Enhanced photocatalytic CO₂ reduction via S atom-promoted carbon nitride complexed with imidazolium-based ionic liquids: Achieving superior selectivity

The utilization of carbon nitride (C₃N₄) in photocatalytic CO₂ reduction faces obstacles due to significant charge recombination of photogenerated electron-hole pairs, limited light-harvesting capability, and a constrained surface area. In this study, S-C₃N₄/IL composites were synthesized by molecular docking of ionic liquids (IL) through S atoms in layered carbon nitride under environmental conditions by simple self-assembly technology to achieve efficient photocatalytic CO₂ reduction. Contrasted with pristine C₃N₄ (1.1 μmol g^-1h⁻¹), S-C₃N₄/IL demonstrated over a 300-fold increase in the rate of CO₂ reduction to CO (310 µmol g^-1h⁻¹), achieving an approximate 100 % selectivity. Moreover, the possible reaction mechanism was proposed by in-situ infrared and DFT calculations. The remarkable photocatalytic efficacy of S-C₃N₄/IL can be ascribed to multiple factors: 1) augmented visible light absorption range and enhanced interfacial compatibility due to sulfur doping, fostering improved interaction between the amphiphilic ionic liquid and the hydrophobic C₃N₄; 2) suppressed char + ge recombination, facilitating increased generation of abundant free radicals; 3) the Coulombic interaction between the IL's anionic and cationic components with the formed electron-hole pairs, thereby impeding their recombination. Besides, ultraviolet photoelectron spectroscopy, fluorescence lifetime support these results. DFT calculation suggests S doping also plays a critical role in reducing free energy of *COOH formation. In addition, ¹³C isotope experiment confirms the carbon source of CO from CO₂. This proposition paves the way for enhancing the photocatalytic prowess of carbon nitride in CO₂ reduction.