A nanoscale sheet-shaped graphite phase carbon nitride and a preparation method and application thereof

Abstract

This invention discloses a method for preparing nanoscale sheet-like graphitic carbon nitride, the resulting nanoscale sheet-like graphitic carbon nitride, and its applications, belonging to the field of photocatalytic material synthesis technology. The method for preparing nanoscale sheet-like graphitic carbon nitride disclosed in this invention utilizes nano-hexagonal sheet-like melamine cyanurate as a precursor, leveraging its own two-dimensional stacked structure as a "self-template" to achieve directional in-situ growth of g-C3N4 along the two-dimensional direction. Through a one-step thermal polymerization method, a nanosheet structure with expanded interlayer spacing is directly formed. The nanoscale sheet-like graphitic carbon nitride prepared by the method of this invention exhibits significantly expanded interlayer spacing, high specific surface area, optimized band structure, and good carrier separation efficiency. It demonstrates excellent redox capabilities and reaction efficiency in multiphase photocatalytic systems and can be widely used as a photocatalyst in fields such as the degradation of organic pollutants, photocatalytic water splitting for hydrogen production, and carbon dioxide reduction.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic material synthesis technology, specifically to a high-performance nanoscale sheet-like graphitic carbon nitride photocatalyst, its preparation method, and its application. Background Technology

[0002] Graphitic carbon nitride (g-C3N4), as a typical non-metallic polymer semiconductor, has shown great application potential in photocatalytic degradation of organic pollutants, photocatalytic water splitting for hydrogen production, and carbon dioxide reduction due to its suitable band gap (approximately 2.7 eV), excellent chemical stability, and low raw material cost. Currently, industrial production of g-C3N4 mainly relies on thermal polymerization, typically using small-molecule nitrogen-containing compounds such as dicyandiamide, melamine, or urea as precursors, and carrying out high-temperature polycondensation at 500℃ to 600℃.

[0003] However, traditional small molecule precursors typically form dense bulk structures after thermal polymerization, and the severe π-π interlayer stacking effect results in extremely low specific surface area of ​​the product (usually below 20 m²). 2 The g-C3N4 nanosheets have few exposed catalytic active sites, and photogenerated carriers recombine easily, severely limiting photocatalytic efficiency. Furthermore, post-processing is cumbersome and costly. To improve performance, the industry often uses strong acid exfoliation or organic solvent intercalation to prepare g-C3N4 nanosheets. For example, Chinese patent CN 117658084 discloses an acid etching process for preparing g-C3N4 nanosheets, in which bulk g-C3N4 is added to concentrated hydrochloric acid for etching. Chinese patent CN121244015 discloses an organic solvent intercalation process for preparing g-C3N4 nanosheets, in which bulk g-C3N4 obtained by thermal polymerization of melamine is added to N,N-dimethylformamide for soaking and centrifugation to obtain g-C3N4 nanosheets. However, these methods are not only complex, time-consuming, and have low yields, but they also easily introduce impurities, damage the crystal structure, and cause environmental pollution, greatly limiting the large-scale production and practical application of g-C3N4 materials.

[0004] Therefore, how to directly prepare g-C3N4 nanosheets with both high specific surface area and good structural integrity through a simplified process, avoiding cumbersome post-processing and exfoliation procedures, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of complex preparation processes, low specific surface area, and easily destroyed structure of graphitic carbon nitride (g-C3N4) nanosheets in existing technologies. This invention provides a method for preparing nanosheet-like graphitic carbon nitride using simple raw materials, requiring only a one-step thermal polymerization reaction without post-processing or exfoliation. Furthermore, this invention also provides the graphitic carbon nitride prepared by the described method and its applications.

[0006] In a first aspect, the present invention provides a method for preparing nanoscale sheet-like graphitic carbon nitride that uses simple raw materials, requires only one thermal polymerization reaction without post-processing or exfoliation steps, and yields nanoscale sheet-like graphitic carbon nitride. To achieve this objective, the technical solution adopted by the present invention is as follows: a method for preparing nanoscale sheet-like graphitic carbon nitride, the method comprising the following steps: Nanoscale hexagonal sheet-like melamine cyanurate is thermally polymerized in air to obtain nanoscale sheet-like graphitic carbon nitride. The process conditions for the thermal polymerization reaction are as follows: heating to 400-700°C at a heating rate of 1-10°C / min, holding at that temperature for 2-5 hours, and then naturally cooling to room temperature.

[0007] The method for preparing nanoscale sheet-like graphitic carbon nitride described in this invention uses nano-hexagonal sheet-like melamine cyanurate (MCA) as a precursor. Utilizing its two-dimensional stacked structure formed by intermolecular hydrogen bonds as a self-template, the precursor is grown in situ along the two-dimensional direction and rearranged into graphitic carbon nitride through a one-step thermal polymerization reaction in air, eliminating the need for post-processing exfoliation. This self-templatement mechanism fundamentally suppresses the π-π interlayer stacking effect of traditional bulk materials, allowing the product to naturally form a nanosheet structure with expanded interlayer spacing, significantly increasing the specific surface area and surface active sites. Simultaneously, it greatly simplifies the process flow and avoids damage to the crystal structure caused by chemical exfoliation.

