JPH11116243A - Carbon monoxide sensitive thin titanium dioxide film, gas sensor using same and production of thin titanium dioxide film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a sensor material which acts at low temp., has high gas selectivity to CO and can detect CO with high sensitivity. SOLUTION: A film of a film forming soln. giving a metallic compsn. consisting of 0.01-3 mol.% at least one group VIII element selected from the group consisting of Fe, Co and Ni, 0.05-5 mol.% noble metallic element and the balance Ti is formed on a ceramic substrate, dried, pretreated in an oxidizing atmosphere, fired at 350-600 deg.C in a reducing atmosphere and further heated in an oxidizing atmosphere to obtain the objective CO sensitive thin titanium dioxide film consisting of 0.01-3 mol.% of the selected group VIII element, 0.05-5 mol.% of the noble metallic element and the balance titania.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a titanium oxide thin film which can be suitably used as a gas sensor having a high gas selectivity for carbon monoxide gas, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, a gas sensor for detecting only a specific gas component in air containing a small amount of a specific gas such as hydrogen, carbon monoxide, hydrocarbon, or ammonia has been developed. In such a gas sensor, a sensitivity to a specific gas only, that is, a sensor material having a high gas selectivity is required.Conventionally, several kinds of metal oxides are mixed, or after further adding a metal to the mixture, An attempt was made to synthesize a sensor material with high gas selectivity by a method such as firing.

However, since it is extremely difficult to design a sensor material having high gas selectivity, actually, a material having a relatively large difference in sensitivity between a specific gas and other gases is selected from existing sensor materials. It is currently used. In such a situation, when the properties of the specific gas and other gases are similar, it is difficult to increase only the sensitivity to the specific gas.For example, since carbon monoxide and hydrogen are both reducing gases, It has been difficult to synthesize those that are sensitive only to carbon monoxide and not to hydrogen.

Further, since the gas sensor needs to detect a trace amount of a specific gas, a high gas sensitivity is required for a sensor material. It is generally known that high gas sensitivity can be obtained by adding a small amount of a noble metal to a sensor material. However, when noble metals are added,
Although the sensitivity of the entire sensor is improved, the sensitivity to gas components other than the specific gas is also improved, so that there is a problem that the selectivity to the specific gas is impaired. Furthermore,
The conventional sensor material has a problem that desired sensor characteristics cannot be obtained unless it is heated to a temperature of 300 ° C. or higher.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and operates at a low temperature, has a high gas selectivity to carbon monoxide, and detects it with high sensitivity. It is intended to provide a sensor material to be obtained.

[0006]

Means for Solving the Problems The present inventors have found that a titanium oxide thin film containing a specific Group 8 element and a noble metal element in a predetermined ratio and the balance of titania can be used even in a low-temperature state. Of the present invention can be detected with high sensitivity and high selectivity, and arrived at the present invention. That is,
According to the present invention, at least one group VIII element selected from the group consisting of iron, cobalt and nickel is contained in an amount of 0.01 to 3
The present invention provides a carbon monoxide-sensitive titanium oxide thin film, characterized in that the carbon monoxide-sensitive titanium oxide thin film contains 0.05% to 5% by mole of a noble metal element and the remainder is made of titania.

Further, according to the present invention, iron, cobalt,
At least one kind of 8 selected from the group consisting of nickel;
Group element in 0.01 to 3 mol%, noble metal element in 0.05 to
5 mol%, containing 0.01 to 0.5 mol% of lithium,
A carbon monoxide-sensitive titanium oxide thin film characterized in that the balance is composed of titania is provided.

In the titanium oxide thin film of the present invention, 8
Preferably, the group III element is nickel and the noble metal element is platinum. Further, according to the present invention, there is provided a gas sensor in which the carbon monoxide-sensitive titanium oxide thin film is incorporated in a sensor unit.

Further, according to the present invention, as a composition ratio of the metal element portion, at least one Group VIII element selected from the group consisting of iron, cobalt, and nickel is contained in an amount of 0.01 to 3 mol%, A film-forming solution containing 0.05 to 5 mol% and the balance being titanium is formed on a ceramic substrate, dried, pre-treated in an oxidizing atmosphere, and then dried in a reducing atmosphere. There is provided a method for producing a carbon monoxide-sensitive titanium oxide thin film, characterized by baking at a temperature of up to 600 ° C. and further heating in an oxidizing atmosphere.

