Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas SensorsAnode in Lithium Ion Batteries

J.Phys.Chem.C2010,114,18753–1876118753

SynthesisofMesoporousr-Fe2O3NanostructuresforHighlySensitiveGasSensorsandHighCapacityAnodeMaterialsinLithiumIonBatteries

BingSun, JosipHorvat, HyunSooKim,§Woo-SeongKim,#JunghoAhn,¶andGuoxiuWang*,

SchoolofChemistryandForensicScience,UniVersityofTechnologySydney,Broadway,Sydney,NSW2007,Australia,InstituteforSuperconductingandElectronicMaterials,UniVersityofWollongong,Wollongong,NSW2522,Australia,BatteryResearchCenter,KoreaElectrotechnologyResearchInstitute,Changwon641-120,Korea,DaejungEnergyMaterialsCo.,Ltd,Namdong-gu,Incheon405-820,Korea,andDepartmentofMaterialsEngineering,AndongNationalUniVersity,Gyungbuk760-749,Korea

ReceiVed:March13,2010;ReVisedManuscriptReceiVed:September28,2010

MesoporousR-Fe2O3materialswerepreparedinlargequantitybythesofttemplatesynthesismethodusingthetriblockcopolymersurfactantF127asthetemplate.Nitrogenadsorption-desorptionisothermalmea-surementsandtransmissionelectronmicroscopeobservationrevealedthattheas-preparedmesoporousR-Fe2O3nanostructureshavelargemesoporesinawidesizerangeof5-30nm.IthasbeenfoundthattheMorintransitiondependsonthermalhistoryofmesoporousR-Fe2O3,whichisdrivenbysurfaceanisotropy.SuperparamagneticbehaviorofmesoporousR-Fe2O3isalsoassociatedwithsurfacespinswithblockingtemperaturearound50K.Whenappliedasgassensors,mesoporousR-Fe2O3nanostructuresexhibitedhighgassensitivitytowardaceticacidandethanolgas.Asanodesinlithiumioncells,mesoporousR-Fe2O3materialsshowahighspeci ccapacityof1360mAh/gwithexcellentcyclingstabilityandhighratecapacity.

1.Introduction

Asann-typesemiconductor,hematite(R-Fe2O3)hasattractedagreatdealofattentionfromresearchersindifferent eldsbecauseofitsnontoxicity,lowcost,highstabilityunderambientconditions,andmultiplefunctions.Ithasbeenintensivelyinvestigatedforapplicationsinlithiumbatteries,1,2sensors,3catalysts,4pigments,5andmagneticdevices.6TheperformanceofR-Fe2O3stronglydependsontheparticlesize,morphology,andstructure.Foruseasanodematerialinthelithiumionbattery,thereversibilityoflithiumintercalationinR-Fe2O3dependsstronglyonthestructureandparticlesize.NanosizeR-Fe2O3exhibitsbetterperformancethanmicroscalesamplesasthesmallerparticlesizecangreatlyreducethediffusionlengthofthelithiumions.7,8Fromtheinvestigationsongas-sensingperformance,thesensitivitiesofR-Fe2O3nanospheresaremuchhigherthanthoseofmicrocrystallineR-Fe2O3powders.Theinterconnectedporesinthenanospheresareofbene tforthediffusionofgasandhavebeenprovedtooffermoreactivesitesforgaschemisorption.9Thereisgrowinginterestsinthepreparationofnanomaterialswithspeciallydesignedstructures.NanostructuredR-Fe2O3withone-dimensionalnanowire/nano-tubestructures,10,11two-dimensional ake/ lmstructures,12,13andthree-dimensionalhollow/porousstructures6,14havealreadybeensynthesizedbyavarietyofmethods,suchasthesol-gelmethod,15,16electrostaticspraydeposition,17hydrothermaltreatment,18,19andtemplatemethod.20,21

SincethediscoveryofMCM-41mesoporoussilicain1992,mesoporousmaterialshavebeendevelopedintoanimportant

*Towhomcorrespondenceshouldbeaddressed.E-mail:Guoxiu.Wang@uts.edu.au.Fax:+61-2-95141460.

UniversityofTechnologySydney.

UniversityofWollongong.§

KoreaElectrotechnologyResearchInstitute.#

DaejungEnergyMaterialsCo.,Ltd.¶

AndongNationalUniversity.

classofmaterialsandoccupyaveryimportantpositioninmaterialsscience.22Themesoporestructurecandramaticallyincreasethesurfacearea/volumeratio,whichmakesthemesoporousmaterialsveryusefulinthesurface-relatedapplica-tions.Thereisintenseinterestinpreparingmesoporoustransi-tion-metaloxidesbecauseoftheiruniquecatalytic,magnetic,adsorptive,andelectrochemicalpropertiesinapplicationsascatalysts,23magneticmaterials,24absorbents,25andenergycon-vertionandstoragematerials.26Severalmesoporoustransition-metaloxideshavealreadybeensynthesized,suchasCo3O4,27Mn2O3,24Fe2O3,28TiO2,29andV2O5;30however,itisstillmuchmoredif culttosynthesizemesoporoustransition-metaloxidesthanmesoporoussilica.Thematerialswiththree-dimensionalmesoporousarchitecturesarenormallyobtainedthroughtemplate-directedmethods,whichcanbesimplyclassi edintothehardtemplatemethodandthesofttemplatemethod.Thehardtemplates(usuallymesoporoussilicaorcarbon)normallypossesswell-con nedchannelsandpores,whichstronglyin uencethestructureoftheresultingsolidproduct.Softtemplatesareusuallysurfactants,long-chainpolymers,andviruses,whichfunctionasstructure-directingagentsthatassistintheassemblyofreactingspecies.20Yangetal. rstreportedthesynthesisofaseriesofmetaloxidesbyemployingnonionicpolymersurfactantsastemplates,whichcouldbeeasilyremovedbysolventextractionorthermaltreatment.31,32Thehardtemplatemethodsusuallyinvolveamultistepsynthesisprocess.Meso-poroussilicaorcarbonmustbeprepared rst.Then,theinorganicprecursorsarecombinedwiththetemplatesbyimpregnationorincorporation.The nalproductcanbeobtainedbytemplateremovalafterthesolidspecieshasformedthroughreaction,nucleation,andgrowth.20However,forthesofttemplatemethods,thetemplatescanformbyself-assemblyinliquidsolutionandcanbeeasilyremovedbysolventorcalcination.Therefore,thesofttemplatemethodisgenerallyeasiertobescaledupforlarge-scaleproductionthanthehard

10.1021/jp102286e 2010AmericanChemicalSociety

PublishedonWeb10/15/2010

18754J.Phys.Chem.C,Vol.114,No.44,2010templatemethod.However,itisdif culttocontroltheregularity,poresize,andporestructureusingthesofttemplatemethod.Inthispaper,wereportthesynthesisofmesoporousR-Fe2O3nanostructureswithlargemesoporesbyusingasofttemplatemethodinnonaqueoussolution.Theas-preparedmesoporousR-Fe2O3materialsexhibituniquemagneticproperties,highgassensitivity,andhighreversiblelithiumstoragecapacityinlithiumioncells.

