Chemical and Optical Properties of Small Metal Particles in Aqueous Solution

Although colloidal metals are systems with a huge amount of interface, relatively few surface chemistry studies of these systems have been made. A number of surface reactions on metallic silver particles (4‐nm diameter) are described in the present article and the accompanying changes in the optical properties are discussed. The reactions include the deposition of excess electrons and positive holes, changes in the Fermi level position and reactivity upon adsorption of various molecules, and photochemical reactions. The preparation and properties of oligomeric silver clusters in aqueous solution are also described. The clusters have strong absorption bands, undergo electron transfer with many acceptors, photoemit electrons with large quantum yields, and react eagerly with nucleophiles. The cluster Ag 4 2+ , a special form of dimeric silver, is stable towards oxygen, whereas larger clusters are readily oxidized. The chemical changes associated with the colloid → atom transition are demonstrated by discussing various properties such as the electrochemical potential of silver aggregates as a function of size. Cluster formation in the reduction of other metals (Pd, Cu, Au, Pb) in aqueous solution is also discussed briefly.