Journal of Materials Science & Technology  2020 , 37 (0): 55-63 https://doi.org/10.1016/j.jmst.2019.07.034

Research Article

Synergetic effect of graphene and Co(OH)2 as cocatalysts of TiO2 nanotubes for enhanced photogenerated cathodic protection

Xiayu Luab, Li Liuac*, Xuan Xieac, Yu Cuia, Emeka E. Oguziecd, Fuhui Wangac

a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China
b School of Materials Science and Engineering, University of Science and Technology of China, Shenyang, 110016, China
c Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China
d Electrochemistry and Materials Science Research Laboratory, Department of Chemistry, Federal University of Technology, P.M.B. 1526, Owerri, Nigeria

Corresponding authors:   ∗Corresponding author at: Key Laboratory for Anisotropy and Texture of Materials(Ministry of Education), School of Materials Science and Engineering, NortheasternUniversity, Shenyang, 110819, China.E-mail address: liuli@mail.neu.edu.cn (L. Liu).

Received: 2019-05-14

Revised:  2019-06-26

Accepted:  2019-07-8

Online:  2020-01-15

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

A layer of graphene (GR) particles was successfully deposited at the interface between Co(OH)2 nanoparticles and TiO2 nanotubes, aiming to improve the photoelectrochemical performance of the large-bandgap semiconductor TiO2. The obtained Co(OH)2/GR/TiO2 was extensively characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis absorption spectra and photoluminescence (PL) emission spectra. Electrochemical impedance spectra, photogenerated potential-time (E-t) photocurrent density-time (i-t) and i-E curves and open circuit potential (OCP) curves were measured to investigate the photoelectrochemical activities and photogenerated cathodic protection properties. The results revealed that Co(OH)2/GR/TiO2 exhibits excellent photoelectrochemical and photogenerated cathodic performance due to synergistic effect between Co(OH)2 and graphene. Co(OH)2 and graphene co-modified TiO2 photoanode could provide an effective protection for 304 stainless steel (304SS) in 3.5 wt% NaCl solution for 12 h, which would be promising for future practical applications in the field of marine corrosion protection.

Keywords: TiO2 nanotubes ; Graphene ; Co(OH)2 nanoparticles ; Photoelectrochemical performance ; Photogenerated cathodic protection

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Xiayu Lu, Li Liu, Xuan Xie, Yu Cui, Emeka E. Oguzie, Fuhui Wang. Synergetic effect of graphene and Co(OH)2 as cocatalysts of TiO2 nanotubes for enhanced photogenerated cathodic protection[J]. Journal of Materials Science & Technology, 2020, 37(0): 55-63 https://doi.org/10.1016/j.jmst.2019.07.034

1. Introduction

Steel materials used in marine environments are vulnerable to corrosion, especially localized corrosion due to the presence of chloride in such environments. Many methods have been developed to protect steel from corrosion in marine environments, including coatings and cathodic protection using sacrificial anodes [[1],[2]]. Since Tsujikawa and Fujisawa [3] first discovered that TiO2 could be used as a kind of photogenerated cathodic protection material to protect steel materials, photogenerated cathodic protection method has attracted considerable attention because of its energy conservation and environmentally friendly features [[3], [4], [5]].

Nanostructured TiO2 has continued to arouse enormous interests because of its efficient photoactivity, low cost, low toxicity, high safety and thermal stability [[6], [7], [8], [9], [10]]. In particular, TiO2 nanotubes have been found wide applications in several fields due to their unique one-dimensional architecture and specific physicochemical properties like enlarged surface area and superior chemical stability [[4],[11], [12], [13]]. Despite these remarkable attributes of the nanotube structure, the intrinsic large band gap of TiO2 (e.g. 3.2 eV for anatase) restricts its visible light absorption capacity, which significantly depresses its solar power utilization ratio. Efforts have been made to improve the photocatalytic activity of TiO2 nanotubes through such means as metal or non-metal dopings [[11],[14], [15], [16], [17]] and semiconductor/semiconductor coupling [[18], [19], [20], [21], [22], [23], [24], [25], [26]].

