Phenylacetic acid derivative-modified iron oxide nanoparticles, their manufacturing method, and gas sensor

Abstract

The present invention provides novel particles containing iron oxide, a method for producing the same, and a gas sensor having a gas-sensitive layer using the particles and having good response characteristics. [Solution] Phenylacetic acid derivative-modified iron oxide nanoparticles have the surface of iron oxide nanoparticles modified with a phenylacetic acid derivative, and have an average particle diameter of 10 to 100 nm.The crystal structure of the iron oxide nanoparticles has a γ-Fe2O3 ratio of 80 to 100% and an Fe3O4 ratio of 0% to 20%.

Description

[Technical Field]

[0001] The present invention relates to iron oxide nanoparticles modified with a phenylacetic acid derivative, a method for producing the same, and a gas sensor. [Background technology]

[0002] Gas sensors that use metal oxides (such as tin oxide and zinc oxide) in the gas-sensitive layer that is sensitive to the gas to be detected are widely used industrially. In such gas sensors, the surface of the metal oxide that makes up the gas-sensitive layer functions as a reaction site, so the composition and surface structure of the metal oxide are being studied to improve the performance of gas sensors.

[0003] For example, Patent Document 1 describes a gas sensor having a gas-sensitive layer in which a second layer containing cerium oxide particles is formed on a first layer containing tin oxide. Patent Document 2 describes a gas sensor having a gas-sensitive layer made of Ni-containing SnO2 nanosheets. Patent Document 3 describes a gas sensor having a gas-sensitive layer made of porous ZnO nanobelts. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2024-010596 [Patent Document 2] Japanese Patent Application Publication No. 2024-007259 [Patent Document 3] Japanese Patent Application Publication No. 2023-167934 Summary of the Invention [Problem to be solved by the invention]

[0005] However, gas sensors using iron oxide for the gas-sensitive layer have not been sufficiently studied, and further improvement in response characteristics has been desired.

[0006] The present invention has been made in view of the above circumstances, and has as its object to provide novel particles containing iron oxide, a method for producing the same, and a gas sensor having a gas-sensitive layer containing the particles and having good response characteristics. [Means for solving the problem]

[0007] In order to solve the above problems, the following phenylacetic acid derivative-modified iron oxide nanoparticles, a method for producing the same, and a gas sensor are provided. [1] Phenylacetic acid derivative-modified iron oxide nanoparticles, the surface of which is modified with a phenylacetic acid derivative, The average particle size is 10 to 100 nm, The crystalline structure of the iron oxide nanoparticles has a γ-Fe2O3 content of 80-100% and a Fe3O4 content of 0-20%. Phenylacetic acid derivative-modified iron oxide nanoparticles. [2] The content of the phenylacetic acid derivative in the phenylacetic acid derivative-modified iron oxide nanoparticles is 1 to 9 mass %. The phenylacetic acid derivative-modified iron oxide nanoparticles described in [1] above. [3] For the sensitive layer of gas sensors, The phenylacetic acid derivative-modified iron oxide nanoparticles according to [1] or [2] above. [4] A method for producing phenylacetic acid derivative-modified iron oxide nanoparticles according to [1], comprising the steps of: a hydrothermal synthesis step of obtaining a hydrothermal synthesis product of a phenylacetic acid derivative and an iron precursor; and an annealing step of heat-treating the hydrothermal synthesis product at 100 to 550°C; Including, Method for producing phenylacetic acid derivative-modified iron oxide nanoparticles. [5] The heat treatment temperature in the annealing step is 230 to 290°C. The method for producing phenylacetic acid derivative-modified iron oxide nanoparticles according to [4] above. [6] A substrate; a pair of electrodes formed on the substrate; a gas-sensitive layer formed on the substrate and connecting the pair of electrodes; Equipped with The gas-sensitive layer contains the phenylacetic acid derivative-modified iron oxide nanoparticles of [1] or [2]. Gas sensor. [7] The gas to be detected is at least one of ethanol, acetic acid, acetone, and acetaldehyde. The gas sensor described above in [6]. [8] Operating temperature is 200-240°C; The gas sensor according to [6] or [7] above. [9] It shows a response value (Ra / Rg) of 130 or more to 10 ppm of ethanol. The gas sensor described above in [7].

[10] It shows a response value (Ra / Rg) of 100 or more to 10 ppm of acetic acid. The gas sensor described above in [7].

[11] It shows a response value (Ra / Rg) of 180 or more to 10 ppm of acetone. The gas sensor described above in [7].

