Triazine-hydrazine bond-based efficient water photolysis COF material and preparation method thereof

Abstract

The invention belongs to the technical field of photocatalytic materials, and discloses a triazine-hydrazine bond-based efficient water photolysis COF material as well as a preparation method and application thereof. The material is prepared by carrying out condensation polymerization reaction on benzo [1, 2-b: 3, 4-b ': 5, 6-b'] trithiophene-2, 5, 8-tricarboxaldehyde, 4, 4 ', 4' '-(1, 3, 5-triazine-2, 4, 6-triyl) tribenzaldehyde and hydrazine hydrate. An intramolecular donor-acceptor (D-A) electronic structure is constructed by introducing an electron-rich benzotrithiophene unit and an electron-deficient triazine unit, and the electron-deficient triazine unit and the electron-deficient triazine unit are connected through a full-conjugated azine bond, so that separation and transmission of photo-generated charges are effectively promoted. The preparation method adopts a solvothermal method, and a high-crystallinity product is obtained through acid catalysis and degassing crystallization. The material has high specific surface area and excellent chemical stability, and shows high-efficiency photocatalytic water splitting hydrogen production activity and good cycle stability under the driving of visible light.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic materials technology, specifically to a highly efficient COF material for water splitting based on triazine-hydrazine bonds and its preparation method. Background Technology

[0002] With the growth of global energy demand, developing clean and renewable energy sources has become a key approach to solving energy shortages and environmental problems. Photocatalytic water splitting technology uses solar energy to convert water into hydrogen. Hydrogen, as a high-energy-density clean energy carrier, plays an important role in the future energy system. The performance of the photocatalyst directly determines the efficiency of converting light energy into chemical energy; therefore, developing efficient and stable photocatalytic materials is the core task in this field.

[0003] Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers formed by covalently linked lightweight elements (such as C, H, O, and N). These materials possess characteristics such as high specific surface area, regular pore structure, low density, and designable structural units, showing potential applications in gas adsorption, separation, sensing, and catalysis. In recent years, COF materials have attracted widespread attention in the field of photocatalytic water splitting due to their periodic π-conjugated framework and tunable band structure.

[0004] However, currently reported COF photocatalytic materials still face some limitations in practical applications. On the one hand, most COF materials lack an effective built-in electric field or charge transport channel, causing electrons and holes generated by photoexcitation to recombine easily before migrating to the surface active sites, severely limiting the improvement of quantum efficiency. On the other hand, some COF materials use imine or borate ester bonds for connection. These chemical bonds are prone to hydrolysis under prolonged aqueous light exposure or acid / alkali conditions, leading to framework collapse and activity decay, making it difficult to meet the chemical stability requirements of photocatalytic reactions. Furthermore, single-type structural units often struggle to simultaneously achieve broad spectral absorption and suitable band positions, resulting in low utilization of the solar spectrum. Therefore, introducing specific electron donor-acceptor systems and highly stable bonds through molecular structure design to construct novel COF photocatalysts that combine high charge separation efficiency and excellent chemical stability is a pressing issue in this field. Summary of the Invention

[0005] The technical problem solved by this invention is that existing covalent organic framework (COF) materials have problems such as high recombination rate of photogenerated carriers, narrow visible light utilization range, and insufficient chemical stability in aqueous environment in photocatalytic water splitting applications, resulting in low hydrogen production efficiency and difficulty in recycling.

[0006] To address the above problems, the present invention provides the following technical solution: In a first aspect, the present invention provides a highly efficient photocatalytic COF material for water splitting based on triazine-hydrazine bonds, employing the following technical solution: A highly efficient photocatalytic water splitting COF material based on triazine-hydrazine bonds is a covalent organic framework material prepared by condensation polymerization of raw materials containing the following molar amounts: benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde: 0.8~1.2 parts; 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde: 0.8~1.2 parts; hydrazine hydrate: 2.5~4.5 parts.

