DaltonTransactions

PAPER

Cite this: Dalton Trans., 2015, 44,15491

Received 27th May 2015,Accepted 8th July 2015

DOI: 10.1039/c5dt01985k

www.rsc.org/dalton

Few-layered MoSe2 nanosheets as an advancedelectrode material for supercapacitors

Suresh Kannan Balasingam,a Jae Sung Leeb and Yongseok Jun*c

We report the synthesis of few-layered MoSe2 nanosheets using a facile hydrothermal method and their

electrochemical charge storage behavior. A systematic study of the structure and morphology of the as-

synthesized MoSe2 nanosheets was performed. The downward peak shift in the Raman spectrum and the

high-resolution transmission electron microscopy images confirmed the formation of few-layered

nanosheets. The electrochemical energy-storage behavior of MoSe2 nanosheets was also investigated for

supercapacitor applications in a symmetric cell configuration. The MoSe2 nanosheet electrode exhibited a

maximum specific capacitance of 198.9 F g−1 and the symmetric device showed 49.7 F g−1 at a scan rate

of 2 mV s−1. A capacitance retention of approximately 75% was observed even after 10 000 cycles at a

high charge–discharge current density of 5 A g−1. The two-dimensional MoSe2 nanosheets exhibited a

high specific capacitance and good cyclic stability, which makes it a promising electrode material for

supercapacitor applications.

1. Introduction

Supercapacitors (also known as electrochemical capacitors orultracapacitors) are a class of electrochemical energy storagedevices with merits including high power density, long cyclelife, high reliability, short charging time, and low maintenancecost.1,2 The bottleneck of the state-of-the-art electrochemicalcapacitors (ECs) in the market is their low energy density com-pared with that of commercial lithium-ion batteries(120–170 Wh kg−1).3 In general, ECs can be classified into twomain categories based on their charge storage mechanisms: (i)double-layer capacitors, which are based on an electrostaticcharge storage mechanism (non-faradaic reaction at the inter-face of capacitor electrodes) that leads to the regular double-layer capacitance and (ii) pseudocapacitors, which are basedon a different charge storage mechanism, mainly faradaic inorigin (the passage of charge across the double layer). Thecharge storage mechanism of pseudocapacitors involves elec-trosorption of charged species (from the electrolyte) on theelectrode surface, which further leads to the fast redox reactionat the electrode surfaces. Carbon-based materials are the bestknown double-layer capacitor electrodes; however, their funda-mental flaw is their low energy density values.4 To overcome

this issue, various pseudocapacitance materials such as con-ducting polymers, transition metal oxides (TMOs), and tran-sition metal dichalcogenides (TMDCs) have been intensivelyinvestigated as electrode materials for supercapacitor appli-cations. Of these materials, conducting polymers provide ahigh energy density but exhibit lower cyclic stability, and thetransition metal oxides suffer from reduced electrical conduc-tivity, which leads to poor device performance.

Recently, TMDCs have received much attention fromresearchers because of their 2D sheet-like morphology, higherelectrical conductivity (than the oxides), high surface area andthe multivalent oxidation states of transition metal ions.5–7

Analogous to graphite, TMDCs consists of graphene-like layersof MX2 sheets (where M can be any metal ions of groups IV, Vand VI transition elements and X is a chalcogen, X may be S,Se and Te) which are stacked together by weak van der Waalsforces. Similar to graphene, a monolayer or few layers of MX2

can be exfoliated from their bulk counterpart, which has a verydistinct physical and chemical properties and also has adiverse application in electronic devices, photo transistors,sensors, catalysis, and electrochemical energy storagesystems.8–10 In 2007, the first report on the electrochemicaldouble-layer capacitance measurement of MoS2 nanowall filmswas published by Soon and Loh,11 which opened up the fieldof TMDC-based electrode materials for supercapacitor appli-cations. Later, different morphologies of MoS2, WS2, andtheir composite-based materials were investigated as super-capacitor electrodes.12–25 Other sulfide materials such as, VS2nanosheets were employed as an electrode material for in-plane supercapacitors, CoS2 and NiS sheets were also demon-

aDepartment of Chemistry, School of Natural Science, Ulsan National Institute of

Science and Technology (UNIST), Ulsan 689-798, Republic of KoreabSchool of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), Ulsan 689-798, Republic of KoreacDepartment of Materials Chemistry and Engineering, Konkuk University,

