Formation of titania/silica hybrid nanowires containing linear mesocage arrays by evaporation-induced block-copolymer self-assembly and atomic layer deposition

Article abstract of DOI:10.1002/anie.200700923

Hot-wired silica: Titania-functionalized silica nanowires containing linear arrays of mesocages (see picture) are prepared by a combination of evaporation-induced self-assembly of polystyrene-block-poly-(ethylene oxide) soft templates inside nanoporous hard templates and atomic layer deposition. This combination allows the independent tuning of the internal fine structure of the nanowires and the properties of their outer surface. (Figure Presented)

Full text of DOI:10.1002/anie.200700923

Angewandte
Chemie
DOI: 10.1002/anie.200700923
Functionalized Nanowires
Formation of Titania/Silica Hybrid Nanowires Containing Linear
Mesocage Arrays by Evaporation-Induced Block-Copolymer
Self-Assembly and Atomic Layer Deposition**
Xin Chen,* Mato Knez, Andreas Berger, Kornelius Nielsch, Ulrich Gösele, and Martin Steinhart*
Mesoporous materials are currently being investigated as
versatile support and template structures for a broad range of
applications in the fields of catalysis,[1] electronics,[2] storage,
filtration,[3] nanobiotechnology,[4] and drug delivery.[5] Their
preparation by the self-assembly of structure-directing soft
templates, such as surfactants[6,7] and block copolymers, has
been intensively investigated, and di-and triblock copolymers
containing poly(ethylene oxide) blocks have been used to
produce mesoporous structures with various morphologies
and adjustable pore sizes ranging from a few nanometers to a
few tens of nanometers.[8–12]
Nanoporous hard templates, such as self-ordered porous
alumina,[13] contain arrays of aligned nanochannels with
uniform diameters in the range from about 25 to 400 nm
and lengths of up to several hundreds of micrometers. Such
templates have been used to form nanowires and nanotubes
from a variety of materials.[14] The exploitation of templated
self-assembly processes, such as mesophase formation,[15]
crystallization,[16] liquid/liquid decomposition,[17] and the
microphase separation of block copolymers inside nano-
porous hard templates, allows the rational generation of one-
dimensional nanostructures that exhibit specific mesoscopic
fine structures, which, in turn, determine their properties. In
the case of block copolymers, bulk-like morphologies form in
pores with diameters larger than 100–200 nm.[18,19] Mesopo-
rous silica nanowires are thus accessible by a hierarchical
templating process involving the self-assembly of a block-
copolymer soft template inside a nanoporous hard tem-
plate.[18] Unprecedented morphologies that are substantially
different from the microphase structures of the corresponding
bulk systems form as a result of the two-dimensional confine-
ment imposed by the geometry of nanopores having pore
diameters smaller than about 100 nm.[20–22] Mesoporous
silica[20] and carbon nanowires,[22] for example, with uncon-
ventional pore morphologies, have been obtained by hier-
archical self-assembly.
Only a few reports deal with approaches to the prepara-
tion of silica nanowires containing linear arrays of mesocages
based on the use of Pluronic-type block-copolymer templates,
even though such nanowires represent an interesting platform
for various applications ranging from drug delivery to sensor
technology to the still-challenging fabrication of linear arrays
of functional nanoparticles.[23] Herein we report the prepara-
tion of silica/titania hybrid nanowires that consist of a silica
core containing linear arrays of mesocages and a titania shell
by hierarchical self-assembly of block-copolymer soft tem-
plates inside nanoporous hard templates involving evapora-
tion-induced self-assembly (EISA)[7,10] and subsequent
atomic layer deposition (ALD).[24] The combination of
EISA and ALD is a versatile modular assembly system that
allows the internal mesoscopic fine structure and the proper-
ties of the outer nanowire surface to be tailored independ-
ently. The ALD step, for example, could improve the
biocompatibility of mesoporous nanowires loaded with drugs.
