Study On Photocatalytic Properties Of G-C₃N₄ Sintered At High Temperature And Pressure

The development of chemical industry has improved the living standard of human beings.At the same time,it has also resulted in the generation of a large amount of waste water and waste gas,which has aggravated the pollution to the environment."Gold and silver mountains are not as good as green waters and green mountains."Therefore,environmental protection has also attracted great attention from various countries.Degradation of organic pollutants by photocatalytic technology is an effective method to reduce environmental pollution.In recent years,g-C₃N₄has wide applications in the field of photocatalysis because of its suitable energy band structure（The bandgap is nearly 2.7 e V）.g-C₃N₄,as an important photocatalysis material,it has be used in the degradation of organic pollutants,antibiotics,and hydrogen production by decomposing water.However,the pristine g-C₃N₄still has many shortcomings.Therefore,its photocatalytic efficiency is not high,which limits its wide application.In order to improve the photocatalytic performance of g-C₃N₄,there are different approaches to improve the material performance for g-C₃N₄,including material doping,structure engineering,and semiconductor recombination.However,these methods all inevitably introduce heteroatoms.In this work,we synthesised a series of samples treated with different pressure and temperature,and compare their photocatalytic performance.We also designed different experiments to clarify its catalytic mechanism to improve its photocatalytic performance.We found that high pressure can effectively change the N content of g-C₃N₄and improve its photocatalytic performance.The research contents and conclusions of this paper are as follows:（1）The orginal pristine g-C₃N₄was synthesized by a one-step pyrolysis method.Afther that,the pristine g-C₃N₄was sintered at different temperatures and pressures using a piston-cylinder press.The structure,morphology,and optical properties of all the samples were characterized using different techniques.The results show that the synthesized g-C₃N₄crystallised well with a bandgap of 2.53 e V.Under the excitation wavelength of 320 nm,the sample was found to have a strong fluorescence phenomenon at 447 nm.Thermogravimetric analysis results show that pristine g-C₃N₄decomposes at 675℃,so we sintered the samples at a high temperature below 600℃.The structure of g-C₃N₄did not show obvisous changes with high temperature treatment below 600℃,but the interlayer spacing decreased at600℃.The morphology of the smaples nearly kept the same after high temperature and pressure treatment,all of which were typical non-uniform block particles.The pores existing between the particles are compacted and the particles are crushed,resulting in a slight change in the specific surface area.The fluorescence analysis results show that the fluorescence of the sintered samples is significantly weakened,and the bandgap is significantly reduced at 600℃.（2）Rhodamine B（Rh B）was selected as the targeted contam-inant for the evaluation of the photocatalytic performance of different g-C₃N₄samples.The results show that under the same pressure,the degradation efficiency varies with the temperature.When the temperature is lower than 400℃at 1.0 GPa,the degradation decreases with the increase of temperature;when the temperature is lower than400℃at 1.5 GPa,the degradation efficiency increases and then decreases.At room temperature,the photocatalytic performance of the samples increases with increasing pressure.This is because the factor pressure effectively improves the separation of photogenerated electrons,thereby enhancing the photocatalytic performance.When the samples were treated with both high pressure and high temperature,the pressure has a greater impact on the catalytic effect of the material.Thus the degration efficiency increased with pressure,but when the temperature is too high（>400℃）,which is near to the temperature of phase transition,resulting in structural changes and reduced degradation efficiency.（3）We designed different experiments to explore the mechanism.These experiments includ the active radical quenching experiment,electron paramagnetic resonance（EPR）and X-ray photoelectron spectroscopy（XPS）analysis.The results show that the photocatalytic performance of g-C₃N₄treated by high temperature and high pressure is closely correlated with the ratio of N_2C/N_3C.With the high pressure treatment,the N_2C/N_3Cratio increases significantly,which leads to the structure of g-C₃N₄changing from triazine ring to heptaazine ring,resulting to an improvement of photocatalytic performance.However,with the increase of temperature,the N_2C/N_3Cratio decreases,especially at 1.5 GPa and 400℃,g-C₃N₄changes from heptaazine ring to triazine ring structure.Thus the photocatalytic performance also decreases.Moreover,the free radical activity quenching experiment proved that the main active species in the degradation of Rh B by g-C₃N₄was superoxide anion radical（·O₂^-）.