(120e) Surface Segregation in Alloy Thin Films: A High-Throughput Study

The surface composition of an alloy may differ from the composition of its bulk because of preferential segregation of components that have low surface energy to the alloy surface. Because surface composition influences the rates of elementary molecular processes that occur at surfaces, surface segregation can have a significant impact on the performance of devices based on alloys, including catalysts and separation membranes. Segregation is a complex function of bulk composition and environment. However, because alloy composition varies continuously, accelerated approaches are needed to study segregation over large regions of bulk composition space.

We have developed a novel offset filament deposition tool for preparation of Composition Spread Alloy Films (CSAFs), materials libraries that contain all possible alloy compositions on a single, compact substrate. In this work, we use the tool to prepare Pd_xCu_1-x CSAFs for the study of surface segregation in sulfur-tolerant alloys for hydrogen separation. We apply spatially resolved low energy ion scattering spectroscopy (LEISS) and x-ray photoelectron spectroscopy (XPS) to characterize the films' top surface and near surface compositions as functions of bulk composition (location on substrate) and temperature. We apply the Langmuir-McLean thermodynamic model to the CSAF segregation data for high-throughput estimation of ΔG_seg, the free energy of Cu segregation, as a function of bulk composition. Our results show that Cu segregates to the top surface of the clean alloy at all bulk compositions.

Environmental factors can influence segregation patterns. Thus, we also characterized near- and top-surface compositions of Pd_xCu_1-x CSAFs in the presence of adsorbed sulfur, introduced as H₂S, a common contaminant in process streams derived from fossil fuels. We observed that sulfur induces segregation reversal, drawing palladium atoms back to the alloy's top surface to create energetically favored Pd-S bonds.