Abstract:

Photocathodic protection by TiO₂ semiconductor materials for metals has interested many corrosion researchers for years. However, a pure TiO₂ semiconductor anode can only absorb ultraviolet light and cannot maintain the photocathodic protection in the dark. This has limited its practical applications to a great extent. Overcoming these limitations is significant as well as challenging. Therefore, the objective of this work is to prepare a modified TiO₂ composite film with visible light absorption and charge storage capabilities for application in photocathodic protection. First, we fabricated an ordered TiO₂ nanotube array film on a Ti substrate by electrochemical anodization. Then, we prepared NiO nanoparticles on the film via a hydrothermal reaction to obtain a p-n heterostructured NiO/TiO₂ nanotube array composite film. The properties of the prepared films were investigated by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-Vis absorption spectroscopy, photoluminescence spectroscopy, and photoelectrochemical techniques. The results indicated that the electrochemically anodized TiO₂ film had an anatase phase structure and consisted of vertically ordered nanotubes with an inner diameter of about 80 nm and length of 250 nm. After the NiO nanoparticles were deposited on the film, the TiO₂ nanotube array structure remained intact. The main phase of TiO₂ was still anatase, but the light absorption of the NiO/TiO₂ composite film was extended into the visible region, which was in contrast to that of the simple TiO₂ film. Moreover, the composite film showed lower photoluminescence intensities than the TiO₂ film, implying that a higher charge carrier separation efficiency could be achieved by modification with NiO. Under white light illumination, the photocurrent density of the NiO/TiO₂ composite film in a mixed solution of 0.5 mol·L^-1 KOH and 1 mol·L^-1 CH₃OH reached 176 μA·cm^-2, which was 2 times higher than that of the simple TiO₂ nanotube film, indicating that the composite film had improved photoelectric conversion efficiency and photoelectrochemical properties. The potential of 403 stainless steel (403SS) in 0.5 mol·L^-1 NaCl solution decreased by 380 and 440 mV relative to its corrosion potential when coupled to the TiO₂ film and NiO/TiO₂ composite film, respectively, under white light illumination. This indicated that the heterostructured NiO/TiO₂ film as a photoanode could produce more effective photocathodic protection on the steel as compared with the pure TiO₂ film. Even after 2.5 h of illumination, the composite film could continuously provide photocathodic protection to 403SS for about 15.5 h in the dark, suggesting that the NiO/TiO₂ composite film had a charge storage capability that was significant for its practical applications.

Key words: Anodic oxidation, Hydrothermal treatment, TiO₂ nanotube, NiO, Photoelectrochemistry, Photocathodic protection, Stainless steel