A CNN / Au / MoS2 photocatalyst containing highly dispersed Au, its preparation method and application

Abstract

The present invention provides a CNN / Au / MoS2 photocatalyst containing highly dispersed Au, a preparation method thereof and an application thereof. The CNN / Au / MoS2 photocatalyst comprises three components: graphitic carbon nitride nanosheets, MoS2 nanosheets and atomically sized Au. Au is distributed on the surface of the graphitic carbon nitride nanosheets to form CNN / Au, and the MoS2 nanosheets are distributed on the surface of CNN / Au, and a heterojunction is formed between the three components. The highly dispersed Au atoms can not only promote the large-area uniform growth of MoS2 on the surface of CNN, so that the active reaction sites are fully exposed, but also induce the transformation of the heterojunction between CNN and MoS2 from a type-II heterojunction to a Z-type heterojunction. Some of the Au atoms not covered by MoS2 act as cocatalysts, significantly reducing the catalytic hydrogen production overpotential of the heterojunction system, thereby significantly improving the photocatalytic activity of the catalyst.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalysts, and particularly relates to a CNN / Au / MoS2 photocatalyst containing highly dispersed Au, a preparation method thereof, and an application thereof. Background Art

[0002] The overexploitation and utilization of fossil energy have continuously deteriorated the global environmental situation, and the energy crisis has become increasingly severe. It is urgent to search for and develop green and environmentally friendly alternative energy sources. The photocatalytic water splitting technology for hydrogen production can fundamentally solve the global energy shortage and environmental pollution problems, and thus has important application prospects. However, most current catalysts generally have disadvantages such as low catalytic efficiency and poor stability, and the use of noble metal cocatalysts significantly increases the technical cost of photocatalytic hydrogen production, seriously restricting the popularization and application of the photocatalytic water splitting technology for hydrogen production. Therefore, the development of highly active and low-cost photocatalysts has become a key link in realizing the conversion and application of renewable hydrogen energy.

[0003] In recent years, graphitic carbon nitride (CN) has attracted a great deal of research interest in the field of photocatalytic hydrogen production due to its unique non-metallic properties and visible-light response performance. However, as an organic polymer semiconductor, the inherent limitations of CN in terms of electrical conductivity inevitably lead to its low charge separation efficiency and carrier utilization rate. An effective solution is to combine CN with other semiconductors to form a heterostructure. However, due to the influence of its atomic arrangement and the inherent surface properties of the material, it is difficult to obtain a large-area and closely contacted heterojunction interface between the two semiconductors, and the improvement of the carrier separation efficiency is limited. In addition, most current catalytic systems use noble metals as co-catalysts, and the usage amount of the co-catalyst accounts for about 1% - 3% of the catalyst, which undoubtedly significantly increases the usage cost of the catalyst. Actively seeking suitable strategies to improve the contact quality of the heterojunction interface and reduce the use of noble metal co-catalysts is of great significance for promoting the practical application of photocatalytic hydrogen production technology. Utilizing the strong binding ability between metals and N atoms in carbon nitride to construct a highly dispersed metal-modified carbon nitride system is expected to further improve the hydrogen production performance of carbon nitride catalysts. On the one hand, the strong chemical interaction between metals and N atoms helps the rapid transfer of photo-generated electrons from carbon nitride to the metal surface, thus improving the carrier separation efficiency; on the other hand, the highly dispersed metal atoms provide abundant active sites for the hydrogen production reaction, which is more conducive to the progress of the hydrogen production reaction (J. Alloys Compd., 2023, 936, 168209; Jenergy Fuels, 2021, 35, 6504 - 6526; J. Mater. Chem. A, 2015, 3, 2485 - 2534; Int. J. Hydrogen Energy, 2022, 47, 29295 - 29304; Appl. Catal., B, 2019, 242, 92 - 99; Appl. Catal., B, 2019, 246, 120 - 128; J. Alloys Compd., 2022, 915, 165351; adv. Mater., 2016, 28, 2427 - 2431).

