A nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst and its preparation method

Abstract

This invention relates to a nickel sulfide / cadmium sulfide / sulfur-doped graphitic carbon nitride dual heterojunction composite photocatalyst and its preparation method, belonging to the field of photocatalysis technology. S-doped g-C3N4 nanosheet powder is mixed uniformly with nickel chloride hexahydrate, cadmium acetate, thioacetamide, hexadecyltrimethylammonium bromide, and polypyrrole in ammonia water. The molar ratio of nickel chloride hexahydrate, cadmium acetate, and thioacetamide is (1-5):(1-6):(2-12) to obtain a precursor solution. The precursor solution is then heated at 120-180 °C to obtain a reaction solution. The precipitate in the reaction solution is washed and dried, effectively promoting the separation and transport of photogenerated charges. Furthermore, the dual heterojunction provides a rapid charge transfer pathway for the rapid migration of electrons and holes during the catalytic process, thereby achieving the goal of accelerating the separation of photogenerated charges and ultimately significantly improving the photocatalytic hydrogen evolution activity of g-C3N4.

Description

Technical Field

[0001] This invention belongs to the field of photocatalysis technology, specifically relating to a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst and its preparation method. Background Technology

[0002] Solar energy is an inexhaustible energy source, widely used in various forms such as photothermal conversion, solar photovoltaic power generation, and photochemical reactions. In recent years, due to persistent energy shortages and environmental crises, the development of environmentally friendly photocatalytic semiconductors has become a hot topic. Therefore, the photochemical utilization of solar energy has emerged and continues to receive increasing attention. This mainly includes three aspects: photocatalytic reduction of CO2, photocatalytic water splitting into H2, and photocatalytic degradation of organic pollutants. Among these, the photocatalytic degradation of organic pollutants is of great significance for solving environmental pollution problems. Titanium dioxide (TiO2) is a well-known photocatalytic semiconductor material with a wide band gap (3.2 eV), allowing it to utilize only about 4% of the ultraviolet light in the entire solar spectrum. Therefore, the performance of TiO2 cannot yet meet the needs of industrial applications of solar energy utilization, and the development of new, highly efficient photocatalytic systems is still underway.

[0003] In recent years, graphite-like carbon nitride materials (g-C3N4) have been considered a novel metal-free semiconductor material due to their excellent semiconductor properties and suitable band gap (2.69 eV). g-C3N4 exhibits excellent photocatalytic performance in the photocatalytic splitting of water for hydrogen evolution and the photocatalytic degradation of organic pollutants under visible light. The most common method for preparing g-C3N4 is through thermal polycondensation by directly heating different organic precursors. This method is simple to operate and suitable for mass production. Meanwhile, the main precursors for preparing g-C3N4 include dicyandiamide, melamine, urea, and thiourea, which are much cheaper than the raw materials for preparing other photocatalysts. However, due to the high recombination rate of photogenerated electron-hole pairs, the photocatalytic efficiency of pure g-C3N4 remains limited. Technicians have employed various methods to improve the photocatalytic activity of pure g-C3N4 and to suppress electron-hole recombination as much as possible.

[0004] Typically, constructing heterojunctions with other semiconductors is a promising strategy to extend the absorption edge of g-C3N4 into the visible light region and hinder the recombination of photogenerated carriers therein. However, this strategy also has negative effects, such as reducing the redox potential of photogenerated electron-hole pairs. Using suitable cocatalysts is widely considered a promising solution to enhance the photocatalytic hydrogen evolution capability of g-C3N4, as it provides additional reaction sites and accelerates the photocatalytic surface hydrogen evolution kinetics. Among these cocatalysts, NiS has attracted considerable attention due to its high abundance on Earth, low cost, and excellent H2 evolution performance. However, loading NiS onto the g-C3N4 surface did not significantly improve the photocatalytic hydrogen evolution performance of g-C3N4. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a graphite-like carbon nitride dual heterojunction composite photocatalyst of nickel sulfide / cadmium sulfide / sulfur doped and its preparation method. The composite photocatalyst material is composed of NiS / CdS and Sg-C3N4, wherein NiS, CdS and Sg-C3N4 all exhibit typical nanosheet structures, and NiS and CdS nanosheets self-assemble into a three-dimensional structure composed of stacked nanosheets, which are uniformly loaded on Sg-C3N4 nanosheets.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst includes the following steps:

[0008] S1, S-doped g-C3N4 nanosheet powder is mixed evenly with nickel chloride hexahydrate, cadmium acetate, thioacetamide, hexadecyltrimethylammonium bromide and polypyrrole in ammonia water. The molar ratio of nickel chloride hexahydrate, cadmium acetate and thioacetamide is (1-5):(1-6):(2-12) to obtain the precursor solution.

