Preparation, self-assembly, and mechanistic study of highly monodispersed nanocubes

Article abstract of DOI:10.1021/ja067636w

In this paper, we describe the synthesis and growth mechanism of highly monodispersed platinum nanocubes. The platinum nanocubes are synthesized by the decomposition of a platinum precursor in a hydrogen atmosphere. The morphology and size distribution of the platinum particles formed has been studied with HRTEM. By controlling the concentration of the platinum precursor, we demonstrate that at low concentration, it is possible to grow polydispersed nanocubes with {1,0,0} facets. Increasing the concentration of the precursor changes the growth mechanism, resulting in the formation of highly monodispersed platinum nanocubes. Highly monodispersed platinum nanocubes are formed in a two-step growth mechanism with initial growth of the {1,1,1} facets followed by secondary growth filling the {1,0,0} facets. The particle monodispersity facilitates the formation of long-range arrays of nanocubes.

Full text of DOI:10.1021/ja067636w

Published on Web 02/21/2007
Preparation, Self-Assembly, and Mechanistic Study of Highly
Monodispersed Nanocubes
Jintian Ren and Richard D. Tilley*
Contribution from the School of Chemical and Physical Sciences and the MacDiarmid Institute
of AdVanced Materials and Nanotechnology, Victoria UniVersity of Wellington, P.O. Box 600,
Wellington, New Zealand
Received October 25, 2006; E-mail: richard.tilley@vuw.ac.nz
Abstract: In this paper, we describe the synthesis and growth mechanism of highly monodispersed platinum
nanocubes. The platinum nanocubes are synthesized by the decomposition of a platinum precursor in a
hydrogen atmosphere. The morphology and size distribution of the platinum particles formed has been
studied with HRTEM. By controlling the concentration of the platinum precursor, we demonstrate that at
low concentration, it is possible to grow polydispersed nanocubes with {1,0,0} facets. Increasing the
concentration of the precursor changes the growth mechanism, resulting in the formation of highly
monodispersed platinum nanocubes. Highly monodispersed platinum nanocubes are formed in a two-step
growth mechanism with initial growth of the {1,1,1} facets followed by secondary growth filling the {1,0,0}
facets. The particle monodispersity facilitates the formation of long-range arrays of nanocubes.
direct methanol fuel cell.1
5-18
The performance of platinum
Introduction
nanoparticles in catalytic processes has been found to be highly
dependent on which facets terminate the surface of the
It is well-known that the electrical, optical, and magnetic
properties of metal and bimetallic nanocrystals vary widely with
size and shape. For many applications including catalysis,
electronics, data storage, and biological sensors, large amounts
14,19
particles.
For example, faceted platinum nanoparticles have
been shown to exhibit a higher catalytic activity as compared
1
9
to spherical particles.
1
and well-defined nanocrystals must be produced. While the
There have been several studies to make shaped particles.
However, the formation of monodispersed faceted platinum
nanocubes has yet occur. The general protocol for platinum
nanoparticle synthesis involves the reduction of a platinum
precursor by reducing agents in the presence of a surfactant.1
Faceted nanoparticles of platinum have been formed by reduc-
size control of spherical nanocrystals has been achieved for
many systems, controlling the nanocrystal shape is still a real
synthetic challenge. The fabrication of nanocubes has been
studied for various types of metal and bimetallic nanocrystals
that have structures based on face centered cubic (fcc) crystal
9-23
2-4
5-7
8
9
packing, including platinum,
silver,
palladium, copper,
2
24
tion using hydrogen, sodium borohydride, and polyol
reduction.
gold,10 and FePt.
11-12
2
5-26
In all of these studies, the resultant nonuniform
Platinum nanoparticles have been extensively investigated for
their unique catalytic properties.13-14 Platinum is a useful
industrial catalyst for reducing pollutant gases from the exhausts
of automobiles, producing hydrogen from methane, and in the
sized and irregularly faceted particles could not be self-
24,29
assembled into ordered arrays.
