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

Abstract

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. Two dimensionless groups are found to reflect this competition and interaction: (a) the ratio of bulk to capillary driving forces ( Δ f ˜ ), and (b) ratio of difference in surface energies to the interfacial energy which relates to the three-phase contact angle (θ). The simulated morphologies, namely, core–shell, Janus and inverse core–shell, are found to cluster neatly into distinct regions of the Δ f ˜ -θ space. To connect phase-field simulations with specific BNP systems, the variation of θ with Δ f ˜ as a function of temperature is computed for Ag–Cu using a CALPHAD approach. The computed Δ f ˜ -θ trajectory for Ag–Cu, when superimposed on the morphology map derived from simulations, enabled the prediction of different morphological transitions as a function of temperature. Therefore, by providing an alternative and efficient approach to connect phase-field simulations with CALPAHD, the study demonstrates a unique computational framework that can assist in tailoring nanoparticle morphology through a variation of processing parameters.