Nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material, preparation method and internet environment monitoring system

Abstract

The invention discloses a nitrogen oxide sensor based on an Ag2Te / CeO2 sensitive material, a preparation method and a plant internet environment monitoring system, and belongs to the technical field of semiconductor gas sensors. The sensor is composed of an Al2O3 ceramic tube substrate, an Ag2Te / CeO2 sensitive material and a nickel-chromium heating coil, wherein the two ends of the outer surface of the Al2O3 ceramic tube substrate are provided with parallel and independent annular gold electrodes, the outer surface of the Al2O3 ceramic tube substrate and the gold electrodes are coated with the Ag2Te / CeO2 sensitive material, and the nickel-chromium heating coil is arranged in the Al2O3 ceramic tube substrate. According to the invention, Ag2Te / CeO2 is taken as a sensitive material, so that the adsorption enhancement response of nitrogen oxide molecules at low temperature can be effectively promoted; in addition, the hollow roll structure formed by compounding Ag2Te and CeO2 and the introduction of n-n heterojunction are also beneficial to the response of the sensor. According to the invention, the sensor is combined with the Internet of Things equipment and the Internet service at the same time, and a plant Internet environment monitoring system is developed, so that the system has important application value for constructing intelligent agriculture.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor gas sensors, and in particular relates to a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive materials, a preparation method and a plant internet environment monitoring system. Background Art

[0002] With the rapid population growth, climate change and environmental degradation, food security issues are becoming increasingly prominent. Agricultural greenhouses are considered a promising solution to future food demand and agricultural sustainability, and have become one of the fastest growing industries. Generally speaking, fertilizers and pesticides are applied to increase the yield of greenhouse crops. However, due to the high humidity in the greenhouse, nitrogen fertilizers can produce nitrogen oxides (NO x :NO2 and NO), while insecticides such as imidacloprid (IMD) can produce NO through light radiation. x . NO x It can directly or indirectly damage plant growth and reduce yields. In a relatively closed greenhouse environment, it can also pose health risks to workers. Therefore, it is important to develop and deploy remote real-time monitoring of NO in smart greenhouse agriculture. x Sensors are imperative.

[0003] In the past few decades, chemiresistor gas sensors have become a popular choice for NO due to their high sensitivity, low manufacturing cost, and good stability. x However, a large number of studies have confirmed that humidity has a significant impact on the gas sensing performance of chemiresistor sensors. Therefore, it is necessary to eliminate the interference of ambient humidity on sensor performance and achieve the detection of trace NO x The effective monitoring of NO has become a bottleneck problem that needs to be solved in the development of agricultural greenhouse chemiresistor sensors. In essence, the chemical adsorption and physical adsorption of H2O molecules have an impact on chemiresistor sensors. On the one hand, the chemically adsorbed H2O molecules will compete with the chemically adsorbed oxygen for adsorption on the surface of the sensing material or react with the chemically adsorbed oxygen. Then, the active sites of the gas molecule reaction will be occupied, resulting in the absorption of NO. x The response of the sensor decreases. Therefore, the sensing material should have a low H2O adsorption energy to inhibit the chemical adsorption of H2O molecules. On the other hand, as the humidity increases, H2O molecules will undergo physical adsorption, affecting the resistance of the sensor and thus the response of the sensor. Therefore, the adsorption of H2O molecules has little effect on sensing materials with high conductivity.

[0004] Based on the above considerations, silver telluride (Ag2Te) is a good candidate material. Some literature reports show that with NO x Compared with the adsorption energy of H2O, Te and Ag atoms have lower adsorption energy of H2O. xCompetitive adsorption between molecules hardly affects the sensing performance. In addition, as a narrow bandgap semiconductor with high conductivity, the effect of water on the resistance of Ag2Te can be considered minimal. In addition, heterojunction engineering of chemiresistor sensors has been shown to be an effective way to improve sensing performance. Cerium oxide (CeO2) has abundant oxygen vacancies and excellent redox properties, which is very useful in improving NO x Combining these two materials to form a heterojunction can give full play to their respective advantages and achieve NO in high humidity agricultural greenhouse environments. x Trace monitoring. Summary of the invention

[0005] The purpose of the present invention is to provide a plant internet environment monitoring system constructed based on a nitrogen oxide sensor of Ag2Te / CeO2 sensitive material, which can be used for real-time remote monitoring and information collection to help farmers effectively manage greenhouses.

