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AppliedSurfaceScience273 (2013) 118–121ContentslistsavailableatSciVerseScienceDirectAppliedSurfaceSciencejournalhomepage:/locate/apsuscEffectsofparticlesizeandpHvalueonthehydrophilicityofgrapheneoxideXuebingHua,b,c,YunYua,∗,WeiminHoua,c,JianerZhoub,LixinSonga,∗aKeyLaboratoryofInorganicCoatingMaterials,ShanghaiInstituteofCeramics,ChineseAcademyofScience,Shanghai201800,PRChinaKeyLaboratoryofInorganicMembrane,JingdezhenCeramicInstitute,Jingdezhen333001,PRChinacUniversityofChineseAcademyofSciences,19YuquanRoad,Beijing,100049,PRChinabarticleinfoabstractArticlehistory:Received4December2012Receivedinrevisedform10January2013Accepted28January2013Available online 4 February 2013Keywords:GrapheneoxideHydrophilicityParticlesizepHvalueGraphene-basedmaterialhasattractedextensiveattentionfrombothexperimentalandtheoreticalsci-entifiivativeofgraphene,grapheneoxidehasalsobecomeanattractivematerialandbeeninvestigatedwidelyinmanyareassincepaper,wepreparedgrapheneoxidebythemodifirophilicityofgrapheneoxidewithdifferefindthewatercontactangleofthedif-ferentgrapheneoxidesdecreasesfrom61.8◦to11.6◦,-rayphotoelectronspectroscopy,zetapotentialanddynamiclightscatteringmea-surementsweretakentostudythechemicalsultsshowthehydrophilicityofgrapheneoxideissensitivetoparticlesizeandpHvalue,kprovidesasimplewaystocontrolthehydrophilicityofgrapheneoxidebyadjustingparticlesizeandpHvalue.© 2013 Elsevier B.V. All rights uctionAsaderivativeofgraphene,grapheneoxide(GO)consistsofoxygenatedgraphenesheetswithoxygen-containingfunctionalgroupsontheirbasalplanesandedges[1].Owingtothesegroups,GOshowstheexcellenthydrophilicityanduniformityofdisper-sioninwater,makingitmoreeasilyhandleanddepositthannativegraphene[2].Nowadays,GOhasbeenwidelyinvestigatedinmanyareas,suchasnanoelectronics[3],nanocomposites[4],supercapa-citors[5],environmentalprotection[6],energystorage[7],sensors[8]andsurfactant[9].Hydrophilicityisanimportantpropertyofsolidsurfaceandcanbeaffectedbyboththechemicalcompositionandthegeometricstructureofthesurface[10].Inrecentyears,thestimuli-responsivesurfacewiththetunablehydrophilicityhasgainedmuchattentionbecauseoftheirusefulnessinfundamentalresearchandindus-trialapplications,suchasthecontrollableseparationsystemsandthemicrofluidicdevices[11–13].Thesmartsurfacecanreversiblyswitchitshydrophilicityinresponsetoexternalstimuli,includingpH[14],temperature[15],light[16],electricalfield[17],chemicalsubstances[18],usreportshavedemonstratedthatGOhastheexcellentandtunablehydrophilicity[19–25].However,fewreportshavefocusedindetailontheinfl,weinvestigatetheimpactsofparticlesizeandpHvalueonthehydrophilicityofGObymeasuringthewatercontactangle(CA).BycontrollingtheparticlesizeandpHvalue,wefiaphene-basedmaterialswithcontrol-lablehydrophilicpropertiesprovidationofGO∗.:+862169906167;fax:+862169906171.E-mailaddresses:yunyush@(),lxsong@().WepreparedGOusingthemodifiedHummersmethod[26].Briefly,1gofexpandablegraphiteand0.5gofNaNO3wereplacedinafl,75mlofconcentratedH2SO4wasaddedwithstir-ringinanicewaterbath,rewasobtaineemixturewasheatedto70◦y,100mldeionizedwaterwasaddedslowlywithstirringandthereactionwasheated0169-4332/$–seefrontmatter© 2013 Elsevier B.V. All rights :///10.1016/./AppliedSurfaceScience273 (2013) 