Download Synthesis of Inorganic Nanostructures in Reverse Micelles

Synthesis of Inorganic Nanostructures in Reverse Micelles
Limin Qi
College of Chemistry and Molecular Engineering, Peking University,
Beijing, P.R. China
INTRODUCTION
There is an immense interest in the synthesis of
inorganic nanostructures, which have at least one
dimension between 1 and 100 nm by definition, owing
to their fascinating properties and tremendous potential for technological applications.[1–4] Because the
mechanical, optical, electrical, magnetic, thermal, chemical, and biomedical properties of inorganic nanomaterials are largely dependent on the size, shape,
composition, and architecture of the nanostructures,
many efforts have been devoted to the manipulation
of these factors. In this regard, different approaches,
ranging from physical to chemical methods, have been
exploited for the controlled fabrication of well-defined
inorganic nanostructures. Increasingly, colloidal chemists are contributing to the biomimetic synthesis of
inorganic nanostructures with dimensional, morphological, and architectural specificity by using organized
self-assemblies of surfactants as nanostructured reaction media or templates. This sophisticated approach
is largely inspired by biomineralization, where organized aggregates of biomacromolecules exert exquisite
control over the nucleation, growth, and patterning
of inorganic minerals. In particular, as a typical membrane mimetic system,[5] reverse micelles have been
widely used as nanostructured reaction media or nanoreactors for the biomimetic synthesis of various inorganic
nanostructures. This article gives an overview of the
controlled synthesis of inorganic nanostructures in
reverse micelles, with its attention focused on the latest
advances in the synthesis mechanism, processing method,
and control of the morphology and architecture of
nanostructures. After a general discussion on the reverse
micelle-based synthesis technique, the development in
the synthesis of spherical nanoparticles, nonspherical
nanostructures and their assemblies, and core–shell
nanostructures is summarized.
REVERSE MICELLES AS NANOREACTORS
Surfactants normally contain a hydrophilic head and a
hydrophobic chain, and these amphiphilic molecules
can self-assemble into a rich variety of organized
structures in solution, such as normal and reverse
micelles, microemulsions, vesicles, and lyotropic liquid
crystals.[5] Specifically, reverse micelles are globular
aggregates formed by the self-assembly of surfactants
in apolar solvents, whereas normal micelles are globular
aggregates formed by the self-assembly of surfactants in
water.[6] The structure of reverse micelles is characterized
by a polar core formed by the hydrophilic heads and an
apolar shell constituted by the hydrophobic chains.
Water can be readily solubilized in the polar core to form
water-in-oil (w/o) droplets, which are usually called
reverse micelles at low water content and w/o microemulsions at high water content. Traditionally, microemulsions are defined as thermodynamically stable,
optically transparent dispersions of two immiscible
liquids stabilized by an interfacial surfactant film.[7,8]
The w/o microemulsions normally consist of water
droplets ranging from 5 to 100 nm in size, slightly larger
than reverse micelles, which are typically smaller than
5 nm.[9,10] Although there are several arguments to
strongly support a differentiation between reverse
micelles and w/o microemulsions, the demarcation
line is quite blurred and there is often no clear distinction
between these two systems in the literature related to
nanoparticle synthesis in nanostructured media.[10,11]
Accordingly, in this entry, no such distinction is made
and all systems consisting of nanosized water droplets
stabilized by surfactants and dispersed in oil are termed
as either reverse micelles or w/o microemulsions, regardless of the relative component proportions. Practically,
a reverse micelle solution is often called a w/o microemulsion and a reverse micelle may correspond to a
microemulsion droplet.
The droplet size of reverse micelles can be readily
modulated in the nanometer range by various parameters, in particular, the water–surfactant molar ratio,
W [W ¼ (water)/(surfactant)]. On the other hand,
the droplets are kinetically unstable and a dynamic
exchange process occurs between colliding droplets.
These unique properties together with the specific
adsorption of surfactants on inorganic materials enable
the use of reverse micelles as effective nanoreactors for
the controlled synthesis of inorganic nanostructures
with tailored size, shape, composition, and architecture.
Encyclopedia of Surface and Colloid Science DOI: 10.1081/E-ESCS-120023694
Copyright # 2006 by Taylor & Francis. All rights reserved.
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The synthesis of inorganic nanoparticles in reverse
micelle systems was first demonstrated for monodisperse
metal particles (3–5 nm) by Boutonnet et al.,[12] in
the early 1980s. Since then, reverse micelle- or microemulsion-based synthesis of inorganic nanostructures
has expanded dramatically, and quite a few reviews
dealing with various aspects of the technique are
available.[3,13–28]
Nanoparticle Synthesis in
Reverse Micelles
For a feasible nanoparticle synthesis in reverse micelles,
water–surfactant–oil formulations that give stable microemulsions must be identified. Phase diagrams already
available in the literature for the ternary mixtures of
surfactant–oil–water or the quaternary mixtures of
water–surfactant–cosurfactant–oil are useful for this
purpose. However, the effects of the reactants and products on the domain of stable w/o microemulsions
should be taken into account, which sometimes severely
limits the highest reactant concentrations that can be
used for precipitation reactions. Among the various
surfactants exploited in formulating microemulsions
for nanoparticle synthesis, the anionic double-chained
surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT)
and nonionic polyoxyethylated surfactants [e.g., poly
(ethylene oxide)-(5)-nonylphenyl ether, (NP-5)] appear
to be the most popular. The attraction of AOT and the
nonionic surfactants is partly attributed to the fact that
they can form reverse micelles without the need for
cosurfactants. In addition, irrespective of the nature of
the apolar solvent, the nearly spherical and monodisperse water-containing reverse micelles are formed
by the AOT, the size of which is quite independent of
the surfactant concentration and regulated mainly by
the W value, e.g., at W > 10, the droplet radius, Rd,
can be directly obtained from the W value by the linear
relationship Rd/nm ¼ 0.17 W.[6,28] Generally, the size
of reverse micelles increases with increasing the W value;
however, the size and shape of reverse micelles and their
dependence on the water and surfactant concentrations
are actually system specific. A unique advantage of the
nonionic surfactants is that their use does not involve
the introduction of potentially undesirable counterions;
moreover, the ability to alter the size of the hydrophilic
oxyethylene groups and the hydrophobic alkyl groups
provides additional flexibility in surfactant selections.
In general, a number of factors should be considered
carefully when selecting a suitable surfactant for a
specific synthesis reaction in reverse micelles. For example, the surfactant should be chemically inert with
respect to all other components of the microemulsion,
and the possible interference of the counterions of
ionic surfactants should be taken into consideration.
Synthesis of Inorganic Nanostructures in Reverse Micelles
Notably, in some cases, functionalized surfactants, the
counterions of which serve as one of the reactants,
may be employed for a desirable control of the nanoparticle synthesis in reverse micelles. To illustrate,
Ag bis(2-ethylhexyl) sulfosuccinate, Ag(AOT), can be
used for the preparation of metallic Ag and semiconductor Ag2S nanoparticles in the Ag(AOT) reverse
micelles.
The synthesis of inorganic nanoparticles in reverse
micelles has been achieved mainly by mixing two
identical reverse micelle solutions (microemulsions)
containing the appropriate hydrophilic reactants, i.e.,
a two-microemulsion method. The schematic picture
shown in Fig. 1 illustrates the formation of the nanoparticles of the product (P) from the precipitation
reaction between the two reactants A and B solubilized, respectively, in the water cores of two microemulsions. Upon mixing, the droplets collide, coalesce
to form transient droplet dimers, exchange the water
contents, and disintegrate into droplets, continuously.
Fig. 1 Schematic representation of nanoparticle synthesis in
reverse micelles by mixing two reverse micelle solutions
(microemulsions).
Synthesis of Inorganic Nanostructures in Reverse Micelles
Then the precipitation reaction takes place inside the
nanodroplets, which is followed by nucleation, growth,
ripening, and coagulation of primary particles, resulting
in the formation of the final nanoparticles surrounded
by water and/or stabilized by surfactants. In addition
to the most commonly used two-microemulsion method,
a single-microemulsion method that includes a number
of variations has also been frequently used for the
reverse micelle-based nanoparticle synthesis. For example, one of the reactants (e.g., precipitating or reducing
agent, and alkoxide) can be added directly, in the
form of a solution or a solid, liquid, or gas, to the microemulsion carrying the other reactant. Alternatively, a
microemulsion containing all the reactants can be activated by a trigger (e.g., pulse radiolysis, laser photolysis,
ultrasonication, and temperature elevation) to initiate
the reactions that eventually lead to the nanoparticle
formation. It should be pointed out that, in addition to
the size of the reverse micelles, the dynamic nature of
the reverse micelles plays a key role in the formation
of the final nanoparticles for both the two- and singlemicroemulsion methods.
Because the exchange of aqueous contents between
the microemulsion droplets or the intermicellar material
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exchange is closely related to the formation process of
nanoparticles in reverse micelles, it is necessary to consider how the intermicellar exchange influences various
aspects of the nanoparticle formation (Fig. 2).
