Preparation method and application of a composite photocatalyst doped with carbon quantum dots and Z-type heterojunction
By constructing a Z-shaped heterojunction of NiCo2S4 and CQDs on Mn0.5Cd0.5S, the problem of low separation efficiency of photogenerated electron-hole pairs in Mn0.5Cd0.5S photocatalyst was solved, and the photocatalytic performance was significantly improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-06-12
- Publication Date
- 2026-07-31
AI Technical Summary
Pure Mn0.5Cd0.5S photocatalysts have drawbacks in the photocatalytic hydrogen evolution process, such as low separation efficiency of photogenerated electron-hole pairs, few surface active centers, and poor stability, which affect their photocatalytic performance.
By introducing NiCo2S4 and carbon quantum dots (CQDs), a Z-type heterojunction composite photocatalyst was constructed. NiCo2S4 and Mn0.5Cd0.5S formed a heterojunction, and CQDs acted as an electron transfer bridge to promote the separation and transport of photogenerated charge carriers.
It significantly improves the light capture efficiency and spectral response range of the photocatalyst, enhances the migration and separation of photogenerated electron-hole pairs, strengthens the electron transport rate, and improves the photocatalytic performance.
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Figure CN122479775A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of photocatalytic materials, specifically to a method for preparing and applying a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction. Background Technology
[0002] In recent years, rapid economic development has led to the consumption of large quantities of traditional fossil fuels such as coal, oil, and natural gas. The waste and wastewater generated from the combustion of fossil fuels are discharged into the environment, causing pollution. Solar-driven photocatalytic water splitting technology, as a specific green method for hydrogen recovery, is considered a clean and sustainable approach with enormous application potential. In this process, catalysts play a crucial role. Among numerous photocatalytic materials, bimetallic sulfides are highly effective photocatalysts because the synergistic effect between the two metal ions gives them stable and excellent hydrogen evolution activity. It is worth noting that Mn... 0.5 Cd 0.5 S-based three-component solid solutions have become a research hotspot due to their advantages such as controllable band gap, good optical response, and high Fermi level. However, pure Mn... 0.5 Cd 0.5 S has drawbacks such as low efficiency in separating photogenerated electron-hole pairs, few surface active centers, and poor stability, which affect its photocatalytic hydrogen evolution performance. Summary of the Invention
[0003] This invention aims to solve the existing Mn 0.5 Cd 0.5 To address the issue of poor hydrogen evolution performance in S-type photocatalysis, a method for preparing and applying a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction is provided.
[0004] The present invention discloses a method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction, comprising the following steps:
[0005] 1. Mixing NiCo2S4 and Mn 0.5 Cd 0.5 S was added to anhydrous ethanol, sonicated at room temperature, stirred, and then evaporated to dryness to obtain a solid powder; the solid powder was then vacuum dried to obtain Mn. 0.5 Cd 0.5 S / NiCo2S4 composite material;
[0006] II. Adding carbon quantum dot solution to Mn 0.5 Cd 0.5 In the S / NiCo2S4 composite material, ultrasonic treatment at room temperature, followed by stirring and vacuum drying, yielded CQDs / NiCo2S4 / Mn. 0.5 Cd 0.5 S composite photocatalyst.
[0007] Furthermore, in step one, NiCo2S4 and Mn 0.5 Cd 0.5 The molar mass ratio of S is 1: (0.05~0.1).
[0008] Furthermore, the ultrasonic treatment time in step one is 30-40 minutes.
[0009] Furthermore, the stirring time mentioned in step one is 6 to 8 hours.
[0010] Furthermore, the evaporation to dryness described in step one is carried out in a water bath at 60~65 °C.
[0011] Furthermore, the vacuum drying described in step one is performed at 60~70 ℃ for 12~14 h.
[0012] Furthermore, in step two, the concentration of the carbon quantum dot solution is 0.007~0.015 g / mL.
[0013] Furthermore, in step two, the carbon quantum dot solution reacts with Mn... 0.5 Cd 0.5 The volume ratio of the S / NiCo2S4 composite material is 1: (0.6~1.6).
[0014] Furthermore, the preparation method of the carbon quantum dot solution in step two is as follows:
[0015] Alkali lignin was ultrasonically dissolved in deionized water and stirred until homogeneous. The solution was then transferred to a polytetrafluoroethylene-lined reactor and reacted at 180–185 °C for 12–14 h. The mixture was then allowed to cool naturally to room temperature. Large carbon particles formed by agglomeration were removed by filtration. The filtrate was placed in a dialysis bag and dialyzed at room temperature for 3–5 days, with the water changed every 8 h. After dialysis, the carbon quantum dot solution was obtained by filtration.
[0016] The present invention also provides the application of the composite photocatalyst prepared by the above method in photocatalytic hydrogen production.
