Indium oxide nanofilm, PdO@S-1 molecular sieve membrane, molecular sieve / indium oxide double-layer membrane, double-layer membrane structure gas sensor

Abstract

The application belongs to the technical field of gas sensors, and particularly relates to an indium oxide nanometer film, a PdO@S-1 molecular sieve film, a molecular sieve / indium oxide double-layer film, and a double-layer film structure gas sensor. The indium oxide nanometer film provided by the application has controllable film thickness, good consistency, a porous structure and a large specific surface area, is beneficial to gas diffusion, can realize rapid adsorption and desorption, and provides an effective sensitive material for developing a high-performance NO2 sensor. The nano PdO is loaded in the channel of the S-1 molecular sieve in the application, the stability of the PdO is improved by using the channel confinement effect of the molecular sieve material, the nano PdO agglomeration under high-temperature working conditions is avoided, the stability of the sensor is improved, and the adsorption properties and gas-sensitive reaction activity of NO2 are also improved. The PdO@S-1 molecular sieve film can significantly improve the sensitivity, humidity interference resistance and stability of the indium oxide nanometer film, and has good application prospects in the detection of NO2.

Description

Technical Field

[0001] This invention belongs to the field of gas sensor technology, specifically relating to indium oxide nanofilms, PdO@S-1 molecular sieve membranes and their preparation methods and applications, molecular sieve / indium oxide bilayer membranes and their preparation methods and applications, and bilayer membrane structure gas sensors and their applications. Background Technology

[0002] NO2 is a toxic pollutant gas, reddish-brown to brown in color, with an irritating odor. It irritates the eyes, throat, and respiratory tract, causing difficulty breathing and coughing. Long-term exposure to NO2 can lead to lung disease and exacerbate existing respiratory conditions, causing serious harm to humans and animals. However, low concentrations of NO2 are difficult to detect. Real-time monitoring and reduction of NO2 emissions can not only improve air quality and protect public health but also help reduce environmental problems such as acid rain and photochemical smog. Therefore, developing efficient and reliable NO2 gas sensors for monitoring trace (ppb level) NO2 in the atmospheric environment is of great significance.

[0003] Chemi-resistive gas sensors based on metal-oxide-semiconductor (MOS) semiconductors have shown potential application value in NO2 atmospheric pollutant detection due to their advantages such as low cost, suitability for silicon-based integrated devices, and ease of miniaturization. In₂O₃, as a typical n-type semiconductor, is considered a high-performance sensitive material for NO₂ gas detection. When In₂O₃ comes into contact with the oxidizing gas NO₂, the DC resistance of the In₂O₃ sensitive material increases, converting changes in atmospheric gas concentration into a detectable electrical signal. For example, Chinese patent CN112760603A discloses a porous columnar indium oxide gas-sensitive film with a thickness of 400 nm. At an operating temperature of 150 °C, its sensitivities to 1 ppm, 5 ppm, 10 ppm, 20 ppm, and 50 ppm nitrogen dioxide are 1.47, 3.01, 7.95, 15.09, and 19.7, respectively. However, its sensitivity to trace amounts (ppb level) of NO₂ is not high enough. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide indium oxide nanofilms, PdO@S-1 molecular sieve membranes, their preparation methods and applications, molecular sieve / indium oxide bilayer membranes, their preparation methods and applications, and bilayer membrane structure gas sensors and their applications. The indium oxide nanofilms, molecular sieve / indium oxide bilayer membranes, and bilayer membrane structure gas sensors provided by this invention exhibit high detection sensitivity for trace NO2.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solutions.

[0006] The present invention provides an indium oxide nanofilm with a thickness of 0.5~2.3μm, wherein the indium oxide nanofilm has a porous structure.

[0007] The present invention provides a PdO@S-1 molecular sieve membrane, comprising an S-1 molecular sieve and PdO nanoparticles loaded in the pores of the S-1 molecular sieve.

[0008] Preferably, the thickness of the PdO@S-1 molecular sieve membrane is 10~200μm, and the loading of PdO nanoparticles in the PdO@S-1 molecular sieve membrane is 1~3wt%.

[0009] This invention provides a method for preparing the PdO@S-1 molecular sieve membrane described above, comprising the following steps: (1) A water-soluble palladium source, ethylenediamine and water are mixed and subjected to a complexation reaction to obtain a palladium complex solution; (2) Tetrapropylammonium hydroxide, tetraethyl orthosilicate and water are mixed to obtain a silicon-template mixture; (3) The palladium complex solution and the silicon-template mixture were mixed and subjected to hydrothermal reaction and calcination in sequence to obtain PdO@S-1 molecular sieve; (4) The PdO@S-1 molecular sieve is mixed with isopropanol, coated and sintered to obtain a PdO@S-1 molecular sieve membrane; Steps (1) and (2) are not in any chronological order.

[0010] Preferably, in step (1), the mass ratio of the water-soluble palladium source, ethylenediamine, and water is 1~10:4~40:600~1200; In step (2), the mass ratio of tetrapropylammonium hydroxide, tetraethyl orthosilicate, and water is 3~8:2~6:5~12; the complexation reaction takes 0.5~4.5 h and is carried out under ultrasonic conditions; In step (3), the mass ratio of the water-soluble palladium source in the palladium complex solution to the tetraethyl orthosilicate in the silicon-template mixture is 1~10:200~600; the hydrothermal reaction temperature is 160~180℃ and the time is 36~72h; the calcination temperature is 400~600℃ and the time is 4~8h. In step (4), the sintering temperature is 200~350℃ and the time is 1~3h.

[0011] This invention provides a molecular sieve / indium oxide bilayer membrane, comprising a sensitive layer and a gas sieving / adsorption layer located on the upper surface of the sensitive layer; the sensitive layer is the indium oxide nanofilm described in the above technical solution; the gas sieving / adsorption layer is the PdO@S-1 molecular sieve membrane described in the above technical solution or the PdO@S-1 molecular sieve membrane prepared by the preparation method described in the above technical solution.

