Gas-Phase Synthesis And Chracterization Of Zinc Oxide Micro/Nanomaterials

ZnO, a direct wide bandgap material, exhibits both advanced photoelectric andpiezoelectric properties. As a versatile and smart functional material, ZnO micro/nanomaterialhas a diverse group of growth morphologies including nanowires, nanotubes, nanobelts,hollow spheres and assemblies of these structures, which dominate their applications directly.So, to clearly understand the growth of these ZnO nanomaterials, and to scrutinize theirstructures and properties, are crucial things for achieving their extended applications anddesigning novel functional materials.In this dissertation, various ZnO micro/nanostructures were synthesized by the gas-phaseprocess, and the growth mechanisms and some basic physichemical properties of as-preparedZnO micro/nanostructures were discussed and investigated. In synthesis process, not only wasthe ZnO micro/nanostructures growth investigated in thermodynamic equilibrium system, butalso the effects of the relative growth speed of various facets for ZnO micro/nanostructuregrowth were studied by controlling the processing parameters including evaporation anddeposition temperatures, pressure, working gas, catalysts, resources, substrates, evaporationtime period and carrier gas flux. It has been found that the most common polar surface is thebasal plane of wurtzite zinc oxide. Another two most commonly observed facets for ZnO are{2-1-10} and {01-10}, which are non-polar surfaces and have lower energy than the (0001)facet. The relative change of the surface activity of these growth facets under given conditionsresults in the growth of various ZnO micro/nanostructures.In zinc vapor deposition process, hollow-core ZnO materials, ZnO nanowire/rods and ZnOmultipods were grown in turn with the increase of oxygen partial pressure: (1) In the case ofoxygen partial pressure less than 0.1%, hollow-core ZnO materials were fabricated. Under thein-situ surface oxidation and core evaporation conditions, ZnO shells with differentmorphologies including cages, hollow spheres, and assembly of nanorods into hollow sphereshave been achieved by tuning partial pressure of oxygen, temperature, and working gas flux.The results show that the photoluminisence (PL) spectra of various ZnO shells are closelycorresponding to their shapes at room temperature. Under the vapor transport andcondensation conditions, Zn/ZnO nanocables were grown firstly, and then ZnO nanotubeswere formed by evaporating Zn cores from these nanocables. The growth direction of ZnOnanotubes can be any direction of <2-1-10>, <01-10> and 1-0001] directions. (2) While oxygen partial pressure was in the range of 1-4%, ZnO nanowire/rods with and without wellorientation were the favored ones to grow. It has been found that the different pyrolysisprocesses of fluororesin are the effective methods for tuning the shapes of ZnO nanostructurearrays in this process. By this way, hexagonal-rod, nanoneedle, nanonail, umbralla-likenanostructure and microtube arrays have been fabricated on silicon substrates. The resultsshow that the growth direction of these aligned nanorods is [0001] direction, and their PLproperties depend on their shapes. (3) While the oxygen partial pressure more than 4%, ZnOmultipods were fabricated. As the oxygen partial pressure up to 8%, high-purity ZnOtetrapods (T-ZnO) have been synthesized in large-scale. It has been found that organicmaterials and other foreign materials play leading roles for initiating the growth of multi-armZnO and changing the shape of its arm. In common, the growth of T-ZnO follows the routeincluding the formation of nucleius and the arm epitaxy of T-ZnO along [0002] direction. Inhomogeneous nucleation system, a growth species with Zn-blende structure should be createdbefore the arm epitaxy of T-ZnO. In heterogeneous nucleation system, however, theequilibrium of octagonal geometry would be destroyed, resulting in various ZnO multipods.In carbothermal reduction process, ZnO nanotubes, ZnO nanowires/nanorods and ZnOnanobelts were grown without and with the appearance of oxygen or catalyst, respectively: (1)In a homogeneous nucleation system, Zn/ZnO nanocables were the main products, and ZnOnanotubes were formed by evaporating Zn cores from these nanocables. (2) While ca. 1-2vol.% oxygen appeared in reactor, ZnO nanorod/wires with and without well orientation weregrown following vapor-liquid-solid (VLS) and vapor-solid (VS) mechanisms, respectively.Generally, ZnO nanorods grow along the [0001] direction following the VS mechanism,whereas, the growth direction of nanowires can be changed into <2-1-10> or <01-10>direction with the aid of catalyst. (3) With the assistance of catalysts themselves, nanobeltswere synthesized directly in this process, in which some novel catalysts including CuO, SnO₂,FeAc₂, CoAc₂, ZnAc₂, NiAc₂, ZnC₂O₄ and CUC₂O₄ were testified that they all have highselectivity for growing ZnO nanobelts. Well-aligned ZnO nanobelts have been fabricated onSi wafer with a coating of SnO₂ nanoparticles or CUC₂O₄. At the high deposition temperature,the nanobelts with catalyst particles at their tips can be grown along any direction of <2-1-10>,<01-10> and [0001]. At the low deposition temperature, however, nanobelts with catalystparticles at their bottoms always grow along [0002] direction. It is striking to note thatcomb-like ZnO crystals can be grown by the assistance of catalyst and/or inductive. Thegrowth direction of comb-like ZnO crystals at the low deposition temperature is various,whereas, the <2-1-10> and <01-10> directions are the main growth directions at the highdeposition temperature. In addition, hybrid nanostructures of ZnO/C and ZnO/Cu have been fabricated by a simpletwo-step method. It should be noted that the carbon fibre is the favored ones as substrate togrow the aligned ZnO nanomaterials, and the assistance of catalyst must be needed ingrowing the ZnO nanostructures on carbon nanotubes (CNTs) directly. It has been evidencedthat the temperature dominates the growth of hybrid ZnO/Cu arrays.