First principles investigation on the stabilization mechanisms of the polar copper terminated Cu₂O(1 1 1) surface

The stabilization of the unstable, polar copper terminated Cu₂O(1 1 1) surface by reconstruction and hydroxylation was studied theoretically with static and molecular dynamics calculations at ab initio density functional theory (DFT) level. Surface reconstruction was investigated using extensive finite temperature molecular dynamics (MD) combined with a simulated annealing technique. Both the global minimum energy structure obtained during annealing the system at higher temperature (300 K) and the final 'quenched' structure which was obtained after cooling the system to 0 K show the expected reconstruction of the adsorbate-free surface. The copper atoms in the first layer and oxygen atoms in the second and third layers are markedly displaced, and the atomic planes merge together to form a uniform mixed layer, thereby minimizing the polarity of the surface. Surface hydroxylation by adsorption of OH^- or dissociated water was investigated using static optimization at 0 K. The results show that adsorption is exothermic and that the reconstruction characterizing the annealed OH-free surface does not occur in the presence of adsorbed OH. A surface coverage of 50% results in the surface structure that is the closest to the unrelaxed bulk terminated surface.