Facile synthesis and characterization of hexagonal boron nitride nanoplates by two-step route

Article abstract of DOI:10.1111/j.1551-2916.2011.04752.x

Large quantities of hexagonal boron nitride (h-BN) nanoplates with hexagonal morphologies have been successfully synthesized by a simple two-step route without using any template or catalyst, including combustion synthesis and then annealing processes. Th

Full text of DOI:10.1111/j.1551-2916.2011.04752.x

J. Am. Ceram. Soc., 94 [12] 4496–4501 (2011)
DOI: 10.1111/j.1551-2916.2011.04752.x
© 2011 The American Ceramic Society
ournal
J
Facile Synthesis and Characterization of Hexagonal Boron Nitride
Nanoplates by Two-Step Route
Zhengyan Zhao, Zhigang Yang, Yan Wen, and Yuhua Wang†
Department of Material Science, School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, China
Large quantities of hexagonal boron nitride (h-BN) nanoplates
with hexagonal morphologies have been successfully synthe-
sized by a simple two-step route without using any template or
catalyst, including combustion synthesis and then annealing
processes. The phase content, morphology, and optical proper-
ties of the products have been characterized. It reveals that the
obtained products are pure and of high crystallinity. The UV–
vis absorption spectrum indicates that the h-BN nanoplates
have an optical band gap of ~6.07 eV. Strong violet–blue
photoluminescence (PL) emission with a broad band ranging
from 400 nm to 475 nm has been observed, indicating that the
nanoplates as-grown by this simple route are promising for
application in nanosize optical devices (LEDs, blue-light
source, UV detector, etc.). The phonon lines features are obvi-
ously found in the PL spectra, the phonon frequency involved
in these transitions is consistent with the B–N E2g vibrational
mode, which has been measured by the Raman spectroscopy.
sured by using a UV–vis spectrophotometer and show the
existence of electronic transitions between 4.4 and 6.2 eV.17
Luminescence spectroscopies (photoluminescence, electro-
luminescence, cathodoluminescence, etc.) have also been
employed to investigate the optical properties of BNNTs.18,19
In addition to the BN nanotube structure, a variety of h-BN
nanomaterials have been synthesized, such as nanocones,20
nanocapsules,21 nanohorns,22 nanorods,23 nanowires,24 core/
shell heterostructures,25,26 nanospheres,27,28, hollow nanorib-
bons,29,30 and nanosheets (BNNSs).31–36 Obviously, compared
to its sister one-dimensional (1D) BNNTs, the two-
dimensional (2D) BN nanostructures, such as BNNSs and
nannoplates37,38 have remained much less explored. And the
detailed studies on optical properties of these 2D BN nano-
structures have not yet been performed so far, due to limita-
tion of facial fabrication methods adopted. Classical synthesis
methods utilize special technologies, such as high-temperature
arc plasma,8 a laser in a diamond anvil cell at high nitrogen
pressure (5–15 GPa),9 CVD system,12–14,17 most of the
methods carried out above were at high temperatures (from
1100°C to 2700°C)17,19,21–27; and moreover, in most as-grown
products there contained undesirable impurities (e.g., amor-
phous B particles and metallic or metal oxide impurities).36
Therefore, for the sake of practicality, developing facile low-
cost and large-scale synthetic routes to synthesize 2D h-BN
nanomaterials with high purity, high crystallinity, uniform,
and fine particle size is urgently expected.
The combustion synthesis method is an effective, low-cost
method for production of various industrially useful materi-
als. Significant advantages of this synthesis route include a
template and catalyst-free, low-temperature, facile procedure,
and high yield. Recently, a number of important break-
throughs in this method made it notable for development of
new catalysts and functional nanomaterials with properties
better than those for similar traditional materials.39 Herein,
we report a two-step facile route for growing h-BN nano-
plates of bulk quantities with uniform size distribution. This
method consists of combustion process and subsequent
annealing under nitrogen atmospheric conditions. Significant
advantages of the synthesis route include a template-free,
low-temperature, facile procedure, and high yield. Detailed
structure, morphology, and optical properties of the h-BN
nanoplates achieved by this route have been evaluated.
