title: Surface Segregation of Fe in Pt-Fe Alloy Nanoparticles: Its Precedence and Effect on the Ordered-Phase Evolution during Thermal Annealing.
abstract: 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 FeOxouter shell over the alloy core. By combining both ex situ and in situ approaches to probe the interplay between ordering and surface-segregation phenomena, we illustrate that the surface segregation of Fe precedes the ordering process and affects the ordered phase evolution dramatically. We show that the ordering initiates preferably at the pre-existent Fe-rich shell than the particle core. While the material-specific findings from this study open interesting perspectives towards a controlled phase evolution of Pt-Fe nanoalloys, the characterization methodologies described are general and should prove useful to probing a wide-range of nanomaterials.
authors.affiliation: Department of Materials Science and Engineering, McMaster University Hamilton-L8S 4L8 (Canada) E-mail: gbotton@mcmaster.ca.
authors.forename: Sagar
authors.initials: S
authors.lastname: Prabhudev
authors.affiliation: Department of Materials Science and Engineering, McMaster University Hamilton-L8S 4L8 (Canada) E-mail: gbotton@mcmaster.ca ; Canadian Center for Electron Microscopy Hamilton-L8S 4M1 (Canada).
authors.forename: Matthieu
authors.initials: M
authors.lastname: Bugnet
authors.affiliation: School of Materials Science and Engineering, Shanghai Jiao Tong University Shanghai, 200240 (P.R. China).
authors.forename: Guo-Zhen
authors.initials: GZ
authors.lastname: Zhu
authors.affiliation: National Research Council Canada Ottawa-K1A 0R6 (Canada).
authors.forename: Christina
authors.initials: C
authors.lastname: Bock
authors.affiliation: Department of Materials Science and Engineering, McMaster University Hamilton-L8S 4L8 (Canada) E-mail: gbotton@mcmaster.ca ; Canadian Center for Electron Microscopy Hamilton-L8S 4M1 (Canada).
authors.forename: Gianluigi A
authors.initials: GA
authors.lastname: Botton
keywords: alloy nanoparticles; in situ electron microscopy; iron; phase transition; platinum