A sulfur dioxide gas sensor and a method for manufacturing the same

Abstract

The application is suitable for the technical field of sulfur dioxide gas monitoring, and provides a sulfur dioxide gas sensor and a preparation method thereof, which comprises the following steps: dissolving polyvinylpyrrolidone in N,N-dimethylformamide, stirring, adding a tungsten source, stirring and dissolving, adding a tin source, continuing to stir, obtaining a precursor spinning solution, electrospinning to obtain a composite nanofiber membrane, obtaining a SnO2-WO3 composite nanofiber sensitive material through step-by-step high-temperature calcination, and then assembling to obtain a gas sensor. 3 The application successfully realizes efficient detection of SO2 at room temperature by constructing SnO2-WO3 n-n heterojunction and introducing rich oxygen vacancies, has a wide detection range of 800 ppb-20 ppm for SO2, has a response time of 28 seconds and a recovery time of 122 seconds for 2 ppm SO2, and has good reusability.

Description

Technical Field

[0001] This invention belongs to the field of sulfur dioxide gas monitoring technology, and particularly relates to a sulfur dioxide gas sensor and its preparation method. Background Technology

[0002] In smart agriculture systems, agricultural activities such as soil pyrolysis remediation generate sulfur dioxide (SO2) gas. Even at extremely low concentrations, SO2 can damage crops, inhibit photosynthesis, and threaten human health. Therefore, real-time and accurate monitoring of SO2 is crucial for ensuring agricultural production safety. Currently, commercially available sulfur dioxide gas sensors in the environmental monitoring field are mainly based on metal oxide semiconductor sensitive materials. A typical product structure is as follows: a ceramic substrate (tubular or flat) is coated with a single metal oxide sensitive layer, with gold electrodes at both ends of the sensitive layer to extract resistance signals. The working process of this type of sensor is as follows: first, the sensitive material is heated to 200-400℃ using a heating wire; at this high temperature, oxygen in the air is chemically adsorbed onto the surface of the sensitive material. The oxygen then extracts electrons from the semiconductor conduction band to form oxygen negative ions (such as O2). - O - This leads to an increase in the material's resistance. When SO2 molecules in the gas being measured come into contact with the surface of the sensitive material, SO2 undergoes a redox reaction with oxygen anions, releasing electrons back into the sensitive material, causing a decrease in the material's resistance. The resistance change across the gold electrode is measured in real time by an external circuit, and the SO2 concentration can be deduced from the calibration curve. In addition, the preparation of this type of sensor usually adopts hydrothermal synthesis, template method, or sol-gel method. Sometimes, it is also necessary to introduce noble metals such as gold and platinum for doping modification to improve performance.

[0003] However, the aforementioned high-temperature operating mode results in extremely high device power consumption (typically hundreds of milliwatts to several watts), and the slow heating and cooling process causes the sensor's response recovery time to be as long as several minutes. At the same time, the existing manufacturing process is complex and costly, and batch consistency is difficult to guarantee, which cannot meet the monitoring needs of distributed, low-power, wireless sensor network deployment in vast farmlands. More importantly, this high-temperature sensor has poor selectivity for sulfur dioxide and is easily affected by interfering gases such as ammonia, hydrogen sulfide, and nitrogen oxides in the agricultural environment, often causing false alarms. After a false alarm, manual verification or data retrieval is required, which is inefficient.

