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Title: 1-dimensional ZnO-based nanostructures : synthesis and applications
Other Titles: Yi wei yang hua xin na mi jie gou de he cheng yu ying yong yan jiu
Authors: Lu, Zhenzhen (魯湞湞)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2013
Publisher: City University of Hong Kong
Subjects: Nanostructured materials.
Zinc oxide.
Notes: CityU Call Number: TA418.9.N35 L83 2013
xx, 111 p. : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2013.
Includes bibliographical references.
Type: thesis
Abstract: Zinc oxide (ZnO) is a wide band gap (3.37 eV) semiconductor having a high electron hole binding energy (60 meV) and important applications in electronics, optics, optoelectronics, lasers, and light-emitting diodes. The piezoelectric and pyroelectric properties of ZnO make it a great candidate for sensors, transducers, energy generators, and photocatalysts for hydrogen production. ZnO is also a "green" material that is biocompatible, biodegradable, and nontoxic for medical applications and environmental science. As for one-dimensional (1-D) nanostructures, ZnO has equal importance to silicon-based 1-D nanostructures, and it is playing an increasingly key role in developing nanoscience and nanotechnology. In this thesis, three applications of ZnO nanorod arrays were studied including 1. ZnO nanorod arrays which play the role of electron transfer channel and scaffold in solar cells application; 2. we adopted ZnO nanorod arrays as the template to prepare Si nanotubes which are applied in lithium ion batteries; 3. a new property of ZnO nanorod arrays was studied in the form of nano scattering elements which can be applied in screens and LED fields. Firstly, a facile chemical solution method was reported for the first time to synthesize CdS/CdSe double-sensitized ZnO nanocable arrays. In this method, CdS was prepared using a chemical bath deposition (CBD) method and CdSe was formed on the surface of CdS via an ion exchange reaction. Comprehensive characterization techniques were carried out to study the composition, microstructure and crystallinity of the sensitization layers. The CdS/CdSe ratio was revealed to be controllable by varying the temperature of the ion exchange reaction. It was further demonstrated that sensitization of ZnO nanorods with CdS and CdSe exhibited enhanced light-harvesting abilities. The photovoltaic performance of the double-sensitized ZnO nanocables in photoelectrochemical solar cells was evaluated. An increase in short circuit current was observed, and a power conversion efficiency of 3.23% was achieved. In addition, the formation mechanism of the double-sensitizing layer was also discussed. Secondly, silicon is a promising candidate for electrodes in lithium ion batteries due to its large theoretical energy density (4200 mAh/g). However, its poor capacity retention, caused by pulverization of the Si structure during cycling, restricts its practical application. Recently, silicon nanotube structure has been proved to be capable of accommodating large volume changes associated with lithiation in battery applications. We developed a new method to prepare a nanotubed form of silicon which could be extended to large areas without silane. The prepared electrodes exhibit high initial coulombic efficiencies (i.e., >85%) and stable capacity retention (>75% after 200 cycles), due to an unusual underlying mechanics that is dominated by free surfaces. For further improving its performance, we inserted graphite into the silicon nanotube using a plasma-enhanced chemical vapour deposition (PECVD) method to enhance its conduction with the substrate. The capacity retention could be increased to over 95% after 200 cycles. Herein, we report on the preparation of silicon-graphite nanotubes and their highly reversible lithium storage and excellent high-rate capability. This result suggests that the as-prepared silicon-graphite nanotubes are promising candidates as the anode materials of rechargeable Li-ion batteries. Lastly, large band-gap (>3.3eV) nanomaterials enable nearly total transparency in the range of visible light range as scattering elements. Periodic nanostructures provide the facility to control and direct the flow of light through them. Here, we report on ZnO nanorod array photonic crystals composed entirely of c-axis directional nanorods. The arrangement of this structure, as well as its band properties and length of the nanorods, present opportunities for otherwise unattainable device applications such as large area back projector screens and LED back scattering screens which need a large view angle. We present a nano-structured screen that demonstrates a high transmittance, large view angle and high brightness in the visible spectrum, and show its association with length. Numerical simulations reproduce the experimental observations and indicate that the interval between the nanorods provide the transport passage. Experimentally, we found a transmitted light viewing angle from the ZnO nanorod arrays of about 90°, and high transmittance ratio of about 90%.
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