[0017] The beneficial effects of this invention are:
[0018] This invention successfully prepared NiCo2S4 nano-eggshell spheres and Mn nanospheres via the solubilizing and hydrothermal methods, respectively. 0.5 Cd 0.5 S nanoparticles were used to construct NiCo2S4 / Mn 0.5 Cd 0.5 Z-type heterojunction composites of NiCo2S4 and Mn were developed. Experimental results show that this heterojunction composite exhibits significantly superior photocatalytic activity compared to the single component. Further, NiCo2S4 / Mn... 0.5 Cd 0.5S is combined with carbon quantum dots (CQDs). The excellent light absorption properties of CQDs can improve the light harvesting efficiency and spectral response range of the composite material, thereby driving sufficient photogenerated carrier excitation. Furthermore, CQDs act as electron transfer bridges in Mn... 0.5 Cd 0.5 The rapid conduction channel formed between S and NiCo2S4 not only effectively promotes the migration and separation of photogenerated electron-hole pairs, but also significantly enhances the electron transport rate and suppresses recombination during transport, ultimately improving the efficiency of CQDs / NiCo2S4 / Mn. 0.5 Cd 0.5 S photocatalytic performance. Attached Figure Description
[0019] Figure 1 Here are the XRD patterns of the photocatalyst; where (a) is Mn 0.5 Cd 0.5 (b) shows the XRD spectrum of S, and (c) shows the NiCo2S4 / Mn ratio obtained with different NiCo2S4 addition amounts in Example 1. 0.5 Cd 0.5 XRD spectra of the S composite material, (c) showing the CQDs / NiCo2S4 / Mn composites obtained with different CQDs solution addition amounts. 0.5 Cd 0.5 XRD spectra of S composite photocatalysts;
[0020] Figure 2 The results show the morphology analysis of the photocatalyst; where (a) represents Mn. 0.5 Cd 0.5 (a) is the SEM image of S, (b) is the SEM image of NiCo2S4, and (c) is the SEM image of CQDs / NiCo2S4 / Mn. 0.5 Cd 0.5 SEM image of the S composite photocatalyst, (d) is CQDs / NiCo2S4 / Mn 0.5 Cd 0.5 HRTEM image of the S composite photocatalyst, (e) is CQDs / NiCo2S4 / Mn 0.5 Cd 0.5 EDS surface distribution diagram of the S composite photocatalyst, (f) is the CQDs / NiCo2S4 / Mn 0.5 Cd 0.5 EDS energy dispersive spectroscopy analysis of S composite photocatalyst;
[0021] Figure 3 High-resolution XPS spectra of 20C / 8% NCS / MCS prepared in Example 2; where (a) is Mn 0.5 Cd 0.5XPS measurements of full spectra of S, NiCo2S4 and 20C / 8%NCS / MCS eggshell nanospheres: (b) high-resolution spectrum of C 1s, (c) high-resolution spectrum of Ni 2p, (d) high-resolution spectrum of Co 2p, (e) high-resolution spectrum of Cd 3d, (f) high-resolution spectrum of S 2p, and (g) high-resolution spectrum of Mn 2p.
[0022] Figure 4 The UV-vis DRS absorption spectra of different photocatalysts are shown below; where (a) is the absorbance of u%NCS / MCS composite material at different wavelengths, (b) is the band gap energy of u%NCS / MCS composite material, (c) is the absorbance of zC / 8%NCS / MCS composite photocatalyst at different wavelengths, and (d) is the band gap energy of each c / 8%NCS / MCS composite photocatalyst.
[0023] Figure 5 MS curves of different catalysts at various frequencies are shown; where (a) is the Mn 0.5 Cd 0.5 MS curves of S, (b) MS curve of NiCo2S4, (c) MS curve of 8% NCS / MCS, and (d) MS curve of 20C / 8% NCS / MCS.
[0024] Figure 6 The figures show the Mott-Schottky curves and energy structure diagrams for different catalysts; where (a) represents NiCo2S4 and Mn. 0.5 Cd 0.5 The complete band positions of S, 8%NCS / MCS and 20C / 8%NCS / MCS, (b) shows the detection of hydroxyl radicals by EPR spectroscopy, and (c) shows the detection of superoxide radicals by EPR spectroscopy;
[0025] Figure 7 The results are the photoelectrochemical experiments of different photocatalysts; (a) is the photocurrent response test result, (b) is the electrochemical impedance spectroscopy, (c) is the linear sweep voltammetry test result, and (d) is the photoluminescence spectrum.
[0026] Figure 8 The photocatalytic hydrogen production performance of different photocatalysts without the addition of any metal co-catalysts is shown; (a) is the cumulative H2 plot of composite materials with different NiCo2S4 contents; (b) is the hydrogen production rate of photocatalysts with different CQDs addition amounts; and (c) is the hydrogen production rate of photocatalysts at different times.
[0027] Figure 9The results of the cyclic hydrogen production experiment of 20C / 8%NCS / MCS photocatalyst are shown; (a) is the cyclic hydrogen production stability test diagram; (b) is the XRD pattern before and after the cyclic test of 20C / 8%NCS / MCS. Detailed Implementation
[0028] To solve Mn 0.5 Cd 0.5 Photocatalytic activity (S) suffers from drawbacks such as low efficiency in separating photogenerated electron-hole pairs, few surface active centers, and poor stability. Introducing other semiconductor materials to construct heterojunctions can improve photocatalytic performance and increase hydrogen production efficiency. The Z-scheme electron transfer mechanism maximizes the separation of photogenerated carriers while effectively maintaining the strong redox capabilities of the composite material. NiCo2S4 possesses excellent optical and electrical properties and can replace noble metals as a superior, low-cost co-catalyst to enhance the photocatalytic activity of single semiconductors. NiCo2S4 not only exhibits the conductivity of single metal sulfides Co and Ni but also possesses more metal active centers, which is beneficial for promoting interfacial electron transfer and reducing hydrogen evolution overpotential.
[0029] Carbon quantum dots (CQDs) are a novel type of carbon nanomaterial with a surface rich in carboxyl groups. These carboxyl groups not only endow CQDs with good water solubility but also provide suitable reaction sites for further functionalization and surface passivation of organic, polymer, inorganic, or biomaterials. Furthermore, CQDs exhibit significant application potential in photocatalysis due to their simple preparation process, controllable cost, good stability, environmental friendliness, high quantum yield, and high electron transfer rate. The carbon sources used for synthesizing CQDs are diverse, including biomass derivatives (such as lignin and glucose) and synthetic molecules (such as citric acid and ascorbic acid). Lignin, as a highly valuable renewable aromatic polymer, is a key research focus in the field of biomass resource conversion. Lignin has a high carbon content and a well-developed aromatic conjugated structure, providing a good structural basis for the preparation of CQDs with specific photoelectric properties.
