Topotactic Nitridation Enabled Core-Shell La₄Ti₃(O,N)₁₂@LaTiO₂N for Efficient Photoelectrochemical Water Splitting

Layered perovskite oxynitrides are attractive semiconductors for sunlight-driven water splitting thanks to their tunable electronic structures and structural anisotropy. However, their synthesis remains challenging because nitridation of layered perovskites often causes structural collapse or direct transformation into 3D perovskites, obscuring the nitridation mechanism. Herein, we systematically investigate the nitridation of a robust (111)-plane layered perovskite, La₄Ti₃O₁₂. Neutron diffraction and electron microscopy provide direct experimental evidence that nitridation proceeds topotactically in a gradual and spatially heterogeneous manner, preserving the layered framework over an extended time window and yielding an N-substituted layered perovskite oxynitride before full conversion into the 3D perovskite LaTiO₂N. This previously unrecognized persistence of layered oxynitride enables formation of a layered/3D core-shell heterostructure, La₄Ti₃(O,N)₁₂@LaTiO₂N, in which the layered phase acts as a structural and electronic host framework for formation of favorable heterojunction band alignment. As a result, the core-shell photoanode improves charge carrier dynamics, thereby showing substantially enhanced photoelectrochemical water splitting activity compared with both the single-phase 2D layered and the 3D oxynitrides. These findings identify layered oxynitrides as a key factor governing heterojunction formation and provide direct experimental insight into the nitridation mechanism of layered perovskites, providing a viable pathway toward efficient perovskite-based oxynitrides.