Recent Progress in Mixed‐Anion Materials for Solar Fuel Production

Mixed‐anion materials, consisting of metal cation(s) and more than two anionic species in a single phase, are promising as photocatalysts for water splitting and CO 2 fixation using visible light. Oxynitrides, oxysulfides, and oxyhalides that contain d 0 transition‐metal or d 10 typical‐metal cations have absorption bands in the visible‐light region and band‐edge potentials suitable for these reactions. In general, visible‐light absorption by these materials arises from the p orbitals of anions that are less electronegative than oxygen, forming a valence band above the level of oxygen 2p orbitals. Less electronegative anions in mixed‐anion compounds are essential for visible‐light absorption but are inherently less stable than oxygen, potentially causing oxidative decomposition of the materials during photoreaction. More importantly, mixed‐anion compounds are more difficult to synthesize than single‐anion compounds like oxides, and reducing the density of defects that act as recombination centers for photogenerated charge carriers in the material remains challenging. There is therefore much room for improving the activities of mixed‐anion photocatalysts. Herein, the development of mixed‐anion materials for solar‐to‐fuel conversion over the past 10 years is highlighted with a focus on key milestones in research developments.