Gas Sensing Characteristics of Ultrathin TiO 2− x Films Investigated with XPS, TPD and In Situ Resistance Measurements

Abstract

Ultrathin Pt/TiO 2− 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 Å Pt on top of 65 Å Ti at temperatures of 800–900 K. Scanning electron microscopy (SEM) shows that after the high-temperature oxidation 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 incorporation of surface and lattice oxygen. The sensitivity to reducing gases such as hydrogen or propylene becomes very pronounced after subjecting the Pt/TiO 2− x films to thermal treatment in oxygen in the 500–700 K range. Exposure to hydrogen or propylene at elevated temperatures leads to partial removal of surface oxygen and a decrease in film resistance. The trends in the relative resistance changes of Pt/TiO 2− x films as a function of gas exposure confirm that both platinum and TiO 2− x surface sites contribute to high gas sensitivity. Thermal treatment under a controlled atmosphere alters the interaction between platinum and titania, modifying the relative sensitivity of the discontinuous film to oxidizing and reducing gases. © 1997 by John Wiley & Sons, Ltd.