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 A and 925 A, 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 A) 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 A).