Self-supported helical oxide arrays templated by pore-swollen chiral mesoporous silica

Article abstract of DOI:10.1039/b816538f

Self-supported helical architectures of Ia3d In₂O₃ and Fd3m Co₃O₄ are fabricated for the first time using the pore-swollen chiral silica as template. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b816538f

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Self-supported helical oxide arrays templated by pore-swollen
chiral mesoporous silicaw
Zhe An, Jing He,* Xin Shu and Yixin Wu
Received (in Cambridge, UK) 22nd September 2008, Accepted 19th November 2008
First published as an Advance Article on the web 24th December 2008
DOI: 10.1039/b816538f
ꢀ
ꢀ
2 3
O and Fd3m
Self-supported helical architectures of Ia3d In
Co are fabricated for the first time using the pore-swollen
chiral silica as template.
3
O
4
Size- and shape-controlled nanostructural metal oxides have
continuously captured research interests in the past few years
due to their unique properties and potential applications in
1
‘
nanodevices’. However, to fabricate ordered 3D hierarchi-
2
cally structured metal oxides, which might exhibit special
advantages in adsorption, separation, and heterogeneous
catalysis, is still a challenging mission. Among the diverse
synthetic themes, significant progress has been achieved with
3
the nanocasting route, which produces morphology-
controlled and stable nanostructured integrity more readily
than traditional soft-templating approach. Nanoparticles,
nanowires/rods, and nanostructured networks of metal oxides
could be selectively formed in the void of ordered porous
4
carbon and silica materials. The mesoporous silica with
5
helical pores and helical morphology, discovered recently,
provides the possibilities of producing metal oxides with
unique 3D chiral ordered architecture by the nanocasting
method. Thus in this work, for the first time, chiral meso-
porous silica with tuned pore sizes is synthesized. Self-
Fig. 1 (A) Pore size distribution (inset: XRD patterns) of helical silica
with a pore size of 4.3 (a), 3.0 (b), and 2.0 nm (c); (B) FESEM image
(inset: hexagonal cross-section); (C) HRTEM image; and (D) magnifi-
cation of selected region in (C) of helical silica.
ꢀ
ꢀ
supported chiral architectures of Ia3d In
Co are then fabricated using the enlarged channels of
chiral mesoporous silica as the template.
The chiral mesoporous silica is synthesized following a
2 3
O and Fd3m
3
O
4
The HRTEM micrographs (Fig. 1C and 1D and Fig. S2w)
show clear and well-ordered fringes corresponding to the (100)
plane equidistantly along the axis of the nanobar, character-
istic of chiral channels.
5b
typical procedure previously reported with cetyltrimethyl-
ammonium bromide (CTAB) as the template, sodium silicate
as the silica precursor, and ethyl acetate as the pH adjusting
reagent. Using trimethylbenzene (TMB) as the pore-swelling
After the CTAB template in the mesoporous channels is
7
effectively removed (Fig. S3w) by a microwave digestion
method, indium or cobalt nitrate is used as the precursor to
impregnate the mesoporous silica in ethanol. Then the nitrates
are transformed to oxides by calcination. The pore volume
6
reagent, the mesoporous silicas with pore sizes enlarged
from 2.0 to 3.0 and even 4.3 nm (Fig. 1A) are prepared. The
XRD reflections (Fig. 1A, inset) could all be indexed as 2D
hexagonal p6mm symmetry. A twisted hexagonal rod-like
morphology with an outer diameter of ca. 140–160 nm and a
length of ca. 600 nm is observed in the FESEM image for the
chiral mesoporous silica (Fig. 1B, Fig. S1 and S2w). Two kinds
of chiral channels can be found in the mesoporous silica
samples (Fig. S1w). But the left-/right-handed ratio proves to
be about 1 : 1 by counting the characteristic morphologies of
(
Table 1) of the mesoporous silica with pore sizes of 4.3, 3.0,
and 2.0 nm is estimated to decrease by 66, 57, and 45% due to
the In O encapsulation, and by 76, 58, and 33% because of
the Co O encapsulation. This means that the pore volume
2
3
3
4
Table 1 Textural properties of metal oxide/silica composites
Pore size
5
00 randomly chosen crystallites in the FESEM images.
