Shape-controlled platinum nanocubes and their assembly into two-dimensional and three-dimensional superlattices

Article abstract of DOI:10.1021/jp802081n

Liquid-liquid phase transfer has been used to synthesize platinum nanocrystals with a cubic morphology. By finely tuning the parameters controlling the nucleation and growth processes, nanometric truncated cubes or perfect cubes may be obtained. To our knowledge, this is the first time such shapes are obtained with this procedure. The importance of both the length of the capping agent to control the growth process and the bromide anions as poison for the {111} facet is shown. The low degree of size polydispersity allows these nanocrystals to self-assemble with a long-range ordering in two-dimensional and three-dimensional supraerystals. According to the nanocrystal shape, simple cubic or face-centered cubic supracrystals are observed. It is remarkable to notice that well-faceted supracrystals with sizes on the order of 10 ??m may be obtained. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp802081n

J. Phys. Chem. B 2008, 112, 14583–14592
14583
Shape-Controlled Platinum Nanocubes and Their Assembly into Two-Dimensional and
Three-Dimensional Superlattices†
A. Demortie`re,‡ P. Launois,§ N. Goubet,‡ P.-A. Albouy,§ and C. Petit*,‡
Laboratoire des Mate´riaux Me´soscopiques et Nanome´triques, UMR 7070, UniVersite´ Pierre et Marie
Curie-Paris 6, 4 place Jussieu 75252 Paris Cedex 05, France, and Laboratoire de Physique des Solides, UMR
CNRS 8502, baˆt. 510, UniVersite´ Paris Sud, 91405 Orsay Cedex, France
ReceiVed: March 10, 2008
Liquid-liquid phase transfer has been used to synthesize platinum nanocrystals with a cubic morphology.
By finely tuning the parameters controlling the nucleation and growth processes, nanometric truncated cubes
or perfect cubes may be obtained. To our knowledge, this is the first time such shapes are obtained with this
procedure. The importance of both the length of the capping agent to control the growth process and the
bromide anions as poison for the {111} facet is shown. The low degree of size polydispersity allows these
nanocrystals to self-assemble with a long-range ordering in two-dimensional and three-dimensional
supracrystals. According to the nanocrystal shape, simple cubic or face-centered cubic supracrystals are
observed. It is remarkable to notice that well-faceted supracrystals with sizes on the order of 10 µm may be
obtained.
1. Introduction
explain the shape evolution.24 For instance, adsorption theory
and kinetic models must also be taken into account. Thus, a
The synthesis of nanocrystals with well-controlled shapes is
of great interest to improve their individual and collective
properties. The shape is a major factor in determining the
specific properties in catalysis,1 optics,2 and magnetism.3,4 For
example, a change in the catalytic activity of platinum nano-
crystals has been shown to coincide with a modification of their
shape.5 Indeed, the catalysis effect depends on the facets, edges,
and corners of the nanocrystal.6 The emergence of new methods
to design magnetic nanocrystals such as nanocubes,7–10
nanorods,11–13 and cigar-shaped rods14 has allowed the tuning
of specific individual magnetic properties, such as, for example,
the orientation of the magnetic easy axis and the shape
anisotropy.15 On the other hand, assembling the nanocrystals
into long-range ordered suprastructures requires both a narrow
size distribution and a uniform shape.16 It is further well-known
the lattice point symmetries must be compatible with those of
the elementary unit. It is thus expected that the supracrystal
structure will be dependent upon the shape of the nanocrystals
acting as the elementary building blocks.17 The synthesis of
nanocrystals with original morphologies should thus lead to new
long-range ordered structures and opens up the possibility of
inducing specific collective properties.
new general framework should be established to describe all
parameters governing the growth mechanisms.
In this variety of nanocrystal morphologies, the cubic shape
is of interest because of its peculiar symmetries and the specific
superstructures that may be associated with them.25 In a
pionnering work on nanocubes, El-Sayed et al.26–30 reported the
formation of Pt cubic nanocrystals (11 nm) using the reduction
of a platinum salt (PtCl42-) by H2 in the presence of polyacry-
late. Synthesis of silver nanocubes (100 nm) using a polyol
method has been reported by Xia et al.31,32 Other chemical
methods based on organometallic decomposition have been used
to obtain Fe,12 FePt,33 and FePt3 nanocubes. In past years, a
10
number of synthetic routes have been developed to produce a
large variety of nanocubes, including such compounds as
CeO2,34 PbSe,35 PbS,36 In2O3,37 Co304,38 Fe2O3,9 Fe3O4,39,25 Pd,40
Au,41 and Ag.42 Recently, the synthesis of platinum nanocrystals
with different morphologies presenting the cubic symmetry has
been reported.43 However, the process used, based on the
decomposition of an organometallic precursor at high temper-
ature and under high hydrogen pressure, is not very convenient.
