Atomic structure, stability and formation of surface confined alloys

Abstract

In this work, various bimetallic model surfaces were prepared on Ru(0001) and Pt(111) single crystals and structurally characterized by scanning tunneling microscopy (STM) and Auger-electron spectroscopy (AES) to identify the atomic structures of surface confined alloys, their stabilities and the surface alloy formation process. Furthermore, Monte-Carlo (MC) simulations based on a 2D lattice-gas Hamiltonian (limited to effective pair interactions) provide an adequate energetic description of the surface alloy structures and allow for correlations with density functional theory calculations. The preparation of metastable lateral equilibrated alloys confined to the outermost layer and their characterization with respect to the atom arrangement was successfully performed for the bimetallic systems PtRu/Ru(0001), PdRu/Ru(0001) and AuPt/Pt(111), covering many different compositions in the range of 0 < x < 1 for each system. The MC simulations based on experiment - CuPd/Ru(0001), PtRu/Ru(0001), PdRu /Ru(0001), AuPt /Pt(111) - and DFT energies - AuPt/Pt(111) - reproduce the experimentally found ensemble and ligand statistics with excellent agreement. This structural information could be in several cases correlated with (electro-)catalytic properties of the respective surface alloy. Stability aspects and the surface alloy formation process was studied via various overgrowth experiments - PdRu, PtRu - and the "freezing" of the surface structure after initial metal-metal exchange processes - PtRu/Ru(0001). Based on these experiments a surface alloy formation mechanism is suggested and the relative strong stability towards surface confinement confirmed. The formation of metastable binary surface alloys can be generally predicted by comparing the nearest-neighbour bulk distances and the surface energy of both components.