Ni 微波

Available online at http://wendang.chazidian.com

MaterialsLetters62(2008)2571–

2573

http://wendang.chazidian.com/locate/matlet

Microwave-assistedsynthesisofnickelnanoparticles

WeiXu,KongYongLiew,HanfanLiu,TaoHuang?,ChuntaoSun,YanxiZhao

KeyLaboratoryofCatalysisandMaterialScienceoftheStateEthnicAffairsCommission&MinistryofEducation,HubeiProvince,

South-CentralUniversityforNationalities,Wuhan430074,PRChina

Received5September2007;accepted21December2007

Availableonline5January2008

Abstract

Nickelnanoparticleswithuniform,monomorphicself-assembledflower-likemicrostructureweresynthesizedbyreductionofnickelchloridewithhydrazinehydrateinthepresenceofpolyvinylpyrrolidone(PVP)inethyleneglycolundermicrowaveirradiation.AnappropriateamountofNa2CO3wasnecessaryfortheformationofmonomorphicanduniformNinanoflowers.SmallamountofNaOHandahigherconcentrationofhydrazinewerebeneficialtotheformationofNinanoflowerswithasmallerdiameterandanarrowerdistribution.Thetransmissionelectronmicroscopy(TEM)andX-raydiffractionmeasurementsmanifestedthattheas-synthesizedNinanoflowerwastheassemblesself-organizedbyhundredsofsmallerprimarynanoparticleswithanaveragedimensionofabout6.3nm.©2008ElsevierB.V.Allrightsreserved.

Keywords:Microwave;Nickelnanoparticles;Nanoflowers;Self-assembly;Nanomaterials;Microstructure

1.Introduction

Inrecentdecades,thetransitionmetalnanomaterialshavebeenpaidconsiderableattentionowingtotheirpotentialapplicationsinvariousfieldssuchasmicroelectronics,optoelectronics,catalysis,magneticmaterials,informationstorageandamongothers[1–7].Thephysicalandchemicalpropertiesofthesemetalnanoparticlesarestronglydependentontheirsizeandshape[8–10].Sospecialnanostructurewithwell-definedsizeandshapewasrequired.Manytechniqueshavebeendevelopedforthepreparationofnanosizedmetallicmaterialsanddifferentreducingagentsandstabilizingcompoundswereemployedtocontrolparticlesizeandshape.Itiswell-knownthatnickelnanoparticlesshowremarkablecatalyticandmagneticproperties.Attemptshavebeenmadetoobtainuniformnickelnanoparticleswithacontrolledmeansizeandanarrowdistributionaswellasspecialmorphology.Forinstance,nickelnanoparticlesweresynthesizedbypolyolorortherroutes,usingNi(OH)2orNiCl2asprecursorandpolymerorsurfactantsasprotectiveagents[11–16].Intheabsenceofstabilizers,particleswithasizeintherangeofmicrometerwerealsoobtained[17,18].Inaddition,Zachetal.preparedanickel

?Correspondingauthor.Tel.:+862787195367;fax:+862767842752.E-mailaddress:huangt6628@http://wendang.chazidian.com(T.Huang).

0167-577X/$-seefrontmatter©2008ElsevierB.V.Allrightsreserved.doi:10.1016/j.matlet.2007.12.057

nanocrystalline“snowball”withmeandiametersof20to600nmbytheH2coevolutionmethod[19].Recently,needle-likenickelnanoparticleswerepreparedinthemicroemulsionbyreductionofnickelchloridewithhydrazinehydrate[20].Sofar,however,therehavebeenfewreportsonthenickelnanoparticleswithdifferentmorphologies.

Inthispaper,microwaveirradiationwasemployedtosyn-thesizenickelnanoparticlesbyreductionofnickelchloridewithhydrazinehydrateinthepresenceofpolyvinylpyrrolidone(PVP)andappropriateamountofNa2CO3andNaOHinethyleneglycolsolution.Theas-synthesizednickelnanoparticleswithmono-morphicself-assembledflower-likemicrostructurewerechar-acterizedbytransmissionelectronmicroscopy(TEM)andX-raypowderdiffraction(XRD).Thesynthesisparameterswerealsoinvestigated.2.Experimental

Nickelchloride,polyvinylpyrrolidone(PVP,averagemole-cularweight,MW=30000,FlukaChemicals),ethyleneglycol(EG,AcrosChemicals),hydrazinehydrate(85%)andotherchemicalswereallanalyticalgradeandusedwithoutfurtherpurification.AllthereactantsweredissolvedinEGtomakeupacertainconcentrationofthecorrespondingEGsolution,

