Composite photocatalyst with double heterojunction and preparation method and application thereof

Abstract

The application discloses a composite photocatalyst with a double heterojunction and a preparation method and application thereof, and belongs to the technical field of photocatalytic water decomposition hydrogen production. The catalyst uses reduced graphene oxide with uniformly dispersed superfine Mo2C nanoparticles as a substrate, in-situ loads ZnCdS through a hydrothermal method to obtain ZnCdS@Mo2C / rGO, then grows Co-ZIF in the shape of a rhombic dodecahedron on the surface of the ZnCdS@Mo2C / rGO, and finally etches to obtain a hollow nanocage structure of NiCo-LDH, thereby constructing a NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst. 2+ The synergistic effect of the NiCo-LDH@ZnCdS heterojunction and the ZnCdS@Mo2C / rGO Schottky junction forms a cascade electron transfer channel, accelerates the transfer of photo-generated electrons to Mo2C, inhibits the recombination of photo-generated carriers, and improves the photocatalytic hydrogen production performance.

Description

Technical Field

[0001] This invention relates to a composite photocatalyst with dual heterojunctions, its preparation method, and its application, belonging to the field of photocatalytic water splitting for hydrogen production technology. Background Technology

[0002] With the depletion of fossil fuels and the continued growth of global energy demand, the development of renewable energy sources has become an urgent priority. Hydrogen energy, as a clean and efficient energy source with abundant reserves, safety, no pollution, and high energy density, has become an ideal alternative to fossil fuels. Among existing hydrogen production technologies, solar energy, as a continuous, safe, harmless, and abundant energy source, gives photocatalytic water-to-hydrogen technology a natural advantage. However, problems such as the rapid recombination of photogenerated electrons and holes, the narrow absorption range of sunlight by photocatalysts leading to insufficient utilization, and the rapid reverse reaction of products H2 and O2 on the catalyst surface result in low solar-to-hydrogen conversion efficiency for pure semiconductor photocatalysts, making them uncompetitive. Therefore, developing high-performance photocatalyst materials is currently the biggest challenge in photocatalytic hydrogen production.

[0003] The performance of photocatalysts can be improved through strategies such as morphology control, heterostructure construction, elemental doping, and cocatalyst modification. Among these, cocatalyst modification has been widely studied due to its ability to reduce recombination of photogenerated carriers, provide reaction sites, and lower activation energies. Noble metals, such as Pt, Pd, and Au, are currently the most efficient and stable cocatalysts, but their scarcity and high cost limit their large-scale application. Therefore, developing inexpensive and readily available non-noble metal cocatalysts has become a current research hotspot. Transition metal carbides, with their similar electronic structure to Pt, high conductivity, wide pH range, and good stability, exhibit excellent hydrogen evolution cocatalytic activity, making them ideal alternatives to Pt-based noble metal cocatalysts. Meanwhile, constructing heterojunctions by tightly contacting two semiconductors with interleaved band structures generates a potential difference due to the energy level difference between the semiconductors on both sides of the interface, inducing the formation of a built-in electric field. This accelerates the separation of photogenerated carriers within the space charge region, enhancing the photocatalytic activity. Simultaneously constructing heterojunctions and modifying the cocatalyst to build a dual heterostructure allows for further improvement in photocatalytic hydrogen production activity through the synergistic effect of the heterojunction and the cocatalyst. Especially for the composite of LDH nanosheets with CdS and the cocatalyst, the traditional hydrothermal method is often used to prepare LDH nanosheets. However, the band structure of these nanosheets does not satisfy the transfer of photogenerated electrons from CdS to the LDH nanosheets and then to the cocatalyst. This not only limits the driving force for photogenerated electron transfer but also leaves CdS in the outermost layer, making it susceptible to severe photocorrosion. Summary of the Invention

[0004] To address the limited ability of current transition metal carbide cocatalysts to enhance the performance of semiconductor photocatalytic hydrogen production, this invention provides a dual heterojunction composite photocatalyst that simultaneously possesses a type II heterojunction and a Schottky junction. This invention also provides a method for preparing this catalyst and its applications.

[0005] This invention provides a dual heterojunction composite photocatalyst, NiCo-LDH@ZnCdS@Mo2C / rGO, which simultaneously possesses a type II heterojunction NiCo-LDH@ZnCdS and a Schottky junction ZnCdS@Mo2C / rGO. Unlike traditional hydrothermal methods for preparing LDH nanosheets, this invention utilizes a metal ion etching method to prepare NiCo-LDH nanocages, thereby controlling their band structure to sequentially decrease the Fermi levels of NiCo-LDH, ZnCdS, and Mo2C. In the composite photocatalyst NiCo-LDH@ZnCdS@Mo2C / rGO, the built-in electric field at the NiCo-LDH@ZnCdS interface of the type II heterojunction is aligned with the built-in electric field at the ZnCdS@Mo2C / rGO interface of the Schottky junction. This allows NiCo-LDH to act as a propeller, driving more electrons to transfer along the cascaded electron transfer channels to Mo2C, thereby accelerating electron transfer and improving the utilization rate of photogenerated electrons. At the same time, the high Schottky barrier is maintained, effectively suppressing electron backflow and improving the separation efficiency of photogenerated carriers, thus enhancing the photocatalytic activity. Furthermore, the excellent photocorrosion resistance of the LDH nanocage protects ZnCdS, improving its photostability and thus enhancing the reusability of the composite catalyst.

