Influence of methylimidazole isomers on ferrocene-catalysed nitrogen doped carbon nanotube synthesis

Article abstract of DOI:10.1016/j.jorganchem.2010.02.028

Nitrogen doped multi-walled carbon nanotubes (N-CNTs) were synthesized by the solid state pyrolysis of ferrocenylmethylimidazole or a mixture of ferrocene (FcH)/i-methylimidazole (i = 1, 2 and 4) at 800 °C at different ratios in sealed quartz tubes. Transmission electron microscopy (TEM) images confirmed that the carbon nanotubes (CNTs) obtained were doped with nitrogen to give nitrogen doped multi-walled CNTs (N-CNTs). N-CNTs showed bamboo-like structures for the CNTs produced from both ferrocenylmethylimidazole and the mixtures of FcH/i-methylimidazole at varying ratios. The study revealed that the different imidazoles produced different types/size distributions of shaped carbon nanomaterials (SCNMs) including N-CNTs with different diameters. An investigation of the bamboo structures revealed that the three methylimidazole isomers led to tubes with different individual bamboo compartment distances and different morphologies including different N contents. This confirms that the synthesis of N-CNTs is determined by fragments (ratios, types) produced by decomposition of reactants at high temperature.

Full text of DOI:10.1016/j.jorganchem.2010.02.028

Journal of Organometallic Chemistry 695 (2010) 1451–1457
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Influence of methylimidazole isomers on ferrocene-catalysed nitrogen doped
carbon nanotube synthesis
Edward N. Nxumalo a, Vongani P. Chabalala a, Vincent O. Nyamori c, Michael J. Witcomb b, Neil J. Coville a,
*
a DST/NRF Centre of Excellence in Strong Materials and Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
b Microscopy and Microanalysis Unit, and DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
c The School of Chemistry, University of KwaZulu-Natal, Durban Centre, Private Bag X54001, Durban 4000, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Nitrogen doped multi-walled carbon nanotubes (N-CNTs) were synthesized by the solid state pyrolysis of
ferrocenylmethylimidazole or a mixture of ferrocene (FcH)/i-methylimidazole (i = 1, 2 and 4) at 800 °C at
different ratios in sealed quartz tubes. Transmission electron microscopy (TEM) images confirmed that
the carbon nanotubes (CNTs) obtained were doped with nitrogen to give nitrogen doped multi-walled
CNTs (N-CNTs). N-CNTs showed bamboo-like structures for the CNTs produced from both ferrocenylme-
thylimidazole and the mixtures of FcH/i-methylimidazole at varying ratios. The study revealed that the
different imidazoles produced different types/size distributions of shaped carbon nanomaterials (SCNMs)
including N-CNTs with different diameters. An investigation of the bamboo structures revealed that the
three methylimidazole isomers led to tubes with different individual bamboo compartment distances
and different morphologies including different N contents. This confirms that the synthesis of N-CNTs
is determined by fragments (ratios, types) produced by decomposition of reactants at high temperature.
Ó 2010 Elsevier B.V. All rights reserved.
Received 4 December 2009
Received in revised form 12 February 2010
Accepted 25 February 2010
Available online 3 March 2010
Keywords:
Nitrogen doped carbon nanotubes
Shaped carbon nanomaterials
Bamboo compartment
Ferrocene
Methylimidazoles
1. Introduction
Doping of CNTs with heteroatoms leads to a change in the CNT
structure. Nitrogen, for example, has profound effects on the nano-
Since the report on the synthesis of carbon nanotubes (CNTs) in
1991 by Iijima [1], many researchers have devoted much time and
effort in developing new strategies for the synthesis of shaped car-
bon nanomaterials (SCNMs). Organometallic complexes have often
been used in the synthesis of these SCNMs that include CNTs [2],
carbon nanofibres (CNFs) [3], carbon nanospheres (CNSs) [4], car-
bon nanocoils [5], etc. The advantages of using organometallic
compounds are that they do not need to have a counterion, they
do not have to be used in the presence of a support and they act
as both a catalyst and as a carbon source to form SCNMs. Initial re-
ports using organometallic complexes such as ferrocene [6], cobal-
tocene [7] and nickelocene [8], under reducing conditions revealed
that the pyrolysis of the metallocenes generated CNTs.
