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Title: Anodic titanium dioxide layers : synthesis, properties and applications
Authors: Huang, Chuanjun
Degree: Ph.D.
Issue Date: 2010
Abstract: Highly-ordered, self-organized TiO₂ nanotube arrays obtained through electrochemical anodization in F⁻ ions containing electrolytes have received considerable attention in the past decade due to their easy fabrication and anticipated high potential for technological application. Although extensive research has been conducted, there is still much to be learned about the fabrications, characterizations and applications of the anodic TiO₂ layer. Research interest is aroused by tailoring of the morphology of the anodic TiO₂nanotube arrays through precisely controlled anodization parameters and carefully prepared electrolytes. TiO₂ layers obtained through anodizing titanium in F⁻ ions containing electrolyte, which have the advantage of attaching onto the metallic substrate, significantly extend the advanced applications of nanocrystalline TiO₂. These applications are strongly related to the semiconductor nature, chemical stability, high surface-to-volume ratio and tunable geometries of these layers. The development of anodic TiO₂ layers with tunable geometry through anodization of titanium and titanium alloys, and the investigation of their properties and applications, have both scientific and practical significance. In this thesis, investigations of the TiO₂ layer formed through anodizing titanium and NiTi alloy in different electrolytes were conducted. In F⁻ ions containing aqueous solution, research emphasis was placed on the effects of duration of anodization, addition of phosphoric acid, and applied potential on the morphology of the TiO₂ layer. Highly-ordered TiO₂ nanotube arrays and hybrid material consisting of TiO₂ nanotube arrays and bamboo-shoot-like structures were developed and examined by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Using selected-area electron diffraction (SAED) and Raman spectroscopy, the as-prepared bamboo-shoot-like structure was determined to be anatase, and through X-ray diffraction (XRD) the as-prepared TiO₂ nanotube array was found to be amorphous. The formation of the bamboo-shoot-like structures was ascribed to ageing induced dielectric breakdown under potentiostatic anodization at special applied potentials in HF solution containing H₃PO₄. For F⁻ ions containing hybrid electrolyte, the influence of applied potentials on the surface morphology of the TiO₂ layer was demonstrated and the critical role of the electrical field was studied. TiO₂ layers including micro-, meso- and macro-porous structures, corresponding to the applied potential, were obtained. The formation of a nanoporous TiO₂ layer under various applied voltages in F⁻ ions containing hybrid solution was attributed to the non-uniform electrical field on the metal surface. For a non-aqueous solution containing NH4HF2 and ethylene glycol, the effects of applied potential on the surface morphology and growth rate of the TiO₂ layer were investigated. Highly-ordered TiO₂ nanotube arrays, which were covered with nanowires, were observed by FESEM. Growth rate related to applied potential was calculated according to tube length and anodization time. It was found that a high applied voltage leads to a high growth rate but loose nanotube array. An optimal applied voltage in view of order of the TiO₂ nanotube arrays and the growth rate is suggested. These anodic TiO₂ layers with their various geometries provided sufficient scope for studying the influence of geometry on their properties, for investigating the associated effect as a support for other functional materials, and for examining the influence of architecture on the cell performance of dye-sensitized solar cells. Anodization of NiTi alloy in diluted HF solution was carried out. A surface oxide layer with enhanced surface roughness was observed through FESEM and scanning probe microscopy (SPM). The composition of the oxide layer was characterized using X-ray photoelectron spectroscopy (XPS). By investigation through electrochemical potentiodynamic in Hanks’ based salt solution, it was found that the surface oxide layer exhibited improved corrosion resistance. The semiconductor property and the geometry of TiO₂ layers attached to a titanium substrate make them a favorable heterogeneous photocatalyst. The photocatalytic properties of the bamboo-shoot-like structure/TiO₂ nanotube arrays hybrid material and the nanoporous TiO₂ layers were examined through decomposition of a kind of non-biodegradable dye-AO7 under weak UV irradiation. The data from photocatalytic measurement were plotted as logarithms of the time-dependent normalized dye concentration and it was found that the photocatalytic activity followed pseudo-first-order kinetics. When investigating the influence of morphology on photocatalytic efficiency, it was found that the hybrid material exhibited higher photocatalytic efficiency than the TiO₂ nanotube arrays. Moreover, the influence of post-annealing treatment on the photocatalytic activity was studied and it was demonstrated that samples annealed at 550 °C resulted in maximum photocatalytic efficiency, which was attributed to the formation of the anatase/rutile junction. The enhanced photocatalytic activity of TiO₂ layers annealed at 550 °C was attributed to the increase in charge-separation. Among various structures including micro-, meso- and macro-porous TiO₂ layers, ordered TiO₂ nanotube arrays exhibit the highest photocatalytic activity in decomposition of dye AO7 under UV illumination. A fixed-bed flow-through photoreactor, which scaled up the decomposition of waste solution, was designed and examined. It was found that the anodic TiO₂ nanotube arrays/Ti is a good alternative to the commonly used TiO₂ nanoparticle photocatalyst.
