Spontaneous Chemical Ordering in Bimetallic Nanoparticles

In most experimental studies on bimetallic nanoparticles the homogeneous alloying is consistently reported or a priori assumed. It is generally believed that at nanoscale, alloying is promoted even for otherwise immiscible systems. In this study we present evidence that Pd–Pt nanoalloys are much more susceptible to segregation than their bulk counterparts. The spontaneous segregation (chemical ordering) of Pd–Pt nanoalloys was evidenced in hydrogen absorption studies and additionally confirmed by using X-ray photoelectron spectroscopy. The results clearly show that phase segregation in bimetallic nanoparticles do occur despite the lack of the miscibility gap in the Pd–Pt phase diagram, negative heat of mixing and small lattice mismatch for both metals. Phase segregation for other bimetallic systems is also discussed. Hydrogen solubility in the investigated nanoparticles is enhanced nearly by 3 orders of magnitude with respect to H solubility in unsegregated Pd–Pt alloys. This superior solubility points out the importance of phase segregation, which is frequently overlooked, yet fundamental for systems designed for heterogeneous catalysis and hydrogen storage