A convenient solid-state reaction route to nanocrystalline TiB2

Article abstract of DOI:10.1016/j.inoche.2003.11.005

Nanocrystalline titanium diboride (TiB₂) powders were successfully synthesized at 650°C by using TiCl₄, Mg and MgB ₂ as the reactants. Transmission electron microscopy images show that typical TiB₂ crystallites are composed of uniform particles with the average grain size of about 40 nm.

Full text of DOI:10.1016/j.inoche.2003.11.005

Inorganic Chemistry Communications 7 (2004) 192–194
www.elsevier.com/locate/inoche
A convenient solid-state reaction route to nanocrystalline TiB2
a
a
a
a
a,
*
Liang Shi a,b, Yunle Gu , Luyang Chen , Zeheng Yang , Jianhua Ma , Yitai Qian
a
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Anhui, Hefei 230026, PR China
b
Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, PR China
Received 14 August 2003; accepted 7 November 2003
Published online: 2 December 2003
Abstract
Nanocrystalline titanium diboride (TiB2) powders were successfully synthesized at 650 °C by using TiCl4, Mg and MgB2 as the
reactants. Transmission electron microscopy images show that typical TiB2 crystallites are composed of uniform particles with the
average grain size of about 40 nm.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis; X-ray powder diffraction; Titanium diboride; Nanocrystals
Titanium diboride (TiB2), like borides of Zr, Cr and
other transition metals, is well known for its high
melting point, outstanding hardness, low specific weight,
high modulus, high chemical stability, good wear resis-
tance and so forth [1–3]. Due to these attractive prop-
erties, TiB2 has extensive applications in many areas.
For example they have been used as high temperature
materials, surface protection materials and wear resis-
tant material [4,5].
Traditionally, TiB2 was prepared by various high
temperature reactions, such as the direct combination of
boron with metal titanium over 1000 °C, reduction of
titanium oxide and boron oxide by carbon, hydrogen
reduction of boron halides and titanium halides. Re-
cently, various efforts have been made to synthesize TiB2
at lower temperatures, control morphology or simplify
reaction procedures. For example, Gu et al. [6] prepared
nanocrystalline titanium diboride via a benzene-thermal
reaction of metallic sodium with amorphous boron
powder and titanium tetrachloride at 400 °C; Axelbaum
et al. [7] reported to synthesize nanometer-sized TiB2
particles by gas-phase combustion reaction of sodium
vapor with TiCl4 and BCl3 in a high-temperature
flow reactor; Bates et al. [8] reported a solution-phase
reaction of NaBH4 and TiCl4 to produce an amor-
phous precursor powder, which was then annealed at
900–1100 °C to obtain nanocrystalline TiB2.
In this work, we report a convenient chemical route
to synthesize nanocrystalline titanium diboride. The
reaction was carried out in an autoclave and can be
described as follows:
TiCl4 þ MgB2 þ Mg ! TiB2 þ 2MgCl2
In a typical procedure, an appropriate amount of
anhydrous TiCl4 (0.03 mol), MgB2 (0.03 mol), excessive
Mg (0.05 mol) and were put into a stainless steel auto-
clave of about 50 ml capacity. The autoclave was sealed
and maintained at 650 °C for 8 h, then cooled to room
temperature. The product was washed with dilute hy-
drochloric acid and distilled water, respectively, to re-
move MgCl2 and other impurities. After drying in
vacuum at 60 °C for 4 h, black powder product was
obtained.
X-ray powder diffraction (XRD) pattern was mea-
sured on a Rigaku Dmax-cA X-ray diffractometer with
ꢀ
Cu Ka radiation (k ¼ 1:54178 A). The morphology was
observed from transmission electron microscopy (TEM)
images taken with a Japan Hitachi H-800 transmission
electron microscope. X-ray photoelectron spectra (XPS)
were recorded on a VGESCALAB MKII X-ray photo-
electron spectrometer, using non-monochromatized Mg
Ka X-rays as the excitation source. The specific surface
area was determined from the nitrogen absorption data
* Corresponding author. Tel.: +86-551-3601589; fax: +86-551-
3607402.
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2003.11.005

