Oxygen-induced step bunching and faceting of Rh(553): Experiment and ab initio calculations

Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh(553) surface, a surface vicinal to (111), undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about ${10}^{-6}\mathrm{mbar}$ and a temperature of $380°\mathrm{C}$ this leads to (331) facets with one-dimensional oxide chains along the steps, coexisting with (111) facets. Further increase of the pressure and temperature results in (111) facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior.

Figure 1

(Color online) (a) Model of the Rh(553) surface. (b) STM image $(200\times 200{\mathrm{\AA }}^2)$ of the clean Rh(553) surface, using a positive sample bias of $2.8\mathrm{V}$ and a tunneling current of $0.3\mathrm{nA}$. (c) LEED pattern from the clean Rh(553). The diffraction spots from the (111) terraces are indicated. (d) SXRD scan from the Rh(553) surface in the $K$ direction with $H=0$ and $L=0.5$ plotted in (553) coordinates.

Figure 2

(a) LEED image of the Rh(553) surface after an oxygen exposure of $5\times {10}^{-6}\mathrm{mbar}$ for $600\mathrm{s}$ at a sample temperature of $380°\mathrm{C}$. The $(1\times 1)$ (large hexagon) and $(2\times 1)$ (small hexagon) from the (111) facets and the $(1\times 1)$ from the oxygen-induced (331) facets are indicated. (b) and (c) SXRD scans from the oxygen-exposed surface in the $K$ direction with $H=0$ plotted in (553) and (331) coordinates, respectively.

Figure 3

(Color online) (a) STM image of the Rh(553) surface after an oxygen exposure of $5\times {10}^{-6}\mathrm{mbar}$ for $600\mathrm{s}$ at a sample temperature of $380°\mathrm{C}$, using a negative sample bias of $V=-0.8\mathrm{V}$ and a tunneling current of $I=0.3\mathrm{nA}$. (b) Zoom-in STM image on a (331) facet ($70\times 30\mathrm{\AA }$, $U=-0.8\mathrm{V}$, $I=0.3\mathrm{nA}$). (c) Simulated STM image of the (331) facets decorated with oxygen atoms in a $(10\times 1)$ pattern according to the model in d. (d) Model of the surface morphology and atomic arrangement (oxygen, red balls; Rh at the step edges, blue balls; Rh, gray balls). Note the one-dimensional oxide formed on the steps of the (331) surface.

Figure 4

HRCL spectra from the Rh(553) surface after an oxygen exposure of $5\times {10}^{-6}\mathrm{mbar}$ for $600\mathrm{s}$ at a sample temperature of $380°\mathrm{C}$. The Rh $3d_{5∕2}$ spectra can be decomposed into components corresponding to onefold, twofold, and fourfold oxygen-coodinated Rh atoms, while the O $1s$ consists of one component.

Figure 5

(a) LEED image of the Rh(553) surface after an oxygen exposure of $5\times {10}^{-3}\mathrm{mbar}$ for $600\mathrm{s}$ at a sample temperature of $550°\mathrm{C}$. The diffraction spots from the $(9\times 9)$ structure on the (111) facets strongly indicate increased step bunching. (b) and (c) SXRD scans from the oxygen-exposed surface in the $K$ direction with $H=0$ plotted in (553) and (111) coordinates, respectively. The peak at $K=-0.89$ in (c) shows the presence of the $(9\times 9)$ surface oxide found on Rh(111) (Ref. 9) on the (111) facets.

Figure 6

(Color online) (a) HRCL spectra from the Rh(553) surface after an oxygen exposure of $5\times {10}^{-3}\mathrm{mbar}$ for $600\mathrm{s}$ at a sample temperature of $500°\mathrm{C}$. The spectra strongly indicate the presence of the $(9\times 9)$ structure on the (111) facets. (b) Model of the completely (111)- and $\left(11\overline{1}\right)$-faceted Rh(553) surfaces with the surface oxide structure on the surface.

Figure 7

(Color online) (a) Faceting of Rh(553) into Rh(331) (blue/grey atoms) and Rh(111) (black atoms), (b)–(d) calculated adsorption structures for O on Rh(553), and (e)–(g) O on Rh(331).

Figure 8

(Color online) Phase diagram comparing $\mathrm{O}∕\mathrm{Rh}\left(553\right)$ to $\mathrm{O}∕\mathrm{Rh}\left(331\right)+\mathrm{O}∕\mathrm{Rh}\left(111\right)$: dark gray (red) (553) phases, light gray (green) (331) phases, and dashed (blue) (111) surface oxide.