Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas S(2)

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