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MaterialsResearchBulletin45(2010)159–164

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MaterialsResearchBulletin

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

Microwave-assistedsynthesisofpalladiumnanocubesandnanobars

YanchunYua,YanxiZhaoa,TaoHuanga,*,HanfanLiua,b,**

KeyLaboratoryofCatalysisandMaterialScienceoftheStateEthnicAffairsCommission&MinistryofEducation,HubeiProvince,CollegeofChemistryandMaterialScience,South-CentralUniversityforNationalities,Wuhan430074,Chinab

InstituteofChemistry,ChineseAcademyofScience,Beijing100080,China

a

ARTICLEINFOABSTRACT

Articlehistory:

Received5June2009

Receivedinrevisedform4September2009Accepted24September2009Availableonline2October2009Keywords:A.Metals

A.NanostructuresB.ChemicalsynthesisC.Electrondiffraction

Microwavewasemployedintheshape-controlledsynthesisofpalladiumnanoparticles.Palladiumnanocubesandnanobarswithameansizeofabout23.8nmwerereadilysynthesizedwithH2PdCl4asaprecursor,tetraethyleneglycol(TEG)asbothasolventandareducingagentinthepresenceofPVPandCTABin80sundermicrowaveirradiation.Thestructuresoftheas-preparedpalladiumnanoparticleswerecharacterizedbytransmissionelectronmicroscopy(TEM),X-raypowderdiffraction(XRD)andultraviolet–visible(UV–vis)absorptionspectroscopy.TheformationofPdBr42ÀduetothecoordinationreplacementoftheligandClÀionsinPdCl42ÀionsbyBrÀionsinthepresenceofbromidewasresponsibleforthesynthesisofPdnanocubesandnanobars.Inaddition,amilderreducingpower,ahigherviscosityandastrongeraf?nityofTEGwerebene?cialtothelargersizesofPdnanocubesandnanobars.

ß2009ElsevierLtd.Allrightsreserved.

1.Introduction

Shape-controlledsynthesisofmetalnanostructureshasbeenpaidmuchattentionfordecadesowingtotheirsize-andshape-dependentproperties[1–5].So,specialnanostructurewithuniformsizeandwell-de?nedshapeisrequiredtoregulatetheirpropertiesforvariousapplications.Asoneofthemoststudiedmetallicnanomaterials,Pdhasattractedmuchinterestforitsapplicationsinmany?elds.Forinstance,ithasbeenwidelyusedasthecatalystsforsomeorganicreactions[6–9],hydrogenation[10,11]andthelow-temperaturereductionofpollutantsexhaustedfromautomobiles[12,13].Pdnanoparticleswerealsousedaselectrocatalystsfordirectalcoholoxidationinalkalinemedia[14–17].Itwasalsousedasacentralcomponentinhydrogensensingduetoitsexceptionalsensitivitytowardhydrogen[18,19].Sincethecatalyticef?ciencyofPdnanoparticleshighlydependsonbothitssizeanditsshape,alotofeffortshavebeendevotedinthepastdecadetosizeandshape-controlledsynthesisofPdnanostructures.

Pdnanoparticleswithvariousmorphologieshavebeenpreparedinthepresenceofsomematerials,suchassurfactants,polymers,coordinatingligand,DNAorRNA,andsometemplates[20–23].Forinstance,polyvinylpyrrolidone(PVP)orsome

*Correspondingauthor.Tel.:+862767842752;fax:+862767842752.

**Correspondingauthorat:CollegeofChemistryandMaterialScience,South-CentralUniversityforNationalities,Wuhan430074,China.Tel.:+862767842752;fax:+862767842752.

E-mailaddresses:huangt6628@http://wendang.chazidian.com(T.Huang),h?iu@http://wendang.chazidian.com(H.Liu).

