Osmium Carbonyl Clusters on Gold and Silver Nanoparticles as

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Osmium Carbonyl Clusters on Gold and Silver Nanoparticles as Models for Studying the Interaction with the Metallic Surface Chunxiang Li, Wai Yip Fan, and Weng Kee Leong* Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed: July 13, 2009; ReVised Manuscript ReceiVed: September 16, 2009

The surface-cluster interactions between four classes of clusters, viz., (i) those which may be expected to interact directly via the cluster core, (ii) those with a spacer carrying a free thiol functional group, (iii) those with a spacer carrying a free alcohol functional group, and (iv) those with a spacer carrying a free carboxylic acid functional group, with metallic substrate and nanoparticles of silver, and to a lesser extent gold, were examined and compared. In most cases, the clusters interact with both types of surfaces (substrate and nanoparticles) via the tethered functional group. However, cluster fragmentation is commonly observed to occur on the metallic nanoparticle surface. 1. Introduction One of the most important developments in catalysis has been the use of organometallic clusters as precursors for the preparation of nanoparticles, particularly of bimetallic nanoparticles.1 The interactions between the organometallic cluster precursor and the surface, as well as the process through which the cluster is transformed into the metallic nanoparticle, are of interest. For example, our previous studies show that both the support and the functional groups on triosmium clusters will affect how they interact.2 However, the study of such interactions directly on a surface is difficult. We have thus been interested in the preparation of nanoparticles of the surface material, with the appropriate organometallic cluster anchored onto it, to serve as molecular models. Ideally, such a model molecular system should be monodispersed and have suitable spectroscopic handles. Some related examples include that of metallic nanoparticles functionalized with borane clusters,3 transition metal carbonyls such as Cr(CO)4(L) and Re(CO)3(L)Br [L ) 2,3bis(2′-pyridyl)pyrazine],,4 as well as the recent synthesis of silver nanoparticles stabilized by the water-soluble organometallic surfactant [Os3 (CO)10(µ-H){µ-S(CH2)10COO}]Na.5 Nanoparticles, however, can have very different properties from bulk material, as the percentage of interfacial atoms is much higher; for particle size e2 nm, the majority of the atoms are located on the surface. Such nanoparticles are often protected by a monolayer of capping agents, which bear a strong resemblance to self-assembled monolayers in that the selfassembly process is controlled by the chemistry rather than the size of the particles. The monolayers on such nanoparticles simultaneously stabilize the reactive surface of the particle and present organic functional groups at the particle-solvent interface. In particular, monolayer-protected clusters of gold and silver show good stability, tunable solubility, and relative ease of characterization; capping agents used have included alkanethiolates, unsaturated carboxylates, amines, and isocyanates.6 * To whom correspondence should be addressed. Current address: Division of Chemistry & Biological Chemistry, Nanyang Technological University, SPMS-04-01, 21 Nanyang Link, SPMS-CBC-06-07, Singapore 637371. E-mail: [email protected].

In the study presented here, we set out to investigate how the interactions of a set of osmium clusters with the gold and silver surfaces compare with their interactions when used as stabilizing agents for gold and silver nanoparticles. This would allow us to assess if metallic nanoparticles protected by osmium clusters can be used as models for osmium clusters supported on metallic surfaces. 2. Experimental Section 2.1. General Procedures. All the cluster compounds were synthesized using the reported procedures.7 The 200 nm gold layer was coated onto glass slides using electron beam evaporation, while the silver substrate was silver foil from commercial sources. All other reagents were purchased from commercial sources and used as supplied without further purification. UV-vis spectra were recorded using a Shimadzu UV-1601 PC spectrometer. TEM images were recorded on a JEOL JEM 3010 TEM at an accelerating voltage of 300 kV. TEM samples were prepared by placing a drop of the nanoparticles onto a carboncoated Cu grid. Mean particle sizes were obtained by measuring the sizes of the nanoparticles in a few randomly chosen areas of the digitized image, each containing approximately 100-200 nanoparticles. Energy dispersive X-ray (EDX) studies were made with an EDX analyzer attached to the TEM. Voltage and spot size were adjusted to make the counts per second in the range of about 1000. Solid IR spectra were obtained using a Shimadzu Prestige-21 Fourier transform infrared spectrometer. ToF-SIMS spectra were recorded on an ION-TOF SIMS 4 instrument, using bunched 69Ga+ ion pulses with an impact energy of 25 keV. 2.2. Preparation of Cluster-Modified Silver Nanoparticles. Typically, a solution of AgNO3 (0.12 mmol) and the osmium cluster (4.1 µmol) in toluene was photolyzed (tungsten lamp) for about 5 h. The brown precipitate obtained was collected by centrifuge and then washed with toluene and water in turn to get rid of any unreacted cluster and AgNO3, respectively. 2.3. Preparation of Cluster-Modified Gold Nanoparticles. A Nd:YAG pulsed laser (1064 nm, 10 ns, 200 mJ/pulse) was focused onto a piece of gold foil placed in ethanol for 5 min, with continuous stirring. A second irradiation (532 nm, 10 ns, 50 mJ/pulse) was then carried out on the solution itself to induce reaggregation to give a narrow range of nanoparticulate size.

