JP2002090324A - Gas sensor and control method for metal oxide thin layer surface state - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain an excellent film forming property and to improve the sensitivity even in lean gas by making the film of a sensor part thin and controlling the fine surface state. SOLUTION: This gas sensor is provided with a substrate and a tin oxide film mainly composed of tin oxide, formed on the substrate. The tin oxide film is obtained by forming a film from a solution containing at least a tin compound and an organic polymeric compound that can chemically modify the tin compound, and then baking it in an oxygen inclusion atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a gas sensor and a method for controlling the surface state of a gas sensor, and more particularly to a highly sensitive gas sensor and a method for controlling the surface state of a metal oxide obtained by arbitrarily controlling the fine structure of a tin oxide thin film.

[0002]

2. Description of the Related Art Since a gas sensor using a semiconductor oxide such as tin oxide can be manufactured without using expensive equipment, a new environment-related industry in many fields including the ceramics industry, the electric machine industry, and the chemical industry. Technology needs. For this reason, the importance of a technique for increasing the sensitivity of a semiconductor thin film gas sensor, such as improvement in selectivity to various gases and improvement in response sensitivity at a low gas concentration, is increasing. Conventionally, as a gas sensor using tin oxide, a gas sensor formed by forming tin oxide having an average primary particle diameter of 35 to 70 nm to a thickness of 10 to 50 μm and further adding at least one of lead and lanthanum is known. (Japanese Patent Laid-Open No. 7-
260729). In this gas sensor, a precipitate of tin hydroxide obtained by adding aqueous ammonia to an aqueous solution of tin tetrachloride is dried, and tin oxide obtained by firing is kneaded with water to form a paste. This paste is applied to an electrode portion of an alumina substrate provided with a comb-shaped electrode of a platinum thin film and a heater, dried and fired to obtain a thick film of tin oxide, thereby forming a gas sensor.

[0003]

However, since a gas sensor using a semiconductor oxide reads a change in resistance due to gas adsorption on the oxide surface, the conventional gas sensor assumes that the starting material serving as the sensor portion is powder. However, there is a problem that it is difficult to obtain a high sensitivity with a small reaction area. In particular, there is a problem that the sensitivity in the case of a gas having a low concentration is not sufficient. Therefore, when a sensitizer or the like is used, its dispersibility becomes a problem, and the second
Even in the case of a sensor having a laminated structure in which layers are laminated, sufficient performance has not been achieved.

As a method of forming a film, in transfer printing, conventionally, a mixture of powder and squeegee oil has been applied as a printing paste, but the roughness of the particle size and the low adhesive strength pose problems. I was In addition, it has been considered that the use of squeegee oil requires the use of an aromatic hydrocarbon organic solvent such as toluene for washing after use.

The present invention has been made to address such a problem, and has excellent film-forming properties. By reducing the thickness of the sensor portion and controlling the state of its fine surface, the sensitivity can be improved even with a rare gas. It is an object of the present invention to provide a gas sensor and a metal oxide thin layer surface state control method suitable for the gas sensor.

[0006]

A gas sensor according to the present invention comprises:
A substrate, comprising a tin oxide film containing tin oxide as a main component formed on the substrate, wherein the tin oxide film contains at least a tin compound and an organic polymer compound that can be chemically modified with the tin compound. It is characterized in that it is obtained by forming a film from a solution containing the solution and then firing in an oxygen-containing atmosphere. In the present invention, the chemical modification means that a tin compound and an organic polymer compound are not simply mixed in a solvent but cause any chemical interaction or a chemical change such as a coordination bond or a covalent bond. State.

Further, the present invention is characterized in that the tin compound is an organic tin compound. The organic polymer compound has a number average molecular weight of 100 to 5,000,000. In addition, the above-mentioned solution contains the above-mentioned tin compound 100 in a solvent.
It is characterized by containing 5 parts by weight of the organic polymer compound and 5 to 100 parts by weight of the organic polymer compound. The solution has a viscosity at 25 ° C. of 0.1 to 10 Pa · s. It is characterized in that hydroxyapatite is coated on the tin oxide film.

The method for controlling the surface state of a thin metal oxide layer of a gas sensor according to the present invention comprises a film forming step of forming a layer containing a metal compound on a substrate, and a firing step of firing the formed layer containing the metal compound. The film forming step is a step of applying and drying using a solution containing a metal compound and an organic polymer compound capable of chemically modifying the metal compound, and the baking step is performed by drying The method is characterized in that the thin film is fired in an oxygen-containing atmosphere.

