JP7711515B2 - Gas Sensors - Google Patents

Description

The present invention relates to a gas sensor.

Research and development into technology that can detect substances present in samples with high sensitivity and selectivity is being actively conducted. If such technology were realized, it would be useful in fields such as managing building ventilation by monitoring harmful gases in the air, managing food freshness by quantifying the amount of putrefactive gases, early detection of disease by detecting disease markers in breath, and preventing terrorist attacks and chemical weapon attacks by detecting minute amounts of toxic gases. For these applications, a highly sensitive detection technology on the order of ppb, or more preferably on the order of ppt, is required, but none has been realized yet.

As a highly sensitive and selective detection technology, a sensor using a material in which recognition molecules, which have a shape and electrostatic potential distribution that selectively interact with the substance to be detected, are immobilized on the surface of a nanocarbon material is being considered (see, for example, Patent Document 1). Nanomaterials, particularly nanocarbon materials, have a large specific surface area, and it is believed that their electronic properties change significantly even with extremely small amounts of target substance, making them suitable for highly sensitive detection technology. As a technology to further enhance the high detection ability of nanocarbon materials, a technology has been reported in which a composite material of carbon nanotubes and metal oxide particles is used in the detection section (see, for example, Non-Patent Document 1).

Gas sensors using nanocarbon materials detect target substances based on changes in contact resistance caused by the adhesion of the target substance to the nanocarbon material surface and changes in resistance caused by carrier injection. As a result, they can be used at room temperature, and are superior to existing gas sensors using metal oxides that require high temperatures for detection.

Another technique proposed for imparting detection selectivity is to laminate a layer containing a cationic polymer as a gas sensor that interacts with the substance to be detected and a graphene oxide layer (see, for example, Patent Document 2).

JP 2008-249705 A JP 2020-134498 A

"Sensors and Actuators: B", (Netherlands), 2012, Vol. 170, pp. 67-74

However, in detection technology using nanocarbon with immobilized target recognition molecules, as described in Patent Document 1, it is difficult to stably maintain the function of the recognition molecules on the nanocarbon material, and it is difficult to immobilize the recognition molecules on the nanocarbon material at high density, so the amount of binding with the target substance does not increase. As a result, there are issues such as a decrease in sensitivity in exchange for selectivity and unsuitability for mass production.

In addition, in the technology described in Patent Document 2, in which a layer containing a gas-sensitive polymer and a nanocarbon material layer are laminated, there is no direct charge transfer between the gas-sensitive material that selectively interacts with the substance to be detected and the nanocarbon material that detects the substance by changing the resistance value, so that only limited effects were obtained in terms of both detection sensitivity and detection selectivity.

Furthermore, as described in Non-Patent Document 1, when a sensor using a nanocarbon material is used at room temperature to detect a target substance, the adsorption and desorption reaction of the target substance takes time. Therefore, there is also the problem of a slow response speed from when the target substance is introduced into the detector until detection is completed, and a slow recovery speed after detection.

The present invention aims to solve these problems by means suitable for industrial production, and to provide a gas sensor that has high detection sensitivity, excellent detection selectivity, and excellent response speed for room temperature detection.

That is, the present invention is a gas sensor having a substrate, a first electrode, a second electrode, a resistor containing a nanocarbon material in contact with the first electrode and the second electrode, and a layer (hereinafter referred to as a "coat layer") containing a polymer material and a metal oxide and/or a metal sulfide in contact with the resistor.

The present invention provides a method for reproducibly producing a gas sensor that is not only highly sensitive but also has excellent selectivity and response speed for each gas type.

FIG. 1 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention; FIG. 1 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention; FIG. 1 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention;

The following describes in detail a preferred embodiment of the gas sensor according to the present invention. However, the present invention is not limited to the following embodiment, and can be modified in various ways depending on the purpose and application.

The gas sensor according to the embodiment of the present invention is a gas sensor having a substrate, a first electrode, a second electrode, a resistor containing a nanocarbon material in contact with the first electrode and the second electrode, and a layer containing a polymer material and a metal oxide and/or a metal sulfide in contact with the resistor (hereinafter referred to as a "coat layer" for convenience in this specification).

A gas sensor is a type of chemical sensor that detects the presence of a specific chemical substance when the test specimen is a gas. Even if the most stable phase of a substance is not a gas phase at room temperature and pressure, it usually has a certain vapor pressure and is contained in a gas sample. In this case, the amount of the substance in the gas sample may be very small. The gas sensor according to the embodiment of the present invention can detect even such small amounts of substances.

The detection of the target substance may be performed by applying a constant voltage between the first and second electrodes and observing the change in the current flowing between the two electrodes when the target substance is exposed to the voltage, or by continuously flowing a constant current between the first and second electrodes and observing the change in the applied voltage when the target substance is exposed to the current. The target substance may also be detected by calculating the resistance value from these voltage-current values and observing the change in the resistance value.

<Coat layer>
The coating layer contains metal oxides and/or metal sulfides, and thereby modifies the characteristics of the resistor in contact with it. Although the mechanism by which this occurs is still unclear, it is believed that the metal oxides and/or metal sulfides modulate the carrier density and the valence band/conduction band structure in the resistor.

