AppliedSurfaceScience
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Aone-potmethodtoprepareN-dopedtitaniahollowsphereswithhighphotocatalyticactivityundervisiblelightYanhuiAoa,b,*,JingjingXuc,SongheZhanga,b,DegangFudaKeyLaboratoryofIntegratedRegulationandResourceDevelopmentonShallowLakes,MinistryofEducation,HohaiUniversity,Nanjing210098,ChinaCollegeofEnvironmentalScienceandEngineering,HohaiUniversity,Nanjing210098,ChinacCollegeofEnvironmentalScienceandEngineering,NanjingUniversityofInformationScienceandTechnology,Nanjing210044,ChinadStateKeyLaboratoryofBioelectronics,SoutheastUniversity,Nanjing210096,ChinabARTICLEINFOABSTRACTArticlehistory:
Received24July2009Receivedinrevisedform9November2009Accepted9November2009Availableonline13November2009Keywords:PhotocatalysisTitaniaHollowspheresN-dopedVisiblelightN-dopedtitaniahollowspheres(NTHS)werepreparedbyaone-pothydrothermalmethodusingureaasprecursorofnitrogen.ThepreparedhollowsphereswerecharacterizedbyX-raydiffraction(XRD),transmissionelectronmicroscope(TEM),scanningelectronmicroscope(SEM),Brunauer–Emmett–Teller(BET),X-rayphotoelectronspectroscopy(XPS)andUV–visdiffusereflectancespectrum(DRS).Thephotocatalyticactivityofas-preparedtitaniahollowsphereswasdeterminedbydegradationofReactiveBrilliantReddyeX-3B(C.I.reactivered2)undervisiblelightirradiation,andwascomparedtonon-dopedtitaniahollowspheresandcommercialP25titania.Resultsindicatedthattheas-preparedNTHSshowedhighestphotocatalyticactivity.ß2009ElsevierB.V.Allrightsreserved.1.IntroductionSemiconductormediatedphotocatalyticoxidationandreductionofferspotentiallyafacileandcheapmethodforremovinginorganicandorganicpollutantsfromwastewaters.Amongvariousoxidesemiconductorphotocatalyst,titaniumdioxidehasprovedtobethemostsuitablecatalystsforwidespreadenvironmentalapplicationbecauseofitsbiologicalandchemicalinertness,strongoxidizingpower,non-toxicityandlong-termstabilityagainstphotoandchemicalcorrosion[1–7].However,itstechnologicalapplicationseemslimitedbyseveralfactors,amongwhichthemostrestrictiveoneistheneedofusinganultraviolet(UV)light,wavelength(l)<387nm,asexcitationsourceduetoitswideband-gap(3.2eVforanatase)[8].Therefore,itcanonlycapturelessthan5%ofthesolarirradianceattheEarth’ssurface.Forthesakeofefficientuseofsunlight,oruseofthevisibleregionofthespectrum,thetechnologyofenlargingtheabsorptionscopeofTiO2maythenappearasanappealingchallengefordevelopingthefuturegenerationofphotocatalysts.ManyworksreportedthatdopingTiO2withnonmetallicelements,suchasnitrogen[9–14],sulphur[15],carbon[16],shifttheopticalabsorptionedgeofTiO2towardlowerenergy,therebyincreasingthephotocatalyticactivityinvisibleregion.Ontheotherhand,weknowthatthemorphologyandmicrostructuresareveryimportantforthephotocatalyticactivityoftitania.Therefore,fabricationoftitaniahollowmicrosphereshasrecentlyattractedenormousattentionbecauseoftheirlowdensity,highsurfacearea,goodsurfacepermeabilityaswellaslargelight-harvestingefficiencies[17].Itisalsoexpectedthathigherenergyconversionefficiencyandphotocatalyticactivitycouldbeachievedusingtitaniahollowmicrospheresasphoto-catalysts.Untilnow,hollownanospheresoftitaniahavebeensynthesizedbyavarietyoftemplatesmethods.Thesacrificialtemplatesincludingmonodispersedsilica[18],carbonspheres[19–22]orpolystyrenespheres[23–26].Inthisarticle,fortheenhancementofvisible-light-drivenphotocatalyticactivityoftitaniaandovercomingthecomplexityoftemplatingmethodsforpreparationoftitaniahollowspheres.Afacileandtemplate-freeroutestoN-dopedtitaniahollowsphereshavebeendeveloped.Then,westudiedtheirapplicationtophotocatalyticdecompositionofReactiveBrilliantReddyeX-3B(C.I.reactivered2)inaqueoussolution.2.Experimental2.1.Samplepreparation*Correspondingauthorat:KeyLaboratoryofIntegratedRegulationandResourceDevelopmentonShallowLakes,MinistryofEducation,CollegeofEnvironmentalScienceandTechnology,HohaiUniversity,Nanjing210098,China.E-mailaddress:andyao@seu.edu.cn(Y.Ao).0169-4332/$–seefrontmatterß2009ElsevierB.V.Allrightsreserved.doi:10.1016/j.apsusc.2009.11.023ToprepareN-dopedtitaniahollowspheres(NTHS),thedetailedprocedurewasasfollowing:first,0.37gofNH4Fand0.6gofureaweredissolvedinto15mLdistilledwatertoformsolutionA.Y.Aoetal./AppliedSurfaceScience256(2010)2754–27582755
Secondly,2.4gofTi(SO4)2wasdissolvedinto20mLdilutedH2SO4(1M)toformsolutionB.Afterwards,solutionAwasmixedwithsolutionBundervigorousstirring.ThemolarratioofNH4F:ur-ea:Ti(SO4)2was1:1:1.Finally,themixedsolutionwastransferredtoa50mLTeflon-linedautoclave,whichwasfilledwithdistilledwaterupto80%ofthetotalvolume.Theautoclavewassealedandkeptat1808Cfor10h.Theproductswerecentrifugated,washedandredispersedinethanolandwaterforthreecycles,respectively.Theobtainedsampleswerethendriedat808Cfor2hundervacuum.Thetitaniahollowspheres(THS)werepreparedforcomparisonbythesamewaywithoutaddingurea.2.2.CharacterizationThestructurepropertiesweredeterminedbyX-raydiffract-ometer(XD-3A,ShimadzuCorporation,Japan)usinggraphitemonochromaticcopperradiation(Cu-Ka)at40kV,30mAoverthe2urange20–808.Thehollowmorphologieswerecharacterizedbytransmissionelectronmicroscopy(TEM,JEM2000EX)andscanningelectronmicroscopy(SEM,Sirion,FEI).TheUV–visabsorptionspectraofthetitaniahollowsphereswereobservedwithShimadzuUV-2100equippedwithanintegratingsphere.BETsurfaceareameasurementswerecarriedoutbyN2adsorptionat77KusinganASAP2020instrument.ThebindingenergywasidentifiedbyX-rayphotoelectronspectroscopy(XPS)withMg-Karadiation(ESCALBMK-II).2.3.PhotocatalyticactivityThephotocatalyticactivityofpreparedsampleswasevaluatedbyphotocatalyticdegradationofReactiveBrilliantReddyeX-3B(C.I.reactivered2)inaqueoussolution.0.1gofsamplewasaddedinto200mLof25mgLÀ1X-3Bsolution.Thesuspensionwasstirredindarkfor30mintoobtainadsorption–desorptionequilibriumofX-3Bbeforeillumination.A290Whalogenlamp(InstrumentalCorporationofBeijingNormalUniversity)withalightfiltercuttingthelightbelow400nmwasusedasvisiblelightsource.Atadefinedtimeinterval,5mLsuspensionwasremoved.AndthentheconcentrationofX-3BwasanalyzedusingtheUV–visspectrophotometerat535nm.3.Resultsanddiscussion3.1.CharacterizationofN-dopedtitaniahollowspheres(NTHS)ThepreparationofNTHSisbasedonthenewlydevelopedchemicallyinducedself-transformationmechanismbyYuetal.