J. Nig. Soc. Phys. Sci. 2 (2020) 205–217

Journal of the
Nigerian Society

of Physical
Sciences

Effect of benzophenone on the physicochemical properties of
N-CNTs synthesized from 1-ferrocenylmethyl

(2-methylimidazole) catalyst

Ayomide Hassan Labuloa,∗, Elijah Temitope Adesujia, Charles Ojiefoh Oseghalea, Elias Emeka
Elemikeb, Adamu Usmana, Akinola Kehinde Akinolac, Enock Olugbenga Darec

aDepartment of Chemistry, Federal University of Lafia, Lafia, Nasarawa State, Nigeria
bDepartment of Chemistry, Federal University of Petroleum, Nigeria

cDepartment of Chemistry Federal University of Agriculture, Abeokuta, Ogun State, Nigeria

Abstract

Vertically-aligned nitrogen-doped carbon nanotubes (v-N-CNTs) were synthesized via the chemical vapour deposition (CVD) technique. 1-
ferrocenylmethyl(2-methylimidazole) was employed as the source of the Fe catalyst and was dissolved in different ratios of acetonitrile/benzophenone
feedstock which served as both the carbon, nitrogen, and oxygen sources. The morphological difference in N-CNTs was as a result of increased
oxygen concentration in the reaction mix and not due to water vapour formation as observed in the oxygen-free experiment, indicating specifically,
the impact of oxygen. Raman and X-ray photoelectron spectroscopy (XPS) revealed surface defects and grafting of oxygen functional groups on
the sidewall of N-CNTs. The FTIR data showed little or no effect as oxygen concentration increases. XPS analysis detected the type of nitrogen
species (i.e. pyridinic, pyrrolic, graphitic, or molecular nitrogen forms) incorporated in the N-CNT samples. Pyrrolic nitrogen was dominant
and increased (from 8.6 to 11.8 at.%) as oxygen concentration increases in the reaction precursor. An increase in N content was observed with
the introduction of a lower concentration of oxygen, followed by a gradual decrease at higher oxygen concentration. Our result suggested that
effective control of the reactant mixtures can manipulate the morphology of N-CNTs.

DOI:10.46481/jnsps.2020.105

Keywords: Chemical vapour deposition, nitrogen-doped carbon nanotubes, 1-ferrocenylmethyl(2-methylimidazole), X-ray photoelectron
spectroscopy

Article History :
Received: 11 May 2020
Received in revised form: 08 August 2020
Accepted for publication: 09 August 2020
Published: 15 November 2020

c©2020 Journal of the Nigerian Society of Physical Sciences. All rights reserved.
Communicated by: B. J. Falaye

1. Introduction

Vertically-aligned carbon nanotubes (v-CNTs) have been
found to be fascinating for various range of applications, such
as catalysis (as catalyst support) [1, 2, 3], electronics [4, 5, 7]

∗Corresponding author tel. no: +234 8062295936
Email address: labulo@yahoo.com (Ayomide Hassan Labulo )

and biological [8, 9, 10] devices. This is as a result of the con-
trollable diameter and surface area of v-CNTs [11] which can
be explored in the fabrication of materials of particular inter-
est. The major drawbacks of v-CNTs are their low selectivity
and reactivity at the surface. These drawbacks can be over-
come by surface functionalization and nitrogen-doping which
tailor their physicochemical properties [12]. Doping of CNTs

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Labulo et al. / J. Nig. Soc. Phys. Sci. 2 (2020) 205–217 206

with heteroatoms, such as B, P, S and N, into the sp2 carbon
framework has been reported [13, 14, 15, 16]. These electron-
rich atoms help fine-tune the electronic properties of CNTs [2].
Also, nitrogen incorporation into CNTs alters the wall thick-
ness, crystallinity and diameter of CNTs [17]. The nitrogen em-
bedded into CNTs can take various forms. The most common
are graphitic-nitrogen, pyrrolic-nitrogen, pyridinic-nitrogen and
molecular N2 stuck in the interior of CNT structures [18, 19,
20]. The nitrogen composition largely depends on the solu-
bility of nitrogen in the catalyst nanoparticle during the reac-
tion at a specified temperature. It has been shown that the type
and concentration of nitrogen obtained depend on the nature
of the catalyst employed (i.e. ferrocene or ferrocenyl deriva-
tives), synthetic temperature, gas flow rate and type of nitrogen-
containing precursors [21, 22, 23].

