Gas-sensitive material, semiconductor gas sensor, and gas detection method

Abstract

[Problem] To provide a gas-sensitive material having excellent temporal stability. [Solution] A gas-sensitive material (1) comprises: a solid acid that is a metal oxide semiconductor; and a solid base that is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium. The solid acid and the solid base are adsorbed.

Description

Gas-sensitive material, semiconductor gas sensor, and gas detection method

[0001] The present invention relates to a gas-sensitive material, and more particularly to a semiconductor gas sensor equipped with this gas-sensitive material, and a gas detection method using this gas-sensitive material.

[0002] Prior art relating to gas-sensitive materials for semiconductor gas sensors is known, for example, Patent Document 1 (Japanese Patent Application Publication No. 2008-241430). Patent Document 1 discloses a gas-sensitive part and a catalyst layer covering the gas-sensitive part.

[0003] The catalyst layer of the gas-sensitive material described in Patent Document 1 is, for example, a metal oxide semiconductor such as tin oxide in which a metal element such as yttrium is dissolved. The catalyst layer prevents the non-detectable gas from reaching the gas-sensitive part by causing a combustion reaction between the non-detectable gas, such as hydrogen or ethanol, and oxygen in the air.

[0004] Japanese Patent Publication No. 2008-241430

[0005] The catalyst layer in Patent Document 1 suffers from decreased catalytic activity each time a combustion reaction occurs on its surface. Therefore, improvement in the time-dependent stability of the gas sensor was desired.

[0006] The present invention was made to solve the above-mentioned problems, and its objective is to provide a gas-sensitive material with good stability over time.

[0007] A gas-sensitive material according to one aspect of the present invention comprises a solid acid, which is a metal oxide semiconductor, and a solid base, which is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium. The solid acid and the solid base are adsorbed together.

[0008] Preferably, the metal oxide semiconductor contains tungsten oxide.

[0009] Preferably, tungsten oxide has a solid base adsorbed on some of the tungsten atoms.

[0010] Preferably, the solid base is present in a proportion of 0.1 mol% to 10 mol% relative to the solid acid.

[0011] A semiconductor gas sensor according to one aspect of the present invention comprises: a gas-sensitive material containing a metal oxide semiconductor as a main component; a heater electrode at least a part of which is embedded in or disposed adjacent to the gas-sensitive material, the heater electrode connecting a first terminal portion and a second terminal portion; and a resistance detection electrode at least a part of which is embedded in the gas-sensitive material, the resistance detection electrode being connected to a third electrode so that a difference in resistance from the gas-sensitive material can be detected. The gas-sensitive material comprises a solid acid that is a metal oxide semiconductor, and a solid base that is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium. The solid acid and the solid base are adsorbed to each other.

[0012] A gas detection method according to one aspect of the present invention uses a gas-sensitive material that comprises a solid acid that is a metal oxide semiconductor, and a solid base that is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium, wherein the solid acid and the solid base are adsorbed to each other, and detects a target gas by detecting the target gas when the gas-sensitive material is heated. The gas detection method comprises: an oxygen adsorption step of heating the gas-sensitive material to about 400°C to cause the gas-sensitive material to adsorb oxygen; a target gas adsorption step of causing the target gas to adsorb onto the oxygen-adsorbed gas-sensitive material; and a target gas desorption step of desorbing the target gas adsorbed onto the gas-sensitive material. An oxygen adsorption time T that is the time required for the oxygen adsorption step ０ satisfies the relationship of the oxygen adsorption time T ０ < 60 sec.

