JP2011075421A - Gas sensing element and gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas sensing element, and a gas sensor having an excellent property such as the high sensitivity, a long lifetime and a low resistance, easily manufactured, excellent in the productivity, and easily obtaining the gas selectivity. <P>SOLUTION: The gas sensing element 1 includes: a three-dimensional grid composite structure 4 in a state that semiconductor particulates 2 and noble metal particulates 3 contact each other; and pores 5 as voids spread in a three-dimensional mesh within the composite structure 4, and communicating with each other, and has a gas selectivity by selecting compositions of the semiconductor particulates 2 and the noble metal particulates 3, and changing a ratio of the semiconductor particulates 2 and the noble metal particulates 3. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a gas sensor and a gas sensor. More specifically, the gas sensor has excellent characteristics such as high sensitivity, long life, and gas selectivity. Further, the gas sensor is easy to manufacture and excellent in mass productivity, and The present invention relates to a gas sensor provided with the gas sensing element.

Conventionally, as a gas sensor that is easy to handle and can be miniaturized, a gas sensor using a semiconductor as a gas sensing element has been proposed and put into practical use.
This gas sensor utilizes the fact that the electrical resistance of the gas detector changes in proportion to the concentration of the gas to be detected when the semiconductor gas detector constituting the gas detector is brought into contact with the gas to be detected. The concentration is measured, and a metal oxide sintered body, which is a semiconductor, is generally used as the gas detection unit.
This gas sensor is used for general gas leak detection, taking advantage of the fact that the gas concentration can be measured with a simpler operation than before. In recent years, new applications have been applied to detection of air pollutant gases, bad breath causing substances, hydrogen gas, and the like. In order to promote the spread to such a new application field, high sensitivity, low power consumption, provision of gas selectivity, and the like have been problems, and studies are being conducted.

As a method of improving the sensitivity and gas selectivity of the gas detection unit, a method of supporting noble metal fine particles on the surface of metal oxide particles that are semiconductors is known.
Here, as the noble metal fine particles to be supported, those having a catalytic effect that enhances the effect between the gas to be detected or oxygen gas in the air and the metal oxide particles are preferable. For example, tin oxide particles as the metal oxide particles When is used, palladium, platinum, gold or the like is preferably used. Since the noble metal fine particles are added as a catalyst, the addition amount is about several percent at most of the mass of the metal oxide particles.
As a method for producing metal oxide particles carrying noble metal fine particles having such a catalytic effect, a raw material containing metal oxide particles and a precursor of noble metal fine particles is hydrothermally reacted under high temperature and high pressure, A method of supporting fine noble metal fine particles on the surface of metal oxide particles has been proposed (for example, Patent Document 1).

In addition, it is known that it is effective to reduce the metal oxide particles constituting the gas detection unit in order to improve the sensitivity of the gas sensor (for example, Non-Patent Documents 1 and 2). When the metal oxide particles are tin oxide particles, the sensitization effect appears when the diameter of the particles is about 10 nm or less, and particularly remarkable when the particle diameter is about 5 nm or less (Non-patent Document 2).
The reason why the sensitivity is improved by reducing the particles as described above can be explained as follows. First, the reason why the electrical resistance changes when the semiconductor gas detection part comes into contact with the gas to be detected is that the gas to be detected removes oxygen bonded to the surface layer of the metal oxide particles and the interparticle bonding part. This is because the conductivity (electrical resistance) of the surface layer and the bonding portion of the metal oxide particles changes. Since the thickness of the surface layer and the bonding part surface layer is constant regardless of the particle diameter of the metal oxide particles, the ratio of the surface layer to the metal oxide particles and the whole interparticle bonding part as the particle diameter decreases. Will increase. Therefore, it is considered that the ratio of the surface layer where the conductivity changes increases and the sensitivity is improved.

  On the other hand, in order to improve the sensitivity of the gas sensor, it has been reported that it is effective to narrow the interval between measurement electrodes attached to the gas sensor to an order of 1 μm or less. This is called a microgap effect. For example, tungsten oxide (Non-Patent Documents 3 and 4), indium oxide (Non-Patent Document 5), and tin oxide (Non-patent Document) are used as metal oxide particles used as a gas detection unit. A report using References 6 and 7) has been reported. Further, in order to apply the above-mentioned microgap effect to a gas sensing element having a conventional electrode spacing, secondary particles of noble metal are dispersed in metal oxide fine particles of the gas sensing element, and the noble metal secondary particles are interposed between the noble metal secondary particles. There has been proposed a gas sensor in which a sensitivity is improved by forming a minute gap of 1 μm or less and using this minute gap as a detection region (Patent Document 2).

  Further, a semiconductor gas sensor is also proposed in which a pair of film electrodes and a heater are provided on an insulating substrate, and a gas detection unit made of a metal oxide such as tin oxide is formed on these film electrodes (Patent Document). 3). In this semiconductor type gas sensor, the gas type showing the maximum sensitivity can be adjusted by changing the relative ratio between the film thickness dimension of the gas detection part and the gap dimension of the film electrode, and the selectivity of the gas to be detected is provided. Can be made.

JP 2008-20411 A JP 2007-78513 A JP 7-140102 A

Supervised by Masao Karabe, "Biosensor Chemical Dictionary", Techno System, August 2007, 475 pages “DENKI KAGAKU”, The Electrochemical Society, December 1990, 58, No. 12, p. 1143 TAMAKI Jun, MIYAJI Akira, MAKINODAN Jiro, OGURA Shunsuke, KONISHI Satoshi, “Effect of micro-gap electrode on detection of dilute NO2 using WO3 thin” film microsensors), Sensors and Actuators B, Vol.108, No.1-2, Page.202 (2005.07) Yudai Kanno, Shingo Hashi, Dao Zumbet, Susumu Sugiyama, Jun Tamaki, “Development of High Sensitivity Nitrogen Dioxide Sensor Using Nano-Gap Comb Electrode”, Chemical Society of Electrochemical Society, 43rd Chemical Sensor Research Meeting , No. 36, issued in March 2007 TAMAKI Jun, NIIMI Jun, OGURA Shunsuke, KONISHI Satoshi, "Effect of micro-gap electrode on sensing properties to dilute chlorine gas of indium oxide" thin film microsensors), Sensors and Actuators B, Vol.117, No.2, Page.353 (2006.10) Tetsufumi Otani, Jun Tamaki, "Evaluation of grain boundary and interface resistance of tin oxide micro gas sensor by complex impedance method", Chemical Society of Electrochemical Society, 43rd Chemical Sensor Research Conference, No. 34, issued in March 2007 Masanori Nakata, Jun Konishi, Jun Tamaki, “Effects of microgap on flammable gas detection characteristics of tin oxide thin film microgas sensor, evaluation of grain boundary and interface resistance”, Chemical Society of Electrochemical Society, 43rd Chemical Sensor Research presentation, No. 35, issued in March 2007

  By the way, in the conventional gas sensing element in which the noble metal fine particles are supported on the surface of the metal oxide particles, in order for the noble metal fine particles to act effectively as a catalyst, the noble metal is converted into ultrafine particles and supported on the surface of the metal oxide particles. There is a need. However, it is difficult to mass-produce gas detectors in which noble metal ultrafine particles are supported on the surface of metal oxide particles because the process for supporting ultrafine particles of noble metal is complicated and requires strict control. There was a problem.

  In addition, in order to form the measurement electrodes attached to the gas sensor at intervals of the order of 1 μm or less, semiconductor technology such as photolithography and focused ion beam processing and MEMS technology are required. Equipment and devices used in the technology are very expensive, and there is a problem that an increase in cost cannot be avoided.

Further, in a gas sensor in which noble metal secondary particles are dispersed in metal oxide fine particles to form a minute gap, when the amount of noble metal secondary particles added is small, the noble metal secondary particle spacing is wide, On the other hand, when the amount of the secondary particles of the noble metal is increased, a minute gap is not formed between the secondary particles of the noble metal, and so-called conductive paths are formed. There was a problem that the measurement electrodes were completely short-circuited, making measurement impossible.
As described above, in this gas sensor, it is necessary to strictly control the amount of addition and dispersion state of the precious metal secondary particles, and even if these are controlled, the characteristics of the obtained gas sensor are likely to vary. There was a problem that it was not suitable for mass production.
Furthermore, this gas sensor can improve the sensitivity, but has no effect on the gas selectivity.

  Further, in a semiconductor type gas sensor in which a gas detection part is formed on a film electrode, the selectivity of the gas to be detected is given by changing the relative ratio between the film thickness dimension of the gas detection part and the gap dimension of the film electrode. Therefore, it is necessary to prepare a plurality of types of gas sensors having different interval sizes for each type of gas to be detected, or to adjust the film thickness of the gas sensing element, which is not suitable for mass production.

  The present invention has been made in order to solve the above-described problems, and has excellent characteristics such as high sensitivity, long life, and low resistance, and is easy to manufacture and excellent in mass productivity. It is an object of the present invention to provide a gas sensor and a gas sensor that can easily impart the property.

  In order to solve the above-mentioned problems, the present inventor conducted intensive studies focusing on the dispersion / contact state of the semiconductor fine particles and the noble metal fine particles used in the gas detection unit of the gas sensor, and as a result, formed the gas sensor. Dispersing precious metal fine particles whose average particle diameter is approximately the same as or close to that of the semiconductor fine particles constituting the gas detection unit in the gas detection unit, thereby achieving high sensitivity and long life, low resistance, easy manufacturing and mass production. The present invention is completed by finding that a gas sensing element excellent in performance can be obtained, and further, by finding that gas selectivity can be easily imparted by changing the volume ratio of semiconductor fine particles and noble metal fine particles. It came to.

  That is, the gas sensor of the present invention is a gas sensor that measures the concentration of the gas to be detected using a change in electric resistance of the gas detector, wherein the gas detector includes semiconductor fine particles and noble metal fine particles, The average particle diameter of the noble metal fine particles is 0.5 to 2 times the average particle diameter of the semiconductor fine particles, and the volume ratio of the noble metal fine particles to the total amount of the noble metal fine particles and the semiconductor fine particles is 0.00. It is characterized by being 5 volume% or more and 40 volume% or less.

