Gas sensor element, gas sensor, and method for measuring hydrogen gas or carbon monoxide gas concentration using the same

Abstract

To provide a gas sensor element capable of detecting flammable gases such as hydrogen gas and carbon monoxide gas, without requiring the use of precious metals or high-temperature conditions, and a hydrogen gas or carbon monoxide gas sensor using the same. [Solution] The gas sensor element is characterized by comprising a diamond-like carbon film, and detecting the presence of the target gas based on the mass change of the film caused by the action of the target gas on the diamond-like carbon film. In a gas sensor element 1 that applies this principle to a quartz crystal microbalance (QCM) sensor, a diamond-like carbon film 13 is formed directly or via an intermediate layer on at least one surface of the counter electrodes 12A sandwiching the quartz substrate 11. A gas sensor using this element exhibits high specificity for hydrogen gas and carbon monoxide gas.

Description

Technical Field

[0001] The present invention relates to a gas sensor element, a gas sensor, and a method for measuring the concentration of hydrogen gas or carbon monoxide gas using the same.

Background Art

[0002] In an environment where flammable gases such as hydrogen gas and carbon monoxide gas are handled, sensors for detecting gas leakage are installed from the viewpoint of accident prevention. As gas sensors used for such applications, catalytic combustion type gas sensors (see, for example, Patent Documents 1 to 3), semiconductor type gas sensors (see, for example, Patent Documents 4 and 5), etc. are known.

[0003] In a catalytic combustion type gas sensor, it is common to use expensive noble metal wires such as platinum and palladium, and depletion of future material resources and cost become problems. Further, in a semiconductor type hydrogen gas sensor, there is a problem that the energy consumption increases because a high temperature of 200°C or higher is required for operation.

[0004] By the way, in recent years, a quartz crystal microbalance (QCM) has been used as a means for detecting an extremely minute mass change. This utilizes the property that in a quartz crystal resonator that resonates when an alternating electric field is applied with a quartz substrate sandwiched between counter electrodes, the resonance frequency shifts in response to a mass change when a substance adheres to the electrode surface. As such an applied technology, for example, in Patent Document 6, a gas sensor for detecting a low-concentration organic compound gas (such as toluene) in which a layer of metal complex-included zeolite is formed on a QCM electrode has been proposed. In this organic compound gas sensor, it has been shown that when an organic compound gas is adsorbed on the metal complex-included zeolite, the vibration frequency of the QCM decreases due to a slight increase in mass (see FIGS. 2 to 4 of Patent Document 6).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] This invention has been made in view of the above circumstances, and aims to provide a gas sensor element capable of detecting flammable gases such as hydrogen gas and carbon monoxide gas, without requiring the use of precious metals or high-temperature conditions, and a hydrogen gas or carbon monoxide gas sensor using the same. [Means for solving the problem]

[0007] The inventors, through diligent research to solve the above problems, discovered that a sensor element in which a diamond-like carbon film is formed on a QCM electrode experiences an increase in resonance frequency when exposed to hydrogen gas or carbon monoxide gas during its resonance. As shown in Patent Document 6, normally, in QCM, the resonance frequency decreases when a substance adheres to the electrode, causing an increase in mass. Therefore, the increase in resonance frequency when exposed to hydrogen gas or carbon monoxide gas is quite unexpected. Furthermore, it was found that the resonance frequency of the sensor element returns to its original value when exposure to hydrogen gas or carbon monoxide gas is removed, indicating that this change is reversible. Further investigation by the inventors led to the conclusion that this increase in resonance frequency was caused by a decrease in mass resulting from the reaction or substitution of nitrogen atoms and adsorbed water molecules contained in the diamond-like carbon film with hydrogen gas or carbon monoxide gas when the film is exposed to these gases. In other words, nitrogen atoms and water molecules from nitrogen gas present in the air are adsorbed on the surface of the diamond-like carbon film. When this film is exposed to hydrogen gas, the adsorbed nitrogen atoms react with the hydrogen gas to form ammonia and are desorbed, or a substitution with hydrogen gas occurs. When exposed to carbon monoxide gas, the adsorbed water molecules react with the carbon monoxide gas to form methanol and are desorbed, or a substitution with carbon monoxide gas (which is thought to have a different adsorption capacity per unit area than water molecules) occurs. It is thought that these desorptions and substitutions cause a decrease in mass. As for reversibility, it is presumed that when exposure to hydrogen gas or carbon monoxide gas is removed, nitrogen gas and water molecules present in the air are re-adsorbed, returning the mass to its original level. In other words, this sensor element can specifically and reversibly increase its resonance frequency in response to hydrogen gas and carbon monoxide gas. The present invention was completed based on these findings and provides the following.

[0008] (1) The present invention is a gas sensor element comprising a diamond-like carbon film, characterized in that it detects the presence of the target gas based on the fact that the gas to be detected acts on the diamond-like carbon film, causing a change in the mass of the film.

[0009] (2) The present invention also relates to the sensor element described in item (1), wherein a diamond-like carbon film is formed directly or via an intermediate layer on at least a portion of a quartz crystal having electrodes on its surface.

[0010] (3) The present invention also relates to a gas sensor element according to item (1) or (2), wherein a diamond-like carbon film is formed directly or via an intermediate layer on at least one surface of a counter electrode sandwiching a quartz substrate.

