Mass Production and PhotocatalyticActivity of Highly CrystallineMetastable Single-Phase Bi20TiO32NanosheetsT E N G F E I Z H O U A N D J U N C H E N G H U *

Key Laboratory of Catalysis and Materials Science of the StateEthnic Affairs Commission & Ministry of Education, HubeiProvince, South-Central University for Nationalities,Wuhan, 430074, People’s Republic of China

Received June 11, 2010. Revised manuscript receivedSeptember 19, 2010. Accepted October 12, 2010.

Highly crystalline metastable bismuth titanate (Bi20TiO32)nanosheets are prepared via a simple green wet chemicalroute for the first time. The Bi20TiO32 photocatalysts werecharacterized by transmission electron microscopy (TEM), fieldemission scanning electron microscopy (FESEM), energydispersive spectrum analysis (EDS), X-ray diffraction (XRD), N2

adsorption-desorption (BET), and UV-vis diffuse reflectancespectroscopy(DRS). Inspiringly,Bi20TiO32 nanosheetsshowedhighphotocatalytic activity for the degradation of nonbiodegradableazo dye under simulated sunlight and visible-light irradiation.The experimental results showed that the photocatalytic activityof the Bi20TiO32 nanosheets was superior to the commercialDegussa P25 TiO2, and demonstrated that the morphology andcrystal structure have a distinct effect on the photocatalyticactivity. The reasons for the high photocatalytic activity and theformation mechanism of Bi20TiO32 nanosheets are alsodiscussed.

Introduction

Environmental and energy issues are very important topicson a global scale. Natural energy, such as sunlight, can beemployed to help curb the damage that polluted wastewaterhas on the environment. Semiconductor photocatalysis offersthe potential technology for complete elimination of toxicchemicals through its efficiency and potentially broadapplicability (1, 2). Various new compounds and materialsfor photocatalysis have been synthesized in the past fewdecades. A successful example is TiO2, a metal oxide oftenused as a catalyst in photochemistry. However, fast recom-bination rate and poor solar efficiency (maximum 5%) havehindered the practical applications of most photocatalysts(3, 4). Therefore, studies on attempting to eliminate thesedrawbacks on photocatalysts have been performed (5). Thesurface structure of photocatalysts plays an important roleon their photocatalysis because the photocatalytic reactionor photoelectron conversion takes place only when photo-induced electrons and holes are available on the surface.Synthesis of nanoscale semiconductor photocatalysts be-comes more attractive because of their different physicaland chemical properties from bulk materials. Meanwhile,most studies report only micro to millimolar amounts of

photocatalysts with intrinsically high cost. Therefore, thedevelopment of more economically viable for the large-scalesynthesis of nanoscale photocatalysts with high catalyticactivity and easy separation is a challenge for researchers(6, 7).

