Near-Surface Defect Structure of CoO in the Vicinity of the CoO/Co₃O₄ Phase Boundary

Abstract

The near-surface defect structure of CoO was studied by “in situ” work function measurements in the temperature range 790 – 905°C at oxygen partial pressure near the equilibrium with Co₃O₄. The studies indicate that the near-surface oxide layer is enriched in cobalt interstitials which are not observed in the bulk. While CoO is oxidized and its non-stoichiometry increases, the observed phenomena in the outer surface layer may be considered within several consecutive regimes:

• 1 The formation of cobalt vacancies,

• 2 The formation of both cobalt vacancies and cobalt interstitials,

• 3 The formation of the Co₃O₄ superficial layer over CoO grains,

• 4 The phase transition CoO → Co₃O₄ in the bulk phase.

Cobalt interstitials which are formed in the outer layer of CoO at higher non-stoichiometry may be considered as precursors of the spinel phase. It has been observed that the CoO → Co₃O₄ phase transition within the near-surface layer takes place at ${p_{{O_2}}}$ still within the existence of the CoO phase field in the bulk. Equilibrium ${p_{{O_2}}}$values corresponding to the bulk phase transition CoO → Co₃O₄ are in a good agreement with literature data.