A semiconductor gas sensor using a semiconductor oxide such as tin oxide reads a change in resistance or the like due to adsorption of a gas to the surface of the semiconductor oxide. The present invention provides a thin film having a large surface area because a chemically modified organic polymer compound is decomposed and volatilized at the time of firing by forming and firing a solution containing a tin compound and an organic polymer compound capable of chemically modifying the tin compound. Can be formed. In particular, by adjusting the number average molecular weight and the amount of the organic polymer compound, the surface micropores of the tin oxide can be arbitrarily controlled from several nanometers to several microns. Further, by setting the viscosity at the time of film formation to a predetermined range, the use as a paste used for transfer printing becomes easy. By coating hydroxyapatite on the tin oxide sensor thin film, the adsorption performance of the test gas is improved, and as a result, the sensitivity of the sensor is improved.

The method for controlling the surface state of a thin metal oxide layer according to the present invention comprises a film forming step of applying and drying a solution containing a metal compound and an organic polymer compound capable of chemically modifying the metal compound; A baking step of baking the pre-baked thin film in an oxygen-containing atmosphere, whereby the surface state of the metal oxide thin layer can be controlled by combining conventionally known techniques.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A gas sensor according to the present invention will be described with reference to FIG. FIG. 1 is a sectional view of a detection unit of the gas sensor.
The gas sensor 1 is provided with platinum electrodes 3, 3 on a substrate 4, and a tin oxide film 2 is formed electrically connected to the platinum electrodes 3, 3. On the back surface of the substrate 4, a heater 5 as a heating means is provided. As the substrate 4, a glass substrate,
Known substrates for gas sensors, such as a silicon substrate, an alumina substrate, and a magnesium oxide substrate, can be used. Further, the gas concentration is measured by detecting a change in electrical characteristics between the platinum electrodes 3 and 3, for example, an electrical resistance value.

The tin oxide film 2 is prepared by preparing a solution containing at least a tin compound and an organic polymer compound capable of chemically modifying the tin compound, and firing a thin film formed from this solution in an oxygen-containing atmosphere. Obtained. The tin compound that can be used in the present invention may be any compound that can be chemically modified in solution with an organic polymer compound described below. For example,
Inorganic tin compounds such as tin chloride, tin nitrate, tin sulfate,
Examples include tin tetraalkoxides such as tin tetraisopropoxide and tin tetraethoxide, and organic tin compounds such as tin 2-ethylhexanoate and tin naphthenate. Of these, organotin compounds having excellent film-forming properties are preferred.

The organic polymer compound may be any compound having a functional group capable of chemically modifying a tin compound in a solution, and may be an oxygen-containing polymer such as polyethylene glycol, polyethylene oxide, polyvinyl alcohol, methyl cellulose and carboxymethyl cellulose. And polyvinylpyrrolidone. Of these, oxygen-containing polymers that can easily modify the tin compound are preferable.

The surface state of the tin oxide film after firing is controlled by the molecular weight of the organic polymer compound that is chemically modified into the tin compound. As the molecular weight increases, the diameter of the fired micropores increases. The number average molecular weight of the organic polymer compound varies depending on the type of the organic polymer compound,
It is preferably from 0 to 5,000,000. Number average molecular weight is
If it is less than 100, the diameter of the micropore becomes small. on the other hand,
If the number average molecular weight exceeds 5,000,000, film formation may be difficult, and a uniform tin oxide film may not be obtained.

As a solvent for dissolving or dispersing the tin compound and the organic polymer compound, an organic solvent, water, or a mixed solvent of an organic solvent and water can be used. In particular, an organic solvent capable of dissolving an organotin compound and an oxygen-containing polymer is preferable, and 2-methoxyethanol and 2-ethoxyethanol, in which dissolution of the oxygen-containing polymer and chemical modification of the organotin compound are likely to occur. And the like are particularly preferred.

A solution containing a tin compound and an organic polymer compound is prepared by mixing 100 parts by weight of a tin compound and 5 to 100 parts by weight of an organic polymer compound in a solvent. Depending on the molecular weight of the organic polymer compound, the amount of the organic polymer compound may be 100
If the amount exceeds the weight part, the surface state after firing will not be uniform,
If it is less than 5 parts by weight, desired surface roughness cannot be obtained. The concentration of the tin compound in the solution is between 0.1 and 2
0 mol / l, preferably 0.25 to 1 mol / l. 0.1mol / l
If it is less than 1, the tin oxide film becomes island-shaped and loses continuity. If it exceeds 20 mol / l, the solubility of the organic polymer compound becomes poor.

