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Centrifugal electrospinning of highly aligned polymer nanofibers over a large
area†
Dennis Edmondson,‡Ashleigh Cooper,‡Soumen Jana, David Wood and Miqin Zhang*
Received 15th June 2012, Accepted 20th July 2012
DOI: 10.1039/c2jm33877g
Well-ordered one-dimensional nanostructures are enabling important new applications in textiles,
energy, environment and bioengineering owing to their unique and anisotropic properties. However,
the production of highly aligned nanofibers in a large area remains a significant challenge. Here we
report a powerful, yet economical approach that integrates the concepts of the parallel-electrode
electrospinning with centrifugal dispersion to produce nanofibers with a high degree of alignment and
uniformity at a large scale. We first demonstrated this approach with polyvinylidene fluoride to show
how experimental parameters regulate fiber properties, and then with chitosan, a natural polymer, and
polyethylene oxide, a synthetic polymer, to illustrate the versatility of the system. As a model
application, we then demonstrated the significance of fiber alignment in improving the piezoelectric
effect for voltage generation. The technique presented here may be used for mass production of aligned
nanofibers of various polymers for a myriad of applications.
Introduction
Nanoscale fibers are widely used in textile, energy, environmental
and bioengineering applications as they exhibit unique optical,
1
electrical,
2
mechanical,
3
and biological
4
properties that are not
found in their bulk counterparts. Some of these applications
require highly ordered, well-aligned fiber architectures in order
to provide the required physical, mechanical, chemical or elec-
trical anisotropy. For example, aligned polymer fibers of various
compositions are able to regulate cell migration, proliferation,
and differentiation, which is critical for tissue engineering.
5,6
Highly aligned polyfluorene-based nanofibers can increase
charge-carrier mobility or enhance photoluminescence in the
fiber alignment direction.
7
Composite electrolyte membranes
with aligned polyimide-based fibers demonstrate greater proton-
conduction for enhanced fuel cell efficiency.
8
Electrospinning has
emerged as a simple, flexible, and versatile technique for creating
many nanofiber-based materials.
9
The production of aligned nanofibers by electrospinning is
commonly achieved by use of specially designed fiber collectors,
most notably, a fast rotating mandrel collector
10,11
or a parallel-
electrode collector.
12
In a rotating collector configuration, the
produced polymer fibers are deposited on and wrapped around a
rotating mandrel.
10,11,13
The degree of fiber alignment largely
depends on the mandrel rotational speed. In a parallel-electrode
configuration, the insulating gap (mostly an air gap) between two
parallel electrodes serves as the fiber collector, and charged fibers
are aligned up across the gap by the electric field near the elec-
trodes that points perpendicularly to the electrode edges;
14
the
length of the aligned fibers is limited by the width of the insu-
lating gap.
15
This configuration bears the advantage that the
fibers can be easily removed from the collector, but the degree of
fiber alignment decreases as the thickness of the fibrous mat
increases due to the reduced electric field strength caused by the
accumulated charge of the deposited nanofibers.
13,16
Alternative
to collector modification, centrifugal dispersion of polymer
solution has been recently explored to produce aligned fibers.
17,18
In this system configuration, a centrifugal force disperses a
polymer–solvent solution through a capillary, which causes
elongation and thinning of the solution jet, and the fiber is
produced with no applied voltage.
Despite these exciting advancements, an electrospinning
system that is suitable for large-scale production of nanofibrous
structures while retaining a high degree of fiber alignment has yet
to be demonstrated. Here, we present a hybrid electrospinning
system capitalizing on the fiber alignment mechanisms of both
the parallel-electrode method and centrifugal dispersion that can
produce highly aligned polymer nanofibers at a large scale. We
first demonstrate the capability of this system using poly-
vinylidene fluoride (PVDF) as a model polymer due to the
favorable piezo-, pyro-, and ferro-electric properties that aligned
PVDF fibers can provide for applications in actuators, transis-
tors, textiles and composites.
19
We then further demonstrate the
versatility of the system with two additional polymers, chitosan
(a natural polymer) and polyethylene (a synthetic polymer), that
Department of Materials Science and Engineering, University of
Washington, 302L Roberts Hall, Box 352120, Seattle, WA 98195, USA.