In this paper, we describe the first utilization of the
liquid-liquid phase transfer method44 to synthesize platinum
with an overall cubic symmetry and a narrow size distribution.
A study of the main parameters governing the synthesis
mechanism has been carried out in order to understand their
specific role. Long-range two-dimensional (2D) or three-
dimensional (3D) self-assemblies are observed, whose super-
lattices depend on the specific shape of the elementary nano-
crystals. These supracrystals, which are micrometricly large,
display well-faceted morphologies. This can be related to the
growth process, which is either homogeneous or heterogeneous.
Several techniques have been used to control the growth of
nanocrystals with anisotropic shapes: growth in colloidal
templates,18,19 growth by selective-adsorption,20 growth in the
presence of external fields,21 and seeded growth.22 The classical
model to understand the morphology of crystals is given in the
framework of the Gibbs-Curie-Wulff theorem,23 which dic-
tates that the external equilibrium shape corresponds to the
minimum surface energy for a given volume. However, several
results revealed recently that, in complex systems such as
colloidal systems, this purely thermodynamic argument cannot
2. Experimental Section
† Part of the “Janos H. Fendler Memorial Issue”.
* To whom correspondence should be addressed.
‡ Universite´ Pierre et Marie Curie.
Chemicals. All products and solvents were purchased from
a variety of sources and used without purification: hydrogen
hexachloroplatinate (IV) hydrate (H2PtCl6) (Aldrich, 99,9%);
§ Universite´ Paris Sud.
10.1021/jp802081n CCC: $40.75
2008 American Chemical Society
Published on Web 09/26/2008

14584 J. Phys. Chem. B, Vol. 112, No. 46, 2008
Demortie`re et al.
with a small focus (copper anode, wavelength: λ ) 0.15418,
focus size: 0.2 × 0.2 mm2, power: 50 kV, 20 mA). The optics
consisted of two parabolic multilayer graded mirrors at right
angles. It delivers a well-defined and intense parallel mono-
chromatic beam. The sample was mounted on a rotation stage,
and the diffraction patterns were recorded onto photostimulable
imaging plates. Vacuum pipes were inserted between the sample
and the imaging plate to reduce air scattering. We used grazing
incidence geometry, and the sample was allowed to oscillate
by a few degrees during exposure.
3. Synthesis of Platinum Nanocubes
A liquid-liquid phase transfer method was used to synthesize
platinum nanocrystals, similar to that described in a previous
publication concerning CoPt nanocrystals.45 However, some
modifications have been brought to the procedure. In the
classical Brust method, the reducing agent is added in the
presence of the coating agent (alkylamine) in the biphasic
solution. One method to increase the nanocrystal size is to
introduce a delay between the beginning of the reduction step
and the addition of the coating agent;46 this is the route presently
followed.
All the different reaction steps were performed in a glovebox
under nitrogen. The platinum complex solution (PtCl6[TDA]2,
C ) 1.55 × 10-3 mol·L-1) was prepared from an aqueous stock
solution of PtCl62- added to a toluene solution of TDAB (C )
2 × 10-2 mol.L-1). The quaternary ammonium serves as transfer
agent of the metallic salt from the aqueous to the toluene
fraction. After complete phase transfer, the aqueous phase was
separated and discarded. The platinum organic solution was
added to 20 mL of toluene, and an aqueous solution of NaBH4
(0.1 mol ·L-1) was slowly added under rapid magnetic stirring.
After 45-180 min of aging time, the coating agent (alkylamine)
was introduced. This delay is presently defined as the ripening
time (τrip) and is discussed later. After overnight stirring (ca.
16 h), the organic and aqueous phases were finally separated.
Extraction of the Nanocrystals. The platinum nanoparticles
were recovered by evaporation of the organic solution using a
rotary evaporator. The residual paste was divided into four, and
an excess of ethanol was added; then the nanocrystals were
precipitated by centrifugation. This sequence allows us to discard
the supernatant containing the excess of octylamine and reaction
byproduct. This procedure was repeated three times, and finally
the colloids were dispersed into 4 mL of toluene. To eliminate
unstable nanocrystals in the solution, a last centrifugation was
performed. The supernatant was collected for further investigation.
Figure 1. TEM images of platinum nanocrystals by adding the capping
agent (A) before the reducing agent, (B) 45 min after introducing the
reducing agent, and (C) 45 min after introducing the reducing agent
but in open air.
tetrakis(decyl)ammonium bromide (TDAB), tetrakis(octyl)am-
monium bromide (TOABr), tetrakis(octyl)ammonium chloride
(TOACl), and sodium borohydride (NaBH4), heptylamine
(C7H15NH2) (Aldrich,99%); hexadecylamine (C16H33NH2) and
octylamine (C8H17NH2) (Fluka, 99%); dodecylamine
(C12H25NH2) (Fluka, 99,5%). Water was purified with a Mil-
lipore system (18.2 MΩ). After synthesis, the nanocrystals were
stored under nitrogen in a glovebox.
