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US 8,696,938B2 34 staticrepulsion.TheresultingGOlayersformedastable dispersioninwater.AccordingtoHoriuchi,etal. 14. single-layergraphenewasdetected. (d)Itmaybenotedthattheapproachofusingelectrostatic repulsiontoseparategrapheneoxidelayerswaspursued 5 anisocyanatetreatment.However,unlesstabilizedby Selectedpolymers,thechemicallymodifiedgraphene shetsobtainedthroughthesemethodstendtoprecipi tateasireversibleagglomeratesduetotheirhydropho bicnature.Theresultingagglomeratesbecameinsoluble inwaterandorganicsolvents. (n)Lietal.34 overcamethisisuebyusingammoniumto adjustthepHvalueofadispersionofchemicallymodi fiedgrapheneshetsinwater,whichservedtomaximize thechargedensityontheresultinggrapheneshets.The resultingelectrostaticforcesactedtostabilizetheaque ousSuspension. (o)SiandSamulski 35 reportedachemicalrouteto aqueoussolutionsofisolatedgrapheneshetsbyreduc inggrapheneoxideinthresteps.(1)pre-reductionof grapheneoxidewithsodiumborohydrideat80°C.for1 htoremovethemajorityoftheoxygenfunctionality;(2) sulfonationwiththearyldiazoniumsaltofsulfanilic acidinanicebathfor2h;and(3)post-reductionwith hydrazine(100°C.for24h)toremoveanyremaining oxygenfunctionality.ThelightlySulfonatedgraphene canbereadilydispersedinwateratreasonableconcen trations (2 mg/mL) in the pH range of 3-10. Isolated grapheneshetspersistinthemixtureofwaterand organicsolventsincludingmethanol,acetone,acetoni trile,thusmakingitpossibletofurthermodifyitssurface foraplicationssuchasreinforcementsincomposites. Thisisaverytediousproces,nevertheles. (p)AnotherverytediousprocessforthepreparationofGO nanoshets,proposedbyBeceril,etal.46,entailed (1)intercalating-oxidizinggraphitewithasolutionof NaNO,andKMnO,inconcentratedHSO for5days; (2)washingtheoxidizedgraphitewith5wt.%HSO in waterandreactingthewashedoxidizedgraphitewitha 30wt.%aqueoussolutionofHO tocompletethe oxidation; (3) removing inorganic anions and other impuritiesthrough15washingcyclesthatincludedcen trifugation,discardingSupernatantliquid,andre-sus pendingtheSolidinanaqueousmixtureof3wt.% HSO and0.5wt.%H2O,usingstiringandultrasoni cation;(4)carryingoutanothersetofcentrifugationand washingproceduresthretimesusing3wt% HClin waterasthedispersionmediumandthenonemoretime usingpurifiedwatertore-suspendthesolid;(5)passing thisSuspensionthroughaweakbasicion-exchangeresin toremoveremainingacid;and(6)dryingtheSuspension toobtainapowder. earlierin1998byLiuandGong 18,asafirststepin theirattempttosynthesizepolyaniline-intercalatedGO. Ina3-Dgraphitecrystal,theinter-layerspacing(L)is 0.335nm,whichisknowntoincreaseto0.6-1.1nmif graphiteisoxidizedtoproduceGO.Further,GO is10 hydrophilicandcanbereadilydispersedinaqueous Solution. (e)Dekanyetal. 19 observedthattheinter-graphene spacinginGOwasincreasedtoL1.23nmwhenGO particlesweredispersedin0.05NNaOHsolution.When 15 dispersedina0.01N NaOH solution,thespacingwas esentialyinfinite,likelyimplyingthatGO wascom pletelyexfoliatedtobecomeadisorderedstructure. (f)Chenetal. 20 exposedGO toatemperatureof1,050° C.for15secondstoobtainexfoliatedgraphite,which wasthenSubjectedtoultrasoniciradiationinamixture solutionofwaterandalcohol. (g)Jangetal.21 thermallyexpandedGICorgraphite oxidetoproduceexfoliatedgraphiteandSubjectedexfo liatedgraphitetomechanicalshearingtreatments,such asbalmiling,toobtainpartialyoxidizedNGPs. (h)Thermalexfoliationasawayofproducingnano-struc turedgraphitewasalsoattemptedbyPetrik22. (i)Thermalexfoliationofintercalatedgraphiteorgraphite oxidewasconductedbyDrzaletal.23 usingmicro waves as a heat source. Microwave energy induced graphiteexfoliationwasdisclosedearlierbyKwon,etal. 24. (i)Aksay,McAlister,andco-workers 7-9,45 alsoused thermalexfoliationofGO toobtainexfoliatedgraphite oxideplatelets,whichwerefoundtocontainahigh proportionofsingle-layergrapheneoxideshets,based ontheBETmethodwithnitrogengasadsorptioninthe drystateandinanethanolsuspensionwithmethylene bluedyeasaprobe. (k)Severalpolarorganiccompoundsandpolymershave been intercalated into inter-graphene or inter-flake spacestoformintercalatedorexfoliatedGO nanocom positese.g.,25.Partialreductionofapolymer-GOtoa polymer-graphenenanocompositealsocouldbeaccom plishedelectrochemicallyorchemically 18.26. 25 30 35 40 45 (1)Preparationofultra-thinfilmsbyalayer-by-layerself assemblyapproachfromGOnanoplateletsandpolymer electrolytesalsohasbeeninvestigated27-31. Approach2:DirectFormationofPristineNanoGraphene Althoughtheoriginalintentofthesestudieswasprima50 rily to fabricate self-assembled GO-poly (ethylene oxide)nanocomposites,theirfirststepalmostalways involvedexfoliationandseparationofGOplatelets.This wasevidencedbytheX-raydifractiondataoftheresult ingstructuresthatshowedcompletedisappearanceof55 thosedifractionpeakscorrespondingtographiteoxide orpristinegraphite27.29. Platelets (q)Withoutgoingthroughachemicalintercalationroute, Mazurkiewicz. 36 claimedtohaveproducedgraphite nanoplateletshavinganaveragethicknessintherange of1-100nm throughhigh-pressuremilingofnatural flakegraphite.However,noevidencewaspresented36 toshowthattrulythinplatelets(e.g.,those<10nmin thicknes)wereproduced. (r)Shioyama 37 preparedapotassium-intercalatedGIC fromhighlyorientedpyrolyticgraphite(HOPG),initi atedinsitupolymerizationofisopreneorstyreneinthe inter-graphenespaces,andthenthermallydecomposed inter-graphenepolymerchainsatahightemperature (500-1,000°C.).Thevolatilegasmoleculesservedto exfoliategraphitelayers,and,aftertheVolatilegas escaped,isolatedgrapheneshetswereobtained.Unfor tunately,Shioyamadidnotdiscusthethicknesofthe isolatedgrapheneshets. (m)Stankovichetal.32 followedtheapproachesof Hirataetal.16toproduceanddispersegraphiteoxide shetsinwatertoobtainstablecoloidaldispersions. Thegraphiteoxidedispersionwasthenreducedwith hydrazine,aprocedurepreviouslyusedbyLiuandGong earlier18,butinthepresenceofpoly(sodium4-sty renesulfonate).Thisprocessledtotheformationofa stableaqueousdispersionofpolymer-coatedgraphene platelets.Stankovichetal.33 furtherdevelopeda methodtoproduceleshydrophilicGOplateletsusing 60 65PDF Image | SCO2 Producing Nanographine Platelets
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