. The morphology andmicrostructure of the Bi2WO6 samples were revealed byTEM images (Figures 3(a) and 3(b)). The panoramic viewshown in Figure 3(a) demonstrates that the as-preparedproduct is composed of homogeneous nanoplates. Closeobservation revealed by high-magnification TEM image(Figure 3(b)) shows that most of these nanosized Bi2WO6 are

10 20 30 40 50 60 70 80

(204

)

(102

)

(400

)

(262

)(1

33)

(202

)

(200

)

Inte

nsi

ty(a

.u.)

(131

) JCPDS 39-256

2θ

Figure 1: The XRD pattern of Bi2WO6 sample prepared by hydro-thermal method.

200 300 400 500 600 700 800

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orba

nce

(a.u

.)

Wavelength (nm)

Figure 2: Diffuse reflectance spectrum of as-prepared Bi2WO6 sam-ple.

nanoplatelike. In addition, the shape and the size can also beobserved clearly, which is platelike with size of about 200 nm.

The absorption spectrum of malachite green was shownin Figure 4(a). As demonstrated, the highest absorptionappeared at 616 nm which was used to monitor the con-centration of malachite green. The effect of the initialconcentration of malachite green on the photocatalyticdegradation rate was first investigated. The concentrationof Bi2WO6 was kept at 1 g·L−1, and the time of irradia-tion was 75 min. The malachite green solutions with theinitial concentrations of 5 mg·L−1, 10 mg·L−1, 15 mg·L−1,and 20 mg·L−1 were prepared, respectively. As shown inFigure 4(b), the degradation efficiency of malachite greendecreased when the initial concentration increased, and thetrend was accelerated when the concentration became higher.This phenomenon could be explained as follows. Malachitegreen molecule is photosensitive. More photons wouldbe absorbed when the concentration increased, leading torelatively low light transmittance. Thus, the depth of light


0.5 μm

(a)

50 nm

(b)

Figure 3: The TEM images of as-synthesized Bi2WO6 nanoplates: (a) low magnification, (b) high magnification.

penetration decreased, and the photocatalytic activity ofBi2WO6 was inhibited. Less Bi2WO6 would be activated, andthe degradation efficiency of dye decreases accordingly [21].

The effect of pH value on the degradation efficiencyof malachite green in the range that the solution possessesa stable color of blue was further investigated. The pHvalue of the original solution of malachite green is 5.3.Diluted hydrochloride acid solution or potassium hydroxidesolution was used to tune the pH value when necessary.The initial concentration of malachite green solution and theconcentrations of the photocatalyst were kept at 10 mg·L−1

and 1 g·L−1, respectively. Various solutions of different pHvalues of 2, 4, 5.3, and 8 were prepared. After 30 minof photocatalytic irradiation, degradation efficiencies werecompared. As illustrated in Figure 5, the degradation effi-ciency of malachite green solution decreased when the pHvalue of the reaction solution increased. The degradationefficiency of the malachite green solution was 98.9% whenpH value was 2, while the degradation efficiency was 30.63%when pH value was 8. The observed dependence may beascribed to the pH value effect on the photocatalyst. Theoxidation of organic compounds involving the diffusion oforganic compound to the particle surface so as to form acomplex firstly, followed by the exchange of electrons withthe reactive surface of oxides has been reported [22]. Sincemalachite green has an anionic configuration, the adsorptionis favored in acidic solution. The prevailing pH value ofthe solution affects the mode and extent of adsorption ofmalachite green on the Bi2WO6 surface and thus, indirectly,affects the photodegradation efficiency of malachite green.On the other hand, in the catalytic process, H+ can enhancethe surface acidity of Bi2WO6 and make the malachite-greenmolecules more prone to interact with Bi2WO6. These effectslead to the observed increase in the degradation efficiencywith decreased pH value.

The cost of photocatalyst was the primary factor con-tributing to the chemical costs of photocatalytic treatment.It is important to minimize the required amount of photo-catalyst. Thus, the investigation of Bi2WO6 concentrationson the degradation of malachite green dye was conducted,which is shown in Figure 6. The initial concentration and thepH value of malachite green solution are kept at 10 mg·L−1

and 2, respectively. The concentrations of Bi2WO6 are setat 0.2 g·L−1, 0.6 g·L−1, 1.0 g·L−1, 1.4 g·L−1, and 1.8 g·L−1,respectively. After 10 min of photocatalytic degradation,the degradation efficiencies were compared. In Figure 6,the photocatalytic degradation performance is relativelyundesirable. The degradation efficiency of malachite greensolution increased with the increase of the concentrationof Bi2WO6 until the concentration of Bi2WO6 reached to1.0 g·L−1, while corresponding degradation efficiency was87.04%. However, the degradation efficiency changed littlewhen the concentration of Bi2WO6 was more than 1.0 g·L−1.This can be explained on the basis that optimum photocat-alyst loading is dependent on initial solute concentration. Ifthe concentration of photocatalyst was increased, the totalactive surface was increased correspondingly, and as a result,the enhanced photocatalytic performance was obtained.However, the increased concentration of photocatalyst wouldhave no effect on promoting the degradation efficiency aftera maximum photocatalyst concentration was imposed. Thismay be ascribed to the increased aggregation of photocatalystat high concentration. Therefore, the optimal concentrationof Bi2WO6 in current case is 1.0 g·L−1.

According to above experiments, the optimal conditionsfor photocatalytic degradation of malachite green are asfollows: the initial concentration of malachite green, pHvalue, and the concentration of Bi2WO6 are 10 mg·L−1, 2,and 1.0 g·L−1, respectively. Figure 7 shows the evolution ofthe degradation of malachite green with irradiation time. It is


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Initial concentration (mg/L)

(A0−A

)/A

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(b)

Figure 4: (a) The absorption spectrum of malachite green. (b)The effect of the initial concentration of malachite green on thedegradation efficiency.

2 3 4 5 6

0.4

0.6

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(A0−A

)/A

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Figure 5: The effect of initial pH value on the degradation effi-ciency.

0 0.5 1 1.5 2

0.2

0.4

0.6

0.8

1

(A0−A

)/A

0

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Figure 6: The effect of the concentration of photocatalyst on thedegradation efficiency.

−5 0 5 10 15 20 25 30 35 40 45 50 55 60 65

0

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P25 in simulate sunlightP25 in visible light

Time (nm)

Bi2WO6 in simulated sunlight and in airBi2WO6 in simulated sunlight and in N2

(A0−A

)/A

0ab

c

d

Figure 7: The photocatalytic degradation of malachite green timeunder optimal conditions (the initial concentration of malachitegreen, initial pH value, and the concentration of photocatalyst are10 mg·L−1, 2, and 1.0 g·L−1, resp.): (a) Bi2WO6 under Xe lamp (asindicated by black square; (b) reference P25 under Xe lamp (asindicated by white triangle, simulated sunlight); (c) reference P25under visible light (as indicated by black triangle); (d) Bi2WO6

under visible light and anoxic condition.

revealed that the degradation of malachite green solution wasfast during the initial 5 min of photocatalytic reaction. Thenit decreased gradually. According to earlier reports, [23–25]the relation between the rate of photocatalytic degradationand the concentration of photocatalyst can be expressedby the Langmuir-Hinshelwood equation: r = −dc/dt =kKc/(1 + Kc). When the concentration of substrate is low(Kc � 1), the Langmuir-Hinshelwood equation can be


expressed as r = −dc/dt = kKc. Thus, the rate of pho-tocatalytic degradation is proportional to the concentrationof photocatalyst. For the degradation of malachite greensolution, the concentration of malachite green graduallydecreased during the photocatalytic degradation. As a result,the photocatalytic degradation is relatively fast during theinitial 5 min. For the malachite green solution with theinitial concentration of 10 mg·L−1 and pH value of 2, thecolor of the solution hardly changed in the dark withoutany photocatalyst. For the same malachite green solution,the degradation efficiency with Bi2WO6 photocatalyst in thedark for 90 min is only 13.6%, which can be attributed tothe adsorption of malachite green on Bi2WO6 photocatalyst.And the degradation efficiency without photocatalyst is13.56% after 15 min under light irradiation, which is dueto the photolysis of malachite green. These comparisonexperiments revealed that the decrease of malachite green ismainly caused by the photocatalytic degradation.

For comparison, a typical commercial TiO2, P25, wasselected as the reference. Under simulated solar light, itexhibited comparable activity as that of the Bi2WO6 (Figure(7b)). However, as indicated by Figure (7c), under visiblelight irradiation P25 exhibited poor photocatalytic activitiesin the photodegradation of malachite green because, itcan only be activated by UV light. In order to checkif oxygen molecules affected the degradation process, thephotocatalytic degradation of malachite green was carriedout under N2-saturated conditions. The result shown inFigure (7d) indicates that under the anoxic condition, thephotodegraded rate was largely suppressed.

4. Conclusion

In conclusion, a visible light-induced Bi2WO6 nanoplatephotocatalyst was hydrothermally synthesized at neutralpH values and at a temperature of 160◦C for 24 h. Theobvious degradation of malachite green was only observedwith Bi2WO6 photocatalyst under light irradiation. Thephotocatalytic performance of the Bi2WO6 sample was foundgreatly influenced by the concentration of photocatalyst,the concentration of malachite green, and the pH valueof the reaction system. The optimum concentration ofphotocatalyst was 1.0 g·L−1 for the degradation of 10 mg·L−1

malachite green solution with the pH value of 2. In addition,the relation between the rate of photocatalytic degradationand the concentration of malachite green can be describedby the pseudo-first-order kinetics, rationalizing in terms ofthe Langmuir-Hinshelwood model.

Acknowledgment

The authors acknowledge the financial support from theKey Laboratory of Pesticide Chemistry of Jiangsu Province(NYXKT201005).

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Research Article

Preparation of Porous F-WO3/TiO2 Films with Visible-LightPhotocatalytic Activity by Microarc Oxidation

Chung-Wei Yeh,1 Kee-Rong Wu,2 Chung-Hsuang Hung,3

Hao-Cheng Chang,2 and Chuan-Jen Hsu4

1 Department of Information System, Kao Yuan University, Kaohsiung 811, Taiwan2 Department of Marine Engineering, National Kaohsiung Marine University, Kaohsiung 811, Taiwan3 Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology,Kaohsiung 811, Taiwan

4 Metal Industries Research and Development Centre, Kaohsiung 811, Taiwan

Correspondence should be addressed to Kee-Rong Wu, [email protected]



Copyright © 2012 Chung-Wei Yeh et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Porous F-WO3/TiO2 (mTiO2) films are prepared on titanium sheet substrates using microarc oxidation (MAO) technique. TheX-ray diffraction patterns show that visible-light (Vis) enabling mTiO2 films with a very high content of anatase TiO2 and highloading of WO3 are successfully synthesized at a low applied voltage of 300 V using electrolyte contenting NaF and Na2WO4 withoutsubsequent heat treatment. The cross-sectional transmission electron microscopy micrograph reveals that the mTiO2 films featureporous networks connected by many micron pores. The diffused reflection spectrum displays broad absorbance across the UV-Visregions and a significant red shift in the band gap energy (∼2.23 eV) for the mTiO2 film. Owing to the high specific surface areafrom the porous microstructure, the mTiO2 film shows a 61% and 50% rate increase in the photocatalytic dye degradation, ascompared with the N,C-codoped TiO2 films under UV and Vis irradiation, respectively.

1. Introduction

Titanium dioxide (TiO2), known for being environmentallyfriendly and chemically stable, is one of the most suit-able semiconductors for several environmental applications,including air purification, water disinfection, hazardouswaste remediation, and, in recent years, the splitting of waterto produce hydrogen [1–3]. Upon irradiation of photonswith energies greater than or equal to the band gap energy(3.2 eV for anatase TiO2) of the photocatalyst, the photogen-erated electrons and holes can either recombine in the bulkor migrate to the surface to initiate various redox reactions.The photocatalytic (PC) reaction is usually accepted to bea surface-oriented phenomenon, regardless of whether itinvolves fine particles or films [1]. The surface area report-edly dominates the PC activities of the porous anodized TiO2

oxides [4]. A high specific surface area favors PC efficiency.Hence, films that have a rough surface with a high surface

area provide superior PC performance. In recent years, sub-stantial effort has been made to improve the PC activities ofTiO2 films [4–10]. Porous TiO2 films with a high surfacearea, such as porous anodized TiO2 oxides [4], nanotubearrays [5, 6], ordered nanopore arrays [7], nanograined thinfilms[8], mesoporous structures[9], and hierarchical micro-/nanoporous structures [10], are of great interest for theirpotential to improve PC activities. Interestingly, in additionto being mechanically stable, the ordered nanopore TiO2

arrays exhibit superior PC and photoelectrochemical (PEC)activities in the degradation of organic compounds againstnanotube counterpart arrays, because of their excellentseparation and the transport properties of photogeneratedelectron-hole pairs. In addition to having a high specific sur-face area, the ordered nanopore TiO2 arrays have a network-framed structure that increases the carrier transport routes.

The use of TiO2 is limited by its wide band gap(∼3.2 eV). Semiconductors with smaller band gaps, such as


CdS (2.4 eV), Fe2O3 (2.3 eV), and Cu2O (2.2 eV), commonlysuffer from photocorrosion in electrolyte solution and rapidrecombination of the photogenerated carriers [1]. Couplinga semiconductor with a high work function with anothersemiconductor can greatly enhance the oxidation of holesphotogenerated in the semiconductor, by increasing theefficiency of carrier separation. Heterostructured oxides,such as TiO2/ITO, TiO2/WO3, CdS/TiO2, and TiO2/SnO2,have been proposed for providing a potential driving forcefor separating photogenerated charge carriers [2, 11–14].Of the coupled semiconductors, ITO is the most populartransparent conductive substrates used in photoanodes andphotocatalysis [2, 12], but it is one of the most expensivematerials and has a detrimental effect on its PEC activitydue to the increase in resistivity of the ITO substrate [15].Among the various heterostructured oxides, TiO2/WO3 hasbeen shown to have applications in photocatalysis [9, 16,17], biophotoelectrodes [18], and anticorrosion [19]. Theimproved PC activity of the TiO2/WO3 particles has beenattributed to the increased surface acidity and improvedcharge separation due to the proper coupling of WOx specieswith TiO2 within the nanoparticles [9, 16, 17, 20]. Theadsorption affinity of reactant molecules on the surface ofthe TiO2/WO3 photocatalyst is a more important factorthan the surface reaction of the photogenerated carriers indetermining PC activity. The increase in the adsorption ofMB solution is more likely to be associated with an increasein the surface area of the TiO2/WO3 particles. However,the enhancement of PC activity under visible-light (Vis)irradiation is reportedly around one tenth of that measuredunder ultraviolet (UV) irradiation [21].

Some researchers have shown that the introduction offluorine (F) atoms into a photocatalytic system effectivelyincreases the Vis PC activity of TiO2 [22–27]. Wu and Chenpresented a high Vis PC efficiency of F-doped TiO2 particu-late thin films assisted by NaF solution. They concluded thatthe presence of F− ions is critical not only to the formationof particulate TiO2 films, but also to the formation of Ti3+

and oxygen vacancies upon F− ion doping, which results ina superior PC efficiency [23]. Moreover, it has been reportedthat the photocatalytic activity and the anatase crystallinityof porous TiO2 films can be enhanced by adding appropriateamount of F− ions into the electrolyte in microarc oxidation(MAO) process [26, 27].

MAO, a plasma-assisted electrochemical process, hasbeen used to fabricate a TiO2 surface layer by applyinga positive voltage to a Ti sheet that is immersed in anelectrolyte [26]. In addition to its short process durationand less-sophisticated equipments, the MAO process isreportedly a potentially good approach for preparing porousand network-structured TiO2 films that adhere robustly toa Ti sheet substrate [26–30]. The main mechanism in MAOinvolves dielectric breakdown, which causes spark dischargeand microarcing. Therefore, porous oxide films form on theanode surface [29]. The porous network is regarded as anopen, wormlike structure on the surface of the film, withrandomly orientated channels. The approach has severalpotential applications, mostly involving biocompatibility,such as biomimetic apatite [31], orthopedic implants with

antimicrobial coatings [30, 32–34], and wear protection[35]. Recently, some related studies have addressed thedegradation of dye [4, 26, 27, 36, 37] or dye-sensitized solarcells (DSSC) [38]. For instance, He et al. recently synthesizedWO3/TiO2 composite films using pulse-MAO at an appliedvoltage of 400–460 V and electrolyte consisting of Na2WO4,NaOH, and NaF. Most recent, porous WO3/TiO2 films werealso fabricated by DC-MAO at an applied voltage of 450 Vusing Na2WO4 and Na3PO4 solution as electrolyte [36].The relatively high applied voltage of the above-mentionedprocesses, however, resulted in a significant increase in thecontent of TiO2 rutile phase, which is well known to bephotocatalytically inferior to the anatase phase in variousapplications [4]. This is because that applying high voltagesresults in not only increasing the content of rutile phase butalso decreasing the amount of WO3 loading or W dopant inthe porous TiO2 film.

One of the advantages of MAO process is the possibilityof incorporating anionic and/or cationic ions into the TiO2

layer on an inexpensive Ti substrate, by controlling the com-position and concentration of the electrolyte [28–30]. How-ever, the MAO TiO2 film is liable to contain some free Ti ele-ments and has large pores and a high pore density. Postalkalior heat treatments have been used to improve the morpho-logical and intrinsic characters of MAO coatings [4, 31, 38].

In this study, porous F-TiO2/WO3 (mTiO2) films withincorporated cationic and anionic ions were formed by anMAO process at a relatively low applied voltage using anelectrolyte with a properly chosen composition. The PCproperties of the as-anodized films were tested by degradingmethylene blue (MB) solution under UV and Vis irradiation.

2. Experimental Procedure

2.1. Sample Preparation. Porous mTiO2 were formed on Tisubstrates (25× 75× 1 mm) using an in-house MAO system,in which the Ti sample was applied as an anode and a stain-less steel container was used as a cathode. The surfaces of theTi substrates were polished using silicon carbide paper andultrasonically cleaned three times for 15 min each time, in100% acetone, 100% ethanol, and distilled water. In a seriesof screening experiments, the electrolyte solutions were NaF(2 g/L) with Na2WO4 (15 g/L), and the applied voltage was300 V. For comparison, an undoped TiO2 film (uTiO2) andan N,C-codoped TiO2 (nTiO2) films both deposited on ITOglass substrates with about the same thickness of 1.8 μm wereprepared using DC magnetron sputtering system. Details ofthe nTiO2 films, which were prepared at a low doping con-centration of N and C (<2%) were presented elsewhere [39].

2.2. Sample Characterization. The crystal structures of thesamples were analyzed using a high-resolution X-ray diffr-actometer (XRD, Rigaku ATX-E) and a Micro-PL/ Ramanspectroscope (Jobin-Yvon T64000). The surface topographyof each sample was determined using an atomic forcemicroscope (AFM, SPI 3800N, Seiko). Surface morphologyand chemical composition of the samples were examinedby using a scanning electron microscope (SEM, JEOL


JSM-6700F) equipped with an energy dispersive X-rayspectrometer (EDS). A transmission electron microscope(TEM, Philips Tecnai 20) was employed for microstructurecharacterization. TEM cross-sectional specimens of thesamples were prepared using a focused ion beam (FEI QuantD 200) at a voltage of 30 kV.

The TiO2 powders from scraping the porous mTiO2 filmwas taken to have a BET (Brunauer-Emmett-Teller) surfacearea measurement. The diffused reflectance of the films wasmeasured by using a UV-vis-NIR spectrometer (Hitachi U-4100) equipped with an integration sphere. Since the Tisubstrates are completely opaque and zero transmittanceto the incident light, the absorption and band gap energy(Eg) of the samples were calculated using the formulastated in [36, 37, 40]. Two light sources, UV lamps (λ ∼365 nm) and blue-light-emitting diodes (BLED, 420 < λ <530 nm), were used to provide irradiated light intensitiesof 2.7 and 12.5 mW/cm2, respectively. The PC activity wasevaluated using aqueous MB solution as model pollution[15]. Aqueous MB solutions without samples were alsoilluminated in the same manner to generate a blank value,and such a solution with samples was not irradiated, as anadsorption test.


3.1. Microstructural Measurements. Figure 1 presents theXRD patterns of as-anodized mTiO2 sample along withthe N,C-codoped TiO2 (nTiO2) and pure TiO2 (uTiO2) forcomparisons. The XRD patterns of mTiO2 reveal a fullydominant anatase TiO2 phase. The peak marked by a triangleis associated with Ti, and may derive from the Ti substrateand from not well-anodized TiO2 oxide, which is the resultof random arcing by MAO [35]. Sample mTiO2 also exhibitsan intense diffraction peak at 2θ = 23.67◦ of crystallineWO3 peaks of (020) plane, as also detailed in the inset inFigure 1. A broad diffraction peak from 2θ = 33◦ to 34.5◦

is assigned to a crystalline WO3 phase [41]. Comparingwith uTiO2 in the inset of Figure 1, the diffraction (101)plane is shifted slightly toward a lower 2θ value, suggestingpossible distortion of the crystal lattice of TiO2 by thetungsten, fluorine, or/and other dopants. However, a smallshift towards higher diffraction angles, due to different ionradii of Ti and W, has been observed in the XRD patternsof the anatase TiO2 phase as W is doped on titania [37].Sample mTiO2 has a slightly broader peak at 2θ = 25.18◦

than the uTiO2, indicating the smaller crystallite size of themTiO2. These findings imply that the substitution of Ti4+

ions for W4+ in the TiO2 lattice to form a nonstoichiometricWxTi1−xO2 solid solution may have occurred in the mTiO2

film [9]. In the case considered herein, the loaded WO3 likelyfavors the formation of the anatase TiO2 phase by the MAOprocess, which can be further studied by acquiring Ramanspectra. Thus, the WO3 phase was loaded with a noticeablecontent on the TiO2 matrix. The amounts of O, Ti, and Wwere found to be 68.27, 25.59, and 6.14 at.%, respectively,whereas the fluorine was too small beyond the detectablelimit of the EDS. Noted also that the relative high amountof tungsten oxides of which could be promoted by addition

mTiO2

nTiO2

uTiO2

23 24 25 26

ITOTiAnatase TiO2

WO3

20 25 30 35 40 45 50 60

2θ(deg)

55

Inte

nsi

ty(a

.u.)

Figure 1: XRD patterns of the mTiO2, nTiO2, and uTiO2 films.

mTiO2

nTiO2

uTiO2

142.4

100 150 200 400 500 600 700

1000 1100900800

Wave number (cm−1)

Inte

nsi

ty(a

.u.)

Figure 2: Raman spectra of the mTiO2, nTiO2, and uTiO2 films.

of fluorine in the electrolyte was reportedly attributable tothe high photocatalytic ability [27, 37].

Raman spectra in the range of 100–1150 cm−1, shownin Figure 2, reveal further information on the structure ofthe films. Sample mTiO2 yields four distinct Raman peaksat 146, 394, 516, and 637 cm−1 with slight broadening, ascompared with the other two samples, which are directlyattributable to the anatase phase [42]. This result is ascribedto the crystallinity of the anatase phase and is consistent withthe XRD patterns. A shift in the high wave number and broadpeak of the Raman spectrum at 146 cm−1 implies that thesubstitution of W ions into the MAO-anodized TiO2 latticeand less well-crystallized TiO2 particles probably occurred.The Raman peak at 810 cm−1 which is associated with the


Inte

nsi

ty(a

.u.)

470 468 466 464 462 460 458 456 454

Binding energy (eV)

464.1

458

457

Ti2p

mTiO2

(a)

Inte

nsi

ty(a

.u.)

Binding energy (eV)

535 534 533 532 531 530 529 528

532.8

531.6

530.7

O1s

mTiO2

(b)

Inte

nsi

ty(a

.u.)

Binding energy (eV)

mTiO2

W4d

240 250 260 270

A

B

C

D

30 32 34 36 38 40

W4 f

(c)

Inte

nsi

ty(a

.u.)

Binding energy (eV)

mTiO2

F1s

690 688 686 684 682 680

(d)

Figure 3: High resolution XPS spectrum of: (a) Ti2p, (b) O1s, (c) W4f/Wd, and (d) F1s core levels of the mTiO2 film.

crystallized monoclinic or orthorhombic WO3 is extremelyweak, as shown in the inset of Figure 2, perhaps becauseof interference with a broad background signal of anataseTiO2 [21]. A relatively broad band at 970 cm−1 is assignedto a terminal W=O stretching vibration in tungsten trioxidehydrates. Additionally, a shift in the high wave number ofthe WO3 Raman spectrum at ∼970 cm−1 reveals that WO3

nanocrystallites had formed along the TiO2 surface or grainboundaries in a relatively high W concentration [9, 16, 43],which was previously conformed by the EDS measurement.

3.2. XPS Analysis. High-resolution XPS measurements wereperformed to elucidate the surface chemical composition andthe oxidation state for the porous mTiO2 film. Curve fittingusing Gaussian distribution function plotted in Figure 3(a),the XPS spectrum of Ti2p core level of the mTiO2 filmshows two well-known peaks at binding energy of ∼459.0 eVand at ∼464.1 eV, corresponding to Ti2p3/2 and Ti2p1/2,respectively, of Ti4+ oxidation state of TiO2. A minor peak

at binding energy of ∼457.0 eV is also observed, which isassigned to Ti3+ of Ti2O3 phase and is in accordance with theXRD result presented in Figure 1. Alternatively, the O1s corelevel binding energies for sample mTiO2 were deconvolutedinto three distinct components, indicating that there aredifferent kinds of O binding states in the mTiO2 film, asshown in Figure 3(b). It is well stated that the peak atbinding energy of 530.7 eV can be assigned as the crystallattice oxygen of Ti–O and W–O binding, while the peaksat binding energy of 531.6 and 532.8 eV represent the oxygenin hydroxyl groups (O–H) and in adsorbed water moleculeson the film [9, 37]. It is worthy to note that the area underhydroxyl groups (O–H) is estimated to be as large as 26.2%,which is ascribed for enhancing photocatalytic activity [44].

As seen in Figure 3(c), though the peak positions ofW4f5/2 (A) and W4f7/2 (B) shift to a higher binding energycompared to the reported values, their oxidation state of theincorporated tungsten species is reportedly assigned as W+6

oxidation state of of pure WO3 [9]. The lower binding energy


NSYSU SEI 10 kv 4, 000 1 μm WD 8.7 mm

(a)

NSYSU SEI 10 kv 37, 000 100 nm WD 8.5 mm

(b)

WD 8.4 mmNSYSU SEI 10 kv 37, 000 100 nm

(c)

6 (μm)

(d)

Figure 4: SEM plain images of (a) mTiO2, (b) nTiO2, and (c) uTiO2 films and (d) FIB projected image of the mTiO2 film, where the squaredcrater at the center was the TEM sample cut from.

shift can also be seen in the W4d peaks of sample mTiO2, asshown in the inset of Figure 3(c). These indicate that samplemTiO2 was loaded with a considerably high amount of WO3

phase. The other two peaks (C and D) are intuitively assignedto other tungsten oxide (i.e., WO2) or some nonstoichio-metric tungsten oxides such as W12O39

n− and impuritiesfrom the precursors used. Nevertheless, some other phasespossibly form, since stoichiometric ion exchange betweenW4+ and Ti4+ may occur [17]. In fact, W4+ can substitute Ti4+

in the lattice of TiO2 due to the similarity in their ion radii;nonstoichiometric solid solution of WxTi1−xO2 forms. Thus,nonstoichiometric solid solution of WxTi1−xO2 can form andleads to produce a tungsten impurity energy level [17]. Someresearchers have reported that the W4f peaks of WO3-loadedTiO2 shift to a lower binding energy [43], but others havefound a backward shift, as compared to that of pure WO3

[37]. This may be due to different methods and precursorsinvolved in sample preparation. This is being under takenfurther investigation in our lab. As seen in Figure 3(d), aweak and broad F1s peak at binding energy of ∼684.9 eV isobserved for sample mTiO2, which is mostly originated fromF− ions physically adsorbed on the TiO2 surface [24]. Parkand Choi have conclude that the adsorbed F− ions induceenhancement in the production of •OH radicals [44] whenthe F-containing compounds were used as TiO2 precursors.

However, no peak around 687.6 eV is observed which isattributed to the doped F atoms in TiO2 [24, 45].

On the basis of the XPS results stated above, it can bebriefly concluded that F− ions is physically adsorbed on theTiO2 surface in a very low concentration and W ions arelikely loaded as a pure WO3 phase along with other tungstenoxides and WxTi1−xO2 composite on the mTiO2 film.

3.3. Morphological and Topographical Observations. PorousTiO2 films with improved connectivity are reportedly impor-tant for adsorbing organic contaminant ions or initiatingvarious redox reactions [29]. Figure 4(a) presents the mor-phological SEM image of sample mTiO2, showing a typicalporous MAO structure with some submicron pores andcolloidal particles on a large cavity. Moreover, Figure 4(d)shows a FIB projected image of the mTiO2, where sponge-like morphology is filled with various micron and submicronpores. These submicron pores were probably formed by thesecondary breakdown of the large pores that were producedby the primary breakdown of arcing [29]. The wall ofthe larger pores includes smaller pores and several poresinside the film of which increases its reactive site surfacearea. On the other hand, as shown in Figures 4(b) and4(c), samples nTiO2 and uTiO2 exhibit a typical columnar-like morphologies deposited by a DC magnetron sputtering


Table 1: Some properties and apparent rate constants of three samples.

Sample Band gap energy, eV Specific surface area, m2/gApparent rate constant, h−1

UV Vis

mTiO2 2.23 1.75 0.53 0.42

nTiO2 2.93 0.16∗ 0.33 0.28

uTiO2 3.25 0.14∗ 0.36 0.06∗

estimated from the AFM measurement [37].

200 nm

Figure 5: Cross-sectional TEM micrograph of the mTiO2 film inwhich the porous TiO2 was formed on a 300 nm-thick TiO2 layersupported by Ti substrate.

technique [15, 39]. Thus, a low specific surface area isexpected, rendering a low active area in contact with aqueoussolution. The cross-sectional TEM micrograph of the mTiO2

film in Figure 5 reveals that a pore diameter ranging from400 to 800 nm was formed on a WO3/TiO2 layer with athickness of about 300 nm. The porous network connectedby many micron pores is regarded as an open with randomlyorientated channels in the porous film.

The specific roughness factor of nTiO2 and uTiO2 films(Figures 4(b) and 4(c)) were previously reported to be about1.3 and 1.2, which were equal to surface areas of about0.16 m2/g and 0.14 m2/g, respectively (Table 1) [15, 39]. ThemTiO2 shows a BET surface area of 1.75 m2/g, which is tentimes greater more than that of the nTiO2 and uTiO2. Thus,extremely rough, network-structured, and well-crystallizedTiO2 films can be obtained by MAO process without anysubsequent heat treatment.

3.4. Optical Properties. Figure 6 shows the diffused reflectionspectra of sample mTiO2 film, displaying broad absorbanceacross the UV-Vis regions. Obviously, sample mTiO2 has abetter optical absorption in the region of 400–700 nm owingto the presence of tungsten oxides and solid solution ofWxTi1−xO2 than the N,C-TiO2 and pure TiO2 [17]. It is wellknown that the absorption edge of the samples shift towardthe Vis region as WO3 is loaded on TiO2 matrix [9, 16–19, 37]. The band gap energies are calculated according to

0.9

0.8

0.7

0.6300 350 400 450 500 550 600 650 700

mTiO2

nTiO2

Wavelength (nm)

Abs

orba

nce

(a.u

.)

Figure 6: Diffused reflection spectra of the mTiO2 and nTiO2 films.

the equation Eg = hc/λ, where Eg is the band gap energy(eV), h is the Planck’s constant (4.136 × 10−15 eV s), c is thevelocity of light (2.99 × 108 m/s), and λ is the wavelength(nm) of absorption onset [36, 40]. The band gap energies are2.23 eV for mTiO2 and 2.93 eV for nTiO2 films, whereas thatis estimated to be 3.25 eV for the pure TiO2 [39]. Althoughsome researchers reported that only F-doping did not causeany significant change in the optical absorption of TiO2

(2.90–2.95 eV [24, 26]), they observed a new absorptionband in the visible range of 400–550 nm in addition toa strong fundamental absorption edge (∼387 nm) of TiO2

[24, 45]. Thus, the shift towards the longer wavelengthsmarkedly originates from the band gap narrowing of TiO2

by coupling with tungsten oxides [37, 43] and possibly acomplex of WxTi1−xO2 [17].

3.5. Photocatalytic Activities. PC activity can be increasedby increasing the apparent reaction rate constant andthe equilibrium adsorption constant of the catalysts [16].Figure 7 plots the degraded concentration of the MB solutionagainst the reaction time of three different samples. Nosignificant MB degradation is observed in a blank substrateunder UV radiation. As expected, Figure 7 clearly revealsthat the adsorption of MB by sample mTiO2 was strongerthan that of samples nTiO2 and uTiO2, due to the cationicnature of MB dye [17]. The MB adsorption capacity ofTiO2/WO3 composites usually increases with the loading


mTiO2nTiO2

uTiO2

−3 −2 −1 0 0.5 1 1.5

10

8

6

4

2

0

MB

con

cen

trat

ion

(mg/

L)

Blank

Time (h)

Figure 7: MB adsorption and degradation of the mTiO2, nTiO2,and uTiO2 films under UV irradiation. Blank and absorption testsare presented for comparisons.

MB

con

cen

trat

ion

(mg/

L)

mTiO2

nTiO2

uTiO2

10

8

6

4

2

01 2 3

Time (h)

0

Figure 8: MB degradation of the mTiO2, nTiO2, and uTiO2 filmsunder BLED irradiation.

of WO3 clusters [16, 20, 46], because the surface acidityof a WO3 monolayer results in strong adsorption of theTiO2/WO3 composites [20]. This result implies that theenhanced MB adsorption capacity of sample mTiO2 isprobably associated with the specific surface area of thesample that were prepared by the MAO process other than bythe formation of WO3 clusters in the TiO2/WO3 composites,suggesting that the MB adsorption capacity is related notonly to the surface acidity of the WO3 clusters, but also tothe specific roughness factor [20, 21, 46].

As listed in Table 1, under UV irradiation, sample mTiO2

exhibits the highest activity of the three samples tested,showing the highest apparent rate constant of 0.53 h−1–61%greater than that, 0.33 h−1, of sample nTiO2. Sample mTiO2,broad absorbance observed in the Vis regions, exhibitssignificant PC activity under BLED irradiation, an apparentrate constant of 0.42 h−1–50% greater than that, 0.28 h−1, ofsample nTiO2, as shown in Figure 8. Sample nTiO2 has anapparent rate constant of 0.36 h−1 and 0.06 h−1 under UVand Vis irradiation, respectively.

The PC activity of TiO2 depends on several factors,including crystallinity [47], surface area [8], crystal orienta-tion [48], surface hydroxyl density, and phase composition[49, 50]. In general, the photogenerated charge carriertransfer to the film surface would be limited by the interfacialdiffusion between crystalline WO3/TiO2 particles, which isfaster than volume diffusion. And the sponge-like porousstructure of the mTiO2 film can make charge carrier easierto reach the surface than a typical planar nTiO2 sample.The porous mTiO2 film exhibits the highest PC activityamong the oxides of interest; surprisingly, only 50%–61%higher in photocatalytic activity obtained from the film withten times greater more in the surface area than others.Qualitatively, the mTiO2 film consists of TiO2 and WO3

crystallites with a higher specific surface area and a lowercrystallinity than the nTiO2 and uTiO2 films. The PC activityof mTiO2 is enhanced by its WO3 crystallites and highspecific surface area but is reduced by its low crystallinityand some impurities, such as Ti and Na elements. Impuritiesact as carrier recombination centers in an oxide and alwaysdetrimentally affect PC activity. In contrast, high crystallinityis associated with a small fraction of crystal defects that actas recombination sites [7–9], allowing the generated chargesto diffuse to the crystallite surface without undergoingrecombination [4]. Thus, improvement of crystallinity is apossible way to increase the film photocatalytic ability.

In addition, though WO3 has a conduction band thatallows for the transfer of photogenerated electrons fromTiO2, its valence band is not positioned properly towardto the coupled TiO2; therefore, effective charge separa-tion cannot be fully obtained in crystalline TiO2/WO3

heterostructures [41]. The formation of WO3 crystallites,randomly distributed along TiO2 nanocrystals, rather thanthe highly adsorbing WO3 monolayer or amorphous WOx,reduces the adsorption capacity of WO3 [20, 41]. Finally, theamount of tungsten loaded onto TiO2 was not optimizedherein, but this factor reportedly is important in determiningPC activity [9, 16, 21, 46].

4. Conclusions

In this study, a porous F-TiO2/WO3 film with crystal-lized anatase TiO2 phase and network-framed structurewas successfully obtained by microarc oxidation withoutsubsequent heat treatment. The F-TiO2/WO3 film exhibitshigh photocatalytic activity in MB degradation under UVand BLED irradiation. The Vis PC efficiency is dominatedby the incorporation of tungsten oxides and possiblya complex of WxTi1−xO2. The specific surface area of the film


is one of the most important parameters in determining theefficiency of the PC process, because a higher specific surfacearea favors the adsorption of more MB molecules on itsactive sites. Thus, the elucidated MAO process is one of themost cost-effective methods for producing films in practicalphotocatalytic applications.

Acknowledgment

The authors would like to thank the National Science Councilof the Republic of China, Taiwan, for financially supportingthis research under Contract No. NSC 98-2221-E-022-004-MY2.

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Research Article

Preparation of TiO2-Fullerene Composites andTheir Photocatalytic Activity under Visible Light

Ken-ichi Katsumata, Nobuhiro Matsushita, and Kiyoshi Okada

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,Kanagawa, Yokohama 226-8503, Japan

Correspondence should be addressed to Ken-ichi Katsumata, [email protected]



Copyright © 2012 Ken-ichi Katsumata et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The development of visible light-sensitive photocatalytic materials is being investigated. In this study, the anatase and rutile-C60

composites were prepared by solution process. The characterization of the samples was conducted by using XRD, UV-vis, FT-IR,Raman, and TEM. The photocatalytic activity of the samples was evaluated by the decolorization of the methylene blue. From theresults of the Raman, FT-IR, and XRD, the existence of the C60 was confirmed in the samples. The C60 was modified on the anataseor rutile particle as a cluster. The C60 didn’t have the photocatalytic activity under UV and visible light. The anatase and rutile-C60

composites exhibited lower photocatalytic activity than the anatase and rutile under UV light. The anatase-C60 exhibited also loweractivity than the anatase under visible light. On the other hand, the rutile-C60 exhibited higher activity than the rutile under visiblelight. It is considered that the photogenerated electrons can transfer from the C60 to the rutile under visible light irradiation.

1. Introduction

Since the photocatalytic activity of a TiO2 was discoveredin 1970s, it has been studied by many researchers becausenontoxic, chemical stable, inexpensive, and widely available[1, 2]. When TiO2 is irradiated by ultraviolet (UV) light,electron (e−) and hole (h+) pairs are generated and produceradical species such as OH radicals and O2

− by reactionwith moisture in the atmosphere, which reduce and oxidizeadsorbates on the surface. These radicals can decomposemost organic compounds and bacteria [3–7]. Therefore, thephotocatalyst has been applied on various industrial fields[8, 9]. However, since the band gap of TiO2 is 3.0–3.2 eV, theTiO2 photocatalyst is activated only by UV light irradiation(<400 nm) [10].

To exhibit the photocatalytic activity under visible light,metal (Cr, V, Fe, Mn, Co, and Ni) doping into Ti site andanion (N, S, and C) doping into O site have been studied[11–20]. These doped TiO2 can be sensitive to visible light,but the oxidative power of holes decreases. Because the holesare generated in a localized narrow band originating from thedopant metal ions or anions in the forbidden band of TiO2.

Recently, it has been reported that TiO2 powders with graftedmetal ions (Cu(II), Cr(III), Ce(III), and Fe(III)) were capableof serving as photocatalysts sensitive to visible light [21–25].This system is called the interfacial charge transfer (IFCT)and has greatly an oxidative decomposition activity.

Among these materials, carbon-supported TiO2 also ex-hibited the photocatalytic activity under visible light [26–30]. Fullerene (C60) has the interesting properties which arethe delocalized conjugated structures and electron-acceptingability. C60 can efficiently promote a rapid photoinducedcharge separation and slow charge recombination. Althoughthe role of C60, accepting the photogenerated electrons fromTiO2 particles, has been demonstrated, a few efforts aremade to utilize the unique properties of C60 to increasethe efficiency of photocatalysis [31, 32]. However, themechanism is not clear in detail.

In this study, TiO2-C60 composites were prepared andcharacterized, and the photocatalytic activity under UVand visible light was evaluated, comparing to the pureTiO2. The mechanism of visible light-sensitive photocatalystwas investigated by using the photodeposition of platinum(Pt).


400 500 600 700 800Wavelength (nm)

Abs

orba

nce

(a.u

.)

Anatase

A1C

A5C

A10C

(a)A

bsor

ban

ce(a

.u.)

400 500 600 700 800Wavelength (nm)

Rutile

R1C

R5C

R10C

(b)

Figure 1: UV-vis spectra of the samples. (a) The anatase, A1C, A5C, and A10C samples, (b) the rutile, R1C, R5C, and R10C samples.

2. Experimental

2.1. Preparation of TiO2-Fullerene Composites. Fullerene(C60) solution was prepared by adding C60 (1, 5, 10 mg)into toluene (30 mL) and mixing for 10 min. TiO2 powder(anatase or rutile; 100 mg) was put in the solution, and itwas mixed at 80◦C for 24 hours. Then, the solution wasevaporated and dried at 90◦C, and the TiO2-C60 compositesamples were obtained. The anatase powder and toluenewere taken from Wako Pure Chemical Industries, Tokyo,Japan. The C60 (carbon cluster; ST) was taken from KantoChemicals Co. Inc., Tokyo, Japan. The rutile powder (MT-150A) was taken from TAYCA Co., Tokyo, japan. The sampleswith different C60-additive amount are denoted in this reportas “AXC or RXC.” A, R, X, and C indicate the anatase, rutile,C60 additive amount, and C60, respectively. For example, A1Cmeans 1 mass % C60 added anatase sample.

2.2. Synthesis of Photodeposited Samples. The prepared TiO2-fullerene composite samples were dispersed in 3 : 7 (v/v)methanol/water solution (20 mL), and then the requiredamount (5 mass%) H2PtCl6 solution was added in. Themixture was irradiated by fluorescent light with cut filter(>420 nm) for 3 hours at room temperature. After irradia-tion, the sample was collected by centrifugation and washedseveral times with distilled water. The obtained paste wasdried at 80◦C.

2.3. Characterization of the Samples. The crystalline phaseswere identified by a high-power X-ray diffractometer (XRD,RINT-TTR3B; Rigaku, Japan) using monochromated CuKα radiation (50 kV–200 mA). The samples were analyzedusing a Raman spectroscopy (RAMANOR T64000; Jobin-Yvon S.A.S., France) with an Ar laser (514.5 nm) operated at

50 mW. UV-visible absorption properties of the samples weremeasured using a UV-visible scanning spectrophotometer(UV-vis, Lambda 35; PerkinElmer Inc., USA). Transmissionelectron microscopy (TEM) observation of the sampleswas performed using a transmission electron microscope(TEM, HF-2000, Hitachi, Japan) operating at 200 kV. ForTEM observation, one drop of the sample, dispersed inwater, was deposited on an amorphous carbon grid. IRmeasurements were performed using a Fourier transform-infrared spectroscopy (FT-IR, JIR-7000, JEOL, Japan).

2.4. Evaluation of Photocatalytic Activity. Photocatalytic ac-tivity of the samples was evaluated by decomposition ofmethylene blue (MB; C16H18ClN3S). The samples wereimmersed in 0.02 mM MB aqueous solution for overnightto saturate the adsorption. After washing by ultrapurewater, a cylinder (φ40 × 30 mm) was contacted on theSiO2 glass (50 × 50 × 2t mm) by using silicone grease,and 0.01 mM MB aqueous solution was poured into thecylinder. And then, the samples (0.05 g) were put into thesolution. Irradiating overhead UV light (1.0 mW/cm2) orfluorescent light (10,000 lx), the absorption spectra of MBwere measured by a UV-vis spectrophotometer.


3.1. Characterization of the TiO2-C60 Composite Samples.Anatase and rutile were white powders because they cannotabsorb the wavelength in visible range. The obtained samplewith C60, however, had a color. With increasing C60-additiveamount, the color of the samples became gray. Figure 1shows the UV-vis absorption spectra of the samples. Theadsorption of anatase and rutile started from <400 nm. Onthe other hand, the sample with C60 adsorbed the photon of


10 20 30 40 50 60

Inte

nsi

ty(a

.u.)

A1C

A5C

A10C

C C C

A

R R RAA

AA

AA

A: anatase

R: rutile

C: C60

Cu Kα 2θ (deg)

(a)

10 20 30 40 50 60

R: rutile

C: C60

Cu Kα 2θ (deg)

Inte

nsi

ty(a

.u.)

R1C

R5C

R10C

C C C

R

R

RR

R

R R

(b)

Figure 2: XRD patterns of the TiO2-C60 composite samples. (a) The A1C, A5C, and A10C samples, (b) the R1C, R5C, and R10C samples.

400800120016002000

Wavenumber (cm−1)

Tran

smit

tan

ce

A10C

A5C

A1C

1429 11821618

(a)

400800120016002000

Wavenumber (cm−1)

1429 11821618

Tran

smit

tan

ce

R10C

R5C

R1C

(b)

Figure 3: IR spectra of the TiO2−C60 composite samples. (a) the A1C, A5C, and A10C samples, (b) the R1C, R5C, and R10C samples.

>400 nm, and the adsorption edge was about 700 nm. This isindicated that the sample with C60 had absorption propertyin visible range. The spectrum shape of the sample with C60

was similar to the C60, and the absorption increased withincreasing C60-additive amount. Therefore, it is consideredthat the absorption in visible range is attributed to the C60.

Figure 2 shows the XRD patterns of the samples. Thecrystalline phases detected in the anatase composite samples

were anatase, rutile, and C60 (Figure 2(a)). Rutile was alittle included in the anatase composite sample. This isattributed to the starting materials (chemical), and it wasnot formed during preparation process of the anatase-C60

composite samples. The C60 peaks of the A1C sample werenot seen clearly, but the peaks appeared in the A5C andA10C samples. In the case of the rutile composite samples,the crystalline phases were rutile and C60, and anatase was


1300 1400 1500 1600 1700


Inte

nsi

ty(a

.u.)

A1C

A5C

A10C

1462

(a)

1300 1400 1500 1600 1700


Inte

nsi

ty(a

.u.)

1462

R1C

R5C

R10C

(b)

Figure 4: Raman spectra of the TiO2−C60 composite samples. (a) The A1C, A5C, and A10C samples, (b) the R1C, R5C, and R10C samples.

