CN117205957B - Preparation method and application of carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material - Google Patents

Abstract

A preparation method and application of a carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material relate to a preparation method and application of a composite photocatalytic material. The invention aims to solve the problems that the utilization efficiency of carbon nitride to visible light is low, the recombination rate of photoinduced carriers is high and the photocatalytic efficiency is severely limited in the photocatalytic process. The method comprises the following steps: 1. preparing a carbon dot solution; 2. preparing a carbon nitride precursor; 3. compounding; 4. calcining; 5. pt is loaded. The carbon nitride-supported carbon dot anchored small-size Pt composite photocatalytic material is used as a photocatalytic material for photocatalytic pyrolysis of water to produce hydrogen. The carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material prepared by the invention has good performance and excellent cycle stability of 1.352mmolh ^‑1g^‑1 of hydrogen production rate of photocatalytic pyrolysis water under the irradiation of visible light. The invention can obtain the carbon-nitride-supported carbon-dot-anchored small-size Pt composite photocatalytic material.

Description

Preparation method and application of carbon nitride-supported carbon dots anchoring small-sized Pt composite photocatalytic material

Technical Field

The invention relates to a preparation method and application of a composite photocatalytic material.

Background Art

Hydrogen energy is considered to be an ideal energy source for solving future energy crises and environmental problems. Photocatalytic water splitting to produce hydrogen is one of the important strategies to alleviate energy crises and environmental pollution problems. Carbon nitride (C ₃ N ₄ ) is an effective semiconductor photocatalyst. Due to its good chemical stability, non-toxicity and high photocatalytic activity, it has been widely studied in the field of photocatalytic hydrogen production. However, limited by the light response range of C ₃ N ₄ , its utilization efficiency of visible light in the photocatalytic process is low and the photoinduced carrier recombination rate is high, which seriously limits its photocatalytic efficiency. Finding a suitable method to broaden its light absorption range and promote the separation of electron-hole pairs is the key to improving photocatalytic performance. Carbon dots have become a hot topic of research due to their adjustable light absorption range, good conductivity, excellent photostability and low toxicity.

Summary of the invention

The purpose of the present invention is to solve the problem that carbon nitride has low visible light utilization efficiency and high photoinduced carrier recombination rate in the photocatalytic process, which seriously limits its photocatalytic efficiency, and to provide a preparation method and application of a carbon nitride-loaded carbon dot-anchored small-sized Pt composite photocatalytic material.

A method for preparing a carbon nitride-supported carbon dot-anchored small-size Pt composite photocatalytic material is specifically completed by the following steps:

1. Add o-phenylenediamine to anhydrous ethanol and stir for a period of time to obtain an o-phenylenediamine anhydrous ethanol solution; transfer the o-phenylenediamine anhydrous ethanol solution to a polytetrafluoroethylene autoclave, and then put it into a vacuum oven at a temperature of 170°C to 200°C for a period of time, and finally cool it naturally to room temperature to obtain a carbon dot solution;

2. Mix the amine source and the phosphorus source with deionized water, place them on a stirrer and stir until a uniform solution is formed, then place the solution in a constant temperature water bath and set the temperature to 80°C to 130°C, keep the temperature at 80°C to 130°C for a period of time to obtain a mixed solution; transfer the mixed solution to a polytetrafluoroethylene autoclave, and then place it in a vacuum oven at a temperature of 150°C to 200°C for a period of time, then cool it to room temperature, collect the precipitate, wash the precipitate, and finally place it in a vacuum oven to dry the precipitate to obtain a carbon nitride precursor;

3. Adding a carbon dot solution to a carbon nitride precursor, and then adding glycerol and anhydrous ethanol to obtain a mixed solution; reflux the mixed solution at a constant temperature for a period of time, and then centrifuging the mixed solution obtained after the reflux to obtain a precipitate; placing the precipitate in a vacuum oven for drying to obtain a composite material;

Fourth, the obtained composite material is fully ground and placed in a porcelain boat, the porcelain boat is placed in a muffle furnace, the muffle furnace is heated, calcined in air for a period of time, and cooled to room temperature to obtain a carbon point-based carbon nitride composite material;

5. Adding H ₂ PtCl ₆ solution to the carbon dot-based carbon nitride composite material to obtain a mixed solution; performing photodeposition on the mixed solution, and finally freeze-drying to obtain a carbon nitride-loaded carbon dot-anchored small-sized Pt composite photocatalytic material.

