Highly sensitive and selective gas sensing material to methylbenzene, methods for preparing the gas sensing material and gas sensor including the gas sensing material

Abstract

Disclosed is a gas sensing material for methylbenzene detection. Specifically, the gas sensing material includes a nanocomposite of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4. The content of Cr in the nanocomposite is from 67.0 at. % to 90.0 at. %, based on the sum of the contents of Cr and Zn atoms. The gas sensing material is highly selective to methylbenzenes over other gases and is highly sensitive to methylbenzenes. Also disclosed are methods for preparing the gas sensing material. The methods facilitate control over the composition of the gas sensing material and enable rapid synthesis of the gas sensing material at low temperature. Also disclosed is a gas sensor including the gas sensing material.

Claims

1. A gas sensing material for methylbenzene detection, comprising a nanocomposite of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4 wherein the content of Cr in the nanocomposite is from 67.0 at. % to 90.0 at. %, based on the sum of the contents of Cr and Zn atoms.

2. A gas sensor for methylbenzene detection, comprising a gas sensing layer composed of the gas sensing material according to claim 1.

3. A method for fabricating a gas sensor for methylbenzene detection, comprising a) mixing the gas sensing material according to claim 1 with deionized water to prepare a paste, b) coating the paste on a substrate, and c) drying and annealing the coated substrate to form a gas sensing layer.

4. The method according to claim 3, wherein the substrate is an Au electrode-patterned alumina substrate and the coating is performed by drop coating.

5. The method according to claim 3, wherein the drying is performed at 70 C. to 90 C. for 1 hour to 2 hours and the annealing is performed at 400 C. to 600 C. for 0.5 hour to 24 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

(2) FIG. 1 is a flowchart schematically illustrating a method for preparing a gas sensing material for methylbenzene detection according to one embodiment of the present invention;

(3) FIG. 2 shows the results of X-ray diffraction analysis for gas sensing materials prepared in Comparative Examples 1 to 3 and Examples 1 to 3;

(4) FIG. 3 shows SEM images of the secondary particle structures of gas sensing materials prepared in Comparative Examples 1 to 3 and Examples 1 to 3;

(5) FIGS. 4a to 4c are TEM images of gas sensing materials prepared in Example 1 and Comparative Examples 1 and 3, respectively;

(6) FIGS. 5a to 5f show the dynamic gas sensing characteristics of gas sensing materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 toward toluene, xylene, and ethanol (each 5 ppm) at an operating temperature of 275, respectively;

(7) FIG. 6 shows the gas response of gas sensors fabricated in Examples 1 to 3 and Comparative Examples 1 to 3 to toluene, xylene, and ethanol gases at different operating temperatures;

(8) FIG. 7 shows the gas sensing characteristics of a gas sensor fabricated in Example 1 to ethanol, xylene, toluene, benzene, HCHO, and CO at an operating temperature of 275;

(9) FIG. 8 shows the results of X-ray diffraction analysis for a Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite, which was synthesized in the same manner as in Example 1 except that the content of Cr was changed such that [Cr]/{[Cr]+[Zn]} was 78.2 at. %, as measured by ICP analysis;

(10) FIG. 9 shows the gas sensing characteristics of a gas sensor fabricated in Example 1 to xylene with varying concentrations of the gas;

(11) FIG. 10 compares the xylene response of an inventive gas sensing material to xylene at different concentrations with those of gas sensing materials reported in the literature;

(12) FIG. 11 compares the selectivity of a gas sensing material prepared in Example 1 to xylene over ethanol with those of gas sensing materials reported in the literature;

(13) FIG. 12 shows the gas response of a gas sensor (275) fabricated in Example 1 to 1 ppm xylene gas at relative humidities of 20%, 50%, and 80%; and

(14) FIG. 13 shows the gas sensing characteristics of a Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite sensor containing 68.0 at. % of Cr, which was synthesized based on spray pyrolysis in Example 4, toward toluene, xylene, and ethanol gases (each 5 ppm) at 275.

DETAILED DESCRIPTION OF THE INVENTION

(15) The present invention will now be described in more detail.

(16) The present invention is directed to a gas sensing material for the detection of methylbenzenes as major indoor environmental pollutant gases with very high sensitivity and outstanding selectivity, methods for preparing the gas sensing material, and a gas sensor including the gas sensing material.

(17) A gas sensing material for methylbenzene detection according to the present invention includes a nanocomposite of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4. Particularly, the content of Cr in the nanocomposite is from 67.0 at. % to 90.0 at. %, based on the sum of the contents of Cr and Zn atoms.

(18) The Cr.sub.2O.sub.3 content of the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite is a very important factor determining the sensitivity and selectivity of the gas sensing material. It is thus important to quantitatively control the Cr.sub.2O.sub.3 content.

(19) If the content of Cr in the nanocomposite is less than 78.2 at. %, the gas responses of the gas sensing material to methylbenzene gases and the selectivities of the gas sensing material to methylbenzene gases over other gases may be lowered, which can be seen from the results in Examples 1 to 3 that follow. Meanwhile, if the content of Cr in the nanocomposite exceeds 90.0 at. %, the proportion of Cr.sub.2O.sub.3 increases over the entire region of the sensing material, and as a result, the gas response of the sensing material to methylbenzene gases is lowered because electric conduction occurs through Cr.sub.2O.sub.3 with the lower resistance and gas response.

