THE EQUILIBRIUM SHAPE AND SURFACE-ENERGY ANISOTROPY OF CLEAN PLATINUM

We have measured the equilibrium shape of clean platinum by monitoring the changes in the shape of a series of micrometer-sized platinum droplets during annealing at 1200°C in 10-7 Torr of oxygen. The droplets start out with a rounded shape and a wavy surface. Upon annealing for 24 h or more, the particles are converted to nearly spherical shapes with distinct facets in the (100) direction and smaller facets in the (111) direction. At equilibrium, the facets only cover 16% of the particle's surface. The rest of the surface is stepped or kinked. The equilibrium particle shape is also independent of particle size for particles in the size range explored here (d > 1 micrometer). Initially, the particles show a series of hill and valley structures around the (100) facets. However, the hill and valley structures disappear after a long anneal. After a long anneal, the particles also assume an epitaxial relation with the support. From the data, we estimatethat the surface energy of platinum only varies by about 92 erg/cm2 with crystal face, with Pt(100) having the lowest surface energy, Pt(111) having a slightly higher surface energy, and all of the other faces of platinum having essentially the same surface energy. By comparison, previous workers have shown that heats of adsorption of simple gases often vary by 350 erg/cm2 with crystal face. The implication, therefore, is that in platinum the variations in surface energy with orientation are small enough that particle shape control in supported metal catalysis is feasible. Consequences to the stability of various single crystal faces are also discussed. © 1990.