Sensitive membrane

Abstract

This sensitive membrane for detecting volatile compounds, the membrane containing an n-type metal oxide semiconductor and cerium oxide, 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 cerium oxide. [Item 2] The sensitive film according to Item 1, wherein the content of the cerium oxide is 0.1 mol % or more and 10 mol % or less with respect to the total content of the n-type metal oxide semiconductor and the cerium oxide. [Item 3] The sensitive film according to Item 1 or 2, wherein the thickness is 30 nm or more and 1000 nm or less. [Item 4] The sensitive film according to any one of Items 1 to 3, wherein the sensitive film is a porous film. [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 or more and 100 nm or less and cerium oxide particles having a particle size of 5 nm or more and 100 nm or less; (ii) applying the dispersion liquid onto a substrate and baking the dispersion 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; the 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.

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

[0007] 1 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention, and FIG. 2 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention, and FIG. 3 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention, and FIG. 4 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention, and FIG. 5 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention, and ｓ / R ０ 1 is a graph showing the R of the sensitive film 300 seconds after dimethyl sulfide-containing air was introduced into the gas sensor. ｓ / R ０ The graph shows the R of the sensitive film 300 seconds after air containing 1 ppb of dimethyl sulfide was introduced into the gas sensor. ｓ / R ０ The figure plots R of the sensitive film against the thickness of the sensitive film for the sensitive films with cerium oxide contents of 0, 0.1, 4.8, and 9.6 mol% (total content ratio of indium oxide and cerium oxide). ｓ / R ０ The R of the indium oxide film was plotted for 300 seconds after air containing 1 ppb, 10 ppb, 100 ppb, or 1000 ppb of dimethyl sulfide was introduced into the indium oxide film. ｓ / R ０ 1 is a graph showing the R of the indium oxide film 300 seconds after dimethyl sulfide-containing air was introduced into the gas sensor. ｓ / R ０ is plotted against the dimethyl sulfide concentration.

[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 cerium oxide.

[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 cerium oxide.

[0014] The content of cerium 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 cerium 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.

[0015] The content of cerium 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 cerium oxide. When the content of cerium oxide is within the above range, volatile compounds can be detected with higher sensitivity.

[0016] 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.

[0017] 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.

[0018] In a more preferred embodiment, the sensitive membrane of the present invention is a sensitive membrane for detecting dimethyl sulfide.

[0019] In a preferred embodiment, the thickness of the sensitive film of the present invention is 30 nm or more and 1000 nm or less. If the thickness of the sensitive film of the present invention is in this range, the volatile compound can diffuse throughout the sensitive film, thereby 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.

[0020] In a more preferred embodiment, the thickness of the sensitive film of the present invention is 100 nm or more and 700 nm or less, and in a further preferred embodiment, the thickness of the sensitive film of the present invention is 200 nm or more and 500 nm or less.

[0021] 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.

[0022] 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.

[0023] The median 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 median 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.

[0024] 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.

[0025] 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. ０and the sensitive film resistance value R when exposed to the target gas. ｓ By measuring this, the target gas can be detected.

[0026] 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 cerium 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.

[0027] 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.

[0028] [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.

[0029] 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 cerium 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.

[0030] 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.

[0031] The particle size of the n-type metal oxide semiconductor particles and the cerium 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 can be made smaller, thereby improving the detection sensitivity of the target gas.

[0032] The n-type metal oxide semiconductor particles and cerium oxide particles having particle sizes within the above ranges may be commercially available or may be produced by known methods, such as those described in WO 2021 / 171724.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] [Gas Sensor] The gas sensor of the present invention will now be described.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] <Fabrication of Gas Sensor> Cerium oxide was added to a dispersion liquid containing dispersed indium oxide particles to a content of 9.6 mol% (ratio of the total content 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 electrode-equipped main surface of the substrate. The mixture was baked at 500°C for 10 minutes in an oxygen atmosphere to fabricate a gas sensor having the configuration shown in Figure 1.

