Out-of-equilibrium supported Pt-Co core-shell nanoparticles stabilized by kinetic trapping at room temperature

• Published in: European physical journal. Applied physics Vol. 97; p. 56
• Main Authors: Khelfane, Hocine, Andreazza-Vignolle, Caroline, Ramos, Aline Y., Penuelas, José, Sauvage, Thierry, Andreazza, Pascal
• Format: Journal Article
• Language: English
• Published: EDP Sciences 2022
• Subjects: Condensed Matter; Materials Science; Physics; Core-shell nanoparticles; GISAXS; Anomalous X-ray scattering; Nanoalloys; Co-Pt alloy
• ISSN: 1286-0042; 1286-0050
• DOI: 10.1051/epjap/2022220027

The chemical stability of supported CoPt nanoparticles in out-of-equilibrium core-shell configurations was investigated mainly by anomalous grazing incidence small angle X-ray scattering (AGISAXS) in association with combined transmission electron microscopy and X-ray absorption spectroscopy. CoPt nanoparticles were prepared at room temperature by ultrahigh vacuum atom beam deposition using two different routes: simultaneous deposition of the two metals (CoPt) or sequential deposition. In this last case, Co deposition on a Pt-core (Pt@Co) and the reverse configuration (Co@Pt) are explored. In the Pt@Co case, our experimental analysis of 2.5 nm particles shows the stability of a Pt rich-core (80% Pt) surrounded by a two-monolayers-thick Co shell. In the reverse case, the core-shell structure is also stabilized, while the codeposited sample leads to an alloyed structure. These results suggest that the growth kinetics can trap the thermodynamically non-favorable core-shell structure even for this system which has a high alloying tendency. Besides the lack of atom mobility at room temperature, this stabilization can also be associated with core strain effects. Post thermal treatment of core-shell samples induces a structural transition from the core-shell configuration to the equilibrium alloyed configuration. This study demonstrates that the element-selective scattering technique, AGISAXS is highly efficient for the extraction of chemical segregation information from multi-component supported nanoparticles, such as core-shell structures, up to ultimate small sizes.