PreparationofTractablePlatinum,Rhodium,andRutheniumNanoclusterswithSmallParticleSizein
OrganicMedia
YuanWang,*JiawenRen,KaiDeng,LinlinGui,andYouqiTang
StateKeyLaboratoryforStructuralChemistryofUnstableandStableSpecies,InstituteofPhysicalChemistry,PekingUniversity,Beijing100871,China
ReceivedJanuary31,2000.RevisedManuscriptReceivedApril4,2000
Intheabsenceofanyusualprotectiveagent,stableplatinum,rhodium,andrutheniummetalnanoclusterswithsmallparticlesizeinorganicmediaareeffectivelypreparedbyheatingcorrespondingmetalhydroxidecolloidsinethyleneglycolcontainingNaOHforthefirsttime.TheaveragediametersofthePt,Rh,andRunanoclustersdeterminedbymeansoftransmissionelectronmicroscopyareinarangefrom1to2nm.Theparticlesizedistributionineachcolloidalsolutionissharp,within2nmwide.Studiesonthepreparationconditionsandsomepropertiesofthe“unprotected”Ptmetalnanoclustershavebeencarriedout.ByadjustingpH,itisconvenienttoseparatethePtnanoclusterasaprecipitatefromglycolsolvent,andtheprecipitatedPtnanoclusterscaneasilybe“dissolved”inmanyorganicsolventstoformtransparentPtnanoclustersolutionswithhighconcentrationintheabsenceofusualprotectiveagents.The“unprotected”PtnanoclusterscanalsobeeasilytransformedtovariousprotectedPtnanoclusterswiththesamePtcoresandcanbeextractedintotoluenebyformingPPh3-modifiedPtclusters.
Introduction
Thepreparationandpropertiesofnarrowlydistrib-utednanoscopicmetalcolloidsormetalnanoclustershavebeenintensivelystudiedbecauseoftheintensescientificinterestinexploringthepropertiesofsmallmetalparticles,andbecauseofanticipationinnewtechnologicalapplications.1Theycanbeusedasquan-tumdotsforunderstandingthequantumsizeeffectanddesigningnewopticandelectricmaterials.2,3Thetran-sitionmetalnanoclustersalsoserveasabridgebetweenhomogeneousandheterogeneouscatalystsandprovidenewopportunitiesforcatalysis.4Theyarenotonlyactiveandselectivecatalystsforvariousorganicreactionssuchashydrogenation5andhydration,6butalsoactivecatalystsforvisiblelight-inducedhydrogenevolution.7
*Correspondingauthor.Telephone:+86-10-62758575.E-mail:wangy@ipc.pku.edu.cn.
(1)Forexample:(a)Schmid,G.Chem.Rev.1992,92,1709.(b)Schmid,G.,Ed.ClustersandColloids;VCH:Weinheim,1994;andreferencestherein.(c)Furlong,D.;Launikons,A.;SasseW.J.Chem.Soc.,FaradayTrans.I1984,80,751.(d)Chen,C.;Serizawa,T.;Akashi,M.Chem.Mater.1999,11,1381.(e)LeBars,J.;Specht,U.;Bradley,J.S.;Blackmond,D.Langmuir1999,15,7621.(f)Goia,D.;Matijevic,E.ColloidsandSurfacesA:1999,146,139.(2)Scho¨n,G.;Simon,U.ColloidPolym.Sci.1995,273,101.
(3)(a)Chen,S.;Ingram,R.;Hostetler,M.;Pietron,J.;Murray,R.;Schaaff,T.;Khoury,J.;Alvarez,M.;Whetten,R.Science1998,280,2098.(b)Elghanian,R.;Storhoff,J.;Mucic,R.;Letsinger,R.;Mirkin,C.Science1997,277,1078.(c)Andres,R.P.;Bielefeld,J.;Henderson,J.;Janes,D.;Kolagunta,V.;Kubiak,C.;Mahoney,W.;Osifchin,R.Science1996,273,1690.
(4)Lewis,L.N.Chem.Rev.1993,93,2693.
(5)(a)Lewis,L.N.J.Am.Chem.Soc.1990,112,5998.(b)Toshima,N.;Kuriyama,M.;Yonezawa,Y.;Kushihashi,K.;Asakura,K.J.Phys.Chem.1991,95,7448.(c)Bonnemann,H.;Braun,G.Angew.Chem.,Int.Ed.Engl.1996,35,1992.(d)Zuo,X.;Liu,H.;Liu,M.TerahedronLett.1998,39,1941.