10.1021/jp9066185 CCC: $40.75  2009 American Chemical Society Published on Web 10/06/2009

Osmium Carbonyl Clusters on Au and Ag Nanoparticles Depending on the number of nanoparticles produced, an ethanol solution of the osmium clusters solution (∼20 mL, 1.7 × 10-5 M) was then added to the gold nanoparticle dispersion (24 mL, ∼5 × 10-5 M). After stirring for some time, the yellow solution was centrifuged to afford a yellow supernatant and a black solid. The solid was dried and kept in the dark; it can be redispersed in ethanol for characterization. 2.4. Preparation of Cluster-Modified Silver or Gold Substrates. The gold-plated glass slides were cleaned in ethanol, immersed in a mixture of H2SO4/H2O2 (3:1, v/v) at 398 K for 5 min to remove any organic contaminants, and then rinsed with deionized water before drying in air.8 The silver substrates were cleaned in ethanol, followed by reduction under a flow of H2 gas at 100 °C. The cleaned substrate was then soaked in a dichloromethane solution of the cluster (3 mg in 5 mL) for 1 d, washed with dichloromethane, and then dried in vacuo for 8 h.

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Figure 1. ToF-SIMS spectrum (negative ion mode) of 1-modified silver substrate.

3. Results and Discussion The preparation of gold or silver compounds containing triosmium clusters with heterometallic bonds has been reported by several groups.9 The gold starting materials used are gold phosphine cationic units, and they tend to attach to the osmium core via a µ3 or µ2 bonding mode. When several gold phosphine cationic units are coordinated to the osmium framework, Au-Au bonds may be formed, depending on the distance between the gold phosphine cationic units. It is thus clear that gold atoms show good affinity to the osmium core. Although less is known of osmium-silver clusters, these also show a similar bonding mode.10 It may therefore be expected, a priori, that the interaction of osmium clusters with the metallic gold and silver surfaces would exhibit similarities. On the other hand, we have also found that, with clusters containing a spacer group, the interaction between the cluster and the surface can depend on the nature of the functional group present on the spacer as well as the nature of the surface.2 We have thus examined the surface-cluster interactions of four classes of clusters, viz., (i) those that may be expected to interact directly via the cluster core [Os3(CO)10(µ-H)(µ-OH), 1, and Os3(CO)10(µ-H)2, 2], (ii) those with a spacer carrying a free thiol functional group [Os3(CO)10(µ-H)[µ-S(CH2)2SH], 3; Os3(CO)10(µH)[µ-S(1,3-C6H4)SH], 4; and Os3(CO)10(µ-H)[µ-S(CH2)8SH], 5], (iii) those with a spacer carrying a free alcohol functional group [Os3(CO)10(µ-H)[µ-S(CH2)11OH], 6, and Os3(CO)10(µ-H)[µO(CH2)4OH], 7], and (iv) those with a spacer carrying a free carboxylic acid functional group (Os3(CO)10(µ-H)[µS(CH2)2COOH], 8; Os3(CO)10(µ-H)[µ-S(1,3-C6H4)COOH], 9; and Os3(CO)10(µ-H)[µ-S(CH2)10COOH], 10]. The bulk of our study has been with metallic silver (substrate and nanoparticles); the study of metallic gold was with a more limited set (clusters in groups i and ii). 3.1. Clusters That May Interact Directly via the Cluster Core. Substrate. In an earlier study, we have demonstrated that ToF-SIMS appears to be a good characterization technique for clusters on surfaces; if the surfaces are properly washed, the technique allowed us to affirm that any species detected in the ToF-SIMS spectrum were those of surface-anchored species.11 The association of 1 with the silver substrate was confirmed by the presence of triosmium cluster species in the ToF-SIMS spectrum; the negative ion spectrum showed peaks attributable to the series of molecular ions [Os3(CO)n]- (n e 12) and to fragments having silver atoms, viz., [Os3(CO)11(µ-H)Ag]- (m/z 988), [Os3(CO)10(µ-H)Ag2]- (m/z 1070), and [Os3(CO)10(µH)Ag3]- (m/z 1178) (Figure 1). For 2, however, fragments containing the surface atoms could not be detected; the ToF-

