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The effect of sintering and Pd-doping on the conversion of CO to CO2 on SnO2 gas sensor materials
Sensors and Actuators B 44 (1997) 446 – 451
The effect of sintering and Pd-doping on the conversion of CO to CO2 on SnO2 gas sensor materials L. Delabie a,*, M. Honore´ a, S. Lenaerts a, G. Huyberechts a, J. Roggen a, G. Maes b a
b
Interuni6ersity Microelectronics Centre, Kapeldreef 75, B-3001, Leu6en, He6erlee, Belgium K.U. Leu6en, Department of Chemistry, Celestijnenlaan 200F, B-3001, Leu6en, He6erlee, Belgium Accepted 12 May 1997
Abstract The principal aim of this work is to study the effect of the processes of sintering and Pd doping of SnO2 gas sensor materials on the conversion of CO to CO2. For this purpose, the gas phase above screen printed sensor material is investigated using FTIR spectroscopy, while surface area, porosity and particle size measurements are performed on the SnO2 powders. During sintering, larger agglomerates of primary particles are formed, which results in a larger conversion degree of CO. The effect of Pd doping of the tin dioxide film on the CO conversion is more pronounced. The transformation of CO starts at a lower temperature and the conversion degree increases remarkably. © 1997 Elsevier Science S.A. Keywords: Sintering; Pd doping; SnO2 gas sensor; CO; CO2
1. Introduction Tin dioxide is a n-type semiconductor with broad forbidden energy gap. This material is suitable for gas sensing applications because reaction of reducing gases with adsorbed oxygen species induces a conductivity change. The physicochemical sensor mechanism can be described by two separate reaction series [1]. The first one consists of so-called ‘sensitivation reactions’ in which the SnO2 material is activated in air by an increased concentration of conductance electrons in surface states associated with ionosorbed oxygen species O2− and O − . These adsorbed species create an excess electron concentration in the contact zone between the material domains, which results in a conductivity decrease due to the potential barrier between the negative surface charges and the positive donor ions in the bulk of the material. In the second reaction sequence, surface oxygen species react with reducing gases from the surrounding atmosphere, e.g. CO, NH3 or CH4. Physisorption of the reducing molecule is followed by reaction with O − species, the enthalpy of the latter reaction allowing the transfer of the released * Corresponding author. Tel.: + 32 16 281262; fax: + 32 16 281501; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 7 0 - 6
electron to the conductance band of SnO2, and the desorption of the product molecule. Despite several proofs for the suitability of SnO2 for sensor applications [2], the SnO2 gas sensor still suffers from lack of specificity, sensitivity and stability. During the preparation process of the thick film gas sensor, a variety of process variables can influence the properties of the tin dioxide material and, as a consequence, also those of the sensor [3]. In this work, the effect of sintering time and subsequent cooling down rate on the particle size and the porosity of the powder, as well as on the conversion degree of CO to CO2 above the screen printed thick films, is studied. According to the recent literature, doping of tin dioxide with palladium has a positive effect on the CO sensitivity [4–10]. This leads to a second aspect investigated in this work: the effect of Pd doping on the CO to CO2 conversion degree.
2. Experimental Pure SnO2 was prepared following the procedure generally used for gas sensor materials [3]: by precipitation by ammonia of a SnCl4.5H2O water solution, followed by washing and calcination. This calcination
L. Delabie et al. / Sensors and Actuators B 44 (1997) 446–451
process is carried out at a temperature of 550°C, for 6 h. The screen printed films were obtained by printing a paste of the SnO2 powder mixed with a polymer solution on an alumina substrate, similar to the screen printing process used for fabricating the thick film SnO2 sensor. The gas phase above the screen printed SnO2 was investigated using FTIR spectroscopy. The IR spectra were collected using a Mattson Galaxy 7001 spectrometer equipped with a liquid N2 cooled semiconductor MCT detector. An Aabspec 2000 sample module allowing variation of the surrounding atmosphere, as well as a controlled temperature increase of the sample up to 400°C during collection of the spectrum was used. Temperature measurement of the sample was performed using a chromel/alumel thermocouple. A gas mixture of 4 vol % CO in dry air was used during the experiments. The gas flow was monitored by means of massflow controllers (MKS 147B) regulated by a gas flow control unit. A preliminary heating step up to 400°C was performed on the sample before each spectroscopic experiment. During this heating step the physisorbed H2O and most of the isolated and H-bonded surface OH groups disappeared from the SnO2 surface [11]. Surface area, pore size and pore volume of the powders were obtained by evaluation of the nitrogen adsorption–desorption data from a Micromeritics ASAP 2000 apparatus. The particle size distribution was obtained from laser diffraction particle size analysis using a Malvern Mastersizer X instrument.