(6)Toshima,N.;Wang,Y.Langmuir1994,10,4574.
Generallyspeaking,transitionmetalnanoclusterswithsmallparticlesize(1-3nm)havehighcatalyticactivity,goodlighttransparency,andobvioussize-dependentproperties.
Topreparestablenoblemetalcolloidswithsmallparticlesize(1-3nm)andnarrowsizedistribution,protectiveagentssuchassurfactants,polymers,ororganicligandswereusuallyused.Ontheotherhand,adiluteaqueouscolloidalsolutionofsmallPtnanoclus-ter(Pt<100ppm)stabilizedbyadsorbed[PtCl6]2-andcitratehasbeenprepared.1cThepreviousstrategiesforthepreparationofsmallmetalnanoclustershavebeengreatlypromotingthedevelopmentofmetalcolloidornanoclusterchemistry.1Howeverastable,“unprotected”smallmetalnanoclusterinorganicmediawitharea-sonablemetalconcentrationshouldbeofgreathelpforresearchintheactiveandprospectivefieldofmetalnanoclusterchemistry.
Someresultsofthepreparationofnanoscopicmetalcolloidsinorganicmediawithouttraditionalprotectiveagentshavebeenreported,8-10theproductscanbecalled“unprotected”metalcolloids.“Unprotected”metalcolloidsdonotmeanthatthemetalnanoparticlesaretrulybare.Infact,theyarestabilizedbysolventsor
(7)Toshima,N.;Kuriyama,M.;Yamada,Y.;Hirai,H.Chem.Lett.1981,1981,193.Duonghong,D.;Norgarello,E.;Gratzel,M.J.Am.Chem.Soc.1981,103,4685.Kurihara,K.;Kizling,J.;Stenius,P.;Fendler,J.H.J.Am.Chem.Soc.1983,105,2574.
(8)Curtis,A.;Duff,D.;Edwards,P.;Jefferson,D.;Johnson,B.;Kirkland,A.;Wallace,A.J.Phys.Chem.1988,92,2270.
(9)Cardenas-Trivino,G.;Klabunde,K.;Brock,D.,E.Langmuir1987,3,986.
(10)(a)Esumi,K.;Tano,T.;Meguro,K.Langmuir1989,5,268.(b)Tano,T.;Esumi,K.;Meguro,K.J.ColloidInterfaceSci.1989,133,530.
10.1021/cm0000853CCC:$19.00©2000AmericanChemicalSociety
PublishedonWeb05/25/2000
TractablePt,Rh,andRuNanoclusterssimpleanionsadsorbedonthem,orbyboth.Curtis8andco-workerspreparedan“unprotected”coppercolloidinmethanolbythereductionofcopper(II)saltswithhydrazinehydrate.Thecoppercolloidalparticleshaveameandiameterof13.3nm.Klabunde9andco-workerspreparedstablePdcolloidswithaveragediametersof6-8nminacetoneorpropanolbyusingtheatomclusteringprocess.ThesePdcolloidalparticlesare“living”inthesensethattheycanbeusedtoproducemetalfilm.Esumi10reportedthepreparationofstablepalladiumcolloidswithaveragePdparticlesizesof8-10nmfromthethermaldecompositionofpalladiumcomplexesinmethylisobutylketone.Toourknowledge,reportsonthechemicalpreparationofstable“unpro-tected”Pt,Rh,andRumetalnanoclusterswithparticlesizeslessthan3nminorganicmediahavesofarbeenscarce.
Inthispaper,wereportaneffectivepreparationmethodforthestable“unprotected”Pt,Ru,andRhmetalnanoclusterswithsmallparticlesizeandnarrowsizedistributioninorganicsolventsforthefirsttime.Theresultant“unprotected”Ptnanoclusterinethyleneglycol(glycol)caneasilybeseparatedfromthesolventasaprecipitatethatis“soluble”inmanykindsoforganicsolvents.Moreover,thesemetalnanoclustershaveavaluableadvantagethatmanypolymersororganicligandscaneasilymodifythemtoformvariousprotectedmetalnanoclusterswiththesamemetalcore.
