METHOD OF MANUFACTURING METAL OXIDE GAS SENSOR FUNCTIONALIZED BY MULTICOMPONENT ALLOY NANOPARTICLE-PEROVSKITE COMPOSITE CATALYST

Abstract

Provided are a composite structure, in which metal nanoparticle-perovskite oxide is bound to metal oxide supports (i.e., sensing materials), and a preparation method thereof. The composite structure has improved durability, in which metal nanoparticles uniform in size are evenly distributed on the surface of perovskite oxide. Provided is also a high-performance gas sensor having excellent target gas detection performances by including the composite structure.

Claims

1. A composite structure, wherein metal nanoparticle-perovskite oxide is bound to metal oxide supports, and the metal nanoparticle-perovskite oxide has a form in which metal nanoparticles are ex-solved on the surface of the perovskite oxide.

2. The composite structure of claim 1, wherein the metal nanoparticle-perovskite oxide is represented by the following Chemical Formula 1:
La.sub.1-aCa.sub.aFe.sub.1-bX.sub.bO.sub.3-d  [Chemical Formula 1] X is one or more selected from the group consisting of Ni, Co, and Cu, in Chemical Formula 1, a is 0.1 to 0.9, b is 0.01 to 0.1, and d is 0 to 1.

3. The composite structure of claim 1, wherein the metal oxide support is a compound represented by the following Chemical Formula 2:
Y.sub.eO.sub.f  [Chemical Formula 2] in Chemical Formula 2, Y is W, Sn, Zn, Fe, or Ti, e is 1 to 3, and f is 1 to 4.

4. The composite structure of claim 1, wherein the metal oxide support is in the form of nanofiber.

5. A method of preparing a composite structure, the method comprising: a step (step 1) of mixing a metal nanoparticle precursor with a perovskite oxide precursor; a step (step 2) of preparing a solid solution by annealing the mixture of the step 1; a step (step 3) of obtaining metal nanoparticle-perovskite oxide by heat-treating the solid solution of the step 2 in a reducing atmosphere; and a step (step 4) of obtaining the composite structure in which metal nanoparticle-perovskite oxide is bound to the metal oxide supports by performing electrospinning and oxidative heat treatment of a mixture of the metal nanoparticle-perovskite oxide of the step 3 and a precursor of the metal oxide support.

6. The method of claim 5, wherein, in the step 4, a template structure is further mixed.

7. The method of claim 5, wherein the metal nanoparticle precursor includes one or more selected from the group consisting of cobalt (Co) salts, nickel (Ni) salts, and copper (Cu) salts.

8. The method of claim 5, wherein the perovskite oxide is a compound represented by the following Chemical Formula 3:
La.sub.1-aCa.sub.aFeO.sub.3-d  [Chemical Formula 3] in Chemical Formula 3, a is 0.1 to 0.9, and d is 0 to 1.

9. The method of claim 5, wherein, in the step 1, the metal nanoparticle precursor and the perovskite oxide are mixed at a molar ratio of 1:9 to 1:99.

10. The method of claim 5, wherein, in the step 4, the metal oxide support is a compound represented by the following Chemical Formula 2:
Y.sub.eO.sub.f  [Chemical Formula 2] in Chemical Formula 2, Y is W, Sn, Zn, Fe, or Ti, e is 1 to 3, and f is 1 to 4.

11. The method of claim 5, wherein, in the step 3, one or more reducing atmosphere gases of a H.sub.2/Ar mixed gas, a H.sub.2/H.sub.2O mixed gas, a CO/CO.sub.2 mixed gas, and a H.sub.2/N.sub.2 mixed gas are used.

12. The method of claim 5, wherein a volume ratio of the H.sub.2/Ar mixed gas, the H.sub.2/H.sub.2O mixed gas, the CO/CO.sub.2 mixed gas, or the H.sub.2/N.sub.2 mixed gas is 1/99 to 99/1.

13. The method of claim 5, wherein the heat treatment temperature of the step 3 is 300° C. to 1000° C.

14. A gas sensor comprising the composite structure, in which metal nanoparticle-perovskite oxide is bound, of claim 5.

15. The gas sensor of claim 14, wherein an operating temperature of the gas sensor is 200° C. to 450° C.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0088] FIGS. 1(a) to 1(e) show schematic illustrations of a process of preparing a composite structure according to one embodiment of the present invention;

[0089] FIG. 2(a) shows results of scanning electron microscopy (SEM) and FIGS. 2(b) to 2(d) show transmission electron microscopy (TEM) for observing the size and distribution of ex-solved metal nanoparticles according to the heat treatment temperature and the type of element according to Experimental Example of the present invention;

[0090] FIGS. 3(a) and 3(b) show results of measuring sensitivity of a gas sensor according to Experimental Example of the present invention; and

[0091] FIGS. 4(a) and 4(b) shows results of measuring the gas sensor according to heat treatment according to Experimental Example of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0092] Hereinafter, the actions and effects of the present invention will be described in more detail with reference to the specific exemplary embodiments. However, these exemplary embodiments are only for illustrating the present invention, and the scope of the present invention is not limited thereto.

