WO1992017483A1

WO1992017483A1 - Hydrogen sulfide gas sensor and precursor compound for manufacture of same

Info

Publication number: WO1992017483A1
Application number: PCT/US1992/002418
Authority: WO
Inventors: Tommie L. Royster, Jr.; Gustavo R. Paz-Pujalt; Dilip Jumar Chatterjee; Carl A. Marrese
Original assignee: Eastman Kodak Company
Priority date: 1991-03-29
Filing date: 1992-03-24
Publication date: 1992-10-15

Abstract

A tungsten carboxylate precursor compound useful for coating interdigitated electrodes used in hydrogen sulfide gas sensors is disclosed. A method of coating electrodes with the precursor compound using a precise solution casting technique such as spin-coating or casting, dip-casting or spray-casting techniques is also described. Electrodes which are solution coated with the precursor compound may be used to fabricate superior quality chemiresistor sensors for use in hydrogen sulfide gas sensing devices.

Description

HYDROGEN SULFIDE GAS SENSOR AND PRECURSOR COMPOUND FOR MANUFACTURE OF SAME

Technical Field xnis invention relates to hydrogen sulfide gas sensors. In particular, it relates to a chemiresistor coating for electrodes used in hydrogen sulfide gas sensors.

Background Art

Hydrogen sulfide is a highly toxic gas. Although the gas has a strong odor, it has the insidious ability to temporarily deaden the human sense of smell. Thus, the health of unknowing victims exposed to toxic levels of hydrogen sulfide gas may rapidly deteriorate without prior warning. Excessive and prolonged exposure can result in death. Therefore, there is an important benefit in being able to detect the presence of hydrogen sulfide gas in the environment.

In addition to the above environmental concerns, the presence of sulfide during the production of photographic products may directly affect the quality of the product. The extent to which one can control the sulfide content in either the atmosphere or in the photographic product manufacturing process depends on the ability to measure it. Therefore, the detection and quantitative analysis of sulfides, even at trace amounts (i.e., ng/ml) , must be precise.

A chemiresistor sensing device generally contemplates the use of a power supply transmitting current through a sensor which contains a semiconductor material, such as a metal oxide. The semiconductor material behaves as a chemiresistor. A chemical influence can be caused by the ambient gas interacting with the semiconductor material and can be monitored by a change in the resistance or conductance of the material by the use of electrodes which transmit the change in conductance to a monitor or detector means, such as a voltmeter.

Chemiresistor gas sensors using semiconductor materials comprised of thin film metal oxides, such as tungsten oxide, have shown good sensitivity for detecting reducing gases, such as hydrogen, anhydrous ammonia, hydrazine, propane, butane, methyl alcohol, ethyl alcohol and hydrogen sulfide (H₂S) .

Chemiresistor sensors which incorporate thin films of tungsten oxide as the sensing material have been known to respond selectively and sensitively to hydrogen sulfide gas. The exposure of tungsten oxide to hydrogen sulfide gas results in a decrease in the resistance of the sensing metal oxide. A measurement of the decrease in the resistance of the sensing metal oxide can be used to determine the concentration of the hydrogen sulfide gas. Certain known chemiresistor sensors comprise a resistor layer, such as a heater resistor, an electrical connection to the heater, a support layer, such as an alumina substrate, a conductor layer (often composed of interdigitated electrodes) and a deposited chemical sensing layer most frequently comprised of tungsten oxide (See for example, Jones et al., U.S. Patent No. 4,822,465).

The manner in which the tungsten oxide semiconductor material is applied to the electrodes is of particular importance because the microstructure resulting from the method or technique of depositing the tungsten oxide layer can affect both the selectivity and sensitivity of the tungsten oxide layer to hydrogen sulfide gas.

The sensors described in Willis et al., U.S. Patent No. 4,197,089, describe hydrogen sulfide gas sensors with improved selectivity to hydrogen sulfide gas, which comprise a chemically formed sensor film of tungsten trioxide produced by decomposing a droplet of tungsten salt contained in solution and deposited on the sensor. The patent also discloses a physically formed sensor film of tungsten trioxide which is produced by sintering tungsten trioxide in the powder form on the electrode surface.

