JP6315686B2 - Novel stannic oxide material, synthesis method thereof, and gas sensor material - Google Patents

Description

  The present invention relates to a novel stannic oxide material suitable for a gas sensor and the like, and a synthesis method thereof.

  The inventors have worked on synthesizing monodisperse nanoparticles such as metal oxides in the liquid phase. For example, in Patent Document 1 (JP2011-126746), the synthesis of ink of transparent conductive nanoparticles in which cubic ITO crystallites are monodispersed is disclosed for an ITO electrode in a liquid crystal display or the like.

  Regarding stannic oxide, which is a gas sensor material, Patent Document 2 (JP5234664) discloses synthesizing stannic oxide nanoparticles. For example, SnIV acetylacetonate dichloride is dissolved in a water insoluble solvent such as dibenzyl ether, dispersed in water to form micelles, supplied with oxygen from an oxidizing agent such as trimethylamine oxide, and refluxed at 280 ° C. Thus, it is disclosed that stannic oxide nanoparticles are obtained. However, although the stannic oxide crystallite obtained in this way is not rod-shaped and can be said to be close to a cubic shape, the corners of the particles are taken, and the stannic oxide crystallite of the present invention is in the form Is different.

JP2011-126746 JP5234664

  An object of the present invention is to provide a novel stannic oxide material, a synthesis method thereof, and a gas sensor material.

The stannic oxide material of the present invention is composed of rod-shaped SnO ₂ crystallites, the side surface of the rod is the (110) plane of SnO ₂ crystal, and the SnO ₂ crystallite is in a monodispersed state. In the stannic oxide of the present invention, individual crystallites are primary particles, and secondary particles are not developed, that is, stannic oxide crystallites are monodispersed. The surface of the crystallite is mainly the (110) plane, and the properties of the (110) plane appear as surface properties. For example, when a gas sensor material is used, characteristics such as high sensitivity specifically to ethanol occur.

The stannic oxide material of the present invention is hydrothermally synthesized by heating a strong acidic aqueous solution containing Sn ⁴⁺ ions. And by selecting the conditions for hydrothermal synthesis,
A monodispersed SnO ₂ crystallite that is rod-shaped and has the (110) face of the SnO ₂ crystal exposed on the side of the rod,
And hydrothermal synthesis of a monodispersed SnO ₂ crystallite having a cubic shape and a surface of the (001) plane of SnO ₂ crystal.

For example, when triethanolamine (TEOA: N- (C ₂ H ₄ OH) ₃ ) or the like is present in an aqueous solution during hydrothermal synthesis, rod-shaped SnO ₂ crystallites are produced, and ethylenediamine (EDA: NH _2- When CH ₂ —CH ₂ —NH ₂ ) or monoethanolamine (MEA: NH ₂ — (C ₂ H ₄ OH)) is present in an aqueous solution, cubic SnO ₂ crystallites are formed.

  The preferred temperature for hydrothermal synthesis is, for example, 160 ° C.-250 ° C., and the reaction time is 1 hour-120 hours. Further, strong acidity means that the pH at room temperature is less than 1, and the nanoparticle has a beautiful rod shape or cubic shape and is monodispersed by hydrothermal synthesis under strong acidity. When the pH exceeds 1, cubic corners are formed, the variation in the form of nanoparticles increases, and secondary particles in which primary particles are bonded to each other are generated. For hydrothermal synthesis of rod-like crystallites, the pH is preferably 0.6 or less and 0 or more, and in the presence of triethanolamine, for example, pH is 0.4 and rod-like crystallites are generated, and 0.8 is a cubic crystallite. Produces. For example, it is made strongly acidic with hydrochloric acid, but nitric acid or the like may be used. Organic amino compounds such as triethanolamine are pH adjusters, and when the pH is adjusted with NaOH, the corners of cubic particles can be taken.

Further, the present invention is a gas sensor material comprising rod-shaped SnO ₂ crystallites, the side surfaces of which are (110) faces of SnO ₂ crystals, and the SnO ₂ crystallites are monodispersed. This gas sensor material has high ethanol sensitivity in heated and dry air, and is suitable for detection of ethanol in exhaled air. The gas sensor material is formed into a film, for example, a pair of electrodes are connected, and a heater for heating is provided to form a gas sensor. Further, this SnO ₂ may carry a catalyst such as Pd, Pt, Au or the like.

