In₂S₃@TiO₂/In₂S₃ Z-Scheme Heterojunction with Synergistic Effect for Enhanced Photocathodic Protection of Steel

Abstract

In this work, a TiO₂/In₂S₃ heterojunction film was successfully synthesized using a one-step hydrothermal method and applied in the photocathodic protection (PCP) of 304SS. The octahedral In₂S₃ and In₂S₃@TiO₂ nanoparticles combined and coexisted with each other, with In₂S₃ quantum dots growing on the surface of TiO₂ to form In₂S₃@TiO₂ with a wrapping structure. The composite photoelectrode, which includes TiO₂ with a mixed crystalline phase and In₂S₃, exhibited significantly enhanced PCP performance for 304SS compared with pure In₂S₃ and TiO₂. The In₂S₃@TiO₂/In₂S₃ composites with 0.3 g of P25 titanium dioxide (P25) showed the best protection performance, resulting in a cathodic shift of its OCP coupled with 304SS to −0.664 V_AgCl. The electron transfer tracking results demonstrate that In₂S₃@TiO₂/In₂S₃ forms a Z-scheme heterojunction structure. The enhanced PCP performance could be attributed to the synergistic effect of the mixed crystalline phase and the Z-scheme heterojunction system. The mixed crystalline phase of TiO₂ provides more electrons, and these electrons are gathered at higher energy potentials in the Z-scheme system. Additionally, the built-in electric field further promotes the more effective electrons transfer from photoelectrode to the protected metals, thus, leading to enhanced photoelectrochemical cathodic protection of 304SS.

1. Introduction

Corrosion of metals is common in nature because of a thermodynamically favorable spontaneous reaction. The harsh marine environment can accelerate the metal ions’ dissolution, and promote the development of corrosion. Environmental pollution, engineering safety accidents, and economic losses caused by corrosion have caused great harm to human activities. Photoelectrochemical cathodic protection is considered as one of the most promising and environmentally friendly strategies for protecting metals in service, especially in marine environments with sufficient sunlight. This is due to the non-sacrificial anode and the utilization of solar energy [1,2]. In this technology, the photogenerated electrons produced by semiconductor photoanodes are transferred to the coupled metal to prevent corrosion, helping it achieve a thermodynamically stable state. The photoelectric characteristics of semiconductors directly determine the photoelectrochemical cathodic protection effect on metals, and the semiconductors (or photoelectrodes) play a crucial role in the entire protection process [3,4].

Since the report of photoelectrochemical cathodic protection of copper using TiO₂ by Yuan and Tsujikawa in 1995 [5], titanium dioxide (TiO₂), one of the most important n-type semiconductors, has been widely investigated due to its efficient photocatalytic properties, good stability, low toxicity, and cost-effectiveness. A series of TiO₂-based photoanodes have been fabricated and applied in photocathodic protection for metals [6,7,8,9]. However, because of low light-harvesting efficiency, the high recombination efficiency of photogenerated carriers, and an insufficiently negative Fermi level, developing a highly efficient semiconductor photoanode for photoelectrochemical cathodic protection on metals remains a significant challenge [10,11].

Many efforts have been made to modify TiO₂ [12], such as morphology control, elemental doping, and heterojunction construction, to improve the performance of photoelectrochemical cathodic protection [13,14,15,16]. Among these methods, the combination of TiO₂ with narrow-band-gap semiconductors to form a heterostructure has shown to be an effective approach for enhancing light absorption and carrier separation efficiency. In this case, indium sulfide (In₂S₃) stands out as an ideal candidate due to its narrow band gap (2.1~2.2 eV) and a Fermi level that is more negative than that of TiO₂ [17,18,19]. Huang et al. reported the synthesis of a type II In₂S₃/TiO₂ heterostructure to enhance activity in CO₂ photocatalytic reduction [20]. Park et al. prepared a nanostructured type II In₂S₃/TiO₂ photoanode using the hydrothermal method for efficient photoelectrochemical water splitting [21]. While type II In₂S₃/TiO₂ materials have exhibited good photocatalytic performance in CO₂ reduction and water oxidation, they are not ideal for the photocathodic protection of 304 stainless steel (304SS) [22]. In addition to type II photocatalysts, the Z-scheme photocatalyst system is another heterojunction structure that consists of two semiconductors mimicking the natural photosynthesis of green plants. The electrons in the higher conduction band and the holes in the lower valence band would participate in photocathodic protection [23,24,25]. Z-scheme heterostructures can achieve more efficient charge separation and more negative reduction potential for photogenerated electrons, demonstrating their evident superiority compared with type II photocatalysts. However, there are few reports on Z-scheme In₂S₃/TiO₂ materials for photocathodic protection. Furthermore, P25 TiO₂, a commonly used commercial photocatalyst, is known to exhibit more excellent photocatalytic performance than anatase TiO₂ or rutile TiO₂ due to its mixed crystalline phase effect. However, the underlying influence of the mixed crystalline phase in P25 TiO₂ on charge transfer in Z-scheme systems remains unclear.

