Synthesis of graphene-basednanosheetsviachemicalreduction of exfoliated graphite oxide

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Carbon45(2007)1558–1565www.elsevier.com/locate/carbonSynthesisofgraphene-basednanosheetsviachemicalreductionofexfoliatedgraphiteoxideaaaaSashaStankovich,DmitriyA.Dikin,RichardD.Piner,KevinA.Kohlhaas,cccAlfredKleinhammes,YuanyuanJia,YueWu,b,*a,*SonBinhT.Nguyen,RodneyS.RuoffaDepartmentofMechanicalEngineering,NorthwesternUniversity,2145SheridanRd.,Evanston,IL60208-3133,USAbDepartmentofChemistry,2145SheridanRd.,Evanston,IL60208-3113,USAcDepartmentofPhysicsandAstronomyandCurriculuminAppliedandMaterialsSciences,UniversityofNorthCarolina,ChapelHill,NC,27599-3255,USAReceived17January2007;accepted19February2007Availableonline6March2007AbstractReductionofacolloidalsuspensionofexfoliatedgrapheneoxidesheetsinwaterwithhydrazinehydrateresultsintheiraggregationandsubsequentformationofahigh-surface-areacarbonmaterialwhichconsistsofthingraphene-basedsheets.Thereducedmaterialwascharacterizedbyelementalanalysis,thermo-gravimetricanalysis,scanningelectronmicroscopy,X-rayphotoelectronspectroscopy,NMRspectroscopy,Ramanspectroscopy,andbyelectricalconductivitymeasurements.Ó2007ElsevierLtd.Allrightsreserved.1.Introductionlets/nanoplateletsfromtheexpandedmaterial(ballmillingorexposuretoultrasound)hasbeenrecentlydemonstratedGraphite-likenanoplateletshaverecentlyattracted[6–13].Althoughthissimplemethodhasbeenappliedattentionasaviableandinexpensivefillerincompositeonalargescaletocommerciallyavailablesulfuricacid-materials[1–3]thatcanbeusedinmanyengineeringappli-intercalatedgraphite,itneverresultsincompleteexfoliationcations,giventheexcellentin-planemechanical,structural,ofgraphitetothelevelofindividualgraphenesheets.Thethermal,andelectricalpropertiesofgraphite[4].Theseextentofthermalexpansion(andthereforetheplateletexcellentpropertiesmayberelevantatthenanoscaleifthickness)isdependentonthetypeofgraphiteusedandgraphitecanbeexfoliatedintothinnanoplatelets,andevenontheintercalationprocedure[14,15].Withfewexceptionsdowntothesinglegraphenesheetlevel[5].[3],thegraphitenanoplateletsobtainedviathisprocesstyp-Graphitenanoplateletshaveoftenbeenmadefromicallyconsistofhundredsofstackedgraphenelayersexpandedgraphite,whichinturnwasproducedfrom(assumingthatthethicknessofonelayerisequaltothegraphiteintercalationcompoundsviarapidevaporationinterlayerseparationingraphite,0.34nm)andaverageoftheintercalantatelevatedtemperatures.Forexample,between30and100nminthickness.rapidthermalexpansionofsulfuricacid-intercalatedInadditiontothethermalexpansionroute,thedelami-graphite,followedbyasuitabletreatmenttoproduceplate-nationofintercalatedgraphitecansometimesbeachievedbyinducingagas-producingchemicalreactionwithinitsinterlayergalleries(chemicalexpansion).Forexample,alow-temperaturechemicalexpansionroutetographite*Correspondingauthors.Fax:+18474917713(S.T.Nguyen);Fax:+1nanoplateletsandnanoscrolls,basedonpotassium-interca-8474913915(R.S.Ruoff).E-mailaddresses:stn@northwestern.edu(S.T.Nguyen),r-ruoff@latedgraphite,hasbeenreported[16,17].However,thisnorthwestern.edu(R.S.Ruoff).approachcouldnotbereproducedinourlaboratoryeven0008-6223/$-seefrontmatterÓ2007ElsevierLtd.Allrightsreserved.doi:10.1016/j.carbon.2007.02.034 