[0004] Existing metal oxide semiconductor gas sensors, such as single-material sensors like pure tungsten oxide, cannot effectively detect sulfur dioxide at room temperature. This is because gas adsorption and redox reactions in these sensors require activation energy provided by high temperatures. At room temperature, the material's response is extremely low or even nonexistent. High-temperature operation is considered an unavoidable technical characteristic of metal oxide gas sensors, and those skilled in the art generally believe that only through heating can sufficient sensitivity and response speed be obtained. Although existing research has attempted to lower the operating temperature through nanostructures and noble metal doping, heating above 100°C is still required, making true room-temperature operation impossible. Summary of the Invention

[0005] The purpose of this invention is to provide a sulfur dioxide gas sensor and its preparation method, thereby addressing the problems mentioned in the background section.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] On one hand, the present invention provides a method for preparing a sulfur dioxide gas sensor, comprising the following steps:

[0008] S1. Preparation of precursor spinning solution: Dissolve polyvinylpyrrolidone in N,N-dimethylformamide, heat and stir to obtain a transparent and viscous polymer solution. Then, add a tungsten source to the polymer solution and stir to dissolve it completely. Next, add a tin source under continuous stirring and continue stirring. Then let it stand and degas to obtain a uniform, transparent and viscous precursor spinning solution.

[0009] S2. Electrospinning: The precursor spinning solution is injected into a syringe for electrospinning. Under the action of a high voltage electric field, the jet flows through stretching, bending, and solvent evaporation, and is collected on a receiver to obtain a continuous and uniform composite nanofiber membrane.

[0010] S3. Drying and stepwise high-temperature calcination: The composite nanofiber membrane is dried by blowing air, and then stepwise high-temperature calcination is carried out in an inert gas atmosphere. After completion, it is naturally cooled to room temperature to obtain SnO2-WO3 composite nanofiber sensitive material.

[0011] S4. Sensor Assembly: Grind the SnO2-WO3 composite nanofiber sensitive material into a uniform slurry, coat it onto the surface of a ceramic substrate with gold electrodes, and weld leads to make a gas sensor. Alternatively, cut the SnO2-WO3 composite nanofiber sensitive material into regular shapes, connect copper foil at both ends with conductive gel to serve as electrodes, and make a gas sensor.

[0012] On the other hand, the present invention also provides a sulfur dioxide gas sensor, which is prepared by the above-described preparation method.

[0013] On the other hand, the present invention also provides an application of a sulfur dioxide gas sensor in the detection of sulfur dioxide at room temperature.

[0014] Compared with the prior art, the specific beneficial effects of the present invention are as follows:

[0015] This invention utilizes a tight composite of SnO2 nanoparticles and WO3 nanofibers to create a sensitive material. Both are n-type metal oxide semiconductors. Upon contact, the two phases can spontaneously form a SnO2-WO3 n-type heterojunction. The specific interface formation mechanism is as follows: SnO2 and WO3 have different intrinsic work functions, with the Fermi level of WO3 being lower than that of SnO2. After the two phases are tightly bonded, the Fermi levels on both sides of the interface cannot maintain equilibrium. Free electrons in SnO2 will migrate across the interface to the WO3 side. After the electrons migrate out, ionized donor positive charges remain in the SnO2 interface region, while excess electrons accumulate in the WO3 interface region. Electron depletion layers are formed simultaneously at the interface of the two phases. Charge migration continues until the Fermi levels of the two phases tend to converge, and the charge exchange process reaches equilibrium. A stable built-in electric field is generated at the interface, thereby forming an n-type heterojunction interface with a double depletion layer structure.

[0016] The heterojunction interface is key to the efficient detection of sulfur dioxide at room temperature in the sensor prepared by this invention. When the sensor is placed in an air environment, oxygen molecules in the air capture free electrons on the surface of the sensitive material and within the depletion layer, forming adsorbed oxygen ions, further thickening the interface depletion layer and causing the sensor to exhibit a high baseline resistance. When SO2, the analyte gas, is introduced into the test chamber, gas molecules diffuse along the porous fiber network to the heterojunction active interface, where they undergo a redox reaction with the adsorbed oxygen on the surface. The captured electrons are released back into the semiconductor conduction band, reducing the thickness of the interface depletion layer and consequently lowering the sensor resistance. The built-in electric field generated at the heterojunction interface can reduce the activation energy required for the SO2 redox reaction, allowing the gas-sensitive reaction to occur without external high-temperature heating, thus eliminating the dependence of traditional metal oxide sensors on high-temperature operating conditions. Simultaneously, the interwoven porous nanofiber structure prepared by electrospinning provides abundant gas diffusion channels, fully exposing heterogeneous active sites and synergistically improving the sensor's response amplitude, response recovery rate, and cycle stability.

