Triazine-heptazine homojunction carbon nitride as well as preparation method and application thereof

Abstract

The invention relates to the field of photocatalytic materials, and discloses triazine-heptazine homojunction carbon nitride as well as a preparation method and application thereof. According to the preparation method, dicyandiamide, potassium chloride and lithium chloride are taken as raw materials, the high-crystallinity triazine-heptazine homojunction carbon nitride is prepared by a one-step method by limiting the heating rate and calcining time in a vacuum environment and controlling the proportion of triazine to heptazine structures, and the high-crystallinity triazine-heptazine homojunction carbon nitride is applied to a photocatalytic depolymerization reaction of lignin, so that the high-crystallinity triazine-heptazine homojunction carbon nitride is obtained. The obvious catalytic activity and selectivity are embodied. The method is simple to operate, mild in reaction condition and easy to synthesize, and has a certain industrial application prospect. The triazine-heptazine homojunction carbon nitride prepared by the method disclosed by the invention has shorter carrier lifetime of 4.03 ns and lower exciton binding energy of 18.1 meV. The obtained triazine-heptazine homojunction carbon nitride is used as a photocatalyst, and in the reaction of depolymerizing a lignin model compound 2-phenoxy-1-phenethyl alcohol, the conversion rate is as high as 98%.

Description

Technical Field

[0001] The present invention relates to the field of photocatalytic materials, and more specifically, to a triazine-heptazine homojunction carbon nitride and its preparation method and application. Background Art

[0002] Utilizing photocatalytic technology to achieve efficient depolymerization of lignin is one of the important directions in the current research on the high-value utilization of biomass resources. Lignin is a natural aromatic polymer with a complex structure and high stability, which is widely present in plant cell walls in nature. Traditional lignin conversion methods generally have problems such as high energy consumption, harsh reaction conditions, and serious environmental pollution. In contrast, photocatalytic depolymerization of lignin has significant advantages such as mild reaction conditions, no need to add expensive reagents, and a green and environmentally friendly process, and is thus considered a green conversion path with great development potential.

[0003] During the photocatalytic depolymerization process, the photocatalyst is excited by absorbing solar energy, generating photogenerated electrons and holes, which then undergo redox reactions with lignin and its model compounds to break carbon-carbon bonds or carbon-oxygen bonds, producing high-value small molecule products such as acetophenone and phenyl formate. However, one of the bottlenecks in photocatalytic reaction efficiency is that photogenerated electrons and holes are prone to recombination, resulting in energy loss and significantly reducing the reaction efficiency. Therefore, how to improve the separation and migration efficiency of photogenerated carriers and thus enhance the solar energy conversion efficiency is the core scientific problem and technical difficulty in this technical field.

[0004] Based on this, means such as constructing heterojunctions, homojunctions, introducing surface defects, and in-situ doping have been widely explored to regulate the bandgap structure, interfacial electric field, and electron migration path of photocatalysts. Among them, constructing a homojunction with a good interfacial structure (such as a triazine / heptazine homojunction) can form an effective carrier separation channel inside the material, reduce the recombination probability, and significantly enhance the photocatalytic activity. Therefore, developing new photocatalytic materials with high crystallinity, adjustable structure, and strong interfacial activity is the key breakthrough for realizing efficient and sustainable conversion of lignin.

[0005] The prior art CN119500212A discloses a carbon-doped carbon nitride photocatalyst with a triazine / heptazine homojunction structure prepared from melamine and 2,4,6-triaminopyrimidine as raw materials through supramolecular assembly and molten salt calcination, which has excellent photogenerated carrier separation ability and photocatalytic benzyl alcohol oxidation performance. This method requires first mixing the melamine and 2,4,6-triaminopyrimidine powder, obtaining a supramolecular precursor through steps such as calcination, ultrasonic dispersion, and rotary evaporation, and then calcining the obtained precursor with molten salt to obtain the triazine-heptazine homojunction.

