Synthesis of Monodisperse WO3

SynthesisofmonodisperseWO3·2H2Onanospheresbymicrowave

hydrothermalprocesswithL(+)tartaricacidasaprotectiveagent

QingjunSun,JianminLuo,ZhengfengXie,JideWang,XintaiSu⁎

CollegeofChemistryandChemicalEngineering,XinjiangUniversity,Urumqi830046,China

Received6September2007;accepted30January2008Availableonline5February2008

Abstract

MonodispersecrystallineWO3·2H2O(H2WO4·H2O)nanosphereshavebeenpreparedbyL(+)tartaricacid-assistedmicrowavehydrothermal(MH)processforthefirsttime.TheX-raypowderdiffraction(XRD)patternindicatedthattheproductwasingoodagreementwiththestandardJCPDSdataforWO3·2H2O.Theparticlesizesandshapeshavebeenmeasuredbytransmissionelectronmicroscopy(TEM)andscanningelectronmicroscopy(SEM)techniques.TheeffectsofthemassratiosofL(+)tartaricacidtoNa2WO4·2H2OonthemorphologyofWO3·2H2Ohavebeeninvestigated.Finally,TheformationmechanismforL(+)tartaricacid-assistedMHsynthesisoftheWO3·2H2Onanosphereswasalsodiscussedindetail.©2008ElsevierB.V.Allrightsreserved.

Keywords:Tungsten(VI)oxidedihydrate;Nanomaterials;Monodispersenanospheres;Microwavehydrothermal

1.Introduction

Tungsticacidarewellknowncompoundswithgeneral

formulaWO3·nH2O(n=1/3,1/2,1or2),whichareimportant

basicmaterialsfordisplays,passiveandactivesensordevices

[1].Theyarealsousedforthefabricationoftungstenfilaments

forincandescentlamps[2,3],asalloyingcomponentinheavy

metalsandfortheproductionoftungstencarbideinhardmetals

[4,5].TheWO3·nH2Onanoparticlesornanocrystalliteshave

beensynthesizedbyvarioustechniquessuchasacidprecipita-

tionmethod[6],sol–gelmethod[7],microemulsionmethod

[8],ion-exchangemethod[9,10],andhydrothermalroute

[11,12].Amongthem,synthesisunderhydrothermalconditions

canprovideadirect,one-steproutetopreparenanosizedoxide

materials,andbecomeanattractivemethod.However,this

methodusuallyrequiresprolongedreactiontimeformorethan

10hevenforseveraldays.Forexample,CsabaBalázsietal.

[12]synthesizedthetungstentrioxidedihydrate(WO3·2H2O)

byahydrothermalrouteat120°Cformorethan20h.However,

tothebestofourknowledge,fewstudieshavefocusedonthesynthesisorself-organizationofnanospheresofWO3·2H2O.It

isexpectedthatclearisotropicdispersionoftungsticacidcanbe

preparedandconvenientlyusedinmorewidefieldssuchas

sensordevices,catalyst,andelectrochromicmaterialsetc.

Inthispaper,wereportarapidandshape-controllablemicro-

wavehydrothermal(MH)synthesisofWO3·nH2Onanospheres

withL(+)tartaricacidasaprotectiveagent.Thismethodre-

quiredaveryshortsynthesistime,andthereactionprocesswas

alsoverysimple.

2.Experimental

2.1.Materials

Allofthechemicalreagentsusedintheexperimentwereof

analyticalgrade.Microwavereactionwasperformedinado-

mesticmicrowaveoven(GlanzG8027TL-2,frequency2.45GHz,

maximumpower800W).

2.2.PreparationofWO3·nH2Onanospheres

Thedetailedsynthesizingprocesswasasfollows.Inatypi-

calsynthesis,6gofNa2WO4·2H2Oand1g(or0.5g,0

gAvailable online at www.sciencedirect.com

MaterialsLetters62(2008)2992–

2994www.elsevier.com/locate/matlet

⁎Correspondingauthor.Tel./fax:+8609918582807.E-mailaddress:suxintai827@163.com(X.Su).

