Incorporation Of Nitrogen Vacancy On Photophysical And Photocatalytic Hydrogen Generation Of G-C₃N₄

Photocatalytic decomposition of water for hydrogen production is an ideal way to convert solar energy into hydrogen gasification chemical energy.The key of research on photocatalytic hydrogen production is to develop photocatalytic materials with high activity,high stability and wide spectral response.Graphite carbon nitride?g-C₃N₄?has become a research hotspot in the field of photocatalysis in recent years due to its excellent performance among many reported photocatalytic hydrogen production materials.However,the photocatalytic hydrogen production efficiency of g-C₃N₄ is low due to its small specific surface area,poor conductivity and high photogenerated electron-hole pair composite.Among many measures to improve the photocatalytic hydrogen production performance of g-C₃N₄,post-heat treatment can adjust the band gap and broaden the light response range of g-C₃N₄ by the introduction of nitrogen vacancies,thus to significantly improve the hydrogen production performance.At present,there is no effective way to efficiently introduce nitrogen vacancies in g-C₃N₄ material.In addition,heat treatment destroy structural bonds of g-C₃N₄ while produces nitrogen vacancies,which reduces the crystallinity of g-C₃N₄ material and is not conducive to the improvement of hydrogen production performance.In order to solve the above problems,this paper will optimize the performance of g-C₃N₄ material from two aspects:The introduction of nitrogen defects efficiently in g-C₃N₄ material and the regulation of nitrogen vacancy concentration in NH₃ atmosphere and improve the crystallinity of g-C₃N₄.The main research contents are as follows:1.Efficient introduction of nitrogen vacancies into porous g-C₃N₄ to improve photocatalytic hydrogen production performance by rapid heat treatment.The small specific surface area of g-C₃N₄ synthesized by high temperature thermal polymerization of precursors such as melamine lead to a less exposure of N atoms in the air which is not conducive to the formation of nitrogen vacancies in the oxidation reaction.Porous g-C₃N₄ has large specific surface area and high surface atomic concentration,which is conducive to the formation of nitrogen vacancies during heating.According to the above ideas,we used nano-silica?12 nm?as template and dicyandiamide as precursor material to prepare porous g-C₃N₄ material with large specific surface area.And the photophysical and photocatalytic hydrogen production performance was studied then.XRD analysis showed that rapid heat treatment reduced the crystallinity of the samples.The results of XPS and EPR show that rapid heat treatment results in a large number of nitrogen vacancies in porous g-C₃N₄.Ultraviolet-visible spectra show that the introduction of nitrogen vacancies into porous g-C₃N₄ enhances the absorption of long wavelength spectra and narrows the band gap.PL spectra and electrochemical experiments show that the introduction of nitrogen vacancies enhances the charge transfer efficiency and reduces the recombination rate of photogenerated electron-hole pairs.It was found that the porous g-C₃N₄ after heat treatment at 650 ^oC for 10 min had the highest photocatalytic hydrogen production performance(30?mol·h^-1),which was four times as high as that without heat treatment,and three times higher than the traditional g-C₃N₄ after heat treatment under the same conditions,which indicates the introduction of high concentration nitrogen vacancies in porous g-C₃N₄ material by heat treatment is beneficial to the improvement of hydrogen production performance.2.Controlling nitrogen vacancy in g-C₃N₄ in ammonia atmosphere for enhanced photocatalytic hydrogen generation.The research on nitrogen defect of heat treated graphite oxide phase carbon nitride?g-C₃N₄?and its photocatalytic hydrogen production performance has attracted much attention.However,the existence of high and uncontrollable nitrogen vacancy concentration destroys the crystal structure of g-C₃N₄ thus to reduce the crystallinity of g-C₃N₄ and leads to a low photocatalytic hydrogen production efficiency.Based on the above problems,this paper use the decomposition of urea to product NH₃ atmosphere by heat to restrain the over-oxidation of N element.The g-C₃N₄ prepared by dicyandiamide as precursor was mixed with urea of different content for rapid heat treatment,and its phase composition,micro-morphology and optical absorption were measured by X-ray diffraction?XRD?and scanning electron microscopy?SEM?,etc.The results show that the addition of urea reduces the concentration of nitrogen vacancies and improves the crystallinity of g-C₃N₄ materials.Under optimum urea addition,the visible light photocatalytic hydrogen production rate of g-C₃N₄ is 6.5?mol·h^-1,which is three times as fast as that of the sample without urea addition.The results show that NH₃produced by in-situ decomposition of urea inhibits the oxidation degree of nitrogen atom during heat treatment,thus to improve the crystallinity of g-C₃N₄,and further improves the photocatalytic hydrogen production performance.3.Preparation of thin layer carbon nitride from thiocyanate and its heat treatment properties.Traditional g-C₃N₄ is usually prepared by condensation of nitrogen-rich precursors with ammonia as the leaving group,which limits the degree of polymerization,which lead to a small specific surface area.Therefore,a carbon nitride?CN?with large and thin layer was synthesized through thermal polycondensation of thiocyanate with thiohydrogen?SH?as the leaving group in this experiment.XRD and FT-IR spectra showed that the synthesized carbon nitride was g-C₃N₄ and the crystallinity of g-C₃N₄ was reduced by heat treatment,but the structure of the sample did not change during heat treatment.XPS results show that nitrogen vacancies have been successfully introduced into CN by the heat treatment process.UV-vis spectra show that the absorption of visible light of CN is much higher than that of g-C₃N₄.The introduction of nitrogen vacancies in heat treatment enhances the absorption of long wavelength spectra,and narrows the band gap.PL spectra and electrochemical experiments show that nitrogen vacancies reduce the recombination efficiency of photogenerated electron-hole pairs.The experimental results of photocatalytic hydrogen production show that the hydrogen production performance of CN synthesized with thiocyanate as precursor(6?mol·h^-1)is much higher than that of bulk g-C₃N₄(0.3?mol·h^-1),and the hydrogen production performance of the sample is doubled by heat treatment(13?mol·h^-1).Generally speaking,the use of thiocyanate can synthesize large area thin layer g-C₃N₄ with excellent performance in a short time,and nitrogen vacancies can be efficiently introduced through heat treatment to improve photocatalytic hydrogen production performance.