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MRB-5933;No.ofPages7

MaterialsResearchBulletinxxx(2012)xxx–xxx

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MaterialsResearchBulletin

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ControlledsynthesisofconcavetetrahedralpalladiumnanocrystalsbyreducingPd(acac)2withcarbonmonoxide

HaiZhua,QuanChia,YanxiZhaoa,ChunyaLia,HeqingTanga,JinlinLia,TaoHuanga,*,HanfanLiua,b

KeyLaboratoryofCatalysisandMaterialsScienceoftheStateEthnicAffairsCommission&MinistryofEducation,HubeiProvince,CollegeofChemistryandMaterialsScience,South-CentralUniversityforNationalities,Wuhan430074,Chinab

InstituteofChemistry,ChineseAcademyofScience,Beijing100080,China

a

ARTICLEINFO

ABSTRACT

Articlehistory:

Received15February2012

Receivedinrevisedform17May2012Accepted14June2012Availableonlinexxx

Keywords:A.Metals

A.NanostructuresB.Chemicalsynthesis

C.Electrochemicalmeasurements

COreducingstrategytocontrolthemorphologiesofpalladiumnanocrystalswasinvestigated.ByusingCOasareducingagent,uniformandwell-de?nedconcavetetrahedralPdnanocrystalswithameansizeofabout55Æ2nmwerereadilysynthesizedwithPd(acac)2asaprecursorandPVPasastabilizer.Thestructuresoftheas-preparedPdnanocrystalswerecharacterizedbytransmissionelectronmicroscopy(TEM),X-raypowderdiffraction(XRD),ultraviolet–visible(UV–vis)absorptionspectroscopyandelectrochemicalmeasurements.TheresultsdemonstratedthatCOwasthemostessentialfortheformationoftheconcavetetrahedralPdnanostructures.Themorphologiesandsizesofthe?nalproductscanbewellcontrolledbyadjustingthe?owrateofCO.ThemostappropriateCO?owrate,temperatureandtimefortheformationoftheidealconcavetetrahedralPdnanocrystalswas0.033mLsÀ1,1008Cand3h,respectively.

ß2012ElsevierLtd.Allrightsreserved.

1.Introduction

Shape-controlledsynthesisofnoblemetalnanostructureshasbeenpaidmuchattentionfordecadeowingtotheirpotentialapplicationsinmany?eldssuchascatalysis[1–4],photonics[5,6],optoelectronics[7,8],plasmonics[9,10],microelectronics[11,12],informationstorage[13],sensing[14–17],biologicallabeling[18],andamongothers.Theuniquepropertiesofmetallicnanoparticlesarestronglydependentontheirsizesandshapes,aswellastheircompositions,crystallinitiesandstructures.So,specialnanos-tructureswithuniformsizesandwell-de?nedshapesarerequiredtotunetheirpropertieswithagreaterversatilityforvariousapplications.Uptonow,differenttechniquessuchastemplate-directed[19,20],solventthermal[21],microwavedielectricheating[22,23],chemicalandelectrochemicalmethods[24,25]havebeendevelopedforthepreparationofmetallicnanoparticles,andmanyreducingagentswereemployedtocontrolparticlesizesandmorphologies.

IncaseofPd,ithasattractedmuchinterestduetoitsextraordinaryproperties.Ithasbeenwidelyusedascatalystsforsomeorganicreactions[26–28]andlowtemperaturereductionofpollutantsexhaustedfromautomobiles[29,30]becauseofits

*Correspondingauthorat:CollegeofChemistryandMaterialsScience,South-CentralUniversityforNationalities,Wuhan430074,China.Tel.:+862767843521;fax:+862767842752.

