Gas sensor and method of manufacturing the same

Abstract

To provide a gas sensor with superior sensor characteristics.SOLUTION: A gas sensor is provided, comprising a gas sensitive layer containing diamond particles and a metal oxide.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to gas sensors. [Background technology]

[0002] Gas sensors for detecting gases have been developed for some time. For example, Patent Document 1 proposes a gas sensor including a composite having a substrate and a metal oxide provided on the surface of the substrate, with the ratio of the surface area to the area of ​​the portion where the metal oxide is provided on the surface being 5.5 to 2840. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-053804 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been a demand for detecting low concentrations of gases and odors, and therefore there is a need for further improvement in the responsiveness of gas sensors. However, the technology of Patent Document 1 leaves room for further improvement in terms of sensor characteristics such as responsiveness and selectivity. Therefore, there is a demand for gas sensors with excellent sensor characteristics. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] (1) According to one embodiment of the present disclosure, there is provided a gas sensor. The gas sensor includes a gas-sensitive layer containing diamond particles and a metal oxide. The gas sensor of this embodiment has excellent sensor characteristics due to the gas-sensitive layer containing diamond particles and a metal oxide.

[0007] (2) In the gas sensor described in (1) above, the gas-sensitive layer may have a first layer containing the metal oxide and a second layer formed on a surface of the first layer and containing the diamond particles. The gas sensor of this embodiment has excellent sensor characteristics.

[0008] (3) In the gas sensor according to (1) or (2), the metal oxide may contain tin oxide. The gas sensor of this embodiment has better sensor characteristics.

[0009] (4) In the gas sensor described in (3) above, the tin oxide may have a dendritic structure, which provides the gas sensor with even better sensor characteristics.

[0010] (5) In the gas sensor according to any one of (1) to (4), the diamond particles may have an average primary particle diameter of 1 nm or more and 10 nm or less. The gas sensor of this embodiment has excellent sensor characteristics.

[0011] (6) In the gas sensor according to any one of (1) to (5), the gas to be detected may be at least one gas selected from the group consisting of a combustible gas, a reducing gas, an oxidizing gas, a combustion-supporting gas, and water vapor. The gas sensor of this embodiment has excellent sensor characteristics.

[0012] (7) In the gas sensor according to any one of (1) to (6), the gas to be detected may be at least one selected from the group consisting of acetaldehyde, hydrogen, normal octane, isoprene, ethanol, nitrogen dioxide, meta-xylene, toluene, para-xylene, acetone, and ethylene. The gas sensor of this embodiment has excellent sensor characteristics.

[0013] (8) According to another aspect of the present disclosure, there is provided a method for manufacturing a gas sensor. This method for manufacturing a gas sensor includes a first step of forming a first layer containing tin oxide having a dendritic structure by immersing a substrate on which an electrode has been formed in a liquid containing a tin oxide precursor, and a second step of forming a second layer containing diamond particles by applying a solution in which diamond particles are dispersed to the surface of the first layer. This method for manufacturing a gas sensor can manufacture a gas sensor with excellent sensor characteristics.

[0014] (9) In the method for manufacturing a gas sensor described in (8) above, the tin oxide precursor may be tin fluoride. According to this embodiment of the method for manufacturing a gas sensor, a gas sensor having more excellent sensor characteristics can be manufactured.

[0015] The present invention can be realized in various forms, such as a sensor device including a gas sensor, a method for manufacturing a sensor device, a gas monitoring device using a gas sensor, a gas monitoring method, an acetaldehyde sensor, a hydrogen sensor, a normal octane sensor, etc. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a perspective view schematically illustrating the configuration of a main part of the gas sensor. [Figure 2] 1A to 1C are process diagrams illustrating an example of a method for manufacturing a gas sensor. [Figure 3] FIG. 2 is an explanatory diagram showing a scanning electron microscope image of the gas-sensitive film in the gas sensor of Example 1. [Figure 4] FIG. 2 is an explanatory view showing a transmission electron microscope image of a cross section of the gas sensor of Example 1. [Figure 5] FIG. 2 is an explanatory diagram showing an HAADF-STEM image of a cross section of the gas sensor of Example 1. [Figure 6] FIG. 1 is an explanatory diagram showing responsiveness to acetaldehyde. [Figure 7] FIG. 10 is an explanatory diagram showing responsiveness to hydrogen. [Figure 8] FIG. 10 is an explanatory diagram showing the responsiveness to normal octane. [Figure 9] FIG. 1 is an explanatory diagram showing responsiveness to isoprene. [Figure 10] FIG. 1 is an explanatory diagram showing responsiveness to ethanol. [Figure 11] FIG. 1 is an explanatory diagram showing responsiveness to nitrogen dioxide. [Figure 12] FIG. 10 is an explanatory diagram showing the responsiveness to meta-xylene. [Figure 13] FIG. 1 is an explanatory diagram showing responsiveness to toluene. [Figure 14] FIG. 10 is an explanatory diagram showing the responsiveness to paraxylene. [Figure 15] FIG. 1 is an explanatory diagram showing responsiveness to acetone. [Figure 16] FIG. 1 is an explanatory diagram showing responsiveness to ethylene. DETAILED DESCRIPTION OF THE INVENTION

[0017] A. Embodiment FIG. 1 is a perspective view schematically illustrating the configuration of a main part of a gas sensor 100 according to an embodiment of the present disclosure. The gas sensor 100 is a sensor device that detects a desired gas. The gas sensor 100 of this embodiment is a semiconductor sensor. The gas sensor 100 includes a substrate 10 and a gas-sensitive layer 20.