S.Stankovichetal./Carbon45(2007)1558–15651559withaduplicationoftheexpensiveultra-high-intensitywereportacomprehensivestudyofthisreducedmaterialultrasonicequipmentreportedtherein.byelementalanalysis,X-rayphotoelectronspectroscopyGivenourinterestinthepreparationofgraphene-based(XPS),gasadsorption,solidstateNMRspectroscopy,materials[18–20],wesetouttodevelopageneralandRamanspectroscopy,thermo-gravimetricanalysis(TGA),reproducibleapproachforthepreparationofgrapheneSEM,andelectricalconductivitymeasurements.sheetsfromgraphite.Afternumerousfailedattemptstocreategraphene-basedsheetsviagraphiteintercalation2.Experimentalcompounds,wedecidedtousegraphiteoxide(GO)asonepossiblerouteformeetingthischallenge.Ourbasic2.1.MaterialsandmethodsstrategyinvolvedthecompleteexfoliationofGOintoindi-GOwaspreparedfrompurifiednaturalgraphite(SP-1,30-lmnominalvidualGOsheetsfollowedbytheirin-situreductiontopro-particlesize,BayCarbon,BayCity,MI)bytheHummersmethod[22].duceindividualgraphene-likesheets[19,20].Herein,weSEMimageswereobtainedwithafieldemissiongunscanningelectrondescribethedetailedprocessforthereductionofexfoliatedmicroscope(LEO1525,CarlZeissSMTAG,Oberkochen,Germany).GOsheetswithhydrazineandthecharacterizationoftheSamplesforAFMimagingwerepreparedbydepositingcolloidalsuspen-sionsofGOonfreshlycleavedmicasurfaces(TedPellaInc.,Redding,resultingmaterial.Inparticular,wepresentevidencetoCA).AFMimagesweretakenonaMultiTaskAutoProbeCP/MTScan-supporttheclaimthatGOcanbecompletelyexfoliatedningProbeMicroscope(VeecoInstruments,Woodbury,NY).Imagingintoindividualgrapheneoxidesheetsandthatchemicalwasdoneinnon-contactmodeusingaV-shape‘‘Ultralever’’probeBreductionofsuchsheetscanfurnishgraphene-likesheets.(B-dopedSiwithfrequencyfc=78.6kHz,springconstantsk=2.0–GOisproducedbytheoxidativetreatmentofgraphite3.8N/m,andnominaltipradiusr=10nm,ParkScientificInstruments,Woodbury,NY).AllimageswerecollectedunderambientconditionsatviaoneofthreeprincipalmethodsdevelopedbyBrodie50%relativehumidityand23°Cwithascanningrasterrateof1Hz.[21],Hummers[22],andStaudenmeier[23],respectively.SurfaceareaanalysiswasperformedwithaMicromeriticsASAP2010Itstillretainsalayeredstructure,butismuchlighterinAnalyzer(MicromeriticsInstrumentCorporation,Norcross,GA).Thecolorthangraphiteduetothelossofelectronicconjuga-sampleswereoutgassedat3mTorrand150°Cfor24hpriortoanalysis.tionbroughtaboutbytheoxidation.AccordingtotheElementalanalysesandKarl–FishertitrationwereperformedbyGalbra-ithLaboratories(Knoxville,TN).XPSmeasurementswereperformedmostrecentstudies[24–29],GOconsistsofoxidizedgraph-usinganOmicronESCAProbe(OmicronNanotechnology,Taunusstein,enesheets(or‘grapheneoxidesheets’)havingtheirbasalGermany)withamonochromatedAlKaradiation(hm=1486.6eV).TGAplanesdecoratedmostlywithepoxideandhydroxylwasperformedunderanitrogenflow(100mL/min)usingaTAInstru-groups,inadditiontocarbonylandcarboxylgroupsmentsTGA-SDT2960onsamplesizesfrom5to6mg,andthemasslocatedpresumablyattheedges(Lerf–Klinowskimodel).wasrecordedasafunctionoftemperature.Thesampleswereheatedfromroomtemperatureto800°Cat5°C/min.ToavoidthermalexpansionofTheseoxygenfunctionalitiesrenderthegrapheneoxidelay-theGOduetorapidheating,GOsampleswerealsoheatedfromroomersofGOhydrophilicandwatermoleculescanreadilytemperatureto800°Cat1°C/min.intercalateintotheinterlayergalleries.GOcanthereforeSolid-stateFT-NMRspectrawererecordedonaChemagneticCMXbealsothoughtofasagraphite-typeintercalationcom-400instrumentequippedwitha4-mmmagicanglespinning(MAS)probepoundwithbothcovalentlyboundoxygenandnon-cova-atamagneticfieldof9.4T.Theneatsamples(28mgeach)werespunat9.4kHztoaveragetheanisotropicchemicalshifttensor.Spectrabasedlentlyboundwaterbetweenthecarbonlayers.Indeed,onfreeinductiondecayswithmoderatedecouplingpowerwereaveragedrapidheatingofGOresultsinitsexpansionanddelamina-over18,000scanswitharecycledelayof8s.The90°pulseis2.5lsastioncausedbyrapidevaporationoftheintercalatedwaterdeterminedbyacquisitiononadamantane.Solidadamantane(38.3ppm)13andevolutionofgasesproducedbythermalpyrolysisofwasalsousedastheexternalreferenceforCchemicalshiftbasedon1theoxygen-containingfunctionalgroups[30].SuchthermaltheTMSscale.Ramanspectrawererecordedfrom200to2000cmonaRenishaw2000ConfocalRamanMicroprobe(RhenishawInstruments,treatmenthasrecentlybeensuggestedtobecapableofpro-England)usinga514.5-nmargonionlaser.ducingindividualfunctionalizedgraphenesheets[30].Bynature,GOiselectricallyinsulating(seebelow)and2.2.ReductionofexfoliatedGOwithhydrazinehydratethuscannotbeused,withoutfurtherprocessing,asacon-ductivenanomaterial.Inaddition,thepresenceoftheoxy-Inatypicalprocedure,GO(100mg)wasloadedina250-mLround-genfunctionalgroupsmakesGOthermallyunstable,asitbottomflaskandwater(100mL)wasthenadded,yieldinganinhomoge-undergoespyrolysisatelevatedtemperatures[31,32].Nota-neousyellow-browndispersion.ThisdispersionwassonicatedusingaFisherScientificFS60ultrasonicbathcleaner(150W)untilitbecameclearbly,ithasbeendemonstratedthattheelectricalconductiv-withnovisibleparticulatematter.Hydrazinehydrate(1.00mL,ityofGO(andpresumablyitsthermalstabilityaswell)can32.1mmol)wasthenaddedandthesolutionheatedinanoilbathatberestoredclosetothelevelofgraphitebychemicalreduc-100°Cunderawater-cooledcondenserfor24hoverwhichthereducedtion[33–36].SuchreductionsofGO,however,havenotGOgraduallyprecipitatedoutasablacksolid.Thisproductwasisolatedbeenstudiedingreatdetail.Tothatend,wehaveexaminedbyfiltrationoveramediumfrittedglassfunnel,washedcopiouslywithwater(5·100mL)andmethanol(5·100mL),anddriedonthefunnelthechemicalreductionofexfoliatedgrapheneoxidesheetsunderacontinuousairflowthroughthesolidproductcake.withseveralreducingagentsandfoundhydrazinehydrate(H2NNH2ÆH2O)tobethebestoneinproducingverythin2.3.Samplepreparationandmeasurementofelectricalconductivitygraphene-likesheets,consistentwithpreviousreports[31,32].High-resolutionscanningelectronmicroscopyTheelectricalconductivityofthegraphiticpowderswasmeasuredat(SEM)alsoprovideduswithevidenceofthinsheets.Heredifferentapparentdensities(createdbycompressingthesamplestovarying 1560S.Stankovichetal./Carbon45(2007)1558–1565degrees)byamethodsimilartothatdescribedintheliterature[37].Briefly,exfoliatedbytheultrasonictreatmentatconcentrationsagivenquantityofpowderwaspouredintoaglasstube(ID=5mm)andof1mg/mLinwateralwaysrevealedthepresenceofsheetsmanuallycompressedbetweentwocopperplungersthatfitcloselytothewithuniformthickness(1nm;anexampleisshownintubeID.ADCpowersupply(Agilent6613C,AgilentTechnologies,SantaClara,CA),picoamperemeter(Keithley6485,KeithleyInstruments,Fig.1).Thesewell-exfoliatedsamplesofGOcontainedCleveland,OH),andmultimeter(HP34401A,Hewlett–Packard,CA)nosheetseitherthickerorthinnerthan1nm,leadingtowereconnectedtomeasureDCconductivitybyatwo-probemethod.aconclusionthatcompleteexfoliationofGOdowntoindi-Adigitalmicrometer(MitutoyoCorporation,Kanagawa,Japan)wasusedvidualgrapheneoxidesheetswasindeedachievedundertomeasuretheheightofthepowdercolumnateachcompressionstep.theseconditions.Whileapristinegraphenesheetisatomi-Inthepresentwork,valuesforelectricalconductivityforpristinegraphite,GO,andreducedGO,weredeterminedbyfittingtheexperimen-callyflatwithawell-knownvanderWaalsthicknessofk0.34nm,grapheneoxidesheetsareexpectedtobetaldatatotheequation:rc=rh[(//c)/(1/c)],derivedfromthegeneraleffectivemedia(GEM)equation[38,39]withanassumptionthat‘thicker’duetothepresenceofcovalentlyboundoxygentheconductivityofthelow-conductivephase(air)iszero.Inthisexpres-andthedisplacementofthesp3-hybridizedcarbonatomssionrcistheconductivityofthecompositemedium,rhand/arethecon-slightlyaboveandbelowtheoriginalgrapheneplane.ductivityofthehighly-conductivephaseandtheirvolumefraction,respectively,/cisthepercolationthreshold,andkisacriticalexponentFromXRDexperiments,theintersheetdistanceforGOrelatedtothepercolationthresholdandtotheshapeoftheparticles.Thisvarieswiththeamountofabsorbedwater,withvaluessuchformulahasidenticalformasinthegeometricalpercolationmodel(GPM)as0.63nmand0.61nmreportedfor‘‘dry’’GOsamples[40],withtheexceptionofthecriticalexponent,k,whichmayhavevalues(completedryingofGOisprobablyimpossible[28])todifferentfromtheuniversalvalueof2asdeterminedforthe3-dimensional1.2nmforhydratedGO[43].IfthesevaluescouldbeGPM.ThetwofittingparametersintheGEMequationarerhandk.Theper-regardedasthe‘‘thickness’’ofahydratedindividualGOcolationthreshold,/c,isdeterminedasaratiooftheapparentpowderlayer,giventheuniformityoftheobservedthicknessesindensitybeforecompression,dp,andtheapparentdensityoftheparticles,ourGOmaterialsandthatsheetsone-half(oranyother3dg.Thebulkdensityofgraphite(2200kg/m)hasbeenusedinallcasesasinverseintegervalue,suchasonethird,etc.)ofthemini-avaluefordg.ThisassumptionhasbeenproventobecorrectwithinanmumthicknessobtainedbyAFMareneverobserved,theaccuracyof5%bymeasuringtheapparentdensityofpowdersamplesthatwerecompressedatapressureof300MPa.GOsheetsobservedbyAFMrepresentfullyexfoliatedgrapheneoxidesheets.(Wenotethatheightssignificantly3.Resultsanddiscussionsmallerthan1nmareobservableinourAFMexperiment;anexampleofwhichistheobservationofC12amine3.1.ExfoliationofGOinwateradsorbedontofreshlycleavedmicafromthevaporphase.ThismakesthemicahydrophobicinjustafewminutesandAnimportantpropertyofGO,broughtaboutbythewhenanewcleanAFMtipisbroughtintocontactwiththehydrophilicnatureoftheoxygenatedgraphenelayers,ismonolayer,someoftheC12aminewicksupthetipanditseasyexfoliationinaqueousmedia.Asaresult,GOread-depletesthemonolayer.Theresultingvoidsleftinthefilmilyformsstablecolloidalsuspensionsofthinsheetsinwaterarearound0.2nmdeep.Thus,ifplateletswiththicknesses[41,42].Afterasuitableultrasonictreatment,suchexfolia-thatarelessthanthoseobservedfortheGOsheetsamplestioncanproducestabledispersionsofverythingraphenediscussedhereinwerepresent,theywouldbereadilyoxidesheetsinwater[18,19]Thesesheetsare,however,dif-detected.)ferentfromgraphiticnanoplateletsorpristinegraphenesheetsduetotheirlowelectricalconductivity.3.2.ReductionofexfoliatedGOInourwork,sufficientlydilutecolloidalsuspensionsofGOpreparedwiththeaidofultrasoundareclear,homog-Duringthehydrazinereductionofgrapheneoxidesheetsenous,andstableindefinitely[18].AFMimagesofGOdispersedinwater,thebrown-coloreddispersionturnsFig.1.Anon-contactmodeAFMimageofexfoliatedGOsheetswiththreeheightprofilesacquiredindifferentlocations. S.Stankovichetal./Carbon45(2007)1558–15651561blackandthereducedsheetsaggregateandeventuallypre-350/g)3cipitate[18].Theprecipitationofthereducedsheetsoccurs,300NitrogenAdsorptionIsothermpresumablyduetotheirbecominglesshydrophilicasa250resultofoxygenremoval(seediscussionofelementalanal-desorption200ysis,NMR,andXPSbelow)andthusincreasedincompat-atSTP150ibilitywiththeaqueousmedium.Assolvationoftheadsorption100reducedsheetsdecreases,theintersheethydrophobicinter-50actionscausethemtoaggregateaswellasadheretotheAdsorptioncapacity(cm0hydrophobicsurfaceoftheTeflon-coatedstirbar.Allof00.20.40.60.81ourattemptstoredispersetheminwaterorobtainstableRelativepressuredispersionsofthereducedsheetsinanumberoforganicFig.3.Nitrogenadsorptionanddesorptionisothermsforreducedsolvents(dimethylformamide,tetrahydrofuran,toluene,exfoliatedGO.Theshapeoftheisothermindicatesthatthematerialchloroform,orchlorobenzene)failed.containsbothmicroandmesopores[53].3.3.CharacterizationofreducedGO3-dimensionalstructureofthesheets(seeabove)