E-mailaddress:huangt6628@http://wendang.chazidian.com(T.Huang).

exceptionalsensitivityandremarkableadsorbingcapacitytowardhydrogen[31].Pdnanoparticleswerealsousedaselectrocatalystsfordirectalcoholoxidation[24,25,32,33].Sincethecatalyticef?ciencyofPdnanoparticleshighlydependsonbothitssizeanditsshape,alotofeffortshavebeendevotedtothefabricationofuniformPdnanostructureswithcontrollablesizesandwell-de?nedmorphologies.Sofar,variousmorphologicalPdnanopar-ticleshavebeenpreparedbyusingdifferentmethods.ThemostrepresentativePdnanostructuresarerods[34–38],wires[21],sheets[39],cubes/bars[40,41],tetrahedral[42,43],octahedral[44],andmultipod[45]aswellasicosahedra[23,46,47]andtetrahexahedra[24,25].Inthepreparingprocess,polyols(suchasethyleneglycol,tetraethyleneglycol)[23,36,47],citricacid[46],ascorbicacid[48,49],vitaminB1[50],alkylamine[51,52],formaldehyde[53]aswellasCOgas[39]weregenerallyusedasreducingagents.Meanwhile,polyvinylpyrrolidone(PVP),alkylammoniumionsorothersurfactantswerethemostwidelyusedmediatingmaterialstoserveassurface-regulatingagentsorstabilizers.However,theformingmechanismsforsomenanos-tructureshavenotyetbeenrevealed.

Moreover,mostoftheas-obtainednanocrystalsweregenerally?atorconvexshapes.Recently,thoughconcavepolyhedralPdnanocrystals[53]weresynthesizedinthepresenceofformalde-hyde,itisonlyalimitedsuccessforthemtodate.Thereportindicatedthattheformationoftheconcavetetrahedral/trigonalbipyramidalPdnanocrystalswasdependentonthealdehydegroup[53].However,theformationmechanismhasbeenstillunclear.Infact,decompositionofformaldehydemayproduceCO

0025-5408/$–seefrontmatterß2012ElsevierLtd.Allrightsreserved.http://wendang.chazidian.com/10.1016/j.materresbull.2012.06.048

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andH2.Therefore,itcanbeassumedthattheformationoftheconcavePdnanostructureswouldberelatedtoCOmolecules.Ifso,concavepolyhedralPdnanocrystalsshouldbeobtainedinthepresenceofCO.Indeed,ourrecentexperimentcon?rmedthisidea.Herein,thesynthesisofconcavetetrahedralPdnanocrystalswasdemonstratedbyusingCOasareducingagent,Pd(acac)2asaprecursorandPVPasastabilizer.Theformationmechanismwasalsoexploredpreliminarily.

AdvanceX-rayDiffractometeremployingCuKaradiationwith40kVand50mA.Ultraviolet–visible(UV–vis)absorptionspectraweremeasuredonaLambdaBIO35spectrophotometer.

2.3.Electrochemicalmeasurements

2.Experimental

2.1.Materials

Pd(acac)2(99%)waspurchasedfromAcrosChemicals.PVP(averagemolecularweight,Mw=30,000)andN,N-Dimethylfor-mamide(DMF)werepurchasedfromSinopharmChemicalReagentCo.,Ltd.(Shanghai,China).Allreagentswereofanalyticalgradeandusedasreceivedwithoutfurtherpuri?cation.HighpurityCO(99.999%)andH2(99.999%)wereused.

Pd-modi?edworkingelectrodeswerefabricatedbydepositingethanolicdispersionofpuri?edconcavetetrahedralPdnanocrys-talsontoaglassycarbonelectrodefollowedbynaturaldrying.Asaturatedcalomelelectrode(SCE)andaplatinumfoilwereusedasthereferenceandcounterelectrode,respectively.Firstly,toinvestigatetheCOadsorptiononthefreshly-preparedconcavetetrahedralPdnanocrystals,theCOstrippingvoltammetrywasrecordedin0.1MH2SO4atasweeprateof2mV/swithoutintroducinganyadditionalCO.Thenasecondpotentialscanningwasfollowedatthesamesweeprate.Afterthat,COgas(99.999%)wasbubbledfor15minthroughthe0.1MH2SO4solutioninwhichthemodi?edelectrodewasimmersedbeforemeasurements.Themodi?edelectrodewasquicklytransferredintoafresh0.1MH2SO4solutionandtheCOstrippingvoltammetrywasrecordedonceagain.

