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Gas sensitivity measurements on NO2 sensors based on lead phthalocyanine thin films
Sensors and Actuators
Gas Sensitivity Measurements Thin Films A. HEILMANN, Department
M. MfSLLER
Microelectronics
on NO, Sensors Based on Lead Phthalocyanine
and C. HAMANN
of Physics and Electronic
V. LANTTO
511
B, 4 (1991) 51 I-513
Devices,
University
of Technology,
Chemnitz
(F.R.G.)
and H. TORVELA Laboratory,
University
of Oulu, SF-90570
Oulu (Finland)
Abstract The conductance response of NOz sensors based on lead phthalocyanine thin films to NOz, CO, SO2 and to mixtures of these gases has been studied. The sensor response is measured between 100 ppb and 100 ppm of NOz in a carrier gas which is either Nz or synthetic air. No selective sensitivity is observed to CO or SO, individually. If CO is present with the NOz, the conductance response to NOz decreases. Some of the measurements might be interpreted as an increase of NOz sensitivity as a result of SOZ exposure along with N02. In mixtures containing 5000 ppm of both CO and SOZ, 1 ppm of NO2 can be detected.
1. Introduction The polluting influence of NO, on the environment means that there is a need to decrease its emission from power plants, chemical industries and automotive vehicles. Hence the monitoring of NO, concentrations in air has become increasingly important. Detection of NO, is also necessary in some chemical industries where a suitable process measurement technique is required. Lead phthalocyanine (PbPc) is an organic p-type semiconductor which shows changes in its electrical conductivity as a result of adsorption of oxidizing gases [ 11. Single-crystal as well as thin-film samples show sensitivity to 02, Cl2 or NOz [2]. PbPc is capable of 0925-4005/91/$3.50
detecting concentrations in the ppb range. It is insensitive to CO, and to many hydrocarbons [ 1, 31. There have been investigations on the sensitivity to NO2 and Cl* individually [4-71, but for practical applications of sensors based on PbPc, it is also necessary to know the behaviour of the sensors in the presence of other gases in addition to NO*. Some sensitivity measurements of a PbPc thin-film sensor to NOz in the case when CO or SO2 or both were present in addition to NOz are presented here. 2. Experimental The sensor chip (1.5 mm x 1.5 mm) contained comb-like gold electrodes. The gap between the electrodes was 18 pm with a transverse length of 2.7 mm. The chip carrier was of a common type. The thickness of the vacuum-sublimated PbPc film was about 200 nm. The sensor current with a constant d.c. voltage of 6.5 V was measured as a function of time. An experimental set-up based on the flowthrough principle [ 81 was used to investigate the gas sensitivity of the sensor. The sensor was mounted in a test chamber inside an oven. Different mixtures of NO2 with CO or SO2 in a carrier gas (high purity N2 or synthetic air) were obtained through a gas blender (Signal, Series 850). The flow rate of the gas mixture to the test chamber was maintained at 1 l/min. 0 Elsevier Sequoia/Printed in The Netherlands
nA
NOzand SO, nsynth T=160'C
200
air
I
0 Fig. 1. Current response with time of the sensor to different CO concentrations in synthetic air at 190 “C in the presence of 100 ppm of NO*.
3. Results and Discussion Before the initial heating, the current of the sensor in the pure carrier gas at room temperature was less than 100 pA. At working temperatures (160 or 190 “C), this current increases to values between 1 and 3 nA. Some response measurements to NO* with this sensor have already been reported [7,9]. No selective sensitivity was observed to CO alone. As the amount of CO was increased from 100 ppm to l%, no change in the current was detected. Figure 1 shows the influence of CO in the presence of NO*. Six different concentrations of CO were allowed to flow along with 100 ppm of NO,. A decrease of the sensor current as a result of CO exposure can be clearly seen. Some changes due to the flow system can only be assumed as a reason for this decrease at very high CO concentrations. It may also be possible that CO was chemisorbed on the sensor surface, replacing the NO2 adsorbate. When CO was removed, the current did not return to the initial value before the CO exposure. Further use of the sensor was nearly impossible, since the sensor current was unstable and the sensitivity to NOz low. It seems possible that very high concentrations of CO together with NOz may have destroyed the sensor. The sensor was not at all sensitive to SO2 alone. Figure 2 shows the behaviour of the
600
1200
MOO
2400 time
3000
3600
I
1
I
4800
Fig. 2. Current response with time of the sensor to different SO, concentrations in synthetic air at 160 “C in the presence of 1 ppm of NO*.