It is generally accepted that the water contents of
microemulsion droplets are exchanged rapidly (on a
microsecond scale) through droplet collision, fusion,
and dimer fission, with the fusion step as the ratedetermining step. The exchange rate can be characterized
by a second-order, intermicellar exchange rate constant,
kex, expressed in terms of the droplet concentration in the
continuous oil medium while the fission rate of the fused
dimers can be characterized by a first-order, fission rate
constant, kfiss. Around room temperature, kex is generally
between 106 and 108 dm3/mol/s for the AOT reverse
micelles, while it is between 108 and 109 dm3/mol/s
for the reverse micelles formed by nonionic surfactants
of the poly(ethylene oxide) (PEO) monoalkyl ether (CiEj)
type.[19] On the other hand, the droplet encounter rate
constant, ken, is approximately 1010 dm3/mol/s at room
temperature for microemulsion droplets dispersed in a
typical oil like n-heptane, which indicates that only
0.01–1% of binary droplet collisions result in droplet
fusion and intermicellar exchange for the AOT reverse
Fig. 2 Mechanisms of nanoparticle formation in reverse micelles via intermicellar exchange.
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micelles, which correspond to relatively rigid droplets.
Generally, chemical reactions of the hydrophilic reactants within the nanodroplets are very fast with a
reactant rate constant, kr, much larger than kex, indicating that the intermicellar material exchange is the
rate-determining step in the overall reaction leading to
nanoparticle formation. It may be pointed out that many
ideas based on bulk nucleation and growth models may
not be valid any more for the particle formation in these
dynamic compartmentalized media. However, if the
involved reaction is very slow (e.g., the hydrolysis of
tetraethoxysilane to produce silica), the intermicellar
exchange does not play an important role in the particle
formation, and the reaction proceeds in the same way
as it does in the bulk. In this case, the most simple
bulk ideas of nucleation and growth can be applied to
explain the experimental results by using a pseudophase
model.[29]
Autocatalysis and ripening are two typical ways of
nanoparticle growth in microemulsion droplets, which
also manifest the considerable effects of intermicellar
exchange.[25] As the reaction takes place in reverse
micelles, more and more droplets could contain products and reactants simultaneously. The reaction inside
nanodroplets can be catalyzed by the surface of an
existing product particle, and a large particle has a
greater probability of acting as a catalyst owing to its
large surface. Therefore, if the reaction is autocatalytic,
the exchange of reactants between the two colliding
droplets containing a product particle would result in
the reaction occurring on the existing particle or the
growth of the particle.
The Ostwald ripening is another typical growth
process of nanoparticles formed in reverse micelles.
The ripening theory assumes that the larger particles
will grow by condensation of material, coming from
the smaller particles that solubilize more than the large
ones. In microemulsions, this growth may take place
through the fused dimers, and a small particle formed
inside a droplet can be transferred to another one
carrying a larger particle if the size of the channel
connecting the two coalescing droplets is larger than
the small particle. This indicates that the surfactantfilm flexibility plays a fundamental role in the ripening
contribution to the growth process because the interdroplet exchange of the growing particles is inhibited
by the inversion of the film curvature in the fused
dimers. It is interesting to point out that there is an
important difference between the autocatalysis and
ripening mechanisms: the autocatalysis depends on
the reactant exchange whereas the ripening depends
on the product exchange.
Once the nanoparticles attain their final size, they
could be located within the water cores of the droplets
that are stabilized by a surfactant film. However, in
most cases, the surfactant molecules may be attached
Synthesis of Inorganic Nanostructures in Reverse Micelles
directly to the particle surface, thus stabilizing the
nanoparticles and protecting them from further growth.
In this case, the whole surfactant film can be adsorbed
onto the particle by an ‘‘implosion’’ mechanism when
a droplet containing a particle with a comparable size
coalesces with an empty droplet. This stabilization
mechanism is favored when the surfactant film is
relatively rigid. For a highly flexible surfactant film, the
particle may approach the droplet interface and some of
the surfactant molecules may be adsorbed onto the
particle leaving the droplet, which is followed by an
inversion of the film curvature and the adsorption of a
whole surfactant film onto the particle. When the surfactant film is very flexible, the film is able to adjust to the
particle size during the particle growth, possibly leading
to the formation of particles much larger than the
droplet size.
Finally, it must be emphasized that under specific
conditions, the inorganic particles formed inside reverse
micelles may undergo further growth or aggregation to
form final particles that are significantly larger than their
parent nanodroplets. In particular, inorganic nanowires
with lengths up to tens of micrometers can be produced
in reverse micelle solutions because of the complex interplay between the dynamic nature of the parent micelles,
the growth habit of the inorganic materials, and the
specific interfacial organic–inorganic interactions, which
will be discussed in more detail in the part on the
synthesis of nonspherical and complex nanostructures.
Control of Nanoparticle Size
In most cases, spherical nanoparticles are obtained in
reverse micelles where the surfactant-stabilized water
nanodroplets play the role of nanoreactors. It was
initially assumed that the nanodroplets could be used
as templates to control the final size of the particles
obtained in reverse micelles. However, later research
has shown that there is no direct correlation between
the droplet size and the particle size, although the
droplet size does seem to have a great influence on
the final particle size in many cases. Nevertheless, the
combination of the droplet size with other parameters
including intermicellar exchange rate, surfactant film
flexibility, and reactant concentration can exert a
delicate control over the final particle size.
Basically, there are two different mechanisms for
the particle-size control in the microemulsion-based
synthesis, i.e., the microemulsion-controlled mechanism and the surfactant-controlled mechanism. In the
microemulsion-controlled mechanism, the particle size
varies as a function of various microemulsion parameters, such as the droplet size, intermicellar exchange
rate, and reactant concentration. On the contrary,
in the surfactant-controlled mechanism, the particle
Synthesis of Inorganic Nanostructures in Reverse Micelles
1) Droplet size: In principle, the particle size
increases with the droplet size. As the droplet size is
approximately proportional to the water-to-surfactant
ratio, W, for most reverse micelles, a nearly linear
increase of the particle size with W has been observed
for the microemulsion-based synthesis of a variety of
inorganic nanoparticles. As an example, in Fig. 3, the
variation of the size of Cu nanoparticles obtained in
the AOT reverse micelles at various water contents
has been shown.[14] It suggests an increase in the
particle size on increasing the ratio, W, from 1 to 10
or on increasing the droplet size, approximately, from
0.6 to 3 nm, i.e., the particle size varies from 2 to 13 nm
when iso-octane is used as the bulk solvent, whereas
the particle size varies from 1 to 7 nm when cyclohexane is used as the bulk solvent. It is worth noting that
on increasing the water content, the particle size
reaches a plateau at a certain water content, e.g.,
at W ¼ 10 for the iso-octane system. This has been
originally explained in terms of the water structure in
the droplet, as an increase in the water content induces
a gradual change from ‘‘bound’’ to ‘‘free’’ water in the
droplets. Recently, a plausible explanation based on
the transition from the microemulsion-controlled
particle size to the surfactant-controlled particle size
14
Average Diameter (nm)
size is controlled by the adsorption or capping of
surfactants on the particles inside the microemulsion
droplets, and it is independent of the key microemulsion parameters. The size control by surfactant
adsorption has been mostly observed in the preparation of metal catalyst nanoparticles because of the
favored adsorption of surfactants on catalysts. For
example, the first report on the microemulsion-based
nanoparticle synthesis has revealed that the particle
size of Pt, Rh, Pd, and Ir remains almost constant
(3–5 nm), independent of the water amount and reaction concentration.[12] In this case, the particles are
stabilized by the surfactants, and the role of microemulsions in the particle size control does not seem
obvious because ligands or capping agents may be used
directly in bulk solution to inhibit the particle growth
and obtain similar results. Therefore, the following
discussion will be focused on the microemulsioncontrolled mechanism for the particle-size control in
the microemulsion-based technique.
The microemulsion-controlled particle size and the
particle size distribution (PSD) depend on the particular combination of all the relevant microemulsion
parameters, and the effects of various factors on the
final particle size are actually system specific. However,
according to a range of experimental results, the effects
of several key parameters on the particle size can
be generally summarized, especially for the spherical
nanoparticle synthesis based on the simple precipitation
or reduction reaction occurring within water droplets:
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12
10
8
6
4
2
0
0
2
4
6
8
10
12
14
16
W
Fig. 3 Variation of the size of Cu nanoparticle synthesized
in AOT reverse micelles formed by different bulk solvents
with the W value. Bulk solvent: () iso-octane, (G) cyclohexane. (From Ref.[14].)
with increasing droplet size has been proposed for
the tendency to reach a plateau.[25]
2) Surfactant film flexibility: Generally, the particle
size increases with the surfactant film flexibility. In
reverse micelles, the intermicellar exchange process is
governed by the dimer stability, which depends on
the interdroplet attractive interaction, and by the
surfactant film flexibility, which depends on the type
of surfactant, cosurfactant, and oil used as well as on
the droplet size. In principle, the film flexibility can
be increased by several means: increasing oil molecular
weight, increasing the cosurfactant concentration,
approaching the instability boundary of microemulsions, etc. Particularly, in the water–AOT–iso-octane
reverse micelles, the replacement of iso-octane by
cyclohexane induces a considerable decrease in the film
flexibility, resulting in a decrease in kex by one order of
magnitude. Therefore, at a fixed water content, the size
of the Cu nanoparticles is smaller when the bulk
solvent is cyclohexane instead of iso-octane (Fig. 3).
However, the overall influence of kex on the particle
size is dependent on the relative proportion of different
growth mechanisms (e.g., autocatalysis and ripening),
which is relevant to the film flexibility, and on the
reactant nature and concentration.
