Preparation and application of ammonia gas sensor based on iron-doped zinc oxide modified molybdenum oxide

Abstract

The invention provides preparation and application of an ammonia gas sensor based on iron-doped zinc oxide modified molybdenum oxide. Through an improved material preparation process, firstly, molybdenum disulfide (MoS2) microspheres are synthesized by adopting a hydrothermal method, then zinc-iron hydrotalcite and the MoS2 microspheres are compounded by adopting an in-situ coating technology, and the composite material is formed by calcining at 300-450 DEG C for 5 hours. In the assembly process of the sensor, the composite material is uniformly coated on the surface of an Al2O3 ceramic tube after being subjected to ultrasonic dispersion, and is aged for 12 hours in an air environment of 200-400 DEG C to optimize the stability, and finally, a nickel-chromium alloy heating wire and a Pt electrode lead are packaged through a 250 DEG C tin soldering process. Under the optimal working temperature, the sensitivity of the sensor to 20ppm NH3 is 1.5, the response time is 60s, the recovery time is 21s, the lowest detection limit is as low as 500ppb, and the sensor has the advantages of quick response, high sensitivity and long-term stability, is suitable for real-time monitoring of ammonia volatilization in an agricultural nitrogen fertilizer application area, early warning of a livestock and poultry breeding closed space and leakage detection of an industrial pipeline, and has a wide application prospect. And an efficient solution is provided for accurate detection of the ammonia concentration of 1-500ppm and multi-scene safety management and control.

Description

Technical field:

[0001] The present invention relates to the field of nanocomposite gas sensors and environmental monitoring technology, and specifically relates to the preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum oxide. This sensor can be widely used in the detection of ammonia concentration in industrial safety, agricultural environmental monitoring, food processing quality control and water treatment. Background technology:

[0002] Ammonia is an unavoidable byproduct of nitrogen fertilizer application in modern agriculture, and its efficient detection technology is directly related to the sustainability of food production and ecological and environmental safety. Excessive use of nitrogen fertilizers has led to an increase in the amount of ammonia volatilization from farmland around the world. According to data from the Food and Agriculture Organization of the United Nations, the value of nitrogen lost each year exceeds US$15 billion, and it also causes multiple chain problems such as PM 2.5 pollution, eutrophication of water bodies, and respiratory diseases of practitioners. However, existing ammonia sensors have exposed a series of technical shortcomings when dealing with complex agricultural scenarios: traditional electrochemical or metal oxide sensors generally have a long response time, making it difficult to capture the transient concentration fluctuations of ammonia during fertilization machinery operations or open-air composting, resulting in a lag in precise control; the coexistence of high concentrations of interfering gases such as CO2 and CH4 in the farmland environment makes the false alarm rate of commercial semiconductor sensors as high as 30%. More seriously, although high-precision optical sensors can achieve ppb-level detection, their unit cost of more than US$500 and professional maintenance requirements greatly limit their large-scale application in developing countries.

[0003] In response to the above challenges, the present invention has achieved a revolutionary breakthrough through material design and process innovation based on Fe-ZnO@MoO3 composite materials. This material uses the two-dimensional conductive network of MoS2 and the layered adsorption structure of zinc-iron hydrotalcite to construct efficient electron transmission channels and ammonia molecule capture sites, and combines the directional built-in electric field formed by band engineering regulation to significantly enhance the specific adsorption capacity of ammonia.

[0004] The core value of this invention lies in opening up the entire technical chain from material innovation to agricultural scene implementation. In the field of precision fertilization, the sensor network can link intelligent agricultural machinery to achieve variable fertilization, reducing excessive nitrogen fertilizer use by 30%-50%; at the environmental supervision level, high-density monitoring data provides a scientific basis for regional ammonia emission flux accounting, and assists in the formulation of carbon-nitrogen coordinated emission reduction policies; in terms of labor safety, its 25ppm threshold warning function can effectively protect against ammonia exposure risks in confined spaces such as greenhouses and livestock and poultry farms. Compared with optical technologies such as TDLAS, this solution has overwhelming advantages in cost, power consumption and ease of use, marking a key turning point for ammonia detection technology from laboratory precision to practical application in complex scenarios. Summary of the invention:

