Sensitive membrane

Abstract

This sensitive membrane for detecting volatile compounds, the membrane containing an n-type metal oxide semiconductor and a metal oxide and having a thickness of 30nm-1000nm, inclusive, makes it possible to detect volatile compounds with greater sensitivity.

Description

Sensitive membrane

[0001] The present invention relates to a sensitive membrane.

[0002] Some volatile compounds contained in human breath (e.g., dimethyl sulfide) are known to be one of the components that cause bad breath. As a method for detecting components that cause bad breath, such as dimethyl sulfide, from human breath, for example, the method described in Patent Document 1 is known.

[0003] Japanese Patent Application Laid-Open No. 2003-75384

[0004] An object of the present invention is to provide a sensitive membrane for detecting volatile compounds with higher sensitivity.

[0005] The present invention includes the following aspects: [Item 1] A sensitive film for detecting volatile compounds, comprising an n-type metal oxide semiconductor and a metal oxide, and having a thickness of 30 nm to 1000 nm. [Item 2] The sensitive film according to Item 1, having a thickness of 100 nm to 700 nm. [Item 3] The sensitive film according to Item 1 or 2, which is a porous film. [Item 4] The sensitive film according to any one of Items 1 to 3, wherein the content of the metal oxide is 0.1 mol % to 10 mol % with respect to the total content of the n-type metal oxide semiconductor and the metal oxide. [Item 5] A method for producing the sensitive film according to any one of Items 1 to 4, comprising the following (i) and (ii): (i) preparing a dispersion liquid containing n-type metal oxide semiconductor particles having a particle size of 5 nm to 100 nm and metal oxide particles having a particle size of 5 nm to 100 nm; (ii) applying the dispersion liquid onto a substrate and baking the coating liquid to form a sensitive film. [Item 6] A gas sensor comprising: a substrate; an electrode disposed on a first main surface of the substrate; a sensitive film according to any one of items 1 to 4 formed on the first main surface so as to cover the electrode; and a heater disposed on one of the main surfaces of the substrate so as to be in contact with the substrate. [Item 7] The gas sensor according to [6], further comprising an insulating film disposed on the first main surface around the sensitive film, wherein the thickness of the sensitive film is thinner than the thickness of the insulating film.

[0006] According to the present invention, a sensitive membrane for detecting volatile compounds with higher sensitivity can be provided.

[0007] Fig. 1 is a cross-sectional view of a gas sensor according to an embodiment of the present invention. Fig. 2 is a cross-sectional view of a gas sensor according to an embodiment of the present invention. Fig. 3 is a cross-sectional view of a gas sensor according to an embodiment of the present invention. Fig. 4 is a cross-sectional image of a gas sensor according to the present invention. Fig. 5 is a cross-sectional image of a gas sensor according to the present invention. Fig. 6 is a cross-sectional view of a gas sensor according to an embodiment of the present invention. Fig. 7 is a cross-sectional image of a gas sensor according to the present invention. Fig. 8 is a cross-sectional image of a gas sensor according to the present invention. ０ / R ｓ (A) is a diagram plotting the R of the sensitive film against the thickness of the sensitive film, and (B) is a diagram plotting the R of the sensitive film against the void area of ​​the sensitive film 300 seconds after dimethyl sulfide-containing air was introduced into the gas sensor. ０ / R ｓ The graph shows the R 300 seconds after air containing 10 ppb of hydrogen sulfide was introduced into the palladium oxide supported gas sensor. ０ / R ｓ The graph shows R 300 seconds after hydrogen sulfide-containing air with concentrations of 1 ppb, 10 ppb, 100 ppb, and 1000 ppb was introduced into the gas sensor. ０ / R ｓ The graph shows the R 300 seconds after air containing 10 ppb of methyl mercaptan was introduced into the copper oxide supported gas sensor. ０ / R ｓ The graph shows the R 300 seconds after air containing methyl mercaptan at concentrations of 1 ppb, 10 ppb, 100 ppb, and 1000 ppb was introduced into the gas sensor. ０ / R ｓ is plotted against the above concentrations.

