Rapid synthesis in ionic liquids of room-temperature-conducting solid microsilica spheres

Article abstract of DOI:10.1002/anie.200501446

(Figure Presented) Take the rough with the smooth: Conducting silica microspheres with different morphologies - rough, dotted, and smooth (see picture) - have been rapidly synthesized in an ionic liquid under various experimental conditions. The conducting property of the silica particles is attributed to the presence of entrapped molecules of ionic liquid and water.

Full text of DOI:10.1002/anie.200501446

Communications
Conducting Materials
DOI: 10.1002/anie.200501446
Rapid Synthesis in Ionic Liquids of
Room-Temperature-Conducting
Solid Microsilica Spheres**
David S. Jacob, Augustine Joseph,
Somashekarappa P. Mallenahalli,
Sangaraju Shanmugam, Shirly Makhluf,
Jose Calderon-Moreno, Yuri Koltypin, and
Aharon Gedanken*
Herein, we report the rapid synthesis of monodispersed
conducting solid microsilica spheres by hydrolyzing tetrame-
thylorthosilicate (TMOS) and tetraethylorthosilicate (TEOS)
in an ionic liquid, 1-butyl-3-methylimidazolium hexafluoro-
phosphate, at room temperature. We also report the effect of
varying experimental conditions such as the temperature and
pH of the solutions on the synthesis and morphology of the
silica spheres.
Metal oxide spheres have attracted much attention in
fields such as optical materials and catalysts, as well as other
[
1–5]
nanotechnologies.
useful means to prepare metal oxide spheres and small
The sol–gel method is one of the most
[
6]
particles with high densities. Stöber et al. developed an
excellent method to prepare monodispersed silica spheres
from silicon alkoxides in alcoholic solutions. This process
was extended to the preparation of various metal oxides such
[
7]
[
8–10]
as Al O , TiO , and ZrO .
While the original Stöber
2
3
2
2
technique yields silica spheres with sizes in the range of 200–
7
2
00 nm, a modified Stöber method yields larger spheres of 1–
mm. However, it is difficult to prepare monodispersed
[
11]
silica spheres larger than 2 mm in alcoholic basic media at
[
7,11]
room temperature,
while the synthesis of silica spheres in
acidic media has been reported to produce spheres as well as
[
12]
irregularly shaped particles of 10–60 mm in size. Controlling
the size, shape, monodispersity, and yield of the desired
product has become a challenge for material chemists.
[
*] D. S. Jacob, Dr. A. Joseph, Dr. S. P. Mallenahalli, Dr. S. Shanmugam,
S. Makhluf, Prof. Y. Koltypin, Prof. A. Gedanken
Department of Chemistry and
Kanbar Laboratory for Nanomaterials
Bar-Ilan University Center for Advanced Materials and
Nanotechnology
Bar-Ilan University
Ramat-Gan, 52900 (Israel)
Fax: (+97)235-351-250
E-mail: gedanken@mail.biu.ac.il
Prof. J. Calderon-Moreno
Universitat Politecnica de Catalunya
EPSC-PMTA-201
Av. Canal Olimpic s/n
Castelldefels, Barcelona 08860 (Spain)
[
**] We thank Dr. Yakov Langsam for SEManalyses, Michael Riboch for
AFMmeasurements, and Dr. Boris Markovsky for conductivity
measurements.
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Chemie
Room-temperature ionic liquids (RTILs) have received
much attention as solvents, and significant progress has been
made in their applications to biphasic reactions, chemical
synthesis, electrochemistry, catalysis, polymerization, bioca-
talysis, liquid–liquid separations, extractions, dissolution,
biopolymer, molecular self-assembly, and interfacial synthe-
CÀH stretching and ring-stretching vibration bands that
confirmed the presence of the entrapped ionic liquid as well
as OÀH stretching bands from water molecules. The entrap-
ped ionic liquid remained intact in the silica spheres even
after several washings with acetone. The D and G bands in the
Raman spectrum indicate the carbon content of the sample
annealed at about 10008C.
[
13,14]
sis.
Recently, an ionic liquid was used as both a solvent
[
15]
and a template in the synthesis of zeolites. Supermicropo-
rous lamellar and well-defined inverse opal microstructured
silicas have been prepared in ionic liquids, which serve as
The size distribution of the microspheres was confirmed
by SEM, which revealed diameters in the range of 50–85 mm.
The surface area measured for the as-prepared sample was
[
16,17]
2
À1
templates.
Hollow TiO2 microspheres have been pre-
3.17 m g , while that for the calcined sample ranged from 8–
2
À1
[12]
pared in ionic liquids, and it was reported that their size was
influenced by the rate of stirring and the reaction temper-
12 m g , which is much higher than reported values. The
increase in surface area is a result of the evaporation or
decomposition of the entrapped molecules of water and ionic
liquid inside the silica spheres; upon removal of the entrapped
species, empty pores are left behind. Thermogravimetric
analysis of the silica spheres showed a weight loss of about
9.6% at 1508C due to the removal of water, whereas a weight
loss of 17% at 5508C was attributed to the decomposition and
removal of the entrapped ionic liquid, as confirmed by Raman
spectroscopy.
[
14]
ature. Elsewhere, mesoporous silica nanoparticles of var-
ious morphologies that contained an ionic liquid were used as
a controlled-release delivery nanodevice to discharge anti-
[
18]
bacterial ionic liquids against Escherichia coli K12.
Moreover, owing to their diverse electrochemical proper-
ties, ionic liquids have been used as electrolytes for electro-
chemical devices such as lithium secondary batteries, electric
double-layer capacitors, dye-sensitized solar cells, fuel cells,
[
19]
and actuators. Several important properties of ionic liquids
make them attractive substitutes for volatile organic sol-
The proposed mechanism for the formation of the differ-
ent morphologies of the spheres is shown in Figure 1. As a
[
20]
vents. The properties of RTILs can be controlled over a
wide range through variation of both the anion and cation as
well as the chain lengths of the substituent groups. An
increasing chain length in the cation alters the melting point
of the ionic liquids and increases their viscosity and hydro-
[
21]
phobicity. Varying the anion also affects the property of