2.ExperimentalSection

2.1.PreparationofMesoporousr-Fe2O3.Inatypicalsynthesis,thetriblockcopolymer(HO(CH2CH2O)106(CH2-CH(CH3)O)70(CH2CH2O)106H)(F127)andironnitratewereusedasthetemplateandinorganicsource,respectively.F127blockcopolymer(1g)wasdissolvedinamixtureofpropanol(5g)andethyleneglycol(EG;5g).Tothissolution,0.01molFe(NO3)3·9H2Owasaddedwithvigorousstirringover2h.Theresultingsolsolutionwasthenagedinairat40°Cfor7daysandthenwasgraduallyheatedto150°C(1°C/min)andwasmaintainedat150°Cfor24h.Finally,thesampleswerefurthersinteredat400°Cfor5htoremovetheblockcopolymersurfactant(heatingrate:1°C/min).

2.2.Structural,Optical,andMagneticCharacterization.Thecrystalstructureandphaseoftheas-preparedmesoporousR-Fe2O3werecharacterizedbyX-raydiffraction(XRD,GBCMMA)usingCuKRradiationwith2θrangingfrom20°to70°.RamanspectrawerecollectedonaJOBINYvonHoribaConfocalMicoRamanspectrometermodelHR800with632.8nmdiodelaserexcitationona300lines/mmgratingatroomtemperature.Thebandgapenergyoftheas-preparedmesopo-rousR-Fe2O3wascalculatedviaultraviolet-visible(UV-vis)spectroscopyonaShimadzuUV-1700spectrophotometer.Themorphologyandmicrostructureswerecharacterizedbytrans-missionelectronmicroscopy(TEM)andhigh-resolutionTEM(HRTEM,JEOL2011).N2adsorption-desorptionmeasure-mentswereconductedusingaQuantachromeAutosorbanalyzerat77Kwiththesamplesdegassedat120°Covernightundervacuumbeforemeasurements.Themagneticmomentofme-soporousR-Fe2O3wasmeasuredwithavibratingsamplemagnetometer(VSM)optionofaQuantumDesignPhysicalPropertiesMeasurementSystem(PPMS).Thetemperaturedependenceofthemagneticmomentswasmeasuredinthreedifferentconsecutiveregimes:duringwarmingafterzero- eld-coolingofthesample(ZFCW),duringsubsequent eldcooling(FCC),and nally,duringwarmingafter eldcooling(FCW).FortheZFCWmeasurements,thesamplewascooledinzero eldfrom305to5K,amagnetic eldwasapplied,andthemagneticmomentwasmeasuredinconstant eldasthetemperaturewassweptto305K.TheFCCmeasurementswereperformedbysubsequentlycoolingthesampleto5Kwithoutchangingthe eld.TheFCWmeasurementswereperformedduringafurthersweepofthetemperaturefrom5to305Kwithoutchangingthe eldaftertheFCCmeasurement.Thesweeprateofthetemperaturewas2K/min.Severaltemperaturescanswereperformedatdifferentmagnetic elds.Magnetichysteresisloopsweremeasuredatselectedtemperatureswithasweeprateofthe eldof50Oe/s.

2.3.Gas-SensingMeasurement.Gas-sensingpropertiesoftheas-preparedmesoporousR-Fe2O3weremeasuredbyacomputer-controlledWS-30Agas-sensingmeasurementsystem.TheschematicsofthedeviceanditsoperatingprinciplesareshowninFigureS-1oftheSupportingInformation(SI).

2.4.ElectrochemicalCharacterizations.Topreparetheworkingelectrode,theas-preparedmesoporousR-Fe2O3powder(50wt%),acetyleneblack(40wt%),andpoly(vinylidene

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Figure1.(a)X-raydiffractionpatternoftheas-preparedmesoporousandcommercialR-Fe2O3.(b)Ramanspectrumofas-preparedmeso-porousR-Fe2O3andahematitereferencespectrum.

uoride)(PVDF,10wt%)weremixedinN-methyl-2-pyrrolidone(NMP)toformaslurry.Theresultantslurrywaspastedontocopperfoilwithabladeandwasdriedat100°Cfor12h2undervacuumconditionsfollowedbypressingat200kgcm-.Electrochemicalmeasurementswerecarriedoutusingtwo-electrodecoincellswithlithiummetalasthecounterandreferenceelectrode.TheCR2032-typecellswereassembledinanargon- lledglovebox.Theelectrolytesolutionwas1MLiPF6dissolvedinamixtureofethylenecarbonate(EC)anddimethylcarbonate(DMC)withavolumeratioof1:1.Cyclicvoltam-metry(CV)wascarriedoutonaCHI660C1electrochemistryworkstationwithascanrateof0.1mVs-from0.005to3.0Vinatwo-electrodesystem.Thecharge-dischargemeasurementswerecarriedoutatambienttemperatureatdifferentcurrentdensitiesinthevoltagerangefrom0.005to3.0V.

3.ResultsandDiscussion

3.1.StructureandMorphologicalAnalysis.Figure1ashowsthepowderX-raydiffraction(XRD)patternoftheas-preparedmesoporousR-Fe2O3andthecommercialR-Fe2O3powder(Sigma-Aldrich).Alldiffractionpeakscanbeindexedtothestandardhematite(R-Fe2O3)crystalstructure(JCPDScardnumber33-0664)10indicatingthatapureandhighlycrystallineproducthasbeenobtainedbycalciningironoxideprecursor.TheaveragecrystalsizeofmesoporousR-Fe2O3wascalculatedtobeabout20.6nmonthebasisofthebroadeningofthe(104)diffractionpeakusingtheScherrerformula.RamanspectroscopyisapowerfultooltoidentifythecrystalphaseofFe2O3.Figure

SynthesisofMesoporousR-Fe2O3NanostructuresFigure2.UV-visspectrumofmesoporousR-Fe2O3.

