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The Impact of the Nature of the Electrode Material on SnO2 Thick Film Sensor Performance: Influence on Oxygen Adsorption
Available online at www.sciencedirect.com
Procedia Engineering 47 (2012) 514 – 517
Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland
The Impact of the Nature of the Electrode Material on SnO2 Thick Film Sensor Performance: Influence on Oxygen Adsorption S. Ranka *, S. Hafnera, N. Barsana, U. Weimaraa a
Tübingen University, Faculty of Science, Department of Chemistry, Institute of Physical and Theoretical Chemistry, Auf der Morgenstelle 15, 72076 Tübingen, Germany
Abstract SnO2 thick film sensors on Al2O3 substrates with two different electrode materials, i.e., gold and platinum, were investigated concerning their sensing properties towards CO and H2 in dry air. Electrical measurements differ significantly in response behavior based on the nature of the electrode. By varying only the oxygen concentrations and measuring the change in resistance, a strong dependence of the interaction of oxygen is detected whether gold or platinum electrodes were used. These observations lead to the conclusion that temperature-dependent oxygen adsorption processes, as well as a direct catalytic conversion of the test gas, catalyzed by the electrode material strongly influence sensor performance characteristics. © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense © 2012 Published by Elsevier Ltd. Sp. z.o.o. Keywords: SnO2, gas sensor, electrode material
1. Introduction It is well documented that besides enabling a resistance measurement, noble metal electrodes on a thick film sensor system have an impact on the sensing performance [1][2]. It can be purely electric in nature – the electrode gap determines the number of grain-grain boundaries – but there is also an important chemical influence due to the catalytic processes at the three phase boundary: electrode/metal oxide/ambient atmosphere. In this contribution we are presenting findings obtained from electrical measurements performed in different ambient conditions and catalytic conversion experiments. They were performed on sensors
* Corresponding author. Tel.: +49-7071-2978768; fax: +49-7071-295960. E-mail address: [email protected]
1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. doi:10.1016/j.proeng.2012.09.197
S. Rank et al. / Procedia Engineering 47 (2012) 514 – 517
based on undoped SnO2 and with Au and Pt electrodes and on the corresponding powder mixtures. Based on them we are proposing the interaction with ambient oxygen and the catalytic conversion of CO as the determining factors. 2. Experimental 2.1. Sample preparation Undoped SnO2 layers (aprox. ~ 50 ȝm thick) were deposited on alumina substrates, provided with Au or Pt electrodes, by screen-printing. The starting material was synthesized via a sol-gel method, with a final calcination temperature of the obtained Sn(OH)4 of 1000°C [3]. After drying the printed layers overnight, the sensors went through an annealing process in a belt oven with 4 different temperature zones (300°C - 400°C –600°C – 450°C). The backside of the alumina substrate is provided with a heater structure which allows running the sensor at the desired temperature. 2.2. Measurement techniques DC resistance measurements were used to characterize the sensing performance (by using a Keithley DMM199 multimeter). Four sensors were placed in series inside a Teflon chamber and tested at 200°C / 250°C / 300°C / 350°C using a 200 sccm total gas flow. Each measurement consisted of exposures to two different test gas sequences – 30, 50, 100, 200 ppm H2 & 25, 75, 125, 250 ppm CO in dry air. In the beginning, two hours of stabilization time was provided. An example is shown in Fig. 1. Same setup has been used to investigate the sensor response upon exposure to increasing oxygen concentrations. The measurements were carried out in dry nitrogen, wherein every 24 hours, the oxygen Fig. 1. Time dependence of the resistance during exposure to concentration was increased (1% - 2%). The oxygen CO and H2 in dry condition at 250°C. concentration has been monitored by an oxygen analyzer (ZIROX SGM). The catalytic conversion of CO to CO2 was measured on powder mixtures of SnO2 and micrometric gold (1 wt.%) and platinum (1 wt.%) powders, a model system corresponding to the metal oxide electrode interface. The exhaust gas composition has been analyzed by using an IR spectrometer (INNOVA 1310). 3. Results 3.1. Electrical measurements In Fig. 2, the sensor signal (R0/Rgas) is plotted as a function of temperature, for the two different types of sensors, in the case of exposure to 250 ppm CO and 200 ppm H2, respectively. For carbon monoxide in dry air the sensor provided with gold electrodes shows higher signals than one provided with platinum electrodes at the operation temperatures of 200°C and 250°C. Above 250°C, the sensor response is similar. Hydrogen measurements also indicate a higher response for the case of gold electrodes at lower
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Fig. 2. Sensor signal (R0/R) of SnO2 thick film layers supported with different electrode materials (Au & Pt) as a function of operational temperature. (left) 250ppm CO in dry air (right) 200ppm H2 in dry air.
