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CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS

VOL. 41, 2014

A publication of

The Italian Association
of Chemical Engineering

www.aidic.it/cet
Guest Editors: Simonetta Palmas, Michele Mascia, Annalisa Vacca
Copyright © 2014, AIDIC Servizi S.r.l.,
ISBN 978-88-95608-32-7; ISSN 2283-9216

MoS2 Nanosheets for HER and LIB

Maria Sarno, Anna Garamella, Claudia Cirillo*, Paolo Ciambelli

Department of Industrial Engineering and Centre NANO_MATES, University of Salerno
Via Giovanni Paolo II ,132 - 84084 Fisciano (SA), Italy
clcirillo@unisa.it

Very high surface area thin nanosheets of MoS2, with also low lateral dimension, as well as high number of
edges sites, were prepared via a solvent free, easily controllable and scalable process. The syntheses
were performed by thermolysis of ammonium thiomolybdates (NH4)2MoS4 in a continuous flow reactor,
monitored with a mass spectrometer. The precursor decomposition was promoted in the temperature
range 25-400°C, obtaining amorphous MoS2. Nanosheets were obtained thought a successive annealing,
while material order can be improved by progressively increasing temperature and time. It was found that
the annealing time at the end of the first step determines a reduction of the final nanosheets lateral size
and number of layers. The samples were characterized by Raman Spectroscopy, Scanning (SEM) and
Transmission (TEM-EDS) Electron Microscopy, thermogravimetric analysis (TG-DTG-MS), X-ray
diffraction (XRD).

1. Introduction
MoS2 nanostructure have generated intense scientific interest owing to their promising electronic and
mechanical properties (Ramakrishna Matte et al., 2010), in the area of energy conversion and storage.
MoS2 has been widely investigated as catalysts for electrocatalytic or photocatalytic hydrogen evolution
reaction (HER) in aqueous solution and as efficient electrode material for lithium ion batteries (LIBs) (Zhao
et al., 2013; Stephenson et al., 2014). Even if bulk MoS2 is not active for the HER, it has been forecasted
by using density functional theory calculation (Hinnemann et al., 2005) an excellent electrocatalytic activity
(tafel slope 55–60 mVdec−1), linearly dependent by the number of edge sites, for MoS2 nanocatalyst
(Jaramillo et al., 2007) prepared on an Au substrate. The same authors (Bonde et al., 2008) prepared later
a more commercially relevant nanocatalyst, MoS2 nanoparticles on a Toray carbon paper, finding higher
tafel slope (120 mV/dec) and exchange current density. Further information about the electrocatalytic
activity of MoS2 comes form the paper of Merki et al. (Merki et al., 2011). They prepared amorphous thin
films of MoSx (about 1-2 μm thick), finding that the real catalyst was amorphous MoS2, that exhibits a Tafel
slope of 40 mVdec−1. More recently, engineering the MoS2 structure, to have an high surface area
mesostructure, exposing a large fraction of edge sites, a tafel slope of 50 mV/dec, has been found
(Kibsgaard et al., 2012).
The reported results clearly demonstrate that the catalytic activity of MoS2 toward the HER is very
promising and closely associated with the different morphology of the prepared nanostructures. It deserves
to be further investigated to definitely elucidate the effect of the support through a systematic comparison
of the same nanostructured MoS2 on a support and alone, and clarified the effect of order of the
nanostructured materials prepared through a simple and scalable process.
MoS2 is a conductive material characterized by a distinctive layered structure that makes it favorable for
reversible Li+ intercalation/deintercalation (Xiao et al. 2010). The electrochemical performance of MoS2 as
a LIB electrode was believed to be significantly influenced by morphology, structure and particle size. The
Li diffusion path could be significantly shortened in nanostructured MoS2 improving the performance. As a
nanostructure materials MoS2 can exist in a diverse range of morphologies and microstructures. These
include fullerene like MoS2, MoS2 nanotubes, MoS2 nanowires, nanoribbons and nanosheets. In particular,
with regards to this specific application, hydrothermal synthesized MoS2 nanoflakes (Feng et al., 2009) and
amorphous MoS2 nanoflowers (Li et al., 2009) (prepared by an hydrothermal method), and nanotubes
(Dominko et al., 2002) (prepared by a mixture of C60, used as promoter, and MoS2 powder at 1010 K and

