Synthesis, structure, and properties of mesoporous B/C/N microspheres

Article abstract of DOI:10.1002/zaac.200900377

Reaction of low surface area carbon with a mixture of urea and boric acid at 930 °C yields a composition close to BC₄N with a graphitic structure. BC₄N was characterized by electron energy loss spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. BC₄N is a porous ceramic with a surface area of 428 m²·g^-1, and shows a CO₂ uptake of 40 wt-%. The layered structure of BC ₄N involves a random distribution of boron, carbon, and nitrogen atoms and shows high thermal stability up to 1000 °C. A comparative analysis of the structure and properties of BC₄N and graphene using first-principles pseudopotential based density functional theoretical calculations is presented. The calculations predict it to be an insulator. The bulk modulus of BC₄N exhibits an interesting dependence on the ordering of boron and nitrogen on the graphene lattice.

Full text of DOI:10.1002/zaac.200900377

ARTICLE
DOI: 10.1002/zaac.200900377
Synthesis, Structure, and Properties of Mesoporous B/C/N Microspheres
[
a]
[b]
[b]
Kalyan Raidongia, K. P. S. S. Hembram, Umesh V. Waghmare,
[a]
[a,c]
M. Eswaramoorthy, and C. N. R. Rao*
Dedicated to Professor Arndt Simon on the Occasion of His 70 Birthday
Keywords: Boron; BC N; Ceramics; Graphitic material; Carbon dioxide fixation
4
Abstract. Reaction of low surface area carbon with a mixture of urea distribution of boron, carbon, and nitrogen atoms and shows high ther-
and boric acid at 930 °C yields a composition close to BC N with a mal stability up to 1000 °C. A comparative analysis of the structure
graphitic structure. BC N was characterized by electron energy loss and properties of BC N and graphene using first-principles pseudopo-
spectroscopy, X-ray photoelectron spectroscopy, transmission electron tential based density functional theoretical calculations is presented.
microscopy, Raman spectroscopy, and X-ray diffraction. BC N is a The calculations predict it to be an insulator. The bulk modulus of
BC N exhibits an interesting dependence on the ordering of boron and
N involves a random nitrogen on the graphene lattice.
4
4
4
4
2
–1
porous ceramic with a surface area of 428 m ·g , and shows a CO
2
4
uptake of 40 wt-%. The layered structure of BC
4
Introduction
tained by the hot filament CVD method [11]. CVD [12, 13],
laser ablation [14, 15], ion beam assisted deposition [16], and
magnetron sputtering [17–19] were employed to deposit
B C N films on solid substrates. A crystalline composition of
Several efforts to prepare materials containing boron, carbon,
and nitrogen atoms (B C N ) with different compositions were
x
y
z
x
y
z
carried out. One of the objectives of such studies is to tune
and alter the properties of graphene through doping with a
suitable element such as boron and nitrogen [1]. Whereas gra-
phene exhibits fascinating electronic and vibrational properties
and has a great potential for a variety of applications, its poten-
tial for use as a semiconductor in nanoelectronic circuits is
limited because of its vanishing bandgap. Nanotubes contain-
ing boron, carbon, and nitrogen of variable compositions were
reported [2]. For example, nanotubes with separated layers of
carbon and boron nitride were prepared by the arc discharge
method, laser ablation and chemical vapor deposition (CVD)
B CN is reported to occur on direct nitridation of B C with
4
4
4
nitrogen in the 1600 to 2250 °C range [20, 21]. Amorphous
B CN was obtained by the pyrolyzation of a xerogel consist
4
4
of B-trichloroborazene and bis(trimethylsilyl)carbodiimide
22, 23]. Attempts have been made to prepare high surface
area BCN by heating mesoporous carbon with B O at high
[
2
3
temperatures in a nitrogen environment [24]. Porous carbon
materials doped with boron and nitrogen are reported to be
formed by the pyrolysis and carbonization of gels containing
boric acid and citric acid in a nitrogen atmosphere [25], the
compositions varying between of BC N and BC N. A solid
16
6
[
3–7]. B C N nanotubes were also obtained by the pyrolysis
x y z
with the composition of BC N seems to be formed on the ther-
4
of a BH -trimethylamine adduct [8]. Nanotubes of graphitic
3
mal decomposition of the BH –C H N complex at high tem-
3
5
5
carbon and carbon nitride were employed as templates to ob-
tain BCN nanotubes of varying compositions [9, 10]. Single-
walled BCN nanotubes with indefinite compositions were ob-
peratures [26].
Nitridation of boric acid and carbonization of saccharose in
molten urea is reported to give a material with the bulk compo-
sition of BC N, but the material was not adequately character-
4
*
Prof. Dr. C. N. R. Rao
Fax: +91-802-208-2760
E-Mail: cnrrao@jncasr.ac.in
ized with respect to composition and properties [27]. Reaction
of amorphous carbon nanotubes with boric acid and urea was
shown to yield nanotubes with the approximate composition
[
[
[
a] Chemistry and Physics of Materials Unit
Jawaharlal Nehru Centre for Advanced Scientific Research
Jakkur P.O.
of BC N [28]. It was our desire to find a means of carrying
4
out a large scale synthesis of BC N with desirable properties.
4
Bangalore 560064, India
b] Theoretical Sciences Unit
In order to accomplish this, we made use of amorphous carbon
spheres, obtained from the hydrothermal treatment of glucose,
as the starting material. By the reaction of the carbon spheres
with boric acid and urea at high temperatures we obtained bulk
quantities of BC N with interesting properties. BC N is found
Jawaharlal Nehru Centre for Advanced Scientific Research
Jakkur P.O.
Bangalore 560064, India
c] New Chemistry Unit and CSIR Centre of Excellence in
Chemistry
4
4
Jawaharlal Nehru Centre for Advanced Scientific Research
Jakkur P.O.
to be a ceramic with high thermal stability, containing both
micro and mesopores.
Bangalore 560064, India
Z. Anorg. Allg. Chem. 2010, 636, 30–35
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
30