Seoul 143-701, Republic of Korea. E-mail: yjun@konkuk.ac.kr; Tel: +82-2450-0440

This journal is © The Royal Society of Chemistry 2015 Dalton Trans., 2015, 44, 15491–15498 | 15491

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7. View Article Online

View Journal | View Issue

strated as active materials for supercapacitor electrodes due totheir large surface area and enhanced structural stability.When compared to sulfides, selenides are rarely studied forcapacitive properties.26–31 Moreover, recent reports on MoSe2based anode materials for lithium ion and sodium ion bat-teries showed them as promising electrodes for energy storageapplications. Shi et al.32 reported mesoporous crystallineMoSe2 using silica SBA-15 as a hard template which showed ahighly reversible lithium storage capacity of 630 mA h g−1, Koet al.33 synthesized yolk–shell structured MoSe2 microspheresvia selenization of MoO3 microspheres, which demonstrated ahigh sodium ion storage capacity of 433 mA h g−1. Wanget al.34 also synthesized MoSe2 nanoplates using the thermal-decomposition process, which could deliver high sodium ionstorage capacity of 513 mA h g−1. The enhanced energy storagecapacity of the two dimensional MoSe2 electrode materialsrender our attention to investigate this material for supercapa-citor applications. When compare to the bulk MoSe2, few-layered sheets show high surface area and unique electronic(quasiparticle bandgap) properties due to interlayer couplingand screening effects.35 Generally few-layered MoSe2nanosheets are synthesized via a CVD or rapid thermal proces-sing method which are very expensive and user-hostile tech-niques for the synthesis of large quantity of electrodematerials.36–39 In this work, we have synthesized a large quan-tity of few-layered MoSe2 nanosheets using a facile hydro-thermal method, and their electrochemical capacitance wasmeasured in aqueous electrolyte (H2SO4) under a symmetriccell (two-electrode) configuration. We obtained a maximalspecific capacitance of 199 F g−1 at a scan rate of 2 mV s−1 andalso for the long-term stability test, the capacitance retentionof approximately 75% was observed over 10 000 cycles, whichshowed it as a suitable material for electrochemical capacitors.

2. Experimental2.1. Material synthesis

The precursors, sodium molybdate (Na2MoO4·2H2O), seleniumpowder (Se), and sodium borohydride (NaBH4) were purchasedfrom Sigma Aldrich (ACS grade) and used without further purifi-cation. In a typical hydrothermal synthesis, precisely 1.32 g ofNa2MoO4·2H2O, 1.24 g of Se, and 0.2 g of NaBH4 were weighedand dissolved in 80 ml of DI water. The mixture was continu-ously stirred until a red-colored solution was obtained, which isan indication of uniform distribution of the selenium metalpowder. The as-prepared precursor solution was transferredinto the autoclave, sealed, and then heated at 200 °C for 12 h inan electric oven. The autoclave was then naturally cooled downto room temperature. After that, the MoSe2 nanosheets were fil-tered off and washed with DI water several times to remove theresiduals and then dried at 40 °C for a few hours.

2.2. Material characterization

The morphology of the as-prepared sample was examinedusing high-resolution transmission electron microscopy

(HR-TEM) (JEOL). The crystal structure and elemental compo-sition of MoSe2 was confirmed by X-ray diffraction (XRD,Bruker) measurements using Cu Kα emission (λ = 1.5406 Å) in10–90° range with a step size of 0.02°, Raman spectroscopy(WITec) using 532 nm laser excitation, after calibrating theRaman shift with a silicon reference at 521 cm−1, and X-rayphotoelectron spectroscopy (XPS) (Thermo Fisher, UK).

