| In the 21st century, environmental pollution and energy shortage are two major challenges for human during sustainable development. As a kind of clean, cheap, renewable and inexhaustible energy, solar energy is considered to be the best available energy. How to make use of solar energy efficiently and quickly, and make solar energy transformation and storage come true are two main targets for people to try. Semiconductor photocatalytic technology can utilize solar energy to chemical conversion and storage, and use the degradation of organic pollutants and clean surface technology to solve the problem of environmental pollution. And photocatalytic decomposition of water to produce renewable energy hydrogen, is an effective way to solve the shortage of fossil energy. Therefore, photocatalytic technology will be one of the means to solve the energy and environment.Semiconductor materials can absorb solar energy that equal to or greater than the band gap (E= 1240 /λ,), thus electron in valence band is emitted, and transfer to the conduction band, at the same time, a hole is produced in the valence band. Photo-induced electron and hole respectively react with eletron and hole scavenger, and produce highly reactive radical, to achieve the aim of oxidation of organic pollutants, photocatalytic water splitting to product hydrogen, and reduction of CO2. Photocatalytic materials generally have the following characteristics:(1) the chemical stability; (2) to absorb the ultraviolet, visible light and infrared parts of sunlight; (3) with high activity; (4) environmental friendly and non-poisonous. The two main factors of affecting photocatalytic performance of semiconductor materials include narrow scope of light absorption and low carrier separation efficiency. To expand the light absorption of the photocatalytic materials and make use of the solar energy efficiently can make the photocatalytic materials to produce more photo-induced electrons and holes. And photocatalytic materials with high separation efficiency of carriers would have high quantum efficiency, leading more photo-induced electrons and holes to participate in photocatalytic reaction more efficiently. Therefore, to explore new type of semiconductor photocatalytic materials has important practical and strategic significance.The relationship between structure and photocatalytic performance of semiconductor materials is inseparable. Microstructure, particle size and particle self-assembly methods of semiconductor materials can make an impact on specific surface area and reaction active site of semiconductor materials, thus affect the photocatalytic performance of semiconductor materials. In addition, the atomic structure of elements (including nuclear charge number and extranuclear electron distribution, etc.) can also affect the attraction of nucleus to extranuclear electron and the transmission of photo-induced electron, so as to make the semiconductor materials with similar structures show different photocatalytic performance. Two-dimensional materials due to the unique two-dimensional layered structure and large specific surface area, can promote the transmission of photo-induced electrons and holes between the layers, which is beneficial to suppress carrier compound. To a certain extent, two-dimensional materials possess good photocatalytic performance.In 2004, scientist Geim, etc, from Manchester University in British first reported monolayer graphite flake can be obtained successful from graphite, that is graphene. From then on, two-dimensional material because of special structure and high specific surface area, cause great research interest and broad attention of scholars at home and abroad. Graphene is a new type of element carbon material after fullerene and carbon nanotube and the structure system of the carbon material is perfected on the dimension. Basic structure unit of graphene is composed of stable carbon six-member ring, which is connected with carbon atoms with sp2 hybridization, and carbon atom through the strong σ and relatively weak π keys connected to adjacent carbon atom, with periodic honeycomb lattice structure. In addition, graphene is a semiconductor with a band gap of zero, attributed to π electronic in its valence band and π* electronic in the conduction meet in K and K’position of Fermi level, and the velocity of movement of electrons in graphene is very fast, reached 1/300 of the speed of light. Graphene with special electronic structure, possess a lot of unusual electrical properties, such as electronic-hole phenomenon, the bipolar electric field effect, high carrier mobility and unperishable conductivity, etc. In addition, the graphene also showed excellent chemical properties, optical properties, thermal properties and mechanical properties. Therefore, two-dimensional materials represented by graphene, including transition metal sulfides, boron nitride and transition metal oxides etc. have extensive and important applications in photocatalysis field, and become new research hotspot.Germanium-based compounds is a new type of two-dimensional layered photocatalytic materials. In 1915, the first time scientists have discovered the semiconductor properties of germanium. Germanium (Ge) as a kind of excellent semiconductor material with high carrier mobility, belongs to the main group element of IVA, is predicted to the most promising semiconductor material to replace silicon. Two-dimensional germanium-based compounds have a narrow band gap (1.5-1.7 eV), with a wide scale to absorb visible light. And the basic structure of germanium-based compounds is two-dimensional layer structure