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Ag structures in a propane atmosphere
Materials Characterization 58 (2007) 740 – 744
Gas-sensing characteristics of undoped-SnO2 thin films and Ag/SnO2 and SnO2/Ag structures in a propane atmosphere J. Aguilar-Leyva ⁎, A. Maldonado, M. de la L. Olvera Departamento de Ingeniería Eléctrica, CINVESTAV-IPN, Apdo. Postal 14-740, México, D.F., 07000, Mexico Received 27 June 2006; received in revised form 10 October 2006; accepted 16 November 2006
Abstract The gas-sensitivity properties of SnO2 thin films as well as Ag/SnO2 and SnO2/Ag structures in an atmosphere containing propane (C3H8) were examined. The SnO2 and Ag films were deposited by a spray pyrolysis technique and vacuum evaporation, respectively. Soda-lime glass was used as the substrate for all the structures in this study. The sensors were measured in a propane atmosphere with different gas concentrations (50, 100, 200, 300, 400, and 500 ppm) at different operation temperatures (22, 100, 200, and 300 °C). The Ag/SnO2 structure, sintered a 400 °C for 1 h, showed the highest sensitivity, of the order of 400 at an operation temperature of 200 °C. The results show that the addition of Ag in the tin oxide films effectively acts as a catalyst in propane sensors and confirm the potential feasibility of using Ag as a catalyst in SnO2-based propane sensors. © 2006 Elsevier Inc. All rights reserved. Keywords: Thin films; Gas sensors; Gas propane; Tin oxide
1. Introduction The use of liquefied petroleum gas (LPG) has increased with the increase of the world population and industrialization. As a result the number of accidental explosions due to gas leakage has increased. According to the Mexican LPG mixture the main components of LPG are propane (C3H8 = 60%) and butane (C4H10 = 40%). Chemical sensors offer excellent capabilities in monitoring the environment as well as preventing accidents in domestic and industrial fields. Since 1962 Seiyama et al., as other researchers, have researched on the detection of different toxic pollutants and combustible gases around the world [1–5]. Semiconductor oxides such as tin oxide (SnO2) [4–7] ⁎ Corresponding author. Tel.: +52 55 5061 3784; fax: +52 55 5061 3978. E-mail address: [email protected] (J. Aguilar-Leyva). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.11.016
and zinc oxide (ZnO) [8] have been widely employed in gas-sensing applications. A few studies have been carried out on other oxides, such as FeO and CeO2 [9,10]. SnO2 sensors have many advantages over other sensors, such as high sensitivity at low gas concentrations, low operation temperature, simple design, and low cost of tin sources. However, other problems such as the low selectivity and low stability are associated with SnO2 sensors. These disadvantages may be overcome by using suitable additives, mainly noble metals such as Ag, Pt, Pd, Au and Ru [11–20]. An important aspect about the gas-sensing devices is the cost of manufacturing and raw materials, which are directly associated with the selection of deposition techniques [16,17]. It is a challenge to process new sensing materials using inexpensive techniques. One cost-effective technique for growing semiconductor oxides is the spray pyrolysis technique. It grows high quality thin
J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
Fig. 1. Cross-section view of the structures produced: (a) SnO2 film, (b) SnO2/Ag, and (c) Ag/SnO2.
films with acceptable reproducibility. These thin films are much cheaper than the volumetric materials, which are sintered semiconductor oxides and are used in the commercial sensors. The operation principle of a thin film gas sensor is based on the modulation of the electrical surface resistance, which is developed as a consequence of the chemical and electronic interaction between the semiconductor oxide surface and the surrounding oxidizing or reducing atmosphere. As a result of this interaction, a change in the electrical resistance is produced at the semiconductor surface. The magnitude of the sensitivity of the film is estimated by means of this surface resistance change [18–20]. In this study, we are interested in sensing the propane gas, C3H8, because it is one of the main constituents in the Mexican mixture of LPG gas. Three different systems, SnO2 films and the SnO2/Ag and Ag/SnO2 structures on soda-lime glass substrates were synthesized and characterized.
