TiO2 Nanotube Arrays with Engineered Geometries: Growth, Characterization and Study of Selected Interfaces

Abstract

Titanium dioxide (TiO2) is a wide band gap, robust, n-type semiconductor whose transformation into vertically-oriented nanotube arrays makes them suitable electron percolation pathways for vectorial charge transfer togerther with high surface area. Growth of self-organized TiO2 nanotube arrays as a membrane or on non-native substrates is critical for exploiting the full potential of this nanostructure in a variety of applications. Although regular TNAs on native substrates constitute one of the most potent semiconductors for a variety of applications, to date, less research has been conducted on engineering the geometry of such structures for photon management. In this dissertation, a fundamental investigation into TNA-based photon management by periodically modulated titanium dioxide nanotube arrays (PMTiNTs) and double layer TNAs on native substrates was carried out. The present work shows that bilayer film stacks consisting of nanotubes with small (~60 nm) and large (~200 nm) diameters, respectively improve the optical absorption. In the second part of this thesis, wherein the geometry of these unique nanostructures is engineered, galvanostatic pulse- generated modulation was used. The highly ordered PMTiNTs have been reported, which constitute bottom-up fabricated one-dimensional photonic crystals. As a last phase of the synthesis process, highly sophisticated vacuum deposition based techniques were used to investigate the influence of a variety of deposition rates and pressures at room temperataure on anodic nanotube growth formation on non-native substrates. In the third phase of the work, X-ray and ultraviolet photoelectron spectroscopic studies were used to investigate the interfacial band alignment for photocatalytic charge separation in TiO2 nanotube arrays coated with CuPt nanoparticles. In the last phase, low energy surfaces repellent to a wide spectrum of liquids by functionalizing TiO2 nanotubes arrays using monolayers of two different fluorinated hydrocarbon molecules were studied.