E-mail: mzhang@u.washington.edu; Fax: +1 206-543-3100; Tel: +1 206-
616-9356
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm33877g
‡ These authors contributed equally to this work.
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have shown a wide range of applications in medicine, biotech-
nology and food industries.
Experimental
Centrifugal electrospinning system setup
The CE system includes a syringe-needle-spinneret positioned on
a rotating hub driven by a variable speed electric motor (Amtek,
Monrovia, CA). The spinneret is connected to an A30, 30 kV dc
voltage power supply (Ultravolt, Ronkonkoma, NY) and
centered in a non-conductive cylindrical housing with a diameter
that can be varied from 2 to 4 feet. Eight aluminum plates
(electrodes), each attached to a grounded, 0.5 inch diameter
aluminum rod, are secured to the non-conducting housing,
concentrically surrounding the central hub (Fig. 1).
Polymer solutions for electrospinning
Polyvinylidene fluoride tetrafluoroethylene and polyvinylidene
fluoride (PVDF) (Arkema Corporation, King of Prussia, PA)
were mixed at a weight ratio of 70/30. The polymer mixture was
dissolved in dimethyl formamide (DMF)–acetone at a weight
ratio of 60/40 to create final polymer concentrations between 20
and 27.5 wt%. To aid in the dissolution, the PVDF solution was
refluxed at 80 C for 30 min.
Centrifugal electrospinning setup characterization
A 20 wt% PVDF solution was electrospun using a 25 gauge
needle (0.26 mm ID), at a voltage of 12 kV dc, and a spinneret–
collector distance of 20 cm. As a control, a low pressure of 0.1 psi
was applied to the syringe to drive the flow of polymer solution
which was electrospun without spinneret rotation to illustrate
static dispersion and parallel electrode deposition. The same
solution was then electrospun with the spinneret rotated at 100,
200, 300 and 400 rpm. To determine the influence of the polymer
concentration on the fiber diameter, the PVDF concentration
was varied between 20 and 27.5 wt% in the DMF–acetone
solution. From these tests, a rotational speed of 200 rpm was
chosen as the spinneret speed.
Fiber characterization
Fibers were retrieved from the gaps of electrodes, sputter-coated
with Au/Gd for 30 seconds at 18 mA, and imaged with a SEM
(JOEL JSM 7000F) at an operating voltage of 5 kV. Fast Fourier
transform (FFT) was performed using ImageJ (NIH, Bethesda,
Maryland, USA) on a representative image to determine the fiber
alignment. Specifically, an image was uploaded into ImageJ
software and FFT analysis produced a pixel intensity image
based on the frequency and direction of the fibers. The FFT
images were normalized to a vertical axis with a baseline value of
zero and radial pixel summing was performed using an oval
prolife plug-in.
20
The FFT data were plotted over 180as the
FFT image is symmetric about the horizontal axis.
To demonstrate the piezoelectric functionality of PVDF fibers,
fibers were electrospun from a 20 wt% PVDF in DMF–acetone
solution containing 3 wt% tetrabutylammonium chloride
(TBAC), which increases the solution conductivity to increase
the electrospinnability and effectively increases the b-phase
formation to contribute to the piezoelectric effect.
19
Samples
were electrospun across a three-inch gap to form aligned fibers,
and randomly oriented fibers were collected from the collector
plate. Samples were also retrieved from a stationary dispersion
condition (without spinneret rotation) across the same four-inch
gap to illustrate a conventional parallel-electrode configuration.
The sample dimensions were 0.74 mm
2
by 25 mm long. The fiber
specimens were encased in PDMS, forming a cantilever beam
with exposed fibrous ends. The specimens were tested according
to the schematic in Fig. 6a. Electrodes were connected to the
exposed fibers via conductive silver epoxy and connected to an
Agilent 34420A NanoVolt meter that transmits voltage data to a
computer running LabVIEW software via a custom vi program.
The fiber specimen/PDMS samples were clamped to a support
Fig. 1 Centrifugal electrospinning (CE) system for large-area production of aligned polymer nanofibers. (a) Schematic illustration of the system
configuration. (b) Photograph of the CE system with deposited PVDF nanofibers. (c) Electrospun PVDF fibers deposited across a four-inch gap between
two grounded electrodes.