[0006] The nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material described in the present invention is composed of an Al2O3 ceramic tube substrate with parallel and independent annular gold electrodes at both ends of the outer surface, a sensitive material coated on the outer surface of the Al2O3 ceramic tube substrate and the gold electrodes, and a nickel-chromium heating coil placed in the Al2O3 ceramic tube substrate; the sensitive material is Ag2Te / CeO2 sensitive material, and is prepared by the following steps:

[0007] (1) First, 480-720 mmol of sodium hydroxide (NaOH) is slowly added to 65-70 mL of deionized water in multiple times (3-8 times), and stirred for 30-60 minutes under ice bath conditions to form a uniform solution;

[0008] (2) adding 4 to 10 mmol of cerium nitrate (Ce(NO3)3·6H2O) to 10 to 15 mL of deionized water, stirring at 20 to 30°C for 30 to 60 minutes to form a uniform solution, then adding the solution dropwise to the solution obtained in step (1), and continuing to stir in an ice bath for 60 to 80 minutes to form a uniform solution;

[0009] (3) transferring the solution obtained in step (2) to a stainless steel autoclave lined with polytetrafluoroethylene, reacting at 160-180° C. for 12-24 hours; cooling naturally to room temperature, centrifuging the reaction product, and washing the resulting precipitate with deionized water and ethanol alternately several times until the pH of the supernatant after centrifugation is 7, and then drying under vacuum at 60-80° C. for 10-12 hours to obtain a light yellow CeO2 powder;

[0010] (4) adding 43-344 mg of CeO2 powder obtained in step (3) to 30-40 mL of anhydrous ethylenediamine and stirring at 20-30° C. for 0.2-1.0 hour to form a uniform solution, adding 160-180 mg of silver nitrate (AgNO3) and 70-90 mg of tellurium dioxide (TeO2) in sequence, and continuing stirring for 0.5-2.0 hours;

[0011] (5) 8 to 15 mL of hydrazine hydrate (N2H4·H2O, 85%, w / w%) is quickly added to the solution obtained in step (4), and the stirring is continued at 20 to 30° C. for 20 to 30 hours; the obtained solution is centrifuged, and the obtained precipitate is washed alternately with deionized water and ethanol several times until the pH value of the supernatant after centrifugation is 7, and then dried under vacuum at 60 to 80° C. for 10 to 12 hours to obtain a black solid powder, which is the Ag2Te / CeO2 sensitive material;

[0012] The method for preparing a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material described in the present invention comprises the following steps:

[0013] (1) 10-15 mg of Ag2Te / CeO2 sensitive material was placed in an agate mortar, and then 1-2 mL of deionized water was dripped into it, and the mixture was fully mixed and ground into a paste slurry; then the slurry was dipped and evenly coated on the surface of an Al2O3 ceramic tube substrate with parallel and independent annular gold electrodes at both ends of the outer surface to form a 10-30 μm thick sensitive material film, and the sensitive material completely covered the annular gold electrodes and the surface of the Al2O3 ceramic tube; the length of the Al2O3 ceramic tube was 4-4.5 mm, the outer diameter was 1.2-1.5 mm, and the inner diameter was 0.8-1.0 mm; the width of a single annular gold electrode was 0.4-0.5 mm, the distance between the two gold electrodes was 0.5-0.6 mm, the thickness of the gold electrode was 0.1-0.2 mm, and a platinum wire was led out from the gold electrode, and its length was 4-6 mm;

[0014] (2) baking the Al2O3 ceramic tube obtained in step (1) at 50°C to 80°C for 15 to 30 minutes, and after the sensitive material is dried, passing a nickel-chromium heating coil with a resistance value of 25 to 35Ω through the interior of the Al2O3 ceramic tube as a heating wire, and finally welding the device to a hexagonal tube socket according to a side-heated gas sensor, thereby obtaining a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material, whose optimal operating temperature is 65°C;

[0015] In specific application, the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor is first measured under various known concentrations of nitrogen oxides, and then the responsivity of the sensor under these concentrations is calculated, and the relationship curve of "sensor responsivity-nitrogen oxide concentration" is established through the computer origin software; then the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor under unknown concentrations of nitrogen oxides is measured, and then the obtained responsivity is substituted into the relationship curve of "sensor responsivity-nitrogen oxide concentration" through the computer origin software to calculate the nitrogen oxide concentration, thereby realizing the monitoring of the nitrogen oxide concentration in the surrounding environment.

