Pd-coated ZIF-8 / CQDs / FeHCF ternary composite film gas-sensitive material and preparation method and application thereof

Abstract

The invention relates to the technical field of composite material preparation, in particular to a Pd-coated ZIF-8 / CQDs / FeHCF ternary composite film gas sensitive material as well as a preparation method and application thereof. ZIF-8 in the ternary composite film gas-sensitive material provides a high specific surface area and a regular microporous framework, and is used for preliminary adsorption and enrichment of VOCs molecules; pd nano-particles are uniformly distributed inside and outside ZIF-8 pore channels, high catalytic activity sites are provided, and activation and reaction of toluene molecules are accelerated; the CQDs construct a rapid electron transport network, so that the interface conductivity is remarkably improved; feHCF is used as a Prussian blue analogue, so that the mechanical stability and the moisture resistance of the film are enhanced, and a multi-phase interface is provided. According to the Pd-coated ZIF-8 / CQDs / FeHCF ternary composite film gas-sensitive material disclosed by the invention, an adsorption-catalysis-electric conduction three-in-one multi-phase composite strategy is adopted, the bottlenecks of a traditional sensor gas-sensitive material in specific surface area, electric conductivity and stability are broken through, a brand new implementation path is provided for rapid detection of ppb-level VOCs (Volatile Organic Compounds) represented by methylbenzene, and the gas-sensitive material has a wide application prospect. And the method has good expansibility and industrialization potential.

Description

Technical Field

[0001] This invention relates to the field of composite material preparation technology, and in particular to Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive materials, their preparation methods and applications. Background Technology

[0002] With the rapid development of industries such as ultra-large-scale integrated circuits (ULSI), high-end displays, new energy, and medical electronics, the requirements for online detection and control of trace volatile organic compounds (VOCs) in cleanrooms and critical process environments are becoming increasingly stringent. VOCs, represented by toluene, are highly volatile and non-polar molecules, easily adsorbed or condensed on optical and semiconductor surfaces during processes such as extreme ultraviolet lithography, chemical vapor deposition, photolithography development, and cleaning, causing attenuation of specular reflectivity, optical distortion, electrical performance drift, and even a decrease in breakdown voltage. As process nodes gradually advance to 3nm or even below 2nm, the industry is placing stringent requirements on the detection sensitivity and response speed of VOCs, represented by toluene, at the ppb level, while also demanding that sensors remain stable under long-term operation in complex environments.

[0003] However, traditional detection methods mostly rely on large instruments such as chromatography, mass spectrometry, and ion mobility spectrometry, or single-function chemical sensors, which have problems such as high cost, slow response, large equipment size, and difficulty in online continuous monitoring. Therefore, domestic and foreign efforts to focus on online VOCs detection have focused on the development of novel gas-sensitive materials such as metal oxide semiconductors (MOS), carbon-based materials, and metal-organic frameworks (MOFs). For example, patent CN113512062A discloses a CuIP2S type complex VOC fluorescence sensing material. MOS materials such as SnO2, In2O3, ZnO, and WO3 are simple to prepare and inexpensive, but their response signals are weak at trace concentrations (as shown in the literature "An, Dongmin Wang, Qiong Tong, Xiaoqiang Lian, Xiaoxue Zou, Yunling Li, Yan. ZnO-enhanced In2O3-based sensors for n-butanol gas[J]. CERAMICS INTERNATIONAL, 2019, 45(6)"). Carbon-based materials such as graphene, carbon nanotubes, and carbon quantum dots (CQDs) have high electron mobility, which can accelerate electron conduction, but they lack sufficient adsorption sites and catalytic activity, resulting in limited response to nonpolar VOCs. MOF materials have tunable pore size and high specific surface area, which can achieve pre-concentration of target molecules, but they have poor conductivity, limited mechanical strength, and are prone to structural collapse under humid and hot conditions, resulting in insufficient sensing stability.

[0004] Therefore, developing gas-sensitive materials that combine high sensitivity, rapid response, good selectivity, and environmental stability has become a key technological direction that urgently needs to be broken through. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material, its preparation method and application, which adopts a three-in-one multiphase composite strategy of "adsorption-catalysis-conductivity" to solve the problems of low sensitivity, slow response and insufficient sensing stability in the prior art.

[0006] The present invention solves the above-mentioned technical problems through the following technical means:

[0007] In a first aspect, the present invention provides a method for preparing a Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material, comprising the following steps:

[0008] To prepare Pd@ZIF-8, a Pd NPs ethanol dispersion was mixed with 2-methylimidazole, and then added to methanol. The mixture was sonicated for 8-13 min to obtain solution A. Zinc nitrate hexahydrate was dissolved in methanol to obtain solution B. Solution B was poured into solution A with stirring. The mixture was reacted at 25-80℃ for 6-12 h. The precipitate obtained from the first reaction solution was collected by centrifugation and washed to obtain the Pd@ZIF-8 core-shell structure. The resulting Pd@ZIF-8 was dispersed in methanol to obtain a Pd@ZIF-8 methanol dispersion.

[0009] To prepare the FeHCF / N-CQDs composite carrier, FeHCF precipitate was dispersed in anhydrous ethanol, N-CQDs ethanol solution was added, and the reaction was stirred at room temperature for 2.0-3.0 h. The resulting second reaction solution was centrifuged and washed, and the final precipitate was redispersed in anhydrous ethanol, vortexed and sonicated to obtain FeHCF / N-CQDs ethanol dispersion.

[0010] To construct the thin film, Pd@ZIF-8 methanol dispersion was added dropwise to FeHCF / N-CQDs ethanol dispersion, and the mixture was sonicated for 10-20 min. The resulting mixture was purified by centrifugation, and the final precipitate was dispersed in anhydrous methanol to obtain a ternary dispersion. After cleaning and drying the interdigitated electrode, the ternary dispersion was dropped onto the center of the interdigitated electrode, and spin-coated according to the following procedure: First, spin-coating at a low speed of 700-900 rpm for 5-10 s to spread the ternary dispersion and cover the entire interdigitated electrode. Second, spin-coating at a medium speed of 1500-2500 rpm for 20-40 s to form a preliminary film. Third, spin-coating at a high speed of 2500-3500 rpm for 10-20 s. The coated electrode was then vacuum-dried at 60-80℃ for 2-5 h to obtain the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film.

