Probing Shallow Defect States‐Induced Photocarrier Behavior of G‐C ₃ N ₄ on a Femtosecond Timescale Toward Photocathodic Protection

Abstract Defect engineering, while beneficial for g‐C ₃ N ₄ photocatalysis, often impairs charge transfer if improperly controlled. Herein, shallow defect states are successfully introduced into S‐doped and C vacant g‐C ₃ N ₄ (CN‐ES) via a dual‐solvent‐assisted synthetic approach, aiming to maximize the photocarrier transport superiority. Notably, these elevated defect energy levels reduced the photoexcitation time to the femtosecond scale and dramatically shortened the average charge relaxation to 35.84 ps, significantly accelerating the charge transfer kinetics of g‐C ₃ N ₄ . Importantly, the shallow defect states are critical to evoke a moderate electron‐trapping ability as reflected by shortening the electron long‐lived time from almost nanosecond time‐scale into 323.78 ps, and thus acted as a temper electron reservoir to enhance photocarrier separation efficiency. Additionally, non‐radiative recombination is also suppressed due to the shallow defect states, yielding the slowest pseudo‐first‐order rate constant (0.053 s ⁻¹ ). Theoretical calculations further elucidated that this optimized photocarrier transfer stems from an enhanced polarized electric field and asymmetrical charge distributions of highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO). Consequently, in a large‐scale application test, CN‐ES achieved excellent photocathodic protection for commercialized 304 stainless steel mesh, demonstrating outstanding long‐term open‐circuit potential retention of 96.5% from an initial −0.577 V.