ActaPhys.鄄Chim.Sin.,2008,24(7)：1283-1286
物理化学学报(WuliHuaxueXuebao)
1283
www.whxb.pku.edu.cn
具有可见光响应的C、N共掺杂TiO2纳米管光催化剂的制备
石
摘要：
健1,鄢李
226007;
军1
2
蔡云法2
(1南通大学化学与化工学院,江苏南通
浙江大学环境工程研究所,杭州310027)
为了提高二氧化钛对可见光的利用效率,采用等离子体电解方法对二氧化钛实现了C、N共掺杂.掺杂
通过等离子体电解HCONH2、NaNO2、(NH2)2CO产生活性N、C实现.XPS结果表明,C、N掺杂进入了氧化钛晶格.紫外漫反射光谱分析表明,氧化钛对可见光(跃400nm)的响应增强,其光催化降解能力也大大增强.C、N共掺杂TiO2是一种利用太阳能的理想的光催化剂.关键词：光催化;
可见光;O643
二氧化钛;
共掺杂
中图分类号：O644,
FabricationofC,N鄄CodopedTiO2NanotubePhotocatalystswith
VisibleLightResponse
(1SchoolofChemistryandChemicalEngineering,NantongUniversity,Nantong226007,JiangsuProvince,P.R.China;
2
InstituteofEnvironmentalEngineering,ZhejiangUniversity,Hangzhou310027,P.R.China)Abstract：ToenhancetheabsorptioninthevisibleregionforTiO2,Cand/orNdopingofTiO2nanotubewascarriedoutbyanovelmethod———plasmaelectrolysis.ThedopingwasachievedwithactiveCandNproducedfromtheplasmaelectrolysisofHCONH2,NaNO2,or(NH2)2CO.TheresultsofXPSrevealedthatCandNweresuccessfullydopedintothelatticeofTiO2.UV鄄Visdiffusereflectancespectrashowedashifttolongerwavelengthsandanenhancementoftheabsorptioninthevisibleregion(>400nm)fortheC,N鄄codopedTiO2,comparedtothepureTiO2,C鄄dopedTiO2andN鄄dopedTiO2.TheC,N鄄codopedTiO2showedthebestphotocatalyticactivitiesforthedegradationofmethylorangeundervisiblelightirradiation,andwasapromisingphotocatalystforutilizingthesolarenergy.KeyWords：Photocatalysis;
Visiblelight;
Titania;
Codoping
SHIJian1,鄢LIJun1CAIYun鄄Fa2
PhotoinducedoxidationandreductionreactionsatthesurfaceofTiO2particlesorthinfilmshavebeenattractingmuchatten-tioninviewoftheirpossibleapplicationsforphotodegradationofpollutantsorsplittingwaterintohydrogenandoxygen[1,2].However,TiO2canbeonlyactivatedunderUVlightirradiationofwavelengthlessthan387nm.Recently,itwasreportedthatdopingTiO2withnonmetalssuchasnitrogen,carbon,fluorine,andboroncouldinduceitsphotocatalyticactivityundervisiblelight[1,2].Amongthesenonmetals,Nwasfoundtobemoreeffec-tiveindecreasingthebandgapofTiO2bygeneratinganisolatedN2pnarrowbandabovetheO2pvalence[3].Livraghietal.[4]re-portedthatparamagneticcentersN0b(singleatomnitrogencenters
鄢
inthebulkoftitaniumdioxide)playedanessentialroleinthe
absorptionofvisiblelight,inthepromotionofelectronstotheconductionband,andinphotoinducedelectrontransfertore-ducibleadsorbates.Mrowetzetal.[5]foundthattheholesgener-atedonnonmetaldopedTiO2byvisiblephotonshadalowoxi-dizationcapacity,inotherwords,theactivityofthenonmetaldopedTiO2shouldbefurtherenhanced.N鄄dopedTiO2wascom-monlyobtainedbyheatingNH3[3]orionimplantation[6].Howev-er,ammoniagaswashazardous,andionimplantationprocesswascomplicatedandledtoacertaindecreaseofphotoresponseintheUVrange.ThereforeitwasofsignificancetoexploreasafeandefficientmethodforN鄄doping.Inaddition,someresearches
Received:December18,2007;Revised:February11,2008;PublishedonWeb:April25,2008.Correspondingauthor.Email:shijianjs66@163.com;Tel:+86513鄄85015495;Fax:+86513鄄85015501.江苏省青年科学基金(BK2006545)和江苏教委科学基金资助项目
鬁EditorialofficeofActaPhysico鄄ChimicaSinica
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ActaPhys.鄄Chim.Sin.,2008
Vol.24
revealedthatnonmetalscodopedTiO2showedhigherabsortionandhigherphotocatalyticactivitythanN鄄dopedTiO2undervisi-blelight.Fanetal.[7]developedaN,S鄄codopedtitaniaphotocata-lystthroughhydrolysisoftitaniatetrachlorideusingammoniainthepresenceofglacialaceticacidandammoniumsulfate.Lietal.[8]fabricatedN,F鄄codopedTiO2byspraypyrolysisandfoundthedopantwashomogeneouslydistributedthroughoutTiO2.
