Surface Segregation in Compositionally Complex Ag-Au-Pd-Pt Solid Solutions: Insights from High-Throughput Experimentation and Atomistic Simulations

The activity of compositionally complex electrocatalysts depends on their surface composition, which can be different from the volume composition. In solid solutions, surface segregation under vacuum can be estimated based on the surface energy of the constituent elements. Upon exposure to ambient conditions, the surface reactivity of the elements, particularly their tendency to oxidize, is also important. Here, we investigate differences between the surface and volume composition of a model noble metal system, Ag-Au-Pd-Pt, fabricated by co-sputter deposition in the form of thin-film materials libraries (MLs), spanning a compositional range of Ag12-55Au7-50Pd6-60Pt7-58. The volume compositions of 684 measurement areas of these libraries were determined with energy dispersive X-ray spectroscopy (EDX). For each library, a set of nine selected areas was additionally measured by X-ray photoelectron spectroscopy (XPS), to determine the surface compositions, i.e. the first few nanometers of the films. The XPS data reveal surface segregation of Ag up to 8 at.% and Pt depletion of similar magnitude. The results were further validated by large-scale molecular dynamics/Monte-Carlo simulations using accurate machine learning interatomic potentials (MLIP), providing theoretical insights of surface segregation under vacuum conditions across the quaternary composition space.