Interlayer-active layered oxysulfides NaMTiO_2.2S_1.8 (M = Nd, Sm) with an <i>n</i> = 1 Ruddlesden–Popper structure acting as photocatalysts for visible light water splitting

Layered compounds that utilize interlayer space as a reactive field are known as "interlayer-active" compounds and have been gaining attention, particularly in photocatalysis for water splitting. However, most of the reported "interlayer-active" photocatalysts are oxide semiconductors that possess a wide bandgap. Thus, they cannot utilize visible light essential for efficient water splitting. In this study, we synthesized novel Ruddlesden-Popper (RP) (<i>n</i> = 1) layered oxysulfides, NaMTiO_2.2S_1.8 (M = Nd, Sm), by heating "interlayer-active" layered oxides, NaMTiO₄, under H₂S flow. In NaMTiO_2.2S_1.8, the sulfur atoms occupy the apical oxygen sites and contribute to the elevated valence band maximum (VBM) to enable visible light absorption. Additionally, NaMTiO_2.2S_1.8 exhibits both proton exchange and interlayer hydration capabilities as well as photocatalytic activity for hydrogen and oxygen evolution under visible light. Hence, NaMTiO_2.2S_1.8 is the first example of both a <i>n</i> = 1 RP and an "interlayer-active" oxysulfide with the potential for visible-light-driven overall water splitting. The "interlayer-active" RP (<i>n</i> = 1) oxysulfide is expected to find application in various fields beyond photocatalysis by utilizing interlayer reactions such as ion exchange and interlayer hydration.