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Highly Efficient Photocathodic Protection Performance of 2D/2D ZIS@CNNs Composites under Visible Light

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Submitted:

04 July 2023

Posted:

05 July 2023

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Abstract
Low separation efficiency of photogenerated carriers and inefficient utilization of visible light limit the application of g-C3N4 nanosheets (CNNs) in the field of photocathodic protection (PCP). Therefore, this work considered indium zinc sulfide (ZnIn2S4, ZIS) as a visible photosensitizer and an electronic donor to promote its PCP performance. ZIS nanolayers with nano-leaf structures were fabricated on CNNs by a simple hydrothermal method. The synthesized 30% ZIS@CNNs composites significantly enhanced the PCP performance for 316 stainless steel (SS) compared with pure CNNs. Under visible light illumination, the 30% ZIS@CNNs photoelectrode exhibited the largest photoinduced current density of 17.30 μA cm−2 and a photoinduced potential drop of 0.37 V, which was approximately 4 and 7.5 times higher than that of pure CNNs, respectively. The improved protection performance may be attributed to the significant increase in visible light absorption and the great enhancement in the separation efficiency of
Keywords: 
g-C3N4
;  
ZnIn2S4
;  
photocathodic protection
;  
composites
;  
XPS
Subject: 
Engineering  -   Marine Engineering

1. Introduction

Photocathodic protection (PCP) technology has attracted extensive attention because it is a promising green and clean technology in the past decades [1,2,3,4]. This technology takes the advantage of photosensitive characteristics of semiconductor photoanode materials to generate electrons under light with a certain energy level and protects metals from corrosion by transferring electrons to the metal surface and causing a negative shift of the metal’s potential.
As a metal-free semiconductor, g-C3N4 (~2.7 eV) is widely applied in many fields such as photocatalytic hydrogen production [5,6,7], degradation of organic pollutants [8,9,10], and CO2 reduction, etc. [11,12] due to the advantages of visible light response, obvious chemical stability, easy preparation, rich reserves, and strong environmental affinity. Compared with metal-semiconductor, g-C3N4 has incomparable advantages in avoiding secondary pollution [13]. However, the protection of metals by g-C3N4 materials may not be ideal due to its low surface area, rapid recombination of photogenerated electron-hole pairs, and inefficient utilization of visible light [14,15,16,17]. In order to address these issues, various approaches, including nanostructure design [18,19], elemental doping [20,21], and heterojunction construction [22,23], have been applied to modify g-C3N4. Among these strategies, heterojunction construction is recognized as one of the most effective approaches to suppress the carrier recombination and improve the absorption efficiency of visible light, such as NaNbO3/g-C3N4 [24], KBiFe2O5/g-C3N4 [25], Co3O4/g-C3N4 heterojunction [26].
As a unique ternary sulfide n-type semiconductor, ZnIn2S4 (ZIS, 2.34 ~ 2.55 eV) has been widely used in the photocatalytic fields due to its exceptional layered structure, tunable band gap, and excellent visible light absorption properties [27,28,29]. Compared with conventional binary metal sulfides, ZIS has higher photo-corrosion resistance and better chemical stability, which has aroused great attention of researchers in the field of PCP [30]. However, ZIS still has some limitations in its applications, including poor photoelectron mobility and low carrier separation efficiency [31,32,33]. Coupling g-C3N4 with ZIS to construct heterojunction composites is an effective method to enhance their photocatalytic performance under visible light. For example, Dang et al. reported that g-C3N4/ZIS composite showed better photocatalytic hydrogen production activity than pure ZIS and g-C3N4 [34]. Hou et al. reported that g-C3N4/ZIS composite exhibited good photocatalytic degradation of ofloxacin performance and higher photocatalytic hydrogen evolution than pure g-C3N4 and ZIS [35]. However, no application of g-C3N4/ZIS composites in the field of PCP has been reported.
The morphological structure and size of semiconductors play an important role in the process of photogenerated electron-hole pairs. The ultrathin two-dimensional (2D) g-C3N4 nanosheets (CNNs) are regarded as an ideal structure that can significantly enlarge the specific surface area, shorten the transfer distance of photoelectrons, and expose a considerable number of active sites, thus effectively improving the performance of g-C3N4 [36,37]. In addition, it has been reported that the 2D/2D morphology in the heterojunction composites can significantly facilitate electron migration and boost visible light absorption ability [38], which may improve the utilization efficiency of photogenerated carriers and enhance the PCP performance under visible light.
In this work, we fabricated the 2D/2D ZIS@CNNs composites by two-step calcination and hydrothermal method and then coated the composites onto the fluorine-doped tin oxide (FTO) substrates. The microstructures, chemical compositions, and crystalline structures of the composites were identified by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The optical absorption properties were investigated using UV-Vis diffuse reflectance spectroscopy (DRS). The separation efficiency of photogenerated carriers was investigated by PL spectroscopy. The PCP performances of the composites were evaluated by photoinduced open circuit potential (OCP) and photocurrent densities. The PCP mechanism was proposed by analyses of electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) curves.