[0012] This invention provides a method for preparing the molecular sieve / indium oxide bilayer membrane described above, comprising the following steps: (1) A water-soluble palladium source, ethylenediamine and water are mixed and subjected to a complexation reaction to obtain a palladium complex solution; (2) Tetrapropylammonium hydroxide, tetraethyl orthosilicate and water are mixed to obtain a silicon-template mixture; (3) The palladium complex solution and the silicon-template mixture were mixed and subjected to hydrothermal reaction and calcination in sequence to obtain PdO@S-1 molecular sieve; (4) The PdO@S-1 molecular sieve is mixed with isopropanol, coated on the surface of indium oxide nanofilm, and then sintered to obtain a molecular sieve / indium oxide bilayer film. Steps (1) and (2) are not in any chronological order.

[0013] Preferably, in step (1), the mass ratio of the water-soluble palladium source, ethylenediamine, and water is 1~10:4~40:600~1200; In step (2), the mass ratio of tetrapropylammonium hydroxide, tetraethyl orthosilicate, and water is 3~8:2~6:5~12; the complexation reaction takes 0.5~4.5 h and is carried out under ultrasonic conditions; In step (3), the mass ratio of the water-soluble palladium source in the palladium complex solution to the tetraethyl orthosilicate in the silicon-template mixture is 1~10:200~600; the hydrothermal reaction temperature is 160~180℃ and the time is 36~72h; the calcination temperature is 400~600℃ and the time is 4~8h. In step (4), the sintering temperature is 200~350℃ and the time is 1~3h.

[0014] This invention provides a dual-layer film structure gas sensor, including a ceramic planar substrate 1; Interdigitated electrodes 2 and bilayer film 3 are sequentially stacked on the upper surface of the ceramic planar substrate 1, and electrode pads 51 are electrically connected to the interdigitated electrodes 2; the bilayer film 3 is the molecular sieve / indium oxide bilayer film described in the above technical solution or the molecular sieve / indium oxide bilayer film prepared by the preparation method described in the above technical solution. The serpentine heating electrode 4 is located on the lower surface of the ceramic planar substrate 1, and the electrode pad 52 is electrically connected to the serpentine heating electrode 4.

[0015] This invention provides the indium oxide nanofilm described in the above-mentioned technical solutions, the PdO@S-1 molecular sieve membrane described in the above-mentioned technical solutions or the PdO@S-1 molecular sieve membrane prepared by the preparation method described in the above-mentioned technical solutions, the molecular sieve / indium oxide bilayer membrane described in the above-mentioned technical solutions or the molecular sieve / indium oxide bilayer membrane prepared by the preparation method described in the above-mentioned technical solutions, and the application of the bilayer membrane structure gas sensor described in the above-mentioned technical solutions in NO2 detection.

[0016] The In2O3 nanofilm provided by this invention has controllable and consistent film thickness, and possesses a porous structure and large specific surface area, which is conducive to gas diffusion and enables rapid adsorption and desorption. It provides an effective sensitive material for developing high-performance NO2 sensors and improves the detection sensitivity of NO2.

[0017] This invention loads noble metal PdO nanoparticles into the pores of S-1 molecular sieve. By utilizing the pore confinement effect of the molecular sieve material, the stability of the noble metal PdO nanoparticles is improved, and the aggregation of PdO nanoparticles under high-temperature conditions is avoided, thereby improving the stability of the sensor. This invention also utilizes the high catalytic activity of noble metal PdO nanoparticles to improve the gas-sensing reaction activity of the bilayer membrane structure sensor.

[0018] The molecular sieve / indium oxide bilayer membrane provided by this invention combines the advantages of In2O3 nanofilms and PdO@S-1 molecular sieve membranes. The S-1 molecular sieve membrane, which confines and loads PdO nanoparticles, can significantly improve the sensitivity of the gas sensor. Furthermore, by coating the In2O3 sensitive layer with a PdO@S-1 molecular sieve membrane, this invention improves the humidity interference resistance and long-term stability of the In2O3-based NO2 sensor, and effectively addresses the poor interference resistance and susceptibility to light exposure of the In2O3 sensitive material. This allows the bilayer NO2 sensor to meet the needs of various regions and weather conditions. The gas sensor based on the molecular sieve / indium oxide bilayer membrane provided by this invention exhibits high sensitivity, high selectivity, and high stability in NO2 detection, and shows promising application prospects in monitoring NO2 gas in various environments.

[0019] Moreover, the bilayer membrane structure strategy proposed in this invention has a certain degree of universality. Printing molecular sieve membranes onto the surface of oxide semiconductor sensitive membranes can significantly improve the sensitivity, stability, and moisture resistance of semiconductor gas sensors.

[0020] As shown in the test results of the embodiments, the improved double-layer film gas sensor (PdO@S-1 / In2O3 (n-type semiconductor)) of the present invention has a response that is about 4.1 times higher than that of the single-layer In2O3 sensitive film sensor (In2O3 (n-type semiconductor)), with high sensitivity and significant improvements in moisture resistance and long-term stability. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a ceramic planar substrate for a NO2 gas sensor based on a double-layer film structure. In the diagram, a is a schematic diagram of the upper surface structure of the ceramic planar substrate, and b is a schematic diagram of the lower surface structure of the ceramic planar substrate. In the diagram, 1 is the ceramic planar substrate, 2 is the interdigitated electrode, 3 is the double-layer film, 31 is the In2O3 nanofilm, 32 is the PdO@S-1 molecular sieve film, 4 is the serpentine heating electrode, 51 is the upper electrode pad on the upper surface of the ceramic planar substrate, and 52 is the lower electrode pad on the lower surface of the ceramic planar substrate. Figure 2 SEM image of In2O3 nanofilm; Figure 3 TEM image of PdO@S-1 molecular sieve material; Figure 4 SEM images of In2O3(1) type sensor and PdO@S-1 / In2O3(2) type sensor are shown. Among them, a and b are SEM images of In2O3(1) type sensor, c is a cross-sectional SEM image of In2O3(1) type sensor, and d is a cross-sectional SEM image of PdO@S-1 / In2O3(2) type sensor. Figure 5 XRD patterns of In2O3 nanofilm and S-1 and PdO@S-1 molecular sieves are shown, where a is In2O3 nanofilm and b is PdO@S-1 molecular sieve. Figure 6 The sensitivity curves (a) and baseline resistance curve (b) of the In2O3(1) type sensor, In2O3(2) type sensor, PdO@S-1 / In2O3(1) type sensor, and PdO@S-1 / In2O3(2) type sensor to 5ppm NO2 gas are shown. Figure 7 The curves showing the relationship between the NO2 concentration and the resistance change between the gold electrodes for testing NO2 gas at a working temperature of 65℃ using a PdO@S-1 / In2O3(2) type sensor for 5~10000ppb NO2 gas. Figure 8 The curve showing the relationship between the sensitivity S of the PdO@S-1 / In2O3(2) sensor and the NO2 gas concentration in an atmosphere of 5~10000ppb NO2 at a working temperature of 65℃. Figure 9 The resistance change curve between the gold interdigitated electrodes of the PdO@S-1 / In2O3(2) sensor under 5ppm NO2 gas cycling test for 6 cycles at a working temperature of 65℃. Figure 10The response curves of In2O3(2) type sensor and PdO@S-1 / In2O3(2) type sensor to 1ppm NO2 gas at different humidity (35~80 RH%) at an operating temperature of 65℃; Figure 11 The long-term stability curves of the In2O3(2) type sensor and the PdO@S-1 / In2O3(2) type sensor for 1ppm NO2 gas were obtained after half a month at an operating temperature of 65℃. Detailed Implementation

