Au Nanoparticles Encapsulated in Ru Carbonyl Carboxylate Shells

A two-step surface functionalization approach has been used to encase Au nanoparticles in monolayer organometallic Ru-complex shells by the reaction o...
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Langmuir 2006, 22, 7861-7866

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Au Nanoparticles Encapsulated in Ru Carbonyl Carboxylate Shells Suhua Wang and Wee-Sun Sim* Department of Chemistry, National UniVersity of Singapore, Kent Ridge, Singapore 119260, Singapore ReceiVed March 23, 2006. In Final Form: June 14, 2006 A two-step surface functionalization approach has been used to encase Au nanoparticles in monolayer organometallic Ru-complex shells by the reaction of an intermediate surface-bound mercaptopropanoic acid capping species with Ru dodecacarbonyl (Ru3(CO)12) clusters. Vibrational (infrared and Raman) spectroscopy shows that insertion of carboxylate groups into the Ru clusters results in their fragmentation and the formation of a shell of Ru dicarbonyl carboxylate oligomers that remain attached to the Au nanoparticles through the original Au-alkanethiolate bonds. The structural integrity of the metallic nanoparticulate Au cores has been verified by X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy. The organometallic Ru-complex shell may be decomposed thermally to eliminate the mercaptopropanoate and carbonyl groups and leave a mixed phase of Au and RuO2.

Introduction Monolayer-protected nanoparticles with functionalized surfaces are of considerable interest in catalytic, biological, and optoelectronic applications.1-3 Surface functionalization of these nanoparticles can be achieved by changing the capping molecules or modifying their structures, hence altering the surface hydrophilicity or hydrophobicity and surface reactivity. Weakly bound capping molecules may be readily displaced by other molecules that have stronger interactions with the nanoparticle surfaces,4 while more strongly chemisorbed capping molecules are typically replaced using ligand place-exchange reactions.5 The most extensively investigated capping molecules for Au nanoparticles are alkanethiols, because of their ability to spontaneously form self-assembled monolayers on freshly prepared Au surfaces.6,7 The alkanethiol molecules readily chemisorb through Au-S bonding with their hydrocarbon tails pointing outward, forming a chemically inert outer surface layer. By introducing suitable functional groups, either directly into the capping molecules or via ligand exchange, the chemical reactivity of the nanoparticles toward other molecular species can be facilitated. The synthesis and characterization of Au nanoparticles capped using functionalized thiols such as mercaptosuccinic acid (MSA),8 3-mercaptopropanoic acid (MPA),9 p-mercaptophenol,10 and various mercapto bromides11 and amides12 have been documented. Their ω-functionalized-thiolate surfaces possess pendant groups that provide active sites on the nanoparticle peripheries for a variety of coupling reactions to occur. Au nanoparticles stabilized with p-mercaptophenol,10 * Corresponding author. E-mail: [email protected]. (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (4) Warner, M. G.; Reed, S. M.; Hutchison, J. E. Chem. Mater. 2000, 12, 3316. (5) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (6) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem.sEur. J. 1996, 2, 359. (7) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides. G. M. Chem. ReV. 2005, 105, 1103. (8) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075. (9) Shiraishi, Y.; Arakawa, D.; Toshima, N. Eur. Phys. J. E 2002, 8, 377. (10) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (11) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (12) Paulini, R.; Frankamp, B. L.; Rotello, V. M. Langmuir 2002, 6, 2368.

