The Adsorption of Surfactant
Exfoliated Graphene at Interfaces
Alison Sham B. Eng. (Hons), B.Sc. (Hons)
A thesis submitted for the degree of Doctor of Philosophy at The
Australian National University
April 2017
Copyright © Alison Sham 2017
i
Declaration
This thesis is an account of my research conducted in the Department of Applied
Mathematics, College of Physical and Mathematical Sciences, at The Australian National
University, from August 2011 to April 2017. This thesis contains no material submitted for
the award of any other degree or diploma in any university or other tertiary education
institution. To the best of the author’s knowledge, it contains no material previously
published by another person, except where due reference is made in the text. This research
has been supported by an Australian Government Research Training Program (RTP)
Scholarship.
Alison Sham
April 2017
ii
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Acknowledgments
This thesis would not have been possible without the support and encouragement of a
great many people.
First and foremost, I would like to express my sincere gratitude to my supervisor,
Shannon Notley, for his continued support, encouragement and guidance throughout my
PhD. Although Shannon assumed a number of new roles and responsibilities through
the duration of my PhD which required much of his attention, he always took the time
to offer invaluable advice and feedback and was always keen to see me progress, both in
terms of my candidature and as a researcher. I am incredibly grateful to him for his
support and am certain that this thesis would not have been completed without it.
I would also like to thank the other current and former members of my supervisory
panel including Professor Tim Senden, Dr Andrew Fogden and Professor Vince Craig
for their advice and enthusiasm throughout my project. Special thanks go to Vince,
who, in many ways, acted as a surrogate local supervisor during the earlier years of my
candidature when Shannon was in Melbourne.
I would also like to express my appreciation to the staff and students of the Department
of Applied Mathematics for their friendliness, expertise and encouragement throughout
my candidature. Special mention must go to Martina Landsmann, for her knowledge,
and assistance in navigating the administration labyrinth, as well as Tim Sawkins and
Ron Cruikshank for their technical support. Thank you also to my fellow students and
post-docs including Muidh Alheshibri, Hong Jie An, Namsoon Eom, Marie Jehannin, E-
iv
Jen Teh, Jane Qian, Matthew Quinn, Virginia Mazzini, Tom McKay, Rick Walsh and
Tao Wang for their friendship and fond memories.
I am also grateful to the staff and students of the Department of Chemistry and
Biotechnology at Swinburne University of Technology for hosting me during my
research visits to Melbourne. Many thanks also to Deborah Wakeham, Mark Rutland
and the rest of the staff and students at the Division of Surface and Corrosion Science at
KTH Royal Institute of Technology in Stockholm for their expertise and assistance in
performing lateral force microscopy measurements which ultimately and unfortunately,
did not make it into this thesis.
I would also like to thank my friends for their support throughout my PhD. Special
thanks to my friends Ian Donald and Dave Rodgers, for welcoming me into their homes
and hosting me during my many and frequent visits to Melbourne. I would also like to
thank my fellow freshies including Blitzkat, Shrieking Violet, Rumble Bee and Hell
Nino along with the rest of the CRDL family for being part of a welcome, albeit short,
distraction from my PhD. A big thank you also to my Fitsistas, who have been
instrumental in allowing me to reach new heights (and new lows), and have been a great
source of positivity and cheerfulness over the past five years. To all my other friends,
thank you for all the good times we shared. You may not have realised it, but they have
made my PhD just a little bit more bearable.
Last, but not least, I would be nowhere without the love and support of my family, and
of course my Handsome Bookend.
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Abstract
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 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
vi
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.
vii
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.
viii
Publications and Presentations
Publications
I. A. Y. W. Sham and S. M. Notley. A review of fundamental properties and
applications of polymer-graphene hybrid materials. Soft Matter 9, 6645-6653
(2013).
II. A. Y. W. Sham and S. M. Notley. Layer-by-Layer Assembly of Thin Films
Containing Exfoliated Pristine Graphene Nanosheets and Polyethyleneimine.
Langmuir 30, 2410-2418 (2014).
III. A. Y. W. Sham and S. M. Notley. Graphene–polyelectrolyte multilayer film
formation driven by hydrogen bonding. J. Colloid Interface Sci. 456, 32-41
(2015).
IV. A. Y. W. Sham and S. M. Notley. Foam stabilisation using surfactant exfoliated
graphene. J. Colloid Interface Sci. 469, 196-204 (2016).
V. A. Y. W. Sham. and S. M. Notley. Adsorption of organic dyes from aqueous
solutions using surfactant exfoliated graphene, In Preparation.
Oral Conference Presentations
I. A. Y. W. Sham. “Creating Novel Lightweight 3D Carbon Structures”, 28th
Australian Colloid and Surface Sciences Student Conference, Riverwood
Downs, Australia, February 2012
ix
II. A. Y. W. Sham and S. M. Notley. “Functional Graphene-Polyelectrolyte Thin
Films Formed by Hydrogen Bonding”, UK Colloids 2014, London, England,
July 2014
III. A. Y. W. Sham and S. M. Notley. “Functional Graphene – Polyelectrolyte Thin
Films formed by Hydrogen Bonding”, International Conference on Nanoscience
and Nanotechnology - ICONN 2014, Adelaide, Australia, February 2014
IV. A. Y. W. Sham and S. M. Notley. “Effect of Salts on the Stability of Graphene
Particle Stabilised Foams”, International Conference on Nanoscience and
Nanotechnology - ICONN 2016, Canberra, Australia, February 2016
Poster Presentations
I. A. Y. W. Sham and S. M. Notley. “Assembly of Graphene – Polyelectrolyte
Thin Film Coatings using Intermolecular Forces”, 2nd
OzCarbon, Melbourne,
Australia, 2013
II. A. Y. W. Sham and S. M. Notley. “Functional Graphene-Polyelectrolyte Thin
Films Formed by Hydrogen Bonding”, 4th
Graphene Conference, Toulouse,
France, 2014
III. A. Y. W. Sham and S. M. Notley. “Incorporating Graphene in Carbon Based
Foams for Water Treatment Processes”, Australian Colloid and Interface
Symposium 2013, Noosa, Australia, 2013
IV. A. Y. W. Sham and S. M. Notley. “Effect of Salts on the Stability of Graphene
Particle Stabilised Foams”, International Conference on Nanoscience and
Nanotechnology - ICONN 2016, Canberra, Australia, 2016
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Table of Contents
Declaration ........................................................................................................................ i
Acknowledgments .......................................................................................................... iii
Abstract ............................................................................................................................ v
Publications and Presentations ................................................................................... viii
Table of Contents ........................................................................................................... xi
List of Acronyms ......................................................................................................... xvii
Chapter 1 Introduction ................................................................................................... 1
1.1 Potential Large - Scale Graphene Production Methods ................................................ 2
1.2 Positioning and Placement of Surfactant Exfoliated Graphene through Adsorption .... 6
1.3 Utilizing the Behaviour of Graphene at Interfaces ........................................................ 7
1.4 Thesis Outline ................................................................................................................. 9
Chapter 2 Adsorption at Interfaces ............................................................................. 11
2.1 Introduction .................................................................................................................. 11
2.2 Intermolecular Interactions .......................................................................................... 12
2.2.1 Electrostatic Interactions .................................................................................................... 12
2.2.2 Van der Waals Interactions ................................................................................................ 13
2.2.3 Hydrogen Bonding Interactions ......................................................................................... 14
2.3 Surface Interactions...................................................................................................... 15
2.3.1 Van der Waals-Based Interactions ..................................................................................... 16
2.3.2 Electrostatically Charged Surfaces and The Electric Double Layer ................................... 17
2.4 Thermodynamic Properties of Interfaces ..................................................................... 19
2.5 Adsorption Processes and the Gibbs Adsorption Isotherm .......................................... 23
2.6 Species-Specific Adsorption Processes ........................................................................ 24
2.6.1 Surfactant Adsorption ......................................................................................................... 24
2.6.2 Polymer Adsorption ........................................................................................................... 26
2.7 Conclusion .................................................................................................................... 28
Chapter 3 Graphene: Structure, Properties and Production Methods ................... 30
3.1 Introduction .................................................................................................................. 30
3.2 Structure of Graphene .................................................................................................. 31
