JP2000321232A - Composite membrane for gas sensor, gas sensor, and method for manufacturing composite membrane for gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the concentration of carbon dioxide continuously accurately in the vicinity of room temperature. SOLUTION: A composite membrane for a gas sensor includes a conductive polymer 1 such as base-type polyaniline(PAn) and an insulating polymer 2 such as polyvinylalcohol(PVA) as material. Base-type PAn is obtained by subjecting a powder of polyanthranilic acid(PANA) to heat treatment at a predetermined temperature for a predetermined time. The base-type PAn is dissolved in dimethyl sulfoxide(DMSO) and then mixed with a PVA/DMSO solution to create a composite solution. Then the composite solution is cast into a micro platinum comb electrode formed on a glass substrate, and a solvent is removed by vacuum drying to obtain a gas sensor. In this gas sensor, HCO3 is created by the reaction between CO2 gas and water held by PVA, and the created HCO3 is captured in the base-type PAn as a dopant to change the electric conductivity of a composite membrane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a composite film for a gas sensor for detecting the concentration of CO ₂ gas, a gas sensor, and a method for producing the composite film for a gas sensor.

[0002]

2. Description of the Related Art Recently, there is an increasing need to continuously measure carbon dioxide gas in many fields such as environmental protection of airtight spaces, plant cultivation in greenhouses, and monitoring of atmospheric concentration related to environmental problems. ing. To measure the concentration of carbon dioxide, C
An O ₂ sensor is used. The CO ₂ sensor measures the concentration of carbon dioxide by changing the electrical conductivity of a material capable of adsorbing carbon dioxide in the atmosphere.

[0003]

However, most of the conventional CO ₂ sensors require measurement conditions of 400 ° C. or higher, and are required to accurately (particularly, continuously) measure the carbon dioxide concentration near room temperature. Was difficult. The present invention
In view of the above, it is an object of the present invention to provide a composite film for a gas sensor, a gas sensor, and a method for manufacturing a composite film for a gas sensor, which can accurately measure a carbon dioxide concentration particularly at around room temperature with a wide dynamic range and continuously. With the goal.

[0004]

To solve the problems described above SUMMARY OF THE INVENTION According to the first aspect of the present invention, bicarbonate ion (HCO ₃ ^-) and conductive polymer can take in as a dopant, hygroscopic in and a water can be retained for insulating polymer, by reaction with water molecules that are retained by the insulating polymer and carbon dioxide (CO ₂₎ HCO ₃ ^- is produced, by HCO ₃ wherein the conductive polymer ^- the Provided is a composite film for a gas sensor in which the electric conductivity changes according to the amount of intake. In the present invention, H generated by dissociation equilibrium between water and CO ₂ gas held by the insulating polymer is used.
CO ₃ ^- incorporated into the conductive polymer. HCO ₃ ^- to change the electrical conductivity of the composite membrane according to uptake, by measuring the value of electric conductivity, can measure the concentration of CO ₂ gas.

[0005] In the present invention, a material that changes conductivity when HCO ₃ ^- is doped, for example, a basic polyaniline or a derivative thereof can be used as the conductive polymer. The basic polyaniline itself is insulative, but changes into a conductor when HCO ₃ ^- is doped. In the present invention, polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylic acid, which have excellent hygroscopicity, can be used as the insulating polymer. In addition,
For example, the weight ratio of the conductive polymer and the insulating polymer,
When the ratio is approximately 1: 6, the composite film can be made conductive.

According to a second aspect of the present invention, there is provided a gas sensor comprising such a composite film for a gas sensor and a pair of electrodes formed on the composite film for a gas sensor.

Further, according to a third solution of the present invention, a first step of removing a carboxylic acid (—COOH) from polyanthranilic acid by heat treatment at a predetermined temperature and time to form a conductive polymer; A second step of dissolving the conductive polymer in a predetermined solvent, and then mixing the resultant with a solution containing an insulating polymer to form a composite solution; and, after casting the composite solution on a substrate, removing the solvent by a drying treatment. A method of manufacturing a composite film for a gas sensor, the method comprising:

[0008] In the first step, the carboxylic acid (-COOH) is removed by heat-treating the polyanthranilic acid powder at a predetermined temperature and time to produce a basic polyaniline or a derivative thereof. . Further, in the third step, before or after the composite solution is cast on a substrate, an electrode may be formed on the substrate.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A composite membrane for a gas sensor, a gas sensor, and a method for manufacturing a composite membrane for a gas sensor according to the present invention will be specifically described below with reference to the drawings.

