# Facile Synthesis of Highly Aligned Multiwalled Carbon Nanotubes from Polymer Precursors

Authors: Catherine Y. Han; Zhi-Li Xiao; H. Hau Wang; Xiao-Min Lin; Susana Trasobares; Russell E. Cook
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

We report a facile one-step approach which involves no flammable gas, no catalyst, and no in situ polymerization for the preparation of well-aligned carbon nanotube array. A polymer precursor is placed on top of an anodized aluminum oxide (AAO) membrane containing regular nanopore arrays, and slow heating under Ar flow allows the molten polymer to wet the template through adhesive force. The polymer spread into the nanopores of the template to form polymer nanotubes. Upon carbonization the resulting multi-walled carbon nanotubes duplicate the nanopores morphology precisely. The process is demonstrated for 230, 50, and 20 nm pore membranes. The synthesized carbon nanotubes are characterized with scanning/transmission electron microscopies, Raman spectroscopy, and resistive measurements. Convenient functionalization of the nanotubes with this method is demonstrated through premixing CoPt nanoparticles in the polymer precursors.


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## Body

## 1. Introduction

The
discovery of carbon nanotubes (CNTs) by Iijima in 1991 opened up a brand new
era in materials science and nanotechnology [1]. The intrinsic electronic
properties of single-walled carbon nanotubes are such that they may be metallic
or semiconductive depending on their  diameter and the graphitic ring
arrangement around the tube wall [2]. Furthermore, CNTs show exceptionally good
thermal and mechanical properties. It is expected that CNTs could solve the
thermal dissipation problem of nanodevices due to their high thermal conductivity.
CNTs can transport significant  amount of electric current without doping
problem encountered in Si-based FETs (field effect transistors) because of the
graphitic nature, and the covalent bonds among carbon atoms are much stronger.
CNTs are among the most promising materials anticipated to impact future
nanotechnology due to their unique structural and electronic properties, which
have generated great interest for application in a broad range of potential
nanodevices.  The unique properties of
CNTs have led to the study of their use in areas as diverse as sensors [3],
electrochemical actuators [4], nanoelectronics [5], field-emitting flat panel displays
[6], battery [7], scanning probe microscopy tips [8–10], FETs [11, 12],  gas
storages [13], and so forth.Extensive
efforts have been made to control the growth and properties of CNT since their
discovery in 1991 [1].  Large quantities
of carbon nanotubes can now be produced by arc discharge [14], laser ablation
[15], chemical vapor deposition methods [16–20], or flame synthesis
[21–24].  However, the applications of
CNTs have been limited because of problems in catalyst residues, difficulty with
the alignment, diameter uniformity of the nanotubes mixed metallic and
semiconductive properties, and so forth  [21].
One of the most efficient approaches to the production of large areas of
highly ordered, isolated long CNTs with monodispersed tube diameter and length
is based on template-confined growth of CNTs.
The diameter, length, and packing density of CNTs can be ideally
controlled when the nanotube arrays are fabricated in porous anodic aluminum
oxide (AAO) templates [16, 25–27].
Furthermore, porous AAO templates with well-controlled 3D channel
structures such as dumbbell shaped [28], linearly joined [29, 30], Y-branched
[31] as well as novel dendriform have been prepared and used to grow well-aligned arrays of CNTs through the chemical vapor deposition method [32]. Pyrolysis of gaseous hydrocarbons such as  C2H2,
C2H4, and C3H6 in templates can be
achieved with or without catalyst, but both processes require specialized
reaction chamber, various gas supply, careful control of gas flow, pyrolyzing
temperature higher than 650°C,
and costly safety equipments for highly flammable gases.Graphitic
nanotubes have also been synthesized by carbonization of the polyacrylonitrile
[33] and poly(furfuryl alcohol)  [34, 35] within the pores of AAO membranes at
600 and 900C°, respectively.
The synthesis is achieved by carbonization of polymers through a three-step
approach: (1) monomers and initiators are infiltrated into the pores of AAO
template, (2) the polymers are produced inside the pores of AAO template
through polymerization, and (3) The CNTs are produced through carbonization of
the polymers prepared in step (2). The whole process requires carefully
controlled polymerization initiation and it is tedious and time consuming.  No detailed structural characterization was reported
in the first case [33], and bamboo and bubble structure were obtained for the
latter [35].Herein
we report a convenient one-step approach for the preparation of well aligned
carbon nanotube array.  Commercially
available polymers are used for the study.
No catalyst and no monomer initiator are required.  Comparing to pyrolysis of highly combustible
gases, this method is simple and avoids many safety concerns.  In addition, the as-prepared aligned CNTs
contain open pores that are ready for functionalization or filling with
nanoparticles. Polymer nanotubes
prepared with templates through melt-wetting in the 300 to 900 nm pore diameter
region have been reported [36].  Due to
the high viscosity nature of the polymers in general, whether the melt-wetting
method works for much smaller pores has not been addressed.  In this work, we explore templates with pore
diameters between 230 and 20 nm with melt-wetting, and through carbonization at
a moderate temperature we report good quality multi-walled carbon nanotubes
(MWNTs) formation.  Aligned CNTs with
both ends open or with one end open and one end closed are presented here.


