Small-Angle X-ray Scattering Study of Platinum-Containing Hydrogel/Surfactant Complexes

Size distributions of platinum nanoparticles embedded in nanostructured hybrid matrixes and the internal structure of these systems were studied by anomalous and conventional small-angle X-ray scattering. The complexes of polyelectrolyte gels with oppositely charged surfactants were employed as nanostructured matrixes. Two complexes of a cationic gel of poly(diallyldimethylammonium chloride) with anionic surfactants sodium dodecyl sulfate and sodium dodecylbenzene sulfonate were charged with platinum compounds (PtCl4, Na2PtCl6, (NH4)2PtCl4, and H2PtCl6) and reduced with NaBH4 and N2H4×H2O. Fast reduction with NaBH4 yields mostly small Pt nanoparticles with radii about 2 to 3 nm, whereas for N2H4×H2O, a significant amount of large (up to 40 nm) particles is found. The pH of the reaction medium and the Pt ion geometry were found to influence particle nucleation and growth. The internal ordering in gel/surfactant complexes during the nanoparticle formation was characterized from the Bragg peaks in the scattering patterns. Although the magnitude of the peaks diminishes after metal nanoparticle formation, quantitative peak parameters indicate an increase of the degree of order. This suggests that the highly ordered zones in the hydrogels concentrate around the growing nanoparticles thus stabilizing them. Addition of Na2PtCl6 to another complex, an anionic gel of poly(methacrylic acid) with a cationic surfactant cetylpyridinium chloride, results in drastic structure rearrangements. The order observed in the collapsed gel nearly degrades due to a competitive interaction of the negative PtCl6 2- ions with positively charged pyridinium heads and a new micellar-like structure is formed instead. Further Pt ion reduction restores the initial gel structure and yields very large (radii up to 80 nm) metal particles growing outside the areas of the surfactant ordering.