Inspired by the above discussion, in this work, we report a direct Z-scheme In₂S₃@TiO₂/In₂S₃ system to enhance the photoelectrochemical cathodic protection performance and protect 304SS. The In₂S₃@TiO₂/In₂S₃ composite was successfully prepared on the fluorine-doped tin oxide (FTO) glass substrates through a simple one-step hydrothermal method. We investigated the morphologies, crystalline structures, and optical properties of the resulting In₂S₃@TiO₂/In₂S₃ composites. The influence of TiO₂/In₂S₃ heterojunction and crystalline phase of TiO₂ on the PCP performance of the as-prepared composite films for 304SS was also examined. Furthermore, the charge transfer mechanism was explored to clarify the promoting mechanism of In₂S₃@TiO₂/In₂S₃ in photocathodic protection of 304SS.

2. Results and Discussion

2.1. Characterization of the As-Synthesized Samples

The crystal structures of the purchased AT and PT and the hydrothermally obtained IS powders were characterized by XRD and the results are shown in Figure 1a. It is evident that all the observed diffraction peaks of AT samples correspond to the anatase phase of TiO₂ (JCPDS, No. 21-1272) [26]. In addition to the diffraction peaks of the anatase phase, the diffraction peaks at 27.4°, 36.1°, and 54.3° in the XRD pattern of PT samples belong to (110), (101), and (211) lattice planes of the rutile phase (JCPDS, No. 21-1276) [19]. As for the hydrothermally obtained IS samples, the diffraction peaks at 27.4°, 33.2°, 43.6°, and 47.7° can be attributed to the β-In₂S₃ (JCPDS, No. 65-0459) crystal planes of (311), (400), (511), and (440) [27], indicating the successful synthesis of In₂S₃ samples. The XRD patterns of the prepared AT/IS, PT/IS, and IS coatings are shown in Figure 1b. It is worth noting that all the spectra for the three kinds of coatings display diffraction peaks corresponding to the FTO substrates (marked with ●). After deducting the FTO substrate, the diffraction peaks of the IS coatings can be indexed to the characteristic peaks of β-In₂S₃. For the AT/IS and PT/IS coatings, the diffraction peaks ascribed to both β-In₂S₃ and anatase phases are identified. However, there are no detectable peaks of the rutile phase for PT/IS samples, which could be attributed to the low peak intensity of the rutile phase in PT/IS samples.

The morphologies of IS, AT/IS, and PT/IS on FTO substrates were observed by SEM, and the obtained images are shown in Figure 2a–c. The hydrothermally synthesized In₂S₃ samples show octahedron or truncated octahedron shapes, with particle sizes ranging from 200 to 500 nm (Figure 2a). As shown in Figure 2b,c, the AT/IS and PT/IS composites have a similar morphology, with the nanoparticles of TiO₂ dispersed on the surface and around these octahedral materials. Table S1 shows the AT/IS and PT/IS film roughness obtained by the analysis of AFM characterization results (Figure S1). The R_q of AT/IS and PT/IS films are about 120~130 nm. The TEM images of pure In₂S₃ and TiO₂ shown in Figure 2d–f, and the edges and corners of In₂S₃ are clearly seen in Figure 2d, consistent with the results obtained from the SEM image in Figure 2a. The particle sizes of AT and PT samples are both 20~30 nm in Figure 2e,f. Observing the images of TEM for AT/IS and PT/IS composites in Figure 2g,h, the quantum dots with the particle sizes of 2~6 nm appear on the surface of TiO₂. Furthermore, as shown in Figure 2i, there are three different lattice fringes that can be found in the HRTEM of PT/IS samples. The lattice fringe spacings of 0.169 nm and 0.352 nm correspond to the (211) plane of rutile TiO₂ and the (101) plane of anatase TiO₂ [20], while the lattice fringe spacing of 0.268 nm corresponds to the (400) plane of In₂S₃ [25]. These results indicate that the quantum dots are In₂S₃. Additionally, TEM–EDS elemental mapping images of PT/IS composites, shown in Figure 2j, display the distribution of In, S, O, and Ti elements, with no other impurity elements detected. The quantum dots are uniformly dispersed on the surface of TiO₂, forming an In₂S₃@TiO₂ heterostructure. Therefore, these results evidently confirm the formation of the In₂S₃@TiO₂/In₂S₃ heterojunction, indicating the successful preparation of the composite.

