Chapter 1 Introduction to Nanostructured Zinc Oxide
1.5 Motivation behind Our Work
Thanks to its unique properties, substantial effort has been devoted to the development of fabrication and application of nanostructured zinc oxide such as ZnO nanorods, nanowires, nanobelts, and nanotubes. Currently, vapor-phase deposition techniques are the most popular ones for the growth of high-quality ZnO nanorod arrays at high temperatures. The simple physical-vapor-deposition technique generally requires economically prohibitive high temperature of > 800 oC, and the complex chemical-vapor-deposition technique involves expensive substrates, sophisticated equipments and rigorous experimental conditions though the organometallic zinc precursors used can reduce the reaction temperature to 400 oC. Recently, liquid-phase preparation of high-quality ZnO nanorod arrays at low temperatures (e.g. 90-95 oC) has also been achieved through two-step wet- chemical processes including the initial coating of ZnO seed particles on substrates and the subsequent growth of ZnO nanorods through the thermal decomposition of Zn-amide complexes in aqueous solutions. However, it is still a challenge to develop a simple, mild and practical strategy for the low-cost and large-scale/area fabrication of high-quality ZnO nanorod arrays that is in great demand for promising applications.
In this work, we have developed a novel liquid-phase method for fabricating well-aligned ZnO nanorod arrays on arbitrary substrates such as zinc substrates and ZnO film-coated substrates (e.g. glass, silicon, and polymer). This novel synthetic approach also allows further reducing the growth temperature to 65 oC, leading to an effective and low-cost fabrication process for high-quality ZnO nanorod arrays. Simple low-temperature strategy
has been developed for the low-cost and large-area fabrication of ZnO nanorod arrays on zinc substrates through the natural oxidation process of zinc metal in formamide aqueous solution. The one-step wet-chemical approach has exhibited a well-controlled growth of highly oriented and densely packed ZnO nanorod arrays with large-area homogeneity and predictable morphologies such as tunable diameters and identical lengths of nanowires or nanorods. The novel chemical-liquid-deposition process as an analogue of the widely used chemical-vapor-deposition technique has been demonstrated for the near room- temperature production of ZnO nanorod arrays through continuous supply, transport, and thermal decomposition of zinc complexes in liquid phase. Moreover, a simple synthetic procedure for preparing dense arrays of single-crystalline ZnO nanorods on ZnO film- coated substrates (e.g. glass, silicon, and polymer) has also been developed by a soft solution method without the use of metal catalyst. The growth of ZnO nanorods is controllable with designed patterning and a new two-step growth mechanism was proposed by studying the growth kinetics of the ZnO nanorods. This substrate- independent preparation of ZnO nanorod arrays on patterned substrates enables a wide variety of potential applications in electronic and optoelectronic fields. Additionally, large-scale dense arrays of well-aligned hexagonal ZnO nanotubes were grown on zinc foils used in this reaction solution and a competitive growth mechanism is proposed.
This thesis is organized into five chapters. We give a brief introduction and literature review of nanostructured zinc oxide in Chapter 1, followed by experimental section in Chapter 2. In Chapter 3, we present near room-temperature production of diameter-tunable ZnO nanorod arrays through natural oxidation of metal zinc. In Chapter 4, we demonstrate
nanorod/nanotube arrays. The obtained nanoproducts were characterized by means of XRD, SEM, and TEM, and their growth mechanisms were further proposed. In the last chapter, we give an overall conclusion and deliver some issues for future work.
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