[0011] Furthermore, the preparation method of the Pd NPs ethanol dispersion is as follows:

[0012] Sodium citrate was added to ultrapure water and stirred to dissolve, thus obtaining a sodium citrate solution; sodium chloropalladium was added to pure water and mixed to obtain a sodium chloropalladium solution.

[0013] The sodium citrate solution was heated to 60-80°C in a water bath. A reflux condenser was connected and cooling water was passed through. Sodium chloropalladium solution was added dropwise at a rate of 0.2-0.8 mL / min with stirring. After the addition was completed, stirring was continued and the mixture was refluxed at 60-80°C for 0.5-2 hours. The mixture was then kept under stirring and reflux and allowed to cool naturally to room temperature to obtain the third reaction solution.

[0014] Centrifuge the third reaction solution at 6000~12000 rpm for 10~20 min, discard the supernatant, disperse the precipitate with anhydrous ethanol, centrifuge again at 6000~12000 rpm for 10~20 min, repeat 3 times, add the final precipitate to anhydrous ethanol and disperse by ultrasonication to obtain Pd NPs ethanol dispersion.

[0015] Further, the molar ratio of sodium citrate to sodium chloropalladium is (0.34~1.00):(0.025~0.05).

[0016] Furthermore, the molar ratio of palladium to 2-methylimidazole in the Pd NPs ethanol dispersion is 1:10.

[0017] Furthermore, the molar ratio of 2-methylimidazole to zinc nitrate hexahydrate is (7~8):1.

[0018] Furthermore, the preparation method of the N-CQDs is as follows:

[0019] Citric acid and ethylenediamine were added to the lining of a reaction vessel, followed by ultrapure water. The mixture was stirred until completely dissolved, and the reaction vessel was sealed. The vessel was then placed in a preheated drying oven at 200-240°C and reacted for 2-6 hours. After the reaction was completed, the product was removed and allowed to cool naturally to room temperature. The resulting solid product was diluted with ultrapure water, filtered through a 0.22 μm filter membrane, and then centrifuged at 8000-12000 rpm for 10-20 minutes. The supernatant was collected and dried at 80-120°C to obtain N-CQDs.

[0020] Furthermore, the method for preparing the FeHCF precipitate is as follows:

[0021] Prepare FeCl2 aqueous solution and K3[Fe(CN)6] aqueous solution with concentrations of 0.05~0.15mol / L respectively. According to the molar ratio of FeCl2 to K3[Fe(CN)6] 3:2, simultaneously inject the FeCl2 aqueous solution and K3[Fe(CN)6] aqueous solution into an ice bath and stir the reaction for 0.5~2h. Centrifuge the resulting reaction solution and collect the FeHCF precipitate.

[0022] Furthermore, the centrifugal purification in the thin film construction step is as follows: the mixture is centrifuged at 8000~12000 rpm for 5~15 min, the precipitate is vortexed with anhydrous methanol, and then centrifuged at 8000~12000 rpm for 5~15 min. The methanol washing is repeated a total of 5 times.

[0023] Secondly, the present invention provides a Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material, wherein the ternary composite thin film gas-sensitive material is prepared by the preparation method described in any one of Examples 1-8.

[0024] Thirdly, the present invention provides the application of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material described in the second aspect above in the preparation of toluene concentration detection materials.

[0025] The present invention prepares a Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material, in which ZIF-8 provides a high specific surface area and a regular microporous framework for the initial adsorption and enrichment of VOCs molecules; Pd nanoparticles are uniformly distributed inside and outside the ZIF-8 channels, providing highly catalytically active sites to accelerate the activation and reaction of toluene molecules; CQDs construct a fast electron transport network, significantly improving interfacial conductivity; and FeHCF, as a Prussian blue analogue, enhances the mechanical stability and moisture resistance of the film and provides a multiphase interface.

[0026] The Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material of the present invention adopts a three-in-one multiphase composite strategy of "adsorption-catalysis-conductivity", which breaks through the bottleneck of traditional sensor gas-sensitive materials in terms of specific surface area, conductivity and stability. It provides a new realization path for the rapid detection of VOCs represented by toluene at the ppb level, and has good scalability and industrialization potential. Attached Figure Description

[0027] Figure 1 These are the XRD patterns of the thin films prepared in Examples 1-3 and Comparative Examples 1-3;

[0028] Figure 2These are nitrogen adsorption-desorption curves and pore size distribution curves of the films prepared in Examples 1-3 and Comparative Examples 1-3;

[0029] Figure 3 These are temperature response curves of the thin films prepared in Examples 1-3 and Comparative Examples 1-3;

[0030] Figure 4 These are concentration response curves of the films prepared in Examples 1-3 and Comparative Examples 1-3. Detailed Implementation

[0031] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0032] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0033] The abbreviations used in this application are defined as follows:

[0034] Pd NPs: palladium nanoparticles; VOCs: volatile organic compounds; MOFs: metal-organic frameworks; CQDs: carbon quantum dots; 2-MIM: 2-MIM; N-CQDs: nitrogen-carbon quantum dots; FeHCF: ferrous ferricyanide (Fe3[Fe(CN)6]2).

[0035] The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material of the present invention includes the following steps:

[0036] To prepare Pd@ZIF-8, a Pd NPs ethanol dispersion was mixed with 2-methylimidazole, and then added to methanol. The mixture was sonicated for 8-13 min to obtain solution A. Zinc nitrate hexahydrate was dissolved in methanol to obtain solution B. Solution B was poured into solution A with stirring. The mixture was reacted at 25-80℃ for 6-12 h. The precipitate obtained from the first reaction solution was collected by centrifugation and washed to obtain the Pd@ZIF-8 core-shell structure. The resulting Pd@ZIF-8 was dispersed in methanol to obtain a Pd@ZIF-8 methanol dispersion.

[0037] To prepare the FeHCF / N-CQDs composite carrier, FeHCF precipitate was dispersed in anhydrous ethanol, N-CQDs ethanol solution was added, and the reaction was stirred at room temperature for 1.5-2.5 h. The resulting second reaction solution was centrifuged and washed, and the final precipitate was redispersed in anhydrous ethanol, vortexed and sonicated to obtain FeHCF / N-CQDs ethanol dispersion.

