Design of a One-Dimensional Zn₃In₂S₆/NiFe₂O₄ Composite Material and Its Photocathodic Protection Mechanism Against Corrosion

Abstract

Z-scheme Zn₃In₂S₆/NiFe₂O₄ nanocomposites were prepared by electrospinning and hydrothermal methods, and their photocathodic protection performance was studied on 304 SS and Q235 CS in NaCl solution (3.5 wt.%). The two-dimensional Zn₃In₂S₆ loaded on the one-dimensional NiFe₂O₄ resulted in faster electron migration and enhanced light absorption capability. Moreover, it had been observed through electrochemical testing that the assembly of Zn₃In₂S₆/NiFe₂O₄ heterojunctions improves the efficacy of photocathodic protection. Following illumination, the self-corrosion potentials of 304 SS and Q235 CS coupled with Zn₃In₂S₆/NiFe₂O₄ nanocomposites decreased by 1040 mV and 560 mV, and the photoinduced current densities were 1.2 times and 3.9 times greater than the value of Zn₃In₂S₆. Furthermore, the mechanism of enhanced photocathodic protection performance for Zn₃In₂S₆/NiFe₂O₄ heterojunctions was systematically discussed. XPS and ESR analysis indicated that Zn₃In₂S₆/NiFe₂O₄ composites follow the Z-scheme electron migration path and retain the stronger reduction and oxidation capacity of Zn₃In₂S₆/NiFe₂O₄. Therefore, the Z-scheme heterostructures are responsible for the realization of cathodic protection for carbon steel.

1. Introduction

Steel structures are extensively employed in marine and coastal engineering projects owing to their advantages, such as light self-weights, straightforward and rapid construction processes, and recyclability. Consequently, they are a staple in modern construction engineering [1]. Nevertheless, the prevalent use of low-alloy and low-carbon steels in steel structures makes them susceptible to corrosion, leading to a substantial diminution of the steel structures and their load-bearing capacity, as well as compromising their overall stability and reliability and shortening their service lives [2]. Consequently, the development of innovative anti-corrosion technologies is paramount to effectively prolong the lifespan of equipment, minimize maintenance expenditures, ensure operational safety, and mitigate energy wastage. Photocathodic protection technology is a novel protection method for steel structures [3,4].

As we all know, metal corrosion involves the process by which metal atoms lose electrons and transform into metal compounds. The core principle of cathodic protection lies in furnishing negative electrons to the metal under protection, enabling it to undergo oxidation by oxides and thereby substituting for metal corrosion [5]. Photoelectric cathodic protection technology harnesses the unique photoelectric conversion capabilities of semiconductor materials [6]. By absorbing sunlight, a clean energy source, the technology generates photoelectrons to supply electrons to the protected metal. Furthermore, the semiconductor material can be used repeatedly without causing harm, addressing the issues of energy consumption and environmental pollution associated with traditional cathodic protection techniques. This technology represents an economical, energy-efficient, and environmentally friendly approach to corrosion prevention. The efficacy of this technology primarily hinges on the energy level structure and photoelectric conversion efficiency of the photoanode material. In this realm, TiO₂ [7,8] and ZnO [9,10], widely used photoanode materials, can play a pivotal role in providing photoelectric cathodic protection for metals with lower corrosion rates, such as stainless steel [11,12]. However, due to their insufficiently negative conduction band positions and weak reduction activity, TiO₂ and ZnO photoanodes fail to offer effective cathodic protection to metals prone to corrosion, such as carbon steel [13,14]. Additionally, they belong to the category of wide-bandgap semiconductors that primarily absorb ultraviolet light [15], which is scarce in sunlight. This results in a low utilization rate and photoelectrochemical efficiency of sunlight.

To tackle this challenge, extensive research has been undertaken, encompassing elemental doping [16], morphology control [17,18,19], and heterojunction modification [20,21,22]. Among these strategies, the construction of Z-scheme heterostructures stands out as a particularly effective approach, as it not only enhances the photoelectric conversion efficiency but also boosts the redox activity of the photoanode. Cao, W. et al. [23] fabricated a Bi/BiOBr/TiO₂ nanocomposite material and observed that, under visible light illumination, this photoanode could shift the potential of coupled 316L stainless steel negatively to −430 mV versus SCE. Furthermore, based on electron spin resonance characterization of the Bi/BiOBr/TiO₂ photoanode, they proposed a charge transfer mechanism for the Z-scheme heterojunction. Chang, Y. et al. [24] successfully synthesized TiO₂/In₂S₃ heterojunction thin films using a one-step hydrothermal method. They discovered that, compared to pure In₂S₃ and TiO₂, the composite photoelectrode composed of TiO₂ and In₂S₃ significantly improved the photocathodic protection performance for 304 SS. This enhancement was attributed to the formation of a Z-scheme heterojunction structure within the In₂S₃@TiO₂/In₂S₃ composite. Jin, Z.Q. et al. [25] prepared a Z-scheme polydopamine (CdS/PDA) heterostructure through the sample mixing method. Their study demonstrated that this Z-scheme CdS/PDA heterostructure exhibited excellent photoelectrochemical cathodic protection for 304 SS in a NaCl solution. Notably, when PDA was grown for 10 min, it effectively reduced the open-circuit potential of 304 SS to −0.5V. Xu, S.W. et al. [26] developed a series of TiO₂/Au/CdS nanostructured coatings. Through active species trapping experiments, they revealed that these coatings exhibited Z-scheme heterojunction properties, enabling efficient charge separation and possessing robust reduction and oxidation capabilities. Zinc indium sulfide (Zn_mIn₂S_3+m) is a typical visible-light-responsive catalyst type among ternary metal sulfides, featuring a similar layered structure and excellent photoelectric conversion performance [27]. Specifically, Zn₃In₂S₆ is a direct bandgap semiconductor that crystallizes in a tetragonal structure with a space group of P3m1. In this structure, Zn atoms occupy tetrahedral sites, while In atoms connect tetrahedra and octahedra. Along the c-axis, it consists of a series of S-In-S-Zn-S-In-S-Zn-S-Zn-S atomic layers, which enhance light utilization and conductivity. Zn₃In₂S₆ is a suitable material for constructing heterojunction structures, with a bandgap width of 2.8 eV and a conduction band position close to −0.9 eV, making it promising for providing protection to metals with lower self-corrosion potentials in photocathodic protection [28]. She et al. [29] combined Zn₃In₂S₆ with a traditional TiO₂ material to construct a Z-scheme Zn₃In₂S₆/TiO₂ heterojunction material. Fluorescence emission spectroscopy tests showed that the composite material significantly enhanced the transfer ability of photogenerated carriers. Xu et al. [28] prepared a Zn₃In₂S₆/TiO₂ nanocomposite material for the photocathodic protection of carbon steel. By loading Zn₃In₂S₆, the light absorption range of TiO₂ was extended to the visible light region. The composite material reduced the open-circuit potential of carbon steel by 490 mV under illumination and generated a photogenerated current density of 1.91 mA·cm⁻². In addition, there are studies on the construction of other highly reductive heterostructures, such as Zn₃In₂S₆/g-C₃N₄ [30], Zn₃In₂S₆/BN [31], and polypyrrole/Zn₃In₂S₆ [32]. Additionally, our research found that Zn₂In₂S₅ [33] can serve as a reducing material for constructing a Z-scheme heterojunction. By incorporating Zn₂In₂S₅ into TiO₂, we successfully fabricated a zinc indium sulfide/titanium dioxide (Zn₂In₂S₅/TiO₂) photoelectrochemical cathodic protection coating. Through theoretical calculations and electrochemical experiments, we discovered that the Z-scheme heterojunction formed between Zn₂In₂S₅ and TiO₂ facilitates the separation of photogenerated electrons and holes. Compared to pure Zn₂In₂S₅ and TiO₂, the Zn₂In₂S₅/TiO₂ composite demonstrates a superior photoelectrochemical protection capability for 304 stainless steel.