[0017] The sensor prepared by this invention has a wide detection range of SO2 from 0.8ppm to 15ppm, with a detection limit as low as 0.8ppm. Its response time to 2ppm SO2 is as fast as 28 seconds, and its recovery time is as fast as 122 seconds, far superior to existing room-temperature SO2 sensors. After six repeated tests, the relative standard deviation of the response value is less than 2.01%, demonstrating excellent reproducibility. Furthermore, this invention employs a one-step electrospinning method combined with a controllable calcination process, making the raw materials readily available, the operation simple, and the morphology controllable. It avoids complex hydrothermal synthesis and expensive precious metal doping, facilitating industrial-scale mass production and significantly reducing manufacturing costs. This invention has practical significance in scenarios requiring real-time detection of sulfur dioxide, such as smart agriculture and distributed environmental monitoring. Attached Figure Description

[0018] Figure 1 A flowchart illustrating the fabrication process of a sulfur dioxide gas sensor provided in an embodiment of the present invention;

[0019] Figure 2 A schematic diagram of an electrospinning apparatus provided in an embodiment of the present invention;

[0020] Figure 3 SEM morphology images and EDS elemental surface distribution characterization results of a series of SnO2-WO3 composite nanofiber sensitive materials prepared for the examples are shown below. Specifically, a is a SEM image of 1wt% low-doped SnO2-WO3 composite nanofibers (scale bar 1 μm); b is a SEM image of 2wt% medium-doped SnO2-WO3 composite nanofibers (scale bar 2 μm); c is a SEM image of 5wt% high-doped SnO2-WO3 composite nanofibers (scale bar 3 μm); d is a SEM image of pure WO3 nanofibers (scale bar 5 μm); e is a SEM image of 2wt% medium-doped SnO2-WO3 composite nanofibers (scale bar 20 μm); f is an EDS surface scan mapping of superimposed W, Sn, and O elements; g, h, and i are, respectively, independent surface distribution maps of W, Sn, and O elements. The scale bar for all EDS surface scan images is 1 μm.

[0021] Figure 4 This is a schematic diagram of the band structure and sensing mechanism of the SnO2-WO3 heterojunction in SO2 provided in an embodiment of the present invention;

[0022] Figure 5 Comparison curves of the dynamic response of sensors with different Sn doping levels to SO2 provided in embodiments of the present invention;

[0023] Figure 6 This is a typical single-response curve of the sensor prepared in Example 1 of the present invention for 2 ppm SO2;

[0024] Figure 7The graph shows the repeatability test results of the sensor prepared in Example 1 of this invention under 5 ppm sulfur dioxide.

[0025] Figure 8 This is a diagram showing the detection width of the sensor prepared in Embodiment 1 of the present invention;

[0026] Figure 9 The baseline resistance curve of the sensor prepared in Embodiment 1 of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0029] Example 1: A sulfur dioxide gas sensor, the preparation process of which is as follows: Figure 1 As shown, the specific steps include:

[0030] S1. Preparation of precursor spinning solution:

[0031] First, the high molecular weight polymer polyvinylpyrrolidone (PVP, Mw≈1.3×10⁻⁶) is added. 6 PVP was dissolved in N,N-dimethylformamide (DMF) at a mass-to-volume ratio of 1 g : 8 mL. The mixture was magnetically stirred at 60 °C for 2 hours to obtain a transparent and viscous polymer solution. Then, 0.9 g of ammonium metatungstate (AMT), a tungsten source, was added to the solution and stirred at 40 °C for 1 hour to completely dissolve it. Next, tin chloride dihydrate (SnCl2·2H2O) was added under continuous stirring, with the amount added so that the final SnO2 content of WO3 was 2 wt%. The mixture was stirred for another hour. Finally, the mixture was allowed to stand for 24 hours to remove bubbles, resulting in a uniform, transparent, and viscous precursor spinning solution.

