MesoporousTitaniaSphereswithTunableChamberStuctureandEnhancedPhotocatalyticActivityHexingLi,*,†ZhenfengBian,†JianZhu,†DieqingZhang,†GuishengLi,†YuningHuo,†HuiLi,†andYunfengLu*,‡DepartmentofChemistry,ShanghaiNormalUniVersity,Shanghai200234,China,andChemicalandBiomolecularEngineeringDepartment,UniVersityofCalifornia,LosAngeles,California90095ReceivedMarch28,2007;E-mail:hexing-li@shnu.edu.cn;luucla@ucla.eduManynaturalmaterials,likeseashellandlotusleaf,arecomposedonlywithordinarycompositionbutexhibitfascinatingpropertyowingtotheiruniquestructure.1Suchintricatenaturaldesignshaveinspiredextensiveresearchinsynthesizingmaterialswithcontrolledstructureandmorphology,withexpectationsofachievingnovelorenhancedproperties.2,3Thiswork,forthefirsttime,reportsthesynthesisofhollowtitaniasphereswithtunableinteriorstructureandurchinlikemorphologyandexaminestheirphotocatalyticactivityenhancedbysuchuniquestructure.Hollowspheres,generally,areofgreatinterestforalargespectrumofapplications,suchasadsorbents,deliverycarriers,catalysts,andbiomedicaluses.4-6Theirsynthesisoftenreliesontemplatingapproaches,inwhichhard(e.g.,inorganic,metal,andpolymerparticles)orsoftsacrificialtemplates(e.g.,supramolecularassembliesofsurfactantandpolymer)wereusedtocreateahollowstructure.7-12Thetemplateapproachcanbeeasilyimplemented;however,thecapabilityofconstructingcomplicatedstructureislimitedbytheavailabilityofatemplate.Asanalternative,template-freeapproachesbasedondifferentmechanismswerealsodevelopedtosynthesizehollowsphereswithmorecomplicatedstructure.13-16Tosynthesizethehollowtitaniasphereswithsuchcomplicatedstructureandmorphology,weadaptedatemplate-freeapproach,inwhichatitaniaprecursor,TiOSO4,wassolvothermallyreactedinglycerol,alcohol,andethylether.Judiciouschoiceofthealcoholmolecules(e.g.,methanol,ethanol,andpropanol)andreactiontimeaffordsthesynthesisofsphereswithadjustablemorphology,size,andinteriorstructurethatistunablefromsolid,sphere-in-sphere,tohollow(seeSupportingInformation).Figure1showselectronmicrographsdepictingthestructuralevolutionwithreactiontime.Uniform,smooth,solidsphericalparticles(a)wereobtainedafter1-hourreaction.Smallsurfaceplatelets(b)wereformedafter12handfurthergrowwithtime,forminganurchinlikepricklysurface(c-f).Itwasalsofoundthattheplateletsformedshellsonthesolidcores,creatingacore-shellstructure(c)after1-dayreaction.Interestingly,thesecoresshrinkwithtime,formingauniquesphere-in-spherestructurewithcontinuouslyreducinginnerspheresize(d-f).Theinnerspheresfinallydiminishedafter14-daysofreactionandcreatedahollowspinystructure(f).Accompanyingthisinterestingstructuralevolu-tionisthegrowthoftheoutside-spherediameterfrom1.6to3.1µm(TableS1).Theoutside-spherediametercanalsobetunedbychoosingthealcoholmolecules.Forexample,thespheressynthe-sizedwithmethanol,ethanol,andpropanolundersimilarconditionsfor2daysdisplayasimilarsphere-in-spherestructureandurchinlikemorphology,whiletheiroutside-spherediametersystematicallyincreasesfrom2.0and2.4to5.2µm(FigureS1).Althoughtheas-synthesizedspheresareamorphous,theyturntobehighlycrystalline(anatase)aftercalcinationat550°Cfor3hwhilemaintainingtheirsphere-in-spherestructure,asconfirmedbyX-raydiffraction(XRD),high-resolutionTEM,andselectedareaelectrondiffractionstudies(FiguresS2-3).Suchaspinystructurepossessesmesoporousnetworksevidencedbythenitrogensorptionexperiment(FigureS4).Surfaceareaandporevolumeoftheuncalcinedspheresincreasefrom46to167m2/gand0.11to0.32cm3/g,respectively,whenreactiontimewasincreasedfrom0.5to2days,andremainsimilarorslightlydecreaseforalongerreactiontime(TableS1).Suchporousstructureprovidesefficienttransportpathwaystotheirinteriorvoids,whichiscriticalforcatalyst,delivery,andotherapplications.Moreimportantly,theuniquesphere-in-spherestructureallowsmultireflectionsofelec-tromagneticwaves,suchasultravioletandvisiblelight,withintheirinteriorcavities,endowingthesesphereswithgreatlyenhancedproperties.Todemonstratethisintricatestructure-functioncorrection,weexaminedphotocatalyticactivityofthecalcinedspheresusingphenoldegradationasaprobereaction(Figure2).Thesolidspheres(12h,alsoseeFigure1a)showthelowestactivityof55%whileotherspheresshowsignificantlyhigheractivities.Inparticular,thesphereswithsphere-in-spherestructure(2day,denoteasP2,also†ShanghaiNormalUniversity.‡Universityof