[0011] (4) The present invention is also a gas sensor element according to any one of items (1) to (3), which is a sensor element for hydrogen gas or carbon monoxide gas.

[0012] (5) The present invention also relates to a gas sensor element according to any one of items (1) to (4), wherein the diamond-shaped carbon is nitrogen-containing diamond-shaped carbon.

[0013] (6) The present invention also relates to a gas sensor element according to any one of items (1) to (5), wherein the diamond-like carbon film has a thickness of 100 to 500 nm.

[0014] (7) The present invention also relates to a gas sensor element according to any one of items (1) to (6), wherein the diamond-like carbon film is formed by an aggregate of diamond-like carbon flakes having a diameter of 0.1 to 100 μm.

[0015] (8) The present invention is a gas sensor element according to any one of items (3) to (6), characterized in that the counter electrode is made of aluminum or titanium.

[0016] (9) The present invention is also a gas sensor characterized in that when an alternating electric field is applied to the gas sensor element described in any one of items (3) to (7) to vibrate the crystal substrate at the resonant frequency and the gas sensor element is exposed to the object to be detected, the presence of hydrogen gas and / or carbon monoxide gas in the object to be detected is detected by the rise in the resonant frequency.

[0017] (10) The present invention is characterized in that, while applying an alternating electric field to the gas sensor element according to any one of (3) to (7) and vibrating the quartz substrate at the resonance frequency, exposing the gas sensor element to each of two or more calibration gases containing hydrogen gas or carbon monoxide gas having different known concentrations, thereby obtaining the degree of increase in the resonance frequency of the gas sensor element with respect to each calibration gas and creating a calibration curve; and applying an alternating electric field to the gas sensor element and vibrating the quartz substrate at the resonance frequency, and then applying the amount of increase in the resonance frequency when this is exposed to the detection target to the calibration curve, thereby calculating the concentration of hydrogen gas or carbon monoxide gas in the detection target. The present invention also provides a method for measuring the concentration of hydrogen gas or carbon monoxide gas in a detection target, which includes such a concentration measurement step.

Effects of the Invention

[0018] According to the present invention, there are provided a gas sensor element that can detect hydrogen gas or carbon monoxide gas, which is a combustible gas, without requiring the use of noble metals or high-temperature conditions, and a hydrogen gas or carbon monoxide gas sensor using the same.

Brief Description of the Drawings

[0019] [Figure 1] FIG. 1 is a front view showing the gas sensor element 1 of the present invention. [Figure 2] FIG. 2 is a side view showing the gas sensor element 1 of the present invention. [Figure 3] FIG. 3 is a plot showing the change in resonance frequency when each of the QCM sensors formed with DLC, N-DLC@N10%, or N-DLC@N50% films is exposed to 20% hydrogen gas-containing nitrogen gas or 100% nitrogen gas. (a) is for the QCM sensor formed with the DLC film, (b) is for the QCM sensor formed with the N-DLC@N10% film, and (c) is for the QCM sensor formed with the N-DLC@N50% film. [Figure 4]Figure 4 is a plot showing the frequency change in a QCM sensor with an N-DLC@N50% film formed on it when the hydrogen gas concentration is changed. [Figure 5] Figure 5 is a plot showing the change in resonance frequency when a QCM sensor with a DLC, N-DLC@N10%, or N-DLC@N50% film is exposed to nitrogen gas containing 20% ​​carbon monoxide or 100% nitrogen gas, respectively. (a) is for the QCM sensor with a DLC film, (b) is for the QCM sensor with an N-DLC@N10% film, and (c) is for the QCM sensor with an N-DLC@N50% film. [Figure 6] Figure 6 is a plot showing the frequency change in a QCM sensor with an N-DLC@N50% film formed on it when the carbon monoxide gas concentration is changed. [Figure 7] Figure 7 shows electron microscope images of flaky N-DLC@N90% obtained by peeling and recovering an N-DLC@N90% film formed on a glass substrate with hydrofluoric acid, and then pulverizing it by sonication in methanol. [Modes for carrying out the invention]

[0020] The following describes one embodiment of the gas sensor element, one embodiment of the gas sensor, and one embodiment of the method for measuring the concentration of hydrogen gas or carbon monoxide gas. However, the present invention is not limited in any way to the following embodiments or embodiments.

[0021] [Gas sensor element] The gas sensor element of the present invention comprises a diamond-like carbon film, and is characterized by detecting the presence of the target gas based on the mass change of the film caused by the action of the target gas on the diamond-like carbon film. As described above, when hydrogen or carbon monoxide acts on the diamond-like carbon film, it causes a slight decrease in the mass of the film. By combining this film with a sensor element that detects this slight decrease in mass, a highly sensitive sensor for these gases can be obtained. Preferred examples of such sensor elements include the quartz crystal microbalance (QCM) sensor and the surface acoustic wave (SAW) sensor, but are not particularly limited.

