Synthesis of Three-Layer Perovskite Oxynitride K₂Ca₂Ta₃O₉N·2H₂O and Photocatalytic Activity for H₂ Evolution under Visible Light

Substitution of oxide anions (O^2–) in a metal oxide for nitrogen (N^3–) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden–Popper phase three-layer oxynitride perovskite of K₂Ca₂Ta₃O₉N·2H₂O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K₂LaTa₂O₆N·1.6H₂O. Topochemical ammonolysis reaction of a Dion–Jacobson phase oxide KCa₂Ta₃O₁₀ at 1173 K in the presence of K₂CO₃ resulted in a single-phase layered perovskite, K₂Ca₂Ta₃O₉N·2H₂O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K₂Ca₂Ta₃O₉N·2H₂O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K₂Ca₂Ta₃O₉N·2H₂O modified with a Pt cocatalyst generated H₂ from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H₂ evolution activity of K₂Ca₂Ta₃O₉N·2H₂O was an order of magnitude lower than that of K₂LaTa₂O₆N·1.6H₂O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K₂Ca₂Ta₃O₉N·2H₂O was shorter than that in K₂LaTa₂O₆N·1.6H₂O, which could explain the low photocatalytic activity of K₂Ca₂Ta₃O₉N·2H₂O.