ZnFeAl-layered double hydroxides/TiO2 composites as photoanodes for photocathodic protection of 304 stainless steel

A series of ZnFeAl-layered double hydroxides/TiO2 (ZnFeAl-LDHs/TiO2) composites are synthesized by a combined anodization and hydrothermal method. The structure, surface morphology, photo absorption and photocathodic protection properties of these samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) and electrochemical tests. The unique structure of the ZnFeAl-LDHs reduces the charge carriers recombination, and the visible photoresponse property increase the light harvesting. The XPS study reveals that the electrons in the ZnFeAl-LDHs travel to TiO2, and the ZnFeAl-LDHs/TiO2 composites generate and transfer more electrons to 304 stainless steel (304SS), and exhibits a better photocathodic protection performance than pure TiO2. In addition, after intermittent visible-light illumination for four days, the photoanode still exhibits good stability and durability.

Stainless steels are used in many fields because of their good corrosion resistance. However, due to the effects of chloride ions and marine microorganisms, pitting corrosion easily occurs and the passivation film of stainless steel tends to be destroyed in marine environments, accelerating corrosion 1,2 . Several methods have been proposed to slow down the corrosion rate of stainless steels 3,4 , and photocathodic protection is one of the most innovative methods. TiO 2 is a low cost, non-toxic, highly stable semiconductor. Since the first report of photoelectrochemical water splitting using a TiO 2 electrode under ultraviolet light 5 , the photoelectric effect of TiO 2 has attracted the extensive attention and research [6][7][8][9][10] . In recent years, the photocathodic protection effect of TiO 2 has attracted the interests of scientists. Y. Ohko 11 and T. Imokawa 12 investigated the photoelectrochemical behavior of 304 stainless steels coated with TiO 2 , and Yuan et al. 13 investigated the photocathodic protection of Cu using a TiO 2 coating. Previous researches have indicated that TiO 2 is excited and generates electron-hole pairs under light illumination, and the electrons can be transferred to metals via the conduction band. This allows the potential of metals to be more negative than the corrosion potential, inhibiting the corrosion of the metals 14 . However, the sunlight utilization rate and photon separation rate of TiO 2 are low 15 , which limits the practical applications of TiO 2 . Efforts have been devoted to modifying the band structure of TiO 2 by cation doping or semiconductor doping [16][17][18][19][20] . Semiconductor doping is an effective method to promote photoelectrochemical properties, and the most reported semiconductor materials include WO 3 , CdS and SnO 2 21,22 [23][24][25] , anion exchangers 26 , energy storage materials 27,28 and adsorbents 29 , but they have not been applied in photoelectric anticorrosion of stainless steels. LDHs based on zinc oxides have shown strong visible light absorption 30,31 . The oxo-bridges in Fe-based LDH photocatalysts help to inhibit the recombination of electrons with holes, and extend the diffusion length of the hole [32][33][34] . Additionally, Mantilla discovered that ZnAlFe-LDH materials show semiconductor properties in the UV-vis region after heat treatment 35 . In this work, we synthesized ZnFeAl-LDHs/TiO 2 photoanodes and investigated their photocathodic protection of 304 stainless steels.

