EFFECT OF SULFUR ON OXYGEN-ADSORPTION AND OXIDATION OF THE NI(111) SURFACE

The effect of sulfur (S) on oxygen chemisorption and the initial stages of oxidation of the Ni(111) crystal surface have been studied with Auger electron spectroscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy at room temperature under ultrahigh vacuum conditions. It is found that sulfur strongly retards the oxygen adsorption kinetics, but does not change the oxidation mechanism. The initial chemisorption sticking coefficient (s) is found to follow the relationship s is-proportional-to (1 - 3-theta(s))2, where theta(s) is the S coverage. Oxidation of S precovered surfaces can be described fairly well by an island growth model; the decrease in oxidation rate, relative to that of the S-free surface, can be attributed to the reduction in the density of oxide nuclei. Some experimental observations indicate that the oxide layer is partially removed by S at 440 K. Sulfur is believed to remain at the Ni/NiO interface during oxidation.