A mild solvothermal route to nanocrystalline titanium diboride

Article abstract of DOI:10.1016/S0925-8388(02)01173-8

Titanium diboride (TiB₂) has been synthesized via a benzene-thermal reaction of metallic sodium with amorphous boron powder and titanium tetrachloride at 400°C in an autoclave; the temperature was much lower than that of traditional methods. The X-ray diffraction (XRD) pattern can be indexed with the hexagonal cell of TiB₂ and the lattice constants a = 3.027 and c = 3.228 A?. X-ray photoelectron spectroscopy (XPS) results show the mole ratio of Ti:B as 1:1.766. Transmission electron microscope (TEM) images indicate that the average particle size is in the range of 15-40 nm in diameter.

Full text of DOI:10.1016/S0925-8388(02)01173-8

Journal of Alloys and Compounds 352 (2003) 325–327
L
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A mild solvothermal route to nanocrystalline titanium diboride
*
Yunle Gu , Yitai Qian, Luyang Chen, Fu Zhou
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, PR China
Received 6 June 2002; received in revised form 18 October 2002; accepted 18 October 2002
Abstract
Titanium diboride (TiB ) has been synthesized via a benzene-thermal reaction of metallic sodium with amorphous boron powder and
2
titanium tetrachloride at 400 8C in an autoclave; the temperature was much lower than that of traditional methods. The X-ray diffraction
˚
XRD) pattern can be indexed with the hexagonal cell of TiB and the lattice constants a53.027 and c53.228 A. X-ray photoelectron
2
(
spectroscopy (XPS) results show the mole ratio of Ti:B as 1:1.766. Transmission electron microscope (TEM) images indicate that the
average particle size is in the range of 15–40 nm in diameter.

2002 Elsevier Science B.V. All rights reserved.
Keywords: Transition metal compounds; Ceramics; Chemical synthesis; X-ray diffraction
1
. Introduction
phase reaction of NaBH and TiCl , followed by annealing
4 4
the obtained amorphous precursor at 900–1100 8C. Ax-
elbaum and co-workers [14,15] have developed a gas-
phase combustion process that can directly yield unag-
Borides of transition metals have attracted considerable
interest for their various unique properties. Among these
intermetallic compounds, TiB has a high melting point,
glomerated, low-oxygen TiB nanoparticles by the reaction
2
2
high hardness and strength, excellent wear and corrosion
resistance, which surpass that of titanium nitride and
carbide [1–3]. Titanium diboride is an important material
in diecasting and dry cutting industry for wear, corrosion
and erosion protection [4,5]. As it has a high temperature
stability and good electrical conductivity, titanium diboride
is also a promising interconnection material for integrated
circuits [6].
Traditionally, titanium diboride powder was prepared by
a variety of high temperature methods [7], such as the
direct reactions of titanium, its oxide or hydrides, with
elemental boron over 1000 8C; carbothermal reduction of
titanium oxide and boron oxide; or hydrogen reduction of
boron halides in the presence of the metal or its halides.
Recently, electrochemical syntheses [8,9] and solid-state
reactions [3,10–12] have been developed for preparing
titanium diboride. However, these methods generally can-
not lead to nano-sized products.
of sodium vapor with TiCl4 and BCl . However, the
products were contaminated with metallic titanium and
titanium oxide.
3
Recently, we have developed
a novel route to
nanocrystalline titanium diboride via a solvothermal re-
action of metallic sodium with amorphous boron powder
and TiCl at 400 8C. It can be described as follows,
4
Benzene
2
B 1 TiCl 1 4Na → TiB 1 4NaCl
4 2
400 8C
where TiB formation and crystallization proceeded syn-
2
chronously. In forming nanocrystalline TiB , active nas-
2
cent titanium was generated in reducing TiCl by metallic
4
sodium. The titanium reacted with boron forming titanium
diboride. Benzene serves as reaction medium to control
reaction speed and particle size. By confining the reaction
within an anaerobic environment, the product was free of
oxide impurity. However, sample surface oxidation may
occur during the purification process.
Previously, Bates and co-workers [13] have prepared
nanocrystalline TiB of 5–100 nm via an initial solution-
2
2
. Synthesis and purification procedure
*
Corresponding author. Tel.: 186-551-360-6144; fax: 186-551-360-
592.
E-mail address: ylgu@mail.ustc.edu.cn (Y. Gu).
1
Firstly, 1 g of amorphous boron powder (2 mm, Merck
KGaA), 25 ml of benzene and 5 ml of liquid analytical
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0925-8388(02)01173-8