Examples of metal oxides/sulfides that provide such effects include the following:
Metal oxides:
Zinc oxide, aluminum oxide, yttrium oxide, indium oxide, gallium oxide, calcium oxide, silver oxide, zirconium oxide, tin oxide, strontium oxide, cerium oxide, tungsten oxide, tantalum oxide, titanium oxide, niobium oxide, hafnium oxide, magnesium oxide.
Metal sulfide:
Zinc sulfide, indium sulfide, cadmium sulfide, silver sulfide, tungsten sulfide, bismuth sulfide, molybdenum sulfide.

Among these, those that exhibit semiconductor properties and are likely to effectively interact electronically with nanocarbon materials are preferred.Furthermore, among these, at least one selected from the group consisting of zinc oxide, zinc sulfide, zirconium oxide, tin oxide, cerium oxide, tungsten oxide, and titanium oxide is preferred, and from the viewpoint of compatibility with polymer materials, tin oxide and titanium oxide are particularly preferred.The tin oxide may be SnO, _SnO2 , or a mixture thereof, but _SnO2 is more preferred because it is more semiconductive.

Methods for forming the coating layer include applying and drying a solution or dispersion containing a polymeric material and a metal oxide and/or metal sulfide, and applying a melt, solution or dispersion containing a monomer or prepolymer of the polymeric material and a metal oxide and/or metal sulfide, and polymerizing the monomer or prepolymer to form a polymeric material.

In order to effectively modify the characteristics of the resistor, it is preferable that at least a portion of the metal oxide and/or metal sulfide in the coating layer is in contact with the resistor. The modification of the characteristics of the nanocarbon material contained in the resistor by the metal oxide and/or metal sulfide can be observed as a change in the work function of the resistor. The work function can be measured by obliquely cutting the region in the gas sensor where the resistor and coating layer are present and performing photoelectron spectroscopy measurement in air on the cut piece.

In order to stably and efficiently form contact between the metal oxide and/or metal sulfide and the resistor, it is preferable that the metal oxide and/or metal sulfide are not unevenly distributed in the coating layer. In order to prevent such uneven distribution of the metal oxide and/or metal sulfide in the coating layer, it is preferable that bonds are formed between the polymer material in the coating layer and the metal oxide and/or metal sulfide. Typically, the specific gravity of the metal oxide and/or metal sulfide is greater than the specific gravity of the polymer material, so the metal oxide and/or metal sulfide tends to be unevenly distributed in the direction of gravity during the coating layer formation stage. This does not occur if the two are held together at the molecular level by bonds.

From the viewpoint of obtaining sufficient selectivity and the effect of improving response speed, it is preferable that the amount of metal oxide and/or metal sulfide in the coating layer is large. On the other hand, in order to ensure the crack resistance of the coating layer, the amount should not be too large. In consideration of the balance between these effects, the volume fraction of metal oxide and/or metal sulfide in the coating layer is preferably 10 vol% or more and 60 vol% or less, more preferably 23 vol% or more and 59 vol% or less, even more preferably 30 vol% or more and 58 vol% or less, even more preferably 37 vol% or more and 58 vol% or less, and particularly preferably 45 vol% or more and 55 vol% or less.

The volume fraction _Xv of the metal oxide and/or metal sulfide in the coating layer can be calculated as follows: the density _ρc of the coating layer is calculated from the volume and weight of the coating layer, and the density _ρm of the metal oxide and/or metal sulfide alone and the density _ρp of the polymer material alone not containing the metal oxide and/or metal sulfide are calculated from the following formula: However, when _Xv is expressed as a percentage, it is 100× _Xv vol%.

The metal oxide and/or metal sulfide in the coating layer preferably has a particulate shape. In order for the electronic properties of the metal oxide and/or metal sulfide to be fully exhibited, it is preferable that the number average particle diameter of the particles is large. On the other hand, in order to form a uniform coating layer with good reproducibility, increase the specific surface area of the metal oxide and/or metal sulfide, and obtain a large effect, it is preferable that the number average particle diameter of the particles is small. In consideration of these effects, the number average particle diameter of the metal oxide and/or metal sulfide is preferably 1 nm or more and 500 nm or less, more preferably 5 nm or more and 100 nm or less, even more preferably 10 nm or more and 70 nm or less, and particularly preferably 15 nm or more and 60 nm or less.

Next, the arrangement of the resistor and coating layer will be described. Schematic cross-sectional views of examples of the structure of a gas sensor according to an embodiment of the present invention are shown in Figures 1 to 3.

The gas sensor 10 shown in FIG. 1 has a coating layer 4 on a substrate 5, and a first electrode 2a and a second electrode 2b on the coating layer 4. A resistor 3 is present in the region between the first electrode 2a and the second electrode 2b so as to contact both electrodes. V represents a voltmeter, which measures the voltage between the first electrode and the second electrode. A represents an ammeter, which measures the current flowing between the first electrode and the second electrode. A power source capable of adjusting the voltage is also connected to the circuit including the first electrode and the second electrode. The method of detecting the detection target substance using the gas sensor according to the embodiment of the present invention may be detection based on the change in the voltage value or current value of the resistor when the resistor is exposed to the detection target substance, or detection based on the change in the resistance value or impedance of the resistor calculated from these values.