C. N. R. Rao et al.
ARTICLE
In this article, we present the synthesis and characterization boron atom bonded to carbon and nitrogen, respectively [7, 20,
of BC N and describe some of the properties through a combi- 21, 30, 32]. The weak feature at 193 eV is assigned to a boron
4
nation of experimental and theoretical studies.
atom bonded to oxygen, probably arising from a small amount
of unreacted B O . Based on the capture cross sections and
2
3
intensities of boron, carbon, and nitrogen signal in Figure 2,
we estimated the composition to be B C N or B C N .
Results and Discussion
1.4
4
4
12
3
Whereas the XPS spectra of the product obtained in the present
study look somewhat different from the corresponding spectra
Amorphous carbon spheres prepared [29] by the hydrother-
mal treatment of aqueous glucose solution were used as the
starting material for the synthesis of B C N . Diameters of the
of the BC N nanotubes [28], the main features are comparable.
4
x
y
z
carbon spheres were in the 150 nm to 500 nm range as can be
seen from the FESEM image in Figure 1a. A TEM image of a
typical carbon sphere is shown in Figure 1b. The images reveal
the smooth surface of the spheres. The BET surface area of
2
–1
these carbon spheres was 85 m ·g . The reaction of the carbon
spheres with urea and boric acid at 930 °C gave a product
containing boron, carbon, and nitrogen. The product consisted
of spherical particles as shown in the FESEM image in Fig-
ure 1c, diameters of these spheres remaining in the 150–
5
00 nm range. From the TEM image in Figure 1d, it is obvious
that unlike the smooth surface of carbon spheres, the BC N
4
spheres show contrast variations originating from the presence
of pores at the surface with uneven depth.
Figure 1. (a) FESEM image and (b) TEM image of carbon spheres.
x y z
(c) FESEM image and (d) TEM image of B C N spheres.
XPS core level spectra of the B C N product is shown in
x
y
z
4
Figure 2. XPS of BC N spheres.
Figure 2. The carbon 1s spectrum shows a peak centered at
2
2
86 eV, which can be deconvoluted into two bands at 283 and
87 eV. The band at 283 eV is assigned to a carbon atom
In order to establish the composition of the B C N product,
x
y
z
bonded to boron and the band at 287 eV ascribed to a carbon we carried out electron energy loss spectroscopy (EELS) meas-
atom bonded to another carbon and nitrogen atoms [7, 20, 21, urements on the K edge absorptions bands of boron, carbon,
3
0, 31]. The nitrogen 1s signal can be deconvoluted into two and nitrogen of the entire-sphere as well as different parts of
bands at 399 and 400 eV, of which the former corresponds to the sphere were carried out in a high resolution transmission
a nitrogen atom bonded to boron and the latter to a nitrogen electron microscope. A representative spectrum in Figure 3a
atom bonded to carbon [7, 20, 21, 31, 32]. The boron 1s signal shows the K shell ionization edges around 188, 284, and
appears as a broad feature with shoulders on either side. This 401 eV, which are characteristic of boron, carbon, and nitro-
band can be deconvoluted into three bands centered at 188, gen, respectively. Each core edge fine structure shows two
1
90 and 193 eV, of which the first two can be assigned to a peaks corresponding to the π* and σ* bands, because of the sp2
3
1
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 30–35

