Nitrogen oxide sensor based on ag2te / cuo sensitive material, preparation method and internet environment monitoring system

Abstract

The application relates to a nitrogen oxide sensor based on 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 with parallel and mutually independent annular gold electrodes at two ends of an outer surface, Ag2Te / CeO2 sensitive material coated on the outer surface of the Al2O3 ceramic tube substrate and the gold electrodes, and a nickel-chromium heating coil arranged in the Al2O3 ceramic tube substrate; the Ag2Te / CeO2 sensitive material can effectively promote the adsorption of nitrogen oxide molecules at low temperature and enhance the response; in addition, the hollow roll structure formed after the Ag2Te and CeO2 are compounded and the introduction of n-n heterojunction are also beneficial to the response of the sensor. The application combines the sensor with internet of things equipment and internet services, develops a plant internet environment monitoring system, and therefore has important application value for the construction of intelligent agriculture.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor gas sensor technology, specifically relating to a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material, its preparation method, and a plant internet environment monitoring system. Background Technology

[0002] With rapid population growth, climate change, and environmental degradation, food security has become an increasingly prominent issue. Agricultural greenhouses are considered a promising solution to future food needs and agricultural sustainability, and have become one of the fastest-growing industries. Generally, chemical fertilizers and pesticides are applied to increase greenhouse crop yields. However, due to the high humidity inside greenhouses, nitrogen fertilizer can produce nitrogen oxides (NOx) through the action of soil enzymes and microorganisms. x (NO2 and NO), insecticides such as imidacloprid (IMD) can produce NO through light radiation. x NO x It can directly or indirectly damage plant growth and reduce yield. In the relatively enclosed greenhouse environment, it also poses health risks to workers. Therefore, the development and deployment of NO2 for remote real-time monitoring in smart greenhouse agriculture is crucial. x Sensors are a necessity.

[0003] Over the past few decades, chemical resistance gas sensors have become a popular choice for NO due to their high sensitivity, low manufacturing cost, and good stability. x The dominant technology in sensing is humidity. However, numerous studies have confirmed that humidity has a significant impact on the gas-sensing performance of chemimetric resistors. Therefore, eliminating the interference of ambient humidity on sensor performance is crucial for achieving the detection of trace amounts of NO. x Effective monitoring of NO has become a bottleneck problem that urgently needs to be solved in the development of chemical resistance sensors for agricultural greenhouses. Essentially, the chemical and physical adsorption of H2O molecules affects chemical resistance sensors. On the one hand, chemically adsorbed H2O molecules will compete with chemically adsorbed oxygen for adsorption on the surface of the sensing material or react with chemically adsorbed oxygen. Then, the active sites of the gas molecules will be occupied, leading to reduced sensitivity to NO. x The response decreases. Therefore, the sensing material should have a low H2O adsorption energy to suppress the chemisorption of H2O molecules. On the other hand, with increasing humidity, H2O molecules undergo physical adsorption, affecting the sensor's resistance and thus its response. Therefore, the adsorption of H2O molecules has a relatively small impact on sensing materials with high conductivity.

[0004] Based on the above considerations, silver telluride (Ag₂Te) is a promising candidate material. Some literature reports indicate that, compared with NO… x Compared to other adsorption energies, Te and Ag atoms have lower H2O adsorption energies. Therefore, it is expected that H2O and NO... xCompetitive adsorption between molecules has almost no impact on sensing performance. Furthermore, as a narrow-bandgap semiconductor with high conductivity, water's effect on the resistance of Ag₂Te is negligible. Moreover, heterojunction engineering of chemiluminescence sensors has proven to be an effective way to improve sensing performance. Cerium oxide (CeO₂) possesses abundant oxygen vacancies and excellent redox properties, making it useful for improving NO₂ resistance. x It shows great potential in terms of sensitivity and lowering the detection limit. Combining these two materials to form a heterojunction can leverage their respective advantages to achieve NO detection in high-humidity agricultural greenhouse environments. x Trace monitoring. Summary of the Invention

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

[0006] The present invention discloses 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 its 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 inside the Al2O3 ceramic tube substrate; the sensitive material is Ag2Te / CeO2 sensitive material, and is prepared by the following steps:

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

[0008] (2) Add 4-10 mmol of cerium nitrate (Ce(NO3)3·6H2O) to 10-15 mL of deionized water, stir at 20-30°C for 30-60 minutes to form a homogeneous solution, and then add the solution dropwise to the solution obtained in step (1), and continue stirring under ice bath conditions for 60-80 minutes to form a homogeneous solution.

