石墨烯纳米材料在电化学中的应用

TUTORIALREVIEWwww.rsc.org/csr|ChemicalSocietyReviews

Graphene-basednanomaterialsandtheirelectrochemistry

MartinPumera*

Received2ndMarch2010DOI:10.1039/c002690p

Graphene-basednanomaterialsareintheforefrontofchemicalresearch.Thistutorialreviewprovidesanintroductiontotheirelectrochemistry,itsfundamentalsandapplications.Selectedexamplesofapplicationsinenergystorageandsensingarepresented.Thesyntheticmethodsforpreparinggraphenesaswellastheirmaterialschemistryarethoroughlydiscussed,astheyhaveaprofoundinfluenceontheelectronicandelectrochemicalbehaviorofgraphene-related

nanomaterials.Inherentelectrochemistryandspectroelectrochemistryofgraphenenanomaterialsisdiscussedthoroughly.Importantapplicationinsensingandenergystorageareasarehighlighted.

Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690P1.Introduction

Grapheneisatwo-dimensionalsheetofsp2bondedcarbonatoms,whichcanbeviewedasanextra-largepolycyclicaromaticmolecule.Whilegraphenesheetshavebeenknowntobecomponentsoftraditionalcarbonmaterials,suchasgraphite,orcomponentsofa‘‘new’’classofcarbonmaterials,suchascarbonnanotubes,therouteleadingtothepreparationofsingle-layerplanargraphenesheetsofatomicthicknesswasonlydiscoveredin2004.1Suchtwo-dimensionalcarbonsheetspossessuniqueproperties,namelyballisticconductivity,highelasticity,veryhighmechanicalstrength,highsurfacearea,andrapidheterogeneouselectrontransfer.2Indeed,notjustdoone-layergraphenematerialshaveinterestingproperties,multilayeredgraphenenanostructuresareequallyinterestingandworthyofinvestigation,aswewillshowbelow.

DivisionofChemistry&BiologicalChemistry,SchoolofPhysicalandMathematicalSciences,NanyangTechnologicalUniversity,

21NanyangLink,Singapore637371.E-mail:pumera@ntu.edu.sg,martin.pumera@gmail.com;Fax:+656791-1961

Electrochemistryisadisciplineinwhichchemicalcompositioncanbemanipulatedbyapplyingelectronflowor,viceversa,electronflowcanbegeneratedfromchangingchemicalcomposition.Electrochemicalreactionhappensonthesurfacesofmaterials,whichwecallelectrodes.Electrochemistryhasawiderangeofapplications,rangingfromlaboratorytoindustrialscale,fromdetectionsciencetoenergystorage.3Whileelectrochemistryitselfasascientificdisciplineiscenturiesold,thequestfor‘‘better’’electrodematerialssuitableforthepresentneedsofscienceandsocietyisunlimited.Theword‘‘better’’inthiscontexthasavarietyofmeanings,rangingfromlighter,higherenergystoragecapacity,higherselectivity,highersensitivity,lessbrittle,non-poisonous,pronetopassivation,easilyprocessable,scalable,etc.Lessthanadecadeago,carbonnanotubes(CNTs)emergedasinterestingelectrodematerials.4Researchontheelectrochemistryofgraphene-basednanostructuresbeganveryrecentlyandrapidlygainedattentionduetotheoutstandingpropertiesofgraphenes.5Theaimofthistutorialreviewistoprovideinsightintothebasicelectrochemistryofgraphenesheetsandtoshowthebenefitsofgraphene-basednanomaterialsforvariousimportantapplications.

MartinPumerareceivedhisPhDatCharlesUniversity,CzechRepublic,in2001.Aftertwopostdoctoralstays(intheUSAwithJosephWangandinSpainunderMarieCurieFellowship),hejoinedtheNationalInstituteforMaterialsScience,Japan,in2006foratenure-trackarrangementandstayedthereuntilSpring2008whenheacceptedatenuredpositionatNIMS.In2009,DrPumerareceivedaERC-StGaward.DrPumerajoinedMartinPumera

theNanyangTechnological

University,Singapore,in2010.DrPumerahasbroadinterestsinelectrochemistryandelectrophoresis,inthespecificareasofnanotechnology.HeismemberoftheEditorialboardofElectrophoresis.

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2.Structureofgraphene-basednanomaterials

Theelectronicandelectrochemicalpropertiesofagraphene-basednanomaterialstronglydependonitsstructure.Grapheneisatwo-dimensionalsheetofcarbonatomsconnectedbysp2bonds.Becausethesheetisnotinfiniteandsinceitcontainsterminatingedges,therearetwomainsurfacesofthegraphenesheetforelectronstoparticipateinheterogeneouselectrontransfer(HET):thebasalplaneandtheedge.Wecancategorizegraphene-basednanomaterialsregardingtheirdimensionsinthez-axisandthex–yaxesbecausethedimensionlimitationsinthedifferentaxessignificantlyinfluencetheelectronicpropertiesofgraphene.Cleardistinctionsbetweengraphenenanostructureswithdifferentpropertiescanbemadeaslistedbelow(seeFig.1foranillustration).(i)z-Axisconstrainedgraphenenanomaterials

(a)Single-layergraphenesheetwithlateraldimensionsfromtensofnanometresabove.Single-layergrapheneexhibitsa

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Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690PFig.1Graphenenanomaterials.Thesecanbeconstrainedinaz-axisorinx–y–zaxes.z-Axisconstrainednanomaterialsconsistofvirtuallyinfinitegraphenesheetswithonelayer(A,graphene),twolayers(B,double-layergraphene),threetotenlayers(C,few-layergraphene)andtentohundredlayers(D,grapheneplatelets).Ifthenumberoflayersisevengreater,suchmaterialsshouldbereferredtoasgraphite.Ifwedecidetoconstraingraphenesheetsinthexory-axis(inadditiontoconstraininginthez-axis),wereceivegraphenenanoribbons(A),two-orfew-layergraphenenanoribbons(BandC)orstackedgrapheneplateletnanofibers(D,ifthez-axisisnotconstrained).

zerobandgapandcanbeclassifiedasazerobandgapsemiconductor.1(b)Double-layergraphenewithlateraldimensionsfromtensofnanometresabove.Thishasazerobandgapinitsnativestatebutcanbeopenedbyapplyinganelectricfieldaxiallytowardthebilayer.InterlayerelectronhoppingispresentinboththeA–BandA–Cstackings.6(c)Few-layergraphene(fewerthan10graphenelayers).Mostofthe‘‘bulk’’preparationmethodsforproductionofgrapheneactuallyresultinfew-layergraphene(seediscussioninthefollowingsection).Suchgrapheneismetallic.6(d)Grapheneplatelets(10–100graphenelayers;B3–30nmthick).Suchgrapheneplatelets(alsosometimescallednano-grapheneplateletsorgraphenenanoplatelets)actuallyresemblesmallpiecesofhighlyorderedpyrolyticgraphite(HOPG)7becausetheirelectronicpropertiesarethesameasthoseofHOPG.

