Iridium oxide [1] is relevant as a material for electrochromics [2 - 4], electrocatalysis [5 - 19] and as a pH sensor [20]. A proposed mechanism for the electrochromic properties of the material based on the electronic structure was suggested by Granqvist [3, 4] which implies variations in band filling and therefore also the conductivity associated with intercalation of protons and possibly other ions [21]. For oxygen-evolution electrocatalysts one expects the activity to depend on the binding energy of intermediates involved in the reaction, in turn dependent on the electronic structure.

On this basis we investigated the nature of the electronic structure of electrochemically formed iridium oxide films (EIROF) by in-situ conductivity measurements in an electrochemical cells 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 vs. a reversible hydrogen reference electrode in a solution of 0.5 mol dm^-3 H₂SO₄. The in-situ conductivity measurements are shown in Fig. 1 along with a cyclic voltammogram.

The change in conductivity is also reflected in results of ex-situ current-sensing atomic-force microscopy (CS-AFM) for EIROF electrodes emersed at different potentials. At an emersion potential of 0 V the CS-AFM current-voltage characteristics is non-linear and similar to those of diodes. At an emersion potential of 1.6 V the CS-AFM current-voltage characteristics were approximately linear, consistent with metallic behavior. Mott-Schottky analysis, included in Fig. 1, 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 allows 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.

References

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Captions

Fig. 1. Conductivity under potential control collected for an EIROF on a gold substrate. The figure includes a Mott-Schottky plot and a voltammogram for a film on carbon support collected at a sweep rate of 5 mV s^-1.