[0009] (3) Transfer the solution obtained in step (2) to a stainless steel autoclave lined with polytetrafluoroethylene and react at 160-180°C for 12-24 hours. After naturally cooling to room temperature, centrifuge the reaction product and wash the precipitate with deionized water and ethanol several times until the pH of the supernatant after centrifugation is 7. Then dry it under vacuum at 60-80°C for 10-12 hours to obtain a light yellow CeO2 powder.

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

[0011] (5) Add 8-15 mL of hydrazine hydrate (N2H4·H2O, 85%, w / w%) quickly to the solution obtained in step (4) and continue stirring at 20-30°C for 20-30 hours; centrifuge the obtained solution, wash the precipitate with deionized water and ethanol alternately several times until the pH of the supernatant after centrifugation is 7, and then dry it under vacuum at 60-80°C for 10-12 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 according to the present invention comprises the following steps:

[0013] (1) Take 10-15 mg of Ag2Te / CeO2 sensitive material and place it in an agate mortar. Then add 1-2 mL of deionized water, mix thoroughly and grind to form a paste. Then dip the paste and coat it evenly 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 make the sensitive material completely cover the annular gold electrodes and the surface of the Al2O3 ceramic tube. The Al2O3 ceramic tube is 4-4.5 mm long, 1.2-1.5 mm in outer diameter, and 0.8-1.0 mm in inner diameter. The width of a single annular gold electrode is 0.4-0.5 mm, the distance between two gold electrodes is 0.5-0.6 mm, the thickness of the gold electrode is 0.1-0.2 mm, and a platinum wire with a length of 4-6 mm is drawn out from the gold electrode.

[0014] (2) The Al2O3 ceramic tube obtained in step (1) is baked at 50℃~80℃ for 15~30 minutes. After the sensitive material is dried, a nickel-chromium heating coil with a resistance of 25~35Ω is passed through the inside of the Al2O3 ceramic tube as a heating wire. Finally, the device is welded to the hexagonal tube base according to the side-heated gas sensing element, so as to obtain a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material. Its optimal working temperature is 65℃.

[0015] In practical applications, the resistance of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor is first measured under various known concentrations of nitrogen oxides. Then, the responsivity of the sensor under these concentrations is calculated, and a "sensor responsivity-nitrogen oxide concentration" relationship curve is established using Origin software. Next, the resistance of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor is measured under unknown concentrations of nitrogen oxides. The obtained responsivity is then substituted into the "sensor responsivity-nitrogen oxide concentration" relationship curve using Origin software to calculate the nitrogen oxide concentration, thereby enabling the monitoring of nitrogen oxide concentration in the surrounding environment.

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

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

[0018] The plant internet environment monitoring system based on a nitrogen oxide sensor made of Ag2Te / CeO2 sensitive material, as described in this invention, consists of Internet of Things (IoT) devices and internet services. Its main components and functions are as follows:

[0019] (1) The Internet of Things 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, a voltage divider circuit is formed by connecting the Ag2Te / CeO2 sensing material between the two gold electrodes of the sensor in series with a grounded resistor R1 of known resistance. A voltage VCC = 5V is applied across 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 ADC module of the nitrogen oxide sensor measures the analog voltage signal across the series resistor R1 and converts it into a digital voltage signal V. ADC Then, by pre-writing a program into the ESP32 chip according to formula R... 传感器 = (5 × R1) ÷ V ADC –R1, calculates the digital signal R of the resistance value of the Ag2Te / CeO2 sensing material between the two gold electrodes of the sensor. 传感器 The ESP32 chip transmits the digital signal R of the resistance value via a Wi-Fi module. 传感器 Wireless transmission to Message Queuing Telemetry Transport (MQTT) broker service (this invention uses the MQTT broker service provided by EMQ's EMQX platform);

[0021] (2) Internet Service: Users log in to the web client interface of the message queue telemetry transmission agent service on different mobile devices or computers to obtain real-time monitoring data from the sensor, namely the digital signal R of the resistance value between the two gold electrodes of the sensor. 传感器 Furthermore, the sensor's responsivity can be calculated using the responsivity formula. By substituting this into the relationship curve of "sensor responsivity - nitrogen oxide concentration", an estimated concentration of nitrogen oxides can be obtained, thus providing information support for farmers to implement greenhouse management measures such as ventilation.

[0022] This invention uses Ag2Te / CeO2 as the sensing material. On the one hand, Ag2Te's weak adsorption energy and high conductivity for H2O molecules are beneficial for improving the sensor's moisture resistance. On the other hand, CeO2 exposes a (100) polar crystal plane with abundant 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 Ag2Te and CeO2 composite and the introduction of an n-heterojunction are also beneficial to the sensor response. The combined effect of these aspects gives the sensor good moisture resistance and significantly improves the sensor's response at low temperatures. The sensor used in this invention is a commercially available tubular structure sensor, which has a simple manufacturing process and is conducive to mass production in industry. At the same time, by combining the sensor with Internet of Things (IoT) devices and Internet services, a plant Internet environmental monitoring system has been developed, thus having important application value for building smart agriculture.

[0023] The plant internet environmental monitoring system based on a nitrogen oxide sensor made of Ag2Te / CeO2 sensitive material prepared in this invention has the following advantages:

[0024] 1. This invention utilizes 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 low in cost.

[0025] 2. The nitrogen oxide sensor prepared by this invention achieves high responsivity at low temperature (65℃) 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 high humidity environments.

[0026] 3. This invention develops a plant internet environment monitoring system, which has important application value for building smart agriculture. Attached Figure Description

[0027] Figure 1 SEM morphology images of the Ag2Te / CeO2 sensitive material prepared in Example 1 of this invention;

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

[0029] Figure 3 : A schematic diagram of the plant internet environment monitoring system constructed based on the nitrogen oxide sensor of Ag2Te / CeO2 sensitive material as described in Example 2 of this invention;

[0030] Figure 4 The real-time curve of the resistance value of the sensor prepared in Example 1 as a function of nitrogen dioxide concentration at an operating temperature of 65°C;

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

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

[0033] Figure 7 The graph shows the changes in responsivity and resistance of the sensor prepared in Example 1 at an operating temperature of 65°C with relative humidity.

[0034] Figure 8 The plant internet environment monitoring system described in this invention provides a real-time curve of the resistance between the two gold electrodes of the sensor after 96 hours of nitrogen fertilizer application in a simulated agricultural greenhouse environment as described in Example 3.

[0035] Figure 9 The plant internet environment monitoring system described in this invention provides a real-time curve of the resistance between the two gold electrodes of the sensor after 96 hours of pesticide application in a simulated agricultural greenhouse environment as described in Example 4.

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

[0037] like Figure 2 As shown, the nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material described in Embodiment 1 of the present invention consists of an Al2O3 ceramic tube substrate with parallel and independent annular gold electrodes at both ends of its 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 inside the Al2O3 ceramic tube; the Ag2Te / CeO2 sensitive material must completely cover the surface of the Al2O3 ceramic tube and the annular gold electrodes.

