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MgTixOy multiphase-heterojunction film with high efficiency for photoelectrochemical cathodic protection
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A novel TiO2 nanotube arrays/MgTixOy multiphase-heterojunction film with high efficiency for photoelectrochemical cathodic protection Chang Fenga,b,c,d,e, Zhuoyuan Chena,c,d,e,*, Jiangping Jinga,d,e, Mengmeng Suna,d,e, Guiying Lua,b,d,e, Jing Tiana,b,d,e, Jian Houc a
Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China University of Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100049, China c State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Wenhai Road, Qingdao 266237, China d Center for Ocean Mega-Science, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China e Open Studio for Marine Corrosion and Protection, Pilot National Laboratory for Marine Science and Technology (Qingdao), No. 1 Wenhai Road, Qingdao, 266237, China b
ARTICLE INFO
ABSTRACT
Keywords: A. TiO2/MgTixOy A. Multiphase-heterojunction film B. SKP C. Photoelectrochemical cathodic protection C. Stability
Heterojunction engineering, as a rising star in the photoelectrochemical cathodic protection (PECCP) field, contributes to promote the separation of the photoinduced electrons and holes. In this paper, the TiO2/MgTixOy multiphase-heterojunction film was prepared and its PECCP performance was studied. Due to a good energy band alignment gradient formed between the multiphases, the separation efficiency of the photoinduced electrons and holes generated by TiO2/MgTixOy are dramatically enhanced, leading to its good PECCP performance and high stability. SKP is, for the first time, used to study the surface work function of the TiO2/MgTixOy multiphase-heterojunction film for characterizing its PECCP performance.
1. Introduction As a promising green technology, photocatalysis and photoelectrochemistry are extensively studied and used in the fields of environment and energy [1–6]. As an important branch of photocatalysis and photoelectrochemistry, the photoelectrochemical cathodic protection (PECCP) technology uses the separated photoinduced electrons generated by semiconductor materials and transfers them to the coupled metal to provide cathodic protection. This is an effective way to protect metallic materials using solar energy. During the PECCP process, the photoelectric conversion semiconductor material will not be consumed and thus this technology will not pollute the environment. Meanwhile, the control synthesis of the photoelectrode is relatively simple and the cost is low [7,8]. Therefore, the PECCP technology is a promising, green and environmentally friendly corrosion protection technology with a great application potential [9,10]. TiO2 [11,12], ZnO [13,14], g-C3N4 [15], SrTiO3 [16,17] etc. are common semiconductor materials with good PECCP performance. However, a single photoelectric conversion semiconductor material tends to have a fast recombination rate of the photogenerated electrons and holes, enabling less photoinduced electrons to be effectively utilized for PECCP. Therefore, further modifications of the photoelectric conversion semiconductor material is of great significance to realize the sustainable and effective utilization of the ⁎
excited photogenerated electrons for PECCP. As an effective way to significantly inhibit the recombination of the photogenerated electrons and holes, heterojunction engineering has been reported to achieve effective separation of the photogenerated electrons and holes [18–20], and to efficiently transfer the photogenerated electrons to the coupled metals, thereby enhancing the PECCP performance of semiconductor materials. The reported heterojunction systems, such as SrTiO3/TiO2 [21,22], Bi2X3/TiO2 (X is S or O) [23,24], In2O3/TiO2 [25], SnO2/TiO2 [26], WO3/TiO2 [27,28], Ag2S/ TiO2 [29], Ni2S3/TiO2 [30], N-doped TiO2/TiO2 [31], BiVO4/TiO2 [32], ZnInS/TiO2 [33], Co3O4/ZnO [34], ZnxMg1-xO/ZnO [35], g-C3N4/ ZnO [36], TiO2/ZnO [37], g-C3N4/In2O3 [38], etc., show significant improvements in the PECCP performance. Although the establishment of heterojunctions can significantly improve the PECCP performance of composite heterojunction materials, its long-term stability is still a big challenge. Bu et al. have systematically reported the PECCP mechanism for steel using the SrTiO3/TiO2 composite photoelectrodes [21]. However, the stability of the photoelectrodes applied for the PECCP showed obvious shortcomings [21]. Sun et al. have investigated the enhanced PECCP performance of the In2O3/TiO2 composite [25]. The stability testing was still an important part to be further verified. Kuang et al. have designed a dual-functional ZnxMg1-xO solid solution nanolayer modified ZnO tussock-like nanorods to apply for the PECCP [35]. The
Corresponding author. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.corsci.2020.108441 Received 18 October 2019; Received in revised form 31 December 2019; Accepted 4 January 2020 0010-938X/ © 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Chang Feng, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2020.108441
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stability of the performance was an important factor that bothers them in choosing the best photoelectrode. Accordingly, how to design a photoelectrode with high PECCP activity and stability is still an urgent issue to be solved. As can be well known, the establishment of multiphase heterojunctions can effectively accelerate the separation of the photogenerated charge carriers [39–43]. A well band alignment formed among the multi-heterojunctions can provide a larger charge carrier transfer gradient [39], which can significantly inhibit the secondary recombination of the photogenerated electrons and holes. And, this provides an idea to design an efficient and stable photoelectrode applied for PECCP. In addition, the scanning kelvin probe (SKP) testing system is considered to be a novel micro-electrochemical technology which can measure the surface work function (WF) of a material. It has been widely used in the fields of metallic corrosion, coatings, solar cells and photocatalysis [44–51]. On one hand, SKP technology can be used to record the surface potential of the material and observe the potential distribution of different components in the micro-region. On the other hand, the surface potential distribution measured by SKP technology can be transformed into surface WF, which can be used to analyze the capability of the surface electrons escaping from the material and study the application in electrochemistry and photoelectrochemistry. Hua et al. have used the SKP technology to prove that the diffusion rate of hydrogen in (001) and (101) grains of 304 SS is faster than that in (111) grains, and hydrogen is proved to be trapped at the phase boundary between austenite and martensite [52,53]. Li et al. explored the reasons for the improvement of the photocatalytic performance of Ag-modified TiO2 through SKP technology [44]. Their results showed that low WF made the Ag-modified TiO2 be easier to escape electrons, and thus accelerated the separation of photogenerated charge carriers, leading to the enhancement of the photocatalytic performance of the Ag-modified TiO2. Although SKP technology has been applied in various research fields, there is no report concerning about the SKP analysis in the field of the PECCP. In view of this, the introduction of the SKP technology into the area of the PECCP will play a positive role for comprehensively and profoundly understanding the effect of the photoelectrodes on the PECCP process. MgTixOy is considered to be a wide band gap semiconductor material with good photocatalytic performance [40,54]. The wide band gap can make the photoinduced electrons and holes generated by MgTixOy be difficult to recombine, therefore, the photogenerated charge carriers can efficiently transfer in the process of the photocatalytic reactions. Furthermore, MgTixOy has a more negative conduction band potential than TiO2, which is beneficial to PECCP. Meanwhile, a well band alignment can be formed between TiO2 and MgTixOy, therefore, the TiO2/MgTixOy multiphase heterojunctions enable the photogenerated charge carriers to migrate directionally. In this way, more photogenerated electrons can be separated and participate in the PECCP reactions, thereby greatly improving the PECCP performance and stability of the TiO2/MgTixOy multiphase heterojunction system. In the present paper, a novel TiO2/MgTixOy multiphase-heterojunction film was prepared and reported for the first time, and its PECCP performance and stability for 304 SS were studied. SKP technique was used for the first time to study the significantly enhanced PECCP performance of the prepared TiO2/MgTixOy multiphase-heterojunction film. The lowest surface WF of the TiO2/MgTixOy multiphase-heterojunction film makes it be easier to escape the electrons, and higher amount of photogenerated electrons can be produced for protecting 304 SS under simulated solar light illumination. In addition, the establishment of multiphase heterojunctions makes the TiO2/ MgTixOy films have durable stability for the PECCP. This study further enriches the means of characterizing the photoelectrodes, and provides an important theoretical basis for understanding the promotion of the PECCP performance and the stability of the photoelectrodes.
