Facile Fabrication of CuO Modified TiO₂ Heterostructure for Enhanced Photocathodic Corrosion Protection of 304 Stainless Steel

Abstract

In recent years, protecting stainless steel from corrosion has become crucial, particularly in harsh environments. The present study focuses on improving the photocathodic corrosion resistance of 304 stainless steel (304SS) by fabricating TiO₂/CuO composite coatings using the spin coating technique with varying CuO weight percentages. Structural characterization through X-ray diffraction (XRD) confirmed the presence of the anatase phase of TiO₂ and the successful integration of CuO. Raman spectroscopy demonstrated redshifts in the TiO₂ characteristic peaks, suggesting changes in bond lengths attributed to CuO incorporation. These findings were further corroborated by Fourier-transform infrared (FTIR) spectroscopy. Surface characterization showed uniform, porous coatings with pore sizes ranging from 75 to 200 nm, which contributed to improved barrier properties. UV–visible diffuse reflectance spectroscopy (UV-DRS) demonstrated enhanced visible light absorption in the heterostructures. Mott–Schottky analysis confirmed improved charge carrier density and favorable band alignment, facilitating efficient charge separation. The electrochemical performance was evaluated in 3.5% NaCl solution under dark and light environments. The results demonstrated that the TiO₂/CuO heterostructure significantly enhanced electron transfer and suppressed electron-hole recombination, providing adequate photocathodic protection. Notably, under illumination, the TiO₂/CuO (0.005 g) coating achieved a corrosion potential of −255 mV vs SCE and reduced the corrosion current density to 0.460 × 10⁻⁶ mA cm⁻². These findings suggest that TiO₂/CuO coatings offer a promising, durable, and cost-effective solution for corrosion protection in industries such as oil, shipbuilding, and pipelines.

1. Introduction

Corrosion of stainless steel, particularly 304SS, has garnered significant attention due to its widespread use. Its versatility makes it ideal for applications ranging from offshore appliances and shipbuilding to household items like refrigerators and sinks and industrial uses such as piping and machinery. 304 stainless steel (304SS) is commonly utilized in offshore applications due to its superior corrosion resistance and ease of fabrication [1]. Despite these advantages, it remains susceptible to localized corrosion, such as pitting and crevice corrosion, particularly in marine environments or chloride-rich solutions. This degradation occurs when chloride ions penetrate and disrupt the passive oxide film on the steel surface, resulting in the initiation of pits that can significantly compromise the structural integrity of the material [2,3].

Several strategies have been employed to address this, such as anodizing, applying protective coatings, and surface treatments [4]. Recently, the photocathodic corrosion protection technique has emerged as an effective method to prevent the material from chloride ion incursion. The photocathodic protection for metals has gained increasing attention as a promising and environmentally friendly technology [5,6,7]. This method leverages the ability of certain semiconductor materials to generate photoelectrons when exposed to light, providing additional protection against corrosion [8]. A significant challenge in photocathodic corrosion protection is identifying suitable photoactive semiconductors that can withstand long-term environmental conditions.

Yuan and Tsujikawa initially demonstrated TiO₂ as a photoanode for corrosion protection in 1995, where it successfully protected copper surfaces [9]. Since then, TiO₂ has become a prominent material in photocathodic corrosion protection applications. Over the past 30 years, extensive research has focused on employing various semiconductors—such as TiO₂, SrTiO₃, In₂O₃, and ZnO—for the photocathodic protection of metals [6,9,10,11,12,13,14,15]. Among these, TiO₂ is the most widely utilized, owing to its exceptional photoelectrocatalytic behavior, excellent chemical stability, strong oxidative capability, and environmental friendliness. These characteristics make TiO₂ an ideal material for corrosion prevention, particularly in seawater or chloride-rich solution applications, where corrosion is a significant concern [16,17,18,19,20,21,22,23,24].

However, TiO₂ is limited by its wide band gap, which confines its light absorption primarily to the ultraviolet region and its quick charge recombination rate [25,26]. Researchers have explored various modifications to TiO₂ to improve its photocatalytic activity and corrosion resistance, further advancing the field of photocathodic protection. To improve the photocatalytic efficiency of TiO₂, extensive research has focused on strategies such as doping and combining it with narrow-bandgap semiconductors. Materials like ZnSe, CdS, WO₃, graphene, Mn-Cr, Ag₂S, Zn₃In₂S, α-Fe₂O₃, and V₂O₅ have been explored for this purpose [27,28,29,30,31,32,33,34,35].

Among these, copper oxide (CuO), with a relatively narrow band gap of approximately 1.8 eV [36], is particularly attractive due to its strong absorption in the visible light region and its frequent use in photocatalytic applications. As a p-type semiconductor, CuO can establish a built-in electric field at the interface when paired with an n-type semiconductor, facilitating charge separation and enhancing photocatalytic activity. This p-n heterostructure accelerates the directional movement of photo-generated electrons and holes, enhancing charge carrier separation and improving photocathodic corrosion protection [37,38]. However, the TiO₂/CuO heterostructure has been extensively explored in fields such as photocatalytic organic pollutant degradation, CO₂ reduction, and hydrogen production [39,40,41,42]. Despite this extensive exploration, their use in photocathodic corrosion protection remains largely unaddressed. Most of the available literature focuses on the photocatalytic performance of these materials, with limited reports detailing their application as protective coatings for metallic substrates. Therefore, integrating CuO with TiO₂ in a heterostructure presents substantial potential for developing advanced materials that effectively overcome the individual limitations of CuO and TiO₂ [43,44,45]. The utility of TiO₂/CuO systems extends beyond traditional photocatalysis and introduces a new pathway for sustainable corrosion protection of metals under illumination.

