Polaron conductivity in α-Fe2O3 quenched by adsorbed NO2

Abstract

Polaron-mediated charge transport in α–Fe 2 O 3 plays a central role in its performance as a gas-sensing material, yet the atomistic interaction between surface adsorbates and polarons remains insufficiently understood. Here, density functional theory with Hubbard-U correction (DFT + U) combined with nudged elastic band calculations is used to investigate polaron formation, migration, and quenching at the Fe-terminated α–Fe 2 O 3 (0001) surface. The calculated activation energy for small-polaron hopping in bulk α–Fe 2 O 3 is found to be 0.12 eV, in excellent agreement with experimental measurements, confirming the validity of the computational approach. Slab calculations show that migration of the polaron from bulk to the surface lowers the energy by 0.12 eV, indicating preferential localization of charge carriers at the gas–solid interface. Adsorption of NO 2 induces substantial electron transfer (0.72 e - ) from the oxide to the molecule, eliminating the localized Fe 2+ polaron state and thereby suppressing polaronic conductivity. These results provide a direct microscopic explanation for the resistance increases of hematite-based sensors upon exposure to oxidizing gases. More broadly, the study establishes how surface adsorption can modulate charge transport α–Fe 2 O 3 through control of polaron populations, offering design principles for improved iron oxide gas sensors.