2. Experimental section 2.1. Preparation of the photoelectrodes The reagents used in the experiments were all purchased from Sinopharm Chemical Reagent Co., Ltd without further purification. The anatase TiO2 nanotube arrays (NTAs) (TiO2(A)) photoelectrode was prepared by a two-step anodization method according to the previous reports [55,56]. A cleaned titanium (Ti) sheet was used as the anode and a Pt electrode was used as the cathode, which was placed in parallel in the electrolyte of ethylene glycol (0.35 wt% NH4F, 10 wt% H2O). A constant potential (60 V) was applied for anodic oxidation for 1 h at room temperature. After 10 min of ultrasonic cleaning in 10 wt% HCl solution, the obtained Ti sheet was rinsed with deionized water for several times. The above-mentioned process of anodic oxidation was repeated one more time. After that, the sample was put into deionized water and ultrasonically cleaned for 30 S. The sample was dried, and then was annealed at 450 °C for 3.5 h to prepare the TiO2(A) photoelectrode. The TiO2/MgTixOy photoelectrode was prepared as follows. The magnesium hydroxide was firstly deposited onto the surface of the TiO2(A) photoelectrode by a multi-potential step method using a CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). A three-electrode configuration was applied in the deposition process, in which the prepared TiO2(A) was used as the working electrode, the Pt electrode worked as the counter electrode, and the saturated calomel electrode (SCE) acted as the reference electrode, respectively. 0.1 M Mg(NO3)2 solution was used as the electrolyte. During the deposition process, the step potentials were −1.5 and −1.2 V, respectively, and the corresponding time was 5 s and 0.5 s. After 8 cycles of deposition, the obtained sample was washed with deionized water and alcohol. Subsequently, the prepared sample was thermally treated in a muffle furnace at 700 °C for 2 h to obtain the TiO2/MgTixOy photoelectrode. As can be well known, the phase transformation of TiO2 occurs during high temperature treatment (≥ 550 °C), resulting in the coexistence of anatase and rutile TiO2 (TiO2(R)) [57–59]. Therefore, as a comparison, the prepared TiO2(A) photoelectrode was also thermally treated in a muffle furnace at 700 °C for 2 h to obtain the TiO2(A)/TiO2(R) photoelectrode. 2.2. Characterization of the prepared photoelectrodes X-ray diffractometer (XRD, D/max-500, Rigaku Co., Tokyo, Japan) was used to characterize the crystal structures of the prepared photoelectrode. Fourier transform infrared (FT-IR) spectra were tested using a Fourier transform infrared spectroscopy (FT-IR, Thermo-Nicolet 8700, Thermo Electron Scientific Inc., USA) at room temperature. UV–vis diffuse reflectance spectrophotometer (U-41000; HITACHI, Tokyo, Japan) was used to analyze the optical absorption properties of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes. Photoluminescence (PL) spectra of the prepared photoelectrodes were measured with a fluorescence spectrometer (PL, Microconfocal Raman Spectrometer, Horiba Jobin Yvon LabRAM HR800, 325 nm, France). Field emission scanning electron microscopy (FE-SEM, ZEISS, ULTRA 55, Germany) was used to analyze the micromorphologies of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes. The elemental composition and mapping of the prepared photoelectrode were analyzed by an X-ray energy dispersive spectrometer (EDS, Oxford, UK). The microstructures of the prepared TiO2/MgTixOy photoelectrode and interfacial information of different phase components were observed by field emission transmission electron microscope (FE-TEM, Tecnai G2 F20, FEI Company, USA).
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2.3. Photoelectrochemical (PEC) performance, PECCP performance and SKP measurements
diffraction peaks in the XRD pattern of TiO2(A) are attributed to the characteristic ones of the Ti substrate [55,56]. For the XRD pattern of TiO2(A)/TiO2(R), the coexistence of anatase and rutile phases of TiO2 can be clearly observed after high temperature treatment at 700 °C. In addition to the diffraction peaks of Ti substrate and anatase TiO2, the corresponding rutile phases of TiO2 (JCPDS No. 21-1276) are observed at 27.6°, 36.1°, 41.2°, 44.1, 54.3°, 56.7°, 64.1°and 68.9°, which are assigned to the (110), (101), (111), (210), (211), (220), (310) and (301) crystal planes, respectively. For the XRD pattern of TiO2/MgTixOy, the weak characteristic peaks of MgTiO3 and MgTi2O5 can be observed. It may be due to the relatively small amount of MgTixOy deposited on the surface of TiO2 tube orifice. Although the intensities of the diffraction peaks are weak, it can be clearly seen that the diffraction peaks are located at 19.1°, 21.2°, 23.9°, 35.5°, 40.6° and 63.8°, respectively. These diffraction peaks correspond well to the (003), (101), (012), (104), (110) and (214) crystal planes of rhombohedral MgTiO3 (JCPDS no. 060494). In addition, the diffraction peaks at 32.7° and 48.6° are observed, which are assigned to the (230) and (331) crystal planes of orthorhombic structure of MgTi2O5 (JCPDS no. 35-0792). The XRD results suggest that MgTixOy was successfully modified on the surface of TiO2. Fig. 1B displays the FT-IR spectra of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy. The FT-IR spectra of TiO2(A) and TiO2(A)/TiO2(R) show a high degree of consistency and the broad band below 750 cm−1 is mainly from the stretching vibration mode of Ti-OTi in TiO2 [63,64]. No other characteristic peaks were observed, indicating the high purity of TiO2(A) and TiO2(A)/TiO2(R). For the FT-IR spectrum of TiO2/MgTixOy, another broad region at approximately 1400−1550 cm−1 can be observed, which comes from the Ti–carboxylic complexes and hydroxyl group [65–67]. The FT-IR results indicate that the carboxyl and hydroxyl groups can be easily adsorbed on the surface of TiO2(A)/TiO2(R), which facilitates the contact between the photoelectrode and the electrolyte solution. Fig. 2 shows the SEM images of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy, respectively. As shown in Fig. 2A, TiO2(A) exhibits a good NTA structure. The orifices are closely arranged with a diameter of approximately 80 nm. Fig. 2B shows the SEM image of the TiO2(A)/ TiO2(R) photoelectrode. The surface of TiO2(A)/TiO2(R) is much smoother than that of TiO2(A), which may be due to the formation of anatase and rutile TiO2 after the high temperature treatment. For the SEM image of TiO2/MgTixOy displayed in Fig. 2C, some nanoparticles obviously appear at the orifices and are well coated on the surface of the orifice of TiO2 NTAs. This may be the surface-modified MgTixOy nanoparticles. Fig. 2D shows the SEM image of the cross section of TiO2/MgTixOy. The height of the nanotubes can be measured as approximately 4 μm. The basic structure of the nanotubes has not been destroyed after high temperature treatment at 700 °C, and the tubular structure remains intact. Fig. 3A shows the SEM image and the corresponding EDS spectrum of TiO2(A). In addition to Ti and O elements, no other impurity elements are observed. The atomic mass percentages of Ti and O were
The light source used for the PEC and PECCP performance measurements is 150-W Xe lamp (PLSSXE300, Changtuo Co. Ltd., Beijing, China). A simulated solar illumination is obtained by adding an AM1.5 G filter and adjusting the light intensity of this light source to 100 mW⋅ cm−2. The PEC performance was tested in a three-electrode cell system using the CHI660D electrochemical workstation. The variations of the current densities of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes were measured using the prepared photoelectrode as the working electrode, the platinum electrode as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. The bias voltage was set as 0 V (vs Ag/AgCl) and the electrolyte was 0.1 M Na2SO4 solution. The variations in the current densities and the mixed potentials, and the polarization curves of the galvanic couple of the 304 SS electrode and the prepared photoelectrodes were measured to characterize the PECCP performance. The galvanic couple of the 304 SS electrode and the prepared photoelectrodes was used as the working electrode. The Ag/AgCl (saturated KCl) electrode and the platinum electrode served as the reference and counter electrode, respectively. Both the 304 SS electrode and the prepared photoelectrodes are placed in 3.5 wt% NaCl solution and the bias voltage was set as 0 V (vs Ag/ AgCl) during the testing process. Intermittent simulated solar light (100 mW, AM 1.5 G) was illuminated on the surface of the photoelectrodes. Electrochemical impedance spectroscopy (EIS) tests were performed at open circuit potential over the frequency range between 105 and 10-1 Hz, with an AC voltage magnitude of 5 mV. The polarization curves were measured using the CHI660D electrochemical workstation with a scan rate of 1 mV⋅s-1 from -400 to 400 mV (vs open circuit potential). The surface WFs of the prepared photoelectrodes were analyzed using SKP (VersaSCAN, Ametek). The tungsten probe with the diameter of 250 μm was acted as the reference and detection probe. The testing area on the surface of the prepared photoelectrodes was 1 × 1 mm2 and the scanning rate was set as 50 μm⋅s−1 with sensitivity of 500 μV. The surface WFs of the photoelectrodes can be calculated based on the following formula [60–62]: WF (Sample) = WF (Tungsten) + ΔW (photoelectrode)/1000. Among them, WF (Tungsten) is the WF of the reference electrode with standard value of 4.55 eV for tungsten; WF (Sample) is the surface WF for the prepared photoelectrode, ΔW (photoelectrode) is the surface potential of the photoelectrode obtained from the SKP measurement. 3. Results and discussion Fig. 1A shows the XRD patterns of the prepared TiO2(A), TiO2(A)/ TiO2(R) and TiO2/MgTixOy. For the XRD pattern of TiO2(A), the diffraction peaks at 25.5°, 37.9°, 48.4°, 54.2°and 55.3° are observed, corresponding to the (101), (004), (200), (105) and (211) crystal planes of standard anatase TiO2 (JCPDS No. 21-1272), respectively. The other
Fig. 1. XRD patterns and FT-IR spectra of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy. 3
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Fig. 2. SEM images of (A) TiO2(A), (B) TiO2(A)/TiO2(R) and (C) TiO2/MgTixOy, and (D) SEM image of the cross section of TiO2/MgTixOy.
