Local charge polarization by introducing a cyanamide group and sulfur dopant with accelerated exciton dissociation and promotion of charge separation for improving CO2 photoreduction performance

Abstract

Photocatalytic CO₂ reduction based on amorphous graphitic carbon nitride (GCN) has been severely restricted by the huge coulomb attraction between photoexcited electrons and holes (e⁻ and h⁺), as well as rapid recombination of photogenerated charge carriers. This exciton effect and sluggish charge separation dynamic originate from the symmetric electron cloud density. Herein, a two-step process (copolymerization and molten-salt treatment) with a heteroatom dopant and group introduction in crystalline GCN was developed to regulate charge distribution and construct local charge polarization (LCP). The as-prepared catalyst exhibited an improved CO₂ photoreduction performance with a CO product yield of 132.12 μmol g⁻¹ and CO selectivity of 95%. Experimental and theoretical calculations demonstrated that the cyanamide (–CN) group and a sulfur (S) dopant in heptazine conjugated ring led to the redistribution of photogenerated charge to form anisotropic charge distribution. These phenomena reduced the exciton binding energy and promoted exciton dissociation into free e⁻ and h⁺. Furthermore, a new lowest unoccupied molecular orbital (LUMO) was created that was lower than the LUMO state of unhybridized heptazine units, which accelerated charge carrier separation and extended the optical absorption range, finally improving the CO₂ photoreduction performance. This work presents an effective strategy to construct LCP in a catalyst via regulating electric and crystalline structures.