ω-bromoalkanethiolates,11 ω-carboxylic acid-alkanethiolates13 and ω-hydroxyl-alkanethiolates13 have been successfully reacted with propanoic anhydride, amines, alcohols, and carboxylic acids, respectively, thus demonstrating that suitably functionalized metal nanoclusters can serve as the building blocks for more complex molecular architectures. Studies utilizing this sequential coupling methodology are almost exclusively based on well-established organic reactions such as ester and amide coupling with incoming organic molecular species.14 There exists, however, a large database of organometallic reactions that has hitherto remained untapped that could potentially be used to attach novel inorganic and organometallic fragments to the monolayer-protected nanoparticle surfaces. This work presents a detailed investigation of one such organometallic system. The carboxyl group is known to react with metal carbonyl clusters such as trirutheniumdodecacarbonyl (Ru3(CO)12) to form Ru carbonyl carboxylate complexes. The reaction between carboxylic acids and Ru3(CO)12 was first extensively investigated by Lewis et al.,15 who found that formic, acetic, propanoic, and decanoic acids yield polymers with molecular formula [Ru(RCOO)(CO)2]2n. These polymers possess a bridging carboxylate structure that is characterized by two infrared absorption bands at ∼1550 and 1400 cm-1, which are respectively assigned to the asymmetric and symmetric OCO stretching vibrations. Subsequent research on the structures and properties of bridging Ru carbonyl carboxylate complexes and their derivatives16-21 have led to the discovery that these compounds can act as catalysts or catalyst precursors for the hydroformylation of olefins,22 the (13) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (14) Shon, Y. S.; Choo, H. C. R. Chim. 2003, 6, 1009. (15) Crooks, G. R.; Johnson, B. F. G.; Lewis, J.; Williams, I. G.; Gamlen, G. J. Chem. Soc. A 1969, 2761. (16) Bianchi, M.; Frediani, P.; Matteoli, U.; Menchi, G. J. Organomet. Chem. 1983, 259, 207. (17) Rotem, M.; Shvo, Y.; Goldberg, I.; Shmuell, U. Organometallics 1984, 3, 1758. (18) Suss-Fink, G.; Herrmann, G.; Morys, P. J. Organomet. Chem. 1985, 284, 263. (19) Bohle, D. S.; Vahrenkamp, H. Inorg. Chem. 1990, 29, 1097. (20) Rotem, M.; Goldberg, I.; Shmuell, U.; Shvo, Y. J. Organomet. Chem. 1986, 314, 185. (21) Salvini, A.; Frediani, P.; Rivalta, E. Inorg. Chim. Acta 2003, 351, 225. (22) Jenck, J.; Kalck, P.; Pinelli, E.; Siani, M.; Thorez, A. J. Chem. Soc., Chem. Commun. 1988, 1428.

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addition of carboxylic acids to alkynes,23 the hydrogenation of carbon monoxide to methanol and ethylene glycol,24 and the chemioselective hydrogenation of CdC double bonds in the presence of CdO double bonds.25 Metal nanoparticles functionalized with organometallic complexes are potential precursors for the preparation of nanoparticle bimetallic systems, which possess enhanced catalytic activity and selectivity when compared to their monometallic counterparts.26,27 Such bimetallic catalysts are typically formed by bulk alloying or physical surface deposition involving the two metals of interest.28 Recent efforts have been focused on the synthesis of mixed metal clusters as precursors for nanoparticle bimetallic systems because the metallic framework in these compounds can serve as a template for the formation of homogeneously sized catalytic clusters.29 In this work, we have utilized the known reactivity between carboxyl groups and Ru3(CO)12 to develop a methodology for the attachment of Ru dicarbonyl groups to well-defined Au nanoparticles using a mercaptopropanoate linker under mild conditions. Such compounds may have novel properties because they bridge the gap between isolated dissimilar metal nanoparticles and nanoparticle bimetallic alloy systems. Experimental Section Chemicals. HAuCl4‚3H2O (Alfa Aesar, 99.99%), NaBH4 (Fluka, >96%), MPA (Adrich, 99+%) and Ru3(CO)12 (Adrich, 99%) were used as purchased. All reactions were carried out in either Millipore water, toluene (AR grade) or ethanol (AR grade). Preparation of Sodium Mercaptopropanoate (NaMP)-Capped Au Nanoparticles. The synthesis of NaMP-capped Au nanoparticles was adapted from a previously reported procedure for an MSAcapped species.30 Briefly, under vigorous stirring, 3.60 mL of freshly prepared aqueous NaBH4 (1.0 M) was added dropwise to 30 mL of a water-ethanol mixture containing 180 mg of HAuCl4‚3H2O (1.5 × 10-2 M) and 240 µL of MPA (9 × 10-2 M) in an ice-water bath. After the reduction was complete (solution pH ) 8.3), the flocculent black precipitate that deposited was collected by centrifugation and washed thoroughly with water-ethanol mixtures. It was then dispersed in water to form a red-purple suspension, which was further purified by dialysis against pure water for 6 h to remove residual inorganic ions. The dialyzed suspension was concentrated to ∼5 mL at 45 °C under reduced pressure and then dried by lyophilization followed by evacuation under dynamic vacuum (0.05 Pa) for 24 h. Finally, about 85 mg of NaMP-capped Au nanoparticle powder was obtained, which is readily soluble in water to form a stable red solution. Preparation of MPA-Capped Au Nanoparticles. The attachment of NaMP on the Au nanoparticle surfaces makes them insoluble in organic solvents, which limits their usage to aqueous solutions. To facilitate the reactivity with Ru3(CO)12 and improve the compatibility between the Au nanoparticle surfaces and organic solvents, the capping NaMP species was transformed into acidic form through ion exchange. A 60 mg portion of the as-prepared Au nanoparticles was dispersed in 15 mL of water and then dialyzed against aqueous HCl (0.2 M) for 5 h. The process produced acidified Au nanoparticles, which were capped with MPA. The MPA-capped Au nanoparticles were dialyzed again, but against pure water, to remove excess H+ and Cl- ions. The final aqueous suspension was dried by lyophilization, and about 50 mg of black powder was obtained. (23) Rotem, M.; Shvo, Y. Organometallics 1983, 2, 1689. (24) Dombek, B. D. J. Am. Chem. Soc. 1980, 102, 6855. (25) Frediani, P.; Bianchi, M.; Salvini, A.; Guarducci, R.; Carluccio, L. C.; Piacenti, F. J. Organomet. Chem. 1995, 498, 187. (26) Scott, R. W. J.; Wilson, O. M.; Oh, S.; Kenik, E. A.; Crooks, R. M. J. Am. Chem. Soc. 2004, 126, 15583. (27) Guczi, L. Catal. Today 2005, 101, 53. (28) Stytsenko, V. D. Appl. Catal. A 1995, 126, 1. (29) Hermans, S.; Khimyak, T.; Raja, R.; Sankar, G.; Thomas, J. M.; Johnson, B. F. G. In Nanotechnology in Catalysis; Zhou, B., Hermans, S., Somorjai, G. A., Eds.; Kluwer: New York, 2004; Vol. 1, p 33. (30) Wang, S.; Sato, S.; Kimura, K. Chem. Mater. 2003, 15, 2445.

