Experimental High-Resolution Observation of the Truncated Double-Icosahedron Structure: A Stable Twinned Shell in Alloyed Au–Ag Core@Shell Nanoparticles

Abstract

Given the binary nature of nanoalloy systems, their properties are dependent on their size, shape, structure, composition, and chemical ordering. When energy and entropic factors for shapes and structure variations are considered in nanoparticle growth, the spectra of shapes become so vast that even metastable arrangements have been reported under ambient conditions. Experimental and theoretical variations of multiply twinned particles have been observed, from the Ino and Marks decahedra to polyicosahedra and polydecahedra with comparable energetic stability among them. Herein, we report the experimental production of a stable doubly truncated double-icosahedron structure (TdIh) in Au−Ag nanoparticles, in which a twinned Ag-rich alloyed shell is reconstructed on a Au−Ag alloyed Inodecahedral core. The structure, chemical composition, and growth pathway are proposed on the basis of high-angle annular darkfield scanning transmission electron microscopy analysis and excess energy calculations, while its structural stability is estimated by large-scale atomic molecular dynamics simulations. This novel nanostructure differs from other structures previously reported.

Ag@Au and Au@Ag Core–Shell Nanoparticles: Insights on the Growth Process and Interactions with Small Molecules

“…Collectively, the observed edge contractions, expansion of central plane atoms, and increase in Δc5∠ strongly suggest a tendency of NPs to assume a more rounded configuration as additional layers are added. These findings align with observations from TEM images reported by Shankar et al and aberration-corrected HAADF by Cruz and co-workers …”

“…Our findings showed that it is generally easier to add gold layers onto silver cores than the other way around, which was consistent across different NP sizes, aligning with previous experimental data and simulations for AuAg alloy NPs by Andolina et al, Cruz and co-workers, and Gromoff et al, that overall suggested that gold surface segregation can be observed in vacuum while silver surface segregation is usually found in more reactive conditions.…”

“…For instance, in silver crystals, atoms are arranged in a FCC structure, and this is anticipated for silver-based NPs. However, small clusters are not constrained by crystalline periodicity, allowing for the observation of irregular solids such as icosahedral structures or asymmetric systems. ,, Additionally, they may form multiple twinned particles resembling decahedra or icosahedra, alongside various habit variations . Since our theoretical framework is based on DFT, a systematic study considering different reconstructions and possible segregation patterns is beyond our scope.…”

“…Since our theoretical framework is based on DFT, a systematic study considering different reconstructions and possible segregation patterns is beyond our scope. These kinds of study are more suitable for empirical or mixed simulation approaches, such as the ones performed by Andolina et al, Cruz and co-workers and Gromoff et al and Gould and co-workers . The diverse range of shapes and sizes possible for metallic NPs can render simulation attempts based on DFT impractical.…”

“…Besides its importance, most of the studies so far address AuAg alloy NPs properties and their growth process − in contrast to AuAg core–shell NPs. While experimental works rely on different techniques, such as aberration-corrected scanning transmission electron microscopy, Aberration-corrected high-angle annular dark-field (HAADF), high-resolution TEM, atomically resolved high-angle annular-dark-field scanning TEM, and energy-dispersive X-ray spectroscopy experiments, simulations are based on the doubly nudged elastic band algorithm coupled to empirical potentials deep neural-network potential from small Au and Ag clusters using density functional theory (DFT), and molecular dynamics and molecular-dynamics and Monte Carlo simulations . In summary, these works agree that a multitude of geometric arrangements coexist within energy landscapes, potentially sharing similar energies and separated by transition barriers that can be overcome at room temperature …”