Organometallic clusters as precursors of metallic nanoparticles 2

Chapter Organometallic complexes and nanomaterials During the past few decades, a new research area has emerged-nanomaterials This new field gained recognition with the first review was written by Amin Henglein in 1989 Nanomaterials exhibit unique properties and improved performance determined by their size, surface structure and inter-particle interaction The most important of these is the size effect, which pertains to the evolution of structural, thermodynamic, electronic, spectroscopic, electromagnetic and chemical features of these finite systems Some of the important applications and technologies of nanomaterials include the following: (1) catalysis; (2) biological, such as gene targeting/drug targeting; (3) optoelectronics; and (4) energy storage and conversion Catalysis was perhaps the first field to take advantage of nanotechnology This included the design and fabrication of catalysts, enhancement of catalytic activity or selectivity, and reduction in cost of catalysts Heterogeneous catalysts constituted nanoparticles (1-100 nm) can harness the large surface area-to-volume ratio and the larger number of active binding sites resulting from more defects present in NPs A major process in a heterogeneous catalytic reaction is the interaction of the chemical reactants with surface sites of the catalysts This interaction depends on the surface properties of the catalysts For example, a gold octamer (Au8) is adsorbed rather strongly on Mg (100) surfaces containing oxygen vacancy F-centres and it shows catalytic activity in CO oxidation However, gold octamers on an F-centre–free MgO (001) surface are essentially inactive for the combustion reaction Traditional preparation of naked metal nanoparticles on support using reduction of their salts is very advantageous in terms of the exposure of the surface to catalytic reactions However, it is lacking in control over the size, shape and stability of the nanoparticles Deliberate tailoring of nanoparticles size, shape and surface could lead to improved or new catalytic properties Indeed, this was inspired by the surprising discovery that while bulk gold is an inactive catalyst, nanosized gold in the nm range exhibited excellent catalytic activity towards CO oxidation, at or even below room temperature Metal carbonyl clusters may serve as precursors to highly active small particulates and catalysts because they have the following advantage: (1) metal catalysts prepared from carbonyls are generally highly dispersed; (2) there are no halide ions which often poison the metal catalysts; (3) NPs may retain the nuclear integrity of their precursors As metal clusters can be viewed as metal islands surrounded by a carbonyl sheath, they provide ideal building blocks for the development of nanoscale metallic islands separated from each other by ligands, and the aggregates would reflect the structure of the original clusters; (4) it is possible to prepare compositionally homogeneous bimetallic catalysts from heteronuclear clusters 1.1 Synthesis of nanomaterials from organometallic complexes The synthesis of nanomaterials and assembling them into ordered arrays to render them functional and operational are crucial aspects of nanoscience With organometallic complexes as precursors, there are two main methods for their conversion into metallic nanoparticles One involves the pyrolysis of the organometallic precursors supported on a porous material, and the other involves thermolysis in a hot solution in the presence of surfactants Size control and dispersity of NPs are usually attained by conducting the reactions at high temperature, which ensures a high rate of nanoparticle nucleation and growth In the case of the second method, capping ligands which form a self-assembled monolayer on the nanoparticles, can also be used to mediate particle growth 1.1.1 Organometallic complexes supported on porous materials In a typical preparation process, the metal cluster is loaded onto a porous support by slurrying with a solvent After removal of the solvent, gentle heating will remove the ligand and leave the metallic core as supported metal Two methods are commonly used to anchor carbonyl clusters onto a support surface One is the direct interaction of the cluster with surface hydroxyl groups (Figure 1.1) The other method is to anchor the cluster onto the surface via a functional group (Figure 1.2) 10 Micro/mesoporous cavities and channels of porous materials such as zeolites and layered clays have been used as the ultimate reaction vessel in