Construction Of Iron-Based Semiconductor Heterojunction Modified By Carbon Quantum Dots And Investigation Of Photocatalytic Performance

Iron-based semiconductor materials have many advantages,such as visible light excitation,easy availability of raw materials,and environmental friendliness,making them promising for applications in fields such as photocatalytic degradation of pollutants.However,the high recombination rate of photogenerated carriers（electrons and holes）and weak redox performance of single iron-based photocatalytic materials have limited their widespread application.In this study,we aimed to develop and utilize iron-based photocatalytic materials for the degradation of organic pollutants.We used TiO₂or g-C₃N₄,which have already achieved industrial-scale production,to construct composite heterostructures with iron-based photocatalytic materials as the core.This improved the defects of single iron-based photocatalytic materials.We then used carbon quantum dots（CQDs）to modify the composite heterostructures and construct ternary composite systems,which significantly enhanced the migration ability of photogenerated electrons in the structure and achieved a significant increase in visible light photocatalytic activity of iron-based composite photocatalytic materials.Therefore,we constructed ternary composite structures using typical iron-based materials such asa-Fe₂O₃,narrow-bandgap iron oxides（ZnFe₂O₄and BiFeO₃）with different crystal structures,and iron-based MOFs materials（MIL-101（Fe））with porous structures and large specific surface areas as the core.We studied the preparation process,physicochemical properties,and the correlation between the optical absorption characteristics,photocatalytic activity,and construction efficiency of the ternary composite materials.We also investigated the migration mechanism of photogenerated electrons（e^-）and holes（h⁺）during visible light photocatalysis.The main contents of this article are as follows:（1）A composite material of CQDs/TiO₂/a-Fe₂O₃was synthesized via the multi-step hydrothermal process.The composite demonstrated a significantly enhanced ability for photocatalytic degradation of methylene blue（MB）under irradiation withl>420 nm,compared to pure TiO₂and TiO₂/a-Fe₂O₃.The study investigated the visible light catalytic reaction mechanism of CQDs/TiO₂/a-Fe₂O₃,where the CQDs were found to capture and transfer photo-induced electrons more efficiently through Ti-O-C bonds and Fe-O-C bonds.The O₂trapped the electrons in CQDs,producing·O₂^-radicals,which played a crucial role in the photocatalytic process.（2）A ternary composite photocatalytic material,consisting of CQDs/ZnFe₂O₄/TiO₂,was synthesized through a sequential synthetic approach using narrow-bandgap spinel-structured ZnFe₂O₄as the core.The study revealed that the heterojunctions constructed by ZnFe₂O₄and TiO₂could significantly enhance the utilization of visible light,compared to the monomers.The photocatalytic degradation performance of the material was investigated for organic pollutants,namely Methylene Blue（MB）,Rhodamine B（Rh B）,and Methyl Orange（MO）,under light irradiation conditions withl>420 nm andl>290 nm.The inclusion of CQDs in the composite resulted in the formation of bonding structures of Zn-O-C,Fe-O-C,and Ti-O-C,which offered additional pathways for the migration of photo-generated electrons within the composite photocatalyst.This increased the separation efficiency of photo-generated e^-and h⁺and resulted in better visible light catalytic activity.（3）A ternary composite structure was constructed using narrow-bandgap perovskite-structured BiFeO₃as the core.The CQDs/BiFeO₃/g-C₃N₄composite photocatalyst with good photocatalytic activity was synthesized by studying the synthesis process parameters of BiFeO₃materials,the optimal BiFeO₃/g-C₃N₄composite ratio,and the optimal amount of CQDs added.The photo-generated electrons and holes in the BiFeO₃/g-C₃N₄composite material were found to follow the Z-scheme heterojunction structure.The optimized composite material of CQDs/BiFeO₃/g-C₃N₄exhibited excellent photocatalytic degradation performance towards MB,Rh B,and MO under light irradiation conditions withl>420 nm.This finding provides evidence of the crucial role played by CQDs in facilitating the migration of electrons（e^-）within the composite material structure.（4）A ternary composite structure was constructed using MIL-101（Fe）with a MOFs structure as the core,and CQDs/MIL-101（Fe）/g-C₃N₄was synthesized.The optimal synthesis parameters for MIL-101（Fe）and MIL-101（Fe）/g-C₃N₄were studied step by step,resulting in the successful preparation of MIL-101（Fe）/g-C₃N₄composite material with good photocatalytic activity.The photocatalytic activity was further enhanced by modifying the composite material with CQDs.The degradation rate of MB by the ternary composite material CQDs/MIL-101（Fe）/g-C₃N₄was significantly improved compared to MIL-101（Fe）and MIL-101（Fe）/g-C₃N₄,with enhancements of124.7 times and 1.3 times,respectively.Moreover,the composite material exhibited the best photocatalytic degradation performance among the four tested iron-based composites for Rh B and MO.Based on the positions of CQDs in the structure,CQDs/MIL-101（Fe）/g-C₃N₄composite material was proposed to have both Type-II and Z-scheme heterojunction models.The enhancement mechanism of CQDs for the migration and separation of visible light generated charge carriers in the ternary composite material was discussed.