Gas sensing mechanism of TiO2-based thin films

Two classes of thin film gas sensors have been studied: TiO2 doped with Cr or Nb and TiO2-SnO2 mixed systems. Thin films have been prepared by the reactive sputtering from mosaic targets. It is demonstrated that titanium dioxide doped with Nb and Cr should be considered as a bulk sensor. Its performance is governed by the diffusion of point defects, i.e. very slow diffusion of Ti vacancies for TiO2: 9.5 at% of Nb and fast diffusion of oxygen vacancies in the case of TiO2: 2.5 at% Cr sensor. The corresponding response times are 55 min for TiO2: 9.5 at% of Nb and 20 s for TiO2: 2.5 at% Cr. In turn, sensors based on TiO2-SnO2, particularly those of the SnO2-rich composition, belong to the group of surface sensors. (C) 2004 Elsevier Ltd. All rights reserved.