33.19 % and 66.81 %, which fits well with the element composition of TiO2. Fig. 3B shows the SEM image and the corresponding EDS results of TiO2/MgTixOy. Ti, O and Mg elements are clearly observed with the atomic percentages of 24.13 %, 67.91 % and 7.96 %, respectively. As shown in the corresponding EDS mapping results, Ti element covers the whole scanning area due to the Ti substrate. While, O and Mg elements correspond well to the SEM image shown in Fig. 3B. The tube orifice is clearly observed in the EDS Mg and O mappings, indicating that Mg and O elements are well dispersed on the surface of the TiO2 nanotube orifices. The difference between the SEM images in Fig. 3 and those in Fig. 2 is mainly due to the fact that EDS is tested under high excitation
voltages, which will break down the surface components and make the surface morphology be difficult to be observed. The composition and interfacial information of the TiO2/MgTixOy multiphase heterojunctions are further studied by HRTEM. Fig. 4A presents the TEM image observed at low magnification. The structure of nanotubes can be clearly observed, which corresponds well to the SEM image of TiO2/MgTixOy shown in Fig. 2. The edge of the nanotubes was further enlarged, as shown in Fig. 4B. The nanostructures with different lattice fringes are found and form the heterojunctions on the surface of the nanotube. The observation results are further magnified at the selected two rectangular areas. Fig. 4C corresponds to the rectangular
Fig. 3. SEM images and the corresponding EDS results of (A) TiO2(A) and (B) TiO2/MgTixOy. 4
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Fig. 4. TEM (A) and HRTEM (B, C, D) images of TiO2/MgTixOy multiphase heterojunctions.
area I. The lattice fringes of TiO2(R), MgTiO3 and MgTi2O5 can be observed with the values of 0.29, 0.27 and 0.33 nm, respectively. Furthermore, the lattice fringes of TiO2(A), TiO2(R), MgTiO3 and MgTi2O5 can be clearly observed corresponding to the rectangular area II, as shown in Fig. 4D. Combining with the physical characterization results shown in Figs. 1–4, it can be proved that MgTixOy is successfully coated on the surface of TiO2 NTAs, and the multiphase heterojunctions are formed. Fig. 5 shows the UV–vis absorption spectra of the TiO2(A), TiO2(A)/ TiO2(R) and TiO2/MgTixOy photoelectrodes. As can be found, the light absorption threshold of TiO2(A) is observed at 382 nm. For the TiO2(A)/TiO2(R) photoelectrode, the light absorption threshold has a slight red shift compared with that of TiO2(A), which may be caused by the formation of TiO2(R) with a narrower band gap. In addition to having the same absorption threshold as TiO2(A)/TiO2(R), the TiO2/ MgTixOy photoelectrode has an additional absorption threshold at approximately 330 nm, indicating the successful synthesis of MgTixOy on the surface of TiO2 NTAs.
The PECCP performance of the prepared photoelectrodes are characterized by measuring the photoinduced current densities and the photoinduced potential drops, and the results are shown in Fig. 6. Fig. 6A shows the variations in the galvanic current densities between the 304 SS electrode and the prepared photoelectrodes in 3.5 wt% NaCl solution under intermittent simulated solar light illumination. Positive excitation current densities are obtained under light illumination, indicating that the photoinduced electrons generated by the photoelectrodes transfer to the coupled 304 SS electrode and provide the PECCP for it. The photoinduced current density of a prepared photoelectrode, which is provided for protecting the coupled metallic electrode, is obtained by subtracting the stable galvanic current density between the protected metal electrode and the photoelectrode in the dark from that under light illumination. As shown in Fig. 6A, the photoinduced current density of TiO2(A) is 9 μA·cm−2 and that of TiO2(A)/TiO2(R) is 23 μA·cm−2. While, that of TiO2/MgTixOy is 49 μA·cm−2, which is 5.4 and 2.1 times of that of TiO2(A) and TiO2(A)/TiO2(R). The results shown in Fig. 6A indicate that TiO2/MgTixOy can produce the most photogenerated electrons and can provide the most electrons needed for the cathodic protection of the coupled 304 SS. Fig. 6B shows the variations of the mixed potential of the 304 SS electrode coupled with the prepared photoelectrodes in 3.5 wt% NaCl solution under intermittent simulated solar light illumination. The mixed potentials of the 304 SS electrode coupled with the prepared photoelectrodes immediately shift to negative direction once the light is switched on, demonstrating that the prepared photoelectrodes can provide the PECCP for the coupled 304 SS electrode. The photoinduced mixed potential drop is the mixed potential of the 304 SS electrode coupled with the prepared photoelectrode under light illumination minus that in the dark. As shown in Fig. 6B, the photoinduced mixed potential drops of TiO2(A), TiO2(A)/ TiO2(R) and TiO2/MgTixOy are -230, -250 and −320 mV, respectively. The TiO2/MgTixOy exhibits the maximum photoinduced mixed potential drop, indicating its excellent PECCP performance. The photoinduced mixed potential drop results shown in Fig. 6B are similar to the photoinduced current density results shown in Fig. 6A, and both of them prove that TiO2/MgTixOy has the best PECCP performance. Meanwhile, in Fig. 6B, the stability of the prepared photoelectrodes is evaluated by measuring the potential variations during the long duration of light illumination. As shown in Fig. 6B, the mixed potential of the 304 SS electrode coupled with the prepared photoelectrodes is
Fig. 5. UV–vis absorption spectra of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/ MgTixOy photoelectrodes. 5
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Fig. 6. (A) The variations in the galvanic current densities between the 304 SS electrode and the prepared photoelectrodes and (B) the variation of the mixed potentials of the galvanic couple of the 304 SS electrode and the prepared photoelectrodes in 3.5 wt% NaCl solution under intermittent simulated solar illumination.
stabilized for 900 s in the dark. Subsequently, the light is switched on and illuminated for 2 h on the photoelectrodes. The mixed potentials of the 304 SS electrode coupled with the TiO2(A) and TiO2(A)/TiO2(R) photoelectrodes change from -0.37 and -0.42 V to -0.33 and -0.39 V, showing a slight decrease in their PECCP performance during the light illumination. However, for TiO2/MgTixOy, the mixed potential of the 304 SS electrode coupled with the TiO2/MgTixOy photoelectrode does not have significant change during the 2-h light illumination. The results shown in Fig. 6B indicate that there is no obvious attenuation of the PECCP performance of the TiO2/MgTixOy photoelectrode during the long duration of light illumination, demonstrating that the TiO2/ MgTixOy photoelectrode possesses high stability and has good application prospects in the field of PECCP. The polarization curves of 304 SS electrode and 304 SS electrode coupled with the TiO2, TiO2(A)/TiO2 and TiO2/MgTixOy photoelectrodes in the absence and presence of simulated solar light illumination are shown in Fig. 7. The corrosion potential of the 304 SS was about −154 mV (vs. Ag/AgCl) before coupling with the prepared photoelectrodes in 3.5 wt% NaCl solution, however, those of 304 SS positively shift to -102, -101 and −113 mV after coupling with TiO2, TiO2(A)/TiO2 and TiO2/MgTixOy photoelectrodes in the dark, respectively. Under simulated solar light illumination, significant negative potential shifts are obtained for 304 SS coupling with different photoelectrodes, indicating the good PECCP performance of the prepared photoelectrodes. Under the simulated solar light illumination, the corrosion potentials of 304 SS coupling with TiO2, TiO2(A)/TiO2 and TiO2/ MgTixOy are -353, -426 and −486 mV, respectively. These results are in
good agreement with the previous PECCP tests shown in Fig. 6, which further indicates the excellent PECCP performance of the TiO2/MgTixOy photoelectrode. SKP technology is used to further characterize the capability of the electrons escaping from the surface of the prepared photoelectrodes. The surface WF, which refers to the minimum energy required to escape an electron from the surface of the photoelectrode, can be easily calculated from the surface potential of the photoelectrodes measured by SKP [68,69]. The smaller the WF is, the easier it is for electron to escape, i.e. for electrons to flow out of the photoelectrode and participate in the reactions [70]. Fig. 8A displays the surface potential distributions of different photoelectrodes measured by SKP. As can be found in Fig. 8A, the potential fluctuation of the prepared individual photoelectrode is smaller than ± 50 mV around the median value, suggesting that the surface of which is fairly flat and uniform. By comparing the surface potentials of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes, TiO2(A) has the most positive surface potential distribution with an average value of approximately 990 mV. TiO2(A)/ TiO2(R) has a surface potential of approximately 830 mV. While, TiO2/ MgTixOy has the most negative potential of approximately 690 mV, which is 300 and 140 mV lower than those of TiO2(A) and TiO2(A)/ TiO2(R), respectively. Fig. 8B shows the surface WFs of the prepared photoelectrodes obtained from the results shown in Fig. 8A. The surface WFs of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes are 5.54, 5.38 and 5.24 eV, respectively. The lowest surface potential distribution and surface WF of TiO2/MgTixOy indicate that the multiphase heterojunctions can easily excite the electrons and transfer them to the surface of 304 SS to provide effective protection for it. Fig. 9A shows the variations in the current densities of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes in 0.1 M Na2SO4 solution under intermittent simulated solar illumination. As shown in Fig. 9A, all of the prepared photoelectrodes shows positive excitation current densities under light illumination. The photoinduced current density of the TiO2(A) photoelectrode is 0.12 mA⋅ cm−2, and that of the TiO2(A)/TiO2(R) is 0.16 mA⋅ cm−2. However, the photoinduced current density of the TiO2/MgTixOy photoelectrode is 0.29 mA⋅ cm−2, which is approximately 2.4 times and 1.8 times of the TiO2(A) and TiO2(A)/ TiO2(R) photoelectrodes, respectively. The generation of photoinduced current is a key factor that affects the PECCP performance of the photoelectrode. The TiO2/MgTixOy photoelectrode has the highest photoinduced current density, indicating that it has the best PECCP performance. The electrochemical impedance spectroscopy and the photoluminescence spectroscopy of the prepared photoelectrodes were measured to evaluate the photogenerated charge carrier migration ability and the recombination ability of the photogenerated electrons and holes. The relevant results are shown in Figs. 9B and 9C. As shown in Fig. 9B, the TiO2/MgTixOy photoelectrode has the smallest surface resistance than TiO2(A) and TiO2(A)/TiO2(R), indicating that the TiO2/ MgTixOy has the fastest electron mobility. In addition, as shown in
Fig. 7. The polarization curves of the 304 SS electrode and the 304 SS electrode coupled with the TiO2, TiO2(A)/TiO2 and TiO2/MgTixOy photoelectrodes in the absence and presence of simulated solar light illumination. 6
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Fig. 8. (A) The surface potential distributions of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes measured by SKP technique; and (B) the surface WFs of the prepared photoelectrodes transformed from the SKP results in Fig. 8A.