Previous studies discussing the formation of faceted platinum
nanocubes include the seminal discussions by El-Sayed and co-
(
1) Heath, J. R. Acc. Chem. Res. 1999, 32, 388-388.
(15) Bell, A. T. Science 2003, 299, 1688-1691.
(
2) Ahmadi, T.; Wang, Z. L.; Green, T.; Henglein, A.; El-Sayed, M. A. Science
(16) Schulz, J.; Roucoux, A.; Patin, H. Chem. ReV. 2002, 102, 3757-3778.
(17) Pino, L.; Recupero, V.; Beninati, S.; Shukla, A. K.; Hegde, M. S.; Bera, P.
Appl. Catal., A 2002, 225, 63-75.
1
996, 272, 1924-1296.
(3) Yu, R.; Song, H.; Zhang, X.; Yang, P. J. Phys. Chem. B 2005, 109, 6940-
6
943.
(18) Williams, K. R.; Burstein, G. T. Catal. Today 1997, 38, 401-410.
(19) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343-1348.
(20) Duff, D.; Edwards, P.; Johnson, B. J. Phys. Chem. 1995, 99, 15934-15944.
(21) Zhong, X. H.; Feng, Y. Y.; Lieberwirth, I.; Knoll, W. Chem. Mater. 2006,
18, 2468-2471.
(22) Teng, X. W.; Liang, X. Y.; Maksimuk, S.; Yang, H. Small 2006, 2 (2),
249-253.
(23) Miyazaki, A.; Nakano, Y. Langmuir 2000, 16, 7109-7111.
(24) Zhao, S.; Chen, S.; Wang, S.; Lie, D.; Ma, H. Langmuir 2002, 18, 3315-
3318.
(25) Herricks, T.; Chen, J.; Xia, Y. N. Nano Lett. 2004, 4, 2367-2371.
(26) Chen, J.; Herricks, T.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2589-
2592.
(27) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem.
B 1998, 102, 3316-3320.
(28) Teng, X.; Yang, H. Nano Lett. 2005, 5, 885-891.
(29) Yang, J.; Deivaraj, T. C.; Too, H.; Lee, J. Y. J. Phys. Chem. B 2004, 108,
2181-2185.
(
4) Yamada, M.; Kon, S.; Miyake, M. Chem. Lett. 2004, 34, 1050-1051.
(
5) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2005,
4
4, 2154 -2157.
(
6) Yu, D.; Yam, V. W. J. Am. Chem. Soc. 2004, 126, 13200-13201.
(
7) Wiley, B.; Sun, Y. G.; Mayers, B.; Xia, Y. N. Chem.sEur. J. 2005, 11,
4
54 - 463.
(
8) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z. Y. Nano Lett.
005, 5, 1237-1242.
2
(
9) Zhou, G. J.; Lu, M. K.; Yang, Z. S. Langmuir 2006, 22, 5900-5903.
(
(
10) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176-2179.
11) Chen, M.; Kim, J.; Liu, J. P.; Fan, H. Y.; Sun, S. H. J. Am. Chem. Soc.
2
006, 128, 7132-7133.
(
(
12) Shukla, N.; Liu, C.; Roy, A. G. Mater. Lett. 2006, 60, 995-998.
13) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2004, 126, 7194-
7
195.
(
14) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2003, 107, 12416-
1
2424.
10.1021/ja067636w CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 3287-3291
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3287

A R T I C L E S
Ren and Tilley
workers,27 who stressed the importance of the initial shape of
the platinum nuclei. More recently, Xia and co-workers and
Teng and Yang discussed the importance of control of the
growth kinetics in platinum particle morphology.2
5-26,28
To maximize the packing density of nanocrystals, it is
desirable that they have a uniform size distribution to enable
the particles to assemble into long-range ordered arrays.
Typically, a size distribution of 5% is required for the formation
of ordered nanocrystal arrays.30 The 2-D self-assembly or 3-D
structured superlattices of these nanocubes open up the pos-
1
1-12
sibilities of fabricating novel nanodevices and templating.
