Synthesis, characterization and properties of novel BN nanocages from a single-source precursor

Article abstract of DOI:10.1016/j.cplett.2005.02.041

In this Letter, we report the synthesis of hexagonal crystalline BN powders with novel cage-like structures from a single-source precursor NH ₄BF₄ at 400-600 °C for 12 h. The obtained samples are characterized by XRD, XPS, TEM and HRTEM. Experiments reveal that the as-prepared BN powders exist in multiwall nanocages with dimensions from about 20 to 200 nm and have very good anti-oxidation properties at high temperatures.

Full text of DOI:10.1016/j.cplett.2005.02.041

Chemical Physics Letters 405 (2005) 229–233
www.elsevier.com/locate/cplett
Synthesis, characterization and properties of novel BN
nanocages from a single-source precursor
a,b
Zeheng Yang , Liang Shi , Luyang Chen , Yunle Gu , Peijun Cai ,
a
a
a
a
a
Aiwu Zhao , Yitai Qian
a,
*
a
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China
b
School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, China
Received 9 December 2004; in final form 5 January 2005
Abstract
In this Letter, we report the synthesis of hexagonal crystalline BN powders with novel cage-like structures from a single-source
precursor NH BF at 400–600 °C for 12 h. The obtained samples are characterized by XRD, XPS, TEM and HRTEM. Experiments
reveal that the as-prepared BN powders exist in multiwall nanocages with dimensions from about 20 to 200 nm and have very good
4
4
anti-oxidation properties at high temperatures.
Ó 2005 Elsevier B.V. All rights reserved.
1
. Introduction
have been reported for the preparation of boron nitride,
such as arc discharge [7], electron-beam irradiation [8],
chemical vapor deposition (CVD) [9], laser ablation
[10], pyrolysis method [11,12] and some other methods
[13–15]. Up to now, besides particles, BN powders with
tube-like [7,15], sphere-like [16], wire-like [17], cocoon-
like [18], tassel-like and tree-like shapes [19] have been
achieved.
Boron nitride (BN) is a structural analog of carbon,
existing in cubic (diamond-like), hexagonal (graphite-
like), turbostratic, and amorphous forms [1]. These
structure characteristics offer BN various properties,
such as supreme hardness (for cubic BN), low density
(
for hexagonal BN), high temperature stability, high
thermal conductivity, high melting point, low dielectric
constant and chemical inertness. Boron nitride has
found wide applications in many areas. For instance,
it can be used as cutting tools, grinding abrasive, refrac-
tory, lubricants, electronic devices, electrical insulators
and so on [2–4].
Since the discoveries of carbon fullerenes and nano-
tubes [5,6], boron nitride has attracted intensive interests
of researchers. Based on its structural similarity to car-
bon, BN was expected to have fullerene and nanotube
morphologies, too. In fact, various synthesis methods
Inspired by the previous researches [20,21], in this
Letter, we report a simple single-source precursor ap-
proach to synthesize hexagonal crystalline BN powders
with novel cage-like structures. The reaction was carried
out in an autoclave via the pyrolysis of NH BF in
4
4
metallic Na flux in a temperature range of 400–600 °C.
Compared to the previous methods, this simple route
has the following advantages: (1) both B and N sources
from a single cheap safe precursor NH BF ; (2) without
4
4
using rigorous synthesis conditions or complicated
apparatus or air-sensitive or toxic reagents (BBr3,
Li N, NaN , NaNH , etc.). The obtained BN nanocages
3
3
2
*
exhibit very good anti-oxidation properties at high
temperatures.
Corresponding author. Fax: +86 551 3607402.
E-mail address: zehengy@mail.ustc.edu.cn (Y. Qian).
0
009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.02.041