[12] A response value (Ra / Rg) of 110 or more for 10 ppm of acetaldehyde. The gas sensor described above in [7]. [Effects of the Invention]

[0008] The phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention have novel properties and can be suitably used in the gas-sensitive layer of a gas sensor. According to the method for producing the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention, the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention can be stably produced. The gas sensor of the present invention has good response characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram illustrating one embodiment of a gas sensor of the present invention. [Figure 2] FIG. 1 shows the results of X-ray diffractometry of phenylacetic acid derivative-modified iron oxide nanoparticles (Example 1) annealed at 230° C. [Figure 3]FIG. 1 shows a scanning electron microscope image of the precursor of phenylacetic acid derivative-modified iron oxide nanoparticles (without annealing treatment). [Figure 4] FIG. 1 shows the particle size distribution of the precursor of phenylacetic acid derivative-modified iron oxide nanoparticles (without annealing treatment). [Figure 5] FIG. 1 shows a scanning electron microscope image of phenylacetic acid derivative-modified iron oxide nanoparticles (Example 1) after annealing at 230° C. for 16 hours. [Figure 6] FIG. 1 shows the particle size distribution of phenylacetic acid derivative-modified iron oxide nanoparticles (Example 1) after annealing at 230° C. for 16 hours. [Figure 7] FIG. 1 shows the results of thermogravimetry of phenylacetic acid derivative-modified iron oxide nanoparticles. [Figure 8] 1 is a graph showing the operating temperature dependence of the sensor response value of a gas sensor using the phenylacetic acid derivative-modified iron oxide nanoparticles of Example 1 to 500 ppb of acetone. [Figure 9] FIG. 1 shows the results of X-ray diffractometry of phenylacetic acid derivative-modified iron oxide nanoparticles (Example 2) annealed at 290° C. [Figure 10] FIG. 1 shows a scanning electron microscope image of phenylacetic acid derivative-modified iron oxide nanoparticles (Example 2) after annealing at 290° C. for 3 hours. [Figure 11] FIG. 1 shows the particle size distribution of phenylacetic acid derivative-modified iron oxide nanoparticles (Example 2) after annealing at 290° C. for 3 hours. [Figure 12] 1 is a graph showing the sensor response (Ra / Rg) to a plurality of target gases for a gas sensor using phenylacetic acid derivative-modified iron oxide nanoparticles (Example 2) after annealing at 290° C. for 3 hours. [Figure 13] 1 is a graph showing the sensor response (Ra / Rg) to a low concentration of acetone for a gas sensor using phenylacetic acid derivative-modified iron oxide nanoparticles (Example 2). [Figure 14] 1 shows the results of measurement with an X-ray diffractometer of iron oxide nanoparticles (Comparative Example 1) after annealing pretreatment at 260° C. [Figure 15] FIG. 1 shows a scanning electron microscope image of iron oxide nanoparticles (Comparative Example 1) before annealing treatment. [Figure 16] FIG. 1 shows a scanning electron microscope image of iron oxide nanoparticles (Comparative Example 1) after annealing at 260° C. for 8 hours. [Figure 17] FIG. 1 shows the particle size distribution of iron oxide nanoparticles (Comparative Example 1) after annealing at 260° C. for 8 hours. [Figure 18] 10 is a graph showing the operating temperature dependence of the sensor response value to 10 ppm of acetone of a gas sensor using iron oxide nanoparticles (Comparative Example 1) after annealing at 260° C. for 8 hours. [Figure 19] 1 is a graph showing the sensor response (Ra / Rg) to a plurality of target gases for a gas sensor using iron oxide nanoparticles (Comparative Example 1) after annealing at 260° C. for 8 hours. [Figure 20] 1 is a graph showing the sensor response (Ra / Rg) to a low concentration of acetone for a gas sensor using iron oxide nanoparticles (Comparative Example 1) after annealing at 260° C. for 8 hours. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, one embodiment of the phenylacetic acid derivative-modified iron oxide nanoparticles, the method for producing the same, and the gas sensor of the present invention will be described.

[0011] (Phenylacetic acid derivative-modified iron oxide nanoparticles) The phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention have the surface of the iron oxide nanoparticles modified with a phenylacetic acid derivative.

[0012] The phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention are preferably used for the sensitive layer of a gas sensor.

[0013] The crystal structure of iron oxide nanoparticles has a ratio of γ-Fe2O3 of 80 to 100% and a ratio of Fe3O4 of 0 to 20%.