I. Introduction
ECENTLY, much interest has been focused on nanostruc-
tures due to their peculiar, fascinating properties and a
R
wide range of interesting potential applications which, in
many cases, shall be more fruitful than for the corresponding
bulk counterparts. Since the first report of carbon nanotubes
(CNTs) in 1991,1 theoretical and experimental studies on
nanostructures of similar honeycomb-like networks have
been stimulated intensely. Isostructural to graphite, hexago-
nal boron nitride (h-BN) is one of the most important III–V
group materials with in-plane trigonal sp2 bonding, in which
B and N atoms substitute for C atoms in a graphitic-like
sheet with almost no change in atomic spacing.2,3 In compar-
ison to the carbon system, BN is a semiconductor with a
wide band gap near 6 eV. Moreover, it possesses excellent
mechanical properties and thermal conductivity, and is much
more thermally and chemically stable than GaN and AlN;
the interesting optical properties of h-BN make it a candidate
for new ultraviolet-light-emitting devices.4,5 As the main
member of BN nanomaterials, BN-nanotubes (BNNTs) have
been synthesized and investigated as a potential candidate
for a nanosized luminescent material.6,7
The BNNTs were first reported by Chopra et al. via an
arc-discharge route.8 After that, considerable efforts have
been devoted to BNNTs through laser heated,9 carbothermal
synthesis methods,10 “amide” routes,11 chemical vapor depo-
sition (CVD) synthesis methods,12–14 plasma-based15 and
ball-milling techniques,16 in the past decade. The optical
properties of single-walled BNNTs have been directly mea-
II. Experimental Procedure
The combustion synthesis and annealing processes were used
to fabricate the h-BN nanoplates in this work. Boric acid
(H3BO3, AR, Sinopharm Chemical Reagent Co., Ltd,
China), urea ((NH2)2CO, AR, Shantou Xilong Chemical
Reagent Co., Ltd, China), sodium azide (NaN3, AR, Tianj-
ing Fucheng Chemical Reagent Co., Ltd, China), and ammo-
nium chloride (NH4Cl, AR, Sinopharm Chemical Reagent
Co., Ltd.) were used as raw materials. In a typical process,
H3BO3 (0.05 mol), NaN3 (0.1 mol), NH4Cl (0.1 mol), and
(NH2)2CO (0.125 mol) were dissolved in 10mL de-ionized
E. Dickey—contributing editor
Manuscript No. 28767. Received October 13, 2010; approved June 14, 2011.
This work is supported by the National Science Foundation for Distinguished
Young Scholars (No. 50925206).
†Author to whom correspondence should be addressed. e-mail: wyh@lzu.edu.cn
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Characterization of Hexagonal Boron Nitride Nanoplates
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water to form a viscous gel-like solution. After stirring for a
short while, the solution filled in an alumina crucible was
transferred to an electric muffle furnace and ignited at 600°C
in atmosphere for 5–10 min. The resultant product was
washed with de-ionized water and absolute ethyl alcohol sev-
eral times, and dried in vacuum at 80°C. The precursor pow-
ders were then annealed between 1000°C and 1400°C in a
nitrogen atmosphere for 6 h for obtaining the final products.
The phase identification of samples was carried out by a
Rigaku D/Max-2400 X-ray diffractometer with Ni-filtered
CuKa radiation. Scanning electron microscopy (SEM) images
were taken by a Hitachi S-4800 scanning electron microscopy.
Transmission electron microscopy (TEM) studies were carried
with a Tecnai G2 F30 instrument operating at 300 kV. Typi-
cally, the h-BN nanoplates sample obtained at 1400°C was
dispersed in de-ionized water and then was put into the ultra-
sonic bath for 4 h; the resultant slurry was centrifuged at
3000 rpm for 20 min; precipitate and fractions were removed,
and the supernatant containing the h-BN nanoplates was col-
lected for TEM analysis. Raman scattering spectra were
obtained at room temperature using a Horiba Jobin Yvon
LABRAM-HR800 laser micro-Raman spectrometer with a
532 nm laser. The laser power was about 1 mW, and pro-
vided a typical spatial resolution < 1 lm and spectral resolu-
tion better than 1 cmÀ1 in peak position. The instrument was
calibrated with the peak of Si 520 cmÀ1. Fourier transform
infrared spectroscopy (FTIR) analysis was performed on a
Nicolet AVATAR 360 Fourier transform infrared spectra
spectrometer in KBr pellets. Diffuse reflectance spectra (DRS)
of samples were collected on finely ground samples by an
UV–vis spectrophotometer (PE lambda950) using BaSO4 as a
reference in the range of 200–700 nm and the reflection
spectra were converted to the absorbance spectra by the
Kubelka–Munk method. The photoluminescence (PL) spectra
were measured by a FLS-920T fluorescence spectrophotome-
ter with Xe 900 (450W xenon arc lamp) as the light source.
h-BN and amorphous BN. Because of its randomness and
partially disordered properties, t-BN does not have an accu-
rate crystal structure, and its lattice constants are usually
determined by reference to h-BN. In an X-ray diffraction
pattern, t-BN can be indexed on the basis of the diffraction
maxima of well-defined h-BN, although some reflections of
h-BN alter their shape and other reflections are missing. The
most characteristic changes caused by the layer disorder in
turbostratic relative to h-BN are seen in a broadening of the
(002) reflection at 2h = 26.76°, with the appearance of a less
intense, broad peak at 2h = 42.54° corresponding to the
two-dimensional (10) reflection [merged from the (100)
and (101) reflections for h-BN] and the disappearance of
higher indexed reflections (004) and (110)above 2h = 50°.
