Surface Oxidation and Reduction of CuO and Cu2O Studied Using XPS and XAES

TLDR

XPS and X‑ray‑excited Auger electron spectroscopy were used to monitor surface composition changes of CuO and Cu₂O during vacuum annealing, oxidation, and hydrogen reduction, including reoxidation by heating at 800 K in 1×10⁻⁴ mbar O₂ and regeneration by vacuum annealing at the same temperature. Prolonged annealing of CuO produced a thick Cu₂O surface layer, vacuum annealing of Cu₂O yielded a thin Cu metal film, and both oxides reduced to surface metal when heated in 1×10⁻⁴ mbar H₂ at 400 K, with the extent of reduction depending on prior thermal history, while hydrogen‑reduced samples reoxidized upon vacuum annealing, demonstrating lattice‑oxygen diffusion.

Abstract

Using XPS and x-ray-excited Auger electron spectroscopy (XAES), we have studied the variation in surface composition of CuO and Cu2O with a variety of high-vacuum treatments, including vacuum annealing, oxidation and hydrogen reduction. Prolonged annealing of CuO results in the formation of a thick layer of Cu2O at the surface whilst vacuum annealing of Cu2O produces a thin (possibly one monolayer) film of Cu metal. Both bulk Cu2O and the thick Cu2O film generated from vacuum-annealed CuO were oxidized to CuO by heating at 800 K in 1×10-4 mbar O2, the original surface being regenerated with vacuum annealing at the same temperature. Both CuO and Cu2O are reduced to metal at the surface by heating in 1×10-4 mbar hydrogen at 400 K. In the case of CuO, the extent of reduction varies with the thermal history of the sample, with prolonged vacuum annealing producing a more reducible surface. Hydrogen-reduced CuO and Cu2O were both reoxidized on vacuum annealing, demonstrating the diffusion of lattice oxygen to the surface.