[0006] The prior art CN115463682A discloses an S-type crystallized carbon nitride homojunction photocatalytic material prepared from melamine, lithium chloride, and potassium chloride. By constructing a triazine-heptazine crystal phase interface electric field through asynchronous crystallization and electrostatic self-assembly, efficient separation of photogenerated electrons is achieved, significantly enhancing its CO2 reduction performance under visible light irradiation. This method involves separately calcining melamine, bulk carbon nitride, triazine carbon nitride, and heptazine carbon nitride obtained by calcining molten salts, and then electrostatically self-assembling the two crystalline forms of carbon nitride into triazine-heptazine carbon nitride.

[0007] The preparation method of the above prior art has a relatively complex raw material system and often requires the use of more than two different types of organic precursors. At the same time, the synthesis process involves multiple operations such as calcination, ultrasonic treatment, and assembly, with a cumbersome process and poor reproducibility. In addition, some methods have high requirements for the reaction environment and are difficult to scale up and apply industrially. More importantly, due to the dependence of the structure regulation means on external auxiliary steps, there is still room for improvement in the photocatalytic performance of the obtained materials. Summary of the Invention

[0008] In order to overcome the defects of the above prior art, such as complex raw material system, cumbersome preparation process, many steps, poor photocatalytic performance, and difficulty in achieving efficient large-scale application, the present invention provides a method for preparing triazine-heptazine homojunction carbon nitride;

[0009] Another object of the present invention is to provide a triazine-heptazine homojunction carbon nitride;

[0010] Another object of the present invention is to provide an application of the triazine-heptazine homojunction carbon nitride.

[0011] To solve the above technical problems, the technical solution of the present invention is as follows:

[0012] A method for preparing triazine-heptazine homojunction carbon nitride, comprising the following steps: heating and calcining dicyandiamide to obtain bulk carbon nitride; mixing the bulk carbon nitride, potassium chloride, and lithium chloride, and performing heat treatment under vacuum conditions, followed by filtration and washing to obtain triazine-heptazine homojunction carbon nitride; the temperature of the heat treatment is 500-600 °C, and the time is 1-3 h.

[0013] Preferably, the temperature of the heat treatment is 550-600 °C, and the time is 1-3 h.

[0014] Preferably, the temperature of the heat treatment is 550 °C, and the time is 1 h.

[0015] Furthermore, the heating rate during the heat treatment is 6-8 °C / min.

[0016] Preferably, the heating rate during the heat treatment is 6 °C / min.

[0017] Furthermore, the mass ratio of bulk-phase carbon nitride, potassium chloride, and lithium chloride is 1:5-7:4-6.

[0018] Furthermore, the temperature of the heating and calcination is 300-600 °C, and the time is 2-6 h.

[0019] Preferably, the temperature of the heating and calcination is 300-550 °C, and the time is 4-6 h.

[0020] Preferably, dicyandiamide is placed in a crucible and heated and calcined in a muffle furnace to obtain bulk-phase carbon nitride.

[0021] Preferably, the obtained triazine-heptazine homojunction carbon nitride mixed with the molten salt is dispersed in pure water, filtered and washed until the ionic strength of the washing liquid is 0, and then dried at 60 °C to obtain triazine-heptazine homojunction carbon nitride.

[0022] A triazine-heptazine homojunction carbon nitride is prepared by the preparation method.

[0023] An application of the triazine-heptazine homojunction carbon nitride is used for preparing a photocatalytic material.

[0024] Furthermore, the photocatalytic material includes a photocatalyst.

[0025] Furthermore, the photocatalytic material is used for photocatalytic depolymerization of lignin.

[0026] Furthermore, the photocatalyst is used for preparing acetophenone and / or phenyl formate.

[0027] A photocatalytic material is prepared from the triazine-heptazine homojunction carbon nitride.