2.3. Cell fabrication and electrochemical measurements

The as-synthesized electroactive material (MoSe2), carbonblack and poly(vinylidene fluoride) were mixed in a mass ratioof 80 : 10 : 10 to obtain a slurry and then coated on the stain-less steel (SS) substrate using a brush. An exactly 1 cm2 area ofMoSe2 nanosheet-coated SS substrate was used as a single elec-trode. Mass loading of each electrode is around 4 mg cm−2.Two electrodes were sandwiched together with Whatman filterpaper as a separator. The assembled electrodes were placed ina test cell rig, and a few drops of 0.5 M sulfuric acid wereadded as the electrolyte. The test cell was sealed with anO-ring and then left for a few minutes to ensure the uniformsoaking of the electrodes into the electrolyte solution beforethe electrochemical measurements. Electrochemical experi-ments were performed using a potentiostat/galvanostat (Bio-logic/VSP) at room temperature. Cyclic voltammetry (CV)curves were obtained at various scan rates (2, 5, 10, 25, 50, 75,100, and 125 mV s−1) in a potential window of 0–0.8 V. Electro-chemical impedance spectroscopy (EIS) measurements wereperformed over the frequency range of 0.1 Hz–100 kHz with anAC amplitude of 10 mV. Galvanostatic charge–discharge (CD)curves were recorded at various current densities (0.10, 0.25,0.50, 0.75, 1, 2, 3, 4, and 5 A g−1) in a potential window of0–0.8 V.

2.4. Calculation of capacitance

The measured device capacitance (Cm, F g−1) can be calculatedbased on the CV measurements using the following equation:

Cm ¼ÐIðVÞdvvmΔV

ðF g�1Þ ð1Þ

where m is the total mass of the electroactive material in bothpositive and negative electrodes (g), v is the scan rate (V s−1),ΔV is the potential window (V), and

ÐIðVÞdv is the integral

area of the CV loop.From the CD curves, the measured device capacitance (Cm)

can be computed as:

Cm ¼ IΔtmΔV

ðF g�1Þ ð2Þ

where I is the discharge current (A), and Δt is the dischargetime (s).

The specific capacitance (Cs) of the single electrode can bedetermined as follows:

Cs ¼ 4� Cm ðF g�1Þ ð3Þwhere Cm is obtained either from CV or CD curves given byeqn (1) and (2).

Paper Dalton Transactions

15492 | Dalton Trans., 2015, 44, 15491–15498 This journal is © The Royal Society of Chemistry 2015

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7.

View Article Online

3. Results and discussion

The crystal structure and phase purity of the as-preparedMoSe2 nanosheets were characterized using X-ray diffraction.Fig. 1 presents the XRD pattern of the MoSe2 nanosheets.Diffraction peaks of the as-synthesized MoSe2 nanosheetsappear at 2θ = 13.3°, 31.6°, and 56°, which can be assigned tothe (002), (100), and (110) planes of the hexagonal phase ofMoSe2 (JCPDS no. 29-0914), respectively. The broadness of the(002) peak indicates the formation of few-layered MoSe2nanosheets.12,40

Raman spectroscopy was employed to further study thestructure of the as-synthesized MoSe2 nanosheets. The typicalpeaks of MoSe2 nanosheets at 239 cm−1 and 287.11 cm−1 areobserved in the spectra (Fig. 2), which corresponds to the out-of-plane A1g mode and in-plane E2g

1 mode of MoSe2, respect-ively.41 For bulk MoSe2, the A1g mode appears at 242 cm−1.

The downward red shift of approximately 3 cm−1 (A1g mode @239 cm−1) indicates the formation of few-layered MoSe2. TheE2g

1 mode of bulk MoSe2 appears at 286 cm−1; however, theblue shift of this in-plane mode observed in our study alsoconfirms the few-layered nanosheet formation.37,42

The chemical composition and oxidation state of theelements were investigated using X-ray photoelectron spectro-scopy. Mo 3d3/2 and Mo 3d5/2 peaks (Fig. 3a) appeared atbinding energies of 232 and 228.8 eV, respectively, which con-firms the +4 oxidation state of molybdenum.43 In addition, the3d peaks of selenium split into two well-defined peaks (Fig. 3b)such as 3d3/2 and 3d5/2, which appeared at binding energies of55.2 and 54.5 eV, confirming the −2 oxidation state of seleniumin MoSe2 nanosheets.44 From the atomic concentration table,the mole ratio of Mo : Se was calculated to be 1 : 2.

The surface morphology of the as-synthesized MoSe2 wasinvestigated using HR-TEM. Fig. 4 presents HR-TEM images ofthe as-synthesized MoSe2 nanostructure. The low-magnifi-cation TEM images (Fig. 4a and b) show MoSe2 nanosheets.Fig. 4c provides clear evidence of the formation of few-layered

Fig. 1 XRD spectrum of MoSe2 nanosheets.

