Background

304 stainless steel (304SS) is widely used in various industries for its good corrosion resistance and fabricability. However, this material easily deteriorates from pitting corrosion in seawater and chloride-containing solutions [1, 2]. Recently, photocathodic protection for metals has received growing attention from scientists worldwide as a promising and green technology [3–7]. TiO2 has been extensively investigated as a photoanode for the cathodic protection because of its high chemical stability, low cost, and nontoxicity [8–11]. However, its wide bandgap (~3.2 eV for anatase) restricts its application because of its exclusive activity only under UV irradiation (3–5% of the solar spectrum) [12, 13]. The recombination of photogenerated electrons and holes in the dark results in a low photo-quantum efficiency of TiO2. To overcome these defects, TiO2 nanotube arrays with large specific surface areas were synthesized [14–16] and various strategies were developed to expand its absorption to the visible light range. These strategies include coupling with narrow-bandgap semiconductors (ZnSe, WO3, SnO2, CdS, and Ag2S) [17–21], metals (Ag, Au, Cu, and Bi) [22–24], and nonmetals (N, F, and graphene) [25–27]. Bi2S3 is an attractive material because of its narrow bandgap (E g = 1.3 eV) and high photo-to-electron conversion efficiency [28]. The Bi2S3/TiO2 heterostructure can reduce the recombination of photogenerated electrons and holes, and this effect would benefit the photoelectric performance of materials [29–32]. However, no research has been reported on the photogenerated cathodic protection property of Bi2S3/TiO2 nanotubes. Successive ionic layer adsorption and reaction (SILAR) is a promising technique with low cost and simple equipment, which can synthesize continuous and compact film at room temperature [33]. In this study, Bi2S3/TiO2 nanotube films served as photoanode for preventing 304SS corrosion. In the fabrication of the films, Bi2S3 nanoparticles were prepared by the SILAR method. The morphology, structure, and optical absorption property were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV–visible (UV–vis) diffuse reflectance spectra, respectively. The influence of Bi2S3 deposition cycles on the photoelectrochemical and photocathodic protection properties of the composite films was also investigated in our work.

Methods

TiO2 nanotubes were first fabricated by anodizing Ti foil in ethylene glycol electrolyte comprised of 0.5 wt% NH4F and 6 vol% H2O for 1.5 h and annealing at 450 °C for 1.5 h in air. Then, Bi2S3/TiO2 nanocomposites were prepared through the alternate immersion of TiO2/Ti substrate in the anionic and cationic precursor solutions at room temperature. The cationic precursor solution was composed of 0.01 M Bi(NO3)3 dissolved in 50 ml of acetone. Meanwhile, the anionic precursor solution was composed of 0.01 M Na2S dissolved in 50 ml of methanol. The TiO2/Ti substrate was first dipped into the cationic precursor solution for 20 s, and then dipped into the anionic precursor solution for 20 s, rinsed, and dried in air. The Bi2S3 synthesized in 10, 20, and 30 deposition cycles were assigned as BST-10, BST-20, and BST-30, respectively.

The morphology of the samples was investigated by SEM (Hitachi S-4800, Japan). The structure of the samples was examined by XRD (Bruker AXSD8 Advance, Germany). The UV–vis diffuse reflectance spectra were obtained on an UV–vis diffuse reflectance spectrophotometer (Hitachi UH4150, Japan). Photoelectrochemical experiments were conducted using a potentiostat/galvanostat (PARSTAT 2273, USA) at room temperature with a Xe lamp (PLS-SXE300C, China) as the visible light source. The open-circuit potential (OCP) of different coupled photoelectrodes were investigated in a double-cell system (Fig. 1a). A TiO2 or Bi2S3/TiO2 nanotube photoelectrode was placed in a photoanode cell containing a mixed 0.1 M Na2S and 0.2 M NaOH solution, whereas 304SS was placed in a corrosion cell containing 3.5 wt% NaCl solution. The Pt foil, saturated calomel electrode (SCE), and coupled electrode of TiO2 and 304SS electrode served as the counter electrode (CE), reference electrode (RE), and working electrode (WE), respectively. Photocurrent curves were measured in 0.2 M Na2SO4 solution using an electrochemical workstation (CHI 1010C, China). The TiO2 or Bi2S3/TiO2 composite photoelectrode, SCE, and Pt foil served as the WE, RE, and CE, respectively (Fig. 1b).

