Chemistry Reference
In-Depth Information
Fig. 6 Atomically resolved STEM-HAADF and BF images of the Au-Rh interface. Simulta-
neously acquired (a) STEM-HAADF and (b) STEM-BF images of an Au
core
Rh
shell
nanorod, with
(c) line intensity profiles taken as indicated in (a) over 4 atomic columns (70 pixels) and a single
atomic column (16 pixels). Reproduced from [
49
]
the end of an AuRh rod. Although atomic columns are clearly resolved, there is no
clear Z-contrast intensity variation as expected for Au (
Z
46).
Instead, random intensity variation from column to column is seen, as indicated by
arrows in Fig.
6a
, or more clearly by the intensity profiles shown in Fig.
6c
.This
result can be interpreted as a sign of intermixing or alloying at the interface. Further
evidence can be seen below from the comparative study of Pd on Au nanorods. In
addition, the atomic column spacing from the centre to the edge of the nanorod remains
consistent with the Au lattice spacing, despite the 7% lattice mismatch between Rh and
Au (see Table
1
). Furthermore, there is no phase contrast feature in the AuRh BF image,
suggesting that any mismatch strain may have been relieved by intermixing at the Au-
Rh interface. The sequential method followed in synthesising these nanorods means
that any mixing should have happened during or immediately after Rh deposition.
In contrast to AuRh, Z-contrast intensity variation is seen for AuPd nanorods, as
shown earlier in Fig.
4
. Here, comparable atomically resolved DF and BF images
taken from a corner of an AuPd nanorod are displayed in Fig.
7a, b
, where an abrupt
contrast change is apparent that is consistent with the presence of a strain contrast
feature in the BF image. This suggests that Au and Pd are well segregated, although
Au and Pd are readily miscible in the bulk [
37
]. Core-shell segregation has been
observed in other studies of AuPd nanoparticles synthesised using similar
seed-mediated sequential methods [
35
,
38
,
81
].
Given that Rh and Pd have comparable atomic numbers, 46 vs 47, the clear
difference in Z-contrast imaging obtained for these two systems highlights the
difference in metal-metal interaction at the interfaces. The comparative approach
adopted in this study of two systems with similar relative differences in key
structural and elemental parameters (Table
1
) highlights the important interplay
between energetics, kinetics and thermodynamics in bimetallic nanostructure for-
mation and shows the considerable potential that exists for structural manipulation
through controlling kinetic parameters during synthesis. However, the driving force
responsible for the interfacial structure of any given system cannot easily be
determined by simply considering these parameters in isolation, so we have used
¼
79) and Rh (
Z
¼
