The Separation Of Photogenerated Carriers And Photocatalytic Activity Influenced By Crystal Fundamental Factors

In the wake of 21st century, the arising of economy benefits human material life but causes serious damage to the ecological environment at the same time. As the coal, oil, natural gas and other fossil fuels are wearing away, human beings are facing intractable energy crisis. Simultaneity, the environmental pollution results from consumption of fossil energy seriously threaten our existential circumstance. Therefore, it is an urgent task to deal with the energy crisis and environmental pollution for sustainable development. Photocatalytic technology, which is a new approach for transfer the low density solar energy to chemical energy, gathers the chemical energy for splitting water, decomposing pollutants and reducing CO2 etc. Photocatalysis might become key technology to realize sustainable development due to the particular advantages in solving energy crisis and controlling environment pollution. However, there are two bottle-necks in development of photocatalytic technology. Take the traditional photocatalyst TiO2 for instance, the band gap of TiO2 is about 3.2 eV which utilization rate of solar energy is low and only respond to ultraviolet light. Besides, the photoinduced electrons and holes easily recombine in TiO2 semiconductor which decreases quantum efficiency. The two disadvantages limit the practical application of photocatalytic technology. For benefit mankind in the future, it is crucial and promising to exploit photocatalytic materials. Recently, considerable attention paid to enhance the separation efficiency of photogenerated carriers in photocatalytic process and then increase the photocatalytic activity. The researches about promoting the separation efficiency always focus on doping, loading and adjusting band structure in wide bandgap semiconductors (TiO2, ZnO, ZnS etc).Nevertheless, there are many influence factors in separation efficiency of photoinduced carriers. Different from other’s research, we make an intensive of fundamental constituent of the crystals, including crystal structure units, band structure of crystals, crystal defects and anisotropy of crystal and so on, and find several approaches to realize and enhance the separation efficiency of photogenerated carriers by adjusting the fundamental constituent of the crystals. Then the photocatalytic activity and absorption range are promoted. In this thesis, we first study different crystal structure units in some Cu compounds, and find the electrons transition process between two different crystal structure units; second, we make a deep study on band structure of crystal semiconductors, and form valance state heterojunction by choosing suitable semiconductors. The formed structure can efficiently restrain the recombination of photoinduced carriers; Crystal defect could form defect levels in semiconductor which are able to capture the photogenerated electrons or holes. We achieve broader light absorption material and restrain the recombination of electrons and holes by controlling the defect concentration in the semiconductor; at last, we control the growth speed of crystal by kinetic method and obtain crystals with different facets exposed. The migration rate of electrons and holes in different facets varies considerably, which greatly enhances the separation efficiency between electrons and holes.In chapter one, we briefly introduced the background, principal mechanism, main applications and latest progress of semiconductor photocatalysts. Next, we showed the present methods to solve the recombination of photogenerated electrons and holes in photocatalytic process. We recommend the adjustment of fundamental constituent in the crystals, including crystal structure units, band structure of crystals, crystal defects and anisotropy of crystal, to restrain the recombination of photogenerated electrons and holes. Finally, we summarized the significance, research ideas and the outline of this thesis.In chapter two, we studied the influence of the separation of photogenerated electrons and holes over different crystal structure units in several crystals.(1) Cu2(OH)PO4 microcrystals were obtained via simple hydrothermal method. There are two crystal structure units, CuO4(OH)2 octahedra and CuO4(OH) trigonal bipyramids, in Cu2(OH)PO4. The d-d transition of Cu ions leads to strong absorption in near-infrared region. Subsequently, we found the Cu2(OH)PO4 showed a certain photocatalytic activity under near-infrared light irradiation, which due to the electron transfer between different crystal structure units in Cu2(OH)PO4. Transition metals compounds could absorb near-infrared light when they form abound of different coordination polyhedron. We could expand the responded region to near-infrared by choose crystals with suitable structure units.