The synthesis of the silica/titania hybrid nanowires is
shown schematically in Figure 1. The mesoporous silica cores
were obtained with polystyrene-block-poly(ethylene oxide)
(PS-b-PEO) soft templates and tetraethyl orthosilicate
(TEOS) as the silica source. Homogeneous precursor sol-
utions containing TEOS, PS-b-PEO, 0.1m HCl, ethanol, and
toluene (weight ratio 1:0.2–0.5:0.1–0.3:40–60:0–15) were
prepared as described elsewhere[10,12] and infiltrated into
nanoporous alumina hard templates with pore diameters of 35
or 60 nm (Figure 1a). Evaporation of the volatile solvents
(ethanol and toluene) resulted in the onset of a microphase
separation where the polar component PEO segregates to the
oxidic pore walls and encases the hydrophobic phase con-
taining the PS blocks. The silica source (TEOS) selectively
segregates into the PEO-rich phase. The PS phase self-
assembles into a regular array of domains having the form of
slightly elongated spheres dispersed in a PEO/TEOS matrix
(Figure 1b). After gelation at room temperature for 24 h and
then at 1108C for 24 h, the PS-b-PEO soft template was
removed by calcination at 5508C for 6 h (Figure 1c). The
silica nanowires thus obtained, which were released from the
hard template by a wet-chemical etching step with aqueous
[*] Dr. X. Chen
National Laboratory for InfraredPhysics
Shanghai Institute of Technical Physics
Chinese Academy of Sciences
Shanghai, 200083 (China)
Fax: (+86)21-6583-0374
E-mail: xinchen@mail.sitp.ac.cn
Dr. X. Chen, Dr. M. Knez, Dr. A. Berger, Prof. K. Nielsch,
Prof. U. Gösele, Dr. M. Steinhart
Max Planck Institute of Microstructure Physics
Weinberg 2, 06120 Halle/Saale (Germany)
Fax: (+49) 345 5511223
E-mail: steinhart@mpi-halle.de
[**] This work was funded by the German Research Foundation (priority
program 1165 “Nanowires andNanotubes”, STE 1127/6-2) andby
the Volkswagen Foundation (thematic impetus: “Interplay between
Molecular Conformations andBiological Function”, Az. I/80 780).
Additional support by the German Ministry of Education and
Research (project no. FKZ 03N8701 andFKZ 03X5507) andthe
NSFC (no. 10334030), as well as technical help from S. Grimm and
K. Sklarek, is gratefully acknowledged.
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Figure 1. Diagram showing the preparation of titania/silica hybrid
nanowires containing linear mesocage arrays by hierarchical templat-
ing. a) A homogeneous precursor solution containing PS-b-PEO as a
soft template is infiltratedinto a self-orderedporous alumina hard
template. b) Evaporation of ethanol induces the formation of spherical
PS domains in a PEO/TEOS matrix by EISA. c) Gelation and calcina-
tion yieldsilica nanowires containing linear arrays of spherical meso-
cages. d) The silica nanowires are released by wet-chemical etching of
the hardtemplate ande) coatedwith titania by ALD.
phosphoric acid (Figure 1d), were found to contain linear
arrays of elongated spherical mesocages.
Figure 2a shows a representative transmission electron
microscopy (TEM) image of released silica nanowires
(length: 9.5 mm; diameter: 25–30 nm). These nanowires
were prepared with symmetric PS(9500)-b-PEO(9500)[25] as
a soft template and a hard template with a pore diameter of
35 nm and a pore depth of 10 mm. The diameters of these
silica nanowires are slightly smaller than those of the nano-
pores in the hard template owing to shrinkage during the
calcination step. The nanowires contain highly regular, linear
arrays of mesocages with a period of about 25 nm. These
mesocages are elongated, extending about 15 nm in the
direction of the fiber axis and about 20 nm in the transversal
direction (Figure 2b).
Figure 2. TEM images of releasedsilica nanowires containing a single
row of mesocages obtainedwith PS(9500)- b-PEO(9500) as the soft
template anda porous alumina hardtemplate (pore diameter: 35 nm;
pore depth: 10 mm): a) low magnification; b) high magnification.
