Electronic Structure and Growth of Electrochemically Formed Iridium Oxide Films

Liudmila Ilyukhina,Svein Sunde,Richard G Haverkamp

doi:10.1149/2.1351714jes

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 in H2SO4 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 the CS-AFM current-voltage characteristics are non-linear and similar to those of diodes. At an emersion potential of 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 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.

Highlights

IntroductionIridium oxide[1] is relevant as a material for electrochromics,[2,3,4] electrocatalysis[5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] and as a pH sensor.[20]
Recent work indicates that the number of electrons in the d-band of transition metal oxides is an important descriptor of catalytic activity,[24,25] other aspects of the electronic structure may be important as well.[26]
Indirect or ex-situ evidence of such switching is available for anodically formed iridium oxide films (AIROF) in the literature,[31,32] but direct evidence is scant

Summary

Introduction

Iridium oxide[1] is relevant as a material for electrochromics,[2,3,4] electrocatalysis[5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] and as a pH sensor.[20]. Anodically formed iridium oxide films (AIROFs) display distinct semiconducting behavior in the reduced form and metallic properties in the oxidized form.[32] A similar transition was not observed for iridium oxide synthesized by hydrolysis.[17] Electrochemically formed iridium oxide films (EIROFs)[34,35] are interesting forms of iridium oxide from the perspective of making thin iridium oxide layers for high utilization of iridium.

Results

Discussion

Conclusion