The fabrication of a carbon nanotube transparent conductive film by electrophoretic deposition and hot-pressing transfer.

Abstract

A super-flexible single-walled carbon nanotube (SWCNT) transparent conductive film (TCF) was produced based on a combination of electrophoretic deposition (EPD) and hot-pressing transfer. EPD was performed in a diluted SWCNT suspension with high zeta potential prepared by a pre-dispersion-then-dilution procedure using sodium dodecyl sulfate as the surfactant and negative charge supplier. A SWCNT film was deposited on a stainless steel anode surface by direct current electrophoresis and then transferred to a poly(ethylene terephthalate) substrate by hot-pressing to achieve a flexible SWCNT TCF. The SWCNT TCF obtained by this technique can achieve a sheet resistance of 220 Omega/sq with 81% transparency at 550 nm wavelength and a strong adhesion to the substrate. More importantly, no decrease in the conductivity of the SWCNT TCF was detected after 10 000 cycles of repeated bending. The result indicates that the EPD and hot-pressing transfer technique is an effective approach for fabricating a carbon nanotube TCF with excellent flexibility.

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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 20 (2009) 235707 (7pp)
The fabrication of a carbon nanotube
transparent conductive film by
electrophoretic deposition and
hot-pressing transfer
doi:10.1088/0957-4484/20/23/235707
Songfeng Pei, Jinhong Du, You Zeng, Chang Liu and
Hui-Ming Cheng
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China
E-mail: jhdu@imr.ac.cn and cheng@imr.ac.cn
Received 20 February 2009, in final form 7 April 2009
Published 19 May 2009
Online at stacks.iop.org/Nano/20/235707
Abstract
A super-flexible single-walled carbon nanotube (SWCNT) transparent conductive film (TCF)
was produced based on a combination of electrophoretic deposition (EPD) and hot-pressing
transfer. EPD was performed in a diluted SWCNT suspension with high zeta potential prepared
by a pre-dispersion-then-dilution procedure using sodium dodecyl sulfate as the surfactant and
negative charge supplier. A SWCNT film was deposited on a stainless steel anode surface by
direct current electrophoresis and then transferred to a poly(ethylene terephthalate) substrate by
hot-pressing to achieve a flexible SWCNT TCF. The SWCNT TCF obtained by this technique
can achieve a sheet resistance of 220 ?/sq with 81% transparency at 550 nm wavelength and a
strong adhesion to the substrate. More importantly, no decrease in the conductivity of the
SWCNT TCF was detected after 10000 cycles of repeated bending. The result indicates that the
EPD and hot-pressing transfer technique is an effective approach for fabricating a carbon
nanotube TCF with excellent flexibility.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Carbon nanotube (CNT) transparent conductive films (TCFs)
have attracted great interest since the pioneer report by Wu
et al [1]. Research works [2–6] have shown that CNT TCFs
possess high optical transparency and electrical conductivity,
which can be comparable to traditional indium tin oxide
(ITO) film.Moreover, CNT TCFs are far more flexible
and environmentally resistant and friendly than ITO film; in
addition to that carbon is a much more resource than indium.
Therefore, it is anticipated that CNT TCFs will not only be a
potential replacement for ITO films in optoelectronic devices
but that they will also find applications in durable transparent
electrodes for flexible and wearable displays. Consequently,
CNT TCFs are considered to be one of the most promising
fields for the industrial application of CNTs.
Several methods, including direct growth [4], filtration
transfer [1, 7], dip-coating [8], and spray-coating [5, 9, 10],
have been reported for preparing CNT TCFs. Though direct
growth by chemical vapor deposition (CVD) can produce CNT
film with a perfect structure and high conductivity [4], it is
difficulttomake largearea uniform filmsdue tothelimitedsize
of CVD equipment. Among other methods, Wu et al [1] used
filtration transfer to achieve a low sheet resistance of 30 ?/sq
with 70% transparency, but this method is limited by the size
of the filtrators and is not suitable for large area production
of CNT TCFs [2]. Dip-coating and spray-coating can directly
make CNT films on large area transparent substrates (both
rigid and flexible) with good transparency and conductivity.
But the adhesion between CNT films and substrates is still
problematic. So far, there is no efficient method to allow the
continuous production of flexible CNT TCFs with desirable
adhesion to substrates for repeated bending.
0957-4484/09/235707+07$30.00
© 2009 IOP Publishing LtdPrinted in the UK
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Nanotechnology 20 (2009) 235707 S Pei et al
Figure 1. Schematic diagram showing the preparation of SWCNT TCFs by EPD and hot-pressing transfer: (a) deposition of a SWCNT film
on the surface of a stainless steel anode by DC electrophoresis; (b) transfer of the SWCNT film from the stainless steel anode to a PET
substrate by hot-pressing; (c) post-treatment of the film by nitric acid to remove residual SDS.