[0048] (Example: Evaluation of Sensor Response) In the evaluation of the sensor response of the gas sensor of the present invention, the resistance value R of the gas sensor was measured in an environment of 25° C. and 80% RH in air without the presence of dimethyl sulfide, which is the target gas. ０ The resistance value R of the sensitive film when a predetermined concentration of dimethyl sulfide is introduced into the above environment is ｓ is measured over time, and R ｓ / R ０ The sensor response of the gas sensor of the present invention was evaluated by checking the ｓ / R ０ The smaller the value of R, the higher the sensitivity of the gas sensor.) The vertical axis (sensor response) in the following Figs. 3 to 8 represents the R ｓ / R ０ Represents.

[0049] <Evaluation of Sensitivity of Gas Sensor> FIG. 3 shows the R ｓ / R ０ Even when the dimethyl sulfide concentration is as low as 1 ppb, the R ｓ / R ０ The change in R was confirmed, demonstrating that the target gas can be detected even at low concentrations. ｓ / R ０ Since the value is stable, it is clear that the target gas can be detected after 10 seconds.

[0050] <Dependence of Gas Sensor on Target Gas Concentration> FIG. 4 shows the R ｓ / R ０ The graph shows the R plotted against the above concentrations. ｓ / R ０ It was confirmed that the concentration of dimethyl sulfide in the gas sensor of the present invention was reduced, demonstrating that the gas sensor of the present invention can quantitatively detect volatile compounds such as dimethyl sulfide.

[0051] <Evaluation of Sensitive Film Thickness Dependence> FIG. 5 shows the relationship between R and the thickness of the sensitive film when evaluation was performed in air containing 1 ppb of dimethyl sulfide using the same procedure as in the "Evaluation of Sensor Response" above, for sensitive films of various thicknesses (20 nm, 227 nm, 467 nm, and 628 nm). ｓ / R ０ It was shown that the sensitivity of the gas sensor can be improved with an increase in the thickness of the sensitive film. When the thickness of the sensitive film is 20 nm, R ｓ / R ０ is close to 1 and varies from measurement to measurement, making the sensitivity of the gas sensor unstable. However, if the thickness exceeds 20 nm (if it is 30 nm or more), the sensitivity of the gas sensor improves and becomes stable, which is preferable. If it is 200 nm or more, R ｓ / R ０ is more preferable because it is sufficiently lower than 1.

[0052] <Amount of cerium oxide in the sensitive film> Sensitive films were manufactured using the same procedure as in the "Manufacturing of gas sensors" above, so that the cerium oxide content was 0, 0.1, 4.8, or 9.6 mol% (ratio of the total content of indium oxide and cerium oxide). Figure 6 shows the relationship between the R and the cerium oxide content when sensitive films with various cerium oxide contents were evaluated in air containing 1 ppb of dimethyl sulfide using the same procedure as in the "Evaluation of sensor response" above. ｓ / R ０ 6 shows that the sensitivity of the gas sensor can be improved with an increase in the content of cerium oxide.

[0053] (Comparative Example: Evaluation of Indium Oxide Film) Figure 7 shows the results of evaluation of an indium oxide film performed using the same procedure as in Figure 3. Unlike the sensitive film of the present invention (i.e., an indium oxide + cerium oxide film), almost no change in resistance value was observed when air containing 1 ppb or 10 ppb of dimethyl sulfide was introduced into the indium oxide film. Furthermore, when air containing 100 ppb or 1000 ppb of dimethyl sulfide was introduced into the indium oxide film, only a slight change in resistance value was observed, and the results were accompanied by noise.

[0054] FIG. 8 is a diagram showing the results of evaluation of a film of simple indium oxide, performed in the same manner as in the evaluation shown in FIG.

[0055] 10, 20 Gas sensor 11, 21 Substrate 12, 22 Electrode 13, 23 Sensitive film 14, 24 Heater 25 Insulating film

Claims

1. A sensitive film for detecting volatile compounds, comprising an n-type metal oxide semiconductor and cerium oxide.

2. The sensitive film according to claim 1, wherein the content of the cerium oxide is 0.1 mol % or more and 10 mol % or less with respect to the total content of the n-type metal oxide semiconductor and the cerium oxide.

3. The sensitive film according to claim 1 or 2, having a thickness of 30 nm or more and 1000 nm or less.

4. The sensitive membrane according to any one of claims 1 to 3, which is a porous membrane.

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 cerium 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.

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