Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall

Formation of Hollow Nanocrystals Through theNanoscale Kirkendall EffectYadong Yin, et al.

Science 304, 711 (2004);DOI: 10.1126/science.1096566

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REPORTSFormationofHollowNanocrystalsThroughtheNanoscaleKirkendallEffectYadongYin,,ez,StevenHughes,a,vsatos*HollownanocrystalscanbesynthesizedthroughamechanismanalogoustotheKirkendallEffect,inwhichporesformbecauseofthedngwithcobaltnano-crystals,weshowthattheirreactioninsolutionwithoxygenandeithersulfurorseleniumleadstothefocessprovidesageneralroueextensionoftheprocessyieldedplatinum–cobaltoxideyolk-shellnanostructures,solidmaterialsareimportantinmanyareasofmodernscienceandtechnology,in-cludingionexchange,molecularseparation,catalysis,chromatography,microelectronics,andenergystorage(1–3).Notableexamplesaremicroporous(Ͻ2-nm)zeolitesandmeso-porous(2-to50-nm)litytomanipulatethestructureandmorphologyofporoussolidsonananometerscalewouldenablegreatercon-trolofthelocalchemicalenvironment(4–6).Wedemonstratethatnanoscaleporescande-velopinsidenanocrystalswithamechanismanalogoustovoidformationintheKirkendallEffect,inwhichthemutualdiffusionratesoftwocomponentsinadiffusioncoupledifferbyaconsiderableamount(7).Wechoosecobaltnanocrystalsasastartingmaterialtoshowthathollownanocrystalsofcobaltoxideandchalcogenidescanbesuccessful-lysynthesizedthroughthereactioeenknownformorethanhalfacenturythatporositymayresultfromdif-ferentialsolid-st1947,SmigelkasandKirk,copperandzincinbrass,astheresultofthedifferentdiffusionratesofthesetwospeciesatanelevatedtemperature(7).Thisphenome-non,nowcalledtheKirkendallEffect,wasthefirstexperimentalproofthatatomicdiffusiondirectionalflowofmatterisbal-DepartmentofChemistry,UniversityofCaliforniaatBerkeley,andMaterialsScienceDivision,LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA.*Towhomcorrespondenceshouldbeaddressed.E-mail:alivis@edbyanoppositeflowofvacancies,ionalmaterialflowsalsoresultfromcoupledreaction-diffusionphenomenaatsolid/gasorsolid/liquidin-terfaces,leadingtodeformation,voidfor-mation,orbothduringthegrowthofmetaloxideorsulfidefilms(8,9).Thesevoidsareusuallyexplainedbyoutwardtransportoffast-movingcationsthroughtheoxidelayerandabalancacemotionandtheforma-tionofporeshavebeenstudiedbecauseofonthereproducibilityandre-theirimpactliabilityofsolders,passivationlayers,dif-fusionbarriers,etc.,esproducedatametal-metaldiffusioncoupleornearthemetal-oxideinterfaceofagrowingoxidedonotyieldmonodisperse,ervedvol-umefractionforporesisalsbserva-tionsareadirectresultofthelargevolumeofmaterialthatvacanciescandiffuseintoandthelargenumberofdefectswithwhichtheycanreact(10).Ifthefaster-diffusingspeciesisconfinedintoananocrystalcore,thenetrateofvacancyinjectionshouldincreasemarkedly,becauseofthehighsurthesmallvolumeofatransformingnanocrystal,usstudiesontheinterdiffusionof30-␮mpowderswithlay-eredcompositionshowedalargevolumefrac-tionofpores,butthegeometryanddistributionoftheporeswerenotuniform,probablybe-causeofaggregationandthebulk-likedimen-sionoftheparticles(11).Considerableprogresshasrecentlybeenmadeinsynthesizingcolloidalnanocrystalswithwell-controlledsize,shape,andsurfaceproperties(12–14).Employingsuchhigh-qualitynanocrystalsasthestartingmaterials,itshouldbepossrofchemicalmethodshavebeendevelopedtosynthesizeuniformcobaltnanocrystalsinsolution(12,15).Furthermore,-causecobaltisthemajorcomponentinoneclassofsuperalloys,itshigh-temperatureox-idationandsulfidationhavebeenwellstudied(16,17).Itisknownthatoxidationandsul-fidationofbulkcobaltundervaporathightemperaturearemainlycontrolledbyoutwarddiffusionofcobaltcations(18).Thismodeofat-baltsulfidehollownanospheresweresynthe-sizedinonepotbyimmediateinjectionofasolutionofsulfurino-dichlorobenzeneintoahotcobaltnanocrystaldispersion(Fig.1A)thatwaspreparedbyliteraturemethods(15,19).At455K,thereactionbetweencobaltandsulfurcompleteswithinafewseconds,lowparticlesareverystableinsolution,suggestingthatthechemicaltransformationofthesurfashedwithmethanol,thesurfactantlayerwasremoved,anditwadflowofcobaltthroughthesul-fideshellresultedinsupersaturationofva-cancies,whichcondensedtoformasingleholeineachnanoparticle(Fig.1,BandD).Twostablecobaltsulfidephaseswereob-served,linnaeite(Co3S4)andcobaltpent-landite(Co9S8),dependingonthesulfur-to-cobaltmolarratiousedinthesynthesis.X-raypowderdiffraction(XRD)patternsinFig.1Eshowtheevolutiono9S8wastheonlysulfidephaseobservedwhenthemolarra-tiowaslowerthan9:8,whereasCo3S4also3S4wasobtainedwhenthemolarratioofsulfurtocobaltwasabove3:edistributionofthesulfidsperse,hollownano-crystalsself-assembledintoorderedhexag-onalarrangementswhenevaporatedslowlyonthesurfaceofacarbon-coatedtransmis-sionelectronmicroscopy(TEM)ENCEVOL30430APRIL2004711Downloaded

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ndingisconsistentwithbulkproducehollownanocrystals,sulfidationstudies(18),inwhichitisob-ofnanocrystalstookϳenanocrystalsdidnotformsuperlat-servedthatanincreasedsulfurvaporpressureFigure2,AtoD,showstheevolutionoftheticesasreadilyascobaltnanocrystalsdo,leadstoinjectionofmorecationvacanciesmorphologyofthenanocrystalswithtimeprobablybecauseofadiminishedvanderintothegrowingsulfideandenhancestheasanOWaalsforce(19).Assembliesofhollownano-parabolicrateconstantforsulfidegrowth.2/showsparticlespresentadistincttopologyoforderedForbulkcobalt,theratesoftheaccessibilityofare3to4ordersofmagnitudelowerthanafterthestartoftheOporesfromtheoutside,theyfallbetweenmeso-thoseofsulfidationabove750K(18).Thisofparticles2/Arflow(Fig.2E).Thesolutionstilldisplayedporousmaterialswithaccessiblechannelsandisalsotrueundertheconditionsweusedtoweakferrofluidicresponsetoa1-Tmagnetvoidlatticesinwhichporesareconfinedinacontinuousmatrix(20).Kinematicaldiffractionsimulations(fig.S1)indicatedthattheXRDpeaksaretoobroadtobeconsistentwithasinglecrystalshellofthedimensionsobservedbyTEM(21).WeobtainedsatisfactoryfitstothedatainFig.1E,panels(d)and(g),byassumingϳ4-nm,ro-graphs(Fig.1D)ofthesamesampleshowthattheaverageouterdiameterofthehol-lowCo9S8nanocrystalsisϳ-sonabsconfirmedbyhigh-resolutionTEM(HRTEM),whichshowsthatbothCo9SandCohollownion1.(A)TEMimageofcobaltnanocrystalssynthesizedby(Fig.1C).Thearrangementofthedomainsintoof0.54gofCo(CO)the28in3mlofo-dichlorobenzeneisanalogoustoticrystallinestructureim-opliespossibleapplicationsofthesehollowa-dichlorobenzenesulfideo-dichlorobenzenephaseat455K.