substrates (∼ 10 × 25 × 1 mm) at a constant deposition temperature of 300 °C, with an accuracy of ± 1 °C. In order to analyze the effect of the thickness on the sensitivity properties of the SnO2 films the film thickness was varied, and measured by a KLA Tencor P15 profilmeter, which has a resolution of 0.1 nm. For the SnO2/Ag and Ag/SnO2 structures, a Sn film with a constant thickness of ∼40 nm was used. Ten milligrams of Ag were evaporated onto or under the SnO2 films to manufacture the SnO2/Ag and Ag/ SnO2 structures by a vacuum evaporation technique. The final Ag films have an average thickness around ∼ 7 nm. Fig. 1 shows a schematic diagram of the three arrays manufactured. The ohmic contacts were manufactured on the film surface using a silver conductive adhesive fluid. All the arrays performed were annealed in a vacuum atmosphere at 400 °C, during 1 h. Fig. 2 shows a diagram of the measurement experimental set-up used for the electrical characterization. The electrical resistance of the films was measured in a vacuum-chamber containing propane gas by a Keithley 2001 multimeter using two tungsten electrodes. These measurements were performed at different propane concentrations and operation temperatures, which were electronically controlled. The sensitivity of the sensors S was calculated using the following mathematical expression: S ¼ ðRg −R0 Þ=R0 where, R0 is the resistance of the sensor in the presence of air and Rg is the resistance in the presence of the test gas, in this case propane gas.
2. Experimental Tin oxide thin films were prepared from an alcoholic solution with a 0.2 M concentration of stannic chloride (SnCl4·5H2O) dissolved in ethanol. This starting solution was chemically sprayed over soda-lime glass
741
Fig. 2. Schematic diagram of the measurements of set-up.
742
J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
3. Results and discussion 3.1. SnO2 film Fig. 1a shows a schematic diagram of this system. Fig. 3 shows the sensitivity variation of a SnO2 film as a function of propane concentration in the range of 5 to 500 ppm at different operation temperatures, namely, 22, 100, 200, and 300 °C. The sensitivity values vary with both the operation temperature and the gas concentration. The best response is presented at 200 °C, and it is increased as the propane concentration increases. The highest sensitivity value obtained was of the order of 13 which was present at 500 ppm. The low sensitivity values and saturation phenomenon observed at low propane concentrations at different temperatures are thought to arise at low operation temperatures (lower than 200 °C, the optimum operation temperature) where the available energy is not enough to carry out the absorption–desorption reactions on the surface, and then the stabilization of surface electronic charge occurs. Whereas at temperatures higher than 200 °C, the surface has already become saturated and consequently no more absorption–desorption reactions can be developed. Fig. 4 shows the effect of film thickness on the sensitivity for a SnO2 film, measured in an atmosphere of 500 ppm of propane and an operation temperature of 200 °C. It is observed that the sensitivities of SnO2 films are strongly dependent on the thickness. It is evident that the thinner films present higher sensitivity values. This result is associated with both a higher surface electronic activity in thin films as compared with thick films and a higher electrical contribution of the surface with respect to the volume.
The SnO2/Ag structure was obtained from a SnO2 thin film (40 nm) deposited on a Ag thin film, previously evaporated on a soda-lime glass substrate. Fig. 1b shows a schematic diagram of this array. Fig. 5 shows the sensitivity variation of this structure as the C3H8 concentration is increased, at different operation temperatures. The sensitivity is low in all the cases, and has a very small increase as the gas concentration increases. The optimum operation temperature was 200 °C, giving rise to a maximum sensitivity on the order of 3 for a gas concentration of 100 ppm. At low operation temperatures a similar behavior to the SnO2 films was presented. It was a surprising fact that the sensitivities obtained in this structure were lower than those obtained in a single
Fig. 3. Sensitivity variation as a function of C3H8 concentration for a SnO2 film at different operation temperatures.
Fig. 5. Sensitivity variation as a function of C3H8 concentration at different operation temperatures for a SnO2/Ag structure.
Fig. 4. Sensitivity variation as a function of the thickness for a SnO2 film, measured at 500 ppm of propane and an operation temperature of 200 °C.
