Multivariate Analysis of Light-Activated SMOX Gas Sensors.

Chemoresistive gas sensors made from SnO2, ZnO, WO3, and In2O3 have been prepared by flame spray pyrolysis. The sensors' response to CO and NO2 in darkness and under illumination at different wavelengths, using commercially available LEDs, was investigated. Operation at room temperature turned out to be impractical due to the condensation of water inside the porous sensing layers and the irreversible changes it caused. Accordingly, for sensors operated at 70 °C, a characterization procedure was developed and proven to deliver consistent data. The resulting data set was so complex that usual univariate data analysis was intricate and, consequently, was investigated by correlation and principal component analysis. The results show that light of different wavelengths affects not only the resistance of each material, both under exposure to the target gases in humidity and in its absence, but also the sensor response to humidity and the target gases. It was found that each of the materials behaves differently under light exposure, and it was possible to identify conditions that need further investigations.

Proof of Concept for Operando Infrared Spectroscopy Investigation of Light-Excited Metal Oxide-Based Gas Sensors.

Light-excitation of semiconducting metal-oxide (SMOX)-based gas sensors is a promising means to lower their operation temperature, thereby reducing power consumption, which would allow for their broader application. Despite increased research interest in light-excited gas sensors, progress has been slow because of a lack of mechanistic understanding. Notably, significant differences between light-excitation and, the better understood, thermal-excitation mechanisms have been identified. Insights from operando spectroscopic studies have been key to understanding the surface chemistry that determines the performance of thermally activated SMOX, but they have not yet been performed on illuminated sensors. Here, for the first time, we demonstrate that it is possible to perform operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements on sensors under illumination. We demonstrate the benefits of the approach and show that under light illumination the splitting of water on the WO3 surface is responsible for the increased resistance of the sensor during exposure to high humidity.