The story of C60 demonstrates the importance of ther-modynamically metastable substances that have applicationsin many fields. So researchers seek to understand how tocreate new substances. One strategy is to explore materialswith thermodynamic metastability, that is, to search formaterials seemingly unfavored by their higher energy, yetpersist due to barriers that impede conversion to lower energyforms (8). Bismuth titanates belong to a complicated systemthat includes several different phases such as Bi2Ti2O7,Bi2Ti4O11, Bi4Ti3O12, Bi12TiO20, and Bi20TiO32. The Bi-Ti-Osystem have drawn great interest due to their physicalproperties and technological applications (9-11). For ex-ample, Murugesan et al. reported that Bi2Ti2O7 nanorodsexhibit photocatalytic activity toward hydrogen generationand degradation of a textile dye under UV-visible light (12).Zhou et al. demonstrated the potential of Bi12TiO20, as avisible-light photocatalyst for the oxidation of methanol toCO2 (13). Mesoporous bismuth titanates photodegradationof DCP under visible-light irradiation has been reported inKong’s communication (14). Because different bismuthtitanate phases are formed depending on different chemicalcompositions and processing conditions, so highly crystallineand single phase bismuth titanate are difficult to obtain.Metastable Bi20TiO32 is a photo active member of the bismuthtitanates family that has the potential to possess the desiredqualities of a photocatalyst. Quenching method is the mostcommon synthesis route for this material. Due to hightemperatures, this route typically results in an irregularmorphology and large agglomerated particles as well as alow surface area (15). And the properties of nanomaterialsdepend not only on their composition but also on theirstructure, morphology, phase, shape, size, distribution, andspatial arrangement (16-19). The controlling over morphol-ogy and size of nanostructures has become a fundamentalissue in the design and synthesis of nanomaterials. It hasbeen reported that a strong electrostatic interaction betweeninorganic crystal faces and organic species results in mor-phological evolution. Along with fundamental studies, webelieve that the next challenge will be the new chemical routesfor the preparation of nanostructured and functional na-nomaterials. Meanwhile, the wet chemical synthesis andmorphological control of metal-oxide nanomaterials havenot been fully demonstrated in recent studies, especially interms of two-dimensional (2D) nanostructures. An improvedstrategy for the simultaneous controlling of chemical reaction,oxidation state, crystal phase, and morphology is requiredfor the next stage of materials chemistry. Therefore, thedevelopment of a synthesis of metastable two-dimensional(2D) nanostructures that is cost-effective, with mild condi-tions, suitable for large-scale production, with high catalyticactivity, and easy separation represents a critical challengeto the practical application of these nanomaterials.

Herein, in our study, for the first time, highly crystallinemetastable phase bismuth titanate (Bi20TiO32) nanosheets,produced on a gram-scale, were synthesized on the basis ofour previous work (20-22). The synthesis of metastableBi20TiO32 phase on a gram-scale could not be fabricated bythe traditional method. So wet-chemical methods were usedin our study since they can be controlled from the molecularprecursor to the final product to give highly pure andhomogeneous materials. These methods allow for lower

* Corresponding author phone: 86 27 67841302; e-mail:junchenghuhu@hotmail.com.

Environ. Sci. Technol. 2010, 44, 8698–8703

8698 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 22, 2010 10.1021/es1019959 2010 American Chemical SocietyPublished on Web 10/27/2010

reaction temperatures control over the size and morphologyof the particles, and preparation of metastable phases (23).The morphology and size of the formed Bi20TiO32 nanosheetscan be easily tuned by varying the experimental parameters.Its photocatalytic activity for degradation of dyes in aqueoussolution under simulated sunlight or visible-light irradiationhas been evaluated.

Experimental SectionMaterials and Reagents. Bismuth nitrate (Bi(NO3)3 ·5H2O)was obtained from Tianjin Kermel Chemical ReagentsDevelopment Centre (Tianjin, China). Methanol, ethanol,anhydrous acetic acid, benzyl alcohol, poly(vinyl alcohol)(PVA), potassium hydroxide, brilliant red X3B, and urea wereprovided by Sinopharm Chemical Reagent Co., Ltd. (Shang-hai, China). Titanium isopropylate was purchased from AlfaAesar. All chemicals were analytical grade reagents. DegussaP25 (∼80% anatase and ∼20% rutile) was purchased fromDegussa (China) Co., Ltd. Deionized and doubly distilledwater was used in this work.

Sample Preparation. In a typical synthesis procedure,9.70 g Bi(NO3)3 ·5H2O was dissolved in 60 mL anhydrous aceticacid. After the Bi(NO3)3 ·5H2O dissolved completely, 4.26 gTitanium isopropylate was slowly added under vigorousmagnetic stirring at room temperature. Then 200 mLmethanol was injected under stirring until the white gelati-nous fluid became transparent. Urea and 7.8 g benzyl alcoholwas added to the mixture in the end. After stirring for 1 h,the solution was transferred into an autoclave and thereaction mixture was purged with 1 MPa N2 3×, and then apressure of 1 MPa N2 was imposed before initiating heating,then heated to 180 °C, maintained for 2 h, then to 240 °C,and then held for an additional 2 h; finally, the vapor insidewas vented. After the supercritical fluid drying (SCFD), ablack powder was collected, rinsed several times with absoluteethanol and distilled water and subsequently calcined in airwith a ramp rate of 3 °C/min to 300 °C, then maintained at300 °C for 5 h. Bismuth titanate nanowires with the growthdirection is [010] were synthesized and used for comparison(see Supporting Information for the detailed syntheticprocedure) (24).

