Nanoscale properties of planar and faceted iridium(210)

In the presence of a thin layer of adsorbed oxygen, the Ir(210) surface exhibits morphological instability and, upon annealing, becomes covered with nanometer-sized pyramids with facets of {lcub}311{rcub} and (110) crystallographic orientation.; Electron diffraction and temperature programmed desorption experiments show that complete faceting requires 0.5 monolayers of oxygen coverage and annealing temperature of 600 K. For lower temperatures and coverages, faceted and planar surfaces coexist. Oxygen desorption above 950 K causes irreversible facet destruction.; In previous adsorbate-induced faceting studies, it has not been possible to remove the adsorbate while preserving the faceted structure. We have accomplished this for O/Ir(210) via two low-temperature chemical methods: catalytic carbon monoxide oxidation and water formation. The clean faceted surface is an excellent substrate for model catalytic studies; it is stable for temperatures below 600 K and irreversibly reverts to planar above 600 K.; Real-time low energy electron microscopy shows uniform facet nucleation and growth on the oxygen-covered Ir(210) surface and facet relaxation of the clean faceted surface. Planar and faceted structures coexist between 850 K and 1150 K in the presence of gaseous oxygen.; Atomic-resolution scanning tunneling microscopy shows that {lcub}311{rcub} facets are atomically smooth. The (110) facets consist of unreconstructed terraces separated by steps and of a complicated superstructure formed on the same facet. The facet atomic structure is consistent with models based bulk iridium properties and with first-principle simulations. Facets grow with annealing temperature increase, their average size ranging from 5 nm to 14 nm.; Acetylene and ammonia decomposition exhibit structure sensitivity over the planar and faceted surface. Ammonia decomposition additionally exhibits size effects in experiments on faceted surfaces with different average facet size. Subtle differences in core-level high resolution soft x-ray spectra suggest that surface morphology, rather than electronic structure changes, is responsible for the structure and size effects.; This work is the first adsorbate-induced faceting study where the preparation of a clean elemental faceted substrate has been accomplished. It paves the way for revealing the mechanisms of substantial mass transport in surface transformations, and is an excellent substrate for the study of structure sensitivity and size effects in catalytic reactions.