Carbon nitride-loaded carbon dots anchored small-sized Pt composite photocatalyst materials are used as photocatalytic materials to photocatalytically split water to produce hydrogen.

Principle of the present invention:

The present invention rationally constructs a carbon dot/C ₃ N ₄ composite material, which can effectively broaden the light absorption range of the material and promote the rapid separation of photogenerated carriers, which is of great significance for improving the photocatalytic hydrogen production efficiency of the material. In addition, the carbon dot surface has variable functional groups, which can effectively disperse the metal co-catalyst and improve the efficiency of the co-catalyst in the photocatalytic process. Therefore, it is very important to construct a suitable carbon dot/C ₃ N ₄ composite material, use the functional groups on the surface of the carbon dot to anchor the small-sized Pt co-catalyst material, broaden the light absorption range of the material, improve the carrier separation efficiency, and effectively improve the efficiency of the co-catalyst to achieve efficient photocatalytic hydrogen production.

Advantages of the present invention:

First, the carbon dots used in the present invention have unique structural and physicochemical properties, such as low toxicity, excellent photostability, photoinduced electron transfer characteristics and a wide visible light response range, which opens up new possibilities for developing catalysts with better photocatalytic activity;

Second, the present invention can realize the controllable synthesis of composite photocatalytic materials anchored with carbon dots as catalysts by regulating variables such as material feed ratio and heat treatment heating rate;

3. The present invention can synthesize a porous and few-layer composite material by simply reflowing and calcining. Compared with the traditional synthesis method, the composite material synthesis method is relatively simple and environmentally friendly, and this method can save a lot of experimental materials;

4. When the volume ratio of carbon dot solution to porous few-layer carrier (precursor) is 0.5mL:0.6g, the carbon nitride-loaded carbon dot-anchored small-size Pt composite photocatalytic material prepared has a good performance of 1.352mmolh ^- 1g ^-1 and excellent cycle stability in photocatalytic water splitting hydrogen production rate under visible light irradiation. Compared with the control experiment: the porous few-layer material without carbon dot solution has a hydrogen production rate of 0.729mmolh ^- 1g ^-1 in photocatalytic water splitting under visible light irradiation. It can be seen that the prepared composite photocatalytic material with carbon dots anchored as catalyst plays an important role in improving the hydrogen production rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a SEM image of the carbon nitride precursor prepared in step 1 of comparative example 1;

FIG2 is a SEM image of a carbon nitride-supported carbon dot-anchored small-sized Pt composite photocatalytic material prepared in Example 1;

FIG3 is a graph showing the ultraviolet-visible light absorption spectra of the carbon nitride prepared in step 3 of comparative example 1 and the carbon dot-based carbon nitride composite material prepared in step 4 of example 1;

Figure 4 is a graph showing the hydrogen production rate of photocatalytic water splitting under simulated sunlight irradiation. In the figure, 1 is the carbon nitride-loaded carbon dot-anchored small-sized Pt composite photocatalyst material prepared in step five of Example 1, and 2 is the carbon nitride composite photocatalyst material prepared in step four of Comparative Example 1.

DETAILED DESCRIPTION

The following examples further illustrate the content of the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention all fall within the scope of the present invention.

