Surface defect-rich g-C3N4/TiO2 Z-scheme heterojunction for efficient photocatalytic antibiotic removal: rational regulation of free radicals and photocatalytic mechanism

Photocatalytic degradation has been considered as an eco-friendly method for the removal of harmful antibiotics. Considering that the fundamental aspect of antibiotic photodegradation is a free radical reaction on the catalyst surface, boosting the generation of surface active free radicals is a direct and effective approach to promote the surface antibiotic oxidation reaction. However, rational regulation of the surface free radicals of photocatalysts is still a big challenge, and has also been rarely investigated. Herein, surface defect engineering was employed to introduce two different surface defect structures (i.e., two-coordinated nitrogen vacancies (N-2C) on g-C3N4 and oxygen vacancies on TiO2) on the surface of a g-C3N4/TiO2 Z-scheme photocatalyst by one-step hydrogenation treatment, aiming to effectively modulate the surface active free radicals that directly dominate the surface redox reaction. The resulting defect-rich hydrogenated g-C3N4/TiO2 (H-CN/TiO2) Z-scheme heterojunction produced much more OH and O-2(-) free radicals on the surfaces of TiO2 and g-C3N4, respectively, which would greatly facilitate the surface redox reactions in photocatalytic processes. As expected, the H-CN/TiO2 Z-scheme heterojunction exhibited obviously improved activity in photodegradation of tetracycline hydrochloride compared with those of defect-free photocatalysts. Meanwhile, the effect of defect structures on the surface free radicals of the H-CN/TiO2 photocatalyst was well explored and the photocatalytic mechanism was also discussed. This work paves the way for the regulation of free radical formation via surface defect engineering to further improve the photocatalytic activities of Z-scheme heterostructures.