SIMS spectrum of 2 on the silver substrate showed the ionic fragments [Os3(CO)n(µ-O)]- (n e 10) and [Os3(CO)n]- (n e 9). Similar observations were made for the clusters on gold substrates, although it appears to be somewhat easier to observe fragments containing gold atoms. For example, ions attributable to [Os3(CO)10(µ-H)(Au)]- (cluster of peaks at about m/z 1047), Os3(CO)10(µ-H)(O)(Au)]- (m/z 1064), and [Os3(CO)10(µH)(Au)Os3(CO)10(µ-H)]- (m/z 1897) could be clearly observed in the spectrum of 1 on the gold substrate. The IR spectra in the carbonyl region for the various osmium clusters deposited on silver or gold substrates are given in Figure 2. These IR patterns are similar to those for clusters of the general formula Os3(CO)10(µ-H)(µ-X) in the solid state10,12 and suggest that the surface species present in all these are structurally similar and of this form. For 1 on both silver and gold substrates, in particular, there is an additional peak or shoulder at ∼2060 cm-1, but more importantly, the highest frequency band is blue-shifted to >2120 cm-1. This may be indicative of fragmentation of the clusters to mononuclear species on the surface.2,13 Nanoparticles. Silver nanoparticles containing only osmium clusters on the surface and not protected by any other surfactants can be prepared by the photolysis (tungsten lamp) of AgNO3 in a toluene solution of the osmium cluster. These modified silver nanoparticles can be redispersed in water, ethanol, or dichloromethane. In contrast, similar irradiation (24 h) of finely ground AgNO3 in toluene did not show any color change nor an absorption peak in the UV-vis spectrum, indicating that no silver particles were formed in the absence of the osmium clusters. The surface plasmon band energy of nanoparticles is sensitive to the electronic and optical properties of the nanoparticle surface and its protecting monolayer. Uncapped silver colloids in water exhibit an absorption maximum at 390 nm,14 and silver nanoparticles protected by different surfactants show red shifts that vary with the functional group of the surfactant used.15 This red shift also increases with the particle size, with the geometrical shape playing a major role as well.16 The UV-vis absorption spectra (in toluene) of 1 and 2 and of the corresponding cluster-modified silver nanoparticles are shown in Figure 3. While the spectrum of 1 shows a peak at ∼310 nm and another broad peak at 350-450 nm, respectively, that for the 1-modified silver nanoparticles shows a very broad hump at around 330-700 nm, with a maximum at ∼460 nm. Similar red shifts were observed for 2, and this may be attributed to adsorption of the osmium clusters. The apparent absence of

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Figure 2. IR spectra of silver substrate containing clusters 1 and 2 and of gold substrate containing clusters 1 and 2, respectively (a-d).

Figure 3. Schematic representation of the clusters 1-10 and UV-vis spectra (in toluene). Top left: clusters 1 and 2 (a and b) and their corresponding modified nanoparticles (c and d). Top right: clusters 3-5 (a-c) and their corresponding modified silver nanoparticles (d-f). Bottom left: clusters 6 and 7 (a and b) and their corresponding modified silver nanoparticles (a and d). Bottom right: silver nanoparticles modified with (a) 8, (b) 9, and (c) 10.

absorption bands corresponding to the clusters is probably due to its low concentration on the nanoparticle surface. However, the broadness of the absorbance bands varies; in comparison to that for 1, a smooth curve with a maximum at ∼470 nm was

observed for 2. This is indicative of a smaller and more uniform size distribution for 2 compared to 1. The size distributions of the nanoparticles are corroborated by the TEM images. The TEM images of silver nanoparticles

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Figure 4. TEM images of (a) 1-modified silver nanoparticles (inset: enlarged image of elongated silver nanoparticles) and (b) 2-modified silver nanoparticles (inset: enlarged image of an irregularly shaped silver nanoparticles).

modified with 1 show both small (1.1 ( 0.3 nm) and large (12.1 ( 2.6 nm) as well as some irregularly shaped nanoparticles (Figure 4a). Similarly, silver nanoparticles modified with 2 comprises small, uniformly sized, spherical particles (1.4 ( 0.4 nm) together with some irregularly shaped ones (Figure 4b). Lattice spacing analysis of the HRTEM image of one of the irregularly shaped silver nanoparticles shows that the lattice spacing was 0.228 nm, which corresponds to the (111) planes of cubic silver (JCPDS file no. 893722). Closer inspection of some of the TEM images of the larger nanoparticles provides visual evidence for the adsorption of clusters. The insets to Figure 4 show that the nanoparticles have a halo about 1 nm thick; the triosmium clusters have an approximate width of less than 1 nm. The presence of the osmium clusters on the nanoparticles is also confirmed by the EDX results; both small and big nanoparticles for 1 showed the presence of Ag (characteristic peak at 3 keV) and Os (characteristic peaks at 2, 8, 9, 10, and 12 keV). The negative ion ToF-SIMS spectrum of silver nanoparticles protected by 1 showed intense clusters of peaks that could be attributed to the series of ions [Os3(CO)n(µ-H)]- (n e 9) and [Os3(CO)n(µ-H)(O)]- (n e 8); the molecular ion and fragments containing silver atoms were not observed. For 2, other than the two series of ions [Os3(CO)n(µ-H)]- (n e 10) and [Os3(CO)n(µH)(O)]- (n e 9), the fragment [Os3(CO)10Ag]- was also detected. As for the metal substrates, the IR spectra in the carbonyl region for the osmium cluster-modified nanoparticles showed carbonyl stretching vibrations at about 2122 (m), 2030 (s), and 1947 (m) cm-1. The essentially identical patterns and band maxima indicate that the surface species are similar. The patterns are also similar to those of triosmium cluster species on the ZnO surface,2 as well as those for the metal substrates above, and suggest significant fragmentation of the osmium cluster core on the surface. Gold nanoparticles not stabilized by surfactants can be prepared by laser ablation in ethanol;17 the concentration was estimated to be 5 × 10-5 M (based on the number of gold atoms), by comparison of the plasmon band intensity to that from nanoparticles produced by chemical reduction.18 The modification of the gold nanoparticles as it was titrated with a solution of 1 in ethanol was monitored by in situ UV-vis absorption spectroscopy (Figure 5). A blue shift of the 574 nm peak to 555 nm after the initial addition of 1 was observed; the peak did not shift significantly upon further addition of 1. A similar blue shift to 533 nm was observed with 2. The observed blue shifts suggest interaction between the osmium clusters and Au atoms,19 and the absence of precipitation during the titrations indicates that both clusters are good surfactants for the stabilization of gold nanoparticles.

