Synthesis of Three-Layer Perovskite Oxynitride KCaTaON·2HO and Photocatalytic Activity for H Evolution under Visible Light.

Substitution of oxide anions (O) in a metal oxide for nitrogen (N) 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 KCaTaON·2HO, and the photocatalytic activity is compared with an analogous two-layer perovskite, KLaTaON·1.6HO. Topochemical ammonolysis reaction of a Dion-Jacobson phase oxide KCaTaO at 1173 K in the presence of KCO resulted in a single-phase layered perovskite, KCaTaON·2HO, which belongs to the tetragonal 4/ space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized KCaTaON·2HO has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. KCaTaON·2HO modified with a Pt cocatalyst generated H 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 H evolution activity of KCaTaON·2HO was an order of magnitude lower than that of KLaTaON·1.6HO. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in KCaTaON·2HO was shorter than that in KLaTaON·1.6HO, which could explain the low photocatalytic activity of KCaTaON·2HO.