[0018] The substrate 10 is made of an insulating material. Examples of insulating materials include, but are not limited to, metal oxides and resins. Examples of metal oxides forming the substrate 10 include, but are not limited to, silica (silicon oxide), silicon nitride, and alumina (aluminum oxide). In this embodiment, an electrode pair (not shown) including a first electrode and a second electrode is formed on the surface of the substrate 10. The electrode pair is made of a conductive material. Examples of conductive materials include, but are not limited to, Pt, Ir, Pd, Ag, Ni, W, Cu, and Al. The electrode pair is formed, for example, by interdigital electrodes.

[0019] The gas-sensitive layer 20 is provided so as to cover the surfaces of the electrode pair. The gas-sensitive layer 20 is connected to the first electrode and the second electrode at different positions. When gas molecules to be detected are adsorbed onto the gas-sensitive layer 20 and an oxidation reaction occurs, the electrical resistance of the gas-sensitive layer 20 changes. Therefore, the gas sensor 100 detects gases and odors by detecting this change in electrical resistance. The gas-sensitive layer 20 in the present disclosure contains diamond particles and metal oxide.

[0020] The gas-sensitive layer 20 of this embodiment includes a first layer 21 containing a metal oxide and a second layer 22 containing diamond particles and formed on the surface of the first layer 21. In this embodiment, the first layer 21 is formed on a substrate 10. That is, the first layer 21 and the second layer 22 are formed on the substrate 10 in this order in the thickness direction. In the gas-sensitive layer 20 of this embodiment, gas comes into contact with the surface of the second layer 22 opposite to the surface on which the first layer 21 is formed. Therefore, the gas to be detected comes into contact with the surface of the second layer 22. Because the second layer 22 is a porous structure having gas diffusion paths and gaps, it is thought that gas that reaches the surface of the second layer 22 diffuses through these gaps and comes into contact with the first layer 21.

[0021] The metal oxide contained in the first layer 21 is not particularly limited, and examples thereof include tin oxide (SnO2), titanium oxide (TiO2), and zinc oxide (ZnO). The first layer 21 preferably contains tin oxide as the metal oxide. The tin oxide may be tin oxide doped with another element. The other element is not particularly limited, and examples thereof include Zn, Ti, Ce, and Fe. The first layer 21 preferably contains 70 mass % or more of tin oxide, more preferably 80 mass % or more, and even more preferably 90 mass % or more, and may even contain 100 mass %. The first layer 21 may contain one or more types of metal oxide, and may contain, in addition to the metal oxide, a semiconductor material other than the metal oxide.

[0022] The form of the metal oxide contained in the first layer 21 is not particularly limited, but may be, for example, a sheet or particles. From the viewpoints of reducing the resistance value of the gas sensor 100 and improving gas selectivity, the form of the metal oxide contained in the first layer 21 is preferably a sheet.

[0023] In the present disclosure, the term "sheet-like" refers to a flake-like structure having a thickness of approximately 0.5 nm to 200 nm. The average thickness of the sheet-like metal oxide is preferably 1 nm to 100 nm, more preferably 2 nm to 50 nm, and even more preferably 5 nm to 10 nm, from the viewpoint of optimizing the depletion layer, specific surface area, surface active sites, etc. The average thickness of the sheet-like metal oxide is determined by averaging the thicknesses measured for 30 random sheet pieces in a transmission electron microscope (TEM) image. The sheet-like metal oxide preferably has a dendritic structure. In the present disclosure, the term "dendritic structure" refers to a dendritic crystal structure with multiple branches. It is particularly preferable that the first layer 21 contains tin oxide having a dendritic structure, from the viewpoint of reducing the resistance value of the gas sensor 100 and improving gas selectivity. In the following description, tin oxide having a dendritic structure is also referred to as a "tin oxide nanosheet."

[0024] The in-plane size (average) of the sheet-like metal oxide is, for example, about 1 nm to 1000 μm. In the present disclosure, the "in-plane size of the sheet-like metal oxide" refers to the dimension in the length direction of the crystal structure of the metal oxide. The in-plane size of the sheet-like metal oxide is the average value of the in-plane sizes measured for 30 random sheet pieces in a transmission electron microscope (TEM) image. In tin oxide nanosheets, the in-plane size (average) of the sheet-like metal oxide is often observed to be, for example, about 1 nm to 1 μm, and more often, for example, about 10 nm to 500 nm.

[0025] The average secondary particle diameter (average value of secondary particle diameters) of particulate metal oxides is approximately 0.5 nm to 200 nm, and from the viewpoint of optimizing the depletion layer, specific surface area, surface active sites, etc., it is preferably 1 nm to 100 nm, more preferably 2 nm to 50 nm, and even more preferably 5 nm to 10 nm. The average secondary particle diameter of particulate metal oxides is determined by averaging the particle diameters measured for 30 random particles in a transmission electron microscope (TEM) image. In this disclosure, "particle diameter," including primary particle diameter and secondary particle diameter, refers to the circle-equivalent diameter of a particle (the diameter of a circle or sphere with the same area, volume, etc. as the particle).