5 nm

0.32 nm

R5C

C60

d110 (rutile)

Figure 5: High resolution TEM image of the R5C samples.

not included (Figure 2(b)). The C60 peaks of the R1C samplewere not seen clearly, but the peaks appeared in the R5C andR10C samples. This is the similar tendency in the anatasecomposite samples. In both cases, the values of the full widthat half maximum (FWHM) in the anatase (101) peak (2θ =25.32◦) and rutile (110) peak (2θ = 27.44◦) were almost thesame, respectively. This is indicated that the crystallinity ofthe anatase or rutile composite samples have almost the samecrystallinity, respectively.

Figure 3 shows the IR adsorption spectra of the samplesmeasured in air at room temperature. In the infrared bandsof C60, there are the four most intense lines at 1429, 1183,577, and 528 cm−1 [33, 34]. The absorption bands at 1429and 1183 cm−1 were observed in A5C, A10C, R5C, andR10C samples, but those were not observed in A1C and

R1C samples (Figures 3(a) and 3(b)). The adsorption bandsat 577 and 528 cm−1 were not seen in all samples becausewidely adsorption of TiO2 was at 400–900 cm−1. On theother hand, the adsorption band at 1618 cm−1 was seen inall samples. This is attributed to the bending vibration of theH2O molecule adsorbed on Ti4+ site [35].

The Raman spectra of the samples are shown in Figure 4.The peak at 1642 cm−1 appeared in all samples (Figures 4(a)and 4(b)). Cataldo [36] reported that the Raman spectrumof pure C60 is characterized by several lines, the most intensewhich are the Ag(1) and Ag(2) modes lying, respectively, at496 and at 1469 cm−1. The Ag(2) mode is also referred asthe pentagonal pinch mode. It is considered that the peakobserved at 1642 cm−1 is Ag(2) mode of C60. The existenceof the C60 in A1C and R1C samples could not be detectedby XRD and IR, but it could be confirmed by the Ramanspectra. In A10C and R10C samples, the broad band ataround 1600 cm−1 appeared. It is guessed that the band isattributed to the carbon cluster.

Figure 5 shows the high resolution TEM image of theR5C sample. Considering the space of the lattice fringe,the particle was confirmed to rutile. Some small particleswere observed at the edge of the rutile particle. In the caseof the anatase-C60 composite samples, the small particleswere also observed at the edge of the anatase particles (notshown here). These particles were not the rutile and anataseparticles. It is guessed that these small particles are the C60

particles adhered to the rutile particles. In the TiO2-C60

composite samples, we consider that the C60 particles aredirectly present on the surface of the TiO2 particles.

3.2. Evaluation of the Photocatalytic Activity of the TiO2-C60 Composite Samples. Figure 6 shows the photocatalyticdegradation of MB in the C60. If the C60 has photocatalytic


0 10 20 30 40 50 60

0.2

0.4

0.6

0.8

1


C/C

0

0

(a)

0.2

0.4

0.6

0.8

1

C/C

0

0 60 120 180 240


0

(b)

Figure 6: Changes of the MB concentrations decomposed by the C60 as a function of (a) UV irradiation time (1.0 mW/cm2), (b) visible lightirradiation time (10,000 lx).

0 10 20 30 40 50 60

0.2

0.4

0.6

0.8

1


C/C

0

AnataseA1C

A5CA10C

0

(a)

0 10 20 30 40 50 60

0.2

0.4

0.6

0.8

1


C/C

0

R1CR5CR10C

Rutile

0

(b)

Figure 7: Changes of the MB concentrations decomposed by the samples as a function of UV irradiation time (1.0 mW/cm2). (a) Theanatase, A1C, A5C, and A10C samples, (b) The rutile, R1C, R5C, and R10C samples.

activity, MB is decolorized. Figure 6(a) shows the test underUV irradiation. When UV irradiation time increased, thevariation of the MB concentration was little. This result indi-cates that the C60 has little photocatalytic activity under UVlight irradiation. The MB concentration slightly decreasedwith increasing visible light irradiation time (Figure 6(b)).

However, the decrement of the MB concentration was a little.It is considered that the C60 exhibited little the photocatalyticactivity under UV and visible light.

Figure 7 shows the photocatalytic degradation of MBin the anatase, rutile, and anatase or rutile-C60 compositesamples under UV irradiation. When UV irradiation time


0 60 120 180 2400.5

0.6

0.7

0.8

1


C/C

0

AnataseA1C

A5CA10C

0.9

(a)

0 60 120 180 2400.5

0.6

0.7

0.8

1


C/C

0

0.9

R1CR5CR10C

Rutile

(b)

Figure 8: Changes of the MB concentrations decomposed by the samples as a function of visible light irradiation time (10,000 lx). (a) Theanatase, A1C, A5C, and A10C samples, (b) The rutile, R1C, R5C, and R10C samples.

20 nm

A5C

C60 clusterPt

Anatase

(a)

10 nm

R5C

Rutile

Pt

(b)

Figure 9: TEM images of the A5C and R5C samples after the photodeposition of Pt particles.

increased, the MB concentration of the anatase, A1C, A5C,and A10C samples decreased (Figure 7(a)). Among them,the MB concentration of the anatase decreased drastically.This result indicates that the photocatalytic activity of theA1C, A5C, and A10C samples is lower than that of theanatase. The activity of the A1C sample was higher thanthat of the A5C and A10C samples, and the A5C sampleindicates a similar tendency to the A10C sample. Withincreasing the C60 additive amount, the activity was lowerin the anatase-C60 composite samples. In the case of therutile and rutile-C60 composite samples, the decrement ofthe MB concentration in the rutile became the largest

(Figure 7(b)). This is indicated that the photocatalytic activ-ity of the rutile is higher than that of the rutile with theC60 (R1C, R5C, and R10C samples). The activity of theR10C sample was higher than that of the R1C and R5Csamples, and the R5C sample was higher than the R1Csample. The tendency was different from the anatase-C60

composite samples. The anatase and rutile-C60 compositesamples exhibit lower photocatalytic activity under UVirradiation than anatase and rutile. It is guessed that theanatase and rutile surfaces are covered by the C60, andthe number of absorbed photons in the anatase and rutiledecreases.


Figure 8 shows the photocatalytic degradation of MBin the anatase, rutile, and anatase or rutile-C60 compositesamples under visible light irradiation. In Figures 8(a)and 8(b), the MB concentration of the anatase and rutiledecreased with increasing visible light irradiation time.This result indicates that the anatase and rutile have thephotocatalytic activity under visible light irradiation. Thisis because fluorescent light was used as a light source,so a little UV light was included in the light source. Allsamples exhibited the photocatalytic activity under visiblelight irradiation shown in Figure 8(a). But the activity of theA1C, A5C, and A10C samples was lower than the anatase.This is similar tendency to the case of UV irradiation(Figure 7(a)). On the other hand, the R1C, R5C, and R10Csamples exhibited higher photocatalytic activity than therutile (Figure 8(b)). This result indicates that the rutile-C60

composite samples have greatly the photocatalytic activityunder visible light.

The band structures of the anatase, rutile, and C60 areshown in Scheme 1. In the case of the C60, some researchersreported different band structures [37, 38]. The conductionband (CB) positions of the C60 were higher energy than thoseof the anatase and rutile. Therefore, it is prospective that thephotogenerated electrons transfer from the CB of the C60

to the CB of the anatase and rutile, and the anatase andrutile-C60 composite samples have a higher photocatalyticactivity than the anatase and rutile under visible light shownin Scheme 2. However, the anatase-C60 composite samplesexhibited higher photocatalytic activity under visible lightthan the anatase (Figure 8(a)).

Figure 9 shows the TEM images of the A5C and R5Csamples after conducting the photodeposition of Pt undervisible light irradiation. In the case of A5C sample, thephotodeposited Pt particles were not observed at the anataseparticles but the C60 cluster. This is indicated that thephotogenerated electrons cannot transfer from the C60 tothe anatase (Scheme 3(a)). On the other hand, in the caseof the R5C sample, the photodeposited Pt particles wereobserved at the rutile. This result indicates that the photo-generated electrons can transfer from the C60 to the rutile(Scheme 3(b)). Therefore, it is considered that the rutile-C60

composite samples exhibit the photocatalytic activity undervisible light.

From the band structure shown in Scheme 1, it is possiblethat the photogenerated electrons transfer from the C60 tothe anatase. In this study, however, it did not occur. Itis guessed that the connecting state between the C60 andthe anatase is one of the reasons why the photogeneratedelectrons cannot transfer from the C60 to the anatase. Furtherinvestigation is needed to clear the reasons.

Photocatalytic activity of the C60-rutile composite sam-ple was superior to the C60-anatase sample under visiblelight (Figure 7). On the other hand, the C60-rutile sampleexhibited lower activity than the C60-anatase sample underUV light (Figure 8). In this study, the number of photonsabsorbed to the samples was not uniformed in UV and visiblelights shown in UV-visible absorption spectra (Figure 1). Itis, therefore, difficult to compare the photocatalytic activitiesof the C60-anatase and C60-rutile samples under UV-visible

+3.04

+0.04

−0.16−0.34−0.84

+1.96+1.76

2.3 eV 3.2 eV 3 eV2.6 eV

C6038)C60

37) Anatase Rutile

V versus SHE

VB

(0)H+/H2

CB

(+1.23)O2/H2O

Scheme 1: Schematic illustrations of the band structures of theanatase, rutile, and C60.

e−

VB

CB

Visible light

Reduction

TiO2

C60

Scheme 2: Schematic illustration of the photogenerated electrontransfer model of the TiO2-C60 composite under visible lightirradiation.

Anatase

e−

Pt ion

Pte−

Rutile

PtPt ion

(a) (b)

Scheme 3: Schematic illustrations of the photodeposition model ofPt particles on (a) the anatase-C60 composite and (b) the rutile-C60

composite under visible light irradiation.

light. However, the photocatalytic degradation rate of MBwas greatly different between under UV and visible lights,and the rate under UV light was higher than that undervisible light. It is guessed that the C60-anatase sample exhibitshigher activity than the C60-rutile under UV-visible light.

4. Conclusions

In present study, the anatase and rutile-C60 composites wereprepared, and the photocatalytic activity of the compositeswas investigated by the MB decolorization test. The C60 parti-cles were directly adhered to the surface of the TiO2 particles.When UV light was irradiated, the photocatalytic activity ofthe anatase and rutile-C60 composites became lower than theanatase and rutile particles without the C60. In the case of thevisible light irradiation, the anatase-C60 composite exhibitedalso lower activity than the anatase. However, the rutile-C60


composite exhibited higher activity than the rutile. Fromthe photodeposition of Pt on the composites under visiblelight, the photogenerated electron transfer from the C60 tothe rutile occurred although the electron transfer did notoccur in the anatase-C60 composite. Therefore, the rutile-C60

composite exhibits the photocatalytic activity under visiblelight. The rutile-C60 can be utilized for the new type of visiblelight sensitive photocatalyst.

Acknowledgments

The authors are grateful to Mr. Y. Komatsubata in TokyoInstitute of Technology for the HR-TEM observation. Thiswork was supported, in part, by a “Grant-in-Aid for Coop-erative Research Project of Nationwide Joint-Use ResearchInstitutes on Advanced Materials Development and Integra-tion of Novel Structured Metallic and Inorganic Materials”and “Inamori Foundation.”

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Research Article

Photocatalytic Ethanol Oxidative Dehydrogenation overPt/TiO2: Effect of the Addition of Blue Phosphors

J. J. Murcia,1 M. C. Hidalgo,1 J. A. Navıo,1 V. Vaiano,2 P. Ciambelli,2, 3 and D. Sannino2, 3

1 Instituto de Ciencia de Materiales de Sevilla (ICMS), Consejo Superior de Investigaciones Cientıficas CSIC,Universidad de Sevilla, Americo Vespucio 49, 41092 Sevilla, Spain

2 Department of Industrial Engineering, University of Salerno, Via Ponte Don Melillo, 84084 Fisciano, Italy3 Nano Mates, Research Centre for Nanomaterials and Nanotechnology at Salerno University, University of Salerno,Via Ponte Don Melillo, 84084 Fisciano, Italy

Correspondence should be addressed to D. Sannino, [email protected]



Copyright © 2012 J. J. Murcia et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Ethanol oxidative dehydrogenation over Pt/TiO2 photocatalyst, in the presence and absence of blue phosphors, was performed. Thecatalyst was prepared by photodeposition of Pt on sulphated TiO2. This material was tested in a gas-solid photocatalytic fluidizedbed reactor at high illumination efficiency. The effect of the addition of blue phosphors into the fluidized bed has been evaluated.The synthesized catalysts were extensively characterized by different techniques. Pt/TiO2 with a loading of 0.5 wt% of Pt appearedto be an active photocatalyst in the selective partial oxidation of ethanol to acetaldehyde improving its activity and selectivitycompared to pure TiO2. In the same way, a notable enhancement of ethanol conversion in the presence of the blue phosphors hasbeen obtained. The blue phosphors produced an increase in the level of ethanol conversion over the Pt/TiO2 catalyst, keeping atthe same time the high selectivity to acetaldehyde.

1. Introduction

The selective oxidation of alcohols is one of the most impor-tant reactions in organic chemistry. The conversion of pri-mary alcohols to aldehydes is very important for the synthesisof fine chemicals such as fragrances or food additives [1–6].There is a great industrial interest to convert low alcohols intouseful organic intermediates or products, that is, ethene, di-ethyl ether, or acetaldehyde [7]. Ethanol serves as a feedstockfor acetaldehyde production by oxidative dehydrogenationover pure or mixed oxide catalysts [8–10]. Acetaldehyde is animportant intermediary in organic syntheses, and this com-pound is normally obtained by catalytic ethanol partial oxi–dation [11, 12]. The catalytic oxidation of ethanol has gener-ally been investigated in order to develop catalysts that maxi-mize products, such as acetaldehyde or acetic acid, and mini-mize production of deep oxidation reactions [11].

Different supported metals, such as Mo, Fe, Co, Ni, Cu,Ag, V, and Au, has been studied in the current literature for

the reaction of partial oxidation of ethanol by thermal cataly-sis [11–18]. It is important to note that the selective oxidationof alcohols to either aldehydes or acid in the presence of anoble metal catalyst is of simple work-up procedure, showsa wide applicability to various alcohols, and is an attractive,environment-friendly process [19, 20].

On the other hand, heterogeneous photocatalysis usingsemiconductor oxides has demonstrated to be very effectivein the oxidation of different organic compounds. The hetero-geneous photocatalysis based on TiO2 is an interesting alter-native because this oxide is an effective, photostable, reusable,inexpensive, nontoxic, and easily available catalyst [21]. Inthe past, the majority of the research in the field of photoca-talysis was focused on the use of TiO2 photocatalysts for thepurification of water or gas atmospheres from environmen-tal contaminants [22, 23]. However, partial photocatalyticoxidation in the gas phase has recently attracted great interestdue to the high potential of this technique in green chemistry[24]. Metal deposition on TiO2 has been intensively studied


as a means of reducing electron/hole recombination and en-hancing efficiencies of TiO2 in the photocatalytic degrada-tion of organic compounds [25]. Pt deposited on TiO2 hasbeen reported to improve [26–31], be detrimental [29, 32],or have negligible effects [30] on photocatalysis dependingon many different factors.

Chen et al. [32] studied the photocatalytic partial oxida-tion of ethanol over TiO2 and Pt/TiO2 and found that theselectivity to acetaldehyde (which is more easily oxidized)was enhanced with the platinization as opposed to the higherformation of acetic acid with bare TiO2. These authors ex-plained that loaded on TiO2, Pt can accelerate the oxygenreduction process occurring at the cathodic area, thereby di-minishing the electron accumulation on the surface of TiO2

particles. This would accelerate the oxidation rate of alcohols,which, in fact, is controlled by the cathodic reduction ofoxygen [32, 33]. This process has been often observed notonly for alcohols, but also for other organic compounds.Vorontsov and Dubovitskaya [34] reported also up to a twotimes increase in the rate of ethanol photooxidation at vari-ous Pt loadings.

In previous papers regarding the oxidative dehydrogena-tion of organic compounds using fluidized bed photoreac-tors [17, 35–37], it has been evinced that UV light does notpenetrate in the reactor core volume, since it is mostly ab-sorbed by the catalyst circulating near the irradiated reactorwindows within few millimetres [17]. To improve photontransfer in the reactor core volume, the reactor thickness hasto be reduced, and, moreover, it is possible to mix the pho-tocatalyst with emitting phosphorescent particles as lightcarriers, known generally as phosphors. Recently, some of ushave reported that the addition of phosphors into a fluidizedbed has, therefore, resulted in about doubling of the cat-alytic surface totally irradiated [17], allowing a considerablyincrease of the photocatalytic activity. In this paper, the selec-tive partial oxidation of ethanol to acetaldehyde over Pt/TiO2

photocatalyst was studied. The photocatalytic reactions werecarried out in a gas-solid photocatalytic fluidized bed reactor.The study of the effect of the addition of blue phosphors intothe fluidized bed was also attempted.

2. Experimental

2.1. Synthesis Procedure. TiO2 used as starting material wasprepared by the hydrolysis of titanium tetraisopropoxide(Aldrich, 97%) in isopropanol solution (1.6 M) by the slowaddition of distilled water (volume ratio isopropanol/water1 : 1). Afterward, the generated precipitate was filtered anddried at 110◦C overnight. The powders thus obtained werethen sulphated by immersion in 1 M sulphuric acid solutionfor 1 h and calcinated at 650◦C for 2 h. Sulphate treatmentwas carried out for two reasons. On one hand, previous re-sults have shown that sulphate pretreatment stabilizes ana-tase phase up to high temperatures and protects surface areaagainst sintering [38]. On the other hand, at the calcinationtemperature of 650◦C, the elimination of sulphate groupspromotes the creation of high number of oxygen vacancies,

which have been reported as preferential sites for Pt adsorp-tion [39].

Photodeposition of platinum was performed overthe calcined TiO2 powder using hexachloroplatinic acid(H2PtCl6, Aldrich 99.9%) as metal precursor. Under an inertatmosphere (N2), a suspension of TiO2 in distilled water con-taining isopropanol (Merck 99.8%) which acts as sacrificialdonor was prepared. Then, the appropriate amount ofH2PtCl6 to obtain a nominal platinum loading of 0.5%weight total to TiO2 was added. Final pH of the suspensionswas 3. Photodeposition of platinum was then performed byilluminating the suspension during 120 min with an OsramUltra-Vitalux lamp (300 W) with a sun-like radiation spec-trum and a main emission line in the UVA range at 365 nm.The intensity of the lamp was 140 W/m2. After photodepo-sition, the powders were recovered by filtration and dried at110◦C overnight.

Blue phosphors (model RL-UV-B-Y; Excitation Wave-length: 365 nm; Emission Wavelength: 440 nm; particles di-ameter: 5–10 μm) were provided by DB Chemic. A widecharacterization of this material was carried out, and theseresults are collected in a recent article [17], evidencing thatthe host crystal structure of the phosphors used in this workis ZnS.

2.2. Characterization Techniques. Crystalline phase composi-tion and degree of crystallinity of the samples were estimatedby X-ray diffraction (XRD). XRD patterns were obtained ona Siemens D-501 diffractometer with Ni filter and graphitemonochromator using Cu Kα radiation. Crystallite sizes werecalculated from the line broadening of the main X-ray dif-fraction peaks by using the Scherrer equation. Peaks were fit-ted by using a Voigt function.

Laser Raman spectra of catalyst were obtained at roomtemperature with a Dispersive MicroRaman (Invia, Ren-ishaw), equipped with 785 nm diode-laser, in the range 100–2500 cm−1 Raman shift.

Light absorption properties of the samples were studiedby UV-Vis spectroscopy. The UV-Vis DRS spectra were re-corded by a Perkin Elmer spectrometer Lambda 35. Band-gaps values were calculated from the corresponding Kubelka-Munk functions, F(R∞), which are proportional to the ab-sorption of radiation, by plotting (F(R∞) · hν)2 against hν.

BET surface area and porosity measurements were car-ried out by N2 adsorption at 77 K using a MicromeriticsASAP 2010 instrument.

Chemical composition and total platinum content of thesamples were determined by X-ray fluorescence spectrome-try (XRF) in a Panalytical Axios sequential spectrophotom-eter equipped with a rhodium tube as the source of radiation.XRF measurements were performed onto pressed pellets(sample included in 10 wt% of wax).

Thermogravimetric analysis (TG-DTG) of the sampleswas carried out in air flow with a thermobalance (SDT Q600,TA Instruments), in the range 20◦C–1000◦C at heating rateof 10◦C/min.

Field emission SEM images were obtained in a HitachiS-4800 microscope. The samples were dispersed in ethanol


20 30 40 50 60 70 80

Inte

nsi

ty(a

.u.)

Pt/TiO2

TiO2

2θ

Figure 1: XRD patterns for TiO2 and Pt/TiO2 photocatalysts.

using an ultrasonicator and dropped on a carbon grid. Theplatinum particle sizes were evaluated by TEM, in a micro-scope Philips CM 200.

X-ray photoelectron spectroscopy (XPS) studies werecarried out on a Leybold-Heraeus LHS-10 spectrometer,working with constant pass energy of 50 eV. The spectrom-eter main chamber, working at a pressure < 2 × 10−9 Torr,is equipped with an EA-200 MCD hemispherical electronanalyser with a dual X-ray source working with Al Kα (hυ =1486.6 eV) at 120 W and 30 mA. C 1 s signal (284.6 eV) wasused as internal energy reference in all the experiments. Sam-ples were outgassed in the prechamber of the instrument at150◦C up to a pressure < 2 × 10−8 Torr to remove chemis-orbed water.

2.3. Photocatalytic Tests. Photocatalytic tests were carried outwith a feeding 30 L/h (STP), at ethanol concentration inthe range 0.1–2 vol.%, in helium flow with oxygen/ethanolratio of 2. Temperature and pressure reaction were 60◦C and1 atm, respectively. Oxygen and helium were fed from cylin-ders, helium being the carrier gas for ethanol vaporized froma temperature controlled saturator. Different concentrationsof ethanol in the reaction feed were obtained by changingtemperature and He flow through the saturator. The gas flowrates were measured and regulated by mass flow controllers(Brooks Instrument).

The fluidized bed reactor used in this work was designedfor working with a gas flow rate in the range 20–70 L/h (STP)with a Sauter average diameter in the particles size range50–100 μm, assuring optimal fluidization [17, 35]. It was atwo-dimensional reactor with 40 mm × 6 mm cross-section,230 mm height pyrex-glass walls, and a bronze filter (meanpores size 5 μm) to provide a uniform distribution of fed gas.In order to decrease the amount of transported particles, anexpanding section (50 mm× 50 mm cross-section at the top)and a cyclone specifically designed [36] are located on thetop and at the outlet of the reactor, respectively. The reactorwas illuminated by two UVA-LEDs modules (80 × 50 mm)

0 200 400 600 800


Cou

nts

(a.u

.)

Pt/TiO2

TiO2

Figure 2: Raman spectra of TiO2 and Pt/TiO2 photocatalysts.

positioned in front of the reactor pyrex windows (light inten-sity: 90 mW/cm2). Each module consisted of 40 UV-LEDsemitting at 365 nm (provided by Nichia Corporation). Withthese illumination conditions, the light pathlength in thephotoreactor was about 2 mm. The catalytic bed was com-posed by 1.2 g of catalyst diluted with 20 g of glass spheres(grain size: 70–110 μm) (Lampugnani Sandblasting HI-TECH).

To optimize the composition of fluidizable solid mixture,photocatalytic tests were also carried out in the presence ofblue phosphors. For these tests, the catalytic bed was com-posed by 1.2 g of catalyst and 1.8 g of blue phosphors dilutedwith 20 g of glass spheres. In this way, phosphors were flu-idized with the catalyst, excited by external UVA-LEDs, andemitted their stored energy in the proximity to the catalyst.

Hereon, the catalytic systems are denoted as Pt/TiO2 andPt/TiO2 + Blue P for the setups without and with phosphors,respectively.

Concentrations of inlet reactants and outlet productswere measured by an online mass detector (MS) (QuantraFourier transform ion cyclotron resonance mass spectrom-eter, Siemens) and a continuous CO-CO2 NDIR analyzer(Uras 10, Hartmann and Braun).

Preliminary tests were carried out to check the amountof solid particles elutriated from the reactor by fluidizing thepowders for several hours. Elutriation was negligible, con-firmed also by the stability of catalytic activity during irrad-iation time in the photocatalytic tests. In addition, experi-mental tests to check the fluidization properties were realiz-ed, both in the presence and absence of phosphors. Thesetests showed that the expansion of fluidized bed was the samein both cases, evidencing that the fluidization regime did notchange in the presence of phosphors.


Table 1: Summary of characterization results.

Catalyst SBET (m2/g) Anatase crystallite size (nm) S (SO3%) nOH/g (mol/g) nOH/m2(mol/m2) Ethanol adsorption (mmol/g)

TiO2 58.3 20 0.52 6.6 E−4 8.8 E−6 0.5

Pt/TiO2 59.0 21 0.39 1.2 E−3 2.1 E−5 1.6

Abs

orba

nce

(a.u

.)

1

0.8

0.6

0.4

0.2

0

200 300 400 500 600 700 800

Waveleng ht (nm)

Blue phosphor

Pt/TiO2

TiO2

Figure 3: UV-Vis DRS spectra for TiO2 and Pt/TiO2 photocatalystscompared with phosphors emission.


3.1. Characterization of the Catalysts. The XRD patterns(Figure 1) and Raman spectra (Figure 2) for sulphated TiO2

and Pt/TiO2 photocatalyst showed that anatase is the onlycrystalline phase present in these samples. The stabilizationof anatase phase by sulphate pretreatment of the TiO2 canbe noticed here, as no traces of rutile were found even afterthe high calcination temperature used (650◦C) [38, 40]. Nopeaks corresponding to platinum was detected by XRD dueto the low loading and high dispersion of metal present inthe Pt/TiO2 catalyst. Anatase crystallite sizes of the sampleswere estimated by the Scherrer equation, and the values arepresented in Table 1. As it can be observed, the addition ofplatinum did not induce any significant change in the anatasecrystallite size of TiO2 (20-21 nm).

Figure 3 shows the UV-Vis DRS spectra for Pt/TiO2 cata-lyst and the starting material (sulphated TiO2). No signif-icant differences between the spectra analyzed are visible forwavelengths below 400 nm in which the characteristic sharpabsorption threshold of TiO2 around 350 nm can be observ-ed. For Pt/TiO2 catalyst, in the range between 400 and600 nm, there is the presence of a very broad absorption bandprobably due to an interaction between platinum surfacespecies and titanium dioxide. Figure 3 shows also the UV-VisDRS spectrum of blue phosphors; this sample has absorptionat wavelength of 320 nm and band gap energy of 3.1 eV.

Differences in the absorption properties of the two cat-alysts are markedly evinced by plotting [F(R∞)∗ hν]2 ver-sus hν (Figure 4) and correspond to a decrease in band gap

2

1.5

1

0.5

01 1.5 2 2.5 3 3.5 4 4.5 5

Pt/TiO2 TiO2

hA (eV)(F

(R∞

)·h

A)2

Figure 4: Band gap calculus from UV-Vis DRS spectra for TiO2 andPt/TiO2 catalysts.

300 400 500 600 700

Wavelength (nm)

Ku

belk

aM

un

k(a

.u.)

Pt/TiO2 absorption

emissionsPhosphor

emis

sion

(a.u

.)s

Ph

osph

or

Figure 5: Comparison between Pt/TiO2 absorption and phosphorsemission.

energies, from 3.4 eV for TiO2 to 2.8 eV for Pt/TiO2 sample.This last value implies that the activation energy can be pro-vided by the energy related to the wavelength emission of theselected phosphors (Figure 5).

BET surface area values for the analyzed samples are giv-en in Table 1. As it can be observed, a little increase in theTiO2 SBET value was produced by the process of platinumphotodeposition.


Table 2: XPS results.

CatalystsBinding energy (eV)

Pt0(%) Ptδ+(%)Ti 2p3/2 O 1s

TiO2 458.5 529.8 — —

Pt/TiO2 458.4 529.6 70.2 29.8

Pt

400 nm×120 k

Figure 6: Selected scanning electron image of Pt/TiO2 photocata-lyst.

Total amount of platinum in the Pt/TiO2 photocatalystwas calculated by XRF, being 0.41%. This value is close to thenominal content (0.5%) indicating a high yield for the pho-todeposition process. XRF analysis also showed the presenceof sulphur from the sulphate pretreatment even though theamount of this element was low corresponding just to a0.39%.

Hydroxyls surface densities were evaluated by TG-DTGlosses in the temperature range of 180◦C–300◦C. The analysisrevealed that the hydroxyls groups on the surface of the TiO2

increased with the deposition of Pt. The TG-DTG results(Table 1) also showed that remaining sulphur content onTiO2 decreased with the addition of platinum, probably dueto the synthesis procedure of the Pt/TiO2 catalyst.

The samples were studied by SEM and TEM to obtaininformation about Pt particle size and dispersion. A selectedSEM micrograph for Pt/TiO2 catalyst is presented inFigure 6. As it can be clearly observed, platinum particlesare quite heterogeneously distributed over the oxide surfacewith a poor dispersion, and the metal deposits present differ-ent sizes. TEM was used to estimate the Pt particle size dis-tribution. One selected micrograph is shown in Figure 7together with a representation of the metal particle size dis-tribution estimated by counting and measuring particles in ahigh number of micrographs taken on different areas of thesample. As we can see, Pt particles present a heterogeneousdistribution, with the highest number of particles (ca. 35%)being in the range of particle size of 5-6 nm.

XPS studies for TiO2 and Pt/TiO2 catalyst were alsocarried out, and a summary of the obtained results is shownin Table 2. For the platinized sample, it was especially usefulto analyze the Pt 4f core level (4f7/2 and 4f5/2) (Figure 8).

The region corresponding to Pt peaks may be deconvolutedinto two components: one corresponding to Pt0 at bindingenergy of 74.5 eV and an other assigned to a partially oxidisedPt(δ+) at binding energy of 71.1 eV [41]. In this way, it ispossible to make an estimation of the fraction of Pt in themetallic state (Pt0) and in the oxidized state (Pt(2+)/Pt(4+)).The deconvolution of the peaks as achieved by using theprogram UNIFIT 2009 [42] and the results are presented inTable 2 and in Figure 8. As it can be seen, the major part ofplatinum (ca. 70%) is present on the sample as metallic plat-inum (Pt0), while about the 30% of the metal was not totallyreduced, and it was still present as oxidized forms (Pt(δ+)).The oxidation state of Pt particles on the TiO2 is one of themost important parameters influencing the improvement ofthe photocatalytic activity of TiO2 according to many previ-ous reported results [43, 44], being Pt0 the state of the metalthat appears to provide the highest enhancement of activity.

Regarding the analysis of the O 1 s region, a peak locatedat a binding energy of 529.8 ± 0.2 eV was registered in bothsamples, corresponding to lattice oxygen in TiO2, with abroad shoulder at higher binding energies ascribed to surfacehydroxyl groups. This later shoulder was more pronouncedfor the Pt/TiO2 sample, indicating a higher degree of hydrox-ylation, in agreement with TG-DTG results. On the otherhand, the XPS Ti 2p core level spectra were similar for TiO2

and Pt/TiO2 without significant changes in the binding ener-gy of the peaks (458.5± 0.1 eV), corresponding to Ti4+ as themain component.

From XPS data, O/Ti ratios were calculated for sulphatedTiO2 and Pt/TiO2 samples. For the pure sulphated TiO2,O/Ti value is 1.70, lower than the stoichiometric value (O/Ti= 2), indicating the presence of oxygen vacancies on thesurface of this oxide. It has been reported that sulphated TiO2

presents a lower O/Ti ratio that nonsulphated TiO2, whichindicates that the amount of oxygen vacancies is higher inthe former samples [38]. For Pt/TiO2 the O/Ti ratio is high-er (O/Ti = 1.88), suggesting that the oxygen vacancies arepartially annihilated during the photodeposition process.

3.2. Photocatalytic Tests. The evaluation of the photocatalyticactivity was carried out by following the reaction of ethanoloxidative dehydrogenation in gas phase. All the photocatalyt-ic tests started feeding the reaction gaseous mixture to thereactor in the dark, until the outlet ethanol concentrationreached the equilibrium value, taken as initial value. There-fore, UVA-LEDs were switched on after the establishment ofthe dark adsorption equilibrium of ethanol on the catalystsurface. No reaction products were observed during or afterthe ethanol dark adsorption at 60◦C, indicating that no se-lective ethanol oxidation occurs by thermal catalysis in theused operating conditions [17].


50 nm

(a)

>7

40

35

30

25

20

15

10

5

0

Particle size range (nm)

3-4 4-5 5-6 6-7

ange

(%)

R(b)

Figure 7: Selected transmission electron micrograph and distribution of platinum particle sizes of Pt/TiO2 photocatalyst.

Binding energy (eV)

Pt (4f)7.8

7.52

7.24

6.96

6.68

6.4

85 82 79 76 73 70 67 64

Inte

nsi

ty(k

cou

nts

)

(a)

Binding energy (eV)

Pt (4f)7.8

7.52

7.24

6.96

6.68

6.4

85 82 79 76 73 70 67 64

Pt0

Ptδ+

Inte

nsi

ty(k

cou

nts

)

(b)

Figure 8: XPS Pt 4f core level for Pt/TiO2 photocatalyst.

The photocatalytic activity of the pure (TiO2) and plat-inized oxide (Pt/TiO2) in the presence and absence of bluephosphors was evaluated for the reaction of ethanol dehydro-genation at a concentration of 0.2 vol%. Ethanol photocat-alytic conversion on Pt/TiO2 as a function of the ethanol inletconcentration was also investigated. Results are presented inFigure 9. As it can be seen, the photocatalytic activity of TiO2

is slightly improved by the photodeposition of Pt. Thus, theethanol conversion increased with the addition of platinumreaching a maximum of about 69% at 0.2 vol% of initial etha-nol concentration. At higher ethanol concentration values,conversion progressively decreased going down to 7% for2 vol% of ethanol. According to MS analysis, acetaldehydewas the main product together with low amounts of CO2,

ethylene and crotonaldehyde as byproducts which were alsodetected.

The effect of metal deposition on the photocatalytic ac-tivity of TiO2 has been widely studied [25–30]. Noble metalnanoparticles deposited on the TiO2 surface are known toact as effective traps for photogenerated electrons due to theformation of a Schottky barrier at the metal-semiconductorcontact. These electrons improve the rate of oxygen reduc-tion and inhibit the electron-hole recombination eventhough the improvement of activity in this work has not beenas accused as in other reported literature, being strongly de-pending on the considered substrate [27–31].

As it is also shown in Figure 9, the addition of blue phos-phors notably increased the activity of the Pt/TiO2 catalyst


100

80

60

40

20

00.2 0.1 0.2 0.5 1 2

Eth

anol

conv

ersi

on(%

)

TiO2 Pt/TiO2

TiO2 + lue PB Pt/TiO2 + lue PB

Ethanol inlet concentration (vol%)

Figure 9: Ethanol conversion on Pt/TiO2 photocatalyst as a func-tion of ethanol inlet concentration.

for all the ethanol concentrations studied. In the same man-ner then for the tests without phosphors, total ethanol con-version decreased with the increase of ethanol concentration;however, as it has been said, for all ethanol concentrationsanalyzed, an important improvement of the photocatalyticactivity of Pt/TiO2 catalyst with the addition of phosphorsinto the fluidized bed can be observed. This is due to thephosphors exploited as light carriers inside the photocataly-tic core bed, giving a shorter optical pathlength to the radi-ation. Moreover, the photoactivity was enhanced, becausethe suitable phosphors introduced into the system were ableto transform 365 nm radiation coming from UV-LEDs into440 nm emission and able to photoexcited the fraction ofphotocatalyst in the core reactor volume, otherwise screenedby the photocatalyst itself as previously found for V2O5/TiO2

[17].Blank tests were also carried out under irradiation with

the reactor loaded only with phosphors and glass sphereswithout catalyst showing only negligible ethanol consump-tion and acetaldehyde production. In this test, the ethanolconversion was less than 2%; therefore, the presence of thespecific photocatalyst was necessary for the reaction [17].

In a same way, a blank test with an inlet ethanol concen-tration of 0.2 vol%, using only sulphated pure TiO2, phos-phors, and glass spheres, was carried out, and we have foundan ethanol conversion of 60%. The conversion is lower thanthat obtained without phosphors, because the phosphors actas screening for UV light decreasing the percentage of TiO2

effectively irradiated. This result underlines that Pt speciesare crucial to activate the catalysts at emission wavelength ofphosphors.

With an inlet ethanol concentration of 0.2 vol%, the con-version levels for sulphated pure TiO2, Pt/TiO2, andPt/TiO2 + Blue phosphors corresponded to values of 69%,72%, and 84%, respectively. At higher initial inlet concen-tration, the increase in ethanol conversion in the presence ofblue phosphors clearly evidenced the photon transfer limi-tations in the absence of phosphors that are overcame with

100

80

60

40

20

00.2 0.1 0.2 0.5 1 2

TiO2 Pt/TiO2

Sele

ctiv

ity

toac

etal

dehy

de(%

)

TiO2 + blue P Pt/TiO2 + blue P

Ethanol inlet concentration (vol %)

Figure 10: Acetaldehyde selectivity as a function of ethanol inletconcentration.

Pt/TiO2 photocatalyst and able to catch the light emissioncarried by phosphors.

Selectivity trend for the different systems as a functionof ethanol concentration is shown in Figure 10. It can be no-ticed that addition of Pt to the TiO2 remarkably increased theselectivity to acetaldehyde at the studied ethanol concentra-tion of 0.2 vol%. On the other hand, the selectivity to acetal-dehyde obtained with the Pt/TiO2 photocatalyst is very sim-ilar for the different ethanol concentrations evaluated, andin any case, it is notably higher than the selectivity obtainedwith the pristine TiO2.

These results suggest that the reaction mechanism forethanol conversion follows different pathways when usingpure or platinized TiO2, as it has been already reported bySiemon et al. for different substrates over platinized and non-platinized commercial TiO2 [31]. A hypothesis on the actionof Pt in the reaction mechanism could be related to the al-ready suggested formation of acetaldehydes radicals on TiO2.In fact, considering that in TiO2, ethanol is adsorbed asethoxy specie and the formation of OH radicals under UV ir-radiation by reaction of hydroxyls with positive holes, theabstraction of hydrogen by the adsorbed ethoxy speciesresults in adsorbed acetaldehyde radicals [29]. Acetaldehyderadical could be transformed into adsorbed acetaldehyde byelectron withdrawing by Pt nanoparticles then favouring thedesorption of the product.

In the presence of blue phosphors, the main reactionproduct was also acetaldehyde, and the selectivity to thiscompound was also much higher than the one obtained withthe pure TiO2. The acetaldehyde selectivity of Pt/TiO2 cata-lyst with or without the addition of blue phosphors is verysimilar, increasing slightly with the ethanol concentration,with values ranging 90%–95%.

4. Conclusions

The ethanol oxidative dehydrogenation in gas phase usingsulphated TiO2 and Pt/TiO2 as catalysts has been studied.


An efficient photocatalyst, active and selective to acetalde-hyde in the ethanol dehydrogenation, can be obtained bymodification of TiO2 by photodeposition of platinum nano-particles. We can observe that the photocatalytic activity ofTiO2 and selectivity to acetaldehyde can be improved by pho-todeposition of platinum. In the same way, the effect of theaddition of blue phosphors to photocatalytic bed was eval-uated. Experimental data evidenced that the presence ofphosphors allowed an improving in the photocatalytic activ-ity of Pt/TiO2, limited by the photon transfer, because anotable increase of ethanol conversion was observed.

Acknowledgments

This research was financed by the Spanish Ministerio Cienciae Innovacion (Project no. CTQ2008-05961-CO2-01) andJunta de Andalucıa (Excellence Project no. P06-FQM-1406).J. J. Murcia thanks CSIC for the concession of a JAE grantand for financing the short stay no. 2011ESTCSIC-6717. Theauthors would like also to thank Lampugnani SandblastingHI-TECH for the glass spheres utilized in this work.

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Review Article

Development of Visible Light-ResponsiveSensitized Photocatalysts

Donghua Pei and Jingfei Luan

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, China

Correspondence should be addressed to Jingfei Luan, [email protected]



Copyright © 2012 D. Pei and J. Luan. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The paper presents a review of studies about the visible-light-promoted photodegradation of the contaminants and energyconversion with sensitized photocatalysts. Herein we studied mechanism, physical properties, and synergism effect of the sensitizedphotocatalysts as well as the method for enhancing the photosensitized effect. According to the reported studies in the literature,inorganic sensitizers, organic dyes, and coordination metal complexes were very effective sensitizers that were studied mostly,of which organic dyes photosensitization is the most widely studied modified method. Photosensitization is an important wayto extend the excitation wavelength to the visible range, and therefore sensitized photocatalysts play an important role in thedevelopment of visible light-responsive photocatalysts for future industrialized applications. This paper mainly describes the types,modification, photocatalytic performance, application, and the developments of photosensitization for environmental application.

1. Introduction

Fujishima and Honda reported the first example for watersplitting into hydrogen and oxygen with TiO2 as catalystunder UV illumination in 1972 [1], and subsequently pho-tocatalysis has been a hot topic in many research fields, andmore efficient photocatalysts and photoelectrodes have beenreported in the past years. A number of semiconductors suchas TiO2, ZnO, Fe2O3, CdS, and ZnS have exhibited excellentphotocatalytic performance [2–6]. Among the commonsemiconductor photocatalysts, TiO2 has been used for energyconversion and photodegradation of many contaminants.However, solar energy reaching the surface of the earth andthe available solar energy for exciting TiO2 (λ ≤ 387 nm) arerelatively small which only occupy less than 5% of the wholesunlight. The low solar energy conversion efficiency andthe high charge recombination rate of the photogeneratedelectrons and holes are often two major limiting factorsfor its widely practical applications [2]. In order to utilizethe cheaper visible light from solar energy and enhancethe energy conversion efficiency during the photocatalyticreactions, efforts have been focused on exploring novelmethods to modify TiO2, of which photosensitization is an

important way to excite TiO2 to the wavelength of visiblelight.

Photosensitization can be achieved by a photosensitizerwhich absorbs light energy, transforms the light energy intochemical energy, and transfers it under favorable conditionsto otherwise photochemically unreactive substrates [7].Under appropriate circumstances, photosensitizer can beadsorpted at the semiconductor surface by an electrostatic,hydrophobic, or chemical interaction that, upon excitation,injects an electron into its conduction band [8]. Based on thereported studies in the literatures, inorganic sensitizers [9],organic dyes, and coordination metal complexes [10] are veryeffective sensitizers that are studied mostly, of which organicdyes photosensitization is the most widely studied modifiedmethod.

It is well known that the organic dyes have prominentphotophysical properties [11]. What is more, the structuresof the organic dyes can be changed according to what theyare required by low cost, low toxicity, and easy handlingapproaches [12–14]. In the past years, plentiful organic dyesgot particular attention and had been tested as photosensitiz-ers, such as eosin Y [15–23], riboflavin [24–28], rose bengal[24, 26], cyanine [11, 29], cresyl violet [30], hemicyanine


[12], and merocyanine [31–33]. However, the stability ofpure organic dyes is a notable problem which should besolved emergently [34, 35].

Semiconductors with narrow band gaps which canadsorb visible light have also been exploited as sensitizers.Compared with pure organic dyes, semiconductors showgreater stability, adjustable band gap which can tailoroptical absorption over a wider wavelength range, andthe possibility of exploiting multiple exciton generation toobtain high efficiencies [36]. There are two prerequisitesfor such heterogeneous semiconductor systems to functionefficiently: (i) the band gap of the sensitizer should benear the appropriate value for optimum utilization of solarradiant energy and (ii) its conduction band edge should behigher than that of TiO2 to allow electrons transferring fromthe sensitizers to TiO2 [9]. However, because of the limit inthe light absorption range, the energy conversion efficiencywith the semiconductor sensitizers is much lower than thatwith the dyes sensitizers. Thus efforts have been made tofind new narrow band gap semiconductor with ideal opticalproperties, enough stability, and low toxicity.

In addition to organic dyes and inorganic sensitizers, dyesand coordination metal complexes are efficient photosensi-tizers which have been receiving increasing research atten-tions, of which ruthenium complexes have been widely usedto extend the photoresponse of TiO2 into the visible region[37–39]. Surface photosensitization by organic dyes andcoordination metal complexes via photoinduced sensitizer-to-TiO2 charge transfer shows attractive features, such asregenerative sensitization and the ability for mediating thedegradation of nonvisible absorbing substrates [40]. But thegeneral difficulty in establishing stable surface anchorage ofthe charge-transfer photosensitizers is an important problemwhich requires further solution.

The photosensitization method has been applied to manyfields in recent years, including the visible-light-promotedphotodegradation of the contaminants [24–29], the dye-sensitized solar cell (DSSC) [41, 42], the semiconductor-sensitized solar cells (SSSC) [36], and visible-inducedhydrogen evolution from water [16–23]. The sensitizedphotodegradation process was found to be an effective wayto accelerate the photodecay of contaminants compared withthe direct photolytic process (i.e., no sensitizer involved)[27]. Compared with the conventional photovoltaic solarcell, the DDSC possessing easy and low-cost fabricationtechnology achieved high photon-electron conversion effi-ciency because the dye on the semiconductor electrode(mostly TiO2) absorbed more wide-range light than TiO2,and the photons were converted to electrons [41]. Thus it ismeaningful to carry on further research in visible-inducedphotosensitization method.

In this paper, we will describe the mechanism ofsensitized photocatalysts and various methods for enhanc-ing the photosensitized effects detailedly. Furthermore, thesynergism effect among the participants during the pro-cess of photosensitization is an important factor whichaffects the energy conversion efficiency. The characteristicsand performance of the photosensitizer under visible lightirradiation are quantitatively contrasted. The regenerative

photosensitization system utilizing electron donors is alsodiscussed.

2. The Photosensitization Mechanism

Redox processes are possible mechanisms for photoinducedenergy transfer, which can be illustrated primitively by thefollowing formula:

(S + hν −→ S∗

),

S∗ + M −→ S+ + e− (A),

S∗ + X −→ S+ + X− (B),

S∗ + Z −→ S− + Z+ (C).

(1)

A photochemically excited molecule may donate anelectron to the medium (M, reaction A) or another moleculewhich acts as an acceptor (X, reaction B), or it may act as anelectron acceptor when a suitable electron donor is present(Z, reaction C) [7].

The proposed mechanism of the primary electron path-ways over dye-sensitized semiconductor photocatalyst isillustrated in Scheme 1. In the photosensitization system, dyeS serves as both a sensitizer component and a molecularbridge to connect electron donor D to a metal oxidesemiconductor [38, 43]. The visible light (>400 nm) withthe energy which is lower than the band gap of thesemiconductor photocatalyst but higher than the band gapof the sensitizer molecules (S) which are adsorbed on thephotocatalyst excites the sensitizer, and subsequently theelectrons are injected to the conduction band (CB) of thephotocatalyst, leading to the efficient charge separation at theinterface between the photocatalyst and the sensitizer andproducing the oxidized form of the dye (S+). Subsequentlythe electrons can reduce water to H2 on the reductionsite (Pt mostly) over the photocatalyst in the process ofwater splitting. Similarly, if this process happens at ornear the catalyst surface, a set of reactions in presence ofwater molecules and dissolved oxygen will result in theformation of several active oxygen species such as superoxideanion, singlet oxygen, and hydroperoxyl radical which willparticipate in the degradation reactions during the processof pollutants’ degradation [15, 44]. The original form ofthe sensitizer is reformed by accepting an electron fromthe electron donor such as ethylenediaminetetraacetic acid(EDTA) in the solution, which irreversibly donates electronsand then decomposes [31].

