Flame synthesis of nitrogen doped carbon for the oxygen reduction reaction and non-enzymatic methyl parathion sensor

Article abstract of DOI:10.1039/c6ra10130e

Growing concerns about the economical feasibility of materials synthesis means that simple methodologies to furnish materials are needed. Moreover, the multi-functional activity of these as-prepared materials is of great importance. Hence, here we report nitrogen-doped carbon nanoparticles from a one-step flame synthesis by directly burning pyrrole at room temperature and in an air atmosphere. The as-synthesized N-doped carbon was scrutinized as a cathode material for the oxygen reduction reaction and was also demonstrated in an electrochemical sensor. Furthermore, X-ray photoelectron spectroscopy (XPS) and Raman analysis was carried out to confirm the percentage of nitrogen content, the bonding environment and the disorder of carbon. The as-prepared N-doped carbon exhibits superior electrocatalytic activity towards the ORR compared with a commercial Pt/C catalyst. Moreover, the N-doped carbon modified glassy carbon electrode manifests a sensitive electrochemical response towards the detection of methyl parathion. A linear response was demonstrated by the fabricated sensor across two concentration ranges, from 0.0025 to 1 μM and 1 to 100 μM, with a lower detection limit of 0.068 nM. The proposed method is very simple, low cost and it can be utilized for practical applications to produce carbon materials on a large scale.

Full text of DOI:10.1039/c6ra10130e

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Page 1 of 21
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DOI: 10.1039/C6RA10130E
Flame synthesis of nitrogen doped carbon for the oxygen
reduction reaction and non-enzymatic methyl parathion
sensor
Natarajan Karikalana, Murugan Velmurugana, ShenꢀMing Chena*, Chelladurai Karuppiaha,b
,
K.M. AlꢀAnazic, M. Ajmal Alid, BihꢀShow Lou,e,f*
aElectroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and
Biotechnology, National Taipei University of Technology, No.1, Section 3, ChungꢀHsiao East
Road, Taipei 106, Taiwan (R.O.C).
bDepartment of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road,
Taipei 106, Taiwan (R.O.C).
c
Department of Zoology, College of Science, King Saud University, Riyadhꢀ11451, Saudi
Arabia
d Department of Botany and Microbiology, College of Science, King Saud University, Riyadhꢀ
11451, Saudi Arabia
e Chemistry Division, Center for General Education, Chang Gung University, Taoyuan, Taiwan
(ROC)
f
Department of Nuclear Medicine and Molecular Imaging Center, Chang Gung Memorial
Hospital, Taoyuan, Taiwan (ROC)
*corresponding author: smchen78@ms15.hinet.net (SM Chen)
blou@mail.cgu.edu.tw (BihꢀShow Lou)
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Abstract
Growing the concerns about economical feasibility of materials synthesis needs a simple
methodology to furnish the materials. Moreover, the asꢀprepared materials meet a multiꢀ
functional activity is great importance. Hence, here we report the nitrogenꢀdoped carbon
nanoparticles synthesized from oneꢀstep flame synthesis by directly burning the pyrrole in room
temperature and air atmosphere. The asꢀsynthesized Nꢀdoped carbon was scrutinized as a
cathode material for oxygen reduction reaction and also demonstrated towards the
electrochemical sensor. Furthermore, the xꢀray photoelectron spectroscopy (XPS) and Raman
analysis confirms that the percentage of nitrogen content, bonding environment and the disorder
of carbon. The asꢀprepared Nꢀdoped carbon exhibits a superior electrocatalytic activity towards
the ORR as similar with the commercial Pt/C catalyst. Moreover the Nꢀdoped carbon modified
glassy carbon electrode manifests a sensitive electrochemical response towards the detection of
methyl parathion. The fabricated sensor was demonstrated the two linear concentrations ranging
from 0.0025 to 1 µM and 1 to 100 µM with the lower detection limit of 0.068 nM. The proposed
method is very simple, low cost and it can be utilized for practical applications to produce the
carbon materials in large scale.
Keywords: Methyl parathion, Nitrogen doped carbon, Oxygen reduction reaction, Flame
synthesis, Mechanism of parathion detection.
