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.

Monometallic Au and Pd nanoparticles (NPs) and homogeneous AuPd nanoalloy particles were synthesized in a continuous flow of reactants (HAuCl4, K2PdCl4, NaBH4 and polyvinylpyrrolidone (PVP)) using a microfluidic reactor with efficient micromixers. The obtained ultrasmall NPs were subsequently deposited onto SnO2 supports with different surface area (32.7 and 3.6 m 2 g-1). Samples with 1.0 and 0.1 wt.% metal loading were prepared. After calcination at 380 °C for 1 h the supported NPs aggregated to some extent. SnO2 supported AuPd nanoalloys with low (0.1 wt.%) metal loadings showed the smallest NP diameters (~ 5-7 nm) and the narrowest size distribution among the samples. The gas sensing performance of the materials was investigated at 300 °C in four different gas atmospheres containing either CO, CH4, ethanol or toluene using dry and humid conditions. They exhibited a distinct variation in the response patterns and selectivity toward the test gases depending on composition and metal loading: Au increased the sensor signals compared to pristine SnO2 in all cases and decreased the interference of water vapor; the supported Pd NPs showed a weak response to toluene, strong sensitivity in CO sensing and slightly better response in ethanol sensing in humid air compared to dry air. However, they showed a high selectivity toward CH4 when used in dry air; AuPd alloy particles provided lower sensor signals compared to pristine SnO2 and no remarkable CH4 selectivity, in contrast to the Pd system. Operando diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) indicates a strong band bending in the case of Pd and AuPd NPs, whereas in the case of Au no band bending occured, indicating a strong electronic interaction between the support and Pd-containing NPs (Fermi-level control mechanism), and a weak electronic interaction between SnO2 and Au NPs (spill-over mechanism).

This work presents an operando infrared spectroscopic study of the temperature-dependent water adsorption on pristine SnO2 surfaces and discusses the possible implications on the oxygen ionosorption and gas-sensing mechanism. The impact of water on the sensor resistance, CO-sensing performance, and CO conversion was studied, and the obtained phenomenological results provide the basis for discussing the operando spectroscopic investigation findings. The provided information allows identification of three different water adsorption regimes ranging from physisorption and dominant associative adsorption to mainly dissociative water adsorption. In these regions, water has different impacts on the surface composition, sensor resistance, and gas-sensing performance.

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.

Lead sulfide (PbS) is a p-type semiconductor that is often applied in photodetectors and solar cells. One major problem that researchers are regularly confronted with is the loss of performance when operated in air. The sensitivity to the ambient, however, makes it an interesting material for the application in gas sensors. Although a lot of research has been focused on the aging of PbS, the influencing factors and the associated material changes are still a matter of debate. Resistance measurements of differently prepared PbS samples operated as sensors for the detection of NO2 (a strongly oxidative gas) in combination with surface-sensitive methods (diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy) enabled the investigation of the changes associated with aging and their influence on the sensing mechanism. The aging mechanism was found to be predominantly influenced by the presence of residuals of the stabilizing agent oleic acid that was adsorbed at the surface of the colloidal quantum dots. Additionally, it was found that a byproduct of the consumption of oleic acid enhanced the detected sensor signal.