Scheme 1 also illustrates the possible recombinationpathways and fluorescence decay of excited sensitizer. Backelectron transfer between the photo injected electron and theoxidized sensitizer plays an important role for controllingthe efficiency of net electron transfer [30]. At each branchpoint in the chain, a high quantum yield can be obtainedonly if the forward electron transfer rate (solid arrows) isfaster than the sum of all the reverse rates from the samepoint in the system. For example, in Scheme 1, the forwardelectron transfer from the semiconductor to the hydrogenevolving catalyst must compete effectively with back transfer


•O2−

ΔEg > 3 eV

CB

S+/S∗

e−

e−

e−

D+/D

H2O

H2OH2

O2

O2

S+/Sdye

VBsemiconductor

hA (>400 nm,<3.1 eV, >ES/S+)

Scheme 1: Proposed mechanism of dye-sensitized photocatalysis under visible light irradiation, including forward electron transfer (solidlines) and possible recombination pathways (dotted lines). Reproduced with a perfect scheme copy from [41]. Copyright 2009 AmericanChemical Society.

to the oxidized dyes, and also with electron transfer to thecatalyst for water oxidation. In general, the reverse pathwayshave much greater driving forces than the forward ones, andthis makes the reverse reactions faster [41]. However, dueto the existence of the interface between the dyes and thephotocatalyst, the separated electrons and holes have littlepossibility to recombine again, regardless of the existenceof the charge-capturing species which are mentioned above.This ensures higher charge separation efficiency and betterphotooxidation capacity for the composite [45]. While theCB acts as a mediator for transferring electrons from thesensitizer to substrate electron acceptors on the photocatalystsurface, the VB remains unaffected in a typical photosensiti-zation [41, 46].

The transport of injected charge across sensitized-semiconductor nanocrystallites under visible light irradia-tion is illustrated vividly in Scheme 2. As shown in Scheme 1,during the injected charge’s transit to the collecting surfaceof the reduction site, there is a significant amount ofelectrons which are lost as they recombine with excitedsensitizers at the grain boundaries. The driving force for theelectron transport within the nanocrystalline semiconductorfilm is created from the varying degree of the electronaccumulation. As more electrons accumulate away from thesurface of the reduction site, the quasi-Fermi level is alteredin such a way that a potential gradient is created within thethin film [30].

The study of the interfacial electron transfer betweenmolecular adsorbates and semiconductor nanoparticles ispresently under intense investigation [47]. It is desirable tohave a mechanistic understanding of the molecular factorsthat influence the quantum yield for excited-state electrontransfer to the semiconductor which is a critical parameterfor the production of electrical power.

3. Methods for Enhancing thePhotosensitized Effects

Though many research papers about visible-light photosen-sitization have been reported, there are still many exigentproblems which should be solved. Most of sensitizers sufferfrom a stability problem such as dissolution and the photo-catalytic degradation, an increase of carrier recombinationcenters, or the requirement of an expensive facility andrelatively long reaction time. In addition, several drawbackssuch as deactivation and separation of fine catalyst powdersfrom the aqueous phase after utilization prevent the large-scale applications of this promising method [2].

According to the reports from the literatures, the pho-tosensitization effect not only depended on their chemicalstructure and the employed sensitizer, but also dependedon the experimental conditions such as the concentra-tions of the dissolved oxygen and contaminants [24]. Itwas possible to improve the efficiency of photosensiti-zation if the life time of the sensitizers in the solvent could beincreased by suitable methods such as changing solution pHvalue, adding metal ions as complex agents, and derivatizingthe functional group of the sensitizer [25]. It had been wellrecognized that the electron injection efficiencies of the sen-sitizers upon nanocrystalline wide band-gap semiconductorswere determinant in photosensitization systems, which notonly depended on their respective intrinsic properties suchas energy levels [48] and excited state lifetimes, but alsodepended on the manner in which they were connected [49],such as physically or chemically adsorbed manner, the natureof anchoring groups, and the distance of the dye skeletonfrom the nanocrystalline surface [12, 50]. We will presentthe methods to enhance the photosensitization effect fromseveral aspects below.


S∗S∗ S∗ S∗

e ee

e

Semiconductor

Reductionsite

Visible light

E f

Scheme 2: Transport of injected charge across sensitized semiconductor nanocrystallites under visible light irradiation. Ef′ refers to the

quasi-Fermi level of the semiconductor nanocluster. Reproduced with a perfect scheme copy from [30]. Copyright 1997 American ChemicalSociety.

3.1. Sensitizer

3.1.1. Novel Photosensitizers. Some researchers developedsome novel photosensitizers which exhibited high photocat-alytic activity. Min et al. [2] found that the conjugated poly-mers (CP’s) with extended p-conjugated electron systemsshowed the relatively high photoelectric conversion efficiencyand charge transfer due to their high absorption coefficientsin the visible part of the spectrum, high mobility of chargecarriers, and good stability. The conjugated polymers couldbe separated from the aqueous phase by using simple gravitysettling and be recycled easily. For example, thiopheneoligomer could photosensitize TiO2 to catalyze the degra-dation of phenol under visible light irradiation [51], andEu3+-β-diketonate complexes with a remarkable quantumyield of 43% were excited under visible light irradiationat 440 nm [52], and so on. As the conjugated polymers,TiO2/polyaniline composite nanoparticles also showed goodsedimentation ability and could decant from the suspensionin about 5 min, while the pure TiO2 nanoparticles did notdecant after 2 h [53].

Besides Ru complexes, Os complexes were also effec-tive for sensitizing TiO2 because electron injection intonanocrystalline TiO2 was thought to occur on a subpi-cosecond time scale which restrained the back electrontransfer and thus enhanced the sensitization effect althoughthe excited-state lifetimes for Os complexes were typicallyshorter than those for the analogous Ru complexes. Sauve etal. speculated that the more important reason for this wasthat the ground-state potentials of the Os complexes couldbe readily tuned to less positive potentials by using strongerdonor ligands [10].

Many sensitizers such as Ru complexes [54], Os por-phyrins [10], and Pt complexes [55] had been fixed on thesurface of TiO2 through chemical anchoring groups (e.g.,carboxylate, phosphonate, and catechol linkage). However,such chemical anchoring bond could be made only in aspecific pH value range and was not inherently stable inan aquatic environment. Kim et al. [56] investigated themetalloporphyrins (especially tin(IV)-porphyrin (SnP)) fortheir photochemical activity in various applications, because

the lifetime of photogenerated SnPc• was long enough tosurvive the slow diffusion from the solution bulk to the TiO2

surface, which made the adsorption of SnP on TiO2 not tobe required and the H2 production was active over a widepH value range (pH 3–11), while the dye anchoring ontothe surface of TiO2 was an essential requirement for thevisible light sensitization with Ru complexes. Scheme 3 illus-trates the electron transfer dynamics occurring on SnP andRu(dcbpy)3 sensitized TiO2 particle. Being less expensive,less toxic, and consisting of more abundant elements unlikethe Ru-based sensitizers, SnP could be developed and utilizedas a practical sensitizer for solar chemical conversion.

Kathiravan et al. [8] observed that chlorophyll whichwas extracted from cyanobacteria could act as an effi-cient photosensitizer. Chlorophyll a served as the light-trapping and energy-transferring chromophore in photosyn-thetic organisms. Chlorophylls were effective photoreceptorsbecause they contained a network of alternating single anddouble bonds, and the orbitals could delocalize electronsfor stabilizing the structure and allowing the absorption ofenergy from sunlight. The ground state absorption studyrevealed that there was an interaction of colloidal TiO2

with chlorophyll through carboxyl group. The process ofelectron transfer from the excited state chlorophyll to theconduction band of TiO2 had been confirmed by the decreasein fluorescence lifetime. Thus as a dominant pigment onearth, chlorophyll a could be used as a photosensitizer morecommonly.

3.1.2. Stability of Sensitizer. Most of the photosensitizerssuffered from a stability problem such as dissolution andthe photocatalytic degradation of the dyes [31], and thedeactivation and separation of fine catalyst powders fromthe aqueous phase after utilization, and the large-scaleapplications of this promising method were prevented [2].The easy separation and reusable ability of PAn/TiO2 impliedthat it was potentially employable in the search for photosen-sitizer with easy separation and reusable ability which wereprerequisites for practical applications under mild conditionsuch as natural light and oxygen from air. Based on above


e−

e−

TiO2

TiO2

2H+

2H+

H2

H2

ED/ED+

ED/ED+

SnP∗/SnP−

SnP/SnP−<30μs

(2) slow

>min <ns

(1) fast

(1) fast

SnP(unbound)

<10 ps

RuL3/RuL3

RuL3∗/RuL3

+

+

<10μs μs∼ms

(2) slow

vs RuL3

(anchored)

Scheme 3: Schematic illustration of the electron transfer dynamics occurring on SnP and Ru(dcbpy)3 sensitized TiO2 particle. Reproducedwith a perfect scheme copy from [56]. Copyright Royal Society of Chemistry 2011.

analysis, TiO2 which was sensitized with polyaniline was apromising photocatalyst which should be employed [2].

3.1.3. Modification of Photosensitizers. Up to now, differ-ent strategies have been successfully applied in designingsensitizers, coordination metal complexes especially, whichabsorb over the whole visible spectrum, including liftingthe HOMO (highest occupied molecular orbital) level byincorporating strong σ-donor ligands or lowering the LUMO(lowest unoccupied molecular orbital) level of the anchoringligands. Other crucial factors are high electron injectionefficiency from the metal to ligand charge transfer- (MLCT-)based excited state to the conduction band of the semicon-ductor and a slow back electron transfer or charge recom-bination process. Both these concerns could be addressedsynthetically with appropriate design of an anchoring func-tionality that could covalently bind the nanoparticulateTiO2 surfaces very efficiently [39]. The photophysical andphotoelectrochemical studies revealed that three kinds ofefficiencies, that is, the fluorescence quenching efficienciesof the dyes by colloidal TiO2, the monochromatic incidentphoton-to-current conversion efficiencies (IPCEs) for thedye-sensitized TiO2 electrodes, and the overall photoelectricconversion efficiencies (g) for the dye-sensitized solar cells(DSSCs) based on dye sensitizers, all depended stronglyon the anchoring group types [12]. The anchoring groupeffects are also related with the kind of solvent and thepresence of competing adsorbates, such as electron donorsand electrolytes.

Carboxyl [57], phosphate [37], sulfonate [12], acetyl [7],and silyl [40] functionalities had been demonstrated to beable to form linkage with TiO2 surface as shown in Scheme 4.Stability of these linkages varies in aqueous medium, andsome of these linkages are only stable within certain pHvalue range and certain solvents. Chen et al. showed thatthe combination of carboxyl and hydroxyl as anchoringgroups led to highly efficient IPCEs over a wide spectrumregion with the maximum IPCE of 73.6% [12]. A possibleexplanation was that the combination of the carboxyl and the

hydroxyl led to a complexation reaction of the correspondingdye molecule with Ti4+ ion, which induced the observationof the red shift and the isosbestic point. Moreover, the silylanchoring group seemed to be an ideal surface modificationmoiety for TiO2 owing to the high affinity of the silylfunctionality for the hydroxyl groups on the surface of thesemiconductor and the chemical inertness of the resultantsilyl, Si–O bonds [40].

Because of such stability of covalent linkage considera-tion, researchers explored the possibility of the coordinationmetal complex’s derivatives containing the anchoring groupsas the photosensitizer for TiO2; thus the photosensitizerwas stable against dissociation even at extreme pH value inaqueous medium or in a wide range of organic solvents. Theyattempt to utilize dehydration of carboxyl group of xanthenedyes with amino group of silane-coupling reagent fixed onTiO2 surface leading to a strong chemical fixation of dye onTiO2 particles and conquering the unstableness of the dye-sensitized photocatalyst in water [22]. There were studieswhich indicated that the linkage of ground dye and divorce ofoxidized dye from TiO2 could enhance the electron injectionand hinder the backward transfer and subsequently improvethe photosensitized efficiency [58]. Thus we can preparemore efficient sensitizers that can couple the functions ofa sensitizer, which is bound to the surface of TiO2 and anantenna, which can realize the intramolecular energy transferfrom highly absorbing chromophoric groups by tuningthe molecular components, and thus the photosensitizedefficiency can be enhanced dramatically.

Consequently we know how significantly the anchoringmanner of a sensitizer molecule influences its sensitizationbehavior on a nanocrystalline semiconductor, and the opti-mization on adsorbing groups may result in more efficientsensitizers for photosensitization applications.

Besides the modification of the anchoring group, anothersuccessful strategy for obtaining a broad absorption whichextends throughout the visible region is to utilize a com-bination of sensitizers which complement each other intheir spectral features [11]. A series of preformed BODIPY


Surfacemodifying

agent Surfacemodifying

agent

Carboxyl linkage Silyl linkage

OC

OO

O

O P OOO

Si

Surfacemodifying

agent

Phosphonate linkage

Scheme 4: Some of the most common covalent anchoring groups for surface modification of TiO2 photocatalysts and TiO2 nanocrystallineelectrodes. Reproduced with a perfect scheme copy from [40]. Copyright 2002 Elsevier Science Ltd.

dimers had been investigated by Ventura et al., showingthat the molar absorption coefficient of the dimer wasabout twice with respect to the monomer and makingthese dimers valuable components of complex molecularstructures for light energy conversion [59]. There were alsostudies which showed that the combination of two differentsensitizers was found to exhibit remarkable photosensitizedperformance, the absorption sites of which on the TiO2

surface were different, meaning that there was not overlapof the electronic orbitals of two different sensitizers and itwas difficult to be electron transfer between two sensitizersstochastically [11, 60]. Cosensitization was found to suppressthe aggregation and affect the sensitization performance pro-foundly. In addition, multilayer films with different numbersof sensitizer/metal-doped-TiO2 bilayers [61] obtained higherefficiency, which could lower the charge recombination ratein the photosensitized system.

3.1.4. Concentration of Photosensitizers. We think that theeffect of photosensitization is significantly influenced by thesensitizer concentration which plays a significant role in thenumber of electrons transferring from the excited sensitizerto the conduction band of the semiconductor photocatalyst.The photosensitization effect was enhanced with increasingsensitizer concentration within a certain range. However,with further increasing sensitizer concentration, the photo-sensitization effect was adversely decreased, possibly due toa saturation limit of the sensitizer adsorption sites on thephotocatalyst surface [28]. In addition, the excess sensitizerswhich were dissolved in the reaction solution could beexcited but could not inject the electrons to the conductionband of the photocatalyst for inducing the photocatalyticreaction [20]. Thus we must find the optimal sensitizerconcentration for facilitating the photocatalytic reaction.

3.2. Loaded Metals. As we know, the linkage between thesensitizer molecules by metal ions is able to establish energylevels inside the band gap which lead to significant visiblelight absorption for photocatalyst and overcome the quench-ing and the insulating effect for the photocatalyst to achievevery high light harvesting efficiency and photocatalyticactivity simultaneously. The existing research suggested thatthe combination between dye sensitizers and metal was muchstronger than the combination between dye sensitizers andsemiconductor photocatalysts [21]. This can be achieved byusing coupled semiconductor layers which own appropriateelectron energy levels where the edge of the conduction

band of the first semiconductor is lower than that of thesecond one. There were many researches about the functionof loaded metals, such as Fe3+ [18], Cr3+ [61, 62], and Pt [18,19] in especial, indicating that highly enhanced visible light-induced photocatalytic reaction could be obtained whenthe sensitized photocatalysts were additionally modified bysurface metal deposits.

Pt showed the best activity among the metals whichshould be ascribed to the fact that electron trapping in Ptwas fast enough to compete with the back electron transfer[38, 55]. The existing research showed that the combinationof different size of metal particles could promote thephotosensitized reactions markedly. Chen et al. found thatthe TiO2/large size-CdS/small size-CdS electrode showedenhancement and broadening of the absorption spectrum invisible light region, in comparison with the electrodes whichwere sensitized with single size CdS nanoparticles [36].Chromium(VI)-doped glasses as well as mesoporous silicasare known for their tetrahedral coordination of chromium.Such coordination allows for a special transition undervisible light irradiation: Cr6+=O2− →Cr5+–O1−. In partic-ular, the Cr5+ can possibly donate an electron into thesurrounding TiO2, and O1− can scavenge an electron fromthe surrounding TiO2. In this case, the charge separationwill occur, which will result in a hole and an electron inTiO2. If this process happens at or near the catalyst surface,the charges can interact with the surface hydroxyl groups oradsorbed oxygen to produce active oxygen radicals. Scheme 5describes this process. Davydov et al. [62] investigated theproposed mechanism of photooxidation on TiO2/Cr-(Ti)-MCM-41 and obtained prominent photosensitized effect.

3.3. Electron Donor. The regeneration of the sensitizers in thepresence of suitable electron donors is a prerequisite for thedevelopment of the practical photosensitization application.In order to regenerate the electron-deficient sensitizer inphotocatalytic system, some electron donors, or sacrificialagents, have to be used by adding them to a reactionsolution to sustain the photoreaction cycle. The existingexperiments have shown that the dechlorination rate ofvisible light-induced degradation of carbon tetrachloride ondye-sensitized TiO2 decreased due to the depletion of theRuII-species sensitizers when the reaction proceeded withoutaddition of the electron donor. On the other hand, whenthe electron donor 2-propanol was present, the dechlorina-tion rate remained constant for 6 h of irradiation withoutshowing any signs of deceleration [45]. Notoriously, various


Cr+6 O−2

Visible light

OH−

3.2 eV

TiO2

e−

e−

h+

O2

Cr+5 O−1

•O2−

•OH

hA

Scheme 5: Proposed mechanism of photooxidation on TiO2/Cr-(Ti)-MCM-41. Active Cr6+ species are incorporated in SiO2 matrix.Reproduced with a perfect scheme copy from [62]. Copyright 2001Academic Press.

alcohols and acids can be used as sacrificial electron donorsto regenerate the sensitizer. There were some commonlyemployed electron donors, such as acetonitrile [25, 33],methanol [12, 24], isopropanol [13, 45], cyclohexanone[15], diethanolamine [20], and ethylenediaminetetraaceticacid (EDTA) [32, 63]. We also found that IO3−/I− (orI3−/I−) shuttle redox mediator could act as electron donorssimilarly [33]. Scheme 6 describes potential energy diagramof H2 production from water over dye-sensitized Pt/TiO2

photocatalysts with I− as an electron donor, in which systemthe dye is merocyanine. Moreover, the polarity of the solventhas a significant influence on the photosensitized efficiency.Thus it is necessary to take into account the influence ofthe solvent on the energy potentials of them in constructingan efficient sensitized photocatalysis system in aqueoussolutions.

3.4. Dissolved Oxygen. There is actually an argument aboutthe effect of dissolved oxygen. Shang et al. [45] observedthat the initial dechlorination rates of CCl4 decreased in theorder of N2 > air > O2-saturated system by dye-sensitizedTiO2 under visible light irradiation. The presence of O2

in the suspension lowered photosensitization efficiency bytwo ways: direct quenching of the excited sensitizer andscavenging CB electrons. However, Song et al. [51] confirmedthat oxygen played an important role during the degradationchain reaction because it was responsible for the generationof •O2

−/HO• radicals. We think that whether dissolvedoxygen is needed in the system of photosensitization dependson the pollutants which will be degraded and differentdegradation mechanisms.

3.5. pH Value of Solutions. The adsorbing power of thesensitizers is strongly influenced by the surface charge. Thepositively charged TiO2 surface at acidic conditions stronglyattracts negatively charged sensitizer molecules, while thenegatively charged TiO2 surface at basic conditions attracts

positively charged sensitizer molecules [44, 46]. However,what we want is that the adsorbing power of the sensitizersis unaffected by the pH value of the solution, and it is thedirection of the researches.

4. Apparent Quantum Efficiency ofPhotosensitization

Although it is very difficult to compare the results of thepresent reported studies because the photocatalytic apparentquantum yields appear to vary according to the reactionconditions and the measurement methods. With a view toknowing the current state of sensitization study in the fieldof photocatalysis, here we tried to list several research resultsof apparent quantum yields for hydrogen evolution (Table 1)and degradation rates of organic pollutants (Table 2) accord-ing to the results which were reported in our references.

5. Prospect

Up to now, the most efficient sensitizers of the solar cellare ruthenium polypyridyl complexes. Although the presentstudy demonstrated the potential use of the sensitized semi-conductor photocatalysts for visible light-induced degra-dation of pollutants and energy conversion, the problemssuch as high cost, long-term unavailability, and undesirableenvironmental impact of these noble metal complexes makethis method unsuitable for large-scale industry production.There remains the need for alternative photosensitizerswhich have larger extinction coefficient and extend theabsorption range into the red visible region [11, 14].

Besides the methods for enhancing the effect of photo-sensitization which was mentioned above, some researchersexplored certain new ways by which noteworthy resultswere achieved. For instance, the photodegradation of plastic(PS) could be realized by preparation of PS-(TiO2/CuPc)composite thin films under the sunlight irradiation with littleformation of toxic byproducts [45]. An integrated chemicalsystem was designed for hydrogen evolution which utilizedphotosensitized oxide semiconductors [66]. The system wasspatially organized by a linear channel zeolite into a vectorialarray of electron donor/sensitizer/semiconductor/electronacceptor/catalyst. The ion-exchange properties and size-exclusion effects of the zeolite cause the donor, the sensitizer,and the acceptor to occupy their appropriate places inthe electron transport chain. Li et al. prepared an effi-cient visible-light active photocatalyst of multilayer-Eosin Y-sensitized TiO2 through linkage of Fe3+ between not onlyTiO2 and Eosin Y but also different Eosin Y molecules toform three-dimensional polymeric dye structure [18]. Themultilayer-dye-sensitized photocatalyst was found to ownhigh light-harvesting efficiency and photocatalytic activ-ity for hydrogen evolution under visible light irradiation.Scheme 7 illustrates the multilayer adsorption of Eosin Yvia linkage of Fe3+ on TiO2 for photocatalytic hydrogenevolution.

Considering the possible practical applications of pho-tosensitization systems, we have to make sure that thephotosensitizer or the mediator which was utilized to destroy


Table 1: The photocatalytic hydrogen evolution by sensitized photocatalysts under visible light irradiation.

No. Catalyst Reaction conditions Irradiation typeHydrogenevolution(μmol/h)

Apparentquantumefficiency

References

(1)Eosin Y sensitized1.0 wt% CuO/TiO2

Catalyst: 20 mg; 70 mL 15% diethanol amine(DEA) H2O; DEA as sacrifice electron donors

A 200 W halogenlamp with a cut-offfilter (λ > 420 nm)

10.56 5.1% [16]

(2)Eosin Y sensitized0.5 wt%Pt/N-TiO2-300◦C

Catalyst: 0.100 g; 80 mL 0.79 mol/Ltriethanolamine (TEA) solution as sacrificeelectron donors; pH 7.0; initially N2-saturated

A 400 W highpressure Hg lamp

with a cut-off filter(λ > 420 nm)

Averageabout 80

Unclear [17]

(3)EosinY–Fe3+(1 : 1)–1.0 wt%Pt/TiO2

Catalyst: 0.100 g; 80 mL 0.79 mol/L TEAsolution as sacrifice electron donors; pH 7.0;initially N2-saturated

A metal halide lamp(400 W) with a

cut-off filter(λ > 420 nm)

275 19.1% [18]

(4)

Eosin Y sensitized1.0 wt%Pt/Ti-MCM-41zeolite

Catalyst: 40 mg; 80 mL 15% TEA H2O; TEA assacrifice electron donors; pH 7.0; initially Aratmosphere

A 300 W tungstenhalogen lamp with a


∼10 12.01% [19]

(5)Eosin Y sensitized0.5 wt% Pt/SrTiO3

Catalyst: 0.2 g; 150 mL 15% DEA H2O; DEA assacrifice electron donors; pH 11.6; initially Aratmosphere

A 300 W Xe arc lampwith a cut-off filter

(λ > 400 nm)∼3 Unclear [20]

(6)Eosin Y sensitized1.0 wt% Rh/TiO2

Catalyst: 20 mg; 70 mL, 15% DEA H2O; DEAas sacrifice electron donors; initially Aratmosphere

A 200 W halogenlamp with a cut-offfilter (λ > 420 nm)

14.63 7.10% [21]

(7)Eosin Y sensitized0.1 wt% Pt/TiO2

Catalyst: 0.3 g; 250 mL, 15% DEA H2O; DEA assacrifice electron donors; initially Aratmosphere

A 300 W Xe lampwith a cut-off filter

(λ > 460 nm)

Averageabout 65

10% [22]

(8)

Eosin Y sensitized1.0 wt%Pt/multiwalledcarbon nanotube(MWCNT)

Catalyst: 20 mg; 80 mL, 15% TEA H2O; TEA assacrifice electron donors; initially Aratmosphere

A 300 W tungstenhalogen lamp with a


54.20 12.14% [23]

(9)Merocyaninesensitized 1.0 wt%Pt/TiO2

Catalyst: 50 mg; 100 mL 95% AN-H2O;acetonitrile and I anions as sacrifice electrondonors


(λ > 440 nm)

Averageabout 17

∼2% [33]

(10)

Merocyanine andcoumarin dyessensitized 1.0 wt%Pt/TiO2

Catalyst: 50 mg; 100 mL 95% AN-H2O;acetonitrile and I anions as sacrifice electrondonors


(λ > 440 nm)Unclear

1.8% forM–

Pt/TiO2

and 2.5%for C–

Pt/TiO2

[31, 32]

(11)Ru complex sensitized0.1 wt%Pt/NS-K4Nb6O17

Catalyst: 5.0 mg; aqueous solution (2.0 mL)containing 10 mM EDTA as an electron donor;initially Ar atmosphere

A xenon lamp(300 W) with a cut-off

filter (λ > 420 nm)3.6 10.5% [63]

(12)3.1 wt% WS2

sensitized 1 wt%Pt/TiO2

Catalyst: 0.2 g, 200 mL aqueous solution; Na2Sas the hole scavenger


(λ > 430 nm)2.13 Unclear [9]

(13)

Carboxylate versusphosphonate in Ru-complex-sensitized3.0 wt% Pt/TiO2

Catalyst: 15 mg; EDTA as sacrifice electrondonors


(λ > 420 nm)

Maximaabout 132

22.4% [57]

(14)

Sensitization of TiO2

film withzinc-substitutedcytochrome

EDTA as a sacrificial electron donor100 W tungsten

halogen lamp withfilters (λ > 475 nm)

About 1020for the first20 min of

illumination

10± 5% [64]

(15)

Ru-, Rh-, andIr-doped SrTiO3

loaded with Ptcocatalysts (0.1 wt%)

Catalyst: 300 mg; 380 mL of 10 vol% aqueousMeOH

A 300 W Xe lampwith cut-off filters

(λ > 440 nm)

Maximaabout 117

5.2% at420 nm

[65]


Table 2: The photodegradation effects of target contaminants by sensitized photocatalysts under visible light irradiation.

No. Catalyst Reaction conditions The light sourceTarget

contaminantsDegradation rate References

(1)Ru-complex-

sensitizedTiO2

Catalyst: [TiO2] = 0.5 g/L,Ru-complex = 3 μM; pH = 3;2-propanol as sacrificeelectron donors; [CCl4] =1 mM, N2-saturated

A 450 W Xe-arc lampwith an UV cut-offfilter (λ > 420 nm)

CCl4

Dechlorinationquantum yield,ΦCl− = 10−3

[46]

(2)Ru-complex-

sensitized 0.2 wt%Pt/TiO2

Catalyst: [TiO2] = 0.5 g/L,Ru-complex = 10 μM; pH = 3;0.1 M isopropyl alcohol assacrifice electron donors;[CCl4] = 1 mM, initiallyN2-saturated

A 450 W Xe-arc lampwith an UV cut-offfilter (λ > 420 nm)

CCl4

Initialdechlorination

rates is4.7 μM/min

[38]

(3)Ru-complex-

sensitizedTiO2

Catalyst: 0.3 g; 150 mL of CCl4

saturated aqueous solution(pH 7), containing 5.0 mM ofKI as sacrifice electron donors

A 100 W tungstenlamp (λ > 450 nm)

CCl4

Rate constant is0.446 μm

min−1 g-catalyst−1

[37]

(4)Ru-complex-

sensitizedTiO2

Catalyst: 0.3 g; 150 mL of CCl4

saturated aqueous solution(pH 6.5–7.0), containing5.0 mM of KI as sacrificeelectron donors, initiallyN2-saturated

A 100 W tungstenlamp (λ > 450 nm)

CCl4

Rate constant is0.585 μM min−1 g-

catalyst−1

[40]

(5)Polyaniline-sensitized

TiO2

Catalyst: 100 mg; 50 mL H2Owithout sacrifice electrondonors, [MB] = 10 mg/L

Natural lightirradiation for 90 min

between 11.00 a.m.and 1.00 p.m.

Methyleneblue (MB)

Decolorizationefficiency is 80%

[2]

(6) CuPc sensitized-TiO2

Catalyst: 0.7 wt% CuPc/TiO2;the ratio of TiO2/CuPc to PS is2.0 wt % in the composite film

Three 8 W fluorescentlamps (310 nm < λ <

750 nm)Plastic (PS)

6.9% weight lossfor composite

film after 250 h[45]

(7)Ru-complex-

sensitizedTiO2

Catalyst: [TiO2] = 1 g/L, c(sensitizer) = 1× 10−5 mol/L;[terbutryne] = 2× 10−5 mol/L;pH = 3

A 500 Whigh-pressure xenonlamp with a cut-offfilter (λ > 420 nm)

Herbicideterbutryne

100% after 4 h [44]

(8)1 wt% poly(fluorene-

co-thiophene)(PFT)-sensitized TiO2

Catalyst: 50 mg; 50 mLaqueous phenol solution withan initial concentration of10 mg/L

A 250 W GaI3 lampwith a cut-off filter

(400 nm < λ <700 nm)

Phenol 74.3% after 10 h [51]

the pollutant does not pollute the environment by itself. Thusthere is a growing interest for developing environmentallybenign materials and/or biodegradable materials as thephotosensitizers. Thus, the utilization of natural polymersseems to be especially attractive. Novel photoactive water-soluble modified polymers which were based on starch[67] and polysaccharides [13] were prepared. These poly-meric systems were quite promising photosensitizers fordemonstrating the reaction of organic compounds in anaqueous solution, while the photosensitizers will not result inenvironmental pollution.

6. Conclusions

In this paper, we have enumerated various photosensitizedways which have been reportedly utilized successfully for thedegradation of organic pollutants and energy conversion by

using the visible range of the solar spectrum. Though exten-sive works on this field have been carried out, only significantdevelopments and researches which were completed havebeen referred to in this paper.

According to the studies which were reported in theliteratures, inorganic sensitizers, organic dyes, and coordina-tion metal complexes were very effective sensitizers that werestudied mostly. The method of photosensitization has beenapplied to many fields in recent years, including the visible-light-promoted photodegradation of the contaminants, thedye-sensitized solar cell and semiconductor-sensitized solarcells, visible-induced hydrogen evolution from water. Theproposed mechanism of the primary electron pathways overdye-sensitized semiconductor photocatalyst is illustrated inour paper. There are many methods to enhance the pho-tosensitized effects, and we must develop novel sensitizerswith high absorption coefficients in the visible part of the


+

−

Dye∗

Dye

hA H+

H2

CB

TiO2

e−

e−

e−

I−I−

I3−I3

−

+1.01 (a)

+0.54I3

−/I−

−0.41H+/H2

−1.119 (b)

Pt

Pote

nti

al(V

)

Scheme 6: Potential energy diagram of H2 production from water over dye-sensitized Pt/TiO2 photocatalysts with I− as an electrondonor. HOMO (a) and LUMO (b) energy levels of merocyanine dye derived from CV measurement in DMF solvent containing 0.1 Mtetrabutylammonium perchlorate. Reproduced with a perfect scheme copy from [33]. Copyright 2002 Elsevier Science B.V.

Pt

hA

hA

H+H2

Fe

FeFe

Fe

FeFe

e

e

e

e

e

Eosin

Eosin Eosin

Eosin

Eosin

TiO2 substrate

Scheme 7: Schematic model for multilayer adsorption of Eosin Yvia linkage of Fe3+ on TiO2 for photocatalytic hydrogen evolution.Reproduced with a perfect scheme copy from [18]. Copyright2009 International Association for Hydrogen Energy Published byElsevier Ltd.

spectrum, high mobility of charge carriers, and good stabilityfor the industrialized application in the future.

Acknowledgments

This work was supported by the National Natural Sci-ence Foundation of China (no. 20877040). This work wassupported by a Grant from the Technological SupportingFoundation of Jiangsu Province (no. BE2009144). This work

was supported by a Grant from China-Israel Joint ResearchProgram in Water Technology and Renewable Energy (no. 5).This work was supported by a Grant from New Technologyand New Methodology of Pollution Prevention ProgramFrom Environmental Protection Department of JiangsuProvince of China during 2010 and 2012 (no. 201001).This work was supported by a Grant from The FourthTechnological Development Scheming (Industry) Programof Suzhou City of China from 2010 (SYG201006). This workwas supported by a grant from the Fundamental ResearchFunds for the Central Universities.

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Research Article

Sol-Gel-Hydrothermal Synthesis of the HeterostructuredTiO2/N-Bi2WO6 Composite with High-Visible-Light- andUltraviolet-Light-Induced Photocatalytic Performances

Jiang Zhang,1 Zheng-Hong Huang,1 Yong Xu,2 and Feiyu Kang1

1 Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China2 Department of Technology, Beijing Tongfang Puri-Tech Co., Ltd., Beijing 100083, China

Correspondence should be addressed to Zheng-Hong Huang, [email protected] Feiyu Kang, [email protected]



Copyright © 2012 Jiang Zhang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The heterostructured TiO2/N-Bi2WO6 composites were prepared by a facile sol-gel-hydrothermal method. The phase structures,morphologies, and optical properties of the samples were characterized by using X-ray powder diffraction (XRD), scanningelectron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS),and UV-vis diffuse reflectance spectroscopy. The photocatalytic activities for rhodamine B of the as-prepared products weremeasured under visible and ultraviolet light irradiation at room temperature. The TiO2/N-Bi2WO6 composites exhibited muchhigher photocatalytic performances than TiO2 as well as Bi2WO6. The enhancement in the visible light photocatalytic performanceof the TiO2/N-Bi2WO6 composites could be attributed to the effective electron-hole separations at the interfaces of the twosemiconductors, which facilitate the transfer of the photoinduced carriers.

1. Introduction

In the past decades, TiO2, as an effective photocatalystwith a band gap of 3.2 eV, has been widely investigated inenvironmental remediation and solar utilization, owing toits effectiveness, cheapness, and chemical stability. However,its low visible light response and high photoinduced chargecombination limited the utilization of solar energy. Todate, most of research on photocatalysis was focused onthe development of novel photocatalysts with high visiblelight responsivity. Very recently, Bi2WO6 with high visiblelight responsivity was widely investigated to degrade organicdye waste water [1–3] and decompose volatile gaseouspollutant [4]. Bi2WO6 is regarded as a promising visible lightphotocatalyst for dealing with the environmental problemsin water and air due to its narrow band gap. Unfortunately,the low photocatalytic performance of Bi2WO6 was causedby its poor adsorption property and weak migration ofphotoinduced charge carriers.

To improve the photoactivity of Bi2WO6, hierarchicalnest-like structure [5] and hierarchical flowers [6] with a

hollow structure were constructed by solution self-assemblyof nanoplates, providing high surface area and porous struc-ture for adsorption of organic molecules. As is well known,high photoinduced charge recombination is a harmful factorto photocatalytic activity of Bi2WO6. The “Schottky” barrierbetween the metal and the semiconducting photocatalyst wasconsidered to be an efficient path to improve the interfacialcharge transfer process and accelerate the charge carrierseparation. Bi2WO6/Cu0 [7] with Fenton-like synergisticeffect and Bi2WO6/Ag0 [8] heterojunctions were designed toprevent the electron-hole recombination and improve thephotocatalytic performance. In addition, carbon-modifiedBi2WO6 [9, 10] was also considered to enhance the pho-toactivity due to high specific surface area and high con-ductivity of carbon, accelerating the charge transfer fromphotocatalyst to the liquid-solid interface contacted withorganic pollutants by making use of carbon’s unique electrontransport properties. Heterostructured Bi2WO6-TiO2 com-posite [11, 12] was discovered to possess the synergetic effectbetween TiO2 and Bi2WO6 which leads to an effective charge


carrier separation, exhibiting the outstanding photocatalyticperformance under sunlike irradiation.

In our experiments, nitrogen-doped Bi2WO6 was synthe-sized by a hydrothermal method in the first step. And thennanosized TiO2 particles were coated on the surface of N-doped Bi2WO6 nanoplates by a facile sol-gel-hydrothermalprocess. The influences of nitrogen doping on the struc-ture, optical properties, and morphologies of Bi2WO6 wereinvestigated. And the mechanism of enhanced photocatalyticactivities of TiO2/N-Bi2WO6 was explained by nitrogendoping and the synergetic effect between TiO2 and N-dopedBi2WO6.

2. Experimental

2.1. Synthesis of N-Doped Bi2WO6. All the reagents were ofanalytical purity and were used as received from ShanghaiChemical Company. Similar to the reported experiments[13], in a typical procedure, aqueous solutions of 10 mmolBi(NO3)3·5H2O and 5 mmol Na2WO4·2H2O were mixedtogether, then the mixture was magnetic stirred for twohours at room temperature. Afterward, aqueous solutioncontaining desired amounts of urea (CO(NH2)2) was addedfor N-doped Bi2WO6 with the atomic ratio of N to Bi in 0.5 inthe precursor. Then, the suspension was added into a 50 mLTeflon-lined autoclave up to 80% of the total volume. Thesuspension in the autoclave was heated at 160◦C for 24 h.Subsequently, the autoclave was cooled to room temperature.The products were collected by filtration and then werewashed by deionized water. The samples were then dried at80◦C for several hours and were denoted as N-BWO.

2.2. Preparation of the Heterostructured TiO2/N-Bi2WO6

Composites. The heterostructured TiO2/N-Bi2WO6 compos-ite was synthesized via a sol-gel-hydrothermal process.Briefly, a mixed solution of 20 mmol of Ti(OC4H9)4,183 mmol of ethanol (C2H5OH), and 1 g of the as-preparedN-doped Bi2WO6 was magnetic stirred for 30 minutes,forming a homogeneous mixed suspension. Subsequently,a mixed solution of 183 mmol of ethanol (C2H5OH),75 mmol of H2O, and 2.8 mmol HNO3 was dropped intothe above suspension and magnetic stirred for another30 minutes to get a highly dispersed gelatin. As-obtainedgelatin was aged for a whole night and was transferredinto the autoclave of 50 mL with 15 mL H2O at 160◦Cfor 24 h, denoted as TiO2/N-BWO. The resultant sampleswere calcined at 400◦C for 1 h, 2 h (5◦C/min) to removesolvents and other organic species, which were denoted asTiO2/N-BWO-400/1 h, TiO2/N-BWO-400/2 h, respectively.For comparison, the pristine Bi2WO6was synthesized by ahydrothermal process at 160◦C for 24 h, denoted as BWO.And the pure TiO2 was fabricated via a sol-gel-hydrothermalmethod at 160◦C for 24 h, denoted as H-TiO2.

2.3. Catalyst Characterization. The crystalline phases ofthe as-prepared catalysts were confirmed by powder X-raydiffraction (XRD). The XRD patterns were obtained forthe heterostructured TiO2/N-Bi2WO6 samples by using aRigaku Multiflex diffractometer at 40 kV and 200 mA with

monochromated high-intensity CuKα radiation. The surfacemorphology of the as-synthesized samples was observedby a field emission scanning electron microscope (FE-SEM). High-resolution transmission electron microscopy(HRTEM) was performed on JEOL-2010F operated at200 kV. To prepare the transmission electron microscopy(TEM) sample, a small amount of samples was ultrasonicallydispersed in ethanol. A drop of such suspension was placedon a 200-mesh Cu grid with holey carbon film and driedcompletely in air. The UV-vis diffuse reflectance spectra(DRS) were acquired on a Shimadzu UV-2450 spectropho-tometer with ISR-240A integrating sphere assembly in therange of 200–800 nm. BaSO4 was used as a reflectancestandard.

2.4. Evaluation of Photocatalytic Activity. The photocatalyticperformance of the heterostructured TiO2/N-Bi2WO6 sam-ples was evaluated by decomposing rhodamine B (RhB)under visible and ultraviolet light irradiation at roomtemperature. A 300 W Xe lamp (CEL-HXB UV300, BeijingChina Education Au-light Co., Ltd.) equipped with anultraviolet cutoff filter to provide visible light (λ > 420 nm)was used as the visible light source, and which equipped withan ultraviolet reflected filter to provide ultraviolet light (λ <400 nm) was regarded as the ultraviolet light source. And thedistance between the liquid surface of the suspension andthe light source was set about 7 cm. The photodegradationexperiments were carried out with the sample powder(50 mg) suspended in RhB aqueous solution (100 mL, 4 ×10−5 mol L−1) with constant magnetic stirring. Prior tothe irradiation, the suspensions were magnetically stirredin the dark for 2 h to establish the adsorption/desorptionequilibrium. At the given time intervals, about 4 mL of thesuspension was taken for further analysis after centrifu-gation. RhB photodegradation was analyzed by recordingthe absorbance at the characteristic band at 553 nm as afunction of irradiation time on a UV-vis spectrophotometer(Shimadzu UV 2450).


3.1. XRD Patterns of TiO2/N-Bi2WO6 Composites. Thephase structures of as-prepared samples were investigatedusing powder X-ray diffraction. The XRD patterns for theheterostructured anatase TiO2/N-Bi2WO6 composites areshown in Figure 1(a). For comparison, the XRD patternsof the pure Bi2WO6, N-doped Bi2WO6, and the pure TiO2

are also given. It is observed that the as-prepared pureBi2WO6 powder was in good agreement with the standardorthorhombic phase of Bi2WO6 (JCPDS no. 39–0256), whilediffraction peaks of H-TiO2, corresponding to the standardtetragonal phase of anatase (JCPDS no. 21–1272), wereclearly observed. No new crystal orientations or changes inpreferential orientations of N-doped Bi2WO6 are observedcomparing to pure Bi2WO6, despite of the presence of dopingwith nitrogen. However, the diffracted intensity of (131)crystallographic plane displays an obvious decrease owingto the doping of nitrogen. And the widening diffractionpeak of (131) crystallographic plane of N-doped Bi2WO6


10 20 30 40 50 60 70 80 90

Bi2WO6

Anatase

TiO2/N-BWO-400/2 h

TiO2/N-BWO-400/1 hTiO2/N-BWO

H-TiO2

Inte

nsi

ty

2θ (◦)

Pure BWO(020) (151)

(462)(204)

(193)(083)(262)(133)(202)

(200) (215)

(101)(004)

(200)

(211) (204)

(131)

(a)

24 25 26 27 28 29 30

Bi2WO6

Anatase

Inte

nsi

ty

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TiO2/N-BWO

N-BWO

-400/2 h

TiO2/N-BWO-400/1 h

TiO2/N-BWO

BWO

H-TiO2

(131)

(101)

(b)

Figure 1: XRD patterns of TiO2/N-BWO, TiO2/N-BWO-400/1 h, TiO2/N-BWO-400/2 h, BWO, N-BWO, and H-TiO2.

is induced by the doping of nitrogen in Figure 1(b). Inaddition, the peak position of N-doped Bi2WO6 shiftsslightly toward a lower 2θ value. In the XRD patterns for theheterostructured TiO2/N-Bi2WO6 composites, the anatasephase TiO2 peak of (101) crystallographic plane graduallyappeared and the diffracted intensity of N-doped Bi2WO6

drastically decreased in Figure 1(a). After the heat treatment,the phase compositions of the as-prepared composites do notchange any, indicating that N-Bi2WO6 plays a suppressiverole in the phase transformation from anatase to rutilephase of TiO2. Highly dispersed TiO2 coated on the surfaceof N-Bi2WO6 resulted in the weak peaks corresponding toN-Bi2WO6 and anatase TiO2 which were observed in theheterostructured TiO2/N-Bi2WO6 composites. The averagecrystal grain sizes of the as-prepared products are calculatedfrom (131) crystallographic plane in Figure 1(b) accordingto the Scherrer formula and are summarized in Table 1. It isfound that, for the pure Bi2WO6, the average crystal grainsize is about 20.67 nm, while the average crystal grain sizein N-doped Bi2WO6 and TiO2/N-Bi2WO6 decreased dueto the doping with nitrogen and the suppressive effect ofTiO2 on the growth of N-Bi2WO6. The average crystal grainsize of TiO2 increased owing to hydrothermal reaction andpostcalcination. Since the ionic radius of N3−(0.171 nm) islarger than that of O2− (0.140 nm) and the high electrondensity of N3− ions, the most of N3− ions were doped in thecrystal lattice of Bi2WO6 [14]. Thus, the crystal lattice cellparameter of Bi2WO6 along b-axis direction has a slightlyincrease whereas that along c-axis direction has a slightlydecrease owing to the doping with nitrogen (in Table 2).

3.2. SEM. The morphologies of the as-prepared productswere characterized by SEM. As shown in Figure 2, thepure Bi2WO6 product is composed of the large and highly

Table 1: Crystal sizes of the as-synthesized samples.

Sample D(BWO)(131)/nm D(TiO2)(101)/nm

H-TiO2 — 6.92

BWO 20.67 —

N-BWO 12.82 —

TiO2/N-BWO 12.00 14.24

TiO2/N-BWO-400/1 h 15.86 12.58

TiO2/N-BWO-400/2 h 10.09 21.60

Table 2: Crystal lattice cell parameters of Bi2WO6 in the as-synthesized samples.

Sample a/nm b/nm c/nm

BWO 5.45 16.43 5.44

N-BWO 5.45 16.44 5.44

TiO2/N-BWO 5.44 16.48 5.43

TiO2/N-BWO-400/1 h 5.45 16.51 5.41

TiO2/N-BWO-400/2 h 5.45 16.44 5.41

dispersed nanoplates. The platelike morphology is main-tained after doping with nitrogen though the sheet sizesbecome much lower. Moreover, the small nanoplates becomemore and more aggregated. For comparison, the as-preparedTiO2 is composed of the small anatase spherical aggregatesvia a hydrothermal process. The significant morphologymodification is induced by the incorporation of TiO2 into theN-doped Bi2WO6 structure. No distinct platelike structurein the TiO2/N-Bi2WO6 is observed owing to the coating ofthe spherical TiO2 particles on the surface of nanoplates.After calcination, the TiO2 nanoparticles occur the crystalgrowth and the heterostructured TiO2/N-Bi2WO6 displaysan aggregate trend.


