Catalytic growth of silica nanoparticles in controlled shapes at planar liquid/liquid interfaces

Article abstract of DOI:10.1039/b712260h

The shape of silica nanoparticles is controlled when they are synthesized at liquid/liquid interfaces; the combination of organic and aqueous phases that form the interface can change the shape of silica into a triangle, cube, or rod. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b712260h

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Catalytic growth of silica nanoparticles in controlled shapes at planar
liquid/liquid interfaces
Nurxat Nuraje,a Kai Sub and Hiroshi Matsui*a
Received (in Gainesville, FL, USA) 9th August 2007, Accepted 14th September 2007
First published as an Advance Article on the web 2nd October 2007
DOI: 10.1039/b712260h
The shape of silica nanoparticles is controlled when they are
synthesized at liquid/liquid interfaces; the combination of
organic and aqueous phases that form the interface can change
the shape of silica into a triangle, cube, or rod.
for the assembly of colloidal nanoparticles16–19 but there were
fewer reports on growing inorganic nanoparticles at planar
immiscible interfaces.20 However, experimental studies of
chemical reactions at liquid/liquid interfaces at the micro-
scopic level are still in their infancy and particle growth at
the liquid/liquid interface has not been explored to control
shape.21,22 In fact, the planar liquid/liquid interface has sig-
nificant potential for controlled particle growth since this soft
interface lacks fixed nucleation sites which gives capping ions
freedom to target certain faces of nuclei and the interfacial
tension retards particle growth rates to control their morphol-
ogies.23 The slowing of nuclear kinetics at the interface is
caused by the lower stability of small nuclei due to the change
in interfacial tension between two different phases.24 While the
slower nucleation kinetics have been observed at planar liquid/
liquid interfaces, there are few reports taking advantage of this
to control the shape of nanoparticles at the interfaces. Here,
our strategy for the shape control of nanoparticles is to apply
various positive and negative ions at planar liquid/liquid
interfaces where the nucleation kinetics are slow enough for
these ions to absorb and change the growth rate on particular
faces, and these face-selective ion absorptions tethered the
shape of resulting nanoparticles via hydrolysis and condensa-
tion (Fig. 1). While silica and other nanoparticles were
assembled into various shapes by the surfactant supramole-
cular assembly method in a bulk single-phase solution,25,26
here we report an alternative method for nanoparticle shape
control: the liquid/liquid interfacial nanoparticle growth meth-
od. In this report, we selected silica as a model growth system
In recent years, material scientists have extensively investi-
gated novel ways to control the shape of nanoparticles.1
Controlling shapes of nanoparticles is becoming extremely
important in various practical applications since chemical
and physical properties of nanoparticles can be tunable by
their shapes.2,3 For example, different shapes of nanoparticles
have different absorption properties so that the shape control
can create a new set of colors of the nanoparticles in addition
to the size, useful for medical imaging. For another applica-
tion of shape-controlled nanoparticles in catalysis: when Pt
nanoparticles were grown in a tetrahexahedral shape, their
catalytic activity was enhanced because these nanoparticles
had many high-index facets that served as active sites for
breaking chemical bonds.4 Here, we report a novel method to
control the shape of nanoparticles by reacting precursors at
planar liquid/liquid interfaces. By simply changing the combi-
nation of organic and aqueous solvents that formed the inter-
face, this interfacial growth technique yielded different shapes
of silica nanoparticles as triangles, cubes, rods, or ropes.
In general, non-spherical nanoparticles can be grown by
modulating the relative surface energies between faces of
nuclei.2,5 While various dry and wet methods have produced
nanoparticles in well-defined shapes, wet chemical methods
have an advantage of controlling the shape by influencing
growth rates on specific faces of nuclei at the nucleation stage
via capping of these faces with surfactants, ions, or biomole-
cules.6–15 In all cases, the shape control becomes more effective
when the nucleation and the growth rates slow down so that
these surfactants have enough time to absorb particular faces;
by this face-selective capping, the growth rate for each face is
no longer equal, which could yield non-spherical nanoparti-
cles.
