Photosensitivity and gas sensing mechanisms: validation of an operando DRIFT spectroscopy apparatus for light-activated chemoresistive gas sensors

Despite the large number of published articles, several key questions concerning the light activation of the gas sensitivity in semiconductors are still open. The primary goal of this work is to validate a novel add-on tool for investigating gas–solid interactions at the chemoresistive sensor surface under operating conditions, using operando diffuse reflectance infrared Fourier transform spectroscopy. The innovative apparatus was designed to support a robust analysis of the sensing material properties for photoactivated applications, filling up the current knowledge gap concerning the involved physical and chemical reaction processes. Moreover, the design of a customized sample holder enables in-situ diffuse reflectance Fourier infrared spectroscopy measurements on irradiated functional powders, thereby limiting the signal-to-noise ratio and collecting additional information on chemisorption processes. The validation was conducted on a gold standard material for the photoactivation procedure, namely zinc oxide. The ZnO-based gas sensor was photoactivated at three different wavelengths (385, 468, and 525 nm) and the ZnO functional powder at 385 nm, by using commercial LEDs. The measurements were performed with an oxidizing and a reducing gas, i.e. NO2 and ethanol, along with humidity. The distinctive peaks observed in operando and in-situ absorbance spectra revealed the presence of the target gases at the sensing layer, responsible for the corresponding changes in the sensor signal. These results confirmed the feasibility of employing the proposed setup to investigate photoactivated gas–solid interactions, while the method and setup account for geometry and illumination, ensuring reliable and comparable measurements.