Preparation of MoS2 by a novel mechanochemical method

Article abstract of DOI:10.1016/j.jallcom.2009.11.070

A novel simple process for preparing MoS₂ by a mechanochemical route under the normal temperature and pressure has been developed. In this method, the mechanochemical solid-state reaction was completed in a high-energy planetary ball mill filled with pure argon. The effects of milling time and speed were discussed and the product was characterized by XRD, DSC and TEM. It is found that the as-obtained samples are MoS₂ with short low stacked layers less than 10 nm, which have potential application in the highly active catalysts. Crown Copyright

Full text of DOI:10.1016/j.jallcom.2009.11.070

Journal of Alloys and Compounds 492 (2010) L5–L7
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Preparation of MoS2 by a novel mechanochemical method
Zhuangzhi Wu, Dezhi Wang∗, Aokui Sun
School of Materials Science and Engineering, Central South University, Yuelu Road, Changsha, Hunan 410083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 August 2009
Received in revised form 7 November 2009
Accepted 10 November 2009
Available online 13 November 2009
A novel simple process for preparing MoS2 by a mechanochemical route under the normal temperature
and pressure has been developed. In this method, the mechanochemical solid-state reaction was com-
pleted in a high-energy planetary ball mill filled with pure argon. The effects of milling time and speed
were discussed and the product was characterized by XRD, DSC and TEM. It is found that the as-obtained
samples are MoS2 with short low stacked layers less than 10 nm, which have potential application in the
highly active catalysts.
Keywords:
Molybdenum disulfide
Preparation
Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
Mechanochemical
Semiconductors
Nanomaterials
1. Introduction
reports on the mechanochemical preparation of MoS2 by high-
energy ball milling.
There has been considerable interest in the transition metal
dichalcogenide layered compounds, a group of anisotropic mate-
rials with strong bonding within the layers and weak interlayer
interactions. As one of the members of this family, MoS2 has numer-
ous applications as hydrodesulfurization catalyst [1], solid-state
lubricant [2], electrode in high-energy density batteries [3,4], inter-
calation host to form new materials [5], etc.
Conventionally, the synthesis methods of MoS2 involve thermal
decomposition [6], high temperature, solid-state reaction between
molybdenum oxide and sulfur powders [7], chemical bath deposi-
tion [8], hydrothermal method [3,9,10], spray drying [11], chemical
vapor deposition (CVD) [12], even ␥-irradiation method [13], etc.
Among these methods, nearly all the solid-state methods demand
high temperature (600–1200 ◦C), and most of the aqueous state
methods are performed in a pressurized device at relatively high
temperatures (180–300 ◦C). In our previous work, hexagonal MoS2
nanoparticles were prepared directly in an aqueous solution at a
low temperature (100 ◦C) [14], but the obtained MoS2 had poor
crystallinity and even could not be confirmed by XRD. Therefore, it
is still a challenge to develop a simple novel method to synthesize
MoS2 at low temperatures.
2. Experimental
The typical preparation procedure is as follows. First, 3.4 g (NH4)2MoO4 and
10 g Na2S were dissolved in distilled water. The amorphous precursor of MoS3 was
prepared by adding appropriate amount of hydrochloric acid into the above aqueous
solution. Then, the dried precursor was energetically ball-milled at 400 rpm in an
atmosphere of argon for 24 h in a planetary ball mill. Compared experiments were
also performed at different speeds for different milling time. Stainless steel balls
were used as the milling media. At last, the ball-milled product was washed with
carbon disulfide, hydrochloric acid, alcohol and deionized water for several times
to remove by-product. The final molybdenum disulfide was collected and dried at
60 ◦C for 12 h.
The prepared MoS2 was characterized by X-ray powder diffraction (XRD) anal-
ysis using D/max-2500 system with a Cu K␣ irradiation source (ꢀ = 0.154 nm). The
differential scanning calorimetry (DSC) was also performed by a DSC 200 F3 Maria
calorimeter. The morphology and structure of the final MoS2 were further examined
with transmission electron microscopy (TEM, Tecnai G2 20).
3. Results and discussion
The XRD patterns of the precursor and ball-milled products for
different time in the 2Â range of 10–80◦ are shown in Fig. 1. It can
be seen that the precursor of MoS3 (Fig. 1a) is of amorphous struc-
ture with low broaden peaks and diffuse background, while the
ball-milled samples (Fig. 1b and c) match well with the hexagonal
MoS2 standard positions (JCPDS Card No. 37-1492). The appearance
of (1 0 0) and (1 1 0) peaks reveals the formation of MoS2. More-
over, the (1 0 3) reflection also can be observed with the milling
time prolonged to 24 h. However, the precursor is amorphous, and
almost all the peaks of the final product are broadened because of
the strong effect of shearing and crushing by ball milling. Therefore,
Herein, we report the synthesis of MoS2 via a simple novel
mechanochemical route under the normal temperature and pres-
sure. According to our best knowledge, so far there have been no
∗
Corresponding author. Tel.: +86 731 88877221.
E-mail address: dzwang@mail.csu.edu.cn (D. Wang).
0925-8388/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.11.070

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Z. Wu et al. / Journal of Alloys and Compounds 492 (2010) L5–L7
Fig. 3. DSC curves of products obtained at different milling speeds: (a) 200 rpm; (b)
300 rpm; (c) 400 rpm.
Fig. 1. XRD patterns of the precursor (a) and ball-milled product for different time:
(b) 12 h; (c) 24 h.
As a precursor, the MoS3 in the above reaction is prepared by the
traditional precipitation of ammonium tetrathiomolybdate. Gener-
ally, the thermal decomposition of MoS3 occurs at above 400 ◦C, so
there will be an exothermic peak at about 400 ◦C, indicating the
existence of the precursor. Fig. 2 shows the DSC curves of the as-
obtained products by different milling time. The exothermic peaks
near 400 ◦C are weakened with increase in milling time, and dis-
appear when the milling time is up to 24 h, which means that the
precursor is completely converted into MoS2.
The milling speed also plays a crucial role in the mechanochem-
ical reaction, as shown in Fig. 3. The exothermic peaks near 400 ◦C
are weakened with increase in milling speeds, and disappear when
the milling speed is up to 400 rpm, indicating a completed conver-
sion of MoS3 into MoS2 at a milling speed above 400 rpm.
Fig. 4a and b shows the microstructures of the amorphous pre-
cursor and final product. Obviously, different from the former, the
latter has typical laminar structures of MoS2, while unlike the image
of highly ordered planes of MoS2 along the c-direction which is
expected in the 2H-MoS2, and these structures exhibit domains of
short-range layered arrays stacked in a fairly chaotic fashion [15].
As shown in Fig. 4b, the length and height of the stacked slabs are
less than 10 nm, providing rich active sites for catalysis. Moreover,
the interlayer distance between (0 0 2) lattice planes is 0.62 nm,
which is consistent with that of the literature for the hexagonal
lattice of the MoS2 phase (PDF: 37-1492). If the ball-milled product
is annealed at 850 ◦C for 0.5 h, the short low stacked layers turn to
the typical structures of 2H-MoS2 with long high stacked layers, as
shown in Fig. 4c. It reveals that the prepared MoS2 by ball milling
constitutes a novel morphology and structure between the amor-
phous trisulfide and the thermodynamically stable 2H allotrope.
Fig. 2. DSC curves of products obtained by different milling time: (a) 2 h; (b) 6 h; (c)
12 h; (d) 24 h.
it is very hard to judge whether the precursor has been completely
converted into the final MoS2 from the XRD patterns.
The mechanochemically induced solid-state reaction can be
simply presented by the following reaction:
MoS3 → MoS2 + S
(1)
Fig. 4. TEM images of (a) the precursor; (b) ball-milled product obtained at 400 rpm for 24 h; (c) the annealed sample of (b).

Z. Wu et al. / Journal of Alloys and Compounds 492 (2010) L5–L7
L7
Since active centers of the HDS reaction are located on the edges of
the MoS2 slabs, the short low stacked layers with important amount
of edges can significantly enhance the catalytic activity [7,16,17],
indicating potential applications of the obtained MoS2 in highly
active catalysts.
be used to prepare highly active catalyst because of the high content
of edges and defects induced by ball milling.
Acknowledgments
Generally, the thermal decomposition of MoS3 occurs at above
400 ◦C. However, herein, the MoS2 is successfully prepared by a
mechanochemical method through the decomposition of amor-
phous MoS3 under the normal temperature and pressure. This
abnormal phenomenon can be attributed to the activation of pre-
cursor and the local high temperature by crushing [18]. It is very
difficult to measure the local microscopic temperature during
milling because of the dynamic nature of the milling process. But
the temperature rise can be estimated by observing the microstruc-
tural and/or crystal structure changes during milling. Compared
with the samples obtained from the thermal decomposition of
MoS3, the XRD pattern and layer stacks of the MoS2 prepared by
ball milling are similar to that of MoS2 obtained by thermal decom-
position at 400 ◦C. So the local temperature rise in this case can
be estimated to be about 375 ◦C under a room temperature of
25 ◦C.
This work was supported by the Graduate Degree Thesis Innova-
tion Foundation of Central South University (1960-71131100018)
and Nonferrious Metals Science Foundation of HNG-CSU (Y2008-
01-009).
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In summary, we have successfully prepared MoS2 with short low
stacked layers by a mechanochemical method under the normal
temperature and pressure for the first time. It is also found that
milling time and speed have great effects on the formation of the
final MoS2. If only the milling time and speed are above 24 h and
400 rpm, respectively, the conversion of MoS3 into MoS2 can be
finished completely. Moreover, the local temperature rise in the
ball milling is also estimated to be about 375 ◦C. This strategy can
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