[0008] In the preparation method of nanoscale sheet-like graphitic carbon nitride described in this invention, the heating rate needs to be strictly controlled during the thermal polymerization reaction. The heating rate is one of the important parameters for balancing the pyrolysis kinetics and crystal growth quality of nano-hexagonal sheet-like melamine cyanurate (MCA). The inventors of this application found in experiments that if the heating rate is <1℃ / min, although crystal growth is more complete, energy consumption is high and production efficiency is significantly reduced; if the heating rate is >10℃ / min, the nano-hexagonal sheet-like melamine cyanurate (MCA) undergoes sudden and violent decomposition, failing to undergo orderly condensation, resulting in a large number of amorphous phases or structural defects in the product, severely disrupting the self-templating effect, and ultimately leading to a decrease in specific surface area and photocatalytic activity. Therefore, through repeated experiments, the inventors finally determined that the heating rate of the thermal polymerization reaction in the preparation method of this application is 1–10 °C / min. Within this rate range, the melamine and cyanuric acid units in the nano-hexagonal sheet-like melamine cyanurate (MCA) can dissociate in an orderly manner, and utilize its two-dimensional hydrogen bond stacking structure as a self-template to guide the product to grow in a two-dimensional orientation, thereby obtaining a nanosheet structure with expanded interlayer spacing and good crystallinity. The heating rate can be arbitrarily selected within the range of 1–10 °C / min, for example, including but not limited to any selection or any combination of two of the following: 1 °C / min, 2 °C / min, 3 °C / min, 4 °C / min, 5 °C / min, 6 °C / min, 7 °C / min, 8 °C / min, 9 °C / min, and 10 °C / min.

[0009] In the preparation method of nanoscale sheet-like graphitic carbon nitride described in this invention, the temperature during the thermal polymerization reaction needs to ensure that the nano-hexagonal sheet-like melamine cyanurate (MCA) can completely condense to form graphitic carbon nitride, while maintaining the stability of its two-dimensional nanosheet structure. The inventors discovered in their research that if the temperature is <400℃, the precursor nano-hexagonal sheet-like melamine cyanurate (MCA) undergoes incomplete pyrolysis, leaving a large amount of unconverted intermediates in the product, making it impossible to form a complete g-C3N4 conjugated framework and hindering effective expansion of the interlayer spacing; if the temperature is >700℃, the g-C3N4 framework undergoes severe thermal decomposition and sublimation, resulting in a sharp drop in yield due to carbon and nitrogen loss, and the formed nanosheets will fuse and agglomerate due to excessive sintering, causing a sharp reduction in specific surface area and loss of catalytic activity. Therefore, after repeated experiments, it was found that the optimal temperature for the thermal polymerization reaction is 400–700℃. This temperature range provides sufficient energy to drive the formation and rearrangement of CN bonds, resulting in good crystallinity in the product, while avoiding structural collapse caused by excessively high temperatures. The temperature during the thermal polymerization reaction can be arbitrarily selected within the range of 400–700℃, including but not limited to any selection from 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, and 700℃, or any combination of two of these temperatures.

[0010] In the preparation method of nanoscale sheet-like graphitic carbon nitride described in this invention, the holding time during the thermal polymerization reaction is a crucial parameter to ensure the full progress of the polymerization reaction and the stabilization of the structure. If the holding time is too short, the reaction will not reach thermodynamic equilibrium, the condensation process will be incomplete, resulting in poor crystallinity of the product, insufficient expansion of interlayer spacing, and the presence of many unstable intermediates in the structure. If the holding time is too long, the formed g-C3N4 will undergo thermal etching under prolonged high-temperature conditions, leading to shrinkage of the sheet edges, re-enhancing of interlayer π-π stacking, and even fusion, which in turn leads to a decrease in specific surface area and an increase in the recombination rate of photogenerated carriers, thus reducing photocatalytic efficiency. The inventors of this application have discovered that the holding time during the thermal polymerization reaction needs to be controlled within 2-5 hours. Within this time range, nano-hexagonal sheet-like melamine cyanurate (MCA) molecules can fully complete condensation and recombination to form g-C3N4 crystals with high polymerization degree and few defects, while maintaining the loose structure of the nanosheets. The heat preservation and calcination time can be arbitrarily selected within 2 to 5 hours, for example, including but not limited to any selection of 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, or any range consisting of any two points.

[0011] The nano-hexagonal sheet-like melamine cyanurate used in this invention is commercially available nano-hexagonal sheet-like melamine cyanurate (MCA). Preferably, the effective content of melamine cyanurate in the nano-hexagonal sheet-like melamine cyanurate is ≥99.0 wt%, the residual free melamine is ≤0.3 wt%, and the residual free cyanuric acid is ≤0.3 wt%. When the effective content of melamine cyanurate in the nano-hexagonal sheet-like melamine cyanurate is ≥99.0 wt%, the residual free melamine is ≤0.3 wt%, and the residual free cyanuric acid is ≤0.3 wt%, the integrity of the precursor hydrogen bond assembly structure and the uniformity of the chemical composition are ensured. High-purity raw materials avoid disordered polycondensation or local sintering caused by impurity monomers during thermal polymerization, thereby ensuring the stable performance of the self-templating effect and giving the final product higher crystallinity, more regular nanosheet morphology, and better photocatalytic activity.