In order to investigate the surface elemental composition and chemical state of PT/IS, the XPS analysis was performed. Figure 3 shows the high-resolution XPS spectra of Ti 2p, O1s, In 3d, and S 2p. It can be seen from the Ti 2p XPS spectrum (Figure 3a), two peaks at 458.7 eV and 464.4 eV are observed, corresponding to Ti 2p_3/2 and Ti 2p_1/2, respectively, which can be attributed to Ti⁴⁺ of TiO₂ [11,12,13,14,15,16,17,18,19]. The peaks in the O1s spectrum for PT/IS samples (Figure 3b) can be fitted into two peaks with binding energies centered at 529.9 eV and 531.3 eV. These peaks correspond to lattice oxygen species in TiO₂ and surface hydroxyl groups/absorbed oxygen, respectively [28]. The In 3d XPS spectrum is shown in Figure 3c, with the peaks at 444.7 eV and 452.2 eV corresponding to the In³⁺ 3d_5/2 and In³⁺ 3d_3/2 peaks, respectively, indicating the presence of In³⁺ in PT/IS samples [17]. Figure 3d shows the S 2p XPS spectrum of PT/IS. After further fitting of the S 2p, two peaks of S 2p_3/2 and S 2p_1/2 appear at 161.2 and 162.3 eV, respectively, which is ascribed to S²⁺ of In₂S₃ [20]. The XPS results further confirm the successful synthesis of a composite consisting of TiO₂ and In₂S₃.

The UV–vis DRS spectra of IS, AT, PT, AT/IS, and PT/IS samples are shown in Figure 4a. It is evident that the optical absorption edge for AT and PT is located around 382 nm and 395 nm, respectively. The IS samples exhibit absorption in the visible light region, characterized by an absorption edge at ~585 nm. Upon the incorporation of In₂S₃, both AT/IS and PT/IS present a noticeable enhancement in response in the range of 400~600 nm compared with AT and PT, respectively. This indicates an improved light absorption capability of the AT/IS and PT/IS samples within the visible light region. Notably, the absorption edge of PT/IS has an obvious redshift compared with that of AT/IS, which can be attributed to the mixed crystalline phase of P25. Furthermore, as shown in Figure 4b, the band gap energies of the above samples were calculated based on the (αhv)²-hv curves [29]. The E_g values of AT and PT are ca. 3.25 eV and 3.14 eV, respectively, while that of IS is ca. 2.12 eV. This indicates that Ti elements are successfully doped into the bulk of α-Fe₂O₃, which slightly increases the visible light absorption.

2.2. The Influence of Heterojunction Structure on PT/IS with Mixed Crystalline Phase Photoelectrode

A series of PT/IS photoelectrodes with different amounts of added P25 (0.01 g, 0.03 g, 0.05 g) were prepared and designated as PT(1)/IS, PT(3)/IS and PT(5)/IS, respectively. The trends of OCP change for all the PT/IS photoelectrodes are shown in Figure 5a. All the photoelectrodes show a strong sensitivity to light irradiation, with a dramatic decrease in OCP from off illumination to on illumination. This decrease is attributed to the migration of photogenerated electrons from the photoelectrode to 304SS. When exposed to light, as the amount of P25 added increases from 0 to 0.03 g, the negative shift in the photo-potential for PT/IS photoelectrodes coupled with 304SS becomes more pronounced. This further confirms the positive effect of the heterojunction structure in PT/IS with a mixed crystalline phase, which promotes the fast injection of electrons from PT/IS to 304SS and subsequent cathodic polarization. However, when the amount of P25 added further increases, the negative shift in the photo-potential decreases. This could be attributed to the excessive P25 addition, leading to the aggregation of nanoparticles and possibly becoming recombination centers for electron–hole pairs. In addition, the photocurrent densities of different PT/IS photoelectrodes coupled with 304SS under intermittent solar light are shown in Figure 5b. As observed, all the samples generate positive photocurrents, indicating the migration direction of photogenerated electrons from the photoelectrodes to 304SS. The PT(3)/IS photoelectrode presents the highest photocurrent response, consistent with the results of the above OCP measurements, further emphasizing the important role of the heterojunction in the PT/IS system.

