The heats of adsorption as a function of coverage have been determined for copper adsorption onto several well-defined oxide thin film surfaces at room temperature by microcalorimetric measurements. The heats of adsorption are accurately determined as a function of coverage with resolution of 2% of a monolayer. For all three oxide surfaces investigated, MgO(100), a p(2x1) molybdenum oxide film on Mo(100) and disordered W oxide, the initial heat of copper adsorption is much lower than the heat of sublimation for Cu (337.4 kJ mol(-1)). On MgO(100) the initial Cu heat of adsorption in the first 2-4% of a monolayer is 240 kJ mol(-1) and increases rapidly to the heat of Cu sublimation. Auger spectroscopy shows that Cu grows on MgO(100) as two-dimensional (2-D) islands until approximate to 0.3 monolayers where it switches to the growth of 3-D islands, at which point the heat of adsorption of Cu reaches approximate to 92% of its heat of sublimation. The room temperature sticking probability of Cu on MgO was also investigated as a function of coverage and determined to be > 0.99. On the ordered p(2x1) oxide of molybdenum on Mo(100), the initial Cu heat of adsorption is 287 kJ mol(-1). The heat of adsorption then decreases slightly to 278 kJ mol(-1) in the first 15% of a monolayer, after which it rapidly increases to the heat of sublimation. Similarly, on the disorder W oxide surface the initial heat of Cu adsorption was 280 kJ mol(-1) at 300 K. These results are compared to Pb adsorption on the same oxide thin films and are discussed in the context of important factors influencing metal island growth.
Ranney JT, Univ Washington, Dept Chem, Box 351700, Seattle, WA 98195 USA.
Univ Washington, Dept Chem, Seattle, WA 98195 USA.
ROYAL SOC CHEMISTRY, CAMBRIDGE