Transmission Electron Microscopy (TEM) Measurements.
An electron microscope JEOL 1011 (100 kV) with magnifica-
tions up to 500 000 was used to obtain TEM images and selected
area electron diffraction (SAED) images; the camera length
could be increased to 250 cm to operate in small-angle
conditions. Samples were prepared by allowing one drop of a
solution of nanoparticles in toluene to evaporate under ambient
conditions on a TEM grid. High-resolution TEM (HRTEM)
images were obtained with an HRTEM JEOL 2010 operating
at an acceleration voltage of 200 kV (LaB6).
4. Control of the Shape of the Platinum Nanocrystals
Effect of the Experimental Conditions on the Platinum
Nanocrystals. It is quite difficult to control the size and the
shape of the nanocrystals using the liquid-liquid phase
transfer.44,46,47 Post-treatments such as germination process,46
digestive ripening,48 or heat treatment49 are often proposed to
increase the particle size. Allowing the solution to age prior to
the addition of the agent could be an alternative way, as it has
been demonstrated in the case of gold.50 Figure 1f shows the
platinum nanocrystals obtained by adding octylamine before
(Figure 1A) and 45 min after (Figure 1B) introduction of the
reducing agent. In the first case, the presence of the capping
agent prior the addition of the reducing agent avoid nuclei
growth, then spherical nanocrystals with an average diameter
of 2 nm are observed. In the second case, the nuclei are allowed
to grow in the synthetic batch before the addition of the capping
agent, then triangular and cubic nanocrystals with an average
Scanning Electron Microscopy. The morphology of super-
lattices grown from nanocube solutions was visualized using a
scanning electron microscope (SEM) JEOL 5510LV.
Small-Angle X-ray Diffraction. X-ray diffraction experi-
ments were performed with a rotating anode generator operated

Pt Nanocube Assembly into 2D and 3D Superlattices
J. Phys. Chem. B, Vol. 112, No. 46, 2008 14585
Figure 2. TEM images of platinum nanocrystals synthesized for ripening times of (A) 45 min, (B) 90 min, (C) 120 min, and (D) 180 min.
TABLE 1: Structural Parameters of Platinum Nanocrystals
Depending on the Ripening Time and the Alkylamines
evolution is obtained as the nanocrystals appear more and more
cubic: polydisperse in shape at τrip) 45 min (Figure 2A),
truncated cubes at τrip ) 90 min (Figure 2B), and then perfect
cubes at τrip ) 120 mn, where 90% of the nanocrystals are
perfectly cubic (Figure 2C). The average size is 5.5 nm with a
size polydispersity of 6%. Further increase of τrip yields an
increase in the average size of well-faceted nanocrystals but
without shape uniformity (Figure 2D).
Chains
ripening
time
(min)
alkylamine
chains
average
size (nm)
polydispersity cubic shape
(%)
(%)
0
45
90
120
180
120
120
120
C8
C8
C8
C8
2
10
12
8
6
5
26
13
7
0
60
87
90
40
0
4.3
4.7
5.5
6.7
2.3
3.2
5.2
Influence of the Alkylamine Coating Agent. It is expected
that the nature of the coating agent should play a role in the
final state of the nanocrystals. In the case of gold nanocrystals,
it has been observed that the final size is inversely dependent
on the binding strength of the ligand.50 We have shown in the
case of platinum and CoPt that the interfacial rigidity also plays
an important role concerning the size of the inorganic nano-
crystals obtained by liquid-liquid phase transfer.45,52 Following
these lines, alkylamines with different chain lengths were tested
as the final capping agent, using a similar ripening time in all
cases (120 min). The resulting particles are shown in Figure 3,
and the corresponding data are collected in Table 1. It is shown
that, for longer alkylamin chains, C16H33NH2 (Figure 3A), no
shape control and no evolution of the average size of the
nanocrystals is obtained compared to the case where the capping
agent is added prior to the reducing agent, whereas a huge
polydispersity is observed. Conversely, using short alkylchains,
C8H17NH2 (Figure 3C) or C7H15NH2 (Figure 3D), the shape of
the nanocrystals is cubic (nearly 90%), with a larger average
size (about 5.5 nm) and a very small size polydispersity. If
faceting would develop during the ripening time, it should be
observed in all cases. This clearly demonstrates that faceting
as well as particle growth happen in the presence of the coating
agent. In the work of Saunders et al. on gold nanocrystals, a
size increase was reported with increasing ripening time without
shape effect.49
C8
C16
C12
C7
10
86
size of 4.3 nm are obtained. Indeed, the formation of anisotropic
nanocrystals as observed here is rather remarkable in the case
of the liquid-liquid phase transfer method because it usually
leads to quasi-spherical nanoparticles.47 Figure 1C shows the
platinum nanocrystals obtained in open air. Oxygen appears to
affect the growth process drastically as nanocrystals are now
not clearly faceted and are highly polydisperse in size; some
coalescence could also be observed. This suggests a surface
perturbation of the platinum nanocrystals, which is probably
related to the strong affinity of oxygen with platinum, as
indicated by its action in fuel cell catalysis. In fact, specific
interaction of O2 with small platinum clusters has been
reported.51 It can be supposed that the adsorption of oxygen on
the nanocrystal surface diminishes the ability of TDAB to bind
to it. In these conditions, the passivation effect of TDAB is
lowered, which may lead to uncontrolled growth.
