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Nanogold on powdered cobalt oxide for carbon monoxide sensor
Sensors and Actuators B 96 (2003) 596–601
Nanogold on powdered cobalt oxide for carbon monoxide sensor Ren-Jang Wu a,∗ , Cheng-Hung Hu b , Chuin-Tih Yeh b , Pi-Guey Su c a
Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu 300, Taiwan, ROC b Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC c TZU-HUI Institute of Technology, Ping-Tung County 926, Taiwan, ROC Received 17 February 2003; accepted 25 June 2003
Abstract Powders of Co3 O4 and Au/Co3 O4 were used as sensing material for a solid electrolyte sensor of carbon monoxide. Prepared powders were deposited on the anode of a galvanic cell using a mixed oxide electrolyte of 5% Y2 O3 and 95% ZrO2 . By comparison with other metal oxides, Co3 O4 displayed good response time and high sensitivity at a moderate temperature of 130 ◦ C. Impregnation of nano-sized crystallites of gold improved the signal of the fabricated sensor. The required operation temperature is limited by the conductivity of the solid electrolyte in the cell. © 2003 Elsevier B.V. All rights reserved. Keywords: CO sensor; YSZ; Solid electrolyte; Co3 O4 ; Au/Co3 O4 ; Catalyst
1. Introduction The sensing of CO is essential for maintaining a safe environment. Different sensing techniques, electrochemical [1], optical [2], resisting [3] and potentiometric [4–6], have been explored to detect contamination by CO. Of these, the potentiometric method has the advantages of simplicity in construction and economy in manufacturing. Tan and co-workers [4] initiated the method by construction of a galvanic cell using yttrium stabilized zirconia (YSZ) as the electrolyte and two coated platinum films as the electrodes. Upon deposition of CuO/ZnO on the anode, exposure of the constructed cell to air contaminated with CO may cause a potential of 23 mV (for 1000 ppm CO) between the electrodes. From measured potential, a CO contamination of 5000 ppm or less may be monitored at a working temperature (T) of ∼400 ◦ C. Yamazoe and co-workers evaluated the obtained signal with a mixed potential model [5]. Using a similar technique, they used CdO (anode) and SnO2 (cathode) as sensing materials and tubular YSZ as electrolyte, and improved the signal to 110 mV at 600 ◦ C. However, the signal was significantly interfered with other gases (H2 , CH4 and NO2 ). Mochizuki et al. exempted the interference of H2 , H2 O and NO by a substitution of Pt/Al2 O3 and Pt on laminated thick-film as
∗ Corresponding author. Tel.: +886-3-5732235; fax: +886-3-5726445. E-mail address: [email protected] (R.-J. Wu).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00646-4
the anode and the cathode [6]. Nevertheless, a working temperature as high as 600 ◦ C is still required. In order to save power in the sensing measurement, a low working temperature device is generally pursued. It has been found [7–9] that catalysts Co3 O4 and Au/Co3 O4 displayed a high catalytic activity towards oxidation of CO in air. The catalysis showed a 100% CO conversion at a temperature of 200 ◦ C or less. It is therefore our intention to test whether these catalysts may be used as a sensing material that could be used to monitor contamination by CO in polluted air at moderate temperatures.
2. Experimentation 2.1. Sensor fabrication Commercial 5%Y2 O3 /95%ZrO2 powders were pressed to disks with 6 mm in diameter and 1 mm in thickness and then calcined to 1400 ◦ C. Two Pt electrodes with a diameter of 1.5 mm (Fig. 1a) were attached at the front side of the calcined disk as electrodes and a Pt heating wire (Fig. 1b) was adhered at the back side of the disk as a heating element. The attachments were performed by the screen-printing technique. The ends of the Pt wire were soldered to a power supply to control the temperature of the disk. Powders of different sensing oxides (CoOx , SnO2 , CuO, or TiO2 ) were converted into a paste by mixing with TEOS, glycol and several drops of HNO3 . A portion of the prepared paste was
R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601
597
2.3. Sensing system
YSZ disk Pt film (cathode)
Sensing material (anode)
(a). Electrodes of sensor (the front side)
Gases of 1000 ppm CO, H2 or CH4 in pure air were used as standard gases. The gases may be further diluted to different compositions (from 300 to 1000 ppm) by mixing with pure air. The flow rate of the testing gas during the sensing measurement was fixed at 100 cm3 /min. Electric potentials, between the sensing (anode) and the reference (cathode) electrodes of the prepared sensor, were measured by a 200 l multimeter from Kiethley. 2.4. TPR experimentation
YSZ disk
Platinum wire (b). Heating device (the back side) Fig. 1. Structure of the sensing element: (a) electrodes of sensor; (b) heating device.
