First-Principles Studies on Oxygen-Induced Faceting of Ir(210)

Density functional theory calculations were performed to obtain an atomistic understanding of facet formation on Ir(210). We determined geometries and energetics of clean and oxygen-covered surfaces of planar Ir(210) as well as Ir(311) and two types of Ir(110) surfaces, which are involved in faceting by forming three-sided nanopyramids. Using the energies together with the ab initio atomistic thermodynamics approach, we studied the stability of substrate and facets in the presence of an oxygen environment. Our results show that facets are stable over the entire temperature range at which oxygen is adsorbed on the surface at coverages >/=0.45 physical ML, supporting the picture of a thermodynamic driving force. We also investigated the dependence of the phase diagram on the choice of the exchange-correlation functional and obtained qualitatively the same behavior. Finally, this work helps to better understand reactivity and selectivity of O-covered planar and faceted Ir surfaces in catalysis.