改良hummers合成氧化石墨烯

AnimprovedHummersmethodforeco-friendlysynthesisofgrapheneoxide

JiChen,BowenYao,ChunLi,GaoquanShi

*

DepartmentofChemistry,TsinghuaUniversity,Beijing100084,People’sRepublicofChina

ARTICLEINFOABSTRACT

Articlehistory:Received4June2013Accepted21July2013Availableonline27July2013

AnimprovedHummersmethodwithoutusingNaNO3canproducegrapheneoxidenearlythesametothatpreparedbyconventionalHummersmethod.Thismodi cationdoesnotdecreasetheyieldofproduct,eliminatingtheevolutionofNO2/N2O4toxicgassesandsim-plifyingthedisposalofwastewaterbecauseoftheinexistenceofNa+andNO3Àions.Forthe rsttime,wealsodevelopedaprototypemethodofpost-treatingthewastewatercol-lectedfromthesystemsofsynthesizingandpurifyinggrapheneoxide.ThecontentofMn2+ionsinthepuri edwastewaterwasmeasuredtobelowerthantheguidelinevaluefordrinkingwater.

Ó2013ElsevierLtd.Allrightsreserved.

1.Introduction

Graphenehasauniqueatom-thicktwo-dimensionalstruc-ture,excellentelectronic,mechanical,opticalandthermalproperties[1].Therefore,ithasbeenwidelyexploredfortheapplicationsinelectronics[2],catalysis[3],sensors[4],andenergyconversionandstorage[5,6],etc.Forthesepurposes,themass-productionofgraphenematerialsatlowcostsisoneoftheessentialrequirements.Actually,graphenesheetsalreadyexistinnatureandweneedtoexfoliatethemfromtheirprecursors[7].Theexfoliationofgraphitetographenecanberealizedeitherphysicallyorchemically[1].Amongthevariousmethods,chemicalreductionofgrapheneoxide(GO)toreducedgrapheneoxide(rGO)isuniqueandattractivebecauseofitscapabilityofproducingsingle-layergrapheneinlargescaleandatrelativelylowcost[8].Furthermore,GOandrGOareprocessibleandtheycanbefabricatedorself-assem-bledintomacroscopicmaterialswithcontrolledcompositionsandmicrostructuresforpracticalapplications[9].

GOistheprecursorofrGO;thus,itplaysacrucialroleincontrollingthestructure,propertyandtheapplicationpoten-tialofrGO[10À16].ThepioneeringworkonthesynthesisofGOwasreportedbyBrodiein1859[17].Inthismethod,one

equalweightofgraphitewasmixedwiththreeequalweightsofKClO3andreactedinfumingHNO3at60°Cfor4days.Sta-udenmaierimprovedBrodiemethodbyreplacingabouttwothirdsoffumingHNO3withconcentratedH2SO4andaddingKClO3inmultipleportions[18].Thissmallmodi cationen-ablestheoverallreactioninasinglevessel;thussimplifyingthesynthesismethod.However,thisreactionstillneedsalongtimeof4days.ThemostimportantandwidelyappliedmethodforthesynthesisofGOwasdevelopedbyHummersandOffemanin1958(Hummersmethod)[19].Inthiscase,theoxidationofgraphitewasachievedbyharshtreatmentofoneequalweightofgraphitepowdersinaconcentratedH2SO4solutioncontainingthreeequalweightsofKMnO4and0.5equalweightofNaNO3.TheHummersmethod,atleast,hasthreeimportantadvantagesoverprevioustech-niques.First,thereactioncanbecompletedwithinafewhours.Second,KClO3wasreplacedbyKMnO4toimprovethereactionsafety,avoidingtheevolutionofexplosiveClO2.Third,theuseofNaNO3insteadoffumingHNO3eliminatestheformationofacidfog.

Hummersmethodhasbeenpaidthemostintensiveatten-tionbecauseofitshighef ciencyandsatisfyingreactionsafety.However,itstillhasthefollowingtwo aws:(1)theoxi-

*Correspondingauthor:Fax:+861062771149.

E-mailaddress:gshi@(G.Shi).

