In situ XAFS studies of Au particle formation by photoreduction in polymer solutions

Formation mechanisms of metal particles (gold (Au) particles) in an aqueous ethanol solution of HAuCl₄ with poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction method were investigated by UV-vis, transmission electron microscopy (TEM), and in situ and ex situ X-ray absorption fine structure (XAFS) analysis. The average diameters of the dilute and concentrated Au particles in PVP solution are estimated from TEM to be 106 Å and 925 Å, respectively. XAFS analysis was carried out to elucidate the reduction process of AuCl₄^- ionic species to metallic Au particles for the Au-L₃ edge of the colloidal dispersions of the concentrated Au solutions. In the photoreduction process, the reduction of AuCl₂ ^- species to Au⁰ atoms is a slower process than that of AuCl₄^- to AuCl₂^-, and the reduction of AuCl₂^- to Au⁰ atoms and the association of Au⁰ atoms to form seed Au particles (particle diameter between 5.5 and 30 Å) concurrently proceeds in the short-duration photoirradiation. In addition, in the long-duration photoirradiation, the slow progression of Au particle growth occurs with the association of Au⁰-Au⁰ metallic bonds, resulting in the formation of larger Au particles (particle diameter larger than 500 Å).