118–121119to90◦reactionwasterminatedbyadditionof100mldeionizedwaterand30mlH2O230wt%vetheionsofoxidantandotherinorganicimpurity,wewashedthemixturebyrepeatedvacuumfiith5%HClaqueoussolution,edispersionwascentrifuged(5000rpmfor20min),y,thesupernatantwasdilutedwithdeionizedwatertoatotalvolumeof200mlina500mlflationofdifferentparticlesizesandpHvaluesofGOaqueousdispersionHomogeneousGOaqueousdispersionwascentrifuged(15,000rpmfor80min),thesupernatant(SGO)waskeptandtheremnant(RGO)n,andRGOdispersion(about10ml)werethendropcoatedoncleanglasssubstratesinaclean-roomenvironmentandallowedtodryfor12hat30◦terizationandmeasurementmethodsTheparticlesizedistributionofSGOandRGOaqueousdis-persionwascharacterizedbydynamiclightscattering(DLS,Brookhaven,USA).micalstateofelementsinGOwasanalysedbyX-rayphotoelectronspectroscopy(XPS,MICROLAB310F,ThermoScientific,UK),whichwasequippedwithamonochromaticMgK␣epressureoftheanalyserchamberwaslessthan10−dingenergieswerecalibratedbasedonthecontaminativecarbon(C1s=284.6eV).XPSpeakswerefittedusingGaussian-likecurvesandthebindingenergyvaluesofpeaksweregivenwithanaccuracyof±apotentialofGOwasdeterminedasafunctionofpHusingazetapotentialanalyser(Zeta-PALS,Brookhaven,USA).ThepHvaluewasmeasuredonamodelPHS-25pHMeterInstruction(withtheprecisionof0.1).ThepHvarophilicityoftheobtainedGOwasevaluatedwiththewatercontactangleinstrument(SL200B,Solon,China)ionizedwaterdropletwasdroppedontoGOsurfacefrom5cmbyvibratingthesyringe.A5␮ropsofwaterwereplacedatdifferentlocationsonahorizontalsurface,erCAmelesizedistributionAstheparticlesizemeasurementonDLSwasbasedontheassumptionthattheparticleswerespherical,heless,themeasure-mentsavailableprovidedaquickandeffectiveevaluationofsizedistribution[27].AsshowninFig.1,ticlesizedistributionofSGOvariesfrom50to175nm,whereasforRGO,nominaleffectivediameterofparticleinRGOandSGOdisper-sionisabout90and35nm,ctraXPSwasalsoemployedtocharacterizethesurfacecomposi-tions1sXPSspectra(solidlines)ctraaredecomposedintothreefit-tedpeaks(dashedlines)ngenergyof285.1eVindicatestheexistenceofCCsp2bondsinGO,while287.1eVresultsfromCObonds(epoxyandhydroxylgroups),and289.2eVgivestheevidenceofCObonds(carbonylgroup)formedduringtheoxidation[28].1sXPSspectraof(a)SGOand(b)./AppliedSurfaceScience273 (2013) 118–:edwithRGO,theC1speakwithbindingenergyaround285.1eVofSGOsuppressessignificantly,suggestingthatSGOhtentialFig.3showhestzetapotentialvalueofSGOandRGOdispersionis−47.6mVatpH10.6and−38.3mVatpH10.3,uesaresimilartothosereportedprevi-ously[29].Thezetapotentialprovidesameasureofthemagnitudeandsignoftheeffectivesurfacechargedensityassociatedwiththedoublelayeraroundthecolloidparticle[2].ultsalsosuggestthatthenegativechargesofGOisaconsequenceoftheionizationofthedifferentgroupspresent[30].Andthesurfacechargedensityshoulddirectrelatetotheconcen-trationoftheionizedgroupspresentatdifferentpHvalues[31].WhenthepHisshiftedtoalkaline,theionizablegroups(carboxylicand/orhydroxylgroup)Hvaluechanges,,atlowpHvalue,thesegroupsareprotonated,renderninFig.3,antly,fromFig.5,erwaterCAisinducedinthesamepHvalue,situation,thesmallerparticlesizebecomesthedominantreasonthatintensifidbeexplainedthatthesmallerparticlesize,withthehigheredge-to-arearatio(highersurfaceenergy)andmoreionizablegroupswhichhelpinteractwithwatermolecules,(Fig.2)analysiscanclarifythechangeofgroupdistributionofGOwithdirminetheeffectofparticlesizeonthehydrophilicity,furtherdetaileddescribedbytheYoung-Dupreequationasfollows[21].