3. Results and discussion
3.1. Effect of sintering Sintering of screen printed tin dioxide films constitutes the last step in the thermal treatment of thick film sensors. Sintering of both the powders and the screen printed films was performed in a tube furnace (Lenton), characterised by a maximum central furnace temperature of 850°C. Four different temperature profiles were applied. In each of them a temperature increase from room temperature up to 850°C was realised in 50 min. For the samples further referred to as A30 or A10, the temperature of 850°C was maintained for 30 or 10 min,
447
Fig. 1. Particle size distribution of non-sintered and differently sintered tin dioxide powders. represents a non-sintered sample, " is powder A30 B4, is powder A30 B1 and is powder A10 B1. Error bars represent the standard deviation (3 measurements were performed).
respectively. A second parameter variation was the cooling down rate: B1 refers to a sample that has been cooled down in 5 min, whereas B4 films have been cooled down to room temperature following a linear temperature profile of 60 min. Both the powder and the screen printed film were subjected to the same sintering profile. Fig. 1 demonstrates that the portion of smaller particles (diameter B 1 mm) decreases after sintering while the volumetric fraction of particles larger than 7 mm increases. This can be explained by the pure physical process of sintering in which the SnO2 particles squeeze together and form larger agglomerates. The BET surface area of the non-sintered powder is much higher than that of the sintered powders. The BJH pore surface and the BJH pore 6olume decrease during the process of sintering while the pore diameter increases (Table 1). All these results are consistent with the explanation of the formation of larger agglomerates during sintering, since a collection of large particles has a smaller surface area. Furthermore, although the number of pores is smaller, the pores themselves are larger in the case of larger particles. On the basis of these results no difference can be made between the powders
Table 1 Parameters resulting from BET-surface area and porosity measurements on tin dioxide powders Powdera
BET surface area (m2 g−1)
Pore surface (m2 g−1)
Pore volume (cm3 g−1)
Pore diameter (nm)
Non-sintered A30 B4 A30 B1 A10 B4 A10 B1
25.87 8.41 8.96 11.32 10.07
27.1 7.4 8.1 10.2 9.1
0.154 0.099 0.115 0.133 0.121
22.8 53.2 56.3 52.1 53.1
a
Specifications for the different powders are described in the text.
448
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tration of bulk and surface vacancies. These changes can also effect the catalytic activity of the samples [15].
3.2. Effect of Pd doping
Fig. 2. Catalytic conversion of CO to CO2 at 400°C on non-sintered SnO2 (4% CO in air, flow rate 200 ml min − 1).
that have been cooled down at different rates. In contrast, a distinction can be made between the powders subjected to different sintering times. The longer the powder is kept at the high temperature, the larger the agglomerates are and the smaller the number of pores is. The influence of sintering on the catalytic properties of the material can be investigated using FTIR spectroscopy. Gaseous CO has a rotational – vibrational absorption band in the infrared spectral region centred at 2143 cm − 1 [12]. The mean feature of the infrared spectrum of CO2 is a strong absorption band at 2349 cm − 1 [13] (Fig. 2). The latter absorption, corresponding to the asymmetric stretching n2 mode of CO2, exhibits much less rotational structure than the CO band, which is due to the noticeably smaller rotational constant of CO2 (B= 0.393 cm − 1) compared to CO (B=1.925 cm − 1) [14]. These experiments are performed at different gas flow rates using a sample temperature of 400°C. The area of the CO and CO2 bands are measured by integrating over the whole absorption bands. It is clear that the non-sintered film shows a smaller degree of transformation of CO to CO2 than the sintered films (Fig. 3). This effect is most expressed at small gas flow rates. The difference can be explained by the larger pores in the sintered films, resulting in a larger area of SnO2 in contact with CO. However, the pore accessibility is not the only important parameter, because sufficient contact time (small flow rate) must be ensured in order that a diffusion layer over the SnO2 pore structure may be formed. On the basis of these results, it is rather difficult to differentiate between the differently sintered films. Another possible effect of sintering, beside these morphological changes, is that sintering at high temperature can influence the concen-
Screen printed tin dioxide films were doped with Pd. In this case, the standard temperature profile (IMEC 850A) during sintering was used: the maximum temperature of 850°C was reached with a temperature increase of 36°C min − 1, the films were kept at this temperature for 10 min and the cooling down rate was 42°C min − 1. During doping, the concentration of palladium in the contacting solutions is such as to result in a 1.2 wt.% Pd to SnO2, when complete deposition should occur. For this purpose, we used either PdCl2 or Pd(C5O2H7)2. PdCl2 was dissolved in water, while Pd(C5O2H7)2 was used in chloroform solution. The parameter investigated in this work was the impregnation time: 1, 5, 15, 30, 60, 180 or 1140 min. After the impregnation, the samples were kept at a temperature of 510°C for 20 or 40 min. This step is carried out to allow the dissociation of the inorganic or organic Pd compounds. Again, the catalytic properties of the different samples were studied using FTIR spectroscopy. However, the gas flow rate was kept constant at a value of 20 ml min − 1 in this case. Figs. 4 and 5 illustrate the effect of the impregnation time. At 400°C no difference in CO conversion can be made between the differently doped films. However, the transformation of CO to CO2 is distinctively higher for the Pd doped films than for the non-doped films (Fig. 4). Above a non-doped sample, a conversion of CO of
Fig. 3. Effect of sintering SnO2 on the CO to CO2 conversion measured by FTIR spectroscopy. represents a non-sintered sample, " is powder A30 B4, is powder A30 B1 and is powder A10 B1. Error bars are based on a relative error of integration over the absorption band of 5%.