stillleavesmanyexposedsurfaces.SEMimagesrevealedthatthereducedGOmaterialcon-Elementalanalyses,coupledwithKarl-Fishertitration,sistsofrandomlyaggregated,thin,crumpledsheetscloselyshowanincreaseinC/Oatomicratiointhereducedmate-associatedwitheachotherandformingadisorderedsolidrial(10.3)comparedtothestartingGO(2.7)[18].Karl-(Fig.2a).Thefoldedregionsofthesheets(Fig.2b)wereFisheranalysiswasperformedasGOisahygroscopicfoundtohaveaveragewidthsof2nmbyhigh-resolutionmaterialandanywaterthatmightbetrappedbetweenSEM.Attheresolutionlimitofourinstrument(seecaption(oron)itslayerswouldcontributetoahigheroxygencon-ofFig.2),thesedataagainsuggest,butdonotprove,thetent(watercontentashighas25wt%hasbeenmeasuredinpresenceofindividualsheetsinourreducedGOmaterials.ourGOsamples).ThereducedGOhasamuchlowerwaterTheabsenceofchargingduringtheSEMimagingindi-content(2.8wt%),consistentwithitshydrophobicnature.catesthatthenetworkofgraphene-basedsheetsandtheHence,itcanbedescribedasconsistingofpartiallyoxi-individualsheetsareelectricallyconductive.Thisqualita-dizedgraphenenanoplatelets,giventhatsomeoxygenistiveconclusionwasfurtherconfirmedbyDCelectricalstillretainedafterreduction.Theblackcolorofthemeasurements(seebelow).reducedGOmaterialsalsosuggestsapartial‘re-graphitiza-SurfaceareameasurementofthereducedGOsheetsviation’oftheexfoliatedGO.Inadditiontothedecreasein2nitrogengasabsorptionyieldedaBETvalueof466m/gtheoxygenlevel,thereductionofGOisaccompaniedby(Fig.3).Thishighspecificsurfaceareaispartiallyanindi-somenitrogenincorporationfromthereducingagent(C/cationofthedegreeofGOexfoliationpriortothereduc-N=16.1byelementalanalysis),presumablythroughation.However,itisstilllowerthanthetheoreticalspecificreactionofhydrazinewiththecarbonylgroupsofGOsurfaceareaforcompletelyexfoliatedandisolatedgraph-(seebelow)[18].2enesheets(2,620m/g[20]),potentiallyduetotheAlthoughGOisthermallyunstableandstartstoloseagglomerationofthegrapheneoxidesheetsuponreduc-massuponheatingevenbelow100°C,themajormasslosstion.Whilethisagglomerationcanresultinthepartialoccursat200°C,presumablyduetopyrolysisoftheoverlappingandcoalescingofthereducedsheetsandlow-labileoxygen-containingfunctionalgroups,yieldingCO,ersthesurfaceareaofthebulkmaterials,thecrumpledCO2,andsteam[27,44].Hence,thethermaldecompositionFig.2.(a)AnSEMimageofaggregatedreducedGOsheets.(b)Aplatelethavinganupperboundthicknessatafoldof2nm.AttheresolutionlimitoftheFEGSEMusedhere,thefollowingarerelevantparametersofthemicroscopeandofthespecimen.Ofthemicroscope:spotsizeandfluxofe-beam(e-current,density,solidangle/aperturesize),energyoftheelectronsintheprimarybeam,electromagneticlensalignment,typeofdetectorsusedandtheiroperatingparameters.Ofthesample:geometry(suchasedges,roughness,thickness,andorientationwithrespecttoe-beam),material(suchasdensity,atomicnumber,electricalconductivity,andcomposition),substrateormatrix(whatthespecimenisaffixedtoorapartof).Thereisalsotheroleoftheoperatorexpertise.Thusfor(b)anddiscussionofameasuredvalueforthefoldthicknessofabout2nm,wesuggestaconfidencelimitofroughly±1nm. 1562S.Stankovichetal./Carbon45(2007)1558–1565ofGOcanbeaccompaniedbyavigorousreleaseofgas,Previously,wehaveemployedXPStoanalyzeGOandresultinginarapidthermalexpansionofthematerial.ThisthereducedexfoliatedGO[18].Inbrief,theC1sXPSspec-isevidentbybothalargevolumeexpansionandalargertrumofGO(Fig.6a)clearlyindicatesaconsiderablemassloss(fromflyingGOdebrisintheTGAinstrument)degreeofoxidationwithfourcomponentsthatcorrespondduringamorerapidheatingregime(Fig.4).Ontheothertocarbonatomsindifferentfunctionalgroups:thenon-hand,theremovalofthethermallylabileoxygenfunctionaloxygenatedringC,theCinC–Obonds,thecarbonylC,groupsbychemicalreductionresultsinmuchincreasedandthecarboxylatecarbon(O–C@O)[45,46].AlthoughthermalstabilityforthereducedGO.ApartfromaslighttheC1sXPSspectrumofthereducedexfoliatedGOmasslossbelow100°C,whichcanbeattributedtotheloss(Fig.6b)alsoexhibitsthesesameoxygenfunctionalities,ofadsorbedwater,nosignificantmasslossisdetectedwhentheirpeakintensitiesaremuchsmallerthanthoseinGO.thismaterialisheatedupto800°C.Inaddition,thereisanadditionalcomponentat285.9eV13TheCMASNMRspectraofGOandthereducedGOcorrespondingtoCboundtonitrogen[46].Theseobserva-indicatesignificantstructuralchangeinducedbythereduc-tionsareconsistentwiththeelementalanalysisdata(seetion.InthespectrumofGO(Fig.5),thepeaksat57andabove)andagainindicatebothconsiderablede-oxy-1368ppmrepresenttheCnucleiintheepoxideandhydro-genationbythereductionprocessaswellasnitrogenxylgroups,respectively[24–27].Theresonanceat130ppmincorporation.2belongstotheun-oxidizedspcarbonsofthegraphenenet-Thesignificantstructuralchangesoccurringduringtheworkandthatat188ppmpresumablyarisesfromthecar-chemicalprocessingfrompristinegraphitetoGO,andthen13bonylgroups.IntheCspectrumofthereducedGO,thetothereducedGO,arealsoreflectedintheirRamanspec-peaksfromtheoxygenatedandthecarbonylcarbonsaretra(Fig.7).TheRamanspectrumofthepristinegraphite,absent.Theremainingprominentfeatureistheresonanceasexpected,displaysaprominentGpeakastheonlyfea-1at117ppmthatisbroadenedbychemicalshiftdistribu-tureat1581cm,correspondingtothefirst-orderscatter-tionandcorrespondstovariationsofcarbonatomingoftheE2gmode[47].IntheRamanspectrumofGO,1environments.theGbandisbroadenedandshiftedto1594cm.Inaddi-1tion,theDbandat1363cmbecomesprominent,indicat-2ingthereductioninsizeofthein-planespdomains,1.0possiblyduetotheextensiveoxidation.TheRamanspec-ReducedGOtrumofthereducedGOalsocontainsbothGandDbands10.8(at1584and1352cm,respectively);however,withanincreasedD/GintensityratiocomparedtothatinGO.This2changesuggestsadecreaseintheaveragesizeofthesp0.6GO1oC/mindomainsuponreductionoftheexfoliatedGO[47],andcanbeexplainedifnewgraphiticdomainswerecreated0.4thataresmallerinsizetotheonespresentinGObeforereduction,butmorenumerousinnumber.NormalizedMassRemaining0.2oGO5C/minaC=O0.0C-O(287.8ev)(286.2ev)0100200300400500600700800900C-C(284.8ev)oTemperature(C)C(O)O(289.0ev)Fig.4.NormalizedTGAplotsforGOandthereducedGO.Thedownwardslopesareduetonormalinstrumentdrift.bC-C(284.5ev)GOC-OC-N(286.5ev)(285.9ev)C(O)OC=OReducedGO(289.1ev)(287.8ev)280282284286288290292294250200150100500-50Chemicalshift(ppm,TMSscale)BindingEnergy(eV)13Fig.5.Solid-stateCMASNMRspectraofGO(top)andreducedFig.6.TheC1sXPSspectraof:(a)GO,(b)reducedGO.AdaptedfromexfoliatedGO(bottom).reference[18]. S.Stankovichetal./Carbon45(2007)1558–15651563Table1FittingparametersdescribingpercolationtransportforthethreegraphiticpowdersshowninFig.8Pristinegraphite(SP-1)GOReducedGO/c=dp/dg0.460.380.055rh[S/m]2500±150.0206±0.0022420±200k0.56±0.031.6±0.050.96±0.02over-estimatedduetothelimiteddatarangeavailableasaresultofthelowcompressibilityandpossiblespatialvari-ationsinthedensityofthematerial.MorereliablearetheconductivitydataforthereducedGO,whichcanbemea-suredathigherpowdercompression.Thisconductivity2(2·10S/m)isabout5ordersofmagnitudebetterthantheconductivityofGO,andcloselyapproachesthatofpristinegraphite(approximately10timeslowerthantheconductivityofpristinegraphitepowderatonlyabout10%ofthebulkgraphitedensity).Suchhighintrinsiccon-ductivityofthereducedGOinspiteofthepresenceofoxy-gen(potentialacceptorsites)mightbeanindicationof:(i)quiteconductiveparticle-to-particleinterfaces,whichoftendeterminestheoverallpercolationconductivity;