- Email: [email protected]
HCs gases by non-Nernstian planar sensors using Nb2O5 electrode
Available online at www.sciencedirect.com
Sensors and Actuators B 130 (2008) 514–519
High temperature detection of CO/HCs gases by non-Nernstian planar sensors using Nb2O5 electrode Laure Chevallier ∗ , Elisabetta Di Bartolomeo, Maria Luisa Grilli, Enrico Traversa Department of Chemical Science and Technology, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy Available online 22 September 2007
Abstract In this work, high temperature planar sensors based on tape-cast YSZ layers with Nb2 O5 n-type semiconducting oxide as sensing electrode were studied. The investigated gases were CO and different saturated and unsaturated hydrocarbons. A study on the presence of the metallic electrode under the metal oxide and on the nature of that metallic (Pt or Au) electrode was performed. Thus, different types of sensors were prepared and investigated. The sensors were tested in potentiometric devices at various concentrations of gases in the 500–700 ◦ C temperature range. The most promising sensor in terms of selectivity, sensitivity and stability was Pt/YSZ/Nb2 O5 , without any metallic electrode under the Nb2 O5 powder. In fact, this sensor was found totally selective to propylene with very large EMF response even at temperature as high as 700 ◦ C (−115 mV/decade). © 2007 Elsevier B.V. All rights reserved. Keywords: HCs sensors; Electrical properties; YSZ; Niobia electrode
1. Introduction The challenge for the development of sensors for monitoring and control of combustion gas emissions is crucial for all vehicle producers. For this purpose, reliable and fast solid-state sensors for NOx [1–6], HCs [7–11] and CO [4–7,10] have been investigated. YSZ-based sensors seem to be more easily integrated in the On-Board Diagnostic (OBD) system, present inside the vehicles to monitor all the components related to the anti-pollution processes. At present, high temperature non-Nernstian electrochemical gas sensors are the most suitable for the detection of HCs/CO [4–11]. Various electrode compositions and sensor designs have been investigated and optimized to make sensors selective and sensitive to a specific gas. Nevertheless, reliable sensors to directly measure the gas concentrations in the exhaust pipes of an internal combustion engine (ICE) and thus to be easily used as a feed-back elements in engine control systems are still lacking. This paper is focused on planar YSZ-based sensors with Nb2 O5 n-type semiconducting oxide as a sensing electrode. Nanometric Nb2 O5 powders were synthesized using a sol–gel route [12]. This metal oxide was chosen not only because of its stability at high temperatures up to 750 ◦ C, but also because of ∗
Corresponding author. E-mail address: [email protected] (L. Chevallier).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.09.056
its promising sensing performance as semiconducting material for HCs detection [9,11]. With respect to our previous work [4–6], the aim of this paper is the investigation of different planar sensor configurations in a wide temperature range in the presence of CO/HCs gases. Both the presence of the metallic electrode under the metal oxide and the influence of the nature of that metallic electrode (Au or Pt) were examined. The optimal configuration for obtaining the maximum sensitivity to propylene was then suggested and the cross-sensitivity with different reducing and oxidizing gases was studied. 2. Experimental Yttria-stabilized zirconia tape-cast layers (8YSZ Kerafol, 150 m in thickness, 10 mm × 10 mm) were used as solid electrolyte materials for planar sensors. Two different devices were prepared, as shown in Fig. 1. In the first device (a), two parallel metallic (Pt or Au) electrodes were first deposited on one side of the layer and one of the electrodes was then recovered by the metal oxide powder. In the second one (b), only one metallic electrode was painted on the YSZ and the metal oxide powder was deposited directly on the YSZ, in a parallel finger on the same side. In both devices, Pt or Au commercial paste was used as metallic electrode and fired at 750 ◦ C for 10 min. Thin gold wires were
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Fig. 1. Scheme of the planar sensor with (a) or without (b) a metallic electrode under the Nb2 O5 oxide.
