Gas-sensitive composite material as well as preparation method and application thereof

Abstract

The invention relates to the field of gas sensors, and discloses a gas-sensitive composite material as well as a preparation method and application thereof. The gas-sensitive composite material comprises a conductive flexible composite material layer and a conductive polymer material layer formed on the conductive flexible composite material layer, wherein the conductive flexible composite material layer contains a material formed by compounding cellulose nanofibers and MXene nanosheets; the conductive polymer material is selected from at least one of polyaniline, polypyrrole and polythiophene. The gas-sensitive composite material has good flexibility, mechanical strength and conductivity, the price is low, the preparation process is simple, and when the gas-sensitive composite material is applied to a sensor, the prepared sensor is high in sensitivity, good in stability, wearable and easy and convenient to carry.

Description

Technical Field

[0001] This invention relates to the field of gas sensors, specifically to a gas-sensitive composite material, its preparation method, and its application. Background Technology

[0002] Gas sensors are widely used in chemical industrial parks for gas leak monitoring and safety control, monitoring gaseous pollutants (such as ammonia, amines, and volatile organic compounds) in the environment surrounding chemical production facilities in real time. This significantly improves the safety level of chemical production facilities. However, common gas sensors, such as fixed gas alarms, have limitations such as small monitoring range and immobility; while portable gas sensors are bulky and heavy. Currently, MEMS gas sensors are being used for gas leak monitoring, but MEMS gas sensors are made of rigid materials, making them difficult to integrate with flexible substrates such as skin.

[0003] Currently, gas-sensitive materials can directly convert chemical stimuli into electrical signals. However, resistive gas sensing materials are inherently brittle (metal oxides and conductive polymers) and / or subject to weak interactions between fillers (such as van der Waals interactions between carbon materials), making the assembly of gas-sensitive materials into flexible sensing materials with high mechanical strength a challenge. Therefore, the development of flexible wearable gas sensors is of great significance. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems existing in the prior art and provide a gas-sensitive composite material. This gas-sensitive composite material has good flexibility, mechanical strength and conductivity, is inexpensive and has a simple preparation process. When applied in sensors, the prepared sensors have high sensitivity, good stability, are wearable and easy to carry.

[0005] To achieve the above objectives, the present invention provides a gas-sensitive composite material, which includes a conductive flexible composite material layer and a conductive polymer material layer formed on the conductive flexible composite material layer;

[0006] The conductive flexible composite material layer contains a material composed of cellulose nanofibers and MXene nanosheets.

[0007] The conductive polymer material is selected from at least one of polyaniline, polypyrrole, and polythiophene.

[0008] Preferably, the thickness of the conductive flexible composite material layer is 15-40 μm.

[0009] Preferably, the weight ratio of the cellulose nanofibers to the MXene nanosheets is 1:0.5-2.

[0010] Preferably, the diameter of the cellulose nanofibers is 10-100 nm.

[0011] Preferably, the cellulose nanofibers are selected from at least one of carboxylated cellulose nanofibers, quaternized cellulose nanofibers, sulfonated cellulose nanofibers, and esterified cellulose nanofibers.

[0012] Preferably, the thickness of the MXene nanosheet is 1.5-10 nm.

[0013] Preferably, the maximum diameter of the MXene nanosheets is <1 μm.

[0014] Preferably, the thickness of the conductive polymer material layer is 50-400 nm, and more preferably 100-200 nm.

[0015] A second aspect of the present invention provides a method for preparing a gas-sensitive composite material, the method comprising the following steps:

[0016] (1) A mixed dispersion containing cellulose nanofibers and MXene nanosheets was dried to obtain a conductive flexible composite material layer.

[0017] (2) The polymer monomer, oxidant and water are mixed to obtain a second mixed solution, which is then immersed in the conductive flexible composite material layer obtained in step (1);

[0018] In step (2), the polymer monomer is selected from at least one of aniline, pyrrole and thiophene.

[0019] Preferably, in step (1), the thickness of the conductive flexible composite material layer is 15-40 μm.

[0020] Preferably, in step (1), the weight ratio of the cellulose nanofibers to the MXene nanosheets is 1:0.5-2.

[0021] Preferably, in step (1), the diameter of the cellulose nanofibers is 10-100 nm.

[0022] Preferably, in step (1), the thickness of the MXene nanosheet is 1.5-10 nm.

[0023] Preferably, in step (1), the maximum diameter of the MXene nanosheet is <1 μm.

[0024] Preferably, in step (1), the mixed dispersion containing cellulose nanofibers and MXene nanosheets is obtained by mixing cellulose nanofiber dispersion and MXene nanosheet dispersion.

[0025] Preferably, the volume ratio of the cellulose nanofiber dispersion to the MXene nanosheet dispersion is 1:0.5-2.

[0026] Preferably, the concentration of the cellulose nanofiber dispersion is 0.2-3 mg / mL.

[0027] Preferably, the concentration of the MXene nanosheet dispersion is 1-5 mg / mL.

[0028] Preferably, the cellulose nanofiber dispersion is selected from at least one of carboxylated cellulose nanofiber dispersion, quaternized cellulose nanofiber dispersion, sulfonated cellulose nanofiber dispersion, and esterified cellulose nanofiber dispersion.

[0029] Preferably, the first dispersion medium of the cellulose nanofiber dispersion is selected from at least one of methanol, N,N-dimethylformamide, ethanol, water and heptane.

[0030] Preferably, the second dispersion medium of the MXene nanosheet dispersion is selected from at least one of water, dimethyl sulfoxide, and N,N-dimethylacetamide.

[0031] Preferably, in step (1), the drying conditions include a temperature of 20-60°C and a time of 12-36 hours.

[0032] Preferably, in step (2), the volume ratio of the polymer monomer to the weight ratio of the oxidant is 1 mL: 0.5-1.5 g.

[0033] Preferably, in step (2), the oxidant is selected from at least one of persulfate, hydrogen peroxide, dichromate, ferric chloride and ascorbic acid.

[0034] A third aspect of the present invention provides a gas-sensitive composite material prepared by the above method.

