An oxygen-doped molybdenum disulfide-silver-loaded silver sulfide-carbon nitride composite photocatalyst, a preparation method and application thereof

Abstract

The application discloses an oxygen-doped molybdenum disulfide-silver-loaded / silver sulfide-carbon nitride composite photocatalyst and a preparation method and application thereof. The oxygen-doped molybdenum disulfide-silver-loaded / silver sulfide-carbon nitride composite photocatalyst is a 1T-2HO-MoS2@Ag / Ag2S-TCN composite photocatalyst in chemical composition, and the mass ratio of molybdenum disulfide and silver-loaded / silver sulfide-carbon nitride in the composite photocatalyst is 8%-15%. The silver-loaded carbon nitride is in-situ synthesized through a hydrothermal reaction and high-temperature calcination, and N,N-dimethylformamide is used as a solvent to in-situ synthesize the oxygen-doped molybdenum disulfide-silver-loaded / silver sulfide-carbon nitride composite photocatalyst with 1T crystal faces and 2H crystal faces, so that the composite photocatalyst can efficiently catalyze the reduction of hexavalent chromium. The preparation method is simple in process, stable in structure and high in product purity. The composite photocatalyst can efficiently reduce hexavalent chromium under acidic and near-neutral conditions.

Description

Technical Field

[0001] This invention relates to the field of inorganic photocatalytic materials technology, and in particular, to an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst, its preparation method, and its application. Background Technology

[0002] The electroplating and textile industries generate large amounts of chromium (IV)-containing wastewater during production processes, seriously threatening human and environmental safety. Photocatalysis, an advanced oxidation technology, offers advantages such as being green and efficient, making it a crucial technology for wastewater treatment. Carbon nitride, as a non-metallic semiconductor material, is widely favored by researchers due to its suitable band gap, high chemical and thermal stability, environmental friendliness, and low cost. However, the rapid recombination of photogenerated electrons and holes limits its application. Molybdenum disulfide, with its abundant active groups, is a highly efficient cocatalyst, but its poor conductivity limits its photocatalytic activity, making it difficult to achieve efficient conversion of hexavalent chromium in wastewater. Developing a highly efficient composite photocatalyst with high photoresponse efficiency and effective inhibition of photogenerated electron-hole recombination is crucial.

[0003] Therefore, the industry urgently needs a new type of photocatalyst to achieve efficient reduction of hexavalent chromium. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst, its preparation method, and its application. This invention utilizes melamine, cyanuric acid, and silver nitrate through hydrothermal reaction and high-temperature calcination, using N,N-dimethylformamide (DMF) as a solvent to synthesize an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst with 1T and 2H crystal planes in situ. This forms a composite structure of oxygen-doped molybdenum disulfide, silver, silver sulfide, and carbon nitride with 1T and 2H crystal planes, which can effectively improve visible light response, slow down the recombination of holes and electron pairs, and has the characteristics of high catalytic efficiency and good stability. It can achieve efficient reduction of hexavalent chromium under acidic or near-neutral environments.

[0005] To achieve the above objectives, the present invention provides a method for preparing an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst, comprising the following steps:

[0006] Step 1: Dissolve nitrogen-containing compounds and silver nitrate in water by heating, then carry out the first hydrothermal reaction in an autoclave to obtain a hydrothermal intermediate product, and then calcine it in an inert atmosphere to obtain silver-loaded carbon nitride.

[0007] Step 2: Mix the prepared silver-loaded carbon nitride and N,N-dimethylformamide to obtain solution A; mix ammonium molybdate tetrahydrate, thiourea and N,N-dimethylformamide to obtain solution B; slowly add solution B dropwise to solution A, and carry out a second hydrothermal reaction in an autoclave to obtain the oxygen-doped molybdenum disulfide-silver-loaded / silver sulfide carbon nitride composite photocatalyst.

[0008] Furthermore, in step two, the mass ratio of silver-loaded carbon nitride to N,N-dimethylformamide is 1:70-90; the mass ratio of silver-loaded carbon nitride, ammonium molybdate tetrahydrate, and thiourea is 1:0.3-1.2:0.6-2.2.

[0009] Furthermore, in step one, the nitrogen-containing compound is melamine and cyanuric acid; the mass ratio of melamine, cyanuric acid and silver nitrate is 25-35:25-35:1.

[0010] Furthermore, in step one, the reaction temperature for the first hydrothermal reaction is 180-220℃, and the time is 10-12h; the heating rate to the temperature of the first hydrothermal reaction is 1-3℃ / min.

[0011] Furthermore, in step one, the calcination treatment is carried out in a nitrogen atmosphere, with a heating rate of 3-5℃ / min, a calcination temperature of 500-600℃, and a time of 3-5h.

[0012] Furthermore, in step two, the reaction temperature for the second hydrothermal reaction is 160-200℃, and the time is 18-24h; the heating rate to the second hydrothermal reaction temperature is 1-3℃ / min.

[0013] This invention also provides an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst, prepared by the above method. The chemical composition of the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst is 1T-2HO-MoS2@Ag / Ag2S-TCN composite photocatalyst, wherein the mass ratio of oxygen-doped molybdenum disulfide 1T-2HO-MoS2 and silver-supported / silver sulfide carbon nitride Ag / Ag2S-TCN in the composite photocatalyst is 8%-15%.

[0014] This invention also provides the application of the above-mentioned oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst or the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst prepared by the above method in the reduction of hexavalent chromium in wastewater under acidic and near-neutral conditions.

[0015] Furthermore, the application includes the following steps: adding the oxygen-doped molybdenum disulfide-silver-carrying / silver sulfide carbon nitride composite photocatalyst to wastewater, carrying out a photocatalytic reaction under visible light, and completing the reduction of hexavalent chromium under acidic and near-neutral conditions. The amount of the oxygen-doped molybdenum disulfide-silver-carrying / silver sulfide carbon nitride composite material added to the wastewater is 0.2 g / L-1.2 g / L, and the pH value of the photocatalytic reaction is 2-6.

