| Complex structures, such as hollow,3D hierarchical, Janus and patchy structures, usually endow micro-/nanomaterials with unique and superior properties. For instance, hollow structured micro-/nanomaterials usually have high surface areas, abundant cavities and controllable micro-physiochemical environments.3D hierarchical micro-/nano-materials show high surface areas and porosity. micro-/nanomaterials with Janus and patchy structures have anisotropic physiochemical properties and heterojunctions. Hence, they are promising candidates in applications including catalysis, absorbents, micro-/nanodevices, lithium batteries, biomaterials and sensors. However, the lacking of general, facile and large-scale fabrication techniques for the preparation of micro-/nanomaterials with complex structures is still a great challenge, and thus seriously hinders their applications. In this thesis, we devoted to the development of advanced techniques for the fabrication of metal oxides with complex micro-/nano-structures, such as hollow,3D hierarchical, Janus or patchy structures, and studied their magnetic, photocatalytic and adsorption properties, as well as the relationship between the properties and their complex structures. The main results are given below:In the first part of this thesis, we have established a non-equilibrium heat-treatment approach for the fabrication of metal oxides with different levels of hollow structures, including α-Fe2O3,Fe3O,BaFe12O19,γ-Fe2O3,CoFe2O4and NiFe2O4single-wall, core-shell, double-wall, even double-wall core-shell hollow fibers and particles. In this approach, we have also proposed two novel mechanisms, including the heterogeneous contraction and in-situ generated dense shell-engaged Ostwald ripening, to explain the formation of various hollow structures. In the heterogeneous contraction mechanism, the heating rate (R) in the non-equilibrium heat-treatment can be easily utilized to tune the temperature gradient established in the inner of the fibers and the difference between the cohesive force and the adhesive force at the interface layer between the inner gel and the dense rigid shell in-situ generated by high R. Therefore, the direction of the contraction of the inner gel precursor and the levels of the final hollow structures of metal oxides are realized to control. In the in-situ generated dense shell-engaged Ostwald ripening mechanism, the dense shells generated in-situ during the short-time non-equilibrium pre-treatment procedure direct the outward Ostwald ripening of inner metal oxide nanocrystals, and sequentially create a hollow interior. The non-equilibrium heat-treatment approach based on heterogeneous contraction mechanism is a general approach for the fabrication of hollow micro-/nanomaterials. They can be easily extended to the preparation of other metal oxides, and even metal sulfides with different levels of hollow structures.In the second part of this thesis, on a basis of the simple side-by-side co-electrospray procedure with a subsequent non-equilibrium calcination process, we have for the first time developed an asymmetric shrinkage approach to the fabrication of magnetic y-Fe2O3/TiO2Janus hollow bowls (JHBs) by constructing a precursor solution pair with different gelation rates during the solvents evaporation process. In this approach, the phase with higher gelation rate in the electrospraying Janus droplet firstly gelated into a rigid hemispherical shell, and acted as a framework of the asymmetric contraction of another liquid phase during the solvent evaporation, resulting in a bowl-shaped gel Janus particles. The gel Janus particles, after calcination in the the non-equilibrium heat-treatment procedure, were transformed into the y-Fe2O3/TiO2Janus hollow bowls with both anisotropic morphology and chemical composition. The as-obtained y-Fe2O3/TiO2JHBs have a transition layer of Fe3+-doped-TiO2between the y-Fe2O3and TiO2phases, and show an efficient visible-light photocatalytic activity as well as convenient magnetic separation for water purification.In the third part of this thesis, we have for the first time demonstrated the use of an oppositely-charged twin-head electrospray to fabricate Janus and patchy particles. Various Janus and patchy particles, such as pot-like and snowman-like TiO2/CeO2and TiO2/CdS Janus particles, and TiO2/Ag spherical and bowl-shaped patchy particles, have been successfully fabricated by the proposed method, respectively. In this method, we modulated the collision and coagulation process of the oppositely charged electrospraying droplets to build Janus and patchy particles. By changing the precursor pairs and adjusting the state of opposite charged electrospraying droplets at the collision, we not only can alter their chemical compositions pair into metal oxide/metal oxide, metal oxide/metal sulphide, metal oxide/noble metal, but also control the structures of the particles into the pot-like Janus, snowman-like solid Janus, snowman-like bowl-hollow Janus, spherical patchy, and bowl-shaped patchy structures. Because of the heterogeneous interfaces and morphological and structural characteristics, the as-obtained Janus and patchy particles may have a number of applications including catalytic oxidation of toxic gas, photocatalytic hydrogen production and degradation of organic pollutants in water.In the forth part of this thesis, by using γ-Fe2O3core-hollow shell spheres as substrates, we have proposed a transition defects induced neighboring growth mechanism to build3D γ-Fe2O3/SnO2patchy hierarchical core-in-hollow shell nanostructures (γ-Fe2O3/SnO2PHCNs) in a hydrothermal reaction. In this mechanism, the initial growth of SnO2nanorod on γ-Fe2O3core-in-hollow spheres leads to the formation of high-energy defects at the neighboring area of the interface of SnO2and γ-Fe2O3. The subsequent nucleation and growth of newly formed SnO2is selectively occurred on these high-energy defects, named as neighboring growth. The neighboring growth of SnO2nanorods causes the further expansion of interfacial defects. When the expansion of interfacial defects and neighboring growth of SnO2nanorods are continuously duplicated, the γ-Fe2O3/SnO2PHCNs are finally generated. Thanks to the efficient separation of visible-light induced electron-hole pairs at the γ-Fe2O3/SnO2heteroj unctions and the multireflection of incident light inside of hollow interior, the as-obtained γ-Fe2O3/SnO2PHCNs show an efficient visible-light photocatalytic activity.In the fifth part of this thesis, we developed an one-step template-free method, in which the nucleation, aggregation and growth of the product is mediated by the solvent decomposition, to fabricate iron oxide (Fe2O3) chestnut-like amorphous-core/γ-phase-shell hierarchical nanostructures (CAHNs). The as-obtained Fe2O3CAHNs show a strong adsorption capability for As(Ⅴ) with a maximum adsorption capacity of137.5mg/g attributing to both their high specific surface area and the heterogeneous surface properties. Furthermore, the ferromagnetic property makes them to be easily separated from water by magnetic separation. The adsorption process obeys well the Freundlich isotherm model rather than the Langmuir one, suggesting that a multilayered adsorption occurs on the surface of the Fe2O3CAHNs. Our work may shed light on the design and preparation of high performance3D hierarchically nanostructured adsorbents. Then, we calcined the as-obtained Fe2O3CAHNs at different temperatures in N2atmosphere to generate y-Fe2O3chestnut-like hierarchical nanostructures (CHNs) and Fe3O4CHNs in order to enhanced their magnetic response. The magnetic hysteresis loops measurements show that the y-Fe2O3CHNs and Fe3O4CHNs have a higher Ms comparing to the precursor of Fe3O4CHNs, suggesting the quick response to the applied magnetic field in their magnetic separation process. In addition, the y-Fe2O3CHNs maintain69%of the As(V) adsorption capacity (94.5mg/g) comparing to Fe2O3CAHNs. Hence, the y-Fe2O3CHNs can also be regarded as an ideal absorbent for As(V) removal attributing to their relatively high As(V) adsorption capacity and quick magnetic separation.
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