[0012] Preferably, the particle size of the nano-hexagonal sheet-like melamine cyanurate satisfies D 50 ≤500 nm and D 98≤3μm. Small-sized nanosheets are more likely to maintain two-dimensional growth orientation during pyrolysis, avoiding local overheating and structural collapse caused by the hysteresis of large particles, thus promoting the formation of a uniformly dispersed nanosheet structure in the final product, further improving the specific surface area and reaction mass transfer efficiency. In this application, selecting nano-hexagonal sheet-like melamine cyanurate with the specific particle size ensures that the nano-hexagonal sheet-like melamine cyanurate precursor is heated uniformly and exhibits a significant spatial confinement effect during thermal polymerization. The particle size of the nano-hexagonal sheet-like melamine cyanurate is, for example, D... 50 It can be any selection from 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, or a range formed by any two points, D 98 It can be any choice of 1μm, 1.5μm, 2μm, 2.5μm, 3μm or a range formed by any two points.

[0013] Preferably, the thickness of the nano-hexagonal sheet-like melamine cyanurate is less than 50 nm. The thin-layer structure facilitates the rapid escape of small molecule gases during thermal polymerization, reduces dense interlayer packing, promotes the expansion of the interlayer spacing and the formation of mesoporous structures in the final product, thereby enhancing the separation efficiency of photogenerated carriers and the surface catalytic activity. When the thickness of the nano-hexagonal sheet-like melamine cyanurate described in this application is less than 50 nm, it can introduce a high specific surface area and abundant edge active sites. The thickness of the nano-hexagonal sheet-like melamine cyanurate can be, for example, any selection from 10 nm, 20 nm, 30 nm, 40 nm, and 50 nm, or a range formed by any two of these values.

[0014] The method for preparing meter-scale sheet-like graphitic carbon nitride as described in claim 1 is characterized in that the heating rate of the thermal polymerization reaction process is 1–5 °C / min. Slow heating allows the two-dimensional hydrogen bond network of the nano-hexagonal sheet-like melamine cyanurate (MCA) to gradually transform into a g-C3N4 conjugated framework during pyrolysis, avoiding sudden decomposition and amorphous condensation caused by rapid heating. This ensures a high degree of crystallinity, significantly increased interlayer spacing, and superior light absorption performance and carrier separation capability. A heating rate of 1–5 °C / min provides sufficient time for the dissociation, rearrangement, and condensation of the nano-hexagonal sheet-like melamine cyanurate (MCA).

[0015] Preferably, the temperature of the thermal polymerization reaction is 550℃ to 700℃. The inventors of this application discovered in their research that when the thermal polymerization reaction temperature is 550℃ to 700℃, this specific temperature is the optimal thermodynamic temperature range for the complete condensation polymerization of nano-hexagonal sheet-like melamine cyanurate (MCA) to form g-C3N4. This temperature range ensures that the precursor is fully converted into highly crystalline graphitic carbon nitride, while avoiding skeletal thermal decomposition and interlayer π-π stacking re-enhancement caused by excessively high temperatures. This achieves an optimal balance between crystallinity, interlayer spacing, and specific surface area, significantly improving photocatalytic degradation efficiency.

[0016] Preferably, the holding time for calcination in the thermal polymerization reaction is 3-4 hours. The inventors of this application have discovered that the holding time directly affects the sufficiency of condensation and recombination of nano-hexagonal sheet-like melamine cyanurate (MCA) and the structural stability after condensation and recombination. When the holding time is 3-4 hours, the nano-hexagonal sheet-like melamine cyanurate (MCA) has sufficient time to complete condensation and recombination and structural stabilization after reaching the target temperature. This avoids incomplete reaction and structural defects caused by excessively short holding time, and also prevents excessive thermal etching and sheet fusion caused by excessively long holding time, thus resulting in a product with high polymerization degree, large specific surface area, and excellent structural stability.

[0017] In the preparation method of nanoscale sheet-like graphitic carbon nitride described above, nano-hexagonal sheet-like melamine cyanurate is used as the sole raw material (i.e., no nitrogen-containing compounds other than melamine cyanurate are added as co-precursors). Furthermore, the nano-hexagonal sheet-like melamine cyanurate requires no pretreatment, and the thermal polymerization reaction is carried out under normal pressure. No post-polymerization exfoliation process is required after thermal polymerization. The preparation method of this invention relies on the self-templating effect of the precursor (nano-hexagonal sheet-like melamine cyanurate) to achieve the construction of nanosheet structures from the source, eliminating the cumbersome steps of traditional acid etching, solvent intercalation, or ultrasonic exfoliation. This significantly reduces production costs and environmental pollution, while avoiding damage to the crystal structure during post-processing and ensuring the consistency of product performance.

[0018] In some preferred embodiments, the thermal polymerization reaction can be carried out in a muffle furnace or in other heating equipment capable of providing an air atmosphere, such as a tube furnace or a box furnace. Natural cooling can occur inside or outside the furnace.

[0019] In the preparation method of nanoscale sheet-like graphitic carbon nitride according to the present invention, the nanoscale sheet-like graphitic carbon nitride obtained after the thermal polymerization reaction can be used directly as a photocatalyst without the need for post-processing such as grinding and sieving. Preferably, the nanoscale sheet-like graphitic carbon nitride can be stored at room temperature away from light before use.

[0020] Secondly, the present invention provides a high-performance nanoscale sheet-like graphitic carbon nitride, wherein the nanoscale sheet-like graphitic carbon nitride is prepared by the method described above.

