Fabrication and Photocatalytic Activity of Nanoporous WO Film · annealed samples were determined...

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Copyright copy 2010 American Scientific PublishersAll rights reservedPrinted in the United States of America

Nanoscience andNanotechnology Letters

Vol 2 51ndash57 2010

Fabrication and Photocatalytic Activity ofNanoporous WO3 Film

Xuming Zhang12 Kaifu Huo12lowast Liangsheng Hu2 and Paul K Chu1lowast1Department of Physics and Materials Science City University of Hong Kong Tat Chee Avenue

Kowloon Hong Kong China2The Key State Laboratory Breeding Base of Refractories and Ceramics School of Materials and Metallurgy

Wuhan University of Science and Technology Wuhan 430081 China

Nanoporous tungsten oxide (WO3 films with a thickness of 500 nm and pore size of about80ndash100 nm are fabricated by chemical anodization of a W foil in a 1 M Na2SO4 solution with 05 wtNaF (pH 40) at 40 V The effect of anodization voltage on the morphology of WO3 films was inves-tigated by evaluating the anodization current and the dissolution concentration of W in electrolyteduring anodization process The photoelectrochemical properties and photocatalytic activity of thenanoporous WO3 films are investigated under visible light irradiation The nanoporous WO3 struc-ture is a sensitive visible light photocatalyst that can be used to decompose contamination and ourresults show that the porous film has better photoactivity than a dense structure

Keywords Nanoporous WO3 Photocatalysis Photoconversion Efficiency Anodization

Semiconductor photocatalysts have attracted increasinginterest because of their wide application in photoelec-trochemical (PEC) water splitting and photocatalytic (PC)degradation of toxic chemicals in water and air1ndash4 TiO2 iscurrently the best known and most widely used photocat-alytic material due to its low cost non-toxicity high sta-bility and high efficiency in degradation of pollutants2ndash3

However TiO2 can be activated only by ultraviolet (UV)light because of the wide bandgap energies of the anatase(32 eV) and rutile (30 eV) phases Since UV radiationonly corresponds to less than 6 of the solar energyimpinging the earthrsquos surface2 many researchers haveattempted to red-shift the absorption range in TiO2 bydoping or modification of TiO2 or to develop new visiblelight sensitive photocatalysts in order to utilize the solarspectrum more efficiently35 Tungsten oxide (WO3 is animportant semiconductor with interesting optical and elec-trical properties that have promising applications in photo-electrochromic devices photocatalysis chemical sensorsand so on6 Its lower bandgap of 24ndash28 eV also enablesmore efficient absorption of visible light A nanoporousWO3 film is an attractive photocatalyst because of itslarge surface and nanoporous structures that not only allowmore effective absorption of incident photons but also canminimize the distance between the site of photon absorp-tion and electronhole redox reactions These two factors

lowastAuthors to whom correspondence should be addressed

result in enhanced photocatalytic acitivity Porous WO3

nanostructures have been synthesized by chemical vapordeposition7ndash8 electrodepositions9 and solndashgel method10ndash11

Another possible route to fabricate nanoporous WO3 filmsis electrochemical anodization of a W foil in a suitableelectrolyte Anodization is a commonly used surface treat-ment method especially pertaining to the formation ofporous structures on the surface of metals such as Al TiTa and Fe12ndash15 Although there are several reports on thefabrication of porous WO3 by anodization16ndash21 the pho-tocatalytic activity and photoelectrochemical properties ofnanoporous WO3 nanostructure have not been investigatedin details17 In this work a regular nanoporous WO3 filmwere fabricated on a W foil by electrochemical anodizationof a W foil in a fluorine-containing electrolyte consistingof 1 M Na2SO4 and 05 wt NaF The dissolution of WO3

in the used electrolyte at different anodization voltageswere investigated for the first time by measuring the Wspecies concentration in the electrolyte The morphologystructure and composition of the nanoporous WO3 filmtogether with the photocatalytic activity and photoelectro-chemical properties were determined These nanoporousWO3 films have potential applications in photocatalysischemical sensing and photoelectronchemical devicesTungsten foils (05 mm thick 996 purity) were pur-

chased from Baoji Nanrong Company (Baoji China)Nanoporous WO3 films were fabricated by anodizing Wfoils and the detailed procedures have been described