## 2. Experimental

All
chemicals and polymers are reagent grade from Aldrich Chemicals except the
five-minutes epoxy (Devcon Corp.). Three types of porous anodic aluminum oxide
(AAO) membranes are used as templates in this work. One is a commercially
available membrane with 60μm
thickness and 230 nm pore diameter (Whatman Ltd. , Anodisc 13 mm) [37].  The other two are prepared by anodic oxidation
of high purity aluminum plate (Alfa Aesar, 99.99%, 0.25 mm thick) through a
two-step anodization process that has been reported in  literature [38, 39].
The membranes are 60 μm
thick, and the pore diameters are 50 and 20 nm, respectively.  A typical procedure for polymer carbonization
in AAO membrane is described here.
Either solid or liquid polymers are placed on top of AAO templates
(~25–30 mg polymer per disk). The polymer
over AAO template in an alumina boat is placed in a tube furnace (Lindberg).  Ar is purged for ~30 minutes.  Then the temperature is slowly increased at a
rate of about 2C°/min
from ambient to the desired temperature (typically 600C°).
The temperature is kept constant for 3 hours under Ar flow to fully
carbonize the polymer.  The polymer
slowly melts and spreads into the nanopores of the AAO template to form polymer
nanotubes.  Starting at a very low
temperature of ~400C°,
the polymers are carbonized, and graphitic nanotubes are generated.  Thermal decomposition of small molecules and
polymers is a complex reaction.  We
observe good Raman signals and the presence of MWNT with SEM from all blackened
areas on AAO (vide infra).  Therefore, we estimated the MWNT yield based
on the starting reagent weight and the final weight gain on the AAO
template.  Typical yield is about 1% by
weight.The Raman measurements are carried out at room
temperature with the  use of a Raman microscope spectrometer (Renishaw, Ltd.)
equipped with a HeNe laser (632.8 nm). A180°  reflective geometry with the analyzer polarization
parallel to the incident laser beam polarization  is adopted. The spectrum is
calibrated against a Si wafer standard (520 cm−1).  The spectrum analysis is done with the  use of
Origin software.  The UV-Raman
measurement is carried out with the  use of a frequency-doubled argon ion laser with
excitation at 244 nm.  Typical spectrum
is averaged over 10 scans.  Field-emission scanning
electron microscopy  (FESEM) images  are obtained on  Hitachi S-4700-II field
emission SEM operating with an accelerating voltage of 10 kV. Transmission electron micrographs (TEM) and
electron diffraction patterns are obtained on a Philips CM30T at 200 kV.


## 3. Results and Discussion

A range of polymers from very low molecular weight
of a few hundred to medium size ones of ~450 k (g/mol) has been applied in this
study (Table1).  Regardless whether the
polymer is solid or viscous liquid, upon heating all polymers melt at higher
temperatures and the wetting phenomena between the polymers and AAO templates
occur.  The melts wet the membranes very
well, and this is likely due to the fact that  the cohesive force for complete filling of the
nanopores  is not as strong as the adhesive force for wetting the walls of the
nanopores [36].  This wetting process
leads to AAO membranes that are completely covered  with polymers, and upon
carbonization at temperature beyond 400C°,
the entire membrane is blackened.
However, the faster rate of adhesive force must be properly balanced
with a slower sample heating rate in order to allow sufficient flow and maximize
the carbon nanotube formation.  Typical
sample heating rate in this study is 2C°/min.  When higher heating rate such as 5C°/min is used, the AAO membrane shows
inhomogeneously darkened areas due to fast thermal decomposition of the
melts.  The blackened AAO membranes are
subject to Raman, SEM, TEM, and conductivity measurements.  Control experiment with use of polymers on
flat substrate (nonporous alumina) upon  carbonization fails to yield any CNTs.  Sample characterization, the  effects of
different starting polymers, and preliminary results on incorporation of
nanoparticles are presented below.Table 1
Polymer precursors and the resulting MWNT morphology and resistance.
PolymersMW g/molResistanceProduct
morphologyPoly(bisphenol
A-co-epichlorohydrin) glycidyl ether35520 kΩMWNTsBisphenol A propoxylate
diglycidyl ether45740 kΩMWNTsBisphenol A ethoxylate
dimethacrylate1,7006 kΩMWNTsPoly(ethylene glycol) methyl
ether2 k7 MΩNot
well formedPolyisoprene40 k1 MΩNot
well formed, rubber-likePolystyrene-co-poly(methyl
methacrylate)100 k–150 k10 kΩMWNTs,
some sponge-like featuresPMMA120 k25 kΩMWNTsPolystyrene-b-polybutadiene140 k300 kΩFlat
collapsed tubesPS polystyrene350 k—Complete
decompositionPoly(acrylic acid)450 k500 kΩFlat
collapsed tubes