brushed onto the surface of a Pt electrode which was designated as the anode of sensor after a 1 h calcination at 400 ◦ C. The purpose of calcination treatment was to burn out the glycol binding the paste.
The reductive behavior of cobalt oxide was pursued in a flowing gas of 10% H2 /N2 (in a flow rate of 30 cm3 /min). The gas passed consecutively through the reference side of a thermal conductivity detector (TCD), the sample (in a reactor made from a quartz tubing of 4 mm i.d.), a cell containing silica gel (to remove water formed during reduction) and the detecting side of the TCD. The temperature of the quartz reactor was raised, with a rate of 7 ◦ C/min, from the ambient temperature to 500 ◦ C by an oven.
3. Results and discussion 3.1. TPR characterization of CoOx
2.2. Sensing materials Different oxides were tested as sensing materials. Co3 O4 powders were prepared by a 30 min calcination of 2CoO3 · 3Co(OH)2 ·H2 O (from Merck) at 450 ◦ C. Other oxides, i.e. SnO2 , CuO and TiO2 were commercial products and tested without any calcination treatment. Au/Co3 O4 was prepared by precipitation deposition of aqueous AuCl3 on the Co3 O4 powder by alkalizing the AuCl3 solution with 1 M NaOH(aq) to pH ∼9.
The stoichiometry x of a CoOx sample can be determined by the amount of hydrogen consumption in TPR reduction. Fig. 2 presents the TPR trace obtained from the calcined CoOx sample. A stoichiometry of x = 1.3 was evaluated, which suggested the following reduction reaction: Co3 O4 + 4H2 → 3Co + 4H2 O
(1)
Two reducing peaks showed on 340 and 380 ◦ C. The shoulder peak at 340 ◦ C was assigned to a reduction of Co3 O4 to
Fig. 2. TPR of Co3 O4 in 10% H2 /N2 when temperature raising rate is 7 ◦ C/min.
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Fig. 3. Sensing curve of Au/Co3 O4 for CO sensor at 300 ◦ C.
CoO. The main peak at 380 ◦ C is a signal corresponding to conversion of CoO to metal Co [9]. 3.2. Sensing curves from different metal oxides Fig. 3 presented a CO sensing curve obtained from Au/Co3 O4 at 300 ◦ C. In the ambient air, a potential difference of Eair ∼5 mV was noticed between the anode and the cathode of the sensor. The potential difference (E) increased on exposing the sensor to 1000 ppm CO. Observed E increased over time and reached a plateau (ECO ). The increase had a response time of t90 ∼20 s. The sensitivity of the sensor was therefore defined as: S = ECO − Eair (at CO = 1000 ppm) A sensitivity of S = 121 mV was found in Fig. 3 for Au/Co3 O4 .
The sensing temperature and the materials coated on the anode had a marked effect on the measured sensitivity. Fig. 4 compared the temperature profiles of CO sensitivity from different transition metal oxides (TiO2 , Co3 O4 , CuO, SnO2 , Au/Co3 O4 ). They were insensitive at the ambient temperature but became sensitive on raising the temperature over Ts (starting temperature) that is characteristic to the kind of oxide used as a sensing material at the anode. The sensitivity increased with temperature, reached a maximum at To (optimum working temperature), and curved down at high temperatures.Table 1 listed temperatures of Ts and To observed for different oxides on sensing CO. Among the tested oxides, the SnO2 and Co3 O4 displayed low starting temperatures of To = 260 and 320 ◦ C and short t90 time of 60 s. Co3 O4 is our preferred sensing material because its sensitivity (60 mV) is around one-order higher than that (8 mV) of SnO2 .