Wang and Sim Reaction of the MPA-Capped Au Nanoparticles with Ru3(CO)12. 30 mg of the MPA-capped Au nanoparticles were suspended in a solution containing 3 mg of Ru3(CO)12 in 10 mL of toluene (5 × 10-4 M), and the mixture was purged with N2 for 10 min. It was then heated at 80 °C under a N2 atmosphere for between 1 and 10 h, during which the Ru3(CO)12 reacted with the carboxyl groups of the MPA molecules to produce Ru carbonyl carboxylate complexes capping the Au nanoparticles. After the reaction mixture cooled, the product was collected by centrifugation and then purified in toluene by repeating a redispersion-centrifugation process six times to remove unreacted Ru3(CO)12 and any soluble side products. The black product obtained (25 mg) was dried in a vacuum for 12 h. Analysis of the reaction product by infrared spectroscopy showed that the surface functionalization was essentially complete after a reaction time of 8 h (as discussed in the Supporting Information). Instrumentation and Measurement. Infrared spectra of the samples dispersed in KBr pellets were recorded on a Digilab FTS 3000 spectrometer. Raman spectra were collected at room temperature with a Horiba Jobin Yvon micro-Raman spectrometer at an excitation wavelength of 514 nm, using a spot area of approximately 3 × 102 µm2 and a laser power of about 0.4 mW. For each measurement, the powdered sample was mounted on a glass slide to form a film with a thickness of ∼0.5 mm. X-ray photoelectron spectroscopy (XPS) was performed at room temperature in a VG Scientific ESCA MK II spectrometer with Mg KR radiation. The binding energy scale in the XPS spectra was calibrated by assuming the position of the main Au 4f7/2 peak to be at 84.0 eV with respect to the Fermi level, and the XPS peaks were fitted using components with variable Lorentzian-Gaussian line shapes. Powder X-ray diffraction (XRD) patterns were recorded on a Siemens D5005 diffractometer equipped with a Cu KR radiation source operating at 40 kV and 40 mA. Thermogravimetric analysis (TGA) was carried out on a TA Instruments SDT 2960 simultaneous TGA-DTA instrument using pure N2 as the carrier gas with a flow rate of 30 mL/min and a heating rate of 5 °C/min. The elemental compositions of the nanoparticles before and after thermal decomposition were determined using Perkin-Elmer 2400 CHN/Eurovector 3000 elemental analyzers and a Thermo Jarrell Ash IRIS ICP spectrometer. The morphologies of the nanoparticles and selected-area electron diffraction patterns were observed using a JEOL 2010F transmission electron microscope (TEM) operating at an accelerating voltage of 200 kV.

