Surface segregation of Fe in Pt–Fe alloy nanoparticles: Its precedence and effect on the ordered-phase evolution during thermal annealing

Coupling electron microscopy techniques with in situ heating ability allows us to study phase transformations on the single-nanoparticle level. We exploit this setup to study disorder-to-order transformation of Pt–Fe alloy nanoparticles, a material that is of great interest to fuel-cell electrocatalysis and ultrahigh density information storage. In contrast to earlier reports, we show that Fe (instead of Pt) segregates towards the particle surface during annealing and forms a Fe-rich FeOx outer shell over the alloy core.