Monolayer-Protected Bimetal Cluster Synthesis by Core Metal

Apr 20, 2002 - Bimetallic monolayer-protected nanoparticles have been synthesized by the core metal galvanic exchange reaction of dodecylthiolate ...
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Monolayer-Protected Bimetal Cluster Synthesis by Core Metal 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 exchange reaction of dodecylthiolate monolayer-protected metal (Ag, Pd, Cu) clusters with the more noble metal metal thiolate complexes AuI[SCH2(C6H4)C(CH3)3] and PdII[S(CH2)11CH3)2]. The bimetal nanoparticles produced are stable and can be isolated without core aggregation or decomposition. These new materials have been examined by UV-vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and elemental analysis. Their optical properties reflect bimetal cluster formation by timedependent shifts in the surface plasmon resonance absorbance. Transmission electron microscopy results suggest that the core metal replacement can also effect a change in nanoparticle core size. Formation of bimetallic nanoparticles appears to stabilized the less stable member of the metal pair.

Introduction Nanoparticles containing two metals are of standing interest since they can exhibit catalytic,1-4 electronic,5 and optical properties6-8 distinct from those of corresponding monometal nanoparticles.9-11 Nonsupported bimetallic nanoparticles have been prepared by reductive deposition of one metal onto a nanoparticle of another metal,12-15 by simultaneous reduction of salts of two different metals (with or without protecting ligands being present),16-19 evaporation followed by condensation,20 and †

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. J. Phys. Chem B 2000, 104, 6028. (2) Schmid, G.; West, H.; Mehles, H.; Lehnert, A. Inorg. Chem. 1997, 36, 891. (3) Nashner, M. S.; Frenkel, A. I.; Adler, D. L.; Shapley, J. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1997, 119, 7760. (4) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Brijoux, W.; Bo¨nnemann, H. Langmuir 1997, 13, 2591. (5) Harikumar, K. R.; Ghosh, S.; Rao, C. N. R. J. Phys. Chem. A 1997, 101, 536. (6) (a) Michaelis, M.; Henglein, A.; Mulvaney, P. J. Phys. Chem. 1994, 98, 6212. (b) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (7) Remita, H.; Khatouri, J.; Treguer, M.; Amblard, J.; Belloni, J. Z. Phys. D 1997, 40, 127. (8) Henglein, A.; Brancewicz, C. Chem. Mater. 1997, 9, 2164. (9) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (c) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795-797. (d) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. J. Electroanal. Chem. 1996, 409, 137-143. (10) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (11) Whetten. R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397-406. (12) Rivas, L.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. Langmuir 2000, 16, 9722. (13) Srnova´-Sloufova´, I.; Lednicky´, F.; Gemperle, A.; Gemperlova´, J. Langmuir 2000, 16, 9928. (14) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (15) Chen, Y. H.; Nickel, U. J. Chem. Soc., Faraday Trans. 1993, 89, 2479.

laser-induced melting.21-24 Depending on the preparation method, either alloy or layered (core-shell) nanoparticles can be synthesized.12-24 Among these, monolayer-protected alloy clusters (MPACs) are early examples of bimetallic nanoparticles that could be stably isolated in dry form.16 Both the mole ratio of metals in and on the surfaces of the MPAC cores differed significantly from the metal salt ratio used in the MPAC synthesis.16,17 It would be desirable to prepare MPACs in which the nanoparticle metal content could be conveniently adjusted to any value along a continuum of composition. Galvanic reactions between metal nanoparticles and a salt of a more noble metal are another way to alter the nanoparticle’s constituent metals. Such reactions have, for example, been employed25 to replace Cu or Ag particles encapsulated in dendrimers by more noble metals (Au, Pt, Pd) and to prepare bi- and trimetallic particles.25a Galvanic displacement reactions have also been used to prepare the bimetallic sulfides of Pb-Cd and Zn-Cd26 and Co-Pt core-shell bimetallic nanoparticles by the reaction of a Pt salt with Co nanoparticles.27 This report describes the first example of a galvanic synthesis of bimetallic nanoparticles using reactions of thiolate monolayer-protected metal clusters (MPCs). The (16) Hostetler, M. J.; Zhong, C.-J.; Yen, B. K. H.; Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 9396. (17) Sandhyarani, N.; Pradeep, T. Chem. Mater. 2000, 12, 1755. (18) Han, S. W.; Kim, Y.; Kim, K. J. Colloid Interface Sci. 1998, 208, 272. (19) Schmid, G. Clusters and Colloids; VCH: Weinheim, 1994. (20) Papavassiliou, G. C. J. Phys. F: Met. Phys. 1976, 6, L103. (21) Takeuchi, Y.; Ida, T.; Kimura, K. J. Phys. Chem. B 1997, 101, 1322. (22) (a) Hodak, J. H.; Henglein, A.; Giersig, M.; Hartland, G. V. J. Phys. Chem. B 2000, 104, 11708. (b) Hodak, J. H.; Henglein, A.; Hartland, G, V. J. Phys. Chem. B 2000, 104, 9954. (23) Abid, J.-P.; Girault, H. H.; Brevet, P. F. Chem. Commun. 2001, 829. (24) Chen, Y.-H.; Yeh, C.-S. Chem. Commun. 2001, 371. (25) (a) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (b) Zhao, M.; Crooks, R. M. Chem. Mater. 1999, 11, 3379. (26) Moriguchi, I.; Matsuo, K.; Sakai, M.; Hanai, K.; Teraoka, Y.; Kagawa, S. J. Chem. Soc., Faraday Trans. 1998, 94, 2199. (27) (a) Park, J.-I.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 57435746. (b) Lin, W.; Wiegand, B. C.; Nuzzo, R. G.; Girolami, G. S. J. Am. Chem. Soc. 1996, 118, 5977-5987.