Fig. 9. (A) The variations in the current densities of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes under intermittent light illumination, (B) Electrochemical impedance spectra (EIS) and (C) photoluminescence spectra of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes.
Fig. 10. SEM image of the TiO2/MgTixOy photoelectrode after the PECCP test and the XRD and FT-IR spectra of the TiO2/MgTixOy photoelectrode before and after the PECCP tests.
Fig. 11. The proposed transfer mechanism of the photogenerated charge carriers and the schematic diagram of the PECCP of the TiO2/MgTixOy multiphase heterojunctions. 7
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Fig. 9C, the photoluminescence intensity of TiO2/MgTixOy has a significant decrease compared with that of TiO2(A) and TiO2(A)/TiO2(R), indicating that the secondary recombination of the photogenerated electrons and hole is significantly inhibited for the TiO2/MgTixOy photoelectrode. These results display that the TiO2/MgTixOy photoelectrode can realize the fast transmission of the photogenerated charge carriers, can effectively inhibit the secondary recombination of the photogenerated electrons and holes, and can generate the highest photoinduced current compared with the TiO2(A) and TiO2(A)/TiO2(R) photoelectrodes, therefore, achieve the excellent PECCP performance of the TiO2/MgTixOy photoelectrode. The SEM image of TiO2/MgTixOy after the PECCP tests and the XRD and FT-IR analyses of the TiO2/MgTixOy photoelectrode before and after the PECCP tests are shown in Fig. 10. As shown in Fig. 10A, the micromorphology of TiO2/MgTixOy does not have significant changes after the PECCP tests. Furthermore, the XRD and FT-IR curves of the TiO2/MgTixOy photoelectrode after PECCP tests are highly consistent with those before PECCP tests, as shown in Fig. 10B and C. These results demonstrate that TiO2/MgTixOy photoelectrode has a high stability in the process of PECCP. Fig. 11 shows the proposed transfer processes of the photogenerated charge carriers in the TiO2/MgTixOy multiphase heterojunctions and its PECCP mechanism. The TiO2/MgTixOy photoelectrode has significantly enhanced photogenerated current density and photoinduced mixed potential drop, indicating that the energy band structures of TiO2 and MgTixOy match well and the TiO2/MgTixOy multiphase heterojunctions with a good energy band alignment gradient are formed. The photogenerated charge carriers can migrate directionally under the action of the multiphase heterojunctions, making the photogenerated charge carriers be well transferred. It was reported that the band gaps of MgTiO3, MgTi2O5, TiO2(R) and TiO2(A) are 3.7 eV, 3.4 eV, 3.0 eV and 3.2 eV, respectively, and the conduction band potentials of them are arranged in the following order: MgTiO3 < MgTi2O5 < TiO2(R) < TiO2(A) [40,54,71,72]. The photogenerated electrons can be transferred to TiO2(A) through the conduction band gradient formed by the TiO2/MgTixOy multiphase heterojunctions, and eventually be transferred to 304 SS through the Ti substrate to provide effective protection for it. Meanwhile, the photogenerated holes will be further transferred to the TiO2/MgTixOy multiphase heterojunctions under the action of the heterojunction electric field, and the corresponding oxidation reactions will occur with the electrolyte. Furthermore, the fabrication of the TiO2/MgTixOy multiphase heterojunctions greatly increases the separation efficiency of the photogenerated charge carriers and effectively suppresses the recombination of the photogenerated electrons and holes. In this way, a large number of photogenerated electrons will continuously flow from the TiO2/MgTixOy photoelectrode to the coupled 304 SS electrode under light illumination, thus significantly enhancing the long-term PECCP performance.
benefit to the improvement of the PECCP performance of the TiO2/ MgTixOy multiphase heterojunctions. Furthermore, the good PECCP performance of the TiO2/MgTixOy multiphase heterojunctions is attributed to the establishment of multiphase heterojunction system, which effectively inhibits the secondary recombination of the photogenerated electrons and holes, and accelerates the migration of the photogenerated charge carriers. SKP technology has been used for the first time to study the PECCP performance of the TiO2/MgTixOy multiphase-heterojunction film. The introduction of SKP technology into the PECCP research has greatly enriched the test methods in the field of PECCP. At the same time, the establishment of multiphase heterojunctions provides a new idea for the design of the PECCP film. Data availability All data included in this study are available upon request by contact with the corresponding authors. Declaration of Competing Interest None. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 41576114, 41676069), Key Research and Development Program of Shandong Province (Grant Nos. 2019GHY112066, 2019GHY112085), State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, China (Project No. 614290101011703), and Qingdao Innovative Leading Talent Foundation (Grant No. 15-10-3-15-(39)-zch). References [1] K. Sivula, R. Van De Krol, Semiconducting materials for photoelectrochemical energy conversion, Nat. Rev. Mater. 1 (2016) 15010. [2] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. [3] M.M. Momeni, M. Mahvari, Y. Ghayeb, Photoelectrochemical properties of ironcobalt WTiO2 nanotube photoanodes for water splitting and photocathodic protection of stainless steel, J. Electroanal. Chem. 832 (2019) 7–23. [4] S. Chandrasekaran, L. Yao, L. Deng, C. Bowen, Y. Zhang, S. Chen, Z. Lin, F. Peng, P. Zhang, Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond, Chem. Soc. Rev. 48 (2019) 4178–4280. [5] M.M. Momeni, M. Taghinejad, Y. Ghayeb, R. Bagheri, Z. Song, Preparation of various boron-doped TiO2 nanostructures by in situ anodizing method and investigation of their photoelectrochemical and photocathodic protection properties, J. Iran. Chem. Soc. 16 (2019) 1839–1851. [6] Y. Zhang, Y. Li, D. Ni, Z. Chen, X. Wang, Y. Bu, J.P. Ao, Improvement of BiVO4 photoanode performance during water photo‐oxidation using Rh‐Doped SrTiO3 perovskite as a Co‐catalyst, Adv. Funct. Mater. 29 (2019) 1902101. [7] M.M. Momeni, S.H. Khansari-Zadeh, H. Farrokhpour, Fabrication of tungsten-irondoped TiO2 nanotubes via anodization: new photoelectrodes for photoelectrochemical cathodic protection under visible light, SN Applied Sciences 1 (2019) 1160. [8] M.M. Momeni, Y. Ghayeb, N. Moosavi, Preparation of Ni–Pt/Fe–TiO2 nanotube films for photoelectrochemical cathodic protection of 403 stainless steel, Nanotechnology 29 (2018) 425701. [9] Y. Bu, J.P. Ao, A review on photoelectrochemical cathodic protection semiconductor thin films for metals, Green Energy Environ. 2 (2017) 331–362. [10] J. Jing, M. Sun, Z. Chen, J. Li, F. Xu, L. Xu, Enhanced photoelectrochemical cathodic protection performance of the secondary reduced graphene oxide modified graphitic carbon nitride, J. Electrochem. Soc. 164 (2017) C822–C830. [11] J. Li, C.J. Lin, C.G. Lin, A photoelectrochemical study of highly ordered TiO2 nanotube arrays as the photoanodes for cathodic protection of 304 stainless steel, J. Electrochem. Soc. 158 (2011) C55–C62. [12] C. Lei, H. Zhou, Z. Feng, Y. Zhu, R. Du, Low-temperature liquid phase deposited TiO2 films on stainless steel for photogenerated cathodic protection applications, Appl. Surf. Sci. 257 (2011) 7330–7334. [13] Y. Yang, Y.F. Cheng, One-step facile preparation of ZnO nanorods as high-performance photoanodes for photoelectrochemical cathodic protection, Electrochim. Acta 276 (2018) 311–318. [14] M. Sun, Z. Chen, Y. Bu, J. Yu, B. Hou, Effect of ZnO on the corrosion of zinc, Q235 carbon steel and 304 stainless steel under white light illumination, Corros. Sci. 82 (2014) 77–84.