3) Reactant concentration: It has been found that
both absolute reactant concentrations and relative
reactant concentrations show a considerable influence
on the final particle-size distribution. First, the particle
size increases with reactant concentration for a flexible surfactant film, and the size distribution changes
from unimodal to bimodal for a rigid film. Second,
the particle size decreases as the excess of one of
the reactants increases, until a plateau is reached at
high excesses. This phenomenon has been previously
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explained assuming that the reactant excess implies
a faster nucleation, resulting in smaller particles.
However, recent simulation results have shown that
the reactant excess hardly influences the nucleation
process but greatly influences the growth mechanism,
i.e., the possibility of ripening growth increases with
increasing reactant excess.[25] It indicates that bulk
nucleation and growth models might be invalid for
the current dynamic compartmentalized media.
Because many parameters have different influences
on the final particles size and PSD, and it is experimentally difficult to fix all the other parameters to
study the effect of a given parameter; various computer
simulations have been performed recently to gain more
insight into the problem. In particular, the experimental results listed above correlate rather well with the
Monte Carlo simulations carried out by the López–
Quintela’s group, under the assumption that the
synthesis is performed by mixing two identical microemulsions carrying the reactants, and that the chemical
reaction occurring in the nanodroplets is much faster
than the interdroplet material exchange.[24,25] The
simulations took the following parameters into account:
the film flexibility parameter f, the reactant exchange
parameter k, the reactant concentration, the reactant
excess ratio, the droplet size, the droplet volume fraction,
and critical nucleus. In addition, the possibility of
autocatalysis and the possibility of ripening were considered in the simulations. It should be pointed out that
there have been various modeling efforts that used
different combinations of assumptions regarding initial
reactant distribution, intermicellar exchange rules, and
the finiteness of the nucleation/reaction process. There
are recent efforts to organize all the possible scenarios
into a coherent, unified framework by the use of a fairly
general model.[30]
Developments in Processing Method
It has been shown that reverse micelles can be used as
ideal nanoreactors for the size-controlled synthesis of
nanoparticles of a wide range of inorganic materials
under rather mild conditions. However, the separation
of the product nanoparticles from the microemulsion
constituents, as well as the recovery and recycling
of the organic solvents remains a challenge. Carbon
dioxide is an attractive alternative to organic solvents
because it is nontoxic, nonflammable, inexpensive,
and environmentally benign, and it has low viscosity,
high diffusion coefficient, as well as tunable solvent
properties (e.g., density, compressibility, diffusivities,
and dielectric properties). Water-in-CO2 (w/c) microemulsions can be formed in supercritical (SC) CO2 by
employing specially designed surfactants that contain
a ‘‘CO2-philic’’ fluorinated tail. As seen with many
Synthesis of Inorganic Nanostructures in Reverse Micelles
w/o microemulsions, certain w/c microemulsions
exhibit a spherical droplet structure with a droplet size
directly proportional to the W value. Similarly, reverse
micelles in CO2 may serve as nanoreactors for the
nanoparticle preparation. A major advantage with
w/c microemulsions is the possibility of breaking the
microemulsion by simply controlling the temperature
and pressure of the system, leading to the direct separation of nanoparticles. In addition, the deposition of
nanoparticles can be accomplished in situ on porous
materials, by utilizing the low viscosity and high diffusivities of SC CO2 continuous phase. Moreover, the
solvent properties of SC CO2 can be regulated by the
adjusting temperature and pressure, thereby allowing
for the regulation of the surfactant film flexibility,
intermicellar exchange rate, and consequently, the
final particle size. Therefore, w/c microemulsions have
shown tremendous implications for industrial applications, as an effective reaction media for the controlled
synthesis of inorganic nanoparticles.[31–33]
Johnston and coworkers[34] first reported the
synthesis of CdS nanoparticles in w/c microemulsions
formed by an ammonium carboxylate perfluoropolyether (PFPE–COONH4) surfactant and found that the
particle size increased with the increasing W value. Subsequently, the PFPE–COONH4-based reverse micelles
in SC CO2 were used for the preparation of TiO2[33,35]
and Ag[32,36] nanoparticles. Similarly, Wai and coworkers prepared Ag,[37,38] Cu,[38] Pd,[39] Rh,[40] and
silver halide[41] nanoparticles in w/c microemulsions
formed by a surfactant mixture of AOT and perfluoropolyether-phosphate ether (PFPE–PO4), and they
demonstrated that Pd and Rh nanoparticles produced
in w/c microemulsions can be used as promising catalysts for the hydrogenation reaction in SC CO2.[39,40]
In addition, they have used w/c microemulsions formed
by a fluorinated AOT to release the controlled synthesis
of CdS and ZnS nanoparticles in SC CO2.[42] After the
preparation of nanoparticles in w/c microemulsions,
their removal from the high-pressure reactor is essential.
For the collection of nanoparticles prepared in w/c
microemulsions, there are two basic methods reported
so far, i.e., a reducing pressure method[34] and a rapid
expansion method.[37] However, the collected nanoparticles are tightly aggregated in many cases, and so,
new methods for better collection of nanoparticles with
minimum agglomeration will be explored.
For the traditional microemulsion technique, it is
still a challenge to separate the produced nanoparticles
and deposit them on a solid support. One common
procedure to prepare supported catalysts is to add a
solvent such as tetrahydrofuran (THF), which dissolves
the surfactant and is miscible with both oil and water,
to the microemulsion, thereby resulting in the destabilization of the microemulsion and sedimentation of
the particles. Another commonly used method for the
Synthesis of Inorganic Nanostructures in Reverse Micelles
processing of nanoparticles synthesized in reverse
micelles, such as CdS nanoparticles, is direct recovery
and immobilization by using thiol-modified supports
via chemical bonding.[43] Yet, another deposition
method involves the centrifugation of Pt nanoparticles
followed by redispersion in water by a stabilizing
surfactant, and the addition of alumina support.[44]
A new method has been developed for the deposition
of a large amount of metal particles onto suitable
supports, which is of great interest for industrial application.[45] This method consists of spraying the microemulsion solution containing Au nanoparticles into an
air/acetylene flame, which allows for the preservation
of the original structures of the nanoparticles. More
recently, w/o microemulsions have been used for the
preparation of oxide nanoparticles and their noncovalent attachment onto carbon nanotubes, which is
important for preserving the mechanical and electrical
properties of carbon nanotubes.[46]
It is worth noting that compressed CO2 has been
successfully used as an antisolvent to recover ZnS[47]
and Ag[48] nanoparticles from AOT-based reverse
micelles, leading to the precipitation of well-dispersed
nanoparticles with the surfactant remaining in the
solution. Recently, Au nanoparticles were prepared
in a CO2-induced w/o microemulsion, and the recovery of Au particles was easily accomplished by the
venting of CO2, while the poly(ethylene oxide)-poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–
PEO) block copolymer surfactant remained in the
organic phase.[49] A novel method was recently
reported for the recovery of Ag nanoparticles from
reverse micelles of PEO–PPO–PEO surfactants in
xylene by simply breaking the reverse micelles with
an increase in temperature.[50]
Another development in the microemulsion technique is the microemulsion-mediated hydrothermal/
solvothermal synthesis that involves hydrothermal/
solvothermal treatment of microemulsions at temperatures higher than 100 C. Gan et al.[51,52] first prepared
Mn-, Cu-, and Eu-doped ZnS nanocrystallites in
nonionic w/o microemulsions under hydrothermal
treatment. It was shown that the hydrothermal treatment considerably enhanced the photoluminescence
of the doped ZnS nanoparticles, which was attributed
to the increased crystallinity, surface passivation of
nanocrystals, and forced migration of the impurity
species into the host lattice of ZnS. On the other hand,
nanoparticles of both anatase and rutile types of TiO2
were prepared by hydrothermal treatment of nonionic
microemulsions at 120 C.[53] It had some advantages
over the room-temperature microemulsion technique,
in which amorphous TiO2 particles are usually yielded,
and additional calcinations at much higher temperatures are required to produce crystalline TiO2 particles.
Subsequent investigation suggested that the reaction
6189
temperature (120 C) was much above the existence
region of the starting microemulsion; hence, the microemulsion could not work as a template during the
hydrothermal treatment, although it could work as an
initial nanoreactor during the mixing step.[54] Recently,
nanosized CdS-sensitized TiO2 crystalline photocatalyst, which exhibited high efficiency for the decomposition of methylene blue, was successfully prepared
by a microemulsion-mediated solvothermal method.[55]
Furthermore, it has been demonstrated that under
specific conditions, microemulsion-mediated hydrothermal synthesis can favor the one-dimensional (1-D)
growth of inorganic nanocrystals in microemulsion
media. For example, under hydrothermal treatment,
CdS[56] and BaF2[57] nanorods were fabricated in microemulsions formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB), whereas SnO2[58]
nanorods were produced in microemulsions formed
by the anionic surfactant sodium dodecyl sulfate (SDS).
SYNTHESIS OF SPHERICAL NANOPARTICLES
It is generally accepted that reverse micelles are good
nanoreactors for obtaining spherical nanoparticles.
Until now, the overwhelming majority of literature
concerning microemulsion-based synthesis reports the
preparation of spherical nanoparticles of a variety
of inorganic materials, such as metals, chalcogenides,
oxides, and other precipitates.
Metal and Metal Chalcogenide Nanoparticles
Many of the earliest microemulsion-based nanoparticle
syntheses reported in the literature involved the preparation of metal nanoparticles and metal chalcogenide
semiconductor nanoparticles. One reason may be that
metal and semiconductor nanoparticles, often called
quantum dots (QDs) owing to the quantum size effects,
exhibit novel optical, electronic, magnetic, and chemical properties, and great potential for promising applications.[20] Another reason seems to be that metals and
metal chalcogenides can be easily produced in reverse
micelles by the simple reduction and coprecipitation
reactions occurring in aqueous solution, respectively.
