Construction of S-scheme NH2-MIL-101(Fe)@DQ-TFP core-shell heterojunction for highly efficient visible-light-driven self-Fenton degradation

Modulating localized electronic structures and interfacial charge-transfer pathways in metal-organic framework (MOF)-based heterogeneous catalysts remains a critical challenge for achieving efficient and durable advanced oxidation processes. Herein, a highly crystalline S-scheme NH₂-MIL-101(Fe)@DQ-TFP (NM@DT) core-shell heterojunction was constructed through a microwave-assisted MOF-first in situ growth strategy. Distinct from conventional physical assembly, the anthraquinone-based β-ketoenamine covalent organic framework (COF) shell was integrated with the amino-functionalized MOF core through chemically coupled growth and interfacial hydrogen-bonding interactions, generating a robust core-shell interface. This intimate interfacial coupling induces spontaneous electronic redistribution and charge polarization, which are verified by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and density functional theory calculations, thereby providing the electronic basis for an internal electric field-assisted S-scheme charge-transfer pathway. The optimized NM@DT-3 heterostructure exhibits enhanced photocatalytic tetracycline degradation performance, achieving 90.70% removal efficiency within 180 min without external H₂O₂ addition. The core-shell configuration not only promotes interfacial charge separation but also creates a spatially confined catalytic microenvironment, enabling efficient oxygen activation and reactive oxygen species evolution coupled with localized H₂O₂ activation at adjacent Fe centers. Mechanistic investigations reveal that freely diffusing hydroxyl radicals (·OH) make only a minor contribution under the tested conditions, whereas superoxide radicals (·O₂⁻), photogenerated holes (h⁺), and singlet oxygen (¹O₂) participate in TC oxidation. In this reaction network, ·O₂⁻ acts as a key intermediate linking photocatalytic O₂ reduction to both H₂O₂ formation and a proposed h⁺-mediated ¹O₂ generation pathway. The reduced reliance on conventional bulk ·OH-dependent chemistry is consistent with the retained catalytic activity of NM@DT-3 under alkaline conditions, with a TC removal efficiency of 77.74% at pH 10. Meanwhile, the chemically integrated core-shell structure effectively suppresses iron leaching, with only 0.107 mg L⁻¹ Fe released during the photocatalytic process. This work provides new insights into engineering chemically coupled MOF/COF interfaces for regulating interfacial charge migration and ROS evolution, offering an effective strategy for constructing stable photo-self-Fenton catalysts for environmental remediation.