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mounted on a nanomechanical tester developed in our lab.
21
The
entire testing system was kept in a quiescence state for 120
seconds before initiating beam deflection of the sample. The
cantilever beam was deflected at 9.70 mm min
1
for 30 seconds.
Three separate samples were tested and the results were averaged
and normalized.
X-ray diffraction analysis was performed to determine the
relative crystalline phases that contribute to the PVDF piezo-
electric response. PVDF TBAC samples were prepared for using
powder, films, and fibers in the form of random, stationary
aligned (parallel-electrode) and centrifugally aligned (200 rpm)
configurations. All samples were immobilized on a silicon
substrate to reduce background noise. A Bruker D8 Discover
XRD system with general area detector diffraction systems was
used to probe the two theta range of 16–28. Analysis was per-
formed with Jade software and plotted with relative intensities
for direct comparison of the diffraction peak locations.
Results and discussion
Centrifugal electrospinning system
The primary components of this centrifugal-electrode electro-
spinning (CE) system are a rotating hub hosting a spinneret (or,
syringe-needle) and an array of grounded plate electrodes that
circularly surround the rotating hub (Fig. 1a). The spinneret
powered by a high-voltage power supply dispenses the polymer
solution while the grounded electrodes introduce a fringe elec-
trostatic field facilitating fiber alignment when charged fibers are
deposited across the gaps (i.e., collectors) between the neigh-
boring electrodes (Fig. 1b). In addition to the alignment by the
electrostatic force, the solution jet gains additional momentum
towards the direction of fiber alignment upon exit from the
spinneret due to the centrifugal force created by the rotating
spinneret. As the leading tip of a fiber is deposited on an elec-
trode, the fiber follows a continuous curvilinear path as the
polymer solution is continuously dispersed from the rotating
spinneret with the trailing end deposited on the neighboring
electrode, enhancing the fiber alignment. The length of the
aligned fibers is defined by the width of the gap, and typically,
nanofibers of a few inches in length can be readily produced
(Fig. 1c).
Clearly, as with any electrospinning systems, the formation
and characteristics of nanofibers for each polymer solution (type
and concentration) are strongly dependent on the system oper-
ating parameters, such as the net charge density (or supplied
voltage), width of the electrode gap, and spinneret rotating speed
(for our CE system). Here we have illustrated how these
parameters affect the properties of electrospun nanofibers using
PVDF as a model polymer system.
Spinneret rotating speed
In a conventional electrospinning process, polymer solution in
the reservoir is electrostatically charged and when the applied
electric field gradient overcomes the surface tension of the
polymer solution, a Taylor cone forms at the tip and outside of
the spinneret. The electrostatic force causes a solution jet to form
at the tip of the Taylor cone and the solution jet elongates
towards the collector, wherein the solvent evaporates, producing
charged polymer fibers.
22
In our CE system, a radial force created
by centrifugal dynamics helps stretch the polymer solution that
exits the spinneret tip. If the rotation speed of the spinneret is
zero, the CE system is analogous to a traditional parallel-elec-
trode configuration.
To see how the centrifugal dispersion affects the properties
(diameter, uniformity and alignment) of electrospun fibers, a
series of experiments with sequentially increased rotational speeds
were used to dispense a 20 wt% PVDF solution. As shown in
Fig. 2a, with a stationary spinneret (i.e., the rotating speed ¼0),
produced PVDF fibers were mostly aligned but misaligned fibers
and a few beads were present. As the spinneret rotating speed was
increased, the degree of fiber alignment increased. At the spin-
neret rotating speed of 200–300 rpm, no apparent misalignment
was observed. Further increase in the rotating speed resulted in
decreased fiber yield, and at a rotational speed of 400 rpm, no
fibers could be collected, indicating that a too-high rotational
speed would impede continuous fiber formation for the polymer
solution used here.
