A graphite phase carbon nitride photocatalyst for photocatalytic hydrogen production and a preparation method and application thereof

Abstract

This invention discloses a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production, its preparation method, and its application. The preparation method includes: mixing melamine and hexamethylenetetramine, followed by a supercritical carbon dioxide solvothermal reaction to obtain a hexamethylenetetramine-doped melamine precursor; and calcining the hexamethylenetetramine-doped melamine precursor to obtain the graphitic carbon nitride photocatalyst for photocatalytic hydrogen production. This invention prepares a hexamethylenetetramine-doped melamine precursor using a supercritical carbon dioxide solvothermal method to modify the graphitic carbon nitride photocatalyst. Photocatalytic hydrogen production activity tests using the graphitic carbon nitride photocatalyst prepared by this invention show that the activity is increased by 6.4 times compared to the unmodified form. The preparation method of this invention has the advantages of high reaction efficiency and good performance of the obtained graphitic carbon nitride photocatalyst.

Description

Technical Field

[0001] This invention belongs to the field of photocatalytic hydrogen production technology, specifically relating to a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production, its preparation method, and its application. Background Technology

[0002] Graphitic carbon nitride (g-C3N4) has become a promising visible light photocatalyst due to its unique band structure, stable physicochemical properties, and abundant and readily available raw materials. However, pristine g-C3N4 suffers from limitations such as small specific surface area, narrow visible light absorption range, low electrical conductivity, high recombination rate of photogenerated electrons and holes, and a limited number of interfacial (photo)reactive sites, which restrict its photocatalytic performance and affect its photocatalytic hydrogen production efficiency. Therefore, targeted modification of g-C3N4 is needed to improve its photocatalytic hydrogen production efficiency.

[0003] Among numerous modification methods, sintering after small-molecule doping of precursors to prepare g-C3N4 is a highly effective approach. Modifying precursors such as melamine, urea, and biuret by incorporating specific small molecules and then employing methods like thermal polycondensation, hydrothermal synthesis, and hot solvation gelation can effectively modify g-C3N4. The hydrothermal method, in particular, is widely used due to its simple synthesis process, time-saving, and energy-efficient nature. However, traditional hydrothermal methods typically involve long synthesis times, numerous byproducts, and suffer from problems such as small specific surface area, severe photogenerated electron-hole recombination, and low stripping efficiency.

[0004] Supercritical carbon dioxide, due to its unique solubility and fluidity, is a non-polar solvent that combines the dissolving power of liquids with the diffusion capabilities of gases. It can simultaneously dissolve precursors and doped small molecules, allowing for a more complete reaction in a homogeneous phase, effectively enhancing mass and heat transfer and reaction efficiency. Furthermore, the absence of solvent residue after depressurization eliminates the need for product-solvent separation, greatly simplifying the reaction process and demonstrating broad application prospects. Summary of the Invention

[0005] To address the issues of small specific surface area and poor photocatalytic performance in catalysts prepared by traditional hydrothermal methods in the prior art, the present invention aims to provide a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production, its preparation method, and its applications. The method involves obtaining a hexamethylenetetramine-doped melamine precursor through supercritical carbon dioxide solvothermal treatment, and then preparing a graphitic carbon nitride photocatalyst with excellent visible light photocatalytic hydrogen production performance by calcining the hexamethylenetetramine-doped melamine precursor. Furthermore, the preparation process is simple, efficient, and has a wide range of applications.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production includes the following steps:

[0008] Melamine and hexamethylenetetramine were mixed and subjected to a supercritical carbon dioxide solvothermal reaction to obtain a hexamethylenetetramine-doped melamine precursor.

[0009] A graphitic carbon nitride photocatalyst for photocatalytic hydrogen production was obtained by calcining a melamine precursor doped with hexamethylenetetramine.

[0010] A further improvement of the present invention is that the mass ratio of melamine to hexamethylenetetramine is (4.2-84):1.

[0011] A further improvement of the present invention is that the mass ratio of melamine to hexamethylenetetramine is (4.2-8.4):1.

[0012] A further improvement of the present invention is that the reaction temperature is 40-180℃ and the reaction time is 20-24h.