[0032] Step 2, electrospinning and forming:

[0033] like Figure 2 As shown, the precursor spinning solution was injected into a syringe equipped with a stainless steel needle with an inner diameter of 0.8 mm and fixed on an electrospinning device. The electrospinning process parameters were set as follows: positive voltage +20 kV, negative voltage -3 kV, receiving distance 15 cm, feed rate 0.8 mL / h, ambient temperature 25 ℃, and relative humidity 30%. Under the action of a high voltage electric field, the jet flow was stretched, bent, and the solvent evaporated, and finally a continuous and uniform composite nanofiber membrane was collected on the receiver.

[0034] Step 3, Drying and Stepwise High-Temperature Calcination:

[0035] The collected composite nanofiber membrane was dried in a 120℃ forced-air drying oven for 2 hours to completely remove residual organic solvents. Then, the dried fiber membrane was placed in a tube furnace and subjected to stepwise high-temperature calcination in an inert gas (nitrogen) atmosphere. The calcination program was as follows: first, the temperature was increased to 300℃ at a rate of 2℃ / min and held for 1 hour; then, the temperature was increased to 500℃ at a rate of 2℃ / min and held for 2 hours. During this process, the low-temperature section decomposed and removed the polymer template, and the high-temperature section allowed the tin source and tungsten source to be fully oxidized and crystallized, respectively, and converted into SnO2 and WO3, forming a tightly contacted n-type heterojunction in situ. At the same time, abundant oxygen vacancies were introduced. After the program was completed, the membrane was naturally cooled to room temperature to obtain the SnO2-WO3 composite nanofiber sensitive material.

[0036] Step 4, Sensor Assembly:

[0037] The SnO2-WO3 composite nanofiber sensitive material is ground into a uniform slurry, coated onto the surface of a ceramic substrate (or ceramic tube) with gold electrodes, and wires are welded to form a side-heated or flat-panel gas sensor. Alternatively, the SnO2-WO3 composite nanofiber sensitive material can be directly cut into a regular shape of 2cm×1cm, and copper foil is connected to both ends through conductive gel to serve as electrodes, thus completing the gas sensor assembly.

[0038] Performance Analysis:

[0039] 1. Four groups of samples, including the SnO2-WO3 composite nanofiber sensitive material prepared in step 3 of Example 1, were analyzed, and scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) images were obtained as follows: Figure 3 As shown, Figure 3 As shown in Figure 1ae, the microstructure comparison of pure WO3, 1 wt% low-doped, 2 wt% medium-doped, and 5 wt% high-doped samples intuitively reflects the regulatory effect of Sn doping amount on the microstructure of nanofibers: the surface of pure WO3 fibers is smooth and lacks heterojunction active interfaces; the 1 wt% low-doped sample has too little SnO2 particle loading and insufficient number of heterojunctions; the 5 wt% high-doped sample shows obvious SnO2 particle agglomeration, which blocks fiber pores and reduces gas adsorption sites; while in the 2 wt% SnO2-WO3 composite nanofibers prepared in Example 1, SnO2 nanoparticles are uniformly dispersed on the fiber surface without large-scale agglomeration, and the one-dimensional fibers intertwine to form a through porous network, which can provide sufficient heterojunction interfaces and gas active sites, making it the optimal microstructure sensing structure;

[0040] like Figure 3As shown in Figure 1, the EDS elemental surface scan results of the sample prepared in Example 1 can verify the uniformity of the two-phase composite: the signals of the three elements W, Sn, and O completely cover the fiber region, the single-element mapping profile is highly matched with the fiber morphology, and there is no local element enrichment or loss, proving that SnO2 is uniformly loaded on the WO3 fiber matrix, and the two phases are tightly combined to form a uniformly distributed nn heterostructure, which provides structural support for the excellent cyclic repeatability and gas-sensitive response performance of the sensor.