[0028] The present invention utilizes the polycondensation reaction of a bulk-phase carbon nitride precursor in a molten salt system, controls the ratio of the triazine and heptazine structures of the product by limiting the heating rate and calcination time in a vacuum environment, and prepares highly crystalline triazine-heptazine homojunction carbon nitride by a one-step method. It is applied to the photocatalytic depolymerization of lignin reaction, showing significant catalytic activity and selectivity. Under the specific heating rate and calcination time of the present invention, the triazine structural unit can be rapidly generated and retained in the early stage of high-temperature polycondensation, avoiding its further conversion into a complete heptazine structure, so as to jointly construct a stable homojunction structure with a partial heptazine crystal phase. On the other hand, this specific reaction condition also helps to lock the interfacial structure in the early stage of crystal growth, promote the coexistence of the triazine phase and the heptazine phase at the microscale, and thus construct a crystallized carbon nitride material with a clear interfacial energy band ladder structure.

[0029] By constructing a regular triazine-heptazine homojunction structure, the present invention forms a stable interfacial electric field at the microscopic level, effectively promoting the separation and directional migration of photo-generated electrons and holes, and significantly improving the utilization efficiency of photo-generated carriers. Compared with traditional carbon nitride materials, the catalyst of the present invention has higher crystallinity and better light absorption performance, and can efficiently break the C–C bond in the lignin model compound 2-phenoxy-1-phenylethanol under visible light irradiation to directionally generate high-value products such as acetophenone and phenyl formate. The present invention breaks through the bottleneck problems such as serious carrier recombination and low catalytic efficiency in the prior art, and has significant technological progressiveness and creativity.

[0030] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows:

[0031] The triazine-heptazine homojunction carbon nitride photocatalytic material constructed by the present invention can effectively promote the spatial separation of photo-generated electrons and holes, enhance the migration ability of electrons inside the material, thereby improving the photocatalytic activity and product selectivity. The synergistic effect between the triazine and heptazine structural units not only provides a more reasonable energy band matching, but also forms a built-in electric field at the interface, effectively reducing the exciton binding energy and improving the utilization efficiency of photo-generated carriers. Compared with heptazine carbon nitride and triazine carbon nitride, the triazine-heptazine homojunction carbon nitride material prepared by this method has a higher transient photocurrent response intensity and a lower electrochemical impedance. In addition, the triazine-heptazine homojunction carbon nitride prepared by this method has a shorter carrier lifetime of 4.03 ns, which is much lower than that of heptazine carbon nitride (9.06 ns); a lower exciton binding energy of 18.1 meV, which is much lower than that of heptazine carbon nitride (23.6 meV) and triazine carbon nitride (35.1 meV). In the reaction of photocatalytic depolymerization of the lignin model compound 2-phenoxy-1-phenylethanol, the triazine-heptazine carbon nitride prepared by this method shows the highest conversion rate of 98%. Description of the Drawings

[0032] Figure 1 is the X-ray diffraction (XRD) pattern;

[0033] Figure 2 is the transmission electron microscope (TEM) photograph, a~c are the TEM photographs of Comparative Example 1, Example 1, and Comparative Example 2, and d is the high-resolution TEM photograph of Example 1;

[0034] Figure 3 is the Fourier transform infrared (FT-IR) spectrum;

[0035] Figure 4 is the variable-temperature fluorescence spectrum;

[0036] Figure 5 is the transient photocurrent response spectrum and the electrochemical impedance spectrum;

[0037] Figure 6 are the PL and TRPL spectra;

[0038] Figure 7 is the performance graph of photocatalytic depolymerization of lignin model compound 2-phenoxy-1-phenylethanol. Detailed implementation mode

[0039] The present invention will be further described below in conjunction with the accompanying drawings of the specification and specific embodiments, but the embodiments do not impose any form of limitation on the present invention. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the technical field.

