Asymmetric surfaces endow Janus bismuth oxyhalides with enhanced electronic and catalytic properties for the hydrogen evolution reaction.

The asymmetric halogen surfaces on both sides of Janus Bi 2 O 2 XY lead to the electrostatic potential difference and the staggered band alignment. Bi 2 O 2 XY display more suitable band edge positions and the stronger internal electric field for HER than pristine BiOX. The superior HER performance is demonstrated by the Δ G H calculations. [Display omitted] The electronic and catalytic properties of Janus bismuth oxyhalide (Bi 2 O 2 XY, where X/Y = Cl, Br, or I, and X ≠ Y) for the hydrogen evolution reaction (HER) are evaluated through first-principles calculations. Janus Bi 2 O 2 XY shows an enhanced separation efficiency of electron−hole pairs and an augmented utilization of solar energy due to Janus asymmetry. The asymmetric halogen surfaces on both sides of Janus Bi 2 O 2 XY induce an electrostatic potential difference, which leads to a staggered band alignment. The solar-to-hydrogen (STH) efficiencies of Janus Bi 2 O 2 BrI and Bi 2 O 2 ClI have greatly improved compared to those of pristine BiOBr and BiOCl. Additionally, Janus Bi 2 O 2 XY achieves stronger internal electric fields (IEFs) and a more suitable Gibbs free energy of hydrogen adsorption (Δ G H) than pristine BiOX. Moreover, the halogen layer with a smaller electronegativity in Janus Bi 2 O 2 XY forms a stronger IEF with the oxygen layer; consequently, the Δ G H of terminations value is closer to the ideal value for the HER. The localized edge states in the p -orbital density of states (DOS) projected onto O atoms are responsible for the HER activity of terminations. This work provides a comprehensive understanding of Janus Bi 2 O 2 XY for the HER and provides a strategy for improving photocatalysis. [ABSTRACT FROM AUTHOR]