In this paper, we describe the effect of the platinum precursor
concentration on the synthesis of nanocubes. Under different
concentrations, different growth mechanisms occur to produce
monodispersed and polydispersed platinum particles of varying
sizes. Monodispersed platinum nanocubes have been self-
assembled into ordered arrays. The understanding of the growth
mechanism involved is significant in the development of the
strategies for the formation of faceted particles of other cubic
crystal structured metal or bimetallic systems.
Materials and Methods
Figure 1. Polydispersed platinum nanocubes formed with low precursor
concentrations. (a and b) Low-resolution TEM image of an ensemble of
platinum nanocubes with sizes ranging from 10 to 100 nm. (c) High-
resolution TEM image of a faceted platinum nanocube. Inset is a fast Fourier
transform (FFT) of the particle image. (d and e) Histograms of particle
morphology and particle size distribution, respectively.
In a typical synthesis, 0.1 mmol of platinum precursor, platinum
acetylacetonate (Pt(acac) 97%, Aldrich), was dissolved in toluene, 2
2
or 20 mL, to give 0.05 and 0.005 M precursor concentrations,
respectively. To this solution, 10 equiv of oleylamine (OLA) was added
as the surfactant.
The platinum precursor was then decomposed under a hydrogen
pressure of 3 bar at 70 °C in a pressure reaction vessel (Fischer-Porter
bottle) for 20 h. Samples were purified by the addition of methanol to
flocculate and precipitate the nanocrystals, which were collected by
centrifuging. The samples for TEM studies were prepared by resus-
pending the precipitate in toluene. One drop of the toluene suspension
was put onto a TEM grid and allowed to evaporate slowly under
ambient conditions. The TEM images and diffraction patterns were
taken on a JEOL 2010 operating at an acceleration voltage of 200 keV.
in controlling particle shape and morphology. After optimizing
these conditions, varying the precursor concentration was found
to have the strongest influence on the growth mechanism.
Effect of Low Concentration. To investigate the effect of
precursor concentration, two preparations with 0.05 and 0.005
M precursor concentration were reacted with 10 equiv of the
surfactant oleylamine for 20 h. Typical products formed at
different precursor concentrations are shown in the TEM images
in Figures 1 and 2. These images indicate a change in
morphology as the precursor concentration increases.
Results
At the low concentrations, Figure 1a-c, the majority of the
particles adopts a cubic shape. From high-resolution TEM
images (i.e., the particle shown in Figure 1c), it may be observed
that the particles are perfectly cubic with sharply faceted edges.
This cubic shape has been previously observed for several fcc
In these experiments, a platinum precursor was hydrogenated
under mild hydrogen pressure in a Fischer-Porter bottle (pres-
sure reaction vessel). The method of hydrogenated decomposi-
tion of organometallic precursors to form nanoparticles has been
31-32
successfully used to form metallic and alloy nanoparticles.
2
-10
metals made with solution synthesis.
Particle morphology,
The advantage of this technique over other reduction routes to
nanocrystals is that the formation of nanoparticles generally
occurs at a slow rate, facilitating the formation of anisotropic
and faceted particles.31-32 Previous research has shown that the
most critical variable for the formation of non-spherical mor-
phologies is the choice of precursor.3 Coupled with the choice
of surfactant molecules, reaction times, and concentrations
allows the control of particle growth and thus particle size and
shape.
mean size, and size distribution were determined by measuring
over 500 nanoparticles from different regions of the grid. This
statistical analysis is summarized in the histograms in Figure
1
d,e. The nanoparticle morphology can be separated into three
types, faceted nanocubes, filled octapod nanocubes (described
next), and others (including rods, tetrapods, and irregular
shapes). From the results, it can be seen that the particle
morphology is dominated by faceted cubes that make up ∼80%
of the particles. The cubic nanocrystals are polydispersed and
vary in size from 10 to 100 nm. Such polydispersity is
commonly found in the literature and is in agreement with other
1
The decomposition reactions of platinum acetylacetonate Pt-
(
(
acac)2 were carried out in toluene under a pressure of 3 bar H2
initial pressure at room temperature in the reactor) in the
2-4
5-10
reports on platinum
and other fcc metals.