[0022] This invention provides an indium oxide nanofilm with a thickness of 0.5~2.3 μm, preferably 1.5~2.3 μm, and more preferably 1.8~2 μm; the indium oxide nanofilm has a porous structure. In this invention, the particle size of the indium oxide nanoparticles in the indium oxide nanofilm is preferably 30~200 nm, and more preferably 70~150 nm.

[0023] In this invention, the preferred method for preparing the indium oxide nanofilm includes the following steps: mixing an indium source, ethylene glycol methyl ether, and ethanolamine to obtain an In2O3 sol-gel precursor solution; and sequentially coating, allowing to stand, and sintering the In2O3 sol-gel precursor solution to obtain an indium oxide nanofilm on the substrate surface.

[0024] Unless otherwise specified, all materials and equipment used in this invention are commercially available products in the field.

[0025] This invention mixes an indium source, ethylene glycol methyl ether, and ethanolamine to obtain an In₂O₃ sol-gel precursor solution. In this invention, the indium source preferably includes indium nitrate and / or indium chloride, and the indium nitrate is preferably used in the form of In(NO₃)₃·4.5H₂O. In this invention, the solid-liquid ratio of the indium source, ethylene glycol methyl ether, and ethanolamine is preferably 0.5~2.5g:1~20mL:0.1~2mL, more preferably 1~2g:3~15mL:0.15~1.5mL, further preferably 1.3~1.6g:4~10mL:0.2~0.5mL, and most preferably 1.43g:5mL:0.23mL. In this invention, the mixing preferably includes sequential stirring and settling, the stirring time is 0.5~5h, more preferably 1~2h; the settling time is preferably 5~30h, more preferably 8~12h; and the mixing temperature is preferably room temperature.

[0026] After obtaining the In2O3 sol-gel precursor solution, this invention sequentially coats, allows to stand and dry, and sintersects the In2O3 sol-gel precursor solution to obtain an indium oxide nanofilm on the substrate surface. This invention does not have a specific limitation on the substrate used for coating; any substrate well-known to those skilled in the art can be used, such as an interdigitated electrode. The substrate is preferably subjected to plasma hydrophilic treatment before use. This invention does not have a specific limitation on the conditions for plasma hydrophilic treatment; any plasma hydrophilic treatment conditions well-known to those skilled in the art can be used. In this invention, the coating is preferably spin-coated. In this invention, the temperature for standing and drying is preferably room temperature, and the standing and drying time is preferably 30-50 min, more preferably 40-50 min. In this invention, the number of times the sequential coating, standing and drying, and sintering are performed is preferably 1-7 times, more preferably 2-6 times, and even more preferably 2-3 times. This invention does not have a specific limitation on the amount of In2O3 sol-gel precursor solution used in each coating, as long as an In2O3 film with a thickness of 0.3-1 μm (more preferably 0.85 μm) is obtained. In this invention, the sintering temperature is preferably 300~500℃, more preferably 300~350℃, and the sintering time is preferably 1~3h, more preferably 1~2h.

[0027] This invention prepares In2O3 nanofilms using the sol-gel method. The film thickness is controllable and has good consistency. It also has a porous structure and a large specific surface area, which is conducive to gas diffusion and enables rapid adsorption and desorption. This provides an effective sensitive material for the development of high-performance NO2 sensors.

[0028] This invention provides a PdO@S-1 molecular sieve membrane, comprising an S-1 molecular sieve and PdO nanoparticles loaded in the pores of the S-1 molecular sieve. In this invention, the thickness of the PdO@S-1 molecular sieve membrane is preferably 10-200 μm, more preferably 50-150 μm, and even more preferably 100-120 μm; the loading of PdO nanoparticles in the PdO@S-1 molecular sieve membrane is preferably 1-3 wt%, more preferably 1.5-2.5 wt%.

[0029] In this invention, the preparation method of the PdO@S-1 molecular sieve membrane includes the following steps: (1) A water-soluble palladium source, ethylenediamine and water are mixed and subjected to a complexation reaction to obtain a palladium complex solution; (2) Tetrapropylammonium hydroxide, tetraethyl orthosilicate and water are mixed to obtain a silicon-template mixture; (3) The palladium complex solution and the silicon-template mixture were mixed and subjected to hydrothermal reaction and calcination in sequence to obtain PdO@S-1 molecular sieve; (4) The PdO@S-1 molecular sieve is mixed with isopropanol, coated and sintered to obtain a PdO@S-1 molecular sieve membrane; Steps (1) and (2) are not in any chronological order.

[0030] This invention involves mixing a water-soluble palladium source, ethylenediamine, and water to perform a complexation reaction, yielding a palladium complex solution. In this invention, the water-soluble palladium source preferably includes palladium nitrate and / or palladium chloride; the palladium nitrate is preferably used in the form of Pd(NO3)2·2H2O. In this invention, the mass ratio of the water-soluble palladium source, ethylenediamine, and water is preferably 1~10:4~40:600~1200, more preferably 3~8:10~35:700~1100, further preferably 4~6:20~32:800~1050, and most preferably 5:30:1000. This invention does not have specific limitations on the mixing process; as long as the raw materials are mixed uniformly, such as through ultrasonic mixing. In this invention, the temperature of the complexation reaction is preferably room temperature, and the reaction time is preferably 0.5~4.5 h, more preferably 1~3 h, and further preferably 2 h; the complexation reaction is preferably carried out under ultrasonic conditions.