The stannic oxide of the present invention becomes an ink material for printing a transparent conductive film when doped with antimony or the like in addition to the gas sensor material. Since the shape is constant and there are no secondary particles, a dense film can be obtained and the resistance becomes low. The stannic oxide composed of cubic nanoparticles was white in antimony free, and the resistivity of the molded body was, for example, 2.9 × 10 ⁴ Ω · cm at room temperature. In the hydrothermal synthesis of stannic oxide nanoparticles, when antimony trichloride was added to the reaction solution, Sb-doped stannic oxide nanoparticles could be synthesized. When 2.4 mol% Sb was doped with respect to 1 mol of SnO ₂ , the color tone became blue, and the resistivity at room temperature became, for example, 1.3 × 10 ¹ Ω · cm, and decreased to 1/1000 or less by doping with antimony.

SnO ₂ nanoparticles can be used as a catalyst or a catalyst support. For example, stannic oxide nanoparticles are dispersed in a solution of Ni (acac) ₂ (acac is acetylacetonato) dissolved in 2-propanol, and a reducing agent such as NaBH ₄ is added at 82 ° C. under reflux. Then, a catalyst supporting about 0.5-5 mol% of metal Ni or the like with respect to 1 mol of SnO ₂ was prepared. In this specification, 1 mol% means 0.01 mol with respect to 1 mol of SnO ₂ .

Diagram showing TEM image of rod-shaped stannic oxide nanoparticles Diagram showing XRD pattern of rod-shaped stannic oxide nanoparticles Diagram showing TEM image of cubic stannic oxide nanoparticles Diagram showing XRD pattern of cubic stannic oxide nanoparticles The figure which shows attribution of the exposed surface of a rod-shaped stannic oxide nanoparticle The figure which shows attribution of the exposed surface of cubic stannic oxide nanoparticles Perspective view of gas sensor The figure shows the gas sensitivity (in dry air) of stannic oxide nanoparticles, (a) sensitivity to 200 ppm H2, (b) sensitivity to 200 ppm CO, and (c) to 100 ppm ethanol. (D) shows the sensitivity to 50 ppm of toluene. Shows gas sensitivity (in dry air) of rod-shaped stannic oxide nanoparticles (Pd not supported, Pd 0.5 mol% / SnO ₂ supported, and Pd 1.0 mol% / SnO ₂ supported) In the figure, (a) shows the sensitivity to 200 ppm of H2, (b) shows the sensitivity to 200 ppm of CO, and (c) shows the sensitivity to 100 ppm of ethanol. Diagram showing gas sensitivity of rod-shaped stannic oxide nanoparticles in dry air (a) and wet air (b)

  In the following, an optimum embodiment for carrying out the present invention will be shown.

Synthesis Example 1: Synthesis of rod-shaped tin oxide nanoparticles Dissolve stannic chloride pentahydrate (3.5 g, 10.0 mmol) in a 3.0 mol / L aqueous hydrochloric acid solution at room temperature, and make up the total volume to 100 mL using a volumetric flask. Thus, 100 mL of a 3.0 mol / L hydrochloric acid aqueous solution (Sn hydrochloric acid solution) containing 0.10 mol / L of stannic chloride was prepared. Next, 100 mL of the above Sn solution was cooled to 0 ° C. using an ice bath, and 33 mL (250 mmol) of triethanolamine (manufactured by Wako Pure Chemical Industries, Ltd.) was slowly added dropwise with stirring. After the dropwise addition, ion exchange water was added to the mixed solution to adjust the volume of the mixed solution to 200 mL. At this time, the pH of the solution was 0.40. The obtained mixed solution was dispensed into an autoclave made of tetrafluoroethylene inner cylinder having an inner volume of 23 mL (20 in total), sealed, and then left to stand in an oven at 200 ° C. for 48 hours. The hydrothermal reaction temperature is preferably 160 ° C. or higher and 250 ° C., and the reaction time is preferably 1-120 hours.