FIG. 1 is a diagram schematically showing a composite film for a gas sensor according to the present invention. 1 includes a conductive polymer 1 and an insulating polymer 2 as materials. Such a composite film has such characteristics that the electrical conductivity of the composite film changes depending on the concentration of CO ₂ gas. Therefore, a gas sensor capable of measuring the concentration of CO ₂ gas in the atmosphere can be obtained by forming an electrode on the composite film of FIG. 1 and measuring the electric conductivity.

As the conductive polymer 1, for example, basic polyaniline (PAn) or a derivative thereof can be used. As the material of the conductive polymer 1, an appropriate material such as an ionic polymer such as polyaniline, a heterocyclic polymer such as polypyrrole, or a polyphenylene-based polymer such as polyparaphenylene can be used. Specific examples of the material name include polypyrrole, polyparaphenylene, polythiophene, and polyperinaphthalene. In order to form a composite film, it is necessary to dissolve the conductive polymer 1 in a solvent, and therefore, the conductive polymer 1 preferably has excellent solubility.

Examples of the insulating polymer 2 include polyvinyl alcohol (PVA) represented by the following formula (I) and the following formula (I)
Polyacrylic acid (PAA) shown in I), polyvinylpyrrolidone (PVP) shown in the following formula (III), and the like can be used. The insulating polymer 2 is not limited to these, and an insulating, hygroscopic, and soluble polymer can be used as appropriate.

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In the following embodiments, the case where basic PAn and PVA are used as an example of the material of the composite film will be described. However, other materials of the conductive polymer 1 and the insulating polymer 2 may be appropriately used. .

FIG. 2 is an explanatory view of a method of manufacturing a gas sensor and a composite film for a gas sensor. Hereinafter, the manufacturing method of the gas sensor composite film will be described with reference to this drawing. The amount, temperature, time, and the like of the material can be set as appropriate.

[0015] Polyanthranilic acid (PANA) is known as one of self-doped polyanilines, and is prepared by a chemical polymerization method, similarly to polyaniline (PAN). Specifically, in a sulfuric acid aqueous solution (200 mL) containing 50 mM of anthranilic acid, a monomer represented by the following formula (IV),
0.2 M ammonium persulfate [(NH ₄ ) ₂ S
₂ O ₈ ] (100 mL) was added, and the mixture was stirred at room temperature for 48 hours.

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Further, after filtration and washing, vacuum drying was performed to obtain a PANA powder represented by the following formula (V) (Step S):
1).

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Next, the PANA powder is heat-treated in a helium atmosphere at a predetermined temperature and a predetermined time (for example, 250 ° C. for 2 hours, 280 ° C. for 2 hours, or 280 ° C. for 8 hours) to decarboxylate (-4CO ₂ ). (Step S2). By the above treatment, the basic polyaniline (P) represented by the following formula (VI)
An) was obtained.

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When PAn is obtained in step S2, the PAn is then dissolved in dimethyl sulfoxide (DMSO) and mixed with a solution of polyvinyl alcohol (PVA) of formula (I) in DMSO. To generate a composite solution (step S3). The weight ratio between PAn and PVA in the composite solution is preferably about 1: 6. When the weight ratio is set to this extent, the composite film formed from the composite solution shifts from insulating to conductive.

Next, as shown in FIG.
After casting on a fine platinum comb-shaped electrode 12 formed on a substrate 11 such as glass, the solvent is removed by vacuum drying to produce the gas sensor composite film and the gas sensor 10 (step S4). The substrate 11 and the electrode 12 can be made of appropriate materials and shapes. In addition, the electrode 12
It may be formed after casting the composite solution. By using a measuring device 14 such as a galvanostat (constant current device) to measure the electrical conductivity, current or voltage of the produced gas sensor 10 by a DC two-terminal method or the like, CO 2
_2. Gas concentration can be measured.

Next, the principle of absorbing CO ₂ by the composite film for a gas sensor according to the present invention will be described. FIG. 3 is a diagram illustrating the absorption of CO ₂ by the composite membrane for a gas sensor. Since PVA (insulating polymer 2) in the composite film for a gas sensor has high hygroscopicity, considerable water (H ₂ O) molecules are contained in the composite film. This H ₂ O molecule and CO ₂
By dissociation equilibrium with the gas, hydrogen carbonate ions (HCO ₃ ⁻ ) are generated as in the following formula (VII).