To gain a deeper understanding of the PT/IS heterojunction, the electron transport behavior and electron lifetime were investigated though EIS measurements under light irradiation at an open-circuit voltage. The Nyquist and corresponding Bode phase angle plots of pure PT, IS, and composite PT/IS photoelectrodes are shown in Figure 5c,d. An equivalent circuit model with a two-time constants for EIS data fitting is exhibited in the inset of Figure 5c and the values of fitting circuit parameters are listed in

Table 1. The equivalent circuit model EIS results of different samples.

Samples	Rs (Ω)	CPE1, Y0 (S·secn)	n1 (0 < n ≤ 1)	Rf (Ω)	CPE2, Y0 (S·secn)	n2 (0 < n ≤1)	Rct (kΩ)
IS	1.50	1.81 × 10−7	1.00	19.5	0.78 × 10−5	0.96	8.10
PT(1)/IS	1.51	2.08 × 10−7	1.00	18.0	1.22 × 10−5	0.96	6.95
PT(3)/IS	1.96	3.56 × 10−7	0.94	23.5	3.24 × 10−5	0.92	2.92
PT(5)/IS	1.84	3.00 × 10−7	0.97	20.3	2.03 × 10−5	0.94	4.17
PT	2.68	1.85 × 10−7	0.99	18.2	1.03 × 10−5	0.97	9.03

. R_s, R_f, R_ct, and CPE represent the series resistance, the film resistance, the charge transfer resistance across the interface of the photoanode and electrolyte, and the constant phase element, respectively [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]. The charge transfer resistance across the interface of the photoelectrode and electrolyte in the low-frequency region is an important parameter for evaluating the electrochemical behavior of photoelectrodes. The composite PT/IS reduces R_ct compared to pure PT and IS, indicating a faster charge transfer in the composite PT/IS owing to the formation of a heterojunction structure between TiO₂ with a mixed crystalline phase and In₂S₃. The R_ct value for PT(3)/IS composite is 2920 Ω, which is lower than that of the other PT/IS composites. This result is consistent with the findings of the PEC performance tests, including OCP and photocurrent response test.

Moreover, according to the Bode phase angle plots of the photoelectrode samples under illumination, the lifetime of photogenerated electrons (τ) can be obtained through the following Equation (1):

$$\tau =\frac{1}{2{\pi f}_{max}}$$

(1)

where $f_{max}$ represents the characteristic peak frequency at low frequency, which is the frequency value (Hz) corresponding to the first peak [31,32]. As shown in Figure 5d, the values of $f_{max}$ for PT(3)/IS composite photoelectrodes are smaller than those of pure photoelectrodes and the other PT/IS composite photoelectrodes. This indicates that the lifetime of the photogenerated electrons of PT(3)/IS composite is the longest, facilitating the efficient separation and transfer of photogenerated carriers.

Additionally, the linear sweep voltammetry (LSV) curves of PT, IS, and PT(3)/IS photoelectrodes in 0.15 M Na₂S + 0.1 M Na₂SO₃ solution are exhibited in Figure S2a. As a hole scavenger, Na₂S and Na₂SO₃ can effectively trap the photogenerated holes that arrive at the surface. According to the previous reports [6,7,8,9,10,11,12,13,14,15,16,17,18], the charge separation efficiency can be calculated by the equation: (${\eta }_{sep}=\frac{J^{HS}}{J_{abs}}$), and the results are exhibited in Figure S2b. J_abs represents the photocurrent density converted from the absorbed photons, and J^HS is the photocurrent densities measured in 0.15 M Na₂S + 0.1 M Na₂SO₃ solution. PT(3)/IS presents a remarkably higher charge separation efficiency than PT and IS. This indicates that the heterojunction structure in the PT/IS system contributes to the effective separation of photogenerated charge.

Based on the above results, an adding amount of 0.03 g of TiO₂ was used in the PT/IS and AT/IS samples in this work.

