Novel surfactants for the synthesis of unusual highly ordered lamellar oxides

Article abstract of DOI:10.1021/jp0260999

Highly ordered long-range lamellar oxides were fabricated using a silicone-based graft copolymer as the template. Lamellar oxides have the largest lattice constant known for layered materials reported to date. The results show that silicone surfactants with a highly flexible siloxane chain would lead to the formation of an unusual lamellar mesostructure.

Full text of DOI:10.1021/jp0260999

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VOLUME 106, NUMBER 45, NOVEMBER 14, 2002
LETTERS
Novel Surfactants for the Synthesis of Unusual Highly Ordered Lamellar Oxides
An-Wu Xu†
School of Chemistry and Chemical Engineering, Zhongshan UniVersity, Guangzhou 510275, China
ReceiVed: May 11, 2002; In Final Form: July 30, 2002
Highly ordered long-range lamellar oxides were fabricated using a silicone-based graft copolymer as the
template. Lamellar oxides have the largest lattice constant known for layered materials reported to date. The
results show that silicone surfactants with a highly flexible siloxane chain would lead to the formation of an
unusual lamellar mesostructure.
Different surfactants have been used to organized oxides into
a variety of mesostructured materials, through the mediation of
electrostatic, hydrogen-bonding, dative bonding, and van der
Waals interactions.1-5 Self-assembly of molecules into an
exciting diversity of mesostructured frameworks has attracted
much attention from a wide range of scientific interests and
applications. A number of layered materials such as silicas,
transition metal oxides, and aluminophosphates have been
obtained using surfactants such as primary amines and quater-
nary ammonium ions.6-10 However, lamellar oxide structures
with large interlayer spacings even matching the wavelength
of light have never been reported to date. Recently, block
copolymers have been increasingly used to organize meso-
structured oxides, because the architecture and compositions of
the amphiphilic block copolymers can be rationally adjusted to
control the interactions between the organic and inorganic
species, self-assembly, and the mesophase selection.11 However,
the use of silicone-based copolymer for preparation of oxide
mesostructures has not been reported so far.
SCHEME 1: Primary Structure of Silicone-Based
Copolymers
high surface activity due to the weak cohesive energy (∼25
mN/m) of introduced PDMS chains in comparison with other
surfactants such as PEO-b-PPO-b-PEO and primary amines.12
It is anticipated that the aforementioned unique properties would
endow silicone-polyether surfactants with substantially different
behavior from conventional surfactants or block copolymers.
Silicone copolymer was prepared by hydrosilylation addition
reaction of PEO allyl end-blocked at PEO side to a PDMS
polymer containing pendant Si-H groups using Pt-based
catalyst, as described elsewhere.12
Lamellar silica powders (designated ZSU-L) were prepared
by hydrolysis of tetraethyl orthosilicate (TEOS) in the presence
of nonionic amphiphilic silicone surfactant in acidic aqueous
media (x ) 26, y ) 3, z ) 12; Mn ) 4000, D ) 1.10, purity >
95%). In a typical synthesis, 1 g of silicone copolymer was
dissolved in 60 mL of 1 M HCl, and then 4.5 mL of TEOS
was added to a templating solution. The reaction mixture was
stirred at room temperature for 18 h to obtain the templated
lamellar silica. TEM images were obtained with a JEOL 100CX
Herein we report a novel silicone-polyether copolymer
designed to template self-assembly of silica/surfactant lamellar
mesophases. The silicone copolymer is composed of a poly-
(dimethylsiloxane) (PDMS) backbone and a side chain of poly-
(ethylene oxide) (Scheme 1)
The essential properties of silicone-based copolymer were
its amphiphilic character, low cost and biodegradability, and
† E-mail: cedc17@zsu.edu.cn.
10.1021/jp0260999 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/19/2002