Several methods have been employed in the synthesis of
N-CNTs; namely arc-discharge [24], laser deposition [25] and
chemical vapour deposition (CVD) [26]. Of these, the CVD
technique has been the method commonly used for large-scale
N-CNT synthesis [27]. However, the control of the reaction
conditions in CVD technique is somewhat tricky, as catalyst
poisoning due to limiting carbon diffusion rate and formation
of amorphous carbon on Fe substrate surface is common [28,
29]. Many researchers have reported the introduction of oxygen
[30], water [31, 32] and CO2 [33], ethyl benzoate [34] among
other reaction gases to improve N-CNTs quality and catalyst
activity [35]. However, an excess level of oxygen-containing
species could lead to N-CNTs etching [29, 32, 36]. Recently,
Sakurai et al. [37] reported that the introduction of the oxygen-
containing molecule (e.g. H2O) during CVD synthesis enhanced
the growth of CNTs and prolong catalyst lifetime at temper-
atures above 750 ◦C. This resulted in the removal of amor-
phous carbon through water vapour etching to give a graphitic
nanostructured carbon network [26]. Fatuba et al. [38] also
reported that the addition of oxygen-containing aromatic com-
pounds (i.e. growth enhancer), such as methyl benzoate and
benzaldehyde into the reaction mixture tailored the size and
controlled the N-CNTs wall numbers and alignment [38]. The
essential role of the growth enhancer compared to previously
reported approach (such as H2O), is to control the wall num-
bers, diameters and to reactivate catalyst particles [39, 40, 41].

In this study, we report for the first time, the use of ben-
zophenone in the reactant mixture to modify N-CNTs growth
and morphology. Benzophenone was also employed to improve
the solubility of the ferrocenyl imidazolium catalyst in acetoni-
trile. We elucidate the effect of oxygen on the type of nitrogen
incorporated in N-CNTs. The morphology, surface area and
stability of N-CNTs were studied at varying oxygen concentra-
tion levels.

2. Experimental

2.1. Materials and characterization

Ferrocene (≥ 97%), ferrocenemethanol (98%), 2-methylimidazole
(≥ 98.2%) and sodium borohydride (95%), potassium hydro-
gen phthalate (≥ 99.5%) were obtained from Sigma Aldrich
Ltd. South Africa. Acetonitrile (HPLC grade, 99.9%), toluene
(≥99.5%) and ethanol (98%) were purchased from Merck Chem-
icals South Africa. Nitric acid (55%) and sulphuric acid (98%)
were purchased from Saarchem, South Africa. 10% Hydro-
gen in argon (purchased from AFROX gases, South Africa) was
used as a carrier gas for the synthesis of N-CNTs. Images of the
synthesized N-CNTs were obtained by using scanning electron
microscopy (SEM) (JOEL JEM 1010) and transmission elec-
tron microscopy (TEM) (JOEL JSM 6100). Higher magnifi-
cation images of N-CNTs were obtained from high-resolution
transmission electron microscope (HRTEM). Elemental analy-
sis was conducted on a LECO CHNS elemental analyser. The
crystallinity of the N-CNTs was determined with a Rigaku/Dmax RB
powder X-ray diffractometer using graphite monochromatized
high-density Cu Kα radiation (λ= 0.15406). The thermal stabil-
ities of N-CNTs were determined using a Q SeriesT M Thermal
Analyzer TGA/DSC (Q600). The Fourier transform infrared
(FTIR) spectra of N-CNTs were recorded on a PerkinElmer
Spectrum RX1 FTIR spectrometer by embedding the samples
into KBr pellets. The adsorption-desorption isotherms and sur-
face area of N-CNTs were determined on a Micrometrics Tris-
tar II surface area analyser. The graphitic nature of the N-CNTs
was determined by a Raman spectrometer (DeltaNu Advantage
532TM). Four accumulated spectra were collected to access the
homogeneity of the samples. The synthesized N-CNTs were
purified in nitric acid under microwave irradiation using a CEM
Discover SP microwave instrument. The surface chemical com-
position of N-CNTs was analysed using X-ray photoelectron
spectroscopy (XPS). XPS analysis was conducted on a Quan-
tum 2000 instrument using a monochromated Al Kα source and
charge neutralizer, with a pass energy of 117.4 eV. The peaks
were deconvoluted using CasaXPS programme. The surface
charge on the N-CNTs in ultra-pure water was determined with
a Malvern Zetasizer (NS500). Boehm titration was conducted
to quantify the acidic functional groups on the N-CNT surfaces.
Potassium hydrogen phthalate (KHP) was used as the primary
standard for the standardization of NaOH solutions using phe-
nolphthalein as the indicator [42]. 0.20 g of each N-CNTs sam-
ples were placed in separate bottles. 25 mL NaOH (0.05 M),
Na2CO3 (0.025M) and NaHCO3 (0.05 M) were added to the
bottle, sealed and shaken for 24 h. The solutions were then fil-
tered and titrated against standardized HCl or NaOH [43]. Dif-
ferent functional groups (i.e. phenolic, carboxylic, lactonic and
hydroxyl groups) were calculated based on the amount of acid
or base consumed.

