A method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts and its application

Abstract

This invention relates to the field of photocatalytic materials technology, and in particular to a method for molten salt-assisted synthesis of a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst and its application. The method includes: mixing and separating ferric chloride, hydrochloric acid, and melamine to obtain a solid precursor; subjecting the solid precursor and sodium sulfite to a first-stage heat treatment under a protective atmosphere to obtain a pyrolysis product; mixing the pyrolysis product with molten salt, grinding, and then subjecting it to a second-stage heat treatment under a protective atmosphere to obtain an intermediate; and acid washing and freeze-drying the intermediate to obtain the sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst. This invention utilizes a two-step pyrolysis process and the molten salt template effect to achieve a nanorod-like structure for the photocatalyst, exhibiting both high specific surface area and stable anchorage of iron elements in an atomically dispersed form on the sulfur-doped carbon-nitrogen support. When applied to the photocatalytic water splitting reaction for hydrogen production, it demonstrates excellent catalytic activity, structural stability, and reaction reproducibility.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic materials and catalysis technology, and in particular to a method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts and their applications. Background Technology

[0002] Against the backdrop of global energy structure transformation and climate change response, developing clean and sustainable energy technologies has become an urgent task for the scientific community. Hydrogen energy, as an ideal energy carrier with high calorific value and zero carbon emissions, is considered a key solution to replace fossil fuels. Among these technologies, photocatalytic water splitting, which uses semiconductor photocatalysts to directly convert solar energy into hydrogen energy, shows great application potential due to its mild reaction conditions and green process.

[0003] Among the many photocatalytic materials explored, graphitic carbon nitride (g-C3N4) stands out as one of the most representative non-metallic photocatalysts due to its unique visible light response characteristics, excellent physicochemical stability, abundant constituent elements, and non-toxic and environmentally friendly properties. However, unmodified bulk g-C3N4 suffers from inherent defects such as low specific surface area, limited visible light absorption range, rapid recombination rate of photogenerated electron-hole pairs, and insufficient surface active sites, resulting in low photocatalytic hydrogen evolution efficiency and severely restricting its practical application.

[0004] To overcome these bottlenecks, research mainly revolves around two major strategies. The first is morphology control and elemental doping. Through thermal exfoliation, template methods, or the introduction of non-metallic elements such as sulfur and phosphorus, the aim is to increase specific surface area, optimize band structure, and construct defects to capture charges. Sulfur doping has been proven to effectively broaden the photoresponse range and regulate surface electronic states. The second is supporting co-catalysts. Noble metals (such as Pt) or non-noble metals (such as Fe and Ni) are supported on the g-C3N4 surface to serve as electron-capturing centers and reactive sites. In recent years, single-atom catalysts have attracted much attention due to their extreme atomic utilization and well-defined active center structures. Anchoring inexpensive transition metals such as iron in single-atom form on g-C3N4 supports shows great promise for applications.

[0005] Despite numerous explorations, significant challenges remain in practical preparation, the core of which lies in achieving efficient synergy of multiple functions within a simple and controllable process. Firstly, the contradiction between single-atom dispersion and stability is prominent; during high-temperature pyrolysis, metal precursors are prone to migration and aggregation, leading to the loss of single-atom sites. Secondly, multifunctional modification is difficult to coordinate simultaneously; achieving effective sulfur doping (optimizing electronic structure), high specific surface area structure construction (increasing active site exposure), and stable anchoring of iron single atoms (providing efficient active centers) in one step is a key technical challenge in the preparation of high-performance g-C3N4-based photocatalysts. Thirdly, existing processes are complex and lack universality, mostly involving multiple post-processing steps, the use of expensive templates or complex ligands, resulting in cumbersome processes, high costs, and difficulty in large-scale promotion.

[0006] Therefore, developing a simple, efficient, and scalable preparation strategy that can synergistically achieve sulfur doping, support structure optimization, and stable anchoring of iron single atoms is of great significance for promoting the practical application of high-performance g-C3N4-based single-atom photocatalysts. Summary of the Invention

[0007] The purpose of this invention is to provide a method for synthesizing sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts with molten salt assistance and its application. Through the precise synergy of two-step pyrolysis and molten salt template effect, multi-level structural regulation from the molecular scale to the microscopic morphology is achieved.