Specific implementation method 1: This implementation method is a method for preparing a carbon nitride-supported carbon dot anchored small-sized Pt composite photocatalytic material, which is specifically completed in the following steps:

1. Add o-phenylenediamine to anhydrous ethanol and stir for a period of time to obtain an o-phenylenediamine anhydrous ethanol solution; transfer the o-phenylenediamine anhydrous ethanol solution to a polytetrafluoroethylene autoclave, and then put it into a vacuum oven at a temperature of 170°C to 200°C for a period of time, and finally cool it naturally to room temperature to obtain a carbon dot solution;

2. Mix the amine source and the phosphorus source with deionized water, place them on a stirrer and stir until a uniform solution is formed, then place the solution in a constant temperature water bath and set the temperature to 80°C to 130°C, keep the temperature at 80°C to 130°C for a period of time to obtain a mixed solution; transfer the mixed solution to a polytetrafluoroethylene autoclave, and then place it in a vacuum oven at a temperature of 150°C to 200°C for a period of time, then cool it to room temperature, collect the precipitate, wash the precipitate, and finally place it in a vacuum oven to dry the precipitate to obtain a carbon nitride precursor;

3. Adding a carbon dot solution to a carbon nitride precursor, and then adding glycerol and anhydrous ethanol to obtain a mixed solution; reflux the mixed solution at a constant temperature for a period of time, and then centrifuging the mixed solution obtained after the reflux to obtain a precipitate; placing the precipitate in a vacuum oven for drying to obtain a composite material;

Fourth, the obtained composite material is fully ground and placed in a porcelain boat, the porcelain boat is placed in a muffle furnace, the muffle furnace is heated, calcined in air for a period of time, and cooled to room temperature to obtain a carbon point-based carbon nitride composite material;

5. Adding H ₂ PtCl ₆ solution to the carbon dot-based carbon nitride composite material to obtain a mixed solution; performing photodeposition on the mixed solution, and finally freeze-drying to obtain a carbon nitride-loaded carbon dot-anchored small-sized Pt composite photocatalytic material.

Specific implementation method 2: This implementation method is different from specific implementation method 1 in that: the mass ratio of o-phenylenediamine to anhydrous ethanol in step 1 is 1.5 g: (100 mL to 200 mL); the stirring time in step 1 is 0.5 h to 2 h, and the stirring speed is 100 r/min to 300 r/min; the insulation time in a vacuum oven at a temperature of 170° C. to 200° C. in step 1 is 9 h to 12 h. The other steps are the same as those in specific implementation method 1.

Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that: the amine source in step 2 is melamine; the mass ratio of the amine source in step 2 to the volume of deionized water is 1.0 g: (60 mL to 120 mL); the phosphorus source in step 2 is phosphorous acid; the mass ratio of the phosphorus source in step 2 to the volume of deionized water is 1.2 g: (60 mL to 120 mL). The other steps are the same as those in specific implementation method 1 or 2.

Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that: in step 2, the temperature is kept constant at 80°C to 130°C for 0.5h to 2h; in step 2, the mixed solution is transferred to a polytetrafluoroethylene autoclave and then placed in a vacuum oven at a temperature of 150°C to 200°C for 7h to 12h. The other steps are the same as those of specific embodiments 1 to 3.

Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that: in step 2, the precipitate is centrifugally rinsed 2 to 6 times with deionized water and anhydrous ethanol respectively, and the centrifugal speed is 3000 r/min to 6000 r/min; in step 2, the temperature for drying the precipitate is 45° C. to 65° C. The other steps are the same as those of specific embodiments 1 to 4.

Specific embodiment 6: The difference between this embodiment and specific embodiments 1 to 5 is that: the mass ratio of the carbon nitride precursor described in step 3 to the volume ratio of glycerol is (0.4g-0.8g):5mL; the mass ratio of the carbon nitride precursor described in step 3 to anhydrous ethanol is (0.4g-0.8g):15mL; the mass ratio of the carbon nitride precursor described in step 3 to the volume ratio of the carbon dot solution is (0.4g-0.8g):0.5mL. The other steps are the same as specific embodiments 1 to 5.

