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Role of water vapour in the interaction of SnO2 gas sensors with CO and CH4
Sensors and Actuators B 61 Ž1999. 39–42 www.elsevier.nlrlocatersensorb
Role of water vapour in the interaction of SnO 2 gas sensors with CO and CH 4 R. Ionescu ) , A. Vancu, C. Moise, A. Tomescu National Institute of Materials Physics, P.O. Box MG-7, 76900 Bucuresti, Magurele, Romania Received 30 November 1998; received in revised form 16 July 1999; accepted 19 July 1999
Abstract New aspects of the influence of water vapour in the interaction of SnO 2 gas sensors with different reducing gases are evidenced by an original experimental method. This consists in measuring the conductance transients during and after rapid transitions from dry into humid air containing identical concentrations of methane and CO, respectively. The results indicate that while methane and water compete in reacting with one type of oxygen ions on the surface, CO reacts mainly with another type of surface oxygen, almost insensitive to water. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Tin dioxide; Gas sensors; Humidity
1. Introduction It is well known that water vapour strongly influences both the conductance G 0 in air and the gas response GŽ p R . Ž p R is the partial pressure of a reducing gas in air. of chemoresistive tin dioxide gas sensors w1–3x. We have shown previously w4x that other unwanted phenomena appearing in the operation of chemoresistive thick-film, sintered SnO 2 gas sensors are also due to the action of water vapour. One of these phenomena is ageing, and in attributing it to the influence of water, we relied on the observation that identical G 0 Ž t . time-evolutions occur both during ageing and during the transient regime which follows after a sudden change in the relative humidity of the ambient air from ; 0% Ždry. to 100% Žwet.. Another effect of water vapour reported in Ref. w4x is that of accentuating the deviations from linearity observed at low . calibration curves. p R on the GŽ p 1r2 R Experimental results were explained in Ref. w4x by a tentative model in which we advanced the hypothesis that water reacts dissociatively with one type of lattice oxygen on the SnO 2 surface, but in two different ways, giving rise to two different types of OHy ions Žsurface reaction products.. One of them, OHy A , replaces the reactive oxygen
) Corresponding author. [email protected]
Fax:
q 40-1-4930-267;
e-m ail:
sites without producing any free carrier; the other, OHy B, on the contrary, produces free carriers without blocking the oxygen sites. On the GŽ t . curves obtained at rapid transitions from dry into wet air, the sudden increase in conductance was explained by the generation of OHy B groups, and the subsequent GŽ t . decay by the recombination and trapping — on different paths — of the charged particles generated. A possible recombination path is that of OHy B ions with the lattice oxygen vacancies resulted in the interaction with water. An interesting question is however if water vapour does or does not compete with reducing gases in reacting on the SnO 2 surface with the same oxygen reaction sites. The experiments described in the present paper are aimed to offer an answer to this question.
2. Experimental results The tin dioxide material was prepared by a wet chemical process from ammonia and tin chloride, calcinated at 4508C, and impregnated with 0.4 wt.% Pd. A paste was prepared by mixing the tin dioxide powder with an organic vehicle. Conductance measurements were performed on usual thick-film sensors prepared by painting the paste between
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 7 7 - 4
40
R. Ionescu et al.r Sensors and Actuators B 61 (1999) 39–42
Fig. 1. Conductance transients for several concentrations of methane in air, in rapid changes from ; 0% to 100% relative humidity.
two gold electrodes on cylindrical alumina substrates, and by sintering in air at 5008C. At constant temperature Ž3508C., the samples were kept for several hours in dry air, in which a given partial pressure p R of methane and of CO, respectively, was realised and maintained for f 10 min, then the relative humidity was suddenly Ž0.1 s. increased to 100% without changing p R , while permanently measuring electrical conductance, until a new quasi-stationary state was installed. The procedure was repeated for several p R within the 0–10 4 ppm range. According to our knowledge, this is an
Fig. 2. Conductance transients for several concentrations of carbon monoxide in air, in rapid changes from ; 0% to 100% relative humidity.