[0022] In these sensor elements, a diamond-like carbon film is formed directly or via an intermediate layer on at least a portion of a quartz crystal with electrodes on its surface. For example, in a QCM sensor, counter electrodes are formed on both sides of the quartz substrate, and in a SAW sensor, for example, a comb-shaped electrode is formed on the surface of a spherical quartz crystal. In any of these sensors, a change in the mass of the diamond-like carbon film can be detected as a change in the resonant frequency. Among these, the QCM sensor is particularly preferred because its sensor element structure is relatively simple and miniaturization can be expected. In the following description, a QCM sensor is described as one embodiment of the present invention, but the present invention is not limited in any way to the following embodiment.

[0023] The gas sensor element of this embodiment has a diamond-like carbon film formed directly or via an intermediate layer on at least one surface of the counter electrodes sandwiching a quartz substrate. One embodiment of the gas sensor element of the present invention will be described with reference to the drawings. Figure 1 is a front view of the gas sensor element 1 of the present invention. Figure 2 is a side view of the gas sensor element 1 of the present invention.

[0024] The gas sensor element 1 comprises a quartz substrate 11, electrodes 12A and 12B formed opposite each other on its front and back surfaces, and lead wires 14A and 14B connected to electrodes 12A and 12B, respectively. The quartz substrate 11 is a disc-shaped AT-cut substrate. The electrodes 12A and 12B are made of metals such as gold, aluminum, titanium, platinum, chromium, and nickel, formed by vapor deposition or sputtering. Of these metals, aluminum or titanium is preferred as the material for electrodes 12A and 12B. According to the inventor's findings, using aluminum or titanium as the material for electrodes 12A and 12B can increase sensitivity to the target gas. The reason for this effect is not entirely clear, but it is presumed that when electrodes are formed from these metals, the interfacial defects between electrode 12A and the diamond-like carbon film 13 are reduced, improving their adhesion and increasing the vibration energy of the QCM sensor. From a resource and cost perspective, aluminum is more preferred among these. A diamond-like carbon film 13 (hereinafter also referred to as the diamond-like carbon film 13) is formed on the surface of the electrode 12A. An intermediate layer made of a different material may be formed between the electrode 12A and the diamond-like carbon film 13. In this case, the diamond-like carbon film 13 will be formed on the surface of the electrode 12A via the intermediate layer. The material of the intermediate layer is not particularly limited.

[0025] It is preferable to use a quartz substrate 11 with a resonant frequency of approximately 1 to 40 MHz. By using a quartz substrate 11 with a resonant frequency in this range, the detection sensitivity of hydrogen gas and carbon monoxide gas by QCM can be improved.

[0026] The diamond-like carbon film 13 captures the target of detection in this invention. Diamond-like carbon is also called DLC (Diamond-Like Carbon) and has splices like diamond. 3 Bonds and sp like graphite 2It forms an amorphous (non-crystalline) hard film that also possesses bonds. Therefore, in the Raman spectrum, the diamond-like carbon film 13 exhibits the G band (1340 cm⁻¹) derived from the graphite structure. -1 (Nearby) and the D-band (1590cm) derived from the diamond structure -1 A somewhat large, broad peak is observed in the vicinity. Various known methods can be cited for forming the diamond-like carbon film 13, but for example, a method such as physical vapor deposition (PVD) using a high-frequency magnetron sputtering deposition apparatus with solid graphite as the target can be cited. When forming the diamond-like carbon film 13 by PVD, the atmospheric gas is preferably air, argon, or nitrogen. Alternatively, the diamond-like carbon film 13 may be formed by plasma chemical vapor deposition (CVD) using a hydrocarbon gas such as CH4. When forming the diamond-like carbon film 13 by CVD, hydrogen derived from the source gas is contained inside the diamond-like carbon film 13. The use of a diamond-like carbon film 13 containing hydrogen in this way is also included in the present invention. The thickness of the diamond-like carbon film 13 is preferably 50 nm or more from the viewpoint of obtaining good sensitivity to the gas to be detected, and preferably 500 nm or less from the viewpoint of obtaining good adhesion to the substrate, and particularly preferably 100 nm to 500 nm. Furthermore, the detection sensitivity tends to improve as the thickness of the diamond-like carbon film 13 increases. This improvement in detection sensitivity is thought to be due to the increase in surface roughness, i.e., the increase in surface area, of the diamond-like carbon film 13 as the film thickness increases.

[0027] As described above, the diamond-like carbon film 13 may be formed on the surface of the quartz substrate 11 directly or via an intermediate layer by PVD or CVD. Alternatively, a drop-cast method may be used, in which diamond-like carbon, prepared separately, is crushed into thin flakes, dispersed in a solvent to form a dispersion, and the dispersion is dropped onto the surface of the quartz substrate 11 or an intermediate layer formed on its surface, and the solvent is evaporated to form the diamond-like carbon film 13. The diamond-like carbon film 13 formed in this way is formed by the aggregation of crushed diamond-like carbon flakes. Preferably, the diameter of the diamond-like carbon flakes is about 0.1 to 100 μm. Since the surface area of ​​the diamond-like carbon film 13 formed by the aggregation of diamond-like carbon flakes is larger than that of the film formed by PVD or CVD, hydrogen gas and carbon monoxide gas can be detected more efficiently. Furthermore, as described above, the diameter of the diamond-like carbon flakes is approximately on the order of μm, so the diamond-like carbon film 13 formed by the drop casting method will naturally be thicker than the diamond-like carbon film 13 formed by PVD or CVD (preferably 500 nm or less, as described above).

