Effect of IrO6 Octahedron Distortion on the OER Activity at (100) IrO2 Thin Film

The (100) crystallographic plane is the most active facet of iridium dioxide (IrO2) for the oxygen evolution reaction (OER). Pulsed laser deposition was used to grow (100) IrO2 on a (100) SrTiO3 substrate at deposition temperature ranging from room temperature to 600 degrees C. Detailed structural and morphological characterization was performed using AFM, XRD, and X-ray reciprocal space mapping (RSM) to unravel the geometrical arrangement of [IrO6] octahedra in the (100) IrO2 thin films. It is shown that the symmetry mismatch between the substrate and the epitaxial thin film imposed an orthorhombic distortion of the tetragonal structure of IrO2 and, as a consequence, the [IrO6] geometry is distorted. These data were correlated to the OER characteristics established from electrochemical measurements. DFT modeling was employed to relate differences in surface relaxation of IrO2 films prepared at different temperatures with changes in OER activity. Vacancy formation leads to higher surface stability at temperatures around 500 degrees C, which corresponds to the deposition temperature at which the electrocatalytic activity of (100) epitaxial IrO2 film is maximal.