Whentakingacloserlookatgraphene,itshouldbenotedthatthex-axesparalleledgesexhibitadifferentstructurethandothey-axesparalleledges(seeFig.2).Onehasa‘‘zigzag’’structurewhiletheotherhasan‘‘armchair’’structure.Thisdifferencehasaprofoundinfluenceontheelectronicandelectrochemicalproperties,especiallyinx–yaxesconstrainedgraphenes.

(ii)x–y–zAxesconstrainedgraphenenanomaterials

(a)Single-layergraphenenanoribbons(alsocallednano-grapheneribbons):theelectronicstructureofnanoribbonsisdefinedbyquantumconfinementalongtheshortx-ory-axis.‘‘Zigzag’’graphenenanoribbonsaremetallicwhile‘‘armchair’’nanoribbonscanbemetallicorsemiconducting.Suchnanoribbonscanbeimaginedasopen,single-walledcarbonnanotubes.8(b)Few-layergraphenenanoribbons:suchnanoribbonshaveamorecomplexelectronicstructureandaretypicallymetallicconductors.Theyareoftenpreparedfrommultiwalledcarbonnanotubes.8,9(c)Stackedgrapheneplateletnanofibers(alsocalledplateletgraphitenanofibers):whenthedimensionofthez-axis

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Fig.2Edgesofgraphenesheetscanhave‘‘zigzag’’or‘‘armchair’’

orientation.Theelectrochemicalandelectronicpropertiesofgraphenesdependontheedgeorientation,especiallyifthegraphenenanomaterialisconstrainedinthex-ory-axis.

isgreaterthanthedimensionofthex–yaxes,themultilayernanoribbonsarecalledstackedgrapheneplateletnanofibers.Suchnanofiberspossessanexceptionallyhighratioofexposedgrapheneedgesitesvs.basalplanesites.Sincetheedgesofthegraphenesheetsaretheelectroactivesites(aswewillseeinthefollowingsections),suchstackedgrapheneplateletnanofibersshowsuperiorelectrochemicalperformancewhencomparedtographiteorcarbonnanotubes.10(iii)Dimensionallyunconstrainedgraphenematerials

(a)Graphite,asamultilayermaterialconsistingofindividualgraphenesheets.

(b)Graphiteoxidewasprepared150yearsagobyBrodie11andconsistsofheavilyoxidizedgraphenesheets,whicharelooselyattachedtoeachother.Graphiteoxideisaprecursorforthefabricationofgrapheneviagrapheneoxide.12Inaddition,theedgesofgraphenesheetsoftencontainoxygen-containinggroups,whichinsomecasesareelectrocatalytic.Inothercases,theyinhibitthereactionorevenadsorbtheproductsofelectrochemicalreaction,whichleadstopassivationoftheelectrodes.13Becausethesepropertiesarecloselylinkedtothepreparationandmaterialschemistryofgraphene-basednano-materials,wewilldiscussthesefirstinordertounderstandtheelectrochemicalbehaviorofgraphenenanomaterials.

3.Preparationofgraphene-basednanomaterials

Graphenenanomaterialscanbepreparedbyatop-downapproach—thedecompositionofhigherstructures,mainlygraphitebutalsocarbonnanotubes—toformgraphenenano-structuredmaterialsorbyabottom-upapproach—wherethegraphenesheetsaresynthesizedusuallyfromhydrocarbongas.Thevarietyofmethodsofpreparationcanbedividedintotwomaincategories:(i)physicalmethods9and(ii)chemicalmethods.12Chem.Soc.Rev.,2010,39,4146–4157|4147

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Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690PFig.3Etchingmultiwalledcarbonnanotubesinordertopreparegraphenenanoribbons(GNR).(a)PristineMWCNTusedasthestartingrawmaterial.(b)MWCNTdepositedonaSisubstrateandthencoatedwithaPMMAfilm.(c)PMMA–MWCNTfilmpeeledfromaSisubstrate,turnedoverandexposedtoanArplasma.(d–g)Severalpossibleproductsgeneratedafteretchingfordifferentdurations:GNRswithCNTcoresobtainedafteretchingforashorttimet1(d);tri-,bi-andsingle-layerGNRsproducedafteretchingfortimest2,t3,andt4,respectively(t4>t3>t2>t1)(e–g);PMMAremovedtoreleasetheGNR(h).(Reproducedwithpermission.9Copyright2009NaturePublishingGroup.)

(i)Thephysicalmethodsinclude(a)thehistoricallyfirstmethodbasedonpeelingHOPGlayerswithcellophanetapeuntilasinglelayerofgrapheneisobtained,1(b)epitaxialgrowthofgraphenelayersonasubstrate,14and(c)chemicalvapordeposition(CVD)ofmultilayeredgraphene.7Itisalsopossibletousethismethodtopreparestackedgrapheneplateletnanofiberswithlengthsinmmsize.10Thelast-mentionedphysicalmethodproducesamountsofgraphenenanomaterialssuitableformainstreamelectrochemistry,wherebulkquantitiesarerequired.Inaddition,anothermethodincludedis(d)etchingmultiwalledcarbonnanotubes(MWCNTs)inordertopreparegraphenenanoribbons,whichhavewell-defineddimensions.ThismethodinvolvesdepositionofMWCNTsonasiliconsubstrate,coveringthemwithPMMA,andconsequentlyetchingthembyplasma(seeFig.3).Dependingontheetchingtime,theMWCNTsareopenedandetchedtocreatesingle-layergraphenenano-ribbons,double-layergraphenenanoribbons,andsoon.9(ii)Chemicalmethods:thesemethodsarebasedonatop-downapproach.Chemicalapproachesinclude(a)chemicaloxidationofgraphitetographiteoxideinconcentratednitricacid(Brodie’smethod11)andconsequentthermaldecompositionofthegraphiteoxideinaninertatmosphere.