The gas sensor 20 shown in FIG. 2 has a first electrode 2a and a second electrode 2b on a substrate 5. A resistor 3 is present in the region between the first electrode 2a and the second electrode 2b so as to contact both electrodes. A coating layer 4 is formed so as to cover the first electrode 2a, the second electrode 2b, and the resistor 3. In other words, compared to the gas sensor 10 shown in FIG. 1, the location of the coating layer 4 is different. The rest of the configuration is the same as that of the gas sensor 10 shown in FIG. 1.

The gas sensor 30 shown in FIG. 3 has two coating layers. That is, like the gas sensor 10 shown in FIG. 1, it has a coating layer 4b on the substrate 5, and a first electrode 2a and a second electrode 2b on top of that. Then, in the region between the first electrode 2a and the second electrode 2b, a resistor 3 is present so as to contact both electrodes. Furthermore, a coating layer 4a is formed so as to cover the first electrode 2a, the second electrode 2b, and the resistor 3. The rest of the configuration is the same as the gas sensor 10 shown in FIG. 1.

The gas sensor according to the embodiment of the present invention may have any structure, but in order to prevent the electrical characteristics of the resistor from being unexpectedly changed by the substrate, it is preferable that a coating layer is present between the substrate and the resistor as shown in Figures 1 and 3, and from the viewpoint of ease of manufacture and reducing factors of variation, it is more preferable that the gas sensor contains only one coating layer as shown in Figure 1, and that this coating layer is present between the substrate and the resistor. The structure shown in Figure 1 is also preferable from the viewpoint that the coating layer does not inhibit the adsorption of gas molecules to the resistor.

The configuration shown in Figure 2 is also preferred when the adhesion between the substrate and the nanocarbon material is good.

When the resistor contains multiple nanocarbon material molecules, the configuration shown in Figures 1 to 3 is preferable to the configuration in which each nanowire has a coating, as disclosed in Patent Document 1, because the nanocarbon materials are in direct contact with each other, resulting in good efficiency in forming conductive paths.

The interface between the coating layer and the resistor may be smooth, or may have irregularities in either layer such that one layer penetrates into the other, or the two layers may penetrate each other.

The thickness of the coating layer is preferably 10 nm or more and 1000 nm or less, more preferably 25 nm or more and 750 nm or less, and particularly preferably 50 nm or more and 500 nm or less. By making the thickness within this range, it becomes easier to form a uniform thin film, and the variation during manufacturing of the resistor in contact with the coating layer, i.e., the variation in the resistance characteristics, is reduced. The thickness of the coating layer can be determined by measuring 10 or more points on the surface of the substrate using an optical interference film thickness measuring device and taking the arithmetic average of the measurements.

<Polymer materials>
The coating layer contains a polymer material to ensure its strength. If a current flows through any part other than the resistor, it is difficult to obtain a correct detection result of the detection target substance, so it is preferable that the polymer material used has high insulating properties. In addition, the polymer material may be a thermosetting resin or a thermoplastic resin. Examples of such polymer materials include the following.

Polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate;
Polyimide;
Polyamideimide;
Polyamides such as polyamide 6, polyamide 6,6, polyamide 12, and polyamide 11;
Fluororesins such as polyvinylidene fluoride, polyvinyl fluoride, polytetrafluoroethylene, ethylene tetrafluoroethylene copolymer, and polychlorotrifluoroethylene;
Vinyl resins such as polyvinyl alcohol, polyvinyl ether, polyvinyl butyral, polyvinyl acetate, and polyvinyl chloride;
Epoxy resin;
Xylene resin;
Aramid resin;
Olefin resins such as polyethylene, polypropylene, cycloolefin polymers (cyclic polyolefins), modified polymers thereof, olefin-modified polymers of polyvinyl chloride, polystyrene, etc.;
Polyimide silicone;
Polyurethane (urethane resin);
Polyurea;
Polysiloxane;
Poly(p-xylylene) (parylene);
Melamine resin;
Phenolic resin;
Polyether;
acrylic resins such as polymethyl methacrylate and polyacrylic acid;
and copolymers thereof.

Among these, it is preferable that the polymer material in the coating layer is a polysiloxane compound, because it has a high ability to retain metal oxides and/or metal sulfides, making it easier to obtain the effects of the present invention, and because the layer is easy to form and has high strength.

<Nanocarbon materials>
Nanocarbon materials include fullerene, carbon nanotube (hereinafter referred to as "CNT"), graphene, carbon nanohorn, etc., each of which will be described below. In the present invention, nanocarbon materials may be used in combination with each other. An example of a combination is peapod, in which fullerene is encapsulated inside CNT.

Fullerene is a compound with a polyhedral structure in which carbon atoms are bonded to each other by ^sp2 hybrid orbital interactions. The polyhedron is composed of five-membered rings and six-membered rings. The number of carbon atoms constituting the polyhedron is 60, 70, 74, 76, 78, etc. Fullerene may be used as a single molecule, or in the form of a fullerene nanowhisker in which a plurality of fullerene molecules are assembled, or a fullerene nanofiber in which fullerene nanowhiskers form a hollow structure.

