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CO sensing mechanism with WO3 based gas sensors
Sensors and Actuators B 151 (2010) 103–106
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
CO sensing mechanism with WO3 based gas sensors M. Hübner a,∗ , C.E. Simion b , A. Haensch a , N. Barsan a , U. Weimar a a b
University of Tübingen, Institute of Physical Chemistry, Auf der Morgenstelle 15, 72076 Tübingen, Germany National Institute of Materials Physics, P.O. Box MG-7, 077125 Bucharest-Magurele, Romania
a r t i c l e
i n f o
Article history: Received 9 July 2010 Received in revised form 31 August 2010 Accepted 3 September 2010 Available online 20 October 2010
a b s t r a c t The interaction between CO and WO3 based gas sensors was investigated in operando conditions by using resistance, catalytic conversion and DRIFTS measurements. The experimental results are showing that the sensing involves the reduction of the metal oxide, which is the opposite of the case of SnO2 , the other extremely relevant material for semiconducting oxides based gas sensors. © 2010 Elsevier B.V. All rights reserved.
Keywords: WO3 CO sensing SnO2 Reduction
1. Introduction Along stannic oxide (SnO2 ), tungstic oxide (WO3 ) is one of the most relevant materials for the fabrication of semiconducting metal oxide based gas sensors; those two materials are the only ones used in commercial devices, the former for the detection of reducing gases and the latter for the oxidizing ones [1–3]. They are also the most studied ones; for instance at the last IMCS conference 2008 in Ohio/USA, over 30% of the oral contributions in the field of resistive gas sensors and metal oxides dealt with these two materials. Gas sensing effects recorded as changes in the resistivity of WO3 are known since long [4] and besides the good performance for the detection of NOx [5,6] it has been shown that there is good performance also for the detection of NH3 [7,8], H2 S [9], etc. Keeping in mind how relevant the material is, it is surprising that not too much is known about the sensing mechanism. There are no comprehensive reviews for WO3 as a gas sensing material; it is however known that the material is non-stoichiometric – the semiconducting effect is linked, like also for SnO2 to the donor levels in the band-gap associated to the oxygen vacancies. It is also known that the crystal chemistry of WO3 is extremely complex with two different crystalline phases in the range of temperatures where the gas sensors are normally operated (transition temperature from monoclinic to orthorhombic around 330 ◦ C) [10]. In the case of NO2 detection the mechanism is considered to consist two processes; a dominant – adsorption of NO2 via one of
the oxygen atoms on the surface leading to a negative charged surface state – and a weaker – an oxygen vacancy is filled with one atom of NO2 resulting in NO and a lattice oxygen – one [11]. In the case of reducing gases one either assumes that the reaction is taking place with pre-adsorbed oxygen [12] – mechanism applied by the authors to NH3 detection – or, like it was considered for H2 S [13,14] based on XPS studies, the surface reduction is consider to be at the origin of the sensor effect. However it is not clear if the authors think that this finding is specific for H2 S or a general feature for all reducing gases. Here, one has to note that even if the reaction between the oxide and the gas took place in realistic conditions, the spectroscopic investigation was performed with the oxide at room temperature and in UHV. In most of sensor studies only resistance measurements were used and it is not surprising that the level of basic understanding is by far more limited than in the case of SnO2 . One of the first problem to solve is the role of pre-adsorbed oxygen in sensing, which is relevant for the detection of reducing gases as a possible reaction partner and for the oxidizing one as a possible competitor. In the case of SnO2 we have shown that for CO and H2 there are two interaction paths: a direct reaction with the metal oxide, which does not involve lattice oxygen and a reaction with pre-adsorbed oxygen [15,16]. Here we are examining the sensing of CO with WO3 based sensors as a function of the oxygen background by performing conductance, catalytic conversion and DRIFTS measurements.
2. Experimental details ∗ Corresponding author. Tel.: +49 (0)7071 78766; fax: +49 (0)7071 5960. E-mail address: [email protected] (M. Hübner). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.09.040
The WO3 sensitive material was obtained by a nonaqueous sol–gel synthesis in combination with the structure directing sur-
104
M. Hübner et al. / Sensors and Actuators B 151 (2010) 103–106
Fig. 1. Time dependence of the resistance during CO exposure (10, 30, 70, 100 ppm) in the absence of oxygen at 300 ◦ C. The amount of CO and CO2 in the exhaust is shown in the upper part.
