Gas sensor

Abstract

To provide a gas sensor with high detection sensitivity and excellent detection selectivity and response speed.SOLUTION: A gas sensor includes: a substrate; a first electrode; a second electrode; a resistive element formed between the first electrode and the second electrode; and a layer in contact with the resistive element and containing a polymeric material and a metal oxide and / or a metal sulfide. The resistive element contains a nano carbon material.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a gas sensor.

Background Art

[0002] Research and development on technologies for highly sensitive and highly selective detection of substances present in a sample have been actively conducted. If such a technology is realized, it is considered useful in fields such as building ventilation management by monitoring harmful gases in the air, food freshness management by quantifying the amount of putrefactive gas, early detection of diseases by detecting disease markers in exhaled breath, and prevention of terrorism and chemical weapon attacks by detecting extremely trace amounts of toxic gases. When aiming at these applications, a highly sensitive detection technology in the order of ppb, more preferably in the order of ppt, is required, but none has been realized yet.

[0003] As a highly sensitive and highly selective detection technology, sensors using materials in which a recognition molecule having a shape and an electrostatic potential distribution that selectively interact with a detection target substance is immobilized on the surface of a nanocarbon material have been studied (see, for example, Patent Document 1). Since nanomaterials, particularly nanocarbon materials, have a large specific surface area, it is considered that the electronic physical properties change greatly even by an extremely small amount of target substance, and they are suitable for highly sensitive detection technologies. As a technology for further enhancing the high detection ability of nanocarbon materials, a technology using a composite material of carbon nanotubes and metal oxide particles in a detection unit has been reported (see, for example, Non-Patent Document 1).

[0004] A gas sensor using a nanocarbon material detects a detection target substance based on a change in contact resistance caused by the adhesion of the detection target substance to the surface of the nanocarbon material or a change in resistance value caused by carrier injection. Therefore, it can be used even at room temperature and is superior to existing gas sensors using metal oxides that require high temperatures for detection.

[0005] In addition, as another technique for imparting detection selectivity, a technique has been proposed in which a layer containing a cationic polymer as a gas-sensitive material that interacts with a substance to be detected and a graphene oxide layer are laminated to impart detection selectivity (see, for example, Patent Document 2).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0007]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the detection technique using nanocarbon with a target recognition molecule immobilized as described in Patent Document 1, it is difficult to stably maintain the function of the recognition molecule on the nanocarbon material, and it is difficult to immobilize the recognition molecule on the nanocarbon material at a high density. Therefore, the amount of binding with the target substance does not increase. As a result, there are problems such as a decrease in sensitivity as a trade-off for selectivity and unsuitability for mass production.

[0009] In addition, in the technique of laminating a layer containing a gas-sensitive polymer and a nanocarbon material layer as described in Patent Document 2, since there is no direct charge transfer between the gas-sensitive material that selectively interacts with the substance to be detected and the nanocarbon material responsible for detection by resistance value change, only limited effects have been obtained for both detection sensitivity and detection selectivity.

[0010] Furthermore, as described in Non-Patent Document 1, when a sensor using a nanocarbon material is used to detect a substance to be detected at room temperature, it takes time for the adsorption and desorption reaction of the substance to be detected. Therefore, there is also a problem that the response speed from when the substance to be detected is introduced into the detector until the detection is completed and the recovery speed after detection are slow.

[0011] An object of the present invention is to solve these problems by means suitable for industrial production and provide a gas sensor having high detection sensitivity, excellent detection selectivity, and response speed in room temperature detection.

Means for Solving the Problems

[0012] That is, the present invention is a gas sensor having 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, a metal oxide, and / or a metal sulfide (hereinafter referred to as a "coat layer").

Effects of the Invention

[0013] According to the present invention, it is possible to provide a means for manufacturing a gas sensor that is not only highly sensitive but also excellent in selectivity and response speed to gas species with good reproducibility.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0015] Hereinafter, preferred embodiments of the gas sensor according to the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and can be variously modified and implemented according to the purpose and application.

[0016] The gas sensor according to an embodiment of the present invention includes a substrate, a first electrode, a second electrode, a resistor that is in contact with the first electrode and the second electrode and contains a nanocarbon material, and a layer that is in contact with the resistor and contains a polymer material, a metal oxide, and / or a metal sulfide (hereinafter, referred to as a "coat layer" for convenience in this specification).

[0017] A gas sensor is a type of chemical sensor that detects the presence of a specific chemical substance and refers to a substance whose subject is a gas. Even a substance whose most stable phase is not a gas phase at normal temperature and pressure 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 trace. The gas sensor according to an embodiment of the present invention can also detect such a trace amount of substance.

