Visible-Light-Assisted Silver Ion Reduction through Silver Diammine and Citrate Aggregation, and Silver Nanoparticle Formation

Hexagonal silver nanoparticles are directly formed in a solution of 6.6 mM silver citrate and 132 mM ammonia irradiated by 1.98–2.46 eV visible light. The corresponding silver ion concentration is 19.8 mM, which is several orders of magnitude higher than those employed in other silver-nanoparticle-formation experiments. In the present study, the roles of silver citrate and ammonia on nanoparticle formation are investigated through experiments in which the concentrations of silver citrate (SC) and ammonia (NH3) are altered. Silver nanoparticles are efficiently formed when [SC] is the 1.65–6.6 mM range and the [NH3]/[SC] ratio is ∼8–16. Further, hexagonal nanoplates are dominantly formed when [SC] = 1.65–6.6 mM and [NH3]/[SC] = ∼16. Within this range, hexagonal nanoplate formation is insensitive to solution concentration. Concentrations of SC less than 1.65 mM, or NH3 ≥ 132 mM, inhibit the formation of silver nanoparticles. These observations suggest that aggregates composed of diammine silver complexes and citrate are formed at specific concentration ranges of SC and NH3, and they assist in the photoreduction of silver ions by 1.98–2.46 eV visible light. Furthermore, the lateral growth of platelet seeds is proposed to be the dominant mechanism for the formation of hexagonal nanoplates at [SC] values of 1.65–6.6 mM and [NH3]/[SC] = ∼16.