THE LOW-TEMPERATURE ADSORPTION OF OXYGEN ONTO NI OXIDES

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 (gamma) and two broad (alpha-1, alpha-2) thermal desorption peaks are observed, that exhibit no isotopic mixing during desorption. Alpha-1 and alpha-2 are high-temperature states, with alpha-1 existing to well above room temperature. Both alpha-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 Ni2O3 point defects, with alpha-1 and alpha-2 yielding a combined saturation O2 cover age of 0.11 +/- 0.03 monolayers. The low temperature gamma-peak (125 K) induces a small positive DELTA-phi increase with desorption, suggesting an O2 species with a small positive charge. Possible adsorption sites for oxygen in the gamma-state are step edges and/or domain boundaries. The maximum O2 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 CO3 contamination, and is independent of oxide epitaxy.