Graphene is also called a graphene sheet, and ideally, all carbon atoms are bonded to each other through ^sp2 hybrid orbital interactions to form a hexagonal lattice structure. When many graphenes are stacked, they become graphite. Graphene is a special semiconductor that does not have a band gap. Graphene includes layers of carbon stacked up to several atomic layers. The graphene used in the present invention preferably has a carbon layer of 10 atomic layers or less, more preferably 3 atomic layers or less, and particularly preferably a monoatomic layer.

The method for synthesizing graphene is not particularly limited, but examples include mechanical exfoliation, chemical exfoliation, silicon carbide heating, and thermal chemical vapor deposition.

The presence of graphene on a substrate can be easily confirmed using an optical microscope. Although observation using an optical microscope is a simple method, careful observation makes it possible to distinguish between single-layer, double-layer, and triple-layer graphene. For more detailed analysis, Raman spectroscopy is used.

Carbon nanohorns have a structure in which graphene is rolled into a cone shape. Carbon nanohorns can be synthesized by irradiating graphite with a carbon dioxide laser in an argon gas atmosphere at room temperature. The diameter of the carbon nanohorn is preferably about 2 nm or more and 5 nm or less. Carbon nanohorns form aggregates unless a separation process is performed. In the present invention, the aggregates may be used as they are, or the individual nanohorns may be separated and used.

CNTs include single-walled CNTs, in which one carbon film (graphene sheet) is wound into a cylindrical shape, double-walled CNTs, in which two graphene sheets are wound concentrically, and multi-walled CNTs, in which multiple graphene sheets are wound concentrically. CNTs can be obtained by arc discharge, chemical vapor deposition (CVD), laser ablation, etc.

The above-mentioned examples are given as examples of nanocarbon materials contained in the resistor of the gas sensor according to the embodiment of the present invention, but CNTs are particularly preferred from the viewpoint of production takt and because they have an electronic structure that causes a large change in resistance due to the adsorption of a substance. Among CNTs, it is preferable that the target substance can be accessible to all CNTs, since the principle of detection is based on the change in resistance due to the adsorption of the target substance. In other words, it is preferable to use single-walled CNTs.

Among single-walled CNTs, there are CNTs that exhibit metallic properties and CNTs that exhibit semiconducting properties, depending on how the carbon film is wound. One form of gas sensor according to the present invention is one that detects the change in resistance of a resistor due to the adsorption of the substance to be detected. In this case, it is preferable for the resistor to have more semiconducting properties, as this will increase the detection signal strength.

When considering the productivity of the gas sensor, a method in which a dispersion or suspension of CNTs is applied or electrophoretically arranged in the region between the first and second electrodes is more preferable than a method in which a single CNT is arranged or grown between the electrodes. In this case, multiple CNTs will be present between the electrodes, and it is preferable that the proportion of semiconducting CNTs is high. In other words, it is preferable that the resistor contains multiple single-walled CNTs, and that 80% or more, more preferably 90% or more, and even more preferably 95% or more of the multiple single-walled CNTs are semiconducting single-walled CNTs.

Methods for obtaining CNT with 80% or more semiconducting content can be achieved by known methods. Examples include ultracentrifugation in the presence of a density gradient agent, selectively attaching a specific compound to the surface of semiconducting or metallic CNT and separating them by utilizing the difference in solubility, and electrophoresis or other methods that utilize the difference in electrical properties. The content of semiconducting CNT in the present invention is a value calculated from the ratio of the peak area of metallic CNT to the peak area of semiconducting CNT in the Raman spectrum.

In addition, when a plurality of CNTs are used as a resistor, the density also affects the detection signal strength of the gas sensor. Any density is acceptable as long as the resistance value of the resistor is within a measurable range. From the viewpoint of suppressing instrumental errors during measurement, it is better for the resistance value not to be too large, so it is better for the CNT density to be somewhat large. On the other hand, if the CNTs are densely packed, the inside of the CNT aggregate is less likely to be involved in gas detection, so it is better for the CNT density to be somewhat small. In consideration of the balance between these effects, the CNT density is preferably 40/μm2 or ^more and 500/μm2 or ^less .

The CNT density is calculated as follows: An arbitrary 10 μm square area in the resistor is observed with an atomic force microscope (AFM), and the number of CNTs present in the obtained image is counted.

In the present invention, the length of the CNT is preferably shorter than the distance between the first and second electrodes in the sensor to which it is applied. Specifically, the average length of the CNT depends on the distance between the electrodes, but is preferably 2 μm or less, more preferably 1 μm or less. The average length of the CNT refers to the average value of the lengths of 20 randomly selected CNTs. One method for measuring the average CNT length is to randomly select 20 CNTs from images obtained by AFM, scanning electron microscope (SEM), transmission electron microscope (TEM), etc., and obtain the average length.

Generally, commercially available CNTs have a distribution in length and may contain CNTs longer than the distance between the electrodes, so it is preferable to add a process to make the CNTs shorter than the distance between the electrodes. For example, methods such as acid treatment with nitric acid or sulfuric acid, ultrasonic treatment, or cutting into short fibers using a freeze-pulverization method are effective. In addition, it is even more preferable to use separation using a filter in order to improve purity.

The diameter of the CNTs is not particularly limited, but is preferably 0.5 nm or more and 100 nm or less, and more preferably 1 nm or more and 50 nm or less.