Fig. 2 Raman spectrum of MoSe2 nanosheets showing two distinct A1g

and E2g1 Raman modes.

Fig. 3 (a) High resolution XPS spectra of Mo 3d, (b) Se 3d regions ofMoSe2 nanosheets.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2015 Dalton Trans., 2015, 44, 15491–15498 | 15493

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7.

View Article Online

nanosheets at higher magnification. In addition, the HR-TEMimages of the MoSe2 nanosheets reveal a layered crystal struc-ture with clear crystal lattice fringes of approximately 0.62 ±0.03 nm, corresponding to the standard d-spacing for the (002)basal plane of the hexagonal crystal structure. Notably, theporous nature of the MoSe2 nanosheet morphology provides ahigh surface area for the electrochemical reaction.

The MoSe2-nanosheet-coated stainless steel substrates wereassembled in a two-electrode configuration (a symmetric cellin a parallel-plate geometry, which is typically used in super-capacitor measurements), with Whatman filter paper as theseparator and 0.5 M sulfuric acid as the electrolyte. The elec-trochemical behavior of the MoSe2 electrodes was character-ized using CV, EIS, and CD cycling measurements. Fig. 5ashows the typical CV curves of a symmetric device (MoSe2nanosheets coated on a stainless steel substrate as workingand counter electrodes) at various scan rates (2 mV s−1 to125 mV s−1). All the CV curves retain their nearly rectangularshape at the various scan rates, which confirms the idealEDLC behavior (non-faradaic process, as indicated in eqn (4))of this material with excellent reversibility. Moreover, thebroad peak appearing in both the anodic and cathodic scansindicates the redox behavior of the MoSe2 nanosheets resultingfrom the faradaic electrochemical process, as shown in eqn (5).

Fig. 4 TEM images of MoSe2 nanosheets: (a) and (b) low magnification,and (c) high magnification; the inset shows the FFT pattern.

Fig. 5 (a) Cyclic voltammograms of MoSe2 nanosheets in 0.5 M H2SO4

electrolyte measured at different scan rates ranging from 2 to125 mV s−1. (b) Specific capacitance value calculated from the CV curvesof (a) at different scan rates.

Paper Dalton Transactions

15494 | Dalton Trans., 2015, 44, 15491–15498 This journal is © The Royal Society of Chemistry 2015

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7.

View Article Online

Notably, the redox peaks appeared even at higher scan rateswith the usual peak shift. The symmetric device exhibited ameasured device capacitance (Cm) of approximately 49.7 F g−1

at a scan rate of 2 mV s−1. For the single electrode, a highspecific capacitance of (Cs) 198.9 F g−1 was calculated at thesame scan rate. The high Cs value may be attributed to thecombination of both faradaic and non-faradaic processes ofthe 2D MoSe2 nanosheets:

11

ðMoSe2Þ þHþ þ e� $ MoSe� SeHþ ð4Þ

ðMoSe2Þ surfaceþHþ þ e� $ ðMoSe2 �HþÞ surface ð5ÞThe detailed derivation of the specific capacitance calcu-

