EFFECT OF HYDROGEN FLOW RATE ON THE PRODUCTION OF CARBON
NANOFIBERS BY USING CATALYTIC CRACKING CHEMICAL VAPOR
DEPOSITION METHOD (CC-CVD)
Fahad S. Al-Kasmoul1, Muataz A. Atieh2, Naif M. Al-Abbadi1, Sulaiman I. Al-Mayman3, Bader.A Al-Shehri1
1
Energy Research Institute(ERI), King Abdualaziz City for science and technology (KACST)
P. O. Box 6086, Riyadh 11442, Saudi Arabia, nabbadi@kacst.edu.sa
2
Department of Chemical Engineering, Centre of Nanotechnology, King Fahd University for Petroleum and Minerals (KFUPM)
3
Petroleum and Petrochemicals Research Institute (PAPRI), King Abdualaziz City for science and technology (KACST)
Abstract
Carbon Nanofibers (CNFs) with diameter range from
(40-205 nm) were synthesized by using catalytic
cracking Chemical Vapor Deposition method (CVD).
Supported iron nanoparticles on aluminum oxide
powder (Fe/Al2O3)catalyst was used. The growth of
carbon nanofiber was carried out by using catalytic
decomposition of Ethylene gas at 800 oC as reaction
temperature and 30 min as reaction time. The CNFs
produced was characterized by using Field Emission
Scanning Electron Microscope (FESEM). The
structure of CNFs formed on Fe/Al2O3 was dependent
totally on the amount of the hydrocarbon source and
the hydrogen flow rate.
Figure 1 Schematic diagram of a catalytically grown
carbon nanofiber [www.wtec.org/loyola/nano, 2002].
Carbon nanofibers (CNFs) with helical structure were
synthesized at low temperature by Qin et al. [5]. Using
anodized aluminum template, Tu et.al were
synthesizing an array film of CNFs [6]. Without a
catalyst and at room temperature, CNFs were also
synthesized by the ion beam irradiation of an aromatic
compound [7]. It has been shown [8] that amorphous
CNFs with numerous Fe nanoparticles embedded
therein were generated by hollow cathode glow
discharge decomposition of Fe(C5H5)2 in He. Using
Cu/Ni catalyst film amorphous-CNFs can be grown at
400 oC [9].
In the published works some mechanisms for growth
of CNFs by thermal CVD method were highly
proposed, such as root growth mechanism [10], tip
growth mechanism [11], open-ended growth
mechanism [12], and yarmulke mechanism [13]. In the
root growth mechanism, the catalyst particles adhere
strongly to the substrate surface and remain pinned
during the growth process. In the tip growth
mechanism, due to the weak catalyst–substrate
interaction the catalyst particles are pushed up from
the substrate and are carried at the tips of CNFs. In the
open-ended growth mechanism, the carbon atoms are
added to the open ends of the CNTs. In the yarmulke
mechanism, the carbon atoms form a hemispherical
graphite cap on the catalyst particle and the nanotubes
grown to form the yarmulke. A more detailed
appreciation of these structures can be seen from the
respective 3-D models, where the darker geometric
shapes represent the metal catalyst particles
responsible for generating these conformations[14-17].
Keyword: CNFs, CC-CVD, Iron supported catalyst
and FE-SEM
Introduction
Research on new materials technology is attracting the
attention of researchers all over the world.
Developments are being made to improve the
properties of the materials and to find alternative
precursors that can give desirable properties on the
materials. Nanotechnology, which is one of the new
technologies, refers to the development of devices,
structures, and systems whose size varies from 1 to
100 nanometers (nm). The last decade has seen
advancement in every side of nanotechnology such as:
nanoparticles and powders; nanolayers and coats;
electrical, optic and mechanical nanodevices; and
nanostructured biological materials. Presently,
nanotechnology is estimated to be influential in the
next 20-30 years, in all fields of science and
technology.
Thermal chemical vapor deposition (CVD) method has
been used to grow the carbon nanofiber. Many carbon
sources such as CH4, C2H4, C2H2, C6H6 and CO were
used as sources of carbon atom and 3d metals like Fe,
Co, and Ni are utilized as catalysts for the
decomposition of carbon-containing gases [1-4]. Most
of carbon nanofibers and nanotubes synthesized by
CVD method are crystalline or partially crystalline,
and only a few of them are amorphous.
1
C2H4 gas as hydrocarbon sources and hydrogen gas as
carrier and reactant gas.
Figure 2: High resolution electron micrographs and
schematic representation of carbon nanofibers with
their graphite platelets, (a) "perpendicular" and (b)
"parallel" to the fiber axis [www.wtec.org, 2002].
