Helical carbon and graphitic films prepared from iodine-doped helical polyacetylene film using morphology-retaining carbonization

Article abstract of DOI:10.1021/ja803865e

In this communication, we report a novel preparation of the helical carbon nanofibril-fabricated thin film from the iodine-doped filmy helical polyacetylene through a carbonization process. Carbonization of the helical polyacetylene films by way of iodine doping is found to afford carbon and graphitic films completely preserving morphologies and even helical nanofibril structures. Copyright

Full text of DOI:10.1021/ja803865e

Published on Web 07/26/2008
Helical Carbon and Graphitic Films Prepared from Iodine-Doped Helical
Polyacetylene Film Using Morphology-Retaining Carbonization
,†
‡
§
§
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Mutsumasa Kyotani,* Satoshi Matsushita, Takuro Nagai, Yoshio Matsui, Masaki Shimomura,
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,‡
Akira Kaito, and Kazuo Akagi*
Tsukuba Research Center for Interdisciplinary Materials Science, UniVersity of Tsukuba, Tsukuba, Japan,
Department of Polymer Chemistry, Kyoto UniVersity, Katsura, Japan, National Institute for Materials Science,
Tsukuba, Japan, and National Institute of AdVanced Industrial Science and Technology, Tsukuba, Japan
Received May 31, 2008; E-mail: kyotani@ims.tsukuba.ac.jp; akagi@star.polym.kyoto-u.ac.jp
Carbon films are interesting materials because of their special
physicochemical and mechanical properties. They have been used
1
in various applications such as electrochemical energy storage,
2
,3
2,4
5
field-effect transitors, cell electrodes, catalyst supports, gas-
absorbent materials, and wear-resistant coating. Many carbon
thin films or membranes supported by substrata have been reported
that consist of evaporated amorphous carbon and macroscopically
aligned or random networks of carbon nanotubes. The freestanding
1
b
6
1b
7
carbon thin film, with a peculiar spiral structure such as hierarchical
helical bundles of nanofibrils, is expected to afford novel electrical
and electromagnetic properties. However, it has been considered
difficult to prepare a freestanding carbon thin film through the
carbonization of an organic polymer film. This is because carbon-
8
ization at high temperature causes thermal decomposition and
volatilization of hydrocarbon gases, destroying the morphology of
the original film.
On the other hand, since iodine doping of the polyacetylene (PA)
9
film results in a remarkable increase in electrical conductivity, it
has been generally adopted to increase the electrical conductivity
of a variety of conjugated polymers. Here we will shed light on a
new aspect of this doping by disclosing its indispensable role in
maintaining the nanostructure and morphology of precursors during
carbonization. We have used the nanofibril-fabricated helical PA
film (Figure 1a-c) to prepare the freestanding helical carbon thin
film. Helical PA film, recently developed using a novel polymer-
ization method, has a spiral morphology, in which bundles
consisting of nanofibrils with diameters < 100 nm are twisted and
Figure 1. (a) SEM image of the helical PA film and (b) enlarged one. (c)
Schematic representation of the helical morphology consisting of twisted
bundles of the PA nanofibrils. (d) SEM image of helical graphitic film
prepared through a morphology-retaining carbonization of iodine-doped
helical PA and then heat treatment at 2600 °C.
1
0
concentrically curled.
The helical carbon thin films were prepared by carbonization of
the iodine-doped helical PA film at high temperature. Experimental
details are described in the Supporting Information (SI). The atomic
ratio of doped iodine to carbon in the PA film was ∼0.25. The
thermal behaviors of the undoped and doped PA films during
carbonization were examined using thermogravimetry and dif-
ferential thermal analysis (TG-DTA). The outgases generated during
the heating of the doped PA film up to 500 °C were also examined
using GC-MS.
Figure 2. (a) X-ray diffraction intensity curves and (b) Raman scattering
spectra for the films carbonized at 800 °C (black) and those heat-treated at