[0026] There are no particular limitations on the thickness of the first layer 21. The thickness of the first layer 21 is determined by averaging thicknesses measured at 30 arbitrary points in a transmission electron microscope (TEM) image.

[0027] The second layer 22 contains diamond particles. The second layer 22 preferably contains 70 mass % or more of diamond particles, more preferably 80 mass % or more, and even more preferably 90 mass % or more, and may contain 100 mass %. In addition to the diamond particles, the second layer 22 may contain metal oxides, metals, polymers, etc.

[0028] The shape of the diamond particles is not particularly limited, but examples include nanoparticles or particle aggregates. The diamond particles are preferably nanoparticle-shaped, from the viewpoint of increasing the surface area in contact with gas and improving responsiveness to gas. In this disclosure, "nanoparticle-shaped" refers to particles with an average primary particle diameter of approximately 0.5 nm to 200 nm. The average primary particle diameter (average value of primary particle diameters) of the nanoparticle-shaped diamond particles is preferably 1 nm to 100 nm, more preferably 1 nm to 50 nm, even more preferably 1 nm to 20 nm, even more preferably 1 nm to 10 nm, and even more preferably 2 nm to 8 nm, from the viewpoint of optimizing the depletion layer, specific surface area, surface active sites, etc. The average particle diameter of the diamond particles is determined by averaging the particle diameters measured for 30 random diamond particles in a transmission electron microscope (TEM) image.

[0029] The diamond particles preferably include diamond particles having NV centers in view of high defect content, nitrogen doping, high vacancy content, spin-bearing nitrogen impurities, low purity, etc. Commercially available diamond particles may also be used. Examples of commercially available diamond particles include, but are not limited to, trade name: Nanodiamond, manufacturer code: N1083 (particle size: <10 nm, amino group modification, 3% ethylene glycol dispersion, manufactured by Tokyo Chemical Industry Co., Ltd.) and trade name: Nanodiamond, manufacturer code: N1194 (carboxylic acid group modification, 5% aqueous dispersion, manufactured by Tokyo Chemical Industry Co., Ltd.).

[0030] The film thickness of the second layer 22 is not particularly limited, but from the viewpoint of sensor responsiveness to low-concentration gases, it is preferably 1 nm to 1000 μm, more preferably 1 nm to 100 μm, and more preferably 1 nm to 10 μm. The film thickness of the second layer 22 is determined by averaging thicknesses measured at any 30 points in a transmission electron microscope (TEM) image.

[0031] The coverage, or the proportion of the area of ​​the entire surface of the first layer 21 that is covered by the second layer 22, is preferably 10% or more, more preferably 50% or more, even more preferably 65% ​​or more, and even more preferably 80% or more, from the viewpoint of improving sensor characteristics. The upper limit of the coverage is, for example, 100% or less. This coverage is determined from a transmission electron microscope (TEM) image. A layer other than the second layer 22, i.e., a layer made of a material other than diamond particles, may be present on the surface of the first layer 21. In other words, another gas-permeable layer may be interposed between the first layer 21 and the second layer 22. It is not essential that the second layer 22 be formed as a continuous layer on the surface of the first layer 21. The second layer 22 may be scattered as multiple regions spaced apart from one another on the surface of the first layer 21. The gas-sensitive layer 20 may include other layers in addition to the first layer 21 and the second layer 22.

[0032] The gas sensor 100 of the present disclosure has excellent sensor characteristics. More specifically, the sensor characteristics can be improved compared to, for example, a gas sensor having a single gas-sensitive layer made of tin oxide, unlike the present disclosure. The sensor characteristics can be evaluated, for example, using the response value, i.e., Ra / Rg (resistance change). (Here, if Ra / Rg is less than 1, Rg / Ra is used as the response value.) Ra indicates the resistance value of the gas sensor 100 in air, and Rg indicates the resistance value of the gas sensor 100 in a gas to be detected (target gas). A larger response value indicates a higher responsiveness of the gas sensor 100. Furthermore, a larger value obtained by dividing the response value for a target gas by the response value for a non-target gas at the same concentration indicates a higher selectivity for the target gas. In other words, a higher ratio of resistance change indicates a higher selectivity for that gas.

[0033] From the viewpoint of responsiveness, the gas to be detected by the gas sensor 100 is preferably a gas that exhibits a resistance change of 1.1 or more relative to air, more preferably a gas that exhibits a resistance change of 2 or more, even more preferably a gas that exhibits a resistance change of 5 or more, and even more preferably a gas that exhibits a resistance change of 10 or more.

[0034] The gas to be detected is not particularly limited, but is preferably at least one selected from the group consisting of a combustible gas, a reducing gas, an oxidizing gas, a combustion-supporting gas, and water vapor. The combustible gas is not particularly limited, but examples thereof include acetone, isoprene, toluene, ammonia, hydrogen, and acetaldehyde. The reducing gas is not particularly limited, but examples thereof include ammonia, hydrogen, and normal octane. The oxidizing gas is not particularly limited, but examples thereof include nitrogen dioxide and water vapor. The combustion-supporting gas is not particularly limited, but examples thereof include nitric oxide and nitrogen dioxide. The gas to be detected is not limited to the above gases, but may also be, for example, a biological gas or an indoor odor gas.