[0009] S2, the precursor solution is kept at 120-180 °C to obtain the reaction solution. The precipitate in the reaction solution is washed and dried to obtain a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst.

[0010] Preferably, the S-doped g-C3N4 nanosheet powder in S1 is obtained by the following process:

[0011] Melamine and thiocyanate were dissolved in an ethanol solution at a molar ratio of (1-10):(1-5) to obtain a mixture. The mixture was then evaporated to dryness and calcined at 500-650 °C for 2-6 h under an inert atmosphere. Finally, it was ground to obtain S-doped g-C3N4 nanosheet powder.

[0012] Furthermore, in the ethanol solution, the volume ratio of ethanol to ultrapure water is (0.1-2):(2-5). When the mixture is calcined, the temperature is increased from room temperature at a rate of 2-8 °C / min, and after reaching 500-650 °C, it is calcined at 500-650 °C for 2-6 h.

[0013] Preferably, in S1, the molar ratio of hexadecyltrimethylammonium bromide, polypyrrole, and nickel chloride hexahydrate is (1-3):(0.001-0.05):(1-5).

[0014] Preferably, in S1, the mass ratio of nickel chloride hexahydrate to S-doped g-C3N4 nanosheet powder is (0.01-0.5):(1-10).

[0015] Preferably, in S1, the ratio of S-doped g-C3N4 nanosheet powder to ammonia water is (1-10) g:(40-60) mL, wherein the ammonia water is obtained by diluting analytical grade ammonia water with ultrapure water, and the volume ratio of analytical grade ammonia water to ultrapure water is (1-2):(2-9).

[0016] Preferably, in step S1, nickel chloride hexahydrate, cadmium acetate, thioacetamide, hexadecyltrimethylammonium bromide, polypyrrole, and S-doped g-C3N4 nanosheet powder are first mixed, then added to ammonia water, stirred for 60-100 min, sonicated for 60-150 min, and finally stirred for 30-60 min to obtain the precursor solution.

[0017] Preferably, in step S2, the precursor solution is kept at 120-180 °C for 4-8 h to obtain the reaction solution.

[0018] Preferably, in step S2, the reaction solution is first centrifuged, then washed 3-5 times with deionized water and anhydrous ethanol respectively to obtain a precipitate, and then the precipitate is dried at 60-80 °C for 12-24 h to obtain a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst.

[0019] A nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst prepared by any one of the above methods, wherein cadmium sulfide and nickel sulfide self-assemble into a three-dimensional structure composed of stacked nanosheets, which is uniformly loaded on sulfur-doped graphite-like carbon nitride nanosheets.

[0020] Compared with the prior art, the present invention has the following beneficial technical effects:

[0021] This invention discloses a method for preparing a nickel sulfide / cadmium sulfide / sulfur-doped graphite-phase carbon nitride dual heterojunction composite photocatalyst. The non-metallic element sulfur doping of -g-C3N4 effectively optimizes the band structure of carbon nitride, thereby significantly improving the hydrogen evolution activity of the photocatalyst. The catalyst is then prepared using a one-step hydrothermal method at 120-180 °C in ammonia water. CTAB, as a surfactant, enhances the interfacial interactions between NiS, CdS, and Sg-C3N4. CTAB's unique long-chain structure effectively regulates the morphology, preventing the aggregation of NiS and CdS and significantly increasing the active surface area. Polypyrrole enhances the intrinsic conductivity of NiS / CdS / Sg-C3N4 and, through its own structural complexation with NiS and CdS, strengthens the heterojunction. Alkaline solutions can adjust the pH of the precursor solution. Weak alkalinity does not damage the structure of Sg-C3N4 itself, providing some protection. Simultaneously, the gas generated from ammonia decomposition can effectively regulate the morphology of the composite material during the hydrothermal reaction, promoting the formation of multidimensional morphologies. The conditions are simple and easy to control, resulting in low production costs. This yields a composite material with a dual heterostructure, synthesized in one step, providing a good heterostructure interface. This structure can significantly enhance the photogenerated charge separation efficiency of the catalyst. CdS and NiS self-assemble into a three-dimensional structure composed of stacked nanosheets, uniformly loaded onto Sg-C3N4 nanosheets, forming a three-dimensional nanosheet stacked structure. This structure can enhance the specific surface area of ​​the NiS / CdS / Sg-C3N4 composite photocatalyst, effectively promoting the separation and transport of photogenerated charges. Furthermore, the dual heterostructure interface provides a rapid charge transfer pathway for the rapid migration of electrons and holes during catalysis, thereby accelerating the separation of photogenerated charges and ultimately significantly improving the photocatalytic hydrogen evolution activity of Sg-C3N4.