[0038] like Figure 3 As shown, where Figure 3 Figure a shows the circuit diagram of the plant internet environment monitoring system constructed based on the Ag2Te / CeO2 sensitive material nitrogen oxide sensor described in Embodiment 2 of this invention. This system consists of an IoT device and an internet service. The IoT device comprises 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. In the IoT device, the battery module performs three functions: first, it powers the system circuit to ensure normal operation; second, it powers the heating coil to provide the sensor with a working temperature of 65°C; and finally, it provides 5V to the voltage divider circuit formed by the sensor and the grounding resistor R1 connected in series. The grounding capacitor uses a 100NF capacitance to effectively filter out noise 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 analog voltage signal across the series resistor R1 and converts it into a digital voltage signal V. ADC Then, by pre-writing a program into the ESP32 chip according to formula R... 传感器 = (5 × R1) ÷ V ADC –R1, calculates the digital signal R of the resistance value of the Ag2Te / CeO2 sensing material between the two gold electrodes of the sensor. 传感器The FLASH module is a non-volatile memory; the program within the ESP32 chip is stored in the FLASH memory and can be executed after power is applied. The 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 bidirectional communication line. The TIMER module is an internal timer / counter that controls the ESP32 chip to perform periodic data acquisition and transmission, specifically measuring the voltage signal across the series resistor R1 and the digital signal of the resistance value R1 once per second. 传感器 (transmission). Figure 3 Figure 'b' shows the web client interface of the message queue telemetry transmission agent service. This is a web-based application; when a user opens a browser to access the client, the program is automatically downloaded from the web client's server and begins running. The corresponding service is deployed in the Dcloud service of the web client. Users can use this interface to start or stop the display of real-time data and export data to Excel. The real-time data display box shows the resistance value of the sensitive material between the two gold electrodes of the sensor on the vertical axis and the time on the horizontal axis. Users can obtain real-time monitoring data from the sensor, namely the digital signal R of the resistance value between the two gold electrodes of the sensor. 传感器 Furthermore, the sensor's responsivity can be calculated using the responsivity formula. By substituting this into the relationship curve of "sensor responsivity - nitrogen oxide concentration", an estimated concentration of nitrogen oxides can be obtained, thus 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 responsivity of the sensor prepared in Example 1 increases with the increase of nitrogen dioxide concentration, and it has a lower detection limit (5ppb) and a wider detection range (5ppb~10ppm).

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

[0041] like Figure 6 As shown, the sensor prepared in Example 1 exhibits 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 for monitoring nitrogen oxides in high humidity environments.

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

[0044] like Figure 9 As shown, the plant internet environmental monitoring system developed in Example 2 monitored nitrogen oxides in real time 96 hours after pesticide application in the simulated agricultural greenhouse environment of Example 4. The curves clearly distinguish the different concentrations of nitrogen oxides produced in the simulated agricultural greenhouse after applying different amounts of pesticides. Resistance value (R) g It is positively correlated with the concentration of nitrogen oxides.

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

[0046] Example 1: Synthesis of Ag2Te / CeO2 Sensitive Material and Preparation of Sensor

[0047] 1. First, slowly add 480 mmol of sodium hydroxide (NaOH) in 5 equal portions to a beaker containing 65 mL of deionized water, and stir in an ice bath for 60 minutes to form a homogeneous 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 homogeneous solution, then add the solution obtained in step (1) dropwise, and continue stirring under ice bath conditions for 60 minutes to form a homogeneous solution;

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

[0050] 4. Add 172 mg of CeO2 powder obtained in step (3) to 30 mL of anhydrous ethylenediamine and stir at 25 °C for 0.5 hours to form a homogeneous solution. Then add 170 mg of silver nitrate (AgNO3) and 80 mg of tellurium dioxide (TeO2) to the solution in sequence and continue stirring for 1 hour.

[0051] 5. Add 10 mL of hydrazine hydrate (N2H4·H2O, 85%, w / w%) quickly to the solution obtained in step (4) and continue stirring at 25°C for 24 hours; centrifuge the solution obtained above, and wash the precipitate obtained by centrifugation with deionized water and ethanol alternately several times until the pH of the supernatant after centrifugation is 7. Then dry it under vacuum at 60°C for 12 hours to obtain 344 mg of black solid powder, which is Ag2Te / CeO2 sensitive material. The microstructure is cubic cerium oxide attached to irregular blocky silver telluride to form a hollow roll structure.

