JP3837536B2 - Gas sensor and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensor, a gas sensor manufacturing method, and the like. More specifically, the present invention is obtained by modifying the surface of a metal oxide semiconductor film typified by a tin oxide thin film with an organic compound. The present invention relates to a gas sensor having an action of selectively responding to gas and excellent in gas selectivity and a method for manufacturing the same.
INDUSTRIAL APPLICABILITY The present invention is useful for providing a novel gas sensor based on a new detection principle, a manufacturing method thereof, and the like that are completely different from the detection principle of a gas sensor using a conventional semiconductor oxide.
[0002]
[Prior art]
In general, a gas sensor using an n-type semiconductor oxide such as tin oxide has excellent long-term stability, does not require an expensive device for its production, and requires a complicated circuit to detect a resistance change. For example, it is widely used as a gas leak alarm for home use because it can be downsized. The semiconductor gas sensor can detect the target gas present in the air from a change in sensor resistance caused by a chemical reaction occurring on the surface of the oxide particles constituting the sensor. In an n-type semiconductor typified by tin oxide, oxygen adsorbed on the surface takes electrons from the oxide, and as a result, there are few carriers in the vicinity of the oxide surface, so a depletion layer having high resistance is formed. . Here, for example, when a combustible gas such as hydrogen or methane is supplied, the combustible gas is chemically bonded to oxygen adsorbed on the oxide surface and oxidized. By this oxidation reaction, the oxygen adsorbed on the oxide surface is reduced, the ratio of the depletion layer in the oxide particles is lowered, and the sensor resistance is lowered.
[0003]
Since the semiconductor gas sensor can detect various gases based on such a detection principle, in recent years, in conjunction with the advantages of the semiconductor gas sensor, research for practical application of various gases has been actively conducted. For example, the gas leak alarm is the first practical example of a semiconductor gas sensor, and thereafter, the target is a toxic gas such as CO, a bad odor gas such as ammonia or H ₂ S, an environmental gas such as NOx, and AsH ₃ . It has spread to semiconductor process gases and odorous substances from food. In addition, research on various volatile organic compounds (VOC), which are the causative substances of sick house syndrome, is also being conducted. In these various gas sensors, it is ideal to selectively detect only the gas to be detected. In recent years, with the diversification of needs, sensor applications have expanded, and it has become an increasingly important issue to provide gas selectivity to sensors.
[0004]
However, from the detection principle of the oxide semiconductor gas sensor, in general, in the case of a flammable gas, any flammable gas is oxidized on the surface of the oxide, so that the gas sensor is given selectivity to various gases. It is not easy. Until now, in order to improve gas selectivity, a method of adding other elements (see Patent Documents 1 and 2), a method of laminating an oxide semiconductor thin film and a selective combustion layer thick film as a gas detection unit ( Patent Documents 3 and 4) have been reported. For example, for formaldehyde, which is a kind of VOC gas, by adding silver to tin oxide, it is approximately 50 times as much as toluene. Although some degree of selectivity has been achieved (see Non-Patent Document 1), it is still necessary to improve the gas selectivity, such as requiring a filter for removing the interfering gas.
[0005]
[Patent Document 1]
JP-A-6-258270 [Patent Document 2]
JP-A-11-116243 [Patent Document 3]
JP 2000-292397 A [Patent Document 4]
JP 11-326258 A [Non-patent Document 1]
Electrical Engineering E, 119, 383-389 (1999)
[0006]
[Problems to be solved by the invention]
Under such circumstances, the present inventor has conducted intensive research with the goal of developing a new gas sensor capable of drastically solving the problems in the prior art in view of the prior art. A gas sensor based on a new detection principle using a hydrogen bond between an organic substance and an oxide surface on an oxide semiconductor surface modified with an organic substance having a hydroxyl group as a group was created. To describe this principle in detail, on the surface of the oxide semiconductor modified with the organic substance, when no detection target gas exists, there is a hydrogen bond between the organic substance fixed on the oxide surface and the oxide surface. The situation is created, but in this case, since it is a hydrogen bond between the hydroxyl group of the organic substance and oxygen on the oxide surface, charge is supplied from the organic substance to the oxide surface, and when the oxide is an n-type semiconductor, depletion occurs. There are few layers, that is, a low resistance state.