Fig. 3. Conductance transients from Fig. 1 in relative units.
original method in the experimental investigation of tin dioxide gas sensors. The progressive increase in the initial conductance G 0 in RH f 0% with increasing pCH 4 and pCO is clearly seen in Figs. 1 and 2, respectively. Observable differences between the effect of increasing CH 4 concentrations pCH 4 and increasing CO concentrations pCO Žin the same range. appear when comparing the conductance kinetics in Fig. 1 for CH 4 , with that in Fig. 2 for CO. Still more striking appear these differences in Figs. 3 and 4, where the GŽ t . curves were normalised to their starting values in dried atmospheres. While in Fig. 4, the
Fig. 4. Conductance transients from Fig. 2 in relative units.
R. Ionescu et al.r Sensors and Actuators B 61 (1999) 39–42
Fig. 5. Maximum relative conductance vs. square root of reducing gas concentration.
effect of increasing pCO upon the GŽ t . curves is rather slight, in Fig. 3, the effect of increasing pCH 4 is clearly observable. With increasing pCH 4 the height G M of the maximum conductance progressively decreases Žcurve a in Fig. 5., the initial rate of conductance decay dGrdt < 0 describes a maximum Žcurve a in Fig. 6., the final Žquasi. steady-state conductance Gf progressively decreases, conductance G 0 in RH f 0% ŽFig. 3.. At longer times after the transition, all the GŽ t . curves, for CH 4 as well as for CO, seem to decrease with the same rate.
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the principal mechanism responsible for the initial conducŽ tance decay is the ‘‘recombination’’ of OHy B ions generated by water. with the lattice oxygen vacancies Žgenerated both by water when producing OHy B and by methane — in addition to water — but also in dry atmospheres.. In this process, the ‘‘recombination’’ rate Žproportional to dGrdt < 0 . is proportional to the product of the surface concentrations of the two ‘‘recombination’’ partners. At low pCH 4 , where the concentration of lattice oxygen ions available for the reaction with water to produce OHy B is still high enough to remain practically unaffected by pCH 4 , the ‘‘recombination’’ progressively increases with increasing concentration of oxygen vacancies produced by increasing pCH 4 . At higher pCH 4 , the concentration of OHy B ions decreases drastically because less and less oxygen ions are available for the reaction with water, and thus the ‘‘recombination’’ rate deceases with increasing pCH 4 . As mentioned before, this also explains the progressive lowering of G M with increasing pCH 4 . At longer times after the transition, the final decay rate dGrdt < f becomes independent on pCH 4 . This suggests another ‘‘recombination’’ mechanism, possibly involving chemosorbed oxygen. ŽPrevious, still unpublished experimental results have shown that the final decay rate increases with increasing pO 2 in the ambient atmosphere.. In atmospheres containing CO, the results are quite different ŽFig. 4.. The effect of increasing pCO on both the height of G M and on the initial conductance decay rate dGrdt is very slight Žcurves b in Figs. 5 and 6.. These observations strongly suggest that in contrast to CH 4 , instead of competing with water in reacting on the same oxygen sites, CO rather reacts on other oxygen sites, which are not involved in the reactions with water vapour.
3. Discussion The strikingly different influences of increasing Žin the same range. pCH 4 and pCO , respectively, upon the GŽ t . dependences after transitions from dried into wet atmospheres suggest that CH 4 and CO react with SnO 2 on different types of oxygen reaction sites. The strong influence of increasing pCH 4 on the height of the maximum G M , on the initial rate of conductance decay dGrdt < 0 and on the final, Žquasi. steady-state conductance Gf suggests that CH 4 competes with water in reacting mainly with the same type of oxygen ions on the surface of SnO 2 . The progressive decrease in the height of the maximum G M Žcurve a in Fig. 5. reflects the progressive decay produced by increasing pCH 4 in the concentration of the oxygen ions available for reaction with water. Within the tentative model we proposed in Ref. w4x, the presence of a maximum on the dGrdt < 0 vs. 6pCH 4 curve Žcurve a in Fig. 6. can be understood by considering that
Fig. 6. Slope of the initial conductance decay vs. square root of reducing gas concentration.