Fig.4Exfoliatinggraphiteinordertoproducegraphenenanomaterials.(A)Graphenesheetsbythermaldecompositionofintercalatedgraphite.(B)SchematicrepresentationofintercalationoftetrabutylammoniumionsinlargegraphiteoxidesedimentsandunreactedgraphiteparticlestoobtainmildlyoxidizedgraphenesinglesheetsinDMF.(Reproducedwithpermission.15,16Copyright2007and2009theAmericanChemicalSociety.)

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Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690PThermaldecompositionshouldbecarriedoutin‘‘thermalshock’’conditions(rampofseveralhundred1Cperminute)toachieverapidevolutionofgasesandseparationofthegrapheneoxidesheets15(seeFig.4A).Theresultinggrapheneoxideisfurtherultrasonicatedandoftenchemicallyreduced(usinge.g.,hydrazineasareducingagent)tographenesheets.Anotherapproachis(b)intercalationofsmallmoleculesbetweenthegraphenesheetswithinthegraphiteandconsequentultrasoundseparationofthelayers.Itshouldbenotedthatgraphenesheetstendtorestackandtheyshouldbestabilizedbyaddingasurfactantorothersimilarsubstances.Therestackingismorepronouncedwhenthesuspensionistransferredtothesolidphase.Inaddition,andperhapsmoreimportantly,itshouldbenotedthatover99%ofgraphenenanomaterialspreparedbytheabovetwomethodshaveamultilayerstructure16andshouldnotbecalledgraphene(astheyareoftencalled)butgrapheneplatelets.Onlyveryrecentlyhasamethodproducing>90%puresingle-layergraphenesheetsfrombulkgraphitebeenreported.Thismethodcombinesnitricacidoxidationandsmallion

intercalationwithingraphenesheetswithsubsequentexfolia-tion16(seeFig.4B).Athirdapproachis(c)intercalationoflithiumionsintoMWCNTsorgraphitestructure,whichcausesanincreaseintheinterlayerspacingofMWCNTsbydoubleandconsequentrupture.17Thisresultsinthecreationofgraphenenanoribbons.Anotherapproach(d)tocreatingmultilayergraphenenanoribbonsisto‘‘unzip’’MWCNTsbypermanganateoxidation.18Itshouldbestressedthatcarefulcharacterizationofproducedgraphenenanomaterialsshouldbeperformedsincesmalldeviationsfromthemethod(orstartingmaterialproperties)mightleadtosignificantdifferencesintheproducednanomaterialandconsequentlyinanyelectrochemicalresponse.

4.Materialschemistryofgraphene

Graphene-basednanomaterialscanbereadilyoxidized,eitherchemicallyorelectrochemically.12Theirpropertiesstronglydependontheirstructureandonthenumberandpositionoftheoxygen-containinggroups.Asweobserved,oxidized

kany’smodelshowing(a)surfacespeciesand(b)foldedcarbonskeleton.Fig.5Structuralmodelsofgrapheneoxide(GO).(A)De

(B)Schematicrepresentationofthegrapheneoxidereductionprocedure.Thetwo-stepreductionprocess,followedbyannealing,isaneffectivemethodtoconvertgrapheneoxidesheetsintographene-basedmaterials(whereCCGstandsforchemicallyconvertedgraphene).StartingmaterialshowsstructuralmodelproposedbyAjayan(lefttopB)takingintoaccountthefive-andsix-memberedlactolrings(blue),esterofatertiaryalcohol(pink),hydroxyl(black),epoxy(red),andketone(green)functionalities.Therelativeratiosarelikelytobe115(hydroxylandepoxy):3(lactolO–C–O):63(graphiticsp2carbon):10(lactol/ester/acidcarbonyl):9(ketonecarbonyl).Themodelhereshowsonlythechemicalconnectivityandnotthestericorientationofthesefunctionalities.(Reproducedwithpermission.19,20Copyright2009NaturePublishingGroupand2006theAmericanChemicalSociety.)

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Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690PgrapheneisthemainintermediateproductforthepreparationofgraphenenanomaterialsfromgraphiteorMWCNTs.Sometimes,grapheneoxideitselfisusedasthefinalmaterial,i.e.,forelectrochemicalsensing;veryoftentheoxygen-containinggroupsareusedasstartingpointsforfurtherfunctionalization.Grapheneoxideisapoorconductorandthusitsconversiontographeneismandatoryformostelectro-chemicalapplications.Hereitshouldbementionedthatreducedgrapheneoxideissometimesreferredtointhelitera-tureasgraphene.However,toavoidanymisinterpretationandconfusionwithpristinegraphene,suchmaterialsshouldbereferredtoaschemically/electrochemicallyreducedgrapheneoxide(CR-GOorER-GO).12Somehow,itissurprisingtonotethatthestructureofgrapheneoxideisstillnotwellknown.19SeveralpossiblemodelsbasedonNMR,XRD,IR,andXPSmeasurementsweresuggested.ANMRspectrumshowsthatgrapheneoxidecontainsC–OHgroups,epoxidegroups(1,2-ethers),aromaticdiol,ketonegroups,andlactols(seeFig.5).Moreover,andmoreimportantly,theexistenceofthesegroupsonthebasalplaneofagraphenesheetmeansthatthesheetisnolongerplanar,butthatithasafoldedcarbonskeleton20sincethe

concentrationofisolatedCQCgroupsislikelytobeverylow(seeFig.5A).Inordertoagainestablishadefect-freesp2bondedcarbonstructure,thereductionofgrapheneoxideisneeded.Understandingthestructureofgrapheneoxideiscrucialforsuccessfullyre-establishingthearomaticstructureofgraphene.Severalchemicalmethodshavebeenusedwithmostofthemusinghydrazineasareducingagent.19Alternatively,atwo-stepmethod,focusingeachsteponthe

Fig.6Adsorptionofnicotinamideadenineongraphenesheets.GeometriesforadsorptionofNAD+ona(A)grapheneedgeterminatedbyhydrogenatomsandcontainingone–COOÀgroup,(B)grapheneedgefullyterminatedbyhydrogenatoms,and(C)basalplaneofgraphenesheetviaCar–Parrinellomoleculardynamics.Gray,C;blue,N;red,O;yellow,P;black,H.(Reproducedwithpermission.13Copyright2009Wiley-VCH).

