Sham, Alison Yue Wan
Description
Graphene, a single layer of highly ordered carbon atoms, has
captured tremendous interest within academia and industry
recently. This is due to its unique, two-dimensional structure
which yields a combination of outstanding properties ideal for
enhancing the performance of existing applications and enabling
future disruptive technologies. Nevertheless, the production of
graphene particles devoid of structural defects that can be
easily integrated and used in...[Show more] products and applications on a
commercial scale has proved exceptionally challenging. The first
part of this problem can be addressed using the ultrasonic
exfoliation of graphene in aqueous surfactant solutions, which
offers an established method of producing defect-free graphene
suspensions suited to large scale production processes. As the
graphene exists in the liquid phase, a variety of techniques are
possible for the transfer and integration of the particles into
products and applications. Many of these techniques rely on
adsorption, which involves the net accumulation of molecules from
a liquid at an interface through attractive intermolecular
interactions. The intermolecular interactions inherent to
surfactant-assisted exfoliated graphene have been used to
investigate the prospect of employing such particles in a series
of applications governed by adsorption mechanisms. The outcomes
of these studies are described here in this thesis.
Suspensions of graphene particles were prepared using the
ultrasonic exfoliation of graphite, with continuous surfactant
addition in the presence of the non-ionic surfactant, Pluronic®
F108. The resultant suspensions and particles were characterised
using UV-Visible spectroscopy, Raman spectroscopy, and
transmission electron microscopy. The prepared graphene
suspensions exhibited single and bilayer defect-free particles
ranging up to 1 μm in size. Zeta potential measurements
indicated the graphene particles exhibited with a low residual
negative electrostatic charge originating from edge defects
rather than the adsorbed surfactant. Four studies reflecting
proof-of-concept applications for surfactant stabilised graphene
were then investigated. Each study focused on a different type of
intermolecular interaction or interface involved in the
underlying adsorption process.
In one study, multilayer thin films were sequentially constructed
through the electrostatic layer-by-layer (LbL) deposition of
Pluronic F108 exfoliated graphene and the cationic
polyelectrolyte, polyethyleneimine on silica surfaces. Multilayer
assembly was monitored using a quartz crystal microbalance and
was shown to be strongly influenced by conditions such as
graphene concentration, pH, ionic strength and ionic species.
Consequently, it was shown that the thickness of the films could
be specified by altering the number of layers deposited, while
the viscoelastic properties of the films could be tailored by
altering film deposition conditions.
Hydrogen-bonded multilayer films consisting of Pluronic F108
exfoliated graphene and the weak polyelectrolyte, polyacrylic
acid were also constructed using the LbL technique at low pH. In
this study, quartz crystal microbalance measurements and Raman
spectra suggested a superlinear film growth regime, whilst atomic
force microscopy qualitative nanomechanical mapping measurements
indicated that the mechanical properties of the films differed
with the number of layers adsorbed. The films also underwent
partial deterioration when exposed to aqueous solutions at
neutral and basic pH. Thus, the hydrogen-bonded thin films
demonstrated a series of features appropriate to functional
coatings, such as pH responsiveness, surface roughness and
internal film structures, which could be altered depending on
deposition conditions and number of layers adsorbed.
The third study described foam stabilisation through adsorption
of Pluronic F108 exfoliated graphene at the liquid-air interface.
Particle surface activity was confirmed through surface tension
measurements and was shown using the evolution of bubble size
distributions to cause a reduction in destabilisation mechanisms.
The addition of alkali metal chlorides also enhanced foam
stability by altering the wettability of graphene particles
through changes in the hydration of polyethylene oxide groups
present on the stabilising surfactant. Thus, the high-aspect
ratio particles and adaptable surface interactions possible with
Pluronic F108 exfoliated graphene are able to stabilise and
enhance bubble surfaces in foams.
In the final study, ionic and non-ionic surfactant exfoliated
graphene were used to adsorb ionic organic dyes from solution.
Dye adsorption was maximised when attractive electrostatic
interactions were possible with the graphene particles. In
particular, the adsorption of methylene blue, a cationic dye, by
anionic SDS exfoliated graphene particles was shown to be
exceedingly rapid. This process was shown to be consistent with
the pseudo-second order kinetics model and Freundlich adsorption
isotherm. Consequently, the surface interactions caused by the
presence of surfactant at the graphene particle surface are
sufficient to drive the adsorption of dye from solution.
These results collectively demonstrate the versatility of
surfactant stabilised graphene adsorption at interfaces, which is
made possible by the nature of the stabilising surfactant and
properties inherent to the 2D graphene lattice. They illustrate
the potential for surfactant exfoliated graphene to be used in a
variety of applications such as functional and responsive thin
films, as well as their ability to act as effective foam
stabilisers and carbon-based adsorbents.
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