Figure 5. Absorbance spectra (in ethanol) (a) of gold nanoparticles, (b) of cluster 1, and (c-e) upon addition of 1 to the gold nanoparticles in 0.1 equiv increments.

Figure 6. Infrared spectra (KBr disks) for gold nanoparticles modified with clusters (a) 1 and (b) 2.

There is a difference in the magnitude of the blue shifts observed for 1 and 2. That this is not due to a difference in size of the nanoparticles can be ruled out since the TEM images for the cluster-modified gold nanoparticles showed spherical particles with average sizes of 8.9 ( 1.6 and 8.4 ( 2.2 nm, for 1 and 2, respectively; the EDX spectra served to confirm the presence of both osmium and gold in these particles. Instead, we conjecture that it may reflect a difference in the interaction between the two clusters and the nanoparticle surface. That the interactions are different is corroborated by their IR spectra in the carbonyl region (Figure 6); while that for 1 shows a pattern (2125, 2034, and 1952 cm-1) similar to that for the metal substrates above, the vibrational bands for 2 (2072, 2018, and 1938 cm-1) are more similar (with some overlapping of peaks) to those for the cluster Os3(CO)10(µ-AuPPh3)2 [2067 (w), 2012 (s), 1977 (m), 1965 (w), 1937 (m)].9g The presence of trinuclear cluster species on the surface in both samples was indicated by the negative ion ToF-SIMS spectra, which showed main peaks that can be attributed to the series [Os3(CO)n(µ-H)]- (n e 9) and [Os3(CO)n(µ-H)(O)]- (n e 8). It is thus possible that the two hydride ligands in 2 may have been replaced by isolobal gold atoms.10 3.2. Clusters with a Spacer Carrying a Free Thiol Group. Sulfur compounds have a strong affinity for transition metal surfaces,20 and this has been exploited in self-assembled

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Li et al. TABLE 1: Solid-State IR Spectral Data for Cluster-Modified Silver Nanoparticles (Ag NPs) and the Corresponding Cluster Precursors

Figure 7. TEM images of silver nanoparticles modified with cluster (a) 3 (inset: enlarged image of an irregularly shaped silver nanoparticle), (b) 4 (inset: image of an aggregate), and (c) 5 (the mean particle size of 5.2 ( 1.2 nm is of the particles around the aggregates). 21