<Conjugated Polymer>
In the gas sensor according to the embodiment of the present invention, it is preferable that the resistor further contains a conjugated polymer in addition to the nanocarbon material. This is because the conjugated polymer reinforces the electrical contact between the nanocarbon materials and between the nanocarbon material and the electrode. For example, in the case of CNTs, each CNT is cylindrical, so the contact between the CNTs is an extremely small area. Since CNTs have high electrical conductivity, charge carriers can be sufficiently transferred between CNTs even in a small contact area, but it is more preferable to place a conjugated polymer near the contact point and have it assist in the transfer of charge carriers.

A conjugated polymer is a polymer in which a conjugated system of multiple bonds between atoms is connected within a monomer unit, or within a monomer unit and between adjacent monomer units.

Examples of conjugated polymers include, but are not limited to, polythiophene-based polymers, polypyrrole-based polymers, polyaniline-based polymers, polyacetylene-based polymers, poly-p-phenylene-based polymers, and poly-p-phenylenevinylene-based polymers. The above-mentioned polymers are preferably those in which a single monomer unit is arranged, but those in which different monomer units are block copolymerized, random copolymerized, or graft copolymerized can also be used.

Among the above-mentioned polymers, conjugated polymers having a heterocycle containing a sulfur atom in the repeating unit are preferred, from the viewpoint of the large electron orbital of the conjugated system and the strong interaction with the semiconductor component. Among them, conjugated polymers having a thiophene ring structure in the repeating unit are particularly preferred, since they have strong adhesion to the semiconductor component and a high electron conduction auxiliary effect.

The preferred molecular weight of the conjugated polymer is 800 or more and 100,000 or less in number average molecular weight. In addition, the above-mentioned polymer does not necessarily have to have a high molecular weight, and may be an oligomer consisting of a linear conjugated system.

The conjugated polymer used in the present invention can be synthesized by a known method. For example, a method of coupling thiophene with a thiophene derivative having an alkyl group at the end of the carboxyl group can be used to synthesize a monomer. Specific examples of such a method include a method of coupling a halogenated thiophene derivative with thiophene boronic acid or thiophene boronic acid ester under a palladium catalyst, and a method of coupling a halogenated thiophene derivative with a thiophene Grignard reagent under a nickel or palladium catalyst. By carrying out a polymerization reaction using such a monomer, a polythiophene polymer having an alkyl chain at the end of the carboxyl group as a side chain can be obtained. In addition, a monomer unit can be obtained by coupling a conjugated unit other than thiophene with thiophene in a similar manner. A polymerizable substituent can be introduced at the end of the monomer unit obtained in this way, and polymerization can be carried out under a palladium catalyst or nickel catalyst to obtain a conjugated polymer containing a conjugated unit other than thiophene.

It is preferable to remove impurities such as raw materials and by-products used in the synthesis process from the conjugated polymer used in the present invention. For example, silica gel columnography, Soxhlet extraction, filtration, ion exchange, and chelate methods can be used for this purpose. Two or more of these methods may be combined.

For the purpose of assisting the transfer of charge carriers, it is preferable that the conjugated polymer is attached to at least a part of the nanocarbon material, and if the nanocarbon material is CNT or carbon nanohorn, it is more preferable that it is attached to the sidewall. One method for confirming that the conjugated polymer is attached to the nanocarbon material is to measure the reflection spectrum and confirm that it has changed from the spectrum of the nanocarbon alone. For nanocarbon materials with thin shapes such as fullerenes, CNTs, and carbon nanohorns, it is also possible to directly observe the state in which the conjugated polymer is attached to the nanocarbon material using an AFM.

The term "attachment" here refers to dissimilar substances coming into contact with each other and not easily separating due to intermolecular interactions. Examples of such intermolecular interactions include hydrophobic interactions, π-π electron interactions, cation-π interactions, multiple electrostatic interactions, or multiple hydrogen bonds.

<Substrate>
The material used for the substrate is not particularly limited, and examples thereof include inorganic materials such as silicon wafers, glass, and sintered alumina, organic materials such as aliphatic polyesters, polyethylene terephthalate, polybutylene terephthalate, polycarbonate, polysulfone, polyethersulfone, polyethylene, polypropylene, polystyrene, polyphenylene sulfide, polyparaxylene, polyimide, polyvinyl alcohol, polyvinyl chloride, polyvinylidene fluoride, polysiloxane, polyvinylphenol, and polyaramid, and mixtures of inorganic material powders and organic materials. These materials may be used alone, or a plurality of these materials may be laminated or mixed for use.

In the gas sensor according to the embodiment of the present invention, it is preferable that a heater is provided on the substrate in order to perform repeated gas detection. By heating the gas sensor with the heater after gas detection, the adsorbed gas molecules can be desorbed. In other words, the gas sensor can be regenerated (cleaned) and used repeatedly. The heater used is not particularly limited, but a small, low-power Micro Electro Mechanical Systems (MEMS) heater is preferable.

<Electrode>
Examples of materials used for the first electrode and the second electrode include, but are not limited to, conductive metal oxides such as tin oxide, indium oxide, and indium tin oxide (ITO); metals such as platinum, gold, silver, copper, iron, tin, zinc, aluminum, indium, chromium, lithium, sodium, potassium, cesium, calcium, magnesium, palladium, and molybdenum, and alloys thereof; inorganic conductive substances such as copper iodide and copper sulfide; organic conductive substances such as polythiophene, polypyrrole, polyaniline, and a complex of polyethylenedioxythiophene and polystyrenesulfonic acid; nanocarbon materials such as CNT and graphene; and conductive carbon black. In the first electrode and the second electrode, these materials may be used alone, or a plurality of these materials may be stacked or mixed.