[0008] To achieve the above objectives, the present invention provides a method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts, comprising the following steps: S1. Mix, stir, separate, and dry ferric chloride, hydrochloric acid, and melamine to obtain a solid precursor; S2. Under a protective atmosphere, the solid precursor and sodium sulfite are subjected to a first-stage heat treatment to obtain pyrolysis products; the heating rate of the first-stage heat treatment is 2-5℃ / min, the target temperature is 450-650℃, and the holding time is 3-6h. S3. Mix the pyrolysis product with molten salt, grind it, and then perform a second-stage heat treatment under a protective atmosphere to obtain an intermediate. The heating rate of the second-stage heat treatment is 5-10℃ / min, the target temperature is 450-650℃, and the holding time is 3-6h. S4. The intermediate is acid-washed and freeze-dried to obtain a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst.

[0009] In this invention, the specific mixing process described in S1 is as follows: first, ferric chloride is dissolved in hydrochloric acid to form a homogeneous solution, and then melamine is added.

[0010] In this invention, the preferred mass-to-volume ratio of ferric chloride, hydrochloric acid, and melamine in S1 is 5.4g:90-110mL:2-15g, more preferably 5.4g:100mL:5g; the preferred mass fraction of the hydrochloric acid is 36%-38%, more preferably 37%.

[0011] In this invention, the stirring time in S1 is preferably 20-40 min, more preferably 30 min, the drying temperature is preferably 50-70℃, more preferably 60℃, and the drying time is preferably 12-48 h, more preferably 24 h.

[0012] In this invention, the protective atmosphere in S2 includes nitrogen or argon, and the mass ratio of sodium sulfite to solid precursor is preferably 0.05-0.3:1, more preferably 0.2:1.

[0013] In this invention, the heating rate of the first stage heat treatment in S2 is preferably 2-5℃ / min, more preferably 3.3℃ / min, the target temperature is preferably 450-650℃, more preferably 550℃, and the holding time is preferably 3-6h, more preferably 4h.

[0014] In this invention, the mass ratio of molten salt to pyrolysis product in S3 is preferably 5-15:1, more preferably 10:1, and the molten salt includes potassium chloride and lithium chloride, wherein the mass ratio of potassium chloride and lithium chloride is preferably 1:0.5-1, more preferably 1:0.8.

[0015] In this invention, the protective atmosphere described in S3 includes nitrogen or argon.

[0016] In this invention, the heating rate of the second stage heat treatment in S3 is preferably 5-10℃ / min, more preferably 7.5℃ / min, the target temperature is preferably 450-650℃, more preferably 550℃, and the holding time is preferably 3-6h, more preferably 4h.

[0017] In this invention, the pickling process in S4 includes: immersing the intermediate in hydrochloric acid for 6-12 hours, more preferably 8 hours; the concentration of the hydrochloric acid is preferably 0.5-2 mol / L, more preferably 1 mol / L.

[0018] In this invention, after acid washing in S4, the acid-washed product is rinsed alternately with distilled water and ethanol, and then freeze-dried to obtain a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst; the freeze-drying temperature is -80~-60℃.

[0019] The present invention also provides a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst prepared by the above preparation method. The photocatalyst has a regular nanorod morphology, high specific surface area and atomically dispersed iron active centers, wherein the iron element is stably anchored in the sulfur-doped carbon-nitrogen framework in single-atom form.

[0020] This invention also provides the application of the above-mentioned sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst in the photocatalytic water splitting to produce hydrogen. This photocatalyst can efficiently and stably catalyze the splitting of water to produce hydrogen under simulated sunlight or visible light irradiation.

