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PAPER www.rsc.org/materials | Journal of Materials Chemistry
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Bottom-up tailoring of nonionic surfactant-templated mesoporous silicananomaterials by a novel composite liquid crystal templating mechanism
Qianjun He, Jianlin Shi,* Jinjin Zhao, Yu Chen and Feng Chen
Received 9th April 2009, Accepted 11th June 2009
First published as an Advance Article on the web 15th July 2009
DOI: 10.1039/b907266g
A bottom-up tailoring methodology was developed to successfully regulate the morphology and size of
novel composite micellar bunches with liquid crystal mesophases self-assembled from multivalent metal
ions (ZrIV) and nonionic surfactants (P123), and subsequently the morphologies and dimensions of
liquid crystal-templated mesoporous silica nanomaterials (MSNs). The composite micellar bunches
with liquid crystal mesophases were directly evidenced by TEM imaging and DLS measurements. By
virtue of these morphologically and dimensionally controllable composite micellar bunches as
templates, SBA-15 MSNs were well tailored from long nanorods of 645� 245 nm to short nanorods of
360 � 210 nm, and even to nanoplates of 310 � 740 nm and subsphaeroidal nanoparticles of 230 nm.
Introduction
The morphological and dimensional tailoring of nonionic
surfactant-templated MSNs have attracted increasing interest.
MCM-41-type mesoporous silica nanomaterials (MSNs) with
various controlled morphologies have been synthesized using
ionic surfactants as structure directing agents (SDA) by base-
catalyzed routes. However by acid-catalyzed routes, the
controllable tailoring for nonionic surfactant-templated MSNs
still remains a great challenge, mainly because of the difficulty of
regulating the dimensions and shapes of micelles. Nevertheless,
compared with other mesoporous silica materials, nonionic
surfactant-templated MSNs exhibit some prefect features in the
biomedical field for the load and delivery of nanocrystals,
quantum dots and large molecules such as DNA, siRNA,
proteins and enzymes.1 Recently, SBA-15 microrods (1.5 �1 mm),2 nanorods (500 � 100 nm)3 and nanospheres (300 nm)4
have been obtained by a salt-out method, a dilute solution
approach and a nonionic/ionic surfactant-templated route,
respectively. However, the discretional adjustment of particle
size and morphology of SBA-type mesoporous nanomaterials, as
far as we know, is still far away from satisfactory achievement,
and especially, the corresponding control mechanisms are not
well understood. Therefore, new morphological and dimensional
tailoring methodology for nonionic surfactant-templated meso-
porous nanomaterials needs to be developed.
A clear understanding of the mechanisms relating to the self-
assembly among organic and inorganic precursors and micelles
would ultimately make for a more rational and general approach
to the morphological and dimensional control of mesoporous
nanomaterials.5 There have been two popular mechanisms for
the formation of mesoporous materials well known as the liquid
crystal templating (LCT) mechanism and the cooperative self-
State Key Laboratory of High Performance Ceramics and SuperfineMicrostructure, Shanghai Institute of Ceramics, Chinese Academy ofSciences, 1295 DingXi Rd, Shanghai 200050, P. R. China. E-mail:jlshi@sunm.shcnc.ac.cn; Fax: +86 21 52413122; Tel: +86 21 52412714
6498 | J. Mater. Chem., 2009, 19, 6498–6503
assembly (CSA) mechanism, which were first proposed by Beck
and Stucky, respectively.6 There is a distinct difference between
the two mechanisms: a liquid crystal phase pre-exists prior to the
addition of silicate species according to the LCT mechanism,
however according to the CSA mechanism, the mesostructured
assembly would not occur until silicate species are added to
cooperate with. It can be thought that the mechanism which
would be applied to explaining the formation of a certain mes-
oporous material mainly depends on the micellization accessi-
bility from the used surfactant in a given reaction system. In
principle, both mechanisms can be viewed at three length scales:
the molecular scale which refers to the interactions between
organic and inorganic precursors, the mesoscale or nanoscale
which involves the growth and assembly of micelles, and the
microscale which is related to the morphology and dimension of
final products. It is evident that the interactions between organic
and inorganic precursors on the molecular scale determine the
growth, assembly, dimension and shape of micelles on the
mesoscale or nanoscale, which further influence the final
morphology and dimensions of the mesoporous material on the
nanoscale or microscale.7 This strategy is defined as a bottom-up
methodology of designing and tailoring materials.
