Gas sensor, Ni-containing SnO2 nanosheet and manufacturing method

Abstract

To provide a gas sensor with good response characteristics to gas (low-concentration gas in particular).SOLUTION: A gas sensor is provided, comprising a base material, first and second electrodes provided on a surface of the base material, and a gas sensing layer connected to the first and second electrodes and provided with a Ni-containing SnO2 nanosheet comprising an SnO2 nanosheet and Ni contained in the SnO2 nanosheet.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a gas sensor used for detecting gas, a Ni-containing SnO2 nanosheet used in the gas sensor, and a technique for producing the Ni-containing SnO2 nanosheet. [Background technology]

[0002] The properties of metal oxides are widely known, and they are used in various devices, such as sensors (e.g., gas sensors), batteries, electronic devices, and optical devices. In these devices, the microstructure, composition, crystal structure, surface area, crystallinity, and crystal plane of the metal oxides must be controlled because they have a significant effect on the device properties.

[0003] Here, tin oxide nanosheets have been developed in which the morphology of tin oxide is controlled as a metal oxide. For example, Patent Document 1 proposes a gas sensor using a tin oxide-containing nanosheet formed by an assembly of tin oxide nanosheet pieces. In Patent Document 1, the sensitivity of the gas sensor is improved by increasing the surface area of ​​the tin oxide-containing nanosheet. Specifically, the tin oxide-containing nanosheet of Patent Document 1 has a central sheet portion formed by an assembly of multiple nanosheet pieces made of tin oxide, and a peripheral portion attached to at least both sides of the central sheet portion and formed by an assembly of multiple nanosheet pieces made of tin oxide at a lower assembly density than the assembly density of the multiple nanosheet pieces in the central sheet portion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-106445 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the gas sensor of Patent Document 1 has room for improvement in response characteristics. Specifically, the gas sensor of Patent Document 1 exhibits a sensor response to hydrogen and methane in the range of 5 to 5000 ppm, but does not demonstrate a sensor response to hydrogen and methane below 5 ppm. Also, while the gas sensor exhibits a sensor response to 0.85 ppm nonanal, it does not demonstrate a sensor response to nonanal below 0.85 ppm. Furthermore, there is no mention of sensor response to other gases such as acetone and nitrogen dioxide.

[0006] As described above, there is a demand for a gas sensor that has good response characteristics, particularly for low-concentration gases. In consideration of the above circumstances, the present invention aims to provide a gas sensor that has good response characteristics for gases (particularly low-concentration gases), a Ni-containing SnO2 nanosheet used in the gas sensor, and a method for manufacturing the Ni-containing SnO2 nanosheet. [Means for solving the problem]

[0007] [1] A gas sensor comprising a substrate, a first electrode and a second electrode provided on a surface of the substrate, and a gas-sensitive layer connected to the first electrode and the second electrode and having a Ni-containing SnO2 nanosheet containing a SnO2 nanosheet and Ni contained in the SnO2 nanosheet.

[0008] [2] The gas sensor according to [1], wherein the SnO2 nanosheet contains the Ni as NiO.

[0009] [3] The gas sensor according to [2], wherein in an X-ray diffraction measurement of the Ni-containing SnO2 nanosheet, the integrated intensity of the peak derived from NiO is greater than 0 and less than or equal to 1 when the integrated intensity of the peak derived from SnO2 is taken as 100.

[0010] [4] The gas sensor according to any one of [1] to [3], wherein the activation energy of the Ni-containing SnO2 nanosheet is 0.4 eV or more and 0.9 eV or less.

[0011] [5] A gas sensor according to any one of [1] to [4], in which the ratio of the sensor response (Ra / Rg) to acetone based on the sensor response (Ra / Rg) to hydrogen is 4.0 or more and 15.0 or less.

[0012] [6] Ni-containing SnO2 nanosheets containing SnO2 nanosheets and Ni contained in the SnO2 nanosheets.

[0013] [7] [6] A method for producing Ni-containing SnO2 nanosheets, comprising the step of synthesizing Ni-containing SnO2 nanosheets by adding a tin compound, a Ni compound, and an alkali source compound to a solvent.

[0014] [8] The method according to [7], wherein the solvent is water.

[0015] [9] The method according to [7] or [8], wherein the tin compound is tin fluoride.

[0016]

[10] The manufacturing method according to any one of [7] to [9], wherein the Ni compound is NiCl2·6H2O.

[0017]

[11] The method according to any one of [7] to

[10] , wherein the alkali source compound is urea.

[0018]

[12] The manufacturing method according to any one of [7] to

[11] , wherein the tin compound and the Ni compound are added to the solvent so that the molar ratio of Ni is 1 to 20 when Sn is taken as 100.

[0019]

[13] The manufacturing method according to any one of [7] to

[12] , wherein the tin compound and the alkali source compound are added to the solvent so that the molar ratio of the alkali source compound is 1 to 80, where Sn is taken as 100. [Effects of the Invention]

[0020] The gas sensor according to the present invention can improve the response characteristics to gas (especially low concentration gas). The Ni-containing SnO2 nanosheet according to the present invention can improve the response characteristics of the gas sensor when used in the gas sensor. The manufacturing method according to the present invention can manufacture the Ni-containing SnO2 nanosheet that can improve the response characteristics of the gas sensor in the gas sensor. [Brief explanation of the drawings]