Pure BWO

200 nm

(a)

N-BWO

200 nm

(b)

200 nm

H-TiO2

(c)

TiO2/N-BWO

200 nm

(d)

200 nm

TiO2/N-BWO-400/1 h

(e)

200 nm

TiO2/N-BWO-400/2 h

(f)

Figure 2: Selected SEM micrographs for the as-prepared samples.

100 nm

(a)

20 nm

(b)

20 nm

(c)

10 nm

TiO2 (103)

0.211 nm

0.243 nm

TiO2 (103)0.243 nm

Bi2WO6 (002)

Bi2WO6 (241)

0.27 nm

(d)

Figure 3: TEM photographs for the as-prepared N-BWO (a, b) and TiO2/N-BWO (c, d).

3.3. TEM. To confirm the microstructure of the het-erostructured TiO2/N-Bi2WO6, the TEM photographs ofthe as-synthesized N-BWO and TiO2/N-BWO are shownin Figure 3. These results show that the nitrogen-dopedBi2WO6 sample is composed of highly dispersed and homo-geneous nanoplates, indicating that doping with nitrogendoes not result in the change of the morphologies ofBi2WO6 (in Figure 3(a)). Figure 3(b) reveals that the weak

crystallinity of N-BWO is obtained as a result of dopingwith nitrogen. The microstructure of the TiO2/N-BWOcomposite was further studied by TEM and HRTEM.Figure 3(c) shows the typical TEM image of the TiO2/N-BWO heterostructure, in which many small spherical TiO2

nanoparticles of approximately 5–10 nm with an anatasephase are coated on the surface of the platelike N-dopedBi2WO6 particles. The nanoplates of N-doped Bi2WO6 were


0

2000

4000

6000

8000

10000

0 4 8 12 16 20

Inte

nsi

ty

E (keV)

N-BWOCu

Cu

Cu

OCu

AW W

W

WW

Bi

Bi

Bi

Bi

Bi

BiBi

(a)

0

2000

4000

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Ca

OTi

Cu

WSi

Bi Bi

Ti

Bi

Cu

WCu

W Bi Bi

TiO2/N-BWO

(b)

Figure 4: Energy dispersive spectra for the as-prepared N-BWO and TiO2/N-BWO.

hardly observed in Figure 3(c), indicating that the well-coated anatase TiO2 layer was obtained on the surface ofN-doped Bi2WO6 nanoplates. The representative HRTEMimage of the magnified view of the top right corner area isgiven in Figure 3(d). As can be seen, three sets of differentlattice fringes were clearly observed. The lattice fringes of0.211 nm and 0.270 nm correspond to the (241) and (002)crystallographic planes of nitrogen-doped γ-Bi2WO6, whilethe lattice fringe of 0.243 nm matches well with the (103)crystallographic plane of anatase TiO2. The well-definedfringes and the high crystallinity of the heterostructuredTiO2/N-Bi2WO6 composites facilitate the separation ofthe photoinduced carriers, improving the correspondingphotocatalytic activities [15]. The distinct interface wasobserved between segregated units of TiO2 and N-dopedBi2WO6 nanoparticles, revealing that the heterostructureindeed formed from these two materials.

To further confirm the formation of the TiO2/N-BWOheterostructure, the energy dispersive spectra were measuredto analyze the species and contents of the as-prepared N-BWO and TiO2/N-BWO samples. Figure 4 shows that thereare some peaks of Bi, W, and O elements in the spectrumof N-BWO, while the peak of N element is not observedowing to the slightly doping with nitrogen. For comparison,the peak of Ti element is discovered except the peaks of Bi,W, and O elements. The content of each element is listedin Table 3. The atomic ratio of Bi : W : O is 2.67 : 1 : 2.31which is different from that of pure Bi2WO6, indicatingthat the stoichiometric ratio of Bi2WO6 is changed by thedoping with nitrogen. Moreover, small amount of residualcarbon is also observed in the N-doped Bi2WO6 sample.Different from the situation of N-BWO, the atomic ratioof Bi : W : Ti : O of the heterostructured TiO2/N-BWO is3.61 : 1: 102.87 : 97.74 due to the incorporation of anataseTiO2. The discrepancy of the atomic ratio was induced bythe thick TiO2 coating layer which influenced the detectionsignal of the energy dispersive spectroscopy. This result was

Table 3: Characterization data of EDS for N-BWO and TiO2/N-BWO samples.

N-BWO

Element Weight ratio (%) Atomic ratio (%)

C K 0.43 2.47

O K 0.95 4.11

Al K 0.29 0.74

Cu K 79.23 86.15

W M 4.74 1.78

Bi M 14.36 4.75

Total 100.00

TiO2/N-BWO

Element Weight ratio (%) Atomic ratio (%)

O K 10.61 30.30

Si K 0.63 1.02

Ca K 0.37 0.42

Ti K 33.44 31.89

Cu K 48.59 34.94

W M 1.23 0.31

Bi M 5.14 1.12

Total 100.00

in accordance with the TEM observation of the existenceof TiO2 coating layer on the surface of N-doped Bi2WO6

nanoplates.

3.4. Optical Properties. To evaluate the photoresponsivity,the UV-vis diffuse reflectance spectra of the as-preparedsamples are investigated. As can be seen in Figure 5(a), H-TiO2 exhibits remarkably strong absorption in the ultravioletlight region blow the wavelength of 400 nm. In contrast, theabsorption edge of the pure BWO sample has an obviousred shift to approximate 460 nm in the visible light region


200 300 400 500 600 700

Abs

orba

nce

Wavelength (nm)

BWO TiO2/N-BWO

TiO2/N-BWO

-400/1hTiO2/N-BWO-400/2 hN-BWO

H-TiO2

(a)

2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

BWO

TiO2/N-

N-

BWO

N-BWO

H-TiO2

(eV)

400/2 h

TiO2N-BWO/ -400/1 h

TiO2N-BWO/ -(αhA)1/

2

hA

(b)

Figure 5: (a) UV-vis diffuse reflectance spectra for the as-synthesized samples. (b) The relationship between (ahv)1/2 and photon energy.

and its ultraviolet absorption is weak. In comparison withthe pure Bi2WO6, the N-BWO sample has a slightly redshift of the absorption edge. However, it is interesting thatthe absorption for the ultraviolet light greatly increasedas a result of doping with nitrogen, revealing that the N-BWO sample possesses the excellent ultraviolet and visiblelight photoresponsivities. The absorption plots show allTiO2/N-BWO samples exhibit a slightly blue shift of theabsorption edge and have strong light response in theultraviolet region with the addition of TiO2 composition.In addition, with prolonging the calcination time, theabsorption intensities in the visible light region increasesignificantly, which indicates that there is a synergetic effecton the light absorption between TiO2 and N-doped Bi2WO6.Based on the equation αhυ = A(hυ−Eg)n/2 [16–18], therelationship between (ahv)1/2 and photon energy is shown inFigure 5(b). And the band gaps of the as-prepared sampleswere estimated to be 2.92, 2.67, 2.60, 2.75, 2.73, and2.71 eV in Table 4, corresponding to H-TiO2, BWO, N-BWO,TiO2/N-BWO, TiO2/N-BWO-400/1 h, and TiO2/N-BWO-400/2 h, respectively. From this result, it is deduced thatthe band gap of N-doped Bi2WO6 becomes much narrowerowing to the influence of doping with nitrogen, whereas thatof TiO2/N-BWO composites become much wider due to theintroduction of TiO2.

3.5. Photocatalytic Activities. Figure 6 shows the photocat-alytic activities of the as-prepared samples in the degradationof RhB under both ultraviolet and visible light irradiation.As can be seen in Figure 6(a), when the system wasilluminated with visible light, the pure Bi2WO6 samplesexhibited the lowest photocatalytic activity and the lowestdegradation ratio of RhB. It was interesting that as-preparedH-TiO2 exhibited a higher photocatalytic activity for thedegradation of RhB, which was different from the tradi-tional viewpoint that TiO2 was an effective ultraviolet-light-

Table 4: Estimation of the band gap energy of the as-preparedsamples.

Sample Band gap energy (eV)

H-TiO2 2.92

BWO 2.67

N-BWO 2.60

TiO2/N-BWO 2.75

TiO2/N-BWO-400/1 h 2.73

TiO2/N-BWO-400/2 h 2.71

driven photocatalyst [19, 20]. However, the nitrogen-dopedBi2WO6 displayed a slightly higher photocatalytic activitythan both H-TiO2, and pure BWO. This result proved thatthe enhanced visible light photocatalytic performance wascaused by the doping with nitrogen. In the case of theheterostructured TiO2/N-BWO composite without calcina-tion, the highest photocatalytic activity for the degradationof RhB was obtained, which was probably caused by theeffective separation of photoinduced charge carriers owingto the synergetic effect between TiO2 and N-doped Bi2WO6

[21, 22]. The visible-light-induced photocatalytic activity ofthe heterostructured TiO2/N-BWO composite was higherthan that of the commercial P25. This indicated that theformation of the new heterostructure was beneficial toaccelerating the separation of the photoinduced charges,improving the visible-light-induced photocatalytic activity.A negative influence on photocatalytic activity was inducedby prolonging the calcination time. It was a probable causethat the crystal growth of TiO2 and N-BWO resulted inthe decrease of photocatalytic reactive sites. As all we know,decreased grain radius would be beneficial to reducing therecombination opportunities of the photoinduced electron-hole pairs.


0 20 40 60 80 100 120

0

0.2

0.4

0.6

0.8

1

Time (min)

BWO TiO2/N-BWO

TiO2/N-BWO

-400/2 h

TiO2/N-BWO-400/1 h

N-BWO H-TiO2

P25

(Cad

−C

t)/C

ad

(a)

0

0.2

0.4

0.6

0.8

1

−10 0 10 20 30 40 50 60 70Time (min)

BWO TiO2/N-BWO

TiO2/N-BWO

-400/2 h

TiO2/N-BWO-400/1 h

N-BWO H-TiO2

P25(C

ad−C

t)/C

ad

(b)

Figure 6: Photocatalytic activity for decomposition of rhodamine B (4× 10−5 mol L−1) under (a) visible light (>420 nm) and (b) ultravioletlight (<400 nm) irradiation at room temperature in air over the as-prepared catalysts. Cad: the concentration of RhB after dark adsorption,Ct : the concentration of RhB during the photocatalytic reaction.

The photocatalytic activity was also evaluated underultraviolet light irradiation. As shown in Figure 6(b), N-doped Bi2WO6 exhibited the optimal photocatalytic activityfor the degradation of RhB among the as-prepared samples.The residual concentration of RhB in the solution wasclose to zero after photocatalytic reaction for 40 minutes.It proved that the doping with nitrogen into the crystallattice of Bi2WO6 was beneficial to improving the ultraviolet-light-induced photocatalytic performance. However, it waslower than that of the commercial P25 owing to its weakultraviolet light photoresponsivity and its large crystal size.And H-TiO2 was found to exhibit much lower photocatalyticactivity than N-doped Bi2WO6. However, the photocatalyticactivity of H-TiO2 under ultraviolet light irradiation wasslightly higher than that of TiO2/N-BWO. In the case ofultraviolet light illumination, the photocatalytic activity ofthe heterostructured TiO2/N-Bi2WO6 samples was muchlower than that of N-BWO. Furthermore, the photocatalyticactivity of TiO2/N-Bi2WO6 heterostructure become lowerand lower with prolonging the calcination time under ultra-violet light illumination. This was contrary with the results ofUV-vis diffuse reflectance spectra in Figure 5. These resultsshowed that the ultraviolet-light-driven photocatalytic activ-ities of N-BWO, H-TiO2, and TiO2/N-BWO decrease withthe enhanced ultraviolet light photoresponsivity as shownin Figure 5. Herein, the probable cause of the decreasedphotocatalytic activities was relative to the increased recom-bination opportunities of the photoinduced charges. FromSEM images in Figure 2, the obvious aggregation and thecrystal growth of TiO2 particles on the surfaces of N-doped

Bi2WO6 particles were observed, which was adverse to theeffective photoinduced charge separation. The photocatalyticactivity was influenced greatly by the dispersivity and thecrystal size of catalyst’s particles. A better dispersivity and asmaller crystal size of the catalyst’s particles were beneficialto the separation of the photoinduced electrons and holes. Inthe case of ultraviolet light irradiation, the TiO2 componentin the heterostructured TiO2/N-BWO composite played themain function during the photodegradation of RhB as shownin Figure 9. The increased crystal size of TiO2 would result inthe increased recombination opportunities of the photogen-erated charges consequentially. A thicker TiO2 layer coatedon the surfaces of N-doped Bi2WO6 particles was adverse tothe utilization of the photoactivity of the N-doped Bi2WO6

component. Moreover, the photodegradation for RhB usingN-doped Bi2WO6 as catalyst was incomplete, producinga large number of intermediates. The above-mentionedcauses were responsible for lower photocatalytic activityof the heterostructured TiO2/N-BWO composite withoutcalcination under ultraviolet light irradiation, comparingwith H-TiO2 and N-doped Bi2WO6. Nevertheless, the pureBWO exhibited the lowest photocatalytic activity due to itsweak ultraviolet light photoresponsivity. By comparison, allof the as-prepared samples exhibited higher photocatalyticactivities under ultraviolet light irradiation than that undervisible light irradiation.

To distinguish the photodegradation rate for RhB overthe as-prepared catalysts, the kinetic-fitted curves plotsof photodegradation of RhB under different light sourcewith various wavelength ranges are displayed in Figure 7


0 10 20 30 40 50 60 70 80−10

0

1

2

3

4

5

Time (min)

N-BWO

H-TiO2

P25

TiO2/N-BWO

Pure BWO

Ultraviolet light

400/2 h

TiO2N-BWO/ -400/1 h

TiO2N-BWO/ -

Ln

(CtC

0)/

−

(a)

−0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40 60 80 100 120Time (min)

400/2 h

TiO2N-BWO/ -400/1 h

TiO2N-BWO/ -

N-BWO

H-TiO2

P25

TiO2/N-BWO

Pure BWO

Ln

(CtC

0)

Visible light

/−

(b)

Figure 7: Kinetic fitted curves plots of photocatalytic degradation of RhB (4 × 10−5 mol L−1) under ultraviolet light (<400 nm) (a) andvisible light (>420 nm) (b) irradiation at room temperature in air over the as-prepared catalysts.

Table 5: The values of the photodegradation rate constant k for RhB and linearly dependent coefficient R2.

SampleUltraviolet light Visible light

k (mol L−1 min−1) R2 k (mol L−1 min−1) R2

H-TiO2 6.58 × 10−2 0.927 1.41 × 10−2 0.982

BWO 6.20 × 10−3 0.929 5.97 × 10−3 0.931

N-BWO 7.53 × 10−2 0.929 2.29 × 10−2 0.964

TiO2/N-BWO 6.69 × 10−2 0.928 3.66 × 10−2 0.946

TiO2/N-BWO-400/1 h 5.71 × 10−2 0.899 2.02 × 10−2 0.959

TiO2/N-BWO-400/2 h 3.78 × 10−2 0.905 1.48 × 10−2 0.991

P25 1.55 × 10−1 0.945 2.61 × 10−2 0.982

and the corresponding values of the photodegradation rateconstant k for RhB and linearly dependent coefficient R2 arelisted in Table 5. The–Ln(Ct/C0) linearly increased, whichmeant the photodegradation of RhB under both ultravioletand visible light irradiation over as-prepared samples couldbe described as first-order reaction [16]. The photocatalyticactivity of the as-prepared samples can be also evaluatedby the values of k, that is, the higher the value of k, thebetter the photocatalytic activity [23]. Moreover, the largerthe coefficients R2, the better the linear dependence relation.From the fitted results in Table 5, under ultraviolet lightillumination, the order of the values of k was 1.55 × 10−1 >7.53 × 10−2 > 6.69 × 10−2 > 6.58 × 10−2 > 5.71 ×10−2 > 3.78 × 10−2 > 6.20 × 10−3, corresponding to theslopes of kinetic fitted curves of P25, N-BWO, TiO2/N-BWO,H-TiO2, TiO2/N-BWO-400/1 h, TiO2/N-BWO-400/2 h andpure BWO, respectively. The ultraviolet-light-driven pho-todegradation rate for RhB using the heterostructuredTiO2/N-BWO composite without calcination is much lowerthan that of the commercial P25. As we know, P25 was anexcellent ultraviolet-light-responsive photocatalyst. Hence,the photocatalytic activity of P25 under ultraviolet light

irradiation should be much higher than that under visiblelight irradiation. This viewpoint was proved by the com-parison of Figures 6(a) and 6(b). By contrast, under visiblelight irradiation, the order of the values of k was 3.66 ×10−2 > 2.61 × 10−2 > 2.29 × 10−2 > 2.02 × 10−2 >1.48 × 10−2 > 1.41 × 10−2 > 5.97 × 10−3, correspondingto the slopes of kinetic-fitted curves of TiO2/N-BWO, P25,N-BWO, TiO2/N-BWO-400/1 h, TiO2/N-BWO-400/2 h, H-TiO2, and pure BWO, respectively. By comparison, the pho-todegradation rate for RhB under ultraviolet light irradiationwas higher than that under visible light irradiation. N-doped Bi2WO6 exhibited the highest photodegradation rateunder ultraviolet light irradiation, whereas TiO2/N-BWOexhibited the highest photodegradation rate under visiblelight irradiation. Furthermore, the photodegradation rateof the heterostructured TiO2/N-BWO composite withoutcalcination under visible light irradiation was much higherthan that of the commercial P25. This result indicatedthat the formation of the heterostructure facilitated theeffective separation of the photoinduced charge carriers onthe interface between TiO2 and N-doped Bi2WO6, improvingvisible-light-driven photocatalytic performance.


300 400 500 600 700 800

Abs

orba

nce

Wavelength (nm)

0 min

8 min

16 min

24 min

32 min

40 min

50 min

60 min

70 min

N-BWO

(a)

300 400 500 600 700 800

Wavelength (nm)

0 min

8 min

16 min

24 min

32 min

40 min

50 min

60 min

70 min

TiO2/N-BWO

Abs

orba

nce

(b)

300 400 500 600 700 800

Wavelength (nm)

0 min

8 min

16 min

24 min

32 min

40 min

50 min

60 min

70 min

H-TiO2

Abs

orba

nce

(c)

Figure 8: Temporal evolution of the spectral changes during the photodegradation of RhB over N-BWO, TiO2/N-BWO, and H-TiO2 underultraviolet light irradiation.

3.6. Photocatalytic Mechanism. The temporal evolutions ofthe spectral changes during the photodegradation of RhBover N-BWO, TiO2/N-BWO, and H-TiO2 under ultravioletlight irradiation are shown in Figure 8. As all we know, RhBpossesses a maximal absorption band at 553 nm. If degradedover N-doped Bi2WO6 under ultraviolet light irradiation for32 minutes (in Figure 8(a)), the spectral maximum of RhBshifted from 553 to 531 nm, indicating that the mechanisticpathways of RhB degradation over N-doped Bi2WO6 wasmultisteps process accompanying with the deethylationprocess of the cleavage of the chromophore structure [16]. Inthe case of TiO2/N-BWO (in Figure 8(b)), the deethylationfunction was weaken owing to the incorporation of TiO2

with an anatase phase under ultraviolet light irradiation.Thus, the photodegradation of RhB occurred mainly bydecomposing the pollutants to small molecules of CO2

and H2O. As for H-TiO2 in Figure 8(c), the deethylationphenomenon was hardly seen during the degradation ofRhB under ultraviolet light irradiation. By comparison, thephotodegradation rate of RhB was accelerated by the dopingwith nitrogen into the crystal lattice of Bi2WO6, because thenumber of photoinduced electrons and holes to participatein the photocatalytic reaction was increased remarkably,resulting in the enhanced photocatalytic activity. Underultraviolet light irradiation, the heterostructure of TiO2/N-BWO does not significantly influence the photodegradation


300 400 500 600 700 800

Wavelength (nm)

0 min

8 min

16 min

24 min

32 min

40 min

50 min

60 min

70 min

TiO2/N-BWO UVA

bsor

ban

ce

(a)

300 400 500 600 700 800

Wavelength (nm)

TiO2/N-BWO

0 min

20 min

40 min

60 min

80 min

100 min

120 min

Vis

Abs

orba

nce

(b)

Figure 9: Temporal evolution of the spectral changes during the photodegradation of RhB over TiO2/N-BWO under ultraviolet lightirradiation (a) and visible light irradiation (b).

rate of RhB comparing to both N-BWO and H-TiO2 due tothe strong oxidation performance of H-TiO2 with an intenseultraviolet photoresponsivity.

Based on the above discussions, the as-prepared H-TiO2

and N-BWO were not only used as ultraviolet-light-drivenphotocatalysts but also regarded as visible-light-driven pho-tocatalysts. Therefore, the heterostructure formed betweenH-TiO2 and N-doped Bi2WO6 was expected to possess theexcellent photocatalytic performances as a bifunctional pho-tocatalyst. Thus the heterostructured TiO2/N-BWO com-posite exhibited the excellent photocatalytic activities underboth ultraviolet light and visible light irradiation as shown inFigure 9. Furthermore, the mechanisms of photodegradationof RhB over TiO2/N-BWO were completely distinct underdifferent conditions of light irradiation. In our experi-ments, the composition of TiO2 in the heterostructureplayed an important role in the photodegradation of RhBunder ultraviolet light irradiation (Figure 9(a)), while thecomposition of N-doped Bi2WO6 in the heterostructureplayed an important role in the photodegradation of RhBunder visible light irradiation (Figure 9(b)). In addition, thephotodegradation pathways of RhB were also disparate in thetwo cases. As for ultraviolet-light-driven photodegradation,the molecules of RhB over TiO2/N-BWO were degradedinto small inorganic molecules of CO2 and H2O directly.Different from the case of ultraviolet photodegradation,the decomposition of RhB over TiO2/N-BWO was carriedout step by step via a deethylation process under visiblelight illumination, resulting in the obvious blue shift of themaximal absorption band of RhB. The interface formedbetween H-TiO2 and N-doped Bi2WO6 can facilitate thetransportation of photoinduced charge carriers and suppressthe recombination of photogenerated electrons and holespairs, inducing the enhanced photocatalytic activity undervisible light illumination.

The enhanced visible-light-driven photocatalytic perfor-mance is related to the corresponding energy band structure.The doping with nitrogen resulted in the formation of adopant energy level at the bottom of conduction band ofBi2WO6. The narrowing band gap of N-BWO was beneficialto increasing the transfer rate of electrons to the photocata-lyst surface, promoting photocatalytic reaction. According tothe estimated Eg values of the TiO2 and N-doped Bi2WO6 inFigure 5(b) and the conduction band (CB) and valence band(VB) levels of Bi2WO6 are lower than that of TiO2 [11, 12],the energy band structure of TiO2/N-BWO composite wasschematically shown in Figure 10. The strong interactionbetween H-TiO2 and N-BWO resulted in a synergetic effecton the photocatalytic activity of the heterostructure. Thephotoinduced electron-hole pairs were produced by theexcitation of TiO2/N-BWO under ultraviolet and visiblelight irradiation. The photoinduced electrons transferredfrom the conduction band of H-TiO2 to that of N-dopedBi2WO6 due to the joint of the internal electric fieldsbetween two materials. However, the photoinduced holestransferred from the valence band of N-doped Bi2WO6 tothat of H-TiO2. Thus, the effective photoinduced chargeseparation provided more and more active free radicalsto participate in the photocatalytic degradation of RhB.The novel design of the bifunctional TiO2/N-BWO het-erostructure provided a new approach to develop the visible-light-driven photocatalysts. But N-doped Bi2WO6 was morelikely to be excitated by visible light, being regarded as themain functional component under visible light irradiation.However, TiO2 was more likely to be excitated by ultravioletlight, being regarded as the main functional componentunder ultraviolet light irradiation. Therefore, TiO2/N-BWOcomposite exhibits diverse photodegradation mechanismsfor RhB under different light irradiation conditions.


e−

e−

Eg = 2.6 eV

Eg = 2.92 eV

h+

h+

TiO2 N-Bi2WO6

Figure 10: Schematic diagram for energy band matching andelectron-hole separation.

4. Conclusion

Heterojuncted TiO2/N-Bi2WO6 composites were synthesizedby a facile sol-gel-hydrothermal process. The as-preparedTiO2/N-Bi2WO6 composites display a wide range of lightabsorption due to the introduction of TiO2, and thecorresponding photocatalytic activities against rhodamine Bare slightly improved in comparison with pristine Bi2WO6

and TiO2. It is also found that the existence of TiO2 in theheterojuncted TiO2/N-Bi2WO6 composites plays an impor-tant role in the photocatalytic property. The enhancement inthe visible light photocatalytic performance of TiO2/Bi2WO6

composites can be attributed to the effective electron-holeseparations at the interfaces between TiO2 and N-Bi2WO6,which facilitate the transfer of the photoinduced carriers.The experiments prove that the heterojuncted TiO2/Bi2WO6

composites are promising photocatalysts under visible andultraviolet light irradiation.

Acknowledgment

The authors are grateful for the financial support bythe National High Technology Research and DevelopmentProgram of China (863 Program no. 2010AA064907).

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Research Article

Preparation, Characterization, and Photocatalytic Property ofCu2O-TiO2 Nanocomposites

Longfeng Li and Maolin Zhang

School of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, China

Correspondence should be addressed to Maolin Zhang, [email protected]

Received 3 August 2011; Accepted 22 August 2011


Copyright © 2012 L. Li and M. Zhang. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The Cu2O-TiO2 nanocomposites were successfully synthesized by the homogeneous hydrolysation, followed by the solvothermalcrystallization and ethylene glycol-thermal reduction process, respectively. The obtained products were characterized by meansof X-ray diffraction(XRD), Uv-vis diffuse reflectance spectroscopy, laser particle size analysis, and scanning electron microscopy(SEM), respectively. The photocatalytic performance of Cu2O-TiO2 nanocomposites was evaluated by the degradation of methylorange (MO) as a model compound. The experimental results showed that the prepared Cu2O-TiO2 nanocomposite exhibitedhigher photocatalytic activity for the decomposition of MO than the pure Cu2O and the commercial Degussa P25 TiO2 undervisible light irradiation.

1. Introduction

The semiconductor photocatalysis has been used to miner-alize the organics via a series of intermediates into inorganicsubstances such as H2O and CO2 in the presence of light.To date, many semiconductors have been found to begood photocatalysts to decompose various organics. Amongvarious semiconducting materials, much attention has beengiven to TiO2 [1–4] because of its high photocatalyticactivity, resistance to photocorrosion, chemical and biolog-ical inertness, commercial availability, and inexpensiveness.However, the photoinduced charge carrier in single baresemiconductor particles like TiO2 has a very short lifetimeowing to the high-recombination rate of the photogeneratedelectron/hole pairs, which reduces photocatalytic efficiency.On the other hand, Titania has a large band gap (about 3.2 eVfor anatase phase) and only a small fraction of solar lightcan be absorbed. These hinder the wide-scale engineeringapplications of pure titanium dioxide. Therefore, in order toimprove the photocatalytic activity of TiO2, it is importantto prevent the photoelectron/hole recombination until adesignated chemical reaction takes place on the surfaceof semiconductor particles as well as to extend the lightabsorbing property of TiO2. Some previous researcheshave found that the coupling of two semiconductors can

improve the photoexcited charge separation and enhance thephotocatalytic activity [5–11]. On the other hand, Cu2O is ap-type semiconductor with direct band gap of 2.0 eV and hasa noticeable light absorption capability in the visible-lightregion [12–14]. Accordingly, it is expected to prepare Cu2O-TiO2 nanocomposite with the highly efficient photoexcitedcharge separation, the enhanced photocatalytic efficiencyand the remarkable visible-light photoresponse by couplingTiO2 with Cu2O, which has been reported in the few previousstudies [15–18].

In this study, using the ethylene glycol as the solvent andthe reducing agent, and titanium tetrabutoxide and Cu (II)acetate as precursors, the Cu2O-TiO2 nanocomposite wassuccessfully synthesized by the homogeneous hydrolysation,followed by the solvothermal crystallization treatment andthe ethylene glycol-thermal reduction reaction, respectively.The process would develop a new method for preparingCu2O-TiO2 nanocomposite with the visible-light photocat-alytic activity under mild conditions.

2. Experimental

2.1. Materials and Apparatus. The reagents (titanium tetrab-utoxide, copper (II) acetate monohydrate, glacial acetic acid,


ethylene glycol, and methyl orange) are all analytic reagentgrade. A Bruker D8 Advance X-ray diffractometer with CuKα radiation (λ = 0.15418 nm), the accelerating voltage of40 kV, emission current of 40 mA, and the scanning speed of8◦/min was used to determine the crystal phase compositionand the crystallite size of the coupled oxides prepared. Anda scanning electron microscope (LEO1530VP) was used toobserve the shape and size of the prepared products. TheUV-Vis diffuse reflectance spectroscopy was obtained usinga UV-visible spectrophotometer (TU-1901, Beijing PurkinjeGeneral Instrumental Co., China), and the particle sizeanalysis was performed on a Zetasizer Nano ZS90.

2.2. Synthesis Procedure. All of the chemical reagents usedin the experiments were analytical grade without furtherpurification and treatment. The synthesis procedures ofCu2O-TiO2 were as follows: 0.01 mol of titanium tetrabu-toxide and 0.04 mol of glacial acetic acid were dissolved in50 mL of ethylene glycol. The solution is taken in Teflon-lined stainless-steel autoclave, heated to 120◦C at a rateof 5◦C/min, and kept under the temperature for 2 hr.Uniform hydrolysis of Ti(OBu)4 was accomplished via insitu homogeneous generation of water, which is formed bythe esterification reaction of ethylene glycol with acetic acid.As Result, the amorphous hydrous titanium oxide precursorTiO2·nH2O was obtained, and then the autoclave was heatedup to 200◦C and held at this temperature for 10 hr, and theamorphous precursor was transformed into the nanosizedTiO2 with stable crystal structure. After the autoclave wasallowed cooling to room temperature, the solution of copper(II) acetate in ethylene glycol was added into the autoclavein certain mole ratio of Ti/Cu under stirring condition.The reaction mixture was heated in the temperature rangeof 150–190◦C for 6 hr in order to load Cu2O onto thesurface of TiO2 by ethylene glycol-thermal reduction process.Subsequently, the product Cu2O-TiO2 was separated fromthe solid-liquid mixture by low pressure distillation at 150◦Cand grinned in agate mortar to obtain the powder samples.

2.3. Photocatalytic Activity Measurement. The photocatalyticactivity tests of the obtained Cu2O-TiO2, the pure Cu2O,and the commercial Degussa P25 TiO2 were performed atca. 30◦C in a 250 mL glass reactor, respectively. A 36-Wfluorescent lamp used as a visible light source was placedabove the reaction mixture approximately 10 cm away fromsolution surface. The initial concentrations of MO andphotocatalyst powders were 0.02 and 3 g·L−1, respectively.Prior to irradiation, the suspension was stirred in a darkto establish adsorption-desorption equilibrium. Once theconcentration of MO had stabilized, the reaction mixturewas irradiated, signaling the start of photocatalysis. At giventime intervals, sample was collected, centrifuged, and filteredthrough a 0.2 μm millipore filter. Then the filtrate wasanalyzed on a 722 visible spectrophotometer at 464 nm.

20 30 40 50 60 70 80

2θ (◦)

CuTiO2

190◦C

180◦C

170◦C

160◦C

150◦C

Cu2O

Figure 1: XRD patterns of the Cu2O-TiO2 samples.


3.1. Influence of Thermal Reduction Temperature on PhaseComposition and Crystal Size. To determine the crystal phasecomposition and crystal size of the prepared photocatalyst,X-ray diffraction(XRD) measurements were carried out atroom temperature over the diffraction angle (2θ) 20◦–80◦.Figure 1 showed the XRD patterns of Cu2O-TiO2 powdersprepared by ethylene glycol-thermal reduction at 150, 160,170, 180, and 190◦C for 6 hr, respectively. From Figure 1,we can observe that there is a continuous sharpening andintensifying of the diffraction peaks for Cu2O in Cu2O-TiO2

with increasing thermal reduction temperature, indicatingthat the crystal size of Cu2O increased as the thermalreduction temperature went up. The crystal size of Cu2O indifferent reduction temperature can be calculated accordingto the Scherrer equation. The results showed that the meansizes of Cu2O in Cu2O-TiO2 were 12.7, 23.9, 35.1, 50.1, and79.7 nm at 150, 160, 170, 180, and 190◦C, respectively. Onthe other hand, we can see the diffraction peaks of Cu in theCu2O-TiO2 samples as the thermal reduction temperaturerising to 180◦C and above indicated that part Cu2O wasreduced further to metallic copper.

3.2. Morphology and Size Distribution of Couple Oxides. Inorder to study the morphology of the prepared Cu2O-TiO2,scanning electron microscopy (SEM) was used. Figure 2showed the TEM image of the Cu2O-TiO2 sample preparedat the thermal reduction temperatures of 160◦C. SEMmicrograph indicated that the obtained Cu2O-TiO2 samplewas homogeneously distributed nanocomposite particles,and the particle size of them was in the range of 40−60 nm.

In the present study, the particle size and size distributionof the Cu2O-TiO2 sample prepared at the thermal reductiontemperatures of 160◦C also was measured on a ZetasizerNano ZS90 (Malvern Instrument, Worcestershire, UK). Asshown in Figure 3, the Cu2O-TiO2 particles size was found


1 μm EHT = 3 kVWD = 4 mm

Signal A = SE1 Date: 15 Oct 2010Time: 12:18:17Mag = 20 kX Photo no. = 8063

Figure 2: SEM pattern of Cu2O-TiO2.

40

30

20

10

01 10 100 1000 10000

Nu

mbe

r(%

)

Size (d.nm)

Size distribution by number

Figure 3: Particle size distribution of Cu2O-TiO2.

in the range of 35–105 nm, and the average particle size wasabout 64 nm. The particle size measured by the zetasizer waslarger than that observed in XRD and SEM. Because MalvernInstruments’ Zetasizer used light scattering techniques tomeasure hydrodynamic size of nanoparticles, a increase inparticle size can be the result of particle agglomeration.

3.3. UV-Vis Diffuse Reflectance Spectra. The absorptionspectra of Cu2O, P25 TiO2, and Cu2O-TiO2 prepared atthe thermal reduction temperatures of 160◦C were given inFigure 4. Figure 4 showed that all the samples had a strongabsorption at the wavelength range from 230 to 380 nm.In addition, it can be also observed from Figure 4 that theabsorption spectroscopy of the Cu2O-TiO2 sample was red-shifted compared to that of TiO2, and the Cu2O-TiO2 samplehad obvious absorption in the visible region (>400 nm). Theabsorption wavelength of the Cu2O-TiO2 nanocompositeswas extended to a visible region due to absorption of visiblelight by Cu2O.

3.4. Photocatalytic Activities of Samples. The photocatalyticactivities of the Cu2O-TiO2 samples through the pho-todegradation of MO under the visible light irradiationfor 3 hours were evaluated and were also compared withthat of the commercial Degussa P25 TiO2 powder andthe pure Cu2O powder. The experimental results were

300 400 500 6000.2

0.4

0.6

0.8

1

1.2

1.4

Abs

orba

nce

Wavelength (nm)

Cu2O-TiO2

Cu2OP25 TiO2

Figure 4: UV-Vis diffuse reflection spectra.

1 2 30

5

10

15

20

25

30

35

40

45

50

55

60

Cu2O-TiO2

Cu2O

P25 TiO2

η(%

)

Photocatalysts

Figure 5: Photocatalytic activities of different photocatalysts undervisible-light irradiation (the Cu2O-TiO2 sample was prepared at athermal reduction temperature of 160◦C).

illustrated in Figure 5. Obviously, the synthesized Cu2O-TiO2 showed higher photocatalytic activity than the pureTiO2 and Cu2O under UV-vis light irradiation. The highphotocatalytic activity of Cu2O-TiO2 can be attributed to themore efficient separation of photoinduced hole-electron (h-e) pairs in the Cu2O-TiO2 composite, that is, to say that thephotogenerated holes migrate towards the interface while theelectrons migrate towards the bulk due to Cu2O-TiO2 p-nheterojunction. Meanwhile, the excited electrons on Cu2Ocan also transfer to TiO2 because the conduction band ofTiO2 lies more positive than that of the Cu2O. Therefore, theCu2O-TiO2 composite exhibited much higher photocatalyticactivity than the pure TiO2 and Cu2O.


4. Conclusions

The nanoscale Cu2O-TiO2 couple oxide photocatalyst wassuccessfully prepared and was characterized by X-ray diffrac-tion, laser particle size analysis, and scanning electronmicroscopy, respectively. The characterization results indi-cated that the couple oxide samples consisted of the nano-sized Cu2O and TiO2 phases when the thermal reductiontemperature was not more than 180◦C. The results alsoshowed that the crystal size of Cu2O was obviously affectedby the thermal reduction temperatures, that is, the Cu2Oparticle size increased with increasing thermal reductiontemperature. Besides, there was a phase change from Cu2O toCu in the obtained samples when the thermal reduction tem-peratures were over 180◦C. In addition, the photocatalyticactivity experiment results showed that the couple oxideCu2O-TiO2 exhibited much higher photocatalytic activitythan the pure TiO2 and Cu2O.

Acknowledgments

The authors thank anonymous reviewers very much. Thiswork was supported by the Nature Science Foundation ofAnhui Provincial Education Committee (No. KJ2010A302).

References

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Research Article

Effect of Electronegativity and Charge Balance onthe Visible-Light-Responsive Photocatalytic Activity ofNonmetal Doped Anatase TiO2

Jibao Lu,1 Hao Jin,2 Ying Dai,1 Kesong Yang,3 and Baibiao Huang1

1 School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China2 Advanced Materials and Process Engineering Laboratory, The University of British Columbia, Vancouver, BC, Canada V6T 1Z43 MEMS Department, Duke University, Durham, NC 27708-0300, USA

Correspondence should be addressed to Ying Dai, [email protected]



Copyright © 2012 Jibao Lu et al. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The origin of visible light absorption and photocatalytic activity of nonmetal doped anatase TiO2 were investigated in details in thiswork based on density functional theory calculations. Our results indicate that the electronegativity is of great significance in theband structures, which determines the relative positions of impurity states induced by the doping species, and further influencesthe optical absorption and photocatalytic activities of doped TiO2. The effect of charge balance on the electronic structure wasalso discussed, and it was found that the charge-balance structures may be more efficient for visible light photocatalytic activities.In addition, the edge positions of conduction band and valence band, which determine the ability of a semiconductor to transferphotoexcited electrons to species adsorbed on its surface, were predicted as well. The results may provide a reference to furtherexperimental studies.

1. Introduction

Titanium dioxide (TiO2) is one of the most promisingmaterials due to its low-cost, nontoxic, long-term stabilityand high oxidative power, which can be used in a widerange of technical fields, such as photocatalytic degradationof pollutants and photoelectrochemical conversion of solarenergy [1, 2]. However, because of its large band gap (3.0–3.2 eV), TiO2 is only responsive to ultraviolet irradiation(λ < 378 nm), which only accounts for ca.4% of sunlight. Inorder to extend the photo response of TiO2 into the visibleregion and enhance its photocatalytic activity, a great dealof effort has been made to lower its threshold energy forphotoexcitation as well as improve the electronic propertiesof TiO2. One of the effective strategies is doping TiO2

with nonmetallic elements, such as N [2–10], C [11–14], S[15–19], and B [20–23], all of which showed considerablephotocatalytic activity under visible light.

While most researchers focus on the phenomena ofredshift induced by special species doping, a thorough and

systematical analysis of the modification mechanism of TiO2,especially the essential reasons of causing various electronicstructures doped with different nonmetallic elements, do notcatch due attention of scientists. To explore a common ruleof the band structure modification for nonmetal doping,we systematically compared the doping effects of fournonmetal species (B, C, N, and F) on anatase TiO2. For easycomparison, we only considered the electronic propertiesof structures with substitutional nonmetal X (X = B, C, N,and F) to O in this paper, though the formation energieswere also calculated to reveal the relative stabilities of thedoped structures. To check the influence of charge balance,we have calculated the electronic structures of codopedTiO2 with N : H, C : 2H, and B : 3H, respectively (note thatthe oxygen vacancies, which are not considered here, mayalso form to compensate the unbalanced charge, which caninduce deep defect levels in the band gap [24] and usuallylead to increment of the recombination). In addition, therole of the electronegativity of doping species in the opticalabsorption and photocatalytic activity of anatase TiO2 was


d1

d2

Figure 1: The supercell geometry and the doped position of the anatase TiO2 model. The big gray ball represents Ti atom; the small lightball represents O atom; the dark ball denotes the dopant.

elucidated in this paper, and we have found that the impuritystates move further away from the valence band maximum(VBM) with the increment of the electronegativity differencebetween the substitutional elements and oxygen. Also, theedge positions of conduction band (CB) and valence band(VB) for doped anatase TiO2 were predicted, which shouldserve as a reference to future experimental studies.

2. Computational Details

Our theoretical study was carried out in the framework ofdensity functional theory (DFT) within generalized gradientapproximation (GGA) [27]. The Vienna ab initio SimulationPackage [28, 29] (VASP) was implemented in the calcula-tions, in which the plane-wave basis set, Vanderbilt ultrasoftpseudopotentials and the exchange-correlation functional ofPerdew et al. (PBE) were used [30]. The underestimation ofband gap by DFT calculations does not affect the analysis,since we only care about the relative changes of the gap,and the PBE functional has been demonstrated to be reliableby previous studies [31]. We simulated the nonmetal-dopedTiO2 model using a 48-atom supercell, in which one O atomis substituted by one impurity atom, and this corresponds toan impurity concentration of about 2%, namely, TiO2-xDx

with x = 0.063, (where D is the corresponding dopant). Thesupercell geometry and the doped position of the anataseTiO2 system are shown in Figure 1. The cutoff energy for thebasis set was chosen to be 500 eV, and the surface Brillouinzones were sampled at 2 × 2 × 2 Γ-centered k-points [32].The density of state (DOS) for the bulk TiO2 was performedat the equilibrium volume using the tetrahedron methodwith Blochl corrections for accuracy [33]. Considering thatthe impurity concentration is very low, the volume of su-percell is kept fixed while the atomic positions are fully re-laxed until that all forces are smaller than 10−4 eV/A.


3.1. Electronegativity Effect on the Electronic Structures. Aswe know, electronegativity is the intrinsic property whichmeasures the escaping tendency of electrons from atomicspecies. Based on DFT theory, Parr and coworkers [34]have given the precise definition to the electronegativity of

Table 1: EA, IP, and calculated electronegativity for several species.Unit is eV.

EAa IPb Electronegativity

Ti 0.207 6.820 3.51

B 0.280 8.298 4.29

S 2.077 10.360 6.22

C 1.267 11.260 6.26

H 0.766 13.598 7.18

N −0.072 14.534 7.23

O 1.461 13.618 7.54

Cl 3.614 12.967 8.29

F 3.398 17.422 10.41aReference [25].

bReference [26].

a neutral atom, which is the negative value of the chemicalpotential in the ground state:

χ = −μ = −(∂E

∂N

)v, (1)

where E and N are the ground-state electronic energy andthe number of electrons, respectively. Formula (1) can thenbe rewritten using differential equation:

χ=−(∂E

∂N

)v= E(N + 1)−E(N)+E(N)−E(N − 1)

2≈ I + A

2,

(2)

where I and A represent the ionization potential (IP) andelectron affinity (EA) of the atom. We can see that Formula(2) has the same expression with the Mulliken’s definition ofthe electronegativity [35]. The electronegativities of severalspecies calculated by Formula (2) are listed in Table 1.

When the nonmetallic elements are brought to the bulkTiO2, charges will redistribute until the electrochemical po-tentials of the compound reach equilibrium. To unravel theinherent relationship of the electronic properties inducedby different doping species, which possessed unique elec-tronegativity, the total density of state (TDOS) and projecteddensity of state (PDOS) for the doped TiO2 are calculatedand displayed in Figure 2. For comparison, the TDOS and


0

50

100

0

5

10

Energy (eV)

0

50

100(e)

0

5

10

0

50

100 (d)

0

3

6

0

50

100 (c)

0

3

6

0

50

100

(b)

0

10

20

−6 −5 −4 −3 −2 −1 0 1 2 3 4

Energy (eV)

−6 −5 −4 −3 −2 −1 0 1 2 3 4

0

50

100

(a)

0

50

100

(A) (B)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

C

Cl

N

O

B

F

Ti

(f) (f)

(e)

(d)

(c)

(b)

(a)

2p

2p

2p

2p

2p

3p

3d

Figure 2: Total densities of states (A) and projected densities of states (B) for (a) pure anatase TiO2, (b) F-doped TiO2, (c) Cl-doped TiO2,(d) N-doped TiO2, (e) C-doped TiO2, and (f) B-doped TiO2. The vertical dash line defines the highest occupied level.

PDOS of pure anatase are also calculated and shown inFigures 2(a) and 2(a′). We can see that the edge of the valenceband is mainly contributed by the O 2p atomic orbitals witha large bandwidth of about 5.0 eV, whereas the conductionband is almost entirely composed of empty Ti 3d orbitals.

For the F-doped TiO2, (see Figures 2(b) and 2(b′)), thereplacement of one O atom with one F atom exerts little effecton the band-gap narrowing. The impurity states introducedby F 2p atomic orbitals locate mainly in the lower energyrange (from −5.5 eV to −4.5 eV) of the valence band, whichimplies that only the substitutional F to O cannot lead toany significant shift in the fundamental absorption edgeof TiO2, consistent with the former experimental results[36, 37]. It is noticed that the electronegativity mainlydetermines the properties of the outermost atomic orbitals,which simultaneously influence the impurity states inducedby F doping. Hence, the larger electronegativity of fluorine(Table 1) should be responsible for the lower energy range ofthe impurity states in the valence band.

Similar doping effects also appear in Cl-doped sample.Because the electronegativity of chlorine is just a little largerthan that of oxygen, they have the similar power of attractingelectrons. Accordingly, the impurity states induced by Cl 3p

states should mix with the O 2p states in the valence band.The calculated DOS provides evidence for the distributioncharacter of these states. From Figures 2(c) and 2(c′), we cansee clearly that the band gap reduces little as compared withthe pure anatase TiO2, and the Cl 3p states lie in the wholevalence band, (from −5.5 eV to −0.1 eV), showing strongdelocalization characteristic of Cl− ion.

For substitutional N to O anatase model (see Figures2(d) and 2(d′)), due to the closer electronegativity of N tothat of O, the oxidative power of N is similar to that of Oelement. Accordingly, the N 2p states locate just above orin the VBM. Since some of the impurity states overlap andmix with the VBM, new edge of valence band can be formedinstead of pure O 2p atomic orbitals, which shifts towards thehigher energy and results in the band gap narrowing of about0.43 eV. Consequently, the decrease of the photo-excitationenergy leads to the redshift of absorption and extends thephoto response into visible region. These results are in goodagreement with previous experimental studies [6, 16, 17] andour former theoretical calculations [8, 18].