Fig.7Locationofoxygen-containinggroupsongrapheneoxidesheets.PreferentialadsorptionofPt–amminecomplexesatthecarboxylicgroupsatedgesandstepsofoxidizedgraphenesheets.(a)TEMimagesofPtcomplex/oxidizedgrapheneandtheirsizedistributionsobtainedfromabout300blackspotsobservedintheTEMimages.Ptcomplexclustersattachedtotheedges(redarrow)andsteps(bluearrow).STEM(b)andZ-contrast(c)imagesofPtcomplex/oxidizedgraphene.EDXspectrumofPtcomplex/oxidizedgrapheneatabrightspot(orangearrow)(d)andatanarea(pale-bluearrow)onthefeaturelesssurface(e)intheZ-contrastimage.IntheEDXspectrum(d),peaksoriginatingfromC,K,PtM1,andPtL1.OtherpeaksareassociatedwithCuandAloftheCugriddisk(TEMsampleholder)ordetectorbodiesoftheEDXanalyzer.(Reproducedwithpermission.21Copyright2009theAmericanChemicalSociety.)

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Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690Preductionofdifferentfunctionalgroups,wasestablished.Inthefirststep,thismethodusesNaBH4asareducingagent.19Thisstepeliminatesthevastmajorityofepoxide,alcohol,ketone,lactol,andestergroupsfromthestructureofthegraphenesheet(seeFig.5B).Thesubsequentstep,dehydrationbyconcentratedsulfuricacid,removestherestofthealcoholgroups,leavingonlythecarboxylicgroupsattheedgesofthegraphenesheets.ThesegroupscanfurtherberemovedbyannealinginanAr/H2atmosphere.19Anotherveryimportantissueassociatedwiththepresenceofoxygen-containinggroupsongraphenesheetsistheirinfluenceontheadsorption/desorptionofproductsofelectrochemicalreactionsfromgrapheneelectrodesurfaces.Adsorbedproductsoftenslowtheelectrochemicalreactionandsuchadsorptionisoftenhighlysensitivetothepresenceofoxygen-containinggroupsonthesurface.Thisissuehastobeaddressedonanindividualbasis.Wewishtopresentoneexamplehere.Amajorelementinelectrochemicalenzymebiosensorsandbiofuelcellsemployingdehydrogenaseenzymesisb-nicotinamideadeninedinucleotide(NAD+).Itsadsorptiononmostcarbonmaterialsposesacriticalproblembecauseitleadstoelectrodepassivation.TheadsorptionofNAD+atsp2carbonmaterialswasshownasbeingcausedbythepresenceofoxygen-containinggroups,namelycarboxylicgroups,thatformattheedgesandedge-likedefectsofgraphenesheets.13ThattheadsorptionofNAD+andelectrodepassivationoccursattheedgesandedge-likedefectsofgraphenewasshownthroughtheuseofamperometry,X-rayphotoelectronspectroscopy(XPS),andcyclicvoltammetry.Car–ParrinellomoleculardynamicsprovedthatsignificantinteractionthatagreeswithwhatwasfoundexperimentallyonlyoccurswhenapositiveNAD+ispositionedclosetoagraphenesheetedgethatcontainsa–COOÀgroup.IfNAD+isclosetothebasalplaneofgrapheneorneartheedgesofgraphenesheetsonlysubstitutedwithhydrogen,norelevantinteractionsoccur(Fig.6).

Whilespectroscopicmethodsareusefulforestablishingageneralmodelofgrapheneoxide,theyprovidenoinformationonthelocationoftheoxygen-containinggroups.Thisisanimportantissue,becausetheoxygen-containinggroupsplayamajorroleintheelectrochemistryofgraphene-basednano-materials.Manymodels20suggestaperiodicstructureofgraphite/grapheneoxide.However,thisisprobablynotcorrect,giventhelackofperiodicityfoundexperimentally.Directlocationofoxygen-containinggroupsingraphene-basednanomaterialsshowsthatindeedthereisnoperiodicity,aswediscussbelow.Determiningthepreciselocationofoxygen-containinggroupsingraphene-basednanomaterialsisachallengingtasksinceitisalmostimpossibletoenvisionthemdirectlywithatomicresolution.However,oxygen-containinggroupscanbelabeledby‘‘stains’’(usingthechemistryofoxygen-containingCgroupsandPt(NH3)6orEu(OH)3),whichcanbeviewedunderahighresolutionelectronmicroscope(HR-TEM;Fig.7).21,22Ithasbeenshownthatoxygen-containinggroupsarelocatedclosetotheedgesofgraphenesheets.21Ithasalsobeenshown(usingamultiwalledcarbonnanotubewithalargediameterandlowcurvatureasamodelofaninfinitegraphenesheet)thatoxygen-containinggroupsatthebasalplaneofgraphenearedistributedrandomly,creatingpatches/islandsofatypicalsizeofabout10nm.

Fig.8Electrochemicalreductionofgrapheneoxide.Schematicdiagramoftheexperimentalsetup(AandB)andelectrolyzingprocess(CandD)forelectro-reducingGOfilmsdirectlytoER-GOfilmsonaninsulating(left)orconductive(right)substratebasedonthethreephaseinterlines(3PIs)model.TheblackarrowsindicatethedirectionoftheER-GOfilmformation.Threephases:conductor/insulator/electrolyte;initialthreephases:workingelectrode/GO/electrolyte.Electro-formedthreephases:ER-GO/GO/electrolyte.(E)(a–d)LinearsweepvoltammogramsofGCelectrodeincontactwithB7mm-thickGOfilmsonquartz(5Â4cm2)atpHvaluesof4.12(a),7.22(b),10.26(c),and12.11(d).(e)LinearsweepvoltammogramsofGCelectrodesuspendedinNa–PBS(1M,pH4.12).(fandg)TypicalI–tcurvesfortheelectrolysisofB7mm-thickGOfilmsonquartz(5Â4cm2;curvef)andonGCelectrode(3mmdiameter;curveg)withanappliedpotentialofÀ0.90VinNa–PBS(1M,pH4.12).(Reproducedwithpermission.26Copyright2009Wiley-VCH.)

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However,largerorsmallerstructures(downtoislandsofoneorafewoxygen-containinggroups)werealsoidentified.22Therefore,itisclearthatthemostfeasiblemodelofgrapheneoxideistheonewithamajorconcentrationofoxygen-containinggroupsclosetoitsedgesandwiththeirrandomdistributioninthebasalplaneofthegraphenesheet.