Influence of the Ripening Time. The formation of large gold
nanocrystals by increasing the ripening time has previously been
reported.49 Following a similar approach, we have increased the
ripening time (τrip) from 45 to 180 min; in each case, the reaction
is stopped 16 h after alkylamine addition. The final morphology
of nanocrystals obtained for the different durations is well-
illustrated by the TEM images given in Figure 2. As expected,
the average size increase from 4.3 to 6.7 nm with a decrease of
the size polydispersity (Table 1). However, a surprising shape
Discussion on the Growth Mechanism. At the beginning
of the reduction step, which corresponds to the introduction of
NaBH4, TDAB molecules act as labile capping agents that
stabilize the platinum nuclei by reducing the interfacial energy.
The binding between this protective ligand and the platinum

14586 J. Phys. Chem. B, Vol. 112, No. 46, 2008
Demortie`re et al.
Figure 3. TEM images of platinum nanocrystals synthesized for different alkylamines: (A) hexadecylamine, (B) dodecylamine, (C) octylamine,
and (D) heptylamine (ripening time 120 mn).
surface is electrostatic in nature and considered weak.48 This
configuration is expected to favor further crystal growth by
allowing easy access to the particle surface.49 Thus, it is not
doubtful that the introduction of the coating agent after the
ripening time will drastically slow down the growth process as
a result of its stronger bonding ability. From this point of view,
the effect of the alkylamine chains may seem surprising,
considering that the strength of the interaction with the metal
surface, which is mediated by the amino group, should be the
same for all alkylamines. In fact, the particle size effect may
be explained by a lower solubility in toluene of alkylamines
with longer chains, which favors their adsorption at the particle
interface.52 The coating layer is thus expected to be not only
thicker but also denser when using longer-chain alkylamines.
Such a palisade will reduce the access of new platinum-TDAB
complexes to the particle surface, resulting in smaller average
sizes. This sterical hindrance is expected to be maximal with
the C16 alkylamine, in which only spherical particles with an
average size of 2.3 nm occur (Figure 3A). In this condition,
the nanocrystal growth is prevented, and the final size and shape
are not very different from those in the case where the reducing
agent is added prior to the capping agent. This strongly suggests
that the determination of the nanocrystal shape depends on the
growth of the seed during the following step (maturation time),
where both the capping agent and the TDAB molecules are
present for 16 h under stirring in the same batch prior to the
extraction. Thus TDAB should play a role. The effect of ions
acting as a poisoning agent of specific crystalline facets is often
reported; it induces an anisotropic growth, yielding specific
shape.53,54 In particular, the cetyltrimethylammonium bromide
(CTAB) surfactant is usually used to obtain shape-specific gold55
or silver56 nanocrystals. Ab initio calculations of the optimal
morphology of face-centered cubic (fcc) particles generally take
into account only the denser {111} and {100} surfaces. Pure
{100} faceting gives cubic shapes, whereas {111} and {100}
faceting yields octahedrons. The equilibrium shape is normally
between both faceting types. It has been previously reported
that bromide anions have a high affinity for {100} facets.30,53,54
Thus, the TDAB surfactant, which is present in excess at the
beginning of the synthesis, acts as a labile coating agent that
prevents coalescence and stabilizes the nanoparticles in solution.
Fink et al.46 have shown that, for gold nanocrystals synthesized
by Brust’s method, Br- anions are adsorbed on the crystal and
induced an electrostatic adsorption site, allowing the TDA+
molecules to passivate the nanoparticles. A similar configuration
can be assumed in our case. To prove the influence of the
counterion on the growth process, bromide and chloride tetra-
alkylammoniums have been used as phase transfer agents at
the same concentration: 7.5 × 10-2 mol·L-1 (TOABr and
TOACl). The reactions have been carried out using octylamine
as the final coating agent and τrip ) 120 min. TEM images
displayed in Figure 4 the nanoparticles synthesized in the
presence of TOABr (Figure 4A) and TOACl (Figure 4B). The
nanocrystals obtained in the presence of bromide have clearly
faceted morphologies and are nearly cubic (inset of Figure 4A).
