Controlled Hydrothermal Synthesis and Characterization of Manganese Oxide
|School||Guangdong University of Technology|
|Keywords||Manganese oxide Hydrothermal Crystalline phase control|
As an important functional transition metal oxide, manganese oxide is one of the most attractive inorganic materials, due to its physical and chemical properties and wide applications in catalysis, ion exchange, and particularly, energy storage. However. The main challenge in this area is how to precisely control the crystalline forms. compositions and crystal structures through controlling the synthesis conditions, which may serve as a powerful tool for the tailoring of physical and chemical properties of materials in a controllable way. The main points of this thesis can be summarized as follows:1. In this thesis, we develop a facile hydrothermal route to synthesize a variety of manganese oxides with different morphology without using any surfactant or catalyst. After characterized in details by powder X-ray diffraction (XRD). scanning electron microscopy (SEM), and Raman Spectra, a novel crystalline phase transformation mechanism has been suggested based on the analysis of experimental data. When the reactant is only MnS04and KClO3, the product is the urchin-like y-MnO2. If a certain amount of acetic acid and potassium acetate is added to the reaction system, the product is a-MnO2ultralong nanowires. However, when the K+concentration in the reaction system is not enough, and the product is converted to the rectangular pyramidal (3-MnO2. While is present in the the reaction system large amounts of potassium acetate, but lack of H+, the product is octahedral Mn3O4. And then add a certain amount of acetic acid to the reaction system, the product was transformed from Mn3O4to MnOOH. Additional studies were carried out to investgate the phase-dependent catalytic activity of a-MnO2.β-MnO2, and γ-MnO2by using the catalytic combustion of toluene as probe reaction. The results of H2-TPR and XPS demonstrate that the a-MnO2has a lower reduction temperature as well as large amount of surface oxygen species, so that ignition temperature T10and complete combustion temperature T99are184℃and220℃respectively.2. We develop a facile one-pot hydrothermal route to synthesize ultralong α-MnO2nanowires at relatively low temperature and for short reaction time. By changing the reaction time and analysing the corresponding SEM data, the mechanism for α-MnO2ultralong nanowires was explored. Based on the analysis of experimental data, steric effects, appropriate different of redox potentials and Ostwald ripening are attributed to the formation of ultralong nanowires and nanowire bundles. Because the surface of the MnO2ultralong nanowires is negative, they have a strong adsorption effect for methyl violet dye with a positive charge. The MnO2ultralong nanowires can be recycled through using a simple vacuum filtration and heating method. After5cycles, the methyl violet adsorption rate is still up to99%.3. Free standing membrane with strong mechanical stabilities and flexibility were prepared through a facile vacuum filtration approach and using MnO2ultralong nanowires as precursors. Because the surface of the nanowires are cover with a great number of OH, the free standing membrane is easy to be functionalized by incorporating nanoparticles to achieve novel properties. Meanwhile, the properties of the membrane could be controlled by incorporating different materials to achieve composite membrane. In order to demonstrate the broad applicability of the MnO2membrane, we fabricate a variety of the composite membrane with various novel properties including magnetism and reversibly switchable wettability between hydrophilicity and hydrophobicity through various materials modification, including CoFe2O4nanoparticles and organic triethoxy(octyl)silane (TEOOS). Furthermore, the free standing membrane could also simultaneously be functionalized with two materials to reveal multiple properties.