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2.2. Synthesis of 1-ferrocenylmethyl(2-methylimidazole) (FcMeCH3)

The general procedure described by Pan et al. [44] was used
to synthesize FcMeCH3. Briefly, ferrocenemethanol (1 mM)
and 2-methyl-1H-imidazole (1.1 mM) was refluxed in acetic
acid for 9 h at 60 ◦C. The product formation was monitored us-
ing preparative TLC plates with a solvent system of CH2Cl2/MeOH
(4:1). The product was neutralized with 50% KOH in distilled
water to remove the acetic acid and then puried by column chro-
matography. The final product was washed in Na2SO4 and fi-
nally dried under vacuum to obtain yellow crystals. Detailed
characterization of FcMeCH3 has been reported in our previous
work [45].

2.3. Synthesis of N-CNTs

N-CNTs were synthesized by pyrolyzing FcMeCH3 cata-
lysts in acetonitrile at 850 oC using the CVD method. In the ex-
periment, different concentrations of benzophenone were added
to the reaction mixture to study the effect of oxygen on the
growth of N-CNTs. The CVD procedure and set-up described
by Oosthuizen et al. [46] was followed. Briefly, 0.25 g of
the catalyst was added to 0.5, 1.0, 1.5 and 2.0 g of benzophe-
none to produce 1, 2, 3 and 4 wt.% oxygen, respectively. The
mixture was dissolved in acetonitrile (as carbon and nitrogen
source) to make a 10 g solution . The reactant mixture was in-
jected using a syringe at 0.8 mL min−1 through the quartz tube
placed in a muffle furnace. The mixture was swept through
the tube by 10% hydrogen in argon carrier gas for 100 mL
min−1. After 30 min of reaction, the furnace was allowed to
cool to room temperature, and the product was collected from
the hot region of the furnace. N-CNTs from 1-4 wt.% oxygen
is denoted as N-CNTs-1%, N-CNTs-2%, N-CNTs-3% and N-
CNTs-4%, respectively. N-CNTs-0% was synthesised by dis-
solving FcMeCH3 catalyst in acetonitrile. For comparison, N-
CNTs-Fe was synthesized using ferrocene and toluene as cata-
lyst and solvent, respectively.

2.4. Purification procedure for N-CNTs

N-CNTs were purified using microwave digestion. Briefly,
N-CNTs (0.8 g) were dispersed in nitric acid (6 M) by ultra-
sonic agitation for 45 min. After sonication, each sample was
purified by a microwave assisted irradiation. This was done
by placing 50 mL of the dispersed sample in a thermal resistant
Teflon (Milestone (TFM)) vessel on a sample rotor available for
4 vessels. The microwave was set at 100 W power and ramped
from room temperature to 100 oC for 30 min. After digestion,
the obtained suspension was filtered on 0.1 µm PTFE mem-
brane. The collected solid samples were washed with deionized
water until a neutral pH was obtained. Afterwards, the N-CNTs
were washed with alcohol and dried in an oven at 100 ◦C for 24
h.

3. Results and discussion

3.1. TEM analysis
The morphology of N-CNTs was studied by TEM. The ob-

tained images are shown in Figure 1. The incorporation of ni-
trogen correlated with the bamboo-like structure typical of N-
CNTs [47] (Figure 1a-f). The use of FcMeCH3 as a catalyst
in acetonitrile and benzophenone gave mainly clean N-CNTs
(Figure 1) and in good yield (Table 1). This could be attributed
to the cleaning effect of oxygen as it reacts with amorphous car-
bon to form CO2. N-CNTs and carbon sphere (CS) are obtained
in toluene solvent.