Fig. 1
figure 1

Schematic sketches of experimental devices for photoelectrochemical characterization of OCPs (a) and transient photocurrent curves (b)

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Results and Discussion

The morphologies of Bi2S3/TiO2 heterostructure were observed by SEM (Fig. 2). TiO2 nanotube arrays exhibited a well-ordered, high-density, and uniform tubular structure with an average diameter of 60 nm (Fig. 2a). The Bi2S3 nanoparticles were successfully deposited on TiO2 nanotube surfaces through the SILAR method (Figs. 2b–d). For BST-10, the particles distributed irregularly on the mouth of TiO2 nanotubes (Fig. 2b). When the number of Bi2S3 deposition cycle increased to 20, the Bi2S3 nanoparticles were deposited regularly on the mouth or wall of TiO2 nanotubes with about 15 nm in diameter. After undergoing 30 cycles, the amount of nanoparticles significantly increased, and the formation of agglomeration caused the particles to block the nanotubes.

Fig. 2
figure 2

SEM images of a pure TiO2, b BST-10, c BST-20, and d BST-30

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Figure 3a depicts the XRD patterns of TiO2 and Bi2S3/TiO2. Aside from the diffraction peaks of titanium substrate, the peaks at 25.38°, 38.03°, 48.01°, 54.05°, 55.17°, 62.71°, and 70.44° can be indexed to lattice planes (101), (004), (200), (105), (211), (204), and (220) of anatase TiO2, respectively (JCPDS 21-1272). Besides the TiO2 peaks, the peaks at 27.74° and 32.54° were attributed to lattice planes (211) and (221) of the orthorhombic Bi2S3 (JCPDS 17-0320). For Bi2S3/TiO2 nanocomposites, the increase in diffraction peak intensity of Bi2S3 with the deposition cycles revealed an increased amount of Bi2S3 nanoparticles on the TiO2 nanotubes. This finding is consistent with the SEM results.

Fig. 3
figure 3

XRD patterns (a) and UV–vis diffuse reflectance spectra (b) of pure TiO2 and Bi2S3/TiO2

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The light absorption abilities of the synthesized Bi2S3/TiO2 nanotube films were assessed by UV–vis spectroscopy (Fig. 3b). Figure 3b shows that TiO2 nanotubes absorbed mainly in the UV range with a wavelength of about 380 nm because of the bandgap energy of anatase (3.2 eV). The spectra of Bi2S3/TiO2 exhibit a relatively broad and strong absorption in the visible region, indicating that the Bi2S3/TiO2 nanocomposite is capable of harvesting visible light and acts as a photoanode under visible light [34].

Figure 4a displays the transient photocurrent curves for TiO2 and Bi2S3/TiO2 photoelectrodes under visible light irradiation. The pure TiO2 nanotube photoelectrode shows nearly 0 μA/cm2 because of weak visible light absorption. However, after Bi2S3 nanoparticle sensitization, the transient photocurrent densities of Bi2S3/TiO2 exhibited an obvious increase, implying that the Bi2S3/TiO2 nanocomposite is capable of utilizing visible light and the heterostructure promotes the separation of photogenerated electrons and holes [35]. The transient photocurrent density of BST-20 (249 μA/cm2) was higher than that of BST-10 (134 μA/cm2) and BST-30 (92 μA/cm2), indicating that BST-20 possesses an optimal separation efficiency of the photogenerated electrons and holes.