[0016] The resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor is measured by connecting the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor in series with a grounding resistor R1 of known resistance to form a voltage divider circuit, applying a voltage VCC=5V across the voltage divider circuit, and then measuring the voltage V across the grounding resistor R1. R1 , through the formula R g =(5×R1)÷V R1 –R1, the resistance value R of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor can be obtained g , and then the responsivity of the sensor is calculated using the responsivity formula.

[0017] The sensor's responsivity is defined as the resistance of the sensitive material between the two gold electrodes of the sensor in air (R a ) and the resistance value (R g ) and R a The ratio is S = (|R a -R g | / R a )*100%.

[0018] The plant Internet environment monitoring system constructed based on the nitrogen oxide sensor of Ag2Te / CeO2 sensitive material described in the present invention is composed of Internet of Things devices and Internet services, and its main components and functions are as follows:

[0019] (1) The IoT device consists of a nitrogen oxide sensor, a voltage divider circuit, an ESP32 chip, and a battery module. The ESP32 chip integrates an analog-to-digital converter (ADC) module, a FLASH module, a UART module, a TIMER module, and a Wi-Fi module.

[0020] First, the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor is connected in series with a grounding resistor R1 of known resistance to form a voltage divider circuit, and a voltage VCC = 5V is applied to both ends of the voltage divider circuit; one end of the series resistor R1 is connected to the analog-to-digital converter (ADC) module in the ESP32 chip through the ADC interface, and the other end of the series resistor R1 is grounded; the analog-to-digital converter (ADC) module of the nitrogen oxide sensor measures the voltage analog signal at both ends of the series resistor R1 and converts it into a voltage digital signal V ADC , and then write the program in advance into the ESP32 chip according to the formula R 传感器 =(5×R1)÷V ADC –R1, calculate the resistance value digital signal R of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor 传感器 ; ESP32 chip transmits the resistance value digital signal R through the Wi-Fi module 传感器 Wireless transmission to a message queuing telemetry transport (MQTT) proxy service (the present invention uses the MQTT proxy service provided by the EMQX platform of EMQ Company);

[0021] (2) Internet service: Users log in to the web client interface of the message queue telemetry transmission agent service on different mobile or computer terminals to obtain the real-time monitoring data of the sensor, that is, the digital signal R of the resistance value between the two gold electrodes of the sensor 传感器 The sensor’s responsivity can be further calculated through the responsivity formula, and by substituting it into the relationship curve of “sensor responsivity-nitrogen oxide concentration”, the concentration estimate of nitrogen oxides can be obtained, thereby providing information support for farmers to implement greenhouse management measures such as ventilation.

[0022] The present invention uses Ag2Te / CeO2 as the sensitive material. On the one hand, the weak adsorption energy and high conductivity of Ag2Te for H2O molecules are conducive to improving the moisture resistance of the sensor; on the other hand, CeO2 exposes the (100) polar crystal plane with rich active sites, which can effectively promote the adsorption and enhanced response of nitrogen oxide molecules at low temperatures; in addition, the hollow roll structure formed by the composite of Ag2Te and CeO2 and the introduction of nn heterojunction are also conducive to the sensor response. The combined effect of these aspects makes the sensor have good moisture resistance and greatly improves the sensor's responsiveness at low temperatures. The present invention uses a commercially available tubular structure sensor with a simple manufacturing process that is conducive to industrial mass production. At the same time, the sensor is combined with Internet of Things devices and Internet services to develop a plant Internet environment monitoring system, so it has important application value for building smart agriculture.

[0023] The plant internet environment monitoring system constructed by the nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material prepared by the present invention has the following advantages:

[0024] 1. The present invention uses a simple and environmentally friendly hydrothermal method and a room temperature solution method to prepare Ag2Te / CeO2 sensitive materials. The synthesis method is simple and the cost is low;

[0025] 2. The nitrogen oxide sensor prepared by the present invention achieves high responsiveness at low temperature (65°C) and has an ultra-low detection limit (5ppb) for nitrogen dioxide. It has good selectivity, stability and moisture resistance, and can realize trace detection of nitrogen oxides in a high humidity environment;

[0026] 3. The present invention has developed a plant Internet environment monitoring system, which has important application value for building smart agriculture. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 : SEM morphology photograph of the Ag2Te / CeO2 sensitive material prepared in Example 1 of the present invention;