[0040] Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0041] Example 1

[0042] (1) Place 50 g of dicyandiamide in a crucible, put it in a muffle furnace and heat it to 550 °C at a heating rate of 6 °C / min, and calcine it in an air atmosphere for 4 h. The obtained product is bulk carbon nitride PCN.

[0043] (2) Take 1 g of bulk carbon nitride PCN and grind it evenly with 5.5 g of KCl and 4.5 g of LiCl. Then place the evenly ground mixture in an ampoule, evacuate and seal it. Then place it in a muffle furnace and heat it to 550 °C at a heating rate of 8 °C / min, and keep it warm for 1 h. The obtained product is washed with boiling water and centrifuged, and dried in an oven at 60 °C for 12 hours. The obtained product is triazine-heptazine homojunction carbon nitride powder.

[0044] Example 2

[0045] (1) Place 50 g of dicyandiamide in a crucible, put it in a muffle furnace and heat it to 300 °C at a heating rate of 6 °C / min, and calcine it in an air atmosphere for 6 h. The obtained product is bulk carbon nitride PCN.

[0046] (2) Take 1 g of bulk carbon nitride PCN and grind it evenly with 7 g of KCl and 6 g of LiCl. Then place the evenly ground mixture in an ampoule, evacuate and seal it. Then place it in a muffle furnace and heat it to 600 °C at a heating rate of 6 °C / min, and keep it warm for 3 h. The obtained product is washed with boiling water and centrifuged, and dried in an oven at 60 °C for 12 hours. The obtained product is triazine-heptazine homojunction carbon nitride powder.

[0047] Comparative Example 1

[0048] Take 1 g of melamine and heat it in a muffle furnace at a heating rate of 5 °C / min to 550 °C, and calcine it in an air atmosphere for 4 hours. The obtained product is washed with boiling water and then centrifuged, and dried in an oven at 60 °C for 12 hours. The obtained product is heptazine-based carbon nitride.

[0049] Comparative Example 2

[0050] (1) Place 50 g of dicyandiamide in a crucible, put it in a muffle furnace and heat it at a heating rate of 6 °C / min to 300 °C, and calcine it in an air atmosphere for 6 h. The obtained product is bulk-phase carbon nitride PCN.

[0051] (2) Take 1 g of bulk-phase carbon nitride PCN and grind it evenly with 5.5 g of KCl and 4.5 g of LiCl. Then place the evenly ground mixture in an ampoule, evacuate and seal it, and then place it in a muffle furnace and heat it to 550 °C at a heating rate of 6 °C / min, and keep it warm for 12 h. The obtained product is washed with boiling water and then centrifuged, and dried in an oven at 60 °C for 12 hours. The obtained product is triazine-based highly crystalline carbon nitride powder.

[0052] Comparative Example 3

[0053] (1) Place 50 g of dicyandiamide in a crucible, put it in a muffle furnace and heat it at a heating rate of 6 °C / min to 300 °C, and calcine it in an air atmosphere for 6 h. The obtained product is bulk-phase carbon nitride PCN.

[0054] (2) Take 1 g of bulk-phase carbon nitride PCN and grind it evenly with 5.5 g of KCl and 4.5 g of LiCl. Then place the evenly ground mixture in an ampoule, evacuate and seal it, and then place it in a muffle furnace and heat it to 550 °C at a heating rate of 10 °C / min, and keep it warm for 4 h. The obtained product is washed with boiling water and then centrifuged, and dried in an oven at 60 °C for 12 hours. The obtained product is triazine-based highly crystalline carbon nitride powder.

[0055] Detection method

[0056] Add 5 mg of triazine-heptazine homojunction carbon nitride powder, 20 mg of 2-phenoxy-1-phenylethanol (pp-ol), and 2 mL of solvent (composed of 1 mL of acetonitrile and 1 mL of chloroform) to a 10 mL glass reaction tube. Before starting the reaction, the reactor is evacuated and then purged with O2 for 5 minutes. After irradiation with a 420 nm blue light LED lamp (50 W), the reaction mixture is filtered through a 0.22 μm nylon filter membrane. The obtained compound is analyzed using a gas chromatography-mass spectrometry (GCMS-QP2020NX, Shimadzu).