[0016] Furthermore, in the aforementioned application, the visible light source is a 300W-500W xenon lamp, the distance between the xenon lamp and the surface of the wastewater is 14cm-16cm, the photocatalytic reaction time is 8min-15min, and the concentration of hexavalent chromium in the wastewater is 5mg / L-40mg / L.

[0017] The present invention has the following beneficial effects:

[0018] 1. The present invention provides a method for preparing an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst. First, silver-supported carbon nitride is prepared by a first hydrothermal reaction and high-temperature calcination using melamine, cyanuric acid, and silver nitrate, and dispersed in DMF to obtain solution A. Ammonium molybdate tetrahydrate, thiourea, and N,N-dimethylformamide are mixed to obtain mixed solution B. By stirring, mixed solution B is well dispersed in A. Ammonium molybdate tetrahydrate and thiourea in solution B undergo a second hydrothermal reaction in DMF to generate oxygen-doped molybdenum disulfide, which condenses on the silver-supported carbon nitride. At the same time, a small amount of silver on the silver-supported carbon nitride is converted into silver sulfide, ultimately forming an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst.

[0019] First, the silver-loaded nitrogen carbide prepared in this invention through a first hydrothermal reaction and high-temperature calcination using melamine, cyanuric acid, and silver nitrate has a large specific surface area, providing a good support for the in-situ synthesis of oxygen-doped molybdenum disulfide. Silver-loaded nitrogen carbide is also a photocatalyst with photocatalytic effects, but its photocatalytic reduction ability is weak due to its low conduction band and rapid recombination of photogenerated electrons and holes. This invention uses an in-situ synthesis method to form a composite material, specifically, oxygen-doped molybdenum disulfide is directly generated on the surface of the silver-loaded nitrogen carbide, and some silver can be converted into silver sulfide during the conversion process to form the composite material. During photocatalysis, the heterojunction formed by the two materials is used to achieve the separation of holes and electrons, improving the photocatalytic effect. Furthermore, ammonium molybdate tetrahydrate and thiourea in solution B undergo a second hydrothermal reaction in DMF to generate oxygen-doped molybdenum disulfide, which condenses on the silver-loaded nitrogen carbide. Simultaneously, a small amount of silver on the silver-loaded nitrogen carbide is converted into silver sulfide. Silver sulfide, due to its excellent light absorption and photoelectric properties, promotes the absorption of visible and infrared light and charge separation and migration, which can effectively improve the photocatalytic activity of composite catalysts.

[0020] Secondly, the method of this invention uses N,N-dimethylformamide (DMF) as a solvent to synthesize oxygen-doped molybdenum disulfide in situ, which improves the crystal structure of molybdenum disulfide (promotes the synthesis of the 1T crystal facet) and enhances the catalytic active sites. Therefore, the photocatalytic material provided by this invention has high catalytic efficiency and good stability. This is because DMF is beneficial for promoting the chemical synthesis of molybdenum disulfide (MoS2), which has two phases, 1T and 2H, corresponding to the octahedral metallic phase and the triangular prism semiconductor phase, respectively. Among these two phases, the 1T phase has superior conductivity and dense active sites, while the 2H phase has higher chemical stability and reduction ability, thanks to its stable chemical structure and lower conduction band. Furthermore, the heterogeneous structure resulting from the different crystal faces also increases the carrier density of the material and suppresses charge recombination.

[0021] Furthermore, this invention provides a method for preparing oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalysts, which is simple in process, structurally stable, and produces products with high purity.

[0022] 2. The oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst provided by this invention is chemically composed of a 1T-2H₂O-MoS₂@Ag / Ag₂S-TCN composite photocatalyst, wherein the mass ratio of 1T-2H₂O-MoS₂ to Ag / Ag₂S-TCN in the composite photocatalyst is 8%-15%. Carbon nitride possesses a unique electronic structure and excellent visible light absorption capability. Simultaneously, silver, due to its surface plasmon resonance (SPR) effect and the formation of the Schottky barrier, acts as a co-catalyst, effectively improving the structure of carbon nitride and its response in the visible light range. Molybdenum disulfide, due to the different coordination of Mo and S, possesses a 1T crystal plane and a 2H crystal plane, with the 1T crystal plane exhibiting high conductivity and dense active sites, resulting in better catalytic activity. Compared to the 2.78 eV band gap of silver-supported nitrogen nitride (Ag-TCN), the photocatalytic material of this invention has a minimum band gap of 2.02 eV, exhibiting better light response, shorter reaction time, and higher efficiency. Furthermore, the oxygen-doped molybdenum disulfide-silver-carried / silver sulfide carbon nitride composite photocatalyst provided by this invention can effectively catalytically reduce hexavalent chromium to trivalent chromium under visible light irradiation, exhibiting extremely high catalytic efficiency and good stability in both acidic and near-neutral environments. Simultaneously, the silver-carried carbon nitride provides a large surface area, offering favorable conditions for the in-situ synthesis of oxygen-doped molybdenum disulfide and also for the formation of silver sulfide in the composite material. However, when the proportion of molybdenum disulfide is low, it cannot effectively suppress the rapid recombination of photogenerated electrons and holes, leading to a decrease in photocatalytic efficiency; while increasing the proportion of molybdenum disulfide can cause further agglomeration of the in-situ synthesized oxygen-doped molybdenum disulfide, thereby reducing the active sites of the material and leading to the failure of material synthesis. Therefore, the material ratio of the components in the prepared composite photocatalyst is also crucial.