0008-6223/$-seefrontmatterÓ2013ElsevierLtd.Allrightsreserved./10.1016/j.carbon.2013.07.055

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dationprocedurereleasestoxicgassessuchasNO2andN2O4;(2)theresidualNa+andNO3Àionsaredif culttoberemovedfromthewastewaterformedfromtheprocessesofsynthe-sizingandpurifyingGO.Tourandco-workersimprovedtheHummersmethodbyexcludingNaNO3,increasingtheamountofKMnO4,andperformingthereactionina9:1(byvolume)mixtureofH2SO4/H3PO4[20].Thismodi cationissuccessfulinincreasingthereactionyieldandreducingtoxicgasevolution,whileusingtwiceasmuchKMnO4and5.2timesasmuchH2SO4asthoserequiredbyHummersmethodandalsointroducinganewcomponentofH3PO4tothereactionsystem.

Recently,Baek’sgroupstudiedtheprocessofetchingthebasalplanesofhighlyorderedpyrolyticgraphite(HOPG)withahotmixtureofH2SO4andHNO3[21].Inthiscase,thegraph-enelayersofHOPGwereeffectivelycutandexfoliatedafteralong-termtreatment.ThisobservationindicatesthattheH2SO4/HNO3mixtureusedinHummersmethodactsasachemical‘‘scissor’’andachemical‘‘drill’’forgrapheneplanestofacilitatethepenetrationofoxidationsolution.Ontheotherhand,KMnO4isoneofthestrongestoxidants,espe-ciallyinacidicmedia[22].WiththeassistanceofKMnO4,acompleteintercalationofgraphitewithconcentratedH2SO4canbeachieved,forminggraphitebisulfateinwhicheverysingle-layergrapheneissandwichedbythelayersofbisulfateions[23,24].ThiscompleteintercalationensurestheeffectivepenetrationofKMnO4solutionintographenelayersfortheoxidationofgraphite.Accordingly,KMnO4canalsotaketheroleofNaNO3andthelatterisunnecessaryforthesynthesisofGOusingHummersmethod.Inthisarticle,wedemon-stratethatGOcanbeproducedusinganimprovedHummersmethodwithoutusingNaNO3.ThismethoddecreasesthecostandenvironmentaldutyofGOproduction.

2.

Experimental

2.1.

Synthesisandpuri cationofGO

GOwaspreparedbytheoxidationofnaturalgraphitepowder(325mesh,QingdaoHuataiLubricantSealingS&TCo.Ltd.,Qingdao,China)accordingtoHummersmethodwithamodi- cationofremovingNaNO3fromthereactionformula[19].Typically,graphitepowder(3.0g)wasaddedtoconcentratedH2SO4(70mL)understirringinanicebath.Undervigorousagitation,KMnO4(9.0g)wasaddedslowlytokeepthetemper-atureofthesuspensionlowerthan20°C.Successively,thereactionsystemwastransferredtoa40°Coilbathandvigor-ouslystirredforabout0.5h.Then,150mLwaterwasadded,andthesolutionwasstirredfor15minat95°C.Additional500mLwaterwasaddedandfollowedbyaslowadditionof15mLH2O2(30%),turningthecolorofthesolutionfromdarkbrowntoyellow.Themixturewas lteredandwashedwith1:10HClaqueoussolution(250mL)toremovemetalions.Theresultingsolidwasdriedinairanddilutedto600mL,makingagraphiteoxideaqueousdispersion.Finally,itwaspuri edbydialysisforoneweekusingadialysismembrane(BeijingChemicalReagentCo.,China)withamolecularweightcutoffof8000À14,000gmolÀ1toremovetheremainingmetalspecies.Theresultantgraphiteoxideaqueousdispersionwas

thendilutedto1.2L,stirredovernightandsonicatedfor30mintoexfoliateittoGO.TheGOdispersionwasthencen-trifugedat3000rpmfor40mintoremovetheunexfoliatedgraphite.Forcomparison,GOwasalsopreparedbyconven-tionalHummersmethod[19],andpuri edusingthesamepro-ceduresdescribedabove.TheGOproductspreparedbytheimprovedandconventionalHummersmethodsarenomi-natedasGO1orGO2,respectively.