Characterization. The crystalline structure of the catalystswere characterized by power X-ray diffraction (XRD) em-ploying a scanning rate of 0.05°/s in a 2θ range from 10° to80°, in a Bruker D8. Advance using monochromatized Cu KRradiation. The morphologies and sizes of the samples wereobserved by transmission electron microscopy (TEM), whichwere taken on a Tecnai G20 (FEI Co., Holland) TEM usingan accelerating voltage of 200 kV. Scanning electron mi-croscopy (SEM) was performed with a S4800 Field EmissionSEM (FESEM, Hitachi, Japan) at an accelerating volatage of10 kV. The SEM was linked to an energy dispersive spectrumanalysis (EDS) system. Specific surface areas of samples weremeasured at 77 K by the BET method (N2 adsorption) witha Micromeritics ASAP 2020 instrument. The ultraviolet-visiblediffuse reflectance spectra were measured using the diffusereflection method with a Shimadzu UV-2450 spectropho-tometer. BaSO4 was used as a reflectance standard inultraviolet-visible diffuse reflectance experiments.

Photocatalytic Measurement. The photoinduced de-composition of the organic dyes was carried out with 0.05 gof the catalyst suspended in a dye solution (1.14 × 10-4 mol/L; 50 mL), prepared by dissolving the organic powders indistilled water in a Pyrex glass cell. The optical system for thedegradation reaction included a 350 W Xe lamp withsimulated sunlight (wavelength from 200 to 800 nm) or visiblelight (wavelength >420 nm, use a cutoff filter of 420 nm), anda filter (to prevent infrared irradiation). Before illumination,the suspensions were prepared under an ultrasonic waterbath for 10 min and magnetically stirred in the dark for ∼40

min to ensure establishment of an adsorption/desorptionequilibrium of dyes on the sample surfaces. The suspensionsincluding the sample powders and dyes were sampled everyfew minutes. The sample powders were then separated bycentrifuging and the dye solutions were analyzed. The pH ofthe solutions was kept constant (∼7.0) during the reactions.The experiments were carried out at room temperature inair. The concentration of the organic dyes was determinedby monitoring the height of the maximum of the absorbancein ultraviolet-visible spectra by a UV-vis/NIR spectropho-tometer (UV-2450, Shimadzu). As a comparison, DegussaP25 and bismuth titanate nanowires were also tested undervisible light experiment conditions.

Results and DiscussionCharacterization of Catalysts. Figure 1 shows the X-raydiffraction (XRD) pattern of the nanosheets formed bycalcination at 300 °C. We can clearly observe that it is in goodagreement with the standard data of tetragonal Bi20TiO32

(JCPDS No. 42-0202). This is of great importance becauseit has been proven to be difficult to produce single-phaseBi20TiO32, due to phase transformations (25). The latticeconstants are a ) 7.700 Å and c ) 5.653 Å, and the averagegrain size of the sample is calculated to be about 52.9 nmwith the (201) diffraction peak according to the Scherrerformula. When the nanoparticles are further calcined at 500°C, X-ray diffraction (XRD) patterns show that they aretransformed gradually into the Bi2Ti2O7 phase (see SupportingInformation Figure S1).

