The preparation of hollow carbon spheres by thermal decomposition of poly(tetrafluoroethylene)

Article abstract of DOI:10.1246/bcsj.79.1312

Hollow carbon spheres with in diameter ranging from 50 to 150 nm have been successfully synthesized by the decomposition of poly(tetrafluoroethylene) ([CF₂CF₂]_n, PTFE) at 550°C using NaN ₃ as a defluorination reagent. The formation mechanism of hollow carbon spheres is also proposed. The present study provides a promising route to decompose other plastics that are difficult to degrade in the environment.

Full text of DOI:10.1246/bcsj.79.1312

1312 Short Articles
Bull. Chem. Soc. Jpn. Vol. 79, No. 8, 1312–1313 (2006)
vacuum at 65 ꢁC for 5 h.
The obtained samples were characterized by X-ray powder
The Preparation of Hollow Carbon
Spheres by Thermal Decomposition
of Poly(tetrafluoroethylene)
diffraction (XRD) on a Rigaku Dmax-ꢀA X-ray diffractometer
˚
with Cu Kꢁ radiation (ꢂ ¼ 1:54178 A). The morphology of
nanocrystalline hollow carbon spheres was examined from
transmission electron microscopy (TEM) images taken with
a Hitachi H-800 transmission electron microscope. Raman
spectra were recorded with a Spex 1403 Raman spectrometer
at ambient temperature. Field emission scanning electron
microscopy (FESEM) was performed on a JEOL JSM-6700F
scanning electron microanalyzer.
ꢀ
Peijun Cai, Yuhua Zhang, and Li Feng
School of Chemical Engineering and Technology,
China University of Mining and Technology,
Xuzhou, Jiangsu 221008, P. R. China
X-ray diffraction powder patterns were taken following the
reaction. Figure 1a shows the XRD pattern of the products,
which were not washed. Two sharp reflection peaks can be
Received January 23, 2006; E-mail: pjcai@ustc.edu
˚
indexed to cubic NaF with the lattice parameter a ¼ 4:60 A
(JCPDS 73-1922). Figure 1b is a typical XRD pattern of the
as-prepared products washed with distilled water and absolute
ethanol. No obvious diffraction peak could be detected, which
indicates that low crystalline or amorphous carbon is pro-
duced.
Hollow carbon spheres with in diameter ranging from
50 to 150 nm have been successfully synthesized by the de-
composition of poly(tetrafluoroethylene) ([CF2CF2]n, PTFE)
at 550 ꢁC using NaN3 as a defluorination reagent. The forma-
tion mechanism of hollow carbon spheres is also proposed.
The present study provides a promising route to decompose
other plastics that are difficult to degrade in the environment.
Representative TEM micrographs of the products are shown
in Figs. 2a and 2b. Based on large numbers of TEM observa-
tions, the proportion of hollow carbon spheres in the samples is
estimated to be about 50%. The external diameter of the hol-
The discovery of carbon nanotubes by Iijima in 19911 has
stimulated intense interest in carbon structures. Carbon nano-
tubes and other carbon structures have a wide range of appli-
cations in the fields of conductive and high-strength compo-
sites, semiconductor devices, field emission displays, and gas
storage media.2,3 Previously, carbon spheres had been synthe-
sized by various methods. For example, hollow carbon spheres
of several micrometers in diameter have been formed from
C60 fullerene powder after shock compression up to 57 GPa.4
Wang and Yin produced graphitic carbon solid spheres via a
mixed-valent oxidecatalytic carbonization (MVOCC) process
at 900–1050 ꢁC.5
*
*
a
b
20
30
40
50
60
70
Polytetrafluoroethylene ([CF2CF2]n, PTFE) is one of the
most widely used plastic materials due to its exceptional prop-
erties such as stability against heating, chemical inertness, low
surface tension, weathering, and excellent tribological per-
formance. However, waste PTFE materials may become per-
sistent solid pollutants that are difficult to degrade in the envi-
ronment.
The decomposition of PTFE has been the focus of recent
studies. By reducing sodium azide, hollow carbon spheres
can be produced in this process. To the best of our knowledge,
such a reaction has not yet studied in the literature. The reac-
tion used in our approach can be formulated as follows:
2Theta (Degree)
Fig. 1. XRD patterns of the products: (a) not washed,
(
ꢀ
water and absolute ethanol.
Face-centered cubic NaF); (b) washed with distilled
a
b
[CF2CF2]n þ 4nNaN3
! 2nC(hollow spheres) þ 4nNaF þ 6nN2:
ð1Þ
In a typical experiment, 1 g of PTFE and 2.60 g of NaN3
(the molar ratio NaN3/PTFE is 4.0) powders were placed into
in a stainless steel autoclave of 12 mL capacity. The autoclave
was sealed and maintained at 550 ꢁC for 10 h and then allowed
to cool to room temperature naturally. A dark precipitate was
collected and washed with distilled water and absolute ethanol,
successively. After that, the obtained sample was dried in a
100 nm
200 nm
Fig. 2. TEM images and ED pattern of as-prepared prod-
ucts: (a) typical area; (b) magnified hollow carbon spheres,
(inset) ED pattern of the hollow carbon spheres.
Published on the web August 7, 2006; doi:10.1246/bcsj.79.1312