[0018] The detection of the detection target substance may be performed by applying a constant voltage between the first electrode and the second electrode and detecting a change in the current value flowing between the two electrodes when the detection target substance is exposed thereto, or by continuously flowing a constant current between the first electrode and the second electrode and detecting a change in the applied voltage when the detection target substance is exposed thereto. Further, the resistance value may be calculated from these voltage-current values, and the detection target substance may be detected based on the change.

[0019] <Coat layer> By containing a metal oxide and / or a metal sulfide, the coat layer modifies the characteristics of the resistor in contact therewith. Although there are still unclear points about this mechanism, it is considered that the carrier density and the structure of the valence band / conduction band in the resistor are modulated by the metal oxide and / or the metal sulfide.

[0020] Examples of the metal oxide / metal sulfide that brings such an effect 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 sulfides: Zinc sulfide, indium sulfide, cadmium sulfide, silver sulfide, tungsten sulfide, bismuth sulfide, molybdenum sulfide.

[0021] Among these, those showing semiconductor properties, which can easily have effective electronic interaction with the nanocarbon material, are preferred. Among them, it is preferably at least one selected from the group consisting of zinc oxide, zinc sulfide, zirconium oxide, tin oxide, cerium oxide, tungsten oxide, and titanium oxide. From the viewpoint of compatibility with the polymer material, tin oxide and titanium oxide are particularly preferred. Tin oxide may be SnO, SnO2, or a mixture thereof, but SnO2 is more preferably used because it has more semiconductor properties.

[0022] As a method for forming the coating layer, there may be mentioned a method of applying and drying a solution or dispersion containing a polymer material and a metal oxide and / or a metal sulfide, a method of applying a melt, solution or dispersion containing a monomer or prepolymer of the polymer material and a metal oxide and / or a metal sulfide, and polymerizing the monomer or prepolymer to form a polymer material.

[0023] In order to effectively modify the properties of the resistor, it is preferable that at least a part of the metal oxide and / or metal sulfide in the coating layer is in contact with the resistor. The modification of the properties 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 where the resistor and the coating layer exist in the gas sensor and performing photoelectron spectroscopy measurement on the section in the atmosphere.

[0024] In order to stably and efficiently form the contact between the metal oxide and / or metal sulfide and the resistor, it is preferable that the metal oxide and / or metal sulfide do not exist unevenly in the coating layer. In order to prevent the uneven distribution of the metal oxide and / or metal sulfide in such a coating layer, it is preferable that a bond is formed between the polymer material in the coating layer and the metal oxide and / or metal sulfide. Typically, since the specific gravity of the metal oxide and / or metal sulfide is greater than that of the polymer material, the metal oxide and / or metal sulfide tend to be unevenly distributed in the direction of gravity during the coating layer formation stage. If the two are held together at the molecular level by a bond, such a situation will not occur.

[0025] From the viewpoint of obtaining sufficient selectivity and the effect of improving the response speed, it is preferable that the amount of the 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. Considering the balance of these effects, the volume fraction of the 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, still 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.

[0026] The volume fraction X of the metal oxide and / or metal sulfide in the coating layer v can be obtained as follows. That is, the density ρ of the coating layer is obtained from the volume and weight of the coating layer c , and the density ρ of the metal oxide and / or metal sulfide alone m and the density ρ of the polymer material alone without the metal oxide and / or metal sulfide p are used to obtain it from the following formula. However, when X v is expressed as a percentage, it becomes 100×X v vol%.

[0027]

Equation

[0028] 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 above particles is larger. 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 above particles is smaller. Considering the balance 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, still more preferably 10 nm or more and 70 nm or less, and particularly preferably 15 nm or more and 60 nm or less.

[0029] Next, the arrangement of the resistor and the coating layer will be described. Regarding the structural example of the gas sensor according to the embodiment of the present invention, a schematic cross-sectional view thereof is shown in FIGS. 1 to 3.

[0030] In the gas sensor 10 shown in FIG. 1, a coating layer 4 is provided on a substrate 5, and a first electrode 2a and a second electrode 2b are provided thereon. And a resistor 3 exists in the region between the first electrode 2a and the second electrode 2b so as to be in contact with both electrodes. V represents a voltmeter and measures the voltage between the first electrode and the second electrode. A represents an ammeter and measures the current flowing between the first electrode and the second electrode. Also, a power supply capable of adjusting the voltage is connected to the circuit including the first electrode and the second electrode. As a method for detecting a substance to be detected using the gas sensor according to the embodiment of the present invention, it may be detected by changes in the voltage value or current value of the resistor when the resistor is exposed to the substance to be detected, or the resistance value or impedance of the resistor may be calculated from those values and detected by the changes.

