Sensing of oxidizing and reducing gases by sensors prepared using nanoscale Co3O4 powders: A study through Cu substitution

We demonstrate the sensing-mechanism of both oxidising and reducing gases by nanoscale Co3O4 powders through a strategy of Cu-doping in Co3O4. The sensitivity towards both types of gases arises due to presence of Co2+ and Co3+ cations in the (nearly) normal spinel structure of Co3O4. Pellets made up of nanopowders were employed for the detection of -6.5 ppm CO gas present in either pure N2 or in a mixture of synthetic air as carrier gas (which represents the O2 sensing-gas too). The high sensitivity of Co3O4 nanoparticles to detect -6 ppm CO (in N2) arises due to the high surface area of nanopowders exposing a higher number of octahedral Co3+ cations as adsorption sites, whereas the sensitivity towards O2 arises due to partial presence (less number) of octahedral Co2+. To support this mechanism, octahedrally Cu2+ substituted Co3O4 specimens are investigated. The inactive Cu2+ at the octahedral site changes the unexposed tetrahedral Co2+ into Co3+. The presence of inactive Cu2+ at the octahedral site and the burial of the Co3+ at the tetrahedral sites reduce the adsorption sites for O2, thereby drastically reducing the overall O2-gas sensitivity shown by the Cu-substituted sample, although they have higher surface-area (nanoparticles). (c) 2022 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan.