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TiO2 is a semiconductor material used for photoelectric conversion. To improve their use of light, nickel and silver sulfide nanoparticles were synthesized on the surface of TiO2 nanowires by a simple dipping and photoreduction method. A series of studies of the cathodic protective action of Ag/NiS/TiO2 nanocomposites on 304 stainless steel has been carried out, and the morphology, composition, and light absorption characteristics of materials have been supplemented. The results show that the prepared Ag/NiS/TiO2 nanocomposites can provide the best cathodic protection for 304 stainless steel when the number of nickel sulfide impregnation-precipitation cycles is 6 and the silver nitrate photoreduction concentration is 0.1M.
The application of n-type semiconductors for photocathode protection using sunlight has become a hot topic in recent years. When excited by sunlight, electrons from the valence band (VB) of a semiconductor material will be excited into the conduction band (CB) to generate photogenerated electrons. If the conduction band potential of the semiconductor or nanocomposite is more negative than the self-etching potential of the bound metal, these photogenerated electrons will transfer to the surface of the bound metal. The accumulation of electrons will lead to cathodic polarization of the metal and provide cathodic protection of the associated metal1,2,3,4,5,6,7. The semiconductor material is theoretically considered a non-sacrificial photoanode, since the anodic reaction does not degrade the semiconductor material itself, but the oxidation of water through photogenerated holes or adsorbed organic pollutants, or the presence of collectors to trap photogenerated holes. Most importantly, the semiconductor material must have a CB potential that is more negative than the corrosion potential of the metal being protected. Only then can the photogenerated electrons pass from the conduction band of the semiconductor to the protected metal. Photochemical corrosion resistance studies have focused on inorganic n-type semiconductor materials with wide band gaps (3.0–3.2EV)1,2,3,4,5,6,7, which are only responsive to ultraviolet light (< 400 nm), reducing the availability of light. Photochemical corrosion resistance studies have focused on inorganic n-type semiconductor materials with wide band gaps (3.0–3.2EV)1,2,3,4,5,6,7, which are only responsive to ultraviolet light (< 400 nm), reducing the availability of light. Исследования стойкости к фотохимической коррозии были сосредоточены на неорганических полупроводниковых материалах n-типа с широкой запрещенной зоной (3,0–3,2 EV)1,2,3,4,5,6,7, которые реагируют только на ультрафиолетовое излучение (< 400 нм), уменьшение доступности света. Research on photochemical corrosion resistance has focused on n-type inorganic semiconductor materials with a wide bandgap (3.0–3.2 EV)1,2,3,4,5,6,7 that only respond to ultraviolet radiation (< 400 nm), reduced light availability.光化学耐腐蚀性研究主要集中在具有宽带隙(3.0–3.2EV)1,2,3,4,5,6,7 的无机n 型半导体材料上，这些材料仅对紫外光（< 400 nm）有响应，减少光的可用性。光 化学 耐腐 蚀性 研究 主要 在 具有 宽带隙 宽带隙 宽带隙 (3.0–3.2ev) 1.2,3,4,5,6,6,7 的 无机 n 型 材料 上 ， 这些 材料 仅 对 （<400 nm） 有 有 有 有 有 有 有 有 有 有 有 有 有 有 有 有 有 有响应，减少光的可用性。 Исследования стойкости к фотохимической коррозии в основном были сосредоточены на неорганических полупроводниковых материалах n-типа с широкой запрещенной зоной (3,0–3,2EV)1,2,3,4,5,6,7, которые чувствительны только к УФ-излучению (<400 нм). Research on photochemical corrosion resistance has mainly focused on wide bandgap (3.0–3.2EV)1,2,3,4,5,6,7 n-type inorganic semiconductor materials that are only sensitive to UV radiation. (<400 nm). In response, the availability of light decreases.
