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Electrochimica Acta 132 (2014) 397–403

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

ydrothermal synthesis of molybdenum disulfide nanosheets asupercapacitors electrode material

e-Jing Huang ∗, Ji-Zong Zhang, Gang-Wei Shi, Yan-Ming Liuollege of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China

r t i c l e i n f o

rticle history:eceived 28 January 2014eceived in revised form 1 April 2014ccepted 1 April 2014vailable online 15 April 2014

eywords:olybdenum disulfide

a b s t r a c t

Two–dimensional (2D) transition metal dichalcogenide nanosheet is attracting increasing attention inenergy storage due to unique nanoconstruction and electronic properties. In this work, molybdenumdisulfide (MoS2) nanosheet was prepared by a simple hydrothermal method, and its properties were char-acterized by X–ray powder diffraction, X–ray photoelectron spectroscopy, scanning electron microscopyand transmission electron microscopy. The electrochemical performances of the product were evaluatedby cyclic voltammogram, galvanostatic charge–discharge and electrochemical impedance spectroscopy.The MoS2 nanosheet showed a specific capacitance of 129.2 F g−1 at a current density of 1 A g−1. In

anosheetlectrode materialsigh–performance Supercapacitor

addition, the MoS2 nanosheet electrode showed good cycle property as an excellent electrode materialfor electrochemical capacitors and the specific capacitance had only a slight decrease after 500 cycles(retention of 85.1%). The excellent electrochemical performance was attributed to the unique morphol-ogy of MoS2 nanosheets, which possessed large specific surface area, unique 2D nanostructures and lowequivalent series resistance.

© 2014 Elsevier Ltd. All rights reserved.

. Introduction

Supercapacitor is a new energy storage device, which has manydvantages including long service lifetime, great power density,ast charge and discharge processes, green environmental protec-ion [1–3]. Therefore, it has attracted enormous research interest inhe recent years. Supercapacitor store and release energy based onither the accumulation of charges at the interface between elec-rode and electrolyte (electrical double layer capacitors, EDLC) orast and reversible faradic redox reactions (pseudocapacitors), oroth, depending on the nature of activated materials. In EDLC, theapacitive behaviors of supercapacitors are closely related to theurface area of the formed interface between the electrode andlectrolyte [4,5]. So looking for high-performance electrode mate-ials with large surface area has become a pivotal issue for theupercapacitor development.

Recently, two–dimensional (2D) nanosheets have attracted areat deal of interest because of their promising potential for appli-

ation in nanotechnology, such as graphene, which has high surfacerea, excellent electrical conductivity, good chemical stability andtrong mechanical strength. These properties make graphene an

∗ Corresponding author. Tel.: +86 376 6390611.E-mail address: kejinghuang11@163.com (K.-J. Huang).

ttp://dx.doi.org/10.1016/j.electacta.2014.04.007013-4686/© 2014 Elsevier Ltd. All rights reserved.

attractive candidate for fabricating various functional devices, suchas energy storage, electrochemical sensors, photovoltaics and pho-todetectors [6–8]. The successful fabrication of graphene baseddevices has stimulated new interests in graphene–like 2D lay-ered materials of elements other than carbon to expect to obtainsome unusual properties. Especially, 2D transition–metal dichalco-genides, such as MoS2, VS2, SnS2, CoS2 and WS2, have receivedsignificant attention because they offer many opportunities for fun-damental and technological research in a variety of fields, includingcatalysis, energy storage, sensing, and field–emitting applications[9–12]. Among them, the layered MoS2 is considered to have greatpotential for applications as different devices. For example, Yin et al.reported a new phototransistor based on the mechanically exfoli-ated single–layer MoS2 nanosheet [13]; Sundaram et al. detectedelectroluminescence in single layer molybdenum disulfide (MoS2)field–effect transistors built on transparent glass substrates [14];Wang et al. reported a Integrated Circuits Based on Bilayer MoS2Transistors [15], and Wang et al. constructed a biosensor based onimmobilization of horseradish peroxidase on molybdenum disul-fide nanosheets modified electrode [16]. The MoS2 crystal consistsof a metal Mo layer sandwiched between two S layers, with these

triple layers stacking together to form a layered structure. The lay-ered structure of MoS2 is expected to act as an excellent functionalmaterial because the 2–dimensional electron–electron correla-tions among Mo atoms would aid in enhancing planar electric

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ransportation properties. Indeed, as a graphene analogue, MoS2anosheets exhibit the unique physical, optical and electrical prop-rties correlated with its 2D ultrathin atomic layer structure [17]nd high surface area, which has distinct similarities compared toraphene, making it very interesting for using in nanoelectron-cs, optoelectronics, energy harvesting [18–22], and applying as

promising supporting material to stabilize metal nanoparticlesNPs), forming hierarchical composites [23].

