Mg-Doped Nanosized BaTaO₂N with Long-Lived Charge Carriers toward Efficient Overall Water Splitting

Perovskite-type oxynitrides offer a promising route for sustainable solar-to-hydrogen energy conversion via one-step-excitation photocatalytic overall water splitting (OWS). However, insufficient charge carrier lifetimes and sluggish surface reaction kinetics, stemming from inadequate control over bulk and surface properties, have thus far limited photocatalytic efficiency. Herein, we demonstrate that the particle size, defect states, and surface properties of BaTaO₂N can be effectively tailored by combining precursor engineering with Mg doping, thereby enhancing its OWS activity. Mg-doped BaTaO₂N nanocubes with particle sizes of several tens of nanometers were synthesized, and a solar-to-hydrogen energy conversion efficiency an order of magnitude higher than previously reported for BaTaO₂N-based photocatalysts was achieved with optimized IrO_x and Cr₂O₃/Ru loading. Mechanistic studies reveal that the dual effects of Mg, namely, passivating bulk defects and tuning surface properties, give rise to long-lived charge carriers and efficient transfer of these carriers to uniformly distributed cocatalyst sites. This work demonstrates that precursor engineering combined with Mg doping enables rational bulk-surface coregulation in oxynitride photocatalysts, providing design principles for developing efficient visible-light-driven OWS systems.