[0005] The present invention belongs to the technical field of nanocomposite gas sensors and environmental monitoring, and specifically relates to the preparation and application of an ammonia sensor based on Fe-ZnO@MoO3 composite materials. The present invention successfully develops a high-performance NH3 gas sensor with the ability to rapidly respond to NH3 in the atmospheric environment at room temperature. The sensor exhibits excellent performance, with a sensitivity of up to 1.5 to 20 ppm NH3, a response time of only 60 s, and a recovery time of 21 s. In addition, the sensor also has good selectivity and long-term stability, and its lowest detection limit is about 500 ppb. It can be widely applied to industrial safety and agricultural environmental monitoring to detect the NH3 concentration in a wide range from 1 ppm to 500 ppm, ensuring the safety and health of the production and living environment.

[0006] The preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum trioxide involved in the present invention, the preparation method comprising the following steps:

[0007] Preparation of MoS2 microspheres:

[0008] Dissolve 1.7 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 2.3 g of thiourea (CH4N2S) in 30 mL of pure water, ultrasonically disperse for 30 minutes until completely dissolved, transfer to a reaction kettle with a polytetrafluoroethylene liner, and carry out a hydrothermal reaction at 150 - 300 °C for 24 hours. After the reaction, centrifuge to collect the precipitate, wash it 3 times with deionized water until neutral, dry it in vacuo at 80 °C for 12 hours, and grind to obtain MoS2 microspheres.

[0009] Preparation of Fe-ZnO@MoO3 composite materials:

[0010] (1) Weigh ferric chloride hexahydrate (FeCl3·6H2O) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) according to a molar ratio of 3:1, with a total molar amount of 12 mmol, dissolve them in 30 mL of deionized water, and stir until a homogeneous solution is formed;

[0011] (2) Dropwise add NaOH solution to adjust the pH of the solution to 10, and continuously stir until a light yellow flocculent precipitate is formed;

[0012] (3) Add the MoS2 microspheres prepared in step 1, heat in a water bath at 70 °C for 6 hours to in-situ modify the molybdenum disulfide microspheres with zinc-iron layered double hydroxide;

[0013] (4) After the reaction, centrifuge to separate the precipitate, wash it repeatedly with deionized water until neutral (detected with pH test paper), dry it at 80 °C for 12 hours, and grind to obtain a material with zinc-iron layered double hydroxide wrapping molybdenum disulfide. Place the material in a muffle furnace, and heat it to 300 °C at a heating rate of 2 °C·min -1 and calcine for 5 hours, then cool naturally and grind to obtain Fe-ZnO@MoO3.

[0014] Composite material calcination and sensor assembly:

[0015] (1) Mix the calcined composite material and ethanol in a mass ratio of 1:5, ultrasonically disperse for 30 minutes, and uniformly coat it on the surface of the Al2O3 ceramic tube;

[0016] (2) The coated ceramic tube is sintered and aged in an air environment at 200 - 400 °C for 8 hours;

[0017] (3) Insert a nickel-chromium alloy heating wire into the ceramic tube, weld the heating wire to the Pt electrode leads at both ends of the ceramic tube with tin wire, and complete the preparation of the ammonia sensor after assembling the housing.

[0018] Through the above unique material preparation method of the present invention, through the "in-situ encapsulation" process, zinc-iron layered double hydroxide is directly encapsulated on the surface of MoS2 during the formation process, forming a heterojunction structure. This structure combines the layered adsorption characteristics of layered double hydroxide and the excellent electron transport ability of MoS2, significantly improving the adsorption efficiency and response speed of the material to ammonia. Compared with the traditional physical mixing method, the encapsulation structure provides more active sites and stable interfacial binding, making the sensor outstanding in terms of selectivity and long-term stability. In addition, the calcination process further optimizes the material lattice structure, ensuring that it has high sensitivity and fast response characteristics in the ammonia concentration range of 1 - 500 ppm.