(20) As can be seen from the results in Example 4 that follows, a Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite containing 68.0 at. % of Cr can be synthesized based on spray pyrolysis. Also in this case, the composite has high response and selectivity to methylbenzenes, indicating its applicability to a methylbenzene gas sensor even when the Cr content is 68.0 at. %.

(21) Therefore, the Cr.sub.2O.sub.3 content of the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite in the gas sensing material for methylbenzene detection according to the present invention may be from 67.0 at. % to 90.0 at. %.

(22) The present invention also provides methods for preparing the gas sensing material for methylbenzene detection. Various methods for the production of the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite satisfying the composition defined above can be used to prepare the gas sensing material for methylbenzene detection according to the present invention. Particularly, a method based on galvanic replacement is advantageous in terms of continuous composition control. Another advantage is that homogeneous composites composed of two or more components at a molecular level can be rapidly synthesized at low temperature.

(23) FIG. 1 is a flowchart schematically illustrating a method based on galvanic replacement. Referring to FIG. 1, the gas sensing material of the present invention is prepared by a method including a) preparing a solution including a Zn salt and a carbohydrate, b) subjecting the solution to spray pyrolysis to prepare a ZnO powder having a hollow structure, and c) mixing the ZnO powder with a Cr salt, followed by a galvanic replacement reaction to produce a nanocomposite of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4.

(24) The nanocomposite prepared by the method contains 78.2 at. % to 90.0 at. % of Cr, based on the sum of the contents of Cr and Zn atoms.

(25) In step a), the Zn salt may be ZnO and the carbohydrate may be selected from the group consisting of sucrose, glucose, and a mixture thereof. In step a), a predetermined amount of nitric acid (HNO.sub.3) may be added. The nitric acid serves to assist in dissolving the Zn salt in the form of Zn.sup.2+ in the aqueous solution through pH adjustment.

(26) Subsequently, the solution of the Zn salt and the carbohydrate is subjected to spray pyrolysis. Specifically, in step b), the spray pyrolysis may be performed by spraying the solution at a rate of 5 L/min to 20 L/min into an electric furnace heated to 700 C. to 1000 C. If the spray pyrolysis temperature is lower than 700 C., the carbohydrate is incompletely thermally decomposed, leaving carbon components and residual organics in the composite. Meanwhile, if the spray pyrolysis temperature exceeds 1000 C., the solvent is volatilized very fast from the droplets, and as a result, the hollow structure collapses or the particles grow excessively, which is disadvantageous in gas sensitivity. If the spray rate of the solution is less than 5 L/min, the pyrolysis time increases, and as a result, the hollow structure collapses or the particles grow excessively, which is disadvantageous in gas sensitivity. Meanwhile, if the spray rate of the solution exceeds 20 L/min, the pyrolysis time decreases, and as a result, the carbohydrate is incompletely thermally decomposed, leaving carbon components in the composite.

(27) After the spray pyrolysis, the ZnO powder is mixed with a Cr salt and the mixture is subjected to a galvanic replacement reaction to produce a nanocomposite of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4. Step c) includes i) dissolving the ZnO powder in xylene and heating the solution to 80 C. to 150 C., ii) adding oleylamine and oleic acid to the heated solution and stirring the resulting solution, iii) mixing the stirred solution with a Cr salt, followed by a galvanic replacement reaction.

(28) The heating in step i) is performed to supply thermal energy for the migration of cations in the ZnO and the migration of Cr cations in the solution to initiate the subsequent galvanic replacement reaction. If the heating temperature is lower than 80 C., thermal energy necessary for the subsequent galvanic replacement reaction to proceed is not supplied. Meanwhile, if the heating temperature exceeds 150 C., a large amount of the xylene solvent is volatilized, and as a result, the amount of the xylene remaining in the reactor is considerably reduced, making it difficult for the subsequent galvanic replacement reaction to proceed. In step iii), the Cr salt may be CrCl.sub.2.

(29) Finally, the method of the present invention may further include, after step iii), washing and drying the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite and annealing the dried Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite at 400 C. to 700 C. for 0.2 hours to 16 hours. The annealing is performed to remove a very small amount of residual organic matter and to form Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 oxide interfaces. If the annealing temperature is lower than 400 C., residual organic matter is not sufficiently decomposed or uniform Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 oxide interfaces are not formed. Meanwhile, if the annealing temperature exceeds 600 C., a secondary phase other than the two phases is formed by a reaction between the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 interfaces or the size of the particles increases, resulting in low gas response.

(30) Alternatively, the gas sensing material for methylbenzene detection according to the present invention may be prepared by a method based on a solid-state reaction through calcination of a mixed powder. Specifically, the method includes a) mixing a Zn salt powder with a Cr salt powder and subjecting the powder mixture to ball milling to prepare a mixed powder and b) calcining the mixed powder at 1100 C. to 1300 C. for 4 hours to 6 hours to prepare a solid-state mix in the form of a fine powder.