Cobalt-based materials have been widely researched in the photocatalysis field because of their low cost, high stability, and high catalytic performances [[27], [28], [29]]. Co(OH)2, for instance, has been reported to effectively capture photogenerated holes, as well as enhance the surface reaction kinetics [[30],[31]]. Such attributes can improve the photocatalytic activity of TiO2 nanotubes very well. However, to date, reports on Co(OH)2-modified TiO2 nanotubes as photoanodes in photogenerated cathodic protection are very limited in the literature. Recently, studies in our group [25] have revealed that Co(OH)2-modified TiO2 photoanode can provide effective protection for 304SS after 40 d immersion in 3.5 wt% NaCl solution. However, photogenerated electrons cannot be derived effectively in this system, thereby hindering the catalytic efficiency. Hence, it is necessary to provide a suitable channel for electrons transfer.

Graphene possesses certain features like large surface area, high electrical conductivity, excellent chemical stability and outstanding mechanical flexibility [[32],[33]], which make it a material of choice in the field of photocatalysis [[34],[35]] and photogenerated cathodic protection [[36],[37]]. Indeed, graphene possesses multiple electron channels and photocatalytic reaction centers that could possibly accelerate electron transfer from TiO2 to the bulk solution and further slow the recombination of photogenerated electrons and holes. Graphene could therefore be applied as a co-catalyst for TiO2 to accelerate the electrons transfer process in photogenerated cathodic protection system. To the best of our knowledge, this co-modification approach of graphene and Co(OH)2 on TiO2 photocatalytic materials has not been reported in the literature.

In this work, Co(OH)2 and graphene co-modified TiO2 photoanode material was designed and obtained by depositing a layer of graphene particles on the surface of TiO2 nanotubes, followed by deposition of Co(OH)2 nanoparticles. Surface morphology, crystalline structure, optical properties and photoelectrochemical performance of the Co(OH)2/GR/TiO2 photoanodes were comparatively investigated. In particular, the photoelectrochemical performance of the Co(OH)2/GR/TiO2 photoanode, as well as the cathodic protection performance coupled with 304SS, was also carefully investigated and analyzed in order to deduce the process mechanisms.

2. Experimental

2.1. Chemicals and materials

The Ti foil (99.5%, 0.25 mm thickness) was purchased from Alfa Aesar Chemicals Co., Ltd, (China). The 304SS was obtained from Shanghai Baosteel. Graphene quantum dots were supplied by Nanjing XF NANO Materials Tech Co., Ltd. All reagents used in this study were analytical grade. Deionized water was used for preparing of all aqueous solutions.

2.2. Synthesis procedures

2.2.1. Preparation of TiO2 nanotubes and 304SS electrode

The highly ordered TiO2 nanotubes film on the surface of a Ti foil substrate was synthesized by a traditional anodic oxidation method [38].

The 304SS was cut into test specimens of about 5 mm × 5 mm × 5 mm and ground with 400, 800, 1000 grit wet SiC paper. The specimens were then connected to copper wire, implanted in epoxy resin to obtain the working electrodes with exposed area for testing 5 mm × 5 mm. The electrodes were again polished with 400, 800, 1000 grit wet SiC paper after the epoxy had dried.

2.2.2. Preparation of GR-surface modified TiO2 films

The potentiostatic electrodeposition method was used to deposit graphene particles on the surface of TiO2 nanotubes [39]. Before deposition, graphene quantum dots aqueous solution was sonicated at 700 W for 30 min and tuned pH by adding 1 M NaOH solution. Afterwards, the electrodeposition was conducted at a constant potential of 6 V for 2 h in a two-electrode system, with a Pt foil as counter electrode and the as-obtained TiO2 as working electrode. The as-prepared sample was cleaned with deionized water several times to remove residual surface ions.

2.2.3. Preparation of Co(OH)2/GR-surface modified TiO2 films

Co(OH)2 nanoparticles were deposited on TiO2 and GR/TiO2 nanotubes films by successive ionic layer adsorption and reaction (SILAR) method [40]. The TiO2 and GR/TiO2 films were successively immersed in an aqueous solution of Co(CH3COO)2, deionized water, an aqueous solution of NaOH and deionized water for several cycles. The procedure for the preparation of Co(OH)2/GR/TiO2 nanotubes films is schematically illustrated in Fig. 1.

