JP6687931B2 - SnO2-based gas sensor - Google Patents

Description

  The present invention relates to increasing the sensitivity of SnO2-based gas sensors.

  SnO2 is a typical gas sensor material, and higher sensitivity is required. In this regard, the present inventor studied to replace Sn atom defects on the surface of SnO2 particles with other metal atoms, or to replace oxygen defects on the surface of SnO2 particles with oxygen bonded to metal atoms, and arrived at the present invention.

  The related prior art is shown. Patent Document 1 (Patent 3146111) reports that when ZrO2 is supported on SnO2 in a proportion of 0.01 to 0.5 mol%, the gas sensitivity is improved.

Patent 3146111

  An object of the present invention is to increase the sensitivity of a SnO2-based gas sensor by a new method.

  In the SnO2 gas sensor of the present invention, in a gas sensor including SnO2, a heater, and an electrode, by substituting Zr atoms for Sn atom defects on the surface of SnO2 particles, a part of Sn atoms on the SnO2 surface is replaced by Zr atoms. Alternatively, the oxygen defects on the surface of the SnO2 particles are replaced with oxygen bound to Zr atoms. The concentration of Zr atoms with respect to the total of Zr atoms and Sn atoms is 0.01 at% or more and 0.2 at% or less. In this invention, the ZrO2 particles are not supported on the surface of the SnO2 particles. When Sn atom defects on the surface of SnO2 particles are replaced with Zr atoms, it means that some Sn atoms on the surface of SnO2 are replaced with Zr atoms. Alternatively, the Zr atoms can be introduced to the surface of the SnO2 particle by replacing the oxygen defect on the surface of the SnO2 particle with oxygen bonded to the Zr atom. In this way, Zr atoms are introduced into the surface of the SnO2 particles, and the surface of the SnO2 particles is surface-modified with Zr atoms. Zr atoms are mainly present on the surface of SnO2 particles. Since it is difficult to accurately measure the concentration of Zr atoms on the surface of SnO2 particles, the concentration of Zr atoms is determined by the ratio with respect to the whole SnO2. The concentration of Zr atoms with respect to the total of Zr atoms and Sn atoms is 0.01 at% or more and 0.2 at% or less, for example, 0.02 at% or more and 0.1 at% or less.

ZrO2 particles are not carried by SnO2 particles, Sn atoms are replaced by Zr atoms, or oxygen defects on the surface of SnO2 particles are replaced by oxygen bonded to Zr atoms,
-This is supported by the fact that the peak of ZrO2 cannot be detected by XRD, and that the adsorbed species of oxygen to SnO2 change due to the introduction of Zr atoms. That is, the introduction of Zr atoms changes the adsorbed species of oxygen at 350 ° C. in air from O ⁻ to O ^{2 −} (FIG. 3). Oxygen does not adsorb as O ^{2− on the} surface of ZrO ² at this temperature, so it is not possible to explain if ZrO 2 particles are supported by SnO 2 particles.

  The substitution of Sn atom defects by Zr atoms and the substitution of surface oxygen defects by oxygen bound to Zr atoms can be estimated from the method for manufacturing the gas sensor. The inventor has contacted an aqueous solution of a mixture of HF and ZrO (NO3) 2 with a monodispersed SnO2 powder calcined at 700 ° C., and then washed and calcined to remove Zr atoms or oxygen bound to Zr atoms. It was introduced on the surface of SnO2. When ZrO (NO3) 2 is contacted with SnO2 powder, HF should elute Sn atoms and non-stoichiometric oxygen atoms on the surface of SnO2 particles and replace them with Zr atoms or oxygen bound to Zr atoms. Further, since the washing was performed, the aqueous solution of Zr salt should be removed, and ZrO2 particles should not be formed on the SnO2 surface.

  As gas sensitivities, CO sensitivity and H2 sensitivity in humid air (3vol% H2O) were measured at gas sensor temperatures of 300 ° C and 350 ° C. Both CO and H2 showed markedly improved sensitivity due to surface substitution of Sn atoms with Zr atoms (Fig. 4). As described above, in the present invention, the sensitivity of the gas sensor in a wet atmosphere is improved by substituting a part of Sn atoms on the surface of SnO2 particles with Zr atoms.

Sectional drawing of the gas sensor of an Example Characteristic diagram showing oxygen partial pressure dependence of resistance value in Comparative Example (HF-SnO2) and Example (Zr-SnO2) at 300 ° C Characteristic diagram showing oxygen partial pressure dependence of resistance value in Comparative Example (HF-SnO2) and Example (Zr-SnO2) at 350 ° C Characteristic diagram showing sensitivity to H2 and CO at 300 ° C. and 350 ° C. of Example and Comparative Example

  The best examples for carrying out the present invention are shown below.

SnO2 nanopowder (Zr surface-modified SnO2 nanopowder) in which some of the surface Sn atoms were replaced with Zr was prepared as follows. The SnCl4 aqueous solution (concentration 1M) was neutralized with NH4HCO3, and after stirring, the precipitate was separated by centrifugation to remove free Cl ^- ions. Hydrothermal treatment was carried out at 200 ° C. for 2 hours, dried and then calcined in air at 700 ° C. to obtain SnO 2 powder in which primary particles were monodispersed. The average primary particle size of SnO2 was 16 nm.