Further, according to the present invention, the composition ratio of the metal element portion is at least one group VIII element selected from the group consisting of iron, cobalt, and nickel in a range of 0.01 to 0.01.
A film forming solution containing 3 mol%, a precious metal element of 0.05 to 5 mol%, lithium of 0.01 to 0.5 mol%, and a balance of titanium is formed on a ceramic substrate. After drying and pre-treatment in an oxidizing atmosphere,
There is provided a method for producing a carbon monoxide-sensitive titanium oxide thin film, which comprises firing at 350 to 600 ° C. in a reducing atmosphere and further heating in an oxidizing atmosphere.

In the production method of the present invention, it is preferable that the film forming liquid is a mixture of a precursor solution containing titanium and a noble metal and a TiO ₂ sol or TiO ₂ fine powder,
Preferably, the Group VIII element is nickel and the noble metal element is platinum. Further, a solution of a precursor containing titanium and a noble metal atom and a TiO ₂ sol or TiO ₂ fine powder are mixed with TiO ₂
It is preferable to mix them so that the weight conversion ratio to ₂ is 1: 3 to 3: 1, and the precursor is synthesized mainly using an alkoxide of titanium or a derivative thereof, an amino acid, and a platinum salt. preferable.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention provides at least one group VIII element selected from iron, cobalt and nickel with 0.0
It is a titanium oxide thin film containing 1 to 3 mol%, a precious metal element of 0.05 to 5 mol%, and the rest composed of titania. The thin film having such a structure can detect carbon monoxide with high sensitivity and high selectivity even in a low temperature state.

In the thin film having such a configuration, 0.01 to 0.5 mol% of Li may be further contained, and the remainder may be titania. By adding a small amount of Li,
This is because gas selectivity can be further improved.

In the present invention, the group VIII element is added to impart high gas selectivity to carbon monoxide, and the content in the thin film is 0.01 to 3
It needs to be in the range of mol%. Content is 0.
If it is less than 01 mol%, the effect of increasing gas selectivity is not sufficient, and if it is more than 3 mol%, the effect of increasing the gas sensitivity of the noble metal is diminished.

Of the Group VIII elements, iron, cobalt and nickel are used because a particularly high gas selectivity for carbon monoxide is obtained, and among them, nickel is particularly preferable.

In the present invention, the noble metal is added to enhance gas sensitivity to carbon monoxide,
It is necessary that the content in the thin film is in the range of 0.1 to 5% by weight. Within this range, higher gas sensitivity and gas selectivity can be obtained, but if it is 0.05 mol% or less, it is difficult to obtain sufficient gas sensitivity. In addition, the gas sensitivity has reached a sufficient saturation at 5 mol%, and further addition leads to an increase in cost.

The noble metal used in the present invention is preferably selected from the group consisting of gold, platinum, silver, palladium, rhodium, osmium, iridium, and ruthenium, and two or more of these are used in combination. Is also good. Among these noble metals, platinum is particularly preferable because of its excellent effect of increasing gas sensitivity as compared with other noble metals.

In the present invention, titania is a main component constituting a thin film. When titania is heat-treated in a reducing atmosphere, trivalent titanium ions are formed in the crystal lattice, and when they become tetravalent ions, electrons are emitted, so that an n-type semiconductor is formed.

The thin film of the present invention may contain, in addition to titanium ions, metal ions having a valence greater than tetravalent, for example, niobium ions and tantalum ions up to several mol%. By including these metal ions, the electron conductivity of the semiconductor can be further increased, and the electron conductivity can be maintained at a high temperature.

The above-mentioned thin film is formed by mixing a film forming solution composed of a mixture of the Group 8 element, the noble metal element, and titanium (if necessary, lithium) as required so as to have a predetermined composition ratio. After the film is formed on the substrate, the film is dried, pre-treated in an oxidizing atmosphere, fired in a reducing atmosphere, and further heated in an oxidizing atmosphere.

