Direct Microscopic Proof of the Fermi Level Pinning Gas-Sensing Mechanism: The Case of Platinum-Loaded WO3.

It is widely known that the sensing characteristics of metal oxides are drastically changed through noble metal oxide surface additives. Using operando infrared spectroscopy it was identified that the Fermi level pinning mechanism dominates the sensor response of platinum-loaded WO3. Spectroscopy, however, provides information about the sample only on average. Traditional microscopy offers structural information but is typically done in vacuum and on unheated sensors, very different than the operation conditions of metal oxide gas sensors. Here, state-of-the-art in situ scanning transmission electron microscopy offers spatially resolved information on heated samples at atmospheric pressure in varying gas atmospheres. As a result it was possible to directly couple microscopically observed structural changes in the surface noble metal nanoclusters with IR spectra and sensor responses. On the basis of the findings, the dominant Fermi level pinning mechanism could be validated. The presented work demonstrates the benefits of coupling in situ microscopy with operando spectroscopy in order to elucidate the sensing mechanism of metal oxides.