This study synthesized a TiO₂/CuO heterostructure coating and applied it onto 304 stainless steel (304SS) substrates via spin coating to enhance photocathodic protection under visible light irradiation. The structural and chemical features of the coatings were analyzed using X-ray diffraction (XRD), Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). Additionally, the optical properties and bandgap energies were evaluated using UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), and the semiconductor behavior and flat-band potentials were assessed through Mott–Schottky analysis. Morphological and elemental analyses were conducted using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). Electrochemical techniques systematically evaluated the photocathodic protection performance, including open-circuit potential (OCP), potentiodynamic polarization, and photocurrent measurements under illumination. This combination of analytical and electrochemical techniques enabled a comprehensive evaluation of the corrosion protection capability of the TiO₂/CuO heterostructure. Investigating TiO₂/CuO heterojunctions for photocathodic corrosion protection provides deeper insights into recent advancements in TiO₂/CuO heterostructures and strategies for optimizing their design. Employing TiO₂/CuO heterostructures as photoanodes significantly improves photocathodic corrosion protection by promoting efficient separation of photo-generated charge carriers and facilitating electron transport to the metal surface.

2. Materials and Methods

2.1. Materials

Titanium tetraisopropoxide (Ti(OCH(CH₃)₂)₄), n-butanol (BuOH), and copper nitrate (Cu(NO₃)₂·3H₂O) were sourced from Sigma-Aldrich. All reagents and chemicals used in the synthesis process were of analytical grade and utilized without additional purification. Deionized water was employed in all experimental procedures.

The chemical composition of 304 stainless steel (304SS) is summarized in

Table 1. Weight percentage composition of the 304SS stainless steel alloy (304SS).

GRADE	Elements Composition %
	C	Mn	Si	P	S	Cr	Ni	N	Fe
304SS	0.08	2.0	0.75	0.045	0.030	18	8	0.10	70.995

. The 304SS substrates were cut into rectangular pieces with dimensions of 10 mm × 20 mm × 2 mm for coating and electrochemical analysis. The substrate was polished using silicon carbide abrasive paper of varying grit sizes to remove surface imperfections and achieve a smooth finish. Coarser grits (e.g., 80 or 120 grit) were initially used to eliminate visible surface defects, such as scratches or uneven areas. Finer grits (e.g., 400, 800, 1000, and 2000 grit) were employed, followed by diamond paste and aerosol polish for further refinement. This thorough polishing process resulted in a scratch-free, mirror-like finish on the 304SS substrate, providing an optimal surface for subsequent treatments. After polishing, the substrate was thoroughly rinsed with distilled water, cleaned with acetone, and washed with ethanol.

2.2. Synthesis of TiO₂/CuO Heterostructure

Titanium tetraisopropoxide (TTIP) was used as the metal oxide precursor, and n-butanol served as the solvent in a 2:8 ratio to synthesize TiO₂ via the sol–gel method [46,47]. The TiO₂ sol was prepared by stirring the mixture for 2 h. The TiO₂/CuO heterostructure was synthesized using a simple chemical reduction method [48,49]. Specifically, different amounts of copper nitrate (Cu(NO₃)₂), such as 0.001 g, 0.005 g, 0.01 g, and 0.05 g, were dissolved in methanol to form a CuO solution. Then, the CuO solution was slowly added to the as-prepared TiO₂ sol. The mixture was stirred for 30 min to allow uniform dispersion and interaction. Subsequently, the resulting solution was ultrasonicated for 1 h to ensure homogeneous mixing and to facilitate heterostructure formation.

2.3. Fabrication of TiO₂/CuO Heterostructure Coating on 304SS Alloy

The TiO₂/CuO heterostructure coatings were prepared using a spin-coating method (Apex Instrument, Kolkata, India). A TiO₂/CuO sol was spin-coated onto 304SS substrates, then sintered at 150 °C to form the initial layer. The coatings were designated as TiO₂/CuO (0.001), TiO₂/CuO (0.005), TiO₂/CuO (0.01), and TiO₂/CuO (0.05), based on the respective CuO concentrations incorporated in a 10 mL solution: 1 mg, 5 mg, 10 mg, and 50 mg, respectively. While variations in CuO content influenced corrosion resistance, electrochemical performance, and stability, the overall coating thickness remained unchanged. Multiple spin-coating cycles were performed using the TiO₂/CuO (0.005) sol to examine the effect of coating thickness. A single deposition corresponded to one layer, while two, three, and four consecutive depositions resulted in coatings with additional layers. The resulting coated substrates were sintered at 450 °C in a muffle furnace (Indfurr Superheat Furnaces, Chennai, India) before undergoing electrochemical studies to evaluate their performance. The primary objective was to determine the optimal CuO concentration for enhancing the corrosion resistance and electrochemical stability of the TiO₂/CuO coatings and assess the impact of coating thickness on these properties. A schematic illustration of the TiO₂/CuO heterostructure synthesis and coating process is presented in Figure 1.