[0041] 2. The working process and detection principle of the sensor prepared in step 4 are explained in detail below:

[0042] The sensor operates as follows: The assembled sensor is placed in a gas-sensitive testing system. At room temperature (20-25℃), the SnO2-WO3 composite nanofiber sensitive material is exposed to air. At this time, oxygen is chemically adsorbed on the surface of the material. Due to the built-in electric field at the SnO2-WO3 n-n heterojunction interface, electrons are easily transferred, resulting in a certain baseline resistance (Ra) of the material. When the test gas containing SO2 is introduced, SO2 molecules diffuse into the porous nanofiber network and undergo a redox reaction with the oxygen species adsorbed on the surface of the material. This reaction releases electrons back into the sensitive material, causing the material resistance to decrease (Rg). The resistance change can be measured in real time using a digital source meter, and the response value can be calculated according to the formula S = (Rg-Ra) / Ra × 100%.

[0043] A schematic diagram of the band structure and sensing mechanism of the SnO2-WO3n-n heterojunction in SO2 is shown below. Figure 4 As shown, its core principle is: SnO2 and WO3 are n-type semiconductors, and when they come into contact, they form an n-n heterojunction. Due to the difference in work function, electrons migrate from the side with a higher Fermi level to the other side, forming a built-in electric field and an electron depletion layer at the interface. This built-in electric field significantly reduces the activation energy of SO2 adsorption and redox reactions, allowing the reaction to proceed efficiently at room temperature. At the same time, the oxygen vacancies introduced by calcination provide more active adsorption sites. The two work together to achieve high sensitivity and rapid response to SO2 at room temperature.

[0044] 3. Performance test results:

[0045] like Figure 5 As shown, the dynamic response curves of sensors to SO2 with different Sn doping amounts (0 wt%, 1 wt%, 2 wt%, 5 wt%, the preparation process is the same as in Example 1, and the mass percentage of SnO2 in WO3 is controlled by adjusting the amount of tin source dihydrate added). It can be seen that the sensor with a doping amount of 2 wt% has the highest response value, indicating that this ratio is the optimal doping amount.

[0046] like Figure 6 As shown, the sensor prepared in Example 1 exhibits a typical single-response-recovery curve for 2 ppm SO2. At room temperature, the sensor has a response time of 28 seconds and a recovery time of 122 seconds, demonstrating excellent rapid detection capability.

[0047] like Figure 8 As shown, the detection width curve of the sensor prepared in Example 1 is shown. In the experiment, SO2 concentrations of 0.8ppm, 1ppm, 2ppm, 5ppm, 10ppm and 15ppm were introduced in sequence. The sensor can generate a stable and progressively increasing response signal in the concentration range of 0.8ppm to 15ppm. There is no obvious saturation in the whole range. It has good resolution for both high and low concentrations. It has a wider detection range than traditional high temperature metal oxide sensors and can cover monitoring scenarios of light to moderate sulfur dioxide pollution.

[0048] like Figure 9 As shown, this is a baseline resistance curve of the sensor prepared in Example 1. Baseline resistance is a fundamental indicator of the stable operation of a gas sensor. This curve reflects the resistance change trend of the device when it is placed in a clean air environment for a long time. It proves that the SnO2-WO3 composite nanofiber sensitive material of the present invention has small baseline drift and stable resistance in a room temperature air atmosphere, with no obvious continuous rise or fall phenomenon. The sensor has excellent static working stability and can ensure that the calibration accuracy does not shift during long-term continuous monitoring, thus reducing the data error of long-term monitoring.

