Surface Modification with Metal Hexacyanoferrates for Expanding the Choice of H₂‐Evolving Photocatalysts for Both Fe³⁺/Fe²⁺ Redox‐Mediated and Interparticle Z‐Scheme Water‐Splitting Systems

The construction of Z‐scheme water splitting systems is an effective approach toward harvesting a wide portion of the solar light spectrum; however, the success has often depended on the property of photocatalyst surfaces. This drawback is typified by the limited choice of efficient H₂ evolution photocatalysts (HEPs) (e.g., Rh‐doped SrTiO₃) for Z‐scheme water splitting using Fe³⁺/Fe²⁺ redox couple. The majority of visible light‐responsive materials shows low activity for H₂ production with Fe²⁺ electron donors despite having suitable band levels, probably due to the absence of an effective surface site for oxidizing Fe²⁺. The choice of HEPs for interparticle Z‐scheme systems has also been limited. Herein, an effective strategy for overcoming these limitations is reported: activation of originally inactive materials via surface modification with metal hexacyanoferrate nanoparticles. Photocatalytic H₂ evolution over TaON in aqueous Fe²⁺ solution is drastically enhanced by comodification with indium hexacyanoferrate (InHCF) and Rh–Cr mixed oxide. InHCF promotes Fe²⁺ oxidation to Fe³⁺ utilizing the holes photogenerated in TaON via Fe^III/Fe^II redox cycles, enabling Z‐scheme water splitting with the Fe³⁺/Fe²⁺ redox mediator coupled with an O₂ evolution photocatalyst under visible light. It is also disclosed that InHCF nanoparticles function as effective solid electron mediators for achieving interparticle Z‐scheme water splitting.