Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas S(3)

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).

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

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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