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Applied Surface Science 257 (2011) 4621–4624

Contents lists available at ScienceDirect

Applied Surface Science

journal homepage: www.elsevier.com/locate/apsusc

Photoelectrocatalytic degradation of organic contaminants at Bi2O3/TiO2

nanotube array electrode

Xu Zhao, Huijuan Liu, Jiuhui Qu∗

State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, PR China

article info

Article history:

Received 11 April 2010

Received in revised form

16 December 2010

Accepted 20 December 2010

Available online 24 December 2010

Keywords:

TiO2nanotube array

Bi2O3

Photoelectrocatalysis

Visible light

Photocatalyst

abstract

In the current work, TiO2nanotube array was prepared via electrochemical anode method. Then the Bi2O3

nanoparticles were deposited onto the TiO2nanotube array via dip-coating method from an amorphous

complex precursor. The crystal structures were characterized via X-ray diffraction analysis. Their sur-

face textures were observed via electron-scanning microscope. The prepared composite array electrode

exhibited high photoelectrocatalytic activities towards degrading organic contaminants under visible

light irradiation. High photoelectrocatalytic activities were also exhibited under UV light irradiation. The

catalytic mechanism was discussed based on the analysis of electrochemical and degradation kinetics

results. It is suggestedaP(Bi

2O3)–N (TiO2) junction was formed to increase the catalytic activates. The

stability of the electrode materials was conﬁrmed ﬁnally.

1. Introduction

In recent years, highly ordered TiO2nanotube arrays prepared

by electrochemical anodization have attracted much attention due

to their unique architecture. A TiO2nanotube array can form a uni-

form TiO2ﬁlm on a titanium substrate, which provides an ordered

structure and porous surface, without a decrease in geometric

thickness [1]. Such an infrastructure is especially favorable for mass

diffusion. Therefore, TiO2nanotube arrays show great potential for

practical environmental puriﬁcation [2,3].

As a wide band gap semiconductor, TiO2need to be excited by

an ultraviolet light which is less than 5% of the solar irradiance

at the Earth’s surface. To prepare TiO2photocatalysts with visi-

ble light responsibility, several strategies have been adopted. One

of the approaches is coupling TiO2with other semiconductor with

appropriate band gaps. A large number of coupled polycrystalline or

colloidal semiconductor, in which the particles adhere to each other

in so-called sandwich structures or present a core–shell geome-

try, have been prepared such as SiO2–TiO2, CdS–TiO2, ZnO–TiO2,

SnO2–TiO2, NiO–Bi2O3, etc. Recently, several coupled systems such

as Bi2O3/SiTiO3[4–7] were reported to be efﬁcient under visible

light irradiation. However, the photocatalytic mechanism for the

coupled system has not been systematically investigated, and no

clear evidence was provided for the complete decomposition of

organic pollutants under visible light.

∗Corresponding author. Tel.: +86 10 62849151; fax: +86 10 62923558.

E-mail address: jhqu@mail.rcees.ac.cn (J. Qu).

Bismuth oxide, Bi2O3, due to its high refractive index, dielectric

permittivity, marked photoconductivity and photoluminescence, is

used in a variety of areas, such as sensor technology, optical coat-

ings, and electrochromic materials. As a photocatalyst, Bi2O3is a

p-type semiconductor with conduction and valence band edges of

+0.33 and +3.13 V (vs. NHE), respectively [8]. Under visible light

irradiation, the photogenered electron and hole is able to oxidize

water. And, highly reactive species, such as O2•− and •OH radicals

may be generated, which may act as initiators of oxidation reactions

[8–10].

In the present paper, we report a novel loading of Bi2O3

nanoparticles onto a TiO2electrode with a highly ordered verti-

cally oriented nanotube array prepared by anode oxidation. The

obtained results indicated that organic contaminants can be efﬁ-

ciently degraded by the Bi2O3/TiO2nanotube array electrode under

UV and visible light irradiation. And, the composite electrode exhib-

ited higher catalytic activities towards the degradation of organic

contaminants than the individual Bi2O3or TiO2electrode did.

The enhanced mechanism was discussed. 2,4-Dichlorophenol, a

kind of typical refractory contaminants was used in the present

work.

