Highly efficient photoelectrochemical anticorrosion performance of C₃N₄@ZnO composite with quasi-shell–core structure on 304 stainless steel

Abstract

Carbon nitride@zinc oxide (C₃N₄@ZnO) composite with a quasi-shell–core structure was prepared in this work. The coating of C₃N₄ on ZnO significantly increases the photoelectrochemical anticorrosion performance of ZnO. When the amount of C₃N₄ added is 1 wt%, the C₃N₄@ZnO composite can provide the best photoelectrochemical anticorrosion capability for 304 stainless steel. This improved performance is attributed to the formation of an effective heterojunction electric field on the interface between C₃N₄ and ZnO, which improves the separation efficiency of the photogenerated electron–hole pairs, increases the lifetime of the photoinduced electrons, and enhances the photoelectrochemical anticorrosion performance of ZnO.

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1. Introduction

Corrosion reactions cause the decrease of the Gibbs free energy, therefore corrosion is spontaneous. Corrosion causes disastrous consequences if not attended to. Apart from this, corrosion induces large economic losses and cause serious environmental pollution. Although current technologies, such as organic coating and cathodic protection, can slow down the corrosion rates of metals, they cause a series of problems such as material consumption, energy wastage, and environmental pollution. Therefore, new environment-friendly corrosion protection technologies must be developed.¹ Studies show that the photovoltaic effect of semiconductor materials with comparatively negative conduction band potentials can be used for the photoelectrochemical anticorrosion property on metals. This novel technology is a very promising environment-friendly system, and has been drawing increasing attention. The main advantage of this technology is the use of clean and renewable solar energy for the corrosion protection of metals, and thus, it is considered to be an ideal corrosion protection technology and one of the most promising anticorrosion technologies. Solar energy is used to excite the semiconductor materials and generate photoinduced electrons via the photovoltaic effect. The electrons subsequently migrate to and accumulate on the coupled metal surface. The potential of the metal will then be cathodically polarized and the anodic dissolution reactions of this metal are inhibited, and thereby, the metal becomes cathodically protected. In contrast to the conventional sacrificial anodes in the cathodic protection system, semiconductor materials only act as photo-to-electron conversion centers and are not consumed. They can theoretically act as non-sacrificial photoanodes because the anodic reactions are not the decomposition of semiconductor materials themselves, but the oxidation of water and/or adsorbed organic pollutants by photogenerated holes.²

Recently, the investigations on photoelectrochemical anticorrosion properties are mainly focused on wide bandgap semiconductor materials, such as TiO₂,^2–7 SrTiO₃,^8,9 and ZnO.¹⁰ Studies show that the photoresponse of these materials can be extended to the visible light region by doping with other nonmetallic or metallic elements or compositing with other narrow-bandgap semiconductor materials. In order to improve the photoelectric conversion performance, semiconductors with special nanomorphologies, such as nanotube arrays, nanowires, and three-dimensional network structures, were prepared and studied.¹¹ To solve the continuity issue of anticorrosion effect in the dark, the semiconductor is composited with some materials possessing electronic storage capacity, such as WO₃,¹² SnO₂,¹³ and CeO₂.¹⁴ Studies show that these composites display the anticorrosion effect on stainless steel (SS) in the absence of light.

In a previous work,¹⁵ graphitic carbon nitride (g-C₃N₄) was reported to have great application potential in the areas of corrosion protection and photoelectrochemical anticorrosion because of its remarkable properties, such as good anti-abrasion and erosion properties, comparatively negative conduction band potential, visible light absorption, high performance price ratio, and simple film formation processes. However, there are core issues concerning wider adoption of g-C₃N₄ in photoelectrochemical anticorrosion applications which need to be urgently solved. One of them is that the relatively low valence band potential (i.e., ∼1.3 V vs. NHE), which is slightly more positive than the oxidation potential of water (∼1.229 V vs. NHE), will result in a relatively low oxidation capability and lead to a lower hole consumption rate. That means, the anodic depolarization process of the holes photoinduced by g-C₃N₄ does not occur easily in the commonly used corrosion electrolyte systems, for example 3.5% NaCl, where water is the only hole scavenger.¹⁵ Therefore, in order to improve the applicability of g-C₃N₄ in the field of photoelectrochemical anticorrosion effect on metal, g-C₃N₄ must be modified to increase the depolarization capability of the holes generated by g-C₃N₄ photoelectrode in the process of the photoelectrochemical reactions. Until now, possible modification strategies mainly include the following aspects: compositing with other semiconductors that have more positive valence band potentials; modifying g-C₃N₄ with other nanomaterials that can decrease the energy barrier of anodic depolarization reactions; and positively shifting the valence band potential by doping methods.

