Tungsten trioxide is the second most commonly used semiconducting metal oxide in gas sensors. Semiconducting metal oxide (SMOX)-based sensors are small, robust, inexpensive and sensitive, making them highly attractive for handheld portable medical diagnostic detectors. WO₃ is reported to show high sensor responses to several biomarkers found in breath, e.g., acetone, ammonia, carbon monoxide, hydrogen sulfide, toluene, and nitric oxide. Modern material science allows WO₃ samples to be tailored to address certain sensing needs. Utilizing recent advances in breath sampling it will be possible in the future to test WO₃-based sensors in application conditions and to compare the sensing results to those obtained using more expensive analytical methods.

NO₂-sensing properties of semiconductor gas sensors using porous In₂O₃ powders loaded with and without 0.5 wt% Au (Au/In₂O₃ and In₂O₃ sensors, respectively) were examined in wet air (70% relative humidity at 25 °C). In addition, the effects of Au loading on the increased NO₂ response were discussed on the basis of NO₂ adsorption/desorption properties on the oxide surface. The NO₂ response of the Au/In₂O₃ sensor monotonically increased with a decrease in the operating temperature, and the Au/In₂O₃ sensor showed higher NO₂ responses than those of the In₂O₃ sensor at a temperature of 100 °C or lower. In addition, the response time of the Au/In₂O₃ sensor was much shorter than that of the In₂O₃ sensor at 30 °C. The analysis based on the Freundlich adsorption mechanism suggested that the Au loading increased the adsorption strength of NO₂ on the In₂O₃ surface. Moreover, the Au loading was also quite effective in decreasing the baseline resistance of the In₂O₃ sensor in wet air (i.e., increasing the number of free electrons in the In₂O₃), which resulted in an increase in the number of negatively charged NO₂ species on the In₂O₃ surface. The Au/In₂O₃ sensor showed high response to the low concentration of NO₂ (ratio of resistance in target gas to that in air: ca. 133 to 0.1 ppm) and excellent NO₂ selectivity against CO and ethanol, especially at 100 °C.

In order to increase their stability and tune-sensing characteristics, metal oxides are often surface-loaded with noble metals. Although a great deal of empirical work shows that surface-loading with noble metals drastically changes sensing characteristics, little information exists on the mechanism. Here, a systematic study of sensors based on rhodium-loaded WO₃, SnO₂, and In₂O₃—examined using X-ray diffraction, high-resolution scanning transmission electron microscopy, direct current (DC) resistance measurements, operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and operando X-ray absorption spectroscopy—is presented. Under normal sensing conditions, the rhodium clusters were oxidized. Significant evidence is provided that, in this case, the sensing is dominated by a Fermi-level pinning mechanism, i.e., the reaction with the target gas takes place on the noble-metal cluster, changing its oxidation state. As a result, the heterojunction between the oxidized rhodium clusters and the base metal oxide was altered and a change in the resistance was detected. Through measurements done in low-oxygen background, it was possible to induce a mechanism switch by reducing the clusters to their metallic state. At this point, there was a significant drop in the overall resistance, and the reaction between the target gas and the base material was again visible. For decades, noble metal loading was used to change the characteristics of metal-oxide-based sensors. The study presented here is an attempt to clarify the mechanism responsible for the change. Generalities are shown between the sensing mechanisms of different supporting materials loaded with rhodium, and sample-specific aspects that must be considered are identified.

Beginning with LaFeO₃, a prominent perovskite-structured material used in the field of gas sensing, various perovskite-structured materials were prepared using sol–gel technique. The composition was systematically modified by replacing La with Sm and Gd, or Fe with Cr, Mn, Co, and Ni. The materials synthesized are comparable in grain size and morphology. DC resistance measurements performed on gas sensors reveal Fe-based compounds solely demonstrated effective sensing performance of acetylene and ethylene. Operando diffuse reflectance infrared Fourier transform spectroscopy shows the sensing mechanism is dependent on semiconductor properties of such materials, and that surface reactivity plays a key role in the sensing response. The replacement of A-site with various lanthanoid elements conserves surface reactivity of AFeO₃, while changes at the B-site of LaBO₃ lead to alterations in sensor surface chemistry.

Here we present a novel concept for the selective recognition of different target gases with a multilayer semiconducting metal oxide (SMOX)-based sensor device. Direct current (DC) electrical resistance measurements were performed during exposure to CO and ethanol as single gases and mixtures of highly porous metal oxide double- and single-layer sensors obtained by flame spray pyrolysis. The results show that the calculated resistance ratios of the single- and double-layer sensors are a good indicator for the presence of specific gases in the atmosphere, and can constitute some building blocks for the development of chemical logic devices. Due to the inherent lack of selectivity of SMOX-based gas sensors, such devices could be especially relevant for domestic applications.

In this study, the electrical characteristics and surface reactions of three different kinds of In₂O₃ based gas sensors were investigated under CO exposure in dry and humid air by using DC electrical resistance measurements and Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). For the first time one gains insight into the surface reactions during the exposure to CO and humidity. The sensors show different behavior on the baseline in humidity depending on the material which is due to different kinds of OH-groups. The resistance change under CO and humidity exposure are additive and the molecules compete for the same reaction partner confirmed by the spectroscopy results. Through isotopic exchange experiments, different adsorbates could be identified.