Core to shell switching of the conduction channel on SnO₂ nanowire sensors

A sensor based on random connections of single-crystalline SnO₂ nanowires (NWs) has been fabricated for ethanol detection. The NW length of ∼10 μm leads to the formation of several random node connections between the Ag electrodes. The samples were subjected to thermal treatments under a reducing atmosphere of H₂/Ar, to generate oxygen vacancies at the surface of the NWs. As a result, the core conduction channel of the NWs, where the conduction is based on electron hopping through the potential barriers at the contact nodes, switch to the surface of the NWs by the creation of an impurity band of shallow donors located at 0.347 eV below the conduction band. We suggest that the H₂-rich atmosphere of the thermal treatments induced the formation of interstitial hydrogen (H _i ) and substitutional hydrogen (H_O), which are shallow donors with low formation energy.