ionic liquids; for example, salts based on the
tetrachloroaluminate(iii) anion are extremely water sensitive,
whereas those based on the hexafluorophosphate anion are
[
22]
neutral and extremely hydrophobic.
Through careful
consideration of the conducting properties and hydrophobic-
ity of the ionic liquid, we chose 1-butyl-3-methylimidazolium
+
À
hexafluorophosphate, BMI PF6 , as the solvent for the
preparation of the silica spheres.
The hydrolysis of TMOS and TEOS in a fixed amount of
ionic liquid under different reaction conditions (temperature,
pH) were studied. The products were characterized by small-
angle X-ray diffraction (SAXRD), energy-dispersive X-ray
(
EDX) spectroscopy, mass spectrometry, and Raman spec-
troscopy. An in-depth morphological study of the silica
spheres prepared under different conditions was carried out
by scanning electron microscopy (SEM), and the solid-state
nature of the silica spheres was confirmed from the micro-
tomed cross-section of the sample by transmission electron
microscopy (TEM).
SAXRD results revealed a lack of organization of the
pores for the as-prepared silica spheres, whereas some extent
of order could be observed in a sample that was calcined at
Figure 1. Schematic representation for the formation of different mor-
phologies of silica spheres under different experimental conditions.
result of the hydrophobicity of the ionic liquid, microsized
droplets of aqueous HCl solution are formed under vigorous
stirring. Molecules of the silica precursor (TMOS, TEOS)
enter these aqueous acidic microdroplets and are hydrolyzed,
leading to the formation of microspheres. Aggregated and
overgrown dimeric silica spheres also result from the above
mechanism, as hydrolysis of the silica precursor takes place
inside two attached microdroplets of water.
The formation of silica spheres with rough surfaces
(Figure 1a) depends on the reaction time and the temper-
ature. With a longer reaction time, microdroplets of water
adhere to the fully grown silica spheres and the silica
1
0008C. EDX spectroscopy showed the presence of phos-
phorus and chlorine from the encapsulation of the ionic
+
À
liquid, BMI PF6 , and aqueous HCl solution within the silica
spheres in the as-prepared sample, whereas their absence was
noted in the sample calcined at 10008C. Desorption chemical
ionization mass spectrometry indicated the presence of 1-
butyl-3-methylimidazole (BIM-H) in the sample, while the
Raman spectrum of the as-prepared silica spheres revealed
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Communications
precursor is hydrolyzed in those microdroplets to yield a
conditions (room temperature, 0.1m HCl, 30 minutes), a clear
solution was observed for the TEOS precursor whereas silica
spheres formed in the case of TMOS which suggests that the
rate of hydrolysis of TMOS is faster than that of TEOS. This
difference in rate was also confirmed upon hydrolysis of
TMOS at room temperature with 0.01m HCl for 30 minutes
by which smooth silica spheres were obtained; under the same
reaction conditions with TEOS, a clear solution was obtained.
Thus, variation of the reaction conditions leads to the
formation of different morphologies of silica.
rough surface, as depicted in Figure 2. Changes in the reaction
time, the pH of the aqueous solution, the temperature, and
the hydrophobicity of the ionic liquid (all under stirring) have
To determine whether the silica spheres were hollow or
solid we microtomed the products, and studied the cross-
section of a sample (Figure 5). The TEM image shows a
smooth outer layer and small particles arranged inside to form
a solid microsilica sphere.
Figure 2. Silica spheres with rough surfaces as synthesized from
TMOS at 508C for 1 h in aqueous HCl solution (0.25m).
an important effect on the preparation and the peripheral
morphology of the silica microspheres. By decreasing the
reaction time (Figure 1b,c), silica spheres with smoother
surfaces were obtained as shown in Figures 3 and 4.
Figure 5. TEMimage of the cross-section of single silica spheres.
Control hydrolysis reactions (1 h) in the absence of ionic
liquids were also carried out to study the morphology of the
hydrolyzed products of TMOS (0.5 mL) and TEOS (0.5 mL)
with aqueous HCl solution (0.25m) under vigorous stirring at
room temperature as well as at 508C. Clear solutions were
observed for the reactions performed at room temperature,
but silica gels were observed at 508C, which suggests that the
ionic liquid accelerates the rate of reaction at room temper-
ature and plays an essential role in the formation of the
spheres. A second control reaction (1 h) in the presence of the
ionic liquid but in the absence of aqueous HCl solution was
performed to study the morphology of the hydrolysis products
of TMOS (0.5 mL) and TEOS (0.5 mL) under vigorous
stirring at 508C. In both cases, dustlike particles (1-micron
particles as well as aggregates) were observed. These results
demonstrate that in the presence of aqueous HCl solutions,
the dustlike particles play a significant role by functioning as
seeds for the formation of fully grown silica spheres. These
dustlike silica particles tested positive for antibacterial
activity (E. coli, Gram-negative). Reactions carried out for
shorter periods of time and in the absence of aqueous HCl
solution resulted in clear mixtures, which substantiate the
roles of the acidic solution and the reaction time. The above
findings demonstrate the necessity of the ionic liquid and the
aqueous HCl solution under vigorous stirring in order to
develop silica microspheres.
Figure 3. Silica spheres with a dotted morphology as synthesized from
TMOS at 508C for 1 h in aqueous HCl solution (0.1m).
Figure 4. Silica spheres with smooth morphology as synthesized from
TMOS at 508C for 5 min in aqueous HCl solution (0.25m).
It was observed that the pH of the acidic solutions as well
the temperature affect the rate of hydrolysis. Reactions were
carried out at pH 1–2 (0.25–0.01m HCl) at room temperature
and at 508C under vigorous stirring. Under similar reaction
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Angew. Chem. Int. Ed. 2005, 44, 6560 –6563