In this work, we report the simple hydrothermal synthesis ofD MoS2. The as–prepared MoS2 sample was composed of manyD nanosheets with the thickness of several nanometers. The elec-rochemical performances of the MoS2 nanosheet were furthernvestigated. It was found that the supercapacitor constructed withhe obtained material exhibited a specific capacitance of 129.2 F−1 at a current density of 1 A g−1 and the specific capacitance kept5.1% after 500 cycles, offering as a new supercapacitor based onwo-dimensional materials.

. Experimental

.1. Synthesis of MoS2 nanosheet

The preparation of MoS2 nanosheet was conducted by a facileydrothermal method. Na2MoO4·2H2O was firstly dissolved ineionized water by ultrasonication. The solution was then adjustedo pH 6.5 with 0.1 M NaOH, followed by the addition of L–cysteine.he molar ratio of Na2MoO4·2H2O and L–cysteine was 1:3. Sub-equently, the mixture was stirred for 5 min and transferred to a00 mL Teflon-lined autoclave, sealed and heated in an oven at80 ◦C for 48 h. The product was then collected, washed with waternd ethanol for several times each and dried in a vacuum oven at0 ◦C for 12 h. 0.30 g 0.80 g

.2. Characterization

The morphologies of the synthesized MoS2 nanosheets wereecorded on a JEM 2100 transmission electron microscope (TEM)nd a Hitachi S–4800 scanning electron microscope (SEM). X–rayowder diffraction (XRD) pattern was operated on a JapanigakuD/Maxr-A X–ray diffractometer equipped with graphiteonochromatized high–intensity Cu K� radiation (� = 1.54178 A).

ourier transform infrared spectroscopy (FT–IR) was measured on Bruker–Tensor 27 IR spectrophotometer. Raman spectra wereecorded at ambient temperature on a Renishaw Raman systemodel 1000 spectrometer with a 200 mW argon–ion laser at

n excitation wavelength of 514.5 nm. X–ray photoelectron spec-roscopy (Thermo Electron Corp., USA) was used to analyze theomposition of MoS2. The N2 adsorption–desorption isotherms ofhe samples were measured using NOVA 2000 (Quantachrome,SA) inorder to determine the specific surface areas. The spe-ific surfacearea was calculated from the Brunauer–Emmett–TellerBET) plot of the nitrogen adsorption isotherm.

.3. Electrochemical measurement

To prepare the electrodes for supercapacitors, 75 wt%oS2 active material, 15 wt% carbon black and 10 wt%

oly(tetrafluoroethylene) binder were mixed together and groundhoroughly to obtain a slurry. Then the slurry was pressed ontohe nickel foam substrates (1 cm × 1 cm) and dried at 80 ◦C for

h. For the electrochemical testing in an aqueous electrolyte,hree–electrode test cells were assembled with a working elec-

rode (nickel foam with pasted mixture), a reference electrodeAg/AgCl), and a Pt wire counter–electrode (� = 0.5 mm). 1.0 Ma2SO4 aqueous solution was freshly prepared as the electrolyte.yclic voltammetry (CV) measurements were conducted from–0.8

Acta 132 (2014) 397–403

to 0.2 V vs. Ag/AgCl at 1, 2, 3, 5, 10, 20, 30, 40 and 50 mV s−1 on aCHI 660D Electrochemical Workstation (Shanghai, CH Instruments,China). Electrochemical impedance spectroscopy (EIS) measure-ments were also carried out in the frequency range from 100 kHzto 0.1 Hz at open circuit potential with an ac perturbation of 5 mV.The galvanostatic charge–discharge characteristics were examinedby a chronoamperometry technique on the same electrochemistryworkstation. The specific capacitance (Cs) of electrode material wascalculated according to the following equation [24]:

Cs = It/�Vm (1)

where I, t, �V and m are the constant current (A), discharge time (s),the total potential difference (V) and the weight of active materials(g), respectively.