[0021] The present invention has the following beneficial effects: This invention provides a method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts, comprising the following steps: S1, mixing, stirring, separating, and drying ferric chloride, hydrochloric acid, and melamine to obtain a solid precursor; S2, subjecting the solid precursor and sodium sulfite to a first-stage heat treatment under a protective atmosphere to obtain a pyrolysis product; the heating rate of the first-stage heat treatment is 2-5℃ / min, the target temperature is 450-650℃, and the holding time is 3-6h; S3, mixing the pyrolysis product with molten salt, grinding, and then subjecting it to a second-stage heat treatment under a protective atmosphere to obtain an intermediate; the heating rate of the second-stage heat treatment is 5-10℃ / min, the target temperature is 450-650℃, and the holding time is 3-6h; S4, acid washing and freeze-drying the intermediate to obtain the sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst.

[0022] The core advantage of the method provided by this invention lies in the dual role of molten salt. On the one hand, the molten salt environment promotes the ordered rearrangement of carbon and nitrogen precursors, forming a highly crystalline mesoporous structure, providing an ideal platform for mass transport and light absorption. On the other hand, it stabilizes iron single atoms through coordination and, in conjunction with sulfur doping, modulates the local electronic structure, thereby optimizing the band structure and reaction pathway. The resulting catalyst possesses a uniform mesoporous distribution, high specific surface area, and atomically dispersed active centers, all of which contribute to its excellent photocatalytic performance.

[0023] This invention successfully prepared a sulfur-doped carbon-nitrogen support-supported iron single-atom photocatalyst with nanorod morphology, high specific surface area and uniform mesopores through a molten salt-assisted two-step thermal polymerization strategy.

[0024] Under visible light irradiation, this photocatalyst exhibits extremely high photocatalytic hydrogen evolution activity and cycling stability. Its performance enhancement is attributed to a synergistic effect of multiple factors: the highly crystalline nanorod framework accelerates charge separation and transport; the atomic-level iron centers act as efficient electron traps, significantly suppressing carrier recombination; and sulfur doping further modulates the surface reaction energy barrier, improving the utilization efficiency of photogenerated electrons. Furthermore, the catalyst maintains structural integrity during long-term use, with negligible activity decay, demonstrating its excellent durability.

[0025] This invention not only provides a method for preparing a highly efficient photocatalyst, but also offers a new approach for designing high-performance single-atom photocatalysts. This strategy features a simple process, easily controllable parameters, and is suitable for large-scale production, showing broad application prospects. Multi-scale structural optimization achieved through molten salt engineering provides a feasible solution to overcome the inherent defects of carbon nitride-based materials, thus promoting the development of clean energy technologies.

[0026] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0027] Figure 1 These are the XRD diffraction patterns of the photocatalysts prepared in Examples 1-3 and Comparative Examples 1-4 of this invention; Figure 2 This is a transmission electron microscope image of the photocatalyst prepared in Example 1 of the present invention; in, Figure 2 In the image, 'a' represents a transmission electron microscope (TEM) image. Figure 2 In the image, b is a transmission electron microscope image corrected for spherical aberration. Figure 2 In the diagram, 'c' represents the distribution of C elements. Figure 2 In the diagram, d represents the distribution of element S. Figure 2 In the diagram, 'e' represents the distribution of element N. Figure 2 f in the figure represents the distribution of Fe element; Figure 3 These are performance graphs of photocatalytic hydrogen production obtained from the photocatalysts prepared in Examples 1-3 and Comparative Examples 1-4 of this invention. Figure 4 This is a test graph of the cycle stability of the photocatalyst prepared in Example 1 of the present invention; Figure 5 These are the nitrogen adsorption-desorption curves of the photocatalysts prepared in Example 1 and Comparative Examples 1-4 of this invention; Figure 6 These are the Fourier transform infrared spectra of the photocatalysts prepared in Examples 1-3 and Comparative Examples 1-4 of this invention. Figure 7 These are the photoluminescence spectra of the photocatalysts prepared in Example 1 and Comparative Examples 1-4 of this invention; Figure 8The graph shows the UV-Vis absorption spectra and fitted band gaps of the photocatalysts prepared in Example 1 and Comparative Example 1. in, Figure 8 In the diagram, 'a' represents the ultraviolet-visible absorption spectrum. Figure 8 In the figure, b represents the result of fitting the bandgap width. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0029] Example 1 This embodiment provides a method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts, the specific implementation steps of which are as follows: S1. Dissolve 5.4g of ferric chloride in 100mL of 38% hydrochloric acid and stir to form a homogeneous solution. Then add 5g of melamine and stir for 30min to disperse it fully. Then centrifuge the mixture and dry the obtained solid product at 60℃ for 24h to obtain a solid precursor. S2. Sodium sulfite and solid precursor are mixed at a mass ratio of 0.2:1 and placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 550℃ at a heating rate of 3.3℃ / min and held for 4 hours for the first stage of heat treatment. After natural cooling, the pyrolysis product is obtained. S3. The pyrolysis product is mixed with molten salt (potassium chloride and lithium chloride in a mass ratio of 1:0.8) at a mass ratio of 1:10, ground, and placed in a tube furnace. The mixture is heated to 550°C at a heating rate of 5°C / min under a nitrogen atmosphere and held for 4 hours for the second stage of heat treatment. After natural cooling, the intermediate is obtained. S4. The intermediate was soaked in 1 mol / L hydrochloric acid for 8 hours for acid washing. The acid-washed product was then rinsed alternately with distilled water and ethanol. Finally, it was freeze-dried at -70℃ to obtain a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst.