Any one of the semiconductor fine particles themselves, a bonding surface or contact surface between the semiconductor fine particles, a bonding surface or contact surface between the semiconductor fine particles themselves and the semiconductor fine particles has a gas detection function, and the noble metal fine particles and The bonding surface or contact surface with the semiconductor fine particles preferably has a metal-semiconductor bonding effect.
It is preferable that the gas detection part has gas selectivity by changing a volume ratio between the semiconductor fine particles and the noble metal fine particles.

  The gas sensor of the present invention comprises the gas sensor of the present invention.

According to the gas sensor of the present invention, the gas detection unit includes semiconductor fine particles and noble metal fine particles, and the average particle diameter of the noble metal fine particles is 0.5 to 2 times the average particle diameter of the semiconductor fine particles. Since the volume ratio of the noble metal fine particles to the total amount of the noble metal fine particles and the semiconductor fine particles is 0.5% by volume or more and 40% by volume or less, excellent characteristics such as high sensitivity and long life can be obtained.
Further, this gas detector is easy to manufacture and excellent in mass productivity.
Further, by changing the volume ratio between the semiconductor fine particles and the noble metal fine particles, the gas sensing element is given gas selectivity, so that gas selectivity can be easily imparted.

It is a schematic diagram which shows an example of the basic structure of the gas sensor of one Embodiment of this invention. It is a figure which shows the relationship between the volume ratio of semiconductor fine particles and noble metal fine particles, and electrical conductivity. It is a figure which shows the relationship between the volume ratio of semiconductor fine particles and noble metal fine particles, and to-be-detected gas sensitivity. It is a circuit diagram which shows the measurement circuit for measuring the resistance value of a gas sensor. It is a schematic block diagram which shows an example of the measuring apparatus for measuring the sensitivity of a gas sensor. It is a schematic block diagram which shows another example of the measuring apparatus for measuring the sensitivity of a gas sensor.

An embodiment for carrying out a gas sensor and a gas sensor of the present invention will be described.
This embodiment is specifically described for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[Gas detector]
(Basic structure and measurement principle of gas detector)
The basic structure and measurement principle of the gas detector in the gas sensor of this embodiment will be described.
The basic structure of the gas detection unit is a three-dimensional composite structure in which semiconductor fine particles and noble metal fine particles having an average particle diameter substantially the same or similar are arranged in a substantially lattice shape so as to be dispersed and in contact with each other.

FIG. 1 is a schematic diagram showing an example of a basic structure of a gas detection unit in the gas sensing element of the present embodiment. The gas detection unit 1 uses a change in electrical resistance (or conductivity) of a semiconductor to detect the structure. A gas detector for measuring the concentration of the detection gas, and in a state in which the semiconductor fine particles 2 and the noble metal fine particles 3 having the same or similar average particle diameter are dispersed, the semiconductor fine particles 2, the noble metal fine particles 3, The semiconductor fine particles 2 and the noble metal fine particles 3 are combined or in contact with each other to form a substantially lattice-shaped three-dimensional composite structure 4.
In the following notation, “semiconductor fine particles 2, noble metal fine particles 3, and semiconductor fine particles 2 and noble metal fine particles 3” are abbreviated as “fine particles of semiconductor fine particles 2 and noble metal fine particles 3” and “bond or “Contact” is abbreviated as “bonding (contact)”.

  Since the voids between the fine particles composed of the semiconductor fine particles 2 and the noble metal fine particles 3 in the composite structure 4 are pores 5 communicating with each other, the pores 5 are three-dimensionally formed in the composite structure 4. It will be stretched around in the form of a mesh.

(Gas detection principle and gas selectivity in the gas detector)
The gas detector 1 of the present embodiment can have gas selectivity by changing the volume ratio between the semiconductor fine particles 2 and the noble metal fine particles 3. That is, if the concentration of the noble metal fine particles 3 with respect to the total amount of the noble metal fine particles 3 and the semiconductor fine particles 2 is defined as the concentration of the noble metal fine particles 3, the concentration of the noble metal fine particles 3 is 5% by volume or less and when the concentration exceeds 5% by volume. The type of gas to be measured can be changed.
Therefore, first, the reason for the gas selectivity and its range will be described based on the principle of gas detection.

  First, in the structure of the gas detection unit 1 according to the present embodiment, when the amount of the noble metal fine particles 3 is increased, the increase in the gas detection unit 1 is a bond (contact) between the semiconductor fine particles 2 and the noble metal fine particles 3. Surface. Here, if both the semiconductor fine particles 2 and the noble metal fine particles 3 are spherical and have the same particle diameter, and these particles are uniformly dispersed, the gap between the semiconductor fine particles 2 and the noble metal fine particles 3 is assumed. The amount of the bonding (contact) surface is maximum when the concentration of the noble metal fine particles is 50% by volume.

Therefore, focusing on the metal-semiconductor interface, the metal-semiconductor junction effect, such as the Schottky junction effect, appears at the metal-semiconductor interface due to the difference between the metal work function and the electron affinity of the semiconductor. It is known.
When these bonding effects are manifested, a rectifying action occurs between the metal and the semiconductor. That is, current flows easily from metal to semiconductor, but current hardly flows from semiconductor to metal, or conversely, current flows easily from semiconductor to metal, but current flows from metal to semiconductor. It becomes difficult to flow.

  Here, since the bonding effect between the metal and the semiconductor such as the Schottky bonding effect is manifested by the characteristics of both the metal and the semiconductor, the degree of expression and the amount of expression are the type and combination of the metal and the semiconductor, Further, it is known to change depending on the state of metal or semiconductor. Therefore, if you select a metal that causes a characteristic change by a specific gas and combine this metal and a semiconductor, or select a semiconductor that causes a characteristic change by a specific gas and combine this semiconductor and a metal, The amount of the metal-semiconductor bonding effect that appears at the metal-semiconductor interface varies depending on the presence and amount of the specific gas. Of course, it is also possible to select and combine both metals and semiconductors that cause a characteristic change by a specific gas.

In this way, the type and combination of the semiconductor and the metal are selected, and the electrode made of the selected metal is provided on the substrate made of the selected semiconductor, and the electrical characteristics between the substrate and the electrode (rectification, electrical conductivity, amplification) If the sensitivity or the like is measured, a gas detector whose electrical characteristics change according to the presence or absence and amount of the specific gas can be formed.
Patent documents of such a gas detector include, for example, Japanese Patent Laid-Open Nos. 2001-281213 and 2008-145128.

  As described above, the gas detector manufactured by using the metal-semiconductor bonding effect described in the above-mentioned patent document is similar to an IC chip in which a planar electrode is formed on one semiconductor substrate. A monolithic structure is used, and a relatively complicated circuit is used to measure the electrical characteristics between the substrate and the electrodes, such as rectification, electrical conductivity, and amplification sensitivity.

  On the other hand, the gas detection unit 1 of the present embodiment combines or contacts the fine particles composed of the semiconductor fine particles 2 and the noble metal fine particles 3 in a uniformly dispersed state to form a substantially lattice-shaped three-dimensional composite structure 4. Since it is formed, a metal-semiconductor bonding effect appears on the bonding (contact) surface between the semiconductor fine particles 2 and the noble metal fine particles 3, and even if a rectifying action or an amplifying action occurs, the gas detector 1 as a whole is seen. In this case, the direction of the action is dispersed and averaged in all directions, and the rectifying action and the amplifying action are not exhibited. Therefore, it is difficult to detect with a measurement method used for a gas detector using a conventional metal-semiconductor bonding effect, such as measurement of a rectification state or amplification action.

  Here, in the gas detection unit 1 of the present embodiment, the direction of the rectifying action is dispersed in all directions, so the direction of the reverse rectifying action (direction of action in which the current hardly flows) is also dispersed in all directions. Therefore, when viewed as the gas detection unit 1 as a whole, the resistance value of the gas detection unit 1 increases if a metal-semiconductor bonding effect is exhibited. That is, in order to detect the bonding effect between the metal and the semiconductor in the gas detector 1 of the present embodiment, the resistance value of the gas detector 1 itself may be measured. To detect a specific gas, the gas detector 1 What is necessary is just to measure the resistance value change.

Next, a change in resistance value in the gas detection unit 1 of the present embodiment will be described.
In the gas detection unit 1 of the present embodiment, when the concentration of the noble metal fine particles 3 is increased within the range of 0.5 volume% or more and 40 volume% or less, the resistance value of the gas detection section 1 also increases. Thus, although the amount of the noble metal fine particles 3 having a smaller electric resistance than that of the semiconductor fine particles 2 is increased, the increase in the resistance value has some effect. it is conceivable that.

As this effect, a case where a metal-semiconductor bonding effect such as a Schottky bonding effect is manifested on the bonding (contact) surface between the semiconductor fine particles 2 and the noble metal fine particles 3 in the gas detection unit 1 of the present embodiment is considered. .
As described above, in the present embodiment, these metal-semiconductor bonding effects can be understood as the resistance value of the gas detection unit 1.
Here, when the noble metal fine particles 3 are increased, the bonding (contact) surface between the semiconductor fine particles 2 and the noble metal fine particles 3 is increased, so that the amount of the coupling effect is increased, and as a result, the resistance of the gas detection unit 1 is increased. The value is considered to increase more.

  From these results, it is considered that the Schottky junction effect and the like are also manifested on the bonding (contact) surface between the semiconductor fine particles 2 and the noble metal fine particles 3 in the gas detection unit 1 of the present embodiment.

Next, the reason why the gas selectivity is developed in the gas detection unit 1 of the present embodiment will be described.
First, the case where the concentration of the noble metal fine particles is 0.5 volume% or more and 5 volume% or less will be described for the gas detection unit 1 of the present embodiment.
If the type of the gas to be measured in this range is the first gas to be measured, the type of the first gas to be measured is the same as the conventional semiconductor gas sensor using the semiconductor fine particles 2 for the gas detector 1.