0025-5408/$–seefrontmatterß2009ElsevierLtd.Allrightsreserved.doi:10.1016/j.materresbull.2009.09.028

alkylammoniumionsarethemostwidelyusedmediatingmaterialstoserveassurface-regulatingagentsorstabilizersinmetallicshaped-nanoparticlesynthesis[24,25].Somehydroxylcompoundssuchaspolyols,citricacid,vitaminC,vitaminBaswellassomealkylaminesoralkanolaminecanbeusedasreducingagentsinreactions.Amongthem,ethyleneglycol(EG)isoneofthemostoftenusedreducingagents.Generally,however,mixstructureswithvariousmorphologiesofPdnanoparticleswereproduced.Forinstance,Pdcolloidalnanoparticlescoexistingwithvariousgeometricshapessuchastriangular,pentagonal,hexago-nal,square,andrhombohedralPdaswellastruncatedcubesoroctahedral,decahedra,andicosahedrawereobtainedbyfastreductionofPdsaltwithmethanol[26]andethanol[27].Pdnanobarsandnanorodscouldbepreparedwithhighyieldsandgooduniformitybyusingamodi?edpolyolprocessinwhich[PdCl4]2ÀwerereducedbyEG[28].Recently,Pdnanoparticleswithnanobelt,nanoplate,andnanotreemorphologieswerereadilypreparedwithPdCl2asstartingmaterials,vitaminB1asareducingagentandwaterasasolvent[29].Thepalladiumsphericalnanoparticles,multitwinnedparticles,andsphericalspongelikeparticlesweresuccessfullypreparedbyasonoelectrochemicalmethodinthepresenceofdifferentsurfactantsorpolymers[30].Pdnanostructuresformedbynanowires,nanoplatesor?ower-likeshapesweresynthesizedbyradiolysisorphotoreduction[31].Fivefoldtwinnednanorodsandrightbipyramidsofpalladiumweresynthesizedinanaqueousmediumwithascorbicacidasthereducingagentandinthepresenceofbromide[21]andsingle-crystalnanocubesandnanobarswerealsoobtained[28].Pdicosahedrawithahighyieldof80%wereselectivelyobtainedinanaqueoussolutionwithcitricacidasareducingagent[32].Sofar,

160Y.Yuetal./MaterialsResearchBulletin45(2010)159–164

however,therehavebeenfewreportsonthesynthesisofpolygonalnanocrystalbymicrowavemethod.

Microwavedielectricheatinghasmanyadvantagescomparedwithconventionalheating,suchaspromptstartup,uniformheating,veryshortheatingtime,easyheatcontrol(onandoff),lowcost,etc.Ithasbeenextensivelyappliedtothesynthesisofmetallicnanostructures[33,34].Inthispaper,microwaveirradiationwasemployedtosynthesizePdnanocubesandnanobarswithtetraethyleneglycol(TEG)asbothareducingagentandasolvent,H2PdCl4asaprecursorandPVP/cetyltrimethylammoniumbro-mide(CTAB)asaco-stabilizer.TheeffectofCTABontheshapeofpalladiumnanoparticleswasalsoinvestigated.2.Experimental2.1.Materials

PVP(averagemolecularweight,Mw=360,000,FlukaChemicals),CTAB,CTAC,palladiumchloride(PdCl2,59.0%)(ShanghaiChemicalsCo.,China),TEG(AcrosChemicals)andotherchemicalswerealloftheanalyticalgradeandusedwithoutfurtherpuri?cation.H2PdCl4ÁnH2OwaspreparedbytreatingPdCl2withconcentratedhydrochloricacidat358C,inwhichthemolarratioofHCl/PdCl2was2/1.H2PdCl4ÁnH2OwasdissolvedinTEGbeforeuseandabrownishredhomogeneousH2PdCl4solutionwasobtained,inwhichtheconcentrationofPd(II)waskeptat0.03molLÀ1.PVP,CTAB,CTAC,KBrandKClwerealsodissolvedinTEGtomakeupacertainconcentrationofthecorrespondingsolutioninTEG,respectively.2.2.PreparationofPdnanocubesandnanobars

