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www.rsc.org/xxxxxx ARTICLE TYPE
Supramolecule-mediated synthesis of MoS2/reduced graphene oxide
composite with enhanced electrochemical performance for reversible
lithium storage
Published on 11 February 2015. Downloaded by University of Aberdeen on 15/02/2015 12:42:27.

Jianbo Yea, Lin Maa, Weixiang Chen*a, Yingjie Maa, Feihe Huanga, Chao Gaob and Jim Yang Leec
5 Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x

MoS2/reduced graphene oxide (MoS2/rGO) composites are fabricated through a facile supramolecule-
mediated hydrothermal route. The effects of the supramolecule (pillar[5]arene) on the microstructure and
electrochemical lithium storage performance of the MoS2/rGO composites are investigated. It is found
10 that the MoS2/rGO composites display a wrinkled thin flaky appearance, in which there are a lot of
irregular pores and apertures. Few-layer MoS2 sheets are well dispersed and anchored on the rGO surface.
When evaluated as a host material for lithium storage, the MoS2/rGO composite exhibits much higher
specific capacity of 1050-1140 mAh g-1 with excellent cyclic performance and significantly enhanced
high-rate capability of 815-875 mAh g-1 at a current density of 1000 mA g-1 in comparison with the
15 pristine MoS2. The improved performance can be ascribed to the robust composite structure and the better
synergic effects between few-layer MoS2 and rGO sheets.

ions such as Li, Na and Mg ions.10-13 According to its 4-electron

1 Introduction conversion reaction of the lithiation/delithiation process
(MoS2+4Li++4e=Mo+2Li2S), MoS2 can achieve a higher capacity
Due to their high energy density, high operating voltage, long
than the graphite material. Various nanostructures of MoS2 such
service life and environmental friendliness, lithium ion batteries 50 as nanoflowers and nanoflakes have been studied for
20 (LIBs) are the dominant power source for a diverse range of electrochemical lithium storage.14, 15 Especially, the single- or
portable electronic products. However, their performances such few-layer MoS2 sheets exhibited a highly reversible specific
as energy and power densities need to be continually improved
capacity.3, 4 For example, the capacity contribution of the
for their applications in the electric vehicles and hybrid electric
exfoliated MoS2 nanosheets in the MoS2/PEO composite was as
vehicles. It may be better to achieve such improvements through
55 high as 1023 mAh g-1.4 However, MoS2 nanosheets often suffer
25 electrode materials innovations.1, 2 The performances of LIBs from the inferior rate capability and poor cyclic stability because
largely depend on the physical and chemical properties of of its intrinsic low electrical/ionic conductivity and structural
electrode materials. Graphite is ubiquitously used as anode in destruction during repetitive lithiation/delithiation processes.3, 4 In
commercial LIBs because of its good cycling performance and
order to overcome these drawbacks, the composite of MoS2
low-flat potential profile for lithium intercalation/deintercalation.
60 sheets with highly conductive carbon materials such as carbon
30 However, its low specific capacity (theoretical capacity of 372
nanotubes and graphene have been demonstrated to be effective
mAh g-1) and poor rate-capability limits its application in the approach.5, 6, 16-19 Graphene, a single-layer sheet of sp2 hybridized
next-generation LIBs. In the past decades, extensive work has carbon atoms, possess many unique properties, such as superior
focused on the exploring alternative anode materials with higher electrical conductivity, high charge mobility, large surface area,
capacity, excellent cyclic performance and better rate capability.1,
2
65 inherent flexibility and chemical stability. These outstanding
35 Recently, two-dimensional layered transition-metal
advantages suggest that graphene may be a desirable matrix for
dichalcogenides (TMDs) have attracted great interest in the field
hybrid electrode materials with the improved electrochemical
of electrochemical energy storage and conversion because of their performances. Integrating of graphene not only effectively
unique physical and chemical properties.3-6, 7 Due to their shorter enhances electron conductibility and promotes rapid electron
path length and more channels for lithium ion diffusion and 70 transfer for the electrode reaction, but also prevents MoS2 layers
insertion, these 2D crystals exhibit enhanced electrochemical
from restacking and stabilizes electrode structure.19-21 Moreover,
40

performances as electrode materials for LIBs.3, 4, 8, 9

due to their analogous microstructure and morphology,
MoS2, as a typical example of TMDs, is bound by strong
MoS2/graphene composites can provide a perfect surface-to-
covalent forces within the layers and the adjacent layers are held surface contact between both components, which is believed as
together by van der Waals interactions. These structural features 75 the best model for the construction of 2D/2D heterostructures.
45 enable MoS2 to be used as a host material for the intercalation of Such surface-to-surface contact would exhibit the better structure

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 Journal of Materials Chemistry A Page 2 of 11
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affinity and synergic effects, resulting in a more persistent 2 Experimental

performance enhancement and more possibility of applications
2.1 Synthesis of MoS2/rGO composite
including energy storage/conversion, catalysis and electronic

