alpha-Cr2O3 (1012): surface characterization and oxygen adsorption

The composition and ordering of the Cr2O3 (10 (1) over bar 2) surface has b een studied with X-ray photoelectron spectroscopy, Auger electron spectrosc opy, and low-energy electron diffraction. It has been found that a nearly-s toichiometric, (1 x 1) surface can be prepared by ion bombardment and annea ling in vacuum to 900 K. The results are consistent with a simple non-polar surface termination giving predominantly five-coordinate Cr3+ surface cati ons. Oxygen exposures at 163 K lead to both dissociative and molecular adso rption. Dissociative adsorption dominates, giving an O-terminated surface w ith a saturation coverage of nearly one O atom per surface Cr3+ cation. Dis sociatively-adsorbed oxygen is stable to over 1100 K and is attributed to a terminal chromyl oxygen species (i.e. Cr=O) that caps the single coordinat ion vacancy of the surface cations. On the O-terminated surface formed by d issociative oxygen adsorption, molecular adsorption of O-2 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 ava ilable chromium sites on the ideal, stoichiometric surface. Because of the low coverage, the adsorption sites are attributed to cations at defect site s in the terminating oxide layer. (C) 1999 Elsevier Science B.V. All rights reserved.