L. Delabie et al. / Sensors and Actuators B 44 (1997) 446–451
Fig. 4. Conversion of CO to CO2 at 400°C above differently Pd doped SnO2 films. The white signs represent conversions above samples doped with PdCl2, the black signs represent conversions above films doped with Pd(C5O2H7)2. Samples kept at 510°C for 20 or 40 min are represented by or , respectively. Error bars are based on a relative error of integration over the absorption band of 5%.
34.4% is measured compared to a conversion of about 57% above a doped sample. At 200°C, the conversion of CO is clearly dependent on the impregnation time. The longer the impregnation time is, the larger the conversion of CO to CO2 is (Fig. 5). Again, the difference between the non-doped films and the Pd doped films is large, at this temperature a non-doped sample shows no CO2 formation at all. The lowest degree of conversion is, at each impregnation time, observed for samples impregnated with PdCl2 which are kept at 510°C for only 20 min. When these samples are kept at
Fig. 5. Conversion of CO to CO2 at 200°C above differently Pd doped SnO2 films. The white signs represent conversions above samples doped with PdCl2, the black signs represent conversions above films doped with Pd(C5O2H7)2. Samples kept at 510°C for 20 or 40 min are represented by or , respectively. Error bars are based on a relative error of integration over the absorption band of 5%.
449
Fig. 6. Influence of the temperature on the conversion of CO to CO2 above screen printed SnO2. Black squares represent a non-doped sample, white signs represent a sample doped with PdCl2, black signs represent samples doped with Pd(C5O2H7)2. Samples with an impregnation time of 15 min are represented by and indicates an impregnation time of 60 min. All samples were kept at 510°C for 40 min. Error bars are based on a relative error of integration over the absorption band of 5%.
510°C for 40 min, the conversion degree to CO2 is larger. For films doped with Pd(C5O2H7)2, there is little difference in CO conversion between samples kept at 510°C for different times. Pd(C5O2H7)2 doped films show a higher conversion degree than films impregnated with PdCl2. There are clearly two main factors contributing to the observed differences in conversion degree between the different samples: the amount of palladium in the film and the form under which the dopant is present in the sample. Due to the use of PdCl2 in water solution and Pd(C5O2H7)2 in chloroform solution, a different amount of Pd can be present in a sample, even at the same impregnation time, due to differences in solubility. Secondly, the palladium in the sample can still be present as undissociated PdCl2 or Pd(C5O2H7)2, respectively, and the amount of dissociation can differ between the two Pd compounds. Also the palladium can be present as Pd0 or PdO. Maybe, the latter two points may explain the difference between the films kept at 510°C for a different time (Fig. 5). Further experiments will be performed to gain more insight in these matters. The effect of the sample temperature is analysed in Fig. 6. In case of the doped screen printed films, CO conversion is observed above 150°C, while a non-doped film shows conversion only above 250°C. At all temperatures, the conversion of CO is definitely higher on a doped sample than on a non-doped sample. Above 300°C the conversion above the differently doped films reaches the same value and does not increase any more under the current test circumstances. It is important to note that only the catalytic properties of the different materials have been studied here.
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However, the sensor response is based on electrical changes in the SnO2 material. Sintering can have an influence on the electrical conductivity of the tin dioxide [16,17]. The effect of Pd doping of screen printed tin dioxide films on the sensor response, will be studied by conductivity measurements on the SnO2 sensor. Also, the development of a new measuring device allowing a combination of the measurement of the catalytic properties and the sensor response, is in due course. This device should allow to perform an application oriented, fundamental research of the effect of Pd doping [18].
4. Conclusions Investigations of the effect of the sintering process and of palladium doping of tin dioxide gas sensor materials on the catalytic CO to CO2 conversion has allowed to draw the following conclusions. The sintering process squeezes together different SnO2 grains into larger agglomerates. This results in a smaller number of larger pores and a smaller surface area. The CO to CO2 conversion is increased after the sintering process, which can be explained by the more ‘accessible’ structure of the sintered film. Pd-impregnated tin dioxide thick films show a remarkably higher conversion of CO in comparison to non-doped SnO2 films, and this conversion starts at a lower temperature.
