Thermotropic liquid crystals as templates for anisotropic growth of nanoparticles

Article abstract of DOI:10.1002/anie.201105314

In control: The organization of a thermotropic liquid crystal can be used to control the growth of inorganic nano-objects (like ZnO). When the systems are in their nematic phase state, anisotropic nanoparticles are formed, while in isotropic conditions, isotropic nanoparticles are obtained. Playing with the exact nature of the liquid crystal enables control of the size and the aspect ratio of the nanoparticles (see picture). Copyright

Full text of DOI:10.1002/anie.201105314

Communications
DOI: 10.1002/anie.201105314
Nanoparticles
Thermotropic Liquid Crystals as Templates for Anisotropic Growth of
Nanoparticles
Sarmenio Saliba, Yannick Coppel, Marie-France Achard, Christophe Mingotaud, Jean-
Daniel Marty,* and Myrtil L. Kahn*
Control over size, shape, and composition of nanomaterials is
one of the major concerns in the field of nanoscience today.
This task has induced a tremendous amount of work, in which
methods based on chemical or physical processes were
developed to synthesize such nanosystems. Among the
various strategies, organic molecules and macromolecules,
exhibiting an anisotropic shape or a particular organization,
have been used as templates for controlling the shape and size
of inorganic materials.[1] Literature reveals particularly inter-
esting attempts in synthesizing nanoparticles (NPs) within
mesophases of liquid crystals (LCs).[2] For example, ZnSe
nanomaterials have been synthesized in lyotropic systems
based on amphiphilic triblock copolymers.[3] Depending on
the liquid-crystalline state, quantum dots, nanodisks, or even
nanowires could be obtained. Nanoporous materials (gener-
ally silica) have also been fabricated by true liquid crystal
templating.[4] Whereas the large majority of such research
employs lyotropic LCs, very few publications deal with the
elaboration of nanomaterials within thermotropic ones.[5–8]
Indeed, the development of an in situ procedure to generate
NPs within an LC medium has proved to be quite a
challenging task. Most studies involve the in situ reduction
of metal precursors through oxidation of the LC medium in
order to obtain the desired NPs. For example, the formation
of CuCl nanostructures inside a mixture of an ionic liquid and
a derivative of ascorbic acid has been reported. This approach
resulted in the formation of CuCl nanoplatelets with a
relatively uniform thickness of about 220 nm and in-plane
sizes of 5–50 mm.[5a–c] Glass-forming liquid-crystalline materi-
als acting as a reducing agent were also used to obtain Au
NPs, the size and shape of which depended on both the
amount of precursor content and the LC state.[5d] Isotropic
NPs of gold or silver have also been synthesized by heating
LC materials doped with the corresponding metal salts.[6] In
other cases sputtering[7] or electrodeposition[8] techniques
were used to form NPs in thermotropic systems. However,
none of the examples above have shown a direct relation
between the structure of the LCs and the morphology of the
synthesized NPs. For the few examples that report an
anisotropic growth, structures in most cases have been outside
the nanometer range. To improve such results, we suggest that
three criteria should be fulfilled in order to tailor the NP
morphology using an LC phase; 1) the chemical reaction
leading to the NPs should not disrupt the LC organization.
Thus the LC molecules should not play the role of reactants
(side products should be avoided as much as possible
throughout the NP formation). 2) Interactions between the
LC molecules, the NP precursor, and eventually the synthe-
sized NPs should favor the templating effect of the LC phase.
3) The use of relatively high viscosity LCs should prevent a
fast disruption of the organization during the NP formation.
We have previously described a very simple organome-
tallic method for the formation of zinc oxide NPs with only
cyclohexane as a side product.[9] Such a reaction may fulfill
the first criterion. ZnO was therefore chosen as the inorganic
material for this study. The use of LC compounds that have an
oligomeric or a polymeric structure may fulfill the last two
criteria. Indeed, such compounds may contain various
functionalities (e.g. amine groups[9b,10]) that favor interactions
between the LCs and the ZnO precursors/ZnO NPs). We
have chosen to work with two different types of LCs that
differ by their type of backbone. A first type involves a
trisamine molecule (tris(2-aminoethyl)amine, TREN) as the
backbone and a second type, a hyperbranched polyamido-
amine core (HYPAM). Ideally, the target LC should exhibit a
mesophase (i.e. nematic phase) close to or at ambient
temperature to avoid any precursor thermal breakdown
before NPs are produced. Thus, concerning the previously
mentioned backbones, we chose to branch a phenylbenzoate
mesogenic derivative, namely the 4-(4-acryloyloxybutyloxy)-
phenyl-4’-(methoxy) benzoate (see S1A in the Supporting
Information for characterization details). Figure 1 shows the
envisaged LC molecules.
[*] Dr. S. Saliba, Dr. Y. Coppel, Dr. M. L. Kahn
Laboratoire de Chimie de Coordination, CNRS UPR 8241, Universitꢀ
de Toulouse
Differential scanning calorimetry (DSC) demonstrates
that both compounds present a glass transition (as expected
for oligomeric or polymeric structures) below 08C and a LC
isotropic phase transition around room temperature (see
sections S2 and S3 in the Supporting Information). Polarized
optical microscopy (POM) and small-angle X-ray diffraction
(SAXS) experiments have shown that TREN-LC and
HYPAM-LC present a nematic phase below the clearing
temperature (see sections S2 and S3 in the Supporting
Information). The syntheses of the ZnO NPs in the two
thermotropic LCs were performed as follows: in a typical
205, route de Narbonne, 31077 Toulouse Cedex 04 (France)
E-mail: myrtil.kahn@lcc-toulouse.fr
Dr. S. Saliba, Dr. C. Mingotaud, Dr. J.-D. Marty
Laboratoire IMRCP, CNRS UMR 5623, Universitꢀ de Toulouse
118, route de Narbonne, 31062 Toulouse Cedex 09 (France)
E-mail: marty@chimie.ups-tlse.fr
Dr. M.-F. Achard
Centre de Recherche Paul Pascal
115 Avenue Schweitzer, 33600, Pessac (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105314.
12032
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Angew. Chem. Int. Ed. 2011, 50, 12032 –12035