which the template synthesis of metal nanomaterials can be carried out Figure 1.1 Structural model of Os5C clusters supported on partially dehydroxylated MgO at a defect site determined on the basis of EXAFS spectra and DFT calculations (Adapted from reference 9) Ph2 P N Co Ph2P Co Co PPh2 Co N Ph2P O Si O O Figure 1.2 Anchoring of cobalt clusters on a mesoporous silica matrix There are several parameters which will affect the size and shape of NPs obtained One is the pyrolysis temperature When the temperature is high enough for removal of all the ligands, NPs with the nuclearity of the original clusters will be obtained When the temperature is increased, the NPs obtained will aggregate to form bigger particles For example, Pt NPs of different size and shape were obtained when robust Pt clusters such as [Pt15(CO)30]2- in FSM-16 (28 Å) were pyrolysed under vacuum; the cluster comprised Pt3(CO)6 units with a cross-section diameter of about Å, and the distance between units is about Å The Pt NPs retained the prismatic triangular Pt framework below 70 oC However, spherical aggregates of about 15 Å in diameter were formed at 200 oC 11 A second parameter is the structure and nuclearity of the metal clusters The metal framework can be maintained after decarbonylation of the supported metal carbonyl clusters For example, the decarbonylation of [Ir4(CO)12]/γ-Al2O3, 12 [Pt15(CO)30]/ MgO, and [Os10C(CO)24]/MgO, appeared to take place without significant changes in the metal framework Similar results were observed when [Pt18(CO)36]2- in FSM-16 (48 Å), or [Ru12C2(CO)16Cu4Cl2]2- and [Ag3Ru10C2(CO)28Cl]2- in MCM-41, were pyrolysed 13 The structure and shape of the support are also important For example, tubular anodized aluminum oxide (AAO) has recently been used as templates in the synthesis of RuO2 tubes (Figure 1.3) 14 This is usually carried out by allowing a solution of the precursors to deposit onto the inner walls of the support before heating to decompose the organometallic precursor Nanotubes or nanowires can be obtained depending on the precursor used Thus Ichikawa reported that Pt nanowires formed in the mesoporous channels of FSM-16 by the photoreduction of H2PtCl6/FSM-16 in the presence of water and 2-propanol; the mesoporous channels in FSM-16 played a templating role for the fabrication of the Pt nanowires by ensuring a one-dimensional elongation of the Pt crystal 15 Sometimes neither nanotubes or nanowires were obtained and instead the NPs were uniformly located and aligned in the ordered mesoporous channels of the templates, an example being the Pt nanoparticles formed in FSM-16 (28 Å) at 200 oC mentioned above Figure 1.3 SEM image showing the hollow cores of the RuO2 nanotubes (Adapted from reference 14) 1.1.2 Decomposition of organometallic complexes in a hot solution Metal carbonyl complexes represent the most common organometallic precursor for thermal decomposition of organometallic complexes in the presence of a surfactant A typical synthesis is depicted in Scheme 1.1 A solution of the surfactant is heated to reflux temperature under Ar or N2, and a solution of the organometallic complex is rapidly injected The progress of the decomposition is monitored by IR spectroscopy The surfactants used have included acids, trioctylphosphine oxide (TOPO), amines, polymers, or their mixtures (Figure 1.4) For example, Fe NPs were produced by the thermal decomposition of Fe(CO)5 in a TOPO solution containing oleic acid, 16 and in the presence of poly(styrene) functionalized with tetraethylenepentamine, 17 which act to passivate the produce NPs Injection of precursors into surfactants Driven by decomposition Interaction with coordinating solvent (1) Rapid Nucleation Precursors supply depleted (2) Growth of Nuclei (3) Growth of Terminates Coated Particles Scheme 1.1 P O P O O P Co P O O O P P Figure 1.4 Co NPs protected by TOPO Sometimes, a new polymorph can be obtained For example, cobalt has two polymorphs - close-packed hexagonal (hcp) and face-centered cubic (fcc) Thermal decomposition of Co2(CO)8 in a hot toluene solution containing TOPO produces a new polymorph, ε-Co NPs 18 Similar reactions in o-dichlorobenzene in the presence of various ligands allow for morphological control 19 The method can also be used to prepare NPs of alloyed transition metals Typically, two metal precursors are decomposed in tandem to produce solid solution NPs, or sequentially to give core-shell NPs 20 For example, the simultaneous decomposition of Fe(CO)5 and Mo(CO)6 in the presence of bis-ethylhexylamine and octanoic acid in refluxing dioctyl ether