When ferrocene is used together with a carbon source, the Fe/C
ratio has a major impact on whether CNTs or CNSs are the major
products formed in the reaction [9]. Low catalyst concentrations
favour CNT production and the absence of a catalyst leads only
to CNSs and amorphous materials [10]. Increasing the Fe/C ratio
leads to an increase in the diameter of the nanotubes produced
due to the formation of larger iron nanoparticles at higher metal
concentration [9]. An increased Fe/C ratio also leads to a higher
yield of iron filled CNTs [9a].
tube morphology [11,12]. The presence of nitrogen in the growth
environment is a favourable condition for the formation of ‘bam-
boo-like’ CNTs, but it is not a necessary condition [13–15]. N-dop-
ing of CNTs can be achieved by in situ N addition during CNT
formation or post-synthesis doping of undoped CNTs. The doping
of CNTs has been reported previously [16]. The routine method
for the in situ synthesis of nitrogen doped CNTs (N-CNTs) is the
chemical vapour deposition (CVD) method using nitrogen contain-
ing compounds [17]. Ammonia [18] or any volatile nitrogen-com-
pound containing an organic component can be used as an N
source. Most experiments performed using nitrogen gas or ammo-
nia in a CVD process generate vertically aligned CNTs [19]. Films of
vertically grown N-CNTs from different substrates have also been
obtained by the pyrolysis of a mixture of ferrocene and melamine
[20]. Li et al. [21] have shown that it is possible to grow high qual-
ity vertically oriented N-CNT arrays over an alumina substrate on a
rough surface. Qian et al. [22] successfully synthesized well-
aligned N-doped multi-walled carbon nanotube arrays over a large
area, on quartz and silicon wafers, by a floating catalyst CVD meth-
od at fairly low temperature (600 °C) using pyridine–ferrocene
mixtures. The nitrogen incorporated nanotubes have smaller outer
diameters but larger inner diameters compared with CNTs grown
from a xylene–ferrocene mixture under similar conditions [22].
N-CNTs have recently shown great potential as catalyst sup-
ports for Pt–Ru nanoparticles in the anodic oxidation of methanol
in direct methanol fuel cells [23]. They have also been used in
* Corresponding author. Tel.: +27 11 717 6738; fax: +27 11 717 6749.
E-mail address: neil.coville@wits.ac.za (N.J. Coville).
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.02.028

1452
E.N. Nxumalo et al. / Journal of Organometallic Chemistry 695 (2010) 1451–1457
standard Schlenk techniques. Silica Gel 60 was used for column
chromatography. Commercial dichloromethane was initially dried
over calcium chloride, and then distilled from calcium hydride.
Preparation of anhydrous diethyl ether was achieved by drying
over fresh sodium wire. Ferrocene and ferrocene carboxaldehyde
were purchased from Strem Chemicals, Inc. (USA). The three iso-
mers (1-methylimidazole, 2-methylimidazole and 4-methylimid-
azole) were purchased from Sigma–Aldrich.
H
N
H
N
N
N
N
N
(a)
(b)
(c)
Fig. 1. Structures of the N sources used during the formation of SCNMs: (a) 1-
methylimidazole; (b) 2-methylimidazole and (c) 4-methylimidazole.
2.2. Instrumentation
environmental applications, in the aqueous phase, as adsorbents of
organic and inorganic compounds and ions [24]. A recent review
described theoretical studies involving carbon nanostructures that
have been substitutionally doped with transition metals and nitro-
gen atoms [12]. Literature reports reveal that the incorporation of
N into CNTs results in an enhancement in conductivity [25] and
improvement of transport and field emission properties of CNTs
[26]. N-doped single-walled CNTs (SWCNTs) synthesized in large
scale using an electric arc discharge method show that the band
gap of SWCNTs can be tuned by varying the degree of N insertion
[27]. This is due to the electron donor ability of the nitrogen atom
that leads to the formation of a n-type semiconductor [28].
