A sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states, its preparation method and applications

Abstract

The application belongs to the technical field of marine metal corrosion prevention, and particularly relates to a sulfur-doped / carbon vacancy g-C3N4 with a shallow defect state and a preparation method and application thereof. The application is prepared by (1) precursor preparation, wherein ethylene glycol is used as a solvent, and small-size hexagonal melamine cyanuric acid prisms are obtained through a solvothermal reaction of melamine / cyanuric acid; and (2) defect generation, wherein molten sulfur is used as a post-heating solvent to perform a precursor thermal polycondensation reaction, and the g-C3N4 material with a sulfur-doped / carbon vacancy modification and a shallow defect state is obtained. The g-C3N4 material with a shallow defect state obtained by the application exhibits excellent sunlight capturing capacity and photo-generated carrier separation efficiency. When coupled with 304 stainless steel in a simulated marine environment, the open circuit potential of the 304 stainless steel can be negatively shifted from -0.192 V to -0.640 V, and the open circuit potential retention rate is still as high as 86.9% after 7200 seconds.

Description

Technical Field

[0001] This invention belongs to the field of marine metal corrosion protection technology, specifically relating to a sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states, its preparation method, and its application. Background Technology

[0002] Metal corrosion, especially seawater corrosion of metal equipment in marine environments, not only significantly reduces the load-bearing capacity and service life of marine steel structures, inducing catastrophic failures, but also causes a huge waste of metal resources, posing a major challenge to my country's sustainable development. Among various marine metal corrosion protection technologies, photocatalytic cathodic protection (PCP) based on photocatalysts is a promising anti-corrosion strategy that can significantly reduce the metal oxidation rate and even prevent metal corrosion. The effectiveness of photocatalytic cathodic protection largely depends on the electronic band structure of the photocatalyst itself; the higher its utilization rate of sunlight, the better the anti-metal corrosion effect. Traditional photocatalysts such as TiO2, CdS, and CdSe metal semiconductors have problems such as large band gaps, severe photocorrosion, or toxicity during synthesis, which limit their application in PCP. g-C3N4, as a non-metallic photocatalyst, has a narrow band gap (approximately 2.7 eV) and excellent physicochemical stability, and has become a hot material in the field of PCP technology for combating marine metal corrosion. However, the bulk g-C3N4 material suffers from insufficient visible light absorption and severe recombination of photogenerated carriers, resulting in a limited number of photogenerated electrons suitable for photogenerated cathode protection and low solar energy utilization, thus limiting its effectiveness in marine metal corrosion protection. Therefore, it is necessary to modify the surface and interface of g-C3N4 material, focusing on optimizing its electronic band structure.

[0003] Defect engineering, aiming to optimize the electronic band structure of g-C3N4 while simultaneously enhancing its solar energy harvesting capacity and photogenerated carrier separation efficiency, has attracted widespread research attention. Studies have shown that defects such as vacancies (C, N vacancies) and dopants (B, C, O, S, P, and halogens) in g-C3N4 can effectively narrow the band gap, increase the conduction band bottom (CBM), and enhance electronic polarization due to electronegativity differences. These defects can respectively contribute to enhanced visible light absorption, accelerated electron migration rate, and promoted charge transfer processes in g-C3N4. In recent years, researchers have increasingly focused on utilizing the additional energy levels (called defect states or subbandgap states) introduced between the CBM and the valence band top (VBM) to improve the photocatalytic performance of g-C3N4-based materials. These defect states not only lower the photoexcitation energy threshold but also act as temporary electron pools to accommodate electrons migrating from the CBM, promoting carrier transport / separation and significantly increasing the number of carriers in g-C3N4. This, in turn, significantly increases the number of active electrons participating in the redox reactions in subsequent photogenerated cathodic protection. While defect states demonstrate significant advantages in photogenerated cathodic protection and other photocatalytic applications, their excessive depth can severely negate the benefits of defect modulation. Specifically, deep defect levels possess excessively strong electron-trapping capabilities, preventing electrons from rapidly transferring to the catalyst surface. This leads to severe recombination of photogenerated carriers (electron-hole) in both the bulk and surface phases, significantly reducing solar energy utilization. Therefore, to maximize the photogenerated cathodic protection effect of defect modulation on metals in marine environments, the g-C3N4 photocatalyst should be designed with caution. This involves not only utilizing defect engineering to narrow the bandgap and accelerate charge transfer kinetics, but also avoiding the introduction of deep defect states and focusing on the construction of shallow defect states to enhance photogenerated carrier separation efficiency and further improve the corrosion resistance of metals in marine environments. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states, its preparation method, and its applications.