2.3. The Photocathodic Protection (PCP) Performance of PT/IS

The photocathodic protection performance of the PT/IS samples was evaluated by measuring the photoinduced open-circuit potential (OCP) curves. For comparison, the pristine AT, PT, IS, and composite AT/IS were also measured under the same conditions. The time-dependent photoinduced OCP of the five photoelectrodes coupled with the 304SS is presented in Figure 6a. Under illumination, the potentials of IS, AT, PT, AT/IS, and PT/IS are −336 mV, –379 mV, −519 mV, and −664 mV, respectively. Meanwhile, the corresponding potential drops are 106 mV, 139 mV, 242 mV, and 372 mV, respectively. It can be observed that the composite photoelectrodes exhibit a more negative shift in potential compared with the pure photoelectrodes. This indicates that the formation of heterojunction structure of TiO₂ and In₂S₃ promotes the effective separation and transfer of photogenerated electrons and holes. Furthermore, P25 TiO₂-based photoelectrodes show higher PCP performance than those of anatase, as evidenced by the results of potential drops. This can be attributed to the mixed crystalline phase of TiO₂, which enhances light absorption efficiency and allows the photoelectrode to provide more electrons for transport to 304SS. Notably, the difference in potential drop between PT/IS and PT is more pronounced than that between AT/IS and AT. This is mainly due to the synergistic effect of the mixed crystalline phase of rutile–anatase and the heterojunction structure of TiO₂ and In₂S₃. In the TiO₂/In₂S₃ system, the impact of the mixed crystalline phase on the electrons transport to 304SS may be amplified by the heterojunction structure. It is likely that the mixed crystalline phase provides catalytic hotspots and promotes efficient transfer of photogenerated electrons at the interface of rutile and anatase [33,34]. Moreover, the above study demonstrates that the heterojunction structure of TiO₂/In₂S₃ can further accelerate electron transport to 304SS. Therefore, the PT/IS photoelectrode exhibits the best PCP performance.

Figure 6b displays the photocurrent densities of different photoelectrodes coupled with 304SS under intermittent solar light. The order of photocurrent densities for all the photoelectrodes is as follows: AT < PT < AT/IS < PT/IS. The composite photoelectrodes present higher photocurrent densities compared to pure TiO₂ photoelectrodes, with the PT/IS photoelectrode achieving the highest photocurrent density. It suggests that more photogenerated electrons for PT/IS photoelectrode are transferred to 304SS because of the synergistic effect of the mixed crystalline phase and heterojunction structure. This results in a more significant negative shift in potentials for 304SS, further confirming the results of OCP.

2.4. The Charge Transfer Mechanism of PT/IS Photoelectrode

To study the synergistic effect of the mixed crystalline phase and heterojunction structure, the transfer process of photogenerated charge carriers was investigated in PT/IS composites under light irradiation. Pt nanoparticles decorated the PT/IS samples using the photo-deposition method. According to the previous reports [35,36], Pt nanoparticles could be selectively deposited at sites where photogenerated electrons flow. Thus, the loading position of Pt nanoparticles can track the transfer direction of photoexcited electrons. As seen in Figure 7, in the TiO₂/In₂S₃ heterojunction structure, Pt nanoparticles were observed to grow on In₂S₃ and were deposited at electron-rich sites through a photoreduction reaction (Pt⁴⁺ + 4e →Pt⁰). The results of TEM and EDS element mapping images support the charge transfer tracking results, indicating that the photogenerated electrons tend to move from TiO₂ to In₂S₃. In other words, TiO₂ and In₂S₃ form a Z-scheme heterojunction in this case.

To further elucidate the charge transfer mechanism, UPS spectra of AT, PT, IS, AT/IS, and PT/IS are presented in Figure 8a–d in order to obtain the Fermi level ($E_F$), the conduction band position ($E_{CB}$), and valence band position ($E_{VB}$). $E_{VB}$, $E_{CB}$, and $E_F$ were calculated according to Equations (2)–(5).

