Controllable Synthesis, Mechanism And Properties Of ZnSe Micro/Nano-Materials

The micro/nano-materials with unique electrical, optical and mechanical properties have the potential applications, including, photonics, electronics, physics, and chemistry. However, the properties of inorganic micro/nano-materials are dependent on their composition, size, shape and crystalline structure. Investigation on the ralationships among these features has attracted increasing attentions in modern science and technology. In this thesis, we have systematically studied and explored the synthesis, the formation mechanisms and the optical property of ZnSe polyhedron shaped nanocrystals and ZnSe microspheres (ZnSe solid and hollow microspheres).1. ZnSe faceted shaped nanocrystals:Large-scaled and monodisperse ZnSe faceted shaped nanocrystals (including cubes, octahedron, and other polyhedron structures) have been prepared via using PVP as the surfactant under a hydrothermal condition.ZnSe microspheres have been prepared without monodisperse nanocrystals by changing the different surfactants, such as, CTAB, PEG 400, and the other hydrothermal reaction conditions are kept constant. PVP is beneficial to improve the dispersity of the ZnSe nanocrystals and holds back their agglomeration into the microspheres. The monodisperse nanocrystals turn into block structure, which is mainly depended on the heating treatment temperature, but the phase composition is not changed.A possible mechanism for the formation of ZnSe nanocrystals has been proposed from nucleation and particle growth, nano-crystalline dispersion theory, a crystal with cubic structure, e.g., here, zinc-blende cubic ZnSe, has highest crystal symmetry. Thus, it has a growth habit to grow into spherical particles and agglomerate many nanocrystals into microspheres. So, the ligands are believed to be strong enough to effectively wrap up as-formed nanocrystals, producing these monodisperse nanocrystals rather than the microspheres by electric double layer, the steric and electrostatic hindrance effect.2. ZnSe microspheres self-assembled from ZnSe polyhedron shaped nanocrystals are prepared by a simple hydrothermal route without any surfactant. The morphologies and the purity of the ZnSe microspheres can be tuned by changing the reaction temperature and time, reaction volume, reducing agent, PH, the Se and Zn sources and the annealing temperature. The results are summarized as follows:(a) The surface roughness of the microspheres can be controlled by tuning the reaction temperature. With the reaction temperature increasing, the size of the nanocrystals is getting larger and the formation of the well-defined faceted (cubes, hexagons, and pentagons) ZnSe nanocrystals is obtained. (b) The reaction time not only affects the surface roughness of the microspheres, but also changes the size and the purity of the products. The longer the reaction time, the larger, the more regular and uniform microspheres are obtained; the impurities with reaction time gradually decreased until them disappeared. (c) When 50% or 60% of the maximum capacity of the autoclave used was employed in our experiment, most of the products display irregular shape, and only a small amount of them are microspheres. (d) The reducing agent has effects on the morphology of the secondary structural unit. When NaBH4, ethylene glycol, ethylenediamine and triethylamine are selected as a reducing agents, the morphology of ZnSe products obtained that include ZnSe microspheres assembled from nanoplates, nanoflakes; flower-like shape assembled from nanoplates; irregular block structure, respectively. In addition, when oleyl amine is used as the surfactant, a large quantity of the ZnSe hollow microspheres assembled from faceted shaped nanocrystals is produced. (e) The amount of NaOH (PH value) mainly affects the morphology of the product, the morphology of ZnSe microspheres becomes more regular and the the amount of NaOH is increasing. (f) The morphology and uniformity of ZnSe microspheres can be controlled by using different Zn and Se sources, respectively. (g) The morphology of the products isn't depended on heating treatment temperature (600℃).The PL spectra from the ZnSe microspheres show that three emission bands at around 467 nm,470 nm and 600 nm, respectively, and the light-emitting potential of ZnSe microspheres usually resulted from the edge emission or impurities. The efficiency of the photocatalytic degradation of methyl blue is investigated; the degradation ratio of the samples reaches 86.5% after 25 min.The possible mechanisms for the formation of ZnSe nanocrystals self-assembly into microspheres has been proposed:(1) With the ZnSe nanocrystals' continuous growing, their high surface energy results in the self-assembling and agglomerating of many ZnSe nanocrystals into microspheres, with a concomitant decrease of the system free energy. (2) A few of these ZnSe microspheres underwent agglomeration made of individual microspheres, forming a short chain of ZnSe microspheres. We hold the opinion that these ZnSe microspheres' agglomeration may be explained as follows:firstly, the primary particles are obtained via the nucleation and growth through the Ostwald-ripening process; secondly, the primary particles aggregate to produce the adjacent of primary ZnSe particles under Brownian movement and Van der Waals force. These primary particles could work as gathering center for adsorption of other nanoparticles to simultaneously form microspheres, which are attached each other, forming a chain of ZnSe microspheres. (3) If oleyl amine was introduced the solution system, it can combine with as-formed N2, resulting in the formation of many bubble-oil-water layer-by-layer wrapped up structures in the solution system. These structures which can act as a composite and layered "soft templates", are stable in such a solution and could work as gathering center for adsorption of other nanoparticles to form microspheres. In addition, the Ostwald ripening mechanism may also affect the formation of these ZnSe hollow microspheres through the present hydrothermal treatment. During this process, the nanocrystals located in the central part of the initial colloids are smaller or less dense than those from the outer parts. Through the Ostwald-ripening process, the growth of those large crystals from the outer part is believed to be faster and easier than that of those in the central which have a higher solubility, and thus the nanocrystals in the central of the microspheres dissolve and hollow microspheres are prepared.