3.2.1 Properties of Graphene ....................................................................................................... 32
xii
3.2.2 Surface Charge and Intermolecular Forces Present on Graphene ....................................... 33
3.3 Graphene Production Methods ..................................................................................... 34
3.3.1 Current Methods of Graphene Synthesis and Production ................................................... 36
3.4 Liquid Phase Production Methods ................................................................................ 39
3.4.1 Exfoliation of Graphene-Based Derivatives ....................................................................... 40
3.5 Production of Pristine Graphene Particle Dispersions ................................................ 42
3.5.1 Exfoliation of Graphene Using Non-Aqueous Systems ...................................................... 43
3.5.2 Exfoliation of Graphene Using Aqueous Systems .............................................................. 45
3.6 Surfactant-Assisted Exfoliation of Graphene ................................................................ 47
3.7 Properties and Features of Surfactant Exfoliated Graphene ........................................ 53
3.8 Conclusion .................................................................................................................... 56
Chapter 4 Experiment Materials and Techniques ..................................................... 58
4.1 Introduction ................................................................................................................... 58
4.2 Preparation and Characterisation of Graphene Suspensions ...................................... 59
4.2.1 Exfoliating Surfactant ......................................................................................................... 60
4.2.2 Materials ............................................................................................................................. 62
4.2.3 Methods .............................................................................................................................. 62
4.2.4 Characterisation of Graphene Particle Suspensions ............................................................ 63
4.3 Sample Preparation Techniques Involving Adsorption ................................................ 69
4.3.1 Layer-By-Layer Self Assembly .......................................................................................... 69
4.3.2 Dip Coating ......................................................................................................................... 71
4.4 Characterising Samples Prepared Through Adsorption ............................................... 73
4.4.1 Atomic Force Microscopy .................................................................................................. 73
4.4.2 Quartz Crystal Microbalance .............................................................................................. 78
4.4.3 Pendant Drop Technique ..................................................................................................... 84
4.5 Chapter Conclusion ...................................................................................................... 85
Chapter 5 Creating Electrostatically Bonded Multilayer Films with Surfactant
Exfoliated Graphene ..................................................................................................... 87
5.1 Introduction ................................................................................................................... 87
5.2 Background ................................................................................................................... 88
5.2.1 Electrostatically Assembled LbL Thin Films ..................................................................... 89
5.2.2 Benefits and Features of Electrostatic LbL Deposition....................................................... 90
5.2.3 Polyelectrolyte Multilayer Film Growth ............................................................................. 92
5.2.4 Graphene Based Materials in Electrostatic Multilayer Films ............................................. 93
5.3 Materials ....................................................................................................................... 95
5.4 Methods ......................................................................................................................... 95
5.4.1 Preparation of Stock Graphene Suspensions ....................................................................... 95
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5.4.2 Particle Characterisation ..................................................................................................... 95
5.4.3 Adsorption Measurements .................................................................................................. 96
5.4.4 Multilayer Characterisation ................................................................................................ 98
5.5 Results and Discussion ................................................................................................. 98
5.5.1 Film Formation for Extended Films ................................................................................... 98
5.5.2 Effect of pH and Electrolyte on Film Formation .............................................................. 103
5.5.3 Effect of PEI Concentration on Film Formation .............................................................. 107
5.5.4 Effect of Graphene Concentration on Film Formation ..................................................... 109
5.5.5 Characterisation of Films with Extended Numbers of Adsorbed Bilayers ....................... 111
5.6 Conclusion .................................................................................................................. 114
Chapter 6 Assembling Hydrogen-bonded Multilayer Films with Surfactant
Exfoliated Graphene ................................................................................................... 117
6.1 Chapter Introduction .................................................................................................. 117
6.2 Background ................................................................................................................ 118
6.2.1 Advantages of Hydrogen-bonded LbL Self-Assembly .................................................... 120
6.2.2 Applications of Hydrogen-bonded Multilayers ................................................................ 121
6.2.3 Graphene Based Materials in Hydrogen-bonded Multilayer Films .................................. 122
6.3 Materials .................................................................................................................... 124
6.4 Methods ...................................................................................................................... 125
6.4.1 Preparation of Graphene Suspension ................................................................................ 125
6.4.2 Adsorption Measurements – Deposition and Removal of Thin Films ............................. 125
6.4.3 Preparation of Dip Coated Multilayer Films .................................................................... 127
6.4.4 Thin Film Characterisation ............................................................................................... 127
6.4.5 Imaging of Surface Structure of Multilayer Films ........................................................... 128
6.5 Results and Discussion ............................................................................................... 129
6.5.1 QCM Adsorption Measurements ...................................................................................... 129
6.5.2 Removal of Thin Films ..................................................................................................... 134
6.5.3 Thin Films Formed Through Dip Coating ........................................................................ 136
6.5.4 Physical and Mechanical Properties of Thin Films .......................................................... 141
6.6 Conclusion .................................................................................................................. 148
Chapter 7 Foam Stabilization Using Surfactant Exfoliated Graphene ................. 150
7.1 Introduction ................................................................................................................ 150
7.2 Background ................................................................................................................ 151
7.2.1 Destabilising Foams ......................................................................................................... 152
7.2.2 Foam Stabilisation Mechanisms ....................................................................................... 154
7.2.3 Effect of Particle Properties on Foam Stability ................................................................ 156
7.2.4 Graphene-based Materials as Foam Stabilisers ................................................................ 163
xiv
7.3 Materials ..................................................................................................................... 164
7.4 Methods ....................................................................................................................... 165
7.4.1 Preparation of Stock Graphene Suspensions ..................................................................... 165
7.4.2 Characterisation of Graphene Suspensions and Particles.................................................. 165
7.4.3 Preparation of Sample Solutions ....................................................................................... 166
7.4.4 Surface Tension Measurements ........................................................................................ 166
7.4.5 Viscosity Measurements ................................................................................................... 167
7.4.6 Bubble Size Distribution Measurements ........................................................................... 167
7.4.7 Foam Stability Measurements ........................................................................................... 167
7.5 Results and Discussion................................................................................................ 168