[0004] Introducing highly dispersed metal atoms is expected to open up a new way for the development of highly active carbon nitride-based photocatalysts. However, the improvement of the photocatalytic hydrogen production performance of the reported highly dispersed metal-modified carbon nitride materials is relatively limited. This is mainly because the realization of the high dispersion of metals on the carbon nitride surface depends on the control of the ultra-low concentration of metals. If the concentration of the metal precursor is too high, it will lead to their aggregation and the formation of large-sized metal particles, and the photocatalytic hydrogen production performance will instead decline. It is difficult to achieve a significant improvement in the hydrogen production performance of carbon nitride only by introducing highly dispersed metals. Summary of the Invention

[0005] To overcome the above-mentioned drawbacks of the prior art, the present invention provides a CNN / Au / MoS2 photocatalyst containing highly dispersed Au, its preparation method and application. In the catalyst system, highly dispersed Au atoms are controllably introduced between graphitic carbon nitride nanosheets (g-C3N4 nanosheets, abbreviated as CNN) and molybdenum disulfide (MoS2). The Au atoms can not only promote the large-area uniform growth of MoS2 on the surface of CNN, making the active reaction sites fully exposed, but also induce the transformation of the heterojunction between CNN and MoS2 from type II to type Z. Meanwhile, some Au atoms not covered by MoS2 act as cocatalysts, significantly reducing the catalytic hydrogen production overpotential of the heterojunction system, thereby significantly improving the photocatalytic activity of the catalyst.

[0006] The present invention is achieved through the following technical solutions:

[0007] A CNN / Au / MoS2 photocatalyst containing highly dispersed Au includes three components: graphitic carbon nitride nanosheets, MoS2 nanosheets, and atomically sized Au. The atomically sized Au is distributed on the surface of the graphitic carbon nitride nanosheets to form CNN / Au, and the MoS2 nanosheets are distributed on the surface of CNN / Au, and a heterojunction is formed among the three components.

[0008] Preferably, the mass ratio of the graphitic carbon nitride nanosheets to Au is 100:(0.06 - 0.18).

[0009] The preparation method of the above-mentioned CNN / Au / MoS2 photocatalyst containing highly dispersed Au of the present invention includes:

[0010] S1, Adsorb chloroauric acid on the graphitic carbon nitride nanosheets and calcine and reduce it in a hydrogen atmosphere to obtain Au-loaded graphitic carbon nitride nanosheets;

[0011] S2, Disperse the Au-loaded graphitic carbon nitride nanosheets in an aqueous solution containing sodium molybdate dihydrate (Na2MoO4·2H2O) and L-cysteine (HSCH2CH(NH2)CO2H), and carry out a hydrothermal reaction to obtain a CNN / Au / MoS2 heterojunction photocatalyst.

[0012] Preferably, in S1, the preparation method of the graphitic carbon nitride nanosheets is: using dicyandiamide as a raw material, preparing bulk carbon nitride by high-temperature calcination, and obtaining graphitic carbon nitride nanosheets by high-temperature exfoliation of the bulk carbon nitride.

[0013] Furthermore, the preparation method of the graphitic carbon nitride nanosheets specifically includes:

[0014] (1) Under an air atmosphere, heat dicyandiamide to 545 - 555 °C for calcination to obtain bulk carbon nitride;

[0015] (2) Under an air atmosphere, the bulk carbon nitride is heated to 515 - 525 °C for calcination to obtain graphitic carbon nitride nanosheets.

[0016] Further, in step (1), it is heated to 545 - 555 °C at a heating rate of 9 - 11 °C / min and held at 545 - 555 °C for 230 - 250 min; in step (2), the heating rate is 4 - 6 °C / min and the calcination time is 230 - 250 min.

[0017] Further, S1 specifically includes:

[0018] (1) The graphitic carbon nitride nanosheets are dispersed in water, then chloroauric acid trihydrate is added and stirred at room temperature; the resulting solution is subjected to solid - liquid separation, and the obtained precipitate is washed and dried to obtain graphitic carbon nitride nanosheets adsorbed with Au precursors;

[0019] (2) The graphitic carbon nitride nanosheets adsorbed with Au precursors are calcined and reduced under a mixed atmosphere of hydrogen and argon to obtain graphitic carbon nitride nanosheets loaded with highly dispersed Au.

[0020] Preferably, in S1, the calcination temperature is 100 - 120 °C and the calcination time is 110 - 130 minutes.

[0021] Preferably, S2 specifically includes:

[0022] (1) The graphitic carbon nitride nanosheets loaded with Au are dispersed in water, then stirred and ultrasonically treated to obtain a dispersion of CNN / Au;

[0023] (2) Sodium molybdate dihydrate and L - cysteine are dispersed in water to obtain a precursor solution of MoS2;

[0024] (3) Under stirring, the precursor solution of MoS2 is added to the dispersion of CNN / Au, and after continuing to stir evenly, the resulting mixture is subjected to a hydrothermal reaction;

[0025] (4) After the reaction is completed, solid - liquid separation is carried out, and the obtained solid is washed and dried to obtain a CNN / Au / MoS2 heterojunction photocatalyst.