monolayers (SAMs) of thiols on metallic or semiconductor surfaces.22 Alkanethiol SAMs on gold surfaces are probably the most examined and have served as models for self-assembly in general.23 The clusters 3-5 are potentially organometallic analogues of alkanethiols. Nanoparticles. The UV-vis absorption spectra of silver nanoparticles modified with clusters 3-5 exhibit distinct plasmon bands centered around 460, 540, and 570 nm, respectively (Figure 3). The absorption bands for nonanethiol-, octanethioland dodecanethiol-capped silver nanoparticles have been reported to be at 436, 420, and 420 nm, respectively,24 compared to 390 nm for uncapped silver nanoparticles. The large red shifts observed here therefore suggest that the interaction is not merely between the thiolate group and the silver nanoparticles. A second feature of the absorption spectra is the rather weak intensity for the 3-modified silver nanoparticles; only a slight hump is observed. This may be possible if most of the particles obtained are very small and only some of the bigger particles exhibit absorption. Indeed, the TEM images show that it comprises mainly small, spherical particles (1.7 ( 0.4 nm) and some irregularly shaped particles with a shell ∼1 nm thick (Figure 7a); the shells can be attributed to a monolayer of 3. In contrast, the average sizes for 4- and 5-modified silver nanoparticles are 2.7 ( 1.6 and 5.2 ( 1.2 nm, respectively, which should also be compared to the 2120 cm-1, which is probably indicative of fragmentation. This sign of fragmentation appears to be absent for 4-modified silver nanoparticles. For gold nanoparticles, the titration of ethanolic solutions of the clusters against a solution of gold nanoparticles generated photochemically in ethanol was followed photometrically. The behavior among the clusters was very different. For 3, there was immediate precipitation of some black solids, and the absorption bands at 320 and 390 nm (due to 3) increased in intensity, while that at 540 nm (due to gold nanoparticles) decreased in intensity, was red-shifted, and became broader (Figure 8). These observations indicate that 3-modified gold nanoparticles are not stable and that the size of particles increases, leading to precipitation; TEM images of the precipitate show that it consists of spherical particles with a size of 9.3 ( 2.7 nm. For 4-modified gold nanoparticles, the 538 nm band was red-shifted to 550 nm, but there was no precipitate. The red shift indicates that the size of the gold nanoparticles has increased, demonstrating that a triosmium cluster with a free aromatic thiol can stabilize gold nanoparticles.29 This is also verified by the TEM images, which show particle sizes of 9.0 ( 2.0 nm, compared to 8.4 ( 2.5 nm for naked gold nanoparticles. In contrast, 5-modified gold nanoparticles showed an increase in intensity of the bands at 320 and 390 nm (due to 5) while the 540 nm band remained the same throughout the titration; no precipitation was observed. This observation is consistent with earlier reports that long chain thiols increase the stability of capped nanoparticles; the TEM images show spherical particles of 8.5 ( 2.3 nm in size.

Figure 8. UV-vis absorption spectrum as 3 is titrated against gold nanoparticles. Each addition is of 0.2 mmol of 3 (in ethanol) into a solution of gold nanoparticles in ethanol (20 mL of 5 × 10-5 M).

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Figure 10. TEM image of 7-modified Ag NPs.

Figure 9. FTIR spectra of (a) 3-, (b) 4-, and (c) 5-modified gold nanoparticles.

The TEM images also show that the modified gold nanoparticles are surrounded by a shell (see Supporting Information, Figure SD2); the presence of osmium, gold, and sulfur is verified by the EDX spectra. The ToF-SIMS spectra of all the clustermodified gold nanoparticles show a series of ions assignable to [Os3(CO)n(µ-H)(µ-S)]+ (n e 9), and the IR spectra for 3- and 5-modified gold nanoparticles are similar to those of the parent cluster (Figure 9), pointing to interaction via an Au-S bond. The IR spectrum of the 4-modified gold nanoparticles is, on the other hand, different [2127 (m), 2054 (s), 2000 (s) and 1927 (m) cm-1] and suggestive of extensive fragmentation of the cluster core; a shoulder on the lower frequency side of the 2127 cm-1 peak suggests that a species with an Au-S bond is also present. Surface. For silver or gold substrates modified with clusters 3-5, the ToF-SIMS spectra all show the series of fragments [Os3(CO)n(µ-H)(S)]- (n ) 1-10). In most cases, especially for gold, the molecular ion as well as fragments containing metal surface atoms can also be detected. For instance, the spectra of the3-modifiedsubstratesshowionscorrespondingto[{Os3(CO)9(µH)S}Ag{SOs3(CO)9}]- (m/z 1820) for the silver surface and [Os3(CO)10(µ-H)(µ-SCH2CH2SAu)]- (m/z1141)and[{Os3(CO)9(µH)S}2Au]- (m/z 1909) for the gold surface. Although species such as [{Os3(CO)9(µ-H)S}Ag{SOs3(CO)9}]- or [{Os3(CO)9(µH)S}2Au]- may be attributed to rearrangement of the anchored species upon bombardment with the Ga ions, the detection of these ions implies that the triosmium cluster maintains its structural integrity and that the interaction with the silver or gold substrate is via the sulfur atom. The IR spectra in the carbonyl region for these modified substrates [typically, 2108 (w), 2070 (s), 2059 (m, sh), and 2025 (s) cm-1] also point to mainly intact clusters. 3.3. Clusters with a Spacer Carrying a Free Alcohol Group. For the clusters 6 and 7, which have a spacer carrying a free alcohol functional group, studies were carried out only with silver nanoparticles and substrate. The TEM images of silver nanoparticles modified with 6 show a broad range of sizes and shapes, from spherical particles (1.2 ( 0.2 nm) to some bigger particles (∼30 nm) surrounded by a layer of amorphous material, as well as multilayered planar structures up to several hundred nanometers in size. That for silver nanoparticles modified with 7 showed cube-shaped particles, as well as some irregular and spherical particles (Figure 10). The cubic particles were approximately 50 nm in size. Presumably, 7 acted as an orienting protecting agent,30d and the particles may be viewed as superlattices of the face-centered cubic structure of silver.30 The UV absorption spectra of both nanoparticles are consistent