[0013] Preferably, in the gas detection method, a target gas concentration C detected after intermittently repeating the gas detection method for at least n days ｎ satisfies the relationships of formulas (1) to (4): n < m ... (1), (target gas concentration C on n-th day ｎ ) / (target gas concentration C on m-th day ０ ) ≦ ±0.15 ... (2), 2 ≦ n < 365 [days] ... (3), 1 ≦ m ≦ 30 [days] ... (4)

[0014] Preferably, in the gas detection method, when a humidity in the ambient atmosphere is changed from a reference humidity H ０ which is an absolute humidity to a high humidity H １ , a resistance change rate ΔR Ｈ satisfies the relationship of formula (5): ΔR Ｈ = RＨ１ / R Ｈ０ >0.8...(5) However, R Ｈ０ = 1 R Ｈ０ : Resistance value R at standard temperature and standard humidity Ｈ１ : Resistance value when humidity is increased at the standard temperature.

[0015] According to the gas-sensitive material of the present invention, water molecules (in the following description, "water molecules" refers to hydroxyl groups OH dissociated from water molecules) are released from the surface of the gas-sensitive material. － By reducing the adsorption of (including) gas-sensitive materials, the lifespan of the gas-sensitive material can be extended.

[0016] This is a front view diagram illustrating a semiconductor gas sensor according to one embodiment of the present invention. This is a diagram illustrating a chemical reaction on the surface of a gas-sensitive material according to one embodiment of the present invention. (a) to (d) are graphs showing the daily variation of a semiconductor gas sensor according to one embodiment of the present invention. (a) to (d) are graphs showing the initial action of a semiconductor gas sensor according to one embodiment of the present invention. (a) to (d) are graphs showing the effect of humidity on a semiconductor gas sensor according to one embodiment of the present invention.

[0017] Embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and their descriptions will not be repeated.

[0018] <About the Gas Sensor> Referring to Figure 1, the semiconductor gas sensor 10 (hereinafter also referred to as "gas sensor 10") according to this embodiment will be described. In the figure, the up and down direction indicated by arrow A1 is the vertical direction, and the left and right direction indicated by arrow A2 is the horizontal direction.

[0019] The semiconductor gas sensor 10 comprises a gas-sensitive material 1 mainly composed of a metal oxide semiconductor, a heater electrode 2 which is embedded in or positioned close to the gas-sensitive material 1 in at least a portion, and a resistance detection electrode 3 which is embedded in the gas-sensitive material 1 in at least a portion and connected to the heater electrode 2 in a manner that allows for detection of the resistance difference.

[0020] In the semiconductor gas sensor 10, a reaction occurs when a gas to be detected (hereinafter also referred to as "detection target gas") comes into contact with the surface of the gas-sensitive material 1 that covers at least a part of the resistance detection electrode 3. The detection target gas is, for example, VOC (Volatile Organic Compound), acetaldehyde, ethanol, methane, ethylene, hydrogen sulfide, hydrogen, CO, ozone, or the like. The gas-sensitive material 1 needs to be heated to a predetermined temperature to react with the detection target gas, and is heated by a heater electrode 2 embedded in or arranged adjacent to the gas-sensitive material 1. The reaction between the gas-sensitive material 1 and the detection target gas is the transfer of electrons, that is, a redox reaction, and the resistance detection electrode 3 detects a change in resistance value caused by an increase or decrease in electrons in the gas-sensitive material.

[0021] The heater electrode 2 connects a first terminal portion 21 and a second terminal portion 22 that are connected to the outside of the gas sensor 10. The heater electrode 2 is a noble metal wire, and its material, wire diameter and the like are the same as those of known products. When the heater electrode 2 is arranged adjacent to the gas-sensitive material 1, it is sufficient that the position allows the gas-sensitive material 1 to be heated to a predetermined temperature, and the heater electrode 2 is typically arranged in a meandering shape. When the heater electrode 2 is embedded and arranged in the gas-sensitive material 1, the embedded portion of the heater electrode 2 typically has a shape wound into a coil. This allows the gas-sensitive material 1 to be heated efficiently.

[0022] The heater electrode 2 heats the gas-sensitive material 1 to a predetermined temperature. The predetermined temperature is, for example, 200 to 500°C, preferably 300 to 400°C. As a result, part of the oxygen molecules in the atmosphere on the surface of the gas-sensitive material 1 become O ― or O ２－ dissociated into atoms and adsorbed on the surface of the gas-sensitive material 1.