[0007]
On the other hand, when the oxide surface is exposed to an organic substance immobilized on the oxide surface, in particular, a detection target gas that has a strong interaction with its hydroxyl group, the interaction between the organic substance and the detection target gas is prioritized and hydrogen bonding is performed. Is cut off, so that charge supply by hydrogen bonds is eliminated and the resistance value of the oxide increases. That is, the principle of this gas sensor is that the hydrogen bond is controlled by the presence or absence of the detection target gas, and as a result, the resistance value changes to detect the gas. The inventor has devised such a principle, and in a sensor in which a metal oxide semiconductor film is surface-modified with an appropriate organic substance, the resistance value increases with respect to carbon monoxide gas having polarity as described above. The present invention has been completed here.
That is, the present invention provides a novel gas sensor having high gas selectivity based on a new detection principle, a method for efficiently manufacturing the gas sensor, and a component of the gas sensor, which do not depend on the detection principle of the conventional oxide semiconductor sensor described above. It aims at providing the gas detector containing as.
[0008]
[Means for Solving the Problems]
The present invention for solving the above-described problems comprises the following technical means.
(1) A gas sensor including a substrate and a metal oxide semiconductor film formed on the substrate as constituent elements, the surface of which is an organic layer having a functional group capable of hydrogen bonding with the metal oxide semiconductor surface A gas sensor having an action of selectively responding to a polar gas, characterized by being modified.
(2) The gas sensor according to (1), wherein the metal oxide semiconductor film is a tin oxide film containing tin oxide as a main component.
(3) The organic material layer having a functional group capable of hydrogen bonding is formed by chemical modification with a silane coupling agent and further modification with an organic carboxylic acid having a hydroxyl group. The gas sensor according to (1).
(4) The gas sensor as described in (3) above, wherein the silane coupling agent comprises a compound having an amino group.
(5) The gas sensor according to (3), wherein the organic carboxylic acid having a hydroxyl group is benzoic acid having a hydroxyl group.
(6) The organic carboxylic acid having a hydroxyl group is fixed on the surface of tin oxide by dehydration condensation of the carboxyl group of the organic carboxylic acid having a hydroxyl group and the amino group of the silane coupling agent. The gas sensor described.
(7) Any one of the above (1) to (6), which has an action of selectively responding to a gas having polarity at 100 to 200 ° C. by increasing the resistance value. The gas sensor described in 1.
(8) A method of manufacturing a gas sensor including a substrate and a metal oxide semiconductor film formed on the substrate and having an effect of selectively responding to a polar gas, the surface of which is a metal oxide semiconductor A method for producing a gas sensor, comprising modifying an organic layer having a functional group capable of hydrogen bonding with a surface.
(9) The method for manufacturing a gas sensor according to (8), wherein the metal oxide semiconductor film is a tin oxide film containing tin oxide as a main component.
(10) The organic layer having a functional group capable of hydrogen bonding is formed by chemical modification with a silane coupling agent and further modification with an organic carboxylic acid having a hydroxyl group. The manufacturing method of the gas sensor of description.
(11) The method for producing a gas sensor according to (10), wherein the silane coupling agent comprises a compound having an amino group.
(12) The method for producing a gas sensor according to (10), wherein the organic carboxylic acid having a hydroxyl group is benzoic acid having a hydroxyl group.
(13) The above-mentioned (10), wherein the carboxyl group of the organic carboxylic acid having a hydroxyl group and the amino group of the silane coupling agent are subjected to dehydration condensation to fix the organic carboxylic acid having a hydroxyl group on the surface of tin oxide. Gas sensor manufacturing method.
(14) A gas detector including the gas sensor according to any one of (1) to (7) as a component, the gas sensor, a detection circuit that detects a change in a resistance value of the gas sensor as an electrical output, A gas detector comprising: output means for outputting gas information relating to a detection target gas based on an electrical output detected by a detection circuit.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in more detail.
