Photocatalytic Properties of Layered Metal Oxides Substituted with Silver by a Molten AgNO<sub>3</sub> Treatment

K<sub>4</sub>Nb<sub>6</sub>O<sub>17</sub> (BG: 3.67 eV) and Na<sub>2</sub>W<sub>4</sub>O<sub>13</sub> (BG: 3.12 eV) layered oxide photocatalysts with wide band gaps were treated with a molten AgNO<sub>3</sub> to substitute K<sup>+</sup> and Na<sup>+</sup> with Ag<sup>+</sup>, resulting in red-shifts of absorption edges in diffuse reflectance spectra. A part of Na<sup>+</sup> ions in the interlayer of Na<sub>2</sub>W<sub>4</sub>O<sub>13</sub> was substituted with Ag<sup>+</sup> ions by the molten AgNO<sub>3</sub> treatment with keeping the layered structure. Both Ag­(I)-substituted K<sub>4</sub>Nb<sub>6</sub>O<sub>17</sub> and Na<sub>2</sub>W<sub>4</sub>O<sub>13</sub> showed photocatalytic activities for O<sub>2</sub> evolution from aqueous solutions containing a sacrificial reagent utilizing the absorption bands newly formed by the Ag­(I)-substitution. Notably, the Ag­(I)-substituted Na<sub>2</sub>W<sub>4</sub>O<sub>13</sub> produced O<sub>2</sub> under visible light irradiation. When ball-milled Na<sub>2</sub>W<sub>4</sub>O<sub>13</sub> was treated with a molten AgNO<sub>3</sub>, the Ag­(I)-substitution rate increased. The Ag­(I)-substituted Na<sub>2</sub>W<sub>4</sub>O<sub>13</sub> with ball-milling showed higher photocatalytic activity for O<sub>2</sub> evolution than that without ball-milling. Z-schematic water splitting proceeded under visible light irradiation by combining the Ag­(I)-substituted Na<sub>2</sub>W<sub>4</sub>O<sub>13</sub> of an O<sub>2</sub>-evolving photocatalyst with Ru-loaded SrTiO<sub>3</sub> doped with Rh of a H<sub>2</sub>-evolving photocatalyst.