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

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Figure 1. Cartoon diagram of core metal galvanic exchange reactions.

synthesis is quite facile and relies on reactions of dodecylthiolate 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], respectively, as outlined in Figure 1. The bimetallic products are abbreviated as MPACs without inferring as to whether the cores are homogeneous alloy or core-shell compositions. They are stable and isolable in dried form without aggregation of the nanoparticles, like their MPC precursors.10 Experimental Section Tetrachloroauric acid (HAuCl4‚xH2O),28 4-(tert-butyl)benzyl mercaptan,29 AuI[SCH2(C6H4)C(CH3)3],30 and PdII[S(CH2)11CH3)2]31 were synthesized by previously published methods. Silver trifluoromethanesulfonate (Ag(CF3SO3)), copper(II) perchlorate hexahydrate (Cu(ClO4)2‚6H2O), potassium tetrachlo(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.

Langmuir, Vol. 18, No. 10, 2002 3881 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 were prepared using procedures as described previously, with minor modifications. For the ca. 4.2 nm diameter Ag MPCs, 2 equiv of dodecanethiol 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. 3.0 nm diameter Pd MPCs, one-half equiv of dodecanethiol was added to K2PdCl4 in toluene, then the detailed procedure of the previous paper was followed.31 For Cu MPCs, 2 equiv of dodecanethiol and Oct4NBr were added to Cu(ClO4)2‚6H2O in toluene, and the solution was stirred for 10 min before 10 equiv of aqueous NaBH4 was added. After the solution was stirred for an hour, the MPCs were purified by repeated dispersal and centrifugation in ethanol. Since Cu MPCs are not very stable in air,33 they were used in galvanic core metal exchange reactions immediately after purification. Preparation of bimetallic nanoparticles followed a relatively simple procedure. To prepare Ag-Au MPACs, 120 mg of AuI[SCH2(C6H4)C(CH3)3] was added to a solution of 200 mg of Ag MPCs in 40 mL of toluene. The reaction mixture was stirred at room temperature for 26 h, and then concentrated in vacuo. The product was placed on a frit, washed with 2-butanol, ethanol, acetonitrile, and acetone, and then dissolved in toluene, and insoluble salts were removed by filtration. More dilute reaction mixtures (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-vis spectra during the galvanic metal exchange reaction. To prepare Ag-Pd MPACs, 100 mg of PdII[S(CH2)11CH3)2] were added to a solution of 200 mg of Ag MPCs in 40 mL of toluene. For Pd-Au MPACs, 76 mg of AuI[SCH2(C6H4)C(CH3)3] was added to a solution of 116 mg of Pd MPCs in 25 mL of toluene. Transmission electron microscopy (TEM) images of nanoparticles were obtained with a side-entry Phillips CM12 electron microscope operating at 120 keV. Samples were prepared for TEM by casting a single drop of a ∼1 mg/mL hexane cluster solution onto standard carbon-coated (200-300 Å) Formvar films on copper grids (600 mesh) and drying in air for more than 30 min. Three typical regions were imaged at 340000×. Size distributions of the cluster cores were obtained from digitized photographic enlargements with Scion Image Beta Release 2. X-ray photoelectron spectroscopy (XPS) data were obtained on a Physical Electronics Industries model 5500 surface analysis system with an Al KR X-ray source, a hemispherical analyzer, a toroidal monochromator, and a multichannel detector (pass energy, 187.9 eV; resolution, ∼0.3 eV), referencing peak positions to the C1s peak at 284.9 eV. In the XPS data analysis, the peak area ratios of spin-orbit couplets were constrained to their appropriate values (e.g., 2:1 for S2p, 3:2 for Pd3d and Ag3d, and 4:3 for Au4f). The binding energy spacing between each doublet was similarly fixed, to 1.18 eV for S2p, 6.0 eV for Ag3d, 5.26 eV for Pd3d, and 3.67 eV for Au4f. UV-vis spectra of CH2Cl2 solutions in 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 synthesize bimetallic alloy nanoparticles by core metal exchange between dodecanethiolate-protected Ag clusters (C12 Ag MPCs) and HAuCl4 in THF resulted in complete decomposition of the C12 Ag MPCs in less than 10 min. The decomposition was evidenced by a visual color change from dark brown to colorless and by the disappearance of the Ag MPC surface plasmon absorbance, which occurs at (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, 88168820.

