LONG-LIVED NONMETALLIC SILVER CLUSTERS IN AQUEOUS-SOLUTION - A PULSE-RADIOLYSIS STUDY OF THEIR FORMATION

Deaerated solutions of AgClO4 containing (1-5) × 10-4 M sodium polyphosphate and 0.1-1 M alcohol are irradiated with single electron pulses or trains of pulses and the intermediates of silver ion reduction detected by optical absorption measurements. Polyphosphate is found to exert a drastic effect on the reduction as small nonmetallic silver clusters are stabilized. In fact, when single pulses are applied, in which only a few percent of the Ag+ ions are reduced, the cluster Ag42+ is the final product which lives for hours. The elementary steps leading to this cluster are investigated. They include (1) reduction of Ag+ ions by hydrated electrons, (2) complexation of the Ag0 atoms formed to yield Ag2+, and (3) dimerization of the Ag2+ complexes. Reaction 1 occurs rather slowly when Ag+ ions adsorbed on polyphosphate chains are involved. Under the experimental conditions chosen, reactions 1 and 2 occur mainly in bulk solution. Reaction 3 is accelerated by a factor of 10 in the presence of polyphosphate. The effect is understood in terms of the reduction in the dimensionality of reaction space as the Ag2+ complexes are attracted by polyphosphate chains. A small fraction of the Ag2+ escapes dimerization by stabilization on the polyphosphate chains. When pulse trains are applied, up to 100% Ag+ reduction can be achieved. The first clusters, i.e., Ag2+ and Ag42+, are further reduced to yield larger clusters Agnof nonmetallic silver having absorption bands at 300 (n = 3) and 330 nm (n > 3) and in the 345-360-nm range (n ≫ 3). A small amount of colloidal metallic silver is also formed. © 1990 American Chemical Society.