[0038] To construct the thin film, Pd@ZIF-8 methanol dispersion was added dropwise to FeHCF / N-CQDs ethanol dispersion, and the mixture was sonicated for 10-20 min. The resulting mixture was purified by centrifugation, and the final precipitate was dispersed in anhydrous methanol to obtain a ternary dispersion. After cleaning and drying the interdigitated electrode, the ternary dispersion was dropped onto the center of the interdigitated electrode, and spin-coated according to the following procedure: First, spin-coating at a low speed of 700-900 rpm for 5-10 s to spread the ternary dispersion and cover the entire interdigitated electrode. Second, spin-coating at a medium speed of 1500-2500 rpm for 20-40 s to form a preliminary film. Third, spin-coating at a high speed of 2500-3500 rpm for 10-20 s. The coated electrode was then vacuum-dried at 60-80℃ for 2-5 h to obtain the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film.

[0039] In a specific implementation, the preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material of the present invention includes the following steps:

[0040] (1) Preparation of Pd NPs ethanol dispersion

[0041] ① Solution preparation: Accurately weigh 10-30 mg of sodium citrate (Na3C6H5O7·2H2O) into a 50 mL three-necked round-bottom flask, add 10-15.0 mL of ultrapure water, and stir at 300-600 rpm until completely dissolved (about 5 minutes) to obtain a clear and colorless sodium citrate solution. Add sodium chloropalladium to pure water and mix well to obtain a sodium chloropalladium solution with a concentration of 5 mmol / L. Accurately measure 5.0-10 mL of the sodium chloropalladium solution.

[0042] ② Reduction reaction: The sodium citrate solution was heated to 60-80℃ in a water bath. A reflux condenser was connected and cooling water was circulated. Sodium chloropalladate solution was then added dropwise at a rate of 0.2-0.8 mL / min under magnetic stirring at 250-800 rpm. After the addition was complete, the mixture was refluxed at 60-80℃ for 0.5-2 h at 250-800 rpm. After the reaction was complete, the heating was turned off, and the system was allowed to cool naturally to near room temperature while maintaining stirring and reflux, yielding the third reaction solution. The molar ratio of sodium citrate to sodium chloropalladate was (0.34-1.00):(0.025-0.05).

[0043] ③ Product purification: Transfer the third reaction solution to a centrifuge tube and centrifuge at 6000-12000 rpm for 10-20 min. Discard the supernatant, disperse the precipitate with 25-35 mL of anhydrous ethanol, and centrifuge again at 6000-12000 rpm for 10-20 min. Repeat this process three times. Finally, add 10-30 mL of anhydrous ethanol to the precipitate and sonicate to obtain a Pd NPs ethanol dispersion.

[0044] (2) Preparation of Pd@ZIF-8

[0045] The Pd NPs ethanol dispersion prepared in step (1) was mixed with 20-40 mg of 2-MIM and dissolved in 15-30 mL of methanol. The solution was sonicated for 8-13 min to obtain solution A. Separately, 10-20 mg of Zn(NO3)2·6H2O was dissolved in 5-10 mL of methanol to obtain solution B. Solution B was quickly poured into solution A under magnetic stirring at 300-600 rpm, and the mixture was reacted at 25-80℃ for 6-12 h. After the reaction was completed, the first reaction solution was centrifuged at 8000-12000 rpm for 10-15 min, and the precipitate was collected. After washing with anhydrous methanol and centrifuging three times, the obtained Pd@ZIF-8 core-shell structure was redispersed in 10 mL of methanol to obtain Pd@ZIF-8 methanol dispersion for later use. The molar ratio of palladium to 2-methylimidazole in the Pd NPs ethanol dispersion is 1:10. The molar ratio of 2-methylimidazole to zinc nitrate hexahydrate is (7~8):1.

[0046] (3) Preparation of N-CQDs

[0047] Place 1.0–5.0 g of citric acid and 0.3–1.0 mL of ethylenediamine in a 10 mL glass reactor liner, add 3–7 mL of ultrapure water, and magnetically stir at 300–600 rpm until completely dissolved. Then, seal the reactor and place it in a preheated drying oven at 200–240 °C for 2–6 h. Remove and allow to cool naturally to room temperature. Dilute the resulting brownish-red crude product with 40–80 mL of ultrapure water, filter through a 0.22 μm filter membrane to remove large particle agglomerates, then centrifuge at 8000–12000 rpm for 10–20 min, collect the supernatant, and finally dry the supernatant at 80–120 °C to obtain N-CQDs.

[0048] (4) Preparation of FeHCF precipitate

[0049] Prepare 30-60 mL of FeCl2 aqueous solution with a concentration of 0.05-0.15 mol / L and K3[Fe(CN)6 aqueous solution with a concentration of 0.05-0.15 mol / L respectively. Simultaneously inject the FeCl2 aqueous solution and K3[Fe(CN)6 aqueous solution into an ice bath (0-5℃) and react for 0.5-2 h under magnetic stirring at 300-600 rpm. Finally, centrifuge the reaction solution and collect the FeHCF precipitate.

[0050] (5) Preparation of FeHCF / N-CQDs composite carrier

[0051] The FeHCF precipitate prepared in step (4) was dispersed in 5-15 mL of anhydrous ethanol, and 1.5-2.0 mL of N-CQDs ethanol solution with a concentration of 0.15-0.20 mg / mL was added. The mixture was stirred at 300-600 rpm for 2.0-3.0 h at room temperature (25℃) to obtain a second reaction solution. The second reaction solution was transferred to a centrifuge tube and centrifuged at 6000-12000 rpm for 10-20 min. After discarding the supernatant, the precipitate was dispersed in 25-35 mL of anhydrous ethanol and centrifuged again at 6000-12000 rpm for 10-20 min. This process was repeated 3 times. After washing, the precipitate was redispersed in 5-10 mL of anhydrous ethanol, vortexed, and briefly sonicated to obtain the FeHCF / N-CQDs ethanol dispersion.

[0052] (6) Constructing thin films

[0053] ① Mix the solutions: Take 5~10 mL of FeHCF / N-CQDs ethanol dispersion and add 0.5~1.5 mL of Pd@ZIF-8 methanol dispersion dropwise. Sonicate for 10~20 min to obtain a mixture.

[0054] ② Centrifugal purification: Centrifuge the mixture from step ① at 8000~12000 rpm for 5~15 min, disperse the precipitate in 5~10 mL of anhydrous methanol by vortexing, centrifuge again at 8000~12000 rpm for 5~15 min, repeat the methanol washing for a total of 5 times, and finally disperse the precipitate in 5~10 mL of anhydrous methanol to obtain a ternary dispersion.