More precisely, pure TREN-LC and HYPAM-LC are iso-
tropic at 308C and 458C, respectively. At these given temper-
atures, growth control could be achieved: the average size of
the NPs is around 5.4(0.7) nm in the case of TREN-LC and
2.7(0.3) nm for HYPAM-LC. When the experiments were
performed in the nematic phase of the LC compound (i.e. at
lower temperatures), anisotropic ZnO structures were
obtained. As shown in Figure 2c and d, nanoworm-like or
nanowire-like structures were grown in TREN-LC and
HYPAM-LC, respectively. Nanoworms have an average
width of (2.5 Æ 0.2) nm and nanowires (2.7 Æ 0.4) nm. Poly-
disperse lengths that vary from a few nanometers up to
around 100 nm were observed when TREN-LC was used,
while lengths from around 10 to 200 nm were obtained with
HYPAM-LC. High-resolution transmission electron micros-
copy (HRTEM) performed on the as-synthesized nanostruc-
tures clearly showed two orientations. Indeed, some nano-
structures are small enough to present their c axis perpen-
dicular to the grid. Figure 3a corresponds to the observation
Figure 1. Schematic representation of the LC compounds used herein.
experiment, the organometallic precursor, [Zn(C6H11)2]
(noted as [Zn(Cy)2]), was added to a solution of either
TREN-LC or HYPAM-LC in anhydrous THF. Mixing of the
LC with the ZnO precursor in a solvent at the pre-hydrolysis
stage ensured a good material homogeneity. After evapora-
tion of the THF under vacuum, the reaction vial was removed
from inert conditions, closed, immersed in a bath at the
desired temperature, and then opened to the atmosphere. A
few hours after treatment, the resulting products became
luminescent (under UV irradiation) in the yellow region of
the visible spectrum. This is a well-known characteristic of
ZnO NPs, thus confirming their presence. Figure 2 shows
typical TEM micrographs of the ZnO nanostructures, the
morphologies of which depend on the experimental condi-
tions.
Figure 3. a),b) High-resolution TEM micrographs of a TREN-LC/ZnO
nanocomposite (synthesized at 58C) showing two crystal orientations
perpendicular and parallel to the z plane, respectively. Insets corre-
spond to the filtered Fourier transform images (bottom) and the
inverse Fourier transform images (top), allowing a very good recon-
struction of the experimental HRTEM images.
In isotropic conditions, hydrolysis of the [Zn(Cy)2]
precursor led to isotropic NPs, as shown in Figure 2a and b.
of a ZnO with the c axis perpendicular to the grid (inter-
atomic distance equal to 0.320 nm within the accuracy of the
measurement corresponding to the shorter distance in the ab
plane associated with the Zn–Zn distance). Whereas Fig-
ure 3b is a micrograph of a ZnO with its c axis parallel to the
grid (inter-atomic distance equal to 0.265 nm within the
accuracy of the measurement corresponding to the shorter
distance along the c axis associated with the Zn–O distance).
In the latter, we observed a disappearance of the diffracting
planes at the point of curvature, which can be associated with
defects in the atomic packing. This experiment puts the
hexagonal wurtzite structure (a = b = 3.2 ꢀ and c = 5.2 ꢀ,
space group: P6mc) in evidence and demonstrates that the
anisotropic growth proceeds along the c axis. The sample was
further analyzed by energy-dispersive X-ray spectroscopy
(EDX) and the results confirmed the presence of elemental
zinc.
Figure 2. TEM micrographs of ZnO NPs synthesized under various
conditions: a) in TREN-LC at 308C (scale bar: 200 nm); b) in HYPAM-
LC at 458C (scale bar: 50 nm); c) in TREN-LC at 58C (scale bar:
50 nm), and d) in HYPAM-LC at 58C (scale bar: 50 nm).
Clearly and for the first time, the use of thermotropic LC
molecules of increasing size allows the growth control of ZnO
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Communications
nanoobjects. Importantly, when the thermotropic LC mole-
cules are used in their nematic phase state, anisotropic ZnO
nanoobjects are formed, the aspect ratio and the straightness
of which increases when using HYPAM-LC instead of TREN-
LC. Therefore, control over the shape and size of ZnO NPs
can be achieved by selecting the right LC compound and the
LC phase, in which the NP synthesis is performed.
To better understand such LC/ZnO nanocomposites, we
analyzed the nature of possible interactions between the LC
compounds and the ZnO nanocrystals. Taking TREN-LC as
an example, solid-state 13C NMR studies were performed at
