In-situ Construction of Co-TiO₂/SrTiO₃ Heterojunctions with Significantly Enhanced Photocathodic Protection Performance

Metal corrosion poses significant environmental and economic challenges, while traditional electrochemical protection methods, such as sacrificial anodes and impressed current techniques, have inherent limitations. Photocathodic protection technology, in which solar energy is utilized to provide electrons for metal protection, offers advantages, such as environmental friendliness and the absence of external metal components. TiO₂ nanotube arrays (NTAs) have been widely used in photocathodic protection, due to their stability and excellent photocatalytic properties. However, their relatively large bandgap and low efficiency of photogenerated charge carrier separation limit their performance for practical applications. To address this issue, a Co-TiO₂/SrTiO₃ heterojunction was proposed through Co ion doping and SrTiO₃ composite formation, aiming to extend the light absorption range of TiO₂ and improve the separation and transport efficiency of photogenerated charge carriers, thereby enhancing its effectiveness in photocathodic protection applications.

Co-TiO₂ NTAs and Co-TiO₂/SrTiO₃ heterojunction materials were fabricated through electrochemical anodization and hydrothermal methods, respectively. Co-TiO₂ NTAs were prepared by using Ti foil as the anode and Pt mesh as the cathode in an ethylene glycol solution containing 0.5 wt.% NH₄F and 9 mmol·L⁻¹ Co (NO₃)₂·6H₂O, at 40 V for 1 h. The obtained Co-TiO₂ NTAs were then annealed at 450℃ for 3 h. Subsequently, the samples were subjected to a hydrothermal reaction in a solution containing 0.565 g Sr(OH)₂·8H₂O at 180℃ for 1 h to obtain Co-TiO₂/SrTiO₃ heterojunction. The crystal structures of the materials were analyzed using D/MAX2500PC X-ray diffraction (Cu-K_α) and the microstructures were characterized using FEI field-emission scanning electron microscopy (SEM). UV-Vis diffuse reflectance spectra were measured using a spectrophotometer and photoluminescence (PL) spectra were obtained with an F-7000 fluorescence spectrophotometer. Photoelectrochemical properties were evaluated using a CHI660E electrochemical workstation and a dual-electrolyte cell system.

The crystal structure analysis via X-ray diffraction (XRD) confirms that SrTiO₃ and TiO₂ successfully formed composite, without any impurity. The incorporation of Co ions induced lattice distortion of TiO₂, resulting in a slight shift of the diffraction peaks toward lower diffraction angles. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) results further verified that SrTiO₃ is uniformly deposited on the walls of TiO₂ nanotubes, while the EDS results also confirmed the successful doping of Co. Photoluminescence (PL) and UV-visible diffuse reflectance spectroscopy (UV-vis DRS) results demonstrated that the Co-TiO₂/SrTiO₃ composite film exhibited enhanced light absorption and higher efficiency in photogenerated charge carrier separation. Electrochemical test results revealed that the Co-TiO₂/SrTiO₃ composite film significantly enhanced the photoelectrochemical cathodic protection of 304 stainless steel, as compared with other samples, with the largest negative shift in open-circuit potential, reaching −0.58 V, a 420 mV decrease from the self-corrosion potential of 304 SS (−0.16 V). The photocurrent density also reached the highest value of 60 μA·cm⁻². Electrochemical impedance spectroscopy (EIS) and Tafel test results further validated the enhanced photoelectrochemical cathodic protection performance of the Co-TiO₂/SrTiO₃ composite film. Additionally, based on the valence band position calculated using the Schottky model, a schematic diagram of the photoelectrochemical cathodic protection mechanism was proposed.

Co-TiO₂ and TiO₂/SrTiO₃ composite films were successfully prepared, as photoanodes for photoelectrochemical reactions, with a focus on exploring their potential in the photoelectrochemical cathodic protection of 304 stainless steel. It has been demonstrated that the Co-TiO₂/SrTiO₃ heterojunction exhibited enhanced light absorption capacity, thus optimizing the separation efficiency of photogenerated charge carriers, increasing electron transfer rate and significantly raising the photocurrent density. Further photoelectrochemical test results revealed that the composite film coupled with 304 stainless steels exhibited a marked negative shift in the open-circuit potential, as compared with the untreated 304 stainless steel, demonstrating excellent photoelectrochemical cathodic protection performance. These findings indicated that the Co-TiO₂/SrTiO₃ composite film has potential in photoelectrochemical cathodic protection, providing valuable theoretical insights for the development of related materials and photoelectrochemical applications.