In the present work the CNFs were grown on Fe/Al2O3
substrates in 1 atm pressure, 800°C and for 30 min
reaction time by Catalytic Cracking CVD method.
Ethylene (C2H4) was used as carbon source. The
structure and growth mechanism of CNFs were
investigated by using FESEM
Figure 3: Schematic Diagram of CC-CVD
The ethylene gas and hydrogen gas flow rates were
fixed at 0.5 and 0.1 1/min respectively. However the
reaction temperature and the reaction time were fixed
at 800 oC and 30 min respectively. Figure 4(a) shows
the low magnification of CNF produced. It is clearly
indicated that the product is in bundles form with
length reach up to 25 μm.
Experimental
Preparation of the supported catalyst
Fe catalyst supported on Al2O3 was synthesized by
using a conventional impregnation method.
Fe(NO3)2.6H2O was used as a metal sources of Fe
nano catalyst with its loading amount adjusted at 1
wt% for all the catalyst samples. The right amount of
Iron nitrite was dissolved in ethanol and sonicated by
using ultrasonic sonication bath for almost 30 min. The
Al2O3 powder was dissolved also in ethanol and
sonicated for 30 min. The Iron nitrate solution and
Al2O3 solution were mixed and sonicate for the same
time (30 min). The mixture was dried at 50 oC for 6 hr
and grinded by using ball mill.
Production of Carbon Nanofiber
Carbon nanofibers were grown by using catalytic
cracking chemical vapor deposition (CVD) reactor.
The schematic diagram at the apparatus is given in
Figure 3. The reactor is a tubular reactor with quarts
tube (200 mm inside diameter; ID and 600 mm length;
L). C2H4 gas was used as the carbon source and
hydrogen (H2) gas as carrier and reactant gas. The
catalyst was placed in the middle of the reactor and the
mixture of the C2H4 and H2 gases was controlled by
digital gas flow meters. After the growth, the samples
were collected and characterized using Field Emission
Microscope (FESEM)
Figure 4 (a)
Results and Discussion .
A thin film or strand of carbon nanofibers (CNFs) was
produced. The optimal production condition of the
produced high purity CNFs were shown in this study.
The effect of the amount of the carbon source; C2H4
and the flow rate of H2 were investigated. Figure 4
shows the SEM images of the produced CNFs by using
Figure 4 (b)
2
shows the low magnification SEM image of the
product. From this image it is indicated that the
samples are in bulk form and cover the aluminum
oxide particles.
Figure 4 (c)
Figure 5 (a)
Figure 4 (d)
Figure 4: SEM images of CNFs at 0.5 l/min ethylene
gas and 0.1 1/min hydrogen.
Figure 5 (b)
Figures 4(b, c and d) show the high magnification
images of the CNFs produced. The diameter of the
CNFs ranging 40nm - 85nm. Meanwhile, the images
show that these CNFs curly, entangled with each other
and it has network structure. Apart of that it is clearly
observed that the surface of the produced CNFs is
rough and has a lot of cracking points on the surface.
By increasing the flow rate of the ethylene gas to 0.7
and hydrogen gas to 0.3, the diameter of the CNFs
produced was increased dramatically from 50- 100nm
as shown in Figure 5. Figure 5(a) shows the low
magnification of the CNFs. It is clearly observed that,
the products are also in bundles form with 20 μm
length and highly pure. Figures 5(b) and (c) show the
high magnifications of the CNFs. The CNFs produced
are curly, entangled, smooth, and longer and no
network connection has been observed between them.
Figure 5 (c)
Figure 5: SEM images of CNFs at 0.7 l/min ethylene
gas and 0.3 1/min hydrogen.
To study the effect of hydrogen gas, the flow rate of
the ethylene gas was fixed at 0.7 1/min and the
hydrogen flow rate increased to 0.5 l/min. Figure 6(a)
3
Figures 6 (b) and (c) show the high magnifications of
the CNFs produced. It is clearly observed that by
increasing the flow rate of the hydrogen gas, the
diameter of CNFs has been increased sharply up to
205nm. The size of the diameter of CNFs at this
condition ranges from 170 - 250 nm. The structure of
the CNFs produced at this condition is curly, entangled
and it has network connection between them.
Figure 6: SEM images of CNFs at 0.7 l/min ethylene
gas and 0.5 1/min hydrogen.
Conclusion
Catalytic cracking CVD was used to produce high
purity of CNFs. Iron Nano particle supported on
Aluminum oxide powders were used as catalyst.
Ethylene as hydrocarbon source and hydrogen gas as
carrier and reactant gas were used. The reaction
temperature and the reaction time were fixed at 800 oC
and 30 min. The produced CNFs were characterized by
using FESEM.