Results and Discussion Vibrational Spectroscopic Characterization. Figure 1A,B shows the infrared spectra of NaMP and MPA-capped Au nanoparticles, respectively. The bands at 1562 and 1397 cm-1 correspond to the asymmetric and symmetric OCO stretching modes of the carboxylate group of NaMP and disappear upon acidification, transforming into the bands at 1700 and 1242 cm-1 corresponding to the CdO and C-O stretching modes of the carboxyl group of MPA.31 The vibrational band frequencies of the MPA-capped Au nanoparticles are in good agreement with those of liquid-phase MPA,31 albeit with much sharper and wellresolved features due to enhanced monolayer ordering on the Au nanoparticle surfaces. The CdO stretching frequency at 1700 cm-1 is red-shifted from that of the monomeric gas-phase species at 1780 cm-1,32 indicating that significant hydrogen bonding exists between the capping MPA molecules,33 which are expected to form dimeric structures in the condensed phase like other carboxylic acids. Figures 1C and 2A,C respectively show the infrared and Raman spectra of the MPA-capped Au nanoparticles after reaction with (31) Ihs, A.; Liedberg, B. J. Colloid Interface Sci. 1991, 144, 282. (32) Stein, S. E. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 2005 (http://webbook.nist.gov). (33) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 2nd ed.; Academic: New York, 1975; pp 301, 303.

Au Nanoparticles in Ru-Complex Shells

Figure 1. Infrared spectra of (A) NaMP-capped Au nanoparticles, (B) MPA-capped Au nanoparticles, (C) Ru dicarbonyl mercaptopropanoate-functionalized Au nanoparticles, and (D) Ru3(CO)12.

Figure 2. Raman spectra of Ru dicarbonyl mercaptopropanoatefunctionalized Au nanoparticles (A and C) and Ru3(CO)12 (B and D).

Ru3(CO)12, whose corresponding spectra (Figures 1D and 2B,D) consist of three groups of bands at 2150-1950, 600-350, and

