The evolution towards equilibrium of AuCo, AgNi and AgCu nanoparticles takes place through a sequence of complex transformations.

Controlled growth of far-from-equilibrium-shaped nanoparticles with size selection is essential for the exploration of their unique physical and chemical properties. Shape control by wet-chemistry preparation methods produces surfactant-covered surfaces with limited understanding due to the complexity of the processes involved. Here, we report the controlled production and transformation of octahedra to tetrahedra of size-selected platinum nanocrystals with clean surfaces in an inert gas environment. Molecular dynamics simulations of the growth reveal the key symmetry-breaking atomic mechanism for this autocatalytic shape transformation, confirming the experimental conditions required. In-situ heating experiments demonstrate the relative stability of both octahedral and tetrahedral Pt nanocrystals at least up to 700 °C and that the extended surface diffusion at higher temperature transforms the nanocrystals into equilibrium shape.

Real time analysis of Co atom incorporation in Ag nanoparticles, followed almost atom-by-atom by combined X-ray scattering and molecular-dynamics methods: from the sub-surface position to quasi-Janus then core–shell structures.

Pt–Pd nanoparticles are grown in the gas phase by a magnetron-sputtering source and characterized by electron microscopy techniques for both Pt-rich and Pd-rich compositions of the metallic vapor. It is shown that this growth procedure can produce different types of core–shell nanoparticles, in one step, with sizes in the range of 4–10 nm, according to the composition of the vapor being rich either in Pt or in Pd. In all cases, the nanoparticles present intermixed cores containing both Pt and Pd and shells made of the majority element, i.e., of (PtPd)@Pt structure for the Pt-rich vapor and (PtPd)@Pd structure for the Pd-rich vapor. Global searches of the optimal chemical ordering show that none of these structures correspond to equilibrium configurations. On the contrary, these core–shell structures are strongly out-of-equilibrium, being the result of kinetic trapping phenomena. This is verified by molecular dynamics growth simulations which are able to reproduce both the different types of chemical ordering and the variety of geometric shapes found in the experiments.