A 2D/2D g-C3N4/BiOBr S-Scheme Heterojunction for Photocatalytic Oxidative Coupling of Benzylamine and Degradation of Tetracycline: Enhanced Charge Separation and Mechanistic Insights

Efficient interfacial carrier transport and charge separation are critical for maximizing photocatalytic performance. Herein, a 2D/2D g-C3N4/BiOBr heterostructure photocatalyst is constructed using a solvent-free, high-energy ball-milling approach, which results in two-dimensional BiOBr nanoflakes evenly anchored on two-dimensional ultrathin g-C3N4 nanosheets. This tailored architectural design expands the specific surface area, increases active site exposure, and optimizes interfacial contact. For both the visible-light-driven oxidative coupling of primary amines and tetracycline degradation, the g-C3N4/BiOBr composite exhibits enhanced reactivity. The optimized composite (with 5 wt% g-C3N4) achieves an 80.1% yield and 95.2% imine selectivity for N-benzylidenebenzylamine, as well as a removal efficiency of 84.3% for tetracycline. Combined material characterizations and density functional theory (DFT) calculations confirm that the improvement in photocatalytic performance arises from the constructed S-scheme heterojunction across the g-C3N4/BiOBr contact interface. Under this charge transfer mode, photogenerated electrons in the conduction band (CB) of BiOBr selectively recombine with holes in the valence band g-C3N4. The electrons and holes with higher energy are respectively preserved in the CB of g-C3N4 and the VB of BiOBr to drive the corresponding surface reactions. The unique 2D/2D structure of the composite favors mass transfer processes in the reaction system while exposing abundant reactive sites, which can also enhance the photocatalytic performance. In addition, the g-C3N4/BiOBr S-scheme photocatalyst exhibits excellent reusability. This study offers a novel perspective for constructing high-performance S-scheme photocatalysts applied in organic synthesis.