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

Abstract

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 2.5nm particles show 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 maybe 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.

Combined atomistic simulations to explore metastability and substrate effects in Ag–Co nanoalloy systems

“…The idea is therefore to follow the kinetics of the growth process of Ag-Co nanoparticles in two opposite modes of deposition on a substrate: direct sequential deposition (Ag atoms on an initial Co core) and reverse sequential deposition (Co atoms on an initial Ag core). These growth conditions can trap non-equilibrium arrangements due to lack of atomic mobility at moderate temperature, close to room temperature, as we have seen in the CoPt system 17 . However, the driving forces in the Ag-Co system have a strong tendency to converge in the same direction: a phase separation favoring Ag on the surface.…”

“…GISAXS measurements provide morphological features of nanoparticle assembly as size, shape and correlation distance between particles on the substrate, while GIWAXS allows the determination of the atomic structure. 17,19,20 Considering the density of nucleation, the time resolution of the scattering spectrum recording was of one spectrum for each 5 atoms deposited in the capture area of each nanoparticle. In the reverse sequence, the initial Ag particles as seeds showed an icosahedral structure of an average size of 1.9 nm when Co atoms were deposited 18 (see ESI†).…”

“…Small-angle and wide-angle Xray scattering experiments in grazing incidence (in GISAXS and GIWAXS, respectively) were carried out in real time during the atom deposition in situ in the UHV deposition set-up end-station on the SIXS beamline at SOLEIL synchrotron (see ESI † for details). GISAXS measurements provide morphological features of nanoparticle assembly as size, shape and correlation distance between particles on the substrate, while GIWAXS allows the determination of the atomic structure 17,19,20 . Considering the density of nucleation, the time resolution of the scattering spectrum recording was of one spectrum for each 5 atoms deposited in the capture area of each nanoparticle.…”

“…These growth conditions can trap non-equilibrium arrangements due to lack of atomic mobility at moderate temperature, close to room temperature, as we have seen in the CoPt system. 17 However, the driving forces in the Ag–Co system have a strong tendency to converge in the same direction: a phase separation favoring Ag on the surface. The two questions that we try to answer in this paper are: What is the dominant mechanism in this highly immiscible system?…”