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: rabissessur@upei.ca

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

5864 J Mater Sci (2012) 47:5861–5866

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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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