The morphology of the silica nanowires can be adjusted by
varying the pore diameter of the hard template and the
composition of the precursor solutions. Figure 3a shows silica
nanowires (diameter: 50 nm) containing two rows of spherical
mesocages with a diameter of about 15 nm which were
obtained with asymmetric PS(9500)-b-PEO(18000) as a soft
template and a hard template having a pore diameter of
60 nm. The size of the hydrophobic domains in sols containing
amphiphilic block copolymers, such as PS-b-PEO, can be
tuned by the addition of organic co-solvents, such as toluene,
to the precursor solution.[9] This organic co-solvent enriches
the nonpolar PS domains, which therefore swell. Conse-
quently, the use of precursor solutions (weight ratio TEOS/
PS-b-PEO/0.1m HCl/ethanol = 1:0.5:0.25:40) containing
20 wt% toluene yielded silica nanowires with larger meso-
cages than those prepared under the same conditions but
without addition of toluene. For example, the use of precursor
solutions containing PS(9500)-b-PEO(18000) and toluene in
a hard template with a pore diameter of 60 nm resulted in the
formation of mesocages with a size of about 40 nm (Fig-
ure 3b) compared to about 15 nm without toluene (Fig-
ure 3a). The silica nanowires thus obtained with a diameter of
about 50 nm contain only a single mesocage row because of
the increased size of the mesocages. The arrangement of these
mesocages is less regular and their size distribution is
apparently broader than in the case of silica nanowires
prepared from toluene-free precursor solutions. Possible
reasons for this finding may be the occurrence of changes in
the EISA process and a change in the volume ratio between
the polar and nonpolar phases, which may lead to changes in
the equilibrium microphase structure of the sol.
Released mesoporous silica nanowires were further
functionalized in an ALD step to coat their outer surface
with a thin titania layer (Figure 1e). ALD is a versatile
approach for the fabrication of thin films on various
substrates, it involves successive two-step deposition cycles
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Figure 4. Titania/silica hybridnanowires: a) TEM image; b) EDX spec-
trum of a single nanowire.
Figure 3. Morphology tuning with PS(9500)-b-PEO(18000) as the soft
template andporous alumina hardtemplates with a pore diameter of
60 nm: a) TEM image of silica nanowires containing two rows of
mesocages; b) silica nanowires containing a single row of mesocages
with a size of about 40 nm obtained by adding 20 wt% toluene to the
precursor solution. The inset in b) shows a high-magnification image
of two adjacent nanowires.
Ti(OiPr)4 and water as precursors.[28,29] The presence of titania
was confirmed by analytical TEM investigations. As
expected, the characteristic peaks for Ti, O, and Si appear
in the energy-dispersive X-ray (EDX) spectrum recorded
from a single hybrid nanowire (Figure 4b). The carbon and
copper peaks are due to the TEM grid onto which the
nanowires were deposited.
In summary, we have demonstrated that evaporation-
induced self-assembly of a block-copolymer soft template
inside a nanoporous hard template yields silica nanowires
containing linear arrays of mesocages. Combining this hier-
archical self-assembly process with atomic layer deposition
allows the internal mesoscopic fine structure of the nanowires
and the chemical properties of their outer surface to be
tailored independently.
with intermediate purging steps. In the first step the sample is
exposed to the vapor of a first, reactive precursor so that a
layer of the precursor molecules is bound to the substrate
surface. Residual precursor molecules are then removed by
purging. A second gaseous precursor converts the first one
into the target compound in the second step. The overall
thickness of the deposited layer can be adjusted by changing
the number of successive deposition cycles and gives a
precision on the subnanometer scale. Thin inorganic films
have been deposited by ALD on a variety of organic,
biological, and inorganic nanostructures, such as nano-
spheres,[26] nanowires,[27] and plant viruses.[28]
Experimental Section
Figure 4a shows a TEM image of mesoporous silica/
titania hybrid nanowires coated with an approximately 15-
nm-thick outer titania layer. The mesoporous silica cores
were prepared with a precursor solution containing PS(9500)-
b-PEO(18000) as a soft template (weight ratio TEOS/PS-b-
PEO/0.1m HCl/ethanol/toluene = 1:0.5:0.25:50:10) in a hard
template with a pore diameter of 35 nm. The cores were
subjected to 200 ALD cycles with tetraisopropyl titanate
The precursor solutions were prepared according to protocols similar
to those reported elsewhere[10,12] by mixing TEOS (Alfa Aesar), PS-b-
PEO (Polymer Source Inc., Canada), 0.1m HCl, ethanol, and toluene
(weight ratio TEOS/PS-b-PEO/0.1m HCl/ethanol/toluene = 1:0.2–
0.5:0.1–0.3:40–60:0-15). To increase the size of the mesocages,
symmetric PS(9500)-b-PEO(9500) was dissolved in an ethanol/
toluene mixture (weight ratio 5:1), then 0.1m HCl and TEOS were
added (weight ratio TEOS/PS-b-PEO/0.1m HCl/solvent mixture =
1:0.4:0.2:50). The porous alumina templates were immersed in the
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Received: March 1, 2007
Published online: August 2, 2007
Keywords: atomic layer deposition · block copolymers ·
.
materials science · nanostructures · self-assembly
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