DOI: 10.3303/CET1441066

Please cite this article as: Sarno M., Garamella A., Cirillo C., Ciambelli P., 2014, Mos2 nanosheets for her and lib, Chemical Engineering
Transactions, 41, 391-396 DOI: 10.3303/CET1441066

391

10-3 Pa), have been studied as anode materials and have been proven to have high capacity. On the other
hand, at this stage the majority of the structures have not been investigated as electrode materials for
lithium storage, the effect of the material order has not been clarified.
MoS2 nanotubes (Loh et al., 2006), MoS2 nanorods (Ota and Srivastava, 2006), MoS2 nanofibers (Liao et
al., 2001) and MoS2 nanoflakes (Pol et al., 2008), have been synthesized. Moreover, after the discovery
and the consequent enormous attention toward graphene properties and potential application, a growing
research space is currently addressed to other 2-D materials and, among them, to layered inorganic
materials such as dichalcogenides and especially MoS2. In particular, for these specific applications the
preparation of thin nanosheets, with also low lateral dimension, allows to have an high total surface area
exposed, as well as a number of edges sites and short path for Li diffusion. Significant effort has been
devoted to prepare MoS2 thin layers. Several methods have been used to synthesize thin layers of MoS2:
scotch tape assisted micromechanical exfoliation, exfoliation in solution, physical vapor deposition,
hydrothermal synthesis, electrochemical synthesis and sulfurization of oxides of molybdenum. However,
the MoS2 so produced tends to form structures similar to fullerene, zero-dimensional nanoparticles or
nanotubes. A very interesting approach consists in a thermolysis of MoS2 precursors in organic solvent
(Altavilla et al., 2011). However, the products are often amorphous or low-crystalline and shown significant
deficiency of molybdenum and the presence of impurities, such as carbon and oxygen.
Another method for producing thin layers of MoS2 from amorphous to highly crystalline, with large surface
area, consists of a solvent free thermolysis process (Liu et al., 2012). The method is one of the most
scalable, permitting also, for example in presence of a template structure (Sarno et al., 2013; Ciambelli et
al., 2011; Sarno et al. 2012), to prepare very promising carbon/MoS2 composites (Zhao et al., 2013;
Stephenson et al., 2014; Li et al. 2011).
Here we report, the synthesis of MoS2 nanosheets, via a solvent free easily controllable and scalable
process. MoS2 nanosheets have been obtained by thermolysis of ammonium thiomolybdates (NH4)2MoS4
in a continuous flow microreactor fed by nitrogen. The precursor decomposition was promoted in the
temperature range 25-400°C, which permits to obtain amorphous MoS2 sheets, followed by a second step
in the range 25-1100°C to increasingly improve the starting material quality. In particular, the synthesis
process was monitored with a mass spectrometer to follow the evolution of the reactions. Moreover, we
have given much attention to the effect of changing the operating conditions, such as temperature, time
and number of reaction steps, in view of a process/materials optimization. All the samples obtained were
characterized by the combined use of different techniques such as micro-Raman Spectroscopy, Scanning
Electron Microscopy (SEM), Transmission Electron Microscopy – Energy dispersive X-ray spectroscopy
(TEM-EDS), thermogravimetric analysis coupled with a mass spectrometer (TG-DTG-MS), X-ray diffraction
(XRD).