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1. Introduction
Increasing the demand of energy and anthropogenic climate changes induced a need in green
energy and also environmental security.1 Fuel cells are the device which generates electricity by
the electrochemical transformation of hydrogen and oxygen into water.2 It produces energy in a
greener way, however, the fuel cells are suffer from the sluggish oxygen reduction reaction
(ORR) at cathode side.3 So far, platinum or platinum based compounds are mostly employed as
ORR catalysts by its superior activity and stability.4 However, platinum has declined for fuel
cells application due to the high cost and relatively low abundant of platinum. Hence, the recent
studies pursued in nonꢀprecious metals and metalꢀfree catalysts for ORR.5–8 The nonꢀnoble metal
catalysts reveal a better performance towards ORR, though, which are used with the
carbonaceous support materials.9–11 Because, the nonꢀprecious metal catalysts are lacking their
lower surface area and poor electrical conductivity hence the carbons are used to reimburse the
conductivity. Recent studies exhibit the carbon materials are solely engaged in ORR, which are
soꢀcalled metalꢀfree catalysts.12–14 In particular, the Nꢀdoped graphenes are revealing excellent
electrocatalytic responses towards ORR.15
On the other hand, to secure the environmental safety, it is necessary to remove the organic
pollutants mainly carcinogenic and lethal hazardous compounds.16 Especially pesticides, which
are extensively used in agriculture for controlling the insects.17 Methyl parathion (MPꢀ
organophosphorous compound) is one of the commonly used pesticides around the world,
however, it is contaminated the soil and water bodies which affects the aquatic organisms.17
Hence, it is very important to develop a viable method to detect MP in water, foodstuff and soils,
etc. Several analytical techniques were employed to determine the MP such as gas
chromatography (GC), high performance liquid chromatography (HPLC), gas chromatography–
mass spectrometry (GCꢀMS) and spectrometry.18–20 These methods manifest the accurate results
with highly sensitivity however they are time consuming process and very expensive for the
practical applications. Therefore, a rapid, selective and sensitive method still need for the
detection of MP. Incidentally, an electrochemical technique offers an alternative and cost
effective way to detect MP compare to the former methods.21 Here, also graphene or composites
of graphene are used to fabricate the sensor electrode.
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Since the discovery of graphene, vast studies have been devoted to scrutinize the properties of
the grapheneꢀbased nanomaterials and its composites towards the numerous electrochemical
applications.22 Graphene has given an advanced platform to the modern researches by its
remarkable properties and versatility.23 However, the synthesis of graphene or graphene based
materials are lacking by their limited synthetic procedure such as mechanical exfoliation of
graphite, thermal exfoliation of graphite, chemical vapor deposition on silicon wafers and the
reduction of graphene oxide (GO).24–26 So far, identified methods are significantly costly or
questionable to safety, hence, the adaptability of graphene synthesis should be improved for
practical applications. Therefore, the recent studies concentrated on activated carbons (ACs)
because of the simple synthesis process, safety and economically viable.
ACs are synthesized by many sources (e.g., biomass, organic chemical and charcoal) and widely
used as adsorption materials by its high porosity and high surface area. Nowadays, it is employed
in heterogeneous catalysis and electrocatalysis, particularly energy and environmental
applications.27–30 Moreover, heteroatom (boron, nitrogen, phosphorous and sulfur) doped carbons
are exhibiting excellent electrocatalytic activity when compare with undoped carbons.12–15,31 In
which, nitrogen doped carbon acquired tremendous interest in the field of ORR and sensors.7,8,15
Among the various method, flame synthesis has furnished the carbon materials in simple oneꢀ
step and very rapid.32 This method has a great interest and motivated from the recently reported
candle soot synthesis procedure.33,34 Herein, we report the synthesis of nitrogen doped carbon
nanoparticles (NCN) from the direct burning of pyrrole in air atmosphere and the resultant
carbon soot was deposited on a glass plate. Furthermore, the asꢀprepared nitrogen doped carbon
nanoparticles was characterized by various physical characterization and electrochemical
characterization towards the detection of MP and ORR.