presence of surfactant molecules. The reaction conditions of
concentration, temperature, and time were found to be important
Effect of High Concentration. The results of increasing the
precursor concentration to 0.05 M are shown in Figure 2. Lower
resolution images (i.e., Figure 2a,b) indicate a pseudo-cubic
morphology for the platinum nanocubes. Most significantly, an
increase in the precursor concentration also altered the size
uniformity and average diameter of the resultant Pt nanopar-
(
30) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325-4330.
(
31) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004,
3
03, 821-823.
(32) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M.; Casanove, M.;
Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286-4289.
3288 J. AM. CHEM. SOC.
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VOL. 129, NO. 11, 2007

Mechanistic Study of Highly Monodispersed Nanocubes
A R T I C L E S
Figure 2. Monodispersed platinum nanocubes formed with high precursor
concentrations. (a and b) Low-resolution TEM image of self-assembled
highly monodispersed filled octopod nanocubes. Inset is a SAED pattern.
(c) High-resolution TEM image of a faceted platinum nanocube. Inset is a
FFT of the particle image. (d and e) Histograms of particle morphology
and particle size distribution, respectively.
ticles. An indication of the monodispersity of the nanocubes is
their formation into long-range arrays across the TEM grid
(Figure 2b). The selected area electron diffraction (SAED)
pattern (Figure 2b, inset) shows four bright inner (1,1,1)
crescents linked by a 4-fold symmetry. This indicated that the
cubes all have the same crystallographic orientation.
Careful examination of higher resolution images, such as the
one shown in Figure 2c, elucidates that the particles are not
perfectly cubic and do not have sharply faceted edges, unlike
the particles formed at low concentrations. We describe these
types of particles as filled octapod nanocubes (the term filled
octapod originates from the growth mechanism of the particles
discussed next). The spread of particle morphology is sum-
marized in the histogram in Figure 2d. As can be seen from
this histogram, the particle morphology is highly uniform with
Figure 3. Growth mechanism on {1,0,0} and {1,1,1} facets. (a and b)
Models showing the growth on the {1,0,0} facet of platinum to form faceted
platinum nanocubes. (c-e) Models showing the octapod unit cell, HRTEM
image of a typical faceted platinum nanocube. (d, e, and g) Models showing
truncated octahedral nuclei and growth in the {1,1,1} facets. (f) HRTEM
of truncated octahedral nuclei. (h) HRTEM image of a filled octapod
nanocube.
>
90% of the particles being filled octapod nanocubes. The
monodispersity of the filled octapods nanocubes is further
highlighted in the histogram in Figure 2e. The results indicate
that the nanocubes are highly monodispersed, with a mean size
of 10.0 ( 0.5 nm (equivalent to (5%).
facets and the different growth rates on the {1,1,1} and {1,0,0}
2
5-28,42-47
facets.
Discussion of Growth Mechanism. The different shapes
observed with changing concentrations can be understood by
consideration of the growth mechanism of the platinum particles.
At low concentrations, approximately 80% of the particles was
polydispersed faceted cubes. Conversely, at high concentrations,
it was observed that over 90% of the particles have a filled
octapod nanocube morphology with only 2% faceted nanocubes
present. Figure 3 shows high-resolution TEM images taken from
individual Pt particles, illustrating the subtle control of morphol-
ogy when the precursor concentration is adjusted.
(
33) Kirkland, A. I.; Jefferson, D. A.; Duff, D. G.; Edwards, P. P.; Gameson,
I.; Johnson, B. F. G.; Smith, D. J. Proc. R. Soc. London, Ser. A 1993, 440,
589-597.
(34) Wang, Z. L.; Ahmad, T. S.; E1-Sayed, M. A. Surf. Sci. 1997, 380, 302-
310.
(35) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 7, 617-619.
(
(
(
(
(
36) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105,
065-4070.
