20029J. Phys.Chem.C 2009, 113, 20029-20035EfficientPhotocatalyticRemovalof ContaminantbyBisNb,Tai-rO,Nanoparticles underVisibleLight IrradiationShuiming Zhang, Gaoke Zhang,t Shujie Yu, Xiaoguo Chen, and Xiaoyang Zhang*School of Resources and Environmental Engineering,Wuhan University of Technology,122Luoshi Road,Wuhan430070,P.R.China,and StateKeyLaboratoryof Crystal Materials,ShandongUniversity27ShandaNanlu,Jinan250100,P.R.ChinaReceived:August13,2009;RevisedManuscript Received:September26,2009NanosizedBigNb,Tai-Ophotocatalystswerepreparedbyafacileand low-costsol-gel method using stableless toxic Ta2Os, Nb2Os, and Bi(NOs)3·5H2O as the raw materials.The as-prepared samples were characterizedbyX-ray diffraction,transmission electron microscopy,Fourier transformationinfrared spectroscopy,X-rayphotoelectron spectroscopy,and UV-vis diffuse reflectance spectroscopy.The Bi,Nb,Tai-,O, nanoparticlesexhibitedanefficientphotocatalyticactivityinthedecompositionofacidredG(ARG)dyesolutionundervisible light irradiation.Besides decoloring,thetypical sampleBigNbo.cTao.4Oalso showed an excellentphotocatalytic propertyfor the removal of the cyanotoxin,microcystin-LR (MC-LR, an emerging contaminantfromtheContaminantCandidateLists(CCLs1-3)oftheUSEPA).Theexcellentvisiblelightphotocatalyticactivity of the samples was mainly attributed to their narrow band gaps, small particle size, and the oxygenvacanciesonthesurfaceofthecatalysts.Accordingto experimental results,apossiblemechanism of thephotocatalysis overBisNb,Tal-,O,wasproposed.1.Introductionprecursor to the final material to give highly pure and homogeneous materials.In addition, wet routes allowlow reactionRecently, semiconductor photocatalysts have attracted ex-temperature, so that the size of the synthesized particles maytensive attention because of their wide applications in solarbe well controlled.28.29 Several synthesis routes have beenenergy conversion and environmental purification.-4 To date,developed to prepare the nanosized tantalum-orniobium-basedthe remarkable progress of photocatalysis has been mainlycompounds.However,most of these methods use niobium orfocused onTiO--basedmaterialsduetotheirhighphotocatalytictantalum alkoxides as the sources, which have many seriousactivity and stability.5-8However,TiO,is only active in thedrawbacks, such as inflammability,relatively high cost,and greatultraviolet (UV) light range, which accounts for a small fractionsensitivity to moisture. All of these undesirable characteristicsin solar light.From a viewpoint of the efficient utilization oflimit its general application.30 Therefore, it is important tosolar light, a lot of work has been done to develop visible-light-develop nanosized niobates andtantalates photocatalysts whichdriven photocatalysts.One of the efforts in thedevelopment ishave highphotocatalvtic activityin visible lightregion viaamodification of TiO, by doping or ion-implanting methods.9-13facile and low-cost sol-gel method at lower temperature.However,doped materials often suffer from thermal instabilityInthepresentpaper,weusedTa2Os,NbOs,andand the doping process may requireexpensiveion implantationBi(NO,)3-5H,O as the raw materials and successfully synthe-equipment 4 Another approach to realize visible light photo-sized BisNb,Tai-O(x =0-1)solid solution photocatalystscatalysis is to develop new photocatalytic materials independentthrough a facile sol-gel method at lower temperature byofTi215-19 In recent years,afamily of Bi-based oxides haschanging the initial concentration of the reactants.The photo-been found to be very active under visible light irradiation.catalytic properties were evaluated by the degradation of acidwhichisattributedtothehybridizedvalencebandbyO2pandred G (ARG) and microcystin-LR (MC-LR) under visible lightBi 6s so as to narrow the band gap.20-23irradiation.It was demonstrated that the as-obtained samplesMany of niobates and tantalates have also been attracted greatexhibited an excellent visible light photocatalytic acitivity foratention because of their high photocatalytic properties.24-27the degradation of the pollutants.However, most of these photocatalysts are mainly synthesizedby the solid-state (SS) method at high temperature and only2.Experimental Sectionhave photocatalytic activity under the UV light region. The2.1.Preparation.TaOs,Nb,Os,bismuth nitratepentahydratephotocatalyst obtained by the SS method often has small specific(Bi(NO:)3·5H,O), potassium hydroxide (KOH), citric acid (CA),surface area, large particle size,and low adsorbability,whichnitric acid (HNO), oxalic acid (OA), ammonia (NH3-H,O), andresult in low photocatalytic efficiency in the treatment ofethylenediaminetetraacetic acid (EDTA)were used as startingpollutants at very low concentration in the environment.Inchemical reagents.All of the reagents were analytical grade andcontrast,wetchemicalroutesareapromising alternative,sincewereused withoutfurtherpurificationthey can well control thewhole process from themolecularThe Bi,Nb,Tat-,O samples were prepared by a sol-gelmethod.First, TazOs was mixed withKOH according to a molar*Corresponding author: Tel 86-27-87651816; fax 86-27-87887445:ratio of 1:10 and was then sintered at 500 C.The sinterede-mail gkzhang@whut.edu.crsubstanceswereleached withhotdeionizedwater,andafter+ Wuhan University of Technology* Shandong Universityfiltration,a transparent solution was obtained.Thetantalum acid@2009American Chemical Society10.1021/jp907831yCCC:$40.75PublishedonWeb10/26/2009
Efficient Photocatalytic Removal of Contaminant by Bi3NbxTa1-xO7 Nanoparticles under Visible Light Irradiation Shuiming Zhang,† Gaoke Zhang,*,† Shujie Yu,† Xiaoguo Chen,† and Xiaoyang Zhang‡ School of Resources and EnVironmental Engineering, Wuhan UniVersity of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China, and State Key Laboratory of Crystal Materials, Shandong UniVersity, 27 Shanda Nanlu, Jinan 250100, P. R. China ReceiVed: August 13, 2009; ReVised Manuscript ReceiVed: September 26, 2009 Nanosized Bi3NbxTa1-xO7 photocatalysts were prepared by a facile and low-cost sol-gel method using stable, less toxic Ta2O5, Nb2O5, and Bi(NO3)3 · 5H2O as the raw materials. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, Fourier transformation infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis diffuse reflectance spectroscopy. The Bi3NbxTa1-xO7 nanoparticles exhibited an efficient photocatalytic activity in the decomposition of acid red G (ARG) dye solution under visible light irradiation. Besides decoloring, the typical sample Bi3Nb0.6Ta0.4O7 also showed an excellent photocatalytic property for the removal of the cyanotoxin, microcystin-LR (MC-LR, an emerging contaminant from the Contaminant Candidate Lists (CCLs 1-3) of the USEPA). The excellent visible light photocatalytic activity of the samples was mainly attributed to their narrow band gaps, small particle size, and the oxygen vacancies on the surface of the catalysts. According to experimental results, a possible mechanism of the photocatalysis over Bi3NbxTa1-xO7 was proposed. 