2. Experimental

2.1. Preparation and characterization of Bi2O3/TiO2nanotube

array

All chemicals were analytical grade reagents and used without

further treatment. Electrolyte was freshly prepared from deionized

doi:10.1016/j.apsusc.2010.12.099

4622 X. Zhao et al. / Applied Surface Science 257 (2011) 4621–4624

Fig. 1. (a) The XRD patterns for TiO2(A) and Bi2O3/TiO2(B) nanotube composite array electrodes, (b) SEM analysis of TiO2(A) and Bi2O3/TiO2(B) nanotube composite array

electrodes.

water. After chemical polishing, titanium foil (thickness about

250 ␮m, purity 99.4%, Beijing Cuibolin Non-Ferrous Technology

Developing Co., Ltd.) was subjected to potentiostatic anodization

in an electrochemical anodization cell with a platinum cathode

in a 0.5wt% HF+1M H

3PO4electrolyte at ambient temperature.

The potential of 20 V was applied for 30 min. Then the samples

were rinsed with deionized water and annealed at 450 ◦C for 24 h.

Porous Bi2O3nanoparticles were deposited onto the TiO2nanotube

array from an amorphous heteronuclear complex via a dip-coating

method.

X-ray diffraction (XRD) of the composite electrodes was

recorded on a Scintag-XDS-2000 diffractometer with Cu K␣radia-

tion. The morphology of the Bi2O3nanoparticles was characterized

by a JSM 6301 electron-scanning microscope (SEM). The photo-

electrochemical measure employed a basic electrochemical system

(Princeton Applied Research) connected with a counter-electrode

(Pt wire, 70 mm in length with a 0.4-mm diameter), a working elec-

trode (the hybrid electrode, active area of 11 cm2), and a reference

electrode (a saturated calomel electrode (SCE)). Na2SO4solution

(0.12 mg/L) was used as electrolyte solution.

2.2. Degradation experiments

Degradation experiments were performed in a cylindrical

quartz reactor. The reactor, which contained a 60 mL sample solu-

tion allowing 4.5 cm of the supported ﬁlm electrode to be immersed

into the solution, was placed 3 cm in front of a 150 W Xe lamp pur-

chased from the German Osram. The intensity of light, as measured

by an irradiance meter (Instruments of Beijing Normal University)

was c.a. 25 mW/cm2at 4 cm into the reactor, the position where

the electrode was placed. The photoelectrocatalytic (PEC) reac-

tion employed a basic electrochemical system (Princeton Applied

Research) connected with a counter-electrode (Pt wire, 70 mm

in length with a 0.4-mm diameter), a working electrode (hybrid

electrode, active area of 10 cm2), and a reference electrode (a sat-

urated calomel electrode (SCE)). 0.1 M Na2SO4solution was used

as electrolyte solution. Initial concentration of the organic contam-

inants solution was 10 mg/L and was analyzed by reversed-phase

high performance liquid chromatogram (HPLC) with a Hitachi HPLC

apparatus (Diode Array Detector L-2450, Column Oven L-2300, and

Pump L-2130). All experiments were carried out at least in dupli-

cate. The reported values are within the experimental error of ±3%.

3. Results and discussion

3.1. Characterization of the composite ﬁlm electrode

The XRD patterns revealed that all the undoped TiO2and

Bi2O3/TiO2samples displayed well-crystallized anatase phase after

being calcined at 773 K (Fig. 1(a)). The Bi-species are present in a

separated phase of Bi2O3. The peak at 27.42◦is identiﬁed as the

main peak of ␣-Bi2O3with cubic phase. The typical SEM images

X. Zhao et al. / Applied Surface Science 257 (2011) 4621–4624 4623

140120100806040200

200

400

600

800

1000

Z''(ohm)

Z (ohm)

TiO2 electrode

dark

400 nm irradiation

Xe lamp irradiation

9008007006005004003002001000

100

200

300

400

500

600

700

800

900

Xe lamp

400 nm irradiation

-Z" (ohm)

Z (ohm)

Bi2O3/TiO2 electrode

dark

Fig. 2. Effects of applied bias potentials and light irradiation on EIS variation at TiO2

and Bi2O3/TiO2nanotube array electrodes (0.1 M Na2SO4).

of TiO2nanotube and Bi2O3/TiO2nanotube composite samples are

shown in Fig. 1(b). It is clear that TiO2nanotube is successfully fab-

ricated by anodic oxidation and most of the nanotubes are uniform

in size and highly oriented. Interestingly, tubular structures are still

in existence after Bi2O3deposition (Fig. 1(b)). However, the mor-

phology of composite nanotube composite is different from that of

TiO2nanotube. These Bi2O3nanoparticles are deposited on the top

of TiO2nanotube or entered into tube.

