Electronic Structure and Bonding in Co-Based Single and Mixed Valence Oxides: A Quantum Chemical Perspective

The mixed valence cobalt oxide, Co₃O₄, is a potential candidate as a photovoltaic (PV) material, which also exhibits intriguing chemical and catalytic properties. Here, we present a comparative study of the electronic, magnetic, and chemical bonding properties of mixed valence Co₃O₄ (i.e., Co^2+/3+) with the related single valence CoO (i.e., Co²⁺) and Co₂O₃ (i.e., Co³⁺) oxides using density functional theory (DFT). We have employed a range of theoretical methods, including pure DFT, DFT+U, and a range-separated exchange-correlation functional (HSE06). We compare the electronic structure and band gap of the oxide materials, with available photoemission spectroscopy and optical band gaps. Our calculations suggest that the bonding between Co³⁺ and O^2- ions in Co₂O₃ and Co₃O₄ and Co²⁺ and O^2- ions in CoO and Co₃O₄ are rather different. We find that Co₂O₃ and Co₃O₄ are weakly correlated materials, whereas CoO is a strongly correlated material. Furthermore, our computed one-electron energy level diagrams reveal that strong Co-O antibonding states are present at the top of the valence band for all the cobalt oxides, hinting at a defect tolerant capacity in these materials. These results, which give a detailed picture of the chemical bonding in related single and mixed valence cobalt oxides, may serve as a guide to enhance the PV or photoelectrochemical activity of Co₃O₄, by reducing its internal defect states or changing its electronic structure by doping or alloying with suitable elements.