Facile Green Synthesis of BCN Nanosheets as High-Performance Electrode Material for Electrochemical Energy Storage

Article abstract of DOI:10.1002/chem.201505225

Two-dimensional hexagonal boron carbon nitride (BCN) nanosheets (NSs) were synthesized by new approach in which a mixture of glucose and an adduct of boric acid (H₃BO₃) and urea (NH₂CONH₂) is heated at 900 °C. The method is green, scalable and gives a high yield of BCN NSs with average size of about 1 μm and thickness of about 13 nm. Structural characterization of the as-synthesized material was carried out by several techniques, and its energy-storage properties were evaluated electrochemically. The material showed excellent capacitive behaviour with a specific capacitance as high as 244 F g^-1 at a current density of 1 A g^-1. The material retains up to 96 % of its initial capacity after 3000 cycles at a current density of 5 A g^-1.

Full text of DOI:10.1002/chem.201505225

DOI: 10.1002/chem.201505225
Full Paper
&
Energy Storage
Facile Green Synthesis of BCN Nanosheets as High-Performance
Electrode Material for Electrochemical Energy Storage
Indrapal Karbhal,[a, b] Rami Reddy Devarapalli,[a, b] Joyashish Debgupta,[a]
Vijayamohanan K. Pillai,[b, c, d] Pulickel M. Ajayan,[e] and Manjusha V. Shelke*[a, b, c, e]
Abstract: Two-dimensional hexagonal boron carbon nitride
(BCN) nanosheets (NSs) were synthesized by new approach
in which a mixture of glucose and an adduct of boric acid
(H3BO3) and urea (NH2CONH2) is heated at 9008C. The
method is green, scalable and gives a high yield of BCN NSs
with average size of about 1 mm and thickness of about
13 nm. Structural characterization of the as-synthesized ma-
terial was carried out by several techniques, and its energy-
storage properties were evaluated electrochemically. The
material showed excellent capacitive behaviour with a specif-
ic capacitance as high as 244 FgÀ1 at a current density of
1 AgÀ1. The material retains up to 96% of its initial capacity
after 3000 cycles at a current density of 5 AgÀ1.
Introduction
cally stable, lighter-weight BCN compounds, which are more
favourable and promising for various applications.[7–10]
Two-dimensional (2D) nanomaterials have attracted much at-
tention in recent years due to their unique planar structure
with remarkable physical, chemical and electronic properties as
a function of layer thickness. Among 2D materials, especially
graphene and its inorganic analogues, such as h-BN, BCN, tita-
nium carbide, MoS2 and WS2, have been the focus of materials
research.[1–3] Of these materials, graphene is the most studied.
It has a layered structure consisting of sp2-hybridized carbon
atoms arranged in a hexagonal crystal lattice and has unique
optical and electronic properties.[4–6] Layered 2D BCN sheets
have similar structure and mechanical, electronic and thermal
properties to graphene doped with B and N. B and N are
neighbours of C, and their atomic radii (B: 87 pm, C: 67 pm
and N: 56 pm) and electronegativities (B: 2.04, C: 2.55 and N:
3.04) are similar to those of C. Thus, they can undergo strong
covalent bonding with C and can form highly thermodynami-
BCN, which is often conceptually thought of as a hybrid of
graphene and hexagonal BN, has the properties anticipated for
an adduct of these two materials. However, it has various
structural forms with compositions such as BCN, BC2N, BC3N,
BC4N and BC6N, depending on the relative percentages of B
and N.[11–15] Generally, B introduces holes and N introduces
electrons into the carbon lattice, rendering the BCN to a p-
type or n-type narrow-gap semiconductor, respectively.