Inatypicalsynthesis,1mLof0.03molLÀ1H2PdCl4solution,1mLof0.12molLÀ1PVP(inmonomericunit)solutionand1.0mLof0.12molLÀ1CTABsolutionwereaddedintoa50mLroundbottom?ask.Then,7mLofTEGwasaddedtokeepthe?nalvolumeofthemixtureat10mL.Inthereactionsystem,themolarratioofH2PdCl4/CTAB/PVPwas1/4/4.Afterthoroughmixing,thesolutionwasthenputintoamodi?eddomesticmicrowaveoven(Galanz,900W)andheatedfor80swith100%outputofthepower.Thecolorofthesolutionturnedfrompaleyellowtobrownishblack,andpalladiumcolloidwasobtained.Themicrowaveovenwasmodi?edwithoutchangingthepowerasdescribedasinRef.[34]:awater-cooledcondenseroutsidetheoven’scavitywasconnectedbyaglassjointofaglassround-bottomed?asksetinside.ATe?onstirrerwassetinthe?askandwasdrivenbyamotor.Thepoweroutputwasadjustedbythemicrowavemachinewitharoutineon–offmannerandthetemperaturedidn’thavetobecontrolled.2.3.Characterization

Ultraviolet–visible(UV–vis)absorptionspectraweremeasuredonaLambdaBIO35spectrophotometer.Transmissionelectronmicroscopy(TEM)andhigh-resolutiontransmissionelectronmicroscopy(HRTEM)imagesweretakenonaFEITecnaiG220transmissionelectronmicroscopyoperatedat200kV.Diffracto-gramsofHRTEMwereobtainedbyFouriertransformation.ThesampleforTEMobservationwaspreparedbyplacingadropofthecolloidaldispersionontoacoppergridcoatedwithaperforatedcarbon?lm,followedbyevaporatingthesolventatambienttemperature.Theaverageparticlesizeandthedistributionweredeterminedfromtheenlargedmicrographsonthebasisofthemeasurementofabout300particles.X-raypowderdiffraction(XRD)patternswererecordedonaBrukerD8advanceX-raydiffractometeremployingCuKaradiationwith40kVand50mA.ThemeansizeofPdnanoparticleswasalsoestimatedbyusingtheScherrer’sequation.

3.Resultsanddiscussion

3.1.UV–visabsorptionspectraofthereactionprocess

Fig.1showsthetime-dependentUV–visabsorptionspectrafortheformationofPdnanoparticlesintheabovetypicalsynthesisprocessfrom0to100s.AsshowninFig.1,atthebeginningthereactionsolutionshowedtwostrongcharacteristicabsorptionpeaksataround300and420nm,correspondingtothatofPdCl42Àion.Inaddition,anotherpeakataround530nmforPdCl42Àionwasalsoobservedbuttooweaktoidentify.Itcanbeseenthatwhenthereactionsolutionwasheatedbymicrowaveirradiationfor40s,thecharacteristicabsorptionpeaksofPdCl42Àionreducedobviously.Whenthereactionproceededfor60s,accompaniedbyaslightblue-shift,thepeakat300nmreducedcontinuously,whilethepeakat420nmdisappearedalmostcompletelywithappearanceofasurfaceplasmonscattering,indicatingthataPdcolloidbegantoform.Withincreasingtheirradiationtime,theplasmonscatteringfurtherenhancedduetotheincreaseofPdnanoparticles.Whenthereactionwascontinuouslyirradiatedbymicrowavefor80s,theplasmonabsorptionreachedthemaximum.Eventhoughtheirradiationwasprolongedto100s,theabsorptionwasasthesameasthatfor80s.Thiscon?rmedthatthereactionhas?nishedcompletelyat80s.3.2.TEMcharacterization

Fig.2showsTEMimagesoftheas-preparedPdnanoparticlesintheabovetypicalsynthesis,inwhichtheprecursorconcentrationwas3mmolLÀ1andthemolarratioofH2PdCl4/CTAB/PVPwas1/4/4.Ascanbeseen,uniformandwell-de?nedPdnanocubesandshortnanobarswereobtained(Fig.2a).Theaveragesizewasabout23.8nmbystatisticcalculation.Fig.2bdisplaysatypicalhigh-resolutionTEMimageofasinglePdnanocube.Theinterplanard

spacingisabout2A

?,correspondingtothatofthe{100}latticeplanesoffccPd.TheinsetofFig.2bshowsaFastFourierTransform(FFT)patternoftheselectedPdnanocube.Thepresenceofthetwodistinctspots,correspondingtotheplanesoffccPd,indicatesthattheas-preparedPdnanocubesaresinglecrystallinestructure.3.3.XRDpatternofPdnanocubesandnanobars

TheXRDpatternofthePdnanocubesandnanobarsobtainedintheabovetypicalexperimentisshowninFig.3.FourcharacteristicpeaksofPdat2u=40.58,46.88,68.48,82.38,correspondingtothe{111},{200},{222},{311}latticeplanes,areobserved.Allthediffractionpeakscanbewell-indexedtofccPdaccordingtotheJCPDScardno.00-001-1201,indicatingthattheas-preparedPdnanocubesandnanobarshaveahighpurityandhigh

crystallinity.

