Electronic Structure of Metal Nanoparticles

Note that, although we did not discuss the fundamental origin of the phenomena (see below) this was a direct proof that in some way the electronic system of metal nanoparticles could not be identified to that of the bulk metal. Moreover, this provided a practical way to evidence nanoparticle size change during a reactive 

The above discussed experiments could not quantitatively evaluate the contribution of the CO ligands in the observed metal core level shifts. To get rid of CO, a direct deposition of nanoparticles on amorphous carbon could appear as a solution. Hence, Ir was evaporated under vacuum and condensed on the substrate. 

The valence band center is marked by a dotted line. The photoemission spectrum corresponding to very small particles is represented in (b). Both the core level and the reduced valence band are shifted to high binding energy by the final state effects with respect to the bulk reference. In (c), a part of the final state effect has been artificially compensated for so that the valence band center (dotted line) and the core level peaks are in the same positions as in (a). In (d), the particle Fermi 

The final conclusion of this discussion is that the core level shifts reported for small particles are the result of both initial and final state effects which could be separated only by a proper analysis. 

Another parameter of importance in metal nanoparticles is the change in interatomic bond length. Extended X-Ray absorption fine structure (EXAFS) showed this on many transition metal nanoparticles with diameters less than 5 nm [89, 90]. On Pd particles with a diameter 1.4 nm, the interatomic distance reduction was about 3%. Note that this is again in hne with surface observation. More generally, surfaces are submitted to tensile stress and ah initio calculations on 2D Pt models show a strong interatomic distance reduction by 6.6% and 9.1% for the (111) and (100) structures, respectively [91]. Of course, in nanopartides, the structure is not purely 2D and the coordination lowering is not as strong. 

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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 ... 

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. 

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. 

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. 

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. 

The composition of the surface-bound species must be considered they contribute to the stability of the dispersions of metal nanoparticles. In the case of electrostatically stabilized dispersions, the techniques to measure the interfacial electronic phenomena, including electrophoresis, electroosmosis, etc., are useful (54). In order to understand the composition (as well as structures) of the chemical species bound in the surface of metal particles, spectroscopic measurements used for common organic substances are used as well as the elemental analysis. 

The formula (11) in view of relations for /ie and /ih describes above-mentioned basic features of size effects in semiconductor crystal. It is important that as against metals, semiconductors show appreciable quantum dimensional effects at the sizes of particles from 3 to lOnm (depending on electronic structure of the semiconductor and sizes of AE0) [20]. Such nanoparticles are usually formed at synthesis of nanocomposite films. 

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