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Highly selective ethanol In2O3-based gas sensor
Materials Research Bulletin 42 (2007) 228–235 www.elsevier.com/locate/matresbu
Highly selective ethanol In2O3-based gas sensor Zili Zhan a,*, Jianwei Lu a, Wenhui Song a, Denggao Jiang a, Jiaqiang Xu b a
b
School of Chemical Engineering, Zhengzhou University, Zhengzhou 450002, China Department of Chemical Engineering, Zhengzhou Institute of Light Industry, Zhengzhou 450002, China Received 9 July 2005; received in revised form 14 April 2006; accepted 8 June 2006 Available online 13 July 2006
Abstract The sensitive composite material was prepared by loading Pt and La2O3 into ultrafine In2O3 matric material (8 nm) synthesized by microemulsion method. A highly selective ethanol gas sensor was developed based on hot-wire type gas sensor, which was sintered in a bead (0.8 mm in diameter) to cover a platinum wire coil (0.4 mm in diameter). The gas sensor was operated by a bridge electric circuit. The influences of La2O3 and Pt additives on C2H5OH sensing properties of In2O3-based gas sensor were discussed. The addition of La2O3 resulted in a prominent selectivity for C2H5OH, and the addition of Pt improved the response rate to C2H5OH without affecting the sensitivity. The temperature and humidity characteristics of the sensor output were also investigated. The selective sensor had low power consumption, significantly minor humidity and temperature dependence, high selectivity and prominent long-term stability. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Semiconductor; A. Nanostructure; B. Chemical synthesis
1. Introduction Ethanol vapour is one of the most popular gases in industry and our daily life, so it is important to detect and control ethanol vapour. A more positive application of an ethanol vapour sensor may be a breath alcohol checker to monitor ethanol vapour in human breath, which is said to be well correlated with the ethanol concentration in the drunk driver’s blood. Therefore, several oxides have been tested for ethanol vapour sensing. Thin films of SnO2 [1], ZnO [2], Bi2O3– MoO3 [3], CdIn2O4 [4] and sintered b-CdSnO3 [5], BaSnO3 [6] are reportedly more or less sensitive to ethanol vapour. Relatively high sensitivity has been reported with electron beam-evaporated SnO2 [7], sintered Pd–La2O3–SnO2 [8,9]. However, all of the C2H5OH gas sensors reported in literature are traditional indirect-heating ceramic-type elements, which have high power consumption (>700 mW) and are easily affected by ambient humidity and temperature [10]. With the development of gas sensors, the invention of C2H5OH gas sensor with low power consumption is becoming more and more important for the application of the portable ethanol alarm. We have been investigating how to upgrade the gas-sensing characteristics of a low power consumption semiconductor gas sensor commercially available [11]. The sensor ought to have high sensitivity, high selectivity, good response properties and desirable long-term stability. As part of this, we tried to improve the ethanol vapour sensing properties of In2O3-based element, and found that the selectivity of the sensor increased with the addition of La2O3, loading with Pt promoted greatly the response rates of the element, giving rise to excellent ethanol vapour-sensing * Corresponding author. Tel.: +86 371 63886501; fax: +86 371 63886501. E-mail address: [email protected] (Z. Zhan). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.06.006
Z. Zhan et al. / Materials Research Bulletin 42 (2007) 228–235
229
Table 1 Compositions of microemulsion system
Aqeous solution Surfactant Cosurfactant Oil phase
Component
Quantity
0.1 mol/L In(NO3)3 Triton X-100 n-Heptyl alcohol n-Octane
50 mL 22.5 g 15 g 25 g
properties in terms of sensitivity, selectivity, response rate and stability. This paper deals with the fabrication of an indium oxide based element. The indium oxide used in present study was prepared by microemulsion method. The hot wire type gas sensor has a simple structure with two terminals connected to a Wheatstone bridge circuit, and indium oxide powder was sintered in a bead to cover a platinum wire coil, which works both as a heater and as an electrode. The sensitive element can be operated with a small power consumption (<150 mW) because of a thermally most favorable structure, with a small thermal capacity and a small thermal loss. A series of elements with various La2O3 and Pt was examined, and the temperature and humidity dependence of the gas sensor output were also investigated, and found to be negligibly small. 