PHYSICOCHEMICAL PROPERTIES OF SMALL METAL PARTICLES IN SOLUTION - MICROELECTRODE REACTIONS, CHEMISORPTION, COMPOSITE METAL PARTICLES, AND THE ATOM-TO-METAL TRANSITION

The study of nanometer and subnanometer colloidal metal particles in aqueous solution complements the investigation of small particles in molecular beams, frozen solutions, and inorganic clusters. The electronic properties of the metal particles are changed by surface modification, for example by chemisorption of a nucleophilic molecule or by deposition of a second metal. The resulting changes in the chemical, photochemical, and optical properties are discussed. Methods are described which enable one to store excess electrons or positive holes on the particles in a controlled manner and to investigate the accompanying changes in the optical properties. Metal particles carrying excess electrons initiate electrochemical reactions such as the reduction of water or the deposition of metals. Concentric bimetallic and trimetallic particles can be synthesized this way. The transition from the atom to the metal can be studied by pulse radiolysis: a known concentration of atoms is generated by a pulse and the development of the metal absorption spectrum is then recorded as a function of time as the atoms coalesce to yield larger particles. It is often observed that the absorption spectrum of the metal is reached after only a few coalescence steps, i.e. at agglomeration numbers of about 10. In the case of silver, two ''magic'' clusters which are not metallic and having long lifetimes (100 s and 15 min) are observed during the coalescence. These clusters have ''giant'' absorption bands at wavelengths distinctly shorter than that of the 380-nm plasmon band of metallic silver particles. The clusters live for hours and days in the presence of polyanions. They have reducing properties and also react eagerly with nucleophilic reagents. They can be fragmented upon illumination and photoemit electrons with quantum yields greater than 0.1. The dependence of the standard redox potential on particle size is also discussed. The atom-to-metal transition manifests itself also in the colloid chemical properties of metal particles (transition from complex formation with anions of oligomeric clusters to double layer formation around larger particles).