One simple synthesis route to whisker-like nanocrystalline boron nitride by the reaction of NaBH4 and NaN3

Article abstract of DOI:10.1016/j.materresbull.2006.08.028

Nanocrystalline boron nitride (BN) was synthesized via a simple route by the reaction of sodium borohydride with sodium azide in an autoclave at 600 °C. X-ray powder diffraction pattern indicated that the product was hexagonal BN, and the cell constant was a = 2.495 A?, c = 6.687 A?. Transmission electron microscopy image showed that it consisted of whisker-like particles with an average size of 200 nm × 20 nm. The product was also studied by FT-IR, XPS and TGA. It has good thermal stability and oxidation resistance in high temperature.

Full text of DOI:10.1016/j.materresbull.2006.08.028

Materials Research Bulletin 42 (2007) 982–988
www.elsevier.com/locate/matresbu
Short communication
One simple synthesis route to whisker-like nanocrystalline
boron nitride by the reaction of NaBH4 and NaN3
a
a
a
a
Jianhua Ma a,b, , Jun Li , Guoxing Li , Yiguang Tian , Jing Zhang ,
*
Jufeng Wu a, Jinyong Zheng a, Hemiao Zhuang a, Tonghua Pan a
a Wenzhou University, Wenzhou, Zhejiang 325035, PR China
b The Key Laboratory of Wenzhou City Pen-making Industry, Wenzhou, Zhejiang 325035, PR China
Received 23 March 2006; accepted 28 August 2006
Available online 2 October 2006
Abstract
Nanocrystalline boron nitride (BN) was synthesized via a simple route by the reaction of sodium borohydride with sodium azide
in an autoclave at 600 8C. X-ray powder diffraction pattern indicated that the product was hexagonal BN, and the cell constant was
˚
˚
a = 2.495 A, c = 6.687 A. Transmission electron microscopy image showed that it consisted of whisker-like particles with an
average size of 200 nm ꢀ 20 nm. The product was also studied by FT-IR, XPS and TGA. It has good thermal stability and oxidation
resistance in high temperature.
# 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; A. Nitrides; B. Chemical synthesis; C. X-ray diffraction
1. Introduction
Boron nitride (BN) is often referred to as ‘‘white graphite’’ because it is a lubricious material with the same platy
hexagonal structure as carbon graphite. Unlike graphite, BN is a very good electrical insulator. It offers very high thermal
conductivity and good thermal shock resistance. BN is stable in inert and reducing atmospheres up to 2800 8C, and in
oxidizingatmospheresto850 8C. Besidesthese, ithas otheradvantageousproperties, suchasextremehardness, chemical
inertness, highmechanicalstrengthandlowdensity, highmeltingpoint,corrosionresistance, andoxidationresistance[1–
3]. Dueto theseproperties, BN becomes an importantmaterial. Ithasgreatpotential applicationsin refractory, lubricants,
protective and optical coatings, electrical insulators, cutting tools, grinding, and abrasive materials, advanced ceramic
composites, and mold release liners, electronic devices, electrical insulators and so on [4–6].
Up to now, intensive research works have been carried out on the synthesis of boron nitride (BN) materials. In
general, BN can be obtained by direct combination of boron and nitrogen [7] and carbothermal reduction of boric
oxide [8]. Moreover, other methods have also been developed to prepare BN. Hirano et al. [9] pyrolyzed (HBNH)3 at
250–700 8C and 100 MPa and obtained cubic BN by subsequent pyrolysis under 6.5 GPa at 1800 8C. Kusunose [10]
reported that BN could be obtained by direct nitriding of boron acid with ammonia gas. Tang et al. [11] synthesized BN
by heating the mixture of boron and iron oxide in a flowing ammonia gas at 1300 8C. Rao and Kaner [12] and
* Corresponding author at: Wenzhou University, Wenzhou, Zhejiang 325035, PR China. Tel.: +86 577 88655170; fax: +86 577 86689508.
E-mail address: mjh820@ustc.edu (J. Ma).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.08.028

