Theoretical Study On Electronic Structures And Related Properties Of Novel Near-Infrared-Activated Photocatalyst:Cu₂（OH）PO₄

At present, environment pollutions and energy crisis have become two of the prominent issues, which seriously impede the development of society and survival of mankind. Therefore, scientists attempt to explore and develop renewable resources without pollutions. Among the solutions, photocatalytic technology has attracted more and more attention due to its potential advantages. For example, it can transform solar energy into chemical energy and degrade organic pollutants. Traditional semiconductor photocatalysts, i.e., TiO2 and SrTiO2, are the most impressive materials because of their low cost, nontoxicity, and long-term stability against chemical corrosion. These photocatalysts can not only split water into hydrogen and oxygen under light irradiation but also have strong oxidation ability to decompose the organic pollutants. However, researchers found that these materials have wide band gaps, resulting in poor utilization of sunlight. Generally, they can only absorb the ultraviolet light (UV-light accounts for 5% of the solar energy). Thus, one of the research highlights in this area is to extend the light absorption of these materials, and a lot of work has been done. On the other hand, exploring new photocatalysts with broaden light absorption is another way to solve the problem.Recently, a great progress has been made in exploring new photocatalysts, some photocatalytic materials with visible light absorption were found. At the same time, a near-infrared (NIR) light activated photocatalyst, copper hydroxyphosphate (Cu2(OH)PO4), was reported experimentally by our group. It strongly absorbs energy in the NIR region and is an effective photocatalyst for the oxidation of 2,4-dichlorophenol in aqueous solution under NIR light irradiation (λ>800 nm). To further improve its photocatalytic performance, we systemically explore its photocatalytic mechanism under NIR light theoretically.In this work, we investigate the related photocatalytic properties of Cu2(OH)PO4 based on the spin-polarized density functional theory calculations and bond-valence theory combining with Debye equation. According to the analysis of calculated results, we gain a better understanding of the material’s photocatalytic mechanism, summarize several valuable conclusions and present a new solution to improve and tune its photocatalytic performance. The results may be beneficial to design and prepare novel photocatalysts. This thesis is divided into four chapters:In chapter 1, a brief introduction into the photocatalytic mechanism of semiconductor photocatalyst is presented and we describe the research background and current situation of Cu2(OH)PO4 in brief.In chapter 2, we introduce the basic framework of density functional theory, the details and development of several exchange correlation functional, the basis set of plane waves and pseudo-potential used in this work, and the first-principles software packages.In chapter 3, we theoretically investigate the NIR light related photocatalytic properties of Cu2(OH)PO4. In particular, the band structure and band decomposed charge density of Cu2(OH)PO4 are calculated to explain the NIR light absorption and the separation and transfer of photogenerated carriers. The calculated effective masses of photogenerated electrons and holes from the top of valence band and bottom of gap band show that the separation and transfer of photogenerated carriers along [011] direction may be more effective than any other directions. A qualitative comparison of carrier transfer abilities along all the directions in the specific planes are given from the three-dimensional band structure. In addition, the calculated net dipole moment for the two basic units of Cu2(OH)PO4, octahedron and trigonal bipyramid, indicates that the macroscopic dipole moment for Cu2(OH)PO4 is zero. However, the distorted octahedron unit has a net dipole moment, which enable one to tune the macroscopic dipole moment by changing the local coordination environments and thus realize more efficient transfer of photogenerated electron.In chapter 4, the contents of the thesis are summarized and a preview for future works is drawn.