Fig. 2. SEM micrographs of the Nb2 O5 sensing electrode before (a) and after (b) the EMF measurements at high temperatures.
used as current collector. For the preparation of the sensing electrode, Nb2 O5 powder was mixed with a screen-printing oil and the slurry thus obtained was painted and then fired at 750 ◦ C. Nanosized Nb2 O5 powders were synthesized using a sol–gel method as described elsewhere [12]. The X-ray diffraction (XRD) analysis was performed using a Philips X-Pert Pro 500 Diffractometer for phase identification of the synthesized oxide. XRD patterns showed only the peaks of hexagonal phase (JXPDS 28-0317). The investigated electrochemical cells were the following: Pt/YSZ/(Pt)Nb2 O5 Pt/YSZ/(Au)Nb2 O5 Pt/YSZ/Nb2 O5
(sensor A)
3. Results and discussion Fig. 2 shows the SEM micrographs of the Nb2 O5 electrode before (Fig. 2a) and after (Fig. 2b) the EMF measurements. Even after 7 weeks of measurements at high temperatures, as high as 700 ◦ C, the grain size was still smaller than 100 nm, about the same size checked before the measurements. Fig. 3 shows the EMF response of sensor A (Pt and niobia(Pt) electrodes) under different concentrations of propylene in air at different temperatures. The measured signal was in the negative direction at all the investigated temperatures. The EMF response at 500 ◦ C was large (−50 mV for 200 ppm of propylene), fast (the response time was less than 10 s) and stable, but the amplitude
(sensor B)
(sensor C)
Microstructures of the powder and of the electrodes of the different sensors were observed by Field Emission Scanning Electron Microscopy (FE-SEM Leo Supra 35). CO/HCs-sensing experiments were carried out in a conventional gas-flow apparatus equipped with a controlled heating facility. The sensors were alternatively exposed to air or CO/HCs (200–1000 ppm in air) at the total flow rate of 200 mL/min, in the 500–700 ◦ C temperature range. At least three specimens were prepared for each type of sensor and each specimen was tested several times to check the reproducibility. The electromotive force (EMF) between the two electrodes of the sensor was measured with a digital multimeter (Keithley 2000). In all the measurements, the metallic electrode was connected to the negative terminal.
Fig. 3. EMF responses vs. time of Pt/YSZ/(Pt)Nb2 O5 sensor to different concentrations of propylene in air in the temperature range 500–600 ◦ C.
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Fig. 4. Sensitivity curves of Pt/YSZ/(Pt)Nb2 O5 sensor under exposure to CO, propane and propylene at 500 ◦ C (a), 550 ◦ C (b) and 600 ◦ C (c).
decreased with increasing the temperature. At 600 ◦ C the EMF response was only −8 mV under 1000 ppm of propylene. Moreover, the sensitivity was also very low: no significant changes of the response were observed with increasing the gas concentrations. At temperatures higher than 600 ◦ C, the response became negligible. The cross-selectivity and cross-sensitivity to CO and propane were checked at all the investigated temperatures. The sensitivity was calculated from the slope of the linear fit of the EMF values versus the gas concentrations in logarithmic scale. Fig. 4 shows the cross-sensitivity curves of sensor A at 500 ◦ C (Fig. 4a), 550 ◦ C (Fig. 4b) and 600 ◦ C (Fig. 4c). At 500 ◦ C the sensitivity to CO was larger than to propylene even though the EMF response to propylene was larger. At higher temperatures the sensor was responding better to propylene than to the two other gases, even though at 600 ◦ C all the sensitivities were very low. Fig. 5 shows the EMF response of sensor B (Pt and niobia(Au) electrodes) under different concentrations of propylene in air at different temperatures. Using gold as metallic electrode under Nb2 O5 seems to enhance the response at higher temperatures. At 600 ◦ C, the EMF response was −20 mV under exposure of 1000 ppm of propylene. The response and recovery times were shorter than 10 s. The cross-selectivity and cross-sensitivity to CO and propane were investigated in the whole temperature range. At lower temperatures (500–550 ◦ C), the sensor was better responding to CO, as shown in Fig. 6a and b. At 600 ◦ C the
sensitivity to CO and propylene became comparable (Fig. 6c) while at 650 ◦ C the sensor was more selective to propylene (Fig. 6d). At all the investigated temperatures, the response to propane was negligible. From the comparison of sensors A and B, it was shown that the presence of two different metallic electrode improved the response to propylene at higher temperatures but it decreased the selectivity to the target gas.