[0019] The preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum oxide according to the present invention are as follows:

[0020] A. Preparation of MoS2 microspheres

[0021] Dissolve 1.7 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 2.3 g of thiourea (CH4N2S) in 30 mL of pure water, ultrasonically disperse for 30 minutes until completely dissolved, transfer to a reaction kettle with a polytetrafluoroethylene inner lining, and carry out a hydrothermal reaction at 150 - 300 °C for 24 hours. After the reaction, centrifuge to collect the precipitate, wash it 3 times with deionized water until neutral, dry it in vacuum at 80 °C for 12 hours, and grind to obtain MoS2 microspheres.

[0022] B. Preparation of Fe-ZnO@MoO3 composite material

[0023] Weigh ferric chloride hexahydrate (FeCl3·6H2O) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) according to a molar ratio of 3:1, with a total molar amount of 12 mmol. Dissolve them in 30 mL of deionized water and stir to form a homogeneous solution. Dropwise add NaOH solution to adjust the pH to 10, and continuously stir until a light yellow flocculent precipitate is formed. Add the MoS2 microspheres prepared in step A, and heat in a water bath at 70 °C for 6 hours to in-situ coat the molybdenum disulfide microspheres with zinc-iron layered double hydroxide. After the reaction, centrifuge to separate the precipitate, wash it repeatedly with deionized water until neutral, dry it at 80 °C for 12 hours and grind it to obtain a zinc-iron layered double hydroxide modified molybdenum disulfide precursor composite material. Heat the precursor material at a heating rate of 2 °C·min -1 to raise the temperature to 200 - 400 °C and calcine for 5 hours. After natural cooling, grind it to a uniform powder to obtain the Fe-ZnO@MoO3 composite material.

[0024] C. Calcination of the composite material and sensor assembly

[0025] Mix the calcined composite material and ethanol according to a mass ratio of 1:5, ultrasonically disperse for 30 minutes, and uniformly coat it on the surface of the Al2O3 ceramic tube. Subsequently, sinter and age in an air environment at 200 - 400 °C for 8 hours to stabilize the material performance. After aging, insert a nickel-chromium alloy heating wire into the ceramic tube, weld the heating wire and the Pt electrode leads at both ends of the ceramic tube with tin wire, and assemble a dust-proof and moisture-proof shell to finally complete the preparation of the ammonia sensor. This sensor can achieve highly sensitive, rapid response and excellent selectivity detection of 1 - 500 ppm NH3.

[0026] The design idea of this invention is as follows: Currently, the mainstream sensitive materials of ammonia sensors have problems such as long response time and poor selectivity. Although doping can improve the performance, it cannot meet the low-cost commercialization requirements. This invention uses the Fe-ZnO@MoO3 composite material. The layered structure of zinc-iron layered double hydroxide is used to increase the adsorption area and provide more active sites. Combining the excellent electrical properties and two-dimensional structure of MoS2 to promote electron transport, the two cooperate to optimize the hole-electron combination, improve the detection performance of the sensor for ammonia, and have a low cost and broad development prospects.

[0027] Application scenario description: The ammonia sensor of this invention can be widely applied in multiple scenarios such as agriculture, industry, and enclosed spaces. Such as Figure 11As shown in the figure, in an agricultural environment, the sensor network is deployed in key areas of farmland to monitor changes in ammonia concentration during fertilization in real time. The monitoring data is sent to the central control platform through a wireless transmission module, and the intelligent agricultural machinery is linked to achieve variable fertilization, accurately control the amount of nitrogen fertilizer, and reduce ammonia volatilization by 30%-50%. At the same time, in closed scenes such as livestock and poultry farms and greenhouses, sensors can monitor NH3 concentration in real time, and trigger an early warning when the concentration exceeds 25ppm to protect the health of employees. In the industrial field, sensors can be integrated into facilities such as pipelines and sewage treatment plants to achieve rapid positioning and emergency response of ammonia leaks, thereby improving production safety.

[0028] The obtained materials were characterized and the performance test results of ammonia sensor were shown in Figure 1-11

[0029] Figure 1 It can be seen that the microscopic morphology of a Fe-ZnO@MoO3 composite material is a scanning electron microscope (SEM) characterization image of the Fe-ZnO@MoO3 composite material obtained in step (2) of Example 1.

[0030] Figure 2 It can be seen that the X-ray diffraction (XRD) characterization diagram of the Fe-ZnO@MoO3 composite material obtained in step (2) of Example 1.