Ramanspectroscopyofgraphenematerialsisapowerfultechniquetocharacterizetheamountofedgeplanes,defectsandcrystallitesizeofgraphenenanomaterials.23Similarly,scanningprobemethodsareveryimportantinprovidingdirectevidencetothesizedistributionandaveragenumberoflayersinthegraphenenanomaterials.2Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690Pconsequentslowingofthereaction(seeFig.8).Theconductivityoftheelectrochemicallyreducedgrapheneoxide(ER-GO)increasedby8ordersofmagnitudeversusGO,andtheO/C

5.Electrontransferatgraphenesheets

Theheterogeneouselectrontransfer(HET)isthefundamentalprocessofelectrochemicalreaction.Thistermreferstoelectrontransferoutoforintothegraphenesheetsfromthesurroundingenvironment.Giventheenormousindustrialimportanceofgraphite-basedelectrodes,thequestionofwhereheterogeneouselectrontransferoccursisofparamountimportance.Therehavebeennumerousapproachestoestablishheterogeneouselectrontransferfromgraphenesheets,usuallyusinghighlyorderedpyrolyticgraphite(HOPG).HOPGhasamultilayergraphenestructurewithminimumdefects.Ithasbeenshownthatwhiletheedge-planeofagraphenesheetexhibitsaHETrateconstant(ke)ontheorderofB0.01cmsÀ1,thebasalplaneiseffectivelyelectro-chemicallyinert,withkblowerthan10À9cmsÀ1.Indeed,agraphenesheetcancontaindefectsinthebasalplane.Insuchacase,thedefectsareusuallyconsideredtobeedge-planesites,sincetheypossessfastHETkinetics.Toexcludethedefectsitesonthebasalplaneofgraphenesheets,anelectro-chemicallyinertmaterial(MoO2)waselectrodepositedonthedefectstopreventthemfromparticipatingintheredoxchemistry.Theresultingdefect-freegraphenesheetsexhibitedakblimitingtozero.24Recently,itwasdemonstratedthattheelectrochemicalresponseofgraphenesheetsisindependentofthenumberoflayersfromasinglegraphenesheettomultilayerstackedgrapheneplatelets.25Importantconsiderationforpracticalapplicationelectro-chemicalapplicationofgrapheneisenormoussurfaceareawhichinprincipleleadstothebenefitpassivationmoreresistantelectrodes,incomparisonwithothercarbon-basedelectrodes.Thisleadstolonger-stabilityofelectrochemicalperformance.

Fig.9Relationshipbetweeninterfacialcapacitanceandnumberoflayersofsolvent-freegraphene.(Reproducedwithpermission.28Copyright2009Elsevier.)

Fig.10Electrochemicaldecorationsofgraphenesheetswithcatalystnanoparticles.NucleationsitesforPdatdefectsattheedgesoftheintactgrapheneislands.(Reproducedwithpermission.29Copyright2008Wiley-VCH.)

6.Inherentelectrochemistryofgraphenenanomaterials

Grapheneoxide(GO)canbereducedbyelectrochemicalmeans.26CyclicvoltammetryofgrapheneoxidesheetsexhibitssignificantreductionwavesstartingatÀ0.60V(vs.Ag/AgClreferenceelectrode),reachingamaximumatÀ0.87V.TheelectrochemicalreductionprocessispHdependent,suggestingthefollowingmechanismofreduction:26GO+aH++beÀ-ER-GO+cH2O.

Thereductionofgrapheneoxidewascarriedoutbyapotentio-statictechnique,withrapidreactionduringthefirst2000sand

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Fig.11Grapheneisanidealmaterialforspectroelectrochemistry.ItistransparentinUV-Visspectraandisaconductor.Transmissionspectraofgraphene-opticallytransparentelectrode(OTE)(B24nmthick,solidblacklineandB8nm,solidgrayline)andindiumtinoxideOTE,dottedblackline,allonquartz.(Reproducedwithpermission.32Copyright2010Wiley-VCH.)

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Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690PFig.12Graphenesheetsareidealmaterialsforenergystorageasultracapacitorsduetotheirlargesurfacearea.(a)SEMimageofchemicallymodifiedgrapheneparticlesurface,(b)TEMimageshowingindividualgraphenesheetsextendingfromchemicallymodifiedgraphenesurface,(c)lowandhigh(inset)magnificationSEMimagesofchemicallymodifiedgrapheneelectrodesurface,and(d)schematicdiagramoftestcellassembly.(Reproducedwithpermission.33Copyright2008theAmericanChemicalSociety.)

ratioofER-GOwas4.23%versus69.2%forGO.Thissignifiesthattheoxygen-containinggroupsweresignificantlyremovedinlargequantitiesandonlythosethatcannotbeelectrochemicallyreduced(e.g.,carboxylicgroups)werepresentinER-GO.26TheelectrochemicalreductioncanalsobeobservedbyRamanspectroscopy.IthasbeenshownthattheintensityofRamanG-banddecreaseswithincreasingnegativepotential,from0toÀ0.6V(vs.SCE),andthentheshiftlevelsoff.Thisshiftcorrespondstoreconstructionofthesp2conjugatednetworkofcarbonsheets.27Itshouldbenotedthatthemechanismofelectrochemicalreductionisnotfullyunderstoodandrequiresfurtherstudy.ReductionofGOwasdemonstratedonvarioussubstrates,frommaterialssuchasnon-conductingsilicaandplastictoconductingITOandglassycarbonmaterials.26,27Ithasbeenshownthatthecapacitanceofgraphenenano-materialsdependsonthenumberoflayers(Fig.9).28Thenumberofstackinglayersofmultilayergraphenesheetswasengineeredbyselectivepost-treatment.Thenumberwasdeterminedaccordingtospecificsurfacearea.Thisisattributedtothedependenceofthespacechargelayercapacitanceofgrapheneonthenumberoflayers,wherethetwofactorsofscreeninglengthandstackingthicknesshaveprimaryinfluence.28Electrochemistrycanbeusedforthedecorationofgraphenesheetswithcatalyticnanoparticles.29Palladiumnanoparticles

Fig.13Electrochemicalsensingongraphenenanoplatelets.(A)(a)and(b)DifferentmagnificationTEMimagesofnanoplatelets;(c)high-resolutionTEMimageofgraphenenanoplateletsshowingnanoflakeswithaknife-edgeorconicalstructurewithopengraphiticplanes;(d)EDXspectrum,showingchemicalcompositionofgraphenenanoplateletfilms.(B)(a)and(b)Cyclicvoltammetryprofilesofgraphenenanoplateletsandbareglassycarbonelectrodes,respectively,insolutionof50mM,pH7.0PBSbuffercontaining1mMascorbicacid,0.1mMdopamine,and0.1mMuricacid.(Reproducedwithpermission.7Copyright2008Wiley-VCH.)

