Defect-Tolerant Conduction Band Formation in SrBi3O4Cl3 through Interlayer Bi 6p Orbital Coupling

Abstract The control of defect-induced electron trap levels is crucial for regulating the reactivity of photogenerated charge carriers. Although electron trapping can suppress rapid electron–hole recombination, excessive deep trapping lowers the energy and mobility of photogenerated electrons, thereby reducing their reactivity. Thus, controlling trap depth is essential for improving photocatalysts and other photofunctional materials. However, rational design principles remain limited. Here, we compare two layered bismuth oxychlorides, SrBiO2Cl and SrBi3O4Cl3, using time-resolved absorption spectroscopy and density functional theory (DFT) calculations. These materials consist of stacked fluorite-like oxide slabs separated by chloride layers: SrBiO2Cl contains [SrBiO2]+ slabs, whereas SrBi3O4Cl3 contains Bi-rich regions in adjacent slabs that face each other across a single chloride layer. This structural contrast provides a suitable platform to examine how interlayer Bi 6p coupling modifies the conduction band minimum (CBM) and affects defect-induced electron trapping. Transient absorption measurements revealed that photogenerated electrons in SrBiO2Cl are predominantly trapped in deep defect states, whereas SrBi3O4Cl3 retains a substantial population of free and/or shallowly trapped electrons in the microsecond time region. DFT calculations indicated that, in SrBiO2Cl, the absence of effective interlayer Bi–Bi coupling results in a CBM composed of relatively isolated Bi 6p states. In this electronic structure, intrinsic defects, particularly anion vacancies, can create Bi 6p-derived dangling-bond-like states that are further stabilized by local relaxation and rehybridization, forming deep electron traps below the CBM. In contrast, SrBi3O4Cl3 forms an energetically stabilized and spatially extended bonding-type Bi 6p-derived CBM through interlayer Bi–Bi coupling across the single chloride layer. Because this bonding-type CBM is already lowered by interlayer Bi–Bi orbital interactions, defect-derived Bi 6p states do not become deeply separated from the conduction band edge even after local relaxation and rehybridization. As a result, intrinsic bulk deep-trap formation is suppressed. This mechanism provides a design principle for defect-tolerant layered bismuth oxychloride photocatalysts.