Paper NS+AS+EM+MI+SP+SS-ThM2
BCC to FCC Phase Transition of Pd_xCu_1-x at Nanoscale

Thursday, November 2, 2017, 8:20 am, Room 19

One of the most interesting characteristics of alloy nanoparticles (NPs) is that they can have different phases from those of the bulk. In the bulk phase diagram of Pd_xCu_1-x, there exists a composition range, 0.35 < x < 0.55, over which a B2 phase (ordered body centered cubic, CsCl structure) is formed at T < 873 K, in spite of the fact that pure Pd and Cu both have face centered cubic (FCC) bulk crystal structures. An experimental methodology has been developed for determining the phase behavior of Pd_xCu_1‑x size and composition spread nanoparticle (SCSNP) libraries. Spatially resolved X-ray photoemission spectroscopy (XPS) was used to map the Cu 2p_3/2 core level shifts (CLS) with respect to the value for pure Cu across composition space on the bulk Pd_xCu_1-x alloy. The result has shown that the Cu 2p_3/2 binding energy decreases monotonically with increasing Pd at.% in the FCC phase. There is additional discontinuous CLS over the composition range from 0.35 to 0.55 Pd at.%, where the B2 phase forms. Therefore, the Cu 2p_3/2 core level binding energy measured by XPS can be used to distinguish between the ordered B2 phase and disordered FCC phase. The Pd_xCu_1-x SCSNP library on a Mo substrate was prepared using a rotatable shadow mask deposition tool previously developed by our group. After annealing the Pd_xCu_1‑x alloy thin film to 700 K, the additional CLS over the composition range, 0.35 < x < 0.55, has been observed at a film thickness > 6 nm, which suggests the formation of B2 phase. However, at a film thickness between 4 – 6 nm, the Cu 2p_3/2 binding energy decreases monotonically across composition space which suggests that only FCC phase exists for alloy films in this thickness range. Because the FCC phase is more densely packed than the B2 phase, the surface tension in this thickness regime can drive a conversion from the ordered B2 phase back to the randomly distributed FCC solid solution. More interestingly, the additional CLS over the composition range from 0.35 to 0.55 Pd at.% reoccurs at a film thickness < 4 nm, which suggests the formation of B2 phase. This observation is the result of dewetting of the Pd_xCu_1‑x NPs after heating at 700 K for 30 mins, and the size of dewetting NPs exceeds 6 nm where the close-packed FCC phase is stabilized. Dewetting of Pd_xCu_1‑x NPs is validated by the appearance of the substrate Mo XPS signal at a film thickness < 4 nm. This comprehensive experimental study of the phase behavior for Pd_xCu_1-x alloy NPs will be correlated with their catalytic activity across composition and size spaces to accelerate the development of alloy NPs for catalytic applications.