Fig. 5. EMF responses vs. time of Pt/YSZ/(Au)Nb2 O5 sensor to different concentrations of propylene in air in the temperature range 500–650 ◦ C.
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Fig. 6. Sensitivity curves of Pt/YSZ/(Au)Nb2 O5 sensor under exposure to CO, propane and propylene at 500 ◦ C (a), 550 ◦ C (b), 600 ◦ C (c) and 650 ◦ C (d).
Fig. 7 shows the EMF responses (Fig. 7a) and the sensitivity curves (Fig. 7b) of sensor C (niobia electrode without metal underneath) at different temperatures to different concentrations of propylene in air. At all the investigated temperatures, the measured EMF signals were again in the negative direction. By increasing the temperature, the response upon switching from air to gas became more stable and faster, but the amplitude of the EMF decreased. However, at 700 ◦ C the intensities were still very high. Under exposure to 1000 ppm of propylene in air, the EMF value of Pt/YSZ/Nb2 O5 was −110 mV, which is still a remarkably large value. The sensitivity curves showed a linear dependence of the EMF responses versus log C and very large sensitivity; in fact at 700 ◦ C, the sensitivity was −115 mV/decade. The results for the Pt/YSZ/Nb2 O5 were found reproducible and the sensor was stable in time: after several days of measurements, the same EMF results were obtained. Among all these different sensors, the sensors Pt/YSZ/Nb2 O5 seemed to be the most suitable to detect propylene at high temperatures. So further investigations were performed on it. The cross-sensitivity was checked by doing the measurements under exposure to different saturated and unsaturated hydrocarbons, CO and NO2 in the temperature range 550–700 ◦ C. At all the investigated temperatures, the sensor was best responding to propylene. Fig. 8 shows the sensitivity curves of sensor C at 700 ◦ C to different gases and Table 1 shows the
different sensitivity values. As expected, the signals were in the negative direction for reducing gases and in the positive one for the oxidizing gas [4–6]. For all the gases, a linear dependence of the EMF responses was observed. The sensitivity to propylene was much higher than the sensitivity to the other gases. A possible explanation of the chemical reaction of propylene can be the following: the presence of specific sites on niobia allows the formation of an intermediate carbocation, then the oxygen species available on the surface promote the oxidation of intermediates to CO2 or acrolein. Finally, the sensitivity was found to be in the order propylene > ethylene > propane > ethane. These findings were in accordance with other studies performed on similar HCs sensors [8,9].
Table 1 Sensitivity values obtained for Pt/YSZ/Nb2 O5 sensor under exposure to CO, NO2 and different HCs at 700 ◦ C Gas
Sensitivity (mV/decade)
NO2 CO Ethane Propane Ethylene Propylene
+24 −29 −30 −44 −81 −115
518
L. Chevallier et al. / Sensors and Actuators B 130 (2008) 514–519
and improved significantly when no metallic electrode under the metal oxide was used. In comparison to the gas response of similar planar sensors for NOx and CO detection previously investigated [4–6], excellent sensing performances were obtained in terms of sensitivity, selectivity and stability at high temperatures up to 700 ◦ C for the Pt/YSZ/Nb2 O5 device. These results indicate that this sensor is a great candidate for monitoring and controlling the exhaust gases in automotive applications. Preliminary “field” tests performed downstream a three-way catalytic converter of a spark ignition engine are very promising. Acknowledgments This work was supported by the Ministry of Education, University and Research (MIUR) of Italy (FIRB Project). The authors gratefully thank Prof. Silvia Licoccia for useful discussions and Cadia D’Ottavi for her technical support in the preparation of niobia powders. References
Fig. 7. EMF responses vs. time (a) and sensitivity curves (b) of Pt/YSZ/Nb2 O5 sensor to different concentrations of propylene in air in the temperature range 550–700 ◦ C.