0167-577X/$-seefrontmatter©2008ElsevierB.V.Allrightsreserved.doi:10.1016/j.matlet.2008.01.093respectively)ofL(+)tartaricacidwasmixedinto100mLde-

ionizedwater,severalmillilitresof6MHClaqueoussolutionwas

introducedintotheaqueoussolution,resultinginayellow

precipitation(adjustthepHvalueto1).After30minofstirring,

themixturewastransferredintoa300mLTefloncontainer,which

wasfilledwithdistilledwaterupto66%ofthetotalvolume,

sealedandtreatedinthedomesticmicrowaveunderpowerof40%

oftotalpowerdensityfor20min.Afterthereactionwas

completed,theresultingsolidproductswerecentrifuged,washed

withdistilledwaterandethanolformorethan6timestoremove

theionspossiblyremaininginthefinalproducts,andfinallydried

at60°Cinairfor60min.

2.3.Characterization

TheobtainedsampleswerecharacterizedbyX-raydiffract-

ometer(XRD)usingaRigakuD/max-gaX-raydiffractometerat

ascanningof2°min−1in2θrangingoffrom10°to80°withCu

Karadiation(λ=1.54178Å).Thetransmissionelectron

microscopy(TEM)analysiswasconductedonamodelHitachi

H-600withanacceleratingvoltageof75kV.Thescanning

electronmicroscopy(SEM)imageswereobtainedonLEO

1450VP.3.Resultsanddiscussion

TheXRDpatternofthesampleisshowninFig.1.AllthereflectionsinthefigurecanbeindexedtoWO3·2H2Othathaslatticeconstantsa=7.500Å,b=6.930Åandc=3.700Å,whichareconsistentwiththevaluesinthestandardcard(JCPDS18-1420).Therefore,WO3·2H2OmusthavebeenobtainedunderMHprocess.Furthermore,itcanbeseenthatthediffractionpeaksarehigherandnarrower,implyingthattheWO3·2H2Ocrystallizeswell.ThecrystallitesizemeasurementswerecalculatedemployingScherrer'sequation,D=kλ/βcosθwhereDisthecrystallitesize,kisaconstant(=0.9assumingthattheparticlesarespherical),λisthewavelengthoftheX-rayradiation,βisthelinewidth(obtainedaftercorrectionfortheinstrumentalbroadening)andθistheangleofdiffraction.TheaveragecrystallitesizeofsamplesfromXRDdataisabout12nm.TosubstantiallyunderstandtheeffectofL(+)tartaricacidontheWO3·2H2Onanostructures,theexperimentsofMHprocesswithdifferentmassratiosofL(+)tartaricacidtoNa2WO4·2H2Owerecarriedout.TypicalTEMimagesofthesamplesareshowninFig.2.Fig.2(a)showstheTEMimageoftheWO3·2H2OnanostructurespreparedbyMHprocesswithoutL(+)tartaricacid.WhenL(+)tartaricacidwasnotused,nanoneedleswereobtainedwithdiametersof20nmandlengthsuptoseveralmicrometers.However,whenthemassratioofL(+)tartaricacidtoNa2WO4·2H2Owasincreasedto0.5:6,ag-gregatednanoneedlesandnanoparticles(Fig.2(b))wereobtained.WhenthemassratioofL(+)tartaricacidtoNa2WO4·2H2Owasincreasedto1:6,homogenousnanosphereswereobtained(Fig.2(c)),displayinguniformmorphologieswithdiametersfrom30to80nm.TheSEMimageofthenanospheresisshowninFig.3,andvisualnanosphereswithsamesizesasintheTEMimagewereobserved.ItcanbeconcludedfromtheTEMandSEMimagesthattheeffectofthemassratioofL(+)tartaricacidtoNa2WO4·2H2OoftheinitialsolutionontheWO3·2H2Onanoparticlemorphologyissignificant.TheL(+)tartaricacidplaysanimportantroleduringtheformationofWO3·2H2OprecipitationandtheMHprocess.Thefinehomogeneousandwell-crystallizedparticleswithregularsphericalmorphologiesareprobablyformedthroughamechanismsimilartothoseproposedfortheformationofCdSeorLa(OH)3nanocrystals[13,14].Bythismechanism,theorganicacidplaysaroleofprotectiveagentandcontrolthegrowthrateofnanoparticlesindifferentdirections.WO3·2H2Oisananisotropicmaterial,andWO3·2H2Onanoneedlesorsquareplateletwereusuallyobtained.ThisresultagreeswellwiththereportofCsabaBalázsi[12].WhentheL(+)tartaricacidisaddedtotheaqueoussodiumtungstatesolution,stabletungsten–tartaricacidcomplexesareformedinthesolution.WiththedropofHCl,tartaricacidisformedagain,andtungsticacidisobtainedsimultaneously.Tungsticacidparticlesarecovered