2.2.Instrumentsandcharacterizations

GOdispersionswerefreeze-driedandusedformorphologicalandstructuralcharacterizations.Ramanspectrawerere-cordedonaRenishawRamanspectrometerwitha514nmla-seratapowerof4.7mW.X-rayphotoelectronspectra(XPS)wererecordedonanESCALAB250photoelectronspectrome-ter(ThermoFisherScienti c)withAlKa(1486.6eV)astheX-raysourcesetat150Wandapassenergyof30eVforhighresolutionscan.UV–visiblespectraweretakenoutbytheuseofaU-3010UV–visiblespectrometer(Hitachi,Japan).Scanningelectronmicrographs(SEM)weretakenoutona eld-emissionscanningelectronmicroscope(Sirion-200,Ja-pan).Theatomicforcemicroscopic(AFM)imagesofGOsheetsweremeasuredusingascanningprobemicroscope(SPM-9600,Shimadzu).ThesamplesusedforSEMandAFMcharacterizationsweredepositedonsiliconwafersandmicasheets,respectively.Fouriertransforminfraredspectros-copy-attenuatedtotalre ectance(FTIR-ATR)spectrawerere-cordedonaFouriertransforminfraredspectrometer(BrukerVertexV70).ThezetapotentialsofGOaqueousdispersionsweremeasuredbytheuseofHORIBANanoparticleanalyzerSZ-100.X-raydiffraction(XRD)wascarriedoutonaD8Ad-vanceX-raydiffractometerwithCuKaradiation(k=0.15418nm,Bruker,Germany).

2.3.

TheremovingofMn2+ionsfromwastewater

Typically,wastewaterwascollectedfromtheprocessof l-tratingGOfromthereactionsystemofimprovedHummersmethod.Successively,20mLofwastewaterwasdilutedandneutralizedbya0.2gmLÀ1KOHsolution.ThepHofthesolu-tionwasadjustedto$10andaprecipitatewasformed.Then,thissystemwaskeptundisturbedovernighttoagetheprecip-itate.Finallythesedimentwas ltrated.TheMn2+ionsinthepuri edwastewater(or ltrate)wastestbyaddingitforsev-eraldropsintoa3mLaqueoussolutionofNa2S2O8(0.1gmLÀ1)followedbyboilingthemixturefor1min.

3.Resultsanddiscussion

GOsamplesweresynthesizedbyusingHummersmethodwithout(GO1)orwith(GO2)usingofNaNO3andpuri edbydialysisandcentrifugation.Theyields(theweightofGOdi-videdbytheweightofgraphitepowder)ofGO1andGO2weremeasuredtobe92%±3%and96%±2%,respectively.Thisre-sultindicatesthatthesolutionofconcentratedH2SO4con-tainingKMnO4iscapableofoxidizinggraphitetoGOinayieldclosetothatofHummersmethodevenwithouttheassistanceofNaNO3.

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Thecomposition,structureandmorphologyofGO1werecharacterizedtobenearlythesametothoseofGO2.Fig.1aistheUV–visiblespectrumoftheaqueousdispersionofGO1.Thespectrumhasamainabsorptionpeakat232nmandashoulderpeakat300nm,whichareattributedtopÀp*tran-sitionofC@CbondsandnÀp*transitionofC@Obonds,respectively.TheoverallfeatureofthisspectrumisidenticaltothatoftheGOsynthesizedusingconventionalHummersmethod(GO2,Fig.S1a)anditsadsorptionpeaksarealsosimilartothoseoftheGOsamplesreportedinliterature[20].ThedispersionofGO1showsaclearyellowcolor,indi-catingasuccessfuloxidationofgraphitetoGO[19].TheC/OatomicratiosofGO1(Fig.1b)andGO2(Fig.S1b)weremea-suredbyXPStobe2.36and2.23,respectively,re ectingtheirsimilardegreesofoxidation.Thesevaluesareamongtherangeof2.1À2.9fortheGOproductsreportedpreviously[19].TheC1sspectrumofGO1(Fig.1c)demonstratesfourtypesofcarbonbonds:C–C/C@C(284.6eV),C–O(286.6eV),C@O(287.8eV),andO–C@O(289.0eV).Thepeakintensitiesofintactcarbon(C–C/C@C)andoxygenatedcarbonatomsinthisXPSspectrumwerecalculatedtobe47.9%and52.1%(Fig.1c),correspondingly.ThosevaluesinthespectrumofGO2weremeasuredtobe46.5%and53.5%,respectively(Fig.S1c).Thisresultfurthercon rmsthattheyhavecompa-rableoxidizationdegrees.ItshouldbenotedherethattheoxidationdegreesofGOproductsvarywiththeirsynthesisconditions[11,15,20].EitherGO1orGO2hasamediumoxi-dationdegreecomparedwiththoseofless[15]andhighlyoxidizedcounterparts[20].ThezetapotentialsofGO1andGO2suspensionsweremeasuredtobeÀ43.8±1.3andÀ45.6±0.6mV,respectively,indicatingtheyarenegativelychargedbecauseofthepresenceofcarboxylgroups.AlthoughGO1hasaslightlyhigherzetapotentialthanthatofGO2,itsvalueisstilllowerthanÀ30mV,providingitwith

