Investigating Nano-structured Catalysts at the Atomic scale by Field Ion Microscopy and Atom Probe Tomography - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 287

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Figure 10.21 (a) Reconstruction of well-defined and well-separated Pt-25.3at%Co

nanoparticles. (b) Analysis of the size-dependence variation in Co-
content.
(Adapted with permission from ref. 14. Copyright (2014) American
Chemical Society).

theoretical spherical diameter on the upper x axis). Within the average
composition of 93.7 5.8 at% Pt and 6.3 5.8 at% Co, a clear and marked
size-dependent variation in composition can be observed. The variations
of Co content are also confirmed by STEM/EDX experiments. These results
suggest that the impregnation method could be improved to get a nar-
rower compositional distribution.
Pt-Co alloys can be magnetic and this feature is dependent on the com-
position (at ambient temperatures, the Curie transition point lies at about 12
at% Co). The electrophoresis method used in this case may thus promote the
deposition of particles with lower Co concentrations. Furthermore, the field
present in the sample preparation may induce the agglomeration in solution
of particles with higher Co concentration. Nevertheless, particles deposited
and analyzed by APT present a broad size and compositional distribution
and the APT analysis remains of interest, although not fully representative of
the total composition of the material.

.

10.3.3.3.3 Analysis of the Intermixing in Ir@Pt Nanoparticles. The syn-
thesis of Ir@Pt core-shell structures is based on an industrial method in
which platinum is deposited selectively over 4 nm Ir nanoparticles. 125 The
characterization of the thickness of the shell and the composition of this
alloy remains challenging with high-resolution TEM due to the similar
atomic weight of the two elements. Within the scope of this study, only
isolated Ir@Pt particles were deposited using the electrophoresis method.
Figure 10.22a shows a typical 3D atom map for this system. Different types
of structure may be found: some clearly shows signs of a core-shell form,
with Pt enrichment in the outer layers, but others consist mainly of Pt