In-situ solution Fe-doping: A versatile pathway to significantly enhance charge separation in CuBi2O4 photocathodes

CuBi 2 O 4 (CBO) photocathodes hold significant promise for efficient photoelectrochemical (PEC) water splitting due to their favorable band gap and theoretical onset potential. However, their practical application is hindered by poor charge separation efficiency. Herein, we introduce a characteristic in-situ solution Fe-doping strategy that markedly improves photoelectrochemical performance of CBO, doubling the photocurrent density and achieving an unprecedented 190 mV anodic shift in the onset potential. By integrating with an electrochemical oxidation post-treatment, a record incident photon-to-current efficiency (IPCE) exceeding 40% at 0.6 V vs. RHE under visible light illumination is achieved. The versatility of the doping strategy is demonstrated across CBO photocathodes synthesized by different methods with various morphologies, grain sizes, and crystallinities. Mechanistic studies reveal that the gradient distribution of Fe 3+ ions generates an internal electric field that facilitates efficient charge separation and increases acceptor density. The strong Fe–O bonding also enhances structural stability against photo-induced corrosion. Notably, our investigation uncovers the non-temperature-dependent nature of CBO photocurrent, indicating that PEC performance enhancement primarily depends on reducing carrier recombination rather than improving bulk conductivity. This work lays the groundwork for future advancements in water splitting performance of CBO photocathodes, offering a complementary strategy to conventional methods for enhancing charge separation efficiency.