[0006] This invention provides a composite photocatalyst with dual heterojunctions. Using uniformly dispersed ultrafine Mo2C nanoparticles on reduced graphene oxide as a substrate, ZnCdS is in-situ supported via a hydrothermal method to obtain ZnCdS@Mo2C / rGO. Furthermore, ZIF-67 is grown on the ZnCdS@Mo2C / rGO surface, and Ni is used for further processing. 2+ A hollow nanocage-like NiCo-LDH structure was obtained through etching, thereby constructing a NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst. Through the synergistic effect of the NiCo-LDH@ZnCdS heterojunction and the ZnCdS@Mo2C / rGO Schottky junction, a cascaded electron transfer channel is formed, accelerating the transfer of photogenerated electrons to Mo2C and suppressing photogenerated carrier recombination, thus improving the photocatalytic hydrogen production performance.

[0007] This invention provides a method for preparing the above-mentioned composite photocatalyst NiCo-LDH@ZnCdS@Mo2C / rGO with dual heterojunctions, comprising the following steps: The first step involved using Mo2C / rGO as a support, adding zinc, cadmium, and sulfur sources, and hydrothermally loading ZnCdS in situ to obtain a ZnCdS@Mo2C / rGO photocatalyst with a Schottky junction. The second step involves adding cobalt nitrate and 2-methylimidazole to grow rhombic dodecahedral Co-ZIF on the ZnCdS@Mo2C / rGO surface; The third step involves adding nickel nitrate and hydrothermally etching Co-ZIF to obtain hollow nanocage-like NiCo-ZIF in situ. This NiCo-ZIF is then combined with ZnCdS to form a heterojunction, resulting in a NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst with a double heterojunction.

[0008] In the above preparation method, the cadmium source is any one of cadmium chloride, cadmium nitrate, and cadmium acetate; the zinc source is any one of zinc chloride, zinc nitrate, and zinc acetate; and the sulfur source is any one of thiourea, sodium sulfide, and thioacetamide.

[0009] The preparation method of the above-mentioned composite photocatalyst NiCo-LDH@ZnCdS@Mo2C / rGO with dual heterojunctions is as follows: (1) 0.01-0.05 g of Mo2C / rGO was ultrasonically dispersed in 30-35 mL of deionized water, and 0.5-3.0 g of cadmium source, 0.5-3.0 g of zinc source, 0.2-3.0 g of sulfur source and 1-10 mL of NaOH solution (4 M) were added. After stirring for 1 h to fully dissolve the cadmium source, the solution was hydrothermally heated at 120-200℃ for 6-24 h to obtain ZnCdS@Mo2C / rGO composite photocatalyst with Schottky junction. (2) Disperse 0.01-0.3 g of ZnCdS@Mo2C / rGO and 0.01-0.15 g of cobalt nitrate hexahydrate in 20 mL of anhydrous methanol by ultrasonication, and label it as solution A; (3) Dissolve 0.01-0.15 g of 2-methylimidazole in another 20 mL of anhydrous methanol and label it as solution B; (4) Mix solution A from step (2) and solution B from step (3), stir at room temperature for 2-24 h, wash with alcohol and dry to obtain Co-ZIF@ZnCdS@Mo2C / rGO; (5) The Co-ZIF@ZnCdS@Mo2C / rGO obtained in step (4) was ultrasonically dispersed in 30 mL of anhydrous ethanol and labeled as solution C; (6) Dissolve 0.003-0.05 g of nickel nitrate hexahydrate in another 10 mL of anhydrous ethanol and label it as solution D; (7) Mix solution C from step (5) and solution D from step (6), and hydrothermally heat at 80-160℃ for 0.5-12 h to obtain NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst with dual heterostructure.

