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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.
Poulston et al. (1996) studied this question.