Fig. 2. Time dependence of the resistance during CO exposure (10, 30, 70, 100 ppm) in the presence of 150 ppm of oxygen. The amount of CO and CO2 in the exhaust is shown in the upper part.
factant P123, as described in [17]. The investigation of gas response tests of the thick film sensors (Film size: 50 m × 7 mm × 4 mm)screen printed on alumina substrates with Pt electrodes and heater – based on that material demonstrated good CO sensing performance. Here, three different oxygen concentrations in nitrogen (5.5 grade) with a gas flow of 500 ml/min were chosen as background conditions: <3 (below the detection limit), 150 and 22,000 ppm. The actual oxygen concentrations were monitored online by using an electrochemical oxygen analyzer (Zirox SGM400). Three sensors were exposed to 4 pulses of CO (10, 30, 70 and 100 ppm) for 3 h; for the recovery after each pulse, 4 h of background atmosphere exposure were allowed. To investigate the formation of reactions products the exhaust was connected to a photo-acoustic gas analyzer (Innova 1312). The in situ DRIFT measurements were carried out in a modified Bruker Vertex 80v spectrometer equipped with liquid nitrogen cooled MCT detector. The optical bench of the spectrometer was evacuated during the whole measurement to a stable vacuum of 1,62 mbars. To operate the gas sensor in real conditions during the infrared measurements, a home made testing chamber with gas access and electrical connectors for the read-out of the resistance was installed into the Harrick Praying Mantis DRIFT unit.
tive explanation could be the possible contribution of the remaining chemisorbed oxygen, from the previous exposure to air, to the CO2 production. We think that this effect is negligible because of the fact that one would expect the remaining chemisorbed oxygen to play a role also in the case of SnO2 , which is not the case. Additionally, if the remaining chemisorbed oxygen would play a role, this will happen just for the first CO exposure, which will eliminate it. The reduction of WO3 can be described by the following reaction (Eq. (1)):
3. Results and discussion Fig. 1 presents the time dependence of the sensor resistance, operated at 300 ◦ C, and of the gaseous composition of the exhaust during CO sensing in the almost total absence of oxygen. The sensor signals are large and the changes of the resistance determined by exposure to CO are completely reversible during the recovery period. Very interestingly, even if the oxygen concentration in the background was very low, one records CO2 formation as a result of the CO exposure. Contrary to these findings, no CO2 generation was observed in former studies [15] and experiments performed in similar conditions – no or very little oxygen – with SnO2 based sensors (not shown here) even if the sensor signals were extremely high. Besides that, the same experiments were performed just simply with uncoated substrates to exclude the possible role of the catalytic conversion on the electrodes; in the same conditions – below 3 ppm O2 – no CO2 generation was recorded. This result indicates that in the case of WO3 , on the opposite of SnO2 , the direct reduction of the material is a possible cause of the sensor signals. An alterna-
COgas + OO → CO2 gas + V•O + e−
(1)
whereas COgas describes the target gas, OO the lattice oxygen of WO3 , CO2 gas the produced carbon monoxide due to surface reduction, V•O the ionized vacancy and e− the released electron. Consequently the ionization of the oxygen vacancy donors determines the decrease of the resistance. To check the validity of the proposed mechanism one can roughly calculate if there is enough lattice oxygen to justify the recorded amount of CO2 in the exhaust. The total mass of WO3 exposed to CO is 30 mg which corresponds to a quantity of 1.3 × 10−4 mol. Exposure to 100 ppm CO over 3 h results in an average CO2 amount in the exhaust around 15 ppm (Fig. 1) leading to a total volume of 1.35 × 10−3 l. By assuming that CO2 being an ideal gas (1 mol of gas corresponds to 22.4 l under normal conditions) one gets to a quantity of produced CO2 during these 3 h of 6 × 10−5 mol. Consequently there should be enough oxygen for the monitored CO2 production. The recovery is controlled by the oxidation reaction with the oxygen still present in the background and therefore, due to its extremely low concentration, that process takes more time. This re-oxidation of the surface decreases the concentration of surface donors (Eq. (2)). 1 gas O + V•O + e− ↔ OO 2 2