[0035] The fourth aspect of this invention provides the application of the above-mentioned gas-sensitive composite material in ammonia detection.

[0036] A fifth aspect of the present invention provides an ammonia sensor, the ammonia sensor comprising the above-mentioned gas-sensitive composite material.

[0037] The sixth aspect of the present invention provides a method for detecting ammonia, the method comprising contacting the above-mentioned gas-sensitive composite material with a mixed gas containing ammonia.

[0038] Preferably, the volume percentage of ammonia in the mixed gas is 0.02-1%.

[0039] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0040] (1) The gas-sensitive composite material described in this invention has good flexibility, mechanical strength and conductivity. This is mainly because cellulose nanofibers have good flexibility and active groups that can interact with MXene nanosheets, thereby constructing a gas-sensitive composite material with good flexibility, mechanical strength and conductivity.

[0041] (2) The gas-sensitive composite material preparation process of the present invention is simple, low-cost, and environmentally friendly;

[0042] (3) The sensor prepared by the gas-sensitive composite material described in this invention has good sensitivity and stability for ammonia. The detectable volume percentage of ammonia is 0.02-1%. This is mainly because the polypyrrole layer contained in the gas-sensitive composite material has good sensitivity. By controlling its thickness, its sensitivity to gas can be further improved. Attached Figure Description

[0043] Figure 1 This is a sample image of the cellulose nanofiber dispersion prepared in Example 1;

[0044] Figure 2 This is a transmission electron microscope image of the cellulose nanofibers prepared in Example 1;

[0045] Figure 3 This is a sample image of the gas-sensitive composite material prepared in Example 2;

[0046] Figure 4 This is a graph showing the change in resistance of the gas-sensitive composite material prepared in Example 3 before and after bending. Detailed Implementation

[0047] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0048] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0049] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating relative importance or implying the number of technical features indicated. Therefore, unless otherwise stated, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature; "multiple" means two or more. The term "comprising" and any variations thereof mean non-exclusive inclusion, where one or more other features, integers, steps, operations, units, components, and / or combinations thereof may be present or added.

[0050] In one aspect, the present invention provides a gas-sensitive composite material, the gas-sensitive composite material comprising a conductive flexible composite material layer and a conductive polymer material layer formed on the conductive flexible composite material layer;

[0051] The conductive flexible composite material layer contains a material composed of cellulose nanofibers and MXene nanosheets.

[0052] The conductive polymer material is selected from at least one of polyaniline, polypyrrole, and polythiophene.

[0053] In a preferred embodiment, the conductive polymer material is selected from polyaniline, polypyrrole, or polythiophene.

[0054] In this invention, a gas-sensitive composite material is formed by in-situ growing a conductive polymer material layer on a conductive flexible composite material layer. The conductive flexible composite material layer contains a material composed of cellulose nanofibers and MXene nanosheets. This material is formed by the interweaving and stacking of cellulose nanofibers and MXene nanosheets. The conductive flexible composite material layer formed in this way exhibits good flexibility and mechanical strength, mainly due to the good flexibility of the cellulose nanofibers and the interaction between their active groups and the MXene nanosheets, resulting in good mechanical strength and conductivity. Furthermore, the conductive polymer material layer formed on the surface of the conductive flexible composite material layer exhibits good gas sensitivity, thus forming the gas-sensitive composite material of this invention.

[0055] In a preferred embodiment, in order to improve the flexibility and sensitivity of the gas-sensitive composite material, the thickness of the conductive flexible composite material layer is 15-40 μm; preferably 20-30 μm.

[0056] In a preferred embodiment, in order to comprehensively improve the conductivity, flexibility and mechanical strength of the gas-sensitive composite material, the weight ratio of the cellulose nanofibers to the MXene nanosheets is 1:0.5-2; preferably 1:1-2; specifically, the weight ratio of the cellulose nanofibers to the MXene nanosheets can be 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8 or 1:2.

[0057] In a preferred embodiment, in order to improve the specific surface area and mechanical properties of the gas-sensitive composite material, the diameter of the cellulose nanofiber is 10-100 nm; preferably 20-80 nm.

[0058] In a preferred embodiment, in order to further improve the mechanical strength of the gas-sensitive composite material, the cellulose nanofibers are selected from at least one of carboxylated cellulose nanofibers, quaternized cellulose nanofibers, sulfonated cellulose nanofibers, and esterified cellulose nanofibers; more preferably, the cellulose nanofibers are selected from at least one of carboxylated cellulose nanofibers, sulfonated cellulose nanofibers, and esterified cellulose nanofibers.

[0059] In this invention, the cellulose fiber can be prepared from cellulose raw materials using methods conventionally used in the art. The cellulose raw materials can be cotton or wood pulp, or commercially available cellulose fibers.

[0060] In this invention, cellulose raw materials are modified using conventional methods in the art. There are no special requirements for the modification method, as long as the cellulose raw materials can be modified by carboxylation, quaternization, sulfonation, and esterification to obtain the corresponding carboxylated cellulose fibers, quaternized cellulose fibers, sulfonated cellulose fibers, and esterified cellulose fibers. Then, the corresponding cellulose nanofibers are obtained by high-pressure homogenization and / or mechanical shearing. More preferably, cellulose fibers are prepared into cellulose nanofibers by high-pressure homogenization.

[0061] In a specific embodiment, the method for preparing the carboxylated cellulose fiber includes: soaking cellulose pulp in water, then adding 2,2,6,6-tetramethylpiperidine oxide (TEMPO) and sodium bromide for mixing, then adding NaClO aqueous solution (1.3-5 mmol) for a first reaction, maintaining the pH of the reaction system at 10-10.5, then performing solid-liquid separation, and washing and filtering the obtained solid material to obtain carboxylated cellulose fiber.

[0062] In a preferred embodiment, the conditions for the first reaction include: a temperature of 15-40°C and a stirring rate of 500-1000 r / min.