Fig. 4. Sensitivities upon various metal oxides in 1000 ppm CO alternated with temperature.
R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601 Table 1 Testing results of 1000 ppm CO upon various metal oxides
3.3. Interference effect with other gases
Metal oxides
Ts (◦ C)
To (◦ C)
Sto (mV)
t90 (s)
CuO TiO2 SnO2 Co3 O4 Au/Co3 O4
390 230 200 170 130
470 370 260 320 300
22 21 8 60 121
300 300 70 60 20
Ts : start working temperature, To : optimum working temperature, Sto : highest sensitivity value; t90 : response time.
An impregnation of 1% gold crystallites (Au/Co3 O4 ) definitely enhanced the performance of cobalt oxides as a sensing material. The impregnation not only doubled the sensitivity to S = 120 mV but also reduced the incubation period to t90 = 20 s. This improvement may be attributed to the high catalytic activity of dispersed gold toward the oxidation of CO [8,9]: 2CO + O2 → 2CO2
599
(1)
Fig. 5 compares the temperature profiles of the fabricated sensitivity of Co3 O4 on three different standard gases. All of these gases showed a similar optimum temperature of To ∼350 ◦ C. Nevertheless, the sensitivity at To varied with the activity of gases tested. The hydrogen and carbon monoxide showed a similar sensitivity of S ∼60 mV. A much lower sensitivity of S = 5 mV was noticed from methane. In Fig. 6, we found that an impregnation of Au increased the sensitivity to these gases by a factor of two. However, their relative sensitivity remained the same. 3.4. Calibration curve of CO sensor Fig. 7 presents calibration curves made in this study for sensing CO by Au/Co3 O4 . Evidently, operation temperature had a significant effect on the performance of the fabricated sensor. A higher sensitivity was noticed at a sensing
Fig. 5. Sensing curves of three different gases upon Co3 O4 with 1000 ppm CO.
Fig. 6. Sensing curves of three different gases upon Au/Co3 O4 with 1000 ppm CO.
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Fig. 7. Au/Co3 O4 varied with CO concentration and temperature.
Fig. 8. Conductivity of YSZ solid electrolyte sensor.
temperature of T = 270 than 250 ◦ C. However, the calibration of 270 ◦ C was unfortunately found to curve down at [CO] > 400 ppm. The calibration curvature may be caused by a saturation of reaction (1) on the surface of the sensing material. Although a relatively low sensitivity was found on lowering the operation temperature to 250 ◦ C, a linear relationship up to [CO] = 1000 ppm was noticed.
tance increased significantly on decreasing the temperature of the electrolyte disk to T < 300 ◦ C. The required operation temperature should be caused by the solid electrolyte (YSZ) used in our galvanic cell. The rate-determining step of measuring electronic potential is the conductivity of oxygen ions in YSZ.
3.5. Conductivity of YSZ solid electrolyte sensor
4. Conclusion
The Au/Co3 O4 used in this study can catalyze reaction (1) at subambient temperatures [9]. It is therefore a surprise that the sensor fabricated by this material had a To ∼320 ◦ C. Fig. 8 showed the temperature profile of YSZ resistance measured from a multimeter. The measured resis-
Co3 O4 is found to be a highly sensitive material for sensing CO. A dispersion of nanogold on Co3 O4 may raise its CO sensitivity from S = 60 to 120 mV and reduce its response time from t90 = 60 to 20 s. A sensing head coated with Au/Co3 O4 could detect CO at temperatures above 130 ◦ C.
