Production of bundles of aligned carbon and carbon-nitrogen nanotubes by the pyrolysis of precursors on silica-supported iron and cobalt catalysts

Article abstract of DOI:10.1016/S0009-2614(00)00437-1

Pyrolysis of acetylene over iron or cobalt nanoparticles well dispersed on silica substrates gives copious yields of aligned carbon nanotube bundles. By carrying out the pyrolysis of pyridine over these catalyst surfaces, good quantities of aligned carbon

Full text of DOI:10.1016/S0009-2614(00)00437-1

2
6 May 2000
Chemical Physics Letters 322 Ž2000. 333–340
www.elsevier.nlrlocatercplett
Production of bundles of aligned carbon and carbon–nitrogen
nanotubes by the pyrolysis of precursors on silica-supported iron
and cobalt catalysts
a,b
a
a,b
a
Manashi Nath , B.C. Satishkumar , A. Govindaraj , C.P. Vinod ,
a,b,)
C.N.R. Rao
a
Chemistry and Physics of Materials Unit and CSIR Centre of excellence in Chemistry, Jawaharlal Nehru Centre For AdÕanced Scientific
Research, Jakkur P.O., Bangalore-560 064, India
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India
b
Received 22 February 2000; in final form 28 March 2000
Abstract
Pyrolysis of acetylene over iron or cobalt nanoparticles well dispersed on silica substrates gives copious yields of aligned
carbon nanotube bundles. By carrying out the pyrolysis of pyridine over these catalyst surfaces, good quantities of aligned
carbon–nitrogen nanotube bundles have been produced. The composition of the carbon–nitrogen nanotubes varies between
C10N and C33N, depending on the catalyst. q 2000 Elsevier Science B.V. All rights reserved.
1
. Introduction
newer methods for making aligned nanotube bun-
dles. Pan et al. w8x reported recently that aligned
carbon nanotubes can be obtained by carrying out
the pyrolysis of acetylene over silica surfaces con-
taining evenly positioned iron–silica nanocomposite
particles while Mukhopadhyay et al. w9x have pro-
Carbon nanotubes and related materials have
gained increasing importance in the last few years
because of their possible technological applications
w1x. Aligned carbon nanotube bundles, which are
specially useful in some of the applications, have
been synthesized using alumina membranes w2,3x,
mesoporous silica w4x and patterned silica substrates
covered with Co w5x. Good quantities of aligned
nanotube bundles have been obtained by the pyroly-
sis of ferrocene and other organometallic precursors
w6,7x. In spite of these efforts, there is still a need for
duced quasi-aligned carbon nanotubes by catalytic
chemical vapour deposition. This led us to explore
whether pyrolysis of simple organic precursor
molecules over metal–silica catalyst surfaces can be
usefully employed for this purpose. With this objec-
tive, we have carried out the pyrolysis of acetylene
over the surfaces of silica-supported iron and cobalt
catalysts prepared by the sol–gel route and obtained
good results. More importantly, we have carried out
the pyrolysis of pyridine over these catalyst surfaces
to obtain carbon–nitrogen nanotube bundles. Car-
)
Corresponding author. Fax: q91-80-846-2766; e-mail:
cnrrao@jncasr.ac.in
0
009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž00.00437-1

