Step-Orientation-Dependent Oxidation: From 1D to 2D Oxides

Using scanning tunneling microscopy and density functional theory, we have studied the initial oxidation of Rh(111) surfaces with two types of straight steps, having $\{100\}$ and $\{111\}$ microfacets. The one-dimensional (1D) oxide initially formed at the steps acts as a barrier impeding formation of the 2D oxide on the (111) terrace behind it. We demonstrate that the details of the structure of the 1D oxide govern the rate of 2D oxidation and discuss implications for oxidation of nanoparticles.

Figure 1

Rh(223) surface after oxygen exposure at ${\mu }_{\mathrm{O}}\approx -1.2\mathrm{eV}$ ($5\times {10}^{-6}\mathrm{mbar}$, $370°\mathrm{C}$), exhibiting a ($9\times 1$) oxide on the step edges. STM images (a) and (b) show (557) and (335) faceted areas, respectively. Interatomic distances are expanded by $\frac{9}{8}$, leading to a $1:1$ coincidence with the ($9\times 9$) oxide at (111) facets [white lines in (b)]. (c) Model of the ($9\times 1$) oxide on a (335) terrace, containing also one row of adsorbed oxygen. Atomic positions were obtained by relaxation by DFT. Yellow (bright) arrows indicate oxygen close to fourfold positions. (d) Comparison of a simulated (upper) and a measured (lower) STM image.

Figure 2

(a) Model of the structure of bare $\{100\}$ and $\{111\}$ steps and a hole. (b) STM image of a sputter-induced hole in the Rh(111) surface. (c) and (d) show DFT-calculated models for the $\{111\}$ and $\{100\}$ steps, respectively, without taking the oxygen on the (111) terraces into account. (e),(f) STM images of oxidized ($5\times {10}^{-6}\mathrm{mbar}$, $370°\mathrm{C}$) steps of the holes. Note the wavy but continuous appearance of the $\{100\}$ step and the vacancies and kinks in the $\{111\}$ step.

Figure 3

STM images ($75\times 75{\mathrm{nm}}^2$) from a Rh surface with sputter-induced holes exposed for 60 s to ${10}^{-4}\mathrm{mbar}$ of oxygen at (a) $250°\mathrm{C}$, (b) $300°\mathrm{C}$, (c) $350°\mathrm{C}$. (d) Width of the oxide for the different step orientations ($A=\{100\}$ and $B=\{111\}$ steps).