A low-temperature coreduction route to boron nitride flakes and hollow spheres

Article abstract of DOI:10.1246/cl.2004.144

Boron nitride flakes and hollow spheres were synthesized by coreduction of NH₄Cl and BBr₃ using sodium as reductant. X-ray powder diffraction (XRD) pattern could be indexed as hexagonal BN. The X-ray photoelectron spectra (XPS) were used to determine the composition ratio, which is B:N = 1:1.05. The transmission electron microscopy (TEM) images showed flake-like and hollow spherical morphology.

Full text of DOI:10.1246/cl.2004.144

144
Chemistry Letters Vol.33, No.2 (2004)
A Low-temperature Coreduction Route to Boron Nitride Flakes and Hollow Spheres
Luyang Chen, Yunle Gu, Liang Shi, Zeheng Yang, Jianhua Ma, and Yitai Qianꢀ
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China,
Hefei, Anhui 230026, P. R. China
(Received November 12, 2003; CL-031087)
Boron nitride flakes and hollow spheres were synthesized by
(TEM) images taken with a Hitachi H-800 transmission electron
microscope. X-ray photoelectron spectra (XPS) were recorded
on a VGESCALAB MKII X-ray photoelectron spectrometer, us-
ing nonmonochromatized Mg Kꢁ X-rays as the excitation
source. Fourier transformation infrared spectroscopy (FTIR)
spectra were obtained using a Shimadzu IR-400 spectrometer
by using pressed KBr disks.
coreduction of NH4Cl and BBr3 using sodium as reductant. X-
ray powder diffraction (XRD) pattern could be indexed as hex-
agonal BN. The X-ray photoelectron spectra (XPS) were used
to determine the composition ratio, which is B:N = 1:1.05.
The transmission electron microscopy (TEM) images showed
flake-like and hollow spherical morphology.
The XRD pattern of the as-prepared BN sample is shown
Figure 1. All of the four peaks at d spacings of 3.354, 2.177,
ꢀ
1.671, 1.252 A can be indexed as hexagonal BN ((002), (100),
There has been growing interest in materials with specific
morphologies because of the expectation of novel properties in
recently years.1,2 The discoveries of fullerene, nanotubes, hollow
spheres as new form of matter in the nanoscale range have
opened a new challenging field in solid state physics, chemistry,
and materials science owing to their potential applications.1,3
Boron nitride (BN) has received considerable attention be-
cause of its advantageous properties such as chemical inertness,
extreme hardness, high thermal conductivity, transparency, and
electrically insulating performance. It has great potential appli-
cation in refractory, lubricants, and cutting tools.4–6 Various
methods were reported for synthesis of boron nitride, such as
the direct reaction of boron and nitrogen, the carbothermic re-
duction of boron oxide and so on.7,8 Recently, Yang et al.9,10 syn-
thesized nanocrystalline cubic BN by pulsed laser-induced liq-
uid–solid interfacial reaction; Tang et al.11 reported that large-
scale BN submicron spherical particles could be synthesized
by chemical vapor deposition; Xu et al.12 synthesized BN nano-
bubes by co-pyrolysis NH4BF4, KBH4, and NaN3 at 450 ꢁC.
Herein, we report a low-temperature route to BN flakes and
hollow spheres by coreduction of ammonium chloride (NH4Cl)
and boron tribromide (BBr3) using metallic sodium as reductant.
The reaction can be described as follows:
(004), (110)). The lattice constants are a ¼ 2:506 and c ¼
ꢀ
ꢀ
6:623 A, close to a ¼ 2:503 and c ¼ 6:661 A (JCPDS card#
73-2095). No impurities such as B2O3 can be detected in the
XRD pattern.
Figure 2 shows the XPS spectrum of the as-prepared h-BN
sample. It indicates that the sample surface consists of nitrogen
and boron, with binding energies of N1s, and B1s at 397.9 and
190.4 eV, respectively. The B1s peak at 190.4 eV and the N1s
peak at 397.9 eV indicate BN, in good agreement with those in
the literature.13 Quantification of B1s and N1s peaks gives an
average B:N atomic ratio of 1:1.05, close to that of BN.
20
30
40
50
60
70
80
350 ꢁ
C
2Theta /degrees
BBr3 þ NH4Cl þ 4Na ꢂꢂꢂ! BN þ 3NaBr þ NaCl þ 2H2
Figure 1. XRD pattern of the BN sample.
In this reaction, nascent nitrogen and boron are generated in the