1bshowstheRamanspectrumoftheas-preparedmesoporousR-Fe2O3alongwithahematitereferencespectrum(RRUFFIDR050300).33ThespectrumindicatesatypicalR-Fe2O3crystalstructure1showingRamanpeaksat224,292,410,496,and613cm-,paredtothepreviousreportonR-Fe2O3microcrystallinepowders,allRamanpeaksofourmesoporousR-Fe2O3areslightlyblue-shifted.34Thismaybeduetothesmallsizeof-1thecomponentnanocrystals.TheRamanpeakaround660cminthespectrumhasbeenreportedinseveralpublishedpapersandisattributedtoalargeamountofdefectsandlocallatticedisorderattheinterfacesandinteriorfaces,whichleadstothereductionofspacesymmetryandtheappearanceofthenewpeak.13,35BoththeXRDandRamanspectroscopymeasurementscon rmedthepurephasenatureofmesoporousR-Fe2O3.Theopticalabsorptionpropertyoftheas-preparedmesoporousR-Fe2O3wasinvestigatedatroomtemperaturebyUV-visspectroscopy(Figure2).Thebandgapofhematitecanbecalculatedbythefollowingequation:

(Rhν)2)B×(hν-Eg)

(1)

whereRistheabsorptioncoef cient,hνisthephotonenergy,Bisaconstant,andEgisthebandgap.The(Rhν)2versushνcurveisshownastheinsetinFigure2.Thebandgapoftheas-preparedmesoporousR-Fe2O3calculatedfromeq1is2.14eV,whichagreeswellwithpreviouslyreportedvaluesof1.9-2.2eVforn-typesemiconductorR-Fe2O3.36,37

Figure3showstheTEMandHRTEMimagesoftheas-preparedmesoporousR-Fe2O3powders.Thepreparedma-terialexhibitsanorderedporousstructurewithdifferentshapesofporesasshowninFigure3a.Poresintheironoxidematerialsoriginatefromtheregionspreviouslyoccupiedbythepolymersurfactants.Aftertheremovalofthesurfactant,somewallsoftheporousstructurecollapseandformawormhole-likemor-phology.Theselectedareaelectrondiffraction(SAED)patternispresentedintheinsetofFigure3ashowingasingle-crystalline-likeSAEDdotpattern.TheHRTEMimageinFigure3bclearlypresentsthelatticefringesofthe(110)and(102)planesofR-Fe2O3correspondingtod-spacingsof0.250and0.388nm.BoththeHRTEMimagesandtheSAEDpatterncon rmtheformationofsingle-crystallinemesoporousR-Fe2O3.Inaddition,regularmesoporestructureswereextensivelyobservedbyTEManalysis,butwealsofoundalargeamountofmesoporousspheres(asshowninFiguresS2andS3oftheSI).

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Figure3.(a)TEMimageofmesoporousR-Fe2O3.Theinsetisthecorrespondingselectedareaelectrondiffraction(SAED)pattern.(b)HRTEMimageofmesoporousR-Fe2O3inwhichtheFe2O3latticecanbeclearlyresolved.

Toexaminethespeci csurfaceareaandtheporesizedistribution,N2adsorption-desorptionisothermmeasurementswerecarriedout.ThecurveshowninFigure4adepictsatypicalH2-typehysteresisloop,whichmakesitclearthatthedistributionoftheporesizesandshapesisheterogeneous.6Thisfurthercon rmedtheTEMresultsshowninFigure3.Theporesizedistribution,obtainedfromtheBarrett-Joyner-Halenda(BJH)method,isshowninFigure4b.Theplotshowsthatthedominantpeaksareinthemesoporousrangewithawidemainpeakaround7nmandasmallnarrowpeakat15nm.38Thetwomainpeaksinporesizedistributionsuggestthatthemesoporesaredistributedoverawiderangeofsizes.ThesurfaceareaestimatedfromtheBrunauer-Emmett-Teller(BET)methodis128m2g-1.Thehighsurfaceareaofthemesoporousstructureswillbeofbene tforimprovementofthesensitivityingassensorsandforincreasedreversiblelithiumstoragecapacityasanodematerialinlithiumionbatteries.

3.2.MagneticPropertiesofMesoporousr-Fe2O3.Figure5showstemperaturedependenceofmagneticmomentintheapplied eldof1T.ForZFCWregime,themomentincreasesstronglywithtemperatureupto50K.Thisisfollowedbyaweakdecreaseofthemomentwithtemperaturefortemperaturesupto180K.Thereisanabruptincreaseofmagneticmomentwithtemperaturebetween180and228K.ThissteepincreasecorrespondstoMorintransition,acharacteristicfeatureofR-Fe2O3.39Magneticmomentdecreaseswithtemperatureabove228K.SimilarfeaturesarealsoobtainedforFCCandFCW

18756J.Phys.Chem.C,Vol.114,No.44,2010Figure4.(a)Adsorption-desorptionisothermsofmesoporousR-Fe2O3and(b)poresizedistributioncalculatedfromthedesorptionisotherm.

Figure5.Temperaturedependenceofmagneticmomentofmesopo-rousR-Fe2O3forzero- eld-cooledmeasurementsuponwarming(ZFCW), eld-cooledmeasurementsuponcooling(FCC),and eld-cooledmeasurementsuponwarming(FCW).

measurements,however,thereisnoincreaseofthemomentwithtemperatureforT<50K.Instead,themagneticmomentslightlydecreaseswithtemperatureupto50Kfollowedbyafurtherdecreasewithadifferentrate.ThereisnooverlapofZFCW,FCC,orFCWbranchesinanymeasuredtemperaturerange.TheMorintransitionoccursatlowertemperaturefortheFCWandFCCmeasurementsthanfortheZFCWones.ThewidthoftheMorintransitionof～40Kindicatesawidedistributionofthecrystalpropertiesinthemesoporoussamples.TheMorintransitionoccursatMorintemperatureTMbecauseofcompetitionbetweendifferentanisotropytermsinR-Fe2O3asdescribedbyArtmanetal.39AntiferromagneticallyorderedspinsinmacroscopicR-Fe2O3crystalsareorientedalongtrigonal

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[111]axisbelowTM.Thesamplehasnonetmagneticmomentatthesetemperatures.Thespinsare ippedintothebasal(111)planeaboveTMandarealsoantiferromagneticallyordered.However,theyareslightlycantedoutoftheperfectantiferro-magneticalignment.Thiscantedcomponentofthespinscouplesferromagnetically,whichgivesrisetoasmallnetmagneticmomentofR-Fe2O3aboveTM.

Thetransitionbetweentheferromagnetic(FM)andantifer-romagnetic(AFM)stateisthe rst-ordermagnetictransition.AlthoughthebulkfreeenergyoftheAFMandFMstatesarethesameatTM,thereisanenergybarrierassociatedwiththetransitionbetweenthetwostates.ChowandKeffershowedthatsurfaceanisotropiesandtheirinteractionwiththebulkstateshavetobeconsideredtoexplainthesetransitions.40Thesurfacestatesdonotcontributeappreciablytothenetmagneticmomentbecausetheyarecon nedtoathinsurfacelayer.However,theyde nethewaytheenergybarrierisovercomeuponthetransitionbetweenthetwostates.Inthecaseofweaksurfaceanisotropy,thetransitionfromAFMtoFMstateduringwarmingoccursatTMofthebulkspins.However,FMtoAFMtransitionduringcoolingoccursatalowertemperaturethanTM.40Forstrongsurfaceanisotropy,itistheopposite:FMtoAFMtransitionoccursatbulkTMuponcoolingandAFMtoFMtransitionoccursataT>TMuponwarming.