[0031] This invention involves mixing tetrapropylammonium hydroxide, tetraethyl orthosilicate, and water to obtain a silicon-template mixture. In this invention, the preferred mass ratio of tetrapropylammonium hydroxide, tetraethyl orthosilicate, and water is 3~8:2~6:5~12, more preferably 5~7:4~5.5:8~11, and even more preferably 6:5:10. In this invention, the mixing temperature is preferably room temperature, and the mixing time is preferably 2~9 hours, more preferably 2~4 hours; the mixing is preferably performed by stirring.

[0032] After obtaining the palladium complex solution and the silicon-template mixture, the present invention mixes the palladium complex solution and the silicon-template mixture, and sequentially performs a hydrothermal reaction and calcination to obtain PdO@S-1 molecular sieve. In the present invention, the mixing temperature is preferably room temperature, the mixing time is preferably 0.5~1.5 h, more preferably 1 h; the mixing is preferably stirred. In the present invention, the mass ratio of the water-soluble palladium source to tetraethyl orthosilicate is preferably 1~10:200~600, more preferably 3~8:300~550, further preferably 4~6:400~520, and most preferably 5:500. In the present invention, the hydrothermal reaction temperature is preferably 160~180℃, more preferably 175~180℃; the hydrothermal reaction time is preferably 36~72 h, more preferably 40~50 h; the hydrothermal reaction is preferably a static hydrothermal reaction.

[0033] Following the hydrothermal reaction, the present invention preferably further includes solid-liquid separation of the obtained hydrothermal reaction liquid, washing the obtained solid product alternately with water and ethanol, drying it, and then subjecting the dried product to subsequent calcination. The present invention does not have specific limitations on the solid-liquid separation; any solid-liquid separation method well known to those skilled in the art can be used, such as filtration, vacuum filtration, or centrifugation. In the present invention, the washing is preferably centrifugal washing, and the number of washing cycles is not specifically limited; washing until neutral is sufficient. In the present invention, the drying temperature is preferably 70~100℃, more preferably 80~90℃, and the drying time is not specifically limited; drying until constant weight is achieved is sufficient.

[0034] In this invention, the calcination temperature is preferably 400~600℃, more preferably 450~500℃, and the calcination time is preferably 4~8h, more preferably 4.5~5h.

[0035] In this invention, the PdO@S-1 molecular sieve is preferably hexagonal nanoparticles, and the side length of the PdO@S-1 molecular sieve is preferably 130~160nm, more preferably 150nm; the diameter of the PdO@S-1 molecular sieve is preferably 200~300nm, more preferably 250nm.

[0036] After obtaining the PdO@S-1 molecular sieve, the present invention mixes the PdO@S-1 molecular sieve with isopropanol, coats it, and then sintersects it to obtain a PdO@S-1 molecular sieve membrane. In the present invention, the mass ratio of the PdO@S-1 molecular sieve to isopropanol is preferably 5~20:20~100, more preferably 8~15:20~50, further preferably 9~12:20~30, and most preferably 1:2. In the present invention, the mixing is preferably achieved through grinding. In the present invention, the sintering temperature is preferably 200~350℃, more preferably 250~350℃; the sintering time is preferably 1~3h, more preferably 2h.

[0037] This invention provides a molecular sieve / indium oxide bilayer membrane, characterized by comprising a sensitive layer and a gas sieving / adsorption layer located on the upper surface of the sensitive layer; the sensitive layer is the indium oxide nanofilm described in the above-described technical solution; the gas sieving / adsorption layer is the PdO@S-1 molecular sieve membrane described in the above-described technical solution or a PdO@S-1 molecular sieve membrane prepared by the preparation method described in the above-described technical solution. In this invention, the thickness of the indium oxide nanofilm is 0.5~2.3 μm, preferably 1.5~2.3 μm, and more preferably 1.8~2 μm.

[0038] This invention provides a method for preparing the molecular sieve / indium oxide bilayer membrane described above, comprising the following steps: (1) A water-soluble palladium source, ethylenediamine and water are mixed and subjected to a complexation reaction to obtain a palladium complex solution; (2) Tetrapropylammonium hydroxide, tetraethyl orthosilicate and water are mixed to obtain a silicon-template mixture; (3) The palladium complex solution and the silicon-template mixture were mixed and subjected to hydrothermal reaction and calcination in sequence to obtain PdO@S-1 molecular sieve; (4) The PdO@S-1 molecular sieve is mixed with isopropanol, coated on the surface of indium oxide nanofilm, and then sintered to obtain a molecular sieve / indium oxide bilayer film. Steps (1) and (2) are not in any chronological order.

[0039] In this invention, the preparation conditions of the molecular sieve / indium oxide bilayer membrane are the same as those of the PdO@S-1 molecular sieve membrane, and will not be repeated here.

[0040] The molecular sieve / indium oxide bilayer membrane preparation process provided by this invention is simple, easy to follow, highly reliable, and has low production cost, making it suitable for mass production in industry.

[0041] This invention provides a double-layer membrane structure gas sensor, the structural schematic of which is shown in the figure. Figure 1 As shown below, in conjunction with Figure 1 A detailed description of the double-layer membrane structure gas sensor is provided.

[0042] The dual-layer film structure gas sensor provided by the present invention includes a ceramic planar substrate 1; Interdigitated electrodes 2 and bilayer films 3 are sequentially stacked on the upper surface of the ceramic planar substrate 1, with an upper electrode pad 51 electrically connected to the interdigitated electrodes 2; the bilayer film 3 is a molecular sieve / indium oxide bilayer film as described in the above technical solution or a molecular sieve / indium oxide bilayer film prepared by the preparation method described in the above technical solution; the molecular sieve / indium oxide bilayer film includes an indium oxide nanofilm 31 in contact with the interdigitated electrodes 2 and a PdO@S-1 molecular sieve film 32 located on the surface of the indium oxide nanofilm 31; the interdigitated electrodes 2 are preferably interdigitated gold electrodes; The serpentine heating electrode 4 is located on the lower surface of the ceramic planar substrate 1, and the lower electrode pad 52 is electrically connected to the serpentine heating electrode 4; the serpentine heating electrode 4 is preferably a serpentine Pt heating electrode.

