ZnO Nanomaterials: Hydrothermal/Solvothermal Preparation And Properties Study

ZnO is a direct wide band gap semiconductor, which has band gap of 3.37 eV and a large binding energy of exciton (60 meV). Because of its diverse and unique semiconducting, piezoelectric, and pyroelectric properties, ZnO, especially ZnO nanomaterials possess unique basic research and applied values in various photoelectric device areas, such as UV-emission devices, field-emission devices, nanogenerators, and solar cells. In this thesis, we based on the controllable chemical synthesize for the size, morphology and surface property of ZnO nanomaterials, mainly studied its size, morphology, surface SiO₂ passivation treatment and surface photochemical reaction dependent properties, that is photoluminescence and surface wettability. Some valuable and original research results were acquired. The details are as following:Uniform ZnO particles with ellipsoidal, needle-like and branched morphologies were prepared by a hexamethylenediamine (HMDA)-assisted hydrothermal method. The formation mechanism of the particles controlled by HMDA were further analyzed, and the morphology-dependent photoluminescent properties of ZnO particles were also studied detailedly. Results indicated that the molar ratio of HMDA/Zn was the key factor to controll the particle morphologies. The branched particles, which prepared at high molar ratio of HMDA/Zn, showing the most intense near-band-edge ultraviolet photoluminescence and the greatest ratio of ultraviolet to visible photoluminescence comparing to other two morphologies particles. This can be well explained by considering the roles of the HMDA in the growth process of ZnO particles. The studies indicated that the ZnO nanomaterials with well ultraviolet emission properties can be prepared through rational controlled the chemical parameters in solution.Uniform and monodispersed ZnO microspheres were successfully prepared through a facile solvothermal method. The formation mechanism of ZnO microspheres in the solvothermal system was studied in-depth, and the surface organic passivation-dependent photoluminescent properties of ZnO microspheres were also studied detailedly. Results indicated that the ethylene glycol solvent with high viscosity and active chelation was the key factor to induce the isotropy self-assemble growth of ZnO nanocrystals. The microspheres exhibitted weak surface defects-dependent visible emission. This can be well explained by the effective passivation of organic materials for the surface defects of ZnO. The studies indicated that the visible emission of ZnO nanomaterials can be effectively controlled through the surface passivation treatment for ZnO.The ZnO@SiO₂ core-shell nanowire arrays were succefully prepared on the glass substrate by combing the hydrothermal method and bioinspired layer-by-layer deposition technique. The tunable growth of SiO₂ shell prepared by layer-by-layer deposition technique was analyzed detailedly, and the surface/interface-dependent photoluminescent properties of core-shell nanowires were also studied further. Results indicated that the thickness of SiO₂ shell can be conveniently controlled by the number of deposition cycles. Comparing with the bare ZnO nanowires, the ultraviolet photoluminescence of ZnO@SiO₂ core-shell nanowire arrays was obvious enhanced and its temperature dependence of ultraviolet emission intensities showed an anomalous behavior. This can be well explained by the influence of SiO₂ shell for the defects existing in the surface/interface of ZnO nanowire array. The studies indicated that the bioinspired layer-by-layer deposition technique is an effective method for the formation of SiO₂ shell on the surface of highly curved ZnO nanowires with shell thickness being well controlled, and enhanced ultraviolet emission can be acquired through the surface SiO₂ passivation treatment in composite ZnO nanomaterials.The ZnO@SiO₂ nanowire arrays were treated with hydrophobic modification and their surface wettabilities have been studied. Results indicated that under the long-term ultraviolet irradiation, different from the photo-induced superhydrophilicity of bare ZnO nanowires, the ZnO@SiO₂ nanowire arrays exhibited durable superhydrophobicity. This can be well explained by the blocking effect of SiO₂ shell on the surface photochemical reactions of ZnO nanowires. Meanwhile, this core-shell nanowire array was also applied on the surface of cotton textile, and obtained a multifunctional textile which exhibited excellent ultraviolet-blocking property together with ultraviolet-durable superhydrophobicity.