[0026] Further, in step (2), the concentration of sodium molybdate dihydrate is 1.8 - 2.2 mmol / L and the concentration of L - cysteine is 3.8 - 4.2 mmol / L.

[0027] Further, in step (3), the rotational speed of magnetic stirring is 400 - 600 r / min and the continuous stirring time is 9 - 11 min.

[0028] Further, the temperature of the hydrothermal reaction in step (3) is 190 - 210 °C, and the reaction time is 23 - 25 hours.

[0029] Further, in step (4), the centrifugation speed is 6500 - 7500 r / min, the centrifugation time is 9 - 11 minutes, and the sample drying temperature is 55 - 65 °C.

[0030] Application of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au in photocatalytic water splitting for hydrogen production.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The CNN / Au / MoS2 catalyst of the present invention is a heterojunction system modified by bifunctional highly dispersed Au atoms. Based on the construction of the heterojunction system, highly dispersed Au atoms are controllably introduced, combining the construction of the heterojunction with the introduction of highly dispersed Au atoms to exert the synergistic effect of the heterojunction and the metal. On the one hand, Au can tightly bind with CNN and MoS2 nanosheets through strong chemical bonds to form a high-quality heterojunction interface, which not only helps to reduce the interfacial migration resistance of carriers but also provides abundant surface active sites for the catalytic reaction. On the other hand, the highly dispersed Au acts as both an electron transfer intermediate and a cocatalyst, inducing an indirect Z-scheme carrier migration process between CNN and MoS2. The photogenerated electrons and holes maintain high reduction and oxidation activities, and water is rapidly reduced to hydrogen through the Au cocatalyst on the catalyst surface, thus significantly improving the hydrogen production performance of the catalyst. At the same time, the characteristics of high dispersion and ultra-small size significantly reduce the demand for the precious metal Au in the catalytic system, and the production and use cost of the catalyst are significantly reduced.

[0033] The present invention defines a suitable mass ratio of CNN to Au, and a catalyst with excellent photocatalytic performance can be obtained. Too high or too low a ratio is not conducive to the improvement of catalytic performance.

[0034] The present invention constructs a CNN / Au / MoS2 heterojunction catalyst by in-situ growing highly dispersed ultra-low concentration Au atoms and MoS2 nanosheets on graphite phase carbon nitride nanosheets through hydrogen calcination reduction and hydrothermal reaction method. The catalyst obtained by this technology has significant advantages in many aspects: (1) In the present invention, Au has a strong binding force with N atoms in CNN and S atoms in MoS2. Therefore, low-concentration Au can be highly dispersed into tiny Au atoms on the surface of CNN through strong chemical bond action. At the same time, it can promote the large-area and close loading of MoS2 through the interaction of Au-S bonds, obtaining a closely contacted interface between CNN and MoS2, greatly reducing the interfacial charge transfer resistance and providing a smooth channel for charge separation. (2) CNN and MoS2 are micron-scale two-dimensional sheet structures, and Au is a highly dispersed atomic-scale structure. The atomic-scale small-sized Au is highly uniformly loaded on the surface of CNN, effectively increasing the proportion of the heterojunction interface between CNN and MoS2, thus promoting the full exposure of surface active reaction sites, which helps to improve the adsorption of reactants on the catalyst surface and the rapid desorption of hydrogen from the catalyst surface. (3) Au on the interface between CNN and MoS2 acts as an electron transport medium to induce the transition of the carrier migration process between CNN and MoS2 from type II to type Z, while Au located on the surface of CNN acts as a cocatalyst to significantly reduce the hydrogen evolution overpotential of the catalytic system, thus effectively sensitizing the evolution of hydrogen. (4) Due to its ultra-small size and high dispersion, the optimal usage amount of highly dispersed Au atoms in the present invention is only 20% of the Au usage amount in traditional catalysts. The introduction of highly dispersed Au atoms can significantly reduce the demand for precious metals in the catalytic system, thus effectively reducing the preparation and usage costs of the catalyst.

[0035] Furthermore, the stirring and mixing time of chloroauric acid in the aqueous solution of CNN has a direct impact on the effective adsorption of Au precursor on CNN. When the mixing time is too short, the Au precursor cannot be fully adsorbed and effectively combined on the surface of CNN, resulting in a decrease in the effective loading amount of Au. When the mixing time is too long, some CNN nanosheets are broken, which is not conducive to maintaining the morphology and catalytic performance of the catalyst.