## 3.1. Raman Characterization

Raman spectroscopy has been used extensively for the
characterization of multi-walled (MWNTs) and single-walled carbon nanotubes (SWNTs).  The polymer/AAO membrane (230 nm) after
carbonization at 600C°
has been subjected to Raman studies (typical polymer, e.g., is bisphenol
A ethoxylate dimethacrylate with Mn ~1700).
After carbonization at 600C°
under Ar over three hours, the AAO membrane is completely black with slight
shiny luster. The Raman spectrum is shown in Figure 1.  The first-order (below 2000 cm−1)
and second-order (2000–4000 cm−1) Raman scattering peaks are clearly
visible with the third-order peak (above 4000 cm−1) noticeable.  The very broad background over the entire
measurement range is due to luminescence from the AAO membrane as indicated in
the AAO Raman spectrum in Figure 1.  Upon
background subtraction, the analyzed Raman spectrum is shown in Figure 2.  The black dots are the experimental Raman
data, and the red trace is the overall fitting with 7 Lorentzian peaks as
indicated individually in blue traces.
The strongest peaks are D- and G-lines at 1341 and 1593 cm−1,
respectively.  The D-line is due to the
disordered sp2
hybridized
carbon, and the G-line is associated with the tangential stretching mode (E2g)
of highly ordered pyrolytic graphite (HOPG) and indicates the presence of
crystalline graphitic carbon in the MWNTs [40, 41].  The second-order Raman peaks corresponded to 2×D  (overtone), and D + G (combination) have also
been observed.  The complete observed
Raman peaks and their assignments  compared to  literature values are listed
in Table 2.  It is worth noting that a
diameter-independent nanotube mode at 430 cm−1 is also consistently
observed in our samples [40].  All
observed Raman modes are in reasonably good agreement with literature values
for MWNTs.  The D-line and associated
higher-order modes are lower than reported values in   literature measured
with different laser excitation (Table 2).
This is likely due to the fact  that the D-line is dispersive and its frequency is
linearly laser excitation energy dependent. We have also carried out UV-Raman measurements
(244 nm laser excitation) on these  samples, and the results are shown in Figure
3.  With UV excitation, the D-line is
very broad and shifts to higher frequency (>1400 cm−1).  Based on the large D/G ratio observed  with
the visible laser excitation, the template confined carbonization of polymers
also produces significant amount of disorder in the MWNTs as compared to those
from the CVD method.  Another observation
worth noting is that when the MWNTs/AAO sample is cut, Raman measurements are
carried out on the cross-section and the laser polarization is applied parallel
and perpendicular to the expected MWNT axis direction, the perpendicular
direction consistently gives ~10% higher Raman peak intensity. This anisotropy
strongly suggests the presence of aligned MWNTs and not disordered graphite.Table 2
Typical Raman scattering peaks for the MWNTs in this study.
Raman
shiftcm−1Raman
shiftcm−1Peak
assignmentsThis
work
(laser
633 nm)Reference
[39]
(laser
515 nm)430450Large
diameter NTs1100110013411351D15931590G2697
(br)27012D29142934D + G319232342G4240
(br, w)42612D + G
(?)Figure 1
Typical Raman spectrum of carbonized polymer embedded in AAO membrane. The
spectrum is signal averaged over 10 scans at room temperature.Figure 2
Analyzed MWNTs Raman spectrum with AAO background subtracted [black
triangular dots (original Raman data), red trace (overall fit), and blue traces
(individual scattering peaks)].  Broad
unassigned peak around 650cm−1 is due to incomplete background
subtraction.Figure 3
UV-Raman spectra of MWNTs in AAO template.
The spectra are measured with frequency-doubled (244 nm) Ar laser
excitation, 1cm−1 resolution, and signal averaged over 10 scans.