[0031] Figure 3 It can be seen that the X-ray photoelectron spectroscopy (XPS) characterization diagram of the Fe-ZnO@MoO3 composite material obtained in step (2) of Example 1.

[0032] Figure 4 It can be seen that the change curve of the response value of the ammonia sensor based on the Fe-ZnO@MoO3 composite material obtained in step (5) of Example 1 to a certain concentration of ammonia changes with temperature. It can be seen from the figure that the optimal working temperature is 350°C.

[0033] Figure 5 It can be seen that the response value change curve of the ammonia sensor based on the Fe-ZnO@MoO3 composite material to different concentrations of ammonia obtained in step (5) of Example 1 is shown.

[0034] Figure 6 The above is the fitting of the response value change curve of the ammonia sensor based on the Fe-ZnO@MoO3 composite material to different concentrations of ammonia obtained in step (5) of Example 1. It can be calculated that the minimum detection limit of the sensor is about 500 ppb. It can be widely used in the field of industrial safety, agricultural environmental monitoring, food processing quality control and water treatment, and can detect NH3 concentrations in a wide range of 1 ppm to 500 ppm.

[0035] Figure 7It is the reproducibility test curve of the ammonia sensor based on the Fe-ZnO@MoO3 composite material obtained in step (5) of Example 1 for the response and recovery of ammonia at a concentration of 20 ppm. During the continuous injection of 20 ppm NH3, the same dynamic response curve shows that the response is stable for six consecutive times and the recovery is good, proving the reproducibility of the sensor.

[0036] Figure 8 It is the response and recovery time curve of the ammonia sensor based on the Fe-ZnO@MoO3 composite material obtained in step (5) of Example 1 for ammonia at a concentration of 20 ppm. As can be seen from the figure, the sensor has a fast response and recovery time, and the recovery performance is good. The response time is 60 s and the recovery time is 21 s.

[0037] Figure 9 It is the response degree of the ammonia sensor based on the Fe-ZnO@MoO3 composite material obtained in step (5) of Example 1 to different gases. As can be seen from the figure, it has high selectivity.

[0038] Figure 10 It is the stability curve of the response degree of the ammonia sensor based on the Fe-ZnO@MoO3 composite material obtained in step (5) of Example 1 to 20 ppm within one month. As can be seen from the figure, a relatively high response value is still maintained after one month, indicating that the material has long-term stability. This ammonia sensor has broad development prospects and can be used in actual production and life.

[0039] Figure 11 It is a schematic diagram of the application scenario of the ammonia sensor described in the present invention in agricultural environment monitoring. The figure shows the system architecture of sensor network deployment in farmland, intelligent agricultural machinery linkage control of fertilization operations, and real-time data transmission to the monitoring platform.

[0040] In summary, the Fe-ZnO@MoO3 composite material prepared by the present invention has significant advantages compared with other common gas-sensitive materials. The zinc-iron layered double hydroxide itself has a unique layered structure, providing the material with rich active sites and a large specific surface area. After being compounded with MoS2, the synergistic effect of the two further expands this advantage. It can be clearly observed through SEM characterization that the composite material forms a microscopic structure conducive to ammonia adsorption and reaction. During the preparation process, the material properties can be effectively adjusted by controlling the raw material ratio.

[0041] The beneficial effects of the present invention are reflected in multiple aspects: First, the preparation method is simple and easy to implement. It mainly adopts conventional steps such as solution reaction and calcination, without the need for complex equipment and processes, and is easy for large-scale production. During the preparation of the Fe-ZnO@MoO3 composite material and the process of their combination, the reaction conditions are mild and the operation is convenient. Second, the cost is low. The raw materials are widely sourced and relatively inexpensive, avoiding the use of expensive precious metals or rare earth elements, and greatly reducing the production cost. In terms of material properties, the obtained Fe-ZnO@MoO3 composite material has a stable structure and excellent adsorption performance. Due to its abundant active sites, it has a strong adsorption capacity for ammonia and can quickly detect the presence of ammonia. After testing, the sensitivity of this sensor to 20 ppm NH3 can reach 1.5, the response time is only 60 s, and the recovery time is 21 s. At the same time, the sensor exhibits good selectivity and can effectively distinguish ammonia from other interfering gases; the long-term stability is also excellent, and its performance is stable and reliable during long-term use, with a minimum detection limit of approximately 500 ppb.