[7] H. Torvela, J. Huusko, V. Lantto, Reduction of the interference caused by NO and SO2 in the CO response of Pd-catalysed SnO2 combustion gas sensors, Sensors and Actuators B 4 (1991) 479 – 484. [8] F.J. Schmitte, G. Wiegleb, Conductivity behaviour of thick-film tin dioxide gas sensors, Sensors and Actuators B 4 (1991) 473 – 477. [9] G. Martinelli, M.C. Carotta, A study of the conductance and capacitance of pure and Pd-doped SnO2 thick films, Sensors and Actuators B 18-19 (1994) 720 – 723. [10] G. Tournier, C. Pijolat, R. Lalauze, B. Patissier, Selective detection of CO and CH4 with gas sensors using SnO2 doped with palladium, Sensor and Actuators B 26 (1-3) (1995) 24 –28. [11] P.G. Harrison, A. Guest, Tin Oxide Surfaces. Part 17. An infrared and thermogravimetric analysis of the thermal dehydration of tin (IV) oxide gel, J. Chem. Soc. Faraday Trans I 83 (1987) 3383 – 3397. [12] G. Herzberg, Molecular Spectra and Molecular Structure, I, 2nd ed., Van Nostrand Reinhold, New York, 1950, p. 127. [13] G. Herzberg, Molecular Spectra and Molecular Structure, II, Van Nostrand Reinhold, New York, 1945, p. 272. [14] G. Herzberg, Molecular Spectra and Molecular Structure, II, Van Nostrand Reinhold, New York, 1945, p. 21. [15] W. Go¨pel, Chemisorption and charge transfer at semiconductor surfaces: implications for designing gas sensors, Prog. Surf. Sci. 20 (1985) 9 – 103. [16] M.S. Dutraive, R. Lalauze, C. Pijolat, Sintering, catalytic effects and defect chemistry in polycrystalline tin dioxide, Sensors and Actuators B 26-27 (1995) 38 – 44. [17] G. Pfaff, Effect of powder preparation and sintering on the electrical properties of tin dioxide-based ceramic gas sensors, Sensors and Actuators B 20 (1994) 43 – 48. [18] K.D. Schierbaum, U. Weimar, W. Go¨pel, R. Kowalkowski, Conductance, work function and catalytic activity of SnO2-based gas sensors, Sensors and Actuators B 3 (1991) 205 – 214.
Acknowledgements Biographies The authors are grateful to the Belgian I.W.T. (L.D.) and N.F.W.O. (G.M.) for fellowships obtained and for financial support (N.F.W.O.).
References [1] P.T. Moseley, J.O.W. Norris, D.E. Williams, Techniques and mechanisms, in: A. Hilger (Ed.), Gas Sensing, IOP, Bristol, 1991, p. 61. [2] G. Huyberechts, M. Van Muylder, M. Honore´, J. Desmet, J. Roggen, Development of a thick film ammonia sensor for livestock buildings, Sensors and Actuators B 18-19 (1994) 296 – 299. [3] M. Honore´, S. Lenaerts, J. Desmet, G. Huyberechts, J. Roggen, Synthesis and characterisation of tin dioxide powders for the realisation of thick film sensors, Sensors and Actuators B 18-19 (1994) 621 – 624. [4] B.S. Chiou, J. Li, J.G. Duh, Characteristics of Pd-doped tin oxide ceramics in response to CO gas, J. Electron. Mater. 17 (6) (1988) 485 – 492. [5] J.G. Duh, J.W. Jou, B.S. Chiou, Catalytic and gas sensing characteristics in Pd-doped SnO2, J. Electrochem. Soc. 136 (9) (1989) 2740 – 2747. [6] S. Semancik, T.B. Fryberger, Model studies of SnO2-based gas sensors: vacancy defects and Pd additive effects, Sensors and Actuators B 1 (1990) 97–102.
Lieselot Delabie obtained a licentiate in Chemistry in 1995 at the K.U. Leuven. She is active in the sensor research at IMEC with main activities in the field of gas sensors. Main interests are in-situ FT–IR spectroscopy, gas–solid interactions and physico-chemical characterisation techniques. Mia Honore´ obtained a degree in industrial chemical engineering from HTI, Oostende in 1974. After research work on low temperature resistor thick film pastes at Barco and K.U. Leuven, she joined IMEC and is active in the development of thick film inks for solar cells, AR coating and superconductors. Current interests are the preparation, characterisation and optimisation of powders and pastes for gas sensor application. Sil6ia Lenaerts obtained a Ph.D. in Chemistry in 1993 at IMEC and the K.U. Leuven. Until 1996, she was active in the sensor research at IMEC with main activities in the field of gas sensors. Main interests are in the field of gas–solid interactions and the application
L. Delabie et al. / Sensors and Actuators B 44 (1997) 446–451
of physico-chemical characterisation techniques, especially in-situ FT–IR spectroscopy, on semiconducting oxides, ceramic materials and organic materials used for gas sensing. Guido Huyberechts obtained a Ph.D. in Chemistry in 1988 at the K.U. Leuven. He joined the sensor research at IMEC in 1987 with main activities in the field of gas sensors. Main interests are in the field of gas –solid interactions and the application and characterisation of ceramic materials, semiconducting oxides and organic materials in gas sensors and microsystems for chemical analysis. Jean Roggen obtained a Ph.D. in Physics in 1977 at the K.U. Leuven. In 1978 he became head of the hybrid technology group within the audio division of Philips.
.
451
In 1984 he moved to IMEC and became head of the microsystems group. Besides chemical gas sensor systems his main interest is in the field of interconnection and packaging of high speed/high density VLSIs and the related reliability problems. Guido Maes obtained a Ph.D. in Chemistry in 1978 at the K.U. Leuven. He obtained the fellowships Research Assistant (1978), Research Associate (1980) and Senior Research Associate (1991) from the Belgian National Science Foundation (NFWO) and was appointed as a part-time Associate Professor (1992) and Professor (1996) at the Faculty of Science of the K.U. Leuven. His main research interests are molecular structures and inter-molecular interactions in isolated conditions (matrix-isolation spectroscopy of nucleic acid bases) and the physicochemical properties of gas sensor materials.