OurmeasurementsareconsistentwiththemodelofChowandKeffer.40ThetransitionsbetweenAFMandFMstatesuponwarmingoccurathighertemperaturesthanuponcooling(Figure5).ThedifferencebetweenZFCWandFCWcouldbeassociatedwithirreversibilitiesofsurfacestates.Magneticanisotropyisde nedbycouplingofspinstothecrystallatticeorder.Intheabsenceofastrongcrystalorder,magneticanisotropydecreases.Becausesurfaceofournanoparticlesisassociatedwithacrystaldisorderthatresultsinunpairedspins,itisreasonabletoassumethatmagneticanisotropyonthesurfaceisweakerthandeeperinsidethenanoparticles.AccordingtothemodelofChowandKeffer,TMisthenobtainedfromthetransitioninZFCWmeasurements.

ThevalueofTMformacroscopicR-Fe2O3is263K,anditdependsonthecrystalsizewhenthecrystalsaresmallerthan～ingthedatareportedbyZysleretal.,41thevalueofTMof209Kforoursamplessuggeststheaveragecrystalsizeofabout20-40nm,whichisconsistentwiththeresultofXRDanalysis.ThesecrystallitesagglomerateintolumpsofafewhundrednanometersinsizeasobservedbyTEMandHRTEManalysis.ThisissimilartothereportonR-Fe2O3particlesobtainedbyreactionof2×10-2MFeCl3and3×10-4MNaH2PO4.41

ThetransitiontemperaturebetweenAFMandFMstatesis eld-dependent,whichwasalsoreportedbyQianetal.42ThetransitiontemperatureinourFCCmeasurementsdecreaseswiththe eldatarateof9K/Tfromitsvalueof191Kat0.01T.This elddependenceoftransitiontemperaturepointstomagneticirreversibilitiesassociatedwithferromagneticorderinginR-Fe2O3thatwasnotdestroyedbywarmingthesampleto305K.ThisisduetoexistenceoftheintrinsicferromagneticorderingaboveTMofArtman-typebutisalsoduetotheunpairedspinsonthesurfaceofR-Fe2O3crystals.Thereisalsointeractionbetweennanoparticlesofagglomerates,whichwilldependonthermalhistoryduetoreorientationofthenanoparticleswithintheagglomerates.

Theotherfeaturesoftheobservedtemperaturedependenceofthemagneticmomentcanbeexplainedintermsofferro-magneticorderingofunpairedsurfacespinsonthetopoftheantiferromagneticmomentofthebulkhematite.Themagnetic

SynthesisofMesoporousR-Fe2O3NanostructuresFigure6.MagnetichysteresisloopsofmesoporousR-Fe2O3measuredatdifferenttemperatures:5,70,and300K.

momentofoursampleat1Tand200Kisabout0.6emu/g.Hematitesinglecrystalhasmagneticsusceptibilityat200Kforparallel-6andperpendiculardirectiontotheeasyaxisofabout3×10and18×10-6emu/(gOe),respectively.43Thisresultsinamagneticsusceptibilityforpowderwithrandomlyorientedcrystalsof13×10-6emu/(gOe).Takingintoaccountthisantiferromagneticsusceptibility,surfacespinscontribute0.47emu/gtothemagneticmomentat200Kand1T(Figure5).Thisissubstantiallymorethantheadditional～0.15emu/gcontributedbytheMorintransitionjustabove200K.TheArtman-typemagneticmomentinmacroscopichematitecrystalsis0.4emu/g,43whichissubstantiallyhigherthantheincreaseby0.15emu/gattheMorintransitionforoursample.Thisisprobablybecauseoursampleconsistsofsomecrystallitessmallerthantheparticularsized0atwhichTMvanishesiftheparticlesizesaresmallerthand0.Thevalueofd0canbeestimatedfromthereportbyZysleratal.tobeabout10nm.41TheMorintemperatureTMdecreaseswithparticlesizeandstrainsintheparticles.44Therefore,particlessmallerthand0donotundergoMorintransitionatthemeasuredtemperatures.Instead,theseparticleshavetheArtman-typeferromagneticcomponentatallmeasuredtemperaturesasshownbyBodkeratal.45Asaresult,theincreaseofthemagneticmomentuponMorintransition,normalizedtothetotalsamplemass,issmallerthanforthesampleswherealloftheparticlesundergoMorintransition.Therefore,thesesub-d0particlescontributetothesuperparamagneticbehaviorbecauseoftheirArtman-typemagneticmoment.Therearealsounpairedsurfacespins.Forparticlessmallerthand0,thesesurfacespinswillcontributetosuperparamagneticbehaviorbecausetheyarecoupledtothecantedArtman-typebulkmagneticmomentatalltemperatures.Couplingoftheunpairedsurfacespinstotheantiferromagneticbulkorderisalsolikelytocontributebecausethiscouplingisdisturbedbythesurfacedisorderforparticlesofanysizes.Thus,bothsurfacespinsandArtman-typemagneticmomentofsub-d0particlescontributetothemagneticmomentof0.47emu/gjustbelow200Kat1T.ThemesoporousR-Fe2O3seemstobeinsuperparamagneticstatefor50K<T<200Kasindicatedbymagnetichysteresisloops(Figure6).TheunpairedsurfacespinsandtheArtman-typecontributionofsub-d0particlesareresponsibleforthissuperparamagneticstate.ThereisastrongincreaseofmagneticirreversibilityforT<50K(Figure5)indicatingyetanothermagnetictransition.AsimilareffectwasalsoreportedbyJiaoetal.28

Mostlikelythiscanbeascribedtotheblockingtransitionat50K,wheresuperparamagneticspinsfreeze-inuponcoolingandgiveanalmostconstantmagneticmoment.Thismomentiszeroifcoolingisinzero eld.WithwarmingupinZFCWmeasurements,themagneticmomentriseswithtemperatureas

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Figure7.Temperaturedependenceofthecoercive eldformesopo-rousR-Fe2O3.

anincreasingnumberofspinsgetsunfrozenbythermalexcitationandgetsalignedinthedirectionofthe eld.Finally,aboveblockingtemperature,superparamagneticsystemoccurs.MeasurementsinFigure5areconsistentwiththispictureasZFCWmagneticmomentincreasesfromalowvalueat5Kuptoitsmaximumvalueattheblockingtemperatureof50K.Above50K,thesampleisinsuperparamagneticstateforH)1Tshowingaverysmallcoercive eldandS-shapedhysteresisloopstypicalofsuperparamagneticparticles(Figures6and7).Thistransitiontemperatureis eld-dependent.Itincreasesfrom53Kat1Tto96Kat0.01T.ThisisalsoconsistentwiththepictureofunfreezingspinsasthetemperatureincreasesinZFCWmeasurements.Ahighermagnetic eldexertslargerforceonmisalignedspins,andalowerenergyofthermalexcitationisrequiredtoaligntheminthedirectionofthe eld.Withthelower eld,higherthermalexcitationsarerequiredforthesameeffect.

