Paper SS+AS-TuA3
Growth and Termination of a Rutile IrO2(100) Layer on Ir(111)

Tuesday, November 8, 2016, 3:00 pm, Room 104E

Iridium oxide is an effective catalyst for promoting electrochemical water splitting and is a promising material for effecting other chemical transformations as well. In this talk, I will discuss our recent investigations of the growth and termination of a crystalline IrO₂(100) film that develops during the oxidation of Ir(111) by gaseous O-atoms. We characterized the oxidation of Ir(111) 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 as the oxygen coverage increases to 1.4 ML (monolayer). Continued oxidation produces a rutile IrO₂(100) layer that reaches a kinetic saturation, under the conditions employed, after the growth of about four atomic layers and decomposes during TPD to yield a sharp O₂ desorption peak at ~770 K. We assert that favorable lattice matching at the IrO₂(100)/Ir(111) interface is responsible for the preferential growth of the IrO₂(100) facet during the initial oxidation of Ir(111), as LEED reveals the formation of a well-defined (6 × 1) coincidence structure. TPD experiments show that CO and H₂O probe molecules bind weakly on the IrO₂(100) surface, and LEISS measurements reveal that the oxide surface is strongly enriched in O-atoms. These characteristics provide evidence that the rutile IrO₂(100) layer is oxygen-terminated, and therefore lacks reactive Ir atoms that can strongly bind molecular adsorbates. Finally, I will discuss our DFT predictions of the stability of so-called on-top and bridging oxygen atoms on rutile IrO₂ and RuO₂ surfaces. The DFT results support the conclusion that IrO₂(100) is oxygen-terminated at the growth temperatures that we employed (< 650 K), and further reveal that on-top oxygen atoms significantly destabilize bridging oxygen atoms on the rutile (100) surfaces; such destabilization is less pronounced on the (110) surfaces. This destabilization may explain our observation that the desorption of on-top oxygen atoms and complete decomposition of the IrO₂(100) film occur over a similar range of temperatures during TPD. Our findings have implications for understanding the generation of rutile IrO₂ layers for model surface chemistry studies.