Recently, €O. Dag and co-workers introduced transition metal
salts to induce the self-assembly of oligo(ethylene oxide)
surfactants into a lyotropic liquid crystal (LLC) system with an
extra small quantity of water, and then used this LLC system as
the template to synthesize mesostructured silica films and
monoliths.8 However, at such a high concentration of surfac-
tants, it is almost impossible to obtain well-dispersed MSNs.
Interestingly, at a relatively low concentration of surfactants,
whether a similar stable LLC system could be obtained or not,
and further, whether this system could be employed to direct the
formation of mesostructured silica nanomaterials is, as far as we
know, not explored and documented yet. In addition, no direct
microscopic imaging evidence before the addition of silicate
precursors is available to support the existence of any liquid
crystal mesophase in aqueous solutions as the template for
directing the formation of ordered mesoporous materials.
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Based on the bottom-up strategy, in the present work multi-
valent metal chlorides (ZrOCl2) were introduced to bind and
induce the oligo(ethylene oxide) surfactant Pluroinc P123
(EO20PO70EO20, Mw z 5800) of a relatively low concentration
to self-assemble into a stable LLC system of composite micelles.
The existence of stable LLC bunches self-assembled from
composite micelles of P123 and multivalent metal ions are clearly
evidenced by direct TEM observations, which are also supported
by direct measurements of micellar sizes by in situ dynamic light
scattering (DLS) analyses. In the present investigated systems,
the addition of multivalent metal ions is inferred to result in the
redistribution of the surface charges of composite micelles on the
molecular scale, subsequently the variation of the size and shape
of micelles, the thickness of double electrode layer and the elec-
trostatic interactions between hydrated micelles and silicate
species on the mesoscale, and finally the controllable tailoring of
the morphology and dimensions of MSNs on the nanoscale. A
series of SBA-15 MSNs with controlled morphologies and
dimensions from long nanorods to short nanorods, even to
nanoplates and smaller subsphaeroidal nanoparticles, have been
obtained. Comparative measurements and analyses of precursor
micelles and the final synthesized mesoporous nanomaterials
approve the high efficiency of the bottom-up tailoring for the
morphologically and dimensionally controlled SBA-15 MSNs.
Experimental
Synthesis of LLC systems and SBA-15 MSNs
In a typical synthetic procedure, a proper quantity (0.0644–0.322 g)
of ZrOCl2$8H2O was completely dissolved into 40 mL of 2 M
hydrochloric acid, and then 0.5 g of Pluroinc P123 was
added. These precursor solutions were continuously stirred for 8 h
at 35� 0.5 �C. The stable LLC systems were obtained, which were
used to synthesize the morphologically and dimensionally
controlled SBA-15 MSNs as follows.
1 g of tetraethyl orthosilicate (TEOS) was added dropwise into
the abovementioned precursor solutions. After 24 h, the as-
synthesized materials were centrifuged. In order to remove
surfactant, the as-synthesized materials were refluxed for 24 h
and repeated three times in a mixed solution of 250 mL ethanol
and 2 mL hydrochloric acid (36–38%) at 78 �C. The products
were then dried overnight at 120 �C in vacuum.