[0028] In this invention, hydrogen gas and carbon monoxide are examples of gases that can be detected. The diamond-like carbon film 13 incorporates nitrogen gas present in the environment during film formation, or adsorbs nitrogen gas present in the air, resulting in nitrogen-containing diamond-like carbon containing nitrogen atoms in its structure. As already mentioned, when hydrogen gas comes into contact with the diamond-like carbon film 13, the nitrogen atoms contained in the structure of the diamond-like carbon film 13 react with the hydrogen gas and are desorbed as ammonia. Also, when carbon monoxide gas comes into contact with the diamond-like carbon film 13, water molecules adsorbed on the diamond-like carbon film 13 react with the carbon monoxide gas and are desorbed as methanol. This desorption of nitrogen atoms and adsorbed water reduces the mass of the diamond-like carbon film 13, and the resonant frequency of the quartz substrate 11 increases. When contact with these gases ends, nitrogen atoms are supplied from nitrogen gas in the air to the defect sites created by the desorption of nitrogen atoms from the diamond-like carbon film 13, and the desorbed adsorbed water is supplied from the air, so the diamond-like carbon film 13 returns to its original mass. As already mentioned, it is also possible that the nitrogen gas and water molecules physically adsorbed on the surface of the diamond-like carbon film 13 undergo a mass change due to mutual substitution with the hydrogen gas or carbon monoxide gas being detected. These mechanisms based on desorption and substitution are all speculations, and it is not necessarily certain whether one or both are correct. In any case, the fact remains that in the gas sensor element 1 of the present invention, the resonant frequency of the quartz substrate 11 rises when the presence of hydrogen gas or carbon monoxide gas is detected, and the resonant frequency of the quartz substrate 11 returns to its original state when its presence is eliminated.

[0029] As described above, the presence of nitrogen atoms contained in the structure of the diamond-like carbon film 13 is particularly important for detecting hydrogen gas. For this reason, it is preferable to use an atmosphere gas containing 10 to 100 vol% of nitrogen-based gas when forming the diamond-like carbon film 13. In this case, it is preferable to use an inert gas such as argon gas as the atmosphere gas excluding the nitrogen-based gas. In addition to nitrogen gas, other nitrogen-based gases that can be used include ammonia gas and other gases that become nitrogen sources through plasma decomposition during film formation. Furthermore, the amount of nitrogen atoms contained in the structure of the diamond-like carbon film 13 can be increased by ion implanting nitrogen atoms into the formed diamond-like carbon film. Diamond-like carbon films into which nitrogen atoms have been intentionally introduced by means such as adding a nitrogen-based gas to the atmosphere gas during film formation or ion implanting nitrogen atoms into the formed diamond-like carbon film are also called "nitrogen-containing diamond-like carbon films." Even if nitrogen atoms are not intentionally introduced as described above, a certain amount of nitrogen atoms will be incorporated into the structure when the unbonded hands present in the formed diamond-like carbon film come into contact with air after film formation, so it will exhibit a certain level of performance as a gas sensor element. Furthermore, a nitrogen-containing diamond-like carbon film, in which nitrogen atoms have been intentionally introduced as described above, and a diamond-like carbon film that has absorbed nitrogen from the air without the intentional introduction of nitrogen atoms (referred to as a "normal diamond-like carbon film" in this paragraph) can be distinguished, for example, by X-ray photoelectron spectroscopy (XPS) analysis. In other words, a nitrogen-containing diamond-like carbon film will have a surface nitrogen content of 2 atom% or more in XPS analysis, while a normal diamond-like carbon film will have a surface nitrogen content of less than 2 atom% in XPS analysis.

[0030] Furthermore, in addition to the nitrogen atoms mentioned above, the structure of the diamond-like carbon film 13 may contain other heterogeneous elements as impurities, such as hydrogen atoms, boron atoms, sulfur atoms, and various metal atoms. The inclusion of these heterogeneous elements as impurities may provide unique effects in terms of sensitivity and the types of gases that can be detected.

[0031] Although an embodiment of a gas sensor element 1, which is an example of the present invention, has been described above, the present invention is not limited in any way to the above embodiment, and can be implemented with appropriate modifications within the scope of the present invention. For example, in the above embodiment, the diamond-like carbon film 13 was formed only on the surface of electrode 12A, but it may be formed on the surfaces of both electrodes 12A and 12B. In this case, it is preferable that the gas responsiveness can be improved as the surface area of ​​the diamond-like carbon film 13 increases. Also, as already mentioned, in the above embodiment, the diamond-like carbon film 13 was formed directly on the surface of electrode 12A, but it may be formed on the surface of electrode 12A via some kind of intermediate layer.

[0032] Furthermore, although the above embodiment uses the diamond-like carbon film 13 as formed by PVD or CVD, post-processing such as plasma treatment or blasting may be performed on the formed diamond-like carbon film 13 in order to increase its surface area. By increasing the surface area of ​​the diamond-like carbon film 13 in this way, the opportunities for the object to be detected to come into contact with the diamond-like carbon film 13 increase, thereby improving the detection sensitivity of the gas sensor element 1. Also, in order to increase the surface area of ​​the diamond-like carbon film 13, pre-processing such as plasma treatment or blasting may be performed on the quartz substrate 11 before forming the diamond-like carbon film 13.

