A facile method to the cube-like MnSe2 microcrystallines via a hydrothermal process

Article abstract of DOI:10.1016/j.ssc.2006.02.006

A convenient hydrothermal process was applied to prepare the cube-like MnSe₂ microcrystallines through the reaction of MnSO ₄·H₂O with Se and NaH₂PO ₂·H₂O in aqueous solution at 160 °C for 12 h. Powder X-ray diffraction (XRD) analysis confirmed that the product was the cubic phase of MnSe₂ with cell parameter a=6.440 . The chemical composition of the MnSe₂ was determined by XPS. The Raman spectrum of MnSe₂ presented the peaks of the Se-Se stretching mode at 232.44 and 266.58 cm^-1. The images of scanning electron microscope (SEM) and transmission electron microscope (TEM) showed the cube-like morphology of the product with the edge length ranging from 20 to 30 μm. The formation mechanism of the MnSe₂ microcrystallines was discussed as well.

Full text of DOI:10.1016/j.ssc.2006.02.006

Solid State Communications 138 (2006) 72–75
www.elsevier.com/locate/ssc
A facile method to the cube-like MnSe microcrystallines
2
via a hydrothermal process
Licheng Wang, Luyang Chen, Tao Luo, Keyan Bao, Yitai Qian
*
Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026,
People’s Republic of China
Received 8 July 2005; accepted 10 February 2006 by J.F. Sadoc
Available online 2 March 2006
Abstract
A convenient hydrothermal process was applied to prepare the cube-like MnSe microcrystallines through the reaction of MnSO $H O with Se
2
4
2
and NaH
2 2 2
PO $H O in aqueous solution at 160 8C for 12 h. Powder X-ray diffraction (XRD) analysis confirmed that the product was the cubic
˚
phase of MnSe with cell parameter aZ6.440 A. The chemical composition of the MnSe was determined by XPS. The Raman spectrum of
2
2
K1
MnSe
transmission electron microscope (TEM) showed the cube-like morphology of the product with the edge length ranging from 20 to 30 mm. The
formation mechanism of the MnSe microcrystallines was discussed as well.
2
presented the peaks of the Se–Se stretching mode at 232.44 and 266.58 cm . The images of scanning electron microscope (SEM) and
2
q 2006 Elsevier Ltd. All rights reserved.
PACS: 61.66.Fn
Keywords: A. Chalcogenides; A. Inorganic compounds; B. Chemical synthesis; D. Crystal structure
1
. Introduction
conducting them in liquid ammonia at room temperature, but
the obtained product was a mixture of amorphous nickel
selenide and crystalline nickel [13]. Then the mixture was
Transition metal dichalcogenides MX (MZMn, Fe, Co,
2
Ni; XZS, Se) with pyrite structures have been paid much
attention due to their electrical, magnetic, and optical
properties [1–5]. A great number of experimental and
theoretical studies have been reported on the so-called
pyrites MX2 during the last decades [6–9]. For their
application in rechargeable battery electrodes [10] and dilute
heated at 300 8C and the crystalline NiSe and NiSe were
2
produced. Recently, hydrothermal synthesis has proved to be a
useful method to produce pyrite crystallines. For example,
NiSe nanocrystallines, CoSe nanorods, FeSe nanocrystal-
2
2
2
lines [14], MnSe nanorods and microcrystallines [15] have
2
been synthesized via the hydrothermal process.
Here, we report a facile hydrothermal method to prepare the
magnetic semiconductors (DMSs) [11], the pyrites MX are
2
cube-like MnSe microcrystallines.
2
very attractive to scientists.
Developing simple and convenient methods to prepare these
pyrites are challenging to chemists. Traditionally, transition
metal dichalcogenides are synthesized by the direct stoichio-
metric combination of elements in evacuated silica tubes [12].
But due to the limit of slow diffusion, the complete reaction
requires intermittent grinding and heating at high temperature
from 500 to 1200 8C. This process needs high energy and long
time. The direct elemental reactions were improved by
2
. Experimental section
All the chemicals were analytical grade and purchased from
Shanghai Chemical Reagents. In a typical procedure,
0.005 mol manganese sulfate monohydrate (MnSO $H O)
4
and 0.01 mol sodium hypophosphite (NaH PO $H O) were
2
2
2
2
dissolved in 40 ml distilled water, then 0.01 mol selenium
metal powder (Se) were added into the solution. The mixture
was transferred into a Teflon-lined autoclave with 60 ml
capacity, which was sealed, maintained at 160 8C for 12 h, and
then cooled to room temperature naturally. At last, the black
product was collected by filtration, washed with distilled water
and ethanol, and then dried at 60 8C in vacuum.
*
Corresponding author.
E-mail address: ytqian@ustc.edu.cn (Y. Qian).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.02.006

