Synthesis of ZnO/Bi 2 S 3 Core/Shell Nanowire Array Photoanodes for Photocathodic Protection of Stainless Steel

: Nanocrystalline Bi 2 S 3 shells were conformally deposited on ZnO nanowire arrays via a successive ionic layer adsorption and reaction approach. Microstructure, optical, and electric prop ‐ erties of the as ‐ prepared ZnO/Bi 2 S 3 core/shell nanowire heterostructures were thoroughly investi ‐ gated using various characterization and electrochemical methods. Compared with the pristine ZnO photoanode ( − 734 mV and 0.57 mA ∙ cm − 2 ), the ZnO/Bi 2 S 3 photoanode with a type ‐ II heterojunction exhibited a more negative shift in the coupled open circuit potential ( − 862 mV) and a higher photo ‐ current density (2.92 mA ∙ cm − 2 ), achieving more effective photocathodic protections for the coupled 304 stainless steel under solar illumination.

Bi2S3 is a n-type semiconductor with a relatively narrow band gap (~1.3 eV) and is able to broaden the optical absorption range of ZnO to the visible part of solar light by hybridization [28,29,37]. More importantly, Bi2S3 possesses appropriate energy band alignments to form a type-II heterojunction with ZnO, which facilitates spatial charge separation via built-in electric field across the interface. Recently, ZnO/Bi2S3 heterostructures have been reported to reduce the recombination of charge carriers and enhance PEC properties [28,29,[37][38][39][40][41][42][43], and moreover Bi2S3 has been also utilized to decorate TiO2 for improving PCP performances [11,18,44]. Therefore, it would be desirable to promote PCP performance of ZnO photoanodes for steels by rational introduction of Bi2S3. Just recently, Zhang et al. reported a Bi2S3/ZnO nanocomposite with a good PCP effect on 304 stainless steel (304 SS) [45]. However, the electrodeposited ZnO nanowires were poorly aligned, and their lengths were only ~300 nm, which dramatically restricted the photoinduced current density for PCP (0.094 mA•cm −2 ). To enhance the light absorption and increase the photoinduced current density, rational construction of Bi2S3/ZnO nanocomposites with the well-aligned ZnO nanowires of sufficient lengths will be promising for PCP.

Photoanode Synthesis
All chemicals were of analytical grade. ZnO nanowire arrays were synthesized by using a seed-assistant hydrothermal method [24]. Firstly, fluorine-doped tin oxide (FTO) conductive glasses were thoroughly cleaned by ultrasonic treatment in acetone, ethanol, and deionized water for 15 min, dried in an electrically heated convection oven at 90 °C, and treated before use with oxygen plasma for 3 min. Then, 100 μL of 25 mM zinc acetate aqueous solution was dropped on the cleaned FTO glasses and dried at 60 °C for 3 min. This drop-dry process was repeated 4 times. To form a thick ZnO seed layer, the precoated FTO glasses were calcined in a muffle furnace at 350 °C for 1 h with a heating rate of 5 °C•min −1 . Lastly, ZnO-seeded FTO glasses were hydrothermally treated in a Teflonlined stainless-steel autoclave filled with 50 mL of aqueous solution containing 2.5 mmol of ZnCl2 and 2.5 mmol of hexamethylenetetramine at 125 °C for 4 h. Deposition of particulate Bi2S3 on ZnO nanowire arrays was realized by a successive ionic layer adsorption and reaction (SILAR) method [11,18,44,46,47]. During each cycle, the substrates were immersed into separate Bi 3+ and S 2-precursor solutions for adsorption and reaction and subsequently rinsed with deionized water after each immersion to remove excess ions and hinder homogeneous precipitation. The absorption, reaction, and rinse times were 50, 60, and 30 s, respectively. The Bi 3+ precursor solution contained 0.125 mmol of Bi(NO3)3 dissolved in 25 mL of ethylene glycol, and the S 2− precursor solution contained 0.625 mmol of Na2S dissolved in 25 mL of deionized water. The substrates were dried at 85 °C for 5 min after each cycle, and optimal ten cycles of SILAR were implemented in total.