The autoclave after the heat treatment was cooled with cold water, and the resulting suspension dispersion (20 in total) was subjected to solid-liquid separation using a centrifuge to separate and recover the solid content. The separation conditions were 18,000 rpm for 10 minutes. 20 mL of ion-exchanged water was added to the collected solid content, and after ultrasonic dispersion treatment, solid-liquid separation was performed by centrifugation under the same conditions as described above. The solid content was washed by repeating this operation twice. The obtained solid content was again ultrasonically dispersed in ion-exchanged water and then freeze-dried to obtain a white powder in which SnO ₂ nanoparticles according to Synthesis Example 1 were monodispersed.

The shape of the SnO ₂ powder obtained in Synthesis Example 1 was observed with a transmission electron microscope (TEM). The obtained TEM image is shown in FIG. From FIG. 1, it was determined that the particle shape of the SnO ₂ powder was rod-shaped. Powder X-ray diffraction measurement of SnO ₂ powder was performed. The results are shown in FIG. 2, and the obtained diffraction pattern coincides with the diffraction pattern of tetragonal rutile SnO ₂ , and it was found that the crystal structure is the same as that of a single composition of tetragonal SnO ₂ . . By comparing the crystallite diameter obtained from the half width of X-ray diffraction with the particle diameter obtained from the TEM image, it was found that the rod-shaped primary particles consist of a single crystallite. No secondary particles were seen in the image, and it was found that primary particles composed of crystallites were monodispersed.

Synthesis Example 2: Synthesis of Cubic Tin Oxide Nanoparticles Stannic chloride pentahydrate (1.75 g, 5.0 mmol) is dissolved in 3.0 mol / L aqueous hydrochloric acid solution at room temperature, and the total volume is adjusted to 50 mL using a volumetric flask. Thus, 50 mL of a 3.0 mol / L hydrochloric acid aqueous solution (Sn hydrochloric acid solution) containing 0.10 mol / L of stannic chloride was prepared. Next, 50 mL of the above Sn solution was cooled to 0 ° C. using an ice bath, and then 4.4 mL (65 mmol) of ethylenediamine (1,2-diaminoethane, manufactured by Wako Pure Chemical Industries, Ltd.) was added dropwise with stirring. After dropping, 0.60 mL of 6.0 mol / L hydrochloric acid aqueous solution was added to the mixed solution, and ion exchange water was added to adjust the volume of the mixed solution to 100 mL. The obtained mixed solution was dispensed into an autoclave made of tetrafluoroethylene inner cylinder having an internal volume of 23 mL (5 bottles in total), sealed, and then left to stand in an oven at 200 ° C. for 48 hours. The hydrothermal reaction temperature is preferably 160 ° C. or higher and 250 ° C., and the reaction time is preferably 1-120 hours.

The autoclave after the heat treatment was cooled with cold water, and the resulting suspension dispersion (total of 5) was subjected to solid-liquid separation using a centrifuge to separate and recover the solid content. The separation conditions were 18,000 rpm for 10 minutes. 20 mL of ion-exchanged water was added to the collected solid content, and after ultrasonic dispersion treatment, solid-liquid separation was performed by centrifugation under the same conditions as described above. The solid content was washed by repeating this operation twice. The obtained solid content was again ultrasonically dispersed in ion-exchanged water and then freeze-dried to obtain a white powder in which SnO ₂ nanoparticles according to Synthesis Example 2 were monodispersed.

The shape of the SnO ₂ powder according to Synthesis Example 2 was observed with a transmission electron microscope (TEM). The obtained TEM image is shown in FIG. From the following, the particle shape of the SnO ₂ powder was determined to be a cubic shape.
The average particle diameter of the SnO ₂ powder was calculated by measuring one side of the cube of 100 SnO ₂ particles (primary particles) that do not overlap on the TEM photograph and calculating the average value. As a result, the average particle size was 12.9 nm. At this time, the standard deviation calculated from the particle size of 55 particles whose average particle size was obtained was 2.0 nm. The coefficient of variation obtained by dividing the standard deviation by the average particle size was 15%.

Powder X-ray diffraction measurement of SnO ₂ powder was performed. The results are shown in FIG. 4, and the obtained diffraction pattern is consistent with the diffraction pattern of tetragonal rutile SnO ₂ , and was found to have the same crystal structure as a single composition of tetragonal SnO ₂ . . Furthermore, a peak appears at 26.5 ° (CuKa1 source) at a 2θ angle. (110) The diffraction peak intensity Int. (110) and half-value width B are calculated for the diffraction peak, and Scherrer's equation Dx = 0.94λ. / Bcosθ (where Dx is the crystallite size, λ is the X-ray wavelength used for the measurement (CuKa1 source), B is the half-value width of the diffraction peak, and θ is the Bragg angle of the diffraction peak). The crystallite diameter of the SnO ₂ powder according to Synthesis Example 2 was determined to be 12.7 nm.

