Synthesis of MoSe2 nanocrystallites by a solvothermal conversion from MoO3

Article abstract of DOI:10.1016/S0025-5408(99)00048-3

Nanocrystalline 2H-MoSe₂ was prepared from MoO₃, N₂H₄·H₂O, and Se in pyridine at 300°C for 12 h by a solvothermal method. A gray-black product was collected and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. The XRD pattern showed that a phase-pure product was prepared. TEM analysis showed that the product was polycrystalline and had plate-shaped particles with a diameter of about 40 nm. The conversion process from MoO₃ to MoSe₂ is discussed.

Full text of DOI:10.1016/S0025-5408(99)00048-3

Materials Research Bulletin, Vol. 34, No. 4, pp. 497–501, 1999
Copyright © 1999 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/99/$–see front matter
PII S0025-5408(99)00048-3
SYNTHESIS OF MoSe2 NANOCRYSTALLITES BY A SOLVOTHERMAL
CONVERSION FROM MoO3
J.H. Zhan, Z.D. Zhang, X.F. Qian, C. Wang, Y. Xie, and Y.T. Qian*
Structure Research Laboratory and Department of Chemistry, University of Science and
Technology of China, Hefei, Anhui 230026, P.R. China
(Refereed)
(Received May 19, 1998; Accepted July 9, 1998)
ABSTRACT
Nanocrystalline 2H–MoSe2 was prepared from MoO3, N2H4⅐H2O, and Se in
pyridine at 300°C for 12 h by a solvothermal method. A gray-black product was
collected and characterized by X-ray diffraction (XRD) and transmission electron
microscopy (TEM) analysis. The XRD pattern showed that a phase-pure product
was prepared. TEM analysis showed that the product was polycrystalline and had
plate-shaped particles with a diameter of about 40 nm. The conversion process
from MoO3 to MoSe2 is discussed. © 1999 Elsevier Science Ltd
KEYWORDS: A. chalcogenides, A. nanostructures, A. layered compounds,
B. chemical synthesis
INTRODUCTION
MoSe2 has a layered sandwich structure, which is isostructural with MoS2 [1]. Layers of
Se–Mo–Se interact with each other by van der Waals forces, which allow adjacent layers to
slide. It is easy for ions such as Liϩ or Naϩ to intercalate in the interlayer gaps, due to a large
d-spacing between two layers. All these properties cause layered MoSe2 to have special
applications in precursors of intercalation superconductor [2], materials for conversion of
sunlight into electrical energy in a photoelectrochemical cell [3–5], and solid-state lubricants
[6]. On the other hand, a material can exhibit different properties when the size of its particles
reach nanometer level. For example, transition-metal (Mo, W) dichalcogenide nanoparticles
outperform their bulk counterparts as solid-state lubricants [7].
*To whom correspondence should be addressed. E-mail: yqian@mail.ustc.edu.cn.
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Traditionally, crystalline molybdenum diselenide was synthesized by solid-state reaction
between a stoichiometric amount of elemental molybdenum and selenium in a sealed
evacuated tube at a temperature of at least 900°C for several days [8]. A metathetical reaction
between high-valent molybdenum halides and alkali–metal selenides proceeds rapidly and
yields crystalline product. This reaction is known as self-propagating high-temperature
synthesis (SHS) [9]. Due to self-produced heat, this reaction system may reach a high
temperature of 1050°C instantly. A similar reaction between molybdenum halides and
alkali–metal selenides in organic solvents has also been explored [10], to produce amorphous
MoSe2 at room temperature. Recently, Feldman et al. [11] reported that MoO3 and H2S can
be converted to nano-MoS2 in a reducing atmosphere of hydrogen at high temperature of
800°C [11], but there is no report on the conversion from MoO3 to nanocrystalline MoSe2 by
this method.
As a developing synthetic technology, solvothermal synthesis has gained much attention
[12,13]. It is generally conducted at intermediate temperatures (100–400°C) in a sealed
system. In this paper, we report the solvothermal synthesis of nanocrystalline MoSe2 by the
conversion of molybdenum trioxide and elemental selenium in a reducing environment of
hydrazine (N2H4).
EXPERIMENTAL
Molybdenum trioxide (99.5%) and selenium (99.95%) in powder form were purchased from
Shanghai Chemical Co. Analytical-grade hydrazine monohydrate (N2H4⅐H2O) and pyridine
were also obtained from this supplier.
A stoichiometric amount of MoO3 and Se powder was put into a stainless steel autoclave
with a 50 ml capacity. Then an appropriate amount of N2H4⅐H2O was added into the tank.
Pyridine was added to fill the autoclave up to 75% of the total volume. The autoclave was
sealed and maintained at 250–300°C for 12 h, then cooled to room temperature naturally. A
gray-black precipitate was collected. After being washed with absolute ethanol and distilled
water, the final product was dried in a vacuum box at 60°C for 4 h.
The final product was characterized by powder X-ray diffraction (XRD) and transmission
electron microscopy (TEM). XRD analysis was conducted on a Rigaku D/max ␥A X-ray
diffractometer at a scanning rate of 0.05° sϪ1 with 2␪ ranging from 10° to 70°, using
graphite-monochromatic Cu K␣ radiation (␭ ϭ 1.54178 Å). A Hitachi H-800 transmission
electron microscope operating at 200 kV was used for TEM analysis. Content of Mo and Se
in this sample was determined by ICP-AES (inductive coupling plasma-atomic emission
spectroscopy) after oxidation of the sample in aqua regia and then dissolution of caustic soda.
RESULTS AND DISCUSSION
Figure 1a shows the XRD pattern of the as-prepared sample. Four broadened peaks, which
correspond to the (002), (100), (103), and (110) planes of 2H–MoSe2, respectively, are
observed. These broadened peaks suggest that there is a small amount of amorphous product.
The TEM microphoto (Fig. 2a) of the sample showed that the MoSe2 particles were in
uniform platelet shape, with an average size of about 40 nm. Also, this microphoto showed
that there was a small fraction of amorphous product. Electron diffraction circles (Fig. 3)
revealed the polycrystallinity of the sample.