peakat2h=10.9°(Fig.2c),correspondingtoad-spaceof0.81nm,andthisvalueisinconsistentwiththatoffreeze-driedGO2(Fig.S2c).ThelargeinterlayerspacingofGO1sheetscanbeattributedtoitsoxygenatedfunctionalgroupsintroducedbytheharshoxidationtreatmentofgraphite[26].

RamanandinfraredspectralstudiesalsodemonstratethatbothGOproductsarestructurallythesame.TheRamanspec-trumofGO1(Fig.2d)orGO2(Fig.S2d)showsaG-bandat$1590cmÀ1andaD-bandat$1350cmÀ1.TheG-bandisasso-ciatedwithgraphiticcarbonsandtheD-bandisrelatedtothestructuraldefectsorpartiallydisorderedgraphiticdomains[27].TheD-bandsinbothspectraarestrong,con rmingthelatticedistortionsofgraphenebasalplanes.Furthermore,theFTIRÀATRspectraofGO1andGO2papers(Fig.2eandS2e)showthefollowingcharacteristicfunctionalgroupsofGO[20,28]:CÀOÀC($1000cmÀ1),CÀO(1230cmÀ1),C@C($1620cmÀ1)andC@O(1740–1720cmÀ1)bonds.TheOÀHstretchingvibrationsintheregionof3600–3300cmÀ1areattributedtothehydroxylandcarboxylgroupsofGOandresidualwaterbetweenGOsheets.Thesehydrophilicoxy-gen-containingfunctionalgroupsprovideGOsheetswithagooddispersibilityinwater[9].

Thermalgravimetricanalysis(TGA)curvesofGO1andGO2arecomparedinFig.3.Bothcurvesexhibitsimilarcharacter-istics:theweightlossbefore100°CiscausedbythereleaseoftrappedwaterbetweenGOsheets[28];thedistinctweightlossbetween200and230°CisattributedtothedecompositionoflessstableoxygenatedfunctionalgroupsonGOsheets[29].Aweakermasslossintherangeof230–700°Cisrelatedtotheremovalofmorestablefunctionalgroups.Thenearlyidenti-calTGAcurvesofbothGOsamplesre ecttheirclosecontentsofoxygenatedgroups.

Post-treatmentofthewastewatercollectedfromthepro-cessesofGOsynthesisandpuri cationiscrucialforcommer-

tobeMn3O4containingasmallamountofMn(OH)2(Fig.5).Theef ciencyofremovingMn2+ionsfromthewastewaterhasbeentestedbytheadditionofthepuri edsupernatant

3natesthegenerationoftoxicgassesandsimpli estheproce-dureofpurifyingwasteliquid,thusdecreasesthecostofGOsynthesis.TheGOproductspreparedbyboththeimprovedandconventionalHummersmethodsarenearlythesamein

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theirdispersibility,chemicalstructures,thicknesses,andlat-eraldimensions.Furthermore,theexclusionofNaNO3doesnotaffecttheyieldoftheoverallreaction.TheimprovedHummersmethoddescribedherecanbeusedtoprepareGOinlargescaleanditisone-steptowardsthesynthesisofgrapheneanditsderivativesthroughenvironmentallyfriendlyapproaches.

Acknowledgements

ThisworkwassupportedbynationalbasicresearchprogramofChina(973Program,2012CB933402),naturalsciencefoun-dationofChina(91027028,51161120361,21274074).

AppendixA.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,at/10.1016/j.carbon.2013.07.055.

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