Investigation of energy band alignments and interfacial properties of rutile NMO2/TiO2 (NM = Ru, Rh, Os, and Ir) by first-principles calculations

Abstract

In the field of photocatalysis, constructing hetero-structures is an efficient strategy to improve quantum efficiency. However, a lattice mismatch often induces unfavorable interfacial states that can act as recombination centers for photo-generated electron–hole pairs. If the hetero-structure's components have the same crystal structure, this disadvantage can be easily avoided. Conversely, in the process of loading a noble metal co-catalyst onto the TiO₂ surface, a transition layer of noble metal oxides is often formed between the TiO₂ layer and the noble metal layer. In this article, interfacial properties of hetero-structures composed of a noble metal dioxide and TiO₂ with a rutile crystal structure have been systematically investigated using first-principles calculations. In particular, the Schottky barrier height, band bending, and energy band alignments are studied to provide evidence for practical applications. In all cases, no interfacial states exist in the forbidden band of TiO₂, and the interfacial formation energy is very small. A strong internal electric field generated by interfacial electron transfer leads to an efficient separation of photo-generated carriers and band bending. Because of the differences in the atomic properties of the components, RuO₂/TiO₂ and OsO₂/TiO₂ hetero-structures demonstrate band dividing, while RhO₂/TiO₂ and IrO₂/TiO₂ hetero-structures have a pseudo-gap near the Fermi energy level. Furthermore, NMO₂/TiO₂ hetero-structures show upward band bending. Conversely, RuO₂/TiO₂ and OsO₂/TiO₂ hetero-structures present a relatively strong infrared light absorption, while RhO₂/TiO₂ and IrO₂/TiO₂ hetero-structures show an obvious absorption edge in the visible light region. Overall, considering all aspects of their properties, RuO₂/TiO₂ and OsO₂/TiO₂ hetero-structures are more suitable than others for improving the photocatalytic performance of TiO₂. These findings will provide useful information for understanding the role and effects of a noble metal dioxide as a transition layer between a noble metal co-catalyst and a TiO₂ photocatalyst.