L. Shi et al. / Inorganic Chemistry Communications 7 (2004) 192–194
193
at liquid nitrogen temperature using the Brunauer–
Ti2p
Ennett–Teller (BET) technique by means of a Microm-
eritics ASAP 2000 apparatus. The components of the
impurities for TiB2 sample were determined by atomic
absorption spectrophotometry performed on a PE-
1100B atomic absorption spectrometer and a Hitachi-
220A UV spectrophotometer.
450
455
460
465
B1s
Fig. 1 shows the XRD pattern of the as-prepared
TiB2 sample, all the peaks can be indexed as hexagonal
TiB2. After refinement, the lattice constants, a ¼ 3:023
ꢀ
ꢀ
A, c ¼ 3:223 A, are very close to the reported value for
ꢀ
ꢀ
TiB2 (a ¼ 3:029 A, c ¼ 3:228 A, JCPDS card, No. 85-
2083). The broadening of the XRD peaks may originate
from the small grain sizes of the TiB2 samples, which is
confirmed by the TEM results. The low shoulder on the
right-hand side of the 100 diffraction peak suggests the
presence of a small amount of TiB. The primary com-
ponents of the impurities for TiB2 sample were (wt%):
Mg, 1.22; Fe, 1.17; Ni, 0.91; Cr, 0.43; Cl, 0.06, which
indicates that each element content is lower than 1 at.%.
Fig. 2 gives the XPS spectra of the as-prepared TiB2
sample. The B1s and Ti2p core-level regions were ex-
amined. The binding energy of B1s is found at 187.4 eV
and Ti2p3 at 454.4 eV. This corresponds well with the
reported binding energies for TiB2 [9,10], confirming the
formation of TiB2. The Ti2p peaks are a little broad and
not very symmetric, which could be attributed to a slight
oxidation of the sample surface. Based on the calcula-
tion of the peak areas, the mole ratio of B/Ti was ob-
tained to be 2.09 and close to the chemical stoichiometry
of TiB2. Small amount of carbon and oxygen were also
detected form the XPS spectra, which may be due to the
absorption of oxygen and carbon on the powder surface.
Fig. 3(a) shows the TEM images of the sample. It can
be seen that typical TiB2 crystallites are uniform parti-
cles with the average grain size of about 40 nm. Fig. 3(b)
shows the selected area electron diffraction pattern of
TiB2, which is consistent with the high crystallinity of
the sample and reveals four clear diffraction rings in
180
184
188
192
Binding energy (eV)
Fig. 2. XPS spectra of the TiB2 sample.
Fig. 3. (a) TEM image and (b) selected area electron diffraction pattern
of the TiB2 sample.
accordance with the (1 0 0), (1 0 1), (0 0 2), (1 1 0) crystal
planes of hexagonal TiB2.
The particle size can be estimated based on the for-
mula D ¼ 6 ꢀ 103/Sq, here D (nm) is the particle size, S
(m2/g) is the specific surface area, q is the density of TiB2
(4.5 g/cm3). This formula assumes that the particles have
a cubic (or spherical) configuration with uniform size
distribution and low surface roughness. The specific
surface area of the TiB2 sample was 25.93 m2/g. Thus,
the particle size D could be obtained to be 51.4 nm.
Taking into account the error induced by the assump-
tion of formula, the D value can be considered to be
consistent with that of TEM result.
According to free energy calculations, our synthetic
route is thermodynamically spontaneously and highly
exothermic (DG° ¼ ꢁ616:44 kJ/mol, DH° ¼ ꢁ671:4 kJ/
mol). The reaction of TiCl4, MgB2 and Mg generates a
great deal heat and results in a high local temperature,
which favors crystallization of TiB2. In addition, ex-
cessive metal Mg (mp ¼ 650 °C) existed as liquid during
20
30
40
50
60
70
80
2θ (degrees)
Fig. 1. XRD pattern of the TiB2 sample.

194
L. Shi et al. / Inorganic Chemistry Communications 7 (2004) 192–194
the reaction process. The molten Mg not only acts as the
reaction medium, but also absorbs the reaction en-
thalpy, which in turn maintains a relative low overall
reaction temperature and avoid the agglomeration of
the particles.
Acknowledgements
This work is supported by the National Natural
Science Foundation of China and the 973 Projects of
China.
It is found that the reaction temperature and time
play important roles on the formation of TiB2. No TiB2
could be obtained if the temperature is below 500 °C. An
optimum reaction temperature for TiB2 is about 650 °C.
An increase of the reaction temperature over 650 °C
resulted in an increase of the particle size. Varying the
reaction time between 4 and 24 h did not significantly
affect the crystallinity or morphology. If the time was
shorter than 2 h, the reaction became very incomplete
and the crystallinity was very poor due to too short
reaction time.
In summary, TiB2 powders with the average grain
size of about 40 nm were successfully synthesized at
650 °C in an autoclave by using TiCl4, Mg and MgB2 as
the reactants. An atomic ratio of B to Ti of 2.09:1.0 was
determined from X-ray photoelectron spectra. In com-
parison with previous routes, the present route allows
for the formation of nanocrystalline TiB2 through a
simpler process. This method may be extended to syn-
thesis other metal borides.
References
[1] J.R. Shappirio, J.J. Finnegan, R.A. Lux, J. Kwiatkowski, H.
Kattelus, M.A. Nicolet, J. Vac. Sci. Technol. A 3 (1985) 2255.
[2] M. Kornmann, R. Funk, Aluminium 53 (1977) 249.
[3] Y. Hwang, J. Lee, Mater. Lett. 54 (2002) 1.
[4] H.O. Pierson, Mater. Manuf. Process. 8 (1993) 519.
[5] M. Jones, A.J. Horlock, P.H. Shipway, D.G. McCartney, J.V.
Wood, Wear 250 (2001) 1009.
[6] Yunle Gu, Yitai Qian, Luyang Chen, Fu Zhou, J. Alloy. Compd.
352 (2003) 325.
[7] R.L. Axelbaum, D.P. DuFaux, C.A. Frey, K.F. Kelton, S.A.
Lawton, L.J. Rosen, S.M.L. Sastry, J. Mater. Res. 11 (1996)
948.
[8] S.E. Bates, W.E. Buhro, C.A. Frey, S.M.L. Sastry, K.F. Kelton,
J. Mater. Res. 10 (1995) 2599.
[9] G. Mavel, J. Escard, P. Costa, J. Castaing, Surf. Sci. 35 (1973)
109.
[10] C. Pfohl, A. Bulak, K.T. Rie, Surf. Coat. Technol. 131 (2000)
141.