ItisrecentlyfoundthatplasmaelectrolysiscanachieveN鄄doping,C鄄doping,andB鄄dopingonaluminaandtitaniumforthewearandcorrosionprotectionorthemedicalapplication[9-11].ThedopingreactionwasachievedwithactiveN,CandBatomspro-ducedfromHCONH2,NaNO2,NaCO3,Na2B4O7,etc.Plasmaelec-trolysishasattractedindustrialattentionbecauseofitsnonline鄄of鄄sighttreatmentandatmosphericworkingenvironment[10,11].However,toourbestknowledge,fewinvestigationsonNorC-dopingofTiO2photocatalystusingplasmaelectrolysisweredone.Inthisarticle,theC,N鄄codopingofTiO2wascarriedoutusingplasmaelectrolysiswiththegoalofenhancingtheabsorp-tioninthevisibleregionforTiO2.TiO2nanotube,whichiscon-sideredtobeapromisingphotocatalystforitshighspecificsurfacearea[12],wasselectedasthepureTiO2.ThedifferencesofTiO2nanotubesbeforeandaftermodificationwereinvestigatedwithSEM,XRD,XPSandUV鄄Vis.ThecatalyticefficiencyofTiO2nanotubesundervisiblelightwasevaluatedbythedegra-dationofmethylorangewastewater.
1Experimental
1.1Fabricationnanotubes
ofsupportedC,N鄄codopedTiO2
TiO2nanotubeswerepreparedbytheelectrochemicalan-odization.Atypicalprocedurewasasfollows[12]:Thehighpuritytitaniumsheet(99%purity)wasobtainedfromHaijiLtd.forTi-tanium&Nickel(ShanxiProvince,P.R.China).TheTiO2nan-otubeplatetwasfirstlymechanicallypolishedwithabrasivepa-pers,washedinanultrasonicbathwithdistilledwaterandace-tone,andthendriedinairatroomtemperature.Electrochemicalexperimentswereconductedusingadirectcurrentpowersup-ply.Titaniumsheetservedasanodicelectrodeandnickelsheetasthecathode.Theelectrolytewas1/12mol·L-1C2H2O4·2H2O+0.5%(w)NHVwerewasusedin4Ftheaqueouspresentsolution,work.Allandtheanodizationanodizationvoltagesexperimentsof25Aftercarriedtheanodization,outatroomthesamplestemperaturewerewithrinsedmagneticindeionizedagitation.wa-teranddried.