(2) As potential NIR-photocatalysts, we examined Cu3(OH)4SO4, Cu4(OH)6SO4 and Cu2(OH)3Cl, all of which possess two different CuOm and Cu’On polyhedra linked with Cu-O-Cu’ bridges, as does Cu2(OH)PO4. Our work shows that Cu3(OH)4SO4 and Cu4(OH)6SO4 are both NIR-photocatalysts, as is Cu2(OH)PO4, but Cu2(OH)3Cl is not. Therefore, the presence of two different CuOm and Cu’On polyhedra linked with Cu-O-Cu’ bridges is not a sufficient condition for a NIR-photocatalyst. Cu3(OH)4SO4, According to the theory and experiments study, Cu4(OH)6SO4 and Cu2(OH)PO4 have acceptor groups (SO4 or PO4) connected to their CuOm and Cu’On polyhedra, but Cu2(OH)3Cl does not. Thus, the presence of acceptor groups linked to the metal oxygen polyhedra, which can act as a "sink" for photogenerated electrons, appears to be a structural feature needed for NIR-photocatalysts. It would be of interest to search for NIR-photocatalysts in transition-metal oxides possessing two different CuOm and Cu’On polyhedra linked with Cu-O-Cu’ bridges and linked to other acceptor groups (e.g., AsO4, MnO4, etc.).In chapter three, we studied different semiconductors which contain changeable valence elements (Cu, Mn), and then form valence states heteroj unction photocatalysts.(1) Cu is abundant reserves, and possesses many different valences. We chose CuO and CuSCN semiconductors to form heteroj unction due to the matching band structure based on the band theory of semiconductors. The different Cu valences in formed CuO/CuSCN valence state heterojunction not only transfer energy between them, but restrain the recombination of photoinduced electrons and holes. During to photocatalytic experiments, CuO/CuSCN exhibited visible light enhanced and ultraviolet light restrained activity. We proved the abnormal effect in terms of theory and experiments.(2) Mn is also abundant and has many different valences around the world. By the means of semiconductors band theory, we chose two different valences Mn compounds, Mn3O4 and MnCO3, which have matching band structures. The photocatalytic experiments over Mn3O4/MnCO3 valence state heterojunction exhibited obvious photo and thermal synergistic effect in degradation of methylene blue and formaldehyde. We deeply studied the mechanism of photo and thermal synergistic effect:the matching band structures of Mn3O4 and MnCO3 is favorable to separate the photogenerated electrons and holes. The lattice oxygen in Mn3O4 captured the photoinduced holes, and then oxidized methylene blue and formaldehyde in the solution. The electric conductivity of lattice oxygen could be enhanced as the temperature rising, which increase the separate efficiency of photogenerated electrons and holes and cause more holes transfer to the surface and participate in catalytic process.In chapter four, we studied the migration of photogenerated electrons and holes influenced by crystal defect. We obtained different S vacancies concentration of ZnS microsphere in hydrothermal process with the addition of NaBH4. The amount of NaBH4 could control the concentration of S vacancies in ZnS sample, and then influenced the visible light absorption. The photocatalytic water splitting experiments displayed that the activities could be enhanced as the amount of S vacancies increased. But the enhancement has a limit. We analyzed the mechanism of electrons transition in terms of theory. The S vacancy could form defect levels near ZnS CB, which could decrease the band gap of ZnS and capture photoinduced electrons. The higher concentration of S vacancies in ZnS microsphere will lead to the recombine center for photogenerated electrons and holes.In chapter five, we controlled the morphologies of crystals to exposed facets via kinetic method, and studied the migration of electrons and holes between different facets which is affected by anisotropy of crystal. We used different complexing agents to control the release speed of Ag+, and adjusted the surface energy of Ag2O via adsorb other anions. Finally, we obtained several Ag2O crystals with different facets exposed. The photocatalytic activities of different Ag2O crystals had great difference. We explained the mechanism from three aspects:first, the surface energy of{100} facet in Ag2O crystals are highest among the three facets which benefits for pollutants adsorption; the weighted average of the effective mass of holes and electrons has larger difference value along the [100] direction, the separated electrons and holes are difficult to recombine along this direction; the suitable redox potentials of the (100) surface. Loading noble metal is also an effective method for enhancing the photocatalytic activity. We utilized the surface plasma effect of Ag nanoparticles to obtaine Ag@Ag3VO4 via simple chemical reduction method. The plasma effect of Ag nanoparticles not only enhanced the light harvest, but also increased the separation efficiency of photogenerated electrons and holes.In chapter six, we summarized our work, list the innovative points, and discussed the problems of this thesis. Finally, we proposed a plan about the future work.In summary, we found the adjustment of fundamental constituent in the crystals, including crystal structure units, band structure of crystals, crystal defects and crystal morphologies, were very effective to restrain the recombination of photogenerated carriers and enhance the photocatalytic activity. The researches have certain significance to photocatalytic field.