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 (pO(2)s). By electrochemical impedance spectroscopy technique, the high temperature conductivity was measured in the pO(2) range from ∼10(-16) to ∼10(-6) Pa at temperatures from 973 to 1223 K and activation energy, ΔE(a), for conduction was estimated as ∼3.23 eV at pO(2) ∼10(-11) Pa. Under reducing atmosphere (pO(2) < 10(-6) Pa), two distinct n-type conduction regimes were observed and corresponding predominant defects are discussed while, at high pO(2) regime (pO(2) >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 ΔE(a)s of 2.79 and 1.82 eV, respectively, without much pO(2) dependence across several orders of pO(2) 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.

New method to prepare polycrystalline meta-thioboric acid, (HBS(2))(3).

A new method to prepare polycrystalline meta-thioboric acid (c-HBS(2)) has been developed and reported. HBS(2) was obtained as a vapor condensate by reacting H(2)S with B(2)S(3) in the vapor phase, and the optimal conditions for this reaction are reported. The X-ray and spectroscopic characterization suggest that the structure of thioboric acid is monoclinic and made up of hexagonal rings formed by trimer units, (HBS(2))(3). The present preparation route is facile, faster than other wet routes of thiolysis, and the reaction requires much lower temperatures, thus avoiding contamination by reactor materials.