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withadiameterofabout7–9nmcanbeelectrochemicallydepositedonthedefectsofgraphenesheetsduetothepreferrednucleationofthepalladiumatvacanciesalongtheedgesoftheintact,nanometre-sizedgrapheneislands(seeFig.10).Theelectrochemicalrouteofsynthesisofcatalystnanoparticlesisveryattractivebecausesuchnanoparticlesnucleateattheelectroactivesitesofcarbonnanomaterials.Thereareawidevarietyofapplicationsforsuchcatalystmodifiedgraphenes.30activatedcarbontoachievealargesurfaceareabutonaflexiblematerial,graphene,whichadjustsitsarrangement

7.Spectroelectrochemistry

Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690PSpectroelectrochemistryenablesustogainadeeperinsightintoredoxreactionbecausethespectroscopicmethodallowsidentificationofparticipatingspecies.31MultilayergraphenesheetsaretransparentintheUV-Visregionevenatthicknessesofabout24nmandpossesshighconductivity.32ItwassuggestedthattheyareagoodreplacementforITOtransparentconductiveelectrodes,whichareoftenusedbyspectro-electrochemists.ThisisbecauseITOelectrodesaremainlytransparentintheVisregionandtheirtransparencyintheUVregiondecreasesdramaticallyincontrasttomultilayeredgraphenewiththicknessesoftenstohundredsofgraphenesheets,whichistransparentinbothregions(seeFig.11).32Grapheneitselfcanbestudiedbyspectroelectrochemistry,similarlytothatofothercarbonnanomaterials.Forexample,electrochemicalreductionofgrapheneoxidetoreducedgraphenesheetscanbeobservedbyRamanspectroscopy.TheshiftoftheG-bandfrom1610cmÀ1to1585cmÀ1indicatesthereductionofgrapheneoxideuponapplicationofapotentialofÀ1V(vs.saturatedcalomelelectrode).278.Electrochemicalapplicationsofgraphene-basednanomaterials

Graphenenanomaterialshaveenormouspotentialinsensing,biosensing,andenergystorageapplications.Theadvantagesofgraphenearethatthesurfaceareaofgraphenesheets(2630m2gÀ1)12ismuchgreaterthanthatofsingle-walledcarbonnanotubes.Graphene-basednanomaterialsavoidtheproblemsassociatedwithcarbonnanotubes,suchasthepresenceofmetalliccatalystimpurities.However,itshouldberememberedthatgraphene-basednanomaterialscandiffersignificantlyifdifferentmethodsofpreparationareusedorevenifsmallvariationsofasinglemethodareemployed.Therefore,anygraphenenanomaterialshouldbethoroughlycharacterizedtoavoidpotentialmisinterpretation.

Theresearchofgraphene-basedultracapacitorsisveryimportantbecauseitisthoughtthattheirenergydensitywillapproximatethatofordinarybatteries.Becauseultracapacitorsstoreenergyatthesurfaceofthematerialintwolayers,grapheneisconsideredanidealcandidatematerialfortheirconstructionbecauseoftheirhighsurfacearea.Modifiedgraphenehasbeenusedasamaterialforelectrodes(seeFig.12forschematicdiagramsofgraphene-basedelectrodesforuseasultracapacitorsandforultracapacitordesign).33Theprincipalinnovationbehindtheconstructionofchemicallymodifiedgrapheneelectrodesisthattheultracapacitorsarenotdependentonarigidporousstructuresuchasthatof

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Fig.14Chemicallyreducedgrapheneoxideforelectrochemicalbiosensingandwiringproteins.I.DNAsensing:(A)DPVsattheGCelectrodeforG(blue),A(orange),T(violet),andC(magenta).(B)DPVsatthegraphite/GCelectrodeforG(blue),A(orange),T(violet),andC(magenta).(C)DPVsattheCR-GO/GCelectrodeforG(blue),A(orange),T(violet),andC(magenta).(D)DPVsforamixtureofG,A,T,andCatCR-GO/GC(green),graphite/GC(red),andGCelectrodes(black).(E)DPVsforssDNAatCR-GO/GC(green),graphite/GC(red),andGCelectrodes(black).(F)DPVsfordsDNAatCR-GO/GC(green),graphite/GC(red),andGCelectrodes(black).Concentrationsfordifferentspecies(A–F):G,A,T,C,ssDNA,ordsDNA:10mgmLÀ1.Electrolyte:0.1M,pH7.0,PBS.II.Wiringproteins:GO-supportedhemeproteinsatthesurfaceofGCelectrodes(oxygenatedfunctionalgroupsatthesurfaceofGOarenotshownforsimplicity).(Reproducedwithpermission.34,35Copyright2009theAmericanChemicalSociety.)

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Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690Pdependingontheelectrolyteused.Chemicallymodifiedgraphenehasaweight-specificcapacitanceofupto135FgÀ1.

Grapheneplateletswerepreparedbycatalyst-freechemicalvapordeposition.Theseplateletshaveahighlygraphitizedknife-edgestructurewithasharpedge2–3nmthick.Near-edgeX-rayabsorptionfinestructurespectroscopyshowsthattheyexhibitapreferredorientationthatisverticalwithrespecttotheSisubstrate.Thesegraphenenanomaterialsdemonstratefastelectrontransfertowardscriticalbio-markers.7Theirperformanceforsensingdopamineinthepresenceofascorbicacidanduricacid,whicharecommoninterferingagents,wasfoundsuperiortothatofothersolidstateelectrodes;itisequivalenttothatofedgeplanepyrolyticgraphite(Fig.13).Grapheneplateletsarebeneficialinthattheycanbeincorporatedintopolymermatriceswithbiorecognitionelements,asopposedtopyrolyticgraphite.Thelargequantityofgraphiticedgeplanes/defectsisessentiallyresponsibleforfasterETkineticsandexpressionofactiveelectrocatalyticandbiosensingproperties.