[0021] As described above, in the preparation method of nanoscale sheet-like graphitic carbon nitride of the present invention, the self-templating effect of nano-hexagonal sheet-like melamine cyanurate forms a two-dimensional structure with expanded interlayer spacing. XRD analysis of the nanoscale sheet-like graphitic carbon nitride prepared by this method confirms that its interlayer spacing is significantly larger than that of traditional bulk materials, and its BET specific surface area can reach 46.77–69.68 m². 2 / g, the structural features exposed more edge active sites, shortening the migration path of photogenerated carriers, combined with sp 2 The high degree of polymerization resulting from the high nitrogen content enables the material to exhibit excellent charge separation efficiency and catalytic activity under visible light.

[0022] Finally, this invention also provides the application of the nanoscale sheet-like graphitic carbon nitride as a photocatalyst in the degradation of organic pollutants, photocatalytic water splitting for hydrogen production, or carbon dioxide reduction. As described above, the expanded interlayer spacing and high specific surface area of ​​the nanoscale sheet-like graphitic carbon nitride of this invention provide abundant surface active sites and shorten carrier migration paths. Simultaneously, the optimized band structure broadens the visible light response range and significantly suppresses the recombination of photogenerated electron-hole pairs. Therefore, it exhibits excellent redox capabilities and reaction efficiency in heterogeneous photocatalytic systems and can be widely used as a photocatalyst in the degradation of organic pollutants, photocatalytic water splitting for hydrogen production, and carbon dioxide reduction, for example, including but not limited to its use as a photocatalyst for the degradation of methylene blue.

[0023] The method for preparing nanoscale sheet-like graphitic carbon nitride described in this invention utilizes nano-hexagonal sheet-like melamine cyanurate as a precursor and its own two-dimensional stacked structure as a "self-template" to achieve directional in-situ growth of g-C3N4 along the two-dimensional direction. Through a one-step thermal polymerization method, no post-processing exfoliation steps (such as strong acid etching, organic solvent intercalation, or ultrasonic exfoliation) are required, directly forming a nanosheet structure with expanded interlayer spacing. This fundamentally avoids the stacking problem of traditional bulk materials, not only simplifying the process flow and reducing the difficulty of process control and production costs, but also avoiding structural damage and environmental pollution problems introduced by chemical reagents.

[0024] The nanoscale sheet-like graphitic carbon nitride prepared by the method of the present invention has significantly expanded interlayer spacing, high specific surface area, optimized band structure and good carrier separation efficiency. It exhibits excellent photocatalytic degradation performance of organic pollutants under visible light and significantly inhibits the recombination of photogenerated electron-hole pairs. Thus, it exhibits excellent redox ability and reaction efficiency in heterogeneous photocatalytic systems and can be widely used as a photocatalyst in the fields of degrading organic pollutants, photocatalytic water splitting to produce hydrogen, and carbon dioxide reduction. Attached Figure Description

[0025] Figure 1 X-ray diffraction patterns of nanoscale sheet-like g-C3N4 prepared in Examples 1, 1, and 2 of this application.

[0026] Figure 2 This is a scanning electron microscope image of the nanoscale sheet-like g-C3N4 prepared in Example 1 of this application.

[0027] Figure 3 This is a scanning electron microscope image of the nanoscale sheet-like g-C3N4 prepared in Example 2 of this application.

[0028] Figure 4 This is a scanning electron microscope image of the nanoscale sheet-like g-C3N4 prepared in Example 3 of this application.

[0029] Figure 5 This is a scanning electron microscope image of the nanoscale sheet-like g-C3N4 prepared in Example 4 of this application.

[0030] Figure 6 This is a scanning electron microscope image of the nanoscale sheet-like g-C3N4 prepared in Example 5 of this application.

[0031] Figure 7 This is a scanning electron microscope image of the nanoscale sheet-like g-C3N4 prepared in Comparative Example 1 of this application.

[0032] Figure 8 This is a scanning electron microscope image of the nanoscale sheet-like g-C3N4 prepared in Comparative Example 2 of this application.

[0033] Figure 9 The N2 adsorption-desorption curves of the nanoscale sheet-like g-C3N4 prepared in Examples 1, 1, and 2 of this application are shown.

[0034] Figure 10The images show the X-ray photoelectron spectra of nanoscale sheet-like g-C3N4 prepared in Examples 1, 1, and 2 of this application. The left image is (a) and the right image is (b).

[0035] Figure 11 The images show the UV-Vis diffuse reflectance spectra of the nanoscale sheet-like g-C3N4 prepared in Examples 1, 1, and 2 of this application. The left image is (a) and the right image is (b).

[0036] Figure 12 The photoluminescence spectra of nanoscale sheet-like g-C3N4 prepared in Examples 1, 1, and 2 of this application are shown.

[0037] Figure 13 The images show the photocatalytic activity of nanoscale sheet-like g-C3N4 prepared in Examples 1, 1, and 2 of this application. The left image is (a) and the right image is (b). Detailed Implementation

[0038] The technical solution and the technical effects achieved by the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0039] Unless otherwise specified, the raw materials and equipment used in the following examples are common in the art and can be prepared by conventional methods in the art or purchased directly.

[0040] The nano-hexagonal flake melamine cyanurate used in the following examples was purchased from Shouguang Puer Chemical Co., Ltd., and is an industrial-grade flame retardant product. The effective content of melamine cyanurate is ≥99.0 wt%, the residual free melamine is ≤0.3 wt%, the residual free cyanuric acid is ≤0.3 wt%, and the D... 50 For ≤0.5μm, D 98 The thickness is ≤3μm, and the sheet thickness is 20~50nm.