7.5.1 Graphene Characterisation ................................................................................................ 168
7.5.2 Surface Activity of Graphene Particles ............................................................................. 169
7.5.3 Bubble Size Distribution ................................................................................................... 170
7.5.4 Foam Stability Measurements ........................................................................................... 173
7.5.5 Effect of Salt Concentration on Foam Stability ................................................................ 177
7.6 Conclusion .................................................................................................................. 180
Chapter 8 Adsorption of Organic Dyes Using Surfactant Exfoliated Graphene .. 182
8.1 Introduction ................................................................................................................. 182
8.2 Background ................................................................................................................. 184
8.2.1 Adsorption by Carbon Based Materials in the Aqueous Phase ......................................... 184
8.2.2 Organic Dye Contaminants ............................................................................................... 185
8.2.3 Adsorption Mechanisms Associated with Porous Carbon Materials ................................ 186
8.2.4 Conditions Influencing Dye Adsorption ........................................................................... 187
8.2.5 Quantitative Modelling of Adsorption Processes.............................................................. 194
8.2.6 Graphene-based Materials as Carbon Adsorbents ............................................................ 201
8.3 Materials ..................................................................................................................... 204
8.4 Methods ....................................................................................................................... 205
8.4.1 Preparation of Stock Graphene Suspensions ..................................................................... 205
8.4.2 Particle Characterisation ................................................................................................... 207
8.4.3 UV-Visible Spectroscopy ................................................................................................. 207
8.4.4 Effect of Filtering Process on Measured Dye Concentration ............................................ 208
8.4.5 Adsorption of Dyes with Surfactant Exfoliated Graphene ................................................ 208
8.4.6 Adsorption Isotherms and Temperature Effects on Dye Adsorption ................................ 209
8.4.7 Adsorption Kinetics and Effect of Contact Time on Dye Adsorption .............................. 210
8.4.8 Effect of pH on Dye Adsorption ....................................................................................... 211
8.5 Results and Discussion................................................................................................ 211
8.5.1 Characterisation of Graphene............................................................................................ 211
8.5.2 Adsorption of Ionic Dyes with Surfactant Exfoliated Graphene....................................... 213
xv
8.5.3 Effect of Surfactant on Dye Adsorption ........................................................................... 214
8.5.4 Effect of Contact Time on Dye Adsorption ...................................................................... 218
8.5.5 Adsorption Kinetics .......................................................................................................... 220
8.5.6 Effect of Temperature on Dye Adsorption ....................................................................... 223
8.5.7 Equilibrium Adsorption Isotherm ..................................................................................... 225
8.5.8 Effect of pH on Adsorption .............................................................................................. 230
8.6 Conclusions ................................................................................................................ 232
Chapter 9 Conclusions and Further Work ............................................................... 235
9.1 Conclusions ................................................................................................................ 235
9.2 Further Work .............................................................................................................. 239
References .................................................................................................................... 243
Appendix ...................................................................................................................... 285
A.1 Literature Review ....................................................................................................... 285
A.2 Assembling Hydrogen-bonded Multilayer Films with Surfactant Exfoliated Graphene ..
.................................................................................................................................... 295
A.2.1 Characterisation of Graphene Suspensions ...................................................................... 295
A.2.2 QCM Multilayer Adsorption Measurements .................................................................... 296
A.2.3 Removal of Multilayer Thin Films ................................................................................... 298
A.2.4 Surface Coverage of Dip-Coated Thin Films ................................................................... 300
A.2.5 Characterisation of Graphene in Dip-Coated Thin Films ................................................. 303
A.2.6 Quantitative Nanomechanical Measurements .................................................................. 304
A.3 Foam Stabilization Using Surfactant Exfoliated Graphene ....................................... 308
A.3.1 Characterisation of Graphene Suspensions ...................................................................... 309
A.3.2 Bulk Rheological Behaviour ............................................................................................ 311
A.3.3 Salt Effects on Foam Stability .......................................................................................... 311
A.4 Adsorption of Organic Dyes Using Surfactant Exfoliated Graphene ........................ 313
A.4.1 Characterisation of Graphene Suspensions ...................................................................... 313
xvi
xvii
List of Acronyms
Acronym Description
AC Alternating current
AFM Atomic force microscope
BET Brunauer-Emmett-Teller
CHAPS 3-[(3-Cholamidopropyl)dimethyl ammonio]-1-Propanesulfonate
CMC Critical micelle concentration
CTAB Cetyltrimethyl ammoniumbromide
CVD Chemical vapour deposition
DBDM n-Dodecyl b-D-maltoside
DC Direct current
DLS Dynamic light scattering
DLVO Derjaguin-Landau-Verwey-Overbeek
DMA N,N-dimethylacetamide
DMEU 1,3-dimethyl-2-imidazolidinone
DMF Dimethylformamide
DMSO Dimethyl sulfoxide
DMT Derjaguin-Muller-Toporov
DNA Deoxyribonucleic Acid
DOC Sodium deoxycholate
EDX Energy dispersive X-ray analysis
FTIR Fourier transform infrared spectroscopy
GBL -butyrolactone
GO Graphene oxide
xviii
HLB Hydrophilic-lipophilic balance
HOPG Highly ordered pyrolytic graphite
HRTEM High-resolution transmission electron microscopy
HTAB Hexadecyltrimethylammonium bromide
LbL Layer-by-layer
LDS Lithium dodecyl-sulfate
MWCO Molecular weight cut-off
NMP N-methylpyrrolidone
NMR Nuclear magnetic resonance
ODCB Ortho-dichlorobenzene
PAA Polyacrylic acid
PAAM Polyacrylamide
PANI Polyaniline
PBA Pyrenebutyric acid
PCA 1-Pyrenecarboxylic acid
PEI Polyethyleneimine
PEO Polyethylene oxide
PMAA Polymethyacrylic acid
PNIPAAm Poly(N-isopropylacrylamide)
PPO Polypropylene oxide
PSD Particle size distribution
PSS Poly(sodium 4-styrenesulfonate)
PVA Polyvinyl alcohol
PVC Polyvinyl chloride
PVP Polyvinylpyrrolidone
xix
PVPON Polyvinylpyrrolidone
QCM Quartz crystal microbalance
QNM Quantitative nanomechanical mapping
rGO Reduced graphene oxide
SAED Selected area electron diffraction
SC Sodium cholate
SDBS Sodium dodecylbenzene-sulfonate
SDC Sodium deoxycholate
SDS Sodiumdodecylsulfate
SEM Scanning electron microscopy
TDOC Sodium taurodeoxycholate
TEM Transmission electron microscopy
TGA Thermogravimetric analysis
TTAB Tetradecyltrimethylammonium bromide
UV-Vis Ultra violet – visible
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
XRR X-ray reflectivity
xx
1
CHAPTER 1
Chapter 1
Introduction
Throughout human history, the development of innovative materials has played a
pivotal role in societal and technological progress. Indeed, whole periods of history
have been defined by such materials. From the Stone Age, to the Bronze Age through
to the Iron Age and beyond, breakthroughs in materials have not only revolutionised
existing technologies, but have also transformed entire disciplines and industries.
Today, the research and development of materials is revealing new prospects for
innovation in areas as diverse as energy, electronics, healthcare, environment and
transport. These innovations not only deliver benefits to society, but often present
attractive commercial opportunities, further fuelling research activities. Thus,
considerable effort and investment is currently being directed towards the study of new
materials with a focus on addressing the great global challenges of the age.
One material that has received substantial interest from both the scientific community
and industry in this regard is Graphene. Graphene also continues to gather attention in
the broader community since it was announced as the topic of the 2010 Nobel Prize in
Physics. Graphene is a macromolecule consisting of a single atomic plane of carbon
atoms. This unique structure results in a combination of exceptional electronic, thermal
and crystallographic properties never before seen in any other material. Hence,
graphene has the potential to be used in a wide range of practical applications including
semiconductor devices, high speed transistors, flexible electronics, solar cells, batteries,
gas sensors, conductive inks and composite materials.