lation of a single electrode and device is given in eqn (1)–(3)(Experimental section). Fig. 5b shows the relationship betweenthe specific capacitance of the MoSe2 nanosheets (a singleelectrode) at various scan rates. By increasing the scan rate upto 20 mV s−1, a sudden drop of the Cs value is observed, andupon further increasing the scan rate up to 125 mV s−1, agradual decrease of the Cs value is recorded. This observationis a common phenomenon in supercapacitors because athigher scan rates, the mass-transport limitation of H+ ions(from the electrolyte) to the interior (bulk) part of the electrodelimits the electrochemical performance, which leads to a lowercapacitance.45–48 The specific capacitance values observed forfew-layered MoSe2 electrodes were compared with the pre-viously reported state-of-the-art MoS2 and WS2 based super-capacitors. Using a similar two-electrode configuration,Winchester et al.49 reported a specific capacitance of 2 mFcm−2 @ 10 mV s−1 scan rate in 6 M KOH electrolyte for liquidphase exfoliated MoS2 layers. The material showed poorcapacitance retention of 68% within 200 cycles in the sameelectrolyte. Cao et al.25 synthesized MoS2 nanosheets formicro-supercapacitors, and a high specific capacitance of 8 mFcm−2 @ 10 mV s−1 scan rate was observed in 1 M NaOH;however, the authors did not show the long-term cycle stabilityof this material. Ratha and Rout12 synthesized WS2 nanosheetsby a facile hydrothermal method and reported a specificcapacitance of 70 F g−1 @ 2 mV s−1 scan rate. Using the samescan rate, our material (few-layered MoSe2 nanosheets) showeda specific capacitance of 198.9 F g−1 which is almost 2.8 timeshigher than the WS2 based materials. Although a recent reporton metallic 1 T phase MoS2 electrode for supercapacitorsmeasured using a three-electrode configuration showed highvolumetric capacitance of around 650 F cm−3 @ 20 mV s−1,their gravimetric capacitance is approximately 190 F g−1 @5 mV s−1. However, using a two-electrode configuration, ourMoSe2 nanosheet electrodes showed a comparable gravimetriccapacitance to the previous report.50

Fig. 6a presents the galvanostatic CD curves of a symmetricdevice at various current density values. The CD curvesare almost symmetric, which indicates the electrochemicalcapacitive behavior with remarkable reversible redox reaction.At the very low discharge current density of 0.1 A g−1, a non-linear shape of the CD curve is observed with the pseudocapa-citive behavior of the electrode material. The symmetric super-

capacitor device exhibited a measured device capacitance of 10.4F g−1, and the single electrode exhibited a specific capacitanceof approximately 41.5 F g−1 at a discharge current density of0.1 A g−1. The rate capability of a single MoSe2 nanosheet elec-trode was investigated under various current densities, asshown in Fig. 6b. Consistent with the CV analysis, the Cs valueof the electrode material decreases at higher current density.As explained in CV, the diffusion rate of the electrolyte ions islimited at higher current density. Therefore, only the surfacearea of the MoSe2 nanosheets is accessible at higher currentdensity. For a lower-current-density CD process, the electrolyteions could penetrate into the interior part of the electrodesurface, thereby resulting in a higher Cs value.

Long cycling life is another important criterion for theapplication of this material in commercial supercapacitors.The cyclic stability of MoSe2 nanosheet electrodes wasmeasured at a high current density of 5 A g−1, as shown inFig. 7a. The capacitance retention is dramatically decreased inthe first few hundred cycles, which may be due to the activesite saturation of the surface of MoSe2 nanosheets during thecharge–discharge process.25 Thereafter, a gradual decrease is

Fig. 6 (a) Cyclic voltammograms of MoSe2 nanosheets in 0.5 M H2SO4

electrolyte measured at different scan rates ranging from 2 to125 mV s−1. (b) Specific capacitance value calculated from the CV curvesof Fig. 5a at different scan rates.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2015 Dalton Trans., 2015, 44, 15491–15498 | 15495

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7.

View Article Online

observed until 10 000 cycles. Notably, a cycling performance ofapproximately 75% capacitance retention is observed evenafter 10 000 cycles, which indicates that MoSe2 nanosheets arepromising electrode materials for high-performance super-capacitors. The remaining 25% capacitance loss may be due tothe chemical instability of MoSe2 in aqueous electrolyte, whichcan be improved by the usage of organic/ionic liquid basednon-aqueous electrolytes. EIS was performed to analyze theinternal resistance and capacitance of the device. Fig. 7b pre-sents a Nyquist plot of the symmetric device at the initial stageand after 10 000 cycles. In the high-frequency region, the x-axisintercept of the semi-circle on the z-real axis represents theelectrochemical series resistance (ESR) of the device, whichwas determined to be 0.43 Ω. The depressed semi-circle in thehigh-to-medium frequency range is modeled by the parallelcombination of an interfacial charge transfer resistance anddouble-layer capacitance. In the mid-frequency region, a

