Azine-based covalent organic frameworks as metal-free visible light photocatalysts for CO2 reduction with H2O

The conversion of CO2 into valuable organic products by means of solar energy is one of the best solutions to both global warming and energy shortage. Covalent organic frameworks (COFs) show a superior thermal and chemical stability due to their covalent bonds, besides, azine-based COFs, an interesting new class of photoactive materials, can combine advantages in CO2 capture and heterogeneous photocatalysis for the conversion of CO2. Here, for the first time two azine-based COFs were used as photocatalysts that catalyzed the reaction of CO2 with H2O into methanol under visible-light irradiation without any sacrificial agents. In addition, these azine-based COFs were more active in the photocatalytic reduction of CO2 with H2O than g-C3N4 and other reported photocatalysts coupling with inorganic semiconductors. Various techniques were used to characterize the synthesized azine-based COFs in combination with molecular simulation to elucidate the differences in the photocatalytic reduction of CO2 between the two azine-based COFs investigated. The current work indicates that azine-based COFs would be ideal metal-free organic semiconductors, which might be developed into new flexible photocatalysts for CO2 reduction due to their tunable composition, structure, and property. Furthermore, the present study would stimulate research and development in COFs-based photocatalysts and other new photocatalysts as energy transducers.