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A new CO2 gas sensing material
Sensors and Actuators B 95 (2003) 266–270
A new CO2 gas sensing material A. Marsal∗ , G. Dezanneau, A. Cornet, J.R. Morante Departament d’Electrònica, Universitat de Barcelona, C\Mart´ı i Franquès, 1, Barcelona 08028, Spain
Abstract A new material for CO2 sensing based on resistive changes is described. Using hydrated LaCl3 as precursor and through two different synthesis routes—simple oxidation and a sol–gel-derived method—LaOCl powders are obtained. The main sensing characteristics of these powders are analysed, with special emphasis on (a) their response to CO2 at a wide range of relative humidities and (b) their cross-sensitivity with CO. Compared with other metal oxide-based materials, lanthanum oxychloride offers a low working temperature and an improved sensor response in both dry and humid atmospheres. © 2003 Elsevier B.V. All rights reserved. Keywords: CO2 gas sensor; LaOCl
1. Introduction The control of CO2 concentration is important in many applications. Non-expensive and robust detection systems are required for air quality, food control and for early fire detection. To date, optical and electrochemical sensors have been used, but the high cost of the former and the unreliability of the latter present serious disadvantages. Solid-state gas sensors based on semiconductor metal oxides may be a promising alternative, since they offer good sensor properties and can be easily mass-produced. Several oxides have been tested, and La-doped SnO2 and BaTiO3 are reported to be the most reliable options for CO2 detection [1–4]. However, comparison of the responses of SnO2 -doped sensors described in these studies reveals discrepancies in the resistance variation. Moreover, in the presence of humidity these sensors reduce drastically their response to CO2 and no interference studies have been performed to test their sensitivity to other gases. Further analysis is required to address these questions and complementary interference analyses should be carried out. In the case of BaTiO3 , its response only becomes appreciable at CO2 concentrations above 1%, so this material is more suitable for high concentration applications. Tests performed to date suggest the need for a more thorough understanding of the interaction of CO2 molecules with the surface of metal oxide semiconductors. The LaOCl surface favours CO2 absorption through the formation of a carbonate on the basis of a lanthanum site. The present study reports an electrical analysis of this gas sensing process. ∗
Corresponding author. Fax: +34-93-402-11-48. E-mail address: [email protected] (A. Marsal). 0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00443-X
Sensor responses to CO2 concentrations ranging from 500 to 5000 ppm at a wide range of relative humidities are presented for this new CO2 sensing material, and its cross-sensitivity with CO is discussed.
2. Experimental Powders were prepared using two synthesis routes. In the first method (labelled A), a simple oxidation of previously dried LaCl3 powder was performed at 750 ◦ C for 4 h. In the second method (labelled B) nanocrystalline powders were prepared by a sol–gel route described elsewhere [5]. Briefly, acrylamide gelification is used to form an organic 3D network in which the solution of La and Cl ions is soaked. The gel obtained is first dried in a microwave oven and a thermal treatment is then applied for 5 h at 600 ◦ C to obtain the final powders. According to XRD analysis, the crystal structure of the only phase observed in Samples A and B matched with LaOCl. Sensors were prepared by depositing a mixture of an organic solvent and LaOCl powders on alumina substrates with screen-printed platinum electrodes. The sensors were then fired at 750 ◦ C in N2 to evaporate the solvent and to ensure the adherence of the sample to the alumina surface.
3. Results and discussion Though the oxidation of LaCl3 to LaOCl and La2 O3 has been described elsewhere [6,7], a temperature study of this transformation was required to determine accurately the
A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
267
Fig. 1. XRD and phase identification of LaCl3 powders at several temperatures.