1. Introduction Recently, semiconductor photocatalysts have attracted extensive attention because of their wide applications in solar energy conversion and environmental purification.1-4 To date, the remarkable progress of photocatalysis has been mainly focused on TiO2-based materials due to their high photocatalytic activity and stability.5-8 However, TiO2 is only active in the ultraviolet (UV) light range, which accounts for a small fraction in solar light. From a viewpoint of the efficient utilization of solar light, a lot of work has been done to develop visible-lightdriven photocatalysts. One of the efforts in the development is modification of TiO2 by doping or ion-implanting methods.9-13 However, doped materials often suffer from thermal instability,9 and the doping process may require expensive ion implantation equipment.14 Another approach to realize visible light photocatalysis is to develop new photocatalytic materials independent of TiO2. 15-19 In recent years, a family of Bi-based oxides has been found to be very active under visible light irradiation, which is attributed to the hybridized valence band by O 2p and Bi 6s so as to narrow the band gap.20-23 Many of niobates and tantalates have also been attracted great attention because of their high photocatalytic properties.24-27 However, most of these photocatalysts are mainly synthesized by the solid-state (SS) method at high temperature and only have photocatalytic activity under the UV light region. The photocatalyst obtained by the SS method often has small specific surface area, large particle size, and low adsorbability, which result in low photocatalytic efficiency in the treatment of pollutants at very low concentration in the environment. In contrast, wet chemical routes are a promising alternative, since they can well control the whole process from the molecular precursor to the final material to give highly pure and homogeneous materials. In addition, wet routes allow low reaction temperature, so that the size of the synthesized particles may be well controlled.28,29 Several synthesis routes have been developed to prepare the nanosized tantalum- or niobium-based compounds. However, most of these methods use niobium or tantalum alkoxides as the sources, which have many serious drawbacks, such as inflammability, relatively high cost, and great sensitivity to moisture. All of these undesirable characteristics limit its general application.30 Therefore, it is important to develop nanosized niobates and tantalates photocatalysts which have high photocatalytic activity in visible light region via a facile and low-cost sol-gel method at lower temperature. In the present paper, we used Ta2O5, Nb2O5, and Bi(NO3)3 · 5H2O as the raw materials and successfully synthesized Bi3NbxTa1-xO7 (x ) 0-1) solid solution photocatalysts through a facile sol-gel method at lower temperature by changing the initial concentration of the reactants. The photocatalytic properties were evaluated by the degradation of acid red G (ARG) and microcystin-LR (MC-LR) under visible light irradiation. It was demonstrated that the as-obtained samples exhibited an excellent visible light photocatalytic acitivity for the degradation of the pollutants. 2. Experimental Section 2.1. Preparation. Ta2O5, Nb2O5, bismuth nitrate pentahydrate (Bi(NO3)3 · 5H2O), potassium hydroxide (KOH), citric acid (CA), nitric acid (HNO3), oxalic acid (OA), ammonia (NH3 ·H2O), and ethylenediaminetetraacetic acid (EDTA) were used as starting chemical reagents. All of the reagents were analytical grade and were used without further purification. The Bi3NbxTa1-xO7 samples were prepared by a sol-gel method. First, Ta2O5 was mixed with KOH according to a molar ratio of 1:10 and was then sintered at 500 °C. The sintered substances were leached with hot deionized water, and after filtration, a transparent solution was obtained. The tantalum acid * Corresponding author: Tel 86-27-87651816; fax 86-27-87887445; e-mail gkzhang@whut.edu.cn. † Wuhan University of Technology. ‡ Shandong University. J. Phys. Chem. C 2009, 113, 20029–20035 20029 10.1021/jp907831y CCC: $40.75 2009 American Chemical Society Published on Web 10/26/2009
20030J.Phys.Chem.C,Vol.113,No.46,2009Zhang et al.sedimentwasobtainedafteradjustingthepHofthesolutionto自(a)CoNAI-2 using nitric acid and was then washed successively untilthe potassium ions were completely removed.The as-prepared人tantalic acid was dissolved in oxalic acid solution, and a(ne) nsuaulBi,Nba,Tao,O,人人tantalum-oxalic acid solution (Ta-OA)was obtained.Theniobium-oxalic acid solution (Nb-OA)was prepared byBi,Nbo,Ta,O,人八Athe hydrothermal method reported in our previous study.31 TheBi,Nb.,Ta.O,concentrations of tantalum and niobium in their solution wereY八Adetermined by inductively coupled plasma-atomic emissionBi,Nbo,Ta,o,人八spectrometry (ICP-AES). Second, Bi(NO3)3·5H,O was addedBi,Tao,into citric acid solution, and then EDTA-ammonia solution wasM八added slowlyinto the solution under continuous stirringuntil203040506070Bi(NOs)3·5H,O was absolutely dissolved and the Bi-CA2Theta (degree)solution was obtained.Third.Ta-OA,Nb-OA.andBi-CA solutionsweremixed(b)together accordingto the composition BisNb,Tai-,O, (x=00.2.0.4.0.6.0.8.and1.0).Theas-obtained solutionwas stirredBi,Nbo,at 80 °C until it became transparent colloidal and was thenheated until the formation of dark-colored, amorphous polymeric(nre) AgsuaulBi,Nba,Ta,,O,precursors occurred.The precursors were subjected to calcina-Bi,Nba.Ta,O,tion at 30o C to remove the organics and then were groundand sintered at 450 C for 4 h.Finally,the BisNb,Taj-,OBi,Nb,Ta.o,nanoparticles were obtained.Bi,Nbo,Ta.,o,2.2. Characterization.The structure and crystallinity of theas-prepared samples were characterized by powder X-rayBi,Tao,diffraction(XRD)onaD/MAX-RB powderX-ray diffracto-meter (Rigaku, Japan) using Cu Kα radiation under the operation25262728930conditionsof 40kv and 50mA.Themorphology and2Theta (degree)microstructure of theas-obtained samples were analyzedbyFigure1.(a)XRD patterns of Bi,Nb,Tai-,Oprepared at 450C fortransmission electron microscopy(TEM)and high-resolution4h.