Photocathodic Protection of 304 Stainless Steel by Bi2S3/TiO2 Nanotube Films Under Visible Light

We report the preparation of TiO2 nanotubes coupled with a narrow bandgap semiconductor, i.e., Bi2S3, to improve the photocathodic protection property of TiO2 for metals under visible light. Bi2S3/TiO2 nanotube films were successfully synthesized using the successive ionic layer adsorption and reaction (SILAR) method. The morphology and structure of the composite films were studied by scanning electron microscopy and X-ray diffraction, respectively. UV–visible diffuse reflectance spectra were recorded to analyze the optical absorption property of the composite films. In addition, the influence of Bi2S3 deposition cycles on the photoelectrochemical and photocathodic protection properties of the composite films was also studied. Results revealed that the heterostructure comprised crystalline anatase TiO2 and orthorhombic Bi2S3 and exhibited a high visible light response. The photocurrent density of Bi2S3/TiO2 was significantly higher than that of pure TiO2 under visible light. The sensitization of Bi2S3 enhanced the separation efficiency of the photogenerated charges and photocathodic protection properties of TiO2. The Bi2S3/TiO2 nanotubes prepared by SILAR deposition with 20 cycles exhibited the optimal photogenerated cathodic protection performance on the 304 stainless steel under visible light.

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][4][5][6][7]. TiO 2 has been extensively investigated as a photoanode for the cathodic protection because of its high chemical stability, low cost, and nontoxicity [8][9][10][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 TiO 2 . To overcome these defects, TiO 2 nanotube arrays with large specific surface areas were synthesized [14][15][16] and various strategies were developed to expand its absorption to the visible light range. These strategies include coupling with narrow-bandgap semiconductors (ZnSe, WO 3 , SnO 2 , CdS, and Ag 2 S) [17][18][19][20][21], metals (Ag, Au, Cu, and Bi) [22][23][24], and nonmetals (N, F, and graphene) [25][26][27]. Bi 2 S 3 is an attractive material because of its narrow bandgap (E g = 1.3 eV) and high photo-toelectron conversion efficiency [28]. The Bi 2 S 3 /TiO 2 heterostructure can reduce the recombination of photogenerated electrons and holes, and this effect would benefit the photoelectric performance of materials [29][30][31][32]. However, no research has been reported on the photogenerated cathodic protection property of Bi 2 S 3 /TiO 2 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, Bi 2 S 3 /TiO 2 nanotube films served as photoanode for preventing 304SS corrosion. In the fabrication of the films, Bi 2 S 3 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 Bi 2 S 3 deposition cycles on the photoelectrochemical and photocathodic protection properties of the composite films was also investigated in our work.

Methods
TiO 2 nanotubes were first fabricated by anodizing Ti foil in ethylene glycol electrolyte comprised of 0.5 wt% NH 4 F and 6 vol% H 2 O for 1.5 h and annealing at 450°C for 1.5 h in air. Then, Bi 2 S 3 /TiO 2 nanocomposites were prepared through the alternate immersion of TiO 2 /Ti substrate in the anionic and cationic precursor solutions at room temperature. The cationic precursor solution was composed of 0.01 M Bi(NO 3 ) 3 dissolved in 50 ml of acetone. Meanwhile, the anionic precursor solution was composed of 0.01 M Na 2 S dissolved in 50 ml of methanol. The TiO 2 /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 Bi 2 S 3 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 TiO 2 or Bi 2 S 3 /TiO 2 nanotube photoelectrode was placed in a photoanode cell containing a mixed 0.1 M Na 2 S 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 TiO 2 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 Na 2 SO 4 solution using an electrochemical workstation (CHI 1010C, China). The TiO 2 or Bi 2 S 3 /TiO 2 composite photoelectrode, SCE, and Pt foil served as the WE, RE, and CE, respectively (Fig. 1b).

