| Abstract
| - Electron-transfer reactions at Ta electrodes covered by a thin (∼2.5 nm) native oxide film, hereafterdesignated as Ta/Ta2O5, have been investigated using scanning electrochemical microscopy (SECM).Oxidation and reduction of soluble redox species (I-, Fe(CN)64-, and Ru(NH3)63+) are localized to randomly-distributed, microscopic sites (radius = 2−50 μm) at the surface of the Ta/Ta2O5 electrode. The electroactivesites display an unusual dependence on the identity of the redox species. Specifically, SECM images showthat some sites are only active for reduction of Ru(NH3)63+ while others, on the same surface, are activefor both the reduction of Ru(NH3)63+ and the oxidation of I-. The rate of I- oxidation at individual siteshas been quantified by SECM and shown to approach the theoretical mass-transport-limited value,suggesting that the oxide film is either absent or very thin (∼1 nm) at these electroactive sites. Theinfluence of the Ta2O5 film thickness on electron-transfer kinetics at individual sites has also beeninvestigated. A sudden decrease in the electron-transfer rate is observed at individual sites during controlledanodic growth of the film. The abrupt decrease in electron-transfer rate at different redox-active sitesoccurs at electrode potentials spanning a ∼1 V range, reflecting differences in the local energetics andkinetics associated with nucleation and growth of the Ta2O5 film.
|