Synthesis of Mesoporous B/C/N Microspheres
hybridization [7]. Based on the EELS data, we got an average 1324 cm–1 and 1600 cm , which are assigned to the D and G
–1
chemical composition of B C N, which is in agreement with bands as in B C N and graphitic materials [34, 35]. The broad-
1.4 3.7
x
y
z
the XPS results. This analysis corresponds approximately to ness of the D band can arise from disorder in the BCN layers.
B C N . For purpose of convenience, we shall indicate the
Reaction between B(OH) and urea, a main synthesis route
4
11
3
3
composition as BC N throughout this paper. In Figure 3b, we for pure BN has been well studied earlier [36, 37]. In a similar
4
show an EELS map of a small portion of BC N sphere, the red, reaction, addition of amorphous carbon to the mixture of
4
green, and blue colors representing boron, carbon and nitrogen B(OH) and urea yields BC N, on two-phase of heating. The
3
4
atoms, respectively. The colors are uniformly distributed initial reactions involved in the formation of BC N are the
4
throughout the sphere suggesting a reasonably homogeneous
nature of BC N. The homogeneity of BC N is also indicated
dehydration of B(OH) and decomposition of urea:
3
4
4
Δ
2
B(OH) → B O + 3H O
3
2
3
2
by the fact that oxidation of BC N at 650 °C in an oxygen
4
Δ
CO(NH ) → NH + HNCO.
environment does not yield BN as it was the found in the case
of B C N nanotubes with separate layers of BN and carbon
2 2
3
x
y
z
The overall stoichiometric reaction for the formation of
[
33]. The XRD pattern and Raman spectrum of BC N do not
4
BC N may be written as:
4
change significantly on oxygen treatment at high temperatures.
B O + 2NH + 8C → 2BC N + 3H O.
2
3
3
4
2
In Figure 5, the thermogravimetric analysis curve of BC N
4
is compared with that of the starting carbon spheres recorded
in air. The carbon spheres get completely oxidized before
7
50 °C just like other carbon materials. The BC N spheres,
4
however, do not show any decomposition up to 1000 °C. This
feature of BC N is noteworthy.
4
Figure 3. (a) EEL spectrum (inset core edge fine structure of nitrogen)
and (b) EELS map of boron (red), carbon (green), and nitrogen (blue)
4
of a small portion of a BC N sphere.
The XRD pattern of the BC N spheres in Figure 4a shows
4
two broad reflections with d spacing of 3.4 Å and 2.1 Å corre-
sponding to the [002] and [100] planes respectively. The pat-
tern is similar to that reported earlier for BCN materials with
different compositions [20, 21, 26–28]. This XRD pattern is
typical of materials with a graphite-like structure with little or
no sheet-to-sheet registry. The interlayer distance (d = 3.4 Å)
4
Figure 5. TGA of BC N spheres and carbon spheres.
of the BC N is slightly larger than that of graphite (3.33 Å)
Nitrogen adsorption–desorption isotherms of BC N at 77 K
4
4
and h-BN (3.36 Å). In Figure 4b we show a TEM image of are shown in Figure 6. The specific surface area of the BC N
4
BC N to reveal the presence of fragments of layered structure. spheres by the Brunauer–Emmett–Teller (BET) method is 428
4
2
–1
The Raman spectrum of BC N, recorded with the 633 nm line m ·g . The isotherm shows type IV behavior with a H3 type
4
from a HeNe laser, shows two strong features centered around hysteresis loop (as per IUPAC classification), associated with
capillary condensation of nitrogen in the mesopores [38]. The
sharp rise in nitrogen adsorption at low relative pressures (p/
p0 < 0.01) indicates the presence of micropores in the material.
The pore size distribution, calculated from the adsorption
branch of the isotherms using DFT (density functional theory)
model confirms the presence of both micropores (1.4 nm) and
mesopores (2.8 nm and 4.5 nm) in the BC N spheres (see inset
4
of Figure 6). The release of gaseous species during the high
temperature reaction appears to be responsible for the meso-
and micropores in the BC N spheres, which in turn give rise
4
to the higher surface area. Thus, the BC N spheres prepared
N. by us exhibit properties of a porous ceramic.
4
Figure 4. (a) XRD pattern of BC
Z. Anorg. Allg. Chem. 2010, 30–35
4
N spheres (b) TEM image of BC
4
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
32

