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.