Re-visiting the O/Cu(111) system - When metastable surface oxides could become an issue!

Surface oxidation processes are crucial for the functionality of Cu-based catalytic systems used for many industrially important chemical reactions. Based on recent findings in XPS and LEED experiments, we assess the stability and population of the experimentally proposed ``8"-structure, a new surface oxide phase, on the Cu(111) surface. Using density-functional theory calculations in combination with {\it ab initio} atomistic thermodynamics and Boltzmann statistical mechanics, we find that the proposed oxide superstructure is indeed metastable and that the population of the ``8"-structure is competitive with the known ``29" and ``44" oxide film structures on Cu(111). We show that the configuration of O and Cu atoms in the first and second layers of the ``8"-structure closely resembles the arrangement of atoms in the first two layers of Cu$_2$O(110), where the atoms in the ``8"-structure are more constricted. Cu$_2$O(110) has been suggested in the literature as the most active low index facet for reactions such as water splitting under light illumination. If the ``8"-structure were to form during a catalytic process, it is therefore likely to be one of the reactive phases.