typical 45° slope is observed for a short region, which isknown as the Warburg impedance due to the frequency depen-dence of ion diffusion/transport in the electrolyte. A shortWarburg region on the plot indicates a short ion diffusionpath and, in turn, effective diffusion of electrolyte ions intothe electrode surface.51 In the low-frequency region, an almost90° straight line is observed, which indicates the excellentcapacitive behavior of the device.52 EIS was performed after10 000 cycles for the post-analysis of the device. The shape ofthe spectrum after 10 000 cycles is similar to the initial shape,except for the diameter of the semi-circle arc in the high-to-medium frequency range. The semi-circle arc of the deviceincreased from 1 to 1.5 Ω after 10 000 cycles, which might bedue to the adhesion loss of some electroactive materials fromthe stainless steel current collector or dissolution of some elec-troactive materials during the repeated charge/dischargeprocess.53–55 The superior electrochemical performance of thefew-layered MoSe2 nanosheets may be attributed to the follow-ing reasons: (1) the 2D sheet-like structure provides a highsurface area and more active sites for the fast reversible redoxreaction on the surface of the electrode and (2) the few-layeredMoSe2 nanosheets stacked together with van der Waals forcesallow the effective intercalation of H+ ions from the electrolyte.

4. Conclusions

In summary, this work reports the facile synthesis of few-layered MoSe2 nanosheets and their capacitive properties in atwo-electrode configuration using aqueous electrolyte. Thesymmetric device exhibited a measured capacitance of approxi-mately 49.7 F g−1, and the single electrode exhibited a specificcapacitance of approximately 199 F g−1 at a scan rate of 2 mVs−1. The excellent capacitive performance of the device ismainly attributed to the graphene-like 2D layered crystal struc-ture, which provides a high surface area and allows effectivediffusion of H+ ions for the fast reversible redox process. Thegood cyclic stability and capacitance retention of approxi-mately 75% are observed even after 10 000 cycles, which indi-cates that MoSe2 nanosheets could be a potential candidate forelectrochemical capacitors.

Acknowledgements

This research was supported by the National Research Foun-dation of Korea (NRF) funded by the Korean government,MSIP/KEIT (2014064020, 10050509).

References

1 C. Yu, P. Ma, X. Zhou, A. Wang, T. Qian, S. Wu andQ. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 17937–17943.

2 A. Ramadoss, B. Saravanakumar, S. W. Lee, Y.-S. Kim,S. J. Kim and Z. L. Wang, ACS Nano, 2015, 9, 4337–4345.

Fig. 7 (a) Specific capacitance retention of the MoSe2 nanosheets as afunction of cycle number, measured by charge–discharge at a highcurrent density of 5 A g−1 in 0.5 M H2SO4 electrolyte. (b) Electrochemicalimpedance spectra of a symmetric device recorded at the initial stageand after 10 000 cycles. The inset shows an enhanced view of the high-to-medium frequency region.

Paper Dalton Transactions

15496 | Dalton Trans., 2015, 44, 15491–15498 This journal is © The Royal Society of Chemistry 2015

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7.

View Article Online

3 M. Li, G. Sun, P. Yin, C. Ruan and K. Ai, ACS Appl. Mater.Interfaces, 2013, 5, 11462–11470.

4 M. Lee, S. K. Balasingam, H. Y. Jeong, W. G. Hong,H.-B.-R. Lee, B. H. Kim and Y. Jun, Sci. Rep., 2015, 5,8151.

5 M. Buscema, J. O. Island, D. J. Groenendijk, S. I. Blanter,G. A. Steele, H. S. J. van der Zant and A. Castellanos-Gomez, Chem. Soc. Rev., 2015, 44, 3691–3718.

6 S.-K. Park, S.-H. Yu, S. Woo, B. Quan, D.-C. Lee,M. K. Kim, Y.-E. Sung and Y. Piao, Dalton Trans., 2013,42, 2399–2405.

7 H. Wang, H. Feng and J. Li, Small, 2014, 10, 2165–2181.8 C. N. R. Rao, U. Maitra and U. V. Waghmare, Chem. Phys.

Lett., 2014, 609, 172–183.9 H. Li, K. Yu, X. Lei, B. Guo, C. Li, H. Fu and Z. Zhu, Dalton

Trans., 2015, 44, 10438–10447.10 M. Pumera, Z. Sofer and A. Ambrosi, J. Mater. Chem. A,

2014, 2, 8981–8987.11 J. M. Soon and K. P. Loh, Electrochem. Solid-State Lett.,

2007, 10, 250–254.12 S. Ratha and C. S. Rout, ACS Appl. Mater. Interfaces, 2013, 5,

