Synthesis of ZnO nanoparticles from microemulsions in a flow(2)

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

[1]Y.N.Xia,P.D.Yang,Y.G.Sun,Y.Y.Wu,B.Mayers,B.Gates,Y.D.Yin,F.Kim,H.Q.

Yan,One-dimensionalnanostructures:synthesis,characterization,andapplications,Adv.Mater.15(2003)353–389.

[2]P.X.Gao,Z.L.Wang,Mesoporouspolyhedralcagesandshellsformedby

texturedself-assemblyofZnOnanocrystals,J.Am.Chem.Soc.125(2003)11299–11305.

[3]X.D.Wang,C.J.Summers,Z.L.Wang,Large-scalehexagonal-patternedgrowth

ofalignedZnOnanorodsfornano-optoelectronicsandnanosensorarrays,NanoLett.4(2004)423–426.

[4]A.Moezzi,A.M.McDonagh,M.B.Cortie,Zincoxideparticles:synthesis,

propertiesandapplications,Chem.Eng.J.185–186(2012)1–22.

[5]P.Yang,H.Yan,S.Mao,R.Russo,J.Johnson,R.Saykally,N.Morris,H.J.Choi,

ControlledgrowthofZnOnanowiresandtheiropticalproperties,Adv.Funct.Mater.12(2002)323–331.

[6]W.I.Park,D.H.Kim,Metalorganicvapor-phaseepitaxialgrowthofvertically

well-alignedZnOnanorods,Appl.Phys.Lett.80(2002)4232–4234.

[7]C.K.Xu,G.D.Xu,Y.K.Liu,G.H.Wang,Asimpleandnovelrouteforthe

preparationofZnOnanorods,SolidStateCommun.122(2002)175–179.

[8]B.D.Yao,Y.F.Chan,N.Wang,FormationofZnOnanostructuresbyasimpl …… 此处隐藏：5706字，全部文档内容请下载后查看。喜欢就下载吧 ……