(31) According to the method based on a solid-state reaction, in step a), the Zn salt powder and the Cr salt powder are mixed in such amounts that the content of Cr is from 78.2 at. % to 90.0 at. %, based on the sum of the contents of Cr and Zn atoms. Within this range, high sensitivity and selectivity to methylbenzene gases can be achieved. Likewise in the previous method, the Zn salt may be ZnO and the Cr salt may be Cr.sub.2O.sub.3.

(32) In step b), the powder mixture is preferably calcined at 1100 C. to 1300 C. for 4 hours to 6 hours. If the calcining temperature is lower than 1100 C., the desired Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 phase is not formed. Meanwhile, if the calcining temperature exceeds 1300 C., a secondary phase other than the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 phase is formed or the size of the particles increases, resulting in low gas sensitivity.

(33) As described above, the gas sensing material for methylbenzene detection according to the present invention can be prepared based on a galvanic replacement or solid-state reaction after the atom contents are adjusted to the predetermined ratio at the initial stage. Alternatively, a ZnCr.sub.2O.sub.4 nanocomposite and a commercial Cr.sub.2O.sub.3 fine powder having a different composition from that defined above are subjected to a solid-state reaction and then the atom contents are adjusted to the ratio defined above, which is described in Example 3 that follows.

(34) Alternatively, the gas sensing material for methylbenzene detection according to the present invention may be prepared by a method based on spray pyrolysis. Specifically, the method includes a) preparing a solution including a Zn salt, a Cr salt, and a carbohydrate and b) subjecting the solution to spray pyrolysis to produce a nanocomposite of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4.

(35) In step a), the Zn may be ZnO, the Cr salt may be CrCl.sub.2, and the carbohydrate may be selected from the group consisting of sucrose, glucose, and a mixture thereof. In step a), a predetermined amount of nitric acid (HNO.sub.3) may be added. The nitric acid serves to assist in dissolving the Zn salt in the form of Zn.sup.2+ in the aqueous solution through pH adjustment.

(36) The subsequent spray pyrolysis is performed as described above. The Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite synthesized based on spray pyrolysis contains 68.0 at. % of Cr. Also in this case, the composite has high response and selectivity to methylbenzenes, indicating its applicability to a methylbenzene gas sensor even when the Cr content is 68.0 at. %.

(37) According to the method based on spray pyrolysis, since droplets containing two ions are converted into oxides without a substantial change in composition, a mixed phase of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4 is formed even when [Cr]/{[Cr]+[Zn]} is 68%.

(38) The present invention also provides a gas sensor for methylbenzene detection, including a gas sensing layer composed of the gas sensing material. Specifically, the gas sensor of the present invention may be fabricated by a method including a) mixing the gas sensing material with deionized water to prepare a paste, b) coating the paste on a substrate, and c) drying and annealing the coated substrate to form a gas sensing layer.

(39) The substrate may be an Au electrode-patterned alumina substrate and the coating may be performed by any suitable technique, such as drop coating.

(40) In step c), the drying may be performed at 70 C. to 90 C. for 1 hour to 2 hours and the annealing may be performed at 400 C. to 600 C. for 0.5 hour to 24 hours.

(41) The present invention will be explained in more detail with reference to the following examples. However, these examples are provided to assist in understanding the invention and are not intended to limit the scope of the invention.

Example 1

Preparation of Inventive as Sensing Material for Methylbenzene Detection and Fabrication of Inventive Gas Sensor for Methylbenzene Detection Including the Gas Sensing Material

(42) (1) FIG. 1 illustrates a method for preparing a gas sensing material for methylbenzene detection and a gas sensor including the gas sensing material according to embodiments of the present invention.

(43) Referring to FIG. 1, first, 8.32 g of zinc oxide (ZnO, 99.99%, Sigma-Aldrich Co. Ltd), 34.20 g of sucrose (C.sub.12H.sub.22O.sub.11, 99.5%, Sigma-Aldrich Co. Ltd), and 20 ml of nitric acid (HNO.sub.3, 60.0%, Samchun Chemical Co.) were mixed in 980 mL of deionized water. The solution was stirred for 1 h. The stirred solution was placed in a nebulizer container made of an acrylic material and the container was mounted in a spray pyrolysis system. A Teflon collection net was installed in a particle collection chamber mounted at the bottom of the spray pyrolysis system. The solution was sprayed in the form of droplets through an oscillator having a frequency of 1.7 MHz under an air atmosphere. At this time, spray pyrolysis was allowed to proceed at a spray rate of 20 L/min into an electric furnace heated to 700. After completion of the reaction, a pure ZnO powder having a hollow structure was collected through the Teflon collection net installed in the particle collection chamber.