ZnO has been widely studied in photoelectrochemical and photocatalytic areas. Compared with TiO₂, which was the first semiconductor material applied in photoelectric conversion, ZnO is both environment-friendly and inexpensive. The electron mobility of ZnO is about two orders of magnitude higher than that of TiO₂.¹⁶ Thus, ZnO may have higher photoelectric conversion capability than TiO₂. Meanwhile, the conduction band potential of ZnO is slightly more negative than that of TiO₂, and as a result, the electrons photogenerated by ZnO can be transferred much more easily to the coupled metal. Therefore, ZnO may be more advantageous than TiO₂ for photoelectrochemical anticorrosion applications. It was reported that ZnO, composited with an appropriate amount of π-conjugated material, such as polyaniline^17,18 and graphene,^19,20 can significantly increase the separation efficiency of the photogenerated electron–hole pairs due to the low electron transfer resistance in ZnO and the heterojunction electric field formed on the interface between them, which in turn vastly improves the photoelectric conversion ability of ZnO. g-C₃N₄ is a kind of π-conjugated material with graphite-like layered structure,²¹ and the force between the layered structures of g-C₃N₄ is much smaller than that between the graphite layers. Thus, it is easy to disperse g-C₃N₄ to ultra-thin single layered structure by ultrasonic dispersion assisted method. Meanwhile, dispersed single-layer C₃N₄ can be assembled very easily on the surfaces of other materials to form coating layers by intermolecular forces.²² Pan et al.²³ prepared C₃N₄@BiPO₄ shell–core structured composite nanomaterial by ultrasonic dispersion assisted method and found that the photocatalytic performance was significantly improved. Zhang et al.²⁴ coated a C₃N₄ cladding layer on the surface of CdS nanowire and they found that the existence of this C₃N₄ cladding layer can significantly enhance the photocatalytic hydrogen production capability of CdS from water splitting.

Based on the above discussions, the incorporation of g-C₃N₄ and ZnO into nanocomposites is a reasonable method for creating good photoelectrochemical anticorrosion performance. In this work, the carbon nitride@zinc oxide (C₃N₄@ZnO) composite with quasi-shell–core nanostructure was prepared by an ultrasonic dispersion assisted method and the photoelectrochemical anticorrosion performance of this composite was studied. Scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to investigate the microstructures and the chemical compositions of this composite. Data from UV/vis diffuse reflectance spectrum, photoinduced open circuit potential (OCP) variation, photoinduced current density, the photoinduced volt-ampere characteristic curve (i–V curve), electrochemical impedance spectroscopy (EIS), and Mott–Schottky curve were collected to study the photoelectrochemical anticorrosion performance of this composite and to analyze the promotion mechanism of the photoelectrochemical anticorrosion performance of this composite.

2. Experimental

2.1 Preparation of mg-C₃N₄

The preparation of mg-C₃N₄ was based on the method used by Goettmann et al.²⁵ First, 1.2 g silicon dioxide (SiO₂) nanopowder with a diameter of approximately 15 nm was dispersed in 20 mL deionized water, and the resulting mixture was ultrasonically vibrated for 30 min. This dispersion liquid was added slowly to a dicyanodiamine solution prepared by dissolving 3 g dicyanodiamine in 20 mL deionized water. The liquid mixture was stirred in a 70 °C water bath until the water in the liquid mixture was completely evaporated. The dried powder was ground in an agate mortar, transferred to a crucible with a lid, and heated at 550 °C for 4 h at a heating rate of 20 °C min⁻¹. The light-yellow powder generated after sintering was added to 100 mL of 4 mol L⁻¹ NH₄F solution and etched for 24 h. The powder was then filtered and washed repeatedly with deionized water. A yellow powder was obtained after drying at 80 °C for 4 h under vacuum. All reagents used in this study were of analytical grades and obtained from Aladdin Reagent Corporation.

2.2 Preparation of C₃N₄@ZnO composite with quasi-shell–core structure

A certain amount of mg-C₃N₄ (accounting for 1, 2, 3 and 5 wt% of the mass of ZnO) was dispersed in 50 mL methanol, followed by sonication for 30 min. C₃N₄ flakelets were formed during the ultrasonic vibration of mg-C₃N₄. After that, 0.2 g ZnO particles, which were bought from Aladdin Reagent Corporation, was added into the mixture, followed by sonication for another 5 min. The resulting liquid mixture was then put in a hood and stirred until the methanol was completely evaporated from the liquid mixture. The C₃N₄@ZnO quasi-shell–core composite materials with different C₃N₄-addition ratios (1–5 wt%) were obtained after heating the dried powder at 300 °C for 30 min at a heating rate of 20 °C min⁻¹. The relevant preparation procedure is shown in Scheme 1.

Scheme 1 Schematic illustration of the relevant preparation process of the C₃N₄@ZnO quasi-shell–core composite.

2.3 Characterizations of the prepared C₃N₄@ZnO quasi-shell–core composite materials

The morphologies and the microstructure of the synthetic products were analyzed using a field emission SEM (Inspect F50, FEI Company, USA) and a HRTEM (Tecnai G2 F20, FEI Company, USA). The crystalline structures and bonding information of the synthetic products were analyzed using XRD (D/MAX-2500/PC⁻¹; Rigaku Co., Tokyo, Japan) and XPS (Axis Ultra, Kratos Analytical Ltd., England). The optical absorption properties were investigated using a UV/Vis diffuse reflectance spectrophotometer (U-41000; HITACHI, Tokyo, Japan).