3.2. Photoelectrochemical properties

Electrochemical impedance spectroscopy (EIS) is an effective

tool for probing the features of surface-modiﬁed electrodes and

was further employed to analyze the semiconducting properties

of the Bi2O3/TiO2nanotube array electrodes. As shown in Fig. 2,

both samples show a pronounced arc (semicircle portion) at higher

frequencies in the EIS plane, whose diameter corresponds to the

electron-transfer resistance controlling the kinetics at the electrode

interface. Signiﬁcant changes in the EIS spectra are observed for the

composite electrodes, following Bi2O3modiﬁcation. In fact, depo-

sition of the Bi2O3nanoparticles on the TiO2nanotube electrode

results in a marked decrease of semicircle diameter, which justi-

ﬁes an analogous decrease of the electron-transfer resistance and

relates directly to the accessibility of the underlying electrode or

the ﬁlm permeability. In this case, the enhanced performance of

the composite TiO2nanotube electrode could originate from the

improvement of charge carrier separation.

1.51.20.90.60.30.0-0.3

0.0

0.1

0.2

0.3

0.4

Current density (mA/cm2)

Applied potential (V SCE)

400 nm irradiation dark

Xe lamp

TiO2

1.51.20.90.60.30.0-0.3

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

dark

400 nm irradiation

Current density (mA/cm2)

Applied bias potential (SCE)

Bi2O3/TiO2Xe lamp

Fig. 3. LSV analysis of TiO2and Bi2O3/TiO2nanotube array electrodes with and

without Xe lamp or visible light irradiation (> 400 nm) (0.1 M Na2SO4).

At the meantime, current response of TiO2and Bi2O3/TiO2

electrode with light irradiation and without light irradiation was

observed (Fig. 3). It can be observed that under Xe lamp irra-

diation, the TiO2and Bi2O3/TiO2electrodes exhibited enhanced

current response compared with that under dark condition. And,

it is observed that no photocurrent was observed under visible

light irradiation for TiO2electrode. By contrast, photocurrent was

observed for Bi2O3/TiO2electrode under visible light irradiation. It

is concluded that it is the Bi2O3nanoparticles that contribute to the

generation of photocurrent under visible light irradiation.

