Morphological control of catalytically active platinum nanocrystals.

Colloidal metallic nanocrystals have been explored for catalytic applications, including fine chemicals synthesis, fuel-cell technology, hydrogen production, and gas sensing. The catalytic activity of a metallic catalyst depends strongly on its surface properties. For instance, hexagonal (111) Pt surfaces are 3–7-times more active than cubic (100) surfaces for aromatization reactions. The reactivity and selectivity of nanoparticles can therefore be tuned by controlling the morphology because the exposed surfaces of the particles have distinct crystallographic planes depending on the shape. A variety of Pt nanostructures, including polyhedra, wires, tubes, dendritic structures, and multipods have been synthesized by regulating growth at specific surfaces or by templating methods. Amphiphilic polymers or surfactants typically stabilize high-energy surfaces of nanoparticles. However, chemical reactions can only occur effectively on catalytically “clean” nanoparticles when the reactants adsorb more strongly to the particle surface than the surfacestabilizing agents do. When the interaction between the stabilizing agent and metal surface is too strong, the catalytic activity is greatly reduced. For instance, the carbonyl group of polyvinylpyrrolidone (PVP) or polyacrylate, which are the most widely used surface-regulating polymers in shapednanoparticle synthesis, interacts strongly with the platinum surface and thus blocks a significant number of active sites. On the other hand, alkylammonium ions have been widely used in synthesizing Au nanoparticles, and their interactions with Pt surfaces are considerably weaker than that of the carbonyl group. Therefore, this class of molecules could serve as ideal surface-stabilizing agents that can regulate the shape of nanoparticles while preserving catalytically active sites.