[0043] In this invention, the thickness of the indium oxide nanofilm 31 is 0.5~2.3μm, preferably 1.5~2.3μm, and more preferably 1.8~2μm.

[0044] The double-layer membrane gas sensor provided by this invention uses a serpentine heating electrode to provide the required operating temperature during operation. NO2 concentration is detected by measuring the DC resistance of the In2O3 nanofilm (In2O3 sensitive layer) in different gases. The working principle is as follows: When the double-layer membrane gas sensor is placed in air, oxygen molecules adsorb onto the surface of the In2O3 sensitive layer, capturing electrons from the conduction band to form oxygen negative ions (O2). - (ads) In this process, O2 acts as an electron acceptor, causing a decrease in the electron concentration and an increase in the resistance of the sensor's sensitive layer. When detecting NO2 gas, the sensor comes into contact with the oxidizing gas NO2, which in turn reacts with ionized adsorbed oxygen (O2). - (ads) Competitive adsorption occurs, as shown in Equation 1. Due to the strong oxidizing property of NO2 gas molecules, NO2 can not only replace the ionic adsorbed oxygen O2 adsorbed on the surface of the sensitive layer, but also... - (ads) It can also adsorb onto oxygen vacancy defect sites on the surface of indium oxide materials, further capturing electrons in the semiconductor conduction band to form NO2. - (ads) This further reduces the electron concentration and increases the sensor's resistance; on the other hand, it reacts with adsorbed oxygen on the surface of the sensitive layer, as shown in Equation 2, where the adsorbed ionic oxygen (O2) - (ads) The indium oxide (IO) reacts chemically with NO2 molecules. During this process, the surface electrons of the IO semiconductor material are depleted, forming an electron-depleted layer on the surface of the sensitive material, which increases the sensor's resistance. The change in the resistivity of the sensitive material is converted into an electrical signal by the sensor and received by the measuring end, thereby achieving the purpose of detecting NO2.

[0045] That is, the DC resistance R between the two gold electrodes is measured by a resistance meter when the resistance of the sensor is basically stable in air and NO2 gas of different concentrations. a (Resistance of the sensor when it is stable in air) and R g (The resistance of the sensor when it is stable in NO2 gas of different concentrations) The sensitivity of the sensor at different concentrations is calculated as S=R g / R a This leads to the establishment of a "concentration-sensitivity relationship curve"; then, the DC resistance between two interdigitated electrodes in an atmosphere of unknown concentration is measured using a resistance meter, and the result is obtained through S=R g / R a Once the sensor sensitivity value at this concentration is calculated, the NO2 gas concentration can be calculated using the "concentration vs. sensitivity curve".

[0046] The PdO@S-1 molecular sieve membrane (gas sieving / adsorption layer, protective layer) can block water vapor molecules in the environment, reducing their interference with the In2O3 sensitive layer and the NO2 gas-sensitive reaction process, thus improving the sensor's stability under different humidity levels. Furthermore, coating the In2O3 sensitive film surface with a molecular sieve membrane helps to fix the active sites of the In2O3 sensitive material, improving the sensor's long-term stability. Simultaneously, utilizing the shape-selective sieving properties of the molecular sieve, it prevents various interfering gases in the air from contacting the indium oxide sensitive layer, allowing only NO2 gas molecules to pass through the molecular sieve membrane and react with the sensitive layer. On the other hand, the PdO nanoparticles confined within the S-1 molecular sieve are beneficial for NO2 adsorption, and their catalytic effect can dissociate NO2 into more reactive nitrogen and oxygen species, lowering the activation energy of the gas-sensitive reaction and improving the sensor's sensitivity to extremely low concentrations of NO2.

[0047] In this invention, the method for fabricating the double-layer membrane gas sensor preferably includes the following steps: The In2O3 sol-gel precursor solution is coated onto the surface of the interdigitated electrode 2 located on the surface of the ceramic planar substrate 1, and sintered after standing to form a planar device with an indium oxide nanofilm 31. The substrate includes a serpentine heating electrode 4 and a lower electrode pad 52 electrically connected to the serpentine heating electrode on the lower surface of the ceramic planar substrate 1; the substrate includes an upper electrode pad 51 electrically connected to the interdigitated electrode 2 on the upper surface of the ceramic planar substrate 1. The PdO@S-1 molecular sieve is mixed with isopropanol, coated onto the surface of the indium oxide nanofilm of the planar device with indium oxide nanofilm, and then sintered to form a PdO@S-1 molecular sieve membrane 32 on the surface of the indium oxide nanofilm, thus obtaining a planar device with a molecular sieve / indium oxide bilayer membrane. The planar device with molecular sieve / indium oxide bilayer membrane is soldered to the device base by means of wires, and then aged to obtain a bilayer membrane structure gas sensor.

[0048] In this invention, the In2O3 sol-gel precursor solution is the aforementioned In2O3 sol-gel precursor solution. In this invention, the preparation methods of the indium oxide nanofilm and the PdO@S-1 molecular sieve membrane are the same, and will not be described in detail here.

[0049] In this invention, the aging temperature is preferably 100~200℃, more preferably 100~150℃, the aging time is preferably 1~3 days, more preferably 2~3 days, and the aging atmosphere is preferably air.

[0050] Meanwhile, the fabrication process of the double-layer membrane structure gas sensor provided by this invention is simple, the steps are convenient, the reliability is high, the production cost is low, and it is suitable for mass production in industry.

[0051] This invention provides the application of the indium oxide nanofilm, the PdO@S-1 molecular sieve membrane, or the PdO@S-1 molecular sieve membrane prepared by the method described in the above-described technical solutions, the molecular sieve / indium oxide bilayer membrane, or the molecular sieve / indium oxide bilayer membrane prepared by the method described in the above-described technical solutions, and the bilayer membrane structure gas sensor described in the above-described technical solutions in NO2 detection. This invention does not specifically limit the source of NO2; any gas containing NO2 pollution is acceptable, such as diesel vehicle exhaust.