In the field of marine corrosion protection, photoelectrochemical cathodic protection technology plays a key role. TiO2 is a semiconductor material with excellent UV light absorption and photocatalytic properties. However, due to the low rate of use of light, photogenerated electron holes recombine easily and cannot be shielded under dark conditions. Further research is needed to find a reasonable and feasible solution. It has been reported that many surface modification methods can be used to improve the photosensitivity of TiO2, such as doping with Fe, N, and mixing with Ni3S2, Bi2Se3, CdTe, etc. Therefore, TiO2 composite with materials with high photoelectric conversion efficiency is widely used in the field of photogenerated cathodic protection. .
Nickel sulfide is a semiconductor material with a narrow band gap of only 1.24 eV8.9. The narrower the band gap, the stronger the use of light. After the nickel sulfide is mixed with the titanium dioxide surface, the degree of light utilization can be increased. Combined with titanium dioxide, it can effectively improve the separation efficiency of photogenerated electrons and holes. Nickel sulfide is widely used in electrocatalytic hydrogen production, batteries and pollutant decomposition8,9,10. However, its use in photocathode protection has not yet been reported. In this study, a narrow bandgap semiconductor material was chosen to solve the problem of low TiO2 light utilization efficiency. Nickel and silver sulfide nanoparticles were bound on the surface of TiO2 nanowires by immersion and photoreduction methods, respectively. The Ag/NiS/TiO2 nanocomposite improves light utilization efficiency and extends the light absorption range from the ultraviolet region to the visible region. Meanwhile, the deposition of silver nanoparticles gives the Ag/NiS/TiO2 nanocomposite excellent optical stability and stable cathodic protection.
First, a titanium foil 0.1 mm thick with a purity of 99.9% was cut to a size of 30 mm × 10 mm for experiments. Then, each surface of the titanium foil was polished 100 times with 2500 grit sandpaper, and then washed successively with acetone, absolute ethanol, and distilled water. Place the titanium plate in a mixture of 85 °C (sodium hydroxide: sodium carbonate: water = 5:2:100) for 90 min, remove and rinse with distilled water. The surface was etched with HF solution (HF:H2O = 1:5) for 1 min, then washed alternately with acetone, ethanol, and distilled water, and finally dried for use. Titanium dioxide nanowires were rapidly fabricated on the surface of titanium foil by a one-step anodizing process. For anodizing, a traditional two-electrode system is used, the working electrode is a titanium sheet, and the counter electrode is a platinum electrode. Place the titanium plate in 400 ml of 2 M NaOH solution with electrode clamps. The DC power supply current is stable at about 1.3 A. The temperature of the solution was maintained at 80°C for 180 minutes during the systemic reaction. The titanium sheet was taken out, washed with acetone and ethanol, washed with distilled water, and dried naturally. Then the samples were placed in a muffle furnace at 450°C (heating rate 5°C/min), kept at a constant temperature for 120 min, and placed in a drying tray.
The nickel sulfide-titanium dioxide composite was obtained by a simple and easy dip-deposition method. First, nickel nitrate (0.03 M) was dissolved in ethanol and kept under magnetic stirring for 20 minutes to obtain an ethanol solution of nickel nitrate. Then prepare sodium sulfide (0.03 M) with a mixed solution of methanol (methanol:water = 1:1). Then, the titanium dioxide tablets were placed in the solution prepared above, taken out after 4 minutes, and quickly washed with a mixed solution of methanol and water (methanol:water=1:1) for 1 minute. After the surface had dried, the tablets were placed in a muffle furnace, heated in vacuum at 380°C for 20 min, cooled to room temperature, and dried. Number of cycles 2, 4, 6 and 8.
Ag nanoparticles modified Ag/NiS/TiO2 nanocomposites by photoreduction12,13. The resulting Ag/NiS/TiO2 nanocomposite was placed in the silver nitrate solution necessary for the experiment. Then the samples were irradiated with ultraviolet light for 30 min, their surfaces were cleaned with deionized water, and Ag/NiS/TiO2 nanocomposites were obtained by natural drying. The experimental process described above is shown in Figure 1.
