Gas sensing based on surface oxidation/reduction of platinum-titania thin films I. Sensing film activation and characterization

Thin Pt TiO 2−x sensing films are characterized with X-ray photoelectron 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 TiO 2−x films are prepared by oxidizing 65 A ̊ Pt 65 A ̊ Ti films in 10 −6 mbar O 2 at 800 to 900 K. The gas sensitivity of the films arises with activation that consists of a reduction step at 750 K followed by a final oxidation treatment at 900 K. Changes in the film stoichiometry during each of the pretreatment and activation steps can be clearly detected with XPS. The onset of gas sensitivity is attributed to oxidation of the titanium metal and the formation of a discontinuous film structure which is revealed in scanning electron microscopy (SEM) images of activated Pt TiO 2−x sensing films. Resistance decreases in the 1 to 10% range are observed for these films following hydrogen or propylene exposure in the 10 −6 to 10 −2 mbar range at T ≤ 700 K. Larger resistance decreases are observed with increasing reducing gas pressure. The resistance decreases are caused by surface reduction at T ≤ 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 film temperatures ≤ 700 K can be correlated with decreases in the surface oxygen concentration too small to be observed by XPS.