Activating and modulating inert basal planes of BiOX (X = I, Br, and Cl) for hydrogen evolution reaction

Bismuth oxyhalides, BiOX (X = I, Br, and Cl), as a new class of promising catalysts for hydrogen evolution reaction (HER), have attracted growing interest from researchers. However, the active sites of BiOX are limited to the edges, leaving a large area of inert basal planes to HER. Herein, the activation of the inert basal planes of monolayer BiOX is explored by the first-principles calculations. It is found that the Gibbs free energies of hydrogen adsorption (ΔGH0) on halogen vacancies (VX, X = I, Br, and Cl) sites are close to the ideal value of 0 eV, indicating that the inert basal planes of BiOX can be activated by VX. With the increasing VX concentration, ΔGH0 decreases and hydrogen is more strongly attached to VX sites. Moreover, the compressive strain can strengthen hydrogen adsorption on VX sites, whereas the opposite role is observed for the tensile strain. Hence, the highest HER of the basal planes can be achieved by the combinations of VX and strain. Furthermore, the resonant states of the Bi-p orbital of Bi atoms on VX sites are responsible for the flexible HER, which can be further determined according to the p-band center of Bi-p orbital. These results provide a comprehensive strategy to activate the inert basal plane and optimize the HER activity for two-dimensional (2D) catalyst.