[0023] The resistance detection electrode 3 is connected to a third electrode 31 that connects to the outside of the gas sensor 10. The resistance detection electrode 3 is a noble metal wire, and its material, wire diameter and the like are the same as those of known products. In the present embodiment, the resistance detection electrode 3 is arranged in the gas-sensitive material 1 so as to pass through substantially the center of the coil-shaped wound heater electrode 2.

[0024] <Regarding the gas-sensitive material> Next, the gas-sensitive material 2 according to this embodiment will be described. The gas-sensitive material 2 comprises a solid acid which is a metal oxide semiconductor and a solid base which is an oxide of a metal element.

[0025] A solid acid is a solid that chemically adsorbs to a solid base and accepts electron pairs from reactants. The solid acid in this embodiment is a metal oxide semiconductor. The metal oxide semiconductor is the main component of the gas-sensitive material 1 and is selected from the group consisting of, for example, tungsten oxide, tin oxide, zinc oxide, and indium oxide. In this embodiment, tungsten oxide WO is selected from the viewpoint of good sensitivity to VOC gases (Volatile Organic Compounds) and good stability over time. ３ This will be explained as an option to choose.

[0026] The solid base is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium. In this embodiment, from the viewpoint of good long-term stability, yttrium Y oxide is selected. ２ O ３ This will be explained as selecting the following. This will allow water molecules (H2O or OH) to reach the surface of the gas-sensitive material. - This can suppress the adhesion of )

[0027] The solid base is included in a ratio of 0.1 mol% to 10 mol% relative to the solid acid. If the ratio is lower than 0.1 mol%, it becomes difficult to obtain the effect of the solid base, i.e., the effect of suppressing the adhesion of water molecules. If the ratio is higher than 10 mol%, the solid base will adhere excessively to the surface of the solid acid, which may reduce the detection sensitivity of the target gas. Preferably, the solid base is included in a ratio of 0.3 mol% to 1.0 mol% relative to the solid acid. Both the solid base and oxygen atoms dissociated from oxygen molecules in the atmosphere need to be adsorbed on the surface of the solid acid. It is precisely because the solid base is included in the above ratio that the sensitivity of the gas-sensitive material 1 can be maintained while suppressing the adhesion of hydroxyl groups dissociated from water molecules in the atmosphere.

[0028] The gas-sensitive material 1 of this embodiment does not have a catalyst layer like the conventional technology. As a result, while having a simple structure, it is possible to prevent the consumption of the catalyst layer due to the combustion of hydrogen gas, etc., and extend the service life of the gas-sensitive material. The gas-sensitive material of this embodiment has a structure that does not easily adsorb water molecules, so even without a catalyst layer, it can maintain a state that is less affected by water molecules for at least 365 days or more.

[0029] (Regarding chemical reactions) Here, with reference to Figure 2, we will explain three types of chemical reactions that occur on the surface of gas-sensitive material 1 and their effects. Figure 2 shows tungsten oxide (solid acid), yttrium (solid base), and oxygen atoms (O ２－ or O ― ), and water molecules (OH here) ― This is an explanatory diagram showing the state in which the substance is adsorbed.

[0030] The first chemical reaction involves the release of water molecules (H₂O or OH₂O). - This is the adsorption of oxygen atoms. Water molecules in the atmosphere adsorb onto the surface of the solid acid. As a result, oxygen atoms cannot be adsorbed, and the sensitivity of the gas-sensitive material decreases. This reaction is especially common on rainy days (days with high humidity), and the variation in the sensitivity of the gas-sensitive material from day to day is called "daily variation."

[0031] The second chemical reaction involves the oxygen atom (O 2- or O ― This is the adsorption of oxygen atoms (O) from oxygen molecules in the atmosphere. ２－ It adsorbs onto the surface of a solid acid by supplying electron pairs to it. Specifically, oxygen atoms adsorb to some of the tungsten atoms in tungsten oxide, which is a solid acid, and dissociate from the tungsten atoms by reacting with the target gas.