One major disadvantage inherent in the above techniques is an inability to precisely manipulate the microstructure of the film by depositing powdered tungsten oxide sensing material and sintering the powder or, placing a drop of an aqueous solution containing a tungsten salt over the electrodes followed by thermal decomposition. Placement of the film on the electrode by these techniques may not be controllable. This imprecision could inhibit the sensitivity and selectivity of the sensing film, which are believed to be directly related to the sensing film's microstructure. Since the method used to deposit the thin film will dictate the microstructure of the metal oxide film and, since the microstructure of the metal oxide film may determine the selectivity and sensitivity toward the reducing gas of interest, the method used to deposit the sensing film is very important to its sensing abilities.

Another method for depositing thin films of tungsten oxide on electrodes is referred to in Jones et al., U.S. Patent No. 4,822,465, which discussed what is known as a radio frequency sputtering technique. This technique contemplates a deposit of the sensing film by sputtering the film on to the electrodes which are, in turn, supported by a substrate. One of the shortcomings of depositing sensing films by sputtering techniques occurs when dopants are added to the sensing film. If it is desirable for the sensing film composition to contain a dopant, it is preferred that there be a uniform mix of sensing compound to the added dopant, to provide consistency in the electrical properties of the film. This type of uniform mix is referred to as uniform doping. Non-uniform doping may result in concentrations in which there is either less than or more than the optimal percentage of dopant in the sputtered thin film.

In addition, the radio frequency sputtering technique inherently introduces varying levels of stress into the thin film which may effect the sensing capability of the thin film. This stress results from the inability of the sputtering technique to deposit the sensing film uniformly over the surface of the electrode. This depositing technique is also inadequate for conforming to irregular substrates. Until development of the present invention, application of tungsten oxide thin films using precise applications, such as spin-casting (also known in the industry as spin-coating) , dip-casting and spray-casting solution techniques (hereinafter collectively referred to as solution casting techniques) , has been unavailable. Until development of the present invention, known technology was unable to provide for the precise and uniform application of tungsten oxide films onto electrodes, because tungsten oxide is insoluble in solvents used in solution casting techniques.

Thus, a need still exists for improving the application of thin film metal oxides and, in particular, tungsten oxide to electrodes contained in hydrogen sulfide and other reducing gas sensors.

Disclosure of Invention

In answer to these unmet needs, a new compound is disclosed which can be thermally decomposed to form tungsten trioxide (W0₃) and, as such, is a precursor of tungsten oxide. The new compound is readily soluble in organic solvents, including aromatic and aliphatic solutions. When this compound is dissolved in organic solvents, it can be applied to the electrodes contained in a hydrogen sulfide gas sensor using a precise solution casting technique. The inventive compound is of the Formula I

wherein W is tungsten, 0 is oxygen, C is carbon, and R, and R_j independently are hydrocarbons. The compound can be referred to as a tungsten carboxylate, and as such, is a precursor to tungsten oxide. In addition, a new method of preparing the tungsten carboxylate precursor is disclosed. A coated electrode, or a plurality of coated interdigitated electrodes, can be fabricated using the novel compound by dissolving the precursor compound in a solvent to form a precursor solution. The controlled coating of the electrodes is then accomplished by coating the electrode with the precursor solution using a standard solution casting technique of the type commonly employed for spin- casting or spin-coating, dip-casting or spray coating or casting. The electrode is then heated by conventional curing means to decompose the tungsten carboxylate precursor which has been deposited thereon by the desired solution casting technique. The decomposition of the uniform thin precursor layer results in a controlled uniform thin layer of tungsten oxide. Once the decomposition occurs, the electrode coating is capable of reacting highly sensitively and selectively to hydrogen sulfide gas.

In its broadest sense, the invention also encompasses a hydrogen sulfide gas sensor having an electrode coated with the novel precursor and a method for fabricating a hydrogen sulfide gas sensor incorporating the coated electrode.

