Preparation Of G-C₃N₄/N-doped CDots Composites And Their Photocatalytic Hydrogen Production Performance

In recent years,graphite carbon nitride(g-C3N4)has attracted extensive attention in the fields of solar energy conversion and environmental remediation because of its suitable energy band structure,high physicochemical stability and simple synthesis method.However,pure g-C3N4 has poor visible light absorption capacity and serious photo-generated carrier recombination,which greatly affects the photocatalytic activity.The recombination of carbon dots(CDs)with g-C3N4 can enhance its light absorption ability and promote the separation of electron hole pairs.Compared with undoped CDs,N-doped CDs(NCDs)have better light absorption capacity and faster charge transfer efficiency.Based on this,in this paper,heterojunction construction and heteroatom doping were used to co modify g-C3N4 to improve its photocatalytic performance,enhance the light absorption capacity and promote carrier separation.Combined with related measurement characterization and theoretical simulation,the main mechanism of performance improvement was investigated.The composite photocatalyst composed of g-C3N4 and NCDs shows better photocatalytic hydrogen production activity.However,there is still a lack of in-depth understanding of the effect of N chemical states on photocatalytic activity.We prepared NCDs doped with pyrrole-N,graphite-N/pyridine-N and pyrrole-N/pyridine-N,and successfully loaded on g-C3N4.Under visible light irradiation,g-C3N4 modified by NCDs containing pyrrole-N(CN/NCDs-en)has the best HER performance,3028 μmol h-1 g-1,which is 5.4 times that of pure CN.The photoelectric characterization results show that the incorporation of NCDs containing pyrrole-N into g-C3N4 can effectively enhance the light absorption ability,promote the separation of electron-hole pairs,and effectively promote the participation of photoexcited electrons in the reaction.In addition,theoretical calculations show that pyrrole-N has the most suitable H adsorption capacity(-1.0 eV)compared with graphite-N and pyridine-N,which is conducive to the formation of H2.In order to further improve the visible light absorption capacity and charge separation efficiency of CN/NCDs,heteroatom doping is employed to capture photogenerated holes to promote the cooperative separation of carrier.We realized the NCDs-loading and the P-doping in g-C3N4 by hydrothermal and calcination.Under visible light irradiation,P-doped CN/NCDs(PCN/NCDs)showed superior HER activity(3731 μmol h-1 g-1),which is 6.7 times that of pure CN.The sub-band gap formed by P-doping improves the absorption of light,and the high electron density of P sites is conducive to hole capture.NCDs also improve light utilization and can be used as electron acceptor to promote charge separation.The synergistic effect of Pdoping and NCDs-loading forms a built-in electric field,which greatly accelerates the separation of carrier and improves the photocatalytic activity.The better visible light absorption ability and faster carrier separation efficiency creates the stronger photocatalytic activity of PCN/NCDs.This modification strategy can be extended to the design of g-C3N4 based photocatalyst,which is helpful for faster charge separation to improve photocatalytic activity.