[0055] ③ Thin film deposition: Immerse the interdigitated electrodes (gold electrodes, finger width / spacing = 20 μm) sequentially in acetone, isopropanol, and ultrapure water, and ultrasonically clean each for 10 minutes. Dry the surface with nitrogen. Then, drop 30-50 μL of ternary dispersion onto the center of the electrode and spin-coat according to the following procedure: First, spin-coat at a low speed of 700-900 rpm for 5-10 seconds to spread the ternary dispersion and cover the entire interdigitated electrode. Second, spin-coat at a medium speed of 1500-2500 rpm for 20-40 seconds to form a preliminary film. Third, spin-coat at a high speed of 2500-3500 rpm for 10-20 seconds. The coated electrode is then vacuum-dried at 60-80℃ for 2-5 hours to obtain the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film.

[0056] The following detailed description of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material and its preparation method according to specific embodiments of the present invention will be provided:

[0057] Example 1

[0058] The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material in this embodiment includes the following steps:

[0059] (1) Preparation of Pd NPs ethanol dispersion

[0060] ① Solution preparation: Accurately weigh 20.0 mg of sodium citrate (Na3C6H5O7·2H2O) into a 50 mL three-necked round-bottom flask, add 15.0 mL of ultrapure water, and stir at 300 rpm until completely dissolved (about 5 minutes) to obtain a clear and colorless sodium citrate solution. Add sodium chloropalladium to pure water and mix well to obtain a sodium chloropalladium solution with a concentration of 5 mmol / L. Accurately measure 5.0 mL of the sodium chloropalladium solution.

[0061] ② Reduction reaction: The sodium citrate solution was heated to 80°C in a water bath, a reflux condenser was connected and cooling water was passed through it, and then sodium chloropalladium solution was added dropwise at a rate of 0.3 mL / min under magnetic stirring at 600 rpm; after the addition was completed, the reaction was refluxed at 80°C at 600 rpm for 1 h; after the reaction was completed, the heating was turned off, and the system was allowed to cool naturally to near room temperature while maintaining stirring and reflux, to obtain the third reaction solution.

[0062] ③ Product purification: Transfer the third reaction solution to a centrifuge tube and centrifuge at 11,000 rpm for 10 min. Discard the supernatant, disperse the precipitate in 30 mL of anhydrous ethanol, and centrifuge again at 11,000 rpm for 10 min. Repeat this process three times. Finally, add 20 mL of anhydrous ethanol to the precipitate and sonicate to obtain a Pd NPs ethanol dispersion.

[0063] (2) Preparation of Pd@ZIF-8

[0064] The Pd NPs ethanol dispersion prepared in step (1) was mixed with 20 mg of 2-MIM and dissolved in 15 mL of methanol. The solution was sonicated for 10 min to obtain solution A. Separately, 10 mg of Zn(NO3)2·6H2O was dissolved in 5 mL of methanol to obtain solution B. Solution B was quickly poured into solution A under magnetic stirring at 600 rpm, and the reaction was carried out at 80 °C for 6 h. After the reaction was completed, the first reaction solution was centrifuged at 8000 rpm for 10 min, and the precipitate was collected. After washing with anhydrous methanol and centrifuging three times, the obtained Pd@ZIF-8 core-shell structure was redispersed in 10 mL of methanol to obtain Pd@ZIF-8 methanol dispersion for later use.

[0065] (3) Preparation of N-CQDs

[0066] 2.0 g of citric acid and 0.5 mL of ethylenediamine were placed in a 10 mL glass reactor liner, and 5 mL of ultrapure water was added. The mixture was magnetically stirred at 500 rpm until completely dissolved. The reactor was then sealed and placed in a preheated drying oven at 200 °C for 4 hours. After reaction, the mixture was removed and allowed to cool naturally to room temperature. The resulting brownish-red crude product was diluted with 40 mL of ultrapure water, filtered through a 0.22 μm filter membrane to remove large agglomerates, and then centrifuged at 11000 rpm for 10 min. The supernatant was collected and finally dried at 105 °C to obtain N-CQDs.

[0067] (4) Preparation of FeHCF precipitate

[0068] Prepare 50 mL each of a 0.1 mol / L FeCl2 aqueous solution and a 0.1 mol / L K3[Fe(CN)6 aqueous solution. Simultaneously inject 30 mL of FeCl2 aqueous solution and 20 mL of K3[Fe(CN)6 aqueous solution into an ice bath (0-5℃) and react for 2 h with magnetic stirring at 600 rpm. Finally, centrifuge the reaction solution and collect the FeHCF precipitate.

[0069] (5) Preparation of FeHCF / N-CQDs composite carrier

[0070] The FeHCF precipitate prepared in step (4) was dispersed in 10 mL of anhydrous ethanol, and 1.5 mL of 0.15 mg / mL N-CQDs ethanol solution was added. The mixture was stirred at 600 rpm for 2.0 h at room temperature (25 °C) to obtain a second reaction solution. The second reaction solution was transferred to a centrifuge tube and centrifuged at 11,000 rpm for 10 min. After discarding the supernatant, the precipitate was dispersed in 35 mL of anhydrous ethanol and centrifuged again at 11,000 rpm for 10 min. This process was repeated 3 times. After washing, the precipitate was redispersed in 10 mL of anhydrous ethanol, vortexed, and briefly sonicated to obtain the FeHCF / N-CQDs ethanol dispersion.

[0071] (6) Constructing thin films

[0072] ① Solution mixing: Take 10 mL of FeHCF / N-CQDs ethanol dispersion and add 0.5 mL of Pd@ZIF-8 methanol dispersion dropwise. Sonicate for 10 min to obtain a mixture.

[0073] ② Centrifugal purification: Centrifuge the mixture from step ① at 11,000 rpm for 10 min, disperse the precipitate in 10 mL of anhydrous methanol by vortexing, centrifuge again at 11,000 rpm for 10 min, repeat the methanol washing for a total of 5 times, and finally disperse the precipitate in 10 mL of anhydrous methanol to obtain a ternary dispersion.