[0052] To further illustrate the present invention, the following descriptions, in conjunction with the accompanying drawings and embodiments, detail the indium oxide nanofilm, PdO@S-1 molecular sieve membrane and its preparation method and application, molecular sieve / indium oxide bilayer membrane and its preparation method and application, and bilayer membrane structure gas sensor and its application. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0053] Example 1 A monolayer film (In2O3(1) type) sensor was prepared using In2O3 nanoparticles as the sensing material. The specific preparation process is as follows: (1) Dissolve 1.43g In(NO3)3·4.5H2O in a mixed solvent of 5mL ethylene glycol methyl ether and 0.23mL ethanolamine, stir for 2h and let stand for 12h to obtain In2O3 sol-gel precursor solution; (2) Place the plasma-treated ceramic device substrate on a flat table. Use a pipette to drop 20µL of In2O3 sol-gel precursor solution onto the surface of the interdigitated gold electrode on the upper surface of the Al2O3 ceramic planar substrate (completely wetting the upper surface of the Al2O3 ceramic planar substrate). Spin coat at room temperature, let stand and dry for 50 min, then pre-sinter at 300℃ for 1 h, and then sinter at 350℃ for 3 h to obtain a planar device with an In2O3 nanofilm grown on a thickness of 0.85μm. Solder the four pads of the ceramic planar to the hexagonal base with wires. Age the obtained device in an air environment at 100℃ for 3 days to obtain a sensor based on a single-layer film structure (denoted as In2O3(1) type). The upper surface of the Al2O3 ceramic planar substrate is provided with an upper electrode pad that is electrically connected to the interdigitated gold electrode, and the lower surface of the Al2O3 ceramic planar substrate is provided with a serpentine Pt heating electrode and a lower electrode pad that is electrically connected to the serpentine Pt heating electrode.

[0054] Example 2 A monolayer film (In2O3(2) type) sensor was prepared using In2O3 nanoparticles as the sensing material. The specific preparation process is as follows: (1) Dissolve 1.43g In(NO3)3·4.5H2O in a mixed solvent of 5mL ethylene glycol methyl ether and 0.23mL ethanolamine, stir for 2h and let stand for 12h to obtain In2O3 sol-gel precursor solution; (2) Place the plasma-treated ceramic device substrate on a flat table. Use a pipette to drop 20 µL of precursor solution onto the interdigitated gold electrode surface on the Al2O3 ceramic planar substrate (completely wetting the Al2O3 ceramic planar substrate surface). Spin-coat at room temperature, let stand and dry for 50 min, and then pre-sinter at 300 °C for 1 h. The spin-coating, standing and sintering pretreatment processes are performed twice. Then sinter at 350 °C for 3 h to obtain a growth of 1.7 μm. A planar device with an In2O3 nanofilm of m thickness was obtained by soldering four pads of the ceramic plane to a hexagonal base with wires. The resulting device was aged in an air environment at 100°C for 3 days to obtain a sensor based on a single-layer film structure (In2O3(2) type). The upper surface of the Al2O3 ceramic planar substrate is provided with an upper electrode pad that is electrically connected to the interdigitated gold electrode, and the lower surface of the Al2O3 ceramic planar substrate is provided with a serpentine Pt heating electrode and a lower electrode pad that is electrically connected to the serpentine Pt heating electrode.

[0055] Example 3 Using In2O3 nanoparticles and PdO@S-1 molecular sieves as sensing materials, a process was prepared. Figure 1 The specific fabrication process of the double-layer membrane (PdO@S-1 / In2O3(1) type) sensor shown is as follows: (1) Dissolve 1.43g In(NO3)3·4.5H2O in a mixed solvent of 5mL ethylene glycol methyl ether and 0.23mL ethanolamine, stir for 2h and let stand for 12h to obtain In2O3 sol-gel precursor solution; (2) Place the plasma-treated ceramic device substrate on a flat table. Use a pipette to drop 20µL of precursor solution onto the interdigitated gold electrode surface on the upper surface of the Al2O3 ceramic planar substrate (completely wetting the upper surface of the Al2O3 ceramic planar substrate). Spin-coat at room temperature, let stand and dry for 50 min, then pre-sinter at 300℃ for 1 h, and then sinter at 350℃ for 3 h to obtain a planar device with an In2O3 nanofilm grown on a thickness of 0.85μm (denoted as In2O3 nanofilm device). The upper surface of the Al2O3 ceramic planar substrate is provided with an upper electrode pad electrically connected to the interdigitated gold electrode, and the lower surface of the Al2O3 ceramic planar substrate is provided with a serpentine Pt heating electrode and a lower electrode pad electrically connected to the serpentine Pt heating electrode. (3) 0.05 g of Pd(NO3)2·2H2O and 0.3 g of ethylenediamine were added to 10 g of deionized water and sonicated at room temperature for 1 h to obtain a metal salt solution; 6 g of tetrapropylammonium hydroxide (TPAOH) and 5 g of tetraethyl orthosilicate (TEOS) were added to 10 g of deionized water and stirred at room temperature for 2 h to obtain a silicon-template agent mixed solution; the metal salt solution was added to the silicon-template agent mixed solution and stirred at room temperature for 1 h, then loaded into a hydrothermal reactor and placed in an oven for hydrothermal reaction at 180 °C for 48 h; after the reaction, the product was washed by centrifugation with water and ethanol alternately until pH=7, the product was dried at 80 °C for 12 h, and then calcined at 500 °C for 4 h to obtain PdO@S-1 molecular sieve; (4) Add 0.1g of PdO@S-1 molecular sieve material to 0.2g of isopropanol and grind it at room temperature for 0.1h until uniform to obtain a slurry. Use screen printing to print the slurry on the In2O3 nanofilm of the In2O3 nanofilm device. Dry it at room temperature for 6h and then sinter it at 350℃ for 3h to obtain a PdO@S-1 molecular sieve membrane with a thickness of 115μm on the surface of the In2O3 nanofilm. Then, use wires to solder the four pads of the ceramic plane to the device base. Age the obtained device in an air environment at 100℃ for 3 days to obtain a double-layer membrane (PdO@S-1 molecular sieve membrane / In2O3 membrane) structure sensor (denoted as PdO@S-1 / In2O3(1) type).