The object of the invention is to improve hydrogen gas sensors. It is a feature of this invention to enable the fabricator of hydrogen sulfide gas sensors to coat interdigitated electrodes and other electrodes with a precursor which, when acted upon by heat, decomposes into tungsten oxide. One advantage of the present invention is the ability to coat electrodes used in hydrogen sulfide gas sensors more precisely, and thereby create -a more consistent microstructure on to the electrode. A further advantage of the invention is the improved substrate conformity of the tungsten oxide thin film over the interdigitated electrodes. If the electrode surface were to contain small indentations or protrusions, these imperfections could be compensated for by precisely applying the precursor compound using solution casting techniques.

A still further advantage of the invention is the improved rheology or "wetting ability" of the compound being deposited on the electrode substrate thereby making it useful in thin film spin-casting or coating, dip-casting and spray techniques, collectively referred to as "solution casting techniques".

A still further advantage of the invention is the ability to uniformly mix a dopant with the precursor compound and apply a uniform mixture of precursor and dopant through the use of a precise solution casting technique.

A still further advantage of the invention is the ability to reduce the stress of a thin sensing film which results from non-uniform application. In particular, it is noted that the stress of thin films deposited by the solution casting technique is significantly less than the stress measured in thin films deposited by use of a radio frequency sputtering technique.

A still further advantage of the invention is the ability to, more efficiently and cost effectively, coat an electrode through the use of solution casting techniques. A still further advantage of the invention is the ability to more precisely fabricate and manipulate tungsten oxide thin films used as sensing films on electrodes for the sensing of hydrogen sulfide gas.

A very precise method for fabricating thin films of tungsten oxide has been afforded by coupling the novel precursor compound with precise solution casting techniques.

Brief Description of Drawings

Figure 1 is a cross-sectional plan view of a chemiresistor sensor according to the present invention. Figure 2 is a schematic diagram showing a thin film of tungsten carboxylate precursor deposited on interdigitated electrodes.

Figure 3 is a graphical representation of a typical relationship between hydrogen sulfide concentration and output voltage of a hydrogen sulfide gas sensor of the type shown in Figure 1.

Modes of Carrying Out The Invention

The objects, features and advantages of the present invention will become more evident as the invention is more fully described herein.

The inventive compound is a novel tungsten carboxylate having the formula

wherein W is tungsten, O is oxygen, C is carbon, and , and ₂ individually are hydrocarbons. There is believed to be delocalized negative charges within the molecule which are represented by the dotted line. In particular, a tungsten carboxylate has been prepared wherein R₁ and _j are individually situated hydrocarbons, such as C^-. The hydrocarbons are aromatic or aliphatic hydrocarbons, but it is believed that R, and R_j individually could be any hydrocarbon chain, so long as the overall solubility and rheology properties of the tungsten carboxylate compound in aliphatic or aromatic hydrocarbon solvents are not significantly changed. The novel tungsten carboxylate takes the form of a blue glassy solid and is sensitive to air and moisture. The compound decomposes in the presence of moisture accordingly:

Fast OW(OOCR)₂ + H₂0 _→ OW(OOCR) (OH) + HOOCR

Slow OW(OOCR)(OH) + H_jO _, OW(OH)₂ + HOOCR

The solubility of the compound in aliphatic and aromatic hydrocarbon makes it useful in solution casting techniques, such as spin-casting, dip-casting and spray-casting.

The inventive tungsten carboxylate may be synthesized according to the following reaction:

RCOOH + Na _, RCOO^* Na* + _H₂

4RC00" Na* + OWC1. _, OW(OOCR), + 4NaCl + Side Product (VI) (IV)

The method of preparing the compounds comprises the steps of reacting an alkali metal with an organic acid to form an acid-salt solution; reacting the acid-salt solution with a solution containing tungsten (VI) oxycholoride in an aromatic solvent to form a reaction mixture; refluxing the reaction mixture to form a tungsten (IV) charboxylate in a refluxed mixture; and extracting the tungsten (IV) carboxylate from the refluxed mixture. Suitable aromatic solvents include toluene and benzene. Suitable alkali metals include sodium, potassium and lithium. Suitable organic acids include 2-ethyl hexanoic acid and 4-phenyl butyric acid or 3-phenyl butyric acid. An example of a sample preparation method of the inventive compound is set forth in Example I below.

