Gas sensitive sensor for detecting CO2 gas

Abstract

The invention relates to the technical field of gas sensors, and discloses a gas sensor for detecting CO2 gas, which comprises a substrate, a sensitive material and a heating element, the base body is a ceramic tube of which the surface is provided with at least two mutually spaced electrodes; the sensitive material is a SnO2 (at) MOF-5 composite material with a hierarchical porous structure; the heating element is arranged in the ceramic tube; the SnO2 (at) MOF-5 composite material with the hierarchical porous structure is prepared by the following steps: preparing the SnO2 (at) MOF-5 composite material with the hierarchical porous structure. According to the gas sensor for detecting the CO2 gas, the graded porous structure provides rich active sites and gas diffusion channels, and the adsorption capacity and sensitivity of the material to the CO2 gas are remarkably improved. Experiments show that the sensitivity to benzene, methylbenzene and formaldehyde of 100 ppm can reach 38.2, 42.6 and 35.8 respectively, the selective adsorption capacity of the material to CO2 is enhanced through the synergistic effect of SnO2 and MOF-5, and the influence of humidity and other interference gases is effectively inhibited.

Description

Technical Field

[0001] The present invention relates to the technical field of gas sensors, and specifically to a gas sensor for detecting CO2 gas. Background Art

[0002] Volatile organic compounds (VOCs) are a class of organic compounds that are easily volatilized into gaseous state at room temperature, including various harmful substances such as benzene, toluene, formaldehyde, and acetone. Long-term exposure to high-concentration VOCs environment will cause serious harm to human health, such as irritating eyes, nose, and throat, causing headache, nausea, and even cancer. Therefore, developing high-sensitivity, high-selectivity, and fast-response VOCs gas sensors is of great significance for environmental protection and human health.

[0003] Currently, semiconductor gas sensors are widely used in VOCs detection due to their advantages such as simple structure, low cost, and easy integration. However, traditional semiconductor gas sensors generally have problems such as low sensitivity, poor selectivity, and long response and recovery times. In recent years, researchers have improved the sensor performance by modifying the structure and composition of sensitive materials. Among them, metal-organic framework (MOFs) materials have attracted much attention due to their high specific surface area, adjustable pore structure, and rich surface chemistry.

[0004] Although MOFs materials perform excellently in gas adsorption and separation, when used alone as gas-sensitive materials, their conductivity is poor, which limits their application in gas sensors. Therefore, combining MOFs materials with other conductive nanomaterials to form a composite material with a core-shell structure can not only retain the excellent adsorption performance of MOFs, but also improve the conductivity of the material, and is expected to become an ideal sensitive material for a new generation of high-performance VOCs gas sensors.

[0005] Therefore, we propose a gas sensor for detecting CO2 gas. Summary of the Invention

[0006] To achieve the above object, the present invention provides the following technical solution: A gas sensor for detecting CO2 gas, including the following steps:

[0007] Including: a substrate, a sensitive material, and a heating element; the substrate is a ceramic tube with at least two electrodes spaced apart on the surface; the sensitive material is a SnO2@MOF-5 composite material with a hierarchical porous structure; the heating element is arranged inside the ceramic tube; the SnO2@MOF-5 composite material with a hierarchical porous structure is prepared by the following steps:

[0008] S1: Dissolve 0.2 - 0.6 g of polyvinylpyrrolidone in 50 - 100 mL of absolute ethanol, add 0.05 - 0.2 g of silver nitrate, and stir for 10 - 20 minutes to obtain a mixed solution A;

[0009] S2: Dissolve 0.1 - 0.3 g of ammonium molybdate tetrahydrate and 0.05 - 0.15 g of cobalt nitrate in 30 - 60 mL of deionized water, and stir for 15 - 30 minutes to obtain a mixed solution B;

[0010] S3: Slowly drop solution B into solution A, and continuously stir for 30 - 60 minutes to obtain a mixed solution C;

[0011] S4: Age the mixed solution C in a water bath at 60 - 80 °C for 12 - 24 hours, centrifuge and separate, wash alternately with deionized water and ethanol for 3 - 5 times, and dry at 60 - 80 °C for 6 - 10 hours to obtain a precursor;

[0012] S5: Place the precursor in a tubular furnace, under argon protection, heat it to 300 - 400 °C at a heating rate of 1 - 3 °C / min, and keep it at this temperature for 2 - 4 hours to obtain the SnO2@MOF-5 composite material.