Here, when the concentration of the noble metal fine particles 3 is 0.5 volume% or more and 5 volume% or less, the change in the resistance value of the gas detection unit 1 based on the same detection principle as the conventional “noble metal catalyst-added semiconductor gas sensor” occurs. It is considered that the gas to be measured is detected as a main factor.
That is, in the bonding (contact) region between the semiconductor fine particles 2 and the semiconductor fine particles 2, the measured gas is adsorbed on these surface layers, or the oxygen adsorbed in advance is desorbed and removed by the detected gas. The electrical resistance of the layer is reduced (conductivity is improved). On the other hand, since the noble metal fine particles 3 have a catalytic effect on the gas to be measured and oxygen adsorption / desorption action, a change in the resistance value of the gas detector 1 is measured as a method for detecting the gas to be measured. Is done.

Even in this region, that is, in the case where the concentration of the noble metal fine particles 3 is 0.5 volume% or more and 5 volume% or less, the Schottky as described above is formed on the bonding (contact) surface between the semiconductor fine particles 2 and the noble metal fine particles 3. It is considered that a metal-semiconductor bonding effect such as a bonding effect occurs. However, since the generation amount is small because the concentration of the noble metal fine particles 3 is low, the action and effect thereof are not manifested, and the change in the resistance value of the gas detector 1 due to the metal-semiconductor bonding effect is negligible. It is thought that.
Even if the concentration of the noble metal fine particles 3 is less than 0.5% by volume, the gas to be measured can be detected, but since the catalytic effect of the noble metal fine particles 3 is almost eliminated, the concentration lower limit of the noble metal fine particles 3 is reduced. The value is 0.5% by volume.

Next, the case where the concentration of the noble metal fine particles is 10% by volume or more will be described.
If the gas to be measured when the concentration of the noble metal fine particles is 10% by volume or more is the second gas to be measured, the type of the second gas to be measured may be the same as the type of the first gas to be measured. But usually it is different.

  As described above, in the present embodiment, the metal-semiconductor bonding effect such as the Schottky bonding effect can be regarded as the resistance value of the gas detection unit 1, and the specific gas, that is, the second measured gas. This can be detected by measuring a change in the resistance value of the gas detector 1.

Here, when the concentration of the noble metal fine particles 3 exceeds 5% by volume, the amount of the bonding (contact) surface between the semiconductor fine particles 2 and the noble metal fine particles 3 increases, so that the bonding effect between these metals and the semiconductor is manifested. It is thought that it will become. In particular, when the concentration of the noble metal fine particles 3 is 10% by volume or more, the influence of the bonding effect between the metal and the semiconductor becomes very large. In the gas detector in this state, the main cause of the change in the resistance value is the metal − This is thought to be due to the effect of bonding between semiconductors.
Also in this region, it is considered that a change in electrical resistance (conductivity) occurs in the bonding (contact) region between the semiconductor fine particles 2 and the surface layer of the semiconductor fine particles. Since the amount of change in resistance value is large, the amount of change in resistance value caused only by the semiconductor fine particles does not become apparent and is considered to be in a negligible range.

  As described above, even if the change in the resistance value of the gas detector 1 is measured by the same measurement method, the main factor of the change in the resistance value in the gas detector 1 changes depending on the concentration of the noble metal fine particles 3. Since they are completely different, different gases can be measured.

First, the main factor of the resistance value change in the gas detector whose concentration of the noble metal fine particles 3 is 5% by volume or less is due to the change in electrical resistance (conductivity) of the bonding (contact) region between the semiconductor fine particles 2 and the surface layer of the semiconductor fine particles. This is a characteristic change of the semiconductor fine particles 2 alone.
Even in this region, the metal-semiconductor bonding effect is considered to have occurred, but since the concentration of the noble metal fine particles 3 is low, the amount of change in the resistance value due to its action is small and does not become obvious, and can be ignored. It is thought that.

On the other hand, in the gas detector with the concentration of the noble metal fine particles 3 of 10% by volume or more, the main factor of the change in the resistance value is due to the bonding effect between the metal and the semiconductor, and in the relationship between the noble metal fine particles 3 and the semiconductor fine particles 2. A characteristic change is detected.
Even in this region, it is considered that the electrical resistance (conductivity) change of the bonding (contact) region between the semiconductor fine particles 2 and the surface layer of the semiconductor fine particles has occurred, but this is caused by the bonding effect between the metal and the semiconductor. Since the amount of change in resistance value is large, the amount of change in resistance value caused only by the semiconductor fine particles does not become apparent and is considered to be in a negligible range.

In the region where the concentration of the noble metal fine particles 3 exceeds 5% by volume and less than 10% by volume, the bonding (contact) region between the semiconductor fine particles 2 and the surface of the semiconductor fine particles are the main causes of the change in the resistance value of the gas detector 1. The change in resistance value of the gas detector 1 is linked to the measurement / detection of a specific gas because the change due to the change in the electrical resistance (conductivity) of the layer and the effect due to the bonding effect between the metal and the semiconductor are arranged side by side. It is difficult to obtain gas selectivity. Therefore, this area is excluded from the measurable area.
However, since the resistance change amount of the gas detector 1 itself can be measured, if the gas selectivity is obtained by changing the type of semiconductor or noble metal or combining with other measurement methods, this region It is possible to measure and detect specific gases.

Further, the reason why the upper limit of the concentration of the noble metal fine particles 3 is 40% by volume will be described.
First, as the concentration of the noble metal fine particles 3 is increased from 5% by volume, the measurement sensitivity of the gas to be measured is also improved. This is thought to be due to an increase in the bonding (contact) surface area between the semiconductor fine particles 2 and the noble metal fine particles 3, thereby increasing the metal-semiconductor bonding effect.

  However, in a state where the bonding (contact) surfaces of the noble metal fine particles 3 are continuous, that is, when a conductive path (conductive path) is formed, most of the current for measurement passes through this conductive path. The sensitivity is lost. In this case, the gas detection part 1 comes to show the characteristic as a metal. In other words, in the gas detection unit 1, the contact between the noble metal fine particles 3 is hindered by the semiconductor fine particles 2, so that a conductive path is not formed, and it is necessary to exhibit properties as a semiconductor.

Here, when the gas detector 1 has a spherical shape in which both the semiconductor fine particles 2 and the noble metal fine particles 3 have the same particle diameter, and the semiconductor fine particles 2 and the noble metal fine particles 3 are uniformly mixed, When the effective medium approximation method is applied, the critical concentration at which the gas detection unit 1 shows a change in the characteristics of the metal and the semiconductor is 50% by volume.
However, in the vicinity of the critical concentration, the effect of the noble metal fine particles 3 becomes too great even if it is a semiconductor characteristic. In the actual gas detection unit 1, the particle shapes of the semiconductor fine particles 2 and the noble metal fine particles 3 are In addition to being completely spherical, the particle diameters of the semiconductor fine particles 2 and the noble metal fine particles 3 have a constant distribution, and the average particle diameters thereof are not the same. Furthermore, the semiconductor fine particles 2 and the noble metal fine particles 3 Considering that there may be a non-uniform portion in the dispersion state, the actual critical concentration is less than 50% by volume.

Here, in order to keep the critical concentration within the range of 40 volume% or more and 60 volume% or less, the average particle diameter of the noble metal fine particles 3 in the gas detection unit 1 is 0.5 times the average particle diameter of the semiconductor fine particles 2. In order to improve the sensitivity of the gas detection unit 1 approximately twice or more as compared with the case where the gas detection unit 1 is composed of only the semiconductor fine particles 2, the noble metal fine particles 3 are required. In addition, the volume fraction of the noble metal fine particles 3 with respect to the total of the semiconductor fine particles 2 needs to be in the range of 0.5 volume% or more and 40 volume% or less.
This has been found by the present inventor as a result of actual verification in consideration of the above points. From this result, the upper limit of the concentration of the noble metal fine particles 3 in the gas detection unit 1 is 40% by volume. It is concluded that

FIG. 2 is a diagram illustrating an example of the characteristics of the gas detection unit 1 of the present embodiment, and is a diagram illustrating the relationship between the volume ratio of the semiconductor fine particles and the noble metal fine particles and the conductivity of the gas detection unit 1.
Here, the resistance value of the gas detection unit 1 increases as the concentration of the noble metal fine particles 3 increases. However, when the concentration exceeds the critical concentration, it rapidly decreases and decreases to a resistance value comparable to that of a metal. In this figure, as an example, the critical concentration is about 50% by volume, but is not limited to this value.
Thus, in the range where the concentration of the noble metal fine particles 3 is lower than the critical concentration, the resistance value of the gas detection unit 1 increases as the concentration of the noble metal fine particles 3 increases, and the semiconductor fine particles in the gas detection unit 1 of the present embodiment. It is considered that the Schottky bonding effect and the like are expressed on the bonding (contact) surface between 2 and the noble metal fine particles 3.

FIG. 3 is a diagram showing an example of the characteristics of the gas detection unit 1 of the present embodiment, and is a diagram showing the relationship between the volume ratio between the semiconductor fine particles and the noble metal fine particles and the detected gas sensitivity.
Curve 1 shows the sensitivity of the first gas to be measured, such as toluene, in the gas detector 1 of the present embodiment, and the bonding (contact) region between the semiconductor fine particles 2 and the surface layer of the semiconductor fine particles 2. This is due to the change in electrical resistance.
The sensitivity of the noble metal fine particles 3 has a constant value even when the concentration is zero, but rapidly increases until the concentration reaches several volume%. This is considered to be a catalytic effect by the noble metal fine particles 3. Furthermore, when the density exceeds several percent, the sensitivity decreases. This is presumably because the catalytic effect of the noble metal fine particles 3 is saturated, while the amount of the semiconductor fine particles 2 decreases.

