Diffusion of gold ions and gold particles during photoreduction processes probed by the transient grating method

The translational diffusion of Au (gold) ions and Au particles during the photoreduction process from AuCl4(-) to metallic Au particles in aqueous ethanol solutions containing poly(N-vinyl-2-pyrrolidone) was investigated by using UV-vis absorption and the laser-induced transient grating (TG) methods. The TG signal of AuCl4(-) solution before photoirradiation was composed of three contributions; the thermal grating, the species grating due to the creation of AuCl2(-), and that due to the depletion of AuCl4(-). Upon photoirradiation, the species grating signal due to AuCl4(-) diminished rapidly and the TG signal due to Au particles appeared within a few minutes and became stronger at a very short time. The subsequent reduction of AuCl2(-) was concomitant with the formation of Au metal particles. The rapid growth of Au(0) atoms into Au particles took place in the short-duration photoirradiation. With the increase of the photoirradiation time, the TG signal was composed of two kinds of Au particle fragments possessing different diffusion coefficients. This is probably due to the pulse-laser induced fragmentation of the larger Au particles. Effects of the polymer on the particle formation were investigated by the concentration dependence of the polymer in the solution. The formation of Au particles by the photoreduction was also compared with that of Pt particles.