A phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material and its preparation method and application

Abstract

The present invention discloses a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, its preparation method, and application, belonging to the technical field of CO2 reduction photocatalytic materials. The present invention uses copper and phosphorus co-doped modified g-C3N4 and then composites it with a phosphazene polymer to prepare a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material. This material can effectively regulate the separation of photogenerated carriers, greatly improving the photocatalytic activity. It has an ultra-high ability to reduce CO2 to CH4, with a CH4 yield of up to 1520.8μmol g ‑1 h ‑1 , which has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic CO2 reduction, and specifically relates to a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, a preparation method and an application thereof. Background Art

[0002] Graphitic carbon nitride (g-C3N4) is a porous carbon material composed of carbon and nitrogen that exhibits excellent photocatalytic performance and chemical stability. Its excellent light absorption properties allow it to convert light energy into chemical energy, leading to applications in catalytic water splitting, organic wastewater treatment, and photocatalytic CO2 reduction. However, g-C3N4 also suffers from certain drawbacks, such as high electron-hole recombination rates, insufficient visible light absorption, low specific surface area, few surface reaction activation sites, slow surface reaction kinetics, and low charge mobility. These shortcomings significantly limit the photocatalytic performance of g-C3N4, particularly its catalytic activity in the photocatalytic reduction of CO2 to methane. To date, many photocatalytic materials, such as TiO2 and Co3O4, have been applied to the photocatalytic reduction of CO2, but their extremely low conversion efficiencies have severely hampered their practical application. Therefore, the search for efficient, stable, and inexpensive photocatalysts has attracted widespread attention. Summary of the Invention

[0003] The present invention aims to provide a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material. This material can better regulate the migration of photogenerated electrons and carrier separation, thereby improving the efficiency of the photocatalytic reduction of CO2 to CH4. Its preparation method is simple, feasible, low-cost, environmentally friendly, and easy to mass-produce.

[0004] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0005] A method for preparing a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, comprising the following steps:

[0006] (1) Melamine was calcined at high temperature for 4 h under nitrogen atmosphere to obtain g-C3N4;

[0007] (2) Prepare a copper salt methanol solution, slowly add the g-C3N4 obtained in step (1) into it while stirring, stir and mix evenly, then heat to 65°C for reflux reaction for 3-4 hours. After the reaction is completed, collect the solid precipitate, vacuum dry it, transfer it to a tube furnace, and calcine it at high temperature for 4 hours under a nitrogen atmosphere to obtain Cu-doped g-C3N4;

[0008] (3) Prepare a methanol solution of phosphate, slowly add the g-C3N4 obtained in step (1) into it while stirring, stir and mix evenly, then heat to 65°C and reflux for 3-4 h. After the reaction is completed, collect the solid precipitate, vacuum dry it, transfer it to a tube furnace, and calcine it at high temperature for 4 h under a nitrogen atmosphere to obtain P-doped g-C3N4;

[0009] (4) The Cu-doped g-C3N4 and P-doped g-C3N4 obtained in the above steps were mixed according to the mass ratio and mechanically ground for 30 min. The mixture was then placed in a nitrogen atmosphere and calcined at high temperature for 2 h. After naturally cooling to room temperature, the product was washed with methanol three times and then dried in vacuum at 80 °C for 24 h to obtain copper and phosphorus co-doped g-C3N4;

[0010] (5) Hexachlorocyclotriphosphazene, trithiocyanate, acetonitrile and dimethyl sulfoxide were added to a hydrothermal reactor in proportion, first subjected to ultrasonic treatment for 1 h, then heated to 180 °C and kept warm for 24-28 h. After the reaction was completed, the product was naturally cooled and washed with deionized water, acetonitrile and ethanol in sequence. After washing, it was vacuum dried at 60 °C for 24 h to obtain a phosphazene polymer.

[0011] (6) The phosphazene polymer obtained in step (5) and the copper and phosphorus co-doped g-C3N4 obtained in step (4) are mixed in a mass ratio, and mechanically ground for 30 minutes to obtain the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material.

[0012] Preferably, the specific method of high-temperature calcination in step (1) is: heating to 500°C at a rate of 10°C / min and then calcining for 4 hours.

[0013] Preferably, the concentration of the copper salt methanol solution in step (2) is 0.1-0.2 mol / L; the usage ratio of the copper salt methanol solution to g-C3N4 is 100 ml: 2-3 g; and the copper salt is copper chloride or copper nitrate.