BCN has very attractive properties, such as suitable bandgap,
luminescence, cathodoluminescence, electroluminescence,
thermoelectricity, low frictional coefficient and high neutron-
absorbing capability. Therefore, it has been widely considered
for catalysis, sensors, neutron absorbers, protective coatings,
gas absorption, electrocatalysts for the oxygen reduction reac-
tion (ORR) and as energy-storage material for batteries and su-
percapacitors. The presence of heteroatoms (B and N) plays
a key role in enhancing the performance in many of these ap-
plications and improving the stability, similar to the cases of
graphene and other 2D materials.[16–18]
[a] I. Karbhal, R. R. Devarapalli, J. Debgupta, M. V. Shelke
Physical & Materials Chemistry Division
CSIR-National Chemical Laboratory (CSIR-NCL)
Pune 411008, MH (India)
However, synthetic methods for the production of BCN are
not well developed. Chemical vapour deposition, pyrolysis, dc
arc discharge and laser ablation are some of the methods re-
ported so far for the preparation of BCN. Almost, all techniques
require high temperature, sophisticated instrumentation or
costly chemicals. Moreover, the very low yields of BCN make
these techniques quite impractical in real-life applica-
tions.[13,19–25] Polymer-precursor-based synthesis of BCN has
been reported by Zhang et al., although there are a few con-
cerns such as inadequate purity and dimensional homogenei-
ty.[26] In contrast to these methods, the present work reports
a facile synthesis of BCN NSs by employing boric acid, urea
and glucose as B, N and C sources, respectively, as high-per-
formance supercapacitor electrode materials.
E-mail: mv.shelke@ncl.res.in
[b] I. Karbhal, R. R. Devarapalli, V. K. Pillai, M. V. Shelke
Academy of Scientific and Innovative Research (AcSIR)
Chennai 600113, TN (India)
[c] V. K. Pillai, M. V. Shelke
CSIR-Network Centre for Solar Energy, CSIR-NCL, Pune (India)
[d] V. K. Pillai
CSIR-Central Electrochemical Research Institute (CSIR-CECRI)
Karaikudi 630006 (India)
[e] P. M. Ajayan, M. V. Shelke
Department of Mechanical Engineering & Material Science
Rice University, Houston 77005 TX (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201505225.
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Scheme 1. Schematic representation of the overall synthetic route for the preparation of h-BCN and h-BN.
The advantages of this method is that, it is green, scalable,
occurs at relatively low temperature (9008C) compared to pre-
viously reported methods,[12,14,27] requires no catalyst and,
above all, uses cheap and readily available precursors. First, an
adduct of boric acid and urea was prepared by heating at
658C. Subsequent heating of this adduct with glucose at
9008C produces BCN sheets. Three different types of BCN com-
posites were synthesized by varying the initial ratio of BN
adduct to glucose and named BCN (3:1), BCN (1:1) and BCN
(1:3). Scheme 1 describes the overall process for the synthesis
of hexagonal BCN (h-BCN). The products were characterized by
Fourier transform infra red (FTIR) spectroscopy, Raman spec-
troscopy, Scanning Electron Microscope (SEM), Transmission
Electron Microscope (TEM), powder X-ray diffraction (PXRD), X-
ray photoelectron spectroscopy (XPS) and Brunauer, Emmett
and Teller (BET) surface area analysis.
discharge operation of the supercapacitor and the consequent
performance deterioration have not been discussed. On the
other hand, the BCN synthesized by the present method
shows a specific capacitance as high as 244 FgÀ1 at a high cur-
rent density of 1 AgÀ1 with much more robust performance
and higher rate capability. For example, it shows excellent sta-
bility and capacitance retention (ꢀ96%) even at a higher cur-
rent density (5 AgÀ1).
Results and Discussion
Figure 1A shows representative PXRD patterns of BCN with dif-
ferent B and N contents. The PXRD pattern of glucose-derived
carbon (obtained by heating glucose in Ar at 9008C) is also
shown for comparison. Interestingly, the (002) and (100) planes
are mostly exposed in all three different types of h-BCN, and
the highest intensity is seen for the peak corresponding to the
(002) plane (JCPDS Card No. 35-1292). More significantly, the
position of this peak shifts slowly towards lower 2q values
with increasing B and N content compared to that of glucose-
derived carbon. This peak appears at 25.88 for glucose-derived
carbon, whereas all the BCN samples show 2q between 24.8
and 25.38 for the (002) plane. The peak positions for BCN
match well with the pattern reported previously.[19] As shown
in Figure 1A, the d spacing of the BCN increases with increas-
ing ratio of BN adduct to glucose, probably because N is small-
er in size than C, and B is more electrophilic in nature and
tends to form stronger covalent bonds with C and N.
