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Investigation of sensing mechanism of Nasicon electrocatalytic sensors in nitrogen dioxide and ammonia
Sensors and Actuators B 189 (2013) 141–145
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Investigation of sensing mechanism of Nasicon electrocatalytic sensors in nitrogen dioxide and ammonia Piotr Jasinski a,∗ , Anna Strzelczyk b , Bogdan A. Chachulski b , Maria Gazda c , Grzegorz Jasinski a a b c
Faculty of Electronics, Telecommunication and Informatics, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Poland Chemical Faculty, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Poland Faculty of Applied Physics and Mathematics, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Poland
a r t i c l e
i n f o
Article history: Available online 22 February 2013 Keywords: Electrocatalytic sensor Nitrogen dioxide Ammonia Sensing mechanism Nasicon
a b s t r a c t In this paper a sensing mechanism of Nasicon electrocatalytic sensor in nitrogen dioxide and ammonia is investigated. Both gases are environmentally hazardous and contain nitrogen atom in the molecule. However, it seems that their sensing mechanism in electrocatalytic sensor could be totally different. Namely, the maximum sensitivity for each gas was obtained at different temperatures. Also, different auxiliary layers are formed for each gas. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Electrocatalytic sensors belong to family of electrochemical sensors with solid state electrolyte. Their sensing mechanism is based on application of periodically changing voltage and acquisition of current response. Obtained in this way current–voltage plot provides fingerprint information about ambient gases, which interact with sensor surface due to electrical excitation. The sensor name, electrocatalytic, is a direct consequence of sensing mechanism, i.e. the electrochemical reactions results from the electric excitation [1,2]. So far relatively simple sensor structures have been investigated. Mostly, these structures consisted of different types of electrolyte and metallic electrodes [3–6]. We have been investigating these sensors before and it seems that Nasicon electrolyte with platinum electrodes is a useful structure for sensing mechanism investigation [6,7]. This structure provides in I–V plot unique response to different gases. So far, the major drawback of electrocatalytic sensor is related to instability due to formation of chemical phases on the sensor surface, which covers and block triple phase boundary. However, it was found that the sensors, which exposed and poisoned by nitrogen dioxide, can be at any time refreshed by short time high temperature treatment [8]. In order to improve sensor stability in different gases it is necessary to better understand the sensors sensing mechanism. This work aims to investigate Nasicon electrocatalytic gas sensor sensing mechanism in nitrogen dioxide and ammonia.
In this paper, Nasicon (Na2.8 Zr2 Si1.8 P1.2 O12 ) sensors with platinum electrodes were prepared and investigated. Namely, Nasicon powder was prepared by the conventional solid-state ball milling method from the zirconium oxide, silicon oxide and trisodium phosphate. The components were mixed in ball mill and annealed at 1373 K (900 ◦ C). Obtained in this way powder was compacted into pellets (∼12 mm in diameter and 1 mm thick) by iso-axial pressing. The pellets were sintered in air for 2 h at 1373 K (1100 ◦ C). Electrodes were made by coating both sides of the pellet with the platinum paste (ESL ElectroScience, ESL 5542) and firing at 1173 K (900 ◦ C) (surface area of 0.5 cm2 ). Measurements were conducted in mixtures of high purity gasses: nitrogen dioxide or ammonia and synthetic air, which were supplied with a constant gas flow of 100 cm3 min−1 maintained by mass flow controllers. The experiments were conducted in a tube furnace, which temperature was controlled with accuracy better than 0.5 K. The measurements were performed using the electrochemical interface Solartron SI 1287. To obtain sensor sensitivity in nitrogen dioxide and ammonia, a triangular voltage excitation signal with the amplitude of 5 V and sweep rate of 50 mV/s was applied to the sensor. The electrical current was recorded during sensor polarization. On the current–voltage plot the peaks with different heights (maximum of the electric current) were visible for different concentrations of ammonia and nitrogen dioxide. This information was used to obtain sensors sensitivity S at different temperatures (1):
∗ Corresponding author. Tel.: +48 58 347 1323; fax: +48 58 347 1757. E-mail addresses: [email protected], [email protected] (P. Jasinski). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.02.044
S=
Imax , c · I0- max
(1)
142
P. Jasinski et al. / Sensors and Actuators B 189 (2013) 141–145
where c is the measured gas concentration (ppm), while I0-max and Imax are the heights of the peaks in the synthetic air and the measured gas, respectively. The I0-max was obtained from reproducible I–V response in synthetic air after the measurement in tested gas. Usually, the reproducible response was obtained within 2 h of voltage cycling. At some temperatures (below 350 ◦ C) for ammonia the I–V response in synthetic air before and after measurement in tested gas was not exactly the same. Namely, the currents were higher than before the measurements, which may suggest that the surfaces of electrodes are covered by some reactions products. The high temperature treatment of the sensor (about 600 ◦ C in synthetic air) refreshes the sensor and the level of currents become low again [8]. To investigate sensors sensing mechanisms XRD pattern of the electrode surface was investigated. Namely, a freshly fabricated Nasicon sensor was exposed to 100 ppm of ammonia or nitrogen dioxide at 473 K (200 ◦ C) or at 623 K (350 ◦ C). At those temperatures the maximum sensitivity was recorded for each gas. When the temperature was stabilized the sensor was polarized (biased) with 1 V for different periods to form a sensing phase on the surface of electrode. The same sensing phase is formed during normal sensor operation, i.e. when the sensor is excited with triangular signal. On current–voltage plot it is reflected by the appearance of peaks due to electrochemical reaction of formation and decomposition of the phases. The surfaces of the sensors were investigated using X-ray diffractometry in order to determine the composition of the formed phase. XRD patterns were recorded with Cu K␣ radiation at room temperature using Philips X’Pert Pro diffractometer. Qualitative phase analysis of diffraction pattern was carried out with ICDD PDF database. The AFM images were collected using Atomic Force Microscope NTegra (NT-MDT).