NiFe₂O₄ has become one of the most popular ferrites in the photocatalytic field due to its excellent photostability and energy band structure. With a relatively narrow bandgap (approximately 2.5 eV [34]), NiFe₂O₄ has a certain light absorption ability in the visible range and good photochemical stability, which enables the broad application of NiFe₂O₄ in the fields of photocatalysis and photocathodic protection. Consequently, NiFe₂O₄ has been widely studied in organic dye degradation and visible photocatalysis [35]. However, NiFe₂O₄ on its own also has the problems of low conductivity [36] and a tendency to agglomerate, which results in a high photogenerated electron–hole pair recombination rate and low photoelectrochemical efficiency in visible light, thereby compromising its photocathodic protection performance.

In this work, the structure of NiFe₂O₄ was modified into one-dimensional (1D) nanofibers via electrospinning to enhance its electron transport capability and charge separation ability. Subsequently, Zn₃In₂S₆ nanosheets were deposited onto 1D NiFe₂O₄ fibers through the hydrothermal method to obtain Zn₃In₂S₆/NiFe₂O₄ heterojunctions. In a 3.5 wt.% NaCl solution, the variations of the open-circuit potential and photocurrent density of the Zn₃In₂S₆/NiFe₂O₄ composites under light illumination were investigated in comparison with individual Zn₃In₂S₆ and NiFe₂O₄. The photoelectrochemical cathodic protection effect of the Zn₃In₂S₆/NiFe₂O₄ composites on 304 SS and Q235 CS was examined. The cathodic protection principle of the Zn₃In₂S₆/NiFe₂O₄ composites for metals was elucidated, and the formation of the Z-scheme heterojunction between Zn3In2S6 and NiFe₂O₄ was demonstrated. This provides research ideas for the construction of stable Z-scheme heterojunction materials and their application in the photoelectrochemical cathodic protection of various metals.

2. Materials and Methods

2.1. One-Dimensional NiFe₂O₄ Nanomaterial Preparation

One-dimensional NiFe₂O₄ nanofibers were prepared by wet electrostatic spinning. Firstly, 10 mmol/L nickel acetate (C₄H₆NiO₄) (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) and 20 mmol/L ferric nitrate hydrate (Fe(NO₃)₃·9H₂O) (Macklin, Shanghai, China) were weighed and mixed sequentially in a beaker. After 15min of stirring, a clarified solution was obtained. Then, 10 wt.% poly(vinylpyrrolidone) (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) was added, and the mixture was stirred continuously for 10 h to form a homogeneous viscous precursor. The precursor solution was loaded into a syringe with a flow rate of 0.5 mL/h. Electrospinning parameters included a voltage of 18 kV, a needle-to-collector distance of 20 cm, and a collector rotation speed of 500 rad/min. After 8 h of spinning, the NiFe₂O₄ fiber membrane was peeled from the collector and calcined at 600 °C (heating rate: 5 °C/min) for 2 h. This process ultimately yielded one-dimensional NiFe₂O₄ nanomaterials. The preparation flowchart is shown in Figure 1.

2.2. Zn₃In₂S₆/NiFe₂O₄ Composite Preparation

Zn₃In₂S₆ was grown on the surface of one-dimensional NiFe₂O₄ nanomaterials via a hydrothermal method. Firstly, 15 mmol/L zinc sulfate (ZnSO₄), 60 mmol/L thioacetamide (C₂H₅NS) (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China), and 10 mmol/L indium chloride (InCl₃) (Macklin, Shanghai, China) were weighed and mixed in a beaker with stirring to obtain the hydrothermal precursor solution of Zn₃In₂S₆. Subsequently, 0.2 g of NiFe₂O₄ nanofibers were weighed and placed in a 10 mL centrifuge tube, to which 5 mL of H₂O was added. The mixture was then subjected to ultrasonic treatment for 30 min to obtain a suspension of NiFe₂O₄ nanofibers. Afterward, the NiFe₂O₄ nanofiber suspension was mixed with the hydrothermal precursor solution of Zn3In2S6 and transferred into a 50 mL polytetrafluoroethylene (PTFE) liner. The liner was then placed in a reaction kettle and subjected to hydrothermal treatment at 180 °C for 16 h. The resulting powder sample was dried at 80 °C for 2 h, and the Zn₃In₂S₆/NiFe₂O₄ nanocomposite was obtained. The preparation flowchart is shown in Figure 1.