[0007] By employing the above technical solution, this invention utilizes the electron-rich benzo[1,2-b:3,4-b':5,6-b']trithiophene (BTT) unit as an electron donor and the electron-deficient 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (TFPT) unit as an electron acceptor. After condensation polymerization, the two units form an alternating donor-acceptor (DA) electronic structure within the molecular framework. This structure generates an internal potential difference in the band structure, driving the transfer of photogenerated electrons from the BTT unit to the TFPT unit, achieving spatial separation of electrons and holes, and reducing the recombination probability of photogenerated charge carriers. Simultaneously, the aldehyde and hydrazine groups in the raw materials react to form azazine bonds (-C=NN=C-). This connecting unit possesses continuous π-electron conjugation characteristics, effectively extending the conjugated system of the BTT and TFPT units in a two-dimensional plane, providing a low-resistance transport channel for photogenerated charges, and improving the kinetic rate of the photocatalytic reaction. Furthermore, the long-range ordered lattice structure formed by monomer self-assembly reduces crystal defects that serve as recombination centers, and its regular channel structure facilitates substrate transport and product diffusion, increasing the effective exposure area of ​​active sites.

[0008] Preferably, the molar proportions of the raw materials are: 1 part of benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde, 1 part of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde, and 3.0~4.0 parts of hydrazine hydrate; the structural feature of benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde is that it has benzotrithiophene as the core skeleton, with an aldehyde functional group attached to its 2, 5, and 8 positions respectively; the 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde consists of a central triazine ring and three phenyl arms with para-aldehyde groups attached.

[0009] By adopting the above technical solution, the reaction raw materials are controlled within the preferred range close to the stoichiometric ratio, which helps to improve the degree of polymerization reaction, reduce the residue of terminal defective groups, ensure the integrity of the product structure, and thus enhance the photocatalytic activity of the material.

[0010] Preferably, the microstructure of the material contains azazine bond (-C=NN=C-) connecting units formed by the condensation of aldehyde and hydrazine groups, and the material is in a crystalline state.

[0011] By employing the above technical solution, the microscopic connection mode and aggregated structure of the material were clarified. The high bond energy of the azazine bond endows the material with chemical stability under acidic and alkaline environments, enabling it to withstand the erosion of media with different pH values ​​in photocatalytic reactions and ensuring the material's recyclability.

[0012] Secondly, the present invention provides a method for preparing a highly efficient photocatalytic water splitting COF material based on a triazine-hydrazine bond, using the following technical solution: A method for preparing a highly efficient photocatalytic water splitting COF material based on triazine-hydrazine bonds includes the following steps: S1. Benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde are dispersed in an organic solvent and ultrasonically treated. S2. Add hydrazine hydrate to the system of step S1 and disperse it evenly by ultrasonication again. S3. Add acid catalyst to the mixture from step S2, degas it, seal it, and react it at a constant temperature of 110~130℃ for 48~96 hours. S4. After the reaction is complete, the solid product is collected, washed, purified and dried to obtain the COF material.

[0013] By employing the above-mentioned technical solution, a controllable synthesis of highly crystalline materials is achieved using a solvothermal method combined with an acid-catalyzed strategy. The principle is that, under acid catalysis, the condensation reaction between aldehyde and amino groups is reversible, allowing the dissociation and reassembly of incorrectly linked or amorphous structures generated in the early stages of the reaction. Through thermodynamic control, this tends towards the formation of an ordered crystal structure with the lowest energy. Simultaneously, a reaction temperature of 110–130 °C and a reaction time of 48–96 hours provide a suitable thermodynamic environment for crystal nucleation and growth, ensuring the full reaction of monomer molecules and the perfect growth of the crystal lattice.

[0014] Preferably, in step S1, the organic solvent is N,N-dimethylformamide or a mixed solvent of 1,4-dioxane and mesitylene; the ratio of the total amount of benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde to the volume of the organic solvent is (0.1~0.3) mmol:3mL.