Figure 2(e) presents that the substitutional C to Oatom in anatase TiO2 introduces one empty and twooccupied localized states in the middle of the gap, which


are contributed by the C 2p states. Experimentally, evidenceof states within the band gap arising from these levels forC-doped titania has been found by XPS [38, 39].

In addition, the band gap between the highest O 2p statesand the lowest Ti 3d states has little change as compared withthe undoped TiO2. Therefore, the observed redshift of opticalabsorption would be mainly attributed to the existence ofimpurity states in the mid gap of TiO2. It is interesting thatthe electronegativity of carbon is close to that of nitrogen (seeTable 1). However, these gap states are located at 0.68, 1.01,and 1.6 eV away from the VBM. We tentatively postulate thatit is mainly due to the higher chemical valence of carbon thanoxygen, which results in the shift of C 2p states towards theCB.

For the case of B-doped TiO2, the substitution of B for Oinduces some new impurity gap states, which are dominatedby the B 2p atomic orbitals. In addition, since the elec-tronegativity of boron is much smaller than that of oxygenbut approaches to that of titanium, the redox potentials ofboron will be close to that of titanium as compared withoxygen, which leads to the impurity gap states lying belowor overlapping with the conduction band, (see Figures 2(f)and 2(f′)). It is well established that the existence of impuritystates in the mid gap can lower the energy barrier in the lightabsorption process. Therefore, these B 2p states should beresponsible for the improvement of visible light absorptionof TiO2.

Additionally, to compare the relative stabilities of anataseTiO2 samples doping with different elements, we calculatedthe formation energies as follows:

Ef = Edoped −(Epure − μO + μD

), (3)

where Edoped and Epure are the total energy of doped and pureanatase supercell, respectively. The symbols μO and μD rep-resent the chemical potential of oxygen and doped elements,respectively. Here we only focus on the so-called oxygen-richsynthesis condition, where the chemical potential for oxygenis related to the chemical potential for gaseous O2 at 1 bar. Wereport our results in Table 2 with the optimized Ti-D bondlengths also listed. It can be noted that the formation energydecreases with the increase of electronegativity of dopant. Inparticular, for the F-doped case, the formation energy has anegative valve, which implies that it is thermodynamicallypreferred. In addition, it is worthwhile to mention thatthe substitutional (for O) doping could be much easierunder oxygen-poor condition due to the lower oxygen partialpressure, while the relative stabilities between the structuresdoped with different ions remain the same as those underoxygen-rich condition.

3.2. Effect of Charge Balance on the Electronic Structures. Tounderstand the effect of charge balance on the electronicstructures, we balance the charge of N, C, and B with one H,two H, and three H atoms, respectively, which is reasonablebecause the doped TiO2 materials were usually processedwith amine solution [3–5], H2 [6], or BH3 atmosphere[20, 21]. In the calculations, one O atom is replaced by N : Hspecies, C : 2H species, and B : 3H species, respectively (see

Table 2: Calculated bond lengths, lattice variation, and defectformation energies.

d1 (A) d2 (A) ΔV/VDefect formation energies

(eV)

Pure 1.990 1.926

B-doped 2.650 2.118 6.6% 7.08

C-doped 2.217 2.010 5.1% 5.94

N-doped 2.043 1.957 4.8% 5.04

Cl-doped 2.519 2.224 6.6% 3.43

F-doped 2.227 2.043 4.9% −0.85

Figure 3). The corresponding DOS of N : H-doped, C : 2H-doped, and B : 3H-doped TiO2 are calculated and shown inFigure 4. Obviously, for the C-doped TiO2, when the chargebalance structure is obtained, three isolated gap states arereplaced by one occupied state, which is located near theVBM, (see Figures 4(c) and 4(c′)). From Figure 4, we cansee clearly that with the decrease of the electronegativitydifference between oxygen and substitutional elements, (N,C, and B), the impurity states become closer to the VBM.In addition, when the charges are balanced, the electrondonors (H) may contribute to the lowering of the energylevels related to the acceptors (N, C, and B), resulting in theimpurity states downward to the VBM, which agrees wellwith the previous studies [40]. Accordingly, the N : H speciesmay enhance the mixing of impurity states induced by N2p states and O 2p atomic orbitals in the VBM, and finallynew valence bands can be formed, which could contributeto the formation of the band-band transition, in favor ofthe visible-light-driven catalytic activities. When it comes tothe B : 3H-codoped case, though the impurity energy levelmoves a little toward the VBM, the isolated states are stilllocalized in the midgap because of the large difference of theelectronegativity between substitutional boron and oxygen.Therefore, based on these understandings, we can draw aconclusion that if the electronegativity of oxygen is close tothat of the doping species, together with the simultaneousformation of charge-balance structure, the doped TiO2 maypossess high photocatalytic activity under visible light.

Additionally, we also studied the N : F codoping effecton the anatase TiO2, which are expected to be efficientto the visible light photocatalytic activity according to theanalysis above. The supercell is simulated by replacing twoO atoms with one N atom and one F atom, respectively (seeFigure 3(d)), and the corresponding structure and DOS areshown in Figure 4(a). It is apparent that the impurity statesinduced by N and F doping locate in different range of thevalence band, which can be ascribed to their unique elec-tronegativities as we discussed above. Obviously, the absorp-tion edge of new band in the visible light originated fromthe doped N atoms rather than doped F atoms, consistentwith the former studies [36, 37]. Furthermore, the impuritystates are fully filled with the electrons at the ground state,which can be attributed to the fact that the doped F− ioncompensates the charge of N3− ion. Therefore, similar tothe case of N : H codoping, the N : F complex results in


1. 30

N

H

2.032 A

2.021 A 7 A

(a)

HH

C

2.138 A

2.068 A 1.097 A

(b)

B

H

H

H

2.307 A

2.105 A 2.192 A

(c)

F N

1.94 A

2.029 A

1.864 A

(d)

Figure 3: The optimal structures for (a) N : H-doped TiO2, (b) C : 2H-doped TiO2, (c) B : 3H-doped, and (d) N : F codoped TiO2.

the formation of the charge-balance structure and couldenhance the mixing of impurity states with the VBM, whichfavors the band-band transition and the photocatalyticactivities under visible light.

3.3. Prediction of Redox Potentials from Atomic Electronegativ-ity. As we know, the ability of a semiconductor to transferphotoexcited electrons to species adsorbed on its surface isgoverned by the band energy position of the semiconductorand the redox potentials of the adsorbate, which is relatedto the electronegativity [1]. Therefore, to thoroughly under-stand the effects of the electronegativity on the doped TiO2,we have calculated the positions of CB and the VB edge usingthe concept of the semiconductor electronegativity, which isdefined as the geometric mean of the electronegativities ofthe constituent atoms:

χ(S) = N√χn

1χs

2· · · χpn−1χqn , (4)

where χn, n, andN are the electronegativity of the constituentatom, the number of species, and the total number of atomsin the compound, respectively [41].

The CB edge position of a semiconductor at the point ofzero charge (E0

CB) can be expressed empirically by

E0CB = χ(S)− Ee − 1

2Eg , (5)

where Eg and Ee are the band gap energy of the semiconduc-tor and the energy of free electrons on the hydrogen scale(∼4.5 eV) [42]. Although this method cannot give preciseabsolute values due to the neglect of structural factors, itmay give a rough estimate of the relative positions of normalhydrogen electrode (NHE), which could provide a referenceto future experimental studies.

Due to the well-known drawback of DFT theory, ourDFT calculations under GGA level give band gaps of about2.2 eV, smaller than the experimental values (3.23 eV) [43].In order to correct the underestimation of band gaps, we em-ployed scissor operator, that is, a systematic upward shiftingof the unoccupied states by a constant amount. In addition,we postulate that the amount of the band gaps underestima-tion would not be affected by doping effect because long-range screening properties of doped TiO2 are expected tobe similar to those in pure TiO2. Therefore, in this paper


0

50

100

150

(a)

0

1.5

3

4.5

6

(b)

0

50

100(c)

0

1.5

3

(d)

0

50

100

0

1.5

3

0

2

4

0

50

100

(B)

Energy (eV)−4 −2 0 2 4

Energy (eV)−4 −2 0 2 4

(A)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

DO

S(a

.u.)

C

N

N

B

F

2p

2p

2p

2p2p

(d)

(c)

(b)

(a)

Figure 4: Total densities of states (A) and projected densities of states (B) for (a) N : F codoped TiO2, (b) N : H-doped TiO2, (c) C : 2H-dopedTiO2, (d) B : 3H-doped TiO2. The vertical dash line defines the highest occupied level.

we use a rigid constant shift of the band gaps for both pureand doped TiO2, namely, 1.03 eV. Consequently, the adjustedband gaps are 3.23 eV for pure TiO2, which is in line with theexperimental values.

According to the strategies mentioned above, the roughCB edge potential of pure anatase TiO2 is −0.27 eV withrespect to NHE. Subsequently the VB edge position is deter-mined as 2.96 eV based on its band gap energy. The correctedband gap energy (Eg) and the corresponding CB and VB edgepositions versus NHE are shown in Figure 4.

It can be found that the electronegativity of doping spe-cies produces certain influence on the oxidative potentialof doped TiO2. Generally, when the impurity states locatein the lower range of the valence band or mixing with theVBM, (see Figures 5(b)–5(g)), the doped TiO2 possess moreoxidative power with the increasing of the electronegativityof the doping species or complex. When it comes to the B : H-codoped, C- and B-doped cases, these materials also obeysuch rule, (see Figures 5(h)–Figure 5(k)). However, sincethere are some isolated states in the midgap, the situationsmay become more complicated, which need further studies.Moreover, it can be known from the results that though

F-doping could not contribute to the optical absorptionspectra, it possesses strong oxidative potential, which couldenhance the redox ability of TiO2. In addition, it has beenstudied that the doped F atoms could reduce the electron-hole recombination rate and further enhances the photocat-alytic activity [36, 44]. Therefore, the N : F-codoped TiO2

may possess high-photocatalytic activity under visible lightirradiation, which makes it an excellent candidate for furtherapplications.

Finally, since surface conditions are of key importance inheterogeneous photocatalysis, a further discussion on howthe findings can be applied to doped surfaces is necessary.On the pure TiO2 surfaces, additional surface states will beintroduced in the electronic structures due to the surfacereconstruction or dangling bonds formed on it, whereas forthe deep inner layers below surfaces the electronic structureretains its bulk character. So that the above findings can berationally applied to the doping of inner layers of surfaces,while when they are applied to the doping of the severaloutermost layers of surfaces, additional calculations andanalysis are needed to give a verification.


3.5

3

2.5

2

1.5

1

0.5

0

−0.5

E versus NHE

CB edge positions

VB edge positions

H+/H

2O

2 /H2 O

(a)(b) (c)

(d) (e) (f) (g)(h) (i) (j)

3.23eV

3.18eV

3.16eV

2.93eV

2.95eV

2.8eV

2.75eV

3.2eV

3.14eV

2.97eV

En

ergy

(eV

)

Pure

N-N : F-F- N : H-C : 2H-

B : 3H- C- B-Cl-

Figure 5: Calculated edge positions of conduction band andvalence band for pure and nonmetal doped anatase TiO2.

4. Conclusions

Our results show that when the electronegativity of thedoping species is larger than that of oxygen, such as F andCl, the impurity states induced mainly by outermost atomicorbitals locate in the lower energy range of the valenceband, which cannot contribute to the redshift of absorption.Whereas when the electronegativity of the doping species issmaller than that of oxygen, the impurity states will lie abovethe VBM. Especially, if the difference is small, such as N, theimpurity states can mix and overlap with the O 2p states,leading to the photo response of TiO2 under visible lightirradiation. In addition, once the charge balance structuresare obtained, the electron donors can lower the impuritystates levels and enhance the mixing of N 2p atomic orbitalswith O 2p states. Also, we have found that the N:F complexmay be more efficient to the visible light photocatalyticactivities. Hence, on the basis of these understandings, wehave proposed a new way to interpret the mechanism ofthe redshift and predict the photocatalytic activities dopingwith different species. Furthermore, based on the definitionof the semiconductor electronegativity, the CB and VB edgepositions have been predicted, which demonstrated that thedoped TiO2 possess favorable redox potentials under visiblelight, making them promising materials for further applica-tions. Based on the above analysis, we conclude that a carefulselection of codopants to yield a charge-balance codopedsystem is an effective way to improve the visible-light-responsive photocatalytic activity of nonmetal doped TiO2.

Acknowledgments

This work is supported by the National Basic ResearchProgram of China (973 program, Grant no. 2007CB613302),National Natural Science Foundation of China under Grantno. 11174180 and 20973102, Natural Science Foundation ofShandong Province under Grant no. ZR2011AM009.

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Research Article

CO2 Reforming Characteristics underVisible Light Response of Cr- or Ag-Doped TiO2 Prepared bySol-Gel and Dip-Coating Process

Akira Nishimura,1 Go Mitsui,1 Katsuya Nakamura,1 Masafumi Hirota,1 and Eric Hu2

1 Division of Mechanical Engineering, Graduate School of Engineering, Mie University, 1577 Kurimamachiya-cho,Tsu 514-8507, Japan

2 School of Mechanical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia

Correspondence should be addressed to Akira Nishimura, [email protected]

Received 25 May 2011; Revised 2 August 2011; Accepted 2 August 2011


Copyright © 2012 Akira Nishimura et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

A Cr- or Ag-doped TiO2 film was prepared by sol-gel and dip-coating process and used as the photocatalyst for CO2 reformingunder the visible light. The ratio of amount of Cr or Ag added to amount of Ti in TiO2 sol solution (R) varied from 0 to 100 wt%or 0 to 50 wt%, respectively. The total layer number of Cr- or Ag-doped TiO2 film (N) coated was changed. The CO2 reformingperformance with the Cr- or Ag-doped TiO2 film was tested under a Xe lamp with or without ultraviolet (UV) light. As a result,when N equals to 1, the concentration of CO which was a product from CO2 reforming was maximized in Cr doping case for R =70 wt% and in Ag doping case for R = 1 wt%, respectively. The best result of concentration of CO = 8306 ppmV, concentration ofCH4 = 1367 ppmV, concentration of C2H6 = 1712 ppmV is obtained when Ntop = 7 with Cr doping in this study.

1. Introduction

Due to mass consumption of fossil fuels, global warmingand fossil fuels depletion have become a serious globalenvironmental problem in the world. After the industrialrevolution, the averaged concentration of CO2 in the worldhas been increased from 280 ppmV to 387 ppmV by 2009.Therefore, it is necessary to develop a new energy productiontechnology with less or no CO2 emission.

It is reported that CO2 can be reformed into fuels, forexample, CO, CH4, CH3OH and H2, and so forth, by usingTiO2 as the photocatalyst under ultraviolet (UV) light illumi-nation [1–10]. If this technique could be applied practically,a carbon circulation system would then be established: CO2

from the combustion of fuel is reformed, using solar energy,to fuels again, and true zero emission can be achieved. ManyR&D works on this technology have been carried out, usingTiO2 particles loaded with Cu, Pd, Pt to react with CO2

dissolved in solution [1, 5, 7, 11–17]. Recently, nano-scaledTiO2 [18–20], porous TiO2 [21], TiO2 film combined with

metal [22, 23], and dye-sensitized TiO2 [24] are developedfor this process. However, the fuel concentration in theproducts achieved in all the attempts so far is still low,ranging from 10 ppmV to 1000 ppmV, to be practicallyuseful [1, 4, 5, 7, 8, 11, 12, 15, 16, 18, 20]. Therefore, thebig breakthrough in increasing the concentration level isnecessary to advance the CO2 reforming technology.

In the applications such as water splitting and purifi-cation of pollutant, the photoresopnse extension of TiO2

to the visible spectrum has been investigated well [25–29].TiO2 by itself can only work under UV light due to its widebandgap of 3.0–3.2 eV, which means that only about 4% ofthe incoming solar energy on the surface can be utilized[30]. On the other hand, the visible light accounts for 43%of whole solar energy [31]. Therefore, if the photoresponseof TiO2 could be extended to the visible spectrum, theCO2 reforming performance of TiO2 technology would beimproved significantly.

Doping with foreign ions is one of the most promisingstrategies for sensitizing TiO2 to visible light and also for


forming charge traps to keep electron-hole pairs separate[32]. The most popular dopants for modification of theoptical and photoelectrochemical properties of TiO2 aretransition metals such as Cr, Fe, Ni, V, Mn, and Cu [28].Choi et al. [33] carried out a systematic investigation ofthe photocatalytic activity of TiO2 doped with 21 differentmetal ions. It was found that doping with metal ionswould introduce additional energy levels in the band gapof TiO2 thus extending the photoresponse of TiO2 into thevisible spectrum. In addition, an optimum concentrationof dopant metal ions exists under specific conditions. Ifthe concentration of dopant excesses the optimum one, thephotocatalytic activity declines because of charge recombi-nation [28]. Many previous reports on metal-doped TiO2

used for photocatalytic degradation reaction of chemicalsunder visible light showed the activity enhancement onlyfor a specific amount of doping ions, otherwise detrimentaleffects occur [34].

Many different techniques have previously been reportedfor metal doping of TiO2 such as wet impregnation [35],hydrothermal deposition [36], RF magnetron sputteringdeposition [24, 26, 37, 38], flame reactor method [39],solidstate reactions [40], ion implantation [41], and pulsedlaser deposition [42]. Recently, the sol-gel method is adoptedfor metal doping of TiO2 well [27–29, 34, 43–46] since thismethod can incorporate dopants into TiO2 lattice, resultingin preparation of the materials with other optical and alsocatalytic properties [47]. In addition, the integration ofdopants into the sol during the gelation process facilitatesdirect interaction with TiO2 by sol-gel method [46].

Although many studies to extend the photoresponse ofTiO2 to the visible spectrum were reported as describedabove, there are only a few reports on its application forpromoting the CO2 reforming purposes [24]. In our previ-ous studies [48–51], the effect of TiO2 thin film preparationconditions in sol-gel and dip-coating process on the CO2

reforming performance under UV light was investigated.In this study, TiO2 sol-gel and dip-coating process with

doping is also adopted in order to extend its photoresponseto the visible spectrum to promote the CO2 reformingperformance. It was reported that the transition metals suchas V, Cr, Mn, Fe, and Ni were effective for the photoresponseextension of TiO2 to the visible spectrum [37]. Accordingto the previous reports [28, 34, 39–41, 52, 53], it can bethought that Cr3+ or Cr6+ ion existing in TiO2 film afterdoping can absorb the light of wavelength from 400 nm to550 nm [28, 34, 39, 41, 52–55]. Therefore, Cr was selected atfirst as the dopant to check the feasibility of promoting CO2

reforming performance of TiO2 in this study. According tothe study which investigated the photocatalytic H2 evolutionfrom water-alcohol mixtures, noble and base materials,including Pt, Au, Pd, Ni, Cu, and Ag, have been reportedto be very efficient for increasing the production of H2 byTiO2 photocatalytic reaction [56]. According to the reactionscheme [1, 5, 6, 11, 16, 50, 51] of CO2 reforming byTiO2 photocatalyst as shown in Figure 1, if a lot of H+ isproduced, the reduction process is promoted, resulting inincrease of the concentration of produced fuel. In addition,the photoresponse extension of TiO2 to the visible spectrum

was obtained by Ni, Cu, and Ag doping in the previousstudy on the photocatalytic H2 evolution from water-alcoholmixtures [56]. TiO2 doped with Ag+ ions absorbs the light ofwavelength from 460 nm to 477 nm [56, 57]. Therefore, Agwas also selected at first as the dopant to check the feasibilityof promoting CO2 reforming performance of TiO2 in thisstudy.

In the present paper, the preparation method for dopingCr or Ag into TiO2 film was developed. The characterizationanalyzed by scanning electron microscope (SEM), electronprobe micro analyzer (EPMA), and X-ray photoelectronspectroscopy (XPS) was conducted. The influence of the ratioof amount of added Cr or Ag to amount of Ti in TiO2 solsolution (R) and of the total coating number of Cr- or Ag-doped TiO2 film (N) on CO2 reforming characteristics underthe condition of illuminating Xe lamp with or without UVlight was also investigated in this study.

2. Experiment

2.1. Preparation of Cr- or Ag-Doped TiO2 Film. Sol-gel anddip-coating process was used for preparing Cr- or Ag-dopedTiO2 film in this study. TiO2 sol solution was made bymixing [(CH3)2CHO]4Ti (purity of 95 wt%, Nacalai TesqueCo.) of 0.1 mol, anhydrous C2H5OH (purity of 99.5 wt%,Nacalai Tesque Co.) of 0.8 mol, distilled water of 0.1 mol, andHCl (purity of 35 wt%, Nacalai Tesque Co.) of 0.008 mol.Cr powders (08819-15, Nacalai Tesque Co., particle sizebelow 74 μm) or Ag powders (30934-92, Nacalai Tesque Co.,particle size below 44 μm) were added into TiO2 sol solution.Copper disc whose diameter and thickness were 50 mm and1 mm, respectively, was dipped into Cr/TiO2 or Ag/TiO2 solsolution and pulled up at the fixed speed (RS) of 0.22 mm/s.Then, it was dried out and fired under the controlled firingtemperature (FT) and firing duration time (FD), resulting inthe fact that Cr- or Ag-doped TiO2 film was fastened on thesurface of copper disc. FT and FD was set at 623 K and 180 s,respectively. In this study, N varied from 1 to 7 for Cr dopingand from 1 to 5 for Ag doping.

2.2. Characterization of Cr- or Ag-Doped TiO2 Film. Thesurface structure and crystallization characteristics of Cr-or Ag-doped TiO2 film were evaluated by SEM (JXA8900R,JEOL Ltd.) and EPMA (JXA8900R, JEOL Ltd.). The EPMAanalysis helps us not only to understand the coating state ofCr- or Ag-doped TiO2 on copper disc but also to measurethe amount of doped Cr or Ag within TiO2 film on copperdisc. Element distribution through thickness direction of Cr-doped TiO2 film was analysed by XPS (PHI Quantera SXM,ULVAC-PHI, INC.) using radiation source of Al radiationwith the pass energy of 224.00 eV, the radiation current of1.0 W, and the acceleration voltage of 15 kV.

2.3. Apparatus and Procedure of CO2 Reforming Experiment.Figure 2 shows that the experimental system setup of CO2

reformer consists of a stainless pipe (100 mm (H.)×50 mm(I.D.)), a copper disc (50 mm (D.)× 1 mm (t.)) coated withCr- or Ag-doped TiO2 film which is located on the teflon


⟨Photocatalytic reaction⟩

⟨Oxidation⟩h+

h+

⟨Reduction⟩CO2

•CH22H+ + 2e−

H2O

CO4H+ + 4e−

H2O

•CH22H+ + 2e−

CH4

•CH2C2H4

C2H6

•CH2 + 2H+ + 2e−

TiO2 + hA (under 380 nm) −→ h+ + e−

H2O −→ •OH + H+

•OH + H2O −→ O2 + 3H+

Figure 1: Reaction scheme of CO2 reforming into fuel by TiO2

photocatalyst (•OH: hydroxy radical, •CH2: carbon radical).

φ50(I.D.)

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Figure 2: Schematic drawing of CO2 reforming experimentalsystem: ((1) Xe lamp; (2) coloured glass filter; (3) quartz glass disc;(4) stainless pipe; (5) gas sampling tap; (6) copper disc; (7) tefloncylinder; (8) valve; (9) CO2 gas cylinder).

cylinder (50 mm (H.)×50 mm (D.)), a quartz glass disc(84 mm (D.)×10 mm (t.)), a coloured glass filter which cutsoff the light of wavelength below 380 nm, SCF-50S-38L,SIGMA KOKI CO., LTD.), Xe lamp (L2175, HamamatsuPhotonics K. K.), and CO2 gas cylinder. The reformer volumefor CO2 charge is 1.25×10−4 m3. Xe lamp is located over thestainless pipe. The light of Xe lamp illuminates the copperdisc coated with Cr- or Ag-doped TiO2 film, which is inserted

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Tran

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Figure 3: Light transmittance data of the coloured glass filter.

into the stainless pipe, through the coloured glass filter andthe quartz glass disc fixed to the top of the stainless pipe.The wavelength of light from Xe lamp is ranged from 185 nmto 2000 nm. The Xe lamp can be fitted with a colouredglass filter to remove UV components of the light. With thefilter, the wavelength of light from Xe lamp is ranged from381 nm to 2000 nm. Figure 3 shows the light transmittancedata of the coloured glass filter to prove the removal ofthe light whose wavelength is below 380 nm. The averagelight intensity of Xe lamp on the copper disc without andwith setting the coloured glass filter is 57.53 mW/cm2 and43.67 mW/cm2, respectively.

In the CO2 reforming experiment, CO2 gas with thepurity of 99.995 vol% was flowed through the CO2 reformeras a purged gas for 15 minutes first. After that, the valveslocated at the inlet and the outlet of CO2 reformer wereclosed. After confirming the gas pressure and gas tempera-ture in the CO2 reformer at 0.1 MPa and 298 K, respectively,the distilled water of 100 μL was injected into the CO2

reformer, and Xe lamp illumination was turned on at thesame time. The water injected vaporized completely in thereformer. Despite of the heat of UV lamp, the temperaturein CO2 reformer was kept at about 343 K during the CO2

reforming experiment. The amount of injected water andthat of CO2 in CO2 reformer was 5.56 mmol and 5.76 mmol,respectively. The gas in CO2 reformer was sampled every24 hours during the CO2 reforming experiment. The gassamples were analyzed by FID gas chromatograph (GC353B,GL Science) and methanizer (MT221, GL Science). Mini-mum resolution of FID gas chromatograph and methanizeris 1 ppmV.


3.1. Analysis of Cr- or Ag-Doped TiO2 Film by SEM andEPMA. Figures 4 and 5 show SEM image and EPMA imageof Cr-doped TiO2 film prepared under the condition of R =1 wt%, respectively. Figures 6 and 7 show SEM and EPMAimage of Ag-doped TiO2 film prepared under the conditionof R = 1 wt%, respectively. These SEM images were taken at1500 times magnification under the condition of accelerationvoltage of 15 kV and current of 3.0×10−8 A. The red lined


10 μm×1500

Figure 4: SEM images of Cr-doped TiO2 film prepared under thecondition of R = 1 wt%.

quadrangle area in Figures 4 and 6 was used for EPMAanalysis shown in Figures 5 and 7, respectively. In Figures 5and 7, the concentration of Ti, Cu, Cr, and Ag in observationarea is indicated by the difference of colour. Light colours,for example, white, pink, and red indicate that the amountof element is large, while dark colours like black, blue, andgreen indicate that the amount of element is small.

The green circle in Figure 4 indicates the existence ofCr particle as shown in Figure 5. Moreover, the red circlesillustrated in Figure 4 indicate that the amount of Cu is alsolarge as pointed out by the white circles in Figure 5. On theother hand, the red circle in Figure 6 indicates the existenceof Ag particle as shown in Figure 7. Furthermore, the yellowcircles illustrated in Figure 6 indicate that the amount of Cuis small and Ti is large as pointed out by white circles inFigure 7. These results represent the following.

(i) Before firing process, Cr/TiO2 or Ag/TiO2 sol solu-tion is adhered on the copper disc uniformly.

(ii) During firing process, the temperature profile ofCr/TiO2 or Ag/TiO2 sol solution adhered on thecopper disc is not even due to the difference ofthermal conductivity of Ti and Cr or Ag. Theirthermal conductivity of Ti, Cr, and Ag at 600 Kis 19.4 W/(m·K), 80.5 W/(m·K), and 405 W/(m·K),respectively [58]. Therefore, the thermal expansionaround Cr or Ag particle and the thermal shrinkagearound the other areas of TiO2 sol occur.

(iii) Because of the thermal stress caused by the unevendistribution of temperature, the cluck around Cr orAg and the shrinkage of TiO2 film around the cluckoccur after firing process. Therefore, a large amountof Cu, which is an element of basis copper disc,around Cr or Ag and a large amount of Ti aroundCr or Ag are observed in Figures 5 and 7.

To evaluate the amount of doped Cr or Ag within TiO2

film quantitatively, the observation area, which is the centerof copper disc, of diameter of 300 μm is analysed by EPMA.

The ratio of Cr or Ag to Ti in this observation area is countedby averaging the data obtained in this area.

Table 1 indicates the relationship between each elementratio and R which is varied from 1 wt% to 100 wt% whenthe N is set at 1 for Cr-doped TiO2 film. From this table, theratio of Cr is increased with increasing R up to 70 wt% sincethe amount of Cr powders added into TiO2 sol solution isincreased. However, the ratio of Cr starts to decrease if the Ris over 70 wt%. The reason might be that when the amount ofCr powders in TiO2 sol solution was too much, the shrinkageof TiO2 film occurred. Therefore, the fixing strength of TiO2

film to copper disc was weakened, resulting in the ratio ofCr being decreased for R over 70 wt%. The ratio of Cr to Tishown in Table 1, which is measured value, is different fromthe calculated R from the Cr powders added into TiO2 solsolution because of the agglomeration of Cr powders in TiO2

sol solution. Although Cr powders in TiO2 sol solution weremixed by magnetic stirrer well and the powders were storedunder no moisture condition before the experiment, it wasstill difficult to prevent it from the agglomeration for such afine particle completely.

Table 2 indicates the relationship between each elementratio and R which is varied from 1 wt% to 50 wt% when theN is set at 1 for Ag-doped TiO2 film. From this table, theratio of Ag is increased with increasing R up to 50 wt% sincethe amount of Ag powders added into TiO2 sol solution isincreased. However, the increase ratio of Ag is not so largecompared to the exact amount of Ag powders added intoTiO2 sol solution. The density of Ag which is 10787 kg/m3

is much larger than the density of Ti which is 4507 kg/m3.Although TiO2 sol solution was mixed by magnetic stirrerwell during dip-coating process, most of Ag particles wereliable to sink in TiO2 sol solution in dip-coating process.Especially, for the case that R is over 10 wt%, the amount ofAg detected by EPMA is so small compared to the amountof Ag powders added into TiO2 sol solution, resulting in thefact that the control of doping a large amount of Ag by sol-gel and dip-coating process is quite difficult. The SEM image,that is, Figure 8, of Ag-doped TiO2 film prepared under thecondition of R = 50 wt% shows that there are many holesand clucks on TiO2 film, which are indicated by pink circles.However, fewer holes and clucks are seen in SEM image ofAg-doped TiO2 film prepared under the condition of R =1 wt% as shown in Figure 6. It is thought that these holesand clucks in Figure 8 are caused by peeling off TiO2 film. Inaddition, from EPMA image of Ag-doped TiO2 film preparedunder the condition of R = 50 wt% shown in Figure 9, itcan be seen that there are some areas where the amountof Cu is large while the amount of Ti is small. These areasis pointed out by white circles, while Ag in surroundingareas are pointed out by red circles. This result is thoughtto be caused by peeling off TiO2 film around Ag particles.However, no such areas where the amount of Cu is largewhile the amount of Ti is small are observed in EPMA imageof Ag-doped TiO2 film prepared under the condition of R =1 wt% as shown in Figure 7. Therefore, it can be concludedthat the peeling off TiO2 film with Ag particles in dip-coatingprocess only occurs under high R condition.


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Ti level Cr levelSample: 09 06 09 1wt

Acc. V 15.0 kVProb C 2.997e−08 AProb diam. (μM) 0

Dwell (MS) 25

Sample: 09 06 09 1wt


Dwell (MS) 25

Sample: 09 06 09 1wt


Dwell (MS) 25

Figure 5: EPMA image of Cr-doped TiO2 film prepared under the condition of R = 1 wt%.

Table 1: Relationship between each element and different R for Cr-doped TiO2 film.

R [wt%] 1 10 20 30 40 50 60 70 80 90 100

Element

Cr [wt%] 5.5 7.1 8.2 11.3 13.2 15.3 18.9 22.4 17.0 18.0 18.6

Ti [wt%] 94.5 92.9 91.8 88.7 86.8 84.7 81.1 77.6 83.0 82.0 81.4

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Table 2: Relationship between each element and different R for Ag-doped TiO2 film.

R [wt%] 1 10 20 30 40 50

Element

Ag [wt%] 0.6 1.4 1.9 2.5 2.6 3.4

Ti [wt%] 99.4 98.6 98.1 97.5 97.4 96.6

Total 100.0 100.0 100.0 100.0 100.0 100.0

3.2. Investigation of the Optimum Doping Ratio of Cr and Ag.Figure 10 shows the concentration changes of CO producedby CO2 reforming along the time under the Xe lamp withUV light on, for several Cr- or Ag-doped TiO2 films preparedunder the different R conditions. In this experiment, COis the only fuel produced from CO2 reforming. Since theconcentration of CO started to decrease after illumination of72 hours for every R, Figure 10 only shows the concentration

up to 72 hours. Before this CO2 reforming experiment, ablank test, that is, running the CO2 reforming experimentwithout illumination of Xe lamp, has been carried out to setup a reference case. No fuel was produced in the blank test asexpected.

According to Figure 10, the concentration of CO isincreased with increasing R up to 70 wt% for Cr-doped TiO2

film. As Table 1 indicates that the ratio of Cr is also increasedwith increasing R up to 70 wt%, the result of CO2 reformingexperiment matches well with the result of EPMA analysis.On the other hand, it is revealed that the concentration ofCO for R = 1 wt% which is larger than that for R = 50 wt%though the amount of Ag detected by EPMA for R = 50 wt%is larger than that for R = 1 wt% as shown in Table 2. Thereason to cause this result might be that TiO2 film was peeledoff with Ag particles in dip-coating process especially underhigh R condition. Since the density of Ag is larger than that ofTi, TiO2 sol solution cannot keep Ag particles on copper disc


10 μm×1500

Figure 6: SEM image of Ag-doped TiO2 film prepared under thecondition of R = 1 wt%.

by its viscosity during dip-coating process. When Ag particlesdrop, TiO2 sol solution or TiO2 film around Ag particlesdrops together. Therefore, the CO2 reforming performanceof R = 50 wt% is lower than that of R = 1 wt%.

To verify the photoresponse extension of TiO2 to thevisible spectrum by Cr or Ag doping, Figure 11 shows theconcentration change of CO with illumination time of Xelamp without UV light for several Cr- or Ag-doped TiO2 filmprepared under different R conditions. In this figure, the datafor R = 0 wt%, 50 wt%, 70 wt%, and 100 wt% are shown forCr-doped TiO2 film. This figure includes the concentrationchange with illumination time of Xe lamp without UV lightmore than 72 hours, since it was thought that the time toattain the peak concentration might be longer than thatunder Xe lamp with UV light [51, 59].

It is seen that the concentration of CO for R = 0 wt%keeps 0 ppmV. Therefore, it means that the prepared TiO2

film without doping does not have the photoresponseability of visible spectrum. On the other hand, Cr- or Ag-doped TiO2 film shows the photoresponse ability of visiblespectrum since the CO is detected. As to the results of Cr-doped TiO2 film, it reveals that R = 70 wt% is the bestthat agrees with the results shown in Table 1 and Figure 10.However, the peak concentration level of CO without UVlight is lower than that with UV light. The results on Ag-doped TiO2 film reveal that R = 50 wt% is better than R= 1 wt% in the experiment without UV light illuminationthough the difference between R = 50 wt% and R = 1 wt%is small.

The following points can be thought to explain theseresults.

(i) The CO2 reforming performance is improved byCr or Ag doping since the photoresponse ability ofvisible spectrum is realized. The light energy whichcan be used for CO2 reforming is increased.

(ii) On the other hand, the CO2 reforming performancecan also be declined by Cr or Ag doping since the

recombination of hole and electron occurs. Thoughit is thought that sol-gel and dip-coating process canincorporate dopants into the TiO2 lattice withoutlattice defect [47], the lattice defect which causesthe recombination of hole and electron might haveoccurred in this study.

(iii) The CO2 reforming performance would be improvedby Cr or Ag doping if the recombination of holeand electron could be prevented. According to theprevious reports [1, 5, 7, 11–17], a metal loadingis usually adopted to prevent the recombination ofelectron and hole under the condition of illuminationof UV light.

Considering the CO2 reforming performance of Cr-doped TiO2 film, the data shown in Table 1, Figures 10 and11 are analyzed based on the above three points as follows.

(I) From Figure 10 that the concentrations of CO forR = 1 wt% and 10 wt% are lower than that for R= 0 wt%. It is thought that the above described (ii)should have occurred in this experiment. Therefore,there is a minimum amount of Cr doping existing inorder to improve the CO2 reforming performance ofTiO2.

(II) As shown in Figure 10, the concentration of CO forR over 30 wt% is bigger than that for R = 0 wt%.If the above described points (i) to (iii) were all ineffect equally, the concentration of CO should notincrease. Therefore, it is thought that the point (i) or(iii) should be dominant until R is up to 70 wt% asshown in Table 1.

(III) It can be seen from Figure 10 that the concentrationof CO for R over 90 wt% is low. The reason is thoughtto be that when R is too high, Cr powders with TiO2

might be removed away from copper disc surface thatis not in contact with the disc during dip-coatingprocess. Although the amount of Cr powders in TiO2

sol solution is increased under high R conditions, theCr in contact with copper disc might be decreasedbecause the amount of Cr powders being removedform copper disc might increase more. Therefore, asa result, the CO2 reforming performance for R over90 wt% is declined.

(IV) Comparing Figure 10 with Figure 11, the concentra-tion of CO under visible light (i.e. without UV light)is lower than that under the UV light. It is thoughtthat Cr doping has two effects on the CO2 reformingperformance: (1) prevention of recombination ofelectron and hole under UV light and (2) extensionof photoresponse ability into visible spectrum. Theresults in Figures 10 and 11 show that the effect ofpreventing the recombination of electron and holeunder UV light is stronger than that of extendingphotoresponse ability to the visible spectrum.

For the effect on the CO2 reforming performance bythe Ag-doped TiO2 film, the results are shown in Table 2,Figures 10 and 11. From the results, it can be seen that


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Figure 7: EPMA image of Ag-doped TiO2 film prepared under the condition of R = 1 wt%.

10 μm×1500

Figure 8: SEM image of Ag-doped TiO2 film prepared under thecondition of R = 50 wt%.

the CO2 reforming performance of R = 50 wt% is better thanthat of R = 1 wt% if without UV light illumination, whilethe CO2 reforming performance of R = 1 wt% is better thanthat of R = 50 wt% if with UV light illumination. The CO2

reforming performance without UV light is generally lowerthan that with UV light irrespective of R. For example, whenR = 50 wt% after illumination of Xe lamp of 72 hours, the

CO2 reforming performance without UV light is about thirtyseventh part of that with UV light. The amount of Ag andthe ratio of amount of TiO2 film to doped Ag are importantto improve the photoresponse ability of doped TiO2 film, asshown in Figure 10. Therefore, R = 1 wt% is selected as theoptimum condition for Ag doping in this study as it gives thebest result as shown in Figure 10. Another point to be notedis that, compared with Cr doping, Ag doping is much moreeffective to promote the CO2 reforming performance underthe conditions with or without UV light.

3.3. Effect of Coating Number of Cr- or Ag-Doped TiO2 Filmon CO2 Reforming Characteristics. Since the product of theCO2 reforming is only CO in this experiment, it is thoughtthat the reduction effect of TiO2 is not so strong according tothe reaction scheme shown in Figure 1. From Figure 1, moreelectron and proton are necessary to produce hydrocarbonslike CH4, C2H4, and C2H6. When the reduction effect ofTiO2 is promoted, the concentration of CO which is a pre-product to the hydrocarbons is also increased. The previousstudies [48–51] show that the increase in N is effective topromote the reduction performance of TiO2 photocatalystdue to increase of the amount of TiO2. Under the higherN condition, more electrons are produced by photocatalyticreaction. The effect of N of Cr- or Ag-doped TiO2 film onCO2 reforming performance and photoresponse ability ofvisible spectrum is investigated further below.


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Figure 9: EPMA image of Ag-doped TiO2 film prepared under the condition of R = 50 wt%.

Figure 12 shows the concentrations of products from theCO2 reforming after illumination of Xe lamp with UV lightof 72 hours for several Cr-doped TiO2 film prepared underconditions of different N. In this experiment, two coatingconditions were investigated: Cr is doped in every layer (Nall)and Cr is doped only in the top layer (Ntop), while R isfixed at 70 wt% in doping. Figure 12 indicates that the totalconcentration of products is increased with increasing N,since the amount of Cr-doped TiO2 film coated on copperdisc becomes larger with increasing N. However, the degreeof the increase in N for Ntop cases is higher than that for Nall

cases. In addition, it is seen that CH4 and C2H6 as well asCO are produced more for Ntop cases compared with Nall

cases. This might be caused by the clucks occurring afterfinishing firing process. Each layer that is the base for nextupper layer coating is weaker in Nall cases, resulting in thefact that the uniform coating for each layer is difficult toachieve. However, it is thought that the coating layers beforethe last coating are kept uniform in Ntop cases. The reasonwhy the concentrations of CH4 and C2H6 are increased withincreasing N in Ntop cases is thought to be the increase in theamount of TiO2.

Figure 13 shows the concentrations of products fromCO2 reforming after illumination of Xe lamp with UV lightof 72 hours for several Ag-doped TiO2 films prepared under

Table 3: Relationship between each element and different N for Ag-doped TiO2 film.

N Nall = 1 Nall = 3 Ntop = 3 Ntop = 5

Element

Ag [wt%] 0.6 2.4 0.6 0.6

Ti [wt%] 99.4 97.6 99.4 99.4

Total 100.0 100.0 100.0 100.0

conditions of different N. In this experiment, R is fixedat 1 wt% in doping. And the results of Nall and Ntop areshown in this figure. The data of other hydrocarbons, whichwere detected for every R, are omitted in this figure sincethe values of them are below 50 ppmV. From Figure 13,the best result is when Nall = 1, though EPMA analysisshown in Table 3 reveals that the largest amount of Agdetected is obtained when Nall = 3 under the experimentalconditions. When N is same, the ratio of detected Ag to totalelement for Nall is larger than that for Ntop. Although a goodpromotion of CO2 reforming performance was expected byEPMA analysis, Nall = 3 does not show the good CO2

reforming performance. In addition, the result of Ntop issuperior to the result of Nall. Under the condition of Nall,TiO2 film is liable to be removed by increasing N since


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)

0 wt%Cr-1 wt%Cr-10 wt%Cr-30 wt%Cr-50 wt%

Cr-70 wt%Cr-90 wt%Cr-100 wt%Ag-1 wt%Ag-50 wt%

Figure 10: Comparison of produced concentration of CO amongdifferent R under the condition of illuminating Xe lamp with UVlight.

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0 24 48 72 96 120 144 168 192Illumination time of Xe lamp (h)

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0 wt%

Cr-50 wt%Cr-70 wt%

Cr-100 wt%Ag-1 wt%Ag-50 wt%

Figure 11: Comparison of produced concentration of CO amongdifferent R under the condition of illuminating Xe lamp withoutUV light.

doped Ag particles cause the uneven thin TiO2 film on theformer coated TiO2 film and make TiO2 film weak. SinceAg-doped TiO2 film detaches, the amount of coated TiO2

is decreased. On the other hand, under the condition ofNtop, it is thought that even and steady thin TiO2 film iscoated more easily than Nall. Therefore, the CO2 reformingperformance of Ntop = 3 is better than that of Nall = 3.However, the CO2 reforming performance of Ntop = 5 isworse compared with that of Ntop = 3. It might be a resultthat doped Ag particles cause clucks of TiO2 film during dip-coating process due to thermal stress. In the last coating,the former coated TiO2 film under many N condition isweaker than that under small N condition, since the copperdisc that is harder than TiO2 film separates from the former

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CO

CH4

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Figure 12: Comparison of product by CO2 reforming afterillumination of Xe lamp with UV light of 72 h for several Cr-dopedTiO2 film prepared under conditions of different N .

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Figure 13: Comparison of product by CO2 reforming afterillumination of Xe lamp with UV light of 72 h for several Ag-dopedTiO2 film prepared under conditions of different N .

coated TiO2 film by coating repetition. Therefore, the CO2

reforming performance is worse with increasing N to 5.According to Figure 14 which shows the results without UVlight, the CO2 reforming performances of Ntop = 3 andNall = 3 are inferior to that of Nall = 1. Consequently, asto Ag doping, the N number seems having no impact onCO2 reforming performance under both with and withoutUV conditions. Since the difference of thermal conductivitiesbetween Ag and Ti is larger than that between Cr and Ti, it isthought that the thermal stress becomes larger for Ag dopingwhen N increases. By comparing all the results, the best CO2

reforming was achieved under the condition of the Ntop = 7with Cr doping in this study. Consequently, the large effectof many N on CO2 reforming performance is obtained forCr doping.

After illumination of Xe lamp with UV light of 72hours, the concentration of CO, CH4, and C2H6 canreach 8306 ppmV (= 92.5 mmol/g-catalyst), 1367 ppmV


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Nall = 1Nall = 3Ntop = 3

Illumination time of Xe lamp (h)

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Figure 14: Comparison of concentration change of CO by CO2

reforming with illumination time of Xe lamp without UV light forNall = 1, Nall = 3 and Ntop = 3.

Inte

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)

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300

400

0 200 400 600 800 1000 1200Depth (nm)

Figure 15: Cu element profile through thickness direction of theCr-doped TiO2 film under the condition of Ntop = 7 by XPSanalysis.

(= 15.2 mmol/g-catalyst), and 1712 ppmV (= 19.1 mmol/g-catalyst), respectively, where the method to calculate theamount of product per weight of catalyst is as follows.

The thickness of Cr-doped TiO2 film should be measuredfirst, and then with the known surface area of copper disc of1.96×10−3 m2 and the density of TiO2 of 3900 kg/m3 [60],the mass of TiO2 can be calculated. XPS analysis was usedto measure the thickness of Cr-doped TiO2 film. Figure 15shows the Cu element profile through thickness directionof the Cr-doped TiO2 film in the case of Ntop = 7 by theXPS analysis. XPS spectra of Cu 3p are detected. In this XPSanalysis, the sputtering rate is about 0.8 nm/min. It is seenthat the intensity of Cu is increased dramatically from about600 nm in this case. It alludes to the fact that the basis copperdisc is at the depth of 600 nm; that is, the thickness of theTiO2 film is 600 nm. In this calculation, the weight of Cr hasbeen ignored since Cr was doped in the top layer only.

Con

cen

trat

ion

ofC

O(p

pmV

)

0

20

40

60

80

100

120

140

160

180

Nall = 1

Ntop = 7

Illumination time of Xe lamp (h)

About 3 times

0 24 48 72 96 120 144 168 192

Figure 16: Comparison of concentration change of CO by CO2

reforming with illumination time of Xe lamp without UV light forNall = 7 and Ntop = 7.