[0041] Example 1 An embodiment of the present invention provides nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal sheet-like melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 550 °C at a heating rate of 1 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 4 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain the nano-scale sheet-like g-C3N4 of this embodiment.

[0042] Example 2 An embodiment of the present invention provides nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal sheet-like melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 400 °C at a heating rate of 2 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 2 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain the nano-scale sheet-like g-C3N4 of this embodiment.

[0043] Example 3 An embodiment of the present invention provides nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal sheet-like melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 600°C at a heating rate of 5°C / min for thermal polymerization. The mixture was then calcined at this temperature for 3 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature to obtain the nano-scale sheet-like g-C3N4 of this embodiment.

[0044] Example 4 An embodiment of the present invention provides nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal sheet-like melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 650 °C at a heating rate of 7 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 4 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain the nano-scale sheet-like g-C3N4 of this embodiment.

[0045] Example 5 An embodiment of the present invention provides nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal sheet-like melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 700 °C at a heating rate of 10 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 5 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain the nano-scale sheet-like g-C3N4 of this embodiment.

[0046] Comparative Example 1 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of urea was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 550 °C at a rate of 1 °C / min and maintained for 4 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature to obtain g-C3N4 of this comparative example.

[0047] Comparative Example 2 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of melamine was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 550 °C at a rate of 1 °C / min and maintained for 4 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature to obtain g-C3N4 of this comparative example.

[0048] Comparative Example 3 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal flake melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 550 °C at a heating rate of 0.5 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 4 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature to obtain g-C3N4 of this comparative example.

[0049] Comparative Example 4 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal flake melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 550 °C at a heating rate of 12 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 4 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain g-C3N4 of this comparative example.

[0050] Comparative Example 5 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal flake melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 380 °C at a heating rate of 1 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 4 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain g-C3N4 of this comparative example.

[0051] Comparative Example 6 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal flake melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 720 °C at a heating rate of 1 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 4 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain g-C3N4 of this comparative example.

[0052] Comparative Example 7 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal flake melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 550 °C at a heating rate of 1 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 1 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain g-C3N4 of this comparative example.

[0053] Comparative Example 8 This invention provides a comparative example of nanoscale sheet-like graphitic carbon nitride, which is prepared by the following method: 10 g of nano-hexagonal flake melamine cyanurate (MCA) was weighed and placed in a 100 mL alumina crucible as a carbon and nitrogen source. The crucible was sealed with tin foil and placed in a muffle furnace. The temperature was increased to 550 °C at a heating rate of 1 °C / min for thermal polymerization. The mixture was then calcined at this temperature for 6 h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain g-C3N4 of this comparative example.

[0054] Example of effect 1 Phase and structure testing of nanoscale sheet-like graphitic carbon nitride described in this invention Using the nanoscale sheet-like graphitic carbon nitride from Examples 1-5 and Comparative Examples 1-2 as experimental subjects, XRD patterns were obtained for the nanoscale sheet-like graphitic carbon nitride from Examples 1 and 1-2, and scanning electron microscopy (SEM) morphology was analyzed for the nanoscale sheet-like graphitic carbon nitride from Examples 1-5 and 1-2. The testing methods are as follows: XRD pattern analysis: Analysis was performed using a Bruker D8 Advance X-ray diffractometer (XRD). The test conditions were: Cu target Kα radiation source (λ = 0.15406 nm), tube voltage 40 kV, tube current 40 mA, scanning range 2θ = 10°–50°, scanning step size 0.02°, and scanning speed 5° / min. Before testing, the samples were vacuum-dried at 60°C for 12 h. A suitable amount of sample was then placed in the sample holder, flattened, and tested.

[0055] Scanning electron microscopy (SEM) test: The morphology of the sample was observed using a Hitachi SU8010 field emission scanning electron microscope (SEM).

[0056] The X-ray diffraction patterns of the nanoscale sheet-like g-C3N4 prepared in Example 1, Comparative Example 1, and Comparative Example 2 are attached. Figure 1 As shown.

[0057] Scanning electron microscope (SEM) images of the nanoscale sheet-like g-C3N4 prepared in Examples 1-5 are attached. Figure 2-6 As shown in the attached figures. Scanning electron microscope (SEM) images of the nanoscale sheet-like g-C3N4 nanoscale sheet-like g-C3N4 prepared in Comparative Examples 1 and 2 are respectively attached. Figure 7-8 As shown.

[0058] From the appendix Figure 1 It can be seen that the crystal structure of the g-C3N4 catalyst prepared in Example 1 using nanoscale hexagonal sheet-like MCA as the carbon and nitrogen source is consistent with that prepared using melamine (Comparative Example 2) and urea (Comparative Example 1) as carbon and nitrogen sources. However, compared with Comparative Examples 1 and 2, the (002) peak of the g-C3N4 prepared in Example 1 shifted to the left. According to the Bragg equation (2dsinθ=λ), this shift indicates that the interlayer spacing of g-C3N4 has increased. This means that the g-C3N4 described in Example 1 has a larger interlayer distance, and its conjugated aromatic structure tends to be loose. This loose structure weakens the connection between layers, resulting in a significant increase in interlayer spacing and exposing more active sites. At the same time, the weakening of the interlayer interaction also has a certain regulatory effect on its band structure.