[0032] The third chemical reaction is the adsorption of tungsten atoms (W) and yttrium oxide (Y2O3). Yttrium oxide, a solid base, adsorbs to some of the W in tungsten oxide, which is a solid acid. This significantly alters the molecular structure of the gas-sensing material surface, making it difficult for water molecules to adsorb to W. In this embodiment, water molecules are less likely to adsorb to the surface of the gas-sensing material, and the target gas is more likely to adsorb to oxygen atoms, which react faster, than to tungsten atoms, which react slower.

[0033] The fourth chemical reaction is the adsorption of the gas-sensitive material and the target gas (VOC). The gas-sensitive material in this embodiment consists of tungsten atoms in four states, namely, tungsten atoms that do not adsorb anything (elemental W atoms) and water molecules (H2O or OH). - ) a tungsten atom with adsorbed yttrium oxide (water-adsorbing W atom), a tungsten atom with adsorbed yttrium oxide (solid-base-adsorbing W atom), and an oxygen atom O ２－ or O ― The material contains adsorbed tungsten atoms (oxygen-adsorbing W atoms). Of these, the target gas reacts with elemental W atoms and oxygen atoms adsorbed on the oxygen-adsorbing W atoms. The reaction rate of the target gas is faster with oxygen atoms than with elemental W atoms. In other words, the reaction rate of the target gas increases as the amount of oxygen-adsorbing W atoms and adsorbed oxygen atoms increases. The gas-sensitive material of this embodiment readily forms these oxygen-adsorbing W atoms, allowing for sensitive detection of the target gas.

[0034] (Regarding adsorption) Here, we will explain the "adsorption" between molecules as used in this specification. Adsorption can be broadly classified into chemiadsorption and physicoadsorption. Chemiadsorption mainly includes ionic bonds, covalent bonds, and coordination bonds, while physicoadsorption includes bonds due to intermolecular forces. The adsorption in the four chemical reactions described above is as follows:

[0035] • First chemical reaction (adsorption of water molecules): Water molecules (H₂O) and tungsten atoms are physically adsorbed by intermolecular forces, and the water molecules are broken down into hydroxyl groups (OH). - The tungsten atom and the oxygen atom undergo chemiadsorption. • Second chemical reaction (adsorption of oxygen atom) Oxygen atom O 2－ and O― The adsorption of tungsten atoms W is mainly chemiadsorption. • Third chemical reaction (adsorption of tungsten atoms and yttrium oxide) The adsorption of tungsten atoms W and yttrium oxide Y2O3 is irreversible and is thought to be due to chemiadsorption. • Fourth chemical reaction (adsorption of the target gas and elemental W atoms, or the target gas and oxygen atoms adsorbed on oxygen-adsorbing W atoms) The adsorption of the target gas and oxygen atoms adsorbed on elemental W atoms / oxygen-adsorbing W atoms is due to the transfer of electrons and is chemiadsorption.

[0036] <About the gas detection method> Next, a gas detection method using the gas-sensitive material of this embodiment will be described. The gas-sensitive material of this embodiment comprises a solid acid, which is a metal oxide semiconductor, and a solid base, which is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium, with the solid acid and solid base adsorbed on it. The gas-sensitive material with the adsorbed solid acid and solid base can detect the target gas by heating it to a predetermined temperature.

[0037] The gas detection method of this embodiment comprises an oxygen adsorption step in which a gas-sensitive material is heated to approximately 400 degrees Celsius to adsorb oxygen onto the gas-sensitive material, a target gas adsorption step in which the target gas to be detected is adsorbed onto the gas-sensitive material on which oxygen has been adsorbed, and a target gas detachment step in which the target gas adsorbed onto the gas-sensitive material is detached.

