Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas S(7)

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

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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