[0014] The phenylacetic acid derivative is not particularly limited as long as it has a functional group capable of binding to the surface of iron oxide nanoparticles, and examples thereof include dihydroxyphenylacetic acid, phenylacetic acid bis(carboxymethyl)amine, phenylacetic acid bis(aminomethyl)ethanol, phenylacetic acid aminoethyl, phenylacetic acid propylamine, phenylacetic acid ethyl aminoethanol, phenylacetic acid dimethylaminoethanol, etc. Furthermore, these phenylacetic acid derivatives may be phenylacetic acid derivatives that have been modified by heat.

[0015] The phenylacetic acid derivative-modified iron oxide nanoparticles have an average particle size of 10 to 100 nm, which is the average value of particle sizes measured for several dozen particles randomly from, for example, a transmission electron microscope (TEM) image or a scanning electron microscope (SEM) image.

[0016] The content of the phenylacetic acid derivative in the phenylacetic acid derivative-modified iron oxide nanoparticles is preferably 1 to 9 mass %, more preferably 3 to 7 mass %. When the content of the phenylacetic acid derivative is within this range, the phenylacetic acid derivative-modified iron oxide nanoparticles can form a gas-sensitive layer of a gas sensor with good response characteristics.

[0017] (Method of manufacturing phenylacetic acid derivative-modified iron oxide nanoparticles) The method for producing phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention comprises the steps of: a hydrothermal synthesis step of obtaining a hydrothermal synthesis product of a phenylacetic acid derivative and an iron precursor; and an annealing step of heat-treating the hydrothermal synthesis product at 100 to 550°C; Each step will be explained below.

[0018] In the hydrothermal synthesis step, a hydrothermal synthesis product of a phenylacetic acid derivative and an iron precursor is obtained. The hydrothermal synthesis product is a precursor of the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention, and the surface of the iron oxide nanoparticles is modified with the phenylacetic acid derivative, and the crystal structure of the iron oxide nanoparticles is Fe3O4.

[0019] The phenylacetic acid derivative may be, for example, the above-mentioned compound. The iron precursor may be, for example, iron sulfate, iron chloride, iron nitrate, or the like.

[0020] The method and conditions for hydrothermal synthesis are not particularly limited, but for example, a solvent (preferably water) containing a phenylacetic acid derivative and an iron precursor can be hydrothermally treated at a temperature of 100 to 400°C and a pressure of 1 to 20 MPa. The solvent may be a mixture of water and an organic solvent, and may also contain an acid or a base.

[0021] In the annealing step, the hydrothermal synthesis product obtained in the hydrothermal synthesis step is heat-treated in air at 100 to 550°C. The annealing step changes the crystal structure of the iron oxide nanoparticles, resulting in a γ-Fe2O3 content of 80 to 100% and a Fe3O4 content of 0 to 20%. Furthermore, in the annealing step, approximately 40 to 85% of the phenylacetic acid derivative remains on the surface of the iron oxide nanoparticles. In other words, the annealing step synthesizes the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention, which have an average particle size of 10 to 100 nm and a crystal structure in which the γ-Fe2O3 content is 80 to 100% and the Fe3O4 content is 0 to 20%.

[0022] The heat treatment temperature in the annealing step is preferably 230 to 290°C. When the heat treatment temperature is within this range, the crystal structure of the iron oxide nanoparticles is stably maintained at the above ratio, and the phenylacetic acid derivative remains on the surface of the iron oxide nanoparticles. Furthermore, the heat treatment in the annealing step is preferably carried out in air or a gas containing oxygen.

[0023] When the phenylacetic acid derivative-modified iron oxide nanoparticles are applied to a gas sensor, the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention can be synthesized, for example, by coating a substrate with a precursor of the phenylacetic acid derivative-modified iron oxide nanoparticles and then performing an annealing step, in which case a gas-sensitive layer of the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention is formed on the substrate.

[0024] The method for producing phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention may include known treatment steps other than the above-mentioned hydrothermal synthesis step and annealing step.

[0025] (gas sensor) 1 is a schematic diagram illustrating one embodiment of a gas sensor of the present invention. The gas sensor 1 of the present invention comprises a gas-sensitive layer 2, a substrate 3, and a pair of electrodes 4. In this embodiment of the gas sensor, a heater 5 for heating the sensor and a heater electrode 6 (made of platinum or the like) are provided on the lower surface of the substrate 3.

