Cayanamide Group Functionalized Crystalline Carbon Nitride Aerogel for Efficient CO₂ Photoreduction

For photocatalytic CO2 reduction, traditional amorphous polymeric carbon nitride (PCN) has suffered from fast photoexcited electron-hole recombination and low specific surface area, resulting in low photocatalytic activity. Herein, starting from thermally polymerized PCN, cayanamide groups (-CN) functionalized crystalline carbon nitride (CCN) self-supporting aerogels are obtained through a molten-salt and self-assembly two-step strategy, which realized efficient photocatalytic CO2 reduction into CO as the main reduction products and O-2 as the oxidation product, with CO evolution rate and selectivity reaching 25.7 mu mol g(-1) h(-1) and 93.8%, respectively. It is revealed that the self-supporting porous aerogel structure can enhance the mass transport of reactants and products, and increase the light absorption ability via multiple photon reflection. With charge carrier separation greatly enhanced in CCN aerogels with high crystallinity as revealed by charge carrier dynamics investigations, theoretical calculations and in situ spectral characterizations evidence that the introduced -CN groups would act as the active sites for photoreduction of CO2 into CO, with energy barrier greatly reduced for the formation of COOH* intermediates, the rate-determining step for CO2-to-CO reduction. This work demonstrates a novel and controllable strategy to develop semiconducting polymers with crystal, molecular and morphological structures synergistically modulated for highly efficient and selective photocatalytic CO2 reduction.