2. Experimental section
2.1. Preparation and fabrication of Nitrogen doped carbon-modified electrode
The previously reported flame synthesis method was followed with some modifications to
synthesis the nitrogen doped carbon in the form of soot.32 Herein, 10 mL of pyrrole was taken in
a small beaker and directly burned under air. Obviously, the aromatic organic compounds are
burning with a sooty flame when igniting, afterwards the resultant soot was deposited on a glass
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plate and carefully collecting the sample by scratching. In addition, the synthesis procedure was
given as schematic diagram and shown in scheme 1.
Scheme 1. A schematic representation of the synthesis of nitrogen doped carbon from pyrrole and its
voltammetric response towards the detection of MP.
To fabricate the electrode for ORR and MP sensor, the NCN (44 mg in 5 mL) was dissolved in
DI water and further it is sonicated for 15 min. The homogeneous NCN solution (8 µL) was drop
casted on the wellꢀpolished glassy carbon electrode (GCE) and rotating disk electrode (RDE)
dried in an ambient condition. The NCN modified electrodes were successfully used to observe
the electrochemical behavior towards MP detection and ORR activity.
2.2. Characterization of NCN
The structure and bonding environment of NCN was analyzed by Xꢀray diffraction pattern
analysis (XRD), XPERTꢀPRO (PANalytical B.V., The Netherlands) with CuKα radiation
(λ=1.5406 Å) and Raman spectra (NTꢀMDT, NTEGRA SPECTRA). The surface morphology
and elemental composition were studied by scanning electron microscope (SEM) Hitachi Sꢀ3000
H. Primary electrochemical reactions such as cyclic voltammetry and chronoamperometry were
carried out in CHI 1205B electrochemical analyzer (CH Instruments, USA). The electrocatalytic
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determination of methyl parathion was performed by differential pulse voltammetry (DPV) CHI
900 (CH Instruments, USA). Oxygen reduction reaction and the number of electron transfer
associated with ORR were analyzed by rotating disk voltammogram AFMSRX (PINE
instruments, USA). A conventional three electrode system has been employed for the catalysis
where the GCE is used as a working electrode (0.07 cm2), platinum wire is an auxiliary electrode
and Ag/AgCl/3 M KCl is used as a reference electrode.
3. Results and discussion
3.1. Characterization of structure and surface morphology of NCN
Fig. 1b, shows the XRD pattern of NCN, which reveals a peak at 25.82° (2θ) corresponds to the
(002) plane with the interlayer spacing of 0.344 nm along the c-axis, which is closely matched
35
with the interlayer spacing of pristine graphite (0.34 nm). This result indicates that the asꢀ
prepared NCN was crystallized in the hexagonal ordered unit cell. But the presence of hetero
atoms such as nitrogen and oxygen moieties in the crystal lattice leads the NCN to lower grain
size and disorder. Therefore, the Raman spectrum was employed to assess the disorders in NCN.
Fig. 1d displays a Raman spectrum of NCN which possesses two broad bands at 1352 and 1594
cmꢀ1, the observed peaks are corresponds to the disordered (D) band and graphitic (G) band.36, 37
The ratio of the relative intensities for D and G bands provide the information about the
compound’s crystallinity, the lower D/G ratio of carbons showed better crystallinity. The lower
D band intensity of NCN results the low distortion in hexagonal lattice and also the D/G intensity
ratio of NCN reveals 0.78 which directly proportional to amount of disorder of carbon.38
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Fig. 1. (a) Crystal model (b) XRD patterns (c) TEM image and (d) Raman spectra of asꢀprepared
nitrogen doped carbon.
Fig. 2a displays the scanning electron microscopy (SEM) image of NCN, which shows the
aggregated flake like surface morphology with the average elemental composition of 90.0, 7.1
and 2.9%, corresponding to the C, O and N respectively (Fig. 2e). These relative atomic
percentages of each element distribution in NCN are further confirmed by the elemental mapping
of energy dispersive xꢀray analysis (EDAX) and the results shown in Fig. 2b, c & d. The local
crystallinity and grain boundary of NCN was probed by transmission electron microscopy
(TEM). Fig. 1c shows the TEM image of as prepared NCN, which depicts the aggregated particle
like shape with distinguishable grain boundaries, however in some places it covers the particles
as sheets.