$$E_{VB}=\left(E_{cutoff}-E_{onset}\right)-21.22$$

(2)

$$E_{CB}=E_{VB}+E_g$$

(3)

$$E_F=E_{VB}+E_{onset}$$

(4)

$$E\left(vs.NHE\right)=-4.5eV-E(vs.vacuumlevel)$$

(5)

in which $E_{cutoff}$ and $E_{onset}$ refer to the cutoff binding energy and the onset binding energy, respectively [37,38]. As shown in Figure 8a–d, the E_cutoff and E_onset values of PT, IS, PT/IS, and AT/IS were given. According to Equations (2)–(5), the E_VB, E_CB, and E_F of PT, IS, PT/IS, and AT/IS were calculated and the results are illustrated in Figure 8e. The Fermi levels of PT and IS are 0 V and −0.72 V, respectively. When PT and IS come into contact to form a heterojunction structure, the difference in Fermi levels between PT and IS leads to an interfacial electron transfer to maintain charge equilibrium. In this case, the E_F value of the PT/IS sample becomes −0.23 V. It is noteworthy that this E_F value is more negative than that of AT/IS (−0.19 eV) due to the mixed crystalline phase effect of PT, which facilitates the migration of photogenerated electrons from PT/IS to 304SS.

Based on the discussions above, Figure 9 illustrates a possible charge transfer process in the In₂S₃/TiO₂ composite and the corresponding energy band structure of semiconductors to clarify the mechanism behind the enhanced performance of PT/IS for photocathodic protection of 304SS. The valence band (VB) edge potentials of PT and IS were obtained from UPS spectra, while the conduction band (CB) edge potentials of these semiconductors were calculated using the band-gap data obtained from UV–vis DRS characterization. The difference in electronic structure results in band bending of PT and IS, creating a built-in electric field at the heterojunction interface. As shown in Figure 9, under light irradiation, PT and IS become excited, generating electron–hole pairs in the CB and VB of the semiconductors, respectively. Due to the Z-scheme heterojunction and the built-in electric field, photogenerated electrons from PT can directly and rapidly recombine with photogenerated holes from IS. The mixed crystalline phase effect of PT provides additional photogenerated electrons, consuming more photogenerated holes from IS. This not only reduces the recombination rate of photogenerated carriers from IS, but also facilitates the migration of more electrons from the higher conduction band position of IS (compared with PT) to 304SS. Consequently, the PT/IS photoelectrode shows the best PCP performance, owing to the synergistic effect between the mixed crystalline phase of rutile–anatase and the Z-scheme heterojunction structure of TiO₂ and In₂S₃.

3. Experimental

3.1. Synthesis of In₂S₃@TiO₂/In₂S₃ Heterojunction

All reagents were of analytical grade and used without additional purification. In₂S₃@TiO₂/In₂S₃ films were synthesized on FTO glass (1.5 × 3 cm², 14 Ω/cm², Wuhan Lattice Solar Technology Co., Ltd., Wuhan, China) through a hydrothermal method. Specifically, the precursor solution was prepared by dissolving 6 mM indium nitrate hydrate (>99.9%, In(NO₃)₃∙xH₂O, Aladdin, Shanghai, China) and 12 mM thiourea (>99%, CH₄N₂S, Aladdin, Shanghai, China). Next, 0.03 g of titanium dioxide nanoparticles (>99.5%, P25, Degussa, Frank, Germany) was added to the precursor solution. After 30 min of ultrasound treatment in the dark, the mixture solution and the cleaned FTO substrates were placed in a Teflon-lined autoclave, with the conductive face-down of FTO against the wall of the autoclave and immersed in the solution. The autoclave was then transferred to an oven, heated to 140 °C, and maintained for 24 h. The resulting film materials that adhered to the surfaces of FTO were washed with deionized water, and subsequently dried under vacuum condition at 50 °C for 3 h. The as-prepared In₂S₃@TiO₂ (P25)/In₂S₃ film was denoted as PT/IS.

For comparison, a pure indium sulfide film (denoted as IS) was prepared through a similar process to that of the PT/IS samples using the same precursor solution, with the exception that no titanium dioxide was added to the precursor solution. The In₂S₃@TiO₂ (anatase)/In₂S₃ film (denoted as AT/IS) was also synthesized using a process similar to that of the PT/IS samples, but with the replacement of titanium dioxide (>99.5%, Degussa, Frank, Germany) with anatase titanium dioxide (>99.8%, Macklin, Shanghai, China) in the precursor solution. Additionally, P25 titanium dioxide film (denoted as PT) and anatase titanium dioxide film (denoted as AT) were prepared through a simple spin-coating method. In this method, 0.03 g of titanium dioxide (P25 or anatase) was fully dispersed in 25 mL of deionized water and ultrasonically dispersed. Subsequently, 20 μL of the suspension was spin-coated onto the conductive face of the FTO substrate, and dried under vacuum condition at 50 °C for 3 h.

