Large-scale synthesis of neodymium hexaboride nanowires by self-catalyst

Article abstract of DOI:10.1016/j.ssc.2006.10.001

Large scale NdB₆ nanowires have been successfully fabricated for the first time using a self-catalyst method with Nd powders and boron trichloride (BCl₃) gas mixed with hydrogen and argon. X-ray diffraction, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the samples. Transmission electron microscopy (TEM) reveals that the NdB₆ nanowires are single crystals with cubic structure. Our investigation forms part of a series of studies for finding comparatively inexpensive methods to prepare RB₆ nanomaterials.

Full text of DOI:10.1016/j.ssc.2006.10.001

Solid State Communications 141 (2007) 53–56
www.elsevier.com/locate/ssc
Large-scale synthesis of neodymium hexaboride nanowires by self-catalyst
∗
Qiwei Ding, Yanming Zhao , Junqi Xu, Chunyun Zou
School of Physics, South China University of Technology, Guangzhou 510640, PR China
Received 29 July 2006; received in revised form 20 September 2006; accepted 4 October 2006 by P. Sheng
Available online 18 October 2006
Abstract
Large scale NdB nanowires have been successfully fabricated for the first time using a self-catalyst method with Nd powders and boron
6
trichloride (BCl ) gas mixed with hydrogen and argon. X-ray diffraction, scanning electron microscopy (SEM) and high resolution transmission
3
electron microscopy (HRTEM) were used to characterize the samples. Transmission electron microscopy (TEM) reveals that the NdB nanowires
6
are single crystals with cubic structure. Our investigation forms part of a series of studies for finding comparatively inexpensive methods to prepare
RB nanomaterials.
6
ꢀ
c 2006 Elsevier Ltd. All rights reserved.
PACS: D61.46.+w; 79.70.+q
Keywords: A. NdB ; C. Nanostructures; E. Self-catalyst
6
1
. Introduction
13]. In this reaction, BCl3 is not easily controllable in
experimental operations because of its hygro-instability owing
to an easily absorbing vapor, and in such a situation boric
acid is produced in the reaction between BCl3 and the vapor,
which is very harmful to the following experiment and the
synthesis process has to be halted. In addition, due to economic
considerations, it is difficult to produce RB6 nanowires on a
large scale production if catalysts of platinum or gold are used
in the experiment.
To our knowledge, no results on the preparation and
properties of NdB6 nanowires have been reported yet.
We herein report for the first time the characterization
of the structural properties of large-scale single-crystalline
NdB6 nanowires. The method employed to synthesize NdB6
nanowires was by the direct reaction of rare-earth metal with
BCl3 gas on Si substrates without using Au or Pt as catalysts
throughout the whole growth process. The experiment is based
on following chemical reaction: Nd(s) + 6BCl3(g) + 9H2(g) =
NdB6(s) + 18HCl(g).
Since the discovery of carbon nanotubes in 1991 [1], mate-
rials with one-dimensional nanostructures such as nanotubes,
nanowires and nanorods have attracted great interest in both
scientific and technological areas of research [1–3]. Mean-
while, rare-earth hexaborides RB6 (R = rare earth element) are
also attractive as an interesting class of compounds. The rare
earth hexaborides have been investigated for years as the best
thermionic electron sources due to their low work function, low
volatility at high temperature, high conductivity, high chemical
resistance, and high mechanical strength [4]. All these signif-
icant properties result in a wide range of applications, such as
high-energy optical systems [5–7], sensors for high-resolution
detectors [8], electrical coatings for resistors, and thermionic
materials [4]. Under the assistance of Pt or Au as the catalysts,
LaB6 whiskers and crystals having a sharp tip have been fabri-
cated by Motojima et al. [9] and Givargizov and Obolenskaya
using the following chemical reaction [10,11]:
2
LaCl3(g)+12BCl3(g)+21H2(g) = 2LaB6(s)+42HCl(g).
Under the same experiment procedure, LaB6 and CeB6 single-
crystal nanowires have been synthesized by Zhang et al. [12,
2. Experiment
Commercial BCl3 gas and high-purity metal Nd powders
∗
were used as raw materials. The metal Nd powders were
loaded on the Si substrate. The Si substrate was positioned
Corresponding author. Tel.: +86 20 87111963; fax: +86 20 87112837.
E-mail address: zhaoym@scut.edu.cn (Y.M. Zhao).
0
038-1098/$ - see front matter ꢀc 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.10.001

