Template-Free Hydro/Solvothermal Synthesis, Characterization, and Applications of Metal Oxides Nanomaterials

Metal oxides nanomaterials have the potential for a wide variety of applications such as catalysis, environmental remediation, gas sensors, optoelectronic devices, lithium ion batteries, and energy storage. It is because of their unique catalytic, optical, and electrical properties. Moreover, it has been demonstrated that these properties are strongly dependent on their compositions, phases, shapes, and sizes. Therefore, the studies on structure and morphology controlled synthesis of nanomaterials are of great interest and are actively being pursued.;In this thesis, we present an environmentally friendly and template-free hydro/solvothermal synthetic method for the morphology-controlled synthesis of metal oxides nanomaterials in pure solvent or in mixed solvent systems. Moreover, the effects of the reaction parameters including reactants and their concentration, reaction temperature, time, and reaction medium on the morphology of the target products were investigated systematically in this work. The possible formation mechanisms of these metal oxides nanostructures were also discussed in-depth on the basis of detailed experimental data. In addition, their shape and size-dependent optical properties as well as their applications in environmental remediation were also investigated.;In this work, hexagonal ZnO micro-cups and micro-rings assembled by nanoparticles were obtained by using only one reactant Zn(CH3COO)2˙2H 2O in pure water system. The precursor not only served as the zinc source, but also provided an effective etchant CH3COOH that played a strategic role in the formation of the hollow structures in the ZnO crystals. We also synthesized ZnO nanoparticles with controllable size in a mixed solvent system. The average size of the nanoparticles could be tailored by adjusting the volume ratio of ethanol/ethylene glycol (EG). Mesoporous MgO nanowires and microflowers were prepared by a template-free solution phase synthetic method combined with subsequent calcination. Our results indicated that the reaction medium played a crucial role in the morphological control of the precursor Mg(OH) 2 nanostructures. The high polarity of pure water favored the polar growth of the precursor, resulting in the formation of nanowires with a diameter of 80 nm, whereas water/ethanol mixtures with a lower polarity, at a volume ratio of and below 2:1, generally led to the formation of microflowers composed of nanoplates. Moreover, as the volume ratio of the water/ethanol mixture reduced, both the thickness of the nanoplates and the size of microflowers increased. In addition, the removal capacities of the mesoporous MgO nanostructures for organic dye MO from water were studied and calculated to be 48.9 mg g -1 and 56.8 mg g-1 for MgO nanowires and microflowers, respectively, which were higher than that of commercial MgO powder (13.6 mg g-1). The superior removal performance was attributed to the excellent porous structure and high surface area of the as-prepared MgO nanostructures. In order to improve the removal performance, we had combined these two merits in terms of the adsorption ability of MgO and photocatalytic property of ZnO together by doping ZnO into MgO nanostructures. Thus, mesoporous (ZnO) x(MgO)1-x nanoplates were obtained. The UV-induced degradation of MO indicated that the mesoporous (ZnO)x(MgO)1-x nanoplates with the combinative merits had high photocatalytic performance and would be a promising candidate for environmental remediation. Moreover, their optical properties were also investigated by the UV-vis absorption and room temperature cathodoluminescence (CL) emission spectroscopy. The UV-vis absorption spectra showed the band gap variation of the as-prepared samples, due to the presence of ZnO in the MgO nanostructures. The result indicated that the design of surface structure could produce oxide nanocrystals with controlled optical properties. The CL spectra showed strong broad peaks in visible range from 450 to 700 nm, which implied there were significant oxygen vacancy defects created on the surface of (ZnO)x(MgO)1-x nanoplates.