2.3. Metal Electrode Preparation

A 1 cm × 1 cm × 0.5 cm block of 304 stainless steel (304 SS) and Q235 carbon steel (Q235 CS) was used as the metal electrode. Each block was sonicated in anhydrous ethanol for 15 min to remove surface contaminants, then dried and welded with a 2 mm diameter copper wire. The welded block was placed in a mold and encapsulated with epoxy resin (mixed at a 3:1 mass ratio) to prevent oxidation. After 48 h of curing, the electrode surface was sequentially polished with 220, 800, 1500, and 2000 mesh sandpaper, followed by 0.5 µm diamond paste to achieve a mirror finish. Finally, the polished electrode was stored in anhydrous ethanol until testing.

2.4. Microscopic Characterization Tests

Scanning electron microscopy (SEM) was performed using a Hitachi-SU8220 scanning electron microscope (Hitachi, Tokyo, Japan) with a tungsten electron source at a 15 kV accelerating voltage. X-ray diffraction (XRD) analysis utilized a Rigaku D/max-2000 (Rigaku, Beijing, China) diffractometer in the range of 10–90° at a scanning rate of 5 °/min. Transmission electron microscopy (TEM, HRTEM) was performed using a JEOL JEM-2100 microscope (JEOL Ltd., Tokyo, Japan)to further analyze the microscopic morphology, material composition, and composite condition of the materials. X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific ESCALAB 250Xi spectrometer(Thermo Fisher, Waltham, America) under ultra-high vacuum (<1 × 10⁻⁸ Pa) with K-α radiation (15 kV). Samples underwent broad-spectrum and narrow-spectrum scans, followed by CasaXPS software(Version 2.3.24PR1.0)-based peak deconvolution to determine their elemental composition and oxidation states. Super depth of field microscopy (SDM) tests were performed using a PZ-CS3500 microscope (Pinzhics, Beijing, China) to analyze the degree of coating scratches and flaking.

2.5. Optical Performance Testing

Ultraviolet–visible diffuse reflectance (UV–Vis DRS) was tested using a TU-1901 (Persee, Beijing, China) with the vertical axis set to absorbance intensity (a.u.) and the wavelength range set to 200–1000 nm, and a glass was used to perform a baseline calibration before testing. Photoluminescence spectroscopy (PL) was recorded using a Hitachi F-7000 model fluorescence spectrophotometer (Hitachi, Tokyo, Japan) with a 300 W xenon lamp. The excitation wavelength was fixed at 370 nm and the emission wavelengths were tested in the range of 380–800 nm. The existence of free radicals in the reaction system was captured by ESR testing to determine whether the redox reaction occurred, thereby determining the band potential information of the photoanode and the charge transfer situation and analyzing the reaction mechanism.

2.6. Optical Performance Test

The open-circuit potential test (OCP) was performed using a Gamry Interface 5000E electrochemical workstation (Gamry, Philadelphia, America) with a three-electrode system: an Ag/AgCl reference electrode, a platinum sheet counter electrode, and a working electrode comprising the photoanode connected in parallel to the protected metal to determine the voltage change in the system. The test environment was a 3.5 wt.% NaCl solution and the test light source was a 500 W xenon lamp that irradiated the surface of the photoanode material through the side window of the photodissociation cell in the double electrolysis cell. The protected metal, the platinum sheet electrode, and the reference electrode were placed in the electrolysis cell together, and the ion exchange between the photodissociation cell and the electrolysis cell was carried out through the Nafion proton exchange membrane, with the test light intensity set at 100 mW/cm². The kinetic potential polarization curve and electrochemical ac impedance (EIS) were measured using the same three-electrode system as the open-circuit potential test, and the kinetic potential polarization curve was tested in the range of −0.8 V~0.8 V curves with respect to the open-circuit potential with a scan rate of 1 mV/s, while for the EIS test, the frequency range of the test was set to 10⁵ Hz~10⁻² Hz, and the amplitude of the ac voltage was set to 10 mV. The Mott–Schottky test (MS) was performed using a 3.5 wt.% NaCl solution with a test range of −1 V~0.6 V with respect to the open-circuit potential, and a step height of 10 mV. The connection device for the above test is shown in Figure 2. The photogenerated current density–time test (J-t) was performed in a double electrolytic cell in which the working electrode was connected to the photoanode material, the counter electrode and the ground were connected to the protected metal, and the reference electrodes were also used as the Ag/AgCl electrode. The photoanode material and the protected metal together form the circuit, and the connection device is shown in Figure 2b.

3. Results

3.1. Microscopic Morphology

SEM and HRTEM were used to investigate the microscopic morphologies and compositions of the photoanode materials (Zn₃In₂S₆ (a), NiFe₂O₄ (b), and the Zn₃In₂S₆/NiFe₂O₄ composite), as shown in Figure 3. As shown in Figure 3a, the individual Zn₃In₂S₆ is arranged in a staggered manner with a lamellar structure, a sheet thickness of approximately 30 nm, and intricate porosity. Moreover, two-dimensional (2D) Zn₃In₂S₆ has a smooth surface, which is conducive to light reflection, thereby enhancing its light absorption efficiency. Figure 3b illustrates the preparation of NiFe₂O₄ fibers. The resultant one-dimensional (1D) NiFe₂O₄ nanofibers possessed a uniform diameter of approximately 120 nm, exhibiting a slightly wrinkled surface. This is attributed to the volatilization of organic components during high-temperature calcination. Figure 3c subsequently presents the SEM image of the Zn₃In₂S₆/NiFe₂O₄ composite material. As can be observed in the image, the layers of Zn₃In₂S₆ synthesized using the hydrothermal method are lamellar and overlaid on the surface of one-dimensional NiFe₂O₄, growing in a staggered and compact manner. The Zn₃In₂S₆/NiFe₂O₄ composite material possessed a diameter ranging from 170 nm to 230 nm. This observation serves as evidence that the Zn₃In₂S₆/NiFe₂O₄ composites have been successfully fabricated. The large specific surface area of NiFe₂O₄ fibers offers abundant sites for the growth and attachment of lamellar Zn₃In₂S₆ sheets. Both the 1D nanofibers and the 2D lamellar structures substantially enhanced the electrical conductivity of the composite material, facilitating the rapid transfer of photogenerated electrons. Figure 3d exhibits the HRTEM image of the Zn₃In₂S₆/NiFe₂O₄ composite material. Within the image, two different lattice spacings were observed, 0.32 nm and 0.25 nm, which corresponded to the (102) crystallographic plane [37] of Zn₃In₂S₆ and the (311) crystallographic plane [38] of NiFe₂O₄, respectively. This demonstrates the successful preparation of the Zn₃In₂S₆/NiFe₂O₄ composite material and the intimate contact between Zn₃In₂S₆ and NiFe₂O₄, a condition conducive to the formation of heterojunctions and the transfer of charges between the materials.

