Studies on oxygen chemical surface exchange and electrical conduction in thin film nanostructured titania at high temperatures and varying oxygen pressure

We report on oxygen surface exchange studies in ∼450-nm-thick nanocrystalline titania films with an average grain size of ∼13 nm by electrical conductivity relaxation along with the conductivity measurements at varying temperatures and oxygen partial pressures (pO2s). By electrochemical impedance spectroscopy technique, the high temperature conductivity was measured in the pO2 range from ∼10−16 to ∼10−6 Pa at temperatures from 973 to 1223 K and activation energy, ΔEa, for conduction was estimated as ∼3.23 eV at pO2 ∼10−11 Pa. Under reducing atmosphere (pO2 < 10−6 Pa), two distinct n-type conduction regimes were observed and corresponding predominant defects are discussed while, at high pO2 regime (pO2 >10−6 Pa), ionic conduction appears dominant leading to a conductivity plateau. The surface relaxation was observed to have two independent time constants likely originating from microstructural effects. The surface exchange coefficients are measured as ∼10−8–10−7 m/s and ∼10−9–10−8 m/s for each contribution with ΔEas of 2.79 and 1.82 eV, respectively, without much pO2 dependence across several orders of pO2 range of ∼10−16–10−6 Pa in the temperature range between 973 and 1223 K. The results are of potential relevance to understanding the near-surface chemical phenomena in nanocrystalline titania which is of great interest for energy and environmental studies.