Nucleation and Growth of Metal Nanoparticles during Photoreduction Using In Situ Time-Resolved SAXS Analysis

Formation of metal nanoparticles (rhodium (Rh) and palladium (Pd) nanoparticles) in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by photoreduction was monitored by means of in situ and time-resolved small-angle X-ray scattering (SAXS) measurements. The kinetics of association process (nucleation, growth, and particle coalescence) of metal atoms to produce metal nanoparticles was successfully evaluated by the quantitative SAXS analysis for the number-average of radius (R-0), number of particles (n(p)), standard deviation in radius (sigma(R)/R-0), and total volume (V-p) of metal nanoparticles produced by the photoreduction. The time evolution of particle size, number of particles, and particle size distribution indicated that the formation of Rh nanoparticles predominantly follows an autocatalytic reduction-nucleation process before the onset of Ostwald ripening-based growth. On the other hand, in the formation of Pd nanoparticles, the reduction-nucleation process occurred significantly faster at the early stage of the reaction, and the dominant growth of Pd nanoparticles subsequently proceeded via an Ostwald ripening-based growth mechanism. It was also found that the rate of nucleation and growth during the metal particle formation is strongly affected by the initial metal concentration and the addition of NaCl.