立方块CuO

MaterialsLetters71(2012)32–35

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MaterialsLetters

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

Shape-controlledsynthesisofself-assemblycubicCuOnanostructuresbymicrowave

LeileiGuo,FangTong,HaowenLiu,HanminYang?,JinlinLi

KeyLaboratoryofCatalysisandMaterialsScienceoftheStateEthnicAffairsCommission&MinistryofEducation,HubeiProvince,South-CentralUniversityforNationalities,Wuhan430074,China

articleinfoabstract

Self-assemblythree-dimensional(3D)cubicCuOnanostructureswithameansizeofapproximately230nmweresynthesizedbyheatingcupricacetate(Cu(OAc)2·H2O)aqueoussolutioninthepresenceofpolyvinyl-pyrrolidone(PVP)andcetyltrimethylammoniumbromide(CTAB)undermicrowaveirradiation.TheCuOnanostructureswerecharacterizedbytransmissionelectronmicroscopy(TEM),X-raypowderdiffraction(XRD)andthermogravimetricanalysis(TGA),whichindicatedthattheas-synthesizedcubicCuOnanostruc-tureswereaself-assemblyofmonodispersednanoparticles.TheconcentrationoftheCu(OAc)2,themolarratioofCu(OAc)2/NaOH/PVP,togetherwiththeheatingtimebymicrowavewerefactorsaffectingthesuccess-fulsynthesisofcubicCuOnanostructures.Themechanismandtheformationfactorsoftheself-assemblywerediscussed.

CrownCopyright©2011PublishedbyElsevierB.V.Allrightsreserved.

Articlehistory:

Received31March2011Accepted27November2011

Availableonline11December2011Keywords:

CuOnanoparticlesCubic

MicrowaveNanomaterialsMicrostructure

1.Introduction

Inthepastdecades,theresearchonthesynthesisofvariousnanos-tructures(nanowires,nano?lm,nanorod)hasattractedmuchattentionbecauseoftheirsizeandshape-inducednovelandphysicochemicalproperties[1–3].Mostofthenanoscalematerialsneedsmall-size,nar-rowdistributionandwell-?nedmorphologytoshowtheirbestcharac-ter,thereforemanyresearchersfocusonthedesignandshape-controlofthedesirednanostructuresincludingmetal,semiconductorandmetaloxide,tooptimizethesematerialsforapplications[4,5].

CuO,asanimportantp-typesemiconductorwithanarrowbandgap(1.4eV),hasbeenwidelyappliedinhigh-Tcsuperconductors,solarenergytransformation,magneticstoragemediaandgassensors[6,7].Duetothesesigni?cantapplications,manymethodshavebeendevelopedtoprepareCuOnanostructureswithvariousmorphologies.Zhangetal.synthesizeddendrite-likeCuOnanostructuresbyasimpleethyleneglycol(EG)assistedhydrothermalmethod.Leaf-like,chrysanthemum-likeandrodshapedCuOnanostructureshavebeensynthesizedbymicrowave-assistedapproachinanionicliquid1-n-butyl-3-methylimidazoliumtetra?uoroboratemedium[8,9].How-ever,theshape-controlofself-assemblythree-dimensionalcubicCuOnanostructureshasnotbeenreported.Inrecentyears,usingmonodis-persednanoparticlesasbuildingunitstoassemblewell-de?nedtwo-andthree-dimensional(2Dand3D)super-latticeshasbeenactivelyinvestigatedbecauseoftheirnovelpropertiesandpotentialapplica-tionsinopticalgratings,optical?lters,datastorage,andmicroelectronicdevices[10–12].Hence,itwouldbeofgreatsigni?cancetosynthesize

theself-assembly2Dor3DCuOsuper-latticestoimproveitsperfor-manceintheapplications.