Theapplicationofreducedgrapheneoxidetowardstheelectrochemistryofbiomarkerswasstudiedextensively.FreeDNAbases(adenine,thymine,guanine,andcytosine),oxidase/dehydrogenase-relatedmolecules(hydrogenperoxide/NADH),neurotransmitters,andascorbicacid,uricacid,andacetaminophenwereemployedtostudytheirelectrochemicalresponsesonchemicallyreducedgrapheneoxide(CR-GO).34CR-GOexhibitsasignificantlylargerelectrochemicalresponsetowardstheoxidationoftheseprobesthandographiteorGCelectrodes,ascanbeseeninFig.14I.Whiletheexactreasonforsuchenhancedresponseisnotclear,itislikelytoberelatedtothepresenceofoxygen-containinggroupsonthesurfaceofthereducedgrapheneoxide,whichposeelectrocatalyticproperties.

Grapheneoxidewassuccessfullyemployedinbioelectro-chemistry.35Thisissomehowsurprising,giventhefactthatGOisessentiallyaninsulatingmaterial.However,asnotedabove,theGOusedinsuchprocesseswaslikelynotfullyoxidized,maintainingansp2hybridizednetwork.SuchGOsupportstheefficientelectricalwiringoftheredoxcentersofseveralheme-containingmetalloproteins(cytochromec,myoglobin,andhorseradishperoxidase)totheelectrode(seeFig.14II).Significantly,proteinsretaintheirstructural

Fig.15Stackedgrapheneplateletnanofibersforelectrochemicalsensing.Cyclicvoltammogramsof5mM(A)dopamine,(B)ascorbicacid,(C)uricacid,(D)NADH,and(E)Fe3+(in1MKCl)onstackedgrapheneplateletnanofibers(PGNFs),graphitemicroparticles(GMPs),multiwalledcarbonnanotubes(MWCNTs),EPPG(dottedblackline),andbareGCelectrodes(dashedblackline).Conditions:scanrate,100mVsÀ1;backgroundelectrolyte,50mMphosphatebuffer;pH7.4.(F)Schematicsoforientationofgraphenesheetsonstackedgrapheneplateletnanofibers(left)andmultiwalledcarbonnanotubes(right).(Reproducedwithpermission.4Copyright2010Wiley-VCH.)

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integrityandbiologicalactivityuponformingmixtureswithGO.35ThesefeaturesforetellthepromisingapplicationsofGO/proteincomplexesinthedevelopmentofbiosensorsandbiofuelcells.Becausetheoutputoftheconstructionofthereducedgrapheneoxidenanomaterialsisuncertain,andthattheelectronicpropertiesofgrapheneplatelets(includingtherateofheterogeneouselectrontransfer)nolongerdependontheirthickness,thestudyofmorewell-definedmaterialswouldbedesirableasfaraselectrochemistryandelectrochemicalsensorandbiosensorfabricationareconcerned.Stackedgrapheneplateletnanofibers(PGNFs)arenowavailableinbulkquantitieswhereasgrapheneandgrapheneplateletsarenot.Thenanofibersofthegraphenesheetsareorientedperpendicularlytothelongaxisofthefiber,thereforerevealingalargenumberofedge-planesites.Suchplacementyieldsamaterialthatischemicallyandelectrochemicallyuniquebecauseonlytheedgesareexposedasopposedtoordinarygraphitecrystalsorcarbonnanotubes.Actually,PGNFsarethestructuraloppositesofCNTsinthatonlytheelectroactiveedgesofthegraphenesheetsareexposedtothesolutionwhileCNTsexposemostlyelectrochemicallyinertwalls(seeFig.15).Becausetheelectrochemistryofsp2materialsisdeterminedbyedge-planesites,PGNFsshouldhaveextra-ordinaryelectrochemicalpropertiesasopposedtographitemicroparticlesandcarbonnanotubes.PGNFshaverapidratesofelectrontransferforawidevarietyofcompoundsincludingdopamine,FeCl3,ferrocyanide,ascorbicacid,uricacid,aswellasthereducedformofb-nicotinamideadeninedinucleotide.10Similarly,PGNFsexhibitmuchlargeresponseforfourDNAbasesinssDNArelatedtotheswinefluA(H1H1)virus.36ThepropertiesofPGNFsareelectrochemicallysuperiortothoseofMWCNTsandgraphitemicroparticlesbecauseoftheirhighratioofgraphenesheetedges.10Suchstackedgrapheneplateletnanofiberswerealsousedforelectrochemicalenzymaticdetectionofglucose.37electrochemicalproperties.Dopingorchemicallymodifyinggraphenewithnitrogenandotherheteroatomsisofhighinterestassuchheteroatomscanprovideelectrocatalyticpropertiesandenhancestabilityofdopedgrapheneelectrodeswhichhasgreatpotentialbothforsensingandenergystorage.38Wehopethatwehaveprovidedreferencestothemostsignificanthistoricaldevelopmentsaswelltothecurrentliteratureformoreadvancedreaders.Letusconcludebysayingthatweareatthebeginningofaverylongandexcitingjourneyofdiscoveringtheelectrochemistryofgraphene.

Notesandreferences

1K.S.Novoselov,A.K.Geim,S.V.Morozov,D.Jiang,Y.Zhang,S.V.Dubonos,I.V.GrigorievaandA.A.Firsov,Science,2004,306,666.

2A.K.GeimandK.S.Novosellov,Nat.Mater.,2007,6,183,andreferencestherein.

3ElectrochemicalMethods.FundamentalsandApplications,ed.A.J.BardandL.R.Faulkner,Wiley,2ndedn,2001.

4M.Pumera,Chem.–Eur.J.,2009,15,4970,andreferencestherein.5D.Chen,L.TangandJ.Li,Chem.Soc.Rev.,2010,DOI:10.1039/b923596e,andreferencestherein.

6T.Ohta,A.Bostwick,T.Seyller,K.HornandE.Rotenberg,Science,2006,313,951,andreferencestherein.

7N.G.Shang,P.Papakonstantinou,M.McMullan,M.Chu,A.Stamboulis,A.Potenza,S.S.DhesiandH.Marchetto,Adv.Funct.Mater.,2008,18,3506.