(b)Enlargedpartof theBi,Nb,Tal-,O,XRDpatterns rangingfromtransmission electron microscopy (HRTEM) with a JEM-2100F20=25°-30°electron microscope (JEOL, Japan),using a 200 kV acceleratingvoltage.Theabsorption edgeof theBigNb,Tai-,O, sampleswasa water coolingdevice.Before illumination,the suspension wasmeasured byaUV-vis spectrophotometer(UV2550,Shimadzu,stirred for 30 min in darkness to disperse the catalyst. TheJapan).Thechemicalbondsonthesurfaceofthecatalystsweredetected by the Fourier transform infrared spectroscopy (FT-reactorwasthenirradiatedwithvisiblelightemittedbya300W Dylamp with a 400 nmcutoff filter.A sample of 1mLwasIR) (Nexus, Thermo Nicolet).X-ray photoelectron spectroscopytaken and centrifuged to remove the photocatalyst particles.The(XPS)analysis was carried out by using a ThermoVG Multilabsamples were equally split in 0.2 mL inserts placed in vials2000 spectrometer(UK)withamonochromaticAlKαsourceand analyzed with high-performance liquid chromatographyand a charge neutralizer. All binding energies were referred to(HPLC, Agilent Series 1100)for the quantification of MC-LR.the C1s peak at 284.63eV of the surface adventitious carbonFor the HPLC analysis, the injection volume to a C-18and revised.The generation of 'OH radicals was investigatedDiscoverycolumn(Supelco)at40C was10μL.Themobileby thephotoluminescence (PL)techniquewithterephthalic acid(TA).32 TheOH trapping fluorescence spectra was measuredphaseinisocraticmodewithaflowrateof I mL/minwas amixture of methanol and 0.01% trifluoroacetic acid(TFA)inwith a fluorescence spectrophotometer (RF-5300PC, Shimadzu,waterataratioof60:40.MC-LRwasmeasuredwithaJapan),photodiode array detector at 238 nm.2.3.Photocatalytic Activity Measurement.The photocata-lytic activities of theBigNb,Tai-,O,sampleswere evaluatedby the degradation of ARG and MC-LR under visible light3.Results and Discussionirradiation.For the photocatalytic degradation of ARG, 0.15g3.1.Characterization.Figure1showstheXRDpatternsofof the as-prepared catalyst was added into100mL of ARGthe BigNb,Tar-,O solid solutions prepared by the sol-gelaqueous solution (50 mg/L,pH =6.8).Before illumination,method at 450 °c for 4 h. It is important to note that nothemixturewasstirred for5minindarknesstodispersethereflectionsfromNb2OsTazOs,orother specieswereobservedcatalyst. The reactor was then irradiated with visible lightasdistinct impurities.Thepatterns of BigTaO,andBigNbOemitted by a 300 W Dy lamp with a 420 nm cutoff filter.Thecould be readily indexed toJCPDS(44-0202)and JCPDS (86-concentration of ARG aqueous solution was analyzed using a0875),respectively.Despitethedifferentcomposition,theUV-vis spectrophotometer (UV751GD, China) at its maximumsimilarity of the diffractograms suggests that the compoundsabsorptionwavelengthof505nm.BigNb,Tal-,O(x=0,0.2,0.4,0.6,and0.8)are isostructuralFor the degradation of MC-LR, which was purified from awithBigNbOz.Figure Ib showsthatthe(lll)peak slightlylaboratory culture of Microcystis aeruginosa,0.05g of the as-shifted to smaller angles with the increase in x. The broadpreparedBi,NboTao.O,photocatalystwas suspended in20mLdiffraction peaks of theBisNb,Tai-,Or solid solutions implythe small crystal size of the products.The peak at 28.2°wasof MC-LR solution (pH=5.9± 0.1 with super quality water)with the initial concentration of 3.1 mg/L in a glass reactor.usedfor the calculation of the crystal sizebecause ithad aThe reaction temperature was kept at room temperature usingrelatively strong intensity and did not overlap with the other
sediment was obtained after adjusting the pH of the solution to 1-2 using nitric acid and was then washed successively until the potassium ions were completely removed. The as-prepared tantalic acid was dissolved in oxalic acid solution, and a tantalum-oxalic acid solution (Ta-OA) was obtained. The niobium-oxalic acid solution (Nb-OA) was prepared by the hydrothermal method reported in our previous study.31 The concentrations of tantalum and niobium in their solution were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Second, Bi(NO3)3 · 5H2O was added into citric acid solution, and then EDTA-ammonia solution was added slowly into the solution under continuous stirring until Bi(NO3)3 · 5H2O was absolutely dissolved and the Bi-CA solution was obtained. Third, Ta-OA, Nb-OA, and Bi-CA solutions were mixed together according to the composition Bi3NbxTa1-xO7 (x ) 0, 0.2, 0.4, 0.6, 0.8, and 1.0). The as-obtained solution was stirred at 80 °C until it became transparent colloidal and was then heated until the formation of dark-colored, amorphous polymeric precursors occurred. The precursors were subjected to calcination at 300 °C to remove the organics and then were ground and sintered at 450 °C for 4 h. Finally, the Bi3NbxTa1-xO7 nanoparticles were obtained. 2.2. Characterization. The structure and crystallinity of the as-prepared samples were characterized by powder X-ray diffraction (XRD) on a D/MAX-RB powder X-ray diffractometer (Rigaku, Japan) using Cu KR radiation under the operation conditions of 40 kV and 50 mA. The morphology and microstructure of the as-obtained samples were analyzed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with a JEM-2100F electron microscope (JEOL, Japan), using a 200 kV accelerating voltage. The absorption edge of the Bi3NbxTa1-xO7 samples was measured by a UV-vis spectrophotometer (UV2550, Shimadzu, Japan). The chemical bonds on the surface of the catalysts were detected by the Fourier transform infrared spectroscopy (FTIR) (Nexus, Thermo Nicolet). X-ray photoelectron spectroscopy (XPS) analysis was carried out by using a Thermo VG Multilab 2000 spectrometer (UK) with a monochromatic Al KR source and a charge neutralizer. All binding energies were referred to the C 1s peak at 284.63 eV of the surface adventitious carbon and revised. The generation of • OH radicals was investigated by the photoluminescence (PL) technique with terephthalic acid (TA).32 The • OH trapping fluorescence spectra was measured with a fluorescence spectrophotometer (RF-5300PC, Shimadzu, Japan). 