[0030] Example 2 This embodiment provides a method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts, the specific implementation steps of which are as follows: S1. Dissolve 5.4g of ferric chloride in 90mL of 36% hydrochloric acid and stir to form a homogeneous solution. Then add 15g of melamine and stir for 40min to disperse it fully. Then centrifuge the mixture and dry the obtained solid product at 50℃ for 12h to obtain the solid precursor. S2. Sodium sulfite and solid precursor are mixed at a mass ratio of 0.05:1 and placed in a tube furnace. Under an argon atmosphere, the temperature is increased to 450℃ at a heating rate of 2℃ / min and held for 6 hours for the first stage of heat treatment. After natural cooling, the pyrolysis product is obtained. S3. The pyrolysis product and molten salt (potassium chloride and lithium chloride in a mass ratio of 1:0.5) are mixed at a mass ratio of 1:5, ground, and placed in a tube furnace. The mixture is heated to 650°C at a heating rate of 7.5°C / min under a nitrogen atmosphere and held for 3 hours for the second stage of heat treatment. After natural cooling, the intermediate is obtained. S4. The intermediate was immersed in 0.5 mol / L hydrochloric acid for 6 h for acid washing. The acid-washed product was then rinsed alternately with distilled water and ethanol. Finally, it was freeze-dried at -60℃ to obtain a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst.

[0031] Example 3 This embodiment provides a method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts, the specific implementation steps of which are as follows: S1. Dissolve 5.4g of ferric chloride in 110mL of 37% hydrochloric acid and stir to form a homogeneous solution. Then add 2g of melamine and stir for 20min to disperse it fully. Then centrifuge the mixture and dry the obtained solid product at 70℃ for 48h to obtain a solid precursor. S2. Sodium sulfite and solid precursor are mixed at a mass ratio of 0.3:1 and placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 650°C at a heating rate of 5°C / min and held for 3 hours for the first stage of heat treatment. After natural cooling, the pyrolysis product is obtained. S3. Mix the pyrolysis product with molten salt (potassium chloride and lithium chloride in a mass ratio of 1:1) at a mass ratio of 1:15, grind it, place it in a tube furnace, heat it to 450°C at a heating rate of 10°C / min under an argon atmosphere, hold it at that temperature for 6 hours, carry out the second stage of heat treatment, and obtain the intermediate after natural cooling. S4. The intermediate was immersed in 2 mol / L hydrochloric acid for 12 h for acid washing. The acid-washed product was then rinsed alternately with distilled water and ethanol. Finally, it was freeze-dried at -80℃ to obtain a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst.

