An investigation into the doping and crystallinity of anodically fabricated titania nanotube arrays: Towards an efficient material for solar energy applications

The primary focus of this dissertation was to improve the properties of the anodically fabricated TiO2 nanotube arrays; notably its band gap and crystallinity while retaining its tubular structure unaffected. The underlying hypothesis was that controlling the crystallinity and band gap while retaining the tubular structure will result in an enormous enhancement of the photoconversion capability of the material. To this end, a direct one-step facile approach for the in-situ doping of TiO2 nanotube arrays during their electrochemical fabrication in both aqueous and non-aqueous electrolytes has been investigated. The effect of doping on the morphology, optical and photoelectrochemical properties of the fabricated nanotube arrays is discussed.;In an effort to improve the crystallinity of the anodically fabricated TiO2 nanotube arrays while retaining the tubular morphology, novel processing routes have been investigated to fabricate crystalline TiO 2 nanotube array electrodes. For the sake of comparison, the nanotubes were annealed at high temperature using the conventionally used procedure. The samples were found to be stable up to temperatures around 580 °C, however, higher temperatures resulted in crystallization of the titanium support which disturbed the nanotube architecture, causing it to partially and gradually collapse and densify. The maximum photoconversion efficiency for water splitting using 7 mum-TiO2 nanotube arrays electrodes annealed at 580 °C was measured to be about 10% under UV illumination.;We investigated the effect of subsequent low temperature crystallization step. Rapid infrared (IR) annealing was found to be an efficient technique for crystallizing the nanotube array films within a few minutes. The IR-annealed 7mum-nanotube array films showed significant photoconversion efficiencies (eta=13.13%) upon their use as photoanodes to photoelectrochemically split water under UV illumination. This was related, in part, to the reduction in the barrier layer thickness from 1100 nm for the thermally annealed sample down to 200 nm for the IR-annealed sample under same conditions. These results support the hypothesis that reducing the barrier layer thickness would result in better performance of the material.;Regarding the possibility of low temperature crystallization, this dissertation encompasses the first report on low-temperature synthesis of crystalline TiO 2 nanotube arrays. Nanotube arrays of up to 1.4 mum length using a two-step process have been demonstrated. The two-step process consists of initial treatment of the Ti foil in an oxidizing agent (H2O 2 or (NH4)2S2O8)-containing electrolytes, followed by potentiostatic anodization of the resulting foil in NH4F-containing electrolytes. The as-synthesized crystalline nanotube arrays were successfully tested as anode electrodes for water photoelectrolysis, with performances comparable to samples annealed at high temperatures, and for liquid junction dye (N 719 dye)-sensitized solar cells.;With the motivation of finding an electrolyte composition that might yield better crystalline nanotubes than that obtained in the HCl-containing electrolytes, the effect of using some polyol electrolytes (diethylene, triethylene, tetraethylene and polyethylene glycols) on the crystallinity and morphology of the fabricated TiO2 nanotube arrays was investigated. The study showed that the use of these electrolytes helped to induce partial crystallinity in the formed nanotube arrays with the intensity of anatase (101) peak was found to increase with increasing the molecular weight of the polyol electrolyte.;This thesis reports, for the first time, synthesis of high-aspect-ratio tantalum oxide nanotube arrays via one-step anodization of Ta foil. The use of aqueous electrolytes containing HF:H2SO4 in the volumetric ratios 1:9 and 2:8 results in formation of ordered nanodimpled surfaces with 40-55 nm pore diameters over the potential range 10-20 V. The addition of 5-10% of either ethylene glycol (EG) or dimethyl sulfoxide (DMSO) to the HF and H2SO4 aqueous electrolytes resulted in the formation of Ta oxide nanotube arrays up to 19 mum thick, either securely anchored to the underlying Ta film or as robust free-standing membranes, as dependent upon the anodization time and applied voltage. (Abstract shortened by UMI.)...