Hydroxylation of NiO films: the effect of water and ion bombardment during the oxidation of nickel foils with O2 under vacuum

Abstract The effect of water on the oxidation of nickel surfaces under vacuum was investigated using X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and temperature-programmed desorption (TPD). It was found that the presence of a small amount of water in the gas phase (of H 2 O partial pressures as low as 10 −8 mbar) is enough to create a monolayer of surface hydroxyl groups on top of nickel oxide films. The formation of the OH groups was detected by both XPS and SIMS, and their superficial nature was assessed by angle-resolved XPS and depth profiling experiments. It was also found that the simultaneous ion bombardment of the surface during combined O 2 + H 2 O treatments leads to an enhancement in the rate growth for the NiO film but does not affect the formation of the OH layer in any way other than via sputtering of the surface. The most interesting observation from this work is the fact that adsorbed water appears to induce the reduction of nickel oxides, as it becomes apparent by the thinning of the grown NiO films. Two temperature extremes were observed in relation to this phenomena. On the one end, exposure of nickel substrates to a combination of oxygen, argon ions and water below 200 K results in a decrease in NiO film thickness by about a factor of two, a behavior likely to be associated with the enhancement in water adsorption at those low temperatures. At the other extreme, temperatures above 650 K are also sufficient to trigger the NiO film thinning, in this case even in the absence of argon ions. Two possible explanations for these observations are discussed.