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This HCO ₃ ^- is used as a base type PAn in the composite membrane.
(Conductive polymer 1) is doped. More specifically, as shown, HCO ₃ ⁻ is N ^{+ in} basic PAn.
Combines with ions. Such binding is more likely to occur as the number of empty sites (N atoms) in the composite membrane increases, and HC
The more bonds between O ₃ ⁻ and N ⁺ ions occur, the higher the response sensitivity to CO ₂ gas. As described above, when PANA is heat-treated at 280 ° C. for 8 hours, a base-type PAn having many empty sites is generated, so that CO ₂ can be adsorbed efficiently. The basic PAn is originally insulating, but changes to a conductive state by doping with HCO ₃ ⁻ . That is, when HCO ₃ ⁻ is doped into a composite film containing basic PAn, the electric conductivity changes. Therefore, by measuring the electrical conductivity of the composite membrane, CO ₂
Gas concentration can be measured accurately.

Next, the characteristics of the composite film for a gas sensor and the gas sensor according to the present invention will be described. In examining the characteristics, as an example, a gas sensor was installed in a measurement cell connected to a vacuum system and capable of adjusting the CO ₂ gas concentration and humidity, and measurement was performed.

FIG. 4 is a diagram showing the relationship between the heat treatment conditions of the PANA powder and the electrical conductivity. As shown, the higher the heat treatment temperature and the longer the heat treatment time, the lower the electric conductivity. This is probably because the higher the heat treatment temperature and the longer the heat treatment time, the more the decarboxylation from PANA is promoted.

FIG. 5 shows a Fourier transform infrared absorption spectrum (FTIR) of PANA.
a Red) absorption spectrum. This diagram is an FTIR absorption spectrum diagram when the PANA is heat-treated in step S2 in FIG. 2, where the horizontal axis indicates the wave number (cm ^-1 ) and the vertical axis indicates the transmittance (au). The waveform a in the figure is the spectrum of the unheated sample without heat treatment, the waveform b is the spectrum when heat treatment is performed at 250 ° C. for 2 hours, and the waveform c is the spectrum when heat treatment is performed at 280 ° C. for 8 hours. It is.

Wave numbers 1690 cm ^-1 and 1450 cm seen in waveform a
The absorption of ^-1 is caused by the stretching vibration of C = O and -CO of -COOH. Also, the absorption at wave numbers 1570 cm ^-1 and 1380 cm ^-1 is
It is attributed to the symmetric and asymmetric vibrations of the dissociated -COO-.
On the other hand, absorptions at wave numbers of 1600 cm ^-1 , 1500 cm ^-1 and 1250 cm ^-1 are also found in the general spectrum of chemically synthesized PAn, and quinone (Q) and benzene ( B) It is attributed to the ring stretching vibration and the CN stretching vibration of the unit. Further, the above-mentioned two carboxyl groups (-
COOH, -COO - none 4 absorption peaks of this is due ^to), as the heat treatment temperature becomes higher, and as the heat treatment time becomes longer, decreasing the decomposition desorption. For example, waveform c
In this case, the four absorption peaks described above almost disappear, and the spectrum becomes almost due to only the polymer main chain. That is, carboxylic acid (—COOH) is eliminated from PANA,
This indicates that a base type PAn was generated.

FIG. 6 is an explanatory view showing how the chemical formula changes during the heat treatment of PANA. As shown in the figure, when the heat treatment temperature is 250 ° C., the carboxyl group (—COOH) is eliminated from PANA, and the heat treatment temperature is reduced to 2 ° C.
When the temperature was raised to 80 ° C, the carboxyl group (-COO)
^-) is eliminated, when continued 8h heat treatment at that temperature for additional self-doping carboxyl group (-COO ^-) is eliminated, eventually base form PAn is obtained.

FIG. 7 shows that P at different temperatures and times.
FIG. 6 is a diagram in which a change in electrical conductivity with respect to a CO ₂ gas concentration is plotted for each of a plurality of composite films formed by heat-treating ANA. The triangle plot in the figure is 250 ° C
When heat treatment was performed for 2 hours at 280 ° C.
When the heat treatment is performed for 2 hours at 280 ° C.
3 shows the measurement results when the heat treatment was performed for 8 hours.
When the heat treatment was not performed, there was almost no change in the electrical conductivity with respect to the CO ₂ gas concentration.