working temperatures, even if the changing point is somewhat lower. In summary, at lower temperatures, sensor response towards CO and H2 is better in the case of the gold electrodes sensors, while at higher working temperatures both types of electrodes/ sensors showing similar results. It is worth to note that the measurements were repeated two times for two sensors both type and that the experimental values presented in Figure 2 are the average values. It is well known that the pre-adsorbed oxygen on the surface of the oxide is the reaction partner for both CO and H2 [4]. It means that one of the possible reasons could be the existence of more of it or in a more reactive form for the sensors provided with Au electrodes; some ideas were already presented by Montmeat et al [5]. It was also reported that the Pt electrodes are quite effective in converting CO to CO2 [6]. Accordingly, we decide to investigate both possibilities. For the clarification of the first issue we measured the effect of increasing the oxygen concentration on the resistance of two freshly produced sensors of each type – in order to rule out the memory effect – provided with Au and Pt electrodes. Figure 3 shows the results, which demonstrate that at 200°C the sensors provided with gold electrodes have a much stronger increase in resistance, when exposed to oxygen, than the ones provided with platinum electrodes. At 300°C there are no significant differences in the effect of oxygen exposure (Fig. 4). The assumption that the change in resistance during these measurements is only influenced by the adsorbed oxygen implies a higher oxygen spillover activity [7] of the gold electrodes at lower
Fig. 3. Characteristic curve of resistance plotted versus time for both type of sensors upon exposure to different levels of oxygen in N2 background.
Fig. 4. Normalized sensor signal dependence on different oxygen concentration (1% & 2 %) at 200°C and 300°C based on resistance (R0) given at 100ppm O2.
S. Rank et al. / Procedia Engineering 47 (2012) 514 – 517
temperatures. Due to the higher activity of gold electrodes, the dissociation of oxygen on the noble metal as well as the further diffusion of the ionosorbed oxygen species onto the metal oxide is more efficient. At higher temperatures this effect is no more decisive. 3.2. Catalytic conversion measurements As already mentioned, we already knew that a significant amount of the target gas can be catalytically converted directly by the electrodes without having an effect on the sensor signal. This seems to be indeed the case for platinum, which shows a significantly higher conversion rate, especially at low temperatures; the results of our experiments are shown in Fig. 5. This indicates that, indeed, it is possible that the effective concentration of the gas that reacts with the oxide can be decreased in the case of platinum electrodes.
Fig. 5. Catalytic activity of pure SnO2 powders and powder mixtures of SnO2 + 1wt% gold or platinum.
4. Conclusion and Outlook Within this work we were able to shed light on the processes taking place on the three-phase boundary of a metal oxide thick film sensor equipped with different electrode materials, which can explain the recorded differences in the sensor signal. The catalytic activity of these materials - on the one side the spillover-effect during the oxygen reaction, which is enhanced for the gold electrodes, on the other side the direct catalytic conversion of test gases, which is higher for the platinum electrodes – is considered to be the determining factor. Even if these results are showing a very clear tendency, the contribution of the electrode itself is supposed to be more complex. But for all that, we expect to benefit from spectroscopic measurements which will be carried out on the developed model system.
