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.