Preparation Technology And Photocatalytic Application Of G-C₃N₄ Matrix Composites

Environmental pollution and energy crisis threaten human survival and development,which is a severe problem.Photocatalysis technology has the characteristics of green,environmental protection and mild,which is expected to fundamentally solve the problem.Graphitic carbon nitride(g-C3N4,hereinafter referred to as CN),as a typical two-dimensional nonmetallic semiconductor material,the graphite-like structure and suitable band gap,and can carry out photocatalytic reactions under visible light.It has become a promising photocatalyst for environmental purification and hydrogen production because of its adjustable structure and good physical and chemical stability.However,the photocatalytic activity of pure CN is limited by some intrinsic defects,such as low separation efficiency of photogenerated carriers and small specific surface area.Hence,we systematically investigate the influences of the morphology of CN,the content of Au in CN/Au,and the content of C in CN/C on the photocatalytic degradation rate and photocatalytic hydrogen production rate by ultrasonic exfoliation regulation and heterojunction composite modifications of CN.The main results of this paper are as follows:(1)The effects of the synthetic conditions on the microstructure and photocatalytic performance of CN were systematically investigated.The CN was prepared by the thermal polymerization method.The specific surface area and microscopic morphology of CN can be changed by changing the precursor,calcination process and performing exfoliation regulation.The bulk CN(BCN)was prepared by pyrolyzing urea precursor at 550? for 6h,furnace cooling to 300? and then air cooling has the best photocatalytic degradation activity,and the efficiency of photocatalytic degradation Rhodamine B(RhB)reached 96.63%after illumination of 20 min,the kinetic constant of 0.180 min-1.The efficiency of photocatalytic degradation could be increased from 88.75%to 97.93%for the exfoliated CN nanosheets(CNN)prepared by different cooling processes.In conclusion,the suitable precursors,calcination processes and exfoliation regulation methods can improve the photocatalytic degradation performance.(2)The effect law of Au content on the photocatalytic degradation rate of CN/Au nanocomposites was investigated by controlling the concentration of HAuCl4 solution to change the mass ratio of Au and CN.The experimental results indicated that the degradation efficiency increased firstly and then decreased with the increase of Au content.The variation of introduction of photocatalytic degradation activity with Au content was mainly competed by two factors.First,the introduction of Au could enhance the separation capability of CN photogenerated carriers and improve the photocatalytic degradation performance;Second,Au could act as a charge complex center,increasing the efficiency of photogenerated electron-hole complexation;Meanwhile,part of the active sites of CN would be covered by Au nanoparticles,reducing the photocatalytic activity.Finally,the best photocatalytic degradation performance was achieved when the mass ratio of Au to bulk CN was 1:10,000 with a kinetic constant of 0.212 min-1.(3)The effect law of Au content in CN/Au nanocomposites on the rate of photocatalytic hydrogen production was as follows:the rate of photocatalytic hydrogen production gradually increased with the increase of Au content.When the mass ratio of Au to bulk and flake CN is 1:100,the average hydrogen production rate is the highest,which are 1050.6 ?mol/g/h and 1825.4 ?mol/g/h,respectively,almost 70 and 6 times as high as that of pure sample.For photocatalytic hydrogen production,the activity of Au as a co-catalyst mainly comes from the hot electron transfer,which was generated by the surface plasmon resonance(SPR)effect of Au nanoparticles.This promotes the formation of electrons and holes and the effective separation of charges,enhancing the performance of photocatalytic hydrogen production.(4)The carbon content in the samples was changed by setting different carbon plating times,and the effect law of carbon content in CN/C nanocomposites on the photocatalytic degradation and hydrogen production rates were studied.It was found that the photocatalytic hydrogen production and degradation performance of CN/C nanocomposites show the same variation tendency with the change of carbon content.The performance first decreases,then increases and then decreases again with the increase of carbon content.This mainly depends on the competition of two factors.First,the introduction of carbon could enhance the separation capability of CN photogenerated carriers and improve the photocatalytic activity;Second,carbon acts as a charge complex center,increasing the efficiency of photogenerated electron-hole complexation;Meanwhile,part of the active sites of CN would be covered by carbon particles,reducing the photocatalytic activity.The best photocatalytic performance of bulk and flake CN/C nanocomposites is achieved as the carbon plating times is 3min,with the kinetic constants of photocatalytic degradation of 0.296 min-1 and 0.242 min-1,respectively;The average hydrogen production rates were 374.3 ?mol/g/h,1542.4 ?mol/g/h.