Organometallic clusters as precursors of metallic nanoparticles 2

Indeed, this was inspired by the surprising discovery that while bulk gold is an inactive catalyst, nanosized gold in the 5 nm range exhibited excellent catalytic activity towards CO oxi

Chapter 1 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 1 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.2 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.3

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 5 nm range exhibited excellent catalytic activity towards CO oxidation, at or even below room temperature.4

Metal carbonyl clusters may serve as precursors to highly active small particulates and catalysts because they have the following advantage:5

(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,6 and the aggregates would reflect the structure of the original clusters;

(4) it is possible to prepare compositionally homogeneous bimetallic catalysts from heteronuclear clusters.7

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.8

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).9 The other method is to anchor the cluster onto the surface via a functional group (Figure 1.2).10Micro/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)

Ph2P

Ph2PN

SiO

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 8 Å, and the distance between units is about 3 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

(1) Rapid Nucleation (2) Growth of Nuclei (3) Growth of Terminates Coated Particles

Precursors supply depleted Driven by decomposition

Interaction with coordinating solvent

Scheme 1.1

Co

P P

P O

O O

O O

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,27produces 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

CdSe

P P

P O

O O

O O

O O

ZnS 5-10 min

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

ruthenium dicarbonyl carboxylate oligomers tethered to the gold surfaces through alkanethiolate linkages (Figure 1.6).32 For Ag NPs, capping agents have included Cr(CO)4dpp (2,3-bis(2’-pyridyl)pyrazine), 33 Re(CO)3BrL (L = dpp, bpm (2,2’-bipyrimidine), and bpy (2,2’-bipyridine)),34 and the water soluble triosmium cluster Os3(CO)10(μ-H)[μ-S(CH2)10COONa)] 35 Such organometallic compound- modified NPs can serve as molecular models as well as potential precursors, for bimetallic catalysts

Figure 1.6 Au NPs capped by ruthenium dicarbonyl carboxylate oligomers (Adapted form reference 32)

1.2 Self-assembly of organometallic clusters on the surface

Surface chemistry is the study of surfaces under controlled conditions and not real surfaces, due to the extreme complexity that arises from real surfaces One aspect of interest is the interaction of adsorbents with surfaces, which can be easily prepared via self-assembly Self-assembled monolayers (SAMs) have been an intriguing area of research over the past twenty years because of the unique opportunities they provide

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.37These 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

Et C

O

C

Et

(CH2)3Si

O O O oxide surface

Et C

O

C

Et

(CH2)3SiCl3OHOHOH oxide surface +

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