Synthesis And Size Control Of Transition Metal Oxide Hierarchical Structures

Hierarchical micro-/nanostructures are mentioned as the higher dimension structures composed of low dimensional nano-building blocks. Recently, transition metal oxide hierarchical structures have attracted great attention due to their unique morphologies, novel structures and the resulting materials properties. However, the structural collapse and morphological changes may occur during the preparation of transition metal oxide hierarchical structures. Moreover, the size control of the hierarchical structures remains a significant challenge. In this regard, the work presented here was focused on the preparation of transition metal oxide hierarchical structures. The hierarchical structures of various metal oxides in the first transition period were prepared by the sol-gel process and the hydro/solvothermal process, and the mechanisms of the formation processes ware elucidated. The work lays a solid foundation for the application of hierarchical structures in the fileds such as photocatalysis, energy storage and conversion, and wastewater treatment.The work was focused on the metal oxides in the first transition period. The sol-gel process and hydro/solvothermal process were employed to prepare the hierarchical structures, such as mesoporous TiO2, TiO2 microspheres, sponge-like Mn3O4 structures, ZnO hollow microspheres, flower-like microspheres of layered nickel hydroxy compounds and hematite mesocrystals. The size control of these hierarchical structures was also investigated. The work includes:Firstly, mesoporous TiO2 and TiO2 microspheres were prepared by the sol-gel process. Effects of the templates on the hierarchical structures were investigated. Highly crystalline mesoporous TiO2 was prepared in the presence of poly(oxyethylene-23) lauryl ether (Brij-35) surfactant. The pore size distribution of the samples became wider as the amount of Brij-35 increased. The sample with large pore size and wide pore size distribution showed higher photocatalytic activity. The cetyltrimethylammonium bromide (CTAB) was also used to prepare mesoporous TiO2. TiO2 microspheres with tunable particle size in the range of 0.3-3 μm were prepared by a facile non-aqueous sol-gel process on the basis of ester elimination mechanism. The size of TiO2 microspheres was controlled by the amount of methanol in the starting solution. The size of TiO2 microspheres decreased as the volume of methanol increased.The CTAB can also be used to control the size of TiO2 microspheres. As the amount of CTAB increased, the size of TiO2 microspheres dramatically decreased.Secondly, sponge-like Mn3O4 structures, ZnO hollow microspheres, flower-like microspheres of layered nickel hydroxy compounds and hematite mesocrystals were prepared by the hydro/solvothermal process. The transformation between the precursors and the coresponding metal oxides was investigated. Manganese formate is a kind of metal-organic frameworks. The sponge-like Mn3O4 structures composed of Mn3O4 nanocrystals were prepared by the decomposition of manganese formate at 200 ℃. The morphology of sponge-like Mn3O4 structures was mostly retained from the morphology of manganese formate precursor. The solvothermal process was used to control the morphology of sponge-like Mn3O4. The large sponge-like structures exhibiting crystallographic symmetry were prepared under solvothermal treatment for a long time. There are two distinct sites of zinc ions in zinc formate dihydrate. Novel hierarchical hollow ZnO microspheres, composed of the large ZnO crystals on the outer surfaces and the small ZnO crystals on the inner surfaces, were prepared by a chemically induced self-transformation process. The size of the microspheres was affected by the concentration of formic acid. Two sizes of ZnO crystals in hierarchical ZnO microspheres were associated with the two distinct sites of zinc ions in zinc formate dihydrate. Both of nickel hydroxychloride and β-Ni(OH)2have layered brucite crystal structure. Hierarchical flower-like nickel hydroxychloride microspheres were prepared by a facile template-free solvothermal process. The flower-like nickel hydroxychloride microspheres were composed of crumpled nanosheets with random orientation. As the reaction time increased, the crystal size of the nickel hydroxychloride decreased and the thickness of the nanosheets within the microspheres also decreased. The flower-like β-Ni(OH)2 microspheres were prepared by nickel hydroxychloride microspheres treated with high concentration of KOH. The β-Ni(OH)2 microspheres prepared with higher concentration of KOH showed better electrochemical performance, while the β-Ni(OH)2 microspheres prepared with lower concentration of KOH showed better adsorption activity for acid fuchsine. There are tunnel-shaped holes occupied by Cl ions in the β-FeOOH crystal structure. The pseudocubic hematite mesocrystals and spindle-shaped hematite mesocrystals was prepared by the transformation of β-FeOOH during the hydro/solvothermal process. The size of pseudocubic hematite mesocrystals were well-controlled by the ratio of ethanol. As the ratio of ethanol increased, the particle size of pseudocubic hematite mesocrystals increased from 0.55 to 1.5μm without losing their internal mesocrystalline structure. The spindle-shaped hematite particles were prepared in the presence of ethylene glycol. The size of spindle-shaped hematite particle was affected by the concentration of KOH and formic acid.