Sputtered thin-film pH electrodes of platinum, palladium, ruthenium, and iridium oxides

Abstract 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 platinum, palladium, ruthenium, and iridium metal targets in argon-oxygen atmospheres is used to produce 1 μm thick electrodes on alumina and silicon substrates. The structure of the deposits is determined and compared to the fully crystalline structure developed after annealing at 420 °C. The pH response is measured for pH 2-11 versus a glass electrode. Exposures of up to 24 h at pH 2–11 are used to study the stability under harsh conditions. Palladium and platinum oxides are found to be less stable than ruthenium oxide. X-ray photoelectron spectroscopy studies are used to determine the surface chemical state and verify the stoichiometry of the sensing surface. The stability of thin-film RuO 2 indicates the usefulness of further testing at high temperatures and wider pH ranges.

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