[0013] A further improvement of the present invention is that the reaction temperature is 100-180°C and the reaction time is 20 hours.

[0014] A further improvement of the present invention is that the reaction is carried out under a pressure of 14–20 MPa.

[0015] A further improvement of the present invention is that the calcination temperature is 520-550℃ and the time is 4-6h.

[0016] A further improvement of this invention is that it is carried out at 5°C for 5 minutes. -1 Heat to the calcination temperature.

[0017] A graphitic carbon nitride photocatalyst for photocatalytic hydrogen production has a specific surface area of ​​8.6–14.4 m². 2 g -1 .

[0018] Application of a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production in photocatalytic hydrogen production.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention utilizes the combined solubility of supercritical carbon dioxide and the diffusion ability of a gas to simultaneously dissolve the precursor melamine and the small-molecule hexamethylenetetramine, allowing for a more complete and thorough reaction in a homogeneous phase. The resulting graphitic carbon nitride photocatalyst exhibits a large specific surface area, porous structure, narrow band gap, and weak carrier recombination. Furthermore, the unique structure of hexamethylenetetramine allows it to be used as a small molecule to dope the melamine precursor, altering its microstructure and significantly increasing the specific surface area of ​​the prepared graphitic carbon nitride photocatalyst for hydrogen production. This also results in a more porous structure within the prepared photocatalytic hydrogen production catalyst, effectively improving its performance under visible light catalytic hydrogen production. The use of supercritical carbon dioxide as a solvent avoids the use of harmful organic solvents, making it environmentally friendly and possessing broad application value.

[0021] Photocatalytic hydrogen production tests revealed that the visible light catalytic hydrogen production activity of the graphitic carbon nitride photocatalyst prepared according to this invention is 501 μmol h⁻¹. -1 g -1 It is 6.4 times that of unmodified graphitic carbon nitride photocatalyst. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a supercritical carbon dioxide solvothermal reaction system.

[0023] Figure 2 The bar chart shows the visible light hydrogen production rate of graphitic carbon nitride synthesized in Examples 1, 2, 3 and Comparative Examples 1 and 2.

[0024] Figure 3 The stability diagram of photocatalytic hydrogen production of graphitic carbon nitride synthesized in Example 1 is shown.

[0025] Figure 4 X-ray diffraction patterns of graphitic carbon nitride synthesized in Examples 1, 2, 3 and Comparative Examples 1 and 2;

[0026] Figure 5 The images are scanning electron microscope (SEM) images of the graphitic carbon nitride synthesized in Comparative Example 1 and Example 1; where (a) is a scanning electron microscope image of the graphitic carbon nitride synthesized in Comparative Example 1, and (b) is a scanning electron microscope image of the graphitic carbon nitride synthesized in Example 1.

[0027] Figure 6 Transmission electron microscope (TEM) images of graphitic carbon nitride synthesized in Comparative Example 1 and Example 1; wherein, (a) is a scanning electron microscope (SEM) image of graphitic carbon nitride synthesized in Comparative Example 1, and (b) is a SEM image of graphitic carbon nitride synthesized in Example 1.

[0028] Figure 7The N2 adsorption-desorption curves of the graphitic carbon nitride synthesized in Examples 1, 2, 3 and Comparative Examples 1 and 2 are shown.

[0029] Figure 8 The UV-Vis absorption spectra of the graphitic carbon nitride synthesized in Examples 1, 2, 3 and Comparative Examples 1 and 2 are shown.

[0030] Figure 9 The electron paramagnetic resonance spectra of graphitic carbon nitride synthesized in Example 1 and Comparative Examples 1 and 2 are shown.

[0031] Figure 10 The steady-state fluorescence spectra of graphitic carbon nitride synthesized in Examples 1, 2, 3 and Comparative Examples 1 and 2 are shown.

