Alkali metal reactions with Ni(110)-O and NiO(100) surfaces

The interaction of alkali metals (lithium, sodium, potassium) with an Ni(110)-O surface has been studied using X-ray photoelectron spectroscopy(XPS). The reaction at 300 K gives rise to a peroxide species together with a degree of reduction of the oxide to metallic nickel. With sodium an Ni(2p) feature attributed variously to Ni3+ or a final-state effect for Ni2+ is suppressed, consistent with its origins in an intersite screening process in NiO. At higher temperatures, lithium and potassium participate in a solid-state reaction with the oxide in which a trivalent nickel oxidation state (as LiNiO2 and KNiO2) is formed. Analogous reactions with sodium form the divalent nickelate Na2NiO2. Coadsorption of the alkali metal and oxygen on clean Ni(110) provides for optimum nickelate formation, whereas the presence of water inhibits the formation of Ni3+ significantly. Similar experiments on NiO(100) suggest that the widespread practice of using thin oxide films to mimic bulk oxides may not always be justified. (C) 2000 Elsevier Science B.V. All rights reserved.