Bi─O Bridges Trigger Lattice Strain‐Electronic Synergy at Inherent In Sites in ZnIn ₂ S ₄ for Boosting Solar‐to‐H ₂ O ₂ Conversion

Abstract Artificial H 2 O 2 photosynthesis without sacrificial agents represents a promising yet challenging route for sustainable chemical production, hindered by low solar‐to‐chemical conversion (SCC) efficiency (natural photosynthesis is only ∼0.1%). Notably, the abundant inherent active sites within base semiconductors remain substantially underutilized. Here, we incorporate Bi into ZnIn 2 S 4 (ZIS) lattices through atomic‐level Bi─O coordination, activating inherent In sites via synergistic lattice strain and electron rearrangement. Multiscale characterization confirms the formation of BiO 2 S 2 –ZIS with quantified 1.51% lattice elongation. Integrated theoretical calculations and in situ spectroscopic analyses reveal that Bi─O coordination increases electron density at adjacent In sites, which lowers the p ‐band center and enhances carrier separation. Meanwhile, lattice strain strengthens Bi─O orbital hybridization and weakens In─O covalency. Thus, these effects cooperatively optimize carrier dynamics. Then, the O 2 adsorption is Pauling‐type at In site to Yeager‐type adsorption at the In─Bi dual sites. Simultaneously, Bi─O bridges function as proton reservoirs to facilitate *OOH formation and *H 2 O 2 synthesis through enhanced Coulombic interactions. The resulting strain‐electron synergy achieves an unprecedented H 2 O 2 production rate of 6.06 mmol g −1 h −1 and 2.32% SCC efficiency, surpassing all reported inorganic semiconductor photocatalysts. This work demonstrates exceptional photocatalytic performance and establishes a highly effective strategy for inherent site activation.