Mechanistic studies of platinum-titania and platinum-alumina thin films for microchemical gas sensors.

Abstract

Microfabricated gas sensors utilizing ultrathin platinum transducing films offer great promise for meeting current challenges to chemical gas sensing. With the research efforts presented in this thesis, considerable advancements have been made toward understanding and optimizing such sensors. The transducing films consist of evaporated, ultrathin (35-100 A) platinum (Pt) films deposited onto metallic titanium (Ti) layers or alumina (Al$\rm\sb2O\sb3)$ layers. Pt/Ti films are studied in microfabricated gas sensing structures and on macrosamples. Titanium may participate in the response of the Pt/Ti films depending upon its oxidation state. Pt/Al$\rm\sb2O\sb3$ thin films are investigated on macrosamples as alternatives to Pt/TiO$\rm\sb{x}$ films used in the microchemical gas sensor. Alumina was chosen as a thermally stable and chemically inactive support. Stability and gas sensitivity of the transducing films are optimized with thermal pretreatments in oxidizing or reducing atmospheres. A range of compositions and microstructures are obtained from one Pt/Ti film. Films pretreated above 600$\sp\circ$C in O$\sb2$ are the most sensitive of the film types investigated. Oxidation of a Pt/Ti thin film at 600$\sp\circ$C results in the preparation of a mixed Pt/TiO$\rm\sb{2-x}$ film with a discontinuous microstructure. Resistance measurements, photoelectron spectroscopies and temperature programmed desorption are used to identify the surface chemical processes responsible for the electrical response of Pt/TiO$\rm\sb{2-x}$ sensing films. Surface reactions involving chemisorbed oxygen play a dominant role in the transduction mechanism of ultrathin platinum films. The films exhibit 10-100% resistance increases upon exposure to O$\sb2$ in the $1\times10\sp{-8}$ to $1\times10\sp{-5}$ Torr range. Films that are pre-exposed to oxygen are sensitive to reducing gases such as hydrogen, propylene and benzene. Resistance decreases in the 20-60% range are observed with reducing gas exposure in the $10\sp{-6}$ to $10\sp{-5}$ Torr range. The mechanistic studies are concluded with temperature programmed reaction spectroscopy (TPRS) experiments involving benzene and propylene on an oxygen saturated platinum foil and 100 A platinum-alumina film. The hydrocarbons react with oxygen at characteristic temperatures to produce CO$\sb2$ and H$\sb2$O. The reaction proceeds with an initial hydrogen abstraction followed by oxidation of the dehydrogenated hydrocarbon intermediates. The dominant parts of the oxidation reactions are the same for the foil and film indicating that alumina does not play a role in the catalytic behavior of platinum-alumina thin films.