In situ time-resolved XAFS analysis of silver particle formation by photoreduction in polymer solutions

摘要

Formation mechanisms of silver (Ag) particles in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction of AgClO_4 were investigated by using UV-Vis, transmission electron microscopy (TEM), extended X-ray absorption fine structure (XAFS), and in situ energy-dispersive X-ray absorption fine structure (in situ DXAFS) measurements. The average diameter of the Ag particles prepared in the presence and absence of benzoin as a photo-activator was estimated from TEM to be 7.4 and 5.1 nm, respectively, and their particle size distributions ranged from 2 to 15 nm. XAFS analysis indicated that metallic Ag particles were synthesized, and the reduction rate of Ag~+ to Ag~0, the creation of Ag~0-Ag~0 bonds and their particle growth by the association of Ag~0-Ag~0 was regulated by the metal concentration and the inclusion of benzoin. In situ DXAFS measurements were performed in real time to investigate the kinetics of Ag~+ reduction and Ag particle formation. During the induction period the reduction of Ag~+ to Ag~0 occurred, and subsequent nucleation and growth process concurrently proceeded after the induction period. The intermediate small Ag particles (C.N. smaller than about 4) were generated in the nucleation process, and grown up to larger particles (C.N.s equal to 4.5 ± 1.5) in the longer-duration photoreduction.