Ag/NiS/TiO2 nanocomposites have been mainly characterized by field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance in the ultraviolet and visible ranges (UV-Vis). FESEM was performed using a Nova NanoSEM 450 microscope (FEI Corporation, USA). Accelerating voltage 1 kV, spot size 2.0. The device uses a CBS probe to receive secondary and backscattered electrons for topography analysis. EMF was carried out using an Oxford X-Max N50 EMF system (Oxford Instruments Technology Co., Ltd.) with an accelerating voltage of 15 kV and a spot size of 3.0. Qualitative and quantitative analysis using characteristic X-rays. X-ray photoelectron spectroscopy was performed on an Escalab 250Xi spectrometer (Thermo Fisher Scientific Corporation, USA) operating in a fixed energy mode with an excitation power of 150 W and monochromatic Al Kα radiation (1486.6 eV) as an excitation source. Full scan range 0–1600 eV, total energy 50 eV, step width 1.0 eV, and impure carbon (~284.8 eV) were used as binding energy charge correction references. The pass energy for narrow scanning was 20 eV with a step of 0.05 eV. Diffuse reflectance spectroscopy in the UV-visible region was performed on a Cary 5000 spectrometer (Varian, USA) with a standard barium sulfate plate in the scanning range of 10–80°.
In this work, the composition (weight percent) of 304 stainless steel is 0.08 C, 1.86 Mn, 0.72 Si, 0.035 P, 0.029 s, 18.25 Cr, 8.5 Ni, and the rest is Fe. 10mm x 10mm x 10mm 304 stainless steel, epoxy potted with 1 cm2 exposed surface area. Its surface was sanded with 2400 grit silicon carbide sandpaper and washed with ethanol. The stainless steel was then sonicated in deionized water for 5 minutes and then stored in an oven.
In the OCP experiment, 304 stainless steel and an Ag/NiS/TiO2 photoanode were placed in a corrosion cell and a photoanode cell, respectively (Fig. 2). The corrosion cell was filled with a 3.5% NaCl solution, and 0.25 M Na2SO3 was poured into the photoanode cell as a hole trap. The two electrolytes were separated from the mixture using a naphthol membrane. OCP was measured on an electrochemical workstation (P4000+, USA). The reference electrode was a saturated calomel electrode (SCE). A light source (xenon lamp, PLS-SXE300C, Poisson Technologies Co., Ltd.) and a cut-off plate 420 were placed at the outlet of the light source, allowing visible light to pass through the quartz glass to the photoanode. The 304 stainless steel electrode is connected to the photoanode with a copper wire. Before the experiment, the 304 stainless steel electrode was soaked in 3.5% NaCl solution for 2 h to ensure steady state. At the beginning of the experiment, when the light is turned on and off, the excited electrons of the photoanode reach the surface of 304 stainless steel through the wire.
In experiments on the photocurrent density, 304SS and Ag/NiS/TiO2 photoanodes were placed in corrosion cells and photoanode cells, respectively (Fig. 3). The photocurrent density was measured on the same setup as the OCP. To obtain the actual photocurrent density between 304 stainless steel and the photoanode, a potentiostat was used as a zero resistance ammeter to connect 304 stainless steel and the photoanode under non-polarized conditions. To do this, the reference and counter electrodes in the experimental setup were short-circuited, so that the electrochemical workstation worked as a zero-resistance ammeter that could measure the true current density. The 304 stainless steel electrode is connected to the ground of the electrochemical workstation, and the photoanode is connected to the working electrode clamp. At the beginning of the experiment, when the light is turned on and off, the excited electrons of the photoanode through the wire reach the surface of 304 stainless steel. At this time, a change in the photocurrent density on the surface of 304 stainless steel can be observed.
To study the cathodic protection performance of nanocomposites on 304 stainless steel, changes in the photoionization potential of 304 stainless steel and nanocomposites, as well as changes in photoionization current density between nanocomposites and 304 stainless steels, were tested.