3.2. SnO2/Ag structure
J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
Fig. 6. Sensitivity of the Ag/SnO2 structure as a function of C3H8 concentration at different operation temperatures.
layer SnO2 thin film. This poor sensitivity can be attributed to the fact that the Ag was not diffused appropriately into the SnO2 film during the annealing process as was expected. Additionally, it should be considered that the deposition of the SnO2 film, in this case, was performed on a silver film, instead of a glass substrate as was the case for the single layer SnO2 film, which could lead to different physical properties of the film. 3.3. Ag/SnO2 structure In this structure the Ag film was deposited onto a SnO2 film, which was previously deposited on the sodalime glass substrate. Fig. 1c shows a schematic diagram of the corresponding array, Ag/SnO2. The sensitivity behavior with the propane concentration is shown in Fig. 6. This structure showed its maximum response at an operation temperature of
743
Fig. 8. Sensitivity variation as a function of operation temperature for the three systems manufactured; SnO2, SnO2/Ag, and Ag/SnO2 in a C3H8 atmosphere of 500 ppm.
200 °C for 500 ppm of C3H8. The sensitivity magnitude of this structure is higher than that obtained in the other two systems (see Fig. 7). Fig. 8 presents the sensitivity variation as a function of operation temperature for the three different systems manufactured, SnO2, SnO2/Ag and Ag/SnO2. It is clear that the maximum sensitivity is observed in the Ag/SnO2 structure, and it is of the order of 400 for an operation temperature of 200 °C. Based on these observations, we can conclude that the Ag/SnO2 structure shows the best gas-sensing properties among all the arrays. This result suggests that it is a more appropriate way for incorporating the noble catalyst metal (Ag) into the structure, because the adsorption– desorption reactions between the reducing gas and the surface region (including the grain boundaries) increase, which leads to a reduction process on the sensor surface and a consequent decrease of the surface electrical resistance. 4. Conclusions
Fig. 7. Sensitivity of SnO2, SnO2/Ag, and Ag/SnO2 as a function of concentration C3H8 at an operation temperature of 200 °C.
Three arrays, SnO2, SnO2/Ag and Ag/SnO2 for sensing propane gas were prepared using both the spray pyrolysis and thermal evaporation techniques. The results show that the sensitivity of the SnO2 film decreases as the film thickness increases. The Ag/SnO2 structure exhibited the highest sensitivity (400) at an operation temperature of 200 °C and a gas concentration of 500 ppm. This result suggests that the sensing properties of the SnO2 film are improved by the deposition of a Ag film onto a SnO2 film. This structure offers excellent gas-sensing properties and it can be incorporated in C3H8 and other hydrocarbon sensors. A detailed study of the sensing mechanisms of these structures is needed.
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J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
Acknowledgement The authors acknowledge to M. A Luna-Arias and E. Aguilar for the technical assistance. This work was supported by the Consejo Nacional de Ciencia y Tecnología (CONACyT) through the project No. 39487. References [1] Seiyama T, Kato A, Fujiishi K, Nagatani M. A new detector for gaseous components using semiconducting thin films. Anal Chem 1962;34:1502–3. [2] Kohl D. Surface processes in the detection of reducing gases with SnO2-based devices. Sens Actuators 1988;18:71–113. [3] Phani AR, Manorama S, Rao VJ. Preparation characterization and electrical properties of SnO2 based liquid petroleum gas sensor. Mater Chem Phys 1999;58:101–8. [4] Schmid Wolf, Bârsan Nicolae, Weimar Udo. Sensing of hydrocarbons and CO in low oxygen conditions with tin dioxide sensors: possible conversion paths. Sens Actuators B Chem 2004;103:362–8. [5] Heiland G. Homogeneous semiconducting gas sensors. Sens Actuators 1982;2:343–61. [6] Watson J. The tin oxide gas sensor and its applications. Sens Actuators 1984;5:29–42. [7] Trivikrama Rao GS, Madhavendra SS. Tin dioxide based sensors for the detection of liquefied petroleum gas. J Mater Sci Lett 1995;14:529–30. [8] Mitra P, Chartterjee AP, Maiti HS. ZnO thin film sensor. Mater Lett 1998;35:33–8. [9] Sahner Kathy, Moos Ralf, Matam Mahesh, Tunney James J, Post Michael. Hydrocarbon sensing with thick and thin film p-type conducting perovskite materials. Sens Actuators B Chem 2005;108:102–12.
