Gas sensor element and gas sensor

Abstract

To provide a gas sensor element and a gas sensor that can be used even in an environment containing oxygen or water.SOLUTION: A gas sensor element 10 includes a substrate 11, a first electrode 20A disposed on the substrate 11, a second electrode 20B disposed on the substrate 11 apart from the first electrode 20A, a MXene film 30 disposed on the substrate 11 and connecting the first electrode 20A to the second electrode 20B, and a protection film 40 that does not transmit oxygen or water. An outer edge part of the MXene film 30 is covered with at least one of the protection film 40, the first electrode 20A, and the second electrode 20B.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to a gas sensor element and a gas sensor. [Background technology]

[0002] A gas sensor capable of detecting volatile organic compound gases is disclosed in U.S. Pat. No. 10,948,446. This gas sensor uses Ti, a type of two-dimensional layered material called MXene. 3 C 2 T x Here, T represents a functional group or atom that modifies the terminal part of the surface, such as a hydroxyl group, oxygen, or fluorine. This gas sensor exhibits different electrical resistance when it adsorbs volatile organic compound gas and when it is placed under nitrogen flow for a while. The mechanism behind this is explained by Ti 3 C 2 T x The volatile organic compound gas is adsorbed by the functional groups and atoms on the surface of Ti 3 C 2 T x It is believed that this is due to a change in carrier mobility. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Pat. No. 1,094,846 [Non-patent literature]

[0004] [Non-Patent Document 1] A. Lipatov et al. "Effect of Synthesis on Quality, Electronic Properties and Environmental Stability of Individual Monolayer Ti3C2 MXene Flakes" Adv. Electron. Mater. 2016, 2, 1600255. Summary of the Invention [Problem to be solved by the invention]

[0005] By the way, Non-Patent Document 1 states that Ti 3 C 2 T x It has been reported that when an FET using Ti as a channel is left in the air, the resistance increases over time. 3 C 2 T x The increase in resistance is due to the change in the number of carriers caused by gas adsorption, as well as the increase in the Ti 3 C 2 T x It has been suggested that this is also due to the oxidation of Ti in the TiN film. 3 C 2 T x It has been suggested that the oxidation of Ti progresses from the outer edge of the film to the inside. Therefore, if the gas sensor of Patent Document 1 is used for a long period of time in an environment containing oxygen and water, the Ti will be oxidized, and the Ti will become less responsive to the gas. 3 C 2 T x The responsiveness of the resistance may decrease.

[0006] The present disclosure has been completed in view of the above circumstances, and an object of the present disclosure is to provide a gas sensor element and a gas sensor that can be used even in an environment containing oxygen and water. [Means for solving the problem]

[0007] The gas sensor element of the present disclosure is a gas sensor element comprising: a substrate, a first electrode disposed on the substrate, a second electrode disposed on the substrate at a distance from the first electrode, an MXene film disposed on the substrate and connecting the first electrode and the second electrode, and a protective film that is impermeable to oxygen and water, wherein the outer edge of the MXene film is covered by at least one of the protective film, the first electrode, and the second electrode.

[0008] A gas sensor according to the present disclosure includes the above-described gas sensor element and a circuit that connects the gas sensor element to a power source. Effect of the Invention

[0009] According to the present disclosure, it is possible to provide a gas sensor element and a gas sensor that can be used even in an environment containing oxygen and water. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a plan view illustrating a schematic external configuration of the gas sensor according to the first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view taken along line AA in FIG. [Diagram 3] FIG. 3 is a schematic cross-sectional view of a gas sensor according to a modified example of the first embodiment, and corresponds to FIG. 2 of the first embodiment. [Figure 4] FIG. 4 is a perspective view of the gas sensor element according to the first embodiment. [Diagram 5] FIG. 5 is a plan view of the gas sensor element. [Figure 6] FIG. 6 is a cross-sectional view taken along line BB of FIG. [Figure 7] FIG. 7 is a cross-sectional view taken along line CC of FIG. [Figure 8] FIG. 8 is a perspective view showing the MXene film formed on the substrate. [Figure 9] FIG. 9 is a perspective view showing a first electrode and a second electrode formed on a substrate. [Figure 10]FIG. 10 is a plan view of the gas sensor element according to the second embodiment. [Figure 11] FIG. 11 is a cross-sectional view taken along line DD of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. (1) The gas sensor element of the present disclosure comprises a substrate, a first electrode disposed on the substrate, a second electrode disposed on the substrate spaced apart from the first electrode, an MXene film disposed on the substrate and connecting the first electrode and the second electrode, and a protective film that is impermeable to oxygen and water, and the outer edge of the MXene film is covered by at least one of the protective film, the first electrode, and the second electrode.

