High-selectivity gas sensitive element for NO2 gas detection at room temperature and preparation method of high-selectivity gas sensitive element

Abstract

The invention discloses a high-selectivity gas sensitive element for detecting NO2 gas at room temperature and a preparation method of the high-selectivity gas sensitive element. The high-selectivity gas sensitive element comprises an electrode plate and a black titanium dioxide sensing film uniformly loaded on the surface of the electrode plate. The preparation method comprises the following steps: adding TiO2 into a solvent to form slurry, uniformly dispensing the slurry on an electrode plate, then uniformly and slowly heating the electrode plate to remove the solvent, repeating for multiple times, and forming a TiO2 coating with required thickness on the surface of the electrode plate; and annealing the electrode plate coated with the TiO2 coating in vacuum, and forming a black titanium dioxide sensing film on the surface of the electrode plate. The black titanium dioxide sensing film has oxygen vacancies, NO2 detection is achieved at the room temperature, the detection sensitivity is high, the detection limit is low, the selectivity is high, rapid and efficient NO2 detection is achieved, the detection cost is low, and the method is suitable for application and popularization.

Description

Technical Field

[0001] The present invention relates to the technical field of gas sensors, and particularly to a highly selective gas-sensitive element for NO2 gas detection at room temperature and a preparation method thereof. Background Art

[0002] Nitrogen dioxide (NO2) is a common harmful gas, mainly derived from the combustion of fossil fuels, vehicle exhaust emissions, and industrial production processes. In the atmosphere, it not only causes the formation of acid rain but also undergoes a photochemical reaction with volatile organic compounds (VOCs) under sunlight irradiation to generate ozone and fine particulate matter (PM 2.5 ), thus exacerbating air pollution and reducing air quality. More seriously, NO2 poses a significant threat to human health. Long-term exposure to high concentrations of NO2 can lead to respiratory diseases, cardiovascular diseases, and even increase the risk of cancer. In addition, NO2 also has a negative impact on plant growth, inhibiting the yield and quality of crops and damaging the ecosystem. Monitoring Requirements and Technical Challenges With the increasing attention to environmental protection and health, the real-time and accurate monitoring of NO2 concentration in the air has become particularly important. Traditional NO2 detection methods, such as chemiluminescence method, spectrophotometry, etc., although having high sensitivity and accuracy, have the disadvantages of complex equipment, cumbersome operation, high cost, and difficulty in realizing on-site rapid detection. Therefore, developing a sensor for NO2 gas that is efficient, sensitive, and highly selective at room temperature is of great significance for fields such as environmental monitoring, industrial safety, and health protection.

[0003] Although traditional metal oxide semiconductor gas sensors have advantages such as fast response and easy integration, they usually require high-temperature operation, which leads to increased energy consumption and device complexity. In addition, existing sensors all suffer from the problems of lack of gas selectivity and moisture resistance, which prevent the sensors from effectively identifying NO2 in mixed gases and humid environments. Some studies have obtained room-temperature NO2 response and improved the gas selectivity to NO2 to a certain extent by preparing composites or doping, but still cannot obtain the gas recognition function for NO2, and do not have moisture resistance, and the preparation of composites will increase the production cost of the device. Based on this need, the present invention proposes a new room-temperature NO2 gas detection technology, aiming to overcome the deficiencies of the existing technology through innovative material design, achieve high-sensitivity, high-selectivity, and low-detection-limit detection of NO2 gas, and provide a method for improving the response sensitivity, selectivity, and moisture resistance of NO2. Summary of the Invention

[0004] The object of the present invention is to provide a highly selective gas sensor element for NO2 gas detection at room temperature, its preparation method, and a method for improving the response sensitivity, selectivity, and moisture resistance of NO2 detection. The technical problem to be solved is the low selectivity of the existing gas sensor elements of metal oxides to NO2 gas.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A highly selective gas sensor element for NO2 gas detection at room temperature, comprising an electrode sheet and a black TiO2 sensing film uniformly loaded on the surface of the electrode sheet. The black TiO2 sensing film is prepared by annealing a TiO2 coating under vacuum conditions.

[0007] A preparation method of a highly selective gas sensor element for NO2 gas detection at room temperature, specifically comprising the following steps:

[0008] (1) Add TiO2 to a solvent to form a slurry, uniformly drop the slurry on the electrode sheet, and then slowly heat the electrode sheet on a hot plate to remove the solvent. Repeat this process multiple times to form a TiO2 coating with the desired thickness on the surface of the electrode sheet;

[0009] (2) Anneal the electrode sheet coated with the TiO2 coating in a vacuum to form a black TiO2 sensing film on the surface of the electrode sheet.

[0010] Preferably, in step (2), the annealing temperature is 450 - 550 °C, and the annealing time is 2 - 6 hours. More preferably, the annealing temperature is 500 °C, and the annealing time is 4 hours.