A sectional view of an example of the gas sensor of the present invention is shown in FIG. In this gas sensor, a gold electrode 2 is provided on a substrate 1 and is electrically connected to the gold electrode 2 to form a metal oxide semiconductor 3, and an organic layer 4 is formed on the metal oxide semiconductor 3. The gas is detected by detecting an electrical characteristic between the gold electrodes, for example, a change in electric resistance value. As the substrate 1, for example, various glass substrates such as quartz glass, alumina substrates, silicon substrates, magnesia substrates and the like used for gas sensors can be used. The method for producing the metal oxide semiconductor film 3 is not particularly limited, and for example, a known method for thin film production such as a sol-gel method, a coating pyrolysis method, a laser ablation method, or a sputtering method can be appropriately employed. Since the detection principle of the sensor of the present invention is attributed to the oxide surface, the oxide has a high surface area ratio and a polycrystalline thin film composed of small crystal grains is desired.
[0010]
Examples of the present invention to be described later show sensors of the present invention manufactured using tin oxide, which is a typical metal oxide semiconductor. Hereinafter, although the sensor of this invention is concretely demonstrated taking the sensor using a tin oxide film as an example, this invention is not restrict | limited to the following examples. The tin oxide film is produced by spin-coating a solution containing a tin compound on a substrate and heat-treating the solution. The tin compound that can be used in the present invention is not particularly limited as long as it can be dissolved in an appropriate solvent and can be formed into a film from this solution, and preferably, for example, an inorganic salt such as tin chloride or tin nitrate. And tin alkoxides such as tin tetraethoxide, and organic tin compounds such as tin naphthenate. The heat treatment temperature for producing the tin oxide thin film is usually about 300 to 1000 ° C., preferably about 400 to 600 ° C. The firing time is about 10 to 120 minutes, preferably about 20 to 30 minutes. The firing atmosphere is not particularly limited, and preferably, for example, in the atmosphere, an oxidizing atmosphere such as an oxygen atmosphere can be exemplified. The firing means is not particularly limited, and any means such as an electric heating furnace, a gas heating furnace, and a light heating furnace can be employed.
[0011]
The organic layer 4 on the surface of the tin oxide film is formed by a two-stage reaction of treatment with a silane coupling agent containing an amino group and subsequent dehydration condensation reaction with an organic carboxylic acid containing a hydroxyl group. The silane coupling agent used for the chemical modification of the tin oxide film surface is a compound containing an amino group necessary for a dehydration condensation reaction with an organic carboxylic acid, and is fixed to the oxide surface. Such compounds, preferably, for example, 3-aminopropyltrimethoxysilane _{(NH 2 CH 2 CH 2 CH} 2 Si (OCH 3) 3), 3- aminopropylmethyldiethoxysilane (NH ₂ CH ₂ CH ₂ CH ₂ SiCH ₃ (OC ₂ H ₅ ) ₂ ) and the like can be exemplified, but are not limited to these, and can be used in the same manner as long as they have the same effect. An alcohol solution typified by these ethanol is prepared and left for several minutes. This solution is dropped on the tin oxide thin film, allowed to stand for several minutes, and then washed several times with alcohol. Thereafter, it is quickly dried by heating at around 110 ° C. for 10 minutes.
[0012]
All the above operations are performed in the air. In this reaction, first, the alkoxide group of the silane coupling agent is hydrolyzed to a hydroxyl group. On the other hand, the oxide surface reacts with moisture in the air and is covered with a hydroxyl group, but this is dehydrated and condensed with the hydroxyl group of the silane coupling agent, so that a covalent bond is formed between the oxide and the silane coupling agent. And the silane coupling agent is immobilized on the oxide surface. The concentration of the alcohol solution of the silane coupling agent depends on the reaction temperature, but if the concentration exceeds 1.5%, the formed organic layer becomes thick and the interaction with the oxide surface becomes weak. On the other hand, when the concentration is lower than 0.2%, the organic layer formed is reduced, and the number of hydrogen bonds described in the sensor principle is insufficient. The concentration of the alcohol solution of the silane coupling agent is 0.2 to 1.4%, preferably 0.5 to 1.3%.
