A novel route to obtain molybdenum dichalcogenides by hydrothermal reaction

Article abstract of DOI:10.1246/cl.2000.920

Hydrothermal reactions between aqueous Na₂MoO₄, Na₂ESO₃ (E=S, Se) and hydrazine monohydrate at 135 °C for 12 h produce phase-pure MoS₂ and MoSe₂ with good yield of about 90%. X-ray powder diffraction, chemical analysis and X-ray photoelectron spectroscopy were used to characterize the structure and composition of the products annealed at 350 °C. Transmission electron microscopy images show that the samples of MoS₂ and MoSe₂ consist of nano-crystallites with average particle size of 4 nm and 7 nm, respectively.

Full text of DOI:10.1246/cl.2000.920

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Chemistry Letters 2000
A Novel Route to Obtain Molybdenum Dichalcogenides by Hydrothermal Reaction
Rong Fan,†,†† Xianhui Chen,*†,††† and Zuyao Chen††
Structure Research Laboratory, University of Science and Technology of China, Hefei, 230026, P. R. China
†
†
†
Department of Chemistry, University of Science and Technology of China, Hefei, 230026, P. R. China
Department of Physics, University of Science and Technology of China, Hefei, 230026, P. R. China
†
††
(Received May 8, 2000; CL-000446)
Hydrothermal reactions between aqueous Na MoO , using Na SeSO as selenium source. Na SeSO was prepared
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4
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3
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Na ESO (E=S, Se) and hydrazine monohydrate at 135 °C for
by refluxing Na SO and selenium powder in distilled water for
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2 3
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2 h produce phase-pure MoS and MoSe with good yield of
10 h. The precipitates of molybdenum dichalcogenides were
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about 90%. X-ray powder diffraction, chemical analysis and X-
ray photoelectron spectroscopy were used to characterize the
structure and composition of the products annealed at 350 °C.
Transmission electron microscopy images show that the sam-
ples of MoS and MoSe consist of nano-crystallites with aver-
annealed at 350 °C in flowing high-pure N atmosphere for 9 h
2
to yield crystalline products. Chemical analysis for the compo-
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1
sition was conducted according to previous methods.
The precipitates from hydrothermal reaction are amor-
phous molybdenum dichalcogendizes indicated by X-ray dif-
fraction (XRD). The XRD pattern shown in Figure 1a for
annealed MoS can be indexed to hexagonal 2H-MoS structure
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2
age particle size of 4 nm and 7 nm, respectively.
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2
with lattice parameter a = 3.12 Å and c = 12.73 Å, which is
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Molybdenum dichalcogenides have drawn considerable
attention due to their technological importance as high-energy
consistent to that in the literature. The composition of this
sample was determined by chemical analysis to be MoS2.02
XPS spectrum also demonstrated its composition close to its
chemical formula. TEM image shown in Figure 2a for
.
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density battery cathodes, lubricants and hydrodisulfuridation
3
catalysts etc. Their sandwich interlayer structure allowed them
to act as intercalation host to derive novel materials with modi-
fied physical properties. Traditionally, the most straightfor-
annealed MoS indicates that it consists of homogeneous parti-
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4
cles with an average size of 4 nm. While the calculation by
Scherrer’s equation suggests the size in the direction [00l] is
lager than that in the direction [h0l]. This is because the layer
rotation around the axis perpendicular to the basic planes of
(00l), which leads to the lack of constant orientation between
sheets. Due to the high surface Gibbs free energy, these nano-
crystallites prone to agglomerate and are not easy to divide by
ward method to obtain molybdenum dichalcogenides is the stoi-
chiometric combination of metal molybdenum powder and ele-
mental chalcogen in an evacuated silica tube at high-tempera-
ture, however which demands significant energy input and
gives little control in the particle size of products. Besides, a
nano-scaled material can exhibit different properties and often
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performs better than its bulk counterparts in many applications.
supersonic wave. MoSe obtained was also characterized by
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Some low-energy routes to molybdenum disulfides, such as
polar organic solvent based metathesis of molybdenum halides
and alkali-metal sulfides or covalence sulfur sources, have been
XRD pattern shown in Figure1b to derive its structure to be
hexagonal 2H-MoSe type with lattice parameter a = 3.22 Å
2
and c = 13.31 Å. Its composition was determined by chemical
analysis to be MoSe1.95. However, due to the exchange of
Na SeSO and Na SSeO , a problem must be proposed that
explored.6 MoS film has been prepared by deposition reaction
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of molybdenum carbonyls with various sulfur sources in non-
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aqueous solution.7 Kaner et al. reported a rapid solid state syn-
whether the products is pure MoSe or contains large amount of
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thesis of MoS from high valence molybdenum halides and
sodium monosulfide. But the large extherm results in a local
MoS . XPS spectrum shown in Figure 3 further convinced the
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purity of the products of MoSe . The typical spectrum shows
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high temperature up to 1050 °C. Zhan et al. reported a
the existence of Mo, Se and unavoided C, O surface contamina-
tion from absorbed gaseous molecules. Considering the value
of binding energy and relative intensity ratio of different ele-
ments, no other impurity peaks can be detected. The peak area
in Mo 3d core and Se 3d core were measured and calculated to
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solvothermal reaction
of molybdenum trioxide and selenium
powder in pyridine to obtain nano-meter MoS and MoSe .
