Co₃O₄ as p-Type Material for CO Sensing in Humid Air

Nanocrystalline cobalt oxide Co₃O₄ has been prepared by precipitation and subsequent thermal decomposition of a carbonate precursor, and has been characterized in detail using XRD, transmission electron microscopy, and FTIR spectroscopy. The sensory characteristics of the material towards carbon monoxide in the concentration range 6.7-20 ppm have been examined in both dry and humid air. A sensor signal is achieved in dry air at sufficiently low temperatures T = 80-120 °C, but the increase in relative humidity results in the disappearance of sensor signal in this temperature range. At temperatures above 200 °C the inversion of the sensor signal in dry air was observed. In the temperature interval 180-200 °C the sensor signal toward CO is nearly the same at 0, 20 and 60% r.h. The obtained results are discussed in relation with the specific features of the adsorption of CO, oxygen, and water molecules on the surface of Co₃O₄. The independence of the sensor signal from the air humidity combined with a sufficiently short response time at a moderate operating temperature makes Co₃O₄ a very promising material for CO detection in conditions of variable humidity.