[0042] Based on the above advantages, this ammonia sensor can be widely applied in multiple fields such as industrial safety, agricultural environmental monitoring, food processing quality control, and water treatment. It can accurately detect changes in NH3 concentration in the range of 1 ppm to 500 ppm, providing a reliable technical means to ensure the safety and health of the production and living environment. Description of the Drawings:

[0043] Figure 1 It is the scanning electron microscope (SEM) characterization of the Fe-ZnO@MoO3 composite material in Example 1.

[0044] Figure 2 It is the X-ray diffraction (XRD) characterization of the Fe-ZnO@MoO3 composite material in Example 1.

[0045] Figure 3 It is the X-ray photoelectron spectroscopy (XPS) characterization of the Fe-ZnO@MoO3 composite material in Example 1.

[0046] Figure 4 It is the response curve graph of the gas-sensitive element in Example 1 to ammonia at a certain concentration under different working temperatures.

[0047] Figure 5 It is the dynamic response curve graph of the gas-sensitive element in Example 1 to ammonia at different concentrations under the optimal working temperature.

[0048] Figure 6 It is the linear fitting graph of the relationship between ammonia concentration and response value of the gas-sensitive element in Example 1 under the optimal working temperature.

[0049] Figure 7It is the repeatability curve of the gas sensor in Example 1 for 20 ppm ammonia at the optimal operating temperature.

[0050] Figure 8 It is the response and recovery time curve of the gas sensor in Example 1 for 20 ppm ammonia at the optimal operating temperature.

[0051] Figure 9 It is the selectivity diagram of the gas sensor in Example 1 for different kinds of gases at the optimal operating temperature.

[0052] Figure 10 It is the long-term stability test diagram of the gas sensor in Example 1 within one month at the optimal operating temperature.

[0053] Figure 11 It is the schematic diagram of the application scenario of the ammonia sensor based on the Fe-ZnO@MoO3 composite material in Example 1 in farmland environment monitoring. Specific implementation method:

[0054] In order to more clearly and comprehensively elaborate the technical solution of the present invention, demonstrate its advantages, and clarify the invention purpose, the following will be described in detail by listing specific embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of the invention. Based on the embodiments involved in this application, all technical features covered in various embodiments of the present invention are within the protection scope of this application.

[0055] In the following embodiments, if the test method is not specifically specified, the conventional experimental operation methods in the art are used; for the reagents and materials used, if not otherwise specified, they can be conveniently obtained through commercial channels. These conventional methods, reagent and material sources ensure the repeatability and wide applicability of the technical solution of the present invention, enabling those skilled in the art to smoothly conduct relevant experiments and production practices according to the description of the present invention.

[0056] Example 1:

[0057] The preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum oxide include the following steps:

[0058] (1) Dissolve 1.7 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 2.3 g of thiourea (CH4N2 S ) in 30 mL of pure water, ultrasonically disperse for 30 minutes until completely dissolved, transfer to a reaction kettle with a polytetrafluoroethylene inner liner, and carry out a hydrothermal reaction at 150 - 300 °C for 24 hours. After the reaction, centrifuge to collect the precipitate, wash it 3 times with deionized water until neutral (detected with pH test paper), and vacuum dry at 80 °C for 12 hours, then grind to obtain MoS2 microspheres.

[0059] (2) Weigh ferric chloride hexahydrate (FeCl3·6H2O, 9 mmol) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 3 mmol) according to a molar ratio of 3:1, dissolve them in 30 mL of deionized water, stir to form a homogeneous solution, then gradually add NaOH solution to adjust the pH to 10, and continuously stir until a light yellow flocculent precipitate is formed. Add the MoS2 microspheres prepared in step (1), heat them in a water bath at 70 °C for 6 hours to in-situ modify the molybdenum disulfide microspheres with zinc-iron layered double hydroxide. After the reaction, centrifuge to separate the precipitate, wash it repeatedly with deionized water until neutral, dry it at 80 °C for 12 hours and grind it to obtain the precursor composite material. Heat it to 300 °C at a heating rate of 2 °C·min -1 and calcine it for 5 hours, then naturally cool it and grind it to a particle size of ≤5 μm. Obtain the Fe-ZnO@MoO3 composite material.

