Interaction of oxygen with nickel studied by field emission microscopy

Abstract

The reaction of oxygen with atomically clean nickel surfaces was studied by field emission microscopy at temperatures from 78 to 500 K, and oxygen pressures from 10^–10 to 1.0 Torr. Epitaxial nickel single crystal specimens were prepared by vapour deposition onto tungsten field emitter substrates held at 550–700 K in ultra high vacuum (UHV) conditions. On exposure to oxygen, a series of reaction processes was observed prior to the nucleation of oxide. Two stages of intermediate reaction were identified subsequent to chemisorption. The first stage, which occurs homogeneously over certain regions of the surface, appears to involve the introduction of oxygen into the plane of the surface. The second stage is apparently a cooperative process, and involves the migration of oxygen to sub-surface sites. Oxide nucleation is believed to take place after the second stage of reaction, and therefore occurs within a relatively disordered surface layer.