[0063] In a specific embodiment, the preparation method of the quaternized cellulose fiber includes: pulverizing wood pulp and adding it to a NaOH aqueous solution (0.04 g, 80 mL) and letting it stand for 12 h; then filtering and washing it 2-3 times with N,N-dimethylacetamide in a Buchner funnel; placing the treated wood pulp into a round-bottom flask, adding 80 mL of N,N-dimethylacetamide and 0.04 g of NaOH (2% of the wood pulp mass), and then placing it in a 65°C water bath and mechanically stirring for 1 h; then adding the modifier 2,3-epoxypropyltrimethylammonium chloride (2-4 g) and stirring for 8 h; then performing solid-liquid separation; and washing and filtering the obtained solid material to obtain the quaternized cellulose fiber.

[0064] In a specific embodiment, the method for preparing the sulfonated cellulose fiber includes: mixing wood pulp with sulfuric acid solution of different mass fractions (58%-64%), heating at 40-45°C for 1-1.5 hours, then performing solid-liquid separation, and washing and filtering the obtained solid material to obtain sulfonated cellulose fiber.

[0065] In a specific embodiment, the preparation method of the esterified cellulose nanofibers includes: hydrolyzing cellulose fibers with formic acid at a concentration of 88 wt% at a reaction temperature of 85-95℃, then performing solid-liquid separation, and washing and filtering the obtained solid material to obtain esterified cellulose fibers.

[0066] In a preferred embodiment, in order to improve the conductivity and mechanical strength of the gas-sensitive composite material, the thickness of the MXene nanosheets is 1.5-10 nm.

[0067] In a preferred embodiment, the maximum diameter of the MXene nanosheets is <1 μm.

[0068] In a preferred embodiment, in order to improve the sensitivity of the gas-sensitive composite material, the thickness of the conductive polymer material layer is 50-400 nm, preferably 60-200 nm; more preferably 100-200 nm; specifically, it can be 100 nm, 120 nm, 140 nm, 160 nm, 180 nm or 200 nm.

[0069] A second aspect of the present invention provides a method for preparing a gas-sensitive composite material, the method comprising the following steps:

[0070] (1) A mixed dispersion containing cellulose nanofibers and MXene nanosheets was dried to obtain a conductive flexible composite material layer.

[0071] (2) The polymer monomer, oxidant and water are mixed to obtain a second mixed solution, which is then immersed in the conductive flexible composite material layer obtained in step (1);

[0072] In step (2), the polymer monomer is selected from at least one of aniline, pyrrole and thiophene.

[0073] In this invention, there are no special requirements for the thickness of the conductive flexible composite material layer; in a preferred embodiment, in order to improve the flexibility and sensitivity of the gas-sensitive composite material, in step (1), the thickness of the conductive flexible composite material layer is 15-40 μm; preferably 20-30 μm.

[0074] In this invention, there are no special requirements for the method of obtaining the conductive flexible composite material layer of the specified thickness. The conductive flexible composite material layer of the specified thickness can be obtained by adjusting the bottom area of ​​the container holding the mixed dispersion containing cellulose nanofibers and MXene nanosheets. The container holding the mixed dispersion containing cellulose nanofibers and MXene nanosheets can be a petri dish or beaker with different bottom areas.

[0075] In a preferred embodiment, in order to comprehensively improve the conductivity, flexibility and mechanical strength of the gas-sensitive composite material, in step (1), the weight ratio of the cellulose nanofibers to the MXene nanosheets is 1:0.5-2; preferably 1:1-2; specifically, the weight ratio of the cellulose nanofibers to the MXene nanosheets can be 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8 or 1:2.

[0076] In a preferred embodiment, in order to improve the specific surface area and mechanical properties of the gas-sensitive composite material, in step (1), the diameter of the cellulose nanofiber is 10-100 nm.

[0077] In a preferred embodiment, in step (1), the mixed dispersion containing cellulose nanofibers and MXene nanosheets is obtained by mixing the cellulose nanofiber dispersion and the MXene nanosheet dispersion.

[0078] In a preferred embodiment, in order to improve the conductivity, flexibility and mechanical strength of the gas-sensitive composite material, the volume ratio of the cellulose nanofiber dispersion to the MXene nanosheet dispersion is 1:0.5-2; preferably 1:1-2; specifically, the volume ratio of the cellulose nanofiber dispersion to the MXene nanosheet dispersion can be 1:1, 1:1.2, 1:1.4, 1:1.5, 1:1.6, 1:1.8 or 1:2.

[0079] In a preferred embodiment, in order to improve the uniformity and stability of the cellulose nanofiber dispersion, the concentration of the cellulose nanofiber dispersion is 0.2-3 mg / mL; preferably 1-3 mg / mL; specifically, the concentration of the cellulose nanofiber dispersion can be 1 mg / mL, 1.5 mg / mL, 2 mg / mL, 2.5 mg / mL or 3 mg / mL.

[0080] In a preferred embodiment, in order to further improve the mechanical properties of the gas-sensitive composite material, the cellulose nanofiber dispersion is selected from at least one of carboxylated cellulose nanofiber dispersion, quaternized cellulose nanofiber dispersion, sulfonated cellulose nanofiber dispersion, and esterified cellulose nanofiber dispersion.

[0081] In a preferred embodiment, in order to further improve the uniformity and stability of the cellulose nanofiber dispersion, the first dispersion medium of the cellulose nanofiber dispersion is selected from at least one of methanol, N,N-dimethylformamide, ethanol, water and heptane.

[0082] In this invention, the method for preparing the cellulose nanofiber dispersion includes: preparing cellulose nanofiber dispersion by high-pressure homogenization and / or mechanical shearing; preferably, cellulose nanofiber dispersion is prepared by high-pressure homogenization.

[0083] In a preferred embodiment, the conditions for high-pressure homogenization include: a pressure of 50-80 MPa and a time of 5-30 min; more preferably, a pressure of 60-80 MPa and a time of 10-20 min.

[0084] In a preferred embodiment, the cellulose fiber is selected from at least one of carboxylated cellulose fiber, quaternized cellulose fiber, sulfonated cellulose fiber, and esterified cellulose fiber.