L. Wang et al. / Solid State Communications 138 (2006) 72–75
73
The samples were characterized by powder X-ray diffrac-
K1
tion (XRD) employing a scanning rate of 8.00008 min in the
q range from 10 to 708 and using a MAX 18 AHF X-ray
diffractometer (MAC Science) equipped with graphite-mono-
2
˚
chromatized Cu Ka1 radiation (lZ1.54056 A). The X-ray
photoelectron spectra (XPS) were recorded on a VGESCALAB
MKII X-ray photoelectron spectrometer, using a non-mono-
chromatized Mg Ka (1253.6 eV) X-ray in high vacuum
K9
5!10 Pa) as the excitation source. The binding energy
(
values were calibrated from that of C1s (284.6 eV). The Raman
spectrum was recorded on a SPEX-1403 laser Raman
spectrometer by means of back scattering technique at room
temperature. The morphology was observed by scanning
electron microscopy (SEM), which was operated on a Hitachi
S-650 scanning electron microscope. Transmission electron
microscope (TEM) images were performed on a Hitachi Model
H-800 transmission electron microscope, using an accelerating
voltage of 200 kV. The samples for TEM were prepared by
putting a few drops of absolute ethanol solution in which the
crystalline samples were ultrasonically dispersed onto an
amorphous carbon substrate supported on a copper grid and
then allowing the solvent to evaporate at room temperature.
Fig. 2. XRD pattern of the cube-like MnSe
grinded MnSe powders (b).
2
microcrystallines (a) and the
2
(
[
002) which are perpendicular to the three directions [100],
010], [001] which have been presented in Fig. 1. The
as-prepared MnSe2 microcrystallines grow along the three
directions at an equal speed, as a result, the cubic morphology
of the product is obtained. The preferential growth can be
observed at {400} peak as well. Fig. 2(b), which is the XRD
3
. Results and discussions
pattern of the grinded MnSe powders, shows the standard
2
XRD pattern (JCPDS 73-1525).
Fig. 1 shows the pyrite structure of the MnSe micro-
2
The XPS surface analysis of the product is given out in
Fig. 3. The survey spectrum shows the sample consists of Mn
and Se, as well as the absorbed carbon and oxygen on the
sample surface. High-resolution spectra are preformed at Mn
crystallines. The pyrite structure is a face-centered cubic phase
and resembles the NaCl structure. The Mn atoms replace the
Na atoms and occupy all the octahedral sites which are formed
by six nearest Se groups. The dumbbell-shaped Se groups
2
2
replace the Cl atoms, being surrounded by six nearest Mn
atoms as well. Fig. 1 also gives out the growth directions of the
cube-like MnSe microcrystallines.
2
The XRD pattern of the product without being ground is
shown in Fig. 2(a). All the diffraction peaks can be indexed as
the pure cubic MnSe phase. The lattice parameter can be
2
˚
calculated as aZ6.440 A, being close to the standard value
˚
aZ6.417 A (JCPDS 73-1525). Compared to the standard XRD
pattern, the peak of {200} is much stronger, which indicates the
growth direction of the product. In the pyrite structure, the
{
200} equivalent planes contain the three planes (200), (020),
2
Fig. 3. XPS of the MnSe survey spectrum (a) and the high resolution spectrum
2
Fig. 1. Crystal structure of the MnSe .
for Mn (b) and Se (c) core shell regions, respectively.

