Surface-directed and bulk spinodal decomposition compete to decide the morphology of bimetallic nanoparticles

Abstract

An embedded-domain phase-field formalism is used for studying phase transformation pathways in bimetallic nanoparticles (BNPs). Competition of bulk and surface-directed spinodal decomposition processes and their interplay with capillarity are identified as the main determinants of BNP morphology. The former is characterized by an effective bulk driving force ∆ f which increases with decreasing temperature, while the latter manifests itself through a balance of interfacial energies captured by the contact angle θ . The simulated morphologies, namely, core-shell, Janus and inverse core-shell, cluster into distinct regions of the ∆ f -θ space. Variation of θ with ∆ f in the Ag-Cu alloy system is computed as a function of temperature using a CALPHAD approach in which surface energies are estimated from a modified Butler equation. This θ − ∆ f trajectory for Ag-Cu, when superimposed on the morphology map, enables the prediction of different morphological transitions as a function of temperature. Therefore, the study establishes a unique thermodynamic framework coupled with phase-field simulations for predicting and tailoring nanoparticle morphology through a variation of processing temperature.