Effect of Humidity on Light-Activated NO and NO₂ Gas Sensing by Hybrid Materials

Air humidity is one of the main factors affecting the characteristics of semiconductor gas sensors, especially at low measurement temperatures. In this work we analyzed the influence of relative humidity on sensor properties of the hybrid materials based on the nanocrystalline SnO₂ and In₂O₃ and Ru (II) heterocyclic complex and verified the possibility of using such materials for NO (0.25-4.0 ppm) and NO₂ (0.05-1.0 ppm) detection in high humidity conditions (relative humidity (RH) = 20%, 40%, 65%, 90%) at room temperature during periodic blue (λ_max = 470 nm) illumination. To reveal the reasons for the different influence of humidity on the sensors' sensitivity when detecting NO and NO₂, electron paramagnetic resonance (EPR) spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) investigations were undertaken. It was established that the substitution of adsorbed oxygen by water molecules causes the decrease in sensor response to NO in humid air. The influence of humidity on the interaction of sensitive materials with NO₂ is determined by the following factors: the increase in charge carrier's concentration, the decrease in the number of active sites capable of interacting with gases, and possible substitution of chemisorbed oxygen with NO₂^- groups.