Deciphering the CO sensing mechanisms of CeO2-based nanostructured semiconductors: Influence of doping, morphology, and humidity

In this work, we have synthesized undoped and Eu-doped CeO2 nanostructures with different morphologies, which were later used to prepare sensing films via the screen-printing technique. Microscopical analyses revealed that four morphologies were obtained: sphere-like nanoparticles, nanorods, nanopolyhedra, and nanocubes, with different exposed facets. Their structural characterization confirmed the formation of single-phase CeO2, with oxygen vacancies as the predominant structural defect. Scanning electron microscopy showed that the film’s surface was porous and without discontinuities. For the electrical characterization, we varied the operating temperature from 250 to 450°C, the CO concentration from 5 to 50 ppm, and the relative humidity from 0 % to 40 %. Nanostructures with polar (100) facets (nanorods and nanocubes) exhibited higher susceptibility to humidity during the CO sensing, while Eu-doped nanoparticles with predominant nonpolar (111) facets could better detect CO, also in the presence of water vapor. The conductance of the films decreased upon exposure to the analyte, which was attributed to the adsorption of CO molecules as carbonate species and confirmed by operando diffuse reflectance infrared Fourier transform spectroscopy. This work provides new insights into the relationship among Eu-doped CeO2 morphologies, humidity effects, and sensing mechanisms, pointing to promising advancements for CO gas sensors.