The sol-gel synthesis process was used to prepare two types of TiO₂ coating: a cerium-doped TiO₂ sol-gel and plain amorphous TiO₂ prepared separately with different operating parameters. Sigma Aldrich supplied the necessary reagents of analytical grade, which were used without further purification.

Commercially available grades of ASTM 304LSS with a square dimension of 2 cm by 2 cm by 0.5 cm were used as a substrate material. Before the coating process, these 304LSS plates were mechanically polished with SiC emery paper (up to 600 grit) and then degreased with acetone and distilled water in an ultrasonic bath for 10 min. Subsequently, the 304LSS plates were dried and blown with argon gas after cleaning. The precleaned samples were immersed in various known concentrations of Ce-doped TiO₂ and plain TiO₂ sol-gel solution via a commercial dip coating machine. For each layer of coating, the samples were lowered into the sol-gel solution for 1 min and then slowly withdrawn at a regulated speed of 3 mm per min. The samples coated with different concentrations of Ce-doped TiO₂ and plain amorphous TiO₂ coatings were designated as 304LSS/1TO, 304LSS/2TO-10-Ce, 304LSS/2TO-20-Ce, 304LSS/2TO-30-Ce, 304LSS/3TO-20-Ce and subjected to a thermal annealing treatment at the relatively low temperature of 200°C for 1 h in an argon environment (see Table 1) with a heating ramp rate fixed at 5^°C/min for all treatments. A layer of smooth, homogenous, high-quality adhesive coating was obtained over each 304LSS sample after the annealing treatment. To investigate the photocatalytic and charge storage properties of the Ce-doped and plain TiO₂ coatings, ITO glass with the same dimensions of 304LSS was used and coated in the same way as that of the 304LSS samples for pertinent comparative studies (see Table 2). The thermally treated specimens were staged in a desiccator and then used for the photo-electrochemical measurements.

The photo-absorption properties of the prepared TiO₂ and Ce-doped TiO₂ coated samples were measured using an HP8453 (HP Germany) UV-visible spectrometer at a wavelength of 300–500 nm. Glancing angle X-ray diffraction (GAXD) measurements were performed for TiO₂ and Ce-doped TiO₂ samples at the end station of the TLS - 13A beamline of the National Synchrotron Radiation Research Center Facility (NSRRC), Taiwan. The samples were measured at a glancing angle of 0.5° over a 2-theta range of 13°–24°. Furthermore, the chemical states of TiO₂ before and after Ce doping were measured using X-ray photoelectron spectroscopy (XPS, Ulvac- PHI 5000 Versa Probe II) with Al K_α excitation source. The surface morphologies of the prepared substrate samples were observed using field emission scanning electron microscopy (FE-SEM, JEOL JSM-6330F). In addition, the chemical composition of the TiO₂-coated and Ce-doped TiO₂ samples was analyzed using an energy dispersion spectroscope (EDS, Oxford INCA300, England) attached to the SEM. Both TiO₂ and Ce-doped TiO₂ coatings were prepared over the 304LSS/ITO substrates using dip-coating equipment (Sadhu design).

A commercial potentiostat manufactured by Corrtest Instruments was used to execute potential variation monitoring and electrochemical polarization analyses due to the impact of UV illumination on the open circuit potentials (OCPs) and corrosion current density of the multilayered Ce doped TiO_2-coated 304LSS specimen. Briefly, to simulate the ambient marine environment, electrochemical analyses were conducted at room temperature in an electrolyte solution (3.5% sodium chloride, NaCl) under aerated conditions with a pH of 7. The operating parameters for the electrochemical polarization analysis were set up in the potential window of 100mv lower and greater than the obtained equilibrium potential vs. saturated calomel electrode from the open circuit potential monitoring analysis. The potential increment or decrement was set up at 15 mv/min. Figure 2 shows the setup schematic of the custom-made three-electrode photo electrochemical system in which the TiO_2-coated 304LSS acted as a working electrode while the platinum-coated titanium mesh and saturated calomel electrode served as the counter and reference electrode, respectively. Additionally, the system was equipped with a quartz glass window to allow the UV illumination to fall on the 304LSS sample. A 500 W Hamamatsu-made (LC8) mercury-xenon lamp was used as an ideal spot UV light source to stimulate UV light with an intensity of 4,300 mW/cm² and a wavelength of 254 nm. Before the startup of each round of polarization analysis, all samples were immersed in the electrolyte solution for 30 min to allow the equilibrium potential to stabilize.

