The low temperature adsorption of oxygen onto Ni oxides

Abstract

In this paper we report on the low-temperature, reversible chemisorption of diatomic oxygen onto NiO, grown in situ on single-crystal Ni substrates. One narrow (γ) and two broad (α 1, α 2) thermal desorption peaks are observed, that exhibit no isotopic mixing during desorption. α 1, and α 2 are high-temperature states, with α 1 existing to well above room temperature. Both a states induce a large work-function increase during adsorption. This work-function increase with adsorption and the absence of isotopic mixing with desorption are consistent with a negatively charged diatomic oxygen species lying above the NiO plane. The adsorption is believed to occur at Ni 2O 3 point defects, with α 1 and α 2 yielding a combined saturation O 2 coverage of 0.11 ± 0.03 monolay The low temperature y peak (125 K) induces a small positive Δφ increase with desorption, suggesting an O 2 species with a small positive charge. Possible adsorption sites for oxygen in the γ state are step edges and/or domain boundaries. The maximum O 2 coverage of this state is 0.12 ± 0.03 monolayers and diminishes with substrate annealing. For all three states, adsorption is not associated with OH or CO 3 contamination, and is independent of oxide epitaxy.