2. Experimental
MoS2 nanosheets has been prepared by thermolysis of ammonium thiomolybdates (NH4)2MoS4 (Sigma
Aldrich, purity of 99.99%; 0.25g) in a N2 environment, thought two steps of conversion: (i) from room
temperature to 400°C + isotherm for 10 min (MoS2_1) or 60 min (MoS2_2); followed by (ii) a cooling from
400°C to room temperature; and finally (iii) a step from room temperature to 1100°C + isotherm for 30 min
(MoS2_3 from MoS2_1 and MoS2_4 from MoS2_2) or 75 min (MoS2_5 from MoS2_2).
The annealing was carried out in a continuous flow microreactor, consisting in a quartz tube (16 mm
internal diameter, 300 mm length), the precursor was loaded on a sintered support (Ciambelli et al., 2011;
Sarno et al., 2013). All the samples obtained were characterized by the combined use of different
techniques. Transmission electron microscopy (TEM) images were acquired using a FEI Tecnai electron
microscope operated at 200 KV with a LaB6 filament as the source of electrons, equipped with an EDX
probe. Scanning electron microscopy (SEM) images were obtained with a LEO 1525 microscope. Raman
spectra were obtained at room temperature with a micro-Raman spectrometer Renishaw inVia with a 514
nm excitation wavelength (laser power 30 mW) in the range 100-3000 cm

-1. Optical images were collected
with a Leica DMLM optical microscope connected on-line with the Raman instrument. For all the sample
about 40 measurements have been carried out. The laser spot diameter was about 10 µm. XRD
measurements were performed with a Bruker D8 X-ray diffractometer using CuKα radiation.
Thermogravimetric analysis (TG-DTG) at a 10 K/min heating rate in flowing air was performed with a
SDTQ 500 Analyzer (TA Instruments) coupled with a mass spectrometer.

392

3. Results and discussion
The thermolysis of ammonium thiomolybdates (NH4)2MoS4 has been extensively studied (Berntsen et al.,
2008; Brito et al., 1995; Leist et al., 1998). It has been reported that (NH4)2MoS4 thermolysis in an N2
environment resulted in the conversion of (NH4)2MoS4 to MoS3, releasing ammonia and hydrogen sulfide
to the gas phase ((NH4)2MoS4 → 2NH3 + H2S + MoS3) at lower temperatures, followed by a conversion of
MoS3 to highly disordered MoS2 (MoS3 → MoS2 + S), the product remained amorphous up to around
350°C, while crystallinity set in at 400°C when first vague lines appeared on the diffractograph. To improve
the MoS2 quality, it is rational to increase the thermolysis temperature. Although a measure of agreement
is evident in the previous studies, there is ambiguity in the transition temperatures, nor systematic
nanostructures synthesis studies have been performed. We develop the syntheses in two steps, the first
one between 25-400°C, to obtain disordered MoS2, further annealed for 10 or 60 min, the second step
between 25 to 1100°C, followed by 30 or 75 min of annealing to further improve the degree of order. We
prepared two different kinds of materials: low crystallinity samples at 400°C and high crystallinity samples
at 1100°C, and following the evolution of the precursor thermolysis with the help of a thermogravimetric
analyser. In particular, in Figure 1a the thermogravimetric (TG-DTG) and temperature profiles, during the
test simulating the MoS2_5 synthesis conditions, characterized by the two longer isotherm steps, are
reported as function of time. It is possible to distinguish five temporal phases: (I) from room temperature to
400°C; (II) 1h in isotherm at 400°C; (III) from 400°C to room temperature (not followed by the
thermobalance); (IV) from room temperature to 1100°C; (V) 630 min in isotherm at 1100°C. After an initial
weight loss due to water, at temperatures between 150-250°C the DTG profile shows a sharp weight loss
due to NH3 and H2S release, as clearly indicated by the corresponding total ion current (TIC) of the most
intense mass fragments peaks: m/z = 15, 16, 17 form NH3 and 32, 33, 34 from H2S (reaction 1). The
NH3 and H2S release completing at 360°C. At 170°C a sulphur release starts, with a maximum at 380°C,
compatible with the evolution of reaction 2. A further 3 wt.% is lost in the 60 min isothermal step at 400°C.
During the further temperature increase (phase IV) and starting from 400°C a sulphur release (m/z=32)
happens. A weight loss is also registered during the high temperature isotherm, due to an intrinsic MoS2
thermal instability (Spalvins, 1987) and the unavoidably few trace of oxygen, with SO2 (m/z=64) release
and contemporaneous formation of oxides.To follow the crystallization, XRD patterns were recorded on the
decomposition products after heating at specific temperatures (Figure 1b). Both MoS2_1 and MoS2_2
exhibit the loss of the original precursor structure and the typical x-ray pattern of amorphous MoS2, the
(002) reflection results broader and less intense if compared with the rest of the spectrum for sample
annealed for 60 min, indicating an exfoliation tendency for longer time annealing. After the second step of
thermolysis the typical spectrum of 2H-MoS2 is shown for both MoS2_3 and MoS2_4, a very broad (002)
reflection is shown in the spectrum of MoS2_4 (Berntsen et al., 2008; Leist et al., 1998), while MoS2_3
exhibits a more pronounced sawtooth (002) peak, sharper and less intense if compared with MoS2_1. At
increasing high temperature annealing time, the peak at about 13° (2Θ) becomes more and more
pronounced and sharper, indicating a progressive in-plane restacking, also the typical MoO2 peaks appear
timidly, likely due to an initial oxidation phenomenon. Similar to the case of graphene, Raman
spectroscopy can be employed to characterize the thickness of MoS2 nanolayers (Castellano-Gomez et
al., 2012; Lee et al., 2010). The bulk MoS2 shows bands at 407.5 and 382 cm