3.2. Material Composition

XRD analysis provides information regarding the crystal surface, as well as the degree of crystallinity of the photoanode material’s crystals. Figure 4a presents the XRD curves of Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄ composite materials, respectively. The sharp diffraction peaks observed in all materials indicated a high degree of crystallinity, while the absence of impurity peaks suggested high sample purity. Among the diffraction peaks observed for Zn₃In₂S₆, there were diffraction peaks at 28.23°, 32.73°, 46.91°, and 55.66°, which correspond to the (102), (014), (110), and (022) crystal planes of Zn₃In₂S₆ (JCPDS NO. 80-0835), respectively, indicating the successful preparation of Zn₃In₂S₆. The diffraction peak curve of NiFe₂O₄ showed diffraction peaks at 30.30°, 35.69°, 37.33°, 43.38°, 53.83°, 57.38°, and 63.02°, which correspond to the (220), (311), (222), (400), (422), (511), and (440) crystal planes of NiFe₂O₄ (JCPDS NO. 86-2267), respectively, indicating the successful preparation of NiFe₂O₄. The XRD curves of the Zn₃In₂S₆/NiFe₂O₄ composite material exhibit characteristic peaks that correspond to the crystal planes of both Zn₃In₂S₆ and NiFe₂O, thereby confirming the successful composition of Zn₃In₂S₆ and NiFe₂O₄ in the same location. In summary, the X-ray diffraction results demonstrated the successful fabrication of Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄ photoanode materials with enhanced crystallinity and high purity through the experimental process. In addition, the grain sizes D of NiFe₂O₄ and Zn₃In₂S₆ before and after the composition were estimated using the Debye–Scherrer equation [39,40]:

D = Kλ/βcosθ

(1)

where λ = 1.5418 Å for target Cu, K = 0.94, θ is the diffraction angle, and β is the full width at half maximum (FWHM). The calculation results showed that the grain sizes of NiFe₂O₄ and Zn₃In₂S₆ before compositing were 46.16 nm and 34.65 nm, respectively. After compositing, the grain size of NiFe₂O₄ decreased to 40.87 nm, while the grain size of Zn₃In₂O₆ increased to 43.99 nm.

XPS is capable of chemically analyzing the material surface to determine the elemental composition and chemical state of the material. Figure 4b shows the spectral peaks of the elements Zn, In, S, Ni, and Fe in Zn₃In₂S₆, as well as in the Zn₃In₂S₆/NiFe₂O₄ composite samples, demonstrating the successful formation of the composite of the two materials. In addition, the high-resolution XPS scans of the orbitals of the elements C, O, Fe, In, Zn, and S enable the determination of the states of the elements in the material, as well as the electron transfer information. The characteristic curve of C 1s is represented in Figure 4c, and the fitting results showed that the characteristic peak at 284.8 eV was the adventitious carbon reference, and the characteristic peaks at 286.0 eV and 288.5 eV were C-O and C=O [41], respectively. The characteristic curves of O 1s are shown in Figure 4d, where the characteristic peak at 529.7 eV corresponded to O²⁻ ions in NiFe₂O₄, O-C at 530.8 eV, and O=C at 531.7 eV. The characteristic peaks at 531.7 eV correspond to O=C. Figure 4e shows NiFe₂O₄ and Zn₃In₂S₆/NiFe₂O₄, where the peaks at 711 eV and 724 eV correspond to the Fe 2p_3/2 and Fe 2p_1/2 spin-orbit peaks, respectively. The characteristic peaks of In 3d are shown in Figure 4f, and the characteristic peaks at 444.9 eV and 452.6 eV correspond to In 3d_5/2 and In 3d_3/2, respectively. In Figure 4g, the Zn 2p_3/2 and Zn 3p_1/2 orbital peaks are observed at 1021.9 eV and 1045.1 eV, respectively. In Figure 4h, 162.5 eV corresponds to the S 2p_3/2 orbital and 163.7 eV to the S 2p_1/2 orbital. The combined XRD and XPS results further demonstrate the successful preparation of Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄ composites, and are in agreement with the results obtained for the SEM and HRTEM tests. In addition, from the fine high-resolution XPS spectra of elemental orbitals, it can be found that the binding energies of the elements Fe and In are significantly shifted, indicating the existence of strong electronic interactions and electron transfer between Zn₃In₂S₆ and NiFe₂O₄, where the binding energy of the Fe element is elevated in Zn₃In₂S₆/NiFe₂O₄ composites and the binding energy of the In element is decreased in Zn₃In₂S₆/NiFe₂O₄ composites. This indicates that electron transfer from NiFe₂O₄ to Zn₃In₂S₆ and aggregation on the surface of Zn₃In₂S₆, suggesting a charge transfer mechanism for Zn₃In₂S₆/NiFe₂O₄ materials following a Z-scheme heterojunction [42].

3.3. Optical Properties

Most sunlight is visible light, but due to the wide bandgap width, some semiconductors have a weak ability to absorb sunlight, resulting in insufficient photoelectric conversion abilities. The UV–Vis DRS test was conducted to evaluate the light absorption capabilities of Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄. The results of the test are displayed in Figure 5a. From the figure, it was observed that the single materials Zn₃In₂S₆ and NiFe₂O₄ exhibited superior absorption capabilities below 475 nm and 596 nm, respectively. After NiFe₂O₄ was composited onto Zn₃In₂S₆, the optical absorption threshold of Zn₃In₂S₆ is red-shifted and broadened to 630 nm. This indicates that the heterojunction resulting from the contact between the two materials has the ability to modify the band structure of the composite, narrowing the forbidden band width of Zn₃In₂S₆ and enhancing its optical absorption performance. Consequently, photons with lower energy can be absorbed, sunlight can be better utilized, and photoelectric conversion performance can be further enhanced, all of which are beneficial for enhancing photoelectric cathodic protection performance.

