To evaluate the material’s suitability for the application in thick film gas sensors, the temperature dependent interaction of indium tin oxide (ITO) with oxygen, carbon monoxide and additional gas species was investigated. For that purpose, a combination of techniques has been used, where gas adsorption measurements on nanoscale powders are complemented by electrical conductivity studies of screen printed thick films.
It was found that, after exposure to ambient air, the surfaces of both powder and films are covered with significant amounts of carbon dioxide, water and oxygen derived adsorbates. While these adsorbates have an impact on the interaction with other gas species and may also alter the electrical conductivity of the film, their desorption proceeds well beyond 600°C. Varying the oxygen partial pressure in the measuring gas, on the other hand, does not only affect adsorbate coverage but also the material’s oxygen stoichiometry, which in turn translates to changes in the electrical film conductivity. Chemisorption analysis on ITO powders exposed to carbon monoxide concentrations of up to 2000 ppm in helium both with and without oxygen addition suggest catalytic properties towards conversion to carbon dioxide. Corresponding measurements of electrical film conductivity under carbon monoxide exposure indicate that this could be mainly due to interaction with oxygen species pre-adsorbed on the film surface.