A Sillén Oxyhalide SrBi₃O₄Cl₃ as a Promising Photocatalyst for Water Splitting: Impact of the Asymmetric Structure on Light Absorption and Charge Carrier Dynamics

Abstract

Bismuth-based oxyhalides with layered Sillén(–Aurivillius) structures have attracted significant attention as photocatalysts. Recent studies have unveiled a part of the structure–property relationship of the materials; however, it has not been fully understood. In the present study, we investigated a Sillén-type oxyhalide SrBi₃O₄Cl₃ with single and double halogen layers. Interestingly, SrBi₃O₄Cl₃ showed a visible light response up to ∼460 nm, whereas SrBiO₂Cl and BiOCl with single and double halogen layers, respectively, did not. Rietveld refinement and STEM-EDX mapping determined the asymmetric Bi occupation in the fluorite [Sr₀.₅Bi₁.₅O₂] layer of SrBi₃O₄Cl₃, which was derived from the coexistence of the halogen layers. DFT calculations and Madelung potential calculations showed that the asymmetric Bi occupation affords both the Bi–Bi interaction across the single halogen layer and the electrostatic destabilization of Cl in the double halogen layer, probably leading to the narrow bandgap of SrBi₃O₄Cl₃. Another merit of possessing the two different halogen layers was revealed by time-resolved microwave conductivity measurements as well as DFT calculations; the spatial separation of the conduction band minimum and valence band maximum based on the coexistence of the halogen layers would promote charge carrier separation. Visible-light-driven Z-scheme water splitting was accomplished using a RuO₂-loaded SrBi₃O₄Cl₃ sample as an O₂-evolving photocatalyst. This study provides another option for engineering band structures and promoting the charge carrier separation of layered oxyhalides for efficient water splitting under visible light.