3.3. Degradation of organic contaminants

Under UV and visible light irradiation, degradations of tar-

get organic contaminants were performed in the processes of

individual electrolysis (1.5 V), individual photocatalysis, and photo-

electrocatalysis process. Under visible light irradiation, as shown in

Fig. 4(a), organic contaminants can be photocatalytically degraded

using the composite ﬁlm electrode; it can also be degraded via the

electro-oxidation process at the bias potential of 1.5 V. Clearly, the

degradation rate of organic contaminants was the highest under the

PEC process with the same bias potential. Thus, it can be concluded

that a sort of synergetic effect occurs during the PEC process in the

degradation of organic contaminants. Additionally, no concentra-

tion variation was observed in the presence of the ﬁlm electrode

without visible light irradiation. Thus, it was concluded that the

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AppliedSurfaceScience257(2011)4621–4624ContentslistsavailableatScienceDirectAppliedSurfaceSciencejournalhomepage:www.elsevier.com/locate/apsuscPhotoelectrocatalyticdegradationoforganiccontaminantsatBi2O3/TiO2nanotubearrayelectrodeXuZhao,HuijuanLiu,JiuhuiQu∗StateKeyLaboratoryofEnvironmentalAquaticChemistry,ResearchCenterforEco-EnvironmentalSciences,ChineseAcademyofSciences,Beijing,100085,PRChinaarticleinfoArticlehistory:Received11April2010Receivedinrevisedform16December2010Accepted20December2010Availableonline24December2010Keywords:TiO2nanotubearrayBi2O3PhotoelectrocatalysisVisiblelightPhotocatalystabstractInthecurrentwork,TiO2nanotubearraywaspreparedviaelectrochemicalanodemethod.ThentheBi2O3nanoparticlesweredepositedontotheTiO2nanotubearrayviadip-coatingmethodfromanamorphouscomplexprecursor.ThecrystalstructureswerecharacterizedviaX-raydiffractionanalysis.Theirsur-facetextureswereobservedviaelectron-scanningmicroscope.Thepreparedcompositearrayelectrodeexhibitedhighphotoelectrocatalyticactivitiestowardsdegradingorganiccontaminantsundervisiblelightirradiation.HighphotoelectrocatalyticactivitieswerealsoexhibitedunderUVlightirradiation.Thecatalyticmechanismwasdiscussedbasedontheanalysisofelectrochemicalanddegradationkineticsresults.ItissuggestedaP(Bi2O3)–N(TiO2)junctionwasformedtoincreasethecatalyticactivates.Thestabilityoftheelectrodematerialswasconﬁrmedﬁnally.©2011ElsevierB.V.Allrightsreserved.1.IntroductionInrecentyears,highlyorderedTiO2nanotubearrayspreparedbyelectrochemicalanodizationhaveattractedmuchattentionduetotheiruniquearchitecture.ATiO2nanotubearraycanformauni-formTiO2ﬁlmonatitaniumsubstrate,whichprovidesanorderedstructureandporoussurface,withoutadecreaseingeometricthickness[1].Suchaninfrastructureisespeciallyfavorableformassdiffusion.Therefore,TiO2nanotubearraysshowgreatpotentialforpracticalenvironmentalpuriﬁcation[2,3].Asawidebandgapsemiconductor,TiO2needtobeexcitedbyanultravioletlightwhichislessthan5%ofthesolarirradianceattheEarth’ssurface.ToprepareTiO2photocatalystswithvisi-blelightresponsibility,severalstrategieshavebeenadopted.OneoftheapproachesiscouplingTiO2withothersemiconductorwithappropriatebandgaps.Alargenumberofcoupledpolycrystallineorcolloidalsemiconductor,inwhichtheparticlesadheretoeachotherinso-calledsandwichstructuresorpresentacore–shellgeome-try,havebeenpreparedsuchasSiO2–TiO2,CdS–TiO2,ZnO–TiO2,SnO2–TiO2,NiO–Bi2O3,etc.Recently,severalcoupledsystemssuchasBi2O3/SiTiO3[4–7]werereportedtobeefﬁcientundervisiblelightirradiation.However,thephotocatalyticmechanismforthecoupledsystemhasnotbeensystematicallyinvestigated,andnoclearevidencewasprovidedforthecompletedecompositionoforganicpollutantsundervisiblelight.