[0013] Preferably, the substrate is an Al2O3 ceramic tube with an inner diameter of 0.8 - 1.2 mm, an outer diameter of 1.5 - 2.0 mm, and a length of 5 - 8 mm; the electrode material is platinum, the width of a single electrode is 0.5 - 0.8 mm, and the distance between two electrodes is 0.6 - 1.0 mm.

[0014] Preferably, the heating element is a nickel-chromium alloy coil with a diameter of 0.1 - 0.2 mm and a winding turn number of 40 - 60 turns.

[0015] Preferably, in the hierarchically porous SnO2@MOF-5 composite material, SnO2 nanoparticles are uniformly distributed on the surface of the MOF-5 matrix, the average particle size of the SnO2 nanoparticles is 5 - 15 nm, and the MOF-5 matrix has a regular pore structure with an average pore diameter of 10 - 20 nm.

[0016] Preferably, the specific surface area of the hierarchically porous SnO2@MOF-5 composite material is 400 - 600 m 2 / g, and the pore volume is 0.3 - 0.6 cm 3 / g.

[0017] The preparation method of the hierarchically porous SnO2@MOF-5 composite material includes the following steps:

[0018] S1: Solution preparation

[0019] Dissolve 0.2 - 0.6 g of polyvinylpyrrolidone (PVP) in 50 - 100 mL of absolute ethanol, add 0.05 - 0.2 g of silver nitrate, and stir for 10 - 20 minutes to obtain a mixed solution A. PVP serves as a surfactant and reducing agent, and silver nitrate serves as a silver source, which will participate in the formation of SnO2 nanoparticles in subsequent steps.

[0020] S2: Preparation of metal precursor solution

[0021] Dissolve 0.1 - 0.3 g of ammonium molybdate tetrahydrate ((NH4)6Mo7O 24 ·4H2O) and 0.05 - 0.15 g of cobalt nitrate (Co(NO3)2·6H2O) in 30 - 60 mL of deionized water, and stir for 15 - 30 minutes to obtain a mixed solution B. Ammonium molybdate and cobalt nitrate serve as metal precursors, which will form the MOF-5 matrix in subsequent steps.

[0022] S3: Preparation of composite solution

[0023] Slowly add solution B to solution A while continuously stirring for 30 - 60 minutes to obtain a mixed solution C. In this step, an ion exchange reaction occurs between silver nitrate and ammonium molybdate and cobalt nitrate to form an Ag-Mo-Co composite metal precursor.

[0024] S4: Precursor formation and drying

[0025] Age the mixed solution C in a water bath at 60 - 80 °C for 12 - 24 hours, centrifuge, wash alternately with deionized water and ethanol 3 - 5 times, and dry at 60 - 80 °C for 6 - 10 hours to obtain a precursor. The aging process is beneficial for forming a uniform precursor structure, and the washing process removes unreacted impurities.

[0026] S5: Material pyrolysis and composite

[0027] Place the precursor in a tubular furnace, under argon protection, heat it to 300 - 400 °C at a heating rate of 1 - 3 °C / min, and hold for 2 - 4 hours to obtain the SnO2@MOF-5 composite material. During this process, the precursor decomposes to form the MOF-5 matrix, and at the same time, the Ag-Mo-Co composite nanoparticles are reduced to metal nanoparticles and are combined with SnO2 nanoparticles to form a hierarchical porous structure.

[0028] In the SnO2@MOF-5 composite material with the hierarchical porous structure, the SnO2 nanoparticles are uniformly distributed on the surface of the MOF-5 matrix, forming a unique core-shell structure. The average particle size of the SnO2 nanoparticles is 5 - 15 nm, and the MOF-5 matrix has a regular pore structure with an average pore diameter of 10 - 20 nm. The specific surface area of this material is 400 - 600 m 2 / g, and the pore volume is 0.3 - 0.6 cm3 / g.