[0059] From the appendix Figure 2-8As can be seen, the g-C3N4 prepared using melamine as the carbon and nitrogen source (Comparative Example 2) exhibits a typical particulate morphology; while the g-C3N4 prepared using urea and nanoscale hexagonal plate-like MCA as carbon and nitrogen sources (Comparative Example 1) both exhibit a plate-like structure. Specifically, the g-C3N4 catalyst prepared using urea as the carbon and nitrogen source consists of stacked small thin plates, while the g-C3N4 catalyst prepared using nanoscale hexagonal plate-like MCA as the carbon and nitrogen source consists of larger, intact plates, exhibiting a distinct multilayer structure with significantly increased interlayer spacing, consistent with XRD analysis results. This significantly increased interlayer distance effectively increases the specific surface area of ​​the g-C3N4 catalyst, thus providing a richer array of catalytic active sites.

[0060] Example 2 Specific surface area and pore size testing of nanoscale sheet-like graphitic carbon nitride described in this invention This study uses nanoscale sheet-like graphitic carbon nitride from Examples 1-5 and Comparative Examples 1-8 as test objects. Nitrogen adsorption-desorption performance, surface elemental composition and chemical state analysis, optical absorption characteristics analysis, and photogenerated carrier separation efficiency analysis were performed on them. The specific analytical methods are as follows: Nitrogen adsorption-desorption performance analysis: The analysis was conducted using a Micron ASAP 2460 fully automated specific surface area and porosity analyzer. Before testing, the samples were vacuum degassed at 120 °C for 6 h to remove adsorbed moisture and impurities. N2 physical adsorption-desorption tests were performed at liquid nitrogen temperature (77 K), and the specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The N2 adsorption-desorption curves obtained from Example 1 and Comparative Examples 1-2 are attached. Figure 9 As shown in Table 1, the specific surface area and pore volume test results of nanoscale sheet-like graphitic carbon nitride in Examples 1-5 and Comparative Examples 1-8 are shown below.

[0061] Surface elemental composition and chemical state analysis: Measurements were performed using a Thermo Fisher Scientific K-Alpha X-ray photoelectron spectrometer with monochromatic Al Kα rays (hv = 1486.6 eV) and a power of 150 W. Prior to testing, the samples were cleaned by argon ion sputtering to remove surface contaminants (carbon). The binding energy was corrected using C 1s (284.8 eV) as the standard, and CN = C (sp) was calculated through peak fitting. 2 ), N-(C)3(sp 3 ) and NH X The content of and calculate sp² / (sp³+NH X The ratio is shown in the attached X-ray photoelectron spectroscopy (XPS) spectra obtained from Example 1 and Comparative Examples 1-2. Figure 10As shown in Table 2, the XPS element content data obtained from the tests of Example 1 and Comparative Examples 1-2 are shown below.

[0062] Optical absorption characteristics analysis: Tests were performed using a Shimadzu UV-2600 UV-Vis-NIR spectrophotometer (Japan), with BaSO4 as the standard white plate as a reference. The test wavelength range was 200–800 nm. The UV-Vis diffuse reflectance spectra (UV-Vis DRS) obtained from Examples 1 and Comparative Examples 1-2 are attached. Figure 11 As shown.

[0063] Photogenerated carrier separation efficiency analysis: Tests were performed using a Hitachi F-4600 fluorescence spectrophotometer (Japan). The excitation source was a 150 W xenon lamp, the excitation wavelength was set to 320 nm, the scanning range was 350–600 nm, and the slit width was 5.0 nm. The photoluminescence spectra (PL) obtained from Example 1 and Comparative Examples 1-2 are attached. Figure 12 As shown.

[0064] Table 1. Specific surface area and pore volume of Examples 1-5 and Comparative Examples 1-8 Table 2. XPS elemental content data obtained from tests in Example 1 and Comparative Examples 1-2. Depend on Figure 9 As can be seen, the N2 adsorption-desorption isotherm of the sample exhibits a distinct hysteresis loop, classifying it as Type IV according to BDDT classification, indicating that the catalyst possesses a typical mesoporous structure. Data in Table 1 show that, compared to Comparative Examples 1 and 2, the specific surface area of ​​Example 1 is significantly increased, confirming effective nanosheet exfoliation during the preparation process, consistent with the XRD and SEM analysis results. The increased specific surface area enhances the electron migration capacity of the material surface and provides more catalytic reaction sites.

[0065] As shown in Table 1, the BET specific surface area of ​​Example 1 (MCA raw material) is as high as 69.68 m². 2 / g, far superior to Comparative Example 2 (11.17 m 2 / g) and Comparative Example 1 (40.23 m 2 / g), and the pore volume is also the largest. This is related to Figure 9 The N2 adsorption-desorption curves were consistent with those obtained from the study, demonstrating that g-C3N4 prepared by the MCA self-templating method has a larger specific surface area and abundant mesoporous structure, which is beneficial for exposing more active sites.

[0066] As shown in Table 1 for Examples 1, 3, 4, and 5, the specific surface area initially decreased slightly and then increased with increasing reaction temperature, but remained at a high level. This indicates that within the temperature range of this invention (400~700℃), the increase in temperature did not destroy the loose structure caused by the self-templating effect; instead, it further increased the porosity through thermal etching. In Comparative Example 5 (380℃), due to the low temperature, the condensation reaction was incomplete, leaving residual intermediates, resulting in a loose structure and a small specific surface area. In Comparative Example 6 (720℃), the temperature was too high, leading to severe thermal decomposition and lamellar sintering of the g-C3N4 framework, resulting in a renewed tight bonding between the layers and a significant decrease in specific surface area.

