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Fig. 9 STEM-HAADF images of AuPt nanorods. (a) Bare Au NR seeds and (b)to(e) Pt-coated
Au NRs with Pt:Au ratios, respectively, of 0.05, 0.25, 0.50 and 0.75. The 3D intensity profiles from
an as-grown Au NR (
arrowed
) and a Pt-coated NR (
arrowed
) are inset in (a) and (b), respectively.
Reproduced from [
83
]. Copyright 2012 American Institute of Physics
6.3 Synthetic Manipulation of Pt on Au Nanorod Growth
As shown in Table
1
, the lattice constant of Pt is very similar to that of Pd and its
average surface energy lies between those of Rh and Pd. These factors determine
similar surface segregation properties at equilibrium as in the AuPd and AuRh
systems, namely, that surfaces are expected to be enriched in Au. Pt, like Rh, has a
much higher cohesive energy than Au, whereas Pd has a similar cohesive energy.
With respect to its miscibility properties in the bulk, the AuPt system is interesting
because although Au and Pt are immiscible for most compositions, they form solid
solutions in the very Au-rich part of the phase diagram [
77
]. As a result of this, AuPt
can be said to bridge the miscible AuPd system and the more immiscible AuRh
system.
Our own study and the work of others conducted on AuPt nanorods indicates a
tendency for clustered growth of Pt, with a marked preference for Pt growth on the
nanorod end facets [
83
,
91
]; examples from our work are shown in Fig.
9
. Growth of
Pt (or indeed Rh) on the outside of an Au NR represents a kinetically controlled
product rather than the thermodynamically most stable product. Our study has
shown that marked changes in the morphology of Pt-coated Au NRs were observed
over the course of 18 months when stored under ambient conditions; see Fig.
10
[
83
]. It seems that, unlike the AuRh and AuPd systems discussed in Sect.
6.1
above,
the Au-Pt interface is not stable. Similar morphology changes were observed for
AuPt NRs with different amounts of Pt coating. This process can be accelerated by
annealing AuPt nanorods at 200
C[
83
]. It is possible that the mechanism of the