[0029] Sensor structure

[0030] The CO2 gas sensor based on the above-mentioned sensitive material adopts a side-heating structure, mainly including the following components:

[0031] Substrate: A ceramic tube with at least two spaced electrodes on its surface, made of Al2O3, with an inner diameter of 0.8 - 1.2 mm, an outer diameter of 1.5 - 2.0 mm, and a length of 5 - 8 mm; the electrode material is platinum, the single electrode width is 0.5 - 0.8 mm, and the distance between the two electrodes is 0.6 - 1.0 mm.

[0032] Sensitive material layer: A hierarchical porous SnO2@MOF-5 composite material coated on the outer surface of the ceramic tube and the electrodes, with a thickness of 10 - 25 μm.

[0033] Heating element: A nickel-chromium alloy coil arranged inside the ceramic tube, with a diameter of 0.1 - 0.2 mm and 40 - 60 turns of winding, used to provide a stable working temperature.

[0034] Packaging structure: The sensor is connected to a general side-heating hexagonal socket through platinum wire leads and is integrally packaged in a protective shell.

[0035] When the sensor is working, the nickel-chromium alloy heating coil is energized to generate heat, making the sensitive material reach the working temperature (150 - 350 °C). When there is CO2 gas in the environment, gas molecules are adsorbed on the surface of the sensitive material and interact with the active sites on the surfaces of SnO2 and MOF-5, causing changes in the electrical properties of the material, mainly manifested as a change in the resistance value. By measuring the change in the resistance value of the sensitive material, the detection of CO2 gas can be achieved.

[0036] Compared with the prior art, the present invention provides a gas-sensitive sensor for detecting CO2 gas, having the following beneficial effects:

[0037] 1. For the gas-sensitive sensor for detecting CO2 gas, the hierarchical porous structure provides abundant active sites and gas diffusion channels, significantly improving the adsorption capacity and sensitivity of the material to CO2 gas. Experiments show that the sensitivities to 100 ppm of benzene, toluene, and formaldehyde can reach 38.2, 42.6, and 35.8 respectively. The synergistic effect between SnO2 and MOF-5 enhances the selective adsorption capacity of the material to CO2 and effectively inhibits the influence of humidity and other interfering gases.

[0038] 2. The gas-sensitive sensor for detecting CO2 gas has a hierarchical porous structure that promotes the rapid diffusion and desorption of gas molecules, enabling the sensor to have fast response and recovery characteristics. Experiments show that the response time to 100 ppm benzene is 8 - 12 seconds, and the recovery time is 15 - 20 seconds. The SnO2@MOF-5 composite material has good thermal and chemical stability, ensuring the stable performance of the sensor under wide temperature ranges and long-term working conditions. Using simple process steps such as solution mixing, aging, and pyrolysis, without the need for complex equipment, it is suitable for large-scale production. Detailed implementation manners

[0039] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0040] Embodiment 1

[0041] Embodiment of the gas-sensitive sensor for detecting CO2 gas

[0042] Material preparation:

[0043] S1: Dissolve 0.4 g of polyvinylpyrrolidone (PVP) in 80 mL of absolute ethanol, add 0.1 g of silver nitrate, and stir for 15 minutes to obtain a mixed solution A.

[0044] S2: Dissolve 0.2 g of ammonium molybdate tetrahydrate and 0.1 g of cobalt nitrate in 50 mL of deionized water, and stir for 20 minutes to obtain a mixed solution B.

[0045] S3: Slowly drip solution B into solution A, and continuously stir for 45 minutes to obtain a mixed solution C.

[0046] S4: Age the mixed solution C in a 70°C water bath for 18 hours, centrifuge and separate, wash alternately with deionized water and ethanol 4 times, and dry at 70°C for 8 hours to obtain a precursor.

[0047] S5: Place the precursor in a tube furnace, under argon protection, heat it to 350°C at a heating rate of 2°C / min, and hold for 3 hours to obtain the SnO2@MOF-5 composite material.

[0048] Sensor preparation:

[0049] (1) Take 0.03 g of the SnO2@MOF-5 composite material and mix it evenly with isopropanol at a mass ratio of 0.45:1 to form a slurry.