sensor when different amounts of SOS were introduced along with the NOz. At the start of exposure an additional increase of the sample current was observed when 50 ppm of SO2 was introduced into the stream containing 1 ppm of NOz. This suggests that the presence of SO, enhances the sensitivity of the sensor to NOz. This, however, has not been confirmed. Figure 3 shows that 1 ppm of NO, can certainly be detected in the presence of 100, 500 and 2000 ppm of SO,. An increase in the sensitivity to 1 ppm of NO2 with increasing concentrations of SOz is also seen in addition to the normal sensor shift. After a long period of use of this sensor, a slight increase in
'0
Fig. 3. Current response with time of the sensor to 1 ppm of NO, in synthetic air at 160 “C in the presence of different amounts of SO,.
513
NO>.SOz andCO
I”
The development of sensor systems for monitoring environmental pollution is an important task. These measurements show that there are some possibilities to detect small amounts of NO2 in reactive industrial emission gases and to create a sensor system for process monitoring.
synth air
T= 160 'C
Acknowledgements 0
I
I
1
600
1200
1800
2LOO 3000 time
3600
4200
s
5100
Fig. 4. Current response with time of the sensor to 1 ppm of NO, in synthetic air at 160 “C in the presence of different amounts of both CO and SOz.
the current was found at the start of the SO2 flow. One reason for this may be the adsorption of NO2 on the inner walls of the flow system. A small sensitivity to SO2 as a result of a long running time of the sensor might also be possible. Further investigations, however, have to be conducted in order to study the complex behaviour of the sensor when SO2 is present in addition to NO*. Possible catalytic reactions between NO2 and SO2 on the film surface have also to be considered. Figure 4 shows that small amounts of NO2 in CO/S02/N02 mixtures can be detected. A concentration of 1 ppm of NO2 was added to streams containing 100 ppm of CO and SO*, 500 ppm of CO and 400 ppm of SO2 and 5000 ppm of CO and S02. 0.1% of NO2 in the total active gas concentration could be detected with certainty. The detection of 0.01% of NO2 was also possible, but the sensitivity was low.
4. Conclusions NO2 sensors based on PbPc thin films show a good selective sensitivity to N02. Some sensitivity changes in the presence of high amounts of CO or SO2 are not yet fully understood and need a more detailed study.
The authors would like to thank Matthan (FPRI) for editing the text.
Jacob
References B. Bott and T. A. Jones, A highly sensitive NO, sensor based on electrical conductivity changes in phthalocyanine films, Sensors and Actuators, 5 ( 1984) 42-53.
P. B. M. Archer, A. V. Chadwick, J. J. Mieasik, M. Tamizi and J. D. Wright, Kinetic factors in the response of organometallic semiconductor gas sensors, Sensors and Actuators, 16 (1989) 379-392. H. Mockert, D. Schmeisser and W. Giipel, Lead phthalocyanine (PbPc) as a prototype organic material for gas sensors: comparative electrical and spectroscopic studies to optimize O2 and NO, sensing, Sensors nnd Actuators, 19 (1989) 159- 176. T. A. Jones, B. Bott and S. C. Thorpe, Fast response metal phthalocyanine-based gas sensors, Sensors and Actuators,
17 (1989) 467-474.
J. Unwin and P. T. Walsh, An exposure monitor for chlorinated hydrocarbons based on conductometry using lead phthalocyanine films, Sensors and Actuators, 18 (1989) 45-57. M. Miiller, G. Bratke, J. Glrtig, M. Starke and C. Hamann, NO, Gas Sensor auf der Basis von Bleiphthalocyanin, Wiss. Zeitschrift der TU Karl-MarxStadt, 31 (1989) 393-398. C. Hamann, G. Kampfrath
and M. Mtiller, Gas and humidity sensors based on organic active thin films, Sensors and Actuators B, 1 (1990) 142- 147. P. Romppainen, H. Torvela, J. VaPnlnen and S. Lepplvuori, Effect of CH,, SO, and NO on the CO response of an SnO,-based thick-film gas sensor in combustion gases, Sensors and Actuators, 8 (1985) 27 l-279.
A. Heilmann, V. Lantto, H. Torvela and C. Hamann, Gas sensitivity measurements on lead-phthalocyanine thin film sensors, Rep. S. 103, Department of Electrical Engineering, University of Oulu, Oulu, Finland, 1989, 26 pp.