(44) 0.03 g of the hollow ZnO powder was dissolved with stirring in 15 ml of xylene (C.sub.6H.sub.4(CH.sub.3).sub.2, ACS reagent 98.5%, Sigma-Aldrich Co.) and heated to 90. Thereafter, to the heated solution were added 0.75 g of oleylamine (C.sub.18H.sub.35NH.sub.2, 70%, Sigma-Aldrich Co.) and 0.14 g of oleic acid (C.sub.17H.sub.33COOH, 90%, Sigma-Aldrich Co.). Stirring was continued until the resulting solution become homogenous. Next, 0.37 ml of a 2 M aqueous solution of chromium (II) chloride (CrCl.sub.2, 99.99%, Sigma-Aldrich Co.) was added to the stirred solution. By the addition of the chromium (II) chloride, [Cr]/{[Cr]+[Zn]} reached 81.9 at. %, as measured by ICP analysis. Thereafter, a galvanic replacement reaction was carried out over 2 h. The reaction solution was washed, dried, and annealed at 500 over 2 h, giving a Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite.

(45) (2) The nanocomposite was mixed with deionized water, drop coated on an Au electrode-patterned alumina substrate, dried at 90 for 2 h, and annealed at 400 for 2 h, completing the fabrication of a gas sensor for methylbenzene detection.

(46) The gas sensor was placed in a gas detection chamber made of a quartz tube. Pure air or a mixed gas were alternately fed into the chamber. A change in the resistance of the gas sensor was measured in real time. The gas concentration was adjusted to an optimal concentration through an MFC. The gas concentration in the gas detection chamber was changed by rapidly feeding the gas into the gas detection chamber through a 4-way valve. The total flow rate in the gas detection chamber was fixed to 200 sccm such that the temperature of the gas sensor was maintained constant despite the rapid change in gas concentration.

Example 2

Preparation of Inventive Gas Sensing Material for Methylbenzene Detection and Fabrication of Inventive Gas Sensor for Methylbenzene Detection Including the Gas Sensing Material

(47) (1) A chromium oxide powder (Cr.sub.2O.sub.3, powder, 98% metals basis, Sigma-aldrich Co.) and a zinc oxide powder (ZnO, nanopowder, <100 nm 99.9% metals basis, Sigma-aldrich Co.) were mixed in such amounts that the same ratio as described in Example 1 was reached ([Cr]/{[Cr]+[Zn]}=81.9 at. %, as measured by ICP analysis). The powder mixture was subjected to ball milling for 24 h. The mixed powder was calcined at 1100 C. for 4 h (a solid-state reaction method) to produce a Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 solid-state mix in the form of a fine powder.

(48) (2) The fine powder was mixed with deionized water, drop coated on an Au electrode-patterned alumina substrate, dried at 90 C. for 2 h, and annealed at 400 C. for 2 h, completing the fabrication of a gas sensor. Thereafter, the gas sensing characteristics of the sensor was measured by the same method as described in Example 1.

Example 3

Preparation of Inventive Gas Sensing Material for Methylbenzene Detection and Fabrication of Inventive Gas Sensor for Methylbenzene Detection Including the Gas Sensing Material

(49) (1) First, a ZnO powder having a hollow structure was produced in the same manner as in Example 1. 0.03 g of the hollow ZnO powder was dissolved with stirring in 15 ml of xylene (C.sub.6H.sub.4(CH.sub.3).sub.2, ACS reagent 98.5%, Sigma-Aldrich Co.) and heated to 90. Thereafter, to the heated solution were added 0.75 g of oleylamine (C.sub.18H.sub.35NH.sub.2, 70%, Sigma-Aldrich Co.) and 0.14 g of oleic acid (C.sub.17H.sub.33COOH, 90%, Sigma-Aldrich Co.). Stirring was continued until the resulting solution become homogenous. Next, 0.18 ml of an aqueous solution of chromium chloride (CrCl.sub.2, 99.99%, Sigma-Aldrich Co.) was added to the stirred solution. By the addition of the chromium chloride, [Cr]/{[Cr]+[Zn]} reached 77.9 at. %, as measured by ICP analysis. Thereafter, a galvanic replacement reaction was carried out over 2 h. The reaction solution was washed, dried, and annealed at 500 C. over 2 h, giving a single-phase ZnCr.sub.2O.sub.4 nanocomposite (Comparative Example 3).

(50) Next, the single-phase ZnCr.sub.2O.sub.4 nanocomposite was mixed with a commercial chromium oxide powder (Cr.sub.2O.sub.3, powder, 98% metals basis, Sigma-Aldrich Co.) in such amounts that the same ratio as described in Example 1 was reached ([Cr]/{[Cr]+[Zn]}=81.9 at. %, as measured by ICP analysis). The powder mixture was subjected to ball milling for 24 h. The mixed powder was calcined at 1100 C. for 4 h (a solid-state reaction method) to produce a Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 solid-state mix in the form of a fine powder.

(51) (2) The fine powder was mixed with deionized water, drop coated on an Au electrode-patterned alumina substrate, dried at 90 C. for 2 h, and annealed at 400 C. for 2 h, completing the fabrication of a gas sensor. Thereafter, the gas sensing characteristics of the sensor was measured by the same method as described in Example 1.