[0030] Specific Implementation Method 1: The preparation method of the composite photocatalyst doped with carbon quantum dots and Z-type heterojunction in this implementation method includes the following steps:
[0031] 1. Mixing NiCo2S4 and Mn 0.5 Cd 0.5 S was added to anhydrous ethanol, sonicated at room temperature, stirred, and then evaporated to dryness to obtain a solid powder; the solid powder was then vacuum dried to obtain Mn. 0.5 Cd 0.5 S / NiCo2S4 composite material;
[0032] II. Adding carbon quantum dot solution to Mn 0.5 Cd0.5 The S / NiCo2S4 composite material was ultrasonically treated at room temperature, then stirred and vacuum dried to obtain CQDs / NiCo2S4 / Mn. 0.5 Cd 0.5 S composite photocatalyst.
[0033] Specific Implementation Method Two: In step one of this implementation method, NiCo2S4 and Mn 0.5 Cd 0.5 The molar mass ratio of S is 1:(0.05~0.1). Other steps and parameters are the same as in Specific Implementation Method 1.
[0034] Specific Implementation Method 3: The ultrasonic treatment time in step one of this implementation method is 30-40 minutes. Other steps and parameters are the same as in Specific Implementation Method 1 or 2.
[0035] Specific Implementation Method Four: The stirring time in step one of this implementation method is 6-8 hours. Other steps and parameters are the same as in any of Specific Implementation Methods One to Three.
[0036] Specific Implementation Method 5: In this implementation method, the evaporation to dryness described in step one is carried out in a water bath at 60~65 °C. Other steps and parameters are the same as in any of Specific Implementation Methods 1 to 4.
[0037] Specific Implementation Method Six: The vacuum drying described in step one of this implementation method is performed at 60~70℃ for 12~14 hours. Other steps and parameters are the same as in any of Specific Implementation Methods One to Five.
[0038] Specific Implementation Method Seven: In step two of this implementation method, the concentration of the carbon quantum dot solution is 0.007~0.015 g / mL. Other steps and parameters are the same as in any of Specific Implementation Methods One through Six.
[0039] Specific Implementation Method Eight: In step two of this implementation method, the carbon quantum dot solution and Mn 0.5 Cd 0.5 The volume ratio of the S / NiCo2S4 composite material is 1:(0.6~1.6). Other steps and parameters are the same as in any of the specific embodiments one to seven.
[0040] The embodiments of the present invention will be described in detail below. The following embodiments are implemented based on the technical solution of the present invention, and detailed implementation schemes and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.
[0041] Example 1:
[0042] The preparation method of the composite photocatalyst doped with carbon quantum dots and Z-type heterojunction in this embodiment includes the following steps:
[0043] I. Preparation of carbon quantum dot solutions
[0044] 0.5 g of alkali lignin (AL) was ultrasonically dissolved in 50 mL of deionized water. The solution was magnetically stirred at 500 r / min for 30 min, then transferred to a 100 mL polytetrafluoroethylene-lined reactor and reacted at 180 ℃ for 12 h. After natural cooling to room temperature, the solution was filtered through a 0.22 μm aqueous microporous membrane sand core funnel to remove large carbon particles formed by agglomeration. The filtrate was placed in a dialysis bag (molecular weight cutoff 3000 Da) and dialyzed at room temperature for 3 days, with water changed every 8 h. After dialysis, a brownish-yellow carbon quantum dot solution was obtained by filtration and stored at 4 ℃. A portion of the CQDs solution was freeze-dried for subsequent characterization.
[0045] II. Mn 0.5 Cd 0.5 Preparation of S
[0046] 2 mmol Cd(CH3COO)2·2H2O, 2 mmol Mn(CH3COO)2·4H2O, and 6 mmol thioacetamide were dissolved in 30 mL of deionized water. The mixture was sonicated for 30 min and stirred for 1 h until homogeneous, turning the solution yellow. The yellow solution was placed in a 50 mL polytetrafluoroethylene-lined autoclave and heated at 160 °C for 24 h. After the reaction was complete, the mixture was cooled to room temperature, centrifuged at 6000 r / min for 2 min, and washed three times each with deionized water and anhydrous ethanol. Finally, it was vacuum dried at 60 °C for 12 h to obtain Mn. 0.5 Cd 0.5 S particles.
[0047] III. Preparation of NiCo2S4
[0048] First, 0.25 mmol Co(NO3)2·6H2O and 0.125 mmol Ni(NO3)2·6H2O were dissolved in a mixed solvent and ultrasonically stirred to obtain a transparent pink solution. The mixed solvent consisted of 8 mL glycerol and 40 mL isopropanol. The transparent pink solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor and heated at 180 °C for 6 h. After the reaction, the brown precipitate was centrifuged at 6000 r / min, washed with anhydrous ethanol, and dried in an oven at 60 °C for 12 h to obtain the nickel-cobalt precursor. Then, 30 mg of the nickel-cobalt precursor was dispersed in 30 mL of ethanol containing 50 mg of thioacetamide. The mixed solution was transferred to a 50 mL high-pressure reactor and heated at 200 °C for 5 h. After the reaction, the mixture was allowed to cool naturally to room temperature, and the reaction product was washed three times with anhydrous ethanol. Finally, it was vacuum dried at 60 °C for 12 h to obtain NiCo2S4 nanospheres.
[0049] IV. NiCo2S4 / Mn 0.5 Cd 0.5 Synthesis of S composite materials
[0050] Weigh 100 mg Mn 0.5 Cd 0.5 S and a certain amount of NiCo2S4 were added to 40 mL of anhydrous ethanol, and the mixture was ultrasonically treated at room temperature for 30 min and stirred for 6 h. The mixture was then evaporated to dryness in a 60 °C water bath to obtain a solid powder. The solid powder was then vacuum dried at 60 °C for 12 h to obtain u%NiCo2S4 / Mn. 0.5 Cd 0.5 S binary composite material, where u% represents the molar percentage of NiCo2S4 added in NiCo2S4 and Mn. 0.5 Cd 0.5 7%, 8%, and 9% of the total mass of S moles are denoted as 7% NCS / MCS, 8% NCS / MCS, and 9% NCS / MCS, respectively.