[0012] With this configuration, the outer edge of the MXene film is covered with at least one of the protective film, the first electrode, and the second electrode, so that the outer edge of the MXene film can be prevented from reacting with oxygen or water and being oxidized, thereby preventing a decrease in the responsiveness of the MXene film to adsorbed gases due to oxidation of the MXene film.

[0013] (2) In the above (1), it is preferable that one end of the MXene film is arranged between the substrate and the first electrode, and the other end of the MXene film is arranged between the substrate and the second electrode.

[0014] According to this configuration, the first electrode and the second electrode can cover part of the outer edge of the MXene film, so that the area where the protective film is formed can be made small.

[0015] (3) In the above (1) or (2), the protective film is Al 2 O 3 , AlO x , SiN.

[0016] (4) A gas sensor according to the present disclosure includes the gas sensor element according to any one of (1) to (3) above, and a circuit connecting the gas sensor element to a power source.

[0017] [Details of the First Embodiment of the Present Disclosure] Specific examples of the first embodiment of the present disclosure will be described with reference to Figs. 1 to 9. The present disclosure is not limited to these examples, but is intended to include all modifications within the scope of the claims and meaning equivalent thereto. In this specification, the configuration of the gas sensor element and the gas sensor will be described with the Z-axis positive direction as the upward direction, the Z-axis negative direction as the downward direction, and the XY plane direction as the horizontal direction, but the gas sensor element and the gas sensor may be arranged differently in the actual use of the gas sensor element and the gas sensor. In this specification, "orthogonal" and "parallel" also include arrangements that are recognized as substantially orthogonal and parallel.

[0018] <Gas sensor> 1 and 2, the gas sensor 1 of the first embodiment includes a housing 2 and a circuit board 3 accommodated in the housing 2. The gas sensor element 10 of the first embodiment is disposed on the circuit board 3. The surface of the circuit board 3 is parallel to the horizontal direction. The up-down direction is perpendicular to the surface of the circuit board 3.

[0019] As shown in FIG. 2, the housing 2 is provided with an intake port 2A for taking in surrounding gas and an exhaust port 2B for exhausting gas from inside the housing 2 to the outside. The intake port 2A is disposed on the upper wall of the housing 2 so as to face the gas sensor element 10 in the vertical direction. The exhaust port 2B is provided in a portion of the housing 2 separated from the gas sensor element 10 (here, the lower wall). A filter 4 is attached to the rim of the intake port 2A so as to close the intake port 2A. The filter 4 is adapted to introduce surrounding gas into the housing 2 while preventing foreign matter such as dust from entering the housing 2. The filter 4 may be configured so as not to transmit a specific type of gas (e.g., water vapor, etc.).

[0020] A fan, pump, etc. (not shown) are disposed near the exhaust port 2B. By operating the fan, pump, etc., it becomes possible to take in gas from the air inlet 2A into the housing 2 and to exhaust the gas from the housing 2 to the outside through the exhaust port 2B, as shown by the arrows. Note that if the structure is such that gas can be exhausted from the housing 2 to the outside through the exhaust port 2B, it is not necessary to dispose the fan or pump.

[0021] A portion of the circuit board 3 is disposed outside through an opening 2C provided in the housing 2. A connection portion 3A is provided on the portion of the circuit board 3 disposed outside the housing 2. An external power source (an example of a power source) (not shown) is connected to the connection portion 3A.

[0022] The gas sensor element 10, a microcomputer (not shown), and the like are mounted on a portion of the circuit board 3 that is disposed inside the housing 2. The circuit board 3 has a circuit (not shown) that electrically connects the connection portion 3A, the gas sensor element 10, and the microcomputer, and the like. The microcomputer controls gas detection by the gas sensor element 10 using power from an external power source.

[0023] The power source connected to the circuit board 3 may be, for example, a battery 5. As shown in FIG. 3, the battery 5 may be disposed inside the housing 2 and mounted on the circuit board 3. In this case, the circuit board 3 does not need to include a connection portion 3A that is connected to an external power source. The entire circuit board 3 may be disposed inside the housing 2.