[0008] The sensitive film of the present invention is a sensitive film for detecting volatile compounds, which contains an n-type metal oxide semiconductor and a metal oxide and has a thickness of 30 nm or more and 1000 nm or less.

[0009] The sensitive film of the present invention contains an n-type metal oxide semiconductor.

[0010] The n-type metal oxide semiconductor is not particularly limited, but examples thereof include indium oxide, yttrium oxide, tin oxide, tungsten oxide, zinc oxide, titanium oxide, strontium titanate, zinc ferrite, copper tungstate, barium titanate, and barium stannate.

[0011] As the n-type metal oxide semiconductor, one of the above compounds may be used alone or two or more of them may be used in combination, but it is preferable to use one of them alone.

[0012] In a preferred embodiment, the n-type metal oxide semiconductor is indium oxide.

[0013] The sensitive film of the present invention contains a metal oxide.

[0014] The metal oxide is not particularly limited, but examples thereof include cerium oxide, palladium oxide, copper oxide, aluminum oxide, calcium oxide, iron oxide, magnesium oxide, lead oxide, nickel oxide, chromium oxide, cadmium oxide, manganese oxide, silver oxide, diantimony trioxide, triiron tetroxide, strontium oxide, barium oxide, scandium oxide, gallium oxide, rubidium oxide, zirconium oxide, niobium oxide, molybdenum oxide, technetium oxide, ruthenium oxide, rhodium oxide, silver oxide, potassium oxide, sodium oxide, lithium oxide, rubidium oxide, cesium oxide, hafnium oxide, tantalum oxide, rhenium oxide, osmium oxide, iridium oxide, spinel, zirconia, and lanthanum strontium manganite.

[0015] As the metal oxide, one of the above compounds may be used alone or two or more of them may be used in combination, but it is preferable to use one of them alone.

[0016] In a preferred embodiment, the metal oxide contained in the sensitive film of the present invention is cerium oxide. In another preferred embodiment, the metal oxide contained in the sensitive film of the present invention is palladium oxide. In another preferred embodiment, the metal oxide contained in the sensitive film of the present invention is copper oxide.

[0017] The content of the metal oxide in the sensitive film is not particularly limited, and may be, for example, 0.01 mol% or more, 0.05 mol% or more, 0.10 mol% or more, 0.50 mol% or more, 1.0 mol% or more, 2.0 mol% or more, or 3.0 mol% or more, relative to the total content of the n-type metal oxide semiconductor and the metal oxide, or may be, for example, 30 mol% or less, 20 mol% or less, 15 mol% or less, 10 mol% or less, 8.0 mol% or less, or 5.0 mol% or less.

[0018] The content of the metal oxide is preferably 0.1 mol % to 10 mol %, more preferably 2.0 mol % to 10 mol %, and even more preferably 3.0 mol % to 8.0 mol %, based on the total content of the n-type metal oxide semiconductor and the metal oxide. When the content of the metal oxide is within the above range, volatile compounds can be detected with higher sensitivity.

[0019] The sensitive membrane of the present invention can be used to detect various volatile compounds. In this disclosure, the term "volatile compound" refers to various compounds that can be gaseous at approximately 400°C. Examples of such volatile compounds include, but are not limited to, volatile sulfur compounds (dimethyl sulfide, hydrogen sulfide, methyl mercaptan, etc.), methane, ethane, isobutane, propane, ethylene, propylene, butylene, acetylene, cyclopropane, benzene, toluene, ethylbenzene, dichloromethane, vinyl chloride, phosgene, chlorofluorocarbons, ozone, fluorine, carbon monoxide, carbon dioxide, methyl ether, acetone, isoprene, ethanol, ammonia, monomethylamine, dimethylamine, trimethylamine, formaldehyde, and acetaldehyde.

[0020] In a preferred embodiment, the sensitive membrane of the present invention is a sensitive membrane for detecting a volatile sulfur compound, preferably dimethyl sulfide, hydrogen sulfide or methyl mercaptan.