2
Nevertheless, mass producing graphene in a form which can be easily integrated and
used in applications has proved one of the most challenging obstacles towards the
widespread, practical implementation of graphene. Large-scale graphene production
methods commonly result in carbon monolayers either along the surface of medium or
within a medium, for example particles suspended in a liquid dispersion or supported by
solid substrate. In order to incorporate graphene in an application, it is often critical to
transfer the graphene layer from the medium to a target interface. Alternatively,
graphene can also be utilised by attracting chemical species towards the surface of the
graphene sheets. Both processes involve a net attractive force between the surface of
the carbon monolayer and the target surface or molecule. In a liquid medium, the
process through which attractive intermolecular forces cause the net accumulation of
particles or molecules at a surface is known as adsorption. Adsorption is affected by the
chemical nature of the interface or molecule, together with the presence and effect of
functional groups on the graphene surface, which appear as a by-product of the specific
graphene production method employed. This thesis investigates how intermolecular
interactions along the surface of graphene particles may be used to facilitate adsorption
phenomena of practical importance. This will be achieved using graphene particles
generated in the liquid phase through an existing production method suited to large-
scale implementation.
1.1 Potential Large - Scale Graphene Production Methods
The market for graphene applications now and in the short term, is currently limited by
the development of appropriate large-scale graphene production methods. These
methods are critical in fulfilling future demand, with conservative forecasts indicating
the global market for graphene materials will reach 3800 t/yr in 2026, totalling an
3
estimated worth of $220m.1 At present, there are three main methods with the potential
to produce graphene in commercial quantities.2 These include chemical vapour
deposition (CVD), growth on silicon carbide and liquid phase exfoliation of graphite.
The first two methods are bottom-up approaches, where carbon-based precursors are
used to construct individual graphene sheets atom-by-atom. For instance, the creation
of graphene layers through CVD involves the deposition of gaseous hydrocarbons onto
substrates such as copper and nickel at high temperatures3, while the growth of
graphene on silicon carbide involves thermal decomposition of the carbide substrate
under ultra-high vacuum.4 The third method, liquid phase exfoliation, is an example of
a top-down approach. Top-down approaches use the natural structure of graphite,
which is composed of highly stratified graphene monolayers, in order to obtain
individual graphene sheets. In particular, liquid phase exfoliation techniques generally
use ultrasonication to facilitate the delamination of graphene layers from graphite in
aqueous and non-aqueous environments.5 Popular graphene-based derivatives such as
graphene oxide (GO) and reduced graphene oxide (rGO) are often used synonymously
with graphene and are easily produced using scalable top-down approaches in the liquid
phase. However, despite a common carbon monolayer backbone, it is misleading to
refer to GO and rGO as graphene. These materials demonstrate distinct mechanical,
electrical and material properties compared to graphene, which are caused by the
presence of defects and oxygenated functional groups throughout the carbon lattice. In
contrast, “pristine graphene” is defined by graphene nanoparticles 1- 10 layers thick
with minimal defects and functionalisation.6 Thus, liquid phase exfoliation, together
with CVD and the graphene growth on silicon carbide have emerged as well established
methods of producing pristine graphene in small-scale laboratory settings and have
shown early promises of scalability for large-scale commercial processes.
4
However, widespread implementation of graphene is not only driven by the ability to
produce graphene on a mass-scale, but a combination of additional factors. In
particular, the successful introduction of graphene into products and applications
requires simple, cost effective methods of producing graphene. Furthermore, the ability
to integrate graphene on a mass scale also depends on identifying the appropriate
production method for the intended application. For instance, both CVD and graphene
growth on silicon carbide are well suited to the production of large, individual, high
quality graphene sheets, which can be precision patterned and transferred for subsequent
use in high-performance electronic and photonic devices (Figure 1.1). In contrast,
liquid phase exfoliation excels at generating bulk quantities of irregularly sized, single
or few-layer graphene sheets which remain dispersed in the liquid phase and are ideal
for use in applications such as functional coatings and composites. Thus, it also is
necessary to consider both the method of graphene production and quality of the
resultant graphene in terms of size, number of layers and atomic defects for each
application. Finally, in order to integrate graphene into mass-scale practical
applications, it is critical that production methods are compatible with the handling and
transfer of graphene for specific applications. The systems studied in this thesis reflect
potential applications which require bulk-grade graphene particles dispersed in the
liquid phase.
5
Figure 1.1: Current methods of producing graphene with the potential for mass production.
Graphene varies widely in terms of size, quality and price depending on the production method
employed and therefore certain production methods are more suited to producing graphene for
specific applications (Adapted by permission from Macmillan Publishers Ltd: Nature,
Novoselov, K. S.; Falko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K., A
roadmap for graphene. Nature 2012, 490, (7419), 192-200.), Copyright 2012).
One ideal method of preparing bulk-grade graphene particles is through the surfactant
assisted ultrasonic exfoliation of graphite. This technique is a liquid phase exfoliation
method which employs surfactants in association with ultrasonic waves to effect
separation of graphene particles from graphite. The surfactant acts to stabilise the
carbon monolayers in aqueous conditions by adsorbing along the particle surface,
causing steric interactions which prevent reaggregation of the graphene layers. As a
result, the process is capable of producing high-concentration suspensions of pristine
graphene particles in water. Unlike other graphene production methods, surfactant
assisted ultrasonic exfoliation has the benefit of being both facile and cost-effective,
requiring only inexpensive starting materials and non-specialised equipment in order to
produce graphene particles. The process is also non-hazardous and environmentally
6
friendly compared to liquid phase exfoliation processes performed in the presence of
non-aqueous solvents. Furthermore, the aqueous phase provides a versatile medium
with which to store, manipulate and transfer graphene particles.
1.2 Positioning and Placement of Surfactant Exfoliated
Graphene through Adsorption
The presence of a liquid medium provides major advantages in the transfer and
integration of surfactant exfoliated graphene particles into practical systems, by
permitting adsorption of the particles at interfaces. Adsorption is an important
interfacial process which forms the basis of a myriad of every-day products and
applications. It involves the net accumulation of particles or molecules from a liquid or
vapour phase onto an interface through attractive intermolecular interactions. An
interface is generally defined as the region between two chemically distinct and
identifiable phases of matter. Thus, adsorption provides an efficient method of
transferring particles to an interface in the absence of competing intermolecular
interactions, such as those applied by a transfer substrate. It can also be employed using
a range of industrially relevant, mass manufacturing techniques such as immersion dip
coating, spin coating and spray coating on a variety of interfaces. However, the
capacity for adsorption relies heavily on the intermolecular interactions acting between
the particle surface and the interface, which are determined by the nature of the
interface and particle surface chemistry. This project focuses on using the
intermolecular interactions inherent to surfactant exfoliated graphene to investigate the
potential of these particles in a series of applications governed by adsorption
mechanisms.
7
1.3 Utilizing the Behaviour of Graphene at Interfaces
In this thesis, the adsorption phenomena of surfactant exfoliated graphene will be
investigated through four different case studies. Each study was chosen in order to
illustrate the fundamental adsorption behaviour of surfactant exfoliated graphene with
respect to a different interface and mechanism of adhesion. The studies were also
carefully selected to reflect potential applications for surfactant exfoliated graphene in
order to explore whether the particles could address practical requirements specific to
each application. These case studies include:
Electrostatically Assembled Multilayer Thin Films
The ability to create thin films and coatings that incorporate graphene is highly
desirable, as it provides a potential route for producing amongst other applications,
novel conducting films, or hybrid composites with enhanced mechanical properties. In
this study, multilayer thin films containing surfactant exfoliated graphene were
assembled using layer-by-layer deposition, which typically involves the alternating,
sequential adsorption of oppositely charged species onto a substrate. Here, the cationic
polyelectrolyte, polyethyleneimine (PEI) and anionic graphene particles served as
charged species. The study serves as an example of adsorption of surfactant exfoliated
graphene at a solid-liquid interface using electrostatic interactions.