[0031] In the gas sensor 20 shown in FIG. 2, a first electrode 2a and a second electrode 2b are provided on a substrate 5. And in the region between the first electrode 2a and the second electrode 2b, a resistor 3 exists in contact with both electrodes. The coat layer 4 is formed so as to cover the first electrode 2a, the second electrode 2b, and the resistor 3. That is, compared with the gas sensor 10 shown in FIG. 1, the location where the coat layer 4 exists is different. Other configurations are the same as those of the gas sensor 10 shown in FIG. 1.

[0032] The gas sensor 30 shown in FIG. 3 has two coat layers. That is, similar to the gas sensor 10 shown in FIG. 1, it has a coat layer 4b on the substrate 5, and on top of that, a first electrode 2a and a second electrode 2b. And in the region between the first electrode 2a and the second electrode 2b, a resistor 3 exists in contact with both electrodes. Further, a coat layer 4a is formed so as to cover the first electrode 2a, the second electrode 2b, and the resistor 3. Other configurations are the same as those of the gas sensor 10 shown in FIG. 1.

[0033] The gas sensor according to the embodiment of the present invention may have any structure, but in order to avoid unexpected changes in the electrical characteristics of the resistor due to the substrate, as shown in FIGS. 1 and 3, it is preferable that the coat layer exists between the substrate and the resistor. From the viewpoint of ease of manufacturing and reduction of variation factors, as shown in FIG. 1, it is more preferable that only one coat layer is contained and the coat layer exists between the substrate and the resistor. The structure shown in FIG. 1 is also preferable from the viewpoint that the coat layer does not inhibit the adsorption of gas molecules to the resistor.

[0034] Also, the configuration shown in FIG. 2 is a preferable configuration when the adhesion between the substrate and the nanocarbon material is good.

[0035] When the resistor contains a plurality of nanocarbon material molecules, the configurations shown in FIGS. 1 to 3 are more preferable than the form in which each nanowire has a coating as disclosed in Patent Document 1, because there is direct contact between the nanocarbon materials and good conductive path formation efficiency can be obtained.

[0036] The interface between the coating layer and the resistor may be smooth, or may be in a form where one layer penetrates into the other layer due to the presence of irregularities in either layer, or may be in a form where they penetrate each other.

[0037] 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 setting the thickness within this range, it becomes easier to form a uniform thin film, and the variation during the manufacture of the resistor in contact with the coating layer, that is, the variation in resistance characteristics, is reduced. The thickness of the coating layer can be obtained from the arithmetic mean by measuring 10 or more points in-plane on the substrate using an optical interference type film thickness measuring device.

[0038] <Polymer material> The coating layer contains a polymer material to ensure its strength. Since it is difficult to correctly obtain the detection result of the substance to be detected when current flows through parts other than the resistor, the polymer material used is preferably one with high insulation. Also, the polymer material may be a thermosetting resin or a thermoplastic resin. Examples of such polymer materials are as follows.

[0039] 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; Fluorine resins 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, and polymers obtained by olefin-modifying polyvinyl chloride, polystyrene, etc.; Polyimide silicone; Polyurethane (urethane resin); Polyurea; Polysiloxane; Poly(p-xylene) (parylene); Melamine resin; Phenolic resin; Polyether; Acrylic resins such as polymethyl methacrylate and polyacrylic acid; And copolymers thereof.

[0040] Among these, from the viewpoint of easily obtaining the effects of the present invention due to the high ability to hold metal oxides and / or metal sulfides, and easy layer formation and high strength, the polymer material in the coating layer is preferably a polysiloxane compound.

[0041] <Nanocarbon material> Examples of nanocarbon materials include fullerenes, carbon nanotubes (hereinafter referred to as "CNT"), graphene, carbon nanohorns, etc., which will be described below. In the present invention, nanocarbon materials may be used in combination. An example of the combination is a peapod in which a fullerene is encapsulated inside a CNT.

[0042] Fullerenes are compounds with a polyhedral structure in which carbon atoms are bonded by sp 2 hybrid orbital interactions. The polyhedron is composed of five-membered rings and six-membered rings. The number of carbon atoms constituting the polyhedron includes 60, 70, 74, 76, 78, etc. Fullerenes may be used individually, or in the form of fullerene nanowhiskers in which a plurality of fullerene molecules are aggregated, or fullerene nanofibers in which fullerene nanowhiskers form a hollow structure.

[0043] Graphene, also called a graphene sheet, ideally has all carbon atoms bonded to each other through sp 2 hybrid orbital interactions and has a hexagonal lattice structure. When multiple layers of graphene are stacked, it becomes graphite. Graphene is a special semiconductor without a bandgap. Graphene is also called including those with carbon layers stacked up to several atomic layers. The graphene used in the present invention preferably has 10 atomic layers or less of carbon layers, more preferably 3 atomic layers or less, and particularly preferably a single atomic layer.

