Band gap engineering of NaTaO3 using density functional theory: a charge compensated codoping strategy

Abstract

In this theoretical study, we employ a codoping strategy to reduce the band gap of NaTaO₃ aimed at improving the photocatalytic activity under visible light. The systematic study includes the effects of metal (W) and nonmetal (N) codoping on the electronic structure of NaTaO₃ in comparison to the effect of individual dopants. The feasibility of the introduction of N into the NaTaO₃ crystal structure is found to be enhanced in the presence of W, as indicated by the calculated formation energy. This codoping leads to formation of a charge compensated system, beneficial for the minimization of vacancy related defect formation. The electronic structure calculations have been carried out using a hybrid density functional for an accurate description of the proposed system. The introduction of W in place of Ta leads to the appearance of donor states below the conduction band, while N doping in place of oxygen introduces isolated acceptor states above the valence band. The codoping of N and W also passivates undesirable discrete midgap states. This feature is not observed in the case of (Cr, N) codoped NaTaO₃ in spite of its charge compensated nature. We have also studied charge non-compensated codoping using several dopant pairs, including anion–anion and cation–anion pairs. However, this non-compensated codoping introduces localized states in between the valence band and the conduction band, and hence may not be effective in enhancing the photocatalytic properties of NaTaO₃. The optical spectrum shows that the absorption curve for the (W, N)-codoped NaTaO₃ is extended to the visible region due to narrowing of the band gap to 2.67 eV. Moreover, its activity for the photo decomposition of water to produce both H₂ and O₂ remains intact. Hence, based on the present investigation we can propose (W, N) codoped NaTaO₃ as a promising photocatalyst for visible light driven water splitting.