[0033] [Gas sensor] Next, an embodiment of the gas sensor of the present invention will be described. The gas sensor of the present invention is characterized in that when an alternating electric field is applied to the gas sensor element 1 to vibrate the crystal substrate 11 at its resonant frequency and the gas sensor element 1 is exposed to the object to be detected, the presence of hydrogen gas and / or carbon monoxide gas in the object to be detected is detected by the increase in the resonant frequency of the crystal substrate 11.

[0034] As described above, when the gas sensor element 1 is exposed to hydrogen gas or carbon monoxide gas, the mass of the diamond-like carbon film 13 decreases very slightly, and this slight decrease in mass is measured as a change in the resonant frequency of the quartz substrate 11, i.e., detected by QCM. As also previously mentioned, when the gas sensor element 1 is exposed to hydrogen gas or carbon monoxide gas, the resonant frequency of the quartz substrate 11 increases, and when the exposure is removed, the resonant frequency of the quartz substrate 11 returns to its original value. The gas sensor of the present invention utilizes this property of the gas sensor element 1 to detect the presence of hydrogen gas or carbon monoxide gas.

[0035] The gas sensor of the present invention is configured such that lead wires 14A and 14B, connected to electrodes 12A and 12B of the gas sensor element 1, are connected to an AC power supply, applying an AC electric field to a crystal substrate 11 via electrodes 12A and 12B, and is connected to a frequency counter or the like capable of measuring changes in frequency.

[0036] The frequency of the AC potential applied to the crystal substrate 11 is preferably around 1 to 40 MHz. Therefore, the crystal substrate 11 is selected to have a resonant frequency within this frequency range. The voltage of the AC potential applied to the crystal substrate 11 is preferably around 5 to 20 V, and more preferably around 5 to 10 V.

[0037] Furthermore, various types of devices with opposing electrodes formed on a quartz substrate are commercially available for QCM applications, and these can be obtained and used as appropriate. Such commercially available products can be obtained from companies such as Tama Device Co., Ltd. and Piezo Parts Co., Ltd.

[0038] The gas sensor of the present invention detects the presence of hydrogen gas or carbon monoxide gas by exposing the gas sensor element 1 contained therein to the gas to be measured. Specifically, when hydrogen gas or carbon monoxide gas comes into contact with the gas sensor element 1, the resonant frequency of the quartz substrate 11 contained therein increases. Therefore, when this increase in resonant frequency is detected, the presence of hydrogen gas or carbon monoxide gas is notified to the user by means of light, sound, an instrument indicating the frequency increase, etc. Since this change in resonant frequency is reversible, the resonant frequency returns to its original state when hydrogen gas or carbon monoxide gas is no longer present, and the above notification stops at that time. As already mentioned, this frequency increase is based on the specific reaction of the diamond-like carbon film 13 to hydrogen gas and carbon monoxide gas, so the gas sensor of the present invention has high selectivity for hydrogen gas and carbon monoxide gas.

[0039] [Method for measuring the concentration of hydrogen gas or carbon monoxide gas in the target substance] A method for measuring the concentration of hydrogen gas or carbon monoxide gas in a target object, using the gas sensor element of the present invention described above, is also part of the present invention. The gas sensor element 1 of the present invention increases its resonant frequency in the presence of hydrogen gas or carbon monoxide gas, and a proportional relationship exists between the amount of increase in the resonant frequency and the concentration of hydrogen gas or carbon monoxide gas. That is, if the hydrogen gas concentration or carbon monoxide gas concentration is plotted on the horizontal axis and the amount of increase in the resonant frequency is plotted on the vertical axis, the plot will be approximately a straight line. Therefore, by preparing two or more samples of known hydrogen gas concentrations or two or more samples of known carbon monoxide gas concentrations, investigating the amount of increase in the resonant frequency in these samples, and plotting the results as change in resonant frequency - hydrogen gas concentration or change in resonant frequency - carbon monoxide gas concentration, a calibration curve for hydrogen gas concentration or carbon monoxide gas concentration can be created. The method for measuring the concentration of hydrogen gas or carbon monoxide gas in a target object of the present invention utilizes this property of the gas sensor element 1.

[0040] In other words, the method for measuring the concentration of hydrogen gas or carbon monoxide gas in a target for detection according to the present invention is characterized by comprising: a calibration curve creation step of creating a calibration curve by applying an alternating electric field to the gas sensor element 1 of the present invention to vibrate a crystal substrate 11 at a resonant frequency, and exposing the gas sensor element 1 to each of two or more calibration curve creation gases containing hydrogen gas or carbon monoxide gas of different known concentrations, thereby determining the degree of increase in the resonant frequency of the gas sensor element 1 for each calibration curve creation gas; and a concentration measurement step of calculating the concentration of hydrogen gas or carbon monoxide gas in the target for detection by applying the amount of increase in the resonant frequency when the gas sensor element 1 is exposed to the target for detection while an alternating electric field is applied to the gas sensor element 1 to vibrate a crystal substrate 11 at a resonant frequency, to the above calibration curve. [Examples]

[0041] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to the following examples.

