SPUTTERED THIN-FILM PH ELECTRODES OF PLATINUM, PALLADIUM, RUTHENIUM, AND IRIDIUM OXIDES

Thin-film metal oxides have been investigated for use as pH electrodes . These materials may have potential for measuring pH under conditions that are not favorable for glass electrodes. Reactive sputtering of p latinum, palladium, ruthenium, and iridium metal targets in argon-oxyg en atmospheres is used to produce 1 mu m thick electrodes on alumina a nd silicon substrates. The structure of the deposits is determined and compared to the fully crystalline structure developed after annealing at 420 degrees C. The pH response is measured for pH 2-11 versus a gl ass electrode. Exposures of up to 24 h at pH 2-11 are used to study th e stability under harsh conditions. Palladium and platinum oxides are found to be less stable than ruthenium oxide. X-ray photoelectron spec troscopy studies are used to determine the surface chemical state and verify the stoichiometry of the sensing surface. The stability of thin -film RuO2 indicates the usefulness of further testing at high tempera tures and wider pH ranges.