3. Results and discussion

3.1. Characterization of the MoS2 nanosheet

It is reported that Mo(VI) can be easily reduced in solution byreductant such as NaBH4, SO2, and H2S etc [25–27]. The sulfuriza-tion reagent L–cysteine will easily decompose and form H2S duringhydrothermal procedure [28]. Thus, MoS2 might be obtained by thereaction of Na2MoO4 and sulfurization reagent without using addi-tional reductant. On the basis of the literature [25–27], the reactionroutes could be expressed as follows:

HSCH2CHNH2COOH + H2O → CH3COCOOH + NH3 + H2S (1)

4MoO42− + 9H2S + 6CH3COCOOH

→ 4MoS2 + SO42− + 6CH3COCOO− + 12H2O (2)

The morphology of the as-prepared MoS2 sample was identifiedby SEM, TEM and high-resolution TEM (HRTEM). The SEM imagesof the MoS2 product are showed in Fig. 1A. It displays the MoS2sample has a uniform flower–like structure with mean diameter<200 nm. From Fig. 1B, it can be clearly observed that many irregularMoS2 nanosheets with thickness of several nanometers aggregatedtogether and assembled into the nanoflowers, indicating that theMoS2 sample has large specific surface areas. The as–synthesizedMoS2 nanosheet was also analyzed by TEM. As shown in Fig. 1 C, thenanosheets of MoS2 stack together and display a typical crinkly andrippled structure. Fig. 1D shows the layer-structure MoS2 productsoverlap each other with the mean value of distance between thetwo lattice fringes of 0.63 nm.

Because the Raman scattering is very sensitive to themicrostructure materials, it is also used to characterize the synthe-sized MoS2 (as shown in Fig. 1E). The characteristic peak positions of379 cm−1 and 404 cm−1 due to the in–plane E1

2g and out–of–planeA1g vibration modes indicates the presence of monolayer MoS2 [29].The in–plane E1

2g mode originates from opposite vibration of two Satoms with respect to the Mo atom while the A1g mode is in respectof the out–of–plane vibration of only S atoms in opposite directions[30].

The synthesized MoS2 was further confirmed by FT–IR measure-ments. The FT–IR spectrum of the product is shown in Fig. 1F. Thepeak observed at 1610 cm−1 and 1400 cm−1 are C = C stretchingdeformation of quinoid and benzene rings, respectively [31], andthe peaks at 1150 cm−1 and 1000 cm−1 are attributed to the aro-matic C–H in–plane bending. The band at 850 cm−1 is assigned to

the out–of–plane deformation of C–H in the 1, 4–disubstituted ben-zene ring [32] and the band at 3420 cm−1 is attributed to the OHvibration. These carbon containing groups in the samples may bedue to the residual of the precursor L–cysteine and intermediate

K.-J. Huang et al. / Electrochimica Acta 132 (2014) 397–403 399

Fig. 1. SEM (A, B) and TEM (C, D) images of the MoS2 nanosheets at two scales; Raman spectra (E), FT–IR spectra (F), XRD patterns (G) and XPS survey spectra (H) of the MoS2

nanosheets.

400 K.-J. Huang et al. / Electrochimica Acta 132 (2014) 397–403

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Fig. 2. (A) Nitrogen adsorption isotherm and (B) pore size dis

roducts during the hydrothermal reaction process. The bands atbout 600 cm−1 is assigned to Mo–S vibration [33].

The as-synthesized MoS2 sample was also studied by XRD pat-erns, which could be readily indexed to the hexagonal phase of

oS2 consistent with the standard powder diffraction file of MoS2JCPDS 37–1492). As shown in Fig. 1G, no characteristic peaks fromther impurities are observed in the XRD pattern such as MoS3,ndicating that the sample was highly pure.

The MoS2 nanosheet was further characterized by XPS to verifyts chemical composition. As shown in Fig. 1H, the predominantlements in the sample are Mo, S, C and O. The experimentalesults of XPS indicated the atomic ratio of Mo to S was about

: 2.

Nitrogen adsorption-desorption isotherms are further used tonvestigate the pore structure and specific surface area of MoS2anosheet and the commercial MoS2 (Fig. 2A). The commercial

ig. 3. CV curve of the MoS2 nanosheets at 10 mV s−1 in 1.0 M Na2SO4 (A); CV curves of th0, 50 mV s−1) (C) in 1.0 M Na2SO4.

ion of the MoS2 nanosheet (a) and the commercial MoS2 (b).

MoS2 and MoS2 nanosheet exhibit reversible type II isotherms,which indicate non-porous material. The specific surface areas ofthe as-prepared MoS2 nanosheet and the commercial MoS2 werecalculated to be 21.1 m2 g−1 and 7.5 m2 g−1, respectively. Obviously,the specific surface area of MoS2 nanosheet was much higher thanthe pristine MoS2. Fig. 2B shows the pore size distribution of thecommercial MoS2 and MoS2 nanosheet. MoS2 nanosheet has a poresize distribution of 3-12 nm.

