A simple approach to the synthesis of BCN graphene

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A simple approach to the synthesis of BCN graphene with high capacitance
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2015 Nanotechnology 26 045402
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Nanotechnology
Nanotechnology 26 (2015) 045402 (7pp)
doi:10.1088/0957-4484/26/4/045402
A simple approach to the synthesis of BCN
graphene with high capacitance
Shuo Dou1, Xiaobing Huang2, Zhaoling Ma1, Jianghong Wu1 and
Shuangyin Wang1
1
State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical
Engineering, Hunan University, Changsha, 410082, People’s Republic of China
2
College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde,
People’s Republic of China
E-mail: [email protected]
Received 27 July 2014, revised 20 November 2014
Accepted for publication 27 November 2014
Published 6 January 2015
Abstract
Boron and nitrogen co-doped graphene (BCN graphene) was prepared through a simple thermal
annealing approach in the presence of a single compound, melamine diborate. The as-prepared
samples were characterized by scanning electron microscopy, transmission electron microscopy,
x-ray photoelectron spectroscopy, Raman spectra and electrochemical techniques. The BCN
graphene shows signiﬁcantly enhanced speciﬁc capacitance due to boron and nitrogen codoping, which permits higher overall doping levels.
S Online supplementary data available from stacks.iop.org/NANO/26/045402/mmedia
Keywords: graphene, doping, supercapacitor
(Some ﬁgures may appear in colour only in the online journal)
1. Introduction
addition to nitrogen, boron doped carbon materials were also
reported as the electrode materials of supercapacitors, showing enhanced performance [7, 8, 15]. Our previous work
showed that N and B co-doping of vertically aligned carbon
nanotubes could effectively enhance the speciﬁc capacitance
due to the synergistic capacitive effect [8].
Graphene with sp2 hybridized carbon atoms has attracted
signiﬁcant attention due to its unique properties and promising applications in a variety of ﬁelds including energy storage
and conversion, sensors, and electronic devices [16–19]. For
supercapacitors, various types of graphene have been investigated to improve the speciﬁc capacitance. Fan et al treated
graphite oxide with a slow heating rate using Mg(OH)2
nanosheets as template to get a supercapacitor electrode
materials with ultrahigh speciﬁc gravimetric and volumetric
capacitances of 456 F g−1 and 470 F cm−3 [20]. Wang et al
prepared MnO2-graphene foam and CNT-graphene foam by
solution casting and subsequent electrochemical methods to
assemble ﬂexible all-solid-state asymmetric supercapacitor.
The asymmetric supercapacitor can be cycled reversibly in a
high-voltage region of 0–1.8 V and exhibit high energy
density, remarkable rate capability, reasonable cycling
Supercapacitors, having a high power density, quick charge/
discharge (CD) rate and long life-cycle, are promising power
sources for portable systems and automotive applications [1].
Supercapacitance arises normally from the electrical doublelayer charge storage with or without additional pseudocapacitance [2–6]. Due to their high surface area and excellent
conductivity, carbon nanomaterials such as carbon nanotube,
graphene and ordered mesoporous carbon are considered as
typical electrode materials for supercapacitors and much
attention has been paid to improving their structure and
properties for supercapacitor applications [2–6]. Both theoretical calculations and experimental characterizations have
proved that chemical doping with foreign atoms such as
nitrogen element can achieve this goal [7–14]. It has been
assumed that nitrogen doping changes the electron donor/
acceptor characteristics of carbon depending on the type of
the groups formed between the nitrogen and carbon atoms,
contributing to the enhancement of capacitance property [8].
The heteroatoms on carbon materials would provide acid/base
characteristics for improved pseudocapacitance [11]. In
0957-4484/15/045402+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
S Dou et al
Nanotechnology 26 (2015) 045402
performance and excellent ﬂexibility [21]. Heteroatom doping
of graphene could further extend its potential applications. Of
the most typical materials, nitrogen doped graphene (N-graphene) was prepared through chemical vapour deposition,
thermal annealing, hydrothermal treatment, chemical synthesis etc, showing promising supercapacitive performance
[7, 10, 11]. Besides, boron doped graphene (B-graphene) was
also demonstrated to be a good supercapacitor electrode
material. For example, Han et al prepared B-graphene by
reduction of graphene oxide (GO) with a borane-tetrahydrofuran under reﬂux via a one-pot synthesis using a liquid
process and was studied as supercapacitor electrode materials,
showing enhanced performance over the undoped graphene
due to the presence of B [12]. Manthiram et al prepared Bgraphene through a Fried-Ice way for supercapacitor applications [22]. In order to further improve the performance of
doped graphene, dual atoms co-doped graphene or carbon
nanotubes have been extensively investigated for electrocatalysis and supercapacitor [7, 17, 23–27]. Müllen et al
prepared boron and nitrogen co-doped graphene (BCN graphene) in the presence of ammonia boron triﬂuoride by a
hydrothermal method, showing improved supercapacitor
performance [7]. Previously, we also prepared BCN graphene
by thermally annealing GO with boric acid under the gas ﬂow
of ammonia and argon [17]. The as-obtained BCN graphene
showed signiﬁcant electrochemical performance as the efﬁcient metal-free electrocatalysts for oxygen reduction reaction. However, this technique involves the use of toxic
ammonia with safety concerns. Therefore, the development of
a safe and efﬁcient technique for the preparation of BCN
graphene is still a challenge.
