Abstract: Despite its industrial dominance, the century-old Haber-Bosch process for ammonia (NH₃) synthesis suffers from sustainability issues stemming from high energy consumption and significant carbon emissions. Photocatalytic nitrogen fixation presents a promising alternative but is hindered by rapid charge recombination, poor N₂ activation, and limited environmental adaptability. Herein, we report a sulfur vacancy (S_V)-rich ZnIn₂S₄/CeO₂ S-scheme heterojunction, synthesized via a one-pot solvothermal method, that integrates dual engineering of defects and interfacial charge modulation. Characterized by femtosecond transient absorption (fs-TA) spectroscopy and electrostatic potential calculations, the S-scheme charge transfer establishes an interfacial built-in electric field (BEF) that spatially separates charge carriers while preserving strong redox potentials. Moreover, the sulfur vacancies serve as electron-rich sites, lowering the energy barrier for NN dissociation and extending light absorption into the near-infrared region. Isotopic labeling confirms atmospheric N₂ as the nitrogen source for NH₃ production, while in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) monitors key *NH_X intermediate formation. The optimized heterojunction photocatalyst achieves an NH₃ production rate of 462 µmol g^-1 h^-1 in N₂ atmosphere and maintains 27 % efficiency (123 µmol g^-1 h^-1) in air. This work provides a universal strategy for designing defect-coupled heterojunctions that reconcile high efficiency with environmental robustness, paving the way for sustainable solar-driven ammonia synthesis.

Key words: Nitrogen fixation, S-scheme heterojunction, Sulfur vacancies, CeO₂, Photocatalysis