Formation of metal nanoparticles (silver (Ag) and palladium (Pd) nanoparticles) in water/alkane/sodium bis­(2-ethylhexyl) sulfosuccinate (AOT) water-in-oil (w/o) microemulsions consisting of AOT, water, and organic solvent (n-heptane, n-octane, n-decane, and isooctane) by photoreduction was monitored by means of a combination of in situ quick X-ray absorption fine structure (QXAFS) and small-angle X-ray scattering (SAXS) measurements. The stages of reduction–nucleation and the association process (aggregative particle growth and Ostwald ripening) of metal atoms to produce metal nanoparticles were discriminated in the course of the photoreduction time. In situ QXAFS results indicated that the aggregative growth follows a sigmoidal profile described both by the solid-state kinetic model, specifically the Avrami–Erofe’ev model, and by the chemical-mechanism-based kinetic Finke–Watzky model. The SAXS results clearly showed that the formation of Ag and Pd nanoparticles preferentially follows a reduction–nucleation and subsequent aggregative particle growth before the starting of Ostwald ripening-based growth. The rate of nucleation and aggregative growth in the formation of Pd nanoparticles tends to be higher than that in the formation of Ag nanoparticles, resulting in the larger particle size of Ag nanoparticles. Another interesting finding is that the growth rate and final particle size is inversely related where a decrease in growth rate corresponds to larger particle sizes.

3-Dimensional atomic-scale structure of metal nanoparticles obtained by RMC-based simulations using HEXRD and EXAFS data.