Oxygen-doped carbon nitride photo-anode and preparation method and application thereof

Abstract

The invention relates to an oxygen-doped carbon nitride photo-anode and a preparation method and application thereof, and belongs to the technical field of photoelectrochemical cathode protection. The preparation method of the oxygen-doped carbon nitride photo-anode comprises the following steps: dissolving melamine and cyanuric acid in a solvent I to obtain a precursor solution, centrifuging the precursor solution, collecting precipitate, washing the precipitate, and drying to obtain a precursor; mixing the precursor with a solvent II to obtain slurry, coating a substrate with the slurry, and drying to obtain a precursor film; and calcining the precursor film and melamine in inert gas to obtain the composite material. According to the oxygen-doped carbon nitride photoanode and the preparation method thereof, the carbon nitride photoanode with different oxygen doping amounts is obtained by regulating and controlling the proportion of cyanuric acid to melamine, and the oxygen-doped carbon nitride photoanode is prepared by introducing part of an oxygen-containing structure into a carbon nitride skeleton through condensation and rearrangement of cyanuric acid and amino while the completeness of the carbon nitride triazine skeleton is kept.

Description

Technical Field

[0001] This invention belongs to the field of photoelectrochemical cathodic protection technology, specifically relating to an oxygen-doped carbon nitride photoanode, its preparation method, and its application. Background Technology

[0002] Carbon nitride, as a non-metallic semiconductor photocatalyst, shows promise in the field of metal photoelectrochemical cathodic protection due to its suitable band structure, good chemical stability, and visible light response characteristics. The principle behind carbon nitride in metal photoelectrochemical cathodic protection lies in the fact that, under visible light irradiation, photogenerated electrons from carbon nitride can be injected into the protected metal, causing cathodic polarization and thus inhibiting metal corrosion at its source. This technology is directly driven by solar energy, requiring no external power or sacrificial anode, aligning with the principles of green and sustainable development.

[0003] However, applying carbon nitride to practical photoelectrochemical cathodic protection systems still faces a series of severe challenges. The inherent defects of carbon nitride materials, such as easy recombination of photogenerated carriers, limited specific surface area, and narrow visible light absorption range, severely limit its quantum efficiency. Furthermore, the preparation of existing carbon nitride photoanode films typically relies on first synthesizing powder and then forming a film through spin coating, a "powder-recombination" route that has limitations in terms of film density and interfacial adhesion. In contrast, direct in-situ growth holds promise for obtaining carbon nitride films with more complete structures and better adhesion, but currently, there are still significant bottlenecks for oxygen-doped systems: at higher film-forming temperatures, oxygen-doped precursors have poor thermal stability and are more prone to volatilization and decomposition, leading to significant internal stress in the film during formation, causing cracking, peeling, and even localized detachment. In 2022, Zhai Yunpu's team prepared oxygen-doped porous rod-shaped carbon nitride by adding formic acid to a supramolecular precursor. Oxygen doping significantly improved its ability to separate photogenerated carriers, and the prepared catalyst exhibited high photocatalytic activity. However, it still has limitations in practical applications. While formic acid can introduce more oxygen atoms, when used in thin film preparation, the excessive introduction of oxygen atoms can cause the film to peel off and crack during high-temperature calcination. Summary of the Invention

[0004] The first objective of this invention is to provide a method for preparing oxygen-doped carbon nitride photoanodes to solve the technical problem of thin film peeling and cracking during high-temperature calcination of existing oxygen-doped carbon nitride photoanodes.

[0005] A second objective of this invention is to provide an oxygen-doped carbon nitride photoanode.

[0006] A third objective of this invention is to provide an application of an oxygen-doped carbon nitride photoanode.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing an oxygen-doped carbon nitride photoanode includes the following steps: S1: Melamine and cyanuric acid are dissolved in solvent one to obtain a precursor solution. The precursor solution is centrifuged to collect the precipitate. The precipitate is washed and dried to obtain the precursor. S2: Mix the precursor with solvent II to obtain a slurry, coat the slurry onto the substrate, and dry to obtain a precursor film; S3: The precursor film from S2 is calcined with melamine under an inert gas to obtain the product.

[0008] Furthermore, in S1, the molar ratio of melamine to cyanuric acid is 1:0.5-3, and 50-100 mL of solvent one is added for every 0.01 mol of melamine; in S2, 0.5-1.5 mL of solvent two is added for every 0.5 g of the precursor.

[0009] Furthermore, solvent one is water; solvent two is ethylene glycol.