[0032] In the diagram, 1 is a carbon dioxide gas cylinder, 2 is a valve, 3 is a carbon dioxide booster pump, 4 is a supercritical carbon dioxide solvothermal high-pressure reactor, 5 is a pressure gauge, 6 is a stirrer, 7 is a pressure relief valve, 8 is a temperature sensor, 9 is an electric heater, and 10 is carbon dioxide. Detailed Implementation

[0033] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0034] The photocatalytic hydrogen production using graphitic carbon nitride photocatalyst in this invention is carried out using a supercritical carbon dioxide solvothermal reaction system. See [link to relevant documentation]. Figure 1 The supercritical carbon dioxide solvothermal reaction system includes a carbon dioxide tank 1. The outlet of the carbon dioxide tank 1 is connected to the supercritical carbon dioxide solvothermal high-pressure reactor 4 via a valve 2 and a carbon dioxide booster pump 3. The top of the supercritical carbon dioxide solvothermal high-pressure reactor 4 is equipped with a pressure gauge 5, a stirrer 6 and a pressure relief valve 7. The bottom of the supercritical carbon dioxide solvothermal high-pressure reactor 4 is equipped with a temperature sensor 8. The supercritical carbon dioxide solvothermal high-pressure reactor 4 is equipped with an electric heater 9. The supercritical carbon dioxide solvothermal high-pressure reactor 4 is filled with carbon dioxide 10.

[0035] The present invention discloses a method for preparing a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production, comprising the following steps:

[0036] At room temperature, melamine and hexamethylenetetramine are ground to obtain a mixed powder at a mass ratio of (4.2–84):1. The mixed powder is added to a supercritical carbon dioxide solvothermal high-pressure reactor in a supercritical carbon dioxide solvothermal reaction system. After sealing, 5 MPa of carbon dioxide is introduced into the supercritical carbon dioxide solvothermal high-pressure reactor to purge the air inside the reactor. The supercritical carbon dioxide solvothermal reaction is carried out at 40–180 °C for 20–24 h. During the reaction, the pressure inside the supercritical carbon dioxide solvothermal high-pressure reactor is kept constant at 14–20 MPa to obtain a hexamethylenetetramine-doped melamine precursor.

[0037] After supercritical carbon dioxide solvent heat treatment, the carbon dioxide gas is released through a pressure relief valve, leaving no solvent residue and eliminating the need for further separation of the product and solvent.

[0038] Hexamethylenetetramine-doped melamine precursor powder was placed in a covered crucible, and then the crucible was placed in a muffle furnace for calcination, with a heating program of 5°C / min. -1 The temperature was raised to 520–550℃ and held for 4–6 hours, and then naturally cooled to obtain a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production.

[0039] In all embodiments of this invention, the supercritical carbon dioxide solvothermal treatment of the photocatalyst precursor is carried out in this system.

[0040] The specific surface area of ​​the graphitic carbon nitride photocatalyst for photocatalytic hydrogen production prepared in this invention is 8.6–14.4 m². 2 g -1 .

[0041] The application of the graphitic carbon nitride photocatalyst for photocatalytic hydrogen production prepared in this invention in photocatalytic hydrogen production.

[0042] Example 1

[0043] This embodiment provides a method for preparing a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production, including:

[0044] Step 1: Mix and grind 4.2g of melamine and 500mg of hexamethylenetetramine (HMTA) for 20min, and place the mixed sample in a supercritical carbon dioxide (scCO2) solvothermal high-pressure reactor. After sealing, introduce 5MPa of carbon dioxide into the supercritical carbon dioxide solvothermal high-pressure reactor to purge the air. Maintain constant pressure for 5min to check the airtightness of the device, and then purge the gas in the reactor.

[0045] Step 2: Pump carbon dioxide at 13 MPa into the supercritical carbon dioxide solvothermal high-pressure reactor described in Step 1, and place the supercritical carbon dioxide solvothermal high-pressure reactor in a 140°C oil bath to raise the temperature. After the temperature stabilizes, adjust the pressure inside the reactor to 14 MPa and maintain it for 20 hours.

[0046] Step 3: Allow the system from Step 2 to cool naturally and release the pressure. Collect the sample, grind it, and weigh 2g of the sample into a covered crucible. Then, place the crucible in a muffle furnace for calcination, setting a heating program of 5℃ / min. -1 The temperature was raised to 520℃ and held for 4 hours, and then naturally cooled to obtain a graphitic carbon nitride photocatalyst, denoted as CN-HMTA-scCO2-100; in CN-HMTA-scCO2-T, T represents the supercritical carbon dioxide solvothermal treatment temperature in ℃.