California.Figure1.SEMandTEM(insets)imagesofthetitaniaspheressynthesizedfor(a)1/24,(b)0.5,(c)1,(d)2,(e)7,and(f)14days,showingtransitinginteriorstructurefromdense,tosphere-in-sphere,tohollowandsurfacemorphologyfromsmoothto

prickly.PublishedonWeb06/16/2007

84069J.AM.CHEM.SOC.2007,129,8406-840710.1021/ja072191cCCC:$37.00©2007AmericanChemicalSocietyseeFigure1d)showanactivityashighas93%.Sinceallthesespherespossesssimilarcrystallinestructure(FigureS2)andsurfacearea(e.g.,32m2/gforthesolidspheresand62m2/gforP2,seeTableS1),suchdramaticactivityenhancementisraisedfromtheuniquesphere-in-spherestructure.Infact,destroyingthesphere-in-spherestructurebygrindingthespheresslightlyincreasesthesurfaceareafrom62to68m2/g;however,thecrushedsampleshowedadramaticallydecreasedactivityof57%,similartothatofthesolidspheres.Asschematicallyillustratedintheinset,webelievethatasphere-in-spherestructurewithanappropriateinnerspherediameterallowsmultiplereflectionsofUVlightwithintheinteriorcavity,allowingmoreefficientuseofthelightsourceandthereforeofferinganimprovedcatalyticactivity(FigureS7).Thespheressynthesizedwith7-dayreaction(Figure1e)displayaloweractivity(87%)thanP2,possiblybecauseoftheirsmallerinnerspheresthatreducereflectionwithintheirinteriorcavity.Consistently,thehollowspheres(14day,alsoseeFigure1f)showevenloweractivity(83%)becauseoftheevenlessreflections.Theaboveresultsunambiguouslysuggestthatsuchuniquestructureprovidestitaniawithgreatlyenhancedphotocatalyticactivity.Moreimportantly,thismultiple-reflectionconceptmaybegeneralizedtothedesignandfabricationofnovelmaterialswithenhancedpro-pertiesformicroelectronics,optoelectronics,andotherapplications.Theformationofsuchspheresmayinvolveaggregationoftitaniabuildingclustersintospheresandtheirsubsequentreaction,dissolution,andre-depositionprocess.Duringthesolvothermalcondition,etherifyingreactionsbetweenalcoholandglycerolproducewatercontinuously.Titaniabuildingclustersmaybegeneratedthroughalcoholysisreactionorhydrolysis-condensationreactionsofTiOSO4.As-formedclustersaggregateandreactformingsolidspheresthatcontainalargenumberofhydrolyzableligands(e.g.,-SO4,glyceroxy,andethoxygroups)possiblyduetoslowreactionkinetics.Withincreasingreactiontime,waterisproducedcontinuouslythroughtheetherifyingreactionsandreactswiththespheres,leadingtothedissolutionandrearrangementofthesurfacebuildingclusters.Asaresultfromthisprocess,surfaceplates(Figure1b)wereinitiallyformedandgrownintoathinshelllayer(Figure1c).Continuationofthisprocessgraduallydissolvesthecorespheres,sequentiallycreatingtheobservedsphere-in-sphereandhollowstructure.TheoccurrenceofetherifyingreactionswasclearlyprovedbyGC-MASandNMRstudies,evidencedfromtheformationofethyletherandtheethersfromethanolandglycerol(FigureS5).ItisworthpointingoutthatreactingTiOSO4withglycerolintheabsenceofethanolcouldnotleadtoanyprecipitant,possiblybecauseofastericeffectthathinderstheetherifyingoralcoholysisreactions.Similarly,thepresenceofhydrolyzableligandswithinthesphereswasalsoconfirmedbyTGA-DTAanalysis.AsshowninFigureS6,thespheresafter1-hourreactionshowmultipleexothermicpeaksandaweightlossof60%;whilethespheresafter2-dayreactiononlyshowasingleexothermicpeakandalowerweightlostof49%.ConsistentwiththeTGAanalysis,energydispersiveX-rayspectroscopy(EDXS)analysissuggestsanincreasingTianddecreasingcarboncompositionwithincreasingreactiontime(TableS2).Thepresenceofthesehydrolyzablemoietiesallowsredisso-lutionandre-depositionofthebuildingclustersuponfurtherreactions.Therefore,theformationoflargerspheresisexpectedfrombuildingclusterscontainingmoreandlargerremovalligands.Indeed,asshowninFigureS1,increasingthecarbon-chainlengthofthealcoholslowsthealcoholysisandetherifyingreactionsandproducesbuildingclusterswithmoreremovalligands,whichleadstotheformationoflargerspheres.ItisalsoworthmentioningthatreplacingTiOSO4withothertitaniumsources,suchastetrachloride,tetrabutyltitanate,andtetraisopropyltitanate,onlyresultedinsolidparticles.Thisispossiblyattributedtothesulfuricacidproducedfromthealcoholysisreactionorthesulfuricgroupitselfthatcatalyzestheetherifyingreactions.Meanwhile,wediscoveredthatuseofethyletherasacosolventisessentialtoensuretheformationofsuchauniquestructure.Theoptimummolarratioofethylether/TiOSO4wasaround11,andonlydenseparticleswereobtainedforanethylether/TiOSO4ratiohigherthan20orlessthan5.Moredetailedinvestigationisunderway.Insummary,wehavedemonstratedthesynthesisofphotocata-lytichollowtitaniasphereswithuniqueurchinlikemorphologyandtunableinteriorstructure.As-createdsphere-in-spherestructureendowsthesphereswithgreatlyenhancedphotocatalyticactivitypossiblyattributedtomultiplereflectionsofUVlightwithinthesphereinteriorvoids.Thisworkprovidesanovelpathwaytothesynthesisofhollowsphereswithcomplicatedstructure,offeringanewmaterialplatformforcatalyst,microelectronic,andotherapplications.Acknowledgment.ThisworkissupportedbytheNationalNaturalScienceFoundationofChina(20377031),ScienceandTechnologyMinistryofChina(2005CCA01100),ShanghaiLeadingAcademicDisciplineProject(No.T0402),NationalScienceFoundation(CAREER),andSandiaNationalLaboratories.SupportingInformationAvailable:Detailedinformationonthespheresynthesis,activitycharacterization,XRD,sorption,GC-MAS,NMR,TEM,TGA,andEDSstudies.ThismaterialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.References(1)Douglas,T.Science2003,299,1192.(2)Lu,Y.F.;Fan,H.Y.;Stump,A.;Ward,T.L.;Rieker,T.;Brinker,C.J.Nature1999,398,223.(3)Cha,J.N.;Stucky,G.D.;Morse,D.E.;Deming,T.J.Nature2000,403,289.(4)Nakashima,T.;Kimizuka,N.J.Am.Chem.Soc.2003,125,6386.(5)Caruso,F.;Caruso,R.A.;Mo¨hwald,H.Science1998,282,1111.(6)Yin,Y.D.;Rioux,R.M.;Erdonmez,C.K.;Hughes,S.;Somorjai,G.A.;Alivisatos,A.P.Science2004,304,711.(7)Mitchell,D.T.;Lee,S.B.;Trofin,L.;Li,N.C.;Nevanen,T.K.;Soderlund,H.;Martin,C.R.J.Am.Chem.Soc.2002,124,11864.(8)Dinsmore,A.D.;Hsu,M.F.;Nikolaides,M.G.;Marquez,M.;Bausch,A.R.;Weitz,D.A.Science2002,298,1006.(9)Caruso,R.A.;Schattka,J.H.;Greiner,A.AdV.Mater.2001,13,1577.(10)Valtchev,V.Chem.Mater.2002,14,956.(11)Qi,L.M.;Li,J.;Ma,J.M.AdV.Mater.2002,14,300.(12)Peng,Q.;Dong,Y.;Li,Y.Angew.Chem.,Int.Ed.2003,42,3027.(13)Guo,C.W.;Cao,Y.;Xie,S.H.;Dai,W.L.;Fan,K.N.Chem.Commun.2003,700.(14)Afanasiev,P.;Bezverkhy,I.J.Phys.Chem.B2003,107,2678.(15)Yin,Y.;Rioux,R.M.;Erdonmez,C.K.;Hughes,S.;Somorjai,G.A.;Alivisatos,A.P.Science2004,304,711.(16)(a)Yang,H.G.;Zeng,H.C.J.Phys.Chem.B2004,108,3492.(b)Chang,Y.;Teo,J.J.;Zeng,H.C.Langmuir2005,21,1074.(c)Liu,B.;Zeng,H.C.J.Am.Chem.Soc.2004,126,16744.(d)Teo,J.J.;Chang,Y.;Zeng,H.C.Langmuir2006,22,7369.

JA072191CFigure2.Comparisonofphotocatalyticactivitiesofthetitaniasphereswithsolid,sphere-in-sphere,andhollowstructure.Insetshowsaschematicillustrationofmultireflectionswithinthesphere-in-spherestructure.COMMUNICATIONS

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