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.