[0036] Furthermore, the hydrogen reduction temperature and time have a direct impact on the formation of highly dispersed Au atoms. In a large number of previous experiments, when the calcination temperature is too low or the calcination time is too short, the chloroauric acid precursor cannot be fully reduced to Au, while too high a calcination temperature will cause Au to aggregate to form larger-sized Au nanoparticles, resulting in a decrease in catalytic performance.

[0037] The CNN / Au / MoS2 photocatalyst described in the present invention is mainly applied to the photocatalyst system for photocatalytic water splitting to produce hydrogen. In a specific embodiment of the present invention, under simulated visible light irradiation of 420-780 nm, the hydrogen production activity of the CNN / Au / MoS2 photocatalyst reaches 8.3 mmol / g / h, which is 14 times that of pure CNN / MoS2, and shows good stability in the hydrogen production test of 4 cycles. When the catalytic activity is equivalent to that of the optimal CN / MoS2 system modified with conventional Au nanoparticles, the amount of Au precursor used in the present invention is effectively reduced by 90%, significantly reducing the development and use costs of the catalyst while ensuring the hydrogen production activity of the catalyst, and is more conducive to promoting the practical application of photocatalytic hydrogen production technology. Description of the Drawings

[0038] Figure 1 It is the morphology diagrams of CNN, MoS2 and CNN / Au / MoS2 samples;

[0039] Figure 2 It is for CNN / Au 0.12 / MoS2 EDS element distribution mapping diagram;

[0040] Figure 3 It is the EDS element distribution mapping diagram of CNN / / MoS2;

[0041] Figure 4 It is for CNN / Au 0.12 / MoS2 XPS diagram of Au;

[0042] Figure 5 It is the UV-visible absorption spectra diagram of CNN loaded with different amounts of Au;

[0043] Figure 6 It is for CNN, MoS2, CNN / Au 0.12 、MoS2 / Au 0.12 、CNN / MoS2, and the photocatalytic hydrogen production performance diagrams of samples when the proportion of Au in CNN / Au / MoS2 catalyst is 0.08%, 0.12%, 0.16% and 1.2% respectively;

[0044] Figure 7 It is the photocatalytic hydrogen production performance diagrams of samples when the proportion of Au in CNN / Au / MoS2 catalyst is 1.0%, 1.2% and 1.4% respectively; [[ID='39']]

[0045] Figure 8 It is for CNN / Au 0.12 / MoS2 photocatalytic stability test diagram. Detailed Embodiments

[0046] To enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0047] The CNN / Au / MoS2 photocatalyst containing highly dispersed Au of the present invention includes three components: CNN, MoS2, and Au. Trace amounts of Au with atomic-scale size are highly uniformly dispersed and tightly bound on the surface of CNN, and MoS2 is further uniformly loaded on the surface of CNN / Au in a large area, forming a tightly bound heterojunction among the three.

[0048] By mass ratio, the ratio of CNN to Au is: CNN:Au = 100:(0.06 - 0.18).

[0049] By mass ratio, the ratio of CNN to MoS2 is: CNN:MoS2 = 100:(0.06 - 0.18).

[0050] CNN is a two-dimensional nanosheet with a size of 0.5 - 1 μm; MoS2 is an aggregated two-dimensional nanosheet with a size of 400 - 600 nm.

[0051] A preparation method of the CNN / Au / MoS2 photocatalyst with excellent photocatalytic performance of the present invention uses dicyandiamide as a raw material to prepare graphitic carbon nitride nanosheets, reduces chloroauric acid to highly dispersed Au atoms on the surface of the graphitic carbon nitride nanosheets by hydrogen reduction method, and further grows MoS2 nanosheets by hydrothermal synthesis method to construct a heterojunction system, obtaining a CNN / Au / MoS2 photocatalyst with a large-area tightly contacted interface.

[0052] The specific preparation method includes:

[0053] 1) CNN preparation step:

[0054] (1) A certain amount of dicyandiamide is heated to 545 - 555 °C at a heating rate of 9 - 11 °C / min and maintained at 545 - 555 °C in air for 230 - 250 min to obtain bulk carbon nitride (BCN);

[0055] (2) 0.5 - 0.8 g of BCN is calcined in air at 515 - 525 °C for 230 - 250 min to obtain CNN.