[0021] [Figure 1] 1 shows diffraction patterns obtained by powder X-ray diffraction measurement for Examples 1 to 4 and Comparative Example 1. [Figure 2] 1 shows diffraction patterns obtained by powder X-ray diffraction measurement for Examples 1 to 4 and Comparative Example 1 after heating in the atmosphere. [Figure 3] 1 shows scanning electron microscope images of the surfaces of gas sensors according to Examples 1 to 4 and Comparative Example 1. [Figure 4] 10 is a transmission electron microscope image of a cross section of the gas sensor according to Example 3 after heating in the atmosphere. [Figure 5] 1 is a graph showing the sensor response (Ra / Rg) of the gas sensors to 40 ppt and 500 ppt of acetone for the gas sensors according to Examples 1 to 4 and Comparative Example 1. The sensor operating temperature for Comparative Example 1 and Example 1 is 250°C, and the sensor operating temperature for Examples 2 to 4 is 225°C. [Figure 6] 1 is a graph showing the sensor response (Ra / Rg) of gas sensors to acetone at multiple concentrations in the range of 40 to 500 ppt for the gas sensors according to Examples 1 to 4 and Comparative Example 1. The sensor operating temperature for Comparative Example 1 and Example 1 is 250°C, and the sensor operating temperature for Examples 2 to 4 is 225°C. [Figure 7]1 is a table showing the sensor response (Ra / Rg) of the gas sensors to acetone at multiple concentrations in the range of 40 to 500 ppt for the gas sensors according to Examples 1 to 4 and Comparative Example 1. The sensor operating temperature for Comparative Example 1 and Example 1 is 250°C, and the sensor operating temperature for Examples 2 to 4 is 225°C. [Figure 8] 1 is a graph showing the sensor response (Ra / Rg) to multiple gases (acetone, ethanol, isoprene, acetaldehyde, toluene, paraxylene, hydrogen, and ammonia) for Examples 2 and 3 and Comparative Example 1. The sensor operating temperature for Comparative Example 1 was 250°C, and the sensor operating temperature for Examples 2 and 3 was 225°C. The measurement conditions were a gas concentration of 500 ppb and a gas flow rate of 500 sccm. [Figure 9] 1 is a table showing the resistance values ​​in air of the gas sensors according to Examples 1 to 4 and Comparative Example 1. The resistance values ​​(Ω) of the gas-sensitive layer are shown at 175°C, 200°C, 225°C, 250°C, 275°C and 300°C. DETAILED DESCRIPTION OF THE INVENTION

[0022] The gas sensor according to the present invention is a sensor device for detecting a desired gas, and specifically includes an insulating substrate, an electrode pair (a first electrode and a second electrode), and a gas-sensitive layer.

[0023] The electrode pair is formed on the surface of the substrate. The gas-sensitive layer is connected to the first electrode and the second electrode at different positions. For example, the gas-sensitive layer is provided so as to cover the surface of the electrode pair (the surface opposite the substrate). That is, the gas-sensitive layer is provided over the surfaces of the first electrode and the second electrode and over the surface of the substrate between the first and second electrodes. The substrate is made of any known material (e.g., silicon material, semiconductor material, resin material, etc.). Similarly, the first electrode and the second electrode are made of any known conductive material (e.g., Pt, Ir, Pd, Ag, Ni, W, Cu, Al, etc.).

[0024] The gas-sensitive layer according to the present invention has a layer containing Ni-containing SnO2 nanosheets. The content of the Ni-containing SnO2 nanosheets in the gas-sensitive layer is not particularly limited, but is, for example, 50% by mass or more and 100% by mass or less.

[0025] The Ni-containing SnO2 nanosheet includes a SnO2 nanosheet and Ni contained in the SnO2 nanosheet. Specifically, the Ni-containing SnO2 nanosheet is a structure in which SnO2 nanosheets are the main component and Ni is contained in the SnO2 nanosheets. The SnO2 nanosheet is a layered structure in which SnO2 is arranged two-dimensionally.

[0026] The thickness of the Ni-containing SnO2 nanosheet is, for example, 1 nm or more and 10 nm or less. For gas sensors, it is important that the Ni-containing SnO2 nanosheet has many reaction sites with target gas molecules, a high resistance change rate, and electrical conductivity. Therefore, the thickness of the Ni-containing SnO2 nanosheet is preferably within the above range. Similarly, for gas sensor applications, the in-plane size (sheet size) of the Ni-containing SnO2 nanosheet is preferably, for example, 10 nm or more and 500 nm or less. In the layer containing the Ni-containing SnO2 nanosheet in the gas-sensitive layer, multiple Ni-containing SnO2 nanosheets form aggregates.

[0027] SnO2 nanosheets contain, for example, Ni as NiO. NiO is encapsulated in the SnO2 nanosheets. However, in addition to the structure in which Ni is contained in the SnO2 nanosheets as NiO, Ni may also be contained in the SnO2 nanosheets as metallic Ni or nickel oxide (Ni2O3) other than NiO. Ni may also be contained in the SnO2 nanosheets in a state in which it covers the surface of the SnO2 nanosheets, or in a state in which it is doped into SnO2.

[0028] The Ni content (content of metallic Ni) in the Ni-containing SnO2 nanosheets is, for example, greater than 0% by mass and less than 3% by mass, preferably greater than 0% by mass and less than 1% by mass. As will be described in detail in the Examples, although Ni can be observed using the EDS method in transmission electron microscope observations, the amount is not large enough to be quantified. Considering the quantification limit, the Ni content is estimated to be at most about 3% by mass. Furthermore, considering the equipment used and the measurement environment, the Ni content is estimated to be at most about 1% by mass, more strictly speaking. Taking these factors into consideration, the Ni content in the Ni-containing SnO2 nanosheets is set within the above range. The lower limit of the Ni content in the Ni-containing SnO2 nanosheets is more preferably 0.01% by mass or more, even more preferably 0.05% by mass or more, and most preferably 0.1% by mass or more.

[0029] In the diffraction pattern determined by X-ray diffraction measurement of Ni-containing SnO nanosheets, a peak derived from SnO and a peak derived from Ni are confirmed. When NiO is contained as Ni, the integrated intensity (peak area) of the peak derived from NiO when the integrated intensity of the peak derived from SnO is taken as 100 is, for example, greater than 0 and less than 10, preferably greater than 0 and less than 3, more preferably greater than 0 and less than 1, even more preferably 0.01 to 0.85, and most preferably 0.1 to 0.85.

[0030] The peak derived from SnO2 here refers to the peak with the strongest intensity when multiple peaks derived from SnO2 are present, and is a peak located in the diffraction angle 2θ range of 26° to 27°. The peak derived from Ni is the peak with the strongest intensity, and is a peak located in the diffraction angle 2θ range of 41° to 44°. This peak derived from Ni is more often located in the diffraction angle 2θ range of 42° to 43.5°. This is the strongest peak of NiO, and for example, the strongest peak of NiO listed in PDF card number: 03-065-2901 is located at 43.1°.