[0044] The synthesis method of graphene is not particularly limited, and examples include mechanical exfoliation method, chemical exfoliation method, silicon carbide heating method, or thermal chemical vapor deposition method, etc.

[0045] The presence of graphene on a substrate can be simply confirmed by an optical microscope. Although observation by an optical microscope is a simple method, by observing carefully, it is also possible to distinguish single-layer, bilayer, and trilayer graphene. Raman spectroscopy is used for more detailed analysis.

[0046] Carbon nanohorns have a structure in which graphene is rolled into a conical 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 nanohorns is preferably about 2 nm or more and 5 nm or less. Carbon nanohorns form aggregates when not subjected to a separation process. In the present invention, they may be used as aggregates or separated one by one and used.

[0047] Examples of CNTs include single-layer CNTs in which a single carbon film (graphene sheet) is wound cylindrically, bilayer CNTs in which two graphene sheets are wound concentrically, and multi-layer CNTs in which multiple graphene sheets are wound concentrically. CNTs can be obtained by an arc discharge method, a chemical vapor deposition method (CVD method), a laser ablation method, etc.

[0048] Examples of the nanocarbon materials contained in the resistor of the gas sensor according to the embodiments of the present invention include those described above. From the perspective of production tact and having an electronic structure that causes a large resistance change due to substance adsorption, CNT is particularly preferred. Among CNTs, due to the principle of detection by resistance change caused by adsorption of the detection target substance, it is preferable that the detection target substance can approach all CNTs. That is, it is preferable to use single-walled CNTs.

[0049] Among single-walled CNTs, depending on the way the carbon film is wound, there are CNTs showing metallic properties and CNTs showing semiconductor properties. As one form of the gas sensor according to the present invention, a device that detects by resistance change of the resistor due to adsorption of the detection target substance can be considered. In this case, it is preferable that the resistor has more semiconductor properties because the detection signal intensity becomes larger.

[0050] Considering the productivity of the gas sensor, a method of applying or arranging a dispersion or suspension of CNTs in the region between the first electrode and the second electrode by coating or electrophoresis is more preferable than a method of arranging or growing a single CNT between the electrodes. In this case, a plurality of CNTs will exist between the electrodes, and it is preferable that the proportion of semiconductor-type CNTs is higher. That is, it is preferable that the resistor contains a plurality of single-walled CNTs, and among the plurality of single-walled CNTs, 80% or more, more preferably 90% or more, and even more preferably 95% or more are semiconductor-type single-walled CNTs.

[0051] As a method for obtaining CNTs with 80% or more of semiconductor type, known methods can be used. For example, a method of ultracentrifugation in the coexistence of a density gradient agent, a method of selectively attaching a specific compound to the surface of semiconductor-type or metallic-type CNTs and separating using the difference in solubility, a method of separating by electrophoresis using the difference in electrical properties, etc. can be mentioned. The content ratio of semiconductor-type CNTs in the present invention is a value calculated from the ratio of the peak area of metallic-type CNTs to the peak area of semiconductor-type CNTs in the Raman spectrum.

[0052] Also, when using CNTs consisting of multiple strands as a resistor, its density also affects the detection signal intensity of the gas sensor. Any density is acceptable as long as the resistance value of the resistor is within the measurable range. From the perspective of suppressing equipment errors during measurement, it is better if the resistance value is not too large, so a relatively large CNT density is preferable. On the other hand, when the CNTs are densely packed, it becomes difficult for the inside of the CNT aggregate to participate in gas detection, so a relatively small CNT density is preferable. Considering the balance of these effects, the CNT density is preferably 40 strands / μm 2 or more and 500 strands / μm 2 or less.

[0053] The CNT density is determined as follows. That is, a 10-μm square region arbitrarily selected in the resistor is observed with an atomic force microscope (AFM), and the number of CNTs present in the obtained image is counted for calculation.

[0054] In the present invention, the length of the CNT is preferably shorter than the distance between the first electrode and the second electrode in the applied sensor. Specifically, the average length of the CNT depends on the electrode distance, 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. As a method for measuring the average length of the CNT, a method of randomly selecting 20 CNTs from the images obtained by an AFM, a scanning electron microscope (SEM), a transmission electron microscope, etc. (TEM) and obtaining the average value of their lengths can be mentioned.

[0055] Generally, commercially available CNTs have a length distribution and may contain CNTs longer than the electrode distance. Therefore, it is preferable to add a step of making the CNTs shorter than the electrode distance. For example, methods such as acid treatment with nitric acid, sulfuric acid, etc., ultrasonic treatment, or cutting into short fibrous shapes by a freeze pulverization method are effective. Also, using separation by a filter in combination is more preferable in terms of improving purity.