2. Experimental

2.1. Materials

Chemicals: melamine (C3N3(NH2)3)), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), thioacetamide (TAA), indium nitrate tetrahydrate (In(NO3)3∙4H2O), sodium citrate dihydrate (C6H5O7Na3·2H2O) were all purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). All reagents are analytically pure.
FTO glasses (7 Ω cm−2, 10 × 20 × 2.2 mm) were purchased from Guluo Glass Co. Ltd, (Luoyang, China). The glasses were ultrasonically cleaned in deionized water, acetone, and anhydrous ethanol for 30 min in sequence. Then these substrates were dried naturally at room temperature.

2.2. Synthesis of CNNs

CNNs were fabricated by a two-step calcination method (Figure 1). First, 5 g melamine was calcined at 520 ℃ for 4 h with a heating rate of 5 ℃·min−1 in a tube furnace. Afterward, the resultant sample was kept calcining (heating rate: 2.5 ℃/min) at 550 ℃ for 2 h. The light yellow CNNs were collected (Figure S1).
ZIS nano-leaves were fabricated onto CNNs by a simple hydrothermal method. Firstly, precursor solutions were obtained by adding 0.1 mmol Zn(NO3)2∙6H2O, 0.2 mmol In(NO3)3∙4H2O, and 0.6 mmol sodium citrate dihydrate to the beaker filled with 20 mL deionized water and stirring vigorously for 0.5 h at room temperature. Secondly, different weights of CNNs were added into the precursor solution. The above solution was first treated by ultrasonication for 10 min and then stirred for 3 h to obtain a white suspension. Finally, 0.8 mmol TAA was added to the above suspension, sonicated for 10 min, and churned for 30 min. Then, the above solution was poured into a 30 mL Teflon-lined stainless-steel autoclave, and heated in an oven at 160 °C for 1 h. After cooling down, it was washed with deionized water and anhydrous ethanol, respectively, then collected by centrifugation and dried at 60 °C for 24 h. ZIS@CNNs composites (Figure S1) with different proportions of ZIS were obtained and marked as 20% ZIS@CNNs, 30% ZIS@CNNs, and 40% ZIS@CNNs composites, respectively. As a reference, pure ZIS nano-leaves were also synthesized under similar conditions without CNNs.

2.3. Fabrication of ZIS@CNNs photoelectrodes

The treated FTO was sealed with tape, and only a 10 × 15 mm conductive surface was left as the working surface. 20 mg ZIS@CNNs composite was added to a mixed solution containing 10 μL naphthol and 1 mL anhydrous ethanol, and sonicated for 15 min. A uniform suspension was obtained. 40 μL of the above solution was applied drop by drop to the conductive surface of the FTO, so that the suspension was distributed as uniformly as possible on the surface of the FTO (about 0.528 mg cm−2). Then the FTO was dried in the air atmosphere and transferred into an oven to heat at 100 ℃ for 2 h to obtain ZIS@CNNs photoelectrodes.

2.4. Characterization

The crystal structures of CNNs, ZIS, and ZIS@CNNs were detected by XRD diffractometer (Rigaku Ultima IV) with Cu Kα radiation. The elemental compositions and bonding information of the synthetic products were analyzed using XPS (Thermo ESCALAB 250XI Al Kα as the X-rays). The microstructure and chemical composition of the composite were identified using a high-resolution transmission electron microscope (HRTEM, JEM 200CX TEM). UV-Vis diffuse reflectance spectrometer (Shimadzu UV-3600i Plus, Japan), was used to measure the absorption capacity of materials using BaSO4 plates used as a reference. Photoluminescence (PL, FLS-980) was used to measure the photogenerated carrier separation efficiency of materials, with an excitation wavelength of 320 nm. The fourier transform infrared spectroscopy (FT-IR, Thermo Scientific Nicolet iS20, America) was tested at the wavelength range of 400 ~ 4000 cm–1.

2.5. Electrochemical tests

An electrochemical workstation (CHI760E, Chenhua Instrument Co., Ltd., Shanghai, China) was used to test the electrochemical properties of the fabricated photoelectrodes. The H-type electrochemical cell was selected for electrochemical performance testing, which was filled with a 3.5 wt% NaCl solution and a mixture solution of 0.1 M Na2S and 0.2 M NaOH in the corrosion chamber and photochemical chamber, respectively. Conductivity between electrolyte solutions in two electrolytic cells was performed through the Nafion membrane (N-117, DuPont Co., America). The ZIS@CNNs photoanode was placed in the photochemical chamber, whereas the 316 stainless steel (SS) electrode with an active area of 1 cm2 was placed in the corrosion chamber as the test subject. A 300W Xe lamp (PLS-SXE300, Perfect-light Co., Ltd., Beijing, China) was used to be the light source (Figure S2a). The light illuminated on the photoelectrode through a quartz window with a diameter of 30 mm on the side of the photochemical chamber. A 420 nm filter was used to filter out light below 420 nm. During the measurements of OCP, EIS, and Tafel, the photoanode and 316 SS were connected by wires as working electrode (WE). The saturated calomel electrode (SCE) and Pt sheet were regarded as reference electrode (RE) and counter electrode (CE), respectively. The EIS tests were conducted under light and performed at a frequency range of 10−2 ~ 105 Hz, with an alternating current amplitude of 5 mV. The Tafel test was conducted under light and performed at range of −0.25 ~ 0.25 V (vs. OCP). During the measurement of photocurrent densities, the photoanode and 316 SS were connected to WE and ground electrode, respectively, and RE and CE were connected in a short circuit (Figure S2b). The M-S tests were conducted under the light and measured with a frequency of 1000 Hz. A conventional three-electrode system was used for this testing with 0.1 mol L−1 Na2SO4 solution as the electrolyte solution. The photoelectrode, SCE, and Pt sheet were taken as the WE, RE, and CE, respectively.

