Coupled Ferroelectric–Photoelectrochemical in Water Reduction Over BiFeO ₃ Thin Film Heterostructure Modulated by Rare‐Earth Doping

Abstract Ferroelectric polarization plays a key role in tuning charge dynamics and surface stoichiometry in photoelectrochemical (PEC) water reduction. In this work, p‐type ferroelectric BiFeO ₃ (BFO) thin film heterostructures are modified by rare‐earth (RE) doping (Nd, Sm, and Gd) to explore their influence on electrical leakage, ferroelectric polarization, charge transfer efficiency, and PEC performance. The heteroepitaxial method is used to grow upward‐polarized BFO (111) films doped with 7% Nd, 10% Sm, and 7% Gd. Among the RE‐doped BFO, the 7% Gd‐doped BFO (BGFO) particularly reveals a ∼12% decrease in oxygen vacancies (V _O ) compared to the BFO in X‐ray photoelectron spectroscopy (XPS) O 1 s examinations. The density functional theory (DFT) calculations further confirm the lowest defect of V _O in the BGFO. Consequently, Gd doping is tremendously effective in reducing leakage current by ∼2‐order, enhancing the subsequent remnant polarization from 33 µC cm ⁻² (BFO) to 83 µC cm ⁻² (BGFO). Meanwhile, an inhibited charge recombination in BGFO improves the lifetime of photoexcited charge carriers by ∼1.8‐fold. These improvements collectively facilitate charge migration across the BGFO/electrolyte interface in PEC activities, resulting in a ∼2.8‐fold increase in incident photon‐to‐current efficiency (IPCE) and a ∼1.5‐fold enhancement in photogenerated charge transfer density.