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34
M. Nath et al.rChemical Physics Letters 322 (2000) 333–340
bon–nitrogen nanotubes were first synthesized by
Sen et al. w10x by the pyrolysis of pyridine over
inner diameter. placed in a furnace operating at a
temperature in the 700–9008C range, depending on
the requirement. The flow rates of the gases were
controlled using UNIT mass flow controllers. Frag-
mented sections of the calcined catalyst were first
cobalt nanoparticles. Carbon–nitrogen nanotubes or
nanofibres have since been prepared by means of a
microwave-excited plasma of C H –N mixtures
2
2
2
w11x, reactivermagnetron sputtering of graphite w12x,
pyrolysis of melamine on laser-patterned FerNi sub-
strates w13x and pyrolysis of ferrocene-melamine
mixtures w14x. Some of these preparations yielded
fully or partially aligned nanotubes. The present
study reports good quantities of aligned carbon–
nitrogen nanotubes obtained by pyridine pyrolysis
over the catalysts. These n-type nanotube bundles
are likely to be more useful because of their desir-
able electronic properties.
2
. Experimental
The iron–silica catalyst surfaces were prepared by
employing two procedures. In procedure 1, 2 ml of
tetraethylorthosilicate ŽTEOS. was mixed with 2 ml
absolute ethanol and 9 ml of 1.5 M aqueous solution
of FeŽNO . , and stirred for 20 min. This corre-
3
3
sponds to a loading of 1.04 at.% of iron on silica.
Few drops of concentrated HF solution were added
to the mixture and stirred for another 20 min, to
achieve slow gelation. The resulting gel was de-
posited on glass substrates and dried in an oven at
6
08C for 12 h. The gel so deposited fragmented into
2
smaller sections of 2–8 mm area; it was then
calcined at 4508C for 1 h under vacuum Ž10 Torr.
y3
and reduced in hydrogen at 5008C for 2 h. In proce-
dure 2, iron acetylacetonate was used as the metal
precursor. A desired quantity of the acetylacetonate
Ž1.5 mmol. was dissolved in 100 ml methanol and
then mixed with TEOS Ž2 ml. and the gel obtained
by a procedure similar to the one mentioned earlier.
This procedure was also employed to prepare
Corsilica catalysts starting with cobalt acetylaceto-
nate. The catalyst samples were examined by scan-
ning and transmission electron microscopy ŽSEM
and TEM. using LEICA S440i and JEOL JEM-3010
Fig. 1. SEM image of the Fersilica catalyst Žobtained from
procedure 1 employing iron nitrate. showing the uniform distribu-
tion of Fe nanoparticles. Žb. TEM image showing the uniform
sized Fe nanoparticles obtained by the sol–gel process followed
by calcination and reduction. Žc. SEM image of the Fersilica
catalyst obtained by using iron acetylacetonate Žprocedure 2..
microscopes, respectively.
In order to prepare bundles of aligned carbon
nanotubes, acetylene was pyrolysed over the catalyst
surfaces. The pyrolysis set-up consisted of stainless
steel gas flow lines, fitted to a quartz tube Ž25 mm

M. Nath et al.rChemical Physics Letters 322 (2000) 333–340
335
Table 1
Conditions for the synthesis of aligned nanotubes
Catalyst preparation Carbon nanotubes
Carbon–nitrogen nanotubes
Flow rates Žsccm. Pyrolysis temperature Duration Flow rates Žsccm. Pyrolysis temperature Duration
C H
Ar
Pyridine ArrH2
2
2
Procedure 1
Procedure 2 ŽIron.
Procedure 2 ŽCobalt. 15
15
15
85
85
85
7008C
7008C
7008C
1 h
30 min
30 min
30
40
45
120 ŽAr. 9008C
60 ŽH2 . 9008C
55 ŽH2 . 9008C
1.5 h
10 min
30 min
placed in the quartz tube and reduced under hydro-
gen atmosphere at 5008C for 2 h. The temperature of
the furnace was then raised to 7008C under a slow
ramp rate and the pyrolysis of acetylene carried out
with argon as the carrier gas. Aligned carbon–nitro-
gen nanotube bundles were prepared by a similar
procedure except that pyridine was pyrolysed on the
catalyst surface at 9008C. Typical pyrolysis condi-
tions for the production of carbon and carbon–
nitrogen nanotubes, are listed in Table 1.
The aligned nanotubes were observed by scanning
and transmission electron microscopy. For TEM
Fig. 2. SEM images of Ža. aligned carbon nanotubes growing vertically from the iron–silica catalyst Žprepared by procedure 1., obtained by
the pyrolysis of acetylene, and Žb. and Žc. of aligned nanotubes in side view. Žd. TEM image of the carbon nanotubes obtained with the
Fersilica catalyst.