As shown, the heat treatment temperature of PANA is high.
And the heat treatment time becomes longer, the CO
_TwoThe result showed that the response sensitivity to gas became higher. Ingredient
Physically, PANA and PVA heat-treated at 280 ° C for 8 hours
And the electrical conductivity of the composite film is 50 to 10^FourGas up to ppm
5.95 × 10 with respect to concentration^-FiveFrom 6.04 × 10 ^-3
Scm^-1Up to two orders of magnitude. CO_Two
Humidity dependence of electrical conductivity of composite membranes at zero gas concentration
Sex is hardly seen from 0 to 100% RH (5
× 10^-FiveScm^-1).

FIG. 8 is a diagram showing the humidity dependence of the electric conductivity of the gas sensor. The horizontal axis in the figure is the concentration of CO ₂ gas (ppm), the vertical axis is the electric conductivity (Scm ⁻¹ ), the open circle plot in the figure is the electric conductivity when the humidity is 30% RH, and the rhombic plot is The electric conductivity when the humidity is 50% RH, and the triangular plot shows the electric conductivity when the humidity is 70% RH. As shown in the drawing, it can be seen that the lower the humidity, the greater the change in electrical conductivity and the higher the sensitivity as a gas sensor.

FIG. 9 is a graph showing the relationship between the humidity and the electric conductivity of the gas sensor. The horizontal axis represents the humidity (%).
RH), and the vertical axis indicates electric conductivity (Scm ^-1 ). FIG.
(A) shows that ordinary water (slightly CO
When using dissolved _2), FIG. 9 (B), a comparison of the case of using the water completely degassed ordinary water and CO ₂ when adjusting the constant humidity, respectively .

In FIG. 9A, a white circle plot shows a measurement result when the atmosphere of the composite film is humidified, and a black circle plot shows a measurement result when the atmosphere is dehumidified. Here, P
The figure shows a case where ANA is heat-treated at 280 ° C. for 8 hours, and ordinary water (in which CO ₂ is slightly dissolved) is used to give a certain humidity. The humidity dependence of the electrical conductivity of this composite membrane is hardly observed from 0 to 40% RH (5 × 1
0 ^-5 Scm ^-1 ), rising from 40% or more with humidity to 95%
% Was 3 × 10 ⁻³ Scm ⁻¹ . This is due to the effect of the CO ₂ being slightly dissolved in normal water. Therefore, the same measurement as in FIG. 9A was performed using water in which CO ₂ was degassed from water used when applying a constant humidity, and compared with the case where normal water was used.

In FIG. 9B, open circle plots and black circle plots are obtained by measuring electric conductivity while changing humidity using water using CO ₂ degassed water. On the other hand, open triangle plots, closed triangle plots, open square plots, and closed square plots respectively show measurement results when ordinary water is used. In addition, open circle plot, open triangle plot,
White square plots are obtained when the atmosphere of the composite film is humidified and measured, and black circle plots, black triangle plots, and black square plots are obtained when the atmosphere is dehumidified and measured. Also,
Open triangle plots and solid triangle plots are obtained when PANA was not heat-treated.
The white and black square plots indicate that PANA was 28.
This is the case where heat treatment was performed at 0 ° C. for 8 hours.

As is clear from a comparison between FIG. 9B and FIG. 9A, C
When O ₂ was degassed, a low and constant electric conductivity (3 × 10 ⁻⁵ Scm ⁻¹ ) was obtained regardless of humidity. That is, the electric conductivity of the basic PAn does not depend on the humidity,
It turns out that it responds only to ₂ .

FIG. 10 is a diagram showing the response of the gas sensor. The horizontal axis represents time, and the vertical axis represents current. This figure shows a change in current detected by the gas sensor when a mixed gas of CO ₂ and H ₂ O is intermittently supplied to the gas sensor. In the figure, the upper arrow indicates the gas supply start time, and the lower arrow indicates the gas supply stop time. It took 29 seconds from the start of the gas supply until a current of 90% of the equilibrium value flowed, and it took 60 seconds from the stop of the gas supply until the current flowing to the gas sensor became zero. As can be seen from FIG. 10, the composite membrane for a gas sensor of the present invention can measure the concentration of the CO ₂ gas with good responsiveness even when the gas is intermittently supplied. In other words, the composite membrane for a gas sensor of the present invention can accurately measure the CO ₂ gas even if the measurement is continuously performed.

