Photocatalytic Performance Of Graphitic Carbon Nitride Prepared By Ammonium Salt Assisted Method And Its Isotype Heterogeneous Junction Composites

As a new type of nonmetal semiconductor photocatalytic material,g-C₃N₄ has the advantages of appropriate forbidden band width and valence band position,excellent physical and chemical stability,abundant and cheap raw materials,simple and easy operation of the preparation method,etc.Since Wang Xinchen et al reported in 2009 that graphitic carbon nitride can decompose water under the irradiation of visible light to generate hydrogen,numerous researchers have devoted themselves to the research on g-C₃N₄,which has since opened the prelude of the research on g-C₃N₄.After that,research results and reports on g-C₃N₄ emerge one after another.However,pure g-C₃N₄ not only has small specific surface area,weak visible light response and high recombination rate of photogenerated electron holes,which leads to its unsatisfactory photocatalytic performance.In order to improve its photocatalytic performance,researchers have mainly modified it from the following three aspects:micro-morphology adjustment,broadening the visible light response range and improving the separation efficiency of photo-generated carriers.In order to overcome the shortcomings of g-C₃N₄,the visible light catalytic performance of g-C₃N₄ was improved by adjusting its micro-morphology,increasing its specific surface area and inhibiting its photogenerated carrier recombination rate.Firstly,the yield of g-C₃N₄from urea was increased by adding ammonium sulfate,and g-C₃N₄ material with spherical structure was prepared.Melamine was used as precursor and ammonium sulfate was used as bubble template to prepare mesoporous g-C₃N₄ nanosheet material.Finally,urea and melamine were mixed and then the g-C₃N₄ isotype heterogeneous junction material was obtained in one step by thermal polymerization method,which greatly improved the separation efficiency of photogenerated electron-holes and successfully prepared g-C₃N₄catalyst with higher visible photocatalytic activity.At the same time,X-ray diffraction（XRD）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,nitrogen adsorption and desorption test（BET）,ultraviolet-visible diffuse reflectance absorption spectrum（UV-Vis）and photoluminescence spectroscopy（PL）were used to characterize and analyze the prepared materials.Their catalytic degradation performance to simulated pollutants under visible light irradiation was investigated,and the photocatalytic mechanism was discussed.（1）Ammonium sulfate and urea are mixed as precursors to prepare g-C₃N₄,which greatly improves the yield of g-C₃N₄.When the amount of ammonium sulfate added is 1.00 g,the yield of g-C₃N₄ is the largest,which is 3.6 times as high as that of calcined urea alone.When the amount of ammonium sulfate added is 3.00 g,the yield is 2.2 times of that of calcined urea alone,but the photocatalytic degradation activity of RhB is the same as that of g-C₃N₄ prepared by urea alone as precursor.The main reason for the increase in the yield of g-C₃N₄ is that when ammonium sulfate is added,the activation energy of urea to form melem is reduced,so that more melem is generated and further converted into g-C₃N₄.The addition of ammonium sulfate reduces the preparation cost of g-C₃N₄ by 45%,expands the visible light absorption range of g-C₃N₄,changes the degradation path of g-C₃N₄ to rhodamine B（RhB）,and provides possibility for its practical application.（2）Mesoporous graphitic carbon nitride（mpg-C₃N₄）nanosheets were prepared by one-step thermal polymerization using melamine as precursor and ammonium sulfate as bubble template.The mass ratio of melamine to ammonium sulfate has a great influence on the photocatalytic performance of graphitic carbon nitride nanosheets.When the mass ratio is1:1,the prepared 5-CN has the best catalytic degradation activity.Under visible light irradiation,the degradation rate of RhB is as high as 92.6%within 60min,and the visible photocatalytic degradation rate constant of RhB is 9.2 times that of bulk g-C₃N₄.The results of active species capture experiments confirm that·O₂^-and h⁺are the main active species in the process of catalytic degradation of RhB.In addition,mpg-C₃N₄ nanosheets have good recycling stability and still have high photocatalytic activity after five cycles of recycling.The visible light catalytic performance of mpg-C₃N₄ nanosheets is improved due to its high specific surface area,developed pore structure,high efficiency of photogenerated carriers separation and ultrathin layer nanosheet structure.After that,the visible light catalytic degradation activities were compared for several common simulated pollutants,including rhodamine B,methylene blue,methyl orange and phenol.the degradation efficiency of rhodamine B>methylene blue>methyl orange≈phenol was found,and a reasonable explanation was put forward.（3）Since graphitic carbon nitride prepared by different precursors has different molecular and energy band structures,g-C₃N₄ itself can also construct heterojunction to inhibit recombination of photogenerated electrons and holes.Using melamine and urea as precursors,g-C₃N₄ isotype heterojunction composites were prepared by a simple high temperature calcination method.When the mass ratio of urea to melamine is 7:3,the prepared7-CN has the best visible light photocatalytic activity.Under visible light irradiation,the degradation rate of RhB is as high as 97.62%after 40min.Its degradation paths to RhB are·O₂^-and h⁺,resulting in the decomposition of conjugated chromophore and a series of intermediate products generated in the degradation process.On the one hand,the improvement of photocatalytic performance of the composite material is due to the large number of pore structures in the composite material,which increase the active sites of photocatalytic reaction,and on the other hand,due to the formation of type III heterojunction between u-CN and m-CN,which effectively improves the separation efficiency of photogenerated electrons and holes in the composite material.The innovations of this paper are as follows:（1）For the first time,the yield of graphitic carbon nitride from urea is increased by adding ammonium sulfate,and the addition amount of ammonium sulfate is further adjusted to successfully prepare g-C₃N₄ with spherical shape;（2）For the first time,ammonium sulfate is used as a bubble template and melamine is used as a precursor to prepare g-C₃N₄ nanosheets by a one-step calcination method.According to the different degradation rates of rhodamine B,methylene blue,methyl orange and phenol,a reasonable explanation is proposed.（3）The g-C₃N₄ isotype heterojunction composite with high visible light catalytic activity was successfully prepared by high temperature calcination method using urea and melamine for the first time.