[0032] Comparative Example 1 This comparative example provides a method for preparing a bulk carbon nitride photocatalyst, specifically including the following steps: S1. Weigh 5g of melamine and place it in a tube furnace. Under a nitrogen atmosphere, heat the sample to 550℃ at a heating rate of 3.3℃ / min and hold for 4 hours for heat treatment. After the heat treatment, allow the sample to cool naturally to room temperature in the atmosphere to obtain a bulk carbon nitride photocatalyst.

[0033] Comparative Example 2 This comparative example provides a method for preparing an iron-supported, doped sulfur nitride carbon photocatalyst, the specific steps of which are as follows: S1. Dissolve 5.4g of ferric chloride in 100mL of 38% hydrochloric acid and stir to form a homogeneous solution. Then add 5g of melamine and stir for 30min to disperse it fully. Then centrifuge the mixture and dry the obtained solid product at 60℃ for 24h to obtain a solid precursor. S2. Sodium sulfite and solid precursor were mixed at a mass ratio of 0.2:1 and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 550℃ at a heating rate of 3.3℃ / min and held for 4 hours for the first stage of heat treatment. After natural cooling, the pyrolysis product was obtained. S4. The pyrolysis product was soaked in 1 mol / L hydrochloric acid for 8 hours for acid washing. The acid-washed product was then rinsed alternately with distilled water and ethanol. Finally, it was freeze-dried at -70℃ to obtain a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst.

[0034] Comparative Example 3 This comparative example provides a method for preparing a carbon sulfide-doped photocatalyst, which is basically the same as the preparation method provided in Example 1, except that sodium sulfite is not added in S2.

[0035] Comparative Example 4 This comparative example provides a method for preparing an iron-supported carbon nitride photocatalyst, which is basically the same as the preparation method provided in Example 1, except that ferric chloride is not added in S1, and melamine is directly mixed with hydrochloric acid.

[0036] Test example: The photocatalysts obtained in Examples 1-3 and Comparative Examples 1-4 were subjected to XRD tests, and the results are as follows: Figure 1 As shown. From Figure 1As can be seen, Comparative Example 1 exhibits characteristic diffraction peaks of typical graphitic carbon nitride (g-C3N4) at approximately 13.1° and 27.2°, corresponding to the in-plane ordered arrangement (100 crystal plane) and interlayer π-π stacking (002 crystal plane) of the 3-s-triazine structural units, respectively. In contrast, in Examples 1-3 and Comparative Examples 3-4 prepared with molten salt assistance, the (100) crystal plane diffraction peaks all show significant low-angle shifts to approximately 8.0°, while the (002) peak shifts to approximately 27.9°. According to Bragg's law, this change indicates that the introduction of molten salt effectively modulates the interlayer stacking distance of the carbon-nitrogen framework. This phenomenon confirms that molten salt plays a structure-guiding role in the preparation process, promoting the transformation of the carbon-nitrogen matrix towards a more ordered and compact stacking mode.

[0037] It is noteworthy that the intensity of the (002) diffraction peaks in all samples with introduced iron (Examples 1-3 and Comparative Examples 2 and 4) was significantly reduced, and no diffraction peaks belonging to iron oxide or metallic iron phases were observed. This strongly suggests that the iron species did not form an independent crystalline phase, but was successfully embedded in the carbon-nitrogen framework in an atomically dispersed form. The introduction of iron atoms disrupted the original long-range order of the carbon-nitrogen support, significantly increasing structural defects and disorder, which typically facilitates the exposure of more active sites.

[0038] The photocatalyst prepared in Example 1 was observed using transmission electron microscopy, and the results are as follows: Figure 2 As shown. From Figure 2 As can be seen, Example 1 successfully prepared a uniform nanorod morphology, and clear lattice fringes can be observed in the high-resolution transmission electron microscopy (TEM) image. Numerous isolated bright spots are clearly visible in the aberration-corrected high-angle annular dark-field image. Statistical results confirm that single atoms (spacing > 2.8 nm) account for approximately 90% (single atom to dimer ratio of 192:20), directly proving that iron exists in a highly dispersed single-atom form. The uniform distribution maps of C, N, S, and Fe elements further confirm that each component has achieved successful doping and uniform recombination at the atomic scale.

