Controllable Synthesis, Characterization and Properties of Zinc-based Micro/nanometer Superstructures
|Keywords||Superstructures Zn-based materials controllable synthesis hydrothermal method assembly properties|
The properties of inorganic micro/nano-superstructures are dependent on their composition, size, shape and crystalline structure. Investigation on the relationships among these features has attracted increasing attentions in modern science and technology. Zn-based materials superstructures are important for the potential applications, including, photonics, electronics, physics, and chemistry. In this thesis, we have systematically studied and explored the controllable synthesis, assembly, and properties of Zn-based materials superstructures, which include ZnO, Zn silicates, Ag/ZnO and Cu/ZnO composite materials.1. ZnO 3-dimensiaonal (3D) superstructures with controlled morphology and size, including flower-like, star-like, sphere-like and sea urchin-like shapes, are fabricated by a simple hydrothermal method without any surfactant or template. The gas sensing examinations show that the different Zn-based superstructures have different sensitivity to ethanol, and the sensitivity of the radial-like ZnO 3D flower-like superstructures to ethanol can reach 11.3.The films made of the ZnO 3D flower-like superstructures on the glass are fabricated by a simple hydrothermal method using the surfactant, and the surface roughness of the ZnO 3D superstructure films can be conveniently controlled by changing the different surfactants and dosage of Zn sources, with excellent reproducibility. The photoluminescence (PL) spectra of the ZnO 3D superstructure films show that their PL properties are dependent on the surface roughness of as-synthesized ZnO superstructure films.2. Zn4Si2O7(OH)2·H2O (ZSO) 3D hollow flower-like hierarchical superstructure films deposited on the Si substrate are firstly prepared by a simple hydrothermal route. Each hollow superstructure is consisted of secondary rod bundles which are assembled by many primary rods. The morphologies of the ZSO 3D superstructures can be tuned by changing the concentration of reactant, reaction time and temperature, surfactant, the Zn sources and annealing temperature. The results are summarized as follows:(a) The morphology of the secondary rod bundles within the ZSO 3D superstructures can be conveniently controlled from micrometer-sized prism to sub-micrometer slice, sub-micrometer rods and nanorods by only tuning the concentration of Zn2+. (b) The morphology and structure of the ZSO 3D superstructures are related to the reaction time; with the temperature increasing, the rod size of the bundles is getting smaller and the molar ratio of the ZSO:ZnO is increasing. (c) The reaction temperature not only affects the shape of the superstructures, but also changes their compositions. The higher the temperature, the more remarkable is the hierarchical feature of the secondary rod bundle, and the smaller is the size of the primary rods in the secondary bundles, and the higher is the molar ratio of the ZSO:ZnO within a superstructure. (d) The surfactant has effects on the morphology of the secondary rod bundles within the ZSO hierarchical superstructures; it realizes further hierarchy of the secondary bundles as well as their transformation to single non-hierarchical secondary rod. (e) The hierarchical structures of the secondary rod bundles, which realize the transformation from their united secondary bundles to a single secondary micrometer-sized rod, or to a single secondary micro- or nanometer-sized rod with its diameter varying, can be controlled by using different Zn sources. (f) The crystal structure is mainly depended on heating treatment temperature; below 500℃, the crystal structure of the ZSO is not changed, and at 950℃, orthorhombic ZSO superstructure can be transferred into monoclinic Zn2SiO4.The PL spectra from the orthorhombic ZSO 3D superstructures films show that the stronger the intensity of the PL spectra, the smaller is the diameter of primary rod in secondary rod bundles, and the bigger is the ZSO surface roughness. The investigation on the wettability of the ZSO superstructures films shows that the angle of contact (CA) is mostly relied on the surface shape, and the CA of the ZSO superstructure films varying with their surface shape. It is found that the CA of the ZSO superstructure films consisted of the secondary rod bundles is the highest and reach to 143.1.3. Ag/ZnO 3D superstructures are prepared by the dip coating process, using as-synthesized ZnO 3D supperstructures and AgN03 as the source materials; these composite superstructures show excellent gas sensitivity to methyl alcohol. Ag+ on the surface of the ZnO submicrorods exits in a form of AgNO3 or Ag2O after being dried at～80℃. Annealing at 300℃, AgNO3 or Ag2O is transformed into metal Ag (nanoparticles) and the rectangular holes form on the surface of the ZnO rods, suggesting that the surface of the ZnO submicrorods can be etched by metal Ag nanoparticles. The etching is enhanced with the increase of the concentration of Ag+ solution and reaction time, and it is also dependent on the heating treatment temperature. The Ag/ZnO 3D superstructures by annealing at 300℃can improve their selectivity to methyl alcohol, and their sensitivity is up to 9.3.The Cu/ZnO 3D flower-like superstructures with excellent photocatalysis property can be fabricated by doping method. These morphologies of the Cu/ZnO 3D superstructures can be varied by tuning the reactant concentration. When the content of Cu2+ is below 5%, the Cu2+ mainly change the amount and shape of the ZnO rods inside the ZnO 3D superstructure; the doping of Cu2+ leads to the increase of the rods in ZnO 3D superstructure, companying the formation of the step-like shape at the rod top. When the molar ratio of Zn2+: Cu2+ is 7:1, Cu2+ is loaded on the surface of ZnO rods, forming CuO/ZnO core-shell rods. When the molar ratio of Zn2+:Cu2+ is 3:1, the mixture of the CuO and ZnO flower-like rods is obtained. When the molar ratio of Zn2+:Cu2+ is 3:4, the Zn doped CuO urchin-like superstructures are fabricated. The PL intensity of the Cu/ZnO 3D superstructures will decrease with the increasing doping of Cu2+. The Cu/ZnO 3D structures fabricated with the molar ratio of Zn2+ Cu2+ of 7:1 exhibit a higher photocatalytic activity. The efficiencies of the photocatalytic degradation of methyl orange and direct shy blue 5B are investigated; the degradation ratio of the samples reaches 84.7% and 77.6%, respectively.4. In situ electron-beam irradiation in a transmission electron microscope is performed to fabricate different shape, controllable size, monodispersed, and high pure Zn nanoparticles by using Zn powders as the source material. As-synthesized Zn nanocrystals display various regular geometrical shapes, including rectangle, rhombus, triangle, and hexagon, with a diameter of 10-30 nm. During the formation of Zn nanoparticles, a convergent electron beam is focused on an raw Zn particle, and then cause partial melting and evaporation of the particle and subsequent nucleation and growth of Zn nanoparticles on the C film; the higher the dose of the electron beam, the smaller the diameter of the electron beam, the smaller is the diameters of Zn nanoparticles; while the smaller particles are merged each other under a longer period of the irradiation, resulting in the particles’growth.