The effect of sintering and Pd-doping on the conversion of CO to CO2 on SnO2 gas sensor materials L. Delabie a,*, M. Honore´ a, S. Lenaerts a, G. Huyberechts a, J. Roggen a, G. Maes b a
b
Interuni6ersity Microelectronics Centre, Kapeldreef 75, B-3001, Leu6en, He6erlee, Belgium K.U. Leu6en, Department of Chemistry, Celestijnenlaan 200F, B-3001, Leu6en, He6erlee, Belgium Accepted 12 May 1997
Abstract The principal aim of this work is to study the effect of the processes of sintering and Pd doping of SnO2 gas sensor materials on the conversion of CO to CO2. For this purpose, the gas phase above screen printed sensor material is investigated using FTIR spectroscopy, while surface area, porosity and particle size measurements are performed on the SnO2 powders. During sintering, larger agglomerates of primary particles are formed, which results in a larger conversion degree of CO. The effect of Pd doping of the tin dioxide film on the CO conversion is more pronounced. The transformation of CO starts at a lower temperature and the conversion degree increases remarkably. © 1997 Elsevier Science S.A. Keywords: Sintering; Pd doping; SnO2 gas sensor; CO; CO2
1. Introduction Tin dioxide is a n-type semiconductor with broad forbidden energy gap. This material is suitable for gas sensing applications because reaction of reducing gases with adsorbed oxygen species induces a conductivity change. The physicochemical sensor mechanism can be described by two separate reaction series [1]. The first one consists of so-called ‘sensitivation reactions’ in which the SnO2 material is activated in air by an increased concentration of conductance electrons in surface states associated with ionosorbed oxygen species O2− and O − . These adsorbed species create an excess electron concentration in the contact zone between the material domains, which results in a conductivity decrease due to the potential barrier between the negative surface charges and the positive donor ions in the bulk of the material. In the second reaction sequence, surface oxygen species react with reducing gases from the surrounding atmosphere, e.g. CO, NH3 or CH4. Physisorption of the reducing molecule is followed by reaction with O − species, the enthalpy of the latter reaction allowing the transfer of the released * Corresponding author. Tel.: + 32 16 281262; fax: + 32 16 281501; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 7 0 - 6
electron to the conductance band of SnO2, and the desorption of the product molecule. Despite several proofs for the suitability of SnO2 for sensor applications [2], the SnO2 gas sensor still suffers from lack of specificity, sensitivity and stability. During the preparation process of the thick film gas sensor, a variety of process variables can influence the properties of the tin dioxide material and, as a consequence, also those of the sensor [3]. In this work, the effect of sintering time and subsequent cooling down rate on the particle size and the porosity of the powder, as well as on the conversion degree of CO to CO2 above the screen printed thick films, is studied. According to the recent literature, doping of tin dioxide with palladium has a positive effect on the CO sensitivity [4–10]. This leads to a second aspect investigated in this work: the effect of Pd doping on the CO to CO2 conversion degree.
2. Experimental Pure SnO2 was prepared following the procedure generally used for gas sensor materials [3]: by precipitation by ammonia of a SnCl4.5H2O water solution, followed by washing and calcination. This calcination
L. Delabie et al. / Sensors and Actuators B 44 (1997) 446–451
process is carried out at a temperature of 550°C, for 6 h. The screen printed films were obtained by printing a paste of the SnO2 powder mixed with a polymer solution on an alumina substrate, similar to the screen printing process used for fabricating the thick film SnO2 sensor. The gas phase above the screen printed SnO2 was investigated using FTIR spectroscopy. The IR spectra were collected using a Mattson Galaxy 7001 spectrometer equipped with a liquid N2 cooled semiconductor MCT detector. An Aabspec 2000 sample module allowing variation of the surrounding atmosphere, as well as a controlled temperature increase of the sample up to 400°C during collection of the spectrum was used. Temperature measurement of the sample was performed using a chromel/alumel thermocouple. A gas mixture of 4 vol % CO in dry air was used during the experiments. The gas flow was monitored by means of massflow controllers (MKS 147B) regulated by a gas flow control unit. A preliminary heating step up to 400°C was performed on the sample before each spectroscopic experiment. During this heating step the physisorbed H2O and most of the isolated and H-bonded surface OH groups disappeared from the SnO2 surface [11]. Surface area, pore size and pore volume of the powders were obtained by evaluation of the nitrogen adsorption–desorption data from a Micromeritics ASAP 2000 apparatus. The particle size distribution was obtained from laser diffraction particle size analysis using a Malvern Mastersizer X instrument.