optimum oxidation temperature for the formation of LaOCl. A temperature programmed XRD experiment was performed to obtain the transformation of hydrated LaCl3 initial powders to other species as a function of temperature. Fig. 1 shows the spectra obtained in this study and the identification phases at several temperatures. The evolution of spectra as a function of temperature starts with hydroxides and several hydrated compounds but at 300 ◦ C most have been converted to simple LaCl3 . From 500 to 800 ◦ C the progressive oxidation of LaCl3 to LaOCl takes place. Finally, above 800 ◦ C oxychlorides begin to transform to La2 O3 . The Pt peaks observed in the whole spectra are due to the support on which the powders were placed. As a result of this XRD experiment, the firing temperature for the simple oxidation method was set to 750 ◦ C to ensure the total conversion of LaCl3 to LaOCl and to prevent an La2 O3 phase. In the sol–gel method, the temperature was set to only 600 ◦ C because it is assumed that LaOCl is formed from trapped ions and does not follow the chemical route described. Generally, the presence of chlorides in samples is due to inefficient heat treatment. The subsequent decomposition of these compounds or even Cl ion mobility causes poor time stability. However, LaOCl—reported in the 1990s to be a catalytic material for hydrocarbon oxidation [6,7]— shows high stability even at 800 ◦ C. In fact, neither base line drift nor time stability problems were detected during sensor testing.
Sensor resistance was measured in the presence and absence of oxygen at different temperatures. The fact that the resistance increased in absence of oxygen in the measured range indicates that our material is a p-type semiconductor and that its resistance will also increase in presence of reducing gases such as CO. Fig. 2 shows the temperature dependence of the sensor response to 2000 ppm of CO2 and to 200 ppm of CO in dry air. These data show that the optimised temperature for CO2 response is around 300 ◦ C for Sample A and 260 ◦ C for Sample B; both values are slightly lower than those reported in the literature (Table 1). The slight difference in the temperature of Sensors A and B can be ascribed to the synthesis Table 1 Sensor response to 2000 ppm CO2 and operating temperature of reported studies on metal oxides CO2 gas sensors Reference
Material
Operating temperature (◦ C)
Sensor response
[1] [2] [3] [12] [4] This work (Sample B) This work (Sample A)
SnO2 –La2 O3 SnO2 –La2 O3 SnO2 –LaOCl BaTiO3 –CuO BaTiO3 –CuO–La2 O3 LaOCl
400 400 425 500 550 260
1.4 1.6 1.4 2.5a 4a 3.4
LaOCl
300
1.6
a
Response to 1% CO2 .
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A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
Fig. 2. Response of Sensors A and B to 200 ppm of CO and 2000 ppm of CO2 at several temperatures.
methods. In fact, the optimum temperature depends to an extent on the morphology of the samples and consequently, on the preparation method. The highest RCO2 /Rs.a. ratio was 1.6 for Sample A and 3.4 for Sample B. This latter value is significantly better than those reported by other authors using SnO2 or BaTiO3 in the same conditions. As regards the sensor temperature response to CO and CO2 , we observed the same curve shape, but the response to CO presents a smaller dependence and a less defined maximum than the response to CO2 . As will be discussed later, the fact that the optimum tempera-
ture is the same for both gases suggests that the response to CO is not due to the gas itself, but to its conversion to CO2 . The temperature of the sensors was set to 300 and 260 ◦ C, respectively, and their response in dry air was examined. The range of concentrations studied was 0–5000 ppm for CO2 and 0–400 ppm for CO. These results are plotted in Fig. 3, which presents the sensor response on a log–log scale. Data were fitted to the expression: R = R0 (1 + K[Cx ])−β
Fig. 3. Response of Sensors A and B to several concentrations of CO and CO2 in dry air.
(1)
A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
269
Fig. 4. Response to 200 ppm of CO and 2000 ppm of CO2 versus humidity.