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

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

[0054] Example 2: Construction and Application of a Plant Internet Environmental Monitoring System Based on a Nitrogen Oxide Sensor Using Ag2Te / CeO2 Sensitive Material

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

[0056] 2. Internet service: First, the digital signal obtained in step (1) is transmitted by the Wi-Fi module of the Internet of Things device to the message queuing telemetry transport (MQTT) proxy service (this 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 agricultural greenhouses. Users can then take greenhouse management measures such as ventilation to ensure the normal growth of plants.

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

[0058] Under sunlight and high humidity conditions, dissolve different amounts of nitrogen fertilizer (urea) in 10 mL of deionized water and then evenly add it to the soil containing cucumber seedlings (soil area 750 cm²). 2 In a simulated greenhouse (maintaining 25℃ and 60% relative humidity) after nitrogen fertilizer addition, a plant internet-based environmental monitoring system was used to monitor nitrogen oxide levels in the simulated greenhouse after 96 hours. First, an 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 removed from the simulated greenhouse and placed in the air. The entire monitoring process took 200 seconds. The system output measurement data is as follows: Figure 8As shown, the different concentrations of nitrogen oxides produced in the simulated agricultural greenhouse after applying different amounts of nitrogen fertilizer can be clearly distinguished. Based on the system monitoring data, it can be estimated that after applying 1.0g, 5.0g, and 10.0g of urea for 96 hours, the nitrogen oxide concentrations in the simulated greenhouse are 4ppb, 14ppb, and 59ppb, respectively.

[0059] Example 4: Monitoring of nitrogen oxides after pesticide application 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 containing cucumber seedlings (soil area 750 cm²). 2 In a simulated greenhouse (maintaining 25℃ and 60% relative humidity) after nitrogen fertilizer addition, a plant internet-based environmental monitoring system was used to monitor nitrogen oxide levels in the simulated greenhouse after 96 hours. First, an 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 removed from the simulated greenhouse and placed in the air. The entire monitoring process took 200 seconds. The system output measurement data is as follows: Figure 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. Based on the system monitoring data, it can be estimated that after applying 0.5g, 1.0g, and 2.0g of imidacloprid for 96 hours, the nitrogen oxide concentrations in the simulated greenhouse are 4ppb, 10ppb, and 35ppb, respectively.

Claims

1. A nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material, which is composed of an Al2O3 ceramic tube substrate with parallel and independent ring-shaped gold electrodes on 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 is slowly added to 65-70 mL of deionized water in multiple portions, and stirred under ice bath conditions for 30-60 minutes to form a uniform solution; (2) 4-10 mmol of cerium nitrate is added to 10-15 mL of deionized water, stirred at 20-30°C for 30-60 minutes to form a uniform solution, then the solution is added dropwise to the solution obtained in step (1), and continues to be stirred under ice bath conditions for 60-80 minutes to form a uniform solution; (3) the solution obtained in step (2) is transferred to a polytetrafluoroethylene-lined stainless steel autoclave, and reacted at 160-180°C for 12-24 hours; after natural cooling to room temperature, the reaction product is centrifuged, the obtained precipitate is washed with deionized water and ethanol alternately for several times until the supernatant pH=7 after centrifugation, and then dried at 60-80°C under vacuum for 10-12 hours to obtain a light yellow CeO2 powder; (4) 43-344 mg of the 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, and after a uniform solution is formed, 160-180 mg of silver nitrate and 70-90 mg of tellurium dioxide are sequentially added, and continue to stir for 0.5-2.0 hours; (5) 8-15 mL of 85 w / w% of hydrazine hydrate is quickly added to the solution obtained in step (4), and continues to be stirred at 20-30°C for 20-30 hours; the obtained solution is centrifuged, the obtained precipitate is washed with deionized water and ethanol alternately for several times until the supernatant pH=7 after centrifugation, and then dried at 60-80°C under vacuum for 10-12 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 according to claim 1, characterized in that: The Al2O3 ceramic tube has a length of 4-4.5 mm, an outer diameter of 1.2-1.5 mm, and an inner diameter of 0.8-1.0 mm; the single annular gold electrode has a width of 0.4-0.5 mm, the distance between the two gold electrodes is 0.5-0.6 mm, and the thickness of the gold electrode is 0.1-0.2 mm, and a platinum wire is led out on the gold electrode, and the length of the platinum wire is 4-6 mm.