[0057] Analysis description

[0058] From Figure 1It can be clearly observed that Comparative Example 1 exhibits diffraction peaks belonging to the heptazine phase, while Comparative Examples 2-3 exhibit multiple diffraction peaks belonging to the triazine phase. Example 1 simultaneously presents the diffraction characteristics of both the triazine and heptazine phases, indicating the coexistence of the two crystalline phases of triazine and heptazine in the material and the construction of a homojunction system with a stable structure and clear interface, thus verifying the successful construction of the triazine-heptazine-based crystalline carbon nitride homojunction. Compared with the comparative examples, the diffraction peaks of Example 1 are sharper and the peak shapes are more symmetric, indicating that its crystal structure is more regular and has a higher crystallinity. The above results show that when the reaction time exceeds a certain threshold (greater than 3 hours), the heptazine structure not only does not further enrich, but gradually transforms or degenerates, and finally forms a highly crystalline carbon nitride material mainly composed of the triazine structure, resulting in the inability to establish the triazine-heptazine homojunction structure. In addition, when the heating rate is too fast, the thermal polymerization reaction in the system is too intense, resulting in the rapid polymerization and further condensation of the triazine structure, forming a highly crystalline carbon nitride structure mainly composed of triazine units, making it difficult to retain the heptazine units in the system, thus unable to construct a homojunction structure with the coexistence of triazine and heptazine, leading to the simplification of the structure. If the heating rate is too low, the triazine structure in the reaction system is easily completely converted into heptazine, and the polycrystalline phase structure cannot be retained.

[0059] Figure 2 As can be seen, the three groups of samples all exhibit a six-sided rhombic stacked morphology with a certain regularity, indicating that the carbon nitride matrix material has typical layered structural characteristics. Compared with the heptazine-based carbon nitride of Comparative Example 1 ( Figure 2 a) and the triazine-based carbon nitride of Comparative Example 2 ( Figure 2 c), Figure 2 the triazine-heptazine homojunction carbon nitride of Example 1 shown in b exhibits a denser and more ordered stacked structure, indicating its higher crystallinity and structural uniformity. Figure 2 Regular lattice fringes can be clearly observed in d, indicating that Example 1 has a good crystal structure inside. Two typical lattice plane spacings (0.29 nm and 0.33 nm) are measured, corresponding to the characteristic lattice planes of the triazine and heptazine phases respectively, further confirming the coexistence relationship of the triazine and heptazine structural units inside the material and the formation of a stable and clearly defined homojunction structure, further proving the successful construction of the triazine-heptazine homojunction carbon nitride material. Figure 3 In , Example 1 simultaneously exhibits characteristic absorption peaks related to various functional groups of the triazine structure (Comparative Example 2) and the heptazine structure (Comparative Example 1), verifying the coexistence of the triazine and heptazine structural units in Example 1, and further indicating the successful construction of the triazine-heptazine homojunction carbon nitride.

[0060] Since the increase in temperature triggers a thermally activated non-radiative recombination process, it shows that excitons are more easily dissociated into free carriers at high temperatures. Figure 4In the experiment, when the temperature increased from 77K to 277K, the fluorescence intensity of all three samples decreased, but the exciton binding energies of Comparative Examples 1 and 2 were 23.6meV and 35.1meV, respectively, compared to 18.1meV in Example 1. The excitons in Example 1 dissociated more easily, indicating that the internal electric field created by the triazine-heptazine homostructure can greatly promote the dissociation of excitons into free carriers, thereby increasing the number of free carriers.