a
b
3. Results Nitrogen dioxide and ammonia contain nitrogen atom and both gases are environmentally hazardous. However, it seems that sensing mechanism of electrocatalytic sensor is not the same. In Fig. 1, the response of the sensor to different concentration of nitrogen dioxide at 200 ◦ C (Fig. 1a) and different concentration of ammonia at 350 ◦ C (Fig. 1b) are presented. The I–V plot presented in Fig. 1a shows only one sharp peak for positive and negative voltages. This reflects one step formation and/or decomposition reaction occurring at the triple phase boundary. The I–V plot for ammonia at 350 ◦ C (Fig. 1b) reveals broader peak, which seems to consist of two components. This may indicate either a two-step process or two different reactions at the triple phase boundary. As presented in Fig. 2, at those temperatures about the maximum sensitivity for each gas was obtained. Based on the results shown in Fig. 2, it is evident, that the sensitivity level and the temperature of the maximum sensitivity are different. Namely, for the nitrogen dioxide the maximum sensitivity of 3.4 ppm−1 was obtained at about 175 ◦ C, while for ammonia of 0.1 ppm−1 at about 350 ◦ C. The sensitivity in the ammonia is much lower than in the nitrogen dioxide, which can be explained by different catalytic activity of electrodes towards specific electrochemical reaction. In is worth noting that for ammonia mixed potential potentiometric Nasicon sensor the optimum sensing temperature was found to be between 300 ◦ C and 350 ◦ C [9], which in accordance with current findings. The operation temperature of Nasicon potentiometric nitrogen dioxide sensors reported in the literature scatter, i.e. 400 ◦ C [10] and 100 ◦ C [11]. To investigate sensors’ sensing mechanism, freshly fabricated sensor structures were polarized at different temperatures in 100 ppm of nitrogen dioxide and ammonia. The electrodes of the sensors were investigated by XRD in order to determine phases,
Fig. 1. Response of the sensor to different concentration of nitrogen dioxide at 200 ◦ C (a) and different concentration of ammonia at 350 ◦ C (b).
which were formed on the electrodes as the result of sensor electrical excitation. Fig. 3 presents XRD pattern of the sensor exposed to 100 ppm of nitrogen dioxide at 200 ◦ C and polarized with 1 V for 7 h. It was found that on the negatively polarized electrode the chemical compound was formed (see Fig. 3a), which does not exist on positively polarized electrode. The chemical compound was determined to be sodium nitrate or sodium nitrite
Fig. 2. Sensitivity plot of sensor at different temperatures.
P. Jasinski et al. / Sensors and Actuators B 189 (2013) 141–145
143
Fig. 3. XRD diffraction patterns of both sides of the sensors biased with the voltage of 1 V in 100 ppm of NO2 for 7 h at 200 ◦ C (a) and negative polarization sides of the sensor at 200 ◦ C and 350 ◦ C (b). N – Nasicon, Z – zirconium oxide, and Pt – platinium.
and it is expected that this compound is formed and decomposed during normal sensor operation. Unfortunately, sodium nitrate and sodium nitrite show nearly the same XRD pattern, so it is impossible to distinguish between them. Fig. 3b presents XRD pattern of the sensor exposed to100 ppm of NO2 for 7 h at 200 ◦ C and 350 ◦ C. The XRD reflection related to NaNO2 /NaNO3 layer is much higher at 200 ◦ C than at 350 ◦ C. The sensitivity of the sensor at 200 ◦ C is much higher than at 350 ◦ C, what is in agreement with the hypothesis that the NaNO2 or NaNO3 is responsible for sensing mechanism of electrocatalytic sensors. The XRD pattern of the sensors exposed to 100 ppm of nitrogen dioxide at 200 ◦ C and polarized with 1 V for different times was also investigated. Namely, the freshly fabricated sensors were polarized for 10 min, 1, 2, 5 and 15 h. These durations of polarization were selected, because the electric charge was doubled during subsequent polarizations. Therefore, one may expect that the thickness of the formed layer is doubled for the subsequent durations of polarizations. Fig. 4 presents XRD diffraction patterns of the sensors’ negative polarization side. It can be seen that with the increase of the polarization durations the height of the peak related Fig. 5. AFM images of negative (a) and positive (b) polarization sides of the sensors biased with the voltage of 1 V in 100 ppm of NO2 for 15 h at 200 ◦ C.
