Mg-assisted autoclave synthesis of RB6 (R = Sm, Eu, Gd, and Tb) submicron cubes and SmB6 submicron rods

Article abstract of DOI:10.1002/ejic.200901015

Submicron crystalline rare-earth hexaborides (RB_6; R = Sm, Eu, Gd, and Tb) have been successfully prepared by a facile one-step solid-state reaction of RCl₃·OH₂O, B₂O₃, and Mg powder in an autoclave at the relatively low temperature of 500°C. By controlling the reaction, conditions, submicronsized cubes (RB₆) and rod- and needlelike SmB₆ are obtained. The possible growth mechanism of the 1D SmB₆ structures has also been discussed. The XRD patterns of the products show that all of the hexaborides can be indexed to a cubic phase with high crystallinity and high purity. The field-emission scanning electron microscopy (FESEM) and TEM images display their cube-, rod-, and needlelike morphologies. The selected-area electron diffraction (SAED) patterns reveal the single-crystalline nature of the products.

Full text of DOI:10.1002/ejic.200901015

FULL PAPER
DOI: 10.1002/ejic.200901015
Mg-Assisted Autoclave Synthesis of RB (R = Sm, Eu, Gd, and Tb)
6
Submicron Cubes and SmB Submicron Rods
6
Maofeng Zhang,*[a,b] Ying Jia, Guogen Xu, Pengfei Wang, Xiaoqing Wang,
[a]
[a]
[b]
[b]
[
b]
[a]
[b]
Shenglin Xiong, Xuanjun Wang, and Yitai Qian*
Keywords: Rare earths / Hexaborides / Magnesium / Solid-state reactions / Crystal growth
Submicron crystalline rare-earth hexaborides (RB
Eu, Gd, and Tb) have been successfully prepared by a facile
one-step solid-state reaction of RCl ·6H O, B , and Mg
powder in an autoclave at the relatively low temperature of
00 °C. By controlling the reaction conditions, submicron-
6
sized cubes (RB ) and rod- and needlelike SmB are ob- phologies. The selected-area electron diffraction (SAED) pat-
6
; R = Sm,
structures has also been discussed. The XRD patterns of the
products show that all of the hexaborides can be indexed to
a cubic phase with high crystallinity and high purity. The
field-emission scanning electron microscopy (FESEM) and
TEM images display their cube-, rod-, and needlelike mor-
3
2
2 3
O
5
6
tained. The possible growth mechanism of the 1D SmB
6
terns reveal the single-crystalline nature of the products.
Introduction
submicron-sized rare-earth hexaborides were synthesized at
[
22]
900 °C using metal acetate precursors. Since the proper-
Rare-earth hexaborides have attracted much attention
because of their various peculiar properties such as high
melting point, high hardness, high chemical stability, super-
ties of a material can be significantly changed by their par-
ticle size due to quantum effects, itis expected that micro-
[7,8,23]
structured RB , such as nanowires
and nanoobel-
6
[
1,2]
[3–5]
conductivity,
magnetic properties,
high efficiency
[24]
isks, can give rise to exceptional properties. In short, be-
cause hexaborides are of great technological importance, re-
searchers are trying to explore low reaction temperature,
low cost, and simple and convenient synthetic methods for
the preparation of these microstructured compounds.
Following these demands, we have recently proposed and
adopted the simple and novel technique of Mg-assisted
autoclave solid-state reactions (reaction under autogenic
pressure at elevated temperature). By using this method,
thermionic emission, and narrow-band semiconductivity,
[
6–10]
and so forth.
Therefore, rare-earth hexaborides have
great technical importance and their synthesis is significant.
