Synthesis, Properties, Modifications, and

.2007,107,2891−29592891TitaniumDioxideNanomaterials:Synthesis,Properties,Modifications,andApplicationsXiaoboChen*†LawrenceBerkeleyNationalLaboratory,andUniversityofCalifornia,Berkeley,California94720ReceivedMarch27,−2Mesoporous/emicalModification:O2Nanomaterials:-DopedTiO2Nanomaterials:al-DopedTiO2Nanomaterials:ThirdGeneration*Correspondingauthor.E-mail:XChen3@.†E-mail:SSMao@.28995289628982909291129/d/nces293229322938293829uctionSinceitscommercialproductionintheearlytwentiethcentury,titaniumdioxide(TiO2)hasbeenwidelyusedasapigment1andinsunscreens,2,3paints,4ointments,toothpaste,1972,FujishimaandHondadiscoveredthephenom-enonofphotocatalyticsplittingofwateronaTiO2electrodeunderultraviolet(UV)light.6-8Sincethen,enormouseffortshavebeendevotedtotheresearchofTiO2material,whichhasledtomanypromisingapplicationsinareasrangingfromphotovoltaicsandphotocatalysistophoto-/electrochromicsandsensors.9-12Theseapplicationscanberoughlydividedinto“energy”and“environmental”categories,manyofwhichdependnotonlyonthepropertiesoftheT,withinorganicandorganicdyes)nentialgrowthofresearchactivitieshasbeenseeninnanoscienceandnanotechnologyinthepastdecades.13-17Newphysicalandchemicalpropertiesemergewhenthesizeofthematerialbecomessmallerandsmaller,anddownto10.1021/cr0500535CCC:$65.00©2007AmericanChemicalSocietyPublishedonWeb06/23/2007

2892ChemicalReviews,2007,Vol.107,ChenisaresearchengineeratTheUniversityofCaliearchinterestsincludephotocatalysis,photovoltaics,hydrogenstorage,fuelcells,environmentalpollutioncontrol,careerstaffscientistatLawrenceBerkeleyNationalLabrentresearchinvolvesthedevelopmentofnanostructuredmaterialsanddevices,heteamlcellentreviewsandreportsonthepreparationandpropertiesofnanomaterialshavebeenpublishedrecently.6-44Amongtheuniqueproper-tiesofnanomaterials,themovementofelectronsandholesinsemiconductornanomaterialsisprimarilygovernedbythewell-knownquantumconfinement,andthetransportproper-tiesrelatedtophononsandphotonsarelargelyaffectedbythesizeandgeometryofthematerials.13-16Thespecificsurfaceareaandsurface-to-volumeratioincreasedramati-callyasthesizeofamaterialdecreases.13,21ThehighsurfaceareabroughtaboutbysmallparticlesizeisbeneficialtomanyTiO2-baseddevices,asitfacilitatesreaction/interactionbetweenthedevicesandtheinteractingmedia,whichmainlyoccursonthesurface,theperformanceofTiO2-baseddevicesislargelyinfluencedbythesizesoftheTiO2buildingunits,ostpromisingphotocatalyst,7,11,12,33TiO2mate-rialsareexpectedtoplayanimportantroleinhelpi2alsobearstremendoushopeinhelpingeasetheenergycrisisthrougheffectiveutilizationofsolarenergybasedonphotovoltaicandwater-splittingdevices.9,31,32Ascontinuedbreakthroughshavebeenmadeinthepreparation,modifica-tion,andapplicationsofTiO2nanomaterialsinrecentyears,especiallyafteraseriesofgreatreviewsofthesubjectinthe1990s.7,8,10-12,33,45webelievethatanewandcompre-hensivereviewofTiO2nanomaterialswouldfurtherpromoteTiO2-basedresearchanddevelopmentefforts,wefocusonrecentprogressinthesynthesis,properties,modifications,thesesofTiO2nanomaterials,includingnanoparticles,nanorods,nanowires,parationsofmesopo-rous/nanoporousTiO2,TiO2aerogels,opals,ewingnanoma-terialsynthesis,wepresentatypicalprocedureandrepre-sentativetransmissionorscanningelectronmicroscopyimagestogiveadirectimpresailedinstructionsoneachsynthesis,uctural,thermal,electronic,andopize,shape,andcrystalstructureofTiO2nanomate-rialsvary,notonlydoessurfacestabilitychangebutalsothetransitionsbetendenceofX-raydiffractionpatternsandRamanvibrationalspectraonthesizeofTiO2nanomaterialsisalsosummarized,astheycouldhelptodeterminethesizetosomeextent,althoughcorrelatioiewofmodificationsofTiO2nanomaterialsismainlylimitedtotheresearchrelatedtothemodificationsoftheopticalpropertiesofTiO2nanoma-terials,sincemanyapplicatingorsensitization,itispossibletoimprovetheopticalsenmental(photocatalysisandsensing)andenergy(photovoltaics,watersplitting,photo-/electrochromics,andhydrogenstorage)applicationsarereviewedwithanemphasisoncleanandsustainableenergy,sincetheincreas-ingenergydemandandenvironmentalpodamentalsandworkingprinciplesoftheTiO2nanoma-terials-baseddevicesarediscussedtofacilitatetheunder-stand−GelMethodThesol-gelmethodisaversatileprocessusedinmakingvariousceramicmaterials.46-50Inatypicalsol-gelprocess,acolloidalsuspension,orasol,isformedfromthehydrolysisandpolymerizationreactionsoftheprecursors,whichareusualltepolymerizationandlosslmscanbeproducedonapieceof

Tlwillformwhenthesoliscastintoamold,andthewetgyporousandextremelylow-densitymaterialcalledanaerogelisobtcfiberscanbedrawnfromineanduniformceramicpowdersareformedbyprecipitation,spraypyrolysis,roperconditions,2nanomaterialshavebeensynthesizedwiththesol-gelmethodfromhydrolysisofatitaniumprecusor.51-78Thisprocessnormallyproceedsviaanacid-catalyzedhydrolysisstepoftitanium(IV)alkoxidefollowedbycondensa-tion.51,63,66,79-91ThedevelopmentofTi-O-Tichainsisfavoredwithlowcontentofwater,lowhydrolysisrates,-dimensionalpolymericsmationofTi(OH)senceofalargequantityofTi-OHandinsufficientdevelopmentofthree-dimenypackedfirst-orderparticlesareyieldedviaathree-dimensionallydevel-opedgelskeleton.51,63,66,79-91FromthestudyonthegrowthkineticsofTiO2nanoparticlesinaqueoussolutionusingtitaniumtetraisopropoxide(TTIP)asprecursor,itisfoundthattherateconstantforcoarseningincreaseswithtemper-atureduetothetemperaturedependenceoftheviscosityofthesolutionandtheequilibriumsolubilityofTiO2.63Second-aryparticlesareformedbyepitaxialself-assemblyofprimaryparticlesatlongertimesandhighertemperatures,andtrageTiO2nanoparticleradiusincreaseslinearlywithtime,inagreementwiththeLifshitz-Slyozov-Wagnermodelforcoarsening.63HighlycrystallineanataseTiO2nanoparticleswithdifferentsizesandshapescouldbeobtainedwiththepolycondensationoftitaniumalkoxideinthepresenceoftetramethylammoniumhydroxide.52,62Inatypicalprocedure,titaniumalkoxideisaddedtothebaseat2°Cinalcoholicsolventsinathree-neckflaskandisheatedat50-60°Cfor13daysorat90-100°darytreatmentinvolvingautoclaveheatingat175and200°hesol-gelmethodontheformationofTiO2nanoparticlesofdifferentsizesandshapesbytuningthereactionparameters.67-71Typically,astocksolutionofa0.50MTisourceispreparedbymixingTTIPwithtriethanolamine(TEOA)([TTIP]/[TEOA])1:2),cksolutionisdilutedwithashapecontrollersolutionandthenagedat100°Cfor1dayandat140°areuseminesincludeTEOA,diethylenetri-amine,ethylenediamine,trimethylenediamine,phologyoftheTiO2nanoparticlesChemicalReviews,2007,Vol.107,2showsrepresentativeTEMimagesoftheTiO2nanoparticlesunderdifferentinitialpHconditionswiththeshapecontrolofTEOAat[TEOA]/[TIPO])aryamines,suchasdiethylamine,andtertiaryamines,suchastrimethylamineandtriethylamine,actascomplexingagentsofTi(IV)iopeoftheTiO2nanoparticlecanalsobetunedfromround-corneredcubestosharp-edgedcubeswithsodiumoleateandsodiumstearate.70TheshapecontrolisattributedtothetuningofthegrowthrateofthedifferentcrystalplanesofTiO2nanoparticlesbythespecificadsorptionofshapecontrollerstotheseplanesunderdifferentpHconditions.70Aprolongedheatingtimebelow100°Cfortheas-preparedgelcanbeusedtoavoidtheagglomerationoftheTiO2nano-particlesduringthecrystallizationprocess.58,72ByheatingamorphousTiO2inair,largequantitiesofsingle-phaseana-taseTiO2nanoparticleswithaverageparticlesizesbetween7and50nmcanbeobtained,asreportedbyZhangandBanfield.73-77MuchefforthasbeenexertedtoachievehighlycrystallizedandnarrowlydispersedTiO2nanoparticlesusingthesol-gelmethodwithothermodifications,suchasasemicontinuousreactionmethodbyZnaidietal.78andatwo-stagemixedmethodandacontinuousreactionmethodbyKimetal.53,54Byacombinationofthesol-gelmethodandananodicaluminamembrane(AAM)template,TiO2nanorodshavebeensuccessfullysynthesizedbydippingporousAAMsintoaboiledTiO2solfollowedbydryingandheatingprocesses.92,93Inatypicalexperiment,aTiO2solsolutionispreparedbymixingTTIPdissolvedinethanolwithasolutioncontainingwater,acetylacetone,simmersedintothesolsolutionfor10minafterbeingboiledinethanol;thenitisdriedinairandcalcinedat400°templateisremovedina10wt%cinatiemperature,anatasenanorodscanbeobtained,esizeoftheAAMtemplatecanbeusedtocontrolthesizeoftheseTiO2nanorods,whichtypi-ently,thesizedistributionofthefinalTiO2nanorodsislrtoobtainsmallerandmono-sizedTiO2nanorods,ly,trophoreticdepositionofTiO2colloidalsuspensionsintotheporesofanAAM,orderedTiO2nanowirearrayscanbeobtained.94Inatypicalprocedure,TTIPisdissolvedinethanolatroomtemperature,andglacialaceticacidmixedwithdeionizedwaterandethanolisaddedunderpH)umisusedastheanode,2solisdepositedintotheporesoftheAMMunderavoltageof2-5Vandannealedat500°issolvingtheAAMtemplateina5wt%NaOHsolution,rto

2894ChemicalReviews,2007,Vol.107,gesofTiO2nanoparticlespreparedbyhydrolysisofTi(OR)tedwithpermissionfromChemseddine,A.;Moritz,.1999,tedfromSugimoto,T.;Zhou,X.;Muramatsu,dInterfaceSci.2003,259,53,Copyright2003,ateTiO2nanowiresinsteadofnanorods,2nanotubescanalsobeobtainedusingthesol-gelmethodbytemplatingwithanAAM95-98andotherorganiccompounds.99,100Forexample,whenanAAMisusedasthetemplate,athinlayerofTiO2solonthewalloftheporesoftheAAMisfirstpreparedbysuckingTiO2solintotheporesoftheAAMandremovingitundervacuum;TiO2narocedurebyLeeandco-workers,96aTTIPsolutionwaspreparedbymixingTTIPwith2-propanoland2,heAAMwasdippedintothis

geofanatasenanorodsandasinglenanorodcomposedofsmallTiOMiao,L.;2nanoparticlesornanograins(inset).ReprintedfromTanemura,S.;Toh,S.;Kaneko,K.;Tanemura,2004,264,246,Copyright2004,tedwithpermissionfromLiu,S.M.;Gan,L.M.;Liu,L.H.;Zhang,W.D.;Zeng,.2002,14,on,itwasremovedfromthesolutionandplacedundwashydrolyzedbywatervaporoveraHClsolutionfor24h,air-driedatroomtemperature,andthencalcinedinafurnaceat673Kfor2handcooledtoroomtemperaturewithatemperaturerampof2°C/O2nanotubeswereobtainedaftertheAAMwasdissolvedina6MNaOHsolutionforseveralminutes.96Alternatively,TiO2nanotubescouldbeobtainedbycoatingtheAAMmembranesat60°Cforacertainperiodoftime(12-48h)withdiluteTiF4underpH)2.1andremovingtheAAMafterTiO2nanotubeswerefullydeveloped.97Figure4herscheme,aZnOnanorodarrayonaglasssubstratecanbeusedasatemplatetofabricateTiO2nanotubeswiththesol-gelmethod.101Briefly,TiO2solisChemicalReviews,2007,Vol.107,TiO2nanotubearray;tedwithpermissionfromQiu,J.J.;Yu,W.D.;Gao,X.D.;Li,chnology2006,17,tedonaZnOnanorodtemplatebydip-coatingwithaslowwithdrawingspeed,thendriedat100°Cfor10min,andheatedat550°Cfor1hinairtoobtainZnO/nanorodtemplateisetched-upbyimmersingtheZnO/TiO2nanorodarraysinad5showsatypi2nanotubesinherittheuniformhexagonalcross-sectionalshapeandthelengthof1.5µoncentrationoftheTiO2solisconstant,well-alignedTiO2