[0010] This invention provides a dual heterojunction composite photocatalyst, NiCo-LDH@ZnCdS@Mo2C / rGO, which simultaneously possesses a type II heterojunction NiCo-LDH@ZnCdS and a Schottky junction ZnCdS@Mo2C / rGO. First, ZnCdS is hydrothermally loaded onto the Mo2C / rGO cocatalyst to construct a ZnCdS@Mo2C / rGO Schottky junction. Then, rhombic dodecahedral Co-ZIF is grown on the surface of ZnCdS@Mo2C / rGO, and Ni is introduced into the substrate. 2+ Hollow nanocage-like NiCo-LDH was obtained through hydrothermal etching and then used to construct a type II heterojunction NiCo-LDH@ZnCdS with ZnCdS. Through rational design of the catalyst structure, the built-in electric fields at the interfaces of the NiCo-LDH@ZnCdS heterojunction and the Schottky junction ZnCdS@Mo2C / rGO are aligned. Under the synergistic effect of the NiCo-LDH@ZnCdS heterojunction and the ZnCdS@Mo2C / rGO Schottky junction, NiCo-LDH acts as a propulsion mechanism, driving more photogenerated electrons from ZnCdS to transfer to Mo2C / rGO. Simultaneously, the high Schottky barrier of ZnCdS@Mo2C / rGO prevents electron backflow, forming a cascaded electron transfer channel, suppressing recombination of photogenerated carriers, and improving the separation efficiency and effective utilization of photogenerated electrons, thereby enhancing the photocatalytic water splitting performance of ZnCdS for hydrogen production. Moreover, the hollow nanocage-like structure of NiCo-LDH can prevent the aggregation of layered bimetallic hydroxides, fully exposing more active sites, which is beneficial for electron transfer.

[0011] This invention provides the application of the aforementioned composite photocatalyst NiCo-LDH@ZnCdS@Mo2C / rGO with dual heterojunctions in photocatalytic water splitting for hydrogen production. A 300 W xenon lamp (with a 400 nm filter) was used as the light source, and the reaction was carried out in a sacrificial system of 100 mL of 0.35 M Na2S / 0.25 M Na2SO3. The hydrogen production rate of pure ZnCdS was 36.1 mmol g. –1 h –1 The hydrogen production rate of the ZnCdS@Mo2C / rGO Schottky junction was 52.8 mmol g. –1 h –1 The hydrogen production rate of the NiCo-LDH@ZnCdS type II heterojunction was 51.7 mmol g. –1 h –1 The hydrogen production rate of NiCo-LDH@ZnCdS@Mo2C / rGO, which simultaneously possesses both type II heterojunctions and Schottky junctions, is 90.1 mmol g. –1 h –1The efficiency is 1.7 times that of ZnCdS@Mo2C / rGO Schottky junction and 2.5 times that of pure ZnCdS, which fully demonstrates the superiority of the catalyst of this invention.

[0012] The beneficial effects of this invention are: (1) The establishment of the double heterojunction in NiCo-LDH@ZnCdS@Mo2C / rGO not only improves the transfer of electrons at the interface between NiCo-LDH@ZnCdS and ZnCdS@Mo2C / rGO through the built-in electric field at the interface, but also allows the two built-in electric fields to work synergistically because their electric field directions are consistent. This allows NiCo-LDH to act as a propulsion mechanism, driving more disordered electrons in ZnCdS to be transferred in an orderly manner to the ZnCdS@Mo2C / rGO interface, and then to Mo2C / rGO for hydrogen production through reduction. At the same time, the rectification effect of the ZnCdS@Mo2C / rGO Schottky heterojunction can prevent electrons transferred to Mo2C / rGO from flowing back to ZnCdS, promoting the cascade transfer of electrons along the NiCo-LDH→ZnCdS→Mo2C path, optimizing the utilization rate of photogenerated carriers, and thus improving the reaction activity. (2) Hollow nanocage-like NiCo-LDH was prepared by etching ZIF with metal salts. Morphologically, this effectively avoids the shortcomings of easy agglomeration and stacking of layered bimetallic hydroxides, resulting in a larger specific surface area and fully exposing more active sites, which is beneficial for electron transfer. In terms of band structure, its band structure can be tuned, making its Fermi level significantly higher than that of ZnCdS, constructing a type II heterojunction, and promoting electron transfer. Ultimately, the hydrogen production rate of the composite photocatalyst reached 90.1 mmol g. –1 h –1 It has 2.5 times the hydrogen production performance of pure ZnCdS.

[0013] To illustrate the structure and photocatalytic hydrogen evolution performance of the composite photocatalyst with dual heterojunctions prepared in this invention, further explanation is provided in conjunction with the accompanying drawings. Attached Figure Description

[0014] Figure 1 The XRD and FT-IR spectra of the Mo2C / rGO, ZnCdS@Mo2C / rGO, Co-ZIF@ZnCdS@Mo2C / rGO and NiCo-LDH@ZnCdS@Mo2C / rGO samples prepared in Example 1 are shown.

[0015] Figure 2 SEM images of the Mo2C / rGO, ZnCdS@Mo2C / rGO, Co-ZIF@ZnCdS@Mo2C / rGO and NiCo-LDH@ZnCdS@Mo2C / rGO samples prepared in Example 1.

[0016] Figure 3 The image shows the HR-TEM spectrum of the NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst prepared in Example 1.

[0017] Figure 4 The graph shows a comparison of the photocatalytic hydrogen production performance of Examples 1-5.

[0018] Figure 5 The graph shows a comparison of the photocatalytic hydrogen production performance of Example 1 and Comparative Examples 1-4.

[0019] Figure 6 This is a comparison chart of the photocatalytic hydrogen production performance of Example 1 and Comparative Example 5. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and all materials publicly cited herein and cited by them are incorporated herein by reference.