[0022] Furthermore, solid-state sintering can yield porous S-doped g-C3N4 nanosheets, which can effectively increase their specific surface area and active sites during photocatalytic hydrogen evolution reaction. Attached Figure Description

[0023] Figure 1 The image shows the X-ray diffraction pattern of NiS / CdS / Sg-C3N4 prepared in Example 1 of this invention.

[0024] Figure 2 The image shows the scanning pattern of NiS / CdS / Sg-C3N4 prepared in Example 1 of this invention at 1 μm.

[0025] Figure 3 The image shows a TEM image of NiS / CdS / Sg-C3N4 prepared in Example 1 of this invention at 20 nm.

[0026] Figure 4The hydrogen evolution performance of Sg-C3N4, CdS / Sg-C3N4 and NiS / CdS / Sg-C3N4 prepared in Example 1 of this invention under visible light for four hours is shown in the graph. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0028] This invention discloses a method for preparing a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst, comprising the following steps:

[0029] Step 1: S-doped g-C3N4 nanosheets are obtained by one-step calcination in a tube furnace.

[0030] First, the precursor was prepared using an oil bath method, specifically as follows:

[0031] Melamine and cyanuric acid were added sequentially to a beaker containing an ethanol solution in a molar ratio of (1-10):(1-5), wherein the volume ratio of anhydrous ethanol to ultrapure water was (0.1-2):(2-5). The mixture was stirred for 2-6 hours, and then heated in an oil bath at 90-120 ℃. After the solution was completely evaporated, a mixed solid A was obtained.

[0032] Step 2: Solid A is spread evenly in a ceramic boat and calcined in a tube furnace under an inert atmosphere at a temperature of 500-650 °C for 2-6 h. The temperature is increased from room temperature at a rate of 2-8 °C / min. S doping occurs during the formation of carbon nitride. The temperature of 500-650 °C ensures that melamine and trithiocyanate can be completely polymerized, thus ensuring the successful preparation of S-doped carbon nitride.

[0033] Step 3: After calcination, the sample is naturally cooled in the furnace and ground in a mortar for 40-120 min to obtain yellow powder B, i.e., flake-shaped S-doped g-C3N4.

[0034] Step 4: Prepare nickel chloride hexahydrate, cadmium acetate, thioacetamide, hexadecyltrimethylammonium bromide (CTAB), and polypyrrole in a molar ratio of (1-5):(1-6):(2-12):(1-3):(0.001-0.05). Add powder B and mix. The mass ratio of nickel chloride hexahydrate to powder B is (0.01-0.5):(1-10). Add 40-60 mL of ammonia water (corresponding to 1 g of powder B), which is obtained by diluting analytical grade ammonia water with ultrapure water. The volume ratio of ammonia water (AR) to ultrapure water is (1-2):(2-9). Stir on a magnetic stirrer for 60-100 min, then sonicate for 60-150 min, and finally stir for 30-60 min. Then quickly pour the mixture into the lining of a 100 mL reaction vessel.

[0035] Step 5: Set the oven operating temperature to 120-180 °C. Once the oven temperature reaches the set temperature, place the sealed reactor inside and maintain the temperature for 4-8 hours. CTAB, as a surfactant, enhances the interfacial interactions between NiS, CdS, and Sg-C3N4. CTAB's unique long-chain structure effectively regulates morphology, preventing the aggregation of NiS and CdS and significantly increasing the active surface area. Polypyrrole enhances the intrinsic conductivity of NiS / CdS / Sg-C3N4 and, through its own structure, complexes NiS and CdS, reinforcing the heterojunction. The alkaline solution adjusts the pH; weak alkalinity does not damage the structure of Sg-C3N4, providing some protection. Simultaneously, the gas generated by ammonia decomposition effectively regulates the morphology of the composite material during the hydrothermal reaction, promoting the formation of multidimensional morphologies. After the oven temperature drops to room temperature, remove the reactor for cooling. The cooled reaction solution is poured out and centrifuged, and then washed 3-5 times with deionized water and anhydrous ethanol respectively to obtain a precipitate. The precipitate is then placed in an oven at 60-80 °C for 12-24 h to obtain solid C, which is the NiS / CdS / Sg-C3N4 photocatalyst.