[0085] In a specific embodiment, the preparation method of the carboxylated cellulose nanofiber dispersion includes: subjecting carboxylated cellulose fibers to high-pressure homogenization to obtain the carboxylated cellulose nanofiber dispersion.

[0086] In a specific embodiment, the preparation method of the quaternized cellulose nanofiber dispersion includes: homogenizing the quaternized cellulose fibers under high pressure, dialysis for 5-7 days, and then filtering with a Buchner funnel to obtain the quaternized cellulose nanofiber dispersion.

[0087] In a specific embodiment, the preparation method of the sulfonated cellulose nanofiber dispersion includes: homogenizing the sulfonated cellulose fibers under high pressure, and then performing dialysis treatment to obtain the sulfonated cellulose nanofiber dispersion.

[0088] In a specific embodiment, the preparation method of the esterified cellulose nanofiber dispersion includes: subjecting the esterified cellulose fibers to high-pressure homogenization to obtain the esterified cellulose nanofiber dispersion.

[0089] In this invention, the MXene nanosheets are Ti3C2T. x The MXene nanosheets can be prepared by means of fabrication or can be commercially available MXene nanosheets.

[0090] In a preferred embodiment, in step (1), the thickness of the MXene nanosheet is 1.5-10 nm.

[0091] In a preferred embodiment, in step (1), the maximum diameter of the MXene nanosheet is <1 μm.

[0092] In this invention, the raw materials for preparing MXene nanosheets are selected from at least one of Ti3AlC2, Ti3ZnC2 and Ti3AlCl2.

[0093] In a preferred embodiment, in order to improve the uniformity and stability of the MXene nanosheet dispersion, the concentration of the MXene nanosheet dispersion is 1-5 mg / mL; preferably 1-3 mg / mL; more preferably 1-2 mg / mL; specifically, it can be 1 mg / mL, 1.2 mg / mL, 1.4 mg / mL, 1.5 mg / mL, 1.6 mg / mL, 1.8 mg / mL or 2 mg / mL.

[0094] In a preferred embodiment, the second dispersion medium of the MXene nanosheet dispersion is selected from at least one of water, dimethyl sulfoxide, and N,N-dimethylacetamide.

[0095] In a preferred embodiment, the method for preparing the MXene nanosheet dispersion includes:

[0096] A1: The raw material (e.g., Ti3AlC2) is mixed with a solution containing fluoride ions and hydrogen ions and then subjected to the fourth reaction, followed by solid-liquid separation to collect the precipitate;

[0097] A2: The precipitate collected in step A1 and the second dispersion medium are subjected to a second mixing and dispersion.

[0098] In a more preferred embodiment, the method for preparing MXene nanosheets further includes washing, vacuum filtration, drying, ultrasonication, and solid-liquid separation after solid-liquid separation, wherein the solid-liquid separation, washing, drying, and ultrasonication are all conventional methods in the art.

[0099] In this invention, there are no special requirements for the solution containing fluoride ions and hydrogen ions, as long as the fluoride ions and hydrogen ions in the solution can remove Al atoms or Zn atoms from the raw materials. For example, it can be hydrofluoric acid, or a mixed solution of inorganic acid and fluoride salt.

[0100] In this invention, there are no particular limitations on the conditions for the second mixing and dispersion of MXene nanosheets and the second dispersion medium, as long as a uniformly dispersed MXene nanosheet dispersion can be obtained. For example, the second mixing and dispersion can be carried out by known methods such as stirring and sonication.

[0101] In a preferred embodiment, in step (1), the drying conditions include: a temperature of 20-60°C and a time of 12-36h; preferably, a temperature of 20-50°C and a time of 12-24h; more preferably, a temperature of 30-40°C and a time of 12-20h.

[0102] In a preferred embodiment, step (2) further includes adjusting the pH of the second mixed solution to 2-6 before immersing the conductive flexible composite material layer obtained in step (1); preferably, the pH of the second mixed solution is adjusted to 3-5.

[0103] In this invention, there are no special requirements for the substance used to adjust the pH value of the second mixed solution to 3-5, as long as it can adjust the pH value of the second mixed solution to 3-5; for example, it can be an acidic solution.

[0104] In a preferred embodiment, the acid in the acidic solution is an inorganic acid; more preferably, the acid in the acidic solution is at least one of hydrochloric acid, sulfuric acid, and perchloric acid; specifically, the acid in the acidic solution is hydrochloric acid.

[0105] In a preferred embodiment, in step (2), the volume ratio of the polymer monomer to the weight ratio of the oxidant is 1 mL: 0.5-1.5 g; preferably 1 mL: 0.5-1.2 g.

[0106] In this invention, in step (2), under the action of the oxidant, the polymer monomer can be polymerized in situ on the surface of the conductive flexible composite material layer to grow into a conductive polymer material layer. The conductive polymer material layer is wrapped around the surface of the conductive flexible composite material layer, thereby forming the gas-sensitive composite material described in this invention. Moreover, this method is simple and low in cost.

[0107] In this invention, there are no special requirements for the amount of polymer monomer and conductive flexible composite material layer used, as long as the conductive polymer material layer formed after the polymer monomer is polymerized and grown in situ on the surface of the conductive flexible composite material layer can wrap the conductive flexible composite material layer.

[0108] In a preferred embodiment, the amount of polymer monomer used can result in a conductive polymer material layer with a thickness of 50-400 nm on the surface of the conductive flexible composite material layer. Preferably, the thickness of the conductive polymer material layer is 60-200 nm; more preferably, the thickness of the conductive polymer material layer is 100-200 nm.

[0109] In a preferred embodiment, in step (2), the polymerizing monomer is selected from aniline, pyrrole, or thiophene.

[0110] In a preferred embodiment, the in-situ polymerization of monomers on the surface of the conductive flexible composite material layer is carried out under stirring; more preferably, the in-situ polymerization temperature is 20-40°C, the in-situ polymerization time is 60-120 min, and the stirring rate is 50-100 r / min.