Since the synthesis of metal nanoparticles in microemulsions was first reported by Boutonnet et al.[12] in
the early 1980s, the microemulsion technique has been
successfully used for the preparation of a number of
metal nanoparticles, including Pt, Rh, Pd, Ir, Au, Ag,
Cu, Fe, Co, Ni, and Bi nanoparticles as well as their
alloy nanoparticles, mainly because of the potential
application of the products as catalysts.[1,26,27] For
the reduction reaction occurring in reverse micelles,
the reducing agent must be stable in an aqueous
6190
environment and not react with the other components
of the reverse micelle system. Specifically, both CTAB
and AOT are stable against hydrogen, but stronger
reducing agents such as borohydride ions can be problematic. As bubbled H2 gas results in slow reduction
for many metals, particularly at room temperature,
NaBH4 and hydrazine are most commonly employed
in metal nanoparticle synthesis. The reduction reaction
for most metals is usually straightforward, although
metal borides (e.g., Ni2B and Co2B) may result from
the reduction of metal ions with NaBH4 sometimes.[59]
The preparation of metal nanoparticles is usually
implemented by mixing two microemulsions with one
containing metal ions and the other containing the
reducing agent. Similarly, nanoparticles of substitutional alloys can be prepared if different metal ions
are dissolved in one reverse micelle solution prior to
mixing, provided the two metals are miscible in the
metallic state.
Originally, stable dispersions of monodisperse Pt
nanoparticles (3 nm) were produced by reducing H2PtCl6
with hydrazine in a w/o microemulsion consisting
of penta(ethylene oxide) dodecyl ether (C12E5)-hexanewater.[12] By a similar method, Pt nanoparticles also were
prepared in a microemulsion formed by C12E4, and the
evolution of their optical properties with reaction time
was related to the mechanism of the formation and
stabilization of Pt particles.[60] Recently, in situ x-ray
absorption spectroscopy (XAS) was successfully applied
to explore the formation of Pt nanoparticles at the early
stages within AOT reverse micelles, which confirmed the
Pt4þ ! Pt2þ ! Pt0 reduction sequence and provided
a detailed insight into the mechanism of the nucleation
and growth of Pt particles.[61] Similarly, Ag nanoparticles
were synthesized by reducing AgNO3 with NaBH4 in
reverse micelles,[62] whereas bimetallic Au/Pt[63] and
Pt/Ru[64] alloy nanoparticles were produced by reducing
appropriate mixed metal ions with N2H4. Notably, Bi
nanocrystallites were fabricated by reducing BiOClO4
with NaBH4 in AOT reverse micelles under the Ar protection.[65] In addition to the normal metals salts, functional surfactants, such as Cu(AOT)2 and Ag(AOT),
were introduced as metal precursors for the synthesis
of Cu[66] and Ag[67] nanoparticles in AOT reverse
micelles, to avoid any potential interference from the
counterions. It has been shown that water content, bulk
solvent, droplet concentration, and the addition of cosurfactants largely influence the Cu nanoparticle formation
from Cu(AOT)2 in reverse micelles.[68,69] However, in the
case of Co nanoparticle synthesis from the Co(AOT)2
reverse micelles, the amount of the reducing agent
NaBH4 was one of the key parameters in controlling
the nanoparticle-size distribution, and Co nanoparticles
with high monodispersity could be obtained when there
was a high volume of reducing agent and the reverse
micelles were destroyed.[70]
Synthesis of Inorganic Nanostructures in Reverse Micelles
As a typical semiconductor material exhibiting an
absorption spectrum in the visible range, CdS is the
first chalcogenide semiconductor prepared in reverse
micelles;[71,72] subsequently, nanoparticles of a variety
of chalcogenide semiconductors including metal sulfides, selenides, and tellurides have been reported. For
the preparation of CdS nanoparticles, Cd2þ ions can
be supplied from normal cadmium salts or functional
surfactants like Cd(AOT)2[73] whereas S2 ions can
be supplied from Na2S or H2S. In general, it is desirable to deoxygenate by bubbling with N2 through
reverse micelle solutions containing aqueous reactants
to avoid colloidal sulfur precipitation. In most cases,
AOT reverse micelles are employed for the CdS nanoparticle preparation; however, it was observed that in a
quaternary CTAB microemulsion, the presence of cosurfactant (n-pentanol) allowed a facile modulation of the
CdS particle size, size distribution, and crystallinity.[74]
Recently, an extreme sensitivity of the CdS nanoparticle
formation in reverse micelles to the preparation procedure was observed, e.g., a seemly trivial change in the
solution volume resulted in substantial changes in the
crystal structure and optical absorption of the CdS
nanoparticles.[75] In a similar way, nanoparticles of other
metal sulfides (e.g., ZnS, PbS, and Ag2S) and mixed
sulfides (e.g., Cd1yZnyS and Cd1yMnyS) can also be
synthesized.[14] It is interesting to note that ultra-small
ZnS nanoparticles (several angstroms) with a high
nanoparticle concentration were synthesized recently by
a novel solid–solid reaction between Na2S and ZnSO4
confined in anhydrous AOT reverse micelles.[76]
Compared with the synthesis of sulfide nanoparticles,
the synthesis of metal selenide/telluride nanoparticles
generally requires more rigorous preparation conditions
because the selenium/tellurium precursors are usually
highly oxidizable or water-sensitive. For example, CdSe
nanoparticles were prepared by adding a solution of
[bis(trimethylsilyl) selenium (Me3Si)2Se] in heptane to a
degassed, AOT reverse micelle solution containing
Cd2þ ions,[77] whereas MoSe2 and WSe2 nanoparticles
were synthesized by adding H2Se to a degassed, anhydrous reverse micelle solution containing MoCl4 or
WCl4.[78] By mixing the two degassed AOT reverse
micelles containing, respectively, Na2Te and Cd(AOT)2,
CdTe nanoparticles were produced;[79] on the other
hand, Bi2Te3 nanoparticles were fabricated by adding
a hexane solution of (Me3Si)2Te under N2 to a AOT
reverse micelles containing BiOClO4.[80]
In recent years, the self-organization of colloidal
nanoparticles into ordered nanoparticle self-assemblies,
such as two-dimensional (2-D) and three-dimensional
(3-D) nanoparticle superlattices, has attracted an
intense interest because self-assembled nanoparticles
combine the unique properties of individual nanoparticles with collective properties owing to interactions
between the particles.[81] The microemulsion technique
Synthesis of Inorganic Nanostructures in Reverse Micelles
has shown great potential for obtaining 2-D and 3-D
nanoparticle superlattices as a controlled PSD is
essential for the ordered self-assembly of nanoparticles,
and reverse micelles provide effective nanoreactors for
monodisperse nanoparticles. In particular, Pileni[81]
has prepared a variety of 2-D and 3-D superlattices
from spherical metal and semiconductor nanocrystals
produced in AOT reverse micelles. In all the cases, a
capping agent like thiododecane is introduced to the
reverse micelle solution containing the nanoparticles
to passivate the particle surface and extract the particles
from reverse micelles. A size-selected precipitation
process is usually employed to further decrease the
polydispersity, and hence favor the self-organization
of the nanoparticles into well-defined superlattices. In
addition, the deposition procedure, nanoparticle concentration, substrate, and alkyl chain length of the
capping agent play key roles in the formation of nanoparticle superlattices. The first 2-D and 3-D superlattices
were observed with Ag2S nanoparticles extracted from
AOT reverse micelles by thiododecane addition.[82]
Fig. 4 shows typical TEM images of the 3-D superlattices with a fcc structure, which consisted of highly
monodisperse Ag2S nanoparticles (14% size distribution)
of different sizes. Formation of 2-D and 3-D superlattices of Ag metal nanoparticles has been achieved by
the same technique.[83] For the formation of large-scale
3-D superlattices of Co nanoparticles, the nanoparticles
with a diameter of 8 nm were coated with lauric acid and
deposited at an applied magnetic field.[84] Meanwhile,
Martin et al.[85,86] have investigated the formation of
2-D and 3-D superlattices of Au and Pt nanoparticles
synthesized in anhydrous, nonionic reverse micelles
and capped with alkanethiols, and found that the gap
between the particles in the coherent domains increased
with the increasing thiol chain length.