[0056] Also, the diameter of the CNT is not particularly limited, but is preferably 0.5 nm or more and 100 nm or less, more preferably 1 nm or more and 50 nm or less.

[0057] <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 to reinforce the electrical contact between the nanocarbon materials and between the nanocarbon material and the electrode by the conjugated polymer. For example, in the case of CNT, since each CNT has a cylindrical shape, the contact between CNTs is an extremely small area. Since CNTs have high conductivity, charge carriers can be sufficiently transferred between CNTs even in a small contact area, but it is more preferable to arrange a conjugated polymer near the contact point and assist in the transfer of charge carriers there.

[0058] A conjugated polymer is a polymer in which a conjugated system of multiple bonds between atoms is continuous within a monomer unit or between a monomer unit and adjacent monomer units.

[0059] Examples of the conjugated polymer include polythiophene-based polymers, polypyrrole-based polymers, polyaniline-based polymers, polyacetylene-based polymers, poly-p-phenylene-based polymers, poly-p-phenylene vinylene-based polymers, etc., but are not particularly limited. Among the above-mentioned polymers, those in which a single monomer unit is arranged are preferably used, but those obtained by block copolymerization, random copolymerization, or graft copolymerization of different monomer units are also used.

[0060] Among the above-mentioned polymers, from the viewpoint of a large conjugated electron orbit and a large interaction with the semiconductor component, a conjugated polymer having a heterocyclic ring containing a sulfur atom in the repeating unit is preferable. Among them, a conjugated polymer having a thiophene ring structure in the repeating unit is particularly preferable because of its strong adhesion to the semiconductor component and high electron conduction assisting effect.

[0061] The preferred molecular weight of the conjugated polymer is 800 or more and 100,000 or less in terms of number average molecular weight. Also, the above-mentioned polymer does not necessarily have to be of high molecular weight, and it may be an oligomer consisting of a linear conjugated system.

[0062] The conjugated polymer used in the present invention can be synthesized by known methods. To synthesize a monomer, for example, a method of coupling thiophene and a thiophene derivative having an alkyl group with a carboxy group at the terminal can be mentioned. Specific examples thereof include a method of coupling a halogenated thiophene derivative and thiophene boronic acid or a thiophene boronic acid ester under a palladium catalyst, and a method of coupling a halogenated thiophene derivative and a thiophene Grignard reagent under a nickel or palladium catalyst. By performing a polymerization reaction using such a monomer, a polythiophene-based polymer having an alkyl chain with a carboxy group at the terminal as a side chain can be obtained. Also, a monomer unit can be obtained by coupling a conjugated unit other than thiophene and thiophene in the same manner. By introducing a polymerizable substituent at the terminal of the monomer unit thus obtained and allowing the polymerization to proceed under a palladium catalyst or a nickel catalyst, a conjugated polymer containing a conjugated unit other than thiophene can be obtained.

[0063] 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. As the method, for example, a silica gel column chromatography method, a Soxhlet extraction method, a filtration method, an ion exchange method, a chelate method, etc. can be used. Two or more of these methods may be combined.

[0064] For the purpose of assisting the transfer of charge carriers, the conjugated polymer is preferably attached to at least a part of the nanocarbon material. When the nanocarbon material is CNT or carbon nanohorn, it is more preferably attached to their side walls. As a means of confirming that the conjugated polymer is attached to the nanocarbon material, there is a method of measuring the reflection spectrum and confirming that it has changed from the spectrum of the nanocarbon alone. For thin-shaped nanocarbon materials such as fullerenes, CNTs, and carbon nanohorns, the state of the conjugated polymer attached to the nanocarbon material can also be directly observed by AFM.

[0065] The "attachment" mentioned here refers to the situation where different substances come into contact with each other and are not easily separated by intermolecular interactions. Examples of such intermolecular interactions include hydrophobic interactions, π-π electron interactions, cation-π interactions, multiple electrostatic interactions, or multiple hydrogen bonds.

[0066] <Substrate> The material used for the substrate is not particularly limited. For example, inorganic materials such as silicon wafers, glass, and alumina sintered bodies, 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, polyvinyl phenol, and polyaramide, or mixtures of inorganic material powders and organic materials can be mentioned. These materials may be used alone, or a plurality of these materials may be laminated or mixed and used.

[0067] In the gas sensor according to the embodiment of the present invention, in order to repeatedly detect gas, it is preferable that a heater is provided on the substrate. By heating the gas sensor with the heater after gas detection, adsorbed gas molecules can be desorbed. That is, the gas sensor can be regenerated (cleaned) and used repeatedly. The heater to be used is not particularly limited, but a Micro Electro Mechanical Systems (MEMS) heater that is small and has low power consumption is preferable.

