High-resolution electrochemical, electrical, and structural characterization of a dimensionally stable Ti/TiO2/Pt electrode

The electrochemical and electrical properties of dimensionally stable Ti/TiO2/Pt electrodes, prepared by the galvanostatic oxidation of Ti to form a porous, micrometer-thick layer, followed by the galvanostatic deposition of Pt, have been investigated. Conventional potential sweep methods have been used, together with higher resolution techniques, including scanning electrochemical microscopy (SECM), combined SECM-atomic force microscopy (AFM), conducting-AFM (C-AFM), and focused ion beam (FIB) microscopy. At potentials positive of the flatband potential (E-PB) of n-TiO2, voltammetry suggests the surface activity is low. SECM substrate generation-tip collection mode studies demonstrate that the current which flows under these conditions is due to the presence of both "metallic'' (facile electron transfer) sites and lower activity (retarded electron transfer) zones. Correlation of the local current with high-resolution topographical maps, using SECM-AFM, has shown that both types of sites are associated with Pt deposits. Cross-sectional structural characterization of the substrate electrodes using FIB demonstrates that the Pt deposits, formed within the microporous Ti/TiO2 surface layer, are separated from the underlying metallic Ti substrate by a TiO2 barrier layer of thickness in the range similar to0 to 100 nm. In areas where the Pt is able to make contact with the underlying Ti, the resulting ohmic contact effectively forms a microscopic Pt electrode. In areas where a barrier layer exists, either variations in the thickness or defects are likely to be responsible for the observed heterogeneities in surface conductivity and corresponding electrochemical activity. (C) 2002 The Electrochemical Society.