Most previous methods for the preparation of rare-earth
hexaborides need either high temperature or harsh reaction
conditions, such as the direct solid-phase reaction of a lan-
thanide or its oxides with elemental boron at around
[
1,11–13]
1
800 °C;
ides with boron at 1700 °C;
high-temperature synthesis from rare-earth oxides and bor-
the floating-zone method of rare-earth ox-
[5,14,15]
the high-pressure and
rare-earth hexaborides with the general formula of RB (R
6
=
La, Ce, Pr, and Nd) consisting of nanocubes and nano-
[
16]
on at 1600 °C; the carbothermal reduction of lanthanide
oxide and boron at 1500 °C;[17] the solution method in
molten aluminum at about 1300 °C;[ and the chemical
particles have been successfully synthesized in the tempera-
[25]
ture range from 400 to 500 °C.
In the present study, we
18]
extend the synthetic method of LaB nanocubes and nano-
6
[
7,8,19]
vapor deposition (CVD) method at around 1150 °C.
However, there are only a few papers reporting the synthesis
of hexaborides below 1000 °C. For example, LaB crystals
were synthesized from molten-salt electrolysis at 850 °C.
[26]
particles
to the preparation of RB (R = Sm, Eu, Gd,
6
and Tb) submicron materials, using cheap and easily avail-
able metal chloride precursors, B O , and metallic magne-
6
2
3
[
20]
sium powder as the raw materials. It is unique that the pres-
ent work produced 1D hexaborides, which is different from
our previous work. The structure and morphologies of the
products have been characterized by means of X-ray dif-
fraction (XRD), selected-area electron diffraction (SAED),
field-emission scanning electron microscopy (FESEM),
transmission electron microscopy (TEM), and high-resolu-
Also, LaB thin films were deposited by using the pulsed-
6
laser deposition (PLD) technique at 850 °C.[21] Recently,
[
[
a] Section 503, Xi’an High Technique Institute,
Xi’an 710025, P. R. China
E-mail: mfzhang@ustc.edu.cn
b] Hefei National Laboratory for Physical Sciences at the Micro- tion transmission electron microscopy (HRTEM). To the
scale and Department of Chemistry, University of Science and
Technology of China,
best of our knowledge, no such study has ever been re-
ported on the low-temperature, low-cost, and simple chemi-
Hefei 230026, P. R. China
E-mail: ytqian@ustc.edu.cn
cal synthesis of 1D hexaborides.
Eur. J. Inorg. Chem. 2010, 1289–1294
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M. Zhang, Y. Qian et al.
FULL PAPER
Results and Discussion
25-0932). No other impurity peaks of B, MgO, or B
were detected, which indicates the high purity of the as-
O
2 3
The crystal structures and the phase purity of the materi- obtained samples.
als were determined by XRD. Typical XRD patterns of the
SmB , EuB , GdB , and TbB products are presented in
6
6
6
6
Figure 1. The strong and sharp reflection peaks reveal that
the as-prepared materials are highly crystalline. The diffrac-
tion peaks were carefully indexed and assigned to the lattice
planes of (100), (110), (111), (200), (210), (211), (220), (300),
The Formation of Hexaboride Submicron Cubes
The morphologies and microstructure of the samples
were examined by FESEM. Figure 2 shows the FESEM
images of hexaborides prepared at 500 °C for 12 h. It is evi-
dent that all the products have a cubic morphology. The
particle size distribution for SmB and EuB samples ranges
(
310), and (311) for the corresponding “d” spacings. All the
peaks can be readily indexed to a cubic crystal system
space group: Pm3m (221)] of hexaborides with calculated
lattice constants (a) of (4.133Ϯ0.001) Å (SmB6),
[
6
6
from approximately 150 nm to approximately 400 nm, as
obtained by measuring the size of a large number of well-
defined particles. However, GdB and TbB crystals show
(
(
4.190Ϯ0.001) Å (EuB ), (4.107Ϯ0.001) Å (GdB ), and
6
6
4.105Ϯ0.001) Å (TbB ), which are very close to the re-
6
6
6
ported data (JCPDS card nos.: 24-1120, 40-1308, 24-1082,
more or less uniform-sized particles with particle size ran-
ges from approximately 300 nm to approximately 500 nm.
