Layered Perovskite Oxychloride Bi₄NbO₈Cl: A Stable Visible Light Responsive Photocatalyst for Water Splitting

Mixed anion compounds are expected to be a photocatalyst for visible light-induced water splitting, but the available materials have been almost limited to oxynitrides. Here, we show that an oxychrolide Bi₄NbO₈Cl, a single layer Sillen–Aurivillius perovskite, is a stable and efficient O₂-evolving photocatalyst under visible light, enabling a Z-scheme overall water splitting by coupling with a H₂-evolving photocatalyst (Rh-doped SrTiO₃). It is found that the valence band maximum of Bi₄NbO₈Cl is unusually high owing to highly dispersive O-2p orbitals (not Cl-3p orbitals), affording the narrow band gap and possibly the stability against water oxidation. This study suggests that a family of Sillen–Aurivillius perovskite oxyhalides is a promising system to allow a versatile band level tuning for establishing efficient and stable water-splitting under visible light

Valence Band Engineering of Layered Bismuth Oxyhalides toward Stable Visible-Light Water Splitting: Madelung Site Potential Analysis

A layered oxychloride Bi₄NbO₈Cl is a visible-light responsive catalyst for water splitting, with its remarkable stability ascribed to the highly dispersive O-2p orbitals in the valence band, the origin of which, however, remains unclear. Here, we systematically investigate four series of layered bismuth oxyhalides, BiOX (X = Cl, Br, I), Bi₄NbO₈X (X = Cl, Br), Bi₂GdO₄X (X = Cl, Br), and SrBiO₂X (X = Cl, Br, I), and found that Madelung site potentials of anions capture essential features of the valence band structures of these materials. The oxide anion in fluorite-like blocks (e.g., [Bi₂O₂] slab in Bi₄NbO₈Cl) is responsible for the upward shift of the valence band, and the degree of electrostatic destabilization changes depending on building layers and their stacking sequence. This study suggests that the Madelung analysis enables a prediction and design of the valence band structures of bismuth and other layered oxyhalides and is applicable even to a compound where DFT calculation is difficult to perform