Controlled Synthesis Of Metal Oxides Nanomaterials By A Two-phase Interface Reaction And Their Applications

In recent years, many researches have demonstrated that three-dimensional (3D) hierarchical structured metal oxides nanomaterials composed of low-dimensional nanoscale building blocks could exhibit enhanced physicochemical properties compared to their bulk counterparts and have extremely important potential applications, especially for energy storage and conversion, catalytic, gas sensors, wastewater treatment and so on. In addition, both theoretical calculations and experimental studies have demonstrated that the properties of crystalline materials, such as catalytic reactivity and selectivity, sensing sensitivity, lithium insertion/extraction rate etc., strongly depends on the degree of exposure of high-energy crystal facets. Crystalline materials with high-energy facets are usually more active than crystal facets with low surface energies and could exhibit surface-enhanced chemical activity, such as conspicuous catalytic activity. Thus, controllable synthesis of complicated three-dimensional hierarchical structured metal oxides nanomaterials and crystalline materials with high percentage of high-energy crystal facets is expected to significantly improve the performance of nanomaterials. With the development of varieties of synthetic methods, although significant achievements have been made on controllable synthesis of inorganic nanomaterials with3D hierarchical structure and micro/nanocrystal materials enclosed by high-energy facets, there is still much work need to be done. For example, controllable preparation of3D hierarchical structured inorganic nanomaterials and micro/nanocrystal with high-energy facets by a facile and general wet-chemical synthetic method is still a major challenge. Moreover, further investigated the formation mechanisms of complex3D hierarchical structured metal oxide nanomaterials and crystalline materials with high-energy facets and their related physicochemical properties are also very important, which are not only expected to achieve design synthesis but can also get high performance materials.In this thesis, we aim to fabricate complex3D hierarchical structured and high-energy facets exposed metal oxide nanomaterials by a facile liquid-phase synthesis method and further clarify their formation mechanisms and investigate their applications. We used a simple two-phase interface reaction method under hydrothermal conditions to fabricate metal oxide nanomaterials with controllable morphologies and high-energy facets. The samples’ compositions, the morphologies, the assembly manner of the3D hierarchical structures, the exposure of high-energy crystal facets, the magnetic properties, the specific surface area and pore-size distribution of the samples were characterized by X-ray diffraction analysis (XRD), Mossbauer spectrum analysis, field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), Fourier transform infrared spectra (FT-IR), a vibrating sample magnetometer (VSM) and a surface area analyzer. The electrochemical performances of the electrode materials were performed using an electrochemical workstation and a battery tester. Ultraviolet and visible (UV-vis) spectrophotometer was employed for the analysis of the concentration changes of targeted pollutants.The main results and conclusions are listed as follows:1.3D hierarchical a-Fe2O3microstructures with different morphologies were successfully synthesized through a simple liquid-liquid biphasic interfacial reaction route under the hydrothermal condition, in which Fe(acac)3was used as Fe source.(1) Durian-like a-Fe2O3precursor (the composition is a mix-phase of a-Fe2O3and α-FeOOH) hollow spheres constructed by polyhedral nanoparticles were successfully fabricated when NaOH was used as alkali source and coexisted with0.5g PVP in aqoeous solution. In the absence of PVP, only solid spheres were obtained. When prolonged the reaction time, the morphologies of precursors evolved from solid spheres to hierarchical core-shell structures and finally hierarchical hollow spheres. The Ostwald ripening mechanism was proposed to explain the formation of the hierarchical hollow spheres. Moreover, the precursors were finally transformed to a-Fe2O3by calcination at air and the products maintained its original morphology and size unchanged.(2) When NaOH was replaced by urea and the amount of PVP was1.0g, flowerlike a-Fe2O3hierarchical architectures consisted of many nanorods bundles were successfully synthesized via a one-step benzene-water biphasic interfacial reaction route. In the absence of PVP, the products consisted of a small number of nanorods and irregular nanoparticles. When the amount of PVP was no more than0.5g, the products are flowerlike structures constructed by nanorods. On the basis of a series of time-dependent experiments, a PVP-induced Ostwald ripening mechanism was proposed to explain the formation of such unique3D hierarchical structures. During the formation process of nanorod bundles building blocks, PVP molecules might serve as a potential crystal face inhibitor to promote the anisotropic growth of the nanorods along [001] direction by selectively adsorbed on some specific planes of a-Fe2O3crystal and effectively restrict the lateral growth along [110] direction. Because of the effect of the strong van der Waals attraction, these adsorbed PVP molecules on the side of the nanorods probably simultaneously serve as a linking agent that bridges the adjacent nanorods together and form side-by-side nanobundles along [001] direction.