2. Experimental 2.1. The preparation of nano-In2O3 Triton X-100 (chemical regent) as surfactant, n-heptyl alcohol (chemical regent, 98.5%) as cosurfactant, n-octane (chemical regent) as continuous oil phase and distilled water were used to form inverse microemulsion. Indium nitrate (In(NO3)34.5H2O, AR, 99.5%) as inorganic reactant and ammonia gas as a precipitant to prepare In2O3 particle. The compositions of microemulsion system used in the experiment were shown in Table 1. The transparent inverse microemulsion system was prepared by dispersing the 50 mL aqueous phase into the Triton X-100/n-octane/n-heptyl alcohol mixture (62.5 g) while being agitated violently using a magnetic stirrer, then ammonia gas was slowly injected into microemulsion to control the pH 8.5. After the reaction, acetone was added to break the microemulsion structure to cause sedimentation of the indium hydroxide particles synthesized in microemulsion. The precipitate was separated in a centrifuge at 5000 rpm for 15 min, then it was washed three times with acetone and ethanol respectively and with distilled water as the following to remove any oil and surfactants from the particles. The precipitate was then dried at 120 8C for 12 h. The precursors were then calcined at 500 8C for 2 h for complete conversion of hydroxide into In2O3 particles. 2.2. Fabrication of element Nanometer Indium oxide was mixed with Pt, La2O3 and some trace dopants (Al2O3, SiO2, etc.), and ground into fine gun-like material. The paste was distributed to a platinum wire coil (0.4 mm in diameter) to form sensitive elements (0.8 mm in diameter). Likewise, a-Al2O3 powder in paste was also sintered in a bead as reference elements. The sensitive elements and reference elements were air-dried for 2 h, then heated at 750 8C for 2 h in a furnace. A gas sensor was fabricated by a reference element and a sensitive element according to their same resistance of sensitive
Fig. 1. The structure of the sensitive element.
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Fig. 2. A bridge electric circuit for operating ethanol selective gas sensor. D: sensitive element; C: reference element. R1 and R2: fixed resistance; W: resistance potentiometer; E: sensor voltage.
element and reference element. The structure of the sensitive element was shown in Fig. 1. Then the gas sensor was aged for 240 h. The sensor was operated by use of a bridge circuit shown in Fig. 2. The sensor operating temperature was set at about 250 8C by the bridge voltage (sensor voltage) E. The sensor output was read as the bridge out V of potential difference between the points of (1) and (2) shown in Fig. 2. D and C are sensitive element and reference element, respectively. The gas-sensing properties of the gas sensor were tested with HW-30 gas-sensing testing instrument (Henan Hanwei electronics Co. LTD). Gas sensitivity (DV) is defined by the difference between the sensor output in a sample gas balanced with air (Vg) and in a clear air (Va): DV ¼ V g V a In clear air, the sensor output (Va) was adjusted to zero by adjusting the resistance potentiometer (W), so the sensitivity: DV = Vg 3. Results and discussion 3.1. The morphology of indium oxide particles Fig. 3(a) is a TEM (JEM-2000, Japan) micrograph of the In2O3 prepared by reverse microemulsion method. Most particles are spherical, particles size distributed in a narrow range from 5 to 13 nm, as shown in Fig. 3(b), the average diameter of the particles is about 8 nm. 3.2. The effects of La2O3 and Pt on the gas-sensing characteristics The traditional indirect-heating gas sensors using semiconductor oxides such as SnO2 detect an objective gas in air from a change in electrical resistance caused by the adsorption and/or reaction of gases. Although the structure of the hot-wire gas sensors is different from traditional indirect-heating element, it is expected that such principle is also suitable to hot-wire gas sensor. There have been many reports showing that the gas-sensing properties of a semiconductor sensor are greatly affected by the additions of a noble metal or a metal oxide [9] to the sensor element. Thus the effects of La2O3 and Pt on the gas-sensing characteristics were studied.
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Fig. 3. (a) The TEM micrograph of the In2O3 prepared by reverse microemulsion method. (b) The size distribution histogram of In2O3 particles.