Fig. 4. XRD diffraction patterns of the sensors’ negative polarization side. The sensors are biased with the voltage of 1 V in 100 ppm of NO2 for 10 min, 1, 2, 5 and 15 h at 200 ◦ C.
to NaNO2 /NaNO3 increases. In case of the polarization durations of 10 min, 1 and 2 h, the peak is barely visible, while for 5 and 15 h its existence is unambiguous. It can be expected that for the shorter polarization times the NaNO2 /NaNO3 is formed within the pores of platinum. For the longer polarization times the pores are probably partially filled, what forces the NaNO2 /NaNO3 to grow on exposed for X-rays surface of platinum. Therefore, there is significant peak intensity difference between shorter and longer polarization times. The surface of the sensors after exposition to 100 ppm of nitrogen dioxide at 200 ◦ C and polarized with 1 V for 15 h was examined by atomic force microscopy. The AFM images of surfaces of the negative and positive polarization sides are presented in Fig. 5. It is evident that both surfaces show different microstructure. The surface of negative polarization side shows nanostructures of about 60 nm in diameter, which is expected to be NaNO2 /NaNO3 . Those nanostructures are not visible on the surface of positive polarization side. Contrary to the sensor exposed to nitrogen dioxide, in ammonia the surface of the negative (and positive) sensor’s electrode is
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P. Jasinski et al. / Sensors and Actuators B 189 (2013) 141–145
Fig. 7. Response of the sensor to 80 ppm of ammonia and 80 ppm of nitrogen dioxide at 350 ◦ C. Fig. 6. XRD diffraction pattern of the sensors’ negative polarization side. The sensors are biased with the voltage of 1 V in 100 ppm of NH3 for 15 h at 350 ◦ C. N – Nasicon, Z – zirconium oxide, and Pt – platinum.
free from any traces of NaNO2 /NaNO3 both at 200 ◦ C and at 350 ◦ C. Instead, small traces of sodium peroxide were detected on the surface of negative polarization side at 350 ◦ C (see Fig. 6). At 200 ◦ C the sodium peroxide is not detected, but at the same time the sensitivity of the sensor is lower than at 350 ◦ C. Therefore, it can be concluded that sensing mechanism of electrocatalytic sensor is different in nitrogen dioxide and ammonia. Namely, in nitrogen dioxide the sodium nitrate/nitrite is formed on the sensor electrode, while in ammonia the sodium peroxide. The following reaction can be proposed in nitrogen dioxide: 2NaNO3 → 2Na+ + 2e− + 2NO2 + O2 +
−
NaNO3 → Na + e + NO2
(2) (3)
According to their physical properties, the NaNO2 decomposes at 271 ◦ C, while the NaNO3 at 380 ◦ C. It can be expected that below 270 ◦ C the mixture of sodium nitrate and sodium nitrite is formed and decomposed at the triple phase boundary. Above 271 ◦ C only NaNO3 is expected as a result of nitrogen dioxide electrochemical decomposition. Ono et al. [12] has investigated reaction product of Nasicon amperometric NO2 sensor with gold electrodes at 150 ◦ C. Based on chronopotentiometry experiments he has concluded that both NaNO2 and NaNO3 are formed on sensing electrode in nitrogen dioxide. Moreover, the NaNO2 is formed as a primary product, while NaNO3 as secondary product converted from NaNO2 . In case of the ammonia based on X-ray diffactogram reactions (4) and (5) can be proposed:
However, sodium oxide (Na2 O), which was not detected in this study, reacts violently with water existing during ammonia decomposition what suggest that this oxide is less stable than sodium peroxide. Ono et al. [12] during his chronopotentiometry experiments in synthetic air at 200 ◦ C revealed formation NaOx on the sensing electrode. He ascribed this oxide to sodium peroxide. As presented in Fig. 1b, the double peak is visible in I–V plot for ammonia at 350 ◦ C. In Fig. 7 is presented an I–V plot of the sensor in 80 ppm of ammonia and in 80 ppm of nitrogen dioxide at 350 ◦ C. It is visible that the peak existing in nitrogen dioxide can be matched to one of the peaks existing in ammonia. Therefore, one may expect chemical decomposition of ammonia to nitric oxide (8) [14]: 4NH3 + 5O2 → 4NO + 6H2 O,
(8)
which may further lead to the formation NaNO3 at the triple phase boundary according to reaction (9): NO + Na+ + e− + O2 → NaNO3 .
