Electrochemical Characterization of Liquid Phase Exfoliated Two

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Electrochemical Characterization of Liquid-Phase Exfoliated 2D Layers of Molybdenum Disulfide Andrew Justin Winchester, Sujoy Ghosh, Simin Feng, Ana L Elias, Tom Mallouk, Mauricio Terrones, and Saikat Talapatra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am4051316 • Publication Date (Web): 20 Jan 2014 Downloaded from http://pubs.acs.org on January 22, 2014

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ACS Applied Materials & Interfaces

Electrochemical Characterization of Liquid-Phase Exfoliated 2D Layers of Molybdenum Disulfide Andrew Winchester1, Sujoy Ghosh1, Simin Feng2, Ana Laura Elias2, Tom Mallouk3, Mauricio Terrones2,3,4, Saikat Talapatra1*

1

Department of Physics, Southern Illinois University, Carbondale, Illinois 62901, United States

2

Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania

State University, University Park, Pennsylvania 16802, United States 3

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania

16802, United States 4

Department of Materials Science and Engineering, The Pennsylvania State University,

University Park, Pennsylvania 16802, United States

KEYWORDS 2D layered materials, Molybdenum disulfide, liquid phase exfoliation, electrochemical, charge storage.

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ABSTRACT

We report on the electrochemical charge storage behavior of few-layered flakes of molybdenum disulfide (MoS2) obtained by liquid phase exfoliation of bulk MoS2 powder in 1dodecyl-2-pyrrolidinone (N12P). The specific capacitances of the exfoliated flakes obtained using 6M KOH aqueous solution as an electrolyte were found to be an order of magnitude higher than bulk MoS2 (~ 0.5 mFcm-2 and ~ 2 mFcm-2 for bulk and exfoliated MoS2 electrodes respectively). The exfoliated MoS2 flakes also showed significant charge storage in different electrolytes, such as organic solvents (1M tetraethylammonium tetrafluoroborate in propylene carbonate

(Et4NBF4

in

PC))

and

ionic

liquids

(1-butyl-3-methylimidazolium

hexafluorophosphate (BMIM-PF6)). The values of specific capacitances obtained using Et4NBF4 in PC and BMIM-PF6 were ~ 2.25 mFcm-2 and ~ 2.4 mFcm-2, respectively. The analysis of Electrochemical Impedance Spectroscopy (EIS) using an equivalent circuit modeling was performed in order to understand the charge storage mechanism of these exfoliated MoS2 flakes using different electrolytes. Our findings indicate that liquid phase exfoliation methods can be used to produce large quantities of electrochemically active, 2D layers of MoS2 and can act as an ideal material in several applications related to electrochemistry.

INTRODUCTION Molybdenum disulfide (MoS2) is an indirect band gap semiconductor belonging to the group VI family of transition metal dichalcogenides (TMDs). The crystalline structure of this material, consisting of stacked sheets of S-Mo-S, gives rise to various physico-chemical properties ranging from solid lubricants to catalysts. For example, crystals of MoS2 have been known to be a photoactive material; early work by H. Tributsch established that MoS2 crystals, along with other

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semiconducting TMDs such as MoSe2 and WSe2, could be used as stable photoelectrochemical electrodes.1,

2

Subsequent work on polycrystalline thin films of MoS2 showed comparable

photoelectric response similar to the single crystals.3,

4

More recently, methods to produce

thinner films with larger yields via intercalation, exfoliation, and chemical means have been explored.5-10 MoS2 has also been evaluated as a photocatalyst for the production of H2 gas, with potential for becoming a lower cost replacement for platinum.6, 11-13 As described above, bulk crystals of MoS2 are known to have an indirect band gap of ~1.21.3eV,14,

15

which has lead to much of the initial photoelectrical research on this material.

However, due to its layered structure it is possible to exfoliate this system into fewer layers and observe significant changes in its physical and chemical properties. For example, it has been shown both theoretically and experimentally that by isolating individual layers of the crystal, significant changes in the band structure of MoS2 are observed. In particular, a direct band gap of ~1.7-1.9eV is opened in mono-layered MoS2.16-18

Therefore, it is important to study the

influence of the number of layers and the addition of edges in MoS2 on the photocatalytic efficiency of evolving hydrogen gas.11, 13 Recently, it has been demonstrated that by increasing the available surface area from the bulk crystal to few or single layers of MoS2, it is possible to observe enhanced charged storage in lithium ion battery systems19,

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and supercapacitors.21-23

Therefore, it is very important to control the number of layers, the amount of reactive edges, and defects within these semiconducting TMDs in order to fabricate the next generation of energy storage and harvesting systems. In this work we study fundamental electrochemical properties of MoS2 exfoliated into few layer stacks by examining the behavior of the resulting material in different electrolytes. Our findings indicate that upon exfoliation, the electrochemical charge storage capacity of these

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layered systems increases by an order of magnitude when compared to their bulk counterpart when measured in different electrolytes (KOH, Et4NBF4 in PC and BMIM-PF6,). We obtained a maximum value of specific capacitance of ca. 2.4 mFcm-2 with BMIM-PF6 as electrolyte (applied voltage window of 3.5 Volts).

Electrochemical impedance spectroscopy (EIS) in

conjunction with circuit analysis indicates that in an aqueous electrolyte (e.g. KOH), nearly pure capacitive behavior is observed in the charge storage mechanism, whereas for polymers and other ionic electrolytes diffusive behavior is more prevalent.

Figure 1. (a) Optical image of a dispersion of exfoliated MoS2, (b) UV-VIS spectrum of exfoliated MoS2 showing characteristic peaks with the corresponding value in eV. Inset displays Tauc plot with linear extrapolation, giving a direct band gap of ~1.75 eV, (c) TEM image of a stack of thin flakes of exfoliated MoS2, (d) TEM image of a small, few layer flake of MoS2 with inset electron diffraction pattern showing crystalline structure.

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Experimental The few-layered MoS2 dispersions were obtained via a liquid phase exfoliation technique, as reported by Coleman, et al.24, 25 Commercial MoS2 powder (Sigma Aldrich,