Electrochemical Measurements
For photoelectrochemical measurements, a system comprised of an electrochemical workstation (CS350H, CorrTest, Wuhan, China) and a three-electrode cell was adopted (see the schematic diagram in our previous work [24]). The as-prepared photoanodes (1 cm × 1 cm) served as the work electrode, Pt foil as the counter electrode, and Ag/AgCl as the reference electrode. An aqueous solution of 0.2 M NaOH was used as the electrolyte, and 0.1 M Na2S was used as the hole scavenger. The simulated irradiation source was a 500 W xenon lamp (CHF-XM500, Perfectlight, Beijing, China) equipped with an AM 1.5 G filter, and the light intensity was fixed at 100 mW•cm −2 using an optical power meter (PL-MW2000, Perfectlight, Beijing, China). The open circuit potential (OCP) measurements were conducted in a H-type electrochemical cell separated with a Nafion membrane between two chambers [24]. The as-prepared photoanodes were electrically connected with 304 SS placed in another chamber containing 3.5 wt. % NaCl aqueous solution. Electrochemical impedance spectroscopy (EIS, 10 −2~1 0 5 Hz) and Mott-Schottky measurements (100 Hz) were performed on a Gamry Interface 1010E electrochemical workstation with 10 mV alternating current amplitude under dark.