Using the Kubelka–Munk equation [43], the value of the forbidden bandwidth corresponding to the photoanode can be calculated from the UV–visible diffuse reflectance spectrum:

$${\left(\alpha h\nu \right)}^{1/n}=A(h\nu -E_g)$$

(2)

where α is the light absorption coefficient, h is Planck’s constant,$\nu$ is the frequency of light, A is a constant, E_g is the band gap of the semiconductor, and n represents the type of electron transition in the semiconductor (n = 1/2 when the material is a direct bandgap semiconductor; n = 2 when the material is an indirect bandgap semiconductor). From Figure 5b, the band gaps of Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄ are obtained as 2.72 eV, 2.41 eV, and 2.32 eV, respectively.

The photoluminescence (PL) intensity of the photoanode material was measured to verify the effect of the Zn₃In₂S₆/NiFe₂O₄ composite on the separation efficiency of photogenerated electron–hole pairs. Due to the inefficient separation of the photogenerated electron–hole pairs, the electrons and holes compounded on the semiconductor surface after excitation will produce the luminescence or heat phenomenon. Therefore, in the fluorescence spectrum, the peak intensity represents the photoluminescence intensity. The material exhibiting a lower photoluminescence intensity indicates that its photogenerated electron–hole composite rate is lower. From Figure 3c, it can be seen that the strongest fluorescence peaks of NiFe₂O₄ and Zn₃In₂S₆/NiFe₂O₄ appear at 456 nm and 453 nm, respectively. The red-shift of the strongest peak position of Zn₃In₂S₆/NiFe₂O₄ indicates a reduced band gap after material compositing. The highest photoluminescence intensity in NiFe₂O₄ indicates its poor charge separation efficiency, while limited electron transfer further restricts its protective performance. On the other hand, Zn₃In₂S₆/NiFe₂O₄ has a lower photoluminescence intensity, indicating that the heterojunction formed between Zn₃In₂S₆ and NiFe₂O₄ can promote the separation between photogenerated electron–holes pairs and enhance the photoelectric conversion performance of the material.

Based on the results of the Mott–Schottky curves, further analysis was conducted on the energy band structure of the photoanode materials and the information on the variation of the concentration of photogenerated carriers. As can be seen in Figure 5d, the tangent slopes of Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄ are all greater than 0 within the tested range, exhibiting the characteristics of n-type semiconductors. The intersection of the curve’s approximately straight segment with the abscissa axis denotes the flat band potentials [44] of the material. For n-type semiconductors, the flat band potential is close to its conduction band position. Generally, the conduction band potential is approximately 0.1 eV~0.3 eV more negative than the flat band potential.

Table 1. Energy band structure information for Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄.

Photoanode	Flat Charged Level (V vs. Ag/AgCl)	Conductive Tape (eV)	Bandgap Width (eV)
Zn3In2S6	−0.80	−0.80	2.72
NiFe2O4	−0.28	−0.28	2.41
Zn3In2S6/NiFe2O4	−0.63	−0.63	2.32

shows the flat band potentials of Zn₃In₂S₆, NiFe₂O₄, and Zn₃In₂S₆/NiFe₂O₄ are −0.80 eV, −0.28 eV, and −0.63 eV, respectively. The valence band positions can be calculated to be 1.92 eV, 2.13 eV, and 1.69 eV, respectively. The obtained energy band structure information is summarized in Table 1. The positions of the flat band potentials of Zn₃In₂S₆/NiFe₂O₄ are close to Zn₃In₂S₆, showing the characteristics of Z-scheme semiconductors. From the change in the flat band potential, it can be observed that the doping of Zn₃In₂S₆ reduced the flat band potential of NiFe₂O₄, indicating that the Zn₃In₂S₆/NiFe₂O₄ composite has a more negative conduction band potential compared to NiFe₂O₄ alone. The greater negative conduction band potential provides a stronger electronic driving force for electrons transferred to the protected metal. Thus, the negative shift of the flat band potential indicates that the doping of Zn₃In₂S₆ can effectively improve the electron transfer performance of NiFe₂O₄. In addition, the carrier concentration can be determined from the slope of the tangent. The slope of the tangent is inversely proportional to the carrier concentration. A smaller tangent slope indicates a higher carrier density. In Figure 5d, the Zn₃In₂S₆/NiFe₂O₄ composite has the smallest slope of the straight-line segment, and therefore the highest carrier concentration. This further illustrates that the Zn₃In₂S₆/NiFe₂O₄ composite improves the separation efficiency of the photogenerated electron–hole pairs and can provide more electrons to the protected metal.

3.4. Photochemical Protection Performance and Mechanism Analysis

Since photocathodic protection is based on the transfer and accumulation of photogenerated electrons on the metal surface, OCP (open-circuit potential) and J-t (current–time) tests are used to analyze the potential changes on the surface of different coupled metal electrodes and the magnitude of the generated photogenerated current to evaluate the photocathodic protection performance of composite photoanode materials. Figure 6 shows the open-circuit potential changes on the metal electrode surface under illuminated and dark conditions for NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ composites with 304 SS and Q235 CS in a simulated marine environment. In Figure 6a, upon illumination, due to the formation of a Schottky barrier between the photoanode material and the metal, the electrons transfer and accumulate, causing polarization, which is manifested as a rapid negative shift of the open-circuit potential. It can be seen that the open-circuit potential of 304 SS coupled with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S ₆/NiFe₂O₄ composite materials under dark conditions is −40 mV, −160 mV, and −60 mV, respectively. After illumination, the open-circuit potentials of the coupled metals decrease to −50 mV, −1050 mV, and −1100 mV, respectively, with potential negative shifts of 10 mV, 890 mV, and 1040 mV, respectively. The potential of the coupling metal with the Zn₃ In₂S₆/NiFe₂O₄ composite coupled with 304 SS exhibits the largest negative shift in open-circuit potential, indicating that more photogenerated electrons are accumulated on its surface. This suggests that when Zn₃In₂S₆ is composited with NiFe₂O₄, the composite enhances the light absorption range and improves the material’s ability to utilize light due to the semiconductor’s composition. At the same time, the heterojunction formed between the two heterojunctions improves the separation ability of photogenerated electron–hole pairs, reduces the photogenerated electron–hole recombination of the semiconductor material itself, and enables more electrons to gather on the surface of the protected metal.

