Synthesis And Characterization Of Novel Manganese Oxides

Manganese dioxide is an important functional oxide with distinctive propertiesand wide applications in catalysts, ion-sieves, magnetic properties, and batteries, dueto its rich resource, low cost production and non-toxicity. The property of manganeseoxides is upsubjected to its structures and morphologies. However, the structure andmorphology is determined by synthesis method, different novel morphologymanganese oxides were synthesized with three methods in this dissertation.Mesoporousγ-MnO₂ nanospheres were synthesized by self-assembly method.Mesoporousγ-MnO₂ nanospheres with surface area as high as 232m²/g have beensynthesized by reflux method without the use of template or cross-linking reagents,and we use H₂O₂ to reduce HMnO₄ under room temperature. The reaction was carriedout at low temperature in a short time. The structure and morphology, formationprocess, particle size, surface area and pore size distribution have been investigated byX-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electronmicroscopy (TEM) and brunauer emmet teller (BET). On the basis of the aboveobserved, it is possible to interpret the formation mechanism of theγ-MnO₂nanospheres structures as follows: The permanganate reacts with H₂O₂γ-MnO₂flakes were form, because of their thickness of several nanometers and high inteffacialtension that they can not stand independently. In the process of preparation notemplate was used, so the synthesis products must be formed by self-assembly, drivenby strong surface energy. Thus loose irregular nanospheres were formed in the earlyage, with the reaction went on, more manganese dioxides nanoflakes were adsorbedinto the loose spheres and improve its morphology into perfect spherical particle. BETtest indicates the surface area of the prepared mesoporousγ-MnO₂ is 232m²/g, 4～7times of that of EMD, it is the largest specific area among chemical synthesizedmanganese dioxide so far. Electrochemical measurements of lithium-ion batteriesshowed that the prepared mesoporousγ-MnO₂ electrode exhibits high initial capacity,excellent high rate discharge performance and cycling reversibility. Electrochemistrytesting showed that a high capacity of 230.5mAh/g was obtained in the first discharge process, with excellent cycling performance with an average capacity fading of nomore than 0.2% per cycle. After the 50th cycle, theγ-MnO₂ nanospheres cathodecould still deliver a capacity of 199.8mAh/g.Prismatic single crystalγ-MnOOH is synthesized by hydrothermal methodthrough oxidizing Mn(CH₃COO)₂ with HMnO₄ at 150℃. The advantage of oxidizingagent HMnO₄ is that no other metallic ions coming into the product structures. Thesynthesized samples were characterized by XRD, SEM, TEM, HRTEM and TG, it isfound that the formation process of single crystalγ-MnOOH includes two primaryevolution stages: (1) theγ-MnOOH whiskers are initially formed, and (2) the whiskersgrow to prismatic single crystalγ-MnOOH by an adsorption-growth process, (11(?))plane grows faster than other planes, because the surface energy of each crystal planeis different, the different growth rate makes the whiskers changed to prismatic crystal.The hydrothermal reaction time is the major factor that influences crystal morphology.TG results show that the single crystalγ-MnOOH is transferred to MnO₂ at 350℃,γ-MnOOH is a good precursor to prepare manganese dioxide with the samemorphology asγ-MnOOH.β-MnO₂ nanobelts was synthesized by template method. CTAB, as a cationsurfactant can combine with the growth unit i.e. [Mn(OH)₃]^- due to the action ofcoulomb force. Therefore, CTAB can absorb the surface of the growth unit. Due tothe presence of CTAB cation surfactant, the surface energy of some manganesedioxides crystal plane is reduced, moreover, the hydrophobic group around thecircumference of the CTAB restrict the development of manganese dioxides, then thegrowth ofβ-MnO₂ nanobelts are preferential. Finally,β-MnO₂ nanobelts are obtained.As a kind of green and environment-benign materials, MnO₂ will surely play amore important role in catalysts and chemical power sources. Research and characterof manganese dioxides by different methods have very important and positivemeaning in improving the electrochemical properties of manganese dioxides materialand the development of its catalysts properties, and etc.