Synthesis of Ag-Au Nanoparticles by Galvanic Replacement and Their Morphological Studies by HRTEM and Computational Modeling

Synthesis of Ag-Au Nanoparticles by Galvanic Replacement and Their Morphological Studies by HRTEM and Computational Modeling HindawiPublishingCorporation JournalofNanomaterials Volume2011,ArticleID374096,5pages doi:10.1155/2011/374096 ResearchArticle SynthesisofAg-AuNanoparticlesbyGalvanic ReplacementandTheirMorphologicalStudiesby HRTEMandComputationalModeling 3 1 4 ManuelRamos,1,2DomingoA.Ferrer, RussellR.Chianelli, VictorCorrea, 4 2 JosephSerrano-Matos, andSergioFlores 1 MaterialsResearchandTechnologyInstitute,UT-ElPaso,500WUnivesityAve.,BurgesHallRm.303,ElPaso,TX79902,USA 2 DepartamentodeF??sicayMatem?aticas,UniversidadAut?onomadeCiudadJu?arez,Cd.Ju?arez,ChihuahuaC.P.32300,Mexico 3 MicroelectronicsResearchCenter,UniversityofTexasatAustin,Austin,TX78758,USA 4 DepartamentodeCienciasyTecnolog??a,UniversidadMetropolitana,SanJuan,PR00928,PuertoRico CorrespondenceshouldbeaddressedtoManuelRamos,maramos1@miners.utep.edu Received13October2010;Revised22November2010;Accepted27December2010 AcademicEditor:ShaogangHao Copyright?2011ManuelRamosetal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense, whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited. Bimetallic nanoparticles are important because they possess catalytic and electronic properties with potential applications in medicine,electronics,andchemicalindustries.Agalvanicreplacementreactionsynthesishasbeenusedinthisresearchtoform bimetallicnanoparticles.Thecompletedescriptionofthesynthesisconsistsofusingthechemicalreductionofmetallicsilvernitrite (AgNO3)andgold-IIIchloridehydrate(HAuCl)saltprecursors.Thenanoparticlesdisplayroundshapes,asrevealedbyhigh- resolutiontransmissionelectronmicroscope(HRTEM).Inordertobetterunderstandthecolloidalstructure,itwasnecessaryto employcomputationalmodelswhichinvolvedthesimulationsofHRTEMimages. 1.Introduction facets,whereasdecahedronhasmoderateinternalstrainand smallerfacetsmadeof(111)and(100)planes.Thefollowing Synthesisandcharacterizationofnanocrystalshavebeena (111) < isconcludedregardingmonometallicnanoparticles:γγ(100)<γ(110)asindicatedbyLeeandMeisel[9]. researchtopicofhighinterestinrecentdecadesduetotheir potential application in medical (cancer imaging), optical Previoustheoreticalworkindicatesthattheadditionof physics,catalysis,engineeredmaterials,andelectronics[1– asecondmetal,whensynthesizingnanoparticles,canleadto 6]. Achievement of speci,c particle morphology depends asigni,cantchangeonitsphysical-chemicalproperties,as solelyonrightcombinationofprecursors,aswellassuitable re,ectedalsoonparticlemorphology(i.e.,core-shell,spher- selectionoftemperatureandcappingagents[7]. ical,andtruncated-icosahedral).Verylittleisknownabout Presently,onecan,ndseveralarticleswherefullexplana- bimetallic nanoparticles in terms of its crystallographic structure,shape,andlocationofbimetallicprecursors,which tionsareincludedinchemicalsynthesistechniquestoattain speci,c particle morphologies, along with their potential canattractattentionwhenstudyingbimetallicsystems. applications [8]. Monometallic nanoparticles are assumed Inordertounderstandthedi,erencebetweenbimetallic tohavethreebasicshapes:decahedral,cubo-octahedral,and nanoalloy and bulk systems, Yonezawa and Toshima pro- icosahedral.Nanoparticlesgeometryandfacetsaremadeout posedthatsomebimetallicnanoalloys(i.e.,Au-Ag,Au-Pd) of(111)planesasobservedinicosahedron;andisattributed seemtoexistduetomiscibilitygapsatcertaincompositions to lowest surface energy γ(111) of nucleation in (111)- ratio(i.e.,20%,30%,and10%)provokingtheformationof plane;thisimpliesalargeinternalcore-strainvalues.Cubo- ananoalloy[10].Nanoalloyformationcouldbeattributed to the di,erences in atomic radii and electron migration octahedronpresentsnointernalcore-strainandsigni,cant largesurfaceenergyconstitutedprimarilyby(111)and(100) allowingatomstoaccommodate,showingshellperiodicity 2 JournalofNanomaterials (a) (b) 002 022 022 002 (c) (d) Figure1:(a)20nmresolutionHRTEMimageofsphericalshapeAg-Aunanoparticles.(b)5nmresolutionHRTEMimageshowinglattice distances.(c)Selectareadi,ractionwith(022),(022),and(002)principalre,ections.(d)InverseFastFourierTransformofSADpresented in(c). 100 (i.e.,onionarraylayers)asobservedbyconventionalelectron C. Then a separate second solution that consisted of microscopytechniques[11]. 240mg of gold? -III chloride hydrate (HAuCl) dissolved in ? 