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11714 J. Phys. Chem. B, Vol. 106, No. 45, 2002
Letters
Figure 1. TEM image of an ultrathin section of the as-made lamellar
silica powders (ZSU-L) templated from silicone surfactant in strong
acidic media (1 M HCl).
Figure 2. TEM image of an ultrathin section of the as-synthesized
lamellar silica monoliths and films using silicone surfactant as the
template.
operated at 100 kV. The samples were embedded in epoxy resin,
and ultramicrotomed for TEM measurements.
The lamellar structure of ZSU-L is clearly seen from the TEM
images of an ultrathin section of the as-synthesized sample
(Figure 1). The hybrid silicas are constructed of bilayer
aggregates of the surfactant being sandwiched by thick silica
walls that are arranged parallel to each other. The interlayer
distance measured from the micrograph is about 190 ((10) nm,
which shows to be much larger than those of all previously
synthesized and natural layered materials.6-10 To the best of
our knowledge, highly ordered long-range lamellar silica
templated from silicone surfactant has the largest periodic
interlayer spacing known for lamellar materials. The lamellar
structures extend to the length of micrometer scale through layer
propagation without curvature, as shown in Figure 1. The
splitting of silica walls is the result of damage caused by
untramicrotoming. TEM observation of the sample under various
tilting angles did not show evidence for any mesostructured
framework other than lamellae.
Even with substantial changes in the concentration of silicone
surfactant and HCl, lamellar silica is also retained, suggesting
that silicone surfactants favor the formation of lamellar materials.
It is unprecedented that lamellar silica has an extremely large
interlayer distance. There would exist a different templating
mechanism between ZSU-L and SBA-15 or MCM-41. It is very
likely that special properties of silicone copolymer would be
responsible for the formation of lamellar structure with a large
lattice constant. Both silicone surfactant and the Pluronics family
(such as PEO20PPO70PEO20, P123) are amphiphilic supra-
molecules with the same hydrophilic headgroup PEO. The only
difference is that silicone surfactant contains a hydrophobic
PDMS chain but P123 has a hydrophobic PPO tail. The PDMS
chain is more flexible than the PPO chain in the Pluronics family
or the hydrocarbon chain in alkyltrimethylammonium salts
(CTA+), because the bond angle (Si-O-Si) is significantly
wider (∼143°) and the bond length (Si-O) (0.165 nm) longer
than comparable C-O-C (114°, 0.142 nm) bonds or C-C-C
(109°, 0.140 nm). Thus, the obstacle to rotation is very low
(rotation barrier: 0.8 kJ/mol) and the Si-O bond can freely
rotate and tilt.12 That is the reason even very long PDMS chain
surfactants are in the liquid state at room temperature. In
contrast, the hydrocarbon surfactants tend to be in a solid state
at room temperature because Krafft temperatures for long and
linear hydrocarbon-chain surfactants are high.12 We suggest that
silicone surfactants with more flexible chains (unrestricted
supramolecular chain configuration) than conventional hydro-
carbon surfactants or copolymer (restrictive supramolecular
chain configuration) would be responsible for the formation of
this unusual lamellar silica mesophase.
Figure 3. TEM images of an ultrathin section of the as-synthesized
lamellar TiO2 and ZrO2 templated from silicone surfactant in non-
aqueous solutions.
To demonstrate silicone surfactant for favoring the formation
of lamellae, we fabricated transparent silica monoliths and films
by the sol-gel method13 using silicone surfactant as the
template. Lamellar silica monoliths and films were prepared
over a wide composition range of 0.02 mol of TEOS:0.4-2.5
g of silicone surfactant:0.08-0.30 mol of H2O:(0.4-12) × 10-4
mol of HCl:0.25-1.5 mol EtOH. In a typical synthesis, 1 g of
silicone surfactant was dissolved in 15 mL of ethanol, and the
solution was acidified to pH 2 using hydrochloric acid. The
added water content is equal to, or higher than, that required
for the stoichiometric hydrolysis of the silica presursor. Then
4.16 g of TEOS was added to a templating solution. The reaction
mixture solution was transferred to a vessel for reaction in air
at room temperature for 24 h to obtain the templated lamellar
product. Transparent and continuous silica films were prepared
by the dip-coating process using the sol solution. The lamellar
feature of silica is clearly shown in Figure 2. The lattice constant
of lamellar silica measured from Figure 2 is about 170 ((10)
nm. The results further supply strong evidence that silicone
surfactants favor the formation of unusual lamellar meso-
structure. Other lamellar oxide monoliths and films such as TiO2
and Nb2O5 can be obtained using silicone surfactant as the
template and corresponding metal alkoxides as precursors.
Yang et al.14 prepared a series of hexagonal metal oxide
mesophases using the Pluronics family such as P123 as the
template in nonaqueous solutions. However, in this study, only
lamellar metal oxides such as TiO2 and ZrO2 were obtained by
using the silicone surfactant as the template in nonaqueous
solutions by the same method described by Yang et al.14 The
lamellar structure of TiO2 and ZrO2 is clearly shown in Figure
3A and B, respectively. The interlayer distance of lamellar TiO2
and ZrO2 measured from Figure 3A,B is about 230 ((10) and
170 ((10) nm, respectively. The present results support the
proposal that oxide mesophases are governed by supramolecular
chain configuration; i.e., unrestricted chain configuration (such

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Letters
J. Phys. Chem. B, Vol. 106, No. 45, 2002 11715
References and Notes
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as silicone surfactants) is responsible for the formation of the
zero-curvature (planar) lamellar silica mesophase, and restrictive
chain configuration (such as Pluronics family, C16TA+) is
responsible for high curvature hexagonal mesophase.15 Other
metal oxides such as SnO2 and Al2O3 with similar lamellar
structure can also be obtained using silicone surfacant as the
template and corresponding anhydrous metal chlorides as
precursors in nonaqueous solutions.
In conclusion, we have demonstrated that novel silicone-based
copolymer can successfully template the assembly of the hybrid
layered oxide mesophase with the largest interlayer spacings
for the first time. The synthesis of unusual lamellar oxide
materials with silicone surfactants as the template illustrates the
novelty of this synthetic strategy. We suggest that the un-
restricted supramolecule chain configuration of siloxane may
be responsible for the formation of these unusual lamellar oxide
materials. Such hybrid oxides with long-range ordered lamellar
structure are of interest from the viewpoint of biomineralization
and may find potential applications. The possibility of producing
other oxide materials with unusual mesostructures using this
novel method is also intriguing. It will enrich our ability to create
highly ordered inorganic/organic mesostructure with larger
periodic spacing using the bottom-up approach.
Acknowledgment. Support from the Guangdong Province
“The Tenth Five-Year Plan” Key Project (20010185C) is
gratefully acknowledged.