Graphitic Carbon Nitride-based Photocatalytic Materials Performance Optimization And First Principle Study

With the rapid development of modern society,industrialization has not only brought about more convenience for human life but also exacerbated the contradiction between nature and human being.The energy shortage and environmental pollution have become two huge global challenges.Photocatalytic technology,which could realize the conversion from solar to chemical energy,is currently expected as a new ideal solution for wastewater purification and energy production.As a research hotspot,this technology has been widely applied in the fields of H2 evolution,organic pollutants degradation,sterilization and disinfection.During the photocatalytic process,it is important to choose a non-toxic and environmentally friendly photocatalyst.As an organic polymer semiconductor,graphitic carbon nitride（g-C₃N₄）has attracted great attentions for the unique two-dimensional layered structure,high stability and the response to visible light.However,pure g-C₃N₄ exhibits poor photocatalytic performance due to the fast recombination of photo-induced carriers and insufficient sunlight absorption,which limits its practical applications.In this paper,we have made a considerable amount of effort to improve the photocatalytic activity in terms of the optimization on light absorption range,photo-generated charge-carriers separation and mobility of g-C₃N₄.Among these strategies,element doping and heterojunction construction are considered as promising ways to design high-performance photocatalysts using for organic pollutants degradation and H2 production.Moreover,the photocatalytic reaction process mainly happens on the surface of the semiconductor or the interface of the heterojunction,in which the charge distribution and charge transfer direction determine the photocatalytic efficiency and performance.These are microscopic scale information involved with electron migrations,which are difficult to be observed in experiments.Therefore,we have utilized first principle calculations based on density functional theory to offer important information from the levels of atoms,such as crystal structure and electronic structure of the photocatalyst,generation,migration,and energy levels of the photo-generated charge carriers.It is beneficial to comprehensively understand the photocatalytic mechanism by combining the experimental with theoretical investigations.The thesis includes following four parts:（1）The metal-free photocatalyst S-C₃N₄ was prepared by using a one-step thermal treatment method with thiourea source.In comparison with g-C₃N₄,S-C₃N₄ presents the improved photocatalytic efficiency for Rh B and Cr⁶⁺ photo-degradation due to stronger absorption and charge carrier separation ability.Furthermore,the geometric structure,band structure,work function,and the band-edge potential of S-C₃N₄ were also investigated through the first principle calculations.Our work provides a theoretical reference for nonmetal doping based on g-C₃N₄.（2）From the perspective of band structure engineering,Ti doped g-C₃N₄ has been successfully synthesized by means of heating the mixture of tetrabutyl titanate and melamine.The photocatalytic ability of Ti/C₃N₄ is greatly enhanced in comparison with that of g-C₃N₄.The first principle calculations suggest that Ti atoms enter into g-C₃N₄ units and link two adjacent layers which improve the carrier separation capability of Ti/C₃N₄.Then we proposed the photocatalytic mechanism according to the calculated band-edge potential and the trapping experiments of active species in the photocatalytic proces.The results may provide a new avenue for designing other photocatalysts through doping methods.（3）The fundamental physical properties of monolayer black phosphorus（BP）have been theoretically investigated,which exhibit high carrier mobility and visible light absorption capacity.Density functional theory calculations were also carried out to characterize the electronic and photocatalyic properties in C₃N₄/BP heterojunction photocatalyst.The results indicate that the C₃N₄/BP heterojunction is a typeⅠheterojunction,in which the interficial built-in electric field and band bending promote the space separation of photo-generated carriers.In addition,the valence band potential of monolayer black phosphorus becomes more negative in the process of forming heterojunction,which leads to a stronger reduction capability.The novel 2D/2D heterojunction of metal-free C₃N₄/BP exhibits significant potential application in visible-light photocatalytic H2 evolution activity.（4）The binary layered structure C₃N₄/BiOCl heterojunction photocatalysts have been successfully synthesized via two-step calcination-hydrothermal method.Compared with pure g-C₃N₄ or BiOCl,the C₃N₄/BIOCl photocatalysts present much superior Rh B degradation and H₂ evolution reaction ability due to strong interfacial interaction,excellent carriers separation,and well-matched band structure.The origin of photocatalytic efficiency improvement is identified by performing a combination of theoretical and experimental methods.In particular,focusing on the electrons transfer mechanism of g-C₃N₄ and BiOCl before and after contacting,we have confirmed a quantitative band offsets as well as bandedge potentials,which are decisive conditions during the photocatalytic reactions.The calculated band offsets agree well with experimental results.Our work not only offers a theoretical support for fundamental photocatalytic reaction,but also provides a promising way to design heterojunction photocatalyst with proper band-edge potential and strong redox ability.