4. Conclusions In the present paper, a highly efficient and stable TiO2/MgTixOy multiphase-heterojunction film was prepared and its PECCP performance for 304 SS was studied. MgTixOy are uniformly dispersed on the surface of TiO2 NTAs. MgTiO3, MgTi2O5, TiO2(R) and TiO2(A) contact well and form a multiphase-heterojunction system. The photoinduced current density of the TiO2/MgTixOy multiphase-heterojunction film is 49 μA·cm−2 under the simulated solar light illumination, which is 5.4 and 2.1 times of that of TiO2(A) and TiO2(A)/TiO2(R). The photoinduced mixed potential drops of the TiO2/MgTixOy multiphase-heterojunction film is −320 mV, while, those of TiO2(A) and TiO2(A)/ TiO2(R) are -230 and -250 mV, respectively. Meanwhile, the TiO2/ MgTixOy multiphase-heterojunction film exhibits extremely high PECCP stability. During the 2 h of light illumination, the PECCP of the TiO2/MgTixOy multiphase-heterojunction film shows no weakening trend. The lower surface WF makes the TiO2/MgTixOy multiphaseheterojunction film be much easier to escape electrons, which is of great 8
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A novel TiO2 nanotube arrays/MgTixOy multiphase-heterojunction film with high efficiency for photoelectrochemical cathodic protection Chang Fenga,b,c,d,e, Zhuoyuan Chena,c,d,e,*, Jiangping Jinga,d,e, Mengmeng Suna,d,e, Guiying Lua,b,d,e, Jing Tiana,b,d,e, Jian Houc a
Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China University of Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100049, China c State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Wenhai Road, Qingdao 266237, China d Center for Ocean Mega-Science, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China e Open Studio for Marine Corrosion and Protection, Pilot National Laboratory for Marine Science and Technology (Qingdao), No. 1 Wenhai Road, Qingdao, 266237, China b
ARTICLE INFO
ABSTRACT
Keywords: A. TiO2/MgTixOy A. Multiphase-heterojunction film B. SKP C. Photoelectrochemical cathodic protection C. Stability
Heterojunction engineering, as a rising star in the photoelectrochemical cathodic protection (PECCP) field, contributes to promote the separation of the photoinduced electrons and holes. In this paper, the TiO2/MgTixOy multiphase-heterojunction film was prepared and its PECCP performance was studied. Due to a good energy band alignment gradient formed between the multiphases, the separation efficiency of the photoinduced electrons and holes generated by TiO2/MgTixOy are dramatically enhanced, leading to its good PECCP performance and high stability. SKP is, for the first time, used to study the surface work function of the TiO2/MgTixOy multiphase-heterojunction film for characterizing its PECCP performance.
1. Introduction As a promising green technology, photocatalysis and photoelectrochemistry are extensively studied and used in the fields of environment and energy [1–6]. As an important branch of photocatalysis and photoelectrochemistry, the photoelectrochemical cathodic protection (PECCP) technology uses the separated photoinduced electrons generated by semiconductor materials and transfers them to the coupled metal to provide cathodic protection. This is an effective way to protect metallic materials using solar energy. During the PECCP process, the photoelectric conversion semiconductor material will not be consumed and thus this technology will not pollute the environment. Meanwhile, the control synthesis of the photoelectrode is relatively simple and the cost is low [7,8]. Therefore, the PECCP technology is a promising, green and environmentally friendly corrosion protection technology with a great application potential [9,10]. TiO2 [11,12], ZnO [13,14], g-C3N4 [15], SrTiO3 [16,17] etc. are common semiconductor materials with good PECCP performance. However, a single photoelectric conversion semiconductor material tends to have a fast recombination rate of the photogenerated electrons and holes, enabling less photoinduced electrons to be effectively utilized for PECCP. Therefore, further modifications of the photoelectric conversion semiconductor material is of great significance to realize the sustainable and effective utilization of the ⁎
excited photogenerated electrons for PECCP. As an effective way to significantly inhibit the recombination of the photogenerated electrons and holes, heterojunction engineering has been reported to achieve effective separation of the photogenerated electrons and holes [18–20], and to efficiently transfer the photogenerated electrons to the coupled metals, thereby enhancing the PECCP performance of semiconductor materials. The reported heterojunction systems, such as SrTiO3/TiO2 [21,22], Bi2X3/TiO2 (X is S or O) [23,24], In2O3/TiO2 [25], SnO2/TiO2 [26], WO3/TiO2 [27,28], Ag2S/ TiO2 [29], Ni2S3/TiO2 [30], N-doped TiO2/TiO2 [31], BiVO4/TiO2 [32], ZnInS/TiO2 [33], Co3O4/ZnO [34], ZnxMg1-xO/ZnO [35], g-C3N4/ ZnO [36], TiO2/ZnO [37], g-C3N4/In2O3 [38], etc., show significant improvements in the PECCP performance. Although the establishment of heterojunctions can significantly improve the PECCP performance of composite heterojunction materials, its long-term stability is still a big challenge. Bu et al. have systematically reported the PECCP mechanism for steel using the SrTiO3/TiO2 composite photoelectrodes [21]. However, the stability of the photoelectrodes applied for the PECCP showed obvious shortcomings [21]. Sun et al. have investigated the enhanced PECCP performance of the In2O3/TiO2 composite [25]. The stability testing was still an important part to be further verified. Kuang et al. have designed a dual-functional ZnxMg1-xO solid solution nanolayer modified ZnO tussock-like nanorods to apply for the PECCP [35]. The
Corresponding author. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.corsci.2020.108441 Received 18 October 2019; Received in revised form 31 December 2019; Accepted 4 January 2020 0010-938X/ © 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Chang Feng, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2020.108441
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stability of the performance was an important factor that bothers them in choosing the best photoelectrode. Accordingly, how to design a photoelectrode with high PECCP activity and stability is still an urgent issue to be solved. As can be well known, the establishment of multiphase heterojunctions can effectively accelerate the separation of the photogenerated charge carriers [39–43]. A well band alignment formed among the multi-heterojunctions can provide a larger charge carrier transfer gradient [39], which can significantly inhibit the secondary recombination of the photogenerated electrons and holes. And, this provides an idea to design an efficient and stable photoelectrode applied for PECCP. In addition, the scanning kelvin probe (SKP) testing system is considered to be a novel micro-electrochemical technology which can measure the surface work function (WF) of a material. It has been widely used in the fields of metallic corrosion, coatings, solar cells and photocatalysis [44–51]. On one hand, SKP technology can be used to record the surface potential of the material and observe the potential distribution of different components in the micro-region. On the other hand, the surface potential distribution measured by SKP technology can be transformed into surface WF, which can be used to analyze the capability of the surface electrons escaping from the material and study the application in electrochemistry and photoelectrochemistry. Hua et al. have used the SKP technology to prove that the diffusion rate of hydrogen in (001) and (101) grains of 304 SS is faster than that in (111) grains, and hydrogen is proved to be trapped at the phase boundary between austenite and martensite [52,53]. Li et al. explored the reasons for the improvement of the photocatalytic performance of Ag-modified TiO2 through SKP technology [44]. Their results showed that low WF made the Ag-modified TiO2 be easier to escape electrons, and thus accelerated the separation of photogenerated charge carriers, leading to the enhancement of the photocatalytic performance of the Ag-modified TiO2. Although SKP technology has been applied in various research fields, there is no report concerning about the SKP analysis in the field of the PECCP. In view of this, the introduction of the SKP technology into the area of the PECCP will play a positive role for comprehensively and profoundly understanding the effect of the photoelectrodes on the PECCP process. MgTixOy is considered to be a wide band gap semiconductor material with good photocatalytic performance [40,54]. The wide band gap can make the photoinduced electrons and holes generated by MgTixOy be difficult to recombine, therefore, the photogenerated charge carriers can efficiently transfer in the process of the photocatalytic reactions. Furthermore, MgTixOy has a more negative conduction band potential than TiO2, which is beneficial to PECCP. Meanwhile, a well band alignment can be formed between TiO2 and MgTixOy, therefore, the TiO2/MgTixOy multiphase heterojunctions enable the photogenerated charge carriers to migrate directionally. In this way, more photogenerated electrons can be separated and participate in the PECCP reactions, thereby greatly improving the PECCP performance and stability of the TiO2/MgTixOy multiphase heterojunction system. In the present paper, a novel TiO2/MgTixOy multiphase-heterojunction film was prepared and reported for the first time, and its PECCP performance and stability for 304 SS were studied. SKP technique was used for the first time to study the significantly enhanced PECCP performance of the prepared TiO2/MgTixOy multiphase-heterojunction film. The lowest surface WF of the TiO2/MgTixOy multiphase-heterojunction film makes it be easier to escape the electrons, and higher amount of photogenerated electrons can be produced for protecting 304 SS under simulated solar light illumination. In addition, the establishment of multiphase heterojunctions makes the TiO2/ MgTixOy films have durable stability for the PECCP. This study further enriches the means of characterizing the photoelectrodes, and provides an important theoretical basis for understanding the promotion of the PECCP performance and the stability of the photoelectrodes.