[0051] V. CQDs / NiCo2S4 / Mn 0.5 Cd 0.5 Synthesis of S composite photocatalysts
[0052] Take the 8% NiCo2S4 / Mn prepared in step four 0.5 Cd 0.5 50 mg of S binary composite material was added to 15 mL of carbon quantum dot solution, and then anhydrous ethanol was added to make the total volume of the mixture 40 mL. The mixture was ultrasonically treated at room temperature for 30 min and stirred for 6 h, and then vacuum dried at 60°C for 12 h to obtain the ternary composite photocatalyst 15C / 8%NCS / MCS.
[0053] Example 2:
[0054] The difference between this embodiment and Embodiment 1 is that: in step five, 8% NiCo2S4 / Mn is taken. 0.5 Cd 0.5 50 mg of the S binary composite material was added to 20 mL of carbon quantum dot solution, and then anhydrous ethanol was added to bring the total volume of the mixture to 40 mL. This yielded a ternary composite photocatalyst 20C / 8% NCS / MCS.
[0055] Example 3:
[0056] The difference between this embodiment and Embodiment 1 is that: in step five, 8% NiCo2S4 / Mn is taken. 0.5 Cd 0.550 mg of the S binary composite material was added to 25 mL of carbon quantum dot solution, and then anhydrous ethanol was added to bring the total volume of the mixture to 40 mL. This yielded a ternary composite photocatalyst of 25C / 8% NCS / MCS.
[0057] The effects of the composite material prepared in this embodiment were verified as follows:
[0058] (I) Crystal Structure Analysis
[0059] The crystal structures of the composite material prepared in step four and the composite photocatalyst prepared in step five were tested using XRD, and the results are as follows: Figure 1 As shown. Figure 1 a shows Mn 0.5 Cd 0.5 The XRD spectrum of S. The peaks at 29.60°, 34.30°, 49.30°, and 61.39° correspond to the (111), (200), (220), and (222) crystal planes of MnS PDF#06-0518, respectively. The peaks at 26.45°, 30.64°, 43.88°, and 51.98° belong to the (111), (200), (220), and (311) crystal planes of CdS PDF#75-0518. The coexistence of diffraction peaks indicates that MnS is formed. 0.5 Cd 0.5 It is a solid solution of S, not a mixture of CdS and MnS. Figure 1 b. The characteristic peaks of the NiCo2S4 crystal were observed to correspond to JCPDS card number 20-0782. The diffraction peaks at 31.47°, 38.19°, 50.30° and 55.12° correspond to the (311), (400), (511) and (440) crystal planes of cubic NiCo2S4. There were no impurity peaks, which proved that the successfully synthesized NiCo2S4 has good crystallinity. Figure 1 b has significant Mn content in its NCS / MCS composite material. 0.5 Cd 0.5 S characteristic peak. Characteristic peaks of the (311) and (440) crystal planes of NiCo2S4 were also observed. This fully demonstrates the properties of NiCo2S4 and Mn. 0.5 Cd 0.5 S coexists in the system. The optimal addition amount of NiCo2S4 was determined to be 8% by comparing the photocatalytic hydrogen production rates of various catalysts. Further composite modification was then carried out by introducing CQDs into the 8% NCS / MCS catalyst, such as... Figure 1As shown in Figure c, compared to pure 8% NCS / MCS, the characteristic diffraction peaks of C / 8% NCS / MCS are basically consistent with those of pure 8% NCS / MCS, and no strong characteristic peaks of carbon were found. This indicates that adding different amounts of CQDs did not change the crystal structure of 8% NCS / MCS, and 8% NCS / MCS maintained its structural integrity. This may be due to the relatively low loading of CQDs, or it may exist in an amorphous or uniformly dispersed form on the surface of 8% NCS / MCS, without affecting the crystal structure of 8% NCS / MCS.
[0060] (II) Morphological Analysis
[0061] Figure 2 For Mn 0.5 Cd 0.5 SEM images of S, NiCo2S4, and 20C / 8% NCS / MCS. (See image.) Figure 2 As can be observed in a, Mn 0.5 Cd 0.5 The S solid solution is in the form of irregular nanoparticles with an average size of about 50 nm. Figure 2 The NiCo2S4 shown in b has an eggshell nanosphere structure with a relatively smooth surface and an average diameter of about 100-200 nm. Figure 2 c shows that when Mn 0.5 Cd 0.5 The coupling of S with NiCo2S4 and the addition of CQDs resulted in a rougher surface for the eggshell nanospheres. Compared to pure NiCo2S4, the general morphology remained largely unchanged, but the close contact between the two made it difficult to observe the CQDs. Therefore, the microstructure of the 20C / 8% NCS / MCS composite photocatalyst was observed and analyzed using transmission electron microscopy. Figure 2 As can be seen from d, Mn 0.5 Cd 0.5 NiCo2S4 nanospheres and numerous CQDs particles with diameters less than 5 nm are tightly adhered to the S surface. In the HRTEM image, three different types of lattice fringes were also observed and analyzed: a lattice spacing of 0.288 nm corresponds to the (311) crystal plane of NiCo2S4, while a lattice spacing of 0.309 nm corresponds to Mn. 0.5 Cd 0.5 The (101-111) crystal plane of S has a lattice spacing of 0.208 nm, corresponding to the (100) crystal plane of CQDs. Furthermore, it is also through... Figure 2 The mapping of e showed that C, Mn, Cd, Ni, Co, and S were uniformly distributed in 20C / 8% NCS / MCS, through... Figure 2 The EDS energy dispersive spectroscopy (EDS) plot of f shows the mass percentage of local elements in the region. This indicates that NiCo2S4 and CQDs were successfully loaded onto Mn. 0.5 Cd0.5 S surface, and NiCo2S4, CQDs and Mn were found 0.5 Cd 0.5 There is a tight contact between S and a clear interface structure, which further proves that a heterojunction has been successfully constructed.