Multilayer Thin Films Assembled using Hydrogen Bonding
Forming multilayer thin films with hydrogen bonding interactions rather than
electrostatic forces provides another interesting opportunity to investigate the adsorption
of surfactant exfoliated graphene at a solid-liquid interface. In this study, multilayer
films containing surfactant exfoliated graphene were again assembled layer-by-layer,
8
this time through the alternating, sequential adsorption of complementary chemical
species. Here, the polyelectrolyte polyacrylic acid (PAA) and graphene particles
exfoliated with the surfactant Pluronic® F108 served as the two complementary
components, with the polyelectrolyte and the polyethylene oxide portion of the
surfactant facilitating hydrogen bonding interactions. Although hydrogen-bonded
multilayer thin films are generally more difficult to construct than their electrostatic
counterparts, these films can often provide extended functionality by responding to
environmental conditions such as pH, temperature and humidity. Consequently,
hydrogen-bonded multilayer films are emerging as possible candidates in a range of
functional thin film applications including humidity sensing, fuel cell membranes and
the controlled load and release of active compounds.
Graphene as an Aid to Foam Stabilisation
Highly stable liquid-air foams form the basis of a range of industrial processes and also
serve as an indicator of perceived quality and efficacy in many consumer products. In
this study, surfactant exfoliated graphene particles were used to stabilise air-in-water
foams against collapse. Foams generally exist as a dispersion of air bubbles surrounded
by a continuous liquid phase and require adsorption of surface active materials at the
air–liquid interface in order to stabilise the bubble surface. The ability for surface
active materials such as particles to stabilise the air-liquid interface through adsorption
is highly dependent on the physical properties of the particles and the chemical nature of
the particle surface. Surfactant exfoliated graphene particles pose a unique research
opportunity in this regard given the unique planar structure of the particles. Thus, the
study was selected to illustrate the adsorption of surfactant exfoliated graphene at an air-
liquid interface.
9
Graphene as an Adsorbent for Organic Molecules in Water
Adsorption of water-soluble contaminants onto carbon-based materials is a key
processing technology used in many municipal and industrial wastewater treatment
operations, and water purification systems. In this study, surfactant exfoliated graphene
particles were used to remove ionic organic dyes from aqueous solutions through
adsorption. As adsorption processes are driven by attractive surface interactions, the
ability for carbonaceous materials to adsorb small, organic molecules such as dyes from
aqueous environments is often highly dependent on surface parameters such as surface
area, surface chemistry and surface charge. Surfactant exfoliated graphene particles
offer significant advantages compared to other carbon-based materials in this respect,
providing high surface areas and the ability to tailor particle surface chemistry and
charge. Here, the surface chemistry and charge of the graphene particles was altered
through the use of cationic, anionic and non-ionic surfactants during the graphene
exfoliation process. Thus, the study provides an example of molecular adsorption at the
solid-liquid interface formed by the surfactant exfoliated graphene surface and
surrounding liquid.
1.4 Thesis Outline
This thesis is divided into nine chapters. Chapter 2 gives a brief introduction to
adsorption phenomena and the thermodynamic properties associated with interfaces.
Chapter 3 provides general background to graphene, in particular large-scale graphene
production methods. In Chapter 4, a discussion of the surfactant assisted exfoliation
method used to generate the exfoliated graphene in this study is provided. The chapter
also gives an overview of techniques used to prepare samples through the adsorption of
surfactant exfoliated graphene onto substrates. The techniques used to characterise the
10
samples are also discussed in this chapter. In Chapter 5 and Chapter 6, the production
of graphene-containing multilayers formed through electrostatic and hydrogen bonding
is discussed. Chapter 7 then details experiments using of surfactant exfoliated graphene
particles as an aid to foam stabilisation. Later, Chapter 8 describes the adsorption of
organic dyes from solution using graphene suspensions. Chapter 9 concludes the thesis
with a summary of important findings and a discussion regarding further work in the
area.
11
CHAPTER 2
Chapter 2
Adsorption at Interfaces
2.1 Introduction
An understanding of interfaces and adsorption processes are two complementary
concepts central to this thesis. Interfaces are briefly defined as the boundary separating
two distinct phases of matter. Adsorption meanwhile, is a surface phenomenon that
plays a critical role in a broad spectrum of physical, chemical and biological processes.
It is often a dynamic process which involves the net accumulation of molecules at an
interface from a liquid or vapour phase. Thus, the concentration of adsorbed species at
an interface, known as the surface excess concentration, is higher than that of the
adjacent bulk phases. The key driver for adsorption is the minimisation of the surface
tension or surface energy at the interface. Many interfaces inherently possess high
surface energies, caused by unbalanced intermolecular forces at the boundary of
adjacent phases. Thus, the types of intermolecular forces acting at an interface dictate
the different adsorption behaviours observed.
This chapter provides a short introduction to adsorption phenomena and the
thermodynamic properties associated with interfaces. It begins with a description of the
different types of intermolecular interactions encountered in later chapters, followed by
a discussion of the thermodynamic principles related to interfaces. A general,
thermodynamic treatment of adsorption processes is then given. The chapter concludes
with a description of the specific types of adsorption processes relevant to this thesis.
12
2.2 Intermolecular Interactions
Intermolecular interactions are the fundamental attractive and repulsive forces acting
between neighbouring molecules. In this thesis, a variety of intermolecular interactions
are employed in order to demonstrate the adsorption behaviour of surfactant exfoliated
graphene particles. These include electrostatic interactions, Van der Waals forces and
hydrogen bonding interactions. Electrostatic interactions derive from the Coulombic
force between charged chemical species whilst the Van der Waals force and hydrogen
bonding interactions originate from quantum mechanical effects. All three types of
interactions can be used to facilitate coupling with adjacent surfaces and molecules,
with their range of influence being determined to a large extent by the nature of
chemical species involved in the interaction, in addition to their molecular mobility.
2.2.1 Electrostatic Interactions
Electrostatic intermolecular interactions appear exclusively as a consequence of
Coloumbic attraction and repulsion. These forces arise from interactions between
electric fields, which emanate from a range of species including charges, ions, dipoles
and charged surfaces separated by a distance, 𝑟. Often, electrostatic forces manifest as
strong, long range intermolecular interactions, decaying at a rate of 1
𝑟 for isolated
charge-charge interactions or 1
𝑟4 for interactions between charges and induced molecular
dipoles which experience free rotation, such as those in bulk solutions. Examples of
electrostatic intermolecular interactions include those between the cationic
polyelectrolyte, polyethyleneimine and negatively charged graphene particles discussed
in Chapter 5.
13
2.2.2 Van der Waals Interactions
Van der Waals forces are weak, ubiquitous intermolecular interactions which arise as a
consequence of temporal fluctuations in the electronic polarisation of molecules. They
are comprised of three different types of polar interactions which include London
dispersion forces (instantaneous dipole-instantaneous dipole), Keesom interactions
(permanent dipole-permanent dipole) and Debye interactions (permanent dipole-
induced dipole).