(2)
By increasing the concentration of oxygen in the background (150 ppm) one record a decrease of the sensor signals and an increase of the generated CO2 (see Fig. 2). The magnitude of the sensor signal depends on the equilibrium established between the generation of oxygen vacancies and their cancellation. The decrease of the signal is therefore easy to understand because the healing of
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function changes measurements and the role of water vapor will be examined. Acknowledgments The authors are thankful to the German Research Foundation (DFG) for the financial support provided through the SPP 1299 – “Das Haut-Konzept”. The authors are also thankful to the reviewer who asked for the CO2 balance calculation as an additional validity check and, generally, for the useful suggestions provided by the reviewers and the editor during the reviewing process. References
Fig. 3. DRIFT absorbance spectra for WO3 sensor in the absence of oxygen, in the upper part (red), and in dry synthetic air, in the lower part (blue), at 300 ◦ C during exposure to 250 ppm of CO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
the oxygen vacancies is enhanced by the presence of more oxygen in the background so that the equilibrium is established at a lower surface donor concentration. The increase of the CO2 concentration can be explained by a possible catalytic conversion taking place on the Pt heater and electrodes of the substrate [18]. A spectroscopic proof for the reduction of the material under CO exposure in the absence, as well as in the present of oxygen, is provided by DRIFTS. Fig. 3 shows the absorbance spectra, calculated by taking as reference the single channel spectra acquired in the absence of CO, corresponding to the exposure to 250 pm CO in the absence of oxygen (upper part) and in dry air condition (for the experimental details on the way in which the spectra are acquired and used, see [19]). One observes in both conditions, presence and absence of oxygen, the reduction of the intensity of the bands at 2063 and 1854 cm−1 assigned to various overtones and combination modes of the oxygen and tungsten bond in the lattice of the oxide (W O) [20,21]. Besides that, the presence of CO2 and CO in the pores of the metal oxide can be slightly seen in dry air. In the absence of oxygen the formation of CO2 cannot be detected by the DRIFT measurement due to the very high absorbance of free charge carriers (also the reason for the huge difference in the absorbance between both conditions) which overlaps with the CO2 band; nevertheless, the generation of CO2 was proven by the data presented in Fig. 1. This fact proves that in the case of WO3 there is a surface reaction of CO that consumes lattice oxygen and which is important even in normal conditions for gas sensors applications, namely in air. Accordingly, we consider that for the understanding of reducing gas sensing the described material reduction of WO3 has to be taken into consideration. 4. Conclusion and outlook The CO detection with WO3 sensors most probably implies the reduction of the material. In this, the sensing very much differs from the one in which SnO2 based materials are used and this fact has to be consider in all attempts to model the gas effects. In the next steps in the investigation of reducing gases interaction with tungsten oxide, additional operando methods will be used, e.g. work