[0062] (III) Elemental composition, i.e., valence state analysis
[0063] To further investigate its elemental composition and chemical valence state, XPS was used to analyze Mn. 0.5 Cd 0.5 S, NiCo2S4 and 20 C / 8% NCS / MCS eggshell nanospheres were characterized and analyzed. Figure 3 a shows Mn 0.5 Cd 0.5 XPS measurements of the full spectrum of S, NiCo2S4, and 20C / 8%NCS / MCS eggshell nanospheres. Peaks for C 1s, Ni 2p, Co 2p, Cd 3d, Mn 2p, and S 2p were observed in the XPS measurement of the full spectrum of 20C / 8%NCS / MCS, demonstrating the successful synthesis of the ternary composite material. Figure 3 bg shows the different elements in Mn 0.5 Cd 0.5 High-resolution XPS spectra of S, NiCo2S4, and 20C / 8%NCS / MCS eggshell nanospheres. (Example: ...) Figure 3 b shows the high-resolution spectrum of C 1s, with peak positions calibrated to C 1s 284.8 eV. It can be fitted to three peaks located at 284.8, 286.2, and 287.0 eV, attributed to C=C, CO, and C=O bonds, respectively. For the high-resolution XPS spectrum of Ni 2p (… Figure 3 c) It is divided into two spin-orbit double peaks and two vibrational satellite peaks. The two peaks at 870.85 and 852.51 eV correspond to Ni. 2+ 2p 1 / 2 and Ni 2+ 2p 3 / 2 The peaks at 875.76 and 856.71 eV correspond to Ni, respectively. 3+ 2p 1 / 2 and Ni 3+ 2p 3 / 2 Ni 2+ 2p1 / 2 and Ni 2+ 2p 3 / 2 The binding energies of the secondary peaks correspond to 881.02 and 861.74 eV, respectively. Figure 3 In d, the two peaks at 797.71 and 781.70 eV belong to Co. 2+ 2p 1 / 2 and Co2+ 2p 3 / 2 The binding energies are 794.03 and 778.55 eV, respectively, corresponding to Co 3+ 2p 1 / 2 and Co 3+ 2p 3 / 2 The peaks at 803.10 and 786.15 eV represent Co 2p, respectively. 1 / 2 and Co 2p 3 / 2 The satellite peak. In Figure 3 The high-resolution XPS spectrum of Cd 3d in eV shows two strong peaks at 412.32 and 405.51 eV, respectively, which are attributed to Cd 3d. 3 / 2 and Cd3d 5 / 2 This proves Cd 2+ The existence of. In Figure 3 The S 2p XPS spectrum of NiCo2S4 in f indicates that the energies at 162.07 and 160.47 eV correspond to S 2- S 2p 1 / 2 and S 2p 3 / 2 164.04 and 163.21 eV correspond to S 2- S 2p 1 / 2 and S 2p 3 / 2 Furthermore, the bimodal peaks observed at 168.18 and 169.53 eV belong to sulfate, which is due to the redox reaction during the synthesis of NiCo2S4. In Mn 0.5 Cd 0.5 During the synthesis of S, the valence state of S remains unchanged. Therefore, Mn 0.5 Cd 0.5 The S 2p XPS spectrum of S is only at 163.11 (S 2- 2p 1 / 2 ) and 161.90 eV (S 2- 2p 3 / 2 There are two peaks at ( ), and no sulfate and Peak. In Figure 3 In g, the peaks at 652.73 and 641.26 eV are attributed to Mn 2p 1 / 2 and Mn 2p 3 / 2 This proves Mn 2+ The presence of [a specific element, likely a specific element or component] is observed in eggshell nanospheres. The characteristic peaks of Ni, Co, Cd, Mn, and S elements are similar to those of NiCo2S4 and Mn. 0.5 Cd 0.5 The characteristic peaks of S are consistent with those of NiCo2S4 and Mn. 0.5 Cd 0.5Compared to S, the peaks of Ni 2p and Co 2p are slightly positively shifted, while the peaks of Cd 3d and Mn 2p are slightly negatively shifted. This is attributed to the decrease in electron density of NiCo2S4 in the composite material due to electron transfer at the heterojunction interface, while the electron density of paraelectrons is slightly higher; this change in surface electron density is caused by electron transfer. Figure 3 In b, it can be compared that the peak value of C in 20 C / 8% NCS / MCS is higher than that of Mn. 0.5 Cd 0.5 S and NiCo2S4 indicate that CQDs were successfully loaded in the composite material.
[0064] (iv) Photoelectrochemical analysis
[0065] Effective visible light absorption is crucial for solar photocatalysts. The light-harvesting ability of different catalysts was investigated using UV-Vis DRS absorption spectroscopy, such as... Figure 4 As shown. In 4a, NiCo2S4 exhibits strong light absorption even without an absorption edge due to its metallic properties. Mn 0.5 Cd 0.5 S exhibits strong visible light absorption below 550 nm. Mn 0.5 Cd 0.5 After surface modification with NiCo2S4 eggshell nanospheres, u%NCS / MCS eggshell nanospheres exhibited NiCo2S4 and Mn content. 0.5 Cd 0.5 The overall absorption performance of S. Compared with single-component Mn. 0.5 Cd 0.5 Compared to S, the light absorption intensity of the composite material in the 580-800 nm range increases with increasing NiCo2S4 content. This is attributed to the hollow structure of NiCo2S4, which reflects and fully absorbs visible light, indicating that NiCo2S4 has a significant effect on improving the long-wavelength visible light absorption of NCS / MCS composite materials. Figure 4 b shows the NiCo2S4 and Mn calculated using the Tauc plot method. 0.5 Cd 0.5 The band gap energies of S and 8% NCS / MCS are 2.16, 2.33, and 2.21 eV, respectively. Further introduction of CQDs resulted in a red shift of the catalyst's absorption edge and an increase in absorption intensity, such as... Figure 4 As shown in c, the band gap of 20C / 8%NCS / MCS calculated using the Tauc plot method is 2.07 eV. Figure 4 d). By comparing the band gap widths of catalysts obtained by constructing heterojunctions and introducing CQDs, the band gap width gradually decreases. This indicates that by constructing heterojunctions and introducing CQDs, zC / 8% NCS / MCS achieves more efficient visible light absorption.