[0005] A method for preparing sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states includes the following steps: (1) Preparation of precursor: Melamine and cyanuric acid were dissolved in water respectively, mixed and stirred vigorously for 3 hours, centrifuged and dried to obtain white powder; the powder was dispersed in ethylene glycol (EG) solvent, placed in a high-pressure reactor, heated and reacted, and after cooling to room temperature, the obtained white substance was collected by centrifugation, washed with deionized water and dried to obtain melamine cyanuric acid (MCA) supramolecular precursor MCA-E; (2) Defect generation: The precursor is annealed in molten sulfur solvent under a semi-sealed air atmosphere. Specifically, the MCA-E obtained in step (1) and sulfur powder are mixed evenly and annealed at 5 °C·min. -1Sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states was obtained by heating at a rate of 550 °C and holding for 4 hours.

[0006] Preferably, the molar ratio of melamine to cyanuric acid in step (1) is 1:1.

[0007] Preferably, the heating reaction temperature in step (1) is 180°C and the reaction time is 10 hours.

[0008] Preferably, the centrifugation speed in step (1) is 5000 rpm and the centrifugation time is 5 minutes.

[0009] Preferably, the mass ratio of MCA-E to molten sulfur solvent in step (2) is 1:1.

[0010] Preferably, the heating rate in step (2) is 5 °C·min. -1 .

[0011] The present invention also provides the application of the above-mentioned sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states in marine metal corrosion protection.

[0012] The beneficial technical effects of this invention are as follows: The g-C3N4 material with shallow defect states exhibits excellent solar energy capture capability and photogenerated carrier separation efficiency. When coupled with 304 stainless steel in a simulated marine environment, this material demonstrates significantly enhanced metal corrosion resistance: it can negatively shift the open-circuit potential of 304 stainless steel from -0.192 V to -0.640 V, and its open-circuit potential retention rate remains as high as 86.9% after 7200 seconds. This photogenerated cathodic protection effect is far superior to that of bulk g-C3N4 and other deep defect level g-C3N4 materials. Attached Figure Description