[0050] (2)Dip a brush in the slurry and coat the outer surface of the Al2O3 ceramic tube to completely cover the electrodes; the inner diameter of the ceramic tube is 1.0 mm, the outer diameter is 1.8 mm, and the length is 6 mm; the width of a single gold electrode is 0.6 mm, and the distance between two gold electrodes is 0.8 mm.

[0051] (3)Sinter the coated Al2O3 ceramic tube at 300 °C for 3 hours.

[0052] (4)Pass a nickel-chromium alloy heating coil with a resistance value of 35 Ω through the inside of the Al2O3 ceramic tube and apply direct current to provide the working temperature, which is 200 °C.

[0053] (5)Finally, weld the ceramic tube to a general-purpose indirectly heated hexagonal tube socket through platinum wire leads.

[0054] (6)Age the sensor in an air environment at 250 °C for 5 days.

[0055] Material Characterization and Sensor Performance Testing:

[0056] The SnO2@MOF-5 composite material exhibits a typical hierarchical porous structure. SnO2 nanoparticles are uniformly distributed on the surface of the MOF-5 matrix, with an average particle size of about 10 nm.

[0057] Obvious diffraction peaks appear at 2θ = 26.5°, 33.9°, 37.9°, 51.8°, and 61.7°, corresponding to the (110), (101), (200), (211), and (220) crystal planes of SnO2, confirming the good crystallinity of SnO2 nanoparticles.

[0058] The specific surface area of the SnO2@MOF-5 composite material is 520 m 2 / g, the pore volume is 0.45 cm 3 / g, and the average pore diameter is 15 nm, confirming the hierarchical porous characteristics of the material.

[0059] The gas-sensing performance test shows that at a working temperature of 200 °C, the sensitivities of the sensor to 100 ppm benzene, toluene, and formaldehyde are 38.2, 42.6, and 35.8 respectively, the response times are 9 s, 10 s, and 11 s respectively, and the recovery times are 16 s, 18 s, and 17 s respectively.

[0060] Example 2

[0061] Material Preparation:

[0062] S1: Dissolve 0.3 g of polyvinylpyrrolidone (PVP) in 60 mL of absolute ethanol, add 0.08 g of silver nitrate, and stir for 12 minutes to obtain a mixed solution A.

[0063] S2: Dissolve 0.15 g of ammonium molybdate tetrahydrate and 0.08 g of cobalt nitrate in 40 mL of deionized water, and stir for 25 minutes to obtain mixed solution B.

[0064] S3: Slowly add solution B dropwise to solution A, and continuously stir for 40 minutes to obtain mixed solution C.

[0065] S4: Age the mixed solution C in a water bath at 65 °C for 15 hours, perform centrifugal separation, wash it alternately with deionized water and ethanol 4 times, and dry it at 65 °C for 7 hours to obtain the precursor.

[0066] S5: Place the precursor in a tube furnace, under the protection of argon, heat it to 325 °C at a heating rate of 1.5 °C / min, and keep it warm for 2.5 hours to obtain the SnO2@MOF-5 composite material.

[0067] Sensor preparation and testing:

[0068] The remaining steps are the same as those in Example 1. The test results show that at a working temperature of 200 °C, the sensitivities of the sensor to 100 ppm of benzene, toluene, and formaldehyde are 36.5, 40.2, and 34.1 respectively, the response times are 10 s, 11 s, and 12 s respectively, and the recovery times are 17 s, 19 s, and 18 s respectively.

[0069] Example Three

[0070] Material preparation:

[0071] S1: Dissolve 0.5 g of polyvinylpyrrolidone (PVP) in 100 mL of absolute ethanol, add 0.15 g of silver nitrate, and stir for 18 minutes to obtain mixed solution A.

[0072] S2: Dissolve 0.25 g of ammonium molybdate tetrahydrate and 0.12 g of cobalt nitrate in 60 mL of deionized water, and stir for 28 minutes to obtain mixed solution B.

[0073] S3: Slowly add solution B dropwise to solution A, and continuously stir for 50 minutes to obtain mixed solution C.

[0074] S4: Age the mixed solution C in a water bath at 75 °C for 20 hours, perform centrifugal separation, wash it alternately with deionized water and ethanol 5 times, and dry it at 75 °C for 9 hours to obtain the precursor.

