Targeted regulation of exciton dissociation in graphitic carbon nitride by vacancy modification for efficient photocatalytic CO2 reduction

The excitons are known to exist in graphitic carbon nitride (g-C3N4), but the contribution of excitons to the photocatalytic process has received only sporadic attention. Targeted regulation of excitons dissociation into free charges is an effective measure to improve the utilization of carriers and enhance photocatalytic activity. Here we designed the ultra-thin g-C3N4 nanosheets modified with rich N vacancies (Nv-rich-CN) through molecular self-assembly and molecular intercalation strategies. N vacancies can effectively dissociate excitons into free charges. The electron concentration of the Nv-rich-CN is 3.24 times that of bulk g-C3N4 (CN). Benefiting from the enhanced carrier utilization, Nv-rich-CN showed superior activity for CO2 photoreduction under visible light irradiation. Femtosecond transient absorption spectroscopy revealed a photophysical model of exciton dissociation. Theoretical calculation results explained that the essential reason for the vacancy to promote exciton dissociation may be that the lack of local order provided a driving force for exciton dissociation. Besides, N vacancies acted as active sites in the process of CO2 reduction, promoting the adsorption and activation of CO2 by the photocatalyst. In situ diffuse reflectance infrared Fourier transform spectroscopy revealed the change of intermediate products during the reduction of CO2. This work focuses on the contribution of excitons in the photocatalytic process and provides a novel idea for enhancing the carrier utilization of photocatalysts.