[0066] To investigate the electron transfer pathway, the Mott-Schottky method was used to analyze NiCo2S4 and Mn. 0.5 Cd 0.5 The positions of the conduction band and valence band in S, 8% NCS / MCS, and 20C / 8% NCS / MCS. According to... Figure 5 MS curves of various catalysts at different frequencies, NiCo2S4, Mn 0.5 Cd 0.5 The Mott-Schottky curves for S, 8% NCS / MCS, and 20C / 8% NCS / MCS have positive slopes, indicating that they are all n-type semiconductors. Figure 5 The ad shows NiCo2S4 and Mn 0.5 Cd 0.5 The flat band potentials of S, 8% NCS / MCS, and 20C / 8% NCS / MCS are -0.53, -0.83, -0.99, and -1.20 eV, respectively. Generally, the conduction band potential (Ecb) of an n-type semiconductor is 0.2 V negative than Efb, but in a 0.5 M Na₂SO₄ solution at pH 7, since E(vs. NHE) = E(vs. Ag / AgCl) + 0.197 V, NiCo₂S₄, Mn 0.5 Cd 0.5 S, 8% NCS / MCS and 20C / 8% NCS / MCS E CB The values are -0.53, -0.83, -0.99, and -1.20 eV (vs. NHE). Based on... Figure 5 b and Figure 5 As shown in d, NiCo2S4, Mn 0.5 Cd 0.5 The band gap energies of S, 8% NCS / MCS, and 20C / 8% NCS / MCS are 2.16, 2.33, 2.21, and 2.07 eV, respectively. According to E... VB =E CB +Eg yields NiCo2S4 and Mn 0.5 Cd 0.5 The valence band potentials of S, 8% NCS / MCS, and 20C / 8% NCS / MCS are 1.63, 1.50, 1.22, and 0.87 eV (Vs NHE), respectively. Analysis of these results shows that NiCo2S4 reacts with Mn... 0.5 Cd 0.5 The heterostructure constructed by S can effectively adjust the band structure, promoting the separation and transport of photogenerated carriers. Furthermore, the band structure of the composite photocatalyst was optimized by loading CQDs, making the conduction band position more negative, thus further enhancing the catalyst's ability to reduce photogenerated electrons.
[0067] To clarify the carrier transfer pathways in the 8% NCS / MCS composite material, band structure analysis and electron spin resonance (EPR) radical trapping experiments were used for verification. Band structure Figure 6 a showcased NiCo2S4, Mn 0.5 Cd 0.5 Complete band positions of S, 8% NCS / MCS and 20C / 8% NCS / MCS, NiCo2S4, Mn 0.5 Cd 0.5 The CB / VB ratios for S, 8% NCS / MCS, and 20C / 8% NCS / MCS are -0.53 / 1.63, -0.83 / 1.50, -0.99 / 1.22, and -1.20 / 0.87 eV, respectively. (Compare with H...) + The reduction potential of O2 / H2O (0.00 eV vs. NHE) and the oxidation potential of O2 / H2O (+0.82 eV vs. NHE) show that the band positions of all four photocatalysts meet the thermodynamic requirements for photocatalytic hydrogen splitting, indicating that they are theoretically capable of driving photocatalytic hydrogen splitting.
[0068] Based on this, 5,5-dimethyl-1-pyrrolidone-N-oxide (DMPO) was used as a free radical scavenger, and the superoxide radicals generated in the reaction system were detected by EPR spectroscopy. The presence of hydroxyl radicals (·OH) was investigated to verify the actual charge transfer pathway. No radical signals were detected in any of the catalysts under dark conditions, indicating that photogenerated carriers could not be generated without photoexcitation. However, after 5 minutes of illumination, Mn... 0.5 Cd 0.5 DMPO- was observed in both S and 8% NCS / MCS. The characteristic peak of the signal, while pure NiCo2S4 does not have this signal (e.g., Figure 6 (as shown in b), and 8% NCS / MCS The signal strength is significantly stronger than that of Mn. 0.5 Cd 0.5 S indicates that its photogenerated electrons have a stronger reducing ability. More importantly, from Figure 6 As can be seen from c, a stronger DMPO-·OH signal intensity than NiCo2S4 was detected in 8% NCS / MCS, while in Mn 0.5 Cd 0.5 No DMPO-·OH signal was detected in S.
