Predicting the Band Structure of Mixed Transition Metal Oxides: Theory and Experiment

Fuel production from sunlight using mixed metal oxide photocatalysts is a promising route for harvesting solar energy. While photocatalytic processes can operate with high efficiency using UV light, it remains a challenge of paramount importance to drive them with visible light. Engineering the electronic energy band structure of mixed metal oxides through judicious control of atomic composition is a promising route to increasing visible light photoresponse. The goal of this paper is to develop a simple mixed metal oxide band structure (MMOBS) method to predict the electronic band structure of mixed metal oxides. Several materials in the Ti-Fe-O system that span the composition spectrum were considered in this study: anatase TiO2, Fe-doped anatase TiO2, ilmenite TiFeO3, Ti-doped hematite alpha-Fe2O3, and hematite alpha-Fe2O3. The predictions by the MMOBS method for the Ti-Fe-O system were tested and confirmed using first-principles density functional theory (DFT) calculations and experimental UV-visible absorption spectroscopy measurements. The band gap energy of the anatase-based compounds decreases with increasing Fe content until Fe and Ti are present in approximately the same concentration, and then the band gap energy remains constant and equal to that of hematite (similar to 2.0 eV), independent of Ti content. The positions of the conduction and valence bands, which are critical to driving photocatalytic reactions, are also predicted using the MMOBS method. Finally, the applicability of the MMOBS method to the rational design of photocatalysts for reduction-oxidation reactions in water splitting is discussed.