[0013] Figure 1 A schematic diagram of the preparation of sulfur-doped / carbon-vacancy g-C3N4 by the dual-solvent modulation method, wherein (a) is step 1: precursor modulation in ethylene glycol (EG) solvent; and (b) is step 2: thermally induced defect modulation in molten sulfur solvent. Figure 2The images shown are SEM / TEM images obtained in Example 1 and Comparative Examples 1-3, where (a) is the SEM image of the precursor MCA-E obtained in Example 1; (b, c) are the SEM / TEM images of CN-ES obtained in Example 1; (d) is the SEM image of the precursor MCA-H obtained in Comparative Example 1; (e, f) are the SEM / TEM images of CN-HS obtained in Comparative Example 1; (e) is the SEM image of the precursor MCA-HE obtained in Comparative Example 2; (f, g) are the SEM / TEM images of CN-HES obtained in Comparative Example 2; and (a') is the SEM image of CN-E obtained in Comparative Example 3. Figure 3 The X-ray diffraction (XRD) patterns of g-C3N4 obtained in Example 1 and Comparative Examples 1-4 are shown, where (a) is the XRD pattern of BCN, CN-HS, CN-HES and CN-ES; and (b) is the XRD pattern of CN-E. Figure 4 The N2 adsorption-desorption isotherms of g-C3N4 obtained in Example 1 and Comparative Examples 1-4 are shown in the inset (pore size distribution), where (a) are the N2 adsorption-desorption isotherms of BCN, CN-HS, CN-HES and CN-ES (inset (pore size distribution); and (b) is the N2 adsorption-desorption isotherm of CN-E (inset (pore size distribution)). Figure 5 Electron paramagnetic resonance (EPR) spectra of g-C3N4 obtained in Example 1 and Comparative Examples 1, 2, and 4; Figure 6 The S2p XPS core level spectra of g-C3N4 obtained in Example 1 and Comparative Examples 1-2 are shown, where (a) is CN-ES; (b) is CN-HS; and (c) is CN-HES. Figure 7 The UV-Vis diffuse reflectance absorption spectra obtained in Example 1 and Comparative Examples 1-4 and the (Kubelk-Munk method) αhν ) 2 vs. hν Figure 1 shows the UV-Vis diffuse reflectance absorption spectra of BCN, CN-HS, CN-HES, and CN-ES; (b) shows the UV-Vis diffuse reflectance absorption spectra of BCN, CN-HS, CN-HES, and CN-ES based on the Kubelk-Munk method. αhν ) 2 vs. hν Figure; (c) shows the UV-Vis diffuse reflectance absorption spectrum of CN-E; (d) shows the CN-E spectrum based on the Kubelk-Munk method. αhν ) 2 vs. hν picture; Figure 8 The Mott-Schottky curves of g-C3N4 obtained in Example 1 and Comparative Examples 1-4 are shown, where (a) represents BCN and CN-ES; (b) represents CN-HS; (c) represents CN-HES; and (d) represents CN-E. Figure 9 This is a schematic diagram of the electronic band structure of g-C3N4 obtained in Example 1 and Comparative Examples 1-4; Figure 10 The images show the femtosecond transient absorption spectra (fs-TAS) of g-C3N4 obtained in Example 1 and Comparative Example 1, where (a) is a pseudo-color image of the femtosecond transient absorption spectra (fs-TAS) of CN-HS and (b) is a pseudo-color image of CN-ES; (c) is a fs-TAS of CN-HS and (d) is a fs-TAS of CN-ES at different delay times; (e) is a CN-HS; (f) is a kinetic decay and fitting curve of CN-ES at 650 nm; and (g) is a schematic diagram of charge carrier dynamics of CN-HS and CN-ES. Figure 11 The open circuit (OCP) curves (under illumination) of the photoelectrode coupled to the 304 stainless steel electrode of g-C3N4 obtained in Example 1 and Comparative Examples 1-4 are shown. Among them, (a) is the OCP curve of the photoelectrode coupled to the 304 stainless steel electrode of BCN, CN-HS, CN-HES, and CN-ES; and (b) is the OCP curve of the photoelectrode coupled to the 304 stainless steel electrode of CN-E. Figure 12 The photocurrent response curves of g-C3N4 photoelectrode coupled with 304 stainless steel electrode obtained in Example 1 and Comparative Examples 1-4 under intermittent sunlight are shown. Among them, (a) is the photocurrent response curve of BCN, CN-HS, CN-HES, and CN-ES photoelectrode coupled with 304 stainless steel electrode under intermittent sunlight; (b) is the photocurrent response curve of CN-E photoelectrode coupled with 304 stainless steel electrode under intermittent sunlight. Figure 13 Tafel curves (under illumination) of g-C3N4 photoelectrode coupled to 304 stainless steel electrode obtained in Example 1 and Comparative Examples 1-4, wherein (a) are Tafel curves of BCN, CN-HS, CN-HES, and CN-ES photoelectrodes coupled to 304 stainless steel electrode; and (b) are Tafel curves of CN-E photoelectrode coupled to 304 stainless steel electrode. Figure 14 OCP stability test (under illumination) of the 304 stainless steel electrode coupled to the photoelectrode of g-C3N4 obtained in Example 1 and Comparative Examples 1, 2 and 4. Figure 15XPS core level spectra of (a) Fe2p, (b) Cr2p, (c) Ni2p, and (d) O1s of the 304 stainless steel electrode coupled with CN-ES obtained in Example 1 before and after PCP testing, and (e) surface optical microscopy images. Detailed Implementation