TiO2nanotubeswithsizesof30mm伊20mm伊0.8mmwereusedassubstratematerial.Forplasmaelectrolysistreatment,apulsepowersupplywasemployed,andthetitaniumplatewasusedasacathodewhileagraphiteplatewasusedasananodeinanelectrolyticcell.AsolutionofHCONH2(5-10g·L-1,NaNO2(5-10g·L-1),or(NH2)2CO(2-10g·L-1),andotherproprietaryingredients(addedprimarilyforadjustmentofelectricalcon-ductivity)werechosenastheelectrolytes.TheCand/orNdop-ingwasachievedbasedonthefollowingreactions[9-11].
plasma鄄plasma鄄HCONHelectrolysis
NHdischarge
2邛3+CO邛[N]+[C]+H2+CO2
plasma鄄
plasma鄄
NaNOelectrolysis
discharge
2+H2O邛NH3+CO2plasma鄄
邛[N]+H2+CO2
plasma鄄
(NHelectrolysis
discharge
2)2CO+H2O邛NH3+H2+CO2pulsefrequency邛[N]+H2+CO2
Theappliedvoltageandwere600Vand100Hz,respectively.Theelectrolytebathwaswater鄄cooledanditstemperaturewasmaintainedlowerthan30益.Thedischargetimewas15min.Afterthedopingtreatment,theobtainedsampleswerewashedwithdistilledwateranddriedatroomtemperature,andpartsofthemweresubsequentlyannealedat500益for1h.1.2Characterization
ForthemorphologycharacterizationofTiO2nanotubes,afield鄄emissionscanningelectronmicroscopy(FE鄄SEM,JSM鄄5600LV)wasused.ThecrystalstructureofthesampleswasidentifiedbyX鄄raydiffraction(XRD,Rigaku,D/max鄄rA)usingadiffractometerwithCuK琢radiation,performedoverangularrangesof2兹=20毅-80毅,scannedataspeedof0.02(毅)·s-1andstepsof0.02毅.TheelementalcompositionofTiO2nanotubeswasdeter-minedbyX鄄rayphotoelectronspectroscopy(XPS,PerkinElmer,aRBDupgradedPHI鄄5000CESCAsystem)withMgK琢radiation(h淄=1253.6eV),theX鄄rayanodewasrunat250Wandthehighvoltagewaskeptat14.0kVwithadetectionangleof54毅.UV鄄Visabsorptionspectraofsampleswereobtainedus-ingaUV鄄Visspectrophotometer.Absorptionspectrawererefer-encedtoBaSO1.3Photocatalytic4.
degradationofmethylorangesolutionThephotocatalyticreactionexperimentwasperformedina0.5Lcylindricalglassreactor,wherea300WXelamp(I0=0.6mW·cm-2)verticallyinsertedinacentralquartzglasstubewasused.Theirradiationbelow400nmwasremovedby2mol·L-1NaNO2solution.TheringedTiO2nanotubeswereputinthere-actoraroundtheglasstube.Theinitialconcentrationofmethylorange(themodelpollutant)was20mg·L-1.Theconcentrationofmethylorangewasmonitoredbythespectrophotometer(463nm).Theremovalwascalculatedas[1-(c/c0)]伊100%,wherec0istheinitialconcentrationofmethylorange,cistheconcentrationofmethylorangeduringreaction.
2Resultsanddiscussion
Fig.1showsSEMimagesofTiO2nanotubesbeforeandafterC,N鄄codoping.Itwasclearlyseenthatthediameterofsupport-edTiO2nanotubeswasapproximately30-40nm,thelengthwasapproximately930nm.AfterC,N鄄codoping,thenanotubularstructureofTiO2nanotubekeptitsintegritywithnosignificantmorphologicalchange.Fig.2depictsXRDpatternsofTiO2nan-otubesbeforeandafterC,N鄄codoping.Itisseenthatthecrys-tallinestructuresofpureTiO2nanotubesandC,N鄄codopedTiO2nanotubesarebothanatase(2兹=25.34毅)andtherearenodiffrac-tionpeakscorrespondingtoothercrystalssuchasbrookite,ru-tileandTiN.ThisindicatedthatCandNdopingdidnotchangethecrystalstructureofTiO2.Moreover,thelatticeparameters(a,c)ofpureTiO2nanotubesandC,N鄄codopedTiO2nanotubeswere
No.7SHIJianetal.：FabricationofC,N鄄CodopedTiO2NanotubePhotocatalystswithVisiblelightResponse1285
Fig.2XRDpatternsofTiO2nanotubesbeforeandafterdoping
Fig.1SEMimagesofsamples
(a1)topviewimageofTiO2nanotubes;(a2)cross鄄sectionimageofTiO2nanotubes;(b1)topviewimageofC,N鄄codopedTiO2nanotubes;(b2)cross鄄sectionimageofC,N鄄codopedTiO2nanotubes(0.381nm,0.943nm)and(0.380nm,0.942nm),respectively.The
unchangedcrystalstructurewasreasonablebecauseonlytraceamountofCandNwasdopedandevenlydispersedintothelat-ticeofTiO2.