[0010] Furthermore, the preparation method of the slurry in S2 includes: grinding the precursor for 15 to 25 minutes, then adding the solvent II and continuing to grind for 2 to 5 minutes to obtain the slurry.

[0011] Furthermore, the drying temperature for obtaining the precursor in S1 is 50–70 °C; the drying temperature for obtaining the precursor film in S2 is 80–120 °C.

[0012] Furthermore, the coating thickness of the slurry coated on the substrate in S2 is 0.1 to 0.3 mm, and the substrate is FTO.

[0013] Furthermore, the calcination procedure described in S2 is as follows: heating to 500-550°C at a heating rate of 2-5°C / min, and calcining for 2-6 hours; the inert gas is nitrogen.

[0014] An oxygen-doped carbon nitride photoanode is prepared by the method described above, wherein the oxygen doping content in the oxygen-doped carbon nitride photoanode is 2.25 to 2.97%.

[0015] Application of an oxygen-doped carbon nitride photoanode in the field of photoelectrochemical cathodic protection of metals.

[0016] Furthermore, the metal is 304 stainless steel.

[0017] The beneficial effects of this invention are: This invention obtained carbon nitride photoanodes with different oxygen doping levels by controlling the ratio of cyanuric acid to melamine. While maintaining the integrity of the carbon nitride triazine framework, cyanuric acid introduced some oxygen-containing structures into the carbon nitride framework through condensation and rearrangement with amino groups, thus preparing oxygen-doped carbon nitride photoanodes. The O-MC supramolecular precursor film showed no cracking or detachment during high-temperature calcination. This invention achieves bandgap modulation of carbon nitride through oxygen doping, effectively suppressing charge recombination and improving cathodic protection capabilities.

[0018] This invention develops a controlled synthesis strategy for cyanuric acid, which achieves oxygen doping in graphitic carbon nitride through the pre-assembly of cyanuric acid and melamine and subsequent calcination.

[0019] When the oxygen-doped carbon nitride photoanode of this invention is applied to the cathodic protection of metallic 304 SS, the open-circuit potential reaches -0.73 V under illumination, and it can generate 28.16 μA cm⁻¹. -2 With a current density of up to 300 mV, the photogenerated voltage drop can reach a maximum of 300 mV, and the open circuit potential retention rate is 87.71% during a stability test lasting up to 12 hours. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the fabrication process of oxygen-doped carbon nitride photoanodes. Figure 2 Characterization diagrams of the photoanodes of Examples 1-6 and Comparative Example 1 are shown, where a is the XRD pattern of the photoanodes of Examples 1-4 and Comparative Example 1, b is the XRD pattern of the photoanodes of Examples 1 and 5-6, c is the FTIR pattern of the photoanodes of Examples 1-4 and Comparative Example 1, and d is the FTIR pattern of the photoanodes of Examples 1 and 5-6. Figure 3 The image shown is a SEM image of the photoanode in Comparative Example 1, where the scale bar for a is 50 μm, for b is 20 μm, for c is 2 μm, and for d is 2 μm. Figure 4The images shown are characterization diagrams and EDS analysis diagrams of the photoanodes in Examples 1-4. Specifically, a is the SEM image of the photoanode from Example 2 (scale bar: 50 μm), b is the SEM image of the photoanode from Example 2 (scale bar: 20 μm), c is the SEM image of the photoanode from Example 2 (scale bar: 2 μm), d is the SEM image of the photoanode from Example 3 (scale bar: 50 μm), e is the SEM image of the photoanode from Example 3 (scale bar: 20 μm), f is the SEM image of the photoanode from Example 3 (scale bar: 2 μm), g is the SEM image of the photoanode from Example 1 (scale bar: 50 μm), h is the SEM image of the photoanode from Example 1 (scale bar: 20 μm), i is the SEM image of the photoanode from Example 1 (scale bar: 2 μm), j is the SEM image of the photoanode from Example 4 (scale bar: 50 μm), k is the SEM image of the photoanode from Example 4 (scale bar: 20 μm), and l is the SEM image of the photoanode from Example 4 (scale bar: 2 μm). μm; m is the EDS total spectrum of the photoanode of Example 1, n is the C element distribution of the photoanode of Example 1, o is the N element distribution of the photoanode of Example 1, and p is the O element distribution of the photoanode of Example 1.