2.4. Characterization

Characterization of the samples was performed using various analytical techniques. Raman spectra were collected at room temperature using an optical microscope equipped with a WITec Raman spectrophotometer (WITec GmbH, Baden-Wurttemberg, Germany). The instrument was calibrated with a Si(100) standard at a wavelength of 532 nm before spectral acquisition. The chemical bonding characteristics of the TiO₂/CuO heterostructures were examined using Fourier-transform infrared (FTIR) spectroscopy in attenuated total reflectance (ATR) mode, employing a JASCO 6800-FV-BB spectrometer (JASCO Corporation, Tokyo, Japan). Surface topography was analyzed using atomic force microscopy (AFM) on an Asylum Research MFP-3D Origin AFM (Oxford Instruments, Abingdon, UK) operated in tapping mode. Silicon nitride cantilevers with a spring constant of 0.15 N/m and a resonance frequency of 300 kHz were used for imaging, which was conducted under ambient conditions at room temperature. The surface morphology of the coatings was further observed using scanning electron microscopy (SEM) with a Carl Zeiss instrument (Oberkochen, Germany) operated at an accelerating voltage of 10 kV. The electrochemical performance was evaluated using an Origaflex OGF05A electrochemical workstation (Origalys Electrochem SAS, Rillieux-la-Pape, France) in a standard three-electrode configuration. Photocathodic protection behavior was studied using a 150 W Xenon Short Arc Lamp integrated into the SS50AAA-TP/EM Solar Simulator (Moorpark, California, United States) (Class AAA, ASTM E927-2010 [47], IEC 60904 9 Ed. 2.0), equipped with an AM1.5G filter. The illuminated area was 50 mm × 50 mm, with a calibrated light intensity of 100 mW/cm² (1 sun). The AM1.5G filter ensured a spectral range of 300–800 nm, simulating natural sunlight under global tilt conditions. Open circuit potential (OCP) measurements were carried out in 3.5 wt% NaCl solution, using the coated 304SS electrode (1 cm² exposed surface area) as the working electrode, a saturated calomel electrode (SCE) as the reference, and a platinum foil as the counter electrode. OCP was monitored under dark and illuminated conditions for 30 min to ensure stabilization. After achieving a stable potential, electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 100 kHz to 10 mHz with a 10 mV AC amplitude, lasting approximately 7–15 min. Potentiodynamic polarization (Tafel) measurements were then conducted by scanning the potential from −250 mV to +250 mV versus OCP at a scan rate of 1 mV/s, each taking 10–20 min. Finally, chronoamperometry was performed at a fixed potential of 0.2 V under alternating light and dark conditions, with 300 s intervals per cycle, for 30 min to assess the photocurrent response and stability. All electrochemical tests were repeated three times to confirm the reproducibility of the results.

3. Results

3.1. Raman Analysis

The Raman spectra revealed surface modifications induced by vibrational modes, and the analysis was performed to demonstrate the phases and impurities detected in the TiO₂/CuO heterostructure. Figure 2 shows strong Raman signals within the 140 to 800 cm⁻¹ range, which is commonly used to identify heterostructures. For anatase TiO₂, six distinctive Raman-active modes were observed at 144 cm⁻¹ (E_g), 197 cm⁻¹ (E_g), 397 cm⁻¹ (B_1g), 518 cm⁻¹ (A_1g + B_1g), and 640 cm⁻¹ (E_g), substantiating the presence of the anatase TiO₂ phase [48,49]. In the TiO₂ Raman spectrum, the active vibrational modes included (i) symmetric vibration of O–Ti–O, corresponding to the doubly degenerate E_g modes at 144 and 640 cm⁻¹, and (ii) symmetric bending vibration of O–Ti–O, associated with the B_1g mode at 397 cm⁻¹. Additionally, the A_g peak at 245 cm⁻¹, attributed to CuO, further confirmed the presence of anatase TiO₂-specific peaks, as reported in the literature [50,51]. The CuO caused shifts in the Raman peaks associated with the B_1g and A_1g vibration modes, confirming the structural phases of TiO₂ and CuO in the TiO₂/CuO heterostructure. The incorporation of CuO altered the structural and vibrational features of the TiO₂ lattice, suggesting significant interaction between the two oxides. Such interactions are likely to generate surface defects that may function as photoactive sites, thereby improving charge separation efficiency [52]. As observed, the Raman spectrum of CuO at 245, 356, and 582 cm⁻¹ corresponded to A_g and B_g modes, respectively [53]. However, in the TiO₂/CuO heterostructure analysis, CuO-related signals were not prominently detected, likely due to signal suppression by the stronger TiO₂ peaks. This was ascribed to the enhanced crystallinity or greater abundance of TiO₂ within the heterostructure. The absence of detectable CuO peaks further supported the successful formation of the TiO₂/CuO heterostructure. Similar peaks were observed across the heterostructures with different CuO weight percentages, confirming the consistent presence of anatase TiO₂ crystallites. At lower CuO contents, CuO-related signals were more challenging to detect due to the dominant Raman response of TiO₂, which highlighted the successful integration of CuO into the heterostructure [54].

Additionally, a slight red shift was recorded in the Raman peaks of the TiO₂/CuO heterostructure compared to pure TiO₂. Specifically, the E_g mode of TiO₂ shifted from 144 cm⁻¹ to 151 cm⁻¹, indicating the formation of the TiO₂/CuO heterostructure [49]. A magnified view of the 245 cm⁻¹ and 356 cm⁻¹ regions reveals more distinctly the evolution of CuO-related vibrational modes. These regions were carefully analyzed to examine the acceptable structural variations induced by the incorporation of CuO at varying concentrations. The shift and broadening of these peaks with increasing CuO content suggest enhanced coupling effects and phonon confinement within the heterostructure, which may be crucial in improving photocathodic performance.

3.2. Fourier Transformer Infrared Spectroscopy

The FTIR spectra of TiO₂, CuO, and TiO₂/CuO heterostructures with varying CuO concentrations (TiO₂/CuO (0.001), TiO₂/CuO (0.005), TiO₂/CuO (0.01), and TiO₂/CuO (0.05)) coated on 304SS are presented in Figure 3. A broad peak between 3500 and 3400 cm⁻¹, visible in Figure 3, arises due to the O-H stretching mode of H₂O, indicating the presence of surface-adsorbed water and hydroxyl groups on the surface of pure CuO and TiO₂. Additionally, a peak near 1600 cm⁻¹, attributed to the O-H bending mode of water, appears in the spectra of these pure samples [55,56]. A prominent band below 1000 cm⁻¹, observed at 663.51 cm⁻¹, corresponds to the metal–oxygen (M-O) bond, signifying both Cu–O and Ti–O bonds. This strong absorption band confirms effective chemical interactions between CuO and TiO₂ in the heterostructures, contributing to enhanced charge transfer and stability, which are essential for photocathodic protection. The spectra show that the region between 1000 and 400 cm⁻¹ displays peaks associated with the symmetric stretching of the Cu–O and Ti–O bonds. Increasing CuO concentration in the heterostructures gradually decreases the peak intensities, indicating robust interactions between CuO and TiO₂. These findings confirm the successful formation of TiO₂/CuO heterostructures on 304SS, which are expected to enhance photocathodic protection efficiency by facilitating electron transfer and corrosion resistance [57,58].