On fig. 4 shows changes in the open circuit potential of 304 stainless steel and nanocomposites under visible light irradiation and under dark conditions. On fig. 4a shows the influence of NiS deposition time by immersion on the open circuit potential, and fig. 4b shows the effect of silver nitrate concentration on open circuit potential during photoreduction. On fig. 4a shows that the open circuit potential of the NiS/TiO2 nanocomposite bonded to 304 stainless steel is significantly reduced at the moment the lamp is turned on compared to the nickel sulfide composite. In addition, the open circuit potential is more negative than that of pure TiO2 nanowires, indicating that the nickel sulfide composite generates more electrons and improves the photocathode protection effect from TiO2. However, at the end of exposure, the no-load potential rises rapidly to the no-load potential of stainless steel, indicating that nickel sulfide does not have an energy storage effect. The effect of the number of immersion deposition cycles on the open circuit potential can be observed in Fig. 4a. At a deposition time of 6, the extreme potential of the nanocomposite reaches -550 mV relative to the saturated calomel electrode, and the potential of the nanocomposite deposited by a factor of 6 is significantly lower than that of the nanocomposite under other conditions. Thus, the NiS/TiO2 nanocomposites obtained after 6 deposition cycles provided the best cathodic protection for 304 stainless steel.
Changes in OCP of 304 stainless steel electrodes with NiS/TiO2 nanocomposites (a) and Ag/NiS/TiO2 nanocomposites (b) with and without illumination (λ > 400 nm).
As shown in fig. 4b, the open circuit potential of 304 stainless steel and Ag/NiS/TiO2 nanocomposites was significantly reduced when exposed to light. After surface deposition of silver nanoparticles, the open circuit potential was significantly reduced compared to pure TiO2 nanowires. The potential of the NiS/TiO2 nanocomposite is more negative, indicating that the cathodic protective effect of TiO2 improves significantly after Ag nanoparticles are deposited. The open circuit potential increased rapidly at the end of the exposure, and compared to the saturated calomel electrode, the open circuit potential could reach -580 mV, which was lower than that of 304 stainless steel (-180 mV). This result indicates that the nanocomposite has a remarkable energy storage effect after silver particles are deposited on its surface. On fig. 4b also shows the effect of silver nitrate concentration on the open circuit potential. At a silver nitrate concentration of 0.1 M, the limiting potential relative to a saturated calomel electrode reaches -925 mV. After 4 application cycles, the potential remained at the level after the first application, which indicates the excellent stability of the nanocomposite. Thus, at a silver nitrate concentration of 0.1 M, the resulting Ag/NiS/TiO2 nanocomposite has the best cathodic protective effect on 304 stainless steel.
NiS deposition on the surface of TiO2 nanowires gradually improves with increasing NiS deposition time. When visible light strikes the surface of the nanowire, more nickel sulfide active sites are excited to generate electrons, and the photoionization potential decreases more. However, when nickel sulfide nanoparticles are excessively deposited on the surface, excited nickel sulfide is reduced instead, which does not contribute to light absorption. After the silver particles are deposited on the surface, due to the surface plasmon resonance effect of the silver particles, the generated electrons will be quickly transferred to the surface of 304 stainless steel, resulting in excellent cathodic protection effect. When too many silver particles are deposited on the surface, the silver particles become a recombination point for photoelectrons and holes, which does not contribute to the generation of photoelectrons. In conclusion, Ag/NiS/TiO2 nanocomposites can provide the best cathodic protection for 304 stainless steel after 6-fold nickel sulfide deposition under 0.1 M silver nitrate.
