Enhanced Photocathodic Protection Performance of TiO₂/NiCo₂S₄ Composites for 304 Stainless Steel

Abstract

To address the corrosion of 304 stainless steel in marine environments, TiO₂/NiCo₂S₄ composite photoanodes were fabricated via anodic oxidation and hydrothermal methods. X-ray diffraction, scanning electron microscope, energy-dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy analyses indicated the growth of hexagonal NiCo₂S₄ particles on anatase TiO₂ nanotube arrays, forming a type-II heterojunction. Spectroscopy of ultraviolet-visible diffuse reflectance absorption showed that NiCo₂S₄ extended TiO₂’s light absorption into the visible region. Electrochemical tests revealed that under visible light, the composite photoanode decreased the corrosion potential of 304ss to −0.7 V vs. SCE and reduced charge transfer resistance by 20% compared to pure TiO₂. The enhanced performance stemmed from efficient electron-hole separation and transport enabled by the type-II heterojunction. Cyclic voltammetry tests indicated the composite’s electrochemical active surface area increased 1.8-fold, demonstrating superior catalytic activity. In conclusion, the TiO₂/NiCo₂S₄ composite photoanode offers an effective approach for marine corrosion protection of 304ss.

1. Introduction

With the continuous advancement of marine construction, 304 stainless steel (304ss) has been widely applied in marine infrastructure [1,2,3]. However, the high salinity of seawater accelerates the corrosion of 304ss, leading to economic losses. Traditional corrosion prevention methods often cause environmental pollution and high costs [4,5]. Recently, photocathodic protection (PCP) technology has emerged as a promising anti-corrosion approach because it can preserve metal by using sun energy [6,7]. When photoanode materials absorb sunlight, their inherent properties generate electrons that transfer to the metal surface, reducing the self-corrosion potential of the metal and thereby achieving protective effects [8].

As a common semiconductor material in PCP, TiO₂ offers advantages such as low cost and easy preparation [9,10]. Generally, the anatase TiO₂ has a bandgap (E_g) of approximately 3.2 eV, which restricts it to absorbing only ultraviolet (UV) light in sunlight, significantly limiting its PCP performance [11]. To address this, modification methods for TiO₂ have been proposed, including noble metal deposition (Au [12], Ag [13]), doping (Zn [14], Cr [15], Cu [16]), and semiconductor composites (PANI [17], ZnCdS [18], WO₃ [19]). Among these, semiconductor composite technology combines the advantages of different semiconductor materials, making it an efficient modification strategy. Additionally, among various TiO₂ structures, the nanotube structure exhibits a specific surface area and provides direct electron transport channels, endowing it with excellent photocatalytic properties [20].

Metal sulfides exhibit broader solar light absorption due to their narrow bandgaps [21,22,23]. Among them, NiCo₂S₄, as a bimetallic sulfide, has been widely applied in photocatalysis, supercapacitors, and other fields because it conducts electricity well, has low electronegativity, and has a large specific surface area [24,25,26]. Additionally, compared to single-metal sulfides such as NiS and CoS₂, NiCo₂S₄ demonstrates better photostability and photoelectrochemical performance, making it a highly promising material in photocatalytic research [21,27,28,29]. However, studies on the application of NiCo₂S₄ in PCP remain scarce. Therefore, TiO₂/NiCo₂S₄ composite photoanodes were prepared to explore their potential in the PCP field.

In this study, diamond-like NiCo₂S₄ particles were synthesized on the surface of TiO₂ nanotube photoanodes via the hydrothermal method. The hydrothermal method features controllable reaction conditions while enabling the products to exhibit excellent dispersibility [30]. The produced TiO₂/NiCo₂S₄ composite photoanodes’ optical characteristics, elemental makeup, and microstructure were all described. The protective effect of TiO₂/NiCo₂S₄ composite photoanodes on 304ss was investigated through a sequence of electrochemical measurements. The PCP mechanism of TiO₂/NiCo₂S₄ composite photoanodes on 304ss was systematically analyzed.