J. Ma et al. / Materials Research Bulletin 42 (2007) 982–988
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Solozhenko et al. [13] prepared crystalline BN powders via a self-propagation route by the reaction of MBF4 (M = K
or Na) with Li3N and NaN3.
Recently, hydrothermal or solvothermal method has been reported to prepare BN. Yu et al. [14,15] synthesized
boron nitride nanocrystals with high crystalline perfection by a hydrothermal synthesis route. Hao et al. [16] prepared
BN nanoparticles by the reaction of Li3N and BBr3 at 250–550 8C via the benzene-thermal methods. Shi et al. [17]
reported that BN nanocrystalline could be synthesized by nitriding of MgB2 with NH4Cl and NH4Cl–NaN3,
respectively. Cai et al. [18] reported one convenient synthesis route to boron nitride nanotube by the reaction of boron
powder, iron oxide, and ammonium chloride at 600 8C in an autoclave. Chen et al. [19] prepared nanocrystalline boron
nitride by the reaction of BBr3 and Mg3N2 at 400 8C in an autoclave.
In this paper, nanocrystalline BN has been synthesized via a convenient route by the reaction of sodium
borohydride (NaBH4) and sodium azide (NaN3) in an autoclave at 600 8C. In this reaction, sodium borohydride is as
the boron source and sodium azide is as the nitrogen source.
2. Experimental
All of the manipulations were carried out in a dry glove box with flowing nitrogen gas. Typically, 0.03 mol
analytical grade sodium borohydride and 0.035 mol sodium azide (a slight excess amount) were put into a stainless
steel autoclave. After sealing under argon atmosphere, the autoclave was heated at 600 8C for 8 h, followed by cooling
to room temperature in the furnace. The obtained product from the autoclave was washed with absolute ethanol to
remove metallic sodium generated in the reaction. After that, the product was washed several times with distilled water
to remove the formed impurities. Finally the product was washed three times with absolute ethanol to remove water.
The final product was vacuum-dried at 60 8C for 12 h. Gray powders were obtained.
The obtained sample was analyzed by powder X-ray diffraction (XRD) on a Rigaku Dmax-gA X-ray diffractometer
˚
using Cu Ka radiation (wavelength l = 1.54178 A). The morphology of the sample was examined from transmission
electron microscopy (TEM) on a Hitachi H-800 transmission electron microscope. X-ray photoelectron spectra (XPS)
were recorded on a VGESCALAB MKII X-ray photoelectron spectrometer, using non-monochromatized Mg Ka X-
rays as the excitation source. Fourier transformation infrared spectroscopy (FT-IR) spectra were obtained using a
Shimadzu IR-400 spectrometer by using pressed KBr disks. The thermogravimetric analysis was performed on a
thermal analyzer (model: TA-50) below 1000 8C in air and nitrogen gases at a rate of 10 8C minꢁ1 to study its thermal
stability and oxidation behavior.
3. Results and discussion
Fig. 1 shows the XRD pattern of the as-prepared sample. There are four obvious diffraction peaks on this pattern.
And all these diffraction peaks ((0 0 2), (1 0 0), (1 0 1), (1 0 2), (0 0 4)) at different d-spacing can be indexed as
˚
hexagonal BN. The refinement gives the cell constants, a = 2.495 A, c = 6.687 A, which are consistent with the values
˚
Fig. 1. XRD pattern of the as-prepared sample.

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Fig. 2. TEM image and SAED pattern of the as-prepared BN sample.
˚
˚
reported in the literature (a = 2.503 A, c = 6.661 A) (JCPDS card no. 73-2095). No evidences of impurities such as B,
B2O3, can be found in this XRD pattern. The broadening nature of the XRD peaks indicated that the grain sizes of the
samples are on a nanometer scale.
The morphology of the prepared BN sample was investigated by transmission electron microscopy (TEM) and
selected area transmission electron diffraction (SAED). The TEM image and the SAED pattern are shown in Fig. 2.
Fig. 3. FT-IR spectra of the BN sample.

J. Ma et al. / Materials Research Bulletin 42 (2007) 982–988
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Fig. 4. XPS spectra of the BN sample.