Figures 1(b)–(d) show XRD patterns of the samples prepared
after annealing at 1000°C, 1200°C, and 1400°C for 6 h in a
tube furnace with a N2 ambient, respectively. Figure 1(b) is
the XRD pattern of the product heated at 1000°C, which
suggests that there is a small quantity of impurity phase
associated with the h-BN. The attribution of the small peak
near 34° has not been clear now, but may be attributed to
the rhombohedral BN (r-BN) phases (104) line (Joint Com-
mittee on Powder Diffraction Standards [JCPDS] card
50-1504). Both h-BN and r-BN are layered materials differing
in the packing of hexagonal BN-networks only. Upon ther-
mal treatment at 1200°C and above this small peak
disappeared, and all the peaks can be assigned as those of
h-BN with the lattice parameters of a = 2.4979 A and
c = 6.6504 A (JCPDS card no. 34-0421). No other crystal-
line phases can be detected in the patterns, which indicates
that under current synthesis conditions the obtained samples
has pure hexagonal structure. The intense diffraction peaks
denote the high crystallinity of the h-BN obtained, as
Fig. 1(d) shown. The (002) peak is intense and sharp (full
width at half maximum [FWHM] ~0.48°], indicating that the
crystallite size is large in the c-axis direction, indicative of
well-stacked layer structures is in the products, which is
matched with that illustrated in the SEM image Fig. 2(b).
Figure 2 shows typical SEM images of the as-synthesized
samples by combustion [Fig. 2(a)] process and annealed sam-
ple at 1400°C for 6 h [Figs. 2(b,c)]. As Fig. 2(a) shows, a
large quantity of thin wall structures can be clearly observed
in the product synthesized by combustion process, but they
have no regular arrangement. After being heated at 1400°C
for 6 h, the samples grow into regular vertically aligned
nanoplates [Fig. 2(b)], though templates have not been used.
From Fig. 2(c), we find some crystalline facets of these
nanoplates forming ~120° with a neighboring facet which
resembles the in-plane hexagonal arrangement of atoms in
h-BN. The size of most as-obtained h-BN nanoplates is in
the range of 300–500 nm [Figs. 2(b,c)], and the thickness
could be roughly measured to be below 30 nm.
III. Results and Discussion
Figure 1 shows X-ray diffraction (XRD) patterns of the sam-
ple prepared by combustion synthesis process and those
annealed at 1000°C, 1200°C, 1400°C for 6 h in a tube fur-
nace with a N2 ambience, respectively. Figure 1(a) is the
XRD pattern of the sample prepared after combustion syn-
thesis process, which matches closely with that described in
the literature for turbostratic BN (t-BN).40–42 It means that
after combustion process the product has t-BN structure.
The t-BN is a partially disordered phase of BN, whose struc-
ture is formally analogous to that of turbostratic carbon
blacks and can be identified as intermediate between the
The TEM was used to further analyze the structures of
the obtained samples. Figure 3 shows the TEM images of
the h-BN nanoplates dispersion sample obtained using the
ultrasonic bath for 4 h in de-ionized water. Figure 3(a) is the
low-magnification TEM image of the samples. The h-BN
nanoplates are almost transparent indicative of their small
thickness, but their thicknesses cannot be accurately deduced
from the image. Notably the lateral sizes of the nanoplates
become smaller compared to pristine ones that are observed
by SEM [Figs. 2(b,c)] because the sonication is able to peel
and cut nanoparticles induced by interactions between sol-
vent molecules and sheets surfaces and by the ultrasonica-
tion-assisted hydrolysis.43 Figure 3(b) is the selected area
electron diffraction (SAED) pattern taken from the region in
Fig. 3(a). We find the diffraction spots form into five diffrac-
tion rings, which correspond to (002), (100), (101), (004),
and (110), which indicate that these nanoplates have high
crystallinity. Figures 3(c) and (d) show the high-resolution
TEM image (HR-TEM) and the enlarged image of the frag-
ment of the nanoplates. The incident electron beam was
Fig. 1. XRD patterns of the BN product by combustion synthesis
process ignited at 600°C in atmosphere for 10 min (a) and the samples
after being annealed in a N2 atmosphere for 6 h at 1000°C (b),
1200°C (c), and 1400°C (d).