[1] T. Inoue, K. Ohtsuka, Y. Yoshida, Y. Matsuura, Y. Kajiyama, Metal oxide semiconductor NO2 sensor, Sensors and Actuators 25 (1–3) (1995) 388–391. [2] T. Sauerwald, Investigation of surface processes affecting a multi signal generation of tin oxide and tungsten oxide gas sensors, PhD thesis, Justus-LiebigUniversität Gießen http://geb.uni-giessen.de/geb/volltexte/2008/5500/. [3] D. Briand, L. Guillot, S. Raible, J. Kappler, N.F. de Rooij, Highly Integrated Wafer Level Packaged MOX Gas Sensors, Digest of technical papers TRANSDUCERS 2007 & Eurosensors XXI, Lyon, France ISBN 1-4244-0842-3, 2007, pp. 2401–2404. [4] P.J. Shaver, Activated tungsten oxide gas detector, Applied Physics Letters 11 (1967) 255. [5] M. Akiyama, J. Tamaki, N. Miura, N. Yamazoe, Tungsten oxide-based semiconductor sensor highly sensitive to NO and NO2 , Chemistry Letters (1991) 1611–1614. [6] G. Sberveglieri, L. Depero, S. Groppeli, P. Nelli, WO3 sputtered thin films for NOx monitoring, Sensors and Actuators B 26–27 (1995) 89–92. [7] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Gold-loaded tungsten-oxide sensor for detection of ammonia in air, Chemistry Letters 4 (1992) 639–642. [8] H. Meixner, J. Gerblinger, U. Lampe, M. Fleischer, Thin-film gas sensors based on semiconducting metal oxides, Sensors and Actuators B 23 (2–3) (1995) 119–125. [9] I. Ruokamo, T. Karkkainen, J. Huusko, T. Ruokanen, M. Blomberg, H. Torvela, V. Lantto, H2 S response of WO3 thin film sensors manufactured by Silican processing technology, Sensors and Actuators B 19 (1–3) (1994) 486–488. [10] Landolt, Börnstein: Numerical Data and Functional Relationship in Science and Technology, Volume 17 Semiconductors Subvolume g Physics of Nontetrahedrally Bonded Binary Compounds II, Springer Verlag, Berlin New York, 1982, pp. 166–200, 446, 594. [11] S.L. Baumann, Detektions-Mechanismen auf WO3 bei Einsatz in Verbrennungsabgasen, PhD thesis, Justus-Liebig-Universität Gießen, http://geb.unigiessen.de/geb/volltexte/2004/1432/. [12] E. Llobet, G. Molas, P. Molinàs, J. Calderer, X. Vilanova, J. Brezmes, J.E. Sueiras, X. Correig, Fabrication of highly selective tungsten oxide ammonia sensors, Journal of the Electrochemical Society 147 (2) (2000) 776–779. [13] D.J. Dwyer, Surface chemistry of gas sensors – H2 S on WO3 films, Sensors and Actuators B 5 (1–4) (1991) 155–159. [14] B. Frühberger, M. Grunze, D.J. Dwyer, H2 S interaction with WO3 , XPS studies: surface chemistry of H2 S-sensitive tungsten oxide films, Actuators B 31 (3) (1996) 167–174. [15] S.H. Hahn, N. Barsan, U. Weimar, S.G. Ejakov, J.H. Visser, R.E. Soltis, CO sensing with SnO2 thick film sensors: role of oxygen and water vapour, Thin Solid Films 436 (2003) 17–24. [16] M. Hübner, R.G. Pavelko, N. Barsan, U. Weimar, Influence of oxygen backgrounds on hydrogen sensing with SnO2 nanomaterials, Sensors and Actuators B, Available online 1 February (in press). [17] S. Pokhrel, C.E. Simion, V.S. Teodorescu, N. Barsan, U. Weimar, Synthesis, mechanism, and gas-sensing application of surfactant tailored tungsten oxide nanostructures, Advanced Functional Materials 19 (11) (2009) 1767–1774. [18] J. Kappler, A. Tomescu, N. Barsan, U. Weimar, CO consumption of Pd doped SnO2 based sensors, Thin Solid Films 391 (2001) 186–191. [19] S. Emiroglu, N. Barsan, U. Weimar, V. Hoffmann, In situ diffuse reflectance infrared spectroscopy study of CO adsorption on SnO2 , Thin Solid Films 391 (2001) 176–185. [20] S.M. Kanan, Z. Lu, J.K. Cox, G. Bernhardt, C.P. Tripp, Identification of surface sites on monoclinic WO3 powders by infrared spectroscopy, Langmuir 18 (2002) 1707–1712. [21] G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatti, F. Bregani, Characterization of tungsat–titania catalysts, Langmuir 8 (1992) 1744–1749.
Biographies Michael Hübner received his diploma in chemistry in 2008 from the University of Tübingen. Since 2008 he is working on his PhD in chemistry in the field of metal oxide based gas sensors.
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Cristian E. Simion received his BC (in 2003) and MSc (in 2005) degrees in Theoretical and Condensed Matter Physics from University of Bucharest-Faculty of Physics. Presently he is a PhD student working in the joint collaboration between the gas sensor group at the National Institute of Materials Physics, Bucharest-Romania and Institute of Physical Chemistry, Tübingen University-Germany. His field of interest is metal oxides solid state gas sensors. Alexander Haensch received his diploma in chemistry in 2008 from the University of Tübingen. Since then he is focusing on infrared spectroscopy of chemical gas sensors.
Nicolae Bârsan received in 1982 his diploma in Physics from the Faculty of Physics of the Bucharest University and in 1993 his PhD in Solid State Physics from the Institute of Atomic Physics, Bucharest, Romania. Since 1995 he is a researcher at the Institute of Physical Chemistry of the University of Tübingen and actually is in charge with the developments in the field of metal oxides based gas sensors. Udo Weimar received his diploma in physics 1989, his PhD in chemistry 1993 and his Habilitation 2002 from the University of Tübingen. He is currently the head of Gas Sensors Group at the University of Tübingen. His research interest focuses on chemical sensors as well as on multicomponent analysis and pattern recognition.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
CO sensing mechanism with WO3 based gas sensors M. Hübner a,∗ , C.E. Simion b , A. Haensch a , N. Barsan a , U. Weimar a a b
University of Tübingen, Institute of Physical Chemistry, Auf der Morgenstelle 15, 72076 Tübingen, Germany National Institute of Materials Physics, P.O. Box MG-7, 077125 Bucharest-Magurele, Romania
a r t i c l e
i n f o
Article history: Received 9 July 2010 Received in revised form 31 August 2010 Accepted 3 September 2010 Available online 20 October 2010
a b s t r a c t The interaction between CO and WO3 based gas sensors was investigated in operando conditions by using resistance, catalytic conversion and DRIFTS measurements. The experimental results are showing that the sensing involves the reduction of the metal oxide, which is the opposite of the case of SnO2 , the other extremely relevant material for semiconducting oxides based gas sensors. © 2010 Elsevier B.V. All rights reserved.
Keywords: WO3 CO sensing SnO2 Reduction
1. Introduction Along stannic oxide (SnO2 ), tungstic oxide (WO3 ) is one of the most relevant materials for the fabrication of semiconducting metal oxide based gas sensors; those two materials are the only ones used in commercial devices, the former for the detection of reducing gases and the latter for the oxidizing ones [1–3]. They are also the most studied ones; for instance at the last IMCS conference 2008 in Ohio/USA, over 30% of the oral contributions in the field of resistive gas sensors and metal oxides dealt with these two materials. Gas sensing effects recorded as changes in the resistivity of WO3 are known since long [4] and besides the good performance for the detection of NOx [5,6] it has been shown that there is good performance also for the detection of NH3 [7,8], H2 S [9], etc. Keeping in mind how relevant the material is, it is surprising that not too much is known about the sensing mechanism. There are no comprehensive reviews for WO3 as a gas sensing material; it is however known that the material is non-stoichiometric – the semiconducting effect is linked, like also for SnO2 to the donor levels in the band-gap associated to the oxygen vacancies. It is also known that the crystal chemistry of WO3 is extremely complex with two different crystalline phases in the range of temperatures where the gas sensors are normally operated (transition temperature from monoclinic to orthorhombic around 330 ◦ C) [10]. In the case of NO2 detection the mechanism is considered to consist two processes; a dominant – adsorption of NO2 via one of
the oxygen atoms on the surface leading to a negative charged surface state – and a weaker – an oxygen vacancy is filled with one atom of NO2 resulting in NO and a lattice oxygen – one [11]. In the case of reducing gases one either assumes that the reaction is taking place with pre-adsorbed oxygen [12] – mechanism applied by the authors to NH3 detection – or, like it was considered for H2 S [13,14] based on XPS studies, the surface reduction is consider to be at the origin of the sensor effect. However it is not clear if the authors think that this finding is specific for H2 S or a general feature for all reducing gases. Here, one has to note that even if the reaction between the oxide and the gas took place in realistic conditions, the spectroscopic investigation was performed with the oxide at room temperature and in UHV. In most of sensor studies only resistance measurements were used and it is not surprising that the level of basic understanding is by far more limited than in the case of SnO2 . One of the first problem to solve is the role of pre-adsorbed oxygen in sensing, which is relevant for the detection of reducing gases as a possible reaction partner and for the oxidizing one as a possible competitor. In the case of SnO2 we have shown that for CO and H2 there are two interaction paths: a direct reaction with the metal oxide, which does not involve lattice oxygen and a reaction with pre-adsorbed oxygen [15,16]. Here we are examining the sensing of CO with WO3 based sensors as a function of the oxygen background by performing conductance, catalytic conversion and DRIFTS measurements.
2. Experimental details ∗ Corresponding author. Tel.: +49 (0)7071 78766; fax: +49 (0)7071 5960. E-mail address: [email protected] (M. Hübner). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.09.040
The WO3 sensitive material was obtained by a nonaqueous sol–gel synthesis in combination with the structure directing sur-
104
M. Hübner et al. / Sensors and Actuators B 151 (2010) 103–106
Fig. 1. Time dependence of the resistance during CO exposure (10, 30, 70, 100 ppm) in the absence of oxygen at 300 ◦ C. The amount of CO and CO2 in the exhaust is shown in the upper part.