Example 4

Preparation of Inventive Gas Sensing Material for Methylbenzene Detection and Fabrication of Inventive Gas Sensor for Methylbenzene Detection Including the Gas Sensing Material

(52) (1) First, 2.60 g of zinc oxide (ZnO, 99.99%, Sigma-Aldrich Co. Ltd), 8.36 g of chromium (II) chloride (CrCl.sub.2, 99.99%, Sigma-Aldrich Co.), 34.02 g of sucrose (C.sub.12H.sub.22O.sub.11, 99.5%, Sigma-Aldrich Co. Ltd), and 20 ml of nitric acid (HNO.sub.3, 60.0%, Samchun Chemical Co.) were mixed in 980 mL of deionized water. The solution was stirred for 1 h. The stirred solution was placed in a nebulizer container made of an acrylic material and the container was mounted in a spray pyrolysis system. A Teflon collection net was installed in a particle collection chamber mounted at the bottom of the spray pyrolysis system. The solution was sprayed in the form of droplets through an oscillator having a frequency of 1.7 MHz under an air atmosphere. At this time, spray pyrolysis was allowed to proceed at a spray rate of 20 L/min into an electric furnace heated to 800 C. After completion of the reaction, a Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite ([Cr]/{[Cr]+[Zn]}=68.0 at. %) was collected through the Teflon collection net installed in the particle collection chamber. Thereafter, annealing was conducted under an air atmosphere at 600 C. for 2 h to stabilize the phase and remove residual organic matter.

(53) (2) The fine powder was mixed with deionized water, drop coated on an Au electrode-patterned alumina substrate, dried at 90 C. for 2 h, and annealed at 400 C. for 2 h, completing the fabrication of a gas sensor. Thereafter, the gas sensing characteristics of the sensor was measured by the same method as described in Example 1.

Comparative Example 1

Preparation of Conventional as Sensing Material (Hollow ZnO) for Methylbenzene Detection and Fabrication of Conventional Sensor

(54) (1) A ZnO powder having a hollow structure was produced in the same manner as described in Example 1 (S1-S3 in FIG. 1). The hollow ZnO powder was used as a gas sensing material for methylbenzene detection.

(55) (2) The ZnO powder was mixed with deionized water, drop coated on an Au electrode-patterned alumina substrate, dried at 90 C. for 2 h, and annealed at 400 C. for 2 h, completing the fabrication of a gas sensor. Thereafter, the gas sensing characteristics of the sensor was measured by the same method as described in Example 1.

Comparative Example 2

Preparation of Conventional Gas Sensing Material (Cr2O3 Powder) for Methylbenzene Detection and Fabrication of Conventional Sensor

(56) A commercial chromium oxide powder (Cr.sub.2O.sub.3, powder, 98% metals basis, Sigma-Aldrich Co.) was mixed with deionized water, drop coated on an Au electrode-patterned alumina substrate, dried at 90 C. for 2 h, and annealed at 400 C. for 2 h, completing the fabrication of a gas sensor. Thereafter the gas sensing characteristics of the sensor was measured by the same method as described in Example 1.

Comparative Example 3

Preparation of Conventional Gas Sensing Material (Single-Phase ZnCr2O4 Nanocomposite) for Methylbenzene Detection

(57) (1) A gas sensing material was prepared and a gas sensor was fabricated according to the entire procedure (S1-S6) illustrated in FIG. 1. First, a ZnO powder having a hollow structure was produced in the same manner as in Example 1. 0.03 g of the hollow ZnO powder was dissolved with stirring in 15 ml of xylene (C.sub.6H.sub.4(CH.sub.3).sub.2, ACS reagent 98.5%, Sigma-Aldrich Co.) and heated to 90. Thereafter, to the heated solution were added 0.75 g of oleylamine (C.sub.18H.sub.35NH.sub.2, 70%, Sigma-Aldrich Co.) and 0.14 g of oleic acid (C.sub.17H.sub.33COOH, 90%, Sigma-Aldrich Co.). Stirring was continued until the resulting solution become homogenous. Next, 0.18 ml of a 2 M aqueous solution of chromium chloride (CrCl.sub.2, 99.99%, Sigma-Aldrich Co.) was added to the stirred solution. By the addition of the chromium chloride, [Cr]/{[Cr]+[Zn]} reached 77.9 at. %, as measured by ICP analysis. Thereafter, a galvanic replacement reaction was carried out over 2 h. The reaction solution was washed, dried, and annealed at 500 C. over 2 h, giving a single-phase ZnCr.sub.2O.sub.4 nanocomposite.

(58) (2) The fine powder was mixed with deionized water, drop coated on an Au electrode-patterned alumina substrate, dried at 90 C. for 2 h, and annealed at 400 C. for 2 h, completing the fabrication of a gas sensor. Thereafter, the gas sensing characteristics of the sensor was measured by the same method as described in Example 1.

Experimental Example 1

(59) The gas sensors of Examples 1-3 and Comparative Examples 1-3 were measured for gas sensing characteristics toward ethanol, methylbenzenes (xylene and toluene), benzene, HCHO, and CO, which correspond to indoor environmental gases, at five different temperatures of 250 C., 275 C., 300 C., 325 C., and 350 C. The gas sensors of Examples 1-3 and Comparative Examples 2 and 3 showed p-type oxide semiconductor sensing behavior toward the reducing gases, where their resistance was increased.