FFT analysis was performed on the SEM images to determine
the relative degree of fiber alignment based on the conversion of
the image into frequency spacing (Fig. 2b). A dispersed,
randomly oriented fiber sample exhibits a radially diffuse image
in FFT analysis, while a highly aligned fiber sample exhibits a
high intensity line normal to the fiber orientation. The FFT
images were then analyzed with an oval-plot profile, wherein the
radial intensity was summed and plotted with respect to the angle
of acquisition. As shown in Fig. 2c, the fibers prepared at 200
rpm had the narrowest peak, with the smallest area underneath
the curve, indicating that this rotational speed produced the
highest degree of fiber alignment. Fibers produced from the
stationary spinneret were least aligned.
In addition to the fiber alignment, the fiber morphology
changed with the spinneret rotational speed. At zero or low
(100 rpm) rotational speeds, a beads-on-a-string structure was
observed. With stationary-spinneret dispersion, the high surface
tension of the solution predominated the fiber formation,
wherein the viscoelastic force in the solution resisted changes to
the fiber jet shape, resulting in the beads-on-a-string structure.
23
With increasing spinneret rotational speed, the fiber uniformity
increased and the number of beads in the fiber mat was reduced.
At 200 rpm, no beads were observed, indicating that the
centrifugal force effectively overcame the surface tension. At a
higher speed of 300 rpm, the degree of fiber alignment remained
high, but a few beads reappeared. At an even higher speed
(400 rpm), only beads were produced (data not shown). The bead
formation at these high rotational speeds was likely due to
destabilization of the polymer solution by the increased centrif-
ugal force, which might have prevented the formation of the
Taylor cone and subsequent fiber formation. The destabilization
occurs when the delivery rate of the solution to the spinneret tip
was smaller than the rate at which the solution is removed by
electrostatic and centrifugal forces, leading to non-continuous
fiber formation.
Notably, a decrease in fiber diameter was observed with
increasing spinneret rotating speed (Fig. 2d). The diameter of
fibers produced from the stationary spinneret was 345 nm and
decreased to 224 nm at a spinneret rotating speed of 300 rpm.
The decrease in fiber diameter was attributed to the enhanced
18648 | J. Mater. Chem., 2012, 22, 18646–18652 This journal is ªThe Royal Society of Chemistry 2012
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elongation (and thinning) of the solution jet by the increased
centrifugal force at the fixed solution-feeding rate. Based on these
results, we concluded that a spinneret speed of 200 rpm produces
the most aligned, uniform and smallest diameter fibers from a
20 wt% PVDF solution.
Electrode gap width
The parallel-electrode gap width has a significant impact on the
fiber alignment and dictates the ability of the CE system to
produce long, aligned fibers. The electric field originates radially
from the charged spinneret, directed towards the collector elec-
trodes. Near the electrodes, the electric field lines bend in the
horizontal direction, perpendicular to the electrodes. It has been
demonstrated that the relative magnitude of this horizontal
component of the field increases with increasing gap width, which
favors the fiber alignment across the gap.
16
However, there is a
maximum electrode gap width within which the high-degree of
fiber alignment can be maintained. Above this maximum width,
the degree of fiber alignment decreases due to reduced electric field
strength that aligns charged fibers. As shown in Fig. 3, a 20 wt%
PVDF solution electrospun at 12 kV and 200 rpm spinneret
rotating speed produced highly aligned fibers across both a
one-inch and a four-inch gap (Fig. 3a and b), but the fibers were
not as well aligned on the six-inch gap (Fig. 3c). This was further
illustrated by FFT analysis (Fig. 3d). The four-inch electrode gap
was thus chosen for further studies described below as it maintains
good fiber alignment while producing long fibers.
Polymer concentration
The viscosity of the solution affects the fiber formation, as well as
the resultant fiber morphology and diameter. For a given poly-
mer, the solution viscosity depends on the solvent type and
polymer concentration. There is a concentration range for each
polymer for which continuous nanofibers can be produced. If
the polymer concentration is too low, there is insufficient chain
entanglement to form continuous fibers; conversely, if the
concentration is too high, the resultant high viscosity and surface
tension impede solvent evaporation and jet thinning, resulting in
large fiber diameters.
