Shallow Hole Trapping and Intrinsic Defect Tolerance in Visible-Light Photocatalysts: The Role of Lattice Relaxation...

Understanding the dynamics of photogenerated hole trapping is essential for the rational design of high-performance photocatalysts. While hole trapping in conventional UV-responsive oxides (Type A) is well described by the small-polaron model involving deeply trapped states, this framework fails to account for the sharp absorption features commonly observed in visible-light-responsive materials. Here, using time-resolved transient absorption spectroscopy on tantalum-based oxynitrides (Type B) and representative hybridized oxides such as BiVO₄ (Type C), we show that these sharp spectral features originate from reactive hole states whose energies remain tied to the valence-band edge rather than falling into midgap levels. Our results establish a unified physical picture in which hole trapping is governed by the competition between electronic delocalization and lattice relaxation. We demonstrate that high anion polarizability and/or strong metal-anion d-p hybridization suppresses lattice distortion and kinetically stabilizes shallow, band-edge-proximal hole states. Crucially, this electronic mechanism endows visible-light-responsive photocatalysts with intrinsic defect tolerance as the suppressed lattice relaxation prevents defect sites from acting as deep hole traps. This framework reconciles long-standing discrepancies between the spectroscopic signatures and redox reactivity across different material classes. It further provides design guidelines for developing visible-light-responsive photocatalysts that retain strong oxidative activity.