| Manganese oxides are given wide attention because of their cheap, environmentally friendly, and rich in natural resources. Because manganese oxides have obvious physical and chemical properties, they have been widely used in many fields, such as catalysis, adsorption, sensing, ion exchange, energy storage in lithium ion secondary battery and supercapacitor and so on. It is found that the electronic and optical properties of materials have highly relied on their crystallinity, morphology, dimension and particle distribution. It is of great significance to controllably prepare manganese oxides materials with novel morphologies and to study the relationship between morphology and property. In this thesis, a series of manganese oxide materials with novel morphologies have been hydrothermally prepared, and their formation processes have been studied on the basis of the reaction conditions. Moreover, the electrochemical property of the prepared manganese oxides with novel morphologies has been investigated.The thesis mainly consists of three sections, introduction, experiments and conclusion. The properties, preparation methods and applications of transition metal oxides and manganese oxides with different structures and morphologies were reviewed in Chapter 1. In the Chapter 2, the controllable fabrication of γ-MnOOH with novel hexagram morphology was described and a possible formation process was proposed on the basis of the experimenal results. α-MnO2 with four different morphologies and β-MnO2 with novel morphology were prepared in a reaction system of K2S2O8 and MnSO4 in Chapter Ⅲ, and their electrochemical properties had been investigated. Finally, the research conclusion was given in Chapter Ⅳ.Without the assistance of templates or surfactants, the well-defined single crystal γ-MnOOH with novel hexagram morphology was firstly prepared by hydrothermal treating a suspension of ethyl acetate and KMnO4 at 200 ℃ for 48 h, and its formation process had been investigated on the basis of XRD, FE-SEM, TEM, HRTEM, and SAED analyses. The optimal preparation conditions of γ-MnOOH with novel hexagram morphology were obtained on the basis of the reaction temperature, reaction time and the amount of ethyl acetate. Results indicated that ethyl acetate plays an important role in the formation of y-MnOOH with good crystallinity and uniform hexagram morphology. Ethyl acetate slowly hydrolyzed into acetic acid and caused an acidic reaction environment accompanied with hydrothermal reaction. The acetic acid was adsorbed on the lateral of y-MnOOH with multiple branched nanorods, which caused a soft etching process and the lateral of the multiple branched nanorods became sharper and thinner and finally transformed into y-MnOOH with hexagram morphology. By a topotactic conversion process, β-MnO2 with similar morphology of γ-MnOOH had been obtained by thermal annealing γ-MnOOH with hexapod-like morphology and hexagram morphology.α-MnO2 with urchinlike, microsphere, lycheelike and core-shell structure morphologies had been successfully prepared by changing the reaction temperature, reaction concentration, pH and mental ions such as Fe3+ and Ag+ in the reaction system of K2S2O8 and MnSO4, and their electrochemical property had been investigated. The experimental results indicated that the metal ions in the reaction system influenced the morphology of the obtained materials. α-MnO2 electrode with core-shell morphology exhibited a high specific capacitance of 211 F g-1 at a scan rate of 5 mV s-1, which is much higher than that of other three manganese oxides electrodes with different morphologies. The high capacitance was attributed to the core-shell morphology and the novel structure affected the transport/diffusion path lengths for ions and electrons. In addition, β-MnO2 with grain-like morphology was also obtained by changing the reaction temperature. |
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