Structure and composition of the Pt(s) [6(111) × (111)] step-terrace surface vs. pH and potential in aqueous Br − solutions : Studies by LEED and Auger spectroscopy

Abstract

Experiments were performed in which a well-characterized Pt(s) [6(111)×(111)] surface (consisting of terraces five atoms wide separated by steps of one atom height) was immersed into aqueous CaBr 2 solutions at controlled potential, after which the surface was removed from solution, evacuated and characterized by LEED and Auger spectroscopy. Adsorption of Br was strongest from acidic solutions, displaying a maximum of θ Br = 0.5 (Br per surface Pt atom) near 0.2 V (vs. Ag/AgCl reference). At packing densities near the maximum, the Br layer displayed Pt(s) [6(111)×(111)](3×T) LEED patterns, which were relatively diffuse, indicating imperfect long-range order with a repeat distance of three PtPt distances parallel to the steps. Chemisorbed oxides or hydroxides were formed at relatively positive potentials. A streaked Pt(s) [6(111)×(111)] (9×NT) LEED pattern was found at potentials where oxidation led to similar Br and O/OH packing densities, indicating the presence of a seven-unit repeat distance parallel to the Pt steps and a variable multiple of the terrace width perpendicular to the steps. Retention of Ca 2+ ions by the surface was potential dependent, with a minimum which coincided with the Br maximum, θ Ca = 0.02. Water was retained by the surface due to interaction with Ca 2+ to the extent of 5–15 water molecules per Ca 2+ ion, depending upon the pH and potential. Comparison of these data with similar results for atomically smooth Pt (111) surfaces revealed that bromide was not selectively adsorbed at steps of the Pt surface.