Colloids quasi-metallic

The detection of sharp plasmon absorption signifies the onset of metallic character. This phenomenon occurs in the presence of a conduction band intersected by the Fermi level, which enables electron-hole pairs of all energies, no matter how small, to be excited. A metal, of course, conducts current electrically and its resistivity has a positive temperature coefficient. On the basis of these definitions, aqueous 5-10 nm colloidal silver particles, in the millimolar concentration range, can be considered to be metallic. Smaller particles in the 100-A > D > 20-A size domain, which exhibit absorption spectra blue-shifted from the plasmon band (Fig. 80), have been suggested to be quasi-metallic [513] these particles are size-quantized [8-11]. Still smaller particles, having distinct absorption bands in the ultraviolet region, are non-metallic silver clusters. 

Controlled reduction of cadmium (or lead) ions on surfaces of nanosized silver (or gold) metallic particles results in the formation of double-layer colloids [532-534]. Depending on the coverage, the second layer can vary from being non-metallic clusters to quasi-metallic and metallic colloids. Growth of the second-layer particles can be monitored by absorption spectrophotometry. For... 

As for the terminology, the most frequently used terms are ultrasmall particles and nanocrystals to denote nanoparticles of metals whose diameter is within 2-5 to 50 nm, as well as colloidal crystaUites and subcolloidal particles. The upper threshold of the size of semiconductin nanocrystals in polymer matrices is the condition that allows for the optical homogeneity of compositions (no scattering by the environment at particle size below a quarter of the fight wavelength). The lower boundary is conditioned by the existence of crystalline particles at the interface between the crystalline phase and the quasi-molecular one. Less often the terms molecular aggregation and crystallite clusters are used. 

Plasmon based optical spectroscopy of metal clusters is best described by electrodynamics. To describe the behavior of the plasmon oscillation it is appropriate to apply a quasi-static regime only for a cluster of around 10-20 nm. The static regime assumes that the phase shift in the colloidal particle is small enough to be neglected reducing the cluster oscillation to a simple dipole. 

Catalysts based on colloidal suspensions attracted much attention in recent years, both as supported and as quasi-homogeneous catalysts. These catalysts are prepared by the so-called reduction-deposition method, where a metal is first reduced in solution in the presence of a stabiliser before it is deposited on a heterogeneous support. By using the appropriate reaction conditions, met crystallites of < 10 nm are av able. These catalysts are called nanocatalysts, although the metal crystallite size is not much different from commercial catalysts prepared by traditional methods. However, the metal crystallite size distribution of the nanocatalysts is narrower and they do not contain large metal crystallites that are sometimes observed in traditional catalysts (see right-hand side of Figure 1). 

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