Journal of Materials Chemistry A Accepted Manuscript

nanodevices.22, 23 Our previous works have shown that the 60 Graphene oxide sheets (GOS) were prepared from natural
5 MoS2/graphene composites could be capable to enhance graphite power (Shanghai Colloid Chemical Plant, China) by the
electrochemical performances of LIBs.24-26 Other researchers modified Hummers method,43 the details of which was described
have also demonstrated the enhancements of electrochemical elsewhere.16 The supramolecule (pillar[5]arene, MW= 2269, its
lithium storage properties in MoS2/graphene composites.11, 27, 28 structure as shown in Scheme 1) was synthesized and
Despite the great improvement that has been achieved on the 65 characterized according to the literature.42 Then, the SPM was
10 synthesis MoS2/graphene composites with enhanced employed for the synthesis of MoS2/rGO composites thought
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electrochemical performances, large-scale and efficiently hydrothermal route. In a typical batch, a solution of pillar[5]arene
controllable synthesis of MoS2/graphene composites still remains (0.07 or 0.14 mmol) in 30 mL deionized water was introduced to
a great challenge. More recently, we have presented a solution- the 25 mL suspension of GOS (3.0 mmol, prepared by 36 mg of
processed strategy for facile synthesis of single- and few-layer 70 graphite powder). After stirring for 12 h, 15 mL aqueous solution
15 MoS2/graphene composites with the assistance of cationic of Na2MoO4·2H2O (1.5 mmol) and L-cysteine (L-cys, 7.5 mmol)
surfactants (such as cetyltrimethylammonium bromide, CTAB; was dropped into the suspension under stirring. The concentration
dodecyltrimethylammonium bromide, DTAB).26, 29, 30 These of SPM is 0.001 or 0.002 mol L-1 in 70 mL of hydrothermal
cationic surfactants can modify the surface of graphene oxide solution, which is respectively equivalent to 0.01 or 0.02 mol L-1
sheets (GOS) through electrostatic interactions to promote its 75 CTAB in our previous work.24 The resulting mixture was
20 assembly as well as mediate the charge incompatibility between transferred into a 100 mL Telfon-lined stainless steel autoclave,
GOS and anionic MoO42- (or MoS42-). The microstructure (layer sealed tightly and heated at 240 oC for 24 h. The autoclave was
number) of MoS2 sheets in the composites can be controlled by then cooled to room temperature naturally. The black precipitate
using different cationic surfactants or varying their concentrations was collected by centrifugation, washed with deionized water and
in hydrothermal solution.26, 29, 30 80 ethanol, and dried in a vacuum oven at 80 oC overnight.
25 Supramolecular chemistry is one of the new branches of the Afterwards, the as-prepared precipitate was further annealed at
chemistry.31, 32 The supramolecular compounds, which are 800 oC for 2 h in a stream of 10% hydrogen in nitrogen flowing at
formed via non-covalent interactions between the host and guest, 200 sccm (standard cubic centimeter per minute) to improve the
have found applications in numerous fields such as material crystallinity of MoS2. The MoS2/rGO composite was obtained
science, life science and information science.33-36 As a new class 85 after the heat treatment. Two composites, MoS2/rGO-1 and
30 of supramolecular hosts, pillar[n]arenes have been actively MoS2/rGO-2, were prepared with 0.001 and 0.002 mol L-1 SPM
studied and rapidly developed.37-40 It has been reported that the in hydrothermal solution, respectively. In control experiment, the
amphiphilic pillar[5]arene was used to modify reduce graphene pristine MoS2 was prepared by a similar hydrothermal route and
oxide to prepare rGO-Au nanoparticles for electrochemical heat treatment, except for adding neither SPM nor GOS.
sensor.41 However, to the best of our knowledge, there have been
35 few reports on the suparmolecular system associating with the
LIB application thus far. Herein, we try to employ a water-
soluble supramolecule (SPM) for the facile synthesis of
MoS2/reduced graphene oxide (hereafter MoS2/rGO) composites
based on hydrothermal reduction between Na2MoO4 and L-
40 cysteine (L-cys) in the presence of GOS. As shown Scheme 1, a
supramolecule, named as pillar[5]arene, contains ten
90
trimethylammonium groups, which are shared equally in both the Scheme 1 Schematic of Supramolecule (pillar[5]arene)
upper and lower rims, possesses a rigid symmetrical pillar.42 In
Comparison with the cationic surfactants, the pillar[5]arene can 2. 2 Characterizations of structure and morphology
45 not only better modify the GOS surface via the stronger
X-ray powder diffraction (XRD) patterns were recorded by a
electrostatic interaction, but also exhibit distinct effect on the
Bruker D8 Advance diffractometer with Cu-Kα radiation (λ=
microstructure and electrochemical performances of the
95 0.15405 nm). The crystalline structure and morphology of the
MoS2/rGO composites due to its unique structure and intriguing
samples were characterized using a transmission electron
properties. This work presented a facile synthesis of MoS2/rGO
microscope (high resolution TEM/HRTEM, JEM 2100F) and a
50 composites by SPM-mediated hydrothermal route. It was
SIRION-100 field-emission scanning electron microscope
demonstrated that the composites displays a wrinkled thin flaky
(FESEM, JEOL JSM-6700F). The elemental composition of the
appearance, in which there are a lot of irregular pores and
100 composites was analyzed by energy dispersive X-ray
apertures. Few-layer MoS2 sheets are well dispersed and
spectroscopy (EDS, GENENIS-4000). Thermogravimetric
anchored on the rGO surface. The MoS2/rGO composites exhibit
analysis (TGA) was carried out with a NETZSCH STA 409 PC
55 significantly enhanced electrochemical performances for
apparatus at a heating rate of 2.5 oC min-1 in flow air. The Raman
reversible lithium storage including high specific capacity,
spectra were obtained using a 514 nm excitation laser with a
excellent cyclic stability and enhanced high-rate capability
105 Jobin Yvon Labor Raman HR-800 spectrometer under ambient
conditions.