2. Experimental

2.1. Raw Materials

The reagents used in this study included ammonium fluoride (NH₄F, 99.9%), nickel nitrate hexahydrate (Ni(NO₃)₂⋅6H₂O, 99.0%), cobalt nitrate hexahydrate (Co(NO₃)₂ 6H₂O, 99.0%), thioacetamide (C₂H₅NS, 99.0%), and ethylene glycol (EG, 99.0%). All reagents were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used exactly as supplied. For the duration of the studies, deionized water was utilized. The protected metals were provided by Baoshan Iron & Steel Co., Ltd. (Shanghai, China). The fundamental makeup and composition of 304ss are listed in

Table 1. The fundamental makeup and composition of 304ss [31].

Elements	Cr	Ni	Mn	Si	C	P	S	Fe
Content (wt%)	18.250	8.500	1.860	0.720	0.080	0.035	0.029	70.526

. Titanium foil (>99.99%, 40 × 10 × 0.25 mm) was supplied by Wuhan Research and Experimental Materials Company (Wuhan, China).

2.2. Preparation of Photoelectrodes

Figure 1 shows the preparation process of the TiO₂/NiCo₂S₄ composite photoanode.

Fabrication of TiO₂ Photoanode: First, a 10 mm × 40 mm Ti foil was ultrasonically treated for 5 min in a mixed solution containing 30 mL HNO₃, 30 mL H₂O₂, 12.5 mL H₂O, and 2.25 g NH₄F. The pretreated Ti foil was then subjected to anodic oxidation in a solution composed of 30 mL H₂O, 30 mL EG, and 0.34 g NH₄F, using Pt as the counter electrode (CE), at a voltage of 20 V for 1.5 h. Finally, the anodized Ti foil was heated in a muffle furnace at 450 ℃ for 150 min to obtain the TiO₂ photoanode.

Fabrication of TiO₂/NiCo₂S₄ Photoanode: NiCo₂S₄ particles were synthesized on the surface of TiO₂ via the hydrothermal method according to the previous reported literature [32]. In order to create a precursor solution, Ni (NO₃)₂·6H₂O, Co (NO₃)₂·6H₂O, and C₂H₅NS in 80 mL deionized water at a molar ratio of 1:2:4, followed by 30 min of stirring. The synthesized TiO₂ photoanode was positioned in a Teflon-lined autoclave, and the preparatory solution was introduced. After being sealed, the autoclave was reacted for 15 h at 180 °C. After natural cooling, the resulting photoanode was washed thoroughly and air-dried at room temperature. The contents of raw materials used for preparing different photoanode materials are shown in

Table 2. The raw materials for preparing different TiO₂/NiCo₂S₄ photoanodes.

	Ni (NO3)2·6H2O	Co (NO3)2·6H2O	C2H5NS
TiO2/NiCo2S4-0.25	0.25 mmol	0.50 mmol	1.00 mmol
TiO2/NiCo2S4-0.50	0.50 mmol	1.00 mmol	2.00 mmol
TiO2/NiCo2S4-1.00	1.00 mmol	2.00 mmol	4.00 mmol
TiO2/NiCo2S4-1.50	1.50 mmol	3.00 mmol	6.00 mmol

.

2.3. Characterization

The microscopic nanostructures of the TiO₂ and TiO₂/NiCo₂S₄ photoanodes were observed using a scanning electron microscope (SEM, ZEISS Sigma 360, Carl Zeiss AG, Jena, Germany). X-ray diffraction was used to characterize the crystal structures of the produced samples (XRD, Rigaku SmartLab SE, Rigaku Corporation, Tokyo, Japan). USA’s x-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) was also used to examine the variations in the elemental binding energies of the processed samples. With BaSO₄ powder serving as a calibration baseline, the optical characteristics of the photoanodes were examined using spectroscopy of ultraviolet-visible diffuse reflectance absorption (UV-Vis DRS, Shimadzu UV-3600i Plus, Shimadzu Corporation, Kyoto, Japan).

