Oxygen-Guided Charge Flow in an S-Scheme BaTiO3–In2S3 Heterojunction for Accelerated Photocatalytic Degradation of Doxycycline Hydrochloride

Doxycycline hydrochloride (DH), a persistent antibiotic frequently detected in aquatic environments, remains challenging to eliminate because conventional photocatalysts often suffer from limited visible-light utilization and rapid charge recombination, which suppress reactive oxygen species generation and slow DH transformation. Herein, we develop a visible-light-responsive BaTiO3–In2S3 photocatalyst by constructing an S-scheme heterojunction that steers oxygen-mediated electron transfer for accelerated photocatalytic degradation of DH. Ferroelectric BaTiO3 provides a polarization-associated driving force for carrier separation, whereas In2S3 broadens light harvesting and supplies reductive electrons for oxygen activation. Their work-function mismatch establishes an interfacial electric field that promotes directional carrier migration and suppresses interfacial charge recombination, as evidenced by ex situ/in situ XPS analysis and further supported by DFT analysis. As a result, the BaTiO3–In2S3 heterojunction delivers 59.1% DH conversion with a pseudo-first-order rate constant of 0.02606 min–1, 2.15 times higher than pristine BaTiO3. Quenching and EPR experiments identify ·O2– and h+ as the dominant reactive species, consistent with an S-scheme charge-transfer pathway that preserves high-energy electrons in In2S3 and oxidative holes in BaTiO3. LC-MS analysis suggests three feasible degradation routes, and ECOSAR prediction indicates an overall reduction in aquatic toxicity for several low-molecular-weight intermediates.