in accordance with the model proposed by Clifford and Tuma [8]. In this expression, R and R0 are the resistance values measured with and without the presence of the target gas, Cx is the concentration of the gas to be measured in ppm, and β and K are the fitting parameters. This expression was deduced for n-type semiconductors, but a comparable reasoning can be made for p-type semiconductors as well. Looking at Sample B in Fig. 3, the trend of the CO fitting line is similar to that of CO2 but with a small loss of efficiency. Quantitatively, this translates into highly similar fitting parameters (Table 2). In addition, we found that optimum working temperatures were the same for both gases, so the response observed in presence of CO seems to be due to the CO2 produced during the oxidation of the CO molecule. This appears to be a reasonable explanation for the behaviour observed, but further studies in this direction are needed in order to determine which gas is interacting with the base material. In any case, the response of the sensor to CO was lower than that of SnO2 -based sensors. The latter are more sensitive to CO due to the interaction of this gas molecule with the SnO2 surface [9]. So LaOCl overcomes the typical problem of CO interference in doped SnO2 sensors. Humidity effects were examined in the ranges of 30, 50 and 70% of relative humidity at room temperature Table 2 Resume of parameters in expression (1) obtained from the fitting of experimental points
Sample A Sample B
KCO
βCO
KCO2
βCO2
0.08 0.04
0.10 0.28
0.012 0.11
0.16 0.23
(Fig. 4). A substantial increase in sensitivity to CO2 was found, and smaller changes were observed for response to CO. For Sample A, the ratio Rgas /Rs.a. increased with relative humidity until 50%, at which point the effect of humidity seemed to saturate the sensor response. On the other hand, for Sample B, the greater the relative humidity, the higher the sensor response. So humidity enhances the response of LaOCl to CO2 . Again this behaviour is rather different from that reported for SnO2 [1], in which humidity has the opposite effect, decreasing CO2 sensitivity. In fact, similar behaviour in humid atmosphere was described in a previous study in which SnO2 was doped with LaOCl [10]. There, the material was used as a catalytic additive, playing the role of antenna because of its interaction with CO2 . In that study the sensing mechanisms were analysed under dry air, and carbonates were considered to be key actors in the sensing mechanisms. Rapid formation of phases of variable composition La2 (CO3 )x (OH)2(3−x) has been reported when lanthanum species are in contact with CO2 in a humid atmosphere [11]. The formation of these phases is likely to be more favourable than the simple carbonates, leading to a higher sensor response. Moreover, the formation of these carbonates has been described as reversible, corroborating the suitability of this material for gas sensing applications.
4. Conclusions We present a CO2 gas sensor based on a new material and describe its behaviour when interacting CO as interfering compound. LaOCl performs better than other metal
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A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
oxides, offering a lower working temperature and an enhanced sensitivity to CO2 under humid conditions.
carbonates and their thermal decomposition products, Spectr. Acta 23 (1967) 1909–1915. [12] T. Ishihara, K. Kometani, Y. Mizuhara, Y. Takita, A new type CO2 gas sensor based on capacitance changes, Sens. Actuators B 5 (1991) 97–102.
Acknowledgements The present work was supported by the project FederMicroaire and by the Departament d’Universitats, Recerca i Societat de la Informació of the Generalitat de Catalunya.
References [1] D.H. Kim, J.Y. Yoon, H.C. Park, K.H. Kim, CO2 sensing characteristics of SnO2 thick film by coating lanthanum oxide, Sens. Actuators B 62 (2000) 61–66. [2] N. Mizuno, T. Yoshioka, K. Kato, M. Iwamoto, CO2 sensing characteristics of SnO2 element modified by La2 O3 , Sens. Actuators B 13–14 (1993) 473–475. [3] H.A. Diagne, M. Lumbreras, Elaboration and characterization of tin oxide lanthanum oxide mixed layers prepared by electrostatic spray pyrolysis technique, Sens. Actuators B 3957 (2001) 1–8. [4] M.S. Lee, J.U. Meyer, A new process for fabricating CO2 sensing layers based on BaTiO3 and additives, Sens. Actuators B 68 (2000) 293–299. [5] G. Dezanneau, A. Sin, H. Roussel, H. Vincent, M. Audier, Synthesis and characterisation of La1−x MnO3±δ nanopowders prepared by acrylamide polymerisation, Solid State Commun. 121 (2002) 133– 137. [6] P. Chanaud, A. Julbe, A. Larbot, C. Guizard, L. Cot, H. Borges, A. Giroir Fendler, C. Mirodatos, Catalytic membrane reactor for oxidative coupling of methane. Part 1. Preparation and characterisation of LaOCl membranes, Catal. Today 25 (1995) 225–230. [7] C.T. Au, H. He, S.Y. Lai, C.F. Ng, The oxidative coupling of methane over BaCO3 /LaOCl catalysts, Appl. Catal. A 159 (1997) 133–145. [8] P.K. Clifford, D.T. Tuma, Sens. Actuators 3 (1983) 233. [9] K. Fukui, M. Nakane, CO gas sensors based on Au–La2 O3 loaded SnO2 ceramic, Sens. Actuators B 24–25 (1995) 486–490. [10] A. Marsal, A. Corent, J.R. Morante, Study of the CO and humidity interference in La doped tin oxide CO2 gas sensor, Sens. Actuators B, in press. [11] J.A. Goldsmith, S.D. Ross, Factors affecting the infra-red spectra of some planar anions with D3h symmetry. III. The spectra of rare-earth