Nanosci Nanotechnol Lett 2010 Vol 2 No 1 1941-490020102051007 doi101166nnl20101052 51

Delivered by Ingenta toCity University of Hong Kong

IP 1442142529Fri 23 Jul 2010 011159

Fabrication and Photocatalytic Activity of Nanoporous WO3 Film Zhang et al

previously16 In brief the W foils were mechanicallypolished by SiC sandpapers and then anodized in analkali electrolyte under low voltage to remove the oxidelayer The electrochemical cell had the conventional two-electrode configuration with the tungsten sheet as theworking electrode and a graphite foil as the counter elec-trode (1 cm separation) Anodization of W substrate withthe area of 15 cm2 was carried out at different voltages

2 microm 2 microm

2 microm

500 nm

(a) (b)

(c) (d)

Fig 1 Effect of anodization voltage on the morphology of WO3 nanoporous films (a) 20 (b) 40 (c) 60 V respectively The insets in (a)ndash(c) are thecorresponding SEM images with enlarged magnification and the scale bar in the insets is 500 nm (d) side-view SEM image of nanoprous films shownin (b) the inset is the EDX spectrocopy (e) side-view image of non-porous flat WO3 film formed in non-fluorine ions electrolyte

(20ndash60 V) and a temperature of 298 K for 2 hours ina 100 mL aqueous electrolyte containing 1 M Na2SO4

and 05 wt NaF (pH = 40) In the process of anodiza-tion the anodized current was recorded by the computerconnect with the DC supply power (ITECH IT6123)After anodization the samples were rinsed with a dilutedHF solution and deionized water several times and thendried in air The nanoprous WO3 films initially had an

52 Nanosci Nanotechnol Lett 2 51ndash57 2010

Zhang et al Fabrication and Photocatalytic Activity of Nanoporous WO3 Film

amorphous structure and could be crystallized by anneal-ing at 723 K in air for 3 h at a heating rate of 10 K middotminminus1For comparison non-porous WO3 films were also preparedin 1 M Na2SO4 without fluoride under the same anodiza-tion conditionsThe morphologies of the anodized samples were char-

acterized by field emission scanning electron microscopy(FE-SEM FEI Nova 400 Nano) The thickness of thenanoporous film was estimated directly from the cross-sectional SEM image The as-anodized and subsequentannealed samples were determined by X-ray diffractionwith CuK radiation (= 15416 Aring) (XRD Philips XrsquoPertPro) X-ray photoelectron spectroscopy (XPS PhysicalElectronics 5600) energy-dispersive X-ray spectroscopy(EDS Oxford INCA 200) and UV-Vis-NIR Spectropho-tometer (Lambda 750 UVVisNIR PerkinElmer) TheW-containing species concentration in the electrolyteresulting from the dissolution of WO3 during anodizationwas detected by Inductive Coupled Plasma Emission Spec-trometer (IRIS Advantage ERS)The photocatalytic (PC) activities of the nanoporous and

non-porous WO3 films were evaluated using methyl orange(MO) in an aqueous solution as the probing moleculeA sample with dimensions of 1times1 cm2 was immersed in20 ml of the MO aqueous solution with an initial con-centration of 10 mgL The solution was stirred in darkfor 2 hrs to saturate the photocatalyst with MO The pho-todegradation experiments were performed at the naturalpH of the MO dye The solution was aspirated continu-ously with air during the PC reaction A Xe-lamp (500 W)was used as the visible light irradiation source and thedistance between the light source and samples was main-tained at about 10 cm The change in the MO concen-tration versus PC time was monitored using a UV-Visspectrophotometer (TU-1810SPC Beijing PGENERALBeijing China) at a wavelength of 464 nm and using astandard quartz cuvette of 1 cmThe PEC characteristics of the nanoporous WO3 films