[0060] (3) Mix the calcined composite material and absolute ethanol according to a mass ratio of 1:5, ultrasonically disperse it for 30 minutes, and evenly coat it on the surface of the Al2O3 ceramic tube (tube length 4 mm, outer diameter 1.2 mm, electrode spacing 0.8 - 1 mm). The coated ceramic tube is sintered and aged in an air environment at 200 - 400 °C for 8 hours, and the heating rate is 3 °C·min -1 .

[0061] (4) After aging, insert a nickel-chromium alloy heating wire (diameter 0.1 mm) into the inside of the ceramic tube, and use tin wire (melting point 250 °C) to weld the heating wire to the Pt electrode leads at both ends of the ceramic tube. Strictly control the temperature at 250 °C during the welding process. Fix the component on a dust-proof and moisture-proof base, and complete the preparation of the ammonia sensor after installing the housing.

[0062] (5) Use a sampling needle to inject a certain concentration of ammonia water into the gas sensor analyzer, take the corresponding resistance as the ordinate and time as the abscissa, record the change of resistance, and establish an ammonia response curve, where the response value (S) = R g / R a .

[0063] Example 2:

[0064] The preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum oxide, including the following steps:

[0065] (1) Dissolve 1.7 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 2.3 g of thiourea (CH4N2S) in 30 mL of pure water, ultrasonically disperse it for 30 minutes until completely dissolved, transfer it to a reaction kettle with a polytetrafluoroethylene inner liner, and carry out a hydrothermal reaction at 150 - 300 °C for 24 hours. After the reaction, centrifuge to collect the precipitate, wash it 3 times with deionized water until neutral (detected by pH test paper), vacuum dry it at 80 °C for 12 hours, and grind it to obtain MoS2 microspheres.

[0066] (2) Weigh ferric chloride hexahydrate (FeCl3·6H2O, 9 mmol) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 3 mmol) according to a molar ratio of 3:1, dissolve them in 30 mL of deionized water, stir to form a homogeneous solution, then gradually add NaOH solution to adjust the pH to 10, and continuously stir until a light yellow flocculent precipitate is formed. Add the MoS2 microspheres prepared in step (1), heat in a water bath at 70 °C for 6 hours to in-situ modify the molybdenum disulfide microspheres with zinc-iron layered double hydroxide. After the reaction, centrifuge to separate the precipitate, wash it repeatedly with deionized water until neutral, dry at 80 °C for 12 hours and grind to obtain the precursor composite material. At a heating rate of 2 °C·min -1 to raise the temperature to 350 °C and calcine for 5 hours, and then naturally cool and grind to a particle size of ≤5 μm. Obtain the Fe-ZnO@MoO3 composite material

[0067] (3) Mix the calcined composite material with absolute ethanol according to a mass ratio of 1:5, ultrasonically disperse for 30 minutes, and uniformly coat it on the surface of the Al2O3 ceramic tube (tube length 4 mm, outer diameter 1.2 mm, electrode spacing 0.8 - 1 mm). The coated ceramic tube is sintered and aged in an air environment at 200 - 400 °C for 8 hours, and the heating rate is 3 °C·min -1 .

[0068] (4) After aging, insert a nickel-chromium alloy heating wire (diameter 0.1 mm) inside the ceramic tube, and use tin wire (melting point 250 °C) to weld the heating wire to the Pt electrode leads at both ends of the ceramic tube. Strictly control the temperature at 250 °C during the welding process. Fix the component on a dust-proof and moisture-proof base, and complete the preparation of the ammonia sensor after installing the shell.

[0069] (5) Use a sampling needle to inject a certain concentration of ammonia water into the gas-sensing analyzer, take the corresponding resistance as the ordinate and time as the abscissa, record the change of resistance, and establish an ammonia response curve, where the response value (S) = R g / R a .