Electrophoretic deposition (EPD) is considered to be
a simple and efficient method for producing larger area
CNT films with high homogeneity and proper surface
roughness [11–14]. However, there are very limited reports
aiming to produce CNT TCFs by EPD [12, 13]. The key
issue is that, by using this method, CNT films are obtained
on the surface of a conductive electrode, which is usually
opaque.Therefore it is necessary to develop a method to
transfer the CNT film deposited on a metal electrode to a
transparent substrate without destroying the film and to ensure
strong adhesion between the transferred CNT film and the
substrate.
In this paper we proposed an approach to produce
single-walled carbon nanotube (SWCNT) TCFs based on a
combination of EPD and hot-pressing transfer.
performed in a diluted SWCNT suspension with high zeta
potential prepared by a pre-dispersion-then-dilution procedure
using sodium dodecyl sulfate (SDS) as the surfactant and
a negative charge supplier. A SWCNT film was deposited
on a stainless steel anode surface by direct current (DC)
electrophoresis and then transferred to a poly(ethylene
terephthalate) (PET) substrate by hot-pressing. Finally, the
flexible SWCNT TCF was washed in nitric acid solution to
remove residual SDS. The resultant SWCNT films show an
excellent flexibility,strong adhesion tothe PET substrate, good
transparency and high electric conductivity.
This EPD and hot-pressing transfer approach shows the
following advantages: (1) During EPD, the film thickness can
be easily controlled by varying the deposition time, which
consequently controls the transmittance and conductivity of
the resulting CNT TCFs. (2) Hot-pressing transfer leads
to strong adhesion between the CNT film and the polymer
substrate. Since part of the CNT film can be embedded into
the PET substrate, the CNT TCF displays excellent flexibility.
(3) Both the EPD and hot-pressing techniques are simple,
low cost, and have already been widely used in industry.
Therefore, this approach is good for the continuous production
of homogeneous and large area CNT TCFs with desirable
performance.
EPD was
2. Experimental details
2.1. Materials
SWCNTs produced by a floating CVD method [15] were used
in this study.The as-prepared SWCNTs were purified by
refluxing in 20 wt% hydrochloric acid for 10 h.
Anionic surfactant SDS was purchased from Sigma-
Aldrich Co. and used as-received.
hydrophobic tail of the SDS can adsorb on the CNT surface by
hydrophobic interaction [16], and the hydrophilic head of the
surfactant enables the CNTs to be negatively charged. These
charged CNTs can be attracted and move towards the anode in
an electric field [17]. Thus, SDS serves as both a surfactant and
a negative charge supplier in the CNT suspension.
PET slices with a thickness of 194 μm were commercially
available and used as a flexible transparent substrate.
As a surfactant, the
2.2. Preparation of SWCNT suspension
A pre-dispersion-then-dilution procedure was used to prepare
the SWCNT suspension for EPD. In a typical experiment, SDS
was first dissolved in deionized water with a concentration of
7 mmol l−1, and then 8 mg of purified SWCNTs were added
to 40 ml of the SDS solution and ultrasonicated for 1 h to
get SWCNT suspension. The suspension was centrifuged at
12000 rpm for 5 min to remove SWCNT aggregates and the
supernatant was carefully collected.
supernatantwas dilutedwithdeionizedwater toachieve diluted
SWCNTs with SDS concentrations of 0.5, 1, 1.5, 2, 2.5, 3,
4, 5 and 6 mmol l−1, respectively.
allowed to stand for 12 h, and then their zeta potentials were
measured with a Malvern Zetasizer NanoZS system.
diluted SWCNT suspension with the highest zeta potential was
selected for EPD.
Then the as-collected
The suspensions were
The
2.3. SWCNT TCF preparation by EPD and hot-pressing
transfer
The schematic preparation process of SWCNT TCFs by EPD
combined with hot-pressing transfer is shown in figure 1. As
shown in figure 1(a), the EPD equipment includes a DC power
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resource and a poly(methyl methacrylate) (PMMA) cell that
contains two stainless steel electrodes.
anode was carefully polished and the surface roughness was
less than 20 nm. EPD was carried out by setting a constant
electric field of 100 V cm−1, and the deposition time ranged
from 5 to 60 s. SWCNTs were first deposited on the surface of
the stainless steel anode, which was then withdrawn from the
PMMA cell and dried at room temperature. Subsequently, the
PET film was hot-pressed onto the SWCNT film at 0.5 MPa
and 220◦C, which is slightly lower than the melting point
of PET. After maintaining pressure for 1 min, the PET film
and the stainless steel electrode were naturally cooled to
room temperature. The SWCNT film was transferred intact
onto the PET surface (they can be easily peeled off from
the stainless steel electrode). Finally, the SWCNT TCF was
immersed into 2 mol l−1nitric acid solution for 2 h to remove
residual SDS surfactant, which could dramatically improve its
conductivity [2, 8].