Figure 2 presents typical transmission electron microscopy(TEM) and scanning electron microscopy (SEM) images ofthe Bi20TiO32 nanosheets, which show that the products arecomposed of sheet-like nanostructures. High resolution TEMimages shows the (002) and (220) planes with lattice spacingsof 2.82 and 2.74 Å, respectively. The corresponding fastFourier transform (FFT) indicate the sample belong totetragonal system (Figure 2c inset). According to the abovecharacterizationsandWeisszonelaw,theBi20TiO32 nanosheetshave exposed {110} facets. Three regions-of-interest on onenanosheet were selected to analyze with energy dispersivespectrum analysis (EDS), the position and the ratio of peakshave no significant change, indicate that all elements areevenly distributed on the surface of nanosheets. Benzylalcohol has proven to be a versatile solvent and reactant tocontrol the crystallization and stabilization of nanoparticles(26, 27). Here, benzyl alcohol is also used as a structuredirectingagenttocontrolthesynthesisofBi20TiO32 nanosheets.Transmission electron microscopy (TEM) images reveal themorphology differences of the product synthesized in absenceof benzyl alcohol and Bi20TiO32 nanosheets resulted aftercalcinations (see Supporting Information Figure S2). In theabsence of benzyl alcohol, irregular bismuth titanates with

FIGURE 1. XRD pattern of Bi20TiO32 nanosheets.

VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8699

typical sizes up to ∼200 nm were synthesized. X-ray dif-fraction (XRD) analysis shows that the main phase wasBi12TiO20 (see Supporting Information Figure S1).

The Effect of Urea and Benzyl Alcohol on Morpholo-gies. According to the experimental results mentioned above,benzyl alcohol has a distinct effect both on the morphologyand crystal structure. Besides benzyl alcohol, urea also playsan important role in affecting the morphology. Urea wasfound to be very critical in this synthesis method. In ourexperiment, when no urea was added under the same reactionconditions, bismuth titanate Bi20TiO32 nanosheets with thelarger size was obtained as shown in Figure 2a. In the absenceof benzyl alcohol, no Bi20TiO32 nanosheets were prepared.However, larger nanosheets were obtained in the absence ofurea, and doubling the amount of urea led to smallernanosheets (Figure S2c of the SI). The average grain size ofthe samples are calculated to be 70.5, 52.9, and 39.0 nmusing the (201) diffraction peak according to the Schererformula as the molar ratio of urea/Bi varies from 0 to 1.5 to3 respectively. These results are in agreement with TEMimages (see Figure S2 of the SI). The role of urea is importantto control the size of the nanosheets in the synthesis methodas it provides a steady OH- supply via urea hydrolysis (28).When bismuth nitrate and titanium isopropoxide reacts withmethanol and water to form the Bi-O-Ti precursor, acid isa byproduct, and the accumulation of acid will inhibit thefurther formation of the Bi-O-Ti precursor. However, whenurea is added, the OH- formed by urea hydrolysis neutralizesthe acid and allows the formation of the Bi-O-Ti precursor.High hydrolysis and alcoholysis rates lead to the formationof small Bi20TiO32 nanosheets.

On the basis of the above observations, the formationmechanism shown in Scheme 1 is proposed. In the initialstage, Bi3+ ions are protected by benzyl alcohol moleculeswhich can selectively adsorb onto certain surfaces, formingBi-BZ units, and then Ti4+ ions can attack these units to formBi-O-Ti precursors (Scheme 1). The OH- formed by ureahydrolysis neutralizes the acid and allows the formation of

the Bi-O-Ti precursor. With the increase in concentrationof urea, high hydrolysis and alcoholysis rates lead to theformation of small Bi20TiO32 nanosheets. Several experimentshave been carried out to determine the parameters that aremechanistically important for the formation of the metastablebismuth titanate (Bi20TiO32) nanosheets. Benzyl alcohol hasbeen found to be a successful medium to tailor metal oxideswith well-controlled shape, size, and crystallinity (26, 27). Inthe absence of benzyl alcohol, no nanosheets were formed.Instead, irregular materials were produced, and the crystallinephase is lost. However, when benzyl alcohol is added,metastable bismuth titanate (Bi20TiO32) nanosheets wereformed. Usually, different facets of a single crystal exhibitdistinctive physical and chemical properties. Benzyl alcoholmolecules can selectively adsorb onto certain surfaces of thebismuth titanate and reduce their surface energies. This maybe the possible reason of why a metastable phase was formedin our study. Through the supercritical fluid drying (SCFD)process, the benzyl alcohol can be removed and it wasconfirmed by our previous work (22).

