海洋环境腐蚀与生物污损重点实验室知识库

   TiO₂ has been one of the most important semiconductors because of its good environmental friendliness and chemical stability. Its application in photocathodic protection has attracted great attention because there is no energy consumed and defects in the film do not influence its use. However, given the wide band gap of TiO₂ (3.2 eV), this material can only absorb light with the wavelength less than 380 nm, which limits its practical application. In addition, maintaining cathodic protection in the dark is difficult because of the fast recombination of photo-generated electron–hole (e⁻–h⁺) pairs. To overcome these obstacles, many efforts have been made to modify TiO₂ and its composite films. With semiconductor coupling and noble-metal deposition, Ag/SnO₂/TiO₂, CdSe/TiO₂, CdSe/graphene/TiO₂ and CdTe/graphene/TiO₂ composites were prepared and their photocathodic protection properties were investigated.
   The main motivation of this study is to develop various nano TiO₂ composite and surface modification of TiO₂, and to develop simple-operation sol-gel, photo reduction deposition and cyclic voltammetry for fabrication of narrow-bandgap semiconductor and noble metal on the surface of nano-TiO₂ films, and to investigate its potential application of protection for metals. The correlation between the surface structure of TiO₂-based composite and photoelectrochemical properties was also investigated. The main aims of this study are to entend the lifetime of the photoinduced carriers and enhance the separation efficiency of photoinduced carriers and to improve the photocathodic protection performances of the composite under the dark condition. The mechanism of the photocathodic protection of the modified TiO₂ composite is also discussed in this thesis.
   TiO₂ nanotube arrays with special nano structure were first fabricated by two-time electrochemical anodization method. And then the modified TiO₂ nanotube arrays were prepared. The crystalline style, surface morphology, optical properties and composition were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), UV-vis diffuse reflectance (UV-vis) and X-ray photoelectron spectroscopy (XPS), respectively. The photocathodic protection properties of pure TiO₂ and modified TiO₂ nanotube arrays for 304 stainless steel (304SS) or carbon steel were measured. The main results are listed as following:
1.  The influence of electrolyte temperature and voltage on the photocathodic protection of 304SS was investigated. Results show that TiO₂ nanotube arrays prepared in 25°C electrolyte and 30 V exhibited the best photocathodic protection for 304SS and the highest separation efficiency of photoinduced electron-hole pairs.

2. SnO₂ and Ag were deposited on the TiO₂ nanotube arrays with sol-gel and photoreduction deposition. The effects of SnO₂ film thickness and Ag content on the photocathodic protection of the composite were investigated. The composite which exhibited the best protection performance were selected. Results show that the composites tend to be the most functional when prepared with 20-cycle SnO₂ and 0.10 M AgNO₃ solution which can provide the best photocathodic protection for 304SS. In the Ag/SnO₂/TiO₂, co-sensitization with SnO₂ and Ag nanoparticles leads to high transient photocurrent of 0.2 mA cm^-2 of Ag/SnO₂/TiO₂. The photopotential of the 304SS coupled the composite photoanode was about -980 mV under illumination. The protection can be maintained for more than 8 h in the dark.
3.  The composite film was formed via two-step cyclic voltammetry for CdSe nanoparticles and graphene sheets on the TiO₂ nanotubes prepared by anodization. Results show that the CdSe/RGO/TiO₂ composite film is composed of uniform anatase TiO₂ NT arrays, graphene sheets and cubic CdSe nanoparticles. The average diameter of CdSe nanoparticles were about 25 nm. The photocurrent generated by the CdSe/RGO/TiO₂ composite under visible light can reach approximately 750 μA cm⁻². The CdSe/RGO/TiO₂ composite exhibited good photocathodic protection for 304SS. When the light was off, the protection can be still maintained for more than 12 h.
4.  TiO₂ nanotubes were modified by the CdTe (E_g = 1.5 eV) with high absorption coefficient and graphene with high conductivity. With the combination of photo electrochemistry and corrosion electrochemistry, the photocathodic protection properties of the composite for 304SS were measured. Results show that the photocurrent generated by the CdSe/RGO/TiO₂ composite under visible light can reach approximately 750 μA cm⁻². The CdTe/RGO/TiO₂ composite exhibited good photocathodic protection for 304SS under illumination and in the dark.

5.  The photocathodic protection performances of the composite for the low corrosion-resistant carbon steel with were investigated. CdSe quantum dots were fabricated by cycle voltammetry on the nanotube arrays. The effect of the concentration of SeO₂ in the electrolyte on the morphology and protection of the CdSe/TiO₂ nanotubes was studied. Results show that the composite exhibited good cathodic protection under visible-light illumination and in the dark. When the CdSe concentration was 15 mM, the composite exhibited the best photocurrent response and photocathodic protection.