Crystal facet-dependent p-type and n-type sensing responses of TiO2 nanocrystals

TiO2 nanocrystals with various percentages of exposed {001} facets have been prepared by a hydrothermal approach in the presence of HF. The gas-sensing properties towards ethanol of the TiO2 nanocrystals are examined in detail. The sensors based on the TiO2 nanocrystals exhibit a regular n-type behavior above 250 degrees C and abnormal p-type sensing response at low temperatures (room temperature -120 degrees C). Importantly, both the p-type and n-type sensing responses are revealed to be facet-dependent, demonstrating an obvious increase of response with the increase of the percentage of exposed {001} facets. Other n-type sensing properties including prefactor alpha and detection limit are also found to be {001} facet-dependent. The facet-dependent p-type sensing mechanism is attributed to the increased water molecules on TiO2 surface with increasing {001} percentage, while the facet-dependent n-type sensing mechanism is ascribed to the increased adsorption capabilities of ethanol molecules and surface oxygen species as well as the change of absorbed oxygen ions from O- to O2- with the increase of {001} percentage. The finding here provides some new insights into the facet-dependent gas sensing mechanism, which is beneficial to engineer sensing nanomaterials with enhanced sensing performances by exposing specific crystal facets. (C) 2018 Elsevier B.V. All rights reserved.