3. Results and discussion
3.1. Effect of sintering Sintering of screen printed tin dioxide films constitutes the last step in the thermal treatment of thick film sensors. Sintering of both the powders and the screen printed films was performed in a tube furnace (Lenton), characterised by a maximum central furnace temperature of 850°C. Four different temperature profiles were applied. In each of them a temperature increase from room temperature up to 850°C was realised in 50 min. For the samples further referred to as A30 or A10, the temperature of 850°C was maintained for 30 or 10 min,
447
Fig. 1. Particle size distribution of non-sintered and differently sintered tin dioxide powders. represents a non-sintered sample, " is powder A30 B4, is powder A30 B1 and is powder A10 B1. Error bars represent the standard deviation (3 measurements were performed).
respectively. A second parameter variation was the cooling down rate: B1 refers to a sample that has been cooled down in 5 min, whereas B4 films have been cooled down to room temperature following a linear temperature profile of 60 min. Both the powder and the screen printed film were subjected to the same sintering profile. Fig. 1 demonstrates that the portion of smaller particles (diameter B 1 mm) decreases after sintering while the volumetric fraction of particles larger than 7 mm increases. This can be explained by the pure physical process of sintering in which the SnO2 particles squeeze together and form larger agglomerates. The BET surface area of the non-sintered powder is much higher than that of the sintered powders. The BJH pore surface and the BJH pore 6olume decrease during the process of sintering while the pore diameter increases (Table 1). All these results are consistent with the explanation of the formation of larger agglomerates during sintering, since a collection of large particles has a smaller surface area. Furthermore, although the number of pores is smaller, the pores themselves are larger in the case of larger particles. On the basis of these results no difference can be made between the powders
Table 1 Parameters resulting from BET-surface area and porosity measurements on tin dioxide powders Powdera
BET surface area (m2 g−1)
Pore surface (m2 g−1)
Pore volume (cm3 g−1)
Pore diameter (nm)
Non-sintered A30 B4 A30 B1 A10 B4 A10 B1
25.87 8.41 8.96 11.32 10.07
27.1 7.4 8.1 10.2 9.1
0.154 0.099 0.115 0.133 0.121
22.8 53.2 56.3 52.1 53.1
a
Specifications for the different powders are described in the text.
448
L. Delabie et al. / Sensors and Actuators B 44 (1997) 446–451
tration of bulk and surface vacancies. These changes can also effect the catalytic activity of the samples [15].
3.2. Effect of Pd doping
Fig. 2. Catalytic conversion of CO to CO2 at 400°C on non-sintered SnO2 (4% CO in air, flow rate 200 ml min − 1).
that have been cooled down at different rates. In contrast, a distinction can be made between the powders subjected to different sintering times. The longer the powder is kept at the high temperature, the larger the agglomerates are and the smaller the number of pores is. The influence of sintering on the catalytic properties of the material can be investigated using FTIR spectroscopy. Gaseous CO has a rotational – vibrational absorption band in the infrared spectral region centred at 2143 cm − 1 [12]. The mean feature of the infrared spectrum of CO2 is a strong absorption band at 2349 cm − 1 [13] (Fig. 2). The latter absorption, corresponding to the asymmetric stretching n2 mode of CO2, exhibits much less rotational structure than the CO band, which is due to the noticeably smaller rotational constant of CO2 (B= 0.393 cm − 1) compared to CO (B=1.925 cm − 1) [14]. These experiments are performed at different gas flow rates using a sample temperature of 400°C. The area of the CO and CO2 bands are measured by integrating over the whole absorption bands. It is clear that the non-sintered film shows a smaller degree of transformation of CO to CO2 than the sintered films (Fig. 3). This effect is most expressed at small gas flow rates. The difference can be explained by the larger pores in the sintered films, resulting in a larger area of SnO2 in contact with CO. However, the pore accessibility is not the only important parameter, because sufficient contact time (small flow rate) must be ensured in order that a diffusion layer over the SnO2 pore structure may be formed. On the basis of these results, it is rather difficult to differentiate between the differently sintered films. Another possible effect of sintering, beside these morphological changes, is that sintering at high temperature can influence the concen-
Screen printed tin dioxide films were doped with Pd. In this case, the standard temperature profile (IMEC 850A) during sintering was used: the maximum temperature of 850°C was reached with a temperature increase of 36°C min − 1, the films were kept at this temperature for 10 min and the cooling down rate was 42°C min − 1. During doping, the concentration of palladium in the contacting solutions is such as to result in a 1.2 wt.% Pd to SnO2, when complete deposition should occur. For this purpose, we used either PdCl2 or Pd(C5O2H7)2. PdCl2 was dissolved in water, while Pd(C5O2H7)2 was used in chloroform solution. The parameter investigated in this work was the impregnation time: 1, 5, 15, 30, 60, 180 or 1140 min. After the impregnation, the samples were kept at a temperature of 510°C for 20 or 40 min. This step is carried out to allow the dissociation of the inorganic or organic Pd compounds. Again, the catalytic properties of the different samples were studied using FTIR spectroscopy. However, the gas flow rate was kept constant at a value of 20 ml min − 1 in this case. Figs. 4 and 5 illustrate the effect of the impregnation time. At 400°C no difference in CO conversion can be made between the differently doped films. However, the transformation of CO to CO2 is distinctively higher for the Pd doped films than for the non-doped films (Fig. 4). Above a non-doped sample, a conversion of CO of
Fig. 3. Effect of sintering SnO2 on the CO to CO2 conversion measured by FTIR spectroscopy. represents a non-sintered sample, " is powder A30 B4, is powder A30 B1 and is powder A10 B1. Error bars are based on a relative error of integration over the absorption band of 5%.
