Synthesis of ZnO nanoparticles from microemulsions in a flow type microreactor
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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.Anotherfavorableadvantageofthisapproachisthattheresultantprecipitatesareentrappedinthedroplet,whichisdispersedintheorganicphase,thusavoidingthedepositionofsolidproductonthemicrochannelwalls.
3.1.EffectofZn2+source
ThreeZn2+sourceswereusedintheinvestigationandthesyn-thesiswasconductedunderthefollowingconditions:50°C,2.0MPa,feed owrate2.0mL/min,respectively,NaOH
Journal235(2014)191–197
concentration1.0mol/L,Zn2+concentration0.5mol/L.Fig.5showstheXRDpatternsoftheZnOnanoparticlessynthesizedfromZnSO4,Zn(NO3)2,andZnCl2,respectively.Onlythediffractionpeakschar-acteristicofhexagonalZnOstructurewereobserved,indicatingthatpureZnOcrystalswereobtainedfromthedifferentZn2+sources.Theparticlediameter(D)wascalculatedaccordingtotheDebye–Scherrerequation(D=0.89k/bcosh).Itisfoundthatthecrystallitesizeswere13.0,27.0,and10.4nmforZnCl2,ZnSO4,andZn(NO3)2,respectively.ItsuggeststhattheZn2+sourcemark-edlyaffectstheformationandcrystallizationofZn(OH)2inthemicroreactorsystem.Amongthem,Zn(NO3)2wasthemostsuitable
2+3.2.EffectofZn2+concentration
IntheprecipitationofZn(OH)2,thereactionratedependsonboththeZn2+concentrationandreactiontemperaturefromtheviewpointofreactionkinetics.TheeffectsofZn2+concentrationwereinvestigatedinthesynthesisofZnOnanoparticlesfromZnSO4andZn(NO3)2.Thereactionconditionswereasfollows:feed owratesofM(Zn2+)andM(NaOH),2.0mL/min,respectively;tem-perature,50°C;NaOH/Zn2+molarratio,2.0;pressure,2.0MPa.TheZn2+concentrationwasvariedintherangeof0.3–0.8mol/L.Fig.6illustratestheXRDpatternsofZnOnanoparticlessynthesizedatvariousconcentrationsofZnSO4andZn(NO3)2.Onlyhexagonalcrystallinephasewasdetectableineachsample,regardlessoftheZn2+sourcesandtheirconcentrations.Thedependenceoftheaver-ageparticlesizeontheZn2+sourceandconcentrationisillustratedinFig.7.AsshowninFig.5,theZn2+sourcesigni cantlyaffectedtheparticlesizeoftheobtainedZnOnanoparticles.TheaveragesizesofZnOnanoparticlessynthesizedfromZnSO4were1.2–1.3timesthoseofthenanoparticlesfromZn(NO3)2atthesameZn2+concentrations.Thedifferencemayrelatetothedifferentratecon-stantsofZnSO4andZn(NO3)2withNaOH.Becausetheanionva-lenceofSO2À
ishigherthanthatofNOÀ43,theionicstrengthof
SO2À4
islarger,resultinginadeclineinZn2+activityofZnSO4.ItisindicatedthattheaveragesizeofZnOnanoparticlesalsodependedontheZn2+concentration,decreasingwithincreasingtheZn2+con-centrationforbothZnSO4andZn(NO3)2.Accordingtothecrystalli-zationkinetics[29],thecrystalaveragesize(D)andnucleationrate(B)aredeterminedby:
D¼
4EsVmRTlnS
ð1Þ
"
B¼ZÀ16p
E32
#
cexp
sVmNað2Þ
Y. Wang et al./ Chemical Engineering Journal 235 (2014) 191–197
195
more quickly established and the supersaturation degree is higher, leading to faster nucleation and smaller particle size. 3.3. Effect of temperature Zn(NO3)2 was used as the Zn2+ source, and the concentrations of Zn(NO3)2 and NaOH were 0.5 and 1.0 mol/L, respectively. The synthesis was conducted at a ow rate of 2.0 mL/min and various temperatures (40, 50, 60, and 70°C). It is known that high temperature may lead to de-emulsi cation. To ensure that no de-emulsi cation occurred in the temperature range, M(NaOH) and M(Zn(NO3)2) were fed separately into the microreactor system at the same owrate as in the synthesis runs. No de-emulsi cation of either M(NaOH) and M(Zn(NO3)2) was observed, indicating that both microemulsions were stable under the synthesis conditions. The XRD patterns of ZnO nanoparticles synthesized at different temperatures are shown in Fig. 8. Pure ZnO nanocrystals were obtained in the temperature range investigated. Fig. 9 displays the variation of the average particle size with reaction temperature. A minimum average particle size (10.4 nm) was observed at 50°C when increasing the reaction temperature from 40 to 70°C. At low temperatures, the formation rate of the precipitated precursor was low, and thus the supersaturation degree was low, which was unfavorable to the nucleation but favorable to the growth of the
particles. Although the formation rate of the precipitates was accelerated at high temperatures, the nucleation rate might not be signi cantly increased due to the reduced saturation degree. On the other hand, the growth of the particles might
be enhanced at high temperatures, leading to larger Zn(OH)2 nanoparticles.
