S-scheme heterojunction of crystalline carbon nitride nanosheets and ultrafine WO₃ nanoparticles for photocatalytic CO₂ reduction

Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO₂ conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO₃ nanoparticles to construct the CCN/WO₃ heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO₃ hybrid, featuring strengthened optical capture, enhanced CO₂ adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO₃ catalyst with optimized composition displays a high photocatalytic activity for reducing CO₂ to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H₂ formation. Moreover, it also manifests high stability and fine reusability for the CO₂ conversion reaction. The CO₂ adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO₂^*-, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO₂ reduction mechanism.