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Abstract
1. Introduction to facilitate the strong mechanical strength of the CNTs
has been the key issue in improving the mechanical proper-
parts, ships, aircrafts and satellites. Therefore, the robust
formation of CNTAAl composites is desired.
There have been several research studies aimed at
improving the mechanical strength of Al by incorporating
CNTs [7–11]. CNTs are not easily mixed with Al due to
the large difference between the surface tensions of the
⇑ Corresponding author at: BK21 Physics Division, Department of
Energy Science and Center for Nanotubes and Nanostructured Compos-
ites, Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunk-
wan University, Suwon 440-746, South Korea. Tel.: +82 31 299 6507.
E-mail address: leeyoung@skku.edu (Y.H. Lee).
Available online at www.sciencedirect.com
Acta Materialia 59 (2011) 3313–33
Polymer composites based on carbon nanotubes (CNTs)
have been commonly utilized due to the extraordinary ther-
mal, electrical and mechanical properties of CNTs [1–3].
The composite performance strongly depends on the dis-
persion, length, crystallinity and degree of alignment of
the CNTs, as well as the binding characteristics between
the CNT surface and the host polymer. The percolation
limit of the CNT content in improving electrical conductiv-
ity has ranged widely, from 0.0021 to 9.5 wt.%, mainly due
to the various dispersion conditions of CNTs [4]. Control
of the anchoring of host polymers on the surface of CNTs
ties of composites. Poor control of the anchoring process
often leads to degraded mechanical properties [5].
Mechanically strong metal composites have attracted
interest recently due to green energy requirements [6].
Although aluminum is known as a rust-free light material
and is used in car parts and buildings, its use is still limited
mainly due to its poor mechanical strength as compared to
its iron counterpart. A CNT-based Al composite with
enhanced mechanical strength could be utilized to improve
the fuel efficiency of vehicles by reducing the vehicle weight.
Its applications may be further extended to electronic
The wetting of a metal on carbon nanotubes is fundamentally difficult due to the unusually large difference between their surface ten-
sions and is a bottleneck for making metal–carbon nanotube (CNT) composites. Here, we report a simple method to enhance the wetta-
bility of metal particles on the CNT surface by applying aluminum, which is the material with the largest surface tension. This method
involves two steps: (i) Al nanoparticles are decorated on multiwalled carbon nanotubes by electroplating and (ii) Al powder is further
spread on Al-electroplated CNTs, followed by high-temperature annealing to accommodate complete wetting of the aluminum. The large
surface tension difference is overcome by forming strongAlAC covalent bonds initiated by defects of the CNTs. The decrease in theD-band
intensity, the G-band shift in the Raman spectroscopy and the formation of AlAC covalent bonds, as confirmed by X-ray photoelectron
spectroscopy, were in agreement with our structural model of CNTAvacancyAOAAl determined by density functional calculations.
Crown Copyright � 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.
Keywords: Carbon nanotubes; Aluminum; Electroplating; Interface; Nanocomposite
Improving the wettability of a
Kang Pyo So a,b, Il Ha Lee a,b, Dinh
Seong Chu Lim a,b, Kay Hy
aBK21 Physics Division, Department of Energy Science
Sungkyunkwan Advanced Institute of Nanotechnology,
bR&D Department, Chonju Machinery Re
Received 16 November 2010; received in revised
Available onl
1359-6454/$36.00 Crown Copyright � 2011 Published by Elsevier Ltd. on beh
doi:10.1016/j.actamat.2011.01.061
minum on carbon nanotubes
oc Duong a,b, Tae Hyung Kim a,b,
k An b, Young Hee Lee a,b,⇑
Center for Nanotubes and Nanostructured Composites,
ngkyunkwan University, Suwon 440-746, South Korea
rch Center, Chonju 561-844, South Korea
rm 27 January 2011; accepted 31 January 2011
2 March 2011
www.elsevier.com/locate/actamat
20
alf of Acta Materialia Inc. All rights reserved.
ria
two materials as the surface tension of Al is 955 mN m�1,
which is almost 20 times larger than that of CNTs
(45.3 mN m�1) [12,13]. An additional technical barrier to
formulate CNTAAl composites with high mechanical
strength is the high oxidation capability of Al, which
causes Al particles to be easily oxidized and lose their
metal characteristics. Because of these difficulties, the wet-
tability of Al on the CNT surface has been extremely dif-
ficult to realize.