TheelementcompositionofC,N鄄codopedTiO2nanotubeswasdeterminedbyXPSasshowninFig.3.TheC1scorelevelsofC,N鄄codopedTiO2nanotubesrevealedthreepeaksstructuresatbindingenergiesof284.5,286.5and287.9eV.Thetwofea-turesof287.9and286.5eVwereattributedtoC—NandC—Obands,andthecarbonpeakat284.5eVwasassignedtotheC—Cremainedintheprecursororganiccompound[13].TheseresultsindicatedthatCwasdopedintoTiO2.
TheN1scorelevelsofC,N鄄codopedTiO2nanotubesre-vealedthreepeaksstructuresatbindingenergiesof398.7,400.1and402.0eV.Thepeakat402.0eVwascommonlyattributedto酌鄄N2chemisorbedonTiO2[14,15].Thebindingenergyof398.7eVwasattributedtoC—Nbond.Bycomparisonwithliterature[15],thepeakappearedat400.1eVmightbeNcontainingspeciesin
-thestateofhyponitrite(N2O22),whichwasconsideredtobere-sponsibleforvisiblelightphotocatalysis.Itwascenteredat400.1eV,whichwasavaluegreaterthanthetypicalbindingen-ergyof397.2eVinTiN[13]and,therefore,couldbeattributedtothe
1selectronbindingenergyoftheNatomintheenvironmentofO—Ti—N.ThisshiftinbindingenergycanbeunderstoodbythefactthattheN1selectronbindingenergyishigherwhentheformalchargeofNismorepositive(e.g.,408eVinNaNO3),com-paredtozerooranegativeformalcharge(398.8eVinNH3).ThelocalelectrondensityaroundNislowerformore鄄positiveformalcharges.WhennitrogensubstitutesfortheoxygenintheinitialO—Ti—Ostructure,theelectrondensityaroundNisre-duced,comparedtothatinaTiNcrystal,becauseoftheOatombondedwiththeTiatom.Thus,theN1sbindingenergyofhy-ponitriteishigherthanthatinaN—Ti—NenvironmentwheretheNatomreplacestheOatom.Therefore,wecandeducetheexistenceC—N—ObondsinthefilmsandtheformationofTi—O—N—C.
ThebindingenergiesofTi2p3/2andTi2p1/2wereobservedtobeat458.2and464.2eV,whichwereassignedtoTi4+(TiO2),withapeakseparationof6.0eVbetweenthesetwopeaks.ItshouldbenotedthattheTi2p3/2peak(458.2eV),whichwasno-tablylowerthanthestandarddata(458.7eV).Suchashifttowardlowerbindingenergyindicatedthesuccessfulincorporationofni-trogenandcarbonintotheTiO2lattice.Afterdoping(whentheNatomreplacestheOatomoutofthelattice),thevalencestateoftheTicationcanbereduced.ThebindingenergyoftheTi2p3/2peakshiftstolowerenergieswhenthevalencestateofTi4+isre-ducedtoTi3+andTi2+.Thus,theobservedTi2p3/2bindingenergyafternitrogentreatmentcanbeattributedtotheformationofO—Ti—NbondingbypartiallysubstitutingtheOatomintheTiO2latticewithaNorCatom.ThisshiftsthebindingenergiesoftheTi2pelectronstotheobservedlowervalues.
