Monolayer-Protected Bimetal Cluster Synthesis by CoreMetal Galvanic Exchange Reaction

Young-Seok Shon,†,‡ G. Brent Dawson,§,| Marc Porter,§ and Royce W. Murray*,†

Kenan Laboratories of Chemistry, University of North Carolina,Chapel Hill, North Carolina 27599-3290, and Department of Chemistry,

Iowa State University, Ames, Iowa 50011-3111

Received January 28, 2002. In Final Form: March 15, 2002

Bimetallic monolayer-protected nanoparticles have been synthesized by the core metal galvanic exchangereaction of dodecylthiolate monolayer-protected metal (Ag, Pd, Cu) clusters with the more noble metalmetal thiolate complexes AuI[SCH2(C6H4)C(CH3)3] and PdII[S(CH2)11CH3)2]. The bimetal nanoparticlesproduced are stable and can be isolated without core aggregation or decomposition. These new materialshave been examined by UV-vis spectroscopy, transmission electron microscopy, X-ray photoelectronspectroscopy, and elemental analysis. Their optical properties reflect bimetal cluster formation by time-dependent shifts in the surface plasmon resonance absorbance. Transmission electron microscopy resultssuggest that the core metal replacement can also effect a change in nanoparticle core size. Formation ofbimetallic nanoparticles appears to stabilized the less stable member of the metal pair.

IntroductionNanoparticles containing two metals are of standing

interest since they can exhibit catalytic,1-4 electronic,5and optical properties6-8 distinct from those of corre-sponding monometal nanoparticles.9-11 Nonsupportedbimetallic nanoparticles have been prepared by reductivedeposition of one metal onto a nanoparticle of anothermetal,12-15 by simultaneous reduction of salts of twodifferent metals (with or without protecting ligands beingpresent),16-19 evaporation followed by condensation,20 and

laser-induced melting.21-24 Depending on the preparationmethod, either alloy or layered (core-shell) nanoparticlescanbesynthesized.12-24 Amongthese,monolayer-protectedalloy clusters (MPACs) are early examples of bimetallicnanoparticles that could be stably isolated in dry form.16

Both the mole ratio of metals in and on the surfaces of theMPAC cores differed significantly from the metal salt ratioused in the MPAC synthesis.16,17 It would be desirable toprepare MPACs in which the nanoparticle metal contentcould be conveniently adjusted to any value along acontinuum of composition.

Galvanic reactions between metal nanoparticles and asalt of a more noble metal are another way to alter thenanoparticle’s constituent metals. Such reactions have,for example, been employed25 to replace Cu or Ag particlesencapsulated in dendrimers by more noble metals (Au,Pt, Pd) and to prepare bi- and trimetallic particles.25a

Galvanic displacement reactions have also been used toprepare the bimetallic sulfides of Pb-Cd and Zn-Cd26

and Co-Pt core-shell bimetallic nanoparticles by thereaction of a Pt salt with Co nanoparticles.27

This report describes the first example of a galvanicsynthesis of bimetallic nanoparticles using reactions ofthiolate monolayer-protected metal clusters (MPCs). The

† University of North Carolina.‡ Present address: Department of Chemistry, Western Kentucky

University, Bowling Green, KY 42101.§ Iowa State University.| Present address: Department of Chemistry, San Jose State

University, San Jose, CA 95192.(1) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y.

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Acc. Chem. Res. 2001, 34, 181. (b) Zhao, M.; Crooks, R. M. Chem. Mater.1999, 11, 3379.

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10.1021/la025586c CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 04/20/2002

synthesis is quite facile and relies on reactions of dode-cylthiolate monolayer-protected Ag, Pd, or Cu clusters,and Ag or Cu clusters, with the thiolate complexes AuI-[SCH2C6H4C(CH3)3] and PdII[(S(CH2)11CH3)2], respec-tively, as outlined in Figure 1. The bimetallic productsare abbreviated as MPACs without inferring as to whetherthe cores are homogeneous alloy or core-shell composi-tions. They are stable and isolable in dried form withoutaggregation of the nanoparticles, like their MPC precur-sors.10

Experimental Section

Tetrachloroauric acid (HAuCl4‚xH2O),28 4-(tert-butyl)benzylmercaptan,29 AuI[SCH2(C6H4)C(CH3)3],30 and PdII[S(CH2)11-CH3)2]31 were synthesized by previously published methods.Silver trifluoromethanesulfonate (Ag(CF3SO3)), copper(II) per-chlorate hexahydrate (Cu(ClO4)2‚6H2O), potassium tetrachlo-

ropalladate (K2PdCl4), tetraoctylammonium bromide (Oct4NBr),and 4-(tert-butyl)benzyl bromide were purchased from Aldrich.Water was purified with a Barnstead Nanopure water system,model 4754.

