α-Cr 2 O 3 (101̄2): surface characterization and oxygen adsorption

The composition and ordering of the Cr2O3 (101̄2) surface has been studied with X-ray photoelectron spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction. It has been found that a nearly-stoichiometric, (1×1) surface can be prepared by ion bombardment and annealing in vacuum to 900 K. The results are consistent with a simple non-polar surface termination giving predominantly five-coordinate Cr3+ surface cations. Oxygen exposures at 163 K lead to both dissociative and molecular adsorption. Dissociative adsorption dominates, giving an O-terminated surface with a saturation coverage of nearly one O atom per surface Cr3+ cation. Dissociatively-adsorbed oxygen is stable to over 1100 K and is attributed to a terminal chromyl oxygen species (i.e. CrO) that caps the single coordination vacancy of the surface cations. On the O-terminated surface formed by dissociative oxygen adsorption, molecular adsorption of O2 occurs, giving a weakly-bound species that desorbs at 220 K. The saturation coverage of the molecular species is low at 163 K and corresponds to 2% or less of the available chromium sites on the ideal, stoichiometric surface. Because of the low coverage, the adsorption sites are attributed to cations at defect sites in the terminating oxide layer.