[0015] By adopting the above technical solution, selecting polar solvents or mixed solvents with good solubility in raw materials, and controlling the reaction concentration within a suitable range, the relationship between monomer solubility and nucleation rate is balanced, avoiding the formation of amorphous products due to excessively high concentration or insufficient yield due to excessively low concentration, which helps to obtain crystalline materials with high specific surface area.

[0016] Preferably, in step S3, the acid catalyst is an aqueous acetic acid solution with a concentration of 3-9 mol / L; the volume ratio of the aqueous acetic acid solution to the organic solvent in step S1 is 1:(8-12).

[0017] By adopting the above technical solution, a suitable proton environment is provided by acetic acid of a specific concentration, which not only catalyzes the formation of azazine bonds, but also regulates the reversible equilibrium of the reaction through protonation, avoiding the hydrolysis of linkage bonds due to excessive acidity or the insufficient defect repair ability due to insufficient acidity, thereby ensuring the generation of highly crystalline products.

[0018] Preferably, in step S3, the degassing process employs a liquid nitrogen freezing-vacuuming-thawing cycle, with at least 3 cycles; in step S4, the purification step includes: placing the washed solid product in a Soxhlet extractor and refluxing with anhydrous ethanol as the solvent for at least 24 hours.

[0019] By adopting the above technical solution, the degassing operation removes dissolved oxygen from the system, preventing hydrazine monomers and aldehyde groups from undergoing oxidation side reactions at high temperatures; the Soxhlet extraction step removes unreacted monomers and oligomers adsorbed in the pores, clears the pore structure, and ensures that the final material has intrinsic high porosity.

[0020] This invention provides a highly efficient photocatalytic COF material for water splitting based on triazine-hydrazine bonds and its preparation method. It possesses the following beneficial effects: 1. The BTT-Hz-TFPT-COF material prepared in this invention constructs an intramolecular donor-acceptor (DA) electronic structure by introducing electron-rich benzotrithiophene units and electron-deficient triazine units. This alternating framework structure generates a potential difference within the molecule, effectively driving the transfer of photogenerated electrons from the donor to the acceptor, promoting the spatial separation of photogenerated electrons and holes, thereby reducing the recombination rate of charge carriers and improving the quantum efficiency of photocatalytic reactions.

[0021] 2. This invention utilizes azazine bonds (-C=NN=C-) to connect the structural units, forming a fully conjugated two-dimensional planar framework. This connection method enables the effective expansion of the π-electron system within the framework, providing a low-resistance transport channel for photogenerated charges and improving the charge migration rate. Simultaneously, the high bond energy of the azazine bonds endows the material with excellent chemical stability, allowing it to maintain framework integrity and exhibit high activity retention during long-term cyclic reactions of photocatalytic hydrogen evolution.

[0022] 3. This invention synthesizes a COF material with high crystallinity and a uniform mesoporous structure via a solvothermal method. The highly ordered lattice structure reduces defect sites that serve as recombination centers, while the large specific surface area and regular pore structure facilitate the diffusion and transport of the reaction medium and the evolution of hydrogen products, increasing the effective exposed area of ​​the catalytic active sites. This results in the material exhibiting excellent hydrogen production rates under visible light. Attached Figure Description