[0014] The technical solutions of the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0015] Example 1 A method for preparing sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states is shown in the schematic diagram below. Figure 1 As shown: (1) Precursor preparation: 1.323 g melamine (10.5 mmol) and 1.35 g cyanuric acid (10.5 mmol) were dissolved in water respectively, and then mixed under vigorous stirring for 3 hours; the mixture was centrifuged at 5000 rpm for 5 minutes and dried to obtain a white powder. The powder was dispersed in ethylene glycol (EG), and the solution was transferred to a polytetrafluoroethylene-lined autoclave and subjected to a solvothermal reaction at 180°C for 10 hours; after cooling to room temperature, the white substance was collected by centrifugation at 5000 rpm for 5 minutes, washed with deionized water, and dried in a vacuum oven. The melamine cyanuric acid (MCA) supramolecular precursor MCA-E was obtained. SEM images are shown below. Figure 2 As shown in a, it exhibits a solid hexagonal prism structure with dimensions of 200-500 nm; (2) Defect thermal generation: 1 g of MCA-E and 1 g of sulfur powder obtained in step (1) were ground evenly in a mortar; then, the light yellow mixture was transferred to a muffle furnace and heated at 5 °C·min. -1 The temperature was increased to 550 °C and held for 4 hours; sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states was obtained, named CN-ES. SEM / TEM images are shown below. Figure 2 As shown in (b, c), the original hexagonal prism skeleton is preserved, and it is a hollow prism structure.

[0016] Comparative Example 1 A method for preparing g-C3N4: Replace EG with pure water as the solvent; the remaining steps are the same as in Example 1. The obtained precursor is named MCA-H, and the SEM image is shown below. Figure 2As shown in d, a solid hexagonal prism structure with a size of 40 μm is exhibited; g-C3N4 was obtained and named CN-HS. SEM / TEM images are shown below. Figure 2 As shown in (e, f), due to the strong thermal etching effect of protonated melamine and ambient oxygen, the hexagonal prism structure of the large-size precursor MCA-H gradually collapses during the thermal polycondensation reaction, resulting in CN-HS exhibiting a wrinkled nanosheet morphology.

[0017] Comparative Example 2 A method for preparing g-C3N4: The EG solvent was replaced with a water / EG mixture (volume ratio 1:1), and the remaining steps were the same as in Example 1. The obtained precursor was named MCA-HE, and the SEM image is shown below. Figure 2 As shown in the figure, a solid hexagonal prism structure with a size of 10 μm is revealed; g-C3N4 was obtained and named CN-HES. SEM / TEM images are shown below. Figure 2 As shown in (h, i), it retains the hexagonal prism skeleton of the precursor and is a semi-hollow prism structure.

[0018] Comparative Example 3 In the defect thermal generation step, no sulfur powder was added, and the remaining preparation steps were the same as in Example 1, yielding g-C3N4, named CN-E; SEM images are shown below. Figure 2 As shown in a', the morphology is a partially collapsed hollow hexagonal prism structure.

[0019] Comparative Example 4 Preparation method of bulk g-C3N4 (BCN): Melamine was directly used as a precursor, and the reaction was carried out at 5 °C·min. -1 Heating at a rate of 550 °C and holding for 4 hours yielded the bulk phase g-C3N4, which was named BCN.

[0020] Experimental Example 1 The crystallinity and pore structure characteristics of the defective g-C3N4 synthesized using dual solvent-assisted methods were further analyzed by XRD patterns and N2 adsorption-desorption isotherms. After pyrolysis, compared with the bulk g-C3N4 (BCN), the (002) diffraction peaks of the defective g-C3N4 (CN-ES, CN-HES, CN-HS) and CN-E were significantly weakened and broadened, such as... Figure 3As shown in ab, this indicates that intense thermal stripping occurred during the thermal polycondensation process. The (001) diffraction peak (approximately 12.9°) of CN-ES completely disappeared, while the peak of CN-HS and CN-HES became blurred, indicating a decrease in the orderliness of the crystal structure in the in-plane direction and a higher probability of defect formation. Meanwhile, compared to BCN, the defective g-C3N4 and CN-E MCA precursors showed significantly increased specific surface area (BET) and pore volume due to intense thermal etching, such as... Figure 4 ab and Table 1 are shown.