[0026] The substrate 3 may be, for example, a ceramic substrate (alumina (Al2O3), zirconia (ZrO2), etc.), a silicon substrate, or a glass substrate.

[0027] A pair of electrodes 4 are formed on the substrate 3. Examples of materials for the electrodes 4 include platinum and gold.

[0028] The gas-sensitive layer 2 contains the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention described above. The gas-sensitive layer 2 is formed on a substrate 3 and electrically connects a pair of electrodes 4. The gas sensor 1 of the present invention has the gas-sensitive layer 2 containing the phenylacetic acid derivative-modified iron oxide nanoparticles of the present invention, and thus exhibits a high sensor response to the gas to be detected.

[0029] The thickness of the gas-sensitive layer 2 can be set appropriately depending on the type and characteristics of the gas to be detected, and can be in the range of, for example, about 10 μm to 5 mm.

[0030] For example, a conductive coating film (particle film) can be formed from a paint (dispersion) containing phenylacetic acid derivative-modified iron oxide nanoparticles by a coating method or the like to form the gas-sensitive layer 2 (sensor element). The method for forming the gas-sensitive layer 2 is not particularly limited, and it can be formed using a coating method such as slit coating, spin coating, bar coating, or spray coating. The solvent used in the paint (dispersion) containing phenylacetic acid derivative-modified iron oxide nanoparticles may be any solvent that can disperse the phenylacetic acid derivative-modified iron oxide nanoparticles, and examples of such solvents include water and alcohol. The alcohol may be one or more of isopropanol, ethanol, methanol, n-propanol, isobutanol, n-butanol, etc.

[0031] Furthermore, a binder can be appropriately blended into the gas-sensitive layer 2 (dispersion liquid). The binder is not limited, and examples include one or more types selected from organic binders and inorganic binders. Examples of organic binders include cellulose derivatives, vinyl resins, fluorine-based resins, silicone resins, acrylic resins, epoxy resins, polyester resins, melamine resins, urethane resins, and alkyd resins. Examples of inorganic binders include products obtained by decomposing hydrolyzable silicon compounds such as alkyl silicates, silicon halides, and their partial hydrolyzates; organic polysiloxane compounds and their polycondensates; silica; colloidal silica; water glass; silicon compounds; phosphates such as zinc phosphate; metal oxides such as zinc oxide and zirconium oxide; biphosphates; cement; gypsum; lime; and enamel frit.

[0032] Furthermore, various additives other than the binder may be blended into the gas-sensitive layer 2 (dispersion liquid), such as, for example, a defoaming agent, a crosslinking agent, a curing catalyst, a pigment dispersant, an emulsifier, a film-forming aid, a thickener, a neutralizing agent, and a preservative.

[0033] In the gas sensor 1 of the present invention, the gas to be detected is preferably at least one of ethanol, acetic acid, acetone, and acetaldehyde. These gases are thought to be associated with diseases, and the gas sensor of the present invention exhibits a high sensor response even to low concentrations of these gases contained in biological gases (e.g., exhaled breath and skin gases).

[0034] The gas sensor 1 of the present invention preferably has an operating temperature of 200 to 240° C. When the operating temperature is in this range, the gas sensor 1 of the present invention exhibits higher responsiveness to the gas to be detected.

[0035] Specifically, the gas sensor 1 of the present invention exhibits a response value (Ra / Rg) of 130 or more to 10 ppm of ethanol, a response value (Ra / Rg) of 100 or more to 10 ppm of acetic acid, a response value (Ra / Rg) of 180 or more to 10 ppm of acetone, and a response value (Ra / Rg) of 110 or more to 10 ppm of acetaldehyde.

[0036] The phenylacetic acid derivative-modified iron oxide nanoparticles, the method for producing the same, and the gas sensor of the present invention are not limited to the above embodiments. [Example]

[0037] The phenylacetic acid derivative-modified iron oxide nanoparticles, the method for producing the same, and the gas sensor of the present invention will be described in detail below with reference to examples, although the present invention is not limited to these examples in any way.