[0014] Preferably, the concentration of the methanol solution of phosphate in step (3) is 0.1-0.2 mol / L; the usage ratio of the methanol solution of phosphate to g-C3N4 is 100 ml: 2-3 g; and the phosphate is sodium hypophosphite or sodium phosphite.

[0015] Preferably, the specific method of high-temperature calcination in step (2) and step (3) is: heating to 520°C at a rate of 10°C / min and then calcining for 4 hours.

[0016] Preferably, in step (4), the mass ratio of Cu-doped g-C3N4 to P-doped g-C3N4 is 700-800:1; the specific method of high-temperature calcination is: heating to 300°C at a rate of 10°C / min and then calcining for 2 h.

[0017] Preferably, in step (5), the usage ratio of hexachlorocyclotriphosphazene, trithiocyanic acid, acetonitrile and dimethyl sulfoxide is 1 mol: 3 mol: 25-30 L: 1.5 L.

[0018] Preferably, in step (6), the mass ratio of the phosphazene polymer to the copper and phosphorus co-doped g-C3N4 is 6-8:3.

[0019] The present invention also provides a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material prepared by the above method.

[0020] The present invention also provides the use of the above-mentioned phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material in CO2 reduction to produce CH4.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The heterojunction material in the present invention does not involve complicated synthesis steps. It only requires the use of low-cost and environmentally friendly solvents and a controllable heating process, without the need for cumbersome post-processing processes, to obtain the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material. The preparation process is simple and reliable, and it is easy to mass produce.

[0023] (2) The phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material prepared by the present invention has a high ability to reduce CO2 to CH4 compared with pure C3N4, which produces almost no CH4. The CH4 yield can reach 1520.8 μmol g -1 h -1 , 2.5 times higher than that of phosphazene polymer under the same conditions. This is because the heterojunction composite has a large and strong interface contact, excellent visible light absorption ability, porous structure, spatially separated redox sites, and rapid separation of light-induced charge carriers, resulting in excellent photocatalytic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Figure 2 is the XRD test result diagram of different materials of the present invention;

[0025] Figure 2 TEM image of the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material CT / CuPCN obtained in Example 1 of the present invention;

[0026] Figure 3 This is a test graph of the photocatalytic activity of the catalytic materials obtained in Example 1 of the present invention and the comparative example in the photocatalytic reduction of CO2 to produce methane;

[0027] Figure 4Impedance spectra of catalyst materials of Example 1 of the present invention and the comparative example. DETAILED DESCRIPTION

[0028] The technical solution of the present invention will be further described below with reference to specific embodiments, but is not limited thereto.

[0029] Example 1

[0030] A phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material is prepared by the following method:

[0031] (1) 5 g of melamine was weighed and heated to 500°C at a heating rate of 10°C / min under a nitrogen atmosphere and then calcined for 4 h to obtain g-C3N4;

[0032] (2) Weigh 2.6 g of CuCl2·H2O and 100 mL of methanol and place them in a clean 250 mL round-bottom flask. Ultrasonicate for 1 h to prepare a uniform solution. Then, slowly add 2 g of g-C3N4 while stirring to prevent agglomeration. Then, reflux the mixture at 65 °C for 3 h, cool it, collect the solid precipitate, and dry it in a vacuum oven at 70 °C for 24 h. Then, heat the precipitate to 520 °C under nitrogen atmosphere at a heating rate of 10 °C / min and calcine it for 4 h to obtain Cu-doped g-C3N4, which is recorded as CuCN.

[0033] (3) Weigh 1.32 g of sodium hypophosphite and 100 mL of methanol and place them in a clean 250 mL round-bottom flask. Ultrasonicate for 1 h to prepare a uniform solution. Then, slowly add 2 g of g-C3N4 while stirring to prevent agglomeration. Then, reflux the mixture at 65 °C for 3 h, cool it, collect the solid precipitate, and dry it in a vacuum oven at 70 °C for 24 h. Then, heat the precipitate to 520 °C under nitrogen atmosphere at a heating rate of 10 °C / min and calcine it for 4 h to obtain P-doped g-C3N4, which is recorded as PCN.