ExperimentalSection
Materials.Hydrogenhexachloroplatinate(IV)hexahydrate(H2PtCl6‚6H2O,99%)waspurchasedfromBeijingHongkeChemicalProductsCo.andusedasreceived.RhCl3‚3H2OandRuCl3‚6H2OweresuppliedbyN.E.ChemcatCo.Polyoxyeth-ylene(20)stearylether(C18H37(OCH2CH2)20OH(POSE))andPoly(N-vinyl-2-pyrrolidone)(PVP)(averagemoleculeweight50000)werepurchasedfromAldrichCo.OrganicsolventsandtheotherchemicalswereofanARgradeandwereusedasreceived.
TransmissionElectronMicroscopyMeasurements.Transmissionelectronmicroscopy(TEM)photographsweretakenonaJEM200CXelectronmicroscopeoperatedat120kVwithamagnificationof100000×.High-resolutiontrans-missionelectronmicroscopyphotographsweretakenonaHitachiH-9000electronmicroscopyoperatedat300kVwithamagnificationof400000×.SamplesfortheTEMmeasure-mentswerepreparedbyplacingadropofthemetalnanoclus-tersolution,appropriatelydiluted,onacoppergridcoatedbyapolymerorcarbonfilm.Excesssolutionwasremovedbyadsorbentpaper.Theaveragediameterofthemetalparticleswasdeterminedfromthediametersof300nanoparticlesfoundinanarbitrarilychosenareainenlargedmicrophotographsbyusingamagnifierwithamagnifyingpowerof10.
X-rayDiffractionMeasurements.SamplesforX-raydiffraction(XRD)measurementswerepreparedbyaddingdiluteHClacidintothePtmetalnanoclustersolutionstoprecipitatethemetalnanoclusters.Theprecipitatesweredriedundervacuum.X-raydiffractionpatternswererecordedbyaDmax-2000diffractometer(RigakuCo.).
X-rayPhotoelectronSpectroscopyMeasurement.X-rayphotoelectronspectroscopy(XPS)measurementswerecarriedoutonanESCALAB-5(VGCo.).SampleswerepreparedbyprecipitatingthePtnanoclusterinthecolloidalsolutionwithdilutedHClacid.TheprecipitatedPtnanoclusterwaswashedbydissolvinginacetoneandprecipitatingwithdilutedHClacidforseveraltimesbeforecompletelydryingunderavacuum.ThesamplewasputonanindiumfilmcoveredonaNisampleset,andtheIn3dcorelevelssignalswereusedasaninternalstandard.
Chem.Mater.,Vol.12,No.6,20001623
Preparationof“Unprotected”PlatinumNanoclusters.Alloperationswerecarriedoutunderinertatmosphere(ArorN2).Inatypicalpreparation,aglycolsolutionofNaOH(50mL,0.5M)wasaddedintoaglycolsolutionofH2PtCl6‚6H2O(1.0g,1.93mmolin50mL)withstirringtoobtainatransparentyellowplatinumhydroxideoroxidecolloidalsolutionwhichwasthenheatedat160°Cfor3h,withanArflowpassingthroughthereactionsystemtotakeawaywaterandorganicbyproducts.Atransparentdark-brownhomoge-neouscolloidalsolutionofthePtmetalnanocluster(Pt:3.76g/Lglycol,19.3mmol/L)wasobtainedwithoutanyprecipitate.TheobtainedPtnanoclustersolutionisverystable,noprecipitatewasobservedafterstandingforseveralmonths.Preparationof“Unprotected”RhodiumandRuthe-niumNanoclusters.Stable,transparent,dark-brown,ho-mogeneouscolloidalsolutionsofrhodiumandrutheniummetalnanoclusterswithoutanyprecipitateweresuccessfullypre-paredbyusingaproceduresimilartothatdescribedabove.Atotalof5mLaqueoussolutionofRhCl3‚3H2O(79.0mg,0.30mmol)orRuCl3‚6H2O(95.0mg,0.30mmol)wasaddedinto100mLofglycol.Then5mLofaqueoussolutionofNaOH(0.5M)wasaddedwithstirring.Theotherpreparationconditionswerethesameasthosedescribedabove.
Stable,transparent,dark-brown,homogeneousglycolsolu-tionsofrhodiumandrutheniummetalnanoclusterswithametalconcentrationhigherthan5g/Lcanalsobepreparedbythesimilarmethod.
