Study On The Design, Preparation And Forming Process Of Novel Morphology Manganese Oxide Micro / Nano Materials

Micro/nano-sized transition metal oxides with controlled morphologies have been received special attention due to their unique structure and interesting physicochemical properties. Precise controlling the material directional growth, and its morphology, dimensions and dispersity, the design and controllable preparation of micro/nano materials with novel morphologies will be achieved, which have important academic value and potential applications. As an important transitional material, the manganese oxides are the most atractive materials. It is found that the physicochemical properties of manganese oxides depend on the morphology, exposed special crystal faces and the controlled size, especially appling in catalysis and photoelectric sensing areas. However, it is a challenging subject to design and prepare micro/nano-sized manganese oxide. The purpose is to prepare manganese oxides with controllable morphology, tunable cystal size and dimension, and realize directional growth of manganese oxide along special direction via effective technical routes. In this dissertation, Mn2O3cubes and β-MnO2bipyramid prisms exposed{111} faces have been hydrothermally prepared. Birnessite-type layered manganese oxide with hexagonal nanoplates can be fabricated by topochemical oxidation technique. By using salt template assistance technique, MnO/C composite nanosheets have been obtained. Also their formation processes have been investigated on the basis of the reaction temperature, the reaction time and the amount of morphology-controlled reagents. The research contents are as follows:The H-type layered manganese oxide is hydrothermally treated in tetramethylammonium hydroxide (TMAOH) solution, and it can be exfoliated into manganese oxide nanosheets and then transformed to uniform Mn2O3cube with an average particle size of4μm. By controlling the amount of TMAOH, the hydrothermal treatment temperature and time, the optimized preparation conditions of uniform MnO3cube are obtained. It is found that TMA+ions play an important role in the formation of Mn2O3with good crystallinity and regular cubic morphology. TMA+ions are firstly intercalated into the interlayer of the layered manganese oxide, which causes a short-range swelling and finally leads to the exfoliation of swelled layered manganese into their elementary nanosheets. Some Mn (Ⅲ) on the manganese oxide nanosheets migrate into the solution and Mn (IV) are reduced, and then the exfoliated manganese oxide nanosheets transform to Mn2O3crystal nucleus under suitable hydrothermal condition. Then TMA+ions in the solution are selectively adsorbed on the{100} faces of Mn2O3crystal nucleus to inhibit the growth rate along [100] direction, and even forms Mn2O3particles with cubic morphology.(3-MnO2well-dispersed single crystals (average length-20μm) with growth shape composed by bipyramid prism, are prepared under F-anion assisted hydrothermal method in the reaction system of Mn2+and MnO4-. The optimized preparation conditions of β-MnO2bipyramid prism are obtained by means of adjusting the amount of F-anions, the hydrothermal treatment temperature, and the hydrothermal treatment time, and their formation processes have been proposed. With the hydrothermal reaction proceeding at180℃, the phase transformation changes from the initial nanofiber a-MnO2to tetragonal bipyramid, and finally into bipyramid prism β-MnO2. It is found that the density of the exposed Mn (IV) on several low-index planes of β-MnO2crystal is calculated, and the{111} faces possess the lowest concentration of exposed Mn (IV) among the low surface energy planes of β-MnO2crystals. The F-anions play an important role in controlling the crystal phase and morphology of the obtained materials. The free F-anions in the reaction system are adsorbed preferentially on the{111} faces of β-MnO2crystals, which inhibits the growth rate of the [111] direction, but favors the growth of the{110} faces. The transformation from a-MnO2nanofibers to β-MnO2regular polyhedra can be realized by adjusting the amounts of F-anions.The birnessite-type layered manganese oxide nanoplates with good dispersity and regular hexagonal morphology are obtained by topochemical oxidizing precursor Mn(OH)2in NaCIO solution at room temperature, and its formation process is investigated. The results show that the obtained birnessite-type layered manganese oxide nanoplates inherit the layered structure and regular hexagonal morphology of their precursor Mn(OH)2, which are self-assembled from MnO2particles. The regular hexagonal nanoplates are about150nm in each lateral edge with a thickness of120nm. The binding force between the interlayers changes from Van der Waals force of Mn(OH)2to the electrostatic attraction of birnessite-type layered manganese oxide, and the dispersity and regular morphology of the obtained materials are affected by the precursor Mn(OH)2and the oxidation reaction times. The as-prepared layered manganese oxide was used as supercapacitor electrode and its capacitive performance are investigated by cyclic voltammetry in1mol L-1Na2SO4solution in a voltage of -0.2-0.8V. This layered manganese oxide not only exhibits a high specific capacitance of284F g-1, but also shows excellent cycle stability with96%capacitance retention after2000cycles at a scan rate of20mV s-By using manganese oleate complex (Mn(Oleate)2) as the precursors and NaCl as template, NaCl@MnO/C composites have been prepared through a single heating procedure under Ar atmosphere. By subsequent removal of NaCl template with water, the MnO/C composite nanosheets are obtained. The mophology, particle size and dispersity of MnO nanocrystals are depended on the amount of NaCl template, the heating rate and temperature. The MnO nanocrystals with uniform particle size and good dispersity are prepared by calcinating NaCl@Mn(Oleate)2at600℃, which are embedded in the carbon sheets with the particle size of about34nm. Manganese oleate complex is firstly adsorbed on the surface of NaCl template, and this template provides the growth interface to form MnO/C composite nanosheets. When the embedded MnO nanocrystals are removed by HNO3solution, the regularly spaced carbon networks have remained. The cabon walls existed around the MnO nanocrystals could provide high tensile strength of the composite material in the vertical direction, which effectively prevent MnO nanocrystals from assembling in the charge and discharge process. The special structure would contribute to the as-prepared nanosheets as lithium-ion battery anode material.