[0035] More specifically, the gas to be detected is preferably at least one selected from the group consisting of acetaldehyde, hydrogen, normal octane, isoprene, ethanol, nitrogen dioxide, meta-xylene, toluene, para-xylene, acetone, and ethylene, more preferably at least one selected from the group consisting of acetaldehyde, hydrogen, and normal octane, even more preferably at least one selected from the group consisting of acetaldehyde and hydrogen, and even more preferably acetaldehyde.

[0036] When the gas to be detected is acetaldehyde, from the viewpoint of acetaldehyde selectivity, the gas sensor 100 preferably has a resistance change in response to acetaldehyde divided by a resistance change in response to meta-xylene of 3.5 or more. Similarly, the resistance change in response to acetaldehyde divided by a resistance change in response to toluene of 3.6 or more. Similarly, the resistance change in response to acetaldehyde divided by a resistance change in response to para-xylene of 2.5 or more. Similarly, the resistance change in response to acetaldehyde divided by a resistance change in response to hydrogen of 2.3 or more. Similarly, the resistance change in response to acetaldehyde divided by a resistance change in response to ethylene of 6.9 or more.

[0037] The gas sensor 100 of the present disclosure has excellent sensor characteristics, preventing a decrease in detection accuracy even when the gas to be detected is at a relatively low concentration. Therefore, the gas sensor 100 of the present disclosure is effective for detecting low-concentration gases contained in biogases that are believed to be associated with disease. Examples of biogases include, but are not limited to, exhaled breath and skin gases. Examples of low-concentration gases contained in biogases include, but are not limited to, acetaldehyde, acetone, isoprene, and the like. For example, acetaldehyde has been reported to be correlated with lung cancer and esophageal cancer, and there is a demand for detecting low concentrations of acetaldehyde contained in biogases. Therefore, the gas sensor 100 of the present disclosure can be used, for example, for screening for lung cancer and the like by detecting acetaldehyde in biogases. Furthermore, because health risks posed by gases such as acetaldehyde have been reported, the gas sensor 100 can be used for environmental sensing by continuously sensing acetaldehyde concentrations in living spaces, offices, mobility facilities, public facilities, and the like. Furthermore, since the gas sensor 100 of the present disclosure is a semiconductor type, it is possible to prevent a decrease in detection accuracy even when the gas to be detected has a relatively low concentration.

[0038] Generally, in detecting gases and odors, it is sometimes necessary to construct a sensor array consisting of multiple gas sensors and identify the gases and odors by analyzing the data output from the multiple gas sensors using machine learning or other methods. In such cases, it is desirable to install multiple gas sensors with different gas selectivities in order to obtain diverse sensor signals that are effective in identifying gases and odors. Therefore, various gas sensors with different gas selectivities are required to construct a sensor array. In contrast, the gas sensor 100 disclosed herein has a gas-sensitive layer containing diamond particles and metal oxide, which provides excellent gas selectivity and makes it suitable for such sensor arrays. Furthermore, the gas sensor 100 disclosed herein allows for control of gas selectivity.

[0039] The gas sensor 100 of the present disclosure can be manufactured by any method. For example, first, the first layer 21 is formed on the surface of the substrate 10 on which the first electrode and the second electrode have been formed. Any film formation technique may be used to form the first layer 21. Examples of the film formation technique include, but are not limited to, evaporation, sputtering, chemical vapor deposition, and liquid phase epitaxy. When sheet-shaped tin oxide is used for the first layer 21, the first layer 21 is preferably formed by liquid phase epitaxy. A sheet-shaped tin oxide having a dendritic structure can be formed on the substrate 10 by immersing the substrate 10 in a liquid containing a tin oxide precursor. Examples of the tin oxide precursor include, but are not limited to, tin fluoride and tin chloride. However, tin fluoride is preferred from the viewpoint of synthesizing the sheet-shaped tin oxide.

[0040] Next, a second layer 22 is formed on the surface of the first layer 21. In forming the second layer 22, any film-forming technique may be used using diamond particles. This film-forming technique is not particularly limited, but examples thereof include dispersion liquid dropping, vapor deposition, sputtering, chemical vapor deposition, and liquid phase crystal growth. Any technique may be employed to manufacture the diamond particles, and the diamond particles may be synthesized by, for example, an explosion method, a detonation method, an oxygen-deficient explosion method, or a graphite shock compression polycrystalline diamond synthesis method.

[0041] FIG. 2 is a process diagram showing an example of a method for manufacturing the gas sensor 100. The gas sensor 100 includes a gas-sensitive layer 20 having a first layer 21 containing tin oxide and a second layer 22. The following method is suitable for manufacturing the gas sensor 100. First, the substrate 10 on which the electrodes have been formed is immersed in a liquid containing a tin oxide precursor to form the first layer 21 containing tin oxide and having a dendritic structure (step P110). In the following description, the step of forming the first layer 21 is also referred to as the "first step." Next, a solution in which diamond particles are dispersed is applied to the surface of the first layer 21 to form the second layer 22 containing diamond particles (step P120). In the following description, the step of forming the second layer 22 is also referred to as the "second step." The gas sensor 100 is thus manufactured. According to the manufacturing method shown in FIG. 2, the gas-sensitive layer 20 can be synthesized in an aqueous solution without using a pressure vessel or a high-temperature device. As a result, the manufacturing process of the gas sensor 100 can be prevented from becoming complicated, and the gas sensor 100 can be manufactured easily.