[0070] Example 3:

[0071] Preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum oxide, including the following steps:

[0072] (1) Dissolve 1.7 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 2.3 g of thiourea (CH4N2S) in 30 mL of pure water, ultrasonically disperse for 30 minutes until completely dissolved, transfer to a reaction kettle with a polytetrafluoroethylene liner, and carry out a hydrothermal reaction at 150 - 300 °C for 24 hours. After the reaction, centrifuge to collect the precipitate, wash it 3 times with deionized water until neutral (detected by pH test paper), vacuum dry at 80 °C for 12 hours, and grind to obtain MoS2 microspheres.

[0073] (2) Weigh ferric chloride hexahydrate (FeCl3·6H2O, 9 mmol) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 3 mmol) according to a molar ratio of 3:1, dissolve them in 30 mL of deionized water, stir to form a homogeneous solution, then gradually add NaOH solution to adjust the pH to 10, and continuously stir until a light yellow flocculent precipitate is formed. Add the MoS2 microspheres prepared in step (1), heat in a water bath at 70 °C for 6 hours to in-situ modify the molybdenum disulfide microspheres with zinc-iron layered double hydroxide. After the reaction, centrifuge to separate the precipitate, wash it repeatedly with deionized water until neutral, dry at 80 °C for 12 hours and grind to obtain the precursor composite material. At a heating rate of 2 °C·min -1 to rise to 350 °C and calcine for 5 hours, and then naturally cool and grind to a particle size ≤ 5 μm. Obtain the Fe-ZnO@MoO3 composite material.

[0074] (3) Mix the calcined composite material and absolute ethanol according to a mass ratio of 1:5, ultrasonically disperse for 30 minutes, and evenly coat the surface of the Al2O3 ceramic tube (tube length 4 mm, outer diameter 1.2 mm, electrode spacing 0.8 - 1 mm). The coated ceramic tube is sintered and aged in an air environment at 200 - 400 °C for 8 hours, and the heating rate is 3 °C·min -1 .

[0075] (4) After aging, insert a nickel-chromium alloy heating wire (diameter 0.1 mm) inside the ceramic tube, use tin wire (melting point 250 °C) to weld the heating wire to the Pt electrode leads at both ends of the ceramic tube, and strictly control the temperature at 250 °C during the welding process. Fix the component on a dust-proof and moisture-proof base, and complete the preparation of the ammonia sensor after installing the shell.

[0076] (5) Use a sampling needle to inject a certain concentration of ammonia water into the gas-sensing analyzer, take the corresponding resistance as the ordinate and time as the abscissa, record the change of resistance, establish an ammonia response curve, where the response value (S) = R g / R a .

[0077] Example 4:

[0078] Preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum oxide, including the following steps:

[0079] (1) Dissolve 1.7 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 2.3 g of thiourea (CH4N2S) in 30 mL of pure water, ultrasonically disperse for 30 minutes until completely dissolved, transfer to a reaction kettle with a polytetrafluoroethylene inner lining, and carry out a hydrothermal reaction at 150 - 300 °C for 24 hours. After the reaction, centrifuge to collect the precipitate, wash it 3 times with deionized water until neutral (detected by pH test paper), vacuum dry at 80 °C for 12 hours, and grind to obtain MoS2 microspheres.

[0080] (2) Weigh ferric chloride hexahydrate (FeCl3·6H2O, 9 mmol) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 3 mmol) according to a molar ratio of 3:1, dissolve them in 30 mL of deionized water, stir to form a homogeneous solution, then gradually add NaOH solution to adjust the pH to 10, and continuously stir until a light yellow flocculent precipitate is formed. Add the MoS2 microspheres prepared in step (1), heat in a water bath at 70 °C for 6 hours to in-situ modify the molybdenum disulfide microspheres with zinc-iron layered double hydroxide. After the reaction, centrifuge to separate the precipitate, wash it repeatedly with deionized water until neutral, dry at 80 °C for 12 hours and grind to obtain the precursor composite material. Increase the temperature at a heating rate of 2 °C·min -1 to 450 °C and calcine for 5 hours, then naturally cool and grind to a particle size of ≤5 μm to obtain the Fe-ZnO@MoO3 composite material

[0081] (3) Mix the calcined composite material and absolute ethanol according to a mass ratio of 1:5, ultrasonically disperse for 30 minutes, and uniformly coat the surface of the Al2O3 ceramic tube (tube length 4 mm, outer diameter 1.2 mm, electrode spacing 0.8 - 1 mm). The coated ceramic tube is sintered and aged in an air environment at 200 - 400 °C for 8 hours, and the heating rate is 3 °C·min -1 .