[10] Kim Jin, Han Sang Do, Singh Ishwar, Lee Hi Doek, Wang Jin Suk. Sensitivity enhancement for CO gas detection using a SnO2–CeO2– PdOx system. Sens Actuators B Chem 2005;107: 825–30. [11] Matsubara Shogo, Kaneko Shinichiro, Morimoto Shinji, Shimizu Shoichi, Ishira Tatsumi, Takita Yusaka. A practical capacitive type CO2 sensor using CeO2/BaCO3/CuO ceramics. Sens Actuators B Chem 2000;65:128–32. [12] Phani AR, Pelino M. Effect of noble metals on selective detection of liquid petroleum gas by SnO2. J Mater Res 1998;1:1780–5. [13] Shim Chang-Hyun, Lee Dae-Sik, Hwang Sook-I, Lee Myoung-Bok, Huh Jeung-Soo, Lee Duk-Dong. Gas sensing characteristics of SnO2 thin film fabricated by thermal oxidation of a Sn/Pt double layer. Sens Actuators B Chem 2002;81:176–81. [14] Ménini P, Parret F, Gerrero M, Soulantica K, Erades L, Maisonnat A, Chaudret B. CO response of nanostructured SnO2 gas sensor doped with palladium and platinum. Sens Actuators B Chem 2004;103:111–4. [15] Chaudhary VA, Mulla LS, Vijayamohanan K. Selective gas sensing properties of surface ruthenated tin oxide. J Mater Res 1999;14:185–8. [16] Gupta S, Roy RK, Pal Chowdhury M, Pal AK. Synthesis of SnO2/Pd composite films by PVD route for a liquid petroleum gas sensor. Vacuum 2004;75:111–9. [17] Chang-Hyun S, Dae-Sik L, Sook-I H, Myoung-Bok L, JeungSoo H, Duk-Dong L. Gas sensing characteristics of SnO2 thin film fabricated by thermal oxidation of a Sn/Pt double layer. Sens Actuators B Chem 2002;81:176–81. [18] Moseley PT. Solid state gas sensors. Bristol: Adam Hilger; 1987. p. 71–4. [19] Madau MJ, Morrison SR. Chemical sensing with solid state devices. San Diego CA: Academic Press; 1989. p. 1–12. [20] Kupriyanov LY. Semiconductor sensors in physico-chemical studies, vol. 4. Handbook of sensors and actuators. Amsterdam: Elsevier; 1996. p. 5–9.
Gas-sensing characteristics of undoped-SnO2 thin films and Ag/SnO2 and SnO2/Ag structures in a propane atmosphere J. Aguilar-Leyva ⁎, A. Maldonado, M. de la L. Olvera Departamento de Ingeniería Eléctrica, CINVESTAV-IPN, Apdo. Postal 14-740, México, D.F., 07000, Mexico Received 27 June 2006; received in revised form 10 October 2006; accepted 16 November 2006
Abstract The gas-sensitivity properties of SnO2 thin films as well as Ag/SnO2 and SnO2/Ag structures in an atmosphere containing propane (C3H8) were examined. The SnO2 and Ag films were deposited by a spray pyrolysis technique and vacuum evaporation, respectively. Soda-lime glass was used as the substrate for all the structures in this study. The sensors were measured in a propane atmosphere with different gas concentrations (50, 100, 200, 300, 400, and 500 ppm) at different operation temperatures (22, 100, 200, and 300 °C). The Ag/SnO2 structure, sintered a 400 °C for 1 h, showed the highest sensitivity, of the order of 400 at an operation temperature of 200 °C. The results show that the addition of Ag in the tin oxide films effectively acts as a catalyst in propane sensors and confirm the potential feasibility of using Ag as a catalyst in SnO2-based propane sensors. © 2006 Elsevier Inc. All rights reserved. Keywords: Thin films; Gas sensors; Gas propane; Tin oxide
1. Introduction The use of liquefied petroleum gas (LPG) has increased with the increase of the world population and industrialization. As a result the number of accidental explosions due to gas leakage has increased. According to the Mexican LPG mixture the main components of LPG are propane (C3H8 = 60%) and butane (C4H10 = 40%). Chemical sensors offer excellent capabilities in monitoring the environment as well as preventing accidents in domestic and industrial fields. Since 1962 Seiyama et al., as other researchers, have researched on the detection of different toxic pollutants and combustible gases around the world [1–5]. Semiconductor oxides such as tin oxide (SnO2) [4–7] ⁎ Corresponding author. Tel.: +52 55 5061 3784; fax: +52 55 5061 3978. E-mail address: [email protected] (J. Aguilar-Leyva). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.11.016
and zinc oxide (ZnO) [8] have been widely employed in gas-sensing applications. A few studies have been carried out on other oxides, such as FeO and CeO2 [9,10]. SnO2 sensors have many advantages over other sensors, such as high sensitivity at low gas concentrations, low operation temperature, simple design, and low cost of tin sources. However, other problems such as the low selectivity and low stability are associated with SnO2 sensors. These disadvantages may be overcome by using suitable additives, mainly noble metals such as Ag, Pt, Pd, Au and Ru [11–20]. An important aspect about the gas-sensing devices is the cost of manufacturing and raw materials, which are directly associated with the selection of deposition techniques [16,17]. It is a challenge to process new sensing materials using inexpensive techniques. One cost-effective technique for growing semiconductor oxides is the spray pyrolysis technique. It grows high quality thin
J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
Fig. 1. Cross-section view of the structures produced: (a) SnO2 film, (b) SnO2/Ag, and (c) Ag/SnO2.