[0056] Example 4 Using In2O3 nanoparticles and PdO@S-1 molecular sieves as sensing materials, a process was prepared. Figure 1 The specific fabrication process of the double-layer film (PdO@S-1 / In2O3(2) type) sensor shown is as follows: (1) Dissolve 1.43g In(NO3)3·4.5H2O in a mixed solvent of 5mL ethylene glycol methyl ether and 0.23mL ethanolamine, stir for 2h and let stand for 12h to obtain In2O3 sol-gel precursor solution; (2) Place the plasma-treated ceramic device substrate on a flat table. Use a pipette to drop 20µL of precursor solution onto the surface of the interdigitated gold electrode on the upper surface of the Al2O3 ceramic planar substrate (completely wetting the upper surface of the Al2O3 ceramic planar substrate). Spin-coat at room temperature, let stand and dry for 50 min, and then pre-sinter at 300℃ for 1 h. The spin-coating, standing and sintering pretreatment processes are performed twice. Then sinter at 350℃ for 3 h to obtain a planar device with a 1.7μm thick In2O3 nanofilm (denoted as In2O3 nanofilm device). The upper surface of the Al2O3 ceramic planar substrate is provided with an upper electrode pad electrically connected to the interdigitated gold electrode, and the lower surface of the Al2O3 ceramic planar substrate is provided with a serpentine Pt heating electrode and a lower electrode pad electrically connected to the serpentine Pt heating electrode. (3) 0.05 g of Pd(NO3)2·2H2O and 0.3 g of ethylenediamine were added to 10 g of deionized water and sonicated at room temperature for 1 h to obtain a metal salt solution; 6 g of tetrapropylammonium hydroxide (TPAOH) and 5 g of tetraethyl orthosilicate (TEOS) were added to 10 g of deionized water and stirred at room temperature for 2 h to obtain a silicon-template agent mixed solution; the metal salt solution was added to the silicon-template agent mixed solution and stirred at room temperature for 1 h, then loaded into a hydrothermal reactor and placed in an oven for hydrothermal reaction at 180 °C for 48 h; after the reaction, the product was washed by centrifugation with water and ethanol alternately until pH=7, the product was dried at 80 °C for 12 h, and then calcined at 500 °C for 4 h to obtain PdO@S-1 molecular sieve; (4) Add 0.1g of PdO@S-1 molecular sieve material to 0.2g of anhydrous isopropanol and grind it at room temperature for 0.1h until uniform to obtain a slurry. Use screen printing to print the slurry on the In2O3 nanofilm of the In2O3 nanofilm device. Dry it at room temperature for 6h and then sinter it at 350℃ for 3h to obtain a PdO@S-1 molecular sieve membrane with a thickness of 115μm on the surface of the In2O3 nanofilm. Then, use wires to solder the four pads of the ceramic plane to the device base. Age the obtained device in an air environment at 100℃ for 3 days to obtain a double-layer membrane (PdO@S-1 molecular sieve membrane / In2O3 membrane) structure sensor (denoted as PdO@S-1 / In2O3(2) type).

[0057] Figure 2 This is a SEM image of the In2O3 nanofilm prepared in Example 4. Figure 2 It is known that the diameter of In2O3 nanoparticles is 70~150nm. Many pores are formed between the In2O3 nanoparticles, which increases the surface area of ​​the material, which is beneficial to the adsorption of NO2 gas molecules.

[0058] Figure 3 This is a TEM image of the PdO@S-1 molecular sieve material prepared in Example 4. Figure 3 It can be seen that the PdO@S-1 molecular sieve consists of hexagonal particles with a side length of about 150 nm and a diameter of about 250 nm, and PdO nanoparticles that are relatively uniformly dispersed on the hexagonal PdO@S-1 molecular sieve can be clearly seen. Figure 4 SEM images of In2O3(1) type devices and PdO@S-1 / In2O3(2) type devices are shown, where a and b are SEM images of the In2O3(1) type device, c is a cross-sectional SEM image of the In2O3(1) type device, and d is a cross-sectional SEM image of the PdO@S-1 / In2O3(2) type device. Figure 4 It can be seen that there are some grooves on the surface of the In2O3 nanofilm, and the overall surface is relatively flat; the cross-sectional view of the In2O3(1) type device shows that the thickness of the spin-coated In2O3 nanofilm is 0.85μm; the cross-section of the PdO@S-1 / In2O3(2) type device, that is, the total thickness of the spin-coated In2O3 nanofilm and the molecular sieve membrane is 115μm.

[0059] Figure 5 The XRD patterns of the In2O3 nanofilm and S-1 and PdO@S-1 molecular sieves prepared in Example 4 are shown, where a is the In2O3 nanofilm and b is the PdO@S-1 molecular sieve. Figure 5 It can be seen that the XRD diffraction peaks of the In2O3 nanofilm are consistent with the standard card PDF#71-2195; there is no significant difference in the XRD patterns of PdO@S-1 molecular sieve and S-1 molecular sieve material. The PdO@S-1 molecular sieve spectrum does not have the characteristic diffraction peaks of PdO, indicating that there are no PdO nanoparticles on the surface of PdO@S-1 molecular sieve, which verifies that PdO nanoparticles are encapsulated in situ in the channels of S-1 molecular sieve.

[0060] Test Example 1 The sensors prepared in Examples 1-4 were tested for their sensitivity to 5 ppm NO2 at different temperatures (32℃, 48℃, 65℃, 98℃, and 134℃). The results are shown in Table 1 and... Figure 6 As shown, Figure 6 Figure a shows the sensitivity curve of the sensor prepared in the example to 5 ppm NO2 gas as a function of temperature, and figure b shows the baseline resistance curve of the sensor as a function of temperature. The sensitivity is defined as S = R. g / R a R g R is the resistance between the gold electrodes in NO2 gas. a To clean the resistance between gold electrodes in the air.

[0061] Table 1. Sensitivity and operating temperature data of different types of sensors in a 5ppm NO2 atmosphere.

[0062] From Table 1 and Figure 6 As shown in section a, the In2O3(1) type sensor based on a single-layer In2O3 film structure has the lowest sensitivity, with a maximum sensitivity of 7.9 at 65℃; the sensor based on a double-layer In2O3 film structure (In2O3(2) type) has a higher sensitivity than the In2O3(1) type sensor, with a maximum sensitivity of 727.8 at 65℃; the sensor based on a double-layer film structure (PdO@S-1 / In2O3(1) type) has a maximum sensitivity of 160.5 at 65℃; and the sensor based on a double-layer film structure (PdO@S-1 / In2O3(2) type) has the highest sensitivity, with a maximum sensitivity of 3001.8 at 65℃. Figure 6 As shown in b, the resistance of all four sensors decreases with increasing temperature, which is consistent with the temperature dependence characteristics of semiconductors.