EXAMPLE

Example 1—Co-LCF Oxide

[0093] Cobalt nitrate (Co(NO.sub.3).sub.2) as a Co metal nanoparticle precursor, and lanthanum nitrate, calcium nitrate, and iron nitrate as a La.sub.0.6Ca.sub.0.4FeO.sub.3 perovskite oxide precursor were mixed at a corresponding molar ratio, and sintered at a temperature of 900° C. to dissolve the Co metal in the perovskite oxide. The Co metal nanoparticle precursor and the perovskite oxide precursor were mixed at a molar ratio of 5:95 to allow 0.05 mol of Co to dissolve.

[0094] The perovskite oxide La.sub.0.6Ca.sub.0.4Fe.sub.0.95Co.sub.0.05O.sub.3 particles, into which 0.05 mol of Co was dissolved, were finely milled by high-energy ball milling to increase a specific surface area thereof.

[0095] Thereafter, the transition metal dissolved inside the perovskite oxide lattice was allowed to ex-solve as metal nanoparticles on the surface of perovskite oxide by performing a reductive heat treatment step. The reductive heat treatment step was performed at 600° C., 700° C., and 800° C. for 2 hours under H.sub.2/Ar(4/96, (v/v)) atmosphere to obtain Co-600Ex-LCF (Example 1-1), Co-700Ex-LCF (Example 1-2), and Co-800Ex-LCF (Example 1-3) oxide particles.

Example 2—Ni-LCF Oxide

[0096] Ni-600Ex-LCF (Example 2-1), Ni-700Ex-LCF (Example 2-2), and Ni-800Ex-LCF (Example 2-3) oxide particles were obtained in the same manner as in Example 1, except that nickel nitrate (Ni(NO.sub.3).sub.2) as a Ni metal nanoparticle precursor was mixed with the perovskite oxide precursor at a molar ratio of 5:95 to allow 0.05 mol of Ni to dissolve, thereby preparing La.sub.0.6Ca.sub.0.4Fe.sub.0.95Ni.sub.0.05O.sub.3, in Example 1.

Example 3—CoNi-LCF Oxide

[0097] CoNi-600Ex-LCF (Example 3-1), CoNi-700Ex-LCF (Example 3-2) and CoNi-800Ex-LCF (Example 3-3) oxide particles were obtained in the same manner as in Example 1, except that cobalt nitrate (Co(NO.sub.3).sub.2) and nickel nitrate (Ni(NO.sub.3).sub.2) as a CoNi metal nanoparticle precursor were mixed with the perovskite oxide precursor at a molar ratio of 2.5:2.5:95 to allow each 0.025 mol of Co and Ni to dissolve, thereby preparing La.sub.0.6Ca.sub.0.4Fe.sub.0.95Co.sub.0.025Ni.sub.0.025O.sub.3, in Example 1.

Comparative Example 1—SnO.SUB.2 .Nanofiber

[0098] As Comparative Example 1, SnO.sub.2 nanofibers, in which metal nanoparticle-perovskite oxide were not bound, were prepared.

Comparative Example 2—CoNi-Doped-LCF

[0099] As Comparative Example 2, perovskite oxide (CoNi-Doped-LCF) was prepared, in which CoNi metal nanoparticles were dissolved and remained inside the lattice because the oxidative heat treatment step for ex-solving the CoNi metal nanoparticles was not performed. The perovskite oxide was prepared in the same manner as in Example 3, except for the oxidative heat treatment step.

Experimental Example

[0100] In order to examine how the composite structures of Examples 1 to 3 and Comparative Examples 1 to 2 and performances (gas sensitivity and selectivity) during sensing reaction operating when used as a gas sensor were changed according to the reductive heat treatment process and conditions thereof, the following experiments were performed.

Experimental Example 1—Structure of Metal Nanoparticle-Perovskite Oxide

[0101] Perovskite oxide on which metal nanoparticles were ex-solved according to the reductive heat treatment temperatures of Examples 1 to 3 was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (see FIGS. 2(a) to 2(d)).

[0102] As a result of SEM image analysis, Example 1 showed that the size of the ex-solved Co nanoparticles increased to 11 nm, 20 nm, and 25 nm with increasing reductive heat treatment temperature (x=0 of FIG. 2(a)). In contrast, CoNi heterogeneous alloy nanoparticles showed that although the size of the ex-solved particles increased with increasing temperature, the size increased only to 7 nm, 14 nm, and 18 nm, which were smaller than the size of Example 1, in which Co was used alone (x=0.025 of FIG. 2(a)).