Therefore, the issue remains of how to overcome the
large difference in surface tension and improve the wetta-
bility of Al on the CNT surface, which would be a simple
way to generate CNTAAl composites with high mechanical
strength. In this study, we adopted the strategy of forming
strong AlAC covalent bonding between Al and the CNT
surface. For this, we applied an electroplating method to
coat Al nanoparticles on vertically aligned multiwalled
CNTs, which eventually enhances the wettability of Al on
the CNTs. Strong covalent bonds were formed between
Al and the CNT surface during the electrodeposition. This
covalent bond improves aluminum wettability on the CNT
surface. Defect-associated nucleation of Al on the CNT
surface was observed by Raman spectroscopy and X-ray
photoelectron spectroscopy, and was also supported by
density functional theory (DFT) calculations.
2. Materials and methods
2.1. Synthesis of vertically aligned carbon nanotubes
A thin (less than 100 nm) layer of Ni was deposited onto
a TiN/Si (100) substrate using an RF magnetron sputter.
The chamber was evacuated to a pressure of 1 � 10
�6 Torr and the substrate temperature during the deposi-
tion was maintained at 350 �C by a graphite heater. The
Ni-coated TiN/Si substrate was then moved into a thermal
chemical vapor deposition (CVD) chamber. The CNTs
were grown on Ni-coated TiN/Si substrates using thermal
CVD at 650 �C with a gas mixture with C2H2 (20%) and
Ar (80%). The CVD chamber pressure was held at
5.5 Torr. The CVD chamber was heated by halogen lamps
at a heating rate of 54 �C min�1 until the temperature
reached the growth temperature of 650 �C. The growth
temperature was fixed during the growth time of 20 min.
The experiment procedure has been described in detail else-
where [14].
2.2. Al electroplating
The electrolyte was prepared by dissolving 1.17 M anhy-
drous aluminum chloride into a mixture of two solutions of
tetrahydrofuran (THF):benzene = 8:2. To increase the
solution conductivity and solubility of aluminum chloride,
0.23 M lithium aluminum hydride was added to the electro-
lyte [15]. The electroplating cell consisted of three elec-
3314 K.P. So et al. / Acta Mate
trodes: vertically aligned carbon nanotube as a working
electrode, Pt mesh as a counter electrode and Ag/AgCl as
a reference electrode. N2 gas was flowed through the cell
for more than an hour to remove moisture and oxygen.
The prepared electrolyte was injected into the cell by a med-
ical syringe and the CNT electrode was inserted into the cell
and stabilized for 20 min. The counter electrode was placed
parallel to the working electrode with the reference elec-
trode in the middle. CV measurements were taken with a
scan rate of 50 mV s�1. Electroplating was applied for
5 min at a given applied voltage. The electrolyte was rinsed
with ethanol after electroplating and moisture was removed
by acetone followed by drying in a vacuum oven overnight.
2.3. Al wetting
Al powder (Samjeon Chemicals, Korea) with a size of
75 lm was placed on top of vertically aligned multiwalled
CNTs, which were electroplated by Al as described above.
This structure was then heat-treated at 700 �C for 1 h in a
vacuum furnace (10�5 Torr).
2.4. Measurement
The electroplating was performed using a potentiostat
(Solartron 1287A). The morphology of the electroplated
MWCNTs was characterized by field-emission scanning
electron microscopy (6700F, JEOL) and high-resolution
transmission electron microscopy (TEM; JEOL 2010F
HRTEM, 200 keV). Elemental analysis was obtained by
energy-dispersive X-ray spectroscopy (EDX). AlAC
covalent bonds were characterized by X-ray diffraction
(XRD; Rigaku Rotaflex D/MAX system, Rigaku, Japan,
Cu Ka, 1.54 A˚), X-ray photoelectron spectroscopy (XPS;
ESCA2000, VG Microtech, UK, Al Ka 1486.6 eV) and
Raman spectroscopy (Renishaw, RM1000, 633 nm).