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Q. Ding et al. / Solid State Communications 141 (2007) 53–56
Fig. 2. (a) SEM image of general morphology of the NdB nanowires
6
deposited on a Si substrate; Inset the enlarged SEM image of the tops of the
NdB nanowires; (b) Enlarged SEM image of the NdB nanowires revealing
6
6
that the nanowires tend to grow in an oriented fashion.
Fig. 1. X-ray diffraction patterns of as-synthesized sample.
tips and the no nanoparticles appear on the tips of the NdB6
nanowires, which implies that a no-catalyst growth process is
involved in our experiment. The result of nanoparticles on the
tips of nanowires has previously been seen in metal catalyst
experiments, using Au or Ag catalyze the growth of Si and
GaN nanowires in a VLS growth mode [14]. According to our
result obtained from the SEM images shown in Fig. 2(a), we
can describe the synthesis method of the nanowires as self-
catalyst [15,16]. From Fig. 2(b) we can see that most of the
nanowires are parallel with each other although no template
was used in our synthesis procedure. This means that we can
get the nanowires in an uniform oriented fashion by using this
experimental procedure.
in the quartz boat that was inserted into a horizontal quartz
tube furnace. Before reaction, the atmosphere of the quartz
tube furnace was a mixed gas of 50%H2 + 50%Ar under a
3
3
flow of 40 cm /min. A steady BCl3 flow of 60 cm /min was
started and maintained for 30 min when the temperature of the
tube furnace was raised to an expected furnace temperature
◦
of 1150 C. After the above procedure the quartz tube was
◦
cooled down to room temperature at a rate of 25 C/min under
the mixed gas (50%H2 + 50%Ar) atmosphere. The products
obtained from the Si substrate were first washed with distilled
water to remove the HBO3 which can be produced, after the
experiment, between BCl3 remaining on the substrate and water
in air when the substrate is taken out of the furnace. After
◦
In order to understand the mechanism of structure and
growth of the NdB6 nanowires clearly, the morphology and
microstructure of the product were further characterized by
TEM. For electron diffraction and microscopy observations,
the as-prepared sample was first dispersed in ethanol and then
collected onto a copper grid that was coated with a thin holey
carbon film. Fig. 3(a) shows a typical TEM image of an
individual NdB6 nanowire at a low magnification. A nanowire
with a uniform diameter around 80 nm can be observed from
Fig. 3(a). It is clearly seen from the inset image in Fig. 3(a) that
there is no particle (indicated by an arrow) attached to the tip of
the nanowire as mentioned above, which implys that no extra
catalyst was involved during the synthesis process, and thus,
the growth process in our experiment can be confirmed as self-
catalyst. The selected area electron diffraction (SAED) image
(Fig. 3(c)) taken from the nanowire shown in Fig. 3(a) along
the [120] crystal zone axis shows regular diffraction spots and
confirms that NdB6 nanowires adopt a single crystal structure
and the lattice constant a agrees well with our x-ray diffraction
measurement result. Detailed structural characterization of the
NdB6 nanowires was performed using HRTEM. Fig. 3(b)
shows a typical lattice-resolved HRTEM image of the NdB6
nanowire shown in Fig. 3(a). It reveals that the NdB6 nanowire
drying in vacuum at 60 C for 4 h, the final powder product
was obtained.
The general morphology of the sample was characterized
by a field-emission scanning electron microscope (SEM, LEO
1
530 VP). The phase identification of the sample was made by
XRD (DRIC Y-2000). The microstructure was studied with a
transmission electron microscope (TEM, JEM-2010HR).
3
. Results and discussions
Combining BCl3 gas and Nd at the temperature of
◦
1
150 C, we have successfully obtained NdB6 nanowires. The
composition of the as-synthesized product was examined by
powder X-ray diffraction (Fig. 1). All reflections can be indexed
as a pure cubic phase of NdB6 [space group: Pm3m] and the
peaks corresponding to (hkl) values of (100), (110), (200),
(
210), and (211) are in good agreement with reference to the
unit cell of the cubic structure (JCPDS card No 65-5421),
˚
a = 4.128 A. The XRD result of our samples suggests that
the as-prepared sample is NdB6 with cubic phase.
General overviews of the morphology of the as-synthesized
products are provided by SEM images shown in Fig. 2, from
which we can see that the nanowires obtained on the Si
substrate are large scale and straight along the longitudinal
direction in shape. Fig. 2(a) shows a low-magnification image
of a thick layer of wool-like nanowires. As shown in the
inserted SEM image in Fig. 2(a), the nanowires exhibit sharp
˚
is single crystalline in nature, and the lattice spacing of 4.125 A
corresponds to the d-spacing of the (100) crystal face. In
addition, the HRTEM image confirms that the nanowires grow
along the [100] direction.