More recently there has been an upsurge of interest in the syn-
thesis of CNTs by the pyrolysis of organometallic complexes in a
confined environment e.g. stainless steel autoclaves or sealed glass
vessels, at autogenous pressure especially using Fe and Co metal
catalysts [29]. Undoped CNTs can be produced in high yields by di-
rectly pyrolyzing ferrocene in an autoclave [30]. Zhang et al. [31]
also synthesized larger diameter undoped CNTs through the cata-
lytic decomposition of polypropylene and maleated polypropylene
using Ni as a catalyst in an autoclave at 700 °C. The possible growth
process for CNTs grown using this method was also described.
In this work, we report the N-doping of multi-walled CNTs in a
confined space (sealed quartz vessels) using organometallic pre-
cursors. The use of ferrocenylmethylimidazole as a catalyst for
the synthesis of N-CNTs as well as the use of different ratios of fer-
rocene/i-methylimidazole (i = 1, 2 or 4) (Fig. 1) in a confined space
yielded N-CNTs with different morphologies and several other
SCNMs. Furthermore, we reveal that the growth of N-CNTs is deter-
mined by the fragments produced by decomposition of the reac-
tants and that the different imidazoles yield N-CNTs with
different nitrogen contents.
Raman spectra were measured using the single spectrograph
stage of a Jobin–Yvon T64000 Raman spectrometer. The excitation
source was 636.4 nm from a tunable Spectra-Physics dye laser. The
laser beam was focused onto the sample using the 20Â objective of
an Olympus microscope. The backscattered light was dispersed
using a 600 line/mm grating and detected using a CCD detector.
Thermogravimetric analysis (TGA) measurements were performed
under air on a Perkin–Elmer TGA 7 at a heating rate of 10 °C/min.
TEM analysis was performed on a JEOL JEM-100S Electron Trans-
mission Microscope at 80 kV and on a Philips CM200 (Philips, Eind-
hoven, The Netherlands) equipped with a Gatan Imaging Filter. The
TEM samples were dispersed in methanol using an automated
sonicator and loaded onto a holey Cu grid.
2.3. Synthesis of ferrocene derivatives
2.3.1. Synthesis of ferrocenylmethylimidazole
Ferrocenylmethanol was obtained by the reduction of ferrocene
carboxylaldehyde using an ethereal solution of lithium aluminium
hydride. Ferrocenylmethylimidazole was achieved by reacting
equimolar amounts of ferrocenylmethanol and N,N-carbonylimi-
dazole in anhydrous dichloromethane in an inert atmosphere [32].
2.3.2. Synthesis of SCNMs in sealed tubes
Approximately 100 mg of precursor compounds (ferrocenylme-
thyimidazole or FcH/methylimidazole) were sealed in a quartz
tube (10 cm in length and 1.0 cm internal diameter). The quartz
tube (Fig. 2(a)) was introduced into an electrical furnace and
heated at a rate of 3 °C/min to reach a temperature of 500 °C i.e.
above the precursor decomposition temperature. The tubes were
held at this temperature for at least 30 min to ensure complete
decomposition of reagents. The tube was then heated at a rate of
2 °C/minute to 800 °C and held at that temperature for 12 h before
being gradually cooled to room temperature. A black solid material
was obtained (Fig. 2(b)). A small amount of the black carbonaceous
material was transferred into a mortar and ground using a pestle to
a fine material. The sample was then characterized by TEM, TGA
and Raman spectroscopy.
2. Experimental
2.1. General procedure
The synthesis of ferrocenyl derivatives were performed under
an inert atmosphere of pure argon. This was achieved by using
Fig. 2. Quartz tube reaction vessels (a) before and (b) after reaction.

E.N. Nxumalo et al. / Journal of Organometallic Chemistry 695 (2010) 1451–1457
1453
ucts (amorphous carbon, spheres, fibres, hollow tubes and bamboo
tubes) were obtained from the different reactions and the data is
summarised in Table 1. All the tubes obtained in this study were of
the multi-walled type and TEM images of typical N-CNT products
are shown in Figs. 3 and 4. The product morphology distributions
were estimated from the TEM photographs and while the error
bar was ca. 5–10%, trends in the morphologies can still be noted.
The product ratios indicate that the type of imidazole used did in-
deed impact on the product distribution detected.