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2. 3 Electrochemical Measurements addition, both the MoS2/rGO-1 and MoS2/rGO-2 composites do

not display the (002) peak of the stacked rGO at about
The working electrodes were prepared by casting a slurry of 80
50 2θ=24~25°, indicating that the stacking of rGO sheets does not
wt% active material (the pristine MoS2 or MoS2/rGO composite),

Journal of Materials Chemistry A Accepted Manuscript

10 wt% acetylene black, and 10 wt% polyvinylidene fluorides occur. It is ascribed to that the MoS2 sheets anchored on the
5 binder dissolved in N-methyl-2-pyrrolidinone on a copper foil surface of rGO, which inhibits the rGO sheets from stacking.24, 30
(active material loading of 1.3~1.5 mg). After coating the above
slurry, the electrodes were dried at 110 oC under vacuum
overnight to remove the solvent and compressed subsequently.
Then the two-electrode cells were assembled in an argon-filled
10 glovebox for the electrochemical tests. Li foil was used as the
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counter and reference electrode, and a polypropylene membrane

(Celgard-2300) as the separator. The electrolyte was 1.0 M LiPF6
in a 1:1 v/v mixture of ethylene carbonate and dimethyl
carbonate. Cyclic voltammetry measurements were carried out on
15 an electrochemical workstation (CHI 660B) at a scanning rate of
0.5 mV s-1 over the potential range of 0.005-3.00 V vs. Li+/Li.
Galvanostatic charge/discharge cycles were carried out on a
LAND 2001A Battery Tester between 0.005 and 3.00 V at
various current densities. The specific capacity of lithium storage
20 was calculated based on the mass of the active materials.
Electrochemical impendence spectra (EIS; PARSTAR 2273)
studies were obtained by applying a sine wave with amplitude of
5.0 mV in the frequency range from 200 kHz to 0.01 Hz.

3 Results and discussion

25
Fig. 1 XRD patterns of (a) the pristine MoS2, (b) MoS2/rGO-1 and (c)
MoS2/rGO-2 composites prepared by a SPM-mediated hydrothermal
route after heat treatment. 55
Fig. 2 SEM images of (a) the pristine MoS2, (b) MoS2/rGO-1 and (c)
The crystalline structure of the pristine MoS2 and MoS2/rGO-1 MoS2/rGO-2 composites prepared by SPM-mediated hydrothermal
30 and MoS2/rGO-2 composites were characterized by XRD as process after heat treatment.
shown in Figure 1. All the three samples display diffraction peaks
at 2θ=14.1° 33.1°, 39.4°, 58.6°, 69.1°, which can be readily Figure 2 illustrates the SEM images of the pristine MoS2,
indexed to the (002), (100), (103), (110) and (201) planes of a 60 MoS2/rGO-1 and MoS2/rGO-2 composites. L-cys is commonly
hexagonal structure of MoS2 (JCPDS no. 37-1492). The (002) used to fabricate various metal sulfides nanomaterials such as
35 reflection at 2θ=14.1° is corresponded to a d-spacing of 0.62 nm, MoS2, CuS and SnS2 due to its three significant functional groups
which is in agreement with that of 2H-MoS2. Figure 1 also shows of -NH2, -COOH and –SH.5, 44, 45 L-cys can also be used to
that the pristine MoS2 displays a much sharper (002) reflection reduce GOS to rGO under hydrothermal condition.46 Thus, in this
than two MoS2/rGO composites, indicating the well-stacked 65 work, we employed L-cys as reductant and S-source to prepare
layered structure of the annealed MoS2. However, both MoS2/rGO composites by a SPM-mediated hydrothermal route.
40 MoS2/rGO composites, especially for MoS2/rGO-2, display the As shown in Figure 2(a), the MoS2 displays a flower-like
lower and broader (002) diffraction peak even after annealing, appearance, which agrees with the literature.5 The flower-like
which indicates that the (002) plane growth of MoS2 crystals in MoS2 is assembled by interlaced flakes. In contrast, Figure 2(b, c)
the composites is inhibited.24, 30 The poor and broader (002) peak 70 shows that both MoS2/rGO-1 and MoS2/rGO-2 composites
is also indicative the few-layer MoS2 in the composite. By using display a wrinkled thin flaky appearance, which is similar to the
45 Scherrer's equation, the layer number of MoS2 sheets can be MoS2/rGO prepared by CTAB-assisted hydrothermal method.30 It
estimated to be about 7, 5 and 2 layer, respectively, for the is different that a lot of irregular pores and apertures can be
pristine MoS2, MoS2/rGO-1 and MoS2/rGO-2 composites. In observed from the MoS2/rGO composites, especially from

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MoS2/rGO-2 composite, which may be ascribed to the unique distributions of the pristine MoS2 and MoS2/rGO-2 composite
structure (a rigid symmetrical pillar as shown in Scheme 1) and calculated by BJH method. As demonstrated in Figure S2, both
intriguing properties of SPM. In addition, isolated MoS2 particles the pristine MoS2 and MoS2/rGO-2 composite are characteristic

Journal of Materials Chemistry A Accepted Manuscript

are hardly observed in Figure 2(b, c), which indicates that MoS2 of mesopores. The mesopores of the pristine MoS2 may originate
5 sheets are well in surface-to-surface dispersed and anchored on 40 from the void spaces among the interlaced flakes. It is worth
the surface of the curved rGO sheets. notice that when the pore size ranges from 9 nm to 30 nm,
MoS2/rGO-2 composite exhibits much higher dV/dD value than
the pristine MoS2. Therefore, the mesopores of MoS2/rGO-2
composite are contributed to not only the void spaces between
45 MoS2-rGO heterostructural sheets, but also its numerous pores
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and apertures. Even if these pores and apertures may not produce
significant contribution to the surface area, they can provide more
shortened paths of lithium ion diffusion and more active sites for
lithium ion accommodation, and also facilitate the easy access of
50 the electrolyte, resulting in great improvement in the
electrochemical performances of the MoS2/rGO composite for
reversible lithium storage.47, 48 In comparison with MoS2/rGO
composite prepared by CTAB-assisted hydrothermal route,
MoS2/rGO-2 composite also exhibits much higher dV/dD value
55 when the pore size ranges from 9 nm to 30 nm (seeing Figure S4).