[0021] The sensitive film of the present invention has a thickness of 30 nm or more and 1000 nm or less. A thickness within this range allows volatile compounds to diffuse throughout the sensitive film, further improving the sensitivity of the sensitive film. Furthermore, by setting the thickness to 1000 nm or less, it is possible to prevent cracks from occurring in the sensitive film and to prevent a decrease in the detection accuracy of the target gas.

[0022] In a preferred embodiment, the thickness of the sensitive film of the present invention is 100 nm or more and 700 nm or less. In a more preferred embodiment, the lower limit of the thickness of the sensitive film of the present invention is 150 nm or more, and particularly preferably 500 nm or more.

[0023] In a preferred embodiment, the sensitive membrane of the present invention is a porous membrane. When the sensitive membrane of the present invention is a porous membrane, the surface area of ​​the sensitive membrane is increased compared to a non-porous membrane, and the contact area with air and the target gas is increased, thereby further improving the sensitivity of the sensitive membrane.

[0024] In the present disclosure, a "porous film" refers to a film containing multiple voids, the median void area of ​​which is 900 nm ２ The term "median void area" refers to a film having a void area of ​​900 nm or less. Furthermore, the term "median void area" refers to the median area of ​​all voids present within an analysis range when a polished cross section of the sensitive film is subjected to image analysis within a range of 50 nm x 300 nm. The method for measuring the void area is not particularly limited, and it may be measured by a known method. For example, a porous film has 5 or more voids within the analysis range when a polished cross section of the sensitive film is subjected to image analysis within a range of 50 nm x 300 nm, and the median area of ​​the voids is 900 nm or less. ２ The following is the result.

[0025] The void area of ​​the sensitive membrane of the present invention is preferably 700 nm ２ Less than 400 nm, more preferably ２ More preferably, 300 nm or less ２ If the void area of ​​the sensitive film is within the above range, the surface area of ​​the sensitive film can be further increased, and the sensitivity can be further improved.

[0026] In the gas sensor equipped with the sensitive film of the present invention, when the sensitive film detects the presence of a volatile compound to be detected (hereinafter also referred to as a target gas), the sensitive film detects the target gas by utilizing the phenomenon in which the electrical resistance value of the sensitive film changes. Without being bound by theory, the principle is thought to be as follows.

[0027] When a gas sensor is exposed to air that does not contain the target gas, electron-withdrawing oxygen is adsorbed to the surface of the sensitive film, forming a space charge layer near the surface of the n-type semiconductor contained in the sensitive film. This space charge layer acts as a potential barrier to the movement of electrons between semiconductors, thereby inhibiting the movement of electrons between semiconductors. If a reducing gas such as dimethyl sulfide is introduced onto the semiconductor surface on which the space charge layer is formed, the adsorbed oxygen is consumed, the space charge layer becomes thinner, and the resistance value of the sensitive film decreases. On the other hand, if an oxidizing gas such as fluorine gas is introduced, the space charge layer becomes thicker, and the resistance value of the sensitive film increases. By utilizing this phenomenon, a gas sensor equipped with the sensitive film of the present invention can reduce the sensitive film resistance R when exposed to air that does not contain the target gas. ０ and the sensitive film resistance value R when exposed to the target gas. ｓ By measuring this, the target gas can be detected.

[0028] A gas sensor equipped with the sensitive film of the present invention can have higher sensitivity than conventional gas sensors because the n-type oxide semiconductor in the gas-sensitive film contains a metal oxide. For example, with regard to the detection sensitivity of dimethyl sulfide, one of the components of bad breath, the concentration of dimethyl sulfide contained in human breath is on the order of ppb, making it difficult to detect with the ppm-order sensitivity that is the measurement range of conventional metal oxide semiconductor gas sensors. However, a gas sensor equipped with the sensitive film of the present invention can detect dimethyl sulfide even at low concentrations of dimethyl sulfide in the air, such as about 5 ppb or about 1 ppb.