produces FeMo NPs 21 Sonochemical decomposition in a hot solution of organometallic complexes has also been used to produce agglomerates of NPs, which can be further dispersed by all kinds of surfactants For example, sonication of Fe(CO)5 in a noncoordinating, high boiling solvent such as decalin leads to the formation of agglomerates of polydispersed Fe NPs 22 Reacting these NPs with a variety of functionalized alkanes, alcohols, 23 carboxylic acids, 24 thiols, 25 phosphonic or sulfonic acids, 26 or silanes, 27 produces monolayer-coated NPs To some extent, the size is determined by the surfactants 1.1.3 Surface modification The unique properties of the nanoparticles are attributed to the large number of atoms on the surface For particles in this size regime, a large percentage of the atoms are in or near the surface The interface between the particles and the surrounding media can have a profound effect on the particles’ properties Surface modification of nanoparticles can be divided into three different types based on the nature of the modifying groups: organic, inorganic and organometallic Depending on whether lipophilic or hydrophilic protecting groups are applied for the stabilization, the resulting metal colloids are soluble in organic media or water Organic capping agents play the role of a separating layer in contact with neighboring nanoparticles In particular, polymers have unique properties, including thermal behaviour, processibility, and ability to assemble into ordered structures, which offer the potential for compartmentalizing nanocrystals, directing their assembly and providing a mechanism for charge transfer.3 Polymer-protected colloidal dispersions of metal nanoparticles can be prepared by decarbonylation of an organometallic complex at high temperature in the presence of a surfactant, followed by in-situ synthesis of the polymer using a method employing a microemulsion medium 28 These polymer protected nanoparticles are quite stable and composed of fine particles with a narrow size distribution.29 Other organic compounds used include alkylamines, alkyl acids, thiols, and tetraalkylammonium halides (Scheme 1.3) The attachment of organic molecules to metallic NPs affords an easy way to create chemical functionality on their surfaces Ni : N+Br- Figure 1.5 Capped Ni NPs One of the methods used for surface modification with inorganic compounds is the seed-germ process, 30 that is, the first metal particles are used to grow a second metal on their surfaces The thickness of the second metal layer can be varied over a relatively wide range The outer metal can then be protected by a shell of an appropriate ligand For example, CdSe/ZnS core-shell NPs can be prepared by the reaction of zinc sources with (TMS)2S and CdSe in the presence of TOP (Scheme 1.2) 31 Such core/shell particles play an important role in heterogeneous catalysis The influence of the underlying metal leads to catalytic behaviour which is often significantly different from that of the monometallic species This should be more pronounced if the outer shell is thin; if the thickness of the outer layer is substantial, this must suppress the effect of the inner metal P O P O O P CdSe O O O P P P Zn(CH3)2, (TMS)2S dropwise 5-10 P P O P O ZnS O CdSe P O O P O P Scheme 1.2 The use of organometallic compounds as capping agents is much less common The reaction between Ru3(CO)12 and mercaptopropanoic acid-capped Au NPs leads to 10 towards a basic understanding of self-organization, structure-property relationships, and interfacial phenomena at the condensed surfaces They also represent a good approach to the design of surfaces with molecular dimensions which allow for demonstration of the desired properties 36 Self-assemblies of organometallic complexes on different supports are important because of their relevance to the new and fascinating approach of surface-mediated synthesis and supported catalysts 37 These complexes are also useful for the preparation of heterogeneous catalysts because their final structure and consequently, their catalytic selectivity, can be influenced by the nature of the support and the new structures formed at the surface, with new properties which may be different from that derived from the decomposition of the original cluster 38 1.2.1 Synthesis Supported metal clusters can be formed by adsorption at room or elevated temperatures; Scheme 1.3 shows one such example 39 In some cases, solvent-free deposition of metal clusters onto oxide supports have also been reported; these have the advantage of convenience 40 In general, supported clusters