  A mixed aqueous solution of HF and ZrO (NO3) 2 was mixed with the above SnO2 powder and stirred for 1 hour. The HF concentration was about 5 mass% and the ZrO (NO3) 2 concentrations were three types of 0.05M, 0.1M and 0.4M. After stirring, deionized water was added and the mixture was centrifuged to remove free Zr ions and fluorine ions. Then, it was dried and calcined in air at 600 ° C. to obtain a Zr surface-modified SnO 2 nanopowder. As a comparative example, SnO2 nanopowder was prepared by subjecting it to the same treatment with an HF aqueous solution containing no ZrO (NO3) 2, followed by centrifugation and firing.

  The X-ray diffraction patterns of the Zr surface-modified SnO2 nanopowder and the SnO2 nanopowder of the comparative example were measured. In both cases, only the SnO2 peak was detected, and the ZrO2 peak could not be detected. The Zr atom concentration of each SnO2 powder (the total of the Sn atom concentration and the Zr atom concentration is 100 at%) was measured by the atomic emission method. Table 1 shows the relationship between the charged concentration of Zr salt and the Zr atom concentration. The Zr atom concentration was not proportional to the charged concentration, and only some of the Zr atoms in the mixed aqueous solution of HF and ZrO (NO3) 2 were transferred to SnO2. Furthermore, even when the Zr surface-modified SnO2 nanopowder was observed with an electron microscope, particles corresponding to ZrO2 could not be observed. From the viewpoint of the preparation method, since the centrifugal separation is performed after the contact with the aqueous solution of Zr salt, there should be no droplets of the Zr solution and ZrO2 particles should not be formed.

  These facts indicate that Zr does not support as ZrO2, but Zr replaces Sn defects on the surface of SnO2 particles, or replaces oxygen defects on the surface of SnO2 with oxygen bonded to Zr atoms, and Indicates that it exists. Since SnO2 before surface modification is calcined at 700 ° C, it is unlikely that Zr atoms will diffuse to the central part of the SnO2 particles by subsequent treatment with HF and recalcination at 600 ° C. It is considered to be present on the surface of SnO2 particles. The method for preparing the Zr surface-modified SnO2 nanopowder is arbitrary, and the type of Zr salt to be used is arbitrary.

Table 1
Charged concentration of Zr salt (M) Zr atomic concentration (at%)
0.05 0.03
0.1 0.046
0.4 0.13

  The gas sensor 2 of FIG. 1 was prepared using the Zr surface-modified SnO2 nanopowder. A heater 6 and an electrode 8 were provided on a substrate 4 made of alumina or the like, a SnO2 film 10 was screen-printed so as to cover the electrode 8, and baked at 580 ° C. in air to obtain a gas sensor 2. The thickness of the SnO2 film 10 is about 40 μm. Reference numerals 12 and 13 are pads for the heater 4 and the electrodes 8. The gas sensor may have any structure, for example, a MEMS type, and the heater 6 may be exposed on the surface of the substrate 4 to serve also as the electrode 8.

  The Zr surface-modified SnO2 nanopowder may further carry a noble metal such as Pd, Pt, Au or the like, and the SnO2 film 10 may be mixed with a third component such as alumina or silica. The SnO2 film 10 may be thick or thin. A gas sensor using Zr surface-modified SnO2 nanopowder is represented by Zr-SnO2, and a gas sensor using SnO2 nanopowder treated with an HF aqueous solution containing no Zr is represented by HF-SnO2.

2 and 3 show the oxygen partial pressure dependence of the resistance values of Zr-SnO2 (Zr atomic concentration 0.03 at%) and HF-SnO2 at 300 ° C and 350 ° C. As the inventors have already announced, when the oxygen adsorbing species on the SnO2 surface is O ⁻ , the resistance value is linear to the half power of the oxygen partial pressure, and when O ^{2 −} , the resistance value is the oxygen partial pressure. Is linear to the 1/4 power of. 300 ° C. In the Zr-SnO2, HF-SnO2 both oxygen adsorbed species is O ^- (2), adsorbed species in the Zr-SnO2 at 350 ° C. is changed to O ^2-, but in HF-SnO2 O ^- remains Was (Fig. 3). The fact that oxygen was adsorbed as O ²⁻ at 350 ° C. was the same even when the Zr atom concentration was 0.046 at% and 0.13 at%. Oxygen is not adsorbed as O ²⁻ at 350 ° C. on the surface of ZrO 2 particles. Therefore, the results in FIG. 3 are unexplainable if Zr exists as ZrO2 particles.

FIG. 3 shows that when a part of Sn atoms on the surface of SnO2 particles is replaced with Zr atoms, the adsorption state of oxygen changes. O ^- replacing the more active by O ^2- and gas sensitivity also changes. Figure 4 shows the sensitivity to hydrogen and CO in a humid atmosphere (H2O 3vol%). Zr-SnO2 had a Zr atom concentration of 0.046 at%, but the same was true for other Zr atom concentrations. Surface modification of both hydrogen and CO with Zr atoms increased the sensitivity, especially the CO sensitivity.

2 Gas sensor 4 Substrate 6 Heater 8 Electrode 10 SnO2 film 12, 13 Pad

Claims (1)

In a gas sensor including SnO2, a heater, and an electrode,
Some Sn atoms on the SnO2 surface are replaced by Zr atoms, or oxygen defects on the SnO2 particle surface are replaced by oxygen bound to Zr atoms,
A SnO2-based gas sensor, wherein the concentration of Zr atoms in SnO2 is 0.01 at% or more and 0.2 at% or less with respect to the total of Zr atoms and Sn atoms.