Organometallic clusters as precursors of metallic nanoparticles 4

Chapter 3 Interaction of osmium clusters with metallic nanoparticles and surfaces One aspect of nanoparticles NPs that is different from bulk materials is the percentage of atoms that a

Chapter 3 Interaction of osmium clusters with metallic nanoparticles and surfaces

One aspect of nanoparticles (NPs) that is different from bulk materials is the percentage of atoms that are interfacial For nanoparticles smaller than 2 nm, the majority of atoms are located at the interface between the particle and the surrounding environment, that is, at the surface For this reason, there is a strong resemblance between nanoparticles and self-assembled monolayers (SAMs) Since SAMs form by self-assembly, it does not matter what the size of the particles are, i.e., chemistry controls the process The structure of SAMs, however, differs greatly depending on the curvature of a given surface SAMs on nanoparticles simultaneously stabilize the reactive surface of the particle and present organic functional groups at the particle-solvent interface In particular, monolayer-protected clusters (MPCs) of gold and silver show good stability, tunable solubility and relative ease of characterization; capping reagents for such MPCs have included alkanethiolates, unsaturated carboxylates, amines and isocyanates.1

The results in the previous chapter show that both the support and the functional groups on the triosmium clusters will affect how they interact Furthermore, such interactions are difficult to study We have thus been interested in the preparation of nanoparticles with the appropriate substrates 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,2 transition metal carbonyls such as Cr(CO)4(L)

and Re(CO)3(L)Br (L = 2,3-bis(2'-pyridyl)pyrazine),3 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.4

In this chapter, a set of osmium clusters were deposited onto gold or silver substrates, or used as stabilizing agents for gold or silver NPs We hope that a comparison can be made to examine if the NPs protected by osmium clusters can be used as models for the supported osmium clusters on gold or silver surfaces

3.1 Anchoring of osmium clusters onto gold or silver

The preparation of gold or silver compounds containing triosmium clusters with heterometallic bonds have been reported by many groups.5 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 obvious that the gold atom shows good affinity to the osmium core in all these osmium-gold clusters Osmium-silver clusters also show a similar bonding mode.6 In this section, the deposition of the osmium clusters Os3(CO)10(μ-H)(μ-OH), 1, Os3(CO)10(μ-H)2, 22 and

Os3(CO)10(μ-H)(μ-SCOPh), 33 onto a gold or silver substrate are compared with their

deposition onto Au or Ag NPs

3.1.1 Interaction of osmium clusters with gold or silver nanoparticles

Silver nanoparticles

In order to avoid interference by surfactants, Ag NPs were prepared by the photolysis of AgNO3 in a toluene solution of osmium clusters without any other surfactants The modified Ag NPs can be redispersed in ethanol, water and dichloromethane The surface plasmon (SP) band energy of NPs is sensitive to the electronic and optical properties of the particles’ surfaces and of the protecting monolayers Uncapped silver colloids in water exhibit an absorption maximum at 390

nm,7 and Ag NPs protected by different surfactants show red shifts which depend on the functional group of the surfactant used.8 This red shift increases with the particle size, and the geometrical shape plays a major role in determining the plasmon resonance.9

The UV- vis absorption spectra of 1, 22, 33 and their corresponding modified Ag NPs in toluene are shown in Figure 3.1 The absorption band of 1 in toluene shows a

peak at 308 nm, and another broad peak at 356 - 445 nm, respectively In contrast, the

UV absorption band of the 1-modified Ag NPs shows a very broad hump at around

333 nm to 700 nm, with a maximum at 463 nm This red shift of the absorption band

of 1-modified Ag NPs can be attributed to the adsorption of osmium clusters It is

possible that the broad UV absorbance peak of the Ag NPs was due to a large size or

shape distribution of the Ag NPs formed However, the peak corresponding to 1

cannot be detected, possibly due to the small amount of osmium clusters on the Ag NPs A control experiment in which finely ground AgNO3 in toluene was irradiated

for 24 h with a tungsten lamp did not show any colour change nor an absorption peak

in the UV-vis spectrum, indicating that no silver particles were formed in the absence

of the osmium clusters Thus, 1 induced the formation of Ag NPs The formation of the silver colloids may be attributed to the visible light induced photooxidation of 1

by excited Ag+, resulting the formation of Ag atoms (Ag0) Subsequent agglomeration

of Ag0 produced silver colloids

Wavelength (nm)

a b

c

d e f

Figure 3.1 Absorption spectra of (a) 1-modified Ag NPs, (b) 22-modified Ag NPs, (c) 33-modified Ag NPs and (d-f) the corresponding precursor clusters