PreparationofPPh3ModifiedPtNanoclustersbyUsingthe“Unprotected”PtNanoclustersasBuildingBlocks.Inatypicalexperiment,5mLofatoluenesolutionofPPh3(3.81mmol/L)wasmixedwith20mLofaglycolsolutionofthePtnanocluster(1.90mmol/L)(PPh3/Ptratio:0.50)followedbyadding15mLofethanolintothemixturetoformahomogeneous,red-brownPtnanoclustersolution.Afterstirringfor10min,50mLwaterwasaddedintotheobtainedPtnanoclustersolution.Aliquidintwophaseswasobtained.PPh3-modifiedPtnanoclustersthenstayedintheupperredorganicsolutionleavingacolorlessaqueoussolutioninthebottom.WashingtheorganicnanoclustersolutionwithwaterthreetimesanddryingitwithNaClgiverisetoabrownPtnanoclustertoluenesolution,suggestingthatthePtnanoclus-terhasbeencompletelycapturedbyPPh3throughacoordina-tionactiontothesurfacePtatomsofthePtnanoclusters.
ResultsandDiscussion
Preparationof“Unprotected”NobleMetalNano-lustersHavingSmallParticleSize.Metalnanoclus-tersareusuallypreparedbychemicallyorphysicallyformingmetalatomsinsolutionsfollowedbyanag-gregationprocessofthemetalatomsinthepresenceorabsenceofprotectiveagentssuchaspolymers,surfac-tants,orstrongcoordinationligands.Inthepresenceofprotectiveagents,notonlyaretheformedmetalnanoparticlesstabilizedbutalsotheclusteringprocessesareinfluencedbytheprotectiveagents.11Intheabsenceofprotectiveagent,however,toeffectivelyprepareastable“unprotected”metalnanoclusterwithsmallparticlesizeandnarrowsizedistributioninorganicmedia,asuitablesolventandaneffectivemethodforcontrollingthemetalparticlesizehavetobeused.Thereductionmethod,usingpolyolforthepreparationoffinepowdersofCu,Ni,Co,andsomenoblemetalswithmicrosizefrommetalhydroxides,hadbeenstudiedbyFiglarzandco-workers.12Wehadsucceededinsetting
(11)(a)Hirai,H.;Nakao,Y.;Toshima,N.J.Macromol.Sci.,Chem.1979,A13,727.(b)Ahmadi,T.;Wang,Z.;Green,T.;Henglein,A.;EI-Sayed,M.Science1996,272,1924.
(12)(a)FievetF.;Lagier,J.P.;BlinB.;BeaudoinB.;FiglarzM.SolidStateIonics1988,32-33,198.FievetF.;Lagier,J.P.;Figlarz,M.MRSBull.1989,14,29.
1624Chem.Mater.,Vol.12,No.6,2000Wangetal.
Figure1.TEMphotographsandsizedistributionsoftheplatinum,rhodium,andrutheniumnanoclustersinglycol:(a)Ptnanocluster(0.37gPt/L);(b)Ptnanocluster(3.7gPt/L);(c)Rhnanocluster(0.31g/L);and(d)Runanocluster(0.32g/L).
upamethodforthepreparationofpolymer-protectedCu/PdandCu/Ptalloynanoclustercatalystsbyreducingthecorrespondinghydroxideprecursorswithamodifiedpolyolprocess.6Inthiswork,ethyleneglycolwasusedastheorganicsolventinthepreparationofthe“unpro-tected”metalnanoclustersandmetalhydroxidecolloidalparticleswerepreformedbeforethereductionprocess.Thepresentpreparationprocedureofthe“unpro-tected”noblemetalnanoclusterscanbeexpressedasfollows:
H2PtCl698“unprotected”(2)160°C,Nflow
2
(1)NaOH,pH>12inglycol
Ptnanocluster
Byheatingaglycolsolutionorglycol/watermixturecontainingPt,Rh,orRuhydroxideoroxidecolloidsandNaOHat160°Cfor3h,atransparentcolloidalsolutionofthemetalnanoclusterswasobtainedwithoutanyprecipitate.Themetalcolloidsareverystable,noprecipitationwasobservedafterstandingforatleastseveralmonths.ThepHvalueofthestartingPthydroxidecolloidalsolutionisveryimportantforob-tainingthepresentstable“unprotected”Ptnanocluster
inglycol.Ifitislessthanabout12,insteadofametalcolloidalsolution,aprecipitatewasobtainedduringtheheatingprocessaswehavereportedbefore.6However,theobtainedPtnanoclusteringlycolisverystablewhenthepHwasadjustedtoavaluehigherthan7.