24
In this study, PVDF solutions with polymer concentrations of
20–27.5 wt% were electrospun and evaluated. As shown in Fig. 4,
the fiber diameter and non-uniformity increased as the polymer
concentration increased; the 20 wt% and 22.5 wt% PVDF solu-
tions yielded more uniform fibers with smaller diameters (Fig. 4a
Fig. 2 PVDF fibers produced at various spinneret rotating speeds. (a) SEM images of PVDF nanofibers (scale bars ¼20 mm) deposited across a
four-inch gap from a 20 wt% polymer solution electrospun at 12 kV. (b) FFT analysis illustrating the degree of fiber alignment. (c) Arbitrary pixel
intensity plotted from the radial summation of pixel intensity from the FFT analysis. (d) Fiber diameter distribution with the median diameter
represented by the horizontal line shown in the middle of the bar. The error bars indicate the largest and smallest fiber diameters measured from a
dataset of 50 fibers.
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and b) than 25 wt% and 27.5 wt% solutions (Fig. 4c and d). For
applications where the smallest diameters (thus the greatest
surface area-to-volume ratio) are preferred, the optimal PVDF
concentration range was 20–22.5 wt% for a spinneret rotating
speed of 225 rpm. With the operating parameters chosen from
the above investigations, we were able to produce highly aligned
PVDF nanofibers with a 20 wt% PVDF solution (Fig. 5).
Using the same methodology shown above, well aligned
nanofibers of other polymeric materials can be readily produced
with our CE system (see ESI†).
An illustrative application of aligned nanofibers in
piezoelectricity
To demonstrate the significance of fiber alignment in practical
application, we examined the piezoelectric properties of PVDF
fibers. In a traditional piezoelectric application of PVDF, the
piezoelectric effect is strongly dependent on the crystalline phase
and content (a,b,g,d), and PVDF materials can be mechanically
or thermally manipulated to induce crystalline changes. Elec-
trospinning has been shown to induce poling of PVDF along the
fiber length due to the strong electrical field and mechanical fiber
stretching during the electrospinning process.
25
The aligned fibers
have been particularly favored for better piezoelectric response.
26
Thus, highly aligned PVDF fibers prepared from our CE system
were expected to produce significant piezoelectric responses
compared to those prepared by traditional electrospinning
methods.
Aligned PVDF fibers were prepared across three-inch elec-
trode gaps at an applied voltage of 15 kV and a spinneret rotating
speed of 200 rpm from a 20 wt% PVDF solution with 3 wt%
tetrabutyl ammonium chloride (TBAC). For comparison,
randomly oriented fibers as well as aligned fibers produced by the
parallel electrodes (spinneret rotational speed ¼0) were also
prepared. All fiber samples were 25 mm long with a cross-
sectional area of 0.76 mm
2
. The samples were embedded in
PDMS, connected via electrodes to a voltage-output analyzer
and clamped onto a nanomechanical tester that could produce
controlled strain rates (Fig. 6a). A common method to measure
the piezoelectric response of nanofibers is through a mechanical
bending test.
25,27,28
In this study, the fiber/PDMS specimen was
deflected in a cantilever motion at one end, which caused a
bending strain across the cross-section of the sample and
produced a measurable output voltage. The aligned fibers shown
in Fig. 5b produced an output voltage of 3.04 mV at a strain of
0.10 compared to the only 0.059 mV for the randomly oriented
fibers. Though the individual PVDF fibers in the randomly
Fig. 3 Influence of the electrode-gap width on the PVDF fiber
morphology. SEM images of the PVDF nanofibers from a 20 wt%
polymer solution electrospun at 12 kV with a spinneret rotating speed of
200 rpm and retrieved from (a) one-inch, (b) four-inch, and (c) six-inch
electrode gaps. The scale bars represent 2 mm. Insets in (a), (b), and (c) are
the images from FFT analysis illustrating the degree of alignment. (d)
Arbitrary pixel intensity plotted from the radial summation of pixel
intensity from the FFT analysis.
Fig. 4 Influence of the polymer concentration on the morphology of electrospun PVDF fibers. SEM images of fibers produced from (a) 20, (b) 22.5, (c)
25, and (d) 27.5 wt% PVDF in DMF–acetone. The scale bars represent 2 mm. The PVDF solutions were electrospun at 12 kV and 200 rpm, and the fibers
were collected from a four-inch electrode gap. (e) Size distribution of electrospun PVDF nanofibers as a function of polymer concentration. The error
bars indicate the largest and smallest fiber diameters measured from a dataset of 50 fibers.