Results and Discussion
Crystal phases of the as-prepared ZnO/Bi2S3 core/shell nanowire arrays were analyzed by powder XRD patterns in Figure 1. For the ZnO photoanode, the collected diffraction peaks can be divided into two groups. One group of diffraction peak located at 31.   Figure 2 shows the representative SEM images of ZnO and ZnO/Bi2S3 photoanodes. The ZnO/Bi2S3 photoanode exhibited similar surface morphologies with the ZnO photoanode. Dense nanowires with the lengths between 1.5 and 2.1 μm and the diameters between 35 and 150 nm were vertically aligned and uniformly distributed on the entire FTO substrate. It was noted that the surface of nanowires changes from smooth for the pristine ZnO (Figure 2b) to rough for the Bi2S3 overcoated ZnO (Figure 2e). Obviously, a great number of Bi2S3 nanoparticles [48] are tightly deposited on the outer wall of ZnO nanowires so that ZnO nanowire cores are successfully covered by conformal Bi2S3 shells. The information of microstructures and composition distributions was obtained by HRTEM and EDAX elemental mappings. Figure 3a shows a typical core/shell structured nanowire with a diameter of ~100 nm and a highly porous shell composed of nanoparticles. In the lattice-resolved TEM image (Figure 3b), the lattice fringe with an interplane spacing of 0.261 nm is ascribed to the (002) plane of ZnO from the nanowire core. Importantly, a distinct interplane spacing of 0.33 nm was clearly observed, which can be attributed to the (021) crystal plane of orthorhombic Bi2S3 (JCPDS# 17-0320) [11,18,44,47]. Near the outermost wall of the nanowire, lattice fringes show short-range order and longrange disorder, which is the characteristic of nanocrystalline or amorphous Bi2S3. These analyses support the absence of Bi2S3 diffraction peaks in the XRD pattern. The core/shell nanostructure was further unveiled by STEM and the corresponding elemental mapping images in Figure 3c-g. The Zn and O elements were mostly constrained in the nanowire core, while the Bi and S elements were mostly distributed in the shell region. The merged image of Zn, O, Bi, and S elements in Figure 3h displays that the core/shell nanowire has a shell thickness of ~15 nm. In brief, both HRTEM and EDAX analyses demonstrate the successful coating of the nanocrystalline Bi2S3 shell on the ZnO nanowire core. Chemical compositions and states of the ZnO/Bi2S3 photoanode were studied by Xray photoelectron spectroscopy (XPS). The survey spectrum in Figure 4a reveals the existence of various bind energies from Zn, O, Bi, and S elements. The high-resolution Zn 2p spectrum in Figure 4b displays two peaks centered at 1021.7 and 1044.8 eV, ascribing to Zn 2p3/2 and Zn 2p1/2 of Zn 2+ , respectively. In Figure 4c, the predominant O 1s spectrum can be deconvoluted into two peaks located at 531.1 and 532.4 eV, corresponding to oxygen vacancies and chemisorbed oxygen, respectively [24,49]. In the Bi 4f spectrum (Figure  4d), two main peaks at 158.3 and 163.6 eV are attributed to Bi 4f5/2 and Bi 4f7/2 of Bi 3+ , respectively. The remained peak at 160.9 eV in Figure 4d is related to S 2p3/2. The S 2s peak at 225.3 eV (Figure 4e) further proves the presence of S 2− [50,51]. The appearance of S 2s peak at 232.2 eV is indicative of the formation of sulfates (SO4 2− ), arising from the hydrolysis of sulfides [50]. Therefore, XPS analyses prove the presence of Bi 3+ and S 2− , suggesting the formation of Bi2S3 on the surface of nanowires.  Figure 5a shows the optical absorption properties of ZnO and ZnO/Bi2S3 photoanodes measured via UV-Vis spectroscopy. It was seen that the ZnO photoanode can only absorb UV light with an absorption edge at 385 nm. Obtained from the slope in Tauc plots of (αhv) 2 to photo energy (Figure 5b), the band gap value of the ZnO photoanode was 3.22 eV. By contrast, the absorption edge of the ZnO/Bi2S3 photoanode was extended to 614 nm, indicating an optical band gap of 2.02 eV. Obviously, the ZnO/Bi2S3 photoanode exhibits a significantly enhanced absorption capability in the visible part of solar spectrum when Bi2S3 is composited with ZnO. To understand the PCP performance of the ZnO/Bi2S3 photoanode, the photoinduced open circuit potential (OCP) variation of coupled 304 SS and transient photocurrent responses were measured under intermittent illumination of simulated solar light (AM 1.5 G). As shown in Figure 6a, OCP of 304 SS moderately shifts to the negative direction in the dark while coupled with the ZnO and ZnO/Bi2S3 photoanodes. Once exposed to white light, the ZnO/Bi2S3 photoanode shows a dramatical shift to −862 mV in the couped OCP, suggesting a better PCP performance than the ZnO photoanode (−734 mV). According to the large photoinduced potential drop, it is speculated that the photoelectrons of ZnO/Bi2S3 photoanode excited from its valence band maximum (VBM) to conduction band minimum (CBM) by white light can be transferred to the connected 304 SS for efficient cathodic polarization and protection. Figure 6b exhibits positive photocurrent responses for the ZnO and ZnO/Bi2S3 photoanodes, characteristic of PCP for 304 SS. Importantly, the ZnO/Bi2S3 photoanode (2.92 mA•cm −2 ) presents five times higher photocurrent than the ZnO photoanode (0.57 mA•cm −2 ) in the final of first cycle, demonstrating a better PCP performance than the just-reported ZnO/Bi2S3 nanocomposites (0.094 mA•cm −2 ) [45].   To obtain the flat-band potentials of ZnO and ZnO/Bi2S3 photoanodes, Mott-Schottky plots were recorded at an applied frequency of 100 Hz in the dark. The linear fitting of Mott-Schottky plots in Figure 8 shows positive slopes, typical of n-type semiconductors [52,53]. Based on the x-axis intercepts of the fitted tangent lines, the flat-band potentials of ZnO and ZnO/Bi2S3 photoanodes were estimated to be approximately −0.01 and −0.20 V vs. Ag/AgCl, respectively. Thus, the CBM values of n-type ZnO and Bi2S3 can be considered as −0.21 and −0.40 V, respectively [52,53]. In terms of the optical band gaps, VBM of n-type ZnO and Bi2S3 can be speculated at 3.01 and 1.62 V, respectively. As shown in the inset, a type-II semiconductor heterojunction was constructed between ZnO and Bi2S3, which is beneficial for the spatial charge separation in the ZnO/Bi2S3 photoanode. Clearly, hybridization of Bi2S3 with ZnO not only broadens the absorption spectrum but also increases the charge separation efficiency, which is of great importance to boost PCP performances of the ZnO/Bi2S3 photoanode.
To investigate the charge transfer properties, EIS was used to record Nyquist plots of the ZnO and ZnO/Bi2S3 photoanodes under the dark. Generally, the charge transfer resistance (Rct) can be estimated from the diameter of the semicircle in the high-frequency region, and a smaller radius of impedance arc indicates a more decreased Rct [22,23]. As fitted by EClab software using the proposed equivalent circuit model (see the inset in Figure 9), the Rct value of ZnO/Bi2S3 photoanode was 12.7 kΩ, which was much smaller than that of the ZnO photoanode (50.8 kΩ). Therefore, the ZnO/Bi2S3 photoanode exhibits the significantly improved charge transfer kinetics, resulting in the enhanced PCP performance.  The inset is the proposed equivalent circuit model, in which Rs is the solution resistance; CPE is the capacitance phase element for the semiconductor/electrolyte interface; and Rct is the charge transfer resistance across the interface.
Author Contributions: Methodology, formal analysis, investigation, and data curation, Y.L.; conceptualization, writing, visualization, supervision, project administration, and funding acquisition, S.L. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the "Hundred Talent Program" from Sun Yat-sen University.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of Interest:
The authors declare no conflict of interest.