3. Results and discussion
3.1. Synthesis of ferrocenylmethylimidazole
Ferrocenylmethylimidazole was synthesized from equimolar
amounts of ferrocenylmethanol and N,N-carbonylimidazole in
anhydrous dichloromethane as reported by Simenel et al. [32].
The chemical structures of these two compounds were confirmed
by 1H NMR, 13C NMR and FTIR spectroscopies and XRD patterns.
In these systems there is no flowing H2 since the reactions are
performed in sealed quartz tubes. Thus the formation of amor-
phous carbon is inevitable [35]. The molecules FcH and ferro-
cenylmethylimidazole generally give tubes which are cleaner
compared to those obtained from the FcH/methylimidazole mix-
tures (e.g. the 50/50 mix) under analogous reaction conditions.
Such an observation is also common for floating catalyst systems
[17,36].
While the amorphous carbon and the spheres may have con-
tained nitrogen, it is not possible to determine the nitrogen content
in these materials. In contrast, CNTs that contain nitrogen can
readily be detected by the bamboo morphology that is characteris-
tic of the Fe catalysed synthesis of N-CNTs [37–39]. Thus analysis
of this sub-set of products allows for an analysis of the effect of
nitrogen on the CNT products generated.
3.2. Synthesis of carbon nanotubes
Carbonaceous materials were synthesized successfully by the so-
lid state pyrolysis of ferrocenylmethylimidazole or varying ratios of
mixtures of FcH and methylimidazole in sealed quartz tubes. The
advantage of this procedure is that the effect of high pressure on
product formation can be utilized. Methylimidazoles were chosen
for study as they have a high N content (ꢀ30% by mass), are air-stable
and exist as solids and liquids. Reaction conditions were chosen to
ensure breakdown of the reactants by decomposition. In the closed
environment the reactants will break down into C, H, N and Fe con-
taining species [33]. The conditions will produce ionic species as well
as radicals, molecular species and ions, and the type of reactant and
the reactant ratio could thus impact on the products formed.
In this study the effect of the position of the methyl group in
methylimidazole has been investigated in the synthesis of nitrogen
doped CNTs. In particular, three different methylimidazoles (i.e. 1,
2 or 4-methylimidazole) were chosen for investigation. As these
compounds all have the same elemental composition any differ-
ences in the products formed will relate to the position of the Me
group on the ring backbone. In principle, if they all break down
completely under the reaction conditions then all three isomers
should generate similar ratios and types of carbonaceous products.
Indeed the current mechanism proposed for CNT growth involves
carbon atoms dissolving in a catalyst and re-precipitating to give
CNT growth [34]. The three methylimidazole isomers do, however,
have different physical and chemical properties and hence we may
expect decomposition products and rates to be different.
3.2.2. Effect of FcH/imidazole ratio
The TEM images of CNTs grown from ferrocene (100%) show
hollow tubes (outer diameter = 120 nm and inner diameter =
60 nm) that typically contain encapsulated iron nanoparticles
(Supplementary material, Fig. S1). The data shows that the prod-
ucts obtained are similar to those obtained in a typical floating cat-
alyst flow system (except for the production of more amorphous
material in the closed systems) [40].
The average outer diameters (ODs) of the multi-walled CNTs
(MWCNTs) produced from ferrocenylmethylimidazole were
70 nm, whilst their average internal diameters (IDs) were 30 nm.
These narrower tubes (relative to FcH) are expected [41]. Impor-
tantly, the presence of nitrogen has resulted in the expected forma-
tion of bamboo N-CNTs. Images of the bamboo morphology are
shown in Fig. 4a.
In this investigation, the pyrolysis reactions all gave black car-
bonaceous materials that were then analysed by TEM, TGA and Ra-
man spectroscopy.
Of more interest is the impact of the mixtures of FcH and
methylimidazoles on the products produced. Remarkably the posi-
tion of the ring substituent impacts on both the product distribu-
tion (in particular N-CNT yield) and the product morphology (N-
CNT diameters) (see Table 1). Thus, for a FcH/i-methylimidazole
3.2.1. TEM analysis of CNTs
TEM analysis was used to evaluate the products generated in
the pyrolysis reactions. TEM images showed that different prod-
Table 1
Relative% distribution of CNTs, CNFs, CNSs and a-C with the size distribution and the relative yields produced from the pyrolysis of FcH, ferrocenylmethylimidazole and FcH/i-
methylimidazole at varying ratios.
