Elucidating the Mechanism of Working SnO₂ Gas Sensors Using Combined Operando UV/Vis, Raman, and IR Spectroscopy

Abstract

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.

Application of Raman Spectroscopy to Working Gas Sensors: From in situ to operando Studies

“…The vacancies can be ionized, thereby releasing electrons to the conduction band. In both models, the presence of oxygen (from air) plays an important role, either, as an ionosorbed species or as an oxidation source, but a detailed understanding of the interplay of (sub)surface processes and the sensor signal is still a tremendous challenge [4,5,6]. Part of the complexity of the gas sensor mechanism arises from the fact that, besides the interaction of the target gas with the metal oxide, the analyzed gas may undergo catalytic reactions.…”

“…For a knowledge-based design of better gas sensors, a detailed understanding of their mode of operation on a molecular level will be required. To identify the gas sensing mechanism, operando experiments based on IR, Raman, UV/Vis, and X-ray absorption spectroscopy have been applied [5,6,7,8,9,10,11,12], to relate the sensor response to the structural changes under working conditions. To further enhance the mechanistic understanding of gas sensors, it would be highly desirable to develop operando approaches, which (i) are applicable under realistic operating conditions of the gas sensor, (ii) allow for a direct (real-time) correlation of the sensor response with the spectroscopic signals, and (iii) enable a simultaneous monitoring of the gas-phase composition [4,5,6].…”

“…Despite the potential of Raman spectroscopy for studying gas sensors, only a small number of studies have been published, compared with other characterization techniques (e.g., IR spectroscopy). Previous work published in the context of metal-oxide gas sensors ranges from the (ex situ) use of Raman characterization [13,14,15,16,17,18,19,20] to In situ Raman studies [21,22,23,24,25,26,27,28,29,30,31,32] and operando Raman studies [5,6,33,34,35,36], part of which were done in combination with operando UV-Vis spectroscopy [6,35]. In these studies typical gas sensor materials, such as SnO 2 , WO 3 , and In 2 O 3 were investigated towards a variety of target gases, e.g., H 2 S, CH 4 , H 2 , CO, NH 3 , NO 2 , ethanol (EtOH), and acetaldehyde (acetald.).…”

Oxygen Defects in Nanostructured Metal‐Oxide Gas Sensors: Recent Advances and Challenges†

“…developed an operando approach based on combined UV/Vis, Raman, and FTIR spectroscopy, enabling the sensor response to be related to its structural changes directly under working conditions. [ 56 ] In this multiple spectroscopic experiment, UV/Vis spectra were used to monitor the number of oxygen vacancies in SnO 2 . The work demonstrates with the example of ethanol gas sensing that the resistance is directly correlated with the concentration of oxygen vacancies and the presence of surface species.…”

“…These spectroscopic techniques were developed from an in situ to a powerful operando technique for gas sensor research within a few years. [ 18,56,79,80‐81 ] For example, Chen et al . employed the in situ diffuse reflectance infrared Fourier transform spectroscopy ( in situ DRIFTS) technique…”