Table 1. Summary of the effect of oxygen from benzophenone on the yield of
N-CNTs synthesized by using 1-ferrocenylmethyl[2-methylimidazole] catalyst
in acetonitrile at 850 ◦C

Samples Yield (%)

N-CNTs-0% 74
N-CNTs-1% 68
N-CNTs-2% 63
N-CNTs-3% 61
N-CNTs-4% 58

The N-CNTs yields decrease as the concentration of oxygen
increases due to the formation of CO2 from unreacted carbon
and oxygen. The TEM images of N-CNTs-1% and N-CNTs-
2% (Figure 1a and b) showed a curly tubular structure. This
could be as a result of Fe catalyst left inside the N-CNTs with
smaller diameters [48]. The bamboo compartment of N-CNTs-
1%, N-CNTs-2%, N-CNTs-3% and N-CNTs-4% decreased as
the concentration of oxygen increased (Table reftab2). All N-
CNTs obtained are opened at the tips, while some region along
the tube gave stacked cup-like cones. This suggests that the
bamboo structures were obtained by tip growth mechanism [49].
The cup-like cones appear to be more prominent as the oxygen
concentrations increased (Figure 1c and e). N-CNTs-0% and
N-CNTs-Fe exhibit relatively straight tubes and a wall thick-
ness of ∼15 nm. The wall thickness decreases as the concen-
tration of oxygen from benzophenone increases (Table 2). This
is due to a reduction in the number of corrugated carbon layers
and the closure of tubes which resulted in reduced compartment
distances [50].

Table 2 shows the effect of oxygen on the inner diame-
ter (ID) and outer diameter (OD) of the synthesized N-CNTs.
From the results, it is believed that oxygen plays a vital role
in modulating the morphology and diameters of N-CNT [39].
The OD decrease as the oxygen content in the reaction mix-
tures increases. This is due to the effect of oxygen on the cat-
alyst leading to a decrease in Fe particle size as a result of cat-
alyst migration, sintering, and precipitation processes [51, 52].
It was suggested that oxygen enhances the catalyst activity by
removing amorphous carbon which prevents N-CNTs surface

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Figure 1. TEM images of N-CNTs obtained from (a) N-CNTs-0%, (b)
N-CNTs-1%, (c) N-CNTs-2%, (d) N-CNTs-3%, (e) N-CNTs-4% and
(f) N-CNTs-Fe

poisoning [41]. N-CNTs-1%, N-CNTs-2% and N-CNTs-3%
gave smaller ID (14±7 nm to 16±5 nm). However, larger ID
N-CNTs was obtained for N-CNTs-4% (i.e. 33±8 nm). This
is due to excess oxygen content, leading to etching of the outer
walls which largely affects N-CNTs quality.

Figure 2 shows the HRTEM images of N-CNTs with vary-
ing oxygen contents. An increase in the d002 interlayer spac-
ing of the graphitic carbon was observed as the OD decreases.
The interlayer d-spacing increased from 0.339 nm (N-CNTs-
1%) (Figure 2b) to 0.344 nm (N-CNTs-3%) (Figure 2d). The
increase in the d002 spacing is due to the curvature of smaller
diameter N-CNTs and higher strain caused by the structural
defect on the nanotube walls [53]. Also, the regular bamboo
compartment for N-CNTs-4% (Figure 2f) was destroyed. This
is attributed to supersaturation of molten Fe catalyst particles
with carbon [54]. It could also be as a result of highly reac-
tive oxygen at the surface or within the molten Fe nanoparticles
which form FeO (i.e. Fe + O2 →FeO + O), leading to etching
of the graphitic carbon.

3.2. SEM analysis

The morphology of N-CNTs was analysed using SEM. The
obtained images are shown in Figure 3 (a-f). Figure 3 a-e man-
ifested the effect of oxygen on N-CNTs growth and alignment.
This was as a result of the reaction of oxygen with very reactive
hydrogen radical involved in the hydrocarbon-based growth of
nanotubes [20]. This helps to scavenge unreactive hydrogen
which inhibits the growth of sp2 like graphitic sheets [30]. For
example, the vertical alignment was observed for N-CNTs-1%,
N-CNTs-2% and N-CNTs-3% (Figure 3 b-d) compared to N-
CNTs-Fe (Figure 3f) The alignment was depleted at higher oxy-
gen concentration (as observed in N-CNTs-4%). This could be
attributed to the partial oxidization Fe-catalyst which reduced
catalyst density, leading to reduced N-CNTs nucleation [55].

Figure 2. Eect of oxygen on N-CNT wall thickness and diameters:
HRTEM images of (a) N-CNTs-0%, (b) N-CNTs-1%, (c) N-CNTs-
2%, (d) N-CNTs-3%, (e) N-CNTs-4% and (f) etched wall of N-CNTs-
4%

At moderate oxygen concentration (i.e. N-CNTs-2%), the nan-
otubes walls are free of amorphous carbon (Figure 3c) as com-
pared to N-CNTs-4% (Figure 3e) with more amorphous carbon
and lesser tubes (Table 1).