## 3.2. SEM Characterization

The formation of polymer nanotubes and carbon
nanotubes upon carbonization is directly studied with the  use of field emission
scanning electron microscopy.  A
five-minute epoxy polymer is used for the initial study.  There are two components, the epoxy and
curing agent, after the two components are mixed a drop of polymer is placed on
a 230 nm AAO disk that is glued onto a Cu plate.  The sample is heated at 140C° for 1 hour.  After the polymer is fully cured, the
template is removed by soaking and washing with NaOH solution, and the SEM image
of the remaining polymer is shown in Figure 4(a).
As revealed in the image, the epoxy polymer flows into the 230 nm pores
very well.  From this study also
consistent with that reported in [36], majority of the observed
nanoscale objects are polymer nanotubes.
This is likely due to the stronger adhesive force than the cohesive force.  Since the major   component of the five-minute
epoxy is bisphenol A, a few well-defined small molecular weight bisphenol A related
polymers are used for the polymer nanotube carbonization study (Table 1).  The epoxy coated AAO disk is heat treated at
2C°/min to 500C°, after template removal, the CNT
bundle is shown in Figure 4(b).  The top
view and the close-in side view of these CNTs are shown in Figures 4(c) and 4(d),
respectively.  To explore whether the
molten wetting procedure works for much smaller nanopores, we apply the same
epoxy to home-made AAO membranes with 50 and 20 nm pore diameters.  With the same heating rate at 2C°/min, the samples are heated to 600C° and hold for 3 hours.  The results as shown in Figures 4(e) and 4(f)
indicate that the method indeed works well for small nanopores.(a) The fully crosslinked epoxy
nanotubes from a 5-minute epoxy, heated at 140C° for 1 hour and
released from AAO template by dissolving the alumina in 1 M NaOH solution. (b)
Bundle of CNTs after further carbonization at 500C° and being released from the pores
of AAO template. (c)  Top view of aligned MWNTs showing the pore opening. (d)
Side view of MWNTs with diameter ~230 nm. (e) MWNTs with 50 nm diameter prepared
from AAO template anodized from oxalic acid. (f) MWNTs with 20 nm diameter
prepared from AAO template anodized from sulfuric acid.
(a)(b)(c)(d)(e)(f)For the commercial 230 nm pore diameter AAO disk,
both ends of the nanopores are open.  For
the home-made AAO membranes, before the barrier layer is removed, one end is
open and the other end is closed for all the nanopores [42].  Since open or closed ends play a crucial role
for additional nanotube functionalization and/or tube filling, we look into the
nature of the MWNTs prepared with our home-made AAO membranes in more
details.  The side view  SEM images  in
Figures 4(e) and 4(f) of the MWNTs released from the templates do not provide
conclusive answer.  Therefore we remove
the barrier layer by slow etching and image the back side directly.  We use one piece of the MWNTs/AAO sample and
glue the polymer filling side (open-end side) to a Cu foil.  The sample is soaked in a 1 M NaOH solution
at room temperature, and the sample is studied with the  use of SEM at a fixed time
increment.  After 10 minutes etching, the
barrier layer starts to open.  The SEM
image (Figure 5(a)) shows a large area view of AAO barrier side with imbedded
MWNTs.  After 15 minutes etching, the
alumina barrier layer is removed.
However, the end of each MWNT is still capped (Figure 5(b)).  This is indicative of the materials at the
end of the MWNT  being etched at a much slower rate compared to that of the
alumina barrier layer.  Additional
etching beyond 20 minutes only widens the nanopores, and individual MWNTs are
standing in each pore and leaning toward each other (Figures 5(c) and 5(d)).  As shown in Figure 5(d) the end of each MWNT is
darker in the middle and brighter around the edge and does not appear to be a
solid rod.  Our result supports that during
the melt-wetting process, the melts driven by adhesive force reach the bottom
of the nanopores and form a conformal layer that upon carbonization leads to
MWNTs with one end closed.(a)
SEM image of the barrier side of synthesized AAO membrane with 20 nm pores, Al
removed, filled with carbonized MWNTs (600C°), and etched the barrier layer for
10 min. in 1 M NaOH; (b) after ~15 min. etching MWNTs with caps start to detach
from the pore walls; (c) after 20 min. etching, the pores are widened and MWNTs
are standing in the pores; (d)  same as (c)  with higher magnification, the MWNT
has a grayish center with bright rim indicates the cap still remains.
(a)(b)(c)(d)