Similar results were observed for clusters 22 and 33 except that for the former, the

UV-vis spectrum exhibited a smooth curve with a maximum at 466 nm, while the latter showed a broad peak centered at 523 nm This indicated that the size of the

former Ag NPs were smaller and more uniform than the 1-modified Ag NPs, while the

latter are the largest Thus, it is clear that different osmium clusters lead to different

sizes of Ag NPs, although the factors are unclear

Figure 3.2 shows TEM images of the modified Ag NPs Small spherical particles (1.1 ± 0.3 nm) together with some big particles (12.1 nm ± 2.6 nm) and some

irregularly shaped NPs were observed in the TEM image of 1-modified Ag NPs

(Figure 3.2a) Closer inspection of some of the TEM images of the larger particles provide visual evidence for the adsorption of cluster The inset of Figure 3.2a shows elongated Ag NPs with a layer of about 1 nm thickness surrounding them Since surfactants were not used in the preparation of the NPs, the presence of these shells is proposed to be a multilayer of clusters adsorbed on the silver NPs, as the triosmium cluster has an approximate width of less than 1 nm EDX results from both small and big particles show the presence of Ag (characteristic peaks at 3 keV) and Os

(characteristic peaks at 2, 8, 9, 10 and 12 keV), thus confirming the association of 1

with the Ag particles (Figure 3.3)

Figure 3.3 EDX for Ag NPs protected by 1

Similarly, the TEM image of 22-modified Ag NPs (Figure 3.2b) shows small,

uniformly sized, spherical particles (1.4 ± 0.4 nm) together with some irregularly shaped NPs The inset shows an irregularly shaped Ag NP with a layer of about 1 nm thickness surrounding it, suggesting again the presence of a multilayer of clusters

adsorbed on the Ag NPs The TEM image of 33-modified Ag NPs (Figure 3.2c) shows

that spherical particles with average size about 10.0 nm (± 2.8 nm), together with some bigger particles (~20-50 nm), were obtained EDX analyses of both modified Ag

NPs also show the presence of osmium and silver, and for 33, sulfur as well Thus the 33-modified Ag NPs are bigger than those modified with 1 or 22, and the cluster used

influences the size of Ag NPs formed It is possible that the association of Ag NPs

with 22 is faster than with 1 or 33, thus accounting for the smaller size observed with 22-modified Ag NPs Presumably, the rate association of Ag NPs with clusters is

generally very fast, hence accounting for the small size of most of the Ag NPs obtained

As indicated in the earlier chapter, ToF-SIMS appears to be a good characterization

technique for clusters on surfaces When Os3(CO)12, 3(Os) orRu3(CO)12, 3(Ru) was

coated onto a gold or silver substrate, the molecular ion and its attendant fragmentation pattern was observed in the ToF-SIMS spectrum However, if the substrate was washed with dichloromethane, no signal attributable to the parent clusters or their fragments was detected, thus allowing us to affirm that any species detected in the ToF-SIMS spectrum of cluster-modified Ag NPs were not surface anchored species

The negative ion ToF-SIMS spectrum of Ag NPs protected by 1 is shown in Figure

3.4a The most intense cluster of peaks could be attributed to the series [Os3(CO)x(μ-H)]- (x ≦ 9)and [Os3(CO)x(μ-H)(μ-O)]- (x ≦ 8) The molecular ion,

or fragments, containing silver atoms were not observed The intensities were quite low due to a low concentration of osmium clusters on the NPs However, the presence

of peaks that could be attributed to a triosmium moiety indicated that 1 was anchored onto the Ag NPs successfully For the 22-modified Ag NPs, other than the two series

of ions [Os3(CO)x(μ-H)]- (x ≦10)and [Os3(CO)x(μ-H)(μ-O)]- (x ≦ 9), the fragment [Os3(CO)10Ag]- was also detected (Figure 3.4b) This showed that the osmium clusters were possibly bonded to the Ag NPs