For TbB crystals, in addition to the submicron cubes, some
6
nanoparticle structure is also observed on the surface of the
as-obtained products.
Figure 3 shows TEM images of the as-prepared hexa-
borides. The images confirm that the hexaborides have a
cubic morphology, which is consistent with the FESEM re-
sults. Figure 3 (a) shows a TEM image of a SmB sample.
6
The calculated average particle size of SmB6 is about
200 nm, and the SAED pattern (inset) reveals the single-
crystalline nature of individual SmB submicron cubes. An
6
average particle size of about approximately 200 nm is ob-
served in Figure 3 (b) for EuB crystals, and it clearly shows
6
the 3D nature of a single crystal. Similarly, GdB and TbB6
6
crystals (Figure 3, c and d) have the same cubic morphology
with an average particle size of about 400 nm. By a series
of detailed temperature- and time-dependent experiments,
it was found that a large amount of RB submicron cubes
6
can be obtained under the optimum reaction conditions of
500 °C for 12 h.
Figure 1. XRD patterns of the as-prepared products (a) SmB
EuB , (c) GdB , and (d) TbB
6
, (b)
6
6
6
.
6 6 6 6
Figure 2. FESEM images of the as-prepared (a) SmB , (b) EuB , (c) GdB , and (d) TbB .
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Mg-Assisted Autoclave Synthesis of Submicron RB
6
The Formation of 1D Rod- and Needlelike Hexaboride
In the present preparation process, it was found that the
reaction time plays an important role in the formation of
1D hexaborides. To reveal the intermediate steps in the for-
mation of the 1D structure, a series of detailed time-de-
pendent experiments was carried out at 500 °C. For SmB6
samples, if the reaction time was shorter than 4 h, the reac-
tion was very incomplete and the crystallinity of the prod-
uct was very poor due to the reaction time being too short.
When the reaction time was 12 h, the initial poorly crystal-
line products turned into highly crystalline submicron cubes
(Figure 3, a). However, when the reaction time was 24 h,
the products became rodlike. The size and morphology of
the as-prepared SmB6 samples were examined using
FESEM, TEM, SAED, and HRTEM. Typical images of
the SmB samples are shown in parts a–d of Figure 4. Fig-
6
ure 4 (a) clearly reveals a highly monodisperse distribution
of a rodlike structure that has a width of approximately
150–250 nm and a mean length of approximately 1.5 µm.
Figure 4 (b) shows the TEM image of a single rod, which
clearly discloses the cubic pillar structure morphology. The
SAED pattern (Figure 4, c) reveals the single-crystalline na-
Figure 3. TEM images of the as-prepared (a) SmB
6
(inset: corre-
sponding SAED patterns), (b) EuB , (c) GdB , and (d) TbB .
6
6
6
Figure 4. (a) FESEM (the inset shows an enlarged image), (b) TEM, (c) SAED, and (d) HRTEM images, and (e) an EDX spectrum of
rodlike SmB samples prepared at 500 °C for 24 h.
6
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M. Zhang, Y. Qian et al.
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ture of a single rod with a preferential growth direction other key factor in the formation of 1D structures of RB6.
along the long axis direction. The HRTEM image (Fig- For example, at the lower temperature of 450 °C, only a
ure 4, d) shows that the rodlike SmB are structurally uni- small amount of RB submicron cubes and nanoparticles
6
6
form with interplanar spacings of about 0.408 and could be obtained. However, needlelike SmB was fabri-
6
0.296 nm, which correspond to the (100) and (110) lattice cated at the elevated reaction temperature of 600 °C, as
spacings of SmB , respectively. This structural information shown in Figure 5 (c).