When used as an electrode in the gas sensor according to the present invention, it is preferable to select from gold, silver, platinum, palladium, organic conductive substances, and nanocarbon materials from the viewpoint of stability against corrosive gases, etc.

The width, thickness, spacing, and arrangement of the first and second electrodes can be designed as desired. The width of each electrode is preferably 1 μm or more and 1 mm or less, and the thickness of each electrode is preferably 1 nm or more and 1 μm or less. The spacing between the first and second electrodes is preferably 1 μm or more and 10 mm or less. The planar shape of each electrode is not limited to a rectangle, and may include curves or be comb-shaped. In addition, the width and thickness of each electrode do not have to be the same.

The present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to the following examples.

Among the compounds used, the abbreviations are shown below.
MeSi: Methyltrimethoxysilane SucSi: 3-trimethoxysilylpropylsuccinic anhydride NapSi: 1-naphthyltrimethoxysilane DAA: Diacetone alcohol (also known as 4-hydroxy-4-methyl-2-pentanone)
PGMEA: propylene glycol monomethyl ether acetate.

Example 1
(1) Preparation of a dispersion of CNT to be used in a resistor 0.10 g of poly-3-hexylthiophene (P3HT, manufactured by Sigma-Aldrich, regioregular, number average molecular weight (Mn): 13,000) was added to a flask containing 5 mL of chloroform, and ultrasonic stirring was performed in an ultrasonic cleaner (US-2 manufactured by Iuchi Seiei-do Co., Ltd., output 120 W) to obtain a chloroform solution of P3HT. Next, 0.5 mL of this solution was dropped into a mixed solution of 20 mL of methanol and 10 mL of 0.1 N hydrochloric acid to perform reprecipitation. The solidified P3HT was collected by filtration using a membrane filter (manufactured by PTFE Co., Ltd.: tetrafluoroethylene) with a pore size of 0.1 μm, rinsed thoroughly with methanol, and the solvent was removed by vacuum drying. Dissolution and reprecipitation were performed once more to obtain 90 mg of reprecipitated P3HT.

Next, 1.5 mg of CNT-A (Sigma-Aldrich, (6,5) chirality single-walled CNT, purity 95%) and 1.5 mg of the above P3HT were added to 15 mL of chloroform, and the mixture was ultrasonically stirred for 30 minutes at 250 W output using an ultrasonic homogenizer (Tokyo Rikakikai Co., Ltd. VCX-500) while cooling on ice. After 30 minutes of ultrasonic irradiation, the irradiation was stopped once, 1.5 mg of P3HT was added, and ultrasonic irradiation was continued for another hour. Furthermore, filtration was performed using a membrane filter (pore size 10 μm, diameter 25 mm, Millipore Omnipore membrane) to remove CNT complexes and aggregates with a length of 10 μm or more. 45 mL of dichlorobenzene was added to 5 mL of the obtained filtrate to obtain CNT dispersion A containing CNT-A with P3HT attached (CNT complex concentration in solvent 0.01 g/L).

(2) Synthesis of polymeric material used for coating layer and preparation of coating solution A three-neck flask was charged with 10.90g (0.08mol) of MeSi, 5.25g (0.02mol) of SucSi, 24.84g (0.10mol) of NapSi, and 51.14g of DAA, and an aqueous phosphoric acid solution was added over 10 minutes, in which 0.205g of phosphoric acid (0.50% by mass relative to the charged monomer) was dissolved in 11.16g of water while stirring at room temperature. The flask was then immersed in a 40°C oil bath and stirred for 60 minutes, after which the oil bath was heated to 115°C over 30 minutes. One hour after the start of the temperature rise, the internal temperature of the solution reached 100°C, and the solution was heated and stirred for 2 hours (internal temperature was 100-110°C). The resin solution obtained by heating and stirring was cooled in an ice bath, and then 2% by weight of anion exchange resin and cation exchange resin were added to the resin solution, and stirred for 12 hours. After stirring, the anion exchange resin and the cation exchange resin were removed by filtration to obtain a solution of polysiloxane A. During the temperature increase and the heating and stirring, nitrogen was passed at 0.05 L/min. During the reaction, a total of 19.82 g of by-products, methanol and water, was distilled out. The solid content concentration of the obtained polysiloxane A solution was 33 mass%.

Separately, tin oxide suspension A was prepared by adding 1.0 g of tin (IV) oxide (Kanto Chemical Co., Ltd.) to 100 mL of DAA and ultrasonically stirring the mixture for 3 hours at 250 W output using an ultrasonic homogenizer.

(3) Gas sensor preparation 69.49 g of polysiloxane A solution, 100 ppm of DFX-18 (fluorine-based surfactant, manufactured by Neos Co., Ltd.), 60.77 g of DAA, 36.65 g of PGMEA, and 39.29 g of tin oxide suspension A were mixed and stirred to obtain a homogeneous solution, thereby obtaining polysiloxane A coating solution 1. The obtained polysiloxane A coating solution 1 was spin-coated (800 rpm x 20 seconds) onto a glass substrate (thickness 0.7 mm), and heated at 180°C for 60 minutes using a hot plate to form a coating layer with a thickness of 400 nm. In this case, the volume fraction of _SnO2 in the coating layer was 25 vol%.

