Long-Lived Nonmetallic Silver Clusters in Aqueous Solution: A Pulse Radiolysis Study of Their Formation

Article abstract of DOI:10.1021/j100373a056

Deaerated solutions of AgClO4 containing (1-5)*1E-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 100percent Ag2⁺ reduction can be achieved.The first clusters, i.e.Ag2⁺ and Ag4²⁺, are further reduced to yield larger clusters Agn of nonmetallic silver having absorption bands at 300 (n = 3) and 330 nm (n greater than 3) and in the 345-360 nm range (n much greater than 3).A small amount of colloidal silver is also formed.