Fractal aggregates of the Pt nanoparticles synthesized by the polyol process and poly(N-vinyl-2-pyrrolidone) reduction

Small-angle X-ray scattering was used to characterize the size and aggregation behavior of the Pt nanoparticles synthesized by the polyol process and the unusual poly(N-vinyl-2-pyrrolidone) (PVP) reduction. With formaldehyde (HCHO) as the reduction agent, the Pt nanoparticles synthesized in aqueous solutions with a high PVP/PtCl4 weight ratio were characterized by short rods with a 70% polydispersity in rod length. The size and size distribution of the rod-like Pt nanoparticles (3 nm in rod length and 2 nm in rod diameter) are consistent with the corresponding transmission electron microscopy image. With a comparable PVP/PtCl4 weight ratio in the aqueous solution containing HCHO, the high number density of reduced Pt nanoparticles led to a fractal-like aggregation with a fractal dimension of 2.1 and a correlation length of ~30 nm. We also demonstrated that Pt nanoparticles can be synthesized by PVP reduction at 323 K without HCHO. The particle size and the clustering behavior of the Pt nanoparticles reduced by PVP are closely related to the PVP concentration in the solution. Both the Pt nanoparticles synthesized in the commonly used polyol process and the unusual PVP reduction form fractal-like clusters via the PVP–metal nanoparticle association when the number density of the Pt nanoparticles in the solutions is high.