3.3. UV–vis-Diffusion Reflectance Spectra

The UV–vis diffuse reflectance spectra (UV–vis DRS) of TiO₂, CuO, and TiO₂/CuO heterostructures with varying CuO content (0.001, 0.005, 0.01, and 0.05 wt%) are shown in Figure 4a. Pure TiO₂ exhibits strong absorption in the UV region but limited absorption in the visible range, reflecting its wide band gap and restricted activity under visible light [36]. In contrast, CuO displays significant absorption in the visible region due to its narrow band gap (~1.8 eV), indicating its potential for enhanced solar light utilization [25]. The TiO₂/CuO heterostructures exhibit a noticeable redshift in the absorption edge and a broadened visible light response compared to pristine TiO₂. This shift confirms the formation of heterojunctions between TiO₂ and CuO, facilitating improved light harvesting and charge separation efficiency. The Tauc plots (Figure 4b) estimated the optical band gaps using the transformed Kubelka–Munk function [59]. The band gaps were calculated to be approximately 3.20 eV (a, TiO₂), 1.80 eV (b, CuO), 2.96 eV (c, TiO₂/CuO–0.001), 2.17 eV (d, TiO₂/CuO–0.005), 2.54 eV (e, TiO₂/CuO–0.01), and 2.74 eV (f, TiO₂/CuO–0.05). These results indicate that the TiO₂/CuO heterostructures possess band gap values intermediate between those of TiO₂ and CuO, with the (0.005) CuO composite exhibiting the most significant narrowing [60]. The reported band gaps were experimentally determined by UV–Vis DRS and may vary slightly due to microstructural effects and defect states in the coatings. This suggests an enhanced ability to absorb visible light and implies improved photoelectrochemical performance under solar irradiation.

3.4. Scanning Electron Microscopy

The surface morphology of uncoated 304SS and TiO₂, CuO, and TiO₂/CuO (0.005) coated 304SS substrates was examined via SEM, as depicted in Figure 5. The SEM image of bare 304SS (Figure 5a) reveals grid-like marks resulting from mechanical polishing, indicating a smooth baseline surface with minimal microstructural irregularities at both magnifications [38]. In contrast, the TiO₂-coated 304SS (Figure 5b) appears as a uniform layer at lower magnification. However, at higher magnification, the surface reveals nearly spherical, agglomerated particles forming a microporous structure. This morphology increases surface area and promotes electrolyte interaction, while the uniform coverage suggests strong adhesion and adequate surface protection, making it suitable for catalytic and protective applications.

Similarly, the CuO coated 304SS (Figure 5c) shows a relatively continuous layer at lower magnification. Still, a coarser, irregular particle distribution with large voids and a textured crystalline morphology becomes evident at higher magnification. This structure enhances surface stability and reactivity by facilitating electron transport, which is attributed to CuO’s high porosity and irregular particle shapes.

Herein, incorporating CuO onto TiO₂ modifies the surface morphology due to heterogeneous nucleation, which promotes the formation of more granular, textured features on the TiO₂ matrix. This modified surface morphology contributes to enhanced interfacial contact between the coating and the 304SS substrate, leading to improved mechanical anchoring and better dispersion of photo-generated charge carriers, both of which contribute to the stability of the heterostructure. Additionally, the p-type nature of CuO promotes strong electronic interaction with n-type TiO₂, enabling the formation of a compact, stable heterojunction that resists delamination or surface degradation. The TiO₂/CuO (0.005) coated 304SS (Figure 5d) shows a seemingly uniform surface at lower magnification (1 µm). Still, at higher magnification (200 nm), it reveals a combination of roughly spherical TiO₂ and irregular CuO particles forming an interconnected, textured surface. This morphology improves intraparticle charge transfer and enhances corrosion resistance by promoting electrolyte interaction and electron movement across the coating. Thus, the TiO₂/CuO (0.005) heterostructure provides an optimal balance of porosity, roughness, and stability, supporting improved electrochemical performance and corrosion protection. The interconnected surface texture facilitates sustained photo-generated electron flow, enhancing light absorption and charge separation. These characteristics render the TiO₂/CuO (0.005) coating a potential material for improving the photocathodic protection performance of 304SS [56].

Cross-sectional SEM analysis (Figure 5e) confirms the uniform thickness and structural integrity of the TiO₂/CuO (0.005) coating, with good adhesion to the 304SS substrate. Figure 5f further highlights the compact interfacial zone, supporting strong mechanical and electronic coupling between the coating and the substrate. The cross-sectional EDS elemental mappings (Figure 5g–j) clearly illustrate the stratified distribution of oxygen (O), titanium (Ti), and copper (Cu) across the coating thickness, validating the spatial separation and coexistence of TiO₂ and CuO phases. Plan-view elemental maps (Figure 5k–n) further confirm the homogeneous dispersion of the elements, demonstrating that the heterostructure is compositionally uniform and effectively integrated. Additionally, to assess the mechanical robustness of the TiO₂/CuO (0.005) coating, an adhesion test using the cross-cut tape method (ASTM D3359) [61] was conducted. The coating achieved a 4B rating, reflecting strong adhesion with minimal flaking at the cut intersections. This indicates a mechanically stable interface between the heterostructure and the 304SS substrate. In practical applications, thermal and mechanical stresses may arise due to environmental fluctuations or operational loads. The mismatch in thermal expansion coefficients between CuO and TiO₂ could induce interfacial stress. However, the graded interface and interlocked morphology observed in cross-sectional SEM mitigate this risk by facilitating stress relaxation. Additionally, the strong adhesion and interconnected structure enhance the coating’s resistance to delamination or cracking, supporting its potential for long-term corrosion protection in dynamic environments.