2.2.Methods

Inatypicalsynthesis,160mgPVPand25mgPd(acac)2weredissolvedin10mLDMF.Afterthoroughmixing,theresultinghomogeneousyellowsolutionwastransferredtoaglassthree-necked?ask.The?nalconcentrationofPd(acac)2was8.2mMandthemolarratioofPVP/Pd(acac)2was17/1.Undervigorouslystirring,COgaswasbubbledcontinuouslyintothesolutionata?owrateof0.033mLsÀ1.The?owrateofCOwascontrolledbyusingmass?owcontroller.Followingtheexclusionofair,the?askwasheatedat1008Cfor3hunderatmosphericpressure.Afterbeingcooledtoroomtemperature,theresultingblackhomoge-neousPdcolloidswereprecipitatedbyacetone,separatedbycentrifugationandfurtherpuri?edbyethanol.Underthesameconditions,thereactionwasalsoconductedbyusingpureH2orthemixedCO–H2gasinsteadofCO.

Transmissionelectronmicroscopy(TEM)imagesweretakenonaFEITecnaiG220transmissionelectronmicroscopyoperatedat200kV.ThesampleforTEMobservationwaspreparedbyplacingadropofthecolloidaldispersionontoacoppergridcoatedwithaperforatedcarbon?lm,followedbyevaporatingthesolventatambienttemperature.Theaverageparticlesizeandthedistribu-tionweredeterminedfromtheenlargedmicrographsonthebasisofthemeasurementofabout300particles.X-raypowderdiffraction(XRD)measurementwasrecordedonaBrukerD8

3.Resultsanddiscussion

Fig.1showsTEMimagesofthetypicalPdnanocrystalspreparedusingCOasareducingagentwitha?owrateof0.033mLsÀ1at1008Cfor3h.Ascanbeseen,uniformandwell-de?nedconcavetetrahedralPdnanocrystalswerethepreferentialnanostructure,thoughfollowedbyafewtrigonalbipyramidalshapes.TheaveragesidelengthoftheconcavetetrahedralPdnanocrystalswas55Æ2nm.Theas-obtainedmorphologicalfea-tureswerethesameasthosereportedbyZhengandco-workers[53].Itwasclearlyobservedthateachfaceofthetetrahedronwasexcavatedwithatrigonalpyramidatthecenter.AccordingtoZheng’sstudiesonthismorphology[53],eachcut-outpyramidatthecenterofthetetrahedralPdholdsa{111}facetandthree{110}facetsexposed.Andtheconcavetetrahedralaresinglecrystalline,whiletheconcavetrigonalbipyramidsaresingletwinnedwithastackingfaultalongthe{111}planes.TheconcavetetrahedralPdnanocrystalsobtainedbyZhengwasrelatedtothealdehydegroup[http://wendang.chazidian.combinedwithourresultsherein,theformationoftheconcavetetrahedralPdwasessentiallydependentuponCOmolecules.

Fig.2showstheXRDpatternoftheconcavetetrahedralPdnanocrystalsobtainedinthetypicalexperimentabove.FivecharacteristicpeaksofPdat2u=40.58,46.88,68.48,82.48,86.98

,

Fig.1.TypicalTEMimage(a)andhighmagni?cationTEM(b)oftheconcavetetrahedralPdnanocrystalspreparedwithaCO?owrateof0.033mLsÀ1at1008Cfor3h.

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{111}

{200}

{220}

{311}

{222}

30405060708090

2θ(degree)

Fig.2.XRDpatternsofthetypicalconcavetetrahedralPdnanocrystals.

correspondingtothe{111},{200},{220},{311}and{222}latticeplanes,wereobserved.Allthediffractionpeakscanbewell-indexedtoface-centeredcubic(fcc)PdaccordingtotheJCPDScardNo.00-001-1201,indicatingthattheas-preparedconcavetetrahe-dralPdhadahighpurityandhighcrystallinity.

