Detection of CuAuPd Phase Boundaries Using Core Level Shifts

Abstract

A high throughput study has been conducted of the Cu 2p, Au 4f, and Pd 3d X-ray photoemission spectra obtained from a continuous distribution of CuAuPd alloy samples prepared as a single composition spread alloy film (CSAF). All three elements exhibit shifts of their core level binding energies with respect to their pure states when diluted into the alloy. The Cu 2p core level shift (CLS) exhibits additional shifts over the composition ranges at which the CuAuPd alloy transitions between FCC and B2 phases. This discontinuous CLS has been used to map the extent of the B2 phase across the ternary CuAuPd alloy composition space. The sensitivity of core level binding energies to the alloy phase offers an opportunity to use XPS to study phases in alloy nanoparticles, ultrathin films, and other morphologies that are not amenable to structure determination by diffraction based methods.

Surface Segregation Across Ternary Alloy Composition Space: CuxAuyPd1–x–y

“… The B2 phase is an ordered CsCl structure on a bcc lattice. This B2 phase has been identified in CSAFs using both electron backscatter diffraction and careful measurements of Cu 2p 3/2 core level shifts observed using XPS. ,,, In this work, we find that Cu segregation to the surface is inhibited over a composition range that is roughly aligned with that of the B2 phase. Figure shows that there is a narrow bulk composition region, 0.45 < x Cu < 0.55, over which the surface excess of Cu drops to almost zero.…”

“…The darker one centered at Cu 0.5 Pd 0.5 shows 50% lower LEIS intensity than expected from the pure components, indicating a crystal structure of lower surface atom density, consistent with the existence of the B2 phase in the CuAuPd phase diagram. 39 Ternary Alloy Segregation Equilibrium. The Langmuir−McLean equation has been widely used to describe equilibrium surface segregation in binary alloys.…”

“…In this work, the CuAuPd system was chosen for the ternary segregation study because it is a well-studied alloy system with a known phase diagram 39 and components with very different masses, allowing easy quantification of surface composition using LEIS. Pd alloys catalyze many hydrogenation 41,42 and oxidization 5,43 reactions and can be used as hydrogen p u r i fi c a t i o n m e m b r a n e s .…”

“…In this work, the CuAuPd system was chosen for the ternary segregation study because it is a well-studied alloy system with a known phase diagram and components with very different masses, allowing easy quantification of surface composition using LEIS. Pd alloys catalyze many hydrogenation , and oxidization , reactions and can be used as hydrogen purification membranes. ,− Segregation in PdAu, ,,,− PdCu, ,,,, and CuAu , binary alloys has been described frequently in the literature, and CuAuPd segregation has also been reported at a few discrete compositions. , It is known that under vacuum conditions, Au and Cu in Pd alloys are the surface-enriched elements.…”

“…Composition spread alloy films (CSAFs) are high-throughput libraries that were first developed in the 1950s for determination of alloy phase diagrams . Since then, they have been successfully applied to study the phase diagrams, electronic structures, and many other properties of multicomponent alloy systems. Figure illustrates a ternary CSAF prepared by deposition of three alloy components with composition gradients oriented at 120° .…”