[0047] Example 2

[0048] This embodiment is the same as Embodiment 1, except that the supercritical carbon dioxide solvothermal high-pressure reactor is placed in a 100°C oil bath for heating.

[0049] Example 3

[0050] This embodiment is the same as Embodiment 1, except that the supercritical carbon dioxide solvothermal high-pressure reactor is placed in an oil bath at 180°C for heating.

[0051] Example 4

[0052] This embodiment is the same as Embodiment 1, except that the supercritical carbon dioxide solvothermal high-pressure reactor is placed in a 40°C oil bath for heating.

[0053] Example 5

[0054] This embodiment provides a method for preparing a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production, including:

[0055] Step 1: Mix and grind 4.2g of melamine and 50mg of hexamethylenetetramine for 20min, and place the mixed sample in a supercritical carbon dioxide solvothermal high-pressure reactor. After sealing, introduce 5MPa carbon dioxide into the supercritical carbon dioxide solvothermal high-pressure reactor to purge the air. Maintain constant pressure for 5min to check the airtightness of the device, and then purge the gas in the reactor.

[0056] Step 2: Pump carbon dioxide at 13 MPa into the supercritical carbon dioxide solvothermal high-pressure reactor described in Step 1, and place the supercritical carbon dioxide solvothermal high-pressure reactor in an 80°C oil bath to raise the temperature. After the temperature stabilizes, adjust the pressure inside the reactor to 16 MPa and maintain it for 20 hours.

[0057] Step 3: Allow the system from Step 2 to cool naturally and release the pressure. Collect the sample, grind it, and weigh 2g of the sample into a covered crucible. Then, place the crucible in a muffle furnace for calcination, setting a heating program of 5℃ / min. -1 The temperature was raised to 520℃ and held for 4 hours, and then naturally cooled to obtain a graphitic carbon nitride photocatalyst.

[0058] Example 6

[0059] 4.2g of melamine and 250mg of hexamethylenetetramine were mixed and ground for 20min. The mixed sample was then placed in a supercritical carbon dioxide solvothermal high-pressure reactor. After sealing, 5MPa of carbon dioxide was introduced into the supercritical carbon dioxide solvothermal high-pressure reactor to purge the air. The pressure was maintained for 5min to check the airtightness of the device. Then the gas inside the reactor was purged.

[0060] Step 2: Pump carbon dioxide at 13 MPa into the supercritical carbon dioxide solvothermal high-pressure reactor described in Step 1, and place the supercritical carbon dioxide solvothermal high-pressure reactor in a 120°C oil bath to raise the temperature. After the temperature stabilizes, adjust the pressure inside the reactor to 18 MPa and maintain it for 22 hours.

[0061] Step 3: Allow the system from Step 2 to cool naturally and release the pressure. Collect the sample, grind it, and weigh 2g of the sample into a covered crucible. Then, place the crucible in a muffle furnace for calcination, setting a heating program of 5℃ / min. -1 The temperature was raised to 550℃ and held for 6 hours, and then naturally cooled to obtain a graphitic carbon nitride photocatalyst.

[0062] Example 7

[0063] 4.2g of melamine and 1000mg of hexamethylenetetramine were mixed and ground for 20 minutes. The mixed sample was then placed in a supercritical carbon dioxide solvothermal high-pressure reactor. After sealing, 5MPa of CO2 was introduced into the supercritical carbon dioxide solvothermal high-pressure reactor to purge the air. The pressure was maintained for 5 minutes to check the airtightness of the device. Then the gas inside the reactor was purged.

[0064] Step 2: Pump 13 MPa CO2 into the supercritical carbon dioxide solvothermal high-pressure reactor described in Step 1, and place the supercritical carbon dioxide solvothermal high-pressure reactor in a 160°C oil bath to raise the temperature. After the temperature stabilizes, adjust the pressure inside the reactor to 20 MPa and maintain it for 24 hours.

[0065] Step 3: Allow the system from Step 2 to cool naturally and release the pressure. Collect the sample, grind it, and weigh 2g of the sample into a covered crucible. Then, place the crucible in a muffle furnace for calcination, setting a heating program of 5℃ / min. -1 The temperature was raised to 550℃ and held for 6 hours, and then naturally cooled to obtain a graphitic carbon nitride photocatalyst.

