Metal films and particles on oxide surfaces: Structural, electronic and chemisorptive properties

The interface between metals and oxides is critical in many technologies, including materials science, microelectronics and catalysis. Recent surface science studies of ultrathin vapour-deposited metal films on well defined surfaces of oxides under clean conditions of ultrahigh vacuum are reviewed. The geometric and electronic structure of the metal films and their chemisorption properties are described in detail, with illustrative examples from the author's work. Oxidation of the admetal and efficient spreading occurs in the first monolayer when the admetal is an alkali metal or an early transition metal whose oxide's heat of formation exceeds the enthalpy requirement for reducing the substrate. Repulsive lateral interactions between alkali-metal adatoms leads to a strongly decreasing heat of adsorption with coverage, and depolarization, as on metal substrates. Mid-to-early transition metals form three-dimensional metallic particles and leave part of the oxide surface uncovered, when thermodynamics control the reaction. Nevertheless, kinetic limitations can sometimes lead to nearly full coverage by the first monolayer at low temperatures. Although the first 1-10% of such monolayers is cationic, the remainder is usually nearly neutral, and forms disordered islands which nucleate at defects. Even when such islands are only one atom thick, they have chemisorption properties that are very similar to bulk metal surfaces with a high degree of coordinative unsaturation. This indicates that metal-metal bonding parallel to the surface resembles that in bulk metals, with weaker bonding to the oxide below. Deposition conditions and defect concentration can influence the fractional coverage at which 3D growth begins. Islands that are thicker than a few layers are often crystalline, and expose mainly low-index metal facets whose chemisorption properties resemble those facets. Subtle differences exist between the chemisorption properties of these 2D and 3D metal islands and bulk facets, which could dramatically affect catalytic performance.