[0082] (4) After aging, insert a nickel-chromium alloy heating wire (diameter 0.1 mm) inside the ceramic tube, and use tin wire (melting point 250 °C) to weld the heating wire to the Pt electrode leads at both ends of the ceramic tube. Strictly control the temperature at 250 °C during the welding process. Fix the component on a dust-proof and moisture-proof base, and complete the preparation of the ammonia sensor after installing the shell.

[0083] (5) Use a sampling needle to inject a certain concentration of ammonia water into a gas-sensing analyzer, take the corresponding resistance as the ordinate and time as the abscissa, record the change of resistance, and establish an ammonia response curve, where the response value (S) = R g / R a .

Claims

1. Preparation and application of an ammonia sensor based on iron-doped zinc oxide modified molybdenum oxide, comprising the following steps: (1) Dissolve 1.7 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 2.3 g of thiourea (CH4N2S) in 30 mL of deionized water, ultrasonically disperse for 30 minutes until completely dissolved, transfer the solution to a reaction kettle with a polytetrafluoroethylene inner lining, and perform a hydrothermal reaction at 200 °C for 24 hours; Centrifuge to collect the precipitate, wash it 3 times with deionized water until neutral, vacuum dry at 80 °C for 12 hours, and grind to obtain MoS2 microspheres; (2) Weigh ferric chloride hexahydrate (FeCl3·6H2O) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) according to a molar ratio of 3:1, with a total molar amount of 12 mmol. Dissolve them in 30 mL of deionized water, stir until a homogeneous solution is formed, and gradually add NaOH solution to adjust the pH of the solution to 10. Continuously stir until a light yellow flocculent precipitate is formed; add MoS2 microspheres, heat in a water bath at 70 °C for 6 hours to in-situ modify the MoS2 microspheres with zinc-iron layered double hydroxide; after the reaction, centrifuge to separate the precipitate, centrifuge until neutral, dry at 80 °C for 12 hours, and grind to obtain molybdenum disulfide material encapsulated with zinc-iron layered double hydroxide. Place the material in a muffle furnace and calcine it at a heating rate of 2 °C·min -1 to 300 - 500 °C for 5 h, cool naturally and then grind to obtain Fe-ZnO@MoO3. (3) Mix the composite material and absolute ethanol in a mass ratio of 1:5, ultrasonically disperse for 30 minutes, uniformly coat it on the surface of the Al2O3 ceramic tube, age in air at 200 - 400 °C for 12 hours, then weld a nickel-chromium alloy heating wire and a Pt electrode lead, and assemble a dust-proof housing to complete the preparation of the sensor.

2. The preparation method according to claim 1, characterized in that, In step (1), the total molar amount of ferric chloride hexahydrate and zinc nitrate hexahydrate is 12 mmol.

3. The preparation method according to claim 1, characterized in that, In step (3), the calcination temperature is 300 - 450 °C. The composite material of iron-doped zinc oxide modified molybdenum oxide is obtained by combustion.

4. The preparation method according to claim 1, wherein In step (4), the parameters of the Al2O3 ceramic tube are: tube length 4 mm; outer diameter 1.2 mm; electrode spacing 0.8 - 1 mm; electrode width 0.5 - 1 mm.

5. The preparation method according to claim 1, characterized in that, In step (4), the welding temperature is 250 °C, and the heating wire and the Pt electrode lead are welded with a tin wire.

6. The preparation method according to claim 1, characterized in that, The composite material of iron-doped zinc oxide modified molybdenum oxide has a heterojunction structure, enhancing the electron transport performance of the material.

7. Application of the ammonia sensor according to claim 1 in industrial safety monitoring, livestock and poultry breeding environment monitoring or sewage treatment systems.

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