Metal nanoparticles electronic structure

Bray KL (2001) High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. 213 1-94 Bronstein LM (2003) Nanoparticles Made in Mesoporous Solids. 226 55-89 Bronstrup M (2003) High Throughput Mass Spectrometry for Compound Characterization in Drug Discovery. 225 275-294... 

Recently, however, the development of nanotechnology may provide the changes on the research and development of practical catalysts. As mentioned in the previous section we can now design and synthesize a metal nanoparticle with not only various sizes and shapes, but also with various combinations of elements and their locations. Thus, we can now design the synergetic effect of two elements. In the case of core/shell structured bimetallic nanoparticles, the shell element can provide a catalytic site and the core element can give an electronic effect (a ligand effect) on the shell element. Since only the atoms on the surface can be attached by substrates, the thickness of the shell should be an important factor to control the catalytic performance. 

In Section 2 the general features of the electronic structure of supported metal nanoparticles are reviewed from both experimental and theoretical point of view. Section 3 gives an introduction to sample preparation. In Section 4 the size-dependent electronic properties of silver nanoparticles are presented as an illustrative example, while in Section 5 correlation is sought between the electronic structure and the catalytic properties of gold nanoparticles, with special emphasis on substrate-related issues. 

Advantages of small metal nanoparticles are (i) short range ordering, (ii) enhanced interaction with environments due to the high number of dangling bonds, (iii) great variety of the valence band electron structure, and (iv) self-structuring for optimum performance in chemisorption and catalysis. 

Photoemission has been proved to be a tool for measurement of the electronic structure of metal nanoparticles. The information is gained for DOS in the valence-band region, ionization threshold, core-level positions, and adsorbate structure. In a very simplified picture photoemission transforms the energy distribution of the bounded electrons into the kinetic energy distribution of free electrons leaving the sample, which can easily be measured ... 

Interpretation of the Electronic Structure of Transition and Noble Metal Nanoparticles... 

In the previous Sections (2.1-2.3) we summarized the experimental and computational results concerning on the size-dependent electronic structure of nanoparticles supported by more or less inert (carbon or oxide) and strongly interacting (metallic) substrates. In the following sections the (usually qualitative) models will be discussed in detail, which were developed to interpret the observed data. The emphasis will be placed on systems prepared on inert supports, since - as it was described in Section 2.3 - the behavior of metal adatoms or adlayers on metallic substrates can be understood in terms of charge transfer processes. 

The identification of structure sensitivity would be both impossible and useless if there did not exist reproducible recipes able to generate metal nanoparticles on a small scale and under controlled conditions, that is, with narrow size and/or shape distribution onto supports. Metal nanoparticles of controlled size, shape, and structure are attractive not only for catalytic applications, but are important, for example in optics, data storage, or electronics (c.f. Chapter 5). In order not to anticipate other chapters of this book (esp. Chapter 2), remarks will therefore be confined to few examples. 

Besides electronic effects, structure sensitivity phenomena can be understood on the basis of geometric effects. The shape of (metal) nanoparticles is determined by the minimization of the particles free surface energy. According to Wulffs law, this requirement is met if (on condition of thermodynamic equilibrium) for all surfaces that delimit the (crystalline) particle, the ratio between their corresponding energies cr, and their distance to the particle center hi is constant [153]. In (non-model) catalysts, the particles real structure however is furthermore determined by the interaction with the support [154] and by the formation of defects for which Figure 14 shows an example. 

Electrochemical template-controlled sjmthesis of metallic nanoparticles consists of two steps (i) preparation of template and (ii) electrochemical reduction of metals. The template is prepared as a nano structured insulating mono-layer with homogeneously distributed planar molecules. This is a crucial step in the whole technology. The insulating monolayer has to possess perfect insulating properties while the template has to provide electron transfer between electrode and solution. Probably, the mixed nano-structured monolayer consisting of alkylthiol with cavities which are stabilized by the spreader-bar approach [19] is the only known system which meets these requirements. 

Pacchioni G, Illas F. 2003. Electronic structure and chemisorption properties of supported metal clusters model calculations. In Wieckowski A, Savinova ER, Vayenas CG. editors. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Dekker. 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

A.K. Santra and D.W. Goodman, Size-dependent electronic, structural, and catalytic properties of metal clusters supported on ultrathin oxide films, in Catalysis and Electrocatalysis at Nanoparticle Surfaces, eds. A. Wieckowski, E.R. Savinova, and C.G. Vayenas. Marcel Dekker, New York, 2003, pp. 281-309. 

These studies indicate that the charge transfer at the metal-oxide interface alters the electronic structure of the metal thin film, which in turn affects the adsorption of molecules to these surfaces. Understanding the effect that an oxide support has on molecular adsorption can give insight into how local environmental factors control the reactivity at the metal surface, presenting new avenues for tuning the properties of metal thin films and nanoparticles. Coupled with the knowledge of how particle size and shape modify the metal s electronic properties, these results can be used to predict how local structure and environment influence the reactivity at the metal surface. 

Metal nanoparticles have received much attention in the past because their unique electronic structure makes them interesting materials for nanoelectronics, optics and catalysis [33]. A large body of work has already been published on the preparation [34] and characterization [35] of such particles and will not be the subject of this section. 

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