Curve 2 shows the sensitivity of the second gas to be measured, such as hydrogen sulfide, in the gas detector 1 of the present embodiment, and is caused by a change in resistance value due to a metal-semiconductor bonding effect. is there.
In this case, when the concentration of the noble metal fine particles 3 is zero, the sensitivity is zero. The sensitivity increases as the concentration of the noble metal fine particles 3 increases. This is presumably because the interface between the noble metal fine particles 3 and the semiconductor fine particles 2 increases. Note that the sensitivity decreases before the critical concentration, which is considered to be because the influence of the noble metal fine particles 3 becomes excessive.

When these curves 1 and 2 are compared, in the region where the concentration of the noble metal fine particles 3 is 0.5 volume% or more and 5 volume% or less, the sensitivity of the curve 1 is high. Therefore, the first measured gas is selectively selected. Can be detected. Further, in the region where the concentration of the noble metal fine particles 3 is 10% by volume or more and 40% by volume or less, the sensitivity of the curve 2 is high. Therefore, the second measured gas having a composition different from that of the first measured gas is selectively used. Can be detected.
On the other hand, in the region where the concentration of the noble metal fine particles 3 exceeds 5% by volume and less than 10% by volume, both the sensitivity of the curves 2 and 3 exists to a certain extent, and therefore the gas to be measured can be selectively detected. Is difficult.

(Detailed explanation of basic structure of gas detector)
As described above, the basic structure of the gas detection unit 1 of the present embodiment is obtained by combining (contacting) the fine particles composed of the semiconductor fine particles 2 and the noble metal fine particles 3 in a state of being uniformly dispersed with each other. A dimensional composite structure 4 is formed.

As the semiconductor fine particles 2, inorganic oxide fine particles are suitably used because they are semiconductor materials and the resistance value (or conductivity) needs to change due to the presence of at least the first gas to be measured.
As such inorganic oxide fine particles, for example, metal oxide fine particles containing metals such as tin, antimony, iron, tungsten, zinc, and indium are preferably used, and in particular, tin oxide (SnO ₂ ), oxidation tungsten (WO _3), or the like, may be selected having a high sensitivity to the gas to be detected.

In the gas detection unit 1 using these metal oxide fine particles, the bonding (contact) region between the semiconductor fine particles 2 and the semiconductor fine particles 2 are adsorbed in advance on the bonding (contact) region and the surface layer of the semiconductor fine particles 2. Oxygen that is present is desorbed and removed by the gas to be detected, so that the electrical resistance of the surface layer decreases (conductivity increases).
In addition, you may select the metal oxide microparticles | fine-particles which an electrical resistance falls by to-be-measured gas itself adsorb | sucking.

In addition to the characteristic change (resistance value change) due to the first gas to be measured, the semiconductor fine particles 2 may also have a characteristic change (resistance value change) in the second gas to be measured. If there is also a characteristic change (resistance value change) in the second gas to be measured, the composition of the noble metal fine particles 3 can be arbitrarily selected, and the selection range is widened.
However, when the characteristic change (resistance value change) due to the second gas to be measured is similar to the characteristic change (resistance value change) due to the first gas to be measured, the selectivity of the gas to be measured in the gas detection unit 1 May get worse. From this point, it is assumed that the semiconductor fine particle 2 undergoes a characteristic change (resistance value change) mainly due to the first gas to be measured, and whether the characteristic change (resistance value change) is small with respect to the second gas to be measured. Alternatively, it is preferable that the characteristic change (resistance value change) does not occur (the noble metal fine particles 3 change the characteristic (resistance value change) for the second gas to be measured).

  The shape of the semiconductor fine particles 2 is preferably close to a sphere. This is because when the particles are close to spherical, they are easily dispersed uniformly when dispersed with the noble metal fine particles 3. Also, if the shape is close to a sphere, the area where the fine particles of the semiconductor fine particles 2 and the noble metal fine particles 3 come into contact with each other can be made substantially the same regardless of the particles. This can stabilize the characteristics of the gas detector 1.

  The average particle diameter of the semiconductor fine particles 2 is not particularly limited, but is preferably fine, specifically 3 nm or more and 500 nm or less, more preferably 3 nm or more and 100 nm or less, still more preferably. Is 3 nm or more and 30 nm or less.

  Here, the reason why the particle size of the semiconductor fine particles 2 is preferably small is as follows. First, when detecting the first gas to be measured, by reducing the particle diameter of the semiconductor fine particles 2, the ratio of the surface layer to the entire semiconductor fine particles 2, that is, the region in which the resistance value changes due to the gas to be detected is relatively This is because the amount of change in the electrical resistance (conductivity) of the semiconductor fine particles 2 itself increases. In addition, when the particle diameter of the semiconductor fine particles 2 is reduced, the contact area between the fine particles composed of the semiconductor fine particles 2 and the noble metal fine particles 3 can be reduced, so that the sensitivity of the contact point is improved. Furthermore, since the number of particles per unit volume of the gas sensing element 1 increases, the number of contact points also increases.

That is, for detection of the first gas to be measured, as the particle diameter of the semiconductor fine particles 2 decreases, the amount of change in electrical resistance (conductivity) at each contact point and the semiconductor fine particles 2 itself increases, and the number of particles and the contact Since the score increases, the sensitivity of the gas detection unit 1 is improved.
On the other hand, regarding the detection of the second gas to be measured, the contact area between the semiconductor fine particles 2 and the gas to be measured is increased by reducing the particle diameter of the semiconductor fine particles 2. As a result, the sensitivity of the gas detection unit 1 is improved and the measurement time is shortened.

  Next, the noble metal fine particles 3 are not oxidized in the temperature range from room temperature (25 ° C.) to the operating temperature of the gas sensor, do not react with the semiconductor fine particles 3, and are further sintered. And need to be stable. The operating temperature of the gas sensor is a temperature at which the gas sensing element 1 is heated at the time of gas detection, and is usually selected within a range of 200 ° C. to 400 ° C. depending on the type of gas to be measured and the characteristics of the gas sensing element 1. Is done.

It is preferable to select the noble metal fine particles 3 in which the characteristic change (resistance value change) occurs due to the presence of the second gas to be measured.
As described above, the second detection gas detection principle is the expression and change of the metal-semiconductor bonding effect on the bonding (contact) surface between the semiconductor fine particles 2 and the noble metal fine particles 3. Here, as already described, when the semiconductor fine particle 2 undergoes a characteristic change with respect to the second measured gas, the characteristic change of the semiconductor fine particle 2 is caused by the first measured gas or the second measured gas. It becomes difficult to determine whether it is due to the gas to be measured 2 and the selectivity for the gas to be measured is deteriorated.

Therefore, the first measured gas is detected by the characteristic change (resistance value change) of the semiconductor fine particles 2, and the second measured gas is detected by the characteristic change (resistance value change) of the noble metal fine particles 3. It is preferable to select the type of fine particles to be detected for each gas to be measured because measurement and interpretation of results are easy.
In addition, when the semiconductor fine particle 2 does not change in characteristics in the second measured gas, it is essential that the characteristics of the noble metal fine particles 3 change due to the presence of the second measured gas.

  Examples of such noble metal fine particles 3 include metals or alloys containing one or more selected from the group consisting of gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium. In particular, gold, silver, platinum, and palladium are preferable because they have catalytic characteristics that improve the measurement sensitivity of the semiconductor fine particles 2 to the first gas to be measured.

  The average particle size of the noble metal fine particles 3 is preferably 0.5 times or more and 2 times or less, more preferably 0.8 times or more and 1.25 times or less, more preferably the average particle size of the semiconductor fine particles 2. Is 0.95 times or more and 1.05 times or less, and most preferably about the same size as the average particle diameter of the semiconductor fine particles (approximately 1 time: the average particle diameter is substantially the same). Also, the shape is preferably close to a sphere like the semiconductor fine particles 2.

Here, the reason why the average particle diameter and the shape of the noble metal fine particles 3 are preferably equal to that of the semiconductor fine particles 2 is preferable because the noble metal fine particles 3 and the semiconductor fine particles 2 are easily dispersed uniformly. This is because variation in characteristics such as sensitivity to gas is reduced, and as a result, stability and reliability of characteristics of the gas detection unit 1 are improved.
If the average particle diameter and shape of the noble metal fine particles 3 are the same as those of the semiconductor fine particles 2, the gas selection is performed by adjusting the volume ratio of the semiconductor fine particles 2 and the noble metal fine particles 3 to control the characteristics of the gas detector 1. This is because even in the case of providing the characteristics, the actual measurement value and the theoretical value are approximated, and the control becomes easy.

Further, if the average particle diameter and shape of the noble metal fine particles 3 are the same as those of the semiconductor fine particles 2, as will be described later, at the time of manufacturing the gas sensor 1, from the mixed dispersion liquid containing these noble metal fine particles 3 and the semiconductor fine particles 2. When forming the noble metal fine particle-containing semiconductor fine particle deposit, the semiconductor fine particles 2 and the noble metal fine particles 3 are easily dispersed and mixed easily, and the gas sensing element 1 having good characteristics can be obtained, which is preferable.
When the average particle size of the noble metal fine particles 3 is less than 0.5 times the average particle size of the semiconductor fine particles 2 or exceeds twice the average particle size, the noble metal fine particles 3 and the semiconductor fine particles 2 are difficult to disperse uniformly. Therefore, variations in characteristics such as sensitivity of the composite structure 4 to the gas to be detected increase, and as a result, the stability and reliability of the characteristics of the gas sensor 1 are lowered, which is not preferable.

Both the semiconductor fine particles 2 and the noble metal fine particles 3 may be primary particles, one may be primary particles and the other may be secondary particles, or both may be secondary particles. In the present embodiment, it is only necessary that the semiconductor fine particles 2 and the noble metal fine particles 3 having the same or similar average particle diameter are uniformly dispersed and mixed, and each fine particle is a primary particle or a secondary particle. Is not a problem.
However, when at least one is a secondary particle, it is necessary to maintain the state of a secondary particle in the process of manufacturing the gas detection part 1, and a part is crushed or it is crushed into a primary particle. It is necessary to be careful not to have it. When a part of the particles is crushed, the particle size is further reduced, and the average particle size and shape of one fine particle are different from those of the other fine particle. As a result, the semiconductor fine particles 2 and the noble metal fine particles are obtained. 3 is difficult to uniformly disperse and mix, and sufficient characteristics cannot be obtained.

