S-Scheme Heterojunctions in FeOOH–CN/Ni(OH)₂ with Ni–Fe Synergistic Effects for Enhanced Photocatalysis-Self-Fenton Degradation of Organic Contaminants

The Fenton reaction in advanced oxidation processes is an ideal pathway for the generation of highly reactive hydroxyl radicals (^•OH). However, its application in organic wastewater treatment is limited due to the requirement for acidic pH conditions and the additional introduction of H₂O₂ and Fe²⁺. In this study, FeOOH-CN/Ni(OH)₂ composites were constructed as photocatalysis-self-Fenton catalysts by the amorphous FeOOH, which was uniformly anchored onto the surface of pure g-C₃N₄ (carbon nitride (CN)), and Ni(OH)₂ was assembled on top. In this process, FeOOH-CN/Ni(OH)₂ achieved self-sufficiency in H₂O₂ production while enabling efficient degradation of tetracycline hydrochloride (TCH) over a wide pH range, with a degradation ratio of up to 98.82%, and a first-order reaction rate constant (<i>k</i>) was determined to be 0.04925 min^-1, representing a 10.15 times enhancement compared to the pristine CN. A reasonable S-scheme heterojunction charge migration mechanism was demonstrated based on valence-band X-ray photoelectron spectroscopy (VB-XPS), atomic force microscopy (AFM) equipped with a Kelvin probe force microscope, and density functional theory (DFT) theoretical calculations. Furthermore, degradation experiments demonstrated that the catalyst possesses remarkable degradation efficiency for other contaminants as well. On one hand, this significant improvement can be attributed to the formation of an S-scheme heterojunction between Ni(OH)₂ and CN, which not only increased the specific surface area of the composite but also remarkably enhanced the separation and transfer efficiency of photogenerated charges. On the other hand, the synergistic effect between Ni and Fe effectively facilitated the activation of H₂O₂ and promoted the cycling of Fe³⁺/Fe²⁺. This study provided an efficient visible-light-driven photocatalysis-self-Fenton system, offering novel perspectives for enhancing Fenton-based processes and advancing the application of Fenton reactions in the treatment of organic wastewater.