Characterization of LLC systems
One to three drops of the abovementioned precursor solutions,
prior to the addition of TEOS, were added on a copper net, and
residual liquid was absorbed with a filter paper. Then the copper
net was observed at once on a JEM-2010 electron microscope
attached with an Oxford Link ISIS energy-dispersive spectrometer
operating at 200 kV. Meanwhile, energy dispersive spectra (EDS)
were collected on observed micelles and out of observed micelles.
The mean zeta potentials and sizes of composite micelles in
these precursor solutions prior to the addition of TEOS were
detected at a constant temperature of 35 �C by the DLS method
via a zeta potential and particle-size analyzer (ZetaPlus,
Brookhaven Instrument Corp., Holtsville, N.Y., USA).
This journal is ª The Royal Society of Chemistry 2009
Characterization of SBA-15 MSNs
Transmission electron microscopy (TEM) and scanning electron
microscopy (SEM) images of SBA-15 MSNs obtained on a JEM-
2010 electron microscope operating at 200 kV and a JSM-6700F
electron microscope, respectively. The mean particle dimensions
in a and c directions were calculated from about 100 particles in
a TEM image.
Small-angle X-ray diffraction (SAXRD) data of SBA-15
MSNs were recorded on Rigaku D/Max-2550V diffractometer
using Cu Ka radiation (40 kV and 40 mA) at a scanning rate of
0.4�/min over the range 0.6–5.0� with a step width of 0.002�.
Nitrogen adsorption–desorption isotherms were carried out
on a Micromeritics Tristar 3000 analyzer at 77 K under
a continuous adsorption condition. All samples were pretreated
for 12 h at 120 �C under nitrogen before measurement. Average
pore diameter was calculated from desorption branches of
isotherms by the Barrett–Joyner–Halenda method. Specific
surface area and pore volume were calculated by Brunauer–
Emmett–Teller and Barrett–Joyner–Halenda methods, respec-
tively.
Results and discussion
The interactions among P123, hydronium, and introduced
multivalent metal salts are proposed as follows:
mH3O++P123/[P123$mH3O+]m + (1)
nZrOCl2 þ�
mþ 23n�
H3Oþ þ P123þ nH2O
/n
n�ZrCl2ðH2OÞ4
�2þ$P123$
�m� 4
3n�
H3Oþoðmþ2
3nÞþ
(2)
where the molar ratio of Cl to Zr is assumed to approximately be
2 according to the elemental analyses by EDS (see Table 1, Fig. 1
and Fig. 2).
Comparing eqn (1) and (2), the introduction of ZrOCl2 would
positively charge the micelles of [P123$mH3O+]m + (abbreviated
as H$P123) further, and the average charge density of
{n[ZrCl2(H2O)4]2+$P123$(m�4n/3)H3O+}(m+2n/3) + (abbreviated as
Zr$P123) composite micelles, which were reflected by the values
of (m + 2n/3), would increase with the increase of the ZrOCl2concentration because more [ZrCl2(H2O)4]2 + would be bound to
P123 and consequently the n value would increase. It was pre-
dicted that the mean lengths of rodlike micelles decrease with the
increase of salt concentration as well as their charge densities in
aqueous solutions of low ionic strength.9 In this way, the
dimension and zeta potential of rodlike composite micelles
would be tunable via the addition of ZrOCl2, or other similar
multivalent metal salts, which would determine the dimension,
morphology and dispersivity of SBA-15 MSNs.
Such inspired design and prediction based on the bottom-up
strategy are supported experimentally by TEM imaging and DSL
techniques in the present work. As listed in Table 1, with the
increase of the Zr concentrations, the mean charge densities
(ZP/LDLS) of Zr$P123 composite micelles increase according to
the zeta potential measurements, the mean lengths of rodlike
micelles also decrease according to the particle size measure-
ments by DLS and TEM methods (Fig. 1), and the mean lengths
J. Mater. Chem., 2009, 19, 6498–6503 | 6499
Fig. 1 TEM images of Zr$P123 composite micelles at the different Zr
concentrations: (A) 5 mM, (B) 12.5 mM, (C) 18.8 mM, and (D) 25 mM.
