We report self-sacrificial in situ synthesis of a Z-scheme heterojunction photocatalyst Fe3(PO4)2/MIL–88B(Fe)–NH2 that shares a common metal component, where the Fe fiber substrate serves dually as the structural support and metal precursor. The resulting heterojunction fiber composite, noted as Fe/MIL–NH2@fiber, demonstrates efficient charge separation and reactive oxygen species (ROS) generation, in which MIL–88B(Fe)–NH2 drives superoxide radical ( \(\cdot {\text{O}}_{2}^{ - }\) ) formation while Fe3(PO4)2 contributes to hydroxyl radical (·OH−) production. This study primarily focuses on the removal of formaldehyde (HCHO) as a model volatile organic compound (VOC) to explore how the developed Fe/MIL–NH2@fiber platform achieves a humidity-adaptive removal process. Fe/MIL–NH2@fiber exhibits notable performance in HCHO removal through a synergistic process of adsorption and photodegradation. Humidity plays a crucial role in mediating the interactions between the porous photocatalyst and HCHO, where excessive relative humidity (90% RH) suppresses adsorption but simultaneously enhances photodegradation by facilitating ·OH− generation. Kinetic analyses reveal that adsorption proceeds faster than photodegradation, suggesting that the metal–organic framework (MOF)-integrated heterojunction is an advantageous solution for VOC removal, benefiting from a dual-action mechanism where immediate capture is complemented by permanent oxidative removal. Notably, the developed Fe/MIL–NH2@fiber demonstrated a removal efficiency greater than 85% after three consecutive cycles, highlighting its robustness and potential for long-term applications under varying humidity conditions. This work offers design principles applicable to MOF-integrated photocatalysts for step-forward air purification in practical scenarios.
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