[0021] Table 1. Pore structure characteristics of the prepared g-C3N4

[0022] Experimental Example 2 The carbon vacancies (EPR) and sulfur doping (XPS) of the prepared defective g-C3N4 material were investigated using EPR and XPS tests, respectively. The EPR results are shown below. Figure 5 As shown, compared with BCN, the g signal intensity of CN-HS, CN-HES, and CN-ES samples is significantly weakened at ~2.0042, indicating a reduction in unpaired electrons in carbon, further confirming the presence of carbon vacancies. XPS results are shown below. Figure 6 As shown, after a thermally induced defect formation stage controlled by molten sulfur solvent, CN-ES, CN-HS, and CN-HES all indicate successful sulfur doping, and the S 2p spectra show the S 2p of the CSC bonds. 3 / 2 and S 2p 1 / 2 The peak indicates that sulfur doping has replaced the dicoordinated N sites in the triazine framework. The sulfur doping content of CN-ES is 0.54%, which is higher than that of CN-HS (0.23%) and CN-HES (0.31%) (as shown in Table 2). This may be because the EG-regulated MCA-E precursor has a smaller size, which is conducive to the more complete penetration of molten sulfur and SO2 vapor into the intermediate of the thermal polycondensation process. Through the above analysis, it can be concluded that the dual solvent regulation strategy can effectively introduce sulfur doping / carbon vacancies simultaneously, which may introduce additional defect energy levels in its electronic structure, which is beneficial to improving the corrosion resistance of photogenerated cathodic protection of 304 stainless steel in marine environments.

[0023] Table 2 shows the XPS surface elemental composition (at.%) of the prepared g-C3N4. Note: 1 Atomic percentage (%) 2 C / N represents the carbon-to-nitrogen atomic ratio.

[0024] Experimental Example 3 To investigate the effect of a dual-solvent controlled preparation strategy on the solar energy harvesting ability and photogenerated carrier transfer of sulfur-doped and carbon vacancy-modified CN-ES, ultraviolet-visible diffuse reflectance spectroscopy was first used... Figure 7 a) The optical properties of these g-C3N4 analogs were compared. Specifically, EG and molten sulfur-modified CN-ES exhibited the strongest visible light absorption in the 400-800 nm range, with their maximum absorption band edge extending to 576 nm, similar to CN-HES. In contrast, CN-HS had the narrowest maximum absorption band edge (438 nm), 17 nm lower than BCN, indicating its largest band gap. Through Kubelk-Munk function transformation, CN-ES showed the smallest band gap (2.55 eV) and the shallowest defect state location (1.89 eV). Figure 7 b). CN-HS has the largest bandgap (2.97 eV) and the weakest absorption of visible light, while also exhibiting multiple defect states (2.07 eV and 1.61 eV). Compared to other defective g-C3N4 (CN-ES, CN-HES), this indicates the presence of deeper defect states, which will lead to severe recombination of photogenerated carriers and is detrimental to the utilization of sunlight. Furthermore, the solar energy absorption capacity of the CN-E sample prepared under molten sulfur-free conditions is weaker than that of CN-HES and CN-ES ( Figure 7 (cd) indicates the advantages of the dual-solvent-assisted synthesis strategy in enhancing light absorption and shortening the band gap.

[0025] Experimental measurements of the electronic band structure further confirmed the defect state positions of the CN-ES, CN-HES, and CN-HS samples. Specifically, based on the Mott-Schottky curves (… Figure 8 The CB positions for CN-HS, CN-HES, CN-ES, CN-E, and BCN are -1.69, -1.82, -2.05, -1.70, and -1.79 V (vs. AgCl / Ag), which translate to -1.08, -1.21, -1.44, -1.09, and -1.18 V relative to the reversible hydrogen electrode (RHE). The band gap and defect state values ​​were calculated using UV-Vis diffuse reflectance spectroscopy. Figure 7 (ab), draw a schematic diagram of the band structure () Figure 9The brown and green horizontal lines represent the shallower and deeper defect state energy levels in the band structure, respectively. Interestingly, except for CN-HS, all dual-solvent modulated samples have narrow band gaps, with CN-ES and CN-HES having the smallest band gaps (2.55 eV), indicating a wider solar absorption range. Compared to CN-E without post-heat treatment using molten sulfur, CN-HS, CN-HES, and CN-ES all exhibit additional defect energy levels due to the presence of sulfur dopant and carbon vacancies. Notably, the shallow defect states of CN-ES are located at approximately -1.04 and -0.74 V, with the deepest level being 0.37 V higher than that of CN-HES. In contrast, CN-HS not only has a larger band gap (2.97 eV), but its defect states are located at approximately -0.28 V, making it the sample with the deepest defect energy level from the CBM. This will result in excessively strong electron trapping capabilities, easily leading to severe photogenerated carrier recombination. Worse still, the deep defect level position of CN-HS (approximately -0.33 V) is higher than that of O2 / O2. - The reduction potential (-0.33 V vs. RHE) of the CBM is corrected, meaning that even if strongly bound electrons at this energy level escape, the insufficient reduction potential cannot meet the potential requirements for protecting 304 stainless steel, thus significantly reducing the overall photocathode protection performance of the material. Conversely, CN-ES has the shallowest defect states, and its "deeper" defect states are still sufficiently shallow. This not only lowers the visible light absorption threshold to enhance light absorption but also possesses a moderate electron trapping ability, temporarily accommodating electrons migrating from the CBM before releasing them to the catalyst surface. This greatly improves the separation efficiency of photogenerated carriers, contributing to enhanced photocathode protection performance of 304 stainless steel in marine environments.

