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Ionic Liquids for the Convenient Synthesis of Functional Nanoparticles and Other Inorganic Nanostructures.

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M. Antonietti et al.
Nanostructures
Ionic Liquids for the Convenient Synthesis of Functional
Nanoparticles and Other Inorganic Nanostructures
Markus Antonietti,* Daibin Kuang, Bernd Smarsly, and Yong Zhou
Keywords:
ionic liquids · mesophases · microporous materials ·
nanostructures · sol–gel processes
I
onic liquids are a new class of organic solvents with high polarity and
a preorganized solvent structure. Very polar reactions can be carried
out in these liquid in the absence of or with a controlled amount of
water, and crystalline nanoparticles can be synthesized conveniently at
ambient temperatures. The pronounced self-organization of the
solvent is used in the synthesis of self-assembled, highly organized
hybrid nanostructures with unparalleled quality. The extraordinary
potential of ionic liquids in materials synthesis is described in this
minireview and a physicochemical explanation is given.
Introduction
Ionic liquids (ILs) are organic salts with low melting
points, sometimes as low as 96 8C.[1] ILs have received much
attention in many areas of chemistry and industry due to their
potential as a “green” recyclable alternative to traditional
organic solvents.[2] The ILs are liquid over a wide range of
temperatures, in some cases in excess of 400 8C. Because of
their properties, such as high polarity, negligible vapor
pressure, high ionic conductivity, and thermal stability, ILs
can be used in catalysis,[3, 4] as inert solvents in electrochemistry,[5] for polymer synthesis,[6, 7] and in the adaptation of
enzymatic reactions to organic media.[8] ILs can also be used
to replace water in chemical or technical processes.
Although ionic liquids have found application only
recently in chemistry, they are an old class of substances:
the first description of an IL with a melting point of 12 8C was
published in 1914.[9] The most extensively studied ILs are the
1-alkyl-3-methylimidazolium salts. Newer systems include
species with additional functionality, for example, long-chain
amphiphilic ILs with both lyotropic[10] and thermotropic
liquid crystallinity.[11] Organic reactions can be conducted
with increased selectivity in these ILs.[12] Other liquid-
[*] M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou
Max Planck Institute of Colloids and Interfaces
Research Campus Golm
14424 Potsdam (Germany)
Fax: (+ 49) 331-567-9502
E-mail: pape@mpikg-golm.mpg.de
4988
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
crystalline species with wide phase
regions and a very high dipole moment
and polarizability are described in
ref. [13].
This review will not focus on the
use of ILs in catalysis and organic/
inorganic synthesis,[14] as there are already excellent reviews and books[15] available. Instead we
will describe recent developments in which the advantages of
ILs for materials chemistry and especially for the synthesis of
novel nanostructures have been gradually realized.
Ionic Liquids for the Synthesis of Nanostructures
First, ILs were used in electrosynthesis: various metallic
nanoparticles, such as palladium,[16] iridium,[17] and semiconductor nanoparticles such as stable Ge nanoclusters[18]
have been synthesized. The preparation of Ti nanowires onto
graphite by electroreduction as described by Freyland et al.[19]
is exciting but still requires final proof. In all of these
examples the large operation window for electrochemical
reactions and the high polarity of ILs are exploited.
Very fine and stable noble-metal nanoparticles (Ir0 and
0
Ru , 2.0–2.5 nm in diameter) can also be synthesized in ILs by
chemical reduction.[20] The colloidal system metal-nanoparticle/IL-stabilizer is extraordinarily stable and no ligands are
required; extraordinarily high turnover numbers are achieved
with this system in catalytic hydrogenation.
Besides the large electrochemical window, other advantages of ILs can be considered:
* Although polar, ILs can have low interface tensions which
in addition seems to adapt to the other phase (e.g. for
1-butyl-3-methylimidazolium
tetrafluoroborate
the
g 38 mn m 1 against air[21]). Since low interface tensions
result in high nucleation rates, very small particles can be
generated which undergo Ostwald ripening only weakly.