[0039] Photocatalytic hydrogen evolution (HER) performance testing: (1) Weigh 10 mg of the photocatalyst powder prepared in Examples 1-3 and Comparative Examples 1-4 respectively, add 0.2 mg of chloroplatinic acid (H2PtCl6·6H2O), 16 mL of deionized water and 4 mL of methanol (as sacrificial agent), and sonicate at room temperature for 1 h to obtain a uniformly dispersed photocatalyst suspension.

[0040] The aforementioned photocatalyst suspension was poured into a high-transmittance quartz reactor and connected to the gas supply system. The reactor's internal gas chamber was purged with high-purity nitrogen and then evacuated to remove impurities such as oxygen. This purging-evacuation process was repeated three times. Subsequently, a 300W xenon lamp equipped with an ultraviolet filter (λ≥420nm) was used as a simulated visible light source to vertically irradiate the quartz reactor, initiating the photocatalytic water splitting and hydrogen production reaction, with continuous irradiation for 4 hours.

[0041] During the reaction, approximately 0.5 mL of gaseous product was extracted from the reactor headspace every hour using a gas-tight syringe. The extracted gas was immediately injected into a gas chromatograph equipped with a thermal conductivity detector (TCD), and the hydrogen in the collected gas was qualitatively and quantitatively analyzed using a standard curve method.

[0042] Among them, the results of photocatalytic hydrogen production performance are as follows: Figure 3 As shown. From Figure 3 As can be seen, Example 1 achieved a concentration as high as 45.62 mmol·g under optimized conditions. -1 ·h -1 The hydrogen evolution rate was significantly higher than that of Comparative Example 1 (approximately 63 times that of Comparative Example 2, without molten salt assistance, approximately 11 times that of Comparative Example 2, and several times higher than that of Comparative Examples 3 (without S doping) and 4 (without Fe loading). This result irrefutably demonstrates that the synergistic effect among molten salt assistance, sulfur doping, and single-atom iron loading is crucial for achieving ultra-high photocatalytic activity.

[0043] The photocatalyst prepared in Example 1 was subjected to a cycle stability test, and the results are as follows: Figure 4 As shown. From Figure 4 As can be seen, the catalyst prepared in Example 1 retained approximately 86.6% of its initial activity after five consecutive cycles (a total of 20 hours) of reaction, and no significant structural degradation or loss of active components was observed. This fully demonstrates that the photocatalyst structure constructed by the molten salt strategy possesses excellent chemical and structural stability, meeting the basic durability requirements for practical applications.

[0044] The nitrogen adsorption-desorption curves of the photocatalysts prepared in Example 1 and Comparative Examples 1-4 are as follows: Figure 5 As shown in Table 1, the specific surface area data are as follows.

[0045] Table 1. Specific surface area, porosity, and pore size data

[0046] from Figure 5 As can be seen from Table 1, Example 1 has the largest specific surface area (103.07 m²). 2The specific surface area ( / g) was significantly higher than that of the comparative samples. This is directly attributed to the molten salt acting as a soft template during pyrolysis, guiding the formation of highly porosity nanorod morphologies. The increased specific surface area provides an ideal physical platform for the adsorption, mass transfer, and full exposure of active sites of reactants, which is an important structural basis for achieving high performance.

[0047] The Fourier transform infrared spectra of the photocatalysts prepared in Examples 1-3 and Comparative Examples 1-4 are as follows: Figure 6 As shown. From Figure 6 It can be seen that all samples retained the characteristic vibrational peaks of the g-C3N4 framework. Notably, Examples 1-3 showed peaks at approximately 990 cm⁻¹. -1 A new vibrational peak belonging to a metal-nitrogen coordination (MNC, where M represents Fe, K, Li, etc.) structure appeared, providing direct spectroscopic evidence that iron single atoms are anchored to the carbon-nitrogen support via nitrogen atoms. Meanwhile, the spectral peaks of the sample prepared with molten salt assistance were generally stronger and sharper, further confirming that its structure possesses higher order and crystallinity.

