(Invited) In-Situ Studies of Alloying and Alloy Surfaces in the Electrochemical Environment

Abstract

The development of new functional materials is transforming many technological areas of great interest from electronics and catalysis to detection and bio-sensing. Surface alloying and self-organization phenomena, driven by the surface stress and elastic interactions in metals, are key to these developments. While much has been done theoretically and explored experimentally in ultra-high vacuum (UHV), very little has been exploited in practical applications. Electrochemistry is the ideal science to exploit the physics and chemistry of such functional materials. Alloying between two components in general is driven by negative enthalpy of mixing and the lowering of surface free energy. However, when confined to a few surface layers, it can be observed even in systems that do not form alloys in the bulk phase (so called immiscible systems). UHV studies at elevated temperatures have found that the formation of immiscible surface alloys is a rather common phenomenon in systems with large lattice mismatch and that it is driven by surface stress relaxation and elastic energy. In this talk in-situ surface x-ray scattering studies of alloy formation and alloy surfaces will be presented. This will include alloys prepared by UHV methods, in particular both bulk and surface Pt3Sn(111) alloys, and alloys prepared directly in the electrochemical environment, Pb-Au(111). For the Pt3Sn system, of interest in electrocatalysis due to its tolerance towards CO poisoning, the surface has been studied under UHV conditions and during the adsorption of water and the results are compared to measurements of bulk Pt3Sn alloys in 0.5 M H2SO4 electrolyte under potential control. For the Pb-Au(111) system, resonant x-ray scattering methods are used to identify the onset of surface alloying and correlate the changes in surface stress with the out-of-plane atomic structure.