Introducing additives in semiconducting metal oxides includes, besides the use of filters, dynamic operation procedures and chemometric approaches, the most common way of tuning the sensitivity, selectivity, and stability of chemoresitsive gas sensors. For the vast majority of commercially used gas sensing materials, the introduction of additives is essential and is one of the longest lasting topics in gas sensor research. This Review discusses the different chemical and electrical sensitization mechanisms of additives as well as the role of different structures. Based on state-of-the-art experimental findings, this Review revises and updates the concepts that are used to explain the mechanisms through which the additives influence the performance of typical gas sensing materials, i.e., oxide nanoparticles arranged in a porous layer. The first sections classify the different additive structures, namely, doped or loaded oxides as well as mixtures of oxides, and describe the basic working principle of pristine semiconducting metal oxide gas sensors. The subsequent sections discuss different chemical and/or electrical contributions to the sensitization by additive structures, their mutual influence on each other, and the way they impact the sensing properties. The presented concepts and models are essential for understanding the complex role of additives and provide the basis for a knowledge-based design of gas sensors based on semiconducting metal oxide nanoparticles, which is outlined in a separate section.

This work demonstrates that it is possible to follow the surface chemistry of oxygen on SnO 2 based gas sensing materials using operando Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS). The inherent difficulties, due to the intrinsic properties of the studied oxide and the limitations of the method, were overcome by comparing the results obtained for two different materials and by using of isotopically labeled gases together with the simultaneous measurement of the sensor signals. In spite of the differences in the surface composition and reactivity between the different materials, the experimental results show that the reactive oxygen species are similar in nature and the gas recognition takes place by the interplay of surface reduction and (re-)oxidation.

The presented work unravels the complex structure–function-relationships of Pt-loaded SnO2, namely the sensitization by a Fermi-control mechanism and relation of catalytic activity and gas sensing effect.

This work focuses on two aspects of goldloaded tin dioxide gas sensing materials: The influence of the size and dispersion of the gold on the sensing effect and the investigation into the mechanism at the origin of the improved gas sensing performance. For this purpose, a set of selected and well-characterized gold loaded tin dioxide materials were examined. The results show that the beneficial effect of gold on the CO sensing performance is observed for nanosized as well as for micron-sized gold entities, i.e., the effect is related to Au itself. Nevertheless, the response is strongly enhanced with increasing gold dispersion. Deeper insights into the mechanism of the sensitization, obtained by state-of-the-art operando spectroscopic techniques, indicated that oxygen is adsorbed on gold and transferred to the tin dioxide surface. There, it is bound as a negatively charged, ionic species, which gives additional sites for the interaction with target gases, i.e., enhances the gas sensing performance. These results strongly support the previously proposed oxygen spillover mechanism for gold-loaded tin dioxide.