First-principles Studies Of Carbon- Nitrogen Based Two-dimensional Metal-free Materials

In recent years, two-dimensional (2D) materials have attracted physicist, chemist and materials scientists for their novel properties in the field of electronics and optoelectronics.2D metal-free catalysts have the potential advantages of nontoxicity, easy processing and environmentally friendly properties, to modify 2D metal-free materials is an important method for obtaining metal-free materials with special functions. For experimentalists, it is a trial job and consuming much time. With the rapid development of computer science and technology, the cost of the computation reduce sharply, the first-principles calculations based on density functional theory (DFT) have made breakthrough, which reduce the period of materials design and become an important means in materials science. In this article, we mainly use first-principles calculation method to study photocatalytic and electrocatalytic properties of the 2D metal-free materials.This academic dissertation contains four chapters. In the first part, theoretical framework of the first-principles calculations is briefly discussed. First, we introduce Born-Oppenheimer and Hartree-Fock approximations. Then we start from Thomas-Fermi model, Hohenberg-Kohn theorem and Kohn-Sham equation and go back the development and improvement of the density functional theory. Next, we present common used exchange-correlation functional and effective core potential methods. Last, we introduces the tools used in the calculation such as VASP and Materials Studio. In the second part, we begin to review the electronic structure and the application of graphene, transition metal dichalcogenides, h-BN, phosphorene and so on. Then we look back recent progress of environmentally friendly 2D carbon-nitrogen materials, mainly concentrate on its application in photocatalytic water splitting. Last, we present the application of 2D carbon nitride materials in electrocatalysis and fuel cells.In the third part, based on first-principles calculations, we present a way to improve the photocatalytic efficiency of g-C3N4 by forming a g-C3N4/C2N nanocomposite. Such a nanocomposite combines the photocatalytic activity of g-C3N4, and the wide and strong visible light absorption of C2N. Moreover, the g-C3N4/C2N interface possesses a type-II band alignment with relatively big chemical potential differences, which can promote the separation of photogenerated electron-hole pairs. This study provides new possibilities for the development of highly efficient metal-free photocatalysts.In the fourth part, we begin to introduce 3d transition-metal-doped carbon nitride based catalyst, which exhibits excellent ORR catalytic activity in experiment. Next, we present our model, C2N monolayer embedded by Me (Fe, Co, or Ni), which we call Me@C2N. Based on first principles calculations, we study its thermodynamic process of oxygen reduction reaction. It reveals that when Me (Fe, Co, or Ni) embed in C2N, the adsorption energy of O2 has obviously improved, and every step of the ORR reaction is exothermic process. It shows the Me@C2N has big potential in ORR design and is beneficial to develop new ORR catalytic materials.