To the solution containing the tin compound and the organic polymer compound, various additives usually used for forming a coating film, for example, a leveling agent, a viscosity imparting agent, a stabilizer and the like can be added. In particular, when applying as a printing paste in transfer printing, a viscosity-imparting agent is added to
0.1 Pas to 10 Pas at 25 ° C, preferably 2 to 6 Pa
By adjusting to s, a paste derived from a solution that can be directly used for transfer printing can be produced. The viscosity can be adjusted also by the amount of the organic polymer compound.

A method for forming a tin oxide film will be described. 1) A solution containing a tin compound and an organic polymer compound adjusted to the above viscosity and concentration is applied on a substrate. As a coating method, a known coating method can be adopted. For example, a spin coat method, a dip coat method, a spray coat method, a print coat method such as screen printing and pad printing, an electrodeposition coat method and the like can be mentioned. 2) Dry at 100-150 ° C in air. Next, the solution is applied again according to the above method 1). Then 1
After drying at a temperature of 00 to 150 ° C., this step is repeated a plurality of times, for example, 2 to 20 times, until a predetermined film thickness is obtained. In addition, 100 ~
Pre-baking can be performed after drying at a temperature of 150 ° C. or immediately after application. The pre-firing temperature is 250-35
The temperature is 0 ° C. 3) Finally, in an oxygen-containing atmosphere, exceed 350 ° C and 1000 ° C
The sintering is carried out at the following temperature, preferably in the temperature range of 500 to 800 ° C. In the case of pre-firing, firing is performed at a temperature higher than the pre-firing temperature. If the firing temperature is lower than 350 ° C., the crystallization of tin oxide becomes insufficient. 1000 ℃
If the temperature exceeds the above range, sintering of the tin oxide film is likely to occur, and the surface roughness is reduced. Note that the oxygen-containing atmosphere is preferably in air. 4) The obtained tin oxide film has a thickness of 1 μm or less, pores in nanometer units appear on the surface of the thin film, and the surface area increases. As a result, a high-sensitivity sensor is obtained.

Further, the surface of the tin oxide film has a thickness of several nm to 10 nm.
By forming a degree of hydroxyapatite layer,
A more sensitive double-layered high-sensitivity sensor can be obtained. Since the test gas is more adsorbed by the surface-coated hydroxyapatite layer, the gas is concentrated on the sensor surface, and as a result, a low gas concentration can be detected. The method for forming the hydroxyapatite layer may be a known method, for example, a calcium ion concentration of 0.1 to 50 mmol / l,
There is a method of immersing in a simulated body fluid in which the phosphate ion concentration is adjusted to 0.1 to 20 mmol / l for 2 hours or more. Here, the simulated body fluid is a processing fluid capable of forming a calcium phosphate compound, and is prepared from sodium chloride, sodium hydrogen carbonate, potassium chloride, potassium hydrogen phosphate, magnesium chloride, calcium chloride, sodium sulfate, sodium fluoride, pure water, and the like. You.

In the method for controlling the surface state of a thin metal oxide layer according to the present invention, a method similar to the above-described method for forming a tin oxide film can be used. That is, the present invention can be applied to a metal oxide thin layer capable of forming a semiconductor oxide sensor other than tin oxide, and it is possible to change the pore diameter size of the thin layer surface from several nanometers to several micrometers, which is used for a gas sensor. The surface state of the metal oxide thin layer can be controlled. Examples of metals that can form a semiconductor oxide sensor include zinc, zirconium, tungsten, indium, gallium, lanthanum, strontium, cobalt, iron, silver, and the like. The chemical modification method, film formation method, and firing method can be performed in the same manner as in the case of tin.

[0021]

EXAMPLES Example 1 Tin tetraisopropoxide (Sn) was used as a tin compound.
_{(O-iC 3 H 7)} 4 and), 2-methoxyethanol _{(CH 3 OC 2 H 4 OH} ) as solvent, was prepared polyethylene glycol (number average molecular weight 2000) as the organic polymer compound. The coating solution was prepared in a glove box in a dry nitrogen atmosphere. To a 2methoxyethanol solution of a tin compound adjusted to a concentration of 0.25 mol / l, polyethylene glycol was added at 20% by weight of the tin compound.
By heating and stirring for 1 hour, a uniform coating solution was obtained. The solution was passed through a nylon filter having a mesh size of 0.2 μm to remove impurities.