[27,28],whichisrelatedtofluoride-mediatedself-transformationfromsolidtitaniabylocalizedOstwaldripening.Amongtheprocess,preferentialdissolutionoftheinteriorparticlesiscoupledwiththedepositionoflooselypackednanoparticlesonthesurfaceofexternalshell.AtypicalTEMimageoftheas-preparedNTHSisshowninFig.1(a).Thestrongcontrastbetweenthedarkedgesandbrightcentersindicatesthehollowstructureoftitaniaspheres.Itcanalsobeseenthatthesizeofhollowspheresareca.500nm.Theoutersurfaceofthehollowspheresisrough,whichindicatedtheyarecomposedofalotoflooselypackednanoparticles.Suchahollowstructuremayenhanceadsorptionandmasstransportationofcontaminations,thusenhancethephotocatalyticactivity.ThecorrespondingSEMimageisshowninFig.1(b),itcanalsobeseenthatthesizeofthehollowspheresareabout500nm.FromtheBETmeasurement,thespecificsurfaceareasare213.5m2gÀ1forNTHS,whichismuchhigherthanthevalue(50m2gÀ1)ofP25titania.Thecrystalstructureoftitaniumdioxidegreatlyaffectsphotocatalyticactivity.Amorphoustitaniaseldomdisplaysphoto-activityduetosomenon-bridgingoxygeninbulktitania,whoseTi–Oatomicarrangementdefectscanactasrecombinationcentersofphoto-generatedelectron–holepairs.Additionally,differentcrystalphaseswillalsoaffectphotocatalysis.Generally,theanatasephaseisreportedwithhighphotocatalyticactivity.Therefore,thephasecharacterizationofNTHSwasinvestigatedbyX-raydiffraction(XRD)analysisandtheobtainedpatternisshowninFig.2.Asshowninthefigure,theparticleshadformedanatasephasesincethecharacteristicdiffractionpeaksofanatase(majorpeaks:25.48,38.08,48.08,54.78,63.18)wereevidentinthesample.NootherpeakssuchasN–OandTi–Nappeared.ItmaybeduetothereasonthatthecontentofNwastoolittletobedetectedinthephotocatalyst.Furthermore,thefactofnarrowpeaksindicatesthatthehollowtitaniaspheresareofhighcrystallinity.Thesurfacechemicalcompositionsandchemicalstatesofas-preparedsampleswereinvestigatedusingXPS.TheresultsshowthatNTHScontainsTi,O,NandC.TheCelementcanbeascribedtotheadventitioushydrocarbonfromtheXPSinstrument.Thehigh-resolutionXPSspectraoftheN1sregiononthesurfaceofNTHS(asshowninFig.3)showsthattherearetwoXPSpeaksatabout399.8and396.5eV.Thepeakat396.5eVisassociatedwiththeTi–Nbond[9,29].Whilethepeakat399.8eVisattributedtoN–O,N–Nbond[30].Thehigh-resolutionXPSspectraoftheTi2pregiononthesurfaceofNTHSareshowninFig.4.ItcanbeseenthatTi2p3/2(458.1eV)andTi2p1/2(463.8eV)ofNHTStransfertolowerbindingenergycomparedtopuretitania.ThetransferisascribedtothefactthatthesubstitutionofOintitanialatticebyNelementandformationofO–Ti–Nband[31].Fromalloftheresultswecanconcludethatnitrogenwassuccessfulincorporatedintothetitaniabyourmethod.Fig.5comparestheUV–visdiffusereflectancespectrumofNTHSandP25.Theresultsindicatedthatdopingnitrogenbythismethodcangiverisetoaclearred-shiftintheopticalresponseoftitaniahollowspheres.Furthermore,highervisibleabsorbance(400–700nm)wasobservedforNTHS.FromtheresultswecanFig.1.(a)TEMimageand(b)SEMimageofNTHS.2756Y.Aoetal./AppliedSurfaceScience256(2010)2754–2758Fig.2.XRDpatternsofNTHS.Fig.3.High-resolutionXPSspectrumofN1sregionofNTHS.predictthatNTHSwouldshowhighphotocatalyticactivityundervisiblelightirradiation.3.2.PhotocatalyticactivityInordertoinvestigatethephotocatalyticactivityofas-preparedNTHS,degradationexperimentsofReactiveBrilliantRedX-3BFig.4.High-resolutionXPSspectrumofTi2pregionofNTHS.Fig.5.ThediffusereflectanceUV–visspectraofNTHSandP25.