E11g

Intensity (a.u.)
D
LA
E12g A1g G
(a)

(b)

250 300 350 400 1500 2000

Raman shift (cm-1)

10
Fig. 3 TEM and HRTEM images of (a, b) the pristine MoS2 and (c, d)
MoS2/rGO-1 and (e, f) MoS2/rGO-2 composites prepared via a SPM-
mediated hydrothermal method after annealing.
Fig. 4 Raman spectrum of (a) the pristine MoS2 and (b) MoS2/rGO-2
To further investigate their microstructure, the pristine MoS2, composite after heat treatment; (c) the atomic displacements of two
15 MoS2/rGO-1 and MoS2/rGO-2 composites were characterized by 60 Raman-active modes in the unit cell of the MoS2 crystal.
TEM and HRTEM as shown in Figure 3. Figure 3(a) clearly
The MoS2/rGO-2 composite was further characterized by
shows the interlaced MoS2 flakes. It can further find from Figure
Raman spectroscopy to confirm the few-layer MoS2 as shown in
3(b) that the pristine MoS2 displays a well-defined layered
Figure 4(left). Four mode Raman peaks at 278.8 cm-1 (E11g ),
structure with 5-9 layers and a (002) plane interlayer distance of
335.9 cm-1 (LA), 376.8 cm-1 (E12g) and 403.6 cm-1 (A1g) can be
20 0.62 nm, agreeing well with its XRD analysis. In contrast to the
65 observed from the Raman spectroscopy of the pristine MoS2 in
pristine MoS2, Figure 3 (c, d) and (e, f) show the few-layer MoS2
Figure 4(a).49 While only two lower peaks at 376.8 cm-1 (E12g)
sheets with some defects in both MoS2/rGO-1 and MoS2/rGO-2
and 401.3 cm-1 (A1g) can be found for the MoS2/rGO-2 composite
composites. The layer number of MoS2 sheets is 5-6 and 2-3 layer,
in Figure 4(b, left). The A1g mode arises from the out-of-plane
respectively, which also agree with the XRD analysis.
vibration of just S atoms in the opposite direction along the c-axis
25 The specific surface area and porous nature of the typical two
70 while the in-plane E12g mode is associated with the opposite
samples (the pristine MoS2 and MoS2/rGO-2 composite) are
vibration of the Mo atom versus two S atoms, which is clearly
determined by measuring the N2 adsorption-desorption isotherms
shown in Figure 4(c).49 Investigations have indicated that the E12g
at 77 K (seeing Figure S1 in ESI). The surface areas of the
and A1g mode of MoS2 are sensitive to the number of atomic
pristine MoS2 and MoS2/rGO-2 composite are 14.3 and 35.2 m2
layers.49, 50 Especially, the few-layer MoS2 sheet has a down-shift
30 g-1, respectively. The surface area of MoS2/rGO-2 composite is
75 of A1g peak with the reducing layer number.49 The A1g peak at
closed to the corresponding results of the mesoporous Co3O4
401.3 cm-1 is indicative of few-layer MoS2 sheets for the
sheets/3D-graphene (34.5 m2 g-1)47 and MoS2 nanosheets/carbon
MoS2/rGO-2 composite.50 In addition, the lower intensity of E12g
nanotube paper (32 m2 g-1).48 The higher surface area of these
and A1g mode peaks indicates that the MoS2 sheets in the
composites may be attributed to the incorporation of graphene
composite process more defects and disordered structure
35 (rGO) or carbon nanotubes. Figure S2 shows the pore size
80 compared with the pristine MoS2, which is coincident with the

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XRD and TEM/HRTEM analysis. Figure 4 (b, left) also shows 50 calculated for investigating the electrochemical performances.
that the E11g and LA mode peaks of MoS2 are almost invisible for The synthesis strategy in this work demonstrates that MoS2/rGO
MoS2/rGO-2 composite. The fact is ascribed to that the few-layer composites should contain MoS2 and rGO with a small quantity

Journal of Materials Chemistry A Accepted Manuscript

MoS2 sheets are anchored on the rGO surface.6, 51 of amorphous carbon. From results of EDS and TG analysis, we
5 To further confirm the existence of rGO in MoS2/rGO-2 can calculate the content of MoS2, rGO and amorphous carbon by
composite, the Raman spectrum was collected from 1100 to 2200 55 using the calculating method in our previous work.54 It is
cm-1. Figure 4(right) shows two pronounced Raman peaks at calculated that the content of MoS2/rGO-1 composite is 77.8wt%
1345 cm-1 (D-band) and 1589 cm-1 (G-band) of rGO, which agree of MoS2 and 15.4wt% of rGO with 6.8wt% of amorphous carbon;
with that of hydrazine-reduced-GO52 and hydrothermal-reduced- and the content of MoS2/rGO-2 composite is 75.6wt% of MoS2
10 GO53. The G-band represents the vibration of ordered sp2-carbon and 14.9wt% of rGO with 9.5wt % of amorphous carbon.
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atoms in a 2D hexagonal lattice, while the D-band is ascribed to