Results and Discussion
The morphologies of Bi 2 S 3 /TiO 2 heterostructure were observed by SEM (Fig. 2). TiO 2 nanotube arrays exhibited a well-ordered, high-density, and uniform tubular structure with an average diameter of 60 nm (Fig. 2a). The Bi 2 S 3 nanoparticles were successfully deposited on TiO 2 nanotube surfaces through the SILAR method (Figs. 2b-d). For BST-10, the particles distributed irregularly on the mouth of TiO 2 nanotubes (Fig. 2b). When the number of Bi 2 S 3 deposition cycle increased to 20, the Bi 2 S 3 nanoparticles were deposited regularly on the mouth or wall of TiO 2 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.  The light absorption abilities of the synthesized Bi 2 S 3 / TiO 2 nanotube films were assessed by UV-vis spectroscopy (Fig. 3b). Figure 3b shows that TiO 2 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 Bi 2 S 3 /TiO 2 exhibit a relatively broad and strong absorption in the visible region, indicating that the Bi 2 S 3 /TiO 2 nanocomposite is capable of harvesting visible light and acts as a photoanode under visible light [34].  Bi 2 S 3 /TiO 2 exhibited an obvious increase, implying that the Bi 2 S 3 /TiO 2 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/cm 2 ) was higher than that of BST-10 (134 μA/cm 2 ) and BST-30 (92 μA/cm 2 ), indicating that BST-20 possesses an optimal separation efficiency of the photogenerated electrons and holes. Figure 4b compares the photogenerated OCPs of 304SS coupled with different TiO 2 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 TiO 2 nanotubes to 304SS [36,37]. After the light was off, the OCP of 304SS coupled to pure TiO 2 returned to a value near the free corrosion potential of bare 304SS, indicating the invalid recombination of the photogenerated electrons and holes in the TiO 2 [38]. By contrast, the OCPs of 304SS coupled with Bi 2 S 3 /TiO 2 exhibited a slightly positive shift and stayed far below than the free corrosion potential of bare 304SS. The charges stored in the Bi 2 S 3 /TiO 2 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: TiO 2 (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 Bi 2 S 3 amount. However, the excessive Bi 2 S 3 particles served as the recombination sites of the electrons and holes, which hindered the charge transfer from the Bi 2 S 3 /TiO 2 composite to 304SS.  The X-ray photoelectron spectroscopy (XPS) was measured to investigate the chemical compositions and states of Bi 2 S 3 /TiO 2 (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 (O L ) in TiO 2 . The peak at 531.6 eV was derived from the adsorbed oxygen (O A ). The O A was composed of OH species or weak bonding oxygen on the composite surface. The presence of O A was ascribed to the generation of oxygen vacancy on the sample surface. This suggests that the Bi 2 S 3 /TiO 2 composite exhibits higher photocathodic protection properties than TiO 2 . Figure 6 shows the schematic diagram of the photoelectric conversion and transportation processes in the Bi 2 S 3 /TiO 2 composite. The Bi 2 S 3 nanoparticles can easily absorb the photons in the visible light due to the presence of O A and the suitable bandgap width of Bi 2 S 3 . When the photons were absorbed by the Bi 2 S 3 nanoparticles, the photoexcited electrons were generated and transferred from the conduction band (CB) of Bi 2 S 3 to the CB of TiO 2 . The photogenerated holes were then shifted from the valence band (VB) of TiO 2 to the VB of Bi 2 S 3 . When Na 2 S 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 Bi 2 S 3 /TiO 2 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 TiO 2 .

Conclusions
In summary, Bi 2 S 3 -nanoparticle-decorated TiO 2 nanotubes were successfully synthesized through the electrochemical anodization and SILAR method. The sensitization of Bi 2 S 3 significantly extended the spectral response from UV to the visible region. Consequently, the composite showed higher photocurrents and cathodic protection performance than pure TiO 2 . With increased number of Bi 2 S 3 deposition cycles, the increasing grain size and loading of the Bi 2 S 3 nanoparticles significantly affected the photocathodic protection activity of the Bi 2 S 3 / TiO 2 nanocomposite. The Bi 2 S 3 /TiO 2 nanotubes prepared by SILAR deposition with 20 cycles exhibited the optimal photocathodic protection property.