Surface modification of carbon nanofiber with high degree of graphitization

Article abstract of DOI:10.1021/jp036819r

Mechanical and chemical treatments were found to improve the graphitization extent of graphitized platelet carbon nanofiber, which showed little change in graphitization parameters of X-ray diffraction even after heat treatment at 2800°C. Such treatments also restored the surface area that was significantly decreased through the formation of loop-shaped ends on the surface of carbon nanofibers by the graphitization. Formation, distortion, and removal of loop-shaped ends corresponded to such morphological and structural changes of the platelet carbon nanofiber. Relaxation of high-energy edges of the hexagon at the graphitization and high reactivity of closed loop ends at the treatments were observed under TEM. This study also supports existence of the primary rodlike structural unit in carbon nanofibers.

Full text of DOI:10.1021/jp036819r

J. Phys. Chem. B 2004, 108, 1533-1536
1533
Surface Modification of Carbon Nanofiber with High Degree of Graphitization
Seongyop Lim, Seong-Ho Yoon,* and Isao Mochida
Institute for Materials Chemistry and Engineering, Kyushu UniVersity, 6-1 Kasugakoen, Kasuga,
Fukuoka, 816-8580 Japan
Jun-hwa Chi
Korea Electric Power Research Institute, 103-16 Munji-dong, Yuseong-gu, Daejeon, 305-380 Korea
ReceiVed: September 20, 2003; In Final Form: NoVember 3, 2003
Mechanical and chemical treatments were found to improve the graphitization extent of graphitized platelet
carbon nanofiber, which showed little change in graphitization parameters of X-ray diffraction even after
heat treatment at 2800 °C. Such treatments also restored the surface area that was significantly decreased
through the formation of loop-shaped ends on the surface of carbon nanofibers by the graphitization. Formation,
distortion, and removal of loop-shaped ends corresponded to such morphological and structural changes of
the platelet carbon nanofiber. Relaxation of high-energy edges of the hexagon at the graphitization and high
reactivity of closed loop ends at the treatments were observed under TEM. This study also supports existence
of the primary rodlike structural unit in carbon nanofibers.
Introduction
to confirm the unique structure of platelet CNF. Modification
and removal of closed loop ends in graphitized platelet CNFs
were expected to provide a highly graphitized CNF with novel
surface functions.
Carbon nanofibers (CNFs) have been recognized to be unique
forms of carbon materials.1-4 The diversity of CNFs has been
recognized as variable alignments of laminated c-plane layers
along the fiber axis, which provided typically three types of
CNFs such as platelet (alignment perpendicularly to the fiber
axis), tubular (alignment parallel to the axis), and herringbone
Materials and Method
Platelet CNF (PCNF) was prepared from carbon monoxide
and hydrogen mixed gases over Fe catalyst at 600 °C in a
conventional horizontal tube furnace, as described in a previous
(
alignment angled to the axis) CNF.4
Graphitizable carbon materials are usually highly graphitized
by the heat treatment over 2400 °C to be easily recognized by
means of X-ray diffraction (XRD).5 The platelet CNF was
found to show highly graphitic structure comparable to that of
the graphite even in its as-prepared form.7 However, the
interlayer spacing (d002-spacing), the height of layered stackings
9
work. PCNF was graphitized (GPCNF) at 2800 °C for 10 min
,6
under an argon atmosphere. GPCNF was mechanically milled
by using stainless steel balls (Diameter 2 mm) in ethanol for
-9
72 h (GPCNF-M). GPCNF was treated in 10% HNO3 at ambient
temperature for 24 h (GPCNF-NA).
(Lc(002)), and the mean basal plane diameter (La(110)) of the
Fibers were measured by an X-ray diffractometer (Rigaku
Geigerflex II, Cu KR target, Rigaku Co. Ltd., Japan) to calculate
the crystallographic parameters (d002, Lc(002), and La(110))
platelet CNF assessed by XRD were not much changed even
after the graphitization over 2500 °C, whereas the electron
diffraction patterns of TEM (transmission electron microscope)
indicated certainly better alignment in the graphitized platelet
1
1
according to the JSPS standard procedure. The specific
surface area was measured by the nitrogen isotherm using
SOPTOMATIC 1990 (Fisons Instrument, Italy). High-resolution
TEM (HR-TEM) were performed by using a JEM-2010F
microscope (JEOL, Japan) with an acceleration voltage of 200
kV.
9
CNF. Such graphitization has been reported to induce formation
of concentrically laminated loop ends via closing free edges on
the surface of platelet CNF8,9 as is similar with the case of
graphite based carbon material.10
In this Letter, the present authors treated platelet CNF first
thermally, then mechanically and chemically to follow the
morphological and structural changes by TEM, XRD, and
Raman spectroscopy. Structural changes caused by such treat-
ments were notable in terms of structural understandings of
Results
Table 1summarizes the preparation conditions and some
physical properties of CNFs. PCNF prepared at 600 °C showed
a very high degree of graphitization in its as-prepared form,
the values of interlayer distance (d002) and the height of stacking
9
CNFs. In a previous work, we have proposed that the platelet
CNF is constructed by uniform close-packing of rod-shaped
primary units of 2-3 nm wide and 20-30 nm long, as described
by TEM and STM (scanning tunneling microscope) images and
the models in Figure 1. The peculiar behaviors of the platelet
CNF through a series of treatments in this study were expected
(Lc002) being 0.3363 and 28 nm, respectively. PCNF showed
0
.035 of H/C atomic ratio, suggesting C-H edges on the surface
of as-prepared CNF.
Heat treatment of PCNF at 2800 °C (GPCNF) hardly
improved its graphitization extent, as shown by its d spacing
and Lc in Table 1, whereas no hydrogen was found in the
*
Corresponding author. Phone: +81-92-583-7801. Fax: +81-92-583-
7
798. E-mail: yoon@cm.kyushu-u.ac.jp.
1
0.1021/jp036819r CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/13/2004