[0013]
The organic carboxylic acid used in the second stage reaction is not particularly limited as long as it contains a hydroxyl group, and preferred examples thereof include 3,4-dihydroxybenzoic acid. 3,4-Dihydroxybenzoic acid is dissolved in distilled water to obtain an aqueous solution. Here, 1.2 times equivalent of water-soluble carbodiimide is added to 3,4-dihydroxybenzoic acid, and after stirring sufficiently, it is allowed to stand for 10 minutes. The sample subjected to the silane coupling treatment is immersed in this solution and left for about 20 hours. The sample is washed with distilled water and dried by heating at around 110 ° C. for 10 minutes. All the above operations are performed in the air. In this reaction, an amino group and a carboxyl group undergo a dehydration condensation reaction, and an amide bond is formed, whereby benzoic acid containing a hydroxyl group is bonded to the amino group portion of the silane coupling agent. In this reaction, alcohol such as ethanol can be used as a solvent instead of water. In this case, the reaction proceeds by using dicyclohexylcarbodiimide instead of water-soluble carbodiimide as the dehydrating condensing agent.
[0014]
Usually, a tin oxide sensor responds to a combustible gas such as hydrogen or carbon monoxide by a decrease in resistance value. However, the gas sensor obtained according to the present invention has a resistance value against hydrogen that increases with respect to carbon monoxide at a sensor temperature of 100 ° C. to 200 ° C., although the resistance value decreases with respect to hydrogen in the same manner as a normal tin oxide sensor. The specific response is shown. This is because carbon monoxide, which is a polar molecule, interacts preferentially with the hydroxyl group contained in the organic material layer fixed on the oxide surface, so that the hydrogen formed between the hydroxyl group and the oxide surface. This is because the bond is dissociated, and such an effect cannot be expected for hydrogen which is a nonpolar molecule.
[0015]
That is, according to the present invention, it is possible to provide a gas sensor that shows a selective and specific response only to a gas having polarity among combustible gases. The gas sensor of the present invention is widely applied to detection of gas having polarity. The method for producing a gas sensor by chemical modification of the oxide surface according to the present invention applies the same method to a sensor using a metal oxide semiconductor other than tin oxide in addition to the case of using the above tin oxide. Can do. Examples of the metal that can form the oxide semiconductor sensor include zinc, indium, tungsten, molybdenum, cobalt, antimony, iron, and copper in addition to the tin. In the present invention, a gas sensor in which those oxide films are formed on a substrate can be manufactured and provided.
[0016]
【Example】
EXAMPLES Next, although this invention is demonstrated concretely based on an Example, this invention is not limited at all by the following Examples.
Example 1
(1) Production of tin oxide film A gold thin film comb-shaped electrode as a detection electrode was formed on one side of a quartz glass insulator substrate by a sputtering method. An organic tin solution was applied onto this substrate by spin coating to form a film. The spin coating conditions were 500 rpm for 10 seconds, followed by 2000 rpm for 20 seconds. After the film formation, it was heated and dried in air at 130 ° C. for 30 minutes and further baked at 600 ° C. for 30 minutes. This spin coating and drying / firing treatment were repeated twice to obtain a tin oxide thin film. The terminal portion of the comb electrode was masked so that a tin oxide film was not formed. The thickness of the tin oxide film as observed with a scanning electron microscope was 200 to 300 nm. FIG. 2 is an X-ray diffraction pattern diagram of a tin oxide film produced by the above process, and a peak due to tin oxide appeared, and its generation could be confirmed.
[0017]
(2) Formation of organic layer on tin oxide film surface As a silane coupling agent, 3-aminopropyltrimethoxysilane was used. 0.1 ml of 3-aminopropyltrimethoxysilane was added to 9.9 ml of ethanol, and a uniform solution having a concentration of 1% by volume was prepared by sufficiently stirring. This solution was allowed to stand for 5 minutes, and then a drop was dropped on the tin oxide film using a dropper. After standing in this state for 3 minutes, the sample was washed with ethanol three times and dried by heating at 110 ° C. for 10 minutes.