Fig. 8. Cross-sensitivity curves of Pt/YSZ/Nb2 O5 sensor under exposure to CO, NO2 and different HCs at 700 ◦ C.
4. Conclusions High temperature YSZ-based planar sensors with Nb2 O5 sensing electrode were studied and the effect of the sensors configuration was investigated. The sensing behaviour was found to be strongly influenced by the nature of the metallic electrode
[1] N. Miura, N. Yamazoe, Approach to high-performance electrochemical NOx sensors based on solid electrolytes, in: H. Baltes, W. G¨opel, J. Hesse (Eds.), Sensors Update 6, Wiley-VCH, Weinheim, Germany, 2000, pp. 191–209. [2] N. Miura, S. Zhuiykov, T. Ono, M. Hasei, N. Yamazoe, Mixed potential type sensor using stabilized zirconia and ZnFe2 O4 sensing electrode for NOx detection at high temperature, Sens. Actuators B 83 (2002) 222–229. [3] W. Xiong, G.M. Kale, Novel high-selectivity NO2 sensor incorporating mixed-oxide electrode, Sens. Actuators B 114 (2006) 101–108. [4] A. Dutta, N. Kaabbuathong, M.-L. Grilli, E. Di Bartolomeo, E. Traversa, Study of YSZ-based electrochemical sensors with WO3 electrodes in NO2 and CO environments, J. Electrochem. Soc. 150 (2003) H33–H37. [5] E. Di Bartolomeo, N. Kaabbuathong, M.L. Grilli, E. Traversa, Planar electrochemical sensors based on tape-cast YSZ layers and oxide electrodes, Solid State Ion. 171 (2004) 173–181. [6] E. Di Bartolomeo, M.-L. Grilli, YSZ-based electrochemical sensors: from materials preparation to testing in the exhausts of an engine bench test, J. Europ. Ceram. Soc. 25 (2005) 2959–2964. [7] E.L. Brosha, R. Mukundan, R. Brown, H.F. Garzon, J.H. Visser, M. Zanini, Z. Zhou, E.M. Logothetis, CO/HC sensors based on thin film of LaCoO3 and La0.8 Sr0.2 CoO3−δ metal oxides, Sens. Actuators B 69 (2000) 171–182. [8] T. Hibino, A. Hashimoto, S. Kakimoto, M. Sano, Zirconia-based potentiometric sensors using metal oxide electrodes for detection of hydrocarbons, J. Electrochem. Soc. 148 (2001) H1–H5. [9] T. Hibino, S. Kakimoto, M. Sano, Non-Nernstian behaviour at modified Au electrodes for hydrocarbons gas sensing, J. Electrochem. Soc. 146 (1999) 3361–3366. [10] R. Mukundan, E.L. Brosha, F.H. Garzon, Mixed potential hydrocarbon sensors based on a YSZ electrolyte and oxide electrodes, J. Electrochem. Soc. 150 (2003) H279–H284. [11] J. Zosel, K. Ahlborn, R. Mueller, D. Westphal, V. Vashook, U. Guth, Selectivity of HC-sensitive electrode materials for mixed potential gas sensors, Solid State Ion. 169 (2004) 115–119. [12] S. Licoccia, R. Polini, C. D’Ottavi, F. Serraino Fiory, M.L. Di Vona, E. Traversa, A simple and versatile sol–gel method for the synthesis of functional nanocrystalline oxides, J. Nanosci. Nanotechnol. 5 (2005) 592–595.
