Ultrasensitive gas sensor using Co₃O₄-modified SnO₂

SnO₂ powder was loaded with 0-5 % Co₃O₄ by mass by a ball milling method. The resulting composites were screen printed onto a substrate to test the sensing properties to CO and H₂. It was found that the sensor response, defined as the ratio of electrical resistance in air to that in gas, was greatly promoted with 0.5 or 1.0 % Co ₃O₄ loading, while further loadings (3 or 5 %) gave an adverse effect. For the 1 % Co₃O₄-loaded device, for example, the responses to 100, 50 and 10 ppm CO in air were as large as 375, 181 and 23 at 250 °C, respectively, and those to 50, 10 and 1 ppm H ₂ in air were 568, 181 and 53 at 300 °C, respectively. The electrical resistance in air increased significantly with increasing Co ₃O₄ loading, indicating an electronic interaction between Co₃O₄ and SnO₂. The change of the electronic interaction with a change in the redox state of Co₃O₄ appears to be an origin of the ultra high sensitivity to CO and H₂.