[0028] Figure 2 : A schematic diagram of the structure of a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material as described in Example 1 of the present invention;

[0029] Figure 3 : Schematic diagram of the plant Internet environment monitoring system constructed by the nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material described in Example 2 of the present invention;

[0030] Figure 4 : Real-time curve of the resistance value of the sensor prepared in Example 1 changing with the concentration of nitrogen dioxide at an operating temperature of 65°C;

[0031] Figure 5 : The relationship curve between the response of the sensor prepared in Example 1 and the concentration of nitrogen dioxide at an operating temperature of 65°C;

[0032] Figure 6 : A comparison chart of the selectivity of the sensor prepared in Example 1 to different gases at an operating temperature of 65°C;

[0033] Figure 7 : A curve diagram showing the change of the responsivity and resistance value of the sensor prepared in Example 1 at an operating temperature of 65°C with relative humidity;

[0034] Figure 8 : The real-time curve of the resistance of the sensitive material between the two gold electrodes of the sensor of the plant internet environment monitoring system described in the present invention after applying nitrogen fertilizer in the simulated agricultural greenhouse environment of Example 3 for 96 hours;

[0035] Fig. 9 : The plant internet environment monitoring system described in the present invention is a real-time curve of the resistance of the sensitive material between the two gold electrodes of the sensor in Example 4 after applying pesticides in a simulated agricultural greenhouse environment for 96 hours.

[0036] like Figure 1 As shown, in the Ag2Te / CeO2 sensitive material prepared in Example 1 of the present invention, Ag2Te has an irregular block morphology with a size of 50 to 150 nm and constitutes a hollow roll structure; CeO2 has a cubic structure, exposing the (100) polar crystal plane, with a size of 9 to 50 nm, and is attached to the Ag2Te surface.

[0037] like Figure 2 As shown, the nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material described in Example 1 of the present invention is composed of an Al2O3 ceramic tube substrate with parallel and independent annular gold electrodes at both ends of the outer surface, a sensitive material coated on the outer surface of the Al2O3 ceramic tube and the gold electrodes, and a nickel-chromium heating coil placed in the Al2O3 ceramic tube; the Ag2Te / CeO2 sensitive material needs to completely cover the surface of the Al2O3 ceramic tube and the annular gold electrode.

[0038] like Figure 3 As shown, Figure 3 a is the circuit schematic diagram of the plant Internet environment monitoring system constructed based on the nitrogen oxide sensor of Ag2Te / CeO2 sensitive material described in Example 2 of the present invention, which is composed of an Internet of Things device and an Internet service. The Internet of Things device is composed of a nitrogen oxide sensor, a voltage divider circuit, an ESP32 chip, and a battery module. The analog-to-digital converter (ADC) module, a FLASH module, a UART module, a TIMER module, and a Wi-Fi module are integrated in the ESP32 chip. In the Internet of Things device, the battery module has three functions. First, it supplies power to the system circuit to ensure the normal operation of the equipment. Secondly, it supplies energy to the heating coil to provide a 65°C operating temperature for the sensor. Finally, it provides a 5V voltage to the voltage divider circuit composed of the sensor and the grounding resistor R1 in series. The grounding capacitor adopts a capacitance of 100NF in order to effectively filter out clutter in the signal. One end of the series resistor R1 is connected to the analog-to-digital converter (ADC) module in the ESP32 chip through the ADC interface, and the other end of the series resistor R1 is grounded; the analog-to-digital converter (ADC) module measures the voltage analog signal at both ends of the series resistor R1 and converts it into a voltage digital signal V ADC , and then write the program in advance into the ESP32 chip according to the formula R 传感器 =(5×R1)÷V ADC –R1, calculate the resistance value digital signal R of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor 传感器; FLASH module is a non-volatile memory. The program in the ESP32 chip is stored in FLASH and can be executed after power is turned on; UART (Universal Asynchronous Receiver-Transmitter) module is a serial communication protocol that allows the ESP32 chip to transmit data with other devices through a single two-way communication line; TIMER (timer) module is a timing counter inside the chip (controls the ESP32 chip to achieve periodic data collection and transmission, that is, once a second, the voltage signal across the series resistor R1 is measured and the resistance value digital signal R is calculated. 传感器 transmission). Figure 3 b is the web client interface of the message queue telemetry transmission agent service. The program is a web-side program. When the user opens the browser to access the client, it will automatically download the program from the web client server and start running. The corresponding service has been deployed in the Dcloud service of the web client. Users can start or stop the display of real-time data and export Excel data through this interface. The vertical axis of the real-time data display box shows the resistance value of the sensitive material between the two gold electrodes of the sensor, and the horizontal axis shows the time. Users can obtain the real-time monitoring data of the sensor, that is, the digital signal R of the resistance value between the two gold electrodes of the sensor. 传感器 The sensor’s responsivity can be further calculated through the responsivity formula, and by substituting it into the relationship curve of “sensor responsivity-nitrogen oxide concentration”, the concentration estimate of nitrogen oxides can be obtained, thereby providing information support for farmers to implement greenhouse management measures such as ventilation.

[0039] like Figure 4 As shown, when the device operating temperature is 65°C, the responsiveness of the sensor prepared in Example 1 increases with the increase of nitrogen dioxide concentration, and has a lower detection limit (5 ppb) and a wider detection range (5 ppb to 10 ppm).

[0040] like Figure 5 As shown, the relationship curve between the response of the sensor prepared in Example 1 and nitrogen dioxide at a low temperature of 65°C can be fitted by the ExpAssoc function. When the sensor tests an unknown concentration of nitrogen oxides, the calculated response can be substituted into the above fitting curve to calculate the unknown nitrogen oxide concentration.

[0041] like Figure 6 As shown, the sensor prepared in Example 1 has good selectivity for nitrogen oxides at a low temperature of 65°C.

[0042] like Figure 7 As shown, the sensor prepared in Example 1 has good moisture resistance at a low temperature of 65° C. and can be used to monitor nitrogen oxides in a high humidity environment.

[0043] like Figure 8 As shown, the plant Internet environment monitoring system developed in Example 2 performs real-time monitoring of nitrogen oxides in the simulated agricultural greenhouse environment of Example 3 after nitrogen fertilizer is applied for 96 hours. The curve can clearly distinguish the different concentrations of nitrogen oxides produced in the simulated agricultural greenhouse after applying different amounts of nitrogen fertilizer; the resistance value (R g ) is positively correlated with the concentration of nitrogen oxides.

[0044] like Fig. 9 As shown, the plant Internet environment monitoring system developed in Example 2 performs real-time monitoring of nitrogen oxides in Example 4 after applying pesticides for 96 hours in the simulated agricultural greenhouse environment. The curve can clearly distinguish the different concentrations of nitrogen oxides produced in the simulated agricultural greenhouse after applying different amounts of pesticides. g ) is positively correlated with the concentration of nitrogen oxides.

[0045] The plastic box used to simulate the agricultural greenhouse is 30 cm long, 25 cm wide, 17 cm high in the front and 22 cm high in the back. 2.5 kg of soil is placed in the bottom layer and 5 cucumber seedlings are planted. DETAILED DESCRIPTION

[0046] Example 1: Synthesis of Ag2Te / CeO2 sensitive material and preparation of sensor

[0047] 1. First, add 480mmol of sodium hydroxide (NaOH) into a beaker containing 65mL of deionized water in 5 equal portions and stir in an ice bath for 60 minutes to form a uniform solution;

[0048] 2. Add 4 mmol of cerium nitrate (Ce(NO3)3·6H2O) to a beaker containing 15 mL of deionized water, stir at 25°C for 60 minutes to form a uniform solution, then add the solution obtained in step (1) dropwise, and continue stirring in an ice bath for 60 minutes to form a uniform solution;

[0049] 3. The solution obtained in step (2) was transferred to an 80 mL polytetrafluoroethylene-lined stainless steel autoclave and placed in an oven at 180°C for 24 hours. After the reaction, it was naturally cooled to room temperature. Then, the product was centrifuged and washed alternately with deionized water and ethanol several times until the pH of the supernatant after centrifugation was 7, and dried in a vacuum oven at 80°C for 12 hours to finally obtain 688 mg of light yellow CeO2 powder with a microscopic morphology of a regular cubic structure and exposed (100) polar crystal plane;

[0050] 4. Add 172 mg of CeO2 powder obtained in step (3) into 30 mL of anhydrous ethylenediamine and stir at 25°C for 0.5 hour. After a uniform solution is formed, add 170 mg of silver nitrate (AgNO3) and 80 mg of tellurium dioxide (TeO2) in sequence and continue stirring for 1 hour.