In this work, we developed a novel thermal annealing
method of GO to prepare BCN graphene using a single
dopant chemical, melamine diborate, as the B and N source.
The schematic is illustrated in ﬁgure 1(a). Oda et al [28] have
demonstrated that melamine diborate could be applied to
prepare BCN compound, which we used for synthesis of
BCN graphene in this work. The use of the single B and N
precursor simpliﬁes the synthetic process and makes it safe
without using ammonia as the N source. For comparison,
single atom doped graphene, that is, B-graphene and N-graphene were also prepared in a similar way using boric acid
and melamine as the heteroatom sources, respectively. The
capacitance behaviour of the as-prepared doped graphene was
investigated by cyclic voltammetry and CD techniques and it
was found that BCN graphene shows the highest capacitance
compared to N-graphene, B-graphene and undoped graphene,
contributed by the efﬁcient heteroatom doping. Besides, the
BCN graphene also shows excellent rate capability and high
durability as the electrode materials for supercapacitors.
in ethanol under sonication for 90 min to obtain a homogeneous dispersion. Ethanol was removed by the rotary
evaporation. The collected solid mixtures were dried in
vacuum oven at 60 °C for 24 h. Finally, the doping process
was carried out by annealing GO and melamine diborate at
600, 700, 800, 900 and 1000 °C for 2 h in argon to form BCN
graphene (denoted as BCN-600, BCN-700, BCN-800, BCN900, BCN-1000, respectively). The undoped graphene was
also prepared under the same condition in the absence of
dopants for comparison. Similarly, B-graphene was prepared
by thermally annealing GO (100 mg) in the presence of boric
acid (508 mg) at 700 °C which was denoted as B-graphene;
and N-graphene was prepared by thermally annealing GO
(100 mg) in the presence of melamine (492 mg) at 700 °C
which was denoted as N-graphene.
2.2. Physical characterizations
The morphology of BCN was observed by scanning electron
microscope (SEM, Hitachi, S-4800) and transmission electron
microscope (TEM, JEOL, JEM-2010). X-ray photoelectron
spectroscopic (XPS) measurements were performed on an
ESCALAB 250Xi using a monochromic Al x-ray source
(200 W, 20 eV). The Raman spectra were collected on a
Raman spectrometer (Labram-010) using 632 nm laser. The
Brunauer−Emmmett−Teller (BET) speciﬁc surface area
characterizations of the samples were probed by a nitrogen
adsorption−desorption method at 77 K (SSA-4200).
2.3. Electrochemical test
All the electrochemical tests including cyclic voltammograms
(CVs) and galvanostatic CD were carried out using an electrochemical workstation (CHI 760E, CH Instrument, USA)
with a typical three-electrode. A platinum mesh was used as
counter electrode and saturated calomel electrode as reference
electrode. At ﬁrst, 5 mg electrode material was added into
1 ml ethanol to get a well dispersed suspension (5 mg ml−1)
under sonication. Then, 50 μl naﬁon solution (5 wt%) as a
binder was added into the catalyst suspension. To prepare the
electrode, 20 μl catalyst suspension was dropped onto the
surface of a pre-polished glassy carbon electrode (GCE).
After fully dried at room temperature, the catalyst casted GCE
was used as the working electrode to measure its capacitance
in 1 M H2SO4 aqueous solution saturated with N2.