(60) In contrast, the gas sensor of Comparative Example 1 showed n-type oxide semiconductor sensing behavior toward the reducing gases, where their resistance was reduced. Accordingly, the gas response of each of the gas sensors of Examples 1-2 and Comparative Examples 2-3 showing p-type oxide semiconductor sensing behavior toward the reducing gases was given by R.sub.g/R.sub.a (where R.sub.g is the resistance of the sensor in the gas and R.sub.a is the resistance of the sensor in air). In contrast, the gas response of the gas sensor of Comparative Example 1 was given by R.sub.a/R.sub.g. The response of each gas sensor to xylene (S.sub.xylene) and an interfering gas (S.sub.gas) were measured and the selectivity of the gas sensor to xylene was calculated from the response ratio (S.sub.xylene/S.sub.gas).

(61) When the resistance of the sensor in air reached a constant state (R.sub.a), a test gas (5 ppm ethanol, xylene, toluene, benzene, HCHO or CO) was allowed to flow into a gas chamber to change the atmosphere of the gas chamber. When the resistance of the sensor in the gas was kept constant (R.sub.g), the atmosphere of the gas chamber air was changed by a flow of air. At this time, changes in resistance were measured.

Experimental Example 2

X-Ray Diffraction Analysis for the Inventive Gas Sensing Materials

(62) FIG. 2 shows the results of X-ray diffraction analysis for the gas sensing materials of Example 1 (Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite prepared based on spray pyrolysis), Example 2 (commercial Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 fine powder solid-state mix), Example 3 (commercial Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 fine powder/nanocomposite solid-state mix), Comparative Example 1 (ZnO powder having hollow structure), Comparative Example 2 (commercial Cr.sub.2O.sub.3 fine powder), and Comparative Example 3 (ZnCr.sub.2O.sub.4 nanocomposite).

(63) Referring to FIG. 2, the gas sensing material of Example 1 had a pattern of the nanocomposite in which Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4 were mixed in the predetermined ratio. The X-ray diffraction pattern of the gas sensing material of Example 2 shows that Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4 were mixed in the predetermined ratio. The gas sensing materials of Comparative Examples 1-3 were found to have diffraction patterns corresponding to ZnO, Cr.sub.2O.sub.3, and ZnCr.sub.2O.sub.4, respectively. The X-ray diffraction pattern of the gas sensing material of Example 3 confirms the presence of Cr.sub.2O.sub.3 because Cr.sub.2O.sub.3 was further added to and mixed with ZnCr.sub.2O.sub.4. The X-ray diffraction patterns confirm that the ZnCr.sub.2O.sub.4 composite was produced by galvanic replacement of the mixture of the hollow ZnO and the Cr salt and the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 was produced when the Cr salt was present in a sufficient amount.

Experimental Example 3

Secondary Particle Structures of the Inventive Gas Sensing Materials

(64) FIG. 3 shows SEM images of the secondary particle structures of the gas sensing materials synthesized in Example 1 (Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4), Example 2 (commercial Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 fine powder solid-state mix), Example 3 (commercial Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 fine powder/nanocomposite solid-state mix), Comparative Example 1 (ZnO), Comparative Example 2 (Cr.sub.2O.sub.3), and Comparative Example 3 (ZnCr.sub.2O.sub.4).

(65) Referring to FIG. 3, the gas sensing material (ZnO) of Comparative Example 1 was found to maintain its hollow structure because when the solution containing the metal salt and sucrose was sprayed from the nebulizer for spray pyrolysis, the sucrose was rapidly decomposed at the high temperature in the electric furnace and the metal salt particles helped oxidize the sucrose while maintaining the hollow structure. The gas sensing materials of Example 1 and Comparative Example 3, which were prepared by galvanic replacement of the gas sensing material of Comparative Example 1, failed to maintain their hollow structures during replacement. The collapsed sensing materials existed in the form of oxide particles. Particularly, Cr.sub.2O.sub.3 secondary particles were mixed with ZnCr.sub.2O.sub.4 secondary particles in the nanocomposite of Example 1. In conclusion, the use of galvanic replacement enables rapid low-temperature synthesis of ZnCr.sub.2O.sub.4, which have been synthesized at a high temperature of 1100 C. for 4 h based on a solid-state mixing method.

Experimental Example 4

TEM Images of the Inventive as Sensing Materials

(66) FIGS. 4a to 4c are TEM images of the gas sensing materials prepared in Example 1 and Comparative Examples 1 and 3, respectively.

(67) The image (FIG. 4a) and elemental analysis of the gas sensing material of Example 1 reveal a uniform distribution of Cr and Zn. Lattice imaging reveals that the sensing material is a composite of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4. The image (FIG. 4b) of the gas sensing material of Comparative Example 1 shows that ZnO maintained its hollow structure. The image (FIG. 4c) of the gas sensing material of Comparative Example 3 shows the presence of ZnCr.sub.2O.sub.4 only.