[0021] Figure 5 The XPS spectra of the photoanodes in Examples 1-4 and Comparative Example 1 are shown, where a is the full XPS spectrum, b is the C 1s spectrum, c is the N 1s spectrum, and d is the O 1s spectrum. Figure 6 The UV-vis diagram, Tauc extrapolation diagram, PL spectrum and band structure diagram of the photoanode of Examples 1-4 and Comparative Example 1 are shown, where a is the UV-vis diagram, b is the Tauc extrapolation diagram, c is the PL spectrum and d is the band structure diagram. Figure 7 The images show the UV-vis diagram, Tauc extrapolation diagram, PL spectrum, and band structure diagram of the photoanodes in Examples 1 and 5-6, where a is the UV-vis diagram, b is the Tauc extrapolation diagram, c is the PL spectrum, and d is the band structure diagram. Figure 8 The diagrams show the cathodic protection performance of the photoanodes in Examples 1-4 and Comparative Example 1, where a is the open circuit potential diagram, b is the electroinduced current density diagram, c is the Tafel curve (dark state), and d is the Tafel curve (with light). Figure 9 The LSV curves, Mott-Schottky plots, and EIS plots of the photoanodes of Examples 1-4 and Comparative Example 1 are shown, where a is the LSV curve, b is the Mott-Schottky plot, and c is the EIS plot. Figure 10The diagrams show the cathodic protection performance of the photoanodes in Examples 1 and 5-6, where a is the open circuit potential diagram, b is the electroinduced current density diagram, c is the Tafel curve (dark state), and d is the Tafel curve (with light). Figure 11 The LSV curves, Mott-Schottky plots, and EIS plots of the photoanodes in Examples 1 and 5-6 are shown, where a is the LSV curve, b is the Mott-Schottky plot, and c is the EIS plot. Figure 12 The diagram shows the cathodic protection performance of the photoanode in Example 1, where a is the long-term stability curve, b is a comparison of metallographic microscope images of the metal surface before and after 12 hours of corrosion, c is the open circuit potential diagram under artificial seawater conditions, and d is the current density diagram under artificial seawater conditions. Figure 13 The images shown are physical images of the O-MC supramolecular precursor film and the oxygen-doped carbon nitride photoanode of Example 1, where a is a physical image of the O-MC supramolecular precursor film and b is a physical image of the oxygen-doped carbon nitride photoanode. Detailed Implementation

[0022] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0023] Example 1 Preparation method of oxygen-doped carbon nitride photoanode in Example 1: S1: Dissolve 1.26 g of melamine and 2.58 g of cyanuric acid in 75 mL of water and stir at room temperature for 12 h to obtain an O-MC supramolecular precursor solution; centrifuge the O-MC supramolecular precursor solution and collect the white precipitate; wash the white precipitate twice with ultrapure water and twice with anhydrous ethanol, and then dry it overnight in a vacuum drying oven at 60 ℃ to obtain the O-MC supramolecular precursor.

[0024] S2: Take 0.5 g of O-MC supramolecular precursor and grind it in a mortar for 20 min. Use a pipette to add 1 mL of ethylene glycol to the mortar and continue grinding with the O-MC supramolecular precursor for 5 min until a uniformly mixed slurry is formed.

[0025] S3: After cleaning the FTO, use 3M tape to control the coating area to 1*1 cm. Use a coater to apply the slurry onto the FTO, with a coating thickness of 0.2 mm. After coating, place the FTO on a heating plate and slowly heat it to 100℃ at a rate of 5℃ / min. Maintain this temperature at 100℃ for 20 min to obtain the O-MC supramolecular precursor film. The O-MC supramolecular precursor film is denoted as O-MC / FTO.

[0026] S4: Place the O-MC supramolecular precursor film at one end of a quartz boat, and put 0.5 mg of melamine at the other end of the quartz boat. Cover and seal with aluminum foil, then transfer the quartz boat into a tube furnace and heat it to 550 °C at a heating rate of 3 °C / min in high-purity nitrogen. Calcination for 4 h yields an oxygen-doped carbon nitride photoanode, denoted as OCN1:2. 0.2mm .from Figure 13 It can be seen that the O-MC supramolecular precursor film did not experience any film peeling or cracking after calcination.

[0027] Examples 2-4 The preparation methods of the oxygen-doped carbon nitride photoanodes in Examples 2-4 are largely the same as those in Example 1. The difference between the preparation methods of the oxygen-doped carbon nitride photoanodes in Examples 2-4 and those in Example 1 is that the amount of melamine added in Example 2 is 1.26 g, and the amount of cyanuric acid added is 0.645 g. The oxygen-doped carbon nitride photoanode in Example 2 is designated as OCN1:0.5. 0.2mm .