(9)
This can explain similarities between the ammonia and nitrogen dioxide I–V plots. 4. Conclusions
2Na+ + 2e− + 4NH3 + 4O2 → Na2 O2 + 2H2 + 6H2 O
(4)
In this paper the sensing mechanism of electrocatalytic Nasicon sensor in nitrogen dioxide and ammonia was investigated. It was found that at 200 ◦ C the sensor is sensitive to nitrogen dioxide due to sodium nitrate/nitrite formation. At 350 ◦ C sodium nitrate/nitrite was barely detected. In case of the sensor exposed to ammonia, formation of sodium peroxide was detected. The amount of this phase is lower at 200 ◦ C than at 350 ◦ C, what is in agreement with sensitivity level at those temperatures.
Na2 O2 → 2Na+ + 2e− + O2
(5)
Acknowledgements
The Na2 O2 decomposes at 460 ◦ C, so it can be expected as the product of electrochemical decomposition of ammonia. Differently, in the paper by Liang et al. [13] related to ammonia mixed potential potentiometric sensing, reactions (6) and (7) are suggested to occur: 2NH3 + 3Na2 O (in Nasicon) → N2 + 3H2 O + 6Na+ + 6e− +
−
4Na + 4e + O2 → 2Na2 O (in Nasicon)
This study was supported financially by the Polish Ministry of Science and Higher Education under grant No. N N515 243437. The authors thank Dr. Jakub Karczewski for acquiring AFM images. References
(6) (7)
[1] G. Jasinski, P. Jasinski, A. Nowakowski, T. Zajt, B. Chachulski, Electrocatalytic nitrogen dioxide sensor, Proceedings of SPIE 5505 (2004) 89–94. [2] E.L. Shoemaker, C.U. Kim, M.C. Vogt, CO2 sensing mechanism of an electrocatalytic sensor based on a tunsgen-stabilized bismuth oxide solid electrolyte
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and cyclic voltammetry measurement techniques, Sensors and Actuators B 110 (2005) 89–100. P. Jasinski, A. Nowakowski, W. Weppner, Kinetic studies of Nasicon based sensors with cyclic voltammetry, Sensors and Materials 12 (2000) 89–97. E.L. Shoemaker, M.C. Vogt, F.J. Dudek, Cyclic voltammetry applied to an oxygenion-conducting solid electrolyte as an active electrocatalytic gas sensor, Solid State Ionics 92 (1996) 285–292. E.L. Shoemaker, M.C. Vogt, F.J. Dudek, T. Turner, Gas microsensor using cyclic voltammetry with a cermet electrochemical cell, Sensors and Actuators B 42 (1997) 1–9. G. Jasinski, P. Jasinski, B. Chachulski, A. Nowakowski, Electrocatalytic gas sensors based on NASICON and LISICON, Materials Science Poland 24 (2006) 262–268. G. Jasinski, P. Jasinski, A. Nowakowski, B. Chachulski, Properties of a lithium solid electrolyte gas sensor based on reaction kinetics, Measurement Science and Technology 17 (2006) 17–21. G. Jasinski, P. Jasinski, Solid electrolyte gas sensors based on cyclic voltammetry with one active electrode, in: IOP Conference Series: Materials Science and Engineering: ICC3: Symposium 15: Advanced Engineering Ceramics and Composites, IOP Publishing Ltd., 2011. X. Liang, T. Zhong, H. Guan, F. Liu, G. Lu, B. Quan, Ammonia sensor based on NASICON and Cr2 O3 electrode, Sensors and Actuators B 136 (2009) 479–483. Y. Shimizu, H. Nishi, H. Suzuki, K. Maeda, Solid-state NOx sensor combined with NASICON and Pb–Ru-based pyrochlore-type oxide electrode, Sensors and Actuators B 65 (2000) 141–143. K. Obata, S. Matsushima, NASICON-based NO2 device attached with metal oxide and nitrite compound for the low temperature operation, Sensors and Actuators B 130 (2008) 269–276. M. Ono, K. Shimanoe, N. Miura, N. Yamazoe, Reaction analysis on sensing electrode of amperometric NO2 sensor based on sodium ion conductor by using chronopotentiometry, Sensors and Actuators B 77 (2001) 78–83. X. Liang, G. Lu, T. Zhong, F. Liu, B. Quan, New type of ammonia/toluene sensor combining NASICON with a couple of oxide electrodes, Sensors and Actuators B 150 (2010) 355–359. J.-C. Lou, C.-M. Hung, S.-F. Yang, Selective catalytic oxidation of ammonia over copper–cerium composite catalyst, Journal of the Air & Waste Management Association 54 (2004) 68–76.