edges, defects and disordered carbon. Commonly, the relative
Raman intensity ratio of ID/IG provides a convenient clue to
measure defects in carbon materials. The calculated value of ID/IG
15 is 1.39 for MoS2/rGO-2 composite. The value is higher than that
of L-cys-reduced-GO (1.08-1.17), which may be caused by the
additional amorphous carbon produced from the SPM
carbonization during heat treatment.
The element compositions of two MoS2/rGO composites were
20 identified with repeat energy dispersive X-ray spectroscopy
(EDS) analysis. The MoS2/rGO-1 composite contains Mo
(46.1wt%), S (31.1wt%), C (18.3wt%) and a small quantity of O
(4.5wt%); and MoS2/rGO-2 composite contains Mo (44.5wt%), S
(30.6wt%), C (19.7wt%) and a small quantity of O (5.2wt%). C
25 element is derived from both rGO and amorphous carbon
produced by the carbonization of the SPM during heat treatment.
A small amount of O element is ascribed to the residual oxygen- 60

containing functional group of rGO and amorphous carbon.54

According to the element compositions, the weight percent of
30 MoS2 can be calculated to be 77.2% and 75.1% for MoS2/rGO-1
and MoS2/rGO-2 composites, respectively. The atomic ratios of
S:Mo are 2.02 and 2.06 for MoS2/rGO-1 and MoS2/rGO-2,
respectively, which are very close to the stoichiometry of MoS2. Scheme 2 Schematic of a SPM-mediated hydrothermal synthesis of the
MoS2/rGO composite.

In our recent works, the cationic surfactants (such as CTAB

65 and DTAB) were employed to modify the GOS surface for
preparing the single- and few-layer MoS2/rGO composites based
on the concurrent reduction of Na2MoO4 (or (NH4)2MoS4) and
GOS under hydrothermal treatment or refluxing.29, 30, 56 These
cationic surfactants have shown some ability to control the
70 microstructure (layer number) of MoS2 in the composites. In this
work, SPM-mediated hydrothermal route was developed to
synthesis the MoS2/rGO composites. Based on the above
35 Fig. 5 TGA curves of (a) MoS2/rGO-1 and (b) MoS2/rGO-2 composites. characterization results and the prevailing views, the mechanism
for the SPM-mediated hydrothermal synthesis of MoS2/rGO
The content of MoS2 in the MoS2/rGO composites was further
75 composite could be reconstructed as shown in Scheme 2. The
confirmed by thermo-gravimetric analysis (TGA) in dry air. The
pillar[5]arene with a stable macrocyclic ring contains ten
TGA curves of both MoS2/rGO composites are shown in Figure 5.
trimethylammonium groups shared equally in both the upper and
The one continuous weight loss in the range of approximately
lower rims, thus it can better modify the GOS surface via the
40 290-480oC for the MoS2/rGO composites are attributed to the
stronger electrostatic interaction than the cationic surfactants. A
removal of oxygen-containing groups, the oxidation of MoS2 to
80 suspension of SPM-modified GOS can be easily prepared by
MoO3 and the decomoposition of rGO and amorphous carbon.26 It
mixing GOS and a SPM solution under stirring. The modifying of
was reported that the rGO and amrophous carbon could be
SPM on the GOS surface would render the GOS surface
completely oxidized under 600~700 oC in air,55 hence the finally
positively charged to mediate the charge incompatibility between
45 remaining product of the MoS2/rGO composites was only MoO3.
26 GOS and anion MoO42-. In comparison with cationic surfactants
Thus, the mass fraction of MoS2 in MoS2/rGO-1 and
85 (such as CTAB and DTAB), the pillar[5]arene can not only better
MoS2/rGO-2 composites can be calculated to be around 78.4wt%
modify the GOS surface via the stronger electrostatic interaction,
and 76.2wt%, respectively, which agree well with EDS analysis.
but also exhibit distinct effect on the microstructure and
In addition, the component content of the composites needs to be

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electrochemical property of the MoS2/rGO composite due to its and 3rd cathodic sweep, the two reduction peaks at 0.85 V and
unique molecule structure and intriguing properties. When an 0.34V at the 1st CV disappeared, while other three new reduction
aqueous solution of Na2MoO4 was introduced to the suspension peaks at about 1.88 V, 1.10 V and 0.25 V can be discerned for

Journal of Materials Chemistry A Accepted Manuscript

of SPM-modified GOS, MoO42- could be adsorbed on the SPM- 45 both electrodes. The peak at about 1.88 V is attributed to the
5 GOS surface to form a MoO42--SPM-GOS complex. Under lithiation process of S to form Li2S, while other two poor peaks at
hydrothermal condition, MoO42- anions were reduced to MoS2 by 1.10 V and 0.25 V are ascribed to the association of Li+ ions with
L-cys, simultaneous with the reduction of GOS to rGO. L-cys Mo and lithium insertion in defect sites, respectively, according
plays a dual role of reducing agent and sulfur source during the to the literatures.59-61 In addition, Figure 6 also shows that
hydrothermal process. The fresh hydrothermal product consists 50 MoS2/rGO-2 composite electrode displays higher peak current
10 mainly of MoS2 and rGO with a small amount of SPM. The with larger area than the pristine MoS2, indicating that the former
subsequent heat-treatment in N2/H2 at 800 oC finalizes the
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is better capable for reversible lithium storage than the pristine

synthesis of MoS2/rGO composite. The small amount of SPM in MoS2.
the composite precursor would be carbonized to amorphous
3.5
3rd
carbon during annealing. The rGO and amorphous carbon can (a)
3.0

Voltage/ V vs. Li /Li

15 effectively restrain MoS2 from stacking in c-direction, resulting 2nd 1st

+
2.5
in few-layer MoS2 sheets in the composite. In addition, the e
2.0 charg
carbonization of pillar[5]arene with a rigid symmetrical pillar 1.5 3rd
would produced a lat of pores and apertures in MoS2/rGO 1.0 1st
composite. 0.5 disc
har
ge
2nd
0.0
3.0 0 300 600 900 1200 1500
1st
2.0
(a) Capacity(mAh g )
-1
Current (mA/mg)

