(487n) Surface Characterization and Oxygen Adsorption On α-Cr2O3(0001)

X-ray photoemission has been used to examine an ion bombarded and annealed Cr₂O₃(0001) single crystal surface in vacuum. The surface composition as measured by the XPS O/Cr ratio of the sputtered surface does not show noticeable variations with annealing temperatures from 300 K to 1000 K. The values of the O/Cr ratio, near 1.5, suggest that the surface composition is closer to that of a non-polar Cr-terminated surface than a polar Cr- or O-terminated surface.

When compared to the oxygen dosed surface, the Cr 2p XPS spectrum of the ion-bombarded and annealed surface reveals a lower binding energy contribution consistent with metallic Cr. Oxygen dosing at room temperature removes this feature, but it reappears on heating in vacuum to 800 K or higher. The intensity of the low binding energy feature is close to that predicted for the fraction of signal due to a single layer of surface chromium atoms based on a simple exponential decay model. TPD spectra show O₂ desorption in two features centered around 275 K and 1100 K. The restoration of the metallic Cr signal following the high temperature desorption peak suggests that the reduction is due to the desorption of oxygen species.

Oxygen exposure at higher temperature has shown more effective incorporation of oxygen into the surface. Unlike oxygen dosing at room temperature, the Cr 2p XPS is narrower, and less lower binding energy contributions are found upon annealing at temperatures between 900 K to 1100 K. These results suggest that oxygen exposure at high temperature has resulted in a different form of oxygen which is more fully incorporated as lattice oxygen into the surface. In addition, annealing the ion-bombarded surface for longer time also results in less reduction, suggesting that the slow migration of lattice oxygen from the bulk can reoxidize the reduced surface.

The photoemission observations on the (0001) surface lead to three criteria for the preparation of a nearly-ideal surface ? 1) Surface Cr species should be present in predominantly the Cr³⁺ form, 2) no molecular oxygen should exist on the surface, and 3) no terminal oxygen (Cr=O) species should be present. A recipe for the preparation of a nearly-stoichiometric surface has been developed using oxygen incorporation at high temperature and annealing.