New Insights on the Nanoparticle Growth Mechanism in the Citrate Reduction of Gold(III) Salt: Formation of the Au Nanowire Intermediate and Its Nonlinear Optical Properties

Abstract

The synthesis of gold nanospheres via the reduction of AuCl4- salt by sodium citrate has been revisited, with focus on the mechanism of particle growth. In stark contrast to the widely accepted LaMer nucleation-growth model, the initially formed nanoclusters of about 5 nm diameter self-assemble to form an extensive network of nanowires. The diameter of the nanowires progressively increases in size, and at the same time the connected network is fragmented into small segments before the final spherical particles are formed. We put forward a modified nanocrystal growth mechanism, which is able to explain a number of other experimental observations. By arresting the reduction process by rapid cooling, the 5 nm diameter nanowires could be isolated and were found to be very stable. The nonlinear optical properties of the Au nanowires were investigated and discussed. Highly connected, small diameter nanowires of highly conductive metals may be useful for future electronic applications, e.g., conductive polymer−metal composites with low metal loading.