nanocomposite materials based on chitosan and molybdenum disulfide
TRANSCRIPT
Nanocomposite materials based on chitosan and molybdenumdisulfide
Iskandar Saada • Rabin Bissessur
Received: 29 February 2012 / Accepted: 5 April 2012 / Published online: 24 April 2012
� Springer Science+Business Media, LLC 2012
Abstract Molybdenum disulfide (MoS2) was lithiated
with n-butyllithium to obtain LiMoS2. The exfoliation and
re-stacking properties of LiMoS2 allowed for the facile
intercalation of chitosan into the re-stacked layers of MoS2.
A series of nanomaterials were synthesized, and charac-
terized by powder-X-ray diffraction, thermogravimetric
analysis, and four-probe electrical conductivity measure-
ments. The chitosan–MoS2 nanomaterial at 1:1 mol ratio
displayed an increase in electrical conductivity value by a
factor of 100 with respect to the pristine layered structure.
Introduction
Since the commercialization of the rechargeable lithium
ion power source in the 1990s, there has been an expo-
nential growth in the demand for high energy density, light
weight, and environmentally benign power sources [1].
Lithium ion batteries offer several advantages over tradi-
tional rechargeable cells (e.g., lead–acid and nickel–cad-
mium), and these include higher energy density, longer
shelf-life, and no memory effect [2]. However, a new type
of lithium ion battery has recently emerged, known as the
lithium ion-polymer battery [3]. This battery can be uti-
lized in handheld devices to deliver improved energy
density and safety compared to the traditional lithium ion
battery. Lithium ion-polymer batteries are appealing
because they are in the solid state, exhibit enhanced safety
due to the elimination of toxic liquids in the electrolyte
system, and can be configured into different shapes [4].
From a market perspective, the cost of manufacturing
lithium ion-polymer batteries is anticipated to decrease
with increasing demand, similar to what is currently being
observed for lithium ion batteries [5].
Our research is geared toward the development of
composite cathodes through the utilization of environ-
mentally benign polymers. The polymer utilized in this
work is chitosan (Fig. 1) which is obtained from the
deacetylation of chitin, a compound found in lobster and
crab shells. Lobster and crab shells are bio-wastes that are
produced in abundance. For instance, in Prince Edward
Island alone, the fishing and food industries produce an
estimated amount of 15,000 tons of lobster shells per year
[6]. These bio-wastes can be chemically modified into
chitosan, a potential battery material due to several key
features such as low self discharge, non-toxicity, good
elasticity, and reasonable lithium ion conductivity [4, 7].
Several research groups have already explored the use of
chitosan in solid-state batteries [8–10]. However, it is
noteworthy that chitosan could also be used in other
applications such as in photography, cosmetics, paper fin-
ishing, and in drug-delivery systems [7].
Nanocomposite materials based on chitosan have been
reported in the past. For instance, chitosan has been
intercalated into montmorillonite, and the resulting nano-
composite could be used as an electrode material [11].
Recently, composite films consisting of chitosan and
graphene oxide have been reported, and these have been
shown to possess enhanced mechanical strength [12]. Other
nanocomposites recently synthesized contain chitosan and
silver, which could ultimately be utilized to remove
formaldehyde from air [13].
In this paper, we report on the intercalation of chitosan
into molybdenum disulfide (MoS2). MoS2 is a semiconduc-
tor obtained from molybdenite, the mineral from which
I. Saada � R. Bissessur (&)
Department of Chemistry, University of Prince Edward Island,
Charlottetown, PE C1A 4P3, Canada
e-mail: [email protected]
123
J Mater Sci (2012) 47:5861–5866
DOI 10.1007/s10853-012-6486-z
molybdenum metal is extracted. While molybdenite occurs
naturally, it can also be obtained as a secondary product from
the mining of copper [14]. Its optical, electrical, and exfo-
liating properties [15] have been capitalized in research
fields such as photochemistry and materials science [16, 17],
and as an industrial material for solid-lubrication and
rechargeable batteries [14]. In particular, MoS2 can be used
as an enhanced performing cathode material in lithium ion
batteries due to its ability to react reversibly with lithium
[18], reasonable electrical conductivity [19], and ease of
intercalating polymeric materials in its van der Waals space
[20–23]. In fact, polymer-MoS2 nanocomposites have been
shown to possess enhanced mechanical, electrical, and
thermal properties [24–28], which has fuelled our motivation
to generate novel chitosan–MoS2 nanomaterials. These
nanomaterials were characterized by powder X-ray diffrac-
tion, thermogravimetric analysis, and four-probe electrical
conductivity measurements.
Materials
Low molecular weight chitosan, n-butyllithium (2.5 M
solution in hexanes), and MoS2 were purchased from
Aldrich. These materials were used as received without
further purification.
Instrumentation
Powder X-ray diffraction (XRD) analysis was conducted
using a Bruker AXS D8 Advance diffractometer. It is
equipped with a graphite monochromator, variable diver-
gence slit, and variable antiscatter slit. The samples were
run as pressed pellets on silicon substrates at room tem-
perature, in air, with 2-theta values from 2� to 60�.