4
.3 nm
3.0 nm
2.0 nm
a
b
a
b
a
b
Sample
Silica
S
BET
V
p
S
BET
V
p
S
BET
V
p
State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology, Beijing 100029, PR China.
E-mail: jinghe@263.net.cn; Fax: +86-10-644235385
1100
389
273
1.15
0.39
0.27
1181
370
364
0.98
0.42
0.41
775
209
234
0.49
0.27
0.33
In
Co
2
O
3
/silica
/silica
w Electronic supplementary information (ESI) available: Experimental
details, characterization methods, FT-IR, FESEM, HRTEM, and
XRD. See DOI: 10.1039/b816538f
3
O
4
a
2
BET specific surface area, m g
ꢀ1
b
3
Total pore volume, cm g .
ꢀ1
.
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Chem. Commun., 2009, 1055–1057 | 1055

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occupied by the metal oxides increases obviously with enlarged
pore size. The higher oxide loading is proposed to benefit from
the facile penetration of metal nitrates as precursors into larger
channels of helical mesoporous silica. The TEM image of the
In O /silica hybrid material (Fig. S4(a)w) illuminates the well-
2
3
packed In O3 (dark black) formed abundantly and homo-
2
geneously inside the 4.3 nm chiral channels. Due to high oxide
loadings, a sharp decrease in the reflection intensity of the
(
2 3 3 4
100) plane is observed for both In O /silica and Co O /silica
Fig. 3 Schematic diagram for the formation of self-supported chiral
composites (Fig. S4(b)w), which is attributed to the decrease in
the contrast between the walls and the pores as proposed
metal oxides: (a) chiral mesoporous silica with a pore size of 4.3 nm;
(
b) metal oxide/silica composites; (c) helical mesostructured metal
oxides with self-supported architecture after etching the silica; and
d) the top view of the helical mesostructure consisting of 55 nanowires
8
previously.
Etching the silica with an aqueous solution of NaOH gives
ꢀ
Ia3d In O (JCPDS No. 44-1087) and Fd3m Co O (JCPDS
(
ꢀ
2
3
3
4
along the axis.
No. 42-1467) as illustrated by the XRD patterns in Fig. 2. The
morphologies of both In O and Co O rely highly upon the
2
3
3
4
pore diameter of the helical mesoporous silica (Fig. 2). For the
metal oxides templated by the silica with a pore size of 2.0 nm,
scattered nanoparticles are mainly observed. When the pore
size is enlarged to 3.0 nm, non-continuous oxide nanowires are
formed, arrayed to a topical ordered structure. Swelling the
pore size to 4.3 nm, continuous nanowires are fabricated into
2 3 3 4
self-supported helical bundles for both In O and Co O . The
diameters of continuous nanowires are 3.6–4.0 nm, well in
accord with the pore diameter of their silica template. The
outer diameters of self-supported helical bundles of nanowires
are estimated at around 120 nm for In O and 110 nm for
2
3
Co O , consistent with the outer diameter scale of the silica
3
4
rods (Fig. 1C). The fringes (marked by arrows), which are also
observed in the silica template, appear along the axis of the
nanorod. The nanocasted metal oxides have thus substantially
reproduced the morphology of the silica template. The
adequate filling of template pores with oxides ensures the
formation of self-supported helical arrays. The formation
mechanism of the self-supported helical framework is simu-
lated in Fig. 3. The lattice (Fig. 2(c), insets) shows a spacing of
0
.29 nm for In O and 0.47 nm for Co O , corresponding to
2 3 3 4
ꢀ
the well-crystallized (222) and (111) planes of the Ia3d and
ꢀ
Fd3m groups. The N sorption measurements give BET
2
2
ꢀ1
2
ꢀ1
surface areas of 95 m g and 151 m g for helical Co
and In arrays, which are quite high for metal oxides. The
narrow pore size distribution (Fig. 4) gives the maximum at
.1 and 3.8 nm for self-supported Co and In helical
arrays.