[0038] The gas detection method of this embodiment involves an oxygen adsorption time T, which is the time required for the oxygen adsorption process. ０ (Initial action time) is equal to the oxygen adsorption time T. ０ <Satisfies the relationship of 60 sec

[0039] In this embodiment, when the gas sensor is switched from an off state to an on state at the start of use, the gas-sensing material needs to be heated to desorb water and other gases that have been adsorbed when not in use, and to re-adsorb oxygen atoms through a second chemical reaction. Once a certain amount of oxygen atoms have been adsorbed, the resistance value stabilizes and the sensor becomes detectable. The time required from the start of use until a certain amount of oxygen atoms are adsorbed is called the initial action. It is preferable that the initial action be as short as possible, and that the desorption time of water and other gases is short. In this embodiment, because a solid base is adsorbed by a third chemical reaction in the gas-sensing material, water molecules are less likely to be adsorbed even when not in use (in other words, the first chemical reaction is less likely to occur). This shortens the water desorption time and thus the initial action time.

[0040] In the gas detection method of this embodiment, the target gas concentration C is detected after intermittently repeating a predetermined gas detection method for at least n days. ｎ However, the relationship between equations (1) to (4) is satisfied. The specific gas detection method will be described later in the examples. n < m ... (1) (Detection target gas concentration C on day n) ｎ ) / (Detection target gas concentration C on day m) ０ ) ≤ ±0.15 … (2) 2 ≤ n < 365 [days] … (3) 1 ≤ m ≤ 30 [days] … (4)

[0041] The gas sensor of this embodiment is less prone to water molecule adsorption on the surface of the gas-sensitive material. This prevents an increase in the number of water molecules adhering to the surface of the gas-sensitive material, thus preventing a decrease in sensitivity over time.

[0042] The gas detection method of this embodiment uses the humidity of the ambient atmosphere as a reference humidity H ０ High humidity H when this is the absolute humidity １ The rate of change in resistance ΔR when it is changed to this. Ｈ The relationship in equation (5) is satisfied. ΔR Ｈ = R Ｈ１ / R Ｈ０ >0.8...(5) However, R Ｈ０ = 1 R Ｈ０ : Resistance value R at standard temperature and standard humidity Ｈ１: Resistance value when humidity is increased at the standard temperature.

[0043] The gas sensor of this embodiment has a structure that makes it difficult for water molecules to adsorb onto the surface of the gas-sensitive material due to the adsorption of a solid base. Therefore, the first chemical reaction is less likely to occur even under high humidity conditions, and a stable state can be maintained.

[0044] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0045] <Comparative Test Regarding Daily Variation of Gas Sensors> In this test, the effect of daily variation on gas sensor sensitivity was compared for the example and comparative example by measuring samples of the same concentration ppb intermittently for 365 days.

[0046] (Test Method) Using a gas chromatograph (SGEA-P3-A, manufactured by NISSHA FIS Co., Ltd.) equipped with gas sensors using the gas-sensitive materials of the comparative examples and examples shown in Table 1, predetermined concentrations of acetaldehyde and ethanol were measured. First, the gas-sensitive material temperature was adjusted to 400°C and the device was powered on for 8 hours a day, after which the power was turned off for the rest of the time. This process was repeated. Once a week, standard gases of acetaldehyde (manufactured by Sumitomo Seika Co., Ltd.) and ethanol (manufactured by Taiyo Nippon Sanso Corporation) at predetermined concentrations were injected into the gas chromatograph and measured.

[0047]

[0048] (Test Results) The experimental results of the comparative example will be explained with reference to Figures 3(a) and 3(b). Figure 3(a) shows the results of measuring acetaldehyde at a predetermined concentration intermittently for 365 days using the comparative example's gas sensor. At low concentrations (100 ppb), the daily difference in measured values ​​(daily variation) was large. At medium concentrations (1000 ppb), a decrease in measured values ​​was observed, especially after day 150. At high concentrations (10000 ppb), large daily variations were observed, especially between days 100 and 150. From these results, it was found that the comparative example is not suitable for measuring acetaldehyde at low concentrations (100 ppb). Furthermore, it was found that errors of 100 ppb or more occurred when measuring acetaldehyde at medium concentrations (1000 ppb) and high concentrations (10000 ppb), and that this error increased as the number of days increased. This is thought to be due to the first chemical reaction, namely the adsorption of water molecules from the atmosphere onto the surface of the gas-sensitive material.