3. Results and discussion

3.1. Characterization of ZIS@CNNs composites

Figure 2a-c showed the morphological images of CNNs, ZIS, and ZIS@CNNs composites, respectively. It could be observed from Figure 2a that pure CNNs prepared via two-step calcination method had distinct flaky images and exhibited overlapping and stacking morphologies. As can be seen from Figure 2b that the morphology of pure ZIS synthesized by the hydrothermal method resembles nano-leaves. As shown in Figure 2c, the bottom layer of the ZIS@CNNs composite material is CNNs, and ZIS nano-leaves are evenly attached to its surface. These results indicate that ZIS nano-leaves are successfully grown on CNNs. Figure 2d shows the chemical composition of the ZIS@CNNs composites obtained by EDS testing, which shows the presence of Zn, In, S, C, and N elements.
When CNNs were added to the ZIS precursor solution with dissolved sodium citrate dihydrate, Zn2+ and In3+ in the solution moved rapidly to CNNs, and then the added TAA released S2–, which reacted with Zn2+ and In3 attached to the surface to form ZIS nanoparticles. Under hydrothermal conditions, sodium citrate dihydrate exerted an inhibitory effect on the growth of nanoparticles, and then the nanoparticles gradually became nano-leaves. Eventually, a large number of ZIS nano-leaves accumulated on the surface of CNNs, forming 2D/2D ZIS@CNNs heterojunctions.
The microscopic morphologies of CNNs, ZIS, and 30% ZIS@CNNs were further investigated using TEM. From Figure 3a and b, it was clear that the morphologies of CNNs and ZIS were nanosheets and nano-leaves, respectively. The surface of the 30% ZIS@CNNs composite is rougher than that of pure CNNs due to the deposition of the ZIS nano-leaves onto the surface of the CNNs, which is consistent with the SEM results. Figure 3d displayed the HRTEM image of 30% ZIS@CNNs. The lattice spacing of 0.321 nm corresponds to the (002) crystalline planes of CNNs, and the lattice spacing of 0.315 nm corresponds to the (104) crystalline planes of ZIS (JCPDS No. 49−1562).
Figure 4a,b shown the N2 adsorption–desorption isotherms and pore size distribution curves of CNNs and 30% ZIS@CNNs composite, respectively. According to the adsorption isotherm, the SBET of CNNs was 656.965 m2/g, while the SBET of 30% ZIS@CNNs composite decreased sharply to 47.239 m2/g. The pore size distribution of 30% ZIS@CNNs composite and CNNs were analyzed according to the Barrett-Joyner-Halenda (BJH) method [39], The average pore diameters of 30% ZIS@CNNs composite and CNNs are about 13.05 and 4.38 nm, respectively, indicating that both materials are mesoporous structures.
Figure 5a displayed the XRD patterns of pure CNNs, pure ZIS, and three different ZIS@CNNs composites. It could be obviously seen from Figure 5a that there were two evident characteristic peaks in the XRD patterns of CNNs. The weak peak at 12.9° can be assigned to an in-plane repeating motif of the continuous heptazine network [40,41]. Due to the interlayer-stacking (002) crystal plane of melon networks, there is a prominent and sharp diffraction peak at 27.7° [42]. From the XRD patterns of ZIS, the peak at 28.3°, 47.0°, and 55.8° can be assigned to the diffraction peaks of (104), (110), and (024) hexagonal crystal planes of ZIS (JCPDS No. 49−1562), respectively. For the three different ZIS@CNNs composites, the peak of (002) crystal plane of CNNs overlaps with the peak of (104) hexagonal crystal plane of ZIS, and the remaining two peaks are the characteristic diffraction peaks of ZIS. In addition, due to the introduction of ZIS, the diffraction peak density of CNNs is weakened. However, the peak position of CNNs does not move, indicating that the introduction of ZIS does not change the crystal phase structure of CNNs.
FT-IR spectra were used to characterize the variation of CNNs in the reaction process. As displayed in Figure 5b, several strong peaks appeared in the region of 900–1700 cm−1 are caused by the skeleton vibration of C-N and C=N heterocycles [43]. And the peak located at 808 cm−1 is caused by the breathing vibration of the s-triazine units [44]. The peaks between 3000 and 3500 cm−1 are caused by H2O molecules bound with surfaces of CNNs and ZIS@CNNs and the amino groups formed by the hydrothermal reaction [45]. Compared with the FT-IR spectra of CNNs and three different ZIS@CNNs composites, the peaks of the four curves were similar, which indicates that the introduction of ZIS does not damage the skeleton structure of CNNs. ZIS and CNNs can co-exist well in the hydrothermal reaction. It also shows that CNNs have excellent structural stability.
XPS was considered as an effective means to further study the surface chemical state of the ZIS@CNNs composites. As could be seen in Figure 6a, five elements, Zn, In, N, S, and C, were clearly present in the tested material. Figure 6b shown the high-resolution XPS spectrum of element C. It could be clearly seen that the peaks of CNNs and ZIS@CNNs composites were very similar, which indicates the presence of CNNs in the composites. The strong peak locates at 287.6 eV is assigned to N–C=N and the weaker peak at 284.3 eV is assigned to graphite carbon nitride [46]. The peak of ZIS@CNNs composites negatively shift to 287.5 and 284.2 eV, respectively. The binding energy of ZIS@CNNs composites decreases by 0.1 eV compared to CNNs. This may be attributed to the intense electronic interactions between CNNs and ZIS, where CNNs gain photoelectrons [47]. Figure 6c shown the high-resolution XPS spectrum of element N. Four peaks can be seen in Figure 6c, and the intensity of these four peaks gradually decreased from 398.0 to 403.7 eV. The peak with the highest binding energy at 398.0 eV is caused by the C–N=C bond. The peak at 399.6 eV is caused by N–(C)3 groups. The peak at 400.5 eV indicates the presence of N–H bonds [48]. The weak peak at 403.7 eV is caused by charge buildup due to electron transfer. In comparison with CNNs, the binding energy of element C, N of the ZIS@CNNs composites negatively shift, indicating the state of CNNs gaining photoelectrons. To further analyze the electronic state of ZIS, high-resolution XPS spectra of Zn, In, and S elements were analyzed. It can be seen that the peaks of ZIS and ZIS@CNNs composites are very similar, which indicates the presence of ZIS in the composites. Figure 6d shows the high-resolution XPS spectrum of element S. It could be seen two peaks with different peak widths. The narrow peak at 160.6 eV is the characteristic peak of S 2p3/2 and the broad peak at 161.8 eV is the characteristic peak of S 2p1/2, indicating the S2– chemical state in the ZIS@CNNs composite. Figure 6e shows the high-resolution XPS spectrum of element In, the peaks at 444.2 and 451.6 eV are the characteristic peaks of In 3d5/2 and In 3d3/2, respectively, indicating the In3+ chemical state in ZIS@CNNs composite [49]. As displayed in Figure 6f, the peaks at 1021.3 and 1044.8 eV are the characteristic peaks of Zn 2p3/2 and Zn 2p1/2, respectively, indicating the Zn2+ chemical state in ZIS@CNNs composite [50]. In comparison with ZIS, the binding energy of element S, In, Zn of the ZIS@CNNs composites positively shift, indicating the state of ZIS losing photoelectrons [51]. Combining the above results of the gain and loss of photoelectrons in CNNs and ZIS, it can be judged that photoelectrons flow from ZIS to CNNs.