2.4. Photoelectrochemical Tests

All testing of the produced samples electrochemically was conducted using a CHI760E electrochemical workstation. The setup instructions are shown in Figure 2. An electron exchange was achieved by using a Nafion membrane to join the cell for corrosion to the cell for photoelectrolysis. In the corrosion cell, the electrolyte utilized was 3.5 weight percent NaCl. The ready-made photoanode was further submerged in a photolysis cell containing a mixed solution of 0.1 M Na₂S and 0.2 M NaOH. Na₂S is a powerful hole-trapping substance that can promote the separation of electrons and holes produced by photosynthesis. A 300 W gas bulb (PLS-SXE 300 xenon lamp, Beijing Perfectlight Technology Co., Ltd, Beijing, China) served as the light source. Visible light was vertically irradiated onto the photoanode material through a quartz window.

Figure 2a displays the test device’s wiring for the OCP, EIS, and Tafel curve testing. The 304ss connected to the material of the photoanode acted as the electrode that works (WE), the Pt electrode as the CE, and utilized the reference electrode (RE) as the saturated calomel electrode (SCE). The alternating-current voltage for the EIS test had an amplitude of 5 mV and a frequency range of 10⁵ to 10⁻² Hz; the potential was set as the OCP of the photoanode under light irradiation. The initial potential for the EIS experiment was set to the stable OCP value, which was obtained after connecting the photoanode material to the shielded steel under visible light conditions. During the I-t curve test, the prepared material was connected to the WE, and the 304ss was attached to the ground. The CE and RE were short-circuited with a wire (Figure 2b). In addition, cyclic voltammetry (CV) was used to calculate the electrochemical active surface area (ECSA) at different scan rates. Based on the density functional theory (DFT), the work functions of TiO₂ and NiCo₂S₄ photoanode materials were calculated using the DMol3 module of Materials Studio software (Materials Studio 2023, Accelrys, CA, USA). The convergence criterion for the self-consistent field (SCF) was set to 1.0 × 10⁻⁶ eV, and the k-point grids were set to 6 × 6 × 2 and 8 × 6 × 4 respectively.

3. Results and Discussion

3.1. Morphology and Characterization

Figure 3 shows the SEM images of the prepared TiO₂ and TiO₂/NiCo₂S₄ photoanodes, as well as the energy-dispersive x-ray spectroscopy (EDS) test results of the TiO₂/NiCo₂S₄ photoanode. Figure 3a and 3b show the top and cross-sectional SEM images of the prepared pure TiO₂ nanotubes (NTs). It can be seen that TiO₂ NTs were successfully prepared by anodic oxidation. Figure 3c,d shows the micro-morphology of the TiO₂/NiCo₂S₄ photoanode. It can be found that diamond-like particle structures show up on the nanotubes’ surface. In addition, based on the test findings displayed in Figure 3e–j, it can be found that elements Ti, O, Ni, Co, and S all exist in the TiO₂/NiCo₂S₄ photoanode. The results show that NiCo₂S₄ was successfully deposited on TiO₂ through the hydrothermal method.

The XRD patterns of the TiO₂ and TiO₂/NiCo₂S₄ photoanodes are presented in Figure 4. For the pure TiO₂ photoanode, diffraction peaks at 25.3°, 48.1°, and 62.7° correspond to the (101), (200), and (204) crystal planes of anatase TiO₂ (JCPDS 21-1272), while the remaining peaks are assigned to the titanium substrate [33]. In the TiO₂/NiCo₂S₄ photoanode, besides the characteristic peaks of TiO₂ and Ti, new diffraction peaks emerge at 31.6° and 38.3°, which match the (222) and (400) planes of hexagonal NiCo₂S₄ (JCPDS 20-0782) [34]. Notably, no significant shifts in the TiO₂ peaks are observed compared to the pure TiO₂ sample, indicating that NiCo₂S₄ was successfully composited onto the TiO₂ NTs via the hydrothermal method without altering the host material’s crystal structure.

