Flux Synthesis of Layered Oxyhalide Bi₄NbO₈Cl Photocatalyst for Efficient Z-Scheme Water Splitting Under Visible Light

An oxyhalide photocatalyst Bi 4 NbO 8 Cl has recently been proven to stably oxidize water under visible light, enabling the Z -scheme water splitting when coupled with another photocatalyst for water reduction. We herein report the synthesis of Bi 4 NbO 8 Cl particles via a flux method, testing various molten salts to improve its crystallinity and hence photocatalytic activity. The eutectic mixture of CsCl/NaCl with a low melting point allowed the formation of single-phase Bi 4 NbO 8 Cl at as low as 650 °C. Thus, synthesized Bi 4 NbO 8 Cl particles exhibited a well-grown and plate-like shape while maintaining surface area considerably higher than those grown with others fluxes. They showed three times higher O 2 evolution rate under visible light than the samples prepared via a solid-state reaction. Time-resolved microwave conductivity measurements revealed greater signals (approximately 4.8 times) owing to the free electrons in the conduction band, indicating much improved efficiency of carrier generation and/or its mobility. The loading of RuO 2 or Pt cocatalyst on Bi 4 NbO 8 Cl further enhanced the activity for O 2 evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H 2 -evolving photocatalyst Ru/SrTiO 3:Rh along with an Fe 3+ /Fe 2+ redox mediator, the RuO 2 /Bi 4 NbO 8 Cl is an excellent O 2 -evolving photocatalyst, exhibiting highly effective water splitting into H 2 and O 2 via the Z -scheme.