2572W.Xuetal./MaterialsLetters62(2008)2571–2573

respectively.Inatypicalsynthesis,5mLof0.02MNiCl2solution,1mLof0.1MNa2CO3solution,4mLof1MPVPsolution,0.2mLof0.1MNaOHsolutionand0.6mLof0.87MN2H4·H2Osolutionwereaddedintoa100mLround-bottleflask.Thetotalvolumeofthereactionsolutionwaskeptat20mLbyaddingEG.Themixturesolutionwasthenputintoamodifieddomesticmicrowaveoven(Galanz,900W)andheatedfor60swith50%oftheoutput.Theblackcolloidwasobtainedandthencooledquicklywithice-waterbath.

Thesizeandmorphologyoftheas-synthesizednickelnanoparticlesweredeterminedonaFEITecnaiG220trans-missionelectronmicroscopy(TEM)operatedat200kV.ThesampleforTEMobservationwaspreparedbyplacingadropofthecolloidaldispersionontoacoppergridcoatedwithaperfo-ratedcarbonfilm,followedbyevaporatingthesolventatam-bienttemperature.Theaverageparticlesizeandthedistributionweredeterminedfromtheenlargedmicrographsonthebasisofthemeasurementofabout300particles.

X-raypowderdiffraction(XRD)wasperformedonaBrukerD8AdvanceX-raydiffractometeremployingCu-Kαradiationwith40kVand50mA.ThesampleforXRDmeasurementwasasfollows:thenickelcolloidswereaddedintoacertainamountofacetonitrileandblackprecipitateswereobtained.Aftercen-trifugation,theprecipitatewaswashedwithacetonitrileforseveraltimesandthencoatedonglassplate,followedbyvac-uumdrying.

3.Resultsanddiscussion

Generally,alkalescentmediumisfavorableforhydrazinetoreduceNiCl2.WhentheconcentrationofNiCl2,PVPandN2H4·H2Owere5mM,0.2Mand25mM,respectively,theeffectsofNa2CO3andNaOHontheNinanoparticleswereinvestigated.Itwasfoundthatan

Fig.1.TEMimagesofthesamplespreparedindifferentsystemswithmolarratioofNa2CO3/NiCl2/NaOH/PVP:(A)1/1/0/40;(B)0.5/1/0.2/40;(C)1/1/0.2/40;(D)1/1/0.2/80.Inallcases,[NiCl2]=5mM;[N2H4·H2O]=25

mM.

Fig.2.TEMimagesofthenanoflowers.ThemolarratiosofN2H4·H2O/NiCl2were2/1,10/1,20/1in(A),(B)and(C),respectively;(D)ahigh-magnificationTEMofasinglenanoflower.Inallcases,[NiCl2]=5mM;Na2CO3/NiCl2/NaOH/PVP=1/1/0.2/40(molar

ratio).

appropriateamountofNa2CO3wasnecessaryfortheformationofthePVP-stabilizedNinanoparticlesinethyleneglycol.Themonomorphicflower-likeNinanoparticles(namelynanoflowerornanocluster)withanaveragedimensionof94.6nmwereobtainedwhenthemolarratioofNa2CO3/NiCl2was1/1withoutadditionofNaOH,asshowninFig.1A.However,nonickelcolloidswereformedwhenonlyalittleamountofNaOHwasusedwithoutNa2CO3.Furthermore,accompany-ingwiththeadditionofNa2CO3,theadditionofalittleamountofNaOHwasbeneficialtotheformationofflower-likeNinanoparticleswithasmallerdiameterandanarrowerdistribution.AsshowninFig.1BandC,whenthemolarratioofNa2CO3/NiCl2/NaOHwere0.5/1/0.2and1/1/0.2,themeandiametersoftheas-preparedNinanoflowerswere76.7and60.8nm,respectively.Nevertheless,anexcessofNaOHledtotheformationofbiggernanoflowers.

Moreover,itappearsthatnanoporesseparatemanysmallerprimarynanoparticlesineachnanoflowerasobservedfromtheseTEMimagesinFig.1.Thisindicatesthattheas-synthesizedNinanoflowers,whichcanberegardedasasecondarynanostructure,wereactually

monodispersive

Fig.3.XRDpatternofnickelnanoparticles.[NiCl2]=5mM;Na2CO3/NiCl2/NaOH/N2H4·H2O/PVP=1/1/0.2/5/40(molarratio).