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M. Nath et al.rChemical Physics Letters 322 (2000) 333–340
Fig. 3. SEM images Ža., Žb. and Žc. of the aligned carbon–nitrogen nanotubes obtained by the pyrolysis of pyridine over Fersilica substrates
Žprepared by procedure 1.. In Žd. C 1s and N 1s XPS signals of the nanotubes are shown.
studies, the nanotube samples were ground, dis-
persed in carbon tetrachloride and deposited onto
holey carbon copper grids. In the case of the aligned
carbon–nitrogen nanotubes, the compositions were
examined by X-ray photoelectron spectroscopy ŽXPS.
carried out concurrently with high-resolution elec-
tron microscopy.
3
. Results and discussion
using a VG scientific ESCA LAB V spectrometer.
In Fig. 1a we show a SEM image of the iron–silica
Electron energy loss spectroscopy ŽEELS. was also
catalyst surface, obtained by procedure 1, subjected
Fig. 4. SEM images of aligned nanotubes: Carbon nanotubes grown on Ža. Fersilica surfaces Žprepared by procedure 2., by pyrolysis of
acetylene Žflow rate, 15 sccm. at 7008C for 30 min under a flow of Ar Ž85 sccm., and Žb. on Corsilica surfaces Žby procedure 2. by
pyrolysis of acetylene Žflow rate, 15 sccm. at 7008C for 30 min under Ar Ž70 sccm.. Carbon–nitrogen nanotubes grown on Žc. Fersilica
surface Žobtained by procedure 2. by pyrolysis of pyridine at 9008C for 10 min with Ar Ž40 sccm. as the carrier gas and under a flow of
hydrogen Ž60 sccm., and Žd. on Corsilica surfaces Žby procedure 2. by pyrolysis of pyridine at 9008C for 30 min with Ar Žflow rate, 45
sccm. as the carrier gas under a flow of hydrogen Ž55 sccm.. XPS N 1s signals of the samples from Žc. and Žd. are shown next to the SEM
images.

M. Nath et al.rChemical Physics Letters 322 (2000) 333–340
337
to calcination and reduction. The image shows a
uniform distribution of iron–silica nanoparticles on
the surface. We show a TEM image of the catalyst
surface in Fig. 1b. The image shows fairly uniform