Biographies Andreu Marsal graduated in physics at the University of Barcelona in 2000. He is PhD student at this University, studying structural and gas sensing properties of metal oxide nanopowders for applications in gas-sensor devices. Guilhem Dezanneau received the engineering degree in materials science from the Institut National Polytechnique de Grenoble (INPG), France, in 1996, and the same year the degree in physics from the Université Joseph Fourier, Grenoble, France. He obtained the PhD degree in 2001 from the Laboratoire des Matériaux et du Génie Physique of the CNRS, France. Since Feb. 2001, he occupies a post-doctoral position at the Electronic Department of the University of Barcelona. His research concerns the synthesis, characterization and modelisation of oxide materials for gas sensing and SOFC applications. Albert Cornet graduated in physics at the University of Barcelona in 1977. He received his PhD in 1982 from the University of Barcelona and in 1983 from the University Paul Sabatier of Toulouse. From 1982 until now he has been employed in the Physics faculty of the University of Barcelona, where he has been involved in the research on characterization of semiconductor structures, gas sensors and other advanced materials used in micro or optoelectronic devices. At the present time he is Professor in the Electronics Department and member of the staff of the Electronics Materials and Engineering Laboratory. Joan Ramon Morante received the PhD in physics from the University of Barcelona in 1980. He joined the Department of Applied Physics and Electronics of the same university in 1977, and in 1986 he was appointed full professor of electronics in this department. The same year, he founded the LCMM (Laboratory of Characterization of Materials for Microelectronics). In 1991 was founded EME (Engineering and Electronic Materials research group), now Department of Electronics, which leads. His current research activities and projects are focused on the fields of characterization of electronic materials and processes, micromachined Si-based sensors and actuators, gas sensors and semiconductors devices. He is author or co-author of more than 400 scientific and technical papers.
A new CO2 gas sensing material A. Marsal∗ , G. Dezanneau, A. Cornet, J.R. Morante Departament d’Electrònica, Universitat de Barcelona, C\Mart´ı i Franquès, 1, Barcelona 08028, Spain
Abstract A new material for CO2 sensing based on resistive changes is described. Using hydrated LaCl3 as precursor and through two different synthesis routes—simple oxidation and a sol–gel-derived method—LaOCl powders are obtained. The main sensing characteristics of these powders are analysed, with special emphasis on (a) their response to CO2 at a wide range of relative humidities and (b) their cross-sensitivity with CO. Compared with other metal oxide-based materials, lanthanum oxychloride offers a low working temperature and an improved sensor response in both dry and humid atmospheres. © 2003 Elsevier B.V. All rights reserved. Keywords: CO2 gas sensor; LaOCl
1. Introduction The control of CO2 concentration is important in many applications. Non-expensive and robust detection systems are required for air quality, food control and for early fire detection. To date, optical and electrochemical sensors have been used, but the high cost of the former and the unreliability of the latter present serious disadvantages. Solid-state gas sensors based on semiconductor metal oxides may be a promising alternative, since they offer good sensor properties and can be easily mass-produced. Several oxides have been tested, and La-doped SnO2 and BaTiO3 are reported to be the most reliable options for CO2 detection [1–4]. However, comparison of the responses of SnO2 -doped sensors described in these studies reveals discrepancies in the resistance variation. Moreover, in the presence of humidity these sensors reduce drastically their response to CO2 and no interference studies have been performed to test their sensitivity to other gases. Further analysis is required to address these questions and complementary interference analyses should be carried out. In the case of BaTiO3 , its response only becomes appreciable at CO2 concentrations above 1%, so this material is more suitable for high concentration applications. Tests performed to date suggest the need for a more thorough understanding of the interaction of CO2 molecules with the surface of metal oxide semiconductors. The LaOCl surface favours CO2 absorption through the formation of a carbonate on the basis of a lanthanum site. The present study reports an electrical analysis of this gas sensing process. ∗
Corresponding author. Fax: +34-93-402-11-48. E-mail address: [email protected] (A. Marsal). 0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00443-X
Sensor responses to CO2 concentrations ranging from 500 to 5000 ppm at a wide range of relative humidities are presented for this new CO2 sensing material, and its cross-sensitivity with CO is discussed.
