Synthesis Of Modified TiO₂ Nanotube Arrays And Their Photoelectrocatalytic Activity

The development of environmentally friendly materials and technologies to degrade contaminants and produce clean energy is an effective way to solve the environmental problems and shortage of energy faced by all over the world. Photocatalytsis with TiO2 material has been considered as a potential solution to these two crises because it can utilize solar energy to degrade pollutants and produce organic fuels. TiO2 nanotube arrays (TiO2-NTs) material formed by anodization possesses a large surface area and a single orientation which can improve its adsorption efficiency towards organic pollutants and provide a unidirectional electric channel for superior charge separation, which make it a promising photocatalytic material. Unfortunately, there are some demerits, such as the low utilization efficiency of visible light, the high recombination rate of photoinduced electron-hole pairs, and the poor conductivity, restrain its pratical application in water treatment.In this thesis, a series of TiO2-NTs was prepared by an electrochemical anodization process, then TiO2-NTs based visible-light-driven photocatalysts were prepared by modifying with Plasmonic, doping and semiconductor compositing. The characterization of TiO2-NTs based photocatalysts, such as the crystal, surface morphology and optical property, were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), FT-IR Spectrometer (FTIR), UV-Vis Spectrometer, to indicate the relationship between microscopic structure of materials and photoelectrocatalytic activity. The main work contents are divided into four parts, and the main research and results are as follows:Firstly, a Plasmonic Photocatalyst Ag/AgCl/TiO2-NTs electrode was fabricated and interrogation of its visible-light photoelectrocatalytic activity was carried out. The fabrication of Ag/AgCl/TiO2-NTs electrode was achieved by an electrodeposition process. Compared to base TiO2-NTs, the Ag/AgCl/TiO2-NTs electrode showed an enhanced performance in the photoelectrocatalytic (PEC) degradation of Microcystin-LR under visible light irradiation, and the PEC degradation efficiency up to 92% within 5 h. The reactive oxidative species (ROS) were observed to be generated from the hole (h+) and electron (e) during the PEC process. The mechanism for the PEC degradation of the MC-LR was investigated by the addition of different scavengers and the results showed that the major reactive species generated in-situ were h+, ·OH and O2-, which were actually responsible for the degradation of MC-LR. The effect of operating parameters, such as the initial pH of MC-LR solution and the type of anions, were studied. And the results showed that the degradation of MC-LR was highly promoted with the presence of NaCl in the electrolyte or in an acid medium.Secondly, a self-doped TiO2-NTs electrode was prepared and its visible-light photoelectrocatalytic activity was examinated. A simple electrochemical reduction approach is adopted to enhance the photoelectrochemical performance of TiO2-NTs in terms of the degradation of water contaminants such as Rhodamine B, phenol and E. coli K-12. The results obtained from X-ray diffraction and X-ray photoelectron spectroscopy demonstrate that oxygen vacancies i.e., Ti3+self-doping, were formed in the lattices of TiO2-NTs during the electrochemical reduction process of pristine TiO2-NTs at different negative potentials ranging from-1.2 to-1.5 V. Compared to the pristine TiC^-NTs, the treated TiO2-NTs samples by electrochemical reduction process were found to show enhanced photoelectrocatalytic activity in both the UV and visible regions in the entire potential window tested. Significantly, the photocurrent density of self-doped TiO2 nanotube arrays sample prepared at-1.3 V was 250% higher than that of the pristine TiO2-NTs under visible-light illumination. Impedance analysis revealed that the electrical conductivity of the nanotubes was significantly enhanced after self-doping. In summary, the photoelectrocatalytic activity of the self-doped TiO2 nanotube increased dramatically due to the enhanced electrical conductivity and absorption in the visible region.Thirdly, a Z-scheme g-C3N4-Ti3+/Ti02-NTs electrode was manufactured and its visible-light photoelectrocatalytic activity was tested. A photocatalytic material g-C3N4-Ti3+/TiO2-NTs, was prepared by a facile and viable approach involving a heat treatment followed by an electrochemical reduction step. It was characterized by various instrumental techniques such as X-ray diffraction, Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and UV-vis diffuse reflectance spectra. The photoelectrocatalytic degradation efficiency of phenol was systematically evaluated and found to be highly dependent on the content of the g-C3N4 loaded on the TiO2 nanotubes. With an optimal loading quantity of g-C3N4, the apparent photocurrent density of g-C3N4-Ti3+/Ti02-NTs was four times higher than that of the pristine TiO2 under visible-light illumination. The enhanced photoelectrocatalytic behavior observed for g-C3N4-Ti3+/Ti02-NTs, was ascribed to a cumulative impact of both g-C3N4 and Ti3+which widen the photo responsive behavior of the material into visible region and facilitate the effective charge separation of photo-induced charge carriers.Forthly, a bipolar visible light responsive photocatalytic fuel cell (PFC) system PANI/TiO2-CuO/Co3O4 was built. This PFC system consisted of a PANI/TiO2-NTs photoanode and a CuO/Co3O4 film photocathode, was established with a dual objective of degrading organic water pollutants and generating electric power in the same time. Under illumination, the Fermi level of PANI/TiO2-NTs photoanode was higher than that of CuO/Co3O4 film photocathode, created an interior bias of 0.24V. This interior bias induced the transfer of electrons from the photoanode across the external circuit to the photocathode and combined with the holes produced therein with electric power generation. In this manner, the separation of electron/hole pair was achieved in the photoelectrodes by releasing the holes of PANI/TiO2-NTs photoanode and electrons of CuO/Co3O4 film photocathode. Using this PFC based system, the degradation of an organic water pollutant, Rhodamine B, which removal was obtained to the extent of 68.5% at 4 h reaction time, and simultanously a 86 ?A cm "2 current density was generated.