Bull. Chem. Soc. Jpn. Vol. 79, No. 8 (2006)
Ó 2006 The Chemical Society of Japan 1313
1
4
.
Step1
Step2
Step3
.
.
.
.
.
.
.
.
.
.
.
7
3
6
2
5
Fig. 5. Scheme of synthesis process of hollow carbon
spheres (1-PTFE; 2-NaN3; 3-autoclave; 4-monomer; 5-
Na droplet; 6-carbon cluster; 7-carbon hollow sphere).
posed. The possible process is schematically illustrated in
Fig. 5. In this reaction system, the thermal decomposition of
NaN3 is known to proceed according to the previous report.7
The resultant metallic sodium may form droplets owing to
the heat generated from the exothermic reactions, and these
metallic Na droplets may also act as templates in the formation
process of hollow carbon spheres. Also upon increasing the
reaction temperature to 500 ꢁC, PTFE began to decompose
to monomers8 (Step 1). On the surface of the Na droplet, the
produced monomer C2F4 is reduced into NaF and amorphous
carbon clusters. Similar to the synthesis of other hollow
spheres using emulsion droplets as templates,9 the produced
carbon shell could stabilize the sodium droplets, thus preserv-
ing the spherical shape (Step 2). As the reaction continues, the
amount of sodium becomes depleted and the shell gradually
thickens, finally resulting in hollow carbon spheres (Step 3).
In summary, hollow carbon spheres have been synthesized
by the decomposition of PTFE at 550 ꢁC using NaN3 as a de-
fluorination reagent. The formation mechanism of hollow car-
bon spheres is also proposed. The significance of the present
study embodies the two aspects: One is a route to decompose
plastics, and the other one is a way to prepare hollow carbon
spheres in nano-scale.
Fig. 3. FESEM micrograph of the products.
600
800
1000
1200
1400
1600
1800
2000
Wavenumber (cm-1)
Fig. 4. Raman shifts of amorphous hollow carbon spheres.
low carbon spheres is 50–150 nm and the thickness of the wall
is about 10 nm. The diameter and wall thickness are similar to
that of the hollow carbon spheres reported in the literatures.6
From the contents of Fig. 1, the XRD pattern of the product,
there are no diffraction peaks that can be observed in the ED
(inset of Fig. 2b) of one typical hollow carbon sphere. Field-
emission scanning electron microscope (FESEM) micrograph
of the hollow carbon spheres of the product is shown in
Fig. 3. Large scaled hollow carbon spheres are found within
the diameter range from 50 to 150 nm, in good agreement with
the TEM results.
References
1
2
S. Iijima, Nature 1991, 354, 56.
S. Frank, P. Poncharal, Z. L. Wang, W. A. de Heer, Science
1998, 280, 1744.
3 M. Shim, A. Javey, N. W. S. Kam, H. J. Dai, J. Am. Chem.
Soc. 2001, 123, 1512.
The product was used directly to record the Raman spec-
trum (Fig. 4) at room temperature using a con-focal laser
Raman micro-spectrometer with an argon-ion laser at an exci-
tation wavelength of 514.5 nm. Two broadened bands at
around 1587 and 1346 cmꢂ1 were recorded, which correspond
to the typical Raman peaks of graphitized carbon nano-
spheres. The peak at 1346 cmꢂ1 could be assigned to the vibra-
tions of carbon atoms with dangling bonds in planar termina-
tions of disordered graphite. The peak at 1587 cmꢂ1 (G-band)
corresponds to an E2g mode of graphite and is related to the
vibration of sp2-bonded carbon atoms.
4
Phys. Lett. 2002, 362, 47.
K. Niwase, T. Homae, K. G. Nakamura, K. Kondo, Chem.
5
6
Z. L. Wang, J. S. Yin, Chem. Phys. Lett. 1998, 289, 189.
L. Shi, Y. L. Gu, L. Y. Chen, Z. H. Yang, J. H. Ma, Y. T.
Qian, Chem. Lett. 2004, 33, 532.
T. A. Richter, Energetic Materials 1: Physics and Chem-
7
istry of Inorganic Azides, ed. by H. D. Fair, R. F. Walker, Plenum,
New York, 1977, p. 33.
8
X. G. Yang, C. Li, W. Wang, B. J. Yang, S. Y. Zhang,
Y. T. Qian, Chem. Commun. 2004, 342.
P. J. Cai, Y. J. Tang, L. Zhang, K. Du, C. G. Feng, J. Vac.
Sci. Technol., A 2004, 22, 419.
9
The formation mechanism of hollow carbon spheres is pro-