Fig. 1.   Schematic illustration of preparation of Co(OH)2/GR/TiO2 nanotubes films.

2.3. Characterization techniques

The crystal structure of the samples was evaluated by X-ray diffraction measurements (XRD, X’pert PRO, Panalytical, Netherlands) using a CuKα radiation at 40 kV and 40 mA, with 2θ ranging from 10° to 90°. The surface morphology, length and diameter of the TiO2 nanotubes were examined with a scanning electron microscopy (SEM, INSPECT F50, FEI, USA). The microstructure was studied via a transmission electron microscopy (TEM, JEM-2100 F, JEOL, Tpkyo, Japan) with an acceleration voltage of 200 kV. The surface compositions and chemical states were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB250, Thermo VG, USA) with AlKα radiation and C 1s peak (284.6 eV) as a reference. Raman spectrum was tested by a Raman spectroscopy (JY HR800) with a 633 nm He-Ne laser as excitation source. The light absorption properties were characterized by a diffuse reflectance UV-vis spectrometer (V-770, JASCO, Japan) with a wavelength ranging from 200 m to 900 nm. The fluorescence properties were investigated by a fluorescence spectrophotometer (FLSP-920, Edinburgh Instruments, Britain) under the ultraviolet excitation of 320 nm.

2.4. Photoelectrochemical measurements

All photoelectrochemical tests were performed on a potentiostat Autolab PGSTAT302 N (Metrohm Autolab, The Netherlands). A three-electrode system was used to measure electrochemical impedance spectra, photogenerated potential-time (E-t) photocurrent density-time (i-t) and i-E curves using a Pt foil as counter electrode and a KCl-saturated silver/silver chloride electrode (Ag/AgCl) as reference electrode. Four different test specimens including TiO2, Co(OH)2/TiO2, GR/TiO2 and Co(OH)2/GR/TiO2 were used as working electrodes. For cathodic protection experiments, TiO2 photoanode and the 304SS were connected as the working electrode. All measurements began after a stable OCP was observed. The light source was a 300 W Xe lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.) with power energy density of 100 mW·cm-2. Electrochemical impedance spectra (EIS) tests of the TiO2 specimens were carried out with an AC signal of 10 mV in the frequency range of 100 kHz to 10 mHz. ZSimpWin3.50 software was used to analyze EIS data. The photogenerated i-E characteristics were recorded by scanning the potential from -0.8 to 0.4 V (vs. Ag/AgCl) with a scan rate of 0.02 V s-1.

3. Results and discussion

3.1. Phase structure

Raman spectrum of the graphene quantum dots is shown in Fig. 2. Three peaks at around 1329, 1577 and 2666 cm-1 could be clearly observed, which correspond to the D, G and 2D bands, respectively [[16],[41],[42]]. The characteristic 2D peak of graphene appears in the Raman spectrum, which indicates that it is graphene rather than graphene oxide.

Fig. 2.   Raman spectrum of the graphene quantum dots.

Fig. 3 shows the XRD patterns of TiO2, Co(OH)2/TiO2, GR/TiO2, Co(OH)2/GR/TiO2 and GR, respectively. All TiO2 specimens show the characteristic (101) (004) (200) (105) (204) (215) crystal planes of anatase TiO2 (JCPDS card No. 21-1272). The main characteristic (002) diffraction peak of graphene (JCPDS card No. 41-1487) is detected at around 26°. Interestingly, the graphene peak is not found in the XRD patterns of GR/TiO2 and Co(OH)2/GR/TiO2, probably due to overlap with the (101) diffraction peak of anatase TiO2. The absence of typical diffraction peaks of Co(OH)2 in Co(OH)2/TiO2 and Co(OH)2/GR/TiO2 is mainly due to its small loading amount.

Fig. 3.   XRD patterns of (a) TiO2, (b) Co(OH)2/TiO2, (c) GR/TiO2, (d) Co(OH)2/GR/TiO2 and (e) GR.

