Unique MIL-53(Fe)/PDI Supermolecule Composites: Z‑Scheme Heterojunction and Covalent Bonds for Uprating Photocatalytic Performance

It is important to find an effective way to enhance the photocatalytic efficiency of metal–organic frameworks. In this work, an organic supermolecule perylene diimide (PDI) semiconductor with a carboxyl terminal was added into the synthesis process of MIL-53­(Fe) crystals. The PDI/MIL-53­(Fe) (PM) composite photocatalyst was first obtained. The TC-H photodegradation rate of the most efficient 5PM was nearly 94.08% within 30 min, whose apparent reaction rate constant (k) is 4 times that of PDI and 33 times that of MIL-53­(Fe), respectively. By investigation and characterization, it has been found that PDI nanofibers were dispersed and fixed in MIL-53­(Fe) and bonded to each other by covalent bonds. The radical trap experiments and electron spin resonance analysis illustrated that hydroxyl radical (·OH), superoxide radical (·O2 –), and photogenerated holes (h+) were active species. Combined with the band structure of PDI and MIL-53­(Fe), it is proposed that the PM photocatalyst was a Z-scheme heterojunction mechanism. Therefore, PM photocatalysts showed excellent charge separation and transfer ability. The performance improvement of 5PM is due to enhanced visible light absorption, efficient charge separation, and excellent redox potential. Five cyclic photocatalytic tests and experiments further demonstrate that the 5PM photocatalyst has a promising future for pollutant removal.