TheZFCW,FCC,andFCWcurvesdonotoverlapabove200K.ThereasonforthiscouldbetheirreversibilitiesassociatedwiththeorderingofsurfacespinsthatarestillnotsuppressedatthesetemperaturesaswellasArtman-typeferromagneticorderaboveTM.However,thereisalsoapossibilityofmagneticnanoparticlesrotatinguponthermalcyclingwithintheagglomeratesinadifferentwayforFCC,FCW,andZFCWmeasurements.Thischangestheinteractionbetweenthenanoparticlesandresultsinadditionalirreversibili-tiesthatdependonthewaythemeasurementsareperformed.41AgoodindicationforthiseffectisthatthemagneticmomentforT>200KinFigure5measureduponwarmingdecreaseswithtemperaturewiththesamerateforZCFWandFCWmeasurements.However,magneticmomentmeasureduponcooling(FCC)inthesametemperaturerangeandthesame elddecreaseswithtemperatureatahigherrate.

Thecoercive eldofournanoparticlesatroomtemperatureismuchlargerthanforbulkcrystals(Figure7).Thecoercive eldcanbestronglyaffectedbydistributionofsizesofnanoparticlesaswellasbytheirinteractionwithintheag-glomerates.Hcofsimilarmagnitudetooursampleswasobtainedforhematitenanoparticlesthatweredenselypackedintoagglomerates.Looselypackednanoparticlesgave～10timeslowerHc.42Thissupportsourargumentontheimportanceofnanoparticleinteractiontotheirmagneticproperties.

Magnetichysteresisloopsareinagreementwiththeabovedescription(Figure5).Thehysteresisloopmeasuredat5Kstronglydiffersfromtheonesmeasuredabove50K.Ithasmuchlargercoercive eld,Hc,thanthehysteresisloopsmeasuredbetween50and200K(Figures6and7).Thehysteresisloopsbetween50and200KaredistinctlyS-shapedwithvanishinglylowcoercive eldasexpectedforsuperparamagneticparticles.HcstartsincreasingwiththeonsetofArtman-typeferromag-

18758J.Phys.Chem.C,Vol.114,No.44,2010netismfornanoparticleslargerthand0at～200K(Figure7).SuperparamagneticbehaviorseemstobesuppressedforT>200KbecauseofanadditionalmagneticmomentarisingfromArtman-typeofordering.Theinteractionbetweenthenanopar-ticleswithinthemesoporousstructuresprobablyplaysanimportantroleintheseprocesseseventhoughitsmagnitudeandtheexactmechanismsarestillunknown.TheincreaseofHcandtheappearanceofferromagneticorderattemperatureshigherthantheblockingtemperaturemaysoundconfusing.However,therearetwofamiliesofspinscontributingtothemeasuredmagneticmoment:unpairedsurfacespinsandbulkspinsduetoArtman-typetransition.Whenbothofthemareorderedferromagnetically,thenetmagneticmomentmaybelargeenoughtogivetheobservedferromagneticbehavior.Nanoparticleagglomerationprobablyalsoplaysaroleherethroughmagneticcouplingofnanoparticles.

Therefore,surfacespinsandArtman-typemagneticmomentofsub-d0nanoparticlesdominatethemagneticbehaviorofmesoporousR-Fe2O3below200K.Thereisevidenceforsuperparamagneticbehaviorofthissystemwithblockingtemperaturearound50Kat1T.Morintransitionofparticleslargerthand0occursat200Kgivingadditionalferromagneticcontributionabove200K.Themagneticmomentofthesurfacespinsandsub-d0particleswithArtman-typeorderingisabout3timeshigherthantheoneduetoMorintransitionofparticleslargerthand0at200Kand1T.Thereseemstobeaneffectoforientationofthenanoparticleswithinthemesoporousstructuresonthenetmagneticmoment.Asthisorientationchangeswith eldandtemperature,sodoesthemagneticinteractionbetweenthenanoparticlesgivingamagneticmomentthatdependsonhistoryofthermalcyclinginmagnetic eld.

3.3.Gas-SensingPerformance.Thegas-sensingperfor-manceoftheas-preparedmesoporousR-Fe2O3wasinvestigatedtowardavarietyof ammable,toxic,andcorrosivegasessuchasethanol,acetone,gasoline,heptane,formaldehyde,aceticacid,1-butanol,and2-propanol.Figure8adisplaysthereal-timesensingresponsecharacteristicstowardethanolofasensorbasedontheas-preparedmesoporousR-Fe2O3ataworkingtemper-atureof150°Cand30%relativehumidity(RH).Itcanbeseenthatthevalueoftheoutputvoltageincreasedabruptlyaftertheinjectionofethanolandrecovereditsinitialvalueafterreleaseofthegas.Theresponseandrecoverytimes(de nedasthetimerequiredtoreach90%ofthe nalequilibriumvalue)oftheas-preparedmesoporousR-Fe2O3basedsensorwereonly1-3seachindicatingtheirrapidresponseandgoodreversibility.FromOhm’slaw,theelectricresistanceofthesensorunderwentadecrease(increase)whentheethanolwasinjected(released),whichisthetypicalsensorbehaviorofn-typesemiconductorsensors.Intheambientenvironment,n-typeR-Fe2O3isexpectedtoadsorbbothoxygenandmoisture.TheadsorbedO2-andOH-groupstrapelectronsfromtheconductionbandoftheR-Fe2O3nanocrystalsandincreasetheresistanceofthesensor.Whenthesensorisexposedtoethanol,thetestgasmoleculesarechemiadsorbedattheactivesitesonthesurfaceoftheas-preparedmesoporousR-Fe2O3.Thesemoleculeswillbeoxidizedbytheadsorbedoxygenandthelatticeoxygen(O2-)ofR-Fe2O3atthesensorworkingtemperature(150°C).Inthisprocess,electronswillbetransferredtothesurfaceoftheas-preparedmesoporousR-Fe2O3tolowerthenumberoftrappedelectronsinducingadecreaseintheresistance.10Themagnitudeoftheresponseforasensorbasedontheas-preparedmesoporousR-Fe2O3improveddramaticallywithincreasingtheconcentra-tionoftheethanolfrom5ppmto1000ppmsuggestingthattheas-preparedmesoporousR-Fe2O3hasgoodsensitivitytoward

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Figure8.(a)Real-timeethanolsensingcharacterizationofsensorsbasedontheas-preparedmesoporousR-Fe2O3.(b)Sensitivityofthesensorsbasedonas-preparedmesoporousR-Fe2O3towarddifferentgasesorvapors.