[0018]
Next, 15.4 mg (0.1 mmol) of 3,4-dihydroxybenzoic acid was added to 10 ml of distilled water and dissolved by sufficiently stirring. 18.6 mg (0.12 mmol) of water-soluble carbodiimide corresponding to 1.2 equivalents to 3,4-dihydroxybenzoic acid was added thereto, and the mixture was allowed to stand for 15 minutes after stirring. The tin oxide film treated with the silane coupling agent was immersed in this solution and allowed to stand for 20 hours, and then the sample was washed three times with distilled water and dried by heating at 110 ° C. for 10 minutes. FIG. 3 shows an infrared absorption spectrum of a sample reacted with 3,4-dihydroxybenzoic acid after silane coupling treatment and compared with a sample not subjected to surface treatment. Absorption due to a hydroxyl group appears in the vicinity of 3450 cm ⁻¹ , and it can be confirmed that 3,4-dihydroxybenzoic acid is fixed on the surface of tin oxide by a dehydration condensation reaction with a silane coupling agent. In addition, all the processes relating to the formation of the organic layer on the surface of the tin oxide film were performed in air.
[0019]
(3) Evaluation of sensor characteristics The detection sensitivity of the obtained sensor for hydrogen (H ₂ ), carbon monoxide (CO) gas, and propane (C ₃ H ₈ ) gas was evaluated. The gas concentrations used were all 3% (air dilution), and the measurement temperature was 150 ° C. In the measurement, clean air (1 minute), sample gas (3 minutes), and clean air (16 minutes) were sequentially flowed, and a total of 20 minutes was defined as one cycle. The measurement results are shown in FIG. The vertical axis in the figure is the resistance value normalized by the initial resistance. In response to hydrogen gas, the resistance value of the sensor decreased in the same manner as a sensor using a known tin oxide. This sensor did not respond to propane gas. On the other hand, for carbon monoxide gas, the sensor resistance was increased. This has shown that the sensor of this invention has high selectivity with respect to various combustible gas.
[0020]
Comparative Example 1
A sample having no organic layer on the surface of the tin oxide film was prepared. The manufacturing method was based on the method shown in Example 1, (1) Preparation of tin oxide film. The measurement conditions for sensor characteristics were also the same as in Example 1. FIG. 5 shows detection characteristics of the obtained sensor for various gases. Unlike the case of Example 1, this sensor showed a response that the resistance value of the sensor decreased with respect to hydrogen gas and carbon monoxide gas. In addition, this sensor did not show a clear response to propane gas. This indicates that the organic layer plays an important role in the specific response to carbon monoxide gas in Example 1.
[0021]
Comparative Example 2
A sample was prepared by simply treating the surface of the tin oxide film with a silane coupling agent. The manufacturing method was in accordance with the method shown in Example 1. The measurement conditions for sensor characteristics were also the same as in Example 1. FIG. 6 shows detection characteristics of the obtained sensor for various gases. Unlike the case of Example 1, this sensor showed a response that the resistance value of the sensor decreased with respect to hydrogen gas and carbon monoxide gas. In addition, this sensor did not show a clear response to propane gas. This indicates that benzoic acid having a hydroxyl group contained in the organic material layer plays an important role in the specific response to carbon monoxide gas in Example 1. That is, in Comparative Example 1 and Comparative Example 2 that do not have such a hydroxyl group, a hydrogen bond is not formed, and thus the specific response to carbon monoxide shown in Example 1 cannot be observed.
[0022]
Example 2
A sensor was produced according to the method of Example 1 except that the concentration of the coupling agent during the silane coupling treatment was 0.1% by volume. As a result of measuring the sensor characteristics, this sensor showed a response that the resistance value of the sensor decreased with respect to hydrogen gas and carbon monoxide gas, unlike the result of Example 1.
[0023]
Example 3
A sensor was produced according to the method of Example 1 except that the concentration of the coupling agent during the silane coupling treatment was 0.7% by volume. As a result of measuring the sensor characteristics, this sensor showed a response in which the resistance value decreased with respect to hydrogen gas and the resistance value increased with respect to carbon monoxide gas, similarly to the result of Example 1.
[0024]
Example 4
A sensor was produced according to the method of Example 1 except that the concentration of the coupling agent during the silane coupling treatment was 1.5% by volume. As a result of measuring the sensor characteristics, this sensor showed a response that the resistance value of the sensor decreased with respect to hydrogen gas and carbon monoxide gas, unlike the result of Example 1. The results from Examples 1 to 4 show that the concentration of the coupling agent during the silane coupling process needs to be optimized in order to develop a specific sensor response to carbon monoxide. ing.