[0074] ③ Thin film deposition: The interdigitated electrode (gold electrode, finger width / spacing = 20 μm) was sequentially immersed in acetone, isopropanol, and ultrapure water, and ultrasonically cleaned for 10 minutes in each. The surface was then dried with nitrogen. Then, 50 μL of ternary dispersion was dropped onto the center of the electrode, and spin-coating was performed according to the following procedure: First, spin-coating was performed at a low speed of 800 rpm for 8 seconds to spread the ternary dispersion and cover the entire interdigitated electrode. Second, spin-coating was performed at a medium speed of 2000 rpm for 30 seconds to form a preliminary film. Third, spin-coating was performed at a high speed of 3500 rpm for 20 seconds. The coated electrode was then vacuum dried at 80℃ for 4 hours to obtain the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film, denoted as Pd@ZIF-8 / CQDs / FeHCF-1.

[0075] Example 2

[0076] In this embodiment, the amount of Pd@ZIF-8 ethanol dispersion added in step (6) of Example 1 is changed from 0.5 ml to 1.0 ml, while other conditions remain unchanged. It is denoted as Pd@ZIF-8 / CQDs / FeHCF-2.

[0077] Example 3

[0078] In this embodiment, the amount of Pd@ZIF-8 ethanol dispersion added in step (6) of Example 1 is changed from 0.5ml to 1.5ml, while other conditions remain unchanged. It is denoted as Pd@ZIF-8 / CQDs / FeHCF-3.

[0079] Example 4

[0080] The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material in this embodiment includes the following steps:

[0081] (1) Preparation of Pd NPs ethanol dispersion

[0082] ① Solution preparation: Accurately weigh 10.0 mg of sodium citrate (Na3C6H5O7·2H2O) into a 50 mL three-necked round-bottom flask, add 10.0 mL of ultrapure water, and stir at 400 rpm until completely dissolved (about 5 minutes) to obtain a clear and colorless sodium citrate solution. Add sodium chloropalladium to pure water and mix well to obtain a sodium chloropalladium solution with a concentration of 5 mmol / L. Accurately measure 8.0 mL of the sodium chloropalladium solution.

[0083] ② Reduction reaction: The sodium citrate solution was heated to 60°C in a water bath, a reflux condenser was connected and cooling water was passed through it, and then sodium chloropalladate solution was added dropwise at a rate of 0.2 mL / min under magnetic stirring at 250 rpm; after the addition was completed, the reaction was refluxed at 60°C at 250 rpm for 0.5 h; after the reaction was completed, the heating was turned off, and the system was allowed to cool naturally to near room temperature while maintaining stirring and reflux, to obtain the third reaction solution.

[0084] ③ Product purification: Transfer the third reaction solution to a centrifuge tube and centrifuge at 6000 rpm for 15 min. Discard the supernatant, disperse the precipitate with 25 mL of anhydrous ethanol, and centrifuge again at 6000 rpm for 15 min. Repeat this process three times. Finally, add 10 mL of anhydrous ethanol to the precipitate and sonicate to obtain a Pd NPs ethanol dispersion.

[0085] (2) Preparation of Pd@ZIF-8

[0086] The Pd NPs ethanol dispersion prepared in step (1) was mixed with 32.84 mg of 2-MIM and dissolved in 30 mL of methanol. The solution was sonicated for 8 min to obtain solution A. Separately, 17 mg of Zn(NO3)2·6H2O was dissolved in 5 mL of methanol to obtain solution B. Solution B was quickly poured into solution A under magnetic stirring at 300 rpm, and the reaction was carried out at 25 °C for 12 h. After the reaction was completed, the first reaction solution was centrifuged at 10000 rpm for 12 min, and the precipitate was collected. After washing with anhydrous methanol and centrifuging three times, the obtained Pd@ZIF-8 core-shell structure was redispersed in 10 mL of methanol to obtain Pd@ZIF-8 methanol dispersion for later use.

[0087] (3) Preparation of N-CQDs

[0088] 1.0 g of citric acid and 0.3 mL of ethylenediamine were placed in a 10 mL glass reactor liner, and 3 mL of ultrapure water was added. The mixture was magnetically stirred at 300 rpm until completely dissolved. The reactor was then sealed and placed in a preheated 220 °C drying oven for 2 hours. After reaction, the mixture was removed and allowed to cool naturally to room temperature. The resulting brownish-red crude product was diluted with 50 mL of ultrapure water, filtered through a 0.22 μm filter membrane to remove large agglomerates, and then centrifuged at 8000 rpm for 15 min. The supernatant was collected and dried at 80 °C to obtain N-CQDs.

[0089] (4) Preparation of FeHCF precipitate

[0090] Prepare 50 mL each of a 0.05 mol / L FeCl2 aqueous solution and a 0.05 mol / L K3[Fe(CN)6 aqueous solution. Simultaneously inject 30 mL of FeCl2 aqueous solution and 20 mL of K3[Fe(CN)6 aqueous solution into an ice bath (0-5℃) and react for 0.5 h with magnetic stirring at 600 rpm. Finally, centrifuge the reaction solution and collect the FeHCF precipitate.

[0091] (5) Preparation of FeHCF / N-CQDs composite carrier

[0092] The FeHCF precipitate prepared in step (4) was dispersed in 5 mL of anhydrous ethanol, and 1.5 mL of 0.16 mg / mL N-CQDs ethanol solution was added. The mixture was stirred at 500 rpm for 205 h at room temperature (25 °C) to obtain a second reaction solution. The second reaction solution was transferred to a centrifuge tube and centrifuged at 6000 rpm for 15 min. After discarding the supernatant, the precipitate was dispersed in 25 mL of anhydrous ethanol and centrifuged again at 6000 rpm for 15 min. This process was repeated 3 times. After washing, the precipitate was redispersed in 5 mL of anhydrous ethanol, vortexed, and briefly sonicated to obtain the FeHCF / N-CQDs ethanol dispersion.

[0093] (6) Constructing thin films

[0094] ① Solution mixing: Take 5 mL of FeHCF / N-CQDs ethanol dispersion and add 0.5 mL of Pd@ZIF-8 methanol dispersion dropwise. Sonicate for 15 min to obtain a mixture.

[0095] ② Centrifugal purification: Centrifuge the mixture from step ① at 8000 rpm for 5 min, disperse the precipitate in 5 mL of anhydrous methanol by vortexing, centrifuge again at 8000 rpm for 5 min, repeat the methanol washing for a total of 5 times, and finally disperse the precipitate in 5 mL of anhydrous methanol to obtain a ternary dispersion.