42
R. Ionescu et al.r Sensors and Actuators B 61 (1999) 39–42
4. Conclusions In their interaction with the SnO 2 surface, CH 4 and CO have different privileged reaction partners. CH 4 competes with water in reacting on the same oxygen sites. This competition is reflected in the strong influence of increasing pCH 4 on the GŽ t . curves after transitions from dried into wet atmospheres in which, with increasing pCH 4 : Ži. the maximum G M heights decrease, Žii. the initial decay rate dGrdt describes a maximum. In contrast to CH 4 , CO reacts mainly on oxygen sites which are not involved in the interaction with water. This is reflected in the very slight Ževen negligible for pCO F 100 ppm. influence of increasing pCO on the GŽ t . curves after transitions from dried into wet atmospheres.
w3x N. Barsan, R. Ionescu, The mechanism of interaction between CO and SnO 2 surface — the role of water vapour, Sensors and Actuators B 12 Ž1993. 71–75. w4x R. Ionescu, Ageing and p-type conduction in SnO 2 gas sensors, accepted for publication in Sensors and Actuators B. Radu Ionescu is a senior researcher and head of the Gas Sensors Group in the National Institute of Materials Physics. He graduated in Physics in 1963 in the University of Bucharest and took his PhD degree in 1979, with a thesis on the ‘‘Transport phenomena in V–VI semiconductor compounds’’. In 1985, he started to work in the field of SnO 2 gas sensors. He is the author of more than 50 scientific papers in amorphous and crystalline semiconductors. Ana Vancu is a senior researcher within the Gas Sensors Group. She graduated in Physics in 1959 at the University of Bucharest, then worked as a scientific researcher in the Institute of Physics, where she obtained her PhD degree in 1974, with a thesis on the ‘‘Optical properties of amorphous semiconductors’’. In 1986, she joined the Gas Sensors Group. She is an author of more than 60 scientific papers, contributions in scientific monographies and patents.
References w1x D.E. Williams, in: P.T. Moseley, B.C. Tofield ŽEds.., Solid State Gas Sensors, Chaps. 5.1, Adam Hilger IOP, Publishing, 1987, 71. w2x K. Ihokura, J. Watson, The Stanic Oxide Gas Sensor, CRC Press, Boca Raton, FL, 1994.
Adelina Tomescu is a researcher within the Gas Sensors Group. She graduated in Physics in 1989 at the University of Bucharest, then worked as an assistant researcher in the Institute of Physics, Gas Sensors Group. Since 1998, she has been a PhD student and a researcher in the same group. She is the author of more than 17 scientific papers.
Role of water vapour in the interaction of SnO 2 gas sensors with CO and CH 4 R. Ionescu ) , A. Vancu, C. Moise, A. Tomescu National Institute of Materials Physics, P.O. Box MG-7, 76900 Bucuresti, Magurele, Romania Received 30 November 1998; received in revised form 16 July 1999; accepted 19 July 1999
Abstract New aspects of the influence of water vapour in the interaction of SnO 2 gas sensors with different reducing gases are evidenced by an original experimental method. This consists in measuring the conductance transients during and after rapid transitions from dry into humid air containing identical concentrations of methane and CO, respectively. The results indicate that while methane and water compete in reacting with one type of oxygen ions on the surface, CO reacts mainly with another type of surface oxygen, almost insensitive to water. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Tin dioxide; Gas sensors; Humidity
1. Introduction It is well known that water vapour strongly influences both the conductance G 0 in air and the gas response GŽ p R . Ž p R is the partial pressure of a reducing gas in air. of chemoresistive tin dioxide gas sensors w1–3x. We have shown previously w4x that other unwanted phenomena appearing in the operation of chemoresistive thick-film, sintered SnO 2 gas sensors are also due to the action of water vapour. One of these phenomena is ageing, and in attributing it to the influence of water, we relied on the observation that identical G 0 Ž t . time-evolutions occur both during ageing and during the transient regime which follows after a sudden change in the relative humidity of the ambient air from ; 0% Ždry. to 100% Žwet.. Another effect of water vapour reported in Ref. w4x is that of accentuating the deviations from linearity observed at low . calibration curves. p R on the GŽ p 1r2 R Experimental results were explained in Ref. w4x by a tentative model in which we advanced the hypothesis that water reacts dissociatively with one type of lattice oxygen on the SnO 2 surface, but in two different ways, giving rise to two different types of OHy ions Žsurface reaction products.. One of them, OHy A , replaces the reactive oxygen
) Corresponding author. [email protected]
Fax:
q 40-1-4930-267;
e-m ail:
sites without producing any free carrier; the other, OHy B, on the contrary, produces free carriers without blocking the oxygen sites. On the GŽ t . curves obtained at rapid transitions from dry into wet air, the sudden increase in conductance was explained by the generation of OHy B groups, and the subsequent GŽ t . decay by the recombination and trapping — on different paths — of the charged particles generated. A possible recombination path is that of OHy B ions with the lattice oxygen vacancies resulted in the interaction with water. An interesting question is however if water vapour does or does not compete with reducing gases in reacting on the SnO 2 surface with the same oxygen reaction sites. The experiments described in the present paper are aimed to offer an answer to this question.