2.4 Photoelectrode preparation

A fluorine-doped tin oxide (FTO) conductive glass (13 × 10 mm) was first ultrasonically cleaned with acetone of analytical grade for 5 min, rinsed with deionized water, and then dried under a clean, dry airflow. One longitudinal edge of the conductive side was then carefully covered with insulating tape, with the exposed effective area of the FTO glass measuring 1 cm². In total, 0.01 g as-prepared powder was mixed with 0.1 mL deionized water in an agate mortar, and the mixture was carefully ground for 10 min to form a homogeneous suspension. Then, 0.025 mL as-prepared suspension was evenly distributed onto the exposed area of the conductive side of the FTO glass. The insulating tape on the edge of the FTO glass was removed after the suspension had dried in the air. Finally, the FTO glass deposited with C₃N₄@ZnO powder was heated to 120 °C for 2 h under vacuum. A copper wire was connected to the conductive side of the FTO glass using conductive silver tape. The uncoated parts of the conductive side of the FTO glass were isolated with parafilm after the conductive silver tape had dried.

2.5 Electrochemical/photoelectrochemical measurements

Electrochemical/photoelectrochemical measurements were performed using CHI660D Electrochemical Workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The series of C₃N₄@ZnO quasi-shell–core composite photoelectrodes prepared, Ag/AgCl (saturated KCl), and Pt electrode acted as the working, reference, and counter electrodes, respectively. The potentials are reported on the Ag/AgCl (saturated KCl) scale. The schematic illustrations of the experimental setup for measuring the variations of the photoinduced OCP and current density, which were used to evaluate the photoelectrochemical anticorrosion performance of the prepared composites, were described in detail and shown in Fig. 1 in a previous work from the authors' laboratory.¹⁵ The variations of the photoinduced current density were measured at a bias potential of 0 V, and the variations of the photoinduced OCP and current density were all measured under white light (250 mW cm⁻²) illumination in 3.5 wt% NaCl at 25 °C. The white light was generated by a Xe arc lamp (PLS-SXE300, Beijing Changtuo Co. Ltd., Beijing, China), passed through a flat circular quartz window, equipped on the side of the three-electrode cell, and illuminated on the backside of the photoelectrode.

Fig. 1 SEM images of (a) ZnO, (b) 1 wt% C₃N₄@ZnO quasi-shell–core composite and (c) 5 wt% C₃N₄@ZnO quasi-shell–core composite.

The i–V curves were measured from −0.8 V to 1.0 V with a scan rate of 0.02 V s⁻¹. The i–V curves, Mott–Schottky plots, and EIS tests were all performed in 0.1 mol L⁻¹ Na₂SO₄ at 25 °C. Both Mott–Schottky plots and EIS tests were measured in the dark. EIS tests were performed at OCP over the frequency range between 10⁴ and 10⁻¹ Hz, with an AC voltage magnitude of 5 mV, using 12 points/decade. Mott–Schottky plots were measured at the potential range of −0.6 V ∼ −0.1 V and the frequency of 10 Hz with an AC voltage magnitude of 10 mV. A three-electrode experimental system was employed to measure the i–V curves, Mott–Schottky plots, and EIS results.

3. Results and discussion

The surface morphologies of ZnO and C₃N₄@ZnO quasi-shell–core composites with different mg-C₃N₄ dosages (1% and 5%) were depicted in Fig. 1. It was shown that the ZnO existed as non-uniform short rods and most of them were approximately 1 µm in length and 500 nm in diameter (Fig. 1a). For 1 wt% C₃N₄@ZnO quasi-shell–core composite (Fig. 1b), ZnO particles became more unclear and the surface much smoother compared to those shown in Fig. 1a, demonstrating that C₃N₄ has covered the surface of the ZnO nanoparticles. When the amount of C₃N₄ added increased to 5 wt% (Fig. 1c), it was more difficult to distinguish the ZnO particles from the coating, and the surface became smoother than those shown in Fig. 1b, which may be caused by the coating of C₃N₄ on the surface of ZnO particles. Furthermore, the layered structure of C₃N₄ was not observed, as shown in Fig. 1b and c, indicating that the C₃N₄ added was dispersed as C₃N₄ flakelets with an ultra-thin single layered structure by ultrasonic dispersion assisted method during the preparation of the composite. Therefore, the composite prepared in this work can be expressed as C₃N₄@ZnO composites instead of mg-C₃N₄@ZnO composites, although the carbon nitride materials used were mg-C₃N₄.

Fig. 2 shows the XRD patterns of ZnO and C₃N₄@ZnO quasi-shell–core composites with different mg-C₃N₄ dosages (1%, 2%, 3% and 5%). The diffraction peaks of commercial ZnO used in this work correspond to the standard lead–zinc ore structure. There is no significant difference between the diffraction peaks of the C₃N₄@ZnO quasi-shell–core composites and ZnO. For the C₃N₄@ZnO quasi-shell–core composites, a diffraction peak at 2θ = 27.4° is expected, which is the characteristic index of the interlayer stacking of aromatic series for graphitic materials. However, this diffraction peak was not observed on the XRD patterns of the C₃N₄@ZnO quasi-shell–core composites, as shown in curves (b–e) in Fig. 2. The absence of this diffraction peak can be caused by two reasons. One is that the amount of C₃N₄ added in the composites is very little or the degree of crystallinity of C₃N₄ is not high enough. The other one is the disappearance of the layered structure of mg-C₃N₄ caused by ultrasonic dispersion.