## 3.3. TEM Characterization

Additional sample characterization is carried out using transmission electron microscopy. Figure6(a) shows a TEM image of a 230 nm carbon nanotube bundle which resulted from completely dissolving the AAO template. Electron diffraction patterns of all nanotubes were not of high quality.  Many amorphous domains are observed but occasionally some graphitic characteristics are revealed in some diffraction patterns: the brightest ring corresponds to the 002 reflection of hexagonal graphite and the next continuous ring seen in the diffraction pattern corresponds to the 110 reflection of hexagonal graphite. There is no difference in the intensity in this particular diffracted ring, which suggests that there is no preferred orientation along the a*- or b*-axes [1]. A high-magnification TEM image of a 20 nm CNT prepared from AAO template with slow heating to 600C° is shown in Figure 6(b). The tube wall thickness between 4 and 5 nm is visible from the image. Based on the interplanar separation in graphite (d002 = 3.35 Å), such tube wall thicknesses could accommodate approximately 12–15 graphitic shells [16].(a)  TEM
image of a MWNT bundle made from completely dissolving the AAO template (230 nm pores, 500C°). (b)  High
magnification TEM image of a 20 nm MWNT prepared with slow heating to 600C°.
(a)(b)Another
advantage of the current process is that it can be easily extended to prepare
functionalized nanotubes by premixing any type of nanospecies such as
nanoparticles or nanofibers with the polymer precursor, and then let the
wetting process bring these nanospecies into the template channel to form
various functional nanotubes.  For
example, CoPt nanoparticles with 6 nm diameter [43] are successfully
incorporated into carbon nanotubes with diameters at 230 and 50 nm by this
method (Figures 7(a) and 7(b)).  Some aggregation
is noticed as the incorporated nanoparticles appear to be slightly larger than
6 nm; however, we demonstrate that this procedure brings nanoparticles into
nanopores successfully without major aggregation of the nanoparticles upon
heating, and this is a simple one-step method for such a complicated
nanostructure.  Apparently, the polymer
matrix, viscosity, and low loading all help to avoid nanoparticle aggregation.  Previously nanoparticles may have been loaded by
multistep impregnation followed with reduction to prepare such functionalized
nanostructure [34].(a)  CoPt nanoparticles (dark spots) with 6 nm diameter are successfully
incorporated into 230 nm carbon nanotubes. (b)  CoPt nanoparticles with 6 nm diameter are incorporated into 50 nm MWNTs
with this procedure.
(a)(b)


