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This dissertation, |Synthesis of One-dimensional Tungsten Oxide Nanostructures| by 洪昆權, Kunquan, Hong, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled SYNTHESIS OF ONE-DIMENSIONAL TUNGSTEN OXIDE NANOSTRUCTURES Submitted by Hong Kunquan for the degree of Doctor of Philosophy at The University of Hong Kong in January 2008 Tungsten oxide has been used for various optical devices, sensors and so forth for its outstanding electrochromic, optochromic, gaschromic and electronic properties. One-dimensional (1D) tungsten oxide nanostructures are expected to improve the performance of those devices as well as to open up new applications. o Generally, high temperature (1200-2000 C) and special equipments are required in the synthesis of these tungsten oxide nanostructures, which can be obstacles for a wider application. In this thesis, some new methods have been developed to synthesize 1D tungsten oxide nanostructures. The relevant growth mechanisms were also investigated. Firstly, a simple method was devised to grow tungsten oxide nanostructures by thermal evaporation at low temperature and yet without using catalyst. By o heating tungsten powder at 650 C, and tungsten plate or tungsten powder covered o silicon substrates at 400 C, tungsten oxide (W O ) nanowires with diameter of 18 49 10-50 nm were obtained with high quantity. It was further shown that the presence of a pre-oxide layer on tungsten substrate is essential for high yield W O 18 49 nanowire growth. Under optimal conditions, the diameter of the nanowires can be decreased down to 4-7 nm. Second, when potassium iodide (KI) was introduced in the synthesis, various types of nanostructures or micro-structures, such as tungsten oxide and potassium tungstates (K W O ) micro-walls were obtained, depending 2 3 10 on the temperature of substrate, the gas flow and the pressure in the reactor. It was o found that tungsten oxide nanowires can be synthesized at 400 C in the presence of KI, and the addition of source materials will enhance the growth. Finally, by thermally evaporating tungsten trioxide powder source, tungsten oxide W O nanowires were also synthesized with high yield and on a large 18 49 scale. Depending on the temperature of the source (in the range of 900-1000 C), the diameter of the tungsten oxide nanowires varies from 10 nm to 100 nm, following an exponential relation. Nanowires with controllable diameter are therefore obtainable by this method. Under other conditions, nanowires, nanoplates, nanocrystals and comb-like nanostructures were obtained on silicon substrate. The growth mechanisms of these nanostructures, especially the dependence of nanowire size on source temperature, were further investigated. It is found that the diameter of nanowires increases with growth time but a maximum diameter seems to exist as determined by the source temperature. A theoretical model was used to analyze the growth mechanism for the diameter control where zero-dimensional (0D) and two-dimensional (2D) nucleation contribute to the growth of length and the diameter of the nanowire respectively. DOI: 10.5353/th_b3955855 Subjects: Tungsten oxides - Synthesis Nanostructures

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