Microstructure and CO gas sensing properties of porous ZnO produced by starch addition

Abstract

Effects of corn starch addition on the microstructure and CO gas response were studied by sintering ZnO at 600∼900°C for 3 h in air. The addition of 5 wt% corn starch as the fugitive phase decreased both the grain size and the sintered density of ZnO at all sintering temperatures and thus increased the sensitivity to 200 ppm CO. The increasing relative density and grain size with sintering temperature was accompanied by a decreasing CO gas sensitivity after maximum at 700°C. The temperature showing the maximum CO gas sensitivity decreased with decreasing grain size. Humidity decreased both the hysteresis of electrical conductivity and CO gas sensitivity.

Introduction

The electrical conductivity of ceramic semiconductors may be changed by exposing their surface to the reducing gases such as CO and H₂. Many semiconductor-type gas sensors utilizing this property have been developed based on ZnO, SnO₂, Fe₂O₃, etc. 1, 2, 3, 4, 5. The electrical conductivity change is caused by the reaction of reducing gases with the adsorbed oxygen ion on the semiconductor surface, and thus by a subsequent change in the charge carrier concentration near the surface. The valence state of the adsorbed oxygen ion is known to change with temperature and affects gas response 6, 7, 8. It was reported that the stable oxygen ion is O₂⁻ below 100°C, O⁻ between 100 and 300°C and O⁻² above 300°C [6]. When a reducing gas flows near the n-type ceramic gas sensor materials, the following reaction occurs on the surface of materials and the electrical conductivity of n-type ceramic materials increases [4]. Oxygen ion species is denoted as O⁻ for convenience (Eq. (1)).

$$\text{R+}\text{O}{}^{\mathrm{-}}\text{(}\text{ads}\text{)=R}\text{O}\text{+e}{}^{\mathrm{-}}\text{(R:}\text{reducing gas)}$$

Many efforts are being made to improve the gas sensing properties by making thin and thick film 9, 10or by the addition of catalysts [11].

High sensitivity may be expected for porous specimens which have a large surface area for the gas reaction. The porous specimens are generally made by sintering the sample at low temperature. However, the low sintering temperature does not always result in the required sensitivity and mechanical integrity. Thus SnO₂ is often preferred as a gas sensor since it does not readily sinter. ZnO is less frequently used as a gas sensor partly due to its easy sintering characteristics.

In this study, the porous ZnO was made by adding corn starch in the green body of ZnO and later burning out the corn starch. The microstructure and the CO gas sensing properties were compared with that of ZnO without starch addition. The effect of humidity was also considered 12, 13, 14.