二氧化钛的紫外拉曼光谱研究外文翻译.docx
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二氧化钛的紫外拉曼光谱研究外文翻译.docx
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二氧化钛的紫外拉曼光谱研究外文翻译
附录
UVRamanSpectroscopicStudyonTiO2.I.PhaseTransformationatthe
SurfaceandintheBulk
JingZhang,MeijunLi,ZhaochiFeng,JunChen,andCanLi*
StateKeyLaboratoryofCatalysis,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,
P.O.Box110,Dalian116023,China
ReceiVed:
September16,2005;InFinalForm:
NoVember4,2005
PhasetransformationofTiO2fromanatasetorutileisstudiedbyUVRamanspectroscopyexcitedby325and244nmlasers,visibleRamanspectroscopyexcitedby532nmlaser,X-raydiffraction(XRD),andtransmissionelectronmicroscopy(TEM).UVRamanspectroscopyisfoundtobemoresensitivetothesurfaceregionofTiO2thanvisibleRamanspectroscopyandXRDbecauseTiO2stronglyabsorbsUVlight.TheanatasephaseisdetectedbyUVRamanspectroscopyforthesamplecalcinedathighertemperaturesthanwhenitisdetectedbyvisibleRamanspectroscopyandXRD.TheinconsistencyintheresultsfromtheabovethreetechniquessuggeststhattheanatasephaseofTiO2atthesurfaceregioncanremainatrelativelyhighercalcinationtemperaturesthanthatinthebulkduringthephasetransformation.TheTEMresultsshowthatsmallparticlesagglomerateintobigparticleswhentheTiO2sampleiscalcinedatelevatedtemperaturesandtheagglomerationoftheTiO2particlesisalongwiththephasetransformationfromanatasetorutile.ItissuggestedthattherutilephasestartstoformattheinterfacesbetweentheanataseparticlesintheagglomeratedTiO2particles;namely,theanatasephaseintheinnerregionoftheagglomeratedTiO2particlesturnsouttochangeintotherutilephasemoreeasilythanthatintheoutersurfaceregionoftheagglomeratedTiO2particles.WhentheanataseparticlesofTiO2arecoveredwithhighlydispersedLa2O3,thephasetransformationinboththebulkandsurfaceregionsissignificantlyretarded,owingtoavoidingdirectcontactoftheanataseparticlesandoccupyingthesurfacedefectsitesoftheanataseparticlesbyLa2O3.
1.Introduction
Titania(TiO2)hasbeenwidelystudiedbecauseofitsuniqueopticalandchemicalpropertiesincatalysis,[1]photocatalysis,[2]sensitivitytohumidityandgas,[3,4]nonlinearoptics,[5]photoluminescence[,6]andsoon.ThetwomainkindsofcrystallineTiO2,anataseandrutile,exhibitdifferentphysicalandchemicalproperties.Itiswell-knownthattheanatasephaseissuitableforcatalystsandsupports,[7]whiletherutilephaseisusedforopticalandelectronicpurposesbecauseofitshigh
[8]
dielectricconstantandhighrefractiveindex.IthasbeenwelldemonstratedthatthecrystallinephaseofTiO2playsasignificantroleincatalyticreactions,especially[9-11]
photocatalysis.Somestudieshaveclaimedthattheanatasephasewasmoreactivethantherutilephaseinphotocatalysis[9.,10]
Althoughatambientpressureandtemperaturetherutilephaseismore
[12]thermodynamicallystablethantheanatasephase,anataseisthecommonphaseratherthanrutilebecauseanataseiskineticallystableinnanocrystallineTiO2atrelatively
[13]
lowtemperatures.Itisbelievedthattheanatasephasetransformstotherutilephaseoverawiderangeoftemperatures[.14]Therefore,understandingandcontrollingofthecrystallinephaseandtheprocessofphasetransformationofTiO2areimportant,thoughtheyaredifficult.
Manystudies[13-31]havebeendonetounderstandtheprocessofthephasetransformation[15]
ofTiO2.Zhangetal.proposedthatthemechanismoftheanatase-rutilephasetransformationwastemperature-dependentaccordingtothekineticdatafromX-raydiffraction(XRD).Onthebasisoftransmissionandscanningelectronmicroscopies,Goumaetal.[16]suggestedthatrutilenucleiformedonthesurfaceofcoarseranataseparticlesandthenewlytransformedrutileparticlesgrewattheexpenseofneighboringanataseparticles.Pennetal.[17]suggestedthattheformationofrutilenucleiattwininterfacesofanataseparticlesheatedhydrothermally.
CatalyticperformanceofTiO2largelydependsonthesurfaceproperties,especiallythesurfacephase,becausecatalyticreactiontakesplaceonthesurface.ThesurfacephaseofTiO2shouldberesponsibleforitsphotocatalyticactivitybecausenotonlythephotoinducedreactionstakeplaceonthesurface[32]butalsothephotoexcitedelectronsandholesmightmigratethroughthesurfaceregion.Therefore,thesurfacephaseofTiO2,whichisexposedtothelightsource,shouldplayacrucialroleinphotocatalysis.However,thesurfacephaseofTiO2,particularlyduringthephasetransformation,hasnotbeeninvestigated.Thechallengingquestionsstillremain:
isthephaseinthesurfaceregionthesameasthatinthebulkregion,orhowdoesthephaseinthesurfaceregionofTiO2particlechangeduringthephasetransformationofitsbulk?