[0096] ③ Thin film deposition: The interdigitated electrode (gold electrode, finger width / spacing = 20 μm) was sequentially immersed in acetone, isopropanol, and ultrapure water, and ultrasonically cleaned for 10 minutes in each. The surface was then dried with nitrogen. Then, 30 μL of ternary dispersion was dropped onto the center of the electrode, and spin-coating was performed according to the following procedure: First, spin-coating was performed at a low speed of 700 rpm for 5 seconds to spread the ternary dispersion and cover the entire interdigitated electrode. Second, spin-coating was performed at a medium speed of 1500 rpm for 20 seconds to form a preliminary film. Third, spin-coating was performed at a high speed of 2500 rpm for 10 seconds. The coated electrode was then vacuum dried at 60℃ for 5 hours to obtain the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film.

[0097] Example 5

[0098] The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material in this embodiment includes the following steps:

[0099] (1) Preparation of Pd NPs ethanol dispersion

[0100] ① Solution preparation: Accurately weigh 30.0 mg of sodium citrate (Na3C6H5O7·2H2O) into a 50 mL three-necked round-bottom flask, add 15.0 mL of ultrapure water, and stir at 600 rpm until completely dissolved (about 5 minutes) to obtain a clear and colorless sodium citrate solution. Add sodium chloropalladium to pure water and mix well to obtain a sodium chloropalladium solution with a concentration of 5 mmol / L. Accurately measure 10.0 mL of the sodium chloropalladium solution.

[0101] ② Reduction reaction: The sodium citrate solution was heated to 80°C in a water bath, a reflux condenser was connected and cooling water was circulated, and then sodium chloropalladate solution was added dropwise at a rate of 0.8 mL / min under magnetic stirring at 800 rpm; after the addition was completed, the reaction was refluxed at 75°C at 800 rpm for 1.5 h; after the reaction was completed, the heating was turned off, and the system was allowed to cool naturally to near room temperature while maintaining stirring and reflux, to obtain the third reaction solution.

[0102] ③ Product purification: Transfer the third reaction solution to a centrifuge tube and centrifuge at 12000 rpm for 15 min. Discard the supernatant, disperse the precipitate in 35 mL of anhydrous ethanol, and centrifuge again at 12000 rpm for 15 min. Repeat this process three times. Finally, add 30 mL of anhydrous ethanol to the precipitate and sonicate to obtain a Pd NPs ethanol dispersion.

[0103] (2) Preparation of Pd@ZIF-8

[0104] The Pd NPs ethanol dispersion prepared in step (1) was mixed with 40 mg of 2-MIM and dissolved in 25 mL of methanol. The solution was sonicated for 13 min to obtain solution A. Separately, 20 mg of Zn(NO3)2·6H2O was dissolved in 10 mL of methanol to obtain solution B. Solution B was quickly poured into solution A under magnetic stirring at 500 rpm, and the reaction was carried out at 75 °C for 10 h. After the reaction was completed, the first reaction solution was centrifuged at 12000 rpm for 15 min, and the precipitate was collected. After washing with anhydrous methanol and centrifuging three times, the obtained Pd@ZIF-8 core-shell structure was redispersed in 10 mL of methanol to obtain Pd@ZIF-8 methanol dispersion for later use.

[0105] (3) Preparation of N-CQDs

[0106] 5.0 g of citric acid and 1.0 mL of ethylenediamine were placed in a 10 mL glass reactor liner, and 7 mL of ultrapure water was added. The mixture was magnetically stirred at 600 rpm until completely dissolved. The reactor was then sealed and placed in a preheated 240 °C drying oven for 6 hours. After reaction, the mixture was removed and allowed to cool naturally to room temperature. The resulting brownish-red crude product was diluted with 80 mL of ultrapure water, filtered through a 0.22 μm filter membrane to remove large agglomerates, and then centrifuged at 12000 rpm for 20 min. The supernatant was collected and finally dried at 120 °C to obtain N-CQDs.

[0107] (4) Preparation of FeHCF precipitate

[0108] Prepare 50 mL each of a 0.15 mol / L FeCl2 aqueous solution and a 0.15 mol / L K3[Fe(CN)6 aqueous solution. Simultaneously inject 30 mL of FeCl2 aqueous solution and 20 mL of K3[Fe(CN)6 aqueous solution into an ice bath (0-5℃) and react for 2 h with magnetic stirring at 300 rpm. Finally, centrifuge the reaction solution and collect the FeHCF precipitate.

[0109] (5) Preparation of FeHCF / N-CQDs composite carrier

[0110] The FeHCF precipitate prepared in step (4) was dispersed in 15 mL of anhydrous ethanol, and 2.0 mL of 0.20 mg / mL N-CQDs ethanol solution was added. The mixture was stirred at 300 rpm for 3.0 h at room temperature (25 °C) to obtain a second reaction solution. The second reaction solution was transferred to a centrifuge tube and centrifuged at 12000 rpm for 15 min. After discarding the supernatant, the precipitate was dispersed in 30 mL of anhydrous ethanol and centrifuged again at 12000 rpm for 15 min. This process was repeated 3 times. After washing, the precipitate was redispersed in 10 mL of anhydrous ethanol, vortexed, and briefly sonicated to obtain the FeHCF / N-CQDs ethanol dispersion.

[0111] (6) Constructing thin films

[0112] ① Solution mixing: Take 8 mL of FeHCF / N-CQDs ethanol dispersion and add 1.0 mL of Pd@ZIF-8 methanol dispersion dropwise. Sonicate for 20 min to obtain a mixture.

[0113] ② Centrifugal purification: Centrifuge the mixture from step ① at 12000 rpm for 15 min, disperse the precipitate in 8 mL of anhydrous methanol by vortexing, centrifuge again at 12000 rpm for 15 min, repeat the methanol washing for a total of 5 times, and finally disperse the precipitate in 10 mL of anhydrous methanol to obtain a ternary dispersion.

[0114] ③ Thin film deposition: The interdigitated electrode (gold electrode, finger width / spacing = 20 μm) was sequentially immersed in acetone, isopropanol, and ultrapure water, and ultrasonically cleaned for 10 minutes in each. The surface was then dried with nitrogen. Then, 40 μL of ternary dispersion was dropped onto the center of the electrode, and spin-coating was performed according to the following procedure: First, spin-coating was performed at a low speed of 900 rpm for 10 seconds to spread the ternary dispersion and cover the entire interdigitated electrode. Second, spin-coating was performed at a medium speed of 3500 rpm for 40 seconds to form a preliminary film. Third, spin-coating was performed at a high speed of 2500 rpm for 10 seconds. The coated electrode was then vacuum dried at 60℃ for 5 hours to obtain the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film.