[0042] According to another aspect of the present disclosure, there is provided a sensor device including the gas sensor 100 and a detection unit. The detection unit detects a target gas in response to a current flowing between a first electrode and a second electrode. The detection unit preferably detects the concentration of the target gas in addition to detecting the presence of the target gas. This sensor device can prevent a decrease in gas detection accuracy.

[0043] B. Variations The configuration of the gas sensor 100 in the above embodiment is merely an example and can be modified in various ways. For example, the gas-sensitive layer 20 of the gas sensor 100 in the above embodiment has the first layer 21 containing a metal oxide and the second layer 22 containing diamond particles, but it may also be a mixture of metal oxide and diamond particles. For example, the surface of the first layer 21 containing a metal oxide may be dusted with diamond particles, or more specifically, for example, diamond particles may be dusted on tin oxide having a dendritic structure. That is, in general, the gas sensor 100 may have a gas-sensitive layer 20 containing diamond particles and a metal oxide. [Example]

[0044] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0045] [Example 1] A gas sensor having a gas-sensitive layer with a first layer containing sheet-like tin oxide and a second layer containing diamond particles was manufactured as follows as the gas sensor of Example 1. The gas sensor was observed using a scanning electron microscope and a transmission electron microscope, and the sensor characteristics were evaluated.

[0046] <1> Diamond particles Diamond particles were used, trade name: Nanodiamond, manufacturer code: N1083 (particle diameter: <10 nm, amino group modification, 3% ethylene glycol dispersion, manufactured by Tokyo Chemical Industry Co., Ltd.).

[0047] <2> Synthesis of tin oxide (SnO2) nanosheets A dendritic tin oxide film consisting of multiple tin oxide nanosheets was formed as the first layer on an alumina substrate with interdigital electrodes, followed by a second layer consisting of diamond particles. First, a pair of Pt interdigital electrodes were formed on the surface of the alumina substrate using screen printing. A Pt heater electrode, used to heat the sensor, was also formed on the backside of the alumina substrate. Next, the surface of the alumina substrate with the interdigital electrodes was exposed to vacuum ultraviolet light for 20 minutes to remove adsorbed organic matter and render the surface hydrophilic. The tin oxide nanosheets were formed using liquid-phase crystal growth. Specifically, the alumina substrate was immersed in a 28 mM tin fluoride aqueous solution at 90°C for 30 minutes, forming a thin film consisting of tin oxide nanosheets on the surface of the alumina substrate.

[0048] <3> Gas sensor fabrication the above <2> The above-mentioned tin oxide nanosheets were synthesized on the alumina substrate. <1> The diamond particles were dispersed in distilled water using an ultrasonic cleaner, and the dispersion was then dropped onto the tin oxide nanosheet, which was then dried, thereby coating (modifying) the surface of the nanosheet with diamond particles.

[0049] [Comparative Example 1] As Comparative Example 1, a gas sensor (model number TGS2602) manufactured by Figaro Giken Co., Ltd., which is a typical gas sensor for VOCs (Volatile Organic Compounds), was used to evaluate the sensor characteristics.

[0050] Comparative Example 2 As Comparative Example 2, a gas sensor was manufactured in which the gas-sensitive layer was a layer made of a tin oxide nanosheet, and the sensor characteristics were evaluated. The gas sensor of Comparative Example 2 does not include the layer containing diamond particles, as compared with the gas sensor 100 of Example 1. The gas sensor of Comparative Example 2 is <2> It was prepared by the method described in

[0051] FIG. 3 is an explanatory diagram showing a scanning electron microscope image of the gas-sensitive film in the gas sensor of Example 1. FIG. 3(b) is an enlarged view of FIG. 3(a), and FIG. 3(c) is an enlarged view of FIG. 3(b). Diamond particles shown in white were observed in all of FIGS. (a) to (c). Diamond particles with a particle diameter of approximately 1 to 10 nm were observed in FIG. (c). In FIGS. (a) and (b), the diamond particles were observed to be applied almost uniformly. Furthermore, in FIGS. (a) and (b), it was observed that the second layer 22 made of diamond particles did not contain cracks. Examples of cracks include linear cracks with a length of approximately 1 to 2 μm. In FIGS. (a) to (c), the diamond particles were applied uniformly, and no regions (e.g., regions with a size of approximately 100 nm square) consisting only of tin oxide nanosheets were observed in the first layer 21. In the surface shapes shown in these scanning electron microscope images, the surface coverage of first layer 21 by diamond particles of second layer 22 was estimated to be 80 to 100%. Here, second layer 22 is a porous structure having paths and gaps through which gas diffuses, so it is thought that gas adsorbed on the surface of second layer 22 diffuses through these gaps and comes into contact with first layer 21. For this reason, if the coverage is assumed to be the contact area with gas, it is estimated to be around 70 to 90%, smaller than the above-mentioned 80 to 100%.