ToverifythecriticaleffectofCOonconcavetetrahedralstructure,themixedCO–H2gas(1:1byvolume)wasusedasareducingagentinsteadofpureCOandbubbledintothereactionsystemata?owrateof0.033mLsÀ1underthesameotherconditions.TheresultwasshowninFig.3a.Itcanbeseenthatthedominantproductsweretetrahedralnanocrystalswithanaveragesizeof35Æ2nmaswellastrigonalbipyramids.ThereductionpotentialinvolvingCOisÀ0.12V,sotheCOreductionwasdominantforthemixedCO–H2gas.ThoughtheexistenceofH2interferedwiththereductionbyCOanditsadsorptiononPdsurfacesandresultedintheunobviousconcavityoftheas-obtainednanocrystals,itstillcon?rmedthattheformationoftetrahedralaswellastrigonalbipyramidalnanostructureswithvagueconcavitieswasreallydependentontheuseofCO.SimilarexperimentwasconductedbyusingpureH2instead.AsshowninFig.3b,bycontrast,pompon-likeself-assemblies[54]ofPdnanoparticleswereobtained,andnopolyhedralnanocrystalswereobserved.TheseresultsshowedthatconcavetetrahedralorotherpolyhedralnanostructurescouldnotgenerateintheabsenceofCO.Accordingly,thisalsoprovedourassumptionthatconcavepolyhedralPdnanocrystalsshouldbeobtainedbyusingCOasareducingagentinsteadofformaldehyde.

Sofar,however,itwasreportedthatonlywell-de?nedultrathinhexagonalnanosheetswereproducedbyusingCOasareducing

agent[39].Toexplaintheconcavetetrahedralfeatureobtainedhere,CO?owratewasconsideredespecially.Fig.4showsTEMimagesofPdnanocrystalspreparedatdifferent?owratesofCOgas.Ascanbeseen,withtheincreaseoftheCObubblingratefrom0.033to0.05,0.1,0.3mLsÀ1,theamountofconcavetetrahedradecreased,whileothervariousmorphologiesincreased.WhentheCO?owrateincreasedto0.05mLsÀ1,particleswithuncertainmorphologiesandnonuniformsizeswereobtained,thoughconcavetetrahedralparticleswerestillinthemajority(Fig.4b).Itisnoteworthythatafewnanosheets,accompanyingwithtetrahedralandothermorphologicalparticles,weregeneratedataCO?owrateof0.1mLsÀ1(Fig.4c)andapparenthollownanosheetswereproducedat0.3mLsÀ1(Fig.4d).Moreover,theparticlesizesshrankgraduallyandthedegreeofconcavitydecreased.TheaveragesidelengthoftheconcavetetrahedralPdnanocrystalswas55Æ2,51Æ5,31Æ2nmfrom(a),(b)to(c),respectively.Theseresultsindicatedthatthe?owrateofCOhadagreateffectonmorphologiesandsizesofthe?nalproducts.ArelativelyslowerrateofCOgasinlet,whichmayleadtoaslowerreducingrateandaloweradsorptionofCOonthePdnucleuscrystallite,wasfavorablefortheformationofuniformandwell-de?nedconcavetetrahedralPdnanocrystals(Fig.4a).WhileCOgaswasinletwithafasterrate,ononehand,thereducingratewasincreased,ontheotherhand,theadsorptionofCOonthenucleuscrystallitewasenhancedandtheadsorbingselectivitydecreased.Asaresult,variedmorphologiesofPdnanocrystalswereproducedandtheaverageparticlesizesdecreased.Inaddition,thiseffectofCO?owrateindicatedthatconcavetetrahedralPdnanocrystalsrequiredgrowthunderkineticcontrol.ItisadvantageoustotheformationofconcavetetrahedralPdnanocrystalsataslow?owrateofCOduetokineticcontrol.WiththeincreaseofCO?owrate,irregularsmallernanoparticlesappearedduetoafasterrateofatomicaddition,andevenapparenthollownanosheetsweregeneratedbecauseofthegrowthcon?nementeffectofCOwithafaster?owrate.

Furthermore,http://wendang.chazidian.comparedwiththoseobtainedat1008C,nearlynoconcavetetrahedralPdnanoparticleswereobtainedat808C,asshowninFig.5a,whilemanyirregularapparenthollowhexagonalnanosheetsaccompaniedwithafewsmallerconcavetetrahedralandothermorphologicalparticleswereproducedat1208C,asshowninFig.5c.Thisisbecauseatalowertemperatureboththeformationandgrowthofnucleuscrystallitesloweddowndueto

Fig.3.TEMimagesofPdnanoparticlespreparedusingmixedCO-H2gas(a)andpureH2(b)asareducingagent,respectively,insteadofCOunderthesameotherconditions.