Figure1showstheTEMphotographsandsizedistributionsoftheunprotectedPt,Rh,andRunano-clustersinglycol.TheaverageparticlesizeoftheRhnanoclusteris1.3nmwithasizedistributionof0.7to2.2nm,whilethatoftheRunanoclusteris1.1nmwithasizedistributionalsofrom0.7to2.2nm.ThenarrowlydistributedPtnanoclusters(0.37-10gPt/L)haveanaveragediameterof1-2nm.
InfluenceofMetalConcentrationontheSizeofPtNanocluster.TheaverageparticlesizesofthePtnanoclusterspreparedbythepresentmethodareslightlydependentontheconcentrationofH2PtCl6inthestartingglycolsolution,whichisquitedifferentfromtheusualcasesforthechemicalpreparationof“unpro-tected”metalcolloids.Forexample,Esumi10reportedtheinfluenceofmetalconcentrationonthemetalparticlesizeinthepreparationoforganopalladiumsolsbythermaldecompositionofpalladiumacetateinmeth-
TractablePt,Rh,andRuNanoclustersChem.Mater.,Vol.12,No.6,20001625
Figure2.TEMphotographsoftheplatinumhydroxidecolloidinglycol/water(10/1)andthePtclusterderivedfromreductionofthePthydroxidecolloid:(A)platinumhydroxidecolloidand(B)Ptnanocluster(0.37g/LPt).
ylisobutylketone.WhenthePdconcentrationisabout0.1mmol/L,theaveragePdparticlesizeisabout8nm,whilewhenthePdconcentrationincreasesto1mmol/L,theaveragePdparticlesizechangestobe∼140nm.Howeverinthepresentsystem,an“unprotected”PtnanoclusterpreparedinaH2PtCl6concentrationof51mmol/Lhasanaveragediameterof1.3nm,whichisonlyslightlylargerthanthatofthePtnanoclusterpreparedinaH2PtCl6concentrationof0.37mmol/L(dav)1.1nm).Thisresultshowsthatthepresentmethodisveryeffectiveforpreparing“unprotected”Ptmetalnanoclustershavingsmallparticlesizeandnarrowlysizedistribution.Thereasonthatsmall“unprotected”metalnanoclusterscanbeobtainedinahighmetalconcentrationinthisworkispartiallyduetotheformationofsmallandnarrowlydistributedmetalhydroxidescolloidalparticlesbeforethereductionpro-cessinthepreparation.
InfluenceofSolventinthePreparationontheParticleSizeofthePtNanocluster.ThesolventusedinthepreparationexperimentshasimportantinfluenceontheparticlesizeoftheobtainedPtnano-cluster.Whenthestartingmixturecontains10%ofwater,theparticlesizeoftheobtainedPtnanocluster(0.37gPt/L)increasesobviously.Figure2showstheTEMphotographoftheplatinumhydroxideoroxidecolloidalparticlesinglycol/water(10/1)andthePtnanoclusterderivedfromreductionofthePthydroxidecolloid.Thenarrowlydistributedplatinumhydroxidecolloidalparticleshaveanaverageparticlesizeof4.0nm.ThePtnanoclusterhasanaveragediameterof2.4nm(Figure2B),whichislargerthanthatofthePtnanocluterpreparedatthesamePtconcentrationwithoutadditionofwaterinthestartingmixture(1.1nm,Figure1a).Theseresultssuggestthattheparticlesizeofthe“unprotected”Ptnanoclustercanbecon-trolledtosomeextentbychangingthepreparationconditions.
SeparationandRedispersionofthe“Unpro-tected”PtNanoclusters.The“unprotected”platinumnanoclustersinglycolpreparedinthisworkcanbeconvenientlyseparatedasablackprecipitatebyaddingadiluteaqueoussolutionofHCltoadjustpHofthePtnanoclustersolutiontoavaluelowerthan4.TheprecipitatedPtnanoclustercollectedbycentrifugationcanbe“dissolved”inmanyorganicsolvents,suchas
Figure3.TEMimageofaPtnanoclusterincyclopentanonepreparedbydissolvingaprecipitatePtnanoclusterinthesolvent.
alcohol,ketone,THF,DMF,andDMSO,givingrisetodifferenttransparentcolloidalsolutionsofthe“unpro-tected”Ptnanoclusters.However,theprecipitatedPtnanoclustersarenotsolubleintolueneorwater.ThePtnanocluster“dissolved”insomeketonesoventswasfoundtobeverystable.Forexample,thePtnanoclusterincyclohexanoneorcyclopentanonedoesnotformanyprecipitateafterstandingforatleastseveralmonths.StablecyclohexanoneorcyclopentanonesolutionsofthePtnanoclusters(dav)1-2nm)withaPtconcentrationhigherthan200g/LcanbeeasilyobtainedbydissolvingtheprecipitatedPtnanoclustersinthesolvents.Figure3showstheTEMphotographandsizedistributionofaPtnanoclusterincyclopentanone.ItcanbeseenfromFigure3thatnoobviousaggregationofthePtnano-particlesoccursduringthetransferringprocess.