2. Experimental section 2.1. Preparation of the photoelectrodes The reagents used in the experiments were all purchased from Sinopharm Chemical Reagent Co., Ltd without further purification. The anatase TiO2 nanotube arrays (NTAs) (TiO2(A)) photoelectrode was prepared by a two-step anodization method according to the previous reports [55,56]. A cleaned titanium (Ti) sheet was used as the anode and a Pt electrode was used as the cathode, which was placed in parallel in the electrolyte of ethylene glycol (0.35 wt% NH4F, 10 wt% H2O). A constant potential (60 V) was applied for anodic oxidation for 1 h at room temperature. After 10 min of ultrasonic cleaning in 10 wt% HCl solution, the obtained Ti sheet was rinsed with deionized water for several times. The above-mentioned process of anodic oxidation was repeated one more time. After that, the sample was put into deionized water and ultrasonically cleaned for 30 S. The sample was dried, and then was annealed at 450 °C for 3.5 h to prepare the TiO2(A) photoelectrode. The TiO2/MgTixOy photoelectrode was prepared as follows. The magnesium hydroxide was firstly deposited onto the surface of the TiO2(A) photoelectrode by a multi-potential step method using a CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). A three-electrode configuration was applied in the deposition process, in which the prepared TiO2(A) was used as the working electrode, the Pt electrode worked as the counter electrode, and the saturated calomel electrode (SCE) acted as the reference electrode, respectively. 0.1 M Mg(NO3)2 solution was used as the electrolyte. During the deposition process, the step potentials were −1.5 and −1.2 V, respectively, and the corresponding time was 5 s and 0.5 s. After 8 cycles of deposition, the obtained sample was washed with deionized water and alcohol. Subsequently, the prepared sample was thermally treated in a muffle furnace at 700 °C for 2 h to obtain the TiO2/MgTixOy photoelectrode. As can be well known, the phase transformation of TiO2 occurs during high temperature treatment (≥ 550 °C), resulting in the coexistence of anatase and rutile TiO2 (TiO2(R)) [57–59]. Therefore, as a comparison, the prepared TiO2(A) photoelectrode was also thermally treated in a muffle furnace at 700 °C for 2 h to obtain the TiO2(A)/TiO2(R) photoelectrode. 2.2. Characterization of the prepared photoelectrodes X-ray diffractometer (XRD, D/max-500, Rigaku Co., Tokyo, Japan) was used to characterize the crystal structures of the prepared photoelectrode. Fourier transform infrared (FT-IR) spectra were tested using a Fourier transform infrared spectroscopy (FT-IR, Thermo-Nicolet 8700, Thermo Electron Scientific Inc., USA) at room temperature. UV–vis diffuse reflectance spectrophotometer (U-41000; HITACHI, Tokyo, Japan) was used to analyze the optical absorption properties of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes. Photoluminescence (PL) spectra of the prepared photoelectrodes were measured with a fluorescence spectrometer (PL, Microconfocal Raman Spectrometer, Horiba Jobin Yvon LabRAM HR800, 325 nm, France). Field emission scanning electron microscopy (FE-SEM, ZEISS, ULTRA 55, Germany) was used to analyze the micromorphologies of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes. The elemental composition and mapping of the prepared photoelectrode were analyzed by an X-ray energy dispersive spectrometer (EDS, Oxford, UK). The microstructures of the prepared TiO2/MgTixOy photoelectrode and interfacial information of different phase components were observed by field emission transmission electron microscope (FE-TEM, Tecnai G2 F20, FEI Company, USA).
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2.3. Photoelectrochemical (PEC) performance, PECCP performance and SKP measurements
diffraction peaks in the XRD pattern of TiO2(A) are attributed to the characteristic ones of the Ti substrate [55,56]. For the XRD pattern of TiO2(A)/TiO2(R), the coexistence of anatase and rutile phases of TiO2 can be clearly observed after high temperature treatment at 700 °C. In addition to the diffraction peaks of Ti substrate and anatase TiO2, the corresponding rutile phases of TiO2 (JCPDS No. 21-1276) are observed at 27.6°, 36.1°, 41.2°, 44.1, 54.3°, 56.7°, 64.1°and 68.9°, which are assigned to the (110), (101), (111), (210), (211), (220), (310) and (301) crystal planes, respectively. For the XRD pattern of TiO2/MgTixOy, the weak characteristic peaks of MgTiO3 and MgTi2O5 can be observed. It may be due to the relatively small amount of MgTixOy deposited on the surface of TiO2 tube orifice. Although the intensities of the diffraction peaks are weak, it can be clearly seen that the diffraction peaks are located at 19.1°, 21.2°, 23.9°, 35.5°, 40.6° and 63.8°, respectively. These diffraction peaks correspond well to the (003), (101), (012), (104), (110) and (214) crystal planes of rhombohedral MgTiO3 (JCPDS no. 060494). In addition, the diffraction peaks at 32.7° and 48.6° are observed, which are assigned to the (230) and (331) crystal planes of orthorhombic structure of MgTi2O5 (JCPDS no. 35-0792). The XRD results suggest that MgTixOy was successfully modified on the surface of TiO2. Fig. 1B displays the FT-IR spectra of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy. The FT-IR spectra of TiO2(A) and TiO2(A)/TiO2(R) show a high degree of consistency and the broad band below 750 cm−1 is mainly from the stretching vibration mode of Ti-OTi in TiO2 [63,64]. No other characteristic peaks were observed, indicating the high purity of TiO2(A) and TiO2(A)/TiO2(R). For the FT-IR spectrum of TiO2/MgTixOy, another broad region at approximately 1400−1550 cm−1 can be observed, which comes from the Ti–carboxylic complexes and hydroxyl group [65–67]. The FT-IR results indicate that the carboxyl and hydroxyl groups can be easily adsorbed on the surface of TiO2(A)/TiO2(R), which facilitates the contact between the photoelectrode and the electrolyte solution. Fig. 2 shows the SEM images of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy, respectively. As shown in Fig. 2A, TiO2(A) exhibits a good NTA structure. The orifices are closely arranged with a diameter of approximately 80 nm. Fig. 2B shows the SEM image of the TiO2(A)/ TiO2(R) photoelectrode. The surface of TiO2(A)/TiO2(R) is much smoother than that of TiO2(A), which may be due to the formation of anatase and rutile TiO2 after the high temperature treatment. For the SEM image of TiO2/MgTixOy displayed in Fig. 2C, some nanoparticles obviously appear at the orifices and are well coated on the surface of the orifice of TiO2 NTAs. This may be the surface-modified MgTixOy nanoparticles. Fig. 2D shows the SEM image of the cross section of TiO2/MgTixOy. The height of the nanotubes can be measured as approximately 4 μm. The basic structure of the nanotubes has not been destroyed after high temperature treatment at 700 °C, and the tubular structure remains intact. Fig. 3A shows the SEM image and the corresponding EDS spectrum of TiO2(A). In addition to Ti and O elements, no other impurity elements are observed. The atomic mass percentages of Ti and O were
The light source used for the PEC and PECCP performance measurements is 150-W Xe lamp (PLSSXE300, Changtuo Co. Ltd., Beijing, China). A simulated solar illumination is obtained by adding an AM1.5 G filter and adjusting the light intensity of this light source to 100 mW⋅ cm−2. The PEC performance was tested in a three-electrode cell system using the CHI660D electrochemical workstation. The variations of the current densities of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes were measured using the prepared photoelectrode as the working electrode, the platinum electrode as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. The bias voltage was set as 0 V (vs Ag/AgCl) and the electrolyte was 0.1 M Na2SO4 solution. The variations in the current densities and the mixed potentials, and the polarization curves of the galvanic couple of the 304 SS electrode and the prepared photoelectrodes were measured to characterize the PECCP performance. The galvanic couple of the 304 SS electrode and the prepared photoelectrodes was used as the working electrode. The Ag/AgCl (saturated KCl) electrode and the platinum electrode served as the reference and counter electrode, respectively. Both the 304 SS electrode and the prepared photoelectrodes are placed in 3.5 wt% NaCl solution and the bias voltage was set as 0 V (vs Ag/ AgCl) during the testing process. Intermittent simulated solar light (100 mW, AM 1.5 G) was illuminated on the surface of the photoelectrodes. Electrochemical impedance spectroscopy (EIS) tests were performed at open circuit potential over the frequency range between 105 and 10-1 Hz, with an AC voltage magnitude of 5 mV. The polarization curves were measured using the CHI660D electrochemical workstation with a scan rate of 1 mV⋅s-1 from -400 to 400 mV (vs open circuit potential). The surface WFs of the prepared photoelectrodes were analyzed using SKP (VersaSCAN, Ametek). The tungsten probe with the diameter of 250 μm was acted as the reference and detection probe. The testing area on the surface of the prepared photoelectrodes was 1 × 1 mm2 and the scanning rate was set as 50 μm⋅s−1 with sensitivity of 500 μV. The surface WFs of the photoelectrodes can be calculated based on the following formula [60–62]: WF (Sample) = WF (Tungsten) + ΔW (photoelectrode)/1000. Among them, WF (Tungsten) is the WF of the reference electrode with standard value of 4.55 eV for tungsten; WF (Sample) is the surface WF for the prepared photoelectrode, ΔW (photoelectrode) is the surface potential of the photoelectrode obtained from the SKP measurement. 3. Results and discussion Fig. 1A shows the XRD patterns of the prepared TiO2(A), TiO2(A)/ TiO2(R) and TiO2/MgTixOy. For the XRD pattern of TiO2(A), the diffraction peaks at 25.5°, 37.9°, 48.4°, 54.2°and 55.3° are observed, corresponding to the (101), (004), (200), (105) and (211) crystal planes of standard anatase TiO2 (JCPDS No. 21-1272), respectively. The other
Fig. 1. XRD patterns and FT-IR spectra of the prepared TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy. 3
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Fig. 2. SEM images of (A) TiO2(A), (B) TiO2(A)/TiO2(R) and (C) TiO2/MgTixOy, and (D) SEM image of the cross section of TiO2/MgTixOy.