R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601
References [1] P.D. van der Wal, N.F. de Rooij, M. Koudelka-Hep, Extremely stable nafion based carbon monoxide sensor, Sens. Acuators B: Chem. 31 (1996) 119–123. [2] M. Ando, T. Kobayashi, M. Haruta, Optical CO detection by use of CuO/Au composite films, Sens. Actuators B: Chem. 25 (1995) 851– 853. [3] H. Yamaura, K. Moriya, N. Miuro, N. Yamazoe, Mechanism of sensitivity promotion in CO sensor using indium oxide and cobalt oxide, Sens. Actuators B: Chem. 65 (2000) 39–41. [4] N. Li, T.C. Tan, H.C. Zeng, High-temperature carbon monoxide potentiometric sensor, J. Electrochem. Soc. 140 (1993) 1068– 1073. [5] N. Miuro, T. Raisen, G. Lu, N. Yamazoe, Highly selective CO sensor using stabilized zirconia and a couple of oxide electrodes, Sens. Actuators B: Chem. 47 (1998) 84–91. [6] K. Mochizuki, R. Sorita, H. Takashima, K. Nakamura, G. Lu, Sensing characteristics of zirconia-based CO sensor made by thick-film lamination, Sens. Actuators B: Chem. 77 (2001) 190–195. [7] W.S. Epling, G.B. Holund, J.F. Weaver, S. Tsubota, M. Haruta, Au/MnOx catalytic performance characteristics for low-temperature carbon monoxide oxidation, Appl. Catal. B: Environ. 6 (1995) 117– 126. [8] Y.J. Chen, C.-T. Yeh, Deposition of highly dispersed gold on alumina support, J. Catal. 200 (2001) 59–65.
601
[9] D.E. Wu, Effect of Oxidation State of Cobalt on Catalytic Oxidation of CO Over CoOx and Au/CoOx , Ph.D. Dissertation, National Tsing Hua University, 2000.
Biographies Ren-Jang Wu received a BS in chemistry from National Tsing Hua University in 1986, an MS in chemistry from National Taiwan University in 1988 and a PhD in chemistry from National Tsing Hua University in 1995. His field of interests are chemical sensors, catalysis, nanoscience and chemical standard technology. Cheng-Hung Hu received a BS in chemistry from National Cheng Gung University in 2000, and an MS in chemistry from National Tsing Hua University in 2002. His field of interest is chemical sensors technology. Chuin-tih Yeh received a BS in chemistry from National Taiwan University in 1965 and a PhD in chemistry from Carnegie-Mellon University in 1971. His field of interests are catalysis, nanoscience and fuel-cell technology. Pi-Guey Su received BS degree in chemistry from Soochow University in 1983 and PhD degree in chemistry from National Tsing Hua University in 1998. His field of interest is chemical sensors technology by nanoscience and nanocomposite.
Nanogold on powdered cobalt oxide for carbon monoxide sensor Ren-Jang Wu a,∗ , Cheng-Hung Hu b , Chuin-Tih Yeh b , Pi-Guey Su c a
Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu 300, Taiwan, ROC b Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC c TZU-HUI Institute of Technology, Ping-Tung County 926, Taiwan, ROC Received 17 February 2003; accepted 25 June 2003
Abstract Powders of Co3 O4 and Au/Co3 O4 were used as sensing material for a solid electrolyte sensor of carbon monoxide. Prepared powders were deposited on the anode of a galvanic cell using a mixed oxide electrolyte of 5% Y2 O3 and 95% ZrO2 . By comparison with other metal oxides, Co3 O4 displayed good response time and high sensitivity at a moderate temperature of 130 ◦ C. Impregnation of nano-sized crystallites of gold improved the signal of the fabricated sensor. The required operation temperature is limited by the conductivity of the solid electrolyte in the cell. © 2003 Elsevier B.V. All rights reserved. Keywords: CO sensor; YSZ; Solid electrolyte; Co3 O4 ; Au/Co3 O4 ; Catalyst
1. Introduction The sensing of CO is essential for maintaining a safe environment. Different sensing techniques, electrochemical [1], optical [2], resisting [3] and potentiometric [4–6], have been explored to detect contamination by CO. Of these, the potentiometric method has the advantages of simplicity in construction and economy in manufacturing. Tan and co-workers [4] initiated the method by construction of a galvanic cell using yttrium stabilized zirconia (YSZ) as the electrolyte and two coated platinum films as the electrodes. Upon deposition of CuO/ZnO on the anode, exposure of the constructed cell to air contaminated with CO may cause a potential of 23 mV (for 1000 ppm CO) between the electrodes. From measured potential, a CO contamination of 5000 ppm or less may be monitored at a working temperature (T) of ∼400 ◦ C. Yamazoe and co-workers evaluated the obtained signal with a mixed potential model [5]. Using a similar technique, they used CdO (anode) and SnO2 (cathode) as sensing materials and tubular YSZ as electrolyte, and improved the signal to 110 mV at 600 ◦ C. However, the signal was significantly interfered with other gases (H2 , CH4 and NO2 ). Mochizuki et al. exempted the interference of H2 , H2 O and NO by a substitution of Pt/Al2 O3 and Pt on laminated thick-film as
∗ Corresponding author. Tel.: +886-3-5732235; fax: +886-3-5726445. E-mail address: [email protected] (R.-J. Wu).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00646-4
the anode and the cathode [6]. Nevertheless, a working temperature as high as 600 ◦ C is still required. In order to save power in the sensing measurement, a low working temperature device is generally pursued. It has been found [7–9] that catalysts Co3 O4 and Au/Co3 O4 displayed a high catalytic activity towards oxidation of CO in air. The catalysis showed a 100% CO conversion at a temperature of 200 ◦ C or less. It is therefore our intention to test whether these catalysts may be used as a sensing material that could be used to monitor contamination by CO in polluted air at moderate temperatures.