700 800 900 0

200 400

813 785

b

Mass (m/z)

[Os3(CO)8(μ-OH)]

-[Os3(CO)10Ag]

-813 785 771

757 741

729 701 673

-CO

713 685

-CO

-CO -CO

796

-CO -CO -CO

829

-CO

Figure 3.4 ToF-SIMS spectra (negative ion mode) of Ag NPs modified with 1 (a) and

22 (b)

The appearance of the ToF-SIMS spectrum of 33-modified Ag NPs is very different

(Figure 3.5) Peaks corresponding to triosmium clusters were not observed Instead, intense peaks at m/z = 192, 137, 105, 77 and 28, assignable to Os, SCOPh, COPh, Ph and CO, respectively, were observed An intense peak assignable to Ag was also

observed Careful examination of the spectrum showed that the highest mass fragment could be found at m/z = 616, which may be assignable to a diosmium moiety

Os2(CO)6S2H2, on the basis of the isotopic pattern This may be due to a low level of

clusters on the modified Ag NPs; other evidence (see later) suggest that 33 was

deposited as a trinuclear species thus ruling out decomposition on the surface

COPh

Os

-Figure 3.5 ToF-SIMS spectrum (negative ion mode) of Ag NPs modified with 33

Inset shows the cluster of ion at m/z 192

Figure 3.6 shows the IR spectra in the carbonyl region for the osmium clusters and those of the modified Ag NPs All the modified Ag NPs are characterized by vibrations at about 2125(w), 2032(s) and 1951(s) cm-1 The essentially identical patterns and band maxima indicate that the osmium clusters are likely to be bound to the surfaces in the same fashion The pattern is similar to that of the osmium species

Os3(CO)10(μ-H)(μ-Zn) on the ZnO surface discussed in the previous chapter (Figure 2.11), suggesting significant interaction between the osmium cluster core and the Ag NPs

2200 2150 2100 2050 2000 1950 1900 1850

a b c

d e

Figure 3.7 shows the UV-vis absorption spectra of Au NPs, 1-modified Au NPs and

1 in ethanol The modification of Au NPs was monitored by in-situ UV-vis absorption

spectroscopy A blue shift of the 574 nm peak to 555 nm after the initial addition of 1

to the Au NPs was observed; the peak did not shift significantly upon further addition

of 1 The observed blue shift therefore suggests some interaction between the osmium

clusters and Au atoms.12 No precipitation was observed during the titration, indicating that the osmium clusters are good surfactants for the stabilization of Au NPs

Cluster 1 in ethanol

Au NPs in ethanol

Figure 3.7 Absorbance spectra of 1-modified Au NPs: (a) Au NPs in ethanol, (b) 1 in ethanol, (c-e) 1 added to Au NPs in increments of 0.1 equivalent

In the case of 22, no precipitate was observed in the reaction, indicating that it was

also a good surfactant for the stabilization of Au NPs The absorption band of

22-modified Au NPs showed a blue shift (533 nm) compared to Au NPs prepared by

laser ablation (574 nm) in ethanol (Figure 3.8) Compared to that for 1-modified Au NPs, it was a larger shift, possibly because 22 has two hydrides, both of which can be

possibly replaced by isolobal Au atoms on the Au NPs In contrast, the interaction

between 1 and Au NPs may not be as strong

Figure 3.8 Absorbance spectra of (a) Au NPs and (b) 22-modified Au NPs in ethanol

TEM images for 1 and 22-modified Au NPs show the spherical particles with

average size of 8.9 ± 1.6 and 8.4 ± 2.2 nm, respectively (Figure 3.9) EDX indicates the presence of osmium and gold (Figure 3.10) As for the Au NPs, the negative ion