[0111] In this invention, there are no special requirements for the oxidant in step (2), and any oxidant commonly used in the art is acceptable; in a preferred embodiment, the oxidant is selected from at least one of persulfate, hydrogen peroxide, dichromate, ferric chloride and ascorbic acid.

[0112] In this invention, step (2) further includes washing and drying the obtained product to finally obtain the gas-sensitive composite material of this invention.

[0113] In this invention, there are no special requirements for the drying method; any method conventionally used in the art is acceptable. For example, it can be dried in an oven (10-40°C).

[0114] A third aspect of the present invention provides a gas-sensitive composite material prepared by the above method.

[0115] The fourth aspect of this invention provides the application of the above-mentioned gas-sensitive composite material in ammonia detection.

[0116] A fifth aspect of the present invention provides an ammonia sensor, the ammonia sensor comprising the above-mentioned gas-sensitive composite material.

[0117] The ammonia sensor described in this invention has high sensitivity to ammonia, good thermal stability and chemical stability, and can directly address the problem of ammonia leakage monitoring and detection in the atmospheric environment. It can detect leaked ammonia in the environment with high selectivity and speed at room temperature, reduce the hazards caused by ammonia leakage, and thus ensure the safety of personnel, environment and equipment.

[0118] The sixth aspect of the present invention provides a method for detecting ammonia, the method comprising contacting the above-mentioned gas-sensitive composite material with a mixed gas containing ammonia.

[0119] In a preferred embodiment, the volume percentage of ammonia in the mixed gas is 0.02-1%.

[0120] The following examples further illustrate the gas-sensitive composite material, its preparation method, and its application according to the present invention. These examples are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures; however, the scope of protection of the present invention is not limited to the following examples.

[0121] Unless otherwise specified, the experimental methods used in the following examples and comparative examples are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following examples and comparative examples are commercially available.

[0122] Example 1

[0123] Preparation of carboxylated cellulose nanofiber dispersion C1:

[0124] Wood pulp and 100 mL of water were mixed, and TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol) were slowly added. Then, NaClO solution (2 mmol NaClO) was added, and the first reaction was carried out at 25 °C and 500 r / min. NaOH solution (0.2 M) was added simultaneously to maintain the pH of the reaction system at 10. Solid-liquid separation was then performed, and the obtained solid material was washed with water and filtered to obtain carboxylated cellulose fibers. The carboxylated cellulose fibers were then homogenized under high pressure at 80 MPa for 20 min using water as the primary dispersion medium to obtain a carboxylated cellulose nanofiber dispersion C1 (e.g., ...). Figure 2 As shown in the figure, the concentration of carboxylated cellulose nanofiber dispersion C1 is 2 mg / mL, and the diameter of carboxylated cellulose nanofibers is about 20 nm.

[0125] Preparation of MXene nanosheet dispersion K1:

[0126] A1: Add Ti3AlC2 to an aqueous solution of LiF (50 mg / mL) in HCl, stir at 35°C for 24 h, then separate the solid and liquid by centrifugation, wash with deionized water until the pH of the supernatant is 6, and collect the precipitate.

[0127] A2: The precipitate obtained in step A1 is dispersed in water and subjected to ultrasonic treatment to obtain MXene nanosheet dispersion K1 with a thickness of 1.5 nm and a maximum diameter of less than 1 μm, and a concentration of 1 mg / mL.

[0128] Preparation of gas-sensitive composite material M1:

[0129] (1) Mix 15 mL of carboxylated cellulose nanofiber dispersion C1 and 15 mL of MXene nanosheet dispersion K1, and then dry the mixed material to obtain a conductive flexible composite material layer F1 with a thickness of 30 μm. The drying conditions include: temperature of 30℃ and time of 12h.

[0130] (2) Mix 0.6 mL of pyrrole monomer, 0.69 g of ammonium persulfate and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 4 with hydrochloric acid, and then immerse it in the conductive flexible composite material layer F1 obtained in step (1). Stir to allow the pyrrole monomer to polymerize in situ on the surface of the conductive flexible composite material layer F1. The conditions for in situ polymerization include: temperature of 30°C and time of 65 min. Then wash and dry in an oven at 40°C for 7 h to obtain gas-sensitive composite material M1, wherein the thickness of the polypyrrole layer is 150 nm.

[0131] Example 2

[0132] Preparation of sulfonated cellulose nanofiber dispersion C2:

[0133] Wood pulp was mixed with a 60% sulfuric acid solution and heated at 45°C for 1.5 hours. Solid-liquid separation was then performed, and the resulting solid material was washed with water and filtered to obtain sulfonated cellulose fibers. The sulfonated cellulose fibers were then homogenized under high pressure at 80 MPa for 20 minutes using water as the primary dispersion medium to obtain a sulfonated cellulose nanofiber dispersion C2. The concentration of the sulfonated cellulose nanofiber dispersion C2 was 2 mg / mL, and the diameter of the sulfonated cellulose nanofibers was approximately 20 nm.

[0134] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0135] Preparation of gas-sensitive composite material M2:

[0136] (1) Mix 15 mL of sulfonated cellulose nanofiber dispersion C2 and 15 mL of MXene nanosheet dispersion K1, and then dry the mixed material to obtain a conductive flexible composite material layer F2 with a thickness of 25 μm. The drying conditions include: temperature of 30℃ and time of 12 h.

[0137] (2) Mix 1 mL of pyrrole monomer, 0.69 g of ferric chloride and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 4 with hydrochloric acid. Then immerse the conductive flexible composite material layer F2 obtained in step (1) in the solution. Stir the solution to allow the pyrrole monomer to polymerize in situ on the surface of the conductive flexible composite material layer F2. The conditions for in situ polymerization include: temperature of 30°C and time of 65 min. Then wash the solution and dry it in an oven at 40°C for 7 h to obtain the gas-sensitive composite material M2, wherein the thickness of the polypyrrole layer is 200 nm.