3.2. Light absorption properties

To investigate the absorption properties of CNNs and three different composites for visible light, their UV-Vis DRS was measured [52]. It can be clearly seen from Figure 7a that the absorption edges of the three different ZIS@CNNs composites are significantly red-shifted toward the visible region compared with pure CNNs, and the 30% ZIS@CNNs composite shows the largest absorption threshold, which indicates that the ZIS@CNNs composites have stronger absorption of visible light than pure CNNs, and the 30% ZIS@CNNs composite has the best absorption performance. From the results of TEM and SEM as well as BET, it is obvious that the introduction of ZIS makes the CNNs porous structures as well as the increased area of visible light contact, which allows more visible light to be captured through the pores into the gaps between the CNNs, thus the composites exhibit a strong light capture capability. In addition, the Tauc curves (Figure 7b) are obtained by Kubelka−Munk means [53,54]:
( α h v ) = A ( h v − E g ) n
where the absorption index, the Planck constant, the frequency, and the characteristic constant are defined as α, h, v, and A, respectively. The value of n depends on the transition characteristics of the material, n = 1/2 for the direct band gap and n = 2 for the indirect band gap. The Eg of pure CNNs, ZIS, 20% ZIS@CNNs, 30% ZIS@CNNs, and 40% ZIS@CNNs composites are 2.7, 2.39, 2.63, 2.52, and 2.59 eV, respectively.
The separation efficiency of photogenerated carriers has an important influence on the PCP performance of the material. In order to study the carrier separation efficiency of different materials [55], PL tests were performed which can be seen in Figure 8. The greater the decay of PL peak intensity, the higher the carrier separation efficiency [56]. Compared with pure CNNs, the peak intensities of the three different ratios of composites decayed significantly, with the 30% ZIS@CNNs composite showing the greatest decay. This may be due to the introduction of ZIS, which makes the ZIS@CNNs composites exhibit significant fluorescence quenching. The above results indicate that the recombination efficiency of the photogenerated electron-hole pairs of the composites is significantly reduced, and the 30% ZIS@CNNs composite has the lowest carrier recombination efficiency.

