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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 initioatomistic 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.

The stability of the various facets in oxygen-induced faceting of Re(123̅1) has been studied by low-energy electron diffraction, scanning tunneling microcopy, and synchrotron-based high-resolution X-ray photoemission spectroscopy. When Re(123̅1) is annealed at 800–1200 K in oxygen (10 −7Torr), the surface becomes completely covered with nanometer-scale facets, and its morphology depends on the substrate temperature and oxygen exposure. Especially, the (112̅1) facet competes with the (101̅1) facet in determining the surface morphology, and the stability of each facet relies on oxygen coverage. Using density functional theory, the O−Re binding energies on the facets for various oxygen concentrations are calculated to explain how the oxygen coverage affects the anisotropy of surface free energy, which in turn determines the morphology of the faceted surface.