Biographies Laure Chevallier has a post doct position at the University of Rome Tor Vergata. She has got her PhD in materials for environment and energy in 2007 at the University of Rome Tor Vergata and her science master degree at the High
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Institute ESCOM (Ecole Sup´erieure de Chimie Organique et Min´erale) in CergyPontoise in France. Her research field is on electroceramics for chemical sensors and solid oxide fuel cells.
in physics in 1992. Her research interests include: semiconducting materials, solar cells, electrochemical gas sensors for high temperature applications, screen printing technology, thin film deposition techniques.
Elisabetta Di Bartolomeo is assistant professor at the University of Rome Tor Vergata. She has got her PhD in materials engineering in 1999 and her degree in physics in 1994. She has been working on superconducting materials then on electroceramics for humidity, varistors and gas sensors. More recently, she has working on materials for solid oxide fuel cells at intermediate temperatures.
Enrico Traversa received his “Laurea” (Italian doctoral degree) in chemical engineering from the University of Rome La Sapienza in 1986. In 1988, he joined the University of Rome Tor Vergata where he is currently professor of materials science and technology and Director of the doctorate course in materials for environmental and energy applications. His research is focused on studies and development of solid oxide fuel cells, sensors, gas separation membranes and more recently materials for tissue engineering.
Maria Luisa Grilli has a contract at ENEA. She received her PhD in materials engineering in 2000 at the University of Rome Tor Vergata and her degree
Sensors and Actuators B 130 (2008) 514–519
High temperature detection of CO/HCs gases by non-Nernstian planar sensors using Nb2O5 electrode Laure Chevallier ∗ , Elisabetta Di Bartolomeo, Maria Luisa Grilli, Enrico Traversa Department of Chemical Science and Technology, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy Available online 22 September 2007
Abstract In this work, high temperature planar sensors based on tape-cast YSZ layers with Nb2 O5 n-type semiconducting oxide as sensing electrode were studied. The investigated gases were CO and different saturated and unsaturated hydrocarbons. A study on the presence of the metallic electrode under the metal oxide and on the nature of that metallic (Pt or Au) electrode was performed. Thus, different types of sensors were prepared and investigated. The sensors were tested in potentiometric devices at various concentrations of gases in the 500–700 ◦ C temperature range. The most promising sensor in terms of selectivity, sensitivity and stability was Pt/YSZ/Nb2 O5 , without any metallic electrode under the Nb2 O5 powder. In fact, this sensor was found totally selective to propylene with very large EMF response even at temperature as high as 700 ◦ C (−115 mV/decade). © 2007 Elsevier B.V. All rights reserved. Keywords: HCs sensors; Electrical properties; YSZ; Niobia electrode
1. Introduction The challenge for the development of sensors for monitoring and control of combustion gas emissions is crucial for all vehicle producers. For this purpose, reliable and fast solid-state sensors for NOx [1–6], HCs [7–11] and CO [4–7,10] have been investigated. YSZ-based sensors seem to be more easily integrated in the On-Board Diagnostic (OBD) system, present inside the vehicles to monitor all the components related to the anti-pollution processes. At present, high temperature non-Nernstian electrochemical gas sensors are the most suitable for the detection of HCs/CO [4–11]. Various electrode compositions and sensor designs have been investigated and optimized to make sensors selective and sensitive to a specific gas. Nevertheless, reliable sensors to directly measure the gas concentrations in the exhaust pipes of an internal combustion engine (ICE) and thus to be easily used as a feed-back elements in engine control systems are still lacking. This paper is focused on planar YSZ-based sensors with Nb2 O5 n-type semiconducting oxide as a sensing electrode. Nanometric Nb2 O5 powders were synthesized using a sol–gel route [12]. This metal oxide was chosen not only because of its stability at high temperatures up to 750 ◦ C, but also because of ∗