3.3. PCP effects

In order to test the protection effect of CNNs and three different ZIS@CNNs composites on 316 SS, we conducted OCP tests on the above four materials [57]. As can be seen in Figure 9a, the OCP of all materials dropped steeply as the light source was turned on, and the potential drops of three different ZIS@CNNs composites were significantly higher than that of pure CNNs. In addition, 30% ZIS@CNNs exhibited the largest potential drop (0.37 V), which was 7.4 times higher than that of pure CNNs (0.05 V). After the power was turned off, the OCP first rosed sharply and then slowly and finally stabilized around a certain value. As could be seen from Figure 9a, the stable values of the rebound potentials of both ZIS@CNNs composites and CNNs were lower than the self-corrosion potential of 316 SS (−0.18 V vs. SCE), which may be due to the fact that the electrons in ZIS@CNNs composites and CNNs did not disappear completely after the light was turned off. There are still some electrons are transferred from three different ZIS@CNNs composites and CNNs photoanodes to 316 SS surface, indicating that they still played a continuous protective role for 316 SS in the dark state. In addition, three different ZIS@CNNs composites rebounded to more negative potential values compared with pure CNNs, indicating that the composite had a better continuous protection effect than pure CNNs in the dark state. The potentials of three different ZIS@CNNs composites remained stable after four on-off photocycles, indicating that the composites can provide stable protection to 316 SS for a long time. In addition, to test the protection effect of the composite material on other metals, we conducted OCP tests on 304 SS under the same conditions as 316 SS, and the potential drop compared to 316 SS was shown in the Figure 9e. It can be clearly seen that ZIS@CNNs composites have obvious protection for 304 SS, and 30% ZIS@CNNs composite has much better protection effect for 304 than CNNs. In addition, in comparison with the protection of CNNs and 30% ZIS@CNNs composite for 316 SS, the protection for 304 SS is weaker than 316 SS.
CV curves were used to analyze the electrochemical activity of pure CNNs and three different ZIS@CNNs composites. The CV curves of four materials were tested at different sweep speeds and the results were shown in Figure 10 a-d. It can be seen from Figure 10 that the area enclosed by the CV curves of the three ZIS@CNNs composites was larger than that of the CNNs at the same sweep speed, and the area enclosed by the CV curves of the 30% ZIS@CNNs composite was the largest. The scatter plot of Figure 10e was obtained with the sweep speed as the horizontal coordinate and the current variation of the CV curve as the vertical coordinate. The data were fitted to obtain four straight lines with different slopes, and the slope allows us to obtain the capacitance values of different materials. The larger the slope, the larger the capacitance value [58]. Besides, the electrochemical active area correlated with the size of the capacitance value, the larger the capacitance value, the larger the active area. Therefore, the slope size of the 4 lines in Figure 10e could be used to qualitatively analyze the effect of the size of the electrochemically active area of the 4 materials on the protection performance of the materials. It could be seen from Figure 10e that the slopes of the three different ZIS@CNNs composites were significantly larger than that of CNNs and the slope of the 30% ZIS@CNNs composite was the largest, which indicates that the capacitance value of the three different ZIS@CNNs composites were larger than that of CNNs and the 30% ZIS@CNNs composite had the largest capacitance. Therefore, the electrochemically active areas of three different composites were greater than that of CNNs. 30% ZIS@CNNs composite had the largest electrochemically active area. This is in agreement with the 30% ZIS@CNNs composites exhibiting the optimal protective properties. In addition, the slope of the 40% ZIS@CNNs composite was slightly smaller than that of the 20% ZIS@CNNs composite, which may be attributed to the increase in the proportion of ZIS, which caused a large amount of ZIS to accumulate, resulting in smaller gaps between the nano-leaves, resulting in a reduced active area. However, more heterojunction channels were formed between the ZIS and CNNs in the 40% ZIS@ CNNs composites, which allowed the 40% ZIS@ CNNs composites to exhibit better protective properties than the 20% ZIS@CNNs composites.
The photoanodes generates current under light, therefore, photocurrent density is another effective approach to characterize the protection effect of photoanodes on 316 SS [59]. The i-t curves of 316 SS coupled with CNNs and three different ZIS@CNNs composites were measured. As shown in Figure 9b, when the light source was turned on, the photocurrent densities of all materials increased rapidly to a certain value. It was obvious from Figure 9b that the photocurrent densities of three different ZIS@CNNs composites were larger than that of pure CNNs, and 30% of ZIS@CNNs composites exhibited the largest photocurrent densities (17.30 μA cm–2), which was more than 4 times that of pure CNNs (4.22 μA cm–2). The current drops in the 100 ~ 150 s and 200 ~ 250 s closed-light phases for CNNs and the three different composites were not as rapid as in the open-light phase, but more slowly, and the final steady current was not zero, which may be due to the fact that the carriers generated in the open-light phase did not disappear completely. Combined with the above and OCP results, it can be indicated that three different ZIS@CNNs composites could provide better protection for 316 SS than pure CNNs under light conditions, and the 30% ZIS@CNNs composite had the best protection effect.
Figure 9c shown the Tafel curves of 316 SS coupled with CNNs and three different ZIS@CNNs composites. Table 1 shown the fitted values of the above curves. The order of Ecorr for different materials was that 30% ZIS@CNNs composite < 40% ZIS@CNNs composite < 20% ZIS@CNNs composite < CNNs. The 30% ZIS@CNNs composite has the most negative photogenerated potential, which is consistent with the OCP results. In addition, the order of icorr for different materials is that 30% ZIS@CNNs composite > 40% ZIS@CNNs composite > 20% ZIS@CNNs composite > CNNs. The icorr for 30% ZIS@CNNs composite is more than 3 times that of CNNs. Larger icorr means faster electrode reaction rates [60]. icorr for 30% ZIS@CNNs is the largest, indicating this composite has the highest protection efficiency.
In order to study the electron transfer efficiency inside and between materials, we measured the EIS plots of 316 SS coupled with CNNs and three different ZIS@CNNs composites, and the results are shown in Figure 9d. It could be clearly seen that the impedance arcs of three different ZIS@CNNs composites were smaller than that of CNNs under the light condition, and 30% ZIS@CNNs composite has the smallest impedance arc. This indicates that the ZIS@CNNs composites generate more photogenerated electrons and the formation of heterojunction channels accelerates the electron transfer, which enhances the protection of the material for 316 SS. According to the measured impedance data, the fitted circuit used is Rs (Qf Rf) (Cdl Rct) circuit. The fitting circuit diagram was shown in the internal diagram of Figure 9d. Rf, Rs, and Rct were the internal resistance of the photoanode, the resistance of 3.5 wt% NaCl solution, and the charge transfer resistance, respectively. Qf and Cdl were defined as internal film capacitance of the material and double-layer capacitance, respectively. Table 2 shown the values of the fitting. The smaller the Rct, the easier the charge transfer [61]. The order of Rct of different materials was that 30% ZIS@CNNs (426.5 Ω cm−2) < 40% ZIS@CNNs (556.6 Ω cm−2) < 20% ZIS@CNNs (874.2 Ω cm−2) < CNNs (934.1 Ω cm−2). It is obviously that the Rct of CNNs was about 2 times than that of 30% ZIS@CNNs composite, which means that the transfer efficiency of electrons in 30% ZIS@CNNs composite is higher than that in pure CNNs. Furthermore, 30% ZIS@CNNs composite has the highest electron transfer efficiency.
The stability of the material is an important factor that affects the practical application of the material. Figure 9f shows the XRD patterns of the material before and after the electrochemical test, and it can be seen from the figure that the XRD patterns before and after the reaction are not significantly changed. This indicates that the photoanode material has good stability.

