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Keywords: ZnO Electrodeposition, Pulse Electrodeposition, Hierarchical Nanostructures, Nanotube, Growth Mechanism
Issue Date: 24-Jan-2013
Abstract: In the interest of an efficient, economical and well-suited fabrication method for large-scale production of one dimensional (1D) ZnO nanostructures, the electrodeposition method has been indicated as a promising avenue. The main aim of this work was to study the growth mechanism of ZnO NWs from nitrate-based solutions. A model based on the local concentrations of Zn2+ and OH-, generated from the electrochemical reduction of NO3-, was proposed to explain the growth mechanism of ZnO NWs. The ratio between OH- production and Zn2+ diffusion rates was identified as a crucial parameter, determining the NW growth regime and therefore the nanowire¿s dimensions. The proposed mechanism represents a significant further insight into the ZnO NW electrodeposition from NO3- based solutions and provides a novel strategy to achieve higher deposition efficiencies, in comparison to the O2 based route, while still controlling the diameter of NWs. It is always of great interest to increase the deposition rate and deposition efficiency without compromising the desired morphology. To promote this interest, pulse electrodeposition technique was employed. It was demonstrated that controlling the pulse parameters provides a powerful tool to control the local ratio between [OH-] and [Zn2+] enabling to employ the aforementioned mechanism. A comparison between the morphology and properties of samples deposited by the conventional and pulsed technique reveals that for a similar duration of deposition, the pulsed technique not only provides the opportunity to improve the morphology, structural and optical properties of samples but also reduces chemical consumption. Employing hierarchical assembly of 1D nanostructures may open up promising avenues to meet the growing demand for fabricating pioneering devices with higher efficiencies as these structures not only provide a higher surface area but also combine the features of micro- and nano-structures, exhibiting unique properties. To fabricate such hierarchical structures, it was demonstrated that coating the primary NWs with a seed layer is an essential step. Hence, a straightforward and scalable fully electrochemical deposition process was developed, for the first time, wherein the seed layer was electrodeposited from ionic liquid electrolytes. This process is expected to provide new opportunities for technological applications such as solar cells and photonic devices. Fabrication of nanotubes (NTs) is another avenue which may help to meet the ever-increasing demand for increasing the efficiency of pioneering devices through enlarging the surface area. The common two-step deposition process of NTs is associated with some extra work and additional cost making it less favorable for large-scale production. Moreover, it was demonstrated that the reported single-step electrodeposition of NTs is irreproducible and more likely the hollow structure is only formed at the tips of the arrays. Hence, in pursuit of a straightforward and scalable method, a reproducible single-step was designed and the preliminary experiments demonstrated that that there is an opportunity of fabricating ZnO NTs in a single step. Moreover, it confirms that the dissolution of NRs¿ cores is the prevailing formation mechanism of NTs, rejecting the proposed theory of NTs formation through buildup of the walls.
Appears in Collections:Ph.D Theses (Open)

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