Semiconductor-based photocatalysis by utilizing solar energy for sustainable organic pollutant elimination has been a promising tactic to alleviate environmental issues. Nevertheless, the development of robust and efficient photocatalysts to degrade organic pollutants still faces major challenges because of insufficient charge separation. Here we design and fabricate a heterojunction consisting of copper, carbon-modified TiO₂, and sulfur-doped g-C₃N₄ nanosheets (i.e., S-C₃N₄/Cu/C-TiO₂). The heterostructure affords a remarkable synergistic photocatalysis for tetracycline hydrochloride degradation, achieving an 82.6% removal efficiency within 30 min under visible light irradiation, about 15.4 and 7.3 times higher than that of S-C₃N₄ and C-TiO₂, respectively. The superior performance is attributed to the synergy between Cu doping and the Z-scheme heterojunction, which not only enhances the interfacial electric field effect, facilitating charge separation, but also boosts the redox capability. The charge carrier transfer between Cu/C-TiO₂ and S-C₃N₄ follows a Z-scheme, as verified by trapping experiments, electron spin-resonance spectroscopy, and density functional theory calculations. Furthermore, the tetracycline hydrochloride degradation pathways are enunciated by liquid chromatograph mass spectrometry analysis. This work provides an effective approach for constructing high-performance photocatalysts that have potential in environmental remediation.

Photocatalytic O₂ activation to generate reactive oxygen species is crucially important for purifying organic pollutants, yet remains a challenge due to poor adsorption of O₂ and low efficiency of electron transfer. Herein, we demonstrate that ultrafine MoO_x clusters anchored on graphitic carbon nitride (g-C₃N₄) with dual nitrogen/oxygen defects promote the photocatalytic activation of O₂ to generate ·O₂⁻ for the degradation of tetracycline hydrochloride (TCH). A range of characterization techniques and density functional theory (DFT) calculations reveal that the introduction of the nitrogen/oxygen dual defects and MoO_x clusters enhances the O₂ adsorption energy from −2.77 to −2.94 eV. We find that MoO_x clusters with oxygen vacancies (Ov) and surface Ov-mediated Mo^δ+ (3 ≥ δ ≥ 2) possess unpaired localized electrons, which act as electron capture centers to transfer electrons to the MoO_x clusters. These electrons can then transfer to the surface adsorbed O₂, thus promoting the photocatalytic conversion of O₂ to ·O₂⁻ and, simultaneously, realizing the efficient separation of photogenerated electron–hole pairs. Our fully-optimized MoO_x/g-C₃N₄ catalyst with dual nitrogen/oxygen defects manifests outstanding photoactivities, achieving 79% degradation efficiency toward TCH within 120 min under visible light irradiation, representing nearly 7 times higher activity than pristine g-C₃N₄. Finally, based on the results of liquid chromatograph mass spectrometry and DFT calculations, the possible photocatalytic degradation pathways of TCH were proposed.

Engineering of defects in semiconductors provides an effective protocol for improving photocatalytic N₂ conversion efficiency. This review focuses on the state-of-the-art progress in defect engineering of photocatalysts for the N₂ reduction toward ammonia. The basic principles and mechanisms of thermal catalyzed and photon-induced N₂ reduction are first concisely recapped, including relevant properties of the N₂ molecule, reaction pathways, and NH₃ quantification methods. Subsequently, defect classification, synthesis strategies, and identification techniques are compendiously summarized. Advances of in situ characterization techniques for monitoring defect state during the N₂ reduction process are also described. Especially, various surface defect strategies and their critical roles in improving the N₂ photoreduction performance are highlighted, including surface vacancies (i.e., anionic vacancies and cationic vacancies), heteroatom doping (i.e., metal element doping and nonmetal element doping), and atomically defined surface sites. Finally, future opportunities and challenges as well as perspectives on further development of defect-engineered photocatalysts for the nitrogen reduction to ammonia are presented. It is expected that this review can provide a profound guidance for more specialized design of defect-engineered catalysts with high activity and stability for nitrogen photochemical fixation.