3.3. Thermal studies

The thermal stabilities of N-CNTs with different oxygen
wt.% loading was studied as shown in Figure 4. TGA analy-
sis was measured in air at 25-1000 ◦C to give an idea of the
oxygen content and the purity of the samples. The first mass
loss due to loss of water appears before 100 ◦C. N-CNTs-1%
shows a significant weight loss at 386 ◦C while N-CNTs with
2-4 wt.% oxygen showed a weight loss between 390-530 ◦C.
N-CNTs-0% is the most thermally stable with the decomposi-
tion temperature at 589 ◦C. The oxygen treated N-CNTs started
to decompose at the on-set point between 334 and 430 ◦C (Ta-
ble 3). All N-CNTs showed weight loss after decomposition
above 87% with a residual mass between 9.6-0.5%. From DTG
curves, the maximum mass loss temperature for 1-4% oxygen-
treated N-CNTs is between 392 and 514 ◦C. Further investiga-
tion by Raman, XPS and FTIR analysis was done.

3.4. Crystallinity of N-CNTs

Figure 5 shows the Raman spectra of N-CNTs-0%, N-CNTs-
1%, N-CNTs-2%, N-CNTs-3%, N-CNTs-4% and N-CNTs-Fe.
The two prominent peaks observed at ∼1330 and ∼1573 cm−1

are assigned to the D- and G-bands, respectively. The intensity
ratios of the D- and G-bands (ID/IG ) shows the defect level of
graphitic carbon materials [56, 57, 58]. The ID/IG ratio of N-
CNTs-0% and N-CNT-Fe is 0.74 and 0.66, respectively (Table
4). After introduction of oxygen from benzophenone in the re-
actant precursor, the ID/IG ratio increased to 0.97, 0.93, 0.85 and
0.79 for N-CNTs-1%, N-CNTs-2%, N-CNTs-3% and N-CNTs-
4%, respectively. This is as a result of incorporation of surface

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Table 2. Effect of oxygen on N-CNTs diameter and wall thickness

Oxygen wt. % Ave.
OD±SD
(nm)

Ave.
ID±SD
(nm)

Wall thickness
(nm)

Ave. Compart-
ment distance
(nm)

N-CNTs
(%)

N-CNTs-0% 48±25 38±31 15 18±11 90

N-CNTs-1% 37±31 19±5 11 17±9 85

N-CNTs-2% 33±21 14±7 9 15±8 76

N-CNTs-3% 34±19 16±5 8 13±9 74

N-CNTs-4% 41±15 33±8 7 11±8 65

N-CNTs-Fe 75±16 48±12 14 20±12 83

Table 3. Thermal features of N-CNTs at different oxygen concentration.
Toxidation - refers to the temperature of primary oxidation.

Entry Catalyst On set point
(oC)

Toxidation (oC )

1 N-CNTs-0% 430 572

2 N-CNTs-1% 378 450

3 N-CNTs-2% 397 410

4 N-CNTs-3% 346 428

5 N-CNTs-4% 334 420, 514

6 N-CNTs-Fe 386 392

oxygen functionalities and N atoms which produces more de-
fects and disorders on the graphitic structure of the N-CNTs.
The lower ID/IG ratio in N-CNT-Fe and N-CNTs-0% indicates
that fewer defects are introduced in the carbon lattices due to
less nitrogen atom intrusion into the graphitic carbon network
compared to N-CNTs-1%, N-CNTs-2% and N-CNTs-3%, re-
spectively. The width of the G-band peak also indicates the
level of doping in N-CNTs [59, 60]. Table 4 shows that the G-
band width of N-CNTs with varying concentrations of oxygen
follows the order of N-CNTs-1% > N-CNTs-2% > N-CNTs-
3% > N-CNTs-4% > N-CNTs-0% > N-CNTs-Fe. This result
suggested a possible increase in N-doping at lower oxygen con-
centration.

Table 4. IG /ID ratios of the N-CNTs

Samples D G ID/IG

N-CNTs-0% 1341 1591 0.74

N-CNTs-1% 1342 1601 0.97

N-CNTs-2% 1354 1599 0.93

N-CNTs-3% 1365 1595 0.85

N-CNTs-4% 1369 1590 0.79

N-CNT-Fe 1374 1581 0.66

3.5. Surface chemistry of N-CNTs

Figure 6 shows the FTIR spectra of N-CNTs from 0-4% of
oxygen and N-CNTs-Fe. Peaks at around 2927 and 2625 cm−1

are assigned to the O–H and CH3 stretching vibrations [61], re-
spectively. The prominent band at 2381 cm−1 is assigned to the
characteristic absorbance of CO2 groups [62], while peaks at
1763, 1567 and 1030 cm−1 are assigned to stretching vibrations
of C=O, C=N and C-O functional groups, respectively [63].
The peaks at 1375 cm−1 are assigned to stretching vibrations of
C-NH3 [64]. The presence of C=N and C-N functional group
on the purified N-CNTs indicates the substitution of graphitic
sp2 carbon with nitrogen, leading to the bamboo configuration
observed in TEM images [65]. For N-CNTs-0%, the intensity
of the C=O band peak at 1763 cm−1 was weaker than that from
1-4% oxygen, which becomes broader as the concentration of