2nd
3.5
1.0 3rd 2nd
(b)
3.0 1st 3rd
0.0

Voltage/ V vs. Li+/Li

2.5
-1.0
g e
-2.0
2.0 char
1.5
-3.0
3rd
1.0
-4.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.5 1st disch
20 Potential/V vs. Li+/Li 2nd
arge
0.0
0 300 600 900 1200 1500
3.0 Capacity(mAh g-1)
1 st (b) 55
2.0
Current (mA/ mg)

3.5
2 nd 2nd
1.0 3.0
(c)
3 rd 1st 3rd
Voltage/ V vs. Li /Li

0.0 2.5
+

-1.0 2.0 rge

cha
-2.0 1.5
3rd
-3.0 1.0
disc
-4.0 0.5 1st harg
0.0 0.5 1.0 1.5 2.0 2.5 3.0 2nd
e
0.0
Potential/ V vs. Li+/Li 0 300 600 900 1200 1500
-1
Fig. 6 Cyclic voltammograms of (a) the pristine MoS2 and (b) Capacity(mAh g )
MoS2/rGO-2 composite electrodes at a scan rate of 0.5 mV/s. Fig. 7 Galvanostatic discharge/charge potential profiles of (a) the pristine
MoS2, (b) MoS2/rGO-1 and (c) MoS2/rGO-2 composite electrodes for the
Cyclic voltammetry measurements were firstly conducted in first three cycles at a current density of 100 mA g-1.
25 order to better investigate the redox reactions taking place in the
electrodes. Figure 6 shows the first three cycle voltammograms 60 Figure 7 illustrates the galvanostatic discharge/charge potential
(CVs) of the pristine MoS2 and MoS2/rGO-2 composite curves of the pristine MoS2, MoS2/rGO-1 and MoS2/rGO-2
electrodes for reversible Li+ storage. In the 1st cathodic scan, both composite electrodes for the first three cycles at a current density
electrodes display two reduction peaks at 0.85 V and 0.34 V as of 100 mA g-1. As shown in Figure 7, during the first discharge
30 shown Figure 6. The peak at 0.85 V could be attributed to the (lithiation) process, the three electrodes display two potential
intercalation of Li+ into the MoS2 lattice to form LixMoS2, which 65 plateaus at about 1.1 V and 0.6 V, which agree with their first
leads a phase transformation change from 2H (trigonal prismatic) CVs. The plateau at about 1.1 V is due to the intercalation of Li+
to 1T (octahedral) structure.57, 58 The other peak at 0.34 V could into the MoS2 to form LixMoS2 and another plateau at about 0.6 V
be regarded as the conversion reaction of LixMoS2 into metallic is ascribed to the conversion reaction of LixMoS2 into metallic
35 Mo and Li2S.57, 58 In the 1st reverse anodic scan, a dominant peak Mo and Li2S (Mo nanoparticles embedded in Li2S matrix).62 The
at around 2.35 V can be observed, which is ascribed to the 70 sloping potential curve below 0.5 V could be ascribed to the
oxidation of Li2S to form S.58 The electrode reactions at the 1st formation of a solid electrolyte interphase (SEI) film resulting
CV can be described as: MoS2+xLi++xe−→LixMoS2, LixMoS2+(4- from the electrochemically driven electrolyte degradation.58, 62 In
x)Li→Mo+2Li2S and Li2S-2e-→S+2Li+. Thus, the electrode the 2nd and 3rd discharge curves, there are three potential plateaus
40 material is mainly composed of Mo and sulfur instead of the located at around 2.1 V, 1.2 V and 0.5 V, which also agree with
initial MoS2 at the end of the first CV.59-61 In the following 2nd 75 the corresponding changes of their CV features. In the charge

6 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]
 Page 7 of 11 Journal of Materials Chemistry A
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DOI: 10.1039/C5TA00006H

(delithiation) process, the pristine MoS2, MoS2/rGO-1 and such high capacity of MoS2/rGO composites should been
MoS2/rGO-2 composite electrodes display a pronounced potential 50 attributed to the synergic interaction between few-layer MoS2 and
plateau at 2.3 V, which is consistent with the dominant oxidation rGO sheets.

Journal of Materials Chemistry A Accepted Manuscript

peak at 2.35 V in their CVs. Figure 7 also shows that the pristine
5 MoS2 delivers a discharge capacity of 1180.5 mAh g-1 and a 1600

-1

-1

500 mA g-1
1000 mA g-1
100 mA g
200 mA g
reversible capacity of 882.2 mAh g-1 with a Coulombic efficiency 1400

Capacity (mAh g-1)

-1
of 74.7% in the 1st cycle. For MoS2/rGO-1, the corresponding 1200 100 mA g (c)
values are 1397.2 mAh g-1 and 1002.2 mAh g-1 with a Coulombic 1000
(b)
efficiency of 71.7%; and for MoS2/rGO-2, the corresponding 800
10 values are 1521.5 mAh g-1 and 1072.6 mAh g-1 with a Coulombic 600
Published on 11 February 2015. Downloaded by University of Aberdeen on 15/02/2015 12:42:27.

efficiency of 70.5%. The irreversibility in the first cycle is due to 400

(a)
the irreversible electrode processes such as the formation of the 200 Charge:
Discharge:
SEI film, the electrochemically driven electrolyte degradation and 0
0 10 20 30 40 50 60 70 80
reduction of oxygen-containing groups.54 At the 2nd and 3rd cycle, Cycle number
15 the Coulombic efficiency for the three electrodes increases up to
more than 94%. Fig. 9 Rate capabilities of (a) pristine MoS2, (b) MoS2/rGO-1 and (c)
MoS2/rGO-2 composite electrodes at different current densities.
1600
1400 55 Figure 9 depicts the rate capabilities of the pristine MoS2，
Capacity(mAh g )
-1