[0025]
【The invention's effect】
As described above in detail, the present invention relates to a gas sensor and a method for manufacturing the same, and the following effects are exhibited by the present invention.
(1) According to the present invention, there is provided a new gas sensor based on the detection principle that the formation and dissociation of hydrogen bonds between an organic substance fixed on the oxide surface and the oxide surface is controlled by the detection gas. Can do.
(2) By the gas sensor manufacturing method of the present invention, a gas sensor made of a metal oxide semiconductor film having an organic layer on the surface can be obtained.
(3) The gas sensor of the present invention exhibits a selective and specific response to a gas having polarity.
[Brief description of the drawings]
FIG. 1 is a diagram showing the structure of a gas sensor of the present application.
FIG. 2 is an X-ray diffraction pattern of a surface-treated tin oxide film.
FIG. 3 is a diagram showing infrared absorption spectra of a surface-treated sample and an untreated sample.
FIG. 4 is a diagram showing sensor characteristics of the sensor shown in Example 1 for various gases.
FIG. 5 is a diagram showing sensor characteristics for various gases of the sensor shown in Comparative Example 1;
6 is a graph showing sensor characteristics for various gases of the sensor shown in Comparative Example 2. FIG.
[Explanation of symbols]
1 Substrate 2 Electrode 3 Metal oxide semiconductor part 4 Organic substance part

Claims (14)

A gas sensor including a substrate and a metal oxide semiconductor film formed on the substrate as constituent elements, the surface of which is modified with an organic material layer having a functional group capable of hydrogen bonding with the metal oxide semiconductor surface. A gas sensor having an action of selectively responding to a polar gas. The gas sensor according to claim 1, wherein the metal oxide semiconductor film is a tin oxide film containing tin oxide as a main component. 2. The organic material layer having a functional group capable of hydrogen bonding is formed by chemical modification with a silane coupling agent and further modification with an organic carboxylic acid having a hydroxyl group. The gas sensor described. The gas sensor according to claim 3, wherein the silane coupling agent comprises a compound having an amino group. The gas sensor according to claim 3, wherein the organic carboxylic acid having a hydroxyl group is benzoic acid having a hydroxyl group. 4. The gas sensor according to claim 3, wherein the carboxyl group of the organic carboxylic acid having a hydroxyl group and the amino group of the silane coupling agent are dehydrated and condensed to fix the organic carboxylic acid having a hydroxyl group on the surface of tin oxide. The gas sensor according to any one of claims 1 to 6, wherein the gas sensor has an action of selectively responding to a gas having polarity at 100 to 200 ° C in air by increasing a resistance value. A method of manufacturing a gas sensor including a substrate and a metal oxide semiconductor film formed on the substrate and having a function of selectively responding to a polar gas, the surface of which is a metal oxide semiconductor surface and hydrogen A method for producing a gas sensor, comprising modifying an organic layer having a functional group capable of bonding. 9. The method of manufacturing a gas sensor according to claim 8, wherein the metal oxide semiconductor film is a tin oxide film containing tin oxide as a main component. 9. The gas sensor according to claim 8, wherein the organic substance layer having a functional group capable of hydrogen bonding is chemically modified with a silane coupling agent and further modified with an organic carboxylic acid having a hydroxyl group. Production method. The gas sensor manufacturing method according to claim 10, wherein the silane coupling agent is made of a compound having an amino group. The method for producing a gas sensor according to claim 10, wherein the organic carboxylic acid having a hydroxyl group is benzoic acid having a hydroxyl group. 11. The gas sensor according to claim 10, wherein the carboxyl group of the organic carboxylic acid having a hydroxyl group and the amino group of the silane coupling agent undergo dehydration condensation to fix the organic carboxylic acid having a hydroxyl group on the surface of tin oxide. Method. A gas detector comprising the gas sensor according to any one of claims 1 to 7 as a component, wherein the gas sensor, a detection circuit that detects a change in resistance value of the gas sensor as an electrical output, and detected by the detection circuit. A gas detector comprising: output means for outputting gas information related to the detection target gas based on the electrical output.

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