Q. Ding et al. / Solid State Communications 141 (2007) 53–56
55
Fig. 4. Schematic representation of the growth mechanism for the self-catalyst
growth of NdB nanowires; (A) Nd powders distribute regulary; (B) When the
6
temperature of the furnace reaches the melting point of metal Nd, the metal
Nd powders start to melt and form a large quantity of liquid droplets with
size identical to those of the original solid metal particles; (C) The pyrolyzed
3+
B
from BCl is supposed to be absorbed by the melting metal Nd droplet
3
at some point of the surface, resulting in some metal boride particles, which
serve as nuclei for the following nanowire growth; (D) the growth of metal
boride nanowires grow after the formation of the nuclei and continue as long as
Fig. 3. (a) Low-magnification TEM image of an individual NdB nanowire.
6
Inset the TEM image of the top of a NdB nanowire; (b) HRTEM of NdB
6
6
3+
appropriate quantities of B and the source metal reactants are available.
nanowires, reveals that the nanowires are growing along the [100] direction; (c)
SAED patterns taken on the nanowire along the [120] direction.
neodymium (Nd) powders and boron trichloride (BCl3) gas
mixed with hydrogen and argon. Nanowires with diameters
of about 80 nm and length extending to several micrometers
have a single-crystalline structure and grow along the [100]
axis. As compared with the method used to prepare LaB6
and CeB6 nanowires by Zhang et al. [12,13], the method
employed here to synthesize NdB6 nanowires was produced
by the direct reaction of rare-earth metal with BCl3 gas on
substrates without using Au or Pt as catalyst throughout the
whole growth process. Based on the self-catalyst process,
a growth mechanism is proposed for the formation of the
nanowires. In addition, we believed this approach, which is
based on a simple chemical reaction process, can be readily
extended to the synthesis of RB6 (R = rare earth) and
related boride nanowires. Our investigation forms part of a
series of studies for finding comparatively inexpensive methods
to prepare RB6 nanomaterials. Furthermore, the as-prepared
NdB6 nanostructure offers a promising candidate in future in
thermionic emission, field induced emission, as well as other
electronic devices that required high-performance electron
sources.
With the results obtained from SEM and TEM observations,
it is difficult to think that NdB6 nanowires synthesized here
are dominated by the conventional vapor–liquid–solid (VLS)
growing process proposed for nanofibres 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 [17]. We haven’t observed any metal particles in nanowires
(
which would be evidence supporting the VLS mechanism) in
extensive TEM observations. Here the growth procedure can
be describe as “self-catalytic” growth [15], in other words,
in our experiment the melting metal Nd plays the roles of
both raw material and catalyst simultaneously in the process
of synthesizing NdB6 nanowires. A conceptual description of
the NdB6 nanowires is sketched in Fig. 4, in which, when
◦
the furnace temperatures are up to 1150 C (higher than the
◦
melting point of 1016 C for Nd), a large quantity of liquid
Nd droplets with size close to that of the original solid metal
particles appear (part A to part B in Fig. 4); subsequently, the
pyrolyzed B3 from BCl3 is supposed to be absorbed by the
melting metal at some point of the surface, resulting in there
being some NdB6 nanoclusters, which serve as nuclei for the
following nanowire growth (part B to part C in Fig. 4); the
growth of metal boride nanowires begins and continues as long
as appropriate quantities of B3 and the source metal reactants
are available (part C to part D in Fig. 4); and finally, when the
source metal is exhausted entirely, the growth terminates.
+
Acknowledgements
+
This work was funded by NSFC Grant (No. 50472055)
supported through the NSFC Committee of China and Science
and Technology Foundation supported through the Science and
Technology Bureau of Guangzhou Government respectively.
4
. Conclusions
References
In summary, we have for the first time successfully
fabricated NdB6 nanowires with a self-catalyst method using
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