[0034] (4) 0.3 g Cu-doped g-C3N4 and 0.4 mg P-doped g-C3N4 were mixed and mechanically ground for 30 min. The mixture was then placed in a nitrogen atmosphere, heated to 300 °C at a rate of 10 °C / min, and calcined for 2 h. After cooling to room temperature, the product was washed three times with methanol and then dried in vacuum at 80 °C for 24 h to obtain copper and phosphorus co-doped g-C3N4, which was recorded as CuPCN.

[0035] (5) Weigh 0.35 g of hexachlorocyclotriphosphazene and 0.53 g of trithiocyanate into a 50 mL hydrothermal autoclave, add 28 mL of acetonitrile and 1.5 mL of dimethyl sulfoxide, and ultrasonicate for 1 h. Seal the hydrothermal autoclave and place it in the hydrothermal autoclave. Seal the hydrothermal autoclave and heat it to 180 °C at a heating rate of 10 °C / min for 24 h. After the reaction is cooled, wash the brown precipitate with deionized water, acetonitrile, and ethanol. After washing, dry it in a vacuum oven at 60 °C for 24 h to obtain a phosphazene polymer, which is recorded as CT.

[0036] (6) 0.7 mg of phosphazene polymer was mixed with 0.3 mg of copper and phosphorus co-doped g-C3N4 and mechanically ground for 30 min to obtain the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, which was recorded as CT / CuPCN.

[0037] Example 2

[0038] A phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material is prepared by the following method:

[0039] (1) 5 g of melamine was weighed and heated to 500°C at a heating rate of 10°C / min under a nitrogen atmosphere and then calcined for 4 h to obtain g-C3N4;

[0040] (2) Weigh 4.65 g of Cu(NO3)2·3H2O and 100 mL of methanol and place them in a clean 250 mL round-bottom flask. Ultrasonicate for 1 h to prepare a uniform solution. Then, slowly add 3 g of g-C3N4 while stirring to prevent agglomeration. Then, reflux the mixture at 65 °C for 3 h, cool it, collect the solid precipitate, and dry it in a vacuum oven at 70 °C for 24 h. Then, heat the precipitate to 520 °C under nitrogen atmosphere at a heating rate of 10 °C / min and calcine it for 4 h to obtain Cu-doped g-C3N4, which is recorded as CuCN.

[0041] (3) Weigh 1.32 g of sodium hypophosphite and 100 mL of methanol and place them in a clean 250 mL round-bottom flask. Ultrasonicate for 1 h to prepare a uniform solution. Then, slowly add 3 g of g-C3N4 while stirring to prevent agglomeration. Then, reflux the mixture at 65 °C for 3 h, cool it, collect the solid precipitate, and dry it in a vacuum oven at 70 °C for 24 h. Then, heat the precipitate to 520 °C under nitrogen atmosphere at a heating rate of 10 °C / min and calcine it for 4 h to obtain P-doped g-C3N4, which is recorded as PCN.

[0042] (4) 0.35 g Cu-doped g-C3N4 and 0.5 mg P-doped g-C3N4 were mixed and mechanically ground for 30 min. The mixture was then placed in a nitrogen atmosphere, heated to 300 °C at a rate of 10 °C / min, and calcined for 2 h. After cooling to room temperature, the product was washed three times with methanol and then dried in vacuum at 80 °C for 24 h to obtain copper and phosphorus co-doped g-C3N4, which was recorded as CuPCN.

[0043] (5) Weigh 0.35 g of hexachlorocyclotriphosphazene and 0.53 g of trithiocyanate into a 50 mL hydrothermal autoclave, add 28 mL of acetonitrile and 1.5 mL of dimethyl sulfoxide, and ultrasonicate for 1 h. Seal the hydrothermal autoclave and place it in the hydrothermal autoclave. Seal the hydrothermal autoclave and heat it to 180 °C at a heating rate of 10 °C / min for 24 h. After the reaction is cooled, wash the brown precipitate with deionized water, acetonitrile, and ethanol. After washing, dry it in a vacuum oven at 60 °C for 24 h to obtain a phosphazene polymer, which is recorded as CT.

[0044] (6) 0.6 mg of phosphazene polymer was mixed with 0.3 mg of copper and phosphorus co-doped g-C3N4 and mechanically ground for 30 min to obtain the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, which was recorded as CT / CuPCN.