nanot,above6°Cmin-1,theTiO2coatwilleasilycrackandflakeofffromtheZnOnanorodsduetogreattensilestressbetweentheTiO2coatandtheZnOtemplate,andaTiO2filmwithloose,eandInverseMicelleMethodsAggregatesofsurfactantmoleculesdispersedinaliquidcolloidarecalledmicelleswhenthesurfactantconcentrationexceedsthecriticalmicelleconcentration(CMC).TheCMCistheconcentrationofsurfaclles,thehydrophobichydrocarbonchainsofthesurfactantsareorientedtowardtheinteriorofthemicelle,andthehydro-philicgrocentrationofthelipidpresentinsolutiondetidsformasiidsorganizeinsphericalmicellesatthefirstCMC(CMC-I),intoelongatedpipesatthesecondCMC(CMC-II),andintostackedlamellaeofpipesatthelamellarpoint(LMorCMC-III).TheCMCdependsonthechemicalcomposition,emicellesareformedinnonaqueousmedia,andthehydrophilicheadgroupsaredirectedtowardthecoreofthemicelleswhilethehydrophobicgroupsare

2896ChemicalReviews,2007,Vol.107,snoobviousCMCforreversemicelles,becausethenumberofaggregatesiesareoftenglobularandroughlysphericalinshape,butellipsoids,cylinders,peofamicelleisafunctionofthemoleculargeometryofitssurfactantmoleculesandsolutionconditionssuchassurfactantconcentration,tem-perature,pH,esandinversemicellesarecommonlyemployedtosynthesizeTiO2nanomaterials.10miuesofH2O/surfactant,H2O/titaniumprecursor,ammoniaconcentration,feedrate,andreactiontemperatureweresignificanousTiO2nanoparticleswithdiametersof10-20nmweresynthesizedandconvertedtotheanatasephaseat600°Candtothemorethermodynamicallystablerutilephaseat900°pedTiO2nanoparticleswiththechemicalreactionsbetweenTiCl4solutionandammoniainareversedmicro-emulsionsystemconsistingofcyclohexane,poly(oxyethyl-ene)5nonylephenolether,andpoly(oxyethylene)9nonylephenolether.104TheproducedamorphousTiO2nanoparticlestransformedintoanatasewhenheatedattemperaturesfrom200to750°Candintorutileattemperatureshigherthan750°drolysisoftitaniumtetrabutoxideinthepresenceofacids(hydrochloricacid,nitricacid,sulfuricacid,andphosphoricacid)inNP-5(IgepalCO-520)-cyclohexanereversemicellesatroomtemperature.110Thecrystalstructure,morphology,andparticlesizeoftheTiO2nanoparticleswerelargelycontrolledbythereactioncondi-tions,andthekeyfactorsaffectingtheformationofrutileatroomtemperatureincludedtheacidity,thetypeofacidused,-glomerationoftheparticlesoccurredwithprolongedreactiontimesandincreasingthe[H2O]/[NP-5]and[H2O]/[Ti-(OC4H9)4]itableacidwasapplied,entativeTEMimagesofthtudycarriedoutbyLimetal.,TiO2nanoparticleswerepreparedbythecontrolledhydrolysisofTTIPinreversemicellesformedinCO2withthesurfactants-ammoniumcarboxylateperfluoropolyether(PFPECOONH4+)(MW587)andpoly(dimethylaminoethylmethacrylate-block-1H,1H,2H,2H-perfluorooctylmeth-acrylate)(PDMAEMA-b-PFOMA).106Itwasfoundthatthecrystallitesizepreparedinthepresenceofreversemicellesincreasedaseitherthemolarr2nanomaterialspreparedwiththeabovemicelleandreversemicellemethodsnormallyhaveamorphousstructure,ar,thisprstallinityofTiO2nanoparticlesinitially(synthesizedbycontrolledhydrolysisoftitaniumalkoxideinreversemicellesinahydrocarbonsolvent)couldbeimprovedbyannealinginthepresenceofthemicellesattemperaturesconsiderablylowegesoftheshuttle-likeandround-shaped(inset):Zhang,D.,Qi,L.,Ma,J.,Cheng,.2002,12,3677(/10.1039/b206996b).magesofaTiOReprintedwithpermissionfromLin,2nanoparticleafterannealing.J.;Lin,Y.;Liu,P.;Meziani,M.J.;Allard,L.F.;Sun,.2002,124,entinthesolidstate.108ThisprocedurecouldproducecrystallineTiO2nanoparticleswithunchangedphysicaldimensionsandminimalagglomerationandallowsthepreparationofhighlycrystallineTiO2nanoparticles,asshowninFigure7,hodThesolmethodherereferstothenonhydrolyticsol-gelprocessesandusuallyinvolvesthereacti,ametalalkoxideoranorganicether.111-119TiX4+Ti(OR)4f2TiO2+4RX(1)TiX4+2RORfTiO2+4RX(2)

