Observation of nanometer-sized electro-active defects in insulating layers by fluorescence microscopy and electrochemistry

We report a method to study electro-active defects in passivated electrodes. This method couples fluorescence microscopy and electrochemistry to localize and size electro-active defects. The method was validated by comparison with a scanning probe technique, scanning electrochemical microscopy. We used our method for studying electro-active defects in thin TiO₂ layers electrodeposited on 25 μm diameter Pt ultramicroelectrodes (UMEs). The permeability of the TiO₂ layer was estimated by measuring the oxidation of ferrocenemethanol at the UME. Blocking of current ranging from 91.4 to 99.8% was achieved. Electro-active defects with an average radius ranging between 9 and 90 nm were observed in these TiO₂ blocking layers. The distribution of electro-active defects over the TiO₂ layer is highly inhomogeneous and the number of electro-active defect increases for lower degree of current blocking. The interest of the proposed technique is the possibility to quickly (less than 15 min) image samples as large as several hundreds of μm² while being able to detect electro-active defects of only a few tens of nm in radius.