The photocurrent density value represents the separating power of photogenerated electrons and holes, and the greater the photocurrent density, the stronger the separating power of photogenerated electrons and holes. There are many studies showing that NiS is widely used in the synthesis of photocatalytic materials to improve the photoelectric properties of materials and to separate holes15,16,17,18,19,20. Chen et al. studied noble-metal-free graphene and g-C3N4 composites co-modified with NiS15. The maximum intensity of the photocurrent of the modified g-C3N4/0.25%RGO/3%NiS is 0.018 μA/cm2. Chen et al. studied CdSe-NiS with a photocurrent density of about 10 µA/cm2.16. Liu et al. synthesized a CdS@NiS composite with a photocurrent density of 15 µA/cm218. However, the use of NiS for photocathode protection has not yet been reported. In our study, the photocurrent density of TiO2 was significantly increased by the modification of NiS. On fig. 5 shows changes in the photocurrent density of 304 stainless steel and nanocomposites under visible light conditions and without illumination. As shown in fig. 5a, the photocurrent density of the NiS/TiO2 nanocomposite increases rapidly at the moment the light is turned on, and the photocurrent density is positive, indicating the flow of electrons from the nanocomposite to the surface through the electrochemical workstation. 304 stainless steel. After the preparation of nickel sulfide composites, the photocurrent density is greater than that of pure TiO2 nanowires. The photocurrent density of NiS reaches 220 μA/cm2, which is 6.8 times higher than that of TiO2 nanowires (32 μA/cm2), when NiS is immersed and deposited 6 times. As shown in fig. 5b, the photocurrent density between the Ag/NiS/TiO2 nanocomposite and 304 stainless steel was significantly higher than between pure TiO2 and the NiS/TiO2 nanocomposite when turned on under a xenon lamp. On fig. Figure 5b also shows the effect of the AgNO concentration on the photocurrent density during photoreduction. At a silver nitrate concentration of 0.1 M, its photocurrent density reaches 410 μA/cm2, which is 12.8 times higher than that of TiO2 nanowires (32 μA/cm2) and 1.8 times higher than that of NiS/TiO2 nanocomposites. A heterojunction electric field is formed at the Ag/NiS/TiO2 nanocomposite interface, which facilitates the separation of photogenerated electrons from holes.
Changes in the photocurrent density of a 304 stainless steel electrode with (a) NiS/TiO2 nanocomposite and (b) Ag/NiS/TiO2 nanocomposite with and without illumination (λ > 400 nm).
Thus, after 6 cycles of nickel sulfide immersion-deposition in 0.1 M concentrated silver nitrate, the photocurrent density between Ag/NiS/TiO2 nanocomposites and 304 stainless steel reaches 410 μA/cm2, which is higher than that of saturated calomel. electrodes reaches -925 mV. Under these conditions, 304 stainless steel combined with Ag/NiS/TiO2 can provide the best cathodic protection.
On fig. 6 shows surface electron microscope images of pure titanium dioxide nanowires, composite nickel sulfide nanoparticles, and silver nanoparticles under optimal conditions. On fig. 6a, d show pure TiO2 nanowires obtained by single-stage anodization. The surface distribution of titanium dioxide nanowires is uniform, the structures of nanowires are close to each other, and the pore size distribution is uniform. Figures 6b and e are electron micrographs of titanium dioxide after 6-fold impregnation and deposition of nickel sulfide composites. From an electron microscopic image magnified 200,000 times in Fig. 6e, it can be seen that the nickel sulfide composite nanoparticles are relatively homogeneous and have a large particle size of about 100–120 nm in diameter. Some nanoparticles can be observed in the spatial position of the nanowires, and titanium dioxide nanowires are clearly visible. On fig. 6c,f show electron microscopic images of NiS/TiO2 nanocomposites at an AgNO concentration of 0.1 M. Compared to Figs. 6b and fig. 6e, fig. 6c and fig. 6f show that the Ag nanoparticles are deposited on the surface of the composite material, with the Ag nanoparticles uniformly distributed with a diameter of about 10 nm. On fig. 7 shows a cross section of Ag/NiS/TiO2 nanofilms subjected to 6 cycles of NiS dip deposition at an AgNO3 concentration of 0.1 M. From high magnification images, the measured film thickness was 240-270 nm. Thus, nickel and silver sulfide nanoparticles are assembled on the surface of TiO2 nanowires.
Pure TiO2 (a, d), NiS/TiO2 nanocomposites with 6 cycles of NiS dip deposition (b, e) and Ag/NiS/NiS with 6 cycles of NiS dip deposition at 0.1 M AgNO3 SEM images of TiO2 nanocomposites (c , e).
Cross section of Ag/NiS/TiO2 nanofilms subjected to 6 cycles of NiS dip deposition at an AgNO3 concentration of 0.1 M.