Interfaces between two immiscible liquids, such as the
surfaces of droplets, have been shown to be ideal platforms
a Department of Chemistry, City University of New York, Hunter
College and The Graduate Center, 695 Park Ave, New York, NY
10065, USA. E-mail: hmatsui@hunter.cuny.edu; Fax: þ1 (212) 772
5332; Tel: þ1 (212) 650 3918
Fig. 1 Illustration of the scheme to grow silica in various shapes at
planar liquid/liquid interfaces. The aqueous phase contains ions that
not only catalyze the condensation but also cap a specific face of the
nuclei to slow particle growth at the interface.
b Department of Chemistry, City University of New York, College of
Staten Island, 2800 Victory Blvd., Staten Island, NY 10314, USA.
E-mail: sukai2002us@yahoo.com; Fax: þ1 (718) 982 3910;
Tel: þ1 (718) 982 2000
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since its growth mechanisms in acid and base catalyses are well
characterized. While previously the interfacial synthesis strat-
egy has been applied to grow polymers in controlled morpho-
logies,21,27 the application of interfacial growth to inorganic
nanoparticle syntheses in controlled shapes has not been
explored extensively.
metres in length were readily formed toward the aqueous layer
(Fig. 2b). While these nanowires formed at the liquid/liquid
interface were not branched out as observed in the homo-
geneous sol–gel system, the formation of the similar elongated
structure between them indicates that there is only a weak
influence on the shape control of the silica nanoparticles with
the HCl/butanol interfacial system. This is due to fast silica
condensation with the acid catalyst and under these conditions
ions at the interface have little time to influence the shape of
the silica nanoparticles by absorbing on specific faces of the
silica nuclei.
In this planar liquid/liquid interfacial growth of silica, the Si
precursor, tetraethyl orthosilicate (TEOS), and catalyst were
loaded in different layers; TEOS in an organic layer and the
catalyst in an aqueous layer. As the alkoxide precursor from
the organic phase contacted and reacted with the catalyst at
the liquid/liquid interface, silica particles were formed by
hydrolysis and condensation. The intermediate species con-
densed from silanol [Si(OH)n]m clusters were hydrophilic and
therefore the growth of silica nanoparticles was driven in the
direction from the organic layer to the aqueous layer. The
interfacial experiments were conducted under five different
conditions with TEOS as the precursor. In the interfacial
syntheses with acid catalysis, HCl or HNO3 solution was
employed as the aqueous phase and n-butanol as the organic
phase. In the interfacial syntheses with base catalysis, we
applied NaOH or NH4OH as the aqueous phase and
n-butanol as the organic phase. Chloroform was also applied
as the organic phase to further slow down the growth rate of
silica at the liquid/liquid interface.
However, a striking difference was observed when the
organic solvent containing the TEOS precursor was changed
from n-butanol to chloroform for the interfacial silica growth
with acid catalysis. In this case, the TEOS precursor in the
chloroform phase in the bottom layer was hydrolyzed at the
interface by the acid catalyst HCl that originated from the
aqueous phase in the top layer, which is the reversed config-
uration of the HCl/butanol interface. This HCl/CHCl3 inter-
face produced triangle-shaped silica nanoparticles as shown in
Fig. 2c. The triangular nanoparticle was 200 ꢁ 20 nm on each
side. It should be noted that there were areas on the TEM
grids where the spherical form of silica was more dominant.
The electron diffraction pattern (inset of Fig. 2c) showed the
polycrystalline nature of the nanoparticles and these silica
nanoparticles could contain the a-quartz domain based on
this diffraction pattern. Although the condensation was still
the rate-determining step in the acid catalysis interface, the
aprotic organic solvent, CHCl3, made a significant impact on
the nucleation kinetics on specific nucleus faces otherwise silica
nanoparticles would not grow in a particularly defined shape.