[0056] 2) CNN / Au preparation step:

[0057] (1) Disperse 95 - 105 mg of CNN into 23 - 27 mL of deionized water, then add 125 - 131 μL of chloroauric acid trihydrate with a concentration of 0.02 g / mL, and stir at room temperature for 11.5 - 12.5 hours. Centrifuge the resulting solution at a speed of 6500 - 7500 r / min for 9 - 11 min, then pour off the supernatant. Wash the lower precipitate with deionized water three times and dry it overnight at 55 - 65 °C to obtain graphitic carbon nitride nanosheets adsorbed with Au precursors.

[0058] (2) Grind the graphitic carbon nitride nanosheets adsorbed with Au precursors to obtain a pale yellow powder. Place the powder in a quartz boat and calcine and reduce it in a tubular furnace under an atmosphere of H2(5%) / Ar(95%). The calcination temperature is 100 - 120 °C, the heating rate is 1 - 3 °C / min, and the calcination time is 110 - 130 min to obtain highly dispersed Au-loaded graphitic carbon nitride nanosheets, and the sample is labeled as CNN / Au.

[0059] 3) Preparation steps of the catalyst CNN / Au / MoS2 containing highly dispersed Au:

[0060] (1) Disperse 18 - 22 mg of CNN / Au in 45 - 55 mL of deionized water, then stir and ultrasonically treat it for 13 - 17 min to obtain a dispersion of CNN / Au.

[0061] (2) Disperse sodium molybdate dihydrate and L-cysteine in 18 - 22 mL of deionized water to obtain a precursor solution of MoS2. Specifically, the concentration of sodium molybdate dihydrate is 1.8 - 2.2 mmol / L, and the concentration of L-cysteine is 3.8 - 4.2 mmol / L.

[0062] (3) Add the precursor solution of MoS2 to the dispersion of CNN / Au under magnetic stirring at a speed of 400 - 600 r / min and continue stirring for 9 - 11 min. Then transfer the mixture to a 100 mL Teflon-lined stainless steel autoclave and heat and react it in a forced-air drying oven. Specifically, the reaction temperature is 190 - 210 °C, and the reaction time is 23 - 25 hours.

[0063] (4) Centrifuge the obtained product at a speed of 6500 - 7500 r / min for 9 - 11 minutes, then pour off the supernatant, wash it three times with deionized water and ethanol, and dry it overnight at 55 - 65 °C. The obtained product is the CNN / Au / MoS2 photocatalyst.

[0064] The CNN / Au / MoS2 photocatalyst containing highly dispersed Au described in the present invention can be applied to photocatalytic water splitting for hydrogen production, and the photocatalytic hydrogen production activity is 2.9 - 8.3 mmol / g / h.

[0065] Example 1:

[0066] Prepare CNN / Au according to the preparation method proposed by the present invention 0.12 / MoS2 heterojunction photocatalyst, the mass ratio of CNN to Au is 100:0.12:

[0067] (1) A certain amount of dicyandiamide is heated to 550 °C at a rate of 10 °C / min in an air atmosphere and maintained for 4 hours to obtain bulk carbon nitride, and then further cooled to 520 °C and calcined for 4 hours to obtain CNN;

[0068] (2) Disperse 100 mg of CNN into 25 mL of deionized water, then add 128 μL of chloroauric acid trihydrate with a concentration of 0.02 g / mL, and stir at room temperature for 12 hours. The obtained solution is centrifuged at 7000 r / min for 10 min, and then the supernatant is discarded. The lower precipitate is washed 3 times with deionized water and dried overnight at 60 °C to obtain graphite phase carbon nitride nanosheets adsorbed with Au precursors.

[0069] (3) Grind the graphite phase carbon nitride nanosheets adsorbed with Au precursors to obtain a pale yellow powder. Place the powder in a quartz boat and calcine it in a tube furnace with H2(5%) / Ar(95%), the calcination temperature is 110 °C, the heating rate is 2 °C / min, and the calcination time is 120 minutes to obtain highly dispersed Au-loaded graphite phase carbon nitride nanosheets. The sample is labeled as CNN / Au.

[0070] (4) Disperse 20 mg of CNN / Au in 50 mL of deionized water, then stir and ultrasonically treat for 15 min to obtain a dispersion of CNN / Au.

[0071] (5) Prepare 20 mL of MoS2 precursor solution, and the concentrations of sodium molybdate dihydrate and L-cysteine in the precursor solution are 2 mmol / L and 4 mmol / L respectively.

[0072] (6) Add the MoS2 precursor solution to the dispersion of CNN / Au under magnetic stirring at a speed of 500 r / min and continue to stir for 10 min. Then transfer the mixture to a 100 mL Teflon-lined stainless steel autoclave and heat and react in a forced air drying oven. The reaction temperature is 200 °C and the reaction time is 24 hours.

