Atomic Imaging of the Surface Termination and Reconstruction of Low and High Index Iridium Oxide Surfaces and Insights into Their Facet-Dependent Oxygen Evolution Activities

Resolving the atomic surface structure, particularly surface termination or reconstruction, is essential for understanding the catalytic properties of metal oxides. Although rutile phase iridium dioxide (IrO₂) is the state-of-the-art electrocatalyst for the oxygen evolution reaction (OER) in water splitting, the atomic-level surface structures of IrO₂ remain largely unexplored, limiting our understanding of its facet-dependent OER activities. Herein, we perform aberration-corrected integrated differential phase contrast scanning transmission electron microscopy of the low- and high-index surface structures of spindle-shaped IrO₂ nanorods and reveal distinct surface terminations and/or reconstructions on different surfaces. Notably, the (110) surface shows a predominantly top-/bridge-oxygen termination and high structural stability without obvious surface reconstruction. In contrast, the (001) and (101) surfaces, where all surface Ir atoms are coordinatively unsaturated, undergo significant reconstruction. Additionally, a high-index (321) surface composed of (110) terraces is identified and exhibits a distinct [IrO] surface termination, indicating a weaker binding energy between Ir with O. Density functional theory calculations reveals weakened oxygen-binding energies on both the reconstructed (101) surface and the high-index (321) surface, predicting substantially lower limiting OER overpotentials compared to the (110) surface. These findings provide an important structural basis for understanding the OER activities of IrO₂ surfaces.