[0028] In Example 3, the amount of melamine added was 1.26 g, and the amount of cyanuric acid added was 1.29 g. The oxygen-doped carbon nitride photoanode in Example 3 was designated as OCN1:1. 0.2 mm .

[0029] In Example 4, the amount of melamine added was 1.26 g, and the amount of cyanuric acid added was 3.87 g. The oxygen-doped carbon nitride photoanode in Example 4 was designated as OCN1:3. 0.2mm .

[0030] Examples 5-6 The preparation methods of the oxygen-doped carbon nitride photoanodes in Examples 5-6 are largely the same as those in Example 1. The difference between the preparation methods of the oxygen-doped carbon nitride photoanodes in Examples 5-6 and those in Example 1 is that the coating thickness of the slurry on FTO in Example 5 is 0.1 mm. The oxygen-doped carbon nitride photoanode in Example 5 is designated as OCN1:2. 0.1 mm .

[0031] In Example 6, the coating thickness of the slurry on FTO was 0.3 mm. The oxygen-doped carbon nitride photoanode in Example 6 was designated as OCN1:2. 0.3 mm .

[0032] Comparative Example 1 The preparation method of the carbon nitride photoanode of Comparative Example 1 includes the following steps: 1 g of melamine was placed in a quartz ceramic boat, covered, and calcined in a muffle furnace at a heating rate of 3 °C / min to 550 °C for 4 h to obtain carbon nitride. 5 mg of carbon nitride was weighed, ground, and then 20 μL of 5% Nafion solution and 1 mL of anhydrous ethanol were added. The mixture was ultrasonically dispersed for 30 min to obtain a carbon nitride dispersion. A clean FTO glass was used, with a coating area of ​​1*1 cm, and placed on a heating plate at 80 °C for drop coating. 20 μL of the carbon nitride dispersion was transferred using a pipette and evenly drop-coated onto the FTO. After the surface dried, the next drop coating was performed, repeating this process 5 times. Finally, the glass was placed in a vacuum drying oven at 60 °C for later use to obtain a carbon nitride photoanode, denoted as CN. pure The Nafion solution is a DuPont D520 membrane solution.

[0033] from Figure 2 As can be seen from ab, CN in Comparative Example 1 pure The XRD pattern shows two key diffraction peaks: the main diffraction peak is located at approximately 27.5°, corresponding to the (002) crystal plane, originating from the periodic arrangement of heptaazine units; the other, weaker diffraction peak is located at approximately 13.4°, corresponding to the (100) crystal plane, corresponding to the interlayer stacking of the conjugated aromatic system. Furthermore, the oxygen-doped carbon nitride photoanodes of Examples 1-6, due to the presence of FTO, exhibit characteristic diffraction peaks of FTO at 26.5°, 33.8°, 37.9°, 51.7°, 61.6°, and 65.6°, indicating that the FTO structure remained stable and undamaged during calcination. The oxygen-doped carbon nitride photoanodes of Examples 1-6 retained the same diffraction peaks near 27.5° and 13.4°, indicating that even with the addition of cyanuric acid, the resulting structure is still predominantly carbon nitride. This demonstrates that oxygen doping enters the carbon nitride framework, the main framework is preserved, and the framework is not completely destroyed; moreover, the coating thickness does not affect the intrinsic structure of carbon nitride.

[0034] from Figure 2 As can be seen from the CD, the photoanodes of Examples 1-6 and Comparative Example 1 all retained the position located at 800 cm. -1 The characteristic peaks in the vicinity are attributed to the respiratory vibrations of the triazine ring. At 1200 cm⁻¹ -1 Up to 1700 cm -1 The strong absorption bands in the region remain clearly visible, corresponding to the typical tensile vibrations of the CN / C=N skeleton of aromatic heterocycles. Furthermore, at 3000 cm⁻¹... -1 Up to 3400 cm -1 The broad peaks within the range are attributed to the stretching vibrations of NH / OH, possibly originating from the adsorption of hydroxyl groups on the carbon nitride surface and the hydrogenation of nitrogen atoms. These results further confirm that the heptaazine ring framework structure of g-C3N4 remains intact.

[0035] from Figure 3 It can be seen that CN pure It exhibits irregular blocky stacked aggregates, lacks obvious lamellar network stacking characteristics, and has a relatively rough and dense surface. This dense aggregation severely limits the specific surface area and hinders the exposure of active sites.

