Theoretical Investigation of Ta2O5, TaON, and Ta3N5: Electronic Band Structures and Absolute Band Edges

Early transition metal oxides, nitrides, and oxynitrides have attracted a great deal of interest because of their potential applications in photovoltaics and photocatalysis. In this work, a systematic investigation is conducted of the electronic band structures of the Ta2O5 polymorphs,beta-Ta3N5 and beta-TaON, which are crucial for the understanding of their photocatalytic properties, based on state-of-the-art first-principles approaches. The calculated results imply that many-body perturbation theory in the GW approximation can overcome the severe underestimation of the band gap caused by standard density functional theory (DFT) in the local and semilocal approximations and provide a quantitative agreement with experiment. The effects of the electron-phonon coupling on the electronic band structure are considered by the Frolich model, and especially for epsilon-Ta2O5, a strong electron-phonon coupling is predicted as a result of small high- frequency dielectric constants and large effective masses. Based on an analysis in terms of the phenomenological ionic model, the band-gap difference between three compounds can be physically attributed to not only the well-known energy difference between the O 2p and N 2p orbitals, but also the influences of the Madelung potential on the conduction-band-energy. By comparing the calculated absolute band edge positions to the redox potentials for water reduction and oxidation, all three of the compounds are predicted to have potential photocatalytic properties for unassisted water splitting. In addition, we also analyzed the stability and band gaps of different Ta2O5 polymorphs and found that the beta-Ta2O5, the phase commonly used in theoretical studies, is actually unstable and its unusually small band gap can be attributed to the strong overlap of neighboring atomic orbitals. On the other hand, epsilon-Ta2O5, which is much less well studied compared to beta-Ta2O5, leads to calculated properties that are much niore consistent with the experimental findings for Ta2O5 in general. The theoretical analysis and findings presented in this work have general implications for the understanding of the electronic band structures of other early transition metal compounds.