C. N. R. Rao et al.
ARTICLE
configuration (i) has a relatively more homogeneous distribu-
tion of boron and nitrogen, it is highest in energy. On the other
hand, configuration (iv) is highly heterogeneous in the sense
BN forms a continuous region (phase segregated), and lowest
in energy among the four configurations. From the cohesive
energies (Table 1), it is clear that BC N layers are less stable
4
than pristine graphene by –2.5 to –6 %, and the configuration
(
i) of BC N is only 0.3 eV per atom higher in energy than the
4
configuration (iv). In agreement with weakened binding, the
graphene lattice expands by up to 0.4 % upon substitution of
boron and nitrogen at 33 %, with the precise value depending
on the ordering of boron and nitrogen. The configurations of
the samples synthesized by us would depend on the kinetics
and other experimental factors such as temperature that affect
the growth. Clearly, configurational entropy would favor a ho-
N spheres mogeneous distribution of boron and nitrogen, as in configura-
tion (i), which may be the most stable form if the samples
are grown at high temperatures. Luo et al. [39] show that the
Figure 6. Nitrogen adsorption-desorption isotherm of BC
4
(
inset pore size distribution).
hexagonal BC N structure stacked with one B–N layer and two
4
C–C layers has the lowest total energy among all constructed
CO adsorption of the BC N spheres measured at 298 K and
2
4
4
0 bar shows 9 wt-% uptake (Figure 7a). The uptake, however,
goes up to 40 wt-% of CO at 195 K and 1 atmosphere. The
2
adsorption and desorption isotherms (Figure 7b) follow exactly
the same path, showing no hysteresis. The adsorption isotherm
does not exhibit saturation even at p/p = 1. Methanol adsorp-
0
tion measurements on the BC N spheres at room temperature
4
and low pressures were carried out. The BC N spheres ad-
4
sorbed about 200 mL (28 wt-%) of methanol at p/p = 0.9.
0
The H adsorption isotherm of the BC N spheres at 77 K and
2
4
5
1
0 bar rises sharply and reaches saturation at 10 bar, with
.1 wt-% of H adsorption. At 77 K and 1 bar atmospheric
2
pressure, the BC N spheres adsorb 0.8 wt-% of H .
4
2
To determine possible structures of a BC N layer, we carried
4
out first-principles calculations by considering different order-
ing of boron and nitrogen substituted for one third of carbon
atoms in the 6 × 6 unit cells (a supercell) of graphene, and
relaxing the atomic positions and the periodic cell parameter a
to minimum energy (see Figure 8). Whereas there are numer-
ous possible configurations, we consider four [labeled (i) to
4
Figure 8. (left) Energy as a function of lattice constant of BC N for
configurations with different ordering of boron and nitrogen on carbon
lattice, and (right) structures of configurations with the highest (top)
and lowest energy (bottom) of the four. Note that the energies are of
(
iv)] of them, in which boron is substituted for carbon on one
sublattice and for nitrogen on another. This ensures that no
two boron (or nitrogen) atoms are nearest neighbors. Whereas the whole supercell that contains 72 atoms.
Figure 7. CO
2 4
adsorption-desorption isotherm of BC N spheres (a) at 298 K and 40 bar (b) at 195 K and 1 atm.
3
3
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 30–35