geofTiOofTiCl2nanoparticlesderivedfromreactionHRTEM4andTTIPinTOPO/heptadecaneat300°tedwithpermissionfromTrentler,T.J.;Denler,T.E.;Bertone,J.F.;Agrawal,A.;Colvin,.1999,121,doxidegroupscanbeprovidedbytitaniumalkoxidesorcanbeethodbyTrentlerandColvin,119ametalalkoxidewasrapidlyinjectedintothehotsolutionoftitaniumhalidemixedwithtrioctylphosphineoxide(TOPO)inheptadecaneat300°Cunderdryinertgasprotection,riesofalkylsubstituentsincludingmethyl,ethyl,isopropyl,andtert-butyl,thereactionratedramaticallyincreasedwithgreaterbranchingofR,ionofXyieldedacleartrendinaverageparticlesize,sednucleophilicity(orsize)untofpassivatingagent(TOPO)oninpureTOPOwasslowerandresultedinsmallerparticles,whilereactionswithoutTOPOweremuchquickerandyieldedmixturesofbrookite,rutile,8showstypicalTEMimagesofTiO2nanocrystalsdevelopedbyTrentleretal.119InthemethodusedbyNiederbergerandStucky,111TiCl4wasslowlyaddedtoanhydrousbenzylalcoholundervigorousstirringatroomtemperatureandwaskeptat40-150°cipitatewascalcinatedat450°ctionbetweenTiCl4andbenzylalcoholwasfoundsuitableforthesynthesisofhighlycrystallineanatasephaseTiO2nanoparticleswithnearlyuniformsizeandshapeatverylowtemperatures,suchas40°ticlesizecouldbeselectivelyadjustedintherangeof4-8nmwiththeappropriatethermalconditionsandaproperchticlegrowthdependedstronglyontemperature,andloweringthetitaniumtetrachlorideconcentrationledtoaconsiderabledecreaseofparticlesize.111Surfactantshavebeenwidelyusedinthepreparationofavarietyofnanoparticleswithgoodsizedistributionanddispersity.15,16Addingdifferentsurfactantsascappingagents,suchasaceticacidandacetylacetone,intothereactionmatrixChemicalReviews,2007,Vol.107,etshowsaHRTEMofaTiO2withpermissionfromCozzoli,P.D.;Kornowski,A.;Weller,.2003,125,psynthesizemonodispersedTiO2nanoparticles.120,121Forexample,ScolanandSanchezfoundthatmonodispersenonaggregatedTiO2nanoparticlesinthe1-5nmrangewereobtainedthroughhydrolysisoftitaniumbutoxideinthepresenceofacetylacetoneandp-toluenesulfonicacidat60°C.120Theresultingnanoparticlexerosolscouldbedispersedinwater-alcoholoralcoholsolutionsatconcentrationshigherthan1Mwithoutaggregation,whichisattributedtothecomplexationofthesurfacebyacetylacetonatoligandsandthroughanadsorbedhybridorganic-inorganiclayermadewithacetylacetone,p-toluenesulfonicacid,andwa-ter.120Withtheaidofsurfactants,differentsizedandshapedTiO2nanorodscanbesynthesized.122-130Forexample,thegrowthofhigh-aspect-ratioanataseTiO2nanorodshasbeenreportedbyCozzoliandco-workersbycontrollingthehydrolysisprocessofTTIPinoleicacid(OA).122-126,130Typically,TTIPwasaddedintodriedOAat80-100°Cunderinertgasprotection(nitrogenflow)andstirredfor5min.A0.1-2Maqueousbasesolutionwasthenrapidlyinjectedandkeptat80-100°esemployedincludedorganicamines,suchastrimethylamino-N-oxide,trimethylamine,tetramethylammoniumhydroxide,tetrabut-ylammoniumhydroxyde,triethylamine,reaction,bychemicalmodificationofthetitaniumprecursorwiththecarboxylicacid,(in4-6h)crystal-lizationinmildconditionswaspromotedwiththeuseofsuitablecatalysts(tertiaryaminesorquaternaryammoniumhydroxides).AkineticallyoverdrivengrowthmechanismledtothegrowthofTiO2nanorodsinsteadofnanoparticles.123TypicalTEMimagesoftheTiO2nanorodsareshowninFigure9.123Recently,Jooetal.127andZhangetal.129reportedy,amixtureofTTIPandOAwasusedtogenerateOAcomplexesoftitaniumat80°Cin1-octadecene.

