A time resolved study of the response of a WO3 gas sensor to NO2 using AC impedance spectroscopy

The transient response of a tungsten oxide (WO3) based gas sensor to 1.5 ppm nitrogen dioxide (NO2) was studied using AC impedance spectroscopy. It was found that the time taken for the sensor to re-equilibrate fully at 300 °C after the introduction of the target gas was 5 h. Changes in the material’s resistance, capacitance and grain boundary barrier height in response to the introduction of the NO2, were characterised by analysis of AC impedance spectra using a combination of Nyquist and complex modulus plots. The grain boundary resistivity was found to increase substantially on introduction of the NO2, together with a small, associated increase in the bulk resistivity. The grain boundary capacitance remained constant, within experimental error, throughout the equilibration period. The grain boundary barrier height was found to increase from 212 in air to a stable final value of 291 meV after the gas was introduced.