3.2. SEM images

SEM images of the prepared TiO2 and GR/TiO2 specimens are shown in Fig. 4. The anatase TiO2 specimen prepared by anodization is comprised of well-ordered and uniformed tubular structure. This kind of nanotube structure can provide larger surface area, more reaction sites and show a better photogenerated cathodic protection performance. The average inner diameter of the unmodified TiO2 nanotubes is approximately 100 nm (Fig. 4(a)), and the length is about 16.8 μm (Fig. 4(b)), which is large enough to accommodate the graphene and Co(OH)2 nanoparticles. After electrodeposition of graphene on TiO2, continuous and uniform graphene particles are successfully deposited on the surface of the TiO2 nanotubes without blocking the pores (Fig. 4(c)), which can obviously improve the mobility of the photogenerated carriers.

Fig. 4.   SEM images of (a) top view, (b) cross-section view of TiO2 nanotubes and (c) top view of GR/TiO2.

3.3. TEM observation

TEM images of the prepared TiO2 and Co(OH)2/GR/TiO2 nanotubes are shown in Fig. 5(a) and (b). The inner diameter of the TiO2 nanotubes is about 100 nm (Fig. 5(a)), which is consistent with the SEM results. Moreover, the wall of the unmodified TiO2 is very straight. Fig. 5(b) shows the morphology of Co(OH)2 and graphene co-modified TiO2 nanotubes, which is different from that of unmodified TiO2 nanotubes. The tube wall of Co(OH)2/GR/TiO2 nanotubes is not smooth and exhibits a wavy margin and rippled surface (Fig. 5(b)). STEM image of Co(OH)2/GR/TiO2 nanotubes and corresponding EDS mapping images are shown in Fig. 5(c)-(f). Ti and O are two basic compositions of TiO2 nanotubes. Thus, Ti and O elements converge at the tube wall of the nanotubes so that a clear shape of the nanotubes can be observed in Fig. 5(d) and (e). In addition, weak signals of Co element on the surfaces of the nanotubes, revealing that Co(OH)2 is deposited on the surface of TiO2.

Fig. 5.   TEM images of (a) TiO2, (b) Co(OH)2/GR/TiO2 and (c) STEM image of Co(OH)2/GR/TiO2, corresponding (d) Ti element, (e) O element and (f) Co element EDS mapping images.

3.4. XPS analysis

The chemical composition of the specimens has been studied by XPS. Fig. 6(a) shows the Ti 2p spectra (including Ti 2p1/2 and Ti 2p3/2 peaks) of TiO2, GR/TiO2, and Co(OH)2/GR/TiO2 nanotubes films. The binding energies of 464.2 eV and 458.5 eV are caused by Ti4+ in unmodified TiO2. After the modification by graphene, the Ti 2p1/2 and Ti 2p3/2 peaks change to 464.6 eV and 458.9 eV, respectively. Ti-O bonding and the surface absorbed O-H bonding [[35],[41],[43]] also show a positive shift after the modification by graphene (Fig. 6(b)). It is clear that the chemical state of Ti has been changed significantly due to the presence of graphene. Compared with GR/TiO2, the Ti 2p and O 1s spectra of Co(OH)2/GR/TiO2 do not change significantly. The C 1s spectrum of the GR/TiO2 is deconvoluted into three strong peaks at 284.6, 287.5 and 289.0 eV (Fig. 6(c)), which correspond to the C=C (C-C), C-O and O = C-OH functional groups [[44], [45], [46]], respectively. This suggests that graphene is successfully deposited onto the surface of the TiO2 nanotubes by electrodeposition method. The Co 2p spectrum of Co(OH)2/GR/TiO2 (Fig. 6(d)) indicates that Co(OH)2 is also deposited onto the surface of TiO2 successfully.

Fig. 6.   XPS spectra of (a) Ti 2p and (b) O 1s peaks of TiO2, GR/TiO2 and Co(OH)2/GR/TiO2, (c) C 1s peak of GR/TiO2 and (d) Co 2p peak of Co(OH)2/GR/TiO2.

