Self-catalyst growth of single-crystalline CaB6 nanostructures

Article abstract of DOI:10.1016/j.jssc.2007.06.027

Large-scale calcium hexaboride (CaB₆) nanostructures have been successfully fabricated with self-catalyst method using calcium (Ca) powders and boron trichloride (BCl₃) gas mixed with hydrogen and argon. X-ray diffraction (XRD), scan

Full text of DOI:10.1016/j.jssc.2007.06.027

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Journal of Solid State Chemistry 180 (2007) 2577–2580
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Self-catalyst growth of single-crystalline CaB6 nanostructures
Ã
Junqi Xu, Yanming Zhao , Chunyun Zou, Qiwei Ding
School of Physics, South China University of Technology, Guangzhou 510640, PR China
Received 27 February 2007; received in revised form 1 June 2007; accepted 8 June 2007
Available online 7 July 2007
Abstract
Large-scale calcium hexaboride (CaB6) nanostructures have been successfully fabricated with self-catalyst method using calcium (Ca)
powders and boron trichloride (BCl3) gas mixed with hydrogen and argon. X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED) were used to characterize the
compositions, morphologies, and structures of the samples. Our results show that the nanowires are highly single crystals elongated
preferentially in the [1 1 0] direction. The growth mechanism based on the self-catalyst process is simply discussed.
r 2007 Elsevier Inc. All rights reserved.
PACS: D.61.46.+w; 79.70.+q
Keywords: CaB6; RB6; Nanowires; Nanotubes; Nanostructures
1. Introduction
Therefore, CaB6 has attracted considerable contemporary
interest from both fundamental and practical viewpoint.
Recently, CaB6 single-crystal nanowires have been syn-
thesized by Xu et al. [15] using the following chemical
reaction: CaO (s)+3B2H6 (g) ¼ CaB6 (s)+H2O+8H2. How-
ever, in the above procedure, B2H6 is characterized by both
high toxicity and an explosive behavior, which make the
whole experiment process dangerous. In addition, the low
reaction pressure (ꢀ155 mTorr) and catalyst (Ni) are also
needed, which make the synthesis procedure complicated.
The possibility of searching a relatively safe boron source
instead of B2H6 as original material and using self-catalysts
method to synthesize CaB6 nanostructures at normal
pressure are the motivations for this work.
In this article, we report the self-catalyst synthesis of
CaB6 nanostructures by the direct reaction of alkaline-
earth metal with BCl3 and H2 gas on Si substrates without
using Ni as catalyst throughout the whole growth process.
Further details about the experiments and discussion will
be given in the following sections.
Since the discovery of carbon nanotubes in 1991 [1], one-
dimensional (1D) nanomaterials in the form of tubes,
wires, and rods have attracted much attention in the past
decade because of their interesting geometries, novel
properties, and potential applications [2,3]. Many material
systems including carbon [1,4,5], semiconductors [2,6,7],
oxides [3,8,9], nitrides [10,11], carbides [12,13], and borides
[14,15] have been successfully fabricated by a variety of
methods. Calcium hexaboride (CaB6), one of the alkaline
earth cubic hexaboride, has been attracting a great deal of
attention as a material for structures exposed to high
temperatures, for surface protection, and for wear-resistant
parts used in a corrosive environment, due to its favorable
properties such as high melting point (2373 K), high
hardness, chemical stability, and high electrical conductiv-
ity [16–18]. It is especially worth noticing that studies have
been carried out to determine the feasibility of using CaB6
and its composites as a potential material for improvement
of the abrasion resistance of bricks for converter [19,20].
2. Experimental procedure
Ã
The CaB6 nanostructures were synthesized in a conven-
tional tube furnace with a horizontal quartz tube. The reaction
Corresponding author. Fax: +86 20 8711 2844.
E-mail address: zhaoym@scut.edu.cn (Y. Zhao).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.06.027

