Synthesis Of Conjugated Polymer Semiconductor Photocatalyst And Study Its Structure-Property Relationship

Semiconductor photocatalysis has attracted tremendous attention for their promising potential to settle intractable energy and environment issues using abundant solar energy. Splitting of water into hydrogen fuel and degradation of environmental pollutants are the two important aspects for photocatalytic applications. To date, the focus in this field lies in the development of novel efficient photocatalysts. For the practical large-scale application, an ideal photocatalyst would be cheap, sustainable, stable and efficiently working with visible light to utilize the main component of solar spectrum. Polymeric photocatalysts hold great promise owing to their abundant sources, low-cost fabrication and easy processibility. However, there have been few reports on the development of polymer photocatalysts in contrast with their inorganic counterparts. The existing bottlenecks of polymer semiconductor photocatalysts are small visible-light absorption due to wide bandgap, limited photocatalytic activity due to low mobility of photogenerated charge carriers, poor oxidative stability due to high HOMO energy level. Consequently, develop a visible-light-driven, efficient and stable polymeric photocatalytic system is still an inviting prospect and highly desired.In this dissertation, we report a new type of polymer photocatalyst based on crystalline polyimide (PI) synthesized via solid-state thermal condensation of amine and dianhydride monomers. The synthesis is environmentally-friendly, cost-effective and easy to scale up. Moreover, PI exhibits efficient and stable photocatalytic activity for H2production and organic pollutants degradation under visible light irradiation.Based on the polyimide structure, we investigate the influence of condensation temperature and monomer molar ratio on electronic band structure of PI regarding the photocatalytic activity. It is found that the bandgap of PI gradually decreases with increasing condensation temperature and the sample with moderate bandgap exhibits the highest H2evolution activity. It is also found that the bandgap of amine-terminated PI is smaller than that of anhydride-terminated PI and thus shows a higher photocatalytic activity for H2evolution under visible light irradiation. But the crystallinity of anhydride-terminated PI is higher than that of amine-terminated PI and thus shows a higher photocatalytic activity for H2evolution under full arc light irradiation. Moreover, owing to the stronger photooxidation capability, the anhydride-terminated PI exhibits preferential activity for the photodegradation of methyl orange (MO).Based on the polyimide structure, we also investigate the influence of chemical structure of amine and dianhydride monomers on electronic band structure of PI regarding the photocatalytic activity. It is found that the PI synthesized by melem and PMDA exhibits excellent photocatalytic activity for H2production and organic pollutants degradation under visible light irradiation. The visible-light H2evolution rate of this PI is20.6μmol/h, which is about three times as high as that of graphitic carbon nitride (g-C3N4,7.0μmol/h). The visible-light MO degradation rate of this PI is about22times as high as that of N-doped TiO2.Finally, we develop an effective strategy for the design of polymer photocatalyst with enhanced photooxidation capability, that is, introduce electron-withdrawing substituent into polymer framework to decrease the valance band (VB) level. By incorporating electron-deficient pyromellitic dianhydride (PMDA) constituent into the network of g-C3N4, the photooxidation property is greatly enhanced. The modified photocatalyst features preferential activity for water oxidation over water reduction in comparison to g-C3N4. Moreover, the active species involved in the photodegradation of MO switches from photogenerated electrons to holes after band structure engineering.