改良hummers合成氧化石墨烯

第第页改良hummers合成氧化石墨烯AnimprovedHummersmethodforeco-friendlysynthesisofgrapheneo*ide

JiChen,BowenYao,ChunLi,GaoquanShi

*

DepartmentofChemistry,TsinghuaUniversity,Beijing100084,People’sRepublicofChina

ARTICLEINFOABSTRACT

Articlehistory:Received4June2022Accepted21July2022Availableonline27July2022

AnimprovedHummersmethodwithoutusingNaNO3canproducegrapheneo*idenearlythesametothatpreparedbyconventionalHummersmethod.Thismodicationdoesnotdecreasetheyieldofproduct,eliminatingtheevolutionofNO2/N2O4to*icgassesandsim-plifyingthedisposalofwastewaterbecauseoftheine*istenceofNa+andNO3ions.Forthersttime,wealsodevelopedaprototypemethodofpost-treatingthewastewatercol-lectedfromthesystemsofsynthesizingandpurifyinggrapheneo*ide.ThecontentofMn2+ionsinthepuriedwastewaterwasmeasuredtobelowerthantheguidelinevaluefordrinkingwater.

2022ElsevierLtd.Allrightsreserved.

1.Introduction

Graphenehasauniqueatom-thicktwo-dimensionalstruc-ture,e*cellentelectronic,mechanical,opticalandthermalproperties[1].Therefore,ithasbeenwidelye*ploredfortheapplicationsinelectronics[2],catalysis[3],sensors[4],andenergyconversionandstorage[5,6],etc.Forthesepurposes,themass-productionofgraphenematerialsatlowcostsisoneoftheessentialrequirements.Actually,graphenesheetsalreadye*istinnatureandweneedtoe*foliatethemfromtheirprecursors[7].Thee*foliationofgraphitetographenecanberealizedeitherphysicallyorchemically[1].Amongthevariousmethods,chemicalreductionofgrapheneo*ide(GO)toreducedgrapheneo*ide(rGO)isuniqueandattractivebecauseofitscapabilityofproducingsingle-layergrapheneinlargescaleandatrelativelylowcost[8].Furthermore,GOandrGOareprocessibleandtheycanbefabricatedorself-assem-bledintomacroscopicmaterialswithcontrolledcompositionsandmicrostructuresforpracticalapplications[9].

GOistheprecursorofrGO;thus,itplaysacrucialroleincontrollingthestructure,propertyandtheapplicationpoten-tialofrGO[1016].ThepioneeringworkonthesynthesisofGOwasreportedbyBrodiein1859[17].Inthismethod,one

equalweightofgraphitewasmi*edwiththreeequalweightsofKClO3andreactedinfumingHNO3at60Cfor4days.Sta-udenmaierimprovedBrodiemethodbyreplacingabouttwothirdsoffumingHNO3withconcentratedH2SO4andaddingKClO3inmultipleportions[18].Thissmallmodicationen-ablestheoverallreactioninasinglevessel;thussimplifyingthesynthesismethod.However,thisreactionstillneedsalongtimeof4days.ThemostimportantandwidelyappliedmethodforthesynthesisofGOwasdevelopedbyHummersandOffemanin1958(Hummersmethod)[19].Inthiscase,theo*idationofgraphitewasachievedbyharshtreatmentofoneequalweightofgraphitepowdersinaconcentratedH2SO4solutioncontainingthreeequalweightsofKMnO4and0.5equalweightofNaNO3.TheHummersmethod,atleast,hasthreeimportantadvantagesoverprevioustech-niques.First,thereactioncanbecompletedwithinafewhours.Second,KClO3wasreplacedbyKMnO4toimprovethereactionsafety,avoidingtheevolutionofe*plosiveClO2.Third,theuseofNaNO3insteadoffumingHNO3eliminatestheformationofacidfog.

Hummersmethodhasbeenpaidthemostintensiveatten-tionbecauseofitshighefciencyandsatisfyingreactionsafety.However,itstillhasthefollowingtwoaws:(1)theo*i-

*Correspondingauthor:Fa*:+861062771149.

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

0008-6223/$-seefrontmatter2022ElsevierLtd.Allrightsreserved./10.1016/j.carbon.2022.07.055

226

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dationprocedurereleasesto*icgassessuchasNO2andN2O4;(2)theresidualNa+andNO3ionsaredifculttoberemovedfromthewastewaterformedfromtheprocessesofsynthe-sizingandpurifyingGO.Tourandco-workersimprovedtheHummersmethodbye*cludingNaNO3,increasingtheamountofKMnO4,andperformingthereactionina9:1(byvolume)mi*tureofH2SO4/H3PO4[20].Thismodicationissuccessfulinincreasingthereactionyieldandreducingto*icgasevolution,whileusingtwiceasmuchKMnO4and5.2timesasmuchH2SO4asthoserequiredbyHummersmethodandalsointroducinganewcomponentofH3PO4tothereactionsystem.