ToF-SIMS spectrum of 1-modified Au NPs showed main peaks that can be attributed

to the series [Os3(CO)x(μ-H)]- (x≦9) and [Os3(CO)x(μ-H)(μ-O)]- (x ≦ 8) These

indicated that 1 or 22 was anchored onto the Au NPs successfully

100 nm

100 nm

a

50 nmb

Figure 3.9 TEM images of (a) 1-modified Au NPs, (b) 22-modified Au NPs

Figure 3.10 EDX for 22-modified Au NPs

Figure 3.11 shows the IR spectra in the carbonyl region of modified Au NPs

Cluster 1-modified Au NPs were bonded to Au NPs via a bridging Au because they

had similar patterns and vibrational bands as those described in the previous chapter (vibrational frequencies at about 2125, 2034 and 1952 cm-1) However, the vibrational

bands for 22-modified Au NPs were 2072, 2018 and 1938 cm-1 The peak at 2018

cm-1 was the most intense These values are similar to those of Os3(CO)10(μ-AuPPh3)2

(2067w, 2012s, 1977m, 1965w, 1937m),13 with the exception of some overlapping

peaks Thus, it is possible that both hydrides of 22 were replaced by gold atoms

Figure 3.11 FTIR spectra for (a) 1-modified Au NPs and (b) 22-modified Au NPs

3.1.2 Deposition of osmium clusters onto silver or gold surfaces

The detection of the presence of triosmium cluster species in the ToF-SIMS spectra

of 1-modifed Ag substrate confirmed the association of 1 with the Ag atoms (Figure

3.12) The negative ion spectrum showed the peaks attributed to the molecular ion, to the series [Os3(CO)x]- (x ≦ 12) and to the fragments having silver atoms, [Os3(CO)11(μ-H)Ag]- (m/z 988), [Os3(CO)10(μ-H)Ag2]- (m/z 1070), [Os3(CO)10(μ-H)Ag3]- (m/z 1178)

In most cases, however, ToF-SIMS only served to verify that triosmium species were on the surfaces; fragments containing the surface atoms could not be detected

For instance, the ToF-SIMS spectrum of 22-modified Ag substrate showed the ionic

fragments [Os3(CO)x(μ-O)]- (x ≦ 10) and [Os3(CO)x]- (x ≦ 9), while the fragments [Os3(CO)x(μ-H)(μ-S)]- (x ≦ 9) were observed in the spectrum of

33-modified Ag substrate

-908880852

824

796768

-CO -CO -CO -CO -CO

Figure 3.12 ToF-SIMS spectrum (negative ion mode) of 1-modified Ag substrate

On the other hand, the ToF-SIMS spectra of 1- and 22-modified Au substrates

suggested the formation of a layer of triosmium clusters on the gold surface primarily via a μ-Au bonding mode (Scheme 3.1); fragments containing Au atoms, such as, [Os3(CO)10(μ-H)(μ-Au)]- (cluster of peaks at about m/z 1047), Os3(CO)10(μ-O Au)(μ-H)]- (m/z 1064) and [Os3(CO)10(μ-H)(μ4-Au)(μ-H)Os3(CO)10]- (m/z 1897),

could be clearly observed in the spectrum of 1-modified Au substrate The spectrum

of 22-modified Au substrate is shown in Figure 3.13

Ag/AuScheme 3.1

[Os3(CO)9(μ-H)(μ-O)]

-[Os3(CO)10(μ-H)(μ-Au)]

-[Os3(CO)7(μ-H)(μ-Au)3]

-Figure 3.13 ToF-SIMS spectrum (negative ion mode) of 22-modified Au substrate

The IR spectra in the carbonyl region for the various osmium clusters deposited on

Ag or Au surfaces are given in Figure 3.14 The vibrational bands of the cluster-modified Ag substrates (2115w, 2028s and 1945s cm-1) suggested that the

clusters were bonded to the Ag surface via a μ-Ag For 1-modified Ag substrate, the

presence of a peak at 2056 cm-1 is possibly due to another surface species, viz.,

Os3(CO)10(μ-H)(μ-OAgn) This species is likely to be derived from the reaction of 1 with oxidized Ag substrate Clusters 1 and 22 are also likely to be bonded to the Au

substrate via a μ-Au bonding mode However, a hump at 2068 cm-1 for 1 on the Au

substrate indicated the presence of Os3(CO)10(μ-H)(μ-OAun) (Scheme 3.2) All these analyses showed that structure of the osmium cluster species deposited onto silver or gold, be in nanoparticles or substrates, are mostly the same

