Current state of knowledge on the metal oxide based gas sensing mechanism

Semiconducting metal oxide based gas sensors are a highly attractive option for use in a widespread array of applications as they can be miniaturized, are highly robust and inexpensive. This review focuses on SnO2, In2O3 and WO3, three commonly used metal oxides that are known to show a diversity in sensing behavior. In order to optimize the sensing behavior, it is imperative that the mechanism is understood. It is unanimously agreed upon that adsorption of oxygen and the resulting charge layer play a central role in the sensor response. Surface oxygen had proven to be illusive experimentally. Recent insights gained from operando methods, sophisticated surface science analysis tool and theoretical calculations are discussed. Additionally, these studies have revealed that the surface reactions are far more complicated than initially assumed. It is found that other adsorbates beyond oxygen can influence the sensor response. Depending on the site, the formation of adsorbates can have different effects on the sensor depending on the reaction site, e.g. hydroxyl group formation from humidity can result in the removal of negative charge or be electroneutral. The importance of understanding the effect of adsorbates becomes particularly evident when examining the chemically complicated volatile organic compounds, e.g. ethanol and acetone. In this review simplified schematics based on insights from theoretical and experimental studies are developed, so that the surface chemistry can be presented in an understandable manner despite its complexity.