London dispersion forces are a type of weak, attractive, intermolecular force that exists
between all molecules, regardless of chemical composition. They arise from the
instantaneous dipole moment caused by oscillating electron charges, and are able to
interact with the electronic polarisability of other molecules by propagating an
oscillating electric field. Affected molecules redistribute their electron cloud in
response to this electric field, forming an instantaneous dipole that propagates a
corresponding in-phase electric field. The combination of these fields results in an
overall attractive intermolecular interaction. Dispersion forces are short range
interactions which remain effective for distances of tens of nanometres, decaying at a
rate of 1
𝑟6 for isolated molecules and for bulk materials. Dispersion forces appear
throughout the adsorption experiments in this thesis and also occur in graphite. More
specifically, dispersion forces are the primary attractive forces that hold the graphene
sheets in the lamellar structure and arise chiefly due to the system of delocalised π-
electrons along the carbon monolayer. They are also responsible for π-π interactions,
which occur between neighbouring π-electron rich systems such as aromatic molecules.
Dipole-dipole (Keesom) and dipole-induced dipole (Debye) interactions are two other
components of the van der Waals force, which exist as a result of at least one polar
14
group being present. The Keesom force occurs when the permanent electric fields
surrounding at two molecular dipoles interact, causing either repulsion or attraction
depending on molecular orientation. In contrast, Debye interactions are caused by
electric fields emanating from permanent dipoles, which perturb the distribution of
electron clouds surrounding nearby molecules, inducing polarization. Both Keesom and
Debye interactions are stronger than dispersion forces, and decay at a rate of 1
𝑟6
assuming the molecules are in an environment, such as a bulk liquid phase in which
they are free to rotate.
2.2.3 Hydrogen Bonding Interactions
Hydrogen bonding is a special category of dipole-dipole (Keesom) interactions that
involves molecules containing hydrogen being directly bound to highly electronegative
atom such as fluorine, oxygen or nitrogen. The proximity of the hydrogen atom to the
electronegative species in these molecules causes an overall shift in the electron cloud
away from the hydrogen atom. As a consequence, the bond between the two atoms
becomes highly polarised, with the hydrogen atom acquiring a partial positive charge
while the adjacent atom acquires a corresponding negative charge. The dipole is then
able to interact with dipoles present in other molecules, leading to attractive hydrogen
bonding interactions. The resultant interactions are generally strong and remain
effective over short ranges, decaying at a rate of 1
𝑟2. The interactions are also highly
orientation dependent, and therefore exist for only short periods in solutions due to
rapid molecular motion. A variety of systems involving hydrogen bonding interactions
appear in this thesis, including those between the polyelectrolyte, PAA, and the
polyethylene oxide groups present on the surfactant Pluronic F108 discussed in Chapter
6.
15
Hydrogen bonding is also an important interaction that influences the intermolecular
forces acting in aqueous systems. The hydrogen bonds formed between neighbouring
water molecules in a pure liquid are highly directional and give rise to a structured, yet
dynamic molecular network. In aqueous solutions however, the presence of solutes
often affects the local structuring of water molecules. For instance, polar solutes are
able to associate with water molecules through favourable dipole-dipole interactions,
promoting solubility of these species in water. In contrast, solutes containing non-polar
groups tend to either aggregate in the bulk or accumulate at interfaces in order to
minimise interactions between polar and non-polar species whilst maximising the
energetically favourable hydrogen bonding interactions between water molecules. This
behaviour is known as the hydrophobic effect and is responsible for a variety of
colloidal and surface phenomena examined in this thesis, including the surface affinity
of surfactants and micelle formation.
2.3 Surface Interactions
In bulk materials, the effect of intermolecular interactions between individual molecules
combines to exert a total cooperative interaction between surfaces, or between particles
and surfaces. These attractive interactions drive adsorption in a number of studies
presented in this thesis, and include those responsible for the adsorption of surfactant
exfoliated graphene particles from solution onto an interface. Van der Waals
interactions and electrostatic based interactions are the main interactions acting between
surfaces and are considered as individual interactions in this section. Derjaguin-
Landau-Verwey-Overbeek (DLVO) theory7, 8
, which describes the combined effect of
such interactions, is not considered here as it often assumes the effect of electrostatic
repulsion between two similarly charged surfaces. DLVO theory is therefore more
16
relevant to colloidal stability than adsorption processes. Indeed, DLVO, along with
steric stabilisation, are critical factors in enabling the creation of stable colloid graphene
suspensions9.
2.3.1 Van der Waals-Based Interactions
Lifshitz theory provides an approximate form of the interaction energy between pairs of
macroscopic bodies which interact solely through van der Waals forces. The theory
ignores molecular structure effects, instead treating bulk phases as continuous media
and using bulk properties in order to avoid the complexity associated with pairwise
additivity of molecular interactions. Given the typical lateral dimensions of graphene,
the adsorption of graphene particles to a surface by van der Waals forces may be
modelled using a system of two interacting infinite, flat surfaces across a medium.
Using Lifshitz theory, the van der Waals interaction energy between two infinite, flat
surfaces, 1 and 3, across a medium, 2, is given by the equation:
𝑉123 = −𝐴123
12𝜋𝐷2
2.1
Where:
𝑉123 = van der Waals interaction energy
𝐴123 = Hamaker constant for the system.
𝐷 = Separation distance between surfaces 1 and 3
𝐴123 is characteristic of the system and differs for each combination of medium and
bulk material employed. It is typically determined from bulk dielectric properties as a
function of frequency. The corresponding van der Waals interaction force for the
system is the derivative of the interaction energy and is given by:
17
𝐹123 = −𝐴123
6𝜋𝐷2
2.2
Where:
𝐹123 = van der Waals interaction force
This form of the van der Waals interaction energy and force assumes the interaction
between the bodies is non-retarded, that is, the speed of light is assumed constant
throughout all media and is valid for all separations. Given the two-dimensional nature
of the graphene particles, both the van der Waals interaction energy and force involved
in the adsorption of graphene particles at a substrate surface are likely to be smaller than
those given by Equations 2.1 and 2.2.
2.3.2 Electrostatically Charged Surfaces and The Electric Double Layer
When immersed in liquids with high dielectric permittivity, most surfaces will acquire a
net charge as a result of the ionisation of surface groups such as hydroxyl, carboxyl and
silanol functional groups. Ionisation is driven by the dissociation of these groups or the
adsorption of ionic species from solution onto the surface. Consequently, the
magnitude of the surface charge can be modified by altering the pH or ionic strength of
the solution. The resulting surface charge gives rise to an electric field, which changes
the distribution of ions in the surrounding medium. More specifically, it attracts counter
ions towards the surface in order to neutralise the surface charge, whilst repelling co-
ions away into solution. The Stern model10
can be used to describe the resulting
distribution of ions and is visualised as a compact layer of counter ions rigidly attached
to the surface, coupled with a diffuse outer layer of ions (Figure 2.1). The inner layer of
ions is strongly adsorbed to the surface through electrostatic interactions and is known
18
as the Stern or Helmholtz layer. In contrast, the outer layer is only loosely associated
with the charged surface and is subject to a combination of electrostatic forces and
thermal motion effects. Together, these two regions constitute the electric double layer.
Figure 2.1: Schematic of the electric double layer created when a surface is immersed in
solution. The potential decreases linearly with distance from the surface up until the slipping
plane. Beyond this plane, the potential decays exponentially until the bulk solution is reached.
As a consequence of the surface charge, an electrokinetic potential exists throughout the
electric double layer. In the Stern layer, the potential decreases linearly from the
surface with increasing distance until the slip plane, is reached. The potential at the
slipping plane is known as the zeta potential. Beyond the slipping plane, the potential
decreases exponentially, reaching zero at the imagined boundary between the double
layer and the bulk solution.
