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

Ferroelectric semiconductors show huge potential in photocatalytic overall water splitting (POWS), while achieving strong polarization remains challenging. Herein, we develop bipolar-axis intergrowth ferroelectrics Bi₇Ti₄NbO₂₁ (iBTN) with colossal polarization intensity and favorable reaction thermodynamics for efficient and stable POWS. Compared to conventional unipolar-axis ferroelectrics Bi₃TiNbO₉ and Bi₄Ti₃O₁₂ 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 iBTN, establishing ultra-strong orthogonal polarization fields. Thus, iBTN achieves the lowest exciton binding energy (43.62 meV), highest density of states, ultra-low electron effective mass (0.010 m₀), 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, iBTN exhibits state-of-the-art POWS rates among pristine ferroelectric photocatalysts, with stoichiometric H₂ and O₂ evolution rates of 73.31 and 37.34 µmol·h^-1, respectively. Outdoor tests present a stable POWS activity of iBTN 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.