3. Results and discussion
During the thermal annealing at high temperature, the
decomposed B and N could be successfully incorporated into
graphene. To investigate the structural morphology of the asobtained BCN graphene, the SEM images were collected for
BCN-700, as shown in ﬁgure 1(b). It can be seen from
ﬁgures 1(b) and S1 (in the supplementary data, available at
stacks.iop.org/NANO/26/045402/mmedia) that graphene-like
nanosheets are observed for BCN graphene, conﬁrming that
the doping process reserves the intrinsic morphology of
2. Methods
2.1. Materials preparation
GO was prepared by modiﬁed Hummers method [29]. GO
(100 mg) and melamine diborate (1000 mg) were ﬁrst mixed
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Nanotechnology 26 (2015) 045402
a)
Annealing
GO
C
Melamine diborate
B
o
c)
Intensity / a.u.
b)
N
ID/IG=1.1894
graphene
ID/IG=1.2041
B-graphene
ID/IG=1.3356
N-graphene
ID/IG=1.4003
BCN-700
500
1000
1500 2000 2500
Raman shift / cm-1
3000
Figure 1. (a) Schematic of preparation BCN graphene; (b) SEM image of BCN graphene obtained by thermal annealing at 700 °C; and (c)
Raman spectra of BCN graphene, and undoped graphene.
graphene, which is also supported by the TEM image in
ﬁgure S1. The speciﬁc surface area of the doped graphene
materials were characterized by the BET method. Figure S2
shows the nitrogen adsorption/desorption isotherms of BCN700, B-graphene and N-graphene. The BET speciﬁc area of
BCN-700 is 84.9 m2 g−1. Compared to B-graphene
(76.7 m2 g−1) and N-graphene (99.7 m2 g−1), the BCN-700
exhibits a very similar speciﬁc surface area to that of the B, N
single-doped graphene samples.
Raman spectrum is an efﬁcient tool to investigate the
electronic properties of carbon-based materials [30]. In
ﬁgure 1(c), we collected the Raman spectra of BCN-700, Bgraphene, N-graphene and undoped graphene. As can be seen
from the Raman spectra, the D band and G band are located
around 1330 cm−1 and 1580 cm−1, respectively. It has been
found that the G band arises from the bond stretching of all
sp2-bonded pairs, while the D band is associated with the sp3
carbon of defect sites. In the Raman spectra of carbon-based
materials, ID/IG is an interesting indicator of the defect level.
It can be seen from ﬁgure 1(c) that doped graphene shows
higher ID/IG ratio than undoped graphene (1.19) owing to the
incorporation of defects by B- or/and N-doping. By comparing the ID/IG values of BCN-700 (1.40), N-graphene
(1.34), and B-graphene (1.24), it is found that BCN-700
shows the highest ID/IG ratio, indicating more defects generated by the dual-atom co-doping. The BCN graphene
obtained at different temperatures was also characterized by
Raman to observe the temperature effect on its electronic
properties. It can be observed from ﬁgure S3 that, as the
annealing temperature increases from 600 to 1000 °C, the ID/
IG ratio of BCN graphene decreases from 1.43 to 1.33. The
decrease in the ID/IG ratio with the temperature increase may
be due to the improved graphitic degree of the BCN graphene
caused by reduction effect and ‘self-repairing’ of the graphene
layer at higher annealing temperature [33].
To analyze the composition of the as-prepared materials
and the chemical states of the key elements (doped heteroatoms) in the materials, XPS was employed. As expected,
BCN-700 shows the C, B, and N signals indicating the successful doping of B and N into graphene (ﬁgure 2(a)). The
presence of an O 1s peak in the BCN-700 is possibly due to
the residues of the incompletely removed oxygen-containing
functional groups of GO. The XPS survey also gives the
contents of B (3.95 atom%) and N (19.73 atom%) in the
BCN-700. Correspondingly, 1.72 atom% of B and 9.4 atom%
of N has been doped into graphene in the B-graphene and Ngraphene, respectively. It could be observed that the codoping in BCN-700 could signiﬁcantly increase the doping
content of the both B and N by forming the B–N bond. The
chemical states of C, N and B in BCN graphene were further
investigated with ﬁne-scanned XPS spectra. The XPS C1s
spectra given in ﬁgure 2(b) clearly show an almost total loss
of the oxygen component above 286 eV, while the carbon
peak at 284.6 eV becomes more asymmetric and broadened
due to B and N incorporation into the sp2 network of graphene upon annealing with melamine diborate [31]. As shown
in ﬁgure 2(b), the XPS C1s peak could be deconvoluted into
ﬁve peaks at around 284.0, 284.6, 285.2, 286.4 and 288.0 eV,
attributed to C–B, C=C, C–N or C–C, C=O, and O–C=O
bonds, respectively. We further examined the B1s XPS peak,
as shown in ﬁgure 2(c). It could be found that the B1s XPS
could be deconvoluted into three sub-peaks, attributed to BC,
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Nanotechnology 26 (2015) 045402
Figure 2. (a) The survey scan of XPS on BCN graphene, B-graphene, and N-graphene; high resolution of (b) C1s XPS peak; (c) B1s XPS
peak and (d) N1s XPS peak of BCN-700.
