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73Journal of Materials Chemistry,0-7997
PAPERwww.rsc.org/materia；Phasetransitionandmorpho；BinZhao,FengChen,*Yancha；Received18thMay2010,Acce；Asystematicmorphological；compositionandcrystallit；Introduction；Titaniumdioxideis
　PAPERwww.rsc.org/materials|JournalofMaterialsChemistryPhasetransitionandmorphologicalevolutionoftitania/titanatenanomaterialsunderalkalescenthydrothermaltreatment?BinZhao,FengChen,*YanchaoJiaoandJinlongZhangReceived18thMay2010,Accepted23rdJune2010DOI:10.497dAsystematicmorphologicalphasediagramoftitania/titanatewasdeterminedbythemostimportantsynthesisparametersofreactiontemperatureandNaClconcentrationviaafacileone-stepalkalescentapproach.X-RaydiffractionandRamanspectroscopywereusedtogenerallyassessthephasecompositionandcrystallitesizeofdifferenttitania/titanatesamples,whichwerealsousedtoinvestigatethephasetransitionbehaviorinconnectionwiththesynthesisparameters.Scanning/transmissionelectronmicroscopywasemployedtocharacterizethemorphology,size,andlatticeplane,andtofurtherelucidatethemorphologicalevolutionoftheresultingproducts.AccordingtotheOstwald’ssteprule,aphasetransitionmechanismwasproposedbasedonourcurrentsystematicexperimentalresults,whichrevealsthephasetransitionsfromlayeredhydrogentitanatetoanataseorbrookiteunderhydrothermaltreatment.ThebrookiteCanatasecompositeexhibitedexcellentphotocatalyticactivityinthephotodegradationofmethylorange,phenolandsalicylicacidsolutionunderUVirradiation.IntroductionTitaniumdioxideisawell-knownfunctionalmaterial,whichisconsideredtobeanearlyidealphotocatalystforenvironmentalprotectionandremediationprocedures,suchasairandwaterpuri?cation,waterdisinfection,andhazardouswasteremedia-tion,duetoitspropertiesofsuperiorphotoactivity,highpho-tostability,andnon-toxicity.1C6Thechemicalreactivityandphotocatalyticbehavioroftitania/titanatenanomaterialsisstronglyin?uencedbythecrystallinephase,grainsize,morphology,microstructureandotherphysicochemicalcharac-teristics.5C8TiO2existsinthreenaturalcrystallinepolymorphs:anatase(I41/amd),rutile(P42/mnm)andbrookite(Pbca),9whichcanbemodeledasanedgeandcorner-linkedstructurewithTicationscoordinatedoctahedrallybyoxygenanions.10Theanatasepolymorphisgenerallyreportedtoexhibitbetteref?-ciencyforphotocatalysisandsolarenergyconversionbecauseofthelowrecombinationrateofitsphotogeneratedelectronsandholes.11Lietal.8reportedthatthebrookitenanoplates,whichweresynthesizedviaaredoxrouteunderhydrothermalcondi-tions,exhibitedthehighestphotocatalyticactivityunderthesamesurfaceareaofloadedTiO2.Inaddition,itwasreportedthatthecompositeoxidemadebytwotitaniaphasesshowedhigherphotocatalyticactivityasaresultofthesynergisticeffectbetweendifferentTiO2polymorphs.12,13Morphology,size,andphasestructurecontrolledsynthesisoftitania/titanatenanocrystalliteshaslongbeenoneofthemainthemesinTiO2research.ThemorphologyandtextureplayKeyLaboratoryforAdvancedMaterialsandInstituteofFineChemicals,EastChinaUniversityofScienceandTechnology,130MeilongRoad,Shanghai,CA,200237,P.R.China.E-mail:Fengchen@.;Fax:+86-21-;Tel:+86-21-?Electronicsupplementaryinformation(ESI)available:XRDpatternsofallsamplesobtainedinthecurrentwork,photocatalyticdegradationcurvesofMOunderUVirradiation,Ramanspectraandthespeci?