First-principles Study Of G-C₃N₄ Photocatalytic Materials

First-principles density functional theory is a quantum mechanics method based on the complexity of multi-electron wave function, which focus on studying electronic structure and is widely used in physical and chemical sciences. First-principles theory has a very sound foundation in physics, and there are a large amount of soft-wares combined with density functional calculations modular interface, which enhanced the importance of the density functional theory. Besides, with the development of large-scale cluster computing applications and accelerated processing speed, improved accuracy, modified theoretical model, first-principle calculations becomes much reliable compared with the experimental value. Moreover, theoretical calculation method can analysis microscopic object, which can help understanding the reactions mechanism and changes of the materials.g-C3N4 is a kind of inorganic non-metallic semiconductor material, which is consisted of two lighter elements carbon and nitrogen. Recently, it has been widely used in water pollution treatment, clean energy production and storage, fuel cells and sensors. The unique geometric and electronic structure determine g-C3N4 the specific performances, such as optical, electrical, thermodynamic and mechanical properties. The researches of g-C3N4 mainly focus on these properties, but subject to the high standard of experimental methods and technical means, it’s hard to obtain the needed data. Therefore, using first-principle density functional calculations to research the properties of g-C3N4 is especially important. In this paper, g-C3N4 was chosen as the research object, including bulk g-C3N4 and g-C3N4 sheet. We studied the structures and properties of g-C3N4 from the following three aspects:1. The nonmetallic elements B, P and S were doped in g-C3N4, and the change laws of the electronic and optical properties of the doped systems were studied. We found that the band gap decreased and band structure become more compact after doping, a new band was produced between the conduction band and the valence band, which is conducive to the electronic transition. The dielectric function curve, the optical absorption coefficient spectra, the electron energy loss spectroscopy and the optical conductivity spectra of pure g-C3N4 and doped structures were also studied. The results showed that the light response range of P and S doped systems was still mainly in the ultraviolet region, which implied the doped scheme has little effect on improving the light absorption properties of g-C3N4. However, the light absorption curve of the B-doped system extended to the infrared region, which enhanced the light absorption capacity of g-C3N4 greatly.2. The geometric, electronic, optical, elastic and thermodynamic properties of g-C3N4 under high pressure were investigated. The results show the lattice constants of g-C3N4 gradually decreased with the pressure increasing. Besides, there is no chemical bond, only a weak interaction of van der Waals forces between the layers of g-C3N4. Therefore, the interlayer distance decreased particularly with the pressure increasing. It was found that the change of band gap follows a linear equation y=-0.0241x+2.7098, wherein x represents the external pressure, and y stands for the energy band gap. The absorption coefficients and responding absorption edge are enlarged as the applied pressure increased. The absorption edge of g-C3N4 will extend to 680 nm under 40 GPa. The elastic properties of g-C3N4 under external pressures were examined, the results showed that g-C3N4 became more brittle and turned from brittle material to ductile material absolutely at 35 GPa. In addition, the variation of thermodynamic properties (including vibrational internal energy, entropy and heat capacity) at different pressure were discussed.3. The geometric, electronic and optical properties, thermodynamic stability, and work function of Li-doped g-C3N4 monolayer were investigated by constructing g-C3N4 monolayer sheet models. The results were compared with pristine g-C3N4 monolayer, it showed that the Li atoms were preferentially substituted the open-hollow sites of g-C3N4 and the doped structures distorted seriously. The Li-doped structure enable light absorption range of g-C3N4 monolayer sheet shift to infrared region, and the greatest redshift showed as the Li doping concentration reached 7.14 at.%.