Crystallization Regulation And Photocatalytic Hydrogen Production Performance For Graphitic Carbon Nitride

Converting solar energy into hydrogen energy through semiconductor photocatalysts is one of the important ways to solve the energy crisis.Therefore,how to effectively improve the photocatalytic conversion efficiency of solar energy has become an important issue in semiconductor photocatalytic technology.Graphitic carbon nitride（g-C₃N₄）is a new type of polymer semiconductor photocatalytic material,which has excellent characteristics such as stable physical and chemical properties,visible light absorption,simple preparation and wide and cheap raw material sources,causing great interest from the researchers.However,there are still some problems in conventional g-C₃N₄ prepared by direct calcination at high temperatures using nitrogen-containing precursors;On the one hand,traditional g-C₃N₄ usually presents a bulk structure with a small specific surface area,which leads to a shortage of active sites during the photocatalytic reaction;On the other hand,amorphous or semi-crystalline g-C₃N₄ is tend to be obtained due to the difficulty of intermediate product transfer and diffusion during conventional thermal polymerization,leading to incomplete polymerization and lattice defects.The incomplete polymerization and low degree of crystallization hinder the separation and transfer of photoexcited carriers,resulting in generally low photocatalytic activity,which problems greatly limit the application of g-C₃N₄.Therefore,improving the photocatalytic hydrogen production performance of g-C₃N₄ by using simple methods has become a key issue to be solved urgently.In this paper,aiming at the problem of low photocatalytic activity of g-C₃N₄,the crystal structure regulation and microscopic morphology design were carried out.Firstly,highly crystalline g-C₃N₄ was prepared by controlling the quality of dicyandiamide in a closed environment with spontaneous high pressure,and the mechanism of crystallization degree and hydrogen production rate changing with the quality of dicyandiamide was obtained;On this basis,the inverse opal porous g-C₃N₄ was prepared innovatively by co-deposition method,and g-C₃N₄ with highly crystalline inverse opal porous structure was prepared by increasing the mass fraction of cyanamide.It is further proved that the regular and ordered three-dimensional structure of inverse opal porous g-C₃N₄ and the improvement of crystallization degree can synergistically enhance the photocatalytic hydrogen production performance.The specific research contents are as follows:（1）Effects of thermal polymerization intermediate concentration on g-C₃N₄crystallization and photocatalytic hydrogen production performance.Highly crystalline g-C₃N₄ was prepared by thermally polymerizing different masses of dicyandiamide（1.6-2.2 g）under spontaneous ultra-high pressure（1.4-6.2 MPa）in a closed environment and adjusting the concentration of different gaseous intermediates.When the amount of dicyandiamide was 1.8 g,the interlayer stacking distance of g-C₃N₄ was reduced to 3.19?,correspondingly,its photocatalytic hydrogen production rate reached 9241.3μmol/（g·h）under simulated sunlight irradiation,which is about 2.5 times that of traditional g-C₃N₄.When the mass of dicyandiamide is kept constant（1.0 g）,the crystallization degree and hydrogen production performance of g-C₃N₄ are not significantly affected by increasing the initial pressure（0.8-1.2 MPa）.It is proved that the concentration of intermediate products rather than the initial gas pressure is the key factor determining the performance of g-C₃N₄ photocatalytic hydrogen production.This study clarifies the effect of the concentration of intermediate gaseous products on the hydrogen production performance during the thermal polymerization process,and provides a useful reference for improving the intrinsic photocatalytic hydrogen production performance of g-C₃N₄.（2）Preparation of inverse opal structure g-C₃N₄ material by co-deposition method and its hydrogen production performance.In this study,a simple silica microsphere/cyanamide solution co-deposition method was adopted by changing different centrifugation rates to control deposition effect of the microspheres in the dispersion through which a porous g-C₃N₄ material with an inverse opal structure was successfully synthesized.The test analysis results show that when the gravity deposition rate is 5500 r/min and the mass fraction of cyanamide solution is 50%,the inverse opal structure g-C₃N₄（CN5500）is prepared,and its specific surface area is 32 m²/g.Under the irradiation reaction of simulated sunlight,the hydrogen production rate of CN5500 is4137.2μmol/（g·h）,which is more than three times that of bulk g-C₃N₄（1209.3μmol/（g·h））,and shows good photocatalytic hydrogen production cycle stability.The inverse opal structure porous-g-C₃N₄ can not only provide more active sites for photocatalytic reactions,but also is conducive to the multiple reflection of incident light and the mass transfer of reaction substrates and products during photocatalytic hydrolysis benefiting from its penetrating macroporous structure,which will be helpful to the improvement of the photocatalytic hydrogen production rate.（3）Preparation of highly crystalline inverse opal structure g-C₃N₄ and its hydrogen production performance.In order to further improve the crystallization performance of inverse opal porous g-C₃N₄,this study successfully prepared highly crystalline inverse opal porous g-C₃N₄ by increasing the mass fraction of cyanamide solution in the colloid solution and maintaining the deposition effect of microspheres in the dispersion.The experimental results show that the photocatalytic hydrogen production rate increases firstly and then decreases with the increase of the mass fraction of cyanamide solution.When the mass fraction of cyanamide in the solution is 70%,the prepared highly crystalline inverse opal porous g-C₃N₄（CN5500-70%）has a hydrogen production rate of7217.0μmol/（g·h）,which is about 7 times that of bulk g-C₃N₄.This is because its interlayer charge transfer rate of the charges is enhanced by improving the crystallization degree of the porous inverse opal g-C₃N₄,and the photocatalytic hydrogen production rate is effectively improved due to the synergistic effect of the porous structure of inverse opal and high crystallinity at the same time.