Correlations between structure and chemical composition on oxidized (Pt, Ni)₃Al(1 1 1) surfaces

We have investigated the fully-oxidized surface that forms on (Pt, Ni)₃Al(1 1 1) at temperatures ranging from 300 to 1000 K and at oxygen pressures of ca. 10^-6 to 10^-7 Torr, using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Based on X-ray photoelectron spectroscopy data that were published previously, oxidation temperatures below 700 K at these pressures produces a mixture of surface Al₂O₃ and NiO, with NiO being the predominant oxide. At 800-1000 K, pure Al₂O₃ exists. In this work, STM data from a sample containing 10 at% Pt show that oxidation causes an increase in roughness, relative to the clean surface. The apparent roughness correlates directly with NiO content, both of which reach a maximum at an oxidation temperature of 500 K. The oxide surface is smoothest when it consists of pure Al₂O₃, i.e. after oxidation at 800-1000 K. In terms of crystallinity, LEED data show that the Al₂O₃ which forms at 1000 K is ordered, but its structure on the Pt-containing samples is different than reported previously for the Pt-free surface. This is true despite the structure of the clean surface being unperturbed by Pt, based on STM and LEED. The different structure of the oxide probably relates to the fact that the oxide is also thinner in the presence of Pt. The change in oxide structure undoubtedly correlates with a change in stability and adhesion of the oxide, both of which are of paramount importance in industrial applications.