Despite its being a highly important material, BCN has not
been widely explored in the field of energy. Recently, Wang
et al. have shown BCN/graphene to be a good electrocatalyst
for the ORR.[17] Lei et al. have synthesized BCN NSs in a molten
mixture of LiCl and KCl using NaBH4 and urea as B and C, N
sources, respectively, for application in Li-ion batteries.[28] Very
few reports are available on energy-storage applications of
BCN. Vertically aligned BCN nanotubes are reported to show
a specific capacitance of 321 FgÀ1 at a current density of
0.2 AgÀ1.[29] However, the current density applied to achieve
this capacitance is very low in terms of practical applications,
in which supercapacitors are required to charge and discharge
at very high rates. More significantly, the possible destruction
of the alignment by sustained electric field during the charge/
Raman spectroscopy was also used for structure analysis
(Figure 1B). All the as-obtained BCN NSs samples show two
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Figure 1. A) PXRD pattern of BCN NSs having BN:C ratios of 1:1, 1:3 and 3:1 as well as glucose-derived carbon. B) Raman spectra of BCN NSs and glucose-de-
rived carbon. C) FTIR spectra of BCN and glucose-derived carbon (Glucose Carbon); inset: enlarged view of the IR spectra showing the CꢁN stretching band.
D)–F) Representative core-level C 1s, N 1s and B 1s XP spectra of BCN (3:1) showing the presence of CÀC, CÀB, CÀN and BÀN bonds (open circles: raw data,
solid line: overall fit, coloured areas: deconvoluted peaks).
characteristic peaks, namely, the D or defect-induced band and
G or graphitic band at 1325 and 1572 cmÀ1, respectively. The D
and G bands are redshifted by about 10 cmÀ1 compared to
those of glucose-derived carbon. Redshifting of both D and G
bands of BCN compared to that of glucose-derived carbon in-
dicates heterogeneity in BCN because of the presence of BÀC,
CÀN and BÀN bonds. Since, N doping introduces electrons and
B doping holes into the carbon matrix, both processes result in
changes in the electronic structure of carbon matrix, which
lead to changes in the polarization in the material and cause
shifts in the Raman spectra. The D-band shift is attributed to
the introduction of new types of disorders due to the new fine
structures in the system, whereas shifts in the G band are due
to weakening of CÀC bonds by elongation or contraction,
which results in changes in the electronic structure. Further,
the codoping of B and N in the carbon matrix introduces
excess electron carriers and thereby causes the redshift com-
pared to glucose-derived carbon. This is also evident in the ID/
IG ratio, which is higher in BCN than in glucose-derived carbon.
For comparison, Raman spectra of the BN adduct and h-BN are
shown in Figure S1 of the Supporting Information. In the case
of the BN adduct no peak was detected, but in the case of h-
BN one distinct peak appeared at 1371 cmÀ1(Supporting Infor-
mation, Figure S1).[19,30,31]
Figure 1C shows superimposed FTIR spectra of BCN NSs and
glucose-derived carbon to reveal the common features. For ex-
ample, the peak at 785 cmÀ1 is due to the presence of BÀN
bonds in BCN, and the broad peak between 1100 and
1250 cmÀ1 is due to the C=N stretching vibration. In contrast,
both of these peaks are absent in the spectrum of glucose-de-
rived carbon. However, the inset of Figure 1C shows the CꢁN
stretching band for BCN at 2170 cmÀ1, which confirms the for-
mation of a ternary BCN compound.[25]
XPS is a valuable tool for confirming the presence of BÀC,
CÀN, BÀN and CÀC bonds in BCN. The survey spectrum of BCN
(3:1) is shown in Figure S2 of the Supporting Information.
Since XPS is a surface analysis technique, we observed the
presence of B, C, N and O in the survey scan. The presence of
O in the survey scan could be due to moisture adsorbed on
the surface owing to the strongly oxophilic nature of B in the
material. From the deconvolution of the corresponding peak
of C, information on the bonding of C was obtained. Accord-
ingly, Figure 1D shows a C 1s signal that can be deconvoluted
into four peaks at 283.9, 284.8, 286.1 and 287.8 eV, which are
assigned to CÀB, CÀC, CÀN and CÀO bonds, respectively. Fig-
ure 1E shows the N 1s signal, which is the sum of three peaks
at 398.1, 399.4 and 400.5 eV. These peaks correspond to NÀB,
graphitic NÀC, and pyridinic NÀC bonds, respectively. Figure 1F
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Figure 3. AFM images of BCN (3:1) and corresponding height profiles.
