New Visible-Light-Driven H₂- and O₂‑Evolving Photocatalysts Developed by Ag(I) and Cu(I) Ion Exchange of Various Layered and Tunneling Metal Oxides Using Molten Salts Treatments

Ag­(I) and Cu­(I) ions were exchanged for alkali ions in various wide-band-gap metal oxides with layered and tunneling structures for sensitization to visible light. Crystal structures of many Cu­(I)-ion-exchanged materials were similar to those of Ag­(I)-ion-exchanged materials when the same host materials were used. The ion-exchanged materials possessed new absorption bands at longer wavelength regions than their host materials because of contributions of Ag 4d and Cu 3d orbitals to the valence band maxima of the ion-exchanged materials. Some Ag­(I)-ion-exchanged materials, Ag­(I)-A₂SrTa₂O₇ (A = Li, K), Ag­(I)-K₂SrNb_0.2Ta_1.8O₇, Ag­(I)-K₂CaNaNb₃O₁₀, and Ag­(I)-KLaNb₂O₇ showed activities for photocatalytic O₂ evolution from an aqueous solution containing a sacrificial electron acceptor under visible-light irradiation. When Ruddlesden–Popper-type layered perovskite metal oxides consisting of Ti­(IV) or Ta­(V) in the perovskite slabs and K­(I) in the interlayers were used as a host material, the Cu­(I)-ion-exchanged materials showed high activities for photocatalytic H₂ evolution from an aqueous solution containing sacrificial electron donors under visible-light irradiation. On the other hand, when the other host materials were used as a host material, almost all Cu­(I)-ion-exchanged materials hardly showed the activities. The Cu­(I)-ion-exchanged metal oxides with a tunneling structure (Li₂MSr₆O₁₄ (M = Na₂, Sr, Ba, Pb)) instead of layered materials were successfully synthesized and responded to visible light.