Promoting Photodegradation Efficiency via a Heterojunction Photocatalyst Combining with Oxygen Direct and Fast Diffusion from the Gas Phase to Active Catalytic Sites

The low solubility of oxygen in solution is the main obstacle for the biodegradation of organic pollutants in wastewater. To address this problem, inspired by the degradation mechanism of aerobic bacteria toward organic pollutants, a novel photodegradation system was presented and operated by a heterojunction photocatalyst combining with a hydrophobic triphase interface, allowing oxygen to directly diffuse from the gas phase to active catalytic sites submersed in polluted solutions. Especially, the heterojunction photocatalyst was fabricated by graphitic carbon nitride nanosheets (C₃N₄ NS) sensitized with 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP). The resulting photocatalyst was coated on a certain part of the commercial superhydrophobic carbon paper (CP) and submersed in the polluted wastewater, while the other part of hydrophobic CP (without coating with C₃N₄-TCPP nanocomposite) was exposed to air to form a gas-liquid-solid tri-phase photodegradation system. With this system, the photodegradation rate was 10-fold higher than that of a conventional liquid/solid diphase system in oxygen-saturated solutions. This was, on one hand, due to the abundant oxygen on the surface of a photocatalyst coming from the fast and direct diffusion from the gas phase through the superhydrophobic nanoporous part of CP. On the other hand, the hybrid C₃N₄-TCPP nanocomposite enhanced the light absorption efficiency under simulated sunlight irradiation and restrained the recombination of photogenerated electron-hole. Moreover, the triphase photodegradation system was stable in aqueous solutions for a long time and can be reused almost without attenuation for five cycles, which provided a great potential to be utilized for practical wastewater treatment.