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As the 600 nm-class photocatalyst, BaTaO 2 N is one of the promising candidates of the perovskite-type oxynitride family for photocatalytic water splitting under visible light. The oxynitrides are routinely synthesized by nitriding corresponding oxide precursors under a high-temperature NH 3 atmosphere, causing an increase in the defect density and a decrease in photocatalytic activity. To improve the photocatalytic activity by reducing the defect density and improving the crystallinity, we here demonstrate an NH 3 -assisted KCl flux growth approach for the direct synthesis of the BaTaO 2 N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO 2 N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO 2 N crystals could only be grown with the solute concentrations of ≥ 10 mol % using the KCl flux, and the solute concentration of 10 mol % was solely favorable to directly grow cube-like BaTaO 2 N crystals with an average size of about 125 nm and exposed {100} and {110} faces at 950 °C for 10 h. The time-and temperature-dependent experiments were also performed to postulate the direct growth mechanisms of cube-like BaTaO 2 N submicron crystals. The BaTaO 2 N crystals modified with Pt and CoO x nanoparticles showed a reasonable H 2 and O 2 evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH 3 -assisted KCl flux method.

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Photocatalytic overall water splitting on (oxy)nitrides under visible light is one of the interesting approaches to fulfill the growing demand for clean and renewable energy. The improvement of the fabrication method is however important for reducing the defect density of (oxy)nitride crystals. The present study aims to investigate the direct growth of the LaTiO 2 N (LTON) crystallites with less defect density by an NH 3 -assisted flux method and to demonstrate the visible-light-induced photocatalytic water oxidation activity in relation to their crystallite morphology. Single-phase LaTiO 2 N crystallites (average size of 120 ± 39 nm) in round shape with smooth surface and high crystallinity were grown by an NH 3 -assisted flux method using the KCl flux with the solute concentration of 5 mol % at 950 °C for 10 h. The photocatalytic water oxidation activity of bare and CoO x -loaded LaTiO 2 N crystallites grown directly by an NH 3 -assisted flux method (1-step-LTON) was evaluated under visible light by comparing with the LaTiO 2 N crystallites fabricated by a two-step method (2-step-LTON), converting La 2 Ti 2 O 7 to LaTiO 2 N by high-temperature nitridation. Within the first 2 h of the photocatalytic water oxidation half-reaction, the O 2 evolution rates of bare and CoO x -loaded 1-step-LTON crystallites were 82 µmol·h -1 and 204 µmol·h -1 , respectively, which are much higher than that of bare and CoO x -loaded 2-step-LTON crystallites (37 µmol·h -1 and 177 µmol·h -1 ) due to less defect density of the LaTiO 2 N crystallites achieved by a direct fabrication route using KCl flux. An NH 3 -assisted flux growth is a promising route for the direct fabrication of the LaTiO 2 N crystallites with less defect density that is beneficial for the enhancement of photocatalytic water oxidation half-reaction.

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The doping of foreign cations and anions is one of the effective strategies for engineering defects and modulating the optical, electronic, and surface properties that directly govern the photocatalytic O 2 and H 2 evolution reactions. BaTaO 2 N (BTON) is a promising 600 nm-class photocatalyst because of its absorption of visible light up to 660 nm, small band gap (E g = 1.9 eV), appropriate valence band-edge position for oxygen evolution, good stability under light irradiation in concentrated alkaline solutions, and nontoxicity. Although the photocatalytic and photoelectrochemical water-splitting efficiencies of BaTaO 2 N have been progressively improved, it is still far from the requirements set for practical applications. Here, we employ a 5% B site-selective doping of aliovalent metal cations (Al 3+ , Ga 3+ , Mg 2+ , Sc 3+ , and Zr 4+ ) to enhance sacrificial visible light-induced photocatalytic H 2 and O 2 evolution over BaTaO 2 N. The results of physicochemical characterizations reveal that no significant change in crystal structure, crystal morphology, and optical absorption edge is observed upon cation doping. Therefore, the difference observed in O 2 and H 2 evolution during the photocatalytic reactions over pristine and doped BaTaO 2 N photocatalysts is explained by examining optical, electronic, and surface properties. Also, molecular dynamics (MD) is used to gain insights into the respective effect of cation doping on adsorption energy of water molecules and formed intermediates (H* for H 2 evolution and HO*, O*, and HOO* for O 2 evolution) on the BaTaO 2 N surfaces terminated with TaO 6 , TaN 6 , and TaO 4 N 2 octahedra. Finally, the experimental reaction rates for H 2 and O 2 evolution are correlated well using a linear energy−performance relationship, elucidating the doping and surface-termination trends observed in the BaTaO 2 N photocatalysts.