2.3. Photocatalytic Activity Measurement. The photocatalytic activities of the Bi3NbxTa1-xO7 samples were evaluated by the degradation of ARG and MC-LR under visible light irradiation. For the photocatalytic degradation of ARG, 0.15 g of the as-prepared catalyst was added into 100 mL of ARG aqueous solution (50 mg/L, pH ) 6.8). Before illumination, the mixture was stirred for 5 min in darkness to disperse the catalyst. The reactor was then irradiated with visible light emitted by a 300 W Dy lamp with a 420 nm cutoff filter. The concentration of ARG aqueous solution was analyzed using a UV-vis spectrophotometer (UV751GD, China) at its maximum absorption wavelength of 505 nm. For the degradation of MC-LR, which was purified from a laboratory culture of Microcystis aeruginosa, 0.05 g of the asprepared Bi3Nb0.6Ta0.4O7 photocatalyst was suspended in 20 mL of MC-LR solution (pH ) 5.9 ( 0.1 with super quality water) with the initial concentration of 3.1 mg/L in a glass reactor. The reaction temperature was kept at room temperature using a water cooling device. Before illumination, the suspension was stirred for 30 min in darkness to disperse the catalyst. The reactor was then irradiated with visible light emitted by a 300 W Dy lamp with a 400 nm cutoff filter. A sample of 1 mL was taken and centrifuged to remove the photocatalyst particles. The samples were equally split in 0.2 mL inserts placed in vials and analyzed with high-performance liquid chromatography (HPLC, Agilent Series 1100) for the quantification of MC-LR. For the HPLC analysis, the injection volume to a C-18 Discovery column (Supelco) at 40 °C was 10 µL. The mobile phase in isocratic mode with a flow rate of 1 mL/min was a mixture of methanol and 0.01% trifluoroacetic acid (TFA) in water at a ratio of 60:40. MC-LR was measured with a photodiode array detector at 238 nm. 3. Results and Discussion 3.1. Characterization. Figure 1 shows the XRD patterns of the Bi3NbxTa1-xO7 solid solutions prepared by the sol-gel method at 450 °C for 4 h. It is important to note that no reflections from Nb2O5, Ta2O5, or other species were observed as distinct impurities. The patterns of Bi3TaO7 and Bi3NbO7 could be readily indexed to JCPDS (44-0202) and JCPDS (86- 0875), respectively. Despite the different composition, the similarity of the diffractograms suggests that the compounds Bi3NbxTa1-xO7 (x ) 0, 0.2, 0.4, 0.6, and 0.8) are isostructural with Bi3NbO7. Figure 1b shows that the (111) peak slightly shifted to smaller angles with the increase in x. The broad diffraction peaks of the Bi3NbxTa1-xO7 solid solutions imply the small crystal size of the products. The peak at 28.2° was used for the calculation of the crystal size because it had a relatively strong intensity and did not overlap with the other Figure 1. (a) XRD patterns of Bi3NbxTa1-xO7 prepared at 450 °C for 4 h. (b) Enlarged part of the Bi3NbxTa1-xO7 XRD patterns ranging from 2θ ) 25°-30°. 20030 J. Phys. Chem. C, Vol. 113, No. 46, 2009 Zhang et al
J.Phys.Chem.C,Vol.113,No.46,200920031Photocatalytic Removal of Contaminant by Bi,Nb,Taf-,O(a)20nm51/Figure 2. TEM images of Bi,Nbo.Tao.4O, nanoparticles: (a) TEM image; (b) HRTEM image; (c) SAED pattern.prepared sample exists in Nb5+,22.38 The binding energy at 30.1diffraction peaks. The average crystal size of the as-preparedsamples was about 15 nm,as estimated bytheScherrer equation.and 27.2 eVfor Ta 4fs2 and Ta 4fin (Figure 3c)can be assignedto the Tas+ state.39Figure 2 shows the morphology and microstructure of theUV-vis diffuse reflectance spectra of the BigNb,Tar-Ortypical sampleBisNbo.6Tao.4Or.On thebasis of TEM observa-tions inFigure 2a,theparticle sizeof the Bi,Nbo.gTao.Osamplephotocatalysts prepared by the sol-gel method are shown inis only about 12-15 nm, which agrees well with the valueFigure 4.All the samples have photoabsorption from UV lightestimated from theXRDpattern.TheHRTEM image(Figureto visible light, and the wavelengths of the absorption edges2b)shows thelatticefringes with an interplanar spacing of 0.316are around 443 nm.For a crystalline semiconductor,the opticalnm correspondsto the(111)planeof theBi;Nbo.6Tao4O,crystalband gap is determined by the following equation using theoptical absorption data near the band edge40.41Theselectiveareaelectron diffraction(SAED)pattern inFigure2c can be indexed to (311), (220), (200), and (111) planes, whichindicates the particles in Figure 2a were formed by aggregatingahu = A(hu - E,)n2some nanosized single crystals.To investigate the chemical state and the surface composi-where a, , A, and Eg are the absorption coefficient, lighttions, XPS studies have been carried out. The XPS result showsthe existenceofBi 4f.Nb3d.and Ta4f peaks inthefrequency,proportionalityconstant,and bandgap,respectively.Bi,Nbo.Tao.O,sample (seeFigure3).FromFigure3a, theBiIn the equation, n decides the characteristics of the transition4f region can be fitted intofour peaks157.6,159.6,163.1,andinasemiconductor,thatis,directtransition(n=)orindirect164.9 eV.The binding energy value of Bi3+ state in some Bi-transition (n =4).The values of n and Egwere determined bycontaining compounds is about in the range of 158.2-159.6thefollowing steps: (1)plot ln(ahu) vs ln(hu-E,), using aneV for Bi 4fin334 which is very close to Bi2+ state. In theapproximate value of Eg,and then determine the valueofn withstructure of the compound Bi,Nb..Tao.Oz,there exist twothe slope of the straightest line near the band edge; (2) plotdifferent Bi0 distances,ranging from 1.94 to 2.61A.35.36 The(ahv)2/nvs hy and then evaluatetheband gapE,by extrapolatingtwo strong peaks taken for the Bi region at 164.9 and 159.6 eVthe straightest line to the hy axis intercept. Following thisare assigned to the Bi 4fsn and Bi 4fin peaks of Bi3+(1) in themethod,thevalue of nfor BigNb,Ta-,Owas estimated tobeshort Bi-O bonds.The lower binding energy peaks (163.1,2. This means that the optical transitions for the present157.6eV)correspond totheBi4fsnandBi4frn2peaksof thecompounds are indirectly allowed.The values of the band gapsBi3+(2)in the long Bi-O bonds.The lower binding energyfor the catalysts were estimated to be around 2.80eV.With anpeaks (163.1, 157.6 eV) also indicated the possibility appearanceincreasing amount of Ta substitution, the absorbance onset ofof Bi+3-xat the surface of BisNbo.6Tao.aOnanoparticles,whichthe as-obtained samples showeda slight shift to shortermay be dueto a deficiency in oxygen and an enhancedwavelengths.The results indicate that the BisNb,Tai-Oconcentration of oxygen vacancies in the vicinity of bismuthnanoparticles have a suitable band gap for photocatalyticcations,37 resulting in the higher quantum efficiency of photo-decomposition of organic contaminants under visible lightcatalysis. The peaks with binding energy of 207.9 and 209.8irradiation.eV,as displayed in Figure3b,arefor the Nb 3ds2 and Nb 3da/23.2.PhotocatalvticProperties.Thephotocatalyticactivitiesregion, respectively,which indicates that niobium in the as-of theBigNb,Tai-,O samples wereevaluated via thedegrada-