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Fig. 4. Conversion of CO to CO2 at 400°C above differently Pd doped SnO2 films. The white signs represent conversions above samples doped with PdCl2, the black signs represent conversions above films doped with Pd(C5O2H7)2. Samples kept at 510°C for 20 or 40 min are represented by or , respectively. Error bars are based on a relative error of integration over the absorption band of 5%.
34.4% is measured compared to a conversion of about 57% above a doped sample. At 200°C, the conversion of CO is clearly dependent on the impregnation time. The longer the impregnation time is, the larger the conversion of CO to CO2 is (Fig. 5). Again, the difference between the non-doped films and the Pd doped films is large, at this temperature a non-doped sample shows no CO2 formation at all. The lowest degree of conversion is, at each impregnation time, observed for samples impregnated with PdCl2 which are kept at 510°C for only 20 min. When these samples are kept at
Fig. 5. Conversion of CO to CO2 at 200°C above differently Pd doped SnO2 films. The white signs represent conversions above samples doped with PdCl2, the black signs represent conversions above films doped with Pd(C5O2H7)2. Samples kept at 510°C for 20 or 40 min are represented by or , respectively. Error bars are based on a relative error of integration over the absorption band of 5%.
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Fig. 6. Influence of the temperature on the conversion of CO to CO2 above screen printed SnO2. Black squares represent a non-doped sample, white signs represent a sample doped with PdCl2, black signs represent samples doped with Pd(C5O2H7)2. Samples with an impregnation time of 15 min are represented by and indicates an impregnation time of 60 min. All samples were kept at 510°C for 40 min. Error bars are based on a relative error of integration over the absorption band of 5%.
510°C for 40 min, the conversion degree to CO2 is larger. For films doped with Pd(C5O2H7)2, there is little difference in CO conversion between samples kept at 510°C for different times. Pd(C5O2H7)2 doped films show a higher conversion degree than films impregnated with PdCl2. There are clearly two main factors contributing to the observed differences in conversion degree between the different samples: the amount of palladium in the film and the form under which the dopant is present in the sample. Due to the use of PdCl2 in water solution and Pd(C5O2H7)2 in chloroform solution, a different amount of Pd can be present in a sample, even at the same impregnation time, due to differences in solubility. Secondly, the palladium in the sample can still be present as undissociated PdCl2 or Pd(C5O2H7)2, respectively, and the amount of dissociation can differ between the two Pd compounds. Also the palladium can be present as Pd0 or PdO. Maybe, the latter two points may explain the difference between the films kept at 510°C for a different time (Fig. 5). Further experiments will be performed to gain more insight in these matters. The effect of the sample temperature is analysed in Fig. 6. In case of the doped screen printed films, CO conversion is observed above 150°C, while a non-doped film shows conversion only above 250°C. At all temperatures, the conversion of CO is definitely higher on a doped sample than on a non-doped sample. Above 300°C the conversion above the differently doped films reaches the same value and does not increase any more under the current test circumstances. It is important to note that only the catalytic properties of the different materials have been studied here.
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However, the sensor response is based on electrical changes in the SnO2 material. Sintering can have an influence on the electrical conductivity of the tin dioxide [16,17]. The effect of Pd doping of screen printed tin dioxide films on the sensor response, will be studied by conductivity measurements on the SnO2 sensor. Also, the development of a new measuring device allowing a combination of the measurement of the catalytic properties and the sensor response, is in due course. This device should allow to perform an application oriented, fundamental research of the effect of Pd doping [18].
4. Conclusions Investigations of the effect of the sintering process and of palladium doping of tin dioxide gas sensor materials on the catalytic CO to CO2 conversion has allowed to draw the following conclusions. The sintering process squeezes together different SnO2 grains into larger agglomerates. This results in a smaller number of larger pores and a smaller surface area. The CO to CO2 conversion is increased after the sintering process, which can be explained by the more ‘accessible’ structure of the sintered film. Pd-impregnated tin dioxide thick films show a remarkably higher conversion of CO in comparison to non-doped SnO2 films, and this conversion starts at a lower temperature.