[0022] Equivalent technologies of the specific embodiments described herein that are readily apparent to those skilled in the art through routine experimentation are included in this application.

[0023] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the instruments and equipment used in the following examples are all standard laboratory instruments and equipment; unless otherwise specified, the experimental materials used in the following examples were all purchased from regular biochemical reagent stores.

[0024] (a) The following example illustrates the preparation of composite photocatalysts supported on dual transition metal carbide co-catalysts. Example 1

[0025] 0.015 g of Mo₂C / rGO was ultrasonically dispersed in 30 mL of deionized water. 1.32 g of zinc acetate dihydrate and 1.60 g of cadmium acetate dihydrate were added, and the mixture was stirred for 10 min to ensure complete dissolution in the Mo₂C / rGO solution. Then, 1.1 g of thioacetamide and 6 mL of 4 M NaOH solution were added, and the mixture was stirred for 1 h. The resulting solution was then transferred to a high-pressure reactor and hydrothermally heated at 180 °C for 12 h to obtain ZnCdS@Mo₂C / rGO.

[0026] 0.1 g ZnCdS@Mo2C / rGO and 0.044 g cobalt nitrate hexahydrate were ultrasonically dispersed in 20 mL of anhydrous methanol and labeled as solution A. Simultaneously, 0.05 g 2-methylimidazole was dissolved in another 20 mL of anhydrous methanol and labeled as solution B. Solutions A and B were rapidly mixed and stirred at room temperature for 2 h. After centrifugation, washing with alcohol, and drying, the Co-ZIF@ZnCdS@Mo2C / rGO composite was obtained.

[0027] The Co-ZIF@ZnCdS@Mo2C / rGO obtained above was ultrasonically dispersed again in 30 mL of anhydrous ethanol and labeled as solution C. Simultaneously, 0.005 g of nickel nitrate hexahydrate was ultrasonically dissolved in 10 mL of anhydrous ethanol and labeled as solution D. Solutions C and D were mixed and stirred until homogeneous. Then, the mixture was transferred to a high-pressure reactor and hydrothermally heated at 90 °C for 1 h to obtain a hollow nanocage-like NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst.

[0028] Figure 1 XRD (a) and FT-IR (b) spectra of Mo2C / rGO, ZnCdS@Mo2C / rGO, Co-ZIF@ZnCdS@Mo2C / rGO, and NiCo-LDH@ZnCdS@Mo2C / rGO samples prepared in Example 1. The spectra show that the hydrothermally prepared ZnCdS is a ZnCdS solid solution, not a physical mixture of ZnS and CdS. After growing Co-ZIF on the ZnCdS@Mo2C / rGO surface, a distinct Co-ZIF peak appeared, while in Ni... 2+ After etching, the ZIF peak disappeared, proving that the ZIF structure was destroyed and NiCo-LDH was formed, indicating the successful preparation of NiCo-LDH@ZnCdS@Mo2C / rGO.

[0029] Figure 2SEM images of Mo2C / rGO (a), ZnCdS@Mo2C / rGO (b), Co-ZIF@ZnCdS@Mo2C / rGO (c), and NiCo-LDH@ZnCdS@Mo2C / rGO (d) prepared in Example 1 are shown. The images clearly show that the Mo2C / rGO surface is smooth and has a wide lateral dimension; after loading ZnCdS, the surface is coated with a layer of nanoparticles; further, after growing Co-ZIF on the ZnCdS@Mo2C / rGO surface, a distinct rhombic dodecahedral structure appears, and the surface is smooth; through Ni... 2+ After etching, it became a rough hexagon, proving the destruction of ZIF.

[0030] Figure 3 The image shows the HR-TEM spectrum of the NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst prepared in Example 1. The crystal planes with lattice spacings of 0.32 nm, 0.22 nm, and 0.28 nm correspond to the ZnCdS (111) crystal plane, the Mo2C (101) crystal plane, and the NiCo-LDH (100) crystal plane, respectively, further confirming the successful preparation of NiCo-LDH@ZnCdS@Mo2C / rGO. Example 2

[0031] 0.01 g of Mo₂C / rGO was ultrasonically dispersed in 30 mL of deionized water. 1.0 g of zinc nitrate hexahydrate and 1.0 g of cadmium nitrate tetrahydrate were added, and the mixture was stirred for 10 min to ensure complete dissolution in the Mo₂C / rGO solution. Then, 0.5 g of thiourea and 4 mL of 4M NaOH solution were added, and the mixture was stirred for 1 h. The resulting mixture was then transferred to a high-pressure reactor and hydrothermally heated at 120 °C for 6 h to obtain ZnCdS@Mo₂C / rGO.

[0032] 0.05 g ZnCdS@Mo2C / rGO and 0.01 g cobalt nitrate hexahydrate were ultrasonically dispersed in 20 mL of anhydrous methanol and labeled as solution A. Simultaneously, 0.01 g 2-methylimidazole was dissolved in another 20 mL of anhydrous methanol and labeled as solution B. Solutions A and B were rapidly mixed and stirred at room temperature for 12 h. After centrifugation, washing with alcohol, and drying, the Co-ZIF@ZnCdS@Mo2C / rGO composite was obtained.