Since one of the most interfering gases on C2H5OH is LPG, a series of elements with various La2O3 loading up to 17 wt.% were subjected to measurements of the sensitivity and selectivity to 100 ppm C2H5OH and 2000 ppm LPG, the amount of Pt was fixed at 4.5 wt.%. The results were shown as a function of La2O3 content in Fig. 4(a) and (b). As seen from Fig. 4(a), the addition of La2O3 brought about an extensive decrease in sensitivity to 100 ppm C2H5OH and 2000 ppm LPG. The highest sensitivity to 100 ppm C2H5OH was attained at a loading of 0 wt.%, but the sensitivity to 2000 ppm LPG was 100 mV. Fig. 4(b) showed the selectivity to C2H5OH gas against LPG gas defined as the ratio of SEtOH to SLPG gas sensitivity, where SEtOH and SLPG are the sensitivity to 100 ppm C2H5OH and 2000 ppm LPG, respectively. The SEtOH/SLPG passed a peak at 1 wt.% La2O3, then decreased with an increase in the amount of La2O3. Though the selectivity to 100 ppm was best at 1 wt.%, the response time (about 20 s) was too long to apply. In order to ensure the response time was less than 10 s and the sensitivity to 100 ppm C2H5OH was more than 100 mV, the La2O3 loading appeared to be optimized at 1–1.5 wt.%. So the La2O3 loading was fixed at 1.35 wt.% in following experiment. In order to improve the response and recovery characteristics, the effect of the additives amount of Pt was investigated as shown in Fig. 4(c). The addition of Pt improved the response kinetics to C2H5OH without affecting the sensitivity significantly. The amount of Pt appeared to be optimized at 4.5 wt.%, the recovery time was about 30 s. The role of Pt is considered to optimize the oxidation activity of the sensing material, but this point is open to future investigations, and we cannot illustrate the abnormal characteristic of 1.5 wt.% Pt loading. As a result, the element, Pt
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Fig. 4. (a) The relationship between La2O3 content and the sensitivity to 100 ppm C2H5OH and 2000 ppm LPG; the Pt content was fixed at 4.5 wt.%. (b) The relationship between La2O3 content and the selectivity of element; The Pt content was fixed at 4.5 wt.%. (c) The effects of Pt on the sensitivity and recovery time of the Pt–La2O3 (1.35 wt.%)–In2O3 element in 100 ppm C2H5OH.
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Fig. 5. The measured sensitivity of the sensor to various gases as a function of gas concentration.
(4.5 wt.%)–La2O3 (1.35 wt.%)–In2O3, had desirable kinetics and sensitivity characteristics. In the following paper, except for special interpretation, the operating voltage (E) was fixed at 3 V, the sensing element was Pt (4.5 wt.%)– La2O3 (1.35 wt.%)–In2O3 element. 3.3. Gas sensing characteristics of the sensor In general, the sensitivity of the sensor increases with increasing concentration of sample gas. As shown in Fig. 5, the measured sensitivity was around 120 mV for 100 ppm ethanol gas, and this increased to 290 mv for 1000 ppm. This high sensitivity is good enough for practical application. With regard to practical use, a gas sensor should provide good selectivity to other gases and a rapid response in detecting the existence of ethanol. The sensor prepared in the above way possessed these basic requirements. For example, as shown in Fig. 5, for 2000 ppm LPG gas and 500 ppm gasoline (90#), the sensitivity were only 27 and 80 mV, respectively. The response transients of the gas sensor on turning on and off the 100 ppm ethanol for the Pt–La2O3–In2O3 element were shown in Fig. 6. The output voltage was 123 mV and the time for 90% of full response to 100 ppm ethanol was 7 s. The rate of recovery was slower, the recovery time was around 30 s.
Fig. 6. The response transients of sensor elements to switching on and off the 100 ppm ethanol in air at operating voltage 3 V.
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Fig. 7. The humidity dependence of the sensor output to 100 ppm ethanol and in clear air.
Fig. 7 shows the humidity dependence of the sensor output characteristics at 20 8C. The gas sensor output was adjusted to zero in air at 20 8C and 50%RH. As shown in Fig. 7, with increasing the relative humidity, the sensor output increased smoothly both in air and in 100 ppm C2H5OH. The drift of sensor output in air was less than 10 mV from 20 to 80%RH. When the alarm level is initially set at the sensor output to 100 ppm C2H5OH (123 mV) at 20 8C and 50%RH, the drift of sensitivity in 100 ppm C2H5OH also less than 10 mV, that is to say, the change of sensitivity was within 10% comparing with the setting alarm level. Fig. 8 shows the temperature dependence of the sensor output characteristics at the fixed relative humidity 40%RH. The sensor output was adjusted to zero in air at 25 8C and 60%RH. The change of sensor output is less than 10 mV both in clean air and in 100 ppm C2H5OH in the range of 0–40 8C. As discussed above, the temperature dependence and humidity dependence of the sensor output were both negligibly smaller compared with indirect-heating gas sensor [10]. The ambient temperature and humidity dependence of the sensor’s sensitivity is a major disadvantage for detecting ethanol vapour as well as other reducing gases [12–14], which is mostly caused by water vapour affecting the conductance of the sensor element. The hot-wire gas sensor was fabricated by a reference element and a sensitive element as shown in Fig. 2. Humidity increase not only the conductivity of sensitive element but also that of reference element, thus the interference of the water vapour with the sensor output was suppressed by the compensation of the reference element. For the same reason, the influence of ambient temperature on sensor output is also small.