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J. Ma et al. / Materials Research Bulletin 42 (2007) 982–988
In figure, the sample shows that it consists of whisker-like particles that have an average diameter of 20 nm and an
average length of about 200 nm. These whisker-like particles exhibits slightly agglomerated particle morphology due
to the ultrafine size. The SAED pattern of BN samples is inserted in Fig. 2. In this pattern, two obvious diffraction rings
can be found. These rings can be indexed as (0 0 2) and (1 0 0). And the SAED pattern of the sample can also confirm
the crystallinity of BN, in which the diffraction ring diameters again correspond well to the hexagonal BN structure.
Fig. 3 shows the FT-IR spectra of the whisker-like BN sample at room temperature. Two strong peaks around 1379
and 802 cmꢁ1 could be observed clearly for the sample, which are assigned to the B–N stretching vibrations and B–N–
B bending vibrations, respectively [20,21]. A weak peak around 3405 cmꢁ1 could be attributed to the moisture
absorbed on the surface of the sample.
Further evidence for the formation of BN can be obtained from the XPS of the sample. Fig. 4 shows the results of
the XPS spectra. They show that the sample surface consists of nitrogen, boron, carbon and oxygen, with binding
energies of N 1s, B 1s, C 1s and O 1s at 398.7, 191.5, 284.40 and 532.40 eV, respectively. The C 1s and O 1s peaks
indicate that there exists a small amount of impurity elements such as C and O due to the adsorption of CO2, H2O and
O2 on the surface of the sample. The B 1s peak at 191.5 eV and the N 1s peak at 398.7 eV indicate BN, in good
agreement with those in the literature [22]. The quantification of B 1s and N 1s peaks confirmed that the atomic ratio of
B:N was 1.01:1, which closely agrees with the stoichiometric composition of BN.
To investigate the thermal stability and the oxidation resistance of the as-prepared nanocrystalline BN, the
thermogravimetric analysis (TGA) was carried out at temperatures below 1000 8C under flowing air or nitrogen. Fig. 5
shows the TGA curves of the sample. From the TGA curve (a), heated under flowing air, we can find that the weight of
the product has not changed significantly below 600 8C. A slight weight loss indicates that the sample might adsorb a
little water on the surface. But the quantity of the adsorbed water is very small. As the temperature keeps on rising from
700 to 1000 8C, there is an obvious weight gain during the process, indicating that the BN sample is oxidized into B2O3
and nitrogen oxides. The TGA curve (b) is carried out under flowing nitrogen. We can find that the weight of the
product has not changed significantly below 1000 8C except a slight water weight loss on the surface. These indicate
that the prepared BN has excellent thermal stability and anti-oxidation property that will make it particularly useful for
high temperature applications.
The influences of the reaction temperature and time on the formation of nanocrystalline BN were investigated. An
optimum reaction conditions for the formation of nanocrystalline BN was 600 8C for 8 h. If the reaction temperature was
lower than 500 8C, the reaction was incomplete and the crystallinity of the as-prepared BN was lower. But if the
temperature was above 700 8C, the particles size increased. So the reaction temperature we chose was 600 8C. If the
reactiontimewasshorterthan4 h, thereactionwasincompleteandthecrystallinityofBNwaslower. Varyingthereaction
time in the range of 5–10 h at 600 8C did not significantly affect the crystallinity and particle size of the as-prepared BN.
In our present route, with the temperature increasing, excessive NaN3 decomposed (decomposition point: 300 8C)
generating N2 and molten Na (melting point: 98 8C) based on the overall equation (Eq. (1)) [23]. During the
Fig. 5. TGA curves of the as-prepared BN sample.

J. Ma et al. / Materials Research Bulletin 42 (2007) 982–988
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decomposition process, nitrogen atoms may be generated [24]. These nitrogen atoms are regarded as the nitrogen
source in synthesizing BN. The NaBH4 began to decompose with increasing temperature (>500 8C) [25] as expressed
in Eq. (4). The newly formed nitrogen atoms are so active that they can react directly with the BH3 to produce BN and
H2. The excessive molten Na may act as a medium for the transition of nitrogen and electron in the reaction process to
control the reaction speed and the particles’ size. And the high pressure in the autoclave may help to reduce the
reaction temperature. The possible process could be illustrated as follows
2NaN3 ! 3N2 þ 2Na
N3 ! N2 þ N þ e þ energy
Naþ þ e ! Na
(1)
(2)
(3)
ꢁ
NaBH4 ! BH3 þ NaH
2NaH ! 2Na þ H2
2BH3 þ 2N ! 2BN þ 3H2
According to the above possible processes, the total reaction can be described as following
(4)
(5)
(6)
NaBH4 þ NaN3 ! BN þ 2Na þ 2H2 þ N2
(7)
4. Conclusion
Nanocrystalline BN has been successfully prepared via a simple route by the reaction of sodium borohydride with
sodium azide in an autoclave at 600 8C. The product has the hexagonal BN structure. It consists of whisker-like
particles with an average size of 200 nm ꢀ 20 nm. The excessive molten Na may act as a medium for the transition of
nitrogen and electron in the reaction process to control the reaction speed and the particles’ size. And the high pressure
in the autoclave may help to reduce the reaction temperature. The product has good thermal stability and oxidation
resistance in the high temperature.
Acknowledgment
This material is based upon work funded by Zhejiang Provincial Natural Science Foundation of China under Grant
no. Y404292.
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