with the broad range of shapes and sizes, with a broad absorption band extending from about 400 to 800 nm for 6-modified Ag NPs and a similar broad absorption band starting from about 300 nm, for 7-modified Ag NPs, respectively (Figure 3). Evidence of triosmium cluster moieties in both modified nanoparticles is apparent from the ToF-SIMS spectra; that for 6 showed clusters of peaks assignable to triosmium carbonyl fragments in the 800-900 mass range, while a cluster of peaks assignable to the fragment [Os3(CO)9(µ-H)(µ-O)]- was discernible for 7. The νCO pattern for 6 [2123 (w), 2111 (w), 2058 (s), 2024 (s), 1994 (s) cm-1] is consistent with bonding of the cluster to the silver nanoparticles via the OH group and some fragmentation, but that for 7 [2108 (w), 2057 (s), 2019 (s), 1973 (w, br)] showed only the former interaction. For the silver surface with 6 anchored, the ToF-SIMS spectrumshowedfragmentscorrespondingtotheseries[Os3(CO)n(µH)(µ-S)]+ (n ) 1-10, parent ion at m/z 885) and the ion [Os3(CO)10(µ-H){µ-S(CH2)11OAg}]- (m/z 1105). For the silver surface with 7, the series of fragments corresponding to [Os3(CO)n(µ-H)]- (n ) 1-10, parent ion at m/z 853) and [Os3(CO)n(µ-H)(µ-O)]- (n ) 1-6, parent ion at m/z 757) were observed. The presence of CH2 stretches in the IR spectrum of the 6-modified Ag substrate confirmed that the alkyl chain was still intact. However, the CO stretching bands [2063 (s), 2022 (s), 1954 (m), and 1884 (s) cm-1 for 6; 2067 (s), 2022 (m), 1889 (s), and 1804 (s) cm-1 for 7] are unusual and not readily accounted for, which does point to a very different interaction with the surface compared to nanoparticles. 3.4. Clusters with a Spacer Carrying a Free Carboxylic Acid Group. The UV-vis spectra of silver nanoparticles modified with the clusters 8-10 show adsorption bands that are red-shifted by 110-140 nm from that of the uncapped nanoparticles, providing clear evidence for the adsorption of osmium clusters on the nanoparticles (Figure 3).14,31 The presence of osmium clusters on the silver nanoparticles was also confirmed by the EDX spectra, which indicated the presence of sulfur, silver, and osmium. Tri- and diosmium species can be observed in the ToF-SIMS spectra of 9- and 10-modified silver nanoparticles, respectively, but severe overlapping of the peaks prevented further identification of the surface species. The TEM images show spherical particles with average sizes of 2.9 ( 0.6 and 37 ( 11 nm, for 9- and 10-modified silver nanoparticles, respectively, with aggregation into larger particles. For the silver nanoparticles containing 8, however, there are many different shapes; there is a small amount of rod-shaped nanoparticles, but the most abundant are large, network-like structures made up of aggregates of plates (Figure 11). There are ∼1 nm gaps between these nanoplates, consistent with the presence of a layer of the cluster; the molecular length of 8 is ∼1.2 nm. Presumably, neighboring nanoplates are connected by the osmium clusters; the triosmium core is bound to the silver surface of one nanoplate, and its carboxylic acid group is bound to that of another nanoplate.32 The IR spectra of all the cluster-modified silver nanoparticles are consistent with the presence of such an interaction. For

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Figure 11. TEM images of 8-modified silver nanoparticles: (a) network-like Ag structures (inset: enlarged image of modified Ag NPs surrounding by a layer of amorphous material), (b) enlarged image of network-like structures, and (c) rod-shaped 8-modified silver nanoparticles.