[0031] X-ray diffraction measurement can be performed, for example, under the following conditions. Source: Cu·Kα X-ray tube current: 15mA X-ray tube voltage: 40 kV Scanning range: 2θ=2.0~80.0° Scan speed: 20,000° / min Scan step: 0.02°

[0032] As described above, the presence of Ni in Ni-containing SnO2 nanosheets can be confirmed by X-ray diffraction measurement. However, the method for confirming the presence of Ni is not limited to X-ray diffraction measurement. For example, the presence of Ni in Ni-containing SnO2 nanosheets can be confirmed by elemental analysis such as EDS using a transmission electron microscope, observation of electron beam diffraction patterns using a transmission electron microscope, electron energy loss spectroscopy (EELS) using transmission electron microscope observation, elemental analysis using inductively coupled plasma (ICP) optical emission spectroscopy, X-ray photoelectron spectroscopy, X-ray fluorescence analysis, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), fluorescence spectroscopy, or XAFS (X-ray absorption fine structure) measurement.

[0033] The activation energy in the Ni-containing SnO2 nanosheets is, for example, greater than 0.4 eV and equal to or less than 0.9 eV.

[0034] The activation energy is calculated using the Arrhenius equation shown in the following equation (1). σ=Aexp(-Ea / kBT) (1) In equation (1), σ is the conductivity, A is the frequency factor, Ea is the activation energy, kB is the Boltzmann constant, and T is the absolute temperature.

[0035] By taking the logarithm of both sides of equation (1), the activation energy is calculated from the Arrhenius plot (vertical axis: lnσ, horizontal axis: 1 / T) determined using the following transformed equation (2). The Arrhenius plot is determined using the resistance values ​​(Ω) of the gas sensor at multiple temperatures (e.g., 175°C, 200°C, 225°C, 250°C, 275°C, and 300°C) within a predetermined temperature range (e.g., a range of 175 to 300°C). lnσ=(-Ea / kB)×(1 / T)+lnA ···(2)

[0036] The gas sensor according to the present invention can improve response characteristics (sensor response (Ra / Rg)) compared to a configuration in which a tin oxide nanosheet containing no Ni is used in the gas-sensitive layer. Ra is the resistance value of the gas sensor in air, and Rg is the resistance value of the gas sensor in the gas to be detected. The air used to measure Ra is a mixed gas of 20% oxygen and 80% nitrogen. Ra / Rg refers to the rate of change in electrical resistance (rate of resistance change) when switching from air to the gas to be detected.

[0037] The gas sensor according to the present invention can detect various gases (e.g., combustible gases, reducing gases, oxidizing gases, and combustion-supporting gases). Specifically, the gas sensor according to the present invention can detect one or more of acetone, ethanol, isoprene, acetaldehyde, toluene, paraxylene, hydrogen, and ammonia gas.

[0038] The gas sensor of the present invention exhibits a good sensor response (Ra / Rg) even for low-concentration gases (500 ppt or less). For example, the sensor response (Ra / Rg) for a 40 ppt gas can be 1.25 or higher. Therefore, the sensor is also effective for detecting low-concentration gases (e.g., acetone, ethanol, acetaldehyde, ammonia, etc.) contained in biological gases (e.g., exhaled breath and skin gases) that are thought to be related to disease.

[0039] Furthermore, the gas sensor according to the present invention exhibits high gas selectivity. For example, the ratio of the sensor response (Ra / Rg) to acetone relative to the sensor response (Ra / Rg) to hydrogen is, for example, 4.0 or more and 15.0 or less. The operating temperature of the gas sensor is preferably 100 to 400°C, more preferably 200 to 300°C.

[0040] The present invention can also be conceived as a sensor device including the gas sensor exemplified above and a detection unit that detects the presence (and concentration) of a target gas to be detected in accordance with the current flowing between the first electrode and the second electrode. The sensor device according to the present invention can detect gas with high accuracy.

[0041] In addition to gas sensors, Ni-containing SnO2 nanosheets can be used in various devices such as molecular sensors, solution sensors, battery materials, and artificial photosynthetic materials.

[0042] The method for producing Ni-containing SnO2 nanosheets according to the present invention includes the following steps (hereinafter referred to as the "synthesis step"). The synthesis step is a step in which a tin compound, a Ni compound, and an alkali source compound (base) are added to a solvent to synthesize Ni-containing SnO2 nanosheets. The addition of the alkali source compound increases the pH, which has the advantage of accelerating the synthesis. However, the addition of the alkali source compound is not essential.

[0043] Specifically, in the synthesis process, a tin compound, a Ni compound, and an alkali source compound are added to a solvent, and then the mixture is maintained at a predetermined temperature (e.g., 0 to 100°C) for a predetermined time (e.g., about 10 minutes to 24 hours) to synthesize Ni-containing SnO2 nanosheets.

[0044] The solvent is not particularly limited, but examples thereof include water, alcohols such as methanol and ethanol, organic solvents such as acetone, hexane and toluene, and acids such as acetic acid and formic acid. Among these, water is preferred as the solvent. Furthermore, both "highly polar solvents (hydrophilic)" with high polarity and "lowly polar solvents (hydrophobic)" with low polarity can be used as the solvent. Polar solvents include protic polar solvents and aprotic polar solvents, and both can be used.

[0045] Other examples of solvents include acetaldehyde, acetic anhydride, acetonitrile, acetophenone, acetylacetone, allyl alcohol, ethanolamine, aniline, benzaldehyde, benzene, benzyl alcohol, benzyl benzoate, 1-butanol, 2-butanol, methyl ethyl ketone, butyl acetate, tert-butyl alcohol, dibutyl ether, carbon disulfide, chloroform, epichlorohydrin, o-cresol, m-cresol, p-cresol, cyclohexane, cyclohexanol, 1,2-dichloroethane, dichloromethane, diethyl carbonate, diethylene glycol, diethylene glycol monobutyl ether, butyl carbitol acetate, diethylene glycol monoethyl ether, diethylene glycol monoethyl ether acetate, diethylene glycol monomethyl ether, diethyl ether, dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, 1,4-dioxane, ethyl acetate, ethyl benzoate, 2-chloroethanol, ethylene glycol, 1,2-dimethicone, ethane, 2-butoxyethanol, ethylene glycol monobutyl ether acetate, 2-ethoxyethanol, ethylene glycol monoethyl ether acetate, ethylene glycol monomethyl ether acetate, ethyl formate, 2-ethylhexanol, ethyl acetoacetate, ethyl propionate, formamide, furfuryl alcohol, glycerin, heptane, 1-hexanol, ligroin, 2,6-lutidine, 2-methoxyethanol, methyl acetate, 2-methyl-2-butanol, 3-methyl-1-butanol ethanol, isoamyl acetate, methyl propionate, triethanolamine, nitrobenzene, nitromethane, n-octane, 1-octanol, 2-octanol, pentane, 1-pentanol, 3-pentanol, n-pentyl acetate, petroleum benzine, phenol, 1-propanol, n-propyl acetate, propylene glycol, propylene oxide, propylene oxide, n-propyl ether, pyridine, 1,1,2,2-tetrachloroethane, tetrachloroethylene, tetrahydrofuran, tetralin, 1,1,Examples of the solvent include 2-trichloroethane, trichloroethylene, triethylamine, trifluoroacetic acid, xylene, o-xylene, m-xylene, and p-xylene. Two or more of the above-listed solvents may be mixed and used.

