From 1D to 3D Ru Nanostructures on a Pt Stepped Surface as Model Systems in Electrocatalysis: UHV-STM and XPS Study

A Ru-decorated Pt single crystal vicinal to the (111) plane was studied using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in ultrahigh-vacuum environment. Pt(332) vicinal surface was used, and different coverages of Ru (theta(Ru)) were deposited. STM images show that Ru initial growth is highly influenced by the stepped nature of the surface. For instance, 1D and 2D nanostructures grow decorating the steps at low coverage (theta = 0.34), while at higher coverage a second layer is stabilized and bilayer (3D) growth sets in. The size and shape of the Ru nanostructures differ from those reported for Ru on Pt(111) due to the high density of steps that induces anisotropic growth and delays the monolayer to bilayer growth transition. Sample annealing at 623 K promotes further anisotropy and an increase in the amount of 3D structures. XPS suggests that no bulk alloying (bulk diffusion) of Ru occurs at this temperature. Additionally, our findings are used to discuss the different electrocatalytic behavior reported for some PtRu systems and to explain tendencies observed for Ru-decorated Pt nanoparticles toward the CH3OH and CO electrooxidation reactions. (AU)