[0023] 3. The oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst provided by this invention performs photocatalytic reduction of hexavalent chromium under visible light. This is because the photocatalytic material prepared by this invention has a low band gap, enabling it to respond under visible light and excite electron-hole pairs. The heterojunction of the composite material promotes the separation of electrons and holes, thus reducing hexavalent chromium. Under near-neutral conditions (pH=6), the reduction rate of hexavalent chromium provided by this invention can reach 94.6% after 30 minutes of reaction, and under acidic conditions (pH=2), the reduction rate can reach 99.4% after 10 minutes of reaction. This is because when the pH is acidic, it is conducive to promoting the separation of photogenerated electrons and holes, and the positively charged surface of the composite photocatalyst material is conducive to the adsorption of hexavalent chromium, promoting the photocatalytic reduction reaction. However, when the pH is alkaline, the trivalent chromium generated by photocatalysis forms chromium hydroxide under alkaline conditions and deposits on the catalyst, reducing the active sites of the catalyst and thus reducing the catalytic effect.

[0024] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0025] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0026] Figure 1 X-ray diffraction patterns of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 1-3;

[0027] Figure 2 The images shown are scanning electron microscope (SEM) images of the composite photocatalysts prepared in Example 1, Comparative Examples 1-3, and Comparative Example 5; wherein, Figure 2 (a) is a SEM image of the composite photocatalyst prepared in Comparative Example 1. Figure 2 (b) is a SEM image of the composite photocatalyst prepared in Comparative Example 2. Figure 2 Image (c) shows the SEM image of the composite photocatalyst prepared in Comparative Example 3. Figure 2 Image (d) is a SEM image of the composite photocatalyst prepared in Example 1. Figure 2 Image (e) is a SEM image of the composite photocatalyst prepared in Comparative Example 5.

[0028] Figure 3 The UV-Vis diffuse reflectance spectra of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 1-3 are shown below.

[0029] Figure 4The Tauc curves of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 1-3 are shown.

[0030] Figure 5 XPS spectra of the composite photocatalysts prepared in Example 1 and Comparative Example 1; wherein Figure 5 (a) shows the Ag 3d spectra of Example 1 and Comparative Example 1. Figure 5 (b) is the S2p spectrum of Example 1. Figure 5 (c) is the Mo 3d spectrum of Example 1. Figure 5 (d) is the O1s spectrum of Example 1;

[0031] Figure 6 The photocatalytic performance curves of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 2-4 under visible light irradiation with λ>420nm for the reduction of hexavalent chromium are shown.

[0032] Figure 7 The graph shows the effect of pH on the reduction of hexavalent chromium by the composite photocatalyst prepared in Example 1 under visible light irradiation with λ>420nm, after adjusting the pH to 2, 4, and 8 for wastewater containing hexavalent chromium with a pH of approximately 6.

[0033] Figure 8 The image shows the photocatalytic performance of the composite photocatalyst prepared in Example 1, which undergoes a four-cycle reduction of hexavalent chromium. Detailed Implementation

[0034] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered by the claims.

[0035] This invention provides a method for preparing an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst, comprising the following steps:

[0036] (1) Nitrogen-containing compounds and silver nitrate are dissolved in water and subjected to a first hydrothermal reaction in an autoclave. Then, they are calcined at high temperature in a tube furnace under a nitrogen atmosphere to obtain silver-loaded carbon nitride (Ag-TCN).

[0037] (2) Mix Ag-TCN and N,N-dimethylformamide (DMF) to obtain solution A;

[0038] (3) Ammonium molybdate tetrahydrate ((NH4)6Mo7O 24 Solution B is obtained by mixing *4H2O), thiourea (CH4N2S), and DMF;

[0039] (4) Slowly add solution B to solution A, stir, and then put it into a high-pressure reactor for a second hydrothermal reaction to obtain oxygen-doped molybdenum disulfide-silver-silver-carbon nitride composite photocatalyst.

[0040] This invention involves dissolving a nitrogen-containing compound and silver nitrate in water, conducting a first hydrothermal reaction in an autoclave, and then calcining at high temperature in a tube furnace under a nitrogen atmosphere to obtain Ag-TCN. In this invention, the nitrogen-containing compound is preferably melamine or cyanuric acid. This invention does not have specific requirements regarding the source of the nitrogen-containing compound and silver nitrate; commercially available products well-known in the art can be used. In a specific embodiment of this invention, the mixing operation is as follows: dissolving the nitrogen-containing compound and silver nitrate in deionized water at 60°C with vigorous stirring; the stirring rate is preferably 500-1000 rpm, more preferably 800 rpm, and the time is 50-80 min, more preferably 60 min. In this invention, the above mixture is placed in an autoclave for a first hydrothermal reaction. The temperature of the first hydrothermal reaction is preferably 180-220°C, more preferably 190-210°C, and most preferably 200°C; the time is preferably 10-12 h, more preferably 11-12°C, and most preferably 12 h. The heating rate (and the heating rate of the drying oven) to the temperature of the first hydrothermal reaction is preferably 1-3℃ / min, more preferably 2℃ / min; the time of the first hydrothermal reaction is calculated from the time the temperature is reached; then, the furnace is placed in a tube furnace under a nitrogen atmosphere for calcination. The heating rate of the tube furnace is preferably 3-5℃ / min, more preferably 5℃ / min, the calcination temperature is preferably 500-600℃, more preferably 550℃, and the calcination time is preferably 3-6h, more preferably 3-5℃, and most preferably 4h, to obtain silver-loaded carbon nitride.

[0041] In this invention, after the first hydrothermal reaction, the resulting hydrothermal reaction solution is preferentially cooled, filtered, and dried sequentially. The cooling method is not particularly demanding; methods well-known in the art can be used to cool to room temperature, such as natural cooling. Similarly, the filtration method is not particularly demanding; methods well-known in the art can be used. The drying temperature is preferably 50-70℃, more preferably 60℃, and the drying time is preferably 8-12 hours, more preferably 10 hours. The drying method is not particularly demanding; methods well-known in the art can be used to ensure the required temperature and time. After drying, a solid sample is obtained.

[0042] After calcination, the resulting solid is preferably cooled. The present invention does not have particular requirements for the cooling method; methods well known in the art, such as natural cooling, can be used. After cooling, silver-loaded carbon nitride is obtained.