films with acceptable reproducibility. These thin films are much cheaper than the volumetric materials, which are sintered semiconductor oxides and are used in the commercial sensors. The operation principle of a thin film gas sensor is based on the modulation of the electrical surface resistance, which is developed as a consequence of the chemical and electronic interaction between the semiconductor oxide surface and the surrounding oxidizing or reducing atmosphere. As a result of this interaction, a change in the electrical resistance is produced at the semiconductor surface. The magnitude of the sensitivity of the film is estimated by means of this surface resistance change [18–20]. In this study, we are interested in sensing the propane gas, C3H8, because it is one of the main constituents in the Mexican mixture of LPG gas. Three different systems, SnO2 films and the SnO2/Ag and Ag/SnO2 structures on soda-lime glass substrates were synthesized and characterized.
substrates (∼ 10 × 25 × 1 mm) at a constant deposition temperature of 300 °C, with an accuracy of ± 1 °C. In order to analyze the effect of the thickness on the sensitivity properties of the SnO2 films the film thickness was varied, and measured by a KLA Tencor P15 profilmeter, which has a resolution of 0.1 nm. For the SnO2/Ag and Ag/SnO2 structures, a Sn film with a constant thickness of ∼40 nm was used. Ten milligrams of Ag were evaporated onto or under the SnO2 films to manufacture the SnO2/Ag and Ag/ SnO2 structures by a vacuum evaporation technique. The final Ag films have an average thickness around ∼ 7 nm. Fig. 1 shows a schematic diagram of the three arrays manufactured. The ohmic contacts were manufactured on the film surface using a silver conductive adhesive fluid. All the arrays performed were annealed in a vacuum atmosphere at 400 °C, during 1 h. Fig. 2 shows a diagram of the measurement experimental set-up used for the electrical characterization. The electrical resistance of the films was measured in a vacuum-chamber containing propane gas by a Keithley 2001 multimeter using two tungsten electrodes. These measurements were performed at different propane concentrations and operation temperatures, which were electronically controlled. The sensitivity of the sensors S was calculated using the following mathematical expression: S ¼ ðRg −R0 Þ=R0 where, R0 is the resistance of the sensor in the presence of air and Rg is the resistance in the presence of the test gas, in this case propane gas.
2. Experimental Tin oxide thin films were prepared from an alcoholic solution with a 0.2 M concentration of stannic chloride (SnCl4·5H2O) dissolved in ethanol. This starting solution was chemically sprayed over soda-lime glass
741
Fig. 2. Schematic diagram of the measurements of set-up.
742
J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
3. Results and discussion 3.1. SnO2 film Fig. 1a shows a schematic diagram of this system. Fig. 3 shows the sensitivity variation of a SnO2 film as a function of propane concentration in the range of 5 to 500 ppm at different operation temperatures, namely, 22, 100, 200, and 300 °C. The sensitivity values vary with both the operation temperature and the gas concentration. The best response is presented at 200 °C, and it is increased as the propane concentration increases. The highest sensitivity value obtained was of the order of 13 which was present at 500 ppm. The low sensitivity values and saturation phenomenon observed at low propane concentrations at different temperatures are thought to arise at low operation temperatures (lower than 200 °C, the optimum operation temperature) where the available energy is not enough to carry out the absorption–desorption reactions on the surface, and then the stabilization of surface electronic charge occurs. Whereas at temperatures higher than 200 °C, the surface has already become saturated and consequently no more absorption–desorption reactions can be developed. Fig. 4 shows the effect of film thickness on the sensitivity for a SnO2 film, measured in an atmosphere of 500 ppm of propane and an operation temperature of 200 °C. It is observed that the sensitivities of SnO2 films are strongly dependent on the thickness. It is evident that the thinner films present higher sensitivity values. This result is associated with both a higher surface electronic activity in thin films as compared with thick films and a higher electrical contribution of the surface with respect to the volume.
