Structure of a thin oxide film on Rh(100)

The initial oxidation of Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction, surface x-ray diffraction, scanning tunneling microscopy, and density functional theory. We report a structural study of an oxygen induced structure displaying a $c(8\times 2)$ periodicity at an oxygen pressure above ${10}^{-5}\mathrm{mbar}$ and using a sample temperature of $700\mathrm{K}$. Our experimental and theoretical data demonstrate that this structure is due to the formation of a thin surface oxide with a hexagonal trilayer $\mathrm{O}-\mathrm{Rh}-\mathrm{O}$ structure.

Figure 1

(Color online) Image of the oxide induced LEED pattern at $146\mathrm{eV}$ after an oxygen exposure of $5\times {10}^{-5}\mathrm{mbar}$ for $600\mathrm{s}$ at a sample temperature of $700\mathrm{K}$. Solid lines indicate the Rh(100) substrate pattern, dashed lines the hexagonal overlayer induced by the oxygen exposure, and dotted lines the $c(8\times 2)$ pattern from the combined hexagonal oxide layer and squared substrate.

Figure 2

STM image of the $c(8\times 2)$ structure revealing a close-to-hexagonal surface structure. The longer range undulation due to the coincidence between the hexagonal oxygen induced overlayer and the square Rh(100) substrate is also revealed resulting in a $c(8\times 2)$ unit cell as indicated. The tunneling parameters were $-0.11\mathrm{V}$ and $1\mathrm{nA}$.

Figure 3

HRCL spectra obtained from the $c(8\times 2)$ structure. The Rh $3d_{5∕2}$ level reveals three components corresponding to the bulk of the Rh crystal, the interface between the Rh crystal and the $c(8\times 2)$ structure and highly oxygen coordinated Rh atoms. The O $1s$ spectrum shows two clearly discernible peaks of approximately the same intensity demonstrating the presence of two kinds of O atoms with different chemical surroundings in the $c(8\times 2)$ structure.

Figure 4

(Color online) (a) and (b) The resulting model of the $c(8\times 2)$) structure. (c) The simulated STM image of the $c(8\times 2)$ structure. The $c(8\times 2)$ cell and the bright triangels are indicated. (d) The experimental STM image after Fourier transform filtering, which averages over a few cells and thereby reduces the noise.

Figure 5

Comparison between the theoretical and experimental Rh $3d$ and O $1s$ core level shifts for the $c(8\times 2)$ surface oxide. For Rh the core level shifts are referred to the Rh bulk value, whereas for oxygen the O $1s$ binding energy difference between the surface and the interface O layer is shown. “A” indicates the calculated initial state shifts, “B” the calculated shifts including final state effects, and “C” shows the experimental shifts. In the calculation differently coordinated atoms in the same layer have different binding energies with the range of the calculated shifts indicated by bars.

Figure 6

Top and side view of the structural model of the oxygen induced $c(8\times 2)$ structure on Rh(100) as determined by ab initio calculations and quantitative LEED. The oxygen atoms in on-top positions of the Rh substrate are marked by thick black circles. Atom designations in parentheses refer to equivalent positions ($c1m1$ symmetry). Small arrows in the magnified part indicate the directions of the lateral displacements of the ${\mathrm{O}}_{2n}$ and ${\mathrm{Rh}}_{1n}$ atoms.

Figure 7

Comparison between experimental (black) and calculated (gray) LEED $I\text{−}V$ spectra of the best-fit model of the oxygen induced $c(8\times 2)$ structure on Rh(100) $(R_{\mathrm{Pe}}=0.16)$.

Figure 8

(Color online) Measured structure factors from some fractional and integer order rods of the $c(8\times 2)$ structure (symbols). Calculated structure factors using the model as calculated by DFT (dotted line), as obtained by LEED assuming an equidistant lattice in $x$ and $y$ (dashed line), and by combining the in-plane coordinates from DFT with the out-of-plane coordinates from LEED (full line).

Figure 9

(Color online) Calculated phase diagram of oxygen on Rh(100). $(2\times 2)$ and $(2\times 2)\text{−}2\mathrm{O}$ refer to chemisorbed oxygen with oxygen coverages of $1∕4$ and $1∕2$ ML, respectively, with the $(2\times 2)\text{−}2\mathrm{O}$ phase denoting the structure suggested by Baraldi et al. (Ref. 15), where oxygen is adsorbed in threefold hollow sites. The chemical potential is related to the temperature and the oxygen partial pressure $p$ through the ideal gas equation ${\mu }_{\mathrm{O}}(T,p)={\mu }_{\mathrm{O}}(T,p^0)+1∕2k_{B}T\ln (p∕p^0)$ (Ref. 46).