References [1] U. Jain, A.H. Harker, A.M. Stoneham & D.E. Williams. Effect of electrode geometry on sensor. J of Electroceramics 7 2000; pp. 143–16. [2] N. Barsan, U. Weimar. Conduction Model of Metal Oxide Gas Sensors. J of Electroceramics 7 2001. pp. 143–167 [3] J. Kappler. Characterisation of high-performance SnO2 gas sensors for CO detection by in situ techniques. Dissertation. University of Tuebingen. Shaker Verlag 2001. [4] N. Barsan, M. Schweizer-Berberich, W. Göpel. Fundamental and practical aspects in the desgin of nanoscaled SnO2 gas sensors: a status report. rFresenius J Anal Chem 365 1999. pp. 287–304 [5] P. Montmeat, J.-C. Marchand, R. Lalauze, J.-P. Viricelle, G. Tournier, C. Pijolat. Physico-chemical contribution of gold metallic particles to the action of oxygen on tin dioxide sensors. Sensors and Actuators B 95 2003. pp. 83-89 [6] J. Kappler, A. Tomescu, N. Barsan, U. Weimar. CO consumption of Pd doped SnO2 based sensors. Thin Solid Films 391 2001. pp. 186–191 [7] D. Kohl. The Role of Noble Metals in the Chemistry of Solid-state Gas Sensors. Sensors and Actuators B 1 1990. pp. 158– 165
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Procedia Engineering 47 (2012) 514 – 517
Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland
The Impact of the Nature of the Electrode Material on SnO2 Thick Film Sensor Performance: Influence on Oxygen Adsorption S. Ranka *, S. Hafnera, N. Barsana, U. Weimaraa a
Tübingen University, Faculty of Science, Department of Chemistry, Institute of Physical and Theoretical Chemistry, Auf der Morgenstelle 15, 72076 Tübingen, Germany
Abstract SnO2 thick film sensors on Al2O3 substrates with two different electrode materials, i.e., gold and platinum, were investigated concerning their sensing properties towards CO and H2 in dry air. Electrical measurements differ significantly in response behavior based on the nature of the electrode. By varying only the oxygen concentrations and measuring the change in resistance, a strong dependence of the interaction of oxygen is detected whether gold or platinum electrodes were used. These observations lead to the conclusion that temperature-dependent oxygen adsorption processes, as well as a direct catalytic conversion of the test gas, catalyzed by the electrode material strongly influence sensor performance characteristics. © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense © 2012 Published by Elsevier Ltd. Sp. z.o.o. Keywords: SnO2, gas sensor, electrode material
1. Introduction It is well documented that besides enabling a resistance measurement, noble metal electrodes on a thick film sensor system have an impact on the sensing performance [1][2]. It can be purely electric in nature – the electrode gap determines the number of grain-grain boundaries – but there is also an important chemical influence due to the catalytic processes at the three phase boundary: electrode/metal oxide/ambient atmosphere. In this contribution we are presenting findings obtained from electrical measurements performed in different ambient conditions and catalytic conversion experiments. They were performed on sensors
* Corresponding author. Tel.: +49-7071-2978768; fax: +49-7071-295960. E-mail address: [email protected]
1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. doi:10.1016/j.proeng.2012.09.197
S. Rank et al. / Procedia Engineering 47 (2012) 514 – 517
based on undoped SnO2 and with Au and Pt electrodes and on the corresponding powder mixtures. Based on them we are proposing the interaction with ambient oxygen and the catalytic conversion of CO as the determining factors. 2. Experimental 2.1. Sample preparation Undoped SnO2 layers (aprox. ~ 50 ȝm thick) were deposited on alumina substrates, provided with Au or Pt electrodes, by screen-printing. The starting material was synthesized via a sol-gel method, with a final calcination temperature of the obtained Sn(OH)4 of 1000°C [3]. After drying the printed layers overnight, the sensors went through an annealing process in a belt oven with 4 different temperature zones (300°C - 400°C –600°C – 450°C). The backside of the alumina substrate is provided with a heater structure which allows running the sensor at the desired temperature. 2.2. Measurement techniques DC resistance measurements were used to characterize the sensing performance (by using a Keithley DMM199 multimeter). Four sensors were placed in series inside a Teflon chamber and tested at 200°C / 250°C / 300°C / 350°C using a 200 sccm total gas flow. Each measurement consisted of exposures to two different test gas sequences – 30, 50, 100, 200 ppm H2 & 25, 75, 125, 250 ppm CO in dry air. In the beginning, two hours of stabilization time was provided. An example is shown in Fig. 1. Same setup has been used to investigate the sensor response upon exposure to increasing oxygen concentrations. The measurements were carried out in dry nitrogen, wherein every 24 hours, the oxygen Fig. 1. Time dependence of the resistance during exposure to concentration was increased (1% - 2%). The oxygen CO and H2 in dry condition at 250°C. concentration has been monitored by an oxygen analyzer (ZIROX SGM). The catalytic conversion of CO to CO2 was measured on powder mixtures of SnO2 and micrometric gold (1 wt.%) and platinum (1 wt.%) powders, a model system corresponding to the metal oxide electrode interface. The exhaust gas composition has been analyzed by using an IR spectrometer (INNOVA 1310). 3. Results 3.1. Electrical measurements In Fig. 2, the sensor signal (R0/Rgas) is plotted as a function of temperature, for the two different types of sensors, in the case of exposure to 250 ppm CO and 200 ppm H2, respectively. For carbon monoxide in dry air the sensor provided with gold electrodes shows higher signals than one provided with platinum electrodes at the operation temperatures of 200°C and 250°C. Above 250°C, the sensor response is similar. Hydrogen measurements also indicate a higher response for the case of gold electrodes at lower
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S. Rank et al. / Procedia Engineering 47 (2012) 514 – 517
Fig. 2. Sensor signal (R0/R) of SnO2 thick film layers supported with different electrode materials (Au & Pt) as a function of operational temperature. (left) 250ppm CO in dry air (right) 200ppm H2 in dry air.
