| Ever since Fujishima discovered the photoassisted splitting of H2O by TiO2 in 1972, the photoelectric applications of TiO2 have been increased as its well photoelectric properties and chemical stability. One of the applications is photoaided cathodic protection which can prevent metals from corrosion. The working principle of photoaided cathodic protection is that when a metal coated with a semiconductor film exposed to illumination, the electrons of valence band transit to conduction band which were excitated by incident photon and electron-hole pairs formed. The photogenerated electrons can be transferred to the metal substrate since its low potential. This behavior induces the density of electron on the metal surface up which macroscopic performance is the electrode potential of the substrate more negative than its corrosion potential. So the photoanode does not consume and reusable during this process of the corrosion protection that is the main advantage of TiO2.However, the electron-hole pairs excitated by incident photon are apt to recombination. TiO2 will lose the photoaided corrosion protection as soon as the illumination disappears. In order to overcome this limitation, this research focuses on the modification of TiO2 and SnO2 which can inhibit the recombination of photo-generated electron-hole pairs, and try to get different effects of compound modification in different ways and quantities. All of these are to find a best mode to sustain the cathodic protection performance of TiO2 in dark. Research contents and results are as follows:1. An improved sol-gel method was used for preparation of TiO2, SnO2 and the composite sol of them at room temperature. Then built some certain thickness films on 304 stainless steel and slide glass substrate by dip-coating technology, such as TiO2 nano-film, mixed TiO2-SnO2 composite nano-film, layer-stacke TiO2/SnO2 composite nano-film. These sol and nano-films were characterized by laser particle size analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and UV-visible light photometer (UV-Vis). The results showed that the particle size distribution of the sol was narrow, with no large particles of reunion. So the sol could be stored for long time as their well stability. The dried gel showed good crystallization and the films were smooth after sintered at 450℃. After compound SnO2 by mixed modification, the microcosmic feature of composite nano-film was further optimized.2. Electrochemical and photoelectrochemical method of corrosion were used for studying the photoelectric properties and resistance to corrosion behavior of TiO2 film and composite nano-films in simulated seawater. Open circuit potential-time curves of 304 stainless steel substrate coated with pure TiO2 nano-films showed the corrosion potential dropped sharply under UV light. However, the potential will be back up to the initial value as soon as the light shut off. Both potentiodynamic polarization curves and electrochemical impedance spectra showed that the substrate could be protected well from corrosion by nano-films under UV illumination or not. When there was UV light, the nano-films as photo-anode provided photogenerated electrons for 304 stainless steel substrate, so the potential of substrate showed a significant decline, that was in the cathodic protection area. When there was no illumination, the nano-films as physical barrier impeded the interactions between the substrate and surrounding. However, this protect performance was weaker than photogenerated cathodic protection.3. After shut off the UV illumination, the potential of 304 stainless steel coated with mixed TiO2-SnO2 composite semiconductors rised smaller and smaller while the Sn/Ti molar ratio increased. When the ratio of Sn and Ti was 1:1, it showed the best modified result. This phenomenon could be explained by the following reason:SnO2 stored photogenerated electrons from the conduction band of the TiO2 during the UV illumination period, and then slowly released these electrons into the surface of substrate in the absence of light. So the potential could be continue controlled at a lower value. Furthermore, the Tafel polarization curves and electrochemical impedance spectrum also proved that the corrosion resistance of the nano-films would be better along with the increasing of ratio of Sn and Ti.4. Layer-stacked TiO2SnO2 composite nano-film showed the optimal photoassisted cathodic protection compared with pure TiO2 nano-film and mixed TiO2-SnO2 composite nano-film. If there were 3 layers SnO2 at the bottom, layer-stacked TiO2/SnO2 composite nano-film had the best photoelectric corrosion effect.
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