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J. Xu et al. / Journal of Solid State Chemistry 180 (2007) 2577–2580
2578
can be expressed as: Ca(s)+6BCL3(g)+9H2(g) ¼ CaB6(s)
+18HCl(g). The synthetic route can be described as
follows. Firstly, high-purity Ca metal powder (commercial
99.9% and with particle size less than 0.05 mm), 0.2 mg in
weight, was well dispersed with a thin thickness over a Si
substrate (8 ꢁ 8 mm in size). Then the Si substrate with Ca
powders on it was loaded in a quartz boat, and the boat
was placed in the center of the long quartz tube within
a horizontal tube furnace. After the quartz tube was
evacuated and filled with nitrogen for three times, the
quartz tube was heated under a mixed gas (50% H2+50%
Ar) flow of about 120 ml/min. When 1000 1C was reached,
the BCl3 flow of 30 ml/min was introduced in to the quartz
tube and kept for 20 min. Finally, the power was switched
off and the furnace cooled to the room temperature under
a H2/Ar flow of 30 ml/min. After cooling, a grey or dark
layer was found on the silicon substrate.
Fig. 2(a) shows a typical SEM image of the synthesized
large quantity CaB6 nanostructures on a silicon substrate.
The SEM image (Fig. 2(b)) provides more detail about the
morphology of the products. As can be seen from Fig. 2(b),
there are two types of morphologies in the CaB6 structures.
One of the morphologies is rectangular shaped plate-like
crystals with width of several hundred nanometers, which
is the predominant morphology, and the other are
nanowires with diameter within 60–100 nm, whose yields
are relatively less. Most nanostructures have a smooth
surface, and are straight along their axes. No catalyst
particles were found on the tips of the nanostructures,
which implies that a no catalyst growth process is involved
in our experiment.
A typical low-magnification TEM image of the produced
CaB6 nanowires is shown in Fig. 3(a). The nanowire is
about 100 nm in lateral dimensions and more than 1 mm in
length. The nanowire has smooth surfaces and their tips,
shown in the low-right inset of Fig. 3(a), are always
terminated with a flat surface. Selected-area electron
diffraction (SAED) was used to determine the lattice
structure of the nanowire. The diffraction pattern appeared
identical as we moved the SAED aperture along the entire
nanowire, and Fig. 3(d) shows a typical SAED pattern
taken along [1–10] zone axis of cubic structure. It reveals
The products were characterized and analyzed using
X-ray diffractometer with CuKa radiation, LEO 1530 VP
field-emission scanning electron microscopy (SEM) and
JEOL JEM-2010 transmission electron microscopy (TEM).
3. Results and discussion
The phase identification of the product was carried out
by X-ray powder diffractometer. A step scan mode was
adopted with a scanning step of 0.021 and a sampling time
of 2 s. Fig. 1 shows the X-ray diffraction (XRD) patterns of
the as-prepared CaB6 product. The XRD patterns of the
product were successfully indexed with a cubic lattice using
the program Dicvol, and the lattice parameters were
further least-squares refined by the program PIRUM as
˚
a ¼ 4.14 A which agrees well with the data from reference
(JCPDS card: no. 89-4304). Our XRD analysis results
suggest that the as-prepared product is the CaB6 with cubic
phase.
Fig. 1. X-ray diffraction patterns of the sample obtained from Si
substrate.
Fig. 2. (a) SEM micrograph of as-synthesized CaB6 nanostructures.
(b) SEM image showing an enlarged view of the CaB6 nanostructures.

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Ca
Cu
O
Si
Cu
Ca
Cu
0
2
4
6
8
10
X-ray energy (KeV)
Fig. 3. (a) TEM micrograph of a portion of a nanowire; the insert showing the tip of the nanowire. (b) A representative HRTEM image of the nanowire.
(c) HRTEM image of the area of the rectangle in Fig. 3(b) showing a crystallized structure and preferential growth along the [110] direction. (d) The
representative SAED pattern recorded along the [1–10] zone axis. (e) EDS spectrum recorded from the nanowire.
that the CaB6 nanowires adopt single crystal structure
which has a primitive cubic structure and the lattice
constant a agrees well with our XRD measurement result.
To elucidate more details of the atomic structure of the
CaB6 nanowires high-resolution TEM (HRTEM) was also
employed. A representative HRTEM image of the nano-
wire is shown in Fig. 3(b), from which we can see the lattice
spacings and an amorphous layer with 5–10 nm thickness
surrounding the nanowire. In order to get the clear fringe
spacings, the area of the rectangle in Fig. 3(b) was
magnified to Fig. 3(c). The lattice spacing of 0.41 and
0.29 nm correspond to the d-spacing of the (001) and (110)
crystal faces, respectively. From the HRTEM image, a
right angle was formed between the (001) and (110) crystal
faces, which is in good agreement with the theoretical value
90.01. The atom-resolution image also confirms that the
nanowire is parallel to the [110] crystallographic direction
of the CaB6 nanowire. EDS technique was also used to
study the chemical composition of the catalyst. Fig. 3(e) is
a representative EDS spectrum recorded from the nano-
wire, and the results show that, besides the Ca component,
there is little amount of Si, O, Cu in the nanoswires.

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2580
Because the samples used to analyze the compositions were
taken from thin film of nanowires, we think that there may
be contamination of our samples with Si substrate (due to
etched by BCl3), especially at the positions that contacted
with Si substrate and O (due to the little oxidization of
Ca powder before reaction). For the copper composition, it
should have come from the Cu grid. It is difficult to detect
the B component because of the small atomic number of
light elements, and thus based on the analysis of EDS
spectrum, it is difficult to confirm the exact compositions of
the nanostructures with CaB6 or not.
One question that needs attention is how the CaB6
nanostructures grow. The TEM analysis shows that the
nanostructures may not be dominated by the conventional
vapor–liquid–solid (VLS) mechanism proposed for nano-
fibres grown by a catalyst-assisted process, in which a
transition metal particle is capped at the tip of the fibre
and serves as the active catalytic site [21]. We have not
observed any metal particles in nanostructures (which
would be the evidence for supporting the VLS mechanism)
in extensive TEM observations. Here the growth procedure
can be described as ‘‘self-catalytic’’ growth [22–24],
that is to say, in our experiment, the melting metal
Ca plays the role of reactant and catalyst simultaneously.
For detailed description of the growth process for the
CaB6 nanostructures, one can refer to our recent work
on PrB6 nanowires, LaB6 nanowires, and CeB6 nano-
wires [22–24].
Acknowledgments
The authors thank Dan Jiang and Chaolun Liang the
Sun Yat-sen University for valuable discussion with
the TEM results. This work was supported by the National
Nature Science Foundation of China (NSFC, Grants
nos.: 50472055).
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