[0026] Test Example 4 To gain a deeper understanding of the influence of deep and shallow defect states on the photocarrier transfer dynamics of g-C3N4 samples under sunlight, femtosecond transient absorption spectroscopy (fs-TAS) measurements were performed on CN-ES (shallow defect state) and CN-HS (deep defect state) using a 340 nm laser as the pump excitation source. Figure 10As shown in Figures ab, CN-HS exhibits both negative and positive absorption signals in the 480-660 nm range. The strong and broad negative absorption signal in the 440-610 nm range is attributed to ground-state bleaching or stimulated emission (SE), indicating rapid electron-hole recombination, which is detrimental to solar energy utilization in metal photocathode protection. Furthermore, the weak positive signal in the 610-660 nm range indicates weak excited-state absorption (ESA) of electrons, holes, or electron-hole pairs, which is also unfavorable for metal photocathode protection performance. In contrast, CN-ES shows the opposite trend, with a significantly weakened negative signal in the 460-570 nm range (down to -1.45 mOD) and a significantly enhanced positive signal in the 570-660 nm range (above 6.0 mOD) (Figures 10c-d). These signals qualitatively indicate that photocarrier recombination in CN-ES with shallow defect states and sulfur doping / carbon vacancies is suppressed, providing sufficient electrons for the protection of 304 stainless steel.

[0027] Subsequently, the lifetimes of different photocarrier transfer processes were calculated, and the dynamic decay trajectories were fitted using a triple-exponential model at ESA (650 nm). Figure 10(e, f) further confirms the key role of shallow defect states in promoting photocarrier separation. Here, τ1, τ2, and τ3 correspond to the processes of "hot" electrons cooling to the conduction band bottom (CBM), electrons being captured by shallower defect states, and electrons being captured by deeper defect states, respectively. Compared with CN-HS, which has deep defect energy levels, CN-ES, which has shallow defect energy levels, exhibits significantly accelerated photocarrier transfer dynamics not only in the electron excitation and "cooling" (de-excitation) stages but also in the capture stage: i) During excitation, the ESA signal response time of CN-ES is drastically shortened to within 300 fs, which is 2.4 times that of CN-HS, indicating a rapid photogenerated carrier process separating from the ground state. ii) When hot electrons migrate from high energy levels to the bottom of the CBM (S1 state), the migration rate of CN-ES (420 fs) is 6.8 times that of CN-HS (2.86 ps), shortening the overall electron transfer time. iii) During the capture process, electrons are first captured by shallower defect states and then by deeper defect states, but the probability of annihilation by "holes" near the deeper defect states is higher. Specifically, for shallower defect states, the τ2 of both samples is low (44.51 ps for CN-HS and 6.86 ps for CN-ES). However, the long lifetime τ3 of the two samples differs significantly, with CN-HS exhibiting a τ3 as long as 891.06 ps (approaching nanoseconds), indicating severe nonradiative recombination of photocarriers. The long lifetime τ3 of CN-ES is reasonable (323.78 ps), consistent with previous reports. The reasonable τ3 value further indicates that shallow defect states have a moderate electron-capturing ability, serving as both a temporary electron pool and preventing photocarrier recombination, making it easier for electrons to participate in subsequent photocathode redox reactions. Notably, CN-HS exhibits a negative SE transient at approximately 56.76 ps after excitation ( Figure 10 c) This is due to the radiative recombination of high-energy electrons and holes, while electrons in deep defect states are in a "dormant" state and cannot be transported to the outer surface, resulting in the loss of a large number of photogenerated electrons. Conversely, the SE signal of CN-ES disappears completely (Fig. 10f), and the average lifetime (τ) ave The photocathode spectroscopy time (PS) was significantly reduced to 35.84 ps, indicating that photocarrier recombination was significantly suppressed and ground-state electrons were largely consumed at the CN-ES surface. These effects enable g-C3N4 with shallow defect states to have faster electron transfer dynamics, which is beneficial to improving the photocathode protection performance of the metal.

