Study On The Low-Temperature Solution-based Synthesis And Formation Mechanism Of ZnS And ZnO Nanomaterials

Currently, the research of ZnS and ZnO namomaterials has already become one of the center subjects of semiconductor materials research areas. In the short space of a few years time, the research of ZnS and ZnO namomaterials has made remarkable progress. The method for preparation of ZnS and ZnO nanomaterials can be divided into physical methods and chemical methods, among which, the chemical solution method is favorable because it is simple, cheaper, and need no expensive apparatus. But the chemical process is complicated and the reaction mechanism is not well understood.This dissertation is interested in synthesis of ZnS and ZnO namomaterials via low-temperature chemical solution route, controlling the morphology of the products by adjusting the reaction conditions. Meanwhile, the growth process and formation mechanism of these inorganic crystals is another focus in our research.The dissertation is divided into four parts to describe our research:1. A mild and simple solvothermal technique has been developed to synthesize uniform nanowire bundles and aligned nanorods of wurtzite ZnS using hydrazine hydrate (N₂H₄·H₂O) as the solvent. The nanowires/nanorods in the bundle, growing along the [001] direction, are aligned not only in length direction but also in crystallography orientation. The phase- and shape-controlled synthesis has been realized by tuning the volume ratio of hydrazine hydrate and distilled water. The effect of reaction conditions on the phase and morphology of the resulting nanocrystals was investigated. Based on the experimental results, the formation mechanism and the role of the solvent were discussed. Meanwhile, the optical properties of the as-synthesized nanowire bundles and aligned nanorods of wurtzite ZnS were characterized by UV-vis absorption and Photoluminescence spectra. This solvothermal technique could be extended to access ZnS with other novel morphologies and other nanomaterials. The present work proves that II-VI semiconductor nanocrystals with controllable phase and morphology can be achieved via solvothermal route by selecting some appropriate solvent.2. Novel self-encapsulated ZnS micron core-shell spheres have been synthesized via a simple hydrothermal method. The formation process of self-encapsulated micron core-shell spheres was investigated. Based on the time-dependent experimental results, the possible formation mechanism was discussed. The as-synthesized self-encapsulated ZnS micron core-shell spheres were also characterized by Photoluminescence spectrum. This method may be used for preparing other materials with self-encapsulated core-shell morphology.3. A facile chemical solution route has been developed to synthesize ZnO structures with high hierarchy at room temperature. By controlling the reaction conditions, such as the feeding amount of NaOH and (or) reaction time, complex microflowers, column-like, spherical, and flower-like hierarchical structures can be selectively prepared at room temperature. Time-dependent experiments were conducted at the standard synthesis conditions of the complex microflowers. Based on the experimental results, a possible mechanism for the formation of ZnO complex microflowers and shape evolution with time were discussed. The optical properties of ZnO complex microflowers were characterized by room temperature Photoluminescence and Raman spectra. It is reasonable to expect that this facile room temperature route can easily be scaled up to prepare ZnO and other materials with various unusual structures for important applications in the large field of nanotechologies.4. Novel ZnO three-dimension hierarchical structures have been synthesized via a simple hydrothermal route. On each assembly, there are a number of uniform highly density nanorods radially grown as multiple rows in a parallel manner, which is different from that the elongated building blocks grow radially from a center. The growth process was followed and a series of parallel experiments were performed to study the formation mechanism and control factors of this unique ZnO hierarchical structures. Meanwhile, the as-synthesized ZnO three-dimension hierarchical structures were also characterized by Photoluminescence spectrum.