Bi₄AO₆Cl₂ (A = Ba, Sr, Ca) with Double and Triple Fluorite Layers for Visible-Light Water Splitting

Layered oxyhalides containing double or triple fluorite layers are promising visible-light-responsive water-splitting photocatalysts with unique band structures. Herein, we report on the synthesis, structure, and photocatalytic property of Bi₄BaO₆Cl₂ (I4/mmm) with alternating double (Bi₂O₂) and triple (Bi₂BaO₄) fluorite layers, which was extracted from the crystallographic database on the basis of Madelung potential calculations. Rietveld refinements from powder X-ray and neutron diffraction data revealed the presence of cationic disorder between Bi₂O₂ and Bi₂BaO₄ layers, leading to electrostatic stabilization. DFT calculations suggested that photogenerated electrons and holes flow through the double and triple layers, respectively, which may suppress electron-hole recombination. We expanded this double-triple system to include Bi₄CaO₆Cl₂ and Bi₄SrO₆Cl₂ with orthorhombic distortions and different degrees of cationic disorder, which allow band gap tuning. All the double-triple compounds Bi₄AO₆Cl₂ showed stable water-splitting photocatalysis in the presence of a sacrificial reagent.