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Synthesis and Characterization of TiO2 Nanowire and Nanotube Arrays for Increased Optoelectronic Functionality
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- Author / Creator
- Mohammadpour, Arash
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The n-type semiconducting, vertically oriented TiO2 nanotube and nanowire arrays constitute a mechanically robust, high surface area, easily functionalized architecture with vectorial electron percolation pathways and have been the focus of interest for variety of applications including but not limited to solar cells, water photoelectrolysis and photocatalysis, hydrogen sensors, drug delivery, stem cell differentiation and glucose sensors. Novel applications and better performance in present applications require innovative and more complex TiO2 nanostructures. We reported on fabrication of multipodal TiO2 nanotubes. Multipodal refers to the nanotubes with more than one leg and are desirable for applications relying on differential surface functionalization and volume filling of individual legs. Capillary and hydrogen bonding forces dominant on the micro- and nanoscale strong enough to bend the TiO2 nanotubes by tens of degrees are generated during the imbibition of electrolyte into and out of the intertubular spaces between adjacent tapered nanotubes. These forces were exploited to develop a mechanism we call “nanotube combination” to produce multipodal nanotubes. Very large diameter TiO2 nanotubes with inner diameters as large as 900 nm, were also generated which surpass the largest inner diameter reported thus far for anodically formed self-organized TiO2 nanotubes by a factor of 2.5. Such nanotubes with pore diameters comparable with the optical wavelength make it possible to perform unprecedented resonance scattering and effective medium regime studies. Large diameter nanotube arrays were simulated using the finite difference time domain (FDTD) method, and the simulated optical properties were compared to those measured experimentally. Inefficient absorption of red/near-IR light by sensitized TiO2 nanostructures limits the efficiency of light harvesting involved applications they are employed in, and utilizing Förster resonance energy transfer (FRET) is promising for resolving the problem. However, FRET efficiencies benefit strongly from the deterministic placement of chromophores. We tested the possibility of FRET phenomenon in nanoporous anodic aluminum oxide (AAO) by encapsulating Alq3 molecules into the nanvoids in the pore wall fissures and coating carboxyfluorescein onto the surface of the walls. Due to the efficient FRET for such a chromophore placement in AAO, such deterministic positioning might also be advantageous to apply to TiO2 nanotubes. We performed the first direct measurement of charge carrier mobility in TiO2 nanowire arrays. We reported an effective electron drift mobility of 1.9 × 10-5 cm2V-1s-1 in rutile nanowire arrays directly measured using the time of flight and space charge limited current techniques. In addition, we measured an equilibrium free electron concentration of ~1014 cm-3 and a trap density of 3.5 × 1016 cm-3 in rutile nanowires. These results point to the importance of reducing traps to improve charge transport in rutile nanowires. We also introduced magnetic field for the first time to the process of electrochemical anodization of TiO2 nanotube arrays and demonstrated its advantages in addressing limitations of the conventional anodization method. The use of magnetic fields provides the possibility of anodic growth of TiO2 nanotubes through anodization of discontinuous Ti films which is unprecedented. It expands the possibilities of employing TiO2 nanotube arrays in complex devices such as MEMS, lab-on-a-chip and microchips.
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- Graduation date
- Fall 2014
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.