Fig. 5 Highly aligned PVDF fibers produced by the CE system with a
20 wt% polymer solution at 15 kV and 225 rpm.
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oriented specimen were likely poled, the randomly oriented
poling directions in the membrane’s bulk macrostructure
(Fig. 6b) resulted in a negligible collective.
Importantly, the partially aligned (Fig. 6c) fibers produced by
stationary dispersion (0 rpm, i.e. the parallel electrode method)
representing the traditional parallel-electrode method produced
0.11 mV voltage outputs. The introduction of a centrifugal
dispersion at 200 rpm enhances piezoelectric response by 27
fold (Fig. 6d) through fiber alignment and molecular poling
alignment, demonstrating the potential of the CE technique to
produce highly aligned, functional materials for a variety of
applications.
XRD analysis was performed to characterize the changes in
crystalline phases, which correlate with the piezoelectricity of
PVDF, by the introduction of the centrifugal dispersion force via
the CE system. As shown in Fig. 6e, the original, as-received
powder form of the PVDF material exhibited peaks at 18.5and
20.3which are associated with the a-
29
and g-phases of PVDF.
30
In the preparation of the electrospinning solution, PVDF and
TBAC were dissolved in a DMF–acetone solvent and refluxed at
80 C. To demonstrate the effect of the solvent and refluxing on
the piezoelectric properties, a film sample was characterized with
a single, diffuse g-phase peak, which corroborates previous
results of PVDF in DMF–acetone solution crystalline phases.
31
With the introduction of the electrospinning force, b-phase peaks
appear at 20–22, as shown by the randomly oriented fiber
spectra, stationary aligned and centrifugally aligned fibers. The
high voltage applied to the electrospinning solution changes the
molecular conformation of PVDF and the stretching of the
polymer jets results in initial mechanical stretching, which causes
nucleation of the b-phase. As all of the electrospun fibers,
regardless of dispersion or collection methods, were composed of
only the b-phase, there was no significant impact of the centrif-
ugal dispersion on the polymorphism of PVDF. However, as the
proposed CE system produced more highly aligned nanofibers
than the traditional parallel-electrode method, the resultant
PVDF nanofiber responses were enhanced due to the macro-
scopic anisotropy of the fibers which coincided with the b-phase
polarity.
Conclusions
Well-aligned nano-architectures are required in many engi-
neering and biomedical applications to develop sophisticated
electrical, chemical or biological devices. Several approaches
have been used to modify the collectors of electrospinning
systems in order to produce aligned nanofibers. A conventional
parallel-electrode electrospinning system can produce nanofibers
with a high degree of alignment, but only over a small area. A
centrifugal dispersion system, on the other hand, can produce
nanofibers over a large area, but the degree of nanofiber align-
ment is limited. This study presents a new approach to the
electrospinning and demonstrates the fabrication of highly
aligned and uniform nanofibers over a large area (with fiber
length up to several inches). It should be noted that here we only
illustrated the basic principle and methodology of the CE system
setup and demonstrated how the nanofibrous structure can be
modulated by a few key system parameters, and in no way
exhausted all the variables that may play a significant role. Other
important parameters include the distance between the spinneret
and collector, applied voltage, and solution-feeding rate. Also
important is the interplay of these variables. The piezoelectric
effect of the highly aligned PVDF fibers produced by the CE
method demonstrated their potential broad application. Even for
a given polymer, different applications require different material
properties and thus different system setups. Fortunately, our CE
system provides the flexibility to easily adjust the key parameters.
For instance, the eight collector gaps between the grounded
electrodes in our system can be set to different widths so that the
effect of the electrode gap width on the fiber production and
properties can be revealed by a single run.
Acknowledgements
This work was supported in part by Kyocera Professor
Endowment. The authors would like to thank Vincent Casmirri
and Coleen Caputo of Arkema Corporation for supplying the
polyvinylidene fluoride and polyvinylidene fluoride tetrafluoro-
ethylene chemicals and Tanner Edmondson for fabrication and
assembly assistance.
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18652 | J. Mater. Chem., 2012, 22, 18646–18652 This journal is ªThe Royal Society of Chemistry 2012
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