Growth of oxygen-induced nanoscale-pyramidal facets on Rh(210) surface

Oxygen-induced nanometer scale faceting of the atomically rough Rh(210) surface has been studied using Auger electron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). When the Rh(210) surface is annealed at temperature $\ge 550\text{K}$ in oxygen (pressure $\ge 2\times {10}^{-8}\text{Torr}$), it becomes completely covered with nanometer-scale facets. LEED studies reveal that the faceted surface is characterized by three-sided nanoscale pyramids exposing one reconstructed (110) and two {731} faces on each pyramid. STM measurements confirm the LEED results and show that the average facet size ranges from 12 to 21nm when changing annealing temperature from 800 to 1600 K. Moreover, atomically resolved STM images show that the (110) face of faceted Rh(210) exhibits various reconstructions ($1\times n$, $n=2–4$) depending on oxygen coverage. Faceted Rh(210) is a potential template for studies of structure sensitive reactions.

Figure 1

Hard-ball model of Rh(210) surface (top and side view) showing four exposed layers.

Figure 2

LEED patterns from planar and faceted Rh(210) surfaces at different incident electron-beam energy $\left(E_{\text{e}}\right)$: planar surface at (a) $E_{\text{e}}=60\text{eV}$, (b) $E_{\text{e}}=20–110\text{eV}$; faceted O/Rh(210) surface at (c) $E_{\text{e}}=60\text{eV}$, (d) $E_{\text{e}}=20–50\text{eV}$, (e) $E_{\text{e}}=56–84\text{eV}$, and (f) $E_{\text{e}}=90–120\text{eV}$. The primitive unit cell of the LEED pattern in (a) is labeled together with its main symmetry directions in real space.

Figure 3

Evolution of the LEED spots from faceted O/Rh(210) surface with annealing temperature (a) 400 K, (b) 450 K, (c) 500 K, (d) 550 K, (e) 600 K, (f) 650 K, (g) 750 K, and (h) 850 K at incident electron-beam energy 74 eV. The LEED spot in (a) corresponds to the $\left(-1,-1\right)$ beam from the planar O/Rh(210) surface based on the notation in Fig. 2a.

Figure 4

(a) LEED pattern from the faceted surface at incident electron-beam energy $E_{\text{e}}=70\text{eV}$ and (b) LEED pattern from the planar surface at incident electron-beam energy 160 eV.

Figure 5

(Color online) STM images of a fully faceted O/Rh(210) surface prepared by predosing 10 L oxygen followed by annealing in ${\text{O}}_2$ $\left(4\times {10}^{-8}\text{Torr}\right)$ at 850 K for 2 min and then cooling in oxygen to room temperature. Scan parameters are: (a) $3000\times 3000{\text{Å}}^2$ ($V=2\text{V}$ and $I=1\text{nA}$), (b) $500\times 500{\text{Å}}^2$ after the $X$ slope taken ($V=2\text{V}$ and $I=1\text{nA}$), and (c) $250\times 220{\text{Å}}^2$ after the $X$ slope taken ($V=1\text{V}$ and $I=0.5\text{nA}$). The azimuthal angles of edge lines between different facets are also shown.

Figure 6

(Color online) (a) STM image of (731) facet of faceted O/Rh(210) $\left(104\times 84{\text{Å}}^2\right)$ ($V=1.2\text{V}$ and $I=0.5\text{nA}$). (b) Cross-section profile of line 1. (c) Cross-section profile of line 2.

Figure 7

(Color online) STM images of the (110) facet from faceted O/Rh(210) prepared by predoing 10 L oxygen followed by annealing in oxygen at 850 K for 2 min and then cooling to room temperature in oxygen. Oxygen pressures are: (a) $2\times {10}^{-8}\text{Torr}$, (b) $4\times {10}^{-8}\text{Torr}$, and (c) $6\times {10}^{-8}\text{Torr}$. The sizes of the images are (a) $250\times 250{\text{Å}}^2$, (b) $105\times 85{\text{Å}}^2$, and (c) $94\times 50{\text{Å}}^2$ ($V=1.2\text{V}$ and $I=0.5\text{nA}$). The cross-section profiles of the lines along [001] in images (a)–(c) are represented in (d)–(f).

Figure 8

(a) Schematic of single pyramid from the faceted Rh(210) surface. (b) Hard-ball model of Rh(731) surface and (c) Hall-ball models of $\text{Rh}\left(110\right)\left(1\times n\right)\left(n=2,3,4\right)$ surfaces.