(C.I.reactivered2)werestudiedundervisiblelightandresultsareshowninFig.6.Theblankexperimentwithoutcatalystswasalsoinvestigated.Andthevaluecanbeneglectedwithabout2%ofconversionafter2hirradiation.AsshowninFig.6,itpresentshighphotocatalyticactivityforthedegradationofX-3Binaqueoussolution;about72%ofX-3Bisdegradedwithin120min.WhileforP25andTHS,thedegradedpercentare13and31%,respectively.Furthermore,weinvestigatedthekineticsofX-3BdegradationbydifferentsamplesthroughLangmuir–Hinshelwoodmodel:ÀdCkrKaCdt¼1þKaC(1)where(ÀdC/dt)isthedegradationrateofX-3B,CistheX-3Bconcentrationinthesolution,tisreactiontime,krisareactionrateconstant,andKaistheadsorptioncoefficientofthereactant.KaCisnegligiblewhenvalueofCisverysmall.Asaresult,Eq.(1)canbedescribesafirst-orderkinetics.SettingEq.(1)attheinitialconditionsofthephotocatalyticprocedure,whent=0,C=C0,itcanbedescribedasfollows:lnC0C¼kappÂt(2)wherekappisapparentrateconstant,chosenasthebasickineticparameterforthedifferentphotocatalysts,sinceitenablesonetoFig.6.VariationsoftheUV–visabsorptionspectraofX-3BwiththeirradiationtimeinthepresenceofNTHS.Y.Aoetal./AppliedSurfaceScience256(2010)2754–27582757
Fig.7.KineticsofX-3BdisappearanceinthepresenceofNTHS,THSandP25.determineaphotocatalyticactivityindependentofthepreviousadsorptionperiodinthedarkandtheconcentrationofX-3Bremaininginthesolution[32].Thevariationsinln(C0/C)asafunctionofirradiationtimearegiveninFig.7.Theobtainedapparentrateconstantskappare0.011,0.001and0.0031minÀ1forNTHS,P25andTHS,respectively.ItcanbeseenthatNTHSshowsmuchhigherphotocatalyticactivityforX-3Bdegradationthanothertwosamples.ThesuperiorphotocatalyticactivityisassociatedwiththecooperativedopingeffectsofNonthephysicochemicalproper-tiesaswellasthehollowstructureofNTHS.Firstofall,itcanbeseenfromtheUV–visdiffusereflectancespectrumofNTHSthatitresultsinaintenseincreaseinabsorptioninthevisiblelightregionandared-shiftintheabsorptionedge.Theband-gapnarrowingoftitaniabyN-dopingleadstoenhancedphoto-catalyticactivityofNTHSundervisiblelight.Becausetheprepareddopedsamplescanbeactivatedbyvisiblelight,thusmoreelectronsandholescanbegeneratedandparticipateinthephotocatalyticredoxreactions.Secondly,theNTHSwithlargesurfaceareaandporousstructuresmayalsoplayanimportantroleindeterminingthephotocatalyticactivityoftheas-preparedsample.Largerspecificsurfaceareaprovidesmoresurfaceactivesitesfortheadsorptionofcontaminationmoleculesthusmakesthephotocatalyticprocessmoreefficient.Furthermore,thehollowsphericalstructuresallowmultireflectionsofirradiationlightwithintheirinteriorcavities,endowingthemwithenhancedlight-harvestingandthuswithhigherphotocatalyticactivity[33].4.ConclusionsInthepresentarticle,wereportasimpleone-potmethodforthepreparationofN-dopedtitaniahollowspheres.Theas-preparedtitaniahaslargespecificsurfaceareaandshowsaclearred-shiftintheopticalresponse.Thephotocatalyticactivityofas-preparedhollowtitaniasphereswasdeterminedbydegradationofReactiveBrilliantReddyeX-3B(C.I.reactivered2)undervisiblelightirradiation,andcomparedtocommercialP25titania.ResultsshowthatthephotocatalyticactivityofNTHSenhancedalot.Thisapproachprovidesagreen,simpleandeconomicalmethodforpreparationofN-dopedtitaniawithhollowstructuresandhighvisiblelightresponsiveactivity.Acknowledgments
WearegratefulforgrantsfromtheNationalKeyBasicResearchon‘‘WaterPollutionControlandGovernance’’ofChina(No.2008ZX07101-008),theNationalNaturalScienceFoundationofChina(No.50830304),theBasicResearchProjectofJiangsuprovince(BK2008041)andChinaPostdoctoralScienceFoundation(No.20090450133).References
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