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Labulo et al. / J. Nig. Soc. Phys. Sci. 2 (2020) 205–217 210

Figure 3. SEM images of (a) N-CNTs-0%, (b) N-CNTs-1%, (c) N-CNTs-2%, (d) N-CNTs-3%, (e) N-CNTs-4% and (f) N-CNTs-Fe

oxygen increases. The increase in the intensity of the C=N peak
at 1567 cm−1 for N-CNTs from 1-4% oxygen can be related to
the increase in nitrogen-doping level, which correlates with Ra-
man analysis results (Table 4). The results of Boehm titration

of N-CNTs-0%, N-CNTs-1%, N-CNTs-2%, N-CNTs-3%, N-
CNTs-4% and N-CNTs-Fe are shown in Table 5. According to
this method, NaHCO3, Na2CO3 and NaOH, neutralize carboxyl
groups, carboxyl groups and lactones; and carboxyl groups, lac-

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Labulo et al. / J. Nig. Soc. Phys. Sci. 2 (2020) 205–217 211

Figure 4. (a) TGA curves and (b) DTA of purified N-CNTs synthesized from 0-4% wt. oxygen

Figure 5. Raman spectra of N-CNTs

tones and phenols, respectively. Therefore, different functional
groups can be calculated from the volume of acid and bases
used. The acid functional groups on N-CNTs-1%, N-CNTs-
2%, N-CNTs-3% increases a little as the oxygen concentration
increases compared to N-CNTs-0% and N-CNTs-Fe, while the
amount of basic functional groups significantly increases. This
indicates that the oxygen functionalities on the surface of N-
CNTs synthesized in the presence of oxygen are more basic
than N-CNTs synthesized in acetonitrile only [66]. From the
results, N-CNTs-1%, N-CNTs-2%, N-CNTs-3% contains high
concentration of basic group (≥ 1.025 mmol/g) (Table 5). Addi-
tionally, N-CNTs-2% has the highest concentration of the phe-
nolic group.

Zeta potential (ζ ) measurement provides information on
the adsorption of ions (H+ and OH−) from aqueous suspension
and dispersibility which lead to the formation of net charge on

Figure 6. FTIR spectra of N-CNTs

the N-CNTs [67]. These net charges lead to the formation of
the electrical double layer which stabilizes the suspension and
prevents particle aggregation. The properties of nanoparticles
are largely affected by their colloidal stability. Nanoparticles
with zeta potential less than -25 mV or above +25 mV are
said to have a high degree of stability [67]. Table 6 shows
the variation in the zeta potential of N-CNTs-0%, N-CNTs-
1%, N-CNTs-2%, N-CNTs-3%, N-CNTs-4% and N-CNTs-Fe
nanofluids. Our result showed that the zeta potential follows the

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Table 5. Boehm titration of N-CNTs
Samples Acidic groups (mmol/g) Basic groups

(mmol/g)

Phenolic Carboxylic Lactonic

N-CNTs-0% 0.766 0.813 0.070 0.889

N-CNTs-1% 0.085 1.025 0.680 1.025

N-CNTs-2% 0.181 1.142 0.0826 1.542

N-CNTs-3% 0.062 0.851 0.348 1.416

N-CNTs-4% 0.016 0.664 0.529 0.784

N-CNTs-Fe 0.0860 0.612 0.481 0.741

Table 6. Zeta potentials of N-CNTs in ultrapure water

Samples Zeta potential (mV)

N-CNTs-0% -37.6

N-CNTs-1% -51.4

N-CNTs-2% -57.0

N-CNTs-3% -54.0

N-CNTs-4% -43.2

N-CNTs-Fe -38.8

order N-CNTs-2% > N-CNTs-1% > N-CNTs-3% > N-CNTs-
Fe > N-CNTs-0% > N-CNTs-4%. The zeta potential increases
as the concentration of oxygen increases but drops sharply at
higher oxygen concentration. According to this measurement,
the oxidized N-CNTs are negatively charged in the aqueous
phase as a result of oxygen-containing functional group ion-
ization [68].