Fig. 8. The temperature dependence of the sensor output to 100 ppm ethanol and clean air.
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Fig. 9. The long-term stability of the sensor.
In addition, the selective ethanol gas sensor had a prominent long-term stability in a room air operation for 300 days, as show in Fig. 9. The long-term drift is less than 10 mV. 4. Conclusion The selective ethanol gas sensor commercially available based on the hot-wire type gas sensor was developed in this paper. The sensor had high sensitivity and selectivity to ethanol, quick response and recovery properties and small influence by the ambient temperature and humidity. The addition of La2O3 and Pt can obviously improve its gas-sensing properties. The ultrafine particles of the sensing material and compact structure determined the advantage of low power consumption of 150 mW, which was much lower than that of the indirect-heating-type ethanol sensor. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
L. Promsong, M. Sriyudthsak, Sens. Actuators B 24–25 (1995) 504–506. B.B. Rao, Mater. Chem. Phys. 64 (2000) 62–65. N. Hykaway, W.M. Sears, R.F. Frindt, S.R. Morrison, Sens. Actuators 15 (1988) 105–118. Y.F. Dong, W.L. Wang, K.J. Liao, Sens. Actuators B 67 (2000) 254–257. T. Zhang, Y. Shen, R. Zhang, Mater. Lett. 23 (1–3) (1995) 69–71. S. Tao, F. Gao, X. Liu, O.T. Sorensen, Sens. Actuators B 71 (2000) 223–227. Y.K. Fang, J.J. Lee, Thin Solid Film 169 (1989) 51–56. T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B 9 (1992) 63–69. J. Tamaki, T. Maekwa, S. Matsushima, N. Miura, N. Yamazoe, Chem. Lett. (1990) 477–480. N. Baˆrsan, J.R. Stetter, M. Findlay, W. Go¨pel, Sens. Actuators B 66 (2000) 31–33. Z. Zili, J. Denggao, S. Wenhui, C. Jian, J. Transducer Technol. 22 (5) (2003) 31–36. D.S. Vlachos, P.D. Skafidas, J.N. Avaritsiotis, Sens. Actuators B 24–25 (1995) 491–494. N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Surf. Sci. 86 (1979) 335–344. G. Martinelli, M.C. Carotta, L. Passari, L. Tracchi, Sens. Actuators B 26–27 (1995) 53–55.
Highly selective ethanol In2O3-based gas sensor Zili Zhan a,*, Jianwei Lu a, Wenhui Song a, Denggao Jiang a, Jiaqiang Xu b a
b
School of Chemical Engineering, Zhengzhou University, Zhengzhou 450002, China Department of Chemical Engineering, Zhengzhou Institute of Light Industry, Zhengzhou 450002, China Received 9 July 2005; received in revised form 14 April 2006; accepted 8 June 2006 Available online 13 July 2006
Abstract The sensitive composite material was prepared by loading Pt and La2O3 into ultrafine In2O3 matric material (8 nm) synthesized by microemulsion method. A highly selective ethanol gas sensor was developed based on hot-wire type gas sensor, which was sintered in a bead (0.8 mm in diameter) to cover a platinum wire coil (0.4 mm in diameter). The gas sensor was operated by a bridge electric circuit. The influences of La2O3 and Pt additives on C2H5OH sensing properties of In2O3-based gas sensor were discussed. The addition of La2O3 resulted in a prominent selectivity for C2H5OH, and the addition of Pt improved the response rate to C2H5OH without affecting the sensitivity. The temperature and humidity characteristics of the sensor output were also investigated. The selective sensor had low power consumption, significantly minor humidity and temperature dependence, high selectivity and prominent long-term stability. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Semiconductor; A. Nanostructure; B. Chemical synthesis
1. Introduction Ethanol vapour is one of the most popular gases in industry and our daily life, so it is important to detect and control ethanol vapour. A more positive application of an ethanol vapour sensor may be a breath alcohol checker to monitor ethanol vapour in human breath, which is said to be well correlated with the ethanol concentration in the drunk driver’s blood. Therefore, several oxides have been tested for ethanol vapour sensing. Thin films of SnO2 [1], ZnO [2], Bi2O3– MoO3 [3], CdIn2O4 [4] and sintered b-CdSnO3 [5], BaSnO3 [6] are reportedly more or less sensitive to ethanol vapour. Relatively high sensitivity has been reported with electron beam-evaporated SnO2 [7], sintered Pd–La2O3–SnO2 [8,9]. However, all