diffraction peaks. The average crystal size of the as-prepared samples was about 15 nm, as estimated by the Scherrer equation. Figure 2 shows the morphology and microstructure of the typical sample Bi3Nb0.6Ta0.4O7. On the basis of TEM observations in Figure 2a, the particle size of the Bi3Nb0.6Ta0.4O7 sample is only about 12-15 nm, which agrees well with the value estimated from the XRD pattern. The HRTEM image (Figure 2b) shows the lattice fringes with an interplanar spacing of 0.316 nm corresponds to the (111) plane of the Bi3Nb0.6Ta0.4O7 crystal. The selective area electron diffraction (SAED) pattern in Figure 2c can be indexed to (311), (220), (200), and (111) planes, which indicates the particles in Figure 2a were formed by aggregating some nanosized single crystals. To investigate the chemical state and the surface compositions, XPS studies have been carried out. The XPS result shows the existence of Bi 4f, Nb 3d, and Ta 4f peaks in the Bi3Nb0.6Ta0.4O7 sample (see Figure 3). From Figure 3a, the Bi 4f region can be fitted into four peaks 157.6, 159.6, 163.1, and 164.9 eV. The binding energy value of Bi3+ state in some Bicontaining compounds is about in the range of 158.2-159.6 eV for Bi 4f7/2, 33,34 which is very close to Bi2+ state. In the structure of the compound Bi3Nb0.6Ta0.4O7, there exist two different Bi-O distances, ranging from 1.94 to 2.61 Å.35,36 The two strong peaks taken for the Bi region at 164.9 and 159.6 eV are assigned to the Bi 4f5/2 and Bi 4f7/2 peaks of Bi3+(1) in the short Bi-O bonds. The lower binding energy peaks (163.1, 157.6 eV) correspond to the Bi 4f5/2 and Bi 4f7/2 peaks of the Bi3+(2) in the long Bi-O bonds. The lower binding energy peaks (163.1, 157.6 eV) also indicated the possibility appearance of Bi+3-x at the surface of Bi3Nb0.6Ta0.4O7 nanoparticles, which may be due to a deficiency in oxygen and an enhanced concentration of oxygen vacancies in the vicinity of bismuth cations,37 resulting in the higher quantum efficiency of photocatalysis. The peaks with binding energy of 207.9 and 209.8 eV, as displayed in Figure 3b, are for the Nb 3d5/2 and Nb 3d3/2 region, respectively, which indicates that niobium in the asprepared sample exists in Nb5+. 22,38 The binding energy at 30.1 and 27.2 eV for Ta 4f5/2 and Ta 4f7/2 (Figure 3c) can be assigned to the Ta5+ state.39 UV-vis diffuse reflectance spectra of the Bi3NbxTa1-xO7 photocatalysts prepared by the sol-gel method are shown in Figure 4. All the samples have photoabsorption from UV light to visible light, and the wavelengths of the absorption edges are around 443 nm. For a crystalline semiconductor, the optical band gap is determined by the following equation using the optical absorption data near the band edge40,41 where a, V, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively. In the equation, n decides the characteristics of the transition in a semiconductor, that is, direct transition (n ) 1) or indirect transition (n ) 4). The values of n and Eg were determined by the following steps: (1) plot ln(ahV) vs ln(hV - Eg), using an approximate value of Eg, and then determine the value of n with the slope of the straightest line near the band edge; (2) plot (ahV) 2/n vs hV and then evaluate the band gap Eg by extrapolating the straightest line to the hV axis intercept. Following this method, the value of n for Bi3NbxTa1-xO7 was estimated to be 2. This means that the optical transitions for the present compounds are indirectly allowed. The values of the band gaps for the catalysts were estimated to be around 2.80 eV. With an increasing amount of Ta substitution, the absorbance onset of the as-obtained samples showed a slight shift to shorter wavelengths. The results indicate that the Bi3NbxTa1-xO7 nanoparticles have a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation. 3.2. Photocatalytic Properties. The photocatalytic activities of the Bi3NbxTa1-xO7 samples were evaluated via the degradaFigure 2. TEM images of Bi3Nb0.6Ta0.4O7 nanoparticles: (a) TEM image; (b) HRTEM image; (c) SAED pattern. ahV ) A(hV - Eg) n/2 Photocatalytic Removal of Contaminant by Bi3NbxTa1-xO7 J. Phys. Chem. C, Vol. 113, No. 46, 2009 20031
20032Zhang et al.J.Phys.Chem.C,Vol.113,No.46,20091.6Bi4fin(a)(ne)ouguosavN1.4 -Bi4fsneue1.2 -10oeo10.80.6Wavelength (nm)0.40.20.0156158160162168164166300400500600Binding Energy (eV)Wavelength (nm)7(b)Nb3d.a6Nb3d,neaea5-a20A12.800+2042082122062102.02.53.03.54.04.51.55.0Binding Energy (eV)hv (eV)Figure 4. UV-vis diffuse reflectance spectra of the as-preparedTafn(c)BigNb,TaiO, samples.Arrow direction:x=0.0.2,0.4,0.6,0.8,and1.0.(e sre700 C for 4 h (SS-BigNbo.6Tao.4O).42 It was found thatTa4fsnBigNbo.6Tao.Or prepared by the sol-gel method was about 6times fasterthan SS-BigNboTao.Oforthedecomposition ofARG.This may be attributed to the large particle size of SS-BigNbo.6Tao.gOr,which results in a poor dispersion of catalystin the solution. As can be seen in Figure 5b, all of theBi,Nb,Ta,-O,samples showtheexcellentphotocatalytic activ-ity under visible light irradiation. With the increase of x, the22242628303234activity of the as-obtained catalysts slightly increased. AmongtheBisNb,TaO,-,Ophotocatalysts,BisNbo.6Tao.O shows theBindingEnergy (eV)highest activity.Figure 3. High-resolution XPS spectra of Bi,Nbo.6Tao4O, nanopar-Figure 6 displays the temporal evolution of the spectralticles: (a) Bi 4f: (b) Nb 3d: (c) Ta 4f.changes during the photodegradation of ARG over theBigNbo.6Tao.O, sample.The absorption spectrum of the originaltion of ARG and MC-LR under visible light irradiation.Figuresolution shows three distinctive peaks at 215,330, and 505 nm5displays thephotodegradationofARGover theBigNb,Tai-,Owhich correspond to the structure of the benzene ring,thesamples under different conditions.As shown in Figure 5a,anaphthalene ring, and the nitrogen to nitrogen double bondblank experiment in the absence of the photocatalyst butunder(-N-N-),respectively.Thedecreaseof absorption peaks ofvisible light irradiation showed that the photolysis of ARG wasacid red G at 505 nm means that the nitrogen to nitrogen doublenegligible.However,with Bi,Nbo.6Tao.4O as the photocatalyst,bond was destroyed. The specific peaks of the benzene ring97%of ARG isdecolorizedafter120min,showing theexcellentand naphthalene ringat215 and330nm become smootherphotocatalytic activity of BigNbo.6Tao.4Ounder visible lightgradually during the degradation processes,which indicated thatirradiation. The adsorption of ARG on theBigNbo.Tao.4Othe catalyst not only destroyed the chromophore of ARG butalso decomposed the benzene ring and naphthalene ring partly.26sample in the dark was also checked; the concentration of ARGwas only decreased 23% after 120 min, suggesting that theFigure 7 shows the FT-IR spectra of the BigNbo.6Tao.4OrdecolorizingofARGismainlycausedbyphotodegradationbutnanopowdersbeforeandafterthephotocatalyticreaction ofARGdye.Thebroadbandsbetween450and1000cm-intwonotadsorption.Forcomparison,thephotocatalyticpropertiesof P25 and SS-BigNbo.6Tao.O, were also evaluated.Afterspectra are mainly assignable toBi-O, M-O(M=Nb orTa),and M-O-M stretching vibrations.43-45 The bands at aboutirradiation by visible light for 120 min, the degradation rate ofARGbyP25wasonly30%.ThepureBigNbo.6Tao.Ocom3430and1635cm-1can beascribedtowatermoleculespound was prepared by a conventional solid-state method atadsorbed on the surface of the catalysts.The band at 1385-1390