The effect of oxygen on the porosity of N-CNTs-0%, N-
CNTs-1%, N-CNTs-2%, N-CNTs-3% and N-CNTs-4% was char-
acterized by BET analysis. The nitrogen-adsorption isotherms
of all N-CNTs are of type IV with different hysteresis loops in
the high-pressure regions (P/Po = 0.7–1), suggesting the pres-
ence of mesoporous structure [69]. As shown in Table 7, the
surface areas of N-CNTs follows the order: N-CNTs-2% > N-
CNTs-1% > N-CNTs-3% > N-CNTs-Fe > N-CNTs-0% > N-

Table 7. BET surface area and pore volume of N-CNTs-0%, N-CNTs-1%, N-
CNTs-2%, N-CNTs-3%, N-CNTs-4% and N-CNTs-Fe

Samples Surface area (m2g−1) Pore volume (cm3 g−1)

N-CNTs-0% 95 0.37

N-CNTs-1% 127 0.35

N-CNTs-2% 130 0.57

N-CNTs-3% 122 0.53

N-CNTs-4% 89 0.39

N-CNTs-Fe 110 0.46

CNTs-4%. This indicates that the surface area of the N-CNTs
can be modified by the introduction of oxygen into the reactant
mixture.

3.6. Elemental analysis

The elemental composition and the bonding environment of
the C, O and N species were determined by XPS analysis, and
the result is presented in Table 8. Figure 7 shows the high-
resolution N 1s energy region of selected N-CNTs (N-CNTs-
0%, N-CNTs-3%, and N-CNTs-Fe). The deconvolution of the
spectra gave three distinct N 1s peaks centred at 398.50, 400.18
and 401.20 eV assigned to pyridinic, pyrrolic and graphitic ni-
trogen, respectively [70]. A steady increase in the level of
nitrogen-doping was observed during the CVD synthesis, fol-
lowed by a gradual decrease due to an increase in oxygen con-

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Labulo et al. / J. Nig. Soc. Phys. Sci. 2 (2020) 205–217 213

Figure 7. XPS N 1s spectra of (a) N-CNTs-0%, (b) N-CNTs-3% and
(c) N-CNTs-Fe

centration (Table 8); a result consistent with Raman and TGA
data. Compared with N-CNTs-0% and N-CNTs-Fe, the N-
CNTs-3% gave higher pyrrolic-nitrogen species which could
be attributed to the presence of active site caused by the lower
amount of oxygenated species on the graphitic carbon frame-
work [71]. At a low oxygen concentration in benzophenone,
pyridinic-N species was formed (Table 8). At a high oxygen
content, pyrrolic-N was obtained [72]. This may be due to the
change in the elemental ratio (C: N: O) in the precursor mixture.
The amount of nitrogen incorporated into N-CNTs obtained in
our study is higher compared to other studies [34, 73]. This
was attributed to the higher amount of nitrogen contained in
the ferrocenyl imidazolium catalyst. Additionally, the decrease
in nitrogen content could be ascribed to the presence of O in
benzophenone which we believe inhibits nitrogen incorporation
into N-CNTs. Deconvolution of O 1s spectra of N-CNTs-0%,
N-CNTs-3% and N-CNTs-Fe peaks gave two bands centred at
531.26 and 533.40 eV assigned to C=O and C-O [74], respec-
tively. The elemental analysis results (Table 8) corroborate XPS
result with increased nitrogen-doping triggered by addition of
varying amount of oxygen.

Table 9 shows the detailed analysis of C 1s peaks of N-
CNTs-0%, N-CNTs-3% and N-CNTs-Fe. The deconvoluted
C 1s peaks produced five components at 284.3, 285.8, 287.0,
287.9 and 289.4 eV, assigned to C=C, C-C, hydroxyl, carbonyl
and carboxyl functional groups, respectively [75, 76]. From the

Figure 8. XRD patterns of the N-CNTs

XPS analysis, the carboxyl and carbonyl functional groups in-
crease as the sp2 carbon decreases. For example, the atomic
percentage of C=O increased from 3.8 (N-CNTs-0%) to 8.8%
(N-CNTs-3%). The oxidation of C=C is confirmed by an in-
crease in C-C components, which led to the formation of new
functional groups on N-CNT surfaces.