The manner in which tin glycol was chemically modified with tin tetraisopropoxide was examined by Fourier transform infrared spectrum (FT-IR) measurement. The result is shown in FIG. FIG. 2 is an FT-IR spectrum. As shown in FIG. 2, the peak (indicated by △ in the figure) due to —CO—stretching vibration observed in the spectrum of polyethylene glycol (indicated by PEG in the figure) alone was the dried coating film (indicated by △ in the figure). Sn-Gel with P in the figure
EG), disappears or becomes broad, and a new peak (indicated by a star in the figure) appears. It can be seen that the polar portion of the polyethylene glycol molecule and tin tetraisopropoxide have an interaction such as coordination bond.

An alumina (Al ₂ O ₃ ) substrate (13 mm long × 8 mm wide ×
1 mm thick) was prepared. The above coating solution was applied on this substrate by a spin coating method (2000 rpm, 20 sec) to form a film. Immediately after the film formation, preliminary firing was performed at 350 ° C. This spin coating and pre-baking
This was repeated twice and then baked at a temperature of 800 ° C. for one hour to obtain a tin oxide (SnO ₂ ) thin film.

FIGS. 3 and 4 show the sintering temperature, the presence of organic substances, and the process of crystallization, respectively. Fig. 3 shows the drying and baking temperature and FT-IR spectrum of the coating film applied on the substrate. At a temperature of 300 ° C or more, the peaks based on the stretching vibration and the bending vibration observed in the organic compound disappear. I understand. FIG. 4 shows the results of X-ray diffraction (XRD) based on differences in the drying and baking temperatures of the coating film. At a temperature of 300 ° C., tin oxide crystals (110), (101), (21)
1) Although the peak based on the crystal plane is small and indicates that crystallization has not completely progressed, it can be seen that tin oxide crystals are formed at a temperature of 400 ° C. Figure 5 shows 800 ° C
-Ray diffraction (XRD) after baking the substrate at the temperature of
As a result, a peak (indicated by S in the figure) based on the tin oxide crystal was observed together with the alumina of the substrate (indicated by C in the figure) and platinum of the electrode (indicated by Pt in the figure).
Crystallization of nO ₂ ) was confirmed.

FIG. 6 shows the shape of the obtained tin oxide thin film in the depth direction. FIG. 6 is a three-dimensional surface diagram with a side length of 1000 nm obtained by an atomic force microscope (AFM).
(A) shows the tin oxide thin film obtained in Example 1 and FIG.
(B) is shown as Comparative Example 1, and is a tin oxide thin film produced by the same method as in Example 1 except that polyethylene glycol was not used. Difference between the maximum height and the minimum height (Z _max ) in the uneven portion formed on the tin oxide thin film
Is 28.02 nm in Example 1 and 25.16 nm in Comparative Example 1.
Met. Further, the difference in height in the depth direction (Z _depth ) between the adjacent uneven portions was 6 to 12 nm in Example 1 and 3 to 4 nm in Comparative Example 1. From this, it was observed that in Example 1, the unevenness was twice or more intense than in Comparative Example 1 in the depth direction. This indicates that the combustion / foaming due to the addition of polyethylene glycol contributes not only to the particle size but also to the miniaturization of the unevenness in the depth direction (high surface area).

A gas sensor shown in FIG. 1 was prepared using the obtained tin oxide thin film, and the detection sensitivity to carbon monoxide (CO) gas was evaluated. The concentration of CO gas used is 392pp
m (diluted with nitrogen) and the measurement temperature was 500 ° C. The thickness of the tin oxide thin film formed on the alumina substrate on which the platinum comb electrodes were formed was about 900 nm. The measurement schedule was (a) clean air (5 minutes), (b) CO gas (20
Minute), (c) Clean air (75 minutes) was flowed sequentially, and the total of 100 minutes was defined as one cycle. FIG. 7 shows the measurement results. Comparative Example 1 is an example in which a tin oxide thin film having a thickness in the depth direction of 3 to 4 nm was used because the above polyethylene glycol was not added. The sensitivity (S _{air / co} ) of the tin oxide thin film of Example 1 to CO gas was 1.493, whereas the sensitivity (S _{air / co} ) of Comparative Example 1 was 1.418, and the sensitivity of Example 1 was increased. It was confirmed that. Further, the actually measured resistance value of Example 1 was larger than that of Comparative Example 1. The above results are considered to be due to the increase in the surface area of the thin film due to combustion and foaming of the added organic polymer.

[0027]

Example 2 Polyethylene oxide was further added to the coating solution obtained in Example 1 to reduce the viscosity to 2 Pa · s or more.
It was adjusted to 6 Pa · s. Using this solution, a thin film was formed by a transfer printing method. Thereafter, a tin oxide thin film was obtained in the same manner as in Example 1 to produce a gas sensor. The obtained gas sensor showed the same high sensitivity as in Example 1.