XPS was utilized to identify each element’s valence states in TiO₂/NiCo₂S₄ and the interfacial electronic interactions within the composite material. Figure 5 shows the XPS results of the TiO₂ NTs and TiO₂/NiCo₂S₄ photoanodes. Figure 5a presents the full scan spectrum of the TiO₂/NiCo₂S₄ photoanode, indicating the presence of Ti, O, Ni, Co, and S elements. As shown in Figure 5b, the XPS results of the Ti element in the TiO₂ and TiO₂/NiCo₂S₄ photoanodes are presented. In the pure TiO₂ photoanode, the two peaks displayed by Ti element at 458.8 eV and 464.5 eV are Ti 2p3/2 and Ti 2p1/2, respectively. The doublet separation of Ti 2p is 5.7 eV, which is in good agreement with the reported value. The above results indicate that the valence state of Ti remains +4 [35,36]. In the TiO₂/NiCo₂S₄ photoanode, the two peaks of Ti are at 458.7 eV and 464.4 eV, respectively. Moreover, compared with the pure TiO₂, the corresponding binding energy of the Ti element in the TiO₂/NiCo₂S₄ photoanode shifts to a lower energy position, suggesting that TiO₂ gains electrons after being modified by NiCo₂S₄. As shown in Figure 5c, two characteristic peaks appear at 530.0 eV and 531.8 eV, which can be attributed to lattice oxygen (O_L) and oxygen vacancies (O_A), respectively [37]. The separation of photogenerated electrons and holes in the composite is facilitated by the creation of O_A [38]. Figure 5d shows the XPS results of the Ni element in the TiO₂/NiCo₂S₄ photoanode. The peaks at 855.7 eV and 875.2 eV correspond to the 2p_3/2 and 2p_1/2 orbitals of Ni³⁺, while the peaks at 853.0 eV and 870.4 eV correspond to the 2p_3/2 and 2p_1/2 orbitals of Ni²⁺. The other two peaks may be satellite peaks caused by Ni 2p [39]. In Figure 5e, the peaks at 778.2 eV and 793.7 eV can be attributed to Co³⁺, and the two peaks centered at 780.7 eV and 798.2 eV can be attributed to Co²⁺. Similarly, the remaining two peaks are satellite peaks caused by Co 2p [40]. Figure 5f shows the characteristic peaks of the S element in the TiO₂/NiCo₂S₄ photoanode. The characteristic peaks centered at 161.0 eV and 162.0 eV are a part of the S 2p_3/2 and S 2p_1/2 of S²⁻, respectively, and the peaks at 162.8 eV and 165.2 eV are a part of the S 2p_3/2 and S 2p_1/2 of unsaturated S₂²⁻. Additionally, the characteristic peak at 168.4 eV results from the oxidation reaction during the synthesis of NiCo₂S₄ [32].

To assess the photoanode materials’ capacity to absorb light, UV-Vis DRS tests were carried out on the TiO₂, TiO₂/NiCo₂S₄ photoanodes, and pure NiCo₂S₄. The results are presented in Figure 6a. As the image illustrates, the absorption cut-off edge of the pure TiO₂ photoanode is approximately at 375 nm. Moreover, the absorption edge of pure NiCo₂S₄ is around 640 nm, while the TiO₂/NiCo₂S₄ photoanode exhibits an absorption edge at 405 nm. Through the modification of TiO₂ with NiCo₂S₄, the absorption range of the photoanode has redshifted. In addition, the band gap energy (E_g) values of pure TiO₂, TiO₂/NiCo₂S_4, and NiCo₂S₄ were determined through Equation (1):

$${(\alpha \mathrm{h}v)}^2=A(\mathrm{h}v-E_{\mathrm{g}})$$

(1)

In the formula, the absorption coefficient (α), Planck constant (h), photon frequency (ν), and characteristic constant (A) are represented by their respective symbols [41,42]. The E_g of pure TiO₂, TiO₂/NiCo₂S₄, and NiCo₂S₄ were calculated as 3.31 eV, 3.06 eV, and 1.94 eV, respectively (Figure 6b), which are consistent with the values reported in previous literature [2,32].

OCP is a critical indicator for evaluating the PCP performance of photoanodes. Under sporadic visible light, the OCP values of 304ss combined with various photoanodes were measured [43], as shown in Figure 7a. When exposed to visible light, the OCP of 304ss coupled with pure TiO₂ and pure NiCo₂S₄ was −0.60 V vs. SCE and −0.48 V vs. SCE, respectively. Notably, when 304ss was coupled with the composite photoanodes, their OCP values were all lower than those when coupled with pure TiO₂ and pure NiCo₂S₄. Among them, under illumination, TiO₂/ NiCo₂S₄-0.50 resulted in the most negative OCP of 304ss, reaching −0.70 V vs. SCE. This may be attributed to the formation of TiO₂ and NiCo₂S₄ heterojunction electric fields in the composite material, which transferred more photogenerated electrons to the surface of 304ss, thereby reducing the corrosion potential of 304ss. In the dark, the OCP values of 304ss coupled with composite photoanodes were all lower than those with pure TiO₂. These findings indicate that TiO₂/NiCo₂S₄ composite photoanodes provide superior PCP performance for 304ss compared to pure TiO₂.