The doped PA film is almost completely carbonized at temper-
atures above 800 °C. The thermal behavior observed in the TG-
DTA curve for the doped PA film is quite different from that of
the undoped PA film (Figure S1). No indication of the thermal
decomposition is observed in the DTA curve for the doped PA
film. Surprisingly, the fibrous morphology of the original structure
remains unchanged even after the carbonization and then the heat
2
600 °C after the carbonization (red).
treatment (Figure 1d). In addition, the weight loss of the films due
to carbonization at 800 °C is very small, amounting to only 20%
of the weight of the film before iodine doping. Thus the nanofibril-
fabricated carbon film is obtained through the carbonization of the
doped PA film. X-ray diffraction of the carbon film prepared from
the doped PA film at 800 °C has no crystalline reflection (Figure
2a). The Raman scattering spectrum of the carbon film shows a
strong, broad peak at 1350 cm
structure (D-band), together with a comparable peak at 1580 cm
†
University of Tsukuba.
‡
Kyoto University.
-1
§
attributed to the disordered
NIMS.
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AIST.
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0880 9 J. AM. CHEM. SOC. 2008, 130, 10880–10881
10.1021/ja803865e CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. (a) Structural model of the iodine-doped PA. (b) Hypothetical
molecular model of the specimen formed on the way to the dehydrogenation
reaction from the iodine-doped PA film shown in image a. (c) Hexagonal
carbon bond network formed at 800 °C after passing through the model
shown in image b. (Enlarged figures are given in Figures S2 and S3.)
2
corresponding to the structure of the sp hexagonal carbon bond
network (G-band) (Figure 2b).
1
a
Figure 4. TEM images of (a) the dispersed helical graphitic nanofibrils
and (b) a single helical graphitic nanofibril with its electron diffraction
pattern (inset). (c) High resolution TEM image of the helical graphitic
nanofibril. (d) Schematic representation of the helical structure consisting
of graphitic nanofibrils. (Enlarged figures are given in Figures S4-S7.)
It is apparent that the iodine doping prevents the PA film from
thermally decomposing at high temperature. According to the
1
1
structural model of the iodine doped PA film, polyiodide ions
-
-
such as I
3
and I
5
are situated between the PA chains, forming a
charge transfer complex (Figure 3a). Iodine tends to react with
films prepared from the iodine-doped PA films are stable under
atmospheric conditions. It is therefore expected that these carbonized
films might exhibit intrinsic functions characteristic of the helical
structure and that they might be also be used in practical carbon
materials.
1
2
hydrogen at high temperature. Since an outgassing of hydrogen
iodide has been detected by GC-MS during heating of the doped
PA film, it can be assumed that hydrogen contained in the doped
PA is removed to some extent as hydrogen iodide from the PA
chains and that the PA chains partially cross-link between the
neighboring chains (Figure 3b). Furthermore, most of the hydrogens
are removed with increasing temperature. As a result, the networks
Acknowledgment. This work was supported by a Grant-in-Aid
for Science Research in a Priority Area “Super-Hierarchical
Structures” from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan, and partly supported by “Nano-
technology Support Project” of the MEXT.
2
of sp hexagonal carbon bonds are formed during carbonization
(
Figure 3c). These results indicate that the carbon film prepared
from the iodine-doped PA film at 800 °C exists in an almost
amorphous state.
Supporting Information Available: Experimental details, iodine
doping, and carbonization. These materials are available free of charge
via Internet at http://pubs.acs.org.
The carbon film prepared at 800 °C can be further graphitized
by heat treatment at 2600 °C, as shown in Figure 2. The two peaks
-
1
at 1350 and 1580 cm become sharp in the Raman spectrum
Figure 2b). The sharp diffraction peak corresponding to the (002)
(
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JA803865E
J. AM. CHEM. SOC. 9 VOL. 130, NO. 33, 2008 10881