[0011] Preferably, the TiO2 in step (1) is nano-TiO2 with a burr structure on its surface. Its preparation method is specifically as follows: First, add (NH4)2TiF6 and H3BO3 to water to form a mixed solution, and then immerse ZnO in the above solution at room temperature until a white product appears at the bottom of the solution. Centrifuge and wash to obtain nano-TiO2 with a burr structure on its surface. The molar ratio of (NH4)2TiF6, H3BO3, and ZnO is 1:(2.8 - 3.3):(0.2 - 0.25).

[0012] The solvent described in step (1) includes but is not limited to deionized water and ethanol.

[0013] In step (1), the heating temperature of the electrode sheet on the hot plate is 20 - 40 °C lower than the boiling point of the used solvent. At this temperature, the electrode sheet can be slowly heated to avoid too fast or too high heating temperature, and defects such as cracks are likely to appear in the sensing film.

[0014] Preferably, the number of repetitions in step (1) is 2 - 8 times.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) The preparation method is simple, and a black TiO2 sensing film can be obtained only by annealing under vacuum conditions; (2) The black TiO2 sensing film has more oxygen vacancies, can detect NO2 at room temperature, has high detection sensitivity and low detection limit, and the detection limit is as low as the sub-ppb level; (3) It shows excellent selectivity to NO2 gas and can effectively distinguish other common gases; (4) It realizes the rapid and efficient detection of NO2, has low detection cost, and is suitable for popularization and application. Description of the Drawings

[0016] Figure 1 Are the X-ray diffraction (XRD) patterns of W-TiO2 and B-TiO2.

[0017] Figure 2 Are the SEM images of W-TiO2 and B-TiO2.

[0018] Figure 3 Are the electron paramagnetic resonance (EPR) spectra of W-TiO2 and B-TiO2.

[0019] Figure 4 In (a) is a comparison chart of the selectivity of W-TiO2 and B-TiO2 to NO2 gas and other gases; (b) is the response of W-TiO2 to 0.5 ppm NO2 under different humidity conditions; (c) is the response of B-TiO2 to 10 ppm NO2 under different humidity conditions. Detailed Embodiments

[0020] The following is further described in conjunction with the drawings and specific embodiments.

[0021] Example 1

[0022] A preparation method of a highly selective gas sensor element for NO2 gas detection at room temperature, comprising the following steps:

[0023] (1) First, (NH4)2TiF6 and H3BO3 are added to water to form a mixed solution, and then ZnO is immersed in the above solution at room temperature for 20 - 50 hours. A white product appears at the bottom of the solution. The molar ratio of (NH4)2TiF6, H3BO3, and ZnO is 1:3:0.2. The product is separated from the reaction solution by centrifugation and rinsed three times with deionized water (DI) to finally obtain nano-TiO2 with a burr structure on the surface.

[0024] (2) TiO2 is mixed with deionized water to form a slurry, and 3 μL of the slurry is evenly drop-coated on an electrode sheet (such as an alumina ceramic substrate). After drop-coating, the electrode sheet is heated on a hot plate at 70 °C for 5 minutes to remove moisture, and this process is repeated three times to form a TiO2 coating on the surface of the electrode sheet.

[0025] (3) The electrode sheet coated with TiO2 coating was annealed at 500 °C in air for 4 hours to obtain white TiO2, denoted as W-TiO2.

[0026] Example 2

[0027] In this example, all steps except step (3) are the same as those in Example 1.

[0028] (3) The electrode sheet coated with TiO2 coating was annealed at 500 °C in vacuum for 4 hours to obtain black TiO2, denoted as B-TiO2.

[0029] The X-ray diffraction (XRD) patterns of W-TiO2 and B-TiO2 are as Figure 1 shown. All diffraction peaks in the XRD patterns of the two samples can be clearly attributed to the anatase structure of TiO2 (JCPDS card number 21-1272), and no additional peaks and phases are observed. Compared with the two, the diffraction peaks of B-TiO2 are more obvious, proving that its crystallinity is better.

[0030] Figure 2 SEM of W-TiO2 and B-TiO2. After annealing in different environments, the SEM of the obtained products is basically the same, and the burr-like structure of TiO2 is retained in both annealed W-TiO2 and B-TiO2, which improves the specific surface area of the surface of the gas sensor element.

[0031] Electron paramagnetic resonance (EPR) spectroscopy tests were carried out on W-TiO2 and B-TiO2. As Figure 3 shown, B-TiO2 shows a strong EPR signal at g = 2.004, which is identified as electrons trapped in oxygen vacancies, while this signal can hardly be observed in W-TiO2, confirming that the oxygen vacancies in B-TiO2 are more than those in W-TiO2.

