Aqueous and Non-aqueous Dispersions of Graphene and Boron Nitride Nanosheets : NMR Measurements and Molecular Dynamics Simulations
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Arunachalam, Vaishali
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Preface
Ever since the discovery of graphene in 2004, there has been considerable interest in
two dimensional (2D) nanomaterials due to their distinctive properties and the prospect
of potential applications. A 2D-nanomaterial may be obtained from the bulk layered
material by procedures that can overcome the van der Waals attractive force that hold
adjacent layers together. Historically this was first achieved by micro-mechanical cleavage
by the deceptively simple procedure of peeling atomically thin single layers from the bulk
material using scotch tape. The procedure, unfortunately, is not scalable and consequently
alternate procedures, both top-down as well as bottom-up, have been extensively explored.
One of the simplest methods to obtain defect-free 2D nanosheets is the sonication assisted
liquid phase exfoliation of the bulk layered material in a suitable solvent.
The role of the solvent is crucial to the liquid phase exfoliation process, as the formation of stable dispersions require that the exfoliated sheets, produced on sonication,
be prevented from re-aggregating. A wide range of solvents, solvent mixtures and
surfactant solutions have been investigated and solvent systems that favour formation
of stable dispersions identified. Much of the current understanding of the role of the
solvent is based on phenomenological models, matching surface energies of the solvent
and the layered material so as to minimize the surface tension between the two. What has
remained elusive, however, is a molecular perspective of the nature of interactions between
the solvent and the exfoliated nanosheet. This the focus of the present study. This thesis
reports results of investigations on dispersions of graphene in aqueous and non-aqueous
media as well as dispersions of boron nitride in water using solution and solid-state
Nuclear Magnetic Resonance (NMR) spectroscopy aided by Molecular Dynamics (MD)
simulations for interpreting the experimental observations.
The thesis is organized as five chapters with Chapter 1 providing a brief overview
of 2D nanomaterials with focus on graphene and boron nitride (BN); their properties
and applications. The chapter discusses the methods for obtaining graphene and BN
nanosheets with emphasis on the sonication assisted liquid phase exfoliation approach.
The chapter also provides a brief review of the phenomenological models that have
been advanced to understand the stability of dispersions of 2D nanomaterials in different
solvents. The stability of the nanosheet dispersions require that solvent or ligand molecules
be in close association with the nanosheets with properties and mobilities quite different
from those of the bulk solvent molecules. The challenge for in-situ measurements is to
be able to probe the bound or associated solvent/ ligand molecules in the presence of a
large excess of the bulk. NMR methods from the solution chemists toolbox are known to
provide methodologies that can distinguish bound ligand molecules from those in the bulk
and are, therefore, ideal techniques for investigating nanosheet dispersions. In particular
transfer Nuclear Overhauser Effect Spectroscopy (tr-NOESY) as well as Rotating-frame
Overhauser Effect Spectroscopy (ROESY) are well suited for systems where bound and
free solvent/ ligand molecules are in continuous exchange. This chapter also provides a
brief overview of the NMR experiments used in studying nanosystem-ligand interactions.
The results from NMR measurements provide a spectroscopic signature of solvent nanosheet
interactions in the dispersions and in conjunction with Molecular Dynamics
(MD) simulations provide a molecular level understanding of the stability of the dispersions
and the role of the solvent. The MD simulation methodology used in this study are
discussed in Chapter 2 along with the computational tools employed in the thesis.
Graphene is perhaps the most studied 2D nanomaterial and its distinctive properties
has paved the way for the commercial use of graphene-based materials in a variety of
applications. Sonication of bulk graphite in an organic solvent or aqueous surfactant
solutions has been considered a simple and scalable route for the production of defect
free graphene nanosheets. In aqueous solutions the interaction of surfactant chains with
the graphene sheets is crucial to the stability of the dispersion. In Chapter 3, 1H
two-dimensional Nuclear Overhauser Effect spectroscopy (NOESY) and classical MD
simulations have been used to probe these interactions in graphene dispersions stabilized
by the cationic surfactant cetyltrimethylammonium bromide (CTAB). It is shown from
the presence of intense negative transfer-NOESY cross peaks that the surfactant chains
are quasi-bound to the graphene sheets and undergo rapid exchange with free surfactant
ligands present in the dispersion. A surprising feature of the NOESY is the presence of
cross-peaks between groups that are separated by more than 5_A along the chain even
between protons of the `head' group of the CTA surfactant chain and protons of the `tail'
methyl group. This observation of apparent very short separation of protons of distal
groups of the surfactant chain corroborated reects the arrangement adopted by the
surfactant chains in the quasi-bound state in the dispersion. Classical MD simulations of
the dispersion provides a simple interpretation of these observations. The simulations
show that surfactant CTA chains lie at on the graphene sheets adopting a random
arrangement with the head of one chain in close proximity to the tail of another chain.
This arrangement can give rise to cross peaks in the NOESY between groups that are
apparently far separated along the chain. One of the most efficient organic solvents for
the sonication assisted liquid phase exfoliation of graphite is N-methyl-2-pyrrolidone
(NMP). Much of the success of phenomenological models based on surface energies has
been correctly predicting that NMP would be good solvent because its surface energy and
that of graphite are comparable. A molecular level understanding of the interaction of
NMP and graphene sheets in the dispersion is, however, not available. In Chapter 4, it
is shown that NMR methods can provide a spectroscopic signature for these interactions.
The 2D ROESY NMR shows significant differences in the spectra of graphene dispersions
in NMP and the pure solvent. MD simulations of a graphene sheet immersed in NMP
solvent molecules show that these differences arise because of induced layering of solvent
molecules in the vicinity of the sheet. The arrangement facilitates lowering of the
rotational correlation time of the NMP molecules near the surface of the graphene sheet
that are easily captured in the experimental two-dimensional ROESY NMR and which
manifests as enhanced cross-peak intensities as compared to the bulk solvent.
Among the graphene analogues boron nitride nanosheets has been considered the
closest because of the similarities in structures of hexagonal BN and graphite as well as the
positions of the respective elements in the periodic table. Their aqueous dispersibilities
are, however, very different. While graphene does not exfoliate or form stable dispersions
in water the hydrophobic BN forms stable dispersions on sonication in water, without
the need for surfactants or stabilizers. In Chapter 5, it is shown from zeta potential
measurements that the sheets are positively charged and the stability of the dispersions
are electrostatic in origin. The observations indicate that BN reacts with water on
sonication. Ab initio (Car-Parinello) MD simulations and reactive force-field (ReaxFF)
MD simulations were performed to understand the reactivity, and the origin of the
stability of the aqueous dispersions of BN. The simulations showed that water molecules
dissociate at the edges of the BN sheets leading to the to the formation of NH bonds
with the release of OH into the bulk. The simulations explain why the dispersions are
basic and the exfoliated BN nanosheets in the dispersion positively charged. 1H and 11B
solid-state NMR spectroscopy were used to identify the chemical species as predicted by
the MD simulations. The combination of MD simulations and NMR measurements are
able to provide a comprehensive understanding of the origin of the aqueous dispersibility
of the hydrophobic BN nanosheets. The results are summarized in Chapter 6.
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