The SnO2/Ag structure was obtained from a SnO2 thin film (40 nm) deposited on a Ag thin film, previously evaporated on a soda-lime glass substrate. Fig. 1b shows a schematic diagram of this array. Fig. 5 shows the sensitivity variation of this structure as the C3H8 concentration is increased, at different operation temperatures. The sensitivity is low in all the cases, and has a very small increase as the gas concentration increases. The optimum operation temperature was 200 °C, giving rise to a maximum sensitivity on the order of 3 for a gas concentration of 100 ppm. At low operation temperatures a similar behavior to the SnO2 films was presented. It was a surprising fact that the sensitivities obtained in this structure were lower than those obtained in a single
Fig. 3. Sensitivity variation as a function of C3H8 concentration for a SnO2 film at different operation temperatures.
Fig. 5. Sensitivity variation as a function of C3H8 concentration at different operation temperatures for a SnO2/Ag structure.
Fig. 4. Sensitivity variation as a function of the thickness for a SnO2 film, measured at 500 ppm of propane and an operation temperature of 200 °C.
3.2. SnO2/Ag structure
J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
Fig. 6. Sensitivity of the Ag/SnO2 structure as a function of C3H8 concentration at different operation temperatures.
layer SnO2 thin film. This poor sensitivity can be attributed to the fact that the Ag was not diffused appropriately into the SnO2 film during the annealing process as was expected. Additionally, it should be considered that the deposition of the SnO2 film, in this case, was performed on a silver film, instead of a glass substrate as was the case for the single layer SnO2 film, which could lead to different physical properties of the film. 3.3. Ag/SnO2 structure In this structure the Ag film was deposited onto a SnO2 film, which was previously deposited on the sodalime glass substrate. Fig. 1c shows a schematic diagram of the corresponding array, Ag/SnO2. The sensitivity behavior with the propane concentration is shown in Fig. 6. This structure showed its maximum response at an operation temperature of
743
Fig. 8. Sensitivity variation as a function of operation temperature for the three systems manufactured; SnO2, SnO2/Ag, and Ag/SnO2 in a C3H8 atmosphere of 500 ppm.
200 °C for 500 ppm of C3H8. The sensitivity magnitude of this structure is higher than that obtained in the other two systems (see Fig. 7). Fig. 8 presents the sensitivity variation as a function of operation temperature for the three different systems manufactured, SnO2, SnO2/Ag and Ag/SnO2. It is clear that the maximum sensitivity is observed in the Ag/SnO2 structure, and it is of the order of 400 for an operation temperature of 200 °C. Based on these observations, we can conclude that the Ag/SnO2 structure shows the best gas-sensing properties among all the arrays. This result suggests that it is a more appropriate way for incorporating the noble catalyst metal (Ag) into the structure, because the adsorption– desorption reactions between the reducing gas and the surface region (including the grain boundaries) increase, which leads to a reduction process on the sensor surface and a consequent decrease of the surface electrical resistance. 4. Conclusions
Fig. 7. Sensitivity of SnO2, SnO2/Ag, and Ag/SnO2 as a function of concentration C3H8 at an operation temperature of 200 °C.
Three arrays, SnO2, SnO2/Ag and Ag/SnO2 for sensing propane gas were prepared using both the spray pyrolysis and thermal evaporation techniques. The results show that the sensitivity of the SnO2 film decreases as the film thickness increases. The Ag/SnO2 structure exhibited the highest sensitivity (400) at an operation temperature of 200 °C and a gas concentration of 500 ppm. This result suggests that the sensing properties of the SnO2 film are improved by the deposition of a Ag film onto a SnO2 film. This structure offers excellent gas-sensing properties and it can be incorporated in C3H8 and other hydrocarbon sensors. A detailed study of the sensing mechanisms of these structures is needed.