[0067] As can be seen from Example 1 and Comparative Examples 3-4 in Table 1, the heating rate has a direct impact on the performance of nanoscale sheet-like graphitic carbon nitride. Although the specific surface area of ​​Comparative Example 3 (0.5 ℃ / min) is still acceptable, it has shown a decreasing trend, proving that excessively slow heating not only leads to increased energy consumption and is detrimental to industrial production, but also results in decreased performance. In Comparative Example 4 (12 ℃ / min), due to excessively rapid heating, the precursor underwent sudden and violent decomposition, gas escape was obstructed, leading to pore collapse and a significant reduction in specific surface area.

[0068] As can be seen from Example 1 and Comparative Examples 7-8 in Table 1, the holding time has a direct impact on the performance of nanoscale sheet-like graphitic carbon nitride. In Comparative Example 7 (holding time 1 h), the condensation reaction was insufficient due to the short holding time, resulting in incomplete structural development and a smaller specific surface area. Although the specific surface area of ​​Comparative Example 8 (holding time 6 h) increased somewhat, it was still lower than that of Example 1, indicating that excessively long holding times can lead to unnecessary thermal etching and edge fusion, resulting in a loss of specific surface area.

[0069] Depend on Figure 10 As can be seen in (a), the C 1s spectra of the catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 can all be resolved into three characteristic peaks after peak fitting. The peaks with binding energies at 284.3 eV, 285.9 eV, and 287.7 eV are attributed to the standard reference carbon, C-NH, and C-NH, respectively. x and sp 2 - Bonded carbon ((N-)₂C=N). From Figure 10 (b) It can be seen that the corresponding N 1s spectrum can be analyzed into three peaks, with binding energies located around 398.2 eV, 399.5 eV and 400.6 eV, respectively, corresponding to sp 2 Hybridized nitrogen (C=NC), sp 3 Hybrid nitrogen (N-(C)3) and amino species (NH4+) x ), where sp 2 Hybridized nitrogen (C=NC) and sp 3The presence of the characteristic peak of hybrid nitrogen (N-(C)3) confirms the formation of a typical g-C3N4 structure.

[0070] Depend on Figure 10 It was also observed that, compared to Comparative Examples 1 and 2, most of the fitted peaks in the N1s and C1s spectra of the g-C3N4 catalyst prepared in Example 1 shifted slightly towards lower binding energies, indicating a decrease in the binding energy between C and N atoms. This decrease in binding energy means a relative weakening of the CN bond strength, making the g-C3N4 catalyst prepared in Example 1 more easily etched by NH3 during synthesis to form a rich mesoporous structure and expose more active sites.

[0071] As shown in Table 2, Example 1 had the highest sp² hybrid nitrogen (CN=C) content (79.9%), and the sp² / (sp³+NH) ratio was also the highest. X The largest ratio (3.98) indicates a more complete conjugated structure, which is beneficial for the transport of photogenerated carriers. This is consistent with... Figure 10 The results of XPS high-resolution spectral analysis were consistent, proving that the MCA self-templating method promoted the polymerization of g-C3N4, forming a more complete conjugated system.

[0072] from Figure 11 As can be seen in (a), compared with Comparative Examples 1 and 2, the ultraviolet absorption edge of g-C3N4 prepared in Example 1 shows a significant red shift, indicating that its visible light response range has been broadened. Typically, the red shift of the semiconductor's light absorption edge is closely related to the narrowing of its bandgap (Eg). The Eg value of the g-C3N4 sample was calculated based on the Tauc method. The results are as follows... Figure 11 As shown in Figure (b), the band gaps of g-C3N4 prepared in Example 1, Comparative Example 1, and Comparative Example 2 are 2.52 eV, 2.54 eV, and 2.58 eV, respectively. This indicates that the g-C3N4 catalyst prepared using nanoscale hexagonal sheet-like MCA as a carbon and nitrogen source has a relatively narrower band gap. The reduction in band gap helps to lower the energy required for photogenerated electrons to transition from the valence band (VB) to the conduction band (CB), thereby enhancing the material's ability to capture and utilize visible light and providing favorable conditions for improving its photocatalytic efficiency.

[0073] The PL spectra of the g-C3N4 catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 were measured at an excitation wavelength of 365 nm as follows: Figure 12As shown, the emission peak at approximately 462 nm is the characteristic fluorescence peak of g-C3N4, originating from the recombination of its band-edge electron-hole pairs. Comparison shows that the fluorescence intensity of g-C3N4 prepared in Example 1 is significantly lower than that in Comparative Examples 1 and 2, indicating that it has the lowest recombination rate of photogenerated carriers. This result demonstrates that g-C3N4 prepared using nanoscale hexagonal sheet-like MCA as a carbon-nitrogen source possesses higher carrier separation capability, which is beneficial for more photogenerated electrons and holes to participate in surface redox reactions, thus providing an important guarantee for enhancing its photocatalytic activity.