ThesepropertiesmayprovideaconvenientwaytothefabricationofvariouscompositefunctionalmaterialscontainingsmallPtnanoclustersandarehelpfultostudiesonthestabilizingmechanismforthe“unpro-tected”metalnanoclustersindifferentsolvents.
CharacterizationoftheMetalNanoclusters.X-raydiffraction(XRD)measurementswerecarriedoutontheprecipitatedPtnanocluster(preparedinaconcentrationof3.7g/LPt)(Figure4).Thediffraction
1626Chem.Mater.,Vol.12,No.6,2000Figure4.X-raydiffractionpatternoftheprecipitatedPtnanocluster(3.7g/L).
linesof(111)(0.2269nm),(200)(0.1971nm),(220)(0.1388nm),and(311)(0.1178nm)ofmetallicPtcanbeclearlyobservedinthediffractionpatternofthePtnanocluster,provingthatthePtnanoclusterhasafcccrystalstructure.13TheaveragePtparticlesizecalcu-latedfromthehalfwidthofthediffractionline(111)is∼larger2nm,particleswhichagreesinthewellPtwithnanoclustertheTEM(althoughresults.Thetheamountissmall)shoulddominatethecontributiontothediffractionsignals.
X-rayphotoelectronspectroscopic(XPS)analyseswerecarriedoutonadriedPtnanocluster(driedPt,3.7g/L)sample.Beforedrying,thePtnanoclusterwaswashedbyprecipitatingindiluteHClacidand“dissolving”inacetoneforseveraltimestoremoveglycolandotherimpurities.ThespectrumofthedriedPtnanoclusterisquitesimilartothatofpurePtmetal,signalsfromO1sandCl2plevelscannotbedetectedintheexpandedspectra.ThebindingenergiesofPt4f7/2and4f5/2ofthedriedPtnanoclustershaveavalueof70.9and74.2eV,respectively.TheXPSresultsprovethatthePtnano-clusterhasavalencestateofmetallicPt,14rulingoutthepossibilitythatthereexistsplatinumoxideinthePtnanoclustersurface.XPSspectroscopymeasurementswerealsocarriedoutforadriedRhnanoclusterandadriedRunanocluster.TheRh3d5/2andRu3d5/2bindingenergiesofRhandRuinthecorrespondingmetalnanoclustershaveavalueof307.1and280.0eV,respectively,indicatingthatbothRhandRuinthemetalnanoclustersareinametallicstate.
Forunderstandingthehighstabilityoftheprepared“unprotected”metalnanoclusters,GC-MSmeasure-mentswerecarriedoutonaglycolsolutionofthePtnanoclustercontainingNaOHtoanalyzethevolatilespeciesinthePtnanoclustersolutionthatmayaffectthestabilityofthePtnanocluster.ThesamplefortheGC-MSmeasurementwaspreparedasfollows:ThePtnanoclusteringlycolwasprecipitatedbyaddingdiluteHClacid,theprecipitatedPtnanoclustercollectedbycentrifugationwaswashedbydissolvinginethanolandprecipitatingwithdiluteHClacidforseveraltimes.TheobtainedprecipitatedPtnanoclusterwasfinally“dis-(13)McClune,W.F.PowderDiffractionFileAlphabeticalIndexInorganicPhase;JCPDS:Swarthmore,PA,1980.
(14)PracticalSurfaceAnalysisbyAugerandX-rayPhotoelectronSpectroscopy;Briggs,D.,Seach,M.,Eds.;JohnWiley&Sons:NewYork,1983.