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

∼420 nm. The instability can be attributed to either the presence of excess etchant chloride anion or the absence of excess thiolate ligands. A second, successful, core metal galvanic exchange reaction was based on a Au(I)-thiolate complex. Most Au(I)-thiolate complexes are polymeric and once precipitated become insoluble in organic solvents.30,34 The tert-butyl group is however known to help solubilize organic and organometallic compounds in organic solvents.35 We chose the polymeric complex AuI[SCH2(C6)H4C(CH3)3] and reacted it with C12 Ag MPCs. Spectra observed during the reaction in dichloromethane are shown in Figure 2; the corresponding color change was from dark brown to dark purple over a time period of ∼50 h. The result, by comparison to previous observations in which Au is reduced onto Ag colloids,6b qualitatively indicates incorporation of Au atoms into the Ag MPCs. (Such a reaction has been used by Crooks et al.25 for galvanic replacement of metal nanoclusters entrapped in dendrimers.) The spectral observations and elemental analyses (vide infra) confirm the Au incorporation. The reaction is thermodynamically expected based on the more positive reduction potential of gold relative to silver salts. The intense 420 nm16 surface plasmon (SP) absorption band of the C12 Ag MPCs (Figure 2) red-shifts during the reaction with AuI[SCH2(C6H4)C(CH3)3] and reaches 465 nm in 26 h and 496 nm in 52 h. The spectra shift continuously as Au is incorporated into the nanoparticle core, or more particularly onto the core surface since the SP band maximum is known to reflect the surface composition of a metal nanoparticle.16,18,36,37 Monolayerprotected Au clusters exhibit10 weak SP bands at ca. 520 nm; even at long reaction times the MPAC maximum (Figure 2) remains at higher energy than the “pure” Au MPC SP band position. The appearance of a single absorption band shows that mixed bimetallic nanoparticles are formed rather than monometal nanoparticles. These galvanic metal replacement results are similar to those made by Mulvaney6b in (nonreplacement) experiments in which Au metal was reduced onto Ag colloidal nanoparticles. The metal replacement reaction of Pd(S(CH2)11CH3)2 complexes with C12 Ag MPCs is again expected from the difference in reduction potentials between Pd and Ag (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.