were evaluated using a three electrode PEC cell with thesame Xe lamp The samples were insulated with epoxyresin leaving an open area of 1times 1 cm2 as the workingphotoanode An AgAgCl electrode served as the refer-ence electrode and a platinum foil was the counter elec-trode All the photocurrent experiments were performedon a CHI650c potentiostat (CH Instruments Inc ShanghaiChina) by means of linear sweep voltammetry at a scan-ning rate of 001 Vs and a 05 M Na2SO4 solution wasused as the electrolyteFigure 1 displays the SEM images of the tungsten

oxide films anodized at different voltages and the mor-phology changes according to the anodization voltage At20 V very little etching can be observed and some non-porosified patches are still visible With the anodizationvoltage increased to 40 V a regular nanoporous structure isformed on the entire surface of W foil However a higher

anodization voltage of 60 V leads to an over-etched layerand the pores tend to ldquogrowrdquo together The highly orga-nized nanoporous film formed at 40 V has an averagepore size of 80ndash100 nm and wall thicknesses of 6ndash12 nmThe thickness of the nanoporous film is about 500 nmas indicated by the side-view SEM image in Figure 1(d)EDS suggests that the nanoporous film mainly consistsof W and O (Fig 1(d) inset) For comparison a non-porous WO3 film is also prepared by anodization of a Wfoil for 30 min in 1 M Na2SO4 without fluoride at thesame anodization voltage and the side-view image revealsthat the thickness of the non-porous film is about 450 nm(Fig 1(e))The effect of anodization voltage on the morphology of

WO3 films was also investigated by evaluating the anodiza-tion current and the dissolution concentration of W speciesin electrolyte during anodization process by ICP analysesFigure 2 shows that the anodization current (j versus time(t under different anodization voltages exhibiting a clas-sic characteristic shape including the dip-rise-gradual fallthat has been observed for the formation of nanoporousor nanotube arrays of metal oxide via anodization1622ndash24

Initially the current density transient of the flat W surfacedecreased rapidly due to the formation of a barrier oxidefilm on the W metal and then the slightly increases incurrent density stem from the solubility of oxides in elec-trolytes containing F ions resulting in the formation ofa partially porous structure The current density increasedagain beyond the minimum value as a consequence of thethinning barrier oxide layer at the pore bottom After thata steady current density was attained by maintaining anequilibrium state between the oxide formation rate and thedissolution rate of the grown oxide The overall currentdensity observed in jndasht plots increased with elevation ofanodization voltages This may be attributed to an accel-erated dissolution of the formed tungsten oxide as solu-ble fluoride-complexes with the higher voltage Figure 3shows the concentration of the dissolved tungsten in elec-trolyte versus anodization voltage with the reaction time

Fig 2 jndasht curves under different voltages during anodization of W foilin a electrolyte consisting of 1 M Na2SO4 and 05 wt NaF (pH= 4)

Nanosci Nanotechnol Lett 2 51ndash57 2010 53

Fig 3 The dissolved concentration of W species versus different volt-age after anodization of W foil for 2 h in a 100 mL electrolyte

of 2 h It is clearly seen that the concentration of Wincrease with the anodization voltage The W concentra-tion in a 100 ml electrolyte for anodization of 2 h under20 V is 403 mgL which increased to 4152 mgL under40 V At 60 V the W concentration in 100 ml electrolyteis 48 mgL The dissolute W species in electrolyte isbelieved to the dissolution of tungsten oxide on surfaceof W foil The increase of W species in electrolyte withthe voltages suggests that the dissolution of the formedtungsten oxide as soluble fluoride-complexes acceleratedwith the elevation of voltage Thus over-etching occurat higher voltage due to the faster dissolution of theformed oxide layer2526 and the irregular nanoporous nano-structures are formed at higher voltage such as 60 V in thispaperFigure 4 depicts the XRD patterns of the anodized tung-

sten oxide films before and after annealing in air at 723 Kfor 3 h Before annealing only peaks corresponding tothe W substrate are apparent revealing that the anodizednanoporous film is amorphous (curve 1) After annealingthe XRD patterns of the nanoporous film (curve 2) can beindexed to a monoclinic phase of WO3 (JPCD card No43-1035) XPS is further used to confirm the WO3 compo-sition Figure 5(a) shows the survey XPS results acquired