The major difference in applying CHCl3 as the organic layer is
the silica growth rate at the interface. In order for the silica
growth to occur, silanols from the organic layer of chloroform
need to cross the interface toward the aqueous layer and
undergo the condensation; however CHCl3 could retard the
transfer from the organic to the aqueous layers since this
aprotic solvent hydrogen bonds to electrophilic deprotonated
silanols at the acidic interface.28 Due to this reduced rate
for the overall condensation reaction, chlorine ions at the
interface have enough time to selectively adsorb onto certain
faces of the silica nanoparticles at the nucleation, which
controls the shape of the resulting particles. Previously, the
adsorption of chlorine ions onto the h111i face produced
triangular Cu nanoparticles,29 and the triangular silica nano-
particles grown at the interface between the CHCl3 organic
layer and the HCl aqueous layer could undergo the same
shape-control mechanism. It should be noted that the control
experiment in Fig. 2a, which failed to grow the shaped
particles in a bulk solution without the interface, suggests that
the interface plays an important role on the particle shape; if
those particles are simply grown by means of the salt con-
densation during the drying process, we should observe the
trianglular particles when the bulk solution of precursor and
catalyst was dried.
With the acid catalysis, hydrolysis of the TEOS precursor
without an interface by stirring the solution yielded branched
silica wires as shown in Fig. 2a. This branched wire growth
without any particular shape control was also observed in
sol–gel growth methods in homogeneous solutions containing
acid catalysis.26 However, the shape of the silica particles was
changed as the precursor was hydrolyzed at the liquid/liquid
interface between the organic phase and the aqueous phase.
When the TEOS precursor was in the n-butanol (top layer)
and the acid catalyst HCl was in the aqueous phase (bottom
layer), silica nanorods of 40 nm in width and several micro-
Fig. 2 TEM images of (a) silica grown with no interface by stirring
the aqueous solution of HCl, (b) silica grown at the HCl/butanol
interface, (c) silica grown at the HCl/CHCl3 interface (right: magnified
TEM image, scale bar ¼ 100 nm). Scale bar ¼ 200 nm. Insets show the
electron diffraction patterns.
When the TEOS precursor was hydrolyzed in homogeneous
media with base catalysis without the interface by stirring the
solution, large polydispersed silica particles were grown as
shown in Fig. 3a. This type of amorphous particle growth
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interface failed to influence the shape of the silica nano-
particles.
In summary, negative and positive ions at the liquid/liquid
interface could change the growth rates on specific faces of
silica nanoparticles under the slow nucleation conditions and
this interfacial growth technique yielded different shapes of
silica nanoparticles as triangles, cubes, rods, or ropes by
simply changing the combination of organic and aqueous
solvents that formed the interface. The aim of this study is
to demonstrate that the particle shape can be changed at the
liquid/liquid interface when the shape cannot be controlled
under the same conditions in the bulk single-phase solution.
The interfacial synthesis has an advantage in slowing the
particle growth, which increases the probability of surfactants
and ions influencing the resulting shapes. Silica was used as a
model in this study, and this interfacial method will become
more practical if the same shape control can be achieved for
more important materials such as quantum dots. In theory,
this interfacial synthesis should be applicable to control the
shapes of various inorganic nanoparticles whose precursors
can be dissolved in organic solutions and whose catalysts or
reducing agents can be dissolved in aqueous solutions. There-
fore, this interfacial growth method has potential to tether the
shape of inorganic nanoparticles without using complex multi-
ple synthetic processes. It should be noted that this method
enables one to not only tether nanoparticle shapes but also
provide means to investigate the effect of solvents on funda-
mental inorganic nanoparticle growth mechanisms under con-
trolled environments. And last, we would like to emphasize
that our interfacial method can be served as an alternative
method to the existing colloidal synthesis for the shape control
of nanoparticles, and there is potential that our interfacial
method could be applied to controlling the shape of some
systems whose shape is difficult to change by traditional
colloidal methods. And the combination of those two methods
could enhance the precision of the shape control.