[0036] Example 1:

[0037] This invention discloses a method for preparing a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst, comprising the following steps:

[0038] Step 1: S-doped g-C3N4 nanosheets can be obtained by one-step calcination in a tube furnace. First, the precursor is prepared using an oil bath method. Specifically, melamine and trithiocyanate are added sequentially to a beaker containing an ethanol solution at a molar ratio of 8:3, where the volume ratio of anhydrous ethanol to ultrapure water is 1:4. The mixture is then stirred for 5 hours. Finally, the mixed solution is heated in an oil bath at 100 °C until the solution is completely evaporated to obtain solid A.

[0039] Step 2: Spread solid A evenly in a ceramic boat and calcine it in a tube furnace under an inert atmosphere. The calcination temperature is 500 °C, the holding time is 6 h, and the heating rate is 5 °C / min.

[0040] Step 3: After calcination, the sample is naturally cooled in the furnace and ground in a mortar for 60 min to obtain yellow powder B, namely flake-shaped S-doped g-C3N4.

[0041] Step 4: Mix nickel chloride hexahydrate, cadmium acetate, thioacetamide, hexadecyltrimethylammonium bromide (CTAB), and polypyrrole in a molar ratio of 2:3:5:1:0.01. Then add powder B and mix. The mass ratio of nickel chloride hexahydrate to powder B is 0.02:5. Add 50 mL of ammonia water (corresponding to 1 g of powder B), where the volume ratio of ammonia water (AR) to ultrapure water is 1:9. Stir on a magnetic stirrer for 90 min, then sonicate for 60 min, and finally stir for 30 min. Then quickly pour the mixture into the lining of a 100 mL reaction vessel.

[0042] Step 5: Set the oven operating temperature to 140 °C. Once the oven temperature reaches the set temperature, place the sealed reaction vessel inside and maintain the temperature for 5 hours. After the oven temperature drops to room temperature, remove the reaction vessel and cool it. Pour out the cooled reaction solution, centrifuge, and wash it three times each with deionized water and anhydrous ethanol to obtain a precipitate. Then, place the precipitate in an oven at 80 °C for 14 hours to dry, yielding solid C, which is the NiS / CdS / Sg-C3N4 photocatalyst.

[0043] The photocatalytic effect of NiS / CdS / Sg-C3N4 was tested using a LabSolar 6A fully automated photocatalytic evaluation system (mainly used for photocatalysis, photoelectrocatalysis, electrocatalysis, etc.). The specific test procedure was as follows: 30 mg of the composite photocatalyst and 15 mL of triethanolamine were weighed and sequentially placed into a glass reaction vessel containing 85 mL of deionized water. The mixture was stirred and irradiated with a 300 W xenon lamp for 4 hours.

[0044] Example 2:

[0045] This invention discloses a method for preparing a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst, comprising the following steps:

[0046] Step 1: S-doped g-C3N4 nanosheets can be obtained by one-step calcination in a tube furnace. First, the precursor is prepared using an oil bath method. Specifically, melamine and trithiocyanate are added sequentially to a beaker containing an ethanol solution in a 5:5 molar ratio, where the volume ratio of anhydrous ethanol to ultrapure water is 0.5:3. The mixture is then stirred for 6 hours. Finally, the mixed solution is heated in an oil bath at 90 ℃ until the solution is completely evaporated to obtain solid A.

[0047] Step 2: Spread solid A evenly in a ceramic boat and calcine it in a tube furnace under an inert atmosphere. The calcine temperature is 550 °C, the holding time is 4 h, and the heating rate is 5 °C / min.

[0048] Step 3: After calcination, the sample is naturally cooled in the furnace and ground in a mortar for 60 min to obtain yellow powder B, namely flake-shaped S-doped g-C3N4.

[0049] Step 4: Mix nickel chloride hexahydrate, cadmium acetate, thioacetamide, hexadecyltrimethylammonium bromide (CTAB), and polypyrrole in a molar ratio of 1:4:5:2:0.02. Then add powder B and mix. The mass ratio of nickel chloride hexahydrate to powder B is 0.1:4. Add 60 mL of ammonia water (corresponding to 1 g of powder B), where the volume ratio of ammonia water (AR) to ultrapure water is 1:8. Stir on a magnetic stirrer for 60 min, then sonicate for 120 min, and finally stir for another 60 min. Then quickly pour the mixture into the lining of a 100 mL reaction vessel.