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Biographies Piotr Jasinski obtained MSc in electronics in 1992 from the Gdansk University of Technology (GUT), Poland. Working at GUT, he received PhD in 2000 and DSc in 2009. Between 2001 and 2004 post doctoral fellow at Missouri University of Science and Technology, while between 2008 and 2010 an assistant research professor. Currently is an associate professor at Gdansk University of Technology working in the field of electroceramics. Anna Strzelczyk received her MS degree in chemical technology in 2010 from the Gdansk University of Technology, Poland. Since 2010 she is a PhD student in Department of Chemical and Process Engineering, Gdansk University of Technology, Poland. Her main research interests are related to the electrochemical toxic gas sensors and humidity sensors. ´ Bogdan Chachulski received his PhD in physical chemistry from the Gdansk University of Technology in 1977. Since 1982 he is appointed as an assistant professor ´ University of Technology. He is of chemical and process engineering at the Gdansk currently active in the area of electrochemical sensor development for air monitoring. Maria Gazda received her PhD in experimental physics from the University of Gdansk, Poland. Currently she is an associate professor at faculty of applied physics and mathematics of Gdansk University of Technology. Her research interests focus on the structural and electrical properties oxide materials and their applications in gas sensors and fuel cells. Grzegorz Jasinski received MS and PhD degrees in electronics from the Gdansk University of Technology (GUT), Poland, in 2001 and 2008, respectively. Since then he has been working as assistant professor at GUT. In his research he is focused on construction of various gas sensors, electronic nose, multisensor systems, and applications of various numeric methods to analyze sensor responses.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Investigation of sensing mechanism of Nasicon electrocatalytic sensors in nitrogen dioxide and ammonia Piotr Jasinski a,∗ , Anna Strzelczyk b , Bogdan A. Chachulski b , Maria Gazda c , Grzegorz Jasinski a a b c
Faculty of Electronics, Telecommunication and Informatics, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Poland Chemical Faculty, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Poland Faculty of Applied Physics and Mathematics, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Poland
a r t i c l e
i n f o
Article history: Available online 22 February 2013 Keywords: Electrocatalytic sensor Nitrogen dioxide Ammonia Sensing mechanism Nasicon
a b s t r a c t In this paper a sensing mechanism of Nasicon electrocatalytic sensor in nitrogen dioxide and ammonia is investigated. Both gases are environmentally hazardous and contain nitrogen atom in the molecule. However, it seems that their sensing mechanism in electrocatalytic sensor could be totally different. Namely, the maximum sensitivity for each gas was obtained at different temperatures. Also, different auxiliary layers are formed for each gas. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Electrocatalytic sensors belong to family of electrochemical sensors with solid state electrolyte. Their sensing mechanism is based on application of periodically changing voltage and acquisition of current response. Obtained in this way current–voltage plot provides fingerprint information about ambient gases, which interact with sensor surface due to electrical excitation. The sensor name, electrocatalytic, is a direct consequence of sensing mechanism, i.e. the electrochemical reactions results from the electric excitation [1,2]. So far relatively simple sensor structures have been investigated. Mostly, these structures consisted of different types of electrolyte and metallic electrodes [3–6]. We have been investigating these sensors before and it seems that Nasicon electrolyte with platinum electrodes is a useful structure for sensing mechanism investigation [6,7]. This structure provides in I–V plot unique response to different gases. So far, the major drawback of electrocatalytic sensor is related to instability due to formation of chemical phases on the sensor surface, which covers and block triple phase boundary. However, it was found that the sensors, which exposed and poisoned by nitrogen dioxide, can be at any time refreshed by short time high temperature treatment [8]. In order to improve sensor stability in different gases it is necessary to better understand the sensors sensing mechanism. This work aims to investigate Nasicon electrocatalytic gas sensor sensing mechanism in nitrogen dioxide and ammonia.