Optical Properties and Photocatalytic Activities. TheUV-vis diffuse reflectance spectra of the calcined bismuthtitanate (Bi20TiO32) are shown in Figure 3. Compared withDegussa P25, the synthesized bismuth titanate demonstratesa significant increase in photoabsorption and a shift of theabsorption edge to longer wavelengths in the visible lightregion, indicating that the samples have potential ability forphotocatalytic decomposition of organic contaminants undervisible-light irradiation, which is consistent with the yellowishcolor of the sample. It is important to note that the absorbancein the visible light region is not at a significant loss in UVabsorbance which is typically true for transition metal dopedTiO2 (29, 30). The degradation of X3B as a representativemodel pollutant was chosen to evaluate the photocatalyticperformance of the Bi20TiO32 nanosheets. Figure 4 shows theUV-vis spectra of the X3B solution after simulated sunlight(200 nm < wavelength < 800 nm) irradiation for various timeperiods in the presence of Bi20TiO32 nanosheets. In both cases,

FIGURE 2. (a-c) TEM images of the Bi20TiO32 nanosheets with different resolutions, the inset is a high resolution image and thecorresponding FFT; (d) SEM image of the Bi20TiO32 nanosheets; (e) EDS result of the corresponding zone in SEM.

8700 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 22, 2010

the irradiation caused a significant decrease of the absorptionpeak at 535 nm, and is associated with a diminishing of thetypical purple color of the X3B solution. In the absence ofphotocatalyst, the concentration of X3B remains virtuallyunchanged. With the addition of 50 mg bismuth titanate asphotocatalysts, the concentration of X3B decreases consid-erably rapidly, indicating that bismuth titanate Bi20TiO32 is

an active photocatalyst for the degradation of organicpollutants in simulated sunlight. The extent of the decreasein concentration strongly depends on the morphology of thephotocatalyst, with about 58.5%, 73.5%, and 97.1% of X3Bbeing decomposed after 30 min over irregular bismuthtitanate, bismuth titanate nanowires with the growth direc-tion is [010] and bismuth titanate nanosheets, respectively(Figures 5 and 6). It is clear that the concentration of X3Bdecreased more rapidly for the nanosheets than that for theirregular bismuth titanate, indicating a higher photocatalyticactivity of the nanosheets. For comparison, the photocatalyticdegradation rate of X3B under visible light also has beenevaluated. After visible-light irradiation for 90 min, about87.6%, 98.7%, and 79.0% of X3B were decomposed on bis-muth titanate with molar ratio of urea/Bi from 0, 1.5, to 3,respectively. For Degussa P25, only 64.8% of X3B is decom-posed. For comparison, the photocatalytic degradation rateof X3B over bismuth titanate nanowires was measured andit was only 57.1% (Figure 7). The higher photocatalytic activityof Bi20TiO32 may have a close relationship with the meta-stability and facet-oriented. The differences in activity of thevarious Bi20TiO32 may be connected with their morphologies.The BET surface area was measured to be 16.3, 19.9, and 40.2m2 ·g-1 for the molar ratio of urea/Bi from 0, 1.5, to 3,respectively. It is well-known that a higher surface areaincreases the number of active sites and promotes the

SCHEME 1. Schematic Illustration of the Proposed Formation Mechanism of Bi20TiO32 Nanosheets

FIGURE 3. UV-vis diffusive reflectance spectrum of Bi20TiO32nanosheets. Bismuth titanate nanosheets with molar ratio ofurea/Bi from 0(b), 1.5(a), to 3 (c), Degussa P25 (d).