230
Z. Yang et al. / Chemical Physics Letters 405 (2005) 229–233
2
. Experimental
The reagents NH BF and metallic Na were of ana-
4
4
lytical purity from Shanghai Chemicals Company. In a
typical procedure, 3.0 g NH BF and 3.0 g granular
4
4
c
metallic Na (excessive) were put in a stainless steel auto-
clave of 50 mL capacity. The autoclave was sealed and
heated in an electric furnace at 400–600 °C for 12 h, then
cooled to room temperature naturally. The product was
washed first with absolute alcohol to remove the exces-
sive Na, then, rinsed with distilled water to remove the
residual NaF and some other impurities. After being
dried in vacuum at 70 °C for 4 h, a white powder prod-
uct was obtained.
b
a
10
20
30
40
50
60
70
80
2
θ (degrees)
X-ray powder diffraction (XRD) patterns were re-
corded on a Japan Rigaku Dmax-cA X-ray diffracto-
meter with Cu Ka radiation (k = 1.54178 A). The
Fig. 1. XRD patterns of the prepared samples. (a) Sample A: prepared
at 400 °C, 12 h; (b) Sample B: prepared at 500 °C, 12 h; (c) Sample C:
prepared at 600 °C, 12 h.
˚
transmission electron microscopy (TEM) images were
taken with a Hitachi H-800 transmission electron micro-
scope. The high-resolution transmission electron micros-
copy (HRTEM) observation and the selected area
electron diffraction (SAED) pattern were performed
with a 200 kV electron microscope on JEOL-2010. X-
ray photoelectron spectroscopy (XPS) measurements
were performed on a VGESCALAB MKII X-ray pho-
toelectron spectrometer with an exciting source of Mg
Ka = 1253.6 eV. Thermogravimetric analysis (TGA)
was carried on a Shimadzu TGA-50H thermogravimeter
analyzer and the sample was heated from room temper-
ature to 1000 °C at a rate of 10 °C/min in a steady flow
of air.
Fig. 2 displays the TEM and HRTEM morphologies
of sample C, which was prepared at 600 °C for 12 h. It
can be seen that the obtained sample exists in nano-
cages (Fig. 2a). Most of the cages are box-like, which,
to our knowledge, has not been reported for BN yet
and some are hollow sphere-like. The dimensions of
the cages range from about 20 to 200 nm and the thick-
ness of the wall range from about 4 to 10 nm. Fig. 2b is
the high-resolution TEM images of sample C. The cor-
responding selected area electron diffraction (SAED)
patterns of sample C (inset in Fig. 2b) indicate that
the product is crystallized. From the patterns, the
reflections of (002) and (101) of hexagonal BN crystal-
lites can be observed. Fig. 2c is the HRTEM images of
the box-like nanocage (marked by arrow 1 in Fig. 2b).
Fig. 2d shows the HRTEM images of two connected
cages sharing the same wall (marked by arrow 2 in
Fig. 2b), which is quite common in the TEM image
(see Fig. 2a) and probably happens when the two cages
grow side by side. The walls of the nanocages consist of
12–16 layers, but the wall shared by two cages may
have more layers. The fringe spacing is about 0.33
nm, which is close to the interplanar spacing of the
(002) lattice planes of h-BN.
3
. Results and discussion
The composition and crystallinity of the as-synthe-
sized BN samples were examined by XRD. The XRD
patterns of the obtained products are shown in Fig. 1.
Fig. 1a–c are the XRD patterns of samples A–C, which
were prepared at 400, 500 and 600 °C, respectively, for
1
2 h. It can be seen that with increasing reaction temper-
ature the crystallization of the BN samples becomes bet-
ter. At 400 and 500 °C, the (100) and (101) peaks still
overlap (see Fig. 1a,b), indicating the existence of an
incompletely ordered h-BN structure [22]. The (100)
and (101) peaks begin to split at 600 °C (see Fig. 1c).
All the reflections of sample C can be indexed as hexag-
Fig. 3 gives the XPS spectra of Sample C. The C1s
line of carbon contamination appears at 284.75 eV
and all binding energies are corrected accordingly. The
survey spectrum of sample C (Fig. 3a) indicates the pres-
ence of B and N elements, and there exists a small
amount of impurity elements such as C and O due to
the adsorption of CO , H O and O on the surface of
˚
onal BN with lattice constants of a = 2.5022 A and
˚
c = 6.6675 A, which are close to the reported values of
˚
˚
h-BN (a = 2.50399 A, c = 6.6612 A, JCPDS card, No.
3-2095). No noticeable peaks of impurities are ob-
7
2
2
2
served in the XRD patterns. From the XRD patterns,
the prominence of the (002) plane can be observed,
which may indicates the existence of well-stacked lay-
ered structures in the hexagonal BN samples [23,24].
the sample. The binding energies centered at 398.30 eV
for N1s (Fig. 3b) and 190.70 eV for B1s (Fig. 3c) are
in good agreement with the reported values of bulk
BN [25]. Quantification of B1s and N1s peaks gives