To verify the effect of photoresponse extension of TiO2 tothe visible spectrum for Ntop = 7 in Cr coating, Figure 16shows the results obtained with illumination time of Xelamp without UV light. The data for Nall = 1 underR = 70 wt% is also shown in this figure for comparison.Although CH4 is produced from the experiment forNtop = 7,the concentration is below 10 ppmV. Therefore, Figure 16shows the concentration of CO only. The peak value ofconcentration of CO for Ntop = 7 is about 3 times as large asthat of Nall = 1. This proves that in the case of Ntop = 7, thephotoresponse ability of visible spectrum is also promotedby the increase of the amount of TiO2 and the convertedCr ion-like Cr3+ brought by encouragement of reductionperformance of photocatalyst. Given that the concentrationof product for R = 0 wt% is at such a low level as shown inFigures 10 and 11, it can be concluded that CO2 reformingperformance of TiO2 is promoted dramatically by Cr doping.In the research by Ozcan et al. [24] which tried to extend thephotoresponse of TiO2 to the visible spectrum by Pt loading,the amount of product from CO2 reforming without UVlight was 104 times less than what we produced in this study.Therefore, Cr doping by sol-gel and dip-coating processis effective to promote the CO2 reforming performance ofTiO2.

4. Conclusions

Based on the experimental results, the following conclusionscan be drawn from this study.

Both Cr doping and Ag doping can promote the CO2

reforming performance by TiO2. The optimum R when N= 1 for Cr doping and Ag doping is 70 wt% and 1 wt%,respectively. Since the density of Ag is much larger thanthe density of Ti, the amount of Ag doped on TiO2 film islower compared to the actual amount of Ag powders addedinto TiO2 sol solution, resulting in the fact that the good


CO2 reforming performance is not obtained under large Rcondition. The promotion of CO2 reforming performanceby Cr or Ag doping comes from two aspects: (1) thephotoresponse ability being extended to visible spectrum bythe doping, and (2) doping preventing the recombination ofelectron and hole if there is UV light. The latter is strongerthan the former. The best result obtained is in the case ofNtop = 7 with Cr doping under the investigated conditions inthis study.

Acknowledgment

The authors gratefully acknowledge the financial supportfrom Tanikawa Fund Promotion of Thermal Technology.

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Research Article

Visible-Light Photodegradation of Dye on Co-Doped TitaniaNanotubes Prepared by Hydrothermal Synthesis

Jung-Pin Wang,1 Hsi-Chi Yang,2 and Chien-Te Hsieh3

1 PhD Program of Technology Management, Chung Hua University, Hsinchu 300, Taiwan2 Department of Construction Management, Chung Hua University, Hsinchu 300, Taiwan3 Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan

Correspondence should be addressed to Chien-Te Hsieh, [email protected]

Received 3 May 2011; Revised 6 July 2011; Accepted 8 August 2011


Copyright © 2012 Jung-Pin Wang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Highly porous Co-doped TiO2 nanotubes synthesized from a hydrothermal treatment were used to photodecompose methyleneblue (MB) in liquid phase under visible light irradiation. The anatase-type titania nanotubes were found to have high specificsurface areas of about 289–379 m2/g. These tubes were shown to be hollow scrolls with outer diameter of about 10–15 nm andlength of several micrometers. UV absorption confirmed that Co doping makes the light absorption of nanotubes shift to visiblelight region. With increasing the dopant concentration, the optical band gap of nanotubes became narrower, ranging from 2.4 eVto 1.8 eV, determined by Kubelka-Munk plot. The Co-doped nanotubes exhibit not only liquid-phase adsorption ability, but alsovisible-light-derived photodegradation of MB in aqueous solution. The synergetic effect involves two key factors in affecting thephotocatalytic activity of Co-doped titania nanotubes under fluorescent lamp, that is, high porosity and optical band gap. Themerit of the present work is to provide an efficient route for preparing Co-doped TiO2 nanotubes and to clarifying their adsorptionand photocatalytic activity under fluorescent lamp.

1. Introduction

Tubular form of TiO2 has attracted considerable research forits potential applications in organic light-emitting diodes,photocatalysts, gas sensors, and high-effect solar cells [1–6],among others. To enhance efficient photocatalytic applica-tion, it is generally recognized to increase the active surfacearea of TiO2 nanostructures [1, 5]. Recently, various titaniananotubes have been synthesized by using hydrothermaltreatment [2, 7–9], soft-chemical synthesis [4], and self-organization combined with electrochemical method [1,3]. The as-prepared titania nanotube is of mesoporosityand high aspect-ratio structures with nanocrystalline walls,showing promising candidates for use in photocatalytic(e.g., de-NOx catalyst) [5] and photoelectrochemical (e.g.,electrodes of dye-sensitized solar cell) systems [10].

Hydrothermal treatment is an efficient chemical route,capable of preparing highly porous TiO2 nanotubes. Earlywork has investigated the hydrothermal synthesis of crys-talline titania particles to produce high-purity titania-based

nanotubes with an average diameter of ca. 10 nm [7–9]. Thisability to fabricate the TiO2 nanotubes is expected to posi-tively impact realistic applicability. Apart from high porosity,one of vital concerns on the photocatalytic efficiency of TiO2

nanostructures is based on solar light conversion. Analyzedon solar light spectrum, visible light accounts for 45% ofenergy from the solar radiation, whereas UV light is lessthan 3%-4% [11]. This reflects that only 3%-4% solar lightcan be applied according to the band gap of anatase-typeTiO2 of 3.0–3.2 eV [3]. One possible way for improving theefficiency in visible range is to narrow band gap or splitinto several subgaps of TiO2. Pioneer studies have reporteddoping of TiO2 with suitable species such as metal ions suchas Pt, V, Ni, Mn, Cr, or Fe [11–14], or nonmetals such as Natoms [8, 15–18]. They have been demonstrated to improvethe photocatalytic reactivity toward organic molecules undervisible light illumination. Recently, the Co doping intoTiO2 nanocatalysts has been confirmed to exhibit superiorphotodegradation capability under visible light irradiation[19, 20]. However, to our knowledge, there are few reports


on in situ doping on TiO2 nanotubes for enhancement ofvisible-light photocatalytic efficiency.

It is believed that a most straightforward approach formetallic doping would be ion implantation. Although thisdoping technique efficiently introduces species into TiO2

compact layers, it unfortunately accompanies the structuraldamage of crystallites that may strongly reduce the photonconversion efficiency [1]. Moreover, the ion implantationis often expensive based on economic viewpoint. Withinthe above scope, this study intends to develop a simplehydrothermal synthesis of Co-doped titania nanotubes withcobalt nitrate +NaOH aqueous solutions. TiO2 nanotubeswith various amounts of Co dopants were synthesized toexamine their photocatalytic efficiencies under visible lightirradiation. The as-grown TiO2 nanotubes were also charac-terized by high-resolution transmission electron microscope(HR-TEM), UV absorption, N2 physisorption, and visible-light-derived photocatalysis of methylene blue (MB). In thisstudy, the relationship between the visible-light photocat-alytic activity and the amount of Co content has beenexplored. The merit of the present work is to provide anefficient route for preparing highly porous TiO2 nanotubesand to clarifying their adsorption and photocatalytic activityunder fluorescent lamp.


2.1. Preparation of Co-TiO2 Nanotubes. Titania-based nan-otubes synthesized by hydrothermal synthesis have beenreported elsewhere [2, 7–9]. The TiO2 precursor used inthis study was commercial TiO2 nanopowders (P25, DegussaAG), consisted of ca. 30% rutile and ca. 70% anatase incrystalline phase. In the nanotube preparation, 2 g of theTiO2 powder was mixed with 100 mL of 10 M NaOH +0.01 M Co(NO3)2 aqueous solution, followed by thermaltreatment of the mixture at 135◦C in a Teflon-lined autoclavefor 24 hr. Here, four molar ratios of Co to Ti were set at 1,3, 5, and 7 mol%, respectively. We subjected the precipitatefrom filtration to pH-value regulation by mixing it with 1 Lof 0.1 N HNO3 solutions. To obtain anatase-type titania, thepH value of the slurry was adjusted to 1.6 by HNO3 washing[7]. The final products were obtained by the filtration withsubsequent drying at 110◦C overnight.

2.2. Characterization of As-Grown Nanotubes. An UV spec-trometer (Varian Cary100) was applied to analyze thereflectance spectra of titania samples, ranged from 200 nmto 800 nm in wavelength. The phase identification of TiO2

nanotubes was characterized by XRD with Cu Kα radiationusing an automated X-ray diffractometer (Philip PW 1700).HR-TEM (JEOL JEM-6500F) was used for morphologicalobservations of the Co-TiO2 nanotubes. Specific surfaceareas and pore volumes of the derived nanotubes weredetermined by gas adsorption. An automated adsorptionapparatus (Micromeritics, ASAP 2000) was employed forthese measurements. Adsorption of N2, as a probe gas, wasperformed at −196◦C. Nitrogen surface areas and microporevolumes of the samples were determined from Brunauer-Emmett-Teller (BET) and Dubinin-Radushkevich (DR)

equations, respectively. The amount of N2 adsorbed atrelative pressures near unity (P/P0 = 0.98 in this work) hasbeen employed to determine the total pore volume, whichcorresponds to the sum of the micropore and mesoporevolumes. The peak pore diameter of nanotubes can beestimated according to pore size distribution, determinedfrom Barret-Joyner-Halenda (BJH) method.

2.3. Liquid-Phase Adsorption and Photodegradation. Adsorp-tion experiments of MB were conducted by placing a certainamount of TiO2 adsorber and 100 cm3 of the preparedaqueous solution into a glass-stoppered flask. The flask wasput in a constant-temperature shaker bath, with a shakerspeed of 100 rpm. The adsorption temperature and periodemployed here were 40◦C and 5 hr, respectively. Preliminaryexperiments had shown the adsorption process attainedequilibrium in 5 hr for all TiO2 samples used in the presentstudy.

After liquid-phase adsorption, MB photodegradation onTiO2 nanotubes was carried out to examine the photocat-alytic reactivity under visible illumination. The photocat-alytic decomposition of MB solutions was characterized byan UV-visible spectrometer (Shimadzu UV-2550). Based onthe Beer-Lambert law [15], the concentration of MB aqueoussolution is linearly proportional to the absorbance of mea-sured spectrum in the concentration range around 30 mg/L.The MB-adsorbed titania slurries were also illuminated at40◦C, employing a 13 W fluorescent lamp. To ensure no UVlight illumination, the irradiation from the fluorescent lampwas filtered through an UV cut filter (Newport FSQ-GG400), which allows the visible light >400 nm pass through thefilter. The incident intensity of illumination from the visiblelight was set at 750 μW/cm2.


3.1. Textural Characteristics of Co-TiO2 Nanotubes. The as-synthesized nanotube samples were designated as Co-1-TNT,Co-3-TNT, Co-5-TNT, Co-7-TNT, respectively, accordingto preparation of different Co/Ti molar ratios. In Figure 1,the microstructure of Co-TiO2 nanotubes are illustratedby HR-TEM images. As shown in Figure 1, the as-grownCo-TiO2 nanotubes are hollow with outer diameters of ca.10–15 nm, inner diameters of ca. 5–10 nm, and lengths ofseveral micrometers. Both the ends are opened, which isextremely critical for their adsorption and photocatalysiscapability. The as-grown Co-TiO2 nanotubes are generallyhomogeneous that is, narrow tubular size distribution. Itcan be observed from Figure 1 that the nanotubes are scrolls(unlike carbon nanotubes), showing unequal number ofwalls on both of tube sides. The typical nanotube axis isroughly along the [100] direction of the anatase crystals, asillustrated in the inset of Figure 1(a).

Figure 2 shows the typical isotherms of N2 adsorptiononto the titania nanotubes with different amounts of Codopant prepared from the hydrothermal treatment. Theseisotherms are found to have the hysteresis behavior withinhigh pressure of 0.6–0.98, reflecting that the products aremainly mesoporous. The pore structures of the titania


50 nm

[100][001]

(a)

20 nm

(b)

20 nm

(c)

Figure 1: Typical HR-TEM image of (a) Co-1-TNT, (b) Co-3-TNT, and (c) Co-5-TNT nanotubes, prepared by the hydrothermal synthesis.The inset is the high magnification image on sidewall of the nanotube (sample: Co-1-TNT).

samples determined according to the adsorption data arealso collected in Table 1. These atomic Co concentrationsin titania crystallites were measured by electron diffractionspectroscopy (EDS) collected, as shown in Table 1. Incomparison, the titania nanotubes (TNT) without any Codoping is used as a reference. The specific surface areas in arange of 289–379 m2/g are much higher than for the startingmaterial (P25), which has a surface area of ca. 50 m2/g. Itcan be observed from this table that the titania nanotubesare highly mesoporous.

As shown in Table 1, the specific surface area is found todecrease with the doping amount. This change is probablyattributed to the introduction of cobalt atoms in tita-nia crystalline structure, thus resulting in different titaniananotubes. Indeed, this transformation of TiO2 precursorto anatase-type titania nanotube has been well examinedand reported elsewhere [2, 7]. However, the formation ofCo-doped titania nanotubes prepared from hydrothermalsynthesis is rarely discussed. Upon NaOH treatment, someof Ti–O–Ti bonds are broken, forming an intermediatecontaining Ti–O–Na, Ti–O–Co, and Ti–OH. This indicatesthat Co and Na atoms would occupy some broken Ti–Obonds of the TiO2 precursor simultaneously, leading to theformation of lamellar fragments that are intermediate phasein the formation process of the nanotube material. Theseintermediates would proceed with rearrangement to formsheets of edge-sharing TiO6 octahedra with Co2+, Na+, andOH− intercalated between the sheets. Since the bond distanceof Co–O is larger than that of Na–O (ca. 0.23 nm in NaOH)and H–O (ca. 0.15 nm in H2O), the intercalation with Co2+

ions would result in a larger interlayer distance than that withNa+ and H+. Then the sheets would be scrolled to becomenanotubes after HCl washing [2]. The rolling of the sheetsreduces the number of surface dangling bonds, and thuslowers its system energy [8]. The presence of Co ions in these

Table 1: Surface characteristics of Co-TiO2 nanotubes determinedfrom N2 physisorption at −196◦C.

Materials CCoa SBET

b Vtc Pore size distribution

type (ions/cm3) (m2/g) (cm3/g) Vmicrod (%) Vmeso

e (%)

P25 — 45.8 0.121 0.016 (16) 0.102 (84)

TNT 0 376 0.929 0.118 (13) 0.811 (87)

Co-1-TNT 1.90× 1020 379 0.934 0.117 (13) 0.816 (87)

Co-3-TNT 4.31× 1020 350 0.975 0.118 (12) 0.857 (88)

Co-5-TNT 5.59× 1020 341 0.932 0.117 (12) 0.815 (88)

Co-7-TNT 7.31× 1020 289 0.685 0.118 (17) 0.567 (83)aCCo: cobalt atomic concentration determined from EDS analysis.bSBET: specific surface area computed using BET equation.cVt : total pore volume estimated at a relative pressure of 0.98.dVmicro: micropore volume determined from DR equation.eVmeso: mesopore volume determined from the subtraction of microporevolume from total pore volume.

sheets would probably cause surface heterogeneity, formingdifferent curvatures of nanotubes. This can be attributed toa fact that the Co doping affects the surface characteristicsof as-grown nanotubes. The pore size distributions of thesenanotubes are depicted in Figure 3, which were calculatedfrom their N2 adsorption isotherms, using BJH method.These distributions are found to have one, two, or threepeaks in mesopore size range. The peak pore sizes range from8 to 15 nm, identical with the HR-TEM observation. Thisimplies that both tips of nanotubes are opened and theirinner cavities are accessible to N2 gas molecules. It can beinferred that the inner cavities in nanotubes possess a majorcontribution to the total pore volume.

3.2. XRD and UV Absorption of Co-TiO2 Nanotubes. TheXRD patterns of as-grown titania samples with different


0

0

0.2 0.4 0.6 0.8 1Relative pressure (P/P0 )

100

200

300

400

500

600

700

Vol

um

ead

sorb

ed(c

m3

/g,S

TP

)800

(a)

0

0


100

200

300

400

500

600

700

Vol

um

ead

sorb

ed(c

m3

/g,S

TP

)

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(b)

0

0


100

200

300

400

500

600

700

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um

ead

sorb

ed(c

m3 /g

,ST

P)

(c)

0

0


100

200

300

400

500

600

700

Vol

um

ead

sorb

ed(c

m3 /g

,ST

P)

(d)

Figure 2: Adsorption (solid symbol)/desorption (empty symbol) isotherms of N2 onto titania nanotubes at −196◦C: (a) Co-1-TNT; (b)Co-3-TNT; (c) Co-5-TNT; (d) Co-7-TNT.

amounts of Co dopant are shown in Figure 4. It is knownthat the precursor P25 has ca. 70% anatase and ca. 30%rutile phases. It can be seen from XRD patterns that therepresentative peaks are anatase [101], [004], [200], [105],and [204] diffractions at scattering angles (2θ) of 25.3◦,36.6◦, 48.0◦, 55.0◦, and 62.6◦, respectively. It is obviousthat the rutile of P25 has been transferred to anatase. Theanatase phase with a longer c axis has been reported to bethe preferred phase in TiO2 nanotubes [7, 8]. Interestinglyenough, the peak intensity is found to increase with thedopant concentration. Generally, the vague peaks reflect thesmall number of crystalline layers due to the small wallthickness of the tubes. This implies that the doping of Co may

affect the rolling of the sheets; that is, different curvatures ofrolled sheets would dominate the wall thickness of tubes.

The XRD cannot identify the low amount of Co dopantin titania nanotubes due to its detection limit. However,three diffraction peaks (101), (004), and (200) appear a slightshift after the introduction of Co dopant (see Figure 4),resulting in the small change of lattice parameters. Accord-ingly, this result reflects that the Co dapants insert into thecrystalline TiO2 structures without any Co or cobalt oxidecoatings. The lattice parameters and interlayer distances ofvarious titania nanotubes based on the XRD patterns arecollected and listed in Table 2. It can be seen that undopedTiO2 nanotubes have lattice parameters (a- and c-axis) of


10 100 1000

10 100 1000

10 100 1000

10 100 1000

Pore diameter, D (A)

0

0.1

0.2

0.3

0

0.1

0.2

0.3

0

0.1

0.2

0.3

0

0.1

0.2

0.3

dV/d

log

D(c

m3/g

A)

Co-1-TNT

Co-3-TNT

Co-5-TNT

Co-7-TNT

Figure 3: Pore size distributions of various Co-doped TiO2 nanotubes determined from BJH method.

0.3710 and 0.9502 nm, respectively, in the unit cell basedon the tetragonal Bravais lattice. In comparison with theCo-doped titania nanotubes, there exists derivations of0.65%−2.05% (a-axis) and 0.44%−0.78% (c-axis), and thedeviation increases with the amount of Co dopants. Afterthe Co doping, the interlayer distances of d(200) and d(004)

gradually extend, confirming the intercalation of Co dopantsinto the anatase-type crystals.

To inspect the concentration of Co dopant, an EDStechnique was employed to measure atomic ratios of as-synthesized titania samples. Table 1 shows the atomic ratiosof the nanotubes determined form EDS analysis. The weightpercentage of Co dopant to TNT for all Co-doped titaniananotubes has an order as follows: Co-1-TNT (0.19%) < Co-3-TNT (0.43%) < Co-5-TNT (0.56%) < Co-7-TNT (0.73%).Generally, the atomic ratio of Co is found to be slightly lowerthan the operating ionic concentration in the precursor. Thedecrease can be attributed to the replacement of metal ionswith protons during acid washing.

Figure 5 shows the diffuse reflection spectra of the titaniananotubes as a function of wavelength. It is well known

Table 2: Lattice parameters and interlayer distances of TiO2

nanotubes with different amounts of Co dopants, based on the XRDpatterns.

Materials c (nm) a (nm) d(200) (nm) d(004) (nm)

P25 0.9508 0.3708 0.1864 0.2377

TNT 0.9502 0.3710 0.1865 0.2380

Co-1-TNT 0.9544 0.3734 0.1867 0.2386

Co-3-TNT 0.9556 0.3748 0.1874 0.2389

Co-5-TNT 0.9568 0.3756 0.1878 0.2392

Co-7-TNT 0.9576 0.3786 0.1893 0.2394

that anatase-type TiO2 crystalline structure has a strongabsorption edge below ca. 380 nm [11]. The absorption edgesof Co-doped TiO2 nanotubes show a shift to visible-lightregion (i.e., >400 nm in wavelength). After that, an obviouspeak is found within the wavelength region 550–650 nm,in where the peak height is found to increase with thedopant concentration. The absorption peak at 550–650 nmcan be ascribed to the formation of impurity energy level


20 30 40 50 60 702θ (deg)

Inte

nsi

ty(a

.u.)

Co-7-TNT

Co-5-TNT

Co-3-TNT

Co-1-TNT

Anatase

(101)

(004) (200)(105) (204)

Figure 4: XRD patterns for Co-doped TiO2 nanotubes preparedfrom hydrothermal treatment.

200 400 600 800Wavelength (nm)

0

1

2

3

4

Diff

use

refl

ecta

nce

,F(R

)

Co-1-TNT

Co-3-TNT

Co-5-TNT

Co-7-TNT

Figure 5: Diffuse reflection spectra for TiO2 nanotubes withdifferent Co dopant concentrations.

within the band gap. This proves that the hydrothermaldoping technique has modified the UV-visible absorptioncharacteristics of titania catalysts.

Theoretically, the absorption spectrum used for the cal-culation of band gap can be expressed in terms of F(R),

Kubelka-Munk (KM) model. The diffuse reflectance, R, ofthe titania sample is related to the KM function by [21–23]

F(R) = (1− R)2

2R= α

S, (1)

where α and S represent the absorption coefficient andthe scattering coefficient, respectively. The KM model isfrequently used to estimate the absorption band gap basedon the diffuse reflection spectra [21, 23]. The optical bandenergy of titania samples can be evaluated by using a linearityplot of [F(R) · hν]1/η versus hν, in which hν is the energy ofthe incident photon and the exponent η depends on the typeof optical transition caused by photon absorption [23]. Anexcellent fitting was proposed by using the KM plots (η =2) for the metal-oxide nanocrystals. As shown in Figure 6,the linearity KM plots were extended and intersected withx-axis, that is, the energy of the incident photon. The inter-section represents the optical band gap of titania samples.It can be seen that all magnitudes of band gaps are smallerthan 3.2 eV, and the optical band gap shows a decreasingtrend with the Co dopant concentration: Co-1-TNT (2.3 eV)> Co-3-TNT (2.1 eV) > Co-5-TNT (2.0 eV) > Co-7-TNT(1.9 eV). The above results disclose two messages: (i) theCo dopants effectively make the band gap narrower and (ii)the variation of band gap depends on dopant concentration.Figure 7 shows a quantitative description of the band gap asa function of dopant concentration in Co-TiO2 nanotubes.A gradual decreasing relationship between the optical bandgap and the number of Co dopants confirms that thehydrothermal synthesis of Co doping in the TiO2 nanotubesleads to a pathway of “communicated electrons” betweenthese crystals.

3.3. Adsorption and Visible-Light Photocatalysis of MB on Co-TiO2 Nanotubes. To examine the photocatalytic efficiencyof titania samples, a possible mechanism for removingMB on Co-TiO2 nanotubes is taken into account, whichconsists of (i) liquid-phase adsorption of MB and (ii)photocatalysis of adsorbed MB under fluorescent lamp.Numerous studies have proposed the similar photocatalyticmechanism in describing decomposition of various organicson TiO2 and ZnO nanostructures [24–29]. Accordingly, theadsorption capability of MB on titania nanotubes playsan important role in affecting the photocatalytic efficiency.Thus, the active sites for MB adsorption are believed tobe governed by the surface structure of titania nanotubes.To figure out adsorption and photocatalysis effects, anadsorption experiment in complete darkness is carried outfirst. The adsorption capacity as a function of time forall titania nanotubes is shown in Figure 8. An adsorptionequilibrium of MB adsorption within 2 hr is reached, and theadsorption capacities show an order as follows: Co-1-TNT(110 mg/g) > TNT (101 mg/g) > Co-3-TNT (97 mg/g) > Co-5-TNT (91 mg/g) > Co-7-TNT (70 mg/g). These adsorptioncapacities of titania nanotubes are much higher than that ofnanoparticles [30]. This order is generally followed by themagnitude of specific surface area of nanotubes, indicating


1.5 2 2.5 3 3.5 4 4.5hA (eV)

0

1

2

3

4

[F(R

)·hA]1/

2

Co-1-TNT

Co-3-TNT

Co-5-TNT

Co-7-TNT

Figure 6: KM plots for TiO2 nanotubes with different Co dopantconcentrations.

1 2 3 4 5 6 7 8

1.8

2

2.2

2.4

Ban

dga

p(e

V)

Cobalt concentration, CCo × 1020 (ions/cm3)

Figure 7: Variation of optical band gap with Co dopant concentra-tion in TiO2 nanotubes.

that the nanotubes offer different numbers of active sites forthe MB adsorption in liquid phase.

As for MB-titania interaction, the electrostatic attractionplays a crucial role in the physical adsorption [31]. Theadsorption process can be tentatively expressed in thermo-dynamic term as

TNT∗ + MB ←→ TNT∗(MB) (R1)

0 5 10 15 20 25 30 35 40 45Time (hr)

0

40

80

120

160

200

MB

rem

oval

capa

city

(mg/

g)

Adsorption Photocatalysis

Co-1-TNT

Co-3-TNT

Co-5-TNT

Co-7-TNT

P25

TNT

Figure 8: Liquid-phase adsorption of MB followed by visible-lightphotocatalysis kinetics of as-prepared product of Co-doped TiO2

nanotubes at 40◦C.

where MB represents the adsorbates in liquid phase, TNT∗

the available adsorptive sites on titania nanotubes, and TNT∗

(MB) the adsorptive sites occupied by MB molecules. Thesum of TNT∗ and TNT∗ (MB) numbers is thus the totalnumber of sites, capable of adsorbing the MB molecule atmonolayer coverage (R1). The adsorptive surface coverageΘads, that is, the fraction of BET area covered by MBmolecules, can be evaluated by assuming that the areaoccupied by a MB molecule is estimated to be 130 A2 [31].

The calculated adsorptive coverage Θads for the nan-otubes ranges from 60% to 70%, proving the presence ofsurface heterogeneity for MB adsorption. The adsorptivecoverage shed a clue that explores (i) the normalizationof adsorptive areas due to different porosities of Co-dopedTNTs and (ii) the number of active sites for adsorbingMB molecules from the initial aqueous solution. Since allCo-TiO2 nanotubes has alike physically surface structure(e.g., mesopore fraction and pore size distribution), thisdifference among Θads values is thus attributed to surfaceheterogeneity, contributed from the presence of Co dopants.Figure 9 shows the variation of surface coverage Θads withCo dopant concentration. The adsorptive surface coverage(Θads) smaller than 100% reflects that MB cannot completelywet titania surface in the adsorption system. Generally, theliquid-phase adsorption in the monolayer region is differentfrom that in the multiplayer region, and the surface coveragepossibly relates to hydrolyzed surface area and existence of–OH group over titania nanotubes. Basically, TiO2 surfacefavors hydrophilic behavior [4], whereas metallic cobaltappears a more hydrophobic characteristic. Pan et al. have


0

0.2

0.4

0.6

0.8

1

Act

ive

surf

ace

cove

rage

θads

θphoto

1 2 3 4 5 6 7 8

Cobalt concentration, CCo × 1020 (ions/cm3)

Figure 9: Variations of adsorptive surface coverage and photo-catalyzed surface coverage with Co dopant concentration in TiO2

nanotubes.

revealed the enhanced hydrophobicity of Co-doped TiO2

nanocrystals [32]. However, it is worth noting that the MBmolecules in aqueous solution are prone to be hydrates,favoring the surface of hydrophilic nanotubes. Accordingly,the adsorption capacity shows a gradual decreasing functionof the amount of Co dopant. Thus, more Co doping wouldlead to the better water repellency, probably diminishingadsorptive surface coverage of MB in aqueous solution.

Visible-light photocatalysis of MB on Co-TiO2 nan-otubes is conducted after attaining adsorption saturation,and the photocatalytic kinetics are also shown in Figure 8.As expected, original TNT appears a little visible-lightphotocatalytic capability (<3 mg/g), whereas these Co-TiO2

nanotubes display photocatalytic ability in decomposingorganic dyes under fluorescent lamp. This proves that thehydrothermal synthesis of Co-TiO2 tubes is an efficientapproach in enhancing not only specific surface area butalso photocatalysis performance under fluorescent lamp.Accordingly, the synergetic effect involves two key factorsin affecting the photocatalytic activity of Co-doped titaniananotubes under fluorescent lamp, that is, high porosity andoptical band gap. However, these kinetic curves appear aslower MB removal rate in comparison with the liquid-phaseadsorption, since the photocatalytic equilibrium takes a longperiod of 40 hr. The visible-light photocatalytic effectivenessseems to be less significant than the adsorption.

The adsorption process has been illustrated as (R1). Thephotocatalysis of MB molecules under visible illuminationcan be considered as a surface-catalyzed reaction, whichdepends on the number of adsorptive sites and opticalband gap of TS catalysts. It is generally recognized thatconduction band electrons (e−) and valence band holes (h+)

are generated on the surface of photocatalysts when theaqueous catalyst suspension is illuminated by light with anenergy higher than the band gap energy [33], as expressed in(R2). Holes can react with water adhering to the surface ofCo-TiO2 nanotubes to form highly reactive hydroxyl radicals(OH•), as shown in (R3). Oxygen acts as an electron acceptorby forming a superoxide radical anion (O•2

−), as shown in(R4). The suspension of superoxide radical anions may actas oxidizing agents or as an additional source of hydroxylradicals via the subsequent formation of hydrogen peroxide,as shown in (R5)–(R7). The strong oxidants associated withhydroxyl radicals react with adsorbed MB molecules, andmake the blue solution colorless, as shown in (R8). Sincedecomposition reaction of the MB is composed of severalsteps, (R8) is just a simplified form [33].

TiO2 + hν −→ e− + h+ (R2)

H2O + h+ −→ OH • + H+ (R3)

O2 + e− −→ O•2− (R4)

O•2− + H+ −→ HO2• (R5)

2HO2• −→ H2O2 + O2 (R6)

H2O2 + O•2− −→ OH • + O2 + OH− (R7)

OH • + MB+ −→ colorless compound (R8)

Accordingly, this reaction (R2) enables the promotion ofredox ability of the photogenerated electron-hole pairs, bycarrying out the following sequences. The Co-doped crystalsare capable of acquiring the excitation energy from visibleirradiation. This is attributed to the fact that its narrow bandgap easily obtains the electron-hole pairs over the nanotubesunder fluorescent lamp, thus, leading to the photocatalyticactivity.

The photocatalytic reaction is basically a surface reactionthat is assumed to take place on the adsorptive site. If thisassumption is correct, the surface coverage of photocatalysissites should be the same as that of adsorptive sites basedon equal apparent rate constants. According to the photo-catalytic capacity at 40 hr visible illumination, the surfacecoverage for MB photocatalysis (Θphoto) is evaluated forproving the above assumption. The relation of calculatedΘphoto values versus Co dopant concentration is also plottedin Figure 9. It can be seen that the two surface coverages,Θads and Θphoto, cannot match each other, indicating thatthe assumption fails. The smaller value of Θphoto (i.e.,within 20%–40%) shows that only 50%–60% MB-adsorbedsites would be photocatalyzed under fluorescent lamp. Thisresult can be attributed to two possible explanations. Thefirst one is that the source of visible-light irradiation inthis study is too weak to photogenerate enough numberof electron/hole pairs, thus partially photocatalyze someMB-adsorbed sites. Secondly, the photocatalysis is a paralleland complicated chemical reaction (R2)–(R8), showing ahigher energy barrier than the physical adsorption needed.Generally, the physical adsorption of MB would take placeon nonspecific sites, whereas the photocatalysis generates


only on some specific adsorptive sites. On specific sites,the photoexcited energy required (1.8–2.4 eV) is one or twoorder higher than the physisorption. Additionally, the visiblephotocatalysis of MB takes a very long period in comparableto the physisorption, as shown in Figure 8. This means thatthe serial of photocatalytic reactions (R2)–(R8), includingphotoinduction, electron/hole generation, radical formation,and MB decomposition, thus become a rate-determiningstep during MB decomposition process. Thus, this mayinduce a lower surface coverage for photocatalysis of MBunder fluorescent lamp.

As shown in Figure 9, the value of Θphoto, is also adecreasing function of the Co dopant concentration. Again,this confirms that the optical band gap is not the keyfactor in affecting the visible photocatalytic performance.In this work, 1 at. % Co dopant concentration in titaniacrystalline structure is competence for visible photocatalysis.This couple of decline relationship in Figure 9 can beascribed to that total amount of adsorptive sites significantlyaffects the number of photocatalyzed sites. Accordingly,titania nanotubes with a large number of pores are expectedto facilitate the liquid-phase adsorption capacity, whichalso induces the enhancement of the visible photocatalysiscapability.

4. Conclusions

The present work showed efficient Co doping of highlyporous TiO2 nanotubes by hydrothermal synthesis. Thespecific surface areas of Co-doped TiO2 nanotubes werefound to have a great value of ca. 289–379 m2/g. Thesetubes were shown to be hollow scrolls with a typicalouter diameter of about 10–15 nm, inner diameter 5–10 nmand length of several micrometers. The titania nanotubeshad an anatase-type crystalline structure. UV absorptionanalysis reflected that these titania nanotubes show a strongabsorption in the visible range and narrow optical bandgap within 1.8–2.4 eV, according to the KM plots. Wehave confirmed that the Co-TiO2 nanotubes displayed aphotocatalysis ability in decomposing organic dyes underfluorescent lamp. The visible photocatalysis would take along period of 40 hr, indicating that the visible photocatalysisis a rate-determining step during the MB removal process.On the basis of the results, the hydrothermal synthesis ofCo-TiO2 tubes is an efficient approach in enhancing notonly specific surface area, but also photocatalysis capabilityunder fluorescent lamp. This novel titania nanostructure,Co-doped titania nanotubes, can improve the photocatalyticefficiency in a variety of photocatalysis applications becauseof the combination effect of a high porosity with a visible-light-derived photocatalysis.

Acknowledgment

The authors are very grateful for the financial support fromthe National Science Council of the Republic of China underthe contracts NSC 100-2120-M-155-031, NSC 100-2221-E-155-078, and NSC 99-2632-E-155-001-MY3.

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Research Article

Photocatalytical Properties and TheoreticalAnalysis of N, Cd-Codoped TiO2 Synthesized byThermal Decomposition Method

Hongtao Gao,1, 2 Bing Lu,3 Fangfang Liu,3 Yuanyuan Liu,3 and Xian Zhao1

1 State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China2 Department of Chemistry and Chemical Engineering, Jining University, Qufu 273155, China3 College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China

Correspondence should be addressed to Hongtao Gao, [email protected]

Received 19 June 2011; Revised 1 August 2011; Accepted 2 August 2011


Copyright © 2012 Hongtao Gao et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

N, Cd-codoped TiO2 have been synthesized by thermal decomposition method. The products were characterized by X-raydiffraction (XRD), scanning electron microscope (SEM), UV-visible diffuse reflectance spectra (DRS), X-ray photoelectronspectroscopy (XPS), and Brunauer-Emmett-Teller (BET) specific surface area analysis, respectively. The products represented goodperformance in photocatalytic degradation of methyl orange. The effect of the incorporation of N and Cd on electronic structureand optical properties of TiO2 was studied by first-principle calculations on the basis of density functional theory (DFT). Theimpurity states, introduced by N 2p or Cd 5d, lied between the valence band and the conduction band. Due to dopants, the bandgap of N, Cd-codoped TiO2 became narrow. The electronic transition from the valence band to conduction band became easy,which could account for the observed photocatalytic performance of N, Cd-codoped TiO2. The theoretical analysis might providea probable reference for the experimentally element-doped TiO2 synthesis.

1. Introduction

Due to its ability to decompose harmful organic pollutantscompletely [1, 2], nano-TiO2 has attracted much interestin the last few decades as an environmental purificationphotocatalyst. However, the band gap of the TiO2 is inthe range of 3.0–3.2 eV (3.2 eV for anatase and 3.0 eV forrutile), and only the UV fraction of solar light (about 3–5%) is effective for inducing photoactivity [3]. It has beenrealized that doping played a dramatic role in shifting theabsorption edge to a lower energy region and increasingthe photocatalytic activity in the visible light region [4–7]. A substantial amount of research work have focused onimproving the absorption of visible light (400–800 nm) bynonmetal doping with N [5, 7–9], C [4, 6, 10], S [11], F [12],I [13], B [14], P [15], and so forth.

Metal doping can significantly reduce the band gap andpromote electronic excitation under visible light irradiation.Umebayashi et al. [16] reported that electron localization

and migration by the dopant played an important role inlight response of TiO2. Mn-doping modified the electronicstructure of rutile TiO2 and improved the catalytic per-formance [17]. Andronic et al. [18] reported that therewas a linear correlation between the band gap energyof the Cd-doped TiO2 films and dyes photodegradationefficiency. Cd-doped mesoporous titania had high visible-light photocatalytic activities [19]. Whether as interstitialatom or lattice atom displacement, metal doping introducesimpurity states between valance band (VB) and conductionband (CB), which act as electrons and holes recombinationcenters and can capture most of the charge carrier. So,the metal ion doping plays a limit role in improving thephotocatalytic activity of TiO2.

Due to doping, there always exist unfavorable factors,such as oxygen vacancy, brought by charge imbalance foreither single metal or nonmetal element doping. However,the bielement doping is likely to maintain charge balancethrough charge compensation in the crystals. Wen et al. [20]


discussed the effect of bielement doping and the calcina-tion temperature on the microstructure and photocatalyticactivity of I, F codoped TiO2. Nonionic surfactant was usedas template to prepare N, F codoped TiO2 for degradationof microcystin [21]. Xu et al. [22] found that there was anew electronic level in the structure of Ce, C codoped TiO2

and pointed out that Ce doping could delay recombinationof electrons and holes, shift the absorption to the redlight region and enhance its photocatalytic activity. Tan etal. [23] studied the mechanism of light absorption andphotocatalytic properties of Mo, N codoped TiO2 andpointed out that oxygen vacancies played an important rolein improving the photocatalytic performance of TiO2. Dueto the strong synergistic effect of W and N, the electronicstructure was changed with the band gap narrowing andthe optical absorption increasing [24]. Pingxiao et al. [25]studied preparation and photocatalysis of TiO2 nanoparticlesdoped with nitrogen and cadmium, and the results of thestudy showed that nitrogen and cadmium codoping causedthe absorption edge of TiO2 to shift to the visible-lightregion. At present, the theoretical research of N, Cd-codopedanatase TiO2 (101) surface has not been seen reported.

In our work, bare TiO2, Cd-doped, N-doped and N,Cd-codoped TiO2 photocatalysts have been synthesizedby thermal decomposition method. The products werecharacterized by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), UV-visible diffuse reflectance spectra(DRS), and X-ray photoelectron spectroscopy (XPS), respec-tively. The photocatalytic activities of samples were studiedon the degradation of methyl orange (MO). First-principlecalculations based on density functional theory (DFT) wereperformed to probe the effect of N and Cd incorporationon electronic structure and optical properties of TiO2. Tothe best of our knowledge, this is the first theoreticalexplanation to rationalize the gap narrowing mechanism andthe substitutional and adsorptive roles of N and Cd dopingin surface-state anatase. The theoretical calculations wereused to account for the experimental observation which washigh photocatalytic performance of N, Cd-codoped TiO2 fororganic pollutants degradation.

2. Experimental

2.1. Photocatalyst Preparation. The photocatalyst series havebeen synthesized by thermal decomposition method, usingdodecylamine as nitrogen source and cadmium nitrate(Cd(NO3)2) as Cadmium source. Isopropanol and tetra-n-butyl titanate (TTNB) were added into 250 mL round-bottom flask in turn according to a certain proportion,then 100 mL acetic acid solution (pH = 2) was added withmagnetic stirring to form transparent solution. After then,dodecylamine and Cd (NO3)2 was added into the solutionaccording to certain molar ratio. The obtained solution wasthen transferred to a 250 mL three-neck boiling flask. Afterhaving been stirred vigorously for 24 h, it was heated inthe paraffin bath, with the temperature rising gradually to120◦C at a rate of 10◦C/h and temperature heating for twohours, until a white precipitate appeared. The precipitate waswashed with ethanol until pH = 7 and dried at 60◦C for 24

hours, forming a white solid composite. Finally, the sampleswere calcined at certain temperature in muffle furnace for 2hours. Bare TiO2, Cd-doped TiO2, and N-doped TiO2 wereprepared in the same way.

2.2. Characterization. The crystalline phase of the pow-ders that evolved after calcination was examined by XRDwith a Rigaku D/MAX-2000 diffractometer, using Cu Kαirradiation (λ = 0.154056 nm) at 45 kV and 40 mA. Thecrystallite size (D) was estimated from the width of linesin the XRD pattern according to the Scherrer equation.A scanning electron microscope (SEM) JSM-6300 (JEOLLtd, Japan) was used to investigate the surface morphologyof the sample. In addition, UV-vis diffuse spectra weremeasured at room temperature with a UV-vis spectrometer(Cary-500, Varian Co.). Nitrogen adsorption-desorptionisotherms at 77 K were measured using a QuantacheromeNove 1000e system, and the Brunauer-Emmett-Teller (BET)surface area was calculated from the linear part of BETplot. X-ray photoelectron spectroscopy (XPS) measurementswere performed with the ESCALAB 250 Microprobe System(ThermoFisher SCIENTIFIC), using the Mg K Line of a300 W Mg X-ray tube as a radiation source at 15 kV. All thebinding energies were referenced to the C 1s peak at 284.8 eVof the surface adventitious carbon.

2.3. Photocatalytic Activity Measurements. Photocatalyticactivity of photocatalysts was evaluated by the degradation ofMO, which was performed in an SGY-I photochemical reac-tor (Nanjing, Stonetech. EEC Ltd. Nanjing, China). A quartzcylinder (50 × 450 mm) was placed inside the reactor andilluminated with 300 W high-pressure mercury lamp. Foreach condition, a certain amount of powders was added into500 mL of aqueous solution of MO (20 mg·L−1). A magneticstirrer was located at the bottom of the quartz cylinder sothat a homogeneous TiO2 suspension could be maintainedthroughout the reaction. The solutions containing photocat-alysts were stirred mechanically in the dark for 30 min toensure adsorption-desorption equilibrium between MO andphotocatalyst powders. The photocatalytic experiment wasrepeated under the identical reaction conditions to confirmthe reproducibility. During the experimental process, 5 mL ofaqueous suspension was taken from the quartz cylinder afterspecific intervals, centrifuged, and filtered through 0.45 μmmillipore filter to monitor the degradation of MO dye. UVspectrophotometer (UV-vis Cary50; Varian Co.) was used tomonitor changes in the spectral intensity distribution of thedye.


3.1. XRD Analysis. The phase structure, crystallite size, andcrystallinity of TiO2 play an important role in photocatalyticactivity, and many studies have confirmed that anatasephase of titania shows higher photocatalytic activity thanbrookite or rutile phase [26]. The XRD patterns of TiO2

photocatalysts calcined at 450◦C were showed in Figure 1.There existed sharp diffraction peaks, which lay at 25.4◦,37.9◦, 48.0◦ and 53.9◦, corresponding to (101), (004),


10 20 30 40 50 60 70

2θ (◦)

N, Cd-TiO2

N-TiO2

Cd-TiO2

Pure TiO2

Inte

nsi

ty(a

.u.)

Figure 1: XRD patterns of photocatalysts calcined at 450◦C.

(200), (105) anatase crystal plane diffraction, respectively.There were no apparent diffraction peaks at 27.5◦ and 54.5◦

in the XRD patterns, indicating the rutile phase did not existin the sample. It indicated that the prepared products wereanatase-TiO2 monophase and element-doping did not affectthe crystalline phase of TiO2 catalyst.

It was also seen in Figure 1 that the intensity of themaximum diffraction peak (2θ = 25.4◦) of the bare TiO2, Cd-TiO2, N-TiO2, and N, Cd-codoped TiO2 decreased in turn.The average size of crystallites was estimated based on thebroadening of (101) peak at 2θ = 25.4◦ according to Scherrerequation. The crystal diameter of bare TiO2, Cd-TiO2, N-TiO2 and N, Cd-TiO2 was 14.56 nm, 12.55 nm, 13.34 nm,and 12.60 nm, respectively. It indicated that element-dopinginhabited the crystal grain growth and particles aggregation,resulting in larger specific area. Small particle size couldshorten the route of an electron migrates from the conduc-tion band of the TiO2 to its surface, while large surface areacould provide more active sites and absorb more reactivespecies. Additionally, there was no new crystalline phase forCd-TiO2, N-TiO2, and N, Cd-TiO2, illustrating that the Nand Cd doping did not change the catalysts phase.

3.2. SEM Analysis. Figure 2 showed SEM micrograph ofthe calcinated samples at 450◦C. The SEM images showedthat nanoparticles were uniform (12∼15 nm), global andslightly agglomerated. Further observation indicated thatthe morphology of samples was very rough and might bebeneficial to enhancing the adsorption of reactants.

3.3. UV-Vis DRS Analysis. Figure 3 illustrated the UV-vis DRS of the photocatalysts. Compared to the strongabsorption in the UV region, the absorption in the visibleregion was relatively weak for all the photocatalysts. Both inthe UV region (<400 nm) and visible region, the absorptionsof as-prepared photocatalysts were all stronger than that ofpure TiO2. The intensity of absorption of pure TiO2, Cd-TiO2, N-TiO2, and N, Cd-doped TiO2 increased in turn,and the absorption intensity of N, Cd-codoped TiO2 was thestrongest. The band gap (Eg) of the photocatalysts could becalculated for practical purposes by the following equation[27]

Eg = 1240λ

, (1)

8.0 kV ×80, 000 100 nm WD 7.3 mmSEINONE

Figure 2: SEM images of N, Cd-codoped TiO2 calcined at 450◦C.

a: pure-TiO2

b: Cd-TiO2

c: N-TiO2

d: N, Cd-TiO2

200 300 400 500 600 700 800

Wavelength (nm)

1.6

1.7

1.8

2

1.9

Abs

orpt

ion

abcd

Figure 3: DRS of photocatalysts: a: pure TiO2, b: Cd-doped TiO2,c: N-doped TiO2, and d: N, Cd-codoped TiO2.

where λ is the absorbance wavelength. The band gaps ofTiO2 catalysts were shown in Table 1. The Eg of pureTiO2, Cd-codoped, N-doped, and N, Cd-codoped TiO2

was 3.20 eV, 3.16 eV 3.15 eV, and 3.08 eV, respectively. Theband gaps of as-prepared photocatalysts were all smallerthan that of pure TiO2. The absorption edges of Cd-dopedTiO2 (392.4 nm), N-doped TiO2 (394 nm), and N, Cd-codoped TiO2 (402.6 nm) were larger than that of pure TiO2

(387.5 nm), which indicated that the observation of opticalbands in the visible range (400–550 nm) could be ascribedto doping. The absorption in the visible region might beinduced by a subband-gap transition corresponding to theexcitation from the valence band to the impurity band [9].The absorption of N, Cd-codoped TiO2 in the visible light(λ = 402.6 nm) was the maximum, indicating the codopingof N and Cd might have a synergistic effect on enhancing thephotocatalytic activity.

3.4. Surface Area Analysis. The photocatalytic activity ofphotocatalyst is relative to the number of active surfacesites, the surface properties of photocatalysts often have


Table 1: Absorption edges and the band gaps of samples.