1534 J. Phys. Chem. B, Vol. 108, No. 5, 2004
Letters
Figure 1. Primary structural unit of platelet CNFs: STM and TEM images of its packing in CNF, TEM images of a mechanically separated
structural unit, and suggested models.
TABLE 1: Preparation Conditions and Some Physical
Properties of Platelet CNFs Used in This Study
surfaces, respectively, whereas the surfaces of GPCNF and
GPCNF-M were wholly covered by the loop ends. The order
-1
XRD properties
of the ratio, I1355/I1580 (the intensity ratio of the 1355 cm band
surface
-
1
against the 1580 cm band) followed PCNF > GPCNF-NA
> GPCNF ≈ GPCNF-M, which reflected the extent of free
d002
Lc(002) area
2
samples
PCNF
preparation
as prepared
H/Ca (nm)
(nm)
(m /g)
1
2
-1
0.035 0.3363
graphitized at 2800 °C 0.000 0.3363
28
32
91
32
45
66
edges in TEM images. The 1620 cm bands of PCNF and
GPCNF-NA were more emphasized than those of GPCNF and
GPCNF
GPCNF-M
GPCNF-NA 10% HNO3 for 24 h
ball-milling for 72 h
0.3360
0.3356
40
137
12
GPCNF-M, indicating edge-rich planes. The full width at half-
-1
maximum of the 1580 cm band, ∆ν(1580), decreased slightly
in the following order: PCNF > GPCNF > GPCNF-M ≈
GPCNF-NA, which is probably related to their graphitization
a
The H/C ratio means the atomic ratio of hydrogen per carbon.
elemental analysis, indicating no C-H edge. The surface area
12,13
extent.
2
decreased markedly from 91 to 32 m /g.
The milling of GPCNF increased slightly the graphitization
extent and surface area, as shown in Table 1. HNO3 treatment
of GPCNF recovered the surface area up to 66 m /g, improving
significantly the graphitization extent, as shown by 0.3356 nm
of d002 and 137 nm of Lc(002).
Discussion
2
In the present study, the surface of PCNF was observed under
high-resolution TEM following a series of treatments such as
graphitization, mechanical milling and chemical oxidation to
clarify its microstructure. The heat treatment at 2800 °C
(graphitization) induced the closed loop ends on the surface of
TEM photographs of CNFs are shown in Figure 2. The edges
of c-plane layers in the PCNF (Figure 2a) were closed to loop
ends with concentrically laminated multilayers through graphi-
8
,9
PCNF as reported previously and observed with other carbon
8
,9
10
tization at 2800 °C in Figure 2b, as previously reported. It
must be noted that every rod-shaped unit in its pile was covered
with a cap as seen in Figure 1. Ball-milling distorted the
alignment of the loop-ended structural units, as shown in Figures
materials. Such loop ends have been proposed to be formed
through folding of some planar hexagons at their edges, which
looks like a multiwalled nanotube as described by Moriguchi
1
0
et al. However, uniform closed loop ends suggests that their
formation is not accidental but based on the regularity in the
substructure of PCNF. We have demonstrated in a previous
2
c and 3c, where some space appeared between the loop-ended
structural units, possibly leading to an increase of the surface
area. Comparing high magnification micrographs of Figure 3b,c,
the semispherical head of the loop end was seen to swell slightly
in size, or probably the interspacing of c-plane layers became
denser after ball-milling treatment. In Figures 2d and 3d, the
acid treatment definitely cut off the loop ends, consequently
exposing the free edges. From the electron diffraction patterns
9
work that there exists a primary structural unit like a nanosized
rod in PCNF, and the concentrically laminated loop end was
found as a cap of nanosized carbon rod formed by graphitization.
The nano rod consists of 8-10 pieces of hexagonal planes. The
edges of such a set of planes appear to form regularly a closed
loop end. The edges of the planes within the rod are connected
with corresponding ones to form the concentric loops of
hexagonal planes.
The heat treatment at higher temperatures than 2000 °C
removes surface C-H bonds and stacks more densely hexagonal
layers of PCNF prepared at 600 °C, forming chemically active
(subimages of Figure 2a,b), improvement of the graphitization
extent was confirmed after the heat treatment. Such well-aligned
stacking appears to be preserved through milling and acid
treatments as far as TEM was concerned (Figure 3b-d).
The recovery of free edges after the acid treatment was
confirmed by Raman spectra of the CNFs, as shown in Figure
1
0
dangling sites on the edges. Thus, such edges must be
stabilized by bonding each other in the manner as described
above, even though such bonding must cause a large strain or
4. As shown in TEM photographs of Figures 2 and 3, the PCNF
and GPCNF-NA exposed the edges on whole and major

