Sensing properties of chemically sprayed TiO2 thin films using Ni, Ir, and Rh as catalysts

The purpose of this work is double, to analyze the influence of (i) the addition of different catalysts and (ii) the implementation of different procedures to introduce them in the titanium dioxide (TiO₂) thin films, in order to improve the film sensitivity for detecting oxygen. For reaching these objectives, TiO₂ thin films were deposited on alumina substrates by the ultrasonic spray pyrolysis (USP) technique employing titanium(IV) oxide acetylacetonate (TiO(acac)₂) as a titanium precursor, and pure methanol as a solvent. Iridium, nickel, and rhodium were the three catalysts used, which were incorporated by impregnation and USP onto the TiO₂ thin films surface. The electrical characterization, consisting in surface resistance measurements of the films, in a mixture of oxygen and zero-grade air, was performed using interdigitated gold electrodes contacted on the alumina substrates. From these, the film response or resistance change of the TiO₂ films was estimated. Single TiO₂ thin films measured at 400 °C displayed a response of the order of 0.02 and 0.18 to oxygen of 1000 and 10,000 ppm, respectively, whereas TiO₂ thin films using impregnated rhodium proved to have the highest response to O₂, with a value in the order of 2.5 to a concentration of 1000 ppm of O₂ diluted in zero-grade air at an optimal operating temperature of 250 °C.