33.19 % and 66.81 %, which fits well with the element composition of TiO2. Fig. 3B shows the SEM image and the corresponding EDS results of TiO2/MgTixOy. Ti, O and Mg elements are clearly observed with the atomic percentages of 24.13 %, 67.91 % and 7.96 %, respectively. As shown in the corresponding EDS mapping results, Ti element covers the whole scanning area due to the Ti substrate. While, O and Mg elements correspond well to the SEM image shown in Fig. 3B. The tube orifice is clearly observed in the EDS Mg and O mappings, indicating that Mg and O elements are well dispersed on the surface of the TiO2 nanotube orifices. The difference between the SEM images in Fig. 3 and those in Fig. 2 is mainly due to the fact that EDS is tested under high excitation
voltages, which will break down the surface components and make the surface morphology be difficult to be observed. The composition and interfacial information of the TiO2/MgTixOy multiphase heterojunctions are further studied by HRTEM. Fig. 4A presents the TEM image observed at low magnification. The structure of nanotubes can be clearly observed, which corresponds well to the SEM image of TiO2/MgTixOy shown in Fig. 2. The edge of the nanotubes was further enlarged, as shown in Fig. 4B. The nanostructures with different lattice fringes are found and form the heterojunctions on the surface of the nanotube. The observation results are further magnified at the selected two rectangular areas. Fig. 4C corresponds to the rectangular
Fig. 3. SEM images and the corresponding EDS results of (A) TiO2(A) and (B) TiO2/MgTixOy. 4
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Fig. 4. TEM (A) and HRTEM (B, C, D) images of TiO2/MgTixOy multiphase heterojunctions.
area I. The lattice fringes of TiO2(R), MgTiO3 and MgTi2O5 can be observed with the values of 0.29, 0.27 and 0.33 nm, respectively. Furthermore, the lattice fringes of TiO2(A), TiO2(R), MgTiO3 and MgTi2O5 can be clearly observed corresponding to the rectangular area II, as shown in Fig. 4D. Combining with the physical characterization results shown in Figs. 1–4, it can be proved that MgTixOy is successfully coated on the surface of TiO2 NTAs, and the multiphase heterojunctions are formed. Fig. 5 shows the UV–vis absorption spectra of the TiO2(A), TiO2(A)/ TiO2(R) and TiO2/MgTixOy photoelectrodes. As can be found, the light absorption threshold of TiO2(A) is observed at 382 nm. For the TiO2(A)/TiO2(R) photoelectrode, the light absorption threshold has a slight red shift compared with that of TiO2(A), which may be caused by the formation of TiO2(R) with a narrower band gap. In addition to having the same absorption threshold as TiO2(A)/TiO2(R), the TiO2/ MgTixOy photoelectrode has an additional absorption threshold at approximately 330 nm, indicating the successful synthesis of MgTixOy on the surface of TiO2 NTAs.
The PECCP performance of the prepared photoelectrodes are characterized by measuring the photoinduced current densities and the photoinduced potential drops, and the results are shown in Fig. 6. Fig. 6A shows the variations in the galvanic current densities between the 304 SS electrode and the prepared photoelectrodes in 3.5 wt% NaCl solution under intermittent simulated solar light illumination. Positive excitation current densities are obtained under light illumination, indicating that the photoinduced electrons generated by the photoelectrodes transfer to the coupled 304 SS electrode and provide the PECCP for it. The photoinduced current density of a prepared photoelectrode, which is provided for protecting the coupled metallic electrode, is obtained by subtracting the stable galvanic current density between the protected metal electrode and the photoelectrode in the dark from that under light illumination. As shown in Fig. 6A, the photoinduced current density of TiO2(A) is 9 μA·cm−2 and that of TiO2(A)/TiO2(R) is 23 μA·cm−2. While, that of TiO2/MgTixOy is 49 μA·cm−2, which is 5.4 and 2.1 times of that of TiO2(A) and TiO2(A)/TiO2(R). The results shown in Fig. 6A indicate that TiO2/MgTixOy can produce the most photogenerated electrons and can provide the most electrons needed for the cathodic protection of the coupled 304 SS. Fig. 6B shows the variations of the mixed potential of the 304 SS electrode coupled with the prepared photoelectrodes in 3.5 wt% NaCl solution under intermittent simulated solar light illumination. The mixed potentials of the 304 SS electrode coupled with the prepared photoelectrodes immediately shift to negative direction once the light is switched on, demonstrating that the prepared photoelectrodes can provide the PECCP for the coupled 304 SS electrode. The photoinduced mixed potential drop is the mixed potential of the 304 SS electrode coupled with the prepared photoelectrode under light illumination minus that in the dark. As shown in Fig. 6B, the photoinduced mixed potential drops of TiO2(A), TiO2(A)/ TiO2(R) and TiO2/MgTixOy are -230, -250 and −320 mV, respectively. The TiO2/MgTixOy exhibits the maximum photoinduced mixed potential drop, indicating its excellent PECCP performance. The photoinduced mixed potential drop results shown in Fig. 6B are similar to the photoinduced current density results shown in Fig. 6A, and both of them prove that TiO2/MgTixOy has the best PECCP performance. Meanwhile, in Fig. 6B, the stability of the prepared photoelectrodes is evaluated by measuring the potential variations during the long duration of light illumination. As shown in Fig. 6B, the mixed potential of the 304 SS electrode coupled with the prepared photoelectrodes is
Fig. 5. UV–vis absorption spectra of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/ MgTixOy photoelectrodes. 5
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Fig. 6. (A) The variations in the galvanic current densities between the 304 SS electrode and the prepared photoelectrodes and (B) the variation of the mixed potentials of the galvanic couple of the 304 SS electrode and the prepared photoelectrodes in 3.5 wt% NaCl solution under intermittent simulated solar illumination.
stabilized for 900 s in the dark. Subsequently, the light is switched on and illuminated for 2 h on the photoelectrodes. The mixed potentials of the 304 SS electrode coupled with the TiO2(A) and TiO2(A)/TiO2(R) photoelectrodes change from -0.37 and -0.42 V to -0.33 and -0.39 V, showing a slight decrease in their PECCP performance during the light illumination. However, for TiO2/MgTixOy, the mixed potential of the 304 SS electrode coupled with the TiO2/MgTixOy photoelectrode does not have significant change during the 2-h light illumination. The results shown in Fig. 6B indicate that there is no obvious attenuation of the PECCP performance of the TiO2/MgTixOy photoelectrode during the long duration of light illumination, demonstrating that the TiO2/ MgTixOy photoelectrode possesses high stability and has good application prospects in the field of PECCP. The polarization curves of 304 SS electrode and 304 SS electrode coupled with the TiO2, TiO2(A)/TiO2 and TiO2/MgTixOy photoelectrodes in the absence and presence of simulated solar light illumination are shown in Fig. 7. The corrosion potential of the 304 SS was about −154 mV (vs. Ag/AgCl) before coupling with the prepared photoelectrodes in 3.5 wt% NaCl solution, however, those of 304 SS positively shift to -102, -101 and −113 mV after coupling with TiO2, TiO2(A)/TiO2 and TiO2/MgTixOy photoelectrodes in the dark, respectively. Under simulated solar light illumination, significant negative potential shifts are obtained for 304 SS coupling with different photoelectrodes, indicating the good PECCP performance of the prepared photoelectrodes. Under the simulated solar light illumination, the corrosion potentials of 304 SS coupling with TiO2, TiO2(A)/TiO2 and TiO2/ MgTixOy are -353, -426 and −486 mV, respectively. These results are in
good agreement with the previous PECCP tests shown in Fig. 6, which further indicates the excellent PECCP performance of the TiO2/MgTixOy photoelectrode. SKP technology is used to further characterize the capability of the electrons escaping from the surface of the prepared photoelectrodes. The surface WF, which refers to the minimum energy required to escape an electron from the surface of the photoelectrode, can be easily calculated from the surface potential of the photoelectrodes measured by SKP [68,69]. The smaller the WF is, the easier it is for electron to escape, i.e. for electrons to flow out of the photoelectrode and participate in the reactions [70]. Fig. 8A displays the surface potential distributions of different photoelectrodes measured by SKP. As can be found in Fig. 8A, the potential fluctuation of the prepared individual photoelectrode is smaller than ± 50 mV around the median value, suggesting that the surface of which is fairly flat and uniform. By comparing the surface potentials of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes, TiO2(A) has the most positive surface potential distribution with an average value of approximately 990 mV. TiO2(A)/ TiO2(R) has a surface potential of approximately 830 mV. While, TiO2/ MgTixOy has the most negative potential of approximately 690 mV, which is 300 and 140 mV lower than those of TiO2(A) and TiO2(A)/ TiO2(R), respectively. Fig. 8B shows the surface WFs of the prepared photoelectrodes obtained from the results shown in Fig. 8A. The surface WFs of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes are 5.54, 5.38 and 5.24 eV, respectively. The lowest surface potential distribution and surface WF of TiO2/MgTixOy indicate that the multiphase heterojunctions can easily excite the electrons and transfer them to the surface of 304 SS to provide effective protection for it. Fig. 9A shows the variations in the current densities of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes in 0.1 M Na2SO4 solution under intermittent simulated solar illumination. As shown in Fig. 9A, all of the prepared photoelectrodes shows positive excitation current densities under light illumination. The photoinduced current density of the TiO2(A) photoelectrode is 0.12 mA⋅ cm−2, and that of the TiO2(A)/TiO2(R) is 0.16 mA⋅ cm−2. However, the photoinduced current density of the TiO2/MgTixOy photoelectrode is 0.29 mA⋅ cm−2, which is approximately 2.4 times and 1.8 times of the TiO2(A) and TiO2(A)/ TiO2(R) photoelectrodes, respectively. The generation of photoinduced current is a key factor that affects the PECCP performance of the photoelectrode. The TiO2/MgTixOy photoelectrode has the highest photoinduced current density, indicating that it has the best PECCP performance. The electrochemical impedance spectroscopy and the photoluminescence spectroscopy of the prepared photoelectrodes were measured to evaluate the photogenerated charge carrier migration ability and the recombination ability of the photogenerated electrons and holes. The relevant results are shown in Figs. 9B and 9C. As shown in Fig. 9B, the TiO2/MgTixOy photoelectrode has the smallest surface resistance than TiO2(A) and TiO2(A)/TiO2(R), indicating that the TiO2/ MgTixOy has the fastest electron mobility. In addition, as shown in
Fig. 7. The polarization curves of the 304 SS electrode and the 304 SS electrode coupled with the TiO2, TiO2(A)/TiO2 and TiO2/MgTixOy photoelectrodes in the absence and presence of simulated solar light illumination. 6
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Fig. 8. (A) The surface potential distributions of TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes measured by SKP technique; and (B) the surface WFs of the prepared photoelectrodes transformed from the SKP results in Fig. 8A.