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In the present paper, we first report an aqueous solution
based route to molybdenum dichalcogenides from Na MoO
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and Na ESO (E=S, Se) assisted by hydrazine. It gives obvious
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advantages in comparison with previous low-energy methods:
1) it is not necessary to prepare unstable or air-sentitive precur-
(
sor of high valence halides; (2) water, a convenient and green
solvent, is more favored than toxic organic solvents in industri-
al applications. Our experiment procedure are presented below.
A stoichiometric amount of Na MoO and Na S O was
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put into a Teflon-lined autoclave and dissolved with 35% aque-
ous N H solution into a clear, colorless solution. This auto-
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clave was sealed and maintained at 135 °C for 12 h. The black
solid precipitate was collected by filtering, washed with dis-
tilled water and ethanol, and dried at 60 °C for 3 h. The synthe-
sis of MoSe was carried out in an analogous procedure by
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Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
921
and the stoichiometry of starting materials has been observed.
When N H is absent, brown MoS wet gel can be obtained,
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which can partly convert to MoS by reducing with hydrazine.
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Thus we infer that MoS cluster may be the intermediate and
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the reaction procedure can be explained as follows:
However, for a shortened reaction period of 1 h, only product of
MoS forms in a smaller yield. Thus, the second step of reduc-
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tion with N H is a rapid one.
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The sample of MoS2 and MoSe2 are homogeneous
nanoparticles with average size of 4 nm and 7 nm, respectively,
which are much smaller than that of the samples prepared by
previous solid-state reactions or solvothermal process at 300
°C. Usually, the hydrothermal reactions involve a solid–liquid
interaction procedure, in which solid reagents may act as nucle-
ation core for the growth of products. But in our experiment,
the initial reaction medium is a clear solution of Na S O ,
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Na MoO and N H , which may contributes to the homoge-
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neous nucleation of the product of MoS . Moreover, a greatly
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lowered reaction temperature gives better control of particle
size.
In summary, MoS and MoSe were prepared successfully
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by a hydrothermal reduction of aqueous Na S O and Na MoO
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assisted by N H at 135 °C for 12 h, followed by annealing at
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50 °C in an inert atmosphere. The mechanism of these
hydrothermal reactions and morphology control are investigat-
ed. To our knowledge, many low-temperature solution routes
to transition metal dichalcogenides are explored, but all of them
are conducted in non-aqueous medium, even moisture in air
will influence the formation of products significantly. We first
developed an aqueous solution based route to molybdenum
dichalcogenides, and it could be applicable for preparing other
technologically important transition metal dichalcogenides.
give the ratio of Mo to Se to be 1: 1.90. Infrared spectrum of
MoSe obtained recorded in a KBr matrix shows no IR
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–
1
absorbence in the region 930–940 cm for Mo–O vibration.
This confirmed the good purity of MoSe obtained without
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amorphous MoO . Actually, reduced MoO usually forms in an
x
x
acid condition, while our hydrothermal reactions were carried
out in weak alkaline media. The TEM image shown in Figure
This work was supported by the grant of Natural Science
Foundation of China.
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b for annealed MoSe shows that it comprises of homogenous
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nanoparticles. The average size is about 7 nm calculated by
Scherrer’s equation in the direction of [001]. These hydrother-
mal reactions can be related below:
References
1
J. Rouxel and R. A. Brec, Rev. Mater. Sci., 16, 137 (1986).
J. R. Lince and P. D. Fleischauer, J. Mater. Res., 2, 827
2
(1987).
3
4
R. R. Chianelli, Catal. Rev. — Sci. Eng., 26, 361 (1984).
S. Lemaux, A. S. Golub, P. Gressier, and G. Ouvrard, J.
Solid State Chem., 147, 336 (1999).
Besides, we also explored one-step hydrothermal prepara-
tion from Na MoO , Na SO and selenium powder assisted by
5
6
7
8
9
L. Rapoport, Y. Bibik, Y. Feldman, M. Homyonfer, S. R.
Cohen, and R. Tenne, Nature 387, 791 (1997).
D. M. Schleich and M. J. Martin, J. Solid State Chem., 64,
359 (1986).
G. Chatzitheodorou, S. Fiechter, M. Kunst, and H.
Tributsch, Mater. Res. Bull. 23, 1261 (1988).
P. R. Bonneau, R. F. Jarvis, Jr., and R. B. Kaner, Nature,
349, 510 (1991).
2
4
2
3
N H . The final product proves to be phase-pure MoSe , too.
2
4
2
Some minor unreacted Se powder can be removed readily due
to its density less than that of water.
In order to understand the possible mechanism of these
hydrothermal reactions, we changed the amount of starting
reagents while maintaining other factors constant, such as tem-
perature, reaction period, etc. If Na S O is absent, the initial
J. H. Zhan, Z. D. Zhang, X. F. Qian, C. Wang, Y. Xie, and
2
2
3
clear solution of Na MoO and N H gives no solid precipitate
Y. T. Qian, Mater. Res. Bull., 34, 497 (1999).
2
4
2
4
after hydrothermal process. With the increasing of the amount
of Na S O , phase-pure MoS forms without any other solid by-
10 J. H. Zhan, Z. D. Zhang, X. F. Qian, C. Wang, Y. Xie, and
Y. T. Qian, J. Solid State Chem., 141, 270 (1998).
2
2
3
2
product. So, the formation of MoS is chemically favored in
11 R. Belcher and A. J. Nutten, “Quantitative Inorganic
2
this pathway. No function between the nature of final products
Analysis,” Butterworths, London (1970), Vol. 3.