Figure 2. A) SEM and B) TEM images of BCN (3:1) showing sheet-like mor-
phology. C) and D) High-resolution TEM images of a portion of the BCN
sheet revealing lattice fringes (E) and corresponding fast Fourier transform
pattern (F)
shows the B 1s signal, which can be deconvoluted into three
peaks due to BÀC, B=C and BÀN bonds at 189, 190.7 and
191.7 eV, respectively. All the peaks thus suggest the formation
of BÀC, CÀC, CÀN and BÀN bonds.[29]
Figure 2A shows a representative SEM image of as-synthe-
sized BCN (3:1). The high-resolution (HR) TEM image in Fig-
ure 2B reveals a sheet-like structure of the material. Lattice
fringes are visible at higher magnification, and the d spacing
calculated from the lattice fringes is 0.35 nm, which matches
well with that of the (002) plane of hexagonal BCN in the XRD
pattern (JCPDS No. 35-1292). On the other hand, the inset of
Figure 2D clearly shows two interconnecting planes. The angle
between these two planes is 658. The fast Fourier transform
image in Figure 2F indicates formation of a hexagonal struc-
ture.[32] SEM and TEM images of BCN (3:1), BCN (1:1) and BCN
(1:3) and glucose-derived carbon are shown in Figures S3 and
S4 of the Supporting Information.
Figure 4. SEM image (A) and elemental mapping images for of B (B), C (C)
and N (D).
Cyclic voltammetry (CV) and galvanostatic charge/discharge
measurements were done by using 1m H2SO4 as electrolyte in
a two-electrode system. Figure 5A shows the CV curves of glu-
cose-derived carbon and BCN (3:1), BCN (1:1) and BCN (1:3) at
a typical scan rate of 100 mVsÀ1 in 1m H2SO4 solution. The CV
measurements indicate a higher capacitance of BCN (3:1) com-
pared to BCN (1:1), BCN (1:3) and glucose-derived carbon. Glu-
cose-derived carbon shows a lower current and a smaller
width of the CV curve implying low-capacitance behaviour.
The capacitance increases systematically with increasing
amount of BN adduct and, interestingly, the capacitance seems
to be the highest for the case of BCN (3:1). This is also con-
firmed by Figure 5B, which shows typical galvanostatic charge/
discharge curves at 1 AgÀ1 for all BCN compositions and glu-
cose-derived carbon and demonstrates that BCN (3:1) has
In addition, SEM imaging of as-prepared BCN NSs was per-
formed, and elemental-mapping images for B, C and N are
shown in Figure 3B–D, respectively. They indicate uniform dis-
tribution of B, C and N in the BCN NSs.
Atomic Force Microscope (AFM) imaging was performed to
determine the thickness and size of the as-prepared BCN NSs
(Figure 4). The size of the BCN NSs is about 1 mm and their
thickness is about 13 nm.
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Figure 5. A) CV curves of glucose-derived carbon and BCN NSs with different compositions at 100 mVsÀ1 B) Galvanostatic charge/discharge curves of glucose-
derived carbon and BCN NSs. C) CV curves of BCN (3:1) at scan rates of 10, 20, 50 and 100 mVsÀ1. D) Galvanostatic charge/discharge curves of BCN (3:1) at
current densities of 1, 2, 3, 4 and 5 AgÀ1 in 1m H2SO4. E) Specific capacitance of glucose-derived carbon and BCN NSs with different compositions at current
densities of 1, 2, 3, 4 and 5 AgÀ1. F) Stability evolution of BCN (3:1) at a current density of 5 AgÀ1 in 1m H2SO4
a higher specific capacitance than the other BCN NSs and glu-
cose-derived carbon.