[0033] The Co-ZIF@ZnCdS@Mo2C / rGO obtained above was ultrasonically dispersed again in 30 mL of anhydrous ethanol and labeled as solution C. Simultaneously, 0.003 g of nickel nitrate hexahydrate was ultrasonically dissolved in 10 mL of anhydrous ethanol and labeled as solution D. Solutions C and D were mixed and stirred until homogeneous. Then, the mixture was transferred to a high-pressure reactor and hydrothermally heated at 80 °C for 0.5 h to obtain a hollow nanocage-like NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst. Example 3

[0034] 0.03 g of Mo₂C / rGO was ultrasonically dispersed in 30 mL of deionized water. 2.0 g of zinc chloride and 2.0 g of cadmium chloride were added, and the mixture was stirred for 10 min to ensure complete dissolution in the Mo₂C / rGO solution. Then, 1.5 g of sodium sulfide and 8 mL of 4 M NaOH solution were added, and the mixture was stirred for 1 h. The resulting mixture was then transferred to a high-pressure reactor and hydrothermally heated at 200 °C for 24 h to obtain ZnCdS@Mo₂C / rGO.

[0035] 0.15 g of ZnCdS@Mo2C / rGO and 0.07 g of cobalt nitrate hexahydrate were ultrasonically dispersed in 20 mL of anhydrous methanol and labeled as solution A. Simultaneously, 0.08 g of 2-methylimidazole was dissolved in another 20 mL of anhydrous methanol and labeled as solution B. Solutions A and B were rapidly mixed and stirred at room temperature for 24 h. After centrifugation, washing with alcohol, and drying, the Co-ZIF@ZnCdS@Mo2C / rGO composite was obtained.

[0036] The Co-ZIF@ZnCdS@Mo2C / rGO obtained above was ultrasonically dispersed again in 30 mL of anhydrous ethanol and labeled as solution C. Simultaneously, 0.01 g of nickel nitrate hexahydrate was ultrasonically dissolved in 10 mL of anhydrous ethanol and labeled as solution D. Solutions C and D were mixed and stirred thoroughly. Then, the mixture was transferred to a high-pressure reactor and hydrothermally heated at 160 °C for 12 h to obtain a hollow nanocage-like NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst. Example 4

[0037] 0.02 g of Mo₂C / rGO was ultrasonically dispersed in 30 mL of deionized water. 1.50 g of zinc acetate dihydrate and 1.50 g of cadmium acetate dihydrate were added, and the mixture was stirred for 10 min to ensure complete dissolution in the Mo₂C / rGO solution. Then, 1.1 g of thiourea and 5 mL of 4 M NaOH solution were added, and the mixture was stirred for 1 h. The resulting solution was then transferred to a high-pressure reactor and hydrothermally heated at 160 °C for 16 h to obtain ZnCdS@Mo₂C / rGO.

[0038] 0.12 g of ZnCdS@Mo2C / rGO and 0.06 g of cobalt nitrate hexahydrate were ultrasonically dispersed in 20 mL of anhydrous methanol and labeled as solution A. Simultaneously, 0.06 g of 2-methylimidazole was dissolved in another 20 mL of anhydrous methanol and labeled as solution B. Solutions A and B were rapidly mixed and stirred at room temperature for 2 h. After centrifugation, washing with alcohol, and drying, the Co-ZIF@ZnCdS@Mo2C / rGO composite was obtained.

[0039] The Co-ZIF@ZnCdS@Mo2C / rGO obtained above was ultrasonically dispersed again in 30 mL of anhydrous ethanol and labeled as solution C. Simultaneously, 0.007 g of nickel nitrate hexahydrate was ultrasonically dissolved in 10 mL of anhydrous ethanol and labeled as solution D. Solutions C and D were mixed and stirred until homogeneous. Then, the mixture was transferred to a high-pressure reactor and hydrothermally heated at 120 °C for 2 h to obtain a hollow nanocage-like NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst. Example 5

[0040] 0.015 g of Mo₂C / rGO was ultrasonically dispersed in 30 mL of deionized water. 2.0 g of zinc acetate dihydrate and 2.0 g of cadmium acetate dihydrate were added, and the mixture was stirred for 10 min to ensure complete dissolution in the Mo₂C / rGO solution. Then, 1.5 g of sodium sulfide and 4 mL of 4 M NaOH solution were added, and the mixture was stirred for 1 h. The resulting mixture was then transferred to a high-pressure reactor and hydrothermally heated at 140 °C for 24 h to obtain ZnCdS@Mo₂C / rGO.