Thermogravimetric analysis (TGA) was performed on a
TA Q500 instrument. Samples were run in air and nitrogen
atmosphere, with a heating rate of 10 �C up to 650 �C.
Room temperature electrical conductivity measurements
were performed by using the co-linear four point probe
technique. Powdered samples were compacted into circular
disks, with a diameter of 1.27 cm and thickness ranging
from 2.79 9 10-2 to 3.15 9 10-2 cm. The measurements
were done with a Jandel Multi Height Probe instrument
(Linsdale, UK), connected to a Keithley 2000 Multimeter
(USA). Electrical contacts with the disks were made by
four equally spaced tungsten carbide probes (tip
radius:100 lm), with a spacing of 1.0 mm. A current
(I) flows through the outer probes, and DV is the voltage
developed across the inner probes. The ratio DV/I was
recorded at several positions near the center of each disk,
and averaged values were recorded. The resistivity (q) of
the disk was then determined by the formula [29],
q ¼ pw
ln 2
DV
I
� �ave
where w is the average thickness of the disk in cm. The
conductivity (r) in S/cm was then determined by taking the
reciprocal of the resistivity.
Preparation of LiMoS2
LiMoS2 was prepared as described in the literature [14].
MoS2 (2.31 g, 1.14 9 10-2 mol) was weighed into a
125 mL Erlenmeyer flask and transferred to a nitrogen
atmosphere glove box. 30 mL of dried pentane was added,
followed by dropwise addition of 17 mL of n-butyl lithium
(2.5 M). The reaction was left to stir mechanically for
3 days. The black powder obtained was filtered, washed
with excess pentane then dried under suction and stored in
the dry box for future use.
Intercalation of chitosan into MoS2 (1:1 molar ratio)
Chitosan (0.116 g, 6.18 9 10-4 mol) was weighed in a
25 mL Erlenmeyer flask. 10 mL of 1 % acetic acid (v/v) was
added to the flask, and the chitosan was allowed to dissolve
for a period of 12 h. LiMoS2 (0.105 g, 6.28 9 10-4 mol)
was added to a 125 mL Erlenmeyer flask, followed by the
addition of 10 mL of de-ionized water, and the resulting
suspension was sonicated for 2 h. Once sonication was
complete, the dissolved chitosan was added to the exfoliated
LiMoS2 and the reaction was left to stir mechanically for
36 h. The reaction mixture was filtered under suction, and the
collected product was stored in a vacuum dessicator.
A similar synthetic methodology was used for the
preparation of nanocomposite materials consisting of 1:2
and 1:4 molar ratios of MoS2 to chitosan. However, with
increasing amount of chitosan, a larger volume of 1 %
acetic acid was used to fully dissolve the polymer.
Results and discussion
Powder X-ray diffraction characterization
Powder X-ray diffraction confirms the successful interca-
lation of chitosan into MoS2. As shown in Fig. 2, the X-ray
2
2 2 n
Fig. 1 Structure of chitosan
5862 J Mater Sci (2012) 47:5861–5866
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diffractograms reveal an increase in interlayer spacing
values with respect to pristine MoS2 (d-value: 6.2 A),
consistent with intercalation of the polymer. The XRD data
are summarized in Table 1. The schematic diagram shown
in Fig. 3 illustrates the proposed arrangement of the
chitosan chain within the MoS2 layers. With the aid of
Spartan’08 (Wavefunction Inc., Irving CA), an estimate of
the molecular dimension of a single chain of chitosan was
found to be *3.6 A. Based on this estimated dimension
and on the observed interlayer expansions shown in
Table 1, we propose a single layer of the chitosan sand-
wiched between the layers of MoS2. However, a helical
conformation of the polymer chain cannot be ruled out.
It is noteworthy that there is little difference in the
interlayer spacing value of the intercalate with increasing
amount of chitosan, and this indicates that the layered
structure can accommodate a maximum amount of the
polymer which is roughly 1:1, in terms of mole ratio of the
polymer with respect to the layered host system. Any
higher ratio used, led to externally lying polymer in addi-
tion to the intercalated polymer. The X-ray diffraction
pattern also depicts that the intercalation is complete as no
MoS2 phase can be detected. For comparison, the XRD of
pristine MoS2 is shown in Fig. 4. The XRD of bulk
chitosan, shown in Fig. 5, reveals the amorphous character
of the biopolymer.
From the X-ray diffraction pattern of the intercalates,
the average crystallite size of the nanocomposites were
determined by using the Scherrer formula [30] and the
results are given in Table 1. Compared to pristine MoS2
which has a crystallite size of 742 A, a considerable
decrease in the average crystallite size of the intercalates
was observed. This is due to the fact that once the layers
are exfoliated, they re-stack with the polymer chain trapped
in between, resulting in turbostratic structures similar to the
V2O5 xerogels layered system [31]. It is interesting to note
that addition of acetic acid to the exfoliated layers does not
result in its intercalation, as evidenced by the powder
pattern of the dried product which reveals a similar inter-
layer spacing value as the pristine MoS2, suggesting that
neither water and/or acetic acid molecules are intercalated
in the gallery space of the layered host system.