In this paper, Nasicon (Na2.8 Zr2 Si1.8 P1.2 O12 ) sensors with platinum electrodes were prepared and investigated. Namely, Nasicon powder was prepared by the conventional solid-state ball milling method from the zirconium oxide, silicon oxide and trisodium phosphate. The components were mixed in ball mill and annealed at 1373 K (900 ◦ C). Obtained in this way powder was compacted into pellets (∼12 mm in diameter and 1 mm thick) by iso-axial pressing. The pellets were sintered in air for 2 h at 1373 K (1100 ◦ C). Electrodes were made by coating both sides of the pellet with the platinum paste (ESL ElectroScience, ESL 5542) and firing at 1173 K (900 ◦ C) (surface area of 0.5 cm2 ). Measurements were conducted in mixtures of high purity gasses: nitrogen dioxide or ammonia and synthetic air, which were supplied with a constant gas flow of 100 cm3 min−1 maintained by mass flow controllers. The experiments were conducted in a tube furnace, which temperature was controlled with accuracy better than 0.5 K. The measurements were performed using the electrochemical interface Solartron SI 1287. To obtain sensor sensitivity in nitrogen dioxide and ammonia, a triangular voltage excitation signal with the amplitude of 5 V and sweep rate of 50 mV/s was applied to the sensor. The electrical current was recorded during sensor polarization. On the current–voltage plot the peaks with different heights (maximum of the electric current) were visible for different concentrations of ammonia and nitrogen dioxide. This information was used to obtain sensors sensitivity S at different temperatures (1):
∗ Corresponding author. Tel.: +48 58 347 1323; fax: +48 58 347 1757. E-mail addresses: [email protected], [email protected] (P. Jasinski). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.02.044
S=
Imax , c · I0- max
(1)
142
P. Jasinski et al. / Sensors and Actuators B 189 (2013) 141–145
where c is the measured gas concentration (ppm), while I0-max and Imax are the heights of the peaks in the synthetic air and the measured gas, respectively. The I0-max was obtained from reproducible I–V response in synthetic air after the measurement in tested gas. Usually, the reproducible response was obtained within 2 h of voltage cycling. At some temperatures (below 350 ◦ C) for ammonia the I–V response in synthetic air before and after measurement in tested gas was not exactly the same. Namely, the currents were higher than before the measurements, which may suggest that the surfaces of electrodes are covered by some reactions products. The high temperature treatment of the sensor (about 600 ◦ C in synthetic air) refreshes the sensor and the level of currents become low again [8]. To investigate sensors sensing mechanisms XRD pattern of the electrode surface was investigated. Namely, a freshly fabricated Nasicon sensor was exposed to 100 ppm of ammonia or nitrogen dioxide at 473 K (200 ◦ C) or at 623 K (350 ◦ C). At those temperatures the maximum sensitivity was recorded for each gas. When the temperature was stabilized the sensor was polarized (biased) with 1 V for different periods to form a sensing phase on the surface of electrode. The same sensing phase is formed during normal sensor operation, i.e. when the sensor is excited with triangular signal. On current–voltage plot it is reflected by the appearance of peaks due to electrochemical reaction of formation and decomposition of the phases. The surfaces of the sensors were investigated using X-ray diffractometry in order to determine the composition of the formed phase. XRD patterns were recorded with Cu K␣ radiation at room temperature using Philips X’Pert Pro diffractometer. Qualitative phase analysis of diffraction pattern was carried out with ICDD PDF database. The AFM images were collected using Atomic Force Microscope NTegra (NT-MDT).
a
b
3. Results Nitrogen dioxide and ammonia contain nitrogen atom and both gases are environmentally hazardous. However, it seems that sensing mechanism of electrocatalytic sensor is not the same. In Fig. 1, the response of the sensor to different concentration of nitrogen dioxide at 200 ◦ C (Fig. 1a) and different concentration of ammonia at 350 ◦ C (Fig. 1b) are presented. The I–V plot presented in Fig. 1a shows only one sharp peak for positive and negative voltages. This reflects one step formation and/or decomposition reaction occurring at the triple phase boundary. The I–V plot for ammonia at 350 ◦ C (Fig. 1b) reveals broader peak, which seems to consist of two components. This may indicate either a two-step process or two different reactions at the triple phase boundary. As presented in Fig. 2, at those temperatures about the maximum sensitivity for each gas was obtained. Based on the results shown in Fig. 2, it is evident, that the sensitivity level and the temperature of the maximum sensitivity are different. Namely, for the nitrogen dioxide the maximum sensitivity of 3.4 ppm−1 was obtained at about 175 ◦ C, while for ammonia of 0.1 ppm−1 at about 350 ◦ C. The sensitivity in the ammonia is much lower than in the nitrogen dioxide, which can be explained by different catalytic activity of electrodes towards specific electrochemical reaction. In is worth noting that for ammonia mixed potential potentiometric Nasicon sensor the optimum sensing temperature was found to be between 300 ◦ C and 350 ◦ C [9], which in accordance with current findings. The operation temperature of Nasicon potentiometric nitrogen dioxide sensors reported in the literature scatter, i.e. 400 ◦ C [10] and 100 ◦ C [11]. To investigate sensors’ sensing mechanism, freshly fabricated sensor structures were polarized at different temperatures in 100 ppm of nitrogen dioxide and ammonia. The electrodes of the sensors were investigated by XRD in order to determine phases,
Fig. 1. Response of the sensor to different concentration of nitrogen dioxide at 200 ◦ C (a) and different concentration of ammonia at 350 ◦ C (b).
which were formed on the electrodes as the result of sensor electrical excitation. Fig. 3 presents XRD pattern of the sensor exposed to 100 ppm of nitrogen dioxide at 200 ◦ C and polarized with 1 V for 7 h. It was found that on the negatively polarized electrode the chemical compound was formed (see Fig. 3a), which does not exist on positively polarized electrode. The chemical compound was determined to be sodium nitrate or sodium nitrite
Fig. 2. Sensitivity plot of sensor at different temperatures.