[0045] Example 3

[0046] A phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material is prepared by the following method:

[0047] (1) 5 g of melamine was weighed and heated to 500°C at a heating rate of 10°C / min under a nitrogen atmosphere and then calcined for 4 h to obtain g-C3N4;

[0048] (2) 3.06 g of CuCl2·H2O and 100 mL of methanol were weighed and placed in a clean 250 mL round-bottom flask. Ultrasonication was performed for 1 h to prepare a uniform solution. Then, 2 g of g-C3N4 was slowly added under stirring to prevent agglomeration. The mixture was refluxed at 65 °C for 3 h, cooled, and the solid precipitate was collected and dried in a vacuum oven at 70 °C for 24 h. Then, the precipitate was heated to 520 °C under a nitrogen atmosphere at a heating rate of 10 °C / min and calcined for 4 h to obtain Cu-doped g-C3N4, which was recorded as CuCN.

[0049] (3) Weigh 2.52 g of sodium phosphite and 100 mL of methanol and place them in a clean 250 mL round-bottom flask. Ultrasonicate for 1 h to prepare a uniform solution. Then, slowly add 2 g of g-C3N4 while stirring to prevent agglomeration. Then, reflux the mixture at 65 °C for 3 h, cool it, collect the solid precipitate, and dry it in a vacuum oven at 70 °C for 24 h. Then, heat the precipitate to 520 °C under nitrogen atmosphere at a heating rate of 10 °C / min and calcine it for 4 h to obtain P-doped g-C3N4, which is recorded as PCN.

[0050] (4) 0.4 g Cu-doped g-C3N4 and 0.5 mg P-doped g-C3N4 were mixed and mechanically ground for 30 min. The mixture was then placed in a nitrogen atmosphere, heated to 300 °C at a rate of 10 °C / min, and calcined for 2 h. After cooling to room temperature, the product was washed three times with methanol and then dried in vacuum at 80 °C for 24 h to obtain copper and phosphorus co-doped g-C3N4, which was recorded as CuPCN.

[0051] (5) Weigh 0.35 g of hexachlorocyclotriphosphazene and 0.53 g of trithiocyanate into a 50 mL hydrothermal autoclave, add 30 mL of acetonitrile and 1.5 mL of dimethyl sulfoxide, and ultrasonicate for 1 h. Seal the hydrothermal autoclave and place it in the hydrothermal autoclave. Seal the hydrothermal autoclave, and then heat the hydrothermal autoclave to 180 °C at a heating rate of 10 °C / min for 24 h. After the reaction is cooled, wash the brown precipitate with deionized water, acetonitrile, and ethanol. After washing, dry it in a vacuum oven at 60 °C for 24 h to obtain a phosphazene polymer, which is recorded as CT.

[0052] (6) 0.8 mg of phosphazene polymer was mixed with 0.3 mg of copper and phosphorus co-doped g-C3N4, and mechanically ground for 30 min to obtain a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, which was recorded as CT / CuPCN.

[0053] A phosphazene polymer / copper-doped g-C3N4 heterojunction photocatalytic material is prepared by the following method:

[0054] (1) 5 g of melamine was weighed and heated to 500 °C at a heating rate of 10 °C / min under a nitrogen atmosphere and then calcined for 4 h to obtain g-C3N4;

[0055] (2) Weigh 2.6 g of CuCl2·H2O and 100 mL of methanol and place them in a clean 250 mL round-bottom flask. Ultrasonicate for 1 h to prepare a uniform solution. Then, slowly add 2 g of g-C3N4 while stirring to prevent agglomeration. Then, reflux the mixture at 65 °C for 3 h, cool it, collect the solid precipitate, and dry it in a vacuum oven at 70 °C for 24 h. Then, heat the precipitate to 520 °C under nitrogen atmosphere at a heating rate of 10 °C / min and calcine it for 4 h to obtain Cu-doped g-C3N4, which is recorded as CuCN.

[0056] (4) Weigh 0.35 g of hexachlorocyclotriphosphazene and 0.53 g of trithiocyanate into a 50 mL hydrothermal autoclave, add 28 mL of acetonitrile and 1.5 mL of dimethyl sulfoxide, and ultrasonicate for 1 h. Seal the hydrothermal autoclave and place it in the hydrothermal autoclave. Seal the hydrothermal autoclave and heat it to 180 °C at a heating rate of 10 °C / min for 24 h. After the reaction is cooled, wash the brown precipitate with deionized water, acetonitrile, and ethanol. After washing, dry it in a vacuum oven at 60 °C for 24 h to obtain a phosphazene polymer, which is recorded as CT.