(B)TEMimageofthe(5synthesizedml)intocobaltbythenanocrystalinjectionofsulfurinnanocrystalsasnanoscalereactors,because(A).Co/Sphase(C)molarHRTEMratioimagesof9:solutionwith3S4particlesweresynthesizedfromthecobaltsampleshownin3Ssmallmoleculesmaybeabletopenetrateanothersynthesizedasin(B),but4with(left)aandCo:SComolar9S8(right).ratio(ofD)9:geofthecobaltsulfiystalscobaltInallinstancesofsulfidation,wefound(c)andsample(bthathadanaveragediameterofϳ11nm.(CoE)9XRDS8particlespatternsstartedof(a)cobaltfromthatthediameteroftheholeinthecenterofrepresent9:7,(d)peaks9:8,from(e)to9:10,h)cobaltcobalt,(f)CO9:11,sulfide(g)synthesized9:12,and(h)with9:entThedots,Co/Striangles,molarratios:and(b)squares9:5,9S8,andCO3S4phases,ocrystalswas40to70%oftheinitialparticlesize(startingwithcobaltparticleswithasizedistributionof7%,asinglesynthesisyieldedahole-sizedistri-butionof13%).Ifsulfurtransportthroughthegrowingshellwerenegligible,asshownforbulksulfidationbymarkerexperiments(18),icantinwardsulfurtransportcouldoccurthroughgraisopossiblethatinwardrelaxationoftheholeoccurs,duetoannihilatiy,theestimationoftheionofCoOhollownanocryssedsulfurconcentrationin-atcreasedholesizeandenhancedoutwardO455ml/min)toastreamofO2/Armixture(1:4involumeratio,growthoftheshell,.(A(A)tobeing0Dmin,)TEMblownthroughacobaltcolloidalsolution(B)images30min,of(C)the80solutionsmin,andafter(D)flow210of2/:HRTEMofaCoOmobilityratherthansulfurmobilitywas(a)and0diamondsmin,(b)(E2.5)XRDrepresentmin,patterns(c)peaks5.5min,ofthesampleobtainedfromthesolutionafterflowofOhollow2/Arforfrom(d)cobalt10min,and(e)CoO30min,phases,(f),and(g)nloaded

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REPORTSatthatpoint,ϳ3hoursforthecobaltcorestobecompletelyconsumed;centralporeswereclearlydistinguishableforallnanocrystalsunderTEM,ulations(fig.S1)suggestamulti-crystallinestructurewithacrystaldomainsizeofϳ3nm,inagreementwiththeHRTEMobservations(Fig.2,inset).Theevolutionofhollowmorphologyisbestilsystems,annihilationofexcessvacanciesatdislocationsandboundariescanproducestressesthatleadtotheformationofcracksneartheinterface;thecracksthenactasnucleiforthefurthercondensationofsu-persaturatedvacancies(11).Althoughtheexactmechanismislikelytobedifferent,innanocrystalsvoidsalsobegintodevelopandmergeattheboundary(Fig.3).Thehighdefectcontentandsurfaction,asva-canciesdiffuseinward,theywilleactionproceedsintime,morecobaltatomsdiffuseouttotheshell,andtheaccompanysultsintheformationofbridgesofmaterialbe-tweridgesprovideafasttransportpathforoutwaarphenomenonwasobservedforbulkpowders(11).Thegrowthratheporevolumeseemedtoformduringthefirstfewminutes,whereasittookϳybebecause,asthebridgesarealsoconsumedduringthereaction,asmallercionofCoSehollownano-crystalswithtimebyinjectionofasuspen-sionofseleniumino-dichlorobenzeneintoacobaltnanocrystalsolutionat455K,fromtop-lefttobottom-right:0s,10s,20s,1min,2min,/Semolarratiowas1:,ationofdisk-shapedcobaltnanocrystals(21)wasobservedtore-sultintheformationofhollownanodiskswithcylindricalpores,indicatingthatspheinarystud-iesonoxidationofironnanospheresandsulfidationofcadmiumnanospheresalsore-sultedinhollowstructures,tically,themobilitiesofthereactinggsolidnanocrystalscontainingonereactantinacomparativelydilutesolutioncreatesanad-ditionalasymmetrythatmayfavorthecre-ationofhollowstructures:Therelativelylargechangeintheconcentrationofthecorematerialbetweenthecoreandthesolutionprovi,numerouscombinationsofreactantsmaybeexpectedtoproducevarioushollownano-structuresofinsulators,semiconductors,treportontheformationofgoldnanoboxesmayinvolvethesamemechanismatsomestage,althoughthedi-mensionofthestructuresproducedisanor-derofmagnitudelarger(22).Hollownanocrystalsofferpossibilitiesinmaterialdesignforapplicationsincatalysis,nanoelectronics,nano-optics,drugdeliverysys-tems,andasbuildingblocksforlightweightstructuralmaterials(23–25).Forexample,ac-curatefixationofthecatalystwithinthepores,combinedwithotheremergingtechniquesofchemicalcontrol(26),n-stratetheuseofhollownanocrystalsincataly-sis,westudiedtheirfunctionasnanoreactors,@CoOyolk-shellnanostructurewassynthesized,inwhichtepswereinvolvedinthepreparationofthesenanoreactors:thesynthesisofplatinumseedsbyamodified“polyol”pro-cess(27),thedepositionofcobaltonplatinumtoformPt@Cocore-shellnanocrystals,andtheFig.4.(A)Platinumnanocrystalspreparedbytheinjectionofasolutionof0.15gofplatinumacetylacetonatein5mlofo-dichlorobenzeneintoarefluxingbathof10mlofo-dichlorobenzenethatcontained0.3gof1,2-hexadecanediol,0.1mlofoleicacid,0.1mlofoleylamine,utionwasthenheatedforanother120min.(B)WeformedPt@CoOyolk-shellnanostructuresbyinjecting1.08gCo2(CO)8in6mlofo-dichlorobenzeneintotheplatinumnano-crystalssolution,andfollowedbytheoxida-tionoftheproductparticlesbyblowingastreamofO2/Armixture(1:4involumera-tio,120ml/min)ENCEVOL30430APRIL2004713Downloaded

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transformationofcobaltintoCoOhollowstruc-tures(28).Figure4Ashowsatypicalsampleofplatinumparticleswithanaveragediameterofϳositionofcobaltontoplatinumatthereactiontemperatureyieldednoalloy,onlyPtcore/Coshellparticles,dationreactionremovedcobaltatomsawayfromtheplatinumparticlesurface,leadingtotheformationofaplatinumyolk/CoOshellstructure(Fig.4B).NofreeplatinumparticleswerefoundbyTEMinspec-tionofthePt@dcontrolthesizeofPt@CoOparticlesbychangingthediameterartodetermineifthePt@CoOma-terialswereactiveasheterogeneouscatalysts,wechosethehydrogenationofethyleneasamodelreaction,becauseiumisoneofthemostactivemetalsforthisreaction,whereastheactivityofmetalliccobaltisϳ2ordersofmagnitude

REPORTSlower(29).s,,.A352,rogenationofethylenewasstudiedatatmo-nanocrystalswereinactiveforethylenehy-(2003).sphericpressureinadifferentiallyoperatedplugflowdrogenation(30),,.33,637(2000).,,¨hwald,g,.,and208to353K(sample-dependent).2H4,150TorrofHChem.107,7351(2003).umacetylacetonatewasreducedwithalong-normalizedpermoleofsurfaceplatinumspeciestemperaturesϾscontainingchainpolyoltoformuniformplatinumnanoparticlesϪ1(Ptaturnoverfrequency(moleculePtssϪ)toobtains1).platinumwithoutpretreatmentwereactiveinthepresenceofsurfactantssuchasoleicacid,MolesofPtswasdeterminedbyDϭ1.13/d,whereDistheplatinumdispersion[theratioofPtforColeylamine,eofthe2H4hydrogenationattemperaturesasplatinumparticleswastunedfrom1to10nm,(umparticlet)]ewasdeterminedfromnumberquencyforethaneformationat227KwasCo2(CO)8wastheninjectedintothehotsolutionandaverageTEMmeasurements.