[0066] Comparative Example 1

[0067] At room temperature, 2g of melamine was weighed and placed in a covered crucible. The crucible was then placed in a muffle furnace for calcination, with a heating program of 5℃ / min. -1 The temperature was raised to 520℃ and held for 4 hours, and then naturally cooled to obtain a graphitic carbon nitride photocatalyst, abbreviated as CN.

[0068] Comparative Example 2

[0069] Step 1: Mix and grind 4.2g of melamine and 500mg of hexamethylenetetramine (HMTA) for 20 minutes. Weigh 2g of the mixed sample and place it in a covered crucible. Then, place the crucible in a muffle furnace for calcination, setting the temperature program to 5℃ / min. -1 The temperature was raised to 520℃ and held for 4 hours, and then naturally cooled to obtain a graphitic carbon nitride photocatalyst, denoted as CN-HMTA-GRI.

[0070] Performance evaluation:

[0071] The photocatalytic hydrogen production test method was as follows: 20 mg of photocatalyst was dispersed in 80 mL of a 10% (v / v) triethanolamine aqueous solution, and 720 mL of a chloroplatinic acid hexahydrate aqueous solution with a platinum content of 0.7691 g / mL (the platinum content in the added chloroplatinic acid hexahydrate aqueous solution was 3% of the added graphitic carbon nitride content) was added. Before illumination, the reactor was purged with argon gas for 20 min to remove oxygen from the system. Then, a magnetic stirrer was turned on, and the xenon lamp power was switched on. The photocatalytic hydrogen production activity of the synthesized graphitic carbon nitride was evaluated by detecting the amount of hydrogen produced in the photocatalytic system.

[0072] Figure 2 The bar chart shows the visible light hydrogen production rates of graphitic carbon nitride synthesized in Examples 1, 2, 3, and Comparative Examples 1 and 2. From... Figure 2 As can be seen, the samples prepared by treating the precursor with supercritical carbon dioxide at temperatures of 100℃, 140℃, and 180℃ all exhibited higher photocatalytic hydrogen production activities than the photocatalysts of Comparative Example 1 and Comparative Example 2. This indicates that the method of introducing hexamethylenetetramine small molecules and melamine doping into the precursor through supercritical carbon dioxide solvothermal treatment to modify graphitic carbon nitride is an effective means to improve the photocatalytic performance of graphitic carbon nitride photocatalysts.

[0073] Figure 3 This is a stability diagram of photocatalytic hydrogen production from the graphitic carbon nitride synthesized in Example 1. According to... Figure 3 It can be seen that within a 20-hour reaction time, the photocatalyst of the present invention exhibits good photocatalytic hydrogen production stability and photocatalytic hydrogen production activity.

[0074] Figure 4 The images show X-ray diffraction patterns of the graphitic carbon nitride synthesized in Examples 1, 2, 3, and Comparative Examples 1 and 2. Figure 3 It can be seen that the X-ray diffraction patterns of the graphitic carbon nitride synthesized in Examples 1, 2, 3 and Comparative Examples 1 and 2 are basically consistent, indicating that the supercritical carbon dioxide solvothermal treatment of the precursor and the doping of hexamethylenetetramine in the precursor did not change the structure of the synthesized graphitic carbon nitride.

[0075] Figure 5 Images (a) and (b) are scanning electron microscope (SEM) images of the graphitic carbon nitride synthesized in Comparative Example 1 and Example 1, respectively. (a) is an SEM image of the graphitic carbon nitride synthesized in Comparative Example 1, and (b) is an SEM image of the graphitic carbon nitride synthesized in Example 1. Figure 4 It can be observed that the graphitic carbon nitride has an aggregated layered structure, while the graphitic carbon nitride obtained by introducing hexamethylenetetramine small molecules and melamine doping into the precursor through supercritical carbon dioxide solvothermal treatment according to the present invention has significantly more wrinkles and pores. This indicates that the supercritical carbon dioxide solvothermal treatment precursor of the present invention and the doping of hexamethylenetetramine in the precursor have a great modifying effect on the microstructure of graphitic carbon nitride.