Fig.1.UV–visabsorptionspectraofthereactionsystematdifferentstages.

Y.Yuetal./MaterialsResearchBulletin45(2010)159–164161

Fig.2.TEMandhigh-resolutionTEMimagesofPdnanoparticles.(a)TEMimage,theinsetarrowsshowsomeofthenanobars;(b)HRTEMimage,theinsetisafastFouriertransform(FFT)patternoftheselectedPd

nanocube.

TheaverageparticlesizeisestimatedbytheScherrer’sequationtobeabout23.6nm,whichisexcellentlyconsistentwiththeobservationbyTEM.3.4.EffectofCTABandPVP

ItisnoteworthythatanappropriateamountofCTABandPVPplaysanimportantroleincontrollingthemorphologiesofthe?nalPdnanoparticles.Fig.4showsTEMimagesofPdnanoparticleswithdifferentamountsofCTABandPVP.AsshowninFig.4a,anobviousagglomerationwasobservedwithouttheadditionofPVPwhentheotherconditionswerekeptthesameasthatintheabovetypicalexperimentinwhichtheprecursorconcentrationwas3mmolLÀ1andH2PdCl4/CTABmolarratiowas1/4.Whenthereactionsystemcontained12mmolLÀ1ofPVP,inwhichthemolarratioofH2PdCl4/CTAB/PVPwas1/4/4,Pdnanocubesandnanobarswithuniformsizeandwelldispersionwereobtained,asshowninFig.4b.However,whentheconcentrationoftheprecursorandPVPwaskeptunchanged,themorphologiesofPdnanoparticlesvariedwiththeconcentrationofCTAB.AsshowninFig.4candd,mixstructureswithvariousmorphologiesofPdnanoparticleswereproducedwhenthemolarratioofH2PdCl4/CTAB/PVPwas1/3/4and1/10/4,respectively.ItwasfoundthattheabsenceofeitherPVPorCTABinthereactionsystemwouldhardlyproducePdnanocubesandnanobars.TheseresultsshowthattheformationofPdnanocubesandnanobarsdependsonthecoexistenceofCTAB

Fig.3.XRDpatternofthetypicalPdnanocubesandnanobars.

andPVP.WhenPVPconcentrationwas12mmolLÀ1,themostappropriaterangeofCTABconcentrationforthecontrolsynthesisofPdnanocubesandnanobarswas12–24mmolLÀ1.Inotherword,themostappropriaterangeofthemolarratioofH2PdCl4/CTAB/PVPwas1/4/4to1/8/4.

ThesynthesiswastypicallyconductedundermicrowaveirradiationinTEGwithH2PdCl4asaprecursorinthepresenceofPVPandCTAB.TEGservedasbothasolventandareducingagent.NodoubtPVPplaysanimportantroleincontrollingthemorphologiesof?nalPdnanoparticles.ThePVPnotonlyservesasacolloidalstabilizertoeliminaterandomagglomeration,butalsodirectstheparticlestogrowintothewell-de?nedshapesduringthesynthesis[32].ItiswellknownthatCTABiswidelyusedasashapecontrollerinnanostructuresynthesis.Nevertheless,Prof.Xia’sgrouphascon?rmedthatbromideplayedacriticalroleintheformationofPdnanobarsandnanorods[28].TheydemonstratedthatbromidecanchemisorbontothesurfaceofPdseedsandaltertheorderofsurfacefreeenergiesfordifferentfacetssotheformationof{100}surfacecanbegreatlypromotedtogeneratenanocubesandnanobars.Tofurtherunderstandourexperimentalresults,weusedKBr,CTAC,andKClinsteadofCTAB,respectively,inthesamereactionsystemunderthesamecondition.Fig.5a–dshowstheTEMimageoftheas-preparedPdcolloidcorrespondingtoCTAB,CTAC,KBr,andKClsystem,respectively.Obviously,theshapesoftheas-obtainedPdnanoparticlesforCTACsystem(Fig.5b)aredifferentfromthatofCTABsystem(Fig.5a).ThecooperationofCTACandPVPresultedintheformationofanicosahedralPdnanostructure,whichillustratesahexagonalshapeunderTEM,asshowninFig.5b.SimilarresultswerealsoobservedforKBrandKClsystem.Asitcanbeseen,PdnanocubesandnanobarswerealsoobtainedforthereactionsystemwithKBr(Fig.5c)insteadofCTAB,whilePdicosahedrawereobtainedforthatwithKCl(Fig.5d)insteadofCTAC.SynthesisinthepresenceofeitherCTABorKBrallgeneratednanocubesandnanobars,whilethatintheabsenceofbromideorinthepresenceofchloridemainlyproducedicosahedra.SinceCTABandCTAC,aswellasKBrandKCl,haveasamecation,thedifferenceinthemorphologyofPdnanoparticlescanbeattributedtotheirdifferentcounterions,BrÀandClÀ,respectively.ThustheformationofPdnanocubesandnanobarsshouldbeascribedtotheexistenceofBrÀanions.