## 3.4. Nature and Effect of Polymer Precursors

When
a polymer melt or solution is placed on a substrate with high surface energy,
it will spread to form a thin film. Similar wetting phenomena occur if porous
templates are brought into contact with polymer melts or solutions. A nanotube
structure can  be preserved if the wetting process is quenched at the initial
stage since the wall wetting and complete filling of the pores take place at
different time scales [36, 44]. In our experiments, wetting of template wall
happens on a time-scale of a few minutes when any liquid-form epoxy (e.g., a
5-minute epoxy) is used as starting materials. The fully crosslinked epoxy
nanotubes are released from the AAO template by dissolving the alumina (Figure 4(a)).  The epoxy tubes are of uniform diameter and
length, with wall thickness of several tens of nanometers.  It is found that the topography of the epoxy
tubes matches the shape of their hosting pore channels so well that they can be
used to duplicate the internal pore structure faithfully.  Further carbonization process does not change
the morphology of the epoxy nanotubes and their examination by electron
microscopy technique can thus give a more reliable approach to study the pore
structure since the carbonized nanotubes become conducting enough so that no additional
carbon or metal coating is needed for imaging (Figure 8(a)). Since AAO membrane
has recently attracted much attention for its application in nanotechnology, it
is important to know the actual internal pore structure.  AAO templates with pore diameter as small as
20 nm are also used for CNTs growth successfully (Figure 4(f)).  The pore diameters can be increased or reduced
by wet chemical etching [42] and atomic layer deposition method [45–48],
respectively.  The CNTs grown in the AAO
template are very flexible and they can be bent at large angle (>120°)
without being broken.  The CNTs are of
uniform length and the open-ended structure will facilitate their use in
sensing applications as well as tube filling (Figure 8(b)) [3, 49–52].(a)  Carbon
nanotubes of various shapes can be made. Polymer nanotubes after carbonization
at 500C° replicate the morphology of commercial AAO nanopores. (b) The nanotubes are parallel to each other and
perpendicular to the template. They remain aligned when template is removed
slowly. (c) Defects such as bubble
formation in MWNTs may be trapped due to nonideal heat treatment
condition.  PS-co-PMMA is used in this
case. (d) PS-b-poly(butadiene) precursor upon carbonization leads to flat and
collapsed nanotubes.
(a)(b)(c)(d)All
CNTs are of equal height and there is no overgrown problem normally happening in
CVD growth.  The
nanotubes are parallel to each other, perpendicular to the template, and well-ordered to form a periodic triangular close-packed array without extra
processing steps. The tube
density, estimated from the pore density, can be as high as4.4×1010 pores/cm2.  The tube diameter distribution throughout the
array is narrow, typically 10% of the main diameter—much narrower
than heretofore reported using other methods of nanotube array synthesis.  This CNT preparation process works for a
large number of polymers we have tried (Table 1), except for polystyrene and
polybisphenol A carbonate that decompose into styrene and bisphenol A,
respectively [53, 54].  These small
molecules evaporate completely and nothing is left after the carbonization
process.  It is found that carbon
nanospecies with different morphology and/or structure can be produced when different
polymer precursors are carbonized under the same reaction condition. For
example, polystyrene-co-polymethyl mathacrylate resulted in carbon nanotubes with
many bubbles and sponge-like features (Figure 8(c)) and polystyrene-block-polybutadiene
generated CNTs with very thin walls, so that they appear collapsed (Figure 8(d)).  Fully collapsed carbon nanotubes have been
reported in  literature.  For certain
range of tube parameters, a completely collapsed nanotube is energetically more
favorable [55].  A more recent article
reported collapsed nanotubes due to poisoned metal catalyst that led to uneven
carbon concentration and precipitation [56].
Since no catalyst is used in our study, the collapsed nanotubes may be
related to the loss of styrene, a decomposed product from the block copolymers
used that also leads to uneven carbon generation during carbonization.  While there are remaining questions regarding
the collapsed nanotubes formation from other polymers such as poly(acrylic
acid), all observations of MWNTs and other morphology formation from various
polymer precursors are summarized in Table 1.The
transport measurements made on a 50 nm CNT array imbedded in AAO template by a
two probe method show the characteristic of a semiconductor (Figure9). Even though CNTs can be made at temperatures as low
as 400C°, semiconductive behavior is observed only in samples made at
temperatures above 600C°.  The resistance
of MWNTs imbedded in 230 nm AAO membranes prepared at 600C° from different
polymer precursors is measured with a two-probe method across the membranes and
ranges between 6 and 40 KΩ.  Not well formed and collapsed carbon tubes
give much higher resistance values between 300 KΩ and 7 MΩ.  The
results are also listed in Table 1.Figure 9
Resistive measurements made on a 50 nm CNT array (600C°) imbedded
in homemade AAO template by a two probe method show the characteristic of a
semiconductor.


## 4. Conclusion

We
report a simple, fast, one-step approach for the preparation of well aligned
carbon nanotube array.  Starting at very
low temperature, ~400C°,
the polymers are carbonized and graphitic nanotubes are generated.  There is no flammable gas and catalyst
involved, no monomer or polymer initiator is needed.  The MWNTs duplicate  the membrane nanopores
precisely.  For membranes with open ends
on both sides, the resulting MWNTs contain the same open ends.  For AAO membrane with closed barrier at one
side, the resulting MWNTs contain one closed end.  The open or closed ends of the MWNTs have
strong implication for additional nanotube functionalization and/or
filling.  The template imbedded MWNTs of
the same height are also ideal for nanoparticles filling.  We demonstrated that CoPt nanoparticles can
be filled along with the polymer nanotube precursor that can not be done with
the CVD process.  This method also allows
post-synthesis nanoparticle filling.  The
MWNT and functionalized MWNT imbedded AAO membranes may find applications in
catalysis reactions or catalyst support.
The released MWNTs with combined semiconductive property and mechanical
strength may be useful toward sensors and actuators.  Finally, this method of MWNT array synthesis
is not inherently area limited and can be scaled up with proper templates.



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*Source: 562376-2009-06-01.xml*