A combined SIMS-AES/LEED study of the room-temperature oxidation of Ni(110) and Ni(111) surfaces

Abstract

Results of kinetic studies of the interaction of oxygen with Ni(110) and Ni(111) obtained with both SIMS and AES are presented. An initial rapid chemisorption region, accompanied by the appearance of regular overlayers of oxygen, is followed by a region of nucleation and lateral growth of oxide nuclei, which proceeds appreciably faster on Ni(110) than on Ni(111). In-depth growth of oxide was found to be of minor importance on both surfaces at the oxygen pressures used. The final thicknesses of the oxide films, determined using the AES-oxygen uptake curves together with AES data obtained from NiO-bulk material, amount to two layers of NiO(100) on Ni(110) and three layers of NiO(111) on Ni(111). The SIMS yield of negative ions (O-, NiO-, NiO-2) responds very sensitively to the formation and growth of oxide nuclei. The positive ion yields (Ni+, NiO+, Ni+2) show a rather complicated behavior in the region of chemisorption and nucleation; this is tentatively attributed to an interplay of a chemical effect (due to the presence of oxygen) with a geometrical shielding effect (due to an overlayer of oxygen). A very slow increase of the positive ion yields above the exposures necessary to saturate the AES-oxygen signal suggests processes in the oxide film leading to its gradual perfection with respect to geometry and bonding. The results demonstrate that SIMS used in connection with other surface-sensitive methods can yield further important details on the oxidation stages of metal surfaces. On the other hand, reliable information concerning the sensitivity of SIMS yields to different bonding states of oxygen to metal, which is important for an understanding of the SIMS process, can be obtained. © 1978.