(c)
1200 MoS2/rGO-1 and MoS2/rGO-2 composite electrodes at different
1000 current densities. As shown in Figure 9(b, c), MoS2/rGO-1 and
(b)
800
MoS2/rGO-2 composite electrodes not only exhibit higher
600
specific capacity and significantly enhanced rate-capability, but
400 (a)
Charge: 60 also display better cycle stability at different current densities
200
Discharge:
0
than the pristine MoS2. In particular, at a high current density of
0 20 40 60 80 100 1000 mA g-1, MoS2/rGO-2 composite electrode delivers the
Cycle number
reversible capacity as high as 875 mAh g-1 with stable durability.
Fig. 8 Cycling performances for (a) pristine MoS2, (b) MoS2/rGO-1 and The corresponding value for the MoS2 is only 495 mAh g-1.
(c) MoS2/rGO-2 composite electrodes at a current density of 100 mA g-1. 65 Figure 8 also shows that after cycling at different current
20 Figure 8 shows the cycling performances of the pristine MoS2, densities (100, 200, 500 and 1000 mA g-1), both MoS2/rGO-1 and
MoS2/rGO-1 and MoS2/rGO-2 composite electrodes at a current MoS2/rGO-2 composite electrodes can almost completely recover
density of 100 mA g-1. As shown in Figure 8(a), the pristine the original capacity when the current density is set back to 100
MoS2 electrode delivers a reversible capacity of 882-918 mAh g-1 mA g-1, and further testing demonstrates the good conservation of
for the first 15 cycles, which is closed to that (860 mAh g-1) of 70 the capacity. In contrast, as the current density reverses from
25 the exfoliated MoS2 sheets electrode.3 However, after 15 cycles, 1000 mA g-1 to 100 mA g-1, the specific capacity of the pristine
the pristine MoS2 electrode shows a successive capacity fading. MoS2 electrode cannot return to its original value. In addition,
Its reversible capacity gradually descends from 918 mAh g-1 at MoS2/rGO composites prepared in this work also exhibit better
10th cycle to 517 mAh g-1 at the 100th cycle, with a capacity rate-capability in comparison with our previous work30 and other
retention of 56.3%. In comparison with the pristine MoS2, both 75 works.11, 20 For example, the few-layer MoS2/rGO composite
30 MoS2/rGO-1 and MoS2/rGO-2 composite electrodes exhibit prepared by CTAB-assisted hydrothermal route delivered a
higher reversible capacity with excellent cyclic stability as shown reversible capacity of 690 mAh g-1 at 1000 mA g-1.30 The
in Figure 8(b, c). For the span of 100 cycles, the MoS2/rGO-1 and reversible capacity of both MoS2/rGO hybrid paper and MoS2-
MoS2/rGO-2 composite electrodes hardly display capacity fading, coated 3D-rGO networks was about 600 mAh g-1 at the same
indicating their excellent durability. The MoS2/rGO-1 and 80 current density as anode of LIBs.11, 20
35 MoS2/rGO-2 composite electrodes can retain the reversible Many works have demonstrated significantly enhanced
capacity of 1020-1050 mAh g-1 and 1080-1140 mAh g-1, performances of MoS2/rGO composites prepared by different
respectively, which are superior to that of MoS2/rGO hybrid methods for electrochemical energy storage and conversion.19, 20,
28, 30
paper (890 mAh g-1)20 and CNT@MoS2 sheets (862 mAh g-1).63 These excellent electrochemical properties might be
In addition, we find that the specific capacity of MoS2/rGO 85 attributed to the reasonable material selection and compositing.
40 composites is higher than the sum of the contribution from three Because the few-layer MoS2 sheets display analogous
components (MoS2, rGO and amorphous carbon). Our previous microstructure and morphology to the rGO, MoS2/rGO
work showed that the specific capacity of amorphous carbon and heterostructures would exhibit better synergic effects between
L-sys-reduced GO is 190 and 780 mAh g-1, respectively.54 This both components, resulting in the great enhancement in
work showed that the capacity of the MoS2 is 900 mAh g-1. Based 90 electrochemical performance. The rGO can serve as a perfect
45 on the content of each component of the composites, the specific matrix for the growing, dispersing and anchoring of few-layer
capacities of MoS2/rGO-1 and MoS2/rGO-2 composites can be MoS2 nanosheets. Due to its superior conductivity and excellent
calculated to be 833 and 815 mAh g-1, respectively, which are flexibility, graphene (or rGO) not only enhances electron
lower than their experimental results (1050-1140 mAh g-1). Thus, conductibility and promote rapid electron-transfer for electrode
95 reaction, but also effectively buffers the volume change of

This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 7
 Journal of Materials Chemistry A Page 8 of 11
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DOI: 10.1039/C5TA00006H

electrochemical active material and stabilizes electrode structure leading to a great reduction in contact resistance. Moreover, rGO
during the repetitive charge/discharge process. Therefore, the can effectively promote rapid electron transfer due to its superior
enhanced reaction kinetics with higher reversible capacity and conductivity and high charge mobility. It was reported that the

