Reduction of oxidized nickel surfaces

The behavior of oxidized nickel surfaces was studied with X-ray photoelectron spectroscopy (XPS) during a heating process. Different surface states were prepared by oxidizing the surfaces of a nickel single crystal with (111) orientation, and polycrystalline nickel foils in oxygen or water vapor. A nickel oxide layer of thickness 2 ML was formed during the oxygen exposure. Oxidation in water vapor at 30 mbar yielded an additional nickel hydroxide layer. During the heating experiments (300-900 K), the surfaces of the oxidized samples were reduced to metallic nickel. The reduction process of the nickel surfaces oxidized in oxygen was supported by hydrogen exposure (10(-6) mbar). At low hydrogen pressures the reduction of the polycrystalline nickel surface took place at temperatures lower than the reduction of the oxidized single crystal surface. In experiments at 10(-6) mbar hydrogen no significant difference could be observed between the two samples. Defecting the single crystal surface by ion etching before oxidizing yielded no change in reduction behavior. On the surfaces oxidized in water vapor, the nickel hydroxide was reduced at low temperatures in a first step whereas the nickel oxide remained unreduced. The reduction of the remaining nickel oxide took place at the same temperatures as the reduction of the surfaces oxidized in oxygen. (C) 2000 Elsevier Science B.V. All rights reserved.