[0029] Furthermore, while conventional metal oxide semiconductor gas sensors have had difficulty detecting target gases contained in high-humidity air, such as human breath, the gas sensor equipped with the sensitive film of the present invention can detect odorous components such as dimethyl sulfide even in high-humidity environments, such as 80% RH.

[0030] [Manufacturing Method] A method for manufacturing the sensitive membrane of the present invention will be described below, but the method for manufacturing the sensitive membrane of the present invention is not limited to the following method.

[0031] In one embodiment, the sensitive film of the present invention can be produced by a method comprising the following steps (i) and (ii): (i) preparing a dispersion containing n-type metal oxide semiconductor particles having a particle size of 5 nm or more and 100 nm or less and metal oxide particles having a particle size of 5 nm or more and 100 nm or less; and (ii) applying the dispersion onto a substrate and baking the applied dispersion to form a sensitive film.

[0032] The n-type metal oxide semiconductor is not particularly limited, but examples thereof include indium oxide, yttrium oxide, tin oxide, tungsten oxide, zinc oxide, titanium oxide, strontium titanate, zinc ferrite, copper tungstate, barium titanate, barium stannate, etc. Furthermore, one or more precursor compounds that can be converted into the above-described n-type metal oxide semiconductors by chemical reactions occurring during the steps (i) to (ii) (e.g., during firing) may be used.

[0033] The metal oxide is not particularly limited, and examples thereof include cerium oxide, palladium oxide, copper oxide, aluminum oxide, calcium oxide, iron oxide, magnesium oxide, lead oxide, nickel oxide, chromium oxide, cadmium oxide, manganese oxide, silver oxide, diantimony trioxide, triiron tetroxide, strontium oxide, barium oxide, scandium oxide, gallium oxide, rubidium oxide, zirconium oxide, niobium oxide, molybdenum oxide, technetium oxide, ruthenium oxide, rhodium oxide, silver oxide, potassium oxide, sodium oxide, lithium oxide, rubidium oxide, cesium oxide, hafnium oxide, tantalum oxide, rhenium oxide, osmium oxide, iridium oxide, spinel, zirconia, and lanthanum strontium manganite.

[0034] The particle size of the n-type metal oxide semiconductor particles and the metal oxide particles is preferably 5 nm to 50 nm, more preferably 5 nm to 20 nm, inclusive. If the particle size of these particles is within this range, the pore size of the porous sensitive membrane after film formation will be smaller, and the detection sensitivity of the target gas can be improved.

[0035] The n-type metal oxide semiconductor particles and metal oxide particles having particle sizes within the above ranges may be commercially available or may be produced by known methods. For example, the method described in WO 2021 / 171724 may be used as a known method for producing such particles.

[0036] The type of solvent for the dispersion is not particularly limited, and any organic solvent that can volatilize without reacting with the particles during the steps (i) and (ii) above may be used.

[0037] The type of the substrate is not particularly limited, but may be, for example, a sapphire substrate, a glass substrate, an alumina substrate, a Si substrate, a GaN substrate, or the like.

[0038] The method for applying the dispersion onto the substrate is not particularly limited, and may be any known method such as dispensing, inkjet, spin coating, dip coating, spray coating, roll coating, blade coating, or slit coating.

[0039] After the dispersion liquid is applied to the substrate, a drying step may be performed before baking, in which the substrate is heated on a hot plate or the like to volatilize the solvent. The conditions for the drying step are not particularly limited, but the drying step may be performed, for example, at a temperature range near the boiling point of the solvent. By drying at such a temperature, the solvent is removed from the substrate on which the dispersion liquid is applied. The drying step may also be performed by heating on a hot plate or the like, or by using infrared rays, microwaves, electromagnetic induction, or the like.

[0040] Furthermore, after the drying step, the dried film may be overcoated with the dispersion liquid and dried repeatedly, allowing the thickness of the sensitive film after baking to be adjusted as desired.