Dodecanethiolate-protected Ag,32 Pd,31 and Cu clusters wereprepared using procedures as described previously, with minormodifications. For the ca. 4.2 nm diameter Ag MPCs, 2 equiv ofdodecanethiol and Oct4NBr were added to Ag(CF3SO3) in toluene,followed immediately by a 10-fold molar excess of aqueous NaBH4,and then the solution was allowed to stir for 3 h. For the ca. 3.0nm diameter Pd MPCs, one-half equiv of dodecanethiol was addedto K2PdCl4 in toluene, then the detailed procedure of the previouspaper was followed.31 For Cu MPCs, 2 equiv of dodecanethioland Oct4NBr were added to Cu(ClO4)2‚6H2O in toluene, and thesolution was stirred for 10 min before 10 equiv of aqueous NaBH4was added. After the solution was stirred for an hour, the MPCswere purified by repeated dispersal and centrifugation in ethanol.Since Cu MPCs are not very stable in air,33 they were used ingalvanic core metal exchange reactions immediately afterpurification.

Preparation of bimetallic nanoparticles followed a relativelysimple procedure. To prepare Ag-Au MPACs, 120 mg of AuI-[SCH2(C6H4)C(CH3)3] was added to a solution of 200 mg of AgMPCs in 40 mL of toluene. The reaction mixture was stirred atroom temperature for 26 h, and then concentrated in vacuo. Theproduct was placed on a frit, washed with 2-butanol, ethanol,acetonitrile, and acetone, and then dissolved in toluene, andinsoluble salts were removed by filtration. More dilute reactionmixtures (17 mg of Ag MPCs and 10.6 mg of AuI[SCH2(C6H4)C-(CH3)3] in 50 mL of CH2Cl2) were employed to observe UV-visspectra during the galvanic metal exchange reaction. To prepareAg-Pd MPACs, 100 mg of PdII[S(CH2)11CH3)2] were added to asolution of 200 mg of Ag MPCs in 40 mL of toluene. For Pd-AuMPACs, 76 mg of AuI[SCH2(C6H4)C(CH3)3] was added to asolution of 116 mg of Pd MPCs in 25 mL of toluene.

Transmission electron microscopy (TEM) images of nanopar-ticles were obtained with a side-entry Phillips CM12 electronmicroscope operating at 120 keV. Samples were prepared forTEM by casting a single drop of a ∼1 mg/mL hexane clustersolution onto standard carbon-coated (200-300 Å) Formvar filmson copper grids (600 mesh) and drying in air for more than 30min. Three typical regions were imaged at 340000×. Sizedistributions of the cluster cores were obtained from digitizedphotographic enlargements with Scion Image Beta Release 2.X-ray photoelectron spectroscopy (XPS) data were obtained ona Physical Electronics Industries model 5500 surface analysissystem with an Al KR X-ray source, a hemispherical analyzer,a toroidal monochromator, and a multichannel detector (passenergy, 187.9 eV; resolution, ∼0.3 eV), referencing peak positionsto the C1s peak at 284.9 eV. In the XPS data analysis, the peakarea ratios of spin-orbit couplets were constrained to theirappropriate values (e.g., 2:1 for S2p, 3:2 for Pd3d and Ag3d, and4:3 for Au4f). The binding energy spacing between each doubletwas similarly fixed, to 1.18 eV for S2p, 6.0 eV for Ag3d, 5.26 eVfor Pd3d, and 3.67 eV for Au4f. UV-vis spectra of CH2Cl2 solutionsin quartz cells were acquired on an ATI UNICAM spectrometer.Elemental analyses were performed by Galbraith Laboratories.

