Bipolar‐Axis Intergrowth Ferroelectrics for Efficient and Stable Photocatalytic Overall Water Splitting

ABSTRACT Ferroelectric semiconductors show huge potential in photocatalytic overall water splitting (POWS), while achieving strong polarization remains challenging. Herein, we develop bipolar‐axis intergrowth ferroelectrics Bi 7 Ti 4 NbO 21 ( i BTN) with colossal polarization intensity and favorable reaction thermodynamics for efficient and stable POWS. Compared to conventional unipolar‐axis ferroelectrics Bi 3 TiNbO 9 and Bi 4 Ti 3 O 12 with symmetric stacking of structural units, the asymmetric stacking structure simultaneously induces prodigious dipole moments superimposed along the a ‐axis (3793.53 D) and interlayer dipole moments along the c ‐axis (106.39 D) within i BTN, establishing ultra‐strong orthogonal polarization fields. Thus, i BTN achieves the lowest exciton binding energy (43.62 meV), highest density of states, ultra‐low electron effective mass (0.010 m 0 ), and exceptionally high electron‐hole effective mass ratio ( m e / m h = 400), enabling synergistic enhancement across the entire photogenerated carrier dynamics process of “generation‐separation‐transport”. Simultaneously, ferroelectric polarization optimizes surface catalysis, allowing favorable adsorption characteristics and low POWS reaction energy barrier. Consequently, i BTN exhibits state‐of‐the‐art POWS rates among pristine ferroelectric photocatalysts, with stoichiometric H 2 and O 2 evolution rates of 73.31 and 37.34 µmol·h −1 , respectively. Outdoor tests present a stable POWS activity of i BTN for 50 h in 10 days, with a solar‐to‐hydrogen efficiency reaching 0.11%, demonstrating considerable practical potential. The development of multipole‐axis intergrowth ferroelectrics unlocks a new path toward efficient POWS.