Fig. 4
figure 4

Photoresponse spectra (a) and OCP variations (b) of pure TiO2 and Bi2S3/TiO2 under intermittent irradiation

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Figure 4b compares the photogenerated OCPs of 304SS coupled with different TiO2 nanotubes. When the light was on, the potentials of coupled electrodes all shifted negatively within a few seconds. This effect may be attributed to the cathodic polarization of 304SS which results from the excited photoelectrons transfer from TiO2 nanotubes to 304SS [36, 37]. After the light was off, the OCP of 304SS coupled to pure TiO2 returned to a value near the free corrosion potential of bare 304SS, indicating the invalid recombination of the photogenerated electrons and holes in the TiO2 [38]. By contrast, the OCPs of 304SS coupled with Bi2S3/TiO2 exhibited a slightly positive shift and stayed far below than the free corrosion potential of bare 304SS. The charges stored in the Bi2S3/TiO2 composite were released and again transferred to 304SS in the dark. The negative shift of potentials is reportedly an important parameter for evaluating the separation efficiency of photogenerated charges [39, 40]. The increased negative shift of the potentials indicates the increased effectiveness of the cathodic protection of photoanodes. Under visible light, the shift of potentials can be ranked in the following order: TiO2 (150 mV vs. SCE) < BST-30 (534 mV vs. SCE) < BST-10 (572 mV vs. SCE) < BST-20 (662 mV vs. SCE). Hence, BST-20 possesses the optimal photocathodic protection property for 304SS. This result may be due to the fact that the active sites and light harvesting increased with the rising Bi2S3 amount. However, the excessive Bi2S3 particles served as the recombination sites of the electrons and holes, which hindered the charge transfer from the Bi2S3/TiO2 composite to 304SS.

The X-ray photoelectron spectroscopy (XPS) was measured to investigate the chemical compositions and states of Bi2S3/TiO2 (BST-20). The XPS survey spectra revealed the existence of Bi, S, Ti, and O components, in addition to C contaminants (Fig. 5a). As shown in Fig. 5b, the XPS peaks of O 1s at 529.7 eV were analyzed from the lattice oxygen (OL) in TiO2. The peak at 531.6 eV was derived from the adsorbed oxygen (OA). The OA was composed of OH species or weak bonding oxygen on the composite surface. The presence of OA was ascribed to the generation of oxygen vacancy on the sample surface. This suggests that the Bi2S3/TiO2 composite exhibits higher photocathodic protection properties than TiO2.

Fig. 5
figure 5

XPS survey spectra of the synthesized Bi2S3/TiO2 (a) and high-resolution XPS spectra of O 1s of TiO2 and Bi2S3/TiO2 (b)

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Figure 6 shows the schematic diagram of the photoelectric conversion and transportation processes in the Bi2S3/TiO2 composite. The Bi2S3 nanoparticles can easily absorb the photons in the visible light due to the presence of OA and the suitable bandgap width of Bi2S3. When the photons were absorbed by the Bi2S3 nanoparticles, the photoexcited electrons were generated and transferred from the conduction band (CB) of Bi2S3 to the CB of TiO2. The photogenerated holes were then shifted from the valence band (VB) of TiO2 to the VB of Bi2S3. When Na2S served as a hole-trapping agent, the photogenerated charges were effectively separated. The electrons were finally transferred to the 304SS electrode, and the potential of the 304SS electrode negatively shifted. The 304SS was prevented from corrosion by Bi2S3/TiO2 under visible light. Therefore, the more efficient separation of the photogenerated charges in the composite would accelerate the oxidation and reduction reactions and, hence, generate a higher photocathodic protection activity than TiO2.

Fig. 6
figure 6

Schematic representation of the electron transfer processes in Bi2S3/TiO2

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Conclusions

In summary, Bi2S3-nanoparticle-decorated TiO2 nanotubes were successfully synthesized through the electrochemical anodization and SILAR method. The sensitization of Bi2S3 significantly extended the spectral response from UV to the visible region. Consequently, the composite showed higher photocurrents and cathodic protection performance than pure TiO2. With increased number of Bi2S3 deposition cycles, the increasing grain size and loading of the Bi2S3 nanoparticles significantly affected the photocathodic protection activity of the Bi2S3/TiO2 nanocomposite. The Bi2S3/TiO2 nanotubes prepared by SILAR deposition with 20 cycles exhibited the optimal photocathodic protection property.