Chemistry Letters Vol.33, No.10 (2004)
1237
morphology of aggregates of palladium nanoparticles changes
from spherical to wire-like or network with increasing the
added amount of HCl. These experimental results suggest that
the added amount of HCl to methanol solution has a significant
influence on the morphology of aggregates of palladium
nanoparticles.
(b)
(a)
The formation of the wire-like aggregation could be
interpreted through the simulation results of particle growth by
irreversible aggregation of particles.10 Since exposed ends of
clusters of the particles grow more rapidly than the interior,
the resulting cluster has a chainlike structure.
100 nm
100 nm
Figure 2. TEM images of the reaction mixture after (a) 10 min
and (b) 6 h from addition of the G1.0 PAMAM dendrimer.
From the fact that the addition of HCl to the solution result-
ed in the wire-like aggregates, the driving force for assembly
could be electrostatic interaction and hydrogen bonding between
ladium(II) acetate and the resulting mixture was stirred at room
temperature for 10 min, one drop of the solution was placed on a
carbon grid for TEM analysis. Only spherical colloids with low
contrast were observed (Figure 2a). No individual palladium
nanoparticles were identified as black dots. After an aging time
of 6 h, the spherical colloids stuck together and started directed
aggregation growth (Figure 2b). A careful look at the aggregates
enables to distinguish individual palladium nanoparticles as a
black dot, indicating that reduction of the palladium ions pro-
ceeded in the colloids. When the aging time was further pro-
longed to 24 h, the wire-like assemblies consisted of palladium
nanoparticles were observed. These results indicate that the pri-
mary colloids consisting of PAMAM dendrimers and palladium
ions with a diameter of 40 nm were formed at the early stage of
the reaction and aggregated into wire-like assemblies in a
directional manner. The reduction of the palladium ions by
methanol proceeded slowly in the colloids. The shape of the
wire-like assemblies was kept during the reduction process of
palladium ions. Consequently, the palladium nanoparticles were
incorporated into the wire-like colloids composed of PAMAM
dendrimers.
Addition of NaBH4 as a strong reducing agent to a freshly
prepared methanol solution of palladium(II) acetate and
PAMAM dendrimers resulted in the immediate change of color
from yellow to black, which is characteristic for formation of
palladium nanoparticles. The TEM investigation of the obtained
product showed no wire-like aggregates but irregular aggregates
of palladium nanoparticles. It is assumed that the wire-like
colloids were destroyed during the rapid collision of metal
atoms. This result indicates that the weak reducing agent such
as methanol is a prerequisite for the formation of the wire-like
colloids of the palladium nanoparticles.
2À
ammonium groups of the dendrimer and PdCl4 anion formed
by ligand exchange.11,12 Two control experiments were carried
out. Addition of HBr instead of HCl also showed the same result
like using HCl. In contrast, addition of HNO3 showed no wire-
like aggregates. This would be due to the lack of palladium
anions (PdX42À, X = ligand). These results support that the hy-
drogen bonding between halogen anions and the ammonium
groups play an important role in this system.
In conclusion, we have demonstrated the self-organization
of the metal nanoparticles into wire-like aggregates by dendri-
mers. The present method offers unique possibilities for design-
ing new synthetic routes of controlled organized structure based
on the metal nanoparticles. The investigations for fine-tuning of
the product structure by controlling experimental conditions and
different generation of the dendrimers will be reported. We hope
that this concept represents a powerful and general strategy for
the creation of highly structured multifunctional materials.
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Published on the web (Advance View) August 28, 2004; DOI 10.1246/cl.2004.1236