  As described above, the gas detector manufactured using the metal-semiconductor bonding effect such as the conventional Schottky bonding effect forms a planar noble metal electrode on one semiconductor substrate. A monolithic structure similar to that of an IC chip. Therefore, the electrode shape or the electrode itself is made thin so that the electrode or semiconductor substrate and the gas to be measured are in contact with each other, but there is basically a limitation due to the monolithic structure.

  On the other hand, the gas detection unit 1 of the present embodiment combines (contacts) the fine particles composed of the semiconductor fine particles 2 and the noble metal fine particles 3 in a state of being uniformly dispersed with each other, and has a substantially lattice-like three-dimensional composite structure 4. Furthermore, since the voids between the fine particles are formed as pores 5 communicating with each other, the contact between the gas to be measured and the semiconductor fine particles 2 and the noble metal fine particles 3 is improved as compared with the conventional monolithic structure. The characteristics of the semiconductor fine particles 2 and the noble metal fine particles 3 as a whole are changed uniformly and rapidly with respect to the gas to be measured. As a result, the characteristic change amount increases and the change speed increases, so that the sensitivity increases and the measurement time can be shortened.

  In addition, although this gas detection part 1 is a sintered compact in which each fine particle which consists of the semiconductor fine particle 2 and the noble metal fine particle 3 is normally couple | bonded weakly, the coupling | bonding and contact of these fine particles in this embodiment are carried out. Is not limited to a sintered body. For example, the fine particles composed of the semiconductor fine particles 2 and the noble metal fine particles 3 are in contact with each other (not sintered). A structure bonded with a porous insulating substance may be used.

"Manufacturing method of gas detector"
The manufacturing method of the gas detection part of this embodiment is not particularly limited, and a conventionally used manufacturing method can be applied.
However, it is preferable that both the semiconductor fine particles and the noble metal fine particles have a nanometer size because the improvement of characteristics such as sensitivity can be obtained by making the semiconductor fine particles into a nanometer size. Here, when handling nanometer-sized fine particles, a dispersion liquid in which these fine particles are dispersed in a dispersion medium in advance is prepared, and the fine particles are mixed and dispersed using this dispersion liquid, and then applied and dried. It is effective to form a film.
Here, a method for manufacturing a gas detection unit using a dispersion of semiconductor fine particles and noble metal fine particles will be described.

(Semiconductor fine particles)
The semiconductor fine particle is a semiconductor material whose characteristic (resistance value) changes due to the presence of at least the first gas to be measured, and the characteristic (resistance value) changes depending on the presence of the second gas to be measured as necessary. Any inorganic oxide fine particles that have been conventionally used as a gas detector are suitable.
As the inorganic oxide fine particles, for example, metal oxide fine particles containing metals such as tin, antimony, iron, tungsten, zinc, and indium are preferably used, and in particular, tin oxide widely used as a sensor material. _(SnO 2), tungsten oxide (WO _3), or the like, may be selected having a high sensitivity to the gas to be detected.

  The shape of the semiconductor fine particles is preferably close to a sphere. This is because, if it is close to a sphere, it is easy to disperse uniformly when dispersed with the noble metal fine particles. In addition, if the shape is close to a sphere, the area of the part where each fine particle consisting of semiconductor fine particles and noble metal fine particles can be made substantially the same regardless of the particles, the sensitivity of the contact point can be made uniform, The characteristics of the gas detector 1 are stabilized.

  The average particle size of the semiconductor fine particles is not particularly limited, but is preferably fine, specifically 3 nm or more and 500 nm or less, more preferably 3 nm or more and 100 nm or less, and still more preferably. It is 3 nm or more and 30 nm or less.

  Here, the reason why the particle diameter of the semiconductor fine particles is preferably small is as follows. First, when detecting the first gas to be measured, by reducing the particle diameter of the semiconductor fine particles, the ratio of the surface layer to the entire semiconductor fine particles, that is, the region where the resistance value varies depending on the gas to be detected, is also relatively increased. This is because the amount of change in electrical resistance (conductivity) of the semiconductor fine particles themselves increases. In addition, when the particle diameter of the semiconductor fine particles is reduced, the contact area between the fine particles of the semiconductor fine particles and the noble metal fine particles can be reduced, so that the sensitivity of the contact point is improved. Furthermore, since the number of particles per unit volume of the gas sensing element 1 increases, the number of contact points also increases.

That is, for the detection of the first gas to be measured, as the particle size of the semiconductor fine particles decreases, the amount of change in electrical resistance (conductivity) at each contact point or semiconductor fine particle itself increases, and the number of particles or contact points increases. Since it increases, the sensitivity of the gas detection part 1 improves.
On the other hand, in the detection of the second gas to be measured, the contact area between the semiconductor fine particles and the gas to be measured is increased by reducing the particle diameter of the semiconductor fine particles, so that the characteristic change of the semiconductor fine particles can be performed uniformly and quickly. As a result, the sensitivity of the gas detector 1 is improved and the measurement time is shortened.

In order for the dispersion liquid in which the semiconductor fine particles are dispersed in the dispersion medium to exist stably, the average dispersed particle diameter of the semiconductor fine particles is preferably 100 nm or less.
Here, if the average dispersed particle diameter of the noble metal fine particles described later is also 100 nm or less, a mixed dispersion in which the semiconductor fine particles and the noble metal fine particles are uniformly dispersed in the same dispersion medium can be stably obtained. And a composite deposit composed of noble metal fine particles can be obtained in a good state, which is preferable.
Furthermore, if the shape is nearly spherical and the average particle diameter is 3 nm or more and 100 nm or less, the size and shape of the voids between the fine particles in the gas sensing body obtained by the manufacturing method of the present embodiment are determined by the gas to be detected. It can be suitable for passing.

The semiconductor fine particles are preferably metal oxide fine particles obtained by a hydrothermal synthesis method.
Considering that the semiconductor fine particles are dispersed in a dispersion, particularly an aqueous dispersion, the surface is preferably hydrophilic. However, if the hydrothermal synthesis method is used, the surface is highly hydrophilic and the composition is high. This is because fine metal oxide fine particles having a uniform particle diameter can be easily obtained.

(Precious metal fine particles)
The noble metal fine particles need not be oxidized in the temperature range from room temperature (25 ° C.) to the operating temperature of the gas sensor, and need only be stable without reacting with the inorganic oxide fine particles. There is no need to sinter.
The operating temperature of the gas sensor is a temperature at which the gas sensing element is heated at the time of gas detection, and is normally selected as appropriate depending on the type of measurement gas and the characteristics of the gas sensing element within a temperature range of 200 ° C. to 400 ° C. .

In addition, as the noble metal fine particles, it is preferable to select a noble metal whose characteristics change due to the presence of the second gas to be measured. This is because the change due to the first gas to be measured is a change in the resistance value of the semiconductor fine particles, and the change due to the second gas to be measured is the resistance value due to the metal-semiconductor bonding effect caused by the characteristic change of the noble metal fine particles. This is because it is preferable to separate the target fine particles for each gas to be measured because the measurement and interpretation of the results are easier.
In particular, when the semiconductor fine particles do not cause a characteristic change with respect to the second measured gas, it is essential that the noble metal fine particles cause a characteristic change with respect to the second measured gas.

  As such noble metal fine particles, one or more selected from the group of gold, silver, platinum, palladium, rhodium, iridium, ruthenium and osmium can be used. In particular, gold, silver, platinum, and palladium are preferable because they have catalytic characteristics that can improve measurement sensitivity. In addition, when using 2 or more types of noble metals, the mixture of fine particles of each noble metal may be sufficient, and you may use as an alloy.

The average particle size of the noble metal fine particles is preferably 0.5 to 2 times the average particle size of the semiconductor fine particles, more preferably 0.8 to 1 and 1 times the average particle size of the semiconductor fine particles. .25 times or less, more preferably approximately the same as the average particle diameter of the semiconductor fine particles.
Also, the shape is preferably close to a sphere like the semiconductor fine particles.

Also in this noble metal fine particle, in order that a dispersion liquid exists stably, it is preferable that the average dispersed particle diameter of a dispersed particle is 100 nm or less.
If the average particle diameter of the noble metal fine particles is 0.5 to 2 times the average particle diameter of the semiconductor fine particles, the semiconductor fine particles and the noble metal fine particles are easily and uniformly dispersed in the same dispersion medium. And a dispersion having excellent dispersibility can be obtained. As described above, if the semiconductor fine particles and the noble metal fine particles are uniformly dispersed in the same dispersion medium, the noble metal fine particle-containing semiconductor fine particle deposit obtained by removing the dispersion medium from the dispersion can also be used as a semiconductor. The fine particles and the noble metal fine particles can be uniformly dispersed, and a gas sensor having good characteristics with no variation in characteristics can be obtained.

Furthermore, when the average particle diameter and shape of the noble metal fine particles are equivalent to those of the semiconductor fine particles, a dispersion liquid containing these fine particles is prepared at the time of manufacturing the gas sensing element, and noble metal fine particle-containing semiconductor fine particle deposits are produced from this dispersion liquid. Is preferable because the semiconductor fine particles and the noble metal fine particles are easily dispersed uniformly and a good gas sensing element can be obtained.
Even when adjusting the volume ratio between the semiconductor fine particles and the noble metal fine particles to control the characteristics of the gas detector, if the average particle diameter and shape of the semiconductor fine particles and the noble metal fine particles are the same, the approximation to the theoretical value is possible. It is preferable because it can be easily controlled.

