Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas S(8)

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.