(ii)averydenseconductivenetworkwithmanycross-linkedconnec-tions;and(iii)possibleionicchannelsofchargetransferacrossthesample.Thislastmechanismisdefinitelyadom-inantoneintheelectricalconductionofGOpowderanda0500100015002000reasonforitshighsensitivitytotemperatureandenviron--1mentalhumidity.Incontrast,theconductivityoftheRamanShift(cm)reducedGOisbasedprimarilyoninter-particlecontactsFig.7.TheRamanspectraofSP-1graphite(top),GO(middle),andtheandismuchlesssensitivetoenvironmentalconditions.reducedGO(bottom).Westressherethattheaforementionedexperimentallydeterminedconductivitiesgreatlydependonthemeasure-Todeterminetheextenttowhichthechemicalreductionmentparameters,fromtheheightandcross-sectionalareaofexfoliatedGOrestorestheelectricalpropertiesoftheofthesampledcolumnandtheappliedpressuretothepar-graphiticnetwork,wemeasuredtheroom-temperatureticlesurfaceareaandaspectratio,environmentaltempera-electricalconductivitiesofcompressed-powdersamplesofture,andhumidity.Thelasttwoareespeciallyimportantinthepristinegraphite,GO,andthereducedGOat30%rel-measurementsofGOconductivityduetoitshydrophilicativehumidity(Fig.8).Theresultsofthefitstotheexper-nature.ItisthereforenotstraightforwardtoobtainanimentaldataaresummarisedinTable1.Theextrapolated‘absolutevalue’ofpowderconductivitysothatourresultsintrinsicconductivity,rh,ofthereducedGOisprobablycouldbecomparedwithothersreportedintheliterature.4103.4.Possiblemechanismsforthechemicalreduction310ofGObyhydrazineParentgraphite2,(S/m)10σcWhileresultsfrombothelementalanalysisandXPS1ReducedGOclearlyshowthatreductionoftheexfoliatedGOresults1013inconsiderableremovalofoxygen(seeabove),ourC-310MASNMRdataadditionallysuggestthatthereduction/GOConductivityde-oxygenationofGOalsoresultsinsignificantrestoration-4102ofthespcarbonsites.Thesameconclusioncanbereached-5throughtheelectricalconductivitymeasurementssincethe100.00.10.40.50.60.70.8observedincreaseinconductivityuponreductionofGOVolumefractionφrequiresthatconductivepathwaysofconjugatedcarbonFig.8.Conductivityasafunctionofthevolumefractionofthreedifferentatomsbere-established.However,itisnotchemicallygraphiticpowders:pristinegraphite(SP-1),GO,andreducedGO.obvioushowtreatmentofGOwithhydrazinecanleadto 1564S.Stankovichetal./Carbon45(2007)1558–1565deoxygenationandtheincreaseinunsaturationthattroscopicexperimentsduringthereduction,arenecessaryaccompaniesre-graphitization(seebelow).todeconvolutethechemicalpathwaysofGOreduction.IftheLerf–Klinowskimodel(seeabove)isassumedtobecorrect,mostoftheoxygenfunctionalitiesinGOshould4.Conclusionbepresentintheformofeitherhydroxylorepoxidegroups[24–27].Additionally,GOisbelievedtoalsocontainaInconclusion,reductionofexfoliatedgrapheneoxidenumberofcarbonyl-containingoxygenfunctionalitiessuchsheetsinwaterwithhydrazineresultsinamaterialwithaslactones,anhydrides,andquinones.Thepresenceofgraphiticcharacteristicsthatarecomparabletothoseoftheselattermoietiescanbeusedtoexplaintheincorpora-pristinegraphite.Onthenanoscale,thiscarbon-basedtionofnitrogenintothereducedGO,sincehydrazinematerialconsistsofthingraphene-basedsheetsandpos-canreactwithanhydridesandlactonestoformhydrazidessessesahighspecificsurfacearea.Thecharacterizationofandwithquinonestoformhydrazones[48].However,onlythereducedGOindicatesthatthehydrazinetreatmenthydrazoneformationresultsintheremovalofoxygen.Fur-resultsintheformationofunsaturatedandconjugatedcar-therreductionofthehydrazonetoyieldadeoxygenatedbonatoms,whichinturnimpartselectricalconductivity.2sp-carboncanalsooccurifitissituatedadjacenttoanAssuch,reducedgrapheneoxidesheetsmayfinduseinaepoxide(Whartonreaction)[49].ThepossibilityofavarietyofapplicationssuchashydrogenstorageandasWolff–Kishner-typereductionofthecarbonylgroupsanelectricallyconductivefillermaterialincomposites.seemsunlikelygiventhelowtemperatureandlowbasicityofthereactionmedia.Nevertheless,thesethreeprocessesAcknowledgementscannotaccountforalltheremovedoxygenduringhydra-zinereductionascarbonylgroupsarenotbelievedtobeSupportfromNASA(Award#NCC-1-02037)throughthedominantoxygenfunctionalitiesinGO.Thekeyques-theUniversityResearch,EngineeringandTechnologytion,therefore,ishowoxygenisremovedfromtheepox-Institute(URETI)onBio-inspiredMaterials(BiMat)isidesandhydroxyls.appreciated.WeacknowledgetheuseofinstrumentsinHydrazineisknowntoreadilyring-openepoxidesandtheNorthwesternNUANCE(supportedbyNSF-NSEC,formhydrazinoalcohols[50].Thoughsuchareactionpath-NSF-MRSEC,KeckFoundation,thestateofIllinois,wayispossibleinGOuponhydrazinetreatment,itcannotandNorthwesternUniv.)facility.bethedominantonesincering-openingoftheepoxideswithhydrazineinitselfwouldnotresultinanyoxygenReferencesremoval.Further,theepoxidesinGOarepresumablyqua-ternaryandthusarenotsusceptibletoadirectnucleophilic[1]DrzalLT,FukushimaH.Graphitenanoplateletsasreinforcementsattackduetostericreasons.Finally,themodestextentofforpolymers.PolymPrepr(AmChemSoc,DivPolymChem)observednitrogenincorporationalsorendersthispathway2001;42(2):42–3.unlikelytobetheonlymodeofepoxidereactioninGO.It[2]DrzalLT,FukushimaH.Exfoliatedgraphiteasanano-reinforcementforpolymers.IntSAMPESympEx2003:1635–42.ispossible,however,thattheinitialderivativeproducedby[3]FukushimaH,DrzalLT.Acarbonnanotubealternative:Graphitetheepoxideopeningwithhydrazinereactsfurtherviathenanoplateletsasreinforcementsforpolymers.AnnTechConf–SocformationofanaminoaziridinemoietywhichwouldthenPlastEng2003:2230–4.undergothermaleliminationofdiimidetoformadouble[4]KellyBT.Physicsofgraphite.London:AppliedScience;1981.bond(Fig.9)[51,52].Suchareactionmightbefurtherdri-[5]KotovNA.Materialsscience:Carbonsheetsolutions.Nature2006;442(7100):254–5.veninGObyre-establishmentoftheconjugatedgraphene[6]ChenG,WuD,WengW,YanW.Dispersionofgraphitenanosheetsnetwork.inapolymermatrixandtheconductingpropertyofthenanocom-Ultimately,ouranalysisstillleavesopenthequestionofposites.PolymEngSci2001;41(12):2148–54.how,orevenif,thehydroxyloxygenisremoved.Theanswer[7]ChenG,WuD,WengW,WuC.Exfoliationofgraphiteflakeanditstothisintriguingqueryconstitutesafertileresearchareananocomposites.Carbon2003;41(3):619–21.[8]ChenG,WengW,WuD,WuC.PMMA/graphitenanosheetsthatawaitsfurtherinvestigationsbybothexperimenterscompositeanditsconductingproperties.EurPolymJandtheorists.AtheorythatconfirmstheLerf–Klinowski2003;39(12):2329–35.modelofGOandlendssupporttotheNMR-evaluated[9]ChenG,WuC,WengW,WuD,YanW.Preparationofpolystyrene/proportionsoftetra-substitutedepoxidemoietiesvsothergraphitenanosheetcomposite.Polymer2003;44(6):1781–4.oxygen-functionalities,beforeandafterreduction,would[10]ChenG,WengW,WuD,WuC,LuJ,WangP,etal.Preparationandcharacterizationofgraphitenanosheetsfromultrasonicpowder-beasignificantadvancement.Inaddition,thoroughiso-ingtechnique.Carbon2004;42(4):753–9.tope-labelingstudies,coupledtosophisticatedin-situspec-[11]KwonO,ChoiS,ParkK,KwonY.Thepreparationofexfoliatedgraphitebyusingmicrowave.JIndEngChem(Seoul,RepubKorea)OHO2003;9(6):743–7.+H2NNH2[12]WengW-G,ChenG-H,WuD-J,YanW-L.HDPE/expandedNH-H2ONNH-N2H2HN2graphiteelectricallyconductingcomposite.ComposInterf22004;11(2):131–43.Fig.9.Aproposedreactionpathwayforepoxidereductionwith[13]WengW,ChenG,WuD,ChenX,LuJ,WangP.Fabricationandhydrazine.characterizationofnylon6/foliatedgraphiteelectricallyconducting 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