[0036] from Figure 4 It can be seen that the bulk structure of the oxygen-doped photoanode begins to disintegrate. O-CN1:0.5 0.2 mm It exhibits a flower-like stacked nanosheet structure, which is relatively dense. With oxygen doping, the dense nanosheets begin to transform into thinner and more distinct layers, O-CN 1:1 0.2 mm The edges of the layers become thinner and more elongated, indicating that the oxygen-containing gas released during calcination triggers the peeling of the bulk layers. When O-CN1:2 0.2 mm At this stage, the nanosheets expand further, exhibiting a layered, lamellar stacked structure. This structure significantly increases the specific surface area, providing more adsorption sites for reactants. With further oxygen doping, O-CN1:3 0.2 mm The layered lamellar crystals evolved into larger lamellar structures, and more aggregates appeared, leading to the collapse of the layered lamellar structure. The changes in the morphology of the photoanodes in Examples 1-4 indicate that oxygen doping plays a crucial role in optimizing the microstructure of carbon nitride, providing a structural basis for subsequent performance improvements. Furthermore, EDS was used to modify the O-CN1:2... 0.2 mm Elemental analysis is performed using photoanodes, from Figure 4 As can be seen from mp, O-CN1:2 0.2 mm It contains three elements: C, N, and O. These elements are in the O-CN1:2 ratio. 0.2 mm The surface is uniformly distributed.

[0037] The EDS elemental distribution of the photoanodes in Examples 1-4 and Comparative Example 1 is shown in Table 1.

[0038] Table 1. EDS elemental distribution of photoanodes in Examples 1-4 and Comparative Example 1

[0039] from Figure 5 As can be seen from b, after introducing oxygen doping, the C 1s spectra of the oxygen-doped carbon nitride photoanodes in Examples 1-4 all exhibit a novel strong peak at 288.9 eV. This peak belongs to a carbonyl-related species (C=O), directly proving that oxygen atoms have successfully bonded into the carbon framework. From Figure 5 c. As can be seen, the N 1s high-resolution spectra of Examples 1-4 and Comparative Example 1 exhibit high stability, with minimal variation in peak positions and component ratios, indicating that oxygen doping treatment did not significantly alter the basic coordination environment of nitrogen atoms. From Figure 5As can be seen, the O 1s spectra of Examples 1-4 changed significantly compared to Comparative Example 1: A new characteristic peak appeared at 531.8 eV in the oxygen-doped carbon nitride photoanodes of Examples 1-4, attributed to the C=O bonds within the lattice. This corroborates the appearance of a new peak at 288.9 eV in the C 1s spectrum, confirming the occurrence of lattice oxygen doping. Secondly, the C-OH peak, originally located at 532.1 eV, shifted significantly to 532.8 eV. This binding energy shift can be attributed to an electron-induced effect: the abundant C=O groups introduced into the lattice have strong electron-withdrawing properties, reducing the electron cloud density of the conjugated framework, thereby altering the electronic environment of the surface C-OH species connected to them, causing their binding energy to shift towards higher energies. This phenomenon further confirms the synergistic existence of surface hydroxyl modification and bulk lattice doping.

[0040] from Figure 6 It can be seen that CN in Comparative Example 1 pure The oxygen-doped carbon nitride photoanodes of Examples 1-4 all exhibited significant absorption in the 300-450 nm range, with similar absorption edge positions. The CN of Comparative Example 1 was obtained by extrapolating the Tauc curves. pure The optical bandgap (Eg) of the oxygen-doped carbon nitride photoanodes in Examples 1-4 is 2.76-2.80 eV, indicating that oxygen doping did not significantly alter the intrinsic bandgap of carbon nitride. Photonic PL (PL) measurements showed that the lowest peak PL intensity was observed when the doping ratio reached 1:2, indicating the lowest recombination rate of photogenerated electrons and holes. Based on the bandgap results and... Figure 9 The model's Schottky curve is derived from c. Figure 9 The band structure diagram of d, from Figure 6 As can be seen from d, oxygen doping alters the band structure of carbon nitride, providing a foundation for subsequent cathodic protection performance.

[0041] from Figure 7 It can be seen that when the coating thickness is 0.2 mm, the photoanode has the lowest PL peak intensity and the lowest recombination rate of photogenerated electrons and holes.