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FIG. 1
Powder X-ray diffraction patterns for (a) the sample prepared by the reaction of MoO3, Se,
and N2H4⅐H2O in pyridine at 300°C for 12 h; (b) the sample after annealing in nitrogen
atmosphere at 500°C for 2 h; and (c) MoO2 prepared by the reaction between MoO3 and
N2H4⅐H2O at 300°C for 12 h.
FIG. 2
TEM image showing (a) the main morphology of the as-prepared sample and (b) the
aggregation of the sample after annealing at 500°C.

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FIG. 3
Electron diffraction pattern for the as-prepared sample.
When the sample was annealed in nitrogen atmosphere at 500° for 2 h, all diffraction peaks
could be indexed to hexagonal MoSe2 (space group P63/mmc) (Fig. 1b). After refinement, the
calculated cell constants (a ϭ 3.29, c ϭ 12.93Å) were close to those reported on JCPDS card
29-914. Annealing may have led to aggregation of the sample, as shown in Figure 2b.
The content of Mo and Se for this sample was MoSe2.07, as determined by ICP-AES
experiments. The sample annealed in nitrogen atmosphere had an obviously different com-
position.
2H–MoSe2 nanoparticles were formed by the reactions between MoO3 and Se in a
reducing environment of hydrazine. The formation of MoSe2 undergoes a bi-step conversion
from MoO3 to MoO2 and from MoO2 to MoSe2. If no elemental Se is present in the system,
MoO3 is rapidly reduced to monoclinic MoO2 by N2H4 (Fig. 1c). When MoO2 is in place of
MoO3, it also is converted to MoSe2.
Hydrazine acts as a reducing reagent in the conversion process of MoO3. If we assume H2
to be an intermediate, there are two possible routes for the decomposition of hydrazine [14].
It can decompose thermally to produce N2, H2, and NH3 (N2H4 3 1/2 N2 ϩ NH3 ϩ 1/2 H2);
however, at sufficiently high temperatures, N2 and H2 are the only products (N2H4 3 N2 ϩ
2H2). In this work, the reaction was conducted at 300°C. This may be the main reaction.
MoO3 is converted to MoSe2 via MoO2 in the reducing environment of N2H4. The
conversion process may be schemed as follows:
2MoO3 (s) ϩ N2H4 (l) 3 2MoO2 (s) ϩ N2 (g) ϩ H2O (l)
(1)
(2)
MoO2 (s) ϩ N2H4 (l) ϩ 2Se (l) 3 MoSe2 ϩ N2 (g) ϩ 2H2O (l)
Pyridine has an important role in the conversion of MoO3 to MoSe2. Although there is no
report that elemental selenium can dissolve in pyridine, it is possible that pyridine can cause
a small fraction of selenium to dissolve in the solution phase at 300°C. On the other hand,
pyridine activates the surface MoO3 and Se powder effectively and provides a fresh surface
for the reaction.
The effect of temperature on the conversion of MoO3 to MoSe2 was also studied. Lower
temperature leads to more amorphous product. Higher temperature is helpful in crystalliza-
tion of the product, but may lead to decomposition of the solvent pyridine. Thus, we selected
a moderate temperature range (250–300°C), which is below the critical temperature
(346.8°C) of pyridine [15].

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CONCLUSIONS
MoO3 and Se were converted to 2H–MoSe2 nanocrystallites in the environment of N2H4⅐H2O
via the pyridine-thermal route, in the temperature range of 250–300°C. The product yield
obtained was good (90%) according to the amount of Se. Solvothermal conversion perhaps
affords a simple route to synthesize nanometer-sized 2H–MoSe2. The conversion of MoO3
and Se to MoSe2 is a two-step process via MoO2. Pyridine plays an important role in the
conversion of MoO3 to MoSe2. It activates Se, provides fresh surfaces for the reaction, and
promotes the reaction.
ACKNOWLEDGMENT
This work is supported by Chinese National Foundation of Natural Science Research and the
National Manometer Materials Climbing Program.
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