74
L. Wang et al. / Solid State Communications 138 (2006) 72–75
Scheme 1. Chemical reactions in the experiment.
tallines which have desquamation phenomenon on the surfaces
15b]. The smooth surfaces demonstrate that the product is well
crystallized.
[
The possible chemical reactions to form the cube-like
MnSe microcrystallines are presented in Scheme 1. In the first
2
2
K
reaction, Se is reduced to Se
H O. Secondly, the obtained Se combines with Se to form
by the reductant NaH PO $
2 2
2
K
2
2K
Se , like the formation of polysulfide ions in aqueous solution.
2
Fig. 4. Raman spectrum of MnSe
2
at room temperature.
2K
2C
At last, the produced Se2 reacts with the Mn to precipitate
the MnSe as the final product. During the whole process,
2
and Se core regions. The binding energy of Mn2p3 is at
40.95 eV and that of Se3d is at 54.95 eV, which are in good
reaction (1) is the crucial step for the formation of the well-
crystallized cubic MnSe microcrystallines. In reaction (1), the
6
2
agreement with the reported value in the literature [14a,16].
The Raman allowed modes in the pyrite structure are mainly
caused by the intraionic stretching modes of the dumbbell-
q
positive electrode potential 4 Se/Se
2K
is K0.92 eV and the
negative electrode potential 4 H PO /H PO is K0.499 eV,
q
3
3
3
2
2
K
that is, the reductant NaH PO cannot reduce Se to Se
2
2
shaped Se units. The intraionic Se–Se stretching mode of
2
spontaneously at room temperature (298 K). The whole system
needs high temperature to conquer the high activation energy
Ea of the reaction (1) and make the reaction happen. So,
reaction (1) determines the lowest reaction temperature.
Meanwhile, it should be pointed out that the high activation
pyrite-type compounds exhibits the information about the
strengths of the intraionic Se–Se bonds. In our experiment, the
peaks of the Se–Se stretching mode is observed at 232.44 and
K1
266.58 cm in Fig. 4, being close to the values in the previous
reports [17].
2
K
energy Ea of reaction (1) makes the formation of Se
very
slow, as a result, the formations of Se and the MnSe nuclei
Fig. 5 shows the SEM and TEM images of the as-prepared
MnSe microcrystallines. It can be observed that the sample is
2
2
K
2
2
become very slow as well. The slow growth of the MnSe2
nuclei benefits the growth of the well-crystallized cube-like
composed of a number of cube-like microcrystallines. The
edge length of the MnSe microcrystallines is in the range from
2
3
0 to 30 mm and the volume is from 8000 to 27,000 mm . The
MnSe microcrystallines. Other reductants such as ethylene-
2
2
surfaces of the cube-like MnSe microcrystallines are very
diamine and sodium sulfite anhydrous are also used to reduce
Se in reaction (1), but the cube-like MnSe microcrystallines
2
2
smooth, which is quite different from the MnSe microcrys-
2
cannot be produced. So, the reductant NaH PO is not
2
2
substituted due to its suitable reducing ability.
Temperature is an important factor for the system, when the
temperature is lower than 140 8C, the reaction (1) cannot
happen and the precipitates are still selenium powder. To some
extent, 140 8C is the lowest temperature determined by the high
activation energy Ea of reaction (1). When the temperature is
higher than 180 8C, a kind of so-called MnP O white-gray
x
y
precipitates which is discussed below begin to turn out. If the
system is heated to 200 8C for 24 h, the black MnSe2
microcrystallines will disappear and the white-gray MnP O
x
y
precipitates will become the only product. In general, the
temperature should be controlled in the range of 140–180, and
160 8C is an appropriate temperature to prepare the cube-like
MnSe microcrystallines. As another important factor, time
2
should be as long as 12 h in order to make the reactions
complete at 160 8C.
According to the previous reports, the obtained MnSe will
2
transform into a-MnSe, which is the thermodynamic stable
phase when the temperature is increased. However, the so-
called MnP O white-gray precipitates turn out instead of
x
y
a-MnSe in our experiment. This phenomenon may be due to
C
the following two reasons. First, H can be obtained in
Fig. 5. SEM (a)–(c) and TEM (d) images of the MnSe
2
.

L. Wang et al. / Solid State Communications 138 (2006) 72–75
75
microcrystallines and the reductant NaH PO2 cannot be
2
replaceable. The convenient hydrothermal synthetic method
can be applied to synthesize other transition metal selenides.
Acknowledgements
Financial supports from National Natural Science
Foundation of China are appreciated greatly.
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2
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