Diffusion of platinum ions and platinum nanoparticles during photoreduction processes using the transient grating method

The photoreduction process of PtCl ₆ ^2- to Pt nanoparticles in poly(N-vinyl-2-pyrrolidone) solutions upon UV light irradiation was investigated by monitoring the change in the diffusion coefficient (D). The D values of chemical species during UV irradiation was measured by the laser-induced transient grating (TG) method. The TG signal of the PtCl ₆ ^2- solution before UV irradiation was composed of three kinds of contributions, the thermal grating, the species grating due to the creation of PtCl ₄ ^2-, and the species grating due to the depletions of PtCl ₆ ^2-. Upon UV irradiation of the solution, the species grating signal due to PtCl ₆ ^2- diminished and then the TG signal of Pt nanoparticles gradually appeared. This result indicates that the gradual clustering of Pt ⁰ atoms into Pt nanoparticles occurs after all PtCl ₆ ^2- ions are photochemically reduced to PtCl ₄ ^2- and subsequently transformed to Pt ⁰ atoms with a short delay. With increasing time of the UV irradiation, the TG signal intensity increased, while D of the Pt nanoparticles did not change. This suggests that the number of Pt nanoparticles increases, but the size of the Pt nanoparticles with the polymer layer is unchanged, in the course of the UV irradiation.