have metal centers bonded to oxygen atoms of the support A well known structure is Os3(CO)10(μ-H)(μ-OSi≡), where Si≡ represents the silica surface 41 12 Et O O Et Et C Et o N C Et 25 C Et N Ru N N Et Et Et Et C N dry CHCl3 N N Et Et N N Ru Et Et N C Et Et (CH2)3 (CH2)3 Si O O O SiCl3 + OHOH OH oxide surface oxide surface Scheme 1.3 Adsorption of clusters on supports may also be facilitated via a ligand that has a chemical functionality (headgroup) with a specific affinity for a surface There are a number of headgroups that are known to bind to specific metals, metal oxides, and semiconductors, including -OH, -COOH, -SH, -NH2, -SeH, -N≡C, -CH=CH2, -SiH3, -P=O, and -CSSH 42 The adsorption of carboxylic acids on metal or metal oxides is an acid-base reaction, and the driving force is the formation of a surface salt between the carboxylate anion and a surface metal cation 43 Self-assembled monolayers (SAMs) of alkylchlorosilanes and alkylalkoxysilanes require hydroxylated surfaces as substrates for their formation The driving force for these SAMs is the formation of polysiloxane, which is connected to the surface silanol groups via Si-O-Si bonds Sulfur and selenium compounds have a good affinity to transition metal surfaces, such as gold, and silver (Figure 1.7), 44 although the reason for the good affinity of sulfur and selenium to transition metals is still not clear now 45 13 Fe Fe Fe Se Au Figure 1.7 Adsorption of [Fe3(CO)9(μ3-Se)]- on gold surface Non-metallic supports that have been used include silica gel, 46 porous silica, 47 and other materials such as ITO and MoS2 48,49 Figure 1.8 shows an example for an ITO surface 50 In comparison, there are few literature reports on the interaction of clusters with metallic supports Metallic supports can be used to provide bimetallic alloys via complete reduction of the supported organometallic species in a hydrogen environment Os ITO O O Si O NC Os Os Os Os O ITO O Si O Os H N H Figure 1.8 Adsorption of osmium clusters on ITO 1.2.2 Characterization techniques Despite the numerous methods available, determination of the structure of a surface 14 organometallic species remains challenging Because single crystal X-ray diffraction cannot be performed for a surface organometallic species, other techniques must be relied upon in order to obtain an idea about the bonding situation around the metal centers In general, successful characterization of a supported metal cluster requires a combination of techniques The characterization techniques for supported clusters include TEM, STM, AFM, XPS, EXAFS, Raman, IR spectroscopy and ToF-SIMS, with varying degrees of success.46 Comparison of the infrared spectra with known compounds is one of the more generally useful identification methods for supported metal carbonyl clusters For example, Li et al reported that a triosmium moiety was bonded to a nitrile group on functionalized silica surface by comparison of the IR spectrum of the surface species with the known compound Os3(CO)10(μ-H)(μ-CO)[Et4N] (Figure 1.9) Os Os L Os Os oxide surface Os Os L H L = NC(CH2)3Si- Figure 1.9 Proposed surface osmium species (left) and molecular structure of the analogue (right) Similarly, Roberto et al have prepared Os3(CO)10(μ-H)(μ-OH) via surface-mediated synthesis on silica, and they have proposed that the surface-anchored precursor was Os3(CO)10(μ-H)(μ-OSi≡), based on the similarity of their IR spectra However, there 15 are difficulties associated with the identification of surface species in this way, including the difficulty of obtaining a reasonable surface spectrum, broadened peaks in the spectrum due to the non – uniform surface species and the shifted peaks as well because the ligands provided by the surfaces may not be similar to those in the reference compounds 51 Furthermore, this method cannot be applied in cases where there are no known molecular analogues The closely related Raman spectroscopy can also be employed in a similar manner although it has mainly been used to study metal-metal bonds in supported clusters For example, three bands at 80 (br), 119 (br) and 160 (s) cm-1 were assigned to three Os – Os stretching modes in the investigation of Os3(CO)12 deposition on γ–Al2O3 52 Indirect evidence of the presence of metal clusters can also be obtained from the observation of vibrational bands due to bridging CO ligands, which can be observed more readily in the Raman spectum Whitmire reported that the binding energy of the thiolate ligand of a triiron cluster on the gold surface was eV lower than that of the original cluster, from which he concluded