,G.Dobrik,P.LambonandS.Biro,Nat.Nanotechnol.,8L.Tapaszto

2008,3,397,andreferencestherein.

9L.Jiao,L.Zhang,X.Wang,G.DiankovandH.Dai,Nature,2009,458,877,andreferencestherein.

10A.Ambrosi,T.SasakiandM.Pumera,Chem.–AsianJ.,2010,5,266,andreferencestherein.

11B.C.Brodie,Philos.Trans.R.Soc.London,1859,149,249.

12D.R.Dreyer,S.Park,C.W.BielawskiandR.S.Ruoff,Chem.Soc.Rev.,2010,39,228,andreferencestherein.

13M.Pumera,R.Scipioni,H.Iwai,T.Ohno,Y.MiyaharaandM.Boero,Chem.–Eur.J.,2009,15,10851.

14C.Berger,Z.Song,T.Li,X.Li,A.Y.Ogbazghi,R.Feng,Z.Dai,A.N.Marchenkov,E.H.Conrad,P.N.FirstandW.A.deHeer,J.Phys.Chem.B,2004,108,19912.

15A.Yu,P.Ramesh,M.E.Itkis,E.BekyarovaandR.C.Haddon,J.Phys.Chem.C,2007,111,7565.

16P.K.Ang,S.Wang,Q.Bao,J.T.L.ThongandK.P.Loh,ACSNano,2009,3,3587,andreferencestherein.

17A.G.Cano-Marquez,F.J.Rodrıguez-Macıas,J.Campos-Delgado,C.G.Espinosa-Gonzalez,F.Tristan-Lopez,D.Ramırez-Gonzalez,D.A.Cullen,D.J.Smith,M.TerronesandY.I.Vega-Cantu,NanoLett.,2009,9,1527,andreferencestherein.

18D.V.Kosynkin,A.L.Higginbotham,A.Sinitskii,J.R.Lomeda,A.Dimiev,K.PriceandJ.M.Tour,Nature,2009,458,872,andreferencestherein.

19W.Gao,L.B.Alemany,L.CiandP.M.Ajayan,Nat.Chem.,2009,1,403,andreferencestherein.

20T.Szabo,O.Berkesi,P.Forgo,K.Josepovits,Y.Sanakis,

kany,Chem.Mater.,2006,18,2740,andD.PetridisandI.De

referencestherein.

21R.Yuge,M.Zhang,M.Tomonari,T.Yoshitake,S.IijimaandM.Yudasaka,ACSNano,2008,2,1747.22M.Pumera,Chem.–AsianJ.,2009,4,250.

23M.S.Dresselhaus,A.Jorio,M.Hofmann,G.DresselhausandR.Saito,NanoLett.,2010,10,751.

24T.J.Davis,M.E.HydeandR.G.Compton,Angew.Chem.,Int.Ed.,2005,44,5251,andreferencestherein.

25M.PumeraandM.G.Shuhua,Chem.–AsianJ.,2010,DOI:10.1002/asia.201000437.

26M.Zhou,Y.Wang,Y.Zhai,J.Zhai,W.Ren,F.WangandS.Dong,Chem.–Eur.J.,2009,15,6116.

27G.K.RameshaandS.Sampath,J.Phys.Chem.C,2009,113,7985.28D.-W.Wang,F.Li,Z.-S.Wu,W.RenandH.-M.Cheng,Electrochem.Commun.,2009,9,1729.

Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690P9.Conclusions

Aswehaveshowninthistutorialreview,eventhoughresearchongraphene-basednanomaterialsstartedrelativelyrecently,anenormousamountofefforthasbeenmadeinordertounderstandtheelectronicandelectrochemicalbehaviorofgrapheneaswellasitschemistry.Suchrapidprogresswaspossibleonlybecauseoftheaccumulationofknowledgeandthedevelopmentoftechniquesforlearningofpreviousresearchongraphiteandcarbonnanotubes.Althoughwehaveabasicunderstandingoftheelectrochemistryofgraphene-relatednanomaterialsandthereareseveralapplications(includingcommercializedones)oftheelectro-chemistryofgraphenes,muchremainstobedonetoelucidatethemoresubtledetailsofgrapheneelectrochemistrysuchastheroleofthestructureofgrapheneoxideonitselectro-chemistry,theroleofthespecificoxygen-containinggroupsattheedgesofgraphenesheets,andthemechanismoftheoxidation/reductionofthegraphenesheetsthemselves.Theimportanceofdetailedcharacterizationofgraphene-basednanomaterialspriortotheiremploymentaselectrodescannotbeoveremphasizedbecauseevensmallvariationsinthemethodsofpreparationleadtographeneswithdifferent

4156|Chem.Soc.Rev.,2010,39,4146–4157

Thisjournalis

󰀂c

TheRoyalSocietyofChemistry2010

View Online

29R.S.Sundaram,C.Gomez-Navarro,K.Balasubramanian,

M.BurghardandK.Kern,Adv.Mater.,2008,20,3050.30P.V.Kamat,J.Phys.Chem.Lett.,2010,1,520.

31L.KavanandL.Dunsch,ChemPhysChem,2007,8,974.

32C.M.Weber,D.M.Eisele,J.P.Rabe,Y.Liang,X.Feng,L.Zhi,

K.Mullen,J.L.Lyon,R.Williams,D.A.BoutandK.J.Stevenson,Small,2010,6,184.

33M.D.Stoller,S.Park,Y.Zhu,J.AnandR.S.Ruoff,NanoLett.,

2008,8,3498.34M.Zhou,Y.ZhaiandS.Dong,Anal.Chem.,2009,81,5603.35X.Zuo,S.He,D.Li,C.Peng,Q.Huang,S.SongandC.Fan,Langmuir,2010,26,1936.

36A.AmbrosiandM.Pumera,Phys.Chem.Chem.Phys.,2010,DOI:10.1039/c0cp00213e.

37V.Vamvakaki,K.TsagarakiandN.Chaniotakis,Anal.Chem.,2006,78,5538.

38Y.Wang,Y.Shao,D.W.Matson,J.LiandY.Lin,ACSNano,2010,4,1790.

Downloaded by Lanzhou University on 08 August 2011Published on 09 July 2010 on http://pubs.rsc.org | doi:10.1039/C002690PThisjournalis

󰀂c

TheRoyalSocietyofChemistry2010Chem.Soc.Rev.,2010,39,4146–4157|4157