Movsesyan, Liana (2017)
Template-assisted growth and characterization of ZnO-based nanowire arrays and 3D networks.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
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Item Type: | Ph.D. Thesis | ||||
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Type of entry: | Primary publication | ||||
Title: | Template-assisted growth and characterization of ZnO-based nanowire arrays and 3D networks | ||||
Language: | English | ||||
Referees: | Trautmann, Prof. Dr. Christina ; Jaegermann, Prof. Dr. Wolfram | ||||
Date: | 2017 | ||||
Place of Publication: | Darmstadt | ||||
Date of oral examination: | 6 October 2016 | ||||
Abstract: | In recent years, research on the fabrication of semiconductor nanowires has attracted increasing interest in various fields of research. Especially, the successful synthesis of ZnO micro- and nanoscale structures has paved the way to numerous applications for devices including nanogenerators, sensors, solar and fuel cells. This work focuses on the synthesis of ZnO nanowire arrays and nanowire networks by means of electrochemical deposition in etched ion-track membranes. Three different geometries of nanowire-based structures are discussed: arrays of vertically aligned (1) cylindrical ZnO nanowires and (2) ZnO nanocones as well as (3) mechanically stable three-dimensional (3D) ZnO and ZnO/TiO2 nanowire networks. To establish a reproducible growth process of ZnO in track-etched membranes, the electrodeposition parameters of vertically aligned cylindrical ZnO nanowires are adjusted by independently varying the applied potential during the electrodeposition, the pore diameter of the membranes, and the concentration of the electrolyte (Zn(NO3)2·6H2O). The influence of these parameters on the morphological and crystallographic properties of the nanowires is analysed by means of high resolution scanning electron microscopy (HRSEM) and X-ray diffraction (XRD). ZnO nanocone arrays with µm-size bases and nm-size tips are fabricated applying two growth approaches: base-to-tip and tip-to-base. The arrays are analysed by SEM in terms of mechanical stability. An important part of this thesis is the design and synthesis of 3D ZnO nanowire networks. The number density and diameter of the nanowires in the network are optimized in order to obtain mechanically stable 3D building blocks exhibiting a high surface area. The crystallographic properties are studied by XRD, high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and high-angle annular dark-field (HAADF) imaging. Knowledge drawn from these investigations is crucial for the implementation of these 3D nanoarchitectures into devices, including photodiodes and -electrodes. Furthermore, the durability of ZnO-based nanowire networks in aqueous environment under applied potential and illumination is tested. In particular, they are employed as photoanode model systems for photoelectrochemical water splitting. The measurements are performed on pure ZnO and core/shell ZnO/TiO2 nanowire networks and compared to those of their film counterparts. The role of the TiO2 layer as a protection layer against photo(electro)chemical corrosion of ZnO as well as the efficient charge separation and transport from nanowires to the electrolyte is highlighted. By using X-ray photoelectron spectroscopy (XPS) the contamination and chemical composition on the surface of the networks are quantified before and after photoelectrochemical measurements. Additionally, the Schottky barrier heights formed at the ZnO-Au and TiO2-Au contacts are theoretically determined from the values of the work function and the valence band position. The work presented in this thesis shows how the outstanding flexibility of the ion-track technology combined with electrochemical deposition leads to new options for fabrication of complex ZnO-based nanowire structures. The possibility to tune the length, diameter and orientation allows the production of free-standing, high aspect ratio structures which are promising for a wide range of applications, including energy conversion and storage. |
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URN: | urn:nbn:de:tuda-tuprints-54424 | ||||
Classification DDC: | 500 Science and mathematics > 500 Science 500 Science and mathematics > 530 Physics 500 Science and mathematics > 540 Chemistry |
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Divisions: | 11 Department of Materials and Earth Sciences > Material Science > Ion-Beam-Modified Materials | ||||
Date Deposited: | 02 Mar 2017 10:11 | ||||
Last Modified: | 09 Jul 2020 01:17 | ||||
URI: | https://tuprints.ulb.tu-darmstadt.de/id/eprint/5442 | ||||
PPN: | 400085046 | ||||
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