[0068] <Electrode> Examples of the materials used for the first electrode and the second electrode include 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 polyethylene dioxythiophene and polystyrene sulfonic acid; nanocarbon materials such as CNT and graphene; and conductive carbon black, but are not limited thereto. In the first electrode and the second electrode, these materials may be used alone, or a plurality of these materials may be laminated or mixed and used.

[0069] When used as an electrode in the gas sensor according to the present invention, from the viewpoint of stability against corrosive gases and the like, it is preferably selected from gold, silver, platinum, palladium, organic conductive substances, and nanocarbon materials.

[0070] The width, thickness, interval, and arrangement of the first electrode and the second electrode can be arbitrarily designed. 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 interval between the first electrode and the second electrode 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 a curve or may be in a comb shape or the like. Also, the width and thickness of each electrode do not have to be the same.

Example

[0071] Hereinafter, the present invention will be described more specifically based on examples. Note that the present invention is not limited to the following examples at all.

[0072] Among the compounds used, those using abbreviations are shown below. MeSi: Methyltrimethoxysilane SucSi: 3-Trimethoxysilylpropyl succinic anhydride NapSi: 1-Naphthyltrimethoxysilane DAA: Diacetone alcohol (alias: 4-Hydroxy-4-methyl-2-pentanone) PGMEA: Propylene glycol monomethyl ether acetate.

[0073] Example 1 (1) Preparation of a dispersion of CNTs used for the resistor 0.10 g of poly-3-hexylthiophene (P3HT, manufactured by Sigma-Aldrich, regioregular, number average molecular weight (Mn): 13000) was added to a flask containing 5 mL of chloroform, and ultrasonic stirring was performed in an ultrasonic cleaner (US-2 manufactured by Seiei Do Co., Ltd., output 120 W) to obtain a chloroform solution of P3HT. Next, this solution was dropped by 0.5 mL each into a mixed solution of 20 mL of methanol and 10 mL of 0.1 N hydrochloric acid for reprecipitation. The solidified P3HT was filtered and collected by a membrane filter (PTFE: tetrafluoroethylene) with a pore size of 0.1 μm, thoroughly washed with methanol, and then the solvent was removed by vacuum drying. Dissolution and reprecipitation were performed once again to obtain 90 mg of reprecipitated P3HT.

[0074] Next, 1.5 mg of CNT-A (manufactured by 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 ultrasonic stirring was performed for 30 minutes at an output of 250 W using an ultrasonic homogenizer (VCX-500 manufactured by Tokyo Rika Kikai Co., Ltd.) while ice-cooling. When the ultrasonic irradiation had been carried out for 30 minutes, the irradiation was stopped once, 1.5 mg of P3HT was added, and ultrasonic irradiation was further carried out for 1 hour. Further, filtration was performed using a membrane filter (pore diameter 10 μm, diameter 25 mm, Omnipore membrane manufactured by Millipore) to remove CNT composites and aggregates having 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 to which P3HT was attached (CNT composite concentration in the solvent: 0.01 g / L).

[0075] (2) Synthesis of polymer material used for coating layer and preparation of coating solution 10.90 g (0.08 mol) of MeSi, 5.25 g (0.02 mol) of SucSi, 24.84 g (0.10 mol) of NapSi, and 51.14 g of DAA were charged into a three-necked flask, and while stirring at room temperature, an aqueous phosphoric acid solution in which 0.205 g (0.50% by mass with respect to the charged monomers) of phosphoric acid was dissolved in 11.16 g of water was added over 10 minutes. Thereafter, the flask was immersed in an oil bath at 40 °C and stirred for 60 minutes, and then the temperature of the oil bath was raised to 115 °C over 30 minutes. One hour after the start of temperature increase, the internal temperature of the solution reached 100 °C, and heating and stirring were carried out for 2 hours from there (internal temperature: 100 to 110 °C). After the resin solution obtained by heating and stirring was cooled in an ice bath, 2% by weight of an anion exchange resin and a cation exchange resin were added to the resin solution, respectively, and stirring was carried out for 12 hours. After stirring, the anion exchange resin and the cation exchange resin were filtered off to obtain a solution of polysiloxane A. During the temperature increase and heating and stirring, nitrogen was flowed at 0.05 L / min. A total of 19.82 g of by-products, methanol and water, distilled out during the reaction. The solid content concentration of the obtained polysiloxane A solution was 33% by mass.

[0076] Separately, 1.0 g of tin(IV) oxide (manufactured by Kanto Chemical Co., Inc.) was placed into 100 mL of DAA, and ultrasonic stirring was performed for 3 hours at an output of 250 W using an ultrasonic homogenizer to prepare tin oxide suspension A.