The film forming solution in the present invention contains at least one Group 8 element selected from iron, cobalt, and nickel in a composition ratio of metal element portions of 0.01 to 3 mol% and a noble metal element of 0.05 to 0.05 mol%. Other constitutions are not particularly limited as long as the content is 5 mol% and the balance is titanium, and can be prepared by various methods. For example, it can be prepared by a method of mixing a precursor solution containing titanium, a noble metal, or a Group 8 element with a TiO ₂ sol at a ratio according to the present invention, or a method of adding a salt of a noble metal or a Group 8 element to a TiO ₂ sol. ,
A method in which a precursor containing titanium and a noble metal and a TiO ₂ sol are mixed at a specific ratio and a predetermined ratio of a salt of a Group 8 element is added is preferable because the highest gas sensitivity and gas selectivity can be obtained. In the case where lithium is contained, a film forming solution can be prepared in a similar manner.

In the present invention, the precursor containing titanium and a noble metal refers to a precursor in which a titanium atom and a noble metal atom are connected to one molecule via a chemical bond. When the titanium ion and the noble metal ion are only dissolved in the solvent, the relative positional relationship changes with time because the titanium ion and the noble metal ion can move freely,
The precursor is clearly different in that the relative positional relationship between the titanium atom and the noble metal atom is predetermined.

The precursor can be synthesized by using an organic metal reagent having a reactive functional group and utilizing a technique used for organic synthesis. That is, for an organic titanium compound containing a titanium atom and the functional group, an organic noble metal compound containing a noble metal atom and the functional group or a reactive noble metal compound is used, and both functional groups or a functional group and a noble metal compound are used. Can be synthesized by reacting with each other.

Further, a precursor may be synthesized by bonding a titanium compound and a noble metal compound or the like via a compound having a plurality of functional groups. Specifically, a method of bonding an alkoxide of titanium or an alkoxide derivative in which a part of an alkoxide is substituted with an acetylacetonate group (hereinafter, referred to as AcAc) to a platinum oxide via an amino acid may be used. .

In this method, a carboxyl group of an amino acid is reacted with an alkoxide group of an alkoxide or an alkoxide derivative to bond the two, and then an amino group of the amino acid is reacted with and bound to a platinum ion to form a platinum atom and a titanium atom. Can be obtained in one molecule. The amino acid used in the reaction is not particularly limited, but lysine or proline, which has high solubility in solvents such as methanol and ethanol, is preferably used.

Since this reaction is carried out in a solvent in which these compounds are dissolved, the precursor is obtained as a solution containing the precursor. The concentration of the solution at the time of precursor synthesis is not particularly limited. For example, when lysine or proline is used, a titanium concentration of about 0.05 to 1 mol / l can be used. It can be used as it is as a body solution.

As described above, the thin film of the present invention may contain up to several mol% of niobium ions, tantalum ions, etc., but several mol% in the organic titanium compound.
By adding a niobium-tantalum alkoxide or salt to synthesize a precursor, the ions can be easily introduced into the thin film.

The TiO ₂ sol to be mixed with the precursor is composed of fine TiO ₂ particles, a solvent for dispersing the particles, and a small amount of a dispersant. By using the TiO ₂ sol, TiO ₂ is a dispersion of fine TiO ₂ particles
The sol plays the role of aggregate in the drying and heating process after film formation,
Even after the heating, a gap required as a sensor is retained in the thin film.

Therefore, a pore structure suitable for a sensor can be formed in the thin film, and a thin film having excellent sensor characteristics such as gas sensitivity and gas selectivity can be obtained. Further, since the reaction between the gas and the thin film is activated, the sensor can be operated at a lower temperature than before.

The primary particle diameter of the TiO ₂ sol particles is preferably in the range of 5 to 100 nm. 5n
If it is smaller than 100 m, it is difficult to perform the role of aggregate,
If the diameter is larger than nm, the diameter of generated pores becomes large, and the gas sensitivity is likely to be reduced.

Further, if the pH of the TiO ₂ sol is significantly different from the pH of the precursor, the sol becomes unstable and is likely to settle. Therefore, the pH of the TiO ₂ sol solution and the pH of the precursor need to be approximately the same. . The allowable pH difference between the two depends on various conditions. For example, when an acidic solution is used as a precursor, the pH difference between the two may be 3 or less.