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Vol. 94, No. 12
(a)
(b)
(c)
Fig. 2. SEM images of the sample fabricated by combustion synthesis process (a) and the sample annealed in a N2 atmosphere for 6 h at
1400°C (b, c).
(a)
(b)
(c)
(d)
Fig. 3. TEM images of the sample that annealed at 1400°C. Low-magnification image (a). Selected area electron diffraction pattern (b). High-
resolution TEM images of the sample at different magnifications (c, d).
along the [002] direction, thus perpendicular to the (002)
crystal face. It is well known that h-BN is structurally simi-
lar to graphite in nature, except for the different stacking
sequences of atomic planes. Highly crystallized graphite pos-
sesses a Bernal (AB) stacking sequence, while h-BN is
stacked with the B atoms above and beneath the correspond-
ing N atoms. From Fig. 3(c) highly ordered alternating dots
(lattice fringes) can be clearly observed. The periodicity
between each two neighboring white dots is 0.22 nm. This
value is equal to the (100) lattice constant of h-BN, which is
consistent with that measured from the XRD pattern
(Fig. 1). The hexagonal atomic arrangement of the nano-
plates can be seen from the HR-TEM image in Fig. 3(d), the
distance between two neighboring dots in the hexagon
reveals that the spacing between B and N atoms is ~1.43 A,
which is quite close to the well-known length of a B–N bond
of 1.44 A.
Raman and FTIR spectroscopy, in which phonons are
excited by inelastic scattering of light or light absorption,
respectively, are convenient tools to investigate the phonon
features and phase purity of samples. Figure 4 shows the
high-frequency Raman spectrum of the products. As it is
known, only two characteristic Raman active B–N vibrational
modes, the in-phase E2g mode (50 cmÀ1) and counter-phase
E2g mode (1367 cmÀ1), exist for h-BN.44 The low-frequency
rigid-layer mode in any of the prepared samples is not obser-
vable due to strong electronic scattering and a broad lumines-
cence background. As shown in Fig. 4, a dominant sharp
Raman peak at 1369cmÀ1 has been found, which corresponds
to counter-phase BN vibrational mode (E2g) within BN sheets.
The peak of as-prepared h-BN nanoplates is up-shifted about
2 cmÀ1 compared with the well-known one (1367 cmÀ1) of
bulky h-BN. The FWHM of the peak is ~20cmÀ1, which is
wider than that observed in h-BN single crystals obtained by

December 2011
Characterization of Hexagonal Boron Nitride Nanoplates
4499
Fig. 4. Raman spectrum of the sample annealed at 1400°C.
Fig. 6. UV–vis spectrum of as-prepared sample annealed at 1400°C.
Fig. 5. FTIR spectrum of the sample annealed at 1400°C.
high-pressure and high-temperature synthesis (9.1 cmÀ1)5 and
that observed in pure BNNTs (13 cmÀ1).17 An up-shift and
broadening of the E2g mode are typical for small h-BN crystal-
lites.44 However, this value is smaller than that of the pyrolytic
Fig. 7. Excitation PL spectrum (kem = 406 nm) of the sample
annealed at 1400°C.
BN (~30 cmÀ1 45 and the BNNTs (27 cmÀ1) synthesized at
)
1200°C46; and is close to the BN nanosheets prepared by
microwave plasma chemical vapor deposition (MPCVD)
(~19 cmÀ1).34 It suggests that quality of the nanoplates from
this simple route is relatively good.
More information of the lattice vibrations and composi-
tion can be gained through FTIR characterization. Figure 5
shows the typical FTIR spectrum of the products, in which
two strong characteristic peaks positioned at 1378 and
811 cmÀ1 are observed. The broad peak at 1378 cmÀ1 is
ascribed to the E1u (B–N stretching vibration mode perpen-
dicular to the c-axis) modes of h-BN; while the absorption
band of the sharper, weaker peak at 811 cmÀ1 could be
attributed to the A2u (B–N–B bending vibration mode paral-
lel to the c-axis).47 The peak around 3428 cmÀ1 can be
attributed to the water molecules absorbed on the surface of
the samples. No obvious absorption peaks associated with
the starting materials and carbon are observed.