Fig. 2 EDS spectra on and out of Zr$P123 composite micelles under
TEM as shown in Fig. 1A.
Table 1 The dependencies of the mean surface charge density, the size of micelles and the dimension of SBA-15 MSNs on Zr concentration
CZra (mM) 0 (mM) 5 (mM) 12.5 (mM) 18.8 (mM) 25 (mM)
Ron(Zr/Cl)b — 0.50 0.52 0.49 0.50
Rout(Zr/Cl)b — 0.09 0.13 0.22 0.36
LDLSc (nm) 2158 2012 490 376 238
ZPd (mV) 100 108 122 108 95ZP/LDLS (mV nm�1) 0.046 0.054 0.25 0.29 0.40LM–TEM
e (nm) — 1900 � 800 550 � 250 400 � 100 270 � 40LS–TEM
f (nm) 730 � 100l 645 � 65 310 � 30 360 � 45 �230DM–TEM
e (nm) — 250 � 50 300 � 50 250 � 60 220 � 20DS–TEM
f (nm) 290 � 50 245 � 35 740 � 50 210 � 20 �230d100
g (nm) 8.8 9.2 9.4 9.4 9.6DBJH
h (nm) 5.3 5.1 5.0 4.9 4.9wi (nm) 3.5 4.1 4.4 4.5 4.7Sj (m2 g�1) 684 706 785 799 845Vk (cm3 g�1) 1.0 0.8 0.8 0.9 0.8
a The total Zr concentration in precursor solutions. b The molar ratio of Zr to Cl on micelles (Ron(Zr/Cl)) or out of micelles (Rout(Zr/Cl)) determined byEDS under TEM as shown in Fig. 1. c The mean size of micelles determined by DLS measurements at 35 �C. d The zeta potential per micelle calculatedfrom the minimal common divisor of zeta potential values of precursor solutions measured at 35 �C using a zeta potential analyzer. e The average length(LM–TEM) and diameter (DM–TEM) of micellar bunches calculated from TEM images as shown in Fig. 1. f The average length (LS–TEM) and diameter(DS–TEM) of SBA-15 MSNs calculated from TEM images as shown in Fig. 3. g The (100) plane spacing determined by SAXRD as indicated byFig. 5. h The average pore size of SBA-15 MSNs calculated from desorption branches of nitrogen adsorption–desorption isotherms by the Barrett–Joyner–Halenda method. i The thickness of mesoporous walls of SBA-15 MSNs determined by d100 and DBJH. j Specific surface area of SBA-15MSNs calculated by the Brunauer–Emmett–Teller and Barrett–Joyner–Halenda method. k Pore volume calculated by the Barrett–Joyner–Halendamethod. l The mean size of the subassembly of conglutinated SBA-15 micro-rods.
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of SBA-15 MSNs also correspondingly decrease according to
TEM and SEM observations (Fig. 3 and Fig. 4), as predicted.
The successful direct TEM imaging of the composite micelles
and the micelle self-assembled bunches, which is of novelty and
vital importance for revealing the self-assembly and tailoring
mechanism, should be attributed to the relatively strong binding
interactions among Zr$P123 composite micelles which enhance
the micelle electron density due to the enrichment of Zr on
micelles. Such an enrichment of ZrIV ions is supported by the
elementary analyses by EDS under TEM (Table 1, Fig. 1 and
6500 | J. Mater. Chem., 2009, 19, 6498–6503
Fig. 2). In spite of the different Zr concentrations in precursor
solutions, the molar ratios of Zr to Cl on composite micelles
almost keep constant at around 0.5, which are always remarkably
higher than those out of composite micelles (Table 1). Compar-
atively in the absence of ZrIV ions, the micelles of H$P123 cannot
be directly observed by TEM owing to the very low contrast
against the carbon membrane support for TEM imaging.
