Simple synthesis of mesoporous boron nitride with strong cathodoluminescence emission

Article abstract of DOI:10.1016/j.jssc.2011.01.040

Mesoporous BN was prepared at 550 °C for 10 h or so via a simple reaction between NaBH₄ and CO(NH₂)₂. X-ray diffraction demonstrates the formation of t-BN with lattice constants a=2.46 and c=6.67 A. High-resolution transmi

Full text of DOI:10.1016/j.jssc.2011.01.040

Journal of Solid State Chemistry 184 (2011) 859–862
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Simple synthesis of mesoporous boron nitride with strong
cathodoluminescence emission
Xiang-Lin Meng, Ning Lun, Yong-Xin Qi, Hui-Ling Zhu, Fu-Dong Han, Long-Wei Yin,
n
n
Run-Hua Fan, Yu-Jun Bai , Jian-Qiang Bi
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Mesoporous BN was prepared at 550 1C for 10 h or so via a simple reaction between NaBH4 and
Received 8 September 2010
Received in revised form
CO(NH
2
)
˚
2
. X-ray diffraction demonstrates the formation of t-BN with lattice constants a¼2.46 and
c¼6.67 A. High-resolution transmission electron microscopy displays a lot of porous films in the
2
2 January 2011
2
ꢀ1
product, which possesses a high surface area of 219 m g and a pore size primarily around 3.8 nm
Accepted 31 January 2011
Available online 20 February 2011
tested by nitrogen adsorption–desorption method. The mesoporous BN exhibits a strong luminescence
emission around 3.41 eV in the cathodoluminescence spectra, a high stability in both morphology and
structure, and good oxidation resistance up to 800 1C. The byproducts generated during the reaction are
responsible for the formation of the mesoporous BN.
Keywords:
Boron nitride
Nanostructure
&
2011 Elsevier Inc. All rights reserved.
Density functional theory
1
. Introduction
carbon and carbon molecular sieves as templates, respectively.
Mesoporous BN was synthesized at temperatures above 1150 1C
using mesoporous carbon [1,16] or silica as templates [21].
Though the porous BN obtained by the templating method
possesses relatively high surface areas, the high reaction tem-
perature and intricate processing are the main drawbacks that
embarrass the mass production of the porous BN with low cost.
Pyrolysis of borane-based molecular precursors has also been
adopted to fabricate porous BN. Borek et al. [4] produced porous
BN by vacuum pyrolysis of poly (4, 6-borazinylamine) at
600–1200 1C. Lindquist et al. [22] fabricated BN aerogels which
exhibited excellent thermal stability even when heated to
1500 1C in argon atmosphere. Wood and Paine [23] synthesized
hollow spherical BN, and spherical BN powders [24] beyond
800 1C. BN foams were prepared by thermolyzing molecular
precursors up to 1800 1C [19] or from borane hydrazine by self-
propagating high-temperature synthesis [25]. Several other
approaches have also been reported. Lin et al. [26] synthesized
Porous materials are extensively used as catalyst supports, gas
adsorbents, sensors, for chemical purification and separation due
to their high surface area and high porosity [1–9]. Traditional
2 3 2
porous oxide supports such as Al O , SiO , and zeolite have been
widely utilized, whereas the low thermal conductivity and high
sensibility to moisture may sometimes result in some disadvan-
tages, such as causing severe metal sintering on the hot spots, and
making the catalyst be covered with water at low tempera-
ture [10–13]. Activated carbon is another economical porous solid
that has been widely employed [14]. However, carbon materials
can be easily oxidized in air atmosphere above 300 1C [15],
restricting their applications under oxidative atmosphere at high
temperatures.
Boron nitride (BN) has some unique properties, such as high
thermal conductivity, hydrophobility, good optical properties,
excellent resistance to chemical attack and high stability towards
oxidation [4,5,12,15], making porous BN particularly applicable as
gas absorbents [4,5], and catalyst supports at high temperatures
under harsh conditions [6,7,16,17]. Recently, the preparation of
porous BN has received growing attention. Templating method is
so far the most common route to fabricate porous BN using
carbon or silica as templates. Disordered porous BN [18] and
ordered BN mesostructure [19,20] were obtained using activated
2
ꢀ1
h-BN powders with a surface area of 152 m g at 1000 1C using
KBH , NH Cl and KCl as reagents. In view of the present
4
4
researches, it is of great significance to explore some simple
approaches to the porous BN with high surface areas at relatively
low temperatures.
As has been known, BN is a promising material for optoelec-
tronic applications. The first observation of an intense far-ultra-
violet (UV) excition emission enables BN to be employed in new
light-emitting devices [27,28], and thus has attracted increasing
interest in recent years. Cathodoluminescence (CL) spectroscopy
is an effective method to measure the optical property of BN with
n
Corresponding author. Fax: +86 531 88392315.
E-mail addresses: byj97@126.com (Y.-J. Bai), bjq1969@163.com (J.-Q. Bi).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.01.040

