Synthesis of ZnO nanoparticles from microemulsions in a flow

SynthesisofZnOnanoparticlesfrommicroemulsionsina owtype

microreactor

articleinfoabstract

Zincoxide(ZnO)nanoparticlesweresynthesizedfrommicroemulsionsinamicrochannelreactorsystem.Themicroemulsionsprovidecon nedspaceforthereactants,whichisfavorableforcontrollablereactionandnucleation,thusavoidingtheformationoflargeparticles.Inaddition,themicroemulsionspreventthedepositionofZnOparticlesonthewallofthemicrochannelsofthereactor.ThreeZn2+sources(Zn(NO3)2,ZnSO4,andZnCl2)weretestedinthesynthesisofZnOnanoparticles.Amongthem,Zn(NO3)2showedbestperformance,yieldingZnOparticleswiththesmallestaveragegrainsize.TheeffectsofZn2+concentration,reactiontemperature,andfeed owrateontheaverageparticlesizeofZnOnanoparticleswereinvestigated.Atoptimalconditions,ZnOnanoparticleswithaveragesizeof16nmwereobtained.ThesynthesizedZnOnanoparticleswerecharacterizedbyscanningelectronmicroscope(SEM),X-raydif-fraction(XRD),UV–visabsorptionspectroscopy,andalaserparticlesizeanalyzer.

Ó2013ElsevierB.V.Allrightsreserved.

Articlehistory:

Received26April2013

Receivedinrevisedform27August2013Accepted4September2013

Availableonline12September2013Keywords:

MicrochannelreactorMicroemulsionZincoxideNanoparticle

1.Introduction

ZnOisanimportantsemiconductormaterialwithextensiveapplicationsinelectronics,photoelectronics,sensors,andopticaldevices[1–4].ThephysicalpropertiesofZnOnanoparticlesarestronglydependentontheparticledimensions,includingmorphol-ogyandgrainsizedistribution.Twotypesofsyntheticapproaches,vapor-phasesynthesisandsolution-phasesynthesis,havebeendevelopedtofabricateZnOnanoparticles.Thevapor-phaseap-proaches,suchasvapor–liquid–solidgrowth[5],chemicalvapordeposition[6],thermaldecomposition[7],andthermalevapora-tion[8],havetheadvantageofsimpleoperationandhigh-qualityproducts,butgenerallyrequirehightemperaturesandexpensiveCorrespondingauthor.Address:SchoolofChemicalEngineering,Dalian

UniversityofTechnology,Dalian116024,PRChina.Tel.:+8641184986121.

E-mailaddress:wangyao@(Y.Wang).1385-8947/$-seefrontmatterÓ2013ElsevierB.V.Allrightsreserved./10.1016/j.cej.2013.09.020

equipments.Solution-phaseapproachesaremorepromisingduetothelowreactiontemperature,lowcost,andhighef ciency.However,inthelaterapproach,ZnO owersandwhiskerswithlargesize(>100nm)areoftenobtained,andthesubsequentsedi-mentationorcalcinationleadstotheaggregationofZnOparticles.Inaddition,thesynthesisinabatchreactorisnoteffectiveinalargescaleproduction.Therefore,newmethodswhichfacilitatethenucleation,growth,andparticlesizedistributionin

thesynthe-sisofZnOnanoparticlesarehighlydesirable[9].

Themicroemulsionhasfoundgreatapplicationsinthesynthe-sisofnanomaterials[10–12].Inthemicroemulsionapproach,thereactantsinaqueoussolutionarecon nedintheextremelysmalldroplets,inwhichauniformnucleationoccurs.Additionally,the

microemulsionhelpstocontrolthesizeandshapeoftheparticles,preventingthenanoparticlesfromaggregation.Nevertheless,themicroemulsionmethodsuffersfromlowyieldofnanoparticlesandthedif cultyofde-emulsi cation.Asaconsequence,the

reactorperformanceisgenerallylowwhenthesynthesistakesplaceinabatchreactor.

Recently,microchannelreactorshavebeenutilizedforproduc-ingnano-sizedparticles,includingmetalsandalloys[13–18],me-talsalts[19,20],metaloxides[21],polymers[22],mesoporousmaterials[23],andzeolites[24].The owtypemicrochannelreac-torsareabletointensifythemassandheattransfersaswellasthemixing.Thehighsurface-to-volumeratiointhemicrochannelreactorsisfavorabletoenhancetheresponsetimeandmaintainisothermalconditions.Becausetheconcentrationsofreactantsandtemperaturearehomogeneousinthereactionzone,theob-tainedparticlesareuniformandreproducible.

Whenasinglephaseisinvolved,thevelocitydistributioninamicrochannelissubstantiallybroadenedalongthe owdirection.Guntheretal.[25]comparedthewellmixingef ciencychaoticmixerwithaliquid–liquidtwophasemixer,andfoundthat,whenthe uidwasmixedcompletely(P95%),thelengthofthechannelrequiredforthetwo-phase owwas2–3timesshorterthanforthesingle-phase ow.Thecomputational uiddynamics(CFD)simula-tionsindicatethattheenhancementofmasstransfercanbeinter-pretedintermsofaninternalcirculation owwithintheplugs.Asaconsequence,narrowparticlesizedistributioncouldbeobtainedinthesynthesisofnanoparticlesduetotheenhancedmixingandthenarrowresidencetimedistributioninthesegmentedliquid–li-quid ow[26].Anotherimportantissueinthesynthesisofsolidmaterialsinamicrochannelisthattheformedparticlesmaynucle-ateanddepositonthemicrochannelwalls,leadingtorunaway

Inthepresentpaper,ZnOnanoparticlesweresynthesizedbymixingtheZn2+-containingwater-in-oilmicroemulsionwiththeNaOH-containingoneinamicromixerfollowedbysubsequentreactionintherelaytube(Fig.1).Thesynthesisconditionswereoptimized,andtheobtainedZnOnanoparticleswerecharacterized.2.Experimental2.1.Synthesis

Allofthechemicalswereofanalyticalgrade,andusedwithoutfurtherpuri cation(TianjinKermelChemicalReagentCo.Ltd.).De-ionizedwaterwasobtainedfromawaterpuri cationsystem.Themicroemulsionswerepreparedinthefollowingway.N-butanol,cetyltrimethylammoniumbromide(CTAB),andn-octaneweremixedatamassratioof1.0:1.2:4.4toformanorganicphase.CTABservedasthesurfactant,whereasn-butanolastheco-surfac-tant.AnaqueoussolutionofZn2+(Zn(NO3)2,ZnSO4,andZnCl2)werepreparedbydissolvingthesaltinwaterunderstirring.ThesolutionofNaOHwaspreparedinasimilarway.Themicroemul-sionofZn2+(denotedasM(Zn2+))wereobtainedbyaddingtheaqueoussolutionofZn2+intotheaboveorganicphaseundervigor-ousstirringwithanaqueousmassfractionof15%,andthemixturewasstirreduntilitbecametransparent.ThemicroemulsionofNaOH(denotedasM(NaOH))waspreparedbythesameprocedurewiththesameaqueousmassfraction.

nedspace.Therefore,thesynthesisreactionwillterminatewhennoZn2+sourceisavailable,thusmakingthefastreactioncontrolla-bleandpreventingtheformationofsubstantiallylargerparticles.Anotherfavorableadvantageofthisapproachisthattheresultantprecipitatesareentrappedinthedrople …… 此处隐藏：6191字，全部文档内容请下载后查看。喜欢就下载吧 ……