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J. Aguilar-Leyva et al. / Materials Characterization 58 (2007) 740–744
Acknowledgement The authors acknowledge to M. A Luna-Arias and E. Aguilar for the technical assistance. This work was supported by the Consejo Nacional de Ciencia y Tecnología (CONACyT) through the project No. 39487. References [1] Seiyama T, Kato A, Fujiishi K, Nagatani M. A new detector for gaseous components using semiconducting thin films. Anal Chem 1962;34:1502–3. [2] Kohl D. Surface processes in the detection of reducing gases with SnO2-based devices. Sens Actuators 1988;18:71–113. [3] Phani AR, Manorama S, Rao VJ. Preparation characterization and electrical properties of SnO2 based liquid petroleum gas sensor. Mater Chem Phys 1999;58:101–8. [4] Schmid Wolf, Bârsan Nicolae, Weimar Udo. Sensing of hydrocarbons and CO in low oxygen conditions with tin dioxide sensors: possible conversion paths. Sens Actuators B Chem 2004;103:362–8. [5] Heiland G. Homogeneous semiconducting gas sensors. Sens Actuators 1982;2:343–61. [6] Watson J. The tin oxide gas sensor and its applications. Sens Actuators 1984;5:29–42. [7] Trivikrama Rao GS, Madhavendra SS. Tin dioxide based sensors for the detection of liquefied petroleum gas. J Mater Sci Lett 1995;14:529–30. [8] Mitra P, Chartterjee AP, Maiti HS. ZnO thin film sensor. Mater Lett 1998;35:33–8. [9] Sahner Kathy, Moos Ralf, Matam Mahesh, Tunney James J, Post Michael. Hydrocarbon sensing with thick and thin film p-type conducting perovskite materials. Sens Actuators B Chem 2005;108:102–12.
[10] Kim Jin, Han Sang Do, Singh Ishwar, Lee Hi Doek, Wang Jin Suk. Sensitivity enhancement for CO gas detection using a SnO2–CeO2– PdOx system. Sens Actuators B Chem 2005;107: 825–30. [11] Matsubara Shogo, Kaneko Shinichiro, Morimoto Shinji, Shimizu Shoichi, Ishira Tatsumi, Takita Yusaka. A practical capacitive type CO2 sensor using CeO2/BaCO3/CuO ceramics. Sens Actuators B Chem 2000;65:128–32. [12] Phani AR, Pelino M. Effect of noble metals on selective detection of liquid petroleum gas by SnO2. J Mater Res 1998;1:1780–5. [13] Shim Chang-Hyun, Lee Dae-Sik, Hwang Sook-I, Lee Myoung-Bok, Huh Jeung-Soo, Lee Duk-Dong. Gas sensing characteristics of SnO2 thin film fabricated by thermal oxidation of a Sn/Pt double layer. Sens Actuators B Chem 2002;81:176–81. [14] Ménini P, Parret F, Gerrero M, Soulantica K, Erades L, Maisonnat A, Chaudret B. CO response of nanostructured SnO2 gas sensor doped with palladium and platinum. Sens Actuators B Chem 2004;103:111–4. [15] Chaudhary VA, Mulla LS, Vijayamohanan K. Selective gas sensing properties of surface ruthenated tin oxide. J Mater Res 1999;14:185–8. [16] Gupta S, Roy RK, Pal Chowdhury M, Pal AK. Synthesis of SnO2/Pd composite films by PVD route for a liquid petroleum gas sensor. Vacuum 2004;75:111–9. [17] Chang-Hyun S, Dae-Sik L, Sook-I H, Myoung-Bok L, JeungSoo H, Duk-Dong L. Gas sensing characteristics of SnO2 thin film fabricated by thermal oxidation of a Sn/Pt double layer. Sens Actuators B Chem 2002;81:176–81. [18] Moseley PT. Solid state gas sensors. Bristol: Adam Hilger; 1987. p. 71–4. [19] Madau MJ, Morrison SR. Chemical sensing with solid state devices. San Diego CA: Academic Press; 1989. p. 1–12. [20] Kupriyanov LY. Semiconductor sensors in physico-chemical studies, vol. 4. Handbook of sensors and actuators. Amsterdam: Elsevier; 1996. p. 5–9.