[0023] Figure 1 This is a flowchart of the preparation method of the present invention; Figure 2 The X-ray powder diffraction patterns of the two materials BTT-Hz-TFPT-COF and BTT-Hz-TFB-COF of this invention are shown below. Figure 3 The infrared spectra of the two materials BTT-Hz-TFPT-COF and BTT-Hz-TFB-COF of this invention are shown below. Figure 4 The diagram shows the photocatalytic water splitting performance of the two materials, BTT-Hz-TFPT-COF and BTT-Hz-TFB-COF, of this invention. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Please see the appendix Figure 1 - Appendix Figure 4 The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0026] Benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde (BTT), with the molecular formula C15H6O3S3 and a molecular weight of 330.40, is a yellow or orange solid powder with a purity ≥97%. This compound is characterized by a core skeleton of benzo[1,2-b:3,4-b':5,6-b']trithiophene, with an aldehyde functional group attached to positions 2, 5, and 8, exhibiting a planar rigid conjugated structure. 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (TFPT), with CAS number 443922-06-3 and the molecular formula C24H15N3O3, has a purity ≥97%. This compound consists of a central triazine ring and three phenyl arms with para-aldehyde groups attached. 1,3,5-Tris(4-formylphenyl)benzene (TFB), CAS No. 118688-53-2, purity ≥98%. Hydrazine hydrate (N2H4·H2O), N,N-dimethylformamide (DMF), glacial acetic acid, triethanolamine (TEOA), chloroplatinic acid hexahydrate, and anhydrous ethanol are all commercially available analytical grade reagents that meet national or industry standards.

[0027] Preparation Example 1: This preparation example provides a BTT-Hz-TFPT-COF material, comprising the following steps: 33.0 mg (0.1 mmol) of benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde (BTT) and 39.3 mg (0.1 mmol) of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (TFPT) were placed in a pressure-resistant glass tube, and 3 mL of N,N-dimethylformamide (DMF) was added. The mixture was sonicated at 40 kHz for 30 min. Subsequently, 18 µL (approximately 0.3 mmol) of 85% hydrazine hydrate was added, and the mixture was sonicated again for 30 min. 0.3 mL of 6 mol / L acetic acid aqueous solution was added to the mixture, and after thorough mixing, the mixture was degassed three times by a liquid nitrogen freezing-evacuation-thawing cycle. The tube was then sealed with a flame under vacuum. The sealed glass tube was placed in a 120℃ oven and allowed to react for 72 h. After the reaction was completed, the tube was cooled to room temperature, filtered, and the precipitate was collected. The precipitate was washed three times with DMF and anhydrous ethanol, and then extracted by reflux with anhydrous ethanol for 24 h in a Soxhlet extractor. Finally, the tube was dried in a 60℃ vacuum drying oven for 12 h to obtain an orange-yellow powdered BTT-Hz-TFPT-COF material.

[0028] Preparation Example 2: This preparation example provides a BTT-Hz-TFPT-COF material, aiming to support the lower limit of the reaction temperature and the generalization of the mixed solvent system. The steps include: placing 33.0 mg (0.1 mmol) of BTT and 39.3 mg (0.1 mmol) of TFPT in a pressure-resistant glass tube, adding 3 mL of a 1:1 volume ratio mixture of 1,4-dioxane and mesitylene, and ultrasonically dispersing until homogeneous. Adding 18 µL (approximately 0.3 mmol) of 85% hydrazine hydrate and ultrasonically dispersing again. Adding 0.3 mL of a 3 mol / L aqueous acetic acid solution to the mixture, and degassing and sealing the tube as described in Preparation Example 1. The sealed glass tube was placed in an oven at 110 °C and allowed to react for 96 h. The post-treatment steps were the same as in Preparation Example 1, yielding an orange-yellow powdered BTT-Hz-TFPT-COF material.

[0029] Preparation Example 3: This preparation example provides a BTT-Hz-TFPT-COF material, aiming to support the generalization of the upper limit of reaction temperature and the upper limit of acid catalyst concentration. The steps include: placing 33.0 mg (0.1 mmol) of BTT and 39.3 mg (0.1 mmol) of TFPT in a pressure-resistant glass tube, adding 3 mL of LDM, and ultrasonically dispersing until uniform. Adding 25 µL (approximately 0.4 mmol) of 85% hydrazine hydrate, and ultrasonically dispersing again. Adding 0.3 mL of 9 mol / L acetic acid aqueous solution to the mixture, and degassing and sealing the tube as described in Preparation Example 1. The sealed glass tube is placed in a 130°C oven and allowed to react for 48 h. The post-treatment steps are the same as in Preparation Example 1, yielding an orange-yellow powdered BTT-Hz-TFPT-COF material.