3.2. Electrochemical performance of MoS2 nanosheet

The double-layer capacitive performances of the MoS2

nanostructures were evaluated by CV and galvanostaticcharge/discharge tests. Fig. 3A shows the CV curve of the MoS2nanosheet measured in 1.0 M Na2SO4 in the potential windowof–0.8 to 0.2 V at the scanning rate of 10 mV s−1. The CV curve

e MoS2 nanosheets at different scan rates (1, 2, 3, 5, 10 mV s−1) (B) and (10, 20, 30,

K.-J. Huang et al. / Electrochimica Acta 132 (2014) 397–403 401

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2cific capacitance with cycle number at 1 A g−1 in 1.0 M Na2SO4 andreveals that the MoS2 nanosheet electrode has good cycle propertyas an excellent electrode material for electrochemical capacitors

Fig. 5. Nyquist plots of the MoS2 nanosheet electrode in 1.0 M Na2SO4 in the fre-

ig. 4. (A) Galvanostatic charge/discharge curves of MoS2 nanosheets at different cut different current densities (1, 1.5, 3, 5 and 10 A g−1) in 1.0 M Na2SO4.

eveals the quasi-rectangular shape of the MoS2 nanosheet,ndicating typical double–layer capacitance.

For further research the properties of the MoS2 nanosheet,ig. 3B shows the CV curves at various scan rates (1, 2, 3, 5, 10 mV−1). These CV curves exhibit a nearly rectangular mirror-imageurrent response on voltage reversal without obvious redox peaks,roving that the electrode is charged and discharged at a pseudo-onstant rate over the complete voltammetric cycle. From anothererspective, the almost ideal rectangular CV curves from MoS2lectrode reflect small contact resistance due to close connectionetween the MoS2 nanosheets and the nickel foam. It is also notedhat the cathodic peaks shift positively and the anodic peaks shiftegatively with the increase of the scan rate from 1 to 10 mV s−1,hich is possibly due to the resistance of the electrode. This may

e because at high scan rates, the movement of electrolyte ionss limited and only the outer active surface is utilized for chargetorage due to the time constraint.

Fig. 3 C shows the CV curves at the higher scan rates (10, 20, 30,0, 50 mV s−1). The rectangular CV shape remains very well event a high scan rate of 50 mV s−1, suggesting the MoS2 nanosheetlectrode possesses excellent rate capability. Besides, the specificapacitance of MoS2 nanosheet has a lower specific capacitance atigh scanning rate than that at low scanning rate. The possible rea-on can be explained as followed: high scanning rate corresponds tohe high–rate charge/discharge process, which prevents the acces-ibility and movement of ions to inside of the electrode leading theess utilization of electroactive species in the electrode. Meanwhile,

ith the scan rate increasing, the effective interaction between theons and the electrode is greatly reduced because of the resistancef MoS2 and the deviation from rectangularity of the CV becomesbviously.

Galvanostatic charge/discharge curves of the MoS2 nanosheetlectrode were recorded with various current densities to furthervaluate the electrochemical performance. As shown in Fig. 4And Fig. 4B, these typical triangular-shape charge/discharge curvesxhibit good symmetry and fairly linear slopes at different currentensities, again demonstrating the ideal capacitive characteristic.he specific capacitance of the MoS2 nanosheet electrode usingischarge curve (Eq. (1)) is calculated to be 129.2 F g−1 at 1.0

g−1, and still maintains at 73.8 A g−1 when the current den-ity increases by as much as 10 times (10 A g−1), which can bettributed to the advantageous features of the layered structure. Inddition, the initial voltage loss (i.e. IR drop) observed on the dis-harge curves is small even at high current densities, indicating fast

–V response and low internal resistance of supercapacitors. Suchlectrode material with high specific surface area and good elec-rical conductivity connecting the electroactive sites usually showood capacitive performances.

densities (1, 1.5, 3, 5 and 10 A g−1); (B) Specific capacitance of the MoS2 nanosheets

The EIS was measured in the frequency range of 0.1–100,000 Hzat open circuit potential with an ac perturbation of 5 mV (Fig. 5).The measured impedance spectra was analyzed using the CNLSfitting method based on the equivalent circuit, which is given inthe inset of Fig. 5. The Nyquist plot of an ideal supercapacitor iscomprised of a vertical line, while appearing a semicircle at highfrequency region is indicative of interfacial charge transfer resis-tance. As shown in Fig. 5, the equivalent series resistance (ESR) ofMoS2 nanosheet is about 3.1 �, which is mainly from the significantcharge transfer resistance among its nanosheets. Obstruction of ionmovement or increased ion diffusion path lengths will result inincreased impedance in low frequency range. It also can be seen thatthe straight line of the MoS2 is almost vertical, which is closely to anideal capacitor. These observations may be attributed to that MoS2nanosheet has high charge density at the electrolyte solution whichresults in high resistance of ion transfer and therefore low capaci-tance. This is the reason the MoS2 nanosheet show low resistance incapacitive part. All the above mentioned reasons, the as-preparedMoS2 nanosheet exhibited good supercapacitive performance.

