ZnO sol–gel derived porous film for CO gas sensing

Fine powder of ZnO was synthesized by the sol–gel method, which was then calcined followed by spin coating on an alumina substrate and tested for CO gas sensing. With variation in the calcination temperature, the shape of gel powder changed into various morphologies: sheet, needle, and sphere. The grain growth rate increased above 700 °C and the larger grains showed higher degree of crystallization. The organic element in gel powders evaporated below 300 °C. The temperature dependence of the electrical conductance showed the sigmoidal shape, but the temperature range of the constant conductance narrowed with the decrease in the calcination temperature. The optimum sensing properties were observed for the specimen calcined at 500 °C, and it degraded with the increase in calcination temperature because of the larger grain size that limits the surface area for gas–solid reaction.