[0057] (5) 0.7 mg of phosphazene polymer was mixed with 0.3 mg of copper-doped g-C3N4 and mechanically ground for 30 min to obtain the phosphazene polymer / copper-doped g-C3N4 heterojunction photocatalytic material, which was recorded as CT / CuCN.

[0058] Comparative Example 2

[0059] A phosphazene polymer / phosphorus-doped g-C3N4 heterojunction photocatalytic material is prepared by the following method:

[0060] (1) 5 g of melamine was weighed and heated to 500 °C at a heating rate of 10 °C / min under a nitrogen atmosphere and then calcined for 4 h to obtain g-C3N4;

[0061] (2) Weigh 1.32 g of sodium hypophosphite and 100 mL of methanol and place them in a clean 250 mL round-bottom flask. Ultrasonicate for 1 h to prepare a uniform solution. Then, slowly add 3 g of g-C3N4 while stirring to prevent agglomeration. Then, reflux the mixture at 65 °C for 3 h, cool it, collect the solid precipitate, and dry it in a vacuum oven at 70 °C for 24 h. Then, heat the precipitate to 520 °C under nitrogen atmosphere at a heating rate of 10 °C / min and calcine it for 4 h to obtain P-doped g-C3N4, which is recorded as PCN.

[0062] (3) Weigh 0.35 g of hexachlorocyclotriphosphazene and 0.53 g of trithiocyanate into a 50 mL hydrothermal autoclave, add 28 mL of acetonitrile and 1.5 mL of dimethyl sulfoxide, and ultrasonicate for 1 h. Seal the hydrothermal autoclave and place it in the hydrothermal autoclave. Seal the hydrothermal autoclave and heat it to 180 °C at a heating rate of 10 °C / min for 24 h. After the reaction is cooled, wash the brown precipitate with deionized water, acetonitrile, and ethanol. After washing, dry it in a vacuum oven at 60 °C for 24 h to obtain a phosphazene polymer, which is recorded as CT.

[0063] (4) 0.7 mg of phosphazene polymer was mixed with 0.3 mg of phosphorus-doped g-C3N4 and mechanically ground for 30 min to obtain the phosphazene polymer / phosphorus-doped g-C3N4 heterojunction photocatalytic material, which was recorded as CT / PCN.

[0064] Performance Testing

[0065] The phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material CT / CuPCN obtained in Example 1 of the present invention, comparative example 1, comparative example 2, g-C3N4 obtained in step (1) of Example 1, and phosphazene polymer CT obtained in step (5) of Example 1 were used as test materials to test the XRD patterns of different materials. The specific results are shown in FIG. Figure 1 .like Figure 1 As shown, it can be seen that the prepared CT / CuPCN sample shows a weak diffraction peak, which is consistent with the diffraction peak of CT and CuPCN samples, and there is no impurity peak, which proves that the CT / CuPCN sample is preliminarily synthesized.

[0066] The phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material CT / CuPCN obtained in Example 1 of the present invention was subjected to TEM morphology test. Figure 2 .from Figure 2 As can be seen in the information, the CT / CuPCN sample shows a structure with multiple thin layers covering each other. This structure has large and strong interfacial contacts, excellent visible light absorption ability, porous structure, spatially separated redox sites, and rapid separation of photoinduced charge carriers.

[0067] The photocatalytic CO2 reduction performance of different materials was tested by using the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material CT / CuPCN obtained in Example 1 of the present invention, Comparative Example 1, Comparative Example 2, g-C3N4 obtained in step (1) of Example 1, and the phosphazene polymer CT obtained in step (5) of Example 1 as test materials. The specific results are shown in FIG. Figure 3 As shown. Figure 3As can be seen, the CT / CuPCN sample prepared in Example 1 of the present invention exhibited excellent photocatalytic activity by catalyzing the reduction of CO₂ to CH₄ at a yield of 1520.8 μmol / g under visible light irradiation. This yield was significantly greater than that of Comparative Examples 1 and 2, and significantly greater than the sum of the yields of Comparative Examples 1 and 2. This demonstrates that the heterojunction photocatalytic material formed by combining copper and phosphorus co-doped g-C₃N₄ with a phosphazene polymer exhibits significant synergistic effects in the catalytic reduction of CO₂ to CH₄.