3.4. Mechanism analysis

To determine the band edge positions of different materials and the VB and CB positions of CNNs and ZIS, and to elucidate the mechanism of electron transfer. The M-S curves of the five materials were measured. It could be seen from Figure 11 that the slopes of all the curves are positive, which indicates that the experimentally prepared ZIS, CNNs, and the three different ZIS@CNNs composites are n-type semiconductors [62]. Compared with CNNs, the flat-band potentials (Efb) of composites have a negative shift, and 30% ZIS@CNNs composite has the most negative shift. It indicates that more photogenerated electrons can be transferred from the 30% ZIS@CNNs composite to the 316 SS surface for their protection than the other materials. It could be seen from Figure 12 that the Efb of CNNs was –0.6 V vs. SCE, and ECB could be calculated by the following formulae:
E f b ( vs .   NHE ) = E f b ( vs .   SCE ) − 0.24
E C B = E f b ( vs .   NHE ) + 0.20
so the ECB of CNNs is −0.56 eV. Similarly, the ECB of ZIS is −0.83 eV [63,64]. EVB of the semiconductor photoanodes can be calculated by the formula:
E V B = E f b − E C B
So that the EVB of CNNs and ZIS were calculated as 2.14 and 1.56 eV, respectively. Besides, to further confirm the EVB, the XPS valence band spectra were tested. It could be seen from Figure 11f that the EVB of CNNs and ZIS were 2.15 and 1.57 eV, respectively, which were consistent with the M-S results. Obviously, the ECB of ZIS was more negative than that of CNNs, so the photogenerated electrons can be transferred from ZIS to CNNs surface and move to the 316 SS surface. Therefore, the ZIS@CNNs composites generate more electrons than pure CNNs.

4. Conclusions

In summary, three different 2D/2D ZIS@CNNs composites were prepared by calcination and hydrothermal method. According to the results of SEM, TEM, and BET, the compounding of 2D CNNs and 2D ZIS leads to a large number of micropores on the surface of CNNs, resulting in a significant increase in visible light absorption efficiency and generating lots of high-speed nanochannels for charge transfer. In addition, UV-Vis DRS shows that the absorption band edges of composites have a red shift that results in an increase in the absorption of visible light by the material. Photocatalysis is a complex process of carrier generation, separation, compounding, and transfer. The result of PL shows that due to the introduction of ZIS, the separation efficiency of photogenerated carriers of ZIS@CNNs composites is greatly improved. EIS shows that photogenerated carriers could rapidly transfer from the ZIS@CNNs photoanodes to the surface of 316 SS. Therefore, the introduction of ZIS brings a significant increase in both carrier separation and transfer efficiency, which lead to the improved PCP efficiency of ZIS@CNNs nanocomposite under visible light. The photocurrent density of the 30% ZIS@CNNs composite can reach 17 μA cm−2 and the photogenerated potential drop to about −0.58 V, indicating the effective protection of the composite. In conclusion, 2D/2D ZIS@CNNs composite is a very friendly photoanode material to protect the metal from corrosion in the ocean environment.

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org.

Author Contributions

Experiment, Writing - original draft, Picture drawing, review & editing, Weitao Li.; Experiment, Zhanyuan Yang.; Resources, Yanhui Li.; Resources, Pengfei Zhang.; Writing - review & editing, Supervision, Funding acquisition, Hong Li.

Funding

This work was funded by the National Natural Science Foundation of China (No. 51801109) and the Science and Technology Support Plan for Youth Innovation of Colleges in Shandong Province (No. DC2000000891).

