Local charge polarization by introducing cayanamide group and sulfur dopant with accelerated exciton dissociation and promoted 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 hole (e- and h+), as well as rapid recombination of photogenerated charge carrier. This exciton effect and sluggish charge separation dynamic are originating from the symmetric electron could density. Herein, copolymerization and molten-salt treatment two-step process with heteroatom dopant and group introducing in crystalline GCN was developed to regulate charge distribution, constructing local charge polarization (LCP). The as-prepared catalyst exhibits an improved CO2 photoreduction performance with CO product yield of 132.12 μmol∙g-1 and CO selectivity of 95%. Experimental and theoretical calculations demonstrated that cayanamide (-CN) group and sulfur (S) dopant in heptazine conjugated ring successfully causes the redistribution of photogenerated charge to form anisotropic charge distribution, reducing the exciton binding energy and promoting exciton dissociation into free e- and h+. Furthermore, a new lowest unoccupied molecular orbital (LUMO) is created lower the LUMO state of unhybridized heptazine units, which accelerates charge carrier separation and extends the optical absorption range, finally improving the CO2 photoreduction performance. This work presents an effective strategy to construct LCP in catalyst via regulating the electric and crystalline structure.