11427–11433.13 E. G. Da Silveira Firmiano, A. C. Rabelo, C. J. Dalmaschio,

A. N. Pinheiro, E. C. Pereira, W. H. Schreiner andE. R. Leite, Adv. Energy Mater., 2014, 4, 1301380.

14 C. Hao, F. Wen, J. Xiang, L. Wang, H. Hou, Z. Su,W. Hu and Z. Liu, Adv. Funct. Mater., 2014, 24, 6700–6707.

15 H. Tang, J. Wang, H. Yin, H. Zhao, D. Wang and Z. Tang,Adv. Mater., 2015, 27, 1117–1123.

16 G. Sun, X. Zhang, R. Lin, J. Yang, H. Zhang and P. Chen,Angew. Chem., Int. Ed., 2015, 54, 4651–4656.

17 M. Liu, G. Li and X. Chen, ACS Appl. Mater. Interfaces, 2014,6, 2604–2610.

18 B. Hu, X. Qin, A. M. Asiri, K. A. Alamry, A. O. Al-Youbi andX. Sun, Electrochem. Commun., 2013, 28, 75–78.

19 B. Hu, X. Qin, A. M. Asiri, K. A. Alamry, A. O. Al-Youbi andX. Sun, Electrochim. Acta, 2013, 100, 24–28.

20 K. J. Huang, J. Z. Zhang, G. W. Shi and Y. M. Liu, Electro-chim. Acta, 2014, 132, 397–403.

21 K. J. Huang, L. Wang, Y. J. Liu, Y. M. Liu, H. B. Wang,T. Gan and L. L. Wang, Int. J. Hydrogen Energy, 2013, 38,14027–14034.

22 X. Zhou, B. Xu, Z. Lin, D. Shu and L. Ma, J. Nanosci. Nano-technol., 2014, 14, 7250–7254.

23 K. Gopalakrishnan, S. Sultan, A. Govindaraj andC. N. R. Rao, Nano Energy, 2015, 12, 52–58.

24 A. Ramadoss, T. Kim, G.-S. Kim and S. J. Kim, NewJ. Chem., 2014, 38, 2379–2385.

25 L. Cao, S. Yang, W. Gao, Z. Liu, Y. Gong, L. Ma, G. Shi,S. Lei, Y. Zhang, S. Zhang, R. Vajtai and P. M. Ajayan,Small, 2013, 9, 2905–2910.

26 J. Feng, X. Sun, C. Wu, L. Peng, C. Lin, S. Hu, J. Yang andY. Xie, J. Am. Chem. Soc., 2011, 133, 17832–17838.

27 Q. Wang, L. Jiao, H. Du, J. Yang, Q. Huan, W. Peng, Y. Si,Y. Wang and H. Yuan, CrystEngComm, 2011, 13, 6960–6963.

28 J. Yang, X. Duan, Q. Qin and W. Zheng, J. Mater. Chem. A,2013, 1, 7880–7884.

29 L. Zhang, H. B. Wu and X. W. Lou, Chem. Commun., 2012,48, 6912–6914.

30 B. T. Zhu, Z. Wang, S. Ding, J. S. Chen and X. W. Lou, RSCAdv., 2011, 1, 397–400.

31 K.-J. Huang, J.-Z. Zhang and Y. Fan, Mater. Lett., 2015, 152,244–247.

32 Y. Shi, C. Hua, B. Li, X. Fang, C. Yao, Y. Zhang, Y.-S. Hu,Z. Wang, L. Chen, D. Zhao and G. D. Stucky, Adv. Funct.Mater., 2013, 23, 1832–1838.

33 Y. N. Ko, S. H. Choi, S. B. Park and Y. C. Kang, Nanoscale,2014, 6, 10511–10515.

34 H. Wang, X. Lan, D. Jiang, Y. Zhang, H. Zhong,Z. Zhang and Y. Jiang, J. Power Sources, 2015, 283, 187–194.

35 A. J. Bradley, M. M. Ugeda, F. H. da Jornada, D. Y. Qiu,W. Ruan, Y. Zhang, S. Wickenburg, A. Riss, J. Lu, S.-K. Mo,Z. Hussain, Z.-X. Shen, S. G. Louie and M. F. Crommie,Nano Lett., 2015, 15, 2594–2599.