The value of (average particle diameter determined from TEM photograph) / (crystallite diameter determined from XRD diffraction peak) was 1.0. This indicates that the apparent average particle diameter is equal to the crystallite diameter. In other words, it was suggested that SnO ₂ nanoparticles synthesized by this method have high crystallinity.

Assignment of growth surface of rod-shaped tin oxide nanoparticles By high-resolution TEM observation, attribution of the exposed surface of the rod-shaped tin oxide nanoparticles synthesized in Synthesis Example 1 was performed. The results are shown in FIG. The spacing between the surfaces parallel to the major and minor axes of the rod was 0.335 nm and 0.316 nm, respectively. These interplanar spacings were extremely close to the 0.3351 nm and 0.3187 nm lattice spacings of the (110) and (001) planes of tetragonal rutile SnO ₂ , respectively. From the above, it was attributed that the exposed surface on the side surface of the rod-shaped tin oxide nanoparticles was the (110) surface.

Assignment of growth surface of cubic tin oxide nanoparticles By high-resolution TEM observation, the exposure surface of the cubic tin oxide nanoparticles synthesized in Synthesis Example 2 was assigned. The results are shown in FIG. The lattice spacing in each of the a-axis and b-axis directions was 0.335 nm, which coincided with the lattice spacing of the (110) plane of tetragonal rutile SnO ₂ of 0.3351 nm. At this time, it was attributed that two (001) planes perpendicular to (110) were exposed surfaces of cubic tin oxide nanoparticles.

Palladium nanoparticles supported on tin oxide nanoparticles The palladium nanoparticles were supported on the tin oxide nanoparticles prepared in Synthesis Examples 1 and 2 according to the following procedure. First, palladium chloride (0.177 g, 1.0 mmol) was dissolved in 0.10 mol / L hydrochloric acid, and the total volume was adjusted to 10 mL using a volumetric flask, so that 0.10 mol / L hydrochloric acid aqueous solution containing 0.10 mol / L of palladium chloride was used. Was prepared. Subsequently, the tin oxide nanoparticles (250 mg, 0.60 mmol) prepared in Synthesis Example 1 or 2 were weighed, added to 10 mL of ion exchange water, and an aqueous dispersion of tin oxide nanoparticles was obtained using an ultrasonic disperser. Next, a 0.10 mol / L aqueous hydrochloric acid solution containing 0.10 mol / L of palladium chloride prepared previously was added. The amount added at this time was 30 mL or 60 mL, so that the supported amount of palladium on tin was 0.50 or 1.0 mol%, respectively.

  After adding 0.50 mL of ethanol to the obtained mixed dispersion, 1.0 mol / L hydrochloric acid aqueous solution was added dropwise so that the pH of the dispersion was 2.0. At this time, the pH of the dispersion was monitored with a pH meter. The dispersion thus obtained was transferred to a quartz test tube, a magnetic stirrer was added, and the test tube was sealed using a septum cap. Next, the contents of the test tube were irradiated with a high-pressure mercury lamp for 2 hours at room temperature. At this time, the mixture was vigorously stirred using a magnetic stirring bar. After irradiation with a high-pressure mercury lamp, the dispersion was subjected to solid-liquid separation using a centrifuge. After the obtained solid phase was recovered, the solid phase was washed by repeating dispersion in ion-exchanged water and centrifugal sedimentation twice. The target solid phase-supported tin oxide nanoparticles were prepared by drying the solid phase thus obtained in an oven at 60 ° C. for one day.

Gas Sensitivity of Tin Oxide Nanoparticles Using the tin oxide nanoparticles prepared in Synthesis Examples 1 and 2 and the Pd-supported tin oxide nanoparticles, a gas sensor 2 shown in FIG. 7 was prepared and the gas sensitivity was examined. The nanoparticle powder was mixed with distilled water in an agate mortar to prepare a paste. Using this, a sensor film 6 (film thickness 20 μm) was applied to the alumina substrate 4 on which the comb-shaped gold electrode 8 was attached by a screen printing machine. The line width of the comb-shaped electrode 8 was 180 μm, and the electrode interval was 90 μm. 10 is a platinum wire. The printed film 6 was heat-treated in an electric furnace at 350 ° C. for 10 hours while circulating synthetic air.