[0051] 5. 10 mL of hydrazine hydrate (N2H4·H2O, 85%, w / w%) was quickly added to the solution obtained in step (4), and the mixture was stirred at 25°C for 24 hours; the solution was centrifuged, and the precipitate obtained by centrifugation was washed alternately with deionized water and ethanol several times until the pH of the supernatant after centrifugation was 7, and then dried at 60°C in vacuum for 12 hours to obtain 344 mg of black solid powder, which was the Ag2Te / CeO2 sensitive material, and the microscopic morphology was a hollow roll structure formed by cubic cerium oxide attached to irregular blocky silver telluride;

[0052] 6. Take 10 mg of the Ag2Te / CeO2 powder obtained in step (5) and put it into a mortar. Then, add 1 mL of deionized water, mix thoroughly and grind to form a paste slurry. Then, take a small amount of the slurry and evenly coat it on the surface of an Al2O3 ceramic tube with parallel and independent annular gold electrodes at both ends of the surface to form a 30 μm thick sensitive material film, and make the sensitive material completely cover the annular gold electrode and the surface of the Al2O3 ceramic tube. The length of the ceramic tube is 4 mm, the outer diameter is 1.2 mm, and the inner diameter is 0.8 mm. The width of a single annular gold electrode is 0.4 mm, the distance between the two gold electrodes is 0.5 mm, and the thickness of the gold electrode is 0.1 mm. A platinum wire is led out from the gold electrode, and its length is 5 mm.

[0053] 7. Bake at 60°C under an infrared lamp for 30 minutes. After the sensitive material is dry, a nickel-chromium heating coil with a resistance value of 28.5Ω is passed through the inside of the Al2O3 ceramic tube as a heating wire. Finally, the above device is welded and packaged according to the general indirect heating gas sensor to obtain a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material.

[0054] Example 2: Construction and application of a plant internet environment monitoring system based on a nitrogen oxide sensor made of Ag2Te / CeO2 sensitive materials

[0055] 1. The nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material prepared in Example 1 is connected to an Internet of Things device. The battery (voltage 3.7V) in the Internet of Things device supplies energy to the sensor nickel-chromium heating coil and the device circuit through the power management chip (IP5305), ensuring that the sensor operates at an operating temperature of 65°C and the normal operation of the entire system. After the sensor is in operation, the resistance value changes according to the nitrogen oxide concentration in the surrounding environment. The analog-to-digital converter (ADC) module in the Internet of Things device measures and collects the voltage value analog signal between the sensor and the grounding resistor R1 and converts it into a voltage value digital signal V ADC , then the program written into the chip in advance is used according to the formula R 传感器 =(5×R1)÷V ADC –R1, calculate the resistance value digital signal R of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor 传感器 , and then transmit the digital signal wirelessly through the Wi-Fi module;

[0056] 2. Internet service: First, the digital signal obtained in step (1) is transmitted by the Wi-Fi module of the IoT device to the message queuing telemetry transport (MQTT) proxy service (the present invention uses the MQTT proxy service provided by the EMQX platform of EMQ Company). Users can obtain real-time monitoring data through the web client interface on different mobile terminals to obtain information on nitrogen oxide concentration in the agricultural greenhouse. Users can use this to perform greenhouse management measures such as ventilation to ensure the normal growth of plants.

[0057] Example 3: Monitoring of nitrogen oxides after application of nitrogen fertilizer in a simulated agricultural greenhouse environment

[0058] Under sunlight and high humidity conditions, different amounts of nitrogen fertilizer (urea) were fully dissolved in 10 mL of deionized water and then evenly added to the soil (soil area 750 cm2) where cucumber seedlings were planted. 2 ) to simulate a greenhouse after adding nitrogen fertilizer (maintaining 25°C and 60% relative humidity). After 96 hours, the plant Internet environment monitoring system based on the nitrogen oxide sensor of Ag2Te / CeO2 sensitive material began to monitor the nitrogen oxides in the simulated greenhouse. First, the IoT device connected to the nitrogen oxide sensor was placed in the air. After the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor stabilized, the IoT device was placed in the simulated greenhouse. After the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor stabilized again, the IoT device was taken out of the simulated greenhouse and placed in the air. The whole monitoring process took 200 seconds, and the measurement data output by the system is as follows: Figure 8As shown in the figure, different concentrations of nitrogen oxides produced in the simulated agricultural greenhouse after applying different amounts of nitrogen fertilizer can be clearly distinguished. According to the system monitoring data, it can be estimated that after applying 1.0g, 5.0g, and 10.0g of urea for 96 hours, the concentrations of nitrogen oxides in the simulated greenhouse were 4ppb, 14ppb, and 59ppb, respectively.