Fig. 2 XRD patterns of (curve a) ZnO, (curve b) 1 wt% C₃N₄@ZnO quasi-shell–core composite, (curve c) 2 wt% C₃N₄@ZnO quasi-shell–core composite, (curve d) 3 wt% C₃N₄@ZnO quasi-shell–core composite and (curve e) 5 wt% C₃N₄@ZnO quasi-shell–core composite.

In order to further study the C₃N₄ loaded on the ZnO surface, ZnO and C₃N₄@ZnO quasi-shell–core composites with 1 and 5 wt% of mg-C₃N₄ were selected for HRTEM investigations. Fig. 3a is the HRTEM image of ZnO, from which the ZnO (002) crystal plane can be clearly observed and the surface of ZnO is not adsorbed by other substances.

Fig. 3 HRTEM images of (a) ZnO, (b) 1 wt% C₃N₄@ZnO quasi-shell–core composite and (c) 5 wt% C₃N₄@ZnO quasi-shell–core composite.

However, for 1 wt% C₃N₄@ZnO quasi-shell–core composite, a coating layer with the thickness of approximately 1 nm is observed on the surface of ZnO, as shown in Fig. 3b. Meanwhile, the distinctness of ZnO (002) crystal plane is obviously reduced, which can be attributed to the formation of C₃N₄ coating on the surface of ZnO. Fig. 3c is the HRTEM result of 5 wt% C₃N₄@ZnO quasi-shell–core composite, from which a coating layer with the thickness of approximately 4 nm is observed on the surface of ZnO. The clarity of the ZnO (002) crystal plane is further reduced compared with that shown in Fig. 3b. The HRTEM results demonstrate that C₃N₄ was successfully coated on the surface of ZnO and the thickness of the coated C₃N₄ layer increases with the increase of the amount of mg-C₃N₄ added.

The chemical states of elements C and N of the coating layer on the ZnO surface were characterized using XPS and the relevant results of 1 wt% C₃N₄@ZnO quasi-shell–core composite are shown in Fig. 4. Fig. 4a shows the total survey spectrum. The binding energy peaks corresponding to Zn, O, C and N are observed in Fig. 4a. Fig. 4b shows the C1s XPS core level spectrum and the peak with the binding energy of 284.6 eV is the characteristic peak of C1s. Fig. 4c shows the N1s XPS core level spectrum. A weak binding energy peak at 398.6 eV is observed, which is the characteristic peak of N1s. Compared with C1s, the characteristic peak of N1s at 398.6 eV is much weaker. The molar ratio of C to N in C₃N₄ is 3 : 4, thus, the difference of the intensities of these two peaks are expected to be little. In the present work, only 1 wt% C₃N₄ was coated onto the ZnO surface. In principle, the intensities of the characteristic peaks of both C1s and N1s should be very weak. However, the intensity of C1s peak is much stronger than that of N1s, as shown in Fig. 4b and c. The reason for this phenomenon could be the adsorption of CO₂ onto 1 wt% C₃N₄@ZnO quasi-shell–core composite, resulting in the excessively high intensity of the C1s characteristic peak, as shown in Fig. 4.

Fig. 4 XPS spectra of 1 wt% C₃N₄@ZnO quasi-shell–core composite. (a) shows the total survey spectrum. (b) shows the C1s XPS core level spectrum. (c) shows the N1s XPS core level spectrum.

Fig. 5 shows the UV/Vis diffuse reflectance spectra of ZnO and the C₃N₄@ZnO quasi-shell–core composites with different C₃N₄ dosages. The absorption band-edge of ZnO is approximately 380 nm, as shown in curve Fig. 5a, which is consistent with the standard absorption threshold of ZnO.²⁶ After introducing C₃N₄, the absorption band-edge of the C₃N₄@ZnO quasi-shell–core composite red-shifts to >400 nm and the optical absorption intensity in the ultraviolet region at wavelength less than 380 nm declines with the increase of the amount of C₃N₄ added, indicating that C₃N₄ competes with ZnO for the absorption of the incident photons. The absorption capacity of the coated C₃N₄ is enhanced with the increase of the thickness of the coated C₃N₄ layer, leading to the gradual decrease of the absorption capacity of the C₃N₄@ZnO quasi-shell–core composites in the UV region.

Fig. 5 UV/Vis diffuse reflectance spectra of ZnO (curve a); 1 wt% C₃N₄@ZnO quasi-shell–core composite (curve b); 2 wt% C₃N₄@ZnO quasi-shell–core composite (curve c); 3 wt% C₃N₄@ZnO quasi-shell–core composite (curve d) and 5 wt% C₃N₄@ZnO quasi-shell–core composite (curve e).