3.5. UV-vis absorption spectra

The optical absorption properties of the TiO2 specimens were investigated by UV-vis absorption spectra (Fig. 7). Unmodified TiO2 shows the characteristic spectrum with its fundamental absorption sharp edge rising at 396 nm, corresponding to band gap energy of 3.13 eV. GR/TiO2 displays almost the same absorption edge as that of unmodified TiO2 nanotubes, indicating that the deposition of graphene does not change the band gap of TiO2 [[47],[48]]. However, GR/TiO2 exhibits a broad background absorption in the visible light region due to the addition of graphene, which can be seen clearly in the visible absorption spectra (Fig. 7(b)). In addition, the absorption peak appears in the visible light region may be attributed to the small quantity of surface oxygen-containing groups like -COOH and -OH or the interaction between TiO2 and graphene. Because of the presence of Co(OH)2, both Co(OH)2/TiO2 and Co(OH)2/GR/TiO2 have slight red shifts of absorption edge at 445 nm, corresponding to band gap energy of 2.79 eV. Moreover, the Co(OH)2/GR/TiO2 shows the largest enhanced visible light absorption performance due to the synergistic effect of graphene and Co(OH)2.

Fig. 7.   UV-vis (a) and visible (b) absorption spectra of the TiO2, GR/TiO2, Co(OH)2/TiO2 and Co(OH)2/GR/TiO2.

3.6. PL emission spectra

The PL signal, originating from the process of the recombination of photogenerated electrons and holes, can indirectly reflect the separation, migration and transmission of photogenerated electrons and holes in semiconductors. Therefore, in order to gain insights on the recombination and separation of photogenerated electrons and holes of the TiO2 nanotubes, PL spectra (Fig. 8) measurements were carried out for TiO2, GR/TiO2, Co(OH)2/TiO2 and Co(OH)2/GR/TiO2, respectively. Two rather strong PL emission peaks at ∼ 434 nm and 469 nm and a relative low peak at 563 nm are detected, which can be attributed to oxygen-related defects like oxygen vacancies on the surface area of the nanotubes [49]. The TiO2 nanotubes show the highest fluorescence intensity which means the highest recombination rate of photogenerated electrons and holes. The intensity of the two main peaks decreases sharply after modified by graphene and Co(OH)2, and a slight blue shift is also observed. Moreover, Co(OH)2/GR/TiO2 film shows the lowest fluorescence intensity, which suggests the lowest recombination of the photogenerated electrons and holes. Therefore, the co-modification of TiO2 nanotubes by graphene and Co(OH)2 can significantly promote the separation of photogenerated electrons and holes and enhance photocatalytic efficiency.

Fig. 8.   PL spectra of TiO2, GR/TiO2, Co(OH)2/TiO2 and Co(OH)2/GR/TiO2 nanotubes films.

3.7. Electrochemical impedance spectra

In order to study the surface properties of the electrodes and their interactions with solution, the impedance responses of TiO2, GR/TiO2, Co(OH)2/TiO2 and Co(OH)2/GR/TiO2 nanotubes photoelectrodes were monitored under simulated sunlight irradiation. The obtained impedance profiles (in Nyquist and Bode modulus formats) are as shown in Fig. 9(a) and (b). The equivalent circuits used for modeling the impedance data are shown in Fig. 9(c) (TiO2, GR/TiO2 and Co(OH)2/TiO2) and Fig. 9(d) (Co(OH)2/GR/TiO2), where Rs, Qdl, Rct, Qf, Rf and W represent electrolyte resistance, constant phase element (reflects double-layer capacitance), charge transfer resistance, constant phase element (reflects oxide film capacitance), oxide film resistance and Warburg diffusion [50], respectively.

Fig. 9.   Nyquist (a) and Bode (b) plots of TiO2, GR/TiO2, Co(OH)2/TiO2 and Co(OH)2/GR/TiO2 nanotubes photoelectrodes tested in 3.5 wt% NaCl solution under illumination; Schematic of the equivalent circuit obtained by EIS result fitting: (c) TiO2, GR/TiO2 and Co(OH)2/TiO2; (d) Co(OH)2/GR/TiO2.