3.7. Powder XRD pattern studies

The XRD profiles of N-CNTs (i.e. 0-4 wt.% oxygen) and
N-CNTs-Fe showed the crystalline nature of N-CNTs (Figure
8). All diffraction patterns showed the formation of (002) crys-
talline carbon plane (i.e. 26◦), indicative of CNTs formation
[77]. Other peaks at 44.5◦, 49.1◦ and 77.6◦ correspond to (100),
(221) and (401) reflections of the graphite structure of N-CNTs,
respectively. The weak peaks at 37.6◦ and 43.5◦ are assigned to
Fe3C and Fe2O3, respectively, which are stuck inside the core
of N-CNTs [78, 79]. The XRD diffraction pattern for N-CNTs
showed a decrease in the intensities of (002) peaks as the align-
ment increases, particularly, from N-CNTs-1% to N-CNTs-3%.
Also, the diffraction peak intensities of the (002) plane for N-
CNTs-1%, N-CNTs-2% and N-CNTs-3% are weaker than those
of N-CNTs-0% and N-CNTs-Fe. This shows that N-CNTs-Fe
and N-CNTs-0% have fewer structural defects since N-doping
create faults in the graphitic layers. This result agrees with the
Raman results (Table 4). The interlayer d-spacing increases
from 0.339 to 0.352 nm as oxygen concentration increases (Ta-
ble 10). The increase in the d002 spacing is due to curvature of
smaller diameter N-CNTs and higher strain caused by the struc-
tural defect on the nanotube walls. This result is consistent with
d002 spacing obtained from HRTEM analysis.

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Labulo et al. / J. Nig. Soc. Phys. Sci. 2 (2020) 205–217 214

Table 8. Relative atomic concentration and nitrogen species distribution from elemental and XPS analysis

Elemental analysis XPS analysis

Samples C
(at.%)

O
(at.%)

N
(at.%)

C
(at.%)

O
(at.%)

N
(at.%)

Pyrrolic
(at.%)

Pyridinic
(at.%)

Graphitic
(at.%)

Nitrogen
molecule
(at. %)

N-CNTs-0% 80.36 11.62 8.02 72.72 8.36 9.92 8.60 2.30 0.70 0.30

N-CNTs-3% 71.82 13.04 15.14 77.00 9.40 13.36 11.80 1.24 0.80 0.20

N-CNTs-Fe 79.35 15.35 7.30 84.54 7.72 8.27 7.50 0.27 0.60 0.10

Table 9. Intensities of C 1s peaks

Samples 284.3 eV 285.8 eV 287.0 eV 287.9 eV 289.4 eV

C=C sp2 (%) C-C sp3 (%) C-O (%) C=O (%) COOH (%)

N-CNTs-0% 75.5 5.9 13.6 3.8 1.2

N-CNTs-3% 75.3 4.7 11.4 8.8 0.8

N-CNTs-Fe 76.9 4.2 14.8 2.5 1.6

Table 10. X-ray structural parameters of N-CNTs-0%, N-CNTs-1%, N-CNTs-
2%, N-CNTs-3%, N-CNTs-4% and N-CNTs-Fe. d002 values are obtained
from HRTEM and correlated with those from the XRD analysis

Entry Samples d002
values
(nm)

Intensity
of C002
peaks

FWHM
at C002
peaks

Crystalline
size
(nm)

1 N-CNT-Fe 0.348 488.41 2.446 3.06

2 N-CNT-0% 0.333 181.49 1.461 1.17

3 N-CNT-1% 0.339 129.72 1.548 1.41

4 N-CNT-2% 0.340 256.25 2.400 2.54

5 N-CNT-3% 0.344 247.13 2.616 2.61

6 N-CNT-4% 0.352 185.92 2.637 1.91

4. Conclusion

This study presented the role of oxygen and nitrogen-doping
as a promising method to improve the physicochemical proper-
ties of N-CNTs. This has critical implications for reproducibil-
ity in N-CNT synthesis, particularly on the effect of oxygen in
diameter and wall thickness control. It can be concluded that
the introduction of an appropriate amount of oxygen promotes
N-CNTs growth with clean walls and reduced diameters. From
XPS analysis, pyrrolic-N was predominantly incorporated into
the crystalline CNT structure at a high oxygen concentration.
Nitrogen-doping was further confirmed by TGA analysis and
Raman spectroscopy. Lastly, the understanding of the effect
of oxygen species on the morphology and surface area of N-
CNTs during synthesis is critical in vast numbers of industrially
promising supported metal nanoparticles catalyst design.

Acknowledgments

This research was financially supported by the National Re-
search Foundation (NRF) South Africa. We are grateful to the
School of Chemistry and Physics, University of KwaZulu-Natal
(UKZN) for creating a conducive research laboratory for this
work. Ayomide is grateful to Prof. Vincent Nyamori, Prof.

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Labulo et al. / J. Nig. Soc. Phys. Sci. 2 (2020) 205–217 215

Bernand Omondi and Mrs Rashidat Labulo for proofreading
this manuscript.

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