Recently,Baek’sgroupstudiedtheprocessofetchingthebasalplanesofhighlyorderedpyrolyticgraphite(HOPG)withahotmi*tureofH2SO4andHNO3[21].Inthiscase,thegraph-enelayersofHOPGwereeffectivelycutande*foliatedafteralong-termtreatment.ThisobservationindicatesthattheH2SO4/HNO3mi*tureusedinHummersmethodactsasachemical‘‘scissor’’andachemical‘‘drill’’forgrapheneplanestofacilitatethepenetrationofo*idationsolution.Ontheotherhand,KMnO4isoneofthestrongesto*idants,espe-ciallyinacidicmedia[22].WiththeassistanceofKMnO4,acompleteintercalationofgraphitewithconcentratedH2SO4canbeachieved,forminggraphitebisulfateinwhicheverysingle-layergrapheneissandwichedbythelayersofbisulfateions[23,24].ThiscompleteintercalationensurestheeffectivepenetrationofKMnO4solutionintographenelayersfortheo*idationofgraphite.Accordingly,KMnO4canalsotaketheroleofNaNO3andthelatterisunnecessaryforthesynthesisofGOusingHummersmethod.Inthisarticle,wedemon-stratethatGOcanbeproducedusinganimprovedHummersmethodwithoutusingNaNO3.ThismethoddecreasesthecostandenvironmentaldutyofGOproduction.

2.

E*perimental

2.1.

SynthesisandpuricationofGO

GOwaspreparedbytheo*idationofnaturalgraphitepowder(325mesh,QingdaoHuataiLubricantSealingSTCo.Ltd.,Qingdao,China)accordingtoHummersmethodwithamodi-cationofremovingNaNO3fromthereactionformula[19].Typically,graphitepowder(3.0g)wasaddedtoconcentratedH2SO4(70mL)understirringinanicebath.Undervigorousagitation,KMnO4(9.0g)wasaddedslowlytokeepthetemper-atureofthesuspensionlowerthan20C.Successively,thereactionsystemwastransferredtoa40Coilbathandvigor-ouslystirredforabout0.5h.Then,150mLwaterwasadded,andthesolutionwasstirredfor15minat95C.Additional500mLwaterwasaddedandfollowedbyaslowadditionof15mLH2O2(30%),turningthecolorofthesolutionfromdarkbrowntoyellow.Themi*turewaslteredandwashedwith1:10HClaqueoussolution(250mL)toremovemetalions.Theresultingsolidwasdriedinairanddilutedto600mL,makingagraphiteo*ideaqueousdispersion.Finally,itwaspuriedbydialysisforoneweekusingadialysismembrane(BeijingChemicalReagentCo.,China)withamolecularweightcutoffof800014,000gmol1toremovetheremainingmetalspecies.Theresultantgraphiteo*ideaqueousdispersionwas

thendilutedto1.2L,stirredovernightandsonicatedfor30mintoe*foliateittoGO.TheGOdispersionwasthencen-trifugedat3000rpmfor40mintoremovetheune*foliatedgraphite.Forcomparison,GOwasalsopreparedbyconven-tionalHummersmethod[19],andpuriedusingthesamepro-ceduresdescribedabove.TheGOproductspreparedbytheimprovedandconventionalHummersmethodsarenomi-natedasGO1orGO2,respectively.

2.2.Instrumentsandcharacterizations

GOdispersionswerefreeze-driedandusedformorphologicalandstructuralcharacterizations.Ramanspectrawerere-cordedonaRenishawRamanspectrometerwitha514nmla-seratapowerof4.7mW.*-rayphotoelectronspectra(*PS)wererecordedonanESCALAB250photoelectronspectrome-ter(ThermoFisherScientic)withAlKa(1486.6eV)asthe*-raysourcesetat150Wandapassenergyof30eVforhighresolutionscan.UV–visiblespectraweretakenoutbytheuseofaU-3010UV–visiblespectrometer(Hitachi,Japan).Scanningelectronmicrographs(SEM)weretakenoutonaeld-emissionscanningelectronmicroscope(Sirion-200,Ja-pan).Theatomicforcemicroscopic(AFM)imagesofGOsheetsweremeasuredusingascanningprobemicroscope(SPM-9600,Shimadzu).ThesamplesusedforSEMandAFMcharacterizationsweredepositedonsiliconwafersandmicasheets,respectively.Fouriertransforminfraredspectros-copy-attenuatedtotalreectance(FTIR-ATR)spectrawerere-cordedonaFouriertransforminfraredspectrometer(BrukerVerte*V70).ThezetapotentialsofGOaqueousdispersionsweremeasuredbytheuseofHORIBANanoparticleanalyzerSZ-100.*-raydiffraction(*RD)wascarriedoutonaD8Ad-vance*-raydiffractometerwithCuKaradiation(k=0.15418nm,Bruker,Germany).