The photocurrent density curves of several photoanodes connected to 304ss under sporadic visible light are shown in Figure 7b. Upon light irradiation, the current increased for all photoanodes, indicating electron transfer from the photoanodes to the 304ss surface. Significantly, TiO₂/NiCo₂S₄-0.50 exhibited the highest photocurrent density, demonstrating that NiCo₂S₄ modification enhances electron transfer efficiency [44]. This result indicates the superior protective effect of TiO₂/NiCo₂S₄-0.50 on 304ss.

To evaluate the stability of the TiO₂/NiCo₂S₄ composite photoanode, long-term stability tests were conducted by coupling it with 304ss, as shown in Figure 7c,d. Figure 7c reveals that the TiO₂/NiCo₂S₄ photoanode can continuously protect 304ss under illumination and maintain protection for over 10 h in the dark, indicating that the TiO₂/NiCo₂S₄ composite photoanode can provide continuous protection for 304ss [45]. Figure 7d shows the SEM image of the TiO₂/NiCo₂S₄ photoanode after long-term testing, with no observable structural changes compared to the initial morphology. These results indicate the excellent stability of the composite photoanode.

Figure 8a displays the Nyquist plots of 304ss coupled with pure TiO₂ and TiO₂/NiCo₂S₄ composite photoanodes under illumination. The inset shows an enlarged view of the high-frequency region. As indicated, the impedance arc radius of 304ss coupled with TiO₂/NiCo₂S₄ is reduced, suggesting that TiO₂/NiCo₂S₄ exhibits higher electron transfer efficiency compared to pure TiO₂. Further analysis of the experimental data was performed using Z_view software (Z_view 3.0.0.22, Scribner Associates Inc, Southern Pines, NC, USA), with fitting based on the equivalent circuit shown in Figure 8b. The fitting quality was evaluated by chi-squared (χ²) values of about 10⁻³~10⁻⁴ [46]. Therefore, the fitted data in this paper is reliable. The fitted impedance parameters are listed in

Table 3. EIS curve fitting findings for 304ss linked photoanode materials under illumination.

Samples	Rs	Rf	Qf		Rct	Qdl		χ2
	(Ω·cm2)	(Ω·cm2)	Y01 (10−5·Ω−1·cm−2·sn)	n1	(Ω·cm2)	Y02 (10−5·Ω−1·cm−2·sn)	n2	10−4
TiO2	5.914	3.255	92.073	0.90669	1263 ± 8	125.77	0.77939	52.8
TiO2/NiCo2S4	7.705	25.08	23.100	0.76651	999 ± 5	266.68	0.71192	9.28

, where R_s represents the solution resistance, R_f the surface resistance, R_ct the charge transfer resistance, and Q_f/Q_dl the film/electrical double-layer capacitances [47]. The R_ct value of TiO₂/NiCo₂S₄ was 20% lower than that of pure TiO₂, which further indicates that the NiCo₂S₄ significantly enhances the electron transfer efficiency.

To further investigate the PCP mechanism of TiO₂/NiCo₂S₄ composite photoanodes on 304ss, Tafel curves of 304ss coupled with pure TiO₂ and TiO₂/NiCo₂S₄ photoanodes were measured under both illumination and dark conditions, as shown in Figure 8c. Electrochemical parameters derived from Tafel fitting are listed in

Table 4. Tafel curve fitting findings for 304ss linked photoanode materials under both light and dark conditions.