Experimental Example 5

Dynamic Gas Sensing Characteristics of the Inventive as Sensing Materials Toward Toluene, Xylene, and Ethanol (Each 5 ppm)

(68) FIGS. 5a to 5f show the dynamic gas sensing characteristics of the gas sensing materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 toward toluene, xylene, and ethanol (each 5 ppm) at an operating temperature of 275 C., respectively.

(69) The gas sensing material having a hollow structure synthesized in Comparative Example 1 was found to show n-type gas sensing behavior toward toluene, xylene, and ethanol. The gas sensing materials other than the gas sensing material of Comparative Example 1 showed p-type gas sensing behavior.

Experimental Example 6

Gas Responses of the Inventive as Sensors to Toluene, Xylene, and Ethanol Gases at Different Operating Temperatures

(70) FIG. 6 shows the gas responses of the gas sensors fabricated in Examples 1 to 3 and Comparative Examples 1 to 3 to toluene, xylene, and ethanol gases at different operating temperatures.

(71) The gas responses of the gas sensor including the hollow ZnO synthesized in Comparative Example 1 to the gases were found to increase with increasing operating temperature. Particularly, the gas sensor of Comparative Example 1 showed a high response of 65.4 to 5 ppm ethanol gas.

(72) The gas sensor fabricated based on galvanic replacement in Example 1 showed responses of 70.7, 18.9, and 2.6 to xylene, toluene, and ethanol (each 5 ppm) at 275 C., respectively, demonstrating its high responses to the methylbenzene gases. Particularly, considering that the response of the gas sensor of Example 1 to xylene gas was 27.2 times higher than that to ethanol gas, the gas sensor is advantageous in selective methylbenzene detection, which demonstrates that the gas sensor can be sufficiently used for the detection of indoor pollutant gases.

(73) The gas sensors of Example 2 and Comparative Examples 2-3 showed relatively low gas responses of 10. The response of the gas sensors of Example 2 and Comparative Examples 2-3 to methylbenzenes were 3. The gas sensors of Examples 2-3 had lower responses than the gas sensor of Example 1 but showed sufficiently high selectivities to xylene. Accordingly, the gas sensors of Examples 2-3 are expected to be applicable to practical use. In contrast, none of the gas sensors of Comparative Examples 2-3 showed selectivities to the gases.

(74) These results conclude that Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 has high selectivity to xylene irrespective of its synthesis method so long as the Cr content is in the range of 78.2-90.0 at. %. Particularly, it can be concluded that the use of galvanic replacement is more effective for higher sensitivity and selectivity.

Experimental Example 7

Gas Sensing Characteristics of the as Sensor of Example 1 to Ethanol, Xylene, Toluene, Benzene, HCHO, and CO at Operating Temperature of 275

(75) FIG. 7 shows the gas sensing characteristics of the gas sensor fabricated in Example 1 to ethanol, xylene, toluene, benzene, HCHO, and CO at an operating temperature of 275 C. FIG. 7 reveals high selectivities of the gas sensor of Example 1 to the methylbenzenes over ethanol. In addition, the gas sensor of Example 1 showed very high selectivities to the methylbenzenes over other indoor environmental gases.

(76) The high selectivities of the gas sensor of Example 1 are believed to be because the composition of the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite synthesized based on galvanic replacement ([Cr]/{[Cr]+[Zn]}=81.9 at. %, as measured by ICP analysis) is advantageous in detecting methylbenzenes. Therefore, galvanic replacement is an improved approach to maximize the catalytic activities of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4 for the oxidation of xylene and toluene.

(77) When the gas sensing results obtained by the gas sensor of Example 1 were compared with those obtained by the gas sensors of Comparative Examples 2-3, it can again be confirmed that the composition of the composite prepared in Example 1 ensures high xylene selectivity and sensitivity, which could not be achieved by single-phase Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4.

(78) The gas sensing material synthesized based on galvanic replacement in Example 1 was found to be more advantageous in detecting methylbenzenes than Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 synthesized based on a solid-state synthesis method in Example 2. The enhanced methylbenzene response and selectivity of the gas sensing material of Example 1 are thought to arise from galvanic replacement for the synthesis of the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite in the form of a fine powder. That is, the replacement reaction proceeds uniformly in the fine powder to synthesize a single-phase ZnCr.sub.2O.sub.4 fine powder having a small particle size at low temperature and allows the formation of homogeneous p-p heterojunctions over the entire region of the sensing material to maximize the electrical sensitivity, which enhance the methylbenzene sensitivity and selectivity of the gas sensing material. The same can also be found in Example 3. This provides evidence for the formation of more homogenous p-p heterojunctions in the ZnCr.sub.2O.sub.4 fine powder having a smaller size.

Experimental Example 8

Cr Content of the Inventive Cr2O3/ZnCr2O4 Nanocomposite

(79) A Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite was synthesized in the same manner as in Example 1, except that the content of Cr was changed such that [Cr]/{[Cr]+[Zn]} was 78.2 at. %, as measured by ICP analysis. FIG. 8 shows the results of X-ray diffraction analysis for the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite.