Synthesis of Mesoporous B/C/N Microspheres
BC N structures. However, we do not seem to have B–N and graphite type layered structure and gives an X-ray diffraction
4
C–C layers.
pattern and Raman spectrum expected of such structure. It has
a high propensity for CO uptake (up to 40 wt-%). First-princi-
2
Table 1. Lattice parameter (a), bulk modulus (B), and cohesive energy ples density functional theoretical calculations were used to
4
(Ecoh) of graphene and BC N in different configurations.
compare the structure and properties of BC N and graphene.
4
Configuration
a /bohr
B /GPa·m
E
coh /eV per atom
It is found that BC N is stable and that electronic structure and
4
elastic (bulk) modulus are sensitive to the ordering of boron
and nitrogen atoms. An energy gap ranging from 0.17 to
BC
BC
BC
BC
4
4
4
4
N (i)
28.36
28.28
28.28
28.24
28.04
1.81
2.06
1.66
1.61
1.74
8.63
8.81
8.84
8.93
9.16
N (ii)
N (iii)
N (iv)
1.1 eV is exhibited by various configurations of BC N augurs
4
well for its possible applications as a semiconductor in nanoe-
lectronics.
Graphene
The electronic structure of BC N is sensitive to the ordering
4
Experimental Section
Amorphous carbon spheres were prepared by hydrothermal treatment
of a saturated solution of glucose [29]. In a typical procedure, an aque-
of boron and nitrogen (Figure 9). Whereas a small gap of
about 0.17 eV opens up in configuration (i) because of differ-
ent on-site potentials felt on the two sublattices of graphene,
configuration (iv) exhibits a larger bandgap of 1.1 eV. A de- ous solution of glucose (20 mL, 0.5 m) was sealed in a 22 mL Teflon
tailed examination of the band structure reveals that both con- seal autoclave and heated to 180 °C for 6 hours. The resulting black
figurations (i) and (iv) exhibit a direct bandgap. The larger colored solution was centrifuged to obtain a solid that was washed
with ethanol for several times by using sonication and centrifugation.
bandgap of configuration (iv) correlates well with its greater
The dark brown solid obtained as the final product after oven drying
structural stability.
at 50 °C for 12 h was found to contain carbon spheres. This material
x y z
was used as the starting material for the synthesis of B C N spheres.
A mixture of boric acid (0.4 g), urea (4.7 g), and amorphous carbon
spheres (1 g) in distilled water (40 mL) was heated to 60 °C until the
solution became viscous. The product was separated by centrifugation
and dried in air at 40 °C overnight. The dried sample was thermally
treated at 930 °C for 10 h in a nitrogen atmosphere, and cooled to
room temperature. The product was subsequently heated in an NH
3
atmosphere at 1000 °C for three hours to give black-colored BC
4
N
spheres.
Characterization
X-ray diffraction (XRD) patterns were recorded at 25 °C with a Rich-
Siefert 3000-TT diffractometer employing Cu-K radiation. X-ray pho-
toelectron spectroscopy (XPS) measurements were carried out with an
ESCALAB MKIV spectrometer employing Al-K radiation
1486.6 eV). Electron energy loss spectra (EELS) were recorded with
α
4
Figure 9. Electronic density of states as a function of energy for BC N
in configurations (i) and (iv), and graphene.
α
(
a transmission electron microscope (FEI, TECNAI F30) equipped with
an energy filter for EELS operating at 300 kV. Raman spectra were
recorded with a LabRAM HR with a 633 nm line from HeNe laser.
The bulk modulus of each configuration was determined us-
ing the second derivative of energy with respect to unit cell
area from the energy curves. Configurations (i) and (ii) with a Thermogravimetric analysis was performed with a Mettler Toledo Star
homogeneous distribution of boron and nitrogen atoms exhibit system. Low pressure gas adsorption properties were measured using
a higher bulk modulus (by 10 %) than that of graphene or of a QUANTACHROME AUTOSORB-1C surface area analyzer. Surface
areas were obtained from nitrogen adsorption–desorption isotherms at
7 K. High-pressure gas sorption isotherm measurements were carried
the energetically favorable configurations (iii) and (iv). Quali-
7
tatively, the BC N synthesized by us is a harder material rela-
4
out on BELSORP-HP, BEL JAPAN high pressure instrument. The
dead volume of the sample cell was measured with helium gas of
tive to graphite.
9
9.999 % purity. Non-ideal correction for hydrogen gas was made by
applying virial coefficients at the respective measurement temperature.
The adsorption isotherm of MeOH (at 298 K) was measured in the
Conclusions
It was possible to synthesize a ceramic of the composition gaseous state by using BELSORP-aqua volumetric adsorption instru-
ment from BEL, Japan.
BC N starting with amorphous carbon spheres, obtained from
4
the hydrothermal treatment of glucose. The reaction of amor-
phous carbon spheres with urea and boric acid yields bulk
Computational Methods
quantities of BC N spheres. The boron, carbon, and nitrogen
4
atoms are homogeneously distributed in the structure. BC N is We used plane wave self consistent field (PWSCF) [40] implementa-
4
a porous ceramic with high thermal stability. BC N has a tion of density functional theory (DFT), with a generalized gradient
4
Z. Anorg. Allg. Chem. 2010, 30–35
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
34

C. N. R. Rao et al.
ARTICLE
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ionic cores and valence electrons of boron, carbon and nitrogen. Kohn–
Sham wave functions are represented with a plane wave basis with an
energy cutoff of 25 Ry and charge density with a cutoff of 150 Ry. To
facilitate a direct comparison, both BC N and graphene were simulated
4
using 6 × 6 unit cells in the ab plane and a periodic supercell with a
vacuum of 16 Å in the third direction. Integration over the Brillouin
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