  On the other hand, when the average particle diameter of the noble metal fine particles is less than 0.5 times the average particle diameter of the semiconductor fine particles or exceeds twice the average particle diameter, the semiconductor fine particles and the noble metal fine particles are difficult to disperse uniformly. As a result, the actually measured value of the characteristic is greatly deviated from the theoretical value, which makes it difficult to control the characteristic.

As a method for producing the noble metal fine particles, a method in which a reducing agent is added to an aqueous solution containing a salt of the noble metal to reduce and precipitate noble metal ions in the aqueous solution is suitable.
Considering that the noble metal fine particles are also dispersed in an aqueous dispersion, the surface is preferably hydrophilic. However, if the noble metal fine particles are formed in an aqueous solution, the surface has high hydrophilicity and fine noble metal. This is because the fine particles can be easily obtained.

(Dispersion)
This dispersion liquid is a dispersion liquid containing semiconductor fine particles and noble metal fine particles, and there is no particular limitation as long as the semiconductor fine particles and noble metal fine particles are uniformly dispersed.
However, as the components contained in the dispersion, it is preferable that the solid components excluding the semiconductor fine particles and the noble metal fine particles are as small as possible, and the total ratio of the semiconductor fine particles and the noble metal fine particles in the total solid components in the dispersion is 95 volumes. % Or more.

The reason is that in the “noble metal fine particle-containing semiconductor fine particle deposit” obtained from this dispersion, it is preferable that the solid component excluding the semiconductor fine particles and the noble metal fine particles is as small as possible.
In the gas detection unit of the present embodiment, in the detection of the first gas to be measured, the bonding (contact) region between the semiconductor fine particles, and in the detection of the second gas to be measured, the bonding (contact) between the semiconductor fine particles and the noble metal fine particles. ) Area determines the sensitivity of the gas sensor. Here, when solid components other than the semiconductor fine particles and the noble metal fine particles form a film on the surface of the semiconductor fine particles and the noble metal fine particles, the contact between the semiconductor fine particles and between the semiconductor fine particles and the noble metal fine particles is inhibited, and the sensitivity of the gas detection unit is lowered. Since there is a possibility of causing it, it is not preferable. In particular, if the amount of solid components other than the semiconductor fine particles and the noble metal fine particles exceeds 5% by volume, there is a risk of a decrease in sensitivity and the influence cannot be ignored.

Examples of the solid component excluding the semiconductor fine particles and the noble metal fine particles include a binder component and a surfactant. These are added for the purpose of facilitating the production of the coating film, but as described above, the proportion in the total solid component in the dispersion must be less than 5% by volume.
For example, when a hydrolyzate of triethoxysilane that is an inorganic binder component is added, silica that is an insulator is generated from the hydrolyzate. Here, if the ratio (silica conversion) of the hydrolyzate of triethoxysilane in the total solid component is 5% by volume or more, the resistance value of the gas detector may increase or the sensitivity may decrease. Is not preferable.

As such a dispersion containing semiconductor fine particles and noble metal fine particles, an aqueous dispersion is preferable.
Inorganic oxide fine particles and noble metal fine particles, which are semiconductor fine particles, are inherently hydrophilic in surface and have the property of being stably dispersed in an aqueous dispersion medium. Therefore, if an aqueous dispersion medium is used, a good dispersion liquid in which the semiconductor fine particles and the noble metal fine particles are uniformly dispersed without using a dispersant or a surface treatment agent and no defects such as aggregates occur can be obtained. Because. In addition, since noble metal fine particle-containing semiconductor fine particle deposits with good uniformity can be obtained from a good dispersion, a gas sensor with improved characteristics can be obtained.

That is, it is preferable that the surface of the semiconductor fine particles and the surface of the noble metal fine particles have hydrophilicity. From the viewpoint of obtaining a stable dispersion while maintaining the hydrophilicity of these fine particles, an aqueous system using an aqueous dispersion medium is used. A dispersion is preferred.
In particular, as described above, if the metal oxide fine particles produced by hydrothermal synthesis method or noble metal fine particles obtained by reducing metal ions, the hydrophilicity of the surface is further improved. The compatibility with the medium is better and more suitable.

  On the other hand, when using an organic dispersion medium, particularly a nonpolar dispersion medium, instead of an aqueous dispersion medium, the surface of the semiconductor fine particles or the surface of the noble metal fine particles should have an affinity for the nonpolar dispersion medium. In order to achieve this, it is necessary to surface-treat these surfaces with a surface treatment agent or the like to form a hydrophobic film on these surfaces. However, since the film formed on the surface of the semiconductor fine particles or the surface of the noble metal fine particles by the surface treatment is insulative, and further, the heat treatment step in the manufacturing method of the present embodiment does not necessarily require a high temperature. Etc. are likely to remain. If this film remains, there is a high possibility of deteriorating the characteristics of the gas sensor, which is not preferable.

Examples of a method for dispersing these semiconductor fine particles and noble metal fine particles in an aqueous dispersion medium include dispersion by a ball mill, a stirring mill, a jet mill, a vibration mill, an attritor, a high-speed mill, a hammer mill, and the like.
As described above, the semiconductor fine particles obtained by the hydrothermal synthesis method and the noble metal fine particles obtained by reducing the metal ions are in a state of being present in the aqueous reaction solution at the time of production. Therefore, if this reaction solution is replaced with water to obtain a dispersion, a dispersion can be obtained without drying the fine particles, and aggregation of the fine particles due to drying can be prevented and the dispersion step can be omitted. Since it is possible, it is more preferable.
As described above, a dispersion liquid containing semiconductor fine particles and noble metal fine particles in which semiconductor fine particles and noble metal fine particles are dispersed in water can be obtained.

(Generation of semiconductor fine particle deposits containing noble metal fine particles)
By removing the dispersion medium from the dispersion containing the semiconductor fine particles and the noble metal fine particles, a noble metal fine particle-containing semiconductor fine particle deposit is generated.
More specifically, a dispersion containing the above-mentioned semiconductor fine particles and noble metal fine particles is applied onto a substrate to form a coating film, and this coating film is dried to produce a noble metal fine particle-containing semiconductor fine particle deposit. .

The substrate is not particularly limited as long as it has heat resistance and electrical insulation higher than the temperature used in the heat treatment step described below, but a ceramic substrate made of silica, alumina, etc., a glass made of quartz glass, etc. A substrate is preferably used.
As a coating method, for example, various printing methods such as a screen printing method are preferable. In addition, with the recent progress in miniaturization and thinning of gas detectors, the amount of dispersion (paint) used is also decreasing, so a small amount of dispersion (paint) is applied with good position controllability. It is preferable to use a device that can be used, for example, a microdispenser, an ink jet device, or the like.

The coating film thus obtained is dried to produce a noble metal fine particle-containing semiconductor fine particle deposit.
The drying method is not particularly limited, and a normal heat dryer or the like may be used.
The heating temperature for heat drying is preferably 20 ° C. or more and 300 ° C. or less, more preferably 30 ° C. or more and 150 ° C. or less.
Note that high temperature drying exceeding 300 ° C. or rapid heat drying is not preferable because the dispersion medium may be rapidly evaporated to cause expansion or deformation of the coating film.
In particular, when an aqueous dispersion medium is used, it is preferable to dry or heat dry at 100 ° C. or lower. Moreover, in order to accelerate | stimulate drying, you may perform vacuum drying or vacuum heat drying.
As described above, a noble metal fine particle-containing semiconductor fine particle deposit can be generated.

(Formation of gas detector)
-Heat treatment of noble metal fine particle-containing semiconductor fine particle deposit The noble metal fine particle-containing semiconductor fine particle deposit obtained as described above is heat-treated, and each fine particle composed of semiconductor fine particles and noble metal fine particles is bonded (weakly sintered) to form fine particles. By fixing the gap, a gas detection unit including a semiconductor fine particle and a noble metal fine particle and having a stable structure is obtained.
The heat treatment temperature and time may be any temperature and time at which the semiconductor fine particles and the noble metal fine particles are bonded (weakly sintered). However, as the sintering progresses, the contact surface (sintered surface) between the fine particles composed of the semiconductor fine particles and the noble metal fine particles expands, so that although the strength of the gas detector itself is improved, the sensitivity is lowered, which is not preferable. . Therefore, the heat treatment temperature and time are preferably as low as possible and as short as possible within a range where the obtained gas sensor is not damaged or deteriorated due to physical strength, for example.

In addition, the heat treatment temperature needs to be equal to or higher than the operating temperature of the gas sensor to which the gas sensing element obtained thereby is applied. When the heating temperature is lower than the operating temperature of the gas sensor, the state of the insulating substance changes during the operation of the gas sensor, and as a result, the characteristics of the gas detector change and in some cases deteriorate, which is not preferable.
Considering these points, it is preferable that the heat treatment temperature is about 400 ° C. to 600 ° C., and the heat treatment time is about 30 minutes to 2 hours.

Note that the heating atmosphere is good in the air because the semiconductor fine particles are oxides and the noble metal fine particles are not easily oxidized.
In addition, this heat treatment step may be performed continuously with the previous step of heat drying. For example, after the noble metal fine particle-containing semiconductor fine particle deposit is dried at 100 ° C., the temperature is raised to 400 ° C. May be performed.

As described above, it is possible to obtain the gas detection unit of the present embodiment in which the fine particles composed of the semiconductor fine particles and the noble metal fine particles are bonded or fixed to each other to have a stable structure.
A highly sensitive and durable gas sensor can be obtained by attaching measurement electrodes and heaters to the gas detector thus obtained and performing necessary packaging and casing.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited by these Examples.

[Examples 1 and 2 and Comparative Example 1]
The gas sensor substrate A used in Examples 1 and 2 and Comparative Example 1 was prepared as follows.
A comb-shaped electrode made of gold (Au) having a height of 0.6 mm, a width of 0.5 mm, and an electrode width of 0.15 mm is spaced 0.1 mm on the surface of an alumina substrate having a length of 1.5 mm, a width of 1.5 mm, and a thickness of 0.2 mm. The detection electrodes opposed to each other were formed by screen printing. Next, a heater for heating the gas sensor was formed on the back surface of the alumina substrate on which this electrode was formed, and a gas sensor substrate A was obtained.

