Growth and termination of a rutile IrO2(100) layer on Ir(111)

Abstract

We investigated the oxidation of Ir(111) by gas-phase oxygen atoms at temperatures between 500 and 625K using temperature programmed desorption (TPD), low energy electron diffraction (LEED), low energy ion scattering spectroscopy (LEISS) and density functional theory (DFT) calculations. We find that a well-ordered surface oxide with (√3×√3)R30° periodicity relative to Ir(111) develops prior to the formation of a rutile IrO2(100) layer. The IrO2(100) layer reaches a saturation thickness of about four oxide layers under the oxidation conditions employed, and decomposes during TPD to produce a single, sharp O2 desorption peak at ~770K. Favorable lattice matching at the oxide-metal interface is likely responsible for the preferential growth of the IrO2(100) facet during the initial oxidation of Ir(111), with the resulting coincidence lattice generating a clear (6×1) moiré pattern in LEED. Temperature programmed reaction spectroscopy (TPRS) experiments reveal that CO and H2O molecules bind only weakly on the IrO2(100) surface and LEISS measurements show that the oxide surface is highly enriched in O-atoms. These characteristics provide strong evidence that the rutile IrO2(100) layer is oxygen-terminated, and thus lacks reactive Ir atoms that can strongly bind molecular adsorbates. Oxygen binding energies predicted by DFT suggest that on-top O-atoms will remain adsorbed on IrO2(100) at temperatures up to ~625K, thus supporting the conclusion that the rutile IrO2 layer grown in our experiments is oxygen-terminated. As such, the appearance of only a single O2 TPD peak indicates that the singly coordinate, on-top O-atoms remain stable on the IrO2(100) surface up to temperatures at which the oxide layer begins to thermally decompose.