csurfaceareasofthemostimportantsamplesobtainedat180??C.SeeDOI:10.497dacrucialparameterinthedeterminationoftheproperticonsequently,titania/titanatenanomaterialswithspecialmorphologieshaveattractedconsiderableattentionfortheiroptical,catalytic,andmechanicalapplications.14C17Devel-opingfabricationmethodsforphotocatalyticactivenano-structures,morespeci?callycontrollingtheincipientstagesofcrystalgrowth,isofvitalimportancebecauseitisanimportantchallengeinmodernnanotechnologies.8Becauseoftheimportanceinmanykeytechnologiesoftitania/titanatenanomaterials,thesynthesisofhighlycrystallinetitania/titanatenanoparticleswithaspeci?edcrystallinephase,controlledsizeandspecialmorphologyhasbeenanimportantresearchfocusinthe?eldofmaterialschemistry.6,7,18,19C21ManysynthesistechniqueshavebeenadoptedinthepreparationofnanostructuredTiO2/titanatematerials,amongwhichsolCgelmethodassociatedsolvothermaltreatmenthasbeendrawingmuchattentionbecauseavarietyofreactionparameters,suchasreactantconcentration,solventandadditivetype,solutionpH,agingtime,reactiontemperature,reactiontimeandmixingtechniquecanbeconvenientlyadjustedinthesolvothermalprocesses.6,8,22C27Phaseselection/compositionandmorphologicalformation/evolutionofthenanoparticlesarestronglyin?uencedbytheseexperimentalfactors.27Besidesthesolid-statemethod,layeredtitanatenanomaterials(nanosheetsandnanowires)aregenerallysynthesizedthroughanalkaliwetmethod.28Asolidtitaniumsource(P25,anataseorrutile)wastreatedinconcentratedaqueousalkaliunderhydro-thermaltreatment5,29,30orre?uxingreaction.31,32Hydrogentita-natewasthenobtainedviaprotonatingtheas-preparedalkalititanates,whichinvolvedstirringthealkalititanatesindiluteacidforhoursinordertoremovetheresidualalkaliionscompletely.33,34TiO2CBnanowires,anatasenanorodsandmicroscalerutilecanbeobtainedbyheatingthehydrogentita-natenanowiresatdifferentannealingtemperatures.5,35ThecrystallinestructureofanataseissimilartothatofbrookiteTiO2soanataseisusuallyassociatedwiththebrookiteproductinthesynthesisprocess,8whichisthecriticalchallengetosynthesizeapurebrookiteproduct.Recently,analkalescentrouteforthesynthesisoftitania/titanatewasproposedinourpreviousworks,inwhichaqueousammoniawasusedtofacilitatetheformationoflayeredtitanateandaffectthephasetransitionoftitania.28,36Layeredhydrogentitanatenanosheetsweresynthesizedinaqueousammoniaunderhydrothermaltreatmentusingtetrabutyltitanate(TBOT)asastartingmaterial.28Then,purebrookiteTiO2nano?owersweresynthesizedbyaddingsodiumchloride(NaCl)intotheammoniahydrothermalsystem.36However,itwasaninitialandtentativework,sothephasetransitionandmorphologicalevolutionareundiscoveredandstillunderquestioninthenovelalkalescentroute.Therefore,anoverallandsystematicinvestigationisurgentforthealka-lescenthydrothermalsystemandafurtherexplorationisalsonecessaryto?ndthephasetransition,formationmechanismandphotocatalyticapplication.Inthiscontribution,withtheassistanceofconcentratedammonia,whichcanserveasustainedalkalescentenvironment,aTisourcewasrapidlyhydrolyzedtoproducealayeredtitanatestructure.Then,thecrystallinetransformationfromlayeredtitanatetopureanataseorbrookitewasachievedowingtotheunstablepropertiesoflayeredtitanateunderalkalescenthydro-thermaltreatment.Asystematicinvestigationofthealkalescentapproachwasthuselaboratedbyobservingtheeffectsofreac-tiontemperaturesandtheconcentrationsofastrongelectrolyteofNaCl.Themorphologicalphasediagramofanatase,brookiteandhydrogentitanatenanostructues,aswellastheirphoto-catalyticperformance,waspresented.AccordingtotheOst-wald’ssteprule,aphasetransitionmechanismwasproposedbasedonourcurrentsystematicexperimentalresults,whichrevealsthephasetransitionsfromlayeredhydrogentitanatetoanataseorbrookiteunderhydrothermaltreatment.ThebrookiteCanatasecompositeexhibitedthehighestphotocatalyticactivityamongthecurrenttitania/titanateproductsinthepho-todegradationexperimentofMOsolutionunderUVirradiation.MaterialscharacterizationAllproductsandintermediateprecursorsinthisreaction,includinganatase,brookiteandhydrogentitanatenano-structures,werecharacterizedbyanumberofdifferentmeth-odologies,includingX-raydiffraction(XRD),?eldemissionscanningelectronmicroscopy(FESEM),transmissionelectronmicroscopy(TEM),high-resolutiontransmissionelectronmicroscopy(HRTEM),selectedareaelectrondiffraction(SAED),Ramanspectroscopyandnitrogenadsorption-desorp-tion.Crystallographicandpurityinformationonhydrogentitanate,brookiteandanataseTiO2nanostructureswereobtainedusingpowderXRD.Theas-preparedsamplesweredepositedontotheglassslidesaftercentrifugationandsubsequentair-dryingprocedures,andthenthediffractionpatternsofthesematerialswerecollectedusingapowderdiffractometer(RIGAKUD/max2550)operatinginthere?ectionmodewithCuKaradiationatascanrateof0.02??2qsà1.Theaveragesizesoftheanatasenanoparticleswereestimatedfromthehalf-peakwidthsathalftheheightofthediffractionpeaksfromthe(101)planeoftheanatase(2q?25.3??),usingtheScherrerEquation.ThesizeandmorphologyofthebrookiteTiO2samplewascharacterizedusingFESEM(FEISIRION200)atacceleratingvoltagesof5kV.Speci?cally,powdersofbrookiteTiO2nano-structuresweremountedontoconductivecarbontapes,whichwerethenattachedontothesurfacesofSEMbrassstubs.ThesamplewasthenconductivelycoatedwithgoldbythesputteringmethodtominimizethechargingeffectsunderFESEMimagingconditions.BothTEMandHRTEMwereinvestigatedusingaJEOLJEM2100Finstrumentoperatingat200kVandequippedwithanEDXfacility,aswellaswiththepotentialofperformingSAED.TopreparetheHRTEMspecimens,thepowdersampleswere?rstdispersedultrasonicallyinanhydrousethanol.Onedropofthesuspensionwasplacedonacarbon?lmsupportedonacoppergridandallowedtodryinairbeforethespecimensweretransferredontothemicroscope.PhotocatalyticactivityThecatalyticactivityunderUVlightwasmonitoredthroughthephotodegradationofmethylorange(MO),phenolandsalicylicacid.UVirradiationwascarriedoutusinga300Whigh-pressuremercurylamp,whichwassurroundedbyaquartzjackettoallowforwatercooling.Photocatalystpowder(0.10g)wasaddedinto100mLaqueousMO(10mgLà1),phenol(20mgLà1)orsalicylicacid(20mgLà1)solution,andmagneticallystirredinthedarkfor30minbeforeUVilluminationwasconducted.TheabsorbanceofthecorrespondingtargetorganicswasmonitoredbymeasuringwithaUV-visspectrophotometer(VarianCary100).ExperimentalMaterialspreparationThetypicalprocedureforthesynthesisofpurebrookitenano-structuresproposedinourpreviouswork36wasasfollows:7.8mltetrabutyltitanate(TBOT)wasdirectlyhydrolyzedinasolutionof1.10gsodiumchloride(NaCl)and68mlaqueousammonia(NH3$H2O).Thetotalvolumeofthesolutionwasadjustedto75ml,sothattheconcentrationofNaClwasabout0.25M.Afterstirringforashorttime,theresultingsuspensionwastransferredtoaTe?on-linedautoclaveandheatedto180??Cfor24h.Thepowdersobtainedwerewashedanddriedat50??Cfor24h.Inthecurrentwork,thereactiontemperaturesandtheconcentrationsofstrongelectrolyteofNaClwereregulatedcorrelatedtocontrolthemorphology,size,andphasestructureoftheTiO2/hydrogentitanatenanocrystallites.Therefore,aseriesofsamples,suchasanatase,brookiteTiO2andhydrogentitanatewithdifferentmorphologieswereobtainedbyalteringtheconcentrationofNaCl(0.00C1.50M)andalteringthehydrothermaltemperaturesfrom20to200??C,correlatively.ResultsanddiscussionPhasetransitionsFig.1presentsthecrystallinephasedistributionofTiO2/hydrogentitanatenanostructures,whicharedominatedbythehydrothermaltemperaturesandtheconcentrationsofstrongelectrolyteofNaCl.Fig.S1(ESI?)showsalloftheXRDpatternsFig.1Thecrystallinephasedistributeddiagramoftitania/hydrogentitanatenanostructuresdominatedbythehydrothermaltemperaturesandtheNaClconcentrations.ofsamplesinthecurrentwork.AsdepictedinthebottompartofFig.1,samplesofamorphousammoniumtitanate,whichshouldbeexpressedas(NH4)yH2xàyTiO2+x,wereobtainedatlowhydrothermaltemperature(#80??C),sincenodistinctdiffrac-tionpeaksexistedinFig.S1(ESI?).TheXRDpatternofthesampleobtainedin1.50MNaClat80??Cshowsaplausibleweakpeakatabout10??,indicatingtheinitialformationofalayeredhydrogentitanatestructure.ThemiddlepartofFig.1,wherethehydrothermaltempera-turesrangesfrom100to160??C(exceptthesampleobtainedin0.00MNaClat160??C),istheregionoflayeredhydrogentita-nate.Allsamplesobtainedat100??Cwereofalayeredhydrogentitanatestructure,butthediffractionpeaksshowninFig.S1(ESI?)arenotverydistinct,inwhichonlythreeweakpeaksat2qz10??,28??and48.5??emergedintheircorrespondingXRDpattern(Fig.S1,ESI?).ThelayeredhydrogentitanatestructurewithhighdegreeofcrystallinityisformedinhigherhydrothermaltemperatureandNaClconcentration.Samplesobtainedat140and160??C(exceptthesampleobtainedin0.00MNaClat160??C)givetypicalXRDpatternsofhydrogentitanateH2Ti2O5$H2O(JCPDS47-0124).ThelayeredstructurecouldbeproducedandmaintainedbyNH4+ionswithouttheassistanceofNaClunderhydrothermaltreatmentat100C140??C.Theelectrolyteseemedtofacilitatetheformationoftitanatelayers,ashydrogentitanatewasfabricatedatonly80??Candmaintainedupto200??CinthepresenceofahighconcentrationofNaCl(1.50M).TheproductsobtainedwiththeassistanceofNaCl(0.12C1.50M)at160??CwerelayeredhydrogentitanateH2Ti2O5$H2Owithhighcrystallinity.However,comparedtoNa+ions,NH4+ionisaweakelectrolyteandcannotsupplyenoughpositivechargestostabilizethenegativelychargedtita-natelayersat160??Cwithoutaddinganystrongelectrolyte.Therefore,thedisappearanceofthediffractionpeakat2qz9.6??inFig.S1a3(ESI?)indicatedthattheNH4+ionsshouldbedeintercalatedin0.00MNaClat160??C,whichsubsequentlycausedthecombinationoflayersandtheformationofanataseTiO2.Whenthetreatedtemperaturerisesto180or200??C,thecrystallinephaseoftheTiO2productschangedfromanatasetobrookitetohydrogentitanateH2Ti2O5$H2OwithincreasingNaClconcentrationinthehydrothermalsystem,asrevealedatthetopofFig.1.Theproductsobtainedat180??C(Fig.S1a2,ESI?)and200??C(Fig.S1a1,ESI?)arealsoalmostpureanatase(JCPDS21-1272)withouttheassistanceofNaCl.Theparticlesizesofanataseproductsobtainedat160,180and200??CcalculatedbytheDebyeCScherrerequationfromXRDdataare7.3,18.0and27.1nm,respectively,whichindicatesthatanatasenanoparticlegrowslargerwiththeelevationofhydrothermaltemperature.ThebrookiteCanatasecompositewasobtainedbyaddingalittlestrongelectrolyteofNaCl(lessthan0.25Mat180??C).TheintenseanatasepeaksdecreasedwithincreasingNaClconcentrationfrom0.00to0.25M.ThebrookiteCanatasecompositepresentedat200??C(Fig.S1b1,Fig.S1c1,ESI?)hasalargerrangeofNaClconcentration(CNaCl&0.50M),whichalsoshowsthephasetransitionfromanatasetobrookitewithincreasingNaClconcentration.PurebrookiteTiO2(JCPDS:29-1360)wasobtainedwithaNaClconcentrationofabout0.25Mat180??C(Fig.S1c2,ESI?)and0.50C0.75Mat200??C(Fig.S1d1,Fig.S1e1,ESI?).WhentheconcentrationofNaClwasfurtherincreased,theionicintensityinthereactionsystemshouldbefurtherenhancedsothatthelayeredstructuresareinclinedtobemaintainedunderhydrothermaltreatment.AsdepictedinFig.S1(ESI?),boththeintensi?cationofthediffractionpeakat2qz9.8??(themainpeakofhydrogentitanateH2Ti2O5$H2O)andthedecreaseofthediffractionpeakat2qz30.8??(thecharacteristicpeakofbrookite)indicatethephasetransitionfrombrookitetohydrogentitanateH2Ti2O5$H2Owiththeincreaseofconcentra-tionofNaCl(0.25M&CNaCl&1.50M)at180??C.Additionally,asimilarphasetransitionalsooccursat200??CinconnectionwiththeNaClconcentrationrangingfrom0.75to1.50M(0.75M&CNaCl&1.50M).Finally,purehydrogentitanateH2Ti2O5$H2OwasobtainedwhentheconcentrationofNaClinthereactionsystemwasnolessthan1.50MNaClat180or200??C.Threepuresamples(anatase,brookiteandhydrogentita-nate)obtainedat180??CwerefurthercharacterizedbyRamanspectroscopy(Fig.S2,ESI?).AllthreesamplesshowedpureRamanbandswithoutcontamination.ThesynthesizedbrookiteshowsstrongRamanpeaksatabout156(A1g),245(A1g),287(B3g),320(B1g),365(B2g)and637cmà1(A1g),8theanataseexhibitscharacteristicscatteringsat144(Eg),397(B1g),517(A1g)and639cmà1(Eg),37,38whilesynthetichydrogentitanateH2Ti2O5$H2Opresenteduniquepeaksatabout275,382,441and708cmà1.29,39MorphologicalevolutionThemorphologicalevolutioninthecurrentworkwaseluci-datedbytheTEM,HRTEMandFESEMimagesandcorre-spondingSAEDpatterns.TheamorphousTispecies,anatase,anataseCbrookitecomposite,brookite,brookiteChydrogentitanatecompositeandhydrogentitanatearepresentedinFigs2C5.Samplesofamorphousammoniumtitanate,whichshouldbeexpressedas(NH4)yH2xàyTiO2+x,wereobtainedatlowhydrothermaltemperature(T#80??C,exceptthesampleobtainedin1.50MNaClat80??C).TEMcharacterizationofFig.2TEMimages(aandb)andthecorrespondingSAEDpattern(theinsetofFig.2b)oftheamorphousproductobtainedwithoutNaClat80??C.TEM(c)andHRTEM(d)imagesofthehydrogentitanateproductobtainedwithoutNaClat120??C.Fig.4TEM(a)andHRTEM(thebottomrightinsetofFig.4a)imagesoftheanataseCbrookitecompositeproductobtainedin0.12MNaClat180??C.FESEM(b),TEM(candd),HRTEM(thetoprightinsetofFig.4d)imagesofthepurebrookiteproductobtainedin0.25MNaClat180??C.Fig.3TEM(a),HRTEM(bandthetoprightinsetofFig.3a)andcorrespondingSAEDpattern(thebottomrightinsetofFig.3a)oftheanataseproductobtainedwithoutNaClat160??C.TEM(c),HRTEM(d)andthecorrespondingSAEDpattern(thebottomleftinsetofFig.3c)oftheanataseproductobtainedwithoutNaClat180??C.Fig.5ATEMimage(a)ofthebrookiteChydrogentitanatecompositeproductobtainedin0.75MNaClat180??C.TEM(b)andHRTEM(candd)imagesofthepurehydrogentitanateproductobtainedin1.50MNaClat180??C.Fig.2ashowedthatthemorphologyoftheamorphousproductwasincompactmicrostructuralsphereswithanaveragesizeof100nm,whichwereassembledbyrulelesssheet-like?ocs(Fig.2b).ThecorrespondingSAEDpatternlocatedinthebottomrightcornerofFig.2bindicatestheproductwasamorphous.Hydrogentitanateissynthesizedwhenthehydrothermaltemperatureisequaltoorgreaterthan100??C.AsdepictedintheTEMimageofFig.2c,thehydrogentitanateproductadoptsincompact,tangledand?oc-likenanosheets.ThedistinctlayeredfringesinFig.2dcorrespondwellwiththe(200)latticeplanesofhydrogentitanateH2Ti2O5$H2O.However,suchfringesarenotquiteregular,butshowseveredistortion,whichindicatesanimperfectstackoftheruleless?oc-likemonolayernanosheets.TheimperfectstackwasfurtherimprovedwithincreasingthehydrothermaltemperaturesandaddingNaCl.Slab-likehydrogentitanatenanosheetswithhighlyorderedlayersareobtainedinmoreconcentratedNaClandathighertemperatures(T$160??C),whichareelucidatedlaterintheTEMandHRTEMimagesofFig.5.PureanataseTiO2wasobtainedathighhydrothermaltemperatures(T$160??C)withouttheassistanceofNaCl.Fig.3ashowsaTEMimageofananataseproductobtainedat160??C,whichshowsthemonodisperseanatasenanopatcileswithanaveragesizeofabout7nm.AnisolatedanatasenanoparticleshowninthetoprightinsetofFig.3apresentsthesinglecrys-tallinenatureduetoitsdistinctlatticefringes,correspondingwellwiththe(101)planesofanataseTiO2.Themeasuredanglebetweenthetwotypesofdistinctlatticefringesindexingtothe(101)and(004)planesofanataseTiO2is68.1??(Fig.3b),whichalsocorrespondswellwiththecalculatedvalueof68.3??.ThecorrespondingSAEDpatternlocatedinthebottomrightinsetofFig.3aindicatesthenanocrystallinestateoftheproduct.Puresingleanatasenanopartciclesobtainedat180??C,shownintheTEMimageofFig.3c,indicatethegraingrowthoftheproductwithincreasinghydrothermaltemperature.TheanatasenanoparticlesinFig.3Chaveanaveragesizeofabout20nm.ThecorrespondingSAEDpatternlocatedinthebottomleftinsetofFig.3candtheHRTEMimageofFig.3dshowthatpureanataseTiO2nanoparticlesobtainedat200??Cshouldhavealargerparticlesizeandhighercrystallinity.Fig.4ashowstheTEMimageofaproductsynthesizedin0.12MNaClat180??C,whichisproducedbytheaggregationofnanoparticlesandunshapednanorods.Themarkedareaisfurthermagni?edintheHRTEMimageofthebottomrightinsetofFig.4a,wheretheclearlatticefringesshouldcorrespondtothe(101)planesofanataseTiO2.WiththeassistanceofXRDpatterns(Fig.S1,ESI?)andTEMimages(Fig.4),wecaninferthattheproductisabrookiteCanatasecomposite.Fig.4bCdareFESEM,TEMandHRTEMimagesofthebrookiteTiO2nano?owersobtainedinthetypicalprocedure(0.25MNaCl,180??C).TheFESEMimageinFig.4bshowsthatbrookite?owerclustersareaggregatedbyhierarchicalbranchesofpetal-likenanorods.TheTEMimage(Fig.4c)showsthatthelengthofasinglenanorodisabout800nm,anddozensofsuchnanorodsareaggregatedtoformasingle?owercluster.Fig.4dshowsthatthediameterofthenanorodsisabout40nm.Thedistinctlatticespacingsofthenanorodinthreedifferentorien-tationscorrespondwellwiththe(120),(231),(320)planesofbrookiteTiO2.Similarly,thesampleobtainedinmoreconcentratedNaClat180or200??CshouldbeconsideredasthebrookiteChydrogentitanatecomposite.TheTEMimageofFig.5ademonstratestheinferenceabove,inwhichtherepresentativeproductobtainedin0.75MNaClat180??CiscomposedofthebrookiteTiO2