Journal of Materials Chemistry A Accepted Manuscript

better cyclic stability can be realized in MoS2/rGO composite rGO electrode exhibited a very high electron-transfer rate, which
5 prepared in this work. In comparison with other MoS2/rGO and 50 was more than 10-fold faster than that of graphite.64 Thus, the
MoS2/CNT composites, MoS2/rGO composites prepared by a composite of few-layer MoS2 with rGO also great reduces the
SPM-mediated hydrothermal route not only display a wrinkled electron-transfer resistance. The Rct of MoS2/rGO-2 electrode is
thin flaky appearance, but also form which a lot of irregular pores only 25.2% of that of the pristine MoS2 electrode. In addition, the
and apertures can be observed. These pores and apertures numerous pores and apertures of MoS2/rGO-2 composite not only
10 increase exposing edges and defects, resulting in the more 55 shortens diffusion paths of lithium ion, but also facilitates the
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shortened paths for lithium ion diffusion, more active sites for easy access of the electrolyte, thus further improves kinetics
lithium accommodation and more space for electrolyte access. performance of reversible lithium storage.
Therefore, MoS2/rGO composites not only delivers high
reversible specific capacity with the excellent cyclic stability, but 4 Conclusions
15 also exhibits better rate-capability than other MoS2/rGO and
In summary, we have successfully fabricated MoS2/rGO
MoS2/CNT composites reported elsewhere.5, 20, 28
60 composites through a SPM-mediated hydrothermal route. The
MoS2/rGO composites display a wrinkled thin flaky appearance,
from which a lat of pores and apertures can be observed. Few-
layer MoS2 sheets are well dispersed and anchored on the rGO.
The MoS2/rGO composites exhibit much better electrochemical
65 properties than the pristine MoS2 electrode for reversible lithium
storage. In particular, the MoS2/rGO-2 composite electrode
exhibits a high reversible specific capacity as high as 1150 mAh
g-1 with a capacity retention of almost 100% after 100 cycles, and
significantly enhanced rate capability of 875 mAh g-1 at high
70 current density of 1000 mA g-1. The improved performance can
be attributed to the robust composite structure and the better
synergic effects between few-layer MoS2 and rGO sheets. The
present results also turn out that the SPM-mediated hydrothermal
route can be employed for the facile synthesis of MoS2/rGO
75 composites and the composites prepared as such holds promise as
Fig. 10 (a) Nyquist plots of the pristine MoS2 and MoS2/rGO-2 composite anode materials for high performance LIBs.
20 electrodes obtained by applying a sine wave with amplitude of 5.0 mV
over the frequency range from 200 kHz to 0.01 Hz; (b) equivalent circuit
model of the studied system. CPE is the constant phase element.
Acknowledgments
This work is financially supported by the International Science
In order to better understand the advantages of the MoS2/rGO
and Technology Cooperation Program of China
composites, the electrochemical impedance spectra (EIS) was
80 (2012DFG42100), the Natural Science Foundation of China
25 employed to investigate the electrochemical reaction kinetics of
(21173190 and 21473156), the Doctoral Program of Higher
the both electrodes after 10 cycles as shown in Figure 10(a).
Education of China (2011010113003), the International Science
Figure 10(b) elucidates the equivalent circuit model of EIS. Re
and Technology Cooperation Program of Zhejiang Province
denotes the internally ohmic resistance of the test battery,
(2013C24011). Lee JY is also grateful for financial aid from
corresponding to the intercept of high-frequency semicircle at
85 A*STAR Project 1220203049 (R279-000-370-305).
30 Zreal axis; the semicircle in high frequency could be attributed to
the resistance Rf and constant phase element CPE1 of the SEI
Notes and references
film; the semicircle in medium frequency range is classically
a
assigned to the charge transfer resistance Rct and constant phase Department of Chemistry, Zhejiang University, Hangzhou 310027, P.R.
element CPE2 of the electrode/electrolyte interface; the inclined China. Fax: +86 571 87951895; Tel: +86 571 87952477; E-mail:
weixiangchen@zju.edu.cn
35 line in the low frequency region corresponds to Warburg b
90 Department of Polymer Science and Engineering, Zhejiang University,
impedance (Zw) associated with the lithium ion diffusion Hangzhou 310027, P. R. China.
process.24, 30 By the data fitting using Zview software, the Re, Rf c
Department of Chemical and Biomolecular Engineering, National
and Rct of the pristine MoS2 electrode are calculated to be 3.2 Ω, University of Singapore, 10 Kent Ridge Crescent, 119260, Singapore.
29.8 Ω and 67.9 Ω, respectively. The corresponding values for † Electronic Supplementary Information (ESI) available: [N2 adsorption-
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40 MoS2/rGO-2 electrode are 2.4 Ω, 4.3 Ω and 17.1 Ω, respectively. MoS2/rGO-2 compoiste]. See DOI: 10.1039/b000000x/
Therefore, EIS demonstrates the significant reduction in the ‡ Footnotes should appear here. These might include comments relevant
contact resistance of the overall electrode and charge-transfer to but not central to the matter under discussion, limited experimental and
resistance for the lithiation/delithiation reaction after the spectral data, and crystallographic data.
incorporation of graphene. As shown in Figure 2(c), the rGO 100

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 Page 11 of 11 Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA00006H

Graphical Abstract

Journal of Materials Chemistry A Accepted Manuscript

A supramolecule-mediated hydrothermal route was for the first time developed for the
facile synthesis of MoS2/rGO composites, which exhibit significantly enhanced
electrochemical performance for reversible lithium storage.
1600

-1

-1
-1

1000 mA g-1
100 mA g
200 mA g
500 mA g
1400

Capacity (mAh g-1)

Published on 11 February 2015. Downloaded by University of Aberdeen on 15/02/2015 12:42:27.

-1
MoS2/rGO-2
1200 100 mA g

1000
800 MoS2/rGO-1

600
400 MoS2
200 Charge:
Discharge:
0
0 10 20 30 40 50 60 70 80
Cycle number