FIGURE 4. The absorbance spectra changes of X3B solution inthe presence of Bi20TiO32 nanosheets (urea/Bi ) 1.5) undersimulated sunlight (200 nm < λ < 800 nm) irradiation.

FIGURE 5. Photodegradation of X3B (X3B: 1.14 × 10-4 M) undersimulated sunlight irradiation. (a) catalyst-free, (b) light-free, (c)bismuth titanate synthesized without benzyl alcohol, (d) bismuthtitanate nanowires, and (e) bismuth titanate synthesized withbenzyl alcohol.

VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8701

separation efficiency of the electron-hole pairs in photo-catalytic reactions, resulting in a higher photocatalytic activity(31, 32). However, in our study, the relatively low photo-catalytic performance of the bismuth titanate when the molarratio of urea/Bi ) 3 in comparison with the other samplesis attributed to its smaller sheet size, leading to a decreasedproportion of active facets (33), which suggests that the overallphotocatalytic activity of the nanosheets is more related toits surface structure rather than its specific surface area.

In conclusion, a simple green wet chemical synthesis tomass produce highly crystalline metastable bismuth titanate(Bi20TiO32) nanosheets is presented. The Bi20TiO32 nanosheetsshowed higher photocatalytic activity for the degradation ofnonbiodegradable azo-dye as compared to commercialDegussa P25. Morphology and size of the formed Bi20TiO32

nanosheets can be easily tuned by varying the experimentalparameters. The higher photocatalytic activity of Bi20TiO32

may have a close relationship with the metastability. Themethod described herein presents a new way to synthesizenot only Bi20TiO32 nanosheets, but also a family of metastablematerials.

AcknowledgmentsThis work is supported by National Natural Science Founda-tion of China (20803096), South-Central University forNationalities (YZZ 08002, KYCX090004E) and the Scientific

Research Foundation for the Returned Overseas ChineseScholars, State Education Ministry.

Supporting Information AvailableDetail synthetic procedure of the bismuth titanate nanowires.XRD pattern of bismuth titanate, TEM images, reuse andrecycling of bismuth titanate nanosheets with molar ratio ofurea/Bi from 0, 1.5, to 3 are shown in Figures S1-S4. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited(1) Daskalaki, V. M.; Antoniadou, M.; Puma, G. L.; Dimitris, I. K.;

Lianos, P. Solar light-responsive Pt/CdS/TiO2 photocatalystsfor hydrogen production and simultaneous degradation ofinorganic or organic sacrificial agents in wastewater. Environ.Sci. Technol. 2010, 44 (19), 7200-7205.

(2) Lee, K.; Kim, D.; Roy, P.; Paramasivam, I.; Birajdar, B. I.; Spiecker,E.; Schmuki, P. Anodic formation of thick anatase TiO2 meso-sponge layers for high-efficiency photocatalysis. J. Am. Chem.Soc. 2010, 132, 1478–1479.

(3) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R.Oxidative power of nitrogen-doped TiO2 photocatalysts undervisible illumination. J. Phys. Chem. B 2004, 108, 17269–17273.

(4) Yu, J. C.; Zhang, L. Z.; Zheng, Z.; Zhao, J. C. Synthesis andcharacterization of phosphate mesoporous titanium dioxidewith high photocatalytic activity. Chem. Mater. 2003, 15, 2280–2286.

(5) Hu, J. S.; Ren, L. L.; Guo, Y. G.; Liang, H. P.; Cao, A. M.; Wan,L. J.; Bai, C. L. Mass production and high photocatalytic activityof ZnS nanoporous nanoparticles. Angew. Chem., Int. Ed. 2005,44, 1269–1273.