[0061] Figure 5 Among them, all carbon nitride materials showed significant photocurrent response under illumination conditions, and the order of response intensity was: Example 1> Comparative Example 2> Comparative Example 1. The triazine-heptazine homojunction carbon nitride in Example 1 showed the highest current response value, indicating that the internal homojunction structure of Example 1 can effectively enhance its photoelectric response intensity. The migration and transport process of photogenerated carriers was further explored by electrochemical impedance spectroscopy (EIS), and the reduction in impedance arc radius means that the charge transfer resistance is reduced, the interface charge transfer rate is accelerated, and the carrier separation efficiency is improved. The sample in Example 1 showed the smallest charge transfer resistance, which confirms that the internal homojunction structure of the sample can effectively accelerate the charge transfer rate, improve the charge utilization efficiency, and thus significantly enhance the photocatalytic performance.

[0062] Figure 6 The TRPL spectrum provides an analysis of the carrier lifetime in the sample. The average carrier lifetime τ in Example 1 after fitting is m It is about 4.03ns, which is significantly better than the comparative example 1 (τ m =9.6ns), indicating that photogenerated excitons can separate and migrate more rapidly within the homojunction structure of Example 1, rather than undergoing radiative recombination within the material. This is attributed to the enhanced effect of the triazine-heptazine homojunction within the Example 1 sample on the photogenerated carrier separation mechanism. The triazine structure, with its delocalized π-electron conjugated system, provides photogenerated electrons to its more electrophilic heptazine neighbors, accelerating the transport rate of photogenerated electrons and improving the efficiency of photogenerated carrier separation.

[0063] Figure 7 The performance graph shows the photocatalytic depolymerization of the lignin model compound 2-phenoxy-1-phenylethanol. The graph demonstrates that the triazine-heptazine homojunction enhances electron transfer and effectively separates photogenerated charge carriers. Example 1 exhibits significantly enhanced photocatalytic activity, with a 98% conversion of PP-OL and yields of the main products, acetophenone and phenyl formate, reaching 76.5% and 61%, respectively. In comparison, the conversion and yields of Comparative Examples 1 and 2 are significantly lower.

[0064] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A preparation method of triazine-heptazine homojunction carbon nitride, characterized in that, Comprising the following steps: Dicyandiamide is heated and calcined in an air atmosphere to obtain bulk carbon nitride; after mixing the bulk carbon nitride, potassium chloride, and lithium chloride, heat treatment is carried out under vacuum conditions, and after filtration and washing, triazine-heptazine homojunction carbon nitride is obtained; the temperature of the heat treatment is 500-600 °C, and the time is 1-3 h.

2. The preparation method of the triazine-heptazine homojunction carbon nitride according to claim 1, characterized in that, The heating rate during the heat treatment is 6-8 °C / min.

3. The preparation method of the triazine-heptazine homojunction carbon nitride according to claim 1, wherein The mass ratio of bulk carbon nitride, potassium chloride, and lithium chloride is 1:5-7:4-6.

4. The preparation method of the triazine-heptazine homojunction carbon nitride according to claim 1, characterized in that, The temperature of the heating and calcination is 300-600 °C, and the time is 2-6 h.

5. A triazine-heptazine homojunction carbon nitride, characterized in that, Prepared by the preparation method according to any one of claims 1-4.

6. Use of the triazine-heptazine homojunction carbon nitride according to claim 5, characterized in that, For preparing a photocatalytic material.

7. Use of the triazine-heptazine homojunction carbon nitride according to claim 6, characterized in that, The photocatalytic material comprises a photocatalyst.

8. Use of the triazine-heptazine homojunction carbon nitride according to claim 6, characterized in that, The photocatalytic material is used for photocatalytic depolymerization of lignin.

9. The application of the triazine-heptazine homojunction carbon nitride according to claim 6, characterized in that, The photocatalytic material is used for preparing acetophenone and / or phenyl formate.

10. A photocatalytic material, characterized in that, Prepared from the triazine-heptazine homojunction carbon nitride according to claim 5.

Images