Electronic structure of graphite oxide and thermally reduced

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CARBON49(2011)1362–1366availableatwww.sciencedirect.comjournalhomepage:www.elsevier.com/locate/carbonElectronicstructureofgraphiteoxideandthermallyreducedgraphiteoxideabc,daaaaDaZhan,ZhenhuaNi,WeiChen,LiSun,ZhiqiangLuo,LinfeiLai,TingYu,ca,*AndrewThyeShenWee,ZexiangShenaDivisionofPhysicsandAppliedPhysics,SchoolofPhysicalandMathematicalSciences,NanyangTechnologicalUniversity,Singapore637371,SingaporebDepartmentofPhysics,SoutheastUniversity,Nanjing211189,ChinacDepartmentofPhysics,FacultyofScience,NationalUniversityofSingapore,2ScienceDrive3,Singapore117542,SingaporedDepartmentofChemistry,FacultyofScience,NationalUniversityofSingapore,3ScienceDrive3,Singapore117543,SingaporeARTICLEINFOABSTRACTArticlehistory:WepresenttheelectronicstructureevolutionfromgraphiteoxidetothermallyreducedReceived24August2010graphiteoxide.MostfunctionalgroupswereremovedbythermalreductionasindicatedAccepted1December2010byhighresolutionX-rayphotoelectronspectroscopy,andtheelectricalconductivityAvailableonline4December2010increased6orderscomparewiththeprecursorgraphiteoxide.X-rayabsorptionspectros-copyrevealsthatthethermallyreducedgraphiteoxideshowsseveralabsorptionpeakssimilartothoseofpristinegraphite,whichwerenotobservedingraphiteoxideorchemi-callyreducedgraphiteoxide.Thisindicatesthebetterrestorationofgraphiticelectronicconjugationbythermalreduction.Furthermore,thesignificantincreasedintensityofRaman2Dbandofthermallyreducedgraphiteoxidecomparedwithgraphiteoxidealsosuggeststherestorationofgraphiticelectronicstructure(porbital).Theseresultsprovideusefulinformationforfundamentalunderstandingoftheelectronicstructureofgraphiteoxideandthermallyreducedgraphiteoxide.Ó2010ElsevierLtd.Allrightsreserved.1.Introductioncalapplications[7,10].Forexample,RGOfilmwasusedastransparentandconductiveelectrodesfordye-sensitizedso-Graphiteoxide(GO)isafunctionalgroupsrichcarbonaceouslarcell[8]andultra-capacitorapplications[11].layeredmaterial[1,2].Itisanon-stoichiometriccompoundAlthoughtherecentprogressonprocessingRGOfilmforconsistsofvariousfunctionalgroups.Theepoxyandhydro-variouspracticalapplicationshavebeenreported[7,8,10–12],xylgroupsrandomlyinterspersedonthetopandbottomsur-thefundamentalunderstandingoftheelectronicstructurefacesofeachgraphenesheet,whilecarboxylandcarbonylofthiscarbon-basedcompositeisstilllacking.SoftX-raygroupsnormallylocateatedges[1–4].Despitethepresenceabsorptionspectroscopy(XAS)isaveryreliabletooltoinves-offunctionalgroups,thelayeredstructureofGOiswelltigatetheelectronicstructure(unoccupiedstatesaboveFermipreservedwithABstackingorder[3,5].GOisinsulator,butlevel)ofcarbon-basedmaterials[13–16].However,reportsonreducedgraphiteoxide(RGO)isgoodconductorwithconduc-electronicstructureofGOandRGOprobedbyXASareverytivity4–6ordershigherthanthatofGOduetotherestorationlimited[17–19].Toourbestknowledge,therearenorelevant2ofsp-carbonnetworks[1,6–9].RGOcanbepreparedintolargereportsonthermallyreducedgraphiteoxide(tRGO)evensizefilmswithhighconductivityandtransparencyforpracti-thoughitisconsideredasthemostsimpleandeffective*Correspondingauthor:Fax:+6567957981.E-mailaddress:zexiang@ntu.edu.sg(Z.Shen).0008-6223/$-seefrontmatterÓ2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.carbon.2010.12.002 