Synthesis And Photocatalytic Performance Researches Of Porous Graphitic Carbon Nitride

As an inexhaustible green power,solar energy can be used for photocatalytic hydrogen evolution and degradation of organic pollutants,thus to effectively solve the energy crisis and environmental pollution.Therefore,it is the aim in photocatalytic research to explore and synthesize low cost and efficient photocatalysts.With the advantages like visible light response,high physical and chemical stability,easy synthesis,graphite carbon nitride?g-C₃N₄?is considered to be a nonmetal photocatalyst possessing great potential.By modulation of structure and composition,the photocatalytic efficiency of g-C₃N₄ can be further improved.This thesis designs new strategies to modify g-C₃N₄by the introduction of channels and heteroatoms doping.It is a main task to study the effect of modulation on light absorption ability by simulations and calculations.Furthermore,the effects on optical properties,photoelectric properties and photocatalytic performances caused by modulation are studied.Millimeter-scale porous silica sphere is adopted as hard template to synthesize a meso/macroporous g-C₃N₄ millimeter-scale sphere?MCN?with 3D continuous channels.The obtained MCN sample possesses a pore size distribution at 20-80 nm,whose skeleton is slender as 20 nm,thus the BET specific area can reach 58 m² g^–1.By finite-different time-domain?FDTD?optical simulation,the effects of the channels introduction on the light absorption intensity and distribution are evaluated,which is stronger at the edge of and on the inner wall of the channels than that without channels under the same condition.Meanwhile,the bigger the hole size is,the more obvious this enhancement is.The 3D continuous meso/macroporous channels not only expand the effective reaction area,improve the mass transfer,but also enhance the light absorption ability,reduce the recombination of photo-generated electrons and holes,accelerates the diffusion of active charges.These enhancements,together with large number of-NH₂ and defects caused by the growth-confined effect,promote the H₂ generation and phenol degradation over MCN-560 under visible light irradiation,which is 7 times and 3.2 times respectively higher than that over pristine g-C₃N₄.It is the first time using soft polyurethane?PU?sponge as a soft template to prepare hierarchically porous g-C₃N₄?FCN?.The existence of micron-scale pores at 1-2?m and nano-scale pores at about 50 nm are proved in FCN,and the specific surface area can also reach 59 m² g^–1.The formation process of the two kinds of pores is illustrated by TG test.Further the heating rate of 5 ^oC min^–1 is affirmed as the most appropriate condition to synthesize FCN material with higher surface area and without carbon residue.According to the FDTD simulation results on FCN,the micron-scale pores promote the light delivering to the internal of nano-scale channel,thus enhance the whole absorption capacity of light energy.FCN samples especially FCN-5 possess good optical properties and photoelectric properties,thus FCN-5 shows superior H₂ generation rate and phenol degradation efficiency,which is 8 times and 3.5 times respectively higher than that over pristine g-C₃N₄.The mechanisms of photocatalysis over FCN-5 are further explored by different controlling experiments,which is a complex process with the photogenerated active electrons as main species.It is an efficient method to enhance photocatalytic performance to combine the heteroatom doping with the pore creation.S doped meso/macroporous g-C₃N₄ with continuous channels?SMCN?is synthesized by in situ method using porous silica spheres as template and thiourea as raw material.The results of DFT calculation show that the doped S atom has an impact on the crystal structure of g-C₃N₄ thus changes its optical absorption constant.Based on DFT calculation results of optical parameters,FDTD simulations further illustrate the effect of the structure and optical parameter changes on the light energy absorption,and the results illuminate that S doping and the pore creation can both enhance light absorption ability,and the combination of the two can further improve the utilization rate of light energy.SMCN not only improves its optical absorption ability,but also accelerates photoelectric separation rate to a certain extent.Therefore,the photocatalytic phenol degradation efficiency and H₂ generation rate over SMCN are greatly improved,which is 15 times and 3.3 times respectively higher than that over pristine g-C₃N₄.The mechanisms of photocatalysis over SMCN are explored by different controlling experiments,which is a complex process with the photogenerated active electrons as main species too.C doped hierarchically porous carbon nitride sphere?NMC?is prepared using porous silica sphere as hard template and aniline as raw materials.Depended on a large number of micropores and meso/macropores exist in NMC,the accessible surface area can be expanded,the mass transfer can be improved and the light absorption ability can be enhanced.The existence of various nitrogen species can enhance the wettability of NMC samples.Coupling with the enhanced conductivity by a large amount of graphite-phase carbon and the accelerated photoelectric separation rate,NMC samples show good degradation efficiency of phenol under visible light and UV light respectively,which is 3times and 7 times higher than that over pristine g-C₃N₄.