The solvent of the TiO ₂ sol includes
Water, alcohol and the like can be suitably used, and as a dispersant, a polymer surfactant and the like can be suitably used. The concentration of the TiO ₂ sol is not particularly limited. For example, when sol particles having a particle size of about 10 to 60 nm are used, it is preferable to use one having a concentration of about 10 to 30% by weight.

Instead of the TiO ₂ sol, it is also possible to use TiO ₂ fine powder having a similar particle diameter. In this case, the fine powder and the precursor solution are mixed,
If a solution in which TiO ₂ particles are uniformly dispersed is used, the same effect as that of the TiO ₂ sol can be obtained. If necessary, a dispersant or a solvent may be added, or mechanical mixing may be performed.

In the present invention, the precursor solution and TiO
The mixing ratio with the ₂ sol or the TiO ₂ fine powder is preferably in the range of 1: 3 to 3: 1 in terms of the weight conversion ratio to TiO ₂ . If the mixing ratio is not in this range, it is difficult to form pores that are suitable for a sensor and that allow gas to quickly enter.

As the ceramic substrate, quartz glass,
Any of amorphous glass such as low soda silica glass, quartz, single crystal alumina, single crystal such as silicon crystal, and polycrystal such as alumina can be used. Among them, the use of low soda silica glass is preferable because better sensor characteristics, specifically, high gas sensitivity and high gas selectivity can be obtained. Although film formation on a substrate can be performed by various methods, spin coating and dip coating are preferable because a thin film can be formed uniformly and easily.

The substrate is dried after forming a precursor solution. Drying can be performed under heating, but it is preferable to perform natural drying at room temperature from the viewpoint of uniform drying. After drying, the pretreatment is performed in an oxidizing atmosphere, for example, in an atmosphere containing air, oxygen, and an oxidizing gas.

In the present invention, the pretreatment means an organic component contained in the precursor and titanium, a noble metal, a group VIII element,
This refers to a treatment performed to remove inorganic components other than lithium and oxygen. At this time, it is preferable to include an appropriate amount of water vapor in the oxidizing atmosphere, since hydrolysis of the organic titanium compound and the noble metal compound and removal of organic components derived from these compounds can be sufficiently performed.

The pre-treatment is preferably performed in a temperature range of 300 to 500 ° C. If the temperature is lower than 300 ° C., it is difficult to sufficiently remove the organic components, so that carbon tends to remain in the subsequent reduction treatment. If the temperature is higher than 500 ° C., the noble metal grains are likely to grow, making it difficult to obtain fine noble metal particles. It is. The time for the pre-treatment should be appropriately determined depending on the pre-treatment temperature, but usually may be performed for several minutes to several hours.

After the pretreatment, firing is performed in a reducing atmosphere to sufficiently reduce the noble metal and suppress grain growth of the noble metal. The firing temperature is higher than the pretreatment temperature,
Specifically, it is preferable to perform firing at 350 to 600 ° C. If it is lower than 350 ° C., reduction to noble metal and TiO ₂
The desired properties cannot be obtained due to insufficient reduction of
If the temperature is higher than 600 ° C., the sintering of the TiO ₂ particles proceeds, so that the gas sensitivity decreases due to an increase in the pore diameter and a decrease in the pore volume. The firing time depends on the firing temperature, but may be from several minutes to several hours, as in the pretreatment. Note that an atmosphere containing a reducing gas such as hydrogen can be used as the reducing atmosphere.

After firing in a reducing atmosphere, 200 to 5 times in an oxidizing atmosphere for the purpose of controlling the oxidation state of the Group 8 element.
Heat at 00 ° C. for about 0.1 to 10 hours. In this case, if the heating temperature is lower than 200 ° C., it is difficult to change the oxidation state, and if it is higher than 500 ° C., it is difficult to obtain fine noble metal particles, which is not preferable. Note that as the oxidizing atmosphere, an atmosphere containing an oxidizing gas such as oxygen or ozone can be used.

The thickness of the thin film obtained after heating can be adjusted by the concentration of the precursor solution, film forming conditions, for example, the number of film formation, the pulling speed in dip coating, the number of spin rotation in spin coating, and the like. Further, it is also possible to adjust the film thickness by repeating the operation of performing the preliminary treatment after forming the film. 500 film thickness
When the diameter is smaller than nm, the sintering and grain growth of the titanium oxide particles are suppressed. Therefore, it is preferable in that the grain growth of the noble metal particles is easily suppressed. It is more preferable because it becomes possible.

