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 degrees 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 nanoscale, opening up significant possibilities for catalyst optimization. Rietveld refinements and microstructural line profile analysis show that the fcc unit cell dimensions follow Vegard's law within a good approximation. On the other hand, crystallite size, microstrain, phase content, and hcp c/a ratio depend nonlinearly on x but show some correlation to the bulk phase diagram. Elemental mapping shows the nanoparticles to be homogeneous, but in some cases, fcc hcp phase boundaries and modulations in the elemental distribution were observed. All samples below x < 0.3 exhibit a spherical morphology. At higher ruthenium content, x >= 0.3, another morphology emerges with elongated particles together with the dominating spherical mophology. The TEM average particle sizes range from 5.0(8) to 10.4(7) nm.