Evidence for Seed-Mediated Nucleation in the Chemical Reduction of Gold Salts to Gold Nanoparticles

Central to the concept of seed-mediated growth of nanoparticles is that small nanoparticle seeds serve as nucleation centers to grow nanoparticles to a desired size. We have examined this common assumption in a model system, the wet chemical synthesis of gold nanoparticles via reduction of a gold salt, by transmission electron microscopy and electronic absorption spectroscopy. We find that changing the seed concentration does affect the size of the product nanoparticles, but the method of reagent addition drastically affects the outcome even more. For fast addition of reducing agent, the presence of seeds appears to promote the formation of more seeds instead of growth. The observed nucleations are drastically enhanced (99%) compared to particle growth. For slow addition of reducing agent, the seeds do grow, but the product nanoparticle's degree of homogeneity in shape is compromised. For higher concentrations of seeds, nanoparticle growth is better controlled for slow addition of reducing agent compared to fast addition of reducing agent. We propose a mechanistic step to explain the commonly observed size distribution.