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

Abstract

An oxyhalide photocatalyst Bi₄NbO₈Cl 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 Bi₄NbO₈Cl 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 Bi₄NbO₈Cl at as low as 650 °C. Thus, synthesized Bi₄NbO₈Cl 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 O₂ 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 RuO₂ or Pt cocatalyst on Bi₄NbO₈Cl further enhanced the activity for O₂ evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H₂-evolving photocatalyst Ru/SrTiO₃:Rh along with an Fe³⁺/Fe²⁺ redox mediator, the RuO₂/Bi₄NbO₈Cl is an excellent O₂-evolving photocatalyst, exhibiting highly effective water splitting into H₂ and O₂ via the Z-scheme.