example, the VCdO band at 1704 cm-1 in 8 is shifted to 1558 cm-1 in the nanoparticles, with another band at 1380 cm-1, which is indicative of a monodentate carboxylato binding;33 the VCO (2119, 2033, and 1942 cm-1) is indicative of some cluster fragmentation. The presence of the clusters on the silver substrate surface is clearly indicated in the negative ion ToF-SIMS spectra of substrates containing cluster 8-10. Cluster of peaks ascribable to [Os3(CO)n(µ-H)(µ-S)]- (n ) 1-10, parent ion at m/z 885) are observable and, for 8 and 9, fragments such as [Os3(CO)10(µS-COOH)]- and [Os3(CO)9(µ-H){µ-S-COOAg}]- as well. The IR spectrum for the silver substrate with 8 is different from that for the other two; the CO stretching pattern [2120 (m), 2032 (s), 1958 (w) cm-1] is consistent with fragmentation of the cluster core; the COO- stretching band at 1726 cm-1, which is slightly shifted from the 1704 cm-1 value for the cluster 8 itself, suggests the presence of an uncoordinated -COOH group. For 9 and 10, this VCdO band is red-shifted by more than 100 cm-1, while the characteristic carbonyl stretches showed slight (up to about 8 cm-1) or no blue shifts; they interact with the silver surface primarily via the carboxylate group. 4. Concluding Remarks Our study suggests that the structure of the osmium cluster species deposited onto silver or gold nanoparticles do not always correspond to that for the corresponding metallic substrate. The mode of interaction between the cluster and the surface depends on the nature of the functional group used to bind to the surface and the identity and nature (nanoparticles or substrate) of the surface. In most cases, the clusters tend to interact with the metal surface via the functional group, viz., -SH, -OH, or -COOH. However, for nanoparticles, fragmentation of the osmium cluster core appears to be fairly common and is probably a result of their higher surface energy. Furthermore, it is also clear that there is no one characterization method that will offer a precise descriptor of the nature of the interactions. The picture that emerges is that the interactions are usually rather complex and that each characterization method at best favors observation of a particular species or type of interaction. Acknowledgment. This work was supported by an A*STAR grant (Research Grant No. 022 109 0061) and one of us (C.L.) thanks the University for a Research Scholarship. Supporting Information Available: ToF-SIMS, solid-state IR, absorption and EDX spectra, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgley, P. A. Acc. Chem. Res. 2003, 36, 20.

Li et al. (2) Li, C.; Leong, W. K. J. Colloid Interface Sci. 2008, 328, 29. (3) Schmid, G.; Pugin, R.; Meyer-Zaika, W.; Simon, U. Eur. J. Inorg. Chem. 1999, 2051. (4) (a) Tan, H.; Wong, L.; Lai, M. Y.; Kiruba, G. S. M.; Leong, W. K.; Wong, M. W.; Fan, W. Y. J. Phys. Chem. B 2005, 109, 19657. (b) Vlckova, B.; Matejka, P.; van Outersterp, J. W. M.; Snoeck, Th. L.; Stufkens, D. J. Inorg Chem 1994, 33, 2132. (5) Ahmed, M. O. E.; Leong, W. K. J. Organomet. Chem. 2006, 691, 1055. (6) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (b) Manna, A.; Imae, T.; Iida, M.; Hisamatsu, N. Langmuir 2001, 17, 6000. (c) Wang, W.; Chen, X.; Efrima, S. J. Phys. Chem. B 1999, 103, 7238. (7) (a) Nicholls, J. N.; Vargas, M. D. Inorg. Synth. 1990, 28, 232. (b) Kaesz, H. D. Inorg. Synth. 1989, 28, 238. (c) Roberto, D.; Lucenti, E.; Roveda, C.; Ugo, R. Organometallics 1997, 16, 5974. (d) Hanif, K. M.; Kabir, S. E.; Mottalib, M. A.; Hursthouse, M. B.; Malik, K. M. A.; Rosenberg, E. Polyhedron 2000, 19, 1073. (e) Li, C.; Leong, W. K. J. Organomet. Chem. 2008, 693, 1292. (f) Ainscough, E. W.; Brodie, A. M.; Coll, R. K.; Coombridge, B. A.; Waters, J. M. J. Organomet. Chem. 1998, 556 (1-2), 197. (8) O’Dwyer, C.; Lesegno, B. V. D.; Weiner, J. Langmuir 2004, 20, 8172. (9) (a) Molecular Clusters; Lewis, J., Raithby, P. R., Eds.; Wiley-VCH: New York, 1999; Vol. 1, p 348. (b) Drake, S. R.; Johnson, B. F. G.; Lewis, J. Chem. Soc., Dalton Trans. 1989, 505. (c) Johnson, B. F. G.; Khattar, R.; Lewis, J.; Raithby, P. R. Chem. Soc., Dalton Trans. 1989, 1421. (d) Bradford, C. W.; Bronswijk, W. V.; Clark, R. J. H.; Nyholm, R. S. Inorg. Phys. Theor. 1970, 2889. (e) Johnson, B. F. G.; Kaner, D. A.; Lewis, J.; Raithby, P. R.; Taylor, M. J. Polyhedron 1982, 1, 105. (f) Li, Y.; Wong, W.-T. Eur. J. Inorg. Chem. 2003, 2651. (g) Burgess, K.; Johnson, B. F. G.; Kaner, D. A.; Lewis, J.; Raithby, P. R.; Azman, S. N.; Mustaffa, B. S. J. Chem. Soc., Chem. Commun. 1983, 455. (h) Li, Y.; Pan, W.-X.; Wong, W.-T. J. Cluster Sci. 2002, 13 (2), 223. (i) Akhter, Z.; Gallagher, J. F.; Lewis, J.; Raithby, P. R.; Shields, G. P. J. Organomet. Chem. 2002, 231237, 614. (10) Bruce, M. I.; Horn, E.; Matisons, J. G.; Snows, M. R. J. Organomet. Chem. 1985, 286, 271. (11) Li, C.; Lai, M. Y. D.; Leong, W. K. J. Organomet. Chem. 2005, 690, 3861. (12) (a) Johnson, B. F. G.; Kaner, D. A.; Lewis, J.; Raithby, P. R. J. Organomet. Chem. 1981, 215, C33. (b) Burgess, K.; White, R. P. Inorg. Synth. 1990, 27, 209. (13) (a) Bhirud, V. A.; Iddir, H.; Browning, N. D.; Gates, B. C. J. Phys. Chem. B 2005, 109, 12738. (b) Psaro, R.; Dossi, C.; Ugo, R. J. Mol. Catal. 1983, 21, 331. (c) Psaro, R.; Ugo, R.; Zanderighi, G. M. J. Organomet. Chem. 1981, 213, 251. (d) Deeba, M.; Gates, B. C. J. Catal. 1981, 67, 303. (14) (a) Van Hyning, D. L.; Klemperer, W. G.; Zukoski, C. F. Langmuir 2001, 17, 3120. (b) He, S.; Yao, J.; Jiang, P.; Shi, D.; Zhang, D.; Xie, S.; Pang, S.; Gao, H. Langmuir 2001, 17, 1571. (15) (a) Chen, S.; Fan, Z.; Carroll, D. J. Phys. Chem. B 2002, 106, 10778. (b) Zhao, S.; Chen, S.; Li, D.; Ma, H. Colloid Surf. A 2004, 242, 145. (c) Chaki, N. K.; Sudrik, S. G.; Sonawane, H. R.; Vijayamohanan, K. Chem. Commun. 2002, 76. (16) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 16, 6755. (17) Simakin, A. V.; Voronov, V. V.; Kirichenko, N. A.; Shafeev, G. A. Appl. Phys. A: Mater. Sci. Process. 2004, 79 (4-6), 1127–1132. (18) Hu, J.; Zhang, Y.; Liu, B.; Liu, J.; Zhou, H.; Xu, Y.; Jiang, Y.; Yang, Z.; Tian, Z. J. Am. Chem. Soc. 2004, 126, 9470. (19) Kudanalli, D. J. Synthesis and Characterization of CdSe and In2S3 Nanoparticles and Their Surface Modification Using Ru(bipy)2Cl2 Complex; University Microfilms International, Oklahoma State University, 2002. (20) (a) Dubois, L. H.; Nuzzo, R. G. Ann. Phys. Chem. 1992, 43, 437. (b) Ulman, A. J. Mater. Educ. 1989, 11, 205. (21) (a) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (b) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (c) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (22) (a) Yan, C.; Zharnikov, M.; Golzhauser, A.; Grunze, M. Langmuir 2000, 16, 6208. (b) Sheen, C. W.; Shi, J.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (23) Muller-Meskamp, L.; Lussrm, B.; Karthauser, S.; Waser, R. J. Phys. Chem. 2005, 109, 11424. (24) (a) He, S.; Yao, J.; Jiang, P.; Shi, D.; Zhang, H.; Xie, S.; Pang, S.; Gao, H. Langmuir 2001, 17, 1751. (b) Rosermary, M. J.; Pradeep, T. J. Colloid Interface Sci. 2003, 268, 81. (25) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (26) Liang, G. L.; Noid, D. W.; Sumpter, B. G.; Wunderlich, B. J. Phys. Chem. 1994, 98, 11739.