The diameter of the Nyquist impedance arcs of the four test specimens, which corresponds to the Rct decreases in the order: TiO2 (4609 Ω cm2) > Co(OH)2/TiO2 (2265 Ω cm2) > GR/TiO2 (1925 Ω cm2) > Co(OH)2/GR/TiO2 (100.8 Ω cm2). Moreover, the |Z|0.01Hz of the Co(OH)2/GR/TiO2 (363.3 Ω cm2) is the smallest among these four specimens as shown in Fig. 9(b). Thus, the lowest Rct and |Z|0.01Hz obtained from the ternary Co(OH)2/GR/TiO2 nanotubes photoelectrode indicates the fastest rate of charge carriers separation and transfer, which could result in an improved photoelectrochemical performance. The EIS results show that decorating both Co(OH)2 and graphene to TiO2 nanotubes enhances transmission and separation efficiency of charge carriers beyond that modification by either of the additives. This indicates a synergistic effect between Co(OH)2 and graphene in improving charge carriers transport in TiO2 nanotubes.

3.8. Photoelectrochemical performance of the electrodes

To explore photoelectrochemical performance of these as-prepared TiO2 photoelectrodes, a 400 s-cycle test was done 5 times (200 s with illumination and 200 s without illumination). Fig. 10 shows the photogenerated potential variations of TiO2, Co(OH)2/TiO2, GR/TiO2 and Co(OH)2/GR/TiO2 nanotubes photoelectrodes in 3.5 wt% NaCl solution, which is relative to a KCl-saturated Ag/AgCl electrode. Photogenerated potential is an important parameter to evaluate the separation rate of photogenerated electrons and holes. The more negative shift of photogenerated potential indicates that the photoelectrodes possess a better photoelectrochemical performance. The results show that the photogenerated potential of unmodified TiO2 photoelectrode shifts immediately to -250 mV with illumination, and back to a relatively positive value without illumination. The photogenerated potential of the Co(OH)2/TiO2 photoelectrode quickly reaches -340 mV with illumination, and still remains at -330 mV stable after 2000s immersion in 3.5 wt% NaCl solution under chopped illumination. The GR/TiO2 photoelectrode exhibits a remarkable negative shift (reaches -405 mV at first cycle with illumination) compared with that of the unmodified TiO2. The potential of the Co(OH)2/GR/TiO2 photoelectrode drops to -413 mV under illumination and still remains stable at -395 mV, which is a significant improvement over TiO2, Co(OH)2/TiO2 and GR/TiO2 photoelectrodes.

Fig. 10.   Time-based photogenerated potential for unmodified TiO2 nanotubes and Co(OH)2, GR modified TiO2 nanotubes photoelectrodes in 3.5 wt% NaCl solution under chopped illumination.

Fig. 11 shows the photogenerated i-t curves of TiO2, Co(OH)2/TiO2, GR/TiO2 and Co(OH)2/GR/TiO2 nanotubes photoelectrodes in 3.5 wt% NaCl solution under chopped illumination. Among all the TiO2 nanotubes photoelectrodes, the Co(OH)2/GR/TiO2 photoelectrode exhibits the highest transient excitation current density in the whole cycling process, which means that the Co(OH)2/GR/TiO2 photoelectrode can generate the most electrons, due essentially to the synergistic effect of graphene and Co(OH)2.

Fig. 11.   Time-based photocurrent density curves for unmodified TiO2 nanotubes and Co(OH)2, GR modified TiO2 nanotubes photoelectrodes in 3.5 wt% NaCl solution under chopped illumination.

To further investigate the photoelectrochemical performance of the TiO2 photoelectrodes, photogenerated i-E curves of the photoelectrodes were tested in 0.1 M Na2SO4 solution under chopped illumination. As shown in Fig. 12, the Co(OH)2/GR/TiO2 photoelectrode generates the highest photocurrent density in the entire scan range, indicating that co-modification of Co(OH)2 and graphene enhances the photoelectrochemical performance of TiO2. The results of the photogenerated i-t curves and i-E curves demonstrate that the photoelectrochemical performance of TiO2 is improved after the co-modification of Co(OH)2 and graphene.

Fig. 12.   Photocurrent vs. applied potential of the TiO2, GR/TiO2, Co(OH)2/TiO2 and Co(OH)2/GR/TiO2 photoelectrodes in 0.1 M Na2SO4 solution under chopped illumination.