2898ChemicalReviews,2007,Vol.107,gesofTiO2nanorodswithlengthsof(A)12nm,(B)30nm,and(C)16nm.(D)npartsCandD:tedwithpermissionfromZhang,Z.;Zhong,X.;Liu,S.;Li,D.;Han,.,.2005,44,ectionofapredeterminedamountofoleylamineat260°CledtovarioussizedTiO2nanorods.129Figure10showsTEMimagesofTiO2nanorodswithvariouslengths,and2.3nmTiO2nanoparticlespreparedwiththismethod.129Inthesurfactant-mediatedshapeevolutionofTiO2nano-crystalsinnonaqueousmediaconductedbyJunetal.,128itwasfoundthattheshapeofTiO2thesiswasaccomplishedbyanalkylhalideeliy,adioctylethersolutioncontainingTOPOandlauricacidwasheatedto300°ctionauricacidconcentrations,bullet-anddiamond-shapednanocrystalswereobtained;athigherconcentrations,rod-shapelet-anddiamond-shapednanocrystalsandnanorodswereelon-gatedalongthe[001]2nanorodswerefoundtosimultaneouslyconverttosmallnanoparticlesasafunctionofthegrowthtime,asshowninFigure11,duetotheminimizationoftheoverallsurfaceehermalMethodHydrothermalsynthesisisnormallyconductedinsteelpressurevesselscalledautoclaveswithorwithoutTeflonlinersundercontrolledtemperatureand/peraturecanbeelevatedabovetheboilingpointofwater,peratureandtheamountofsolutioethodthatisoupshaveusedthehydrothermalmethodtoprepareTiO2nanoparticles.131-140Forexample,TiO2nanoparticlescanbeobtainedbyhydrothermaltreatmentofpeptizedprecipitatesofatitaniumprecursorwithwater.134Theprecipitateswerepreparedbyaddinga0.5Misopropanolsolutionoftitaniumbutoxideintodeionizedwater([H2O]/[Ti])150),andthentheywerepeptizedat70°Cfor1hinthepresenceoftetraalkylammoniumhydroxides(peptizer).Afterfiltrationandtreatmentat240°Cfor2h,theas-obtainedpowderswerewashedwithdeionizedwaterandabsoluteethanolandthendriedat60°hesameconcentrationofpeptizer,tilTEMimagesofTiO2nanoparticlesmadewiththehydrothermalmethodareshowninFigure12.134Inanotherexample,TiO2nanoparticleswerepreparedbyhydrothermalreactionoftitaniumalkoxideinanacidicethanol-watersolution.132Briefly,TTIPwasaddeddropwisetoamixedethanolandwatersolutionatpH0.7withnitricacid,andreactedat240°2nanoparticles

pendentshapeevolutionofTiO(a)0.25h;(b)24h;(c)ar)ted2nanorods:withpermissionfromJun,Y.W.;Casula,M.F.;Sim,J.H.;Kim,S.Y.;Cheon,J.;Alivisatos,.2003,125,sizedunderthisacidicethanol-waterenvironmentweremesoftheparticleswerecontrolledtotherangeof7-25nmbyadjustingtalReviews,2007,Vol.107,tedfrom2nanoparticlespreparedbytheYang,J.;Mei,S.;Ferreira,.C2001,15,183,Copyright2001,tedwith2nanorodspreparedwiththepermissionfromZhang,Q.;Gao,ir2003,19,sTiO2nanoparticles,edTiO2nanorodsbytreatingadiluteTiCl4solutionat333-423Kfor12hinthepresenceofacidorinorganicsalts.141,143-146Figure13showsatypicalTEMimageoftheTiO2nanorodspreparedwiththehydrothermalmethod.141Themorphologyoftheresultingnanorodscanbetunedwithdifferentsurfactants146orbychangingthesolventcomposi-tions.145AfilmofassembledTiO2nanorodsdepositedonaglasswaferwasreportedbyFengetal.142TheseTiO2nanorodswerepreparedat160°Cfor2hbyhydrothermaltreatm2nanowireshavealsobeensuccessfullyobtainedwiththehydrothermalmethodbyvariousgroups.147-151Typically,TiO2nanowiresareobtainedbytreatingTiO2whitepowdersina10-15MNaOHaqueoussolutionat150-200°14showstheSEMimagesofTiO2nanowiresandaTEMimageofasinglenanowirepreparedbyZhangandco-workers.150TiO2nanowirescanalsobepreparedfromlayeredtitanateparticlesusingthehydrothermalmethodasreportedbyWei