2. Experimental results The tin dioxide material was prepared by a wet chemical process from ammonia and tin chloride, calcinated at 4508C, and impregnated with 0.4 wt.% Pd. A paste was prepared by mixing the tin dioxide powder with an organic vehicle. Conductance measurements were performed on usual thick-film sensors prepared by painting the paste between
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 7 7 - 4
40
R. Ionescu et al.r Sensors and Actuators B 61 (1999) 39–42
Fig. 1. Conductance transients for several concentrations of methane in air, in rapid changes from ; 0% to 100% relative humidity.
two gold electrodes on cylindrical alumina substrates, and by sintering in air at 5008C. At constant temperature Ž3508C., the samples were kept for several hours in dry air, in which a given partial pressure p R of methane and of CO, respectively, was realised and maintained for f 10 min, then the relative humidity was suddenly Ž0.1 s. increased to 100% without changing p R , while permanently measuring electrical conductance, until a new quasi-stationary state was installed. The procedure was repeated for several p R within the 0–10 4 ppm range. According to our knowledge, this is an
Fig. 2. Conductance transients for several concentrations of carbon monoxide in air, in rapid changes from ; 0% to 100% relative humidity.
Fig. 3. Conductance transients from Fig. 1 in relative units.
original method in the experimental investigation of tin dioxide gas sensors. The progressive increase in the initial conductance G 0 in RH f 0% with increasing pCH 4 and pCO is clearly seen in Figs. 1 and 2, respectively. Observable differences between the effect of increasing CH 4 concentrations pCH 4 and increasing CO concentrations pCO Žin the same range. appear when comparing the conductance kinetics in Fig. 1 for CH 4 , with that in Fig. 2 for CO. Still more striking appear these differences in Figs. 3 and 4, where the GŽ t . curves were normalised to their starting values in dried atmospheres. While in Fig. 4, the
Fig. 4. Conductance transients from Fig. 2 in relative units.
R. Ionescu et al.r Sensors and Actuators B 61 (1999) 39–42
Fig. 5. Maximum relative conductance vs. square root of reducing gas concentration.
effect of increasing pCO upon the GŽ t . curves is rather slight, in Fig. 3, the effect of increasing pCH 4 is clearly observable. With increasing pCH 4 the height G M of the maximum conductance progressively decreases Žcurve a in Fig. 5., the initial rate of conductance decay dGrdt < 0 describes a maximum Žcurve a in Fig. 6., the final Žquasi. steady-state conductance Gf progressively decreases, conductance G 0 in RH f 0% ŽFig. 3.. At longer times after the transition, all the GŽ t . curves, for CH 4 as well as for CO, seem to decrease with the same rate.
41
the principal mechanism responsible for the initial conducŽ tance decay is the ‘‘recombination’’ of OHy B ions generated by water. with the lattice oxygen vacancies Žgenerated both by water when producing OHy B and by methane — in addition to water — but also in dry atmospheres.. In this process, the ‘‘recombination’’ rate Žproportional to dGrdt < 0 . is proportional to the product of the surface concentrations of the two ‘‘recombination’’ partners. At low pCH 4 , where the concentration of lattice oxygen ions available for the reaction with water to produce OHy B is still high enough to remain practically unaffected by pCH 4 , the ‘‘recombination’’ progressively increases with increasing concentration of oxygen vacancies produced by increasing pCH 4 . At higher pCH 4 , the concentration of OHy B ions decreases drastically because less and less oxygen ions are available for the reaction with water, and thus the ‘‘recombination’’ rate deceases with increasing pCH 4 . As mentioned before, this also explains the progressive lowering of G M with increasing pCH 4 . At longer times after the transition, the final decay rate dGrdt < f becomes independent on pCH 4 . This suggests another ‘‘recombination’’ mechanism, possibly involving chemosorbed oxygen. ŽPrevious, still unpublished experimental results have shown that the final decay rate increases with increasing pO 2 in the ambient atmosphere.. In atmospheres containing CO, the results are quite different ŽFig. 4.. The effect of increasing pCO on both the height of G M and on the initial conductance decay rate dGrdt is very slight Žcurves b in Figs. 5 and 6.. These observations strongly suggest that in contrast to CH 4 , instead of competing with water in reacting on the same oxygen sites, CO rather reacts on other oxygen sites, which are not involved in the reactions with water vapour.