[0069] Combining the above EPR results with band structure data, the traditional type II heterojunction charge transfer path can be ruled out. If the type II path is followed, photogenerated electrons will transfer from Mn... 0.5 Cd 0.5The electrons transfer from the CB (-0.83 eV) of S to the CB (-0.53 eV) of NiCo2S4. At this point, the electrons are located on the CB of NiCo2S4, and this potential (-0.53 eV) is lower than that of O2 / Its reduction potential (-0.33 eV) is more negative, theoretically enabling it to reduce O2 to... However, if this is the case, 8% NCS / MCS The signal strength should be similar to that of NiCo2S4 (no signal) or Mn. 0.5 Cd 0.5 S is similar, however, the present invention observes The signal strength is significantly higher than that of Mn. 0.5 Cd 0.5 S indicates that the photogenerated electrons did not transfer to NiCo2S4, but remained in Mn. 0.5 Cd 0.5 On the CB of S; simultaneously, if following the type II pathway, holes will migrate from VB (1.63 eV) of NiCo2S4 to Mn. 0.5 Cd 0.5 S's VB (1.50 eV), due to Mn 0.5 Cd 0.5 The potential of S at VB (1.50 eV) is negative to ·OH / OH. - The oxidation potential of Mn is 1.99 eV, which theoretically makes it impossible to oxidize H2O to ·OH, while experimental results show that Mn... 0.5 Cd 0.5 No ·OH signal was found in S, but ·OH signal was detected in NiCo2S4 and 8% NCS / MCS, indicating that holes did not migrate to Mn. 0.5 Cd 0.5 S, instead of being generated by oxidation, remains on the VB of NiCo2S4. Therefore, in the 8% NCS / MCS composite material, photogenerated electrons and holes are retained on Mn. 0.5 Cd 0.5 In the CB of S and the VB of NiCo2S4, the charge transfer pathway conforms to the characteristics of a Z-type heterojunction. Further introduction of CQDs shifts the CB potential of 20C / 8%NCS / MCS negatively to -1.20 eV, further enhancing its reduction capability. This confirms that CQDs, acting as electron mediators, optimize the Z-type charge transfer pathway, thereby significantly improving the photocatalytic hydrogen evolution performance.
[0070] For Mn 0.5 Cd 0.5 A series of photoelectrochemical experiments were conducted using S, u%NCS / MCS, and various C / 8%NCS / MCS. The instantaneous photocurrent response under visible light irradiation (λ>420 nm) was used to study the effect of NiCo2S4 and CQDs on Mn. 0.5 Cd 0.5The influence of S-photocurrent response was investigated, and photocurrent measurements were performed. The results are as follows: Figure 7 As shown in a. NiCo2S4 / Mn 0.5 Cd 0.5 The photocurrent density of the S-shell nanospheres increases with increasing NiCo2S4 content. This photocurrent intensity is likely due to the formation of the heterojunction, which promotes the effective separation and transfer of photogenerated carriers. Further introduction of CQDs significantly enhances the photocurrent intensity of zC / 8%NCS / MCS, indicating that the synergistic effect of the heterojunction and CQDs further strengthens the photocurrent response. This demonstrates that the ternary composite catalyst has a stronger ability in the excitation and transfer of photogenerated carriers, while effectively suppressing electron-hole recombination, which is beneficial for carrier movement on the catalyst surface. Among them, 20C / 8%NCS / MCS exhibits the highest photocurrent response, indicating that it has the best transport and separation of photogenerated carriers.
[0071] Electrochemical impedance spectroscopy, such as Figure 7 As shown in b, the arc radius corresponds to the charge transfer resistance efficiency at the electrode-electrolyte interface. With Mn 0.5 Cd 0.5 With increasing S content, the arc radius decreases, and the Nyquist curvature of 8% NCS / MCS is greater than that of pure Mn. 0.5 Cd 0.5 S decreases. Further, by adding different amounts of CQDs, it was observed that the arc radius decreased, and the Nyquist curvature of 20C / 8%NCS / MCS was the smallest. This indicates that the heterojunction formed by the introduction of NiCo2S4 and the CQDs improved the migration of photogenerated electrons, optimized the migration path, reduced the resistance in the charge transfer process, and promoted hydrogen evolution of the composite material.
[0072] To investigate the precipitation kinetics of H2, linear sweep voltammetry was performed on the material. Figure 7 In section c, the hydrogen evolution overpotential of the catalyst was illustrated using linear sweep voltammetry. (The text then abruptly shifts to a seemingly unrelated topic: "and Mn...") 0.5 Cd 0.5 Compared to S, u%NCS / MCS has a lower overpotential, exhibiting Mn 0.5 Cd 0.5 The rapid electron transfer at the S-NiCo2S4 interface. The low overpotential of the 20C / 8%NCS / MCS may be due to the CQDs in Mn. 0.5 Cd 0.5Rapid electron transfer from the S-NiCo2S4 interface to the surface of the active species leads to the reduction of H2O molecules. This result confirms the significant impact of CQDs on the HER activity of the NCS / MCS heterojunction. It also demonstrates the crucial role of CQDs in promoting charge transfer in the heterojunction, further explaining the enhanced activity of the composite material. In conclusion, the effective separation and transfer of photogenerated charges are key factors for the high hydrogen production activity of 20C / 8% NCS / MCS.
[0073] Photoluminescence spectroscopy was used to investigate photogenerated carriers in Mn. 0.5 Cd 0.5 Transfer and separation efficiencies in S, 8% NCS / MCS, and 20C / 8% NCS / MCS composites. At an excitation wavelength of 360 nm, all samples exhibited a maximum emission peak near 623 nm, as shown in the figure. Figure 7 As shown in d, strong fluorescence intensity indicates that a large number of excited electrons return to the ground state, leading to rapid recombination of electrons and holes. A decrease in fluorescence intensity has a certain inhibitory effect on electron-hole recombination. The lower the peak value, the greater the intensity, and the better the support separation efficiency of the photocatalyst. Due to self-trapped excited radiative recombination, Mn... 0.5 Cd 0.5 S exhibits a strong emission peak. This clearly confirms the rapid transfer and slow recombination of photogenerated charges. The peak intensity of all composite materials is lower than that of the single materials, with 8% NCS / CIS showing a decrease in luminescence intensity. This may be due to the heterojunction promoting Mn... 0.5 Cd 0.5 This is due to the interfacial charge transfer between S and NiCo2S4. This efficient charge transfer has a decisive advantage for photocatalytic hydrogen evolution. Among them, 20C / 8%NCS / MCS has the lowest fluorescence intensity, indicating that CQDs help promote carrier separation efficiency and further improve electron transport efficiency, which is consistent with its high PHE activity. These results can be further confirmed by photoelectrochemical response.