[0076] Figure 6 Images (a) and (b) are transmission electron microscopy (TEM) images of the graphitic carbon nitride synthesized in Comparative Example 1 and Example 1, respectively. Image (a) is a scanning electron microscopy (SEM) image of the graphitic carbon nitride synthesized in Comparative Example 1, and image (b) is a SEM image of the graphitic carbon nitride synthesized in Example 1. Figure 5 As can be observed, the graphitic carbon nitride obtained by supercritical CO2 solvothermal treatment in the precursor to achieve modification and calcination of hexamethylenetetramine small molecules and melamine doping is thinner and has more wrinkles and pores, which is consistent with the scanning electron microscope images.

[0077] Figure 7 The figures show the N2 adsorption-desorption curves of the graphitic carbon nitride synthesized in Examples 1, 2, 3, and Comparative Examples 1 and 2. It can be clearly observed from the figures that the N2 adsorption-desorption curves of the graphitic carbon nitride synthesized in Examples 1, 2, 3, and Comparative Examples 1 and 2 are all Type IV curves, exhibiting significant hysteresis loops, indicating that the synthesized graphitic carbon nitride are all mesoporous adsorbent materials. Furthermore, compared to the graphitic carbon nitride synthesized in Comparative Examples 1 and 2, the specific surface area (5.7 m²) is significantly higher. 2 g -1 6.9m 2 g -1The specific surface area of ​​graphitic carbon nitride obtained by treating the precursor with supercritical carbon dioxide solvothermal treatment at temperatures of 100°C, 140°C, and 180°C according to the present invention, and doping with hexamethylenetetramine small molecules and melamine, is 11.4 m², respectively. 2 g -1 14.4m 2 g -1 8.6m 2 g -1 The improvement in both parameters indicates that the graphitic carbon nitride obtained by supercritical carbon dioxide solvothermal treatment in the precursor, which modifies hexamethylenetetramine small molecules and melamine doping, generates more hollow and macropores, thus obtaining a larger specific surface area and providing more reaction sites for photocatalytic reactions.

[0078] Figure 8 The figures show the UV-Vis absorption spectra of the graphitic carbon nitride synthesized in Examples 1, 2, 3, and Comparative Examples 1 and 2. It can be clearly observed from the figures that the UV-Vis absorption spectra of the graphitic carbon nitride synthesized in Examples 1, 2, 3, and Comparative Examples 1 and 2 all have an absorption edge around 450 nm corresponding to the π→π* electronic transition. Furthermore, unlike the UV-Vis absorption spectra of the graphitic carbon nitride synthesized in Comparative Examples 1 and 2, the UV-Vis absorption spectra of the graphitic carbon nitride obtained by supercritical carbon dioxide solvothermal treatment at temperatures of 100°C, 140°C, and 180°C in the precursor to achieve hexamethylenetetramine small molecule doping and melamine modification show a significant red shift, and a distinct new absorption band corresponding to the n→π* electronic transition is formed in the 450-800 nm range. This indicates that the graphitic carbon nitride structure obtained by supercritical carbon dioxide solvothermal treatment in the precursor to achieve hexamethylenetetramine small molecule doping and melamine modification and calcination contains more unpaired electrons.

[0079] Figure 9 The images show the electron paramagnetic resonance (EPR) spectra of graphitic carbon nitride synthesized in Example 1 and Comparative Examples 1 and 2. Figure 9 It can be seen that the graphitic carbon nitride synthesized in Example 1 and Comparative Examples 1 and 2 all have a characteristic single Lorentz line and the same g value (2.0012), which is due to the sp in the π-conjugated aromatic ring. 2This is caused by the unpaired electrons of the hybridized carbon atoms. Furthermore, in Example 1 of this invention, the graphitic carbon nitride obtained by supercritical carbon dioxide solvothermal treatment in the precursor to achieve doping modification with hexamethylenetetramine and melamine exhibits a significantly stronger spin characteristic peak than the graphitic carbon nitride synthesized in Comparative Examples 1 and 2. This indicates that the graphitic carbon nitride structure obtained by supercritical carbon dioxide solvothermal treatment in the precursor to achieve doping modification with hexamethylenetetramine and melamine, and calcination, generates more unpaired electrons, consistent with the UV-vis analysis results.