The surface of the
2.4. Characterization of the SWCNT TCFs
The microstructure of SWCNT TCFs was observed by
a scanning electron microscope (SEM; Nova Nano SEM
463).The thickness of the SWCNT films deposited
on the stainless steel anode surface was measured by an
atomic force microscope (AFM; Multimode NanoScope IIIa,
Vecco, operated in tapping mode).
measurements were performed in a four-probe configuration
with an electrochemical workstation (Solartron 1260/1287).
Transparency of the SWCNT TCFs at 550 nm was evaluated
with a UV–vis spectroscope (Jasco V-550).
of the CNT TCFs was measured by repeatedly bending the
film with a home-made two-point bending device. The radius
of curvature was set to 5 mm, and the frequency of bending
was once per second. Conductivity of the film center, where
the strain is maximal, was measured after relaxing the sample
at different intervals. Meanwhile, after repeated bending for
fixed cycles the films were cut to strips with a dimension of
3 mm × 20 mm and their yield strength was measured with
a Hounsfield H5K-S materials tester. The thickness of the
PET both with and without SWCNT film is 166 μm after hot-
pressing.
Electrical conductivity
The flexibility
3. Results and discussion
3.1. Zeta potential of the SWCNT suspension
EPD is achieved via the motion of charged particles dispersed
in a suitable solvent towards an electrode under an applied
electric field [14]. It is well known that SDS-coated CNTs
dispersed in water show a colloidal nature [18, 19], and
the stabilization mechanism of SDS-coated CNT colloids is
electrostatic repulsion between adjacent surfactant ion-coated
CNTs. The stabilizing effect is closely related to the surface
charge of the CNTs. This charge can be measured in the form
of the absolute value of the zeta potential (|ζ|), the electrical
potential at the edge of the coated colloid. In order to achieve
an electrostatically stabilized CNT suspension for successful
EPD, a high |ζ| value is required. It has been reported that
Figure 2. |ζ| of SWCNT suspensions with different SDS
concentrations prepared by a pre-dispersion-then-dilution procedure.
increasing the SDS concentration is an efficient means to
increase the |ζ| [19].
causes a serious electrode reaction [12] and the deposited CNT
film would be buried by deposited SDS [17]. As a result,
the CNT film thus obtained adheres tightly to the surface
of the electrode and is difficult to transfer to a transparent
substrate. Therefore, it is necessary to have a CNT suspension
with a lower SDS concentration but relatively higher |ζ| for
successful EPD and transfer of CNT films.
Figure 2 shows the |ζ| of the SWCNT suspension with
different SDS concentrations prepared by a pre-dispersion-
then-dilutionprocedure. It isclear that theSWCNT suspension
with a SDS concentration of 7 mmol l−1, the as-collected
supernatant, has a high |ζ| of 50 mV, and the other suspensions
prepared by diluting this supernatant with deionized water can
maintain this value or even increase the |ζ| with the decrease
of SDS concentration. This phenomenon may be because
the dilution process keeps most of the absorbed negatively
charged surfactant molecules on the surface of the SWCNTs
but decreases the concentration of positively charged counter-
ions in water. These results indicate that pre-dispersion-then-
dilutionefficiently produces a CNTsuspensionwitha lowSDS
concentration but a relatively high |ζ|. The diluted SWCNT
suspension with a SDS concentration of 1.5 mmol l−1and
a peak |ζ| value of 78.8 mV was used in this study for the
SWCNT TCF preparation.
However, a high SDS concentration
3.2. SWCNT TCFs produced by EPD and hot-pressing
transfer
Figure 3 shows the SWCNT films (a, c, e) and partly
transferred films (b, d, f) on the stainless steel anode surface
deposited for different periods. It can be seen that the SDS-
coated SWCNTsare directlydepositedonthe electrode surface
and there is no obvious SDS film formed. The density of
the SWCNT network increases with increasing deposition
time, and the SWCNTs connect with one another to form
an interweaving film.Such a structure not only provides
highly conductive pathways but also enables the film, with
good internal structure, to be transferred whole. The partly
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(a)(b)
(c) (d)
(e) (f)
Figure 3. SWCNT films (a, c, e) and corresponding partly transferred films (b, d, f) on the stainless steel anode surface obtained by (a, b) 5 s,
(c, d) 20 s and (e, f) 40 s.
transferred films (b, d, and f) show a clear boundary between
the transferred area and the original film. Linear scanning
by Raman spectroscopy and SEM across the transferred area
on the stainless steel anode surface showed no SWCNTs left,
indicatingthatthe SWCNT filmscan be completelytransferred
from the stainless steel anode surface by the hot-pressing
transfer process.
