Synthesis And Properties Of Manganese Oxides Micro/Nanostructured Materials Via Nonaqueous Solvent Routes

Fabrication of micro/nanoscale inorganic materials with special morphology and size has been a focus in areas of materials science. The rational design and synthesis of advanced micro/nanostructured materials with controllable morphology and diverse compositions has attracted tremendous interests in the fields of nanoscience and nanotechnology. As an important functional metal oxide, recently many efforts have been put on the synthesis of uniformly sized and shape-controlled particles of manganese oxide due to their potential applications in high-density magnetic storage media, catalysis, ion exchange, molecular adsorption and electronics, and hence on the study of the physical and chemical properties of nanoparticles.This paper is focused on the controlled synthesis of manganese oxide nanostructures through liquid-phase chemistry routes. In addition, formation mechanism and properties of as-obtained micro/nanostructures were also investigated. The detailed information of the paper is listed as follows:1. The size-controlled synthesis of uniform MnO3 octahedra assembled from nanoparticles and their catalytic propertiesUniform Mn2O3 octahedral nanoparticles were synthesized by a mediated N,N-dimethylformamide (DMF) solvothermal route. On the basis of a time-dependent experiment, we propose that the Mn2O3 octahedra were formed through oriented aggregation of primary nanocrystals. Meanwhile, poly(vinyl-pyrrolidone) (PVP) was applied as a surfactant to facilitate the oriented aggregation of small Mn2O3 nanoparticles into octahedral crystallites. By tuning the amount of Mn(NO3)2, particles with average sizes 1?m to 300 nm, with a narrow size distribution, could be fabricated. Interestingly, at the low concentration limit (?500?L), Mn2O3-Mn3O4 composite, Mn3O4, and MnCO3 nanostructures can be formed by tuning the concentration of Mn(NO3)2. It is thought that the DMF solvent worked as a weak reducing agent and the reduction of the original products Mn2O3 nanocrystals were overdone for the time spent. The catalytic test results show that the as-obtained Mn2O3 octahedra exhibited desirable CO catalytic oxidation properties and the surface texture and particle size significantly affected the catalytic activity. By contrast, the larger Mn2O3 octahedral nanoparticles prepared at a lower concentration of Mn(NO3)2 exhibited relatively high activities. The improved catalytic activities may be ascribed to the unique structural and textural characteristics, such as higher specific surface area which could expose more active sites for catalytic oxidation of CO and interconnecting structure which will favor the adsorption of reactants and desorption of products and thus facilitate the CO oxidation process.2. Controllable synthesis of monodisperse Mn3O4 and Mn2O3 nanostructures via a solvothermal routeA facile one-step solvothermal route was developed to synthesize monodisperse Mn3O4 and Mn2O3 nanostructures with the introduction of poly(vinyl-pyrrolidone)/stearic acid (PVP/SA)mixture. H2O2 played a key role in the determination of the products Mn3O4 and Mn2O3. The synthesis parameters for nanostructured MnOx such as surfactant and reaction time were investigated, along with their influences on morphology and composition. The morphology evolution of the Mn3O4 and Mn2O3 reveals that the nanostructures formed via two distinct mechanisms of nucleation and growth of nanocrystals. Based on the time-dependent experiments, the formation of the Mn3O4 and Mn2O3 crystals can be attributed to a precipitation-dissolution-recrystallization-ripening and aggregative attachment process, respectively.3. Facile synthesis of MnCO3 hollow dumbbells and their conversion to manganese oxideManganese carbonate (MnCO3) hollow dumbbells were synthesized via a polyol process. Based on the structural analysis of the samples obtained at different reaction times, a mechanism of nucleation-growth-aggregation-ripening was proposed to account for the formation of the hollow dumbbells. In addition, it was noted that an amount of H2O played crucial roles in determining the final morphology of the products. Moreover, the manganese oxide has also been obtained from the MnC03 crystals after thermal transformation in laboratory air, and the phase of final product could easily be controlled to be either MnO2 or Mn2O3, simply by altering the calcination conditions. The manganese oxide powder products possessed mesoporosity and essentially preserved the pristine morphology of the MnCO3 precursor. Specific surface areas for the MnO2 and Mn2O3 are 97.4 and 34.7 m2/g, respectively. The surface area shrinkage of Mn2O3 is probably due to the heat-treatment at a higher temperature through which the internal mesopores merged.4. Shape-controlled synthesis of manganese oxide nanoplates by a polyol-based precursor routeManganese oxide nanoplates with different shapes have been prepared based on an ethylene glycol-mediated route. The first step consists of precipitating manganese alkoxide precursor in a polyol process from the reaction of manganese acetate with ethylene glycol. During this process, the morphologies of the prepared precursor could be tuned from disc-shaped to hexagonal nanoplates by introducing different organic additives. The second step involves the treatment of the precursor under different conditions. Crystalline Mn2O3 with the same morphology was readily obtained by calcination of the manganese alkoxide precursor. Furthermore, Mn3O4 nanoplates could be obtained by immersing the precursor into the deionized water.5. Shape-controlled synthesis of Mn-Co complex oxide nanostructures by a polyol-based precursor route and their catalytic propertiesA polyol-based precursor route was developed to synthesize Mn-Co complex oxide with well-defined morphologies, in which ethylene glycol (EG) was treated with metal acetates in the presence of PVP. By varying the reaction temperature, the as-obtained precursor was readily regulated its morphologies, which could vary from nanospheres to hierarchically stacked nanoplates. Additionally, the ratio of Mn-acetate to Co-acetate in the solution played a key role in the formation of the nanostructured precursor with different Mn/Co atomic contents. On the basis of the experimental results, a possible growth mechanism for the nanostructured precursor was proposed. MnCo2O4 and CoMn2O4 could be obtained from their precursors without changing the morphologies by a simple calcination procedure. The synthetic methodology appears to be general and promises to provide an entryway into other complex or composite oxide materials with various nano/microstructures. As an example of potential applications, the as-obtained Mn-Co mixed oxide nanomaterials were used as catalyst in CO oxidation, and showed an excellent catalytic activity. In our work, however, the catalytic performance of the CoMn2O4 nanostructures is remarkably better than that of the MnCo2O4 despite the fact that the specific surface area of CoMn2O4 is lower. We conclude that the improved catalytic activities may be ascribed to the unique structural characteristics such as higher Mn/Co atomic ratio owned by the CoMn2O4 catalyst which could expose more active sites for catalytic oxidation of CO.