Next, gold was vacuum-deposited onto the coating layer by resistance heating to a film thickness of 50 nm, and then photoresist (product name "LC140-10cP", manufactured by Rohm and Haas Co., Ltd.) was spin-coated (1000 rpm x 20 seconds) on top of the gold, and dried by heating at 100°C for 10 minutes. The photoresist film thus produced was pattern-exposed through a photomask using a parallel light mask aligner (PLA-501F, manufactured by Canon Inc.), and then shower-developed for 60 seconds with ELM-D (product name, manufactured by Mitsubishi Gas Chemical Co., Ltd.), a 2.38% by mass TMAH aqueous solution, using an automatic developing device (AD-2000, manufactured by Takizawa Sangyo Co., Ltd.), and then washed with water for 30 seconds. The film was then etched for 5 minutes with AURUM-302 (product name, manufactured by Kanto Chemical Co., Ltd.), and then washed with water for 30 seconds. The resist was removed by immersing the substrate in AZ Remover 100 (product name, manufactured by Merck Performance Materials Co., Ltd.) for 5 minutes, washed with water for 30 seconds, and then heated and dried at 120°C for 20 minutes to form the first and second electrodes.

The width of both electrodes was 100 μm, and the distance between the electrodes (electrode distance) was 10 μm. 400 pL of the CNT dispersion A prepared in (1) above was dropped onto the substrate on which the electrodes were formed using an inkjet device (manufactured by Cluster Technology Co., Ltd.), and heat treatment was performed on a hot plate under a nitrogen gas flow at 150°C for 30 minutes to form a resistor. By using the above procedure, gas sensor A with the structure shown in Figure 1 was produced.

(4) Gas sensor evaluation The fabricated gas sensor A was placed in a glass tube with a diameter of 5 cm, and the gas sensor was exposed to the gas to be measured by mixing the gas to be measured into the air flow while air was flowing as a carrier gas at a flow rate of 1 L/min. The response to the gas was evaluated by measuring the resistance value using a Keithley DMM6500 and calculating the rate of change in resistance value when the gas sensor was exposed to the gas to be detected using the following formula.
(Resistance change rate [%])=100×|R−R ₀ |÷R ₀
Where:
R: resistance value after exposure to the gas to be detected R ₀ : resistance value before exposure to the gas to be detected The measurements were carried out at room temperature (25° C.).

The response speed was evaluated as follows: When the gas sensor is exposed to the gas to be detected for a sufficiently long time, the resistance value converges to a certain value. The time required from the start of gas exposure to reaching 95% of the convergence value was defined as the gas response speed τ (min).

The measurement gases used were 1 ppm ammonia ( _NH3 ) and nitrogen dioxide ( _NO2 ). Ammonia is a typical reducing gas, and nitrogen dioxide is a typical oxidizing gas. As an index of detection selectivity for these gases, the resistance change rate due to the gas with the larger resistance change rate upon exposure divided by the resistance change rate due to the gas with the smaller resistance change rate (detection ratio) was used.

The evaluation results for gas sensor A are shown in Table 1. The resistance change rate was larger when exposed to _NH3 than when exposed to _NO2 , and the gas sensor _A had NH3 detection selectivity.

Example 2
Instead of using 60.77 g of DAA and 39.29 g of tin oxide suspension A, 100.03 g of DAA and 0.033 g of titanium oxide ( _TiO2 ) suspension (20.9 wt %, manufactured by JGC Catalysts and Chemicals Co., Ltd.) in which DAA is used as a solvent were used to prepare polysiloxane A coating solution 2 in the same manner as in Example 1 (3), and gas sensor B was fabricated. In this case, the volume fraction of _TiO2 in the coating layer was 25 vol %.

The evaluation results for gas sensor B are shown in Table 1. The resistance change rate was larger when exposed to _NH3 than when exposed to _NO2 , and the gas sensor _B had NH3 detection selectivity.

Example 3
(1) Synthesis of polymeric material used for coating layer 10.90g (0.08mol) of MeSi, 5.25g (0.02mol) of SucSi, 24.84g (0.10mol) of NapSi, 188.74g of titanium oxide (TiO ₂ ) suspension (20.9wt%), and 90.88g of DAA were charged into a three-neck flask, and an aqueous phosphoric acid solution in which 0.205g of phosphoric acid (0.50% by mass relative to the charged monomer) was dissolved in 11.16g of water was added over 10 minutes while stirring at room temperature. After that, the flask was immersed in a 40°C oil bath and stirred for 60 minutes, and the oil bath was heated to 115°C over 30 minutes. One hour after the start of the temperature rise, the internal temperature of the solution reached 100°C, and it was heated and stirred for 2 hours from there (internal temperature was 100-110°C). After the resin solution obtained by heating and stirring was cooled in an ice bath, 2% by weight of anion exchange resin and cation exchange resin were added to the resin solution and stirred for 12 hours. After stirring, the anion exchange resin and cation exchange resin were filtered and removed to obtain a solution of polysiloxane with _TiO2 particles bonded thereto (polysiloxane B solution). During the temperature increase and heating and stirring, nitrogen was passed at 0.05 L/min. A total of 121.19 g of by-products, methanol and water, was distilled out during the reaction. The solid content concentration of the obtained polysiloxane B solution was 33% by mass.