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Fig. 2. (a) SEM image of NCN, (bꢀd) EDAX elemental mapping of NCN which illustrates the
distribution of components, carbon (b) oxygen (c) and nitrogen (d). (e) The bar diagram of
atomic percentage of each elements of NCN.
Xꢀray photoelectron spectroscopy (XPS) is a reliable tool used to analyze the elemental
composition and bonding environment of the compounds. As depicted in Fig. 3a, the XPS survey
of the NCN possesses three major peaks centered at 286.5, 399.5, and 533.8 eV, which are
corresponding to the C1s, N1s, and O1s, respectively. The relative elemental composition of
C1s, N1s, and O1s is calculated from XPS as 89.5, 2.9, and 7.15%, respectively, this value is
closely matched with EDAX analysis. The high resolution N1s, O1s, and C1s spectra are
considered to understand the bonding environment of the elements. Fig. 3b shows the high
resolution spectra of C1s which further deconvoluted into four peaks, the peak at 285.5 eV is
related to the sp2 carbon or graphitic carbon.
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Fig. 3. (a) XPS survey of as prepared nitrogen doped carbon. High resolution XPS spectra with
curve fitting for (b) C1s peak (c) N1s peak and (d) O1s peak.
The peak at 286.1 eV is related to the sp3 carbon where the carbon bonding with nitrogen as C−N
or in other words the graphitic nitrogen bond.39 The peak at 286.9 is related to C=O and peak at
289.7 eV considerable to N–C=O bond. This observation confirms the presence of nitrogen atom
into the carbon framework. Furthermore, a high resolution N1s spectrum was taken to account
for the clear explanation. Fig. 3c depicts the N1s spectra of NCN, which contains three different
bonding environments. The peaks at 398.73, 399.3, and 400.0 eV are assigned to pyridinic,
pyrrolic, and graphitic nitrogen bonding. The crystal model of NCN is shown in Fig. 1a, which
clearly illustrates the information about different environment of nitrogen in carbon framework.
In which, the pyridinic and pyrrolic nitrogen atom deviates the symmetry of C=C, hence the
voids are created in uniform carbon lattice. The highꢀresolution O1s spectra in Fig. 3d also
shows the evidence for the presence of nitrogen by N–C=O bonding which is in agreement with
C1s spectra. Oxygen also doped in carbon as C–O or –C=O, that is clearly observed by the peak
at 532.7 eV which may play a good performance by increasing defects, disorder or local electron
density around O atoms.
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3.2. Electrochemical behavior of MP
To explain the mechanism of MP determination, cyclic voltammetry (CV) was used to
characterize the electron transfer process concerned in reduction of MP. Fig. 4a shows the CVs
obtained for NCN modified and bare GCE in 0.1 M PBS (pH = 7.0) in the presence of 50 µM
MP at a scan rate of 50 mV/s. These CVs were recorded only for first cycles. In the cathodic
sweep, a highly intensive irreversible reduction peak (systemꢀI, see scheme 2a) was observed at ꢀ
0.55 V for NCN modified GCE corresponding to the reduction of the nitro group in MP to
arylhydroxylamine40 whereas bare GCE has taken 50 mV (ꢀ0.6 V) overpotential to reduce the
MP. During the reverse scan, the primary reduced product (systemꢀI, scheme 2a) at an electrode
surface was oxidized into nitrosobenzene (systemꢀII, scheme 2a) at ꢀ0.03 V, further this oxidized
product was reduced in the second cathodic sweep at ꢀ0.06 V (see scan rate).41 For comparison,
we also recorded a CV using the same electrodes in the absence of MP, it can be clearly seen that
there were no peaks arising out for bare and NCN modified GCE (Fig. 4a). In addition, the
obtained CV curves for the absence of MP provide the information about the background
current. Here, the bare GCE depicts the reduction behavior at ꢀ0.75 vs Ag/AgCl and provides
low background current due to the low capacitance, on the other hand, NCN provides high
background current by its superior high capacitance (due to high surface to volume ratio).
