Subscriber access provided by DUESSELDORF LIBRARIES
Article
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 19
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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.
ACS Paragon Plus Environment
1
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 19
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
ACS Paragon Plus Environment
2
Page 3 of 19
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
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,
20
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
ACS Paragon Plus Environment
3
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 19
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.
ACS Paragon Plus Environment
4
Page 5 of 19
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
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,
