Theoretical Investigations on the Surface Stability of CuBi2O4 and CuFeO2

Abstract

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. Currently, not much is known of the surfaces of CuBi2O4 and CuFeO2, although surface properties are crucial for a better understanding of the photocatalytic performance. 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. Beyond that, the Bader charge analysis of surfaces with Cu-rich terminations yields charge fluctuations among multiple surface layers, giving rise to lower stability of Fe-deficient surfaces. In contrast, both materials’ Cu-deficient surface terminations cover greater stability regions. The density of states shows surface states at the valence band maximum and conduction band minimum for cation-deficient surface terminations which can enable the separation of photogenerated electron-hole pairs. These effects could facilitate or complicate higher absorption efficiency for cation-deficient surfaces. Our results emphasize the importance of surface terminations for a better understanding of electronic properties and exhibit further theoretical findings of CuFeO2 and CuBi2O4