3.5. Atomic Force Microscopy

From Figure 6a, the AFM 2D topography of bare 304 stainless steel shows a smooth surface with minimal surface irregularities. The average roughness (R_a) is 20 nm, indicating a polished surface with minor scratches due to diamond polishing. Figure 6b shows the TiO₂-coated 304SS surface, demonstrating a uniform coating with a slightly porous texture. The surface roughness increases somewhat due to the porous nature of TiO₂, with an average roughness (R_a) of 36 nm. In Figure 6c, the CuO coating on 304SS reveals a refined surface structure, indicating significant surface modification and an increased average roughness (R_a) of approximately 48 nm due to the presence of CuO crystals. Figure 6d shows the TiO₂/CuO (0.005) heterostructure coating, featuring a well-defined, smooth, and uniform surface, where both TiO₂ and CuO phases contribute to the surface texture of 304SS, with an average roughness (R_a) of 42 nm. Adding CuO to the TiO₂ coating improves the surface topography, enhancing the heterostructure stability on the 304SS substrate [61,62].

The line profile analysis highlights the height and depth profiles of the coated heterostructures. Figure 6e–h displays the line profiles for bare 304SS, TiO₂, CuO, and the TiO₂/CuO heterostructure, respectively. These profiles reveal sub-micrometer to nanometer-scale pores, with peak-to-peak height variations ranging from 25 nm to 200 nm. The AFM line profile analysis suggests that the activity of the thin film is strongly dependent on particle curvature, with crests indicating the sample’s dimensional features and troughs corresponding to pore locations [63,64]. From Figure 6e, the line profile of bare 304SS shows surface wavy and uneven features with smooth transitions. It reveals a relatively flat surface with minor fluctuations, confirming the low roughness. From Figure 6f, the TiO₂ coating shows distinct peaks and valleys in the line profile, indicating a rough and uneven surface. The profile suggests that the TiO₂ film has a slightly porous structure, which correlates with the increased roughness observed in the 2D topography. From Figure 6g, the CuO line profile reveals more significant surface features, indicating the presence of larger CuO crystals. The profile shows deep valleys and higher peaks, suggesting the more textured nature of the CuO surface. The analysis of AFM height profile images quantitatively evaluated particle sizes and surface features using Gwyddion 2.63 software. At least five different surface regions per sample were analyzed to ensure statistical reliability, and the average values for roughness and peak-to-peak height were calculated. The AFM line profiles (Figure 6e–h) highlight the topographical variations across samples. These profiles indicate nanometer to sub-micrometer-scale features, with estimated particle sizes ranging from 75 nm to 200 nm and peak-to-peak height differences from 25 to 200 nm, as shown in Figure 6h, which may enhance electrolyte interaction, supporting improved photocathodic and corrosion-resistant performance on 304SS [61,63,64]. Although increased surface roughness enhances electrolyte contact, it does not reduce corrosion resistance if the surface architecture is well controlled. In this study, the TiO₂/CuO coatings exhibit nanoscale roughness and optimized porosity, which improve light absorption and facilitate efficient charge transfer under illumination. At the same time, the uniform and compact surface structure acts as a barrier, preventing electrolyte penetration and localized corrosion. Thus, the tailored morphology supports enhanced photocathodic performance and corrosion protection of 304SS.

3.6. Open Circuit Potential

The open-circuit potential (OCP) was monitored over time to examine the corrosion resistance and passivation behavior of TiO₂/CuO heterostructure coatings on 304 stainless steel (304SS) in a 3.5% NaCl solution under dark and illuminated conditions, each for 30 min [38]. Figure 7a represents the OCP values as a function of immersion time for the uncoated 304SS, CuO, TiO₂, and TiO₂/CuO heterostructures with varying CuO concentrations (TiO₂/CuO (0.001), TiO₂/CuO (0.005), TiO₂/CuO (0.01), and TiO₂/CuO (0.05)) coated on 304SS under dark and light conditions. The presented values correspond to the final stabilized OCP values obtained after 30 min of continuous exposure. Under dark conditions, the pristine 304SS exhibited an OCP value of –243 mV (vs SCE), indicating the formation of a protective passivation layer. Pure CuO showed a slightly positive shift, with OCP values of −241 mV (vs. SCE), while TiO₂ exhibited a more positive change from −236 mV (vs. SCE). In comparison, the TiO₂/CuO heterostructures showed varying results: the TiO₂/CuO (0.001) and TiO₂/CuO (0.005) coatings displayed positive shifts to approximately −234 and −231 mV (vs. SCE), suggesting adequate corrosion protection, while the TiO₂/CuO (0.01) and TiO₂/CuO (0.05) coatings exhibited negative potential shifts to approximately −237 and −242 mV (vs. SCE), likely due to defects or increased porosity in the coatings. Among the heterostructures, TiO₂/CuO (0.005) provided the most stable and positive OCP, indicating superior corrosion resistance in dark conditions. Under light conditions, the OCP of pristine 304SS remained almost unchanged at −243 mV, as it lacks a photocathodic effect and is not inherently photoactive. Pure CuO showed a significant negative shift to −245 mV, while TiO₂ shifted more negatively to −248 mV, indicating a moderate photocathodic effect. TiO₂/CuO (0.001) and TiO₂/CuO (0.005) exhibited the most significant negative shifts, reaching −250 mV and −253 mV, respectively, thereby enhancing photocathodic protection. TiO₂/CuO (0.01) and TiO₂/CuO (0.05) showed smaller shifts to −249 mV and −247 mV, likely due to defects in the coatings and excessive CuO content, which introduces recombination centers for electrons and holes, which hinder charge transfer from the TiO₂/CuO composite to the 304SS substrate. TiO₂/CuO (0.005) provided optimal photocathodic protection under light [38,65,66,67].