8.3ϫ10Ϫ3sϪ1(31),ght,d,e,ionofthePt@Conanocrys-Dumesic,.127,342(1991).theratesof3.5ϫ10Ϫ2sϪ1measuredonatalswasperformedafewminutesafterweintro-0.04%Pt/´chetforthevaluablediscussions.2catalyst(32)and1.7ϫ10Ϫ2sϪ1measuredonpureplatinumpowders(0.2(1:4incarbonylvolumebyratio,blowing120ml/min)astreamintoofOSupportedbytheAirForceOfficeofScientificRe-the2/temwaskeptsearchunderawardno.F49620-01-1-0033;bytheto1.6␮mindiameter).stablecolloidalDirector,OfficeofEnergyResearch,OfficeofScience,indicatethattherea-DivisionofMaterialsSciences,-inumparticles,soly,thePt@-AC03-anol,washedwithtolueneandmethanolthreetimes,76SF00098;andbytheFordMotorCompanylnitrogenadsorption/BerkeleyCatalysisCenter(R.M.R.).tionmeasurementonthepowderat77KSupportingOnlineMaterialThegrainboundariesontheshellaretheshowedatypeIVisothermwithtypeH2hysteresis,/cgi/content/full/304/5671/711/withDC1mostprobableentrypointsforethyleneand/gaandBrunauer-Emmet-Telleratotalporevolumeofsurface0.0676areacmof65m23/ai,sis(Wiley,NewYork,1994).9February2004;accepted16March2004Incomparisontocatalystssupportedonoth-ermesoporousmaterials,theisolationofcata-lystnanoparticleswithinsolidshellsshouldminimizesecondaryreactionoftheproductsPrmore,anysynergisticinteractionsbetweencatalystandsupportcanbemoreeffi-DeclinesandBehavioralRiskcientlyusedwhurner*,,uo,a,,.3,111(1998).n,er,k,Science283,is963(1999).Ugandapreventableprovidesifpopulationstheclearestareexamplemobilizedthattohumanavoidimmunodefi(HIV)3.A.-,¨ller,,h,¨sele,.11,483(1999).n,ntier-Giscard,h,ateda60%hasreductionshownina70%casualdeclineinHIVprevalencesinceDespitetheearlylimited1990s,resources,linkedKaliaguine,.222,299(2001).(AIDS),Nature417,813(2002).,r,rthy,,peaboutsubstantialacquiredcondomimmunodefi.121,9467(1999).skas,dall,171,successresponsesapproaches,norHIVotrandpromotionsimilarbe-of130(1947).nall,.103,619(1956).changesisinequivalentglobalHIV/AIDStoavaccineinterventionof80%,tin,,vityofSolids,on,,ProjectionsoftheHIVpandemicpaintainThailand,s,Eds.(Chapman&Hall,London,1972),bleakpictureforglobalhealth(1,2).Never-callyacceptedsincetheearly1990s(9).pp.283–s,becausemostcasesofHIVoccurThen,in1994–1995,,.16,147(1981).er,ActaMet.22,923(1974).throughconsensualsexualintercourse,,,i,bleifpopulationsarewarnedandmo-prevalenceamongyoungerpregnantwomen,.30,545(2000).al.,Nature404,59(2000).l.,Nature425,487(2003).spitesuccessesfromthisapproach,,an,atos,Scienceentlyunrelentingexpansionofthepandemic(10–15).TheUgandanevidenceisstill291,2115(2001).hasservedtoemphasizeaneedforthepro-viewedwithcaution,,lski,.11,365(1977).,Cobalt30,19(1966).motionofmoreeffectiveresponses(3–5).initsevaluation(16–19).,ewski,icz,kbehaviorsandinfectionratesWereviewedpopulation-levelHIVandSci.33,2617(1998).droppedsub,an,atos,nNorthAmericaandEuropeintheingcountriestoevaluatethevalidityandLett.78,2187(2001).m,ef,,omid-1980s(6–8)..8,1(2001).acknowledgedsu,t,ez,-terventions(20,21).satos,.124,12874(2002).,,Science298,2176(2002).,,¨hwald,Science282,versity,Cambridge,,CambridgeUni-incidencewasdeclininginUgandabythelate1111(1998).*To1980s(22–24).By1995,z,,.E62,6957(2000).mail:whomrandstoneburner@respondenceshouldbeaddressed.E-lanloaded

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