CARBON49(2011)1362–136613632methodtoachievebestrestorationofsp-carbonstructuresC-Ocomparewithnormalchemicallyreducedgraphiteoxide(a)2SP(tRGO)[6,7].Inthiswork,X-rayphotoelectronspectroscopy(XPS),conductivitymeasurementandRamanspectroscopywereusedtoassistXAStoinvestigateelectronicstructureevolutionfromGOtotRGO.2.ExperimentalC=OGraphiteoxidewassynthesizedfromnaturalgraphitepowderbyconventionalmodifiedHummersmethod[5].Twopiecesofpaper-likestructuredGOfilmswithsizeof5·5mmand282284286288290292294thicknessof1lmwereprepared[20].OneoftheGOfilms(b)2wasreducedbyH2/Ar(1:2)airflowat950°Cfor30min(tRGO).SPTheheatingandcoolingrateswerekeepingat5°C/min.Thecarbon-relatedchemicalspeciesofGOandtRGOwerecharac-terizedbyhighresolutionXPSusingKratosAxisUltraDLD(delaylinedetector)spectrometerequippedwithamonochro-maticAlKaX-raysource(1486.6eV)withresolutionof0.1eV.TheconductivityofGOandtRGOfilmswasmeasuredbyC-OKeithley2612Aelectricalmeasurementsystem.Ramanspec-C=Otrawererecordedby(1)WITECCRM200system(excitationla-serenergyis2.33eV);(2)Renishawsystem(excitationlaserenergiesare1.58and3.81eV,respectively).Thenearedge282284286288290292294X-rayabsorptionfinestructure(NEXAFS)measurementswereBindingEnergy(eV)carriedoutattheSurface,Interface,andNanostructureScience(SINS)beamlineoftheSingaporeSynchrotronLightFig.1HighresolutionXPSspectraofGO(a)andtRGO(b).Source,usingtotal-electronyield(TEY)modeandlinearTheblacksolidlinesareoriginalspectra;thedeconvolutedp-polarizedlightwithphotonenergyresolutionof0.1eV.dashedlinesinred,greenandblueareassignedtosp2-C,CO(hydroxylandepoxygroups),C@O(carboxylgroups),3.Resultsanddiscussionrespectively.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionHighresolutionXPSspectraofC1sregionofGOandtRGOareofthisarticle.)showninFig.1,whilethedetailedinformationontheportionofdeconvolutedcomponentsisshowninTable1.ItcanbeseenclearlythatGOshowsthreemostprominentdeconvoluted2Table1TheportionofchemicalspeciesforGOandtRGOcomponents,withoneofthemassignedtosp-C1s(atbasedonXPSresults.285eV),andtheothertwoassignedtospeciesofC–O(hydro-xylandepoxy)andC@O(carboxyl)withbindingenergyof287GOtRGOand289eV,respectively[1,3,7].Thisclearlyshowshighde-sp2(C–C)%45±574±5greeoxidationofGO.FortRGO,mostchemicallyattachedfunc-C–O%41±514±3tionalgroupsweresuccessfullyremovedbythermalreduction,C@O%14±212±22andtheportionofsphybridizedcarbonspeciesobviouslyin-creasedasshowninFig.1b.Furthermore,thefullwidthofhalf2maximum(FWHM)ofspcarbonpeakisverysmall(1eV),material,whileDbandisactivatedonlyifdefectsparticipateanditissimilartothatofthepreviouslyreportedtRGO[7].thedoubleresonanceRamanscatteringnearKpointofBrill-I–VcurvesofGOandtRGOareshowninFig.2.Thediffer-ouinzone[22].AlthoughchemicalcomponentsofGOissignif-4entialconductivityofGOis10S/cmatbiasvoltageof3V,icantlydifferentfromthatoftRGO,RamanspectrashowwhiletheconductivityoftRGOis100S/cm,approximately6neglectablechangesontheintensityratioofID/IGbetweenordershigherthanthatofGO.ThedramaticenhancementofGOandtRGO(Fig.3).Therefore,ourresultsindicatethatthe2conductivityoftRGOalsoinferstheremovalofoxygenatedaveragesizeofspdomainsdoesnotchangesignificantlyfrom2functionalgroupsandtherestorationofsphybridizedcar-GOtotRGOastheintensityratioofID/IGisnormallyusedfor2boncomponent,whichisinconsistentwiththeXPSresults.estimatingthespdomainsizeofgraphite-basedmaterials.RamanspectraofGOandtRGOareshowninFig.3.ThereItisreasonablethatthoughthermalreductioncanremovearetwomainprominentpeaksforbothGOandtRGOsamples,thefunctionalgroupsfromGO,theexfoliationofGOisinevi-11assignedtoGband(1580cm)andDband(1310–1430cm,tableduringthethermalreduction[1,6],andthesetwofactors2dependsontheexcitationlaserenergy),whichareE2gvibra-makethespdomainsizeofGOdoesnotchangesignificantlytionalmodein-planeandA1gbreathingmode,respectivelyafterreduction.BesidesGandDbands,therearetwoRaman2[21].GbandisRamanactiveforsphybridizedcarbon-basedbandswithweakerintensitycalled2DandD+Glocateat 