[0046] The alkali source compound (base) is not particularly limited as long as it is a compound that releases an alkali component upon hydrolysis. Examples of alkali source compounds include urea, ethylenediamine, and hexamethylenetetramine (CH 12 N4), sodium hydroxide, potassium hydroxide, barium hydroxide, ammonia, calcium hydroxide, sodium carbonate, sodium borate, potassium carbonate, methylamine, dimethylamine, ethylamine, diethylenetriamine, triethylenetetramine, dimethylhydrazine, methylhydrazine, potassium sulfide, sodium sulfide, caustic alkalis, sodium aluminate, sodium oxide, tetramethylammonium hydroxide, soda lime, pyrrolidine, hydrazine, potassium oxide, sodium hydrogen sulfide, tetraethylenepentamine, 1,3-dimethylbutylamine, 1,2-dimethylhydrazine, N-ethylpiperidine, N-methylpiperidine, piperidine, ethanolamine, piperazine, aminopyridine, rubidium hydroxide, lithium hydroxide, cesium hydroxide, ammonium sulfide, amines or polyamines, ammonium polysulfide, vinylpyridine, sodium metasilicate, a mixture of sodium borohydride and sodium hydroxide, etc. Two or more of the above-listed alkali source compounds may be mixed and used. Among these, the alkali source compound (base) is preferably urea.

[0047] Examples of Sn compounds include, but are not limited to, tin fluoride (SnF), tin chloride (SnCl), tin chloride (SnCl), tin nitrate, potassium stannate, sodium stannate, tin difluoroborate, tin pyrophosphate, tin sulfide (SnS or SnS), tin sulfate (SnSO), tin dioxide hydrate, tin chloride dihydrate, tin chloride pentahydrate, and other tin inorganic salts. Two or more of the above-listed Sn compounds may be mixed and used. Among these, tin fluoride is preferred from the viewpoints of the product, reaction rate, crystallinity of the product, and by-products when chemically reacted with water.

[0048] The Ni compound is not particularly limited, but examples thereof include nickel chloride hexahydrate (NiCl2·6H2O), nickel oxide, nickel hydroxide, nickel(II) hydroxide, nickel hydroxide 0.75 hydrate, and Ni(OH)2·0.75H2O (JCPDS No. 38-0715), nickel, nickel(II) fluoride, nickel(II) fluoride tetrahydrate, nickel(II) iodide, hexaamminenickel(II) iodide, nickel boride, nickel silicide, nickel(II) chloride, nickel(II) chloride hexahydrate, nickel(II) chloride hydrate, nickel(II) chloride (dimethoxyethane adduct), hexaamminenickel(II) chloride, bis(triphenylphosphine)nickel(II) chloride, methallylnickel chloride dimer, nickel(II) nitrate hexahydrate, nickel(II) nitrate solution, nickel(II) acetate tetrahydrate, nickel(II) hydroxyacetate, nickel(II) sulfide, nickel(II) sulfate hydrate, nickel(II) sulfate hexahydrate, sulfur Nickel(II) ammonium salt hexahydrate, nickel(II) amidosulfate tetrahydrate, nickel(II) carbonate, nickel(II) carbonate (basic) hydrate, nickel(II) bromide, nickel(II) bromide trihydrate, nickel(II) bromide hydrate, nickel(II) lactate tetrahydrate, nickel(II) formate dihydrate, nickel(II) citrate hydrate, nickel(II) oxalate dihydrate, nickel(II) sulfamate, nickel sulfamate hydrate, nickel(II) amidosulfate hydrate, nickel(II) cyclohexanebutyrate, nickel(II) 2-ethylhexanoate, nickel(II) trifluoromethanesulfonate, nickel(II) naphthenate, nickel(II) acetylacetonate, 2,Nickel(II) 4-pentanedionate, Nickel(II) cyanide tetrahydrate, Nickel(II) trifluoroacetate tetrahydrate, Nickel(II) tetrafluoroborate hexahydrate, Nickel(II) basic carbonate, Nickel(II) perchlorate hexahydrate, Nickel(II) acetylacetonate hydrate, Potassium hexafluoronickelate(IV), Potassium tetracyanonickelate(II) hydrate, Nickel tungsten oxide, Nickel molybdenum oxide, Nickel iron oxide, Nickel chromite, Nickel chromium, Nickel telluride, Lanthanum nickel, LiNiPO4, Nickel dimethylglyoxime, Nickel(II) phthalocyanine, Nickel(II) Acetylacetonate hydrate, nickel(II) trifluoroacetylacetonate dihydrate, nickel(II) 1,3-bis(diphenylphosphino)propane chloride, nickel 2-methoxyethoxide, bis(triphenylphosphine)nickel(II) iodide, tetrabutylphosphonium bis(1,2-benzenedithiolato)nickel(III)ate, bis(tricyclohexylphosphine)nickel(II) chloride, bis(N,N'-di-t-butylacetamidoynato)nickel(II), bis(1,5-cyclooctadiene)nickel, bis(cyclopentadienyl)nickel, also known as nickelocene, bis(triphenylphosphine)dicarbonylnickel, bis(ethylcyclopentadienyl)nickel, bis(isopropylcyclopentadienyl)nickel, bis(pentaerythritol), (tetramethylcyclopentadienyl)nickel, bis(tetramethylcyclopentadienyl)nickel, potassium tetracyanonickel(II) monohydrate, dichloro[1,2-bis(diphenylphosphino)ethane]nickel(II), dichlorobis(triphenylphosphine)nickel(II), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)nickel(II), bis(triphenylphosphine)nickel dicarbonyl, bis(dibutyldithiocarbamate)nickel(II), bis(triphenylphosphine)nickel(II) bromide, meso-tetraphenylporphinenickel(II), chlorobis(triphenylphosphine)nickel(II), dichlorobis(triphenylphosphine)nickel(II), trifluoroacetylacetonate nickel,Examples of nickel compounds include nickel chloride dihydrate, 1,2-bis(diphenylphosphino)ethane nickel chloride, hexafluoroacetylacetonato nickel(II) hydrate, dichloro[bis(1,3-diphenylphosphino)propane]nickel(II), [1,3-bis(diphenylphosphino)propane]dichloronickel(II), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)nickel(II), and dichloro[1,1'-bis(diphenylphosphino)ferrocene]nickel(II). Two or more of the above-listed nickel compounds may be mixed and used. Among these, nickel chloride hexahydrate (NiCl·6H O) is preferred from the viewpoints of reactivity with Sn compounds and SnO, reaction products, reaction rate, and by-products.