Samples	Ecorr (V vs. SCE)	icorr (μA cm−2)	βc (mV dec−1)	βa (mV dec−1)
Light TiO2	−0.565	41.05	456.8	569.1
Light TiO2/NiCo2S4	−0.642	50.40	649.1	696.0
Dark TiO2	−0.505	46.72	476.7	549.6
Dark TiO2/NiCo2S4	−0.617	48.27	605.4	684.3

. According to the data, the corrosion potential (E_corr) of 304ss exhibits a significant negative shift when coupled with both pure TiO₂ and TiO₂/NiCo₂S₄ photoanodes, regardless of light conditions. Notably, the most substantial negative shift occurs when 304ss is coupled with TiO₂/NiCo₂S₄, indicating superior PCP performance compared to pure TiO₂. This finding is consistent with the OCP test results. In addition, when the TiO₂/NiCo₂S₄ composite photoanode is coupled with 304ss, the i_corr is larger than that of the TiO₂ photoanode, which indicates that more electrons are generated at the TiO₂/NiCo₂S₄ interface than at the TiO₂ interface. Furthermore, as can be seen from Table 4, the β_c values of photoanode materials coupled with 304ss are all less than their corresponding β_a values, indicating that the corrosion process is under anodic control. Among them, the TiO₂/NiCo₂S₄ photoanode coupled with 304ss shows the largest β_c value, suggesting that the corrosion rate of this coupling system is the lowest [37].

CV tests were conducted to compare the electrochemical activity of TiO₂ and TiO₂/NiCo₂S₄ photoanodes. The CV curves of TiO₂ and TiO₂/NiCo₂S₄ photoanodes recorded at six distinct scan speeds are displayed in Figure 9a,b. The enclosed area of the CV curves for the TiO₂/NiCo₂S₄ photoanode was significantly larger than that of the TiO₂ photoanode. Figure 9c presents the scatter plot of current density vs. scan rate, with linear fitting slopes indicating a much steeper increase for TiO₂/NiCo₂S₄ compared to pure TiO₂. This indicates that NiCo₂S₄ modification enhances the ECSA of TiO₂ [43,48]. The ECSA is calculated using Equations (2) and (3).

$$C_{\mathrm{dl}}={\displaystyle \frac{I_{\mathrm{c}}}{V}}$$

(2)

$$ECSA={\displaystyle \frac{C_{\mathrm{dl}}}{C_{\mathrm{s}}}}$$

(3)

where C_dl corresponds to the double-layer capacitance of the optical anode, C_s denotes the electrolyte capacitance, V denotes the scanning speed, and I_c denotes the charging current at various scanning speeds [49]. Since the electrolyte’s capacitance value is almost constant within the same system, the capacitance value may be used to qualitatively describe the ECSA for various photoanodes [50]. This improvement may be attributed to the heterogeneous spin-state mixing of Ni and Co atoms in NiCo₂S₄, which induces lattice distortion and exposes more active sites favorable for catalytic reactions.

To further evaluate the photoanode performance, i-V tests were performed on TiO₂ and TiO₂/NiCo₂S₄ photoanodes in 0.1 M Na₂SO₄ solution exposed to sporadic exposure to visible light irradiation. As shown in Figure 9d, the onset bias potential of TiO₂ was approximately −0.42 V vs. SCE under illumination, while that of TiO₂/NiCo₂S₄ shifted negatively to −0.73 V vs. SCE. The negative shift in the onset potential indicates that NiCo₂S₄ modification of pure TiO₂ facilitates electron transfer [51].

3.2. Mechanism of the TiO₂/NiCo₂S₄ Composites

Mott-Schottky measurements were performed on pure TiO₂ and NiCo₂S₄ to determine their band structures and semiconductor types [52], as shown in Figure 10. The Mott-Schottky curves’ positive slopes for both materials indicate they are n-type semiconductors. Furthermore, TiO₂ and NiCo₂S₄ were found to have flat band potentials (E_fb) of −0.28 V and −0.88 V vs. SCE [26], respectively. Using Equation (4) [53], the E_fb of TiO₂ and NiCo₂S₄ relative to the normal hydrogen electrode (NHE) were obtained as −0.04 V and −0.64 V, respectively. The E_fb and conduction band potentials (E_CB) of n-type semiconductors can be converted using Equation (5) [53]. The E_CB of TiO₂ and NiCo₂S₄ are –0.24 V and –0.84 V, respectively.