-1 due to the A1g and E’2g
modes (Li et al., 2012). As reported by Lee et al. (Lee et al., 2010), the frequency difference between the
two most prominent Raman peaks depends monotonically on the number of MoS2 layers. In particular, in
Figure 2 number of significant Raman spectra are reported for MoS2_4, MoS2_3 and MoS2_5, most of
MoS2_4 and MoS2_3 sheets have lower than 6 layers, while MoS2_5 sheets are decidedly thicker, in
agreement with the XRD observation. It is worth to notice the presence in MoS2_3 of sheets consisting of
more than 10 layers if compared with MoS2_4.
For comparison, MoS2_1, MoS2_3 and MoS2_4 micrographs are shown in Figure 3, it is evident in the
resolution limits of the instrument, a delamination of the MoS2_1 powders after the second step of thermal
treatment (compare Figures 3d and 3e). Finally, MoS2_4 consists of lower size powders with a rougher
surface. Figure 3g shows the complete N2 adsorption-desorption isotherm of MoS2_3, it presented a well-
developed porous structure with a type IV isotherm, which had an obvious hysteretic loop with a
desorption step above the relative pressure of 0.4.

393

Figure 1. TG-DTG profiles, during the test simulating the MoS2_5 synthesis conditions, and the
corresponding TIC (a). XRD diffraction patterns of the precursor and samples after heating at the different
temperatures (b).

360 380 400 420 440

A
1g

In
te

n
si

ty
(

a
.u

Wavenumber (cm
-1
)

E1
2g MoS2_3

360 380 400 420 440

A
1g

In
te

n
s
ity

(
a

.u
.)

Wavenumber (cm
-1
)

E1
2g

MoS2_4

360 380 400 420 440

A
1g

In
te

n
si

ty
(

a
.u

Wavenumber (cm
-1
)

E1
2g MoS2_5

18
19
20
21
22
23
24
25
26
27

0 2 4 6 8 10 12

Number of layers

Fr
eq

ue
nc

y
di

ff
er

en
ce

(
cm

-1
)

c. Lee et al. (2010)

MoS2_4

MoS2_3

MoS2_5

>10

Figure 2. Raman Spectra of MoS2_3, MoS2_4, MoS2_5 and the frequency difference between A1g and
E’2g Raman modes

Figure 3 SEM images of MoS2_1 (a), MoS2_3 (b) and MoS2_4 (c) - 200X, scale bar 100 μm. SEM images
of MoS2_1 (d), MoS2_3 (e) and MoS2_4 (f) - 1000X, scale bar 10 μm. N2 adsorption-desorption isotherm
of MoS2_3 (g).