228C on 1) the TREN-LC molecule alone, 2) the TREN-LC/
[Zn(Cy)2] mixture, and 3) the TREN-LC/ZnO NPs nano-
composites. For the TREN-LC molecule alone, broad and
complex signals were observed, confirming that TREN-LC is
spatially heterogenous (see section S2 in the Supporting
Information). Addition of [Zn(Cy)2] (pre-hydrolysis state)
has an effect on the line shape of NMR signals, a behavior that
is even more pronounced upon NP formation. Briefly, the
carbonyl signals (d13C between 150 to 180 ppm) and the
methylene signals in the a position with respect to amine
functionalities (d13C between 40 to 45 ppm), show an
important broadening in the presence of NPs. On the
contrary, some aromatic (d13C between 110 to 130 ppm), the
methoxy (d13C = 56 ppm), and the OCH2CH2CH2CH2O sig-
nals (d13C = 26 ppm) are sharpened. This result can be
tentatively explained by an interaction of TREN-LC mole-
cules with ZnO NPs through the nitrogen atoms and carbonyl
functions. This interaction increases the local spatial hetero-
geneity for these atoms close to the ZnO surface (signal
broadening). The atoms farther from the surface can expe-
rience a more homogeneous environment or an increase in
mobility (signal sharpening). The multiple interactions with
nitrogen atoms and carbonyl groups may be used to explain
the stability of ZnO in such LC hosts. Whatever the exact
nature may be, these experiments demonstrated that we have
interactions of the LC with the ZnO precursor and the ZnO
NPs, as expected.
DSC, POM, and SAXS experiments were also carried out
to investigate the LC behavior of the newly synthesized LC/
ZnO nanocomposites. Figure 4a is a thermogram profile for
TREN-LC/ZnO that presents a thermodynamically stable
mesophase, therefore visible on both heating and cooling
ramps. The LC transition peak occurs at 378C and a change in
heat capacity linked to a Tg could be observed at À18C. Since
pure TREN-LC exhibits the LC state between À22.0 and
9.68C (see S2 in the Supporting Information), we can clearly
conclude that the presence of ZnO NPs shifts the mesophase
temperature window towards higher temperatures. The
thermo-optical behavior observed between crossed polarizers
was in agreement with the previously mentioned DSC results.
In other words, birefringent textures corresponding to a
nematic phase could be observed within the mesophase
temperature range (Figure 4b). This texture then turns darker
as it approaches the isotropic state. In addition, SAXS
measurements showed diffuse rings at d = 4.5 ꢀ, correspond-
ing to the inter-mesogenic distance in a nematic LC phase
(Figure 4c). Similar results were obtained for HYPAM-LC/
ZnO composites (section S3 in the Supporting Information).
Figure 4. a) DSC thermogram for TREN-LC/ZnO at a rate of
58CminÀ1; b) POM image for TREN-LC/ZnO at 208C, and c) a typical
SAXS pattern for the nanocomposite.
Such experiments demonstrate that the final composite
presents a nematic LC phase. Due to the high reactivity of
the ZnO precursor, monitoring of the LC state before ZnO
formation cannot be performed. However, the previous data
showed that the nematic state does exist in most stages of the
reaction, the most important being at reaction completion.
The results reported above allow us to conclude that ZnO
nano-objects can be fabricated in thermotropic LCs by a
straightforward organometallic method and that the use of a
nematic phase favors the well-sought-after anisotropic growth
of metal oxide NPs. Moreover, a direct correlation between
the structural characteristics of the LC and the morphology of
the nanostructures was demonstrated. The novel LC/NP
composites present, at the same time, LC properties and
optical properties originating from ZnO. This in situ strategy
paves the way for new LC/NP composites of controllable and
stable properties. Influence of more “complex” LC phases on
NP size and shape is currently under study.
Experimental Section
Materials, characterization techniques, and methods are described in
the Supporting Information. The synthesis of liquid crystal molecules
was adapted from literature,[11] whereas the synthesis of the HYPAM
core was carried out following previously published work by our
group.[12]
Received: July 28, 2011
Revised: September 26, 2011
Published online: October 17, 2011
Keywords: anisotropic growth · liquid crystals · nanoparticles ·
.
organometallic chemistry · zinc oxide
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