[0059] Example 4: Monitoring of nitrogen oxides after application of pesticides in a simulated agricultural greenhouse environment

[0060] Under sunlight and high humidity conditions, different amounts of insecticide (imidacloprid) were fully dissolved in 10 mL of deionized water and then evenly added to the soil (soil area 750 cm) where cucumber seedlings were planted. 2 ) to simulate a greenhouse after adding nitrogen fertilizer (maintaining 25°C and 60% relative humidity). After 96 hours, the plant Internet environment monitoring system based on the nitrogen oxide sensor of Ag2Te / CeO2 sensitive material began to monitor the nitrogen oxides in the simulated greenhouse. First, the IoT device connected to the nitrogen oxide sensor was placed in the air. After the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor stabilized, the IoT device was placed in the simulated greenhouse. After the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor stabilized again, the IoT device was taken out of the simulated greenhouse and placed in the air. The whole monitoring process took 200 seconds, and the measurement data output by the system is as follows: Fig. 9 As shown, the different concentrations of nitrogen oxides produced in the simulated agricultural greenhouse after applying different amounts of pesticides can be clearly distinguished. According to the system monitoring data, it can be estimated that after applying 0.5g, 1.0g, and 2.0g of imidacloprid for 96 hours, the concentrations of nitrogen oxides in the simulated greenhouse were 4ppb, 10ppb, and 35ppb, respectively.

Claims

1. A nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material, comprising an Al2O3 ceramic tube substrate with parallel and independent annular gold electrodes at both ends of the outer surface, a sensitive material coated on the outer surface of the Al2O3 ceramic tube substrate and the gold electrodes, and a nickel-chromium heating coil placed in the Al2O3 ceramic tube substrate; characterized in that: The sensitive material is Ag2Te / CeO2 sensitive material, and is prepared by the following steps: (1) First, 480-720 mmol of sodium hydroxide was slowly added to 65-70 mL of deionized water in multiple portions, and stirred for 30-60 minutes under ice bath conditions to form a uniform solution; (2) adding 4 to 10 mmol of cerium nitrate to 10 to 15 mL of deionized water, stirring at 20 to 30° C. for 30 to 60 minutes to form a uniform solution, then adding the solution dropwise to the solution obtained in step (1), and continuing to stir in an ice bath for 60 to 80 minutes to form a uniform solution; (3) transferring the solution obtained in step (2) to a stainless steel autoclave lined with polytetrafluoroethylene, reacting at 160-180° C. for 12-24 hours; cooling naturally to room temperature, centrifuging the reaction product, and washing the resulting precipitate with deionized water and ethanol alternately several times until the pH of the supernatant after centrifugation is 7, and then drying under vacuum at 60-80° C. for 10-12 hours to obtain a light yellow CeO2 powder; (4) 43-344 mg of CeO2 powder obtained in step (3) is added to 30-40 mL of anhydrous ethylenediamine and stirred at 20-30° C. for 0.2-1.0 hour to form a uniform solution, and 160-180 mg of silver nitrate and 70-90 mg of tellurium dioxide are added thereto in sequence. Continue stirring for 0.5 to 2.0 hours; (5) 8 to 15 mL of 85 w / w % hydrazine hydrate is rapidly added to the solution obtained in step (4), and stirring is continued at 20 to 30 ° C for 20 to 30 hours; the resulting solution is centrifuged, and the resulting precipitate is alternately washed with deionized water and ethanol several times until the pH of the supernatant after centrifugation is 7, and then dried under vacuum at 60 to 80 ° C for 10 to 12 hours to obtain a black solid powder, which is the Ag2Te / CeO2 sensitive material.

2. The nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material as claimed in claim 1, characterized in that: The length of the Al2O3 ceramic tube is 4 to 4.5 mm, the outer diameter is 1.2 to 1.5 mm, and the inner diameter is 0.8 to 1.0 mm; the width of a single annular gold electrode is 0.4 to 0.5 mm, the distance between two gold electrodes is 0.5 to 0.6 mm, the thickness of the gold electrode is 0.1 to 0.2 mm, and a platinum wire is led out from the gold electrode, and its length is 4 to 6 mm.