(3) When ammonia water was chosen as alkali source, cauliflower-like a-Fe2O3microstructures constructed by nanoparticle-based buds were successfully fabricated through a one-step toluene-water biphasic interfacial reaction route. Similarly,2.0g PVP also served as crystal face inhibitor and linking agent during the formation process of nanoparticle-based buds building blocks. The BET surface area and pore volume of the synthesized cauliflower-like a-Fe2O3were31.57m2g-1and0.134cm g-1, respectively. A bimodal distribution with a narrow distribution was centred at3.3nm and a wide distribution centred at41nm. 2. Monodispersed rhombic dodecahedral Fe3O4nanocrystals enclosed by{110} high-energy facets were successfully fabricated through a one-step toluene-water biphasic interfacial reaction route, in which Fe(acac)3was used as Fe source and hydrazine hydrate was simultaneously used as reducing agent and alkali source. Keeping the volume of oleic acid in toluene at1-2mL played a key role in the formation of monodispersed rhombic dodecahedral Fe3O4nanocrystals. Based on FT-IR analysis, C17H33COO-might be selectively adsorbed on the{110} facets of Fe3O4during the initial growth stage of Fe3O4crystal and consequently the growth rate along the [110] direction was retarded and the surface energy of the bounded {110} planes was reduced, which allowing them to survive during the process of crystal growth. The sizes of these prepared rhombic dodecahedral nanocrystals could be controlled in the range from60to100nm by adjusting the Fe(acac)3concentration in the range of0.025-0.035M and the volume of hydrazine hydrate in the range of10-15mL. The magnetic measurement revealed that the saturation magnetization (Ms) of the obtained rhombic dodecahedral Fe3O4nanocrystals was86emu/g at room temperature.3. Using titanium n-butoxide as Ti source and at a high volume ratio of hydrochloric acid to water of9:1, large quantities of ultrathin single-crystal anatase TiO2nanosheets with82%exposure of{001} facets were fabricated through a one-step toluene-water biphasic interfacial reaction route. These obtained anatase TiO2nanosheets have an average side length of-92nm and thickness of-10nm. The high concentration of HC1played dual roles:to retard hydrolysis of the titanium precursor and might serve as stabilizer to reduce the surface energy of high-energy{001} facets. It was revealed that the anatase TiO2nanosheets could only be prepared through the two-phase interface reaction (toluene, n-hexane and cyclohexane all can be used as organic phase solvents) at high HC1concentration (the volume ratios of hydrochloric acid to water no less than8.5:1.5) and the reaction time no less than5fours.4. When the cauliflower-like a-Fe2O3microstructures were used as adsorbents, they showed good reusability and excellent adsorption performances for organic dye and heavy metal ions in wastewater. The calculated maximal adsorption capacity were71.63mg g-1for Congo red,17.27mg g-1for Cr(VI) and32.54mg g-1for Pb(II), respectively. Under visible-light irradiation and in the presence of H2O2, the cauliflower-like a-Fe2O3microstructures also exhibited obvious visible-light photocatalytic activity, which may be attributed to the generation of highly oxidative hydroxyl radicals by Fenton reaction between Fe2+and H2O2. Nearly100%of rhodamine B (RhB) molecules were degraded for the prepared cauliflower-like a-Fe2O3while about40%of RhB molecules were degraded by the commercial a-Fe2O3powders within the same visible-light irradiation time.5. When used as peroxidase mimetics, these obtained rhombic dodecahedral Fe3O4nanocrystals showed enhanced peroxidase-like catalytic activity than spherical Fe3O4nanoparticles and commercial Fe3O4powders in the presence of H2O2and have very good recycling performance. The calculated relative activity per unit specific surface for tetramethylbenzidine (TMB) oxidation is2.3and7.6times of the spherical Fe3O4nanoparticles and commercial Fe3O4powders and the removal efficiency per unit specific surface for methylene blue (MB) degradation is5.6and5.9times of the spherical Fe3O4nanoparticles and commercial Fe3O4powders, respectively. When tested as anode materials for lithium-ion batteries, they delivered a high initial discharge capacity of1147mA h g-1at the current density of0.2C. After100cycles, a reversible discharge capacity of362mA h g-1was retained, higher than that of the commercial Fe3O4powders (118mA h g-1) under the same test conditions. At a higher rate of1C, a reversible capacity of191mA h g-1was delivered after more than130charge-discharge cycles. In addition, the rhombic dodecahedral Fe3O4nanocrystals electrode also manifested an enhaned rate capability than the commercial Fe3O4powders at current rates of0.2-4C, especially at high current rate.6. When tested as anode materials for lithium-ion batteries, these as-prepared ultrathin anatase TiO2nanosheets showed excellent cycling performance and capacity retention. At a rate of1C (1C=170mA h g-1), a reversible capacity of143.6mA h g-1was retained after100charge-discharge cycles. Even at a high current rate of10C, a reversible capacity of101.9mA h g-1could still be delivered, demonstrating a magnificent high-rate performance. Under the same test conditions, the discharge capacities of P25TiO2nanoparticles were only107.4and32.3mA h g-1after100cycles when the current rate were0.2and10C, respectively. They manifested an irreversible capacity loss of only12.5%and high Coulombic efficiency of above98%after the10th cycle in the subsequent cycles at1C. Furthermore, they also showed a good rate capability at current rates of0.2-10C. The excellent electrochemical properties can be attributed to the ultrathin feature along [001] direction and a high percentage of exposed{001} facets of the anatase TiO2nanosheets.