Fig. 2 XRD of intercalated
products. a 1:4, b 1:2, c 1:1
Table 1 Summary of XRD data
Mole ratio of
MoS2 to chitosan
Interlayer
spacing (A)
Interlayer
expansion (A)
Crystallite
diameter (A)
A (1:1) 11.00 4.80 85
B (1:2) 10.87 4.67 93
C (1:4) 10.61 4.41 80
2 n2
2
Fig. 3 Proposed lamellar arrangement of chitosan–MoS2 nanocom-
posites
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Thermogravimetric analysis characterization
Thermogravimetric analysis in air was used to further
characterize the nanocomposite materials. As shown in
Fig. 6, the decomposition of the nanocomposites occurs in
three weight loss steps: the first corresponds to the
desorption of co-intercalated and/or adsorbed water mole-
cules, the second weight loss corresponds to the degrada-
tion of chitosan from the surface and/or edge of the layered
structure, and the final weight loss corresponds to the
combustion of the intercalated chitosan. The final phase
produced at 650 �C was identified as MoO2 by powder
X-ray diffraction (Fig. 7). This is in contrast to what has
been previously reported for TGA of MoS2-intercalated
phases in air which were shown to produce MoO3 phase at
that temperature [23]. From the TGA, the stoichiometry of
the intercalates was determined and the results are summa-
rized in Table 2. The TGA data shows that increasing the
amount of chitosan in the reaction mixture does not lead to
further intercalation of the polymer, and this is consistent
with the XRD data which show minimal change in d-spacing
values. For instance, although the stoichiometric ratio in
Lin
(Cou
nts)
0
100
200
300
400
500
600
700
800
900
1000
2-Theta - Scale
2 10 20 30 40 50 60
Fig. 4 XRD of pristine MoS2
Lin
(Cou
nts)
0
100
200
300
400
2-Theta - Scale
2 10 20 30 40 50 60
Fig. 5 XRD of pristine
chitosan
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C compared to B is increased by two fold, the intercalated
amount of chitosan increases by only 0.3.
Electrical conductivity characterization
The nanomaterials showed an enhancement in electrical
conductivity when compared to pristine MoS2 and the data
are summarized in Table 3. The electrical conductivity of
pristine MoS2 was measured to be 5 9 10-7 S/cm, and
pure chitosan showed no conductivity value. However,
chitosan–MoS2 nanocomposite at 1:1 mol ratio displayed
an increase in electrical conductivity value by a factor of
100. On the other hand, chitosan–MoS2 nanocomposite at
1:4 molar ratio showed a less dramatic increase (only by a
factor of 2), which is explained by the presence of the large
amount of the insulating polymer. The increased electrical
conductivity of the nanocomposites can only be explained
by a structural transformation that takes place in the MoS2
during the intercalation process. Prior to intercalation,
MoS2 possesses a trigonal prismatic (D3h) geometry and is
a semiconductor [23]. Upon intercalation with n-butyl
lithium, the MoS2 layers are reduced, resulting in an
octahedral (Oh) geometry. The modification to Oh-MoS2
confers electrical conductivity since the system becomes
metallic [23]. Upon exfoliation and re-stacking the octa-
hedral geometry of the MoS2 layers is maintained.
Conclusions
We have demonstrated the successful insertion of chitosan
into MoS2 by means of powder X-ray diffraction data.
TGA shows that a significant amount of the polymer has
been intercalated in the gallery space of the layered host.
The observed electrical conductivity of the nanocomposites
make them attractive as potential cathodes in lithium ion
batteries.
Fig. 6 TGA of chitosan–MoS2 nanocomposites in air
Lin
(Cou
nts)
0
100
200
300
400
2-Theta - Scale
2 10 20 30 40 50 60
Fig. 7 XRD on product C (1:4)
after TGA in air up to 650 �C
Table 2 Calculated stoichiometry of reaction ratios based on TGA
(MoS2: Chitosan mole ratio) Stoichiometry
A (1:1) (H2O)1.5 (ChitOut)0.4 (ChitIn)0.5 MoS2
B (1:2) (H2O)3.7 (ChitOut)1.3 (ChitIn)1.1 MoS2
C (1:4) (H2O)5.0 (ChitOut)2.2 (ChitIn)1.4 MoS2
J Mater Sci (2012) 47:5861–5866 5865
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Acknowledgements The authors would like to thank the Natural
Sciences and Engineering Research Council of Canada (NSERC),
Canada Foundation for Innovation (CFI), Atlantic Innovation Fund of
Canada (AIF), and the University of Prince Edward Island (UPEI) for
financial support.
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Table 3 Electrical conductivity for reaction ratios and pristine materials
Sample (molar ratio) Measured thickness (cm) Measured resistance (X) Resistivity (X cm) Conductivity (S/cm)
Pristine MoS2 1.52 9 10-2 3.02 9 107 2.08 9 106 5 9 10-7
Pristine chitosan 3.15 9 10-2 – – –
1:1 2.79 9 10-2 1.52 9 105 1.93 9 104 5 9 10-5
1:4 3.10 9 10-2 6.38 9 106 8.97 9 105 1 9 10-6
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