[0050] Step 5: Set the oven operating temperature to 160 °C. Once the oven temperature reaches the set temperature, place the sealed reaction vessel inside and maintain the temperature for 4 hours. After the oven temperature drops to room temperature, remove the reaction vessel and cool it. Pour out the cooled reaction solution, centrifuge, and wash it with deionized water and anhydrous ethanol at least three times each to obtain a precipitate. Then, place the precipitate in an oven at 70 °C for 10 hours to obtain solid C, which is the NiS / CdS / Sg-C3N4 photocatalyst.

[0051] The photocatalytic effect of NiS / CdS / Sg-C3N4 was tested using a LabSolar 6A fully automated photocatalytic evaluation system. The specific test procedure was as follows: 30 mg of the composite photocatalyst and 15 mL of triethanolamine were weighed and sequentially placed into a glass reaction vessel containing 85 mL of deionized water. The mixture was stirred and irradiated with a 300 W xenon lamp for 4 hours.

[0052] Example 3:

[0053] This invention discloses a method for preparing a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride dual heterojunction composite photocatalyst, comprising the following steps:

[0054] Step 1: S-doped g-C3N4 nanosheets can be obtained by one-step calcination in a tube furnace. First, the precursor is prepared using an oil bath method. Specifically, melamine and trithiocyanate are added sequentially to a beaker containing an ethanol solution at a molar ratio of 6:4, where the volume ratio of anhydrous ethanol to ultrapure water is 1:5. The mixture is then stirred for 4 hours. Finally, the mixed solution is heated in an oil bath at 120 °C until the solution is completely evaporated to obtain solid A.

[0055] Step 2: Spread solid A evenly in a porcelain boat and calcine it in a tube furnace under an inert atmosphere. The calcination temperature is 600 °C, the holding time is 2 h, and the heating rate is 2 °C / min.

[0056] Step 3: After calcination, the sample is naturally cooled in the furnace and ground in a mortar for 50 min to obtain yellow powder B, namely flake-shaped S-doped g-C3N4.

[0057] Step 4: Mix nickel chloride hexahydrate, cadmium acetate, thioacetamide, hexadecyltrimethylammonium bromide (CTAB), and polypyrrole in a molar ratio of 2:5:7:3:0.05. Then add powder B and mix. The mass ratio of nickel chloride hexahydrate to powder B is 0.3:8. Add 60 mL of ammonia water (corresponding to 1 g of powder B), where the volume ratio of ammonia water (AR) to ultrapure water is 2:5. Stir on a magnetic stirrer for 60 min, then sonicate for 120 min, and finally stir for another 60 min. Then quickly pour the mixture into the lining of a 100 mL reaction vessel.

[0058] Step 5: Set the oven operating temperature to 150 °C. Once the oven temperature reaches the set temperature, place the sealed reaction vessel inside and maintain the temperature for 6 hours. After the oven temperature drops to room temperature, remove the reaction vessel and cool it. Pour out the cooled reaction solution, centrifuge, and wash it three times each with deionized water and anhydrous ethanol to obtain a precipitate. Then, place the precipitate in an oven at 60 °C for 15 hours to dry, yielding solid C, which is the NiS / CdS / Sg-C3N4 photocatalyst.

[0059] The photocatalytic effect of NiS / CdS / Sg-C3N4 was tested using a LabSolar 6A fully automated photocatalytic evaluation system. The specific test procedure was as follows: 30 mg of the composite photocatalyst and 15 mL of triethanolamine were weighed and placed sequentially into a glass reaction vessel containing 85 mL of deionized water. The mixture was then irradiated with a 300 W xenon lamp for 4 hours.

[0060] Figure 1The horizontal axis represents the 2θ angle, and the vertical axis represents the diffraction peak intensity. At 27°, they correspond to the (002) crystal plane of g-C3N4, and NiS / CdS / Sg-C3N4 can accurately correspond to NiS PDF#12-0041 and CdS PDF#41-1049, indicating that the NiS / CdS / Sg-C3N4 photocatalyst has been successfully prepared.

[0061] Figure 2 Among them, NiS, CdS and Sg-C3N4 all exhibit typical nanosheet structures. Sg-C3N4 exhibits a porous nanosheet structure, while NiS and CdS nanosheets also self-assemble into unique three-dimensional structures made up of stacked nanosheets, uniformly loaded on Sg-C3N4 nanosheets.