DOI: 10.1002/anie.200460091
Angew. Chem. Int. Ed. 2004, 43, 4988 –4992
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Ionic Liquids
*
*
*
*
Low interface energies for larger objects can be translated
into good stabilization or solvatization of molecular
species. Obviously, the IL structures “adapts” to many
species, as it provides hydrophobic regions and a high
directional polarizability which be oriented parallel or
perpendicular to the dissolved species. Put simply:
reactions in ILs are like reactions in a pure “universal”
ligand.
Owing to the high thermal stability of ILs, reactions can be
conducted at temperatures well beyond 100 8C in nonpressurized vessels.
ILs facilitate inorganic synthesis from very polar starting
materials under ambient conditions and under anhydrous
or water-poor conditions. In this way, hydroxide or
oxihydrate formation and the coupled generation of
amorphous species can be suppressed, as low amounts of
water drive the mass balance to completely condensed
systems, which are usually directly crystalline.
The most important advantage of ILs, however, is an
unconventional and very rare property that cannot be
emphasized sufficiently: ILs form extended hydrogenbond systems in the liquid state[22] and are therefore highly
structured.[23, 24] ILs are therefore “supramolecular” solvents. Solvent structuration is the molecular basis of most
molecular recognition and self-organization processes,
with water being the most prominent and pronounced
example.[25] This special quality can be used as the
“entropic driver” for spontaneous, well-defined, and
extended ordering of nanoscale structures.
3. Sol–Gel Reactions in Water-Poor Ionic Liquids
First work on inorganic sol–gel reactions focussed on the
formation of silica aerogels. It turned out that such aerogels
can be dried without a supercritical drying procedure.[26] This
again speaks for a very low interface tension of the binary
system and coupled low capillary forces. It is, however, even
more interesting to make crystallizable species by sol–gel
reactions in water-poor reaction media. Zhou et al. hydrolyzed titanium tetrachloride in 1-butyl-3-methylimidazolium
tetrafluoroborate with some reaction water (water-poor
conditions) in a low-temperature synthesis (at 80 8C).[27]
Anatase powders consisting of 2–3-nm-sized particles and
with surface areas of 554 m2 g 1 were obtained, which
assembled to larger, spherical spongelike superstructures.
These experiments look simple but they illustrate the multiple
advantages of ILs. First, sol–gel reactions in water usually
provide amorphous titania, which has to be calcined above
350 8C to result in the desired crystalline anatase. This usually
prevents direct employment of anatase in organic/inorganic
hybrid systems. Also, the nucleation rate of titania in the bulk
is rather low (usually particles with diameters of ca. 20 nm are
obtained). The IL solvent therefore not only facilitates direct
synthesis of crystalline species under ambient conditions, it
also increases the nucleation rate by more than a factor of
1000, owing to its low interface energy and adaptability. Only
with this combination is the delicacy of the resulting
structures possible.
The anatase obtained has a spongelike structure with high
surface area and narrow pore-size distribution, and due to its
increased volume it is easy to handle. This material is
expected to have potential in solar energy conversion,
catalysis, and optoelectronic devices, for example, for the
Markus Antonietti studied chemistry in
Mainz, Germany, and received his PhD for
research conducted under the direction of
Prof. Hans Sillescu. He completed his habilitation on microgels in 1990 and joined the
chemistry faculty at the University of Marburg in 1991. Since 1993 he has headed
the Chemistry Department of the Max
Planck Institute of Colloids and Interfaces in
Golm. His research interests are complex,
functional, and self-organizing soft-matter
systems and hybrid materials.
Bernd Smarsly studied chemistry and
physics at the University of Marburg, Germany, and received a Master degree of Natural Science in 1998. He completed his
PhD studies in 2001 at the Max Planck Institute of Colloids and Interfaces, spent one
year as a postdoc at the University of New
Mexico at Albuquerque, USA, and returned
in 2003 to become group leader for “Mesoporous Materials” at the Max Planck Institute of Colloids and Interfaces.
