Conduction Band and Defect Engineering for the Prominent Visible‐Light Responsive Photocatalysts

Controlling trap depth is crucial to improve photocatalytic activity, but designing such crystal structures has been challenging. In this study, we discovered that in 2D materials like BiOCl and Bi4NbO8Cl, composed of interleaved [Bi2O2]2+ and Cl- slabs, the trap depth can be controlled by manipulating the slab stacking structure. In BiOCl, oxygen vacancies (VO) create deep electron traps, while chlorine vacancies (VCl) produce shallow traps. The depth is determined by the coordination around anion vacancies: VO forms strong σ bonds with Bi-6p dangling bonds below the conduction band minimum (CBM), while those around Cl are parallel, forming weak π-bonding. The strong re-hybridization makes the trap depth deeper. In Bi4NbO8Cl, VCl also creates shallow traps, but VO does not produce deep traps although Bi-6p orbitals are also forming strong σ bonding. This difference is attributed to the difference of the energy level of CBM. In both cases, the CBM consists of Bi-6p orbitals extending into the Cl layers. However, these orbitals are isolated in BiOCl, but those in Bi4NbO8Cl are bonded with each other between neighboring [Bi2O2]2+ layers. This unique bonding-based CBM prevents the formation of deep electron traps, and significantly enhances H2 evolution activity by prolonging the lifetime of highly reactive free electrons.