More important is that these rodlike Zr$P123 composite
micelles unexpectedly self-assemble into a kind of novel mono-
dispersed bunches in a certain order along a micellar longitudinal
c direction (see the inset of Fig. 1A). The intermicellar spacings
keep almost constant at 26 nm which is remarkably wider than
the (100) plane spacing of SBA-15 MSNs (Table 1 and Fig. 5),
and there is a very low contrast between two micelles which can
clearly be observed by TEM imaging especially in the inset of
This journal is ª The Royal Society of Chemistry 2009
Fig. 3 TEM images of SBA-15 MSNs synthesized at the different Zr
concentrations: (A) and (B) 0 mM, (C) 5 mM, (D) 12.5 mM, (E) 18.8 mM,
and (F) 25 mM.
Fig. 4 SEM images of SBA-15 MSNs synthesized at different Zr
concentrations: (A) 0 mM, (B) 5 mM, (C) and (D) 12.5 mM, (E) 18.8 mM,
(F) 25 mM. All scale bars are 1 mm except that of (C) (100 nm).
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Fig. 1A, implying no precipitation of zirconia in any form, which
was supported by the fact that all these precursor solutions prior
to the addition of silicate species are stable, transparent and clear
over two months. From Fig. 6, all SBA-15 MSNs have the
classical type-IV adsorption–desorption isotherms with type-H1
hystereses and well-defined step loops at relative pressures of
0.5–0.8, which also suggests that they possess uniform meso-
porous structures in accordance with the results obtained from
TEM images (Fig. 3) and SAXRD data (Fig. 5). As listed in
Table 1, they all possess a high surface area, large pore volume
and large mesopores.
Furthermore, with the increase of the Zr concentration, the
composite micellar bunches became more uniform in length with
their extra tails getting shorter or even disappeared, the bunches
became subsphaeroidal at 25 mM of relatively high Zr concen-
tration. Consequently and correspondingly, a kind of sub-
sphaeroidal mesoporous nanoparticles with a minimal
dimension of about 230 nm in the present reaction system was
obtained. Therefore, we strongly believe that these composite
micellar bunches are in a stable LLC state10 in the investigated
precursor solutions prior to the addition of silicate species. After
the addition of TEOS, these composite micellar bunches or LLC
mesophases direct the formation of SBA-15 MSNs and regulate
their dimensions and morphologies, because the dimensions in
This journal is ª The Royal Society of Chemistry 2009
either a or c direction of the composite micellar bunches and the
SBA-15 MSNs match very well with each other in a ratio of
about 1:1 (or about 2:1 and 1:2 under relatively low Zr concen-
trations of 5 mM and 12.5 mM, respectively) according to the
TEM observations (Fig. 1, Fig. 3 and Table 1). It can be inferred
that this bottom-up tailoring route obeys the LCT mechanism.
The entire tailoring process by this bottom-up route can be
schematically illustrated as shown in Scheme 1.
Noticeably, ZrIV ions play an important role in cutting,
conglutinating or dispersing mesoporous nanoparticles (Scheme
1). In the absence of any multivalent metal ion, SBA-15 nanorods
of about 730 nm in length and 290 nm in diameter were
conventionally conglutinated with each other in both a and c
directions into extra long rods of micrometre scale (Fig. 3A and
B and Fig. 4A). This heavy conglutination is considered a result
of the low charge density on the micelles. However in the pres-
ence of a small quantity of ZrIV ions, the composite micellar
bunches of several micrometres in length and a stable LLC
mesophase (see the inset in Fig. 1A) were assembled by virtue of
the hydrogen bonding and the electrostatic attraction between
Zr$P123 composite micelles and counterpart ions (Scheme 1).