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200-100 cm-1 that can be assigned to the CO stretching, CO bending/Ru-C stretching, and Ru-C bending/Ru-Ru stretching modes, respectively.34 The vibrational spectra of the Ru-complexfunctionalized Au nanoparticles have clearly inherited modified vibrational features of both reactants. The CO stretching bands for the functionalized Au nanoparticles at 2054, 1987, and 1952 cm-1 (infrared)/2058, 1996, 1955, and 1933 cm-1 (Raman) display a profile that is quite distinct from that of pure Ru3(CO)12, but are in excellent agreement with that of the terminal dicarbonyl groups in a series of µ3-bridging Ru dicarbonyl carboxylate complexes.18,20 These compounds readily form oligomers/polymers with a core structural unit of the form [Ru(RCOO)(CO)2]2, in which the Ru dicarbonyl groups are bridged by carboxylate species, and adjacent [Ru(RCOO)(CO)2]2 units are linked by Ru-O-Ru bridges.18,20 The small CO stretching band at 2120 cm-1 in the infrared spectrum indicates the presence of some Ru(CO)3 end groups capping the Ru dicarbonyl chains.20 The low-frequency vibrational spectra may be assigned with reference to the Raman and high-resolution electron energy loss spectra of ruthenium dicarbonyl carboxylate complexes18 and alkanethiolate-covered Au surfaces,35 respectively. Bands at 313, 283, 185, and 142 cm-1 are likely to arise from Ru-Ru stretching modes in the oligomeric/polymeric structure, while the band at 238 cm-1 is due to the Au-S stretching mode at the nanoparticle surfaces. The bands at 572 and 502 cm-1 (infrared)/570 and 510 cm-1 (Raman) can be ascribed to the Ru-C and Ru-O stretching modes related to the bound carbonyl and carboxylate groups, respectively. When compared to the MPA-capped species, the functionalized Au nanoparticles show characteristic infrared absorption bands at 1559 and 1400 cm-1 from the asymmetric and symmetric OCO stretching modes that confirm the conversion of carboxyl groups back into carboxylate groups in accordance with the expected reactivity.15 Ru dicarbonyl carboxylate complexes are known to form tetramers of the form [Ru(RCOO)(CO)2(RCOOH)]2 in which the parent carboxylic acid molecules are incorporated into the structure through dative and hydrogen bonding of the carbonyl and hydroxyl groups, respectively.20 The band at 1625 cm-1 may be assigned to the CdO stretching mode of such bound MPA molecules whose frequency has been lowered because of the coordination of the lone pair electrons of the carbonyl O atom with Ru.20 The CdO stretching shoulder at 1701 cm-1 arises from unreacted capping MPA molecules that remain despite prolonged reaction times. H-bonding is prevalent in mercaptocarboxylic acid-capped Au nanoparticles,30 and it is likely that, in the medium (toluene) used for this work, some of the MPA-capped species can form H-bonded aggregates whose interiors are not accessible to the Ru3(CO)12 molecules for reaction in solution. Surface Chemical Composition and Electronic Structure. The XPS spectra of all the elements expected in the functionalized Au nanoparticles, including Au, Ru, C, O, and S, are clearly observed, as shown in Figure 3. Their XPS curve profiles are similar to those of the MPA and NaMP-capped species (as shown in the Supporting Information) with additional binding energy components in the O 1s and C 1s regions that can be attributed to the presence of the newly grafted Ru dicarbonyl fragments. The O 1s XPS spectrum can be deconvoluted into two component peaks centered at 533.0 and 531.3 eV that are assigned to the O atoms in the carbonyl (CO)36,37 and carboxylate (COO)38 (34) Quicksall, C. O.; Spiro, T. G. Inorg. Chem. 1968, 7, 2365. (35) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (36) Cai, T.; Song, Z.; Chang, Z.; Liu, G.; Rodriguez, J. A.; Hrbek, J. Surf. Sci. 2003, 538, 76.

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Figure 4. TEM images, electron diffraction patterns, and particle size distributions of (A) MPA-capped Au nanoparticles and (B) Ru dicarbonyl mercaptopropanoate-functionalized Au nanoparticles.

Figure 3. XPS spectra of Ru dicarbonyl mercaptopropanoatefunctionalized Au nanoparticles showing the deconvoluted components in the (A) O 1s, (B) Ru 3p, (C) Ru 3d and C 1s, (D) S 2p, and (E) Au 4f regions.

groups, respectively. Their corresponding C 1s peaks are observed at 287.3 and 288.5 eV,36-39 and the C/O atomic ratios calculated from the sensitivity factor-corrected integrated areas are essentially equal to the predicted 1:1 and 1:2 ratios, respectively. The Ru 3d5/2 and Ru 3d3/2 doublet peaks appearing at 281.5 and 285.7 eV overlap with the C 1s hydrocarbon peak of the mercaptopropanoate backbone at 284.4 eV, while the Ru 3p3/2 and Ru 3p1/2 doublet peaks are observed at 462.6 and 484.4 eV, respectively. The Ru 3d5/2/Ru 3p3/2 binding energies are higher than that of metallic Ru at 279.9-280.2/461.2-461.6 eV,36,39-41 but are comparable to that of Ru3(CO)12 molecules adsorbed on Au(111)36 and Co(0001),37 implying that the Ru atoms in the functionalized Au nanoparticles are in a chemical state that has similar Ru-carbonyl coordination. They also fall within the range of binding energies (281.5-282.0/462.2-463.5 eV) of Ru(II) and Ru(III) compounds,40,41 consistent with the [Ru(RCOO)(CO)2]2 structure in which the Ru atoms are expected to have a net positive oxidation state due to charge transfer to the two types of electron-withdrawing ligands. From the integrated areas, the CO/Ru ratio is calculated to be 2:1, as expected for Ru dicarbonyl groups. The calculated COO/Ru ratio of 1.3:1 is, however, somewhat higher than the 1:1 ratio of the ideal [Ru(37) Vaari, J.; Lahtinen, J.; Hautojarvi, P. Surf. Sci. 1996, 346, 11. (38) Dennis, A. M.; Howard, R. A.; Kadish, K. M.; Bear, J. L.; Brace, J.; Winograd, N. Inorg. Chim. Acta 1980, 44, L139. (39) Chakroune, N.; Viau, G.; Ammar, S.; Poul, L.; Veautier, D.; Chehimi, M. M.; Mangeney, C.; Villain, F.; Fievet, F. Langmuir 2005, 21, 6788. (40) Handbook of X-ray Photoelectron Spectroscopy; Moulder, J. F., Stickle, W. F., Sobol, P. E., Bomben, K. D., Eds.; Physical Electronics Division, PerkinElmer: Eden Prairie, MN, 1992; p 115. (41) Shen, J. Y.; Adnot, A.; Kaliaguine, S. Appl. Surf. Sci. 1991, 51, 47.