0.042 and 0.018 cm3 gÀ1 respectively. The higher capacitance of
BCN (3:1) could be due to slightly higher pore volume and the
chemical composition, because BCN (3:1) has a higher content
of B, which plays a key role in incorporating ions on the sur-
face of the electrode, which results in more diffusion and an
increase in capacitance. B acts as an electrophilic centre; due
to the presence of an empty p orbital it can store more charge
by attracting ions electrostatically. Simultaneous N doping
would lead to a combination of holes and electrons to en-
hance capacitance with increasing content of B and N in the
carbon lattice to a certain extent. This combination of heteroa-
toms improves the performance of the material significantly, as
illustrated by the data presented in Figure 5a and b.[33–34] Fig-
ure 5c shows the CV curves of BCN (3:1) at scan rates of 10,
20, 50 and 100 mVsÀ1 in 1m H2SO4 solution, and Figure 5d the
galvanostatic charge/discharge curves of BCN (3:1) at 1, 2, 3, 4
The specific capacitances calculated from charge/discharge
data for BCN (3:1), BCN (1:1), BCN (1:3) and glucose-derived
carbon are 244, 188, 103 and 7 FgÀ1, respectively. The primary
reason for the observed increase in capacitance with increasing
fraction of BN adduct could be determined by measuring the
chemical composition as well as surface area of as synthesized
BCN.
The surface area of the BCN NSs was measured by BET analy-
sis (Supporting Information, Table S1 and Figure S5). The BET
surface area of BCN (3:1) of 314 m2 gÀ1 is similar to that of BCN
(1:1) (343 m2 gÀ1). BCN (1:3) and glucose-derived carbon have
surface areas of 107 and 10 m2 gÀ1, respectively. The pore
volume of BCN (3:1) is 0.18 cm3 gÀ1, whereas BCN (1:1), BCN
(1:3) and glucose-derived carbon show pore volumes of 0.14,
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and 5 AgÀ1. The capacitance of BCN (3:1) is 244 FgÀ1 at a cur-
rent density of 1 AgÀ1 in 1m H2SO4. To demonstrate the rate
capability, Figure 5e shows the capacitance behaviour of glu-
cose-derived carbon and BCN (3:1), (1:1) and (1:3) at current
densities of 1, 2, 3, 4 and 5 AgÀ1. The usefulness of these elec-
trode materials is further confirmed by the data in Figure 5 f,
which reveals durability of BCN (3:1) at a high current density
of 5 AgÀ1 with 96% retention of capacitance and excellent cy-
cling stability, which confirm that BCN is a superior material for
supercapacitor applications.
BCN NSs were studied systematically. SEM and HRTEM images
showed a thin, sheet-like morphology with a hexagonal crystal
lattice, and AFM showed that the sheet size is about 1 mm and
the thickness of the nanosheets is around 13 nm. XPS con-
firmed the presence of CÀC, BÀC, BÀN and CÀN bonds. The ca-
pacitance increased with increasing fraction of the BN adduct
to a certain extent, perhaps due to synergetic effects of codop-
ed B and N. The excellent retention of capacitance (96%) at
a current density of 5 AgÀ1 after 3000 cycles indicates that
BCN NSs have good stability and also high rate capability. BCN
NSs show a consistent specific capacitance of 244 FgÀ1 at a cur-
rent density of 1 AgÀ1, which is superior to those of other
promising materials for supercapacitors.
Electrochemical impedance spectroscopy (EIS) was carried
out for all the BCN NSs, and the corresponding Nyquist plots
are shown in Figure S6 of the Supporting Information. The
straight line in the low-frequency region corresponding to the
diffusion process is due to the capacitive behaviour of the BCN
NSs. However, BCN (3:1) has a straight line in the lower-fre-
quency region and also shows more pronounced capacitive
behavior than the other BCN NSs in this study. The Nyquist
plots also showed that the series resistance of the BCN NSs de-
creases with increasing amount of BN. This might be due to
the more electrophilic nature of the material. Semicircles ob-
served for BCN (3:1) and BCN (1:1) in the higher-frequency
region are due to the electrochemical charge-transfer resist-
ance at the electrode/electrolyte interface. The charge-transfer
resistance of BCN (3:1) is lower than those of BCN (1:1) and
BCN (1:3). From the EIS studies, it can be concluded that BCN
(3:1) shows pronounced capacitive behaviour and lower elec-
trochemical charge-transfer resistance. TEM imaging and XPS
analysis were performed on the BCN nanosheets after electro-
chemical analysis to reveal any possible structural changes in
the BCN NSs. The TEM image and selected-area electron dif-
fraction pattern are shown in Figure S7 of the Supporting In-
formation. The TEM image shows that after electrochemical
studies the sheet-like morphology of BCN is not damaged
much. SAED clearly showed a hexagonal pattern, the same as
obtained for BCN sheets before the electrochemical studies. XP
spectra of BCN nanosheets taken after electrochemical studies
are shown in Figure S8 of the Supporting Information. The XPS
data indicate that the BÀC, BÀN and CÀC bonds remain stable
after electrochemical tests, but an additional peak correspond-
ing to C-OH is observed.