In contrast, the use of chloride generates elongated and
heterogeneous morphologies (inset of Figure 4B). Indeed, the
preferential adsorption of bromide on {100} facets will favor
the fixation of TDA+ species and hence will hinder the growth
of the nanocrystals in the {100} direction. Thus {111} facets
are expected to grow more rapidly and will have a tendency to
disappear, leading to cubic crystals.
The above results suggest the following mechanism. During
the first stage of the reaction, the nuclei growth takes place in
the presence of the weakly binding TDAB surfactant. Compared
with the usual conditions, the absence of the alkylamine, which

Pt Nanocube Assembly into 2D and 3D Superlattices
J. Phys. Chem. B, Vol. 112, No. 46, 2008 14587
Figure 4. TEM images of platinum nanocrystals synthesized from (A)
TOABr and (B) TOACl.
blocks the growth of the nuclei, during the initial reduction
favors a seed-growing mechanism. Then, after the nucleation
stage, the crystalline phase and the initial shape of the nuclei
are determined. Consistent with our results, we can assume
single-crystal truncated octahedral in fcc structure is the shape
of nuclei. This has been observed, when alkylamine is added
under similar experimental conditions, prior to the reducing
agent.52 Then the growth is initiated during the ripening time,
and the seeds grow until the facets become large enough to
bond the capping materials. Whether this process is achieved
depends on the ripening time. Afterward, the injection of the
alkylamine causes a growth slow down for the shorter chains
and a growth stop for the longer chains. The combined action
of the TDAB, which orientates the growth of the {111} facets
and the octylamine, which slows the growth mechanism, is the
key to the platinum nanocube synthesis. Nevertheless, it is
important to note that, if the TDAB concentration (C ) 2 ×
10-2 mol·L-1) in the solution increases significantly (2-3
times), then the Pt nanocrystals do not have a specific cubic
shape but a spherical shape. Thus, there is a maximum
concentration for the formation of the anisotropic shape. Indeed,
in his works on AgBr nanoparticles,56 Sugimoto demonstrated
that the variation of bromide concentration changed the growth
Figure 5. TEM images of platinum nanocrystals self-organized in a
hexagonal network (A) (τrip ) 60 min), a quasi-square network (B)
(τrip ) 90 min), and a square network (C) (τrip ) 120 min). HRTEM
images are given in the insets. Deposition conditions: amorphous
carbon-coated TEM grids, single-drop deposition at a concentration of
1 × 10-4 mol·L-1
.
rate of the {100} and {111} faces. Moreover, we can assume
that, according to the [Br-], the growth regime changes from
thermodynamic growth with a diffusion-controlled mode to
kinetic growth with a reaction-controlled mode. In this last
regime, the nanocube formation is favorable because the growth
rates of the {100} and {111} faces are different. Here, although
the final shape is likely to be kinetically driven, the growth rate
depends on many parameters, and the determination of the
growth regime is not easy. On the basis of studies of varying
the accessible growth parameters, we have discovered the critical
factors in the formation of Pt nanocubes.
5. Self-Organization in 2D and 3D Superlattices
2D Self-Assemblies. As it is well-known, self-assemblies of
nanocrystals require low size and shape polydispersity.57 The

14588 J. Phys. Chem. B, Vol. 112, No. 46, 2008
Demortie`re et al.
Figure 6. Cubic nanocrystals. (A) Wide-angle SAED pattern. (B) Numerical simulation of A. (C,D) TEM images of self-assembled superlattices.
(E) Scheme of cubic superlattices of nanocubes. (F) Size distribution. (G) Small angle. (H,I) Numerical simulation of the small-angle SAED.
organization results mainly from an equilibrium between the
attractive van der Waals forces and the repulsive steric forces.58
As these forces are isotrope, this yields, for spherical nano-
crystals, a 2D hexagonal pattern.57,58
TEM image clearly reveals a square lattice made of platinum
nanocubes. In the case of truncated cubes (Figure 5B, τrip ) 90
min), long-range order is not well-defined. In fact, a transition
occurs between the hexagonal pattern and the quadratic pattern.