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Schematic diagram of the preparation of ZIS@CNNs composites.
Figure 1. Schematic diagram of the preparation of ZIS@CNNs composites.
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Figure 2. SEM image of (a) CNNs, (b) ZIS, (c) 30% ZIS@CNNs; (d) EDS spectrum, and (e) EDS mappings of 30% ZIS@CNNs composite.
Figure 2. SEM image of (a) CNNs, (b) ZIS, (c) 30% ZIS@CNNs; (d) EDS spectrum, and (e) EDS mappings of 30% ZIS@CNNs composite.
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Figure 3. TEM image of (a) CNNs, (b) ZIS, (c) 30% ZIS@CNNs. HRTEM images of (d) 30% ZIS@CNNs composite.
Figure 3. TEM image of (a) CNNs, (b) ZIS, (c) 30% ZIS@CNNs. HRTEM images of (d) 30% ZIS@CNNs composite.
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Figure 4. N2 adsorption–desorption isotherms and pore size distribution curves of (a) CNNs and (b) 30% ZIS@CNNs composite.
Figure 4. N2 adsorption–desorption isotherms and pore size distribution curves of (a) CNNs and (b) 30% ZIS@CNNs composite.
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Figure 5. (a) The XRD patterns (b) FT-IR spectrum of CNNs and three different ZIS@CNNs composites.
Figure 5. (a) The XRD patterns (b) FT-IR spectrum of CNNs and three different ZIS@CNNs composites.
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Figure 6. Full scan survey XPS spectra and high-resolution XPS spectra of (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d and (f) S 2p from CNNs, ZIS, and 30% ZIS@CNNs composite.
Figure 6. Full scan survey XPS spectra and high-resolution XPS spectra of (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d and (f) S 2p from CNNs, ZIS, and 30% ZIS@CNNs composite.
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Figure 7. (a) UV−Vis DRS, (b) plot of (αhν)2 versus hν for the band gap energy of CNNs, and ZIS@CNNs composites with different ZIS content.
Figure 7. (a) UV−Vis DRS, (b) plot of (αhν)2 versus hν for the band gap energy of CNNs, and ZIS@CNNs composites with different ZIS content.
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Figure 8. Steady-state PL spectra of CNNs, and ZIS@CNNs composites with different ZIS content.
Figure 8. Steady-state PL spectra of CNNs, and ZIS@CNNs composites with different ZIS content.
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Figure 9. (a) OCP-t curves, (b) The I-t curves, (c) Tafel curves, and (d) Nyquist plots of 316 SS coupled with CNNs and three different ZIS@CNNs composites. (f) Different potential drops of 304 SS and 316 SS coupled with CNNs and ZIS@CNNs, respectively. (f) XRD patterns of 30% ZIS@CNNs composite before and after the electrochemical test.
Figure 9. (a) OCP-t curves, (b) The I-t curves, (c) Tafel curves, and (d) Nyquist plots of 316 SS coupled with CNNs and three different ZIS@CNNs composites. (f) Different potential drops of 304 SS and 316 SS coupled with CNNs and ZIS@CNNs, respectively. (f) XRD patterns of 30% ZIS@CNNs composite before and after the electrochemical test.
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Figure 10. CV curves of (a) CNNs, (b) 20% ZIS@CNNs, and (c) 30% ZIS@CNNs (d) 40% ZIS@CNNs composite (e) Plots of the current densities of the CV curves under different scanning speeds.
Figure 10. CV curves of (a) CNNs, (b) 20% ZIS@CNNs, and (c) 30% ZIS@CNNs (d) 40% ZIS@CNNs composite (e) Plots of the current densities of the CV curves under different scanning speeds.
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Figure 11. (a)-(e) M-S curves of CNNs, ZIS, and ZIS@CNNs composites, (f) Valence band XPS (VB XPS) spectra of CNNs and ZIS.
Figure 11. (a)-(e) M-S curves of CNNs, ZIS, and ZIS@CNNs composites, (f) Valence band XPS (VB XPS) spectra of CNNs and ZIS.
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Figure 12. Electron-hole pair migration mechanism of ZIS@CNNs composite.
Figure 12. Electron-hole pair migration mechanism of ZIS@CNNs composite.
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Table 1. Fitting values of Ecorr and icorr obtained from the Figure 9c.
Table 1. Fitting values of Ecorr and icorr obtained from the Figure 9c.
Samples Ecorr (V vs. SCE) icorr (μA cm−2)
CNNs −0.271 4.08
20% ZIS@CNNs −0.332 6.46
30% ZIS@CNNs −0.62 13.54
40% ZIS@CNNs −0.434 10.93
Table 2. Fitting values of EIS data.
Table 2. Fitting values of EIS data.
Rs (Ω cm−2) Qf (F·cm−2·10−2) Rf (Ω cm−2) Cdl (F·cm−2·10−2) Rct
(Ω cm−2)
CNNs 4.605 5.411×10−2 948.7 0.01807 934.1
20% ZIS@ CNNs 4.574 4.983×10−2 976.3 0.02 874.2
30% ZIS@ CNNs 4.278 1.803×10−3 1590 3.916×10-4 426.5
40% ZIS@ CNNs 2.709 4.235×10−3 1135.3 2.721×10-4 556.6
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