Scanning electrochemical microscopy of electron transfer reactions at thin films of TiO_2 and Ta_2O_5

摘要

Electron-transfer reactions at titanium and tantalum electrodes covered by a native oxide film (approx2.5 nm thick) have been investigated in aqueous solutions using scanning electrochemical microscopy (SECM). SECM images demonstrate that electron transfer across the oxide film occurs at randomly distributed, microscopic sites having dimensions that range from 2 to 50 #mu#m. A SECM-based method is proposed that allows quantitative evaluation of the current density of individual redox-active sites. The methodology is generally applicable to any heterogeneous surface containing an arbitrary number of randomly positioned electroactive sites, each of different size and/or displaying different electron-transfer kinetics, we show that a small number of microscopic electroactive sites, occupying a remarkably small percentage of the total area, can dominate the overall electrochemical behavior of oxide-covered electrodes.