3.2. Characterization

The X-ray diffraction (XRD) patterns were obtained using a Rigaku SmartLab X-ray diffractometer (Cu Kα radiation) with a scan rate of 6°/min and the 2θ range was from 10 to 80°. The field-emission scanning electron microscope (FE-SEM, GeminiSEM 300, ZEISS, Oberkochen, Germany) and transmission electron microscope (TEM, Tecnai G2 F20 FEI, Oberkochen, Germany) were used to observe the morphology of as-obtained samples. Atomic force microscopy (AFM, Dimension Icon Bruker, Germany) was applied to test the surface roughness of samples. The optical absorption properties within the wavelength range from 300 to 800 nm were determined by using a Shimadzu spectrophotometer equipped with an integral sphere to record diffuse UV–vis absorption spectra of the samples. The X-ray photoelectron spectroscopy (XPS, PHI Quantera II, ULVAC–PHI, Kanagawa, Japan) with an Al Kα X-ray excitation source was carried out to obtain information on the elemental compositions and surface valence states of the samples, in which all the XPS spectra were calibrated according to the C1s binging energy at 284.8 eV. The photoluminescence (PL) spectra were performed on an Edinburgh Instrument FLS 920P fluorescence spectrophotometer (Edinburgh Instruments, Livingston, UK).

3.3. Photoelectrochemical and Photocathodic Protection Measurements

The photoelectrochemical (PEC) and photocathodic protection (PCP) experiments were performed on a CHI660E electrochemical workstation in a typical three-electrode configuration(CHI, Shanghai, China). The prepared semiconductor photoelectrode on FTO substrate served as the working electrode. The available photoelectrode area immersed in the aqueous electrolyte was controlled at 1 cm². A platinum plate (1 × 1 cm²) and an Ag/AgCl electrode were employed as the counter electrode and reference electrode, respectively. The light source used a 300W Xe lamp with an AM 1.5 filter in order to simulate the solar light irradiation, the light intensity was 100 mW/cm². The linear sweep voltammetry (LSV) measurements were performed using a voltage range of −0.6 V to 0.6 V vs. Ag/AgCl with a scanning rate of 10 mV/s under light irradiation. The electrochemical impedance spectroscopy (EIS) measurements were recorded with an amplitude of 10 mV over a frequency range of 10⁻² to 10⁵ Hz under light irradiation, where the photoelectrodes were prepared on the surface of FTO substrate, and the electrolyte was 0.15 M Na₂S + 0.1 M Na₂SO₃ solution. In addition, the PCP performance of the prepared films on FTO substrates was evaluated by measuring the OCP curves and I-t curves of the protected 304SS coupled with the photoelectrode. The corresponding experimental test devices are shown in Figure S3, in which the electrolyte of corrosion cell is a 3.5 wt% NaCl solution while the electrolyte in the photoelectrochemical cell is a 0.15 M Na₂S + 0.1 M Na₂SO₃ solution.

4. Conclusions

In summary, we have successfully synthesized an In₂S₃@TiO₂/In₂S₃ direct Z-scheme heterojunction photoelectrode through a simple one-step hydrothermal method. The obtained photoelectrodes were characterized by XRD, SEM, TEM, XPS, and UV DRS. The In₂S₃@TiO₂/In₂S₃ photoelectrode with a mixed crystalline phase of rutile–anatase exhibited good photoelectrochemical cathodic performance, including a photocurrent response of 68 μA/cm² and a potential drop of 372 mV when coupled with 304SS. The significantly enhanced PCP performance can be attributed to the synergistic effects between the mixed crystalline phase of rutile–anatase and the Z-scheme heterojunction structure of TiO₂ and In₂S₃. The mixed crystalline phase provides a greater number of photogenerated electrons, while the Z-scheme heterojunction and the built-in electric field facilitate more effective transfer and separation of photoelectron–hole pairs. This allows the photogenerated electrons from IS to be transported directly and rapidly to 304SS. This work provides valuable insights into the positive impact of the mixed crystalline phase of TiO₂ in Z-scheme photoelectrode materials systems and offers a reference for the potential application of photocathodic protection for steel.