[0074] Example 3 Photocatalytic effect test of nanoscale sheet-like graphitic carbon nitride described in this invention This example uses the nanoscale sheet-like graphitic carbon nitride prepared in Example 1 and Comparative Examples 1-2 as the test object. The specific test method is as follows: 20 mg of g-C3N4 prepared in Example 1, Comparative Example 1, and Comparative Example 2 were added to 80 mL of methylene blue solution (concentration 10 mg / L), and transferred to a 100 mL double-layered quartz glass reactor. The reactor was water-cooled and the temperature was controlled at 15°C. The mixture was stirred for 30 min in the dark to ensure sufficient contact between the catalyst and the methylene blue solution, reaching adsorption-desorption equilibrium. Then, the mixture was irradiated with an HSX-F300 xenon lamp (300 W, 420 nm cutoff filter). Samples were taken after reacting for a certain period (10, 20, 40, 60, and 100 min), centrifuged (10000 r / min, 10 min / time), and the supernatant was collected for analysis. The absorbance of the methylene blue solution was measured using a UV-1240 UV-Vis spectrophotometer (the maximum absorbance of the methylene blue solution is 665 nm). The photocatalytic degradation rate of methylene blue solution can be calculated according to the Lambert-Beer law: In the formula, D% is the photocatalytic degradation rate; C0 and C are the solution concentrations of methylene blue at the initial time and after a certain period of illumination, respectively.

[0075] This example evaluated the photocatalytic activity of the samples prepared in Example 1, Comparative Example 1, and Comparative Example 2 by degrading a methylene blue (10 mg / L) solution under visible light (λ ≥ 420 nm) irradiation. The results are shown in the attached figure. Figure 13 As shown.

[0076] As attached Figure 13As shown in (a), after 30 min of dark adsorption, the adsorption rates of methylene blue by the g-C3N4 prepared in Example 1, Comparative Example 2, and Comparative Example 1 were 14.6%, 8%, and 3.3%, respectively. After 100 min of visible light irradiation, the degradation rates of methylene blue by the three catalysts reached 70%, 37%, and 16.4%, respectively. The results indicate that the photocatalytic activity of the g-C3N4 catalyst prepared in Example 1 is significantly better than that of the catalysts prepared in Comparative Example 1 and Comparative Example 2, with its methylene blue degradation efficiency increased by more than 2 times. This is mainly attributed to its larger specific surface area, more abundant surface active sites, narrower band gap, and higher carrier separation capability resulting from its layered structure.

[0077] Furthermore, the photocatalytic process was analyzed using a pseudo-first-order kinetic model, and the results are shown in the attached figure. Figure 13 As shown in (b). From the appendix Figure 13 (b) It can be seen that the calculated reaction rate constants k of g-C3N4 prepared in Example 1, Comparative Example 2, and Comparative Example 1 are 0.00977, 0.00353, and 0.00139, respectively. Among them, the g-C3N4 catalyst prepared using nanoscale hexagonal plate-shaped MCA as a carbon and nitrogen source has the highest k value, further confirming that it has the best photocatalytic performance.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. 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 be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing nanoscale sheet-like graphitic carbon nitride, characterized in that, The method includes the following steps: Nanoscale hexagonal sheet-like melamine cyanurate is thermally polymerized in air to obtain nanoscale sheet-like graphitic carbon nitride. The process conditions for the thermal polymerization reaction are as follows: heating to 400-700°C at a heating rate of 1-10°C / min, holding at that temperature for 2-5 hours, and then naturally cooling to room temperature.

2. The method for preparing nanoscale sheet-like graphitic carbon nitride as described in claim 1, characterized in that, The nano-hexagonal sheet-like melamine cyanurate contains an effective melamine cyanurate content ≥99.0 wt%, residual free melamine ≤0.3 wt%, and residual free cyanuric acid ≤0.3 wt%.

3. The method for preparing nanoscale sheet-like graphitic carbon nitride as described in claim 1, characterized in that, The particle size of the nano-hexagonal sheet-like melamine cyanurate satisfies D 50 ≤500 nm and D 98 ≤3μm.

4. The method for preparing nanoscale sheet-like graphitic carbon nitride as described in any one of claims 1-3, characterized in that, The thickness of the nano-hexagonal sheet-like melamine cyanurate is less than 50 nm.

5. The method for preparing meter-scale sheet-like graphitic carbon nitride as described in claim 1, characterized in that, The heating rate of the thermal polymerization reaction process is 1–5 °C / min.

6. The method for preparing nanoscale sheet-like graphitic carbon nitride as described in claim 1, characterized in that, The temperature of the thermal polymerization reaction is 550℃~700℃.

7. The method for preparing nanoscale sheet-like graphitic carbon nitride as described in claim 1, characterized in that, The heat treatment and calcination time in the thermal polymerization reaction is 3 to 4 hours.

8. The method for preparing nanoscale sheet-like graphitic carbon nitride as described in claim 1, characterized in that, The nano-hexagonal sheet-like melamine cyanurate is the only raw material used in its preparation. And / or, the nano-hexagonal sheet-like melamine cyanurate does not require pretreatment before the thermal polymerization reaction; And / or, the thermal polymerization reaction is carried out at atmospheric pressure; And / or, the thermal polymerization reaction is carried out in a muffle furnace; And / or, no post-processing stripping step is required after the thermal polymerization reaction.

9. Nanoscale sheet-like graphitic carbon nitride prepared by the preparation method according to any one of claims 1-8.

10. The application of the nanoscale sheet-like graphitic carbon nitride as described in claim 9 as a photocatalyst in the degradation of organic pollutants, photocatalytic water splitting for hydrogen production, or carbon dioxide reduction.

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