[0063] The sensitivity S of the PdO@S-1 / In2O3(2) sensor was tested at an operating temperature of 65℃ in an atmosphere of 5~10000ppb NO2. The results are as follows: Figure 7 As shown. By Figure 7 It can be seen that when the ambient component of the sensor changes from air to NO2 (the shaded area in the figure represents the NO2 atmosphere), the sensor resistance increases significantly. Furthermore, the change in sensor resistance becomes more pronounced as the concentration of the detected gas NO2 increases; that is, the sensitivity increases with increasing NO2 concentration. The functional relationship between sensor sensitivity and NO2 concentration is presented in the figure. Figure 8 In the middle. By Figure 8 It can be seen that the sensitivity increases with the increase of NO2 concentration, and the sensitivity changes approximately linearly with the concentration in low concentration (5~50ppb) NO2 gas. The lower limit of NO2 concentration that the PdO@S-1 / In2O3(2) sensor can detect is 5ppb, corresponding to a sensitivity of 1.1.

[0064] Figure 9 The graph shows the resistance change between the gold electrodes of the PdO@S-1 / In2O3(2) sensor after six cycles of 5ppm NO2 gas cycling. Figure 9 It can be seen that the baseline resistance of the PdO@S-1 / In2O3(2) sensor and the resistance in the gas fluctuate little in multiple tests, indicating good sensor stability.

[0065] The responses of the In2O3(2) type sensor and the PdO@S-1 / In2O3(2) type sensor to 1ppm NO2 gas at different humidity levels (35~80 RH%) were tested at an operating temperature of 65℃. The results are as follows: Figure 10 As shown. By Figure 10It can be seen that as humidity increases, the response of the In2O3(2) sensor based on a single-layer membrane structure to 1 ppm NO2 decreases from 752.5 (35 RH%) to 81.2 (80 RH%), a decrease of 89.2%; while the response of the PdO@S-1 / In2O3(2) sensor based on a nanoparticle bilayer membrane structure to 1 ppm NO2 decreases from 778.9 (35 RH%) to 501.2 (80 RH%), a decrease of only 35.7%. This indicates that the PdO@S-1 molecular sieve membrane can effectively block the influence of water vapor on the In2O3 sensitive material, improving the device's resistance to humidity interference.

[0066] The long-term stability curves of the In2O3(2) type sensor and the PdO@S-1 / In2O3(2) type sensor to 1ppm NO2 gas were tested at an operating temperature of 65℃ for half a month. The results are as follows: Figure 11 As shown. By Figure 11 It can be seen that the single-layer membrane In2O3(2) sensor and the double-layer membrane PdO@S-1 / In2O3(2) sensor exhibit long-term stability against 1ppm NO2 at an operating temperature of 65℃. Over a period of half a month, the response of the double-layer membrane PdO@S-1 / In2O3(2) NO2 sensor fluctuated within the range of 700~800, remaining relatively stable. In contrast, the response of the single-layer membrane In2O3(2) sensor plummeted from 586.2 to 25.5 on day 7, demonstrating very poor stability. Therefore, the double-layer membrane PdO@S-1 / In2O3(2) sensor exhibits excellent long-term stability.

[0067] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. People can obtain other embodiments based on the present invention without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A molecular sieve / indium oxide bilayer membrane, characterized in that, It includes a sensitive layer and a gas sieving / adsorption layer located on the upper surface of the sensitive layer; the sensitive layer is an indium oxide nanofilm; the gas sieving / adsorption layer is a PdO@S-1 molecular sieve membrane; The indium oxide nanofilm has a thickness of 0.5~2.3μm and has a porous structure. The PdO@S-1 molecular sieve membrane comprises an S-1 molecular sieve and PdO nanoparticles loaded in the pores of the S-1 molecular sieve. The thickness of the PdO@S-1 molecular sieve membrane is 10~200μm, and the loading of PdO nanoparticles in the PdO@S-1 molecular sieve membrane is 1~3wt%.

2. The method for preparing the molecular sieve / indium oxide bilayer membrane according to claim 1, characterized in that, Includes the following steps: (1) A water-soluble palladium source, ethylenediamine and water are mixed and subjected to a complexation reaction to obtain a palladium complex solution; (2) Tetrapropylammonium hydroxide, tetraethyl orthosilicate and water are mixed to obtain a silicon-template mixture; (3) The palladium complex solution and the silicon-template mixture were mixed and subjected to hydrothermal reaction and calcination in sequence to obtain PdO@S-1 molecular sieve; (4) The PdO@S-1 molecular sieve is mixed with isopropanol, coated on the surface of indium oxide nanofilm, and then sintered to obtain a molecular sieve / indium oxide bilayer film. Steps (1) and (2) are not in any chronological order.

3. The preparation method according to claim 2, characterized in that, In step (1), the mass ratio of the water-soluble palladium source, ethylenediamine, and water is 1~10:4~40:600~1200; In step (2), the mass ratio of tetrapropylammonium hydroxide, tetraethyl orthosilicate, and water is 3~8:2~6:5~12; the complexation reaction takes 0.5~4.5 h and is carried out under ultrasonic conditions; In step (3), the mass ratio of the water-soluble palladium source in the palladium complex solution to the tetraethyl orthosilicate in the silicon-template mixture is 1~10:200~600; the hydrothermal reaction temperature is 160~180℃ and the time is 36~72h; the calcination temperature is 400~600℃ and the time is 4~8h. In step (4), the sintering temperature is 200~350℃ and the time is 1~3h.

4. A gas sensor with a double-layer membrane structure, characterized in that, Including ceramic planar substrates (1); Interdigitated electrodes (2) and bilayer film (3) are sequentially stacked on the upper surface of the ceramic planar substrate (1), and an upper electrode pad (51) is electrically connected to the interdigitated electrodes (2); the bilayer film (3) is the molecular sieve / indium oxide bilayer film according to claim 1 or the molecular sieve / indium oxide bilayer film prepared by the preparation method according to any one of claims 2 to 3; The serpentine heating electrode (4) is located on the lower surface of the ceramic planar substrate (1), and the lower electrode pad (52) is electrically connected to the serpentine heating electrode (4).

5. The application of the molecular sieve / indium oxide bilayer membrane according to claim 1, the molecular sieve / indium oxide bilayer membrane prepared by the preparation method according to any one of claims 2 to 3, or the bilayer membrane structure gas sensor according to claim 4 in NO2 detection.

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