| The environmental issues, such as water and air pollution, have attracted much attention in the 21st century. The main reason for these problems could be attributed to the long-term using of fossil fuels, such as coal, oil, and natural gas. In addition, the fossil fuels as limited and non-renewable energies, will be exhausted in next century. Therefore, it is an urgent task to develop and utilize new energies for sustainable development. During the past decades, the huge financial and manpower resources have been invested by many countries to solve the energy crisis and environmental pollution. Photocatalytic technology, which can effectively utilize the solar energy for splitting water, reduction of carbon dioxide and degradation of organic pollutants, provides an important way to solve these issues. Generally, the photocatalytic process can be divided into three main steps:Firstly, the electrons in valence band of one semiconductor can be excited by photons with energy larger than its band gap, leading to the formation of photo-generated carriers; Secondly, the photo-generated carriers transfer from the bulk of semiconductor to its surfaces; Finally, the photo-generated carriers participate in the redox reaction. The tremendous progress has been made for photocatalytic technology during the past decades, however, the photocatalytic activity of current materials is still very low. There are two main reasons restricting the photocatalytic efficiency of materials. Due to the wide band gap, traditional photocatalytic materials such as TiO2 and ZnO, can only respond to the UV light, accounting for 4% of the total solar energy. But the visible and near infrared light (accounting for 53% and 43% of the total solar energy, respectively), cannot be absorbed and utilized effectively. Besides, the photo-generated carriers easily recombine in bulk or surfaces of semiconductors, resulting in a low quantum yield. Therefore, various approaches were employed to extend the light absorption and improve quantum yield of photocatalytic materials. Among these methods, the development of novel photocatalytic materials is an effective way to realize the application of photocatalytic technology. Therefore, a large number of photocatalytic materials have been synthesized and characterized for decades. Bi-based semiconductor materials with the advantages of abundance, stability and environmental friendliness, have high photocatalytic activity in the degradation of organic pollutions. For example, BiOCl has excellent photocatalytic performance in organic degradation, which is comparable to or even better than that of TiO2, but its wide band gap limits the conversion efficiency of solar energy. Bi2WO6 and Bi2MoO6 have visible light response due to the narrow band gap, and the layered structure of these materials is advantageous for the transfer of charge carriers. However, the photocatalytic efficiency of these materials is still below a commercially viable level. In addition, due to the narrow band gap and suitable band edge positions, Ta3Ns and Ag2ZnSnS4 have a promising application in photocatalytic water splitting. Ta3Ns can split water to produce oxygen, while Ag2ZnSnS4 possesses the photocatalytic performance of hydrogen evolution. However, the high recombination rate of photo-generated carriers limits their conversion efficiency of solar energy. Various methods such as doping, constructing heterostructure and the deposition of co-catalyst, were employed to further improve the photocatalytic activity of materials. The introduction of intrinsic defects or extrinsic doping is an effective method to realize the modification for semiconductor materials. The defects or impurity atoms in semiconductors introduce impurity states in band gap, which can reduce the energy for electronic transition and extend the light response in visible or near infrared range. For example, N doped TiO2 has the visible light photocatalytic performance, and the black TiO2 prepared by a hydrogenated method has the light response in the near infrared range. In addition, the conductivity of semiconductor materials can be enhanced by introducing intrinsic defects and extrinsic doping, leading to the improvement of photocatalytic performance. The impurity states induced by surface defects or impurity atoms can serve as trapping centers for photo-generated carriers, suppressing their recombination and enhancing the photocatalytic activity. Moreover, the surface defects and impurities of photocatalytic materials can serve as active sites for the photocatalytic reaction, facilitating the adsorption and dissociation of reactants, or reducing the reaction barrier. Thus, all these factors will improve the photocatalytic activity of materials.In this dissertation, we systematically studied the effects of intrinsic defects and extrinsic doping on the photocatalytic properties of Bi2WO6, BiOCl, Ta3Ns and Ag2ZnSnS4, such as the optical absorption and the transfer of photo-generated carriers. The possible underling mechanisms of some important experimental phenomena were explored. We predicted some new physical and chemical properties of photocatalytic materials after the introduction of intrinsic defects or impurity atoms, and provided some new ideas for the enhancement of photocatalytic activity. In the first chapter, we briefly present the background, the current research progress in this field and the main content of this dissertation. In the second chapter, we introduced the basic concepts and theoretical fundamentals of the first-principle calculations and the related software packages. In the third chapter, we studied the thermal stability and electronic structures of intrinsic defect contained Bi2WO6 and Bi2Mo06, and revealed the important roles of O vacancies on the near infrared photocatalytic properties. In the fourth chapter, we investigated the structure and electronic properties of ultrathin BiOCl (001) nanosheet, and explored the effects of surface defects on the upshift of band edge positions and transfer properties of photo-generated carriers. In the fifth chapter, we studied the defect formation energies, electronic structures and photocatalytic properties of intrinsic defect contained and alkali metal doped Ta3Ns. In the sixth chapter, we investigated the electronic structures and photocatalytic properties of Ag2ZnSe(Si-xSex)4, as well as the manipulation of the light absorption and the transfer properties of charge carriers by tuning the Se concentration. In the seventh chapter, we summarized the main conclusions and innovations of this dissertation, as well as some main problems and future research directions in this field were discussed. The main contents and conclusions of this dissertation are listed as follows.