[0035]

According to the present invention, as described above, for example, C
The reaction between O ₂ gas and water held by PVA causes HC
O ₃ ⁻ is generated, and the generated HCO ₃ ⁻ is taken into a basic PAn as a dopant to change the electrical conductivity of the composite film. Therefore, according to the present invention, the concentration of the CO ₂ gas can be accurately measured by measuring the electric conductivity of the gas sensor composite film. In particular, according to the present invention, at room temperature the concentration of CO ₂ gas can be accurately measured and the concentration measurement of the CO ₂ gas can carried out continuously for a long time.

Further, according to the present invention, by setting the heat treatment temperature and the heat treatment time to appropriate values, the degree of change in the electrical conductivity of the composite film with respect to the CO ₂ gas concentration can be increased, and the sensitivity of the gas sensor can be increased. Can be improved. Also,
The electric conductivity of the basic PAn is constant irrespective of humidity if no CO ₂ is present. Therefore, the CO ₂ gas concentration can be measured with high accuracy even under high or low humidity conditions.

[Brief description of the drawings]

FIG. 1 is a view schematically showing a composite film for a gas sensor according to the present invention.

FIG. 2 is a diagram illustrating a method for manufacturing a gas sensor and a composite film for a gas sensor.

FIG. 3 is a diagram showing a state in which HCO ₃ ⁻ binds to N ⁺ ions in a basic polyaniline.

FIG. 4 is a diagram showing a relationship between a heat treatment temperature and electric conductivity of PANA powder.

FIG. 5 is an FTIR absorption spectrum of PANA.

FIG. 6 is an explanatory view showing a state in which a chemical formula changes in the process of heat-treating PANA.

FIG. 7 is a diagram plotting a change in electric conductivity with respect to a CO ₂ gas concentration.

FIG. 8 is a diagram showing humidity dependency of electric conductivity of a gas sensor.

FIG. 9 is a diagram showing a relationship between humidity and electric conductivity of a gas sensor.

FIG. 10 is a diagram showing responsiveness of a gas sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Conductive polymer 2 Insulating polymer 10 Gas sensor 11 Substrate 12 Electrode 14 Measuring device

Claims (9)

[Claims] 1. A carbon dioxide gas (CO ₂ ) comprising a conductive polymer capable of taking in hydrogen carbonate ions (HCO ₃ ⁻ ) as a dopant, and an insulating polymer capable of retaining water and absorbing water. A composite membrane for a gas sensor, wherein HCO ₃ ⁻ is generated by a reaction with water molecules held by the polymer, and the electric conductivity changes according to the amount of HCO ₃ ^{− taken up} by the conductive polymer. 2. The composite film for a gas sensor according to claim 1, wherein the conductive polymer is one of heat-treated polyanthranilic acid, basic polyaniline, and a derivative thereof. Wherein said conductive polymer, HCO ₃ ^- is the insulative before capturing, HCO ₃ ^- complex for gas sensor according to claim 1 or 2, characterized in that changes to incorporate the electrically conductive film. 4. The composite film for a gas sensor according to claim 1, wherein the insulating polymer is any one of polyvinyl alcohol, polyvinyl pyrrolidone, and polyacrylic acid. 5. The composite film for a gas sensor according to claim 1, wherein a weight ratio of the conductive polymer to the insulating polymer is about 1: 6. 6. A gas sensor comprising: the gas sensor composite film according to claim 1; and a pair of electrodes formed on the gas sensor composite film. 7. A first step of removing a carboxylic acid (—COOH) by performing a heat treatment at a predetermined temperature and time to form a conductive polymer; and dissolving the conductive polymer in a predetermined solvent, Second to form a composite solution by mixing with a solution containing a conductive polymer
A method for producing a composite film for a gas sensor, comprising: a step of, after casting the composite solution on a substrate, removing a solvent by a drying treatment. 8. The method according to claim 7, wherein in the first step, the polyanthranilic acid powder is heat-treated at a predetermined temperature and for a predetermined time to remove the carboxylic acid to produce a basic polyaniline or a derivative thereof. 3. The method for producing a composite film for a gas sensor according to 1.). 9. The composite film for a gas sensor according to claim 7, wherein an electrode is formed on the substrate before or after casting the composite solution on the substrate in the third step. Manufacturing method.