Corresponding author. E-mail address: [email protected] (L. Chevallier).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.09.056
its promising sensing performance as semiconducting material for HCs detection [9,11]. With respect to our previous work [4–6], the aim of this paper is the investigation of different planar sensor configurations in a wide temperature range in the presence of CO/HCs gases. Both the presence of the metallic electrode under the metal oxide and the influence of the nature of that metallic electrode (Au or Pt) were examined. The optimal configuration for obtaining the maximum sensitivity to propylene was then suggested and the cross-sensitivity with different reducing and oxidizing gases was studied. 2. Experimental Yttria-stabilized zirconia tape-cast layers (8YSZ Kerafol, 150 m in thickness, 10 mm × 10 mm) were used as solid electrolyte materials for planar sensors. Two different devices were prepared, as shown in Fig. 1. In the first device (a), two parallel metallic (Pt or Au) electrodes were first deposited on one side of the layer and one of the electrodes was then recovered by the metal oxide powder. In the second one (b), only one metallic electrode was painted on the YSZ and the metal oxide powder was deposited directly on the YSZ, in a parallel finger on the same side. In both devices, Pt or Au commercial paste was used as metallic electrode and fired at 750 ◦ C for 10 min. Thin gold wires were
L. Chevallier et al. / Sensors and Actuators B 130 (2008) 514–519
515
Fig. 1. Scheme of the planar sensor with (a) or without (b) a metallic electrode under the Nb2 O5 oxide.
Fig. 2. SEM micrographs of the Nb2 O5 sensing electrode before (a) and after (b) the EMF measurements at high temperatures.
used as current collector. For the preparation of the sensing electrode, Nb2 O5 powder was mixed with a screen-printing oil and the slurry thus obtained was painted and then fired at 750 ◦ C. Nanosized Nb2 O5 powders were synthesized using a sol–gel method as described elsewhere [12]. The X-ray diffraction (XRD) analysis was performed using a Philips X-Pert Pro 500 Diffractometer for phase identification of the synthesized oxide. XRD patterns showed only the peaks of hexagonal phase (JXPDS 28-0317). The investigated electrochemical cells were the following: Pt/YSZ/(Pt)Nb2 O5 Pt/YSZ/(Au)Nb2 O5 Pt/YSZ/Nb2 O5
(sensor A)
3. Results and discussion Fig. 2 shows the SEM micrographs of the Nb2 O5 electrode before (Fig. 2a) and after (Fig. 2b) the EMF measurements. Even after 7 weeks of measurements at high temperatures, as high as 700 ◦ C, the grain size was still smaller than 100 nm, about the same size checked before the measurements. Fig. 3 shows the EMF response of sensor A (Pt and niobia(Pt) electrodes) under different concentrations of propylene in air at different temperatures. The measured signal was in the negative direction at all the investigated temperatures. The EMF response at 500 ◦ C was large (−50 mV for 200 ppm of propylene), fast (the response time was less than 10 s) and stable, but the amplitude
(sensor B)
(sensor C)
Microstructures of the powder and of the electrodes of the different sensors were observed by Field Emission Scanning Electron Microscopy (FE-SEM Leo Supra 35). CO/HCs-sensing experiments were carried out in a conventional gas-flow apparatus equipped with a controlled heating facility. The sensors were alternatively exposed to air or CO/HCs (200–1000 ppm in air) at the total flow rate of 200 mL/min, in the 500–700 ◦ C temperature range. At least three specimens were prepared for each type of sensor and each specimen was tested several times to check the reproducibility. The electromotive force (EMF) between the two electrodes of the sensor was measured with a digital multimeter (Keithley 2000). In all the measurements, the metallic electrode was connected to the negative terminal.
Fig. 3. EMF responses vs. time of Pt/YSZ/(Pt)Nb2 O5 sensor to different concentrations of propylene in air in the temperature range 500–600 ◦ C.