6
indicated that the growth direction of the sample shown in
Figure 4 (b) was in the Ͻ001Ͼ direction. It is noteworthy
that the SEM and TEM images were obtained from ran-
domly selected areas of the sample and, as such, are repre-
sentative of the overall shapes and sizes of our as-prepared
The Growth Process and Mechanism of 1D Hexaborides
To substantially understand the growth mechanism of
SmB6 submicron rods. Energy-dispersive X-ray (EDX) the 1D hexaborides under the present synthetic route, we
spectroscopy was also used to characterize the composition have systematically surveyed the growth process by analyz-
of the products. EDX analysis of the SmB submicron rods ing the 1D structure of the hexaborides at different stages.
6
revealed the presence of Sm and B (Figure 4, e), both of The whole growth process of the 1D structure can be sche-
which are dominant in the materials. The other elements matically illustrated as shown in Figure 6. Firstly, it pro-
detected in the samples originate from copper grids and the duced an amorphous product. This amorphous product
absorption of oxygen on the powder surface. By comparing acted as the nucleation center for the formation of hexabo-
a series of experiments, it is concluded that the optimum ride submicron cubes. At the elevated temperature of
reaction conditions for large-scale preparation of rodlike 500 °C, it was converted into small crystalline primary par-
SmB are 500 °C for 24 h. However, surfaces of as-synthe- ticles (Figure 6, a). Secondly, following a 3D-oriented at-
6
[
27]
sized 1D SmB6 submicron crystals are not very regular tachment,
the crystalline primary particles were trans-
(shown in the inset of Figure 4, a). Because the products formed into imperfect submicron cubes after an 8 h reac-
were prepared by means of a solid-state reaction, the sur- tion (Figure 6, b). Thirdly, when the reaction proceeded for
faces of the products were not easy to control. If the reac- 12 h, the perfect submicron cubes were obtained (Figure 6,
tion temperature is reduced to around 400 °C, it can be car- c). This was due to the so-called Ostwald ripening pro-
[
28]
ried out in a benzene–thermal reaction system, and smooth cess,
surfaces of the products are then available. Much work is small ones through the diffusion of ions, atoms, or molec-
needed and the relevant research is under way. ules within an assembly of crystallites. Fourthly, as the reac-
With further extension of the reaction time from 24 to tion time was further prolonged to 24 h, the submicron
in which the large particles grew at the cost of the
8 h, the products gradually transformed from rod- to nee- cubes grew through an oriented attachment process,[
29–32]
in
4
dlelike in structure. As shown in Figure 5 (a), rodlike SmB6 which the adjacent particles are self-assembled by sharing a
partly turned into a needlelike structure after it had reacted common crystallographic orientation and docking of these
for 36 h. After 48 h, needlelike SmB with needle lengths of particles at a planar interface. They firstly transformed to
6
up to 2 µm were formed (Figure 5, b). It was found that the short rods (Figure 6, d) and then long rods (Figure 6, e).
lengths of the needles are longer than the lengths of the Finally, after the reaction had proceeded for long enough
rods. This is due to the persistent, oriented growth of the (48 h), the needlelike structure started to predominate in
incomplete product rods. The reaction temperature is an- the products through a transformation process using the
6
Figure 5. FESEM images of SmB samples prepared at (a) 500 °C for 36 h, (b) 500 °C for 48 h, and (c) 600 °C for 24 h.
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Mg-Assisted Autoclave Synthesis of Submicron RB
6
6
Figure 6. Schematic illustration of the growth process of the 1D SmB .
irregular particles as precursors (Figure 6, f). It is clear that Conclusion
the crystalline phase of the nuclei is critical for directing
the intrinsic shapes of the crystals due to its characteristic
symmetry and structure.
In summary, we have developed a facile and straightfor-
ward synthetic method to prepare hexaboride submicron
cubes and 1D rod- and needlelike structures at the low tem-
perature of 500 °C. The XRD and SAED patterns confirm
the high crystallinity, high purity, and the single-crystalline
nature of the products. The FESEM and TEM images
clearly show their cube-, rod-, and needlelike morphologies.