[0042] [Example 1: Preparation of DLC] A diamond-like carbon film was formed on the surface of one electrode of a commercially available QCM sensor (SEN-9E, manufactured by Tama Device Co., Ltd., with a resonant frequency of 9 MHz) using the PVD method. First, the QCM sensor was set in the sample holder on the ground electrode side of the high-frequency magnetron sputtering deposition apparatus, and a solid graphite target (manufactured by Toyoshima Seisakusho Co., Ltd., 99.99% purity), which would be used as the sputtering material, was set on the drive electrode side so that the distance between the QCM sensor and the solid graphite target was 50 mm. The gas pressure inside the deposition apparatus was 3.5 × 10⁻⁶ -4The sample holder was evacuated to a pressure of Pa and heated to 200°C, and argon gas (99.9975% purity) was flowed into the deposition apparatus at a flow rate of 30 sccm. The unit "sccm" stands for standard cubic centimetre / min, representing the gas flow rate (mL) per minute under standard conditions of 0°C and 1 atm. After adjusting the gas pressure inside the deposition apparatus to approximately 4 Pa, a protective plate was placed on the QCM sensor, and pre-putter deposition (cleaning) was performed for 10 minutes at a high-frequency power of 200 W (frequency 13.56 MHz). Then, the protective plate was opened, and diamond-like carbon (DLC) was deposited on one electrode surface of the QCM sensor for 180 minutes. The thickness of the resulting diamond-like carbon film was 119 nm. Furthermore, the nitrogen atom content in the formed film was determined to be 1.5 atom% by X-ray photoelectron spectroscopy (XPS).

[0043] [Example 2: Preparation of N-DLC@N10%] Except for changing the gas flowing through the deposition apparatus to a mixed gas of argon gas (27 sccm) and nitrogen gas (3 sccm) and setting the deposition time to 150 minutes, nitrogen-containing diamond-like carbon (N-DLC@N10%) was deposited on the surface of one electrode of the QCM sensor using the same procedure as in Example 1. The thickness of the obtained nitrogen-containing diamond-like carbon film was 121 nm. Furthermore, the nitrogen atom content in the formed film was determined to be 16.4 atom% by XPS.

[0044] [Example 3: Preparation of N-DLC@N50%] Except for changing the gas flowing through the deposition apparatus to a mixed gas of argon gas (15 sccm) and nitrogen gas (15 sccm) and setting the deposition time to 120 minutes, nitrogen-containing diamond-like carbon (N-DLC@N50%) was deposited on the surface of one electrode of the QCM sensor using the same procedure as in Example 1. The thickness of the obtained nitrogen-containing diamond-like carbon film was 109 nm. Furthermore, the nitrogen atom content in the formed film was determined to be 21.5 atom% by XPS.

[0045] [Hydrogen gas exposure test 1] For each of the QCM sensors obtained in Examples 1-3, each with a DLC, N-DLC@N10%, or N-DLC@N50% film, the change in resonance frequency when exposed to hydrogen gas was investigated using a QCM measuring instrument (THQ-100P-SW model manufactured by Tama Device Co., Ltd.). Each sensor was placed in a measurement chamber through which gas was flowed, and 20% hydrogen gas-containing nitrogen gas and 100% nitrogen gas were alternately flowed into the measurement chamber. The frequency changes at this time were investigated and plotted. The results are shown in Figure 3. Figure 3 is a plot showing the change in resonance frequency when QCM sensors with DLC, N-DLC@N10%, or N-DLC@N50% films were exposed to 20% hydrogen gas-containing nitrogen gas or 100% nitrogen gas, respectively. (a) is for the QCM sensor with the DLC film, (b) is for the QCM sensor with the N-DLC@N10% film, and (c) is for the QCM sensor with the N-DLC@N50% film. As shown in Figure 3, when 20% hydrogen gas was flowed through the QCM sensors with DLC, N-DLC@N10%, and N-DLC@N50% films, a frequency increase of approximately 4 Hz, 9 Hz, and 11 Hz was observed, respectively. Furthermore, when the flowing gas was changed back from 20% hydrogen gas-containing nitrogen gas to nitrogen gas, the frequencies of all these QCM sensors returned to almost their original levels, confirming that this change is reversible. Furthermore, comparing Figures 3(a) to (c), it can be seen that as the nitrogen atom content in the diamond-like carbon film increases, the frequency change upon exposure to hydrogen gas also increases. Note that the hydrogen gas concentration expressed in % means vol%. This is also true below.

[0046] [Hydrogen gas exposure test 2] For the QCM sensor with the N-DLC@N50% film obtained in Example 3, the frequency rise was determined and plotted when the hydrogen gas concentration was changed to 20%, 10%, 5%, 2%, and 1% using a QCM measuring instrument (THQ-100P-SW model manufactured by Tama Device Co., Ltd.). The results are shown in Figure 4. Figure 4 is a plot showing the frequency change in the QCM sensor with the N-DLC@N50% film when the hydrogen gas concentration is changed. As shown in Figure 4, a nearly linear correlation was observed between the hydrogen gas concentration and the frequency rise.