P. Jasinski et al. / Sensors and Actuators B 189 (2013) 141–145
143
Fig. 3. XRD diffraction patterns of both sides of the sensors biased with the voltage of 1 V in 100 ppm of NO2 for 7 h at 200 ◦ C (a) and negative polarization sides of the sensor at 200 ◦ C and 350 ◦ C (b). N – Nasicon, Z – zirconium oxide, and Pt – platinium.
and it is expected that this compound is formed and decomposed during normal sensor operation. Unfortunately, sodium nitrate and sodium nitrite show nearly the same XRD pattern, so it is impossible to distinguish between them. Fig. 3b presents XRD pattern of the sensor exposed to100 ppm of NO2 for 7 h at 200 ◦ C and 350 ◦ C. The XRD reflection related to NaNO2 /NaNO3 layer is much higher at 200 ◦ C than at 350 ◦ C. The sensitivity of the sensor at 200 ◦ C is much higher than at 350 ◦ C, what is in agreement with the hypothesis that the NaNO2 or NaNO3 is responsible for sensing mechanism of electrocatalytic sensors. The XRD pattern of the sensors exposed to 100 ppm of nitrogen dioxide at 200 ◦ C and polarized with 1 V for different times was also investigated. Namely, the freshly fabricated sensors were polarized for 10 min, 1, 2, 5 and 15 h. These durations of polarization were selected, because the electric charge was doubled during subsequent polarizations. Therefore, one may expect that the thickness of the formed layer is doubled for the subsequent durations of polarizations. Fig. 4 presents XRD diffraction patterns of the sensors’ negative polarization side. It can be seen that with the increase of the polarization durations the height of the peak related Fig. 5. AFM images of negative (a) and positive (b) polarization sides of the sensors biased with the voltage of 1 V in 100 ppm of NO2 for 15 h at 200 ◦ C.
Fig. 4. XRD diffraction patterns of the sensors’ negative polarization side. The sensors are biased with the voltage of 1 V in 100 ppm of NO2 for 10 min, 1, 2, 5 and 15 h at 200 ◦ C.
to NaNO2 /NaNO3 increases. In case of the polarization durations of 10 min, 1 and 2 h, the peak is barely visible, while for 5 and 15 h its existence is unambiguous. It can be expected that for the shorter polarization times the NaNO2 /NaNO3 is formed within the pores of platinum. For the longer polarization times the pores are probably partially filled, what forces the NaNO2 /NaNO3 to grow on exposed for X-rays surface of platinum. Therefore, there is significant peak intensity difference between shorter and longer polarization times. The surface of the sensors after exposition to 100 ppm of nitrogen dioxide at 200 ◦ C and polarized with 1 V for 15 h was examined by atomic force microscopy. The AFM images of surfaces of the negative and positive polarization sides are presented in Fig. 5. It is evident that both surfaces show different microstructure. The surface of negative polarization side shows nanostructures of about 60 nm in diameter, which is expected to be NaNO2 /NaNO3 . Those nanostructures are not visible on the surface of positive polarization side. Contrary to the sensor exposed to nitrogen dioxide, in ammonia the surface of the negative (and positive) sensor’s electrode is
144
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Fig. 7. Response of the sensor to 80 ppm of ammonia and 80 ppm of nitrogen dioxide at 350 ◦ C. Fig. 6. XRD diffraction pattern of the sensors’ negative polarization side. The sensors are biased with the voltage of 1 V in 100 ppm of NH3 for 15 h at 350 ◦ C. N – Nasicon, Z – zirconium oxide, and Pt – platinum.