that the triiron clusters were bonded to the gold surface via the sulfur atom A similar conclusion was also drawn by Morneau for a triosmium cluster which was bonded to a gold surface via a thiol ligand 53 These studies made use of the observation that the osmium, silicon and sulfur had similar chemical environments with those in the precursors to deduce that the structure of the surface species could be similar However, osmium clusters with very different structures have been reported to have similar binding energies For example, in the clusters H4Os4(CO)12 and Os3(CO)12, the binding energies for osmium were reported to be at 52.0 and 51.9 16 eV, respectively 54,53 Together with the fact that there is in general very little literature available on XPS binding energies for organometallic compounds, this means that XPS is more useful for identification of the elements present than for the identification of molecular structures Another technique that has potential in providing structural information of the surface species is ToF-SIMS This method provides a mass spectrum of the surface and hence a detailed chemical analysis of the composition and structure of any surface species 55 However, this technique has not yet been exploited for the analysis of supported organometallic complexes Information on the nuclearity of supported species can also be obtained by other methods Clusters with several atoms of a heavy metal are large enough to be imaged by high-resolution TEM instruments Information on the size of the metal core of the supported species can be obtained directly from the image However, information obtainable about the ligands is limited A more useful method for affording some idea on the geometry of the surface-anchored species is EXAFS, which can provide metal-metal coordination numbers However, the interpretation of EXAFS data often involves subtle differences and is based on rather difficult data fitting 1.2.3 Synthesis of molecular models As described above, one way to determine the structure of surface species is by comparing the IR spectra with those of known molecular compounds Hence, the preparation of molecular models is very important There are two methods for the 17 preparation of molecular models One is the adsorption of clusters onto nanoparticles, which has been described in section 1.1.3 The other is exemplified by the synthesis of clusters containing silsesquioxanes, which can be viewed as models for clusters anchored onto a silica support (Figure 1.10) The bonding between the clusters and the silsesquioxanes can be via an oxygen atom or a linker group Ru R O Si Si O O O R Si Si O R O O R O Si R Si O O O Si R Si O S H Ru Ru R Si O O O R Si Si O R Ru S O H Ru Ru O Si R R Si O O O Si R H O Si O R Os Os Os Figure 1.10 Ruthenium and osmium clusters substituted silsesquioxanes Silica is the support most extensively used for modern surface organometallic chemistry because its surface chemistry is relatively simple and well established compared to many other supports As depicted in Figure 1.11, there are different silanol groups on the silica surface: (1) isolated groups (or free silanols), which correspond to 25% of the total surface silanols at 200 °C (2) geminal silanols, which correspond to 13% of the total surface silanols at 200 °C; and (3) vicinal silanols, which correspond to 62% of the total surface silanols at 200 °C 56 18 H H OH O Si O O Si Si O O isolated silanol O O HO O O O OH Si O O vicinal siloxane geminal silanol R OH R OH OH Si R R O O O Si Si Si Si OH O O O R Si Si O O O O R Si R Si O R OH Si Si O O R O R O OH O R Si Si O R O O R O O Si O Si Si O Si O O R O O Si Si O R O R O Si Si Si Si R O O R R R R R Figure 1.11 Representation of surface silanol and siloxane groups in SiO2 (top) and in the corresponding molecular analogues, polyhedral oligosilsesquioxanes (bottom) R represents ligands, such as cyclopentyl, t-Bu, Ph 57 Besides silsesquioxanes, 58 molecular models for the silica surface include the silanols The bonding between the clusters and the silanols is via an oxygen atom, such as [Os3(CO)10(μ-H)(μ-OSiR2R’)] (R = Et, Ph; R’ = Et, Ph, OH, OSiPh2OH) 59 These models are based on the assumptions that: (a) silica can indeed be viewed as a ligand, (b) the reaction of silica with an organometallic compound is highly selective, and (c) the characterization of silica-supported complexes can be achieved with a degree of accuracy and confidence that is approaching that of the corresponding molecular organometallic complexes 19 1.3 Objectives The 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