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Chemie
It is known that silica acts as an insulator and that the
conducting properties of silica can be improved through
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[
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À
dazolium hexafluorophosphate BIM PF is reported as 3.8 ”
6
À2
2
À1 [19]
1
0
Scm mol . The conductivity of the silica spheres was
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À6
À6
À2
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À6
À2
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6
.77 ” 10 Scm . Thus, the presence of entrapped ionic
119.
liquid and water were needed to improve the conductive
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[
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À6
À2
with values of 8.14 ” 10
216 h) measured at 608C. These results are interpreted as a
result of the increased mobility of entrapped molecules of the
(194 h) and 9.76 ” 10 Scm
[
[
[
(
[
24]
ionic liquid.
2139.
The conductivity of the silica spheres will hopefully gain
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special oriented alignment of the molecules of ionic liquid
along the surface. A detailed study of the conductivity of an
individual silica sphere is in progress and will be reported
soon. The current process for the synthesis of silica spheres in
an ionic liquid has opened new directions and opportunities to
explore their application in several fields, however more
research has to be carried out to address the challenges that
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fields.
[
2
[
[
[
[
4583; c) R. Wang, S. Hoyano, T. Ohsaka, Chem. Lett. 2004, 33, 6.
Experimental Section
In a typical synthesis, TMOS (0.5 mL, 0.516 g, 3.38 mmol; Aldrich
+
À
Chemical Co, 99%) and BMI PF (3 g, 0.01 mol; Aldrich Chemical
6
À3
Co., 96%; density = 1.37 gcm ) were stirred in a 15-mL teflon vessel
for 1 min at room temperature to form a homogeneous transparent
solution. Then, 0.01m aq. HCl (0.25 mL) was added under stirring.
(Milli-Q water was used to prepare the acid solution and a 0.2-mm
Whatman Anotop 25 filter was used for filtration). The reaction
mixture was stirred vigorously at room temperature for 30 min and
then quenched by the addition of acetone. The white spheres were
separated by centrifuging at 9000 rpm for 10 min, and the product was
washed with acetone several times to remove excess ionic liquid, then
centrifuged at 9000 rpm for 10 min, and dried under vacuum for 24 h.
The size of the silica spheres obtained ranged from 50 to 85 mm.
Received: April 27, 2005
Published online: September 13, 2005
Keywords: conducting materials · hydrolysis · ionic liquids ·
.
silica · synthesis design
Angew. Chem. Int. Ed. 2005, 44, 6560 –6563
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6563