Specific embodiment 7: The difference between this embodiment and specific embodiments 1 to 6 is that: in step 3, the temperature of the mixed solution being refluxed at a constant temperature is 80°C to 130°C, and the time of the constant temperature reflux is 2h to 6h; in step 3, the mixed solution obtained after reflux is centrifuged 2 to 6 times, the time of each centrifugation is 2min to 6min, and the centrifugation rate is 3000r/min to 6000r/min; in step 3, the precipitate is placed in a vacuum oven for drying, the drying temperature is 45°C to 65°C, and the drying time is 8h to 12h. The other steps are the same as those of specific embodiments 1 to 6.

Specific embodiment 8: This embodiment differs from specific embodiments 1 to 7 in that: in step 4, the rate of heating the muffle furnace is 0.5°C/min to 5°C/min; in step 4, the temperature of calcination in air is 300°C to 600°C, and the calcination time is 1h to 4h. The other steps are the same as those of specific embodiments 1 to 7.

Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that: the mass ratio of H ₂ PtCl ₆ to carbon dot-based carbon nitride composite material in the mixed solution described in step 5 is 0.5% to 1.5%; the photodeposition described in step 5 is specifically: irradiation under a xenon lamp using a cut420 filter for 30min to 50min; the freeze-drying temperature described in step 5 is -15℃ to -35℃, and the freeze-drying time is 24h to 48h. The other steps are the same as specific embodiments 1 to 8.

Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that: carbon nitride-supported carbon dots anchor small-sized Pt composite photocatalytic materials are used as photocatalytic materials to photocatalytically split water to produce hydrogen. The other steps are the same as specific embodiments 1 to 9.

The following examples are used to verify the beneficial effects of the present invention:

Example 1: A method for preparing a carbon nitride-supported carbon dot-anchored small-size Pt composite photocatalytic material, which is specifically completed by the following steps:

1. Add 1.5 g of o-phenylenediamine to 150 mL of anhydrous ethanol, and stir at a stirring speed of 250 r/min for 1 h to obtain an o-phenylenediamine anhydrous ethanol solution; transfer the o-phenylenediamine anhydrous ethanol solution to a polytetrafluoroethylene autoclave, and then place it in a vacuum oven at a temperature of 180°C for 12 h, and finally cool it naturally to room temperature to obtain a carbon dot solution;

2. Mix the amine source and the phosphorus source with deionized water, place them on a stirrer and stir until a uniform solution is formed, then place the solution in a constant temperature water bath and set the temperature to 80°C, keep the temperature at 80°C for 1 hour to obtain a mixed solution; transfer the mixed solution to a polytetrafluoroethylene autoclave, and then place it in a vacuum oven at a temperature of 180°C for 10 hours, then cool it to room temperature, collect the precipitate, and rinse the precipitate by centrifugation with deionized water and anhydrous ethanol for 3 times, respectively, at a centrifugal rate of 4000r/min; finally, place the precipitate in a vacuum oven and dry it for 10 hours at a drying temperature of 60°C to obtain a carbon nitride precursor;

The amine source described in step 2 is melamine;

The mass ratio of the amine source described in step 2 to the volume ratio of deionized water is 1.0 g: 100 mL;

The phosphorus source described in step 2 is phosphorous acid;

The mass ratio of the phosphorus source described in step 2 to the volume ratio of deionized water is 1.2 g:100 mL;

3. Add the carbon dot solution to the carbon nitride precursor, and then add glycerol and anhydrous ethanol to obtain a mixed solution; reflux the mixed solution at a constant temperature of 90°C for 3 hours, and then centrifuge the mixed solution obtained after reflux for 3 times, each centrifugation time is 4 minutes, and the centrifugation rate is 4000r/min to obtain a precipitate; put the precipitate into a vacuum oven for drying for 10 hours, and the drying temperature is 60°C to obtain a composite material;

The mass ratio of the carbon nitride precursor described in step 3 to the volume ratio of glycerol is 0.6 g:5 mL;

The mass ratio of the carbon nitride precursor described in step 3 to the volume ratio of anhydrous ethanol is 0.6 g:15 mL;