[0047] [Carbon monoxide gas exposure test 1] For each of the QCM sensors obtained in Examples 1-3, each with a DLC, N-DLC@N10%, or N-DLC@N50% film, the change in resonant frequency when exposed to carbon monoxide gas was investigated using a QCM measuring instrument (THQ-100P-SW model manufactured by Tama Device Co., Ltd.). Each sensor was placed in a measurement chamber through which gas was circulated, and nitrogen gas containing 20% ​​carbon monoxide and 100% nitrogen gas were alternately circulated in the measurement chamber. The frequency changes at this time were investigated and plotted. The results are shown in Figure 5. Figure 5 is a plot showing the change in resonance frequency when QCM sensors with DLC, N-DLC@N10%, or N-DLC@N50% films were exposed to nitrogen gas containing 20% ​​carbon monoxide or 100% nitrogen gas. (a) is for the QCM sensor with the DLC film, (b) is for the QCM sensor with the N-DLC@N10% film, and (c) is for the QCM sensor with the N-DLC@N50% film. As shown in Figure 5, when 20% carbon monoxide gas was passed through, the QCM sensors with DLC, N-DLC@N10%, and N-DLC@N50% films showed frequency increases of approximately 4 Hz, 6 Hz, and 12 Hz, respectively. Furthermore, when the flowing gas was changed back from nitrogen gas containing 20% ​​carbon monoxide to nitrogen gas, the frequencies of all these QCM sensors returned to almost their original levels, confirming that this change is reversible. Furthermore, comparing Figures 5(a) to (c), it can be seen that the frequency change when exposed to carbon monoxide gas increases as the nitrogen atom content in the diamond-like carbon film increases.

[0048] [Carbon monoxide gas exposure test 2] For the QCM sensor with the N-DLC@N50% film obtained in Example 3, the frequency rise was determined and plotted when the carbon monoxide gas concentration was changed to 20%, 10%, 5%, 2%, and 1% using a QCM measuring instrument (THQ-100P-SW model manufactured by Tama Device Co., Ltd.). The results are shown in Figure 6. Figure 6 is a plot showing the frequency change in the QCM sensor with the N-DLC@N50% film when the carbon monoxide gas concentration is changed. As shown in Figure 6, a nearly linear correlation was observed between the carbon monoxide gas concentration and the frequency rise.

[0049] [Examples 4-9; Film Thickness Dependence in N-DLC@N50%] The frequency change when exposed to 20% hydrogen-containing nitrogen gas-containing nitrogen gas was investigated by varying the film thickness of the nitrogen-containing diamond-like carbon film (N-DLC@N50%) formed on the QCM sensor. For this purpose, the nitrogen-containing diamond-like carbon film (N-DLC@N50%) was formed on one electrode surface of the QCM sensor using the same procedure as in Example 1, except that the gas flowing through the deposition apparatus was changed to a mixed gas of argon gas (15 sccm) and nitrogen gas (15 sccm), and the deposition time was set to the time (minutes) shown in Table 1. The film thickness and surface roughness (Rq) of the N-DLC@N50% formed on each QCM sensor are shown in Table 1, and the frequency change when exposed to 20% hydrogen-containing nitrogen gas using the same procedure as in Hydrogen Gas Exposure Test 1 is also shown in Table 1. The surface roughness shown in Table 1 is the root mean square roughness (Rq), which was determined using an atomic force microscope (JEOL Ltd., JSPM-5200) in AC mode with a scan size of 10 × 10 μm, a filter of 0.4 Hz, and a loop gain of 16. The sample size (N) was set to 3 times, and the average value was calculated.

[0050] [Table 1]

[0051] As shown in Table 1, the frequency change when exposed to 20% hydrogen gas-containing nitrogen gas increased as the film thickness of N-DLC@N50% increased. In this case, it is preferable that the film thickness of N-DLC@N50% is 100 nm or more, as this results in a frequency change of 10 Hz or more and good sensitivity. Also, as shown in Table 1, it was confirmed that the surface roughness of the N-DLC@N50% film tended to increase with increasing film thickness. From these findings, it was inferred that this improvement in detection sensitivity was brought about by the increase in the surface area of ​​the N-DLC@N50% film due to the increase in surface roughness. In Example 9, where the film thickness was 946 nm, the adhesion of the film to the QCM sensor was weak, and it was found to be slightly inferior in terms of durability compared to Examples 4-8.

[0052] [Examples 10 and 11: Comparison of N-DLC and flake-type N-DLC] Except for changing the gas flowing through the film deposition apparatus to a mixed gas of argon gas (2 sccm) and nitrogen gas (18 sccm) and setting the film deposition time to 210 minutes, an N-DLC@N90% film was formed on the surface of one electrode of the QCM sensor using the same procedure as in Example 1 to obtain the QCM sensor of Example 10. In other words, the QCM sensor of Example 10 has an N-DLC@N90% film directly formed on the electrode surface of the QCM sensor by the PVD method. Furthermore, an N-DLC@N90% film was formed on a glass substrate using the same procedure as described above, and this film was immersed in an aqueous hydrofluoric acid solution to dissolve the glass substrate, thereby obtaining an N-DLC@N90% film. This film was pulverized by treating it in methanol with 42 kHz ultrasound for 60 minutes, and the flaky N-DLC@N90% was prepared as a methanol dispersion. Figure 7 shows an image of the obtained flaky N-DLC@N90% observed with an electron microscope. As shown in Figure 7, the pulverized N-DLC@N90% exhibited a flaky form with a size of approximately 20-26 μm. 20 μL of this methanol dispersion of flaky N-DLC@N90% was dropped onto a QCM sensor by the drop-cast method, and then the solvent was dried by heating at 100°C for 10 minutes, thereby obtaining the QCM sensor of Example 11 in which a cast film of flaky N-DLC@N90% was formed.

