Reduction and Aggregation of Silver Ions at the Surface of Colloidal Silica

Abstract

The reduction of silver ions at the surface of 40 [angstrom] colloidal silica particles at pH 9 has been studied using the pulse radiolysis technique. The rate of reaction of e[sub eq][sup [minus]] with Ag[sup +] increases linearly with an increase in the Ag[sup +]/SiO[sub 2] ratio until a ratio of 12:1 is reached. At this loading the reaction becomes independent of the silver concentration at the particle surface. Ag[sup 0] and Ag[sub 2][sup +], the first products of silver ion reduction in aqueous solution, have been identified by their absorption spectra as the primary species formed by the reduction of silver ions at the surface of SiO[sub 2]. Within the time window of the experiments ([<=] [mu]s) there is no evidence of larger silver cluster formations (e.g., Ag[sub 4][sup 2+]) that are formed under identical conditions in the absence of silica. Evidently, the silica surface stabilizes the Ag[sub 2][sup +] cation. Irradiation of Ag[sup +]/SiO[sub 2] solutions with trains of electron pulses leads to the formation of two small silver clusters exhibiting spectral bands at 290 and 330 nm and the conventional colloidal particle with its band at approximately 400 nm. 23 refs., 7 figs.