Inthispaper,self-assembly3DcubicCuOnanostructuresweresyn-thesizedbyheatingcupricacetate(Cu(OAc)2·H2O)aqueoussolutioninthepresenceofpolyvinylpyrrolidone(PVP)andcetyltrimethylammo-niumbromide(CTAB)undermicrowaveirradiation.Thenanostruc-tureswerecharacterizedbytransmissionelectronmicroscopy(TEM),X-raypowderdiffraction(XRD)andthermogravimetricanalysis(TGA).Furthermore,theeffectsofsynthesisparametersontheshapecontrolofself-assemblycubicCuOnanostructureshavebeeninvesti-gated,andtheself-assemblymechanismisproposedanddiscussed.2.Experimental

Cupricacetate(Cu(OAc)2·H2O),polyvinylpyrrolidone(PVP,aver-agemolecularweight,Mw=30,000,Kermelchemicals),cetyltri-methylammoniumbromide(CTAB,99.5%,Acroschemicals),ethyleneglycol(EG,Acroschemicals)andsodiumhydroxide(NaOH,82.0%,sinopharmchemicals)wereallofanalyticalgradeandusedwithoutfurtherpuri?cation.Inatypicalsynthesis,2.00mLCu(OAc)2aqueoussolution(0.02molmol·dm?3)and3.00mLPVPaqueoussolution(0.13mol·dm?3)wereaddedintoa50mLround-bottomed?ask,then10.00mLEGsolutioncontaining0.08mMNaOHand0.05mMCTABwasaddedtothe?ask,keepingthemolarratioofCu(OAc)2/NaOH/PVPat1/2/10.Themixedsolutionwasheatedinamodi?eddo-mesticmicrowaveoven(Galanz,900W)whichwasconnectedtoare?uxingsystemfor120s.Thedarkbrowncolloidwasobtainedandthencooledtoroomtemperature.

Thetransmissionelectronmicroscopy(TEM)oftheas-synthesizedCuOnanostructureswasoperatedonaFEITecnaiG220at200kV.ThesampleforTEMobservationwaspreparedbyplacingadropofthe

?Correspondingauthor.Tel.:+862767842752.E-mailaddress:yhm@http://wendang.chazidian.com(H.

Yang).

0167-577X/$–seefrontmatter.CrownCopyright©2011PublishedbyElsevierB.V.Allrightsreserved.doi:

10.1016/j.matlet.2011.11.105

L.Guoetal./MaterialsLetters71(2012)32–3533

Fig.1.XRDpatternofCuOnanoparticles.[Cu(OAc)2]=0.02mol·dm?3,Cu(OAc)2/NaOH/PVP=1/2/10(molarratio),reactiontime=120s.

colloidaldispersionontoacoppergridcoatedwithaperforatedcar-bon?lm,followedbyevaporatingthesolventatambienttemperature.Theaverageparticlesizeandthedistributionweredeterminedfromabout200particlesoftheenlargedmicrographs.

ThesamplewasobtainedbywashingtheCuOsolutionwithabso-luteethanolandcentrifugingat4000rpmfor30min,theprecipitatewasthenspreadonaglassplateanddriedbyinfrared.ThesampleswerecharacterizedbyX-raypowderdiffraction(XRD)onaBrukerD8X-raydiffractometerusingCuKaradiationwith40kVand50mAandscanningrateof0.02s?1inthe2θrange10°–80°.

Thermogravimetricanalysis(TGA)wasconductedonaNETZSCHinstrumentsSTA409usingapuritynitrogenatmosphere(?owrateof100mL/min),withheatingfromroomtemperatureto800°Cataheatingrateof40K/10min.3.Resultsanddiscussion

AtypicalXRDpatternoftheself-assembledCuOsamplewasshowninFig.1.Threediffractionpeakscorrespondingtothe(?111),(111),and(002)latticeplaneswereobservedwhichwasconsistentwiththeliteraturevalues(JCPDS80-1268).Nopeaksofimpurityweredetected,indicatingthatpureCuOnanostructuresweresynthesized.Meanwhile,theaveragesizeofCuOnanoparticleswascalculatedusingScherrerequationtobe5.7nm.However,thesizeofassembledcubicCuOnanostructureswasapproximately230nmfromTEMimagesindicatingsecondarystructuresfromtheassemblyofindividualnanoparticles.