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

salts.25 The color of the MPAC reaction solution changes slowly from dark brown to black in ∼50 h, and the spectra of the solutions change as shown in Figure 3. The intensity of the C12 Ag MPC surface plasmon band at 423 nm slowly decreases during the reaction. No new SP band appears; Pd nanoparticles, owing to the damping effect of the Pd d-d transitions, are known to not exhibit an SP band in UV-vis spectra.31,37 Incorporation of Pd(II) into Ag clusters may diminish the intensity of the Ag SP band in the AgPd MPAC both by displacement from Ag atoms from the MPAC core surface and by exerting an analogous plasmondamping effect on the Ag resonance. The stoichiometry of the metal replacement should be 2:1 as indicated in the cartoon (the cores are shown naked of ligands) in Figure 3, 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 during the reaction and there are only slight changes in the UVvis spectrum. The Pd damping effect may again occur to suppress growth of a Au SP band. However, the suppression of SP bands of group 11 metals by the presence of a group 10 metal in a bimetallic nanoparticle has also been interpreted as subsurface incorporation of the former metal.6a,38,39 Thus, it is possible that Au(0) might have diffused into the Pd nanoparticle interior after core metal exchange, leaving Pd as a main component in cluster surface. Since dodecanethiolate-protected Cu clusters (C12 Cu MPCs) are not very stable in air,33 they were used immediately after preparation in core metal exchange reactions with AuI[SCH2(C6H4)C(CH3)3]. There is no SP band visible (Figure 4) for the Cu MPCs, but as the reaction progresses, a well-defined SP band appears at 532 nm that is very near to that known for Au MPCs. The presence of the Au SP band in the Cu-Au MPAC solutions is a strong indicator that any diffusion of Au into the interior of the MPAC is slow if it occurs at all. By comparison to the Pd-Au MPACs discussed just above, it can be inferred (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. B 1997, 101, 536.

Bimetallic Nanoparticles

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

Figure 5. UV-vis spectra of indicated isolated MPACs in CH2Cl2.

from the Cu-Au MPAC result that diffusion of the galvanically added Au into the Pd MPC core is not the primary 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 scale with relatively dilute solutions (to enable spectral observations). Preparative scale MPAC syntheses were done for 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-

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[SCH2(C6H4)C(CH3)3]. The UV-vis spectra (Figure 5) of isolated Ag-Au, Pd-Au, and Ag-Pd MPACs are almost identical with the dilute solution results of Figures 2 and 3. The isolated MPACs were air stable and obtained in very high yields. Their core sizes and compositions were studied using XPS, elemental analysis, and TEM. X-ray Photoelectron Spectroscopy. XPS provides information about binding energies of inner shell electrons40 of metal and sulfur and surface atomic compositions of 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 binding energies, using C1s as a reference energy. The Table 1 data show that Au4f and Pd3p peaks appear, respectively, in the XPS spectra of Ag-Au and Pd-Au MPACs, and of Ag-Pd MPACs, further confirming incorporation of these metals into the original monometal MPCs. In the MPAC spectra, none of the photoelectron bands could be resolved into multiple different binding energy (BE) states, although in some cases the BE of the initial monometal MPC differed from that seen in the MPAC spectrum. For example, the Pd3d5/2 band appears at 336.2 eV 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 Pd monometal MPCs appears at 161.9, 162.3, and 162.7 eV, respectively. These are perceptibly different energies, but in 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 how different metals influence metal-thiolate bonding in the mixed-metal MPACs. (Previously16 we were able to resolve BE for differently synthesized MPACs into two bands, which were attributed to binding to the different metals.) Irrespective of whether resolution is achieved, the S2p3/2 BE observed in Table 1 suggests that the thiolate bonds in bimetallic MPCs become more like those of Au-SR bonds (i.e., goldlike) in Ag-Au, Pd-Au, and Ag-Pd MPACs. Finally, there are no obvious changes in the Au4f7/2 BE whether the Au is in a monometal Au MPC or

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 peaks are fitted with Gaussians. The solid lines are the summed fits and the dotted lines are the individual fitted components.

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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 MPCs Ag MPC Ag-Au MPAC Ag-Pd MPC Pd-Au MPC Pd MPC Au MPC40

TEMa (diameter, nm) 4.2 ( 1.5 3.0 ( 1.4 3.4 ( 1.5 3.3 ( 1.4 3.0 ( 1.4

Au4f7/2 84.5 84.2 84.3

XPSb (eV) Ag3d5/2 Pd3d5/2 368.1 368.5 368.2

337.4 335.8 336.2

S2p3/2 161.9 162.5 162.3 162.4 162.7 162.3

metal ratio elem anal./rxn feed 1.5/1.1 (Ag/Au) 3.6/2.0 (Ag/Pd) 6.2/1.2 (Pd/Au)

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 a reference.