[0115] Comparative Example 1

[0116] Using the Pd NPs ethanol dispersion prepared in step (1) of Example 1 as the final deposition solution, thin film deposition was carried out according to the thin film deposition steps in Example 1, and finally Pd@ZIF-8 thin film was obtained.

[0117] Comparative Example 2

[0118] The N-CQDs prepared in step (3) of Example 1 were dispersed in ethanol to prepare an N-CQDs ethanol dispersion with a concentration of 0.1 g / L. This dispersion was used as the final deposition solution. Thin film deposition was carried out according to the thin film deposition steps in Example 1 to finally obtain an N-CQDs thin film.

[0119] Comparative Example 3

[0120] The FeHCF precipitate prepared in step (4) of Example 1 was dispersed in ethanol to prepare a FeHCF ethanol dispersion with a concentration of 0.1 g / L. This dispersion was used as the final deposition solution. The FeHCF film was deposited according to the film deposition steps in Example 1 to obtain the final FeHCF film.

[0121] The thin films finally prepared in Examples 1-3 and Comparative Examples 1-3 were used as samples for the following structural characterization and performance testing:

[0122] (1) X-ray diffraction analysis

[0123] The working principle of an X-ray diffractometer is to use X-rays to irradiate a sample, producing a diffraction phenomenon under the influence of the periodic arrangement of atoms within the crystal. By analyzing the diffraction pattern, information such as the phase composition, content, crystallinity, and grain size of the material can be obtained. This invention uses a Bruker D8 Advance X-ray diffractometer (Germany) for testing, with a scanning range of 2–50°, a scanning speed of 10° / min, and a step size of 0.05°. The thin films finally prepared in Examples 1-3 and Comparative Examples 1-3 were tested, and the test results are as follows: Figure 1 As shown. From Figure 1 It can be seen that the Pd@ZIF-8 film completely retains the characteristic peaks of ZIF-8, indicating that the Pd loading did not destroy the ZIF-8 crystal structure. Pure CQDs exhibits amorphous carbon broadening peaks, and pure FeHCF has sharp characteristic peaks at 2θ≈17.4° and 24.6°. The ternary composite system (Pd@ZIF-8 / CQDs / FeHCF-1, -2, -3) retains the characteristic peaks of both ZIF-8 and FeHCF, confirming the successful composite of each component and demonstrating the compatibility of the crystal structure, thus providing a structural basis for the synergistic improvement of gas-sensing performance.

[0124] (2) Specific surface area and pore structure analysis

[0125] Using the films finally prepared in Examples 1-3 and Comparative Examples 1-3 as samples, a Bioder SSA-6000 surface area analyzer was used at a liquid nitrogen temperature of -196°C. The adsorption and desorption isotherms of nitrogen on the materials were precisely measured, and the specific surface area was calculated using the Brunauer-Emmett-Teller (BET) model. The pore size distribution and pore volume were analyzed using the Barrett-Joyner-Halenda (BJH) method, thus comprehensively characterizing the porosity properties of the materials. The test results are as follows: Figure 2 As shown.

[0126] from Figure 2 It can be seen that Comparative Example 1 (Pd@ZIF-8 film) exhibits the highest specific surface area (approximately 1290 m² / g), with a typical microporous isotherm and pore sizes mainly concentrated in the 1.2-2 nm range, indicating that its microporous framework is complete and its gas-accessible surface area is the largest. In the Pd@ZIF-8 / CQDs / FeHCF ternary composite system, the BET specific surface area decreases with increasing CQDs to FeHCF ratio (approximately 937 m² / g in Example 1, approximately 697 m² / g in Example 2, and approximately 581 m² / g in Example 3). Simultaneously, a hysteresis loop appears in the medium-to-high relative pressure range, indicating that the composite process introduces more transport channels while sacrificing some micropores. Comparative Example 2 (pure CQDs film) and Comparative Example 3 (pure FeHCF film) lack a regular pore structure and have the lowest specific surface area.

[0127] (3) Gas sensitivity detection

[0128] The CGS-8 intelligent gas-sensitive analysis system was used to detect the gas-sensitive properties of the material to toluene. The specific steps were as follows: After fixing the prepared sample substrate, the temperature control switch was turned on and the temperature was set, allowing the sensor to adsorb in the air until its resistance stabilized. Then, the test chamber lid was closed. Before testing, the air in the test chamber was dehumidified using an air generator to ensure a stable humidity of 15±5%. Once the resistance stabilized again, the test liquid was injected using a micro-syringe. After the test liquid was heated, it diffused evenly and converted into the target gas. Upon contact with the target gas, the resistance of the gas-sensitive material changed rapidly and eventually stabilized. When the resistance remained relatively stable, the chamber lid was opened for desorption.

[0129]

[0130] Where V1 is the volume of liquid injected into the instrument, V s It refers to the size of the instrument chamber, C. g ρ is the concentration of the target gas, M is the molecular weight of the liquid, ρ is the density of the liquid, and d is the purity of the liquid.

[0131] Using 5 ppb toluene gas as the detection gas, the temperature response curve for gas sensitivity detection is shown in the figure below. Figure 3 As shown in the figure, the concentration response curve is as follows: Figure 4 As shown. From Figure 3 and Figure 4 It can be seen that the response values ​​of different samples at a toluene concentration of 5 ppb all exhibit a "first increase, then decrease" pattern with temperature, showing a good linear growth trend with increasing toluene concentration. At low temperatures, the rates of gas adsorption and catalytic reaction are limited, while at high temperatures, the gas molecules move too quickly, leading to rapid desorption before reacting with the sensor material. Therefore, the response value reaches its peak in the intermediate temperature range. Among them, the response value of the Pd@ZIF-8 / CQDs / FeHCF ternary composite system is significantly higher than that of the single components Pd@ZIF-8, N-CQDs, and FeHCF. This confirms the synergistic effect of the composite material in "adsorption-catalysis-conductivity": Pd nanoparticles provide catalytic active sites to activate the target gas, the porous structure of ZIF-8 enhances gas adsorption, CQDs accelerate electron conduction, and FeHCF stabilizes the composite structure. The four components synergistically amplify the resistance change. Among them, Pd@ZIF-8 / CQDs / FeHCF-2 had the highest peak response value and a more concentrated optimal operating temperature range (approximately 180-200℃), indicating that the proportion of Pd@ZIF-8 in this sample matched the best with CQDs and FeHCF, and the synergistic effect was the most significant.