(RCOO)(CO)2]2 structural unit. These excess carboxylate groups originate from MPA molecules that are either unreacted or coordinated to Ru atoms, as was observed in the infrared spectra. The Au 4f XPS spectrum can be fitted with a pair of doublet peaks at 84.0 and 87.8 eV corresponding to the 4f7/2 and 4f5/2 levels of metallic Au atoms in the nanoparticle cores, as has been reported for other Au nanoparticles capped with alkanethiolate derivatives.42 The unresolved S 2p3/2 and S 2p1/2 doublet peaks at 162.8 and 164.0 eV are consistent with that of alkanethiolate species bonded to Au nanoparticle surfaces and are distinct from the binding energies (163.5 and 164.8 eV) of the parent thiol group.43 Analysis of the peak areas yields a S/Ru ratio of 0.8:1 as opposed to a value of >1:1 calculated for the Ru complex overlayer with a molecular formula of [Ru(S(CH2)2COO)(CO)2)]2n[S(CH2)2COO)]2. This may be rationalized on the basis of the quasi core-shell structure adopted by the functionalized nanoparticles in which the Au cores are sequentially enveloped by S, alkyl, carboxylate, and finally Ru dicarbonyl layers that form an organometallic-like shell. Photoelectrons originating from the innermost S atoms are thus expected to experience significantly more intensity attenuation compared to the Ru atoms near the outer periphery.44 Particle Size Distribution and Crystalline Structure. Typical TEM images, electron diffraction patterns, and XRD patterns of the nanoparticles synthesized in this work are shown in Figures 4 and 5. From the TEM images, it is clear that the nanoparticles are generally spherical and monodisperse with average particle sizes of 13.4 ( 1.7 and 14.0 ( 2.2 nm, respectively, for the MPA- and Ru-complex-functionalized samples. This is consistent with the corresponding Au core sizes of 17.4 and 17.9 nm estimated by applying Scherrer’s equation on the line widths of the XRD peaks,45 which were fitted with Lorentzian profiles and corrected for instrumental broadening effects using a 0.25-mmthick Au foil as a reference. The negligible difference in the observed particle sizes in the TEM images upon functionalization may be explained by noting that the Ru dicarbonyl mercaptopropanoate groups exist in the form of a monolayer on the surfaces of the Au nanoparticles and (42) Zhang, H. L.; Evans, S. D.; Henderson, J. R.; Miles, R. E.; Shen, T. J. Phys. Chem. B. 2003, 107, 6087. (43) Bensebaa, F.; Zhou, Y.; Deslandes, Y.; Kruus, E.; Ellis, T. H. Surf. Sci. 1998, 405, L472. (44) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (45) Taylor, A. X-ray Metallography; Wiley: New York, 1961; p 280.

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Figure 6. TGA curves of (A) Ru dicarbonyl mercaptopropanoatefunctionalized Au nanoparticles, (B) MPA-capped Au nanoparticles, (C) Ru3(CO)12, and (D) RuxSyOz heated in a N2 atmosphere.