in Ag-Au or Pd-Au MPACs. This result is consistent with previous reports5,17,41 on alloy clusters. C12 Pd monometal MPC spectra additionally showed evidence of an oxidized sulfur species. Besides the S2p3/2 photoelectron peak at BE 162.7 eV, two others, at 164.3 and 167.5 eV, could be resolved by deconvolution. These peaks suggest the presence of oxidized sulfur species (e.g., disulfides, sulfinates, and sulfonates) on Pd MPCs. Formation of a Pd-Au MPAC, however, appears to eliminate the oxidized sulfur species from the bimetallic cluster surface, since the MPAC spectra show no evidence of higher BE S2p bands. Similarly, when Ag-Pd MPACs are formed, incorporation of the Pd atoms on the Ag MPC surface does not lead to higher BE S2p bands. This phenomenon is somewhat like that seen in the previous16 MPAC study, in which formation of bimetal nanoparticles seemed to stabilize the less stable member of the pair. In the present results, the Cu-Au MPAC is more air stable than the C12 Cu monometal MPC, also consistent with the above generality. Elemental Analysis and Mechanism of Metal Exchange. The atomic compositions of bimetallic MPAC products of 26 h preparative reactions obtained by elemental analysis are reported in Table 1. The amount (40) Briggs, D.; Seah, M. D. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; John Wiley and Sons: Chichester, 1984. (41) Wu, M.-L.; Chen, D.-H.; Huang, T.-C. Langmuir 2001, 17, 38773883.

of metal exchange is uniformly smaller than that which would be obtained by complete reaction given the mole ratios of metal reactants employed (see Table 1, Anal. vs Feed). That is, according to the 1.1:1 mole ratio of initial C12 Ag MPC and AuI[SCH2(C6H4)C(CH3)3], and a presumably substantial thermodynamic equilibrium constant, nearly all of the Ag atoms in the Ag MPCs should have 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 MPAC reaction products. One must therefore presume that the extent of the galvanic metal exchange reactions was constrained by kinetic factors, such as (a) differences between the rates at which surface versus subsurface atoms of the monometal MPC could be replaced and (b) differences in the rates of metal exchange of atoms at vertex and edge versus terrace sites on the original monometal MPC surface. In other MPC studies where the rates of exchange of one thiolate ligand for another have been examined, it was hypothesized that edge + vertex (“defect”) sites are substantially more reactive40 than terrace sites on the presumably truncated octahedral10 MPC core surface. In Table 1 one sees that the ratio of metals for the Ag-Pd MPAC (3.6) is about the same as that of surface-to-total atoms (the ratio is ca. 3.2 for 4.2 nm diameter nanoparticles), that for the Pd-Au MPACs (6.2) is about the same as that of the total surface defect atoms (ca. 8:1 for 3.0 nm diameter nanoparticles), and that for the Ag-Au MPACs

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(1.5) exceeds both surface/total and surface defect/total ratios.10,42 This dispersity of results indicates that the fraction of exchanged metal atoms may not be as kinetically differentiated as the ligand place exchange reactions43 appear to be. In a simple view, the facility of the metal and of ligand exchange might be hypothesized to be parallel, since one can imagine that reaction steps for metal exchange necessarily involve metal-ligand bond breaking. The results do not support this simplified view. Transmission Electron Microscopy. The determined average core dimensions of MPCs and MPACs are summarized in Table 1. TEM images and histograms of Ag MPCs and Ag-Au MPACs (Figure 7) show that the former has an average core diameter of 4.2 ( 1.4 nm and the latter 3.0 ( 1.4 nm. (The uncertainty in size is simply an average deviation; the histograms are obviously not Gaussian.) Although the MPC samples are obviously rather polydisperse in core size, the size distribution after the galvanic metal exchange reaction is clearly different; the population of smaller clusters in the MPAC material (42) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 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. Langmuir 1998, 14, 17.

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has been increased. The metal exchange reaction seems to result in some etching effect on the nanoparticle sizes. The average core diameter of Ag-Pd MPACs (3.4 ( 1.5 nm) is also clearly smaller than that of Ag MPCs. In this case, there is an explanation alternative to an etching process, namely, that of the 2:1 reaction stoichiometry between Ag0 and PdII; that is, two AgI ions should be released from the nanoparticle core for each Pd atom added to it. Similarly, the modest increase in core size of Pd-Au MPACs relative to Pd monometal MPCs can be attributed to the 2:1 stoichiometry in which two Au atoms are added to the MPAC core for every PdII ion released. These TEM results suggest that core metal galvanic exchange reactions change core dimensions on principles as simple as the reaction stoichiometry between the monometal MPC and the reacting metal salt. Acknowledgment. This research was supported (at UNC) in part by a grant from the National Science Foundation and (at ISU) by the Office of Baric Energy Sciences through the Ames Laboratory Contract W-7405Eng-82. The authors thank Dr. Francis P. Zamborini for providing Pd MPCs. LA025586C