3.3. Effect of scan rate and pH
The scan rate dependence of MP reduction was evaluated to assess the electron transfer process,
hence a different range of scan rate was used from 10 to 100 mV/s and the resultant CVs were
shown in Fig. 4b. It is well known that the reduction peak current was increased systematically
with increasing scan rate, but in our case the secondary redox product peak is observed better
and well shaped. This phenomenon indicates that NCN modified GCE reduced a maximum of
MP via four electron pathway. Therefore the secondary reduced product (systemꢀI, scheme 2a)
remains maximum at the electrode surface further it was produced a redox couple (systemꢀII,
scheme 2a). The peak current of the secondary product redox couple was plotted against the scan
rate and shown in Fig. 4b (inset). From slope value in Fig. 4b (inset), we can find the redox
process is diffusion controlled, however the decrease in primary reduction peak current indicates
the adsorption process for increasing the scan rate. Thus, the present study point out that the MP
reduction was both adsorption and diffusion controlled process. In which the adsorption occurs
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on electrode surface and diffusion contributes to the rate determining step.42 Moreover, the
adsorption of MP on to the electrode surface was confirmed from the scan rate, where a small
peak potential shift was observed in the MP reduction process with increasing scan rate.
Fig. 4. Cyclic voltammograms of (a) bare and NCN modified GCE in presence and absence of
50 µM MP. (b) NCN modified GCE in MP for various scan rates ranging from 10 to 100 mV/s
and the corresponding plot of peak current vs. scan rate in inset. (c) NCN modified GCE in MP
at various pH from 3 to 13 and inset shows the plot of pH vs. peak potential (taking for primary
reduction peak). (d) Differential pulse voltammograms of NCN modified GCE in various
concentrations of MP ranging from 2.5 nM to 100 µM and inset shows the calibration plot for
current vs concentration.
The electrochemical reaction is very sensitive towards the pH of the electrolyte solution which
follows the linear responses for the potential versus pH with the slop of 59 mV (Nernst equation)
for one electron transfer. The effect of pH on the electrocatalytic response of MP was evaluated
for the NCN modified GCE in the pH ranging from 3 to 13. The NCN modified GCE exhibits
the higher current response in pH at 7.0 compare to all other pH investigated (Fig. 4c). A
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decrease in current response was observed for the lower pH due to the H+ ions adsorption on the
electrode surface. As shown in Fig. 4c (inset) the reduction peak potential of MP is linearly
varying depends on the pH in the range of 3 to 13. So, according to the electrocatalytic current
enhancement, the pH 7.0 was selected for further experiments.
3.4. Determination of MP
Differential pulse voltammetry (DPV) is a more sensitive analytical method to determine the
MP. This experiment was carried out to explore the practical viability of NCN modified GCE for
nonꢀenzymatic electrochemical detection of MP, a lethal environmentally hazardous pesticide.
All DPV studies were recorded in 0.1 M PBS (pH = 7.0) by applying the potential window from
ꢀ0.3 to ꢀ0.9 V with optimized pulse amplitude (0.05 V) and pulse width (0.05 s). Fig. 4d shows
the DPV studies of NCN modified GCE from low concentration to high concentration of MP, it
is obvious to see when increases the concentration of MP incrementally the reduction current
also increases systematically. It can be observed from Fig. 4d the reduction potential of MP is
shifting towards negative potential when increase the addition of higher concentrations. This
factor is mainly arising from the (i) adsorption of analyte (MP) in NCN surface confinement or
in other words a thin film formation of analyte on the surface ahead of diffusion layer and (ii)
higher concentration of MP increases the solution resistance hence the tiny amount of ionic
conductivity deviates, it may shift the peak potential. Furthermore, the determination of MP was
evaluated from the calibration plot of reduction current vs. the concentration of MP (Fig. 4d
(inset)) where two linear curves obtained over the concentration range of 0.0025 to 1µM and 1 to
100 µM. From these two linear concentration ranges, the sensitivity of the fabricated sensor was
calculated as 72.4 nA/µM and 10 µA/µM respectively. The fabricated electrochemical sensor
exhibits a low detection limit of 0.068 nM and presents a wide linear concentration range from
the two different concentration ranges of MP. This study confirms the proposed fabricated sensor
was effectively determined the MP over the wide range of concentrations and displays a
foremost platform to the practical applications. In order to evaluate the performance of fabricated
sensor, it was compared with previous reports and provided in (Table 1).44–53 A high linear
concentration range and very low detection limit was observed for the fabricated sensor when
compared with previously reported materials. It can be seen that from the above studies, the
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NCN modified GCE manifests a superior electrocatalytic performance towards the determination
MP.