R = H, OH, Substrate = Ag/Au

R = SCOPh, Substrate = Ag

Os

Os

Os R

22 modified Au NPs

Os Os Os

O H

Ag/Au

Scheme 3.2

3.2 Anchoring of osmium clusters onto silver or gold nanoparticles via a hydrocarbon linker

3.2.1 Anchoring of Os 3 (CO) 10 (μ-H)(μ-S͡ SH)

Sulfur compounds have a strong affinity to transition metal surfaces.14Self-assembled monolayers of thiols on various metals15 or semiconductor surfaces16have attracted great interest over the past few decades mostly because of their possible application as sensors or as active substrates for molecular electronics, and the presence of thiol monolayers provides an excellent protection of the metal surface against oxidation Alkanethiol SAMs on Au surfaces are probably the most examined and have served as a model for self-assembly in general,17 and there has been some interest in aromatic SAMs due to their high stability and better electronic conduction properties.18 As shown in the previous section, gold and silver atoms exhibit a high affinity for the triosmium core It was thus of interest to examine how the osmium clusters Os3(CO)10(μ-H)[μ-S(1,3-C6H4)SH], 5a, Os3(CO)10(μ-H)[μ-S(CH2)8SH], 7a

and Os3(CO)10(μ-H)[μ-S(CH2)2SH], 23a, which all contain a tethered thiol group,

would interact with silver and gold NPs or substrates

We began with an attempt to construct a molecular model of the cluster-substrate

through the reaction of 5a, 7a and 23a with Ph3PAuCl It was hoped that a species of the general formula Os3(CO)10(µ-H)(µ-S͡ SAuPPh3) may be obtained However, we found that the reactions in the presence of NaOMe all afforded the same product

Os3Au2(CO)9(µ3-S)(PPh3)2, 34 Cluster 34 has been completely characterized

including by a single crystal X-ray diffraction study; the ORTEP plot and selected

bond parameters are given in Figure 3.15

Cluster 34 contains an Os3Au2 trigonal bipyramid with an Os3 face capped by an S atom The nine CO ligands are terminally bound, three to each Os atom The Os-Os distances [2.8858(5), 2.8898(6), 3.0118(5) Å] are longer than those in

Os3(CO)9(μ-H)2(μ3-S) [2.764(1), 2.908(1), 2.922(1) Å],19 Os3(CO)10(μ-AuPEt3)2

[2.684(1), 2.760(1), 2.830(1) Å], or Os3(CO)10(μ-AuPPh3)(μ-SCN) [2.863(1), 2.863(1), 2.899(1) Å].20 Attachment of the Au2(PPh3)2 unit results in the Os(3)-Os(4) bond being longer than the other two Os-Os bonds.21 The Au(2) atom shows one short and two longer bonds to the osmium atoms (2.8048(5) Å for Au(2)-Os(5) compared to 2.8601(5) Å and 2.8608(5) Å for the other two) The Os-S distances [2.367(3)-2.379(2) Å] are shorter than those in the cluster Os3(CO)9(μ-H)2(μ3-S), and the Au-Au distance [2.9744(5) Å] is similar those for a number of Au-Os clusters.22, f 5

Figure 3.15 ORTEP diagram and selected bond lengths (Å) and angles (o) for 34

Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms have

been omitted Selected bond lengths (Å) for 34: Au(1)-P(1) 2.290(2), Au(2)-P(2)

2.287(2), Au(1)-Os(3) 2.7925(5), Au(1)-Os(4) 2.8262(5), Au(1)-Au(2) 2.9744(5), Au(2)-Os(5) 2.8048(5), Au(2)-Os(4) 2.8601(5), Au(2)-Os(3) 2.8608(5), Os(3)-S(1) 2.372(2), Os(4)-S(1) 2.379(2), Os(5)-S(1) 2.367(3), Os(3)-Os(5) 2.8898(6), Os(3)-Os(4) 3.0118(5), Os(4)-Os(5) 2.8858(5)

