Investigations Of Photo-generated Cathodic Protection Of The Composite Films Of TiO₂ Nanotubes

Titania (TiO₂) is one of the most important semiconductor oxides because of its excellent photo-electrochemistry, photo-catalysis and good chemical stability. It can be used in many hi-tech applications including photocatalysis, gas sensors, solar cells, cathodic protection and etc. It is of great interest to develop TiO₂ coatings or composites of nano TiO₂ with other dopants on metals for the photogenerated cathodic protection of metals under ultraviolet (UV) or visible light illumination. However, the application of TiO₂ nano coatings serving as anticorrosion measure has been limited by some technical bottleneck, including non uniform and defects for the coating. Especially, due to the wide band gap of TiO₂ (E_g=3.0-3.2eV), it is only excited by ultraviolet light (λ< 390 nm) so that the efficiency of light utilization to solar irradiation is very low, and the recombination rate of photogenerated e^--h⁺ pairs in TiO₂ is high and the quantum efficiency is low. The technical problems result in the unefficient photocathodic protection performance of metals under both visible light illumination and dark conditions.The main goal and motivation of this thesis are to develop various uniform and functional nano TiO₂ composite coatings, and to increase the quantum efficiency of TiO₂ to both UV and visible light excitation. An attempt is made to extend the life of the photo-generated electron-hole pairs and to enhance efficient cathodic protection performances in UV-visible illumination and dark conditions. The mechanism of the photo-generated cathodic protection is also discussed in the thesis. The main results and conclusions are summaried as following:1. The sol-gel method and dip-coating technique have been developed to fabricate theultra fine particles coating of TiO₂, TiO₂-SnO₂ and N-TiO₂ at the room temperature.The size and thickness of TiO₂ are crack-free, ultra purity and controllable, and the preparation method is simple and easy to be reproduced.2. It is shown that the as-prepared nano TiO₂, films on metal own better anticorrosion performances, and the nano TiO₂ films can suppress the recombination of photo-generated electron-hole pairs. The efficient photo-cathodic protection of the N doped TiO₂ nanoparticle films is achieved under UV-visible wavelength spectra regions for the first time. It is indicated that the ultra fine nanoparticle coatings have a good quality of dual functions in anticorrosion of metals, that is, the corrosion protection as the barrier layers and photocathodic protection as a photoanode.3. Highly density and well aligned uniform titanium oxide nanotubes have been fabricated by electrochemical anodic oxidation of a pure titanium substrate in fluorinated electrolyte solutions. The morphologies, composition, structure and photoelectrochemical performances have been studied by using SEM, XRD, UV-Vis, electrochemical methods etc., and the photochemical properties of doped TiO₂ nanotube arrays and their possible applications of anticorrosion for metals have been investigated systemically. It is noted that the TiO₂ nanotubes show a stronger absorption in the ultraviolet (UV) light range, the open-circuit potentials of 316 SS coupled with the TiO₂ nanotubes layers shift negatively under UV light irradiation (λ<380nm).4. The photocathodic protection of the Fe-doped TiO₂-based nanotube layers has been evaluated through the electrochemical measurements under visible light irradiation and dark conditions. The effects of the electrolyte system, anodized time, heat treated temperature on its photoelectrochemical activity and photogenerated cathodic protection performances are emphatically investigated. It is found that the Fe-doped TiO₂ nanotubes show a stronger absorption in the 410-650nm range. The open-circuit potentials of 316 SS coupled with the Fe-doped TiO₂ nanotubes layers shift negatively under visible light irradiation (λ>400nm), and maintain negatively for a period even in dark condition. It is indicated that the Fe-doped TiO₂ nanotube layers are able to function effectively a photogenerated cathodic protection for metals under regular sunlight conditions and remain a durative cathodic protection even in the dark condition.