2. Experimentation 2.1. Sensor fabrication Commercial 5%Y2 O3 /95%ZrO2 powders were pressed to disks with 6 mm in diameter and 1 mm in thickness and then calcined to 1400 ◦ C. Two Pt electrodes with a diameter of 1.5 mm (Fig. 1a) were attached at the front side of the calcined disk as electrodes and a Pt heating wire (Fig. 1b) was adhered at the back side of the disk as a heating element. The attachments were performed by the screen-printing technique. The ends of the Pt wire were soldered to a power supply to control the temperature of the disk. Powders of different sensing oxides (CoOx , SnO2 , CuO, or TiO2 ) were converted into a paste by mixing with TEOS, glycol and several drops of HNO3 . A portion of the prepared paste was
R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601
597
2.3. Sensing system
YSZ disk Pt film (cathode)
Sensing material (anode)
(a). Electrodes of sensor (the front side)
Gases of 1000 ppm CO, H2 or CH4 in pure air were used as standard gases. The gases may be further diluted to different compositions (from 300 to 1000 ppm) by mixing with pure air. The flow rate of the testing gas during the sensing measurement was fixed at 100 cm3 /min. Electric potentials, between the sensing (anode) and the reference (cathode) electrodes of the prepared sensor, were measured by a 200 l multimeter from Kiethley. 2.4. TPR experimentation
YSZ disk
Platinum wire (b). Heating device (the back side) Fig. 1. Structure of the sensing element: (a) electrodes of sensor; (b) heating device.
brushed onto the surface of a Pt electrode which was designated as the anode of sensor after a 1 h calcination at 400 ◦ C. The purpose of calcination treatment was to burn out the glycol binding the paste.
The reductive behavior of cobalt oxide was pursued in a flowing gas of 10% H2 /N2 (in a flow rate of 30 cm3 /min). The gas passed consecutively through the reference side of a thermal conductivity detector (TCD), the sample (in a reactor made from a quartz tubing of 4 mm i.d.), a cell containing silica gel (to remove water formed during reduction) and the detecting side of the TCD. The temperature of the quartz reactor was raised, with a rate of 7 ◦ C/min, from the ambient temperature to 500 ◦ C by an oven.
3. Results and discussion 3.1. TPR characterization of CoOx
2.2. Sensing materials Different oxides were tested as sensing materials. Co3 O4 powders were prepared by a 30 min calcination of 2CoO3 · 3Co(OH)2 ·H2 O (from Merck) at 450 ◦ C. Other oxides, i.e. SnO2 , CuO and TiO2 were commercial products and tested without any calcination treatment. Au/Co3 O4 was prepared by precipitation deposition of aqueous AuCl3 on the Co3 O4 powder by alkalizing the AuCl3 solution with 1 M NaOH(aq) to pH ∼9.
The stoichiometry x of a CoOx sample can be determined by the amount of hydrogen consumption in TPR reduction. Fig. 2 presents the TPR trace obtained from the calcined CoOx sample. A stoichiometry of x = 1.3 was evaluated, which suggested the following reduction reaction: Co3 O4 + 4H2 → 3Co + 4H2 O
(1)
Two reducing peaks showed on 340 and 380 ◦ C. The shoulder peak at 340 ◦ C was assigned to a reduction of Co3 O4 to
Fig. 2. TPR of Co3 O4 in 10% H2 /N2 when temperature raising rate is 7 ◦ C/min.