tion of ARG and MC-LR under visible light irradiation. Figure 5 displays the photodegradation of ARG over the Bi3NbxTa1-xO7 samples under different conditions. As shown in Figure 5a, a blank experiment in the absence of the photocatalyst but under visible light irradiation showed that the photolysis of ARG was negligible. However, with Bi3Nb0.6Ta0.4O7 as the photocatalyst, 97% of ARG is decolorized after 120 min, showing the excellent photocatalytic activity of Bi3Nb0.6Ta0.4O7 under visible light irradiation. The adsorption of ARG on the Bi3Nb0.6Ta0.4O7 sample in the dark was also checked; the concentration of ARG was only decreased 23% after 120 min, suggesting that the decolorizing of ARG is mainly caused by photodegradation but not adsorption. For comparison, the photocatalytic properties of P25 and SS-Bi3Nb0.6Ta0.4O7 were also evaluated. After irradiation by visible light for 120 min, the degradation rate of ARG by P25 was only 30%. The pure Bi3Nb0.6Ta0.4O7 compound was prepared by a conventional solid-state method at 700 °C for 4 h (SS-Bi3Nb0.6Ta0.4O7).42 It was found that Bi3Nb0.6Ta0.4O7 prepared by the sol-gel method was about 6 times faster than SS-Bi3Nb0.6Ta0.4O7 for the decomposition of ARG. This may be attributed to the large particle size of SSBi3Nb0.6Ta0.4O7, which results in a poor dispersion of catalyst in the solution. As can be seen in Figure 5b, all of the Bi3NbxTa1-xO7 samples show the excellent photocatalytic activity under visible light irradiation. With the increase of x, the activity of the as-obtained catalysts slightly increased. Among the Bi3NbxTa01-xO7 photocatalysts, Bi3Nb0.6Ta0.4O7 shows the highest activity. Figure 6 displays the temporal evolution of the spectral changes during the photodegradation of ARG over the Bi3Nb0.6Ta0.4O7 sample. The absorption spectrum of the original solution shows three distinctive peaks at 215, 330, and 505 nm, which correspond to the structure of the benzene ring, the naphthalene ring, and the nitrogen to nitrogen double bond (-NdN-), respectively. The decrease of absorption peaks of acid red G at 505 nm means that the nitrogen to nitrogen double bond was destroyed. The specific peaks of the benzene ring and naphthalene ring at 215 and 330 nm become smoother gradually during the degradation processes, which indicated that the catalyst not only destroyed the chromophore of ARG but also decomposed the benzene ring and naphthalene ring partly.26 Figure 7 shows the FT-IR spectra of the Bi3Nb0.6Ta0.4O7 nanopowders before and after the photocatalytic reaction of ARG dye. The broad bands between 450 and 1000 cm-1 in two spectra are mainly assignable to Bi-O, M-O (M ) Nb or Ta), and M-O-M stretching vibrations.43-45 The bands at about 3430 and 1635 cm-1 can be ascribed to water molecules adsorbed on the surface of the catalysts. The band at 1385-1390 Figure 3. High-resolution XPS spectra of Bi3Nb0.6Ta0.4O7 nanoparticles: (a) Bi 4f; (b) Nb 3d; (c) Ta 4f. Figure 4. UV-vis diffuse reflectance spectra of the as-prepared Bi3NbxTa1-xO7 samples. Arrow direction: x ) 0, 0.2, 0.4, 0.6, 0.8, and 1.0. 20032 J. Phys. Chem. C, Vol. 113, No. 46, 2009 Zhang et al
J.Phys.Chem.C, Vol.113,No.46,200920033PhotocatalyticRemovalofContaminantbyBi,NbTaj-O1.0/ (a)originalE8E!E0.8000.6light offO0photolysis0.4 oSS methodrecovered>191006E1P250.2Aadsorption indarkphotocatalysisEE0.020-200406080100120195Irradiation Time (min)3500150050040003000250020001000100 (b)x0.6Wavenumbers(cm)x=1x=0.8Figure 7. FT-IR spectra of the BigNbo.6Tao4O, catalyst before andx=0.4x=0.2afterthephotocatalytic reaction.x=0803.0photolysis2.5adsorption in dark40photocatalysis201.5-01.0SamplesHlight off light or0.5-Figure 5. (a)Photocatalytic degradation of ARG solution (50 mg/L)of BigNbo.6Tao4O under different conditions. (b)Photocatalyticdegradation of ARG solution overBi,NbTa-,O,nanoparticlesunder0.0-050100150200250visible light illumination within 120 min.Irradiation Time (min)215nmFigure 8. Photocatalytic degradation of MC-LR solution (3.1 mg/L)1.4-at pH =5.9 over the Bi,Nbo.6Tao.4O, sample.1.2Original0min1.0odrinking water has been issued by the World Health Organiza-20 min40mintion.47 To further reveal the photocatalytic properties of the as-10E60 minprepared BiNb,Ta-,O catalysts, MC-LR was selected to90minevaluatethephotocatalyticactivity.Figure8showsthedecrease120minoftheMC-LRconcentrationundervisiblelightinthepresenceof thetypical sampleBi,Nbo.6Tao.4O7.As shown in Figure8,0.2 nophotolysisof MC-LRundervisiblelight inthe absenceofcatalyst is observed. The adsorption of MC-LR on the0.0BisNbo.6TaogOz sample in the dark was also checked.After 240200300400500600700800min,the concentration of MC-LR was decreased 49%.ThisWavelength (nm)result suggests that BigNbo.6Tao.O, has high adsorption abilityFigure 6. UV-vis spectral changes of ARG solution (50 mg/L) duringfor MC-LR,which is in favor of the photocatalytic reaction.the photocatalytic reaction bytheBi,NbosTao.O,sampleundervisibleHowever, with Bi,Nbo.6Tao.4O as the photocatalyst, 94% of MC-light illumination.LR was removedafter visible light irradiation for 240 min,suggesting the BigNbo.6Tao.O photocatalyst exhibits a highphotocatalytic activityfordegradationof MC-LR under visiblecm- is attributed to the vibrations associated with uncoordinatedlight irradiation.NO,ions, which are always exist in the raw material, indicatingInthis work,all the BigNb,Ta-,O solid solutions werethe presence of a trace amount of adsorbed NOin the as-synthesized product.46 As shown in Figure 6, there are noobserved to share a similar phase structure.The superioritysignificant differences between the two spectra and no charac-photocatalyticactivityoftheBisNb,Taj-,Osampleswasmainlyteristic peaks of ARG dye in the spectrum of the recoveredattributed to their narrow band gaps, small particle size, andBisNbo.6Tao.4O catalyst. The FT-IR analysis results confirm thatthe oxygen vacancies on the surface of the catalysts. A smallthe degradation of ARGdye is ascribed to the photocatalysisband gap is effective in generating charge carriers. For therather than physical adsorption and that the as-prepared catalystcatalysts,the hybridization of O 2p and Bi 6s would push upis stable before and after the degradation.the position of the valence band,resulting in the narrow bandMicrocystin-LR (MC-LR) has been regarded as the mostgaps.23 Additionally, for nanoparticles, the diffusion length ofcommon and the most toxic variant among~80 microcystins,electrons and holes (e-/h+) from the bulk to the surface is short,and a provisional guideline value of I μg/L for MC-LR inwhich helps to accelerate the migration rate of e-/h+to the