[0030] Preparation Example 4: This preparation example provides a reference BTT-Hz-TFB-COF material for subsequent performance comparison. The steps include: except that TFPT in the raw materials is replaced with an equimolar amount (39.0 mg, 0.1 mmol) of 1,3,5-tris(4-formylphenyl)benzene (TFB), the other feed ratios, solvent types, catalyst dosage, reaction temperature (120 °C), reaction time (72 h), and post-treatment steps are exactly the same as in Preparation Example 1. Finally, a brownish-yellow powdery BTT-Hz-TFB-COF material is obtained.

[0031] Preparation Example 5: This preparation example provides a reference amorphous BTT-Hz-TFPT polymer for verifying the necessity of a crystalline structure, comprising the following steps: 33.0 mg (0.1 mmol) of BTT and 39.3 mg (0.1 mmol) of TFPT were dissolved in 3 mL of DMF, and 18 µL (approximately 0.3 mmol) of 85% hydrazine hydrate and 0.3 mL of 6 mol / L acetic acid aqueous solution were added. The reaction system was not degassed and sealed; the reaction was carried out directly in an open container with magnetic stirring at room temperature (25 °C) for 24 h. The reaction product was centrifuged, washed with DMF and ethanol, and vacuum dried at 60 °C to obtain a powdery amorphous polymer with a similar color but without obvious crystalline luster.

[0032] Example 1: This example provides a method for photocatalytic water splitting to produce hydrogen using BTT-Hz-TFPT-COF material. The method uses triethanolamine as a hole sacrificial agent and includes the following steps: Weigh 5.0 mg of BTT-Hz-TFPT-COF powder prepared in Example 1 and disperse it in 50 mL of an aqueous solution containing 10% (volume percentage) triethanolamine (TEOA). Sonicate the dispersion for 15 min to form a uniform suspension. Add 38 µL of a 10 mg / mL aqueous solution of chloroplatinic acid (H2PtCl6) to the suspension, so that the theoretical loading of platinum (Pt) co-catalyst is 3 wt% of the photocatalyst mass. Transfer the reaction solution to a top-illuminated photoreactor and connect it to an online trace gas analysis system. Under constant temperature (25°C) circulating condensate, degas the system for 30 min to completely remove dissolved oxygen and air. Use a 300W xenon lamp equipped with a 420 nm cutoff filter as the light source, adjusting the distance between the light source and the liquid surface to 15 cm. The light source was turned on and magnetic stirring was maintained to complete the in-situ photodeposition of platinum within the first hour. Subsequently, the water decomposition reaction was carried out under continuous light irradiation, and the amount of hydrogen produced was detected by online gas chromatograph every hour.

[0033] Example 2: This example provides a method for photocatalytic water splitting to produce hydrogen using BTT-Hz-TFPT-COF material. This method aims to verify the feasibility of a low-loading cocatalyst and ascorbic acid sacrificial agent system, and includes the following steps: Weigh 5.0 mg of BTT-Hz-TFPT-COF powder prepared in Preparation Example 2, disperse it in 50 mL of a 0.1 mol / L ascorbic acid aqueous solution, and ultrasonically disperse it evenly. Add 13 µL of a 10 mg / mL chloroplatinic acid aqueous solution to the suspension, so that the theoretical loading of the cocatalyst Pt is 1 wt% of the photocatalyst mass (lower limit of parameter). The degassing treatment, light source configuration, and temperature control conditions of the reaction system are consistent with those in Example 1. Turn on the light source to perform in-situ photodeposition and photocatalytic reaction, and monitor hydrogen generation using a gas chromatograph.