Figure 5. XRD patterns of (A) MPA-capped Au nanoparticles, (B) Ru dicarbonyl mercaptopropanoate-functionalized Au nanoparticles, (C) Ru dicarbonyl mercaptopropanoate-functionalized Au nanoparticles heated to 600 °C in N2, and (D) RuxSyOz heated to 600 °C in N2.

are thus probably too thin to be distinguishable. The presence of this layer, which can act as a protective shell, may be inferred from the TEM images, which show fusing of the nanoparticle boundaries in the case of the MPA-capped species after 12 min of electron beam irradiation (Figure 4A), but not so for the Rucomplex-functionalized nanoparticles under the same conditions (Figure 4B). The characteristic (111), (200), and (220) XRD peaks, together with the electron diffraction patterns for both the MPA-capped and Ru-complex-functionalized nanoparticles are essentially identical and give a measured lattice constant of ∼4.08 Å that can be attributed to the presence of the face-centered cubic structure of crystalline Au. These results suggest that the surface functionalization has changed the molecular structures of the Au nanoparticle peripheries but did not alter the integrity and crystal structures of the metal cores. Thermal Stability. The thermal stability of the functionalized Au nanoparticles under a N2 atmosphere was studied with TGA, and the resulting thermal decomposition behavior is presented in Figure 6, together with that of a number of reference compounds. There are two major weight-loss processes occurring at 150-300 °C and 450-600 °C (Figure 6A). The first weightloss step occurs in the same temperature regime as that of the thermal degradation of MPA-capped Au nanoparticles (Figure 6B) and Ru3(CO)12 (Figure 6C), while the second weight-loss step is similar to that of RuxSyOz decomposition (Figure 6D).46 Elemental analysis after heating to 300 °C indicates that 80% (46) RuxSyOz was prepared by the quantitative stoichiometric precipitation of AgCl from the aqueous reaction between RuCl3 and Ag2SO4 and recovery of the soluble product.

C, 90% S, and 100% H have been lost from the original structure, consistent with the disintegration of the alkanethiolate capping shell around the Au nanoparticles. Ru3(CO)12 is known to convert into higher nuclearity clusters and carbides,47,48 while Ru carboxylates undergo decarboxylation,49 both at around 290300 °C. Heating to 600 °C leaves a residue that is composed mainly of Au with small amounts of Ru but is essentially free of C and S, as revealed by elemental analysis of the thermal decomposition products. The XRD pattern (Figure 5C) shows much sharper and more prominent Au diffraction peaks, indicating sintering of the nanoparticles and formation of a more welldefined crystal structure as a result of the high-temperature annealing and loss of the capping groups. More interestingly, another set of small but clear peaks, which correspond to the major features of the XRD pattern of RuxSyOz that has been heated to 600 °C as shown in Figure 5D, are also discernible. These new XRD peaks can be attributed to diffraction from the (110), (101), and (211) planes of tetragonal RuO250 and indicate the formation of RuxSyOz, which subsequently transforms into RuO2 at >500 °C during the decomposition of the Ru dicarbonyl mercaptopropanoate groups. Surface Chemistry of Ru Dicarbonyl Mercaptopropanoate Formation on Au Nanoparticles. The results from the characterization of the functionalized Au nanoparticles using the range of techniques used are consistent with a structure in which crystalline (from XRD) metallic (from XPS) Au cores approximately 14 nm in size (from TEM) are capped and linked by mercaptopropanoate moieties to Ru dicarbonyl groups (from infrared and Raman spectroscopy). These conclusions are corroborated by the thermal decomposition behavior (from TGA) (47) Psaro, R.; Fusi, A.; Ugo, R.; Basset, J. M.; Smith, A. K.; Hugues, F. J. Mol. Catal. 1980, 7, 511. (48) Eady, C. R.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc., Dalton Trans. 1975, 2606. (49) Rusjan, M. C.; Sileo, E. E.; Cukiernik, F. D. Solid State Ionics 1999, 124, 143. (50) Haines, J.; Leger, J. M.; Schulte, O.; Hull, S. Acta Crystallogr. B 1997, 53, 880.