The UV visible absorption spectra of the various concentrations of Ce-doped TiO₂ and plain TiO₂ coatings applied over the ITO glass substrates were measured in the wavelength range of 300–500 nm, as shown in Figure 3. Linear extrapolation was employed to determine the absorption onset wavelength of all samples. The absorption spectrum of the plain amorphous TiO₂ coatings shows strong absorption in the deep–UV region and was cut off at the absorption edge at 325 nm, while all the Ce-doped TiO₂ samples showed a progressive red shift of absorption edge with an increase in the concentration of Ce from 10% to 30%. Among the samples, the 30% Ce–doped TiO₂ exhibited the highest absorption edge at 370 nm. The most significant factor attributed to the redshift was the introduction of defect sites accompanying the Ce element in the TiO₂ matrix, which can narrow the bandgap of TiO₂(Luo et al., 2015). The results indicate that doping Ce into the TiO₂ coating significantly reduces band gap energy and effectively inhibits the recombination of the photogenerated electron holes.

To characterize the crystalline nature and structure of the TiO₂ coatings under different concentrations of cerium, glancing angle X-ray diffraction (GAXD) measurements were performed. Plain and Ce-doped samples coated over ITO substrates were annealed at 200°C for 1 h. Figure 4 , shows the GAXD pattern of the various concentrations of the plain and Ce-doped TiO₂ samples plotted in reciprocal space q (q = 4πsin θ/λ), where θ and λ represent the Bragg angle and incident X-ray wavelength (1.03 Å), respectively. It was observed from these XRD results that plain and Ce-doped TiO₂ samples showed no obvious diffraction peaks due either to cerium or TiO₂, except those originating from ITO substrates. Furthermore, the GAXD results indicate that all samples annealed at 200°C remained amorphous irrespective of doping concentrations.

To demonstrate variations in the samples’ chemical states, XPS was carried out. XPS spectra of the plain TiO₂ samples revealed the existence of elements such as Ti, O, and C; no other impurities were found. Figure 5A, further represents the XPS results of Ti 2p spectra for TiO₂ and 30% Ce-doped TiO₂ samples. In the plain TiO₂ samples, the peaks at binding energies of 458.4 and 464.1 eV are attributed to Ti 2p_3/2 and Ti 2p_1/2, respectively. The splitting of Ti 2p doublet around 5.7 eV indicates the presence of Ti⁴⁺ in the TiO₂ samples, which is consistent with other studies (Zhang et al., 2009; Zhu et al., 2017). After doping 30% of Ce into the TiO₂ samples, a shift of about 0.1 eV towards higher binding energy was observed. In addition, Ce 3days XPS spectra as shown in Figure 5B, further confirm the presence of Ce in the 30% doped samples. The existence of multiple peaks around 880–905 eV may be due to the coexistence of Ce³⁺ and Ce⁴⁺ oxidation states in the Ce-doped TiO₂ coating, suggesting that the sample surfaces are not fully oxidized and some oxygen vacancies in the samples might exist (Zhang et al., 2009; Zhang et al., 2012).

The shift of binding energy towards higher values for 30% Ce-doped TiO₂ samples in comparison with plain TiO₂ might account for the surface enrichment of the ceria leading to intermixing or the charge transfer between Ti and Ce atoms (Luo et al., 2015).