[0042]

【Example】

(Example 1) A TiO ₂ sol in which TiO ₂ particles having a primary particle diameter of 30 nm are dispersed in water, and chlorides of Pd, Au, and Pt and nitrates of Ni, Fe, and Co are mixed and dispersed at a ratio shown in Table 1. Then, a film forming solution was prepared. Since the addition ratio is equal to the content ratio after firing, only the content ratio is shown in the table.

After the film forming solution was dip-coated on quartz glass, the film was allowed to stand at room temperature for one day and dried. Thereafter, preheating was performed by heating at 400 ° C. for 1 hour while flowing humidified air having a relative humidity of 90%.

After the preliminary treatment, the resultant was fired in hydrogen at 500 ° C. for 1 hour, and further heated in air at 300 ° C. for 4 hours to obtain a TiO ₂ thin film. In order to evaluate the sensor characteristics of the obtained thin film, a lead wire was attached to the thin film, and electric resistance was measured at 200 ° C. in air and an atmosphere in which 1000 ppm of carbon monoxide gas was added to the air.

Value obtained by dividing the former resistance value by the latter (sensitivity)
Was defined as the gas sensitivity of carbon monoxide gas. Similarly, 100
The gas sensitivity of hydrogen gas was measured using 0 ppm of hydrogen. Further, a value obtained by dividing the gas sensitivity of the carbon monoxide gas by the gas sensitivity of the hydrogen gas is used as a measure of the gas selectivity of the carbon monoxide (that is, the larger the value, the greater the selectivity of the carbon monoxide gas). Will be). As a result, as shown in Table 1, the thin film formed by the method of the present invention exhibited high gas sensitivity of 80 or more and high gas selectivity of 38 or more.

[0046]

[Table 1]

Example 2 Ti (OiPr) ₂ which is an acetylacetonate-substituted product of titanium isopropoxide
8.65% solution of (AcAc) ₂ in isopropanol 8.65
Lysine, a kind of gram and amino acid: NH ₂ (CH ₂ )
₄ CHNH ₂ (COOH) and 0.113 g was reacted in methanol, platinum chloride (H ₂ PtCl ₆ · 6H
₂ O) 0.075 g was added and allowed to further react.

Next, 10 cc of water was added to hydrolyze unreacted alkoxide groups. As a result, a 0.1 mol / l methanol solution of the precursor having a pH of 2 and containing titanium and platinum atoms in the molecule was obtained. On the other hand, a TiO ₂ sol having a pH of 1.5 was prepared by adding nitric acid as a dispersant to TiO ₂ particles having a primary particle diameter of 13 nm and dispersing in water.

The precursor solution and the TiO ₂ sol were mixed at a mixing ratio of 1: 1 in terms of weight to TiO ₂ , and both were mixed. To these mixed solutions, nitrates of Ni, Fe, and Co were added so as to have the contents shown in Table 2 to form a film forming solution, and this was applied onto a Corning # 7059 glass substrate.
Spin coating was performed once at a rotation speed of 500 RPM.
This was dried at room temperature, and then heated at 400 ° C. for 1 hour while flowing humidified air having a relative humidity of 90% to perform a preliminary treatment.

Thereafter, at the temperature shown in Table 2, 3%
Was heated in argon containing hydrogen for 1 hour, and further heated in air at 400 ° C. for 3 hours. The thickness of the obtained thin film was about 90 nm. In order to evaluate the sensor characteristics of the obtained thin film, a lead wire was attached to the thin film.
The gas sensitivity and selectivity of carbon monoxide were evaluated in the same manner as described above. As can be seen from Table 2, all of those obtained according to the method of the present invention exhibited high gas sensitivities of 120 or more and high selectivities of 60 or more.

[0051]

[Table 2]

Example 3 A film forming solution containing a precursor, a TiO ₂ sol, and nickel nitrate was prepared in the same manner as in Example 2-1. Lithium nitrate was further added to this solution at a rate shown in Table 3, and dip coating was performed on a Corning glass (# 7059) substrate at a rate of 50 mm / min.
Similarly, dip coating was performed on a solution to which lithium nitrate was not added. After drying them at room temperature, 500 ° C. while flowing humidified air having a relative humidity of 90%.
For 1 hour to perform a preliminary treatment.