Fig 4 XRD patterns of as-anodized (1) and annealed (2) porous WO3

films The right pattern is partial magnified information in the range of21ndash25 degree

from the annealed nanoprous WO3 film No F or Na con-tamination can be detected The double peaks for W 4f72and 4f52 at 357 and 377 eV (Fig 5(b)) and one peak forW 5p32 at 412 eV could be seen respectively correspondto W (VI) in WO3

2127ndash28 The O peaks could be fitted totwo peaks centered at 5311 and 5317 eV by LorentzianndashGaussian curve fitting which could be attributed to WndashOand WndashOH respectively (Fig 5(c))29 This result sug-gested that the nanoporous WO3 has good hydrophilicperformance

Fig 5 XPS Survey spectra of nanoporous WO3(a) and the fine spectraof W 4f (b) and O 1s (c)

Fig 6 UV-Vis diffuse reflectance spectroscopy of nanoporous WO3

before and after annealing at 723 K for 3 h

The UV-Vis absorption of nanoporous WO3 was alsoinvestigated and shown in Figure 6 The as-anodized sam-ple has an absorption edge at around 380 nm correspond-ing to a band gap of 32 eV Annealing at 723 K for 3 hresults in the absorption edge red-shifted to longer wave-length (lower energy) of around 480 nm (26 eV) and ahigher absorption in the region of visible light could alsobe observed It is suggested that the crystalline WO3 showsbetter photoresponse in visible regionFigure 7(a) exhibits the photocurrent density as a func-

tion of applied potential (vs AgAgCl) measured fromthe annealed nanoporous and non-porous WO3 films in a05 M Na2SO4 solution under Xe lamp irradiation Theaverage photon intensity reaching the sample surface ismeasured to be about 50 mWcm2 and the anodic biaspotential is scanned from minus03 to +15 V at a rate of10 mVs The dark current without Xe lamp irradiat-ing is close to zero and negligible but the photocur-rents measured from the nanoporous WO3 photoanodeincrease dramatically with anodic potential when the biaspotential exceeds 02 V and the photocurrent densityreaches 048 mAcm2 at 15 V bias potential This valueis about three times that observed from the non-porousWO3 film The corresponding photoconversion efficiency() of light energy to chemical energy in the presenceof an external applied potential could be calculated asfollows1730

= (total power outputminuselectrical power otuput)light power input

times100

= jpE0

revminusEappI0

times100

where jp is the photocurrent density (mAcm2 I0 is theintensity of the incident light in mWcm2 E0

rev is the stan-dard reversible potential which is 123 V NHEminus1 jpE

is the total power output and the electrical power output is

Fig 7 Photocurrent (a) and Photoconversation efficiency (b) as a func-tion of measured potential for the WO3 photoanode recorded in (a)05 M Na2SO4 solution under Xe-lamp illumination The average inten-sity of irradiation reaching the sample surface is measured to be about50 mWcm2

jpEapp Eapp = Emeas minusEocp Emeas is the electrode poten-tial (vs AgAgCl) of the working electrode at which thephotocurrent is measured during illumination and Eocp isthe electrode potential (vs AgAgCl) of the same workingelectrode under open circuit conditions and the same illu-mination in the same electrolyte The Eocp values of theself-assembled nanoporous WO3 and the non-porous WO3

films are measured to be 0102 and 0054 V respectivelyThe curves of the photoconversion efficiency as a func-tion of bias potential for the nanoporous and non-porousWO3 are shown in Figure 7(b) A maximum photoconver-sion efficiency of 281 is obtained at 074 V versus SCEfrom the nanoporous film whereas it is 042 at 09 Vversus SCE for the WO3 film The high photoconversionefficiency may be attributed to the large area and improvedlight absorption due to photo multi-scatting in the pores ofthe nanoporous films20