W.Xuetal./MaterialsLetters62(2008)2571–25732573

self-assembliesorganizedbyhundredsofsmallerprimarynanoparticles.Theself-organizationshouldbeascribedtothemutualattractionofmagneticdipoles[15].Furthermore,themeansizesoftheprimarynanoparticleswereabout6±0.5nmonthebasisofTEMmeasurementinallthesecasesabove,showinglittlechangewiththesizesofthenanoflowers.

Inaddition,theeffectofPVPconcentrationonthesizeoftheNinanoflowerwasinvestigatedwhilekeepingtheotherconditionsunchanged.WhenthemolarratioofPVP/NiCl2was20/1,aprecipitatewasformedduetopoorprotectionoffewerPVP.ThoughthedimensionoftheNinanoflowerdecreasedwiththeincreaseofPVPconcentration,themeansizeoftheformedprimarynanoparticleswasindependentonthePVPconcentrationduetotherapidandhomogeneousheatingwithmicrowave.Itwasfoundthattheself-organizationofprimarynanoparticleswaslimitedatahigherPVPconcentrationsothatsmallerself-assembliesofnanoparticleswereformed.AsshowninFig.1CandD,theaveragedimensionoftheNinanoflowersdecreasedfrom60.8nmto55.6nmwhenthemolarratioofPVP/NiCl2increasedfrom40/1to80/1.WhilethemolarratioofPVP/NiCl2increasedto120/1,theNinanoflowersdecreasedslightlyto54.2nmandshowedanobviouslyheterogeneousdistribution(notshown).AnappropriatemolarratioofPVP/NiCl2is40/1fortheformationofNinanoflowers.

Fig.2,inassociationwithFig.1C,showstheeffectofthemolarratioofN2H4·H2O/NiCl2onparticlesize.Thehigh-magnificationTEM,asshowninFig.2D,demonstratesdistinctlytheporousstructureofthesphericalNinanoflowerinwhichrecognizableboundariesamongthosesmallerprimarynanoparticlescanbeseenclearly.ThemeansizeofNinanoflowersdecreasedgraduallyfrom83.7to50.9nmandthereactiontimeshortenedfrom80to50swiththeincreaseofthemolarratioofN2H4·H2O/NiCl2from2/1to10/1,whereasthesizeincreasedto78.8nmwithcontinuouslyincreasingtheN2H4·H2O/NiCl2ratioto20/1.Thismaybeascribedtotheinfluenceofreductionrateonthenucleationandself-assembly.Withtheincreaseofhydrazineconcentration,thereductionratebecamefaster,morenucleiwereproducedandself-organizationledtosmallernanoclusters.WhenthemolarratioofN2H4·H2O/NiCl2increasedcontinuouslyto20/1,thereductionratewasmuchfasterthantheself-organizingrateandrelativelyslowerself-organizationledtolargerself-assemblies.

ToverifytheobservationresultsfromTEM,XRDanalysiswasconducted.Fig.3showsatypicalXRDpatternofthesample.ThreepeaksforNiat2θ=44.5°,51.8°,76.4°,correspondingtothe(111),(200),(222)latticeplanes,wereobserved,revealingthattheresultantparticleswerepureelementalNiwithaface-centeredcubic(FCC)structure.TheparticlediameterwasestimatedbytheScherrer'sequationtobeabout6.3nm.Noobviouspeakofnickeloxideorhydroxidewasdetected.TheparticlesizeobtainedbyXRDisexcellentlyconsistentwiththeobservationbyTEM.BoththeresultsfromTEMandXRDmanifestthattheNinanoflowerisaself-assemblednanostructureorganizedbythesmallprimarynanoparticles.

4.Conclusions

PVP-stabilizedNinanoparticlesweresynthesizedinethyleneglycolbymicrowaveheatingmethodusingnickelchlorideasprecursor,hydrazinehydrateasreducerandPVPasstabilizer.AnappropriateamountofNa2CO3wasnecessaryfortheformationofmonomorphicanduniformNinanoparticleswithflower-likemicrostructure.AlittleamountofNaOHandahigherconcentra-

tionofhydrazinewerebeneficialtotheformationofNinanoflowerswithasmallerdiameterandanarrowerdistribution.Theas-synthesizednickelnanoflowerswithanaveragediameterof50–100nmwereactuallymonodispersiveself-assembliesorganizedbyhundredsofsmallerprimarynanoparticleswithameansizeofabout6.3nm.Acknowledgements

ThisresearchwassupportedbytheNationalNatureScienceFoundationofChina(GrantNo.20673146),theNatureScienceFoundationofHubeiProvince(GrantNo.2006ABA366)andtheScienceFoundationofSouth-CentralUniversityforNationalities(No.YZZ05002).References

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