Silver aggregates of varying size and shape are known to form when aqueous solutions of silver ions and organic acids are exposed to UV and visible light for extended periods, but the products and growth kinetics are often irreproducible from solution to solution. We have undertaken a detailed mechanistic study to identify the source of the variations, with the goal of developing a method to control the size dispersion and morphology of photoproduced nanoparticles. Our results show that trace levels of transition metal ions ([M+n] / [Ag+] &lt; 0.001), particularly di- and tri-valent ions present for example in the starting reagents, play a dominant role in initiating the nanoparticle growth. Some metal-organic complexes formed between these ions and the acids can absorb light through a charge transfer transition, resulting in the production of reduced ions as well as free radicals, both of which can serve to reduce Ag+. Once Ag0 is produced, the growth of silver nanoparticles proceeds rapidly. By using ultrapure reagents and introducing known amounts of the transition metal ions, the size dispersion and shape of the nanoparticles can be controlled. The generality of this mechanism, including the production of other types of nanoparticles, and its possible application to environmental processes will be discussed.

Photoinitiated growth of silver nanoparticles in solutions of organic acids

256th National Meeting (2018)

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