2. Experimental Powders were prepared using two synthesis routes. In the first method (labelled A), a simple oxidation of previously dried LaCl3 powder was performed at 750 ◦ C for 4 h. In the second method (labelled B) nanocrystalline powders were prepared by a sol–gel route described elsewhere [5]. Briefly, acrylamide gelification is used to form an organic 3D network in which the solution of La and Cl ions is soaked. The gel obtained is first dried in a microwave oven and a thermal treatment is then applied for 5 h at 600 ◦ C to obtain the final powders. According to XRD analysis, the crystal structure of the only phase observed in Samples A and B matched with LaOCl. Sensors were prepared by depositing a mixture of an organic solvent and LaOCl powders on alumina substrates with screen-printed platinum electrodes. The sensors were then fired at 750 ◦ C in N2 to evaporate the solvent and to ensure the adherence of the sample to the alumina surface.
3. Results and discussion Though the oxidation of LaCl3 to LaOCl and La2 O3 has been described elsewhere [6,7], a temperature study of this transformation was required to determine accurately the
A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
267
Fig. 1. XRD and phase identification of LaCl3 powders at several temperatures.
optimum oxidation temperature for the formation of LaOCl. A temperature programmed XRD experiment was performed to obtain the transformation of hydrated LaCl3 initial powders to other species as a function of temperature. Fig. 1 shows the spectra obtained in this study and the identification phases at several temperatures. The evolution of spectra as a function of temperature starts with hydroxides and several hydrated compounds but at 300 ◦ C most have been converted to simple LaCl3 . From 500 to 800 ◦ C the progressive oxidation of LaCl3 to LaOCl takes place. Finally, above 800 ◦ C oxychlorides begin to transform to La2 O3 . The Pt peaks observed in the whole spectra are due to the support on which the powders were placed. As a result of this XRD experiment, the firing temperature for the simple oxidation method was set to 750 ◦ C to ensure the total conversion of LaCl3 to LaOCl and to prevent an La2 O3 phase. In the sol–gel method, the temperature was set to only 600 ◦ C because it is assumed that LaOCl is formed from trapped ions and does not follow the chemical route described. Generally, the presence of chlorides in samples is due to inefficient heat treatment. The subsequent decomposition of these compounds or even Cl ion mobility causes poor time stability. However, LaOCl—reported in the 1990s to be a catalytic material for hydrocarbon oxidation [6,7]— shows high stability even at 800 ◦ C. In fact, neither base line drift nor time stability problems were detected during sensor testing.
Sensor resistance was measured in the presence and absence of oxygen at different temperatures. The fact that the resistance increased in absence of oxygen in the measured range indicates that our material is a p-type semiconductor and that its resistance will also increase in presence of reducing gases such as CO. Fig. 2 shows the temperature dependence of the sensor response to 2000 ppm of CO2 and to 200 ppm of CO in dry air. These data show that the optimised temperature for CO2 response is around 300 ◦ C for Sample A and 260 ◦ C for Sample B; both values are slightly lower than those reported in the literature (Table 1). The slight difference in the temperature of Sensors A and B can be ascribed to the synthesis Table 1 Sensor response to 2000 ppm CO2 and operating temperature of reported studies on metal oxides CO2 gas sensors Reference
Material
Operating temperature (◦ C)
Sensor response
[1] [2] [3] [12] [4] This work (Sample B) This work (Sample A)
SnO2 –La2 O3 SnO2 –La2 O3 SnO2 –LaOCl BaTiO3 –CuO BaTiO3 –CuO–La2 O3 LaOCl
400 400 425 500 550 260
1.4 1.6 1.4 2.5a 4a 3.4
LaOCl
300
1.6
a
Response to 1% CO2 .
