Bromine Adsorption and Thermal Stability on Rh(111): A Combined XPS, LEED and DFT Study

The adsorption and thermal stability of bromine on Rh(111) is investigated by synchrotron radiation-based XPS, LEED and DFT. Depending on the coverage, four different superstructures are observed. At low coverages, bromine occupies fcc hollow sites while, at higher coverages, a compression of the overlayer leads to bridge-bound bromine atoms. Br/Rh(111) shows a high thermal stability, which implies strong covalent bonding.

Abstract

This study addresses a fundamental question in surface science: the adsorption of halogens on metal surfaces. Using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (XPS), temperature-programmed XPS, low-energy electron diffraction (LEED) and density functional theory (DFT) calculations, we investigated the adsorption and thermal stability of bromine on Rh(111) in detail. The adsorption of elemental bromine on Rh(111) at 170 K was followed in situ by XPS in the Br 3d region, revealing two individual, coverage-dependent species, which we assign to fcc hollow- and bridge-bound atomic bromine. In addition, we find a significant shift in binding energy upon increasing coverage due to adsorbate-adsorbate interactions. Subsequent heating shows a high thermal stability of bromine on Rh(111) up to above 1000 K, indicating strong covalent bonding. To complement the XPS data, LEED was used to study the long-range order of bromine on Rh(111): we observe a (√3×√3)R30° structure for low coverages (≤0.33 ML) and a star-shaped compression structure for higher coverages (0.33–0.43 ML). Combining LEED and DFT calculations, we were able to visualize bromine adsorption on Rh(111) in real space for varying coverages.

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