Catalyst used to grow CNTs
Fe/H ratio
SCNMs (%)
Diameters (nm)
Yield (mg)
FcH (100%)
0.100
80T, 20a-C
50T, 20F, 30a-C
40T, 60a-C
120(OD), 60(ID)
70(OD), 30(ID)
90(OD), 40(ID)
150(OD), 120(ID)
100(OD), 40(ID)
100 OD, 25 ID
140 OD, 120 ID
120 OD, 100 ID
70(OD), 20(ID)
60(OD), 25(ID)
50(OD), 20(ID)
100 OD, 80 ID
90 OD, 40 ID
78.2
73.0
51.2
67.0
59.1
46.1
65.9
62.5
44.8
47.5
46.1
30.7
30.0
40.4
14.2
37.7
40.2
Ferrocenylmethylimidazole (100%)
FcH/1-methylimidazole (90:10)
FcH/2-methylimidazole (90:10)
FcH/4-methylimidazole (90:10)
FcH/1-methylimidazole (80:20)
FcH/2-methylimidazole (80:20)
FcH/4-methylimidazole (80:20)
FcH/1-methylimidazole (50:50)
FcH/2-methylimidazole (50:50)
FcH/4-methylimidazole (50:50)
FcH/1-methylimidazole (20:80)
FcH/2-methylimidazole (20:80)
FcH/4-methylimidazole (20:80)
FcH/1-methylimidazole (10:90)
FcH/2-methylimidazole (10:90)
FcH/4-methylimidazole (10:90)
0.0714
0.0833
0.0833
0.0833
0.0769
0.0769
0.0769
0.0435
0.0435
0.0435
0.0156
0.0156
0.0156
0.0076
0.0076
0.0076
30T, 5S, 65a-C
30T, 5F, 65a-C
45T, 5S, 50a-C
45T, 5F, 55a-C
35T, 15F, 50a-C
50T, 10F, 40a-C
30T, 10F, 60a-C
5T, 35S, 60a-C
70T, 20F, 10a-C
80T, 10F, 10a-C
10T, 5F, 85a-C
30T, 30F, 40a-C
10T, 40F, 50a-C
5T, 45F, 50a-C
50 OD, 20 ID
80(OD), 40(ID)
70(OD), 40(ID)
35(OD), 10(ID)
T = Tubes, F = CNFs, S = CNSs, a-C = amorphous carbon.

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Fig. 3. HRTEM images of N-CNTs grown from the pyrolysis of (a) FcH/4- methylimidazole (50:50) and (b) ferrocenylmethylimidazole.
Fig. 4. TEM images of N-CNTs showing bamboo structures grown from the pyrolysis of (a) FcH/1-methylimidazole; (b) FcH/2- methylimidazole and (c) FcH/4-
methylimidazole.(50:50).
(50:50) mixture the N-CNT yields decreased in the order i =
1 > 2 > 4. Further, the average tube inner and outer diameters gen-
erally became smaller, in the order i = 1 > 2 > 4. This decrease in
diameter suggests a higher degree of N-doping of the CNTs (in-
crease in N concentration) as has been suggested previously [42].
The product data for the FcH/methylimidazole mixture are most
similar to the products obtained from ferrocenylmethylimidazole.
The degree of N-doping also could be calculated from the % of
the CNTs that had a bamboo structure or an open structure. For
examples, the pyrolysis of FcH/1-methylimidazole gave a degree
of doping of 15% while FcH/2-methylimidazole gave a degree of
doping of 75% at a similar ratio (50:50). FcH/4-methylimidazole
(50:50) and ferrocenylmethylimidazole gave higher degrees of
doping (>95%). This is consistent with the observed tube diameters
as well as the bamboo compartment distances (Figs. 7 and 8).
Other FcH/i-methylimidazole ratios were also used to study the
reaction (90:10; 80:20; 20:80; 10;90). The general trends noted
were:
(i) The trends listed above for the 50:50 ratio hold (in general)
for the other ratios.
(ii) The total yields of all products decreased with increasing
imidazole content.