In order to study the effect of the coated C₃N₄ on the photoelectrochemical anticorrosion performance of ZnO, the OCP of C₃N₄@ZnO quasi-shell–core composites, 304 SS, and their mixed potentials were measured under white light switched on and off intermittently (Fig. 6). As shown by curve Fig. 6a, the OCP of 304 SS electrode in 3.5 wt% NaCl solution is −0.09 V. After coupling with the ZnO thin-film photoelectrode in the dark, the mixed potential decreases to −0.17 V, as shown by curve Fig. 6b. This can be ascribed to the more negative Fermi level of ZnO compared with 304 SS. After coupling with the 1 wt% C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode in the dark, the mixed potential decreases to −0.20 V, as shown by curve Fig. 6c. The OCP of 1 wt% C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode in the dark is −0.31 V, as shown by curve Fig. 6g. After coupling 1 wt% C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode with 304 SS electrode in the dark, the mixed potential shifts positively to −0.20 V. As shown by curves Fig. 6c–e, the mixed potential of the couple of C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode with 304 SS electrode in the dark decreases with the increase of the amount of C₃N₄ added. As we know, both C₃N₄ and ZnO are n-type semiconductors. C₃N₄ has a more negative conduction band potential and a more negative Fermi level potential than that of ZnO. After coating on the ZnO surface, C₃N₄ will pull the Fermi level potential of ZnO to the negative direction, thus leading to the decrease of the OCP of the couple of the C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode with the 304 SS electrode, and the shift increases with the increase of the amount of C₃N₄ added. Under white light illumination, the OCP of the 304 SS electrode coupled with C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode swiftly shifts to the negative direction. After three cycles of on-and-off white light illumination, the photoinduced OCP is stabilized. Table 1 shows the potential data obtained from Fig. 6. E₁ is the stable potential of the 304 SS electrode coupled with C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode after 100 seconds in dark condition. E₂ is the stable potential after 100 seconds light on at the third cycle of illumination. ΔE stands for the potential drop caused by the white light illumination, which is the difference between E₂ and E₁. As shown in Table 1, E₁ decreases with the increase of the addition ratio of C₃N₄ on ZnO in the dark, as discussed above. According to the data of the stable photoinduced potential, E₂, 1 wt% C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode can polarize the potential of 304 SS electrode to −0.40 V under white light illumination, which possesses the optimum anticorrosion potential compared with other addition ratios of C₃N₄ on ZnO. From the data of ΔE, the 304 SS electrode coupled with 1 wt% C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode possesses the largest photoinduced potential drop, i.e., −0.21 V, and ΔE decreases with the increase of the amount of C₃N₄ added. The photoinduced potential drop is decreased to −0.04 V by increasing the amount of C₃N₄ added to 5 wt%.

Fig. 6 Open circuit potential of the Galvanic couple of 304 stainless steel with ZnO (curve b); 1 wt% C₃N₄@ZnO quasi-shell–core composite (curve c); 2 wt% C₃N₄@ZnO quasi-shell–core composite (curve d); 3 wt% C₃N₄@ZnO quasi-shell–core composite (curve e); 5 wt% C₃N₄@ZnO quasi-shell–core composite (curve f) under white light switched on and off intermittently. Curve a is the open circuit potential of 304 SS electrode and curve g is that of 1 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrode in the dark in 3.5 wt% NaCl solution.

Table 1 Photoinduced open circuit potential variations of ZnO and the series C₃N₄@ZnO quasi-shell–core composite photoelectrodes

	ZnO	1 wt% C3N4@ZnO	2 wt% C3N4@ZnO	3 wt% C3N4@ZnO	5 wt% C3N4@ZnO
E1 (V)	−0.17	−0.19	−0.23	−0.26	−0.28
E2 (V)	−0.36	−0.40	−0.35	−0.36	−0.32
ΔE (V)	−0.19	−0.21	−0.12	−0.10	−0.04

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Fig. 7 shows the variations in the photoinduced current densities for the galvanic coupling between the ZnO or C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode and the 304 SS electrode under white light switched on and off intermittently. Positive excitation current densities are obtained under white light illumination. The C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode is connected to the working electrode interface of the potentiostat. Positive excitation currents show that the C₃N₄@ZnO quasi-shell–core composite thin-film electrode acts as an anode and generated anodic current, whereas the 304 SS electrode acts as a cathode and generated cathodic current. The generated anodic current indicates that the energies of the photoinduced electrons are high enough to overcome the energy barriers between the C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode and the 304 SS electrode. The photoinduced electrons can move toward the latter electrode to generate current, and the reduction of oxygen in the electrolyte on the interface between 304 SS and 3.5% NaCl can consume the photoinduced electrons but not those generated from the anodic dissolution of 304 SS, indicating that the 304 SS is cathodically protected under white light irradiation. As shown in Fig. 7, after three cycles of illumination, the photoinduced current gradually stabilizes. The largest photoinduced current density is observed for the galvanic coupling between the 1 wt% C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode and the 304 SS electrode, indicating that the 1 wt% C₃N₄@ZnO quasi-shell–core composite thin-film photoelectrode can provide the best photoinduced anticorrosion effect for 304 SS. The photoinduced current density declines with the further increase of the amount of C₃N₄ added in the C₃N₄@ZnO composites.

Fig. 7 Variations in the current densities for the galvanic couple between 304 SS electrode with ZnO (curve a); 1 wt% C₃N₄@ZnO quasi-shell–core composite (curve b); 2 wt% C₃N₄@ZnO quasi-shell–core composite (curve c); 3 wt% C₃N₄@ZnO quasi-shell–core composite (curve d); 5 wt% C₃N₄@ZnO quasi-shell–core composite (curve e) thin-film photoelectrode under white light switched on and off intermittently in 3.5 wt% NaCl solution.