Wangetal.
solved”inglycolcontainingNaOHtogiverisetoaverystablePtcolloidalsolutionastheoriginalPtnanoclustersolution.IntheGC-MSmeasurementsoperatedatdifferentconditions(inlettemperature:300°C),nootherorganiccompoundexcepttheaddedsolvents,i.e.,glycolandsmallamountethanol,canbedetected.ElementanalyseswerecarriedoutonthedriedPtnanocluster.Thesamplewaswashedbythemethodmentionedabovebeforedryingundervacuum.ThecompositionofCinthedriedPtnanoparticlesislessthan0.8%.ThecharacterizationresultsmentionedabovesuggestthatinthepresentPtnanoclusterglycolsolution,itisglycolandthesimpleanionOH-adsorbedonthePtnanoclustersurfacethatmakethemetalparticlesseparatefromeachothersowell,preventingthePtnanoclusterfromaggregatingevenathightemperature.
PreparationofSomeFunctionalPtNanoclusterProductsbyUsingthe“Unprotected”PtNano-clustersasBuildingBlocks.Thespeciesadsorbedonthepresent“unprotected”metalnanoclusters,i.e.,glycolandthesimpleanion,caneasilyberemovedorreplacedbyvariouscoordinationligandsorprotectiveagents.Thischaractermakesthe“unprotected”PtnanoclusterveryinterestingasabuildingblockmaterialtopreparevariousnanoscopicPtclusterswiththesamePtcoresizeanddifferentmodifyingorprotectiveshellsaswellasmatrixmaterials.
Afteraddinganaqueoussolutionofpolyoxyethylene-(20)stearylether(C18H37(OCH2CH2)20OH(POSE))totheoriginalPtnanoclustersolution(POSE/Ptmoleratiois5),thePtnanoclusterwastransformedtoasurfac-tant-protectedmetalnanoclusterwiththesamePtparticlesize,whichisstableinawiderangeofpH.TEMresultsdidnotshowthechangeinparticlesizeoraggregationofthePtnanocluster.
Poly(N-vinyl-2-pyrrolidone)(PVP)protectednoblemetalnanoclustershavebeenintensivelystudiedinmanyresearchgroupsandexhibitexcellentstabilityandcatalyticpropertiesformanyorganicreactions.ThePVP-protectedmetalnanoclustercatalystsareusuallypreparedbyrefluxinganalcoholsolutionofthecorre-spondingmetalcompoundsinthepresenceofPVP,amethodsetupbyHirai,Toshimaandco-workers.Topreparesmallmetalnanoclusters(1-2nm)withnarrowsizedistribution,amolarratioofPVP(asmonomericunit)tothemetalhigherthan40havetobeused.11aInthiswork,wehavesucceededinpreparingsomePVP-protectednoblemetalnanoclusterswithahighmetal/PVPratio.
TheprocedureforthepreparationofaPVP-protectedplatinumnanoclusterinthisworkisasfollows:The“unprotected”platinumnanoclusteringlycol(6mL,3.7g/L,0.11mmolPt,dav)1.3nm)wasprecipitatedbyaddingHClacid(1M).Theblackprecipitateobtainedbycentrifugationwas“dissolved”inethanol(4mL)toobtainatransparent,dark-browncolloidalsolution.Theplatinumnanocusterinethanolisnotverystableandwillprecipitateafterstandingforseveralhours.After2mLofanethanolsolutioncontaining4.4mgPVP(0.039mmolasmonomericunit)wasaddedtotheabovecolloidalsolutionwithstirring,averystablePVPprotectedplatinumnanoclusters(PVPmonomericunit/
TractablePt,Rh,andRuNanoclustersFigure5.TheTEMimageofaPVP-protectedPtnanoclusterwithahighPt/PVPratio(monomericunitofPVP/Ptmoleratio:0.35)preparedinthiswork,theaveragediameterofthePVP-protectedPtnanoclusteris1.5nm.
Ptmolarratio)0.35)“solution”wasobtained.TEMresults(Figure5)revealedthatnoaggregationoccursduringthisprocess.DriedPVP-protectedPtnanoclus-terswithhighPt/PVPratio(Pt%>80%,dav)1-2nm)canbeobtainedbyremovingsolventundervacuum,whichare“soluble”inwateroralcohol.