[0073] (7) Centrifuge the obtained product at 7000 r / min for 10 minutes, then discard the supernatant, wash it 3 times with deionized water and ethanol, and dry it overnight at 60 °C. The obtained product is CNN / Au 0.12 / MoS2 photocatalyst.

[0074] The morphology of the catalyst is as shown in Figure 1 (c), and its elemental distribution map and XPS map of Au are as shown in Figure 2 and Figure 4 respectively. Its ultraviolet-visible absorption spectrum is as shown in Figure 5 . It can be seen that the ultraviolet-visible light absorption band edge of the sample shows a slight red shift relative to CNN / MoS2, indicating the successful loading of highly dispersed Au, as shown in Figure 6 . The photocatalytic rate of this catalyst is 8.3 mmol / g / h.

[0075] Example 2:

[0076] A CNN / Au 0.06 / MoS2 heterojunction photocatalyst according to an embodiment of the present invention, where the mass ratio of CNN to Au is 100:0.06, and the rest is the same as in Example 1, obtaining CNN / Au 0.06 / MoS2. It can be seen from Figure 5 that the ultraviolet-visible light absorption band edge of the sample shows a slight red shift relative to CNN / MoS2, indicating the successful loading of highly dispersed Au. The photocatalytic hydrogen production activity of this catalytic system is 2.9 mmol / g / h.

[0077] Example 3:

[0078] A CNN / Au 0.18 / MoS2 heterojunction photocatalyst according to an embodiment of the present invention, where the mass ratio of CNN to Au is 100:0.18, and the rest is the same as in Example 1, obtaining CNN / Au 0.18 / MoS2. It can be seen from Figure 5 that the ultraviolet-visible light absorption band edge of the sample shows a slight red shift relative to CNN / MoS2, indicating the successful loading of highly dispersed Au. The photocatalytic hydrogen production activity of this catalytic system is 7.2 mmol / g / h.

[0079] Comparative Example 1:

[0080] Only CNN is prepared, and its preparation method is the same as step (1) in Example 1. The morphology and structure of the obtained sample are as shown in Figure 1 a. The photocatalytic hydrogen production activity of this catalyst is 0.052 mmol / g / h.

[0081] Comparative Example 2:

[0082] Only MoS₂ was prepared. The preparation method was as follows: A certain amount of ammonium molybdate and L-cysteine were added to 20 mL of deionized water to prepare a MoS₂ precursor solution. The concentrations of sodium molybdate dihydrate and L-cysteine in the precursor solution were 2 mmol / L and 4 mmol / L, respectively. Then, the precursor solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and reacted at 200 °C for 24 hours. The pretreatment steps were the same as those in step (7) of Example 1. The obtained pure MoS₂ had a photocatalytic hydrogen production activity of 0.07 mmol / g / h.

[0083] Comparative Example 3:

[0084] Only the CNN / MoS₂ heterojunction catalyst was prepared. CNN was directly mixed with the MoS₂ precursor and subjected to hydrothermal reaction. The preparation method was the same as steps (1), (5), (6), and (7) in Example 1. The elemental distribution of the obtained sample was as Figure 3 shown, and the photocatalytic hydrogen production activity of this catalyst was 0.586 mmol / g / h.

[0085] Comparative Example 4:

[0086] Only the CNN / Au 0.12 heterojunction catalyst was prepared. The preparation method was the same as (1)-(3) in Example 1. The photocatalytic hydrogen production activity of the obtained catalyst was 3.9 mmol / g / h.

[0087] Comparative Example 5:

[0088] Only the MoS₂ / Au 0.12 heterojunction catalyst was prepared. The preparation method of MoS₂ was the same as that in Comparative Example 2, and the Au loading process was the same as steps (2) and (3) in Example 1. The obtained MoS₂ / Au 0.12 heterojunction catalyst had a hydrogen production activity of 0.33 mmol / g / h.

[0089] Comparative Example 6:

[0090] A CNN / Au 1.0 / MoS₂ heterojunction photocatalyst of the embodiment of the present invention, with the mass ratio of CNN to Au being 100:1.0, and the rest being the same as in Example 1, to obtain CNN / Au 1.0 / MoS₂. It can be seen from Figure 7 that the photocatalytic hydrogen production activity of the sample was 7.5 mmol / g / h.