This transition is due to the truncature of the nanocrystals, which
are not totally spherical in shape, as in Figure 5A, nor cubic, as
in Figure 5C. This shows that the arrangement mechanism is
dependent on the crystal shape. This could indeed be expected
from the fundamental rules of crystallography that stipulate that
the object and lattice symmetries must be compatible. Here,
the change of the nanocrystal shape is accompanied by a gradual
increase in the {100} face surface compared to the {111} face,
which favors a face-to-face attachment forming a high inter-
digitation bond.63 The increase of the interaction between
particles with their cubicity is clearly demonstrated by the
variation of the interparticle distances: the interparticle distance
The elaboration of 2D and 3D superlattices is generally
performed by controlled evaporation of the organic solvent from
small polydisperse nanocrystal suspensions in the presence of
a deposition substrate. A large variety of ordered structures in
2D and 3D macroscopic assemblies have been observed this
way.57–63 Here the self-assemblies are obtained by drop deposi-
tion of platinum nanocrystal solution on TEM grids coated by
an amorphous carbon film. As the colloidal nanocrystals have
a narrow size distribution, they spontaneously self-assemble in
2D monolayers (Figure 5). As expected, a hexagonal pattern is
observed for nearly spherical particles, (Figure 5A, τrip ) 60
min). For cubic nanocrystals (Figure 5C, τrip ) 120 min), the

Pt Nanocube Assembly into 2D and 3D Superlattices
J. Phys. Chem. B, Vol. 112, No. 46, 2008 14589
Figure 7. (A,B) SEM images of superlattices in cubic shape obtained with platinum nanocubes. (C) Corresponding X-ray diffraction pattern. (D,E)
SEM images of superlattices in pyramidal shape obtained with truncated nanocubes. (F) Corresponding X-ray diffraction patterns. (G,H) SEM
images of superlattices in triangular shape obtained with truncated nanocubes. (I) Corresponding X-ray diffraction pattern of the superlattices.
is 3 nm (border to border) for quasi-spherical particles, which
corresponds to twice the length of fully extended octylamine
(1.5 nm length). It is 2.6 nm for truncated cubes and 2 nm for
cubes. The increase of the interaction between the {100} faces
thus induces chain interdigitation and a higher compacity of
the network.
7). For this purpose, the technique of controlled evaporation of
a carrier solvent (toluene) from the nanocrystal solution allows
us to obtain organized structures with a high degree of long-
range ordering.60 A volume of 2 mL (C ) 2 × 10-5 mol·L-1
)
of colloidal solution was slowly evaporated (8 days) on a silicon
wafer (5 × 5 mm) placed on the bottom of a glass container of
cylindrical geometry (1 × 5 cm) at room temperature.
3D Supracrystals. It should be noted that the dropwise
deposition method induces defect in the monolayers, as observed
in Figure 5C, where the presence of triangular or elongated
nanocrystals partly induces defect in the self-assembly. This
can be avoided by using an immersion process where the
substrate is immersed in a solution of nanocrystals, which slowly
evaporate under controlled atmosphere. In this condition, a
spontaneous segregation process occurs during the self-assembly
process, which excludes nanocrystals having strong deviation
from the average size and shape.60,61 Hence, 3D superlattices
of cubic and truncated cubic nanocrystals have been grown by
close control of the evaporation conditions (Figures 6–8). For
this purpose, 200 µL of colloidal solution (Cj ) 2 × 10-5
mol·L-1) are slowly evaporated (12 h) in a glass container with
substrate (TEM copper grid coated by amorphous carbon) placed
at the bottom. We also show that micrometer-sized single
supracrystals can be build from nanoparticle solutions (Figure
Case of Cubic Nanocrystals (τrip ) 120′). The 3D structures
obtained by the self-assembly of nanocubes were first investi-
gated by TEM (Figure 6). Panels C and D of Figure 6 are direct
images obtained at different magnifications and reveal the
organization of the 5.5 nm cubic nanocrystals (see histogram
of size in Figure 6F) in well-ordered superlattices. A square-
like arrangement is observed in Figure 6D. The order was
checked by SAED, measured at wide- and small-angles, and is
respectively shown in Figure 6A,G (camera lengths of 80 and
250 cm, respectively). Wide-angle diffraction peaks (Figure 6A)
originate from the scattering by individual nanocrystals, while
small-angle ones (Figure 6G) are associated with their ordering
in a “superlattice”. Numerical simulations of the diffraction
patterns are presented in Figure 6B,H,I. They have been
computed within the kinematical approximation, and they are

14590 J. Phys. Chem. B, Vol. 112, No. 46, 2008
Demortie`re et al.
Figure 8. Truncated cubic nanocrystals. (A) Wide-angle SAED pattern. (C,D) TEM images of self-assembled superlattices. (B,E) Picture showing
the nanocubes positions into the fcc superlattices. (F) Size distribution of the truncated nanocubes. (G) SAED at small angle with a tilt of 30°. (H)
Superlattice with the 4-fold symmetry. (I) Fourier transform of the picture in H.