IthasbeenreportedthatPdCl42Àionisnottherealprecursorspeciesinthereductionprocess[35].Ithasbeenprovedthatthemajorfunctionofbromideinthesynthesisisitschemisorptiononnanocrystalsurfaceanditalsocontributedtotheslow

reduction

162Y.Yuetal./MaterialsResearchBulletin45(2010)159–164

Fig.4.TEMimagesofPdnanoparticleswithdifferentamountsofCTABandPVP.(a)H2PdCl4/CTAB=1/4,withoutPVP;(b)H2PdCl4/CTAB/PVP=1/4/4;(c)H2PdCl4/CTAB/PVP=1/3/4;(d)H2PdCl4/CTAB/PVP=1/10/4.Inallcases,theconcentrationoftheprecursorwas3mmolLÀ1.

duetotheformationofamorestablecoordinatedanionPdBr42Àinthereactionsystemaftertheadditionofbromide.ThepotentialofH2PdCl4islowerthanthatofH2PdBr4.Theadditionofbromidereducedthepotentialofpalladiumionsothatcouldreducethereductionrate.Torealizetheeffectofthecoordinationreplace-mentonthereductionrateandnanocrystalgrowthundermicrowaveirradiation,UV–visabsorptionspectroscopywasemployed.Fig.6showstheUV–visabsorptionspectraofthepalladiumprecursorinTEGwithdifferentadditives.TwocharacteristicabsorptionpeakscorrespondingtothoseofPdCl42Àionwereobservedat260and340nmforH2PdCl4solutioninTEGwithoutanyotheradditiveduetotheligandmetalchargetransfer.Moreover,thoughPVPandCTAC(orKCl)wasintroducedintotheH2PdCl4-TEGsolution,notanychangeoftheabsorptionbandswasobserved.However,whentheH2PdCl4-TEGsolutioncontainedPVPandCTABorKBrwiththesameconcentration,theabsorptionbandsenhancedandaredshiftoccurred.Thetwoligandmetalchargetransferpeaksareshiftedto300and420nm,respectively,andtheredshiftisindependentonPVPforthesameamountofCTABandKBr.TheredshiftoftheabsorptionbandscanbeattributedtothechangeofthepalladiumcomplexfromligandClÀtoligandBrÀ.Inotherword,achangeofthecoordinationbondorligandincoordinatedpalladiumiontookplace.TheseresultsdemonstratedthatthecoordinatedionPdBr42ÀwasformedduetothecoordinationreplacementoftheligandClÀionbyBrÀionafter

theadditionofbromide,whiletheadditionofCTACorKCldidnotbringaboutanychangeofligand.Inaddition,thesameredshiftoftheabsorptionbandforCTABandKBrsystemsindicatesthatthepeakshiftisindependentontheformationofsurfactant-PdBr42ÀcomplexduetotheinteractionbetweensurfactantandPdBr42À,buttheformationofcoordinatedionPdBr42À.ThisresultisconsistentwiththeTEMobservationofsimilarnanocubesandnanobars(Fig.5aandc)forboththeCTABandKBrsystems.Thatistosay,theformationofthePdnanocubesandnanobarsdependsonthereductionkineticsandadsorptionofkineticsoftheBrÀionsonthesurfaceofgrowingPdnanoparticles.TheformationofthecoordinatedionPdBr42Àreducestheredoxpotentialaswellasthereductionrate,whilethepreferentialadsorptionofBrÀionsonthesurfaceofgrowingPdnanoparticlespromotestheformationof{100}facets.ThisisinagreementwiththereportsbyXia’sgroup[21,28].

