A study on singlet oxygen generation for tetracycline degradation via modulating the size of α-Fe₂O₃ nanoparticle anchored on g-C₃N₄ nanotube photocatalyst

Photocatalysis is considered as an effective technique for mitigating ecological risks posed by residual tetracycline (TC). To improve the efficiency of this technique, it is necessary to enable photocatalysts to produce highly reactive species, such as singlet oxygen (¹O₂). However, due to the high activation energy of ¹O₂, photocatalysts can hardly produce ¹O₂ without assistance from external oxidants. Herein, we find that the size-reduced α-Fe₂O₃ nanoparticles (~ 4 nm) that anchored on g-C₃N₄ nanotube (α-Fe₂O₃@CNNT) can spontaneously generate ¹O₂ for degradation of TC. In comparison, only hydroxyl radical (·OH) can be produced by g-C₃N₄ nanotube loaded with ~ 14 nm α-Fe₂O₃ nanoparticles (α-Fe₂O₃/CNNT). Owing to the high reactivity of the ¹O₂ species, the photocatalytic degradation rate (K_app) of TC with α-Fe₂O₃@CNNT (0.056 min⁻¹) was 1.8 times higher than that of α-Fe₂O₃/CNNT. The experimental results and theoretical calculations suggested that reducing the size of α-Fe₂O₃ nanoparticles anchored on g-C₃N₄ nanotube decreased the surface electron density of α-Fe₂O₃, which induces the generation of high-valent Fe(IV) active sites over α-Fe₂O₃@CNNT and turns the degradation pathway into a unique ¹O₂ dominated process. This study provides a new insight on the generation of ¹O₂ for effective degradation of environmental pollutant.