8
60
X.-L. Meng et al. / Journal of Solid State Chemistry 184 (2011) 859–862
various morphologies. The spectra of BN nanosheets were
reported to be between 3.79 and 3.96 eV owing to deep-level
emissions associated with defect-related centers [29]. The CL
spectra of BN nanoribbons are centered at 5.33 eV [30]. For BN
nanotubes, Zhi et al. [31] reported a strong CL peak located
around 3.3 eV and a weak peak at 4.1 eV resulted from the
defects, or B and N vacancies. Han et al. [32] also reported the
CL spectra of BN nanotubes to be between 3.0 and 4.2 eV.
However, to the best of our knowledge, it is presently almost
unknown about the CL property of porous BN.
In this work, we reported a convenient route to synthesize
mesoporous BN at 550 1C or so in the absence of additional
templates using the commonly available cheap reagents of urea
2 2 4
(CO(NH ) ) and sodium borohydride (NaBH ) as raw materials.
The mesoporous BN obtained exhibits a large specific surface area
2
ꢀ1
of 219 m g , a strong luminescence emission around 3.41 eV, a
high stability in both morphology and structure, and excellent
oxidation resistance up to 800 1C.
Fig. 1. XRD pattern of the resulting product.
2
. Experimental
referring to h-BN. The lattice constants of the t-BN obtained are
˚
˚
a¼2.46 A and c¼6.67 A, which agree well with the values
reported in literature [33].
Analytically pure CO(NH
Reagent No. 1 Plant, Tianjin, PR China) and NaBH
2
)
2
(produced by Tianjin Chemical
(produced by
4
A Bruker Vertex 70 spectrometer was used to obtain Fourier
transformation infrared spectra of the resulting products (Fig. S1
in supporting information). Two strong characteristic absorption
Zhanyun Chemical Co. Ltd., Shanghai, PR China) were used as
starting materials. In a typical procedure, 1.53 g (0.041 mol)
ꢀ
1
NaBH
4
and 1.54 g (0.026 mol) CO(NH
2
)
2
were mixed and put into
peaks at 1379 and 800 cm
formation of t-BN [34,37].
give further confirmation for the
a stainless steel autoclave with a capacity of 30 mL. Then the
autoclave was sealed and heated at a rate of 20 1C/min in furnace
to 550 1C and maintained for 10 h. When the autoclave was
cooled to ambient temperature naturally, the products in the
autoclave were collected and washed successively with anhy-
drous ethanol, dilute hydrochloric acid and distilled water several
times to eliminate the residual reagents and byproducts. The final
product was dried at 60 1C for 10 h and white powders were
ultimately obtained. The yield of the resulting product was about
The morphology of the products was investigated by HRTEM,
as shown in Fig. 2. Thin films about 20–30 nm in thickness and
2
0.4–0.6
mm
in areas are the dominant morphology. A mass of
disordered pores with diameters of 3–5 nm can be observed
throughout the films. The inset in Fig. 2a is the lattice fringe
image of one piece of the films, the average distance between the
neighboring fringes is about 0.34 nm which is consistent with the
spacing of (0 0 2) plane of t-BN. In order to understand the
stability of the porous BN, the as-prepared product was heated
in air to 700 1C and held at this temperature for 1.5 h. Fig. 2b
displays the HRTEM image of the heat-treated product, from
which the porous structure is almost the same as that in Fig. 2a,
and the lattice spacing is still about 0.34 nm, confirming the high
stability of the porous BN in both morphology and structure. The
structure stability was also supported by XRD characterization,
which is shown in Fig. S2 in supporting information.
The luminescence property of the porous BN was examined by
FESEM equipped with a CL spectrophotometer. Fig. 3 exhibits the
FESEM image of the product and the corresponding room tem-
perature CL spectrum. The porous BN displays a strong CL
behavior in UV range. The broad emission peaks around 364
and 522 nm in the spectrum correspond to the energy of 3.41 and
2.38 eV, respectively. It is obvious that the emissions are greatly
different from those resulted from pure h-BN single crystal with
perfect crystallinity (215 nm) [27,28], but are similar to those
from BN nanosheets (3.79–3.96 eV) [29], and BN nanotubes
6
5 wt% based on the starting material of NaBH
X-ray powder diffraction (XRD) patterns were obtained on a
Rigaku Dmax-rc diffractometer with Ni-filtered Cu K radiation
V¼50 kV, and I¼80 mA) at a scanning rate of 41/min. The
4
.
a
(
morphology of the products was examined using a Tecnai 20U-
Twin high-resolution TEM (HRTEM) with a point-to-point resolu-
tion of 0.19 nm operating at 200 kV. A CL spectrophotometer
attached to a SU-70 type thermal field emission scanning electron
microscope (FESEM) was used to investigate the optical proper-
ties of the samples.
Nitrogen adsorption–desorption isotherms were carried out at
7
7.3 K on a Quadrasorb SI sorption analyzer. The samples were
outgassed for 8 h at 300 1C under a vacuum in the degas port of
the analyzer. The specific surface area was calculated with the
Brunauer–Emmett–Teller (BET) model, and pore size distribution
was calculated from the adsorption–desorption data using the
Density Functional Theory (DFT) method.
(
3.0–4.2 eV) [31,32]. Comparing with the emission of pure h-BN
3
. Results and discussion
single crystal, the shift of emission could be attributed to the
significant difference in crystallinity, of course, other factors such
as impurities, defects, or B and N vacancies are also inevita-
ble [31,38]. For the porous BN, the CL peaks around 3.41–2.38 eV
could also be ascribed to the deep-level emissions associated with
the defect-related centers (B or N vacancy type defect trapped
states [29,38,39]), because more defects may occur in the porous
structure with large surface areas.
Fig. 1 shows the XRD pattern of the resulting product obtained
from the reaction of NaBH
which is in good agreement with those described in literature for
t-BN [33–36]. The diffraction peak at 2
¼26.71 can be indexed to
the (0 0 2) plane, indicative of the presence of layered structure in
the t-BN. The peak at 2
¼42.41 corresponds to the (10) plane
unresolved (1 0 0) and (1 0 1) planes for h-BN). Owing to the
4 2 2
and CO(NH ) at 550 1C for 10 h,
y
y
(
The BET surface area and pore-size distribution of the porous
BN were characterized by nitrogen adsorption–desorption iso-
therms, as shown in Fig. 4. The isotherms exhibit a hysteresis loop
partially disordered lattice, t-BN does not have an accurate crystal
structure, and its lattice constants are usually determined by