of the C2H5OH gas sensors reported in literature are traditional indirect-heating ceramic-type elements, which have high power consumption (>700 mW) and are easily affected by ambient humidity and temperature [10]. With the development of gas sensors, the invention of C2H5OH gas sensor with low power consumption is becoming more and more important for the application of the portable ethanol alarm. We have been investigating how to upgrade the gas-sensing characteristics of a low power consumption semiconductor gas sensor commercially available [11]. The sensor ought to have high sensitivity, high selectivity, good response properties and desirable long-term stability. As part of this, we tried to improve the ethanol vapour sensing properties of In2O3-based element, and found that the selectivity of the sensor increased with the addition of La2O3, loading with Pt promoted greatly the response rates of the element, giving rise to excellent ethanol vapour-sensing * Corresponding author. Tel.: +86 371 63886501; fax: +86 371 63886501. E-mail address: [email protected] (Z. Zhan). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.06.006
Z. Zhan et al. / Materials Research Bulletin 42 (2007) 228–235
229
Table 1 Compositions of microemulsion system
Aqeous solution Surfactant Cosurfactant Oil phase
Component
Quantity
0.1 mol/L In(NO3)3 Triton X-100 n-Heptyl alcohol n-Octane
50 mL 22.5 g 15 g 25 g
properties in terms of sensitivity, selectivity, response rate and stability. This paper deals with the fabrication of an indium oxide based element. The indium oxide used in present study was prepared by microemulsion method. The hot wire type gas sensor has a simple structure with two terminals connected to a Wheatstone bridge circuit, and indium oxide powder was sintered in a bead to cover a platinum wire coil, which works both as a heater and as an electrode. The sensitive element can be operated with a small power consumption (<150 mW) because of a thermally most favorable structure, with a small thermal capacity and a small thermal loss. A series of elements with various La2O3 and Pt was examined, and the temperature and humidity dependence of the gas sensor output were also investigated, and found to be negligibly small. 2. Experimental 2.1. The preparation of nano-In2O3 Triton X-100 (chemical regent) as surfactant, n-heptyl alcohol (chemical regent, 98.5%) as cosurfactant, n-octane (chemical regent) as continuous oil phase and distilled water were used to form inverse microemulsion. Indium nitrate (In(NO3)34.5H2O, AR, 99.5%) as inorganic reactant and ammonia gas as a precipitant to prepare In2O3 particle. The compositions of microemulsion system used in the experiment were shown in Table 1. The transparent inverse microemulsion system was prepared by dispersing the 50 mL aqueous phase into the Triton X-100/n-octane/n-heptyl alcohol mixture (62.5 g) while being agitated violently using a magnetic stirrer, then ammonia gas was slowly injected into microemulsion to control the pH 8.5. After the reaction, acetone was added to break the microemulsion structure to cause sedimentation of the indium hydroxide particles synthesized in microemulsion. The precipitate was separated in a centrifuge at 5000 rpm for 15 min, then it was washed three times with acetone and ethanol respectively and with distilled water as the following to remove any oil and surfactants from the particles. The precipitate was then dried at 120 8C for 12 h. The precursors were then calcined at 500 8C for 2 h for complete conversion of hydroxide into In2O3 particles. 2.2. Fabrication of element Nanometer Indium oxide was mixed with Pt, La2O3 and some trace dopants (Al2O3, SiO2, etc.), and ground into fine gun-like material. The paste was distributed to a platinum wire coil (0.4 mm in diameter) to form sensitive elements (0.8 mm in diameter). Likewise, a-Al2O3 powder in paste was also sintered in a bead as reference elements. The sensitive elements and reference elements were air-dried for 2 h, then heated at 750 8C for 2 h in a furnace. A gas sensor was fabricated by a reference element and a sensitive element according to their same resistance of sensitive
Fig. 1. The structure of the sensitive element.