[0042] from Figure 8 As can be seen, under illumination, the open-circuit potential (OCP) of the photoanodes in Examples 1-4 and Comparative Example 1 all undergo a significant negative shift. Compared to CN... pure (-0.39 V vs. SCE), the oxygen-doped photoanode exhibits a more negative photopotential, indicating that the introduction of oxygen effectively enhances the driving force of photogenerated electrons, thereby expanding the effective range of photogenerated cathodic protection. Further analysis of the effect of different oxygen doping ratios revealed that the cathodic protection performance of the photoanode first increases and then decreases with increasing oxygen doping concentration. Specifically, O-CN1:2 0.2 mmThe most significant negative potential shift induced by photoanodination in 304 stainless steel was -0.73 V (vs. SCE), confirming the O-CN1:2 ratio. 0.2 mm It exhibits optimal cathodic protection. When the doping ratio is further increased to 1:3, although O-CN 1:3... 0.2 mm It retains its photoresponse characteristics, but its negative potential shift is significantly less than that of O-CN1:2. 0.2 mm This is attributed to the fact that excessive oxygen doping may introduce too many lattice defects, which act as recombination centers for photogenerated carriers, leading to a decrease in effective electron supply and thus weakening the protective effect of the photogenerated cathode. Figure 8 b. It can be seen that the injection current density of the photoanodes in Examples 1-4 is generally higher than that of CN. pure This indicates that oxygen doping effectively suppresses the recombination of photogenerated electrons and holes, significantly enhancing the ability of the photoanode to transport electrons to the metal. O-CN1:2 0.2 mm It exhibits the highest photocurrent response, with a transient current density of 33.5 μA‧cm. -2 The steady-state photocurrent density is 28.4 μA‧cm. -2 This means O-CN1:2 0.2 mm It possesses optimal photogenerated electron supply capability. Tafel data show that, under illumination, the polarization curves of the photoanodes in Examples 1-4 and Comparative Example 1 all exhibit significant response enhancement and negative potential shift. This phenomenon confirms that illumination effectively drives the photoanode to generate photogenerated electrons and applies cathode polarization to the metal end through the wire. The results show that O-CN1:2 0.2 mm It has the most negative self-corrosion potential (Ecorr), which is consistent with the data from OCP.

[0043] from Figure 9 It can be seen that, under intermittent illumination conditions of 50 s on / 50 s off, the photoanodes of Examples 1-4 all exhibit significantly higher performance than CN. pure The photocurrent response was excellent, and the current density was close to zero in the dark state, indicating that the test current mainly originated from the generation and separation of photogenerated carriers, rather than from the electrochemical side reactions of the photoanode itself. In a 0.1 M Na₂SO₄ electrolyte without external bias, the photocurrent density of the photoanodes in Examples 1-4 generally increased with increasing oxygen doping concentration, and peaked at O-CN1:2. 0.2 mm The optimal ratio can be attributed to the improved carrier separation efficiency resulting from the electronic structure modulation and improved interfacial transport induced by oxygen doping. When the ratio is further increased to 1:3, the photocurrent decreases instead. This may be related to the more intense decarboxylation / dehydration and other exothermic side reactions (releasing CO2 / H2O / CO, etc.) triggered by excessive cyanuric acid during calcination polycondensation. This makes the film more prone to pore cracks and reduces the effective contact with FTO, thereby weakening carrier transport and ultimately reducing the photocurrent output. Figure 9 b. Data proves that the photoanodes of Examples 1-4 and Comparative Example 1 are all n-type semiconductors. From Figure 9 c shows that CN pure The photoanode exhibits a stronger impedance response, indicating that its photogenerated carrier migration resistance is significantly greater than that of the oxygen-doped carbon nitride photoanode. With the introduction of oxygen and increasing oxygen doping, the photogenerated carrier migration resistance first decreases and then increases, particularly in the O-CN1:2 ratio. 0.2 mm The smallest overall semicircular radius means that its charge transfer resistance (Rct) is the smallest, the interface charge transfer is faster, and the separation of charge carriers is more efficient, which is consistent with its higher photocurrent output in the LSV curve.