[0038] 1. Synthesis of phenylacetic acid derivative-modified iron oxide nanoparticles and fabrication of gas sensors Example 1 (1) Precursor of phenylacetic acid derivative-modified iron oxide nanoparticles The precursor of phenylacetic acid derivative-modified iron oxide nanoparticles was prepared as follows. First, an aqueous solution (0.1 M each) of iron sulfate heptahydrate (FeSO4·7H2O; Fujifilm Wako Pure Chemical Industries, Ltd.) and 3,4-dihydroxyphenylacetic acid (Combi-Blocks) was prepared, and KOH (5.0 M) was added to adjust the pH to 9.5. The solution was then sealed in a batch reactor and heated for 2 hours using a heater preheated to 300°C to synthesize the precursor of phenylacetic acid derivative-modified iron oxide nanoparticles (hydrothermal synthesis process). The batch reactor was then cooled, and the resulting precursor of phenylacetic acid derivative-modified iron oxide nanoparticles was recovered and centrifuged to remove impurities.The precursor of phenylacetic acid derivative-modified iron oxide nanoparticles was then powdered using a freeze dryer.

[0039] (2) Gas sensor A pair of Pt interdigital electrodes was formed on the surface of the alumina substrate using screen printing. A Pt heater electrode, which was used to heat the sensor, was also formed on the back of the alumina substrate. The powdered precursor of phenylacetic acid derivative-modified iron oxide nanoparticles synthesized in (1) above was dispersed in distilled water using an ultrasonic cleaner to obtain a dispersion. This dispersion was dropped onto the prepared alumina substrate and then dried, thereby coating (modifying) the alumina substrate with the precursor of phenylacetic acid derivative-modified iron oxide nanoparticles. Thereafter, the precursor of the phenylacetic acid derivative-modified iron oxide nanoparticles was subjected to heat treatment (annealing treatment) at 230°C for 30 minutes to 16 hours to synthesize the phenylacetic acid derivative-modified iron oxide nanoparticles on the alumina substrate (annealing step).

[0040] <Example 2> (1) Precursor of phenylacetic acid derivative-modified iron oxide nanoparticles The precursor of phenylacetic acid derivative-modified iron oxide nanoparticles prepared by the same procedure as in Example 1 was used. (2) Gas sensor A gas sensor was fabricated in the same manner as in Example 1, except that an alumina substrate was coated (modified) with a precursor of phenylacetic acid derivative-modified iron oxide nanoparticles and then annealed at 290°C for 3 hours. Phenylacetic acid derivative-modified iron oxide nanoparticles were synthesized on the alumina substrate by the annealing treatment.

[0041] <Comparative Example 1> (1) Iron oxide nanoparticles Iron oxide nanoparticles were produced as follows: First, an aqueous solution (0.1 M) of iron nitrate nonahydrate (Fe(NO3)3·9H2O; Fujifilm Wako Pure Chemical Industries, Ltd.) was prepared, and formic acid (HCOOH; Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the solution to make the concentration 0.02 M. The solution was then sealed in a batch reactor and heated for 1 hour using a heater preheated to 400°C to synthesize iron oxide nanoparticles. The batch reactor was then cooled, and the synthesized iron oxide nanoparticles were collected and centrifuged to remove impurities, followed by pulverization using a freeze dryer. (2) Gas sensor A pair of Pt interdigital electrodes was formed on the surface of the alumina substrate using screen printing. A Pt heater electrode, which was used to heat the sensor, was also formed on the back of the alumina substrate. The powdered iron oxide nanoparticles synthesized in (1) above were dispersed in distilled water using an ultrasonic cleaner to obtain a dispersion liquid. This dispersion liquid was dropped onto the prepared alumina substrate and then dried, thereby coating (modifying) the alumina substrate with iron oxide nanoparticles. Thereafter, the iron oxide nanoparticles were subjected to annealing treatment at 260°C for 1 to 16 hours.

[0042] 2. Various measurements and evaluations <1> Phenylacetic acid derivative-modified iron oxide nanoparticles of Example 1 The phenylacetic acid derivative-modified iron oxide nanoparticles of Example 1 were measured using an X-ray diffractometer, and the results are shown in Figure 2.

[0043] As can be seen from the peak for "without annealing" in Figure 2, the crystal structure of the precursor of the phenylacetic acid derivative-modified iron oxide nanoparticles in Example 1 without annealing was 100% Fe3O4, and the crystal structure of the phenylacetic acid derivative-modified iron oxide nanoparticles after annealing at 230°C (30 min to 16 h) was confirmed to be a mixture of 0% to 20% Fe3O4 and 80% to 100% γ-Fe2O3.