Table 1. XPS analysis of the samples.
Samples
BCN-600
BCN-700
BCN-800
BCN-900
BCN-1000
B (atom%)
N (atom%)
3.08
3.95
2.72
1.04
0.8
21.47
19.73
17.85
6.72
7.89
could be speculated that the reduction effect and ‘selfrepairing’ of the graphene layer at higher annealing temperature increase the graphitic level and make the heteroatoms
difﬁcult to dope into graphene layers.
The doping of B and N atoms results in an improvement
of capacitance to pristine graphene which was conﬁrmed by
the electrochemical measurements. CVs were employed to
investigate the electrochemical behaviour of BCN-700, Ngraphene, B-graphene, and undoped graphene in 1.0 M
H2SO4 aqueous electrolyte solution at a potential interval
from 0 to 0.9 V, as shown in ﬁgure 3(a). The ﬁgure shows the
CV curves of different electrodes at a scan rate of 50 mV s−1,
from which it could be observed that all the doped graphene
electrodes reveal a larger capacitive response than the
undoped graphene electrode. Besides, the dual atoms codoped BCN-700 shows the largest capacitive response compared to B-graphene and N-graphene, implying the co-doping
of B and N might show enhanced capacitance. It should be
pointed out that a more obvious redox couple at 0.38 V of the
anodic scan and 0.32 V of the cathodic scan is observed for Bgraphene, which is due to the higher relative percentage of
BO species, as conﬁrmed by the XPS characterizations in
ﬁgure S3(a) [34]. However, the redox couple does not exist in
BCN graphene, it may be due to the relative low amount of
BN or BCO, and BO bonds. Similarly, for N1s XPS peak, in
addition to the pyridinic N, pyrrolic N, and graphitic N, a
novel C–N–B bond is observed at around 399.2 eV, as shown
in ﬁgure 2(d). The above high-resolution XPS analysis conﬁrms that B and N are covalently bonded with C in graphene
lattice, and also B and N are also partially bonded (B–N
bond) covalently with each other.
Also, we have investigated the doping content of B and
N atoms at different temperatures. As the annealing temperature increases, the content of B and N atom in BCN
graphene samples decreases obviously, as shown in table 1
and ﬁgure S5. It has been also demonstrated that oxygen
groups in GO were removed at high temperature and the
removal process of oxygen provided an active site for heteroatom doping into graphene frameworks [31–33]. And, it
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Nanotechnology 26 (2015) 045402
Figure 3. (a) CV curves of BCN-700, B-graphene, N-graphene and undoped graphene in 1.0 M H2SO4 electrolyte solution at a scan rate of
50 mV s−1; (b) CV curves of BCN-700 at various scan rates up to 1000 mV s−1.
Figure 4. (a) CD curves of BCN-700, B-graphene, N-graphene and undoped graphene in 1.0 M H2SO4 electrolyte solution at a current
density of 0.2 A g−1; (b) CD curves of BCN graphene obtained at different annealing temperatures.
BO species in BCN graphene, as evidenced by the B1s XPS
peak in ﬁgure 2(c), which would suppress the Faradaic
pseudocapacitance behaviour of the BO species. Figure 3(b)
shows the CV curve of BCN-700 electrode at different scan
rates from 50 to 1000 mV s−1; it could be observed that the
rectangular shape of the CV curve is still kept at such a high
scan rate of 1000 mV s−1, due to the unique electronic and
surface structure of BCN graphene [3, 13].