Fig.4showstheUV鄄VisdiffusereflectanceabsorptionspectraofTiO2,C鄄dopedTiO2nanotubes,N鄄dopedTiO2nanotubes,andC,N鄄codopedTiO2nanotube.ItcanbeseenthattheCand/orN
Fig.3HighresolutionXPSspectraofC1s(a),N1s(b),andTi2p(c)
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Fig.4UV-Visdiffusereflectanceabsorptionspectraofsamples
dopedTiO2samplesclearlydisplaydrasticandstrongabsorptioninthevisiblerange.ThisshowsthatthenonmetaldopingwithCorNcanenhancetheabsorptionofTiO2forthevisiblelight.ItshouldbenotedthatC,N鄄codopedTiO2showedhigherabsorp-tionthanC鄄dopedTiO2andN鄄dopedTiO2.Theenhancedab-sorptioninthevisiblelightregionobviouslyhadapositiveef-fectonthephotocatalyticactivityasshowninFig.5.FromFig.5,itwaseasilyseenthattheC,N鄄codopedTiO2hadthehighestcatalyticefficiency,i.e.,thehighestmethylorangeremoval,whichwasobviouslyascribedtotheimprovedabsorptionofthevisiblelight.ItshouldbenotedthatC,N鄄codopedTiO2nanotubes(C:1.4%,N:1.5%,atomfraction)wasthebestphotocatalystamongC,N鄄codopedTiO2nanotubesampleswithdifferentCandNcon-centrations.Moreover,theactivityofthedopedsampleafter5timesreusechangedlittle,suggestingithadagoodstability.ThedopedCatomscouldplayaroleofphotosensitizerlikeorganicdyes,whichcouldbeexcitedandinjectedelectronsintothecon-ductionbandofTiO2;thentherateofelectrontransfertooxygenadsorbedontheTiO2surfaceincreased.Ontheotherhand,theNatomsdopedinTiO2couldalsopromotethiselectrontransfer.Itwasreportedthatnitrogendopingcouldnotonlycreateintra鄄band鄄gapstatesclosetothevalencebandedges,whichinducedvisiblelightabsorptionatthesub鄄band鄄gapenergiesh淄2,butalso
Fig.5ComparisonofphotocatalyticactivitiesofC,N鄄codoped,N鄄doped,C鄄dopedTiO2nanotubesandTiO2
nanotubes
a:TiO2nanotubes;b:C鄄dopedTiO2nanotubes(C:1.1%,atomfraction);c:N鄄dopedTiO2nanotubes(N:1.4%);d:C,N鄄codopedTiO2nanotubes(C:0.7%,N:0.6%);e:C,N鄄codopedTiO2nanotubes(C:0.9%,N:1.3%);f:C,N鄄codopedTiO2nanotubes(C:1.4%,N:1.5%);g:C,N鄄codopedTiO2nanotubes(C:1.4%,N:1.5%)reused5timesshiftitspositionofflat鄄bandpotentialtoahigherlevelthanthatofundopedTiO2,whichincreasedthedrivingforceofelectroninjectedfromcarbonaceousspeciesandacceleratedthereductiveprocessofinterfacialelectrontransfer[16].
3Conclusions
Inthisstudy,anovelmethod—plasmaelectrolysiswasusedforCand/orNdopingofTiO2nanotubeswiththegoalofim-provingtheabsorptioninthevisibleregionforTiO2.TheplasmaelectrolysisprocessdidnotchangethemorphologyandcrystalstructureofTiO2nanotubes.TheresultsofXPSrevealedthatCandNweresuccessfullydopedintothelatticeofTiO2,andtheTi—O—N—CbandwasformedinC,N鄄codopedTiO2.UV鄄Visdiffusereflectancespectrashowedanenhancementoftheab-sorptioninthevisibleregionfortheC,N鄄codopedTiO2,com-paredtothepureTiO2,C鄄dopedTiO2,andN鄄dopedTiO2.TheC,N鄄codopedTiO2showedthebestphotocatalyticactivityintheabatementofmethylorangeundervisiblelightirradiation,andwasapromisingphotocatalystforutilizingthesolarenergy.Acknowledgments：Theauthorswouldliketoacknowl-edgeProf.QingXuforhishelpwiththeSEManalysis.References
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