Synthesis of functional conjugated microporous polymer/TiO₂ nanocomposites and the mechanism of the photocatalytic degradation of organic pollutants

We successfully synthesize six kinds of functional conjugated microporous polymer (CMP) containing amino, hydroxyl, carboxyl and ester groups by a Sonogashira–Hagihara coupling reaction. These CMPs exhibit type-I nitrogen gas sorption isotherms, and their pore widths are approximately 1.28–1.86 nm with a relatively narrow pore size distribution that is attributed to microporous structures. By blending CMPs with TiO₂ under solvothermal conditions, TiO₂ is uniformly coated onto the surfaces of the CMPs, and CMP/TiO₂ nanocomposites are synthesized successfully. The novel nanocomposites show excellent photocatalytic properties for the degradation of organic pollutants, such as methylene blue, ciprofloxacin and tetracycline, under visible light. The degradation rate of organic pollutants is higher than 96.8%, and after 5 recycling cycles, the degradation efficiency is still more than 93%. The effects of the different functional groups of CMPs on the photocatalytic properties are discussed for the first time. CMP/TiO₂ with carboxyl groups show the smallest band gap, highest photocurrent intensity and lowest resistance compared to those of nanocomposites with amino, hydroxyl and ester groups. This result may be caused by the high electronegativity of carboxyl groups, which enhances the charge transfer rate. The photocatalytic mechanism of the CMP/TiO₂ nanocomposites is investigated, and the superoxide anion (·O₂⁻) is the main active species in the photocatalytic degradation process.