Atomic structure of Cu2O(1Â 1Â 1)

Low-energy electron diffraction and scanning tunneling microscopy have been used to probe the surface atomic structure of Cu2O(1 1 1) after various sample preparations. Annealing in oxygen gives a stoichiometric (1 × 1) oxygen terminated surface and further annealing in ultra-high vacuum results in a clear (3×3)R30° reconstruction and surface faceting. Tunneling from filled states in the reconstructed surface reveals a hexagonal pattern of large protrusions, which show an internal structure. The reconstruction is believed to be due to one-third of a monolayer of ordered oxygen vacancies. At areas on the surface where the large features are missing, another smaller type of protrusions is visible, which is associated with the ideal (1 × 1) surface. The relative position of the two types of features gives two possible models of the (1 1 1) surface. In the first model, the (1 × 1) surface is the ideal bulk terminated surface and coordinatively unsaturated oxygen ions are missing in the reconstructed surface. The second model agrees with the first model with the exception that coordinatively unsaturated copper ions in the outmost copper layer are missing in both the (1 × 1) and the reconstructed surface. The latter model is supported by previous surface free energy calculations. Since the undercoordinated copper ions have been suggested to be the catalytic active sites of Cu2O(1 1 1), the presence or absence of these cations could be of great importance for the fundamental understanding of the surface reactivity of Cu2O and of copper-based catalysts.