Photocatalytic Properties of Layered Metal Oxides Substituted with Silver by a Molten AgNO₃ Treatment

K₄Nb₆O₁₇ (BG: 3.67 eV) and Na₂W₄O₁₃ (BG: 3.12 eV) layered oxide photocatalysts with wide band gaps were treated with a molten AgNO₃ to substitute K⁺ and Na⁺ with Ag⁺, resulting in red-shifts of absorption edges in diffuse reflectance spectra. A part of Na⁺ ions in the interlayer of Na₂W₄O₁₃ was substituted with Ag⁺ ions by the molten AgNO₃ treatment with keeping the layered structure. Both Ag­(I)-substituted K₄Nb₆O₁₇ and Na₂W₄O₁₃ showed photocatalytic activities for O₂ evolution from aqueous solutions containing a sacrificial reagent utilizing the absorption bands newly formed by the Ag­(I)-substitution. Notably, the Ag­(I)-substituted Na₂W₄O₁₃ produced O₂ under visible light irradiation. When ball-milled Na₂W₄O₁₃ was treated with a molten AgNO₃, the Ag­(I)-substitution rate increased. The Ag­(I)-substituted Na₂W₄O₁₃ with ball-milling showed higher photocatalytic activity for O₂ evolution than that without ball-milling. Z-schematic water splitting proceeded under visible light irradiation by combining the Ag­(I)-substituted Na₂W₄O₁₃ of an O₂-evolving photocatalyst with Ru-loaded SrTiO₃ doped with Rh of a H₂-evolving photocatalyst.