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Intensity(a.u)

GModelMRB-5933;No.ofPages74H.Zhuetal./MaterialsResearchBulletinxxx(2012)xxx–xxxFig.4.TEMimagesofPdnanocrystalspreparedatdifferent?owratesofCO.(a)0.033mLsÀ1;(b)0.05mLsÀ1;(c)0.1mLsÀ1;(d)0.3mLsÀ1

.

Fig.5.TheeffectsofreactiontemperatureonthemorphologiesofPdnanocrystals.(a)808C;(b)1008C;(c)1208C.Theinsetof(c)showsalocallymagni?edTEMimageofanapparenthollownanosheet.

slowerrateofatomicadditionwithslowreducingrateofPd(acac)2,

whereastheadsorbingrateaswellasthenonselectiveadsorption

ofCOonPdfacetsenhanced.Asaresult,shaped-nanoparticles

werehardtoformbeyondkineticcontrol.Atahighertemperature,

however,theadsorbingrateofCOdecreasedwhiletheselective

adsorptionofCOonPd{111}planesenhanced,andconsequently

afewnanosheetswhicharethermodynamicallyfavorablewere

producedduetoafastergrowingrate.Theapparenthollow

hexagonalnanosheetswereactuallysheet-likeporousstructureswiththickeredges,asshownintheinsetmagni?edTEMofFig.5c.So,anappropriatetemperatureinfavorofkineticcontrolwasrequiredfortheformationofconcavetetrahedralnanostructures.TEMimageofPdnanocrystalsproducedina6hreactionisshowninFig.6.Obviously,notonlydidthenumberofconcavetetrahedralPddecreasesigni?cantly,butalsothedegreeofconcavityfadedout.Ontheotherhand,manydifferentconcavepolyhedralnanocrystalstwinnedwithastackingfaultalongthe{111}wereobtained.Inaddition,thesidelengthofthePd

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Fig.6.TEMimagesoftheas-preparedPdnanocrystalsina6hreaction.

nanocrystalswas44Æ3nm,smallerthanthatobtainedina3hreaction.TheseresultssuggestedthattheoptimumreactiontimeforproducingconcavetetrahedralPdwas3h.Thusitcanbeseenthatalongertimewasunfavorablefortheformationofuniformandwell-de?nedconcavetetrahedralmorphologiesbeyondkineticcontrol.

Interestingly,twinnednanocrystalsproducedwithincreasingthereactiontimeimpliedthataripeningprocesswasunfavorabletothegrowthofconcavetetrahedralPd.Thisabnormalbehavingwascon?rmedbyUV–visabsorptionmeasurement.Fig.7showsthetime-dependentUV–visabsorptionspectraforthereactionprocess.Ascanbeseen,atthebeginningthereactionsolutionshowedtwostrongcharacteristicabsorptionpeaksat268and326nm,correspondingtothatofPd(II)ions.Withthereactionproceeding,thepeakat326nmreducedgradually.Whenthereactionproceededfor3h,whichcorrespondedtothetimeinfavorofconcavetetrahedralPdnanostructures,thepeakat326nmstillremainedobviouslythoughitwasweakenedsigni?cantly,indicatingtheexistenceofPd(II)ions.Anotherpeakwithanenhancementwasobservedat269nm.ThismeantthatPd(II)ionswasnotreducedcompletelyundertheoptimumconditionsfortheidealconcavetetrahedralnanostructure.Accordingtothepeakintensitiesat326nm,theyieldofconcavetetrahedralPdrelatedtotheamountofPdprecursorsat3hwasabout81%.Furthermore,increasingthereactiontimecontinu-ouslyfrom4hto8h,thepeakat326nmdisappearedcompletely,whilethepeakat268nmshiftedto273nm.Theredshiftwasindependentonthetimein4–6hreaction,buttheabsorbanceincreasedgraduallyduetotheenhancementofsurfaceplasmon

1.61.4

1.21.00.80.60.40.20.0Wavelenth/nm

Fig.7.UV–visabsorptionspectraofthereactionsystematdifferentstages.

scattering,implyingthechangesofmorphologies.Thiswasconsistentwiththeobservationonthe6hreaction,asshowninFig.6.Withthereactiontimewasprolongedfrom3h,theconcavetetrahedralnanocrystalsweretransformedintotwinnednanos-tructuresduetoripeningprocessafterthedepletionofmetalprecursor.Consideringtheconcavetetrahedralnanocrystalsarenotthermodynamicallyfavorable,thetwinnednanocrystalswouldberipenedwithastackingalongthe{111}planes,accompanyingwiththedisappearanceofconcavity.Theseresultsdemonstratedthattheformationofthesolidpolyhedraat6hmaybeevolvedfromtheconcaveonesby?llingthecavitiesaswellasripeningduetotheincompletereactionat3h.