Results and Discussion

Synthesis and Spectra. An attempt to synthesizebimetallic alloy nanoparticles by core metal exchangebetween dodecanethiolate-protected Ag clusters (C12 AgMPCs) and HAuCl4 in THF resulted in complete decom-position of the C12 Ag MPCs in less than 10 min. Thedecomposition was evidenced by a visual color change fromdark brown to colorless and by the disappearance of theAg MPC surface plasmon absorbance, which occurs at

(28) (a) Handbook of Preparative Inorganic Chemistry; Brauer, G.,Ed.; Academic: New York, 1965; pp 1054-1059. (b) Block, B. P. Inorg.Synth. 1953, 4, 14-17.

(29) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides,G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

(30) Al-Sa’ady, A. K. H.; Moss, K.; McAuliffe, C. A.; Parish, R. V. J.Chem. Soc., Dalton Trans. 1984, 1609.

(31) Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2001,17, 481.

(32) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.;Heath, J. R. Science 1997, 277, 1978.

(33) Chen, S.; Sommers, J. M. J. Phys. Chem. B 2001, 105, 8816-8820.

Figure 1. Cartoon diagram of core metal galvanic exchangereactions.

Bimetallic Nanoparticles Langmuir, Vol. 18, No. 10, 2002 3881

∼420 nm. The instability can be attributed to either thepresence of excess etchant chloride anion or the absenceof excess thiolate ligands.

A second, successful, core metal galvanic exchangereaction was based on a Au(I)-thiolate complex. MostAu(I)-thiolate complexes are polymeric and once pre-cipitated become insoluble in organic solvents.30,34 Thetert-butyl group is however known to help solubilizeorganic and organometallic compounds in organic sol-vents.35 We chose the polymeric complex AuI[SCH2(C6)-H4C(CH3)3] and reacted it with C12 Ag MPCs. Spectraobserved during the reaction in dichloromethane areshown in Figure 2; the corresponding color change wasfrom dark brown to dark purple over a time period of ∼50h. The result, by comparison to previous observations inwhich Au is reduced onto Ag colloids,6b qualitativelyindicates incorporation of Au atoms into the Ag MPCs.(Such a reaction has been used by Crooks et al.25 forgalvanic replacement of metal nanoclusters entrapped indendrimers.) The spectral observations and elementalanalyses (vide infra) confirm the Au incorporation. Thereaction is thermodynamically expected based on the morepositive reduction potential of gold relative to silver salts.

The intense 420 nm16 surface plasmon (SP) absorptionband of the C12 Ag MPCs (Figure 2) red-shifts during thereaction with AuI[SCH2(C6H4)C(CH3)3] and reaches 465nm in 26 h and 496 nm in 52 h. The spectra shiftcontinuously as Au is incorporated into the nanoparticlecore, or more particularly onto the core surface since theSP band maximum is known to reflect the surfacecomposition of a metal nanoparticle.16,18,36,37 Monolayer-protected Au clusters exhibit10 weak SP bands at ca. 520nm; even at long reaction times the MPAC maximum(Figure 2) remains at higher energy than the “pure” AuMPC SP band position. The appearance of a singleabsorption band shows that mixed bimetallic nanopar-ticles are formed rather than monometal nanoparticles.These galvanic metal replacement results are similar tothose made by Mulvaney6b in (nonreplacement) experi-ments in which Au metal was reduced onto Ag colloidalnanoparticles.

The metal replacement reaction of Pd(S(CH2)11CH3)2complexes with C12 Ag MPCs is again expected from thedifference in reduction potentials between Pd and Ag

salts.25 The color of the MPAC reaction solution changesslowly from dark brown to black in ∼50 h, and the spectraof the solutions change as shown in Figure 3. The intensityof the C12 Ag MPC surface plasmon band at 423 nm slowlydecreases during the reaction. No new SP band appears;Pd nanoparticles, owing to the damping effect of the Pdd-d transitions, are known to not exhibit an SP band inUV-vis spectra.31,37 Incorporation of Pd(II) into Ag clustersmay diminish the intensity of the Ag SP band in the Ag-Pd MPAC both by displacement from Ag atoms from theMPAC core surface and by exerting an analogous plasmon-damping effect on the Ag resonance. The stoichiometry ofthe metal replacement should be 2:1 as indicated in thecartoon (the cores are shown naked of ligands) in Figure3, but this was not analytically established.