[0024] <Gas sensor element> 4 to 6, the gas sensor element 10 includes a substrate 11, a first electrode 20A and a second electrode 20B disposed on the substrate 11, an MXene film 30 disposed on the substrate 11 and connecting the first electrode 20A and the second electrode 20B, and a protective film 40 that is impermeable to oxygen and water. For ease of viewing, the thicknesses (vertical dimensions) of the first electrode 20A, the second electrode 20B, the MXene film 30, and the protective film 40 are illustrated larger than they actually are.

[0025] The substrate 11 has a surface 11A on which the first electrode 20A, the second electrode 20B, and the MXene film 30 are formed. The surface 11A of the substrate 11 is subjected to an insulating treatment. The substrate 11 is made of, for example, SiO 2 / Si substrate. SiO 2 / Si substrate is a Si substrate with an insulating layer of SiO 2 is the substrate 11 formed to a predetermined thickness. The Si substrate may be n-type doped or p-type doped. The Si substrate may be non-doped. The surface 11A of the substrate 11 may be surface-treated with, for example, a self-assembled monolayer (SAM) or the like.

[0026] The first electrode 20A and the second electrode 20B extend in the Y-axis direction. The first electrode 20A and the second electrode 20B are disposed apart in the X-axis direction. The first electrode 20A and the second electrode 20B are made of a conductive metal. The first electrode 20A and the second electrode 20B are impermeable to oxygen and water. The first electrode 20A and the second electrode 20B may be, for example, an Au / Cr film. The Au / Cr film includes a Cr film of several nm and an Au film of several tens of nm laminated on the Cr film. By providing the Cr film, the SiO 2 / The adhesion to the Si substrate is improved. In other words, the Cr film functions as an adhesion layer.

[0027] The first electrode 20A and the second electrode 20B are used to measure the resistance value of the MXene film 30, which will be described below. In detail, a predetermined current is applied between the first electrode 20A and the second electrode 20B by a microcomputer, and the resulting potential difference between the first electrode 20A and the second electrode 20B, i.e., the voltage applied to the MXene film 30, is measured. The resistance value of the MXene film 30 is then calculated.

[0028] The MXene film 30 is a film composed of a two-dimensional layered material called MXene. MXene has the general formula M n+1 X n T xIn the group of substances represented by the formula: M is a transition metal element, X is a light element (at least one of carbon and nitrogen), T is a terminal modification group, n is any natural number, and x is any number including zero. T may not be formed. The MXene film 30 typically has a thickness of several atomic layers, i.e., a thickness of several angstroms. The horizontal length of the MXene film 30 is, for example, several μm.

[0029] It has been reported that when a specific gas (e.g., acetone, ethanol, ammonia, propanal, etc.) is adsorbed on the end face (here, the upper face) of the MXene film 30 perpendicular to the thickness direction (here, the Z-axis direction), the resistance value of the MXene film 30 changes (for example, Patent Document 1). On the other hand, it has been suggested that the oxidation of the metal M of the MXene film 30 progresses from its outer edge when the MXene film 30 is exposed to oxygen or water (for example, Non-Patent Document 1). When the MXene film 30 is oxidized, the change in the resistance value of the MXene film 30 due to the adsorption of a specific gas may become small. Therefore, in this embodiment, the outer edge of the MXene film 30 is covered with the first electrode 20A, the second electrode 20B, and the protective film 40, and a configuration capable of suppressing the oxidation of the MXene film 30 is adopted. Note that the part of the MXene film 30 other than the outer edge is exposed upward, and a specific gas is configured to be accessible to the upper surface of the MXene film 30.

[0030] 6, one end of the MXene film 30 in the X-axis direction is covered by the first electrode 20A, and the other end of the MXene film 30 in the X-axis direction is covered by the second electrode 20B. In detail, the first electrode 20A and the second electrode 20B have a portion formed on the surface 11A of the substrate 11, a portion formed on the MXene film 30, and a portion that connects these portions and covers the end face of the MXene film 30 in the X-axis direction.

