Probing the Structure of a Water-Oxidizing Anodic Iridium Oxide Catalyst using Raman Spectroscopy

Iridium oxide is one of the most important catalysts for water oxidation. The atomic structure of this catalyst remains unknown. We have studied anodically grown iridium oxide catalyst films using Raman spectroscopy. In addition to deuteration and 18O substitution experiments, theoretical models were also constructed using density functional theory to interpret the experimental data. The material was characterized over a large potential range which included that for the oxygen evolution reaction (0.0–1.8 V). The material was found to be composed of [IrO6]n edge-sharing polyhedra (with n ≥ 3). Ir centers are connected to each other via μ-O type oxygen linkages that allow for the Ir centers to electronically couple to each other. The most intense peaks in Raman spectra were characterized by stretching movement of Ir−μ-O bonds in the basal plane of the octahedra coupled to OH bending movements of hydroxyl groups bound to the Ir centers. Oxidation of Ir3+ to Ir4+ at 0.7–1.2 V within a μ-O linked polymeric geometry results in a blue coloration of the material at high potentials. Theoretical calculations indicate that the optical transition responsible for the color is essentially an Ir to Ir charge transfer transition. The active compound that carries out oxygen evolution is resistant to further structure-directing influence of oxidation. In the course of oxidation, it was observed that IrO2 with a rutile structure could form at potentials greater than 1.2 V as a side product of the reaction.