Silica and Metal Oxide Nanoparticles
Metal oxides including silica represent a large class
of inorganic materials that find many scientific and
technological applications. Nanoparticles of a wide
variety of metal oxides, such as silica, titania, zirconia,
iron oxide, and complex oxides, have been synthesized
in reverse micelles through two basic reaction routes,
i.e., precipitation of metal salts and hydrolysis of metal
alkoxides.[18,21] In the first route, precipitation of
hydroxides is typically induced by adding aqueous
alkaline solution (e.g., NH4OH) or a microemulsion
containing the alkaline precipitating agent to a
microemulsion containing metal ions, which is usually
followed by centrifugation and heating to remove
water and/or improve crystallinity. This technique
can be extended to complex oxide systems by the
simultaneous coprecipitation of several metal ions
6191
Fig. 4 Transmission electron microscopy (TEM) images of
an island of Ag2S in a f100g plane of a close-packed facecentered cubic (fcc) structure made of 3 nm (A, B); 4 nm
(C, D); and 5.8 nm (E, F) particles. (From Ref.[14].)
in water nanodroplets. Alternatively, precursor nanoparticles can be prepared first by coprecipitation of
several metal ions in water nanodroplets with a certain
precipitating agent (e.g., oxalic acid), which are decomposed to the desired oxide phase by subsequent calcinations, although this will inevitably result in some
degree of agglomeration. In the second route, an oilsoluble metal alkoxide, M(OR)n, or an anhydrous
reverse micelle solution containing the alkoxide in
the oil phase is added to a water-containing reverse
micelle solution, which results in the hydrolysis and
condensation of the alkoxide producing nanoparticles
of metal hydroxides and oxides. Similarly, complex
metal oxides can be synthesized by the simultaneous
hydrolysis of several different metal alkoxides. In most
cases, the products obtained by alkoxide hydrolysis in
reverse micelles are amorphous or poorly crystallized,
and subsequent calcinations are necessary for obtaining
6192
the desired crystalline nanoparticles of metal oxides
except for silica.
Silica nanoparticles can be prepared in microemulsions by the acidification of aqueous silicate solutions;
however, the available reports on microemulsionmediated synthesis of silica nanoparticles are based
almost exclusively on the base-catalyzed alkoxide
hydrolysis route.[21] Osseo-Asare and Arriagada[87]
reported on the preparation of monodisperse silica
nanoparticles in the size range of 50–70 nm, with standard deviation below 8.5%, by the ammonia-catalyzed
hydrolysis of tetraethyl orthosilicate (TEOS) in a
nonionic w/o microemulsion formed by NP-5. Their
later work showed that the particle size went through a
minimum as W increased at a relatively high ammonia
concentration, which was attributed to an increased
surfactant aggregation number, free water concentration, and intermicellar exchange rate, kex.[88] As the
hydrolysis reaction of TEOS is rather slow and the
final particle size is significantly larger than the corresponding water droplet size, a reverse micellar pseudophase model has been proposed to interpret the growth
kinetics of silica nanoparticles in the microemulsions.[29] The effects of surfactant molecular structure
and type of oil on the silica particle size were also
investigated by Chang and Fogler.[89] The obtained
results suggested that the particle size decreased with
decreasing surfactant-film flexibility, supporting the
general effect of the film flexibility on the particle size.
Recently, highly fluorescent, photostable, and biocompatible organic-dye-doped silica nanoparticles have
been successfully prepared by acid-catalyzed TEOS
hydrolysis in nonionic reverse micelles.[90]
TiO2 nanoparticles, which exhibited aggregates of
anatase or rutile nanoparticles ranging in size typically
from 20 to 50 nm, were synthesized by the precipitation
of TiCl4 with ammonium hydroxide in water droplets of
a nonionic microemulsion and subsequent calcinations
at different temperatures.[91] Actually, the titanium
alkoxide hydrolysis method has been more frequently
used for the microemulsion-based synthesis of TiO2
nanoparticles; however, a post-treatment of hightemperature calcination is usually required to increase
the crystallinity of the nanoparticles. To obtain crystalline TiO2 nanoparticles without the post-treatment of
calcination, hydrothermal treatment of microemulsions
has been applied.[53,54] Alternatively, a novel hot-fluid
annealing technique was developed to synthesize stable
dispersion of monodisperse, crystalline TiO2 nanoparticles, which involved an in situ annealing process
in the presence of reverse micelles.[92] Similarly, ZrO2
nanoparticles can be produced in reverse micelles by
the hydrolysis of zirconium alkoxide instead of titanium
alkoxide. For example, sulfated titania nanoparticles,
which showed a high catalytic activity in n-butane isomerization, were prepared by the addition of zirconium
Synthesis of Inorganic Nanostructures in Reverse Micelles
butoxide to a nonionic w/o microemulsion containing
H2SO4, and subsequent calcination at 873 K.[93] As
another example, cerium oxide nanoparticles were
fabricated by the precipitation of Ce(NO3) 3 with ammonia in cationic CTAB reverse micelles, followed by
calcination.[94]
Extension of the microemulsion technique to
complex oxides has been explored for a wide variety
of materials. For example, crystalline barium hexaaluminate nanoparticles, which exhibited an ultrahigh
surface area, superb thermal stability, and excellent
catalytic performance in methane combustion, have
been successfully synthesized through the hydrolysis
and condensation of barium and aluminum alkoxides
in nonionic w/o microemulsions and subsequent
calcination leading to crystallization.[95,96] It was found
that higher water-to-alkoxide ratio generally resulted
in smaller particle size and increased surface area after
calcination. The method of metal salt precipitation in
reverse micelles has proven to be particularly useful
in preparing mixed metal ferrites; specifically, Zhang
and coworkers[97–100] have prepared nanoparticles
of a series of spinel ferrites including MgFe2O4,
MnFe2O4, CoCrFeO4, and CoCrxFe2xO4 in anionic
reverse micelles formed by dodecylbenzenesulfonate
and toluene. Nanoparticles of various mixed oxides,
such as Fe–Al oxide,[101] Ce–Te oxide,[70] and In–Sn
oxide,[102] can likewise be prepared by coprecipitation
of corresponding mixed metal ions in reverse micelles.
Finally, nanoparticles of many complex oxides including BaFe12O19, YBa2Cu3O7x superconductor, and
perovskite-type mixed oxides have been obtained by
calcination of oxalate precursor nanoparticles produced
through coprecipitation of corresponding mixed metal
ions in w/o microemulsions.[16,21]
Other Inorganic Nanoparticles
In a similar way, microemulsion-based synthesis of
nanoparticles of many other inorganic nanoparticles
can be realized, especially by precipitation reaction
occurring in water droplets. In particular, the synthesis
of silver halide nanoparticles in reverse micelles has
attracted much attention because this simple binary
halide system provides a useful model system for the
in-depth investigation of the microemulsion technique
while silver halide nanoparticles are important for
photographic and electronic industries. Typically, silver
halide, (AgX) (X ¼ Br, Cl), nanoparticles are readily
prepared by mixing two microemulsions containing
solubilized Agþ ions and X ions. Recently, the formation mechanism of uniform AgCl nanoparticles in an
NP-6 reverse micelle system has been investigated
through a new approach that uses a double jet technique, in which microemulsions containing AgNO3 and
Synthesis of Inorganic Nanostructures in Reverse Micelles
KCl were continuously introduced at the same time.[103]
It has been revealed that the AgCl nanoparticles actually grow through Ostwald ripening, and not by the
coagulation process.[104] Recently, AgCl nanoparticles
were prepared by the direct precipitation of silver ions
with the surfactant counterions in the water droplets
of microemulsions formed by dioctyldimethyl ammonium chloride.[105] Furthermore, very small, lightfast
AgCl and AgBr nanoparticles have been synthesized
through a solid–solid reaction performed by mixing
two water-free AOT reverse micelle dispersions containing AgNO3 and KX.[106]
In addition to silver halide, many other inorganic
precipitate systems have been exploited in the microemulsion-based nanoparticle synthesis. To illustrate,
Moulik et al. prepared Cu2[Fe(CN)6][107] and PbCrO4[108]
nanoparticles by precipitation reaction in AOT reverse
micelles, while they synthesized H2WO4[109] nanoparticles
by the reaction of Na2WO4 with HCl in nonionic w/o
microemulsions. They observed that the obtained nanoparticles were spherical on the whole. Sometimes both
the starting materials and the surfactants must be carefully chosen; for example, NH4MnF3 nanoparticles were
produced by the reaction between NH4F and manganese
acetate in reverse micelles formed by an ammoniumsubstituted AOT (NH4-AOT), where the possible
contamination of the desired product by impurity ions
[110]
such as Naþ and NO
3 ions was avoided.
SYNTHESIS AND ASSEMBLY OF
NONSPHERICAL NANOSTRUCTURES
The morphological control of colloidal nanoparticles
is a real challenge, because it is a complex process
requiring a fundamental understanding of the solid
state chemistry, solution chemistry, and interfacial
adsorption and reaction that are involved in crystal
growth or inorganic mineralization. Recently, there
6193
have been great advances in the controlled synthesis of
nonspherical nanoparticles in reverse micelles although
more data are still needed to ascertain the general
principles that determine the particle shape. Another
intriguing issue accompanying the particle-shape control
is the hierarchical assembly of nonspherical nanostructures into ordered superstructures or complex
architectures, which is relevant to the bottom–up
approaches toward future nanodevices and offers opportunities to explore the novel collective properties.
One-Dimensional Nanostructures
One-dimensional nanostructures such as nanowires,
nanorods, nanobelts, and nanotubes have stimulated
intensive interest owing to their unique applications
in mesoscopic physics and fabrication of nanodevices.[4] The majority of the literature concerning 1-D
nanostructure synthesis in reverse micelles deals with
nanowires and nanorods. Essentially, there are two
mechanisms proposed for the microemulsion-mediated
synthesis of inorganic nanowires/nanorods: templatedirected growth and oriented aggregation (Fig. 5).
In the template-directed growth mechanism,
elongated water droplets or interconnected water channels play the role of template to induce the formation
of elongated nuclei, which finally grow into nanorods
with dimensions considerably larger than the templates.