Due to their highly accessible surface, electrochemical behavior and chemical stability, anodic TiO₂ nanotube arrays have attracted considerable interest for energy-storage applications. The electrochemical double layer capacitance (EDLC) of the TiO₂ nanotube arrays obtained from aqueous solution was studied in alkaline and acidic solutions through cyclic voltammetry (CV) and galvanostatic charge-discharge techniques. It was found that the EDLC of the anodic TiO₂ nanotube array was very low and it depended on the ions (OH⁻ or H⁺) in the electrolyte employed. Decorating the anodic TiO₂ nanotube arrays with electroactive materials including NiO-Ni(OH)₂ and RuO₂ was carried out through an electrochemical deposition method for redox supercapacitor application. The redox supercapacitance of the NiO-Ni(OH)₂/anodic TiO₂ nanotube arrays/Ti and RuO₂/anodic TiO₂ nanotube arrays/Ti nanocomposites was examined. In both cases, it was found that the crystalline structure of the anodic TiO₂ nanotube arrays influenced the charge-discharge behavior. Moreover, extension of the potential of the operational windows of the redox supercapacitor was obtained compared with that obtained through the deposition of electroactive materials on metallic titanium substrate. These findings indicate that the anodic TiO₂ nanotube arrays/Ti is a good alternative support material for supercapacitor applications. The semiconductor nature and the geometry of anodic TiO₂ nanotube arrays make them suitable for use as electrode material in photon-to-electric conversion devices, especially dye-sensitized solar cells (DSSCs). Liquid-junction DSSCs using anodic TiO₂ nanotube arrays obtained through anodization in non-aqueous solutions as photoanode were assembled and examined. First, anodic TiO₂ nanotube arrays were fabricated through potentiostatic anodization in ethylene glycol solution with NH₄HF₂, and liquid-junction DSSCs based on the annealed structures were assembled and characterized. The assembling and sealing process was investigated and it was found that the commonly used sealing material, Surlyn film, was not suitable for the liquid-junction DSSCs. Epoxy adhesive slowed down evaporation of the solvents but leakage of the electrolyte was unavoidable. Treatment of the anodic TiO₂ nanotube arrays with TiCl₄ solution was performed and the results demonstrated that the treatment increased photocurrent and consequently enhanced the performance of the cell. Anodic TiO₂ nanotube arrays with special surface nanostructures were fabricated through two-step anodization in non-aqueous electrolyte containing NH₄F and ethylene glycol. It was found that the special surface nanostructure resulted in improvement of cell performance. The electron transport properties of the liquid-junction DSSCs, especially the electron lifetime, were investigated through electrochemical impedance spectroscopy and open-circuit voltage decay approaches. The influence of the addition of a kind of ionic liquid into redox electrolyte was investigated and it was found that it led to an increase in the electron lifetime and enhancement of the cell performance.
Subjects: Hong Kong Polytechnic University -- Dissertations
Pages: xxiii, 237 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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