[0043] After obtaining silver-loaded carbon nitride, this invention involves ultrasonically mixing the silver-loaded carbon nitride with DMF to obtain solution A. In this invention, the preferred mass ratio of silver-loaded carbon nitride to DMF is 1:60-100, more preferably 1:75-95, and most preferably 1:85. This invention does not have specific requirements regarding the source of the DMF; commercially available products well-known in the art can be used. Preferably, this invention involves adding DMF to solution A and ultrasonically mixing; the ultrasonic power is preferably 200-500W, more preferably 300W. This invention uses ultrasound to promote the mixing of silver-loaded carbon nitride in DMF, which is beneficial for subsequent reactions.

[0044] This invention mixes ammonium molybdate tetrahydrate, thiourea, and DMF to obtain solution B. This invention does not have specific requirements regarding the source of ammonium molybdate tetrahydrate, thiourea, and DMF; commercially available products well-known in the art can be used. In a specific embodiment of this invention, the mixing operation specifically involves dispersing ammonium molybdate tetrahydrate and thiourea into DMF under ultrasonication to obtain solution B. In this invention, the mass ratio of ammonium molybdate tetrahydrate, thiourea, and N,N-dimethylformamide is 1:1.5-3.0:20-30. The ultrasonic power is preferably 200-500W, more preferably 300W. Vigorous stirring in this invention promotes the mixing of ammonium molybdate tetrahydrate and thiourea in DMF, which is beneficial for subsequent reactions. Furthermore, the mass ratio of silver-loaded carbon nitride, ammonium molybdate tetrahydrate, and thiourea is 1:0.25-1.25:0.5-2.25. The volume ratio of solution A to solution B is 1:1.

[0045] Solution B is slowly added dropwise to solution A, stirred, and then placed in an autoclave for a second hydrothermal reaction to obtain an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst. In this invention, solution B is slowly added dropwise to solution A, preferably at a rate of 6-12 drops / min, more preferably 8-10 drops / min; the stirring rate is preferably 500-1000 rpm, more preferably 800 rpm, and the time is 20-50 min, more preferably 30 min. The temperature of the second hydrothermal reaction is preferably 160-200℃, more preferably 170-190℃, and most preferably 180℃; the time is preferably 18-24 h, more preferably 20-24℃, and most preferably 24 h. In this invention, the second hydrothermal reaction is preferably carried out in a polytetrafluoroethylene (PTFE) reactor; the temperature of the second hydrothermal reaction is preferably controlled by a drying oven; that is, the mixed solution is placed in the PTFE reactor, and then the PTFE reactor containing the mixed solution is placed in a drying oven for heating. In this invention, the heating rate (and the heating rate of the drying oven) to the hydrothermal reaction temperature is preferably 1-3 °C / min, more preferably 2 °C / min; the hydrothermal reaction time is calculated from the time the temperature required for the hydrothermal reaction is reached. Using N,N-dimethylformamide as a solvent, oxygen-doped molybdenum disulfide is generated in situ through a second hydrothermal reaction, and further forms a heterojunction with Ag-TCN. During the in-situ synthesis, a small amount of silver is converted to silver sulfide, ultimately yielding the oxygen-doped molybdenum disulfide-silver-carrying / silver-sulfide carbon nitride composite photocatalyst. Furthermore, the slow dropwise addition of solution B to solution A in this invention is mainly to ensure thorough mixing and to ensure that ammonium molybdate tetrahydrate and thiourea are uniformly dispersed in the solution. Adding solution A dropwise would make it difficult to ensure thorough mixing because silver-carrying carbon nitride is the main component, accounting for more than 80% of the synthesized material.

[0046] In this invention, after the second hydrothermal reaction, the resulting hydrothermal reaction solution is preferably subjected to sequential cooling, filtration, washing, and drying. The cooling method is not particularly important; methods well-known in the art, such as natural cooling, can be used. Similarly, the filtration method is not particularly important; methods well-known in the art can be used. The washing process involves sequentially using deionized water and ethanol. The number of washing cycles is not particularly important; the goal is to ensure the filter cake is thoroughly cleaned. The drying temperature is preferably 50-70°C, more preferably 60°C, and the drying time is preferably 8-12 hours, more preferably 10 hours. The drying method is not particularly important; methods well-known in the art can be used to ensure the required temperature and time. After drying, an oxygen-doped molybdenum disulfide-silver-carried / silver sulfide-carbon nitride composite photocatalyst is obtained.

[0047] This invention provides a method for preparing the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst described above. The preparation method provided by this invention is simple, and the prepared oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst has high purity.

[0048] This invention provides an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst prepared by the above method. In terms of chemical composition, the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst is a 1T-2HO-MoS2@Ag / Ag2S-TCN composite photocatalyst.

[0049] In this invention, the mass ratio of 1T-2H O-MoS2 and Ag / Ag2S-TCN in the composite photocatalyst is 4%-15%, preferably 10%. The oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst provided by this invention has extremely high catalytic efficiency and good stability.

[0050] This invention provides the application of the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst described in the above-described schemes, or the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst prepared by the methods described in the above-described schemes, in the catalytic reduction of hexavalent chromium. The oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst provided by this invention can effectively catalytically reduce hexavalent chromium to trivalent chromium under visible light irradiation, exhibiting extremely high catalytic efficiency under both acidic and near-neutral conditions, and good stability.

[0051] The following detailed description, in conjunction with embodiments, illustrates the oxygen-doped molybdenum disulfide-silver-carried / silver sulfide carbon nitride composite photocatalyst, its preparation method, and its applications. However, these descriptions should not be construed as limiting the scope of protection of this invention.

[0052] Example 1

[0053] (1) 3g of melamine, 3g of cyanuric acid, and 0.0956g of silver nitrate were dissolved in 60mL of deionized water under vigorous stirring. The solution was heated to 60℃ in a water bath and stirred for 20min to obtain a transparent solution. The solution was transferred to a 100mL autoclave and kept at 200℃ for 12 hours, then allowed to cool naturally to room temperature. The solid sample was filtered and dried, then transferred to a crucible and calcined in a tube furnace at 550℃ for 4h under a nitrogen atmosphere at a heating rate of 5℃ / min to obtain a yellow solid powder. The solid powder was washed with water and ethanol, and then dried in an oven at 60℃ for 12h to obtain silver-loaded carbon nitride solid powder.

