Optical absorption and catalytic activity of subcolloidal and colloidal silver in aqueous solution: A pulse radiolysis study

Abstract

Silver atoms were formed in aqueous Ag 2SO 4 solution by a pulse of high-energy radiation and the subsequent agglomeration processes were followed by recording the corresponding changes in optical absorption. The first product of agglomeration that possesses the absorption band of colloidal silver in the near ultraviolet has only four reduced silver atoms. The absorption maximum is first located at 378 nm and the absorption coefficient is 2.8 × 10 3 g-atom −1 liter cm −1. With increasing agglomeration, the maximum shifts to 380 nm and the absorption coefficient to 1.5 × 10 3 g-atom −1 liter cm −1. Above an agglomeration number of 12, the position of the maximum and the absorption coefficient do not change anymore. In another series of experiments, solutions containing Ag + as well as Cu 2+ were pulse irradiated. Both Ag 0 and Cu + were initially formed at known concentrations. It is shown that Ag + is reduced by Cu + at the growing silver aggregates. However, the reaction requires a certain size of the latter. The critical size lies in the subcolloidal range at about n = 10. Shifts in the redox potential of the Ag + Ag n system with the agglomeration number n are discussed in order to explain this effect.