[0077] (3) Fabrication of gas sensor 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 form a uniform solution, thereby obtaining polysiloxane A coating solution 1. The obtained polysiloxane A coating solution 1 was spin-coated (800 rpm × 20 s) onto a glass substrate (thickness 0.7 mm), and heat treatment was performed at 180 °C for 60 minutes using a hot plate to form a coating layer with a film thickness of 400 nm. In this case, the volume fraction of SnO2 in the coating layer was 25 vol%.

[0078] Next, gold was vacuum-evaporated onto the coating layer to a film thickness of 50 nm by the resistance heating method, and a photoresist (trade name "LC140-10cP", manufactured by Rohm and Haas Co., Ltd.) was spin-coated (1000 rpm × 20 s) thereon and heat-dried at 100 °C for 10 minutes. The fabricated photoresist film 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 (trade name, an aqueous solution of 2.38 mass% TMAH, manufactured by Mitsubishi Gas Chemical Co., Inc.) using an automatic developing apparatus (AD-2000 manufactured by Takizawa Sangyo Co., Ltd.), and then washed with water for 30 seconds. Thereafter, etching treatment was performed with AURUM-302 (trade name, manufactured by Kanto Chemical Co., Inc.) for 5 minutes and then washed with water for 30 seconds. It was immersed in AZ Remover 100 (trade name, manufactured by Merck Performance Materials Co., Ltd.) for 5 minutes to remove the resist, washed with water for 30 seconds, and then heat-dried at 120 °C for 20 minutes to form the first electrode and the second electrode.

[0079] The width of both of these electrodes was 100 μm, and the distance between both electrodes (electrode interval) was 10 μm. 400 pL of the CNT dispersion liquid 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 at 150 °C for 30 minutes under a nitrogen stream on a hot plate to form a resistor. By the above procedure, the gas sensor A having the structure shown in FIG. 1 was fabricated.

[0080] (4) Gas sensor evaluation The fabricated gas sensor A was placed in a glass tube with a diameter of 5 cm, and while flowing air as a carrier gas at a flow rate of 1 L / min, the measurement target gas was mixed into the air stream to expose the gas sensor to the measurement target gas. The response to the gas was evaluated by measuring the resistance value using a Keithley DMM6500 and calculating the rate of change in the resistance value when the gas sensor was exposed to the gas to be detected according to the following formula. (Rate of change in resistance value [%]) = 100 × |R - R0| ÷ R0 Here, R: Resistance value after exposure to the gas to be detected R0: Resistance value before exposure to the gas to be detected That is. This measurement was performed at room temperature (25 °C).

[0081] In addition, the response speed was evaluated as follows. When the gas sensor was exposed to the gas to be detected for a sufficiently long time, the resistance value converged to a certain value. The time required from the start of gas exposure until reaching 95% of the converged value was defined as the gas response speed τ (min).

[0082] As the gases to be measured, 1 ppm of ammonia (NH3) and nitrogen dioxide (NO2) were used. Ammonia is a typical reducing gas, and nitrogen dioxide is a typical oxidizing gas. As an index of the detection selectivity for these gases, the value obtained by dividing the rate of change in resistance due to the gas with the larger rate of change in resistance during exposure by the rate of change in resistance due to the gas with the smaller rate of change in resistance (detection ratio) was used.

[0083] The evaluation results of gas sensor A are shown in Table 1. It showed a larger resistance change rate during NH3 exposure than during NO2 exposure and had NH3 detection selectivity.

[0084] 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 dioxide (TiO2) suspension (manufactured by JGC Catalysts and Chemicals Ltd., 20.9 wt%) using DAA as a solvent were used. In the same manner as in Example 1(3), polysiloxane A coating solution 2 was prepared, and gas sensor B was fabricated. In this case, the TiO2 volume fraction in the coating layer was 25 vol%.

[0085] The evaluation results of gas sensor B are shown in Table 1. It showed a larger resistance change rate during NH3 exposure than during NO2 exposure and had NH3 detection selectivity.