37) Puntes, V. F.; Kristhnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115-
117.
38) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich,
A.; Alivisatos, A. P. Nature 2000, 404, 59-61.
39) Park, S. J.; Kim, S.; Lee, S.; Khim, Z. G.; Car, K.; Hyeon, T. J. Am. Chem.
Soc. 2000, 122, 8581-8582.
4
2
40) Filankembo, A.; Giorgio, S.; Lisiecki, I.; Pileni, M. P. J. Phys. Chem. B
The shape control of platinum nanoparticles has been
2003, 107, 7492-7498.
25-29,42
intensively studied by several research groups.
It is
(41) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am.
Chem. Soc. 2003, 125, 16186-16187.
generally understood that the non-spherical particle shape
originates from a combination of the final stability of different
(
42) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153-1175.
(43) Skriver, H. L.; Rosengaard, N. M. Phys. ReV. B 1992, 46, 7157-7168.
J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3289
9

A R T I C L E S
Ren and Tilley
At low precursor concentrations, low concentrations of
platinum atoms are produced, making the particles grow
relatively slowly from nuclei. The driving force for the particle
shape under these conditions is the diffusion of adatoms to form
faceted surfaces to minimize the final surface energy of the
platinum particles.43-47 In these low concentration reactions,
perfectly cubic faceted particles are formed with sharp right
angular corners and {1,0,0} terminated surfaces, as illustrated
in Figure 3a and as can be observed in the image of a typical
particle shown in Figure 3b. Such faceted cubes have been
similarly observed in other fcc systems.2
-12
From calculations
of small clusters in a vacuum, particles of polyhedral shapes
bound by both {1,0,0} and {1,1,1} faces are expected to have
the lowest energy. However, under these solution conditions, it
is possible that the nuclei form as cubes, and that once formed,
they maintain this shape as they grow due to an energy barrier
in changing into polyhedral shapes, or that faceted cubes bound
by solely {1,0,0} faces are lower in energy as compared to other
polyhedral shapes due to a stronger surfactant to platinum
interaction of the {1,0,0} faces as compared to the {1,1,1} faces.
Such {1,0,0} stabilization has been previously observed by
48
Jefferson and Harris for platinum with strongly binding sulfur.
In higher concentration reactions, a different growth mech-
anism occurs, resulting in the formation of filled octapod
nanocubes. At high precursor concentrations, there are higher
concentrations of reduced atoms present, resulting in the faster
growth of the platinum nanoparticles. Under these conditions,
kinetic control is now dominant. Reactions under kinetic control
have been found to produce particles with highly branched and
pod-like structures.25-29 One of the fundamental basic shapes
for platinum nuclei is the truncated octahedral depicted in Figure
33,42
3c-e.
The HRTEM image of a truncated octahedral shaped
Figure 4. Growth mechanism of filled octapod nanocubes. (a, c, and e)
Models showing the growth of platinum octapods into filled octapod
nanocubes. HRTEM images of (b) a platinum octapod, (d) a partially filled
platinum octapod, and (f) a filled octapod nanocube.
cluster shown in Figure 3f was made at low reaction temper-
atures. Truncated octahedra (also known as cuboctahedra) are
defined when the atoms at the edges of the unit cell are bound
by eight {1,1,1} and six {100} planes. Growth can occur either
at the {1,1,1} facets or at the {1,0,0} facets of the nuclei with
the rate of reduction on the surfaces controlling the final shape.
If the growth rate (G) of the {1,1,1} plane (G{1,1,1}) is
significantly larger than that of the {1,0,0} (G{1,0,0}), then the
nuclei will grow to become an octapod as illustrated in Figure
{
1,0,0} facets has been attributed to the role of the surfactant
3
4-39
binding selectively onto differing energy crystal facets.
However, more recently, several studies for platinum nanoc-
2
5-26
rystals indicate that ions such as Fe(II) or Fe(III)
or silver
2
8
40
acetylacetonate or anions for copper nanocrystals are the
shape determining factor rather than the influence of the
surfactant. In our system, either reason may be credible.