Letters
J. Phys. Chem. B, Vol. 108, No. 5, 2004 1535
Figure 2. HR-TEM images of (a) PCNF, (b) GPCNF, (c) GPCNF-M, and (d) GPCNF-NA. Subimages of (a) and (b) show the electron diffraction
pattern in circled parts of the main images.
Figure 3. HR-TEM images of (a) PCNF, (b) GPCNF, (c) GPCNF-M, and (d) GPCNF-NA, which are magnified from a part of each image in
Figure 1.
tension through formation of the sharp curvature in the
concentric loop alignment of hexagon. Chemical reactivity and
mechanical instability of such loop ends are expected at ambient
temperature.
The mechanical treatment as well as the acid treatment indeed
was found to modify the nanosized structure of GPCNF. Such
changes appear to reflect a local strain at the surface of
individual structural units, which is ascribed to formation of
the closed loop ends. Although the overall alignment of
hexagonal c-planes became certainly better in GPCNF, as shown
under TEM observations and Raman spectroscopy, the formation
of concentrically laminated loop ends from open edges may
inhibit further decrease of interlayer spacing (d002), inducing
defects in graphitic packing of the nanosized structural units
(little increase of Lc). The acidic oxidation cutoff the closed
loop ends on the surface of GPCNF, consequently to improve
overall alignment of hexagonal planes to be more graphitic,
because a nanosized loop end must suffer a high reactivity. The
distortion of the closed loop ends by milling at room temperature
may reflect its high reactivity and tension.

1536 J. Phys. Chem. B, Vol. 108, No. 5, 2004
Letters
loop ends on its surface, which induce a local strain at the
individual surface of primary structural units, limiting the
graphitization extent despite better alignment and overall
stacking. Such closed loop ends are mechanically distorted or
chemically removed. The primary structural units must govern
a series of structural changes in the surface as well as bulk of
the CNF. Surface changes influence the graphitization extent
and simultaneously the surface area of the graphitized CNF.
Acknowledgment. This study was carried out within the
framework of CREST program. The present authors acknowl-
edge the financial support of Japan Science and Technology
Corporation (JST) of Japan.
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19
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