1364CARBON49(2011)1362–13661560thepbandinthegraphiticelectronicstructure,whilethecom-binationmodeofD+Gisinducedbydisorder[22].2Dand1040D+GbandsarenormallyignoredonthestudiesofGOand520RGObecauseoftheweakintensity[1,6,9,19,23].Here,wefoundthatitisveryeasytodistinguishtheelectronicconjuga-GO00tionofGOandtRGObycomparingthesetwobands,inwhichtRGOtheobviousenhancementof2DintensityoftRGOcomparedCurrent(nA)-5-20Current(mA)withGOcanbeclearlyobservedusingvariousexcitationlaser-10-40energies(insetsofFig.3).TheintensityratiosofID/IGandI2D/ID+GarealsoshowninTable2.Thedataalsoshowsthatfrom-15-60-3-2-10123GOtotRGO,theI2D/ID+GchangedmoresignificantlythanID/IG,Votage(V)andthedramaticallyincreasedI2D/ID+GfromGOtotRGOusingvariousexcitationlaserenergiessuggeststhattherecoveryofFig.2IVcurvesofGO(blackline)andtRGO(redline).(ForgraphiticelectronicconjugationfortRGO.interpretationofthereferencestocolourinthisfigureInadditiontoRamanspectroscopy,NEXAFSmeasurementlegend,thereaderisreferredtothewebversionofthisisaneffectivemethodforanalyzingunoccupiedelectronicarticle.)structureofcarbonrelatedmaterials[13–16].CarbonK-edgeNEXAFS(1s–p*and1s–r*locateat285and293eV,respec-tively)spectrainTEYmodeforGOandtRGOareshowninFig.4.Thetwo-dimensionalnatureofgraphiticmaterialissupposedtohavestrongdirectionalityoftheorbitals:rorbi-talsliewithinthebasalplaneandporbitalsaredirectedper-pendiculartothebasalplane.Therefore,XASisangle2dependentforthesp-carbon-basedmaterialswithlayeredstructure.TheNEXAFSforGOandtRGOwereusinglinearpolarizedX-raybeamwithincidentbeamangleofh=90°(per-pendicular)andh=20°(paralleldominate)respecttosubstrate(insetofFig.4).Foridealhighlyorderedpyrolyticgraphite(HOPG)orgraphene,astheEvectorisparalleltothebasalplanewhentheincidentbeamangleis90°,thusthetransitionof1s–p*shouldbequenchedasthepfinalstatescannotbese-lected[14].Inourexperiment,the1s–p*transitionisnotfullyquenchedfornormalincidence(h=90°),buttheintensityisobviouslydecreasedcomparedwithparallelincidence(h=20°)(Fig.4b).ThisissimilartotheresultsreportedrecentlybyLeeetal.[19].However,theincident-angledependenceof1s–p*transitionintensityforGOandtRGOisratherweakcom-paredtothatofHOPG.ThissuggeststhattheGOandtRGOarenotperfectlyflat2-Dmaterialsbutpresentwave-like2-Dstructurewithperiodicfluctuation,probablyoriginatingfromthecoexistenceofdifferentmicro-domains,theratherdisor-deredstackingofGOandtRGOplanescomparedtothatofHOPG,aswellasthedisorderinginducedbytheoxidationpro-cesses[20].Furthermore,thehydroxylandepoxygroupsdis-tributedrandomlyontopandbottomofbasalplaneofeachgraphenelayerwouldpullthebondedcarbonatomsupanddownwithrespecttothebasalplane.For1s–r*transition,theintensitybecomesmuchstrongerfortRGOwhentheinci-dentbeamangleischangedfromh=20°toh=90°,butthereisonlysmallenhancementforGO.ThisisanotherevidencetoshowthatonlysmallamountofrorbitalskeepintactforGObecauseofthestrongoxidation,whiletheintactrorbitalsin-2creaseddramaticallywiththerestorationofsphybridizedFig.3RamanspectraforGOandtRGOexcitedbyvariouscarbonnetworkbythermalreduction.Ashoulderpeakatexcitationlaserenergies(wavelengths):(a)1.58eV(785nm),292eVoftRGOforh=90°shouldbeexcitonicstate,theexis-(b)2.33eV(532nm),(c)3.81eV(325nm).Insetsare2DandtenceofthisexcitonicstateindicatesthattRGOconsistofaD+GrangeforGOandtRGO(backgroundremoved).2largenumberofsphybridizedcarbonatomsasthispeakisusuallyobservedforpristinegraphite[17]andcRGO[19].Here,12700–3000cmasshownininsetsofFig.3.2DbandisRamanweshouldemphasizethatdespitethethermalreductionpro-activeforcrystallinegraphiticmaterialsanditissensitivetocessremovedmostfunctionalgroups,thetRGOcanonlybe