Pure TiO2 Cd-TiO2 N-TiO2 N, Cd-TiO2

λ (nm) 387.5 392.4 394 402.6

Eg (eV) 3.20 3.16 3.15 3.08

important effects on their photocatalytic activity. The BETsurface area of N, Cd-TiO2 (102.67 m2/g) was also higherthan those of Cd-TiO2 (95.63 m2/g), N-TiO2 (81.90 m2/g),and bare TiO2 (69.81 m2/g). It could be concluded fromthe experimental results that element-doping inhabited thecrystal grain growth and particles aggregation. And theaddition of N and Cd played the synergistic effect inmodification the structure of TiO2. The doping led to thecrystal structure distortion, crystallinity reduction, and thespecific surface area increasing, which would enhance theeffective organic adsorption on the surface of catalyst. It alsomade the electronic migration from crystal inner to surfaceeasy. The crystal surface defects could inhibit the electron-hole pairs on the catalyst surface, which would result inthe photocatalytic activity increasing. It also indicated thatthe strong synergistic interaction of N and Cd appeared toplay an important role in driving the excellent photoactivityperformance of the N, Cd-TiO2, which could be seen inSection 3.6.

3.5. XPS Analysis. The XPS survey spectrum of the N, Cd-TiO2 was showed in Figure 4(a). XPS peaks showed thatthe N, Cd-TiO2 photocatalyst contained Ti, O, N, andCd elements and a trace amount of carbon. The presenceof carbon was ascribed to the residual carbon from theprecursor solution and the adventitious hydrocarbon fromthe XPS instrument itself.

Figure 4(b) showed the N1s XPS spectrum of the N, Cd-codoped TiO2. A peak appeared at 405.4 eV and a smallpeak appeared at 399.6 eV, which was ascribed to the Natoms from adventitious N–N, N–H, O–N, or N-containingorganic compounds adsorbed on the surface of TiO2 [5].This analysis indicated that N atoms were incorporated intothe TiO2 crystallattice under our experimental condition.Figure 4(c) showed the Cd 3d XPS spectra of N, Cd-TiO2. Apeak appeared at 405.3 eV and was ascribed to the Cd atoms(Cd 3d5/2) from Cd and CdCO3. The results showed that Cddid not incorporate into the TiO2 crystal lattice but existedin the form of CdCO3. It might also suggest that Cd wasgradually excluded from the Ti-O framework to the surfaceof titania, and hindered the anatase crystallites from growingin size.

3.6. Photocatalytic Activity. Figure 5 showed the relationshipbetween the degradation and the irradiation time for eachphotocatalyst during the photocatalytic degradation of MO.It indicated that there were obvious photocatalytic activitiesunder irradiation for all photocatalysts. Until irradiated for15 minutes, the degradation rates on the MO for bareTiO2, Cd-TiO2, N-TiO2, N, Cd-codoped TiO2 were 88%,95%, 97%, and 99%, respectively. And the degradation rate

corresponding to the N, Cd-codoped TiO2 catalyst wasthe highest. After being irradiated for no more than 30minutes, the degradation rates of as-prepared photocatalystsall almost reached 100% (pure TiO2, Cd-TiO2 N-TiO2, N,Cd-codoped TiO2 were 99.0%, 99.3%, 99.4%, and 99.6%,resp.). The enhanced photocatalytic performance showed N,Cd codoping played a synergistic role in the improvementof the photocatalytic properties of titania. It indicated thatelement codoping was an effective means to improve thephotocatalytic performance of photocatalysts.

4. Theoretical Analysis

4.1. Calculation Models and Methods. First-principles cal-culation based on DFT [28, 29] was performed to explorethe effect of dopants on the electronic structure and opticalproperties of photocatalysts. The anatase (101) surface wasmodeled with a periodically repeated slab. We considered apure TiO2 surface supercell containing 96 atoms, of dimen-sion 2 × 2 in the [101] and [010] directions, respectively,corresponding to a surface area of 10.89 × 7.55 A2 and thenumber of the atom layers was 4. It was named as 2 × 2− 4,as shown in Figure 6(a). The other model was created basedon it. To ensure interaction between the upper and lowerlayers be ignored, the vacuum thickness was set to 10 A.According to the experimental XPS results, one kind of defectsurface was considered, namely, substitutional N, adsorptiveCd model (Ti32CdO63N). The supercell of Ti32CdO63N wasshown in Figure 6(b). Side view of (a) the four-layer relaxedslab, which was used in surface properties calculations andlabeled Ti32O64, (b) Ti32CdO63N was simulated by replacingone oxygen atom with nitrogen atom and adding onecadmium atom on the surface of the supercell.

The calculations in our work have been carried outusing the well-tested CASTEP code [30, 31], which employsplanewave basis sets to treat valence electrons and pseu-dopotentials to approximate the potential field of ionic cores(including nuclei and tightly bond core electrons). Thegeneral gradient approximation (GGA) with PW91 func-tional [32] and ultrasoft pseudopotentials [33] were usedto describe the exchange-correlation effects and electron-ion interactions, respectively. N (2s2 2p3), O (2s2 2p4),Ti (3s2 3p6 3d2 4s2), and Cd (4d10 5s2) electrons wereconsidered as valence states, while the remaining electronswere kept frozen as core states. Pulay density hybrid methodwas used in energy calculations, convergence threshold forself-consistent field was set to 2.0 × 10−6 eV/atom. The k-point sampling of the Brillouin zone was set to 2 × 3 × 1.Fast Fourier change for 50×40×120. The Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm has been used for geom-etry optimizations and the atomic relaxation was carried outuntil all components of the residual forces were less than0.05 eV/A, the energy and the displacement tolerances wereset to 2.0 × 10−5 eV/atom and 2 × 10−3 nm, respectively.In calculation, the geometry models of TiO2 supercellwere optimized firstly, then their electronic structures andoptical properties were calculated. All the calculations wereperformed in the reciprocal space.


1200 1000 800 600 400 200 0−1

0

1

2

3

4

5

6

7

8

×105

O1s

Ti2

p3/2

-Ti2

p1/2

Cd

3d

OK

LL

N1s C

1sCou

nts

(S)

Binding energy (eV)

(a)

410 408 406 404 402 400 398 396 394

405.4 eV

399.6 eV

1800

2000

2200

2400

2600

2800

3000

3200

Cou

nts

(S)

Binding energy (eV)

(b)

Binding energy (eV)

415 410 405 400

1800

2000

2200

2400

2600

2800

3000

3200

Cou

nts

(S)

412 eV

405.3 eV

(c)

Figure 4: (a) The survey XPS spectra of N, Cd-TiO2, (b) N 1s XPS spectra of N, Cd-TiO2, (c) Cd 3d XPS spectra of N, Cd-TiO2.

4.2. Results and Discussions

4.2.1. Impurity Formation Energy and Structural Optimiza-tion. By optimizing the pure anatase TiO2 unit cell, the unitcell parameters were obtained as follows: a = b = 3.8174 A,c = 9.6950 A, dap = 2.0050 A, deq = 1.9540 A, and 2θ =155.917◦. They were in good agreement with experimentalresults [34]: a = b = 3.7848 A, c = 9.5124 A, dap = 1.9799 A,deq = 1.9338 A, and 2θ = 156.230◦. This result implied thatour calculations were reliable and believable.

In order to determine the stabilities of the doped systems,we calculated the formation energies (Ef ) of the doped sys-tems according to the following formula (1)

Ef = ETiO2:D +12EO2 + nETi − ETiO2 −

12EN2 − ECd, (2)

where ETiO2:D and ETiO2 were the total energy of N, Cdcodoped TiO2, pure TiO2 in the same size supercells. EN2 and

EO2 were the energy of N2 and O2 gas molecular, ECd and ETi

were the energy of bulk Cd and Ti metal, respectively, n wasthe number of titanium atoms replaced by Cadmium atomsin the doping system. The calculated results was shown inTable 2. According to the results, we discovered that the Ef ofTi32CdO63N was positive, which indicated that the synthesisof N, Cd codoped TiO2 required energy.

The crystal data of pure and N, Cd codoped TiO2 wereshown in Table 2 In Ti32CdO63N, the Ti–O bond length1.9585 A was longer than the original Ti–O one 1.9543 A.The Ti–N bond length 1.8976 A was shorter than the Ti–O calculated value 1.9585 A, due to the smaller radius of Ncompared to O2− The changes of O–Ti–O(N) bond anglein Ti32CdO63 N were noticeable compared to pure TiO2. Allthese factors could lead to higher dipole moments in TiO6octahedron. Sato et al. [35] found that the local internalfields due to the dipole moment of distorted octahedralpromote the charge separation in the very initial process of


Table 2: Formation energies and optimized structural parameters for pure and N, Cd codoped TiO2.

Ef (eV) Bond length (A) O–Ti–O(N) bond angle (◦)

Ti–O Ti–N —

Pure TiO2 — 1.9543 — 160.503 102.154

Ti32CdO63N 3.630 1.9585 1.8976 163.055 90.353

0 10 20 30 50400

0.2

0.4

0.6

0.8

1

Deg

rada

tion

Pure TiO2

Cd-TiO2

N-TiO2

N, Cd-TiO2

t/min

Figure 5: Photocatalytic degradation of MO by photocatalysts.

Ti atom

O atom

N atom

Cd atom

[101]

[010]

[101]

[

(a) (b)

Figure 6: The model of supercells: (a) Ti32O64, (b) Ti32CdO63N.

photoexcitation. The electron-hole pair can separate moreeasily and recombine slower, and the photocatalysts show abetter photocatalytic performance. This was one reason forN, Cd codoped TiO2 showed a higher activity.

4.2.2. Electronic Structure Analysis. To investigate the effect ofdopants on the electronic structure and properties of TiO2,

the band structure and the partial density of states (PDOS)of the anatase TiO2 were calculated. The band structure forthe bare TiO2 was showed in Figure 7(a) with the Fermienergy being 0 eV on the energy axis. And the PDOS forthe bare TiO2 was presented in Figure 7(b). The PDOS forthe bare TiO2 revealed that the bottom of conduction bands(CB) was mostly composed of Ti 3d states and the top ofvalence bands (VB) was dominated by O 2p states, whichcan be seen in Figure 7(b). It was showed in Figure 7(a)that the valence band maximum (VBM) and the conductionband minimum (CBM) located at G point, which indicatedthat bare TiO2 was a direct-gap semiconductor material. Theminimum gap between VBM and CBM (Eg) was 2.458 eV,which was lower than the experimental value of 3.20 eV.This underestimation of the energy gap was mainly dueto the well-known shortcoming of the exchange-correlationfunctional in describing excited states [36]. However, as akind of effective approximation, its relative calculation valuewas quite exact, and it did not affect theoretical analysis onelectronic structure analysis.

The band structure and PDOS of N, Cd-TiO2

(Ti32CdO63N, substitutional N, adsorptive Cd) werepresented in Figures 8(a) and 8(b), respectively. The bandstructure of TiO2 was modified by N and Cd codoping.The PDOS for the N, Cd-TiO2 revealed that the conductionband consisted of the Ti 3d states mainly, and the valencebands (VB) was dominated by O 2p, Ti 3d, and Cd 4d states,which can be seen in Figure 8(b). As showed in Figure 8(a),there were two kinds of impurity states, which came fromN 2p and Cd 5s, respectively, between valence bands andconduction bands. The impurity state from Cd 5s lied0.655 eV below the bottom of the conduction band, whichcould absorb smaller photon energy and achieved indirecttransition so that the light absorption of TiO2 extended tothe visible light area. While the impurity state from N 2p lied0.244 eV above the top of the valence band, which made itbecome a shallow acceptor level.

Since the shallow acceptor would be act as capture trapfor photoexcited electrons, the impurity states between VBMand CBM could reduce the recombination rate of photoex-cited carriers, which were very crucial for the enhancementof the photocatalysis efficiency. Compared to bare TiO2,both CBM and VBM of Ti32CdO63N shifted to the lowenergy level, which indicated that Ti32CdO63N had strongerredox ability than bare TiO2. The energy gap narrowed from2.458 eV to 2.225 eV, which made the electron transitingfrom valence band to conduction band become easy. All theabove results implied that nitrogen and cadmium codopingresulted in red shift of the optical absorption edge and couldgreatly enhance the photocatalytic activity of TiO2. It was


−1

−0.5

0

0.5

1

1.5

2

2.5

3

3.5

En

ergy

(eV

)

G Q Z GF

O 2p

Ti 3d

Eg = 2.458 eV

(a)

0

20

40

60

80

100

PD

OS

(sta

tes/

eV)

−8 −6 −4 −2 0 2 4 6

Energy (eV)

Ti 3d

O 2p

(b)

Figure 7: Ti32O64: (a) band structure, (b) partial density of state.

−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

N 2p

Cd 5s

Ti 3d

En

ergy

(eV

)

G Q Z GF

Eg = 2.225 eV

(a)

−10 −8 −6 −4 −2 0 2 4

Energy (eV)

20

40

60

80

100

0

PD

OS

(sta

tes/

eV)

O 2p

Ti 3d

Cd 5s

Cd 4d

N 2p

(b)

Figure 8: Ti32CdO63N: (a) band structure, (b) partial density of state.

consistent with the experimentally observed absorption of N,Cd-TiO2 in the visible region.

4.2.3. Optical Properties. In order to make the calculatedresults more correspond with the actual situation, in thecalculation of the optical properties using the “scissoroperators” fixed: 0.742 eV (the difference between band gapof measured and calculated). Photo absorption coefficientwere calculated with function polycrystalline and usingthe photo wavelength as the horizontal axis, the opticalabsorption coefficient as the vertical axis in this paper. Thecalculated optical absorption spectrum diagram was shownin Figure 9. It was showed in Figure 9 that the absorptioncoefficient of N, Cd-TiO2 were higher than that of the bareTiO2 in the wavelength region from 350 nm to 780 nm, andthe optical absorption edge shifted to long-wavelength range.The optical characteristics corresponded to their electronicstructures, modified by N and Cd codoping with the energy

gap reduction and introduction of impurity states betweenthe VBM and CBM. Theoretical calculation results couldaccount for the experimental observation.

5. Conclusions

Bare TiO2, Cd-doped, N-doped, and N, Cd-codoped TiO2

photocatalysts have been synthesized by thermal decom-position. The photocatalysts possessed an anatase crys-talline framework having particle sizes of 10–15 nm. Thecorresponding absorption edge was 387.5 nm, 392.4 nm,394.0 nm, and 402.6 nm, respectively, which indicated theyrepresented photocatalytic activities in the visible region. Natoms were incorporated into the crystallattice, while Cdatoms existed on the crystal surface of TiO2. The codopingof N and Cd might have a synergistic effect on enhanc-ing the photocatalytic activity of TiO2. The first-principlecalculations indicated the energy gap of N, Cd-codoped


200 300 400 500 600 700 8000

1

2

3

4

5

6×104

Abs

orpt

ion

coeffi

cien

t(c

m−1

)

Wavelength (nm)

Ti32CdO63NTi32O64

Figure 9: Absorption spectra of the pure and N, Cd-codopedanatase TiO2.

TiO2 became narrow and local internal fields of codopingenabled photoexcited electron-hole pair’s separation becamevery easy. Excitation from the impurity states of N 2p orCd 5s to the conduction band could account for the opticalabsorption edge shifted toward the low energy level, whichwas consistent with the experimentally observation. N, Cd-codoped TiO2 perform better photocatalytic activity in thevisible light region than the bare TiO2. The theoretical anal-ysis might provide a probable reference for the experimentalsynthesis of new photocatalysts in the future.

Acknowledgments

This work has been cosupported by the Outstanding Adult-Young Scientific Research Encouraging Foundation of Shan-dong Province (Grant no. 2008BS09016) and the ScientificResearch Program of Shandong Province Education Depart-ment (Grant no. J08LC55).

References


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Research Article

Nitrogen-Doped TiO2 Photocatalyst Prepared byMechanochemical Method: Doping Mechanisms and VisiblePhotoactivity of Pollutant Degradation

Yu-Chao Tang,1, 2 Xian-Huai Huang,1 Han-Qing Yu,2 and Li-Hua Tang1

1 Laboratory of Water Pollution Control and Wastewater Reuse, Department of Environmental Engineering,Anhui University of Architecture, Hefei 230022, China

2 School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China

Correspondence should be addressed to Yu-Chao Tang, [email protected]

Received 21 June 2011; Accepted 13 July 2011


Copyright © 2012 Yu-Chao Tang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nitrogen-doped TiO2 (N/TiO2) photocatalysts were prepared using a mechanochemical method with raw amorphous TiO2

as precursors and various nitrogenous compounds doses (NH4F, NH4HCO3, NH3·H2O, NH4COOCH3, and CH4N2O). Thephotocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetric-differential thermal analysis (TG-DTA), and UV-Vis diffuse reflection spectra (UV-Vis-DRS). Their photocatalytic activities wereevaluated with the degradation of p-nitrophenol and methyl orange under UV or sunlight irradiation. The catalysts had a strongvisible light absorption which correspond to doped nitrogen and consequent oxygen deficient. The results of photocatalytic activityshowed the visible light adsorption mechanisms, as the doped nitrogen species gave rise to a mid-gap level slightly above the topof the (O-2p) valence band, but not from the mixed band gap of the N-2p and O-2p electronic levels.

1. Introduction

Photocatalysis by titania (TiO2) has been demonstrated tobe an effective method for removal of pollutants in wateror air during the last three decades. Because of its band-gapenergy, however, pure TiO2 can only be activate by short-wavelength UV light below 387 nm, usually only about 4%of the incoming solar energy on the earth’s surface utilized[1]. Considerable efforts have been made to extend thephotoresponse of TiO2 further into the visible light regionby modifying or using doping technologies in recent years[2–5].

Doping TiO2 with nonmetal atoms such as nitrogen orcarbon has received increasing attention in recent years [6–13]. It was predicted by theory and demonstrated in experi-ments that nitrogen-doped TiO2 exhibits improved catalyticactivity under visible light irradiation [6, 7]. Asahi et al.[6] reported a nitrogen-doped TiO2 by sputtering the TiO2

target in a N2/Ar gas mixture, resulting in a significant shiftof the absorption edge and exhibited excel visible light

activity. Burda et al. [8] reported a nitrogen-doped TiO2 withnitrogen concentrations up to 8% by direct amination of6–10 nm TiO2 particle using triethylamine, and the dopedTiO2 was catalytically active and was able to absorb well intothe visible region up to 600 nm. Usually, the dope of thenitrogen leads to the narrowing of the band gap and im-proves the photocatalytic activity in the visible light region[14]. The nitrogen doping can be attained by various othermethods, such as the heating of TiO2 power in an ammoniaatmosphere [6, 15, 16], the hydrolysis of titanium com-pounds with ammonia solution followed by calcination ofthe resultant precipitates [17], the heating of TiO2 powderwith urea [18], reactive sputtering [19], and the pulsed laserdeposition using a TiN target in a nitrogen/oxygen gasmixture [20]. Development of modified N doped TiO2 pho-tocatalyst with metals, nonmetals, and metal oxides havebeen reviewed by Zhang et al. [21].

Mechanochemical method is a simple method which caneasily realize preparation of nitrogen doping on titania ona large scale. Although the nitrogen-doped TiO2 prepared


by mechanochemical method had been reported before [22,23], but little is known about the structural information andactivities of the catalyst, especially the doping mechanisms.Yin et al. [22] reported that the nitrogen-doped TiO2 by amechanochemical method possessed two absorption edgesof 405 and 550 nm. Chen et al. [23] reported a similarnitrogen-doped TiO2 using the mechanochemical methodin the NH3·H2O solution, but their samples showed a redshift by 20 nm and on one edge only. There should be furtherresearch on the structure and visible-light exciting processof the nitrogen-doped TiO2 prepared by mechanochemicalmethod.

In the present work, nitrogen-doped TiO2 was preparedby ball milling of amorphous TiO2 with various nitrogenoussubstances (e.g., ammoniac salt or ammonium solution)under various prepared conditions. The catalyst was char-acterized by X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), thermal gravimetric-differential ther-mal analysis (TG-DTA), and UV-Vis diffuse reflection spec-troscopy (UV-Vis-DRS). Nitrogen doping mechanism on theamorphous TiO2 and principle of the electron excitation byvisible light irradiation were demonstrated. In addition, itsphotocatalytic activities were tested using model pollutantsunder UV or visible light irradiation.

2. Experimental

2.1. Materials and Methods. TiO2 (CP) was purchasedfrom Shanghai Qianjin Chemical Reagent Company,China. Ammonia solution, NH4F, NH4HCO3, NH3·H2O,NH4COOCH3, and CH4N2O were from Shanghai SuyiChemical Reagent Company, China. Methyl orange, p-nitro-phenol, and phenol were purchased from Shantou XilongChemical Factory and Nanjing Chemical Reagent Factory,respectively. They were used without further purification. Allthe chemical reagents were of analytical grade except TiO2.

2.2. Catalyst Preparation. The preparation of N/TiO2 pho-tocatalyst was carried out in a ball mill (QM-1SP2, NanjingUniversity Instrumental Company). The chemical reagenttitanium dioxide was used as a raw material. Raw titaniumdioxide (about 16.0 g) and NH4F (0.0200 mol, 0.740 g) weremixed in the ball mill tank (100 mL) with 10 mL of deionizedwater. Twelve agate balls of 10.0 mm diameter and forty agateballs of 5.0 mm diameter were introduced. After grinding for180 min at a speed of 580 rpm, the wet powder was dried ata temperature of 105◦C in air for 5 h. The powder was thencalcined at 400◦C (or 500, 600, and 700◦C) for 2 h at 3◦C/minheating rate. During the calcination, the color of the powderchanged from white to gray or slightly yellow depending onthe ammonia salt and its concentration. In order to evaluatethe effect of concentration of ammonia salt on grinding, aseries of different NH4F concentrations (0.37 g, 1.48 g, and3.70 g) were used. The catalysts prepared using NH4F as thenitrogen source was denoted as the NF series.

For comparison, the NH4HCO3 (0.0200 mol, 1.58 g),NH3·H2O (m/V28%, 10.0 mL), NH4COOCH3 (0.0200 mol,1.54 g), and urea (CH4N2O, 0.0200 mol, 1.20 g) were also

used as nitrogen sources. They were denoted as the NC, NH,AA, and UR series, respectively. As a comparison, a raw TiO2

ground with 10.0 mL of deionized water, but no nitrogensource was also prepared. This was denoted as WG. A rawTiO2 was not ground and was denoted as TO.

2.3. Characterization of N/TiO2. X-ray diffraction (XRD)patterns were obtained using a diffraction spectroscopy witha Ni filter and graphite monochromator (X’Pert Pro model,Philips Inc.) at room temperatures. The X-ray source wasCu Ka radiation (λ = 1.54187 A) and a 2θ range from 20◦

to 80◦. The crystal size was determined from the diffractionpeak broadening using the Scherrer equation. High-puritysilicon powder was used as an internal standard to accountfor instrumental line broadening effects during crystal sizeestimation. UV-Visible diffuse reflection spectra (UV-Vis-DRS) were recorded by a UV-240 spectrometer (ShimadzuCo., Japan), and the scan range was from 200 to 800 nm witha 150 mm ϕ integral ball. Standard magnesium oxide wasused as the reference. X-ray photoelectron spectroscopy datawere obtained with an ESCALab250 electron spectrometer(Thermo-VG Scientific Ltd., UK) using 300 W Al Kα radia-tion. The base pressure was approximately 3×10−9 mbar. Thebinding energies were referenced to the C1s line at 284.6 eVfrom adventitious carbon. The thermogravimetric analysis(TG-DTA) was carried out using a differential scanning calo-rimeter (DT-250, Shimadzu, Japan) in air atmosphere witha flow rate of 50.0 mL/min. The scanned temperature rangewas from 100◦C to 700◦C with a heating rate of 10◦C/min.

2.4. Photocatalytic Activity Measurements. To evaluate thephotocatalytic activities of the nitrogen-doped TiO2 pow-ders, the photoreactivity experiments were carried out ina circular reactor containing 100 mg catalyst and 140 mLmethyl orange of 10.0 mg/L (or p-nitrophenol of 10.0 mg/L).Prior to irradiation, the suspensions were stirred in darkfor 15 min to ensure adsorption/desorption equilibrium.After equilibrium, an ultraviolet light lamp with 254 nmwavelength (Jiangyin Lamp Co., China) was used in thephotoreactor. At given irradiation time intervals, aliquots of4.0 mL were sampled and then filtered through a Milliporefilter with a pore size 0.45 μm to remove TiO2 particles.The filtrates were analyzed by recording the adsorption bandmaximum in the UV-Vis spectra of the substances using aspectrophotometer (751 UV-VIS, Shanghai Instrument Co.,China). A UV-light filter was used on the circular reactor toeliminate the ultraviolet when the experiment was carriedout under sunlight (outdoor sunlight in spring in eastern ofChina at northern latitude 31◦ and irradiation intensity wasin the range of 970–1420 μW/cm2).


3.1. XRD Analysis. Figure 1 showed the XRD patterns ofnitrogen-doped TiO2 (NF400, ground with 1.40 g of NH4F),compared nondoped TiO2 (WG400, ground with water),and the raw TiO2 (TO400, raw TiO2 was not treated byground). XRD patterns of these samples showed there were


TO400

20 30 40 50 60 70 80

NF400

GW400

AA

A

AA

A

AA

RR

R R

2θ (deg)

Figure 1: XRD patterns of the nitrogen-doped and nondopedTiO2 (sample ground with 1.40 g of NH4F (NF400), ground withwater (WG400), and the raw TiO2 (TO400); all three samples werecalcined at 400◦C for 2 h; A: anatase, R: rutile).

mixed anatase and rutile in all tested samples. The crystal sizedetermined from the diffraction peak broadening using theScherrer equations was found to be the same, and the crystalphases of the three photocatalysts were identical regardless ofthe prepared conditions. Comparison between WG400 andTO400 suggested the grinding process did not change thecrystal structure of the TiO2. Comparison between WG400and NF400 also implies that the additional ammonia saltdid not affect the crystal structure of TiO2 in the grindingprocess. Comparison between WG400 and TO400 suggestedthe grinding process did not change the crystal structure ofthe TiO2. Yin et al. [22] has reported the grinding processcan change the crystal structure during the milling of P25(a commercial TiO2) in dry condition, but Chen et al. [23]showed a different result in wet condition. Considering ourresults, these may indicate the grinding process in dry orwet was a crucial factor governing the crystal structure ofthe TiO2 prepared. In the wet process, the high mechanicalenergy can be readily transferred through water, but, in thedry process, the energy may easily be concentrated in thelocal region, which accelerates the phase transformationfrom anatase to rutile as the latter is a thermodynamicalstable morphology.

3.2. UV-Vis Diffuse Reflectance Spectra (UV-Vis-DRS). TheUV-Vis-DRS of the nitrogen-doped TiO2 prepared underdifferent conditions are shown in Figures 2–4. All the sampleshad an obvious visible absorption in the range 400–700 nm,but the absorptive intensities were different, depending onthe nitrogen sources used (Figure 2). The highest absorptiveintensities were obtained when milled in NH4F (NF400) andurea (UR400), but weaker in the samples of NH4OOCCH3

(AA400) and ammonia water (NH400), and the weakest inNH4HCO3 (NC400). Except for ammonia water, all othernitrogen sources had the same concentration of 2.00 mol/L(0.0200 mol salt in 10.0 mL water). The difference in absorp-tive intensity, therefore, might not only account for NH4

+

0

20

40

60

80

200 300 400 500 600 700 800

Wavelength (nm)

Refl

ecta

nce

(%)

NC400NH400AA400

NF400UR400

Figure 2: UV-Vis-DRS spectrum of the samples preparedin different nitrogen compounds (NF400:NH4F; NH400:NH3;NC400:NH4HCO3; AA400:NH4OOCCH3; UR400:CH4N2O).

concentration in the system but also the counterpart anions.In the urea sample, there was almost no free NH4

+ or NH3,but it got the similar doped results that suggested that thedoping process occurred not just adsorption of NH4

+ orNH3 on the TiO2 surface [22]. In the mechanical process,the high energy could produce the active surface on TiO2,which would react with nitrogenous substance directly toform chemical adsorbed unstable intermediate, and thisintermediate could form nitrogen doping in the subsequentthermal treatment. In the ammonia water system, the solu-tion pH was 14.0; therefore, the pH was not a crucial factorwhen nitrogen-doped TiO2 was prepared. Li et al. [24] hadreported a TiO2N photocatalyst prepared by treating TiO2

in NH3/Ethanol, and they considered that first dopingprocess was surface adsorption of NH3 molecules and thatnitridation consequently occurred by replacing the oxygenatom in the TiO2 with the nitrogen atom in the NH3 andresulting in the formation of the O–Ti–N and the N–Ti–Nbond. In the case of (NH2)2CO, adsorption of (NH2)2COon surface of TiO2 resulting in the formation of the dopednitrogen may demonstrat chemical adsorption occurringbetween Ti atom and N atom. Nitrogen atom has unpairedelectrons which may be easily adsorbed on Ti4+ by staticelectricity attraction. The most probably existing bond in thecase of (NH2)2CO system may be O2Ti–N–C, and this indi-cated the chemical adsorption occurring between Ti atomand N atom.

Figure 3 shows the UV-Vis-DRS of the samples milled inNH4F and subsequently thermally treated at 400, 500, and600◦C, respectively. When calcined at 500 and 600◦C, thevisible absorption was significantly reduced, indicating thatthe doped nitrogen in TiO2 was not very stable under hightemperatures. A similar result was reported for the catalystprepared by other methods [25]. When thermally treatedat high temperatures, N, O, and Ti atoms can obtain highenergies, and the doped nitrogen (in form of O2Ti–N–Hand/or O2Ti–N, shown in Scheme 1) will be destroyed and


HO

O O

+

+

Heat

Heat

O2

O2

Ti +

+

NH3Mechanochem

H H

H

N

O O

O

O

••Ti

HeatO2

O

O

Ti

Ti

NH

H

H HO

HO

N

O

O

Ti N

O O

+

+

Heat

Heat

O2

O2

Ti +

+

Mechanochem

H C

C

H

N

O O

O

O

••Ti

HeatO2

O

O

Ti

Ti

NH

H

H HO

HO

N

O

O

Ti N

H

H

N

CO2

(a) (b)

Scheme 1: Doping mechanism of nitrogen on titanium dioxide.

Wavelength (nm)

NF600NF500NF400

0

20

40

60

80

200 300 400 500 600 700 800

Refl

ecta

nce

(%)

Figure 3: Effect of calcined temperature on the UV-Vis-DRSspectrum (NF400, NF500, or NF600 represent the NF series catalystcalcined at 400, 500, or 600◦C, resp.).

leave unpaired electrons in the lattice of anatase TiO2. Asthe visible absorption originated from the nitrogen impuritylevel or nitrogen mixed level [26–28], the absorption wasweak when the nitrogen doping was destroyed.

The UV-Vis-DRS of the samples milled at different NH4Fconcentrations is shown in Figure 4. A higher absorbancewas observed for the greater concentrations of NH4F. Com-parison between the NF400-0 samples and sample BLANK(TO400), which was the raw TiO2 directly calcined withoutmilling, found that the UV-Vis-DRS of the two samples werealmost the same. This suggested the mechanical treatmentof TiO2 without nitrogen resources substance would notaffect the visible absorbance of TiO2, and this give the firmerevidence that the visible absorbance of nitrogen-doped TiO2

originated from nitrogen doping.

BLANKNF-0NF-37

NF-74NF-148NF-370

0

20

40

60

80

200 300 400 500 600 700 800

Refl

ecta

nce

(%)

Wavelength (nm)

Figure 4: Effect of NH4F concentration on the UV-Vis-DRS ofthe NF series catalysts (NF-0, NF-37, NF-74, NF-148, and NF-370represent the NF series catalyst calcined in the presence of NH4F;the dosages of NH4F were 0, 0.37, 0.74, 1.48, and 3.70 g, resp.; thesample BLANK was the raw TiO2 directly calcined without milling(TO400)).

For a nitrogen-doped TiO2 sample prepared by othermethods and calculated by the spin-polarized plane wavepseudopotential method, its absorption in the visible lightregion was primarily located between 400 and 500 nm, butTiO2 with the oxygen deficient was above 500 nm [29]. Thephotoabsorption above 500 nm for the milled and thermallytreated TiO2 samples might be originated from oxygendeficiency. For the milled TiO2 (NF400), the active surfaceon TiO2 prepared in the mechanical process could react withnitrogen source substances (in most cases NH4

+ or NH3,except (NH)2CO) to form a reductive intermediate, whichwould inevitably be oxygen deficient on titania in the thermal


1300

1600

1900

2200

2500

Binding energy (eV)

393396399402405408

Cou

nts

(s)

(a)

Binding energy (eV)

6786816846876906936961.4E+04

1.6E+04

1.8E+04

2E+04

2.2E+04

Cou

nts

(s)

(b)

Figure 5: XPS spectra of NF400, (a) N1s and (b) F1s.

process. This might account for the visible light absorptionabove 500 nm. It is noticed that there were no clear onsetof visible light absorption for most samples, and this mayindicate that the light absorption in the visible range maybe mixed with the function of nitrogen doping and oxygendeficient.

3.3. XPS Measurement. The X-ray photoelectron spectro-scopy of NF400, which was calcined for 2 h at 400◦C aftermilled for 3 h in NH4F, is shown in Figure 5. Except forthe titanium and oxygen, a small fraction of nitrogen waslocated at 402.0 eV and 399.5 eV, respectively. This confirmsthat after mechanochemical treatment nitrogen was dopedinto TiO2. The binding energy of 402.0 eV is attributed tomolecular chemical-adsorbed N [6, 29]. It was previouslyreported there were two peaks of chemisorbed N2 moleculeon the TiO2 surface around 400.0 eV and 402.0 eV [6, 26].Previous studies had demonstrated that the binding energyof N1s is located at about 396.0 eV and is attributed tosubstitutional doped nitrogen (N− or N2−), which is in theproximity to the typical binding energy of Ti–N [9, 26, 30–32]. The absence of this peak suggests that there was littlesubstitution doped nitrogen in the sample in the formationof N–Ti–N but that other nitrogen-doped forms must exist[33]. Diwald et al. reported the binding energy of 399.6 eVwas found in the NH3 gas-thermal-treated TiO2 and it wasattributed to the N–H bond [34]. However, the bindingenergy of 399.6 eV of NF400 in our work may be attributedto the N in form of interstitial nitrogen either in N–H orbare N atom form. For the NF400, the precursor had N–H bond and the subsequent thermal treatment temperaturewas sufficiently high to form Ti–N–H bond and can breakN–H bond at even high temperature. The binding energyof 399.5 eV of NF400, therefore, might be attributed to theinterstitial nitrogen bond to Ti, and the structure of thedoped nitrogen in the TiO2 might be as O2Ti–N–H or O2Ti–N corresponding to treated temperature. We also monitoredbinding energy of N1s of the sample UR400 and found it alsolocated at 399.5 eV (Figure 6). These suggested the doping

2000

2100

2200

2300

2400

2500

2600

387390393396399402405

Binding energy (eV)

cou

nts

(s)

Figure 6: XPS spectra of N1s of UR400.

mechanism and the existing nitrogen in the titania dopedby various nitrogen substances were the same. These resultsfurther confirmed the doping process of mechanochemicalmethod was initiated between Ti and N, and the existingformation of nitrogen had no relation with the nitrogen sub-stances.

In the milling process, the active TiO2 surface wasable to react with N atom, but the O atom could not besubstituted by the N atom directly. The unstable nitrogenousintermediate can be transformed by thermal treatment.At high temperature, the hydrogen can react with oxygenand left O2Ti–N between the inter lattice of TiO2. Whentreated at very higher temperature, Ti–N bond would breakand oxidation of nitrogen by O2 happened. The probablydoping mechanism of nitrogen on titanium dioxide underexperimental conditions was shown in Scheme 1.

A small fraction of fluorine located at 684.8 eV was alsofound. It was originated from surface fluoride formed byligand exchange between F− and surface hydroxyl group on


10

10.15

10.3

10.45

10.6

10.75

10.9

(mg)

−0.0004

−0.0003

−0.0002

−0.0001

0

0.0001

0.0002

(mg/

s)

0 150 300 450 600 750

T (◦C)

(a)

−1

0

1

2

3

4

(uV

)

0 150 300 450 600 750

T (◦C)

(b)

Figure 7: TG (a) and DTA (b) of the nitrogen-doped TiO2 (NH400).

TiO2:≡Ti–OH + F− →≡Ti–F + OH− [35]. No sign of dopedfluorine in the lattice of TiO2 was found with binding energycorresponding to 688.5 eV [35, 36]. This suggests the fluorineion could react with Ti4+ through ligand exchange to forma surface fluorinated substance, but not lattice substitutionor interstitial doping. It was also observed that the bindingenergy of Ti and O in the NF400 was 459.0 eV and 530.3 eV,respectively, (not shown). These values were substantiallydifferent from those for pure TiO2, that is, 458.3 eV and531 eV, respectively. The binding energy of Ti4+ in the UR400corresponding to 458.3 eV was very close to that for pureTiO2. This implies the form of Ti and O in the NF400was significantly changed by the chemadsorbed fluorine ion,rather than by the doped nitrogen ion. As the most reactiveatom, the fluorine ion had a considerable effect on theelectrical chemical environment of Ti4+ through electrostaticaction and increasing the binding.

3.4. TG-DTA Analysis. Figure 7 showed the results of DT-TGA measurement of the nitrogen-doped TiO2 (using theprecursor of NH400). Significant weight loss appeared in therange of 300–500◦C, this indicate chemadsorbed unstableintermediate can be destroyed under high temperature. Thestrongest weight loss appeared in the range of 340–360◦C,and an exothermic band appears at which coincidently. Thismeans that the exothermic effect was because intermediateoxidation on TiO2 and water escape from matrix. Ihara et al.[17] reported exothermic reaction start at about 380◦C couldbe due to the oxidation reaction of NH3 or NH2 whichare bonded coordinately onto Lewis acid site with the oxygenreleased from amorphous grain-boundaries by formingoxygen deficient sites. A weak weight loss peak appears in therange of 440–450◦C, and a weak exothermic band appears, atwhich coincidently again, this indicate the hydrogen can bedestroyed under this temperature and formed in a gaseoussubstance. In the heating process, doped nitrogen substancecan react with oxygen (oxygen in air but in the lattice of

TiO2) to form oxidative substance and escape from TiO2

surface.

3.5. Photocatalytic Activity. The photocatalytic degradationresults of p-nitrophenol under ultraviolet and sunlight usingNF400, NF500, NF600, and NF700 as catalysts are shown inFigure 8. The NF400 gave a little higher activity regardlessof the irradiation light treatment. In order to understandhow the doped-nitrogen and oxygen deficiency affected thephotocatalytic activity of the catalyst, a comparison wasmade for the activities under the identical reaction condi-tions, also using the raw TiO2 catalysts which were calcinatedat 400, 500, 600, and 700◦C but not milled. A comparisonamong the four raw TiO2 catalysts (without nitrogen doping)showed the same activities (data not shown). This suggeststhe heating temperature did not affect the photo activity ofthe catalysts and that the higher activity of NF400 was at-tributed to the doped nitrogen, rather than treatment tem-perature. Since the doped nitrogen was not stable, the hightemperature treatment resulted in the loss of nitrogen inTiO2. The NF400 was able to absorb visible light efficiently,but its visible light activity was very low, when consideringthe same performance of the catalyst under UV light. Thisindicates the absorbed visible light by the doped nitrogen or(and) oxygen deficiency could not be efficiently used in thephotocatalytic reaction under the experimental conditions.These results also imply that the visible light absorbancemechanisms may be associated with the doped nitrogenspecies, which could give rise to a mid-gap level slightly abovethe top of the (O-2p) valence band, rather than the mixedband gap by N-2p and O-2p [37]. The electron excited onthe mid-gap level has a lower oxidative ability as comparedto that excited on O 2p [9, 15, 31, 38]. If the visible lightabsorbance is excited on the mixed band gap, the excitedelectron would have same oxidative ability, regardless fromUV light or visible light. The NF400 should have photocat-alytic activity under sunlight, as it had a great visible lightabsorptive ability.


NF400NF500

NF600NF700

0

2

4

6

8

10

C(m

g/L

)

0 15 30 45 60


(a)

NF400NF500

NF600NF700

60 90 120 150 180 210 2400 30


0

2

4

6

8

10

C(m

g/L

)

(b)

Figure 8: Photocatalytic degradation of p-nitrophenol (PNP) using NF400, NF500, NF600, and NF700 as catalysts under (a) ultraviolet and(b) sunlight.

UR400UR500

UR600UR700

0

2

4

6

8

10

C(m

g/L)

0 15 30 45 60


(a)

UR400UR500

UR600UR700

0

2

4

6

8

10

C(m

g/L)

0 15 30 45 60


(b)

Figure 9: Photocatalytic degradation of (a) p-nitrophenol and (b) methyl orange under ultraviolet using UR400, UR500, UR600, and UR700as catalysts.

The photocatalytic degradation results of p-nitrophenoland methyl orange under ultraviolet using UR400, UR500,UR600, and UR700 as catalysts are shown in Figure 9. Thephotocatalytic activities of the four catalysts were of almostthe same levels, when either p-nitrophenol or methyl orangewas used as the model pollutant. These results suggest thatthe photo activity of the nitrogen-doped TiO2 was related tothe prepared conditions.

Figure 10 shows the photocatalytic degradation of methylorange under ultraviolet light using NF400, NF500, NF600,and NF700 as catalysts. The photoactivities of the NF seriescatalysts for methyl orange degradation were very similar tothose of the NF series catalysts for p-nitrophenol degradationunder UV light. The activities of the NF series catalysts

were in order of NF400 > NF500 > NF600 ≈ NF700, formethyl orange or p-nitrophenol degradation. A comparisonbetween this result and those for the UR series catalystsshowed a very different activity order. This indicates that thephoto activities of the nitrogen-doped TiO2 prepared usingthe mechanochemical methods were highly related to thenitrogen sources. In milling process, the active surface of theTiO2 might react with the nitrogen sources. This was con-firmed by the UV-Vis-DRS data shown before.

The photocatalytic activities of NF400, NC400, NH400,AA400, and UR400 under ultraviolet light using p-nitro-phenol as a model pollutant are illustrated in Figure 11. Thephoto activities of the NF400, AA400, and UR400 were ofsimilar levels, but the NH400 had the lowest photocatalytic


0

4

6

8

10

NF400

NF500

NF600NF700

15 30 45 600


2

C(m

g/L

)

Figure 10: Photocatalytic degradation of methyl orange underultraviolet light using NF400, NF500, NF600, and NF700 ascatalysts.


30 45 600 15

UR400NF400AA400

NC400NH400

0

2

4

6

8

10

C(m

g/L)

Figure 11: Comparison of photocatalytic activities of NF400,NC400, NH400, UR400, and AA400 under ultraviolet light usingp-nitrophenol as model pollutant.

activity, attributed to the alkaline conditions in the ball mill-ing process. The lower photoactivity of NC400 was likelyto be associated with the strong adsorption of HCO3

− ontothe active site on TiO2 surface. HCO3

− had very strong ad-sorptive ability on the TiO2, causing a blocking of the activesite and decreasing the photocatalytic activity of degradationstearic acid [39].

The photocatalytic degradation of methyl orange underultraviolet shown in Figure 12 indicates that after the me-chanochemical process, the activity of all the nitrogen-dopedcatalysts was reduced. Comparison between the activitiesof NF400-0 and the BLANK suggests that the ball millingtreatment was the reason for the decreased activity. Thisimplies that the TiO2 surface structure was destroyed by ball


0

2

4

6

8

10

C(m

g/L

)

0 15 30 45 60

NF400-0NF400-37NF400-74

NF400-148BLANK

Figure 12: Effect of NH4F concentration on photocatalytic degra-dation activity of methyl orange under ultraviolet light. (NF400-0,NF400-37, NF400-74, and NF400-48 which represent the qualitiesof NH4F in milling process were 0, 0.37, 0.74, and 1.48 g, resp.;BLANK represents the raw titania directly calcined at 400◦C withoutmilling (TO400).)

milling treatment. Although the visible light absorption wasachieved through the ball milling method, the photocatalyticactivity of the TiO2 was reduced.

4. Conclusions

The nitrogen-doped TiO2 was prepared with mechanochem-ical methods using wet ball milling of raw amorphous TiO2

in different nitrogen compounds. Characterization of thecatalysts demonstrated that the nitrogen-doped TiO2 couldimprove visible light adsorption efficiency. Such an improve-ment was attributed to the fact that the mixed function of thedoped nitrogen and oxygen deficient, and the former speciesgave rise to a mid-gap level slightly above the top of the(O-2p) valence band of TiO2 and latter absorbed the lightwavelength above than 500 nm. However, the TiO2 surfacestructure was destroyed by ball milling treatment, resultingin a reduced photocatalytic activity.

Acknowledgments

This work was supported by the Natural Science Foundationof China (NSFC, no. 50908001) and the Excellent YouthScience and Technology Foundation of Anhui Province (no.10040606Y29). The authors express their sincere thanks toDr. C. Oubre of Rice University for reviewing this paper.

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Research Article

AgBr-Coupled TiO2: A Visible Heterostructured Photocatalystfor Degrading Dye Pollutants

Jianjun Liu, Yingchun Yu, Zhixin Liu, Shengli Zuo, and Baoshan Li

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Correspondence should be addressed to Jianjun Liu, [email protected]

Received 18 May 2011; Accepted 7 June 2011


Copyright © 2012 Jianjun Liu et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A series of AgBr/TiO2 visible photocatalysts with heterojunction structure was synthesized using Ti(OC4H9)4, KBr, and AgNO3 asprecursors. The phase composition, particle morphology and size, microstructures, and absorbance of these photocatalysts werecharacterized by X-ray diffraction, transmission electron microscope (TEM), high-resolution TEM, and UV-vis spectra. It wasfound that the coupled AgBr/TiO2 was an effective photocatalyst to degrade the methylene blue under visible light irradiation,compared with the other noncoupled photocatalysts of AgBr, AgBr/P25, and P25. The photocatalytic activities of AgBr/TiO2

increase first and then decrease with increasing the mass ratio of mAgNO3/mTiO2 and the photocatalyst with the mass ratio of

3.35 has the highest photocatalytic activity. The results showed that the coupled photocatalyst has the particle size of about 15 nmwith homogeneous dispersion and has the strongest absorption in whole UV-vis light region (250∼800 nm) originated from thesynergetic effect of heterostructured AgBr/TiO2. The coupled AgBr/TiO2 photocatalyst can keep stable photocatalytic activity afterfive-circle runs.