2.3.

TheremovingofMn2+ionsfromwastewater

Typically,wastewaterwascollectedfromtheprocessofl-tratingGOfromthereactionsystemofimprovedHummersmethod.Successively,20mLofwastewaterwasdilutedandneutralizedbya0.2gmL1KOHsolution.ThepHofthesolu-tionwasadjustedto$10andaprecipitatewasformed.Then,thissystemwaskeptundisturbedovernighttoagetheprecip-itate.Finallythesedimentwasltrated.TheMn2+ionsinthepuriedwastewater(orltrate)wastestbyaddingitforsev-eraldropsintoa3mLaqueoussolutionofNa2S2O8(0.1gmL1)followedbyboilingthemi*turefor1min.

3.Resultsanddiscussion

GOsamplesweresynthesizedbyusingHummersmethodwithout(GO1)orwith(GO2)usingofNaNO3andpuriedbydialysisandcentrifugation.Theyields(theweightofGOdi-videdbytheweightofgraphitepowder)ofGO1andGO2weremeasuredtobe92%3%and96%2%,respectively.Thisre-sultindicatesthatthesolutionofconcentratedH2SO4con-tainingKMnO4iscapableofo*idizinggraphitetoGOinayieldclosetothatofHummersmethodevenwithouttheassistanceofNaNO3.

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Thecomposition,structureandmorphologyofGO1werecharacterizedtobenearlythesametothoseofGO2.Fig.1aistheUV–visiblespectrumoftheaqueousdispersionofGO1.Thespectrumhasamainabsorptionpeakat232nmandashoulderpeakat300nm,whichareattributedtopp*tran-sitionofC@Cbondsandnp*transitionofC@Obonds,respectively.TheoverallfeatureofthisspectrumisidenticaltothatoftheGOsynthesizedusingconventionalHummersmethod(GO2,Fig.S1a)anditsadsorptionpeaksarealsosimilartothoseoftheGOsamplesreportedinliterature[20].ThedispersionofGO1showsaclearyellowcolor,indi-catingasuccessfulo*idationofgraphitetoGO[19].TheC/OatomicratiosofGO1(Fig.1b)andGO2(Fig.S1b)weremea-suredby*PStobe2.36and2.23,respectively,reectingtheirsimilardegreesofo*idation.Thesevaluesareamongtherangeof2.12.9fortheGOproductsreportedpreviously[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)ando*ygenatedcarbonatomsinthis*PSspectrumwerecalculatedtobe47.9%and52.1%(Fig.1c),correspondingly.ThosevaluesinthespectrumofGO2weremeasuredtobe46.5%and53.5%,respectively(Fig.S1c).Thisresultfurtherconrmsthattheyhavecompa-rableo*idizationdegrees.Itshouldbenotedherethattheo*idationdegreesofGOproductsvarywiththeirsynthesisconditions[11,15,20].EitherGO1orGO2hasamediumo*i-dationdegreecomparedwiththoseofless[15]andhighlyo*idizedcounterparts[20].ThezetapotentialsofGO1andGO2suspensionsweremeasuredtobe43.81.3and45.60.6mV,respectively,indicatingtheyarenegativelychargedbecauseofthepresenceofcarbo*ylgroups.AlthoughGO1hasaslightlyhigherzetapotentialthanthatofGO2,itsvalueisstilllowerthan30mV,providingitwith

peakat2h=10.9(Fig.2c),correspondingtoad-spaceof0.81nm,andthisvalueisinconsistentwiththatoffreeze-driedGO2(Fig.S2c).ThelargeinterlayerspacingofGO1sheetscanbeattributedtoitso*ygenatedfunctionalgroupsintroducedbytheharsho*idationtreatmentofgraphite[26].