ThedifficultyinansweringtheabovequestionswasmainlyduetolackingsuitabletechniquesthatcansensitivelydetectthesurfacephaseofT2i.OUVRamanspectroscopyisfoundtobemoresensitivetothesurfacephaseofasolidsamplewhenthesampleabsorbsUVligh[3t.3]Westudiedthephasetransitionofzirconia(ZrO2)fromtetragonalphasetomonoclinicphasebyUVRamanspectroscopy,visibleRamanspectroscopy,andXRD[3.3]TheseresultsclearlyindicatedthatthesurfacephaseofZrO2isusuallydifferentfromthebulkphaseofZrO2andthephasetransforma-tionofZrO2startsfromitssurfaceregionandthengraduallydevelopsintoitsbulkwhentheZrO2withtetragonalphaseiscalcinedatelevatedtemperatures.
ThesefindingsleadustofurtherinvestigatethephasetransformationinthesurfaceregionofTiO2byUVRamanspectroscopyasTiO2alsostronglyabsorbsUVlight.Inthisstudy,wecomparedtheRamanspectraofTiO2calcinedatdifferenttemperatureswithexcitationlinesintheUVandvisibleregions.XRDandtransmissionelectronmicroscopy(TEM)werealsorecordedtounderstandtheprocessofphasetransformationofTiO2.ItwasfoundthattheresultsofUVRamanspectraaredifferentfromthoseofvisibleRamanspectraandXRDpatterns.TheanatasephaseofTiO2atthesurfaceregioncanremainatrelativelyhighertemperaturesthanthatinthebulkatelevatedcalcinationtemperatures;namely,theanatasephaseintheinnerregionoftheagglomeratedTiO2particlesturnsouttochangeintotherutilephasemoreeasilythanthatintheoutersurfaceregionoftheagglomeratedTi2Oparticles.
Theliterature[15,17,19]proposedthemechanismthatphasetransformationofTiO2mightstartattheinterfacesofcontactinganataseparticles.IftheanataseparticlesofT2iOareseparated,thephasetransformationofTiO2fromanatasetorutilecouldberetardedorprohibited.Jingetal.[34]showedthatLa3+didnotenterthecrystallatticesofTiO2andwasuniformlydispersedontoTiO2intheformoflanthana(La2O3)particleswithsmallsize.Toverifytheaboveassumption,thisstudyalsopreparedtheanatasephaseofTiO2samplecoveredwithLa2O3andcharacterizedtheabovesamplebyvisibleRamanspectroscopyandUVRamanspectroscopy.TheresultsofthetwotypesofRamanspectraareinagreementwitheachotherandshowthattheTiO2particlecoveredwithLa2O3canretainitsanatasephasebothinthebulkandinthesurfaceregionevenaftercalcinationat900C.°
2.ExperimentalSection
2.1.CatalystPreparation.
2.1.1.PreparationofTiO2.TiO2waspreparedbyprecipitationmethod.To100mLofanhydrousethanolwasadded20mLoftitanium(IV)n-butoxide(Ti(OBu)4).Thissolutionwasaddedtoamixturesolutionofdeionizedwaterand100mLofanhydrousethanol.Themolarratioofthewater/Ti(OBu)4was75.Aftertheformedwhiteprecipitatewasstirredcontinuouslyfor24h,itwasfilteredandwashedtwicewithdeionizedwaterandanhydrousethanol.Finally,thesamplewasdriedat100°Candcalcinedinairattemperaturesfrom200to800°Cfor4h,andthencooledtoroomtemperature.
2.1.2.PreparationofLa2O3-CoVeredTiO2(La2O3/TiO2).TheaboveTiO2powdercalcinedat500Cwa°susedasasupport.ThecriticalLa2O3loadingcorrespondingto
2[35,36]monolayercoverageofLa2O3onthegrainsurfaceofTiO2is0.27g/100m2.[35,36]Onthe
2
basisoftheBETsurfaceareaoftheTiO2support(54.3m/g),themonolayerdispersioncapacitycanalsobeexpressedas15wt%La2O3oftheweightofTiO2.La2O3/TiO2samples,containingdifferentamountsofLa2O3(0.5-6wt%)werepreparedbyawetimpregnationmethod.ThesupportwasimpregnatedwithaqueoussolutionofvariousconcentrationsofIanthanumnitrate(La(NO3)36H2O)andsubsequentlystirredinahotwaterbathuntilitwasdried.Afterthesamplewaskeptat110Co°vernight,itwascalcinedat900Cinairfo°r4h.ATiO2samplewaspreparedbycalciningtheTiO2supportat900°Cfor4h(denotedasTiO2-900)forcomparisonwiththeLa2O3/TiO2sample.PureLa2O3wasobtainedbycalciningLa(NO3)36H2Oat550C°for4h.
2.2.Characterization.
2.2.1.UVRamanSpectroscopyU.VRamanspectraweremeasuredatroomtemperature
-1withaJobin-YvonT64000triple-stagespectrographwithspectralresolutionof2cm.Thelaserlineat325nmofaHe-Cdlaserwasusedasanexcitingsourcewithanoutputof25mW.Thepoweroflaseratthesamplewasabout3.0mW.The244nmlinefromaCoherentInnova300Fredlaserwasusedasanotherexcitationsource.Thepowerofthe244nmlineatsamplewasbelow1.0mW.
2.2.2.VisibleRamanSpectroscopy.VisibleRamanspectrawererecordedatroomtemperatureonaJobin-YvonU1000scanningdoublemonochromatorwiththespectral
-1
resolutionof4cm-1.Thelineat532nmfromaDPSS532Model200532nmsingle-frequencylaserwasusedastheexcitationsource.
2.2.3.X-rayPowderDiffraction(XRD),TEM,andUltraViolet-VisibleDiffuseReflectanceSpectroscopy.XRDpatternswereobtainedonaRigakuMiniFlexdiffractometerwithaCuKRrad
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