3.9. Photogenerated cathodic protection performance of the electrodes

From the above results, it is clear that the photogenerated potential of the Co(OH)2/GR/TiO2 electrode (-413 mV) is well below the corrosion potential of 304SS (Ecorr = -170 mV), indicating the great prospects of Co(OH)2/GR/TiO2 for stainless steel cathodic protection in marine environments. In order to experimentally ascertain this corrosion protection properties, the OCP curves of the four kind TiO2 photoelectrodes coupled with 304SS electrode in 3.5 wt% NaCl solution under 12 h long illumination were measured (Fig. 13). The dotted line represents the corrosion potential of 304SS in 3.5 wt% NaCl solution. As expected, the potential of Co(OH)2/GR/TiO2 coupled with 304SS electrode is the most negative at the initial periods due to the synergetic effect of Co(OH)2 and graphene. However, as time progressed, the potential becomes increasingly anodic for all the photoanodes, which can be attributed to the aggressive nature of the chloride ion. The OCP of all the TiO2 photoanodes coupled with 304SS electrode are still more negative than the Ecorr of 304SS after 12 h immersion, which means that the photoanode materials can produce a cathodic protection effect on the 304SS for up to 12 h.

Fig. 13.   OCP curves of the four TiO2 photoelectrodes coupled with 304SS electrode in 3.5 wt% NaCl solution under 12 h long illumination.

3.10. Mechanism of Co(OH)2/GR/TiO2 photoanode

Based on the UV-vis absorption spectra results, it is clear that Co(OH)2 can obviously reduce the band gap and have a little bit visible light enhancement effect. Moreover, graphene on the other hand can enhance visible light absorption. Thus, Co(OH)2/GR/TiO2 shows not only a narrowed band gap, but also the strongest visible light absorption intensity. PL emission spectra show that Co(OH)2/GR/TiO2 specimen possesses the lowest recombination of the photogenerated electrons and holes compared with that of GR/TiO2 and Co(OH)2/TiO2 specimens. Thus, co-modification of Co(OH)2 and graphene does have synergistic effect on improving photocatalytic performance of TiO2, which has also been testified by electrochemical results (EIS, E-t, i-t, i-E).

The photogenerated cathodic protection mechanism of the Co(OH)2/GR/TiO2 photoanode coupled with 304SS is illustrated in Fig. 14. Here, we mainly focus on the separation and transfer process of the photogenerated electrons and holes. Under the simulated sunlight excitation, photogenerated electrons would rapidly transfer from the valence band (VB) to the conduction band (CB), while the photogenerated holes are left in the VB. Accelerated transport of photogenerated electrons from the CB of TiO2 to the surface of 304SS occurs because of the presence of graphene particles, which will lead to the cathodic polarization of 304SS and finally make the potential of 304SS lower than its corrosion potential. Thus, 304SS is successfully protected by Co(OH)2/GR/TiO2 photoanode under illumination. Meanwhile, photogenerated holes in the VB are captured by Co(OH)2 nanoparticles to cause water oxidation reaction by conversion of Co2+ and Co3+. Thus, Co(OH)2 and graphene co-modified TiO2 photoanode gives efficient separation of photogenerated electrons and holes, and suppresses the recombination of photogenerated carriers, which finally provides an effective photogenerated cathodic protection for 304SS.

Fig. 14.   Proposed mechanism for the role of Co(OH)2/GR/TiO2 on the photogenerated cathodic protection for 304SS.

4. Conclusion

Co(OH)2/GR/TiO2 photoanode was successfully prepared by anodic oxidation, electrodeposition, and SILAR methods. Compared with the unmodified TiO2, Co(OH)2/TiO2 and GR/TiO2 photoanodes, the photo-absorption performance, photoelectrochemical performance of Co(OH)2/GR/TiO2 photoanode are the best mainly due to the good electronic conduction of graphene, the holes trapping effect of Co(OH)2, and their synergistic effect. As a result, Co(OH)2/GR/TiO2 photoanode can produce an effective photogenerated cathodic protection for 304SS in 3.5 wt% NaCl solution at least for 12 h.

Acknowledgments

This work was supported financially by the National Natural Science Foundation of China (Nos. 51622106 and 51871049) and the Fundamental Research Funds for the Central Universities (No. 160708001). The authors are very grateful to Prof. Gang Liu in Institute of Metal Research, CAS for his idea of graphene quantum dot modification and technical support for preparation of TiO2 nanotubes.


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