Small-angle x-ray scattering analysis of polymer-protected platinum, rhodium, and platinum/rhodium colloidal dispersions

Formations of poly(N-vinyl-2-pyrrolidone)(PVP)-metal ion (Pt, Rh or Pt/Rh(1/1)) complexes in H2O/C2H5OH (“polymer–metal ion complexes”) before alcohol-reduction and PVP–Pt, Rh, or Pt/Rh(1/1) metal cluster (“polymer–metal cluster”) after alcohol-reduction were studied by small-angle x-ray scattering (SAXS). SAXS intensity of the solution containing polymer–metal ion complexes is higher than that of the reference polymer solution without metal ions, suggesting that concentration fluctuations of polymer segments are enhanced due to the formation of polymer–metal ion complexes. Spatial distributions of metal clusters in colloidal dispersions are different from each other among the cases of Pt, Rh, and Pt/Rh(1/1) colloidal dispersions. The superstructure (greater than 10.0 nm in diameter), whose average sizes highly depend on the metal element employed, are formed. These superstructures are composed of several fundamental clusters with a diameter of ∼2.0–4.0 nm. Colloidal dispersions of Rh form a percolation network of clusters with an average period of ∼6.0 nm inside the higher-order organization of its superstructure. Experimental results concerning the physical aging show the high stability of colloidal dispersions of metal clusters protected by water-soluble PVP.