Equilibrium and Phase Stability of Nanoparticles

Abstract

<p>We explore the effect of size on the phase stability of nanosystems by comparing calculated trends with the annealing behavior of nanoparticles (NPs) initially in a core-shell configuration. The NPs are characterized using a variety of transmission electron microscopy (TEM) techniques.</p> <p>We first theoretically consider the equilibrium within a Au-Pt NP of a given size. When considering the contribution of surface and interface energies, we note the appearance of a restricted composition range of the phase diagram over which the liquid and solid phases cannot coexist in a core-shell configuration. A critical radius of ",42 nm is identified below which the NP is single-phased for any composition. It is demonstrated that both branches of the miscibility gap of the Au-Pt phase diagram shift towards the Au-rich composition with increasing curvature. The magnitude of the shift is found to be strongly correlated with the coupling of nonlinear terms entering the Gibbs energy. The main contribution to the shift arises from the composition-dependent surface energy, calculated by considering the selective adsorption of Au to the surface, evaluated using the available thermodynamic properties of the Au-Pt system.</p> <p>An array of TEM-related analytical methods were developed or adapted for the characterization of individual NPs. In particular, chemical maps with quantitative information from a NP with a spatial resolution of '" 1.2 nm could be achieved, with their corresponding error analysis. We introduce an algorithm to retrieve the radial elemental composition from the projected chemical map of a NP if a spherical symmetry can be assumed and test it with NPs of known structures. We also present a technique to determine the composition of a NP having one of the elements depleting during analysis, and test it experimentally with 5-20 nm Au-Ag NPs. Typically, for every Ag characteristic X-ray detected, one Ag atom is lost to knock-on damage. We discuss the detection limit of the method as a function of NP size and composition.</p> <p>We follow the structural evolution of a ",20 nm Au(core) Pt(shell) NP during annealing at various temperatures between 300 and 800 °e. At low temperatures, interdiffusion occurs between the core and the shell, while at temperatures abovt: ",600 °e, the configuration evolves towards one composed of Au- and Pt-rich spherical caps, separated by a relatively fiat interface. We could measure a 5-10% shift in the composition of each phase with respect to the bulk phase diagram that we assigned to capillarity effect. The shift agrees qualitatively with the calculated trends. The ratio of the surface to the interface energy is measured directly from a TEM micrograph of a segregated NP and is in close agreement with the calculated ones.</p> <p>This work contributes to the understanding of the phase stability of binary NPs. The prospect of extending these studies to NPs of other bimetallic systems while probing their properties seems promising, especially in view of their catalytic, magnetic and optical potential.</p>