Figure 6b shows the photogenerated current density generated by the photoanode under illuminated and dark conditions, further illustrating the charge separation and transfer properties of the photoanode material. It can be observed that under dark conditions, at the equilibrium of the Fermi level between the coupled metal and photoanode, almost no current is generated between the electrodes. Upon illumination, the photocurrent density between the electrodes increases rapidly. However, due to the material’s own defects, the photogenerated current density of the photoanode material begins to decline, eventually stabilizing after a period of time and generating a sharp peak. When 304 SS is coupled with NiFe₂O₄, the photogenerated current density between the two is approximately 0.8 μA·cm⁻², and when coupled with Zn₃In₂S₆, the photogenerated current density reaches approximately 40 μA·cm⁻², which suggests that under illumination, both materials are capable of generating photogenerated electron flows towards the metal surface, providing metal protection. In addition, when coupled with Zn₃In₂S₆/NiFe₂O₄, a photogenerated current density of approximately 50 μA·cm⁻² can be generated between the metal and the photoanode, which is 62.5 times and 1.2 times higher than that of the coupling with NiFe₂O₄ and Zn₃In₂S₆ alone, respectively. This further demonstrates that the composite of the photoanode materials formed a heterojunction that can improve the separation efficiency of photogenerated electron–hole pairs and provide more protective electrons for metals.

Similarly, Figure 6c shows the OCP of Q235 CS coupled with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ composites, respectively. Under dark conditions, the OCP of Q235CS surfaces coupled with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ composites is −610 mV, −650 mV, and −650 mV, respectively. Upon illumination, the potentials change to −580 mV and −1030 mV. The potential drops are −30 mV, 380 mV, and 560 mV, respectively. Among them, the OCP of Q235 CS coupled with NiFe₂O₄ increases upon illumination, suggesting that the conduction band position is insufficient to provide protective properties to the Q235 CS with a more negative Fermi energy level. The Zn₃In₂S₆/NiFe₂O₄ composites still have the largest potential drop, indicating that instead of forming a conventional Type II heterojunction between the heterojunction materials, a Z-scheme heterojunction with a higher redox capacity is formed, which allows the photogenerated electrons passing through the heterojunction to accumulate on the Zn₃In₂S₆ conduction band with a stronger electronic driving force and transfer to the surface of the protected metal.

Figure 6b,d show the magnitude of photogenerated current densities between Q235 CS and NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ under illumination. Upon illumination, photogenerated currents of −3 μA·cm⁻², 9 μA·cm⁻², and 35 μA·cm⁻² were generated between NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ and Q235 CS, respectively. Among them, NiFe₂O₄ does not transfer the photocurrent to the surface of Q235 CS. The photogenerated current density generated by Zn₃In₂S₆/NiFe₂O₄ was 3.9 times that of Zn₃In₂S₆, demonstrating that Zn₃In₂S₆/NiFe₂O₄ for Q235 CS also has better cathodic protection performance.

The corrosion potential (E_corr)/coupling potential (E_coupler) and corrosion current density (I_corr)/coupling current density (I_coupler) of 304 SS or Q235 CS coupled with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ can also be determined using dynamic polarization curves and can likewise determine the photocathodic protection capability of the photoanode material. From Figure 7a,b, it can be seen that under dark conditions, the coupling potentials of 304 SS with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ in the 3.5 wt.% NaCl solution are all near the self-corrosion potentials of 304 SS, at −70 mV, −120 mV, and −210 mV, respectively, which has minimal effects on the corrosion rate of 304 SS. The small variation of the coupling potentials indicates the rebalancing of the Fermi energy levels after the metal is coupled with the photoanodes. Under illumination, the coupling potential of 304 SS with NiFe₂O₄ reaches −90 mV, decreasing by 20 mV, with a coupling current density that reaches 3.98 μA·cm⁻²; the coupling potential of 304 SS with Zn₃In₂S₆ reaches −620 mV, decreasing by 410 mV, with a coupling current density that reaches 5.30 μA·cm⁻²; and the coupling potential of 304 SS with Zn₃In₂S₆/NiFe₂O₄ reaches −780 mV, decreasing by 660 mV, with a coupling current density that reaches 10 μA·cm⁻². Under the influence of light, the photogenerated electrons are enriched on the metal surface, resulting in a decrease in the coupling potential. During the electron migration process, the coupling current density represents the photocurrent’s magnitude. The increase in the coupling current density indicates an increase in the electron concentration during the charge transfer process, which corresponds to the results of the OCP and the photogenerated current densities. Due to limitations in their light utilization and carrier separation efficiencies, NiFe₂O₄ and Zn₃In₂S₆ alone exhibit relatively weaker coupling potential drops and coupling current densities. The Zn₃In₂S₆/NiFe₂O₄ composite photoanode with 304 SS shows the most negative coupling potential and the largest coupling current density, which indicates that its metal surface possesses the largest number of photogenerated electrons, improving its photogenerated carrier separation ability and providing the best photocathodic protection performance.

Figure 7c,d present the corrosion potential/coupling potential and the corrosion current density/coupling current density of Q235 CS coupled with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ in a 3.5 wt.% NaCl solution. The coupling potential under dark conditions indicates that the photoanode material exhibited no significant protective effect on Q235 CS. Under illumination, the coupling potential of Q235 CS with NiFe₂O₄ reached −670 mV, increasing by 10 mV, and the coupling current density reached 8.33 μA·cm⁻²; the coupling potential of Q235 CS with Zn₃In₂S₆ reached −960 mV, with a decrease of 260 mV, and the coupling current density reached 16.81 μA·cm⁻²; the coupling potential of Q235 CS with Zn₃In₂S₆/NiFe₂O₄ reached −1140 mV, with a decrease of 420 mV, and the coupling current density reached 32.48 μA·cm⁻². The composite exhibited the most negative coupling potential and the highest coupling current density when coupled with Q235 CS, which demonstrated that the composite had the best cathodic protection performance, consistent with the OCP and photogenerated current density. All corrosion potential/coupling potential and corrosion current density/coupling current density data are listed in

Table 2. Dynamic polarization curve parameters of 304 SS after coupling with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄.

