Preparation And Modification Of Porous Carbon Nitride Photocatalysts

Inspired by plant photosynthesis,photocatalytic water splitting can convert solar energy into hydrogen energy by using semiconductors as the medium,which is a promising way to solve the problem of energy shortages and environmental pollution.Among various photocatalysts,graphitic carbon nitride（g-C₃N₄）has the advantages of suitable water splitting band gap,cheap raw material,simple preparation process,and strong chemical stability.Compared with the traditional transition-metal-oxide-based photocatalytic materials,g-C₃N₄only contains C,N and a small amount of O and H,without concern for metal scarcity and photo-corrosion problems.However,there are three types of shortcoming for g-C₃N₄,the wide bandgap,localized photogenerated carriers,and less reaction sites,which limit its photocatalytic activity.Although constructing a porous carbon nitride（PCN）by annealing in argon atmosphere,partly solve these problems,still remains a big room for improving the capacity of carrier separation and hole oxidation,to achieve the theoretical efficiency conversion from water to hydrogen.In this study,PCN photocatalytic material was modified by electronic energy band structure tuning and heterostructure constructing.The microstructure,optical properties and electronic structure of the materials were characterized by X-ray Diffractometer,Fourier Infrared Diffractometer,Field Emission Scanning Electron Microscope,Transmission Electron Microscope,UV-vis Spectrophotometer and X-ray Photoelectron Spectroscopy Instrument.The photocatalytic hydrogen and oxygen production activity of the material were assessed through photocatalytic water splitting tests.Following conclusions were obtained.（1）Construction of highly efficient porous carbon nitride photocatalytic material with narrow band gap for oxygen production.Oxygen-containing functional groups were introduced to GCN by impregnating glacial acetic acid.After argon annealing,porous cotton-like carbon nitride was obtained.With the significant increase of the number of pores and the decrease of the average pore size,a large number of catalytic active sites were exposed,which provided more rapid carrier migration channels and promoted photogenerated carrier separation.The band gap decreased from 2.03 e V to1.95 e V,broadening the light absorption range of the material.O element replaced the N element connecting the melon chain,to increase the valence band position from1.44 e V to 1.58 e V,thus enhancing the water oxidation ability.Under visible light irradiation,the hydrogen production rate of sample with a mass ratio of glacial acetic acid to carbon nitride of 7 deposited with 3wt%Pt cocatalyst was 47.96μmol h^-1,which was 1.9 times higher than that of the PCN sample.The oxygen production activity of sample with a mass ratio of glacial acetic acid to carbon nitride of 7deposited with 3wt%Pt cocatalyst reached 46.52 mol in 5 h,which was 1.7 times higher than the PCN sample.（2）Construction of a perfoliate quasi-homogeneous Cd Se/PCN heterojunction with broadened spectrum response.Cd Se quantum dots with average particle size of10 nm and band gap of 1.81 e V were grown in PCN bulk phase by in-situ wet chemical method.The average pore diameter of PCN was 52.9 nm,and Cd Se quantum dots were uniformly dispersed on the surface and pores of PCN.On the premise of broadening the whole range of light absorption,the carrier separation effect of type II heterostructure were fully exerted,by which photogenerated electrons and holes migrated to PCN and Cd Se respectively.Under visible light irradiation,the hydrogen production rate of sample with a Cd Se mass fraction of 60 deposited with3wt%Pt cocatalyst was 192.3μmol h^-1,which was 8.1 times higher than the catalytic activity of PCN（TEOA sacrificial agent）.After adjusting the amount of cocatalyst,he hydrogen production rate of sample with a Cd Se mass fraction of 60%was as high as230.5μmol h^-1 when the amount of Pt cocatalyst was 4wt%.