[0054] (2) First, 400 mg Ag-TCN is dissolved in 35 mL LMF solution by ultrasound to obtain a homogeneous solution, which is solution A.

[0055] (3) Ammonium molybdate tetrahydrate ((NH4)6Mo7O 24 *4H2O (0.3088g) and thiourea (CH4N2S (0.5706g) were ultrasonically dispersed in 35ml of DMF to form solution B.

[0056] (4) Add solution B slowly to solution A at a rate of 10 drops / min and stir for 30 minutes to mix thoroughly. Then transfer the solution to a 100 mL autoclave and maintain it at 180°C for 24 hours, allowing it to cool naturally to room temperature.

[0057] (5) The solid sample was obtained by filtering with filter paper; the sample was washed with anhydrous ethanol and deionized water, and dried at 60°C to obtain a solid powder, which is the oxygen-doped molybdenum disulfide-silver-silver-carbon sulfide composite photocatalyst. The mass ratio of molybdenum disulfide 1T-2HMoS2 to silver-supported / silver-carbon sulfide Ag-TCN is 10%, denoted as 10% 1T-2HO-MoS2@Ag / Ag2S-TCN composite photocatalyst. In this embodiment, the mass ratio of silver-supported carbon nitride, ammonium molybdate tetrahydrate, and thiourea is 1:0.772:1.4265.

[0058] Example 2

[0059] The ammonium molybdate tetrahydrate ((NH4)6Mo7O) from Example 1 was used. 24 The amount of *4H2O* was increased to 0.4632 g, and the amount of thiourea (CH4N2S) was increased to 0.8559 g, with other parameters the same as in Examples 1(1)-(5). The mass ratio of oxygen-doped molybdenum disulfide-silver-carried silver / silver sulfide carbon nitride composite photocatalyst was 15%, denoted as 15% 1T-2H2O-MoS2@Ag / Ag2S-TCN composite photocatalyst. In this example, the mass ratio of silver-carried carbon nitride, ammonium molybdate tetrahydrate, and thiourea was 1:1.158:2.1398.

[0060] Example 3

[0061] The ammonium molybdate tetrahydrate ((NH4)6Mo7O) from Example 1 was used. 24The amount of *4H2O* was reduced to 0.2470 g, and the amount of thiourea (CH4N2S) was reduced to 0.4565 g, with other components remaining the same as in Examples 1(1)-(5). The mass ratio of oxygen-doped molybdenum disulfide-silver-carried silver / silver sulfide carbon nitride composite photocatalyst was 8%, denoted as 8% 1T-2H2O-MoS2@Ag / Ag2S-TCN composite photocatalyst. In this example, the mass ratio of silver-carried carbon nitride, ammonium molybdate tetrahydrate, and thiourea was 1:0.6175:1.1413.

[0062] Comparative Example 1

[0063] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 only includes the same step (1) as in Example 1 to obtain silver-loaded carbon nitride solid powder, without any other steps.

[0064] Comparative Example 2

[0065] The difference between Comparative Example 2 and Example 1 is that: the ammonium molybdate tetrahydrate ((NH4)6Mo7O) from Example 1 was replaced with ammonium molybdate tetrahydrate ((NH4)6Mo7O) 24 The amount of *4H2O* was reduced to 0.1235 g, and the amount of thiourea (CH4N2S) was reduced to 0.2282 g, with other parameters the same as in Example 1. The resulting oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst had a mass ratio of 4% for oxygen-doped molybdenum disulfide 1T-2H O-MoS2 and silver-supported / silver sulfide carbon nitride Ag / Ag2S-TCN, and was designated as a 4% 1T-2H O-MoS2@Ag / Ag2S-TCN composite photocatalyst.

[0066] Comparative Example 3

[0067] The difference between Comparative Example 3 and Example 1 is that the ammonium molybdate tetrahydrate ((NH4)6Mo7O) in Example 1 was replaced with a different one. 24 The amount of *4H2O* was reduced to 0.1853 g, and the amount of thiourea (CH4N2S) was reduced to 0.3424 g, with other parameters the same as in Example 1. The resulting oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst had a mass ratio of 6% for oxygen-doped molybdenum disulfide 1T-2H O-MoS2 and silver-supported / silver sulfide carbon nitride Ag / Ag2S-TCN, and was designated as a 6% 1T-2H O-MoS2@Ag / Ag2S-TCN composite photocatalyst.

[0068] Comparative Example 4:

[0069] The difference between Comparative Example 4 and Example 1 is that silver nitrate in Example 1 was removed, while the rest was the same as in Example 1. The mass ratio of oxygen-doped molybdenum disulfide (1T-2H₂O-MoS₂) to carbon nitride (TCN) in the obtained oxygen-doped molybdenum disulfide-carbon nitride composite photocatalyst was 10%, denoted as 10% 1T-2H₂O-MoS₂@TCN composite photocatalyst.

[0070] Comparative Example 5:

[0071] The difference between Comparative Example 5 and Example 1 is that the ammonium molybdate tetrahydrate ((NH4)6Mo7O) in Example 1 is replaced with ammonium molybdate tetrahydrate ((NH4)6Mo7O) 24 The amount of *4H2O* was increased to 0.5558 g, and the amount of thiourea (CH4N2S) was increased to 1.0271 g, with other parameters the same as in Example 1. The resulting oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst had a mass ratio of 18% for oxygen-doped molybdenum disulfide 1T-2H O-MoS2 and silver-supported / silver sulfide carbon nitride Ag / Ag2S-TCN, and was designated as the 18% 1T-2H O-MoS2@Ag / Ag2S-TCN composite photocatalyst.