The volume ratio of the carbon nitride precursor to the carbon dot solution in step 3 is 0.6 g:0.5 mL;

Fourth, the obtained composite material is fully ground and placed in a porcelain boat, the porcelain boat is placed in a muffle furnace, the muffle furnace is heated at a heating rate of 2°C/min to 500°C, calcined in air for 2h, and cooled to room temperature to obtain a carbon dot-based carbon nitride composite material (CDs/C ₃ N ₄ );

5. adding H ₂ PtCl ₆ solution to the carbon dot-based carbon nitride composite material to obtain a mixed solution; performing photodeposition on the mixed solution, and finally freeze-drying to obtain a carbon nitride-loaded carbon dot-anchored small-size Pt composite photocatalytic material;

The mass ratio of H ₂ PtCl ₆ to the carbon dot-based carbon nitride composite material in the mixed solution described in step 5 is 1%;

The photodeposition described in step 5 is specifically as follows: irradiation for 40 min using a cut420 filter under a xenon lamp (PLS-SXE300);

The freeze-drying temperature in step 5 is -25°C, and the freeze-drying time is 40 hours.

Comparative Example 1: The preparation of carbon nitride composite photocatalytic material is completed according to the following steps:

1. Mix the amine source and the phosphorus source with deionized water, place them on a stirrer and stir until a uniform solution is formed, then place the solution in a constant temperature water bath and set the temperature to 80°C, keep the temperature at 80°C for 1h to obtain a mixed solution; transfer the mixed solution to a polytetrafluoroethylene autoclave, and then place it in a vacuum oven at a temperature of 180°C for 10h, then cool it to room temperature, collect the precipitate, and rinse the precipitate by centrifugation with deionized water and anhydrous ethanol for 3 times, respectively, at a centrifugal rate of 4000r/min; finally, place the precipitate in a vacuum oven for drying for 10h, and the drying temperature is 60°C to obtain a carbon nitride precursor;

The amine source described in step 1 is melamine;

The mass ratio of the amine source described in step 1 to the volume ratio of deionized water is 1.0 g:100 mL;

The phosphorus source described in step 1 is phosphorous acid;

The mass ratio of the phosphorus source described in step 1 to the volume ratio of deionized water is 1.2 g:100 mL;

2. Add glycerol and anhydrous ethanol to the carbon nitride precursor to obtain a mixed solution; reflux the mixed solution at a constant temperature of 90°C for 3 hours, and then centrifuge the mixed solution obtained after reflux for 3 times, each centrifugation time is 4 minutes, and the centrifugation rate is 4000r/min to obtain a precipitate; put the precipitate into a vacuum oven for drying for 10 hours, and the drying temperature is 60°C to obtain a composite material;

The mass ratio of the precursor described in step 2 to the volume ratio of glycerol is 0.6 g:5 mL;

The mass ratio of the precursor described in step 2 to the volume ratio of anhydrous ethanol is 0.6 g:15 mL;

3. Grind the obtained composite material thoroughly and put it into a porcelain boat, put the porcelain boat into a muffle furnace, heat the muffle furnace at a heating rate of 2°C/min to 500°C, calcine in air for 2h, and obtain carbon nitride after cooling to room temperature;

4. adding H ₂ PtCl ₆ solution to carbon nitride to obtain a mixed solution; performing photodeposition on the mixed solution, and finally freeze-drying to obtain a carbon nitride composite photocatalytic material;

The mass ratio of H ₂ PtCl ₆ to carbon nitride in the mixed solution described in step 4 is 1%;

The photodeposition described in step 4 is specifically as follows: irradiation for 40 min using a cut420 filter under a xenon lamp (PLS-SXE300);

The freeze-drying temperature in step 4 is -25°C, and the freeze-drying time is 40 hours.

FIG1 is a SEM image of the carbon nitride precursor prepared in step 1 of comparative example 1;

FIG2 is a SEM image of a carbon nitride-supported carbon dot-anchored small-sized Pt composite photocatalytic material prepared in Example 1;

It can be seen from Figures 1 and 2 that the surface of the carrier has a lamellar structure. The addition of carbon dots can make the surface of the carrier looser and the delamination more obvious.