EXAMPLS.,,1

Working in a conventional dry box, 3.43 g (10.0 mrool) of tungsten (VI) oxychloride was placed in a 200 ml Schlenk flask. Toluene (65 ml) was syringed onto the sample. Freshly cut sodium (0.949 g, 41.3 mmol) was placed into a 250 ml 2- neck flask. The 2-neck flask was connected to the Schlenk flask by a bent elbow. The second neck of the 2-neck flask was stoppered using a rubber septum. Outside of the dry box, 45 ml of 2-ethyl hexanoic acid was syringed onto the sodium and the mixture was heated below the boiling temperature of 113*C until the sodium had completely reacted. The adapter to the Schlenk flask was purged with nitrogen gas before opening the system to a connected bubbler. The 2-ethyl hexanoic acid-salt solution was added to the tungsten (IV) oxychloride solution while stirring at room temperature. Under a purge of nitrogen gas, a condenser was connected to the Schlenk flask. The reaction mixture was refluxed by using an oil bath heated at 125'C. After 16 hours, the solution was cooled to room temperature under a purge of nitrogen. The toluene was removed by vacuum distillation. To remove the excess 2- ethyl hexanoic acid, a dynamic vacuum was used while heating with a 110'C oil bath. The glassy blue product was then extracted from the sodium chloride refluxed mixture with pentane.

The tungsten carboxylate is an ideal precursor for providing a tungsten oxide thin film over electrodes used in hydrogen sulfide gas sensors. It is well known that tungsten oxide is an ideal film for coating of electrodes in hydrogen sulfide gas sensors, because tungsten oxide films have shown good selectivity and sensitivity to hydrogen sulfide gas. The resistance applied to a current passed through the chemiresistor comprised of a tungsten oxide film coating on electrodes decreases when hydrogen sulfide is in the ambient gas. The decrease in resistance is believed to be caused by an exchange mechanism between O"² and S'² with the production of WS₂, which has a greater conductivity than W0₃. The resulting ion exchange between O"² and S"² can be measured by an increase in voltage at a detector device. This is accomplished by having the sensor connected to a standard operational amplifier circuit incorporating the detector device. The decrease in resistance translates into an increase in voltage which is relative to the concentration of the hydrogen sulfide gas. Figure 3 shows the relationship between the concentration of the hydrogen sulfide gas and the increase in voltage of the sensing device caused by the decreased resistance of the chemiresistor. The present invention provides an improved film coating of tungsten oxide on the electrodes used in hydrogen sulfide gas sensors. The film of the novel tungsten carboxylate precursor of the present invention is applied or deposited on the electrodes, preferably arranged in an interdigitated configuration, by a known precise solution casting technique. It is not possible to solution cast tungsten oxide because it is insoluble in solvents typically used in solution casting processes. The inventive tungsten carboxylate precursor can be applied to electrodes, including interdigitated electrodes supported on an inert substrate by precise solution casting techniques. This is possible because the precursor is soluble in the solvents used in solution casting techniques and has necessary rheology or surface wetting properties. The resulting thin film decomposes to tungsten oxide when heated to approximately 350'C by convention curing methods, such as baking.

Figure 1 shows a sensor 22 of the present invention having a substrate support layer 10 made from inert materials, such as aluminum oxide and containing or having mounted thereon conductors or a conductor layer 11, 12, 13, 14. The conductor layer 11, 12, 13 and 14 are made from conducting material, such as gold or palladium. Electrical current can be passed from a standard power supply via a conducting wire or means through the conducting layer 11. The conducting layer 11, is in contact with an adjacent rhodium-oxide based resistor or heater layer 15. The resistor layer 15 generates heat from the conducted current. The current is then passed from the resistor layer 15 to the conductor 14 and via a standard conducting means back to a power supply. On the upper side of the resistor layer 15, there is a silicon-oxide based dielectric layer 16, upon which there is mounted an electrode layer • comprising electrodes 17 and 17'. A sensing film 18 is deposited over the electrodes 17 and 17'

The dielectric layer 16 functions in the sensor 22 to shield the resistor layer 15 from reacting directly with the sensing film 18.

