四菱形

CrystEngComm

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Controlledsynthesisoftetrapod/Mitsubishi-likepalladiumnanocrystals?

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HaiZhu,GanLi,QuanChi,YanxiZhao,HanfanLiu,JinlinLiandTaoHuang*

Received22ndSeptember2011,Accepted21stDecember2011DOI:10.1039/c2ce06247j

Well-de?nedtetrapod/Mitsubishi-likePdnanocrystalswithuniformdimensionsweresuccessfullysynthesizedbyusingPd(acac)2asaprecursorandCOasareducingagentinthepresenceofsodiumacetate.Carboxylateanionssuchasacetateanionsplayedanessentialroleincontrollingthetetrapodmorphologiesofthe?nalproducts.

Considerableattentionhasbeenpaidtonoblemetallicnanocrystalswithuniformsizesandwell-de?nedshapesowingtotheirpotentialapplicationsinmany?eldssuchascatalysis,1photonics,2optoelec-tronics,3microelectronics,4informationstorage,5sensing,6imaging,7surface-enhancedRamanscattering8andphotothermaltherapy.9Alotofefforthasbeendevotedinthepastdecadetoshape-controlledsynthesisofmetalnanomaterials.Uptonow,noblemetalnano-particleswithvariousmorphologieshavebeenobtainedsuchasnanorods,10nanowires,11hexagonalnanosheets,9cnanocubes,12tetrahedra,13octahedra,14icosahedra,15tetrahexahedra16aswellashollowcubes,17concavepolyhedralnanocrystals.18Thecontrolledsynthesisoftheseconventionalshapeshasadvancedremarkably.However,metallicnanocrystalswithwell-de?nedmultipodsorhighlybranchedstructureshavestillbeenataninitialstage.19

Hereinwereportasimplemethodforthehigh-yieldsynthesisofthree-dimensionalbranchedmultipod-shapedPdnanocrystals,anexcitingnewkindofnanostructures.Theas-preparedPdnano-crystalswerepresentintetrapodorMitsubishi-likemorphologies.TherepresentativeTEMimagesoftheas-preparedPdnanoparticlesareshowninFig.1.Ascanbeseen,nearlyalloftheprojectionsoftheas-preparedPdnanoparticlesunderTEMlookliketheMitsubishilogowithwell-de?nedmorphologiesanduniformsizes(Fig.1aandb).Actually,itwasrevealedthatthedominantmorphologicalfeaturewastetrapodsandeachonewasconstructedoffourpodswithtetrahedraldimension,asshowninthehigh-magni?cationTEMimages(Fig.1c),thoughaccompaniedwithafewrealMitsubishi-shapedmorphologies(Fig.S1,ESI?).Becausetheparticlespreferredtolie?atonthesubstrateusingthreeoftheirfourapexes,mostofthe

KeyLaboratoryofCatalysisandMaterialScienceoftheStateEthnicAffairsCommission&MinistryofEducation,CollegeofChemistryandMaterialsScience,South-CentralUniversityforNationalities,Wuhan,HubeiProvince,430074,China.E-mail:huangt6628@http://wendang.chazidian.com;Fax:+86-27-67842752;Tel:+86-27-67843521

?Electronicsupplementaryinformation(ESI)available:Synthesisandcharacterizationoftetrapods/Mitsubishi-likepalladiumnanocrystals,experimentalprocedures,TEMimages,UV-visabsorbancespectra,XRDandXPSspectra.SeeDOI:10.1039/c2ce06247j

particleswereobservedasMitsubishi-likeprojections.ThetoponeinFig.1cshowsthefrontprojectionofasinglePdnanocrystalinwhichoneofthefourpodbrancheswasorientedtowardstheobservationdirection,whilethebottomoneinFig.1cillustratestherearimageprojectedfromthebottomofthetetrahedron.Theaverageapex-to-apexdimensionoftheas-preparedtetrapodPdnanocrystalswasabout37Æ2nm.Thehighresolution(HRTEM)(Fig.1d)measurementshowedlatticefringeswithaninterplanarspacingof0.123nm,whichwasindexedto(311)planesofface-centeredcubic(fcc)Pd.Thetop-leftinsetinFig.1cshowsafastFouriertransform(FFT)patterninwhichtwospotswereobserved.TogetherwithHRTEM,theFFTpatternsuggestedthattheas-preparedPdtetra-podsweresingle-crystallinestructures(Fig.S2,ESI?).

