Fabrication of Single-Crystalline BaTaO₂N from Chloride Fluxes for Photocatalytic H₂ Evolution under Visible Light

BaTaO₂N is an important oxynitride photocatalyst for the splitting of water under visible light. However, the preparation of BaTaO₂N crystals with low defect densities and high crystallinities is challenging via ammonolysis at high temperatures. Thus, we herein report the preparation of BaTaO₂N crystals using an NH₃-assisted flux method. We initially studied the effects of the flux type (RbCl, CsCl, and BaCl₂·2H₂O) on the morphology, crystallinity, and photocatalytic performance of the BaTaO₂N crystals. It was found that the BaTaO₂N crystals grown using a RbCl flux exhibited a significantly higher photocatalytic H₂ evolution rate than those grown using the other two fluxes. In particular, the observed activity was almost twice as high as that reported over BaTaO₂N crystals grown using a KCl flux. Such an excellent activity could be attributed to the lower defect density and higher crystallinity, which were confirmed by UV-vis diffuse reflectance spectroscopy and composition analysis. Subsequently, the effects of the source ratio, solute concentration, reaction temperature, and time on the crystallinity and morphology of the BaTaO₂N crystals grown using a RbCl flux were studied systematically to gain insight into the BaTaO₂N crystal growth mechanism.