The possible growth mechanism of the 1D structured RB6
has also been discussed. The present route has the advan-
tages of mildness, simplicity, and low cost, so we believe
that this approach might be applied to the synthesis of a
new generation of important metal hexaborides.
On the basis of the above experimental observations, we
believe that the present 1D RB structures were produced
6
by a Mg-assisted co-reduction growth mechanism. That is
to say, during the formation process of the 1D hexaboride
structures, Mg acted as a reducing agent. The reaction
[
Equations (1), (2), (3), and (4)] can be described as follows:
RCl
3
·6H
2
O(s) + 3B
RB
2
O
3
(s) + 16.5Mg(s) Ǟ
6
(s) + 1.5MgCl (s) + 15MgO(s) + 6H
2
2
(g) (1)
(2)
B
2
O
3
(s) + 3Mg(s) Ǟ 2B(s) + 3MgO(s)
RCl (s) + 3Mg(s) Ǟ 2R(s) + 3MgCl
R(s) + 6B(s) Ǟ RB (s)
2
3
2
(s)
(3)
Experimental Section
Sample Preparation: All of the chemical reagents (Analytical
Grade) used in this experiment were purchased from National Rea-
6
(4)
This reaction [Equation (1)] is thermodynamically spon- gent Corporation (Shanghai, China) and used without further puri-
Θ
–1
taneous and exothermic (∆ H
= –3656.1 kJmol ). fication. In a typical procedure, an appropriate amount of
r
m
3 2 2 3
RCl ·6H O (0.003 mol), B O (0.009 mol), and an excess amount
Though the present reaction temperature (500 °C) is lower
than the melting point of Mg (648 °C), the Mg powder will
partly transform from the solid state to the liquid state be-
cause of the heat of the reaction that is generated. In the
preparation of hexaborides, as Equation (1) describes, B O3
begins to turn to liquid (melting point 450 °C) in the heat-
of Mg (0.075 mol) were put into a stainless steel autoclave of 25 mL
capacity. Then, the autoclave was sealed under an Ar atmosphere
in a glove box and was maintained at 500 °C for 12 to 48 h, fol-
lowed by being left to cool to room temperature. The products were
2
–1
washed with dilute hydrochloric acid (4 molL ), absolute ethanol,
and distilled water several times, respectively, to remove MgO,
MgCl , and other impurities. The final products were vacuum-dried
2
ing process. With the increase of temperature, more liquid
B O coats the Mg and is reduced to B atoms [Equa- at 60 °C for 6 h.
2
3
tion (2)]. When RCl makes contact with the Mg, it is re-
duced to R atoms [Equation (3)]. The fresh B and R atoms
have a higher activity than commercial boron and rare-
3
Characterization: The phase and composition of the products were
determined by a Rigaku D/Max-γA rotating-anode X-ray dif-
fractometer equipped with monochromatic high-intensity Cu-K
α
earth-metal elements, which enables the formation of RB6 radiation (λ = 1.54178 Å). The morphologies of the samples were
submicron cubes at the low temperature of 500 °C [Equa- observed with a transmission electron microscope (JEOL-2010)
tion (4)]. At the same time, the fast transport of the RB6 with an attached EDX system, which had an accelerating voltage
of 200 kV with a tungsten filament. FESEM images were recorded
on a JEOL JSM-6300F SEM instrument.
species in the high-pressure environment, arising from the
H2 produced during the reaction {about 50 atm [Equa-
tion (1)], estimated by the ideal gas law}, might be responsi-
ble for the growth of the highly crystalline RB 1D struc-
tures. However, as it is difficult to monitor the reaction once
6
Acknowledgments
the system is sealed, much work needs to be done to investi- This work was supported by the National Natural Science Founda-
gate the kinetics of the reaction.
tion of China (no. 20431020), the 973 Project of China (no.
Eur. J. Inorg. Chem. 2010, 1289–1294
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Received: October 18, 2009
Published Online: February 2, 2010
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Eur. J. Inorg. Chem. 2010, 1289–1294