3.2.1.1 Interaction of osmium clusters with silver or gold nanoparticles

Silver nanoparticles

The UV absorption spectra of 5a-, 7a and 23a-modified Ag NPs in toluene exhibit

distinct plasmon bands centered around 458, 542 and 571 nm, respectively (Figure

3.16) However, the intensity for the spectrum of 23a-modified Ag NPs is weak and

only a slight hump is observed This is possibly because most of the particles obtained were very small, and only some of the bigger particles exhibited absorbance The large shifts of the absorption maxima compared to uncapped Ag NPs (390 nm) may

be due to the interaction between the thiolate ions and the Ag atoms of the NPs, or of the triosmium core with the Ag atoms The absorption bands for nonanethiol, octanethiol and dodecanethiol capped Ag NPs have been reported to be at 436, 420 and 420 nm, respectively.23 Thus the large shifts of the absorption maxima observed here suggest the effect of the triosmium core

It was reported that the size of thiol-derived Ag NPs was < 5 nm.24 For

23a-modified Ag NPs, small spherical particles (1.7 ± 0.4 nm) as well as some

irregularly shaped particles with a shell ~1 nm thickness, were observed (Figure

3.17a) The presence of these shells can be attributed to a monolayer of 23a The average size for 5a- and 7a-modified Ag NPs is 2.7 ± 1.6 and 5.2 ± 1.2 nm

respectively These two modified particles also appear to aggregate to form big particles, possibly due to the high affinity of the triosmium core for Ag The formation

of aggregates is also supported by the observation of bands at 542 and 571 nm in the

UV absorption spectra EDX results for all three samples confirm again the presence

of osmium, silver and sulfur elements

The association of 23a with the Ag particles was verified by the peaks due to

[Os3(CO)x(μ-H)(μ-S)]+ (x ≦ 10) and [Os3(CO)10(μ-H)(μ-SCH2CH2SAg)]+ (m/z 1052)

in the negative ion ToF-SIMS spectrum of 23a-modified Ag NPs (Figure 3.18) However, those for the 5a- and 7a-modified Ag NPs only showed the series of

fragments [Os3(CO)x(μ-H)(μ-S)]+ (x ≦ 9)

1000 1500 0

2000

4000

1357 949

801 773

689 744 642 615

1303 885

857 829

Figure 3.18 ToF-SIMS spectrum (negative ion mode) of 23a-modified Ag NPs

The solid-state IR spectral data of the cluster-modified Ag NPs and of the cluster precursors are given in Table 3.1 Two regions of vibrational bands are of particular interest: the C-H stretches and the metal carbonyl stretches The C-H stretching bands can provide information on the ordering of the self-assembled monolayer structure; higher energy is correlated to lower crystallinity or gauche defects of the monolayer The common conformational defect of assembled hydrocarbon chains is a gauche conformation, interrupting a sequence of ordered trans conformations.25 A gauche conformation is caused by an approximately 120o rotation about the backbone bond in the positive or negative directions (Figure 3.19).26 The symmetric and asymmetric

C-H stretching bands for 23a- and 7a-modified Ag NPs appear at about 2928 and

2871 cm-1, and at 2930 and 2856 cm-1, respectively, indicating that the osmium

clusters deposited are not very ordered on the Ag NPs.27 This is in contrast to a well-known fact that short alkyl chains tend to give a more disordered film on flat metal surfaces and nanoclusters It is possible that this is due to the bulky triosmium core.The CO stretching bands, on the other hand, show similar patterns This is consistent with the osmium clusters being bonded to the Ag NPs via the dithiolato ligand while the osmium core remains intact

Figure 3.19 Scheme of the methylene groups position in the case of (a) normal alkyl chain and (b) alkyl chain having a gauche defect

Table 3.1 Solid-state IR spectral data for cluster-modified Ag NPs and the

corresponding cluster precursors

Gold nanoparticles

The stability of the cluster-modified Au NPs was found to be dependent upon the

identity of the cluster used Thus when 23a was titrated against Au NPs which were

photochemically generated in ethanol, some black precipitate was formed immediately The titration could be followed photometrically, and it was observed that

the absorption bands at 321 and 387 nm (due to 23a) increased in intensity while that

at 538 nm (due to Au NPs) decreased in intensity, was red-shifted and became broader

(Figure 3.20) It is clear that 23a-modified Au NPs are not stable in ethanol and the

size of particles increased, leading to precipitation; TEM images of the precipitate showed that the size of spherical particles was 9.3 ± 2.7 nm

