Synthesis-and-exfoliation-of-isocyanate-treated-gr

Synthesisandexfoliationofisocyanate-treatedgrapheneoxidenanoplateletsSashaStankovicha,RichardD.Pinera,SonBinhT.Nguyenb,*,RodneyS.Ruoffa,*aDepartmentofMechanicalEngineering,NorthwesternUniversity,2145SheridanRoad,Evanston,IL60208-3133,USAbDepartmentofChemistry,2145SheridanRoad,Evanston,IL60208-3113,USAReceived17January2006;accepted1June2006Availableonline4August2006AbstractAnumberoffunctionalizedgraphiteoxideswerepreparedbytreatmentofgraphiteoxide(GO)withorganicisocyanates.Theseiso-cyanate-treatedGOs(iGOs)canthenbeexfoliatedintofunctionalizedgrapheneoxidenanoplateletsthatcanformastabledispersioninpolaraproticsolvents.CharacterizationofiGOsbyFT-IRspectroscopyandelementalanalysissuggestedthattheisocyanatetreatmentresultsinthefunctionalizationofthecarboxylandhydroxylgroupsinGOviaformationofamidesandcarbamateesters,respectively.ThedegreeofGOfunctionalizationcanbecontrolledviaeitherthereactivityoftheisocyanateorthereactiontime.Whenusedwithfunctionalizedisocyanates,thedescribedmethodologyallowsfortheelaborationofgrapheneoxidenanoplateletswithdifferentsurfacefunctionalgroups.2006ElsevierLtd.Allrightsreserved.Keywords:Graphiteoxide;Chemicaltreatment;Atomicforcemicroscopy;Infraredspectroscopy;Surfaceproperties1.IntroductionGraphiteoxide(GO),anoxygen-richcarbonaceouslay-eredmaterial,isproducedbythecontrolledoxidationofgraphite[1–3].EachlayerofGOisessentiallyanoxidizedgraphenesheetcommonlyreferredtoasgrapheneoxide[4].Basedonrecentstudies[5–10],GOconsistsofintactgra-phiticregionsinterspersedwithsp3-hybridizedcarbonscontaininghydroxylandepoxidefunctionalgroupsonthetopandbottomsurfacesofeachsheetandsp2-hybrid-izedcarbonscontainingcarboxylandcarbonylgroupsmostlyatthesheetedges.Hence,GOishydrophilicandreadilydispersesinwatertoformstablecolloidalsuspen-sions[4,11–13].Wehavelongbeeninterestedindevelopingandusinggraphene-basednanoplateletsasawell-dispersedfillerforabroadrangeofpolymercomposites.Assuch,wesuspectthatGOwouldserveasanexcellentprecursortoavarietyofgraphene-basednanofillersifitcouldbecompletelyexfoliatedinawiderangeofmedia.Inourhands,GOreadilyexfoliatesinwaterunderamildultrasonictreat-menttoproducetransparentdispersionsthatarestableforatleastmonthsatconcentrationsof1mg/mL[14].Atomicforcemicroscopy(AFM)showsthatthesedisper-sionscontaingraphiteoxidenanoplateletsof1nmuni-formthickness[14].However,sinceGOishydrophilic,itsdirectexfoliationintonon-aqueoussolventsisnotfavored.ThisbehaviorcanpresumablybeattributedtothestronginterlayerhydrogenbondsbetweentheoxygenfunctionalgroupsofadjacentgrapheneoxidelayersinGO.Unlikewater,organicsolventsareunabletopenetratetheinter-layerspacesofGOanddisruptthesehydrogenbonds,whichinturnpreventstheirexfoliation.Ifthedensityofthehydrogenbonddonorgroups,suchashydroxyls,isdecreasedviachemicalfunctionalization,thegrapheneoxidelayerswouldbecomelesshydrophilicandthestrengthofinterlayerhydrogenbondingwillbeattenuated,0008-6223/$-seefrontmatter2006ElsevierLtd.Allrightsreserved.doi:10.1016/j.carbon.2006.06.004*Correspondingauthors.Tel.:[phone];fax:[phone](S.T.Nguyen),fax:[phone](R.S.Ruoff).E-mailaddresses:[e-mail](S.T.Nguyen),r-ruoff@northwestern.edu(R.S.Ruoff).(2006)3342–3347possiblyallowingforexfoliationinorganicsolvents.SuchfunctionalizationofgraphiteoxidehasbeenshowntobepossiblebyLerfetal.,whopreparedandstudiedanumberofchemicallymodifiedgraphiteoxidederivatives[7].Inaddition,Matsuoetal.successfullycarriedoutthesilyla-tionofbutylamine-intercalatedgraphiteoxide[15,16].However,theexfoliationbehaviorofsuchchemicallymod-ifiedgraphiteoxideshasneverbeenstudied.WereporthereinthesynthesisofchemicallymodifiedGOderivativesviatreatmentofgraphiteoxidewitharylandalkylisocyanates.Wealsodemonstrate,forthefirsttime,thatsuchchemicaltreatmentdramaticallyalterstheexfoliationbehaviorofgraphiteoxideandallowsforthecompleteexfoliationofGOintoindividualchemicallyderivatizedgrapheneoxidesheetstobeachievedinorganicsolvents.2.Experimental2.1.MaterialsandmethodsGraphiteoxidewaspreparedfrompurifiednaturalgraphite(SP-1,BayCarbon,Michigan,averageparticlesize30lm)bytheHummers[2]methodanddriedforaweekoverphosphoruspentoxideinavacuumdes-iccatorbeforeuse.4-Isocyanatobenzenesulfonylazidewaspreparedfrom4-carboxybenzenesulfonylazideviaapublishedprocedure[17].AllotherisocyanateswerepurchasedfromSigma–Aldrichandusedasreceived.AllsolventswereHPLCgradeandwerepurchasedfromFisherScientific.AnhydrousN,N-dimethylformamide(DMF)wasobtainedfromaDow-GrubbssolventsysteminstalledbyGlassContours(LagunaBeach,CA)[18].AllmanipulationswiththissolventwereperformedusingstandardSchlenktechniques.AFisherScientificFS60ultrasonicbathcleaner(150W)wasusedinthepreparationofstabledispersionsoffunctionalizedgraphiteoxides.AFMimagesweretakenonanAutoProbeCP/MTScanningProbeMicroscope(MultiTask;VeecoInstruments).Imagingwasdoneinnon-contactmodeusingaV-shape‘‘Ultralever’’probeB(ParkScientificInstruments,B-dopedSiwithfrequencyfc=78.6kHz,springconstantsk=2.0–3.8N/m,andnominaltipradiusr=10nm).Allimageswerecol-lectedunder