X.-L. Meng et al. / Journal of Solid State Chemistry 184 (2011) 859–862
861
Fig. 3. FESEM image (a), the corresponding CL image (b) and CL spectrum (c) of
the porous BN.
Fig. 2. HRTEM images of the as-prepared product (a) and the product heat-treated
in air at 700 1C for 1.5 h (b). The insets in (a) and (b) are the lattice fringe images of
the corresponding products.
at a relative pressure between 0.45 and 0.95, suggesting the
formation of mesoporous BN [40]. The surface area and pore
2
ꢀ1
capacity of the porous BN calculated from Fig. 4a are 219 m g
3
ꢀ1
and 0.32 cm g , respectively. From Fig. 4b, the pore-size dis-
tribution is dominant between 1 and 5 nm with an average pore
size of 3.8 nm.
For better understanding the thermal stability and oxidation
resistance of the mesoporous BN, combined DTA–TGA analysis
was conducted in a SDT Q600 thermal-microbalance apparatus
(TA Instruments Ltd., New Castle, Delaware) at a heating rate of
1
0 1C/min in air atmosphere using Al pans. The TGA–DTA
2 3
O
curves between room temperature and 1200 1C (Fig. S3 in
supporting information) demonstrate that the mesoporous BN
has good thermal stability and oxidation resistance up to 800 1C.
Associated with the morphology depicted in Fig. 2c, besides the
high stability in morphology and structure, the mesoporous BN is
oxidation resistant up to 800 1C, and thus can be utilized as gas
absorbents and catalyst supports at high temperatures under
harsh conditions.
To probe into the effect of reaction temperature and time on
the formation of the mesoporous BN, a series of experiments were
conducted. When the temperature is lower than 450 1C, the
Fig. 4. Nitrogen adsorption–desorption isotherms (a) and DFT pore-size distribu-
tion curve (b) determined from the nitrogen desorption isotherm of the
mesoporous BN.
reaction between NaBH
4 2 2
and CO(NH ) cannot occur, and no BN
can be achieved. When the temperature is above 600 1C, the

8
62
X.-L. Meng et al. / Journal of Solid State Chemistry 184 (2011) 859–862
porous structure hardly changes both in yield and in morphology.
If the holding time is less than 6 h at 550 1C, the reaction is
incomplete and the crystallinity of BN is poorer. The reaction time
over 10 h does not significantly influence the crystallization and
the yield of the mesoporous BN.
According to the XRD pattern of the as-prepared product
without any washing treatments (Fig. S4 in supporting informa-
tion), besides BN, NaCN is also present in the product (Caution:
NaCN is toxic and should be carefully processed). In addition,
when the autoclaves were opened after the reaction, some gases
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In summary, mesoporous BN could be synthesized by the
[
[
simple reaction between NaBH
without using additional templates. The mesoporous structure
with a large surface area of 219 m g
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emission around 3.41 eV in the CL spectra, a high thermal stability
in both morphology and structure, and good oxidation resistance
4 2 2
and CO(NH ) at 550 1C for 10 h
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Shandong Province (Nos. 2009GG10003001, 2009GG10003003),
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Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2011.01.040.