1. Introduction

In recent years, the photocatalysis technology for the puri-fication of contaminated water and the remediation ofpolluted air has attracted significant attention due to itsadvantages of low cost, being environment-friendly, and highefficiency [1, 2]. Among many photocatalysts, titania hasbeen widely studied for its wide application in photocatalysis,solar cells, and hydrogen production because of its nontox-icity, stability in aqueous solution, and no photocorrosion[3]. However, most applications so far are limited to UV lightirradiation because the light absorption edge of pure titaniais less than 380 nm. Moreover, using UV light is expensive,and its content in sunlight accounts for less than 5% [4, 5].Therefore, attempts have been made to increase the TiO2

efficiency by doping with transition metal ions and nonmetalelements, by modification with other semiconductors or bycovering the surface with photosensitizers such as organicdyes and inorganic light absorbers to enhance the opticalabsorption in the visible range. The dye molecules as sensi-tizer can be easily desorbed, resulting in the deterioration ofphotocatalytic activity during the reaction process, and even

degradation of themselves by titania photocatalyst underillumination [6, 7].

It is well known that silver bromide with a band gap of2.6 eV is an inorganic photosensitive semiconductor material[8], which has high photographic sensitivity in the visiblelight region and can be used to modify titania to have visiblelight activity. Hu et al. [9, 10] reported that AgBr/TiO2 visiblelight photocatalysts prepared by deposition-precipitationmethod can efficiently destruct azodyes and bacteria. Zanget al. [11] synthesized an AgBr/TiO2 catalyst and studied itsphotocatalytic activity under simulated sunlight irradiation.In their study, the influence of AgBr content in catalyst andthe incident light intensity on the degradation of methylorange (MO) was also investigated. Li et al. [12] synthe-sized an AgI/TiO2 composite photocatalyst with core/shellnanostructures that exhibited higher photocatalytic activityin the visible region due to its strong light absorption and lowrecombination rate of the electron-hole pairs. Recently, it hasdrawn much attention to form heterojunction photocatalystto enhance the photoactivity of semiconductor catalyst. Theconstruction of heterostructured photocatalysts can promotethe separation of photoexcited electron-hole pairs through


various carrier-transfer pathways and extend the light-respo-nse range by coupling suitable electronic structures in two ormulticomponent semiconductors [13]. Robert [14] reviewedthe TiO2 heterostructured photocatalysts modified by MxOy

and MxOy nanoparticles. Zhang et al. [15] synthesized AgBr-Ag-Bi2WO6 nanojunction photocatalysts with multi-visible-light active components. Ag can improve the separation ofelectron-hole pairs when it was used as electron-transfersystem. The catalyst AgBr-Ag-Bi2WO6 showed high photo-catalytic activity for the degradation of Procion Red MX-5Band pentachlorophenol.

In this study, a nanosized AgBr/TiO2 heterojunction-coupled photocatalyst was synthesized by double-reactionroute using Ti(OC4H9)4 as the precursor. AgBr/TiO2 wasinvestigated in terms of its structure and activity compared toAgBr/P25 which was synthesized by simply depositing AgBron P25 TiO2 through single-reaction route. The influence ofAgBr content on AgBr/TiO2 photoactivity and the cycling lifeof the photocatalyst were also studied.

2. Experimental

Tetrabutyl titanate Ti(OC4H9)4 (Beijing Xingjin ChemicalCo., >98.5%) was used as a titanium precursor in the sol-gel process. Other chemicals and solvents were of analyticalgrade and were used without further purification.

2.1. Preparation of Photocatalysts

2.1.1. Synthesis of AgBr/TiO2 by Double-Reaction Route. AgBrand TiO2 sol precursors were prepared as follows: AgBrsol was obtained by adding 1.0192 g polyvinyl pyrrolidone(PVP) to 0.03 mol/L KBr (200 mL) solution under magneticstirring. Then 200 mL AgNO3 with the same molar concen-tration was added slowly to the solution above, and a lightyellow AgBr sol was obtained. A titania sol was preparedby hydrolyzing tetrabutyl titanate (Ti-(OBu)4) in anhydrousethanol. The mixture of Ti-(OBu)4 and acetylacetone (vol-ume ratio was 5 : 2) [16] was added into an appropriateamount of ethanol and stirred for half an hour to obtain abright yellow titania sol. Then the titania sol was added intothe AgBr sol under vigorous stirring, refluxing at 80◦C for15 hours, during which TiO2 sol precursor was hydrolyzedslowly and deposited on the surface of AgBr nanoparticles.The formed precipitate was separated by centrifugationfollowed by washing with distilled water and ethanol forseveral times. Finally, the as-prepared powder was dried at70◦C and calcined in air at 500◦C for 3 hours to get theAgBr/TiO2 compound photocatalyst. A series of AgBr/TiO2

photocatalysts were prepared by changing the mass ratio ofAgBr to TiO2 (mAgNO3

/mTiO2 = 1, 2, 3, 3.35, 4).

2.1.2. Synthesis of AgBr/P25 by Single-Reaction Route. For thepurpose of comparison, AgBr/P25 composite photocatalystwas also prepared according to the procedure in the previousstudy [9]. A certain amount of P25 TiO2 (Germany’s DegussaCorporation) was dispersed in 200 mL distilled water andstirred for 30 min, then an appropriate amount of KBr and

20 40 60 80

h

g

f

e

d

c

b

a

2θ (◦)

Inte

nsi

ty(a

.u.)

AgBrAnataseRutile

Figure 1: XRD patterns of AgBr (a), TiO2 (b), and AgBr/TiO2 withdifferent mass ratios of mAgNO3

/mTiO2 (c) 1; (d) 2; (e) 3; (f) 3.35 (g)3.5; (h) 4.

PVP was added into it, followed by stirring for another30 min to dissolve all the chemicals adequately. After that,0.03 mol/L AgNO3(200 mL) was added into the mixturesolution. The next step was the same as the synthesis ofAgBr/TiO2. At last, AgBr/P25 photocatalyst with mass ratioof mAgNO3

/mP25 = 3.35 was obtained.

2.2. Characterization of Photocatalysts. X-ray powder diffrac-tion (XRD) patterns were recorded on Rigaku D/MAX-2500 VB2 diffractometer, operated at 40 kV, 200 mA witha Cu target (λ = 1.5406 A). Particle morphology andsize were observed by a transmission electron microscope(TEM) (Hitachi 800) at an accelerating voltage of 200 kV.Microstructures were observed by a high-resolution trans-mission electron microscope (HRTEM) (JEOL-3010) at anaccelerating voltage of 300 kV. The UV-visible absorptionspectra were carried out on a TU-1901(Beijing PurkinjeGeneral Instrument Co., Ltd.) double-beam UV-Vis spec-trophotometer. X-ray photoelectron spectroscopy (XPS)spectra were acquired by a Thermo ESCALAB 250 X-rayphotoelectron spectrometer. Mg Kα radiation was selected asthe X-ray source.

2.3. Evaluation of Photocatalytic Activity. A 500 W Xe lampwas used as the visible light source, and the visible wave-length was controlled through a 420 nm cut filter (LF420,China), which was hanged in a dark box and kept at about15 cm above the liquid surface. Aqueous suspensions ofmethyl blue (MB) dye (100 mL, with an initial concentrationof 10 mg/L) and photocatalyst powder (1.5 g/L) were placedin a beaker. Before irradiation, the suspensions were mag-netically stirred for 30 min under dark condition to estab-lish an adsorption/desorption equilibrium between the dye


170.00 nm

(a)

abto6-6 200 kV ×100000 170.00 nm

(b)

200 kVabto-10-11 90.00 nm200000

(c)

200 kV 100000abto7-5 170.00 nm

(d)

0.288 nm

TiO2

AgBr

0.352 nm

3 nm

(e)

Figure 2: TEM images with different mass ratios of AgBr/P25 with mAgNO3/mP25 (a) 3.35; and AgBr/TiO2 with mAgNO3

/mTiO2 (b) 1; (c) 2; (d)3.35; (e) HRTEM image corresponding to (d).

solutions and the surface of photocatalysts. Under stirring,a small amount of suspension (about 5 mL) was taken outevery 10 min under irradiation and then centrifuged andanalyzed using a 752 spectrophotometer (made in China).The photocatalytic activity of the synthesized photocatalystswas evaluated by plotting the absorbance-time curve andcalculating the specific absorbance of C/C0.


3.1. XRD Characterization. The XRD patterns of AgBr, TiO2,and AgBr/TiO2 synthesized with different mass ratios wereshown in Figure 1. The as-synthesized TiO2 is a mixed crystalphases of anatase and rutile. The contents of the two phaseswere calculated according to the XRD data [17] and givenin Table 1. The phase composition of as-synthesized TiO2 isdenoted as WA (70.8%) and WR (29.2%) as shown in Table 1(Sample b). But in the AgBr/TiO2 system, the content ofanatase increases initially and then decreases with increasingAgBr content which indicates that the excess Br ions maystabilize the metastable anatase phase [18] and be beneficialto the increase of anatase phase up to mAgNO3

/mTiO2 = 3.35,where the anatase phase reaches its maximum content.Besides, there are no any diffraction peaks of metallic Ag in

all the AgBr/TiO2 systems indicating that the fresh catalystsare stable after calcinations and other procedures.

3.2. TEM Characterization. The microstructures of AgBr/TiO2 composite photocatalyst synthesized with differentmass ratios were characterized by TEM and HRTEM asshown in Figure 2. It can be seen that AgBr/P25 shows anagglomerated mixture of AgBr with black color and P25 withlight color (Figure 2(a)). The similar agglomerated mixturecan also be found in Figure 2(b) which shows that the blackAgBr crystallite is about 30∼ 50 nm, and the light TiO2

crystallites agglomerate obviously and are even smaller thanseveral nanometers when the mass ratio is mAgNO3

/mTiO2 = 1(Figure 2(b)). Consequently, while mAgNO3

/mTiO2 = 2, a cou-pled structure of AgBr/TiO2 is formed where it shows a one-to-one corresponding relation of the deep and light coloredparticles in homogeneous dispersion. However, there are afew lonely TiO2 particles because some AgBr particles cannotbe matched with their counterpart of TiO2. In Figure 2(d),the coupled structure of AgBr/TiO2 with smaller size of about15 nm is formed homogeneously and dispersed very well asshown by the composite particles of deep AgBr and lightTiO2 in color. The HRTEM image (Figure 2(e)) of the samplefurther demonstrates that the lattice spacing of the two fixed


Table 1: Synthesis condition, phase composition, and crystalline size of AgBr/TiO2 photocatalysts.

Sample No. AgNO3/TiO2 (wt)Phase composition of TiO2

Anatase/% Crystalline size/nm∗ Rutile/% Crystalline size/nm∗ C/C0

a AgBr — — — — 0.378

b TiO2 70.8 20 29.2 27 0.738

c 1 13.9 28 86.1 36 0.272

d 2 66.7 23 33.3 40 0.073

e 3 86.9 15 13.1 38 0.041

f 3.35 97.1 18 2.9 41 0.029

g 3.5 75 17 25 38 0.076

h 4 87.4 18 12.6 43 0.128

200 300 400 500 600 700 8000

0.2

0.4

0.6

0.8

1

1.2

1.4

e

d

c

b

Abs

orba

nce

Wavelength (nm)

a

Figure 3: UV-Vis spectra of different samples. (a) P25; (b) TiO2; (c)AgBr; (d) AgBr/P25; (e) AgBr/TiO2.

components was determined to be 0.288 nm and 0.352 nm,which is in good agreement with the values for the AgBr(200) plane [JCPDF No. 06-0438] and for the anatase TiO2

(101) plane [JCPDF No. 21–1272], respectively. It was alsoobserved that the close contact between TiO2 and AgBr leadsto the formation of heterojunction microstructure whicheffectively promotes the transfer of photoelectron betweenthe crystal boundary of AgBr and TiO2 [13].

3.3. UV-Vis Characterization. Figure 3 shows the UV-visspectra of different samples. It is seen that P25 (Figure 3(a))and TiO2 (Figure 3(b)) have little or no absorption inthe visible light region (>400 nm), which accords with itsintrinsic absorption properties. However, AgBr has strongabsorption both in UV and visible light that should beassigned to surface plasmon resonance absorption of silverproduced by the photoreduction of AgBr [19]. The compos-ite photocatalysts of AgBr/P25 (Figure 3(d)) and AgBr/TiO2

(Figure 3(e)) have strong visible light absorption, and theabsorbance of AgBr/P25 composite photocatalyst is similar

200 300 400 500 600 700 8000

0.2

0.4

0.6

0.8

1

1.2

e

dA

bsor

ban

ce

Wavelength (nm)

c

b

a

f

Figure 4: UV-vis spectra of AgBr/TiO2 with different mass ratios ofmAgNO3

/mTiO2 (a) 1; (b) 2; (c) 3; (d) 3.35; (e) 3.5; (f) 4.

to the pure AgBr (Figure 3(c)), indicating that the visiblelight absorption of AgBr/P25 is only due to the AgBr withoutforming the coupled structures between them. This wasconfirmed by the TEM photo of AgBr/P25 in Figure 2(a).AgBr/TiO2 (Figure 3(e)) exhibits much higher visible lightabsorbance than AgBr/P25 (Figure 3(d)). It is reasonable thatthe formed heterojunction microstructure between AgBrand TiO2 can construct a channel that is beneficial for theelectron transfer and ion diffusion, and this can promote thepossible doping of Ag+ in TiO2 lattice, leading to the strongvisible light absorption [15, 20].

As shown in Figure 4, AgBr/TiO2 with different massratios of AgNO3 and TiO2 was characterized by the UV-vis spectra. It can be clearly seen that the AgBr/TiO2

has the strongest absorption in the visible region whenmAgNO3

/mTiO2 = 3.35. This result was also related to theformation of special microstructures of this sample includingsmaller particle size, homogeneous dispersion, and inter-face matching semiconductor heterojunction microstructuredemonstrated by TEM and HRTEM (Figure 2).


0 10 20 30 40 50 60 70 80

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

f

e

d

b

c

a

Abs

orba

nce

Reaction time (min)

Figure 5: Photodegradation curves of methylene blue using differ-ent samples (a) P25; (b) TiO2; (c) AgBr; (d) AgBr/P25 (mAgNO3

/mTiO2 = 3.35); (e) AgBr/TiO2 (mAgNO3

/mTiO2 = 3.35); (f) blank.

0 10 20 30 40 50 60 70 80

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Abs

orba

nce

Reaction time (min)

−10

g

c

d

e

f

b

a

Figure 6: Photodegradation curves of methylene blue for AgBr/TiO2 using different mass ratios of mAgNO3

/mTiO2: (a) 1; (b) 2; (c)3; (d) 3.35; (e) 3.5; (f) 4; (g) blank.

3.4. Evaluation of Photocatalytic Activity

3.4.1. Evaluation before and after Combination. Figure 5shows the photodegradation curves of methylene blue (MB)for different samples. The order of visible light activity is asfollows: AgBr/TiO2>AgBr > AgBr/P25 > P25 > TiO2. Gene-rally, TiO2 has almost no visible light activity, and it canonly be activated under UV light irradiation because of itslarge band gap of 3.0∼3.2 eV. The samples of P25 and as-synthesized TiO2 show only little degradation originatedfrom the photolysis of MB. AgBr can be excited by visiblelight and generate electron-hole pairs to exhibit somephotocatalytic activity for degrading MB [8, 19]. However,AgBr/P25 is only the simple mixture of active AgBr andinactive TiO2 which shows poorer photocatalytic activitythan single AgBr. The photocatalytic activity of AgBr/TiO2

with homogeneous coupled structure leading to the strongest

0

20

40

60

80

100

1st 2nd 3rd 4th 5th

Order of runs

Con

vers

ion

ofM

B(%

)

Figure 7: Cycling runs for the photodegradation of methylene blueon AgBr/TiO2 (mAgNO3

/mTiO2 = 3.35).

visible light absorption is higher than AgBr/P25 in theform of the agglomerated mixture. These results suggestthat there is a synergetic effect between the components ofAgBr and TiO2 in the coupled AgBr/TiO2 compared to thesimple mixture of them [12]. The photogenerated electronexcited by narrow band gap semiconductor of AgBr undervisible light irradiation could be directly injected to the TiO2

via the coupled structure, resulting in effective separationof photogenerated electron and hole and enhancing thephotocatalytic activity.

3.4.2. Effect of AgBr Content in AgBr/TiO2 Photocatalyst. Inorder to determine the optimal AgBr content in catalyst,a series of AgBr/TiO2 photocatalysts with different massratios (mAgNO3

/mTiO2 ) were prepared, and their photocat-alytic activities on degradation of MB were compared.It is found that the photocatalytic activity of AgBr/TiO2

increases gradually to maximum with increasing the massratio of mAgNO3

/mTiO2 up to 3.35 and then decreases asshown in Figure 6. The sequence is consistent with thecontent of anatase in AgBr/TiO2 as showed in Table 1implying that the higher content of anatase contributes tothe better photocatalytic activity, which is in accordance witha previous study [21]. Moreover, the homogeneously dis-persed coupled heterojunction microstructures were formedwhile mAgNO3

/mTiO2 = 3.35 which have stronger absorptionin the whole visible region and are beneficial to the furtherimprovement of photocatalytic activity. The decrease ofphotocatalytic activity with further increasing the contentof AgBr in AgBr/TiO2 may result from the fact that theexcessive AgBr particles shorten the average distance betweenthe trapping centers of photogenerated electrons and holesso as to increase the probability of recombination rate ofelectron-hole pairs [9]. More studies are needed to furtherunderstand the interaction mechanism of AgBr content inAgBr/TiO2 photocatalyst.


380 375 370 365 360

373.6 eV

373.3 eV

367.4 eV

Ag 3d5/2

Binding energy (eV)

Ag 3d3/2

367.7 eV

I

II

(a)

80 78 76 74 72 70 68 66 64

Binding energy (eV)

Br 3d

68.3 eVI

II

(b)

Figure 8: XPS spectra of Ag 3d (a) and Br3d (b) for AgBr/TiO2 (mAgNO3/mTiO2 = 3.35). I: before reaction, II: after the 5th reaction cycle.

3.5. Catalyst Stability. The stability of AgBr/TiO2 catalystwith coupled structure is shown in Figure 7 for the degra-dation of MB under visible light irradiation. AgBr/TiO2

was easily recycled by simple filtration without any furthertreatment in these experiments. The photocatalytic activitydid not decrease significantly in the degradation MB afterfive successive cycles under visible irradiation. The high-resolution XPS spectra of the Ag3d and Br3d regions for freshAgBr/TiO2 photocatalyst and AgBr/TiO2 after the 5th runare shown in Figure 8. The Ag 3d5/2 and Ag 3d3/2 peaks areidentified at 367.7 and 373.6 eV before reaction which areattributed to Ag+ in AgBr, and they slightly shifted to low-energy side at 367.4 and 373.3 eV, respectively, after reaction.Probably the Ag+ is interacting with some electronegativeelements of MB dye or intermediate molecules adsorbed onthe surface of photocatalyst after reaction, resulting in a smalldistortion of electron density of Ag+, leading to its small shifttoward lower values [22, 23]. Moreover, the peak of Br 3d at68.3 eV is due to the crystal lattice of Br in AgBr [24]. Theseresults confirm that the heterostructure between AgBr andTiO2 might inhibit silver atoms from forming larger clustersand prevent AgBr decomposition.

Generally speaking, AgBr with high photosensitivityis unstable under light irradiation and can be photode-composed into a metal cluster of Ag through the photo-graphic process. However, once the heterojunction structurebetween AgBr and TiO2 formed, most of the photoexcitedelectrons can be transferred from the conduction bandof AgBr to that of TiO2 that inhibits the reduction ofAgBr and promotes the stabilization of AgBr under lightirradiation [11]. The stable photocatalytic performance ofthe coupled AgBr-TiO2 heterostructures indicates that theAgBr/TiO2 photocatalyst has a better application potential inwastewater treatment using solar energy. However, furtherwork is required to elucidate the mechanism for AgBrstability.

4. Conclusions

Using AgBr as the inorganic photosensitive material, aseries of AgBr/TiO2 composite photocatalysts and AgBr/P25were synthesized by double- and single-reaction procedure.The photocatalytic activity of AgBr/TiO2 synthesized usingdouble-reaction procedure is higher than that of AgBr/P25prepared by the single-reaction procedure. The order ofphotocatalytic activity of the above photocatalysts undervisible light irradiation is as follows: AgBr/TiO2 > AgBr >AgBr/P25 > P25 > TiO2, which is consistent with the orderof the absorbance in UV-vis spectra. While increasing AgBrcontent in the AgBr/TiO2 photocatalyst, the phase contentsof anatase increase initially and then decrease, which isin accordance with the sequence of photocatalytic activity.When the mass ratio mAgNO3

/mTiO2 equals 3.35, the good-matching semiconductor heterojunction microstructures areformed between the interfaces of AgBr and TiO2, whichobviously enhances the absorption of visible light andcontributes to the best photocatalytic activity.

Acknowledgment

The authors gratefully acknowledge the financial supportfrom the National Natural Science Foundation of Chinaunder Grant no. 10972025.

References


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Research Article

Photocatalytic Degradation of Phenolics byN-Doped Mesoporous Titania under Solar Radiation

Priti A. Mangrulkar,1 Sanjay P. Kamble,2 Meenal M. Joshi,1 Jyotsna S. Meshram,3

Nitin K. Labhsetwar,1 and Sadhana S. Rayalu1

1 Environmental Materials Unit, National Environmental Engineering Research Institute (CSIR-NEERI), Nagpur 440 020, India2 Chemical Engineering and Process Development Division, National Chemical Laboratory (NCL), Pashan Road, Pune 411008, India3 P.G. Department of Chemistry, Rashtrasant Tukadoji Maharaj, Nagpur University, Nagpur 440 033, India

Correspondence should be addressed to Sadhana S. Rayalu, s [email protected]

Received 6 January 2011; Accepted 5 March 2011


Copyright © 2012 Priti A. Mangrulkar et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

In this study, nitrogen-doped mesoporous titania was synthesized by templating method using chitosan. This biopolymer chitosanplays the dual role of acting as a template (which imparts mesoporosity) and precursor for nitrogen. BET-SA, XRD, UV-DRS,SEM, and FTIR were used to characterize the photocatalyst. The doping of nitrogen into TiO2 lattice and its state was substantiatedand measured by XPS. The photocatalytic activity of the prepared N-doped mesoporous titania for phenol and o-chlorophenoldegradation was investigated under solar and artificial radiation. The rate of photocatalytic degradation was observed to be higherfor o-chlorophenol than that of phenol. The photodegradation of o-chlorophenol was 98.62% and 72.2%, while in case of phenol,degradation to the tune of 69.25% and 30.58% was achieved in solar and artificial radiation. The effect of various operatingparameters, namely, catalyst loading, pH, initial concentration and the effect of coexisting ions on the rate of photocatalyticdegradation were studied in detail.

1. Introduction

Photocatalysis is a rapidly evolving and efficient technologyfor purification of water. It plays a vital role in degradingseveral toxic organic pollutants into less toxic or nontoxicand safe compounds by their partial and/or by their completemineralization. Photocatalytic degradation process can bedefined broadly as an aqueous phase oxidation process,which is based primarily on the attack of the hydroxylradical, resulting in the destruction of the target pollutantor contaminant compound. Phenol and its derivatives havebeen listed by EPA as priority pollutants [1]. Recently, pho-tocatalytic treatment of organic contaminants over titaniumdioxide has been reviewed by Gaya and Abdullah [2]. It wasreported that TiO2 is the best photocatalyst among all thephotocatalysts. Moreover, TiO2 has been the most widelyused photocatalyst owing to its nontoxicity, high stability,and its cost effectiveness. But there are few drawbacksassociated with it like conventional TiO2 which has very

low surface area (55 m2/g for Degussa P-25 TiO2) and lowporosity and it can be activated only by irradiating withultraviolet light (wide band gap energies, 3.0 eV for rutileand 3.2 eV for anatase) which is around 4–8% of solarradiation that reaches the earth surface. It is therefore ofutmost importance to make TiO2 active in visible region.Sensitization of TiO2 by metal and nonmetal doping is oneof the attempts in this direction. Nitrogen-doped titania hasproved to be an effective means to extend the light absorptionof titania from UV to visible region as in case of nitrogendoped titania there is substitution of lattice oxygen withnitrogen which tends to lower the band gap [3–6]. In recentyears several efforts have been made to improve the surfacearea of titania by synthesizing mesoporous titania whichpossess high surface area and porosity. The preparationof mesoporous titania is reported by using a templatingmethod. Templates are soft like surfactant or block polymeror they could be hard templates like porous silica and carbon[7–10]. Biological templates like insect wings, plant leaves,


bacterial threads, and so forth, have also been reported forthe synthesis of mesoporous titania [11–13]. Mesoporoustitania with visible light absorption ability are promisingas efficient photocatalysts. However, the usual preparationroute of mesoporous titania and nitrogen doping is different,hence it is difficult to synthesize nitrogen-doped mesoporoustitania by a simple route. Synthesizing mesoporous titaniawith nonmetal doping like nitrogen to enhance visible lightactivity seems to be promising area of research. Recently,one step-template-free solvothermal method to synthesizeN-doped mesoporous titania microspheres was reported byChi et al. [14], whereby mesoporous spheres were formedby crystallite aggregation from controlled alcoholysis oftitania precursor and then reacting urea with titania to dopenitrogen in mesostructure. Horikawa et al. [15] also reportedthe synthesis of N-doped mesoporous titania by supercriticaldrying technique.

In continuation with our previous work wherein we hadreported the photo reduction of methyl orange dye by N-doped mesoporous titania, herein we investigated the photooxidation properties of the synthesized photocatalyst. Thephotocatalyst has shown promising activity in phenol and o-chlorophenol photodegradation under visible light.

2. Experimental

2.1. Materials. All the reagents used for experimental studieswere of analytical reagent grade. Methanol, acetic acid,ammonia, phenol, o-chlorophenol, sodium chloride, sodiumcarbonate, and sodium bicarbonate were obtained fromMerck India ltd. Chitosan was procured from ChemchitoIndia. Titanium isopropoxide was obtained from AcrosIndia ltd. The stock solutions of phenol and o-chlorophenolwere prepared in deionized water obtained from Milliporewaters progard2 purification system. Plain solar intensity wasmeasured in Watts per square meter using Lutron LX-102light meter working on photocell principle.

2.2. Method. All the experiments were carried out in acylindrical borosilicate glass reactor of 500 cm3capacity(Figure 1). The reactor was assembled with centrallymounted sparger surrounded by cooling coil. The sameexperimental setup was employed in the present investiga-tion which was previously used by Kamble et al. [16]. Thetemperature of the reaction mixture was maintained nearambient by passing water through the cooling coil. 400 cm3

of solution (phenol/o-chlorophenol) was taken in the reactorand a known weight of the photocatalyst was added to thereactor. The solution was stirred in dark before exposing thereactor to concentrated sunlight from a compound parabolicreflector in order to find out the substrate adsorption effect.The concentration of phenol or o-chlorophenol in thisfiltered sample was treated as the zero time concentration(Ct = 0) in each experiment before exposure to radiation. Airwas sparged in the reactor through central sparger in order tokeep the photocatalyst particles in uniform suspension. Theposition of the reflector was changed after a fixed intervalof time with respect to the position of sunlight in order

Parabolic reflector

Three neck slurry reactor

Water outWater in

Air sparger

Figure 1: Experimental setup of the solar reactor for photocatalyticdegradation experiment.

to maintain a constant band of light around the reactor.Samples were withdrawn at regular intervals of time for theiranalysis.

2.3. Analysis. All the samples were withdrawn using asyringe, centrifuged, and then were immediately filteredthrough 0.45 μ cellulose nitrate membranes. The phe-nol and o-chlorophenol concentration was measured onKnauer HPLC system equipped with Eurospher 100-10C-18 column. The wavelength for detection of phenoland o-chlorophenol was 270 nm and 274 nm, respectively.The mobile phase used for analysis was methanol : water(50 : 50) v/v% and the flow rate was 1.2 cm3 min−1. Theelution time for phenol and o-chlorophenol was 4.2 and8.3 min, respectively. The solar experiments were carried outin the months of January 2008 to April 2008 in Nagpur, India(79◦09′N, 21◦09′E). During this period, the sky was brilliantblue, and the average solar intensity was approximately(±2%) constant at 950 Wm−2 as measured at the groundlevel. The experiments in artificial radiations were alsocarried out during the same period using Tungsten lamp(500 W).

2.4. Synthesis of N-Doped Mesoporous Titania. The synthe-sis of N-doped mesoporous titania has been previouslyreported by us and detailed elsewhere [17]. The ratio ofchitosan and titania was changed to obtain mesoporoustitania with different nitrogen content namely, N-dopedM-TiO2 (1 : 1) and N-doped M-TiO2 (1 : 2) and N-dopedM-TiO2 (1 : 3). The photocatalytic activity of all the threesynthesized photocatalysts was evaluated for o-chlorophenoldegradation. The % degradation of o-chlorophenol undersolar radiation is as shown in Figure 2. It can be seen thathighest percentage degradation was observed in case of N-doped M-TiO2 (1 : 2) followed by N-doped M-TiO2 (1 : 3)and N-doped M-TiO2 (1 : 1). The visible light activity ofthese photocatalysts is attributed to the nitrogen doping.The nitrogen partially replaces oxygen sites in TiO2. Thisreplacement causes oxygen vacancies and amount of Ti3+

(TiO2 is reduced). These defects on the surface or in thebulk suppresses recombination reaction. The presence ofoptimal nitrogen content is crucial because excessive oxygen


0

20

40

60

80

100

N-doped mesotitania (1 : 3)



Ph

otod

egra

dati

on o

f o-c

hlo

roph

enol

(%

)

Figure 2: Comparison of the percentage degradation of o-chlorophenol by N-doped mesoporous titania (1 : 1), N-dopedmesoporous titania (1 : 2) and N-doped mesoporous titania (1 : 3).

2500

2000

1500

500

1000

020 40 60 80

2θ (deg)

Inte

nsi

ty(c

ps)

Figure 3: XRD spectra of N-doped mesoporous titania.

vacancies and Ti3+ may act as recombination centre. Thus,further studies were carried out by N-doped M-TiO2 (1 : 2).


3.1. Characterization of N-Doped Mesoporous Titania. TheBET surface area of the synthesized photocatalyst wasdetermined by specific surface area analyzer (ASAP 2000,micrometics) with nitrogen gas as the adsorbate. The surfacearea of N-doped mesoporous titania was found to be132.36 m2/g which is significantly higher as compared to theconventional titania Degussa P-25 (55 m2/g). The pore sizewas determined by BJH method and the pore size was found

to be 49.68 Á. It can be clearly seen that the photocatalyst ismesoporous in nature and possesses high surface area.

Powder XRD diffraction patterns of N-doped meso-porous titania were carried out on Rigaku Miniflex II desktopX-ray diffractometer with CuKα radiation, as shown inFigure 3. The scanning range of 2θ was set to between 20◦

and 80◦. It can be seen from the X-ray diffraction patternsthat all peaks can be indexed as the anatase phase (25.6◦,37.08◦, 48◦, 55.4◦, and 63.32◦) of titania. The low angle peaks

(1◦–3◦) characteristic of ordered mesoporous structure werenot observed [18].

The Fourier Transform Infrared (FTIR) analysis wascarried on Bruker Vertex-70 by Diffuse Reflectance Accessorytechnique. The FTIR spectra of N-doped mesoporous titaniaare as shown in Figure 4. The peaks corresponding to N–Hstretching (3690.73 cm−1) and NH2 (1573.12 cm−1) indicatethe presence of nitrogen in the synthesized photocatalyst.The peak at 2918.44 cm−1 is attributed to =C–H2 stretchingvibrations, while the peak at 1676.52 cm−1 corresponds toC=C stretching vibrations.

Diffuse reflectance UV-Vis spectra of the synthesizedphotocatalyst was recorded using a Perkin Elmer spectropho-tometer lambda 900 equipped with an integrating sphere.BaSO4 was used as a reference material. The spectrumis as shown in Figure 5. The absorption maxima of N-doped mesoporous titania was found to be 466.66 nm, whichcorresponds to the visible range of the spectrum indicatingthat N-doped mesoporous titania is active in visible light.The prominent red shift of the photocatalyst as comparedto Degussa P-25 TiO2 can be attributed to the presenceof nitrogen. The band gap energy was calculated from theabsorption maxima by using following equation:

Eg (eV) = 1240/λg nm (1)

(see [19]), where λg is absorption maxima. The band gapenergy was found to be 2.65 eV.

Chemical composition analysis using energy dispersiveX-ray (EDX) spectroscopy is as shown in Figure 6. From theEDX spectra the nitrogen content of N-doped mesoporoustitania was found to be 0.72 mass percent.

The morphology of the sample was investigated byscanning electron microscopy (SEM). Figure 7 shows theSEM images of N-doped mesoporous titania with differentmagnifications. As it can be seen from the images, N-doped mesoporous titania particles exhibit an irregularmorphology. The particle size for synthesized mesoporoustitanai was measured by using CILAS-1180 instrument andit was observed to be 3.450 μm.

The X-ray photoelectron spectroscopy measurementswere carried out on VG Microtech ESCA 3000 instrumentat a pressure of >1 × 10−9 Torr. The general scan, C1s,O1s, N1s, and Ti2p core level spectra were recorded withnon-monochromatized Mg Kα radiation (photon energy1253.6 eV) at a pass energy of 50 eV and an electron takeoffangle (angle between electron emission direction and surfaceplane) of 55◦. The overall resolution was 0.2 eV for XPS mea-surements. The core level spectra were background-correctedusing the Shirley algorithm, and chemically distinct specieswere resolved using nonlinear least-squares fitting procedure.Baseline correction and peak fitting for the sample was doneusing the software package XPS peak 41. The core levelbinding energies (BEs) were aligned with respect to the C1s

binding energy of 285 eV.The doping of nitrogen into TiO2 lattice, and its state was

substantiated and measured by XPS. Figure 8 illustrates theO1s, Ti2p, N1s, and C1s spectra of N-doped mesoporous tita-nia (1 : 2). In N1s XPS spectra two signals at BE = 394.2 eV


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=C-H2 stretching

C=C

Figure 4: FTIR spectra of N-doped mesoporous titania.

500400300250 8007006000.10.2

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Figure 5: UV-Vis spectra of N-doped mesoporous titania.

and 400.9 eV can be observed. The peak at 394.2 eV belongsto βN state, and it generally attributes to the presence ofTi–N bond whereby the N atoms replace the oxygen atomsin TiO2 lattice, while the second peak at 400.9 eV may bedue to molecularly adsorbed nitrogen containing compoundon the surface. The spectra obtained are quite consistentwith those observed by Horikawa et al. [15]. The presenceof titania ion in an octahedral environment is indicated bymajor peak which is centred at 462.3 eV in the Ti2p spectra.There is a single major peak that is observed at 534.5 eV incase of O1s, which corresponds to Ti–O bond. In case of Cspectra two major peaks are seen at 285.6 eV and 289.8 eVwhich are indicative of the presence of C–O and C=O bond.The characteristic peak at 281 eV which corresponds to Ti–C

0 1 2 3 4 5 6 7 8 9 10

(keV)

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Figure 6: EDX spectra of N-doped mesoporous titania.

bond was not observed which suggests that carbon atoms donot replace the oxygen atoms in the TiO2 lattice.

Comparison of N-Doped Mesoporous Titania with DegussaP-25 TiO2. The preliminary photodegradation experimentswere carried out under identical set of conditions under


Acc.V

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Magn

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Figure 7: SEM of N-doped mesoporous titania.

solar radiation using commercially available TiO2 (DegussaP-25) and N-doped mesoporous titania. The photocatalyticdegradation activities of this nondoped TiO2 (Degussa P-25) were then compared with those of N-doped mesoporoustitania. It was observed that N-doped mesoporous titaniahas significantly higher activity for both phenol as well aso-chlorophenol as compared to Degussa P-25. The com-parison of N-doped mesoporous titania with Degussa P-25is as shown in Figure 9: It was observed that 28.59 mg ofphenol degraded/g of TiO2 in case of N-doped mesoporoustitania as compared to 24.75 mg phenol degraded/g ofTiO2 for Degussa P-25 in solar radiation, while 40.71 mgo-chlorophenol degraded/g of TiO2 in case of N-dopedmesoporous titania as compared to 24.75 mg o-chlorophenoldegraded/g of TiO2 for Degussa P-25. The difference in thephotodegradation activities in case of o-chlorophenol forTiO2 and N-doped TiO2 is significant as compared to phenol.

(1) In case of N-doped mesoporous titania, there is anintense absorption in the visible light range and thereis a red shift in the case of N-doped TiO2, resulting inthe generation of more photogenerated electrons andholes participating in the photocatalytic reactionsunder visible light. In case of o-chlorophenol thearyl–Cl bond, is a weak bond and it is cleaved withrelative ease which further facilitates the attack of•OH radical and hence o-chlorophenol is degraded toa greater extent. In case of phenol, as it is resonancestabilized, the photodegradation is to a lesser extent.

(2) Adsorption of the substrate on the photocatalyst has amajor role in its photocatalytic degradation. Accord-ing to the mechanism suggested, the attack of •OHradicals takes place on the adsorbed substrate [20].The equilibrium adsorption studies were carried outfor both phenol and o-chlorophenol. In case of dark


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Figure 8: XPS of N-doped mesoporous titania.

adsorption studies, 50 mL (phenol/o-chlorophenol)were mixed with both TiO2 and N-doped TiO2 andshaken for 24 h for equilibrium. The adsorption of o-chlorophenol was 36% on N-doped as compared to15% for TiO2 resulting in more photodegradation.

3.2. Effect of Various Parameters on

Photocatalytic Degradation

3.2.1. Effect of Catalyst Loading. The effect of catalyst loadingon photocatalytic degradation of phenol and o-chlorophenolby N-doped mesoporous titania was studied. The cata-lyst loading was varied from 0.1% w/v to 0.3% w/v. Thephotocatalytic degradation of phenol and o-chlorophenolis as shown in Figures 10(a) and 10(b). It can be seenthat initially the photocatalytic degradation for both phenoland o-chlorophenol increases linearly as the catalyst loadingis increased from 0.1% w/v to 0.2% w/v but when thecatalyst loading is increased further to 0.3% w/v, the rate

of degradation decreases in case of both phenol and o-chlorophenol. However, the increase in degradation rate ismarginal for phenol as compared to o-chlorophenol. Thedecrease in degradation rate with increase in catalyst loadingcan be attributed to the increased opacity of the solutionwhich hinders the light transmission through the solution.As a result of decreased effective light intensity, the photogeneration of electrons and positive holes would be reducedand then the rate of photocatalytic degradation is alsoreduced.

3.2.2. Effect of Initial Concentration. Effect of initial con-centration on the rate of photocatalytic degradation wasstudied at different initial concentrations of phenol and o-chlorophenol and is as shown in Figures 11(a) and 11(b). Itwas found that with the increase in initial concentration ofphenol and o-chlorophenol the rate of degradation decreases.This is because with the increase in concentration theamount of organic species to be degraded increases but the


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otod

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onin

mg/

gof

TiO

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Figure 9: Comparison of N-doped mesoporous titania withDegussa P-25 for photodegradation of o-chlorophenol and phenol.

rate of generation of OH radical, photocatalyst loading, dis-solved oxygen concentration, intensity of illumination andillumination time are constant during the photodegradation.Hence with increase in initial concentration the rate ofphotocatalytic degradation decreases.

3.2.3. Effect of Initial pH. The pH of the solution is an impor-tant factor which affects the degradation rate. The effect ofinitial pH of solution on the photocatalytic degradation ofphenol and o-chlorophenol by N-doped mesoporous titaniawas studied. The pH of initial solution was varied from 3to 11. In acidic range the pH was varied using 0.1 M HClwhile in alkaline range the pH was varied using 0.1 M NaOH.The rate of degradation for both phenol and o-chlorophenolis as shown in Figures 12(a) and 12(b). It can be seenthat in case of both phenol and o-chlorophenol, maximumdegradation was found to be at pH 7. In both acidic as wellas alkaline ranges the rate of degradation decreases. Thiscan be explained from the fact that TiO2 is amphoteric innature and the zero point charge of TiO2 is at pH-6. Atacidic pH, the surface of TiO2 is positively charged while itis negatively charged at alkaline pH. In case of both phenoland o-chlorophenol the photocatalytic degradation decreasesin acidic pH as the adsorption is less at acidic pH, whileat alkaline pH, the negatively charged phenolate ions arerepelled by the negatively charged surface of TiO2 and hencephotocatalytic degradation decreases.

3.2.4. Effect of Coexisting Anions. The industrial effluentmay contain several salts at different concentrations. Thesesalts may exert negative or positive influence on the rate ofphotocatalytic degradation. The effect of various ions likechloride, carbonate, and bicarbonate on the photocatalyticdegradation of phenol and o-chlorophenol were studied byadding 0.1 M solution of their salts to 50 mgL−1 of phenoland o-chlorophenol. The effect of the presence of coexisting

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Figure 10: Effect of catalyst loading on photocatalytic degradation:(a) phenol, (b) o-chlorophenol.

anions on photocatalytic degradation of phenol and o-chlorophenol is shown in Figures 13(a) and 13(b). It can beclearly seen from these figures that the coions have a detri-mental effect on the photocatalytic degradation. In case ofphenol, the photocatalytic degradation in the absence of saltswas 69.5%, while the photocatalytic degradation in case ofsodium chloride, sodium carbonate, and sodium bicarbon-ate was 49.72%, 31.14%, and 35.98%, respectively. Similarly,in case of o-chlorophenol the photocatalytic degradation in


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50 ppm

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Figure 11: Effect of initial concentration on photocatalytic degra-dation: (a) phenol, (b) o-chlorophenol.

the absence of salts was 98.62%, while the photocatalyticdegradation in case of sodium chloride, sodium carbonate,and sodium bicarbonate was 87.09%, 79.34%, and 81.90%,respectively. It can be seen that the presence of chloride,carbonate, and bicarbonate has substantial detrimental effecton photocatalytic degradation of phenolics. The negativeeffect of these anions is attributed to following two reasons:(i) change in pH after addition of salts in aqueous solutionof substrate (from acidic to alkaline).This contributes toinhibition of photocatalytic degradation of phenol and o-chlorophenol (Figures 12(a) and 12(b) also show the sametrend.). (ii) Carbonate and bicarbonate anions basically act

00

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Figure 12: Effect of initial pH on photocatalytic degradation (a)phenol (b) o-chlorophenol.

as hydroxyl radical scavengers, which affect photocatalyticdegradation of phenol and o-chlorophenol. The decrease inthe degradation rate in case of NaCl may be attributed to thepoor adsorption.

3.3. Identification of Intermediates. The intermediatesformed during the photocatalytic degradation of phenol ando-chlorophenol were identified by High Pressure Liquid Chr-omatograph. The intermediates formed in case of photo-catalytic degradation of phenol were catechol (1 to 8 mg/L),hydroquinol (0.2 to 0.7 mg/L), and benzoquinone (0.5to 2 mg/L), while in case of o-chlorophenol the major


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Figure 13: Effect of presence of coexisting ions on photocatalyticdegradation (a) phenol (b) o-chlorophenol.

intermediates formed were chlorohydroquinone (concentra-tion ranges from 0.2 to 0.5 mg/L), catechol concentrationranges from 1 to 16 mg/L), and hydroxyhydroquinol concen-tration ranges from (0.2 to 0.7 mg/L).

3.4. Possible Reaction Mechanism. The reaction taking placeat the surface of the photocatalyst is a redox reaction. Thephotocatalytic degradation of phenol and o-chlorophenoltakes place via •OH radical attack. Several intermediatesare formed during the process of degradation. Generally,after the attachment of two OH groups, the aromatic ringbreaks. The subsequent products are rapidly oxidized to

CO2 and H2O, as evidenced by the absence of aliphaticmoieties [15]. In case of phenol, the •OH radical mainlyattacks the ortho and para position of benzene ring and hencecatechol and hydroquinol were the major intermediatesformed. Generally, intermediates with 2 or at the most 3 OHgroups were observed after which the cleavage of ring takesplace.

In case of o-chlorophenol, the two main reaction path-ways are ortho addition resulting into formation of catecholand hydroxyhydroquinol and para addition which results inthe formation of chlorohydroquinol as the major intermedi-ates.

4. Comparative Evaluation ofPhotodegradation of Phenolics

In general the photocatalytic degradation rate of o-chlorophenol is higher as compared to phenol and may beattributed to the following.

(1) The adsorption of o-chlorophenol is more as com-pared to phenol which in turn leads to high pho-todegradation rate.

(2) The aryl–Cl bond is a weak bond and it is cleavedwith relative ease which further facilitates the attackof •OH radical.

5. Mechanistic Aspects of the PhotocatalyticDegradation of Phenolics

A possible mechanistic pathway illustrating the varioussequence of events taking place at the semiconductor pho-tocatalyst is as follows.

(1) Irradiation of the semiconductor photocatalyst leadsto the generation of electron/hole pair which caneither recombine or dissociate to give a conductionband electron and valence band hole.

(2) Oxygen that is sparged is omnipresent on the surfaceof the catalyst and acts as an electron acceptorwhereas OH− groups and H2O molecules availableas electron donors interact with the photogeneratedhole to yield highly reactive •OH radicals.

(3) The phenol molecule reacts with the highly reactivehydroxyl radical and according to the substitutionrule the •OH radical attacks the ortho and paraposition of the aromatic ring due to the existing OHgroup on it.

(4) The major intermediates in case of phenol arecatechol and hydroquinol.

(5) After the addition of 2-3 OH groups on the aromaticring, the cleavage of ring takes place leading to furthermineralization.

(6) In case of o-chlorophenol, the •OH radical attacks atortho position leading to the formation of hydroxy-hydroquinol, and the cleavage of aryl–Cl and further


addition of •OH radical lead to the formation ofchlorohydroquinol.

(7) Subsequent reactions further lead to mineralizationof intermediates into carbondioxide and water.

6. Conclusions

In the present study, we have explored the photo-oxidationproperties of N-doped mesoporous titania in photodegra-dation of phenolics under visible light. It was observedthat the catalyst shows significant activity in the solarradiation as compared to artificial light. It was seen thatinitial pH and coexisting ions have a significant influence onthe photocatalytic degradation rates. Maximum degradationwas observed at neutral pH while the presence of coionshas a detrimental effect on the degradation rates in caseof both phenol and o-chlorophenol. The detailed studiespertaining to reuse and regeneration of the photocatalystwill be investigated in the latter part of this research work.Further improvement in photocatalytic activity of N-dopedmesoporous titania is envisaged by codoping with nonmetalor metal ions.

Acknowledgments

Financial support from Ministry of Environment and For-est (MOEF), government of India, and Network Project(NWP0022) is greatly acknowledged. The authors are alsothankful to NCL, Pune and JNARDDC, Nagpur for theircooperation during various characterization analyses. One ofthe authors P. A. Mangrulkar, would also take the opportu-nity to sincerely acknowledge the Council of Scientific andIndustrial Research (CSIR) India for granting the SeniorResearch Fellowship. Thanks are also due to Mr. PradeepKumar Doggali for his cooperation and invaluable inputs.

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[17] M. M. Joshi, N. K. Labhsetwar, P. A. Mangrulkar, S. N.Tijare, S. P. Kamble, and S. S. Rayalu, “Visible light inducedphotoreduction of methyl orange by N-doped mesoporoustitania,” Applied Catalysis A, vol. 357, no. 1, pp. 26–33, 2009.

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Development of Visible Light-Responsive Photocatalysts

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