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Fig. 5 EDX maps of Au
(red dots) and Rh (blue dots)
taken from a AuRh nanorod.
The insert is the STEM-
HAADF image of the
nanorod. Reproduced from
[
80
]. Copyright 2012
American Chemical Society
6.1 Characterisation of Interfacial Structures of AuRh
and AuPd Nanorods
The deposition of Rh on Au-seeded nanorods is of interest, partly because of the
contrasting physical and chemical properties of these two elements. While Rh is
catalytically active in the bulk form, Au has only been found to demonstrate
catalytic activity in nanoscale systems. From Table
1
, it can be seen that the large
7% lattice mismatch and the higher bulk cohesive energy and surface energy of Rh
compared with Au suggest a preference for the Rh
core
Au
shell
configuration. This has
indeed been observed for nanoparticles formed on TiO
2
(110) surfaces by physical
vapour deposition of either Rh followed by Au or vice versa [
75
,
76
]. In these
studies, the morphology of the bimetallic nanoparticles was examined using scan-
ning tunnelling microscopy, while the chemical composition was characterised
with low-energy ion scattering. Both techniques are surface sensitive; hence,
information about the metal-metal interaction at the sub-surface region is rather
limited. Despite the complete immiscibility of Au and Rh in the bulk [
77
], chemical
synthesis of both segregated [
78
] and alloyed AuRh NAs [
79
] has also been
reported.
In order to gain mechanistic understanding of metal-metal interactions at the
atomic level, we have applied aberration-corrected STEM, as described in Sect.
4
,
to AuRh nanorods synthesised using a seed-mediated sequential growth method via
a wet chemical route. The main results have been reported in two recent publica-
tions [
51
,
80
]. Figure
5
shows EDX maps of Au and Rh overlaid on the same image.
Despite the limited EDX counts, a clear correlation in the pattern of Au and Rh
signals supports the formation of the Au
core
Rh
shell
structure. As discussed in Sect.
3
,
a system of Rh sequentially deposited onto Au nanorods may not be in thermody-
namic equilibrium. The atomic details at the interface can be revealed by simulta-
neously acquired DF and BF STEM images, shown in Fig.
6a, b
, respectively, from