Methods
Synthesis of TiO 2 nanotubes. TiO 2 nanotubes were synthesized by electrochemical anodization method. ) for 30 s after an ultrasonic cleaning in ethanol and distilled water for 10 min, and then they were rinsed with ethanol and deionized water several times. The anodization process was carried out in an electrolyte system containing 0.44 g of NH 4 F (Sinopharm Chemical Reagent Co., Ltd.), 8 mL of deionized water, and 80 mL of glycol (Sinopharm Chemical Reagent Co., Ltd.) at 20 V for 1.5 h, using a Pt plate (20 mm × 20 mm × 0.3 mm) as the counter electrode and titanium foil as the working electrode. Finally, the samples were annealed at 450 °C for 2 h in air at a heating rate of 5 °C/min. All chemicals were analytical reagent grade and used without further purification.
Preparation of ZnFeAl-LDHs/TiO 2 composites. The ZnFeAl-LDHs/TiO 2 composites were prepared by hydrothermal method. First, 3.0 mmol of Zn(NO 3 ) 2 ·6H 2 O (Sinopharm Chemical Reagent Co., Ltd.), 0.1 mmol of Fe(NO 3 ) 3 ·9H 2 O (Sinopharm Chemical Reagent Co., Ltd.), 0.9 mmol of Al(NO 3 ) 3 ·9H 2 O (Sinopharm Chemical Reagent Co., Ltd.) and 14 mmol of urea (Sinopharm Chemical Reagent Co., Ltd.) were dissolved into 50 ml of deionized water solution, and stirred for 10 min at room temperature. Then, the pH value was adjusted to 3.5 with a NaOH (0.6 M, Sinopharm Chemical Reagent Co., Ltd.) solution and stirred for another 10 min. The solution was transferred to a Teflon-lined autoclave. Finally, the prepared TiO 2 nanotubes were placed in the autoclave at 120 °C for 8 h. The samples were taken out and washed several times with deionized water and ethanol. The above description is the synthetic process of ZnFeAl-LDHs/TiO 2 with total metal concentration of 80 mmol/L. In this experiment, three samples were synthesized with a total metal concentration of 40 mmol/L, 80 mmol/L and 160 mmol/L, and the samples were designated ZnFeAl-LDHs/TiO 2 (a), ZnFeAl-LDHs/TiO 2 (b), and ZnFeAl-LDHs/ TiO 2 (c), respectively.
Characterization. The XRD patterns were recorded on D/Max 2550 diffractometer with Cu Kα radiation in the 2θ range from 10° to 70°. The SEM images were obtained by a HITACHI S-4800 scanning electron microscope. The EDS spectrum were obtained by an Oxford INCA Energy 350 energy dispersive X-ray spectrometer. The UV-vis absorption spectra were recorded with a HITACHI U-4100 spectrophotometer. The XPS data were recorded on a Perkin-Elmer PHI-1600 ESCA spectrometer employing Mg Kα X-rays.