[0138] Example 3

[0139] Preparation of esterified cellulose nanofiber dispersion C3:

[0140] Wood pulp was hydrolyzed at 95℃ with 88wt% formic acid for 6 hours, followed by solid-liquid separation. The resulting solid material was washed with water and filtered to obtain esterified cellulose fibers. The esterified cellulose fibers were then homogenized under high pressure at 50 MPa for 20 minutes using water as the primary dispersion medium to obtain esterified cellulose nanofiber dispersion C3. The concentration of esterified cellulose nanofiber dispersion C3 was 0.3wt%, and the diameter of the esterified cellulose nanofibers was approximately 40 nm.

[0141] MXene nanosheet dispersion K2 was prepared in accordance with the method of Example 1, except that the concentration of MXene nanosheet dispersion K2 was 2 mg / mL;

[0142] Preparation of gas-sensitive composite material M3:

[0143] (1) Mix 15 mL of esterified cellulose nanofiber dispersion C3 and 15 mL of MXene nanosheet dispersion K2, and then dry the mixed material to obtain a conductive flexible composite material layer F3 with a thickness of 30 μm. The drying conditions include: temperature of 30℃ and time of 12h.

[0144] (2) Mix 0.6 mL of pyrrole monomer, 0.69 g of ammonium persulfate and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 3 with hydrochloric acid, and then immerse it in the conductive flexible composite material layer F3 obtained in step (1). Stir to allow the pyrrole monomer to polymerize in situ on the surface of the conductive flexible composite material layer F3. The conditions for in situ polymerization include: temperature of 30°C and time of 65 min. Then wash and dry in an oven at 40°C for 7 h to obtain gas-sensitive composite material M3, wherein the thickness of the polypyrrole layer is 100 nm.

[0145] Example 4

[0146] Preparation of quaternized cellulose nanofibers C4:

[0147] After pulverizing the wood pulp, it was added to an aqueous solution of NaOH (0.04 g, 80 mL) and allowed to stand for 12 h. Then, it was filtered and washed 2-3 times using N,N-dimethylacetamide in a Buchner funnel. The treated wood pulp was then placed in a round-bottom flask, and 80 mL of N,N-dimethylacetamide and 0.04 g of NaOH (2% of the wood pulp mass) were added. The flask was then placed in a 65°C water bath and mechanically stirred for 1 h. Next, 3 g of the modifier 2,3-epoxypropyltrimethylammonium chloride was added and stirred for 8 h. Finally, the mixture was solidified. Liquid separation was performed, and the obtained solid material was washed with water and filtered to obtain quaternized cellulose fibers. Then, the quaternized cellulose fibers were homogenized under high pressure at 50 MPa for 20 min using N,N-dimethylacetamide as the first dispersion medium. After dialysis for 6 days, the mixture was filtered through a Buchner funnel to remove unstripped particles, finally obtaining a quaternized cellulose nanofiber dispersion C4. The concentration of the quaternized cellulose nanofiber dispersion C4 was 2 mg / mL, and the diameter of the quaternized cellulose nanofibers was approximately 50 nm.

[0148] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0149] Preparation of gas-sensitive composite material M4:

[0150] (1) Mix 15 mL of quaternized cellulose nanofiber dispersion C4 and 15 mL of MXene nanosheet dispersion K1, and then dry the mixed material to obtain a conductive flexible composite material layer F4 with a thickness of 35 μm. The drying conditions include: temperature of 30℃ and time of 12h.

[0151] (2) Mix 0.6 mL of pyrrole monomer, 0.69 g of ammonium persulfate and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 4 with hydrochloric acid. Then immerse the conductive flexible composite material layer F4 obtained in step (1) in the solution. Stir the solution to allow the pyrrole monomer to polymerize in situ on the surface of the conductive flexible composite material layer F4. The conditions for in situ polymerization include: temperature of 30 °C and time of 65 min. Then wash the solution and dry it in an oven at 40 °C for 7 h to obtain the gas-sensitive composite material M4, wherein the thickness of the polypyrrole layer is 180 nm.

[0152] Example 5

[0153] Preparation of carboxylated cellulose nanofiber dispersion C5:

[0154] Wood pulp and 100 mL of water were mixed, and TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol) were slowly added. Then, NaClO solution (2 mmol NaClO) was added. The first reaction was carried out at 25 °C and 500 r / min. NaOH solution (0.2 M) was added to maintain the pH of the reaction system at 10. Then, solid-liquid separation was performed. The obtained solid material was washed with water and filtered to obtain carboxylated cellulose fibers. Then, the carboxylated cellulose fibers were homogenized under high pressure at 50 MPa for 20 min with water as the first dispersion medium to obtain carboxylated cellulose nanofiber dispersion C5. The concentration of carboxylated cellulose nanofiber dispersion C5 was 2 mg / mL, and the diameter of carboxylated cellulose nanofibers was about 100 nm.

[0155] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0156] Gas-sensitive composite material M5 was prepared in accordance with the method of Example 1, except that an equal weight of carboxylated cellulose nanofiber dispersion C5 was used to replace carboxylated cellulose nanofiber dispersion C1.

[0157] Example 6

[0158] Preparation of cellulose nanofiber dispersion C6:

[0159] Commercially available cellulose fibers were homogenized under high pressure at 50 MPa for 20 min using water as the primary dispersion medium to obtain cellulose nanofiber dispersion C6, wherein the concentration of cellulose nanofiber dispersion C6 was 2 mg / mL and the diameter of cellulose nanofibers was approximately 20 nm.

[0160] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0161] Gas-sensitive composite material M6 was prepared in accordance with the method of Example 1, except that an equal weight of cellulose nanofiber dispersion C6 was used to replace carboxylated cellulose nanofiber dispersion C1, and an equal volume of aniline was used to replace pyrrole.

[0162] Example 7

[0163] The gas-sensitive composite material M7 was prepared in accordance with the method of Example 1, except that the amount of MXene nanosheet dispersion K1 used was 7.5 mL.

[0164] Example 8

[0165] Carboxylated cellulose nanofiber dispersion C1 was prepared according to the method described in Example 1;

[0166] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0167] Preparation of gas-sensitive composite material M8:

[0168] (1) Mix 15 mL of carboxylated cellulose nanofiber dispersion C1 and 15 mL of MXene nanosheet dispersion K1, and then dry the mixed material to obtain a conductive flexible composite material layer F8 with a thickness of 10 μm. The drying conditions include: temperature of 30℃ and time of 12h.