In the reaction between C12 Pd MPCs with AuI[SCH2-(C6H4)C(CH3)3], the solution color remains blackish duringthe reaction and there are only slight changes in the UV-vis spectrum. The Pd damping effect may again occur tosuppress growth of a Au SP band. However, the suppres-sion of SP bands of group 11 metals by the presence of agroup 10 metal in a bimetallic nanoparticle has also beeninterpreted as subsurface incorporation of the formermetal.6a,38,39 Thus, it is possible that Au(0) might havediffused into the Pd nanoparticle interior after core metalexchange, leaving Pd as a main component in clustersurface.

Since dodecanethiolate-protected Cu clusters (C12 CuMPCs) are not very stable in air,33 they were usedimmediately after preparation in core metal exchangereactions with AuI[SCH2(C6H4)C(CH3)3]. There is no SPband visible (Figure 4) for the Cu MPCs, but as the reactionprogresses, a well-defined SP band appears at 532 nmthat is very near to that known for Au MPCs. The presenceof the Au SP band in the Cu-Au MPAC solutions is astrong indicator that any diffusion of Au into the interiorof the MPAC is slow if it occurs at all. By comparison tothe Pd-Au MPACs discussed just above, it can be inferred

(34) Bachman, R. E.; Bodolosky-Bettis, S. A.; Glennon, S. C.; Sirchio,S. A. J. Am. Chem. Soc. 2000, 122, 7146.

(35) Heo, R. W.; Somoza, F. B.; Lee, T. R. J. Am. Chem. Soc. 1998,120, 1621.

(36) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999,103, 3529.

(37) a) Mulvaney, P. Langmuir 1996, 12, 788. (b) Wood, A.; Giersig,M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 8810.

(38) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz,P.; Brijoux, W.; Bonnemann, H. Langmuir 1997, 13, 2591.

(39) Harikumar, K. R.; Ghosh, S.; Rao, C. N. R. J. Phys. Chem. B1997, 101, 536.

Figure 2. UV-vis spectra in CH2Cl2 solutions of Ag-AuMPACs synthesized by the reaction of C12 Ag MPCs with AuI-[SCH2(C6H4)C(CH3)3].

Figure 3. UV-vis spectra in CH2Cl2 solutions of Ag-PdMPACs synthesized by the reaction of C12 Ag MPCs with Pd-[S(CH2)11CH3]2.

3882 Langmuir, Vol. 18, No. 10, 2002 Shon et al.

from the Cu-Au MPAC result that diffusion of thegalvanically added Au into the Pd MPC core is not theprimary reason for the absence of a Au SP band (i.e.,damping effects are probably the cause).

The experiments above were all done on a small scalewith relatively dilute solutions (to enable spectral obser-vations). Preparative scale MPAC syntheses were donefor 26 h reactions of C12 Ag MPCs with AuI[SCH2(C6H4)C-(CH3)3] and Pd(SC12)2 and of C12 Pd MPCs with AuI-

[SCH2(C6H4)C(CH3)3]. The UV-vis spectra (Figure 5) ofisolated Ag-Au, Pd-Au, and Ag-Pd MPACs are almostidentical with the dilute solution results of Figures 2 and3. The isolated MPACs were air stable and obtained invery high yields. Their core sizes and compositions werestudied using XPS, elemental analysis, and TEM.

X-ray Photoelectron Spectroscopy. XPS providesinformation about binding energies of inner shell elec-trons40 of metal and sulfur and surface atomic compositionsof bimetallic alloy nanoparticles. Figure 6 shows Au4f,Ag3d, and S2p photoelectron spectra for Ag-Au MPACs,and S2p spectra for C12 Ag MPCs. Table 1 gives bindingenergies, using C1s as a reference energy. The Table 1data show that Au4f and Pd3p peaks appear, respectively,in the XPS spectra of Ag-Au and Pd-Au MPACs, and ofAg-Pd MPACs, further confirming incorporation of thesemetals into the original monometal MPCs.

