Alcohol sensing properties of sol–gel prepared Sr(II)-added cobalt aluminate spinel composites

Strontium(II)-added CoAl2O4 composites prepared by sol–gel technique were investigated for the sensing properties to alcohols like methanol, ethanol, isopropanol and butanol. X-ray diffraction, scanning electron microscopy and nitrogen adsorption/desorption isotherms at 77 K were employed to identify the structural phases, surface morphology and BET surface area of the composites, respectively. The composites were prepared with the molar ratios of Co:Sr as 1.0:0.0, 0.0:1.0, 0.8:0.2, 0.6:0.4, 0.4:0.6, and 0.2:0.8, keeping the aluminum molar ratio as constant and were labeled as CoSA1, CoSA2, CoSA3, CoSA4, CoSA5 and SA, respectively. The samples sintered at 900 °C for 5 h, were subjected to dc resistance measurements in the temperature range from 30 to 250 °C to study the alcohol vapor sensing characteristics. The results revealed that the sensitivity factor (Sf) increased with temperature up to the optimum temperature at which maximum sensitivity is obtained and thereafter decreased. The sensitivity factor was found to increase with increase in concentration from 100 to 5000 ppm at the respective operating temperatures. Among the composites prepared CoSA5 showed the maximum sensitivity factor towards alcohols. The sensitivity factor and response and recovery characteristics showed that the order of sensing alcohols by the composites was butanol > isopropanol > ethanol > methanol, which could be explained on the basis of oxidation of alcohols. The stability of the most sensitive composite CoSA5 was also discussed along with the mechanism of sensing. The response and recovery characteristics of CoSA1, CoSA5 and SA to alcohols were also reported.