Experimental Section
Materials
a-d-Glucose was procured from Sigma-Aldrich. Boric acid and urea
were obtained from Lobachem. All chemicals were used as re-
ceived.
Synthesis of BCN nanosheets
Boric acid and urea were used as boron and nitrogen precursors,
respectively, and glucose was used as a primary carbon source.
Typically, for the synthesis of BCN nanosheets, BN adduct was first
prepared by mixing boric acid and urea in a molar ratio of 1:48.
The mixture of urea and boric acid was then slowly heated at 658C
until white needle-shaped crystals of the BN adduct formed.[35,36]
The BN adduct was mixed with glucose in three different weight
ratios (BN:C=1:1, 1:3, 3:1) by grinding in a mortar and pestle. The
mixture was then heated at 9008C for 5 h under argon atmos-
phere. For the comparisons of structure and electrochemical per-
formance, glucose and BNs adduct were heated separately at
9008C to obtain carbon and h-BN respectively.
Characterization
PXRD patterns were recorded with a Phillips PANalytical diffractom-
eter with CuKa radiation (l=1.5406 Š) at a scan speed of 28minÀ1
and a step size of 0.028 in 2q. For measurements, samples were
coated directly on XRD plates. FTIR spectra were taken with
a Bruker Optics ALPHA-E spectrometer with a universal Zn–Se ATR
accessory in the 600–4000 cmÀ1 region or with a Diamond ATR
(Golden Gate). Raman spectroscopy was carried out at room tem-
perature with an HR 800 Raman spectrophotometer (JobinYvon
HORIBA, France) by using monochromatic radiation emitted by an
He–Ne laser (632 nm) operating at 20 mW. The experiment was re-
peated several times to verify the consistency of the spectra. TEM
was carried out with Tecnai F30 FEG instrument operated at an ac-
celerating voltage of 300 kV. Morphology and chemical composi-
tion of h-BCN were examined with Quanta 200 3D FEI scanning
electron microscope. XPS measurements were carried out with
a VG Micro Tech ESCA 3000 instrument at a pressure of >1”
10À9 Torr (pass energy of 50 eV, an electron take-off angle of 608,
and an overall resolution of 1 eV) by using MgKa radiation (hn=
1253.6 eV). The X-ray flux (power 70 W) was kept deliberately low
to reduce beam-induced damage. The spectra were fitted by using
a combined polynomial and Shirley-type background function. An
Autosorb-iQ automatic volumetric instrument was used for low-
pressure volumetric N2 gas adsorption measurements. These meas-
Thus, all the above data suggest the successful preparation
of BCN NSs by employing easily available sources (boric acid,
urea and glucose). As-synthesized BCN NSs were found to be
potentially useful for supercapacitor applications with a very
high specific capacitance of 244 FgÀ1, especially suitable for
hybrid applications at higher power density. However, it is very
difficult to precisely control the exact layer thickness of BCN
NSs prepared by the current method, and it is difficult to
obtain large integrated sheets. A controlled layer-by-layer
bottom-up approach may alleviate these limitations in the
future.
Conclusion
BCN NSs were synthesized with various ratios of BN adduct to
glucose. The structure and supercapacitor performance of the
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Acknowledgements
I.K. and J.D. acknowledge research fellowship from UGC. R.R.D.
acknowledges CSIR for research fellowship. M.S. acknowledges
Indo-US Science and Technology Forum, India for research fel-
lowship. This work was supported by the Council of Scientific
and Industrial Research (CSIR), India through TAPSUN program.
Keywords: boron · doping · electrochemistry · nanoparticles ·
nitrides
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Received: December 31, 2015
Published online on April 13, 2016
Chem. Eur. J. 2016, 22, 7134 – 7140
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