SnO 2 is the most widely used metal oxide gas-sensing material but a detailed understanding of its functioning is still lacking despite its relevance for applications. To gain new mechanistic insight into SnO 2 gas sensors under working conditions, we have developed an operando approach based on combined UV/Vis, Raman, and FTIR spectroscopy, allowing us for the first time to relate the sensor response to the concentration of oxygen vacancies in the metal oxide, the nature of the adsorbates, and the gas-phase composition. We demonstrate with the example of ethanol gas sensing that the sensor resistance is directly correlated with the number of surface oxygen vacancies and the presence of surface species, in particular, acetate and hydroxy groups. Our operando results enable an assessment of mechanistic models proposed in the literature to explain gas sensor operation. Owing to their fundamental nature, our findings are of direct relevance also for other metal oxide gas sensors.

Metal-oxide semiconductors are of great interest for gas-sensing applications. We provide new insights into the mode of operation of ceria-based gas sensors during ethanol gas sensing using combined operando Raman−gas-phase FTIR spectroscopy. Visible Raman spectroscopy is employed to monitor the presence of oxygen vacancies in ceria via F 2g mode softening, while simultaneously recorded FTIR spectra capture the gas-phase composition. Such an experimental approach allowing the direct observation of oxygen vacancies in metal-oxide gas sensors has not been reported in the literature. By systematically varying the gas atmosphere and temperature, we can relate the sensor response to the spectroscopic signals, enabling us to obtain new fundamental insight into the functioning of metal-oxide semiconductor gas sensors, as well as their differences from heterogeneous catalysts.

Understanding the mode of operation of gas sensors is of great scientific and economic interest. A knowledge-based approach requires the development and application of spectroscopic tools to monitor the relevant surface and bulk processes under working conditions (operando approach). In this review we trace the development of vibrational Raman spectroscopy applied to metal-oxide gas sensors, starting from initial applications to very recent operando spectroscopic approaches. We highlight the potential of Raman spectroscopy for molecular-level characterization of metal-oxide gas sensors to reveal important mechanistic information, as well as its versatility regarding the design of in situ/operando cells and the combination with other techniques. We conclude with an outlook on potential future developments.

SnO 2 ist das am häufigsten verwendete Material für Metalloxid-Gassensoren, aber ein detailliertes Verständnis der Funktionsweise steht trotz der Relevanz noch immer aus. Um mechanistische Einblicke in SnO 2-Gassensoren unter Arbeitsbedingungen zu erhalten, haben wir einen operando-Zugang entwickelt, der UV-Vis-, Raman-und FT-IR-Spektroskopie kombiniert, und es erstmals erlaubt, die Sensorantwort mit der Sauerstoffleerstellen-Konzentration im Metalloxid, der Art der Adsorbate und der Gasphasenzusammensetzung zu verknüpfen. Wir zeigen anhand der Ethanol-Gassensorik, dass der Sensorwiderstand direkt mit der Anzahl an Sauerstoffleerstellen an der Oberfläche und der Art der Oberflächenspezies, insbesondere der Acetat-und Hydroxyspezies, korreliert ist. Unsere operando-Ergebnisse erlauben eine Beurteilung der in der Literatur vorgeschlagenen Modelle hinsichtlich der Arbeitsweise von Gassensoren. Aufgrund ihres fundamentalen Charakters sind sie auch für andere Metalloxid-Gassensoren von unmittelbarer Relevanz.

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