2900ChemicalReviews,2007,Vol.107,gesofTiO2nanowireswiththeinsetshowingaTEMimageofasingleTiOdiffraction(SAED)recorded2nanowirewitha[010]tedfromZhang,Y.X.;Li,G.H.;Jin,Y.X.;Zhang,Y.;Zhang,J.;Zhang,.2002,365,300,Copyright2002,.152Intheirexperiment,layer-structuredNa2Ti3O7wasdispersedintoa0.05-0.1MHClsolutionandkeptat140-170°2nanoormationofaTiO2nanowirefromlayeredH2Ti3O7,therearethreesteps:(i)theexfoliationoflayeredNa2Ti3O7;(ii)thenanosheetsformation;and(iii)thenanow-iresformation.152InNa2Ti3O7,[TiO6]octahedrallayersareheldbythestrongstaticinteractionbetweentheNa+cationsbetweenthe[TiO6]octahedrallayersandthe[TiO6]elargerH3+OcationsreplacetheNa+cationsintheinterlayerspaceof[TiO6]sheets,thisstult,+isexchangedbyH+inthediluteHClsolution,henanosheetdoesnothaveinversionsym-metry,osheetssplittoformnanowiresinordertoreleasethestrongstressandlowerthetotalenergy.152ArepresentativeTEMimageofTiO2nanowiresfromNa2Ti3O7isshowninFigure15.152Thehydrothermalmethodhasbeenw1998.153-175Briefly,TiO2powdersareputintoa2.5-20MNaOHaqueoussolutionandheldat20-110°2nanotubesareobtainedaftertheoposedthefollowingformationprocessofTiO2nanotubes.154WhentherawTiO2materialwastreatedwithNaOHaqueoussolution,s-O-TibondswereformedaftertheTi-O-NaandTi-OHbondsreactedwithacidandwaterwhehthedehydra-tionofTi-OHbondsbyHClaqueoussolution,gesofTiO2nanowiresmadefromthelayeredNa2Ti3O7particles,tedfromWei,M.;Konishi,Y.;Zhou,H.;Sugihara,H.;Arakawa,.2004,400,231,Copyright2004,tedwithpermissionfromKasuga,T.;Hiramatsu,M.;Hoson,A.;Sekino,T.;Niihara,ir1998,14,tionbetweentheendsofthesheets,mechanism,theTiO2nanotubeswe16showstypicalTEMimagesofTiO2nanotubesmadebyKasugaetal.153However,Duandco-workersfoundthatthenanotubeswereformedduringthetreatmentofTiO2inNaOHaqueoussolution.161A3Df2Df1DformationmechanismoftheTiO2nanotubeswasproposedbyWangandco-workers.171ItstatedthattherawTiO2wasfirsttransforformationoftheTiO2nanotubes,co-workersfurthersuggested,basedontheirHRTEMstudyasshowninFigure

TitaniumDioxideNanomaterialsFigure17.(a)HRTEMimagesofTiOviewofTiOReused2nanotubes.(b),,,,,,AppliedPhysicsLetters82,281(2003).Copyright2003,AmericanInstituteofPhysics.17,thatTiO2nanotubeswereformedbyrollingupthesingle-layerTiO2sheetswitharolling-upvectorof[001]andattractingothersheetstosurroundthetubes.172Bavykinandco-workerssuggestedthatthemechanismofnanotubeformationinvolvedthewrappingofmultilayerednanosheetsratherthanscrollingorwrappingofsinglelayernanosheetsfollowedbycrystallizationofsuccessivelayers.156InthemechanismproposedbyWangetal.,theformationofTiO2nanotubesinvolvedseveralsteps.176DuringthereactionwithNaOH,theTi-O-Tibondingbetweenthebasicbuildingblocksoftheanatasephase,theoctahedra,wasbrokenandazigzagstructurewasformedwhenthefreeoctahedrassharededgesbetweentheTiionswiththeformationofhydroxybridges,leadingtothegrowthalongthe[100]-dimensionalcrystallinesheetsformedfromthelateralgrowthoftheformationofoxobridgesbetweentheTicenters(Ti-O-Tibonds)inthe[001]directionandrolledupinordertosaturatethesedanglingbondsfromthesurfaceandlowerthetotalenergy,hermalMethodThesolvothermalmethodisalmostidenticalr,thetemperaturecanbeelevatedmuchhigherthanthatinhydrothermalmethod,vothermalmethodnormallyhasbettercontrolthanhy-drothermalmethodsofthesizvothermalmethodhasbeenfoundtobeaversatilemethodfortheChemicalReviews,2007,Vol.107,rograprmissionfromLi,X.L.;Peng,Q.;Yi,J.X.;Wang,X.;Li,.J.2006,12,sisofavarietyofnanoparticleswithnarrowsizedistributionanddispersity.177-179ThesolvothermalmethodhasbeenemployedtosynthesizeTiO2nanoparticlesandnanorodswith/withouttheaidofsurfactants.177-185Forexample,inatypicalprocedurebyKimandco-workers,184TTIPwasmixedwithtolueneattheweightratioof1-3:10andkeptat250°rageparticlesizeofTiO2powderstendedtoincreaseasthecompositionofTTIPinthesolutionincreasedintherangeofweightratioof1-3:10,whilethepalecrystallinephaseofTiO2wasnotproducedat1:20and2:5weightratios.184Bycontrollingthehydro-lyzationreactionofTi(OC4H9)4andlinoleicacid,redispers-ibleTiO2nanoparticlesandnanorodscouldbesynthesized,ly.177ThedecompositionofNH4-HCO3couldprovideH2Oforthehydrolyzationreaction,andlinoleicacidcouldactasthesolvent/ylaminecouldactasacatalystforthepolycondensationoftheTi-O-Tiinorganicnetworktoachieveacrystallineproductandhadlittleinfluenceontheproducts’inlengthsofthecarboxylicacidshadagreatinfluenceontheformationofTiO2,andlong-chainorganicacidswereimportantandnecessaryintheformationofTiO2.177Figure18showsarepresentativeTEMimageofTiO2nanoparticlesfromtheirstudy.177TiO2nanorodswithnarrowsizedistributionscanalsobedevelopedwiththesolvothermalmethod.177,183Forexample,inatypicalsynthesisfromKimetal.,TTIPwasdissolvedinanhydroustoluenewithOAasasurfactantandkeptat250°Cfor20hinanautoclavewithoutstirring.183Longdumbbell-shapednanorodswereformedwhenasufficientamountofTTIPorsurfactantwasaddedtothesolution,duetotheorientedgrowthofparticlesalongthe[001]edprecursortosurfactantweightratioof1:3,theconcentrationofrodsinthenanoparticleassemblyincreaserageparticlesizewassmallerandthesizedistributionwstallinephase,diam-eter,andlengthofthesenanorodsarelargelyinfluencedbytheprecursor/surfactant/enano-

2902ChemicalReviews,2007,Vol.107,rographsandelectrondiffractionpatternsofproductspreparedfromsolutionsattheweightratioofprecursor/solvent/surfactant)1:5:tedfromKim,C.S.;Moon,B.K.;Park,J.H.;Choi,B.C.;Seo,2003,257,309,Copyright2003,reobtainedfromthesolutionwithaprecursor/surfactantweightratioofmorethan1:3foraprecursor/solventweightratioof1:10orfromthesolutionwithaprecursor/solventweightratioofmorethan1:5foraprecursor/surfactantweightratioof1:meterandlengthofthesenanorodswereintherangesof3-5nmand18-25nm,19showsatypicalTEMimageofTiO2nanorodspreparedfromthesolutionswiththeweightratioofprecursor/solvent/surfactant)1:5:3.183Similartothehydrothermalmethod,thesolvothermalmethodhasalsobeenusedforthepreparationofTiO2nanowires.180-182Typically,aTiO2powdersuspensioninan5MNaOHwater-ethanolsolutioniskeptinanautoclaveat170-200°2nanowiresareobtainedaftertheobtainedsampleiswashedwithadiluteHClaqueoussolutionanddriedat60°Cfor12hinair.181tswithdifferentphysicalandchemicalpropertiescaninfluencethesolubility,reactivity,anddiffusionbehaviorofthereactants;inparticular,thepolarityandcoordinatingabilityofthesolventcaninfluesenceofethanolatahighconcentrationnotonlycancausethepolarityofthesolventtochangebutalsostronglyaffectsthe