[0044] The phenylacetic acid derivative-modified iron oxide nanoparticles of Example 1 were observed under an electron microscope. The results are shown in Figures 3 and 5, and the particle size distributions calculated from the respective electron microscope images are shown in Figures 4 and 6. Figure 3 is a scanning electron microscope (SEM) image of the precursor of the phenylacetic acid derivative-modified iron oxide nanoparticles without annealing treatment, and Figure 5 is a scanning electron microscope (SEM) image of the phenylacetic acid derivative-modified iron oxide nanoparticles after annealing treatment at 230°C for 16 hours.

[0045] As can be seen from FIGS. 4 and 6, the average particle size of the phenylacetic acid derivative-modified iron oxide nanoparticles according to Example 1 was in the range of 10 nm to 100 nm.

[0046] Thermogravimetric analysis was carried out on the phenylacetic acid derivative-modified iron oxide nanoparticles used in Example 1. The results are shown in FIG.

[0047] As can be seen from Figure 7, the weight of the phenylacetic acid derivative-modified iron oxide nanoparticles was the largest at 101%, and was 98.7% at 230 °C and 93.4% at 600 °C. Therefore, it is thought that in the sample heated to 230 °C, (98.7 - 93.4) / (101 - 93.4) ​​× 100 = 79.1% of the phenylacetic acid derivative remained on the surface of the iron oxide nanoparticles.

[0048] In addition, the content of phenylacetic acid derivatives in the sample (precursor of phenylacetic acid derivative-modified iron oxide nanoparticles) before annealing (room temperature) was (100-93.4) / 100×100=6.6 (mass%). The content of phenylacetic acid derivatives in the sample after heating at 600°C is 0% by mass. The content of phenylacetic acid derivatives in the sample (phenylacetic acid derivative-modified iron oxide nanoparticles) after heating at 230°C was (98.7-93.4) / 98.7×100=5.4 (mass%). Furthermore, in FIG. 7, when attention is focused on the range of 100° C. to 550° C., the content of the phenylacetic acid derivative in the phenylacetic acid derivative-modified iron oxide nanoparticles in this range is approximately 3 to 7 mass %.

[0049] <2> Gas sensor of Example 1 The gas sensor of Example 1 was evaluated for its sensor response (Ra / Rg) to 500 ppb of acetone when the operating temperature was changed in the range of 140°C to 290°C. Dry air (nitrogen:oxygen = 80:20) was used for Ra, and 500 ppb of acetone gas was used for Rg, and the resistance was measured using a sensor evaluation module. The gas flow rate was controlled using a multi-component gas mixer (MU-3609, manufactured by Horiba, Ltd.), and the total flow rate was 500 cm. 3 min -1 The driving temperature was changed to 140° C., 170° C., 200° C., 230° C., 260° C., and 290° C. The results are shown in FIG.

[0050] As can be seen from Figure 8, the sensor response (Ra / Rg) showed the highest value (21) when the operating temperature was 230°C. It was confirmed that the sensor response (Ra / Rg) was good when the operating temperature was 200 to 240°C.

[0051] <3> Phenylacetic acid derivative-modified iron oxide nanoparticles of Example 2 The phenylacetic acid derivative-modified iron oxide nanoparticles of Example 2 were measured using an X-ray diffractometer, and the results are shown in FIG.

[0052] The crystal structure of the phenylacetic acid derivative-modified iron oxide nanoparticles after annealing at 290°C (3 h) in Figure 9 was confirmed to be a mixture of 0% to 20% Fe3O4 and 80% to 100% γ-Fe2O3.

[0053] The phenylacetic acid derivative-modified iron oxide nanoparticles of Example 2 were observed under an electron microscope. The results are shown in Figure 10, and the particle size distribution calculated from the electron microscope image is shown in Figure 11. Figure 10 is a scanning electron microscope (SEM) image of the phenylacetic acid derivative-modified iron oxide nanoparticles after annealing at 290°C for 3 hours.

[0054] As can be seen from FIG. 11, the average particle size of the phenylacetic acid derivative-modified iron oxide nanoparticles according to Example 2 was in the range of 10 nm to 100 nm.

[0055] The results of thermogravimetric analysis of the phenylacetic acid derivative-modified iron oxide nanoparticles of Example 2 are shown in Figure 7, similar to Example 1. As can be seen from Figure 7, the weight of the phenylacetic acid derivative-modified iron oxide nanoparticles was greatest at 101%, 96.6% at 290°C, and 93.4% at 600°C. Therefore, it is believed that after heating at 290°C, 47.8% (96.6 - 93.4) / (101 - 93.4) ​​x 100 = phenylacetic acid derivative remained on the surface of the iron oxide nanoparticles. That is, the content of phenylacetic acid derivatives in the sample (phenylacetic acid derivative-modified iron oxide nanoparticles) after heating at 290°C is (96.6-93.4) / 96.6×100=3.3 (mass%).

