Morphological evolution in oxygen-induced faceting ofRe(123¯1)

Abstract

We have studied oxygen-induced faceting of the atomically rough $\mathrm{Re}(12\overline{3}1)$ surface by means of Auger electron spectroscopy, low energy electron diffraction, and scanning tunneling microscopy (STM). In contrast to previous faceting studies on other refractory metal surfaces, where simple morphologies of the facets were reported, we find a coverage-dependent morphological evolution of the facets ranging from long sawtooth ridges to complex structures exposing four different facets. The faceting occurs only when oxygen coverage $(\ensuremath{\theta})$ exceeds 0.5 monolayer (ML) and the surface is annealed at $\ensuremath{\ge}700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At low oxygen coverage $(0.5\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l0.7\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$, the $\mathrm{O}∕\mathrm{Re}(12\overline{3}1)$ surface becomes partially faceted upon annealing; further increasing of oxygen coverage $(0.7\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l0.9\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$ causes the surface to become completely faceted, forming long sawtooth ridges along the $[\overline{2}113]$ direction with typical dimensions of $\ensuremath{\sim}8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in width and $g50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in length upon annealing at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The size of the ridges grows with annealing temperature and annealing time, and the distance between the ridges is quite uniform. The two sides of each ridge have $(01\overline{1}0)$ and $(11\overline{2}1)$ orientations, and atomic-resolution STM images reveal that the edge of the ridge is atomically sharp. For $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, a third set of facets, identified as $(10\overline{1}0)$, emerges and truncates the original ridges. With the surface fully covered by oxygen $(\ensuremath{\theta}=1\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$, a fourth facet $(01\overline{1}1)$ also becomes prominent upon annealing. This morphological evolution is accompanied by a reduction of the average ridge length along $[\overline{2}113]$, indicating that the $(11\overline{2}1)$ facet is metastable. Our work demonstrates that even in a simple adsorbate/substrate system, the adsorbate-induced modification of the anisotropy of surface free energy can induce a complex sequence of changes in the surface morphology. The faceted Re surfaces may be model systems to study structure sensitivity in catalytic reactions, and may also provide promising templates to grow nanostructures.