268
A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
Fig. 2. Response of Sensors A and B to 200 ppm of CO and 2000 ppm of CO2 at several temperatures.
methods. In fact, the optimum temperature depends to an extent on the morphology of the samples and consequently, on the preparation method. The highest RCO2 /Rs.a. ratio was 1.6 for Sample A and 3.4 for Sample B. This latter value is significantly better than those reported by other authors using SnO2 or BaTiO3 in the same conditions. As regards the sensor temperature response to CO and CO2 , we observed the same curve shape, but the response to CO presents a smaller dependence and a less defined maximum than the response to CO2 . As will be discussed later, the fact that the optimum tempera-
ture is the same for both gases suggests that the response to CO is not due to the gas itself, but to its conversion to CO2 . The temperature of the sensors was set to 300 and 260 ◦ C, respectively, and their response in dry air was examined. The range of concentrations studied was 0–5000 ppm for CO2 and 0–400 ppm for CO. These results are plotted in Fig. 3, which presents the sensor response on a log–log scale. Data were fitted to the expression: R = R0 (1 + K[Cx ])−β
Fig. 3. Response of Sensors A and B to several concentrations of CO and CO2 in dry air.
(1)
A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
269
Fig. 4. Response to 200 ppm of CO and 2000 ppm of CO2 versus humidity.
in accordance with the model proposed by Clifford and Tuma [8]. In this expression, R and R0 are the resistance values measured with and without the presence of the target gas, Cx is the concentration of the gas to be measured in ppm, and β and K are the fitting parameters. This expression was deduced for n-type semiconductors, but a comparable reasoning can be made for p-type semiconductors as well. Looking at Sample B in Fig. 3, the trend of the CO fitting line is similar to that of CO2 but with a small loss of efficiency. Quantitatively, this translates into highly similar fitting parameters (Table 2). In addition, we found that optimum working temperatures were the same for both gases, so the response observed in presence of CO seems to be due to the CO2 produced during the oxidation of the CO molecule. This appears to be a reasonable explanation for the behaviour observed, but further studies in this direction are needed in order to determine which gas is interacting with the base material. In any case, the response of the sensor to CO was lower than that of SnO2 -based sensors. The latter are more sensitive to CO due to the interaction of this gas molecule with the SnO2 surface [9]. So LaOCl overcomes the typical problem of CO interference in doped SnO2 sensors. Humidity effects were examined in the ranges of 30, 50 and 70% of relative humidity at room temperature Table 2 Resume of parameters in expression (1) obtained from the fitting of experimental points
Sample A Sample B
KCO
βCO
KCO2
βCO2
0.08 0.04
0.10 0.28
0.012 0.11
0.16 0.23
(Fig. 4). A substantial increase in sensitivity to CO2 was found, and smaller changes were observed for response to CO. For Sample A, the ratio Rgas /Rs.a. increased with relative humidity until 50%, at which point the effect of humidity seemed to saturate the sensor response. On the other hand, for Sample B, the greater the relative humidity, the higher the sensor response. So humidity enhances the response of LaOCl to CO2 . Again this behaviour is rather different from that reported for SnO2 [1], in which humidity has the opposite effect, decreasing CO2 sensitivity. In fact, similar behaviour in humid atmosphere was described in a previous study in which SnO2 was doped with LaOCl [10]. There, the material was used as a catalytic additive, playing the role of antenna because of its interaction with CO2 . In that study the sensing mechanisms were analysed under dry air, and carbonates were considered to be key actors in the sensing mechanisms. Rapid formation of phases of variable composition La2 (CO3 )x (OH)2(3−x) has been reported when lanthanum species are in contact with CO2 in a humid atmosphere [11]. The formation of these phases is likely to be more favourable than the simple carbonates, leading to a higher sensor response. Moreover, the formation of these carbonates has been described as reversible, corroborating the suitability of this material for gas sensing applications.
4. Conclusions We present a CO2 gas sensor based on a new material and describe its behaviour when interacting CO as interfering compound. LaOCl performs better than other metal
270
A. Marsal et al. / Sensors and Actuators B 95 (2003) 266–270
oxides, offering a lower working temperature and an enhanced sensitivity to CO2 under humid conditions.
carbonates and their thermal decomposition products, Spectr. Acta 23 (1967) 1909–1915. [12] T. Ishihara, K. Kometani, Y. Mizuhara, Y. Takita, A new type CO2 gas sensor based on capacitance changes, Sens. Actuators B 5 (1991) 97–102.