[0041] 0.1 g ZnCdS@Mo2C / rGO and 0.07 g cobalt nitrate hexahydrate were ultrasonically dispersed in 20 mL of anhydrous methanol and labeled as solution A. Simultaneously, 0.08 g 2-methylimidazole was dissolved in another 20 mL of anhydrous methanol and labeled as solution B. Solutions A and B were rapidly mixed and stirred at room temperature for 6 h. After centrifugation, washing with alcohol, and drying, the Co-ZIF@ZnCdS@Mo2C / rGO composite was obtained.

[0042] The Co-ZIF@ZnCdS@Mo2C / rGO obtained above was ultrasonically dispersed again in 30 mL of anhydrous ethanol and labeled as solution C. Simultaneously, 0.01 g of nickel nitrate hexahydrate was ultrasonically dissolved in 10 mL of anhydrous ethanol and labeled as solution D. Solutions C and D were mixed and stirred until homogeneous. Then, the mixture was transferred to a high-pressure reactor and hydrothermally heated at 140 °C for 6 h to obtain a hollow nanocage-like NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst.

[0043] Comparative Example 1: 0.015 g of Mo₂C / rGO was ultrasonically dispersed in 30 mL of deionized water. 1.32 g of zinc acetate dihydrate and 1.60 g of cadmium acetate dihydrate were added, and the mixture was stirred for 10 min to ensure complete dissolution in the Mo₂C / rGO solution. Then, 1.1 g of thioacetamide and 6 mL of 4 M NaOH solution were added, and the mixture was stirred for 1 h. The resulting solution was then transferred to a high-pressure reactor and hydrothermally heated at 180 °C for 12 h to obtain ZnCdS@Mo₂C / rGO.

[0044] Comparative Example 2: Weigh 1.32 g of zinc acetate dihydrate and 1.60 g of cadmium acetate dihydrate and dissolve them in 30 mL of deionized water. Stir for 10 min to ensure complete dissolution. Then add 1.1 g of thioacetamide and 6 mL of 4 M NaOH solution and stir for 1 h. Transfer the mixture to a high-pressure reactor and hydrothermally heat at 180 °C for 12 h to obtain ZnCdS.

[0045] Comparative Example 3: Weigh 1.32 g of zinc acetate dihydrate and 1.60 g of cadmium acetate dihydrate and dissolve them in 30 mL of deionized water. Stir for 10 min to ensure complete dissolution. Then add 1.1 g of thioacetamide and 6 mL of 4 M NaOH solution and stir for 1 h. Transfer the mixture to a high-pressure reactor and hydrothermally heat at 180 °C for 12 h to obtain ZnCdS.

[0046] 0.1 g ZnCdS and 0.044 g cobalt nitrate hexahydrate were ultrasonically dispersed in 20 mL of anhydrous methanol and labeled as solution A. Simultaneously, 0.05 g 2-methylimidazole was dissolved in another 20 mL of anhydrous methanol and labeled as solution B. Solutions A and B were rapidly mixed and stirred at room temperature for 2 h. After centrifugation, washing with alcohol, and drying, the Co-ZIF@ZnCdS composite was obtained.

[0047] The Co-ZIF@ZnCdS obtained above was ultrasonically dispersed again in 30 mL of anhydrous ethanol and labeled as solution C. Simultaneously, 0.005 g of nickel nitrate hexahydrate was ultrasonically dissolved in 10 mL of anhydrous ethanol and labeled as solution D. Solutions C and D were mixed and stirred until homogeneous. Then, the mixture was transferred to a high-pressure reactor and hydrothermally heated at 90 °C for 1 h to obtain a hollow nanocage-like NiCo-LDH@ZnCdS composite photocatalyst.

[0048] Comparative Example 4: 0.044 g of cobalt nitrate hexahydrate was ultrasonically dispersed in 20 mL of anhydrous methanol and labeled as solution A. Simultaneously, 0.05 g of 2-methylimidazole was dissolved in another 20 mL of anhydrous methanol and labeled as solution B. Solutions A and B were rapidly mixed and stirred at room temperature for 2 h. After centrifugation, washing with alcohol, and drying, Co-ZIF was obtained.

[0049] The Co-ZIF obtained above was ultrasonically dispersed again in 30 mL of anhydrous ethanol and labeled as solution C. Simultaneously, 0.005 g of nickel nitrate hexahydrate was ultrasonically dissolved in 10 mL of anhydrous ethanol and labeled as solution D. Solutions C and D were mixed and stirred thoroughly. Then, the mixture was transferred to a high-pressure reactor and hydrothermally heated at 90 °C for 1 h to obtain hollow nanocage-like NiCo-LDH.

[0050] Comparative Example 5: 0.015 g of Mo₂C / rGO was ultrasonically dispersed in 30 mL of deionized water. 1.32 g of zinc acetate dihydrate and 1.60 g of cadmium acetate dihydrate were added, and the mixture was stirred for 10 min to ensure complete dissolution in the Mo₂C / rGO solution. Then, 1.1 g of thioacetamide and 6 mL of 4 M NaOH solution were added, and the mixture was stirred for 1 h. The resulting solution was then transferred to a high-pressure reactor and hydrothermally heated at 180 °C for 12 h to obtain ZnCdS@Mo₂C / rGO.