Daibin Kuang studied chemistry at the Normal University in Hunan, China, and received his B.S. in 1998. He then moved to
Zhongshan University in Guangzhou, China,
where he completed his M.S. studies in
2000 and his PhD thesis in 2003. Since
September 2003, he has been a research
scientist in the group led by Dr. Smarsly at
the Max Planck Institute of Colloids and Interfaces.
Young Zhou studied chemistry and physics
at the University of Science and Technology
of China (USCT), received his Master degree in 1996, and finished his PhD thesis
there in 2000. Afterwards he worked with
Professor Chujo at the Kyoto University, Japan. He joined the group of Professor Antonietti at the Max Planck Institute of
Colloids and Interfaces as an Alexander von
Humboldt Fellow from 2001 to 2003. Currently he works with Dr. Takayoshi at the
National Institute of Materials Science
(NIMS) at Tsukuba, Japan, as a Japan Science and Technology Fellow.
Angew. Chem. Int. Ed. 2004, 43, 4988 –4992
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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potential one-step synthesis of dye-sensitized titania solar
cells. IL-based quasi-solid-state electrolytes were just recently
employed for such regenerative photoelectrochemical cells
and yielded 7 % energy efficiency, but nanostructure synthesis
still took place by classical means.[28] In another synthetic
application Nakashima et al. reported the preparation of TiO2
hollow microspheres in ionic liquids by means of a so-called
interfacial sol–gel reaction.[29]
The strong surface binding of ILs onto various nanoparticles was employed by Itoh et al., who showed that
hydrophilic and hydrophobic properties of gold nanoparticles
can be tuned by exchange of anions in the IL moiety.[30]
Backed by the same set of advantages of ILs as a reaction
medium, microwave-assisted synthesis of single-crystalline
tellurium nanorods and nanowires have been recently reported.[31]
Solvent self-structuration and supramolecular effects become important when reactions are conducted with higher
concentrations of inorganic reactant. Even standard ILs such as
the 1-butyl-3-methylimidazolium tetrafluoroborate give nicely
nanostructured gels. The reaction of silica[32] gave a spongelike,
bicontinuous phase with a characteristic length of 5 nm. NMR
and Raman spectrometry indicated the IL molecules spontaneously form a double layer by binding to silica. This sounds
unusual for such a small molecule but just reflects the very
strong tendency of the ILs to form extended hydrogen-bonding
networks, in this case an undulating two-dimensional structure.
The liquid structure of ILs and their mixtures with other
solvents are probably organized in a similar fashion.
The self-organization of ILs can be supported by using
amphiphilic species with a longer hydrophobic tail. Again,
due to a combination of hydrogen-bonding networks and
polarity contrast (amphiphilicity), very well-organized lyotropic phases are obtained for both the pure RTILs as well as
their mixtures with water, oils, and reactants. This tolerance of
self-organization against loading is again very unusual, and
even in water it is found only for some special surfactants that
form microemulsions.
Following the reasoning given above, those phases always
interact strongly with surfaces and usually align perpendicular
(homeotropic) to the substrate surface. This is opposite to
water systems where a parallel orientation is preferred, and
can be explained by the very strong polarizability of the
supramolecular IL structure along the aligned hydrogenbonding networks. It can be assumed that similar orientation
effects also exist in the nonamphiphilic ILs, which might be
the reason that they have extraordinary lubrication properties,[33] beyond that predicted from the molecular structure.