[0062] from Figure 3 Grain boundaries and phase interfaces can be observed inside the NiS / CdS / Sg-C3N4 photocatalyst. The lattice spacing of 0.186 nm corresponds to the (131) crystal plane of NiS, while the lattice spacing of 0.356 nm corresponds precisely to the (100) crystal plane of CdS, which is consistent with the XRD results. Therefore, it can be determined that a large number of heterogeneous interfaces exist in the NiS / CdS / Sg-C3N4 photocatalyst, which is of great significance for the exposure of active sites and performance enhancement.

[0063] like Figure 4 As shown, the hydrogen evolution activity of Sg-C3N4 was significantly improved after loading with NiS / CdS. Furthermore, compared to CdS / Sg-C3N4 (without nickel chloride hexahydrate added in step 4 of Example 1), the NiS / CdS / Sg-C3N4 dual heterojunction catalyst achieved a hydrogen evolution rate of 2410 µmol / g within four hours.

Claims

1. A method for preparing a nickel sulfide / cadmium sulfide / sulfur-doped graphitic phase carbon nitride double heterojunction composite photocatalyst, characterized in that, Comprising the following steps: S1, melamine and thiocyanic acid are dissolved in an ethanol solution at a molar ratio of (1-10):(1-5) to obtain a mixed solution, then the mixture after the mixed solution is evaporated to dryness is calcined at 500-650 °C for 2-6 h under the protection of an inert atmosphere, and finally ground to obtain S-doped g-C3N4 nanosheet powder; The S-doped g-C3N4 nanosheet powder is mixed uniformly with nickel chloride hexahydrate, cadmium acetate, thioacetamide, cetyltrimethylammonium bromide, and polypyrrole in ammonia water, the molar ratio of nickel chloride hexahydrate, cadmium acetate, and thioacetamide is (1-5):(1-6):(2-12), the molar ratio of cetyltrimethylammonium bromide, polypyrrole, and nickel chloride hexahydrate is (1-3):(0.001-0.05):(1-5), the mass ratio of nickel chloride hexahydrate to S-doped g-C3N4 nanosheet powder is (0.01-0.5):(1-10), and the ratio of S-doped g-C3N4 nanosheet powder to ammonia water is (1-10) g:(40-60) mL, wherein the ammonia water is obtained by diluting analytical pure ammonia water with ultrapure water, and the volume ratio of analytical pure ammonia water to ultrapure water is (1-2):(2-9), to obtain a precursor solution; S2, the precursor solution is incubated at 120-180 °C for 4-8 h to obtain a reaction solution, the precipitate in the reaction solution is washed and dried to obtain a nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride double heterojunction composite photocatalyst.

2. The method for preparing the nickel sulfide / cadmium sulfide / sulfur-doped graphitic-phase carbon nitride double heterojunction composite photocatalyst according to claim 1, characterized in that, In the ethanol solution, the volume ratio of ethanol to ultrapure water is (0.1-2):(2-5), and when the mixture is calcined, the temperature is increased from room temperature at a rate of 2-8 °C / min, and after reaching 500-650 °C, the calcination is carried out at 500-650 °C for 2-6 h.

3. The method for preparing nickel sulfide / cadmium sulfide / sulfur-doped graphitic-phase carbon nitride double heterojunction composite photocatalyst according to claim 1, characterized in that, In S1, the nickel chloride hexahydrate, cadmium acetate, thioacetamide, cetyltrimethylammonium bromide, polypyrrole, and S-doped g-C3N4 nanosheet powder are first mixed, then added to the ammonia water, stirred for 60-100 min, ultrasonicated for 60-150 min, and finally stirred for 30-60 min to obtain the precursor solution.

4. The method for preparing the nickel sulfide / cadmium sulfide / sulfur-doped graphitic-phase carbon nitride double heterojunction composite photocatalyst according to claim 1, characterized in that, In S2, the reaction solution is first centrifuged, then washed with deionized water and anhydrous ethanol for 3-5 times respectively to obtain a precipitate, and the precipitate is dried at 60-80 °C for 12-24 h to obtain the nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride double heterojunction composite photocatalyst.

5. A nickel sulfide / cadmium sulfide / sulfur-doped graphite-like carbon nitride double heterojunction composite photocatalyst prepared by the method of any one of claims 1-4.

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