[0032] The sensing performances of the sensors based on W-TiO2 and B-TiO2 towards NO2 and several common interfering gases were tested at room temperature. The electrode sheets with W-TiO2 or B-TiO2 loaded on the surface prepared above were placed in a closed space filled with the gas to be measured, and the response of the sensor was measured. The gases to be measured were ethanol, isopropanol, acetone, carbon dioxide (CO2), hydrogen (H2), formaldehyde and NO2. The response of the sensor is defined as follows: when R a > R g , response = (R a - R g ) / R g × 100%; when R a < R g , response = (R g - R a ) / Ra × 100% (R a and R g respectively represent the sensor output resistance before and after gas exposure).

[0033] Figure 4 Figure a is a radar chart comparing the selectivity of W-TiO2 and B-TiO2 to NO2 gas and other gases, summarizing the responses of W-TiO2 and B-TiO2 to ethanol, isopropanol, acetone, carbon dioxide (CO2), hydrogen (H2), formaldehyde and NO2. It is found that the responses of W-TiO2 and B-TiO2 to 3 ppm NO2 are 20.9% and 250% respectively. The response of B-TiO2 is 12 times that of W-TiO2. In addition to the improvement in response, the selectivity of B-TiO2 to NO2 is also significantly improved. B-TiO2 inhibits the responses to the above-mentioned ethanol, isopropanol, acetone, carbon dioxide (CO2), hydrogen (H2), formaldehyde gases and has almost no response to ethanol, isopropanol, acetone, carbon dioxide (CO2), hydrogen (H2), formaldehyde. Compared with W-TiO2, the B-TiO2 sensor shows ultra-high selectivity to NO2 and can realize the identification of NO2 in a mixed atmosphere of 7 gases.

[0034] In addition, the sensing performance of W-TiO2 and B-TiO2 to NO2 under different humidity conditions ( Figure 4 Figures b and c) shows that the response of W-TiO2 to NO2 increases with the increase of humidity ( Figure 4 Figure b). In contrast, the response of B-TiO2 to NO2 is not affected by the moisture in the atmosphere in the range of relative humidity from 30% to 80% ( Figure 4 Figure c). Compared with W-TiO2, B-TiO2 shows significant anti-humidity interference ability when detecting NO2.

Claims

1. A highly selective gas sensor for NO2 gas detection at room temperature, characterized in that, It includes an electrode sheet and a black titanium dioxide sensing film uniformly loaded on the surface of the electrode sheet. The black titanium dioxide sensing film is prepared by annealing a TiO2 coating under vacuum conditions.

2. A preparation method of a highly selective gas sensor element for NO2 gas detection at room temperature, characterized in that, Specifically, it includes the following steps: (1) Add TiO2 to a solvent to form a slurry, uniformly drip the slurry on the electrode sheet, and then uniformly and slowly heat the electrode sheet to remove the solvent. Repeat this process multiple times to form a TiO2 coating with the desired thickness on the surface of the electrode sheet; (2) Anneal the electrode sheet coated with the TiO2 coating in a vacuum to form a black TiO2 sensing film on the surface of the electrode sheet.

3. The preparation method of the highly selective gas-sensitive element for NO2 gas detection at room temperature according to claim 2, characterized in that, In step (2), the annealing temperature is 450 - 550 °C, and the annealing time is 2 - 6 hours.

4. The preparation method of the highly selective gas-sensitive element for NO2 gas detection at room temperature according to claim 2, wherein, The annealing temperature is 500 °C, and the annealing time is 4 hours.

5. The preparation method of the highly selective gas sensor element for NO2 gas detection at room temperature according to claim 2, characterized in that, In step (1), the TiO2 is nano-TiO2 with a burr structure on its surface. Its preparation method is specifically as follows: First, add (NH4)2TiF6 and H3BO3 to water to form a mixed solution, and then immerse ZnO in the above solution at room temperature until white products appear at the bottom of the solution. Centrifuge and wash to obtain nano-TiO2 with a burr structure on its surface. The molar ratio of (NH4)2TiF6, H3BO3, and ZnO is 1:(2.8 - 3.3):(0.2 - 0.25).

6. The preparation method of the highly selective gas-sensitive element for NO2 gas detection at room temperature according to claim 2, characterized in that, The solvent described in step (1) is deionized water and ethanol.

7. The preparation method of the highly selective gas-sensitive element for NO2 gas detection at room temperature according to claim 2, characterized in that, In step (1), the heating temperature of the electrode sheet on the hot plate is 20 - 40 °C lower than the boiling point of the solvent used.

8. A method for improving the response sensitivity, selectivity, and moisture resistance of NO2 detection, characterized in that, Increase the oxygen vacancy content of TiO2 by controlling the annealing conditions.

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