[0049] 4. Repeatability test results:

[0050] To verify the stability and reliability of the sensor in practical applications, the sensor prepared in Example 1 was subjected to a repeatability test. The test method was as follows: the sensor was alternately exposed to 5 ppm SO2 gas and air, and the response-recovery cycle was repeated 6 times. Figure 7 As shown, six consecutive SO2 adsorption-desorption cycles were completed under a 5 ppm sulfur dioxide atmosphere. The response amplitude remained highly stable throughout the multiple cycles, with a relative standard deviation (RSD) of only 2.01%. RSD = (sample standard deviation / average of 6 responses) × 100% (after completing six 5 ppm SO2 adsorption-desorption cycles, all raw response peak data were exported from the testing equipment, and the average of the six responses and the sample standard deviation were calculated. Substituting these values ​​into the RSD calculation formula, the final relative standard deviation of 2.01% was obtained).

[0051] From a mechanistic perspective, the above-mentioned cyclic repeatability results demonstrate that the dynamic electron exchange process at the SnO2-WO3 nn heterojunction interface is highly reversible. In addition, the porous network structure formed by the interwoven fibers can effectively avoid the structural collapse and irreversible chemical adsorption problems that are common in traditional metal oxide semiconductor (MOS) sensors during continuous operation, and can meet the stringent operating conditions required for long-term practical use of the device.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing a sulfur dioxide gas sensor, characterized by, Includes the following steps: S1. Preparation of precursor spinning solution: Dissolve polyvinylpyrrolidone in N,N-dimethylformamide, heat and stir to obtain a transparent and viscous polymer solution. Then, add a tungsten source to the polymer solution and stir to dissolve it completely. Next, add a tin source under continuous stirring and continue stirring. Then let it stand and degas to obtain a uniform, transparent and viscous precursor spinning solution. S2. Electrospinning: The precursor spinning solution is injected into a syringe for electrospinning. Under the action of a high voltage electric field, the jet flows through stretching, bending, and solvent evaporation, and is collected on a receiver to obtain a continuous and uniform composite nanofiber membrane. S3. Drying and stepwise high-temperature calcination: The composite nanofiber membrane is dried by blowing air, and then stepwise high-temperature calcination is carried out in an inert gas atmosphere. After completion, it is naturally cooled to room temperature to obtain SnO2-WO3 composite nanofiber sensitive material. S4. Sensor Assembly: Grind the SnO2-WO3 composite nanofiber sensitive material into a uniform slurry, coat it onto the surface of a ceramic substrate with gold electrodes, and weld leads to make a gas sensor. Alternatively, cut the SnO2-WO3 composite nanofiber sensitive material into regular shapes, connect copper foil at both ends with conductive gel to serve as electrodes, and make a gas sensor.

2. The method for producing a sulfur dioxide gas sensor according to claim 1, characterized by, In S1, the tungsten source is ammonium metatungstate, and the tin source is tin chloride dihydrate.

3. The method for producing a sulfur dioxide gas sensor according to claim 1, characterized by, In S2, the electrospinning parameters are as follows: positive voltage 15-20kV, negative voltage -3kV, receiving distance 15-20cm, feed rate 0.8mL / h, ambient temperature 25℃, and relative humidity 30%.

4. The method for producing a sulfur dioxide gas sensor according to claim 1, characterized by, In S3, the specific process of stepwise high-temperature calcination is as follows: first, the temperature is increased to 300°C at a rate of 2°C / min and held for 1 hour; then, the temperature is increased to 500°C at a rate of 2°C / min and held for 2 hours.

5. The method for producing a sulfur dioxide gas sensor according to claim 1, characterized by, In the SnO2-WO3 composite nanofiber sensitive material, the mass percentage of SnO2 in WO3 is 1 wt%, 2 wt%, or 5 wt%.

6. The method for producing a sulfur dioxide gas sensor according to claim 1, characterized by, In the SnO2-WO3 composite nanofiber sensitive material, SnO2 accounts for 2 wt% of WO3 by mass.

7. A sulfur dioxide gas sensor characterized by comprising: It is prepared using the preparation method described in any one of claims 1-6.

8. An application of the sulfur dioxide gas sensor as described in claim 7 in the detection of sulfur dioxide at room temperature.

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