Designed Synthesis And Property Study Of Manganese Based Oxides Nanomaterials With Varied Valences

Synthesis and application of manganese oxides have attracted great interests in modern science and technology. Because of their special properties, manganese oxides nanomaterials are of considerable importance in technological applications, such as batteries, catalysis, molecular adsorption, ion-exchange and magnetism. Particularly, manganese oxides nanomaterials exhibited excellent degradation property for organic pollutants.In our paper, various manganese oxide nanomaterials were synthesized by a simple aqueous synthesis method at low temperature, including MnOx nanocomposites, birnessite MnO2nanomaterials, Mn3O4/MnOOH nanocomposites and Mn3O4nanomaterials. Then, they were used to degrade RhB in acid condition without light sources and H2O2, and mechanism of degradation reaction was discussed.First, hollow urchin-like MnOx (Mn3O4/MnO2) nanocomposites were successfully synthesized via a simple aqueous synthesis method by using MnCO3as precursors in basic solution. The morphology of final MnOx depends strongly on the structure of intermediate MnC03. Crystal composition of MnOx samples varied with reaction temperature and adding sequence of the reactants. As-prepared sample can degrade RhB effectively and quickly, and the best degradation efficiency of RhB can reach90%. Lower pH value, less amount of sample and higher content of γ-MnO2led to higher degradation efficiency of RhB. Based on the changes of powders’UV-Vis spectrogram before and after degradation, it can be presumed that the mechanism of degradation was redox reaction between MnOx and RhB.Second, birnessite type MnO2nanomaterials were successfully synthesized by using KMnO4as oxidant to oxidize different precursors. The morphology of samples was controlled by the kind of precursors. Birnessite MnO2nanoflowers and nanoflakes were obtained by using Mn2+and Mn(OH)2as precursor respectively. Birnessite MnO2nanoflowers were assembled by nanoflakes. Birnessite MnO2nanoflakes inherited the morphology of Mn(OH)2precursor, and samples with different size can be synthesized by changing amount of NaOH. Composition of as-prepared samples was controlled by the amount of KMnO4. As-prepared samples can degrade RhB effectively and quickly. Moreover, morphology and composition of samples had effect on their degradation properties. Samples with flower-like morphology or with higher average valence of Mn have more excellent degradation properties. The best degradation efficiency of RhB can reach91.7%.Third, Mn3O4/MnOOH nanomaterials were prepared via an aqueous precipitation method in basic condition. The growth of MnOOH associated with Mn3O4. The crystalline phase of MnOOH in samples included y-MnOOH and β-MnOOH. The major phase of sample obtained at30℃was β-MnOOH, and while reaction temperature increased to90℃, the major phase of samples changed to γ-MnOOH.(3-MnOOH as intermediate transformed to γ-MnOOH gradually as reaction time increasing. Samples with different composition such as Mn3O4, Mn3O4/γ-MnOOH, Mn3O4/γ-MnOOH/β-MnOOH, Mn3O4/β-MnOOH, were synthesized by changing the adding method of NaOH and H2O2. As-prepared samples can degrade RhB in acid condition. Samples with more content of β-MnOOH exhibited better degradation properties.Fourth, we focused on synthesis of Mn3O4nanoparticles in basic aqueous solution and studies of samples’synthesis mechanism, composition and degradation property. The morphologies of as-prepared samples were sphere-like and octahedral nanoparticles by using NH3H2O and NaOH as alkali sources respectively. Morphologies and dispersibility of samples were improved by introducing surfactants into the reaction. The composition of Mn3O4sphere-like included MnO-Mn2O3and2MnO-MnO2. The composition of octahedral was MnO-Mn2O3, and it could transform from MnO-Mn2O3to2MnO-MnO2in acid condition because of Mn3+disproportionation. Sphere-like Mn3O4nanoparticles have more excellent degradation property than octahedral Mn3O4nanoparticles, and degradation efficiency of RhB can reach61.3%.Fifth, based on previous results, Mn3O4mic/nanoflakes were prepared in strong basic solution. High OH-concentration was a key factor for the formation of Mn3O4mic/nanoflakes. When mole ratio of OH-and Mn2+was more than10:1, Mn3O4mic/nanoflakes can be obtained. Mn3O4micflakes assembled by nanoparticles can be synthesized via oxidation of Mn(OH)2in strong basic solution, and Mn3O4nanoflakes were prepared via dissolution and recrystallization of Mn3O4crystal in strong basic solution. In addition, Mn3O4nanoparticles and nanorods (prism-like) were obtained by changing reaction temperature and amount of NaOH. Comparing to Mn3O4nanoparticles and nanorods, Mn3O4nanoflakes exhibited more excellent degradation property and the degradation efficiency of RhB can reach68.1%. In summary, our paper reported the synthesis and property of manganese oxides with different valence state. Shape and size of as-prepared samples can be manipulated. The synthesis method was mild and simple. The as-prepared samples have excellent degradation property for organic pollutants, and they can be widely used in environmental treatment.