To investigate the optical properties of the products for
potential applications in UV–blue light emitters, UV detec-
tor, etc., the UV–vis absorption and PL spectra of the sam-
ples have been measured, shown in Figs. 6, 7 and 8. From
UV–vis absorption spectrum (Fig. 6), three absorption lines
are observed, which are approximately located at 204, 225,
and 278 nm. The absorption line centered at 204 nm corre-
sponds to a band gap of ~6.07 eV for these h-BN nano-
plates. This energy is smaller than the reported one
(centered at 6.15 eV) for h-BN micron sized crystals.7 The
low-energy lines at ~225 nm (~5.50 eV) and 278 nm
(4.45 eV) have been observed in single-walled7 and multi-
walled BNNTs48,49 and were further assigned, respectively,
to impurity-levels and near-edge excitonic absorption. These
two lines were not present in the optical absorption spectra
Fig. 8. Emission PL spectrum (kex = 313 nm) of the sample
annealed at 1400°C. The inset shows the phonon replica energies
plotted against the number of phonons emitted.
of bulk h-BN materials and, therefore, were attributed to
the rolling up of h-BN sheets.7 The presence of these two
lines in our work indicates that optical absorption transi-
tions at 4.45 and 5.5 eV are not intrinsic characteristic of
BN-nanotubes, single-wall or multi-wall. Though the nature
of these two peaks is not clear, until now, one preferred sug-
gestion is the presence of defects in the BN-network, such as
vacancies and impurities.50 Based on first-principal quantum-
chemical calculations on the stability and electronic structure
of self-interstitials and vacancies in h-BN structures, the

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Vol. 94, No. 12
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in the optical absorption spectrum of the samples is indica-
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Figures 7 and 8 show representative example of excitation
and emission spectra measured for the synthesized h-BN
nanoplates at room temperature. The excitation spectrum
contains seven UV peaks, which are divided into three sets
named A, B, and C. Excitation set B, which at 289 (4.29 eV),
301 (4.12 eV), and 313 (3.96 eV) nm, could be assigned to
the deep-level transitions of BOÀ ion from the ground state
(1∑+) to the first excited state (2∑+); excitation set A at
267 nm (4.64 eV) might be related to the second excited state
of BOÀ ion.53 It is still difficult to identify the origin of the
peaks of set C at 345 (3.59 eV), 362 (3.42 eV), and 380
(3.26 eV) nm. One possibility is that the excitations are
caused by B or N vacancy-type defect-trapped states.18,29 As
shown in Fig. 8, it displays intense emission between 400 and
475 nm, and tails off toward low energy, when excited at
313 nm, the maximum excitation. The broad emission band
is structured and composed of three peaks centered at 406
(3.05 eV), 430 (2.88 eV), and 459 (2.70 eV) nm, respectively.
The spacing among the three observed peaks is found regu-
lar. We therefore attribute the peaks at 430 (2.88 eV) and
459 (2.70 eV) nm to the phonon replicas of the band at
406 nm (3.05 eV), with respective 1 and 2 emitted phonons.
The plot of the energy of each peak against the number of
phonons is given in the inset of Fig. 8. The slope of the lin-
ear dependence is 0.17 eV (1370 cmÀ1) per phonon, which is
in good agreement with the B–N E2g vibrational mode pho-
non frequency (1369 cmÀ1) of the h-BN nanoplates measured
by Raman spectroscopy as shown in Fig. 4. The origin of
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IV. Conclusion
Large quantities of h-BN nanoplates with hexagonal
morphologies have been successfully prepared by a facile
template- and catalyst- free two-step method, which was car-
ried out at comparatively low-temperature and under normal
pressure conditions. The XRD patterns, Raman, and FTIR
spectra show that the products are pure and have high crys-
tallinity. From SEM images, the synthesized nanoplates have
diameters of 300–500 nm and the thickness is below 30 nm.
Optical properties have been systematically investigated via
UV–vis absorption spectrometry and PL spectrometry. The
UV–vis absorption spectrum indicates that the samples have
a band gap of ~6.07 eV; the presence of the two low-energy
lines at ~225 nm (~5.50 eV) and at 278 nm (4.45 eV) in our
work indicates that optical absorption transitions at 4.45 and
5.5 eV are not intrinsic characteristics of BN-nanotubes, sin-
gle-wall or multi-wall. Strong violet–blue PL emission with a
broad band ranging from 400 to 475 nm has been observed
in PL spectrum, indicating that the h-BN nanoplates
as-grown by this simple route are highly promising for appli-
cation in nanosize optical devices (LEDs, blue-light source,
UV detector, etc.). The phonon replica features are obvious
in the photoluminescence spectroscopy.
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