?owerclustersandmonodispersedhydrogentitanatenanosheets.WhentheconcentrationofNaClinthereactionsystemwasnolessthan1.50MNaCl,purehydrogentitanateH2Ti2O5$H2Oisobtainedafteracidwashingbecausetheconcentrationofstrongelectro-lyteNaClisenoughtomaintainthelayeredstructurewellinthehydrothermalprocess.Fig.5bCdistheTEMandHRTEMimagesofarepresentativehydrogentitanateproductobtainedin1.50MNaClat180??C.TheTEMimageinFig.5bshowsthemonodispersedslab-likenanosheetsinsteadoftheincompact,tangledand?oc-likenanosheets.Thebordersoftheslab-likenanosheetscapturedintheHRTEMimageinFig.5cindicatesthelayerednatureofhydrogentitanateH2Ti2O5$H2O.TheHRTEMimageofFig.5dexhibitstheratherperfectlayeredfringeswithlittledistortion,thespacingsofwhichshouldalsocorrespondwiththe(200)latticeplanesofhydrogentitanateH2Ti2O5$H2O.MechanismdiscussionOurpreviousworksreportedthattheformationofbroookiteTiO2nano?owers36andthephasetransitionfromlayeredhydrogentitanatetoanataseTiO228shouldfollowOstwald’ssteprule.40Inthecurrentwork,animprovedsystematicformationmechanismwashenceproposedbasedonOstwald’ssteprulefortheproductsofanatasenanoparticles,purebrookiteTiO2nano-?owersandhydrogentitanateH2Ti2O5$H2Onanosheets.OnceTBOTisdirectlyaddedandcompulsorilyhydrolyzedinaqueousammoniawith/withoutNaCl,theamorphouslayeredtitanatesformedrapidlybycondensingTiO6octahedralmono-mers(Fig.6aandb).Hydrothermaltreatmentatlowtempera-tures(20C80??C)inducesalimitedoctahedralcondensationandlatticerearrangement,evenforadurationtimeof24h.Thus,theamorphousstateisretainedandtheamorphoushydrogentitanateisproducedafteranacid-washinganddryingprocess(Fig.6j).Whenthehydrothermaltemperatureisequaltoorgreaterthan100??C,amorphoustitanateshouldrearrangethecrystallinelatticestoformorderedlayerstacks,whichresultsinthegenerationoflayeredtitanatestructuresduringthereactionprocess,asshowninFig.6bCe.Theas-obtainedlayeredtitanatescanbeexpressedasNa2àx(NH4)xTi2O5$H2O,inwhichtheproportionofNaisrestrictedbytheconcentrationofNaClinthereactionsystem.Fig.6cshowsthatthelayeredtitanatecanbeintercalatedandmaintainedonlybyNH4+ionswithouttheassistanceofNaCl,whichcanbedenotedastheammoniumtitanate(NH4)2Ti2O5$xH2O,thelayeredstructureofwhichcanendurefor24hatthetemperaturerange100C140??C.TheinterlayersofthetitanateshouldbeintercalatedbymoreNa+ionswithincreasingNaClconcentration.Therefore,Fig.6dandeshowsthelayeredtitanateisintercalatedtogetherbyNa+ionsandNH4+ionsduringthereactionprocess.paredtoNH4+ions,Na+ionsareastrongelectrolytesotheycanstabilizethelayeredstructuremuchbetter.So,theproductsobtainedwiththeassistanceofNaCl(0.12C1.50M)at100C140??Candeven160??Carede?nitelylayeredhydrogentitanatenano-sheets,asshowninFig.6d,eandh.Athighertemperatures(160??C&T#200??C),NaClconcentrationshouldplayamoreimportantroleinphasetran-sitionandmorphologicalevolution.AsshowninFig.1,phasetransitionfromlayeredtitanatetoanatasestartsfrom160??Cwith0.00MNaCl,becausemostoftheammoniamoleculeswouldescapefromtheaqueoussolutionandexistinagaseousstateat160??Corhighertemperaturessothatthereareinsuf?-cientNH4+ionstosupplyenoughpositivechargestostabilizethenegativelychargedtitanatelayers.Consequently,anataseTiO2包含各类专业文献、应用写作文书、外语学习资料、专业论文、各类资格考试、高等教育、中学教育、幼儿教育、小学教育、73Journal 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