[0044] from Figure 10 It can be seen that photoanodes of different thicknesses all exhibit stable photoresponse characteristics, but the negative shift amplitude of the photopotential varies significantly: the negative shift is largest for photoanodes with a thickness of 0.2 mm, while it is relatively weaker for photoanodes with thicknesses of 0.1 mm and 0.3 mm. This is mainly due to the competitive mechanism between coating thickness and light absorption and charge transport: while thinner films are beneficial for rapid carrier migration, they are limited by the light absorption cross-section and the number of effective active sites; thicker films lead to severe carrier recombination and high internal resistance due to increased transport distance, resulting in fewer electrons flowing to 304 SS. Figure 10 As can be seen from b, the injection current density of the 0.2 mm thick photoanode is superior to that of the 0.1 mm and 0.3 mm thick photoanodes. This is because the 0.2 mm thickness effectively balances the contradiction between light absorption and carrier migration distance, ensuring sufficient photogenerated charge generation while avoiding severe bulk recombination and resistive losses caused by excessive film thickness, thus achieving efficient photogenerated cathode protection for 304 SS. From Figure 10 As can be seen from c and d, the 0.2 mm thick photoanode exhibits a better negative potential shift and current response under illumination than the 0.1 mm and 0.3 mm thick photoanodes. The Tafel test results are highly consistent with the open circuit potential test results, mutually confirming the optimal photoelectrochemical cathodic protection performance of the photoanode in Example 1.

[0045] from Figure 11 As can be seen, the photocurrent output exhibits an optimal relationship with thickness: when the film is too thin, photoabsorption and the number of active sites are limited; when the film is too thick, the carrier migration distance within the film increases, and the carrier recombination probability rises, thereby reducing the effective charge separation efficiency. 0.2 mm is the optimal thickness that balances photocurrent and transmission. Figure 11 b. Data proves that the photoanodes of Examples 1 and 5-6 are all n-type semiconductors. Figure 11 c shows that when the coating thickness increases from 0.1 mm to 0.2 mm, the interfacial and bulk phase transport is enhanced, O-CN1:20.2 mm The impedance is lowest at 0.3 mm, and increases further when the thickness is increased to 0.3 mm. This may be because the carrier transport path becomes longer and recombination becomes larger, which hinders the overall charge transport. This indicates that excessively thick films will limit the transport efficiency within the film.

[0046] Application examples Application of oxygen-doped carbon nitride photoanodes in the field of photoelectrochemical cathodic protection of 304 SS The H-type electrolytic cell consists of a corrosion cell and a photocell, separated by a Nafion 117 proton exchange membrane. The photocell has a transparent quartz window. The corrosion cell uses a 3.5 wt% NaCl solution to simulate a marine corrosion environment, while the photocell uses a 0.35 M Na₂SO₃ + 0.25 M Na₂S mixed solution as the electrolyte. In the corrosion cell, the protected metal 304 SS, a platinum sheet, and a saturated calomel electrode serve as the working electrode, counter electrode, and reference electrode, respectively. The photoanode is placed in the photoelectrochemical cell and connected to the protected metal 304 SS by a wire. The light source is a 300 W xenon lamp, with the power density adjusted to 100 mW / cm² under a simulated AM 1.5 G filter (400-800 nm visible light). 2 The photoanode is then irradiated through a quartz window.

[0047] Example of effect 1. The H-type electrolytic cell consists of a corrosion cell and a photocell, separated by a Nafion 117 proton exchange membrane. The photocell has a transparent quartz window. The corrosion cell uses a 3.5 wt% NaCl solution to simulate a marine corrosion environment, while the photocell uses a mixed solution of 0.35 M Na₂SO₃ and 0.25 M Na₂S as the electrolyte. In the corrosion cell, the protected metal 304 SS, a platinum sheet, and a saturated calomel electrode serve as the working electrode, counter electrode, and reference electrode, respectively. The photoanode is placed in the photoelectrochemical cell and connected to the protected metal 304 SS by a wire. The light source is a 300 W xenon lamp, with the power density adjusted to 100 mW / cm² under a simulated AM 1.5 G filter. 2 The photoanode was irradiated through a quartz window. After stabilizing for 20 minutes, the light was continuously turned on for 12 hours. At the end of the test, the pitting corrosion of the 304 SS was observed under a metallographic microscope. A bare 304 SS was immersed in a 3.5 wt% NaCl solution for 12 hours as a control group to observe the pitting corrosion.