Fig. 3 TEM images of (a) silica grown with no interface by stirring
the aqueous solution of NaOH, scale bar ¼ 200 nm, (b) silica grown at
the NaOH/butanol interface, scale bar ¼ 70 nm (right inset: magnified
TEM image, scale bar ¼ 50 nm; left inset: the electron diffraction
pattern), (c) silica grown at the NH4OH/butanol interface, scale bar ¼
200 nm.
without any particular shape control was also observed pre-
viously in homogeneous sol–gel growth methods with
base catalysis.28 However, when silica was grown at the
liquid/liquid interface with the base catalyst, NaOH, in the
aqueous layer and the TEOS precursor in the organic layer of
n-butanol, the shape of the nanoparticles was cubic and the
average size of the cubic nanoparticles was 100 ꢁ 10 nm each
side (Fig. 3b). Some of nanoparticles in Fig. 3b were in a
rectangular form since two cubic nanoparticles aggregated side
by side. The diffraction pattern in Fig. 3b indicates that this
silica nanoparticle is more crystalline as compared to the
triangular particles produced at the HCl/CHCl3 interface,
and the pattern has a close match with a-crystabolite. When
NH4OH was applied as the base catalyst in the aqueous layer,
the polydispersed spherical nanoparticles appeared and their
structure was amorphous. Since polydispersed spherical nano-
particles grown at the NH4OH/butanol interface resemble the
nanoparticles grown without the interface, the NH4OH cata-
lyst at the liquid/liquid interface has no influence on the shape
of the silica. In the case of the base catalysis growth, n-butanol
and chloroform yielded the same particle shapes at the inter-
face. Among the interfaces with the base catalysts, only the
NaOH/butanol interfacial system tethered the particle shape
to cubic because the stronger base NaOH produced higher ion
concentrations at the interface, which can cap certain faces of
the silica nuclei more effectively to control the shape of
resulting silica nanoparticles. Previously, the shape of titania
nanoparticles has been controlled by capping them with
Me4N1. This small cation could stabilize the negative charges
of titania at the nucleation stage via electrostatic interactions
and yielded titania nanoparticles with defined shapes.30
Sodium oleate and sodium stearate could also control the
shape of titania nanoparticles to cubic by capping h100i and
h001i faces.31 For this interfacial growth of silica with base
catalysis, abundant sodium ions at the NaOH/butanol
interface could influence the silica growth on these faces under
the characteristic slow condensation at the liquid/liquid
interface. Since the influence of the NH4OH/butanol interface
on the particle shape of silica is much weaker than the NaOH/
butanol interface, the cationic concentration is also important,
in addition to the slowing growth kinetics, to control
the shape of the nanoparticles at the liquid/liquid interface.
It should be noted that the same phenomenon was observed as
the lower anionic concentration at the weak acid/CHCl3
Experimental
The silica precursor, tetraethyl orthosilicate (TEOS, 98%,
Sigma-Aldrich) was dissolved in the organic solvent, n-butanol
(Fisher) or CHCl3 (Sigma-Aldrich), in a 1 M concentration.
To form the liquid/liquid interface with the n-butanol organic
layer, first a 0.1 M aqueous solution of acid or base, HCl,
HNO3 (Fisher), NH4OH (Sigma-Aldrich), or NaOH (Acros
Organics), was added to a vial, and then 5 ml of the TEOS–
n-butanol solution was added slowly along the wall of the vial
without disturbing the interface. In these interfacial systems,
the organic phase of n-butanol was located as the top layer. To
form the liquid/liquid interface with the CHCl3 organic layer,
first 5 ml of the TEOS–CHCl3 solution was added to a vial,
and then 5 ml of the 0.1 M HCl solution was added slowly
along the wall of the vial without disturbing the interface. In
these interfacial systems, the organic phase of CHCl3 was
located as the bottom layer. After two days, the aqueous layer
containing the silica nanoparticles was washed and centri-
fuged. The silica nanoparticles extracted from the aqueous
layer were then dried on carbon coated copper grids at room
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Acknowledgements
The authors acknowledge gratefully the support from: the US
Department of Energy: DE-FG-02-01ER45935 and the Na-
tional Science Foundation CARRER Award: ECS-0103430.
Hunter College infrastructure is supported by the National
Institutes of Health and the RCMI program: G12-RR-03037.
Part of this work was completed in the Core Facility for
Imaging, Cell and Molecular Biology at Queens College,
CUNY.
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1898 | New J. Chem., 2007, 31, 1895–1898