Thermally Induced Diffusion and Restructuring of Iron Triade (Fe, Co, Ni) Nanoparticles Passivated by Several Layers of Gold

9 pags., 5 figs., 3 tabs.The temperature-induced structural changes of Fe−, Co−, and Ni−Au core−shell nanoparticles with diameters around 5 nm are studied via atomically resolved transmission electron microscopy. We observe structural transitions from local toward global energy minima induced by elevated temperatures. The experimental observations are accompanied by a computational modeling of all core−shell particles with either centralized or decentralized core positions. The embedded atom model is employed and further supported by density functional theory calculations. We provide a detailed comparison of vacancy formation energies obtained for all materials involved in order to explain the variations in the restructuring processes which we observe in temperature-programmed TEM studies of the particles.This research has been supported by the Austrian Science Fund (FWF) under Grant No. P 29893-N36, the FWF and the Christian Doppler Research Association (CDG) under Grant No. PIR8-N34, the Horizon 2020 research program of the European Union under Grant No. 823717-ESTEEM3, and the Spanish Agencia Estatal de Investigacion (AEI) and the Fondo ́ Europeo de Desarrollo Regional (FEDER, UE) under Grant No. MAT2016-75354-P. The authors acknowledge the use of HPC resources provided by the ZID of Graz University of Technology and by the Vienna Scientific Cluster (VSC). Further support by NAWI Graz is gratefully acknowledged. The CESGA supercomputer center (Spain) is also acknowledged for having provided computational resources.Peer reviewe

A coarse-grained Monte Carlo approach to diffusion processes in metallic nanoparticles

A kinetic Monte Carlo approach on a coarse-grained lattice is developed for the simulation of surface diffusion processes of Ni, Pd and Au structures with diameters in the range of a few nanometers. Intensity information obtained via standard two-dimensional transmission electron microscopy imaging techniques is used to create three-dimensional structure models as input for a cellular automaton. A series of update rules based on reaction kinetics is defined to allow for a stepwise evolution in time with the aim to simulate surface diffusion phenomena such as Rayleigh breakup and surface wetting. The material flow, in our case represented by the hopping of discrete portions of metal on a given grid, is driven by the attempt to minimize the surface energy, which can be achieved by maximizing the number of filled neighbor cells

Effects of the Core Location on the Structural Stability of Ni-Au Core-Shell Nanoparticles

7 pags., 6 figs.-- ACS Liveslides presentation: https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.9b05765Structural changes of Ni-Au core shell nanoparticles with increasing temperature are studied at atomic resolution. The bimetallic clusters, synthesized in superfluid helium droplets, show a centralized Ni core, which is an intrinsic feature of the growth process inside helium. After deposition on SiNx, the nanoparticles undergo a programmed temperature treatment in vacuum combined with an in situ transmission electron microscopy study of structural changes. We observe not only full alloying far below the actual melting temperature, but also a significantly higher stability of core-shell structures with decentralized Ni cores. Explanations are provided by large-scale molecular dynamics simulations on model structures consisting of up to 3000 metal atoms. Two entirely different diffusion processes can be identified for both types of core-shell structures, strikingly illustrating how localized, atomic features can still dictate the overall behavior of a nanometer-sized particle.This research has been supported by the Austrian Science Fund (FWF) under grant P 29893-N36 and FWF PIR8-N34, the Spanish Agencia Estatal de Investigacion (AEI) and the Fondo Europeo de Desarrollo ́ Regional (FEDER, UE) under grant no. MAT2016-75354-P, and by the COST Action CM1405 “Molecules in Motion” (MOLIM). The authors would like to acknowledge the use of HPC resources provided by the ZID of Graz University of Technology and by the Vienna Scientific Cluster (VSC). The CESGA super-computer center (Spain) is also acknowledged for having provided computational resources. Further support by NAWI Graz is gratefully acknowledged

Stability of Core–Shell Nanoparticles for Catalysis at Elevated Temperatures: Structural Inversion in the Ni–Au System Observed at Atomic Resolution

We present in situ transmission electron microscopy (TEM) studies of nanoscale Ni–Au core–shell particles on heatable TEM grids. The bimetallic clusters, grown fully inert within superfluid helium nanodroplets to avoid any template or solvent effects, are deposited on amorphous carbon and monitored through a heating cycle from room temperature to 400 °C and subsequent cooling. Diffusion processes, known to impair the catalytic activities of core–shell structures, are studied as a function of the temperature and quantified through fits of a temperature-dependent diffusion constant directly derived from the experiment. After cooling, spatially resolved energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy measurements prove the inversion of the core–shell structure from Ni–Au to Au–Ni. Furthermore, a strong oxidation of the now exposed Ni shell is observed in the latter case. In combination with theoretical studies employing density functional theory, we analyze the influence of oxygen on the observed intermetallic diffusion