3. Discussion The strikingly different influences of increasing Žin the same range. pCH 4 and pCO , respectively, upon the GŽ t . dependences after transitions from dried into wet atmospheres suggest that CH 4 and CO react with SnO 2 on different types of oxygen reaction sites. The strong influence of increasing pCH 4 on the height of the maximum G M , on the initial rate of conductance decay dGrdt < 0 and on the final, Žquasi. steady-state conductance Gf suggests that CH 4 competes with water in reacting mainly with the same type of oxygen ions on the surface of SnO 2 . The progressive decrease in the height of the maximum G M Žcurve a in Fig. 5. reflects the progressive decay produced by increasing pCH 4 in the concentration of the oxygen ions available for reaction with water. Within the tentative model we proposed in Ref. w4x, the presence of a maximum on the dGrdt < 0 vs. 6pCH 4 curve Žcurve a in Fig. 6. can be understood by considering that
Fig. 6. Slope of the initial conductance decay vs. square root of reducing gas concentration.
42
R. Ionescu et al.r Sensors and Actuators B 61 (1999) 39–42
4. Conclusions In their interaction with the SnO 2 surface, CH 4 and CO have different privileged reaction partners. CH 4 competes with water in reacting on the same oxygen sites. This competition is reflected in the strong influence of increasing pCH 4 on the GŽ t . curves after transitions from dried into wet atmospheres in which, with increasing pCH 4 : Ži. the maximum G M heights decrease, Žii. the initial decay rate dGrdt describes a maximum. In contrast to CH 4 , CO reacts mainly on oxygen sites which are not involved in the interaction with water. This is reflected in the very slight Ževen negligible for pCO F 100 ppm. influence of increasing pCO on the GŽ t . curves after transitions from dried into wet atmospheres.
w3x N. Barsan, R. Ionescu, The mechanism of interaction between CO and SnO 2 surface — the role of water vapour, Sensors and Actuators B 12 Ž1993. 71–75. w4x R. Ionescu, Ageing and p-type conduction in SnO 2 gas sensors, accepted for publication in Sensors and Actuators B. Radu Ionescu is a senior researcher and head of the Gas Sensors Group in the National Institute of Materials Physics. He graduated in Physics in 1963 in the University of Bucharest and took his PhD degree in 1979, with a thesis on the ‘‘Transport phenomena in V–VI semiconductor compounds’’. In 1985, he started to work in the field of SnO 2 gas sensors. He is the author of more than 50 scientific papers in amorphous and crystalline semiconductors. Ana Vancu is a senior researcher within the Gas Sensors Group. She graduated in Physics in 1959 at the University of Bucharest, then worked as a scientific researcher in the Institute of Physics, where she obtained her PhD degree in 1974, with a thesis on the ‘‘Optical properties of amorphous semiconductors’’. In 1986, she joined the Gas Sensors Group. She is an author of more than 60 scientific papers, contributions in scientific monographies and patents.
References w1x D.E. Williams, in: P.T. Moseley, B.C. Tofield ŽEds.., Solid State Gas Sensors, Chaps. 5.1, Adam Hilger IOP, Publishing, 1987, 71. w2x K. Ihokura, J. Watson, The Stanic Oxide Gas Sensor, CRC Press, Boca Raton, FL, 1994.
Adelina Tomescu is a researcher within the Gas Sensors Group. She graduated in Physics in 1989 at the University of Bucharest, then worked as an assistant researcher in the Institute of Physics, Gas Sensors Group. Since 1998, she has been a PhD student and a researcher in the same group. She is the author of more than 17 scientific papers.