Effects of gas diffusivity and reactivity on sensing properties of thick film SnO2-based sensors

doi:10.1016/S0925-4005(97)00247-5

Sensors and Actuators B: Chemical

Volume 46, Issue 3, 15 May 1998, Pages 163-168

https://doi.org/10.1016/S0925-4005(97)00247-5 Get rights and content

Abstract

Effects of gas diffusivity and reactivity on gas-sensing properties of porous thick film SnO₂-based sensors have been investigated by employing several kinds of objective gases. The interior of a pure SnO₂ sensor was confirmed to exhibit higher sensitivity to H₂ than the surface. A reverse tendency was observed for CH₄ or C₂H₅OH with larger molecular size than O₂. Excess enhancement in catalytic activity of sensor materials, for example by loading 1.0 wt% Pt or Pd, resulted in deterioration of the interior H₂ sensitivity, while the interior CH₄ or C₂H₅OH sensitivity was enhanced by the Pt or Pd loading. Loading of 1.0 wt% Au led to significant enhancement in both the interior and the surface H₂ sensitivity, irrespective of the small chemical sensitization effect of Au.

Introduction

Numerous efforts have so far been directed to improving the sensitivity of semiconductor gas sensors to an inflammable gas by adding a small amount of a sensitizer, generally a noble metal 1, 2, 3, 4. In most cases, the improved sensitivity is due to the chemical sensitization by noble metals [5], whereas the electronic sensitization is dominant in some cases of SnO₂ loaded with Ag [1]or Pd [6]. In the former type, an objective gas is activated by a noble metal to react with chemisorbed oxygen on the surface of a semiconductive metal oxide as a sensor material. Thus, the sensitivity is mainly controlled by the catalytic activity of a sensor material, or by the reactivity of an objective gas on the surface of the sensor material.

Besides the reactivity, diffusivity of gases (more precisely, difference in diffusivity between an objective and oxygen gas through a porous sensor) was found to be another important factor determining the sensitivity of the interior region of sensors 7, 8, 9. For example, the reason for the higher H₂ sensitivity observed for the interior region of a thick film SnO₂ sensor than the surface region has proved to be attributed to lower diffusivity of O₂ than H₂ into the interior region [7]. Enhancement in H₂ sensitivity of an SnO₂ thin film sensor by the coating with a SiO₂ layer was attributed to the reduced permeation amount of O₂ into the interior of the sensor 8, 9. The limited diffusion of O₂, in comparison with that of H₂, leads to the lowered oxygen partial pressure inside the sensor and then to the decreased coverage of chemisorbed oxygen on the sensor material. This should result in increased sensitivity. Loading of a greater amount of 1.0 wt% Pt than usual to the thick film SnO₂ sensor resulted in a drastic decrease in the interior H₂ sensitivity, while some evidence of chemical sensitization by Pt was observed only at the surface region. In this case, the reduced permeation amount of H₂ into the interior, which was induced by the accelerated combustion of H₂ owing to the markedly enhanced catalytic activity, has proved to be responsible for the drastic decrease in the interior sensitivity [7]. Thus, our previous studies have revealed that H₂ sensing properties of SnO₂-based sensors are determined by the combined effects of the gas diffusivity and gas reactivity.

To get more information on the sensitivity determining factors of thick film SnO₂-based sensors, in the present study effects of gas diffusivity and reactivity on the gas-sensing properties have been investigated systematically by employing several kinds of metal sensitizers and objective gases.

Section snippets

Experimental

A base sensor material employed in this study was pure SnO₂ powder with a surface area of 75.0 m² g⁻¹ supplied by the Catalysis Society of Japan. Tin dioxide powder loaded with 1.0 wt% of a noble metal, such as Pt, Pd or Au, was prepared in the same manner as used for TiO₂-based sensors [10]. Porous thick film sensors having interior and surface electrodes were fabricated on a porous mullite tube of 2.0 mmφ outer diameter and 1.7 mmφ inner diameter, in a manner similar to that reported

H₂-sensing properties

Fig. 1 shows variations in surface and interior sensitivities of four kinds of thick film SnO₂-based sensors with operating temperature. The interior region of a pure SnO₂ sensor was found to be more sensitive to 2.0% H₂ than the surface region at temperatures less than 500°C, as shown in Fig. 1(a). In addition, the temperature (T_M) at the maximum sensitivity (k_M) was observed around 400°C in the interior region. The temperature was lower than another T_M observed for the surface region. Thus,

Conclusions

The interior and surface gas-sensing properties of thick film sensors were found to be markedly dependent upon the kind of objective gases, that is their molecular size relative to O₂, and the catalytic activity of sensor materials. Thus, it was concluded that both the gas diffusivity and the gas reactivity are very important factors determining the gas sensitivity. In addition, the ability of sensor materials for promoting re-adsorption of gaseous oxygen was suggested to be another important

Yasuhiro Shimizu received his B.Eng. degree in applied chemistry in 1980 and Dr Eng. degree in 1987 from Kyushu University. He has been an associate professor at Nagasaki University since 1987. His current research concentrates on the configurational design of intelligent semiconductor gas sensors, in addition to controlling catalytic activity of sensor materials.

Toru Maekawa received his B.Eng. degree in materials science and engineering in 1995 from Nagasaki University and is now a graduate

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Toru Maekawa received his B.Eng. degree in materials science and engineering in 1995 from Nagasaki University and is now a graduate student to get M.Eng. degree.

Yuichiro Nakamura received his B.Eng. and M.Eng. degree in materials science and engineering in 1991 and in 1993, respectively, from Nagasaki University. He now works in Nippon Silica Industrial.

Makoto Egashira received his B.Eng. degree and M.Eng. degree in applied chemistry in 1966 and in 1968, respectively, and Dr Eng. degree in 1974 from Kyushu University. He has been a professor at Nagasaki University since 1985. His recent interests include the development of new chemical sensors, surface modification of ceramics and application of functional inorganic materials.

¹: Paper presented at the 2nd East Asia Conference on Chemical Sensors, Xi'an, P.R. China, 1995.

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Effects of gas diffusivity and reactivity on sensing properties of thick film SnO2-based sensors1

Abstract

Introduction

Section snippets

Experimental

H2-sensing properties

Conclusions

Sensors and Actuators

Sensors and Actuators

Sensors and Actuators B

Sensors and Actuators B

Effects of gas diffusivity and reactivity on sensing properties of thick film SnO₂-based sensors¹

H₂-sensing properties