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Fig. 4. Sensitivity curves of Pt/YSZ/(Pt)Nb2 O5 sensor under exposure to CO, propane and propylene at 500 ◦ C (a), 550 ◦ C (b) and 600 ◦ C (c).
decreased with increasing the temperature. At 600 ◦ C the EMF response was only −8 mV under 1000 ppm of propylene. Moreover, the sensitivity was also very low: no significant changes of the response were observed with increasing the gas concentrations. At temperatures higher than 600 ◦ C, the response became negligible. The cross-selectivity and cross-sensitivity to CO and propane were checked at all the investigated temperatures. The sensitivity was calculated from the slope of the linear fit of the EMF values versus the gas concentrations in logarithmic scale. Fig. 4 shows the cross-sensitivity curves of sensor A at 500 ◦ C (Fig. 4a), 550 ◦ C (Fig. 4b) and 600 ◦ C (Fig. 4c). At 500 ◦ C the sensitivity to CO was larger than to propylene even though the EMF response to propylene was larger. At higher temperatures the sensor was responding better to propylene than to the two other gases, even though at 600 ◦ C all the sensitivities were very low. Fig. 5 shows the EMF response of sensor B (Pt and niobia(Au) electrodes) under different concentrations of propylene in air at different temperatures. Using gold as metallic electrode under Nb2 O5 seems to enhance the response at higher temperatures. At 600 ◦ C, the EMF response was −20 mV under exposure of 1000 ppm of propylene. The response and recovery times were shorter than 10 s. The cross-selectivity and cross-sensitivity to CO and propane were investigated in the whole temperature range. At lower temperatures (500–550 ◦ C), the sensor was better responding to CO, as shown in Fig. 6a and b. At 600 ◦ C the
sensitivity to CO and propylene became comparable (Fig. 6c) while at 650 ◦ C the sensor was more selective to propylene (Fig. 6d). At all the investigated temperatures, the response to propane was negligible. From the comparison of sensors A and B, it was shown that the presence of two different metallic electrode improved the response to propylene at higher temperatures but it decreased the selectivity to the target gas.
Fig. 5. EMF responses vs. time of Pt/YSZ/(Au)Nb2 O5 sensor to different concentrations of propylene in air in the temperature range 500–650 ◦ C.
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Fig. 6. Sensitivity curves of Pt/YSZ/(Au)Nb2 O5 sensor under exposure to CO, propane and propylene at 500 ◦ C (a), 550 ◦ C (b), 600 ◦ C (c) and 650 ◦ C (d).
Fig. 7 shows the EMF responses (Fig. 7a) and the sensitivity curves (Fig. 7b) of sensor C (niobia electrode without metal underneath) at different temperatures to different concentrations of propylene in air. At all the investigated temperatures, the measured EMF signals were again in the negative direction. By increasing the temperature, the response upon switching from air to gas became more stable and faster, but the amplitude of the EMF decreased. However, at 700 ◦ C the intensities were still very high. Under exposure to 1000 ppm of propylene in air, the EMF value of Pt/YSZ/Nb2 O5 was −110 mV, which is still a remarkably large value. The sensitivity curves showed a linear dependence of the EMF responses versus log C and very large sensitivity; in fact at 700 ◦ C, the sensitivity was −115 mV/decade. The results for the Pt/YSZ/Nb2 O5 were found reproducible and the sensor was stable in time: after several days of measurements, the same EMF results were obtained. Among all these different sensors, the sensors Pt/YSZ/Nb2 O5 seemed to be the most suitable to detect propylene at high temperatures. So further investigations were performed on it. The cross-sensitivity was checked by doing the measurements under exposure to different saturated and unsaturated hydrocarbons, CO and NO2 in the temperature range 550–700 ◦ C. At all the investigated temperatures, the sensor was best responding to propylene. Fig. 8 shows the sensitivity curves of sensor C at 700 ◦ C to different gases and Table 1 shows the
different sensitivity values. As expected, the signals were in the negative direction for reducing gases and in the positive one for the oxidizing gas [4–6]. For all the gases, a linear dependence of the EMF responses was observed. The sensitivity to propylene was much higher than the sensitivity to the other gases. A possible explanation of the chemical reaction of propylene can be the following: the presence of specific sites on niobia allows the formation of an intermediate carbocation, then the oxygen species available on the surface promote the oxidation of intermediates to CO2 or acrolein. Finally, the sensitivity was found to be in the order propylene > ethylene > propane > ethane. These findings were in accordance with other studies performed on similar HCs sensors [8,9].
