Hydrogen-Induced Clustering of Metal Atoms in Oxygenated Metal Surfaces

Nearly all metals form spontaneously an oxygenated surface upon exposure to air. Here we demonstrate that the reaction of such an oxygenated surface with hydrogen results in the clustering of metal atoms. Using scanning electron microcopy, X-ray photoelectron spectroscopy and in-situ low-energy electron microscopy, we show that Cu atoms in the oxygenated Cu(110) surface self-assemble into Cu clusters upon the hydrogen induced loss of the chemisorbed oxygen. It is shown that the clustering of Cu atoms occurs preferentially along the upper side of step edges formed by neighboring terraces of the substrate and boundaries of heterophase domains on the same terrace, followed by the spreading across the entire surface as the reaction progresses toward completion. Using density-functional theory calculations, we show that the heterogeneous clustering of Cu atoms is induced by step-crossing barriers that hinder Cu atoms crossing descendent steps, thereby resulting in 3D aggregation of Cu atoms on the upper side of step edges. These results may find broader applicability to tailor the formation of metal clusters for elucidating the intrinsic properties and functionalities of nano clusters.