Synthesis Of G-C₃N₄ Materials And Their Photocatalytic Performance

As the issues of energy and environment become increasingly severe,people begin to pay close attention to the renewable energy sources,for example,solar energy,which is clean and abundant.As a significant technique for solar energy utilization,semiconductor photocatalysis has shown unique advantages.Graphitic carbon nitride（g-C₃N₄）,a metal-free 2D conjugated polymer,has attracted increasing attention due to its low cost,physicochemical stability and visible light-response.However,problems such as small specific surface area,poor conductivity and serious recombination of photogenerated electron and hole have greatly restricted its practical applications.Hence,it’s still a great challenge to prepare g-C₃N₄ with unique structures and high quantum efficiency.Herein,various g-C₃N₄ materials were synthesized using different precursors,furthermore,unique nanostructures and heterostructures of g-C₃N₄-based photocatalysts were created,which show excellent photocatalytic performance.The main results of this thesis are summarized as following:1.Synthesis of various bulk g-C₃N₄ materials through thermal polycondensation using six different precursors.Based on the investigation of morphology,structure,optical property,transfer and separation ability of carrier charges and photocatalytic performance under visible light,we reveals that the g-C₃N₄ materials derived from sulfur-containing precursors possess superior light-absorption property and the ability to suppress the recombination of photon-excited carrier charges,while the urea-derived g-C₃N₄ materials compose of porous nanosheets and exhibit the best photocatalytic degradation performance for methylene blue（MB）under visible light irradiation,with the reaction constant 1.446 h-1 and the rate of MB removal 99.4% within 3h.2.The creation of g-C₃N₄ nanorings（g-CNNRs）.g-CNNRs were unprecedentedly and successfully synthesized through chemical vapor deposition using Si O₂ nanospheres as templates based on the theory of “confinement space”.The g-CNNRs possess uniform-size,monodispersed ring-like structure,large specific surface area,improved visible light（500-800 nm）absorption property,as well as enhanced transfer and separation ability of carrier charges due to their unique nanoring structures.The experimental parameters including the deposition temperature,reaction time,flow rate of carrier gas,as well as the size of SiO₂ nanosphere templates were systematically investigated and optimized.Based on these experiments,we can realize the controllable synthesis of the g-CNNRs.The photocatalytic performance of the as-prepared g-CNNRs for degradation of MB is superior to that of bulk g-C₃N₄ counterpart.g-CNNRs-2 shows a high value reaction constant of 0.27 h-1 in the photocatalytic degradation of MB,which is 5.4 times that of bulk g-C₃N₄ counterpart.3.A facile chemical vapor deposition approach is developed to deposite g-C₃N₄ layers on commercial P25 nanoparticles,realizing a successful combination of the two materials.The prepared g-C₃N₄/TiO₂ nanocomposites exhibit enhanced visible-light absorption characteristics compared with bulk g-C₃N₄ and pristine P25 in the range of 450-600 nm.Moreover,the construction of the heterostructure resulted in the promoted transfer and separation ability of photoexcited electron-hole pairs.The time and temperature of deposition and the flow rate of carrier gas are investigated to optimize the synthetic conditions.MB is used as the model pollutant for photocatalytic degradation reaction to evaluate the photocatalytic performance of the synthesized nanocomposites under visible light irradiation.At 550 oC,with the deposition time of 2 h and the flow rate of carrier gas 200 ml/min,the obtained nanocomposite shows the optimized kinetic rate（k=0.217 h-1）,which is 13.6 times that of pristine P25 and 5.3 times that of the bulk g-C₃N₄ counterparts,respectively.