In contrast, for 7a, while the bands at 317 and 385 nm (due to 7a) increased in

intensity that for the 538 nm band (due to Au NPs) remained the same during titration This observation that no precipitate was formed during the titration is consistent with

earlier reports that long chain thiols increased the stability of capped NPs For 5a, the

band at 538 nm was red shifted to 548 nm, indicating that the size of the Au NPs has increased and demonstrating that a triosmium cluster with a free aromatic thiol can stabilize the Au NPs.28 This was verified by TEM images which show particle sizes of

9.0 ± 2.0 nm, compared to spherical particles of 8.5 ± 2.3 nm for 7a and 8.4 ± 2.5 nm

for naked Au NPs

As for the Au NPs, the TEM images show that the modified Au NPs are surrounded

by a shell, and the presence of osmium, gold and sulfur were verified by the EDX spectra The ToF-SIMS (positive ion mode) spectra of all the cluster-modified Au NPs also show a series of ions due to [Os3(CO)x(μ-H)(μ-S)]+ (x ≦ 9), indicating the

binding of osmium clusters onto the Au NPs The IR spectra for 23a- and 7a-modified

Au NPs are similar to those of the parent cluster (Figure 3.21), thus pointing to

interaction via an Au-S bond However, the IR spectrum of 5a–modified Au NPs is

different; it shows signals at 2127, 2054, 2000 and 1927 cm-1 This pattern is similar

to that for Os3(CO)10(μ-H)(μ-M) (M = Zn, In, Ag, Au) described in the previous chapter (2125, 2034 and 1952 cm-1) Thus it may be that the Au NPs bound to the

cluster core of 5a as well, presumably because of the proximity of the thiol group to

the triosmium core

Figure 3.21 FTIR spectra of (a) 23a-, (b) 5a- and (c) 7a-modified Au NPs

3.2.1.2 Deposition of osmium clusters onto silver or gold substrates

The ToF-SIMS spectra of similarly modified Ag or Au substrates all show the series

of fragments [Os3(CO)x(μ-H)(μ-S)]- (x = 1-10) In some cases, the molecular ion and

fragments containing Au or Ag atoms can also be detected For instance, the spectra of

23-modified substrates show ions corresponding to

[{Os3(CO)9(μ-H)S}Ag{SOs3(CO)9}]- (m/z 1820) for the Ag surface, and [Os3(CO)10(μ-H)(μ-SCH2CH2SAu)]- (m/z 1141) and [{Os3(CO)9(μ-H)S}2Au]- (m/z 1909) for the Au surface (Figure 3.22) Species such as [{Os3(CO)9(μ-H)S}Ag{SOs3(CO)9}]- or [{Os3(CO)9(μ-H)S}2Au]-may be attributed

to rearrangement of the anchored species upon bombardment with the Ga ions Nevertheless, the detection of these ions implied that the triosmium cluster maintained

its structural integrity, and that the interaction with the silver or gold substrate was via

the sulfur atom

In the case of the 7a-modified Au substrate, the fragment

[Os3(CO)10(μ-H){μ-S(CH2)8S-Au)}]+ (m/z 1223) was observed Similarly, the

negative ion mass spectrum of the 5a-modified Au substrate showed

[Os3(CO)10(μ-H){μ-S(C6H4)S}]- (m/z 990), [Os3(CO)10(μ-H){μ-S(C6H4)S)Au}]- (m/z 1187), and [Os3(CO)10(μ-H){μ-S(C6H4)S}Au2]- (m/z 1384)

1000 0

3000 6000 9000 12000

a

0 20000 40000 60000 80000

Mass (m/z)

b

[Os3(CO)9(μ-H)(μ-S)] [Os3(CO)10(μ-H)(μ-S)] -

-[Os3(CO)10(μ-H)(μ-SCH2CH2SAu)] [Os3(CO)10(μ-H)(μ-SCH2CH2S)] -

-[Os3(CO)10(μ-H)(μ-SCH2CH2SAu)]

-[{Os3(CO)10(μ-H)S}Ag{SOs3(CO)9}]

-885

-CO -CO -CO -CO

857

857

829 773

773 801

Figure 3.22 ToF-SIMS spectra of (a) 23a-modified Ag substrate in the positive ion mode (Inset: a fragment in the negative ion mode), (b) 23a-modified Au substrate in

the negative ion mode