[0091] Comparative Example 7:

[0092] A CNN / Au 1.0 / MoS₂ heterojunction photocatalyst of the embodiment of the present invention, with the mass ratio of CNN to Au being 100:1.2, and the rest being the same as in Example 1, to obtain CNN / Au1.2 / MoS2. From Figure 5 it can be seen that the ultraviolet-visible light absorption edge of the sample shows a slight red shift relative to CNN / MoS2. At the same time, an absorption peak characteristic of Au nanoparticles appears near the central wavelength of 600 nm, indicating that when the loading ratio of Au in the sample is increased to 1.2% of the catalyst, larger-sized Au nanoparticles are generated due to the enrichment of a large amount of Au precursors in the sample, rather than continuing to exist in the form of highly dispersed Au. From Figure 7 it can be seen that the photocatalytic hydrogen production activity of the sample is 7.8 mmol / g / h.

[0093] Comparative Example 8:

[0094] A CNN / Au 1.4 / MoS2 heterojunction photocatalyst according to an embodiment of the present invention, with the mass ratio of CNN to Au being 100:1.4, and the rest being the same as in Example 1, to obtain CNN / Au 1.4 / MoS2. From Figure 7 it can be seen that the photocatalytic hydrogen production activity of the sample is 5.1 mmol / g / h.

[0095] As Figure 1 shown, (a), (b), and (c) are transmission electron micrographs of the samples prepared in Comparative Example 1, Comparative Example 2, and Example 1, respectively. It can be seen from the figures that both CNN and MoS2 exhibit two-dimensional sheet-like structures, and the introduction of a small amount of highly dispersed Au in the two semiconductors will not affect their morphologies. Figure 2 Shown is the EDS element distribution mapping diagram of CNN / Au / MoS2, Figure 3 Shown is the EDS element distribution mapping diagram of CNN / MoS2. By comparing Figure 2 and Figure 3 it can be found that the introduction of Au can promote the large-area uniform loading of MoS2 on CNN, effectively reducing harmful agglomeration. Figure 4 Shown is the XPS diagram of Au in CNN / Au / MoS2. From the XPS signal of Au in the figure, it can be determined that Au is successfully loaded in the sample.

[0096] The photocatalytic hydrogen production ability of the samples was tested using a photocatalytic evaluation system (Zhongjiao Jinyuan). During each catalytic reaction, 5 mg of the synthesized sample was ultrasonically dispersed in 20 mL of an aqueous solution containing 10% triethanolamine sacrificial agent, and the resulting suspension was transferred into a 200 mL special reactor for the photocatalytic system. Before the test began, the system was evacuated for 10 - 15 min to ensure that the test was carried out under vacuum conditions. Throughout the experiment, the solution was magnetically stirred to ensure the suspension state of the catalyst, and circulating cooling water was used to maintain the system temperature at 6 °C. The light source was a Xe lamp with a power of 225 W, and the photocatalytic reaction was carried out under visible light irradiation with a wavelength greater than 420 nm. The photocatalytic hydrogen production was monitored in real-time by an online chromatograph, and the sampling point time interval was 30 min. The photocatalytic hydrogen production activities of the catalysts in Examples 1 - 3 and Comparative Examples 1 - 8 are as Figure 6 and Figure 7 shown. By comparing Comparative Examples 1 - 8 with the experimental results of the examples respectively, it can be found that the photocatalytic activities of pure CNN and MoS2 are relatively low. After the two are combined to form a heterojunction, their photocatalytic hydrogen production performance is slightly improved, which is due to the formation of the heterojunction promoting the effective separation of photogenerated electrons; after highly dispersed Au is introduced into CNN and MoS2 respectively, their hydrogen production performance is significantly improved, which is because highly dispersed Au can not only act as a co-catalyst to reduce the hydrogen production overpotential, but also provide abundant active sites for the hydrogen production reaction. The hydrogen production performance of CNN / Au / MoS2 is further greatly improved, which is because the interaction between the heterojunction and Au significantly improves the separation and utilization efficiency of carriers. Within a certain range, when the content of highly dispersed Au increases, the catalytic performance first increases and then decreases. This may be because some Au precursors agglomerate to form larger-sized Au, which destroys the original highly dispersed structure. When the loading ratio of Au is increased to 1.2% of the catalyst (this ratio is a common ratio of co-catalyst to catalyst in a conventional noble metal co-catalyst system), Au in the sample exists in the form of nanoparticles, and its catalytic activity is between CNN / Au 0.18 / MoS2 and CNN / Au 0.12 / MoS2. That is to say, when the Au loading is continued to increase on the basis of Au:CNN = 0.18%, the hydrogen production activity of the catalytic system will be improved. This is because when the concentration of Au increases to a certain extent, the highly dispersed Au in the sample aggregates to form Au nanoparticles, and the Au nanoparticles inject the hot electrons generated by their surface plasmon resonance effect into the catalyst, improving the utilization rate of the catalyst electrons. When the Au loading is further increased to 1.4%, the hydrogen production performance of the sample decreases again. This may be because a large number of large-sized Au nanoparticles occupy the active reaction sites on the catalyst surface, making it difficult for the catalytic reaction to proceed fully. The stability of the sample in Example 1 is as Figure 8 shown. CNN / Au / MoS2 shows good stability in the hydrogen production test for 4 cycles.