[0049] In the synthesis step, a tin compound and a Ni compound are added to a solvent so that the molar ratio of Ni when Sn is taken as 100 is, for example, 1 to 20 (Sn:Ni=100:1-20), preferably 2 to 15, more preferably 5 to 14, and even more preferably 8 to 12. By keeping the molar ratio of Sn to Ni within the above range, a gas sensor can be produced that exhibits a high sensor response to gases (especially acetone) at low concentrations, for example, 40 to 500 ppt.

[0050] In the synthesis step, the tin compound and the alkali source compound are added to a solvent so that the molar ratio of the alkali source compound when Sn is taken as 100 is, for example, 1 to 80 (Sn:alkali source compound=100:1-80), preferably 3 to 70, more preferably 5 to 60, and even more preferably 35 to 55. When the molar ratio of Sn to the alkali source compound is within the above range, a gas sensor can be produced that exhibits a high sensor response to gases (particularly acetone) at low concentrations, for example, 40 to 500 ppt.

[0051] The method for producing Ni-containing SnO2 nanosheets according to the present invention has the advantage that Ni-containing SnO2 nanosheets can be produced by synthesis in an aqueous solution in a simple manner without using a pressure vessel or high-temperature equipment, etc. After the synthesis step, the method may include a step of heating the Ni-containing SnO2 nanosheets at a predetermined temperature (e.g., about 300°C) for a predetermined time (e.g., about 2 hours).

[0052] In the present invention, the Ni content in the Ni-containing SnO2 nanosheets does not correspond to the Ni concentration in the synthesis process, but is much less than the amount expected from the Ni concentration due to some reason. Details of the above will be mentioned in the examples below.

[0053] The gas sensor according to the present invention is manufactured by forming a gas-sensitive layer (a layer containing Ni-containing SnO2 nanosheets) on the surface of a substrate on which a first electrode and a second electrode (electrode pair) are formed. For example, the substrate on which the electrode pair is formed is immersed in a solvent containing a Sn compound, a Ni compound, and an alkali source compound, and maintained at a predetermined temperature (e.g., 0 to 100°C) for a predetermined time (e.g., about 10 minutes to 24 hours), thereby forming a layer containing Ni-containing SnO2 nanosheets on the substrate surface. The method may then include a step of heating the substrate at a predetermined temperature (e.g., about 300°C) for a predetermined time (e.g., about 2 hours). Any known technique may be used to form the electrode pair. [Example]

[0054] The present invention will be described in detail below with reference to examples, although the present invention is not limited to these examples.

[0055] [Example 1] In Example 1, 0.218 g of SnF2 (powder), 0.0066 g of NiCl2·6H2O (powder), and 0.0084 g of urea (powder) were dissolved in 200 mL of distilled water, and the resulting solution was kept at 90°C for 6 hours to synthesize Ni-containing SnO2 nanosheets. In the aqueous solution in Example 1, the molar ratio of Sn:Ni:urea was 100:2:10.

[0056] In the synthesis of Ni-containing SnO2 nanosheets described above, a layer containing Ni-containing SnO2 nanosheets was formed on the surface of a substrate with an electrode pair by immersing the substrate in the prepared solution, and a gas sensor was manufactured by using aluminum oxide (Al2O3) for the substrate and platinum for the electrode pair.

[0057] Specifically, a substrate on which an electrode pair was formed was immersed in a solvent containing an Sn compound and an Ni compound and held at 90°C for 6 hours to form a layer containing Ni-containing SnO2 nanosheets on the substrate surface. Example 1 is indicated as "2%Ni-Sn" in the drawings.

[0058] [Example 2] In Example 2, Ni-containing SnO nanosheets were synthesized by dissolving 0.218 g of SnF (powder), 0.0165 g of NiCl·6H O (powder), and 0.021 g of urea (powder) in 200 mL of distilled water and maintaining the solution at 90°C for 6 hours. In Example 2, the molar ratio of Sn:Ni:urea in the solution was 100:5:25.

[0059] A gas sensor was manufactured by forming a layer containing the synthesized Ni-containing SnO2 nanosheet on the surface of a substrate on which an electrode pair was formed under the same conditions as in Example 1. Example 2 is indicated as "5%Ni-Sn" in the drawings.

[0060] [Example 3] In Example 3, 0.218 g of SnF2 (powder), 0.033 g of NiCl2·6H2O (powder), and 0.042 g of urea (powder) were dissolved in 200 mL of distilled water, and the resulting solution was kept at 90°C for 6 hours to synthesize Ni-containing SnO2 nanosheets. In Example 3, the molar ratio of Sn:Ni:urea in the aqueous solution was 100:10:50.

[0061] A gas sensor was manufactured by forming a layer containing the synthesized Ni-containing SnO2 nanosheet on the surface of a substrate on which an electrode pair was formed under the same conditions as in Example 1. Example 3 is indicated as "10%Ni-Sn" in the drawings.

[0062] [Example 4] In Example 4, Ni-containing SnO nanosheets were synthesized by dissolving 0.218 g of SnF (powder), 0.0495 g of NiCl·6H O (powder), and 0.063 g of urea (powder) in 200 mL of distilled water and maintaining the solution at 90°C for 6 hours. In Example 4, the molar ratio of Sn:Ni:urea in the solution was 100:15:75.