[0053] The frequency changes of the QCM sensors of Examples 10 and 11, prepared using the procedure described above, were investigated when exposed to nitrogen gas containing 20% ​​hydrogen gas using the same procedure as in Hydrogen Gas Exposure Test 1. The results are shown in Table 2. As shown in Table 2, the frequency change of the QCM sensor of Example 11, which was made of a cast film of flake-shaped N-DLC@N90%, was significantly larger when exposed to nitrogen gas containing 20% ​​hydrogen gas compared to the QCM sensor of Example 10, which was made using the PVD method. This is thought to be because, as can be seen from the image in Figure 7, the surface area was improved by making the N-DLC@N90% into flakes, and the amount of hydrogen gas adsorbed increased.

[0054] [Table 2]

[0055] [Examples 12-14; Comparison of electrode materials] For each of the three types of QCM sensors (manufactured by Piezo Parts Co., Ltd., with a resonant frequency of 9 MHz) whose electrode material is gold, titanium, or aluminum, a nitrogen-containing diamond-like carbon (N-DLC@N50%) film was formed on one electrode surface using the same procedure as in Example 1, except that the gas flowing through the film deposition apparatus was changed to a mixed gas of argon gas (10 sccm) and nitrogen gas (10 sccm), and the film deposition time was set to 180 minutes. The thickness of the formed N-DLC@N50% film was 297 nm. For each of these three types of QCM sensors, the frequency change when exposed to 20% hydrogen gas-containing nitrogen gas was investigated using the same procedure as in Hydrogen Gas Exposure Test 1. The results are shown in Table 3.

[0056] [Table 3]

[0057] As shown in Table 3, the degree of frequency change when exposed to nitrogen gas containing 20% ​​hydrogen gas increases or decreases depending on the material of the electrode used to form N-DLC@N50%. Gold is commonly used as the electrode material for this type of electrode, but it can be seen that the sensitivity of the QCM sensor increases when titanium or aluminum is selected as the electrode material for forming the nitrogen-containing diamond-like carbon film. This is presumed to be due to the influence of the adhesion (defects) between N-DLC@N50% and the electrode interface. [Explanation of Symbols]

[0058] 1. Gas sensor element 11 Crystal substrate 12A, 12B electrode 13 Diamond-like carbon film 14A, 14B lead wires

Claims

1. A gas sensor element comprising a diamond-like carbon film, characterized in that it detects the presence of the target gas based on a change in the mass of the film caused by the action of the target gas on the diamond-like carbon film.

2. The sensor element according to claim 1, wherein a diamond-like carbon film is formed directly or via an intermediate layer on at least a portion of a quartz crystal having electrodes on its surface.

3. The gas sensor element according to claim 1, wherein a diamond-like carbon film is formed directly or via an intermediate layer on at least one surface of the counter electrodes sandwiching the quartz substrate.

4. The gas sensor element according to claim 1, which is a sensor element for hydrogen gas or carbon monoxide gas.

5. The gas sensor element according to claim 1, wherein the diamond-shaped carbon is nitrogen-containing diamond-shaped carbon.

6. The gas sensor element according to claim 1, wherein the diamond-like carbon film has a thickness of 100 to 500 nm.

7. The gas sensor element according to claim 1, wherein the diamond-like carbon film is formed by an aggregate of diamond-like carbon flakes having a diameter of 0.1 to 100 μm.

8. The gas sensor element according to claim 3, characterized in that the counter electrode is made of aluminum or titanium.

9. A gas sensor characterized in that, when an alternating electric field is applied to the gas sensor element according to any one of claims 3 to 7, causing the quartz substrate to vibrate at the resonant frequency, the gas sensor element is exposed to the object to be detected, and the presence of hydrogen gas and / or carbon monoxide gas in the object to be detected is detected by an increase in the resonant frequency.

10. A calibration curve creation step of creating a calibration curve by applying an alternating electric field to the gas sensor element according to any one of claims 3 to 7 to vibrate the crystal substrate at a resonant frequency, and then exposing the gas sensor element to each of two or more calibration curve creation gases containing hydrogen gas or carbon monoxide gas of different known concentrations, thereby determining the degree of increase in the resonant frequency of the gas sensor element for each calibration curve creation gas and creating a calibration curve; A method for measuring the concentration of hydrogen gas or carbon monoxide gas in a target to be detected, comprising: a concentration measurement step of applying an alternating electric field to the gas sensor element to vibrate the quartz substrate at its resonant frequency, and then applying the amount of increase in the resonant frequency when this is exposed to the target to be detected to the calibration curve, thereby calculating the concentration of hydrogen gas or carbon monoxide gas in the target to be detected.

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