Figure 7b shows the OCP values for the TiO₂/CuO (0.005) coating with varying consecutive depositions under dark and light conditions. For a single deposition, the OCP value was −231 mV (vs SCE), while two consecutive depositions gave an OCP value of −223 mV (vs SCE). Three consecutive depositions resulted in an OCP value of −233 mV (vs SCE), and four consecutive depositions gave an OCP value of −239 mV (vs SCE) under dark conditions. Under light, the single deposition showed a value of −253 mV (vs SCE), two consecutive depositions had an OCP value of −255 mV (vs SCE), three consecutive depositions showed an OCP value of −252 mV (vs SCE), and four consecutive depositions gave an OCP value of −245 mV (vs SCE). The TiO₂/CuO (0.005) coating with two consecutive depositions exhibited the most positive corrosion potential under dark conditions, indicating enhanced corrosion protection. In contrast, the two consecutive depositions provided the most negative OCP value under light, suggesting optimal photocathodic protection. Based on these results, the TiO₂/CuO (0.005) coating with two consecutive depositions was identified as the optimal configuration for corrosion protection and was further investigated for photocathodic corrosion protection under light illumination [68,69].

As shown in Figure 8, under light irradiation, the OCP values of the TiO₂/CuO-coated 304SS shifted toward more negative potentials, suggesting the generation of photocurrent as a result of photoexcited electrons transitioning from the valence band to the conduction band [70,71]. For instance, the corrosion potential of TiO₂/CuO (0.005) changed from −0.234 V (vs. SCE) to −0.255 V (vs. SCE) under light illumination, highlighting the improved photocathodic corrosion protection provided by the TiO₂/CuO (0.005) coating. The significant variations in the corrosion potential with light ON/OFF conditions observed for the TiO₂/CuO (0.005) coating further indicate its effectiveness in providing photocathodic corrosion protection for 304SS. These results demonstrate the potential of TiO₂/CuO heterostructures in improving corrosion resistance in environments exposed to light irradiation.

3.7. Chronoamperometry

Figure 9 presents the transient photocurrent responses of uncoated 304SS, CuO, TiO₂, and TiO₂/CuO-coated 304SS under visible light illumination. In the absence of light, all samples exhibited a negligible photocurrent density (~0 mA/cm²). A sharp rise in photocurrent density was observed upon lighting, indicating efficient photoresponse and successful migration of photo-generated electrons to the 304SS substrate. The pure CuO electrode generated a photocurrent density of approximately 4.2 mA/cm², whereas pure TiO₂ exhibited a higher value of around 7.6 mA/cm² despite its limited absorption in the visible spectrum [72].

However, after sensitization with CuO nanoparticles, the transient photocurrent densities of TiO₂/CuO coated samples increase significantly, indicating that the TiO₂/CuO nanocomposite can efficiently utilize visible light. The heterostructure formation promotes efficient separation of photo-generated electron-hole pairs, enhancing the photocurrent response. Among the various TiO₂/CuO configurations, the TiO₂/CuO (0.05) photoelectrode exhibits the highest transient photocurrent density, indicating superior charge carrier separation efficiency. Upon visible light irradiation, the photocurrent rises sharply, with the positive current signifying electron transfer from the photoanode to the 304SS substrate. This electron flow drives a Faradaic reaction at the electrode surface, reducing reactive oxygen species in the electrolyte. Consequently, this process mitigates corrosion at the 304SS interface [73,74]. The largest photocurrent of 49 mA/cm² was observed for the TiO₂/CuO (0.005) photoelectrode, indicating its superior photocathodic protection performance and being approximately 5-fold higher than bare 304SS.

Upon illumination, all photoelectrodes showed a notable photocurrent. When the light source was turned off, the photocurrent density returned to its initial value, confirming that light exposure facilitates the separation of valence band electrons and holes in the semiconductor, leading to photocurrent generation. In the dark, the recombination of these charge carriers occurs, preventing the formation of a photocurrent.

3.8. Potentiodynamic Polarization

Figure 10a shows that the polarization curve of uncoated 304SS exhibits no significant shift in corrosion potential (E_corr) or corrosion current density (I_corr) under light ON/OFF conditions. In contrast, Figure 10b–d demonstrates that the CuO, TiO₂, and TiO₂/CuO coated electrodes exhibit more positive corrosion potentials (E_corr) of −232, −228, and −223 mV (vs. SCE), respectively, compared to the 304SS corrosion potential (E_corr) of −234 mV (vs. SCE) under dark conditions. This indicates that the TiO₂/CuO heterostructure coating is an effective barrier layer, protecting the 304SS substrate from corrosion without light [75]. Under light irradiation, as seen in Figure 10b–d, a negative shift in E_corr (−239, −249, and −255 mV vs. SCE) is observed for the CuO, TiO₂, and TiO₂/CuO coated samples. This shift occurs because photoelectrons from the TiO₂/CuO heterostructure are injected into the 304SS substrate, resulting in a significant drop in polarization potential. The corresponding increase in corrosion current density reflects the enhanced photocathodic corrosion protection effect for 304SS. Among the tested samples, the TiO₂/CuO (0.005) coating exhibited the most effective photocathodic corrosion protection under illumination. This improvement is ascribed to the polarization of photo-generated electrons, which enhances electrochemical reactions at the interface. The superior performance of TiO₂/CuO coatings stems from their heterojunction structure, which improves charge separation and inhibits electron-hole recombination [76]. Thus, the TiO₂/CuO-coated 304SS is a corrosion barrier and a photocathodic protection layer.

The corrosion potential and current density for bare 304SS were evaluated as −234 mV (vs. SCE) and 8 × 10⁻² mA/cm², respectively. The TiO₂/CuO (0.005) composite coating demonstrated significantly improved corrosion resistance, with a more positive corrosion potential of −255 mV (vs. SCE) and a drastically reduced corrosion current density of 0.46 × 10⁻⁶ mA/cm². The negative shift in E_corr indicates effective cathodic polarization of the substrate under illumination caused by the injection of photo-generated electrons from the composite layer to the metal surface. Meanwhile, the significant decrease in I_corr is due to the suppression of anodic dissolution, as the continuous supply of electrons from the photoactive coating reduces the driving force for metal corrosion. These results highlight the superior protective properties of the TiO₂/CuO composite layer over uncoated 304SS and substrates coated with CuO or TiO₂ individually.