From the results shown in Fig. 5, C₃N₄ competed with ZnO on the absorption of the incident photons, resulting in the decrease of the optical absorption of the C₃N₄@ZnO quasi-shell–core composites in the UV region. When the thickness of the C₃N₄ coating layer increases, the competitiveness of photon absorption by C₃N₄ is enhanced, resulting in the changes of its photoelectrochemical properties. As we know, the main issue of C₃N₄ in photoelectrochemical anticorrosion property is that its intrinsic valence band potential is very negative, which weakens the oxidizing capability of the photogenerated holes. The photogenerated holes by C₃N₄ are difficult to oxidize the OH⁻ ionized from water in 3.5% NaCl solution, therefore, they cannot be effectively consumed away. Therefore, it is difficult to achieve photoelectrochemical anticorrosion for 304 SS. However, for 1 wt% C₃N₄@ZnO composite, the role of ZnO on photon absorption is still dominant. Moreover, an efficient heterojunction electric field is formed between the interface of ZnO and C₃N₄, which can significantly improve the separation efficiency of the photogenerated electron–hole pairs, and hence can effectively promote its photoelectrochemical anticorrosion performance. The photoelectrochemical anticorrosion performance of C₃N₄@ZnO composites is determined by the amount of C₃N₄ added in the composite. When the amount of C₃N₄ added is low, the photogenerated holes by ZnO can transfer to the surface of C₃N₄ and react with water to eliminate them. Meanwhile, the photogenerated electrons can, through the incomplete coating area on the surface of ZnO, transfer to the nearby ZnO and finally to 304 SS. With the increase of the amount of C₃N₄ added, the incomplete coating area on the surface of ZnO may dramatically reduce, resulting in the increased difficulty for the electrons to transfer to 304 SS and decrease the photoelectrochemical anticorrosion performance to a certain extent. Considering the above reasons, the photoelectrochemical anticorrosion performance of the 1 wt% C₃N₄@ZnO quasi-shell–core composite is considered the best.

In order to study the principle of the promotion of the photoelectrochemical properties of ZnO coated by 1 wt% C₃N₄, the i–V curve, EIS curve, and Mott–Schottky curve were measured under the condition that 304 SS was not coupled with the 1 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrode. Fig. 8 shows the i–V curves of ZnO and the series of C₃N₄@ZnO quasi-shell–core composite photoelectrodes. After the bias reaches −0.4 V, the photoinduced current increases quickly, and the maximum photoinduced current is obtained with the bias of −0.2 V. Afterwards, with the further increase of the bias potential, the photoinduced current declines. Among these series of photoelectrodes, the 1 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrode has the largest photoinduced current, and excessive C₃N₄ will decrease the photoinduced current of C₃N₄@ZnO quasi-shell–core composite photoelectrode, as shown in Fig. 8, demonstrating that 1 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrode possesses the best photoelectric conversion efficiency even without coupling with 304 SS.

Fig. 8 Photoinduced volt–ampere characteristic curves of ZnO photoelectrodes; 1 wt% C₃N₄@ZnO quasi-shell–core composite; 2 wt% C₃N₄@ZnO quasi-shell–core composite; 3 wt% C₃N₄@ZnO quasi-shell–core composite; and 5 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrodes in 0.1 M Na₂SO₄ solution.

Fig. 9 shows the EIS results for the ZnO electrode, 1 wt% C₃N₄@ZnO and 5 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrodes in the dark. The EIS results obtained from Fig. 9 were fitted using the equivalent circuit inserted in the same figure. In this equivalent circuit, R_sol is the solution resistance, CPE is the constant phase angle element and its impedance is equal to (Y₀(jω)ⁿ)⁻¹, where ω is the ac-voltage angular frequency (rad s⁻¹), and Y₀ and n are the frequency-independent parameters. R_T is the electron migration resistance in the thin-film photoelectrode, and the electron transfer and the reaction resistance on the interface between the semiconductor and the electrolyte. As shown in Fig. 9, the measured data are represented by dots with different symbols, while the solid lines are the fitted results using the equivalent circuit provided. The measured data are fitted very well by the equivalent circuit. Table 2 lists the parameters of the EIS data. The R_T value obtained for ZnO is 2.79 × 10⁵ Ω cm², which is approximately twice than that for 1 wt% C₃N₄@ZnO composite, i.e., 1.33 × 10⁵ Ω cm² (Table 2).While, the R_T value obtained for 5 wt% C₃N₄@ZnO composite is 2.10 × 10⁵ Ω cm², which is located between those for ZnO and 1 wt% C₃N₄@ZnO composite. Because of the relatively low intrinsic electronic mobility in Eigen state C₃N₄, the electronic mobility of C₃N₄@ZnO composite, in principle, cannot benefit from the coating of C₃N₄ on ZnO. Therefore, it is considered that the decrease of the R_T value of 1 wt% C₃N₄@ZnO quasi-shell–core composite is mostly due to the formation of the heterojunction electric field on the interface between the ZnO and the coated C₃N₄, which will enhance the electron–hole separation efficiency and accelerate the interface reactions, thus causes the significant decrease of R_T. When the C₃N₄ added is 5 wt%, the electron migration resistance in the thin-film photoelectrode increases again, indicating that the further increase in the thickness of the C₃N₄ coating layer will increase the electron migration resistance and hence decrease the photoelectrochemical performance of the C₃N₄@ZnO composite.