[0072] The composite photocatalysts prepared in Examples 1-2 and Comparative Examples 1-3 were characterized by X-ray powder diffraction. Figure 1 As shown. By Figure 1 It can be seen that as the amount of molybdenum disulfide added increases, the characteristic peak of carbon nitride becomes weaker and weaker, the characteristic peak of molybdenum disulfide becomes more and more obvious, and the characteristic peak of silver sulfide appears in the composite material.

[0073] The morphology of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 1-3 was characterized by scanning electron microscopy, such as... Figure 2 As shown. Figure 2 (a) is a SEM image of the composite photocatalyst prepared in Comparative Example 1. Figure 2 (b) is a SEM image of the composite photocatalyst prepared in Comparative Example 2. Figure 2 Image (c) shows the SEM image of the composite photocatalyst prepared in Comparative Example 3. Figure 2 Image (d) is a SEM image of the composite photocatalyst prepared in Example 1. Figure 2 Image (e) shows a SEM image of the composite photocatalyst prepared in Comparative Example 5. Figure 2It can be seen that the silver-loaded carbon nitride composite photocatalyst obtained in Comparative Example 1 is mainly composed of strip-like structures. The oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalysts obtained in Comparative Examples 2-3, due to their lower molybdenum disulfide content, still mainly exhibit a strip-like structure, but with the increase of molybdenum disulfide content, a distinct lamellar structure appears on the surface. The oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalyst obtained in Example 1 has a strip-like structure with obvious interlaced fine stripes on the surface; the oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalyst obtained in Comparative Example 5 shows agglomeration on the surface with increasing molybdenum disulfide content, exhibiting obvious granular objects. It is evident that silver-loaded carbon nitride can provide a large surface area, offering favorable conditions for the in-situ synthesis of oxygen-doped molybdenum disulfide and also providing conditions for the formation of silver sulfide in the composite material. However, when the proportion of molybdenum disulfide is low (Comparative Examples 2 and 3), it cannot effectively suppress the rapid recombination of photogenerated electrons and holes, resulting in a decrease in photocatalytic effect. On the other hand, when the proportion of molybdenum disulfide increases (Comparative Example 5), it leads to further aggregation of in-situ synthesized oxygen-doped molybdenum disulfide, thereby reducing the active sites of the material and causing the material synthesis to fail.

[0074] The light absorption properties of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 1-3 were characterized by ultraviolet-visible diffuse reflectance spectroscopy, and the results are as follows: Figure 3 As shown. Figure 4 The images show the Tauc curves of the composite photocatalysts from Examples 1-2 and Comparative Examples 1-3. Figure 4It is known that the band gap of silver-loaded carbon nitride is 2.78 eV; the band gap of the 10% oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalyst is 2.02 eV; the band gap of the 15% oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalyst is 2.18 eV; the band gap of the 4% oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalyst is 2.62 eV; and the band gap of the 6% oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalyst is 2.54 eV. In this invention, the oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride composite photocatalyst has a moderate band gap, ensuring sufficient response to visible light while reducing the recombination of photogenerated electrons and holes, thereby significantly improving the catalytic efficiency of the photocatalyst. Silver-loaded nitrogen carbide is also a photocatalyst with photocatalytic effects, but its photocatalytic reduction ability is weak due to its low conduction band and rapid recombination of photogenerated electrons and holes. Compared to the 2.78 eV bandgap of silver-loaded nitrogen nitride (Ag-TCN), the photocatalytic material of this invention has a minimum bandgap of 2.02 eV. Therefore, the photocatalytic material of this invention exhibits better light response, shorter reaction time, and higher efficiency. This invention employs an in-situ synthesis method to form a composite material. Specifically, molybdenum disulfide is directly generated on the surface of silver-loaded nitrogen carbide, while a small amount of silver on the nitrogen carbide is transformed into silver sulfide to form the composite material. During photocatalysis, the heterojunction formed by the two materials achieves the separation of holes and electrons, thereby improving the photocatalytic effect.

[0075] Figure 5 XPS spectra of the composite photocatalysts prepared in Example 1 and Comparative Example 1; wherein Figure 5 (a) shows the Ag 3d spectra of Example 1 and Comparative Example 1. Figure 5 (b) is the S2p spectrum of Example 1. Figure 5 (c) is the Mo 3d spectrum of Example 1. Figure 5 Image (d) shows the O1s spectrum of Example 1. Figure 5 As shown in (a) and (b), compared to the composite photocatalyst material of Comparative Example 1, the composite photocatalyst material of Example 1 has characteristic peaks of Ag and Ag-S, indicating that silver sulfide has formed in the material; Figure 5 As shown in (c) and (d), the composite photocatalytic material of Example 1 exhibits characteristic peaks of Mo-O, indicating successful oxygen doping in the material; Figure 5 As can be seen from (c), the composite photocatalytic material of Example 1 has characteristic peaks of 2H phase and 1T phase MoS2.

