Selective preparation of Bi2O3 visible light-driven photocatalyst by dispersant and calcination

86L.Cheng,Y.Kang/alsostudiedtorealizeselectivesynthesisofBi2O3with

crystallinephase.

Inthiswork,Bi2O3sampleswerefabricatedbyitationtechnique,followedbyconsequentChangesofmorphologyandotherperformancesdispersantandcalcinationparameterswereN-dopedb-Bi2O3wasselectivelyobtainedunderthearationconditions,whichwasalmostthe rsttimeviaAsisknown,dopingofTiO2withnitrogencouldgapandincreaseitsabsorptionforvisiblelight[17],ofBi2O3withnitrogenisrarelyreported,whichanewinsightforcontrollablepreparationofBi2O3performances.

2.Experimentaldetails2.1.SynthesisofBi2O3

AllthechemicalreagentsusedinthisworkareA.R.grade.Bi2O3powderwaspreparedaccordingtothefollowingprocess.Firstly,9.7gBi(NO3)3Á5H2Owasdis-solvedin100mLHNO3(v(HNO3):v(H2O)=1:9)andcertainamount(theinitialdos-agewas1g)ofdispersant(PEG1000,4000or6000)wasaddedintothesolution.Secondly,thesolutionwasadded0.5mol/LofNaOHsolutionwhichwaspreparedbydissolving10gNaOHin500mLdeionizedwaterdropbydropunderstirring.Afterreactingfor2h,thewhiteprecipitatewasseparatedbycentrifugation,washedwithdeionizedwaterandethanolfor vetimes,respectively,driedinanelectricthermostaticdryingovenat80°Candcalcinatedinamuf efurnaceatcer-taintemperature(theinitialtemperaturewas400°C)forcertaintime(theinitialtimewas2h).2.2.Characterization

ThecrystallinephaseofallthesampleswereinvestigatedbyX-raydiffractionanalysis(XRD)onaD/MAX-2500unitwithCuKaradiation(k=1.54056°A).ThesamplemorphologieswererecordedbyFieldEmissionScanningElectronMicro-scope(FESEM)onaNanosem430unit.UV–Visdiffusere ectionspectraofthephotocatalystsweremeasuredonaLambda750SUV/VISSpectrometer.Photolumi-nescencespectra(PL)wereobtainedonaJobinYvonFluorolog3-21 uorescencespectroscopy.AndtheabsorbanceofmalachitegreensolutionwasobtainedonaUV7500UltravioletVisibleSpectrophotometer.2.3.Photocatalyticexperiments

Malachitegreenwasusedasthetargetpollutant.Theexperimentswerecarriedoutaccordingtothefollowingprocess.50.0mgofphotocatalystwasdispersedin100mLmalachitegreensolutionwithconcentrationof10mg/Lin300mLbeaker.Beforeillumination,thesuspensionwasmagneticallystirredinthedarkfor1htoestablishadsorption–desorptionequilibriumofmalachitegreenonthephotocat-alystsurface.Thenthesuspensionwasirradiatedundera200W lamentlamp,whichwaspositionedasideabout5cmawayfromthebeaker.Theabsorbanceofthesolution(centrifugalizedtoseparatethedispersedphotocatalystbeforetesting)wasdetectedevery30min.TheinitialconcentrationandabsorbanceofthesolutionweredenotedbyC0andA0,respectively.Accordingtothelinearrelationshipbe-tweenconcentrationandabsorbance(C/C0=A/A0),theconcentrationofmalachitegreensolutionatdifferenttimecouldbecalculated.

3.Resultsanddiscussion

3.1.EffectofPEGwithdifferentmolecularweight

PEGactsasashapedirectorwhenusedasdispersanttoprepareBi2O3becauseitcanbeabsorbedonthecrystalsurfacesofparticlestoformalayerofmembranetoreduceinterparticleforceandpre-ventparticlesfromdirectcollision,whichmakesparticlesnoteas-ilyaggregate[14].