Fig. 9 EIS spectra of the photoelectrodes prepared by ZnO, 1 wt% C₃N₄@ZnO and 5 wt% C₃N₄@ZnO quasi-shell–core composite and the equivalent circuit for fitting the EIS results obtained in this work in 0.1 M Na₂SO₄ solution.

Table 2 Fitted parameters of the EIS of ZnO, 1 wt% C₃N₄@ZnO and 5 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrodes in 0.1 M Na₂SO₄ solution in the dark

Samples	RSol (Ω cm^2)	n	CPE (Ω^−1 cm^2 s^n)	RT (Ω cm^2)
ZnO	32.10	0.92	1.17 × 10^−5	2.79 × 10^5
1 wt% C3N4@ZnO	28.80	0.90	1.18 × 10^−5	1.33 × 10^5
5 wt% C3N4@ZnO	29.30	0.91	1.19 × 10^−5	2.10 × 10^5

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The conduction band potential and Fermi level potential of C₃N₄ are more negative than those of ZnO. Therefore, after coating with C₃N₄, ZnO is bound to cause the changes in energy level potential and thus a new interfacial electric field will be formed on the interface between C₃N₄ and ZnO. In the present work, Mott–Schottky method was employed to study the relation between capacitance of the space charge region and the applied potential. The description of the specific formula is as follows:

1/C² = 2(eεε₀N_D)⁻¹(E − E_fb − κT/e) (1)

where C is the capacitance of the space charge region in the semiconductor, N_D is the electron carrier density, e is the elemental charge, ε₀ is the permittivity of free space, ε is the relative permittivity of the semiconductor, E is the applied potential, E_fb is the flat band potential, T is the temperature, and κ is the Boltzmann constant.

The flatband potential of a semiconductor material can be determined by extrapolating to C⁻² = 0. The flatband potential of ZnO is approximately −0.38 V (Fig. 10). After coating with 1 and 5 wt% C₃N₄, the flatband potentials of the C₃N₄@ZnO composites are negatively shifted to −0.42 and −0.44 V (Fig. 10), respectively, indicating that trace C₃N₄ causes the negative shift of the flatband potentials of ZnO and the negative shift value increases with the increasing amount of C₃N₄ added in the C₃N₄@ZnO composites. This change in trend is consistent with that of the OCPs at the first 100 s in the dark shown in Fig. 6, from which the OCPs of the C₃N₄@ZnO composites shift to the negative direction with the increasing amount of C₃N₄ added. Because the Fermi level potential of C₃N₄ is more negative than that of ZnO, the new Fermi level of 1 wt% C₃N₄@ZnO composite will be located between those of C₃N₄ and ZnO after coating C₃N₄ on ZnO surface. The negative shift of the Fermi level of ZnO will induce the negative shift of the conduction band potential of ZnO,²⁷ which will, therefore, increase the reduction capability of the photogenerated electrons, and thus improve its photoelectrochemical anticorrosion performance to some extent. Meanwhile, the movement of the Fermi level will lead to the directional migration of electrons and thus the formation of electrical field at the interfaces. Under white light illumination, this interfacial electric field can increase the separation efficiency of the photogenerated electron–hole pairs and the lifetime of the photogenerated electrons, therefore improving the photoelectrochemical anticorrosion performance of C₃N₄@ZnO composite compared to that of ZnO.

Fig. 10 Mott–Schottky plots of ZnO, 1 wt% C₃N₄@ZnO and 5 wt% C₃N₄@ZnO quasi-shell–core composite photoelectrodes in 0.1 M Na₂SO₄ solution in the dark.

Fig. 11 shows schematically the proposed mechanism for the promotion of the photoelectrochemical anticorrosion performance for 304 SS by 1 wt% C₃N₄@ZnO quasi-shell–core composite. After coating 1 wt% C₃N₄ on ZnO surface, the thickness of the C₃N₄ coating layer was found to be approximately 1 nm so C₃N₄ is not the major photon absorption material in this composite. The main photonabsorption effect mainly comes from ZnO. ZnO absorbs light with wavelengths less than 380 nm and generates photoinduced electrons and holes. The difference in Fermi levels between ZnO and C₃N₄ results in the formation of a heterojunction electric field on the interface between these two phases. Usually, the movement of Fermi level will lead to the bending of the energy bands of these two semiconductors. However, the thickness of the C₃N₄ coating layer is only 1 nm, which is much smaller than that of the depletion layer of the semiconductor. The valence band of C₃N₄ remains in a flatband state because it cannot form an effective energy band bending. The photogenerated electrons on the conduction band of ZnO will move to the FTO glass and will finally transfer to the coupled 304 SS to provide an effective cathodic protection. The photogenerated holes left on the valence band of ZnO will transfer to the valence band of C₃N₄ and participate in the oxidation process on the surface of C₃N₄. Therefore, the coating a small amount of C₃N₄ on ZnO surface can increase the separation efficiency of the photogenerated electrons and holes and thus improve the photoelectrochemical anticorrosion performance of ZnO. On the other hand, the coating of C₃N₄ on ZnO surface can improve the surface state of ZnO and allow much easier connection of C₃N₄@ZnO quasi-shell–core composite with other organic coatings, further enhancing its potential for practical applications.