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Fig. 2. A bridge electric circuit for operating ethanol selective gas sensor. D: sensitive element; C: reference element. R1 and R2: fixed resistance; W: resistance potentiometer; E: sensor voltage.
element and reference element. The structure of the sensitive element was shown in Fig. 1. Then the gas sensor was aged for 240 h. The sensor was operated by use of a bridge circuit shown in Fig. 2. The sensor operating temperature was set at about 250 8C by the bridge voltage (sensor voltage) E. The sensor output was read as the bridge out V of potential difference between the points of (1) and (2) shown in Fig. 2. D and C are sensitive element and reference element, respectively. The gas-sensing properties of the gas sensor were tested with HW-30 gas-sensing testing instrument (Henan Hanwei electronics Co. LTD). Gas sensitivity (DV) is defined by the difference between the sensor output in a sample gas balanced with air (Vg) and in a clear air (Va): DV ¼ V g V a In clear air, the sensor output (Va) was adjusted to zero by adjusting the resistance potentiometer (W), so the sensitivity: DV = Vg 3. Results and discussion 3.1. The morphology of indium oxide particles Fig. 3(a) is a TEM (JEM-2000, Japan) micrograph of the In2O3 prepared by reverse microemulsion method. Most particles are spherical, particles size distributed in a narrow range from 5 to 13 nm, as shown in Fig. 3(b), the average diameter of the particles is about 8 nm. 3.2. The effects of La2O3 and Pt on the gas-sensing characteristics The traditional indirect-heating gas sensors using semiconductor oxides such as SnO2 detect an objective gas in air from a change in electrical resistance caused by the adsorption and/or reaction of gases. Although the structure of the hot-wire gas sensors is different from traditional indirect-heating element, it is expected that such principle is also suitable to hot-wire gas sensor. There have been many reports showing that the gas-sensing properties of a semiconductor sensor are greatly affected by the additions of a noble metal or a metal oxide [9] to the sensor element. Thus the effects of La2O3 and Pt on the gas-sensing characteristics were studied.
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Fig. 3. (a) The TEM micrograph of the In2O3 prepared by reverse microemulsion method. (b) The size distribution histogram of In2O3 particles.
Since one of the most interfering gases on C2H5OH is LPG, a series of elements with various La2O3 loading up to 17 wt.% were subjected to measurements of the sensitivity and selectivity to 100 ppm C2H5OH and 2000 ppm LPG, the amount of Pt was fixed at 4.5 wt.%. The results were shown as a function of La2O3 content in Fig. 4(a) and (b). As seen from Fig. 4(a), the addition of La2O3 brought about an extensive decrease in sensitivity to 100 ppm C2H5OH and 2000 ppm LPG. The highest sensitivity to 100 ppm C2H5OH was attained at a loading of 0 wt.%, but the sensitivity to 2000 ppm LPG was 100 mV. Fig. 4(b) showed the selectivity to C2H5OH gas against LPG gas defined as the ratio of SEtOH to SLPG gas sensitivity, where SEtOH and SLPG are the sensitivity to 100 ppm C2H5OH and 2000 ppm LPG, respectively. The SEtOH/SLPG passed a peak at 1 wt.% La2O3, then decreased with an increase in the amount of La2O3. Though the selectivity to 100 ppm was best at 1 wt.%, the response time (about 20 s) was too long to apply. In order to ensure the response time was less than 10 s and the sensitivity to 100 ppm C2H5OH was more than 100 mV, the La2O3 loading appeared to be optimized at 1–1.5 wt.%. So the La2O3 loading was fixed at 1.35 wt.% in following experiment. In order to improve the response and recovery characteristics, the effect of the additives amount of Pt was investigated as shown in Fig. 4(c). The addition of Pt improved the response kinetics to C2H5OH without affecting the sensitivity significantly. The amount of Pt appeared to be optimized at 4.5 wt.%, the recovery time was about 30 s. The role of Pt is considered to optimize the oxidation activity of the sensing material, but this point is open to future investigations, and we cannot illustrate the abnormal characteristic of 1.5 wt.% Pt loading. As a result, the element, Pt
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Fig. 4. (a) The relationship between La2O3 content and the sensitivity to 100 ppm C2H5OH and 2000 ppm LPG; the Pt content was fixed at 4.5 wt.%. (b) The relationship between La2O3 content and the selectivity of element; The Pt content was fixed at 4.5 wt.%. (c) The effects of Pt on the sensitivity and recovery time of the Pt–La2O3 (1.35 wt.%)–In2O3 element in 100 ppm C2H5OH.