[0051] The above-mentioned 0.1 g ZnCdS@Mo2C / rGO, 0.044 g cobalt nitrate hexahydrate and 0.031 g nickel nitrate hexahydrate were ultrasonically dispersed in 40 mL deionized water and stirred at room temperature for 30 min. Then, 0.010 g ammonium fluoride and 0.038 g urea were added, and stirring was continued for another 30 min. The mixture was then transferred to a polytetrafluoroethylene liner and hydrothermally heated at 120 °C for 12 h to obtain NiCo-LDH nanosheets@ZnCdS@Mo2C / rGO.

[0052] (II) Photocatalytic hydrogen production performance of the composite photocatalyst NiCo-LDH@ZnCdS@Mo2C / rGO in this invention The photocatalyst from Example 1 was evaluated using a top-illuminated Pirilux glass reactor. 0.035 g of the photocatalyst was weighed and dispersed in 100 mL of a 0.35 M Na₂S / 0.25 M Na₂SO₃ mixed solution. The solution was placed in the reactor, the reaction apparatus was connected, and the reaction system temperature was maintained at 6°C. The performance of the photocatalytic water splitting for hydrogen production was evaluated using a 300 W xenon lamp (equipped with a 400 nm filter) under magnetic stirring. Before starting the reaction, the reaction system was evacuated for 30 min to completely remove air. The amount of hydrogen produced was quantitatively analyzed using a gas chromatograph equipped with a thermal conductivity detector.

[0053] Using the same method described above, the catalytic hydrogen production performance of the hollow nanocage-structured NiCo-LDH@ZnCdS@Mo2C / rGO photocatalysts obtained in Examples 2-5 was tested. Figure 4 As shown, the NiCo-LDH@ZnCdS@Mo2C / rGO in Example 1 exhibits a hydrogen production performance as high as 90.1 mmol g. –1 h –1 The hydrogen production performance of Examples 2-5 was 60.7, 73.5, 65.3, and 70.6 mmol g, respectively. –1 h –1 The values ​​were all lower than those of Example 1, but still significantly higher than those of Comparative Examples 1-4.

[0054] Using the same method described above, the catalytic hydrogen production performance of the photocatalysts ZnCdS@Mo2C / rGO, ZnCdS, NiCo-LDH@ZnCdS, NiCo-LDH, and NiCo-LDH nanosheets@ZnCdS@Mo2C / rGO obtained in Comparative Examples 1-5 was tested.

[0055] like Figure 5 As shown, the NiCo-LDH nanocages@ZnCdS@Mo2C / rGO of Example 1 exhibit a hydrogen production performance as high as 90.1 mmol / g. –1 h –1 The hydrogen evolution performance of the composite photocatalyst NiCo-LDH nanocages@ZnCdS@Mo2C / rGO was 2.5 times and 1.7 times that of pure ZnCdS and ZnCdS@Mo2C / rGO, respectively, indicating that the composite photocatalyst NiCo-LDH nanocages@ZnCdS@Mo2C / rGO prepared in this invention has excellent photocatalytic water splitting hydrogen evolution performance. Clearly, the hydrogen evolution performance of NiCo-LDH nanocages@ZnCdS@Mo2C / rGO is superior to that of NiCo-LDH nanosheets@ZnCdS@Mo2C / rGO (Comparative Example 5, 60.3 mmol g). –1 h –1 NiCo-LDH nanocages@ZnCdS (Comparative Example 3, 51.7 mmol g) –1 h –1 ZnCdS@Mo2C / rGO (Comparative Example 1, 52.8 mmol g) –1 h –1 ZnCdS (comparative example 2, 36.1 mmol g) –1 h –1The hydrogen production of NiCo-LDH (Comparative Example 4) was almost zero, indicating that the synergistic effect of the NiCo-LDH@ZnCdS heterojunction and the ZnCdS@Mo2C / rGO Schottky junction in NiCo-LDH@ZnCdS@Mo2C / rGO can promote the effective separation of photogenerated electrons and greatly enhance the photocatalytic hydrogen production performance of ZnCdS.

