Angewandte
Metal Nanocrystals
Chemie
tion/oxidation conditions during the polyol synthesis and
2) use of a capping agent much smaller than PVP. In the first
approach, Pd nanocubes with edge lengths up to 50 nm were
prepared by adding FeIII species to a polyol synthesis at 908C.
In this case, the addition of such species was found to enhance
the oxidation of Pd0, thus effectively reducing the number of
seeds formed during the nucleation stage.[78b] At the same
concentration of precursor, the presence of fewer seeds
means a larger size for the final product. Using this approach,
the final size of the Pd nanocrystals could be controllably
increased simply by increasing the concentration of the FeIII
species. Due to the capping effect of PVP, these larger single-
crystalline nanocrystals adopted a cubic shape, with some
slight corner truncation (Figure 13b).
In addition to single-crystal seeds, the Pd system is rich in
twinned structures.[104] As discussed in the beginning of this
section, the retention of twinned seeds produced during
polyol reduction is difficult to achieve due to the intrinsically
corrosive environment. Yet, in a recent study, we found that
citric acid or citrate ions could protect such seeds from
oxidative etching.[66,80] There are two possible mechanisms for
explaining this observation: 1) they can compete with oxygen
adsorption and thus reduce the amount of oxygen immobi-
lized on the Pd surfaces or 2) they can react with and exhaust
the adsorbed oxygen. Regardless of the mechanism, it
becomes possible to stabilize twinned Pd nanocrystals
enclosed by {111} facets—decahedrons and icosahedrons—
and even single-crystal octahedrons, presumably due to the
strong binding of citrate to the {111} facets.
Considering the second approach, Brꢀ was added as a
smaller capping agent and found to promote the development
of {100} facets, allowing for the generation of Pd nanocubes
8 nm in edge length (Figure 13c).[64d] This synthesis was
carried out in water with PVP as a reductant, and both XPS
and EDX analyses suggest that Brꢀ is capable of chemisorb-
ing onto the {100} facets of very tiny Pd nanocrystals. In this
way, the selective chemisorption of Brꢀ alters the order of
surface energies for different facets, allowing for nanocubes,
not Wulff polyhedrons, to be produced at small sizes.
The key to producing one of these nanocrystal shapes to
the exclusion of the others is to control the population of the
various seed types in a synthesis. Amazingly, using a simple
water-based system in which only Na2PdCl4, PVP, and citric
acid were added, we were able to achieve these various seeds
and tune their yields simply by controlling the concentration
and reduction rate of the precursor.[66,80] It has been shown by
simulation that icosahedral, decahedral, and Wulff polyhedral
clusters are favored for Pd at small (N < 100), medium (100 <
N < 6500), and large sizes (N > 6500), respectively.[54] Thus, to
generate Wulff polyhedral seeds, and eventually octahedrons,
a high atomic concentration is required because the number
of Pd atoms in a seed is highly dependent on the atomic
concentration in the solution. Two factors affect the atomic
concentration: 1) the reduction rate and 2) the concentration
of precursor. As citric acid is a mild reducing agent, the Pd
precursor concentration has to be kept relatively high in this
synthesis. When this condition is met, Pd octahedrons are
produced in over 90% yields (Figure 13 f). The same protocol
can be used to generate Pd decahedrons in high yields. In this
case, the concentration of citric acid or citrate ions has to be
increased to ensure sufficient capping of the {111} facets, thus
reducing the surface energy of the {111} facets and compen-
sating for the extra strain energy caused by twinning. For
these reasons, decahedral seeds can be stabilized effectively
and grow into large decahedrons (Figure 14a) even though
the number of atoms might be above the favorable medium
range for decahedral seed generation. Finally, when the
atomic concentration is significantly lowered by reducing the
Pd precursor concentration, icosahedral seeds become favor-
able due to the slow addition of Pd atoms. As a result, Pd
icosahedrons become the major product (Figure 14b).[66,80]
Besides citric acid, ascorbic acid may be used as a reducing
agent. When it was used at 808C in the presence of Brꢀ,
anisotropic growth of decahedral seeds was induced, resulting
in pentagonal nanorods with their side {100} faces presumably
stabilized by Brꢀ.[68] Although there is modest oxidative
etching in this system, these five-fold twinned nanorods can
be stable for a long period of time. In the same synthesis, right
bipyramids were also formed from singly twinned seeds, with
their {100} facets being stabilized with Brꢀ. In general, the
product typically consisted of 30% five-fold twinned nano-
rods with length up to 150 nm, 17% MTPs, 18% singly
twinned right bipyramids, and 35% nanocubes (Figure 14c).
During the course of this work, it was also found that Brꢀ
could initiate anisotropic growth for cubic nanocrystals if the
reduction rate was enhanced by introducing ethylene glycol as
a reductant.[64d] In one example, Pd nanobars enclosed by
{100} facets and with a square cross-section were synthesized
from a reaction containing ethylene glycol, water, PVP, and
KBr (see Figures 13d and e). The aspect ratio of the nanobars
was observed to increase with an increase in the concentration
of ethylene glycol. In a second example, Pd nanoneedles with
a rectangular cross-section of decreasing area were prepared
in water with ascorbic acid and sodium citrate as co-
reductants and CTAB as a capping agent.[103] How and why
such anisotropic growth occurs are yet to be fully understood;
however, localized oxidative etching in the presence of Brꢀ
appears to be involved. As the chemisorbed Brꢀ layer
prevents the further addition of Pd atoms to the Pd nano-
crystal, the surface has to be activated in some way to
continue the growth process. For a cubic nanocrystal,
localized oxidative etching could selectively activate only
one of its six faces for atomic addition. Then, if enough Pd
atoms can be supplied to the etched site by accelerated
reduction, atomic addition will surpass the removal of atoms
caused by etching at that face, breaking cubic symmetry and
eventually leading to the formation of a nanobar. In this case,
a relatively high reduction rate, achieved by having a high
concentration of ethylene glycol in water, is needed to
provide sufficient Pd atoms for continuous growth. For a
Wulff polyhedral nanocrystal, selective activation of an
octagonal face by localized oxidative etching leads to a
nanorod whose side surfaces are a mix of both {100} and {110}
facets. As for the formation of Pd nanoneedles, the exact
mechanism still needs to be resolved. We believe that the Brꢀ
from CTAB may play a critical role, in addition to the capping
effect of the surfactant.
Angew. Chem. Int. Ed. 2009, 48, 60 – 103
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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