TABLE 2: Data Table of Small Angle X-ray Diffraction:
Experimental and Calculated Coordinates of Diffraction
Spots Assuming a Simple Cubic Structure (Cubic Shape)
TABLE 3: Data Table of Small Angle X-ray Diffraction:
Experimental and Calculated Coordinates of Diffraction
Spots Assuming an fcc Structure
Q measured [Å-1
]
Q calculated [Å-1
]
h
k
l
Q measured [Å-1
]
Q calculated [Å-1
]
h
k
l
0.115
0.141
0.163
0.202
0.283
0.115
0.140
0.162
0.199
0.280
1
1
2
1
2
1
1
0
1
2
0
1
0
2
2
0.0985
0.1139
0.1622
0.1893
0.1970
0.2279
0.0988
0.1140
0.1613
0.1891
0.1975
0.2281
1
2
2
3
2
4
1
0
2
1
2
0
1
0
0
1
2
0
simply intended as a guide for the interpretation of the
experimental data.
the observed peaks in Figure 6A correspond to the (200) and
(220) reflections of the platinum nanocrystals. The angular width
of the (200) reflections is 10°, indicating a weak disorientation
of the nanocrystals, one with respect to the other. Indeed the
TEM image in Figure 6D shows that nanocrystal faces are only
parallel in average from a nanocrystal to its neighbor, with some
fluctuation in orientation being allowed. Very weak additional
The wide-angle diffraction pattern in Figure 6A clearly
presents 4-fold symmetry. The calculated pattern in Figure 6B
is obtained assuming that the nanocrystals possess the fcc
structure of bulk platinum (unit cell parameter 0.392 nm), their
[001] axis being parallel to the electron beam. It follows that

Pt Nanocube Assembly into 2D and 3D Superlattices
J. Phys. Chem. B, Vol. 112, No. 46, 2008 14591
diffraction rings, corresponding to the {111} and {200} planes,
can also be seen, corresponding to a small fraction of strongly
misorientated nanocrystals.
D,E) or truncated tetrahedral morphology (Figure 7 G,H). The
typical size of these single crystals is in the range 5-30 µm.
Both morphologies are compatible with 3- and 4-fold sym-
metries, as expected. In the case of pyramids, the angle between
the normal to the substrate and the normal to the faces is close
to 55°. X-ray diffraction patterns reveal punctuated diffraction
rings (Figure 7F). The two dark spots (white arrows) correspond
to the Bragg reflection associated with the main orientation
(200). This is compatible with flat-lying (100) surfaces and (111)
pyramid faces. For the truncated tetrahedra (Figure 7G,H), the
diffraction pattern is given in Figure 7I. Two dark spots are
also observed (white arrows) and correspond to the Bragg
reflection associated with the main orientation (111). This is
compatible with flat lying (111) surfaces and (100) faces of the
tetrahedra. In both cases, the ring radii are perfectly accounted
for, with an fcc lattice of parameter a ) 11.02 nm (see Table
3), which is compatible with the TEM data of the superlattices.
The punctuation of the rings depends upon the substrate area
that is looked at. Simulations show that most single crystals
have their (100) surface flat-lying for Figure 7D,E (pyramid),
while Figure 7G,H corresponds mainly to the (111) orientations
(some low-intensity reflections correspond to a small-proportion
of crystals with flat-lying (220) surfaces (elongated structure
on the left of Figure 7D)). These two majority orientations
correspond to both morphologies seen by SEM.
Therefore, from these results, it is possible to connect the
underlying structure of the nanocrystals to the supercrystal
morphology. Furthermore, information can be deduced from the
growth process of the supracrystals. The nanocrystal superlattice
crystallization occurs either by heterogeneous nucleation on the
substrate or by homogeneous nucleation in solution. The
evaporation conditions determine the crystallization kinetics.
However, a small variation in interparticle attractions leads to
important changes in superlattice nucleation and growth kinet-
ics.64 In the case of truncated nanocubes, the nucleation seems
to be heterogeneous because the structures are supported on
the substrate; as a consequence, the diffraction patterns reveals
strong punctuated diffraction rings. Conversely, for the nanocubes,
the nucleation seems to be homogeneous because the superlat-
tices are in a random position on the substrate; consequently,
diffraction patterns reveal only homogeneous rings. This can
be explained by the difference of the interparticle attraction
forces during the solvent evaporation between the nanocube
sample and the truncated nanocube sample. As shown above,
the face-to-face van der Waals interactions appear stronger in
the case of nanocubes, as a result of the perfect cubic shape.