As a long cycle life is very important for supercapacitors, thecycle charge/discharge test has been employed to examine theservice life of the 2D MoS sample. Fig. 6 shows the variation of spe-

quency range from 0.1 to 100,000 Hz at open circuit potential with an ac perturbationof 5 mV. Inset shows the electrical equivalent circuit used for fitting impedancespectra. Rs: solution resistance; Rct: charge-transfer resistance; Cdl: double layercapacitance; Cps: pseudocapacitance; ZW: Warburg impedance resulting from thediffusion of ions.

402 K.-J. Huang et al. / Electrochimica Acta 132 (2014) 397–403

Table 1Comparison of properties of different MoS2 materials for supercapacitor.

Electrode materials Electrolyte The maximum specific capacitance Final capacitance compared to the initial capacitance Ref.

Polyaniline/MoS2 1.0 M H2SO4 575 F g−1 at 1 A g−1 98% (after 1000 cycles at 1 A g−1) [19]Polypyrrole/MoS2 1.0 M KCl 553.7 F g−1 at 1 A g−1 90% (after 500 cycles at 1 A g−1) [20]MoS2 nanowall films 0.5 M H2SO4 100 F g−1 at 1 mV s−1 - [21]Sphere like MoS2 nanostructures 1.0 M Na2SO4 92.85 F g−1 at 0.5 mA cm−2 −2

Porous tubular C/MoS2 3.0 M KOH 210 F g−1 at 1 A g−1

MoS2 nanosheet 1.0 M Na2SO4 129.2 F g−1 at 1.0 A g−1

Fig. 6. Cyclic performance of the MoS2 nanosheet electrode at 1 A g−1 in1n

atcgaerof

cdFfciIci

Fl

.0 M Na2SO4 electrolyte. The inset shows charge/discharge curves of the MoS2

anosheets.

nd the specific capacitance has only a slight decrease (reten-ion of 85.1% after 500 cycles). The inset of Fig. 6 implies that theharge/discharge process of the MoS2 electrode is reversible. Theood electrochemical performance of the MoS2 nanosheet can bettributed to the 2D nanostructure of the MoS2, which improve thelectrode/electrolyte contact area and favor the diffusion of ions,esulting in a better rate capability. A comparison of the propertiesf the as-prepared MoS2 nanosheet with other electrode materialsor supercapacitor is listed in Table 1 [19–22,34].

To evaluate the durability of the MoS2 nanosheet electrode,yclic voltammetry was used to characterize the long-term charge-ischarge behavior at a constant current density of 1 A g−1.ig. 7 shows a comparison of charge-discharge curves obtainedor the first and 500th cycles, which shows the symmetricharge–discharge curves. The two curves are almost the samen shape, which may be due to the intrinsic of MoS2 nanosheet.n addition, this also indicates voltage drop has no obvious

hanges after 500 cycles, which suggests that the electrochemicalmpedance spectroscopy of the electrode basically unchanged.

ig. 7. Galvanostatic charge–discharge curves of MoS2 nanosheet before and afterong–term cycling at the current density of 1 A g−1.

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93.8% (after 1000 cycles at 0.5 mA cm ) [22]105% (after 1000 cycles at 4 A g−1) [34]85.1% (after 500 cycles at 1 A g−1) This work

4. Conclusion

In summary, we prepare MoS2 nanosheets by a simplehydrothermal method for supercapacitor electrode material. Theas-prepared MoS2 has a net–like structure of interconnectednanoflakes with a thickness of several nanometers. The MoS2nanosheet electrode shows a specific capacitance of 129.2 F g−1.The as-prepared MoS2 nanosheet also exhibits excellent long cyclelife with the capacitance retention of 85.1% after 500 cycles. Thegood exhibition of overall electrochemical performance of theMoS2 nanosheets is due to its large specific surface area and goodconductivity, which facilitate the ion diffusions and interactionswith the electrolyte. The outstanding electrochemical propertiesof the MoS2 nanosheet may lead to potential applications for high-performance supercapacitors.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (U1304214, 21375114).

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