Investigations On Modified Nano TiO₂ Composite Films And Its Photogenerated Cathode Protection Of Metals

Nano TiO₂ thin films,due to their unique photoelectrochemical properties,have been world-wide attractive,and extensively applied in many hi-tech fields,such as photocatalysis,surper-hydrophilic self-cleaning,solar cells and so on.The investigation of TiO₂ for photogenerated cathode protection of metals has received great attention.However,due to the wide band gap of TiO₂ semiconductor,it is only excited by ultraviolet light(λ＜387nm),and only 3-4%solar energy can be utilized. And the recombination rate of photogenerated electron-hole pairs in TiO₂ is high and the quantum efficiency is low,it is usually impossible to offer a distinctly photogenerated cathode protection for metal substrates under dark conditions.In this dissertation,the various techniques have been developed to prepare nano TiO₂ composite films combining with the surface modifications in order to achieve an optimal barrier resistance and efficient photo-generated cathodic protection to corrosion of metals.An environmentally friendly hydrothermal method has been developed to synthesize special nanostructured TiO₂ thin films and their dual functions of barrier resistance and photo-generated cathode protection to corrosion of metals have been investigated.The results have been discussed in terms of the structure of TiO₂ film and its performance of photogenerated cathode protection.The emphases have been made to efficiently last the life of electron-hole pairs in nano TiO₂ composite films by doping and coupling to another semiconductor,and enhance the photogenerated cathode protection for metals in dark.The mechanism of nano TiO₂ composite films for photogenerated cathode protection has been further examined as well.The main results and progresses of this work are outlined as following:1.A sol-gel and dip-coating technique has been developed to prepare nano TiO₂ films modified with anions of nitrogen,sulfur and chlorine supplied on the surface of 316L stainless steel,respectively.When modified with the different anions,the surface morphology,structure and the degree of crystallization of TiO₂ films are remarkably different.The N-modified TiO₂ nano-coatings show higher photocurrent under the same measurement conditions.2.N-modified TiO₂ nano-films are able to act as an optimal barrier layer to metallic substrates in 0.5 M NaCl solution.And N-modified TiO₂ nano-films provide a distinctly photogenerated cathode protection for 316L SS under the white light illumination.When the illumination is stopped,the N-modified TiO₂ nano-films still exhibit a protective barrier layer for metals,showing a dual function in corrosion protection of metals.3.A sol-gel technique is used to prepare CeO₂ doped by B element,which is found to exist in the interstitial space of CeO₂ crystal lattice by forming B-Ce-O bond. Bilayer coatings consisting of a B-doped CeO₂ inner layer and a TiO₂ outer layer are made on 316L SS substrate(316L SS｜B-CeO₂｜TiO₂) by coupling.Based on the photoelectrochemical measurements,the TiO₂｜B-CeO₂ films provide a distinctly photogenerated cathode protection for 316L SS under the white light illumination,and sustain the photoeffect for nearly 7 hours when the illumination is stopped,indicating that TiO₂｜B-CeO₂ films are able to continue photogenerated cathode protection for a long period of time in dark.On the other hand,the TiO₂｜B-CeO₂ films still exhibit a good barrier layer for metals, both protective performances of TiO₂｜B-CeO₂ films can offer better protection for metals in a corrosive environment.4.A simple,low-cost and environmentally friendly hydrothermal method followed by a facile post-treatment has been developed to prepare anatase TiO₂ thin film on the Ti substrate.The morphology and thickness of the as-attained TiO₂ film can be successfully controlled by varying the hydrothermal reaction temperature and time.The TiO₂ films with different surface morphology or thickness show different photocurrent maximum.When the thickness of TiO₂ films is comparable,the net-like structured TiO₂ film obtained with a low-temperature hydrothermal process and the TiO₂ nanotube(TN) array film prepared by electrochemical anodization both offer a distinctly photogenerated cathode protection for 316L SS under the white light illumination.5.A low-temperature hydrothermal method has been developed to prepare TiO₂ thin films with certain crystallization.Different from the common method, namely,calcination at 450℃for 2h,it is facile for a hydrothermal method to attain anatase TiO₂ with high crystallization at a low temperature of 170℃.The surface morphology and the degree of crystallization of the as-attained TiO₂ film can be successfully controlled by varying the temperature and time of hydrothermal reaction.Compared with TiO₂ film derived by calcined at 450℃for 2h,the TiO₂ film derived by hydrothermal reaction at 170℃for 4h exhibits better barrier resistance to corrosion in simulated seawater.