Fig. 10. The average size of ZnO particles as a function of feed ow rate in the synthesis from Zn(NO3)2 at 50°C.
Fig. 8. XRD patterns of ZnO nanoparticles synthesized at different temperatures.
Fig. 9. The average size of ZnO particles synthesized at different temperatures.
Fig. 11. SEM images of ZnO particles synthesized in the microreactor (a) and in the batch reactor (b).
3.4.Effectoffeed owrate
Zn(NO3)2wasusedastheZn2+source,andtheconcentrationsofZn(NO3)2andNaOHwere0.5and1.0mol/L,respectively.Thesyn-thesisreactionwasconductedat50°Cand2.0MPa,andthefeed owratewasvariedfrom2.0to6.0mL/min.ThevariationoftheaveragesizeofZnOnanoparticleswiththefeed owrateisshowninFig.10.TheaverageparticlesizeofZnOdidnotchangemarkedlywithfeed owrate.Atlow owrates(2.0–4.0mL/min),theaver-ageparticlesizewasreducedslightlywiththefeed owrate.Atlow owrates,theincreasedresidencetimeallowedforthecom-pletionofbothsynthesisreactionandcrystallizationinthereac-tionsystem.Athigh owrates,theresidencetimewasprobablylongenoughforthecompletionofsynthesisreaction,because
theobtainedZnOnanoparticleswascomparedwiththoseoftheparticlessynthesizedinthemicroreatorandinthebatchreactor(Fig.12).Itisapparentthatthemicromixersigni cantlyreducedtheparticlesizeandimprovedthesizedistributionofZnOnanoparticles.4.Conclusions
ZnOnanoparticlessynthesizedfrommicroemulsionsinacon-tinuousmicroreactorweresmalleringrainsizeandmorenarrowlydistributedthanthoseinabatchreactor.Themicroemulsionspro-videcon nedspaceforthereactants,whichisfavorableforcon-trollablereactionandnucleation,avoidingtheformationoflargeparticles.Inaddition,themicroemulsionspreventthedepositionofZnOparticlesonthewallofthemicrochannelsofthereactor,
Y.Wangetal./ChemicalEngineeringJournal235(2014)191–197197
andnocloggingofthemicrochannelsoccurredintheinvestigation.ThreeZn2+sources(Zn(NO3)2,ZnSO4,andZnCl2)weretestedinthesynthesisofZnOnanoparticles.ItisfoundthatZn(NO3)2showedbestperformanceinthesynthesis.BothZn2+concentrationandreactiontemperaturesigni cantlyaffectedtheaverageparticlesizesofthesynthesizedZnOnanoparticles,probablyduetotheireffectsonthekineticsofthesynthesisreactionandnucleation.TheaverageparticlesizewasreducedwithincreasingtheZn2+concentration,whereasaminimumaverageparticlesizewasob-served50°Cwhenthereactiontemperaturewasincreasedfrom40to70°C.Thefeed owratedidnotaffectconsiderablytheaver-ageparticlesizeofZnOnanoparticles.However,athigh owrates,largerparticleswereobtained,probablybecausethecrystallizationmightcontinueafterthedropletshadleftthereactionsystem.Thismayimplythatthecrystallizationstepwasslowerthatthesynthe-sisreaction.Acknowledgments
Thisworkwas nanciallysupportedbyNSFC(20773020,20973030,21173033,U1162203),the‘‘863’’Project(2008AA030803),NCET(04-0275),ThePh.D.ProgramsFoundation(MOE,20100041110016),andthe‘‘111’’Project.References
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