[0028] Furthermore, a schematic diagram of the enhanced photocarrier transfer dynamics of CN-ES in fs-TAS is summarized, including solar excitation, electron cooling to the CBM, and electron trapping by defect states at different depths. Figure 10(g) Since long-lived electrons constitute the vast majority (>93%) in both samples, it can be inferred that electrons trapped by deep defect states are crucial for photocathodic protection of metals in aqueous solution. The ideal τ3 (323.78 ps) of CN-ES further indicates that its "deeper" defect states are still located shallow enough to have a moderate electron-trapping ability, which is conducive to photocathodic protection of 304 stainless steel. In contrast, the excessively high τ3 (891.06 ps) of CN-HS indicates that its deeper energy level is located deeper and closer to the lowest unoccupied molecular orbital (LUMO), resulting in an excessively strong electron-trapping ability and exhibiting severe nonradiative recombination, which is detrimental to photocathodic protection of 304 stainless steel.

[0029] Experimental Example 5 The photocathode protection properties of 304 stainless steel (i.e., the corrosion resistance of metals in marine environments): To evaluate the photocathodic protection performance of the prepared g-C3N4 material, a 304 stainless steel electrode was used as the working electrode, an AgCl / Ag electrode as the reference electrode, and a Pt wire as the counter electrode. The g-C3N4 photoelectrode was immersed in a photoelectrochemical cell containing 0.2 M NaOH / 0.2 M NaOH solution, while the 304 stainless steel electrode was placed in a corrosion cell containing 3.5 wt% NaCl. The two cells were connected by a saturated KCl salt bridge. In the photoelectrochemical cell, the g-C3N4 photoelectrodes obtained in Example 1 and Comparative Examples 1-4 were used as the sole working electrodes. Under sunlight irradiation, the open circuit potential (OCP), photocurrent, Tafel polarization curves, and metal corrosion resistance stability were tested using the above-mentioned apparatus.

[0030] like Figure 11 As shown in Figure a, the initial OCP of bare 304 stainless steel is approximately -0.192 V, shifting slightly towards the positive direction within 1650 s. Generally, the more negative the OCP value, the better the photocathode protection effect. All 304 stainless steel electrodes coupled with g-C3N4 exhibit a more negative OCP under illumination than bare 304 stainless steel; even BCN reaches -0.452 V, 260 mV lower than bare 304 stainless steel, indicating that all g-C3N4 materials provide photocathode protection for 304 stainless steel. Specifically, the electrode coupled with CN-E prepared under conditions without molten sulfur has an OCP of -0.488 V (… Figure 11(b) Similar to BCN. For g-C3N4 samples containing defect energy levels, CN-ES shows the best trend in photogenerated cathodic protection, with the most negative and stable OCP (-0.640 V), which is 0.128 V and 0.188 V lower than CN-HES and BCN, respectively. Notably, the initial OCP of the CN-HS photoanode is -0.569 V, followed by a sharp positive shift of 121 mV to -0.448 V, similar to the stable value of BCN. The significant differences in OCP between CN-ES, CN-HS, and other samples indicate that the g-C3N4 defects synergistically regulated by EG and molten sulfur can provide sufficient thermodynamic photogenerated electrons for 304 stainless steel through enhanced solar absorption, photogenerated carrier separation efficiency, and fast charge transfer kinetics.