(1) The photocatalytic activity of materials can be enhanced by extending the range of light response. However, current researches mainly focus on the visible light driven photocatalytic performance, while the photocatalytic activity under near infrared light has not yet attracted much attention despite the fact that this range light constitutes about 43% of solar energy. The recently synthesized Cu2(OH)PO4 possesses the near infrared photocatalytic activity in organic degradation, which is the firstly report on near infrared photocatalyst except for the up-conversion materials. Soon after, the experiment investigation found that the intrinsic defect contained Bi2 WO6 has high near infrared photocatalytic performance. However, the origin of high photocatalytic activity is not clear and deserves to study further. The electronic structures and photocatalytic properties of various intrinsic defects contained Bi2MO6 (M=W, Mo) were investigated by the first principle calculations. The results show that the formation energies of O vacancy defects in Bi2WO6 and Bi2MoO6 are lower compared with others under the O poor/Bi rich condition. The electrons in impurity states induced by O vacancies can be excited to the conduction band after the absorption of near-infrared light. The delocalized feature of impurity states implies that photo-generated holes possess the high mobility. The photo-generated electrons and holes can be separated by different layers, which can reduce their recombination and improve the photocatalytic activity. In addition, due to high mobility of photo-generated holes, O vacancy contained Bi2WO6 may have higher near infrared photocatalytic activity than that of Bi2MoO6.(2) BiOCl has the high photocatalytic performance in the degradation of organic pollutants, but its wide band gap limits the utilization efficiency of solar energy. The two-dimensional nanosheet structures have obvious advantages with respect to their bulk structures due to the more active sites and longer carrier lifetime. Recently, the ultrathin BiOCl (001) nanosheet has been synthesized by experiments, which has higher photocatalytic activity compared with BiOCl nanoplate. However, it is urgent to investigate the microstructure of nanosheet and the effects of surface vacancies on its photocatalytic performance. The surface energies, electronic structures and photocatalytic properties of ultrathin BiOCl (001) nanosheet were investigated based on the density functional theory. The results show that the Cl atomic terminated BiOCl (001) surface possesses lower surface energy than others and is very likely to be introduced in the ultrathin nanosheet. In addition, the Bi vacancies on the surfaces of nanosheet can not only enhance the built-in electric field, promoting the separation of photo-generated carriers, but also enhance its polarity, leading to the significant band edge upshift with respect to BiOCl nanoplate. Our results provide a new insight into the high photocatalytic performance of BiOCl (001) nanosheet.(3) Ta3Ns is a semiconductor with narrow band gap, and the band edge positions straddle the water redox potentials. Due to the n-type conductivity, TasNs is a promising photocatalyst for oxygen evolution or a photoanode for a Z scheme device. However, its high carrier recombination reduces the quantum yield in photocatalytic process. The introduction of intrinsic defects or extrinsic doping can enhance the n-type conductivity of materials, leading to the band bending, which facilitates the transfer of charge carriers from the bulk of semiconductors to their surfaces. The formation energies and electronic structures of intrinsic defect contained and alkali metal doped Ta3N5 were studied based on the density functional theory in detail. The results indicate that the substitution of O for three-coordinated N in Ta3Ns has the lowest formation energy and a shallow donor feature, making a major contribution to its n-type conductivity. From the calculated optical transition levels, the four-coordinated N vacancy in Ta3N5 is responsible for the observed 720 nm sub-band gap optical absorption. In addition, the doping of alkali metals can enhance the n-type conductivity and light response range of Ta3Ns. Meanwhile, the Na doped Ta3Ns is expected to have the higher photocatalytic activity compared with others. These results provide some explanations for recent experimental observations and suggest an effective strategy to improve photocatalytic activity of Ta3N5.(4) Silver-based quaternary chalcogenides have potential applications in solar cell and photocatalytic hydrogen evolution. To increase the utilization of solar energy, the solid solution method allows the precise adjustment of the band gap and band edge positions of the semiconductor materials. However, few previous studies have focused on the effects of composition of solid solution on the separation and transfer of charge carriers. In addition, the superlattice structure can also tune the band gap of semiconductors, and the build-in electric field at the interfaces promotes the separation of charge carriers. We studied the electronic structures and photocatalytic properties of Ag2ZnSn(S1-xSex)4 by the combination of GGA+U and hybrid functional methods. The results show that the Ag2ZnSnS4 and Ag2ZnSnSe4 have small electron effective masses, which facilities the transfer of charge carriers. The effective masses of hole carriers along [100] and [010] directions are very sensitive to stress, which indicates the easy manipulation of the mobility. For Ag2ZnSn(S1-xSex)4 solid solution, the band gap, band edge positions, effective masses of charge carriers, static dielectric constant and exciton binding energies can be manipulated by tuning the Se concentration. In addition, our results suggested that the suitable multilayer structure can improve the photocatalytic activity of the materials by extending light absorption range and carrier mobility.In this dissertation, we have studied the effects of intrinsic defects and extrinsic doping on the photocatalytic properties of several photocatalytic materials, and unveiled the roles of these strategies in extending the light response range and promoting the transfer of charge carriers. These conclusions are of great significance for understanding the roles of defects and impurities in photocatalytic process, and provide an important theoretical foundation for the preparation of highly active photocatalytic materials by experiment. |
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