[0076] Figure 6 The photocatalytic performance curves of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 2-4 under visible light irradiation (λ>420 nm) for the reduction of hexavalent chromium are shown. Figure 6It can be seen that the photocatalytic performance of the composite photocatalysts prepared in Examples 1-2 and Comparative Examples 2-4 was evaluated by reducing hexavalent chromium under visible light irradiation with wavelengths greater than 420 nm. The results are as follows: Figure 6 As shown. Figure 6 The vertical axis represents the ratio of the concentration of unreduced hexavalent chromium to the initial concentration of hexavalent chromium. In the experiment, the initial concentration of hexavalent chromium was 10 mg / L, and the amount of composite photocatalyst used was 0.04 g. Figure 6 It can be seen that the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst prepared in Example 1, without adjusting the reaction pH (the original pH was approximately 6), achieved a hexavalent chromium reduction rate of 94.6% after reacting under visible light for 30 min, exhibiting extremely high photocatalytic activity. The oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst prepared in Example 2, without adjusting the reaction pH (the original pH was approximately 6), achieved a hexavalent chromium reduction rate of 91.0% after reacting under visible light for 30 min, exhibiting extremely high photocatalytic activity. The oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst prepared in Comparative Example 2, without adjusting the reaction pH (the original pH was approximately 6), achieved a hexavalent chromium reduction rate of 59.9% after reacting under visible light for 30 min. The oxygen-doped molybdenum disulfide-silver-carbon sulfide composite photocatalyst prepared in Comparative Example 3, without adjusting the reaction pH (original pH approximately 6), achieved a hexavalent chromium reduction rate of 64.2% after 30 minutes of reaction under visible light. The composite photocatalyst prepared in Comparative Example 4, without adjusting the reaction pH (original pH approximately 6), achieved a hexavalent chromium reduction rate of 42.4% after 30 minutes of reaction under visible light. This demonstrates that when the molybdenum disulfide content is low, the material is primarily composed of silver-loaded / silver-carbon sulfide, leading to rapid recombination of photogenerated holes and electrons due to inefficient migration, resulting in a low hexavalent chromium reduction rate. Furthermore, the lack of silver and silver sulfide resulted in relatively weak light absorption and photoelectric properties in the composite photocatalyst of Comparative Example 4, further reducing the hexavalent chromium reduction rate. The results obtained in Example 3 are similar to those in Examples 1 and 2, and will not be repeated here. In Comparative Example 5, the excessive increase in the proportion of molybdenum disulfide led to further agglomeration of the in-situ synthesized oxygen-doped molybdenum disulfide, thereby reducing the active sites of the material and resulting in poor catalytic performance of the synthesized composite photocatalyst.

[0077] The initial pH of the wastewater containing hexavalent chromium was approximately 6. The pH of the initial wastewater was adjusted to 2 and 4 using dilute sulfuric acid, and then adjusted to 8 using dilute sodium hydroxide solution. The composite photocatalyst prepared in Example 1 was added to the initial wastewater. The photocatalytic performance of the oxygen-doped molybdenum disulfide-silver-silver sulfide-carbon nitride composite photocatalyst of Example 1 was evaluated by reducing hexavalent chromium under visible light with a wavelength greater than 420 nm.

[0078] Figure 7 The graph shows the effect of pH on the reduction of hexavalent chromium in wastewater with an initial pH of approximately 6, and the pH adjusted to 2, 4, and 8 under visible light irradiation (λ>420nm). The results are as follows: Figure 7 As shown, Figure 7 The vertical axis represents the ratio of the concentration of unreduced hexavalent chromium to the initial concentration of hexavalent chromium. In the experiment, the initial concentration of hexavalent chromium was 10 mg / L, and the amount of oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst was 0.04 g. At pH 6, using the composite photocatalyst of Example 1, the reduction rate of hexavalent chromium reached 99.4% after reacting under visible light for 30 min, demonstrating extremely high photocatalytic activity. At pH 2, the composite photocatalyst achieved a reduction rate of 99.4% after reacting under visible light for 10 min, again exhibiting extremely high photocatalytic activity. At pH 8, the composite photocatalyst achieved a reduction rate of 70.5% after reacting under visible light for 30 min. Therefore, pH plays a crucial role in the properties of composite materials. Under acidic and near-neutral conditions, the surface of the composite material is positively charged, which is conducive to the adsorption of hexavalent chromium and promotes photocatalytic reduction. At the same time, acidic conditions are conducive to the separation of photogenerated electrons and holes. The stronger the acidity, the better the photocatalytic effect and the faster the reaction. However, when the pH becomes alkaline, the reduction rate of hexavalent chromium decreases rapidly. This is mainly because trivalent chromium forms chromium hydroxide in an alkaline environment, reducing the active sites of the material.

[0079] The composite photocatalyst prepared in Example 1 was subjected to cycle stability testing, and the specific method is as follows:

[0080] (1) Weigh 40 mg of the oxygen-doped molybdenum disulfide-silver-silver-sulfide carbon nitride material prepared in Example 1 and add it to 50 mL of wastewater containing hexavalent chromium. The initial concentration of hexavalent chromium is 10 mg / L.

[0081] (2) The reaction system (wastewater containing oxygen-doped molybdenum disulfide-silver / silver sulfide carbon nitride material) was subjected to photocatalytic reaction under a 300W xenon lamp without pH adjustment (pH value approximately 6) and with the light source at a distance of 15cm from the screen. Every 5 minutes, 2mL of solution was taken from the reaction system, filtered through a 0.45µm filter in aqueous phase, and the concentration of hexavalent chromium was measured using a UV-Vis spectrophotometer. After 30 minutes of photocatalytic reaction, the xenon lamp was turned off.

[0082] (3) The material and wastewater were separated by a centrifuge at a speed of 8000 rpm. The oxygen-doped molybdenum disulfide-silver-silver nitride composite material after the reaction was collected and cleaned and dried using the method in Example 1 (5), and then added back into 50 mL of wastewater containing hexavalent chromium.

[0083] Repeat steps (2)-(3) three times.

[0084] Figure 8 The image shows the photocatalytic performance of the composite photocatalyst prepared in Example 1, which undergoes a four-cycle reduction of hexavalent chromium. Figure 8 The graph uses the percentage of hexavalent chromium concentration as the vertical axis and time as the horizontal axis. Figure 8 As can be seen, after four cycles, the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride material still exhibits highly efficient photocatalytic performance. The removal efficiencies of hexavalent chromium after four cycles are 96.2%, 91.8%, 85.3%, and 84.1%, respectively. The oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst shows good stability and promising prospects for practical applications. Therefore, the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst provided by this invention not only has extremely high catalytic efficiency and good stability, but also has a simple preparation method and high purity.