[0080] Figure 10 The steady-state fluorescence spectra of graphitic carbon nitride synthesized in Examples 1, 2, 3, and Comparative Examples 1 and 2 are shown. As can be seen from the figures, the graphitic carbon nitride synthesized in Comparative Examples 1 and 2 all exhibit strong characteristic peaks, indicating severe recombination of photogenerated carriers within them. However, the characteristic peak intensities of graphitic carbon nitride obtained by supercritical carbon dioxide solvothermal treatment at temperatures of 100℃, 140℃, and 180℃ in the precursor, followed by melamine doping and calcination, are significantly reduced. This indicates that supercritical carbon dioxide solvothermal treatment of the precursor and the doping of hexamethylenetetramine in the precursor can effectively suppress the recombination of photogenerated carriers and promote their separation.

[0081] The preparation method of the graphitic carbon nitride photocatalyst for photocatalytic hydrogen production in this invention includes: preparing a hexamethylenetetramine-doped melamine precursor using a supercritical carbon dioxide solvothermal method, and then preparing a graphitic carbon nitride photocatalyst with excellent visible light photocatalytic hydrogen production performance by calcining the hexamethylenetetramine-doped melamine precursor. This invention uses a hexamethylenetetramine-doped melamine precursor prepared by a supercritical carbon dioxide solvothermal method to modify the graphitic carbon nitride photocatalyst. Photocatalytic hydrogen production activity tests show that the activity is increased by 6.4 times compared to the unmodified form. The preparation method involved in this invention has the advantages of high reaction efficiency and good performance of the obtained graphitic carbon nitride photocatalyst.

[0082] The above description is only of the preferred embodiment of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All variations made within the scope of the independent claims of the present invention are also within the scope of protection of the present invention.

[0083] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

Claims

1. A method for preparing a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production, characterized in that, Includes the following steps: Melamine and hexamethylenetetramine were mixed and subjected to a supercritical carbon dioxide solvothermal reaction to obtain a hexamethylenetetramine-doped melamine precursor. A graphitic carbon nitride photocatalyst for photocatalytic hydrogen production was obtained by calcining a melamine precursor doped with hexamethylenetetramine.

2. The method for preparing the graphite-phase carbon nitride photocatalyst for photocatalytic hydrogen production according to claim 1, characterized in that, The mass ratio of melamine to hexamethylenetetramine is (4.2~84):

1.

3. The method for preparing the graphite-phase carbon nitride photocatalyst for photocatalytic hydrogen production according to claim 1, characterized in that, The mass ratio of melamine to hexamethylenetetramine is (4.2~8.4):

1.

4. The method for preparing the graphite-phase carbon nitride photocatalyst for photocatalytic hydrogen production according to claim 1, characterized in that, The reaction temperature is 40~180℃, and the reaction time is 20-24h.

5. The method for preparing the graphite-phase carbon nitride photocatalyst for photocatalytic hydrogen production according to claim 1, characterized in that, The reaction temperature is 100~180℃, and the reaction time is 20h.

6. The method for preparing the graphite-phase carbon nitride photocatalyst for photocatalytic hydrogen production according to claim 1, characterized in that, The reaction was carried out at a pressure of 14-20 MPa.

7. The method for preparing the graphite-phase carbon nitride photocatalyst for photocatalytic hydrogen production according to claim 1, characterized in that, The calcination temperature is 520~550℃, and the time is 4~6h.

8. The method for preparing the graphite-phase carbon nitride photocatalyst for photocatalytic hydrogen production according to claim 7, characterized in that, Increase the temperature to the calcination temperature at a rate of 5℃ / min.

9. A graphitic carbon nitride photocatalyst for photocatalytic hydrogen production prepared by the method according to any one of claims 1-8, characterized in that, The specific surface area of ​​the photocatalyst is 8.6~14.4 m². 2 / g.

10. The application of a graphitic carbon nitride photocatalyst for photocatalytic hydrogen production prepared by any one of claims 1-8 in photocatalytic hydrogen production.

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