The resistor layer 15, heats the coated electrodes 17 and 17' to improve sensitivity and selectivity of the sensing film 18, as is commonly done in gas sensor technology.

Electrical current is also passed from a power supply through a conductor 12, to the electrode 17. The current transfers to electrode 17' and is passed through the conductor layer 13 which is connected to a standard operational amplifier circuit with a detector means, of the type known in the art.

The electrode layer 17 and 17', is preferably arranged as interdigitated electrodes which have been coated with the sensing film 18, using a solution casting technique. The sensing film 18, is a tungsten oxide thin film formed from thermally decomposing the novel tungsten carboxylate compound. The sensing film 18, selectively reacts with hydrogen sulfide gas in the ambient atmosphere to cause an increase in the conductance of a current passed through the electrodes 17 and 17'. As such, the film acts as a chemiresistor. Figure 2 shows a schematic representation of the tungsten oxide thin sensing film 18, deposited on a set of interdigitated array of conducting electrodes 17 and 17•. Figure 3 is graphical representation of a typical relationship between hydrogen sulfide concentration and output voltage of a hydrogen sulfide gas sensor 22 of the type shown in Figure 1. The trend shows that there is an increase in output voltage with an increase in hydrogen sulfide gas concentration.

It is believed that tungsten oxide reacts with hydrogen sulfide gas according to the following mechanism

It is also believed that the introduction of oxygen gas (0₂) reacts with the resulting tungsten sulfide (WS₂) to reform the tungsten oxide film.

WS₂ + 0_{2 →} W0₃

The sensors fabricated according to the present invention have improved substrate conformity, a more uniform doping ability, less potential stress in the films and are more conveniently fabricated than known methods.

Accordingly, the preferred embodiments of the invention have been illustrated and described in detail. It is to be understood that numerous changes and variations can be made in the composition and manufacture of the invention without departing from the spirit of the invention, the scope of which is defined by the appended claims.

Claims

Claims:

1. A compound of the formula I

wherein W is tungsten, 0 is oxygen, C is carbon, and R, and R₂ independently are hydrocarbons.

2. The compound according to claim 1 wherein the hydrocarbons are, individually, aliphatic or aromatic hydrocarbons.

3. The compound according to claim 1 wherein the hydrocarbons are saturated hydrocarbons.

4. The compound according to claim 1 wherein the hydrocarbons are C^H^.

5. The method of preparing the compound according to claim 1 comprising the steps of reacting an alkali metal with an organic acid to form an acid-salt solution; reacting the acid-salt solution with a solution containing tungsten (VI) oxychloride in an aromatic solvent to form a reaction mixture; refluxing the reaction mixture to form a tungsten (IV) carboxylate according to claim 1 in a refluxed mixture; and extracting the tungsten (IV) carboxylate according to claim 1 from the refluxed mixture.

6. The method according to claim 5 wherein the aromatic solvent is selected from the group comprising toluene and benzene.

7. The method according to claim 5 wherein the alkali metal is selected from the group comprising sodium, potassium and lithium.

8. The method according to claim 5 wherein the organic acid is selected from the group comprising 2-ethyl hexanoic acid, 3-phenyl butyric acid and 4-phenyl butyric acid.

9. A method of coating an electrode for use in a hydrogen sulfide sensor comprising the steps of dissolving a compound according to claim 1 in a solvent to form a tungsten carboxylate precursor solution; depositing the precursor solution on an electrode using a solution casting technique to form a thin film coating over said electrode; and heating said coated electrode so that the coating decomposes and becomes tungsten oxide.

10. A method according to claim 9 wherein the solvent is an aromatic or aliphatic solvent.

11. A method according to claim 9 wherein the electrode comprises a plurality of interdigitated electrodes.

12. A hydrogen sulfide gas sensor comprising an electrode coated with the compound of claim 1.

13. A hydrogen sulfide gas sensor having an electrode fabricated according to claim 9.

14. A hydrogen sulfide gas sensor comprising a substrate support layer; a conductor layer; a resistor layer; a dielectric layer; and a layer of interdigitated electrodes coated with a compound according to claim 1.