[0056] In addition, such as Figure 6 As shown, the hydrogen production performance of NiCo-LDH nanocages@ZnCdS@Mo2C / rGO is superior to that of NiCo-LDH nanosheets@ZnCdS@Mo2C / rGO (Comparative Example 5), demonstrating the superiority of the NiCo-LDH nanocages@ZnCdS@Mo2C / rGO structure. In NiCo-LDH nanocages@ZnCdS@Mo2C / rGO, the built-in electric field at the interface between the NiCo-LDH nanocages@ZnCdS heterojunction and the ZnCdS@Mo2C / rGO Schottky junction is aligned, causing the NiCo-LDH nanocages to act as a propeller, driving more photogenerated electrons from ZnCdS to transfer to Mo2C / rGO. This not only improves the separation efficiency of photogenerated carriers but also greatly promotes the effective utilization of photogenerated electrons, thereby enhancing the photocatalytic hydrogen production activity. The band structure of NiCo-LDH nanosheets cannot align the direction of the built-in electric field at the interface between the NiCo-LDH nanosheet@ZnCdS heterojunction and the ZnCdS@Mo2C / rGO Schottky junction. Therefore, it cannot promote more photogenerated electron transfer, resulting in weaker hydrogen production performance compared to NiCo-LDH nanocage@ZnCdS@Mo2C / rGO.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite photocatalyst with a dual heterojunction, characterized in that: Using uniformly dispersed ultrafine Mo2C nanoparticles on reduced graphene oxide as a substrate, ZnCdS was in-situ supported on ZnCdS via a hydrothermal method to obtain ZnCdS@Mo2C / rGO. Then, rhombic dodecahedral Co-ZIF was grown on the ZnCdS@Mo2C / rGO surface, followed by Ni... 2+ A hollow nanocage-like NiCo-LDH structure was obtained by etching, and a NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst was constructed. Through the synergistic effect of the NiCo-LDH@ZnCdS heterojunction and the ZnCdS@Mo2C / rGO Schottky junction, a cascaded electron transfer channel was formed, which accelerated the transfer of photogenerated electrons to Mo2C, suppressed the recombination of photogenerated carriers, and improved the photocatalytic hydrogen production performance.

2. A method for preparing the composite photocatalyst with dual heterojunctions as described in claim 1, characterized in that... Includes the following steps: The first step involved using Mo2C / rGO as a support, adding zinc, cadmium, and sulfur sources, and hydrothermally loading ZnCdS in situ to obtain a ZnCdS@Mo2C / rGO photocatalyst with a Schottky junction. The second step involves adding cobalt nitrate and 2-methylimidazole to grow rhombic dodecahedral Co-ZIF on the ZnCdS@Mo2C / rGO surface; The third step involves adding nickel nitrate and hydrothermally etching Co-ZIF to obtain hollow nanocage-like NiCo-LDH in situ. This NiCo-LDH is then combined with ZnCdS to form a heterojunction, resulting in a NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst with a double heterojunction.

3. The method for preparing the composite photocatalyst with dual heterojunctions according to claim 2, characterized in that: The cadmium source is any one of cadmium chloride, cadmium nitrate, or cadmium acetate; the zinc source is any one of zinc chloride, zinc nitrate, or zinc acetate; and the sulfur source is any one of thiourea, sodium sulfide, or thioacetamide.

4. The method for preparing the composite photocatalyst with dual heterojunctions according to claim 2, characterized in that: The specific steps are as follows: (1) 0.01-0.05 g of Mo2C / rGO was ultrasonically dispersed in 30-35 mL of deionized water, and 0.5-3.0 g of cadmium source, 0.5-3.0 g of zinc source, 0.2-3.0 g of sulfur source and 1-10 mL of NaOH solution were added. After stirring for 1 h to fully dissolve the cadmium source, the solution was hydrothermally heated at 120-200℃ for 6-24 h to obtain ZnCdS@Mo2C / rGO composite photocatalyst with Schottky junction. (2) Disperse 0.01-0.3 g of ZnCdS@Mo2C / rGO and 0.01-0.15 g of cobalt nitrate hexahydrate in 20 mL of anhydrous methanol by ultrasonication, and label it as solution A; (3) Dissolve 0.01-0.15 g of 2-methylimidazole in another 20 mL of anhydrous methanol and label it as solution B; (4) Mix solution A from step (2) and solution B from step (3), stir at room temperature for 2-24 h, wash with alcohol and dry to obtain Co-ZIF@ZnCdS@Mo2C / rGO; (5) The Co-ZIF@ZnCdS@Mo2C / rGO obtained in step (4) was ultrasonically dispersed in 30 mL of anhydrous ethanol and labeled as solution C; (6) Dissolve 0.003-0.05 g of nickel nitrate hexahydrate in another 10 mL of anhydrous ethanol and label it as solution D; (7) Mix solution C from step (5) and solution D from step (6), and hydrothermally heat at 80-160℃ for 0.5-12 h to obtain NiCo-LDH@ZnCdS@Mo2C / rGO composite photocatalyst with dual heterostructure.

5. The method for preparing the composite photocatalyst with dual heterojunctions according to claim 4, characterized in that: In step (1), the concentration of the NaOH solution is 4 M.

6. The application of the composite photocatalyst with dual heterojunctions as described in claim 1 in photocatalytic water splitting for hydrogen production.

7. The application according to claim 6, characterized in that: A 300 W xenon lamp light source was used, and the reaction was carried out in a sacrificial system of 100 mL of 0.35 M Na2S / 0.25 M Na2SO3.

8. The application according to claim 6, characterized in that: The composite photocatalyst NiCo-LDH@ZnCdS@Mo2C / rGO with dual heterojunctions can achieve a hydrogen production rate of 90.1 mmol·g. –1 ·h –1 .

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