In order to obtain the speciﬁc capacitance of the electrode
materials, the galvanostatic CD curves were collected with the
voltage windows the same as for the above CV curves on the
four electrode materials. As shown in ﬁgure 4(a), the discharging time of the doped graphene is signiﬁcantly longer
than that of undoped graphene, indicating that the heteroatom
doping offers a much larger capacitance, which agrees well
with the observation with the CV curves. Once again, BCN700 shows longer discharging time, that is, larger capacitance
than B-graphene and N-graphene due to the higher doping
levels of the B and N co-doping. For the galvanostatic CD,
the IR drop of BCN-700 is much smaller than that of Bgraphene and N-graphene, while the undoped graphene
shows largest IR drop. The IR drop is caused by the
equivalent series resistance, which includes electrode
resistance, electrolyte resistance and the interfacial resistance
between the electrode and the electrolyte [35]. The smaller IR
drop on BCN graphene is attributed to the improved electrode
afﬁnity with electrolyte due to the high heteroatom doping
level. The speciﬁc capacitances of the electrode materials
were calculated based on the CD curves in ﬁgure 4(a), the
speciﬁc capacitance for the BCN-700 electrode is 130.7 F g−1
at 0.2 A g−1, which is much higher than N-graphene
(111.3 F g−1), B-graphene (82.9 F g−1) as well as undoped
graphene (77.4 F g−1). It could be deduced that the heteroatom
doping modiﬁed the electronic properties and surface properties of graphene, making the BCN graphene samples more
accessible to store charges for supercapacitors. In addition, B
and N co-doping enables a higher doping level and further
enhances the speciﬁc capacitance of graphene. We also
examined the capacitance performance of BCN graphene
prepared at different annealing temperatures, as shown by the
CD curves in ﬁgure 4(b). It could be found that BCN graphene obtained at 700 °C shows the best capacitance performance over other BCN graphene samples and this is possibly
due to the high doping level of B and N atoms in the graphene
frameworks. However, it is unanticipated that the BCN graphene obtained at 600 °C shows higher doping level than at
5
S Dou et al
Nanotechnology 26 (2015) 045402
Figure 5. (a) Plot of the speciﬁc capacitance of BCN-700, B-graphene, N-graphene and undoped graphene against the current loadings; (b)
durability testing of BCN-700 at a current density of 5 A g−1.
4. Conclusions
700 °C (table 1), but the speciﬁc capacitance of BCN-600 is
much lower than that of BCN-700. In order to understand this
result, we performed further XPS analysis on the samples, as
shown in ﬁgure S5(c). We can see from it that all the N1s
XPS peaks could be deconvoluted into four peaks like BCN700. As the temperature increases, the percentage of B–N
bond decreases. It could be found that BCN-600 has relatively
higher B–N, which is electrochemically inactive, leading to
relatively poor capacitance performance [10, 36].
For an ideal supercapacitor, it should be able to deliver
the same energy under varying operation conditions. Thus, it
is necessary to investigate the capacitance retentions at different current load. Figure 5(a) shows the speciﬁc capacitances against the discharge current loads for the BCN-700
electrode in comparison with the other three electrodes. For
the undoped graphene electrode, the speciﬁc capacitance at a
discharge current density of 5 A g−1 drops by 43.3% of the
capacitance at 0.2 A g−1. However, for the BCN-700, B-graphene, and N-graphene electrodes, the capacitance drops only
by about 26.9%, 37.7% and 41.6%, respectively. This comparison result indicates that the doped graphene electrode
exhibits an improved rate capability while the dual atoms codoped BCN graphene shows the best one. The excellent rate
capability on BCN graphene could be attributed to the
improved interfacial contact between electrode and electrolyte
granting the effective accessibility for electrolyte ions, and the
modiﬁed electronic properties of graphene by the B and N codoping. For the real application in supercapacitor devices, the
stability of the electrode materials is also very critical. The
durability of the BCN graphene electrode was examined by
using the continuous CD cycling tests at a current density of
5 A g−1. Figure 5(b) shows the speciﬁc capacitance retention
of BCN graphene electrode as a function of cycle number. It
could be observed that the speciﬁc capacitance still remains at
97.5% of its initial capacitance after 2000 cycles, indicating
that BCN graphene shows good stability for the supercapacitor application.
In summary, we successfully prepared B and N co-doped
BCN graphene by a simple thermal annealing approach using
a single dopant, melamine diborate, as both B and N sources.
The method is simple and safe without involving ammonia
gas as the toxic N source. Our physical characterizations
demonstrate that the co-doping could signiﬁcantly increase
the doping content of heteroatoms, both B and N atoms, and
further enhance the capacitance performance. The as-prepared
BCN-700 was subjected to the electrochemical testing to
identify its capacitive behaviour as the electrode materials for
supercapacitors with comparison to B-graphene, N-graphene
and undoped graphene. The electrochemical results indicate
that BCN graphene shows the highest speciﬁc capacitance,
good rate capability and good durability. It is believed that the
as-prepared BCN graphene could ﬁnd other potential applications in the ﬁeld of lithium ion batteries, fuel cells, electrocatalysis, sensors, and electronic devices etc.
Acknowledgments
This work was supported by National Natural of Science
Foundation of China (Grant No.: 51402100) Marie Curie
International Incoming Fellowship Programme of European
Commission, Youth 1000 talent Programme of China (Grant
No. 531109020032), and Inter-discipline Programe (Grant
No. 531107040755).
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