[7] H. Torvela, J. Huusko, V. Lantto, Reduction of the interference caused by NO and SO2 in the CO response of Pd-catalysed SnO2 combustion gas sensors, Sensors and Actuators B 4 (1991) 479 – 484. [8] F.J. Schmitte, G. Wiegleb, Conductivity behaviour of thick-film tin dioxide gas sensors, Sensors and Actuators B 4 (1991) 473 – 477. [9] G. Martinelli, M.C. Carotta, A study of the conductance and capacitance of pure and Pd-doped SnO2 thick films, Sensors and Actuators B 18-19 (1994) 720 – 723. [10] G. Tournier, C. Pijolat, R. Lalauze, B. Patissier, Selective detection of CO and CH4 with gas sensors using SnO2 doped with palladium, Sensor and Actuators B 26 (1-3) (1995) 24 –28. [11] P.G. Harrison, A. Guest, Tin Oxide Surfaces. Part 17. An infrared and thermogravimetric analysis of the thermal dehydration of tin (IV) oxide gel, J. Chem. Soc. Faraday Trans I 83 (1987) 3383 – 3397. [12] G. Herzberg, Molecular Spectra and Molecular Structure, I, 2nd ed., Van Nostrand Reinhold, New York, 1950, p. 127. [13] G. Herzberg, Molecular Spectra and Molecular Structure, II, Van Nostrand Reinhold, New York, 1945, p. 272. [14] G. Herzberg, Molecular Spectra and Molecular Structure, II, Van Nostrand Reinhold, New York, 1945, p. 21. [15] W. Go¨pel, Chemisorption and charge transfer at semiconductor surfaces: implications for designing gas sensors, Prog. Surf. Sci. 20 (1985) 9 – 103. [16] M.S. Dutraive, R. Lalauze, C. Pijolat, Sintering, catalytic effects and defect chemistry in polycrystalline tin dioxide, Sensors and Actuators B 26-27 (1995) 38 – 44. [17] G. Pfaff, Effect of powder preparation and sintering on the electrical properties of tin dioxide-based ceramic gas sensors, Sensors and Actuators B 20 (1994) 43 – 48. [18] K.D. Schierbaum, U. Weimar, W. Go¨pel, R. Kowalkowski, Conductance, work function and catalytic activity of SnO2-based gas sensors, Sensors and Actuators B 3 (1991) 205 – 214.
Acknowledgements Biographies The authors are grateful to the Belgian I.W.T. (L.D.) and N.F.W.O. (G.M.) for fellowships obtained and for financial support (N.F.W.O.).
References [1] P.T. Moseley, J.O.W. Norris, D.E. Williams, Techniques and mechanisms, in: A. Hilger (Ed.), Gas Sensing, IOP, Bristol, 1991, p. 61. [2] G. Huyberechts, M. Van Muylder, M. Honore´, J. Desmet, J. Roggen, Development of a thick film ammonia sensor for livestock buildings, Sensors and Actuators B 18-19 (1994) 296 – 299. [3] M. Honore´, S. Lenaerts, J. Desmet, G. Huyberechts, J. Roggen, Synthesis and characterisation of tin dioxide powders for the realisation of thick film sensors, Sensors and Actuators B 18-19 (1994) 621 – 624. [4] B.S. Chiou, J. Li, J.G. Duh, Characteristics of Pd-doped tin oxide ceramics in response to CO gas, J. Electron. Mater. 17 (6) (1988) 485 – 492. [5] J.G. Duh, J.W. Jou, B.S. Chiou, Catalytic and gas sensing characteristics in Pd-doped SnO2, J. Electrochem. Soc. 136 (9) (1989) 2740 – 2747. [6] S. Semancik, T.B. Fryberger, Model studies of SnO2-based gas sensors: vacancy defects and Pd additive effects, Sensors and Actuators B 1 (1990) 97–102.
Lieselot Delabie obtained a licentiate in Chemistry in 1995 at the K.U. Leuven. She is active in the sensor research at IMEC with main activities in the field of gas sensors. Main interests are in-situ FT–IR spectroscopy, gas–solid interactions and physico-chemical characterisation techniques. Mia Honore´ obtained a degree in industrial chemical engineering from HTI, Oostende in 1974. After research work on low temperature resistor thick film pastes at Barco and K.U. Leuven, she joined IMEC and is active in the development of thick film inks for solar cells, AR coating and superconductors. Current interests are the preparation, characterisation and optimisation of powders and pastes for gas sensor application. Sil6ia Lenaerts obtained a Ph.D. in Chemistry in 1993 at IMEC and the K.U. Leuven. Until 1996, she was active in the sensor research at IMEC with main activities in the field of gas sensors. Main interests are in the field of gas–solid interactions and the application
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of physico-chemical characterisation techniques, especially in-situ FT–IR spectroscopy, on semiconducting oxides, ceramic materials and organic materials used for gas sensing. Guido Huyberechts obtained a Ph.D. in Chemistry in 1988 at the K.U. Leuven. He joined the sensor research at IMEC in 1987 with main activities in the field of gas sensors. Main interests are in the field of gas –solid interactions and the application and characterisation of ceramic materials, semiconducting oxides and organic materials in gas sensors and microsystems for chemical analysis. Jean Roggen obtained a Ph.D. in Physics in 1977 at the K.U. Leuven. In 1978 he became head of the hybrid technology group within the audio division of Philips.
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In 1984 he moved to IMEC and became head of the microsystems group. Besides chemical gas sensor systems his main interest is in the field of interconnection and packaging of high speed/high density VLSIs and the related reliability problems. Guido Maes obtained a Ph.D. in Chemistry in 1978 at the K.U. Leuven. He obtained the fellowships Research Assistant (1978), Research Associate (1980) and Senior Research Associate (1991) from the Belgian National Science Foundation (NFWO) and was appointed as a part-time Associate Professor (1992) and Professor (1996) at the Faculty of Science of the K.U. Leuven. His main research interests are molecular structures and inter-molecular interactions in isolated conditions (matrix-isolation spectroscopy of nucleic acid bases) and the physicochemical properties of gas sensor materials.