[0052] FIG. 4 is an explanatory diagram showing a transmission electron microscope image of the cross section of the gas sensor of Example 1. FIG. 4(a) shows a transmission electron microscope image of the cross section of the gas sensor. In FIG. 4(a), an aluminum oxide substrate (Al2O3 substrate) is observed at the bottom of the page, a layer of tin oxide nanosheets is observed in the middle of the page, and a layer of diamond particles is observed at the top of the page. In FIG. 4(a), the thickness of the layer of diamond particles is observed to be a maximum of about 500 μm (the leftmost region of FIG. 4(a)), and regions of tin oxide not covered with diamond particles are also observed (the region near the rightmost region of FIG. 4(a)). In the layer of diamond particles, for example, portions with film thicknesses of about 1 nm, about 10 nm, about 100 nm, about 1 μm, about 10 μm, and about 100 μm are also observed. FIG. 4(b) is an enlarged view showing the region surrounded by the black circle. The black particulate matter is diamond particles, and particularly diamond particles of 1 to 10 nm in size are observed in large numbers.

[0053] Figure 4(c) is a selected area electron diffraction (SAED) image obtained from the area circled in Figure 4(b). As shown in Figure 4(c), three white ring-shaped diffraction lines were observed, and the inner diffraction rings were identified as diffractions with interplanar spacings of 0.207 nm, 0.127 nm, and 0.108 nm, respectively, which were assigned to the {111}, {220}, and {311} crystal planes of cubic diamond. Therefore, Figure 4(c) indicates the presence of diamond particles in the area circled in Figure 4(b).

[0054] FIG. 5 is an explanatory diagram showing a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a cross section of the gas sensor of Example 1. FIG. 5(b) is a mapping image of tin (Sn), carbon (C), and aluminum (Al) in the observation area of ​​FIG. 5(a). FIG. 5(c) is a mapping image of tin (Sn) in the observation area of ​​FIG. 5(a), showing the presence of tin in the white area. FIG. 5(d) is a mapping image of carbon (C) in the observation area of ​​FIG. 5(a), showing the presence of carbon in the white area. FIG. 5(e) is a mapping image of aluminum (Al) in the observation area of ​​FIG. 5(a), showing the presence of aluminum in the white area. FIG. 5(f) is a mapping image of oxygen (O) in the observation area of ​​FIG. 5(a), showing the presence of oxygen in the white area. In Figure 5(a), the thickness of the layer made of diamond particles is observed to be up to about 50 nm (the leftmost region of Figure 5(a)), and there are also areas of tin oxide that are not covered with diamond particles (the region to the right of the center of Figure 5(a)). In the layer made of diamond particles, for example, areas with film thicknesses of about 1 nm and about 10 nm are also observed.

[0055] The gas sensors according to Example 1, Comparative Example 1, and Comparative Example 2 were evaluated for their sensor responses when the concentration of the gas to be detected was varied. The gases to be detected were acetaldehyde, hydrogen, normal octane, isoprene, ethanol, nitrogen dioxide (NO), meta-xylene, toluene, para-xylene, acetone, and ethylene. For acetaldehyde, hydrogen, normal octane, isoprene, ethanol, meta-xylene, toluene, para-xylene, acetone, and ethylene, the sensor resistance (Ra) (unit: Ω) in air and the sensor resistance (Rg) (unit: Ω) in the gas to be detected were measured, and the response value (RESPONSE) was then calculated by Ra / Rg. For nitrogen dioxide (NO), the response value was calculated by Rg / Ra. For Ra, dry air (nitrogen:oxygen = 80:20) was prepared by mixing nitrogen gas and oxygen gas, and this was used for the measurement. Ra was the electrical resistance value of the gas sensor 3600 seconds after the start of exposure to dry air. Rg was measured using the target gas, which was adjusted to 0.5 ppm, 1 ppm, 5 ppm, or 10 ppm by mixing nitrogen gas, oxygen gas, and 25 ppm of target gas (nitrogen base). The oxygen concentration in the target gas after concentration adjustment was 20% by volume. Rg was the electrical resistance value of the gas sensor 3600 seconds after the start of exposure to the target gas. Because a delay occurs in the response of the gas sensor when the gas exposed to the gas sensor is switched, the electrical resistance value was measured after the gas sensor's electrical resistance value stabilized. For the gas sensors of Example 1 and Comparative Example 2, the element temperature was 275°C. For the gas sensor of Comparative Example 1, a heater voltage of 5 V was applied, as described in the instruction manual. In addition, gas selectivity was evaluated for each of the target gases, acetaldehyde, hydrogen, and normal octane, by dividing the response value for acetaldehyde, hydrogen, or normal octane by the response value for each gas at the same concentration.

[0056] Table 1 shows the response values ​​for various detection target gases, the ratio of the response value of Comparative Example 1 to the response value of Comparative Example 2, the ratio of the response value of Example 1 to the response value of Comparative Example 2, and the ratio of the response value of Example 1 to the response value of Comparative Example 1. Table 2 shows the evaluation results of gas selectivity for acetaldehyde, Table 3 shows the evaluation results of gas selectivity for hydrogen, and Table 4 shows the evaluation results of gas selectivity for normal octane.

