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 (V_O) create deep electron traps, while chlorine vacancies (V_Cl) produce shallow traps. The depth is determined by the coordination around anion vacancies: V_O 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, V_Cl also creates shallow traps, but V_O 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.