These oriented phases can be employed for material
synthesis by using sol–gel synthesis with 1-hexadecyl-3methylimidazolium chloride (116). Condensation products of
these oriented phases give almost perfect textures as shown in
Figure 1 for a silica made from a lamellar amphiphilic IL
mesophase.[34] The lamellae or sheets are exactly parallel over
wide areas (see the Fourier transform inset), and even at the
surface a perfect homeotropic organization prevails (see
atomic force micrograph (AFM), Figure 1 b). The real
structure is presumably more complex and must contain
pillars or bridges, as the described structure is stable
4990
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. a) TEM image of 116-templated porous silica prepared in a
sol–gel reaction of 1 g of 116 with effective 1 g of silica at a temperature
of 40 8C. The scale bar is 50 nm. Inset: Corresponding 2D Fourier
transform of the picture. b) Corresponding AFM picture of a surface of
the same material.
throughout the removal of the IL and does not collapse.
Such silicas have essentially the structural characteristics of
clay minerals; however, the chemical composition can be
adjusted freely to meet the requirements of the application at
hand.
4. The Effect of Water
The role of extra water in ILs is complex and depends on
the supramolecular structure of the ionic liquid. It seems to be
safe to state that its structure and chemical reactivity is far
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Ionic Liquids
from that of bulk water, as it is tightly bound and activated in
the H-bonding system of the IL.[19] As a result reactions with
water take place quite rapidly in these systems. On the other
hand, water cannot function here as a solvating ligand since it
is too involved in IL binding; this was dedused, for instance,
from the absence of so-called solvent pores.[30] This is a quite
singular situation for colloid chemistry and material synthesis.
Water modifies the patterns of IL self-organization, and
this is why the structural outcome of such reactions depends
strongly on the water content. The peculiar self-aggregation
behavior of the IL/water system is evident by comparing two
sol–gel-derived IL–silica hybrid materials (with 116), prepared
with varying amounts of water (Figure 2), but the same ratio
typical textures coexisted.[35] The resulting supermicroporous
opals were discussed as optical sensor elements where
reflection contrast depends critically on the absorption of
trace amounts of organic molecules.
5. Conclusions and Outlook
We expect that ILs will find, in addition to organometallic
synthesis, catalysis, and electrochemistry, a fourth area of
application—the synthesis of nanostructured solids, either to
make nanoobjects (e.g. particles and fibers) or for the design
of nanopores and nanochannels in solids. The unique
combination of adaptability towards other molecules and
phases plus the strong H-bond-driven solvent structure makes
ionic liquids potential key tools in the preparation of a new
generation of chemical nanostructures.
Received: March 23, 2004
Published Online: August 27, 2004
Figure 2. X-ray diffractograms for two mesostructured IL (116)/silica
materials prepared from precursors solutions with varying water content (s = 2/l sin q); tha ratio of IL to silica is about 1:1. Sample 1: 2D
hexagonal mesostructure obtained with an excess of water. Sample 2:
Lamellar mesostructure obtained with only stoichiometric amounts of
water and keeping the natural H-bonding network intact. A TEM image
of this structure is shown in Figure 1. The inset shows the higher order
reflections of this species, which go up to the 23rd order.
of IL to silica. While sample 2 was obtained under water-poor
conditions, sample 1 was made with a tenfold excess of water.
The X-ray diffraction patterns indicate that this difference in
the water content affects the self-organization behavior.
Sample 1 represents a 2D hexagonal mesophase, while
sample 2 corresponds to a lamellar structure with a long
period of d = 5.6 nm. The latter sample can be indexed up to
the 23rd interference order, an exceptional perfection for selforganized mesostructures on the nanometer scale. Note that
the sample is liquid, as indicated by the typical halo in the
wide angle. This high order is also reflected in the reaction
products made from such phases (see Figure 1) or seen in
polarized optical microscopy image with textures spanning
several millimeters.
These results demonstrate that ILs can also be applied in
water-rich media. Here, they “only” play the role of a classical
surfactant, however, with a very strong tendency towards selforganization with high order. The combination of polymer
latexes and amphiphilic ILs in water-rich media as templates
for porous silica indeed led to bimodal structures where both
Angew. Chem. Int. Ed. 2004, 43, 4988 –4992
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