Sample	Ecorr or Ecouple (V)	Icorr or Icouple (μA·cm−2)	βa (V decay−1)	βc (V decay−1)
304 SS	−0.16	0.92	3.29	−8.52
NiFe2O4 dark	−0.07	1.19	15.54	−5.21
NiFe2O4 illumination	−0.90	3.98	23.16	−5.49
Zn3In2S6 dark	−0.12	0.63	1.93	−7.28
Zn3In2S6 illumination	−0.62	5.30	8.79	−5.49
Zn3In2S6/NiFe2O4 dark	−0.21	1.34	1.93	−7.28
Zn3In2S6/NiFe2O4 illumination	−0.78	10	2.03	−6.18

and

Table 3. Dynamic polarization curve parameters of Q235 CS after coupling with NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄.

Sample	Ecorr or Ecouple (V)	Icorr or Icouple (μA·cm−2)	βa (V decay−1)	βc (V decay−1)
Q235 CS	−0.68	1.04	13.71	−13.68
NiFe2O4 dark	−0.68	5.01	8.24	−9.25
NiFe2O4 illumination	−0.67	8.33	7.35	−5.17
Zn3In2S6 dark	−0.71	3.71	20.14	−4.92
Zn3In2S6 illumination	−0.96	16.81	11.26	−5.38
Zn3In2S6/NiFe2O4 dark	−0.72	5.24	17.25	−6.84
Zn3In2S6/NiFe2O4 illumination	−1.14	32.48	6.02	−8.55

.

As one of the most commonly used electrochemical test methods, EIS is employed to investigate the charge transfer resistance of semiconductor materials, thereby demonstrating their charge transfer performance. In this context, the dotted and solid lines represent the original and fitted results, respectively. In the fitted circuit, R_c denotes the surface film resistance, with CPE_c being its capacitance. R_ct represents the charge transfer resistance, CPE_dl represents the capacitance of the double layer, and R_s is the solution resistance. The charge transfer resistance can be judged by comparing the curve diameters; in general, as the curve diameter decreases, the charge transfer resistance also decreases accordingly.

Figure 8a,b illustrate the variation of charge transfer performance of Zn₃In₂S₆ loaded with NiFe₂O₄ in a 3.5 wt.% NaCl solution under dark and illuminated conditions, respectively. From the figure, it can be observed that NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ all have higher impedance values under dark conditions, which indicates lower charge transfer capabilities. Under illumination, the impedance values of NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄ decrease significantly, indicating that the light excitation induces electrons to absorb energy at higher energy-excited states, enhancing their activity and facilitating charge transfer. Importantly, Zn₃In₂S₆/NiFe₂O₄ has the smallest charge transfer resistance, thus indicating that the surface electrons of Zn₃In₂S₆/NiFe₂O₄ are more prone to transfer and have the optimal electron transfer performance. This is because the Zn₃In₂S₆/NiFe₂O₄ composite interface forms a built-in electric field, which inhibits the photogenerated electrons from the excited state to the ground state complexed with holes, thereby increasing the concentration of photogenerated electrons and making electron transfer more likely to occur. Thus, more electrons can be transferred to the surface of the protected metal when coupled with the metal, indicating that Zn₃In ₂S₆/NiFe₂O₄ has better cathodic protection performance compared to a single material. The fitted values in the electrochemical impedance spectra are listed in

Table 4. Nyquist curve results for equivalent circuit fittings of NiFe₂O₄, Zn₃In₂S₆, and Zn₃In₂S₆/NiFe₂O₄.

Sample	Rs (kΩ·cm2)	Rct (kΩ·cm2)	CPEc		Rf (kΩ·cm2)	CPEdl
			Yo(SnΩ−1cm−2)	n1		Ydl(SnΩ−1cm−2)	n2
NiFe2O4 dark	13.87	2.3 × 105	5.5 × 10−6	1	5.1	1.2 × 10−5	0.90
Zn3In2S6 dark	14.49	3.7 × 106	1.1 × 10−6	1	48.7	7.8 × 10−6	0.94
Zn3In2S6/NiFe2O4 dark	13.22	1.1 × 107	7.1 × 10−6	0.95	1.7 × 102	3.1 × 10−6	0.95
NiFe2O4 light	15.56	1.0 × 103	5.52 × 10−5	1	1.5 × 104	8.8 × 10−4	0.92
Zn3In2S6 light	17.03	1.9 × 102	3.4 × 10−4	0.76	7.8 × 102	2.9 × 10−3	0.96
Zn3In2S6/NiFe2O4 light	15.15	1.7 × 102	1.7 × 10−5	0.86	1.8 × 104	1.4 × 10−4	1
	13.87	2.3 × 105	5.5 × 10−6	1	5.1	1.2 × 10−5	0.90

.

In order to gain insight into the charge transfer mode between the materials, the heterojunction mechanism of Zn₃In₂S₆/NiFe₂O₄ can be verified by discussing the relationship between the catalytic potentials of the radicals and the conduction band and valence bands of the photoanode in the ESR spectrum. As shown in Figure 9a, the characteristic curves of ·O²⁻ appear in both Zn₃In₂S₆ and Zn₃In₂S₆/NiFe₂O₄ reactive systems, indicating the formation of ·O²⁻ radicals in both systems. However, no ·O²⁻ radicals were generated in the NiFe₂O₄ system. This is because the O₂/·O₂⁻ potential in the reaction system is approximately −0.33 V (vs. NHE) [45], and the conduction band potentials of Zn₃In₂S₆ and Zn₃In₂S₆/NiFe₂O₄ are −0.80 and −0.63 eV, respectively, both of which are lower than the generation potential of ·O²⁻. Therefore, ·O²⁻ is produced in the system. In contrast, the conduction band potential of NiFe₂O₄ is only −0.28 eV, which is insufficient to reduce O₂ to generate ·O²⁻. In addition, the characteristic curve of Zn₃In₂S₆/NiFe₂O₄ showed the highest intensity in the figure, indicating that due to the effect of the heterojunction, electrons accumulate on a certain material, thereby increasing the charge concentration. If the system conforms to the conventional Type II heterojunction, electrons will accumulate on the conduction band of NiFe₂O₄. However, the conduction band reduction of NiFe₂O₄ is insufficient, and the composite material is unlikely to produce the strongest peak in the characteristic curve. Therefore, the system can only follow the transfer law of the Z-scheme heterojunction, where electrons accumulating on Zn₃In₂S₆ exhibit sufficient reducing ability.

