| Titanium dioxide (TiO2) as an inorganic functional material has attracted much attention in the scientific research field. Especially in energy conversion and environmental improvement, such as photoelectrochemical decomposition of water, photocatalytic degradation of pollutants in water, and solar cells, TiO2has shown a great potential of applications. With advances in nanoscience, a various form of TiO2nanomaterials has been made, and their performance is also greatly improved. Compared with other forms of nano-TiO2, TiO2nanotube arrays have a larger specific surface area and better alignment characteristics, and also possess a highly ordered and efficient electronic transmission channel. This novel nanostructure is expected to improve the photoelectric and photocatalytic properties of the TiO2. Since the morphology, crystal structure, and composition of TiO2have a significant impact on its performance, thus in this thesis, TiO2nanotube arrays were studied by exploring new preparation methods, adjusting its morphology, structure, and microstructure, and modifying its chemical and physical properties, and making composites with other narrow band materials. A series of new results have been achieved and the mechanism beneath we elucidate by investigating their photoelectric properties through surface photovoltage spectroscopy and photochemistry performance testing. The main contents include the followings:(1) The morphology and structure of the nanotube arrays are studied and controlled by changing the reaction parameters of the electrolyte, oxidation voltage, oxidation time, oxidation temperature, etc. The results show that the desired oxidation voltage range is different in different electrolyte system, which affects the tube diameter and length of the arrays. In the same electrolyte system, different oxidation voltage can also results in different morphology of the nanotube with different tube diameter and length. A significant impact of different time and temperature on the tube length and diameter are also observed. However, in different electrolyte system, the reaction voltage and time as well as temperature has an optimal range, within which a better morphology of TiO2nanotube arrays can be achieved. On the basis of these optimized conditions with a well-designed post-annealing processing, a series of novel types of TiO2nanotube arrays have been obtained, which include alumina template type, half-wall type, spring types, etc.(2) TiO2nanotube arrays are amorphous structure when prepared by anodic oxidation method under room temperature, which can not satisfy the optoelectronic applications where crystallized TiO2nanotube arrays are required. Traditional crystallization methods of TiO2nanotube array include high temperature annealing in air or other inert gas atmosphere, and the anodic oxidation in the electrolyte at an elevated temperature. However, high temperature processing can not be used to prepare the TiO2nanotube arrays on substrates of low tolerance for temperature, such as polymers, which are needed for many applications. In this research, anodized TiO2nanotube arrays prepared at room temperature were put in water, ethylene glycol and ethanol solution, respectively, to realize the hydrothermal crystallization of TiO2nanotube arrays at a temperature as low as40℃. High quality TiO2nanotube arrays with a single anatase structure have been prepared at40℃with this new method.(3) Two process, anodic oxidation and chemical etching are simultaneously coexisting in the preparation of TiO2nanotube arrays made by anodic oxidation method. The role of chemical etching can not be ignored in the anodic oxidation process with a long reaction time, which can easily lead to split the TiO2nanotubes from tubes to wires, thus forming a tube/wire composite structure on the surface of the nanotubes. The changes of the microstructure may lead to the transformation in the electronic transportation, which in turn leads to some novel performance. In this research, TiO2nanotube/nanowire composite arrays were fabricated on the Ti substrate via anodic oxidation method. Annealing effects on the surface photoelectronic properties of TiO2nanotube/nanowire composite arrays were studied by surface photovoltage spectrum (SPS) and the electrical field induced surface photovoltage spectrum (FISPS). The results showed that the bound excitons exist at the edge of conduction band both in as-prepared and annealed TiO2nanotube/nanowire composite arrays. The build-in field in as-prepared TiO2nanotube/nanowire composite arrays is weak and the bound excitons can inverse antisymmetrically under external field. After being annealed, TiO2nanotube/nanowire composite arrays were crystallized from amorphous structure, the build-in field is increased, but the SPS responses related to the bound excitons could exhibit only under positive field. (4) TiO2can only absorb the ultraviolet part in sunlight due to its large bandgap (3.2eV), but the energy of the ultraviolet part is only about5%of energy of solar spectrum, that makes the photoelectric conversion efficiency of TiO2is low. Compositing with narrow bandgap semiconductor is an important way to expand the TiO2light response range. CuS is a semiconductor material and its band gap is2.0eV. CuS is widely used in the fields of lithium ion batteries, polymer surface modification, superconducting materials, etc. In this research, ordered CuS nanoparticles and TiO2nanotube arrays composites are prepared. The current-voltage curve shows that CuS/TiO2nanotube heteroj unction arrays have obvious rectifying effect. According to the results of SPS and phase spectrum (PS), CuS/TiO2nanotube heteroj unction arrays show p-type semiconductor character and electrons aggregate at the surface in376-600nm, but show n-type semiconductor character and holes aggregate at the surface in300-376nm. The surface photovoltage response is zero at376nm. The photoelectrochemical property of CuS/TiO2nanotube heteroj unction arrays-base photoelectrochemical cell shows0.4%of photoelectricity conversion efficiency under100mW/cm2simulated AM1.5sunlight. After depositing PbS quantum dots on the basis of CuS/TiO2nano tube heteroj unction arrays by successive deposition, the photochemical properties of TiO2nanotube arrays is significantly improved. The CuS/PbS/TiO2nanotube heteroj unction arrays-based photoelectrochemical cell shows1.46%photoelectricity conversion efficiency. |
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