Inatypicalsynthesis,themorphologyandsizeofcubicCuOnanos-tructuresweregreatlyin?uencedbydifferentinitialconcentrationsof

Cu(OAc)2.Fig.2illustratedtypicalTEMimagesofthesynthesizedCuOnanostructureswhentheconcentrationofCu(OAc)2was0.01mol·dm?3,0.02mol·dm?3and0.20mol·dm?3,whichshowedthecubicmorphologywithanaveragesizeofabout40nm,90nmand140nmrespectively.TheTEMimagesindicatedthatthesizeofthesynthesizedCuOnanostructuresincreasedwithincreasingCu(OAc)2concentration,whilethecubicshapewasmaintained.ApossibleexplanationmaybethatthehighconcentrationofCu(OAc)2increasedtherelativerateofatomicstackandcrystalgrowthmorethantherateofnucleation.

PVP,astheprotectionandcappingagent,playsanimportantroleontheshapecontrolofthesynthesisofmetalandmetaloxidenano-particles.Inourstudy,themolarratioofCu(OAc)2andPVPwasalteredtoinvestigatethein?uenceofPVPonthemorphologyandsizeoftheCuOnanostructures.Fig.3arevealedthemorphologyoftheCuOnanostructureswithoutusinganyPVP.Thenanoparticlesshowedirregularshapewithwidesizedistributionandaggregationswereevident,duetothelackofprotectiononthesurfaceofthenano-particles.Theself-assemblycubicCuOnanostructuresformedundertheconditionsofCu(OAc)2/PVP=1/10,asshowninFig.3b.WhentheconcentrationofPVPincreasedtoCu(OAc)2/PVP=1/20,sphericalshapedCuOwerefoundasseeninFig.3c.ThereasonthatthesizeofnanoparticlesdecreasingwiththehigherPVPconcentrationmaybeexplainedasfollows:WhenPVPinthesolutioniswithlowconcen-tration,itonlyplayedasastabilizerwhichcanpreventtheCuOnano-particlesfromagglomeration.AstheconcentrationofPVPincreased,excessPVPencapsulatedtheinitialCuOnanoparticlesandsuppressedthegrowthofthesesmallparticles,whichdecreasedthesizeofinitialnanoparticlesandcausedtheCuOnanostructurestobecomesmaller.Therefore,theformationoftheself-assemblycubicCuOnanostruc-turesrequiredappropriateconcentrationofPVP.

Inaddition,accordingtotheTEMimagesofCuOnanostructuresatdifferenttimes,itwasfoundthatthepreparedcubicCuOnanostruc-turesweresecondarystructuresfromself-assemblyofmanyindivid-ualprimarynanoparticles,asshowninFig.4.TheTEMimagesshowedthecubicCuOwithdifferentshapesat30s,60s,90sand120s.Asthereactionmixturewasheatedundermicrowavefor30s,onlytheaggregatedCuOwasobtained,whilesphere-likeCuOnanos-tructureswereformedwithcontinuousheatingfor60s,thenthesphericalnanoparticlesgrewintonanoparticlesofconnectedindivid-ualcubicCuOnanostructureswithheatingtimefor90s.Finally,theinitialsecondarynanostructurestransformedintotheself-assemblycubicmorphologyafterheatingfor120s.Themechanismofthenano-particlesaggregatingintosphere-likeshapesandthendevelopingintocubicnanostructuresindicatedthatthecubicCuOnanostructuresex-periencedaprocessofself-assembly.Moreover,theself-assemblystructureswerecon?rmedbythefollowingevidences.First,thepri-marynanoparticlesthatservedasbuildingblocksoftheself-assemblyofcubicCuOnanostructureshadnarrowsizedistribution,

(a)(b)(c)

Fig.2.TEMmicrographsofCuOnanoparticleswithdifferentconcentrationsofCu(OAc)2:(a)0.01mol·dm?3;(b)0.02mol·dm?3;(c)0.2moldm?3.Inallcases,Cu(OAc)2/NaOH/PVP=1/2/10(molarratio),reactiontime=120

s.