19
The potential generated by a charged surface provides the driving force for the
adsorption of ionic species along a charged surface. This phenomenon applies to a
range of examples in this thesis, including the adsorption of charged species in
multilayer thin films (Chapter 5 and Chapter 6), and imparting stability into graphene
dispersions through electrostatic repulsion (Chapter 7). The surface charge and
resultant electric double layer also form the theoretical basis for zeta potential
measurements, which are used throughout this thesis to characterise the type of stability
mechanism present in surfactant exfoliated graphene dispersions.
2.4 Thermodynamic Properties of Interfaces
The thermodynamic properties of a system are typically affected by the presence of an
interface, which exhibits fundamental differences in molecular structure compared to
bulk matter phases. An ideal interface can be described as the boundary separating two
distinct, adjoining bulk phases. In bulk phases, cohesive forces between molecules
exist, whereby molecules attract surrounding like molecules through a variety of
intermolecular interactions including van der Waals forces and hydrogen bonding.
However, at interfaces, molecules are only partially surrounded by like molecules and
therefore interact with a smaller number of adjacent like molecules than those in the
bulk. These arrangements are energetically unfavourable compared to the bulk matter
case. There are three common types of interfaces that exist between different types of
phases: solid-liquid, solid-vapour and liquid-vapour. For a liquid-vapour interface, the
unbalanced intermolecular forces at the interface give rise to a net force which acts to
minimise the interfacial area. As a result, work is required in order to relocate a
molecule from the bulk to the interface and increase the interfacial area. The surface
20
tension, 𝛾, is a fundamental quantity which is used to define the work required to
increase the area of a liquid-vapour interface (Equation 2.3):
𝛾 =𝑑𝑊
𝑑𝐴
2.3
Where:
𝛾 = Surface tension (N m-1
)
𝑊 = Work (N m)
𝐴 = Interfacial area (m2)
Similarly, the surface energy of a solid surface is defined in terms of the work per unit
area required to create two separate, flat surfaces from a bulk solid. The work of
cohesion is equal to the cohesive energy per unit area, and is related to the surface
energy of the newly created surface by Equation 2.4:
𝑊 = 2𝛾𝑠 2.4
Where:
𝛾𝑠 = Surface energy (J m-2
)
𝑊 = Work of cohesion per unit area (N m-1
)
Thus, the surface tension parameter for a liquid is equivalent to the surface energy
parameter for a solid.
Although ideal interfaces are useful in defining certain thermodynamic properties such
as the surface tension, further examination of thermodynamic properties is possible
when the real molecular structure of interfaces is considered. Real interfaces do not
21
exist as infinitesimally thin borders between two bulk phases, but instead appear as a
region with a finite thickness of perhaps a few molecules (Figure 2.2). When
proceeding across this region in a direction normal to the interface, the concentration of
one phase decreases whilst the concentration of the adjacent phase increases until it
reaches the bulk concentration. Consequently, real interfaces form an important part of
multi-phase systems and cannot be considered in isolation.
Figure 2.2: Schematic showing the differences between ideal and real interfaces. Ideal
interfaces represent the boundary between two distinct phases. Conversely, real interfaces
appear as a region with a finite thickness of a few molecules.
Thus, a system containing an interface can divided into three areas for the purposes of
studying the thermodynamic properties of a system; two bulk phases 𝛼 and 𝛽, with
volumes 𝑉𝛼 and 𝑉𝛽
, and the interface, 𝜎. Here, the interface is treated using the Gibbs
convention. With the Gibbs convention, the interface is modelled as an infinitesimally
thin geometric surface, known as the Gibbs dividing plane. The Gibbs dividing plane is
used to define the ideal volume of the bulk phases and to calculate surface excess
properties for the system. Excess thermodynamic properties quantify the difference
between the value measured for a real system and an equivalent reference system. In
22
this instance, the surface excess of component 𝑖 is defined as the additional amount of
the 𝑖th material in the system due to the presence of the interfacial region. This value is
calculated with respect to an equivalent reference system with an ideal interface, in
which the bulk concentrations of the two phases remain uniform throughout 𝑉𝛼 and 𝑉𝛽
(Equation 2.5)
𝑁𝑖𝜎 = 𝑁𝑖 − 𝑐𝑖
𝛼𝑉𝛼 − 𝑐𝑖𝛽
𝑉𝛽
2.5
Where:
𝑁𝑖𝜎 = The excess of species 𝑖 in the phase 𝜎 which would have remained in
𝜎 if the bulk composition of 𝛼 and 𝛽 phases had extended to the Gibbs
dividing plane (molecules)
𝑁𝑖 = Total quantity of species 𝑖 in system (molecules)
𝑐𝑖𝛼, 𝑐𝑖
𝛽 = Concentration of species 𝑖 in phase 𝛼,𝛽 (molecules/m
2)
𝑉𝛼 , 𝑉𝛽
= Volume of species 𝑖 in phase 𝛼,𝛽 (m2)
The surface excess concentration can then be defined as the concentration of species 𝑖 at
an interface with the area, 𝐴 through Equation 2.6:
Γ𝑖 =𝑁𝑖
𝜎
𝐴
2.6
Where:
Γ𝑖 = Surface excess concentration of component 𝑖 (molecules m-2
)
Thus, the surface excess and surface excess concentration relies on the appropriate
placement of Gibbs dividing plane. The Gibbs convention states that the correct
23
location of the dividing plane is such that the surface excess concentration of the bulk
phases, Γ𝛼 = Γ𝛽 = 0. An accurate definition of the interface is required in order to
account for adsorption phenomena.
2.5 Adsorption Processes and the Gibbs Adsorption
Isotherm
Adsorption is an important interfacial process with great practical relevance. It is
defined as the net accumulation of chemical species at an interface and occurs when the
surface excess concentration of a component in the interfacial region is greater than that
in the bulk phase. The key driver for adsorption is the minimisation of the surface
tension or surface energy at the interface. During adsorption, the accumulation of
chemical species at the interface causes the number of interfacial molecules subjected to
energetically unfavourable interactions with the adjacent bulk phase to decrease. This
behaviour is described quantitatively by the Gibbs adsorption isotherm. The Gibbs
adsorption isotherm relates the surface tension of an interface and the surface excess
concentration of an uncharged, dilute species at the interface (Equation 2.7):
Γ = −1
𝑅𝑇
𝑑𝛾
𝑑(ln 𝑎)
2.7
Where:
Γ = Surface excess concentration of component 𝑖 (molecules m-2
)
𝑅 = Universal gas constant, 8.315 J K-1
mol-1
𝑇 = Temperature of the system (K)
𝛾 = Interfacial surface tension (N m-1
)
𝑎 = Thermodynamic activity of component 𝑖
24
The Gibbs adsorption isotherm has practical implications for a wide variety of systems
where adsorption processes occur. One type of system encountered throughout this
thesis involving an adsorption process is the adsorption of solutes at an interface from a
solvent. When a solute favours adsorption at an interface, the addition of solute to the
solvent will cause an increase in the surface excess concentration of the solute at the
interface (Γ > 0) according to the Gibbs adsorption isotherm. It will also cause a
reduction in the surface tension of the solvent. In contrast, when a solute avoids the
interface, the surface tension will increase upon addition of the solute, as the solute is
depleted from the surface (Γ < 0).