(6) Howgate, J.; Schoell, S. J.; Hoeb, M.; Steins, W.; Baur, B.; Hertrich,S.; Nickel, B.; Sharp, I. D.; Stutzmann, M.; Eickhoff, M.Photocatalytic cleavage of self-assembled organic monolayersby UV-induced charge transfer from GaN substrates. Adv. Mater.2010, 22, 2632–2636.

(7) Wang, P.; Huang, B. B.; Qin, X. Y.; Zhang, X. Y.; Dai, Y.; Wei, J. Y.;Whangbo, M. H. Ag@AgCl: A highly efficient and stablephotocatalyst active under visible light. Angew. Chem., Int. Ed.2008, 47, 1–4.

(8) Brus, L. Metastable dense semiconductor phases. Science. 1997,276, 373–374.

(9) Hou, J. G.; Kumar, R. V.; Qu, Y. F.; Krsmanovic, D. Controlledsynthesis of photoluminescent Bi4Ti3O12 nanoparticles frommetal-organic polymeric precursor. J. Nanopart. Res. 2009, 11,563–571.

(10) Murugesan, S.; Smith, Y. R.; Subramanian, V. Hydrothermalsynthesis of Bi12TiO20 nanostrucutures using anodized TiO2

nanotubes and its application in photovoltaics. J. Phys. Chem.Lett. 2010, 1 (10), 1631–1636.

(11) Hou, J. G.; Qu, Y. F.; Krsmanovic, D.; Ducati, C.; Eder, D.; Kumar,R. V. Hierarchical assemblies of bismuth titanate complexarchitectures and their visible-light photocatalytic activities. J.Mater. Chem. 2010, 20, 2418–2423.

(12) Murugesan, S.; Subramanian, V. Robust synthesis of bismuthtitanate pyrochlore nanorods and their photocatalytic applica-tions. Chem. Commun. 2009, 5109–5111.

(13) Zhou, J. K.; Zou, Z. G.; Ray, A. K.; Zhao, X. S. Preparation andcharacterization of polycrystalline bismuth titanate Bi12TiO20

and its photocatalytic properties under visible light irradiation.Ind. Eng. Chem. Res. 2007, 46, 745–749.

(14) Kong, L. D.; Chen, H. H.; Hu, W. M.; Zhang, S. C.; Chen, J. M.Mesoporous bismuth titanate with visible-light photocatalyticactivity. Chem. Commun. 2008, 4977–4979.

(15) Cheng, H. F.; Huang, B. B.; Dai, Y.; Qin, X. Y.; Zhang, X. Y.; Wang,Z. Y.; Jiang, M. H. Visible-light photocatalytic activity of themetastable Bi20TiO32 synthesized by a high-temperature quench-ing method. J. Solid. State. Chem. 2009, 182, 2274–2278.

(16) Hu, J. T.; Odom, T. W.; Lieber, C. M. Chemistry and physics inone dimension: synthesis and properties of nanowires andnanotubes. Acc. Chem. Res. 1999, 2 (5), 435–445.

(17) Goldberger, J.; Fan, R.; Yang, P. D. Inorganic nanotubes: a novelplatform for nanofluidics. Acc. Chem. Res. 2006, 39 (4), 239–248.

(18) Zach, M. P.; Ng, K. H.; Penner, R. M. Molybdenum nanowiresby electrodeposition. Science 2000, 290, 2120–2123.

(19) Cao, X. B.; Xie, Y.; Li, L. Y. Spontaneous organization of three-dimensionally packed trigonal selenium microspheres intolarge-area nanowire networks. Adv. Mater. 2003, 15, 1914–1918.

FIGURE 6. The absorbance spectra changes of X3B solution inthe presence of Bi20TiO32 nanosheets (urea/Bi ) 1.5) undervisible-light (λ > 420 nm) irradiation.