consisted of layer of germanium atoms and layer of group atoms, in favour of the transmission of photo-induced electrons and holes and being captured by water molecules between layers, which is beneficial to make separation of the carriers and thus inhibiting composite of photo-induced electrons and holes effectively, so as to improve the photocatalytic properties ultimately. So the research of photocatalytic performance of two-dimensional germanium-based compounds has important strategic significance. This paper is studied and reported for the first time that applying two-dimensional germanium-based compounds in photocatalysis field. And through the study we know that, compared with GeH, GeCH3 because of its larger specific surface area and larger interlayer spacing, leading to the increase of its catalytic reaction activity sites, to the benefit of transmission and transfer of photo-induced carrier faster and more effectively. Moreover, GeCH3 has better thermal stability than GeH, which result in GeCH3 compared with GeH showed higher photocatalytic activity. In this paper, starting from the perspective of exploring new two-dimensional photocatalytic materials, have studied the photocatalytic performance of germanium-based compounds.The specific research content is divided into six chapters:The first chapter is including introduction of semiconductor photocatalysis and exploration of two-dimensional layered photocatalytic materials. Firstly, we introduced the basic concept of semiconductor photocatalysis, analyzed the main problems during photocatalytic process based on photocatalysis principle, and summarized the applications and research progress of photocatalytic technology, mainly including applying in photocatalytic degradation of organic pollutants, photocatalytic decomposition of water and reduction of carbon dioxide, etc. There are mainly two key factors to affect the photocatalytic performance, that is to broaden the light response range of photocatalytic materials and to suppression the composite of photo-induced electronic-hole. Through exploring the new type of photocatalytic materials to apply in photocatalysis field, aims to improve the photocatalytic performance. We discussed the basic structure and applications in photocatalysis of two-dimensional layered materials as well as the structure and research progress of two-dimensional germanium-based compounds (GeH and GeCHs). Finally introduced the significance of selected topic and the research content of this paper.The second chapter is to explore synthesis methods of two-dimensional germanium-based photocatalytic material GeH. Firstly, we synthesis the precursor CaGe2 by two methods, including flux-melting method and high temperature induction melting method respectively. During flux-melting method, we have analyzed the influence of proportion of flux, growth temperature and cooling rate to the structure of precursor CaGe2. During high temperature induction melting method, we analyzed the influence of ratio of reactants, synthesis temperature and time to the structure of synthetic precursor CaGe2. Finally we get the rapid and effective method to synthesis precursor CaGei. Secondly, we make use of ion exchange method to synthesis GeH samples with two-dimensional structure, and through improved method, we have analyzed the influence of synthetic time to the structure of GeH sample. The phase of GeH samples has been analyzed by X-ray diffraction pattern, by use of computational formula of crystalline interplanar spacing and the Bragg equation we get each crystal plane spacing and the corresponding diffraction angle of GeH. The microtopography of GeH samples has been analyzed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the result shows that GeH sample is stacked by the layer structure with different thickness. At the same time, XPS, Raman spectra and FTIR of GeH samples have been tested and analyzed, further proves that the synthetic GeH samples with high purity. Studies have showed that high temperature induction melting method to synthesis of precursor CaGe? combined with ion exchange method at normal temperature to synthesis GeH, reaction time is greatly shortened during the whole synthetic experiment, and the intrinsic microstructure of the samples is retained.The third chapter is the study of photocatalytic properties of two-dimensional germanium-based photocatalytic material GeH. The photocatalytic properties of synthetic GeH samples is characterized by photocatalytic degradation of organic dye rhodamine-B and photocatalytic water splitting to produce hydrogen at room temperature. At the same time, to verify the stability of photocatalytic performance of the synthetic GeH samples as a photocatalyst, we conduct 10 times cyclic degradation of rhodamine B solution and 3 times photocatalytic hydrogen generation experiment respectively. Through the test of Zeta potential and particle size distribution of GeH samples, we studied the potential on the surface of the GeH samples. Optical properties of synthetic GeH samples is characterized by UV-Vis-NIR diffuse reflectance spectra. Through the analysis of the specific surface area of GeH samples, we studied the probable factor which affect the photocatalytic performance. The energy band structure of GeH samples has been calculated by using density functional theory, and we further explores the migration process of photo-induced carriers under visible light irradiation.The fourth chapter is the study of synthesis and photocatalytic properties of two-dimensional germanium-based photocatalytic material GeCH3. Firstly, we explore the the synthetic method