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 Pt/TiO2-x s ensing films to H-2 and C3H6 exposure is discussed. The interaction of these oxygen sensitized films with H-2 and C3H6 is investigated with resistance measurements, X-ray photoelectron spectroscopy and temperat ure programmed desorption. The Pt/TiO2-x sensing films are prepared by evaporation of 65 Angstrom Pt/65 A Ti with subsequent oxidation. Acti vation of these films for gas sensing occurs with a reduction step at 750 K followed by a final oxidation step at 900 K. The Pt/TiO2-x films in their activated state are composed of metallic platinum and titani a in a discontinuous island structure. The Pt/TiO2-x films are quite s ensitive to oxygen exposure in the 550-700 K range. A 10(-5) O-2 expos ure for 1000 s at 660 K causes a threefold increase in the film resist ance. The oxygen exposure enhances the resistivity response of the Pt/ TiO2-x films to reducing gases such that resistance decreases in the 2 0 to 60% range are observed following hydrogen or propylene in the 10( -5) mbar range. XPS measurements indicate that the gas induced resista nce changes are not caused by concomitant changes in the concentration of bulk oxygen vacancies and that the average composition of the sens ing films remains quite stable. Thermal desorption studies confirm tha t extended oxygen exposure in the 550-700 K range results in the incor poration of adsorbed and surface lattice oxygen which can be removed b y reaction with reducing agents. (C) 1998 Elsevier Science B.V.