Effect of NiII ions on the properties of In2O3-based ceramic sensors

The influence of nickel ions on gas-sensitive properties of sol-gel obtained ceramic semiconductor sensors based on In2O3 has been investigated. The addition of Ni-II ions to In2O3 leads to an increase in sensor resistance when compared to undoped In2O3. The increase in resistance is caused by the formation of a Ni-II-In2O3 solid solution structure, By using the electron spin resonance (ESR) spectroscopy, it was found that transition of a part of Ni-II ions to Ni-III takes place in a solid solution structure under high-temperature treatment. The reduction of Ni-III to Ni-I and the formation of [Ni-I-CO] complexes at interaction with CO allow us to assume that the sensitivity of In2O3-NiO sensors to CO at low temperatures (similar to 200 degrees C) is caused by the participation of Ni-III ions in CO oxidation. The activation of CO molecules accompanied by electronic density transfer from CO to nickel occurs in the coordination neighborhood of Ni-III ions. Electronic exchange in the complexes [CO-Ni-III] leads to the reduction of Ni-III to Ni-I ions. As a result, the decreasing electric conductivity of In2O3-NiO sensors in GO-containing atmospheres is observed. In2O3-NiO sensors allow for the detection of CO in the range of concentrations 500-20000 ppm, whereas their sensitivity to hydrocarbons is low. (C) 1998 Elsevier Science S.A. All rights reserved.