[0056] <4> Gas sensor of Example 2 The sensor response (Ra / Rg) was evaluated when the concentration of each target gas was changed for the gas sensor of Example 2. A commercially available gas sensor, TGS2602 manufactured by Figaro Giken Co., Ltd., was used for comparison.

[0057] For Example 2, the temperature of the gas sensor was controlled to 230°C, and for Comparative Example 2, a voltage of 5,000 V was applied, and the resistance value under the flow of the target gas was measured using a sensor evaluation module. For Ra, dry air (nitrogen:oxygen = 80:20) was used, and for Rg, dry air containing the target gas was used, and the resistance value was measured using a sensor evaluation module. As the target gas, multiple gases contained in exhaled breath were selected. Specifically, ethanol, acetic acid, acetone, acetaldehyde, toluene, and hydrogen were used as the target gases. The gas flow rate was controlled using a multi-component gas mixer (MU-3609, manufactured by Horiba, Ltd.), and the total flow rate was 500 cm. 3 min -1 The results are shown in Figure 12.

[0058] As shown in Figure 12, Example 2 had better sensor response (Ra / Rg) for ethanol, acetic acid, acetone, and acetaldehyde than the commercially available products, and it was confirmed that the sensor response (Ra / Rg) improved with increasing concentration.

[0059] The sensor response (Ra / Rg) at 10 ppm for each target gas is shown in Table 1. For ethanol, Example 2 was approximately 6.5 to 7.5 times (130 / 20, 150 / 20) that of the commercial product. For acetic acid, Example 2 was approximately 8.3 times (100 / 12) that of the commercial product. For acetone, Example 2 was approximately 3.5 to 3.7 times (180 / 51, 190 / 51) that of the commercial product. For acetaldehyde, Example 2 was approximately 2.8 to 3.6 times (110 / 39, 140 / 39) that of the commercial product.

[0060] [Table 1]

[0061] The sensor response (Ra / Rg) at lower concentrations of acetone, which showed a particularly good sensor response (Ra / Rg), was further evaluated. The results are shown in Figure 13.

[0062] As shown in Figure 13, when focusing on the sensor response (Ra / Rg) at 100 ppb of acetone, the sensor response (Ra / Rg) of the gas sensor of Example 2 was approximately 2.9 times (7.8 / 2.7) that of the commercially available product, confirming that it exhibited a high response even to low-concentration gases at the ppb level.

[0063] <5> Iron oxide nanoparticles of Comparative Example 1 The iron oxide nanoparticles of Comparative Example 1 were measured using an X-ray diffractometer, and the results are shown in FIG.

[0064] As can be seen from the peaks labeled "No annealing" in Figure 14, the crystalline structure of the iron oxide nanoparticles without annealing in Comparative Example 1 is 100% Fe3O4. Furthermore, as can be seen from the peaks at 2θ = 18° to 19°, the peak intensity after annealing at 260°C (1 hour to 16 hours) is reduced to 47% of the peak intensity without annealing at 260°C, suggesting that the crystalline structure of the remaining iron oxide nanoparticles has changed to α-Fe2O3 or γ-Fe2O3. Therefore, the crystalline structure of the iron oxide nanoparticles after annealing at 260°C (1 hour to 16 hours) is considered to be a mixture of 43% Fe3O4 and 57% α-Fe2O3 or γ-Fe2O3.

[0065] The iron oxide nanoparticles of Comparative Example 1 were observed under an electron microscope. The results are shown in Figures 15 and 16. Figure 15 is a scanning electron microscope (SEM) image of iron oxide nanoparticles without annealing pretreatment, and Figure 16 is a scanning electron microscope (SEM) image of iron oxide nanoparticles after annealing pretreatment at 260°C for 8 hours. The particle size distribution calculated from the electron microscope image in Figure 16 is shown in Figure 17.

[0066] As can be seen from FIG. 17, the average particle size of the iron oxide nanoparticles according to Comparative Example 1 was in the range of 100 nm to 400 nm.

[0067] <6> Gas sensor of comparative example 1 The gas sensor according to Comparative Example 1 was evaluated for its sensor response (Ra / Rg) to 10 ppm of acetone when the operating temperature was changed in the range of 150 to 300° C. The operating temperature was changed to 150° C., 200° C., 220° C., 240° C., 260° C., 280° C., and 300° C. The results are shown in FIG.

[0068] As can be seen from FIG. 18, the sensor response (Ra / Rg) of the gas sensor of Comparative Example 1 exhibited the highest value (23) when the operating temperature was 260°C.

[0069] The gas sensor according to Comparative Example 1 was evaluated for its sensor response (Ra / Rg) when the concentration of each target gas was changed. The measurement was carried out in the same manner as in Example 2. However, the temperature of the gas sensor was controlled to 260°C, and the total gas flow rate was 250 cm 3 min -1 The results are shown in Figure 19.

[0070] As shown in FIG. 19, the sensor response (Ra / Rg) at 10 ppm for each target gas was 16 for ethanol, 11 for acetic acid, 19 for acetone, and 13 for acetaldehyde. Therefore, the sensor response (Ra / Rg) for 10 ppm for each target gas was approximately 8.1 to 9.4 times (130 / 16, 150 / 16) higher for ethanol than for Comparative Example 1. For acetic acid, the response was approximately 9.1 times (100 / 11) higher for Example 2 than for Comparative Example 1. For acetone, the response was approximately 9.5 to 10 times (180 / 19, 190 / 19) higher for Example 2 than for Comparative Example 1. For acetaldehyde, the response was approximately 8.5 to 11 times (110 / 13, 140 / 13) higher for Example 2 than for Comparative Example 1.

[0071] The sensor response (Ra / Rg) was particularly good for acetone, so the sensor response (Ra / Rg) was evaluated at even lower concentrations. The results are shown in Figure 20.

[0072] 20, the sensor response (Ra / Rg) at 100 ppb in Comparative Example 2 was 2.1. Therefore, the sensor response (Ra / Rg) to 100 ppb of acetone in Example 2 was approximately 3.7 times (7.8 / 2.1) that of Comparative Example 1. [Explanation of symbols]

[0073] 1 Gas sensor 2 Gas-sensitive layer 3. Circuit Board 4 electrodes 5. Heater 6 Heater electrode

Claims

1. Phenylacetic acid derivative-modified iron oxide nanoparticles, the surface of which is modified with a phenylacetic acid derivative, The average particle size is 10 to 100 nm, The crystalline structure of the iron oxide nanoparticles is γ-Fe ２ O ３ The ratio of Fe is 80 to 100%. ３ O ４ The proportion of is 0% to 20%. Phenylacetic acid derivative-modified iron oxide nanoparticles.

2. The content of the phenylacetic acid derivative in the phenylacetic acid derivative-modified iron oxide nanoparticles is 1 to 9 mass %. The phenylacetic acid derivative-modified iron oxide nanoparticles of claim 1.

3. For the sensitive layer of gas sensors, The phenylacetic acid derivative-modified iron oxide nanoparticles of claim 1.

4. A method for producing the phenylacetic acid derivative-modified iron oxide nanoparticles of claim 1, comprising the steps of: a hydrothermal synthesis step of obtaining a hydrothermal synthesis product of a phenylacetic acid derivative and an iron precursor; and an annealing step of heat-treating the hydrothermal synthesis product at 100 to 550°C; Including, Method for producing phenylacetic acid derivative-modified iron oxide nanoparticles.

5. The heat treatment temperature of the annealing step is 230 to 290°C. The method for producing the phenylacetic acid derivative-modified iron oxide nanoparticles according to claim 4.

6. A substrate; a pair of electrodes formed on the substrate; a gas-sensitive layer formed on the substrate and connecting the pair of electrodes; Equipped with The gas-sensitive layer comprises the phenylacetic acid derivative-modified iron oxide nanoparticles of claim 1. Gas sensor.

7. The gas to be detected is at least one of ethanol, acetic acid, acetone, and acetaldehyde. The gas sensor according to claim 6.

8. The operating temperature is 200-240°C; The gas sensor according to claim 6.

9. A response value (Ra / Rg) of 130 or more to 10 ppm of ethanol. The gas sensor of claim 7.

10. It exhibits a response value (Ra / Rg) of 100 or more to 10 ppm of acetic acid. The gas sensor of claim 7.

11. It exhibits a response value (Ra / Rg) of 180 or more to 10 ppm of acetone. The gas sensor of claim 7.

12. It exhibits a response value (Ra / Rg) of 110 or more to 10 ppm of acetaldehyde. The gas sensor of claim 7.

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