The MoS2_3 BET surface areas (SA) is 208 m2/g, a very high value if compared with that reported for
conventional MoS2 prepared by thermal decomposition of MoS2 precursors (from 49.3 to 64.4 m

2/g in Leist
et al. (Leist et al.,1998), where the authors speculated that both the heat treatment and the presence of
vacuum are necessary for porous material to be obtained and compared their results with the BET surface

a b c

d e f

20 40 60 20 40 60

MoS2_4

MoS2_2

MoS2_3

precursor

In
te

ns
ity

(
a.

u.
)

In
te

ns
ity

(
a.

u.
)

2 Θ°

103
MoS

112
MoS

110
MoS

105
MoS

2
002
MoS

100
MoS

MoS2_1

006
104
MoS

2 004
MoS

105
MoS

103
MoS

100
MoS

002
MoS

022
MoO

-211
MoO

112
MoS

110
MoS

2
105
MoS

103
MoS

MoS2_5

2 Θ°

002
MoS

100
MoS

112
MoS

110
MoS

Isotherm Plot

100

150

200

250

0,0 0,2 0,4 0,6 0,8 1,0
p/po

V
[c

c/
g]

394

areas of crystalline 5.8 m2/g and restacked 10 m2/g MoS2). BET SA of 40 m
2/g and 77.7 m2/g has been

reported for conventional MoS2 in (Skrabalak et al., 2005) and (Berntsen et al., 2003), respectively. A
solvothermal route was explored in (Berntsen et al., 2003) to obtain a BET of 152.9 m2/g and an ultrasonic
spray pyrolysis in (Skrabalak et al., 2005) to obtain a surface area of 250 m2/g. MoS2_3 has a total pore
volume of 300.01 mm3/g, and a multimodal pore size distribution (BJH Desorption pore distribution)
centred at 2, 3, 7, 10 and 15 nm (pores between exposed surface sheets). It is worth to notice that the
BET SA of MoS2_1 is 2.88 m2/g, while the MoS2_4 SA results 260 m2/g.

Figure 4 TEM images of MoS2_1 scale bar 2μm (a), scale bar 500 nm (b), scale bar 10 nm (c,d). TEM
images of MoS2_2 scale bar 1μm (e). TEM images of MoS2_3 scale bar 50 nm (f), scale bar 20 nm (h),
scale bar 10 nm (i.l). TEM images of MoS2_4 scale bar 50nm (g), scale bar 20 nm (m).

Typical TEM image of the MoS2 samples are shown in Figure 4. MoS2_1 (Figure 4a) is constituted of
larger (tens of microns in size) and smaller few aggregates (1 micron in size); MoS2 layers, 0.62 nm
stacked, are visible in both aggregates (Figure 4c and d). The Figure 4e shows MoS2_2, constituted of
small particles with size of few hundred nanomaters. MoS2_3 (Figure 4f) and MoS2_4 (Figure 4g),
obtained at the end of the second step and after 30 min of annealing, exhibit a different morphology.
MoS2_3 is constituted of larger sheets consisting of few layers or more than 10 layers (inserts of Figure
4h), moreover smaller sheets of few nanometers lateral size with the characteristic MoS2 fringes can be
observed in the figures 4g and 4m for MoS2_4.

4. Conclusions
Very high surface area thin nanosheets of MoS2, with also low lateral dimension were prepared via a
solvent free, easily controllable and scalable process. The precursor decomposition was promoted in the
temperature range 25-400°C, obtaining amorphous MoS2. Nanosheets were obtained thought a
successive annealing, while material order can be improved by progressively increasing temperature and
time. It was found that the annealing time at the end of the first step determines a reduction of the final
nanosheets lateral size and number of layers.

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