(iii) Increasing the Fe/C ratio results to an increase in the diam-
eter of the CNTs. This is related to the formation of larger
iron nanoparticles when a higher catalyst content is used.

E.N. Nxumalo et al. / Journal of Organometallic Chemistry 695 (2010) 1451–1457
1455
(iv) These higher Fe/H ratios (0.100) favour the formation of
more CNTs while at lower Fe/H ratios (0.0076) more amor-
phous material is generated (see Table 1). This is associated
with the formation of more gaseous materials under a more
reducing atmosphere.
produced from 4-methylimidazole have a higher N content; a re-
sult consistent with the tube diameter analysis described above.
The average distance between individual bamboo compart-
ments obtained from the pyrolysis of ferrocenylmethylimidazole
was found to be 45 nm, close to the N-CNT values obtained from
4-methylimidazole (Fig. 6). High N values are expected for the
CNTs produced from ferrocenylmethylimidazole due to the prox-
imity of N group to the Fe catalyst in the reaction [14].
(v) Most samples contained >10% of Fe filled nanotubes.
The products obtained from the three imidazoles are thus dif-
ferent in terms of yields and morphologies. This could be due to
breakdown products produced in the reaction. Clearly the manner
in which decomposition occurs varies with the imidazole used. The
three imidazoles are structural isomers; hence, they have different
physical and chemical properties. The difference in their physical
properties should affect the rate and nature of their decomposition.
3.3. TGA analysis
Thermogravimetric analysis (TGA) of nanotubes under O2 is a
useful technique for studying the thermal stability of carbonaceous
samples. The shape of the TGA curve can give information about
the presence of carbon by-products, such as amorphous carbon
[44,45].
3.2.3. Analysis of the bamboo structures
All samples prepared in this study showed similar oxidation
behaviour in TGA experiments and typical TGA profiles are shown
in Fig. 7. A slight weight loss at ꢀ200 °C is due to the loss of water
and organic volatiles. Decomposition of the CNTs was observed
from 450 to 610 °C. Generally, this accounts for most of the weight
loss (60%). It has been observed that the N-CNTs are generally less
stable than the undoped CNTs (prepared from FcH) and hence
should decompose at lower temperatures [46]. This is reportedly
caused by the presence of a higher defect content in their walls
[47]. Most of the tubes have 5–20% residual weight after oxidation
(Fig. 7). This is attributed to the metal residue (FeOx) that remains
after carbon has been oxidized. This residual weight varies with
In a previous study, we have quantified the N content of CNTs,
using XPS and CHN analysis. In this study this is not possible due to
the large amount of non-CNT products formed. However the
amount of N added to a CNT can be correlated to the distance be-
tween individual bamboo compartments [17]. The larger the bam-
boo compartments the lower the nitrogen content of the N-CNTs
produced when similar reaction conditions are used [43].
The N-CNTs produced from the FcH/i-imidazole 50:50 mixtures
were analysed for their nitrogen content using the procedure de-
scribed above. The distance between bamboo compartments
(Fig. 4) was measured from TEM images (>200 measurements
per sample) for each of the imidazole samples produced. The dis-
tance between the bamboo ‘caps’ within a N-CNT is clearly seen
to vary with the positioning of the methyl group in the imidazole
(Figs. 5 and 6 and Fig. S2 and S3 (Supplementary material)). Tubes
grown from 1-methylimidazole have a larger compartment dis-
tance (avg value = 210 nm) followed by those obtained from 2-
methylimidazole (avg = 95 nm) and those obtained from 4-methyl-
imidazole (avg = 40 nm). These results indicate that the N-CNTs
Fig. 7. TGA curves of CNTs grown from (A) FcH/1-methylimidazole (10:90); (B)
FcH/2-methylimidazole (10:90) and (C) FcH/4-methylimidazole (10:90).
G-band
Fig. 5. Distance between individual bamboo compartments obtained from the
pyrolysis of 1-methylimidazole (50:50).
