Temperature-driven phase transformation and element segregation in Pd-Ru immiscible alloy nanoparticles: Spatial resolving of elements and insights for electrocatalysis

Bimetallic alloys could form three typical structures including solid solution, heterostructure, and intermetallic compound, depending on the interactions between identical and different atoms. Although the trend can be predicted by the types of binary phase diagram, different synthetic protocols will trap the system in various kinetic intermediates among the three typical structures. Herein, we studied the phase evolution and elemental segregation in the alloy nanoparticles of immiscible Pd-Ru before and after thermal annealing. By developing an analysis method of local element segregation (LES) based on the energy dispersive spectroscopy (EDS) mapping signals, we were able to quantify the mixing of Pd and Ru atoms during the gradual phase transition from face-centered cubic (fcc) to hexagonal close packed (hcp). Density functional theory was also applied to calculate the energies of all possible PdRu4 structures (93 fcc models and 267 hcp models), which helps to rationalize the phase transition and element segregation. The annealing process also leads to the change of the electronic structure, which further influences the performance in the electrocatalytic hydrogen evolution reaction. The highest activity of PdRu4-400 was largely attributed to the proper interface between the Pd-rich fcc phase and Ru-rich hcp phase, as revolved by the above methods.