cm-1 is attributed to the vibrations associated with uncoordinated NO3 - ions, which are always exist in the raw material, indicating the presence of a trace amount of adsorbed NO3 - in the assynthesized product.46 As shown in Figure 6, there are no significant differences between the two spectra and no characteristic peaks of ARG dye in the spectrum of the recovered Bi3Nb0.6Ta0.4O7 catalyst. The FT-IR analysis results confirm that the degradation of ARG dye is ascribed to the photocatalysis rather than physical adsorption and that the as-prepared catalyst is stable before and after the degradation. Microcystin-LR (MC-LR) has been regarded as the most common and the most toxic variant among ∼80 microcystins, and a provisional guideline value of 1 µg/L for MC-LR in drinking water has been issued by the World Health Organization.47 To further reveal the photocatalytic properties of the asprepared Bi3NbxTa1-xO7 catalysts, MC-LR was selected to evaluate the photocatalytic activity. Figure 8 shows the decrease of the MC-LR concentration under visible light in the presence of the typical sample Bi3Nb0.6Ta0.4O7. As shown in Figure 8, no photolysis of MC-LR under visible light in the absence of catalyst is observed. The adsorption of MC-LR on the Bi3Nb0.6Ta0.4O7 sample in the dark was also checked. After 240 min, the concentration of MC-LR was decreased 49%. This result suggests that Bi3Nb0.6Ta0.4O7 has high adsorption ability for MC-LR, which is in favor of the photocatalytic reaction. However, with Bi3Nb0.6Ta0.4O7 as the photocatalyst, 94% of MCLR was removed after visible light irradiation for 240 min, suggesting the Bi3Nb0.6Ta0.4O7 photocatalyst exhibits a high photocatalytic activity for degradation of MC-LR under visible light irradiation. In this work, all the Bi3NbxTa1-xO7 solid solutions were observed to share a similar phase structure. The superiority photocatalytic activity of the Bi3NbxTa1-xO7 samples was mainly attributed to their narrow band gaps, small particle size, and the oxygen vacancies on the surface of the catalysts. A small band gap is effective in generating charge carriers. For the catalysts, the hybridization of O 2p and Bi 6s would push up the position of the valence band, resulting in the narrow band gaps.23 Additionally, for nanoparticles, the diffusion length of electrons and holes (e-/h+) from the bulk to the surface is short, which helps to accelerate the migration rate of e-/h+ to the Figure 5. (a) Photocatalytic degradation of ARG solution (50 mg/L) of Bi3Nb0.6Ta0.4O7 under different conditions. (b) Photocatalytic degradation of ARG solution over Bi3NbxTa1-xO7 nanoparticles under visible light illumination within 120 min. Figure 6. UV-vis spectral changes of ARG solution (50 mg/L) during the photocatalytic reaction by the Bi3Nb0.6Ta0.4O7 sample under visible light illumination. Figure 7. FT-IR spectra of the Bi3Nb0.6Ta0.4O7 catalyst before and after the photocatalytic reaction. Figure 8. Photocatalytic degradation of MC-LR solution (3.1 mg/L) at pH ) 5.9 over the Bi3Nb0.6Ta0.4O7 sample. Photocatalytic Removal of Contaminant by Bi3NbxTa1-xO7 J. Phys. Chem. C, Vol. 113, No. 46, 2009 20033
20034J.Phys.Chem.C,Vol.113,No.46,2009Zhang et al.50SCHEME 1:Proposed Mechanism for thePhotocatalyticDegradation of OrganicPollutants overBigNb,Tai-rO7Photocatalysts400120mint0006090 minhv (Vis.60 min40 min20 min0 min0-reactantslegradation products350400450500Wavelength (nm)O,Figure 9. OH trapping PL spectra of BisNba.6Tao.4Or on TA solution(0.5g/L)under visibleirradiation.forBigNbo.6Tao.4Ophotocatalysis.TheO2-couldbeformed85 mL/min N, saturatedbyphotogeneratedelectronreactingdirectlywithO,adsorbed1.00—ARG + isopropanol (10 M)on the surface of the catalyst.The photocatalytic reactivity withARGcontinuous N2 sparging and thus in a stricter anoxic condition0.8 waschecked.As canbeseen inFigureIO.thephotodegradationrate of ARGwas largely prohibited under the anoxic suspension.0.6ightofflight onApparently,thepresence of oxygenisresponsibleforthecsignificant reduction.The presence of oxygen is to primarilyC0.4act as an efficient e-trap,leadingto the generation of Oandpreventing therecombination of e-and h+53On thebasis ofthe discussion above,a possible mechanism for the photodeg-0.2radation of organic pollutants has been proposed (Scheme I).0.0-200204060801001204.ConclusionsIrradiation Time (min)In summary, efficient visible-light-induced BisNb,Tai-,OFigure 10. Photocatalytic degradation of ARG solution (50 mg/L) overnanoparticles were synthesized by a facile and low-cost sol-gelBi,Nbo.Tao.O, nanoparticles under different conditions.method usingstable,lesstoxicTazOs,NbOs,andBi(NO)3-5H2Oas the raw materials.The optical band gaps ofsurface of the nanoparticletoparticipate in the reaction process.48the BigNb,Ta-,O, nanoparticles were estimated tobe about 2.80Furthermore,thecompoundsBisNbOandBisTaObotheV.The average crystal size of the as-prepared samples wascrystallize in a defect fluorite-type structure with distortedonly about 15 nm.These photocatalysts exhibited strongoctahedral coordination geometry and exhibit intrinsic oxygenphotocatalytic activity for the degradation of ARG under visiblevacancies and a disorder in both anionic and cationic lattices.36light irradiation.Moreover,thetypical sampleBisNbo.cTao.OTherefore,during the process of photocatalytic reactions,thealsoshowedexcellentphotocatalvticpropertiesfortheremovaloxygen vacanciesin the structureof compoundsBigNb,Tai-,O7of MC-LR.Kinetic studies by using 'OH trapping PL spectrawhich are isostructural with BigNbOr, may become the centersand the radical scavenger technologies suggest that OH is notto capture photoinduced electrons so that therecombination ofthe dominant photooxidant.TheO2is thepivotal active speciesphotoinducedelectronsandholescouldbeeffectivelyinhibitedforBi,Nb,Ta,-,O,photocatalysis.These results suggested thatresulting in the higher quantum efficiencyof photocatalysis.49.50BigNb,Tar-,O is the prominent materials for photooxidationThe distorted octahedral coordination geometry in the structureof organics in the range of visible light.can also improve the mobility of photogenerated