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Graphene-basednanomaterialsandtheirelectrochemistryMartinPumera2ndMarch2010ChemicalSocietyReviews1.Introduction1.Grapheneisatwo-dimensionalsheetofsp2bondedcarbonatoms,whichcanbeviewedasanextra-largepolycyclicaromaticmolecule.2.Grapheneisa‘‘new’’classofcarbonmaterialandwasonlydiscoveredin2004.3.Suchtwo-dimensionalcarbonsheetspossessuniqueproperties,namelyballisticconductivity,highelasticity,veryhighmechanicalstrength,highsurfacearea,andrapidheterogeneouselectrontransfer.2.Structureofgraphene-basednanomaterials(i)z-AxisconstrainedgraphenenanomaterialsASingle-layergraphenesheet:azerobandgapsemiconductor.BDouble-layergraphene:Thishasazerobandgapinitsnativestatebutcanbeopenedbyapplyinganelectricfieldaxiallytowardthebilayer.CFew-layergraphene(fewerthan10graphenelayers):Suchgrapheneismetallic.DGrapheneplatelets(10–100graphenelayers;B3–30nmthick):Suchgrapheneplateletssmallpiecesofhighlyorderedpyrolyticgraphite(HOPG)becausetheirelectronicpropertiesarethesameasthoseofHOPG.(ii)x–y–zAxesconstrainedgraphenenanomaterials(a)Single-layergraphenenanoribbonstheelectronicstructureofnanoribbonsisdefinedbyquantumconfinement.‘‘Zigzag’’graphenenanoribbonsaremetallicwhile‘‘armchair’’nanoribbonscanbemetallicorsemiconducting.(b)Few-layergraphenenanoribbons:suchnanoribbonshaveamorecomplexelectronicstructureandaretypicallymetallicconductors.(c)StackedgrapheneplateletnanofibersSuchnanofiberspossessanexceptionallyhighratioofexposedgrapheneedgesitesvs.basalplanesites.Sincetheedgesofthegraphenesheetsaretheelectroactivesitessuchstackedgrapheneplateletnanofibersshowsuperiorelectrochemicalperformance(iii)Dimensionallyunconstrainedgraphenematerials(a)Graphite,asamultilayermaterialconsistingofindividualgraphenesheets.(b)Graphiteoxidewasprepared150yearsagobyBrodieandconsistsofheavilyoxidizedgraphenesheets.Graphiteoxideisaprecursorforthefabricationofgrapheneviagrapheneoxide.theedgesofgraphenesheetsoftencontainoxygen-containinggroups,whichinsomecasesareelectrocatalytic.Inothercases,theyinhibitthereactionorevenadsorbtheproductsofelectrochemicalreaction,whichleadstopassivationoftheelectrodes.3.Preparationofgraphene-basednanomaterials(i)physicalmethods(a)thehistoricallyfirstmethodbasedonpeelingHOPGlayerwithcellophanetapeuntilasinglelayerofgrapheneisobtained(b)epitaxialgrowthofgraphenelayersonasubstrate(c)chemicalvapordeposition(CVD)ofmultilayeredgraphene(d)etchingmultiwalledcarbonnanotubes(MWCNTs)inordertopreparegraphenenanoribbons,whichhavewell-defineddimensions.(ii)Chemicalmethods(a)chemicaloxidationofgraphitetographiteoxideinconcentratednitricacidandconsequentthermaldecompositionofthegraphiteoxideinaninertatmosphere.Thermaldecompositionshouldbecarriedoutin‘‘thermalshock’’conditionstoachieverapidevolutionofgasesandseparationofthegrapheneoxidesheets.Theresultinggrapheneoxideisfurtherultrasonicatedandoftenchemicallyreduced(b)intercalationofsmallmoleculesbetweenthegraphenesheetswithinthegraphiteandconsequentultrasoundseparationofthelayers.(c)intercalationoflithiumionsintoMWCNTsorgraphitestructure,whichcausesanincreaseintheinterlayerspacingofMWCNTsbydoubleandconsequentrupture.Thisresultsinthecreationofgraphenenanoribbons.(d)tocreatingmultilayergraphenenanoribbonsisto‘‘unzip’’MWCNTsbypermanganateoxidation.4.Materialschemistryofgraphene•Graphene-basednanomaterialscanbereadilyoxidized,eitherchemicallyorelectrochemically.Theirpropertiesstronglydependontheirstructureandonthenumberandpositionoftheoxygen-containinggroups.(a)Surfacespecies(b)foldedcarbonskeletonRepresentationofthegrapheneoxidereductionprocedureoxygen-containinggroupsongraphenesheetsinfluncetheadsorption/desorptionofproductsofelectrochemicalreactionsfromgrapheneelectrodesurfaces.5.Electrontransferatgraphenesheets•Therehavebeennumerousapproachestoestablishheterogeneouselectrontransferfromgraphenesheets,usuallyusinghighlyorderedpyrolyticgraphite(HOPG).HOPGhasamultilayergraphenestructurewithminimumdefects.6.InherentelectrochemistryofgraphenenanomaterialsGrapheneoxide(GO)canbereducedbyelectrochemicalmeans.Cyclicvoltammetryofgrapheneoxidesheetsexhibitssignificantreductionwavesstartingat-0.60V(vs.Ag/AgClreferenceelectrode),reachingamaximumat-0.87V.TheelectrochemicalreductionprocessispHdependent,suggestingthefollowingmechanismofreduction:GO+aH++be-→ER-GO+cH2O.Ithasbeenshownthatthecapacitanceofgraphenenano-materialsdependsonthenumberoflayer7.SpectroelectrochemistrymultilayeredgrapheneistransparentintheVisregionandtheirtransparencyintheUVregion8.Electrochemicalapplicationsofgraphene-basednanomaterialsSurfacearea2630m2/gbiosensorCR-GOexhibitsasignificantlylargerelectrochemicalresponsetowardstheoxidationoftheseprobesthandographiteorGCelectrodesGOsupportstheefficientelectricalwiringoftheredoxcentersofseveralheme-containingmetalloproteinstotheelectrode.Significantly,proteinsretaintheirstructuralintegrityandbiologicalactivityuponformingmixtureswithGO.Stackedgrapheneplateletnanofibers(PGNFs)9.Conclusions•theroleofthestructureofgrapheneoxideonitselectro-chemistry•theroleofthespecificoxygen-containinggroupsatt