Resolution of Fast and Slow Charging Processes in Ruthenium Oxide Films: An AC Impedance and Optical Investigation

Ellipsometry and ac impedance measurements were employed for in situ investigation of anodic oxide films on Ru and of oxide films formed by thermal decomposition of Ru chloride. An instrumental setup for experiment control, data acquisition, and data analysis in automated measurements of the ellipsometric parameters and of the ac impedance is described. The effects of the applied bias and of proton concentration in solution on the rate of the double injection process of protons and electrons were studied. The charging process in the oxide film could be resolved into a faster mode, attributed to the charging of grain surfaces, and a slower bias dependent mode, which apparently involves incorporation of aqueous protons into oxide grains. The slower mode of charging is associated in hydrous oxide films with an effective diffusion coefficient of 10¹¹-10-¹²cm²s^-1. The overall impedance due to the faster mode of charging in hydrous films in 0.1-1M acid solutions is smaller than 0.1 Ω cm²and this fast mode becomes diffusion controlled in solutions 10³M in acid. However, in relatively dry films, e.g., “as-immersed” thermal oxide films, slower bias dependent rates are found for the fast mode of proton injection, probably due to a bias-dependent diffusion of protons along oxide grain surfaces.