[0074] (v) Photocatalytic hydrogen production test
[0075] Mn was tested under visible light (≥420 nm) irradiation. 0.5 Cd 0.5 Mn with three different mass ratios of S and NiCo2S4 0.5 Cd 0.5 The photocatalytic hydrogen production performance of S / NiCo2S4 eggshell nanospheres and various C / 8% NCS / MCS without the addition of any metal co-catalysts, such as Figure 8 As shown. Figure 8 As shown in figure a, a single NiCo2S4 does not exhibit significant photocatalytic activity, while Mn 0.5 Cd 0.5The hydrogen evolution rate of S reached 41.74 mmol·g within 6 h. −1 However, with the increase of NiCo2S4 content, the NiCo2S4 / Mn ratio decreases. 0.5 Cd 0.5 The hydrogen production rate of S-shell nanospheres was significantly improved. The highest hydrogen production rate of 7% NCS / MCS, 8% NCS / MCS, and 9% NCS / MCS was 103.48 mmol·g⁻¹ within 6 h. -1 However, with further increases in NiCo2S4 content, the hydrogen production rate of 9% NCS / MCS decreased, which may be due to the shielding effect of NiCo2S4 covering Mn. 0.5 Cd 0.5 The surface active sites of S affect Mn 0.5 Cd 0.5 The light absorption of S / NiCo2S4 was further investigated by adding different amounts of CQDs to explore the effect on the catalyst with the highest hydrogen production rate, such as... Figure 8 As shown in b, the introduction of CQDs promoted electron transport in the heterojunction, resulting in a significant increase in the hydrogen evolution rate within 6 h. The hydrogen evolution rates observed at 6 h for the 15C / 8%NCS / MCS, 20C / 8%NCS / MCS, and 25C / 8%NCS / MCS composite photocatalysts were 119.14, 133.58, and 128.72 mmol·g, respectively. -1 The increase in cumulative hydrogen production is attributed to the bridging effect of CQDs, which optimize the transport path within the heterojunction and thus improve the hydrogen production rate. However, when the amount of CQDs exceeds 20 mL, the hydrogen production rate decreases with increasing CQD concentration. This may be because excessive CQD content covers the surface active sites of 8% NCS / MCS, affecting the transport efficiency of photogenerated electrons and thereby reducing the H2 production rate. Figure 8 The average rates of each catalyst can be observed in b and c. When CQDs are not introduced, the rates of NiCo2S4 and Mn... 0.5 Cd 0.5 The optimal S ratio is 8% (1:0.09), with a rate reaching 17.25 mmol·g. -1 ·h -1 Compared to single Mn 0.5 Cd 0.5 The hydrogen production rate increases by approximately two times with the addition of S. While the rate increases by less than one time with the further introduction of CQDs, it still represents a significant improvement. Therefore, the optimal dosage of CQDs is 20 mL.
[0076] (vi) Stability Analysis
[0077] Cyclic hydrogen production experiments were conducted to further investigate the stability of the 20C / 8% NCS / MCS photocatalyst. Figure 9 As shown in Figure a, after five cycles of hydrogen production performance testing, the 20C / 8%NCS / MCS composite photocatalyst still maintained good photocatalytic activity, and the hydrogen production efficiency did not decrease significantly, demonstrating excellent cycle stability. Figure 9 The X-ray diffraction patterns shown in b before and after the cycling experiment also indicate that the diffraction peaks of 20C / 8%NCS / MCS are consistent before and after the reaction. In summary, the 20C / 8%NCS / MCS photocatalyst possesses a relatively stable structure and cycling stability.
[0078] In summary, the introduction of CQDs further optimizes the composite photocatalyst system, mainly through two mechanisms. First, the excellent light absorption characteristics of CQDs enhance the light-harvesting efficiency and spectral response range of the composite material, thereby driving sufficient photogenerated carrier excitation. Second, CQDs act as electron transfer bridges in Mn... 0.5 Cd 0.5 The rapid conduction channel formed between S and NiCo2S4 can not only effectively promote the migration and separation of photogenerated electron-hole pairs, but also significantly improve the electron transport rate and suppress recombination during the transport process, ultimately improving the overall performance of the photocatalytic system.
Claims
1. A method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction, characterized in that, The method includes the following steps: I. NiCo2S4 and Mn 0.5 Cd 0.5 S were added into anhydrous ethanol, ultrasonic treatment at room temperature, followed by stirring, and then evaporated to dryness to obtain solid powder; then the solid powder was vacuum dried to obtain Mn 0.5 Cd 0.5 S / NiCo2S4 composite material; II. The carbon quantum dot solution is added to Mn 0.5 Cd 0.5 S / NiCo2S4 composite material, ultrasonic treatment at room temperature, then stirring, vacuum drying, CQDs / NiCo2S4 / Mn 0.5 Cd 0.5 S composite photocatalyst.
2. The method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction according to claim 1, characterized in that, In step one, NiCo2S4 and Mn 0.5 Cd 0.5 The molar mass ratio of S is 1: (0.05~0.1).
3. The method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction according to claim 2, characterized in that, The ultrasonic treatment time in step one is 30~40 min.
4. The method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction according to claim 3, characterized in that, The stirring time mentioned in step one is 6-8 hours.
5. The method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction according to claim 4, characterized in that, The evaporation to dryness described in step one is carried out in a water bath at 60~65 °C.
6. The method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction according to claim 5, characterized in that, The vacuum drying described in step one is performed at 60~70 ℃ for 12~14 h.
7. The method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction according to claim 6, characterized in that, In step two, the concentration of the carbon quantum dot solution is 0.007~0.015 g / mL.
8. The method for preparing a composite photocatalyst doped with carbon quantum dots and a Z-type heterojunction according to claim 7, characterized in that, In step two, the carbon quantum dot solution and Mn 0.5 Cd 0.5 The volume ratio of the S / NiCo2S4 composite material is 1: (0.6~1.6).
9. The application of the composite photocatalyst prepared by the method according to any one of claims 1 to 8 in photocatalytic hydrogen production.