The contrasting effect of the Ta/Nb ratio in (111)-layered B-site deficient hexagonal perovskite Ba₅Nb_{4-: X}Ta_xO₁₅ crystals on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives

The effect of the Ta/Nb ratio in the (111)-layered B-site deficient hexagonal perovskite Ba₅Nb_4-xTa_xO₁₅ (0 ≤ x ≤ 4) crystals grown by a KCl flux method on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives BaNb_1-xTa_xO₂N (0 ≤ x ≤ 1) was investigated. The Rietveld refinement of X-ray data revealed that all Ba₅Nb_4-xTa_xO₁₅ samples were well crystallized in the space group P3m1 (no. 164). Phase-pure BaNb_1-xTa_xO₂N (0 ≤ x ≤ 1) porous structures were obtained by nitridation of the flux-grown oxide crystals at 950 °C for 20, 25, 30, 35, and 40 h, respectively. The absorption edge of BaNb_1-xTa_xO₂N (0 ≤ x ≤ 1) was slightly shifted from 720 to 690 nm with the increasing Ta/Nb ratio. The O₂ evolution rate gradually progressed and reached the highest value (127.24 μmol in the first 2 h) with the Ta content up to 50 mol% but decreased at 75 and 100 mol% presumably due to the reduced specific surface area and high density of structural defects, such as grain boundaries acting as recombination centers, originated from high-temperature nitridation for prolonged periods. Transient absorption spectroscopy provided evidence for the effect of the Ta/Nb ratio on the behavior and energy states of photogenerated charge carriers, indicating a direct correlation with photocatalytic water oxidation activity of BaNb_1-xTa_xO₂N.