[0063] A gas sensor was manufactured by forming a layer containing the synthesized Ni-containing SnO2 nanosheet on the surface of a substrate on which an electrode pair was formed under the same conditions as in Example 1. Example 4 is indicated as "15%Ni-Sn" in the drawings.

[0064] [Comparative Example 1] In Comparative Example 1, an aqueous solution in which 0.218 g of SnF2 (powder) was dissolved in 200 mL of distilled water was maintained at 90°C for 6 hours to synthesize SnO2 nanosheets that did not contain Ni.

[0065] A gas sensor was manufactured by forming a layer containing SnO2 nanosheets on the surface of a substrate on which an electrode pair was formed under the same conditions as in Example 1. Comparative Example 1 is indicated as "SnO2" in the drawings.

[0066] Under the synthesis conditions of Comparative Example 1 ("SnO2"), the pH was 3.48 when the aqueous solution was prepared, and after 22 minutes it was 2.55. On the other hand, under the synthesis conditions of Example 3 ("10% Ni-Sn"), the pH was 3.40 when the aqueous solution was prepared, and after 22 minutes it was 3.43. From the above, it was confirmed that the synthesis conditions of Example 3 were effective in suppressing a decrease in pH. It is believed that the above effect is due to the addition of urea (powder).

[0067] The following results are obtained for Examples 1 to 4 and Comparative Example 1. <1> ~ <5> The evaluation was carried out as follows. <3> ~ <5> The evaluation was performed on the gas sensor after heating in air at 300°C for 2 hours.

[0068] <1> Powder X-ray diffraction measurement 1 shows the diffraction patterns obtained by powder X-ray diffraction measurement of the Ni-containing SnO2 nanosheets (powder samples) synthesized in Examples 1 to 4 and the SnO2 nanosheet (powder sample) synthesized in Comparative Example 1. In the following description, the Ni-containing SnO2 nanosheets of Examples 1 to 4 and the SnO2 nanosheet of Comparative Example 1 may be simply referred to as powder samples.

[0069] 1, clear diffraction peaks attributable to SnO2 were observed for Examples 1 to 4 and Comparative Example 1. In particular, the strongest diffraction peak attributable to SnO2 was observed around 26.6°.

[0070] In Example 3 (10% Ni-Sn), a small diffraction peak due to NiO was observed around 42.5°. Whether or not the peak was due to NiO was determined using the powder diffraction database (PDF card number: 03-065-2901). This also applies to the following explanation. Note that the diffraction peak due to Ni(OH)2·0.75H2O was unclear because it was observed at approximately the same position as the diffraction peak due to SnO2.

[0071] 2(A) and (B) are diffraction patterns obtained by powder X-ray diffraction measurement for the powder samples of Examples 1 to 4 and Comparative Example 1 after heating in air (300°C, 2 hours). Note that (B) is an enlarged view of (A).

[0072] " <4> As will be described later in the section "Sensor Response," the sensor operating temperature in Comparative Example 1 and Example 1 was 250°C, and the sensor operating temperature in Examples 2 to 4 was 225°C. Therefore, when the sensor was operated, it was heated in the atmosphere. Based on this, in order to evaluate the influence of heating on the powder sample, the powder sample was evaluated by powder X-ray diffraction measurement after being heated in the atmosphere at 300°C. Note that the "integrated intensity of the diffraction peak derived from NiO when the integrated intensity of the diffraction peak derived from SnO2 is set to 100" in the sheet before being heated in the atmosphere (Figure 1) was approximately the same as that after being heated in the atmosphere (Figure 2).

[0073] As can be seen from Fig. 2, similar to Fig. 1, clear diffraction peaks derived from SnO2 were observed for Examples 1 to 4 and Comparative Example 1. In particular, the strongest diffraction peak derived from SnO2 was observed around 26.6°.

[0074] For Example 3, the strongest diffraction peak attributed to NiO was observed near 42.5°. The diffraction peak near 42.5° is attributed to the (200) diffraction line of NiO (PDF card number: 03-065-2901) in the powder diffraction database. The strongest diffraction peak attributed to SnO2 was observed near 26.6°. The diffraction peak near 26.6° is attributed to the (110) diffraction line of SnO2.

[0075] Comparing the integrated intensity of the diffraction peak due to NiO observed around 42.5° with the integrated intensity of the diffraction peak due to SnO2 observed around 26.6°, the integrated intensity of the diffraction peak due to NiO was 0.83 when the integrated intensity of the diffraction peak due to SnO2 was set to 100.

[0076] Considering the integrated intensity of the diffraction peak derived from NiO when the integrated intensity of the diffraction peak derived from SnO2 is set to 100, in Example 3, the Ni content (content of metallic Ni alone) in the Ni-containing SnO2 nanosheets is estimated to be greater than 0 mass% and less than 1 mass%, specifically approximately 0.85 mass%.

[0077] In Examples 1, 2, and 4, no clear peak was observed near 42.5°. Based on the above, it is estimated that the Ni content (content of metallic Ni alone) in the Ni-containing SnO2 nanosheets in Examples 1, 2, and 4 is greater than 0 mass% and less than 1 mass%. <4> Sensor Response" and " <5> From the results of "Activation Energy," it is considered that, similar to Example 3, Examples 1, 2, and 4 also contained Ni in the range of more than 0 mass % and 1 mass % or less.

[0078] <2> Scanning electron microscope observation 3 shows scanning electron microscope images of the surfaces of the gas sensors (after atmospheric heating) according to Examples 1 to 4 and Comparative Example 1. The image in the lower part of FIG. 3 is an enlarged image of the image in the upper part.

[0079] From the scanning electron microscope image in FIG. 3, aggregates of Ni-containing SnO2 nanosheets (SnO2 nanosheets in Comparative Example 1) having a thickness of 1 to 10 nm and an in-plane size of 10 to 500 nm were confirmed for the gas sensors of Examples 1 to 4 and Comparative Example 1.

[0080] <3> Transmission electron microscopy 4A and 4B are transmission electron microscope images of a cross section of the gas sensor (after atmospheric heating) according to Example 3. (a) is a high-angle annular dark field scanning TEM (HAADF-STEM) image, (b) is an elemental mapping image of nickel, (c) is an elemental mapping image of oxygen, and (d) is an elemental mapping image of tin.