CARBON49(2011)1362–13661365Table2IntensityratiosofID/IGandI2D/ID+GforGOandtRGOexcitedbyvariousexcitationlaserenergies.Laserenergy(eV)GO(ID/IG)tRGO(ID/IG)GO(I2D/ID+G)tRGO(I2D/ID+G)1.562.27±0.122.44±0.112.09±0.223.91±0.272.332.11±0.102.01±0.130.70±0.151.52±0.203.811.54±0.051.05±0.060.17±0.050.49±0.1021eVbelow1s–p*(sp).Thispeakispossiblyattributedtoedge-derivedelectronicstateassimilarstructurehasbeenobservedinnanographite[25]andsinglelayergraphene[26],respectively.Athigherphotonenergyrange,the1s–r*transitionaround293eVisassignedtothefinalstateof1s–r1*and1s–r2*atBrillouinzoneregionC–Q[14].Inadditiontothisprominenttransition,XASforsinglecrystallinegraphiteusuallycanre-solveabsorptiontransitionsfromgroundstate1stootherr*subbandstateswhichcontainhighunoccupieddensityofstatesatabout10–30eVabovetheFermilevel[14].Inourexperiment,threeadditionalpeaksC–E,respectively,locateat297.6,302.9and307.6eVareobservedfortRGO(Fig.4a).Thesepeakswerealsoobservedforsinglecrystallinegraphitelocateat297.8,303.5and307.5eV,respectively[14].Theobvi-ousenhancedsignalsforpeaksC–Eatincidentbeamangleofh=90°thanthatofh=20°furtherconfirmsthatC–Eareduetotransitionsfrom1sstatetootherr*subbandstates.Ontheotherhand,therearenosuchnoticeablepeaksforGOandtheywerenotobservedforcRGOasreportedelsewhere[19].Therefore,tRGOpresentsmoresimilarelectronicstructuretopristinegraphitethanthatofcRGO.Thisindicatesthebet-terrestorationofgraphiticelectronicconjugationbythermalreductionthanchemicalreduction.ThebetterrestorationofelectronicconjugationoftRGOcomparedwithcRGOistheba-sicandsignificantevidencetoexplainwhytRGOnormallypresenthigherelectricalconductivitythanthatofcRGO[6,7].4.ConclusionsInconclusion,theelectronicstructureofGOandtRGOissys-tematicallystudied.ItisfoundthatthecomparisonofRaman2DandD+Gbandscanindirectlymonitortheelectronicstruc-Fig.4(a)CK-edgeXASofGOandtRGOforincidentbeamtureofGOandtRGO(porbitals).AngledependentCK-edgeangleofh=20°andh=90°,respectively.(b)TheenlargedNEXAFSmeasurementalsoconfirmedtheeffectiverestorationregionof280291eV.ofthegraphiticelectronicconjugationontRGO.Inadditiontotwoprominenttransitionpeaks1s–p*and1s–r*at285andconsideredasamixtureofthegraphite-likespeciesandother293eV,respectively,someotherpeaks(at297.6,302.9andunspecifiedcomponentsastheweak1s–r*and1s–p*peaks307.6eV)werealsoobserved,whichissimilartothatofpristinecomparetothebackgroundabsorption.graphite.ThisindicatesthatthetRGOcontainssimilarelec-InFig.4b(Enlargedregionof280–291eV),besidesthetronicstructureasthatofgraphite,particularlythein-plane2absorptionpeakOinducedbysp-C1s–p*transitionatr*subbandstates.Theseresultsexplainedthekeyreasons285.3eV,twoadditionalabsorptionpeaksAandBat286.3forthesignificantincreaseofelectricalconductivityoftRGO.and288.1eVcanbeclearlyobservedforGOwithbothinci-dentbeamangleofh=20°and90°,respectively.PeakAcanbeassignedto1s–p*(C–O)transitionasthehydroxylandREFERENCESepoxyfunctionalgroupschemicallyattachedtobasalplaneofeachgraphenelayer[24];peakBcanbeassignedto1s–p*(C@O)astheexistenceofcarboxylandcarbonylgroups[18].[1]StankovichS,DikinDA,PinerRD,KohlhaasKA,ThereisonemorepeakHlocatesat284.3eVforGO,aboutKleinhammesA,JiaY,etal.Synthesisofgraphene-based 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