[0086] Example 3 (1) Synthesis of polymer material used for coating layer Into a three-necked flask, 10.90 g (0.08 mol) of MeSi, 5.25 g (0.02 mol) of SucSi, 24.84 g (0.10 mol) of NapSi, 188.74 g of a titanium dioxide (TiO2) suspension (20.9 wt%), and 90.88 g of DAA were charged. While stirring at room temperature, an aqueous phosphoric acid solution in which 0.205 g of phosphoric acid (0.50% by mass based on the charged monomers) was dissolved in 11.16 g of water was added over 10 minutes. Thereafter, the flask was immersed in an oil bath at 40 °C and stirred for 60 minutes, and then the temperature of the oil bath was raised to 115 °C over 30 minutes. One hour after the start of heating, the internal temperature of the solution reached 100 °C, and heating and stirring were carried out for 2 hours from then (the internal temperature was 100 - 110 °C). After cooling the resin solution obtained by heating and stirring in an ice bath, 2% by weight of an anion exchange resin and a cation exchange resin were each added to the resin solution and stirred for 12 hours. After stirring, the anion exchange resin and the cation exchange resin were filtered off to obtain a solution of polysiloxane with TiO2 particles bonded thereto (polysiloxane B solution). During heating and heating and stirring, nitrogen was flowed at 0.05 L / min. A total of 121.19 g of methanol and water, which are by-products, distilled out during the reaction. The solid content concentration of the obtained polysiloxane B solution was 33% by mass.

[0087] (2) Fabrication and evaluation of gas sensor 69.49 g of the polysiloxane B solution, 100 ppm of DFX-18, 54.48 g of DAA, and 36.65 g of PGMEA were mixed and stirred to form a uniform solution, thereby obtaining polysiloxane B coating solution 1. A gas sensor C was fabricated 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 coat layer was 25 vol%. Next, evaluation was carried out in the same manner as in Example 1(4).

[0088] The evaluation results for gas sensor C are shown in Table 1. A larger resistance change rate was shown upon NH3 exposure than upon NO2 exposure, indicating NH3 detection selectivity.

[0089] Example 4 Instead of using 188.74 g of titanium dioxide (TiO2) suspension (20.9 wt%) and 90.88 g of DAA, 264.24 g and 15.4 g were used respectively, and a solution of polysiloxane combined with TiO2 particles (polysiloxane C) was obtained in the same manner as in Example 3(1). The solid content concentration of the obtained polysiloxane C solution was 41% by mass.

[0090] 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 fabricated 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, the evaluation was carried out in the same manner as in Example 1(4).

[0091] The evaluation results of the gas sensor D are shown in Table 1. It was superior in terms of the resistance change rate during NH3 exposure, the response speed, and the detection selectivity for NH3 compared to NO2, as compared with the gas sensor C.

[0092] Comparative Example 1 A polysiloxane A coating solution 3 was prepared 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 the tin oxide suspension A was not added, and a gas sensor was fabricated. The gas sensor E fabricated by this method is a gas sensor that does not contain metal oxides or metal sulfides in the coating layer.

[0093] The evaluation results of the gas sensor E are shown in Table 1. It can be seen that, compared with the gas sensor according to the present invention, not only is there no effect of improving the detection intensity by metal oxides and / or metal sulfides, but also the response speed is slow and the gas detection selectivity is not exhibited.

[0094] Comparative Example 2 A gas sensor was fabricated 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. The gas sensor F fabricated by this method is a gas sensor that does not contain a polymer material in the coating layer.

[0095] The resistance value of the gas sensor F before exposure to the gas to be detected was above the detection upper limit and could not be measured. This is presumably because the coating layer was fragile and could not stably hold the electrodes and the resistor since it did not contain a polymer material. Therefore, the gas sensor evaluation could not be performed.

[0096]

Table 1

Explanation of symbols

[0097] 2a, 2b electrodes 3 resistor 4, 4a, 4b coating layers 5 substrate 10, 20, 30 gas sensors

Claims

1. 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 “coat layer”), wherein the volume fraction of the metal oxide and / or the metal sulfide in the coat layer is 23 vol% or more and 60 vol% or less, and the metal oxide and / or the 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.

2. The gas sensor according to claim 1, wherein the coat layer is present between the substrate and the resistor.

3. The gas sensor according to claim 1 or 2, wherein the thickness of the coat layer is 10 nm or more and 1000 nm or less.

4. The gas sensor according to any one of claims 1 to 3, wherein at least a part of the metal oxide and / or the metal sulfide in the coat layer is in contact with the resistor.

5. The gas sensor according to any one of claims 1 to 4, wherein the metal oxide and / or the metal sulfide has a particulate shape and its number average particle diameter is 1 nm or more and 500 nm or less.

6. The gas sensor according to any one of claims 1 to 5, wherein the polymer material is a polysiloxane compound.

7. The gas sensor according to any one of claims 1 to 6, wherein a bond is formed between the polymer material and the metal oxide and / or the metal sulfide.

8. The gas sensor according to any one of claims 1 to 7, wherein the nanocarbon material is a carbon nanotube.

9. The gas sensor according to any one of claims 1 to 8, 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 any one of claims 1 to 9, wherein the resistor further contains a conjugated polymer.

11. The gas sensor according to any one of claims 1 to 10, wherein a heater is provided on the substrate.

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