FIGURE 7. Photodegradation of X3B (X3B: 1.14 × 10-4 M) undervisible-light irradiation. (a) catalyst-free, (b) light-free, (c) bis-muth titanate nanowires, (d) Degussa P25, Bismuth titanatenanosheets synthesized with molar ratio of urea/Bi from 0(f),1.5(g), to 3(e).

8702 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 22, 2010

(20) Hu, J. C.; Zhu, K. K.; Chen, L. F.; Kubel, C.; Richards, R. MgO(111)nanosheets with unusual surface activity. J. Phys. Chem. C 2007,111, 12038–12044.

(21) Zhu, K. K.; Hu, J. C.; Christian, K.; Richards, R. Efficientpreparation and catalytic activity of MgO(111) nanosheets.Angew. Chem., Int. Ed. 2006, 45, 7277–7281.

(22) Hu, J. C.; Zhu, K. K.; Chen, L. F.; Yang, H. J.; Li, Z.; Suchopar,A.; Richards, R. Preparation and surface activity of single-crystalline NiO(111) nanosheets with hexagonal holes: Asemiconductor nanospanner. Adv. Mater. 2008, 20, 267–271.

(23) Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. A generalsoft-chemistry route to perovskites and related materials:Synthesis of BaTiO3, BaZrO3, and LiNbO3 nanoparticles. Angew.Chem. Int. Ed. 2004, 43, 2270–2273.

(24) Hou, J.; Qu, Y.; Krsmanovic, D.; Ducati, C.; Eder, D.; Kumar, R.Solution-phase synthesis of single Solution-phase synthesis ofsingle-crystalline Bi12TiO20 nanowires with photocatalytic prop-erties. Chem. Commun. 2009, 3937–3939.

(25) Hou, Y.; Wang, M.; Xu, X. H.; Wang, H.; Shang, S. X.; Wang, D.;Yao, W. F. Bi20TiO32 nanocones prepared from Bi-Ti-O mixtureby metalorganic decomposition method. J. Cryst. Growth. 2002,240, 489–494.

(26) Niederberger, M.; Michael, H. B.; Stucky, G. D. Benzyl alcoholand titanium tetrachloride a versatile reaction system for thenonaqueous and low-temperature preparation of crystallineand luminescent titania nanoparticles. Chem. Mater. 2002, 14,4364–4370.

(27) Niederberger, M.; Michael, H. B.; Stucky., G. D. Benzyl alcoholand transition metal chlorides as a versatile reaction system forthe nonaqueous and low-temperature synthesis of crystallinenano-objects with controlled dimensionality. J. Am. Chem. Soc.2002, 124, 13642–13643.

(28) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan,L. J. Self-assembled 3D flowerlike iron oxide nanostructuresand their application in water treatment. Adv. Mater. 2006, 18,2426–2431.

(29) Hagfeldtt, A.; Gratzel, M. Light-induced redox reactions innanocrystalline systems. Chem. Rev. 1995, 95, 49–68.

(30) Choi, W. Y.; Termin, A.; Hoffmann, M. R. The role of metal iondopants in quantum-sized TiO2: correlation between photo-reactivity and charge carrier recombination dynamics. J. Phys.Chem. 1994, 98, 13669–13679.

(31) Wang, W. W.; Zhu, Y. J.; Yang, L. X. ZnO-SnO2 hollow spheresand hierarchical nanosheets: hydrothermal preparation, forma-tion mechanism, and photocatalytic properties. Adv. Funct.Mater. 2007, 17, 59–64.

(32) Fumiaki, A.; Akira, Y.; Kohei, N.; Masatoshi, O.; Bunsho, O. Visiblelight responsive pristine metal oxide photocatalyst: enhance-ment of activity by crystallization under hydrothermal treatment.J. Am. Chem. Soc. 2008, 130, 17650–17651.

(33) Lee, K.; Kim, M.; Kim, H. Catalytic nanoparticles being facet-controlled. J. Mater. Chem. 2010, 20, 3791–3798.

ES1019959

VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8703