Impedance spectroscopy to identify the conduction mechanisms in WO3 sensors

Knowledge of the conduction mechanisms on gas sensors is necessary to improve the performances of the sensors. The impedance spectroscopy is a powerful method to understand the conduction processes in the metal-oxide gas sensors. The impedance evolution of WO₃ sensors for different concentrations of reducing gas (ethanol) or oxidizing gas (ozone) is studied. The Nyquist responses show that the ozone mainly affects the grain boundaries, whereas ethanol mainly acts on the bulk conduction properties. A comparison between bare WO₃ and palladium (Pd) or gold (Au) activated WO₃ surface shows that Au improves the sensitivity to ethanol, whereas Pd inhibits the sensitivity to both ozone and ethanol. The impedance spectra show a modification of conduction in the grain boundaries, due to the presence of Au and Pd nanoclusters.