Additionally, the results from Figure 10b–d show an appreciable negative shift in E_corr and I_corr values under light ON conditions. This shift indicates that the TiO₂/CuO coating prevents the release of metal ions and efficiently transfers photoexcited electrons to the 304SS surface. These results match the previous open circuit potential and chronoamperometry data, and the polarization technique confirms that the TiO₂/CuO (0.005) coating provides superior photocathodic corrosion protection, making it the optimal coating for enhancing the durability of 304SS in a 3.5% NaCl solution [77,78]. The TiO₂/CuO (0.005) heterostructure coating demonstrated a significant reduction in corrosion rate, decreasing from 169 mm/year (for bare 304SS under illumination) to just 21.6 mm/year. This corresponds to a corrosion protection efficiency of approximately 80%.

3.9. Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS) was used to study the interfacial properties of TiO₂/CuO coatings on 304 stainless steel (304SS). The impedance data were fitted with equivalent circuit models shown in Figure 11, R_S(Q₁R₁) for uncoated 304SS and Rs(Q₁R₁)(Q₂R₂) for coated 304SS. In these circuits, the charge transfer resistance (R₁) and double-layer capacitance (Q₁) at the electrolyte/film interface are connected in parallel and in series with the solution resistance (R_S). For the coated substrate, the Randles circuit was extended with a second parallel branch (R₂Q₂), representing the charge transfer resistance and double-layer capacitance at the film–metal interface due to the heterostructure coating. This arrangement is particularly suitable for systems where charge transfer processes and double-layer capacitance play significant roles. To assess the quality of the fit, the chi-square (χ²) values were calculated for all fittings and found to be below 10⁻³, indicating an excellent fit. The fitted parameters were tabulated in

Table 2. Fitted electrochemical impedance spectroscopy (EIS) parameters for uncoated and coated 304SS under dark and light conditions based on equivalent circuit modeling.

SAMPLE	Rs (Ω cm2)	Q1 × 10−6 (μ Fcm−2)	n1	R1 (Ω cm2)	Q2 × 10−6 (μ Fcm−2)	n2	R2 (Ω cm2)	χ2 × 10−3	Error Bar
BARE	4.05	90.3	0.89	126	-	-	28	0.0048	±1.94
CuO Dark	4.12	88.5	0.86	147	0.04	0.83	43	0.0061	±2.62
CuO Light	3.98	95.2	0.87	71	2.87	0.84	36	0.0053	±2.32
TiO2 Dark	4.27	82.4	0.85	156	0.13	0.87	63	0.0047	±2.16
TiO2 Light	3.45	93.6	0.88	35	5.21	0.89	54	0.0043	±1.95
TiO2CuO Dark	4.53	68.7	0.83	201	1.21	0.82	91	0.0036	±2.11
TiO2/CuO Light	3.09	76.3	0.84	27	9.91	0.84	27	0.0029	±1.45

.

As shown in the Nyquist plots (Figure 12), R_S represents the electrolyte resistance, which remains stable under dark and light conditions. Q₁ and Q₂ reflect surface roughness, charge effects, coating quality, and light-induced charges at the electrolyte/coating and coating/metal interfaces, respectively. Under illumination, both Q₁ and Q₂ slightly increased due to photo-induced surface charges. At the same time, the n values remained close to unity, indicating relatively uniform surfaces with moderate deviation from ideal capacitive behavior. R₁ and R₂ correspond to the charge transfer resistance at the electrolyte/coating and coating/metal interfaces; both are higher in the dark, indicating better corrosion resistance, and decrease under light due to photo-generated electrons enhancing charge transfer. The semicircle in the Nyquist plot corresponds to the charge transfer resistance (R₁) at the interface between the electrode and the electrolyte. The formation of the TiO₂/CuO heterojunction and the improvement in crystallinity significantly reduce R₁, indicating enhanced charge transfer across the interface.

The R₁ values for 304SS, CuO, TiO₂, and TiO₂/CuO heterostructures under dark conditions were 126, 147, 156, and 201 Ωcm², respectively. Under light conditions, the R₁ values were 126, 71, 35, and 27 Ωcm². The reduction in R₁ values is attributed to the injection of photo-generated electrons from the TiO₂/CuO composite into the 304SS electrode, accelerating the electrochemical reaction at the metal–film interface [79]. The second semicircle in the Nyquist plot corresponds to the charge transfer resistance (R₂) at the interface between the coating and the metal substrate. The TiO₂/CuO heterojunction enhances the coating’s barrier properties, resulting in higher R₂ values under dark conditions, indicating improved corrosion resistance. The R₂ values for coated 304SS under dark conditions were 28, 43, 63, and 91 Ωcm² for CuO, TiO₂, and TiO₂/CuO heterostructures, respectively. Under light conditions, these values decreased to 28, 36, 54, and 27 Ωcm², respectively. The decrease in R₂ under illumination is due to photo-generated electrons facilitating charge transfer at the coating/metal interface, accelerating the electrochemical processes at this boundary [80]. Interestingly, the semicircle radius of the TiO₂/CuO heterostructure is much smaller than that of CuO and TiO₂ films, indicating enhanced electron transport efficiency between the film and the metal substrate. These findings confirm that the TiO₂/CuO heterostructure film significantly improves the separation and transfer of photo-generated electron-hole pairs. Consequently, injecting more electrons into the 304SS substrate improves photocathodic corrosion protection under light illumination. Under illumination, the TiO₂/CuO heterostructure injects photo-generated electrons into the 304SS substrate, shifting its corrosion potential cathodically and suppressing anodic dissolution. This is reflected in the decreased R₁ values from EIS and the lower corrosion currents from polarization tests. Together, these results confirm that TiO₂/CuO is an effective barrier and an active promoter of interfacial charge transfer, significantly enhancing photocathodic protection under light [81,82,83,84].

3.10. Mott–Schottky Analysis

Mott–Schottky (MS) analysis was conducted to investigate the flat-band potentials and semiconductor behavior of the pristine and heterostructured samples, as illustrated in Figure 13. The flat-band potentials (V_fb) were extracted from the linear regions of the 1/C² versus potential plots. For pristine TiO₂ (Figure 13a), the V_fb was determined to be approximately –0.39 V (vs. SCE), confirming its n-type semiconductor nature. In contrast, CuO (Figure 13b) exhibited a positive V_fb of +0.52 V (vs. SCE), characteristic of a p-type semiconductor. The TiO₂/CuO (0.005) heterostructure (Figure 13c) displayed dual linear regions, indicating the presence of both n-type and p-type domains, with corresponding flat-band potentials at –0.21 V and +0.89 V (vs. SCE). This dual behavior signifies the formation of a p–n heterojunction, where intimate contact between n-type TiO₂ and p-type CuO facilitates the creation of a built-in electric field that promotes charge carrier separation. The observed shift in the flat-band potentials of the heterostructure indicates enhanced charge separation and improved photoelectrochemical properties due to the formation of a favorable type II heterojunction. This interfacial electronic coupling suppresses recombination and enhances photo-induced electron transfer toward the metal substrate. Therefore, the TiO₂/CuO (0.005) heterostructure is a promising candidate for enhanced photocathodic protection due to its synergistic electronic properties and efficient charge separation mechanism.

3.11. Proposed Photocathodic Protection Mechanism of TiO₂/CuO Heterostructure

The photocathodic protection mechanism of the TiO₂/CuO heterostructure coated on 304 stainless steel is primarily attributed to the formation of a type-II heterojunction, which has been substantiated through a combination of UV–Vis diffuse reflectance spectroscopy (DRS), Tauc plot bandgap estimations, and Mott–Schottky (MS) analysis. Mott–Schottky analysis confirms the formation of a Type-II heterojunction between n-type TiO₂ (flat band ~−0.39 V) and p-type CuO (flat band ~+0.52 V), with shifted potentials in the composite (−0.21 V and +0.89 V). This heterojunction promotes an internal electric field that drives efficient charge separation under visible light.

UV confirms that the pure TiO₂ displayed a wide bandgap of 3.20 eV, limiting its activity to the UV region. CuO, with a narrow bandgap of 1.80 eV, exhibited strong absorption in the visible region. Interestingly, the TiO₂/CuO (0.005) heterostructure showed a reduced bandgap of 2.17 eV, indicating enhanced visible-light absorption due to interfacial interaction and successful hybridization between the two semiconductors. Under visible-light irradiation, electrons in CuO are excited from its valence band (VB) to the conduction band (CB). Due to the Type-II band alignment, these electrons preferentially transfer to the more negative CB of TiO₂. Simultaneously, photo-generated holes in TiO₂’s VB migrate to CuO’s VB. This directional migration of charge carriers across the heterojunction spatially separates electrons and holes, effectively reducing their recombination rate and prolonging their lifetime. The electrons accumulated on TiO₂’s conduction band can then flow to the 304SS substrate, increasing the cathodic current and suppressing the anodic dissolution of the stainless steel [59]. Such a charge transfer pathway is particularly favorable for photocathodic protection applications. The accumulation of electrons on TiO₂ enhances the cathodic current on the 304SS substrate, suppressing its anodic dissolution and improving corrosion resistance under illumination.

In conclusion, the synergistic integration of TiO₂ and CuO in the 0.005 composition leads to a well-aligned Type-II heterojunction [85,86]. The observed shifts in flat band potentials and the narrowed bandgap support this mechanism, establishing the TiO₂/CuO (0.005) system as a promising candidate for visible-light-driven corrosion protection of 304 stainless steel. The schematic representation of the proposed photocathodic corrosion protection mechanism based on the Type-II heterojunction formed between TiO₂ and CuO coated on 304 stainless steel is shown in Figure 14.

4. Conclusions

The TiO₂/CuO heterostructure was synthesized by the sol–gel method and coated onto 304SS to enhance the longevity of seashore appliances. This study successfully demonstrates the photocathodic corrosion protection abilities of the TiO₂/CuO heterostructure under light illumination. UV–visible diffuse reflectance spectroscopy (UV-DRS) confirmed the improved visible light absorption of the TiO₂/CuO heterostructure, enabling efficient utilization of solar irradiation. Mott–Schottky analysis revealed increased charge carrier density and favorable band alignment, facilitating effective separation of photo-generated electron-hole pairs and promoting electron transfer to the 304SS substrate. The Raman spectra of TiO₂/CuO heterostructures on 304SS confirm the formation of TiO₂ and CuO phases, with shifts in the TiO₂ E_g mode indicating strong interaction between the components. FTIR spectra show O-H stretching and metal–oxygen bonds, supporting the heterostructure formation. AFM and SEM images reveal increased surface roughness and uniform morphology, enhancing stability. OCP measurements indicate improved corrosion resistance, with the TiO₂/CuO (0.005) coating showing OCP values of −223 mV in the dark and −255 mV under light. Photocurrent density measurements under visible light reveal that the TiO₂/CuO (0.005) coating exhibits the highest photocurrent (49 mA/cm²), which is 5-fold higher than bare 304SS. These results demonstrate that TiO₂/CuO coatings significantly improve corrosion protection and photocathodic performance, particularly for stainless steel in light-exposed environments.

5. Future Perspective

Integration into offshore appliances: Since 304SS is extensively used in offshore appliances, future research can explore the integration of the TiO₂/CuO heterostructure coating into oil field pipelines and offshore applications. This includes studying the interaction between the coating and evaluating its impact on the overall performance and durability of oil field pipelines and offshore appliances.