reaction of NH4Cl and BBr3 by metallic sodium, and they com-
bine to form hexagonal BN.
B1s
N1s
All the manipulations were carried out in a dry glove box
with Ar flowing. In the typical process, NH4Cl (0.01 mol), BBr3
(0.01 mol), and sodium (0.04 mol) were placed into a stainless
steel autoclave with a quartz liner. Then the autoclave was sealed
and heated at 350 ꢁC for 6 h, followed by cooling to room tem-
perature on standing. The product in the quartz liner was washed
with distilled water and absolute ethanol for several times to re-
move the impurities. The final product was vacuum-dried at
80 ꢁC for 4 h. White powders were obtained.
186 188 190 192 194 196
392 394 396 398 400 402 404
Binding Energy /eV
Binding Energy /eV
Figure 2. XPS spectra of the BN sample.
X-ray powder diffraction (XRD) measurement was carried
out on a Rigaku Dmax-ꢀA X-ray diffractometer with Cu Kꢁ ra-
The FTIR spectrum of the sample at room temperature is
shown in Figure 3. Two strong characteristic peaks locate at
1382 and 816 cmꢂ1, which can be attributed to the in-plane B–
ꢀ
diation (ꢂ ¼ 1:54178 A). The morphology of nanocrystalline
BN was examined from transmission electron microscopy
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.2 (2004)
145
N stretching vibrations and out-of-plane B–N–B bending vibra-
tions, respectively.14,15 The broad absorption peak at 3421 cmꢂ1
constructed by BN polycrystals.
According to free energy calculations, the above BN syn-
thetic reaction is thermodynamically spontaneous and highly
exothermic (ꢁG ¼ ꢂ1220:2 kJmolꢂ1 and ꢁH ¼ ꢂ1246:2
kJmolꢂ1). A great deal of heat generates during the reaction
process and results in an instantaneous local high temperature,
which favors the synthesis of hexagonal BN.
The formation mechanism of BN hollow spheres is pro-
posed. In this reaction system, metallic sodium droplets, which
form owing to reaction heat generated from the exothermic reac-
tion, may act as templates to form BN hollow spheres. When the
reaction temperature reaches 350 ꢁC, NH4Cl (bp 337.8 ꢁC) and
BBr3 (bp 90 ꢁC) begin to vaporize and react with sodium (mp
97.8 ꢁC). Because of the strongly exothermic reaction, the local
temperature near the surface of sodium is very high and the so-
dium vaporizes to form many small droplets. Subsequently, the
coreduction reaction takes place on the surface of the sodium
droplets, which is a key step and may result in the formation
of BN shell on the sodium core. As the reaction continues, the
amount of sodium becomes depleted and the shell gradually
thickens, resulting finally in hollow sphere with a polycrystalline
shell. In order to understand the formation mechanism of BN
hollow spheres, further studies are underway.
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber /cm-1
Figure 3. FTIR spectrum of the BN sample.
may be due to the moisture absorbed on the surface of the sam-
ple.
The TEM images and selected area electron diffraction
(SAED) patterns of as-prepared BN are shown in Figure 4. From
Figure 4a it can be seen that the sample consists of flake-like
morphology. The corresponding SAED pattern (Figure 4b) of
BN flakes is consistent with the single crystalline nature. In Fig-
ure 4c, the sample consists hollow spheres of 50–200 nm in di-
ameter and the shell thickness of approximately 10–20 nm.
The strong contrast between the dark edge and the light center
exhibits its hollow nature. The SAED pattern (Figure 4d) of a
single BN hollow sphere shows two clear diffraction rings corre-
sponding to the (100), (110) crystal planes of hexagonal BN.
This result suggests that the shells of BN hollow spheres are
In summary, we have succeeded in synthesizing hexagonal
BN flakes and hollow spheres of 50–200 nm in diameter by cor-
eduction of NH4Cl and BBr3 using metallic sodium as reductant
at 350 ꢁC. The hollow spheres formation mechanism of metallic
sodium droplets acting as templates is proposed.
This work is supported by the Chinese National Science
Research Foundation.
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Published on the web (Advance View) January 9, 2004; DOI 10.1246/cl.2004.144