The PC activity of the nanoporous WO3 film is alsoinvestigated by studying the degradation in the MO solu-tion under Xe lamp irradiation with a filter to remove UVradiation MO is a non-biodegradable dye used in the tex-tile industry and often considered the standard dye in thedetermination of PC activity The photodegradation exper-iments are performed at the natural pH of MO and the

Fig 8 (a) MO Concentration(C) as a function of illumining time (tunder Xe-lamp with porous and flat WO3 (b) Curves of ln(C0C versusillumining time (t Direct photolysis refers to the degradation under Xe-lamp irradiation without using a photocatalyst

annealed nanoporous or non-porous WO3 samples are illu-minated in a quartz glass cell containing 20 ml of theMO solution with an initial concentration of 10 mgLFigure 8(a) shows the plot of the concentration of MOin the aqueous solution versus irradiation time The pho-todegradation of MO in the presence of the nanoporousWO3 film is obviously enhanced The PC degradationof MO which obeys the first-order reaction kinetics canbe expressed as ln(C0C = kt where k is the apparentrate constant t is the irradiation time and C0 and Care the initial and remaining concentrations of the aque-ous MO solution respectively The k values calculatedfrom the nanoporous and non-porous WO3 are listed inFigure 8(b) which demonstrate the magnitude of the over-all photodegradation rate The k value of the nanoporousWO3 is about 4 times larger than that of the non-porousone disclosing that the nanoprous WO3 films have betterphotocatalytic activity than the non-porous oneRegular self-assembled nanoporous WO3 films have

been produced by simple anodization of W foils in afluorine-based electrolyte The effect of anodization volt-age on the morphology of WO3 films was investigatedby evaluating the anodization current and the dissolu-tion concentration of W in electrolyte during anodizationprocess The self-assembled nanoporous WO3 film that

possesses a large surface area and porous structure exhibitshigher photocatalytic activity and better photoelectrochem-ical performance than the non-porous WO3 film

Acknowledgments This work was financially sup-ported by National Natural Science Foundation of China(NSFC 50902104) City University of Hong Kong Strate-gic Research grant (SRG) No 7008009 Key Project ofChinese Ministry of Education (No 208087) and HubeiProvince Natural Science Foundation (No 2008CDB015)

References and Notes

1 S Malato P Fernandez-Ibanez M I Maldonado J Blanco andW Gernjak Catal Today 147 1 (2009)

2 M D Hernandez-Alonso F Fresno S Suarez and J M CoronadoEnergy Environ Sci 2 1231 (2009)

3 K Shankar J I Basham N K Allam O K Varghese G K MorX J Feng M Paulose J A Seabold K S Choi and C A GrimesJ Phys Chem C 113 6327 (2009)

4 X Qin L Q Jing G H Tian Y C Qu and Y J Feng J HazardMater 172 1168 (2009)

5 X H Tang and D Y Li J Phys Chem C 112 5405(2008)

6 Z Liu T Yamazaki Y Shen D Meng T Kikuta N Nakatani andT Kawabata J Phys Chem C 112 1391 (2008)

7 C G Granqvist Sol Energy Mater Sol Cell 60 201 (2000)8 E Brescacin M Basato and E Tondello Chem Mater 11 314

(1999)9 G Leftheriotis and P Yianoulis Solid State Ionics 179 2192

(2008)10 C Satato M Odziemkowski M Ulmann and J Augustynski

J Am Chem Soc 123 10639 (2001)11 B Yang H J Li M Blackford and V Luca Curr Appl Phys

6 436 (2006)12 H Masuda and K Fukuda Science 268 1466 (1995)13 G K Mor O K Varghese M Paulose K Shankar and C A