Furthermore,itwasfoundthatthered-shiftdistancewasdependentontheconcentrationofbromide.Withtheincreaseofthebromide,thecharacteristicpeaksat260and340nmshiftedgraduallyto300and420nm,asshowninFig.7.WhenthemolarratioofH2PdCl4/KBrwas1/4,withcontinuouslyincreasingtheamountofKBr,theredshiftbecameunobvious.Thismeantthe4-foldBrÀionscansubstitutemostClÀionsinthePdCl42ÀionstoformPdBr42Àions.SimilarphenomenonwasobservedforCTABinsteadofKBr.Thusascanbeseen,theeffectofCTABorKBrcomesfrom

the

Y.Yuetal./MaterialsResearchBulletin45(2010)159–164163

Fig.5.TEMimagesofPdnanoparticles.(a)H2PdCl4/CTAB/PVP=1/4/4;(b)H2PdCl4/CTAC/PVP=1/4/4;(c)H2PdCl4/KBr/PVP=1/4/4;(c)H2PdCl4/KCl/PVP=1/4/4.Inallcases,theconcentrationoftheprecursorwas3mmolLÀ1

.

formationofBrcomplex.ThefunctionofCTABorKBrisjusttoforma

morestableBrcomplextolowertheredoxpotential.Theseresults

con?rmthattheformofprecursorisimportantindeterminingthe

morphologyofPdnanoparticles,andalsoexplainthatwhythe

uniformPdnanocubesandnanobarsproducedonlywiththemolar

Fig.6.UV–visabsorptionspectraofthepalladiumprecursorinTEGwithdifferent

additives.(A)H2PdCl4;(B)H2PdCl4+PVP;(C)H2PdCl4+PVP+CTAC;(D)

H2PdCl4+PVP+KCl;(E)H2PdCl4+PVP+KBr;(F)H2PdCl4+PVP+CTAB;(G)

H2PdCl4+CTAB;(H)H2PdCl4+KBr.Allcontain0.3mmolLÀ1ofH2PdCl4.ratioof1/4/4to1/8/4ofH2PdCl4/CTAB/PVP.ThoughthecompletesubstitutionofallchlorideinthePdCl42Àionsneeds10-foldBrÀions[35],anappropriatelylowconcentrationofbromideismorefavorablefortheformationofsingle-crystalnanocubesandnanobars[28].Inthepresentcase,4-to8-foldBrÀionsaresuitableforPdnanocubesandnanobars.TheseUV–visabsorptionchar-acteristicsareconsistentwiththeTEMobservation.Inaddition,TEGmaybeanotheressentialfactorfortheshape-andsize-controlofPdnanocubesandnanobars.ItwasfoundthatPdnanocubesandnanobarswithmono-morphologyandwelldispersionwerenotobtainedifEGwasusedasasolventandareducingagentinsteadofTEGunderthesameconditions.Thereasonmaybeascribedtothestructure,viscosityandreducingabilityofTEG.Firstly,TEGhasfourethylene-oxygenunitswhichdemonstrateacertainaf?nityformetalionsandmetalnanopar-ticlessothatitcanbeadsorbedonthesurfacesofPdparticlesmorestronglythanEG[36,37].Secondly,TEGhasahigherviscosity(208C,59cPs)thanEG(208C,22.1cPs)[38].Thirdly,TEGhasweakerreducingpowerthanEG[39],thoughitalsohastwoterminalhydroxylgroupssimilartoEG.BothamilderreductionandahigherviscosityofTEGwouldslowdownthenucleationrateandthegrowthrateofPdnanoparticles,andmaybebene?cialtothelargersizesofPdnanocubesandnanobars,whereasthestrongeraf?nityisbene?cialtostabilizingthedispersionofPd

nanoparticles.