The result indicate that the CNFs diameter increase
with the increasing the ethylene gas flow rate, while
the structure vary from rough short, network CNF to
smooth, long CNF. The forms of the product were also
varying under the same condition from bundles to bulk
powder. The promotional effect of hydrogen on carbon
nanofibers formation from the metal catalyzed
decomposition of carbon-containing gas molecules has
been attributed to its ability to convert inactive metal
carbides into the catalytically active metallic state as
well as to prevent the formation of graphitic over
layers on the particle surface. Thus, the catalytic
decomposition of hydrocarbon is highly sensitive to
substrate catalyst, while the hydrogenation of carbon is
relatively less sensitive to catalyst.
For the catalyst, which is not highly active for
decomposition, the hydrogenation reaction becomes
important and the net carbon deposition rate is lowered
by hydrogen gas.
From the experimental results, the effect of hydrogen
acceleration on carbon formation may be interpreted as
that the hydrogen decomposes inactive metal carbides
FeC to form catalytically active Iron Fe metal. Hence,
by increasing the hydrogen flow rate, the active sites
increased which will increase the cracking of the
hydrocarbon and producing more carbon atoms.
Increasing the carbon atoms will give the possibility to
increase the size of the carbon nano-material. It clearly
observed that by increasing the flow rate of the
hydrogen gas the diameter of CNFs has been increased
sharply from 75 nm to 200nm.
Figure 6 (a)
Figure 6 (b)
Acknowledgment
The Authors of the paper would like to extend a great
thank and appreciation to King Abdualaziz City for
Science and Technology; KACST for funding and
supporting the project.
References
[1] M.S. Kim, N.M. Rodriguez and R.T.K. Baker, J.
Catal. 134 (1992), p. 253.
[2] R.T.K. Baker, Carbon 27 (1989), p. 315.
Figure 6 (c)
4
[3] T.V. Reshetenko, L.B. Avdeeva, Z.R. Ismagilov,
V.V. Pushkarev, S.V. Cherepanova, A.L. Chuvilin and
V.A. Likholobov, Carbon 41 (2003), p. 1605.
[4] Y. Qin, L. Yu, Y. Wang, G. Li and Z. Cui, Solid
State Commun. 138 (2006), p. 5.
[5] J.P. Tu, K. Hou, S.Y. Guo and C.X. Jiang,
Mocaxue Xuebao 23 (2003), p. 268.
[6] T. Kimura, H. Koizumi, H. Kinoshita, H.
Takahashi and T. Ichikawa, Jpn. J. Appl. Phys. Part 2
43 (2004), p. L862.
[7] A. Huczko, H. Lange, M. Sioda, Y.Q. Zhu, W.K.
Hsu, H.W. Kroto and D.R.M. Walton, J. Phys. Chem.
B 106 (2002), p. 1534.
[8] K. Aoki, T. Yamamoto, H. Furuta, T. Ikuno, S.
Honda, M. Furuta, K. Oura and T. Hirao, Jpn. J. Appl.
Phys. Part 1 45 (2006), p. 5329.
[9] R.S. Ruoff, D. Qian, W.K. Liu, W. Ding, X. Chen
and D. Dikin, Collect. Tech. Rap. AIAA ASME ASCE
AHS Struct. Struc. Dyn. Mater. 4 (2002), p. 2538.
[10] S. Fan, M.G. Chapline, N.R. Franklin, T.W.
Tombler, A.M. Cassell and H. Dai, Science 283
(1999), p. 512
[11] S. Amelinckx, X.B. Zhang, D. Bernaerts, X.F.
Zhang, V. Ivanov and J.B. Nagy, Science 267 (1995),
p. 635.
[12] S. Iijima, P.M. Ajayan and T. Ichihashi, Phys.
Rev. Lett. 69 (1992), p. 3100.
[13] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T.
Colbert and R.E. Smalley, Chem. Phys. Lett. 260
(1996), p. 471.
[14] Fakhrul,R. A., Muataz, A.A., Iyuke, S.E.,
Dayang Radiah, A.B., AL-khatib, M..F., Elsadiq,
M.update
on
microscopy
and
microanalysis. 2003 14-17.
[15] Muataz, A.A. Fakhrul-Razi, A., , Iyuke,
S.E., Dayang Radiah, A.B., AL-khatib, M..F.,
El-sadiq, M. somche 2003
[16] Iyuke, S.E., Dana, A.A, Fakhrul -Razi, A.,
Muataz A. A., A.B., AL-khatib, M. F.,the 10 t h
apcche congress 17-21 2004.
[17] Muataz A, Nazlia G., Fakhru'l -Razi A.,
Chuah T. G., El- Sadig M., Ahmad F. I.,
Suleyman A. M., and Dayang R. B.
International Conference on MEMS and
Nanotechnology (2006)
5