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.
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.
Alloying processes in nanometre-size Ag@Au and Au@Ag core@shell particles are studied via high resolution Transmission Electron Microscopy (TEM) imaging.
Structural 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 SiN
x
, 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.
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