The oxidizing effect of humidity on WO3 based sensors

Semiconducting metal oxide based gas sensors are used in a wide spectrum of fields, ranging from the detection of hazardous gases within the environment to monitoring air quality. WO3 is the second, after SnO2, most commonly used semiconducting metal oxide in commercial gas sensors. Despite its frequent application, the surface reactions responsible for sensing are largely unknown. Here, for the first time, a mechanism for the surface reaction between WO3 and humidity can be concluded from experimental results. DC resistance measurements and operando diffuse reflectance infrared Fourier transform spectroscopy show an oxidation of the WO3 lattice during humidity exposure. The filling of oxygen vacancies by water explains the effects atmospheric humidity has on WO3 based sensors, specifically the increase in resistance, the higher sensor signals to CO and the lower sensor signals to NO2. These findings are a basis for understanding how sensing occurs with WO3 based sensors.