The resistance values of the gas detectors of Examples 1 and 2 and Comparative Example 1 were measured using the measurement circuit shown in FIG.
In this measurement circuit 11, the gas sensor 12 and the fixed resistor 13 are connected in series, and a voltage of 1 V is applied between the input side terminal 14 of the gas sensor 12 and the output side terminal 16 of the fixed resistor 13. When the voltage between the input terminal 15 and the output terminal 16 of the fixed resistor 13 is measured with a digital multimeter, the applied voltage (1 V), the voltage across the fixed resistor 13, and the fixed resistor 13 From the resistance value, the resistance value of the gas sensor 12 was calculated.

Moreover, the sensitivity of the gas detectors of Examples 1 and 2 and Comparative Example 1 was measured using the measuring apparatus shown in FIG.
The measuring device 21 is formed in a quartz glass rod 22 having an inner diameter of 30 mm for housing and holding the gas sensing element 12, a measurement circuit (not shown) for measuring the resistance value of the gas sensing element 12, and the gas sensing element 12. And a heating circuit (not shown) for heating the gas sensor 12 to the operating temperature by energizing the heater, and the gas sensor 12 when the standard gas 23 and the sample gas 24 are introduced into the glass tube 22, respectively. It is an apparatus for measuring the resistance value of.

"Example 1"
670 parts by mass of stannic chloride (SnCl ₄ .5H ₂ O) was dissolved in 3000 parts by mass of 6 mol / L hydrochloric acid, and 200 parts by mass of a 25% aqueous ammonia solution was added to the obtained aqueous solution while mixing. The obtained precipitate was filtered, washed with deionized water, deionized water was added to adjust the concentration to 1% by mass, and a hydrothermal synthesis raw material was obtained.
This raw material for hydrothermal synthesis was put in an autoclave and heated at 350 ° C. for 5 hours, and the resulting reaction liquid containing the hydrothermal synthesis product was concentrated by an evaporator to obtain an aqueous solution containing tin oxide fine particles.

As a result of measuring the particle diameter of the tin oxide fine particles obtained by hydrothermal synthesis with a transmission electron microscope, the average primary particle diameter was about 10 nm.
The obtained tin oxide fine particles were washed with deionized water, and then dispersed in deionized water to a concentration of 0.2% by mass to obtain a tin oxide fine particle dispersion A.

On the other hand, an aqueous solution containing 0.15 mmol / L chloroauric acid and adjusted to pH 5.7 and an aqueous solution containing 0.15 mmol / L sodium borohydride were mixed in an equivalent amount while maintaining at 20 ° C. Then, the obtained colloidal dispersion was concentrated and purified by an ultrafiltration method to obtain an aqueous solution containing gold fine particles having a dispersion concentration of 30% by mass.
As a result of measuring the particle diameter of the obtained gold fine particles with a transmission electron microscope, the average primary particle diameter was about 10 nm.
The obtained gold fine particles were washed with deionized water, and then dispersed in deionized water to a concentration of 0.2% by mass to obtain a gold fine particle dispersion A.

Next, 99 mL of the tin oxide fine particle dispersion A and 1 mL of the gold fine particle dispersion A were sampled and mixed to obtain a mixed solution A.
Next, 5 μL of this mixed liquid A was sampled and dropped on the comb-shaped electrode of the gas sensing element substrate A, and then heated at 60 ° C. for 3 hours to deposit gold oxide-containing tin oxide fine particles on the gas sensing element substrate A. Form A formed.
Next, the gas detector substrate A on which the gold fine particle-containing tin oxide fine particle deposit A was formed was heated at 400 ° C. for 1 hour in an air atmosphere to obtain the gas detector A of Example 1.

Next, the resistance value of the gas detector A was measured using the measurement circuit 11 shown in FIG.
First, the gas sensor A and the fixed resistor 13 are connected in series, and fixed when a voltage of 1 V is applied between the input side terminal 14 of the gas sensor A and the output side terminal 16 of the fixed resistor 13. The voltage between the input side terminal 15 and the output side terminal 16 of the resistor 13 is measured with a digital multimeter. From the applied voltage (1 V), the voltage across the fixed resistor 13 and the resistance value of the fixed resistor 13, The resistance value of the gas detector A was calculated.
As a result of the measurement, the resistance value of the gas sensing element A was 267 kΩ.

Next, the sensitivity of the gas detector A was measured using a measuring device 21 shown in FIG.
First, the gas sensing element A is fixed in the glass tube 22 and necessary circuits are connected, and then the heater formed on the gas sensing element A is energized to raise the gas sensing element A to 300 ° C. which is the operating temperature. Held. In this state, the standard gas 23 was introduced into the glass tube 22 at 10 mL / second, and the resistance value Ra of the gas detector A was measured. As the standard gas 23, a mixed gas containing 20% by volume of oxygen and 80% by volume of nitrogen and whose relative humidity at a temperature of 25 ° C. was controlled to 40% was used.

Next, the sample gas 24 is introduced into the glass tube 22 at 10 mL / second, and the resistance value Rg of the gas sensing element A after 250 seconds when the resistance value is stabilized is measured. Then, the resistance value Ra and the resistance value are measured. The ratio Ra / Rg of the value Rg was determined, and this ratio Ra / Rg was used as the sensitivity of the gas detector A. As the sample gas 24, the standard gas 23 mixed with 3 ppm each of hydrogen sulfide (H ₂ S), hydrogen (H ₂ ), and toluene (C ₆ H ₅ —CH ₃ ) was used.

Finally, the standard gas 23 was introduced again, and the resistance value of the gas detector A recovered and it was confirmed that there was no abnormality in the measurement.
As a result of the measurement, the sensitivity of the gas detector A was 33 for hydrogen sulfide, 37 for toluene, and 4.3 for hydrogen.

"Example 2"
7 mL of the tin oxide fine particle dispersion A prepared in the same manner as in Example 1 and 3 mL of the gold fine particle dispersion A were collected and mixed to obtain a mixed solution B.
Next, using this mixed solution B, a gas sensor B of Example 2 was obtained in the same manner as Example 1.

Next, the resistance value and sensitivity of the gas sensor B of Example 2 were measured by the same method as in Example 1.
As a result of the measurement, the resistance value of the gas detector B was 1000 kΩ.
The sensitivity of the gas detector B was 100 for hydrogen sulfide, 6.7 for toluene, and 16.7 for hydrogen.

“Comparative Example 1”
Using only the tin oxide fine particle dispersion A produced in the same manner as in Example 1, a gas sensor C of Comparative Example 1 was obtained in the same manner as in Example 1. This gas sensor C does not contain gold fine particles.

Next, the resistance value and sensitivity of the gas detector C of Comparative Example 1 were measured by the same method as in Example 1.
As a result of the measurement, the resistance value of the gas detector C was 90 kΩ.
The sensitivity of the gas detector C was 9.1 for hydrogen sulfide, 4.2 for toluene, and 6.3 for hydrogen.
The results of Examples 1 and 2 and Comparative Example 1 are shown in Table 1.

Comparing Examples 1 and 2 with Comparative Example 1, Example 1 contains 1% by volume of gold fine particles. Therefore, compared to Comparative Example 1 in which the gas detection part is formed only of tin oxide, toluene is used. It can be seen that the sensitivity of is improved. This is considered to be due to the catalytic effect of the gold fine particles. On the other hand, hydrogen sulfide and hydrogen do not show much sensitivity improvement.
In Example 2, since 30% by volume of gold fine particles are contained, hydrogen sulfide and hydrogen are not improved in comparison with Comparative Example 1 in which the gas detection part is formed of only tin oxide, but the sensitivity of toluene is not recognized. It can be seen that the sensitivity to is improved.
As described above, according to Examples 1 and 2, it was found that the gas to be measured can be selected by changing the ratio (ratio) of the gold fine particles in the fine particles.

  Further, when the resistance values of Examples 1 and 2 and Comparative Example 1 are compared, Comparative Example 1 which does not contain gold fine particles is the lowest, followed by Example 1 and Example 2 in the order of increasing the amount of gold fine particles. The resistance value increases with the increase. This confirms that the Schottky bonding effect and the like are manifested on the bonding (contact) surface between the tin oxide fine particles and the gold fine particles in the gas detection unit.

[Examples 3 to 9 and Comparative Examples 2 and 3]
The gas sensor substrate B used in Examples 3 to 9 and Comparative Examples 2 and 3 was produced as follows.
A detection electrode is deposited on the surface of a quartz substrate having a length of 10 mm, a width of 10 mm, and a thickness of 0.2 mm, with comb-shaped electrodes made of gold (Au) having a length of 6 mm, a width of 4 mm, and an electrode width of 1 mm facing each other at an interval of 0.5 mm. A gas sensing substrate B was formed by the method.
In the gas sensor substrate B, since the gas sensor is heated by an external heater, the heater is not formed.

The sensitivities of the gas detectors of Examples 3 to 9 and Comparative Examples 2 and 3 were measured using the measuring apparatus shown in FIG.
This measuring device 31 includes an annular electric furnace 34 including a quartz glass rod 32 having an inner diameter of 30 mm for housing and holding the gas sensor 12, a heater unit 33 for heating the gas sensor 12 to an operating temperature, and a gas sensor. 12 is a device for measuring the resistance value of the gas sensing element 12 when the standard gas 23 and the sample gas 24 are respectively introduced into the annular electric furnace 34. is there. The resistance measuring circuit of the gas detector is the same as that of the first embodiment.

"Example 3"
99.5 mL of the tin oxide fine particle dispersion A produced in the same manner as in Example 1 and 0.5 mL of the gold fine particle dispersion A were sampled and mixed to obtain a mixture D.
Next, 30 μL of this mixed solution D was sampled and dropped on the comb-shaped electrode of the gas sensing substrate B, and then heated at 60 ° C. for 3 hours to deposit gold fine particle-containing tin oxide fine particles on the gas sensing substrate B. Product D was formed.
Next, the gas detector substrate B on which the gold fine particle-containing tin oxide fine particle deposit D was formed was heated at 400 ° C. for 1 hour in an air atmosphere to obtain the gas sensor D of Example 3.

Next, the sensitivity of the gas detector D was measured using a measuring device 31 shown in FIG.
First, the gas detector D is fixed in the glass chamber 32 of the annular electric furnace 34, and necessary circuits are connected. Then, the heater 33 is energized to raise the gas detector D to 300 ° C. which is the operating temperature. Held. In this state, the standard gas 23 was introduced into the glass tube 32 at 10 mL / second, and the resistance value Ra of the gas detector B was measured. As the standard gas 23, the same gas as in Example 1 was used.

Next, the sample gas 24 is introduced into the glass tube 32 at 10 mL / second, and the resistance value Rg of the gas sensing element D after 250 seconds when the resistance value is stabilized is measured. Then, the resistance value Ra and the resistance value are measured. The ratio Ra / Rg of the value Rg was determined, and this ratio Ra / Rg was used as the sensitivity of the gas detector D. As the sample gas 24, the standard gas 23 mixed with ₃ ppm of hydrogen sulfide (H ₂ S) and 50 ppb of toluene (C ₆ H ₅ —CH ₃ ) was used.

Finally, the standard gas 23 was introduced again, and the resistance value of the gas detector D was recovered, and it was confirmed that there was no abnormality in the measurement.
As a result of the measurement, the sensitivity of the gas detector D was 420 for hydrogen sulfide and 10.1 for toluene.

Example 4
99 mL of the tin oxide fine particle dispersion A prepared in the same manner as in Example 1 and 1 mL of the gold fine particle dispersion A were sampled and mixed to obtain a mixed solution E.
Next, using this mixed solution E, a gas sensor E of Example 4 was obtained in the same manner as Example 3.

Next, the sensitivity of the gas detector E of Example 4 was measured by the same method as in Example 3. As a result, the sensitivity of the gas detector E was 534 for hydrogen sulfide and 16.6 for toluene.
Further, the sensitivity of the gas sensor E was measured by the same method as in Example 3 except that the operating temperature was 350 ° C. As a result, the sensitivity of the gas sensor E was 122 for hydrogen sulfide and 6 for toluene. .4.

"Example 5"
98.5 mL of the tin oxide fine particle dispersion A prepared in the same manner as in Example 1 and 1.5 mL of the gold fine particle dispersion A were sampled and mixed to obtain a mixed solution F.
Next, using this mixed solution F, a gas sensor F of Example 5 was obtained in the same manner as Example 3.
Next, the sensitivity of the gas sensor F of Example 5 was measured by the same method as in Example 3. As a result, the sensitivity of the gas sensor F was 560 for hydrogen sulfide and 13.2 for toluene.

"Example 6"
96 mL of the tin oxide fine particle dispersion A prepared in the same manner as in Example 1 and 4 mL of the gold fine particle dispersion A were sampled and mixed to obtain a mixed solution G.
Next, using this mixed solution G, a gas sensor G of Example 6 was obtained in the same manner as Example 3.
Next, as a result of measuring the sensitivity of the gas sensor G of Example 6 by the same method as that of Example 3, the sensitivity of the gas sensor G was 820 for hydrogen sulfide and 10.8 for toluene.

"Example 7"
9 mL of the tin oxide fine particle dispersion A and 1 mL of the gold fine particle dispersion A produced in the same manner as in Example 1 were collected and mixed to obtain a mixed solution H.
Next, using this mixed solution H, a gas sensor H of Example 7 was obtained in the same manner as Example 3.

Next, the sensitivity of the gas detector H of Example 7 was measured by the same method as in Example 3. As a result, the sensitivity of the gas detector H was 1815 for hydrogen sulfide and 2.9 for toluene.
Further, as a result of measuring the sensitivity of the gas sensing element H by the same method as in Example 3 except that the operating temperature was 350 ° C., the sensitivity of the gas sensing element H was 101 for hydrogen sulfide and 1 for toluene. .3.

"Example 8"
7 mL of the tin oxide fine particle dispersion A prepared in the same manner as in Example 1 and 3 mL of the gold fine particle dispersion A were sampled and mixed to obtain a mixed solution I.
Next, using this liquid mixture I, a gas sensor I of Example 8 was obtained in the same manner as Example 3.

Next, the sensitivity of the gas detector I of Example 8 was measured by the same method as in Example 3. As a result, the sensitivity of the gas sensor I was 10716 for hydrogen sulfide and 1.8 for toluene.
Further, as a result of measuring the sensitivity of the gas sensor I by the same method as in Example 3 except that the operating temperature was 350 ° C., the sensitivity of the gas sensor I was 732 for hydrogen sulfide and 1 for toluene. .2.

"Example 9"
6 mL of the tin oxide fine particle dispersion A and 4 mL of the gold fine particle dispersion A prepared in the same manner as in Example 1 were collected and mixed to obtain a mixed solution J.
Next, using this mixed solution J, a gas sensor J of Example 9 was obtained in the same manner as Example 3.

Next, the sensitivity of the gas detector J of Example 9 was measured by the same method as in Example 3. As a result, the sensitivity of this gas sensor J was 2506 for hydrogen sulfide and 1.8 for toluene.
Further, as a result of measuring the sensitivity of the gas sensor J by the same method as in Example 3 except that the operating temperature was 350 ° C., the sensitivity of the gas sensor J was 145 for hydrogen sulfide and 1 for toluene. .1.

  Further, the resistance value of the gas sensing element J is only 366Ω, which is significantly lower than the resistance values of the other embodiments in the range of kΩ order to MΩ order. This indicates that the amount of noble metal fine particles is close to the critical concentration, and it is considered that the sensitivity of hydrogen sulfide has decreased due to the large amount of gold fine particles.

"Comparative Example 2"
Using only the tin oxide fine particle dispersion A produced in the same manner as in Example 1, a gas sensor K of Comparative Example 2 was obtained in the same manner as in Example 3. The gas sensor K does not contain gold fine particles.
Next, the sensitivity of the gas detector K of Comparative Example 2 was measured by the same method as in Example 3. As a result, the sensitivity of the gas sensor K was 85 for hydrogen sulfide and 1.9 for toluene.

“Comparative Example 3”
5 mL of the tin oxide fine particle dispersion A and 5 mL of the gold fine particle dispersion A produced in the same manner as in Example 1 were collected and mixed to obtain a mixed solution L.
Next, using this liquid mixture L, a gas sensor L of Comparative Example 3 was obtained in the same manner as in Example 3.

Next, the sensitivity of the gas detector L of Comparative Example 3 was measured by the same method as in Example 3. As a result, the sensitivity of the gas sensor L was 25 for hydrogen sulfide and 0.9 for toluene.
The resistance value of the gas sensing element L was only 10Ω or less, and exhibited a characteristic as a metal. This indicates that the amount of noble metal fine particles exceeds the critical concentration, which is inappropriate as a gas detector.
The measurement results of the gas sensitivity at Examples 3 to 9 and Comparative Examples 2 and 3 and the gas sensitivity at an operating temperature of 300 ° C are shown in Table 2, and the measurement results of the gas sensitivity at Examples 4 and 7 to 9 at an operating temperature of 350 ° C are shown in Table 2. 3, respectively.

  Comparing Examples 3 to 9 and Comparative Examples 2 and 3, in Examples 3 to 6, since gold fine particles are contained in an amount of 0.5 to 4% by volume, the gas detection part is formed only of tin oxide. It turns out that the sensitivity of toluene is improving with respect to the comparative example 2 which is. This is considered to be due to the catalytic effect of the gold fine particles. On the other hand, no significant improvement in sensitivity is shown for hydrogen sulfide.

In Examples 7 to 9, since gold fine particles are contained in an amount of 10 to 40% by volume, the sensitivity to toluene is not improved, but the sensitivity to hydrogen sulfide is improved. .
That is, the sensitivity is improved by dispersing noble metal fine particles having a particle size substantially the same as the particle size of the semiconductor fine particles in the semiconductor fine particles, and the ratio between the semiconductor fine particles and the noble metal fine particles is adjusted. Thus, it can be seen that it has gas selectivity.

DESCRIPTION OF SYMBOLS 1 Gas sensor 2 Semiconductor fine particle 3 Precious metal fine particle 4 Three-dimensional composite structure 5 Pore 11 Measurement circuit 12 Gas sensor 13 Fixed resistor 14 Input side terminal 15 Input side terminal 16 Output side terminal 21 Measuring apparatus 22 Quartz glass cage 23 Standard gas 24 Sample gas 31 Measuring device 32 Quartz glass bowl 33 Heater part 34 Annular electric furnace

Claims (4)

In the gas detector that measures the concentration of the gas to be detected using the change in the electrical resistance of the gas detector,
The gas detection unit includes semiconductor fine particles and noble metal fine particles,
The average particle diameter of the noble metal fine particles is 0.5 to 2 times the average particle diameter of the semiconductor fine particles,
The gas sensor according to claim 1, wherein a volume ratio of the noble metal fine particles to a total amount of the noble metal fine particles and the semiconductor fine particles is 0.5% by volume or more and 40% by volume or less. Any one of the semiconductor fine particles themselves, a bonding surface or contact surface between the semiconductor fine particles, a bonding surface or contact surface between the semiconductor fine particles themselves and the semiconductor fine particles has a gas detection function,
2. The gas sensor according to claim 1, wherein a bonding surface or a contact surface between the noble metal fine particles and the semiconductor fine particles has a metal-semiconductor bonding effect.   3. The gas sensor according to claim 1, wherein gas selectivity is provided to the gas detection unit by changing a volume ratio between the semiconductor fine particles and the noble metal fine particles.   A gas sensor comprising the gas sensor according to any one of claims 1 to 3.

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