[0081] As can be seen from Figure 4, in Example 3, the shape and size of the SnO2 nanosheets used in the Ni-containing SnO2 nanosheets were observed. Furthermore, contrast images of tin, oxygen, and nickel were observed along the shape of the SnO2 nanosheets. In particular, Figure 3(b) confirmed that Ni was contained throughout the entire SnO2 nanosheets.

[0082] In Examples 1 to 4, Ni was observed using EDS (energy dispersive X-ray spectroscopy) in transmission electron microscope observations, but not in quantities large enough to be quantified. From the above, the Ni content in the Ni-containing SnO2 nanosheets is estimated to be at most about 3% by mass, taking into account the quantification limit. Furthermore, taking into account the equipment used and the measurement environment, the Ni content in the Ni-containing SnO2 nanosheets is estimated to be at most about 1% by mass, more precisely. As described above, the Ni content in the Ni-containing SnO2 nanosheets does not correspond to the Ni concentration used in the production of the Ni-containing SnO2 nanosheets, and for some reason is much less than the amount expected from the Ni concentration used.

[0083] <4> Sensor Response 5 is a graph showing the sensor response (Ra / Rg) of the gas sensors (after atmospheric heating) according to Examples 1 to 4 and Comparative Example 1. The air used for measuring Ra was a mixed gas of 20% oxygen and 80% nitrogen.

[0084] 5(a) shows the sensor response of the gas sensor to 500 ppt of acetone, and FIG. 5(b) shows the sensor response of the gas sensor to 40 ppt of acetone. The sensor operating temperature in Comparative Example 1 and Example 1 is 250°C, and the sensor operating temperature in Examples 2 to 4 is 225°C.

[0085] The sensor response (Ra / Rg) to 500 ppt of acetone was 2.24 in Comparative Example 1, 5.67 in Example 1, 12.30 in Example 2, 21.11 in Example 3, and 3.05 in Example 4. Examples 1 to 4 showed higher sensor responses than Comparative Example 1. Among Examples 1 to 4, the sensor response of Example 3 (10% Ni—Sn) was the highest, approximately 9.50 times that of Comparative Example 1.

[0086] The sensor response (Ra / Rg) to 40 ppt of acetone was 1.10 in Comparative Example 1, 1.41 in Example 1, 1.72 in Example 2, 2.01 in Example 3, and 1.25 in Example 4. As in the case of 500 ppt of acetone, Examples 1 to 4 exhibited higher sensor responses than Comparative Example 1. Among Examples 1 to 4, the sensor response of Example 3 (10% Ni—Sn) was the highest, approximately 1.83 times that of Comparative Example 1.

[0087] Fig. 6 is a graph showing the sensor response (Ra / Rg) to acetone at several concentrations (40 ppt, 60 ppt, 100 ppt, 200 ppt, 500 ppt) within the range of 40 to 500 ppt for the gas sensors (after atmospheric heating) according to Examples 1 to 4 and Comparative Example 1, and Fig. 7 is a table of the sensor responses (Ra / Rg). The sensor operating temperature for Comparative Example 1 and Example 1 was 250°C, and the sensor operating temperature for Examples 2 to 4 was 225°C.

[0088] 6 and 7, Examples 1 to 4 showed higher sensor responses at all concentrations than Comparative Example 1. It was also confirmed that, among Examples 1 to 4, Example 3 (10% Ni—Sn) showed the highest sensor response at all concentrations.

[0089] From the approximation line (y=0.0407x+1.16) of the sensor response of Example 3, it was confirmed that the sensitivity (slope of the approximation curve) of the gas sensor of Example 3 was 0.0407. The sensitivity of the gas sensor of Example 3 was higher than that of Comparative Example 1 as well as Examples 1, 2, and 4. This means that Example 3 exhibits a larger change in sensor response in response to a change in gas concentration.

[0090] 8 is a graph showing the sensor response (Ra / Rg) to multiple gases (acetone, ethanol, isoprene, acetaldehyde, toluene, paraxylene, hydrogen, and ammonia) for Examples 2 and 3 and Comparative Example 1. The sensor operating temperature for Comparative Example 1 was 250°C, and the sensor operating temperature for Examples 2 and 3 was 225°C. The measurement conditions were a gas concentration of 500 ppb and a gas flow rate of 500 sccm.

[0091] The sensor response (Ra / Rg) to acetone was 60.66 for Example 3, 20.79 for Example 2, and 8.90 for Comparative Example 1.

[0092] The sensor response (Ra / Rg) to ethanol was 46.34 in Example 3, 10.15 in Example 2, and 19.45 in Comparative Example 1.

[0093] The sensor response (Ra / Rg) to isoprene was 42.01 for Example 3, 14.89 for Example 2, and 3.65 for Comparative Example 1.

[0094] The sensor response (Ra / Rg) to acetaldehyde was 25.06 in Example 3, 8.69 in Example 2, and 12.06 in Comparative Example 1.

[0095] The sensor response (Ra / Rg) to toluene was 11.83 for Example 3, 6.60 for Example 2, and 1.97 for Comparative Example 1.

[0096] The sensor response (Ra / Rg) to paraxylene was 8.72 in Example 3, 9.17 in Example 2, and 2.83 in Comparative Example 1.

[0097] The sensor response (Ra / Rg) to hydrogen was 4.17 in Example 3, 5.04 in Example 2, and 3.87 in Comparative Example 1.

[0098] The sensor response (Ra / Rg) to ammonia was 2.83 for Example 3, 1.55 for Example 2, and 1.82 for Example 3.

[0099] Examples 2 and 3 had higher sensor responses for all gases than Comparative Example 1. Specifically, the ratios of the sensor responses of Example 2 to those of Comparative Example 1 were 2.34, 0.52, 4.08, 0.72, 3.35, 3.24, 1.30, and 0.85 for acetone, ethanol, isoprene, acetaldehyde, toluene, paraxylene, hydrogen, and ammonia, respectively. The ratios of the sensor responses of Example 3 to those of Comparative Example 1 were 6.82, 2.38, 11.51, 2.08, 6.01, 3.08, 1.08, and 1.55 for acetone, ethanol, isoprene, acetaldehyde, toluene, paraxylene, hydrogen, and ammonia, respectively.