[0048] The photoluminescence spectra of the photocatalysts prepared in Example 1 and Comparative Examples 1-4 are as follows: Figure 7 As shown. From Figure 7 It can be seen that all samples exhibit an emission peak at approximately 467 nm originating from intrinsic exciton recombination of g-C3N4. However, the emission peak intensity of Example 1 was strongly quenched, almost dropping to baseline levels. This indicates that, under the synergistic effect of sulfur doping and iron single atoms, photogenerated electron-hole pairs are effectively separated and transferred, greatly suppressing their unproductive dissipation through radiative recombination. This is the key kinetic reason for achieving ultra-high photocatalytic activity.

[0049] The UV-Vis absorption spectra and fitted bandgap test results of the photocatalysts prepared in Example 1 and Comparative Example 1 are as follows: Figure 8 As shown. From Figure 8 As can be seen, compared with Comparative Example 1, the absorption sideband of Example 1 exhibits a significant redshift, and its optical bandgap decreases from 2.65 eV to 2.55 eV. This is attributed to the band structure modulation caused by sulfur doping and the defect energy levels introduced by iron single atoms. The reduction in bandgap broadens the absorption range of the material for visible light, enabling it to capture and utilize more solar energy, thereby enhancing the initial driving force of photocatalysis.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for molten salt-assisted synthesis of sulfur-doped carbon-nitride support loaded iron monatomic photocatalyst, characterized in that, Includes the following steps: S1. Mix, stir, separate, and dry ferric chloride, hydrochloric acid, and melamine to obtain a solid precursor; S2. Under a protective atmosphere, the solid precursor and sodium sulfite are subjected to a first-stage heat treatment to obtain pyrolysis products; the heating rate of the first-stage heat treatment is 2-5℃ / min, the target temperature is 450-650℃, and the holding time is 3-6h. S3. Mix the pyrolysis product with molten salt, grind it, and then perform a second-stage heat treatment under a protective atmosphere to obtain an intermediate. The heating rate of the second-stage heat treatment is 5-10℃ / min, the target temperature is 450-650℃, and the holding time is 3-6h. S4. The intermediate is acid-washed and freeze-dried to obtain a sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst.

2. The method for the molten salt-assisted synthesis of sulfur-doped carbon-nitride support loaded iron single-atom photocatalysts according to claim 1, characterized in that, The mass-to-volume ratio of ferric chloride, hydrochloric acid, and melamine in S1 is 5.4g:90-110mL:2-15g; the mass fraction of the hydrochloric acid is 36%-38%.

3. The method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts according to claim 1, characterized in that, The stirring time described in S1 is 20-40 min, the drying temperature is 50-70℃, and the time is 12-48 h.

4. The method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts according to claim 1, characterized in that, The mass ratio of sodium sulfite to solid precursor in S2 is 0.05-0.3:

1.

5. The method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts according to claim 1, characterized in that, The mass ratio of molten salt to pyrolysis products in S3 is 5-15:1, and the molten salt includes potassium chloride and lithium chloride.

6. The method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts according to claim 5, characterized in that, The mass ratio of potassium chloride to lithium chloride is 1:0.5-1.

7. The method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts according to claim 1, characterized in that, The pickling process described in S4 includes: immersing the intermediate in hydrochloric acid for 6-12 hours; the concentration of the hydrochloric acid is 0.5-2 mol / L.

8. The method for molten salt-assisted synthesis of sulfur-doped carbon-nitrogen supported iron single-atom photocatalysts according to claim 1, characterized in that, The freeze-drying temperature described in S4 is -80~-60℃.

9. A sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst prepared by the method according to any one of claims 1-8.

10. The application of the sulfur-doped carbon-nitrogen supported iron single-atom photocatalyst according to claim 9 in the photocatalytic water splitting to produce hydrogen.

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