Theoretical investigations on surface stabilities of CuBi2O4 and CuFeO2

CuBi2O4 and CuFeO2 are p-type semiconductors that recently have been suggested as profitable photocathode materials for photo(electro)chemical reactions, such as water splitting or carbon dioxide reduction. There exist a few experimental and theoretical studies, providing a detailed description of the bulk properties of these materials, however, much less is known about the surfaces. In this work, we perform electronic structure simulations using DFT+U to investigate the structures, electronic properties, and thermodynamic stability of CuFeO2 and CuBi2O4 surfaces. The calculations indicate higher stabilities for stoichiometrically terminated (001)–CuBi2O4 and (012)–CuFeO2 surfaces. The density of states and the Bader charge analysis show states above the Fermi level for cation-deficient surface terminations and charge fluctuations among surface atoms, depending on the surface termination. These effects could enable higher absorption efficiency for cation-deficient surfaces. Our results emphasize the importance of surface terminations for a better understanding of electronic properties.