[0041] The conditions for the calcination are not particularly limited, but an atmosphere such as air, oxygen, nitrogen, or argon may be appropriately selected depending on the type of material used, and the calcination time and temperature conditions may also be appropriately adjusted depending on the type of material used. For example, calcination may be carried out by heating at a temperature of 150°C or higher and 1000°C or lower for several minutes to several hours. Furthermore, since long-term calcination makes it difficult to obtain a dense film due to thermal diffusion, calcination may be carried out by inserting the sample into an electric furnace heated to a predetermined temperature or in an RTA furnace using infrared heating. By such calcination, the sensitive film of the present invention can be suitably formed on the substrate.

[0042] [Gas Sensor] The gas sensor of the present invention will now be described.

[0043] The gas sensor of the present invention comprises: a substrate; an electrode disposed on a first main surface of the substrate; any of the above-described sensitive films formed on the first main surface so as to cover the electrode; and a heater disposed on one of the main surfaces of the substrate so as to be in contact with the substrate.

[0044] The type of the substrate is not particularly limited, but may be, for example, a sapphire substrate, a glass substrate, an alumina substrate, a Si substrate, a GaN substrate, or the like.

[0045] The gas sensor of the present invention includes an electrode disposed on the first main surface of the substrate, and a sensitive film of the present invention formed on the first main surface so as to cover the substrate. The amount of target gas adsorbed by the sensitive film can be detected by applying a voltage between the electrodes and detecting a change in electrical resistance between the electrodes. Therefore, the sensitive film only needs to cover the electrodes to the extent that it is conductive. The sensitive film of the present invention can be applied to the substrate using the same procedure as the method described above.

[0046] The gas sensor of the present invention includes a heater disposed on one of the main surfaces of the substrate so as to be in contact with the substrate. The heater is used to heat the sensitive film via the substrate. Heating the sensitive film promotes an adsorption reaction between the sensitive film surface and the target gas, thereby improving the target gas detection sensitivity of the gas sensor.

[0047] In one embodiment, the gas sensor of the present invention includes a heater disposed on a main surface (second main surface) opposite to the first main surface so as to be in contact with the substrate. Fig. 1 is a cross-sectional view showing a gas sensor 10 of the present invention in this embodiment. The gas sensor 10 includes a substrate 11, an electrode 12 disposed on the first main surface, a sensitive film 13 formed on the first main surface so as to cover the electrode 12, and a heater 14 disposed on the second main surface so as to be in contact with the substrate.

[0048] In one embodiment, the gas sensor of the present invention includes a heater disposed on the first main surface so as to be in contact with the substrate. Fig. 2 is a cross-sectional schematic diagram showing a gas sensor 20 of the present invention according to this embodiment. The gas sensor 20 includes a substrate 21, an electrode 22 disposed on the first main surface of the substrate 21, a sensitive film 23 formed on the first main surface so as to cover the electrode 22, and a heater 24 disposed on the first main surface. The gas sensor 20 may also include an insulating film 25 formed so as to cover the heater 24 in order to prevent contact between the sensitive film 23 and the heater 24 during the formation of the sensitive film 23, thereby reducing loss due to heat conduction.

[0049] In one embodiment, the gas sensor of the present invention includes an insulating film on a first main surface and a heater disposed on a main surface (second main surface) opposite the first main surface so as to contact the substrate. FIG. 3 is a cross-sectional schematic diagram showing a gas sensor 30 of the present invention according to this embodiment. The gas sensor 30 includes a substrate 31, an electrode 32 disposed on the first main surface, a sensitive membrane 33 disposed on the first main surface, a heater 34 disposed on the second main surface so as to contact the substrate, and an insulating film 35 disposed on the first main surface. The insulating film 35 is disposed around the sensitive membrane 33 in a top view. The thickness of the sensitive membrane 33 is thinner than the thickness of the insulating film 35 in a cross-sectional view. In a gas sensor having this embodiment, the extension of the sensitive membrane formed by applying a liquid can be limited to the inside of the insulating film 35 during the manufacturing process. This reduces the difference in thickness between the edge and center of the sensitive membrane 33 in a top view, improving reproducibility. In this embodiment, the thickness of the insulating film 35 is preferably 500 nm to 5 μm. In the manufacturing process of the gas sensor, the resistance value of the metal oxide constituting the sensitive film may vary depending on the raw material lot, but the resistance value of the sensitive film 33 can be adjusted by adjusting the width of the insulating film 35 in a top view.