For the measurement of sensor sensitivity, a general flow-type device equipped with an electric furnace (heating device) was used. The electric resistance of the gas sensor 2 was measured by applying a DC voltage of 4 V to a circuit in which the gas sensor 2 was connected in series with the reference resistance and measuring the voltage of the reference resistance. The atmosphere to which the gas sensor 2 is exposed is the synthetic dry air, the synthetic dry air mixed with the test gas of a predetermined concentration, and those which are bubbled in water and humidified. The oxygen sensor and the humidity sensor The humidity was monitored. Further, the gas sensitivity of the gas sensor 2 was measured from the change in electrical resistance due to contact with the test gas. The sensitivity S is, the resistance in air R _a, the resistance in披検gas as R _g, defined S = R _a / R _g.

  FIG. 8 shows the sensitivity of the gas sensor 2 at 250, 300, and 350 ° C. in dry air. The test gas used was 200 ppm hydrogen, 200 ppm carbon monoxide, 100 ppm ethanol, and 50 ppm toluene. The test gas used was diluted with synthetic air. From FIG. 8, it is clear that the gas sensor 2 exhibits a very high gas sensitivity. In particular, rod-shaped tin oxide shows an electric resistance change of 4 digits or more at 250 ° C. with respect to ethanol, and it is understood that it has an overwhelmingly high sensitivity to other gases, that is, high ethanol selectivity.

Palladium nanoparticles were investigated gas sensitivity of the gas sensitivity palladium nanoparticles supported tin oxide loaded with tin oxide nanoparticles. Sensor sensitivity was measured by the method described above. FIG. 9 shows the gas sensitivity of rod-shaped tin oxide nanoparticles and palladium nanoparticles supported thereon. The test gas was 200 ppm hydrogen, 200 ppm carbon monoxide, and 100 ppm ethanol. For carbon monoxide, the gas sensitivity decreased due to palladium loading. This is presumably because the combustion of carbon monoxide on the surface of the sensor film 6 was promoted by palladium. On the other hand, for hydrogen and ethanol, the sensitivity increased due to palladium loading. In particular, with respect to ethanol, an electric resistance change of about 5 digits occurred at 250 ° C., and the ethanol selectivity was further improved by the palladium loading.

  In order to clarify the practicality of this material, the gas sensitivity in wet air (bubbling in water and humidifying) was measured. The test gas was 200 ppm hydrogen, 200 ppm carbon monoxide, 100 ppm ethanol, and 40 ppm formaldehyde. FIG. 10A shows gas sensitivity in dry air, and FIG. 10B shows gas sensitivity at a relative humidity of 45%. Although the gas sensitivity was greatly reduced compared with that in dry air, the sensitivity to ethanol was about 80 at 250 ° C., and it was found that the sensitivity was sufficient with no problem in practical use. It has also been found that high selectivity to ethanol is maintained even in humid air. As described above, since the palladium-carrying rod-shaped tin oxide can detect ethanol without being disturbed by water vapor, the gas sensor 2 is very effective for detecting ethanol in exhaled breath.

2 Gas sensor 4 Alumina substrate 6 Sensor film 8 Gold electrode
10 Platinum wire

Claims (3)

A stannic oxide material comprising rod-shaped SnO ₂ crystallites, the side surfaces of the rod being the (110) plane of SnO ₂ crystals, and the SnO ₂ crystallites being in a monodispersed state. By heating a strongly acidic aqueous solution containing Sn ⁴⁺ ions at a reaction temperature of 160 ° C.-250 ° C. and for a reaction time of 1 hour-120 hours ,
Rod a and the side surface of the rod SnO ₂ crystal (110) plane of SnO ₂ crystallite exposed to hydrothermal synthesis at monodisperse or cubic shape a and the surface of the SnO ₂ crystal (001) plane A method for synthesizing a stannic oxide material, wherein the SnO ₂ crystallite is hydrothermally synthesized in a monodispersed state.   A gas sensor material comprising the stannic oxide material of claim 1.