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R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601
Fig. 3. Sensing curve of Au/Co3 O4 for CO sensor at 300 ◦ C.
CoO. The main peak at 380 ◦ C is a signal corresponding to conversion of CoO to metal Co [9]. 3.2. Sensing curves from different metal oxides Fig. 3 presented a CO sensing curve obtained from Au/Co3 O4 at 300 ◦ C. In the ambient air, a potential difference of Eair ∼5 mV was noticed between the anode and the cathode of the sensor. The potential difference (E) increased on exposing the sensor to 1000 ppm CO. Observed E increased over time and reached a plateau (ECO ). The increase had a response time of t90 ∼20 s. The sensitivity of the sensor was therefore defined as: S = ECO − Eair (at CO = 1000 ppm) A sensitivity of S = 121 mV was found in Fig. 3 for Au/Co3 O4 .
The sensing temperature and the materials coated on the anode had a marked effect on the measured sensitivity. Fig. 4 compared the temperature profiles of CO sensitivity from different transition metal oxides (TiO2 , Co3 O4 , CuO, SnO2 , Au/Co3 O4 ). They were insensitive at the ambient temperature but became sensitive on raising the temperature over Ts (starting temperature) that is characteristic to the kind of oxide used as a sensing material at the anode. The sensitivity increased with temperature, reached a maximum at To (optimum working temperature), and curved down at high temperatures.Table 1 listed temperatures of Ts and To observed for different oxides on sensing CO. Among the tested oxides, the SnO2 and Co3 O4 displayed low starting temperatures of To = 260 and 320 ◦ C and short t90 time of 60 s. Co3 O4 is our preferred sensing material because its sensitivity (60 mV) is around one-order higher than that (8 mV) of SnO2 .
Fig. 4. Sensitivities upon various metal oxides in 1000 ppm CO alternated with temperature.
R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601 Table 1 Testing results of 1000 ppm CO upon various metal oxides
3.3. Interference effect with other gases
Metal oxides
Ts (◦ C)
To (◦ C)
Sto (mV)
t90 (s)
CuO TiO2 SnO2 Co3 O4 Au/Co3 O4
390 230 200 170 130
470 370 260 320 300
22 21 8 60 121
300 300 70 60 20
Ts : start working temperature, To : optimum working temperature, Sto : highest sensitivity value; t90 : response time.
An impregnation of 1% gold crystallites (Au/Co3 O4 ) definitely enhanced the performance of cobalt oxides as a sensing material. The impregnation not only doubled the sensitivity to S = 120 mV but also reduced the incubation period to t90 = 20 s. This improvement may be attributed to the high catalytic activity of dispersed gold toward the oxidation of CO [8,9]: 2CO + O2 → 2CO2
599
(1)
Fig. 5 compares the temperature profiles of the fabricated sensitivity of Co3 O4 on three different standard gases. All of these gases showed a similar optimum temperature of To ∼350 ◦ C. Nevertheless, the sensitivity at To varied with the activity of gases tested. The hydrogen and carbon monoxide showed a similar sensitivity of S ∼60 mV. A much lower sensitivity of S = 5 mV was noticed from methane. In Fig. 6, we found that an impregnation of Au increased the sensitivity to these gases by a factor of two. However, their relative sensitivity remained the same. 3.4. Calibration curve of CO sensor Fig. 7 presents calibration curves made in this study for sensing CO by Au/Co3 O4 . Evidently, operation temperature had a significant effect on the performance of the fabricated sensor. A higher sensitivity was noticed at a sensing
Fig. 5. Sensing curves of three different gases upon Co3 O4 with 1000 ppm CO.
Fig. 6. Sensing curves of three different gases upon Au/Co3 O4 with 1000 ppm CO.
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R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601
Fig. 7. Au/Co3 O4 varied with CO concentration and temperature.
