Synergistic Rh/La Codoping Enables Trap-Mediated Charge Separation in Layered Perovskite Photocatalysts

Two-dimensional layered perovskite oxides have emerged as promising photocatalysts for solar-driven hydrogen evolution. Although doping has been widely employed to enhance photocatalytic performance, its role in modulating the electronic structure and the local chemical environment of these materials remains poorly understood. Here in this study, we investigate the codoping of Rh and La into exfoliated nanosheets of the Dion-Jacobson perovskite KCa₂Nb₃O₁₀ to enhance photocatalytic hydrogen evolution reaction (HER) activity. A substantial increase in H₂ evolution rate, from 12.3 to 69.0 μmol h^-1, was achieved at an optimal doping level of 0.2 wt % Rh and 1.3 wt % La. Comprehensive structural and spectroscopic analyses, including synchrotron techniques and high-resolution microscopy, revealed that Rh³⁺ substitutes Nb⁵⁺ to introduce shallow 4<i>d</i> acceptor states that mediate charge separation, while La³⁺ substitutes Ca²⁺, compensates for aliovalent charge imbalance, and modulates local lattice distortions and oxygen vacancy formation. This codoping strategy enhances charge carrier lifetime and separation efficiency through a trap-mediated mechanism. The observed volcano-shaped activity trend highlights a narrow compositional window, where electronic and structural factors are optimally balanced. These findings establish a mechanistic foundation for defect engineering in layered perovskites and offer a pathway for the rational design of photocatalysts.