$$E_{\mathrm{NHE}}=E_{\mathrm{SCE}}+0.24 \mathrm{V}$$

(4)

$$E_{\mathrm{CB}}=E_{\mathrm{fb}}- 0.2 \mathrm{eV}$$

(5)

$$E_{\mathrm{VB}}=E_{\mathrm{g}}+E_{\mathrm{CB}}$$

(6)

Additionally, combining the E_g values obtained from Figure 6b with Equation (6) [54], the valence band potentials (E_VB) of TiO₂ and NiCo₂S₄ were calculated as 3.07 V and 1.10 V, respectively. The above calculation results are summarized in

Table 5. The calculation results of E_fb, E_CB, and E_VB for TiO₂ and NiCo₂S₄.

	TiO2 (V)	NiCo2S4 (V)
Efb vs. SCE	−0.28	−0.88
Efb vs. NHE	−0.04	−0.64
ECB	−0.24	−0.84
EVB	3.07	1.10

.

As shown in Figure 11, DFT calculation results show that the Fermi levels of the (200) crystal plane of TiO₂ and the (400) crystal plane of NiCo₂S₄. Due to the difference in electron mobilities, when the interfaces of TiO₂ and NiCo₂S₄ come into contact, electrons will transfer between the two materials to achieve dynamic equilibrium of electron mobility. During this electron migration process, an internal electric field forms at the interface contact, leading to band bending and the formation of a Schottky barrier [55]. The work functions of TiO₂ and NiCo₂S₄ are found to be 5.96 eV and 5.49 eV, respectively, by calculating the vacuum level’s difference from the Fermi level. A smaller work function is more conducive to electron spillover [56]. This is consistent with the results regarding the electron gain of Ti element in the XPS tests. Therefore, combined with the energy band structure arrangement of the composite photoanode material, it can be concluded that NiCo₂S₄ can provide more electrons to TiO₂, thereby enhancing the performance of PCP.

Based on the above experimental results, we propose the PCP mechanism of the TiO₂/NiCo₂S₄ photoanode for 304ss. As shown in Figure 12, when visible light irradiates the photoanode surface, both TiO₂ and NiCo₂S₄ absorb sunlight, generating photogenerated excitation of electrons in the CB from the VB. Due to the higher Fermi level of NiCo₂S₄ compared to TiO₂, electrons in the VB of NiCo₂S₄ transfer to the VB of TiO₂. These electrons are then conducted through the external circuit to the 304ss metal surface, thereby providing cathodic protection. Additionally, XPS results indicate a negative shift in the binding energy of Ti in TiO₂ after compositing with NiCo₂S₄, further indicating that TiO₂ gains electrons. This electron transfer process enhances the separation efficiency of photogenerated carriers and improves the PCP performance of the photoanode. Thus, the NiCo₂S₄ modification endows TiO₂ with superior PCP capabilities through the formation of a type-II heterojunction.

4. Conclusions

In this study, TiO₂/NiCo₂S₄ photoanodes were successfully fabricated via anodic oxidation and hydrothermal methods. SEM and EDS results showed the diamond-like NiCo₂S₄ particles were deposited on the TiO₂ NTs’ surface. XRD results indicated the deposition of hexagonal NiCo₂S₄ onto anatase TiO₂.XPS results also indicated the deposition of NiCo₂S₄ontoTiO₂, as well as the direction of electron transfer between TiO₂ and NiCo₂S₄. According to the results of UV-Vis DRS, NiCo₂S₄ alteration improved TiO₂’s capacity to absorb visible light. Electrochemical tests, including OCP, i-t, Tafel, and EIS, demonstrated that the modified photoanodes induced a significant negative shift in the polarization potential of 304ss, reduced charge transfer resistance, and lowered the corrosion potential of 304ss coupled with the photoanodes to −0.70 V vs. SCE. CV results indicated a notably increased ECSA for the TiO₂/NiCo₂S₄ photoanode, further indicating improved photocatalytic activity. The TiO₂/NiCo₂S₄ composite photoanode provided superior PCP performance for 304ss compared to pure TiO₂. This work offers a valuable reference for the modification of TiO₂-based materials in PCP applications.