Modified electrodes
Detection limit
Linear concentration
range (µM)
1.9 to 61
References
(nM)
190
19
44
45
46
47
48
49
50
51
42
52
53
Au/MWCNTs electrode
MWCNTs–Chitosan/GCE
Silicate–CTAB/GCE
ZrO2/CPE
0.19 to 7.6
10
10 to 100
7.6
0.019 to 11
OMC/GCE
7.6
0.09 to 61
pSC6–Ag NPs/GCE
MWCNTs–PAAM/GCE
GdHCF/GNs/GCE
Au ACs/Au
4.0
0.01 to 80
2.0
0.005 to 10
1.0
0.008 to 10
0.65
0.1
0.01 to 80
SHꢀCD/AuNPs/SWCNTs/GCE
ZnO/Ag/MPTMS/OHP
Nꢀdoped carbon/GCE
0.002 to 0.08
0.0025 to 75
0.0025 to 1, 1 to 100
0.07
0.068
Present work
Table 1. Comparison on the performance of NCN modified GCE with other previous reported
works towards the detection of methyl parathion.
3.5. Interference studies
To examine the selectivity of fabricated sensor, the interference studies were carried out for the
potential interfering ions such as other nitro aromatic compounds and common ions. Fig. 5
depicts the interference currents for 10ꢀfold excess of paraxon, fenitrothion, nitrobenzene (NB),
3−
−
pꢀnitrophenol (NP), nitroaniline (NA) and 100ꢀfold excess of Na+, K+, Ca2+, PO4 , NO3 , Cl− in
the presence of constant concentration of 10 µM MP. From the observed results, the fabricated
sensor was showed the interference current less than 5% for other nitro compounds and no
significant current changes for the common ions. The present study stated that the NCN modified
GCE electrode exhibits selective detection towards MP and endorsed that the hired material for
practical applications.
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Fig. 5. Interference studies for NCN modified GCE in 0.1 M PBS containing MP in the presence
of other potential interfering compounds namely (a) paraoxon, (b) fenitrothion, (c) nitrobenzene,
3−
−
(d) pꢀnitrophenol, (e) nitroaniline, (f) Na+, (g) K+, (h) Ca2+, (i) PO4 , (j) NO3 and (k) Cl− in the
concentration of 10 and 100ꢀfold excess with MP for (aꢀe) and (fꢀk) respectively.
3.6. Oxygen reduction reaction on NCN modified GCE
To assess the ORR catalytic activity, the NCN modified GCE was subjected to CV in N2 and O2
saturated 0.1 M NaOH. Fig. 6a exhibits the strong electrocatalytic oxygen reduction curve for
NCN modified GCE at ꢀ0.147 V vs Ag/AgCl in the presence of O2 which is low overpotential
compare to conventional carbon electrodes.7 In contrast, the obtained ORR curve is obviously
different from its background current in N2 saturated 0.1 M NaOH, where NCN modified GCE
shows only double layer capacitance behavior. The performance of NCN modified GCE was
compared with commercially viable and state of the art catalyst: platinum.
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Fig. 6. Cyclic voltammograms of (a) NCN modified GCE in presence of nitrogen and oxygen
saturated 0.1 M NaOH. (b) Commercial Pt/C and NCN modified GCE in the presence of oxygen
saturated 0.1 M NaOH. (c) RDE voltammograms of NCN modified GCE at various rotations. (d)
KLꢀplot for the steady current obtained from the RDE voltammograms for NCN modified GCE.