500mL of deionized water at 100 C with the addition of WepresentasuccessfulchemicalsynthesisfromAuand a mixture of 1% sodium citrate and 50mL of distilled Agsaltprecursorsforbimetallicsphericalshapenanoparti- cles.Bimetallicparticleformationisattributedtoagalvanic water.Finally,bothprecursorsolutionsweremixedtogether replacement reaction and shape. Bimetallic composition andsubjectedtovigorousstirring atconstanttemperature was con,rmed by high resolution transmission electron of 100 C for 1h. The stoichiometric equation for particle ? microscope (HRTEM) results, as well as computational formationofAg-Augalvanicreactionispresentedasfollows: simulationsforreconstructionofHRTEMimages. (1) 3Ag(s)+AuCl???Au(s)+3Ag++4Cl? andseemstobeinagreementwith[12]. 2.Experimental Two precursor solutions were used for chemical synthesis 3.ResultsandDiscussion of bimetallic Ag-Au nanoparticles. The ,rst solution was madedissolving90mgofsilvernitrite(AgNO3)in500mL Particle size, shape, and morphology were studied by of distilled water; later a mixture was added. It was madeHRTEM on an FEI Tecnai TF20 equipped with an STEM with1%sodiumcitratedissolvedin10mLofdistilledwater, unit,high-angleannulardark-,eld(HAADF)detector,and whichwasbroughtandkeptfor1htoboilingtemperature X-Twinlenses.Samplepreparationwasdonebydissolving JournalofNanomaterials 3 (a) (b) Figure2:(a)HRTEMofelbow-likenanoparticle,formedbyaccommodationofthreesmallAg-Aunanoparticles.(b)3Dreconstruction imageof(a)performedbyImageJpackage. EDXmeasurementsofAg-Aunanoparticle nanoparticles, Figure1(b) corresponds to a section of 4000 C Figure1(a)at5nmofresolution,Figure1(d)ispresenting 3500 atomistic distances for [111] and [121] planar directions 3000 EDX with atomistic distances of 0.268nm and 0.278nm for Au 2500 andAgatoms,respectively;selectareaofdi,ractionindicate Ag 2000 (022),(022),and(002)asprincipalplanarre,ections. 1500 Cu Grain boundary was observed for spherical truncated 1000 Au Ag nanoparticles as presented in Figure2(a). Grain boundary 500 Cu Ag Au Ag can be understood in terms of surface energy thermody- 0 namicsandattributedalsototheionicinteractionbetween 5 10 15 20 0 specimensasproposedbyElechiguerraetal.fornanorods Energy(keV) formation [13]. A 3D reconstruction image is presented in Figure2(b); the image was reconstructed using ImageJ Figure3:EDXresultsofAg-Aunanoparticles,CuandCsignals package.Figure3presentsEDXresults;thetwomajorpeak arefromTEMgrid,insetdark,eldscanningtransmissionelectron signals correspond to C/Cu content on TEM di,raction image. grids; gold shows energy intensities at 2keV and 2.6keV, whereas for silver, intensities are observed at 3keV and 3.4keV. Using A ccelrys Materials Studio, a computational 0.5milligramsinisopropanolplacedinanultrasonicbathfor nanoparticle model was done. The model was subjected dispersionofnanoparticleclusters.Onedropofthesolution to TEM simulations using a full dynamical calculation was used for HRTEM on lacey/carbon (EMS LC225-Cu) by multislice method [14]. The TEM simulator is based grids.Operationalvoltagewas200kVinbothdark,eld(DF) n aie(?ibU2), where U and bright ,eld (BF) mode images, with Scherzer defocus i=1 on projected potential f(U) = represents coordinates in reciprocal space (u,v,w). Results conditionatΔfSch = ?1.2(Csλ)1/2.Energy-dispersiveX-ray fromTEMsimulationsarepresentedinFigure4andseem analysis,EDXwasperformedwhileTEMusingasolidangle to be consistent with experimental HRTEM presented on of0.13srdetector. Figure2(a). Atomicpercentageofgoldfoundwasabout13%from EDX results, which was con,rmed from calculated molar concentrationonbothprecursorsolutions;ratiosofAuCl4 4.Conclusion ionswithrespecttosilverwereroughly10%,indicatingthat foreachgoldatomtherearethreesilverneighborspresent. A successful synthesis to produce nanoparticles gold and The percentages were consistent, since lattice parameters silver precursor solutions is presented here. Bimetallic Ag- in both metals are very similar, for Au-lattice ?0.4078nmAunanoparticleswereformedduetoagalvanicreplacement andAg-lattice?0.4086nmfortypicalFCCbulkstructures. reaction,whichconsistsofthemigrationofionicAgandAu atomsfromsaltprecursorsatboilingtemperature.Products Figure1(a) presents two round spherical shapes Ag-Au 4 JournalofNanomaterials (a) (b) (c) Figure4:(a)ComputerassistedAg-Aunanoparticle(Ag-blueandAu-yellow)usedtounderstandHRTEMimagepresentedin2.(b)TEM simulationof(a)(Ag-Aunanoparticles)forcomparisonwithexperimentalHRTEMimages. wereanalyzedbyHRTEMandEDXtechniques.EDXresults Nanotechnology Infrastructure Network (NNIN) Research show energy intensity peaks at 2keV and 2.6keV for goldProgram of the Microelectronic Research Center of UT - and3keVand3.4keVforsilver.Particleshapewasstudied Austin,andtheMaterialsResearchandTechnologyInstitute bycomputationalmodelingforspeci,celbow-likeshapefor ofUT-ElPasoforusageofresearchfacilities. threesmallAg-Aunanoparticles.Themodelwassubjectedto TEM simulations using full dynamical projected potential. 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