D-band
FcH/4-Me-imidazole
CNTs
Ferrocenylimidazole
CNTs
FcH/1-Me-imidazole
CNTs
1200
1300
1400
1500
1600
1700
-1
Wave numbers (cm )
Fig. 6. Distance between individual bamboo compartments obtained from the
Fig. 8. Raman spectra of CNTs grown from ferrocenylmethylimidazole, FcH/1-
pyrolysis of ferrocenylimidazole.
methylimidazole (50:50) and FcH/4-methylimidazole (50:50).

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E.N. Nxumalo et al. / Journal of Organometallic Chemistry 695 (2010) 1451–1457
the amount of FcH used in the synthesis of the CNTs (Table 1)
[48,49]. Of note is the slight weight gain around 400 °C, due to oxi-
dation of the residual Fe catalyst trapped in the tubes.
growth can be controlled by the choice of reactants, as has been
suggested by others [14,33]. Thus a key finding from our experi-
ments is that control of species formed in the reaction can be con-
trolled by the chemical nature of the reactants. Experiments to
further probe the above proposal are currently underway.
3.4. Raman analysis
Raman spectroscopy was used to obtain information about
disorder in the carbon materials. For most spectra, three peaks
are observed: a peak around 1350 cmÀ1 (the D-band), a peak
around 1580 cmÀ1 (the G-band) and a small shoulder at around
1615 cmÀ1. The Raman D-band is associated with disorder and is
a manifestation of an in plane vibrational mode. The G-band orig-
inates from the symmetric vibrations of the Raman active E2g mode
[50]. The small shoulder at around 1615 cmÀ1 is also induced by
disorder.
The intensity ratio (ID/IG) indicates the degree of disorder of the
SCNMs. When samples are compared the ID/IG ratio can establish
which sample contains the most graphitic (most ordered) struc-
ture. Disorder can arise from the range of structures produced as
well as the degree of N-doping of the CNTs. The ratios are displayed
in the Supplementary material (Table S1). Fig. 8 shows the Raman
spectra for samples grown at a similar reactant ratio (50:50). Gen-
erally, a dramatic broadening of the D- and G-band is seen in these
crude samples (compared with the undoped CNT) [43]. Further, the
ID/IG ratio of the samples varied with the FcH/imidazole ratio. This
observation can be correlated to the products obtained from the
three different imidazole isomers. It appears that, in general (and
as expected), reactions with 4-methylimidazole give the largest
ID/IG ratios.
4. Conclusion
A method for the N-doping of CNTs based on the pyrolysis of
ferrocenes and immidazoles in a confined environment has been
described. We have shown that the position of the methyl group
attached to the imidazole can influence the size distribution and
the type of SCNMs produced in the ‘autoclave’ system. An analysis
of the SCNMs and the bamboos structures reveals that the three
structural isomers of methylimidazoles lead to different products;
in particular with N-CNTs that contain different amounts of N as
determined by measurements of bamboo compartments and tube
diameters. The results show that the control of N-CNTs is deter-
mined by fragments produced by decomposition of reactants at
high temperature.
Acknowledgements
The authors wish to thank the DST/NRF Centre of Excellence in
Strong Materials and the University of the Witwatersrand for
financial support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.02.028.
3.5. Mechanism
A mechanism that leads to the formation of N-CNTs with
different morphologies generated from the pyrolysis of imidazole
isomers can be proposed (Fig. 9). When heated to elevated temper-
atures, imidazoles decompose to form radicals, ions and molecular
species (Fig. 9, step 1). Under suitable conditions these decomposi-
tion products will break down further to C and N atoms (step 2),
which then interact with Fe nanoparticles to form N-CNTs and
other carbon species via the classical CNT growth mechanism [34].
The mechanism above does not however fully account for the
different products and product ratios that are formed from the
different imidazole isomers and that could lead to the different
product yields and morphologies. The three imidazole isomers
decompose to give different breakdown products. This is associ-
ated with the different physical and chemical properties and differ-
ent methyl group and C@N positions in the different isomers and
that lead to the formation of the different decomposition products.
This suggests that the radicals, ions and molecular species formed
can interact directly with the Fe nanoparticles (step 4). Presumably
these radicals, ions and molecular species then decompose to C and
N atoms that eventually lead to the N-CNT growth. This would sug-
gest control (via the breakdown products) of CNT morphology by
chemical procedures is possible. This proposal indicates that CNT
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