On fig. 8 shows the surface distribution of elements over the surface of Ag/NiS/TiO2 nanocomposites obtained from 6 cycles of nickel sulfide dip deposition at a silver nitrate concentration of 0.1 M. The surface distribution of elements shows that Ti, O, Ni, S and Ag were detected. using energy spectroscopy. In terms of content, Ti and O are the most common elements in the distribution, while Ni and S are approximately the same, but their content is much lower than Ag. It can also be proved that the amount of surface composite silver nanoparticles is greater than that of nickel sulfide. The uniform distribution of elements on the surface indicates that nickel and silver sulfide are uniformly bonded on the surface of the TiO2 nanowires. X-ray photoelectron spectroscopic analysis was additionally carried out to analyze the specific composition and binding state of substances.
Distribution of elements (Ti, O, Ni, S, and Ag) of Ag/NiS/TiO2 nanocomposites at an AgNO3 concentration of 0.1 M for 6 cycles of NiS dip deposition.
On fig. Figure 9 shows the XPS spectra of Ag/NiS/TiO2 nanocomposites obtained using 6 cycles of nickel sulfide deposition by immersion in 0.1 M AgNO3, where fig. 9a is the full spectrum, and the rest of the spectra are high-resolution spectra of the elements. As can be seen from the full spectrum in Fig. 9a, absorption peaks of Ti, O, Ni, S, and Ag were found in the nanocomposite, which proves the existence of these five elements. The test results were in accordance with the EDS. The excess peak in Figure 9a is the carbon peak used to correct for the binding energy of the sample. On fig. 9b shows a high resolution energy spectrum of Ti. The absorption peaks of the 2p orbitals are located at 459.32 and 465 eV, which correspond to the absorption of the Ti 2p3/2 and Ti 2p1/2 orbitals. Two absorption peaks prove that titanium has a Ti4+ valence, which corresponds to Ti in TiO2.
XPS spectra of Ag/NiS/TiO2 measurements (a) and high resolution XPS spectra of Ti2p(b), O1s(c), Ni2p(d), S2p(e), and Ag 3d(f).
On fig. 9d shows a high-resolution Ni energy spectrum with four absorption peaks for the Ni 2p orbital. The absorption peaks at 856 and 873.5 eV correspond to the Ni 2p3/2 and Ni 2p1/2 8.10 orbitals, where the absorption peaks belong to NiS. The absorption peaks at 881 and 863 eV are for nickel nitrate and are caused by the nickel nitrate reagent during sample preparation. On fig. 9e shows a high resolution S-spectrum. The absorption peaks of the S 2p orbitals are located at 161.5 and 168.1 eV, which correspond to the S 2p3/2 and S 2p1/2 orbitals 21, 22, 23, 24. These two peaks belong to nickel sulfide compounds. The absorption peaks at 169.2 and 163.4 eV are for the sodium sulfide reagent. On fig. 9f shows a high-resolution Ag spectrum in which the 3d orbital absorption peaks of silver are located at 368.2 and 374.5 eV, respectively, and two absorption peaks correspond to the absorption orbits of Ag 3d5/2 and Ag 3d3/212, 13. The peaks in these two places prove that silver nanoparticles exist in the state of elemental silver. Thus, the nanocomposites are mainly composed of Ag, NiS and TiO2, which was determined by X-ray photoelectron spectroscopy, which proved that nickel and silver sulfide nanoparticles were successfully combined on the surface of TiO2 nanowires.