ethanol.Aftereightcyclesbetweentheethanolandfreshair,theoutputvoltagescouldstillrecovertotheirinitialstatesindicatingexcellentreversibility.Figure8bpresentsthesensitiv-ityofasensorbasedontheas-preparedmesoporousR-Fe2O3towardavarietyofgasesatdifferentconcentrations.Thegassensitivity(S)isde nedastheratioofthestationaryelectricalresistanceofthesensorinair(Rair)toitsresistanceinthetestgas(Rgas),thatis,S)RR-Feair/Rgas.Theas-preparedmesoporous2O3exhibitedaveryimpressivesensitivitytowardethanolathighconcentration(1000ppm),whichisbetterthantheperformanceofitsnanosphereandnanowirecounterpartsaswepreviouslyreported.9,10Theimprovementofthesensingper-formanceoftheas-preparedmesoporousR-Fe2O3maybeattributedtothemesoporousstructure.Thehighlyporousstructureleadstohighspeci csurfacearea,whichismeasuredtobe128m2/gresultinginmoreactivesitesforgaschemi-sorption.Theabundantopenporesdistributedinthethree-dimensionalspacecanfacilitatethediffusionofthetestgasandcanimprovethekineticsofboththereactionofthetestgaswithsurfaceadsorbedoxygenandthereplacementofthelatterfromthegasphase.FromFigure8b,wecanalsonotethattheas-preparedmesoporousR-Fe2O3sensoralsohashighsensitivitytowardsomeother ammableandexplosivegasessuchas92#gasoline,2-propanol,1-butanol,heptane,andacetone.However,thesensorcouldbarelydetectheptaneevenathighconcentration(e.g.,1000ppm).Forapplicationinairqualitymonitoring,wealsoinvestigatedthesensingperformanceoftheas-preparedmesoporousR-Fe2O3tosometoxic,corrosive,

SynthesisofMesoporousR-Fe2O3NanostructuresFigure9.Sensitivityofthesensorsbasedonas-preparedmesoporousR-Fe2O3towarddifferentgasesorvaporsataconcentrationof1000ppm.

Figure10.Cyclicvoltammentry(CV)curvesofthecellwith-1anodepreparedfrommesoporousR-Fe2O3(scanningrate:0.1mVsintherangeof0.005-3.0V).

andirritatinggases,suchasformaldehydeandaceticacid.Itisinterestingthattheas-preparedmesoporousR-Fe2O3displayedthehighestresponsivenesstoaceticacid(Figure8b)withSreachingashighasS)185inthepresenceof1000ppmofaceticacidvapor.Thisultrahighsensitivitymaybecausedbythestrongchemisorptionofaceticacidonthesurfaceoftheas-preparedmesoporousR-Fe2O3owingtothestrongcoordina-tionofcarboxyltoFe3+.Forcomparison,Figure9presentsabargraphofthesensitivityofthemesoporousR-Fe2O3sensortowardeighttypesofgasesorvaporsataconcentrationof1000ppm.Wecanclearlyidentifythattheas-preparedmesoporousR-Fe2O3showedthehighestsensitivityandselectivitytowardaceticacidasopposedtoanyothergas.Ontheotherhand,theas-preparedmesoporousR-Fe2O3canselectivelydetectalcoholinthepresenceofother ammableandexplosivegases,suchasacetone,92#gasoline,andheptane.However,thesensorcouldbarelydetectheptaneindicatingthattheas-preparedmesoporousR-Fe2O3basedsensorhasadegreeofselectivitytodifferentgases.

3.4.ElectrochemicalPerformanceinLithiumIonBattery.Theelectrochemicalbehavioroftheelectrodemadefromtheas-preparedmesoporousR-Fe2O3wasevaluatedbycyclicvoltammetry(CV)andgalvanostaticcharge/dischargecy-cling.Figure10showstheCVcurvesofthemesoporousR-Fe2O3anode.Duringthecathodicpolarizationinthe rstcycle,aspikypeakwasobservedat0.65Vwithtwosmall

J.Phys.Chem.C,Vol.114,No.44,2010

18759

peaksappearingat1.0and1.6Vindicatingthefollowingthreelithiationsteps7,8

R-Fe+2O3+xLi+xe-fR-LixFe2O3

(2)

R-Li-xFe2O3+(2-x)Li++(2-x)ef

Li2Fe2O3(cubic)(3)

Li-2Fe2O3(cubic)+4Li++4eT2Fe0+3Li2O

(4)

R-Fe2O3+6Li++6e-f2Fe0+3Li2O

Attheinitialstageoflithiumintercalation(peakI),asmallamountoflithiumcanbeinsertedintothecrystalstructureoftheas-preparedmesoporousR-Fe2O3withoutchangeinthestructure.Inthenextstepoflithiumintercalation(peakII),thehexagonalR-LixFe2O3istransformedtocubicLi2Fe2O3.Thespikypeak(peakIII)correspondstothecompletereductionofironfromFe2+toFe0+andthedecompositionofelectrolyte.Ontheotherhand,intheanodicpolarizationprocess,twobroadoverlappingpeakswererecordedatabout1.7and1.85VcorrespondingtotheoxidationofFe0toFe2+andfurtheroxidizationtoFe3+.46Thecurvesofthesubsequentcyclesaresigni cantlydifferentfromthatofthe rstcycleasonlyonecathodicpeakappearsatabout0.8Vwithdecreasedpeakintensity,whiletheanodicpolarizationonlyshowedonebroadpeakwithalittledecreaseinpeakintensity.Thedifferencebetweenthe rstandthesecondcathodiccurvesisduetoanirreversiblephasetransformationduringtheprocessoflithiuminsertionandextractionintheinitialcycle.Afterthe rstdischargeprocess,R-Fe2O3wascompletelyreducedtoironnanoparticlesandwasdispersedinaLi2Omatrix.47Thedisappearanceofthepeaksat1.6Vand1.0VfromthesecondcathodicprocessindicatesthatthelithiuminsertionreactionandphasetransformationfromhexagonalR-LixFe2O3tocubicLi2Fe2O3areirreversible.46Thedecreaseoftheredoxpeakintensityimpliesthatthecapacityisdecreasedduringcycling.Afterthesecondcycle,theCVcurvesareverystableforthemesoporousR-Fe2O3electrodeindicatingenhancedstabilityduringthelithiationanddelithiationprocesses.