In the oriented aggregation mechanism, precipitation
within spherical water droplets initially results in the
formation of surfactant-encapsulated primary nanoparticles, which subsequently undergo oriented aggregation involving linear attachment, and coalescence
owing to specific interactions of inorganic crystals with
surfactants and/or certain additives, thereby leading to
the growth of single-crystalline nanowires with high
aspect ratios. Besides, the anisotropic growth habit
determined by the inherent crystallographic structure
of a solid material may partially contribute to the
Fig. 5 Schematic illustration of proposed
mechanisms for reverse micelle-based synthesis of inorganic nanowires/nanorods.
6194
growth of 1-D nanostructures in reverse micelle
solutions for certain synthesis systems.
Pileni’s work on preparation of copper nanoparticles
from Cu(AOT)2 microemulsions with varying internal
structures provides a rather persuasive support for the
template-directed growth of 1-D nanostructures in
microemulsions.[15] It was shown that synthesis in a
microemulsion consisting of interconnected water cylinders yielded cylindrical copper nanorods (6.5 nm in width
and 19.8 nm in length) together with some spherical copper nanoparticles, while synthesis in a microemulsion
consisting of spherical water droplets predominantly
produced spherical copper nanoparticles (Fig. 6).
The obtained results demonstrate that the shape of
the water phase of microemulsions at least partially
controls the shape of the produced nanoparticles,
i.e., a template-directed growth exists. Another example of template-directed growth is the synthesis of
CdS nanorods in reverse micelles formed by mixed
surfactants of the anionic AOT and the zwitterionic
phosphatidylcholine (lecithin).[111] It is known that
addition of lecithin to AOT reverse micelles induces a
structural change from spherical reverse micelles to
worm-like, ellipsoidal reverse micelles. Synthesis of
CdS nanoparticles in the ellipsoidal reverse micelles
results in the formation of CdS nanorods (4.1 nm in
width and 50–150 nm in length) with a hexagonal
structure of wurtzite (Fig. 7).
The templating effect of elongated water droplets in
the 1-D nanocrystal growth has been recently confirmed by the ultrasonically induced formation of silver
nanorods and nanofibers in AOT reverse micelles.[112]
Results from small-angle x-ray scattering (SAXS)
revealed that by ultrasonication, the reverse micelles
Synthesis of Inorganic Nanostructures in Reverse Micelles
changed from a spherical to an ellipsoidal structure,
indicating that the reverse micelles played the role
of a template in the formation of the rod- and wireshaped Ag particles.
However, further investigation on the preparation
of copper nanoparticles from Cu(AOT)2 microemulsions with addition of salts suggests that the nanocrystal growth markedly depends on the salt used, even
if the reverse micelle structure does not change with
various salt additions.[113] For example, chloride ions
enable the growth of Cu nanorods with an aspect ratio
controlled by the chloride concentration, whereas bromide ions favor the formation of Cu nanocubes. From
these data, it has been concluded that the 1-D nanocrystal growth is more related to selective adsorption
of ions during the crystal growth than to the nature
of the templates. In other words, the reverse micelle
template is not the key parameter in the anisotropic
nanocrystal growth, while additives and/or impurities
inside the template play the major role in the 1-D
nanocrystal growth.[3] Nevertheless, a nice work on
the synthesis of BaSO4 nanoparticles in cylindrical
reverse micelles formed by polymerizable surfactants
suggested that cylindrical nanoparticles were formed
in the partially polymerized reverse micelles while
spherical nanoparticles were formed in the unpolymerized reverse micelles.[114] This result indicates that the
templating effect of elongated nanodroplets for anisotropic nanoparticles could be considerably enhanced
by employing partially polymerized or more rigid
surfactant films.
On the other hand, there are many examples of 1-D
nanostructure formation in spherical reverse micelles
where there is no obvious templating effect. In this case,
Fig. 6 Transmission electron microscopy images of copper nanoparticles synthesized in Cu(AOT)2 microemulsions of different
shapes: (A) interconnected water cylinders and (B) spherical water droplets. (From Ref.[14].)
Synthesis of Inorganic Nanostructures in Reverse Micelles
6195
Fig. 7 (A) Transmission electron microscopy and (B) high-resolution transmission electron microscopy (HRTEM) images of
CdS nanorods synthesized in ellipsoidal reverse micelles formed by AOT and lecithin. Inset shows the corresponding electron
diffraction pattern. (From Ref.[111].)
the oriented aggregation mechanism is frequently
adopted to explain the formation of nanowires with high
aspect ratios in reverse micelle solutions. Qi et al.[115]
first reported the synthesis of single-crystalline, c-axisoriented BaCO3 nanowires with aspect ratios as large
as 10,000 in nonionic reverse micelles of C12E4 in
cyclohexane. The obtained nanowires were 10–30 nm
in diameter and up to 100 mm in length, which formed
a rather regular 2-D nanowire array in a large area upon
deposition on a Formvar-covered copper grid (Fig. 8).
According to time-dependent TEM observations of
the crystal growth process, a directional (oriented)
aggregation mechanism was initially proposed for
the nanowire formation in reverse micelles. Similarly,
nanowires and nanorods of other water-insoluble
salts including BaSO4,[116] BaCrO4,[117] BaWO4,[118]
CaSO4,[119] AgI,[120] and K3[PMo12O40]
nH2O[121] were
prepared by simple precipitation reactions in reverse
micelles around room temperature. Interestingly,
remarkable helical BaSO4 nanowires together with
bundles of BaSO4 nanofibers with coiled terminus
can be obtained in AOT reverse micelles, sometimes,
as shown in Fig. 9.[122]
Recently, coaligned bundles of vaterite nanowires
were prepared by water-induced crystallization of
alkylbenzenesulfonate-coated amorphous calcium carbonate (ACC) nanoparticles in AOT reverse micelle
solutions, indicating that hybrid 1-D nanostructures
can be assembled by oriented aggregation and transformation of amorphous primary nanoparticles at the
mesoscopic level.[123] It is worth noting that microemulsion-mediated hydrothermal procedure has been
shown to be powerful in the synthesis of a variety
of 1-D nanostructures. Specifically, single-crystalline
nanowires/nanorods of CdS,[56] BaF2,[57] SnO2,[58] and
Ca10(PO4)6(OH)2[124] have been successfully synthesized
in reverse micelles under hydrothermal conditions. In
these cases, an oriented aggregation mechanism is usually
adopted because reverse micelles may be destroyed under
hydrothermal conditions, and they just play the role of a
template for the formation of primary nanoparticles during the mixing step, which undergo subsequent oriented
aggregation to form the final 1-D nanostructures.
Notably, crystalline oxide nanorods can be prepared
directly by alkoxide hydrolysis in microemulsions at
room temperature. To illustrate, shuttle-like TiO2
nanoparticles of the tetragonal rutile structure were
prepared by the HCl-catalyzed hydrolysis of titanium
tetrabutoxide in NP-5 reverse micelles,[125] while rod-like
ZnO nanoparticles of the hexagonal wurtzite structure
were produced by the ammonia-catalyzed hydrolysis of
zinc dibutoxide in NP-6 reverse micelles.[126] Moreover,
g-V2O5 nanorods of the orthorhombic structure were
obtained by direct hydrolysis of a vanadium alkoxide
in AOT reverse micelles.[127] In these cases, the mechanism for the nanorod formation is poorly understood;
however, a continuous reaction and crystallization process seems more reasonable than an oriented aggregation
process. The 1-D growth of nanoparticles may be largely
attributed to the inherent crystal growth habit and/or
specific adsorption of surfactants or ions. In addition,
there are several reports on the crystalline nanorod
synthesis involving the room-temperature synthesis of
amorphous precursor nanoparticles in reverse micelles
6196
Synthesis of Inorganic Nanostructures in Reverse Micelles
Fig. 9 TEM images of BaSO4 nanowires synthesized in
AOT reverse micelles showing (A) closely packed bundle of
nanowires and (B) helical nanowires. (From Ref.[122].)
Fig. 8 (A) Low- and (B) high-magnification TEM images of
BaCO3 nanowires. (From Ref.[115].)
Hierarchical Assemblies of
1-D Nanostructures
followed by crystallization through heat treatment.
In particular, nanorods of rutile SnO2[128] and barium
hexa-aluminate[129] were prepared by the calcination of
the corresponding precursor powders whereas those
of hematite Fe2O3 were fabricated by reflux Fe2O3 gel
powders in tetralin.[130]
Recently, the microemulsion-based procedure has
been successfully applied for the synthesis of inorganic
nanotubes, which represent another important type of
1-D nanostructures. In particular, silica nanotubes,
which have lengths larger than 2 mm, diameters about
150 nm, and a wall thickness of 27 3 nm (Fig. 10),
were fabricated by a reverse-microemulsion-mediated
sol–gel (RMSG) technique.[131] It was proposed that
the addition of FeCl3 favored the formation of cylindrical AOT reverse micelles, which acted as templates
for the hydrolysis and condensation of TEOS around
the surface of the reverse micelles. After washing with
ethanol, AOT was removed and silica nanotubes were
obtained. The diameter of the silica nanotubes was
tunable, which can be done by using different apolar
solvents. More recently, unique titanium phosphate
nanotubes were synthesized via microemulsion-based
solvothermal synthesis in reverse micelles of H3PO4
acidified trioctylamine in benzene; however, the detailed
formation process remained unclear.[132]
Many recent efforts have been focused on the integration of 1-D nanoscale building blocks into ordered
superstructures or complex functional architectures,
which is a crucial step toward the realization of
functional nanosystems. It has been demonstrated that
ordered nanorod superstructures can form by the
self-assembly of uniform nanorods preformed in
reverse micelles. Furthermore, reverse micelles can be
used as nanostructured reaction media for the
direct growth of hierarchical architectures assembled
by primary 1-D nanostructures.