[0169] (2) Mix 0.6 mL of pyrrole monomer, 0.69 g of ammonium persulfate and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 4 with hydrochloric acid, and then immerse it in the conductive flexible composite material layer F8 obtained in step (1). Stir to allow the pyrrole monomer to polymerize in situ on the surface of the conductive flexible composite material layer F8. The conditions for in situ polymerization include: temperature of 30°C and time of 65 min. Then wash and dry in an oven at 40°C for 7 h to obtain gas-sensitive composite material M8, wherein the thickness of the polypyrrole layer is 50 nm.

[0170] Example 9

[0171] Carboxylated cellulose nanofiber dispersion C1 was prepared according to the method described in Example 1:

[0172] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0173] Preparation of gas-sensitive composite material M9:

[0174] (1) Mix 15 mL of carboxylated cellulose nanofiber dispersion C1 and 15 mL of MXene nanosheet dispersion K1, and then dry the mixed material to obtain a conductive flexible composite material layer F1 with a thickness of 30 μm. The drying conditions include: temperature of 30℃ and time of 12h.

[0175] (2) Mix 0.1 mL of pyrrole monomer, 0.69 g of ammonium persulfate and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 4 with hydrochloric acid, and then immerse it in the conductive flexible composite material layer F1 obtained in step (1). Stir to allow the pyrrole monomer to polymerize in situ on the surface of the conductive flexible composite material layer F1. The conditions for in situ polymerization include: temperature of 30°C and time of 65 min. Then wash and dry in an oven at 40°C for 7 h to obtain gas-sensitive composite material M9, wherein the thickness of the polypyrrole layer is 25 nm.

[0176] Example 10

[0177] Preparation of carboxylated cellulose nanofiber dispersion C5:

[0178] Wood pulp and 100 mL of water were mixed, and TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol) were slowly added. Then, NaClO solution (2 mmol NaClO) was added. The first reaction was carried out at 25 °C and 500 r / min. NaOH solution (0.2 M) was added to maintain the pH of the reaction system at 10. Then, solid-liquid separation was performed. The obtained solid material was washed with water and filtered to obtain carboxylated cellulose fibers. Then, the carboxylated cellulose fibers were homogenized under high pressure at 5 MPa for 20 min with water as the first dispersion medium to obtain carboxylated cellulose nanofiber dispersion C5. The concentration of carboxylated cellulose nanofiber dispersion C5 was 2 mg / mL, and the diameter of carboxylated cellulose nanofibers was about 150 nm.

[0179] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0180] Gas-sensitive composite material M10 was prepared in accordance with the method of Example 1, except that an equal weight of carboxylated cellulose nanofiber dispersion C5 was used instead of carboxylated cellulose nanofiber dispersion C1.

[0181] Comparative Example 1

[0182] The implementation is carried out in accordance with Example 1, except that step (2) is omitted.

[0183] Comparative Example 2

[0184] Carboxylated cellulose nanofiber dispersion C1 was prepared according to the method described in Example 1;

[0185] MXene nanosheet dispersion K1 was prepared according to the method described in Example 1;

[0186] Preparation of gas-sensitive composite material M12:

[0187] (1) 15 mL of carboxylated cellulose nanofiber dispersion C1 was dried to obtain a flexible nanofiber layer. The drying conditions included a temperature of 30 °C and a time of 12 h. The flexible nanofiber layer was then placed in a Buchner funnel lined with filter paper, and 15 mL of MXene nanosheet dispersion K1 was added for filtration to obtain a conductive flexible material layer F12.

[0188] (2) Mix 0.6 mL of pyrrole monomer, 0.69 g of ammonium persulfate and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 3 with hydrochloric acid, and then immerse it in the conductive flexible composite material layer F12 obtained in step (1). Stir to allow the pyrrole monomer to polymerize in situ on the surface of the conductive flexible composite material layer F12. The conditions for in situ polymerization include: temperature of 30°C and time of 65 min. Then wash and dry in an oven at 40°C for 7 h to obtain gas-sensitive composite material M12.

[0189] Comparative Example 3

[0190] Carboxylated cellulose nanofiber dispersion C1 was prepared according to the method described in Example 1;

[0191] Preparation of gas-sensitive composite material M13:

[0192] (1) A flexible nanofiber layer was obtained by drying 15 mL of carboxylated cellulose nanofiber dispersion C1. The drying conditions included: temperature of 30℃ and time of 12h.

[0193] (2) Mix 0.6 mL of pyrrole monomer, 0.69 g of ammonium persulfate and 240 mL of water to obtain a second mixed solution. Adjust the pH of the second mixed solution to 3 with hydrochloric acid, and then immerse it in the flexible nanofiber layer obtained in step (1). Stir to allow the pyrrole monomer to polymerize in situ on the surface of the flexible nanofiber layer. The conditions for in situ polymerization include: temperature of 30°C and time of 65 min. Then wash and dry in an oven at 40°C for 7 h to obtain gas-sensitive composite material M13.

[0194] Test case

[0195] (1) The morphology of the carboxylated cellulose nanofibers prepared in Example 1 was characterized by transmission electron microscopy, and the results are as follows: Figure 1 As shown;

[0196] Depend on Figure 1 It can be seen that the diameter of carboxylated cellulose nanofibers is about 20 nm.

[0197] (2) The flexibility of the gas-sensitive composite material M2 prepared in Example 2 was tested, such as... Figure 2 As shown, by Figure 2 It can be seen that the gas-sensitive composite material M2 has good flexibility and can be bent.

[0198] (3) The stability of the gas-sensitive composite material M2 prepared in Example 3 under bending conditions was tested, and the test results are as follows: Figure 4 As shown;

[0199] Depend on Figure 4It can be seen that the resistance of the gas-sensitive composite material described in this invention remains basically unchanged after repeated bending, thus indicating that the gas-sensitive composite material described in this invention has good electrical stability.