In Figure 9b, it can be seen that ·OH radicals appear in both the NiFe₂O₄ and Zn₃In₂S₆/NiFe₂O₄ reaction systems, but are absent in Zn₃In₂S₆. This is because the valence band position of NiFe₂O₄ is 2.13 eV and exceeds that of ·OH/OH⁻ (1.99 eV vs. NHE) [46]. Therefore, the holes in the valence band of NiFe₂O₄ are able to oxidize OH⁻ in the environment to produce ·OH. The formation of the heterojunction between Zn₃In₂S₆/NiFe₂O₄ leads to photogenerated holes accumulating in the valence band of NiFe₂O₄, which has a strong oxidizing capacity, leading to ·OH appearing in the reaction system. In contrast, due to the lower position of the valence band and the weaker oxidizing ability of Zn₃In₂S₆, no ·OH is generated. The highest peak intensity in the Zn₃In₂S₆/NiFe₂O₄ system indicates that the accumulation of photogenerated holes occurs in the Zn₃In₂S₆/NiFe₂O₄ composite material. If the conventional Type II heterojunction structure was followed, the photogenerated holes would ultimately be accumulated in the valence band of Zn₃In₂S₆, which is not consistent with the fact that Zn₃In₂S₆/NiFe₂O₄ has the strongest peak intensity. Therefore, the Zn₃In₂S₆/NiFe₂O₄ composite material should retain the photogenerated electrons in the conduction band of Zn₃In₂S₆, and the photogenerated holes should be transferred to the valence band of NiFe₂O₄, which is consistent with the Z-scheme heterojunction’s charge transfer mechanism.

Figure 10 shows the charge transfer mechanism of the Zn₃In₂S₆/NiFe₂O₄ Z-scheme heterojunction. Z-scheme heterojunctions differ from conventional Type II heterojunctions in that Z-scheme heterojunctions not only enhance the photogenerated electron–hole separation capability of the Zn₃In₂S₆/NiFe₂O₄ composite material but also retain the electron-driving force of the semiconductor with a more negative conduction band potential and the valence band oxidation capacity of semiconductors with a higher valence band potential. This mechanism enables efficient photocathodic protection for various metals with lower self-corrosion potentials in simulated marine environments.

4. Discussion

This paper studies the construction of a Z-scheme heterojunction by modifying one-dimensional NiFe₂O₄ with Zn₃In₂S₆ to enhance the photoelectrochemical cathodic protection ability of the composite material, enabling it to have a significant protective effect on Q235 carbon steel.

(1)

Two-dimensional Zn₃In₂S₆ interlaces with and grows on the surface of one-dimensional NiFe₂O₄ nanofibers, and lattice information corresponding to the NiFe₂O₄ (311) crystal faces and Zn₃In₂S₆ (102) crystal faces can be clearly identified. Furthermore, the diffraction peaks of NiFe₂O₄ and Zn₃In₂S₆ were both found in the XRD curve of Zn₃In₂S₆/NiFe₂O₄. The Zn₃In₂S₆/NiFe₂O₄ composite material was successfully prepared.

(2)

The construction of the energy band of the Zn₃In₂S₆/NiFe₂O₄ composite material led to optical absorption thresholds that had been broadened to 630 nm. Its valence band was 1.69 eV and its conduction band was −0.63 eV. Furthermore, the Zn₃In₂S₆/NiFe₂O₄ composite material has the lowest photoluminescence intensity and charge transfer resistance. It possessed a favorable light utilization capability and an excellent capacity for the separation and transfer of photogenerated electrons.

(3)

Compared with Zn₃In₂S₆ and NiFe₂O₄, the Zn₃In₂S₆/NiFe₂O₄ composite material coupled with both of the metals demonstrates superior photocathodic protection performance. When coupled with 304 SS, the open-circuit potential dropped to 1040 mV, and the photogenerated current density was 50 μA·cm⁻². As for Q235 CS, the open-circuit potential had dropped to 560 mV, and the photogenerated current density was 35 μA·cm⁻².

(4)

The charge transfer pathway between Zn₃In₂S₆ and NiFe₂O₄ was consistent with the mechanism of a Z-scheme heterojunction. The photogenerated electrons and holes were concentrated in the conduction band of Zn₃In₂S₆ and the valence band of NiFe₂O₄, respectively. This charge transfer mechanism preserves the strong redox capability of photogenerated carriers, thereby providing photocathodic protection for metals with low self-corrosion potentials, such as Q235 CS.

The ferroelectric properties of NiFe₂O₄ may cause its conductivity to be greatly affected by temperature. In future work, temperature can be taken as an influencing factor to explore the effect of temperature differences on the photoelectrochemical protection performance of NiFe₂O₄-based Z-scheme heterojunction materials.

5. Conclusions

This paper focuses on metal protection in marine corrosion environments, constructing a stable Z-scheme Zn₃In₂S₆/NiFe₂O₄ nanocomposite based on NiFe₂O₄ and exploring its protective performance and charge transfer mechanism for Q235 CS. In this study, the one-dimensional NiFe₂O₄ modified by the electrospinning method led to an improved charge transfer capability, promoting the formation of a stable Z-scheme heterostructure between Zn₃In₂S₆ and NiFe₂O₄. The formation of the Z-scheme heterojunction effectively enhanced the light absorption capacity of NiFe₂O₄ and increased the separation efficiency of photogenerated carriers. In addition, the highly reductive photogenerated electrons in the Z-scheme heterojunction of the Zn₃In₂S₆/NiFe₂O₄ composite could provide effective protection for Q235 CS. The conclusion of this paper provides a new example for the industrial application of stable Z-scheme heterostructure construction and photoelectrochemical cathodic protection.