∗Correspondingauthor.Tel.:+861062849151;fax:+861062923558.E-mailaddress:jhqu@mail.rcees.ac.cn(J.Qu).Bismuthoxide,Bi2O3,duetoitshighrefractiveindex,dielectricpermittivity,markedphotoconductivityandphotoluminescence,isusedinavarietyofareas,suchassensortechnology,opticalcoat-ings,andelectrochromicmaterials.Asaphotocatalyst,Bi2O3isap-typesemiconductorwithconductionandvalencebandedgesof+0.33and+3.13V(vs.NHE),respectively[8].Undervisiblelightirradiation,thephotogeneredelectronandholeisabletooxidizewater.And,highlyreactivespecies,suchasO2•−and•OHradicalsmaybegenerated,whichmayactasinitiatorsofoxidationreactions[8–10].Inthepresentpaper,wereportanovelloadingofBi2O3nanoparticlesontoaTiO2electrodewithahighlyorderedverti-callyorientednanotubearraypreparedbyanodeoxidation.Theobtainedresultsindicatedthatorganiccontaminantscanbeefﬁ-cientlydegradedbytheBi2O3/TiO2nanotubearrayelectrodeunderUVandvisiblelightirradiation.And,thecompositeelectrodeexhib-itedhighercatalyticactivitiestowardsthedegradationoforganiccontaminantsthantheindividualBi2O3orTiO2electrodedid.Theenhancedmechanismwasdiscussed.2,4-Dichlorophenol,akindoftypicalrefractorycontaminantswasusedinthepresentwork.2.Experimental2.1.PreparationandcharacterizationofBi2O3/TiO2nanotubearrayAllchemicalswereanalyticalgradereagentsandusedwithoutfurthertreatment.Electrolytewasfreshlypreparedfromdeionized0169-4332/$–seefrontmatter©2011ElsevierB.V.Allrightsreserved.doi:10.1016/j.apsusc.2010.12.0994622X.Zhaoetal./AppliedSurfaceScience257(2011)4621–4624Fig.1.(a)TheXRDpatternsforTiO2(A)andBi2O3/TiO2(B)nanotubecompositearrayelectrodes,(b)SEManalysisofTiO2(A)andBi2O3/TiO2(B)nanotubecompositearrayelectrodes.water.Afterchemicalpolishing,titaniumfoil(thicknessabout250␮m,purity99.4%,BeijingCuibolinNon-FerrousTechnologyDevelopingCo.,Ltd.)wassubjectedtopotentiostaticanodizationinanelectrochemicalanodizationcellwithaplatinumcathodeina0.5wt%HF+1MH3PO4electrolyteatambienttemperature.Thepotentialof20Vwasappliedfor30min.Thenthesampleswererinsedwithdeionizedwaterandannealedat450◦Cfor24h.PorousBi2O3nanoparticlesweredepositedontotheTiO2nanotubearrayfromanamorphousheteronuclearcomplexviaadip-coatingmethod.X-raydiffraction(XRD)ofthecompositeelectrodeswasrecordedonaScintag-XDS-2000diffractometerwithCuK␣radia-tion.ThemorphologyoftheBi2O3nanoparticleswascharacterizedbyaJSM6301electron-scanningmicroscope(SEM).Thephoto-electrochemicalmeasureemployedabasicelectrochemicalsystem(PrincetonAppliedResearch)connectedwithacounter-electrode(Ptwire,70mminlengthwitha0.4-mmdiameter),aworkingelec-trode(thehybridelectrode,activeareaof11cm2),andareferenceelectrode(asaturatedcalomelelectrode(SCE)).Na2SO4solution(0.12mg/L)wasusedaselectrolytesolution.2.2.DegradationexperimentsDegradationexperimentswereperformedinacylindricalquartzreactor.Thereactor,whichcontaineda60mLsamplesolu-tionallowing4.5cmofthesupportedﬁlmelectrodetobeimmersedintothesolution,wasplaced3cminfrontofa150WXelamppur-chasedfromtheGermanOsram.Theintensityoflight,asmeasuredbyanirradiancemeter(InstrumentsofBeijingNormalUniversity)wasc.a.25mW/cm2at4cmintothereactor,thepositionwheretheelectrodewasplaced.Thephotoelectrocatalytic(PEC)reac-tionemployedabasicelectrochemicalsystem(PrincetonAppliedResearch)connectedwithacounter-electrode(Ptwire,70mminlengthwitha0.4-mmdiameter),aworkingelectrode(hybridelectrode,activeareaof10cm2),andareferenceelectrode(asat-uratedcalomelelectrode(SCE)).0.1MNa2SO4solutionwasusedaselectrolytesolution.Initialconcentrationoftheorganiccontam-inantssolutionwas10mg/Landwasanalyzedbyreversed-phasehighperformanceliquidchromatogram(HPLC)withaHitachiHPLCapparatus(DiodeArrayDetectorL-2450,ColumnOvenL-2300,andPumpL-2130).Allexperimentswerecarriedoutatleastindupli-cate.Thereportedvaluesarewithintheexperimentalerrorof±3%.3.Resultsanddiscussion3.1.CharacterizationofthecompositeﬁlmelectrodeTheXRDpatternsrevealedthatalltheundopedTiO2andBi2O3/TiO2samplesdisplayedwell-crystallizedanatasephaseafterbeingcalcinedat773K(Fig.1(a)).TheBi-speciesarepresentinaseparatedphaseofBi2O3.Thepeakat27.42◦isidentiﬁedasthemainpeakof␣-Bi2O3withcubicphase.ThetypicalSEMimagesX.Zhaoetal./AppliedSurfaceScience257(2011)4621–4624462314012010080604020002004006008001000Z''(ohm)Z(ohm)'TiO2electrodedark400nmirradiationXelampirradiation90080070060050040030020010000100200300400500600700800900Xelamp400nmirradiation-Z"(ohm)Z(ohm)'Bi2O3/TiO2electrodedarkFig.2.EffectsofappliedbiaspotentialsandlightirradiationonEISvariationatTiO2andBi2O3/TiO2nanotubearrayelectrodes(0.1MNa2SO4).ofTiO2nanotubeandBi2O3/TiO2nanotubecompositesamplesareshowninFig.1(b).ItisclearthatTiO2nanotubeissuccessfullyfab-ricatedbyanodicoxidationandmostofthenanotubesareuniforminsizeandhighlyoriented.Interestingly,tubularstructuresarestillinexistenceafterBi2O3deposition(Fig.1(b)).However,themor-phologyofcompositenanotubecompositeisdifferentfromthatofTiO2nanotube.TheseBi2O3nanoparticlesaredepositedonthetopofTiO2nanotubeorenteredintotube.3.2.PhotoelectrochemicalpropertiesElectrochemicalimpedancespectroscopy(EIS)isaneffectivetoolforprobingthefeaturesofsurface-modiﬁedelectrodesandwasfurtheremployedtoanalyzethesemiconductingpropertiesoftheBi2O3/TiO2nanotubearrayelectrodes.AsshowninFig.2,bothsamplesshowapronouncedarc(semicircleportion)athigherfrequenciesintheEISplane,whosediametercorrespondstotheelectron-transferresistancecontrollingthekineticsattheelectrodeinterface.SigniﬁcantchangesintheEISspectraareobservedforthecompositeelectrodes,followingBi2O3modiﬁcation.Infact,depo-sitionoftheBi2O3nanoparticlesontheTiO2nanotubeelectroderesultsinamarkeddecreaseofsemicirclediameter,whichjusti-ﬁesananalogousdecreaseoftheelectron-transferresistanceandrelatesdirectlytotheaccessibilityoftheunderlyingelectrodeortheﬁlmpermeability.Inthiscase,theenhancedperformanceofthecompositeTiO2nanotubeelectrodecouldoriginatefromtheimprovementofchargecarrierseparation.1.51.20.90.60.30.0-0.30.00.10.20.30.4Currentdensity(mA/cm2)Appliedpotential(VSCE)400nmirradiationdarkXelampTiO21.51.20.90.60.30.0-0.30.000.020.040.060.080.100.120.14dark400nmirradiationCurrentdensity(mA/cm2)Appliedbiaspotential(SCE)Bi2O3/TiO2XelampFig.3.LSVanalysisofTiO2andBi2O3/TiO2nanotubearrayelectrodeswithandwithoutXelamporvisiblelightirradiation(>400nm)(0.1MNa2SO4).Atthemeantime,currentresponseofTiO2andBi2O3/TiO2electrodewithlightirradiationandwithoutlightirradiationwasobserved(Fig.3).ItcanbeobservedthatunderXelampirra-diation,theTiO2andBi2O3/TiO2electrodesexhibitedenhancedcurrentresponsecomparedwiththatunderdarkcondition.And,itisobservedthatnophotocurrentwasobservedundervisiblelightirradiationforTiO2electrode.Bycontrast,photocurrentwasobservedforBi2O3/TiO2electrodeundervisiblelightirradiation.ItisconcludedthatitistheBi2O3nanoparticlesthatcontributetothegenerationofphotocurrentundervisiblelightirradiation.3.3.DegradationoforganiccontaminantsUnderUVandvisiblelightirradiation,degradationsoftar-getorganiccontaminantswereperformedintheprocessesofindividualelectrolysis(1.5V),individualphotocatalysis,andphoto-electrocatalysisprocess.Undervisiblelightirradiation,asshowninFig.4(a),organiccontaminantscanbephotocatalyticallydegradedusingthecompositeﬁlmelectrode;itcanalsobedegradedviatheelectro-oxidationprocessatthebiaspotentialof1.5V.Clearly,thedegradationrateoforganiccontaminantswasthehighestunderthePECprocesswiththesamebiaspotential.Thus,itcanbeconcludedthatasortofsynergeticeffectoccursduringthePECprocessinthedegradationoforganiccontaminants.Additionally,noconcentra-tionvariationwasobservedinthepresenceoftheﬁlmelectrodewithoutvisiblelightirradiation.Thus,itwasconcludedthatthe4624X.Zhaoetal./AppliedSurfaceScience257(2011)4621–46242.52.01.51.00.50.0020406080100VisirradiationRemovalefficiency(100%)Time(hr)PECPCEC2.01.51.00.50.0020406080100Removalefficiency(100%)Time(hr)PECPCECUvirradiationFig.4.RemovalefﬁciencyoftargetpollutantsinPEC,PC,andECprocesseswithvisiblelightorUVlightirradiation.adsorptioneffectcanbeignoredwithinthereactionprocess.Sim-ilarresultswereobservedunderUVlightirradiation.AsshowninFig.4(b),thetargetcontaminantscanbedegradedviaphotocataly-sisprocessandelectrolysis,respectively.And,thedegradationrateoftargetcontaminantswaslargelyimprovedwithPECprocesswiththesamelightirradiationandappliedbiaspotential.3.4.MechanismanalysisIthasbeenreportedthattheTiO2isapotentialphotocatalystindecomposingorganiccontaminantsunderUVlightirradiation.Thus,itisconsideredthatTiO2worksasamainphotocatalystfortheBi2O3/TiO2,whiletheroleofBi2O3isasensitizerabsorbingvis-iblelight.ItisreportedthattheVBofBi2O3islowerby0.7VthanthatofTiO2[11].Undervisiblelightirradiation,theelectronsintheVBofBi2O3areexcitedtoitsCB.Therefore,theVBofBi2O3isren-deredpartiallyvacant,andtheelectronsintheVBofTiO2canbetransferredtothatofBi2O3.Asaresult,holesaregeneratedintheVBofTiO2.Theseaboveprocessesinitiatephotocatalyticoxidationreactions.Therefore,withtheirradiationofvisiblelight,thehetero-juctionsystemcaninduceefﬁcientdegradationoforganics.Intheheterojunctionpreparedinthisstudy,theBi2O3andTiO2nanotubearetightlyboundeachotherinnanosizedlevel.Therefore,theholetransferthroughthejunctionwillbegreatlyefﬁcient.Moreover,theTiO2workingasmainphotocatalystsisuncoatedcompletelybytheBi2O3nanoparticles.Thustheeffectivecatalyticsitesarenotscreenedbytheformationofheterojunctionstructure.AsthebandgapofBi2O3isca.2.85eV,itcanbeexcitedbylightwithwavelengthlessthan435nm.Duetothehighelectron–holerecombinationrateinBi2O3,thephotocatalyticactivityofBi2O3isverylow.BecausethevalencebandofBi2O3islowerthanthatoftitaniumoxide[12],theheterojunctionsformedinthecompositenanotubeﬁlmwillpromotethephotogeneratedholesinBi2O3tobetransferredtotheupperlyingvalencebandsoftitaniumoxide.Thus,thephotocatalyticactivityofthecompositeelectrodewasenhanced.4.ConclusionsTheBi2O3.nanoparticleswaspartlydepositedontoself-organizedTiO2nanotubearraysfromamorphouscomplexviadip-coatingmethod.Ithasbeendemonstratedthesurfacefunc-tionalizationbycoatedBi2O3nanoparticles,providingacompositeverticallyorientednanostructureandexhibitedhighphoto-electrocatalyticactivitiestowardsthedegradationoforganiccontaminantsunderUVandvisiblelightirradiation.AcknowledgmentsThisworkwassupportedbytheNationalNaturalScienceFoun-dationofChina(No.20977103,20837001).References[1]D.Wang,Y.Liu,B.Yu,F.Zhou,W.M.Liu,Chem.Mater.21(2009)1198–1206.[2]Y.S.Chen,J.C.Crittenden,S.Hackney,L.Sutter,D.W.Hand,Environ.Sci.Technol.39(2005)1201–1208.[3]Y.Hou,X.Y.Li,X.J.Zou,X.Quan,G.H.Chen,Environ.Sci.Technol.43(2009)858–863.[4]H.Zhang,S.Ouyang,Z.Li,L.Liu,T.Yu,J.Ye,Z.Zou,J.Phys.Chem.Solids67(2006)2501.[5]Y.Bessekhouad,D.Robert,J.-V.Weber,Catal.Today101(2005)315.[6]A.Hameed,V.Gombac,T.Montini,L.Felisari,P.Fornasiero,Chem.Phys.Lett.483(2009)254–261.[7]A.Hameed,V.Gombac,T.Montini,M.Graziani,P.Fornasiero,Chem.Phys.Lett.472(2009)212–216.[8]S.Y.Chai,Y.J.Kim,M.H.Jung,A.K.Chakrabory,D.Jung,I.W.Lee,J.Catal.262(2009)144–149.[9]A.Hameed,T.Montini,V.Gombac,P.Fornasiero,J.Am.Chem.Soc.130(2008)9658–9659.[10]L.F.Yin,J.F.Niu,Z.Y.Shen,J.Chen,Environ.Sci.Technol.44(2010)5581–5586.[11]Y.H.Ao,J.J.Xu,D.G.Fu,C.W.Yuan,Appl.Surf.Sci.118(2009)382–386.[12]T.P.Gujar,V.R.Shinde,C.D.Lokhande,Mater.Res.Bull.4(2006)1558–1564.

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