Z. Yang et al. / Chemical Physics Letters 405 (2005) 229–233
231
Fig. 2. TEM and HRTEM images of sample C. (a) TEM images of nanocages of sample C: prepared at 600 °C, 12 h. (b) High-resolution TEM
images of sample C. Inset: the corresponding SAED pattern of the h-BN powder. (c) HRTEM images of the box-like nanocage (marked by arrow 1
in Fig. 2b). (d) HRTEM images of two connected cages sharing the same wall (marked by arrow 2 in Fig. 2b).
average B:N atomic ratio of 0.998:1, which closely
agrees with the stoichiometric composition of BN.
The oxidation behavior of the BN sample (sample C)
was investigated at the temperatures below 1000 °C by
TGA, as shown in Fig. 4. Below 700 °C, the weight gain
almost has not changed, which proves that the BN sam-
ple has very good anti-oxidation properties at high tem-
peratures. As the temperature keeps on rising from 700
to 1000 °C, there is an obvious weight gain during the
process, indicating that the BN sample is oxidized into
B O and nitrogen oxides. Nitrogen oxides are carried
The freshly produced BF and NH will react with
3 3
each other to form BN. In metallic Na flux, the process
can be expressed in following equations:
BF þ NH þ HF þ 4Na ¼ BN þ 4NaF þ 2H
ð2Þ
ð3Þ
3
3
2
In total; NH
4
BF
4
þ 4Na ¼ BN þ 4NaF þ 2H
2
The formation of cubic NaF was confirmed by the
XRD analysis of the intermediate product (omitted
here), which did not undergo the washing with absolute
alcohol and distilled water at all.
2
3
In the process, the use of metallic Na is in favor of
forming BN at lower temperature, because the reaction
of HF with Na is thermodynamically spontaneous and
exothermic and the reaction heat could produce higher
local reaction temperatures. For comparison, without
use of Na, we heated NH BF in the autoclave at
away by air. The weight gain from 700 to 1000 °C is
1.69%, which is less than the theoretical weight gain
40.31%) from BN to B O . It can be explained that
3
(
2
3
around 1000 °C B O begins to evaporate [26], which
2
3
counteracts the weight gain from the oxidation of BN.
The excellent anti-oxidation properties will make BN
nanocage powders particularly useful for high tempera-
ture applications.
4
4
6
00 °C for 12 h and no BN was obtained.
As to the formation of the special BN nanocage struc-
tures, we think that boron nitride with hexagonal struc-
ture is similar to graphite and the cell parameters of
As to the reaction mechanism, we consider that at the
reaction temperature, NH BF firstly decomposes into
BF , NH and HF.
4
4
˚
h-BN (a = 2.5022 A and c = 6.6675 A) are close to those
˚
3
3
˚
of graphite (a = 2.464 A, c = 6.708 A). Like graphite, it
˚
2
tends to form sp -like bonded layer-structure. According
NH BF ¼ BF þ NH þ HF
ð1Þ
4
4
3
3

232
Z. Yang et al. / Chemical Physics Letters 405 (2005) 229–233
3
3
2
2
1
1
5000
0000
5000
0000
5000
0000
35
(
a)
N1s
N(KVV)
30
25
O1s
O(KLL)
2
1
1
0
5
0
5
0
5
B1s
200
C1s
5
000
0
0
400
600
800
1000
-
Binding Energy (eV)
0
200
400
600
˚
800
1000
Temperature ( C)
(
b)
3
3
2
2
1
1
500
000
500
000
500
000
N1s
Fig. 4. TGA curve of the h-BN sample prepared at 600 °C, 12 h.
On the other hand, we speculate that during the reaction
process, cubic NaF crystals form first, which exist in
cube-like solids (NaF melting point: 995 °C [28]) in the
system and probably play the role of templates. Later,
h-BN layers form outside the NaF cube-like templates
to yield a core-shell structure. Through the post-
treatment of washing, the NaF solids in the core part
are dissolved and BN nanocages are obtained. With
the aid of the NaF cube-like templates, the novel BN
nanocages with nearly 90° bend box-like structure arise.
However, further investigation is necessary to under-
stand the exact mechanism of forming the box-like BN
nanocages.
5
00
0
3
92
396
400
404
408
412
Binding Energy(eV)
(
c)
B1s
1
000
800
4
. Conclusions
600
Box-like BN nanocages were successfully prepared
400
via a single-source precursor reaction in an autoclave
in the temperature range of 400–600 °C. The sizes of
the cages range from about 20 to 200 nm with multiwalls
and the thickness of the wall range from about 4 to 10
nm. The as-prepared BN sample displays excellent
anti-oxidation properties at high temperatures. So they
might be advantageous to be used as high temperature
materials as well as to find other potential applications.
In addition, this simple single-source precursor ap-
proach might be extended to synthesis of other
compounds.
2
00
0
184
188
192
196
200
Binding Energy (eV)
Fig. 3. XPS spectra of the h-BN sample prepared at 600 °C, 12 h: (a)
survey spectrum; (b) N1s region; (c) B1s region.
to Terrones and co-workers [27], an ordered arrange-
ment of some specific defects with a hexagonal lattice
would lead to closed cage structures. In this work, the
high temperature and high pressure coming from H in
2
Acknowledgements
the sealed autoclave may provide a kind of metastable
growth condition under which specific topological de-
fects would form. Therefore, the lamellar h-BN struc-
tures tend to close up to minimize their surface energy.
Financial supports from National Natural Science
Foundation of China and the 973 Projects of China
are greatly appreciated.

Z. Yang et al. / Chemical Physics Letters 405 (2005) 229–233
233
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