[0097] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art can still modify the specific implementation manners of the present invention or make equivalent replacements. Any such modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are within the scope of the claims of the present invention pending approval.

Claims

1. A CNN / Au / MoS2 photocatalyst containing highly dispersed Au, characterized in that, It includes three components: graphitic carbon nitride nanosheets, MoS2 nanosheets, and atomically sized Au. Au is distributed on the surface of graphitic carbon nitride nanosheets to form CNN / Au, and MoS2 nanosheets are distributed on the surface of CNN / Au, and a heterojunction is formed among the three components; the mass ratio of graphitic carbon nitride nanosheets to Au is 100:(0.06 - 0.18).

2. The preparation method of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au according to claim 1, characterized in that, It includes: S1, adsorbing chloroauric acid on graphitic carbon nitride nanosheets and calcining and reducing in a hydrogen atmosphere to obtain Au-loaded graphitic carbon nitride nanosheets; S2, dispersing the Au-loaded graphitic carbon nitride nanosheets in an aqueous solution containing sodium molybdate dihydrate and L-cysteine, and carrying out a hydrothermal reaction to obtain a CNN / Au / MoS2 heterojunction photocatalyst.

3. The preparation method of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au according to claim 2, characterized in that, In S1, the preparation method of the graphitic carbon nitride nanosheets is as follows: using dicyandiamide as a raw material, preparing bulk carbon nitride by a high-temperature calcination method, and obtaining graphitic carbon nitride nanosheets by a high-temperature exfoliation method for the bulk carbon nitride.

4. The preparation method of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au according to claim 3, characterized in that, The preparation method of the graphitic carbon nitride nanosheets specifically includes: (1) Under an air atmosphere, heating dicyandiamide to 545 - 555 °C for calcination to obtain bulk carbon nitride; (2) Under an air atmosphere, heating the bulk carbon nitride to 515 - 525 °C for calcination to obtain graphitic carbon nitride nanosheets.

5. The preparation method of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au according to claim 2, characterized in that, S1 specifically includes: (1) Disperse the graphitic carbon nitride nanosheets in water, then add chloroauric acid trihydrate and stir for 11.5 - 12.5 hours; perform solid-liquid separation on the obtained solution, and wash and dry the obtained precipitate to obtain graphitic carbon nitride nanosheets adsorbed with Au precursors; (2) Calcining and reducing the graphitic carbon nitride nanosheets adsorbed with Au precursors in a mixed atmosphere of hydrogen and argon to obtain highly dispersed Au-loaded graphitic carbon nitride nanosheets.

6. The preparation method of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au according to claim 2, characterized in that In S1, the calcination temperature is 100 - 120 °C, and the calcination time is 110 - 130 minutes.

7. The preparation method of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au according to claim 2, characterized in that, S2 specifically includes: (1) Disperse the Au-loaded graphitic carbon nitride nanosheets in water, then stir and ultrasonically treat to obtain a dispersion of CNN / Au; (2) Disperse sodium molybdate dihydrate and L-cysteine in water to obtain a precursor solution of MoS2; (3) Add the precursor solution of MoS2 to the dispersion of CNN / Au under stirring, and continue to stir evenly, then carry out a hydrothermal reaction on the obtained mixture; (4) After the reaction is completed, perform solid-liquid separation, and wash and dry the obtained solid to obtain a CNN / Au / MoS2 heterojunction photocatalyst.

8. The preparation method of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au according to claim 2, wherein, In S2, the temperature of the hydrothermal reaction is 190 - 210 °C, and the reaction time is 23 - 25 hours.

9. Application of the CNN / Au / MoS2 photocatalyst containing highly dispersed Au described in claim 1 in photocatalytic water splitting for hydrogen production.

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