[0200] (4) Cut the samples prepared in the comparative example and the example into strips of 20mm×10mm and test the elongation at break of the strips. The test results are shown in Table 1.

[0201] (5) Cut the samples prepared in the comparative example and the example into strips of 20mm×10mm. Then, apply conductive silver paste to the strips and place them in a mixed gas with a volume percentage of 0.02% ammonia. Test their sensing performance at 25°C. The test results are shown in Table 1. The response intensity is defined as the ratio of the difference between the resistance of the sample after contact with ammonia and the initial resistance to the initial resistance, in %;

[0202] (6) Test the response of the sample prepared in Example 1 to the interfering gas, which includes carbon dioxide, methane, carbon monoxide and nitric oxide. Then cut the sample prepared in Example 1 into strips of 20mm×10mm, coat the strips with conductive silver paste, and place them in different concentrations of interfering gas.

[0203] The results showed that, at the same volume concentration, the response intensity of carbon dioxide was only 20% of that of ammonia; the response intensity of methane was only 15% of that of ammonia; the response intensity of carbon monoxide was only 10% of that of ammonia; and the response intensity of nitric oxide was only 18% of that of ammonia.

[0204] Table 1

[0205]

[0206] As can be seen from the results in Table 1, the gas-sensitive composite material described in this invention has good flexibility, mechanical strength and conductivity. When applied in sensors, the prepared sensors have high sensitivity, good stability, are wearable and easy to carry.

[0207] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A gas-sensitive composite material, characterized in that, The gas-sensitive composite material includes a conductive flexible composite material layer and a conductive polymer material layer formed on the conductive flexible composite material layer. The conductive flexible composite material layer contains a material composed of cellulose nanofibers and MXene nanosheets. The conductive polymer material is selected from at least one of polyaniline, polypyrrole, and polythiophene.

2. The gas-sensitive composite material according to claim 1, characterized in that, The thickness of the conductive flexible composite material layer is 15-40 μm; Preferably, the weight ratio of the cellulose nanofibers to the MXene nanosheets is 1:0.5-2.

3. The gas-sensitive composite material according to claim 1 or 2, characterized in that, The diameter of the cellulose nanofibers is 10-100 nm; Preferably, the cellulose nanofibers are selected from at least one of carboxylated cellulose nanofibers, quaternized cellulose nanofibers, sulfonated cellulose nanofibers, and esterified cellulose nanofibers.

4. The gas-sensitive composite material according to claim 1 or 2, characterized in that, The thickness of the MXene nanosheets is 1.5-10 nm; Preferably, the maximum diameter of the MXene nanosheets is <1 μm.

5. The gas-sensitive composite material according to claim 1, characterized in that, The thickness of the conductive polymer material layer is 50-400 nm, preferably 100-200 nm.

6. A method for preparing a gas-sensitive composite material, characterized in that, The method includes the following steps: (1) A mixed dispersion containing cellulose nanofibers and MXene nanosheets was dried to obtain a conductive flexible composite material layer. (2) The polymer monomer, oxidant and water are mixed to obtain a second mixed solution, which is then immersed in the conductive flexible composite material layer obtained in step (1); In step (2), the polymer monomer is selected from at least one of aniline, pyrrole and thiophene.

7. The method according to claim 6, characterized in that, In step (1), the thickness of the conductive flexible composite material layer is 15-40 μm; Preferably, in step (1), the weight ratio of the cellulose nanofibers to the MXene nanosheets is 1:0.5-2.

8. The method according to claim 6 or 7, characterized in that, In step (1), the diameter of the cellulose nanofibers is 10-100 nm.

9. The method according to claim 6 or 7, characterized in that, In step (1), the thickness of the MXene nanosheet is 1.5-10 nm; Preferably, in step (1), the maximum diameter of the MXene nanosheet is <1 μm.

10. The method according to any one of claims 7-9, characterized in that, In step (1), the mixed dispersion containing cellulose nanofibers and MXene nanosheets is obtained by mixing cellulose nanofiber dispersion and MXene nanosheet dispersion; Preferably, the volume ratio of the cellulose nanofiber dispersion to the MXene nanosheet dispersion is 1:0.5-2.

11. The method according to claim 10, characterized in that, The concentration of the cellulose nanofiber dispersion is 0.2-3 mg / mL; Preferably, the concentration of the MXene nanosheet dispersion is 1-5 mg / mL.

12. The method according to claim 10 or 11, characterized in that, The cellulose nanofiber dispersion is selected from at least one of carboxylated cellulose nanofiber dispersion, quaternized cellulose nanofiber dispersion, sulfonated cellulose nanofiber dispersion and esterified cellulose nanofiber dispersion. Preferably, the first dispersion medium of the cellulose nanofiber dispersion is selected from at least one of methanol, N,N-dimethylformamide, ethanol, water, and heptane; Preferably, the second dispersion medium of the MXene nanosheet dispersion is selected from at least one of water, dimethyl sulfoxide, and N,N-dimethylacetamide.

13. The method according to claim 6, characterized in that, In step (1), the drying conditions include a temperature of 20-60°C and a time of 12-36 hours.

14. The method according to claim 6 or 13, characterized in that, In step (2), the volume ratio of the polymer monomer to the weight ratio of the oxidant is 1 mL: 0.5-1.5 g.

15. The method according to claim 14, characterized in that, In step (2), the oxidant is selected from at least one of persulfate, hydrogen peroxide, dichromate, ferric chloride and ascorbic acid.

16. The gas-sensitive composite material prepared by the method of any one of claims 6-15.

17. The application of the gas-sensitive composite material according to any one of claims 1-5 and 16 in ammonia detection.

18. An ammonia gas sensor, characterized in that, The ammonia sensor comprises the gas-sensitive composite material as described in any one of claims 1-5 and 16.

19. A method for detecting ammonia, characterized in that, The method includes contacting the gas-sensitive composite material according to any one of claims 1-5 and 16 with a mixed gas containing ammonia.

20. The method according to claim 19, characterized in that, The volume percentage of ammonia in the mixed gas is 0.02-1%.

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