Osmium Carbonyl Clusters on Au and Ag Nanoparticles (27) Chi, Y. S.; Jung, Y. H.; Choi, I. S.; Kim, Y.-G. Langmuir 2005, 21 (10), 4669. (28) Shaffer, A. W.; Worden, J. G.; Huo, Q. Langmuir 2004, 20, 8343. (29) Price, R. C.; Whetten, R. L. J. Am. Chem. Soc. 2005, 127, 13750. (30) (a) Motte, L.; Pileni, M. P. Appl. Surf. Sci. 2000, 164, 60. (b) Jana, N. R.; Wang, Z. L.; Sau, T. K.; Pal, T. Curr. Sci. 2000, 79, 1367. (c) Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382. (d) Li, D.-G.; Chen, S.-H.; Zhao, S.-Y.; Hou, X.-M.; Ma, H.-Y.; Yang, X.-G. Thin Solid Films 2004, 460, 78.

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18569 (31) Charle, K. P.; Schulze, W. Ber.Bunsen-Ges. Phys. Chem. 1984, 88, 305. (32) (a) Chen, S.; Fan, Z.; Carroll, D. L. J. Phys. Chem. B 2002, 106, 10777. (b) Chen, S.; Carroll, D. L. Nano Lett. 2002, 2, 1003. (c) Chen, S.; Carroll, D. L. J. Phys. Chem. B 2004, 108, 5500. (33) Li, S.-Y.; Tsai, T.-K.; Lin, C.-M.; Chen, C.-H. Langmuir 2002, 18, 5473.

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