Table 1 Sensitivity values obtained for Pt/YSZ/Nb2 O5 sensor under exposure to CO, NO2 and different HCs at 700 ◦ C Gas
Sensitivity (mV/decade)
NO2 CO Ethane Propane Ethylene Propylene
+24 −29 −30 −44 −81 −115
518
L. Chevallier et al. / Sensors and Actuators B 130 (2008) 514–519
and improved significantly when no metallic electrode under the metal oxide was used. In comparison to the gas response of similar planar sensors for NOx and CO detection previously investigated [4–6], excellent sensing performances were obtained in terms of sensitivity, selectivity and stability at high temperatures up to 700 ◦ C for the Pt/YSZ/Nb2 O5 device. These results indicate that this sensor is a great candidate for monitoring and controlling the exhaust gases in automotive applications. Preliminary “field” tests performed downstream a three-way catalytic converter of a spark ignition engine are very promising. Acknowledgments This work was supported by the Ministry of Education, University and Research (MIUR) of Italy (FIRB Project). The authors gratefully thank Prof. Silvia Licoccia for useful discussions and Cadia D’Ottavi for her technical support in the preparation of niobia powders. References
Fig. 7. EMF responses vs. time (a) and sensitivity curves (b) of Pt/YSZ/Nb2 O5 sensor to different concentrations of propylene in air in the temperature range 550–700 ◦ C.
Fig. 8. Cross-sensitivity curves of Pt/YSZ/Nb2 O5 sensor under exposure to CO, NO2 and different HCs at 700 ◦ C.
4. Conclusions High temperature YSZ-based planar sensors with Nb2 O5 sensing electrode were studied and the effect of the sensors configuration was investigated. The sensing behaviour was found to be strongly influenced by the nature of the metallic electrode
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Biographies Laure Chevallier has a post doct position at the University of Rome Tor Vergata. She has got her PhD in materials for environment and energy in 2007 at the University of Rome Tor Vergata and her science master degree at the High
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Institute ESCOM (Ecole Sup´erieure de Chimie Organique et Min´erale) in CergyPontoise in France. Her research field is on electroceramics for chemical sensors and solid oxide fuel cells.
in physics in 1992. Her research interests include: semiconducting materials, solar cells, electrochemical gas sensors for high temperature applications, screen printing technology, thin film deposition techniques.
Elisabetta Di Bartolomeo is assistant professor at the University of Rome Tor Vergata. She has got her PhD in materials engineering in 1999 and her degree in physics in 1994. She has been working on superconducting materials then on electroceramics for humidity, varistors and gas sensors. More recently, she has working on materials for solid oxide fuel cells at intermediate temperatures.
Enrico Traversa received his “Laurea” (Italian doctoral degree) in chemical engineering from the University of Rome La Sapienza in 1986. In 1988, he joined the University of Rome Tor Vergata where he is currently professor of materials science and technology and Director of the doctorate course in materials for environmental and energy applications. His research is focused on studies and development of solid oxide fuel cells, sensors, gas separation membranes and more recently materials for tissue engineering.
Maria Luisa Grilli has a contract at ENEA. She received her PhD in materials engineering in 2000 at the University of Rome Tor Vergata and her degree