[0132] In summary, the Pd@ZIF-8 / CQDs / FeHCF gas-sensitive material prepared by this invention can be used in VOCs gas concentration detection materials, and is especially suitable for low-concentration toluene gas concentration detection materials.

[0133] The foregoing description includes examples from this specification. Of course, for the purposes of describing this specification, it is impossible to describe every conceivable combination of components or methods; however, those skilled in the art will understand that many other combinations and arrangements are possible. Therefore, this specification is intended to cover all such changes, modifications, and variations that fall within the spirit and scope of the appended claims. Furthermore, with regard to the use of the term "comprising" in the detailed description or claims, the term is intended to be inclusive in a manner similar to the term "including," as interpreted when "comprising" is used as a transitional word in the claims.

[0134] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material, characterized in that, Includes the following steps: To prepare Pd@ZIF-8, a Pd NPs ethanol dispersion was mixed with 2-methylimidazole, and then added to methanol. The mixture was sonicated for 8-13 min to obtain solution A. Zinc nitrate hexahydrate was dissolved in methanol to obtain solution B. Solution B was poured into solution A with stirring. The mixture was reacted at 25-80℃ for 6-12 h. The precipitate obtained from the first reaction solution was collected by centrifugation and washed to obtain the Pd@ZIF-8 core-shell structure. The resulting Pd@ZIF-8 was dispersed in methanol to obtain a Pd@ZIF-8 methanol dispersion. To prepare the FeHCF / N-CQDs composite carrier, FeHCF precipitate was dispersed in anhydrous ethanol, N-CQDs ethanol solution was added, and the reaction was stirred at room temperature for 2.0-3.0 h. The resulting second reaction solution was centrifuged and washed, and the final precipitate was redispersed in anhydrous ethanol, vortexed and sonicated to obtain FeHCF / N-CQDs ethanol dispersion. To construct the thin film, Pd@ZIF-8 methanol dispersion was added dropwise to FeHCF / N-CQDs ethanol dispersion, and the mixture was sonicated for 10-20 min. The resulting mixture was purified by centrifugation, and the final precipitate was dispersed in anhydrous methanol to obtain a ternary dispersion. After cleaning and drying the interdigitated electrode, the ternary dispersion was dropped onto the center of the interdigitated electrode, and spin-coated according to the following procedure: First, spin-coating was performed at a low speed of 700-900 rpm for 5-10 s to spread the ternary dispersion and cover the entire interdigitated electrode. Second, spin-coating was performed at a medium speed of 1500-2500 rpm for 20-40 s to form a preliminary film. Third, spin-coating was performed at a high speed of 2500-3500 rpm for 10-20 s. The coated electrode was then vacuum-dried at 60-80℃ for 2-5 h to obtain the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film.

2. The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material according to claim 1, characterized in that, The preparation method of the Pd NPs ethanol dispersion is as follows: Sodium citrate was added to ultrapure water and stirred to dissolve, thus obtaining a sodium citrate solution; sodium chloropalladium was added to pure water and mixed to obtain a sodium chloropalladium solution. The sodium citrate solution was heated to 60-80°C in a water bath. A reflux condenser was connected and cooling water was passed through. Sodium chloropalladium solution was added dropwise at a rate of 0.2-0.8 mL / min with stirring. After the addition was complete, stirring was continued and the mixture was refluxed at 60-80°C for 0.5-2 hours. The mixture was then kept under stirring and reflux and allowed to cool naturally to room temperature to obtain the third reaction solution. Centrifuge the third reaction solution at 6000~12000 rpm for 10~20 min, discard the supernatant, disperse the precipitate with anhydrous ethanol, centrifuge again at 6000~12000 rpm for 10~20 min, repeat 3 times, add the final precipitate to anhydrous ethanol and disperse by ultrasonication to obtain Pd NPs ethanol dispersion.

3. The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material according to claim 2, characterized in that, The molar ratio of sodium citrate to sodium chloropalladium is (0.34~1.00):(0.025~0.05).

4. The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material according to claim 3, characterized in that, The molar ratio of palladium to 2-methylimidazole in the Pd NPs ethanol dispersion is 1:

10.

5. The preparation method of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material according to claim 4, characterized in that, The molar ratio of 2-methylimidazole to zinc nitrate hexahydrate is (7~8):

1.

6. The method for preparing the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material according to claim 1, characterized in that, The preparation method of the N-CQDs is as follows: Citric acid and ethylenediamine were added to the lining of a reaction vessel, followed by ultrapure water. The mixture was stirred until completely dissolved, and the reaction vessel was sealed. The vessel was then placed in a preheated drying oven at 200-240°C and reacted for 2-6 hours. After the reaction was completed, the product was removed and allowed to cool naturally to room temperature. The resulting solid product was diluted with ultrapure water, filtered through a 0.22 μm filter membrane, and then centrifuged at 8000-12000 rpm for 10-20 minutes. The supernatant was collected and dried at 80-120°C to obtain N-CQDs.

7. The method for preparing the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material according to claim 1, characterized in that, The FeHCF precipitate is prepared as follows: Prepare FeCl2 aqueous solution and K3[Fe(CN)6] aqueous solution with concentrations of 0.05~0.15mol / L respectively. According to the molar ratio of FeCl2 to K3[Fe(CN)6] 3:2, simultaneously inject the FeCl2 aqueous solution and K3[Fe(CN)6] aqueous solution into an ice bath and stir the reaction for 0.5~2h. Centrifuge the resulting reaction solution and collect the FeHCF precipitate.

8. The method for preparing the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material according to claim 1, characterized in that, The centrifugal purification in the thin film construction step is as follows: the mixture is centrifuged at 8000~12000 rpm for 5~15 min, the precipitate is dispersed by vortexing with anhydrous methanol, and then centrifuged at 8000~12000 rpm for 5~15 min. The methanol washing is repeated a total of 5 times.

9. A Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material, characterized in that, The ternary composite thin film gas-sensitive material was prepared using the preparation method described in any one of Examples 1-8.

10. The application of the Pd@ZIF-8 / CQDs / FeHCF ternary composite thin film gas-sensitive material as described in claim 9 in the preparation of toluene concentration detection materials.

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