34L.Guoetal./MaterialsLetters71(2012)32–35

(a)(b)(c)

Fig.3.TEMmicrographsofCuOnanostructureswithdifferentmolarratiosofCu(OAc)2/NaOH/PVP:(a)1/2/0;(b)1/2/10;(c)1/2/20.Inallcases,[Cu(OAc)2]=0.02mol·dm?3,re-actiontime=120s.

asshowninFig.4b.Second,Fig.4cshowedthattheas-synthesizedcubicCuOnanostructurescontainedmanyporesandsurfacedefects,whichcanbeconsideredastheresultsoftheself-assembly.Recently,variousmechanismsoftheself-assemblyhavebeenproposed.Zhengetal.assertedthattheself-assemblyofnanoparticlesintotwo-orthree-dimensional(2Dand3D)super-latticesrequiredacontrolledsizedistributionandattractiveforcesuchasvanderWaalsattractionandbalancedinteraction[13].Duringassembly,theprimarynanopar-ticlesweredrawntowardeachotherbytheattractiveforce,andthenformedastablesuperstructureunderthebalanceofstericinteractionbetweenthem.Meanwhile,theself-assemblynanostructureshaveminimumsurfaceandpotentialenergytoformstablenanocrystals.Thoughthemechanismoftheself-assemblyisnotentirelyclear,theself-assemblyof2Dor3Dnanostructuresprovidesaresearchdirectionforthesynthesisofagoodmorphologythatcanbeusedinactualapplicationsinmany?elds.

TheTGAspectrumofCuOnanostructureswaspresentedinFig.5,whichillustratedthemassvariationsofthesampleswithtempera-ture.At25°Cto250°Ctemperaturerange,thespectrumhadonlywispychangethatwascausedbythesmallamountofwaterinthesample.However,thereappearedaweightlossof27.93%overthe256°Cto443°Ctemperaturerange,andanotherweightlossof10.09%overthe443°Cto548°Ctemperaturerange.ThereasonofthetwoobviousweightlossprocessesmaybethedecompositionofthePVPandCTABabsorbedonthenanostructures.Asthetemperatureincreasestoover550°C,thespectrumthatappearedhasnoweightlossandremainedsteadily,whichindicatedthattheorganismdecom-posedabsolutely.TheTGAspectrumshowedthatthePVPandCTAB

(a)(b)

(c)(d)

Fig.4.TEMmicrographsofCuOnanostructuresindifferentreactiontimes:(a)30s;(b)60s;(c)90s;(d)120s.Theinsetof(c)showstheenlargedcubicCuOnanostructures.Inallcases,[Cu(OAc)2]=0.02mol·dm?3,Cu(OAc)2/NaOH/PVP=1/2/10(molarratio).

L.Guoetal./MaterialsLetters71(2012)32–3535

Fig.5.TGAspectrumofcubicCuOnanostructures.

absorbedonthenanostructuresandtheas-synthesizedCuOwerestable.4.Conclusion

Self-assemblythree-dimensional(3D)cubicCuOnanostructuresweresynthesizedbyheatingCu(OAc)2aqueoussolutioninthepres-enceofPVPandCTABundermicrowaveirradiation.Theconcentra-tionofCu(OAc)2andPVPwereimportantfactorsinthesynthesisofself-assemblycubicCuOnanostructures.WhentheconcentrationoftheCu(OAc)2wascontrolledat0.02mol·dm?3,andthemolarratioofCu(OAc)2/NaOH/PVPwasmaintainedat1/2/10,thenheatingby

microwavefor120s,theself-assemblycubicCuOnanostructureswithwellde?nedmorphologyanddispersioncanbeobtained.TheXRDandTEMpatternsindicatedthatthecubicCuOnanostructureswereformedviaaprocessofself-assembly.Theself-assembly3DcubicCuOnanostructuresmaybeusedasnovelmaterialsinvariousapplications.Acknowledgment

ThisresearchwassupportedbytheSpecialFundforBasicScienti?cResearchofCentralColleges,South-CentralUniversityforNationali-ties(ZZY10004).References

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