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

Ultrathin metal films on clean and well-defined oxide surfaces have been prepared by several groups using vapor deposition techniques in ultrahigh vacuum, and the structural, electronic and chemisorptive properties of these films have been characterized using a variety of surface science techniques. Those studies will be reviewed here, and trends in these properties will be identified. While films of mid- or late-transition metals can sometimes be grown in a quasi-layer-by-layer fashion at very low temperatures where kinetics prevail, heating these usually leads to thickening into the thermodynamically preferred structure: three-dimensional metal particles that cover only a fraction of the oxide surface. This occurs in two stages: individual island thickening, then Ostwald ripening. The kinetics of growth, nucleation and thickening will be examined. In some cases, this thermodynamic preference can be shifted to favor complete spreading of the metal film by adding gas molecules. This is driven by the higher adsorption energy of the molecule on the metal sites. Early transition metals which have very stable oxides can partially reduce the substrate oxide, and themselves become oxidized. The chemisorption properties of metal overlayers correlate well with their structural and electronic properties. Sites formed by neutral metal adatoms in these films on oxide surfaces often have chemisorption properties resembling that of some bulk metal crystalline plane. Surprisingly, this is even true for metal islands that are only one atom thick, in which case they resemble a very open or coordinatively unsaturated plane. However, the sites on and in the nearby oxide can alter the final surface chemistry, because adsorbates, especially hydrogen adatoms, can diffuse rapidly from the metal particles to these sites (spillover). While some real advances have already been made, many fundamental questions which are of great importance in oxide-supported metal catalysis and other fields remain to be addressed with this approach.