SURFACE OXIDATION AND REDUCTION OF CUO AND CU2O STUDIED USING XPS ANDXAES

Using XPS and x-ray-excited Auger electron spectroscopy (XAES), we hav e studied the variation in surface composition of CuO and Cu2O with a variety of high-vacuum treatments, including vacuum annealing, oxidati on and hydrogen reduction. Prolonged annealing of CuO results in the f ormation of a thick layer of Cu2O at the surface whilst vacuum anneali ng 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 x 10(-4) mbar O-2, t he original surface being regenerated with vacuum annealing at the sam e temperature. Both CuO and Cu2O are reduced to metal at the surface b y heating in 1 x 10(-4) mbar hydrogen at 400 K. In the case of CuO, th e extent of reduction varies with the thermal history of the sample, w ith prolonged vacuum annealing producing a more reducible surface. Hyd rogen-reduced CuO and Cu2O were both reoxidized on vacuum annealing, d emonstrating the diffusion of lattice oxygen to the surface.