Grimes Sol Energy Mater Sol Cell 90 2011 (2006)14 W Wei J M Macak and P Schmuki Electrochem Commun

10 428 (2008)15 T J Latempa X J Feng M Paulose and C A Grimes J Phys

Chem C 113 16293 (2009)16 H Tsuchiya J M Macak I Sieber L Taveira A Ghicov

K Sirotna and P Schmuki Electrochem Commun 7 295(2005)

17 Y F Guo X Quan N Lu H M Zhao and S Chen Environ SciTechnol 41 4422 (2007)

18 N R Tacconi C R Chenthamarakshan G YogeeswaranA Watcharenwong R S de Zoysa N A Basit and K RajeshwarJ Phys Chem B 110 25347 (2006)

19 A Watcharenwong W Chanmanee N R Tacconi C RChenthamarakshan P Kajitvichyanukul and K RajeshwaJ Electroanaly Chem 612 112 (2008)

20 S Berger H Tsuchiya A Ghicov and P Schmuki Appl Phys Lett88 203119 (2006)

21 M Yang N K Shrestha and P Schmuki Electrochem Commun11 1908 (2009)

22 A Valota D J LeClere P Skeldon M Curioni T HashimotoS Berger J Kunze P Schmuki and G E Thompson ElectrochemActa 54 4321 (2009)

23 N K Allam X J Feng and C A Grimes Chem Mater 20 6477(2008)

24 Y Shin and S Lee Nanotechnology 20 105301 (2009)

25 J M Macak H Tsuchiya A Ghicov K Yasuda R Hahn S Bauerand P Schmuki Curr Opin Solid State Mater Sci 11 3 (2007)

26 K Yasuda J M Macak S Berger A Ghicov and P SchmukiJ Electrochem Soc 154 472 (2007)

27 C S Blackman and I P Parkin Chem Mater 17 1583 (2005)

28 J Y Luo F L Zhao L Gong H J Chen J Zhou Z L Li S ZDeng and N S Xu Appl Phys Lett 91 093124 (2007)

29 Y Kim J Lee H Jeong Y Lee M H Um K M Jeong M KYeo and M Kang J Indust Engin Chem 14 396 (2008)

30 J Akikusa and S U M Khan J Hydrogen Energy 27 863 (2002)

Received 5 March 2010 Accepted 11 April 2010

Delivered by Ingenta toCity University of Hong Kong

IP 1442142529Fri 23 Jul 2010 011159

previously16 In brief the W foils were mechanicallypolished by SiC sandpapers and then anodized in analkali electrolyte under low voltage to remove the oxidelayer The electrochemical cell had the conventional two-electrode configuration with the tungsten sheet as theworking electrode and a graphite foil as the counter elec-trode (1 cm separation) Anodization of W substrate withthe area of 15 cm2 was carried out at different voltages

2 microm 2 microm

2 microm

500 nm

(a) (b)

(c) (d)

Fig 1 Effect of anodization voltage on the morphology of WO3 nanoporous films (a) 20 (b) 40 (c) 60 V respectively The insets in (a)ndash(c) are thecorresponding SEM images with enlarged magnification and the scale bar in the insets is 500 nm (d) side-view SEM image of nanoprous films shownin (b) the inset is the EDX spectrocopy (e) side-view image of non-porous flat WO3 film formed in non-fluorine ions electrolyte

(20ndash60 V) and a temperature of 298 K for 2 hours ina 100 mL aqueous electrolyte containing 1 M Na2SO4

and 05 wt NaF (pH = 40) In the process of anodiza-tion the anodized current was recorded by the computerconnect with the DC supply power (ITECH IT6123)After anodization the samples were rinsed with a dilutedHF solution and deionized water several times and thendried in air The nanoprous WO3 films initially had an

times100

= jpE0

revminusEappI0

times100

= jpE0

revminusEappI0

times100

= jpE0

revminusEappI0

times100

= jpE0

revminusEappI0

times100

Fabrication and Photocatalytic Activity of Nanoporous WO Film · annealed samples were determined...

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