of the GeCH3 samples. Precursor CaGe2 samples are synthesized by induction melting method. Instead of contact ion exchange method, we adopted stir quickly to achieve the purpose of the ion exchange. And through improved method, we have analyzed the influence of synthetic time to the structure of GeCH3 sample. The phase of GeCH3 samples has been analyzed by XRD, by use of computational formula of crystalline interplanar spacing and the Bragg equation we get each crystal plane spacing and the corresponding diffraction angle of GeCH3. The microtopography of GeCH3 samples has been analyzed by using SEM and TEM, and the result shows that GeCH3 sample is stacked by the layer structure with different thickness. At the same time, XPS, Raman spectra and FTIR of GeCH3 samples have been tested and analyzed, further proves that the synthetic GeCH3 samples with high purity. Studies have showed that high temperature induction melting method to synthesis precursor CaGe2 combined with ion exchange method through stir quickly to synthesis GeCH3, reaction time is greatly shortened during the whole synthetic experiment, and the intrinsic microstructure of the samples is retained. The photocatalytic properties of synthetic GeCH3 samples is characterized by photocatalytic degradation of organic dye rhodamine-B and photocatalytic water splitting to produce hydrogen at room temperature, at the same time, to verify the stability of photocatalytic performance of the synthetic GeH samples as a photocatalyst, we conduct ten times cyclic degradation of rhodamine-B solution and three times photocatalytic hydrogen generation experiment. Through the test of Zeta potential and particle size distribution of GeCH3 samples, we studied the potential on the surface of the GeCH3 samples, and further studied the adsorption of GeCH3 samples to several common organic dye. Optical properties of synthetic GeCH3 samples is characterized by UV-Vis-NIR diffuse reflectance spectra. Through the analysis of the specific surface area of GeCH3 samples, we studied the probable factor which affect the photocatalytic performance. The energy band structure of GeCH3 samples has been calculated by using density functional theory, and we further explores the photocatalytic reaction mechanism of GeCH3 samples under visible light irradiation.The fifth chapter is the study for hydrogen production properties from ammonia borane of two-dimensional germanium-based photocatalytic material GeCH3. First of all, dehydrogenation from ammonia borane for synthetic GeH and GeCH3 samples have been conducted under the condition of dark and visible light (λ> 420nm) irradiation, respectively. In order to characterize the stability of catalytic performance of GeH and GeCH3 samples, we conduct three times cyclic dehydrogenation experiment from ammonia borane under visible light irradiation. In order to verify the influence of photochemical reaction to the dehydrogenation from ammonia borane, we have also carried out the contrast experiment for dehydrogenation from ammonia borane under room temperature (25℃) and 40℃ for GeH and GeCH3 samples respectively. The experimental results show that only a small part (16%～21%) of photochemical effects have effect on dehydrogenation from ammonia borane for GeH and GeCH3 samples. In addition, in order to explore the principle of dehydrogenation from ammonia borane, NaHCO3 has been used as the positive charge scavenger to study the mechanism of dehydrogenation from ammonia borane for GeCH3 samples.The sixth chapter is summary and outlook. The main research content and specific conclusions of this paper are analyzed and summarized. The innovation points of thesis have been summarized, and this paper proposes new methods and new ideas. Finally, the main existing problems during the process of research were analyzed and summarized. Moreover, the further research direction and contents is prospected.In conclusion, this thesis works on exploring new two-dimensional layered photocatalytic materials, which have studied the synthesis routes and improved methods for two-dimensional germanium-based photocatalytic material. Precursor CaGe2 have been synthesized step by step by flux melting method and induction melting method. Next, ion exchange method was used to synthesized GeH and GeCH3 samples with two-dimensional layered structure.The microstructure and valence state of GeH and GeCH3 samples have been characterized by use of XRD, SEM, TEM, XPS, Raman spectroscopy and FTIR test measures. The light absorption performance of synthetic GeH and GeCH3 samples have been characterized through the UV-Vis-NIR diffuse reflectance spectra. The photocatalytic performance of synthetic GeH and GeCH3 samples have been characterized through the degradation of various organic pollutants, and photocatalytic water splitting to produce hydrogen and dehydrogenation from ammonia borane. We also have studied the factors including morphology, Zeta potential, pore size distribution, specific surface area and band structure etc. of GeH and GeCH3 samples which affect the photocatalytic performance of materials. And summarized the reasons why GeCH3 shows higher photocatalytic activity compared with GeH. Research work of this paper is helpful to deeply understand the basic principle of two-dimensional layered photocatalytic materials. To explore new type of two-dimensional photocatalytic materials, expand the scope of light absorption, improve the photocatalytic activity, are of important theoretical and realistic significance to solve the two worldwide problems of energy and environment. |
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