Inaddition,PVPalsoplaysanimportantroleincontrollingthemorphologiesof?nalnanoparticles.ThoughPVPcanalsoserveasareducingagentinthesynthesesofmetalnanomaterials[21],noproductwasobservedinthepresenceofPVPwithoutusingCOandanobviousagglomerationwasobservedwithouttheadditionofPVPwhentheotherconditionswerekeptthesameasthoseintheabovetypicalexperiment.So,webelievethatPVPservesonlyasstabilizertoeliminaterandomagglomerationduringthesynthesisthoughitisfoundthattheformationofconcavetetrahedralPdisdependentonthePVP/Pd(acac)2ratio.

Fromtheaboveexperimentalevidences,COwasessentialfortheformationofconcavetetrahedralPdnanocrystals.COplayedanimportantdualroleasabothreducingagentandsurfaceadsorbent.COwasadsorbedonPdsurfacesatthesametimeitreducedPd(II)ion,whilethereactioncouldnotoccurwithoutCO.TheconcavefacesweredevelopedattheverybeginningoftheformationofPdnucleuscrystallite.DuetotheadsorptionofCO,eitheratomicadditionoradatomdiffusionwasrestricted,sothatthemorphologieswouldbecon?http://wendang.chazidian.comoleculeswereadsorbedonsomespecialfacetsassoonasPdnucleuscrystalliteswithdifferentcrystallographicfacetsweregenerated.Tocon?rmthepreferentialadsorptionofCOonspecialfacets,COstrippingvoltammetrymeasurementswereperformed.AsshowninFig.8a,onlyoneCOelectro-oxidation(COox)peakat0.927V(versusRHE)wasobservedforthefreshly-preparedconcavetetrahedralPdnanocrystalsin0.1MH2SO4withoutintroducinganyadditionalCO,whichcanbeassignedtotheCOstrippingonPd(110)facets[55].ThenCOelectro-oxidationpeakdisap-pearedinthefollowedsecondpotentialscanning,asshowninFig.8b.Theseresultscon?rmednotonlythepresenceof{110}facetbutalsothepreferentialadsorptionofCOon{110}facetevenintheproducts.Subsequently,twopeakswereobservedinCOstrippingcurvefortheCO-strippedPd-modi?edelectrodeafterCOdosing,asshowninFig.8c.OnepeakforCOelectro-oxidationappearedat0.925V(versusRHE)againisattributedtoCOstrippingon{110}facets,whileanotherpeakat1.026V(versusRHE)canbewellassignedtotheCOstrippingon{111}facets[55].ThisshowsthatCOcanbeadsorbedontoboth{110}and{111}facetsofconcavePdnanocrystalsafterCOstripping.Thisveri?edthatCOwasadsorbedonlyon{110}facetsforfreshly-preparedconcavePdnanocrystals.AfterCOstripping,COcanbejustadsorbedon{111}besides{110}facetsifdosedwithCO.

Accordingly,theformationoftheconcavePdnanocrystalswasgreatlydependentuponaselectiveadsorptionofCOon{110}facetsinthesynthesisprocess.TheCOadsorptionrelatestoCO?owrateandtemperature.WithalowCO?owrate,COwasadsorbedselectivelyon{110}facetsbyanon-toppattern.Asaresult,thenucleuscrystallitepreferredtogrowalongtheexposed{111}facetsandthecorners,andapyramidwiththreeexposed{110}facetsatthecenterofeach{111}facetofthetetrahedralPdwasgenerated.WithafastCO?owrate,the{110}and{111}facetswerecon?nedandthegrowthpreferredalongtheedgesduetotheadsorptionofCOonboth{110}and{111}facets,andthis

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Absorbance