Toward to branched platinum nanoparticles by polyol reduction: A role of poly(vinylpyrrolidone) molecules

Branched Pt nanoparticles with nanometer sizes have been successfully synthesized by reduction of H2PtCl6·6H2O precursor in ethylene glycol (EG) in the presence of small amounts of NaNO3 and PVP. Morphologies of the Pt nanoparticles can be systematically evolved from regular octahedron, and triangular plate via tri-pod, penta-pod, and octa-pod to multi-pod needle-like shapes only by decreasing concentrations of H2PtCl6·6H2O and NaNO3 at a constant NaNO3/H2PtCl6·6H2O molar ratio and the same PVP concentration. To the best of knowledge, this is the first report for the synthesis of Pt penta-pod. High resolution transmission electron microscope (TEM) observation of the Pt nanoparticles demonstrates that the Pt branches actually extendedly grow out from certain angles of triangular plates, octahedrons, and decahedrons, respectively. Multi-branched needle-like Pt nanocrystals are believed probably to originate from further anisotropic growth of the Pt octa-pods or overlap of small branched Pt nanoparticles. PVP molecules have been found to play an important role in controlling morphologies of the branch-like Pt nanoparticles besides NaNO3. It probably is the cooperated kinetic adsorption and desorption of PVP molecules and various anions on particle surfaces that influence the growth of the Pt nanoparticles. A reasonable growth mechanism has been suggested to explain the evolution of the Pt branches, in which the difference among growth rates along various crystallographic directions of face-centered cubic Pt crystal probably determines final morphologies of the Pt nanocrystals.