[0031] As shown in FIG. 7, the protective film 40 is formed so as to cover both ends of the MXene film 30 in the Y-axis direction. In detail, the protective film 40 has a portion formed on the surface 11A of the substrate 11, a portion formed on the MXene film 30, and a portion connecting these portions and covering the end faces of the MXene film 30 in the Y-axis direction. A part of the protective film 40 may be formed on the first electrode 20A or the second electrode 20B. The protective film 40 is a film that is impermeable to water and oxygen. The protective film 40 has a large resistance value compared to the MXene film 30. The protective film 40 is preferably made of an insulating material. The protective film 40 of this embodiment is made of Al 2 O 3 , AlO x , and SiN. Here, AlO x Al 2 O 3 Aluminum oxides with different compositions (e.g. AlO and Al 2 O) is used to refer to aluminum oxide. x Al is part of it. 2 O 3 may include:

[0032] As described above, the outer edge of the MXene film 30 is covered by at least one of the protective film 40, the first electrode 20A, and the second electrode 20B. That is, the protective film 40, the first electrode 20A, and the second electrode 20B are formed so that the outer edge of the MXene film 30 is not exposed to the outside. This makes it possible to prevent oxygen or water from coming into contact with the outer edge of the MXene film 30 and oxidizing the metal M that constitutes the MXene film 30. Therefore, it is possible to prevent a decrease in the responsiveness of the resistance of the MXene film 30 to the adsorbed gas due to oxidation of the MXene film 30.

[0033] <Method of manufacturing gas sensor element> The gas sensor element 10 has the above configuration. An example of a method for manufacturing the gas sensor element 10 will be described below. First, an MXene film 30 is formed on a substrate 11 (see FIG. 8). There is no particular restriction on the method for synthesizing the MXene film 30. For example, Ti 2CT x is Ti 2 The MXene film 30 can be synthesized by immersing AlC in a fluoride-containing etching solution. The synthesized MXene film 30 may be transferred to the substrate 11 using, for example, tape. The MXene film 30 is fixed to the surface 11A of the substrate 11 by electrostatic attraction between atoms. The MXene film 30 may also be synthesized directly on the substrate 11 by, for example, CVD (chemical vapor deposition), ALD (atomic layer deposition), sputtering, or the like.

[0034] Next, the first electrode 20A and the second electrode 20B are formed on the substrate 11 (see FIG. 9). The first electrode 20A and the second electrode 20B are formed by, for example, vacuum deposition or the like. In detail, the MXene film 30 except for both ends in the X-axis direction is covered with resist, and Cr and Au are sequentially deposited. Thereafter, the resist is lifted off to remove the excess Au / Cr film. As a result, the first electrode 20A and the second electrode 20B are formed at both ends in the X-axis direction of the MXene film 30. Note that the first electrode 20A and the second electrode 20B may be formed using a mask or the like instead of a resist.

[0035] Next, a protective film 40 is formed on both ends of the MXene film 30 in the Y-axis direction (see FIG. 4). 2 O 3 , AlO x Since the protective film 40 is made of one or more materials selected from the group consisting of SiO2, SiN, and SiN, the protective film 40 can be formed by, for example, sputtering, ALD, or the like. In detail, the region where the protective film 40 is not to be formed is covered with a resist, and the protective film 40 is formed. Then, the resist is lifted off to remove excess protective film material, thereby forming the protective film 40. As another method, the protective film 40 is formed on the substrate 11 on which the MXene film 30, the first electrode 20A, and the second electrode 20B are formed, and a resist is applied to the region where the protective film 40 is to be formed. Then, the substrate 11 is immersed in an etching solution containing BHF or hydrofluoric acid to remove unnecessary protective film 40 and remove the resist. With the above, the manufacture of the gas sensor element 10 is completed.

[0036] <Effects of the First Embodiment> As described above, the gas sensor element 10 of embodiment 1 comprises a substrate 11, a first electrode 20A arranged on the substrate 11, a second electrode 20B arranged on the substrate 11 at a distance from the first electrode 20A, an MXene film 30 arranged on the substrate 11 and connecting the first electrode 20A and the second electrode 20B, and a protective film 40 that is impermeable to oxygen and water, and the outer edge of the MXene film 30 is covered by at least one of the protective film 40, the first electrode 20A, and the second electrode 20B.

[0037] With this configuration, the outer edge of the MXene film 30 is covered by at least one of the protective film 40, the first electrode 20A, and the second electrode 20B, and therefore the outer edge of the MXene film 30 can be prevented from reacting with oxygen or water and being oxidized. Therefore, it is possible to prevent a decrease in the responsiveness of the MXene film 30 to the adsorbed gases due to oxidation of the MXene film 30.

