Gas sensing based on surface oxidation/reduction of platinum-titania thin films II. The role of chemisorbed oxygen in film sensitization

The role of chemisorbed oxygen in further sensitizing thin View the MathML source sensing films to H2 and C3H6 exposure is discussed. The interaction of these oxygen sensitized films with H2 and C3H6 is investigated with resistance measurements, X-ray photoelectron spectroscopy and temperature programmed desorption. The View the MathML source sensing films are prepared by evaporation of 65 Å View the MathML source Å Ti with subsequent oxidation. Activation of these films for gas sensing occurs with a reduction step at 750 K followed by a final oxidation step at 900 K. The View the MathML source films in their activated state are composed of metallic platinum and titania in a discontinuous island structure. The View the MathML source films are quite sensitive to oxygen exposure in the 550–700 K range. A 10−5 O2 exposure for 1000 s at 660 K causes a threefold increase in the film resistance. The oxygen exposure enhances the resistivity response of the View the MathML source films to reducing gases such that resistance decreases in the 20 to 60% range are observed following hydrogen or propylene in the 10−5 mbar range. XPS measurements indicate that the gas induced resistance changes are not caused by concomitant changes in the concentration of bulk oxygen vacancies and that the average composition of the sensing films remains quite stable. Thermal desorption studies confirm that extended oxygen exposure in the 550–700 K range results in the incorporation of adsorbed and surface lattice oxygen which can be removed by reaction with reducing agents.