Employing density functional theory (DFT) calculations, the atomic arrangements and energetics of an extensive set of Au–Rh structures in a confined (2×1) surface cell on a Rh(111) substrate were investigated. The energetic preference order of the structures was evaluated with a total energy transformation method accounting for the different chemical compositions of the atoms in the supercell.
Among the studied surface structures an ordered 2x1 surface alloy layer composed of 50% Au and 50%
Rh was identified as the energetically favored structure, which corresponds to an experimentally proposed configuration based on scanning tunneling microscopy (STM) imaging31. The topmost layer of this structure consists of alternating Au and Rh rows, approximately in the same atomic plane, although
Au atoms are slightly protruded. We compared this surface alloy with another candidate structure from STM experiment, an added-Au-row configuration. We characterized the optimized geometries of these two surfaces, and found a slightly lower electron work function for the close-packed alloy surface compared to that of the added-Au-row structure. Bader charge analysis did not reveal significant differences in the atomic charges of surface atoms in the two structures, and in both cases a partial electron transfer from Rh toward Au atoms is observed. By studying Rh and Au adsorption properties, it is found that the preferred adsorption site for the Rh is on-top-Rh on the 2x1 surface alloy, and hollow-Rh on the added-Au-row configuration. For the adsorption of Au the hollow-hollow-Rh site is also favored on the added-Au-row structure, and the bridge-Rh site is preferred on the 2x1 surface alloy. Based on the analysis of the adsorption properties the bonding order preference among Rh and Au species is identified as Rh–Rh > Rh–Au > Au–Au. We arrived at the same conclusion by taking transformed total energies of the set of Au–Rh structures in different ways, and concomitantly analyzing the geometrical arrangements and bonding of the atoms.
Among the structures corresponding to a gold dose of Au=1 ML, the most stable structure is a pseudomorphic single Au layer on top of Rh(111), which is not corrugated. At a gold amount of Au=1.5 ML, the most stable structure is essentially the same found for Au=0.5 ML (alternating Au and Rh rows), but covered with a somewhat corrugated atomic layer of Au.
DFT-based simulated STM images revealed bias-voltage-dependent contrast differences among selected corrugated surface layer structures at various Au coverages, which, however, are difficult to resolve in STM experiments due to the theoretically predicted low apparent corrugations. Experimental results obtained by STM and low energy ion scattering (LEIS) on the thermally induced formation of the ordered Au–Rh surface alloy complement our theoretical study. The presented methods and results are expected
to contribute to the understanding of the formation of metallic surface alloys in various chemical compositions.
Acknowledgments
Financial support of the Slovak Academy of Sciences via the SASPRO fellowship (project No. 1239/02/01), the National Research Development and Innovation Office of Hungary under projects No. K120115, FK124100, GINOP-2.3.2-15-2016-00013, and the BME-Nanotechnology FIKP grant of EMMI (BME FIKP-NAT) are gratefully acknowledged. This work was also supported by the Collegium Talentum 2017 Programme of Hungary.
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