[0103] As a result of TEM analysis, when the reductive heat treatment, i.e., ex-solution was performed at 800° C., formation of Co, Ni, and CoNi alloy nanoparticles on the surface of perovskite oxide was observed (FIGS. 2(b), 2(c), and 2(d)). In particular, it was confirmed that Co, Ni, and CoNi were alloyed with Fe to form nanoparticles. These results indicate that the size, distribution, and composition of metal nanoparticles may be controlled by controlling the reductive heat treatment temperature and the type of dissolved metal element.

Experimental Example 2—Measurement of Sensitivity of Gas Sensor

[0104] A solution, in which the perovskite oxide of Example 1-3, Example 2-3, or Example 3-3, SnCl.sub.2 as a precursor of a metal oxide support (SnO.sub.2), and a poly(vinylpolypyrrolidone) polymer as a template structure were dissolved, was subjected to electrospinning, and then subjected to high-temperature oxidative heat treatment at 500° C. for 1 hour to decompose the polymer. The polymer forming the template structure was mixed in an amount of 100% by weight to 200% by weight with respect to the precursor of the metal oxide support.

[0105] As a result, each composite structure was obtained, in which metal nanoparticle-perovskite oxide was bound to the SnO.sub.2 nanofiber (SnO.sub.2 NFs) supports. The obtained composite structure was dispersed in ethanol, and then coated on an alumina sensor substrate (2.5 mm×2.5 mm) having a gold electrode (width=2.5 μm, gap size=150 μm) to manufacture a gas sensor.

[0106] The SnO.sub.2 nanofiber (Pristine SnO.sub.2 NFs) support of Comparative Example 1, in which perovskite oxide was not bound, was also dispersed in ethanol to manufacture a gas sensor in the same manner as Example 1-3.

[0107] A gas sensor was manufactured in the same manner as Example 1-3, except for using the perovskite oxide of Comparative Example 2, which was not subjected to the reductive heat treatment.

[0108] Thereafter, each gas sensor was stabilized in the humid air (80% RH; relative humidity), and then exposed to 1 ppm to 5 ppm of C.sub.2H.sub.6S while turning on/off in units of 10 minutes to measure the sensing characteristics. The results are shown in FIG. 3(a).

[0109] In FIGS. 3(a) and 3(b), Response represents gas sensing sensitivity and a value of (R.sub.air/R.sub.gas), R.sub.air represents a sensor resistance value in air, and R.sub.gas represents a gas resistance value when exposed to a target gas. Example 3-3, in which CoNi alloy as the metal nanoparticle was used, exhibited the sensitivity up to 160 at 5 ppm of C.sub.2H.sub.6S gas, indicating the highest sensitivity.

[0110] The sensitivity was also measured by exposing to three representative sulfur compound gases, C.sub.2H.sub.6S, CH.sub.3SH, and H.sub.2S while varying the temperatures of the gas sensor of Example 3-3 at 300° C. to 375° C., and the results are shown in FIG. 3(b).

[0111] As a result, the selective sensitivity to C.sub.2H.sub.6S among the gases was excellent, and the selectivity was remarkable at 350° C., indicating the most optimized C.sub.2H.sub.6S gas detection performance.

Experimental Example 3—Comparison of Sensitivity of Gas Sensors According to Reductive Heat Treatment

[0112] It was examined how the gas sensing sensitivity was changed according to the reductive heat treatment to ex-solve metal nanoparticles.

[0113] Sensitivity (Response) was measured using the gas sensors, in which Example 3-3 and Comparative Example 2 were applied respectively, and C.sub.2H.sub.6S, CH.sub.3SH, H.sub.2S, C.sub.3H.sub.6O, CO, NH.sub.3, and CH.sub.4 as target gases for sensing at different concentrations from 1 ppm to 5 ppm, and shown in FIGS. 4(a) and 4(b). A method of calculating sensitivity is the same as in Experimental Example 2.

[0114] Comparative Example 2 (CoNi-Doped-LCF @ SnO.sub.2 NFs), in which the heat treatment process was not performed, showed a rare response to three gases of CO, NH.sub.3, and CH.sub.4, and similar sensitivity to four gases of C.sub.2H.sub.6S, CH.sub.3SH, H.sub.2S, and C.sub.3H.sub.6O, even though the type and gas concentration were different, indicating poor selectivity and response (FIG. 4(a)).

[0115] In contrast, Example 3-3 (CoNi-800Ex-LCF @ SnO.sub.2 NFs) showed improved selective sensitivity to C.sub.2H.sub.6S and CH.sub.3SH, and in particular, showed greatly improved sensitivity to C.sub.2H.sub.6S, indicating excellent selectivity and response (FIG. 4(b)).

[0116] Taken together, it was confirmed that when the reductive heat treatment step of ex-solving metal nanoparticles is included, the sensitivity for target gases may be improved and the selectivity may be also provided. It was also confirmed that gas selectivity may be changed by controlling the type and composition of metal nanoparticles.