36 A. Bachmatiuk, R. F. Abelin, H. T. Quang, B. Trzebicka,J. Eckert and M. H. Rummeli, Nanotechnology, 2014, 25,365603.

37 L. T. L. Lee, J. He, B. Wang, Y. Ma, K. Y. Wong, Q. Li,X. Xiao and T. Chen, Sci. Rep., 2014, 4, 4063.

38 Z. Mutlu, D. Wickramaratne, H. H. Bay, Z. J. Favors,M. Ozkan, R. Lake and C. S. Ozkan, Phys. Status Solidi A,2014, 211, 2671–2676.

39 J. Xia, X. Huang, L.-Z. Liu, M. Wang, L. Wang, B. Huang,D.-D. Zhu, J.-J. Li, C.-Z. Gu and X.-M. Meng, Nanoscale,2014, 6, 8949–8955.

40 B. K. Miremadi and S. R. Morrison, J. Appl. Phys., 1988, 63,4970–4974.

41 T. Sekine, M. Izumi, T. Nakashizu, K. Uchinokura andE. Matsuura, J. Phys. Soc. Jpn., 1980, 49, 1069–1077.

42 P. Tonndorf, R. Schmidt, P. Böuttger, X. Zhang, J. Böurner,A. Liebig, M. Albrecht, C. Kloc, O. Gordan, D. R. T. Zahn,S. Michaelis de Vasconcellos and R. Bratschitsch, Opt.Express, 2013, 21, 4908–4916.

43 H. Tang, K. Dou, C.-C. Kaun, Q. Kuang and S. Yang,J. Mater. Chem. A, 2014, 2, 360–364.

44 H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng,K. Yan, N. Liu and Y. Cui, Nano Lett., 2013, 13, 3426–3433.

45 Y. Gao, G. Pandey, J. Turner, C. Westgate and B. Sammakia,Nanoscale Res. Lett., 2012, 7, 651.

46 W. Chen, R. B. Rakhi and H. N. Alshareef, J. Mater. Chem.,2012, 22, 14394–14402.

47 H. B. Li, M. H. Yu, F. X. Wang, P. Liu, Y. Liang, J. Xiao,C. X. Wang, Y. X. Tong and G. W. Yang, Nat. Commun.,2013, 4, 1894.

48 L. Mai, H. Li, Y. Zhao, L. Xu, X. Xu, Y. Luo, Z. Zhang,W. Ke, C. Niu and Q. Zhang, Sci. Rep., 2013, 3, 1718.

49 A. Winchester, S. Ghosh, S. Feng, A. L. Elias, T. Mallouk,M. Terrones and S. Talapatra, ACS Appl. Mater. Interfaces,2014, 6, 2125–2130.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2015 Dalton Trans., 2015, 44, 15491–15498 | 15497

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7.

View Article Online

50 M. Acerce, D. Voiry and M. Chhowalla, Nat. Nanotechnol.,2015, 10, 313–318.

51 C. Y. Foo, A. Sumboja, D. J. H. Tan, J. Wang and P. S. Lee,Adv. Energy Mater., 2014, 4, 1400236.

52 T. Y. Kim, H. W. Lee, M. Stoller, D. R. Dreyer, C. W. Bielawski,R. S. Ruoff and K. S. Suh, ACS Nano, 2011, 5, 436–442.

53 H.-Q. Wang, Z.-S. Li, Y.-G. Huang, Q.-Y. Li and X.-Y. Wang,J. Mater. Chem., 2010, 20, 3883–3889.

54 J. Yan, T. Wei, W. Qiao, B. Shao, Q. Zhao, L. Zhang andZ. Fan, Electrochim. Acta, 2010, 55, 6973–6978.

55 J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang and F. Wei,Carbon, 2010, 48, 3825–3833.

Paper Dalton Transactions

15498 | Dalton Trans., 2015, 44, 15491–15498 This journal is © The Royal Society of Chemistry 2015

Publ

ishe

d on

20

July

201

5. D

ownl

oade

d by

KO

N-K

UK

UN

IVE

RSI

TY

CE

NT

RA

L L

IBR

AR

Y o

n 14

/09/

2015

00:

54:5

7.

View Article Online