Effect of Ni II ions on the properties of In 2O 3-based ceramic sensors

Abstract

The influence of nickel ions on gas-sensitive properties of sol-gel obtained ceramic semiconductor sensors based on In 2O 3 has been investigated. The addition of Ni II ions to In 2O 3 leads to an increase in sensor resistance when compared to undoped In 2O 3. The increase in resistance is caused by the formation of a Ni II–In 2O 3 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 In 2O 3–NiO sensors to CO at low temperatures (∼200°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 In 2O 3–NiO sensors in CO-containing atmospheres is observed. In 2O 3–NiO sensors allow for the detection of CO in the range of concentrations 500–20 000 ppm, whereas their sensitivity to hydrocarbons is low.