[0050] Examples of the present invention will be specifically described below, but unless otherwise specified in the specification, the examples do not limit the present invention.

[0051] (Evaluation of Sensor Response) In the evaluation of the sensor response of the gas sensor of the present invention, the resistance value of the gas sensor in air without the presence of the target gas in an environment of 25° C. and 80% RH was measured as R ０ The resistance value R of the sensitive film when a target gas of a predetermined concentration is introduced into the above environment is ｓ is measured over time, and R ０ / R ｓ The sensor response of the gas sensor of the present invention was evaluated by checking the ０ / R ｓ The larger the value of R, the higher the sensitivity of the gas sensor.) The vertical axis (Sensor response) in Figs. 4 to 9 shown below is ０ / R ｓ Represents.

[0052] Example 1 <Production of a cerium oxide-supported gas sensor> Cerium oxide was added to a dispersion liquid containing dispersed indium oxide particles so that the content was 10 mol % (the total content ratio of indium oxide and cerium oxide), and the mixture was stirred. A substrate was prepared, with an electrode on one main surface and a heater on the other main surface. The stirred dispersion liquid was applied to the main surface of the substrate with the electrode. The mixture was baked at 500°C for 10 minutes in an oxygen atmosphere to produce a cerium oxide-supported gas sensor having the configuration shown in Figure 1.

[0053] (Observation of Sensitive Film Cross Section) A cross-sectional image of the gas sensor manufactured as described above is shown in Fig. 3. From Fig. 3, it can be observed that the sensitive film of the present invention is a porous film.

[0054] <Evaluation of Film Thickness Dependence of Sensor Sensitivity> FIG. 4A shows the R ０ / R ｓ 4A is a graph plotting R against the thickness of the sensitive film (20 nm, 141 nm, 191 nm, 231 nm, 467 nm, and 628 nm). ０ / R ｓ The value was close to 1.0, and volatile compounds could not be detected with high sensitivity. ０ / R ｓ was a value sufficiently higher than 1.0, and volatile compounds could be detected with high sensitivity.

[0055] <Evaluation of Sensor Sensitivity Dependence on Sensitive Film Gap Area> FIG. 4B shows the R ０ / R ｓ is the gap area of ​​the sensitive film (299 nm ２ , 351 nm ２ , 366 nm ２ , and 396 nm ２ 4B, the void area is 400 nm ２ The sensitive membrane is R ０ / R ｓshowed a value sufficiently higher than 1.0, and volatile compounds could be detected with high sensitivity.

[0056] <Evaluation of the dependency of gas sensor on target gas concentration> In the evaluation of the dependency of sensitive film thickness on the cerium oxide supported gas sensor, the sensitive film with the highest sensitivity (i.e., 628 nm) was used to evaluate the dependency of the gas sensor on the target gas concentration. Figure 5 shows the R ０ / R ｓ Even if the dimethyl sulfide concentration is 1 ppb, R ０ / R ｓ was significantly higher than 1, indicating that the gas sensor can detect the target gas even at a concentration of 1 ppb.

[0057] Example 2 <Production of Palladium Oxide-Supported Gas Sensor> Palladium oxide was added to a dispersion liquid containing dispersed indium oxide particles so that the content was 0.1 mol % (ratio of the total content of indium oxide and palladium oxide), and the mixture was stirred. A substrate was prepared, with an electrode on one main surface and a heater on the other main surface. The stirred dispersion liquid was applied to the main surface of the substrate with the electrode. The mixture was baked at 500°C for 10 minutes in an oxygen atmosphere to produce a palladium oxide-supported gas sensor having the configuration shown in FIG.

[0058] <Evaluation of Film Thickness Dependence of Sensor Sensitivity> FIG. 6 shows the R ０ / R ｓ 6 is a graph plotting R against the thickness of the sensitive film. ０ / R ｓ showed a value significantly higher than 1, and volatile compounds could be detected with high sensitivity.

[0059] <Evaluation of the dependency of gas sensor on target gas concentration> In the evaluation of the dependency of the sensitive film thickness of the palladium oxide supported gas sensor, the sensitive film with the highest sensitivity (i.e., 103 nm) was used to evaluate the dependency of the gas sensor on the target gas concentration. Figure 7 shows the R ０ / R ｓ Even when the hydrogen sulfide concentration is 1 ppb, R ０ / R ｓ was significantly higher than 1, indicating that the gas sensor can detect the target gas even at a concentration of 1 ppb.

[0060] Example 3 <Production of a Copper Oxide-Supported Gas Sensor> Copper oxide was added to a dispersion liquid containing dispersed indium oxide particles so that the content was 0.1 mol % (ratio of the total content of indium oxide and copper oxide) and stirred. A substrate was prepared, with an electrode on one main surface and a heater on the other main surface. The stirred dispersion liquid was applied to the electrode-supported main surface of the substrate. This was baked at 500°C for 10 minutes in an oxygen atmosphere to produce a copper oxide-supported gas sensor with the configuration shown in Figure 1.

[0061] <Evaluation of Film Thickness Dependence of Sensor Sensitivity> FIG. 8 shows the R ０ / R ｓ 8 is a graph plotting R against the thickness of the sensitive film. ０ / R ｓ showed a value significantly higher than 1, and volatile compounds could be detected with high sensitivity.

[0062] <Evaluation of the dependency of gas sensor on target gas concentration> In the evaluation of the dependency of sensitive film thickness of the copper oxide supported gas sensor, the sensitive film with the highest sensitivity (i.e., 313 nm) was used to evaluate the dependency of the gas sensor on target gas concentration. Figure 9 shows the R０ / R ｓ Even when the hydrogen sulfide concentration is 1 ppb, R ０ / R ｓ was significantly higher than 1, indicating that the gas sensor can detect the target gas even at a concentration of 1 ppb.

[0063] 10, 20, 30 Gas sensor 11, 21, 31 Substrate 12, 22 Electrode 32 Electrode 13, 23, 33 Sensitive film 14, 24 Heater 34 Heater 25, 35 Insulating film

Claims

1. A sensitive film for detecting volatile compounds, comprising an n-type metal oxide semiconductor and a metal oxide, and having a thickness of 30 nm or more and 1000 nm or less.

2. The sensitive film according to claim 1, having a thickness of 100 nm or more and 700 nm or less.

3. The sensitive membrane according to claim 1 or 2, which is a porous membrane.

4. A sensitive film according to any one of claims 1 to 3, wherein the content of the metal oxide is 0.1 mol % or more and 10 mol % or less relative to the total content of the n-type metal oxide semiconductor and the metal oxide.

5. A method for producing a sensitive film according to any one of claims 1 to 4, comprising the following steps (i) and (ii): (i) preparing a dispersion containing n-type metal oxide semiconductor particles having a particle size of 5 nm or more and 100 nm or less and metal oxide particles having a particle size of 5 nm or more and 100 nm or less; (ii) applying the dispersion onto a substrate and baking it to form a sensitive film.

6. A gas sensor comprising: a substrate; an electrode disposed on a first main surface of the substrate; a sensitive film according to any one of claims 1 to 4 formed on the first main surface so as to cover the electrode; and a heater disposed on one of the main surfaces of the substrate so as to be in contact with the substrate.

7. The gas sensor according to claim 6, further comprising an insulating film disposed on the first main surface around the sensitive film, wherein the thickness of the sensitive film is thinner than the thickness of the insulating film.

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