Preparation Of Doped G-C₃N₄ And Its Application In Photocatalysis

In recent years,the worldwide environmental pollution and energy shortage have gradually increased.The rapid economic development has led to the increase of chemical product production.The polluted gas produced in the production of chemical products makes the problem of air quality more serious.Serious environmental pollution not only has a great impact on people’s health,but also does not accord with the concept of sustainable development on resource utilization.People have made a lot of efforts in the removal,reduce or transformation of pollutants and constantly made changes.Many attempts have been made in purification methods.According to the action mechanism of pollutants,they can be divided into adsorption pollutants,absorption pollutants and catalytic conversion pollutants.Because the first two methods will cause problems such as adsorbent consumption and secondary release of pollutants,the method of catalytic conversion of pollutants has naturally become a research hotspot.Among them,photocatalytic method is to degrade or convert pollutants into available energy sources by using renewable solar energy.Carbon nitride（g-C₃N₄）is a kind of polymer semiconductor.It has suitable conduction band,valence band position and absorption of visible light.The synthetic process and the synthesized samples are non-toxic.The preparation method is simple.It is an excellent photocatalytic semiconductor material.Because of its high chemical stability,it is very stable in alcohol solution and acid-base solution,which provides good conditions for its practical application.However,its photocatalytic performance is still limited by the rapid electron hole recombination and small specific surface area,which means that the catalytic performance can still be improved.In order to further improve the photocatalytic performance of the material,many methods,such as doping,recombination,manufacturing defects,providing vacancies and changing morphology,are proposed to improve the photocatalytic activity of semiconductors.Aiming at the problems of small specific surface area and less visible light absorption of g-C₃N₄,a modification method of doping g-C₃N₄is proposed in this paper.The specific work is as follows:（1）Preparation,characterization and photocatalytic properties of Sr/Cl doped g-C₃N₄nano materials.Melamine and strontium chloride hexahydrate are mixed in deionized water in a certain mass ratio,stirred until evenly distributed,calcined in air for several hours,and the calcined material is cleaned and dried to obtain Sr/Cl doped g-C₃N₄nano material.Then the samples were analyzed and characterized.XRD,XPS,SEM,nitrogen adsorption desorption and UV-Vis absorption were measured.The phase,surface chemical state,morphology and structure,specific surface area,pore size distribution,UV-Vis absorption capacity and band gap of the samples were analyzed and characterized.It can be seen from the XRD test that there is only the phase structure of g-C₃N₄and no impurity diffraction peak,but the position of the diffraction peak shifts to a large angle,which may be due to the influence of doping on the lattice growth and the reduction of the layer spacing of g-C₃N₄.SEM and mapping results show that C,N,Sr and Cl are evenly distributed in the sample,and the traditional layered stacking structure of g-C₃N₄can be seen.The results of specific surface area and pore size analysis show that the largest specific surface area of the doped sample can be increased to 20.30 m~2g^-1,and it has mesoporous structure.The increase of specific surface area can provide more active sites for catalytic reaction.The PL spectrum test results can see the separation efficiency of electron hole pairs.The test results show that the separation efficiency of electron hole pairs of doped samples is greatly improved compared with that of pure phase g-C₃N₄.The UV-Vis absorption spectrum test is used to evaluate the absorption of UV-Vis light.The visible light absorption range of doped samples is increased and the band gap width is reduced.In addition,the photoelectrochemical properties of the samples were tested.For example,the photocurrent of the doped samples was 3.53 times that of pure g-C₃N₄.The possible mechanism after doping is proposed.Sr ion and Cl ion are doped between g-C₃N₄layers,Sr plays the role of electron transfer,Cl ion plays the role of charge transfer,and the different roles of Sr and Cl promote the separation and transfer of photogenerated electron hole pairs.The results of photocatalytic degradation and photocatalytic nitrogen fixation showed that when the doping amount of Sr Cl₂was 0.1 g,the amount of acetone produced by photocatalytic degradation of isopropanol was 3276.28 ppm,which was 1.4 times that of pure g-C₃N₄.The results of photocatalytic nitrogen fixation showed that the yield of ammonia was 241μmol/g_mat,which is twice that of g-C₃N₄.（2）Preparation,characterization and photocatalytic properties of K/Cl doped g-C₃N₄nano materials.K/Cl doped g-C₃N₄catalyst was prepared by calcining KCl as dopant and melamine as precursor in air atmosphere for several hours.The phase,specific surface area and catalytic activity of the samples were tested and analyzed.The results of specific surface area and pore size distribution show that the specific surface area of doped samples increases accordingly.Compared with pure phase g-C₃N₄,the specific surface area of doped samples can be increased to 18.35 m~2g^-1.The results of UV-Vis absorption analysis show that doping affects the visible light absorption capacity of g-C₃N₄.The doped samples increase the visible light absorption capacity and reduce the band gap to 3.02 e V.The photocatalytic degradation test of the sample shows that its photocatalytic activity is 2.0 times higher than that of pure g-C₃N₄,and the photocatalytic nitrogen fixation performance of the doped sample is 2.86 times higher than that of pure g-C₃N₄.The possible mechanism of double doping is that K ions and Cl ions are doped between layers at the same time,K transmits electrons and Cl transmits holes,which increases the carrier separation ability.The increase of catalytic activity may be due to the increase of specific surface area caused by doping.After illumination,the electrons in the valence band transition to the conduction band,leaving holes in the valence band.The uncomplicated electron holes move to the material surface and undergo redox reaction with pollutants.It can be seen that photocatalysis is a reaction occurring on the sample surface,so the increase of specific surface area has impact on the catalytic activity.The stability of the doped sample CN-K/Cl-0.07 was tested.The same catalyst still showed stable catalytic activity after four cycles of Catalysis under light.