The optical properties of In
The initial position for the sharp decline in the UV diffuse
reflection spectrum is used to discuss the In UV absorption
features. It is estimated at B305 nm (B4.07 eV) for the
3 4
O
2 3
O
4
3
O
4
2 3
O
2
3
O are illustrated in Fig. 5.
2
O
3
Fig. 2 XRD patterns and HRTEM images of In
2
O
3
(left) and Co
right) prepared in mesoporous channels of (a) 2.0, (b) 3.0, and
c) 4.3 nm.
3 4
O
(
(
2 3 3 4
Fig. 4 Pore size distribution of In O (A) and Co O (B) self-
supported helical arrays.
1
056 | Chem. Commun., 2009, 1055–1057
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in the hard-templating method. As shown in Fig. 5B, the
measured PL intensity increases progressively from nanodots
to nanowires and helical arrays, apparently implying the fact
that the better ordered In O nanostructure presents a higher
2
3
PL emission intensity.
In conclusion, it is found in this work that the dimension of
chiral pores of mesoporous silica as a template plays an
important role in controlling the morphologies and then
the optical properties of nanocasted metal oxides. Using
pore-swollen chiral mesoporous silica as a hard template,
Fig. 5 UV diffusion reflection (A) and photoluminescence (B) spectra
of In prepared in mesoporous channels of 2.0 nm (a), 3.0 nm (b),
2 3 3 4
self-supported helical arrays of In O and Co O nanowires
2 3
O
could be fabricated. The whole synthetic procedure might be
extended to the synthesis of other self-supported helical metal
oxides, mixed metal oxides and metal sulfides, etc. The novel
helical self-supported metal oxides are expected to have fasci-
nating application in micro-devices, such as optics, magnetics,
and beyond.
and 4.3 nm (c).
2 3
self-supported In O helical arrays (Fig. 5A(c)) by intersecting
the tangents of the plain and decline parts. Compared with
9a
that of bulk In O (3.65 eV), the bandgap of the In O arrays
2
3
2
3
shows an obvious blue shift, similar to that observed for
nanodots (Fig. 5A(a)) and nanowires (Fig. 5A(b)). This could
be rationally ascribed to the quantum confinement effect, as
The authors are grateful to the financial support from
NSFC, Program for Changjiang Scholars and Innovative
Research Team in University (PCSIRT). They thank
Dr Xin’an Yang (Institute of Physics, Chinese Academy of
Sciences) for the HRTEM measurements.
1
c,9b
proposed previously.
served for In nanorods (300 nm), corundum-type In
nanocubes (ca. 300–310 nm), and nanoporous In
Similar effects have also been ob-
4a
2
O
3
2 3
O
9c
2
O
3
crystal
9
b
clusters (220 nm). The energy shift could be calculated using
1
eqn (1) if its dependence on particle size is assumed;
g
Notes and references
h2
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20 meV for the self-supported In O helical arrays. The
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2 3
average size of the corresponding In O particle is thus
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1
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3.6 nm) form self-supported bundles having a diameter of
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1
3
¨
2
O
3
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(
The photoluminescence (PL) properties
Fig. 5B) of self-supported arrays of nanowires are identical
5
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signals centered at 370 and 470 nm. It is recognized that bulk
2
006, 45, 2088.
1
0a
In
2 3
O exhibits no PL emission. The blue PL emissions, well-
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d
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2
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In O transforms from nanodots to a self-supported helical
1
6, 2374.
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Chem. Commun., 2009, 1055–1057 | 1057