Fig. 8. Conductivity of YSZ solid electrolyte sensor.
temperature of T = 270 than 250 ◦ C. However, the calibration of 270 ◦ C was unfortunately found to curve down at [CO] > 400 ppm. The calibration curvature may be caused by a saturation of reaction (1) on the surface of the sensing material. Although a relatively low sensitivity was found on lowering the operation temperature to 250 ◦ C, a linear relationship up to [CO] = 1000 ppm was noticed.
tance increased significantly on decreasing the temperature of the electrolyte disk to T < 300 ◦ C. The required operation temperature should be caused by the solid electrolyte (YSZ) used in our galvanic cell. The rate-determining step of measuring electronic potential is the conductivity of oxygen ions in YSZ.
3.5. Conductivity of YSZ solid electrolyte sensor
4. Conclusion
The Au/Co3 O4 used in this study can catalyze reaction (1) at subambient temperatures [9]. It is therefore a surprise that the sensor fabricated by this material had a To ∼320 ◦ C. Fig. 8 showed the temperature profile of YSZ resistance measured from a multimeter. The measured resis-
Co3 O4 is found to be a highly sensitive material for sensing CO. A dispersion of nanogold on Co3 O4 may raise its CO sensitivity from S = 60 to 120 mV and reduce its response time from t90 = 60 to 20 s. A sensing head coated with Au/Co3 O4 could detect CO at temperatures above 130 ◦ C.
R.-J. Wu et al. / Sensors and Actuators B 96 (2003) 596–601
References [1] P.D. van der Wal, N.F. de Rooij, M. Koudelka-Hep, Extremely stable nafion based carbon monoxide sensor, Sens. Acuators B: Chem. 31 (1996) 119–123. [2] M. Ando, T. Kobayashi, M. Haruta, Optical CO detection by use of CuO/Au composite films, Sens. Actuators B: Chem. 25 (1995) 851– 853. [3] H. Yamaura, K. Moriya, N. Miuro, N. Yamazoe, Mechanism of sensitivity promotion in CO sensor using indium oxide and cobalt oxide, Sens. Actuators B: Chem. 65 (2000) 39–41. [4] N. Li, T.C. Tan, H.C. Zeng, High-temperature carbon monoxide potentiometric sensor, J. Electrochem. Soc. 140 (1993) 1068– 1073. [5] N. Miuro, T. Raisen, G. Lu, N. Yamazoe, Highly selective CO sensor using stabilized zirconia and a couple of oxide electrodes, Sens. Actuators B: Chem. 47 (1998) 84–91. [6] K. Mochizuki, R. Sorita, H. Takashima, K. Nakamura, G. Lu, Sensing characteristics of zirconia-based CO sensor made by thick-film lamination, Sens. Actuators B: Chem. 77 (2001) 190–195. [7] W.S. Epling, G.B. Holund, J.F. Weaver, S. Tsubota, M. Haruta, Au/MnOx catalytic performance characteristics for low-temperature carbon monoxide oxidation, Appl. Catal. B: Environ. 6 (1995) 117– 126. [8] Y.J. Chen, C.-T. Yeh, Deposition of highly dispersed gold on alumina support, J. Catal. 200 (2001) 59–65.
601
[9] D.E. Wu, Effect of Oxidation State of Cobalt on Catalytic Oxidation of CO Over CoOx and Au/CoOx , Ph.D. Dissertation, National Tsing Hua University, 2000.
Biographies Ren-Jang Wu received a BS in chemistry from National Tsing Hua University in 1986, an MS in chemistry from National Taiwan University in 1988 and a PhD in chemistry from National Tsing Hua University in 1995. His field of interests are chemical sensors, catalysis, nanoscience and chemical standard technology. Cheng-Hung Hu received a BS in chemistry from National Cheng Gung University in 2000, and an MS in chemistry from National Tsing Hua University in 2002. His field of interest is chemical sensors technology. Chuin-tih Yeh received a BS in chemistry from National Taiwan University in 1965 and a PhD in chemistry from Carnegie-Mellon University in 1971. His field of interests are catalysis, nanoscience and fuel-cell technology. Pi-Guey Su received BS degree in chemistry from Soochow University in 1983 and PhD degree in chemistry from National Tsing Hua University in 1998. His field of interest is chemical sensors technology by nanoscience and nanocomposite.