FIG3 is a graph showing the ultraviolet-visible light absorption spectra of the carbon nitride prepared in step 3 of comparative example 1 and the carbon dot-based carbon nitride composite material prepared in step 4 of example 1;

As shown in Figure 3, the addition of carbon dot solution significantly broadens the visible light absorption range of the composite material.

The hydrogen production rates of the carbon nitride-supported carbon dot-anchored small-size Pt composite photocatalyst material prepared in step 5 of Example 1 and the carbon nitride composite photocatalyst material prepared in step 4 of Comparative Example 1 under visible light irradiation by photocatalytic water splitting are shown in FIG4 ;

Figure 4 is a graph showing the hydrogen production rate of photocatalytic water splitting under simulated sunlight irradiation. In the figure, 1 is the carbon nitride-loaded carbon dot-anchored small-sized Pt composite photocatalyst material prepared in step five of Example 1, and 2 is the carbon nitride composite photocatalyst material prepared in step four of Comparative Example 1.

As shown in Figure 4, the carbon nitride-supported carbon dot-anchored small-sized Pt composite photocatalyst prepared in Example 1 has a hydrogen production rate of 1.352 mmolh ^-1 g ^-1 for photocatalytic water splitting under simulated visible light irradiation, while the carbon nitride composite photocatalyst prepared in Example 1 has a hydrogen production rate of only 0.729 _mmolh ^-1 g ^-1 for photocatalytic water splitting under visible light irradiation. Compared with a single porous few-layer _C3N4 photocatalyst, the carbon nitride-supported carbon dot-anchored small-sized Pt composite photocatalyst of the present invention can be increased by more than one time.

Comparative Example 2: This example differs from Example 1 in that the volume ratio of the carbon nitride precursor mass to the carbon dot solution in step 3 is 0.6 g:0.05 mL. Other steps and parameters are the same as those in Example 1.

Comparative Example 3: The difference between this example and Example 1 is that the volume ratio of the mass of the carbon nitride precursor to the carbon dot solution in step 3 is 0.6 g:0.3 mL. The other steps and parameters are the same as those in Example 1.

Comparative Example 2 shows that the carbon nitride-supported carbon dots-anchored small-sized Pt composite photocatalytic material prepared under simulated visible light irradiation has a hydrogen production rate of 0.806 mmolh ^-1 g ^-1 by photocatalytic water splitting.

Comparative Example 3 shows that the carbon nitride-supported carbon dots-anchored small-sized Pt composite photocatalytic material prepared under simulated visible light irradiation has a hydrogen production rate of 1.103 mmolh ^-1 g ^-1 by photocatalytic water splitting.

Claims (6)

1. The preparation method of the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material is characterized by comprising the following steps of:

1. Adding o-phenylenediamine into absolute ethyl alcohol, and stirring for a period of time to obtain an o-phenylenediamine absolute ethyl alcohol solution; transferring the o-phenylenediamine absolute ethanol solution into a polytetrafluoroethylene autoclave, then placing the polytetrafluoroethylene autoclave into a vacuum oven with the temperature of 170-200 ℃ for heat preservation for a period of time, and finally naturally cooling to room temperature to obtain a carbon dot solution;

2. mixing an amine source, a phosphorus source and deionized water, placing the mixture on a stirrer for stirring until the mixture forms a uniform solution, then placing the solution into a constant-temperature water bath kettle, setting the temperature to be 80-130 ℃, and keeping the temperature at 80-130 ℃ for a period of time to obtain a mixed solution; transferring the mixed solution into a polytetrafluoroethylene autoclave, then placing the autoclave into a vacuum oven with the temperature of 150-200 ℃ for heat preservation for a period of time, immediately cooling the autoclave to room temperature, collecting precipitate, cleaning the precipitate, and finally placing the precipitate into the vacuum oven for drying to obtain a carbon nitride precursor;

the amine source in the second step is melamine; the volume ratio of the mass of the amine source to the deionized water in the second step is 1.0g (60 mL-120 mL);