[0057] [Table 1]

[0058] [Table 2]

[0059] [Table 3]

[0060] [Table 4]

[0061] As shown in Table 2, for Example 1, the value obtained by dividing the resistance change to acetaldehyde by the resistance change to meta-xylene was in the range of 3.5 to 4.2. The value obtained by dividing the resistance change to acetaldehyde by the resistance change to toluene was in the range of 3.6 to 6.5. The value obtained by dividing the resistance change to acetaldehyde by the resistance change to para-xylene was in the range of 2.5 to 5.4. The value obtained by dividing the resistance change to acetaldehyde by the resistance change to hydrogen was in the range of 2.3 to 3.0. The value obtained by dividing the resistance change to acetaldehyde by the resistance change to ethylene was in the range of 6.9 to 13.

[0062] 6 to 16 are explanatory diagrams showing responsiveness to various detection target gases. FIG. 6 is an explanatory diagram showing responsiveness to acetaldehyde. FIG. 7 is an explanatory diagram showing responsiveness to hydrogen. FIG. 8 is an explanatory diagram showing responsiveness to normal octane. FIG. 9 is an explanatory diagram showing responsiveness to isoprene. FIG. 10 is an explanatory diagram showing responsiveness to ethanol. FIG. 11 is an explanatory diagram showing responsiveness to nitrogen dioxide. FIG. 12 is an explanatory diagram showing responsiveness to meta-xylene. FIG. 13 is an explanatory diagram showing responsiveness to toluene. FIG. 14 is an explanatory diagram showing responsiveness to para-xylene. FIG. 15 is an explanatory diagram showing responsiveness to acetone. FIG. 16 is an explanatory diagram showing responsiveness to ethylene.

[0063] The results shown in Tables 1 to 4 and Figures 6 to 16 reveal the following: The gas sensor of Example 1 exhibited a higher response to acetaldehyde than the gas sensor of Comparative Example 1 at all concentrations (0.5-10 ppm). Furthermore, since the gas sensor of Example 1 exhibited a higher response than the gas sensor of Comparative Example 2, it was considered that a gas-sensitive layer containing tin oxide nanosheets and diamond particles contributes to the sensor's response to acetaldehyde gas more than a gas-sensitive layer consisting of tin oxide nanosheets alone. Therefore, it can be said that the sensor response to acetaldehyde gas of the gas sensor (Comparative Example 2) consisting of only tin oxide nanosheets in the first layer was improved by the effect of the diamond particles in the second layer as in Example 1. Furthermore, the gas sensor of Example 1 exhibited a response value (Ra / Rg) of approximately 8 to 10 even at low concentrations of 0.5 ppm and 1 ppm, suggesting that it is possible to detect acetaldehyde at concentrations even lower than 0.5 ppm.

[0064] Furthermore, the gas sensor of Example 1 exhibited higher responsiveness to hydrogen at all concentrations (0.5-10 ppm) than the gas sensor of Comparative Example 1, and higher responsiveness to high-concentration hydrogen of 5-10 ppm than the gas sensor of Comparative Example 2. Furthermore, the gas sensor of Example 1 exhibited higher responsiveness to normal octane at 0.5-5 ppm than the gas sensor of Comparative Example 1, and higher responsiveness than the gas sensor of Comparative Example 2 at all concentrations (0.5-10 ppm). Furthermore, the gas sensor of Example 1 exhibited higher responsiveness to isoprene at 0.5 ppm than the gas sensor of Comparative Example 1.

[0065] Furthermore, the gas sensor of Example 1 exhibited higher responsiveness to ethanol and acetone at all concentrations (0.5-10 ppm) than the gas sensor of Comparative Example 1. The reason why the gas sensor of Comparative Example 2 exhibited higher responsiveness than the gas sensor of Example 1 is thought to be because the tin oxide nanosheets in the gas-sensitive layer are more responsive to ethanol and acetone than the diamond particles. The gas sensor of Example 1 exhibited higher responsiveness to nitrogen dioxide (NO2) at 1-10 ppm than the gas sensor of Comparative Example 1. The reason why the gas sensor of Comparative Example 2 exhibited higher responsiveness than the gas sensor of Example 1 is thought to be because the tin oxide nanosheets in the gas-sensitive layer are more responsive to NO2 than the diamond particles.

[0066] The gas sensor of Example 1 exhibited higher responsiveness to each of meta-xylene, toluene, and para-xylene than the gas sensor of Comparative Example 2. Furthermore, the gas sensor of Example 1 exhibited higher responsiveness to ethylene at 5-10 ppm than the gas sensor of Comparative Example 1. The reason why the gas sensor of Comparative Example 2 exhibited higher responsiveness than the gas sensor of Example 1 is thought to be that the tin oxide nanosheets in the gas-sensitive layer are more responsive to ethylene than the diamond particles.