[0049] Figure 3(b) shows the results of intermittently measuring ethanol at a predetermined concentration for 365 days using the comparative example gas sensor. A significant decrease in measured values ​​was observed, particularly after day 150, for both medium (1000 ppb) and high (10000 ppb) concentrations. Generally, the tolerance for accuracy of gas sensors is ±15%. Therefore, it was found that the comparative example gas sensor requires gas adjustment work to recreate the calibration curve around 150 days.

[0050] The experimental results of the example will be explained with reference to Figures 3(c) and 3(d). Figure 3(c) shows the results of intermittently measuring acetaldehyde at a predetermined concentration for 365 days using the gas sensor of the example. According to the example, stable measurement values ​​were obtained from low to high concentrations over the experimental period, i.e., 365 days.

[0051] Figure 3(d) shows the results of intermittently measuring ethanol at a predetermined concentration for 365 days using the gas sensor of the example. According to the example, stable measurement values ​​could be obtained over 365 days in both the case of medium and high concentrations. In other words, it was found that with the gas sensor of this example, gas adjustment work to recreate the calibration curve is unnecessary for at least one year.

[0052] From the above, it was found that the gas sensor of the example is less affected by daily variations than the comparative example, and the replacement frequency can be reduced. This is thought to be due to the adsorption of a solid base onto the W atom by the third chemical reaction, which inhibits the first chemical reaction. Furthermore, because the gas sensor of the example inhibits the first chemical reaction, it can perform the reaction with the target gas by the fourth chemical reaction with high accuracy.

[0053] <Comparative Test Regarding Initial Action of Gas Sensors> In this test, the time required for the resistance value Rs / KOhm (initial action) of the gas sensor to stabilize immediately after startup was compared between the example and the comparative example.

[0054] (Test Method) After a period of one week without power, the gas sensor's heater and power supply were simultaneously turned on. Next, gas sensors using the gas-sensitive materials of the comparative example and example shown in Table 2 were prepared. The resistance value Rs / Kohm was continuously measured immediately after power was applied, and the time until the sensor became usable (time required for oxygen re-adsorption) was measured.

[0055]

[0056] (Test Results) Figure 4 shows the results of measuring the resistance value Rs / KOhm (initial action) immediately after startup of the gas sensors for the comparative example and Examples 1 to 3. Referring to Figure 4, the comparative example showed a smaller slope immediately after startup than the examples, and reached a stable state in about 8 minutes. In contrast, the examples showed a larger slope immediately after startup, and were able to reach a stable state in about 1 minute. In other words, the gas sensor of this example significantly shortened the initial action.

[0057] Based on the above, the gas sensor in the example, due to the solid base produced by the third chemical reaction, exhibits less adsorption of hydroxyl groups originating from the first chemical reaction, thus eliminating the need for hydroxyl group release time. Furthermore, it was found that the second chemical reaction can be started immediately after the gas sensor is activated, shortening the time required for initial action.

[0058] <Comparative test on the effect of humidity> In this test, the difference in resistance value Rs / KOhm due to humidity changes was compared between the example and the comparative example.

[0059] (Test Method) A gas sensor was placed in a measuring chamber with adjustable temperature and humidity, and the gas sensor was maintained at the operating temperature (approximately 400°C) for one hour. After this, the humidity was increased and maintained for one hour, and then the resistance value per unit of absolute humidity was measured. For each of the comparative examples and examples shown in Table 3, the change in resistance value with respect to humidity change was measured.