[0031] Photocurrent monitoring and potentiodynamic polarization are important methods for reflecting the photocatheter protection effect of g-C3N4-based photoelectrodes. Generally, a higher current density indicates smoother charge transfer to the 304 stainless steel surface, and a better metal corrosion inhibition effect. Figure 12 As shown in figure a, CN-ES has the highest photocurrent density (127.7 μA cm⁻¹). -2 ), far exceeding BCN (16.56 μA cm). -2 ) and CN-HS (32.25 μA cm -2 This value is also much higher than CN-E (). Figure 12 b). Furthermore, the Tafel polarization curves show the corrosion potential (E) of bare 304 stainless steel. corr The value is -0.174 V, while the E values ​​for BCN, CN-HS, CN-HES, CN-ES, and CN-E are... corr The values ​​were shifted negatively to -0.430, -0.431, -0.506, -0.613, and -0.482 V, respectively. Figure 13 ab). Among them, E of CN-ES corr The most negative result is consistent with the OCP test results. Figure 11 a). Furthermore, the corrosion current (i) of CN-ES corr The highest (6.37 μA cm) -2 The ratios of CN-ES to bare 304 stainless steel are 3.2 and 219.6 times higher, respectively, indicating that photogenerated electron polarization enhances the interfacial reaction. In summary, CN-ES exhibits the best corrosion resistance to 304 stainless steel, which is mainly attributed to defect modulation. Specifically, sulfur doping / carbon vacancy g-C3N4 with shallow defect energy levels significantly enhances the solar light capture capability and photogenerated carrier separation efficiency.

[0032] The long-term photo-cathode protection stability of defect-state g-C3N4 is crucial for the corrosion resistance of 304 stainless steel; therefore, long-term OCP curve recording tests were conducted. Figure 14 During the 2-hour test, BCN exhibited the lowest OCP retention rate (only 67.2%, increasing from -0.505 V to -0.337 V). Considering the above analysis of the electronic band structure and photocathode protection effect of CN-HS, its lower OCP retention rate (67.5%), similar to BCN, is not surprising. Compared to BCN and CN-HS, CN-ES achieved a final OCP of -0.533 V, with a significantly improved retention rate of 86.9%, indicating the best level of photocathode protection. Furthermore, we characterized the chemical state and morphology of 304 stainless steel before and after coupling with CN-ES using XPS and metallographic microscopy. Figure 15 The study found that the peak shapes and positions of the XPS core energy level spectra of the main metals (Fe, Cr, Ni) and O elements in 304 stainless steel were basically consistent before and after the test, indicating that the chemical composition of the metal samples remained almost unchanged. Furthermore, optical images of 304 stainless steel before and after cycling (…) Figure 15 e) It remains essentially unchanged, exhibiting a stable photocathode protection effect of sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states in the marine environment, i.e., excellent marine metal corrosion resistance.

Claims

1. A method for preparing sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states, characterized in that, The preparation method includes the following steps: (1) Preparation of precursor: Melamine and cyanuric acid were dissolved in water respectively, mixed and stirred vigorously, centrifuged and dried to obtain white powder; the powder was dispersed in ethylene glycol EG solvent, placed in a high-pressure reactor, heated and reacted, and after cooling to room temperature, the obtained white substance was collected by centrifugation, washed with deionized water and dried to obtain melamine cyanuric acid MCA supramolecular precursor MCA-E; (2) Defect generation: The precursor is annealed in molten sulfur solvent under a semi-sealed air atmosphere; the specific steps are to mix and grind the MCA-E obtained in step (1) and sulfur powder evenly, heat to 550 ℃, and keep warm for 4 hours to obtain sulfur doped / carbon vacancy g-C3N4 with shallow defect state.

2. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of melamine to cyanuric acid is 1:

1.

3. The preparation method according to claim 1, characterized in that, The vigorous stirring and mixing time in step (1) is 3 hours.

4. The preparation method according to claim 1, characterized in that, In step (1), the centrifugation speed is 5000 rpm and the centrifugation time is 5 minutes.

5. The preparation method according to claim 1, characterized in that, In step (1), the heating reaction temperature is 180°C and the reaction time is 10 hours.

6. The preparation method according to claim 1, characterized in that, In step (2), the mass ratio of MCA-E to molten sulfur solvent is 1:

1.

7. The preparation method according to claim 1, characterized in that, The heating rate in step (2) is 5 °C·min. -1 .

8. Sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states obtained by the preparation method according to any one of claims 1-7.

9. The application of sulfur-doped / carbon-vacancy g-C3N4 with shallow defect states as described in claim 8 in marine metal corrosion protection.

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