Gas Sensitivity Measurements Thin Films A. HEILMANN, Department
M. MfSLLER
Microelectronics
on NO, Sensors Based on Lead Phthalocyanine
and C. HAMANN
of Physics and Electronic
V. LANTTO
511
B, 4 (1991) 51 I-513
Devices,
University
of Technology,
Chemnitz
(F.R.G.)
and H. TORVELA Laboratory,
University
of Oulu, SF-90570
Oulu (Finland)
Abstract The conductance response of NOz sensors based on lead phthalocyanine thin films to NOz, CO, SO2 and to mixtures of these gases has been studied. The sensor response is measured between 100 ppb and 100 ppm of NOz in a carrier gas which is either Nz or synthetic air. No selective sensitivity is observed to CO or SO, individually. If CO is present with the NOz, the conductance response to NOz decreases. Some of the measurements might be interpreted as an increase of NOz sensitivity as a result of SOZ exposure along with N02. In mixtures containing 5000 ppm of both CO and SOZ, 1 ppm of NO2 can be detected.
1. Introduction The polluting influence of NO, on the environment means that there is a need to decrease its emission from power plants, chemical industries and automotive vehicles. Hence the monitoring of NO, concentrations in air has become increasingly important. Detection of NO, is also necessary in some chemical industries where a suitable process measurement technique is required. Lead phthalocyanine (PbPc) is an organic p-type semiconductor which shows changes in its electrical conductivity as a result of adsorption of oxidizing gases [ 11. Single-crystal as well as thin-film samples show sensitivity to 02, Cl2 or NOz [2]. PbPc is capable of 0925-4005/91/$3.50
detecting concentrations in the ppb range. It is insensitive to CO, and to many hydrocarbons [ 1, 31. There have been investigations on the sensitivity to NO2 and Cl* individually [4-71, but for practical applications of sensors based on PbPc, it is also necessary to know the behaviour of the sensors in the presence of other gases in addition to NO*. Some sensitivity measurements of a PbPc thin-film sensor to NOz in the case when CO or SO2 or both were present in addition to NOz are presented here. 2. Experimental The sensor chip (1.5 mm x 1.5 mm) contained comb-like gold electrodes. The gap between the electrodes was 18 pm with a transverse length of 2.7 mm. The chip carrier was of a common type. The thickness of the vacuum-sublimated PbPc film was about 200 nm. The sensor current with a constant d.c. voltage of 6.5 V was measured as a function of time. An experimental set-up based on the flowthrough principle [ 81 was used to investigate the gas sensitivity of the sensor. The sensor was mounted in a test chamber inside an oven. Different mixtures of NO2 with CO or SO2 in a carrier gas (high purity N2 or synthetic air) were obtained through a gas blender (Signal, Series 850). The flow rate of the gas mixture to the test chamber was maintained at 1 l/min. 0 Elsevier Sequoia/Printed in The Netherlands
nA
NOzand SO, nsynth T=160'C
200
air
I
0 Fig. 1. Current response with time of the sensor to different CO concentrations in synthetic air at 190 “C in the presence of 100 ppm of NO*.
3. Results and Discussion Before the initial heating, the current of the sensor in the pure carrier gas at room temperature was less than 100 pA. At working temperatures (160 or 190 “C), this current increases to values between 1 and 3 nA. Some response measurements to NO* with this sensor have already been reported [7,9]. No selective sensitivity was observed to CO alone. As the amount of CO was increased from 100 ppm to l%, no change in the current was detected. Figure 1 shows the influence of CO in the presence of NO*. Six different concentrations of CO were allowed to flow along with 100 ppm of NO,. A decrease of the sensor current as a result of CO exposure can be clearly seen. Some changes due to the flow system can only be assumed as a reason for this decrease at very high CO concentrations. It may also be possible that CO was chemisorbed on the sensor surface, replacing the NO2 adsorbate. When CO was removed, the current did not return to the initial value before the CO exposure. Further use of the sensor was nearly impossible, since the sensor current was unstable and the sensitivity to NOz low. It seems possible that very high concentrations of CO together with NOz may have destroyed the sensor. The sensor was not at all sensitive to SO2 alone. Figure 2 shows the behaviour of the
600
1200
MOO
2400 time
3000
3600
I
1
I
4800
Fig. 2. Current response with time of the sensor to different SO, concentrations in synthetic air at 160 “C in the presence of 1 ppm of NO*.
