Research On Synthesis、Photocatalytic Properties And Their Mechanisms Of Borates

As human society steps into modern civilization, the development of science and technology always comes with huge resource consumption and serious environmental damage. Therefore, how to solve these two crises has become a hot spot. Photocatalysis is one kind of clear and environmental friendly technology to switch low-density solar energy into high-density chemical energy efficiently which can be used for pollutant degradation, water splitting and CO2 reduction. Since the semiconductor photocatalyst is the key to enhance photocatalytic efficiency, the research in this field has become more significant and valuable to alleviate energy crisis and environmental pollution. In the past decades, a mass of semiconductors that have photocatalytic activity have been reported gradually and under the massy theoretical and experimental foundation, scientists have discovered that the narrow range of light response and low quantum efficiency are two main limitations for photocatalytic technology industrialization. Hence, to solve the two problems above and make the photocatalytic technology into practice in the near future are the great part of the research in this field and the emphasis of this thesis.New photocatalysts development, semiconductor coupling, noble metal modification and element doping are the effective measures to expand light response range or enhance quantum efficiency. However, there come several limitations with the development of photocatalysis. Firstly, although current research on new type photocatalysts has made some great progress and breakthrough in local system, it is still a difficult work to form a field-wide system as well. Secondly, in terms of building interface electric field, the main methods are semiconductor coupling and noble metal modification, but the low intensity and interface defects of interface electric field limit its future development. Thus, small scope and driving force in this method give a limited contribution to photo-generated carriers separation. Finally, element doping for lattice distortion can lead to a huge mass of research to reveal the promotion or inhibition for photocatalysis due to the complicated factors including elements type, doping location, doping ratio and method. Therefore, this paper is focused on building a crystal-wide internal polarization electric field to promote the separation of photo-generated carriers and enhance photocatalytic activity further by improving quantum efficiency, which is owing to its big effect range, strong driving force and low interface defects. This thesis pays emphasis on the synthesis, modification and photocatalytic mechanism of two polarized photocatalysts, Bi2O2[BO2(OH)] and Bi2ZnOB2O6. Moreover, dipole moment of the two materials is given through band and electronic structure analysis and separation mechanism of photo-generated carriers under the influence of internal polar field is also illuminated specifically. The main research content is described in the following:In chapter 1, the fundamental principle, research status and application of semiconductor photocatalysis were introduced. Based on the limitations of photocatalysis, the advantages of polarized borate materials in improving quantum efficiency were discussed in detail, and then the significance of this research and main contents of the thesis were put forward.In chapter 2, the synthesis, growing mechanism and carriers separation mechanism of Bi2O2[BO2(OH)] nanosheets were investigated. Bi2O2[BO2(OH)] was obtained by a hydrothermal method, and nanosheet structure was acquired by adjusting the synthesis conditions. Moreover, the growing mechanism of Bi2O2[BO2(OH)] nanosheets was also confirmed through researching the intermediate products. Then, the electronic structure analysis of Bi2O2[BO2(OH)] indicated its band position and dipole moment, which resulted in a high separation ratio of photo-generated carriers by internal polar field. At last, its high photocatalytic degradation activity confirmed that the internal polarized electric field was indeed an efficient way to improve the quantum efficiency.In chapter 3, Bi2O2[BO2(OH)] nanosheets were modified through different ways to expand its scope of light absorption. On one hand, Bi2S3/Bi2O2[BO2(OH)] compounds were gained by means of ion exchange method. The particle size of Bi2S3 was characterized by XRD and SEM, and the band relationship between Bi2O2[BO2(OH)] and Bi2S3 particles which had a quantum size effect was investigated in detail to prove that this compound system could extend light absorption. On the other hand, M@Bi2O2[BO2(OH)] (M=Ag、Au、Pt) was obtained when a spot of noble metal was introduced in hydrothermal process. The existence and properties of noble metal phases were characterized by XRD, SEM and UV-Vis diffuse reflection. And this method can also enhance the light absorption range effectively. Both samples by these two modification approaches above performed higher photocatalytic activities than the pure Bi2O2[BO2(OH)] nanosheets.In chapter 4, Bi2ZnOB2O6 particles were synthesized, the band structure and carriers separation mechanism of Bi2ZnOB2O6 were affirmed at the same time. Firstly, Bi2ZnOB2O6 particles were obtained through a solid-state sintering process. Then the samples properties were characterized by XRD and SEM. Secondly, the intrinsic photocatalytic degradation activities of Bi2ZnOB2O6 particles were calculated in terms of the samples’ specific surface areas and photocatalytic activities. Finally, structure analysis was performed to determine the band position and dipole moment of Bi2ZnOB2O6. And then, the internal polar field which could promote carriers separation was confirmed to be the primary factor to bring a high photocatalytic activity.In chapter 5, a summary and assessment to all the research in this thesis were presented and achieved conclusions and results were analyzed. Moreover, we discussed potential problems and put forward the promising prospect for the further study.