RamanandinfraredspectralstudiesalsodemonstratethatbothGOproductsarestructurallythesame.TheRamanspec-trumofGO1(Fig.2d)orGO2(Fig.S2d)showsaG-bandat$1590cm1andaD-bandat$1350cm1.TheG-bandisasso-ciatedwithgraphiticcarbonsandtheD-bandisrelatedtothestructuraldefectsorpartiallydisorderedgraphiticdomains[27].TheD-bandsinbothspectraarestrong,conrmingthelatticedistortionsofgraphenebasalplanes.Furthermore,theFTIRATRspectraofGO1andGO2papers(Fig.2eandS2e)showthefollowingcharacteristicfunctionalgroupsofGO[20,28]:COC($1000cm1),CO(1230cm1),C@C($1620cm1)andC@O(1740–1720cm1)bonds.TheOHstretchingvibrationsintheregionof3600–3300cm1areattributedtothehydro*ylandcarbo*ylgroupsofGOandresidualwaterbetweenGOsheets.Thesehydrophilico*y-gen-containingfunctionalgroupsprovideGOsheetswithagooddispersibilityinwater[9].

Thermalgravimetricanalysis(TGA)curvesofGO1andGO2arecomparedinFig.3.Bothcurvese*hibitsimilarcharacter-istics:theweightlossbefore100CiscausedbythereleaseoftrappedwaterbetweenGOsheets[28];thedistinctweightlossbetween200and230Cisattributedtothedecompositionoflessstableo*ygenatedfunctionalgroupsonGOsheets[29].Aweakermasslossintherangeof230–700Cisrelatedtotheremovalofmorestablefunctionalgroups.Thenearlyidenti-calTGAcurvesofbothGOsamplesreecttheirclosecontentsofo*ygenatedgroups.

Post-treatmentofthewastewatercollectedfromthepro-cessesofGOsynthesisandpuricationiscrucialforcommer-

tobeMn3O4containingasmallamountofMn(OH)2(Fig.5).TheefciencyofremovingMn2+ionsfromthewastewaterhasbeentestedbytheadditionofthepuriedsupernatant

3natesthegenerationofto*icgassesandsimpliestheproce-dureofpurifyingwasteliquid,thusdecreasesthecostofGOsynthesis.TheGOproductspreparedbyboththeimprovedandconventionalHummersmethodsarenearlythesamein

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theirdispersibility,chemicalstructures,thicknesses,andlat-eraldimensions.Furthermore,thee*clusionofNaNO3doesnotaffecttheyieldoftheoverallreaction.TheimprovedHummersmethoddescribedherecanbeusedtoprepareGOinlargescaleanditisone-steptowardsthesynthesisofgrapheneanditsderivativesthroughenvironmentallyfriendlyapproaches.

Acknowledgements

ThisworkwassupportedbynationalbasicresearchprogramofChina(973Program,2022CB933402),naturalsciencefoun-dationofChina(91027028,51161120361,21274074).

Appendi*A.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,at/10.1016/j.carbon.2022.07.055.

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AnimprovedHummersmethodforeco-friendlysynthesisofgrapheneo*ide

JiChen,BowenYao,ChunLi,GaoquanShi

*

DepartmentofChemistry,TsinghuaUniversity,Beijing100084,People’sRepublicofChina

ARTICLEINFOABSTRACT

Articlehistory:Received4June2022Accepted21July2022Availableonline27July2022

AnimprovedHummersmethodwithoutusingNaNO3canproducegrapheneo*idenearlythesametothatpreparedbyconventionalHummersmethod.Thismodicationdoesnotdecreasetheyieldofproduct,eliminatingtheevolutionofNO2/N2O4to*icgassesandsim-plifyingthedisposalofwastewaterbecauseoftheine*istenceofNa+andNO3ions.Forthersttime,wealsodevelopedaprototypemethodofpost-treatingthewastewatercol-lectedfromthesystemsofsynthesizingandpurifyinggrapheneo*ide.ThecontentofMn2+ionsinthepuriedwastewaterwasmeasuredtobelowerthantheguidelinevaluefordrinkingwater.

2022ElsevierLtd.Allrightsreserved.

1.Introduction

Graphenehasauniqueatom-thicktwo-dimensionalstruc-ture,e*cellentelectronic,mechanical,opticalandthermalproperties[1].Therefore,ithasbeenwidelye*ploredfortheapplicationsinelectronics[2],catalysis[3],sensors[4],andenergyconversionandstorage[5,6],etc.Forthesepurposes,themass-productionofgraphenematerialsatlowcostsisoneoftheessentialrequirements.Actually,graphenesheetsalreadye*istin