Interfacial Ti–N Bond-Engineered Hierarchical TiO ₂ /Tubular C ₃ N ₄ S-Scheme Heterojunctions for Enhanced Photocatalytic Tetracycline Degradation

Constructing S-scheme heterojunctions is an effective strategy to alleviate the rapid charge recombination of C 3 N 4, yet the photocatalytic performance is still often limited by insufficient interfacial coupling and sluggish charge transfer across the heterointerface. Herein, a hierarchical TiO 2 /tubular C 3 N 4 (TiO 2 /TCN) S-scheme heterojunction was fabricated via an in situ solvothermal method. In this structure, TiO 2 nanospikes are vertically arrayed on the TCN substrate, forming an open hierarchical architecture that exposes more active sites and facilitates mass transport. Meanwhile, interfacial Ti–N bonds are established during in situ growth, thereby enhancing interfacial electronic coupling and charge transfer. Density functional theory (DFT) calculations together with in situ irradiated X-ray photoelectron spectroscopy (XPS) confirm that the built-in electric field across the bonded interface induces a directional S-scheme charge-transfer pathway. This charge-transfer mode promotes the spatial separation of photogenerated carriers while retaining the strongly reducing electrons in TCN and the highly oxidizing holes in TiO 2 . Therefore, the TiO 2 /TCN heterojunction preserves sufficient redox ability to simultaneously produce • O 2 – and • OH radicals as the dominant reactive species. Consequently, the optimized TiO 2 /TCN-2 composite achieves 94.5% tetracycline removal within 40 min and an apparent rate constant of 0.059 min –1, which is 3.93- and 14.75-fold higher than those of the pristine TCN and TiO 2, respectively. These results highlight the importance of interfacial chemical bond engineering in modulating S-scheme charge-transfer behavior for efficient photocatalytic wastewater treatment.