Acknowledgements The present work was supported by the project FederMicroaire and by the Departament d’Universitats, Recerca i Societat de la Informació of the Generalitat de Catalunya.
References [1] D.H. Kim, J.Y. Yoon, H.C. Park, K.H. Kim, CO2 sensing characteristics of SnO2 thick film by coating lanthanum oxide, Sens. Actuators B 62 (2000) 61–66. [2] N. Mizuno, T. Yoshioka, K. Kato, M. Iwamoto, CO2 sensing characteristics of SnO2 element modified by La2 O3 , Sens. Actuators B 13–14 (1993) 473–475. [3] H.A. Diagne, M. Lumbreras, Elaboration and characterization of tin oxide lanthanum oxide mixed layers prepared by electrostatic spray pyrolysis technique, Sens. Actuators B 3957 (2001) 1–8. [4] M.S. Lee, J.U. Meyer, A new process for fabricating CO2 sensing layers based on BaTiO3 and additives, Sens. Actuators B 68 (2000) 293–299. [5] G. Dezanneau, A. Sin, H. Roussel, H. Vincent, M. Audier, Synthesis and characterisation of La1−x MnO3±δ nanopowders prepared by acrylamide polymerisation, Solid State Commun. 121 (2002) 133– 137. [6] P. Chanaud, A. Julbe, A. Larbot, C. Guizard, L. Cot, H. Borges, A. Giroir Fendler, C. Mirodatos, Catalytic membrane reactor for oxidative coupling of methane. Part 1. Preparation and characterisation of LaOCl membranes, Catal. Today 25 (1995) 225–230. [7] C.T. Au, H. He, S.Y. Lai, C.F. Ng, The oxidative coupling of methane over BaCO3 /LaOCl catalysts, Appl. Catal. A 159 (1997) 133–145. [8] P.K. Clifford, D.T. Tuma, Sens. Actuators 3 (1983) 233. [9] K. Fukui, M. Nakane, CO gas sensors based on Au–La2 O3 loaded SnO2 ceramic, Sens. Actuators B 24–25 (1995) 486–490. [10] A. Marsal, A. Corent, J.R. Morante, Study of the CO and humidity interference in La doped tin oxide CO2 gas sensor, Sens. Actuators B, in press. [11] J.A. Goldsmith, S.D. Ross, Factors affecting the infra-red spectra of some planar anions with D3h symmetry. III. The spectra of rare-earth
Biographies Andreu Marsal graduated in physics at the University of Barcelona in 2000. He is PhD student at this University, studying structural and gas sensing properties of metal oxide nanopowders for applications in gas-sensor devices. Guilhem Dezanneau received the engineering degree in materials science from the Institut National Polytechnique de Grenoble (INPG), France, in 1996, and the same year the degree in physics from the Université Joseph Fourier, Grenoble, France. He obtained the PhD degree in 2001 from the Laboratoire des Matériaux et du Génie Physique of the CNRS, France. Since Feb. 2001, he occupies a post-doctoral position at the Electronic Department of the University of Barcelona. His research concerns the synthesis, characterization and modelisation of oxide materials for gas sensing and SOFC applications. Albert Cornet graduated in physics at the University of Barcelona in 1977. He received his PhD in 1982 from the University of Barcelona and in 1983 from the University Paul Sabatier of Toulouse. From 1982 until now he has been employed in the Physics faculty of the University of Barcelona, where he has been involved in the research on characterization of semiconductor structures, gas sensors and other advanced materials used in micro or optoelectronic devices. At the present time he is Professor in the Electronics Department and member of the staff of the Electronics Materials and Engineering Laboratory. Joan Ramon Morante received the PhD in physics from the University of Barcelona in 1980. He joined the Department of Applied Physics and Electronics of the same university in 1977, and in 1986 he was appointed full professor of electronics in this department. The same year, he founded the LCMM (Laboratory of Characterization of Materials for Microelectronics). In 1991 was founded EME (Engineering and Electronic Materials research group), now Department of Electronics, which leads. His current research activities and projects are focused on the fields of characterization of electronic materials and processes, micromachined Si-based sensors and actuators, gas sensors and semiconductors devices. He is author or co-author of more than 400 scientific and technical papers.