NH₃-Assisted Flux Growth of Cube-like BaTaO₂N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity

As the 600 nm-class photocatalyst, BaTaO₂N 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 NH₃ 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 NH₃-assisted KCl flux growth approach for the direct synthesis of BaTaO₂N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO₂N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO₂N 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 BaTaO₂N 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 BaTaO₂N submicron crystals. The BaTaO₂N crystals modified with Pt and CoO_x nanoparticles showed a reasonable H₂ and O₂ evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH₃-assisted KCl flux method. (Figure Presented).