Thecharge-dischargecurvesofthemesoporousR-Fe2O3electrodeduringthe rstandsecondcyclesareshowninFigure11a,andthesearetypicalcharge-dischargecurvesfortransitionmetalanodematerials.47Forthedischargecurveinthe rstcycle,thevoltageinitiallydecreasedquicklytoapproximately1.6VfollowedbyaweakslopecorrespondingtotheinitiallithiuminsertionintotheR-Fe2O3withoutanychangeinthestructure.Thereisalsoawideslopelocatedat1.2-0.85V,wherethereisaphasetransformationfromthehexagonalR-LixFe2O3tothecubicLi2Fe2O3.Anobviousplateauwasobservedat0.85VindicatingthecompletereductionofironfromFe2+toFe0+.ThiselectrochemicalbehaviorisconsistentwiththeresultsoftheCVmeasurement.Thecapacityobtainedabove0.8Vis940mAhg-1(5.6molofLipermoleofR-Fe2O3),whichisveryclosetothetheoreticalcapacityof1007mAhg-1(6molofLipermoleofR-Fe2O3).Afterdischargingthevoltageto0.01V,thetotalspeci ccapacityoftheas-preparedmesoporousR-Fe2O3is1730mAhg-1correspond-ingto10.3molofLipermoleofR-Fe2O3,whichismuchhigherthanthetheoreticalcapacity.Thelargeexcesscapacitycould

18760J.Phys.Chem.C,Vol.114,No.44,2010Figure11.Charge-dischargeperformanceoftheelectrodemadefromtheas-preparedmesoporousR-Fe2O3:(a)thevoltagepro lesofthe rsttwocycles,(b)cyclingperformance.

beascribedtothedecompositionoftheelectrolyteatlowvoltage(generallybelow0.8VvsLi+/Li)toformasolidelectrolyteinterphase(SEI)layerandfurtherlithiumstorageviainterfacialchargingatmetal/Li2Ointerface.48-51Ithasbeendemonstratedinpreviousreportsthatthemorphologyplaysasigni cantroleinthedischargeperformance.12,52Thematerialsthathavesmallparticleswithhighsurfaceareaalwaysyieldhighdischargecapacitiesindicatingthathighsurfaceareacanenhancetheinterfacialchargestorage.Duringthesecondcycle,thedischargecurveonlyshowsaslopeat1.0-0.8V,ually,theslopebehaviorduringthedischargeprocessofmetaloxideanodematerialsisconsideredtorelatetotheirreversibleformationofananocom-positeofcrystallinegrainsofmetalsandamorphousLi2Omatrix.48,53Forthechargecurvesofthe rstandsecondcycles,noobviousplateaucanbeobservedandthechargecapacitiesare1200mAhg-1and1250mAhg-1,respectively,correspond-ingtotheoxidationofFe0toFe2+,withpartoftheFefurtheroxidizedtoFe3+,alongwithsomecontributionfromthereversiblesurfacelayerformedduringthedischargeprocess.ThecyclingperformanceofmesoporousR-Fe2O3electrodeisshowninFigure11b,whichdemonstratesanexcellentcyclingperformance.After50cyclesatacurrentrateof200mAg-1(0.2C),thespeci cdischargecapacityis1293mAhg-1,whichisabout95%ofthesecondcycledischargecapacityandrepresentsa0.1%capacitydroppercycleapartfromthe rstcycle.Theexcellentcapacityretentionshouldberelatedtothemesoporousstructureofthematerials,whichcanaccommodatethevolumechangeoftheLi+insertion/extractionduringthe

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Figure12.Dischargevoltagepro lesoftheelectrodemadeofas-preparedmesoporousR-Fe2O3atdifferentcurrentdensities.Theinsertisthedischargespeci ccapacityatdifferentC-rates.

charge-dischargeprocessesandwhichcanpreventtheactivematerialsfromfallingoffthecurrentcollector.49,54

Wealsoinvestigatedtheelectrochemicalperformanceofme-soporousR-Fe2O3electrodesatdifferentcurrent-densities(asshowninFigure12).Evenatthe2C(2000mAg1)rate,-1weachievedaspeci clithiumstoragecapacityof870mAhg,whichismuchhigherthanthetheoreticalcapacityofgraphite(372mAhg-1).Themesoporousstructurewithhighsurfaceareaprovideshighsurfacecontactwiththeelectrolyteanddecreasesthecurrentdensityperunitarea.Toourknowledge,thisisthebestperformancecomparedwiththepreviouslyreportedelectrochemicalperformanceforR-Fe2O3material.2,11,12,14,17,53Theexcellentcyclingstabilityandhighratespeci ccapacityoftheas-preparedmesoporousR-Fe2O3indicateitsattractivepotentialforuseasananodematerialinlithiumionbatteries.4.Conclusions

Herein,weusedasofttemplatesynthesismethodtosuc-cessfullypreparemesoporousR-Fe2O3.Theas-preparedmaterialshowsahighlyporousstructurewithalargesurfaceareaof128m2g-1.MagneticpropertymeasurementshowsthatsurfacespinsdominatethemagneticbehaviorofmesoporousR-Fe2O3material.Thecontributionofthesurfacespinstomagneticmomentat200Kisabout3timeshigherthantheoneduetoMorintransitionatH)1T.MesoporousR-Fe2O3materialsexhibitsuperparamagneticbehaviorbecauseofsmallparticlesizewithblockingtemperatureat1T eldof50K.TheMorintransitionat～200Kisalsoassociatedwiththesurfacespins.Inthegas-sensingexperiments,theas-preparedmesoporousR-Fe2O3exhibitshighsensitivitiestowardaceticacidandalcoholindicatingitspotentialapplicationinmonitoringcorrosiveand ammablegases.Foruseasanodematerialsinlithiumionbatteries,theas-preparedmesoporousR-Fe2O3showsahighdischargespeci ccapacityof1730mAhg-1forthe rstcycleand1293mAhg-1after50cycles.Thegoodcyclingstabilityandhighratespeci ccapacityindicateitspotentialforapplica-tioninthelithiumionbatteryindustry.

Acknowledgment.Wegratefullyacknowledge nancialsupportfromBezel(Changzhou)NewEnergyScience&TechnologyCo.,Ltd.,DaejungEnergyMaterialsCo.,Ltd.,Korea,andanARCDiscoveryproject(DP07729999)fromtheAustralianResearchCouncil.

SynthesisofMesoporousR-Fe2O3Nanostructures

SupportingInformationAvailable:SchematicdiagramofthegassensorsystemandTEMimagesofmesoporousR-Fe2O3.ThismaterialisavailablefreeofchargeviatheInternetat.

ReferencesandNotes

(1)Jain,G.;Balasubramanian,M.;Xu,J.J.Chem.Mater.2006,18,423.

(2)Wu,C.Z.;Yin,P.;Zhu,X.;OuYang,C.Z.;Xie,Y.J.Phys.Chem.B2006,110,17806.

(3)Gou,X.L.;Wang,G.X.;Kong,X.Y.;Wexler,D.;Horvat,J.;Yang,J.;Park,J.Chem.sEur.J.2008,14,5996.

(4)Hermanek,M.;Zboril,R.;Medrik,N.;Pechousek,J.;Gregor,C.J.Am.Chem.Soc.2007,129,10929.

(5)Feldmann,C.AdV.Mater.2001,13,1301.

(6)Srivastava,D.N.;Perkas,N.;Gedanken,A.;Felner,I.J.Phys.Chem.B2002,106,1878.

(7)Larcher,D.;Masquelier,C.;Bonnin,D.;Chabre,Y.;Masson,V.;Leriche,J.B.;Tarascon,J.M.J.Electrochem.Soc.2003,150,A133.