Remarkably, the NCN modified GCE reveals only 130 mV overpotential compare to commercial
Pt/C catalyst and also shows the similar electrocatalytic current for ORR (Fig. 6b). The ORR
activity of NCN modified GCE electrode furnishes excellent electrocatalytic current compared to
the previously reported Nꢀdoped carbons.8,15 Moreover, the RDE measurements were used to
analyze the ORR kinetics of NCN modified GCE in O2 saturated 0.1 M NaOH. Fig. 6c depicts
the linear sweep voltammetry (LSV) of NCN modified GCE for various rotation rate. The NCN
modified GCE reveals the one step oxygen reduction with the plateau of steady state diffusion
current for all rotations. The kinetic parameters and electron transfer were evaluated by the
Koutecky–Levich (KL) equation. Fig. 6d shows the KL plots for the NCN modified GCE, it
follows the linearity and parallelism which suggested that obtained ORR is firstꢀorder reaction
kinetics with respect to the concentration of dissolved oxygen. In addition, each rotation has an
almost identical electron transfer numbers for ORR at different potentials. The number (n) of
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electron transfer was calculated to be ∼3.98 to 4.02 from the slopes of KL plots for the different
potentials from 0.45 to 0.65 V. This observed electron transfer number suggested that NCN
modified GCE favors a 4eꢀ oxygen reduction process, unlike the other carbonaceous materials.
Hence, it can be used as an alternative material in fuel cells for cost effective and rare earth
abundant platinum.
3.7. Mechanisms of ORR and MP detection
The NCN modified GCE exhibits diversity of applications by its divergent properties due to the
different bonding environment of nitrogen atoms in carbon lattice. As mentioned earlier, the
nitrogen atoms possess two major components in NCN such as pyrrollic and pyridinic nitrogen
(Fig. 1a).
Scheme 2. (a) Plausible mechanism for the reduction of MP. (b & c) Delocalization of the
electron clouds on the carbon atoms in pyridine and pyrrole.
Commonly, pyrrole is known for electron rich aromatic ring and basic in nature which provides
the ground for MP adsorption. The nitro group of MP was easily attracted by Cꢀ3 position in
pyrrole, because that position is highly electron rich in pyrrole (scheme 2c). Following the
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adsorption, the MP was directly reduced to arylhydroxylamine by proton coupled four electron
transfer (PCET) and the asꢀreduced compound was further oxidized into arylnitroso group. In
contrast, bare GCE also following the same reaction pathway; however, the aforementioned
mechanism provides a better understanding about the reaction. In NCN, the pyrrole ring initiates
the reaction, hence the activation energy has reduced, therefore NCN modified GCE reduces MP
at low overpotential comparable to bare GCE.
On the other hand, the electron deficient pyridinic nitrogen furnished the bonding site for the
dissolved oxygen molecule. The sp2 character of nitrogen in pyridine gives the room for a
localized electron hence the ortho and para position of pyridinic group in NCN attains much
electropositive and welcomes the oxygen molecule (scheme 2b). The adsorbed oxygen molecule
was further reduced via four electron transfer, usually unꢀdoped carbons are well known for the
hydrogen peroxide production (via two electron pathway).43 The Nꢀdoped carbons manifest four
electron transfer by the effective adsorption of oxygen molecules and the electrostatic interaction
between the π –orbital and electrons of pyridinic nitrogen. This mechanistic study exhibits how
the NCN modified GCE employed to a diversity of applications and suggests the possible
interactions with both MP and oxygen.
4. Conclusions
In summary, we have demonstrated the simple and reliable method to prepare the nitrogen doped
carbon for an energy and sensor applications. In described method, we follow the flame synthesis
to produce the nitrogen doped carbon from the nitrogen containing organic aromatic compound
namely pyrrole. Besides that, many different heteroatom doped carbons have to be synthesized
from the aforementioned method by varying the precursor compounds. A meticulous evaluation
of the as prepared compound (NCN) exhibits a superior electrocatalytic activity towards the
detection of MP and ORR. Moreover the NCN modified GCE exhibiting the reasonable activity
towards the determination of MP and acceptable level of performance compared to previous
reports. Furthermore, NCN modified GCE reveals a superior electrocatalytic activity towards
ORR as similar with the commercial Pt/C catalyst. Hence, the present study paves a diversity of
applications handled by the single compound and also motivates the metalꢀfree catalyst for
practical applications.
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DOI: 10.1039/C6RA10130E
Acknowledgement
This project was supported by the National Science Council and the Ministry of Education of
Taiwan (Republic of China).
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Title of the content
Schematic representation of the nitrogen doped carbon, the action on methyl parathion and
molecular oxygen by its local electron rich and deficient sites of pyrrole and pyridine.