A striking example is the formation of linear chains
of nanorods by the coupled synthesis and self-assembly
of BaCrO4 nanorods in AOT reverse micelles.[117]
As shown in Fig. 11, the chain-like arrays consist of
uniform, prismatic nanoparticles (16 nm in length and
6 nm in width) that preferentially align side-by-side
and are separated by a regular spacing of 2 nm, consistent with the thickness of an interdigitated layer
of surfactant molecules. Furthermore, rectangular
superlattice of nanorods can form by 2-D assembly
of uniformly sized BaCrO4 nanoprisms. Subsequently,
a novel Langmuir–Blodgett technique was developed
to assemble uniform BaCrO4 nanorods, which were
prepared similarly in AOT reverse micelles, at the
Synthesis of Inorganic Nanostructures in Reverse Micelles
6197
Fig. 10 (A) Scanning electron microscopy; (B) TEM images of silica nanotubes synthesized in AOT reverse micelles; and
(C) schematic representation of the nanotube formation. (From Ref.[131].)
water–air interface.[133] The TEM images presented in
Fig. 12 clearly show the pressure-induced isotropic-2-D,
smectic-3-D nematic phase transitions as well as common
textual defects in the 3-D nematics. As another example,
remarkable doughnut-like aragonite particles showing
sponge-like networks of branch-like nanorods were
fabricated by mesoscale self-assembly and transformation of ACC precursor nanoparticles in AOT reverse
micelles.[134]
Recently, the so-called catanionic reverse micelles
formed by mixed cationic–anionic surfactants have
turned out to be promising nanostructured media
for the controlled synthesis and hierarchical assembly
of 1-D nanostructures. Catanionic reverse micelles
formed by an equimolar mixture of undecylic acid and
decylamine were first employed for the synthesis of
c-axis-oriented, single-crystalline BaWO4 nanowires
with diameters as small as 3.5 nm and lengths up to more
than 50 mm.[135] By addition of a double-hydrophilic
block copolymer, poly(ethylene glycol)-block-poly
(methacrylic acid) (PEG-b-PMAA), to the reverse
micelles, novel penniform architectures assembled from
BaWO4 nanowires were successfully synthesized.[136]
Fig. 13 presents typical TEM images of the obtained
penniform BaWO4 superstructures at different magnifications, which suggests that the obtained penniform
BaWO4 nanostructures consist of c-axis-oriented, singlecrystalline BaWO4 nanowires about 3.5 nm in diameter
and a nearly a-axis-oriented, crystalline BaWO4 shaft
with widths 200–400 nm. It was revealed that the
presence of PEG-b-PMAA favored the formation of
BaWO4 shafts, and the formation of the penniform
superstructures involved an initial formation of rod-like
shaft followed by the gradual growth of nanowires on
both sides of the shafts.
For the unique catanionic reverse micelle system,
the molar ratio (r) between the mixed anionic and
cationic surfactants plays a key role in the synthesis
6198
Synthesis of Inorganic Nanostructures in Reverse Micelles
Fig. 11 The TEM image of ordered chains of prismatic
BaCrO4 nanorods synthesized in AOT reverse micelles. (From
Ref.[2].)
and assembly of 1-D nanostructures. By simply
changing the mixing ratio, r, a variety of novel BaCrO4
nanostructures, such as nanowires, nanobelts, and treelike superstructures consisting of nanobelts, have been
synthesized in the catanionic reverse micelles formed
by undecylic acid, decylamine, and decane.[137] For
example, bundles of BaCrO4 nanowires were obtained
at r ¼ 1, whereas unusual bundles of BaCrO4 nanobelts were obtained at r ¼ 1.4. If the mixing ratio
was further increased, i.e., r ¼ 1.7, hierarchical, treelike BaCrO4 superstructures (10 mm in length), which
consisted of branches with numerous, nearly parallel
nanoleaflets (100 nm in length, 20 nm in width, and
4 nm in thickness) grown on two opposite sides, were
produced (Fig. 14). A two-stage growth mechanism
involving preferential adsorption of undecylic acid on
specific crystal surfaces has been proposed for the formation of the tree-like superstructures.
Nanocubes, Nanoplates, and Their Assemblies
In addition to 1-D nanostructures, nonspherical nanoparticles with other well-defined morphologies, such as
nanocubes and nanoplates, are also of interest, as the
particle shape may have pronounced effects on the particle property and function. Therefore, the controlled
synthesis of these nanoparticles in reverse micelles has
attracted considerable attention, and some interesting
results have been obtained.
In principle, for crystalline solids with a cubic
crystal structure, it is possible to obtain their nanoparticles with a cubic morphology in reverse micelles
under suitable conditions. Monodisperse nanocubes of
hydrophobic Prussian blue [ferric hexacyanoferrate(II)],
Fig. 12 Transmission electron microscopy images of
BaCrO4 nanorod assemblies at the water–air interface at different compression stages: (A) isotropic distribution at low
pressure; (B) monolayer with smectic arrangement at medium pressure; (C) nanorod multilayer with nematic configuration at high pressure; and (D) textual defects within
the nanorod multilayer. Insets in panels (B) and (D) show
the Fourier transformation of the corresponding image
and the nanorod director orientation in the vicinity of disclinations, respectively. (From Ref.[133].)
a typical molecular magnet with a cubic structure, were
first prepared by the photoreduction of [Fe(C2O4)3]3
in the presence of [Fe(CN)6]3 ions in AOT reverse
micelles.[138] Moreover, the obtained nanocubes can
self-assemble into highly ordered 2-D square superlattices. As shown in Fig. 15, after ageing for two days
in the microemulsion reaction media, well-ordered
superlattice structures were assembled from nanocubes
with a mean length of 16 nm, while after two weeks of
aging, the nanocubes were slightly larger (mean 18 nm)
and the domain length of the superlattices was reduced.
Subsequently, this method was adapted to a wide range
of coordination polymer materials, by mixing two AOT
microemulsions instead of the photoreduction process;
for example, molecule-based magnetic nanocubes of
cobalt hexacyanoferrate and chromium hexacyanochromate were routinely prepared.[139] Similarly, uniform
antiferromagnetic KMnF3 nanocubes were synthesized
Synthesis of Inorganic Nanostructures in Reverse Micelles
6199
Fig. 14 Transmission electron microscopy images of treelike BaCrO4 superstructures formed in catanionic reverse
micelles at r ¼ 1.7. Inset shows the corresponding electron
diffraction pattern. (From Ref.[137].)
Fig. 13 (A)–(D) TEM; and (E) HRTEM images of penniform BaWO4 superstructures formed in catanionic reverse
micelles with addition of PEG-b-PMAA. Insets show the corresponding electron diffraction patterns. (From Ref.[136].)
in CTAB reverse microemulsions and their selfassembled 2-D superlattices were observed.[140] It is
noted that polydisperse metallic copper nanocubes were
obtained in Cu(AOT)2 reverse micelles in the presence of
NaBr, indicating that the selective adsorption of Br
ions on the copper f100g faces played a key role in the
Cu nanocube formation.[113]
On bubbling H2S into Cd(AOT)2 reverse micelles,
equilateral, triangular CdS nanoplates with a high
crystallinity were produced.[141] As shown in Fig. 16,
most of the particles are characterized by an angular
shape with an average size of 10 nm, and the triangular
nanoplates are crystallized in a hexagonal wurtzite
structure with a top face of the (001) plane. The thickness of the triangular nanoplates was estimated to be
3–6 nm from the related absorption spectroscopy
study. In this case, there is no direct correlation
between the particle shape and the shape of the parent
water droplets, hence excluding the possibility of the
templating effect. More recently, single-crystalline
silver nanodisks in equilibrium with silver spheres were
produced by the reduction of Ag(AOT) with N2H4 in
AOT reverse micelles.[142] In this case, the nanodisk
size was tuned by the relative amount of reducing
agent whereas their aspect ratios remained in the same
order of magnitude.
Interestingly, a recent work reported the spontaneous
formation of novel, complex structures of calcite CaCO3
nanoplates by the addition of a carbonate-containing
AOT microemulsion to a reverse micelle solution formed
by calcium dodecylbenzenesulfonate under conditions of
high alkalinity.[143] Scanning electron microscopy (SEM)
studies revealed that the CaCO3 superstructure consisted
of stacked array of calcite nanoplates (20 nm in thickness) with a pseudohexagonal morphology (Fig. 17).
The nanoplates varied in width from around 150 nm
at each end of the stack to 1 mm in the central region, to
produce a stacked arrangement with bilateral symmetry.
In contrast to the microemulsion-based synthesis and
assembly of doughnut-shaped particles of interlinked
aragonite nanorods by mesoscale transformation of
ACC nanoparticles,[134] the calcite nanoplate superstructures were produced by the solution-mediated
primary nucleation.
6200
Synthesis of Inorganic Nanostructures in Reverse Micelles
Fig. 16 (A) TEM image of triangular CdS nanoplates
formed in Cd(AOT)2 reverse micelles; (B) high resolution
TEM image of a single nanoplate; and (C) Fourier transformation of the image in panel B. (From Ref.[141].)
Fig. 15 Transmission electron microscopy images of prussian blue nanocubes formed in AOT reverse micelles after
ageing for (A) four days and (B) two weeks. (From Ref.[138].)