[0100] In Example 2, the sensor response to hydrogen was the reference, and the sensor responses to acetone, isoprene, ethanol, paraxylene, acetaldehyde, and toluene were highest, in that order, whereas the sensor response to ammonia was lowest. Specifically, the responses to acetone, isoprene, ethanol, paraxylene, acetaldehyde, and toluene were 4.13, 2.95, 2.01, 1.82, 1.72, and 1.31 times the sensor response to hydrogen, respectively. In contrast, the sensor response to ammonia was 0.31 times the sensor response to hydrogen.

[0101] For Example 3, the sensor responses to acetone, ethanol, isoprene, acetaldehyde, toluene, and paraxylene were high, whereas the sensor response to ammonia was low, relative to the sensor response to hydrogen. Specifically, the responses to acetone, ethanol, isoprene, acetaldehyde, toluene, and paraxylene were 14.55, 11.11, 10.07, 6.01, 2.84, and 2.09 times, respectively, relative to the sensor response to hydrogen. In contrast, the sensor response to ammonia was 0.68 times relative to the response to hydrogen.

[0102] On the other hand, in Comparative Example 1, when the sensor response to hydrogen was used as the standard, the sensor responses were slightly higher in the order of ethanol, acetaldehyde, and acetone, while the sensor responses were lower in the order of ammonia, toluene, paraxylene, and isoprene. Specifically, the sensor responses to ethanol, acetaldehyde, and acetone were 5.03, 3.12, and 2.30 times, respectively, relative to the response to hydrogen. In contrast, the responses to ammonia, toluene, paraxylene, and isoprene were 0.47, 0.51, 0.73, and 0.94 times, respectively, relative to the response to hydrogen.

[0103] The ratio of the sensor response to acetone relative to the sensor response to hydrogen was 2.30 times in Comparative Example 1, 14.55 times in Example 3, and 4.13 times in Comparative Example 2. In Examples 2 and 3, high acetone / hydrogen selectivity of 4.13 to 14.55 times was demonstrated due to the effect of Ni.

[0104] The ratio of the sensor response to isoprene relative to the sensor response to hydrogen was 0.94 times in Comparative Example 1, 10.07 times in Example 3, and 2.95 times in Example 2. Examples 2 and 3 showed high isoprene / hydrogen selectivity of 2.95 to 10.07 times due to the effect of Ni.

[0105] The ratio of the sensor response to toluene relative to the sensor response to hydrogen was 0.51 in Comparative Example 1, 2.84 in Example 3, and 1.31 in Example 2. In Examples 2 and 3, high toluene / hydrogen selectivity of 1.31 to 2.84 was demonstrated due to the effect of Ni.

[0106] The ratio of the sensor response to paraxylene based on the sensor response to hydrogen was 0.73 times in Comparative Example 1, 2.09 times in Example 3, and 1.82 times in Example 2. In Examples 2 and 3, high paraxylene / hydrogen selectivity of 1.82 to 2.09 times was demonstrated due to the effect of Ni.

[0107] <5> activation energy The resistance values ​​of the gas-sensitive layers of the gas sensors of Examples 1 to 4 and Comparative Example 1 were calculated. Fig. 9 shows the resistance values ​​(Ω) in air for the gas sensors of Examples 1 to 4 and Comparative Example 1. Using the resistance values ​​(Ω) of the gas-sensitive layers at 175°C, 200°C, 225°C, 250°C, 275°C, and 300°C shown in Fig. 9, the activation energy was calculated by Arrhenius plot.

[0108] The calculated activation energies were 0.269 eV in Comparative Example 1, 0.500 eV in Example 1, 0.799 eV in Example 2, 0.774 eV in Example 3, and 0.804 eV in Example 4. The activation energies of Examples 1 to 4 were higher than those of Comparative Example 1, and were shown to be characterized by being in the range of 0.4 eV or more and 0.9 eV or less.

[0109] As described above, Examples 1 to 4, which incorporated Ni, had significantly higher activation energies than Comparative Example 1, which did not contain Ni. This means that Ni affects the conduction mechanism (conduction mechanism) of charge carriers. It is believed that such changes in the conduction mechanism contributed to the improved performance of the gas sensor.

[0110] In addition, <4> Sensor response and <5> From the results of the activation energy, it can be said that Ni is also contained in Examples 1, 2, and 4, in which no diffraction peaks derived from NiO were observed in the powder X-ray diffraction measurement.

Claims

1. A substrate; a first electrode and a second electrode provided on a surface of the substrate; connected to the first electrode and the second electrode, ２ Nanosheets and the SnO ２ Ni-containing SnO containing Ni contained in nanosheets ２ A gas-sensitive layer having nanosheets A gas sensor comprising:

2. The SnO ２ The nanosheet contains the Ni as NiO. The gas sensor of claim 1.

3. The Ni-containing SnO ２ In the X-ray diffraction measurement of the nanosheet, SnO ２ The integrated intensity of the peak derived from NiO is greater than 0 and less than or equal to 1 when the integrated intensity of the peak derived from The gas sensor according to claim 2.

4. The Ni-containing SnO ２ The activation energy in the nanosheet is 0.4 eV or more and 0.9 eV or less. The gas sensor of claim 1.

5. The ratio of the sensor response (Ra / Rg) to acetone based on the sensor response (Ra / Rg) to hydrogen is 4.0 or more and 15.0 or less. The gas sensor of claim 1.

6. A Ni-containing SnO comprising an SnO 2 nanosheet and Ni contained in the SnO 2 nanosheet. ２ A method for producing nanosheets, comprising: By adding a tin compound, a Ni compound, and an alkali source compound to a solvent, Ni-containing SnO ２ Nanosheet synthesis process A manufacturing method comprising:

7. The solvent is water The method of claim 6.

8. The tin compound is tin fluoride. The method of claim 6.

9. The Ni compound is NiCl ２ ・6H ２ It is O The method of claim 6.

10. The alkaline source compound is urea The method of claim 6.

11. The tin compound and the Ni compound are added to the solvent so that the molar ratio of Ni is 1 to 20 when Sn is taken as 100. The method of claim 6.

12. The tin compound and the alkali source compound are added to the solvent so that the molar ratio of the alkali source compound is 1 to 80 when Sn is taken as 100. The method of claim 6.

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