Electrochemical
Measurements. An electrochemistry working station (PARSTAT 4000+, Princeton, USA) was used for the electrochemical test of the open circuit potential (OCP) and photocurrent density. The measurement of the OCP was evaluated in a two-cell system includes corrosion cell and photoanode cell. The corrosion cell and photoanode cell are connected, and there is a Nafion membrane at the joint of two cells (Fig. 1). The electrodes in the corrosion cell were 304SS and a reference electrode (RE, saturated calomel electrode), and the solution was 3.5 wt.% NaCl, the electrode in the photoanode cell was ZnFeAl-LDHs/TiO 2 , and the solution was a mixture of 0.1 mol/L Na 2 S (Shanghai TongYa Chemical Technology Development Co., Ltd.) and 0.1 mol/L Na 2 SO 3 (Sinopharm Chemical Reagent Co., Ltd.). In addition, the electrodes, ZnFeAl-LDHs/TiO 2 and 304SS were connected by a wire as the working electrode (WE). The measurement of the photocurrent density was evaluated in a single-cell system with traditional three electrodes (Pt as the counter electrode), and the solution was a mixture of 0.1 mol/L Na 2 S and 0.1 mol/L Na 2 SO 3 . A Xenon lamp (PLS-SXE 300 C, Beijing Perfectlight Company, China) with a 400 nm glass filter was used as the light source device.
Data Availability. All data generated or analysed during this study are included in this published article.
The SEM images of TiO 2 and the ZnFeAl-LDHs/TiO 2 composites are shown in Fig. 3. The TiO 2 nanotubes have an orderly array structure, and the diameter of the nanotube is approximately 70 nm. In the ZnFeAl-LDHs composites, the ZnFeAl-LDHs material is supported on the TiO 2 nanotubes in a lamellar form with the length of 400-800 nm. In ZnFeAl-LDHs/TiO 2 (a), some ZnFeAl-LDHs did not form a lamellar structure and aggregation occurred ( Fig. 3(b)), the aggregation of ZnFeAl-LDHs will inevitably affect the light absorption and the transportation of photoelectrons. In ZnFeAl-LDHs/TiO 2 (b) and ZnFeAl-LDHs/TiO 2 (c), ZnFeAl-LDHs nanoflakes are observed to be cross-distributed on the surface of TiO 2 , and there are more nanoflakes on ZnFeAl-LDHs/ TiO 2 (c) (Fig. 3(d)) than ZnFeAl-LDHs/TiO 2 (b) (Fig. 3(c)), in Fig. 3(c), the ZnFeAl-LDHs nanoflakes on the surface of TiO 2 nanotubes distribute more closely, which may influence the light absorption and electron generation of TiO 2 , then influence the photocathodic protection performance. In addition, the EDS spectrum of ZnFeAl-LDHs/TiO 2 (b) is shown in Fig. 4(a). It can be seen that the sample consisted of Zn, Fe, Al, Ti and O, and the chemical composition agreed well with that of ZnFeAl-LDHs/TiO 2 .
The UV-vis spectra of TiO 2 and ZnFeAl-LDHs/TiO 2 (b) are shown in Fig. 4(b). It can be seen that pure TiO 2 exhibits a steep absorption edge at approximately 380 nm. In addition, a red shift of the absorption edge is observed in ZnFeAl-LDHs/TiO 2 (b), inducing stronger light absorption in the visible region. And the band gap of the two samples were achieved followed the equation,  indicating electron transfer from ZnFeAl-LDHs to TiO 2 38 . The observed peak at 532.5 eV of O 1 s corresponds to the oxygen species in the hydroxide form in the LDH structure 39 . Figure 6(a) shows the OCP-time curves of 304SS coupled to TiO 2 and the ZnFeAl-LDHs/TiO 2 composites under intermittent visible-light illumination. The results show that the corrosion potential of 304SS (Ecorr) is approximately −270 mV, when 304SS is coupled to TiO 2 , the potential decrease to −380 mV when the light is switched on, and the potential increase and is close to the corrosion potential of bare 304SS when the light is off. When 304SS is coupled to the ZnFeAl-LDHs/TiO 2 composites, the potential exhibits more obvious decrease when the light is switched on, and more negative than that of the bare 304SS when the light is off. These results indicate that when 304SS is coupled to the ZnFeAl-LDHs/TiO 2 composites, the visible-light absorption characteristics of the ZnFeAl-LDHs/TiO 2 composites allow the composites to absorb more visible light and generate more carriers. Additionally, the oxo-bridges in ZnFeAl-LDHs/TiO 2 help prevent the recombination of holes with electrons, and the synergistic effect leads to more protection electrons being transferred to the 304SS. (The schematic  illustration is shown in Fig. 7.) ZnFeAl-LDHs material was supported on the TiO 2 nanotubes by hydrothermal method. Under light irradiation, both TiO 2 and ZnFeAl-LDHs can be excited to generate electrons and holes, the electrons of the ZnFeAl-LDHs can transferred to the TiO 2 , and then transferred to 304SS to provide an protection. However, the three samples exhibit different potential changes caused by the amount and morphology of ZnFeAl-LDHs. To verify the stability and durability of the samples, the OCP-time curves of 304SS coupled with ZnFeAl-LDHs/TiO 2 (b) are investigated for 4 days with 4 cycles under intermittent visible light irradiation. Each cycle includes 12 h of light-on and 12 h of light-off. Figure 6(b) shows that the potential decrease to approximately −750 mV under illumination and remain below -700 mV after 12 h. When the light is off, the potential rises to about −500 mV, and is more negative than the corrosion potential of 304SS, which shows that the sample can provide protection in the dark. After 4 days, the sample retains a good protection performance. The results indicate that the ZnFeAl-LDHs/TiO 2 (b) photoanode exhibits good stability and durability and can provide long-term protection for 304SS. Figure 8 shows the photocurrent density curves of TiO 2 and the ZnFeAl-LDHs/TiO 2 composites under intermittent visible-light illumination. The maximum photocurrent density of ZnFeAl-LDHs reaches 138 μA/cm 2 , and the values of all the ZnFeAl-LDHs/TiO 2 composites are larger than that of TiO 2 , which indicate that the ZnFeAl-LDHs/TiO 2 composites generate more electrons.

Conclusions
In summary, ZnFeAl-LDHs/TiO 2 composites with various concentrations of ZnFeAl-LDHs are synthesized. All the composites exhibit better photocathodic protection performances for 304SS than pure TiO 2 which is attributed to the synergistic mechanism of their unique structure and visible-light response property. The composite with a concentration of 80 mmol/L of ZnFeAl-LDHs exhibits the best performance, and the protection potential reaches -760 mV under visible-light illumination, and is lower than the corrosion potential of 304SS in the dark. Moreover, the composites have good stability and durability, this work provides a probable approach for effective and stable photocathodic protection of marine metal.