Fig. 2. Time dependence of the resistance during CO exposure (10, 30, 70, 100 ppm) in the presence of 150 ppm of oxygen. The amount of CO and CO2 in the exhaust is shown in the upper part.
factant P123, as described in [17]. The investigation of gas response tests of the thick film sensors (Film size: 50 m × 7 mm × 4 mm)screen printed on alumina substrates with Pt electrodes and heater – based on that material demonstrated good CO sensing performance. Here, three different oxygen concentrations in nitrogen (5.5 grade) with a gas flow of 500 ml/min were chosen as background conditions: <3 (below the detection limit), 150 and 22,000 ppm. The actual oxygen concentrations were monitored online by using an electrochemical oxygen analyzer (Zirox SGM400). Three sensors were exposed to 4 pulses of CO (10, 30, 70 and 100 ppm) for 3 h; for the recovery after each pulse, 4 h of background atmosphere exposure were allowed. To investigate the formation of reactions products the exhaust was connected to a photo-acoustic gas analyzer (Innova 1312). The in situ DRIFT measurements were carried out in a modified Bruker Vertex 80v spectrometer equipped with liquid nitrogen cooled MCT detector. The optical bench of the spectrometer was evacuated during the whole measurement to a stable vacuum of 1,62 mbars. To operate the gas sensor in real conditions during the infrared measurements, a home made testing chamber with gas access and electrical connectors for the read-out of the resistance was installed into the Harrick Praying Mantis DRIFT unit.
tive explanation could be the possible contribution of the remaining chemisorbed oxygen, from the previous exposure to air, to the CO2 production. We think that this effect is negligible because of the fact that one would expect the remaining chemisorbed oxygen to play a role also in the case of SnO2 , which is not the case. Additionally, if the remaining chemisorbed oxygen would play a role, this will happen just for the first CO exposure, which will eliminate it. The reduction of WO3 can be described by the following reaction (Eq. (1)):
3. Results and discussion Fig. 1 presents the time dependence of the sensor resistance, operated at 300 ◦ C, and of the gaseous composition of the exhaust during CO sensing in the almost total absence of oxygen. The sensor signals are large and the changes of the resistance determined by exposure to CO are completely reversible during the recovery period. Very interestingly, even if the oxygen concentration in the background was very low, one records CO2 formation as a result of the CO exposure. Contrary to these findings, no CO2 generation was observed in former studies [15] and experiments performed in similar conditions – no or very little oxygen – with SnO2 based sensors (not shown here) even if the sensor signals were extremely high. Besides that, the same experiments were performed just simply with uncoated substrates to exclude the possible role of the catalytic conversion on the electrodes; in the same conditions – below 3 ppm O2 – no CO2 generation was recorded. This result indicates that in the case of WO3 , on the opposite of SnO2 , the direct reduction of the material is a possible cause of the sensor signals. An alterna-
COgas + OO → CO2 gas + V•O + e−
(1)
whereas COgas describes the target gas, OO the lattice oxygen of WO3 , CO2 gas the produced carbon monoxide due to surface reduction, V•O the ionized vacancy and e− the released electron. Consequently the ionization of the oxygen vacancy donors determines the decrease of the resistance. To check the validity of the proposed mechanism one can roughly calculate if there is enough lattice oxygen to justify the recorded amount of CO2 in the exhaust. The total mass of WO3 exposed to CO is 30 mg which corresponds to a quantity of 1.3 × 10−4 mol. Exposure to 100 ppm CO over 3 h results in an average CO2 amount in the exhaust around 15 ppm (Fig. 1) leading to a total volume of 1.35 × 10−3 l. By assuming that CO2 being an ideal gas (1 mol of gas corresponds to 22.4 l under normal conditions) one gets to a quantity of produced CO2 during these 3 h of 6 × 10−5 mol. Consequently there should be enough oxygen for the monitored CO2 production. The recovery is controlled by the oxidation reaction with the oxygen still present in the background and therefore, due to its extremely low concentration, that process takes more time. This re-oxidation of the surface decreases the concentration of surface donors (Eq. (2)). 1 gas O + V•O + e− ↔ OO 2 2
(2)
By increasing the concentration of oxygen in the background (150 ppm) one record a decrease of the sensor signals and an increase of the generated CO2 (see Fig. 2). The magnitude of the sensor signal depends on the equilibrium established between the generation of oxygen vacancies and their cancellation. The decrease of the signal is therefore easy to understand because the healing of