Moreover, during the condensation of silicate species, the
composites of the LLC composite micellar bunches and silicate
species would no longer conglutinate with each other along the c
direction due to the positively charged micelle ends, moreover,
the original very long composites could be cut shorter, e.g. in
half, probably owing to the combined effect of shearing strain by
stirring and the further attack from ZrIV ions. It is known that
charges would be distributed near the end caps of rodlike micelles
and that the charge density would increase when rodlike micelles
J. Mater. Chem., 2009, 19, 6498–6503 | 6501
Fig. 5 SAXRD patterns of SBA-15 MSNs synthesized at different Zr
concentrations: (A) 0 mM, (B) 5 mM, (C) 12.5 mM, (D) 18.8 mM, and
(E) 25 mM.
Fig. 6 Nitrogen adsorption–desorption isotherms of SBA-15
MSNs synthesized at different Zr concentrations: (A) 0 mM, (B) 5 mM,
(C) 12.5 mM, (D) 18.8 mM, and (E) 25 mM.
Scheme 1 The bottom-up morphological and dimensional tailoring of
SBA-15 MSNs by the controllable LLC composite micellar bunches.
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are continuously charged.9e Therefore, these highly charged
composites of the LLC composite micellar bunches and silicate
species have a higher end charge density than those not or less
charged by ZrIV ions, which, as we believe, leads to even shorter
micellar bunches/silicate meso-composites along the c direction
and a well-developed dispersivity of SBA-15 MSNs (Fig. 3 and
Fig. 4). Surprisingly, at the slightly increased Zr concentration of
12.5 mM, the shortened LLC composite micellar bunches can be
doubled in the a direction after the assembly with silicate species
so that a kind of mesoporous nanoplate could be obtained. When
the Zr concentrations were further increased, these composites
could not be doubled or cut shorter in either a or c direction. One
can think that these composites are too short in the c direction to
6502 | J. Mater. Chem., 2009, 19, 6498–6503
be further divided, and the greatly enhanced repulsive interaction
owing to the enhanced mean charge density, which is indirectly
supported by the increased thickness of mesoporous walls as
suggested by Table 1, results in the low probability of their
conglutination or growth in both a and c directions. Therefore
short rodlike and even subsphaeroidal mesoporous nanoparticles
have been obtained (Fig. 3E and F and Fig. 4) at relatively high
Zr concentrations of 18.8 mM and 25 mM, respectively.
This journal is ª The Royal Society of Chemistry 2009
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Conclusion
In summary, a liquid crystal templating route for the morpho-
logical and dimensional tailoring of nonionic surfactant-tem-
plated MSNs has been directly evidenced by TEM imaging
assisted by in situ DLS measurements by introducing multivalent
metal ions ZrIV to enhance the image contrast for the first time.
By virtue of introducing multivalent metal salts ZrOCl2, novel
LLC mesophases of metal ion–P123 composite micelles have
been assembled and their morphologies and dimensions can be
well controlled through varying the concentrations of the added
multivalent metal salts, and subsequently the morphologies and
dimensions of SBA-15 MSNs can correspondingly be well
tailored from long nanorods of 645 � 245 nm to short nanorods
of 360 � 210 nm, and even to nanoplates of 310 � 740 nm and
subsphaeroidal nanoparticles of 230 nm. This bottom-up
tailoring strategy can be extended to using other kinds of
multivalent metal salts, surfactants and inorganic resources to
prepare various mesoporous nanomaterials with expected
morphologies and dimensions.
Acknowledgements
We greatly thank Ms Meiling Ruan for the TEM measurements.
We greatly acknowledge financial supports from the National
Nature Science Foundation of China (Grant Nos. 20633090 and
50823007), National 863 High-Tech Program (Grant No.
This journal is ª The Royal Society of Chemistry 2009
2007AA03Z317), Shanghai Rising-Star Program(Grant No.
07QA14061), Shanghai Nano-Science Project (Grant No.
0852nm03900) and CASKJCX Projects (Grant No. KJCX2-
YW-M02 and KJCX2-YW-210).
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