3. A preparation method of a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material according to claim 1 or 2, and the steps are as follows: (1) 10-15 mg of Ag2Te / CeO2 sensitive material is placed into an agate mortar, 1-2 mL of deionized water is then added dropwise, and the mixture is mixed and ground to form a paste-like slurry; then the slurry is dipped to uniformly coat the surface of the Al2O3 ceramic tube substrate with parallel and independent annular gold electrodes at both ends, to form a 10-30 μm thick sensitive material film, and the sensitive material completely covers the annular gold electrode and the surface of the Al2O3 ceramic tube; (2) The Al2O3 ceramic tube obtained in step (1) is baked at 50-80 DEG C for 15-30 minutes, after the sensitive material is dried, a nickel-chromium heating coil with a resistance of 25-35 Ω is passed through the inside of the Al2O3 ceramic tube as a heating wire, finally the device is welded on the hexagonal tube seat according to the side heating type gas sensitive element, thereby obtaining a nitrogen oxide sensor based on Ag2Te / CeO2 sensitive material.

4. The plant internet environment monitoring system based on the nitrogen oxide sensor constructed by the Ag2Te / CeO2 sensitive material according to claim 1 or 2, characterized in that: The monitoring system is composed of an Internet of Things device and an Internet service, (1) The Internet of Things device is composed of a nitrogen oxide sensor, a voltage divider circuit, an ESP32 chip, and a battery module. The ESP32 chip is integrated with 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 ground resistance R1 of a known resistance value to form a voltage dividing circuit, and a voltage of VCC=5V is applied across the voltage dividing circuit; one end of the series resistance 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 resistance R1 is grounded; the voltage analog signal across the series resistance R1 is measured by the analog-to-digital converter ADC module of the nitrogen oxide sensor and converted into a voltage digital signal V ADC , and then the program written in advance in the ESP32 chip is used to calculate the resistance value digital signal R 传感器 of the Ag2Te / CeO2 sensitive material between the two gold electrodes of the sensor according to the formula R 传感器 =(5×R1)÷V ADC –R1; the resistance value digital signal R 传感器 is wirelessly transmitted to the message queue telemetry transport agent service by the Wi-Fi module of the ESP32 chip; (2) Internet service: users log in to the message queue telemetry transport agent service webpage client interface on different mobile terminals or computer terminals to obtain real-time monitoring data of the sensor, that is, the resistance value digital signal R between the two gold electrodes of the sensor 传感器 The response of the sensor is calculated by the response formula, and the concentration of nitrogen oxides is estimated by substituting the relationship curve of "sensor response-nitrogen oxide concentration". This provides information support for farmers to take greenhouse management measures such as ventilation.

5. The plant internet environment monitoring system based on the Ag2Te / CeO2 sensitive material of claim 4, wherein: First, the resistance value of Ag2Te / CeO2 sensitive material between two gold electrodes of the sensor under a plurality of known concentrations of nitrogen oxides is measured, then the responsivity of the sensor under these concentrations is calculated, and the relationship curve of "sensor responsivity-nitrogen oxide concentration" is established by computer origin software; the measurement of the resistance value of Ag2Te / CeO2 sensitive material between two gold electrodes of the sensor is that the Ag2Te / CeO2 sensitive material between two gold electrodes of the sensor is connected in series with a ground resistance R1 of a known resistance value to form a voltage dividing circuit, a voltage of VCC=5V is applied to both ends of the voltage dividing circuit, then the voltage V across the ground resistance R1 is measured R1 , the resistance value R g of Ag2Te / CeO2 sensitive material between two gold electrodes of the sensor is obtained through the formula R R1 =(5×R1)÷V g -R1, and the responsivity of the sensor is calculated through the formula; the responsivity is defined as the ratio of the absolute value of the difference between the resistance value R a of the sensitive material between two gold electrodes of the sensor in air and the resistance value R g of the sensitive material in the measured nitrogen oxide gas to R a , that is, S=(|R a -R g | / R a )*100%.

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

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