Formation Mechanism of Gold Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol Solutions of Polymers Using In Situ Quick Scanning X-ray Absorption Fine Structure and Small-Angle X-ray Scattering

The photoreduction process of [AuCl4](-) precursors in poly(N-vinyl-2-pyrrolidone) solution was investigated by utilizing a combination of in situ quick scanning X-ray absorption fine structure (QXAFS) and small-angle X-ray scattering (SAXS) measurements to elucidate the nucleation and subsequent aggregative particle growth of colloidal gold nanoparticles (Au NPs). The sequence of stages for the reduction-nucleation and association process (autocatalytic surface growth and aggregative particle growth) of Au atoms to generate Au NPs was observed during the photoreduction. QXAFS analysis revealed that the reduction of [AuCl2](-) species to Au-0 atoms is a slower step than that of [AuCl4](-) to [AuCl2](-), and the reduction of [AuCl2](-) to Au-0 atoms and the association of Au-0 atoms to produce Au nucleates concurrently proceeds during short-duration photoirradiation. A sigmoidal curve expressed by the solid-state kinetic model, specifically the Avrami-Erofe'ev model, was applicable to demonstrate the aggregative particle growth. Complementary in situ SAXS results showed that Ostwald ripening based growth of Au NPs is not observed after the termination of the aggregative growth stage in the long-duration photoirradiation.