A Study Of The Relationship Between Structure And Photocatalytic Property In A Series Of Bismuth-based Compounds

Semiconductor photocatalysis can be used in water splitting and organic pollutants degradation, producing renewable hydrogen energy and alleviating environmental pollution, which provides an effective way to solve the energy and environment issues. Semiconductor materials can absorb solar energy that is not less than the bandgap energy and be excited. Electrons transfer from the valence band to the conduction band, leaving holes behind in the valence band. Photogenerated electrons and holes can react with the electron accepters and donors, respectively, producing highly active free radicals to oxidize organic pollutants, split water and reduce carbon dioxide. Photocatalytic materials usually have the following features: (1) can absorb ultraviolet, visible or infrared light; (2) chemically stable; (3) highly active; (4) nontoxic. There are two factors that can affect the photocatalytic properties: light absorption and separation efficiency of carriers. Photocatalytic materials with wide spectrum absorption have a higher rate of solar energy utilization, generating more electron-hole pairs. Photocatalytic materials with high separation efficiency of carriers have higher quantum efficiency, making electrons and holes participate in the photocatalytic reaction effectively. Thus, it has important research significance to broaden the absorption spectra of traditional semiconductors, explore noble materials with narrow bandgap and find new ways to prevent electrons and holes recombination.The photocatalytic properties of semiconductor materials are closely related to their structures. The microstructures of materials, such as morphology, particle size and particle assembly, have an effect on the specific surface area and active sites, and further affect the photocatalytic properties. For polar materials, the polar structure unit arranges in the space periodically, leading a polarization field along a special direction. The polarization field can drive electrons and holes to transfer along the opposite direction, which is beneficial for the separation of electron-hole pairs and the improvement of photocatalytic properties. The nuclear charge number and the configuration of extranuclear electrons can affect the nucleic attraction for extranuclear electrons and the transfer of photogenerated electrons, causing the difference of photocatalytic properties of materials with similar structures. Two semiconductors with matched conduction band and valence band can form type Ⅱ heterostructures, which not only broaden the absorption spectra of wide bandgap semiconductors, but benefit the transfer of electrons and holes between the interfaces and promote charge separation.Bismuth-based compounds are a kind of noble photocatalytic materials. Bismuth element has d10 electronic configuration and the valence band of bismuth-based compounds is composed of Bi 6s and O 2p states, which narrows the bandgap and makes the photocatalytic materials absorb more sunlight. What’s more, bismuth-based layered materials are composed of (Bi2O2)2+ cationic layers and various kinds of subordinate ionic layers, which arrange alternatively and form three-dimensional cystalline structures. The layered structures are beneficial for the charge transfer between layers. Electrons and holes are more easily captured by water molecules, reducing the chance of recombination. Therefore, bismuth-based materials exbit good photocatalytic properties and become a hotspot in photocatalytic field. In this thesis, we start with the two factors that affect the photocatalytic properties and study the relationship of structures and photocatalytic properties of bismuth-based materials. The content of the thesis is divided into five chapters:In the first chapter, we introduced the research background and the basic theory of semiconductor photocatalysis. We introduced the basic concept of semiconductor photocatalysis and the physical and chemical process in the photocatalytic reactions. We introduced the effective ways to improve photocatalytic properties.We summarized the application of photocatalytic technology and the research progress, including organic pollutants degradation, water splitting and carbon dioxide reduction. We talked about the influence of spontaneous polarization on the photocatalytic properties and introduced the concept of polarity and dipole moment. We introduced the microstructures of cystals, layered materials, bismuth-based layered materials, polar structure unit and polar materials. We talked about the improvement of polarization to the separation of electrons and holes. At last, we introduced the significance and content of this thesis.In the second chapter, we synthesized and characterized hierarchical and self-assembly bismuth-based layered photocatalytic materials.Firstly, BiOBr hollow microspheres assembled by nanosheets were synthesized by a simple PVP-assisted biphase solvothermal method. The structure of the products is closely related to the ethylene glycol/ethanol volume ratio and the quantity of additive PVP.The growth process of the products was studied to investigate the formation mechanism of the hollow hierarchical microspheres via observing the morphologies of the products collected in different reaction time. The specific surface area and porosity of the BiOBr product were performed by investigating their nitrogen sorption property. The optical properties of the as-prepared samples were confirmed by the UV-Vis diffuse reflectance spectroscopy. The photocatalytic activity of the BiOBr products was evaluated by photodecomposition of 2,4-DCP under visible light irradiation at room temperature.Secondly, Bi2O2CO3 microspheres were synthesized by a simple hydrothermal method using bismuth citrate and sodium hydrogen carbonate. By observing the phases and morphologies of the products collected in different reaction time, we investigated the formation mechanism of the