Core‐shell‐structured LaTaON2 Transformed from LaKNaTaO5 Plates for Enhanced Photocatalytic H2 evolution

: LaTaON2 is a photocatalyst with intense visible light absorption up to 650 nm, but exhibits low H2 evolution activity owing to uncontrolled facets and high defect densities. In this work, core-shell-structured plate-like LaKNaTaO5 /LaTaON2 was synthesized by nitriding a layered perovskite-type LaKNaTaO5 . The volatilization of K and Na species during the nitridation promoted the rapid transformation of LaKNaTaO5 into LaTaON2 along [010] direction with the plate-like shape retained. This yielded high-quality LaTaON2 shells exposing (010) facets on the lattice-matched LaKNaTaO5 cores. After loading with a Rh co-catalyst, LaKNaTaO5 /LaTaON2 showed photocatalytic H2 evolution activity four times greater than that obtained from conventional irregular-shaped LaTaON2 powders and utilized visible light up to 620 nm. This work provides a novel strategy yielding oxynitrides with well-defined facets and low defect densities by selecting lattice-matched oxide precursors containing volatile components.