Then, at 600 ° C. in ammonia gas for 1 hour
Heating was performed for an hour, and further, heating was performed in air at 450 ° C. for 30 minutes to obtain a TiO ₂ thin film. The thickness of the obtained thin film is about 4
It was 0 nm. In order to evaluate the sensor characteristics of the obtained thin film, gas sensitivity and selectivity of carbon monoxide were evaluated in the same manner as in Example 1. As a result, as shown in Table 3, when a small amount of lithium was added, improvement in gas sensitivity and gas selectivity was observed.

[0054]

[Table 3]

[0055]

EFFECT OF THE INVENTION The titanium oxide thin film of the present invention has a high gas selectivity to carbon monoxide gas even under low temperature conditions, and thus can be suitably used as a sensor material for a gas sensor. Further, according to the production method of the present invention, a titanium oxide thin film having the above characteristics can be easily produced.

Claims (10)

[Claims] 1. An alloy containing 0.01 to 3 mol% of at least one Group 8 element selected from the group consisting of iron, cobalt and nickel and 0.05 to 5 mol% of a noble metal element, with the balance being titania A carbon monoxide-sensitive titanium oxide thin film, comprising: 2. At least one group 8 element selected from the group consisting of iron, cobalt and nickel is contained in an amount of 0.01 to 3 mol%, a noble metal element in an amount of 0.05 to 5 mol%, and lithium in an amount of 0.01 to 5 mol%. A carbon monoxide-sensitive titanium oxide thin film containing 0.5 mol%, with the balance being titania. 3. The carbon monoxide-sensitive titanium oxide thin film according to claim 1, wherein the group VIII element is nickel and the noble metal element is platinum. 4. A gas sensor in which the carbon monoxide-sensitive titanium oxide thin film according to claim 1 is incorporated in a sensor section. 5. The composition ratio of the metal element portion is at least one selected from the group consisting of iron, cobalt, and nickel.
0.01 to 3 mol% of the group 8 element and 0.1 to 0.1% of the noble metal element.
A film forming solution containing 0.5 to 5 mol% and the balance being titanium is formed on a ceramic substrate, dried, pre-treated in an oxidizing atmosphere, and then reduced to 350 to 600 in a reducing atmosphere. A method for producing a carbon monoxide-sensitive titanium oxide thin film, which comprises sintering at ℃ and further heating in an oxidizing atmosphere. 6. The composition ratio of the metal element portion is at least one selected from the group consisting of iron, cobalt, and nickel.
0.01 to 3 mol% of the group 8 element and 0.1 to 0.1% of the noble metal element.
A film forming solution containing 0.05 to 5 mol% and lithium in an amount of 0.01 to 0.5 mol%, with the balance being titanium,
After forming a film on a ceramic substrate, drying, performing a pretreatment in an oxidizing atmosphere, and then performing 350 to 600 in a reducing atmosphere.
A method for producing a carbon monoxide-sensitive titanium oxide thin film, which comprises sintering at ℃ and further heating in an oxidizing atmosphere. 7. The production of a carbon monoxide-sensitive titanium oxide thin film according to claim 5, wherein the film-forming liquid is a mixture of a precursor solution containing titanium and a noble metal and a TiO ₂ sol or TiO ₂ fine powder. Method. 8. The method for producing a carbon monoxide-sensitive titanium oxide thin film according to claim 5, wherein the group VIII element is nickel and the noble metal element is platinum. 9. A method in which a solution of a precursor containing titanium and a noble metal element is mixed with a TiO ₂ sol or a TiO ₂ fine powder so that the weight conversion ratio to TiO ₂ is 1: 3 to 3: 1. Item 10. The method for producing a carbon monoxide-sensitive titanium oxide thin film according to any one of Items 6 to 8. 10. The carbon monoxide-sensitive titanium oxide thin film according to claim 7, wherein the precursor is synthesized mainly using an alkoxide of titanium or a derivative thereof, an amino acid and a platinum salt. Production method.