(80) FIG. 8 reveals that when Cr was added in an amount corresponding to 78.2 at. %, a diffraction phase of Cr.sub.2O.sub.3 was present in a very small amount. In addition, it can be concluded that catalytic activity of Cr.sub.2O.sub.3 can be expected when Cr is present in an amount of 78.2 at. % in the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite. From these results, the lower limit of the content of Cr in the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite can be determined.

Experimental Example 9

Gas Sensing Characteristics of the as Sensor of Example 1 to Xylene with Varying Concentrations of the Gas

(81) FIG. 9 shows the gas sensing characteristics of the gas sensor fabricated in Example 1 to xylene with varying concentrations of the gas. The gas sensor showed high responses of 1.7, 2.0, 3.0, 22.2, and 70.7 to 50 ppb, 100 ppb, 250 ppb, 500 ppb, and 1 ppm ethanol gas at an operating temperature of 275 C., respectively. These results demonstrate high response of the gas sensor to xylene at the concentration level of 250 ppb, suggesting that the gas sensor can be effectively applied to the detection of indoor methylbenzenes at concentrations of 10 ppm, which is the object of the present invention.

(82) The responses of the gas sensing material of Example 1 to xylene at different concentrations and the selectivity of the gas sensing material of Example 1 to xylene over ethanol were compared with those of gas sensing materials reported in the literature. The results are shown in FIGS. 10 and 11. Referring to FIGS. 10 and 11, the gas sensing material of Example 1 showed much higher responses and selectivity to xylene at lower concentrations than gas sensing materials reported in the literature. The gas sensing material of Example 1 was found to effectively detect low concentrations of xylene compared to SnO.sub.2 and CO.sub.3O.sub.4, which are generally known to have high methylbenzene responses. The inventive gas sensor was compared with conventional ZnCr.sub.2O.sub.4 gas sensors reported in the literature. As a result, the conventional gas sensors were found to show limited responses to ethanol and Cl.sub.2 only. It is also difficult to conclude that the selectivities of the conventional gas sensors have significantly high selectivities. In contrast, the inventive Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 nanocomposite prepared based on galvanic replacement showed significantly high responses and selectivities to methylbenzenes. Therefore, the inventive gas sensor is a highly sensitive and selective oxide semiconductor gas sensor to methylbenzenes.

Experimental Example 10

Humidity Stability of the as Sensor of Example 1

(83) FIG. 12 shows the responses of the gas sensor (275 C.) fabricated in Example 1 (Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 ([Cr]/[Cr]+[Zn]=81.9 at %)) to 1 ppm xylene gas under humid conditions (20%, 50%, and 80% RH) rather than in a dry atmosphere.

(84) The sensor of Example 1 did not undergo a reduction in response in a humid atmosphere, i.e. with increasing relative humidity (20%, 50%, and 80% RH), as well as in a dry atmosphere. These results demonstrate that the inventive gas sensor can be applied to the detection of methylbenzenes even in a humid atmosphere, which could not be achieved by conventional oxide semiconductor gas sensors. The high response of the inventive gas sensor to xylene in a humid atmosphere was maintained, suggesting that the inventive gas sensor is suitable for use in daily life.

Experimental Example 11

Cr Contents of the Inventive Gas Sensing Materials for Methylbenzene Detection

(85) Galvanic replacement employed in Examples 1-3 is a method in which oxides are replaced by dissimilar metal salts to form composites. According to this method, there may be a difference between the amount of metal salts added and the amount of metal salts replaced. Thus, the results of ICP analysis for final composites may be significantly different from the amount of metal salts added. The contents of Cr in the Cr-added Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite (Example 1) and the Cr-free single-phase ZnCr.sub.2O.sub.4 composite (Example 3) synthesized based on galvanic replacement were 81.9 at. % and 77.9 at. %, respectively, as measured by ICP analysis.

(86) The Cr content of the Cr-added Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite synthesized based on spray pyrolysis in Example 4 was 68.0 at. %. Since droplets containing two ions were converted into oxides during spray pyrolysis without a substantial change in composition, a mixed phase of Cr.sub.2O.sub.3 and ZnCr.sub.2O.sub.4 was formed even when [Cr]/{[Cr]+[Zn]} was 68%.

(87) FIG. 13 shows the gas sensing characteristics of the Cr.sub.2O.sub.3/ZnCr.sub.2O.sub.4 composite sensor containing 68.0 at. % of Cr, which was synthesized based on spray pyrolysis, toward gases (each 5 ppm) in a dry atmosphere at 275 C. The gas sensing characteristics of the gas sensor toward toluene, xylene, and ethanol gases (each 5 ppm) were measured. The responses of the gas sensor to toluene (S.sub.toluene), xylene (S.sub.xylene), and ethanol (S.sub.ethonal) were found to be 8.1, 18.3, and 2.7, respectively, confirming high responses and selectivities of the gas sensor to the methylbenzenes. These results can lead to the conclusion that the gas sensor can be applied to the detection of methylbenzenes even when the Cr content is 68.0 at. %.