GAS-SENSING CHARACTERISTICS OF ULTRATHIN TIO2-X FILMS INVESTIGATED WITH XPS, TPD AND IN-SITU RESISTANCE MEASUREMENTS

Ultrathin Pt/TiO2-x sensing films are characterized to investigate the adsorption and reaction processes that cause resistance changes. The films are prepared by oxidizing films with a nominal thickness of 65 A ngstrom Pt on top of 65 Angstrom Ti at temperatures of 800-900 K. Scan ning electron microscopy (SEM) shows that after the high-temperature o xidation the film structure becomes discontinuous. The film resistance is very responsive to oxygen exposure in the 500-700 K range, giving an increase in the film resistance. Thermal desorption studies suggest that oxygen exposure in this temperature range causes the incorporati on of surface and lattice oxygen. The sensitivity to reducing gases su ch as hydrogen or propylene becomes very pronounced after subjecting t he Pt/TiO2-x films to thermal treatment in oxygen in the 500-700 K ran ge. Exposure to hydrogen or propylene at elevated temperatures leads t o partial removal of surface oxygen aad a decrease in film resistance. The treads in the relative resistance changes of Pt/TiO2-x films as a function of gas exposure confirm that both platinum and TiO2-x surfac e sites contribute to high gas sensitivity. Thermal treatment under a controlled atmosphere alters the interaction between platinum and tita nia, modifying the relative sensitivity of the discontinuous film to o xidizing and reducing gases. (C) 1997 by John Wiley & Sons, Ltd.