M. Hübner et al. / Sensors and Actuators B 151 (2010) 103–106
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function changes measurements and the role of water vapor will be examined. Acknowledgments The authors are thankful to the German Research Foundation (DFG) for the financial support provided through the SPP 1299 – “Das Haut-Konzept”. The authors are also thankful to the reviewer who asked for the CO2 balance calculation as an additional validity check and, generally, for the useful suggestions provided by the reviewers and the editor during the reviewing process. References
Fig. 3. DRIFT absorbance spectra for WO3 sensor in the absence of oxygen, in the upper part (red), and in dry synthetic air, in the lower part (blue), at 300 ◦ C during exposure to 250 ppm of CO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
the oxygen vacancies is enhanced by the presence of more oxygen in the background so that the equilibrium is established at a lower surface donor concentration. The increase of the CO2 concentration can be explained by a possible catalytic conversion taking place on the Pt heater and electrodes of the substrate [18]. A spectroscopic proof for the reduction of the material under CO exposure in the absence, as well as in the present of oxygen, is provided by DRIFTS. Fig. 3 shows the absorbance spectra, calculated by taking as reference the single channel spectra acquired in the absence of CO, corresponding to the exposure to 250 pm CO in the absence of oxygen (upper part) and in dry air condition (for the experimental details on the way in which the spectra are acquired and used, see [19]). One observes in both conditions, presence and absence of oxygen, the reduction of the intensity of the bands at 2063 and 1854 cm−1 assigned to various overtones and combination modes of the oxygen and tungsten bond in the lattice of the oxide (W O) [20,21]. Besides that, the presence of CO2 and CO in the pores of the metal oxide can be slightly seen in dry air. In the absence of oxygen the formation of CO2 cannot be detected by the DRIFT measurement due to the very high absorbance of free charge carriers (also the reason for the huge difference in the absorbance between both conditions) which overlaps with the CO2 band; nevertheless, the generation of CO2 was proven by the data presented in Fig. 1. This fact proves that in the case of WO3 there is a surface reaction of CO that consumes lattice oxygen and which is important even in normal conditions for gas sensors applications, namely in air. Accordingly, we consider that for the understanding of reducing gas sensing the described material reduction of WO3 has to be taken into consideration. 4. Conclusion and outlook The CO detection with WO3 sensors most probably implies the reduction of the material. In this, the sensing very much differs from the one in which SnO2 based materials are used and this fact has to be consider in all attempts to model the gas effects. In the next steps in the investigation of reducing gases interaction with tungsten oxide, additional operando methods will be used, e.g. work
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Biographies Michael Hübner received his diploma in chemistry in 2008 from the University of Tübingen. Since 2008 he is working on his PhD in chemistry in the field of metal oxide based gas sensors.
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Cristian E. Simion received his BC (in 2003) and MSc (in 2005) degrees in Theoretical and Condensed Matter Physics from University of Bucharest-Faculty of Physics. Presently he is a PhD student working in the joint collaboration between the gas sensor group at the National Institute of Materials Physics, Bucharest-Romania and Institute of Physical Chemistry, Tübingen University-Germany. His field of interest is metal oxides solid state gas sensors. Alexander Haensch received his diploma in chemistry in 2008 from the University of Tübingen. Since then he is focusing on infrared spectroscopy of chemical gas sensors.
Nicolae Bârsan received in 1982 his diploma in Physics from the Faculty of Physics of the Bucharest University and in 1993 his PhD in Solid State Physics from the Institute of Atomic Physics, Bucharest, Romania. Since 1995 he is a researcher at the Institute of Physical Chemistry of the University of Tübingen and actually is in charge with the developments in the field of metal oxides based gas sensors. Udo Weimar received his diploma in physics 1989, his PhD in chemistry 1993 and his Habilitation 2002 from the University of Tübingen. He is currently the head of Gas Sensors Group at the University of Tübingen. His research interest focuses on chemical sensors as well as on multicomponent analysis and pattern recognition.