Fromtheseresults,wecanconcludethattostabilizePtnanoclusterswithsmallparticlesize,ahighPVP/Ptratioasusuallyusedisnotnecessary.AsmallamountofPVP(PVPmonomericunit/Ptratiohigherthan0.35)isenoughtopreventthesmallPtnanocluster(dav)1.5nm)fromaggregatinginwaterorethanol.ThereasonthattheHiraiandToshima’spreparationmethodneedsalargeamountofPVPforpreparingsmallmetalnanoclustersisthatPVPplaysotherimportantrolesinadditiontopreventingthesmallmetalnanoclustersfromaggregatingintheirpreparationprocess,i.e.,coordinationofPVPtometalions,controllingthereductionspeedofmetalionsandinfluencingtheclusteringprocessofmetalatoms.11a
Transferringthe“Unprotected”PtNanoclustersintoToluenebyFormingPPh3CoordinatedPtNanoclusters.Schmidandco-worker1a,bhavepreparedmanymetalnanoclusterswithdefiniteorneardefinitenumbersofmetalatomsandprotectiveligands,suchasAu55(PPh3)12Cl6andPt55(PPh3)12Cl6,whichhavebeenconsideredasQ-particleshavinginvaluablead-vantages.2Ontheotherhand,LiuandToshima15havereportedthataddingalargeamountofPPh3toaPVP-protectedPtmetalnanoclustersolution(themolarratioofPPh3toPtis15)cannotdirectlytransferthePtcolloidalparticlesintotoluene,butformsaflocculateofPtcolloids.Theflocculatecanbe“dissolved”in
(15)Liu,H.;Toshima,N.J.Chem.Soc.,Chem.Commun.1992,1095.
Chem.Mater.,Vol.12,No.6,20001627
butanolorbutanol-toluene(1/9v/v)mixedsolutionbutnotinpuretoluene,implyingthatPVPmaystillremainintheflocculate.
The“unprotected”Ptnanocluster(3.7gPt/Lforexample)inglycolpreparedinthisworkcanbeeasilytransformedtoaPPh3-modifiedPtnanoclusterbyaddingasmallamountofPPh3tothePtnanoclustersolution(PPh3/Ptmolarratiois0.5).TheobtainedPPh3-modifiedPtnanoclustercanbeextractedintotoluene,formingatoluenesolutionofthePPh3-modifiedPtnanocluster.TEMmeasurementsrevealedthatnoag-gregationoccursduringthiscoordinationcapturing-transformingprocess.ThelatticefringescrossingthewholePtnanoparticleofthePPh3-modifiedPtnano-clusterwereobservedinthehigh-resolutionTEMmeasurements.ThefactthatasmallamountofPPh3isenoughfortransferringthenanoclustersintotolueneinthisexperimentcanbeunderstoodsincethespeciesadsorbedonthePtnanoclusteringlycolcaneasilybereplacedbyPPh3,andtheresultantPPh3-modifiedPtnanoclusterdoesnotcontainotherprotectiveagentssuchasPVPthatisinsolubleintoluene.
Conclusion
Wereportinthispaperaneweffectivemethodforthepreparationof“unprotected”Pt,Rh,andRunano-clusterswithsmallparticlesizeinorganicmedia.The“unprotected”Ptnanoclusterswithdifferentparticlesizecanbepreparedbychangingthepreparationconditions.Thehighstabilityofthepresent“unpro-tected”metalnanoclusterswasexplainedbythestabi-lizingeffectsoftheadsorbedsolventandsimpleanions.“Unprotected”PtnanoclustersincyclohexanoneorcyclopentanonewerefoundtobeverystableandaPtnanocluster(dav)1-2nm)cyclohexanonesolutionwithametalconcentrationhigherthan200gPt/Lhasbeensuccessfullyprepared.IthasbeenrevealedthatasmallamountofPVP(Pt/PVPmassratio∼5)isenoughtopreventPtnanoclusterswithsmallparticlesizefromaggregating.The“unprotected”metalnanoclustersdis-solvedindifferentorganicsolventsobtainedinthisworkprovidenewopportunitiesforpreparingmanynewfunctionalmaterialscontainingthecorrespondingmetalnanoclusters.Forexample,theycanbeusedtofabricatecompositefilmsofpolymersorinorganicmetaloxidesandsmallmetalnanoclusterswithhighmetalloading,metalnanoclustercatalystswithdemandingparticlesize,andsurface-modifyingspecies.
Acknowledgment.Theauthorsexpresstheirsin-cerethankstoProfessorHui-zhongHuangatourinstituteandtoMs.Xiu-zhenGaiattheTEMLabora-toryofPekingUniversityfortheirhelpintheXPSandTEMmeasurements,respectively.Thisworkwaspar-tiallysupportedbyaspecialGrant-in-Aid(project29925308)fromtheNationalNaturalScienceFounda-tionofChinaandaGrant-in-Aid(no.99000)fromtheEducationMinistryofChina.
CM0000853
因篇幅问题不能全部显示，请点此查看更多更全内容