3. A method for preparing a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material according to claim 1 or 2, wherein the steps are as follows: (1) 10-15 mg of Ag2Te / CeO2 sensitive material is placed in an agate mortar, and then 1-2 mL of deionized water is dripped into it, and the mixture is fully mixed and ground into a paste-like slurry; then the slurry is dipped and evenly coated on the surface of an Al2O3 ceramic tube substrate with parallel and independent annular gold electrodes at both ends of the outer surface to form a 10-30 μm thick sensitive material film, and the sensitive material completely covers the annular gold electrodes and the surface of the Al2O3 ceramic tube; (2) The Al2O3 ceramic tube obtained in step (1) is baked at 50°C to 80°C for 15 to 30 minutes. After the sensitive material is dried, a nickel-chromium heating coil with a resistance value of 25 to 35Ω is passed through the interior of the Al2O3 ceramic tube as a heating wire. Finally, the device is welded to a hexagonal tube socket according to a side-heated gas sensor, thereby obtaining a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material.

4. A plant internet environment monitoring system based on a nitrogen oxide sensor made of Ag2Te / CeO2 sensitive material, characterized in that: The monitoring system consists of IoT devices and Internet services. (1) The IoT device consists of a nitrogen oxide sensor, a voltage divider circuit, an ESP32 chip, and a battery module. The ESP32 chip integrates an analog-to-digital converter (ADC) module, a FLASH module, a UART module, a TIMER module, and a Wi-Fi module. First, the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor is connected in series with a grounding resistor R1 of known resistance to form a voltage divider circuit, and a voltage VCC = 5V is applied to both ends of the voltage divider circuit; one end of the series resistor R1 is connected to the analog-to-digital converter (ADC) module in the ESP32 chip through the ADC interface, and the other end of the series resistor R1 is grounded; the analog-to-digital converter (ADC) module of the nitrogen oxide sensor measures the voltage analog signal at both ends of the series resistor R1 and converts it into a voltage digital signal V ADC , and then write the program in advance into the ESP32 chip according to the formula R 传感器 =(5×R1)÷V ADC –R1, calculate the resistance value digital signal R of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor 传感器 ; ESP32 chip transmits the resistance value digital signal R through the Wi-Fi module 传感器 Wireless transmission to message queue telemetry transmission agent service; (2) Internet service: Users log in to the web client interface of the message queue telemetry transmission agent service on different mobile or computer terminals to obtain the real-time monitoring data of the sensor, that is, the digital signal R of the resistance value between the two gold electrodes of the sensor 传感器 The sensor's responsivity can be further calculated through the responsivity formula, and by substituting it into the "sensor responsivity-nitrogen oxide concentration" relationship curve, the concentration estimate of nitrogen oxides can be obtained, thereby providing information support for farmers to take greenhouse management measures such as ventilation.

5. The plant internet environment monitoring system constructed based on the nitrogen oxide sensor of Ag2Te / CeO2 sensitive material as claimed in claim 4, characterized in that: First, the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor under various known concentrations of nitrogen oxides is measured, and then the sensor responsivity under these concentrations is calculated, and the relationship curve of "sensor responsivity-nitrogen oxide concentration" is established through the computer origin software; the resistance value of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor is measured by connecting the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor in series with a grounding resistor R1 of known resistance to form a voltage divider circuit, applying a voltage VCC=5V across the voltage divider circuit, and then measuring the voltage V across the grounding resistor R1 R1 , through the formula R g =(5×R1)÷V R1 –R1, the resistance value R of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor can be obtained g , and then the responsivity of the sensor is calculated by the responsivity formula; the responsivity is defined as the resistance value of the sensitive material between the two gold electrodes of the sensor in the air (R a ) and the resistance value (R g ) and R a The ratio is S = (|R a -R g | / R a )*100%.

6. The plant internet environment monitoring system constructed based on the nitrogen oxide sensor of Ag2Te / CeO2 sensitive material as claimed in claim 4, characterized in that: The battery module has three functions: first, it supplies power to the system circuit to ensure the normal operation of the equipment; second, it supplies power to the heating coil to provide the sensor with an operating temperature of 65°C; and finally, it provides 5V voltage to the voltage divider circuit composed of the sensor and the grounding resistor R1 in series; the FLASH module is a non-volatile memory. The program in the ESP32 chip is stored in the FLASH and can be executed after power is turned on; The UART module is a serial communication protocol that allows the ESP32 chip to communicate with other devices through a single bidirectional communication line; the TIMER module is a timing counter inside the chip.

Images