[0067] The gas sensor of Example 1 was found to have particularly high responsiveness to acetaldehyde, normal octane, and hydrogen. Furthermore, with regard to acetaldehyde selectivity, the gas sensor of Example 1 exhibited higher acetaldehyde / meta-xylene ratios, acetaldehyde / para-xylene ratios, and acetaldehyde / ethylene ratios than the gas sensors of Comparative Examples 1 and 2. This indicates that the gas sensor of Example 1 is particularly excellent in acetaldehyde selectivity when meta-xylene, para-xylene, or ethylene is an interfering gas. The gas sensor of Example 1 also exhibited higher acetaldehyde / toluene ratios, acetaldehyde / isoprene ratios, and acetaldehyde / normal octane ratios than the gas sensor of Comparative Example 1. This indicates that the gas sensor of Example 1 is also excellent in acetaldehyde selectivity when toluene, isoprene, or normal octane is an interfering gas.

[0068] Furthermore, since the gas sensor of Example 1 had a higher responsiveness to acetaldehyde gas than to hydrogen gas, it was shown that it was possible to obtain a high response to acetaldehyde gas while suppressing the response to hydrogen gas. This is a useful sensor performance, for example, in applications where acetaldehyde gas is to be detected in the presence of hydrogen gas, which is not the target of detection, as a noise gas (interference gas). Furthermore, since the gas sensor of Example 1 had a higher responsiveness to acetaldehyde gas than to ethylene gas, it was shown that it was possible to obtain a high response to acetaldehyde gas while suppressing the response to ethylene gas. This is a useful sensor performance, for example, in applications where acetaldehyde gas is to be detected in the presence of ethylene gas, which is not the target of detection, as a noise gas (interference gas). From the above, it was found that the gas sensor of Example 1 exhibited high responsiveness and high selectivity, particularly as an acetaldehyde sensor.

[0069] Furthermore, with regard to hydrogen selectivity, the gas sensor of Example 1 exhibited a higher hydrogen / ethylene ratio than the gas sensors of Comparative Examples 1 and 2. This indicates that the gas sensor of Example 1 is particularly excellent in hydrogen selectivity when ethylene is an interfering gas. The gas sensor of Example 1 also exhibited higher values ​​for the hydrogen / meta-xylene ratio, hydrogen / toluene ratio, hydrogen / para-xylene ratio, and hydrogen / isoprene ratio than the gas sensor of Comparative Example 1. This indicates that the gas sensor of Example 1 is also excellent in hydrogen selectivity when meta-xylene, toluene, para-xylene, or isoprene is an interfering gas.

[0070] Furthermore, with regard to normal octane selectivity, the gas sensor of Example 1 exhibited higher normal octane / meta-xylene ratio, normal octane / toluene ratio, normal octane / para-xylene ratio, and normal octane / ethylene ratio (in the range of 0.5-5 ppm) than the gas sensors of Comparative Examples 1 and 2. This indicates that the gas sensor of Example 1 is particularly excellent in normal octane selectivity when meta-xylene, toluene, para-xylene, or ethylene is an interfering gas. The gas sensor of Example 1 also exhibited a higher normal octane / isoprene ratio than the gas sensor of Comparative Example 1. This indicates that the gas sensor of Example 1 is also excellent in normal octane selectivity when isoprene is an interfering gas.

[0071] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Industrial Applicability]

[0072] According to the present disclosure, a gas sensor with excellent sensor characteristics can be provided. This gas sensor can be used, for example, to detect low concentrations of gases contained in biological gases that are thought to be related to diseases, and can therefore be used for disease screening. It can also be used for environmental sensing by sensing the concentrations of gases contained in living spaces, offices, mobility devices, public facilities, etc. [Explanation of symbols]

[0073] 10...substrate, 20...gas sensitive layer, 21...first layer, 22...second layer, 100...gas sensor

Claims

1. A gas sensor comprising: a gas-sensitive layer comprising diamond particles and a metal oxide; Gas sensor.

2. 2. The gas sensor according to claim 1, The gas-sensitive layer comprises: a first layer comprising the metal oxide; a second layer formed on a surface of the first layer and including the diamond particles; having Gas sensor.

3. 3. The gas sensor according to claim 1, The metal oxide includes tin oxide. Gas sensor.

4. 4. The gas sensor according to claim 3, The tin oxide has a dendritic structure. Gas sensor.

5. 3. The gas sensor according to claim 1, The average primary particle diameter of the diamond particles is 1 nm or more and 10 nm or less. Gas sensor.

6. 3. The gas sensor according to claim 1, The gas to be detected is at least one selected from the group consisting of a combustible gas, a reducing gas, an oxidizing gas, a combustion-supporting gas, and water vapor. Gas sensor.

7. 3. The gas sensor according to claim 1, The gas to be detected is at least one selected from the group consisting of acetaldehyde, hydrogen, normal octane, isoprene, ethanol, nitrogen dioxide, meta-xylene, toluene, para-xylene, acetone, and ethylene. Gas sensor.

8. A method for manufacturing a gas sensor, comprising: a first step of forming a first layer containing tin oxide having a dendritic structure by immersing a substrate on which an electrode has been formed in a liquid containing a tin oxide precursor; a second step of forming a second layer containing diamond particles by applying a solution having diamond particles dispersed therein to the surface of the first layer; A method for manufacturing a gas sensor, comprising:

9. 9. The method for manufacturing a gas sensor according to claim 8, the tin oxide precursor is tin fluoride; A method for manufacturing a gas sensor.

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