Boosting exciton dissociation by regulating dielectric constant in covalent organic framework for photocatalysis

Abstract

Polymer semiconductor photocatalysts usually suffer from the high exciton binding energy because of the low dielectric constant. Herein, two crystalline polymer photocatalyst prototypes, neutral covalent organic framework (COF) and ionic covalent organic framework ( i COF), were employed to investigate the exciton effect. After polarization by the ionic sites with high dipole moment, the i COF endows a greatly increased dielectric constant to reach the low exciton binding energy of 23 meV, below the thermal energy under room temperature ( k B T, 26 meV). Thus, enhanced generation, separation, and transport of photogenerated charge carriers over the i COF catalyst resulted in a hydrogen peroxide production rate of 10.01 mmol g -1 h -1 in the photocatalytic oxygen reduction coupling with water oxidation reaction, which is almost 7 times higher than the neutral COF under identical conditions. This work demonstrates a promising way to tune the dielectric properties of polymer photocatalysts in order to regulate the exciton effect and related photocatalytic behavior. • Highly crystalline i COF was obtained in a microwave-assisted solvothermal route • The highly polar ionic sites enhanced the dielectric constant of the COF matrix • Low exciton binding energy was reached to allow spontaneous exciton dissociation • Rapid H 2 O 2 production was observed under visible-light irradiation in pure water Photocatalytic production of high-energy chemicals such as H 2 O 2 plays an essential role in seeking sustainable energy. Organic polymeric semiconductors that can be prepared from the earth’s abundant elements are preferential photocatalysts with advantages such as tunability. Nonetheless, the inherent low dielectric properties of organic polymers result in high exciton binding energies that inhibit the spontaneous exciton dissociation and thus restrict photocatalytic activity. Herein, we demonstrated that polarizing the covalent organic framework by integration of ionic moieties, fabricating i COF, can greatly increase the dielectric constant and thus reduce the exciton binding energy down to k B T (26 meV). The spontaneous exciton dissociation was obtained, accelerating the separation and transport of photogenerated carriers and thus a high H 2 O 2 production rate. This work opens a new avenue to boost the efficiency of organic semiconductors for solar-fuel production. The high polarization of the ionic sites greatly increases the dielectric constant of the COF matrix and thus reduces the exciton binding energy, effectively accelerating the H 2 O 2 production rate under visible-light irradiation.