[0038] In the first embodiment, one end of the MXene film 30 is disposed between the substrate 11 and the first electrode 20A, and the other end of the MXene film 30 is disposed between the substrate 11 and the second electrode 20B.

[0039] According to such a configuration, a part of the outer edge portion of the MXene film 30 can be covered by the first electrode 20A and the second electrode 20B, so that the area where the protective film 40 is formed can be made small.

[0040] In the first embodiment, the protective film 40 is made of Al 2 O 3 , AlO x , SiN.

[0041] The gas sensor 1 according to the first embodiment includes a gas sensor element 10 and a circuit that connects the gas sensor element 10 to a power source.

[0042] [Details of the second embodiment of the present disclosure] A specific example of the second embodiment of the present disclosure will be described with reference to Fig. 10 and Fig. 11. The gas sensor element 110 of the second embodiment includes a substrate 11, a first electrode 120A, a second electrode 120B, an MXene film 130, and a protective film 140. In the gas sensor element 110 of the second embodiment, the arrangement of each member is different from that of the first embodiment, but the materials of each member, the film formation method, etc. are the same as those of the first embodiment.

[0043] 11, the first electrode 120A and the second electrode 120B are formed on the surface 11A of the substrate 11. The MXene film 130 is formed on the substrate 11 on which the first electrode 120A and the second electrode 120B are formed. Therefore, in the second embodiment, unlike the first embodiment, the first electrode 120A and the second electrode 120B do not cover a part of the outer edge of the MXene film 130 (both ends in the X-axis direction).

[0044] In the second embodiment, the first electrode 120A and the second electrode 120B may be a membrane that is permeable to oxygen and water because the first electrode 120A and the second electrode 120B do not cover a part of the outer edge of the MXene film 130. The first electrode 120A and the second electrode 120B do not have to be a conductive metal, and may be made of an organic material such as a conductive carbon material.

[0045] 10, the protective film 140 covers both ends in the X-axis direction and both ends in the Y-axis direction of the MXene film 130. In other words, the protective film 140 covers the entire outer edge of the MXene film 130.

[0046] The effects of the second embodiment are substantially the same as those of the first embodiment, and therefore a description thereof will be omitted.

[0047] <Other embodiments> (1) In the first embodiment, the circuit connecting the gas sensor element 10 to the external power supply is provided on the circuit board 3. However, the circuit may be, for example, an electric wire.

[0048] (2) In the first embodiment, the protective film 40 is Al 2 O 3, AlO x , SiN, or a combination thereof. However, the protective film may be any film that is impermeable to oxygen and water.

[0049] (3) In the first embodiment, the substrate 11 is SiO 2 Although the substrate is a / Si substrate, the substrate may be, for example, a glass substrate, a sapphire substrate, an alumina substrate, a plastic substrate, or the like.

[0050] (4) In the first embodiment, the first electrode 20A and the second electrode 20B are Au / Cr films, but instead of the Cr film, a Ti film or the like may be used as the adhesive layer. Depending on the type of substrate, the first electrode and the second electrode may not have an adhesive layer. The first electrode and the second electrode may be made of, for example, Pt or W instead of Au. [Explanation of symbols]

[0051] 1: Gas sensor, 2: Housing, 2A: Air inlet, 2B: Exhaust port, 2C: Opening, 3: Circuit board, 3A: Connection, 4: Filter, 5: Battery 10, 110: Gas sensor element 11: Substrate, 11A: Surface 20A, 120A: 1st electrode, 20B, 120B: 2nd electrode 30,130:MXene membrane 40,140:Protective film

Claims

1. A substrate; A first electrode disposed on the substrate; a second electrode disposed on the substrate and spaced apart from the first electrode; an MXene film disposed on the substrate and connecting the first electrode and the second electrode; a protective film that is impermeable to oxygen and water; The gas sensor element, wherein an outer edge of the MXene film is covered by at least one of the protective film, the first electrode, and the second electrode.

2. one end of the MXene film is disposed between the substrate and the first electrode; 2. The gas sensor element according to claim 1, wherein the other end of the MXene film is disposed between the substrate and the second electrode.

3. The protective film is made of Al ２ O ３ , AlO ｘ 2. The gas sensor element according to claim 1, wherein the gas sensor element is made of one or more materials selected from the group consisting of SiN and SiN.

4. The gas sensor element according to any one of claims 1 to 3, and a circuit connecting the gas sensor element to a power source.

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