We can assume that this effect is sufficient to induce a
modification of the nucleation mode, and thus favors homoge-
neous nucleation for the nanocubes and then permits the
formation of cubic superlattices in solution. For truncated
nanocrystals, the growth is induced by the substrates and then
they start a specific 2D arrangement such as cubic (see Figure
5), yielding the pyramidal shape, or, for a minority part, they
start from a 2D hexagonal arrangement yielding the triangular
shape. Hence the shape of the nanocrystals controls the growth
process of the supracrystal and thus the shape of the supracrystals.
The surprising square shape of the central spot observed in
Figure 6A is simply the blurred image of the small-angle
diffraction pattern shown in Figure 6G. The superlattice can be
indexed as a primitive cubic, with parameters a ) b ) 6.6 nm
(Figure 6E), in agreement with the TEM image in Figure 6D.
This result is not surprising, as a primitive cubic lattice
corresponds to the dense packing of cubic units (the Wigner-Seitz
cell of a primitive cubic being a cube). Numerical calculations
have been performed to show, within the kinematical ap-
proximation, the influence of the nanocrystal shape on the
reflection intensity. The diffraction patterns (Figure 6H,I) have
been calculated with a form factor equal to the Fourier transform
of a cube that assumes that all nanocubes are in the same
orientation (Figure 6H) and with a form factor equal to the
Fourier transform of a sphere that assumes that the nanocubes
are in a random orientation (Figure 6I). A comparison with the
experimental diagram supports the first hypothesis, in agreement
with Figure 6D. The orientational relationship between the Pt
nanocrystal lattice and the superlattice is thus [001] || [001]s.
Let us now consider 3D supracrystals obtained using the
procedure described above. SEM observations show supracrys-
tals of cubic shape that are either grouped (Figure 7A) or isolated
on the substrate (Figure 7B). Their typical lateral size is in the
range 0.5-2 µm. A structural analysis has been carried out by
small-angle X-ray diffraction in grazing angle geometry. The
diffraction pattern obtained under these conditions (Figure 7C)
shows homogeneous diffraction rings that correspond to ran-
domly oriented supracrystals. As indicated in Table 2, the
position of the diffraction rings is well accounted for using a
simple cubic lattice of parameter a ) 8.19 nm. The difference
of the value extracted from TEM data (a ) 6.6 nm) may be
due to incertitude of the camera length of the electron
microscope and/or to the scale of the supracrystals, which can
be relaxed compared to the multilayers observed by TEM.
Case of Truncated Cubic Nanoparticles (τrip ) 90′). The
data obtained by TEM are summarized in Figure 8. The wide-
angle diffraction pattern in Figure 8A exhibits three diffraction
rings associated with the (111), (200), and (220) reflections of
the fcc platinum structure. Intensity reinforcements of the (200)
ring clearly display 4-fold symmetry showing preferential
alignment of the 100 and 001 axes of the nanocrystals. The
observation of two (111) reflections could be explained either
by a misorientation of part of the superlattice or disorientation
of the truncated nanocubes in the organized structure or by a
combination of both effects. It is likely that the truncation of
nanocubes permits a greater misorientation of nanocrystals than
in the case of perfect nanocubes. A TEM image of the
superlattice is given in Figure 8B,C. A close examination reveals
a square lattice corresponding to an fcc structure, as drawn in
Figure 8B,E with a lattice parameter equal to 11 nm. Attempts
to obtain correct diffraction images of this zone failed because
of misorientation of the carbon grid. By tilting the sample, the
diffraction pattern presented in Figure 8G could be obtained. It
clearly shows that the system has 3- or 6-fold symmetry. This
combined with the 4-fold symmetry of the image presented in
Figure 8H and its Fourier transform (Figure 8I) indicates that
the superlattice has cubic symmetry. This is consistent with the
fcc structure assumed above.
Summary
For the first time, the liquid-liquid transfer method has been
used to synthesize cubic nanocrystals of platinum. The role of
a ripening time between the beginning of reduction and the
introduction of a stable ligand (octylamine) that stops the
reaction is clearly demonstrated. The optimal condition for
nanocube synthesis corresponds to a ripening time of 120 min.
Single supracrystals have been grown from solutions of
truncated cubes using the above-described technique. SEM
images reveal either pyramids with a square base (Figure 7

14592 J. Phys. Chem. B, Vol. 112, No. 46, 2008
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Spherical particles are obtained at ripening times of less than
60 min, with intermediate ripening times leading to truncated
cubic shapes. Moreover, the action of TDAB seems to be
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Acknowledgment. We would like to acknowledge Prof.
J. H. Fendler for the well-founded criticism and council he gave
to this work during his numerous stays in the LMZN at Paris
6. Moreover, he always treated people with great humanity. C.P.
would like to personally acknowledge Prof. Fendler for every-
thing he did for him.
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