Theoretical Studies On Structure Design, Electronic Structure Tuning, And Photocatalytic Properties Of Graphene-Like Carbon Nitrides

The study of graphitic carbon nitrides materials can be traced back to the very early days in 1834. Graphitic carbon nitrides have honeycomb lattices. The structure of graphitic carbon nitrides are similar to that of grapheme, accompanied by many pores in different sizes. However, they have many holes. Recently, interests in polymeric graphitic carbon nitrides were aroused owing to its potential applications in energy conversion, environment protection, and spintronic devices.Graphitic carbon nitrides have abundant polymorphs with different structures. For example, the graphitic carbon nitrides with the chemical formula of C3N4, are the semiconductor, and can serve as photocatalyst that generate hydrogen and oxygen molecules from water using sunlight. While the polymorph with the formula of C4N3 is a spin polarized half-metal which has potential applications in spintronics devicesIt is noteworthy that, compared with the conventional photocatalysts and half-metal materials, graphitic carbon nitrides have the advantages of easy synthesis, low cost and environmental friendliness. Therefore, the study on the relationship between the structures and properties of graphitic carbon nitrides has become a hot topic, which will be helpful for revealing the origins of the excellent performance and improving the photocatalytic efficiency.In this dissertation, we performed first-principles within density functional theory (DFT) to study electronic structures, magnetic properties, mechanical properties and photocatalytic properties of several graphitic carbon nitrides. We focus on the modification of electronic structures under external stress, and improving the photocatalytic efficiency. The results provide theoretical bases for the experimental synthesis of novel graphitic carbon nitrides materials with unique properties.The thesis is organized as follows:Chapter I gives a brief introduction of research background and motivation. Chapter II introduces the theoretical fundamentals used in our research work. Chapters III to VI describe in detail and summarize the work finished during my Ph.D degree studies.The main content and results in this dissertation are listed as follows:1. A stable graphitic carbon nitrides structure (g-C9N7) was predicted theoretically, and its X-ray diffraction pattern was simulated. The stability of the structure was verified using three strategies:formation energy, molecular dynamics simulation and phonon spectrum. Electronic structure calculations show that the material has a spin-polarized ground state with metallic properties. The local magnetic moments tend to form ferromagnetic ordering. When the tensile strain exceeds 2.4%, the g-C9N7 changes from a metal to a half-metal, and 100% of the spin-polarized current can be achieved. The tunable half-metallic properties under tensile strain are quite promising for applications in the spintronics devices.2. Aiming at the application of graphitic carbon nitrides materials in the photocatalytic split of water, we theoretically designed a new type of graphitic carbon nitrides (g-C12N7H3). The X-ray diffraction spectrum was also presented to guide experimental identification. The calculated results show that g-C12N7H3 is a new kind of organic photocatalyst, which can be used to split water to produce H2 and O2. In order to enhance the photocatalytic efficiency, we provided four strategies, i.e., multilayer stacking, raising N atoms, forming g-C9N10/g-C12N7H3 heterojunction, forming graphene/g-C12N7H3 heterojunction. The above results provide theoretical bases for the design of highly efficient organic catalysts.3. We calculated the stability, electronic properties and edge effect of the Ni-embedded graphene nanoribbons (GNRs) with zigzag-shaped edges. The Ni atom alters the sublattice distribution of the spin densities essentially, resulting in symmetrical distribution around the Ni atom. The embedded Ni atom in the GNRs makes the complex possessing metallic properties. The metallic properties arise mainly from Ni 3d orbitals. When Ni atoms are near the edge, structural distortion takes place resulting in tilted-edge structures with low energies. This indicates that the Ni atom prefer to occupy divacancy sites near the edges. The above results provide an effective way to control the electronic structure of graphene nanoribbons.