In the MPAC spectra, none of the photoelectron bandscould be resolved into multiple different binding energy(BE) states, although in some cases the BE of the initialmonometal MPC differed from that seen in the MPACspectrum. For example, the Pd3d5/2 band appears at 336.2eV in the C12 Pd MPC, but at a single higher (337.4 eV)and lower (335.8 eV) BE in Pd-Au and Ag-Pd MPACs,respectively. The S2p3/2 band in C12 Ag, Au, and Pdmonometal MPCs appears at 161.9, 162.3, and 162.7 eV,respectively. These are perceptibly different energies, butin the MPACs the observed S2p3/2 BE is more uniform,162.3-162.5 eV, and not resolved into multiple states.The S2p3/2 BE data potentially provide a window to howdifferent metals influence metal-thiolate bonding in themixed-metal MPACs. (Previously16 we were able to resolveBE for differently synthesized MPACs into two bands,which were attributed to binding to the different metals.)Irrespective of whether resolution is achieved, the S2p3/2BE observed in Table 1 suggests that the thiolate bondsin bimetallic MPCs become more like those of Au-SRbonds (i.e., goldlike) in Ag-Au, Pd-Au, and Ag-PdMPACs. Finally, there are no obvious changes in theAu4f7/2 BE whether the Au is in a monometal Au MPC or

Figure 4. UV-vis spectra of Cu MPCs before and after 3 hof reaction with AuI[SCH2(C6H4)C(CH3)3], forming Cu-AuMPACs.

Figure 5. UV-vis spectra of indicated isolated MPACs in CH2-Cl2.

Figure 6. XPS spectra of (a) Au4f, (b) Ag3d, and (c) S2p regions of Ag-Au MPACs and (d) S2p region of Ag MPCs. The peaksare fitted with Gaussians. The solid lines are the summed fits and the dotted lines are the individual fitted components.

Bimetallic Nanoparticles Langmuir, Vol. 18, No. 10, 2002 3883

in Ag-Au or Pd-Au MPACs. This result is consistentwith previous reports5,17,41 on alloy clusters.

C12 Pd monometal MPC spectra additionally showedevidence of an oxidized sulfur species. Besides the S2p3/2photoelectron peak at BE 162.7 eV, two others, at 164.3and 167.5 eV, could be resolved by deconvolution. Thesepeaks suggest the presence of oxidized sulfur species (e.g.,disulfides, sulfinates, and sulfonates) on Pd MPCs.Formation of a Pd-Au MPAC, however, appears toeliminate the oxidized sulfur species from the bimetalliccluster surface, since the MPAC spectra show no evidenceof higher BE S2p bands. Similarly, when Ag-Pd MPACsare formed, incorporation of the Pd atoms on the Ag MPCsurface does not lead to higher BE S2p bands. Thisphenomenon is somewhat like that seen in the previous16

MPAC study, in which formation of bimetal nanoparticlesseemed to stabilize the less stable member of the pair. Inthe present results, the Cu-Au MPAC is more air stablethan the C12 Cu monometal MPC, also consistent withthe above generality.

Elemental Analysis and Mechanism of MetalExchange. The atomic compositions of bimetallic MPACproducts of 26 h preparative reactions obtained byelemental analysis are reported in Table 1. The amount

of metal exchange is uniformly smaller than that whichwould be obtained by complete reaction given the moleratios of metal reactants employed (see Table 1, Anal. vsFeed). That is, according to the 1.1:1 mole ratio of initialC12 Ag MPC and AuI[SCH2(C6H4)C(CH3)3], and a pre-sumably substantial thermodynamic equilibrium con-stant, nearly all of the Ag atoms in the Ag MPCs shouldhave been supplanted by galvanic exchange with Au.Instead, the Ag:Au ratio in the MPAC product is ca. 1.5:1.Similar statements can be made for the other MPACreaction products. One must therefore presume that theextent of the galvanic metal exchange reactions wasconstrained by kinetic factors, such as (a) differencesbetween the rates at which surface versus subsurfaceatoms of the monometal MPC could be replaced and (b)differences in the rates of metal exchange of atoms atvertex and edge versus terrace sites on the originalmonometal MPC surface.