The final filled octapod cubic structures obtained in the high
concentration reactions can be explained by additional growth.
An initial octapod shape, Figure 4a, forms from growth on the
3
c,d,g. Particles with such an octapod core are the dominant
species at high concentrations, as illustrated in Figure 3h. Our
particles are different to Xia and co-workers,25 who formed
octahedra with growth in the [1,0,0] directions. The contrast
in TEM images of our particles also indicates that they are
octapods and not tetrapods, so they are also different from the
{
1,1,1} facets, and an example observed in the TEM is shown
in Figure 4b. After the initial {1,1,1} growth, the amount of
unreacted platinum precursor in the solution was greatly reduced.
The reaction is now in conditions similar to the low concentra-
tion experiment. Thus, additional deposition of platinum atoms
gold tetrapods formed by Chen and co-workers with growth in
the [1,1,0] directions.41
In solution, the precise reason why growth on the {1,1,1}
facet can be much faster than {1,0,0} growth has been attributed
to several factors. The surface of nanocrystals grown in solution
consists of a monolayer composed of a mixture of surfactant,
solvent, metal precursor, and other ions. The precise roles of
these species are uncertain. Previous studies for platinum showed
that the competition between growths on the {1,1,1} facets and
2
+
from the reduction of Pt ions in the solution predominantly
occurs with adatom diffusion and adsorption. It is energetically
most favorable for these additional atoms to fill the areas
between the octapod branches to produce particles with a final
faceted morphology. In our case, final {1,0,0} faceted cubes
form that are lower in energy than the unfilled octapods, as
pictured in Figure 4c,d. This {1,0,0} growth continues until the
platinum precursor is fully reacted and results in the formation
of the filled octapod nanocubes as depicted pictorially in Figure
4e and in the HRTEM image in Figure 4f.
(
44) Needs, R. J.; Mansfield, M. J. Phys.: Condens. Matter 1989, 1, 7555-
7
563.
(
(
(
(
45) Breeman, M.; Barkema, G. T.; Boerma, D. O. Surf. Sci. 1995, 323, 71-80.
46) Cyrot-Lackmann, F. Surf. Sci. 1969, 15, 535-548.
47) Desjonqueres, M. C.; Cyrot-Lackmann, F. Surf. Sci. 1975, 50, 257-261.
48) Jefferson, D. A.; Harris, P. J. F. Nature 1988, 332, 617.
3290 J. AM. CHEM. SOC.
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VOL. 129, NO. 11, 2007

Mechanistic Study of Highly Monodispersed Nanocubes
A R T I C L E S
In terms of growth, the previous mechanism for the formation
of filled octapod nanocubes can be summarized as follows: (1)
initial reduction leads to the formation of truncated octahedral
nuclei; (2) rapid growth processes at initial high concentrations
with G{1,1,1} [dmt] G{1,0,0} form platinum octapods; and (3)
as the platinum precursor is used up, the concentration drops,
and {1,0,0} facets form.
Conclusion
In conclusion, platinum nanocubes have been synthesized by
the decomposition of a platinum precursor in a hydrogen
atmosphere. Under different concentrations, different growth
processes occur to produce monodispersed and polydispersed
platinum of varying sizes. Monodispersed platinum nanocubes
have been self-assembled into ordered arrays with the same
crystallographic orientation. These results and the elucidation
of the growth mechanism will contribute to strategies for the
formation of monodispersed faceted nanocrystals and their
assembly into future devices.
The monodispersity achieved during the formation of the
filled octapod nanocubes as compared to the faceted nanocubes
can be understood by considering the growth kinetics. The
growth at high concentrations occurs rapidly with the formation
of a high concentration of nuclei throughout the reaction
mixture. The growth of these nuclei on the most catalytically
Acknowledgment. R.D.T. thanks the MacDiarmid Institute
for funding.
27
active {1,1,1} facets is very fast, rapidly producing monodis-
persed octapods uniformly throughout the reaction solution.
These then grow into monodispersed filled octapod cubes.
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