GAS-SENSING BASED ON SURFACE OXIDATION-REDUCTION OF PLATINUM-TITANIA THIN-FILMS I - SENSING FILM ACTIVATION AND CHARACTERIZATION

Thin Pt/TiO2-x sensing films are characterized with X-ray photoelectro n spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) to investigate the mechanism of resistance decreases measured in situ on exposure to hydrogen and propylene, The Pt/TiO2-x films are prepar ed by oxidizing 65 Angstrom Pt/65 Angstrom Ti films in 10(-6) mbar O-2 at 800 to 900 K, The gas sensitivity of the films arises with activat ion that consists of a reduction step at 750 K followed by a final oxi dation treatment at 900 K, Changes in the film stoichiometry during ea ch of the pretreatment and activation steps can be clearly detected wi th XPS, The onset of gas sensitivity is attributed to oxidation of the titanium metal and the formation of a discontinuous film structure wh ich is revealed in scanning electron microscopy (SEM) images of activa ted Pt/TiO2-x sensing films, Resistance decreases in the 1 to 10% rang e are observed for these films following hydrogen or propylene exposur e in the 10(-6) to 10(-2) mbar range at T less than or equal to 700 K, Larger resistance decreases are observed with increasing reducing gas pressure. The resistance decreases are caused by surface reduction at T less than or equal to 700 K with increasing involvement of the sub- surface and bulk oxygen with increasing temperature, UPS measurements indicate that resistance decreases due to reducing gas exposure at fil m temperatures less than or equal to 700 K can be correlated with decr eases in the surface oxygen concentration too small to be observed by XPS. (C) 1998 Elsevier Science B.V.