Topochemical Synthesis and Photocatalytic Activity of Layered K ₂ LaTa ₂ O ₆ N Using CsLaTa ₂ O ₇ Precursor Prepared by Flux Method

K₂LaTa₂O₆N, a Ruddlesden-Popper (RP)-type layered perovskite oxynitride with visible-light-response, was topochemically synthesized from a CsLaTa₂O₇ precursor, which was in prior prepared via a molten-flux method. The synthesis and structure-activity relationship in K₂LaTa₂O₆N were systematically examined using various physicochemical techniques including X-ray diffraction, electron microscopy, and photoconductivity measurements. Increasing the flux amount yielded CsLaTa₂O₇ particles with larger sizes and higher crystallinity, and these features were subsequently transferred to the K₂LaTa₂O₆N. The enhanced crystallinity of the K₂LaTa₂O₆N improved its photoconductivity, likely because of a reduction in the concentration of structural defects and/or vacancies. However, the photocatalytic H₂-evolution activity did not follow this tendency; instead, it correlated positively with the specific surface area of the K₂LaTa₂O₆N. These results indicate that the presence of a certain level of lattice defects in flux-derived K₂LaTa₂O₆N does not strongly affect its photocatalytic activity. The use of oxide precursors with both high crystallinity and a large specific surface area may therefore be crucial for the topochemical synthesis of other RP-type oxynitrides as improved photocatalysts.