In other MPC studies where the rates of exchange ofone thiolate ligand for another have been examined, itwas hypothesized that edge + vertex (“defect”) sites aresubstantially more reactive40 than terrace sites on thepresumably truncated octahedral10 MPC core surface. InTable 1 one sees that the ratio of metals for the Ag-PdMPAC (3.6) is about the same as that of surface-to-totalatoms (the ratio is ca. 3.2 for 4.2 nm diameter nanopar-ticles), that for the Pd-Au MPACs (6.2) is about the sameas that of the total surface defect atoms (ca. 8:1 for 3.0 nmdiameter nanoparticles), and that for the Ag-Au MPACs

(40) Briggs, D.; Seah, M. D. Practical Surface Analysis by Auger andX-ray Photoelectron Spectroscopy; John Wiley and Sons: Chichester,1984.

(41) Wu, M.-L.; Chen, D.-H.; Huang, T.-C. Langmuir 2001, 17, 3877-3883.

Figure 7. Transmission electron micrographs and core size histograms of (a) Ag MPCs and (b) Ag-Au MPACs.

Table 1. MPC and MPAC Results from Transmission Electron Microscopy, XPS, and Elemental Analysis

XPSb (eV)MPCs

TEMa

(diameter, nm) Au4f7/2 Ag3d5/2 Pd3d5/2 S2p3/2

metal ratioelem anal./rxn feed

Ag MPC 4.2 ( 1.5 368.1 161.9Ag-Au MPAC 3.0 ( 1.4 84.5 368.5 162.5 1.5/1.1 (Ag/Au)Ag-Pd MPC 3.4 ( 1.5 368.2 337.4 162.3 3.6/2.0 (Ag/Pd)Pd-Au MPC 3.3 ( 1.4 84.2 335.8 162.4 6.2/1.2 (Pd/Au)Pd MPC 3.0 ( 1.4 336.2 162.7Au MPC40 84.3 162.3

a TEM results, average Au core size from analysis of histogram of TEM images. b The C1s binding energy at 284.9 eV was used as areference.

3884 Langmuir, Vol. 18, No. 10, 2002 Shon et al.

(1.5) exceeds both surface/total and surface defect/totalratios.10,42 This dispersity of results indicates that thefraction of exchanged metal atoms may not be as kineti-cally differentiated as the ligand place exchange reac-tions43 appear to be. In a simple view, the facility of themetal and of ligand exchange might be hypothesized tobe parallel, since one can imagine that reaction steps formetal exchange necessarily involve metal-ligand bondbreaking. The results do not support this simplified view.

Transmission Electron Microscopy. The deter-mined average core dimensions of MPCs and MPACs aresummarized in Table 1. TEM images and histograms ofAg MPCs and Ag-Au MPACs (Figure 7) show that theformer has an average core diameter of 4.2 ( 1.4 nm andthe latter 3.0 ( 1.4 nm. (The uncertainty in size is simplyan average deviation; the histograms are obviously notGaussian.) Although the MPC samples are obviouslyrather polydisperse in core size, the size distribution afterthe galvanic metal exchange reaction is clearly different;the population of smaller clusters in the MPAC material

has been increased. The metal exchange reaction seemsto result in some etching effect on the nanoparticle sizes.

The average core diameter of Ag-Pd MPACs (3.4 ( 1.5nm) is also clearly smaller than that of Ag MPCs. In thiscase, there is an explanation alternative to an etchingprocess, namely, that of the 2:1 reaction stoichiometrybetween Ag0 and PdII; that is, two AgI ions should bereleased from the nanoparticle core for each Pd atom addedto it. Similarly, the modest increase in core size of Pd-AuMPACs relative to Pd monometal MPCs can be attributedto the 2:1 stoichiometry in which two Au atoms are addedto the MPAC core for every PdII ion released. These TEMresults suggest that core metal galvanic exchange reac-tions change core dimensions on principles as simple asthe reaction stoichiometry between the monometal MPCand the reacting metal salt.

Acknowledgment. This research was supported (atUNC) in part by a grant from the National ScienceFoundation and (at ISU) by the Office of Baric EnergySciences through the Ames Laboratory Contract W-7405-Eng-82. The authors thank Dr. Francis P. Zamborini forproviding Pd MPCs.

LA025586C

(42) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir1999, 15, 3782.

(43) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet,R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall,G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray R. W. Langmuir1998, 14, 17.

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