On fig. 10 shows UV-VIS diffuse reflectance spectra of freshly prepared TiO2 nanowires, NiS/TiO2 nanocomposites, and Ag/NiS/TiO2 nanocomposites. It can be seen from the figure that the absorption threshold of TiO2 nanowires is about 390 nm, and the absorbed light is mainly concentrated in the ultraviolet region. It can be seen from the figure that after the combination of nickel and silver sulfide nanoparticles on the surface of titanium dioxide nanowires 21, 22, the absorbed light propagates into the visible light region. At the same time, the nanocomposite has increased UV absorption, which is associated with a narrow band gap of nickel sulfide. The narrower the band gap, the lower the energy barrier for electronic transitions and the higher the degree of light utilization. After compounding the NiS/TiO2 surface with silver nanoparticles, the absorption intensity and light wavelength did not increase significantly, mainly due to the effect of plasmon resonance on the surface of silver nanoparticles. The absorption wavelength of TiO2 nanowires does not significantly improve compared to the narrow band gap of composite NiS nanoparticles. In summary, after composite nickel sulfide and silver nanoparticles on the surface of titanium dioxide nanowires, its light absorption characteristics are greatly improved, and the light absorption range is extended from ultraviolet to visible light, which improves the utilization rate of titanium dioxide nanowires. light that improves the material’s ability to generate photoelectrons.
UV/Vis diffuse reflectance spectra of fresh TiO2 nanowires, NiS/TiO2 nanocomposites, and Ag/NiS/TiO2 nanocomposites.
On fig. 11 shows the mechanism of photochemical corrosion resistance of Ag/NiS/TiO2 nanocomposites under visible light irradiation. Based on the potential distribution of silver nanoparticles, nickel sulfide, and the conduction band of titanium dioxide, a possible map of the mechanism of corrosion resistance is proposed. Because the conduction band potential of nanosilver is negative compared to nickel sulfide, and the conduction band potential of nickel sulfide is negative compared to titanium dioxide, the direction of electron flow is roughly Ag→NiS→TiO2→304 stainless steel. When light is irradiated on the surface of the nanocomposite, due to effect of surface plasmon resonance of nanosilver, nanosilver can quickly generate photogenerated holes and electrons, and photogenerated electrons quickly move from the valence band position to the conduction band position due to excitation. Titanium dioxide and nickel sulfide. Since the conductivity of silver nanoparticles is more negative than that of nickel sulfide, electrons in the TS of silver nanoparticles are rapidly converted to TS of nickel sulfide. The conduction potential of nickel sulfide is more negative than that of titanium dioxide, so the electrons of nickel sulfide and the conductivity of silver rapidly accumulate in the CB of titanium dioxide. The generated photogenerated electrons reach the surface of 304 stainless steel through the titanium matrix, and the enriched electrons participate in the cathodic oxygen reduction process of 304 stainless steel. This process reduces the cathodic reaction and at the same time suppresses the anodic dissolution reaction of 304 stainless steel, thereby realizing the cathodic protection of stainless steel 304. Due to the formation of the electric field of the heterojunction in the Ag/NiS/TiO2 nanocomposite, the conductive potential of the nanocomposite is shifted to a more negative position, which more effectively improves the cathodic protection effect of 304 stainless steel.
Schematic diagram of the photoelectrochemical anti-corrosion process of Ag/NiS/TiO2 nanocomposites in visible light.
In this work, nickel and silver sulfide nanoparticles were synthesized on the surface of TiO2 nanowires by a simple immersion and photoreduction method. A series of studies on the cathodic protection of Ag/NiS/TiO2 nanocomposites on 304 stainless steel was carried out. Based on the morphological characteristics, analysis of the composition and analysis of the light absorption characteristics, the following main conclusions were made:
With a number of impregnation-deposition cycles of nickel sulfide of 6 and a concentration of silver nitrate for photoreduction of 0.1 mol/l, the resulting Ag/NiS/TiO2 nanocomposites had a better cathodic protective effect on 304 stainless steel. Compared with a saturated calomel electrode, the protection potential reaches -925 mV , and the protection current reaches 410 μA/cm2.
A heterojunction electric field is formed at the Ag/NiS/TiO2 nanocomposite interface, which improves the separating power of photogenerated electrons and holes. At the same time, the light utilization efficiency is increased and the light absorption range is extended from the ultraviolet region to the visible region. The nanocomposite will still retain its original state with good stability after 4 cycles.
Experimentally prepared Ag/NiS/TiO2 nanocomposites have a uniform and dense surface. Nickel sulfide and silver nanoparticles are uniformly compounded on the surface of TiO2 nanowires. Composite cobalt ferrite and silver nanoparticles are of high purity.
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