[0028]

Example 3 The oxides obtained in Examples 1 and 2
The thin film is made of sodium ions (Na ⁺) Concentration 139.0mmo
l / l, potassium ion (K⁺) Concentration of 2.8 mmol / l, calcium
Um ion (Ca²⁺) Concentration of 1.8 mmol / l, magnesium
Ion (Mg²⁺) Concentration of 0.5 mmol / l, chlorine ion (Cl
^-) Concentration of 144.0 mmol / l, phosphate ion (PO_Four ³⁺)concentration
For 2 hours in a simulated body fluid adjusted to 1.1 mmol / l
Immersed in the above, hydroxyapatite
To (Ca_Ten(PO_Four)₆(OH)_Two) Coat about 10nm layer
Was. A gas sensor similar to that of Example 1 was manufactured. Got
The gas sensor has a CO gas concentration of 10 ppm (diluted with nitrogen).
7, the sensitivity shown in FIG. 7 shows the same high sensitivity as in Example 1.
did.

[0029]

The gas sensor of the present invention comprises a tin compound,
Since a tin oxide film obtained by forming a film from a solution containing at least an organic polymer compound that can be chemically modified to the tin compound and firing in an oxygen-containing atmosphere is used, the fine surface state after firing with the organic polymer compound is used. Can be a structure having surface micropores that can be arbitrarily controlled from several nanometers to several microns. As a result, a gas sensor having excellent sensitivity even with a rare gas can be obtained.

Since the tin compound is an organotin compound, the number average molecular weight of the organic polymer compound is 100
Since the compounding ratio is set to 100 parts by weight of the tin compound and the organic polymer compound is set to 5 to 100 parts by weight, a coating liquid having excellent film-forming properties can be obtained. Control becomes easier.

Since the viscosity of the coating liquid is adjusted to 0.1 to 10 Pa · s (25 ° C.), it can be easily used as a paste for transfer printing. Further, since the hydroxyapatite is coated on the tin oxide film, the performance of adsorbing the test gas is improved, and the sensitivity of the sensor is further improved.

The method for controlling the surface state of a thin metal oxide layer of a gas sensor according to the present invention comprises the steps of: applying a solution containing a metal compound and an organic polymer compound capable of chemically modifying the metal compound;
It has a film forming process for drying and a firing process for firing the dried thin film in an oxygen-containing atmosphere, so that expensive equipment is not required and the surface state of the metal oxide thin layer can be controlled by conventional techniques. it can. As a result, a highly sensitive gas sensor can be manufactured.

[Brief description of the drawings]

FIG. 1 is a sectional view of a detection unit of a gas sensor.

FIG. 2 is an FT-IR spectrum diagram.

FIG. 3 is a FT-IR spectrum diagram of drying and baking temperatures of a coating film.

FIG. 4 is a diagram showing a drying and baking temperature of a coating film and a process of crystallization of a tin oxide thin film.

FIG. 5 is a view showing a result of X-ray diffraction (XRD) after firing.

FIG. 6 is a three-dimensional surface view by an atomic force microscope (AFM).

FIG. 7 is a diagram showing detection sensitivity to CO gas.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gas sensor 2 Tin oxide film 3 Platinum electrode 4 Substrate 5 Heater

Claims (7)

[Claims] 1. A gas sensor comprising a substrate and a tin oxide film containing tin oxide as a main component formed on the substrate, wherein the tin oxide film comprises a tin compound and a tin compound. A gas sensor obtained by forming a film from a solution containing at least a modifiable organic polymer compound and baking it in an oxygen-containing atmosphere. 2. The gas sensor according to claim 1, wherein said tin compound is an organic tin compound. 3. The number average molecular weight of the organic polymer compound is
The gas sensor according to claim 1 or 2, wherein the number is 100 to 5,000,000. 4. The solution according to claim 1, wherein the tin compound is contained in a solvent.
The gas sensor according to claim 1, comprising 100 parts by weight and 5 to 100 parts by weight of the organic polymer compound. 5. The gas sensor according to claim 1, wherein the viscosity of the solution is adjusted to 0.1 to 10 Pa · s (25 ° C.). 6. The gas sensor according to claim 1, wherein hydroxyapatite is coated on the tin oxide film. 7. A method for controlling a surface state of a thin metal oxide layer of a gas sensor, comprising: a film forming step of forming a layer containing a metal compound on a substrate; and a firing step of firing the layer containing the metal compound. The film forming step is performed using a solution containing a metal compound and an organic polymer compound capable of chemically modifying the metal compound,
A method for controlling a surface state of a thin metal oxide layer, wherein the firing step includes firing the dried thin film in an oxygen-containing atmosphere.