[0060]

[0061] (Test Results) Figure 5 shows the absolute humidity g / m³ of the gas sensors in the comparative example and the example. ３ This graph shows the resistance value R / Rstd per unit area. Comparing comparative example (a) with examples (b) to (d), comparative example (a) had a slope of -0.0267 on the calibration curve, while examples (b) to (d) had a slope of approximately -0.017. This indicates that the slope of the examples was approximately 0.63 times that of the comparative example, showing that water molecules were less adsorbed in the examples than in the comparative example.

[0062] From the above, it was found that the gas sensor of the example is less susceptible to humidity, meaning that water molecules originating from the first chemical reaction are less likely to adsorb onto the surface of the gas-sensing material. This is thought to be because the adsorption of the solid base makes it more difficult for hydroxyl groups to adsorb, thus making it easier to maintain a stable structure.

[0063] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included.

[0064] 1 Gas sensing material, 2 Heater electrode, 3 Resistance detection electrode, 10 Semiconductor gas sensor, 21 First terminal section, 22 Second terminal section, 31 Third terminal section.

Claims

1. A gas-sensitive material comprising a solid acid which is a metal oxide semiconductor and a solid base which is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium, wherein the solid acid and the solid base are adsorbed on the material.

2. The gas-sensitive material according to claim 1, wherein the metal oxide semiconductor comprises tungsten oxide.

3. The gas-sensitive material according to claim 2, wherein the tungsten oxide has the solid base adsorbed on a portion of the tungsten atoms.

4. The gas-sensitive material according to claim 1, wherein the solid base is contained in a proportion of 0.1 mol% to 10 mol% relative to the solid acid.

5. A semiconductor gas sensor comprising: a gas-sensitive material mainly composed of a metal oxide semiconductor; a heater electrode, at least a portion of which is embedded in or positioned close to the gas-sensitive material and connecting a first terminal portion and a second terminal portion; and a resistance-detecting electrode, at least a portion of which is embedded in the gas-sensitive material and connected to a third electrode so as to detect the difference in resistance value with the gas-sensitive material, wherein the gas-sensitive material comprises a solid acid which is a metal oxide semiconductor and a solid base which is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium, and the solid acid and the solid base are adsorbed together.

6. A gas detection method comprising a solid acid which is a metal oxide semiconductor and a solid base which is an oxide of at least one metal element selected from the group consisting of yttrium, zirconium, and magnesium, wherein the solid acid and the solid base are adsorbed on a gas-sensitive material, and the gas detection method comprises an oxygen adsorption step of heating the gas-sensitive material to 400 degrees Celsius to adsorb oxygen onto the gas-sensitive material, a target gas adsorption step of adsorbing the target gas onto the gas-sensitive material on which oxygen has been adsorbed, and a target gas detachment step of detaching the target gas adsorbed onto the gas-sensitive material, wherein the oxygen adsorption time T is the time required for the oxygen adsorption step. ０ The oxygen adsorption time T ０ A gas detection method that satisfies the relationship of 60 sec.

7. The concentration of the target gas C detected after intermittently repeating the gas detection method described in claim 6 for at least n days. ｎ The gas detection method according to claim 6, satisfying the relationship between formulas (1) to (4). n < m ... (1) (concentration of target gas C on day n) ｎ ) / (Detection target gas concentration C on day m) ０ ) ≤ ±0.15 … (2) 2 ≤ n < 365 [days] … (3) 1 ≤ m ≤ 30 [days] … (4) 8. The gas detection method according to claim 6, wherein when the reference humidity H ０ is set as an absolute humidity, the rate of change in resistance ΔR １ when the humidity is changed to high humidity H Ｈ satisfies the relationship of formula (5). ΔR Ｈ =R Ｈ１ / R Ｈ０ >0.8...(5) Provided that, R Ｈ０ =1 R Ｈ０ : resistance value at reference temperature and reference humidity R Ｈ１ : resistance value when the humidity is increased at the reference temperature

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