2.5. DFT calculations
To interpret the experimental data (8,0) carbon nano-
tubes with and without a vacancy were simulated. Al and
O atoms were added on the surface of the nanotubes. All
models were calculated by the DFT implemented by
DMOL3. The numerical atomic basic sets were used with
an orbital radius cut-off of 4.8 A˚. The generalized gradient
approximation (GGA) for exchange and correlation in the
Perdew–Burke–Ernzerh function were employed [16–18].
The Brillouin zone was sampled with a 4 � 4 � 1 irreduc-
ible Monkhorst–Pack k-point grid [19]. The convergence
threshold for calculation of the self-consistent energy was
10�6 Ha. Our model structures were relaxed until the forces
on the atoms were smaller than 0.05 eV A˚�1.
3. Results and discussion
3.1. Schematic diagram of experimental procedure
lia 59 (2011) 3313–3320
Fig. 1 shows a schematic of the experimental procedure
used to improve the wettability of additional aluminum
eria
powder after the electroplating of aluminum on multi-
walled carbon nanotubes (MWCNTs) to form AlACNT
covalent bonds. Vertically aligned MWCNTs synthesized
by thermal chemical vapor deposition (TCVD) on a silicon
substrate were coated with Al nanoparticles by an electro-
plating method. The synthesized MWCNTs had diameters
of 40–50 nm and lengths of 3–5 lm, depending on the
growth time. Al nanoparticles adhered strongly to the
outer MWCNT walls due to the formation of strong AlAC
covalent bonds. Additional Al powder was then spread on
top of the vertically aligned MWCNT film and further
annealed at 700 �C for 1 h to fully accommodate the wet-
ting of additional Al.
3.2. Plating phenomena and morphology analysis
Fig. 2a shows the current–voltage (CV) data obtained
with the aluminum plating solutions. The details of the
solution are described in the experimental section. In the
THF and benzene solvent alone, a negligible current
Fig. 1. Schematic diagram of the Al wetting process on a CNT: (a)
vertically aligned CNT, (b) Al electroplating, (c) loading of Al powder on
top of the vertically aligned CNT and (d) formation of the Al/CNT
composite by heavy Al wetting at 700 �C (10�5 Torr) for 1 h.
K.P. So et al. / Acta Mat
(�4 lA) and linear behavior of the CV data (inset of
Fig. 2b) were observed, confirming that there was insignif-
icant chemical reaction of the solvent with a bias up to
�3 V. The conductivity of the solvent is poor and therefore
no active ionic motion is expected, resulting in simple resis-
tor behavior. By adding AlCl3, the current level increased
to the order of milliamps (Fig. 2b) but still remained low.
AlCl3 was dissolved in the solvent to some degree and Al
ions were further adsorbed on the surface upon bias, but
no apparent reduction peak was observed, confirmed by
the simple capacitive behavior, as shown in Fig. 2b. Actual
coating involving the reduction of Al ions took place when
catalytic LiAlH4 was added [20]. The current level was
increased to 50 mA at �3 V. The reduction of Al ions
was clearly observed and began at �1.5 V, as indicated in
the inset of Fig. 2a.
Figs. 2c and 1d show the current–time transient behav-
ior in the early stage of coating. At a low voltage
(�1.6 V), though still above the reduction voltage, the cur-
rent profile is composed of three steps: step I involves for-
mation of the electric double layer on the electrode,
corresponding to a rapid decrease in current, step II repre-
sents the reduction reaction of ions in the electrode that
gives rise to a gradual increase in current and step III
involves a diffusion-limited current corresponding to the
saturation of nucleation sites for the reduction of ions
[21]. These three steps are represented in Fig. 2c. At a
low bias of �1.6 V, the formation of an electric double
layer of ions took a relatively long time, up to 5 s. The
reduction process of Al ions was also slow, resulting in a
slight increase in the current. With increasing applied volt-
age, the current decreased more rapidly and the reduction
of Al ions occurred more quickly.