(2) Preparation and Evaluation of Gas Sensor 69.49 g of polysiloxane B solution, 100 ppm of DFX-18, 54.48 g of DAA, and 36.65 g of PGMEA were mixed and stirred to obtain a homogeneous solution, thereby obtaining polysiloxane B coating solution 1. Gas sensor C was prepared in the same manner as in Example 1(3) except that polysiloxane B coating solution 1 was used instead of polysiloxane A coating solution 1. In this case, the _TiO2 volume fraction in the coating layer was 25 vol%. Then, it was evaluated in the same manner as in Example 1(4).

The evaluation results for gas sensor C are shown in Table 1. The resistance change rate was larger when exposed to _NH3 than when exposed to _NO2 , and the gas sensor _C had NH3 detection selectivity.

Example 4
A solution of polysiloxane bonded with _TiO2 particles (polysiloxane C) was obtained in the same manner as in Example 3(1), except that 264.24 g of titanium oxide ( _TiO2 ) suspension (20.9 wt%) was used instead of 188.74 g and 15.4 g of DAA was used instead of 90.88 g. The solid content of the obtained polysiloxane C solution was 41 mass%.

A polysiloxane C coating solution 1 was obtained in the same manner as in Example 3(2), except that the polysiloxane C solution was used instead of the polysiloxane B solution. A gas sensor D was produced in the same manner as in Example 1(3), except that the polysiloxane C coating solution 1 was used instead of the polysiloxane A coating solution 1. In this case, the _TiO2 volume fraction in the coating layer was 35 vol%. Then, it was evaluated in the same manner as in Example 1(4).

The evaluation results for gas sensor D are shown in Table 1. Gas sensor D was superior to gas sensor C in the rate of resistance change upon exposure to _NH3 , response speed, and detection selectivity for _NH3 compared to _NO2 .

Comparative Example 1
A gas sensor was produced by preparing Polysiloxane A Coating Solution 3 in the same manner as in Example 1(3), except that 100.06 g of DAA was used instead of 60.77 g of DAA and no tin oxide suspension A was added. Gas sensor E produced by this method is a gas sensor containing neither metal oxide nor metal sulfide in the coating layer.

The evaluation results of gas sensor E are shown in Table 1. Compared to the gas sensor of the present invention, it can be seen that not only is there no effect of improving detection strength due to metal oxides and/or metal sulfides, but the response speed is slow and gas detection selectivity is not expressed.

Comparative Example 2
A gas sensor was produced in the same manner as in Example 1(3), except that a _TiO2 suspension (20.9 wt%) was used instead of the polysiloxane A coating solution 1. Gas sensor F produced by this method is a gas sensor that does not contain a polymer material in the coating layer.

Gas sensor F had a resistance value above the upper detection limit before exposure to the target gas, making it impossible to measure. This is thought to be because the coating layer did not contain a polymeric material and was therefore fragile, and was unable to stably hold the electrodes and resistors. As a result, gas sensor evaluation was not possible.

2a, 2b Electrode 3 Resistor 4, 4a, 4b Coating layer 5 Substrate 10, 20, 30 Gas sensor

Claims (11)

A gas sensor comprising: a substrate, a first electrode, a second electrode, a resistor in contact with the first electrode and the second electrode and containing a nanocarbon material, and a layer in contact with the resistor and containing a polymer material and a metal oxide and/or a metal sulfide (hereinafter referred to as a "coating layer"), wherein a volume fraction of the metal oxide and/or metal sulfide in the coating layer is 23 vol% or more and 60 vol% or less , and the metal oxide and/or metal sulfide is at least one selected from the group consisting of zinc oxide, zinc sulfide, zirconium oxide, tin oxide, cerium oxide, tungsten oxide, and titanium oxide. The gas sensor according to claim 1, wherein the coating layer is present between the substrate and the resistor. The gas sensor according to claim 1 or 2, wherein the thickness of the coating layer is 10 nm or more and 1000 nm or less. The gas sensor according to any one of claims 1 to 3, wherein at least a portion of the metal oxide and/or metal sulfide in the coating layer is in contact with the resistor. A gas sensor according to any one of claims 1 to 4, wherein the metal oxide and/or metal sulfide has a particulate shape and a number average particle size of 1 nm or more and 500 nm or less. 6. The gas sensor according to claim 1 , wherein the polymer material is a polysiloxane compound. 7. The gas sensor according to claim 1, wherein a bond is formed between the polymer material and the metal oxide and/or metal sulfide. 8. The gas sensor according to claim 1, wherein the nano-carbon material is a carbon nanotube. 9. The gas sensor according to claim 1 , wherein the resistor contains a plurality of single-walled carbon nanotubes, and 80% or more of the plurality of single-walled carbon nanotubes are semiconducting single-walled carbon nanotubes. 10. The gas sensor according to claim 1, wherein the resistor further contains a conjugated polymer. The gas sensor according to any one of claims 1 to 10 , wherein the substrate is provided with a heater.