The morphology and surface nature of the various layers of Ce-doped TiO₂ coatings with different Ce contents on the 304LSS substrate were analyzed by SEM, and the results are shown in Figure 6. As expected, every sample with the Ce-doped TiO₂ coating shows an excellent sealable and crack-free intact film covering the sample surface, which is consistent with previous results (Li et al., 2012). It is believed that the addition of cerium ions to the TiO₂ coating restrains crystal growth and contributes to a balance between compactness and chemical stability. Among the Ce-doped TiO₂ coatings, the one with 30% cerium showed a more homogeneous and denser distribution of cerium ions over the surface, and the corresponding EDX result reveals that the prepared coating consisted of Ti, O, and Ce elements with an atomic percent of 11.5%, 2.5%, and 2.5%, respectively. To further improve the stability and corrosion resistance of the 30% Ce-doped TiO₂ coating in the solution containing chloride ions, additional layers of plain amorphous TiO₂ were fabricated over it. Figure 6D shows the surface morphology of the 304LSS/3TO-30-Ce sample substrate. The TiO₂ coverage on the aforementioned sample was continuous and densely distributed on the outer surface of the Ce-doped TiO₂ coating. The cross section morphology and coating thickness of the 304LSS/3TO-30-Ce was determined using focused ion beam (FIB) analysis. The results showed that the thickness of the Ce doped TiO₂ coating was 442 nm and that of the amorphous TiO₂ coating was around 355 nm.

The photo-cathodic protection properties of the Ce-doped TiO₂ and plain amorphous TiO₂ coatings were investigated by OCP measurements in the designated electrolyte solution of 3.5% NaCl for ITO and the 304LSS substrates.

Electrochemical polarization analysis was conducted to investigate the corrosion resistance of a various layers of TiO₂ coated and uncoated 304LSS sample exposed to an aerated 3.5% sodium chloride electrolyte solution in the absence of UV illumination (see Figure 9A). Before each round of polarization analysis, an electrolyte solution was thoroughly purged with air for 20 min. The sample was then immersed in the solution for 3 h until a stabilized equilibrium was reached. The results showed that the electrochemical corrosion potential (ECP) of the TiO₂ coated samples were much higher than that of the uncoated 304LSS sample. The corrosion current density of the uncoated sample was 3.02 × 10⁻⁸ A/cm². In the meantime the 304LSS/1TO sample with only one layer of amorphous TiO₂ coating exhibited a lower anodic current density than the bare one, even in the absence of UV illumination. However, the 304LSS/3TO-30-Ce sample had a significantly higher anodic current density of 1.70 × 10⁻⁵ A/cm². During the polarization analysis the potential scanning direction of the current from the active potential to the noble side led to initiation of Ce³⁺ oxidation to Ce⁴⁺, which resulted in the higher anodic current density for the 304LSS/3TO-30-Ce sample, consistent with what was observed in a previous study (Sonar et al., 2022).

Figure 9B shows the samples’ comparative polarization curves under different conditions in the absence and presence of UV illumination. Table 3 summarizes the test conditions. In the presence of UV illumination, photocatalytic reactions occurred at the surface of the sample, showing a noticeable cathodic shift in electrochemical corrosion potential. This was more active than in the uncoated sample, signifying an accumulation of photogenerated electrons. Accordingly, the corresponding corrosion current density was significantly lowered to 7.15 × 10⁻¹² A/cm². It should be noted that the increase in the anodic current density can be ascribed to the release of the photogenerated electrons from the space charge layer. For the same sample discussed in the preceding section, the ECP change was in good agreement with the OCP variation. To estimate the time-dependent variation in the electrochemical corrosion potential concerning the samples’ corresponding corrosion current density, polarization analyses were conducted at 1 h, 12 h, and 24 h after UV illumination was turned off. The sample’s ECP shows a notable increase in potential after 1 h, which further increased after 10 h and 24 h. This was consistent with the sample’s OCP variations discussed in the previous section. The samples’ respective corrosion densities were estimated to be around 6.10 × 10⁻¹¹ A/cm² after 1 h, and 2.47 × 10⁻⁹ A/cm², 1.06 × 10⁻⁸ A/cm² after 12 h and 24 h respectively. This outcome indicates that the coated sample’s corrosion current density could last one order of magnitude less than that of the uncoated sample, even after 24 h in the absence of UV. This suggests the retention of cathodic protection of the metal alloy by using a specifically designed multilayered TiO₂ coating.