sensor when different amounts of SOS were introduced along with the NOz. At the start of exposure an additional increase of the sample current was observed when 50 ppm of SO2 was introduced into the stream containing 1 ppm of NOz. This suggests that the presence of SO, enhances the sensitivity of the sensor to NOz. This, however, has not been confirmed. Figure 3 shows that 1 ppm of NO, can certainly be detected in the presence of 100, 500 and 2000 ppm of SO,. An increase in the sensitivity to 1 ppm of NO2 with increasing concentrations of SOz is also seen in addition to the normal sensor shift. After a long period of use of this sensor, a slight increase in
'0
Fig. 3. Current response with time of the sensor to 1 ppm of NO, in synthetic air at 160 “C in the presence of different amounts of SO,.
513
NO>.SOz andCO
I”
The development of sensor systems for monitoring environmental pollution is an important task. These measurements show that there are some possibilities to detect small amounts of NO2 in reactive industrial emission gases and to create a sensor system for process monitoring.
synth air
T= 160 'C
Acknowledgements 0
I
I
1
600
1200
1800
2LOO 3000 time
3600
4200
s
5100
Fig. 4. Current response with time of the sensor to 1 ppm of NO, in synthetic air at 160 “C in the presence of different amounts of both CO and SOz.
the current was found at the start of the SO2 flow. One reason for this may be the adsorption of NO2 on the inner walls of the flow system. A small sensitivity to SO2 as a result of a long running time of the sensor might also be possible. Further investigations, however, have to be conducted in order to study the complex behaviour of the sensor when SO2 is present in addition to NO*. Possible catalytic reactions between NO2 and SO2 on the film surface have also to be considered. Figure 4 shows that small amounts of NO2 in CO/S02/N02 mixtures can be detected. A concentration of 1 ppm of NO2 was added to streams containing 100 ppm of CO and SO*, 500 ppm of CO and 400 ppm of SO2 and 5000 ppm of CO and S02. 0.1% of NO2 in the total active gas concentration could be detected with certainty. The detection of 0.01% of NO2 was also possible, but the sensitivity was low.
4. Conclusions NO2 sensors based on PbPc thin films show a good selective sensitivity to N02. Some sensitivity changes in the presence of high amounts of CO or SO2 are not yet fully understood and need a more detailed study.
The authors would like to thank Matthan (FPRI) for editing the text.
Jacob
References B. Bott and T. A. Jones, A highly sensitive NO, sensor based on electrical conductivity changes in phthalocyanine films, Sensors and Actuators, 5 ( 1984) 42-53.
P. B. M. Archer, A. V. Chadwick, J. J. Mieasik, M. Tamizi and J. D. Wright, Kinetic factors in the response of organometallic semiconductor gas sensors, Sensors and Actuators, 16 (1989) 379-392. H. Mockert, D. Schmeisser and W. Giipel, Lead phthalocyanine (PbPc) as a prototype organic material for gas sensors: comparative electrical and spectroscopic studies to optimize O2 and NO, sensing, Sensors nnd Actuators, 19 (1989) 159- 176. T. A. Jones, B. Bott and S. C. Thorpe, Fast response metal phthalocyanine-based gas sensors, Sensors and Actuators,
17 (1989) 467-474.
J. Unwin and P. T. Walsh, An exposure monitor for chlorinated hydrocarbons based on conductometry using lead phthalocyanine films, Sensors and Actuators, 18 (1989) 45-57. M. Miiller, G. Bratke, J. Glrtig, M. Starke and C. Hamann, NO, Gas Sensor auf der Basis von Bleiphthalocyanin, Wiss. Zeitschrift der TU Karl-MarxStadt, 31 (1989) 393-398. C. Hamann, G. Kampfrath
and M. Mtiller, Gas and humidity sensors based on organic active thin films, Sensors and Actuators B, 1 (1990) 142- 147. P. Romppainen, H. Torvela, J. VaPnlnen and S. Lepplvuori, Effect of CH,, SO, and NO on the CO response of an SnO,-based thick-film gas sensor in combustion gases, Sensors and Actuators, 8 (1985) 27 l-279.
A. Heilmann, V. Lantto, H. Torvela and C. Hamann, Gas sensitivity measurements on lead-phthalocyanine thin film sensors, Rep. S. 103, Department of Electrical Engineering, University of Oulu, Oulu, Finland, 1989, 26 pp.