[0048] Figure 12 a System Evaluation O-CN1:2 0.2 mm The continuous electron supply capability and output stability under near-real-world service conditions, with an open-circuit potential retention rate of 87.71% during a 12-hour stability test, demonstrate the effectiveness of O-CN1:2.0.2 mm It exhibits good stability. Metallographic microscopy before and after corrosion showed that the control group bare 304 SS, after immersion in 3.5 wt.% NaCl for 12 h, showed several pitting corrosion patterns. Meanwhile, the coupled O-CN1:2... 0.2 mm After a 12-hour stability test, 304 SS still exhibited a predominantly polished texture, with a significant reduction in dark pitting corrosion. In magnified images, only a small number of smaller, discontinuous, weak pitting features were observed, unlike the typical pitting corrosion characteristics seen in the control group. In the more demanding environment of artificial seawater, O-CN1:2... 0.2 mm The protective effect on 304 SS is very weak. After the test, many discrete pitting corrosion features can still be observed on the surface of 304 SS.

[0049] 2. Both the corrosion cell and the photovoltaic cell of the H-type electrolytic cell use a 3.5 wt% NaCl solution as the electrolyte. Figure 12 Figures c and d are O-CN1:2 0.2 mm Under artificial seawater conditions, O-CN1:2 0.2 mm Even after coupling with 304 SS, an instantaneous negative potential shift and reversible photoresponse characteristics were still observed upon illumination, indicating that the system can still achieve a certain degree of photogenerated electron injection without relying on a sacrificial agent. The potential after illumination was turned on reached -0.352 V vs. SCE, with a negative shift of approximately 202 mV relative to the self-corrosion potential; the corresponding steady-state injection current density was approximately 3.1 μA‧cm. -2 Even in the dark state, it still exhibits a weak reverse current. Compared to the aforementioned sacrificial agent-containing systems, both the negative potential shift and current output are reduced, indicating that the sacrificial agent plays a crucial role in enhancing the continuous output of the photoanode. However, O-CN1:2 0.2 mm The oxygen-doped carbon nitride photoanode prepared in this invention retains a certain protective effect in a more realistic artificial seawater environment, demonstrating its potential for application in real marine environments. Figure 12 Artificial seawater refers to artificially created seawater.

[0050] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. This disclosure is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of this disclosure are indicated by the claims.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. The scope of patent protection of the present invention shall be determined by the claims. Similarly, any equivalent structural changes made based on the content of the present invention's specification shall also be included within the scope of protection of the present invention.

Claims

1. A method for preparing an oxygen-doped carbon nitride photoanode, characterized in that, Includes the following steps: S1: Melamine and cyanuric acid are dissolved in solvent one to obtain a precursor solution. The precursor solution is centrifuged to collect the precipitate. The precipitate is washed and dried to obtain the precursor. S2: Mix the precursor with solvent II to obtain a slurry, coat the slurry onto the substrate, and dry to obtain a precursor film; S3: The precursor film from S2 is calcined with melamine under an inert gas to obtain the product.

2. The method for preparing an oxygen-doped carbon nitride photoanode according to claim 1, characterized in that, In S1, the molar ratio of melamine to cyanuric acid is 1:0.5-3, and 50-100 mL of solvent one is added for every 0.01 mol of melamine; in S2, 0.5-1.5 mL of solvent two is added for every 0.5 g of the precursor.

3. The method for preparing an oxygen-doped carbon nitride photoanode according to claim 1, characterized in that, The first solvent is water; the second solvent is ethylene glycol.

4. The method for preparing an oxygen-doped carbon nitride photoanode according to claim 1, characterized in that, The preparation method of the slurry in S2 includes: grinding the precursor for 15 to 25 minutes, then adding the solvent II and continuing to grind for 2 to 5 minutes.

5. The method for preparing an oxygen-doped carbon nitride photoanode according to claim 1, characterized in that, The drying temperature for obtaining the precursor in S1 is 50–70 °C; the drying temperature for obtaining the precursor film in S2 is 80–120 °C.

6. The method for preparing an oxygen-doped carbon nitride photoanode according to claim 1, characterized in that, The coating thickness of the slurry applied to the substrate in S2 is 0.1 to 0.3 mm, and the substrate is FTO.

7. The method for preparing an oxygen-doped carbon nitride photoanode according to claim 1, characterized in that, The calcination procedure described in S2 is as follows: heating to 500–550 °C at a heating rate of 2–5 °C / min, and calcining for 2–6 h; the inert gas is nitrogen.

8. An oxygen-doped carbon nitride photoanode, characterized in that, The oxygen-doped carbon nitride photoanode is prepared by the preparation method of any one of claims 1 to 7, wherein the oxygen doping content in the oxygen-doped carbon nitride photoanode is 2.25 to 2.97%.

9. The application of an oxygen-doped carbon nitride photoanode as described in claim 8 in the field of photoelectrochemical cathodic protection of metals.

Images