Boosting the photocatalytic CO2 reduction activity of g-C3N4 by acid modification

Graphitic carbon nitride (g-C3N4 denoted as CN) is a typical organic polymeric semiconductor photocatalyst for CO2 reduction to produce solar fuels. In this study, the effect of phosphoric acid (H3PO4) modification on the property and photocatalytic performance of CN towards CO2 reduction was systematically studied. It was found that modification of CN with phosphoric acid (CNP photocatalyst) caused a reduced band gap width and negatively shifted conduction band (CB) potential. The photoreactivity of the optimized photocatalyst (CNP0.75) for CO production rate reaches 35.66 mu molg-1h- 1, which is 4.38 times higher than that of CN sample with a CO evolution rate of only 8.14 mu molg-1h- 1. The enhanced photoreativity of g-C3N4 towards CO2 reduction after phosphoric acid modification is ascribed to the combined effects of enhanced adsorption, stimulated charge separation and improved reduction ability of CB electrons. The positive effect of acid modification on the enhanced photocatalytic activity of CN was further observed by sulfuric acid (H2SO4) modification and hydrochloric acid (HCl) modification, suggesting the versatility of the method. The present study provides a novel way in preparation of high efficient g-C3N4 photocatalyst for CO2 conversion and utilization.