working temperatures, even if the changing point is somewhat lower. In summary, at lower temperatures, sensor response towards CO and H2 is better in the case of the gold electrodes sensors, while at higher working temperatures both types of electrodes/ sensors showing similar results. It is worth to note that the measurements were repeated two times for two sensors both type and that the experimental values presented in Figure 2 are the average values. It is well known that the pre-adsorbed oxygen on the surface of the oxide is the reaction partner for both CO and H2 [4]. It means that one of the possible reasons could be the existence of more of it or in a more reactive form for the sensors provided with Au electrodes; some ideas were already presented by Montmeat et al [5]. It was also reported that the Pt electrodes are quite effective in converting CO to CO2 [6]. Accordingly, we decide to investigate both possibilities. For the clarification of the first issue we measured the effect of increasing the oxygen concentration on the resistance of two freshly produced sensors of each type – in order to rule out the memory effect – provided with Au and Pt electrodes. Figure 3 shows the results, which demonstrate that at 200°C the sensors provided with gold electrodes have a much stronger increase in resistance, when exposed to oxygen, than the ones provided with platinum electrodes. At 300°C there are no significant differences in the effect of oxygen exposure (Fig. 4). The assumption that the change in resistance during these measurements is only influenced by the adsorbed oxygen implies a higher oxygen spillover activity [7] of the gold electrodes at lower
Fig. 3. Characteristic curve of resistance plotted versus time for both type of sensors upon exposure to different levels of oxygen in N2 background.
Fig. 4. Normalized sensor signal dependence on different oxygen concentration (1% & 2 %) at 200°C and 300°C based on resistance (R0) given at 100ppm O2.
S. Rank et al. / Procedia Engineering 47 (2012) 514 – 517
temperatures. Due to the higher activity of gold electrodes, the dissociation of oxygen on the noble metal as well as the further diffusion of the ionosorbed oxygen species onto the metal oxide is more efficient. At higher temperatures this effect is no more decisive. 3.2. Catalytic conversion measurements As already mentioned, we already knew that a significant amount of the target gas can be catalytically converted directly by the electrodes without having an effect on the sensor signal. This seems to be indeed the case for platinum, which shows a significantly higher conversion rate, especially at low temperatures; the results of our experiments are shown in Fig. 5. This indicates that, indeed, it is possible that the effective concentration of the gas that reacts with the oxide can be decreased in the case of platinum electrodes.
Fig. 5. Catalytic activity of pure SnO2 powders and powder mixtures of SnO2 + 1wt% gold or platinum.
4. Conclusion and Outlook Within this work we were able to shed light on the processes taking place on the three-phase boundary of a metal oxide thick film sensor equipped with different electrode materials, which can explain the recorded differences in the sensor signal. The catalytic activity of these materials - on the one side the spillover-effect during the oxygen reaction, which is enhanced for the gold electrodes, on the other side the direct catalytic conversion of test gases, which is higher for the platinum electrodes – is considered to be the determining factor. Even if these results are showing a very clear tendency, the contribution of the electrode itself is supposed to be more complex. But for all that, we expect to benefit from spectroscopic measurements which will be carried out on the developed model system.
References [1] U. Jain, A.H. Harker, A.M. Stoneham & D.E. Williams. Effect of electrode geometry on sensor. J of Electroceramics 7 2000; pp. 143–16. [2] N. Barsan, U. Weimar. Conduction Model of Metal Oxide Gas Sensors. J of Electroceramics 7 2001. pp. 143–167 [3] J. Kappler. Characterisation of high-performance SnO2 gas sensors for CO detection by in situ techniques. Dissertation. University of Tuebingen. Shaker Verlag 2001. [4] N. Barsan, M. Schweizer-Berberich, W. Göpel. Fundamental and practical aspects in the desgin of nanoscaled SnO2 gas sensors: a status report. rFresenius J Anal Chem 365 1999. pp. 287–304 [5] P. Montmeat, J.-C. Marchand, R. Lalauze, J.-P. Viricelle, G. Tournier, C. Pijolat. Physico-chemical contribution of gold metallic particles to the action of oxygen on tin dioxide sensors. Sensors and Actuators B 95 2003. pp. 83-89 [6] J. Kappler, A. Tomescu, N. Barsan, U. Weimar. CO consumption of Pd doped SnO2 based sensors. Thin Solid Films 391 2001. pp. 186–191 [7] D. Kohl. The Role of Noble Metals in the Chemistry of Solid-state Gas Sensors. Sensors and Actuators B 1 1990. pp. 158– 165
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