Exploring the effect of partial B-site Al3+–Mg2+ dual substitution on optoelectronic, surface, and photocatalytic properties of BaTaO2N

Abstract

BaTaO₂N is appraised to be one of the few promising 600 nm-class photocatalysts for solar water splitting. However, the presence of structural defects and low charge separation limits its photocatalytic activity. Compared with mono substitution, dual substitution can be more effective in engineering the structural defects and improving the photocatalytic activity if foreign ions are suitably selected. In this work, we involve a dual-substitution approach to partially substitute Al³⁺ and/or Mg²⁺ for Ta⁵⁺ in BaTaO₂N. By maintaining the maximum concentration of Al³⁺–Mg²⁺ dual substitution at 5%, the effect of the Al³⁺–Mg²⁺ cosubstituent ratio on the optoelectronic, surface, and photocatalytic properties of BaTaO₂N is investigated. The Al³⁺–Mg²⁺ dual substitution leads to the shift of optical absorption edge toward shorter wavelengths, increasing the optical bandgap energy of BaTaO₂N. This effect is more pronounced in the samples with a higher concentration of Mg²⁺ due to the replacement of N³⁻ by a large number of O²⁻ to compensate charge balance. The initial reaction rates for the evolution of O₂ and H₂ reveal the improvement in the photocatalytic activity of BaTaO₂N due to the partial Al³⁺–Mg²⁺ dual substitution. Higher O₂ evolution is observed in the samples with a higher concentration of Mg²⁺, while the H₂ evolution rate significantly relies on the increased concentration of Al³⁺. According to the density functional theory (DFT) calculations, the effective masses of electrons become slightly lower than that of pristine BaTaO₂N after partial Al³⁺–Mg²⁺ (co)substitution, while a contrary tendency is observed for the effective masses of holes. The calculated positions of the valence band maximum and conduction band minimum are aligned with respect to the normal hydrogen electrode (NHE), and partial Al³⁺–Mg²⁺ (co)substituted BaTaO₂N photocatalysts can be promising candidates for visible-light-induced water splitting.