(8)Larcher,D.;Bonnin,D.;Cortes,R.;Rivals,I.;Personnaz,L.;Tarascon,J.M.J.Electrochem.Soc.2003,150,A1643.

(9)Gou,X.L.;Wang,G.X.;Park,J.;Liu,H.;Yang,J.Nanotechnology2008,19,125606.

(10)Wang,G.X.;Gou,X.L.;Horvat,J.;Park,J.J.Phys.Chem.C2008,112,15220.

(11)Chen,J.;Xu,L.N.;Li,W.Y.;Gou,X.L.AdV.Mater.2005,17,582.

(12)Reddy,M.V.;Yu,T.;Sow,C.H.;Shen,Z.X.;Lim,C.T.;Rao,G.V.S.;Chowdari,B.V.R.AdV.Funct.Mater.2007,17,2792.

(13)Tahir,A.A.;Wijayantha,K.G.U.;Saremi-Yarahmadi,S.;Mazhar,M.;McKee,V.Chem.Mater.2009,21,3763.

(14)Wu,Z.C.;Yu,K.;Zhang,S.D.;Xie,Y.J.Phys.Chem.C2008,112,11307.

(15)Woo,K.;Lee,H.J.;Ahn,J.P.;Park,Y.S.AdV.Mater.2003,15,1761.

(16)Dong,W.T.;Zhu,C.S.J.Mater.Chem.2002,12,1676.

(17)Wang,L.;Xu,H.W.;Chen,P.C.;Zhang,D.W.;Ding,C.X.;Chen,C.H.J.PowerSources2009,193,846.

(18)Zeng,S.Y.;Tang,K.B.;Li,T.W.;Liang,Z.H.;Wang,D.;Wang,Y.K.;Qi,Y.X.;Zhou,W.W.J.Phys.Chem.C2008,112,4836.

(19)Lian,J.B.;Duan,X.C.;Ma,J.M.;Peng,P.;Kim,T.I.;Zheng,W.J.ACSNano2009,3,3749.

(20)Cheng,F.;Tao,Z.;Liang,J.;Chen,J.Chem.Mater.2008,20,667.

(21)Liu,J.P.;Li,Y.Y.;Fan,H.J.;Zhu,Z.H.;Jiang,J.;Ding,R.M.;Hu,Y.Y.;Huang,X.T.Chem.Mater.2010,22,212.

(22)Kresge,C.T.;Leonowicz,M.E.;Roth,W.J.;Vartuli,J.C.;Beck,J.S.Nature1992,359,710.

(23)He,X.;Antonelli,D.Angew.Chem.,Int.Ed.2002,41,214.(24)Jiao,F.;Harrison,A.;Hill,A.H.;Bruce,P.G.AdV.Mater.2007,19,4063.

J.Phys.Chem.C,Vol.114,No.44,201018761

(25)Yu,C.C.;Dong,X.P.;Guo,L.M.;Li,J.T.;Qin,F.;Zhang,L.X.;Shi,J.L.;Yan,D.S.J.Phys.Chem.C2008,112,13378.(26)Jiao,F.;Bruce,P.G.AdV.Mater.2007,19,657.

(27)Shaju,K.M.;Jiao,F.;Debart,A.;Bruce,P.G.Phys.Chem.Chem.Phys.2007,9,1837.

(28)Jiao,F.;Harrison,A.;Jumas,J.C.;Chadwick,A.V.;Kockelmann,W.;Bruce,P.G.J.Am.Chem.Soc.2006,128,5468.

(29)Wang,K.X.;Wei,M.D.;Morris,M.A.;Zhou,H.S.;Holmes,J.D.AdV.Mater.2007,19,3016.

(30)Liu,P.;Lee,S.H.;Tracy,C.E.;Yan,Y.F.;Turner,J.A.AdV.Mater.2002,14,27.

(31)Yang,P.D.;Zhao,D.Y.;Margolese,D.I.;Chmelka,B.F.;Stucky,G.D.Nature1998,396,152.

(32)Yang,P.D.;Zhao,D.Y.;Margolese,D.I.;Chmelka,B.F.;Stucky,G.D.Chem.Mater.1999,11,2813.

(33)Laetsch,T.;Downs,R.T.19thGeneralMeetingoftheInternationalMineralogicalAssociation,Kobe,Japan.

(34)Kim,C.H.;Chun,H.J.;Kim,D.S.;Kim,S.Y.;Park,J.;Moon,J.Y.;Lee,G.;Yoon,J.;Jo,Y.;Jung,M.H.;Jung,S.I.;Lee,C.J.Appl.Phys.Lett.2006,89,223103.

(35)Bersani,D.;Lottici,P.P.;Montenero,A.J.RamanSpectrosc.1999,30,355.

(36)Akl,A.A.Appl.Surf.Sci.2004,233,307.

(37)Vayssieres,L.;Sathe,C.;Butorin,S.M.;Shuh,D.K.;Nordgren,J.;Guo,J.H.AdV.Mater.2005,17,2320.(38)Corma,A.Chem.ReV.1997,97,2373.

(39)Artman,J.O.;Murphy,J.C.;Foner,S.Phys.ReV.1965,138,A912.(40)Chow,H.;Keffer,F.Phys.ReV.B1974,10,243.(41)Zysler,R.D.;etal.Phys.ReV.B2003,68,212408.

(42)Qian,H.;Han,G.;Lin,G.;Xu,R.AIChEJ.2008,8,51.(43)Flanders,P.J.;Remeika,J.P.Philos.Mag.1965,11,1271.(44)Muench,G.J.;Arajs,S.;Matijevic´,E.Phys.StatusSolidiA1985,92,187.

(45)Bødker,F.;Hansen,M.F.;Koch,C.B.;Lefmann,K.;Mørup,S.Phys.ReV.B2000,61,6826.

(46)Morales,J.;Sanchez,L.;Martin,F.;Berry,F.;Ren,X.L.J.Electrochem.Soc.2005,152,A1748.

(47)Poizot,P.;Laruelle,S.;Grugeon,S.;Dupont,L.;Tarascon,J.M.Nature2000,407,496.

(48)Jamnik,J.;Maier,J.Phys.Chem.Chem.Phys.2003,5,5215.(49)Maier,J.Nature2005,4,805.

(50)Grugeon,S.;Laruelle,S.;Dupont,L.;Tarascon,J.M.SolidStateSci.2003,5,895.

(51)Balaya,P.;Li,H.;Kienle,L.;Maier,J.AdV.Funct.Mater.2003,13,621.

(52)Jiao,F.;Bao,J.L.;Bruce,P.G.Electrochem.Solid-StateLett.2007,10,A264.

(53)Wu,X.L.;Guo,Y.G.;Wan,L.J.;Hu,C.W.J.Phys.Chem.C2008,112,16824.

(54)Hu,Y.S.;Kienle,L.;Guo,Y.G.;Maier,J.AdV.Mater.2006,18,1421.

JP102286E

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