[0085] In summary, the silver-loaded nitrogen carbide prepared by the hydrothermal reaction and high-temperature calcination of melamine, cyanuric acid, and silver nitrate in this invention has a large specific surface area, providing a good support for the in-situ synthesis of molybdenum disulfide. Silver-loaded nitrogen carbide is also a photocatalyst with photocatalytic effects; however, its photocatalytic reduction ability is weak due to its low conduction band and rapid recombination of photogenerated electrons and holes. This invention uses an in-situ synthesis method to form a composite material, specifically, molybdenum disulfide is directly generated on the surface of the silver-loaded nitrogen carbide, while a small amount of silver on the nitrogen carbide is converted into silver sulfide to form the composite material. During photocatalysis, the heterojunction formed by the two materials achieves the separation of holes and electrons, improving the photocatalytic effect. Furthermore, the method of this invention uses N,N-dimethylformamide (DMF) as a solvent during in-situ synthesis, which can improve the crystal structure of molybdenum disulfide and enhance the catalytic active sites. Therefore, the photocatalytic material provided by this invention has high catalytic efficiency and good stability. This is because DMF facilitates the chemical synthesis of molybdenum disulfide (MoS2), which has two phases, 1T and 2H, corresponding to the octahedral metallic phase and the triangular prism semiconductor phase, respectively. Among these two phases, the 1T phase exhibits superior electrocatalytic performance, while the 2H phase possesses higher chemical stability and reduction capability. This is due to its stable chemical structure and low conduction band. Furthermore, the heterogeneous structure resulting from the different crystal planes also improves the material's carrier density and suppresses charge loading.

[0086] Furthermore, the oxygen-doped molybdenum disulfide-silver-carried / silver sulfide carbon nitride composite photocatalyst prepared by the method of this invention is chemically composed of a 1T-2H₂O-MoS₂@Ag / Ag₂S-TCN composite photocatalyst, wherein the mass ratio of 1T-2H₂O-MoS₂ to Ag / Ag₂S-TCN in the composite photocatalyst is 8%-15%. Compared to the 2.78 eV bandgap of silver-carried nitrogen nitride (Ag-TCN), the photocatalytic material of this invention has a minimum bandgap of 2.02 eV, exhibiting better light response, shorter reaction time, and higher efficiency. The oxygen-doped molybdenum disulfide-silver-silver-carbon sulfide composite photocatalyst provided by this invention can effectively catalytically reduce hexavalent chromium to trivalent chromium under visible light irradiation. This is because the photocatalytic material prepared by this invention has a low band gap, can respond under visible light, and can excite electron-hole pairs. Since the heterojunction of the composite material promotes the separation of electrons and holes, reducing hexavalent chromium, the photocatalyst prepared by this invention not only has extremely high catalytic efficiency in acidic or near-neutral environments, but also has good stability.

[0087] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing an oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst, characterized in that, Includes the following steps: Step 1: Dissolve the nitrogen-containing compound and silver nitrate in water by heating, then carry out the first hydrothermal reaction in an autoclave to obtain a hydrothermal intermediate product, and then calcine it in an inert atmosphere to obtain silver-loaded carbon nitride; the nitrogen-containing compound is melamine and cyanuric acid; the mass ratio of melamine, cyanuric acid and silver nitrate is 25-35:25-35:

1. Step 2: Mix the prepared silver-loaded carbon nitride with N,N-dimethylformamide to obtain solution A; mix ammonium molybdate tetrahydrate, thiourea, and N,N-dimethylformamide to obtain solution B; slowly add solution B dropwise to solution A, and carry out a second hydrothermal reaction in an autoclave. Some of the silver on the surface of the silver-loaded carbon nitride reacts with the sulfur ions released from the decomposition of thiourea to generate silver sulfide in situ, forming a silver-loaded / silver sulfide carbon nitride with Ag / Ag2S coexistence, and finally obtain the oxygen-doped molybdenum disulfide-silver-loaded / silver sulfide carbon nitride composite photocatalyst; The mass ratio of silver-loaded carbon nitride to N,N-dimethylformamide is 1:70-90; the mass ratio of silver-loaded carbon nitride, ammonium molybdate tetrahydrate, and thiourea is 1:0.3-1.2:0.6-2.

2. The chemical composition of the oxygen-doped molybdenum disulfide-silver-carried / silver sulfide carbon nitride composite photocatalyst is 1T-2H O-MoS2@Ag / Ag2S-TCN composite photocatalyst. In step one, the reaction temperature for the first hydrothermal reaction is 180-220℃, and the time is 10-12 hours. The heating rate to the first hydrothermal reaction temperature is 1-3℃ / min; the calcination treatment is carried out in a nitrogen atmosphere, with a heating rate of 3-5℃ / min, a calcination temperature of 500-600℃, and a time of 3-5h; in step two, the reaction temperature for the second hydrothermal reaction is 160-200℃, and the time is 18-24h; the heating rate to the second hydrothermal reaction temperature is 1-3℃ / min.

2. An oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst, characterized in that, The oxygen-doped molybdenum disulfide-silver-silver-carbon sulfide composite photocatalyst prepared by the method described in claim 1 has the chemical composition of 1T-2H₂O-MoS₂@Ag / Ag₂S-TCN composite photocatalyst, wherein the mass ratio of oxygen-doped molybdenum disulfide 1T-2H₂O-MoS₂ to silver-doped carbon sulfide Ag / Ag₂S-TCN in the composite photocatalyst is 8%-15%.

3. The application of the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst prepared by the method of claim 1 or the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide-carbon nitride composite photocatalyst as described in claim 2 in the reduction of hexavalent chromium in wastewater.

4. The application according to claim 3, characterized in that, The application The process includes the following steps: adding the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst to wastewater, and carrying out a photocatalytic reaction under visible light to complete the reduction of hexavalent chromium. The amount of the oxygen-doped molybdenum disulfide-silver-supported / silver sulfide carbon nitride composite photocatalyst added to the wastewater is 0.2 g / L-1.2 g / L, and the pH value of the photocatalytic reaction is 2-6.

5. The application according to claim 4, characterized in that, In the application, the visible light source is a 300W-500W xenon lamp, the distance between the xenon lamp and the surface of the wastewater is 14cm-16cm, the photocatalytic reaction time is 10min-30min, and the concentration of hexavalent chromium in the wastewater is 5mg / L-40mg / L.

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