[0034] Example 3: This example provides a method for photocatalytic water splitting to produce hydrogen using BTT-Hz-TFPT-COF material. This method aims to verify the feasibility of a high-loading cocatalyst and inorganic sacrificial agent system, and includes the following steps: Weigh 5.0 mg of BTT-Hz-TFPT-COF powder prepared in Example 3, disperse it in 50 mL of a 0.1 mol / L sodium sulfite (Na2SO3) aqueous solution, and ultrasonically disperse it evenly. Add 65 µL of a 10 mg / mL chloroplatinic acid aqueous solution to the suspension, so that the theoretical loading of the cocatalyst Pt is 5 wt% of the photocatalyst mass (parameter upper limit). Perform system vacuum degassing according to the operating procedure described in Example 1. Under irradiation with a 300 W xenon lamp (λ>420 nm), maintain the reaction system temperature at 25 °C, and carry out the photocatalytic hydrogen production reaction, recording the amount of hydrogen produced per unit time.

[0035] Comparative Example 1: (Verifying the advantages of introducing a triazine ring to construct a DA system) Compared with Example 1, the difference is that the BTT-Hz-TFB-COF material prepared by Preparation Example 4 (i.e., replacing the triazine ring with a benzene ring) was used as the photocatalyst, and the other test conditions (including the type of sacrificial agent, the loading of the co-catalyst, the light source and the temperature, etc.) were the same.

[0036] Comparative Example 2: (Verifying the necessity of high crystallinity and ordered pores in the material) Compared with Example 1, the difference is that the amorphous BTT-Hz-TFPT polymer (i.e., the same chemical composition but disordered structure) prepared by Preparation Example 5 was used as the photocatalyst, and all other test conditions were the same.

[0037] Comparative Example 3: (Verifying the necessity of covalently bonded polymerization) Compared with Example 1, the difference is that an equimolar amount of a physical mixture of monomers BTT and TFPT (direct grinding and mixing, without chemical reaction) was used instead of the BTT-Hz-TFPT-COF material, and all other test conditions were the same.

[0038] Comparative Example 4: (Verifying the key role of the co-catalyst in the system activity) Compared with Example 1, the difference is that no aqueous solution of chloroplatinic acid was added to the photocatalytic reaction system (i.e. no co-catalyst Pt was loaded), and only the COF material itself was used for photocatalytic reaction. All other aspects are the same.

[0039] Test Example 1: Characterization of Material Microstructure and Physicochemical Properties 1. Experimental Description and Test Methods The structures and porosity properties of the BTT-Hz-TFPT-COF materials obtained in Preparation Examples 1 to 3, the BTT-Hz-TFB-COF material obtained in Preparation Example 4, and the amorphous polymer obtained in Preparation Example 5 were confirmed and analyzed. Powder X-ray diffraction (PXRD) was performed using a Rigaku SmartLab X-ray diffractometer. The test conditions were set as follows: Cu target Kα radiation source (λ = 1.5418 Å), tube voltage 40 kV, tube current 40 mA, continuous scan mode, scan range 2θ 2.0° to 30.0°, step size 0.02°, scan rate 5° / min. The samples were ground and laid flat on a zero-background sample stage. Fourier transform infrared spectroscopy (FT-IR) was performed using a Thermo Fisher Nicoleti S50 infrared spectrometer. The KBr pellet method was used, where dry sample powder and spectrally pure KBr powder were mixed and ground at a mass ratio of approximately 1:50 to 1:100 and then pelleted. The test range was 400 cm⁻¹. -1 Up to 4000cm -1 4cm resolution -1 The sample was scanned 32 times and the air background was subtracted. The nitrogen adsorption-desorption isotherm at 77 K was determined using a Micromeritics ASAP2020 fully automated physical adsorption instrument. All samples were degassed under vacuum at 120 °C for 12 hours before testing. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) model, and the pore size distribution was calculated using the nonlocal density functional theory (NLDFT) method based on the slit pore model of carbon materials.