Fig. 11 Proposed mechanism for the promotion of the photoelectrochemical anticorrosion performance for 304 stainless steel by 1 wt% C₃N₄@ZnO quasi-shell–core composite.

4. Conclusion

The C₃N₄@ZnO composite with quasi-shell–core structure was successfully prepared in this work and it was shown to provide the best photoelectrochemical anticorrosion capability for 304 SS when the C₃N₄ added was 1 wt%. With the further increase in the C₃N₄ added, the photoelectrochemical anticorrosion capability of the C₃N₄@ZnO composite with quasi-shell–core structure declines. According to the experimental results from UV/Vis diffuse reflectance spectra, i–V curves, electrochemical impedance spectra and Mott–Schottky plots, an ultrathin coating layer of C₃N₄ on the surface of ZnO helps to form a heterojunction electric field at the interface between C₃N₄ and ZnO, thereby enhancing the separation efficiency of the photogenerated electron–hole pairs. However, excessive C₃N₄ will compete with ZnO on the absorption of photons, and the photoelectrochemical anticorrosion performance of this composite will significantly decline when C₃N₄ becomes the principal photon absorber. Meanwhile, an overly thick C₃N₄ cladding layer will increase the charge transfer resistance, and thus lower the photoelectrochemical anticorrosion performance of this composite to some extent.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 41376126) and the Hundreds-Talent Program of the Chinese Academy of Sciences (Y02616101L).

Notes and references

1. B. A. Shaw and R. G. Kelly, Electrochem. Soc. Interface, 2006, 24–26.
2. Y. Ohko, S. Saitoh, T. Tatsuma and A. Fujishima, J. Electrochem. Soc., 2001, 148, B24–B28.
3. H. W. Park, K. Y. Kim and W. Y. Choi, J. Phys. Chem. B, 2002, 106, 4775–4781.
4. C. Lei, Y. Liu, H. Zhou, Z. Feng and R. Du, Corros. Sci., 2013, 68, 214–222.
5. Z. Q. Lin, Y. K. Lai, R. G. Hu, J. Li, R. G. Du and C. J. Lin, Electrochim. Acta, 2010, 55, 8717–8723.
6. S. Li and J. Fu, Corros. Sci., 2013, 68, 101–110.
7. S. Li, Q. Wang, T. Chen, Z. Zhou, Y. Wang and J. Fu, Nanoscale Res. Lett., 2012, 7, 227.
8. Y. Ohko, S. Saitoh, T. Tatsuma and A. Fujishima, Electrochem. Solid-State Lett., 2002, 5, B9–B12.
9. Y. Y. Bu, W. B. Li, J. Q. Yu, X. T. Wang, M. L. Qi, M. Y. Nie and B. R. Hou, Acta Phys.-Chim. Sin., 2011, 27, 2393–2399.
10. M. M. Sun, Z. Y. Chen, Y. Y. Bu, J. Q. Yu and B. R. Hou, Corros. Sci., 2014, 82, 77–84.
11. Y. F. Zhu, R. G. Du, W. Chen, H. Q. Qi and C. J. Lin, Electrochem. Commun., 2010, 12, 1626–1629.
12. T. Tatsuma, S. Saitoh, Y. Ohko and A. Fujishima, Chem. Mater., 2001, 13, 2838–2842.
13. R. Subasri and T. Shinohara, Electrochem. Commun., 2003, 5, 897–902.
14. R. Subasri, S. Deshpande, S. Seal and T. Shinohara, Electrochem. Solid-State Lett., 2006, 9, B1–B4.
15. Y. Y. Bu, Z. Y. Chen, J. Q. Yu and W. B. Li, Electrochim. Acta, 2013, 88, 294–300.
16. H. Pan, N. Misra, S. H. Ko, C. P. Grigoropoulos, N. Miller, E. E. Haller and O. Dubon, Appl. Phys. A: Mater. Sci. Process., 2009, 94, 111–115.
17. V. Eskizeybek, F. Sari, H. Gulce, A. Gulce and A. Avci, Appl. Catal., B, 2012, 119, 197–206.
18. J. Singh, A. P. Bhondkar, M. L. Singla and A. Sharma, ACS Appl. Mater. Interfaces, 2013, 5, 5346–5357.
19. T. Xu, L. Zhang, H. Cheng and Y. Zhu, Appl. Catal., B, 2011, 101, 382–387.
20. Y. Y. Bu, Z. Y. Chen, W. B. Li and B. R. Hou, ACS Appl. Mater. Interfaces, 2013, 5, 12361–12368.
21. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
22. X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan and Y. Xie, J. Am. Chem. Soc., 2013, 135, 18–21.
23. C. Pan, J. Xu, Y. Wang, D. Li and Y. Zhu, Adv. Funct. Mater., 2012, 22, 1518–1524.
24. J. Zhang, Y. Wang, J. Jin, J. Zhang, Z. Lin, F. Huang and J. Yu, ACS Appl. Mater. Interfaces, 2013, 5, 10317–10324.
25. F. Goettmann, A. Fischer, M. Antonietti and A. Thomas, Angew. Chem., Int. Ed., 2006, 45, 4467–4471.
26. A. Steinfeld, Int. J. Hydrogen Energy, 2002, 27, 611–619.
27. Z. Zhang and J. T. Yates, Chem. Rev., 2012, 112, 5520–5551.

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