[0068] The present invention also performs impedance tests on Example 1, Comparative Example 1, Comparative Example 2, g-C3N4 obtained in step (1) of Example 1, and phosphazene polymer CT materials obtained in step (5) of Example 1. Figure 4 .like Figure 4 As shown, the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material CT / CuPCN sample prepared in Example 1 has the smallest Enquist radius, which indicates that the charge transfer barrier in the heterostructure is small, allowing for rapid transport and separation of photoinduced charges, and thus has good photocatalytic activity.

[0069] It should be noted that the above embodiments are only some of the preferred embodiments of the present invention, and not all of them. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, characterized in that: It includes the following steps: (1) Melamine was calcined at high temperature for 4 h under nitrogen atmosphere to obtain g-C3N4; (2) preparing a copper salt methanol solution, slowly adding the g-C3N4 obtained in step (1) thereto while stirring, stirring and mixing evenly, heating to 65°C and reflux reaction for 3-4 hours, collecting the solid precipitate after the reaction, vacuum drying, and transferring it to a tube furnace, and calcining it at high temperature for 4 hours under a nitrogen atmosphere to obtain Cu-doped g-C3N4; (3) preparing a methanol solution of phosphate, slowly adding the g-C3N4 obtained in step (1) thereto while stirring, stirring and mixing evenly, heating to 65°C and reflux reaction for 3-4 hours, collecting the solid precipitate after the reaction, vacuum drying, and transferring it to a tube furnace, and calcining it at high temperature for 4 hours under a nitrogen atmosphere to obtain P-doped g-C3N4; (4) The Cu-doped g-C3N4 and P-doped g-C3N4 obtained in the above steps were mixed according to the mass ratio and mechanically ground for 30 min. The mixture was then placed in a nitrogen atmosphere and calcined at high temperature for 2 h. After naturally cooling to room temperature, the product was washed with methanol three times and then vacuum dried at 80 °C for 24 h to obtain copper and phosphorus co-doped g-C3N4; (5) Hexachlorocyclotriphosphazene, trithiocyanate, acetonitrile and dimethyl sulfoxide were added to a hydrothermal reactor in proportion, first subjected to ultrasonic treatment for 1 hour, then heated to 180°C and kept warm for 24-28 hours. After the reaction was completed, the product was naturally cooled and washed with deionized water, acetonitrile and ethanol in sequence. After washing, it was vacuum dried at 60°C for 24 hours to obtain a phosphazene polymer; (6) The phosphazene polymer obtained in step (5) and the copper and phosphorus co-doped g-C3N4 obtained in step (4) are mixed in a mass ratio, and mechanically ground for 30 minutes to obtain the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material.

2. The method for preparing the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 1, characterized in that: The specific method of high temperature calcination in step (1) is: heating to 500°C at a rate of 10°C / min and then calcining for 4h.

3. The method for preparing the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 1, characterized in that: The concentration of the copper salt methanol solution in step (2) is 0.1-0.2 mol / L; the usage ratio of the copper salt methanol solution to g-C3N4 is 100 ml: 2-3 g; and the copper salt is copper chloride or copper nitrate.

4. The method for preparing the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 1, characterized in that: The concentration of the methanol solution of phosphate in step (3) is 0.1-0.2 mol / L; the usage ratio of the methanol solution of phosphate to g-C3N4 is 100 ml: 2-3 g; the phosphate is sodium hypophosphite or sodium phosphite.

5. The method for preparing the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 1, characterized in that: The specific method of high-temperature calcination in step (2) and step (3) is: heating to 520°C at a rate of 10°C / min and then calcining for 4h.

6. The method for preparing the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 1, characterized in that: In the step (4), the mass ratio of Cu-doped g-C3N4 to P-doped g-C3N4 is 700-800:1; the specific method of the high-temperature calcination is: heating to 300°C at a rate of 10°C / min and then calcining for 2h.

7. The method for preparing the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 1, characterized in that: In the step (5), the usage ratio of hexachlorocyclotriphosphazene, trithiocyanate, acetonitrile and dimethyl sulfoxide is 1 mol: 3 mol: 25-30 L: 1.5 L.

8. The method for preparing the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 1, characterized in that: In the step (6), the mass ratio of the phosphazene polymer to the copper and phosphorus co-doped g-C3N4 is 6-8:

3.

9. A phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material, characterized in that: The invention is obtained by adopting the preparation method described in any one of claims 1 to 8.

10. Use of the phosphazene polymer / copper and phosphorus co-doped g-C3N4 heterojunction photocatalytic material according to claim 9 in CO2 reduction to CH4.

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