charge carriersand enhances its visible light photocatalytic properties.3851Acknowledgment.This work was supported by the NationalForthe photocatalytic process,photoinduced electrons orNatural Science Foundation of China (50872013),Nationalholes emerging over a semiconductor directly or indirectly reactBasicResearchProgram of China (973Program)2007CB613302,withO,andOH-toformOand'OHoxidativespecies.52Asand the Key Project of Chinese Ministry of Education (No.shown in Figure 9, the fluorescence intensity at 426 nm is very108164).low,which elucidates that'OH on BisNbo.6Tao.aO catalyst ishardly produced under visible light irradiation. In anotherReferences and Notesexperiment, it wasfound that theaddition of isopropanol,a well-(1)Fox,M.A.;Dulay,M.T.Chem.Reu.1993,93,341known scavenger of OH radicals.53 into the photoreaction(2)Yu,J.C:Yu.J.G.Ho,W.K.Jiang.Z.T.;Zhang,L.Z.Chemsystem did not cause the apparent changes in the degradationMater.2002.14,3808(3) Kapoor, M. P.; Inagaki, S.; Yoshida, H. J. Phys. Chem. B 2005,rate of ARG, as shown in Figure 10.The results indicate that109.9231the freeOH radicals could not bethe main activeoxygen(4) Kaneko, M.; Gokan, N.; Katakura, N.; Takei, Y.; Hoshino, M.species in this photochemical process,which is differentfromChem.Commum.2005,12,1625.TiO2toward photocatalyticdegradationforganicpollutants.545s(5) Martin, S. T.; Lee, A.T.; Hoffmann, M. R. Enuiron.Sci. Technol.1995,29,2567As mentioned above, the sample shows the excellent photo-(6) Adachi, M.; Murata, Y., Takao, J.; Jiu, J.T.; Sakamoto, M; Wang.catalytic activity under visible light irradiation. Therefore, itF.M. J.Am.Chem.Soc.2004, 126,14943.could be postulated that O2is the other pivotal active species(7)Yu, J.C.;Zhang, L.Z.;Yu, J.G.Chem.Mater.2002, 14, 4647
surface of the nanoparticle to participate in the reaction process.48 Furthermore, the compounds Bi3NbO7 and Bi3TaO7 both crystallize in a defect fluorite-type structure with distorted octahedral coordination geometry and exhibit intrinsic oxygen vacancies and a disorder in both anionic and cationic lattices.36 Therefore, during the process of photocatalytic reactions, the oxygen vacancies in the structure of compounds Bi3NbxTa1-xO7, which are isostructural with Bi3NbO7, may become the centers to capture photoinduced electrons so that the recombination of photoinduced electrons and holes could be effectively inhibited, resulting in the higher quantum efficiency of photocatalysis.49,50 The distorted octahedral coordination geometry in the structure can also improve the mobility of photogenerated charge carriers and enhances its visible light photocatalytic properties.38,51 For the photocatalytic process, photoinduced electrons or holes emerging over a semiconductor directly or indirectly react with O2 and OH- to form O2 •- and • OH oxidative species.52 As shown in Figure 9, the fluorescence intensity at 426 nm is very low, which elucidates that • OH on Bi3Nb0.6Ta0.4O7 catalyst is hardly produced under visible light irradiation. In another experiment, it was found that the addition of isopropanol, a wellknown scavenger of • OH radicals,53 into the photoreaction system did not cause the apparent changes in the degradation rate of ARG, as shown in Figure 10. The results indicate that the free • OH radicals could not be the main active oxygen species in this photochemical process, which is different from TiO2 toward photocatalytic degradation of organic pollutants.54,55 As mentioned above, the sample shows the excellent photocatalytic activity under visible light irradiation. Therefore, it could be postulated that O2 •- is the other pivotal active species for Bi3Nb0.6Ta0.4O7 photocatalysis. The O2 •- could be formed by photogenerated electron reacting directly with O2 adsorbed on the surface of the catalyst. The photocatalytic reactivity with continuous N2 sparging and thus in a stricter anoxic condition was checked. As can be seen in Figure 10, the photodegradation rate of ARG was largely prohibited under the anoxic suspension. Apparently, the presence of oxygen is responsible for the significant reduction. The presence of oxygen is to primarily act as an efficient e- trap, leading to the generation of O2 •- and preventing the recombination of e- and h+. 53 On the basis of the discussion above, a possible mechanism for the photodegradation of organic pollutants has been proposed (Scheme 1). 4. Conclusions In summary, efficient visible-light-induced Bi3NbxTa1-xO7 nanoparticles were synthesized by a facile and low-cost sol-gel method using stable, less toxic Ta2O5, Nb2O5, and Bi(NO3)3 · 5H2O as the raw materials. The optical band gaps of the Bi3NbxTa1-xO7 nanoparticles were estimated to be about 2.80 eV. The average crystal size of the as-prepared samples was only about 15 nm. These photocatalysts exhibited strong photocatalytic activity for the degradation of ARG under visible light irradiation. Moreover, the typical sample Bi3Nb0.6Ta0.4O7 also showed excellent photocatalytic properties for the removal of MC-LR. Kinetic studies by using • OH trapping PL spectra and the radical scavenger technologies suggest that • OH is not the dominant photooxidant. The O2 •- is the pivotal active species for Bi3NbxTa1-xO7 photocatalysis. These results suggested that Bi3NbxTa1-xO7 is the prominent materials for photooxidation of organics in the range of visible light. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50872013), National Basic Research Program of China (973 Program) 2007CB613302, and the Key Project of Chinese Ministry of Education (No. 108164). References and Notes (1) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (2) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Chem. Mater. 2002, 14, 3808. (3) Kapoor, M. P.; Inagaki, S.; Yoshida, H. J. Phys. Chem. B 2005, 109, 9231. (4) Kaneko, M.; Gokan, N.; Katakura, N.; Takei, Y.; Hoshino, M. Chem. Commun. 2005, 12, 1625. (5) Martin, S. T.; Lee, A. T.; Hoffmann, M. R. EnViron. Sci. Technol. 1995, 29, 2567. (6) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J. T.; Sakamoto, M.; Wang, F. M. J. Am. Chem. Soc. 2004, 126, 14943. (7) Yu, J. C.; Zhang, L. Z.; Yu, J. G. Chem. Mater. 2002, 14, 4647. Figure 9. • OH trapping PL spectra of Bi3Nb0.6Ta0.4O7 on TA solution (0.5 g/L) under visible irradiation. Figure 10. Photocatalytic degradation of ARG solution (50 mg/L) over Bi3Nb0.6Ta0.4O7 nanoparticles under different conditions. SCHEME 1: Proposed Mechanism for the Photocatalytic Degradation of Organic Pollutants over Bi3NbxTa1-xO7 Photocatalysts 20034 J. Phys. Chem. C, Vol. 113, No. 46, 2009 Zhang et al
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