REDUCTION AND AGGREGATION OF SILVER IONS AT THE SURFACE OF COLLOIDAL SILICA

The reduction of silver ions at the surface of 40 Angstrom colloidal s ilica particles at pH 9 has been studied using the pulse radiolysis te chnique. The rate of reaction of e(aq)(-) with Ag+ increases linearly with an increase in the Ag+/SiO2 ratio until a ratio of 12:1 is reache d. At this loading the reaction becomes independent of the silver conc entration at the particle surface. Ag-0 and Ag-2(+), the first product s of silver ion reduction in aqueous solution, have been identified by their absorption spectra as the primary species formed by the reducti on of silver ions at the surface of SiO2. Within the time window of th e experiments (less than or equal to 200 mu s) there is no evidence of larger silver cluster formations (e.g., Ag-4(2+)) that are formed und er identical conditions in the absence of silica. Evidently, the silic a surface stabilizes the Ag-2(+) cation. Irradiation of Ag+/SiO2 solut ions with trains of electron pulses leads to the formation of two smal l silver clusters exhibiting spectral bands at 290 and 330 nm and the conventional colloidal particle with its. band at similar to 400 nm.. These clusters are stable-in the presence of oxygen for at least sever al weeks. Addition of redox active and reducing agents confirmed that the three spectral bands originate from three different clusters.