Inthispaper,PEG1000,4000and6000wereusedasdispersanttofabricateBi2O3,respectively.Thedispersantdosagewas1.0g,thecalcinationtemperature400°Candthecalcinationtime2h.Fig.1showstheXRDpatternsofallthepreparedBi2O3samples.Itisclearthatwhateverdispersantwasused,allthediffractionpeaksofthesamplewereconsistentwithJCPDScardNo.41-1449,implyingpuremonoclinicphaseBi2O3(a-Bi2O3)wassynthesized.

Thecorrespondinglatticeconstantswere:a=5.850Å,b=8.170Å,c=7.512Å.Thesharppeaksindicatedthatallthesam-pleswerehighlycrystallized.Besides,accordingtotherelationshipbetweenintensityofthestrongestpeak,whichindicated(120)crystalface,withotherpeaks,itcouldbefoundthatthe(120)crystalfaceofBi2O3synthesizedbyPEG4000grewthemostpref-erentially,thesecondwasthatofBi2O3synthesizedbyPEG6000andthelastwasthatofBi2O3synthesizedbyPEG1000.

Thesamples’morphologieswereobservedbyFESEM,asshowninFig.2.ItisevidentthatBi2O3particleswerepeanut-likeforallsamples.TheparticlesizeforPEG1000wasrelativelyuniform,inotherword,thesizedistributionrangewasrelativelynarrow,aboutlengthof428–762nmandwidthof214–333nm.Thiscanbeexplainedasfollows.TheconcentrationofPEG1000wasthelargestunderthesamedispersantdosage,somoreparticlescouldbeenwrappedtoavoidforminglargeclusterscomparedtoPEG4000and6000,resultinginsmallerparticlesizeandmoreuniformsizedistribution.Besides,theBi2O3particlesobtainedusingPEG1000asdispersantweremoreinclinedtoisotropicgrowth(asdis-cussedinXRDpatternssection),whichmadetheirlength–widthratioswerethesmallest.However,ifPEG4000wasused,the(120)crystalfaceofBi2O3particlesgrewthemostpreferentially,thusleadingtotheirlength–widthratioswerethelargest.Inaddi-tion,theconcentrationofPEG4000wasjustaquarterofPEG1000,somultipleparticlesmaybeabsorbedonthesamePEG4000molecularchainandeasilyformedclusters,whichcausedseriousparticleagglomeration.Therefore,theparticlesizewasnon-uni-formandthesizedistributionwaswide.Accordingtostatistics,thelengthrangeofBi2O3particleswasabout285–1409nmandthewidthrange190–371nmforPEG4000,whichmaygiverisetoitssmallestspeci csurfacearea(1.375m2/g,whilethespeci csurfaceareaforPEG1000and6000were1.586m2/gand1.381m2/g,respectively).WhenPEG6000wasused,althoughitsconcentra-tionwasevensmallerthanPEG4000andmultipleparticleswerealsoinclinedtobeabsorbedonthesamePEG6000molecularchain,itslongerchainprobablycurveandtwinetoenwrapparti-cles,whichplayedroleofsterichindrancemoreef cientlythanPEG4000.Besides,thepreferentialgrowthof(120)crystalfaceforPEG6000wasrelieved,implyingthelength–widthratiosofBi2O3particlesweresmallerthanthatofPEG4000,sotheoverallsizedistributionwasrelativelynarrowerthanPEG4000.Bymea-surement,thelengthrangewasabout476–952nmandthewidthrangewas238–357nm.Throughtheabovediscussion,itcouldbeconcludedthatthesizeofBi2O3particlesobtainedbyPEG1000wasthesmallest,whichcontributedtotheirseriousagglomerationinthesubsequentcalcinationprocess,asseeninFig.2(a).

TheUV–Visdiffusere ectionspectraofBi2O3samplesobtainedbydifferentdispersantwerealsomeasured,asshowninFig.3(a).It