Supercritical Flow Synthesis of Pt1–xRux Nanoparticles: Comparative Phase Diagram Study of Nanostructure versus Bulk

The thermodynamic stability of nanocrystals is different from that of bulk systems, and nanoscale phase diagrams are to a large degree unknown. Here we present a systematic investigation of the Pt1–xRux phase diagram through supercritical flow synthesis of Pt1–xRux nanoparticles across the entire compositional range. The synthesis was done in stoichiometric steps of 0.1 using an ethanol–toluene mixture as solvent at 450 °C and 200 bar. The products were characterized by high-resolution synchrotron powder X-ray diffraction, transmission electron microscopy, and elemental mapping of individual particles using energy-dispersive X-ray spectroscopy. The diffraction data revealed a single-phase face-centered cubic (fcc) alloy for x ≤ 0.2, while an additional hexagonal close-packed (hcp) phase emerges as x approaches 1. This behavior deviates significantly from the bulk phase diagram, where a biphasic region is only observed for 0.62 < x < 0.8. Thus, compositional design of Pt–Ru alloys is more flexible on the n...