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M. Nath et al.rChemical Physics Letters 322 (2000) 333–340
sized iron nanoparticles. An analysis of the TEM
images showed that most of the particles had diame-
ters in the 5–20 nm range. In Fig. 2, we show the
SEM images of aligned carbon nanotube bundles
obtained by the pyrolysis of acetylene Žflow rate, 15
sccm; sccm, standard cubic centimeter per minute.
over the iron–silica surface at 7008C for 1 h under
argon flow of 85 sccm. The low magnification image
in Fig. 2a shows the bundles of aligned carbon
nanotubes growing out perpendicularly from the cat-
alyst surface. Fig. 2b,c show the SEM images of
compact aligned nanotube bundles.
In Fig. 3a we show a SEM image of the nan-
otubes obtained from the pyrolysis of pyridine Žflow
rate, 30 sccm. at 9008C for 1.5 h, under Ar Ž120
sccm. flow on the iron–silica catalyst surface. The
image clearly reveals the alignment of the nanotubes,
perpendicular to the catalyst surface. The SEM im-
ages in Fig. 3b,c show side and top views of the
aligned nanotubes, respectively.
nanotube, rather than the growth starting at the sur-
face of the particle. In order to examine this aspect,
we carried out a TEM study of the aligned carbon
nanotubes. In Fig. 2d, we show a typical TEM image
of carbon nanotubes, with diameters in the range of
10–15 nm. The image shows the presence of few
metal particles at the tips of the carbon nanotubes.
Furthermore, the SEM image of the carbon nanotube
bundles in Fig. 2a shows that the nanotubes lift the
metal nanoparticles from the catalyst surface. The
SEM image of carbon–nitrogen nanotube bundles in
Fig. 3a also shows tip-growth. We see metal parti-
cles at the tips of some of the nanotubes in Fig. 3.
In Fig. 4a,b, we show the SEM images of bundles
of carbon nanotubes obtained by the pyrolysis of
acetylene over Fe and Co catalysts prepared from the
acetylacetonates Žprocedure 2.. In Fig. 4c,d, we show
the SEM images of carbon–nitrogen nanotube bun-
dles obtained by pyridine pyrolysis over the Fe and
Co catalysts prepared by procedure 2. XPS analysis
of the carbon–nitrogen nanotubes obtained with these
catalysts shows broad signals due to N 1s around
400 eV. Based on the total N 1s intensities the
In order to characterize the aligned nanotubes
obtained by pyridine pyrolysis, XPS analysis was
carried out. In Fig. 3d we show the C 1s and N 1s
signals from these nanotubes. This C 1s signal is at
compositions of the nanotubes work out to be C N
9
2
85 eV and the N 1s signal around 400 eV. The
and C N, respectively, with the iron and cobalt
1
8
carbon to nitrogen ratio was estimated by taking the
ratio of the integrated peak areas under the C 1s and
N 1s signals and dividing them by the respective
photoionization cross-sections. The average composi-
tion for the aligned carbon–nitrogen nanotube turns
catalysts. However, if we take the intensity of the
signal at 401 eV obtained after spectral decomposi-
tion, the compositions work out to be C N and
1
8
C N, respectively, with the iron and cobalt cata-
3
3
lysts.
The TEM images of some of the carbon–nitrogen
out to be around C N. A Gaussian fit of the N 1s
6
spectrum, however, shows the presence of two peaks
with binding energies of 399 and 401 eV. The 399
eV feature is characteristic of pyridinic nitrogen Žsp2
hybridization., probably present at nanotube ends,
while the peak centered at 401 eV is due to nitrogen
present in the graphene sheets w10x. The latter corre-
nanotubes exhibit interesting features. In Fig. 5 we
present some of the unusual images of these nan-
otubes. The image in Fig. 5a shows a nanotube with
bamboo-shaped morphology. Fig. 5b shows a nan-
otube having a nested cone-shaped cross-section,
while that in Fig. 5c shows a somewhat unusual
morphology involving a conical stacking sequence of
the graphene sheets with intermittent hollow regions.
The high-resolution TEM image in Fig. 5d shows
that the graphene sheets are stacked in the form of
nested cones. Clearly, doping with nitrogen modifies
the morphology of the nanotubes drastically. It seems
that doping favours the formation of quasi-continu-
ous, cylindrical graphene fragments, which in turn
facilitate the growth of the nanotube. It is to be noted
that nitrogen can be present in pyridinic rings present
at the edges of such graphene fragments. The conical
sponds to trivalent nitrogen replacing the carbon in
the hexagonal structure. If we take the intensity of
only the 401 eV feature, the composition turns out to
be C N. EELS measurements confirmed the pres-
1
0
ence of nitrogen in the nanotube preparations. The
compositions estimated from EELS are generally
close to those from XPS w10,15x.
It has been proposed that nanotubes prepared by
precursor pyrolysis using catalyst particles grow by
the tip-growth mechanism w16x. According to this
mechanism, the metal particles stick at the tip of the

M. Nath et al.rChemical Physics Letters 322 (2000) 333–340
339
Fig. 5. Ža.–Žc. TEM images of carbon–nitrogen nanotubes obtained by the pyrolysis of pyridine Žflow rate, 30 sccm. over Fersilica
substrates Žprepared by procedure 1. at 9008C for 1.5 h under Ar Ž120 sccm. flow, showing various morphologies. Žd. HREM image of a
nanotube showing the nested cone-type stacking of graphene sheets in the hollow region.
sections sometimes enclose the catalyst particle as
shown in Fig. 5c, lending some support to the tip-
growth mechanism. The conical curvature in the
hollow regions could be due to the nitrogen doping,
wherein nitrogen is present in the five-membered
rings.

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M. Nath et al.rChemical Physics Letters 322 (2000) 333–340
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otubes. The synthesis of copious quantities of aligned
carbon–nitrogen nanotube bundles by this method is
of significance since they are likely to possess more
desirable electronic properties. The nanotube diame-
ters obtained by us is in the range 15–150 nm, most
of which have a stacked cone morphology. It would
be therefore equally correct to consider them to be
hollow carbon fibres, similar to those reported by
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33
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