[0040] 2. Experimental Data Table 1. Crystal structure and porosity parameters of the materials obtained in each preparation example. ; 3. Conclusions and Mechanism Analysis According to Table 1, infrared spectroscopy shows that the raw materials BTT and TFPT in the product of Preparation Example 1 are at 1680 cm⁻¹. -1 The characteristic absorption peak of the aldehyde group disappears in the vicinity, and the absorption peaks in the 1615-1625 cm⁻¹ range disappear. -1 The presence of C=N double bond stretching vibration peaks indicates a condensation reaction between the aldehyde and hydrazine groups, forming a covalent framework linked by azazine bonds. X-ray diffraction patterns show that the material obtained in Preparation Example 1 exhibits a (100) plane diffraction peak at 2θ = 2.84° and a broad (001) plane peak at 26.12°, belonging to π-π packing, indicating that the material possesses a two-dimensional hexagonal channel structure and layered packing characteristics. The amorphous polymer obtained in Preparation Example 5 shows no obvious diffraction peaks, only a diffuse halo, indicating that the solvothermal method and acid catalysis are necessary for the formation of a long-range ordered crystalline structure. Nitrogen adsorption tests show that the sample in Preparation Example 1 exhibits a type IV reversible adsorption isotherm, with a BET specific surface area of ​​1243.6 m².2 The pore size distribution is concentrated at 1.84 nm, classifying it as a mesoporous material. Compared to the BTT-Hz-TFB-COF prepared in Example 4, the BTT-Hz-TFPT-COF with the introduction of triazine units exhibits a higher specific surface area and pore volume, indicating that the interaction between the triazine units and the benzotrithiophene units is beneficial for maintaining the stability of the pore structure.

[0041] Test Example 2: Performance and Stability Test of Photocatalytic Water Splitting for Hydrogen Production 1. Experimental Procedure The hydrogen production activity of each example and comparative sample was evaluated using a Labsolar-6A all-glass automated online trace gas analysis system. 5.0 mg of photocatalyst powder was weighed and dispersed in 50 mL of an aqueous solution containing 10% (v / v) triethanolamine, and sonicated for 15 min. Aqueous chloroplatinic acid was added to the system; except for Comparative Example 4 (which did not contain this acid) and Examples 2 and 3 (which had specific loading amounts), the theoretical platinum loading for all other samples was 3 wt%. The reaction system was maintained at a constant temperature of 25°C. After vacuum degassing for 30 min, it was irradiated with a 300W xenon lamp equipped with a 420 nm cutoff filter. In-situ photodeposition of the platinum co-catalyst was performed for the first hour, followed by continuous irradiation for 4 hours. Hydrogen production was detected every hour using an online gas chromatograph (TCD detector, argon carrier gas, 5 Å molecular sieve column). Cyclic stability testing was performed using the sample from Example 1. Each irradiation cycle lasted 4 hours, after which vacuum degassing was repeated. The photocatalyst and reaction solution were not replaced, and this was repeated for 4 cycles.

[0042] 2. Experimental Data The average hydrogen production rate (4-hour average) and cycle stability data for each sample are recorded in Tables 2 and 3.