free from any traces of NaNO2 /NaNO3 both at 200 ◦ C and at 350 ◦ C. Instead, small traces of sodium peroxide were detected on the surface of negative polarization side at 350 ◦ C (see Fig. 6). At 200 ◦ C the sodium peroxide is not detected, but at the same time the sensitivity of the sensor is lower than at 350 ◦ C. Therefore, it can be concluded that sensing mechanism of electrocatalytic sensor is different in nitrogen dioxide and ammonia. Namely, in nitrogen dioxide the sodium nitrate/nitrite is formed on the sensor electrode, while in ammonia the sodium peroxide. The following reaction can be proposed in nitrogen dioxide: 2NaNO3 → 2Na+ + 2e− + 2NO2 + O2 +
−
NaNO3 → Na + e + NO2
(2) (3)
According to their physical properties, the NaNO2 decomposes at 271 ◦ C, while the NaNO3 at 380 ◦ C. It can be expected that below 270 ◦ C the mixture of sodium nitrate and sodium nitrite is formed and decomposed at the triple phase boundary. Above 271 ◦ C only NaNO3 is expected as a result of nitrogen dioxide electrochemical decomposition. Ono et al. [12] has investigated reaction product of Nasicon amperometric NO2 sensor with gold electrodes at 150 ◦ C. Based on chronopotentiometry experiments he has concluded that both NaNO2 and NaNO3 are formed on sensing electrode in nitrogen dioxide. Moreover, the NaNO2 is formed as a primary product, while NaNO3 as secondary product converted from NaNO2 . In case of the ammonia based on X-ray diffactogram reactions (4) and (5) can be proposed:
However, sodium oxide (Na2 O), which was not detected in this study, reacts violently with water existing during ammonia decomposition what suggest that this oxide is less stable than sodium peroxide. Ono et al. [12] during his chronopotentiometry experiments in synthetic air at 200 ◦ C revealed formation NaOx on the sensing electrode. He ascribed this oxide to sodium peroxide. As presented in Fig. 1b, the double peak is visible in I–V plot for ammonia at 350 ◦ C. In Fig. 7 is presented an I–V plot of the sensor in 80 ppm of ammonia and in 80 ppm of nitrogen dioxide at 350 ◦ C. It is visible that the peak existing in nitrogen dioxide can be matched to one of the peaks existing in ammonia. Therefore, one may expect chemical decomposition of ammonia to nitric oxide (8) [14]: 4NH3 + 5O2 → 4NO + 6H2 O,
(8)
which may further lead to the formation NaNO3 at the triple phase boundary according to reaction (9): NO + Na+ + e− + O2 → NaNO3 .
(9)
This can explain similarities between the ammonia and nitrogen dioxide I–V plots. 4. Conclusions
2Na+ + 2e− + 4NH3 + 4O2 → Na2 O2 + 2H2 + 6H2 O
(4)
In this paper the sensing mechanism of electrocatalytic Nasicon sensor in nitrogen dioxide and ammonia was investigated. It was found that at 200 ◦ C the sensor is sensitive to nitrogen dioxide due to sodium nitrate/nitrite formation. At 350 ◦ C sodium nitrate/nitrite was barely detected. In case of the sensor exposed to ammonia, formation of sodium peroxide was detected. The amount of this phase is lower at 200 ◦ C than at 350 ◦ C, what is in agreement with sensitivity level at those temperatures.
Na2 O2 → 2Na+ + 2e− + O2
(5)
Acknowledgements
The Na2 O2 decomposes at 460 ◦ C, so it can be expected as the product of electrochemical decomposition of ammonia. Differently, in the paper by Liang et al. [13] related to ammonia mixed potential potentiometric sensing, reactions (6) and (7) are suggested to occur: 2NH3 + 3Na2 O (in Nasicon) → N2 + 3H2 O + 6Na+ + 6e− +
−
4Na + 4e + O2 → 2Na2 O (in Nasicon)
This study was supported financially by the Polish Ministry of Science and Higher Education under grant No. N N515 243437. The authors thank Dr. Jakub Karczewski for acquiring AFM images. References
(6) (7)
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Biographies Piotr Jasinski obtained MSc in electronics in 1992 from the Gdansk University of Technology (GUT), Poland. Working at GUT, he received PhD in 2000 and DSc in 2009. Between 2001 and 2004 post doctoral fellow at Missouri University of Science and Technology, while between 2008 and 2010 an assistant research professor. Currently is an associate professor at Gdansk University of Technology working in the field of electroceramics. Anna Strzelczyk received her MS degree in chemical technology in 2010 from the Gdansk University of Technology, Poland. Since 2010 she is a PhD student in Department of Chemical and Process Engineering, Gdansk University of Technology, Poland. Her main research interests are related to the electrochemical toxic gas sensors and humidity sensors. ´ Bogdan Chachulski received his PhD in physical chemistry from the Gdansk University of Technology in 1977. Since 1982 he is appointed as an assistant professor ´ University of Technology. He is of chemical and process engineering at the Gdansk currently active in the area of electrochemical sensor development for air monitoring. Maria Gazda received her PhD in experimental physics from the University of Gdansk, Poland. Currently she is an associate professor at faculty of applied physics and mathematics of Gdansk University of Technology. Her research interests focus on the structural and electrical properties oxide materials and their applications in gas sensors and fuel cells. Grzegorz Jasinski received MS and PhD degrees in electronics from the Gdansk University of Technology (GUT), Poland, in 2001 and 2008, respectively. Since then he has been working as assistant professor at GUT. In his research he is focused on construction of various gas sensors, electronic nose, multisensor systems, and applications of various numeric methods to analyze sensor responses.