Electronic Structure and Growth of Electrochemically Formed Iridium Oxide Films

Abstract

The nature of the electronic structure of electrochemically formed iridium oxide films (EIROF) is investigated by in-situ conductivity measurements in an electrochemical cell and ex-situ current-sensing atomic force microscopy (CS-AFM). A direct demonstration of changes in the conductivity for electrochemically formed iridium oxide films (EIROF) with the applied potential of EIROF electrodes in an electrochemical cell is presented. The in-situ conductivity shows a single step-like change at a potential of approximately 1.2 V in 0.5 mol dm −3 H 2 SO 4 vs. a reversible hydrogen reference electrode. The change in conductivity is also reflected in results of ex-situ CS-AFM for EIROF electrodes emersed at different potentials. At an emersion potential of 0 V the CS-AFM currentvoltage characteristics are non-linear and similar to those of diodes. At an emersion potential of 1.6 V the CS-AFM current-voltage characteristics are approximately linear, consistent with metallic behavior. Mott-Schottky analysis shows that at low potentials the oxide behaves as a p-type semiconductor with a flatband potential approximately 500 mV below the transition to high conductivity from the in-situ conductivity measurements. These results allow for an interpretation of changes in the relative magnitudes of the III/IV and IV/V (or IV/VI) voltammetric peaks during film growth through a block-release behavior involving space-charge layers in the oxide.