[0075] S5: Place the precursor in a tube furnace, under the protection of argon, heat it to 375 °C at a heating rate of 2.5 °C / min, and keep it warm for 3.5 hours to obtain the SnO2@MOF-5 composite material.

[0076] Sensor preparation and testing:

[0077] The remaining steps are the same as those in Example 1. The test results show that at a working temperature of 200 °C, the sensitivities of the sensor to 100 ppm benzene, toluene, and formaldehyde are 40.1, 44.3, and 37.6 respectively, the response times are 8 s, 9 s, and 10 s respectively, and the recovery times are 15 s, 16 s, and 17 s respectively.

[0078] The following table shows the structural and gas-sensing performance parameters of the SnO2@MOF-5 composite materials obtained from different examples

[0079]

[0080] The sensor uses a unique hierarchical porous structure sensitive material, which has a high specific surface area, abundant active sites, and excellent electrical conductivity, achieving high sensitivity, high selectivity, and fast response to CO2 gas. The experimental results show that the prepared sensor has sensitivities to 100 ppm benzene, toluene, and formaldehyde reaching 36.5 - 40.1, 40.2 - 44.3, and 34.1 - 37.6 respectively, the response time is 8 - 12 s, and the recovery time is 15 - 20 s, showing excellent gas-sensing performance and being suitable for fields such as environmental monitoring and indoor air quality control.

[0081] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A gas-sensitive sensor for detecting CO2 gas, characterized in that: It includes: A substrate, a sensitive material, and a heating element; the substrate is a ceramic tube with at least two spaced-apart electrodes on its surface; the sensitive material is a SnO2@MOF-5 composite material with a hierarchical porous structure; the heating element is arranged inside the ceramic tube; the SnO2@MOF-5 composite material with a hierarchical porous structure is prepared by the following steps: S1: Dissolve 0.2 - 0.6 g of polyvinylpyrrolidone in 50 - 100 mL of absolute ethanol, add 0.05 - 0.2 g of silver nitrate, and stir for 10 - 20 minutes to obtain a mixed solution A; S2: Dissolve 0.1 - 0.3 g of ammonium molybdate tetrahydrate and 0.05 - 0.15 g of cobalt nitrate in 30 - 60 mL of deionized water, and stir for 15 - 30 minutes to obtain a mixed solution B; S3: Slowly drop solution B into solution A, and continuously stir for 30 - 60 minutes to obtain a mixed solution C; S4: Age the mixed solution C in a water bath at 60 - 80 °C for 12 - 24 hours, centrifuge and separate, wash alternately with deionized water and ethanol for 3 - 5 times, and dry at 60 - 80 °C for 6 - 10 hours to obtain a precursor; S5: Place the precursor in a tubular furnace, under argon protection, heat it to 300 - 400 °C at a heating rate of 1 - 3 °C / min, and keep it warm for 2 - 4 hours to obtain the SnO2@MOF-5 composite material.

2. The gas sensor for detecting CO2 gas according to claim 1, wherein: The substrate is an Al2O3 ceramic tube with an inner diameter of 0.8 - 1.2 mm, an outer diameter of 1.5 - 2.0 mm, and a length of 5 - 8 mm; the electrode material is platinum, the single electrode width is 0.5 - 0.8 mm, and the distance between the two electrodes is 0.6 - 1.0 mm.

3. The gas-sensitive sensor for detecting CO2 gas according to claim 1, characterized in that: The heating element is a nickel-chromium alloy coil with a diameter of 0.1 - 0.2 mm and a winding turn number of 40 - 60 turns.

4. The gas-sensitive sensor for detecting CO2 gas according to claim 1, characterized in that: In the SnO2@MOF-5 composite material with a hierarchical porous structure, SnO2 nanoparticles are uniformly distributed on the surface of the MOF-5 matrix, the average particle size of the SnO2 nanoparticles is 5 - 15 nm, and the MOF-5 matrix has a regular pore structure with an average pore diameter of 10 - 20 nm.

5. The gas sensor for detecting CO2 gas according to claim 1, characterized in that: The specific surface area of the hierarchical porous SnO2@MOF-5 composite material is 400-600 m 2 / g, and the pore volume is 0.3-0.6 cm 3 / g.

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