Hydrothermal Synthesis Of Zinc Oxide Nanostructures And Their Applications

Various morphologies of Zinc oxide (ZnO) nanostructure are prepared by hydrothermal reaction in ammonia and zinc chloride solutions. The applications of these ZnO nanostructures for field emitter, glucose biosensor, gas sensor and ultra-violet (UV) switch are investigated. This thesis contains the following works and innovations:1. The ZnO nanowires, nanorods, nanodisk, nanoneedles and nanotubes are synthesized by changing the solution concentration, reaction time, substrate, pH value and growth temperature, and the growth mechanism of these ZnO nanostructures is discussed.2. The growth mechanism of tubular ZnO in solution is investigated in detail.1) We synthesize seven samples at reaction times of 20 min to 9 hours and different temperatures of 95 ℃ to RT by a simple hydrothermal decomposition method carried out in a mixture of ammonia and ZnCl₂ solution. The SEM images of seven samples prepared at different stages clearly elucidate the evolution of tubular ZnO fabricated.2) We show that ZnO tubular structure is originated from an aging process of ZnO rod at a lower temperature (< 75 ℃) and in a shorter time (about 7 hours) due to the preferential chemical dissolution of the metastable Zn-riched (0001) polar surfaces.3) A two-stage growth model is proposed based on the coexistence of the hydrothermal deposition and dissolution of ZnO in the fabricating process. The first growth stage corresponds to the precipitation of ZnO from Zn(NH₃)₄²⁺ at higher temperature. The second stage is an etching process, which happens when the precursor Zn(NH₃)₄²⁺ is depleted at high temperature or the hydrothermaldecomposition is ceased at low temperature.3. ZnO nanotube arrays are prepared by hydrothermal reaction in ammonia and zinc chloride solutions, and the field emission properties are tested. The turn-on field of the field emission is extrapolated to be about 7.0 V/μm at a current density of 0.1 μA/cm². Meanwhile, the emission current densities reach 1 mA/cm² at a bias field of 17.8 V/μm. The field enhancement factor β is estimated to be 910. The field emission of the ZnO nanotubes show good stability. The variation of emission current density is less than 10 % during a 24-hour test under a field of 15 V/μm. The stable FE behavior is attributed to the uniform height of the vertical aligned nanotube arrays.Our ZnO nanotube emitter is comparable to other nanostructured ZnO emitters. Although with a slightly higher turn-on field, the FE stability of our hydrothermal ZnO tubes is better than any other ZnO nanostructures reported in the literatures.In addition, the field emission of the co-axis ZnO nanorods prepared by hydrothermal reaction is also investigated.4. A glucose biosensor is fabricated by immobilizing the glucose oxidase (GO_x) on ZnO nanorods array grown by hydrothermal decomposition. In a PBS buffer with a pH value of 7.4, negatively charged GO_x is immobilized on positively charged ZnO nanorods through electrostatic interaction. At an applied potential of +0.8 V vs. Ag/AgCl reference electrode, ZnO nanorods based biosensor presents a high and reproducible sensitivity of 23.1 μA cm^-2 mM^-1 with a response time of less than 5 s. The biosensor shows a linear response range from 0.01 mM to 3.45 mM and an experiment limit of detection of 0.01 mM. An apparent Michaelis-Menten constant of 2.9 mM shows a high affinity between glucose and GO_x immobilized on ZnO nanorods. Our results demonstrate that ZnO nanorod sample can provide a favorable microenvironment for GO_x loading and stabilize its biological activity due to its large surface area, biomimetic and high electron communication features.5. Gas sensors based on ZnO nanorods grown by hydrothermal decomposition are fabricated. The sensor shows high sensitivity to H₂, NH₃, CO, NO, etc, especially the ideal sensitivity to NH3 at room temperature. This work is still in progress.6. The conductivity of the UV switch based on ZnO nanorods grown by hydrothermal decomposition is investigated. The dependence of the conductivity of ZnO nanorods on UV power is investigated in detail.