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Fig. 5. The measured sensitivity of the sensor to various gases as a function of gas concentration.
(4.5 wt.%)–La2O3 (1.35 wt.%)–In2O3, had desirable kinetics and sensitivity characteristics. In the following paper, except for special interpretation, the operating voltage (E) was fixed at 3 V, the sensing element was Pt (4.5 wt.%)– La2O3 (1.35 wt.%)–In2O3 element. 3.3. Gas sensing characteristics of the sensor In general, the sensitivity of the sensor increases with increasing concentration of sample gas. As shown in Fig. 5, the measured sensitivity was around 120 mV for 100 ppm ethanol gas, and this increased to 290 mv for 1000 ppm. This high sensitivity is good enough for practical application. With regard to practical use, a gas sensor should provide good selectivity to other gases and a rapid response in detecting the existence of ethanol. The sensor prepared in the above way possessed these basic requirements. For example, as shown in Fig. 5, for 2000 ppm LPG gas and 500 ppm gasoline (90#), the sensitivity were only 27 and 80 mV, respectively. The response transients of the gas sensor on turning on and off the 100 ppm ethanol for the Pt–La2O3–In2O3 element were shown in Fig. 6. The output voltage was 123 mV and the time for 90% of full response to 100 ppm ethanol was 7 s. The rate of recovery was slower, the recovery time was around 30 s.
Fig. 6. The response transients of sensor elements to switching on and off the 100 ppm ethanol in air at operating voltage 3 V.
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Fig. 7. The humidity dependence of the sensor output to 100 ppm ethanol and in clear air.
Fig. 7 shows the humidity dependence of the sensor output characteristics at 20 8C. The gas sensor output was adjusted to zero in air at 20 8C and 50%RH. As shown in Fig. 7, with increasing the relative humidity, the sensor output increased smoothly both in air and in 100 ppm C2H5OH. The drift of sensor output in air was less than 10 mV from 20 to 80%RH. When the alarm level is initially set at the sensor output to 100 ppm C2H5OH (123 mV) at 20 8C and 50%RH, the drift of sensitivity in 100 ppm C2H5OH also less than 10 mV, that is to say, the change of sensitivity was within 10% comparing with the setting alarm level. Fig. 8 shows the temperature dependence of the sensor output characteristics at the fixed relative humidity 40%RH. The sensor output was adjusted to zero in air at 25 8C and 60%RH. The change of sensor output is less than 10 mV both in clean air and in 100 ppm C2H5OH in the range of 0–40 8C. As discussed above, the temperature dependence and humidity dependence of the sensor output were both negligibly smaller compared with indirect-heating gas sensor [10]. The ambient temperature and humidity dependence of the sensor’s sensitivity is a major disadvantage for detecting ethanol vapour as well as other reducing gases [12–14], which is mostly caused by water vapour affecting the conductance of the sensor element. The hot-wire gas sensor was fabricated by a reference element and a sensitive element as shown in Fig. 2. Humidity increase not only the conductivity of sensitive element but also that of reference element, thus the interference of the water vapour with the sensor output was suppressed by the compensation of the reference element. For the same reason, the influence of ambient temperature on sensor output is also small.
Fig. 8. The temperature dependence of the sensor output to 100 ppm ethanol and clean air.
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Fig. 9. The long-term stability of the sensor.
In addition, the selective ethanol gas sensor had a prominent long-term stability in a room air operation for 300 days, as show in Fig. 9. The long-term drift is less than 10 mV. 4. Conclusion The selective ethanol gas sensor commercially available based on the hot-wire type gas sensor was developed in this paper. The sensor had high sensitivity and selectivity to ethanol, quick response and recovery properties and small influence by the ambient temperature and humidity. The addition of La2O3 and Pt can obviously improve its gas-sensing properties. The ultrafine particles of the sensing material and compact structure determined the advantage of low power consumption of 150 mW, which was much lower than that of the indirect-heating-type ethanol sensor. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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