Figure 4 shows the surface morphology of the SWCNT
film for 40 s deposition before and after being transferred to
the PET surface. It can be seen from figure 4(b) that the
SWCNTs maintain a uniform distribution and connection with
one another to form a continuous network on the PET substrate
after being hot-press transferred. In order to further verify
the uniformity of the SWCNT film on the PET surface, we
divided the SWCNT film into 30 identical squares of the same
size and measured the sheet resistance of each square. It was
found that the fluctuation of the sheet resistance among all the
30 squares was less than 2.2% of the mean sheet resistance,
which suggests that the SWCNT film on the PET surface is
very uniformly distributed. Figure 4(c), an enlarged view of
figure 4(b), clearly shows that some of the SWCNTs are buried
within the PET substrate. The cross-section of the SWCNT
TCF on the PET substrate shown in figure 4(d) was obtained
from a frozen fracture.It also confirms the embedding of
SWCNT film into the substrate. The transfer of SWCNT films
from the electrode surface to the transparent polymer substrate
is mainly due to the conglutinating effect of polymers in a
viscous state. The adhesive force between SWCNT film and
PET substrate at high temperature is much stronger than the
van der Waals force between the SWCNTs and the electrode
surface, so the SWCNT film can be easily transferred and stick
tightly to the PET substrate. As a result, the film can be bent
or even scrolled without destroying the texture and decreasing
the conductivity of the SWCNT TCF.
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(a)(b)(c)
(d)
Figure 4. SEM images of the SWCNT films. (a) The SWCNT film deposited on the stainless steel anode surface with a deposition time of
40 s. (b) The SWCNT film transferred to a PET surface by hot-pressing after nitric acid washing. (c) An enlarged view of (b).
(d) Cross-section of the SWCNT TCF on a PET substrate.
3.3. Transparency and conductivity of the flexible SWCNT
TCFs
The transparency and conductivity of the flexible SWCNT
TCFs achieved with different EPD times are shown in figure 5.
The inset curve shows change in the SWCNT film thickness
with increase in the deposition time.
the conductivity of the film increases and the transparency
at 550 nm decreases with increasing deposition time. The
SWCNT TCF for 20 s deposition has a thickness of 22 nm,
a sheet resistance of 1.4 k?/sq and a transparency of 91%.
The SWCNT TCF for 40 s deposition with a thickness of
54 nm achieves a sheet resistance of only 20 ?/sq and a
transparency of 81%; which can satisfy the requirement set for
touch screen application [2] and is comparable to most results
for reported for CNT TCFs [11].
increases the thickness and decreases the transparency, but has
little influence on the sheet resistance of the SWCNT film.
Furthermore,three main factors can influence the
conductivityof the SWCNT TCF: (1) the intrinsicconductivity
of the SWCNTs used, (2) the tube–tube contact resistance;
and (3) SWCNT aggregation.
SDS covering the SWCNT surface will increase the contact
resistance, so it is important to remove the residual SDS after
the formation of the film. Geng et al [2, 8] have investigated
the effect of nitric acid washing on the removal of SDS in
the SWCNT network. Their results proved that the increase
of conductivity by acid washing is mainly due to the effective
removal of residual SDS and not to a chemical doping effect.
We have done a similar experiment with our film to verify
this effect and obtained the same result.
post-treatment with nitric acid is beneficial to remove SDS
and increase the conductivity of our film by improving the
connection among SWCNTs. Hot-pressing can cause part of
SWCNT network to become embedded in polymer which may
cause a decrease in net SWCNT film conductivity, because
the adhesion of polymer on the SWCNT surface can increase
It can be seen that
Longer time deposition
In addition, the insulated
Therefore, the
Figure 5. Sheet resistance (? ?) and transparency at 550 nm
wavelength (◦) of the SWCNT TCFs achieved with various EPD
times. All the films were transferred to the PET substrate and
post-treated with nitric acid. The inset curve shows the thickness of
SWCNT films on the electrode surface with increase in the EPD
time.
the contact resistance and the acid washing process cannot
remove the SDS covering the SWCNTs that were embedded
in the substrate. However, most of the network is still free-
standing on the polymer surface and works like a net SWCNT
film. Moreover, the hot-pressing procedure can increase the
network density and thus improve the film conductivity. So the
conductivity of SWCNT TCF formed by hot-pressing transfer
is still very good.
SWCNT aggregation is caused by a re-aggregation
of the dispersed SWCNTs on the stainless steel anode
surface because the EPD occurs via particle coagulation then
deposition on the electrode [14, 20].
repulsion between adjacent SWCNTs significantly decreases
their re-aggregation and then improves the conductivity and
transparency of the obtained SWCNT film.
High electrostatic
The pre-
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