3
26
Y. Gu et al. / Journal of Alloys and Compounds 352 (2003) 325–327
pure TiCl were placed in a quartz tube. The excessive
4
metallic sodium (about 4.5–5.0 g) was freed of kerosene
by washing with benzene. Then, the sodium was added in
the quartz tube. Finally, the quartz tube was put in a
stainless steel autoclave and sealed under argon atmos-
phere. The autoclave has a capacity of some 50 ml. Before
sealing the autoclave, approximately 10 ml of benzene was
put into the autoclave to balance heat and pressure. The
autoclave was heated at 400 8C for 8 h, followed by
naturally cooling to room temperature outside the furnace.
The obtained sample from the quartz glass tube was
washed several times with absolute ethanol, distilled water
and acetone to remove impurities. The final product was
vacuum-dried at 60 8C for 12 h, and 2.145 g of gray
powder product was obtained.
Fig. 2. Wide-scan XPS spectrum of the as-prepared TiB2 sample.
The gray powder product was analyzed by X-ray
photoelectron spectroscopy (XPS) using an ESCAlab MK2
with aluminium Ka rays, powder X-ray diffraction (XRD)
on an X-ray diffractometer (Rigaku rA) using Cu Ka
radiation (wavelength l51.54178 A), and transmission
electron microscopy (TEM) with a Hitachi 800 transmis-
sion electron microscope.
O1s and Ti2s at 32.20, 58.83, 187.68, 285.00, 454.42,
460.00, 530.95 and 561.29 eV, respectively. The B1s peak
at 187.68 eV indicates boron combined with titanium
forming titanium boride; this corresponds to the Ti2p3 / 2 at
454.42 eV, which was considered as the titanium bonding
˚
with boron forming TiB [16]. As the reaction is strongly
2
exothermic, the local temperature may temporarily become
higher than 400 8C. Therefore, the carbon may come
mainly from carbonization of small quantities of benzene
and sample surface adsorption. The O1s peak was not very
symmetric, indicating TiB2 surface oxidation as well as
oxygen adsorption. The mole ratio of Ti:B was calculated
3
. Results and discussion
Fig. 1 shows the XRD pattern of the as-prepared TiB2
sample. All the seven peaks at d-spacing of 3.2274,
2
˚
.6217, 2.0350, 1.6133, 1.5131, 1.3747, 1.2147 A were
as 1:1.766, slightly less than that of TiB . It may be due to
2
indexed as hexagonal TiB2 [(001), (100), (101), (002),
surface enrichment of titanium induced by surface oxida-
tion.
(
3
110), (102), (021)]. The rms error of fit was 9.334
2
5
10 , with the lattice constants a53.027 and c53.228
A transmission electron microscope (TEM) image and
selected area transmission electron diffraction (TED)
pattern are shown in Fig. 3. In Fig. 3a, the material
exhibits slightly agglomerated particle morphology. The
primary particle size was estimated in the range of 15–40
nm in diameter. Some amorphous carbon may cling to and
disperse between the TiB2 primary particles. The TED
˚
˚
A in good agreement with a53.030 and c53.230 A
JCPDS card[ 35-741).
(
The wide-scan XPS spectrum of the as-prepared TiB2
sample is shown in Fig. 2. It shows that the sample surface
consists of titanium, boron, carbon and oxygen, with
binding energies of Ti3p, Ti3s, B1s, C1s, Ti2p3 / 2, Ti2p1 / 2
,
pattern in Fig. 3b confirms the nanocrystallinity of TiB , in
2
which the diffraction ring diameters and intensities again
correspond well to the hexagonal TiB phase.
2
4
. Conclusion
In summary, nanocrystalline TiB of 15 to 40 nm with
2
the hexagonal cell has been successfully prepared via a
benzene-thermal synthetic route by the reaction of metallic
sodium with amorphous boron powder and titanium tetra-
chloride at 400 8C. This study demonstrates that the
solvothermal synthesis can be a mild method for preparing
titanium diboride nanomaterials. In a further study, efforts
will be made to research a better understanding of the
reaction mechanism and to control the reaction kinetics.
Fig. 1. XRD pattern of the as-prepared TiB2 sample.

Y. Gu et al. / Journal of Alloys and Compounds 352 (2003) 325–327
327
Fig. 3. TEM (a, scale bar, 150 nm) and TED (b) images of the as-prepared TiB2 sample.
[
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Acknowledgements
1
Financial support from the Chinese National Foundation
of Natural Science Research is gratefully acknowledged.
5
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