It is worth noting that we did not observe a current
decrease corresponding to step III. This phenomenon is
different from the observed diffusion-limited model [21–
23]. The current decrease in step III is usually observed
when the electrodeposition rate dominates the diffusion
rate of ions so that the ion supply to the nucleation sites
is limited by the overlap of the diffusion zone. However,
vertically aligned carbon nanotubes provide a large surface
area. In this case, no diffusion limit is expected when nucle-
ation is initiated and the reduction speed or current simply
relies on the applied voltage. This behavior was confirmed
by the increased current with increasing applied voltage in
Fig. 2d. Al reduction took place quickly and thus the cur-
rent increased and became saturated at the long time limit.
Fig. 3 shows the XRD pattern obtained after electro-
plating for 5 min to confirm the formation of Al crystals.
As the voltage increased, Al crystal peaks were clearly
observed near 38.5� (111) and 44.7� (200) [24]. This clearly
confirms the reduction of Al to form Al crystals. The pres-
ence of the C(101) peak near 44� even at a high bias voltage
indicates that the crystalline inner walls of the MWCNTs
were intact [25].
The SEM morphologies of the Al-coated MWCNTs are
shown in Fig. 4. The pristine MWCNTs contain carbona-
ceous particles, and nanotubes are entangled on the top
part (Fig. 4a). The size of the Al particles at �3 V was
around 100 nm (Fig. 4b). At �5 V, the carbon nanotubes
were completely covered by Al particles (Fig. 4c). A side
view of the Al-coated MWCNTs at �3 V is shown in
Fig. 4d, demonstrating the uniform deposition of Al parti-
cles, even at the bottom positions. This was confirmed by
the EDX mapping for carbon and aluminum (Fig. 4e and
f). Al particles were uniformly coated over the vertically
aligned MWCNTs, independent of depth.
3.3. Morphology analysis after wetting of additional Al
powder
The morphologies of the pristine MWCNT film are
shown in Fig. 5a–c. Al powder placed at the top of the pris-
tine MWCNTs remained intact even after the melting of
lia 59 (2011) 3313–3320 3315
Al. The side view of the MWCNTs clearly shows no wet-
ting of Al on the surface of the MWCNTs. This is expected
3316 K.P. So et al. / Acta Materialia 59 (2011) 3313–3320
from the large difference between the surface tensions of
the two materials and the hydrophobicity of the CNTs.
Fig. 3. X-ray diffraction analysis after aluminum electroplating at �2 to
�5 V vs. Ag/AgCl. Aluminum crystal patterns are clearly shown after
electrochemical reduction.
Fig. 4. Morphologies after Al electroplating as examined by SEM and EDS
electroplating at (b) �3 V and (c) �5 V for 5 min, (d) side view of the sample
Fig. 2. Cyclic voltammetry and current–time transient analysis: (a) CV charact
a scan range of 0 to �3 V and (b) CV characteristics without LiAlH4. Note
transient for 10 s at �2 V to �5 V vs. Ag/AgCl and (d) the high potential cur
On the other hand, Al powder placed on top of the Al-
coated MWCNTs infiltrated well into the vertically aligned
MWCNTs forest. Consequently, due to this wetting pro-
cess, the MWCNT forest was unevenly shrunk, resulting
in discontinuous cracks in the forest (Fig. 5d). The inset
clearly shows a crack formation near the Al wetting region.