TounderstandtheformingprocessofPdtetrapods,differentcarboxylateswereintroducedintothereactionsystems.TheTEMobservationsrevealedthatthesamepreferentialstructureswithwell-de?nedshapeswereobtainedthoughvariouscarboxylatessuchassodiumformate,sodiumpropionateorsodiumbenzoateaswellassodiumacetatewereused(Fig.2a–d).Inaddition,

the

Fig.1TypicalTEM(aandb)andhighmagni?cationTEM(c)imagesoftheas-preparedtetrapod/Mitsubishi-likePdnanocrystals.Thetop-leftinsetin(c)showsaFFTpattern.(d)High-resolutionTEMimageofapodbranch.

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uniformsizesoftheas-preparedPdnanocrystalsinthesecasesweremuchclosetoeachother(Fig.S3,ESI?).ItshouldbenotedthatthepreferentialstructureswereconcavetetrahedralPdnanocrystalswithadditionofsodiumnitrateinsteadofcarboxy-late,whilehollownanosheetsmixedwithothervariousmorphol-ogiesweregeneratedwithoutusinganycarboxylateorotheracidsalt(Fig.S4,ESI?).Theseresultsimplythatcarboxylateanionsarethekeytothegrowthoftetrapod-shapedPdnanocrystals.Thismaybeattributedtoananisotropiccarboxylate-con?nedgrowthofnanocrystalseedsinthepresenceofcarboxylateanions.Moreover,itcanbeseenthatthesplittingdegreeoffourpodbranchesforeachnanocrystalreducedgraduallyfromformate,acetate,propionatetobenzoate.Thesedifferencesmaybeascribedtothestructuresofthecorrespondingcarboxylates.Withtheincreaseofcarbonatoms,sterichindranceofcarboxylateanionsadsorbedonthespecialsurfacesenhanced.Asaresult,carbox-ylate-con?nedgrowthrateofnanocrystalseedswasweakenedandthebranchingdegreedecreased.

AlthoughtheuseofcarboxylateanionswascriticaltothegrowthoftetrapodPdnanocrystals,COplaysimportantdualrolesbothasareducingagentandasasurfaceadsorbentintheformationoftheuniquePdnanostructures.Interestingly,itwasfoundthattheCO?owratecontrolledthebranchingdegreeofthePdtetrapods.WiththedecreaseoftheCO?owratefrom0.3to0.2and0.1mLsÀ1,thebranchingdegreeofthePdtetrapodsfadedoutgradually(Fig.3aandb).ThissuggestedthattheadsorptionofCOonthespecialfacetswasdependentontheCO?owrate.Moreover,whenCOwasbubbledintothereactionsystemwitha?owrateof0.4mLsÀ1,amixtureofseveralirregularmorphologicalparticleswereobservedduetoafasterreducingrate(Fig.S5,ESI?).Theoptimum?owrateofCOgaswas0.3mLsÀ1fortheidealPdtetrapods(Fig.S5,ESI?).

InadditiontotheCO?owrate,thebranchingdegreeofPdnano-crystalswasdependentonthereactiontemperature.Underthesameotherconditions,atalowertemperature(80??C)theyieldedparticles

Fig.3TEMimagesofPdnanocrystalspreparedatdifferentCO?owratesandtemperatures:(a)0.1mLsÀ1,100??C;(b)0.2mLsÀ1,100??C;(c)0.3mLsÀ1,80??C;and(d)0.3mLsÀ1,120??

C.

wereshapedbetweentetrapodsandconcavetetrahedra(Fig.3c).Thenanoparticlesshowedobvioustetrahedralfeatures,whileanasyn-chronousgrowthwithdifferentdirectionsresultedindistinctivenotchesattheedgesofeachconcavetetrahedron.Itsuggestedthattheformationoftetrapodswasjustderivedfromthedefectiveornotchedconcavetetrahedra.Nevertheless,atahighertemperature(120??C),thereactionpreferredtogenerateconcavetetrahedralPdnanocrystals(Fig.3d).Thisalsorevealedthatthemorphologicalfeatureoftetra-podswasrelatedtothatofconcavetetrahedralstructure(Fig.S6,ESI?).Fromtheotherside,theseresultscon?rmedtheexistenceoffourpodbranchesinatetrapodPdnanocrystal.

http://wendang.chazidian.comparedwith3hreaction,noobviousdifferencesonbothsizesandmorphologiesofthetetrapodPdnanocrystalswereobservedin6h(Fig.4),indi-catingthattheformationoftetrapodPdnanocrystals?nishedin3handwasalmostunaffectedwithmoretime.Thisresultwasalsocon?rmedbyUV-visabsorptionmeasurement(Fig.S7,ESI?).Whenthereactionproceededfor3h,thepeakat326nmwhichcorrespondedtothatofPd(II)ionsdisappearedcompletelyandtheplasmonresonancereachedtothe

maximum.