[0043] ; Table 3 Cyclic stability test data of the sample from Example 1 ; (Note: The activity retention rate is calculated based on the rate of the first round.) 3. Conclusions and Analysis The test data in Tables 2 and 3 show that the average hydrogen production rate of the BTT-Hz-TFPT-COF prepared in Example 1 is 1.24 mmol·h. -1 ·g -1 It is approximately equal to the BTT-Hz-TFB-COF (0.41 mmol·h) in Comparative Example 1. -1 ·g -1The hydrogen production rate of the amorphous polymer (Comparative Example 2) was three times that of the TFPT unit. While both have similar topological structures, the difference lies in the chemical properties of their node units. The triazine ring in the TFPT unit is electron-deficient, acting as an electron acceptor and forming a donor-acceptor electronic structure with the electron-rich benzotrithiophene donor unit. This promotes photogenerated electron transfer and inhibits electron-hole recombination. The hydrogen production rate of the amorphous polymer was only 0.08 mmol·h⁻¹. -1 ·g -1 No significant activity was detected in the physical mixture (Comparative Example 3), indicating that the long-range ordered pore structure formed by covalent bonding is the basis for efficient photogenerated charge transport and exposure of reaction sites. The rate in the unloaded platinum sample (Comparative Example 4) decreased to 0.03 mmol·h. -1 ·g -1 This indicates that the co-catalyst reduced the overpotential of the surface proton reduction reaction. In the cycle stability test, the activity retention rate of the sample in Example 1 was 95.2% after 16 hours of light irradiation, indicating that the azazine bond linkage structure in the material remained stable under the reaction conditions.

[0044] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A highly efficient photocatalytic COF material for water splitting based on triazine-hydrazine bonds, characterized in that, The material is a covalent organic framework material prepared by condensation polymerization of raw materials containing the following molar amounts: benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde: 0.8~1.2 parts; 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde: 0.8~1.2 parts; hydrazine hydrate: 2.5~4.5 parts.

2. The high-efficiency photocatalytic COF material based on triazine-hydrazine bonds according to claim 1, characterized in that, The molar proportions of the raw materials are as follows: benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde: 1 part; 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde: 1 part; hydrazine hydrate: 3.0~4.0 parts; wherein, the structural feature of benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde is that it has benzotrithiophene as the core skeleton, with an aldehyde functional group attached to its 2, 5, and 8 positions respectively; the 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde consists of a central triazine ring and three phenyl arms with para-aldehyde groups attached.

3. The high-efficiency photocatalytic COF material based on triazine-hydrazine bonds according to claim 1, characterized in that, The microstructure of the material contains azazine bond (-C=NN=C-) connecting units formed by the condensation of aldehyde and hydrazine groups, and the material is in a crystalline state.

4. A method for preparing a high-efficiency photocatalytic water splitting COF material based on a triazine-hydrazine bond according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde are dispersed in an organic solvent and ultrasonically treated. S2. Add hydrazine hydrate to the system of step S1 and disperse it evenly by ultrasonication again. S3. Add acid catalyst to the mixture from step S2, degas it, seal it, and react it at a constant temperature of 110~130℃ for 48~96 hours. S4. After the reaction is complete, the solid product is collected, washed, purified and dried to obtain the COF material.

5. The method for preparing a high-efficiency photocatalytic water splitting COF material based on a triazine-hydrazine bond according to claim 4, characterized in that, In step S1, the organic solvent is N,N-dimethylformamide or a mixed solvent of 1,4-dioxane and mesitylene; the total amount of benzo[1,2-b:3,4-b':5,6-b']trithiophene-2,5,8-tricarboxaldehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde in volume ratio to the organic solvent is (0.1~0.3) mmol:3mL.

6. The method for preparing a high-efficiency photocatalytic water splitting COF material based on a triazine-hydrazine bond according to claim 4, characterized in that, In step S3, the acid catalyst is an aqueous acetic acid solution with a concentration of 3-9 mol / L; the volume ratio of the aqueous acetic acid solution to the organic solvent in step S1 is 1:8-12.

7. The method for preparing a high-efficiency photocatalytic water splitting COF material based on a triazine-hydrazine bond according to claim 4, characterized in that, In step S3, the degassing process employs a liquid nitrogen freezing-evacuation-thawing cycle, with at least 3 cycles.

8. The method for preparing a high-efficiency photocatalytic water splitting COF material based on a triazine-hydrazine bond according to claim 4, characterized in that, In step S4, the purification step includes: placing the washed solid product in a Soxhlet extractor and refluxing with anhydrous ethanol as solvent for at least 24 hours.

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