Fig. 9. (A) The variations in the current densities of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes under intermittent light illumination, (B) Electrochemical impedance spectra (EIS) and (C) photoluminescence spectra of the TiO2(A), TiO2(A)/TiO2(R) and TiO2/MgTixOy photoelectrodes.
Fig. 10. SEM image of the TiO2/MgTixOy photoelectrode after the PECCP test and the XRD and FT-IR spectra of the TiO2/MgTixOy photoelectrode before and after the PECCP tests.
Fig. 11. The proposed transfer mechanism of the photogenerated charge carriers and the schematic diagram of the PECCP of the TiO2/MgTixOy multiphase heterojunctions. 7
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Fig. 9C, the photoluminescence intensity of TiO2/MgTixOy has a significant decrease compared with that of TiO2(A) and TiO2(A)/TiO2(R), indicating that the secondary recombination of the photogenerated electrons and hole is significantly inhibited for the TiO2/MgTixOy photoelectrode. These results display that the TiO2/MgTixOy photoelectrode can realize the fast transmission of the photogenerated charge carriers, can effectively inhibit the secondary recombination of the photogenerated electrons and holes, and can generate the highest photoinduced current compared with the TiO2(A) and TiO2(A)/TiO2(R) photoelectrodes, therefore, achieve the excellent PECCP performance of the TiO2/MgTixOy photoelectrode. The SEM image of TiO2/MgTixOy after the PECCP tests and the XRD and FT-IR analyses of the TiO2/MgTixOy photoelectrode before and after the PECCP tests are shown in Fig. 10. As shown in Fig. 10A, the micromorphology of TiO2/MgTixOy does not have significant changes after the PECCP tests. Furthermore, the XRD and FT-IR curves of the TiO2/MgTixOy photoelectrode after PECCP tests are highly consistent with those before PECCP tests, as shown in Fig. 10B and C. These results demonstrate that TiO2/MgTixOy photoelectrode has a high stability in the process of PECCP. Fig. 11 shows the proposed transfer processes of the photogenerated charge carriers in the TiO2/MgTixOy multiphase heterojunctions and its PECCP mechanism. The TiO2/MgTixOy photoelectrode has significantly enhanced photogenerated current density and photoinduced mixed potential drop, indicating that the energy band structures of TiO2 and MgTixOy match well and the TiO2/MgTixOy multiphase heterojunctions with a good energy band alignment gradient are formed. The photogenerated charge carriers can migrate directionally under the action of the multiphase heterojunctions, making the photogenerated charge carriers be well transferred. It was reported that the band gaps of MgTiO3, MgTi2O5, TiO2(R) and TiO2(A) are 3.7 eV, 3.4 eV, 3.0 eV and 3.2 eV, respectively, and the conduction band potentials of them are arranged in the following order: MgTiO3 < MgTi2O5 < TiO2(R) < TiO2(A) [40,54,71,72]. The photogenerated electrons can be transferred to TiO2(A) through the conduction band gradient formed by the TiO2/MgTixOy multiphase heterojunctions, and eventually be transferred to 304 SS through the Ti substrate to provide effective protection for it. Meanwhile, the photogenerated holes will be further transferred to the TiO2/MgTixOy multiphase heterojunctions under the action of the heterojunction electric field, and the corresponding oxidation reactions will occur with the electrolyte. Furthermore, the fabrication of the TiO2/MgTixOy multiphase heterojunctions greatly increases the separation efficiency of the photogenerated charge carriers and effectively suppresses the recombination of the photogenerated electrons and holes. In this way, a large number of photogenerated electrons will continuously flow from the TiO2/MgTixOy photoelectrode to the coupled 304 SS electrode under light illumination, thus significantly enhancing the long-term PECCP performance.
benefit to the improvement of the PECCP performance of the TiO2/ MgTixOy multiphase heterojunctions. Furthermore, the good PECCP performance of the TiO2/MgTixOy multiphase heterojunctions is attributed to the establishment of multiphase heterojunction system, which effectively inhibits the secondary recombination of the photogenerated electrons and holes, and accelerates the migration of the photogenerated charge carriers. SKP technology has been used for the first time to study the PECCP performance of the TiO2/MgTixOy multiphase-heterojunction film. The introduction of SKP technology into the PECCP research has greatly enriched the test methods in the field of PECCP. At the same time, the establishment of multiphase heterojunctions provides a new idea for the design of the PECCP film. Data availability All data included in this study are available upon request by contact with the corresponding authors. Declaration of Competing Interest None. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 41576114, 41676069), Key Research and Development Program of Shandong Province (Grant Nos. 2019GHY112066, 2019GHY112085), State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, China (Project No. 614290101011703), and Qingdao Innovative Leading Talent Foundation (Grant No. 15-10-3-15-(39)-zch). References [1] K. Sivula, R. Van De Krol, Semiconducting materials for photoelectrochemical energy conversion, Nat. Rev. Mater. 1 (2016) 15010. [2] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. [3] M.M. Momeni, M. Mahvari, Y. Ghayeb, Photoelectrochemical properties of ironcobalt WTiO2 nanotube photoanodes for water splitting and photocathodic protection of stainless steel, J. Electroanal. Chem. 832 (2019) 7–23. [4] S. Chandrasekaran, L. Yao, L. Deng, C. Bowen, Y. Zhang, S. Chen, Z. Lin, F. Peng, P. Zhang, Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond, Chem. Soc. Rev. 48 (2019) 4178–4280. [5] M.M. Momeni, M. Taghinejad, Y. Ghayeb, R. Bagheri, Z. Song, Preparation of various boron-doped TiO2 nanostructures by in situ anodizing method and investigation of their photoelectrochemical and photocathodic protection properties, J. Iran. Chem. Soc. 16 (2019) 1839–1851. [6] Y. Zhang, Y. Li, D. Ni, Z. Chen, X. Wang, Y. Bu, J.P. Ao, Improvement of BiVO4 photoanode performance during water photo‐oxidation using Rh‐Doped SrTiO3 perovskite as a Co‐catalyst, Adv. Funct. Mater. 29 (2019) 1902101. [7] M.M. Momeni, S.H. Khansari-Zadeh, H. Farrokhpour, Fabrication of tungsten-irondoped TiO2 nanotubes via anodization: new photoelectrodes for photoelectrochemical cathodic protection under visible light, SN Applied Sciences 1 (2019) 1160. [8] M.M. Momeni, Y. Ghayeb, N. Moosavi, Preparation of Ni–Pt/Fe–TiO2 nanotube films for photoelectrochemical cathodic protection of 403 stainless steel, Nanotechnology 29 (2018) 425701. [9] Y. Bu, J.P. Ao, A review on photoelectrochemical cathodic protection semiconductor thin films for metals, Green Energy Environ. 2 (2017) 331–362. [10] J. Jing, M. Sun, Z. Chen, J. Li, F. Xu, L. Xu, Enhanced photoelectrochemical cathodic protection performance of the secondary reduced graphene oxide modified graphitic carbon nitride, J. Electrochem. Soc. 164 (2017) C822–C830. [11] J. Li, C.J. Lin, C.G. Lin, A photoelectrochemical study of highly ordered TiO2 nanotube arrays as the photoanodes for cathodic protection of 304 stainless steel, J. Electrochem. Soc. 158 (2011) C55–C62. [12] C. Lei, H. Zhou, Z. Feng, Y. Zhu, R. Du, Low-temperature liquid phase deposited TiO2 films on stainless steel for photogenerated cathodic protection applications, Appl. Surf. Sci. 257 (2011) 7330–7334. [13] Y. Yang, Y.F. Cheng, One-step facile preparation of ZnO nanorods as high-performance photoanodes for photoelectrochemical cathodic protection, Electrochim. Acta 276 (2018) 311–318. [14] M. Sun, Z. Chen, Y. Bu, J. Yu, B. Hou, Effect of ZnO on the corrosion of zinc, Q235 carbon steel and 304 stainless steel under white light illumination, Corros. Sci. 82 (2014) 77–84.
4. Conclusions In the present paper, a highly efficient and stable TiO2/MgTixOy multiphase-heterojunction film was prepared and its PECCP performance for 304 SS was studied. MgTixOy are uniformly dispersed on the surface of TiO2 NTAs. MgTiO3, MgTi2O5, TiO2(R) and TiO2(A) contact well and form a multiphase-heterojunction system. The photoinduced current density of the TiO2/MgTixOy multiphase-heterojunction film is 49 μA·cm−2 under the simulated solar light illumination, which is 5.4 and 2.1 times of that of TiO2(A) and TiO2(A)/TiO2(R). The photoinduced mixed potential drops of the TiO2/MgTixOy multiphase-heterojunction film is −320 mV, while, those of TiO2(A) and TiO2(A)/ TiO2(R) are -230 and -250 mV, respectively. Meanwhile, the TiO2/ MgTixOy multiphase-heterojunction film exhibits extremely high PECCP stability. During the 2 h of light illumination, the PECCP of the TiO2/MgTixOy multiphase-heterojunction film shows no weakening trend. The lower surface WF makes the TiO2/MgTixOy multiphaseheterojunction film be much easier to escape electrons, which is of great 8
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