The phosphorus source in the second step is phosphorous acid; the volume ratio of the mass of the phosphorus source to the deionized water in the second step is 1.2g (60 mL-120 mL);

3. adding a carbon dot solution into a carbon nitride precursor, and then adding glycerol and absolute ethyl alcohol to obtain a mixed solution; refluxing the mixed solution for a period of time at constant temperature, and centrifuging the mixed solution obtained after refluxing to obtain a precipitate; placing the precipitate into a vacuum oven for drying to obtain a composite material;

the volume ratio of the mass of the carbon nitride precursor to the glycerol in the third step is (0.4 g-0.8 g) 5mL;

The volume ratio of the mass of the carbon nitride precursor to the absolute ethyl alcohol in the third step is (0.4 g-0.8 g) 15mL;

the volume ratio of the mass of the carbon nitride precursor to the carbon dot solution in the third step is (0.4 g-0.8 g) 0.5mL;

In the third step, the mixed solution is subjected to constant temperature reflux at 80-130 ℃ for 2-6 h;

4. fully grinding the obtained composite material, then placing the ground composite material into a porcelain boat, placing the porcelain boat into a muffle furnace, heating the muffle furnace, calcining in air for a period of time, and cooling to room temperature to obtain the carbon-dot-based carbon nitride composite material;

In the fourth step, the temperature rising rate of the muffle furnace is 0.5-5 ℃/min; in the fourth step, the calcination is carried out in air at the temperature of 300-600 ℃ for 1-4 hours;

5. Adding an H ₂PtCl₆ solution into the carbon dot-based carbon nitride composite material to obtain a mixed solution; performing photo-deposition on the mixed solution, and finally freeze-drying to obtain the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material;

the mass ratio of H ₂PtCl₆ and the carbon dot-based carbon nitride composite material in the mixed solution in the fifth step is 0.5-1.5%;

The photo-deposition in the fifth step is specifically: irradiating for 30-50 min under a xenon lamp by using a cut420 optical filter;

and step five, the freeze drying temperature is-15 ℃ to-35 ℃ and the freeze drying time is 24h to 48h.

2. The method for preparing the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material according to claim 1, wherein the volume ratio of the mass of the o-phenylenediamine to the volume of the absolute ethyl alcohol in the first step is 1.5g (100 mL-200 mL); the stirring time in the first step is 0.5-2 h, and the stirring speed is 100-300 r/min; and in the first step, the mixture is put into a vacuum oven with the temperature of 170-200 ℃ for heat preservation for 9-12 h.

3. The method for preparing the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material according to claim 1, wherein in the second step, the constant temperature is maintained at 80-130 ℃ for 0.5-2 hours; and step two, transferring the mixed solution into a polytetrafluoroethylene autoclave, and then placing the polytetrafluoroethylene autoclave into a vacuum oven with the temperature of 150-200 ℃ for heat preservation for 7-12 h.

4. The method for preparing the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material according to claim 1, wherein in the second step, the precipitate is centrifugally washed 2-6 times by deionized water and absolute ethyl alcohol respectively, and the centrifugal speed is 3000-6000 r/min; and in the second step, the temperature for drying the precipitate is 45-65 ℃.

5. The method for preparing the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material according to claim 1, wherein the mixed solution obtained after reflux in the third step is centrifuged for 2-6 times, the time of each centrifugation is 2-6 min, and the speed of centrifugation is 3000-6000 r/min; and thirdly, placing the precipitate into a vacuum oven for drying, wherein the drying temperature is 45-65 ℃ and the drying time is 8-12 h.

6. The application of the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material prepared by the preparation method as claimed in claim 1, wherein the carbon nitride supported carbon dot anchored small-size Pt composite photocatalytic material is used for photocatalytic pyrolysis of water to produce hydrogen.