SYNTHESIS OF CORE–SHELL
NANOSTRUCTURES
The microemulsion-based synthesis technique is able
to complicate nanostructures by surface coating and
modification to form nanocomposites exhibiting novel
properties and improved functionalities. In particular,
the controlled synthesis of different kinds of core–shell
nanostructures can be realized in reverse micelles. The
procedure usually consists of a two-step process, i.e.,
the formation of uniform core particles with sizes
smaller than the droplet sizes, and the formation of a
shell to coat a single core particle or a nanosized
matrix to incorporate many core particles. Alternatively, core–shell nanostructures may be fabricated by
simultaneous formation of the core and shell materials
in reverse micelles or post-treatment of precursor
nanoparticles formed in reverse micelles.
and ZnS in AOT reverse micelles, and the ZnS–CdSe
core–shell nanoparticles were prepared in a similar
way with an interchange of the Cd and Se reagents with
those of Zn and S.[144] Luminescence spectra indicated
that the growth of ZnS on the CdSe seed resulted in a
significant increase in the luminescence quantum yield
Core–Shell Nanostructures with
a Metal-Containing Shell
Originally, CdSe–ZnS core–shell nanoparticles were
prepared by the successive precipitation of CdSe
Fig. 17 SEM image of stacked lamellar superstructures of
calcite nanoplates formed in reverse micelles. (From
Ref.[143].)
Synthesis of Inorganic Nanostructures in Reverse Micelles
accompanying a blue shift to a narrow band near the
exciton absorption. Subsequent studies have demonstrated that the surface passivation of a semiconductor
core by an inorganic shell material of wider bandgap
could lead to more efficient and photostable luminescent
nanocrystals. In this regard, highly luminescent and
photostable
yellow-light-emitting
ZnS-passivated
CdS : Mn (CdS : Mn/ZnS core–shell) nanocrystals were
recently prepared in AOT reverse micelles.[145,146]
Many metal nanoparticles (e.g., Fe) are susceptible
to rapid oxidation, and this problem can be largely
circumvented by coating the nanoparticles with gold
or other inert metals. In particular, FePtx–Au and
CoPtx–Au nanoparticles with magnetic alloy cores
were prepared by successive reduction of appropriate
metal ions in reverse micelles.[147] Moreover, the synthesis can begin with Au nanoparticles, followed by a
coating with Fe and then followed again by a coating
of Au, creating an Au–Fe–Au onion type nanostructure.[148] Similarly, Fe–Cu–Fe onion type nanoparticles
with antiferromagnetic coupling between Fe layers
were prepared in AOT reverse micelles, which provided
a promising route for understanding the magnetic
interactions at the nanoscope level.[24] Furthermore,
combinations of reduction, oxidation, precipitation,
and hydrolysis reactions can be performed sequentially
in reverse micelles to produce core–shell nanostructures
of other systems including metals–oxides, oxides–metals,
oxides–oxides, and so forth. For example, Fe–iron oxide
core–shell nanoparticles with a metal-core diameter of
approximately 6.1 nm and an oxide shell thickness of
approximately 2.7 nm, which maintained the favorable
magnetic properties of metallic iron and showed a very
good resistance against oxidation, were successfully
synthesized in nonionic reverse micelles.[149]
Core–Shell Nanostructures
with a Silica Shell
Incorporation of inorganic nanoparticles in silica can
provide inorganic nanomaterials with several benefits,
such as high stability against oxidation, degradation,
and coagulation, as well as biocompatibility and
functionality. Considerable attention has been paid
to the synthesis of the core–shell nanostructures with
a silica shell, by using the microemulsion technique.
In particular, Asher and coworkers[150,151] have made
efforts to prepare semiconductor and metal quantum
dots/silica nanocomposites in nonionic w/o microemulsions. The synthesis of monodisperse CdS–silica
nanocomposite spheres (40–300 nm) containing incorporated CdS nanoparticles (approximately 2.5 nm)
was achieved by simultaneous coprecipitation of CdS
in the water droplets during the silica sphere synthesis
through TEOS hydrolysis in the microemulsions.[150]
6201
In a similar way, monodisperse Ag–silica nanocomposite spheres (approximately 100 nm) containing homogeneously dispersed Ag nanoparticles (2 5 nm) were
fabricated by in situ photochemical reduction of silver
ions during the hydrolysis of TEOS in w/o microemulsions.[151] These Ag–silica particles had significant
surface charge and readily self-assembled into crystalline colloidal array photonic crystals that Braggdiffracted light in the visible region.
Among various inorganic core–silica shell nanostructures, silica-coated magnetic nanoparticles have
attracted particular interest owing to their great potential in biomedical and technological applications.
For instance, ultrasmall silica-coated iron oxide nanoparticles (<5 nm) were prepared by the precipitation of
iron oxides with NH4OH in nonionic reverse micelles,
which was followed by NH4OH-catalyzed hydrolysis
of TEOS, resulting in a uniform silica coating, as thin
as 1 nm.[152] Subsequently, tunable superparamagnetic
g-Fe2O3–silica materials were produced by the thermal
annealing of iron oxide–silica composites obtained in
microemulsions with a high content of the iron oxide
precursor.[153] Recently, rod-like e-Fe2O3–silica nanocomposites, exhibiting a giant coercive field (Hc) value
of 2.0 T at room temperature, were successfully synthesized in CTAB reverse micelles by the addition of Ba2þ
ions as a stabilizer for e-Fe2O3.[154] In addition to iron
oxides, other magnetic materials have also been
employed as magnetic cores for the silica coating in
reverse micelles. To illustrate, CoFe2O4 and MnFe2O4
spinel ferrites–silica nanocomposites with tunable magnetic cores have been synthesized by the microemulsion technique.[155] As another interesting example,
monodisperse, air-stable, superparamagnetic, a-Fe–silica
composite nanospheres (50 nm) were prepared by the
NH4OH-catalyzed hydrolysis of TEOS in the presence of
Fe2þ ions in NP-5 reverse micelles, which was followed
by reduction with H2 at 450 C (Fig. 18).[156]
Fig. 18 TEM images of Fe–silica composite nanospheres
synthesized in nonionic reverse micelles. The outer shell is
composed of silica and the dark spots correspond to a-Fe
nanocrystals. (From Ref.[156].)
6202
It is noteworthy that the polymer-coated inorganic
nanoparticles or inorganic–organic nanocomposites,
which offer interesting prospects in various areas because
of their synergistic properties derived from several
components, can also be readily prepared by using the
microemulsion technique. For example, metal oxides–
polymer core–shell nanoparticles were synthesized by
the hydrolysis of modified metal alkoxides in w/o
microemulsions and subsequent atom transfer radical
polymerization initiated by the surface-functionalized
metal oxide nanoparticles.[157] Recently, novel polystyrene-encapsulated Ag nanorods and nanofibers were
synthesized by combination of reverse micelles, gas
antisolvent, and ultrasound techniques.[158] Furthermore,
unusual CdS–polystyrene nanocomposite hollow spheres
were fabricated in reverse micelles by g-radiationinitiated, simultaneous styrene polymerization and CdS
formation.[159]
CONCLUSIONS
Reverse micelles or w/o microemulsions have proved
to be versatile nanostructured reaction media for the
bio-inspired synthesis of a huge variety of inorganic
nanostructures. In particular, reverse micelles can
be employed as effective nanoreactors for the sizecontrolled synthesis of spherical nanoparticles of widely
varied, technologically important, inorganic materials.
Considerable attention has been paid to the precise
control of the final particle size and size distribution,
by simply varying the experimental parameters. Recent
computer simulations are fruitful in elucidating the
effects of various parameters on the particle size;
however, much effort is still needed to adapt this
theoretical method to more complicated systems. On
the other hand, recent studies have demonstrated
that the microemulsion technique is also powerful for
the shape- and architecture-controlled synthesis of
inorganic nanostructures, which represents an exciting
aspect in the microemulsion-based synthesis and opens
new opportunities for the solution synthesis and
assembly of inorganic nanostructures with complex
hierarchy. The development in this aspect, at present,
is still in its infancy and requires a much better
understanding of the underlying mechanisms for the
morphological control and hierarchical assembly.
Moreover, commercial applications of the microemulsion technique call for further improvement in
the processing methods involved in the large-scale
preparation, separation, and deposition of different
nanostructures. In this regard, w/c microemulsions
are particularly attractive as they are environmentally
benign and favorable for easy separation of the produced inorganic nanostructures. It is clear that much
more work needs to be done to realize the full potential
Synthesis of Inorganic Nanostructures in Reverse Micelles
of the reverse micelle-based synthesis of inorganic
nanostructures, and excitement involved in the development of this useful synthesis technique will continue.
ACKNOWLEDGMENTS
Support from the National Natural Science foundation
of China (20325312, 20473003, and 20233010) and the
Foundation for the Author of National Excellent
Doctoral Dissertation of China (200020) is gratefully
acknowledged. The author would like to thank
Prof. Jiming Ma for helpful discussion.
ARTICLES OF FURTHER INTEREST
Electrochemical Reactions in Microemulsions,
p. 2065.
Magnetic Nanoparticles: Preparation and
Properties, p. 3470.
Metallic Colloids, p. 3662.
Microemulsions: High-Pressure Compartmentalization
of Reactants, p. 3919.
Protein Conformation in Reverse Micelle
Environment, p. 5176.
Uniform Inorganic Colloidal Particles Preparation,
p. 6461.
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