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Figure 7. Reaction pathways and structures formed during functionalization of Au nanoparticle surfaces by Ru dicarbonyl mercaptopropanoate groups.

and also the expected surface chemistry of the system as depicted in Figure 7. The structure of the NaMP-capped Au nanoparticles is well established, where the mercaptopropanoate ions are bound to the Au cores through the S atoms (Figure 7A).1,2,7,31 The carboxylate groups remain free and are readily protonated in acidic media to become the carboxyl groups of the MPA-capped species (Figure 7B). This transformation is clearly observed in the corresponding infrared spectra. Ru3(CO)12 reacts with free carboxylic acids (RCOOH) in solution to produce various isostructural complexes which have a similar bridging dicarboxylate core unit containing Ru(CO)2[O-CR-O]2-Ru(CO)2 linkages.15-21 The initial product or intermediate of the reaction is a Ru tricarbonyl carboxylate dimer with the formula of [Ru(RCOO)(CO)3]2, whose infrared spectrum shows a strong characteristic absorption band at ∼2110 cm-1, which is assigned to the CO stretching vibrations of Ru(CO)3 groups.20 The Ru tricarbonyl carboxylate dimer is highly reactive, and the terminal carbonyls can be readily substituted by donor ligands such as phosphines, amines, pyridine or acetonitrile to give complexes of the type [Ru(RCOO)(CO)2L]2.15,16,18 The Ru tricarbonyl carboxylate dimer reacts with carboxylic acids to initially yield asymmetric Ru2(RCOO)2(CO)5RCOOH complexes,20 which can subsequently undergo further reaction by

Wang and Sim

losing terminal carbonyl groups to generate symmetric Ru dicarbonyl carboxylate complexes of the form [Ru(RCOO)(CO)2RCOOH]2,20 or give rise to polymer complexes which have repeating [Ru(RCOO)(CO)2]2 units that are connected by RuO-Ru bridges.17,18,20 The ends of the polymer complexes are capped by carboxylic acid molecules.20 These four products from the reaction between Ru3(CO)12 and carboxylic acids exist in equilibrium, and their relative yields are controlled by the reaction conditions used.20 It is reasonable to expect that the present surface functionalization follows an analogous mechanism, which may be described as follows. The initial reaction between Ru3(CO)12 and the carboxyl groups of Au-bound MPA yields Ru tricarbonyl carboxylate dimers with the formula [Ru(S(CH2)2COO)(CO)3)]2 tethered to the nanoparticle surfaces (Figure 7C). In the absence of other donor ligands, the terminal carbonyl groups can be substituted by adjacently bound MPA molecules to form a symmetric Ru dicarbonyl carboxylate species (Figure 7D, n ) 1). The initially formed Ru tricarbonyl carboxylate dimers can also link up by spontaneously losing terminal carbonyl groups to grow into longer chains of repeating [Ru(S(CH2)2COO)(CO)2)]2 units whose ends are eventually capped by MPA molecules (Figure 7D, n > 1). Unlike the homogeneous solution case, however, it is unlikely that long Ru dicarbonyl carboxylate polymer chains are formed here. This is due to a combination of factors that may include the reduced mobility, greater steric hindrance, and higher probability of end-group termination encountered within the confined areas on the Au nanoparticle surfaces for the relatively short MPA molecules when compared with the free species in solution.13 Using the COO/Ru ratio of 1.3 determined from XPS and the bound MPA/unreacted MPA ratio of 1.5 obtained from the relative absorbances of the respective carbonyl infrared absorption bands at 1625 and 1700 cm-1, the value of n in the polymeric structure is estimated to be 6. Thus, the Au nanoparticles can be regarded as being capped by a shell comprising Ru dicarbonyl carboxylate oligomers that are possibly linked by Ru-O-Ru bridges,15,16,18 as illustrated schematically in Figure 7E.

Conclusion We have demonstrated that Ru-complex-capped Au nanoparticles can be successfully synthesized using sequential surface coupling reactions on suitably functionalized monolayer-protected nanoparticle surfaces. Characterization of the product reveals a pseudo-core-shell-like structure in which Au nanospheres are encapsulated in organometallic shells comprising Ru dicarbonyl carboxylate oligomers tethered to the Au surface through alkanethiolate-linkages. These hybrid metal nanoparticles may serve as useful models and potential precursors for Ru-Au bimetallic catalytic systems. Acknowledgment. We acknowledge financial support for this work from the Agency for Science, Technology and Research (A*STAR) under Grant No. R-143-000-198-305. Supporting Information Available: Infrared spectra of the reaction product of MPA-capped Au nanoparticles and Ru3(CO)12 over a range of reaction times and XPS spectra of NaMP- and MPA-capped Au nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. LA060784F