Fig.2TEMimagesofthePdnanocrystalspreparedinthepresenceofsodiumformate(a),sodiumacetate(b),sodiumpropionate(c)andsodiumbenzoate(d),

respectively.

Fig.4TEM(a)andhighmagni?cationTEM(b)imagesofPdnano-crystalspreparedina6hreactionat100??C.

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TogetherwiththeeffectsoftheCO?owrate,temperatureandreactiontime,wesuggestthatthegrowthcon?http://wendang.chazidian.comparedwithapolyhedral(eitherconvexorconcave)nanocrystal,theas-obtainedbranchedtetrapodPdherehasalargersurfaceareaaswellashighersurfaceenergy,andthusrequiresgrowthunderkineticcontrol.Asaresult,thegrowthcon?nementeffectsofCOandcarboxylateanionsweredominatedbykineticcontrolatlowtemperature,whilethermodynamiccontrolathightemperature.Accordingly,theformationofconcavetetra-hedraisfavoredatahighertemperatureduetothermodynamiccontrol.ThoughitisfavorablefortheformationofbranchedtetrapodPdnanostructuresduetokineticcontrolatlowertemper-ature,itmightleadtoaslowrateofatomicadditionifthetemper-atureistoolow.Thatiswhydefectiveconcavetetrahedraweregeneratedwithabranchedtrendat80??C.So,theformationofbranchedtetrapodPdnanocrystalsrequiresanappropriatetemper-ature.Whenkeeping0.3mLsÀ1oftheCO?owratein3hreaction,carboxylate-con?nedgrowthwasdominantat100??C,andasaresult,thetetrapodPdnanocrystalswerethemainproducts.Atahighertemperature(120??C),CO-con?nedgrowthwasdominant,sothatconcavetetrahedralnanocrystalswerefavorable.Whileatalowertemperature(80??C),thepreferentialstructuresweredefectiveconcavetetrahedralowingtothesynergisticcon?nementofCOandcarboxylateanions.Inaddition,theassociatedformationofMitsu-bishi-likemorphologiesmaybeascribedtocarboxylate-con?nedanisotropicgrowthofnanocrystalseeds.AnadsorptionmodelofCOandAcÀonthefacesoftetrahedralcrystalseedswasproposedtoshowthesegrowthcon?nementeffects(Fig.S8,ESI?).

Thecatalysisactivitiesoftheas-preparedbranchedtetrapodPdnanocrystalswerestudiedbyelectrocatalyticoxidationofformicacid.SmallPdnanoparticleswithameansizeof4.5nm(Fig.S9,ESI?)wereusedasreferenceforcomparison.Fig.5showsthecyclicvoltammetry(CV)curvesfortheelectro-oxidationofformicacid.Themaximumcurrentdensitywasmeasuredtobe9.75mAcmÀ2onthetetrapodPdnanocrystalsat0.16V,whileitwas7.77mAcmÀ2onthesmallPdnanoparticlesat0.1V.Theseresultsrevealthattheas-preparedtetrapodPdnanocrystalsdemonstratedanoutstandingelectrocatalyticactivity.

Insummary,well-de?nedtetrapods/Mitsubishi-likepalladiumnanocrystalswithuniformdimensionsweresuccessfullysynthesizedwithPd(acac)2asaprecursor,PVPasastabilizer,COasareducingagentinthepresenceofsodiumacetateat100??Cfor3hunderatmosphericpressure.Theadditionofcarboxylateanionswascriticaltotheformationofthethree-dimensionaltetrapodmorphologiesofthe?nalproducts.TheformationofthebranchedtetrapodPdnanostructureswasascribedtothecarboxylate-con?nedgrowth.Itis

nodoubtthatthisnewkindofbranchednanostructureswouldbeapromisingcandidateforwideapplicationsincatalysisandothers.ThisresearchwassupportedbytheNationalNatureScienceFoundationofChina(grantno.20673146)andtheScienceFoun-dationofSouth-CentralUniversityforNationalities(grantno.YZZ05002).

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Fig.5CVcurvesforelectro-oxidationofformicacidbythetetrapodPdnanocrystals(a)andsmallPdnanoparticles(b).

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