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C: Physical Processes in Nanomaterials and Nanostructures 2
Understanding Surfactant Stabilization of MoS Nanosheets in Aqueous Dispersions from Zeta-Potential Measurements and MD Simulations Amit Gupta, and Sukumaran Vasudevan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05922 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018
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The Journal of Physical Chemistry
Understanding Surfactant Stabilization of MoS2 Nanosheets in Aqueous Dispersions from Zeta-Potential Measurements and MD Simulations
Amit Gupta and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, INDIA
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ABSTRACT
The sonication assisted exfoliation of MoS2 in aqueous media in the presence of ionic surfactants to give stable dispersions is an attractive procedure for obtaining single or few-layered nano-sheets, as it is easily scalable and does not involve toxic or high boiling solvents. Here we have investigated the origin of the stability of aqueous dispersions of MoS2 nanosheets obtained by sonication in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) by zeta–potential measurements at different ionic strengths and molecular dynamics (MD) simulations. Our measurements show that the dispersions are stabilized by electrostatic repulsive interactions between the delaminated MoS2 nano-sheets, that acquire a positive charge due to the adsorption of the cationic surfactant. MD simulations were performed to understand the interaction between MoS2 nanosheets and the CTAB surfactant chains in the dispersion and the structure and arrangement of the adsorbed surfactant chains. Our simulations are able to reproduce the experimentally measured variation of the zeta potential with ionic strength. In addition, the relative contribution and role of different intermolecular interactions between various components of the dispersion was estimated by simulating the potential of mean force (PMF) between two surfactant adsorbed MoS2 sheets. Based on experiment and simulations we are able to establish that the stability of aqueous dispersions of MoS2 in the presence of an ionic surfactant can be understood based on classical models of charged interfaces.
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INTRODUCTION Atomically thin 2D nanomaterials have attracted increasing interest ever since the discovery of graphene.1 Graphene with its unique properties, distinct from that of the bulk graphite, has spurred the search for new 2D nanomaterials that can be obtained by the exfoliation of layered materials, as well as for more efficient and scalable routes for obtaining them.2–6 The latter is important if wide-scale applications of these materials are to be realized. The original “scotchtape” micro-mechanical cleavage procedure continues to be the gold-standard against which nanosheets obtained by other procedures are compared, but it is limited by its lack of scalability.1,4 One of the simplest procedures for obtaining defect-free single, or few-layer, nanosheets is the sonication assisted liquid-phase exfoliation of bulk layered materials in a suitable medium to obtain stable dispersions of the nanosheets.5,7 Sonication generates cavitation bubbles that on collapse create high-energy jets, which break up the layered crystallites producing exfoliated nanosheets.8,9 The role of the medium is crucial, for the stability of the dispersion depends on how effective it is in preventing re-aggregation of the exfoliated nanosheets.10,11 The choice of the medium has generally been guided by classical models developed for colloidal systems, which balance attractive and repulsive forces between the dispersed species, treating the solvent as a uniform continuum.12–14 ‘Good’ solvents have surface energies comparable with that of the layered material so that the energy difference favors the exfoliated rather than the aggregated bulk state.15 Solvents such as N-methyl-pyrrolidone that fit these criteria have been successfully used to obtain stable dispersions of graphene, MoS2 and other layered compounds by sonication assisted exfoliation.16 Stable dispersions of exfoliated nanosheets in aqueous media can be obtained on sonication in the presence of ionic surfactants or polymers.7,16 The dispersions are stabilized by electrostatic interactions by the presence of surfactant chains adsorbed on the nanosheet.13 Nanosheets of a number of different layered materials, graphene, the transition metal chalcogenides, and
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BN have been obtained by exfoliation using this method. In the case of MoS2 it had been shown from the zeta potential values, that by appropriate choice of the ionic surfactant, stable aqueous dispersions of either positive or negative charged nanosheets can be obtained by sonication assisted exfoliation in aqueous media.17 Positive charged MoS2 sheets are obtained on sonication in the presence of a cationic surfactant, like cetyltrimethylammonium bromide (CTAB), while anionic surfactants, like sodium dodeyl sulfate (SDS), give negatively charged sheets.17 The aqueous dispersions of the MoS2 nanosheets are stabilized by electrostatic repulsion between the sheets. The charge on the sheets is a consequence of the ionic surfactant adsorbed on the surface of the MoS2 nanosheet. 1H NMR spectroscopic measurements showed that surfactant chains are weakly bound to the MoS2 sheets and are in rapid exchange with the free surfactant species present in the dispersion. In situ 2D Nuclear Overhauser spectroscopy (NOESY) measurements of the MoS2 nanosheets in aqueous dispersions provided direct evidence of bound surfactant chains by the occurrence of cross-peaks that exhibit negative transfer NOEs. The cross-peaks indicated that the head-group protons of the surfactant, and protons in the mid-segment and tail of the adsorbed chains are in close spatial proximity, suggesting that the surfactant chains adopt an arrangement where they are randomly adsorbed and lie flat on the basal plane of the MoS2 nanosheets.17 The previous studies had concluded that electrostatic repulsion between the positive charges of the adsorbed CTAB surfactants is primarily responsible for the stability of MoS2 nanosheet dispersions in aqueous CTAB solution. This picture is, however, incomplete, for it does not provide a comprehensive description of how the cetyltrimethylammonium (CTA) surfactant cations, the Br- counterions, and water molecules interact to prevent re-aggregation of the MoS2 nanosheets in the dispersion. Nor does it account for the role of van der Waals and dispersive interactions in the stability of the dispersions. Here we have attempted to provide a more detailed atomic-level description of these interactions and their relative roles and contribution to
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The Journal of Physical Chemistry
the stability of MoS2-CTAB aqueous dispersions. In the present study, we have carried out zeta potential measurements on the MoS2-CTAB dispersions as a function of ionic strength. We have subsequently carried out an all-atom classical molecular dynamics (MD) simulation to study the interaction between MoS2 nanosheets and the CTAB surfactant chains in the dispersion and the structure and arrangement of the adsorbed surfactant chains analyzed. A crucial test was whether the simulations could reproduce the experimentally measured variation of the zeta potential with ionic strength. In addition, we have analyzed the contribution and role of different intermolecular interactions between various components of the dispersion by simulating the potential of mean force (PMF) between two parallel CTAB adsorbed MoS2 sheets as a function of inter-sheet separation.
EXPERIMENTAL DETAILS MoS2 dispersions were prepared by micro-mechanical cleavage by ultrasonication following reported procedures.7 In a typical preparation 100 mg of MoS2 (Sigma Aldrich,
