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Water Dispersible, Positive and Negatively Charged MoS Nanosheets: Surface Chemistry and the Role of Surfactant Binding Amit Gupta, Vaishali Arunachalam, and Sukumaran Vasudevan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00158 • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 9, 2015
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The Journal of Physical Chemistry Letters
Water Dispersible, Positive and Negatively Charged MoS2 Nanosheets: Surface Chemistry and the Role of Surfactant Binding
Amit Gupta, Vaishali Arunachalam and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, INDIA
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ABSTRACT Stable aqueous dispersions of atomically-thin layered MoS2 nanosheets have been obtained by sonication in the presence of ionic surfactants. The dispersions are stabilized by electrostatic repulsion between the sheets and we show that the sign of the charge on the MoS2 nanosheets, either positive or negative, can be can be controlled by the choice of the surfactant Using techniques from solution NMR we show that the surfactant chains are weakly bound to the MoS 2 sheets and undergo rapid exchange with free surfactant chains present in the dispersion. In situ nuclear Overhauser effect spectroscopic measurements provide direct evidence that the surfactant chains lie flat, arranged randomly on the basal plane of the MoS 2 nanosheets with their charged head-group exposed. These results provide a chemical perspective for understanding the stability of these inorganic nanosheets in aqueous dispersions and the origin of the charge on the sheets.
TOC Graphics:
Keywords: Surfactant Exfoliation, Zeta Potential,
1
H NMR, Nuclear Overhauser Effect
Spectroscopy ( NOESY)
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Ever since the discovery of graphene the study of two-dimensional nanosheets obtained by exfoliation of layered materials has emerged as an important sub-discipline within nanosciences.1,2 The unique attributes of graphene, exceptional thermal and electronic conductivity and optical and mechanical properties with potential applications ranging from transparent conductors to biological sensors has catalyzed the search for new materials that can form nanosheets.2–5 In recent years attention has focused on the transition metal dichalcogenides, MX2, that consist of hexagonal layers of metal atoms (M) sandwiched between two layers of chalcogen atoms (X = S, Se or Te). Like graphene these compounds exhibit anisotropic bonding - strong iono-covalent bonding within a sheet but weak van der Waals interactions between the sheets, making exfoliation into individual separate sheets facile.6 What makes these compounds attractive is that they exhibit a diverse range of properties from insulators to metals depending on the combination of chalcogen (S, Se, or Te) and transition metal as well as the coordinationtrigonal prismatic or octahedral- and oxidation state of the metal atoms.7 MX2 compounds therefore offer opportunities for going beyond graphene and opening up new fundamental and technological possibilities in fields like transistors, energy storage, and composites.8–10 Applications would, however, require simple procedures for obtaining nanosheets of the MX2 compounds and it is therefore not surprising that considerable attention has focused on procedures to obtain nanosheets of these materials. Following an identical route as that for graphene, micromechanical cleavage, either using scotch tape or by rubbing, has been used to produce various single-layer nanosheets from bulk BN, MoS2 and NbSe2 layered materials.6 The method produces high quality mono-layers but is limited by low yields. An effective method for obtaining single or few layer 2D nanosheets of MX2 compounds in reasonable yields is by the exfoliation of the lithium-intercalated MX2
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compounds in either water or alcohol. The lithium intercalated compound can be prepared either by chemical reaction using n-butyl lithium or by electrochemical intercalation.11–13 One of the simplest routes to obtain large quantities of nanosheets of the layered MX2, compounds is sonication assisted exfoliation in a suitable solvent or in aqueous solutions.14–16 During sonication the rapid compression and rarefaction of sound waves, create vacuum cavities in the medium that on collapsing result in high pressure jets that peel off the layers.17 The resulting exfoliated sheets would however need to be stabilized either by solvents or surfactants to overcome the cohesive energy between layers that would result in restacking. It has been shown that one way of achieving this is to match the surface free energies of the solvent with that of the material being exfoliated.18,19 The method has been successfully used to fabricate single and multilayer nanosheets of a number of MX2 compounds. Yet another method to stabilize the nanosheets in aqueous media and prevent aggregation, subsequent to sonication, and which is the focus of the present study, is to have surfactants presenting the medium. The surfactant sodium cholate has been used to stabilize a large number of layered crystals including BN, MX 2, and transition metal oxides nanosheets in aqueous media.20 Here we have investigated the exfoliation of layered MoS2 by sonication in aqueous media in the presence of ionic surfactants to give stable dispersions of atomically thin nanosheets in water. The dispersions are stabilized by electrostatic repulsion between the sheets and we show that the sign of the charge on the MoS2 nanosheets, either positive or negative, can be can be controlled by the choice of the surfactant. The nature of the interaction between the surfactant and the inorganic MoS2 nanosheet is crucial. Using techniques from the solution NMR toolbox – diffusion ordered (DOSY) and nuclear Overhauser effect (NOESY) spectroscopy - we have investigated the nature of the surfactant-MoS2 nanosheet interaction and characterized the
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organization and arrangement of the surfactant chains on the surface of the nanosheets. Our results provide a chemical perspective for understanding of the stability of these inorganic nanosheets in aqueous dispersions and the origin of the charge on the sheets. Sonication of MoS2 powder in the presence of either the cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), or the anionic surfactant, sodium dodecyl sulfate (SDS), provide dispersions that are stable over weeks. In a typical experiment(see Supporting Information, S1) 330mg of MoS2 (Sigma Aldrich,