microspheres. In contrast, Bi2O2CO3 nanoplates were hydrothermally synthesized using bismuth nitrate and urea. The photocatalytic activity of the products was evaluated by photodecomposition of RhB under UV-vis light irradiation at room temperature.Thirdly, Bi2O3 octahedrons were synthesized by a mild wet chemical method using bismuth nitrate and ammonium metavanadate. What’s more, Bi2MoO6 microspheres assembled by decahedrons were synthesized through a facile ion exchange method between the BiOBr microspheres and sodium molybdate.In the third chapter, we synthesized and characterized BiOIO3 with internal polar field and other iodates.Firstly, BiOIO3 nanoplates were synthesized by using a simple hydrothermal method. The local dipole moments of the BiO6 pyramids are practically canceled out, but those of the IO3 pyramids are not, thereby leading to a pyroelectric polarization along the c-axis direction. The optical properties of the as-prepared samples were confirmed by the UV-Vis diffuse reflectance spectroscopy. The photocatalytic activity of the products was evaluated by photodecomposition of MO and isopropanol under UV-vis light irradiation at room temperature. To gain insight into a probable reason for this excellent photocatalytic property of BiOIO3, we examine its electronic band structure determined from density functional calculations. The transfer process of photogenerated electrons and holes and the influence of polar field on the charge separation were explored. New ideas that the polar field can promote the separation of electron-hole pairs were proposed. In contrast, the photocatalytic activity of BiFeO3, Bi2O2CO3, BiOX (X= Cl, Br, I), AgIO3 was also evaluated, further studying the role of the heterolayered structure and the internal polar field.Secondly, Ln(IO3)3 (Ln= Ce, Nd, Eu, Gd, Er, Yb) polycrystals were hydrothermally synthesized using lanthanide nitrate or lanthanide oxide and iodic acid as precursors. X-ray diffraction was used to characterize the crystal structures of the Ln(IO3)3 products. The lattice spacing of crystal facets and corresponding diffraction angle were calculated through the formula of lattice spacing and Bragg equation. The optical properties of the as-prepared samples were confirmed by the UV-Vis diffuse reflectance spectroscopy. The photocatalytic activity of the products was evaluated by photodecomposition of MO under ultraviolet light irradiation at room temperature. The differences of photocatalytic properties of Ln-based iodates were studied by analyzing the change of nuclear charge number and the configuration of extranuclear electrons of lanthanide elements.In the fourth chapter, we synthesized and characterized bismuth-based layered heterojunction photocatalytic materials.Firstly, Bi2O2CO3/Bi2S3 heteroj unctions were fabricated through a facile ion exchange method between the B12O2CO3 microspheres and thioacetamide. By controlling the reaction time, Bi2O2CO3/Bi2S3 composites with different Bi2S3 content and size were synthesized. The size of Bi2S3 was calculated on the basis of UV-Vis diffuse reflectance spectroscopy and the effective mass approximation model. The photocatalytic activity of the products was evaluated by photodecomposition of RhB under visible light irradiation at room temperature. The electrons and holes transfer process between the interfaces and free radicals were studied by calculating the band positions of Bi2O2CO3 and Bi2S3.Secondly, C3N4/BiOIO3 composites with heterostructures were fabricated by simply depositing BiOIO3 on the surface of C3N4 at hydrothermal conditions, using bismuth nitrate and potassium iodate as precursors. By controlling the addition of bismuth nitrate and potassium iodate, C3N4/BiOIO3 composites with different molar ratios were synthesized. The optical properties of the as-prepared samples were confirmed by the UV-Vis diffuse reflectance spectroscopy. The photocatalytic activity of the products was evaluated by photodecomposition of MO and 2,4-DCP under visible light irradiation at room temperature. The influence of the deposition of BiOIO3 on the photocatalytic properties and the electrons and holes transfer process between the interfaces were investigated. Because BiOIO3 exhibits good photocatalytic activity under ultraviolet light irradiation, the UV-vis light driven photocatalytic properties of C3N4/BiOIO3 composites were also studied. The electrons and holes transfer process between the interfaces was explored when both C3N4 and BiOIO3 were excited.In the fifth chapter, the summary and prospect were provided. We summarized the research work and the conclusions of this thesis. We discussed the innovation work and new ideas of this thesis. We also analyzed the problems existing in the research work and illuminated the next plan for future work.In summary, this thesis concentrated on broadening the absorption spectra and promoting charge separation of semiconductor materials. The relationship between the structures and photocatalytic properties of bismuth-based materials was investigated. Bismuth-based compounds were synthesized by hydrothermal, solvothermal and ion-exchange methods. The microstructures and chemical states were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy, and so on. The optical properties of the as-prepared samples were confirmed by the UV-Vis diffuse reflectance spectroscopy. The photocatalytic activity of the products was evaluated by photodecomposition of various organic pollutants. We explored the influence of the morphology, polar field and type Ⅱ heterstructure on the photocatalytic properties. We discussed the effective ways to improve the photocatalytic activity of bismuth-based materials. The research work of this thesis is beneficial for the understanding of the basic theory of semiconductor photocatalysis and the improvement of photocatalytic properties of semconductor materials, which has important theoretical significance and value to promote the development of photocatalytic technology and solve the energy and environment issues.