This phenomenon is ascribed to the volume contraction of
eristics of the aluminum plating solution with a scan rate of 50 mV s�1 and
the different current level in the y-axis, (c) the low potential current–time
rent–time transient at �1.6 V to �2 V vs. Ag/AgCl.
melted Al (�22%) during cooling. The side view of the wet
area demonstrates that Al powder infiltrated into the sides
of the MWCNTs forest (Fig. 5e and f). MWCNTs were
embedded in the Al matrix, as shown in the inset of
Fig. 5f. Some of the Al powder still remained on top of
the MWCNTs due to incomplete melting caused by the
preformed aluminum oxide layer.
MWCNTs scratched from the sample were further son-
icated and dropped onto a TEM grid. Al was uniformly
mapping: top views of the (a) pristine MWCNT and Al-MWCNT by
electroplated at �3 V and EDX mapping observations of (e) C and (f) Al.
coated over all of the MWCNTs, as confirmed by EDX
mapping (Fig. 5g–i). MWCNTs still remained intact even
after annealing at 700 �C for 1 h. This is in good agreement
with the previous results showing that MWCNTs were par-
tially degraded, particularly near the defects and edges, but
the graphitized CNT wall remained intact during heat
treatment even at 800 �C for 1 h [26]. This indirectly sug-
gests that our MWCNTs have some defects on their walls
which was also confirmed in the Raman spectrum, as will
be discussed in next section.
Fig. 5. Morphology changes after heavy Al wetting. (a–c) The pristine CNT without Al electroplating: (a) top view, (b) side view and (c) enlarged side view
of the white square in (b). (d–f) The corresponding morphology changes with wetting after Al electroplating at �5 V for 5 min. (g) TEM image after heavy
Al wetting, and element mapping of (h) C and (i) Al.
rms
K.P. So et al. / Acta Materialia 59 (2011) 3313–3320 3317
Fig. 6. AlAC covalent bond analysis: (a) Raman spectroscopy analysis in te
shift (blue), (c) Al2p XPS peak after Al electroplating at �5 V for 5 min a
interpretation of the references to color in this figure legend, the reader is refe
of electroplating voltage, (b) D/G intensity ratio (black) and G-band peak
nd (d) Al2p peak after further heat treatment at 700 �C for 1 h. (For
rred to the web version of this article.)
3.4. AlAC covalent bond analysis by Raman spectroscopy
and XPS
Information on the presence of defects can be found in
the Raman spectra. Fig. 6a shows a D-band near
1330 cm�1 and a G-band near 1590 cm�1 in the pristine
sample, which corresponds to contributions from sp3-like
defects on the CNT wall or amorphous layers and sp2-like
graphitic hexagons, respectively [27]. Interestingly, the
intensity of the D-band and the D/G ratio decreased grad-
ually with increasing electroplating bias, as shown in
Fig. 6b. This implies that the defect density decreased with
Al deposition. This is in contrast with a previous report
that the electrodeposition of Al with amorphous carbons
resulted in an increased D-band intensity [28]. This ambi-
guity will be clarified later when the density functional cal-
culations are discussed. The G-band peak was upshifted
with increasing bias. This is evidence of electron transfer
Fig. 7. The optimized structures (top panel) of (a) (8,0) CNTAvacancy, (b) CNTAvacancyAAl, (c) CNTAvacancyAAlAO and (d) CNTAvacan-
cyAOAAl. The induced charge (bottom panel) due to interaction of CNTAvacancy with (e) Al, (f) Al@O and (g) OAAl. The induced charge is defined as
qinduced = qCNT-adsorbate � qCNT � qadsorbate. The red and blue isosurfaces indicate the regions of charge accumulation and depletion, respectively. The
value of the isosurface is ± 0.007 e A˚�3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
article.)
e
0
0
�
3318 K.P. So et al. / Acta Materialia 59 (2011) 3313–3320
Table 1
DFT calculation results of CNT vacancy with Al atom incorporation.
Configuration Binding energy (eV)
CNTAvacancy
CNTAvacancyAAl (CNTAAl) �6.07 (�1.40)
CNTAvacancyAAlAO (CNTAAlAO) �10.39 (�6.29)
CNTAvacancyAOAAl (CNTAOAAl) �10.50 (�6.20)
The binding energy, amount of electron transfer and
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