2.6 Species-Specific Adsorption Processes
2.6.1 Surfactant Adsorption
The adsorption of surfactants on a surface is a specific type of adsorption process
encountered throughout this thesis. One of the key characteristics of surfactants that
facilitate their adsorption at interfaces is their amphiphilic character. As water is used
in this thesis as a solvent, the surfactant molecules employed here exhibit hydrophilic
groups and a hydrophobic group. These types of surfactants will preferentially adsorb
along a surface with the hydrophilic parts of the molecule maximising contact with the
more hydrophilic phase, while the hydrophobic portion of the molecule will favour
contact with the more hydrophobic phase (Figure 2.3). This arrangement will act to
minimise the energy associated with unfavourable molecular interactions at the
interface, described by Equation 2.7. Consequently, the adsorption of surfactants at a
solid interface results in a lowering of the surface energy. Similarly, the adsorption of
surfactants along the liquid-vapour interface acts to lower the surface tension, which has
an effect on the curvature of the interface.
25
Figure 2.3: Schematic diagrams of (a) the amphiphilic structures of surfactants, (b) adsorption
of surfactants on hydrophobic surfaces, (c) hydrophilic surfaces and (d) at the air-water
interface. The hydrophilicity of the solid surfaces in (b) and (c) are defined relative to the
surrounding liquid.
The effect of surfactant adsorption along the liquid-vapour interface is highly dependent
on the surfactant concentration. At low concentrations, surfactants are largely dispersed
within the liquid phase, with a small proportion of surfactants adsorbed at the interface.
As the surfactant concentration increases, the density of surfactant molecules at the
liquid-vapour increases thereby reducing the surface tension. This reduction in surface
tension allows the formation of a semi flexible, elastic film at the interface. Above a
critical surfactant concentration however, the interface becomes saturated with
surfactant molecules, causing the surface tension to remain invariant. This
concentration is known as the critical micelle concentration (CMC). At surfactant
concentrations above the CMC, ordered surfactant aggregates known as micelles form
(a)
(b)
(c)
(d)
26
in the bulk liquid phase. At equilibrium, surfactant adsorption and desorption at the
liquid-vapour interface occurs at relatively short timescales, with the energy of
attachment usually only several kT per surfactant molecule.11
Thus, adsorption of
surfactants at the liquid-vapour interface can generally be characterised as a dynamic
equilibrium process.
The practical implications of such surfactant adsorption process are apparent in a
number of interfacial phenomena, such as the Gibbs-Marangoni effect. The Gibbs-
Marangoni effect applies to the drainage of thin films containing surfactant, and
involves surface tension gradients acting to stabilise the liquid-vapour interface. As
liquid is drained from the film, the surfactant concentration increases promoting
surfactant adsorption at the interface. Consequently, the surface excess concentration at
the interface increases, driving a local change in surface tension according to Equation
2.7. This in turn, provides a restorative force that opposes drainage of liquid from the
film. The Gibbs-Marangoni effect is a specific adsorption phenomenon encountered in
Chapter 7.
2.6.2 Polymer Adsorption
Another type of process loosely related to surfactant adsorption is the adsorption of
polymers at an interface. Polymers are macromolecules with a chain-like structure that
are comprised of repeating molecular units known as monomers. Polymers consisting
of one type of monomer are known as homopolymers and typically exhibit uniform
chemical interactions along the length of the polymer. In contrast, polymers consisting
of more than one type of monomer are termed heteropolymers and demonstrate different
chemical interactions based on the types of monomers present in the molecule. These
polymers often possess complex chemical structures and can include block co-polymers
27
such as the Pluronic F108 surfactant used in the exfoliation of graphene throughout this
thesis.
Another group of polymers that are used extensively in Chapter 5 and Chapter 6 of this
thesis are polyelectrolytes. Polyelectrolytes are water-soluble polymers comprised of
monomers that possess functional groups capable of dissociating in aqueous solutions,
thereby forming charged polymers. Polyelectrolytes can be divided into two categories,
strong and weak, depending on the extent to which the monomers dissociate. Strong
polyelectrolytes are characterised by monomers which dissociate completely in aqueous
solutions. In contrast, weak polyelectrolytes experience only partial dissociation and
often consist of weakly acidic or basic groups. Consequently, the charge on weak
polyelectrolytes can be modified by altering solution factors such as pH and ionic
strength.12
An example of a weak polyelectrolyte is PAA, which is used in Chapter 6 to
from hydrogen-bonded multilayer thin films through adsorption at the solid-liquid
interface.
When polymers adsorb from solution at a solid-liquid interface, the molecules will
adopt the lowest energy chain extension and conformation possible. Thus, the
adsorption behaviour of polymers at a solid-liquid interface is affected by several
different factors which affect the energy associated with the molecules. One of these
factors is the quality of the solvent, which determines the solubility of the polymer in
solution. Good solvents, which facilitate energetically favourable interactions with
polymer molecules, will support the dispersion of polymers in solution whilst poor
solvents will promote the adsorption of polymers at the solid-liquid interface. Each
polymer molecule may have several binding sites that facilitate adsorption at the
surface. For heteropolymers, the location of these binding sites on the polymer often
corresponds to chemical species which exhibit more energetically favourable
28
interactions with the surface than the solvent. The presence of these binding sites
enabling the formation of looped polymer chains which can then extend into solution.
However, the conformation of the polymer molecules is highly dependent on the
polymer concentration and adsorption time. For instance, low polymer concentrations
and early stages of adsorption result in polymer chains adsorbing in an expanded
configuration along the surface. Higher polymer concentrations and adsorption during
the later stages of the process, results in the polymer layer becoming more densely
packed, with the polymer chains forming only intermittent contacts with the surface,
increasing the likelihood of forming chain loops. The solubility of these loops, and
therefore the conformation of the polymer chains, can be affected by conditions such as
salt type13
, pH14
, ionic strength14
and temperature15
. These factors act to alter the
interactions between polymer segments, as well as interactions between the polymer
chains and the solvent.
2.7 Conclusion
The intermolecular forces acting between molecules, along with the thermodynamic
properties characteristic to interfaces, both play a critical role in describing the origins
of adsorption phenomena. In this chapter, the main types of intermolecular interactions
used to demonstrate the adsorption behaviour of surfactant exfoliated graphene particles
were presented. These included electrostatic, van der Waals and hydrogen bonding
interactions. Electrostatic and van der Waals interactions were not only presented in
terms of individual, pair-wise molecular interactions, but also with respect to their
combined effect in surface-based interactions. This chapter also introduced the
concepts of surface tension and surface energy, which are used to quantify the energy
inherently associated with particular interfaces. The definition of a real interface was
29
then presented using the Gibbs convention. The Gibbs adsorption isotherm, which
relates the surface tension of an interface and the surface excess concentration of a
dilute adsorbed species, was also discussed in this chapter along with examples of
adsorption processes which involve specific chemical species. These concepts not only
form the basis of the surfactant exfoliated graphene production, but also the foundation
of discussion and experiments presented in later chapters.
30
CHAPTER 3
Chapter 3
Graphene: Structure, Properties and
Production Methods
3.1 Introduction
Despite the recent interest associated with graphene, the molecule itself is not a recent
innovation. In fact, graphene occurs naturally as distinct layers in the lamellar structure
of mineral graphite and has been the subject of academic studies for nearly 70 years.16
Nevertheless, single layer graphene was considered an unstable, theoretical structure
until 2004, when Geim and Novosolov documented the first successful isolation of
graphene using the mechanical exfoliation method.17
Since then, graphene has gained
tremendous interest throughout a variety of fields, both in research and industry. As a
result, the literature pertaining to graphene has grown immensely, with close to 12,000
peer-reviewed articles published in 2015 alone.18
This chapter draws upon relevant existing research to provide an introduction to the
surfactant exfoliated graphene particles used in subsequent chapters. The chapter
begins with a general discussion regarding the unique, two-dimensional structure of
graphene and its implications on reactivity and material properties. Next, graphene
production methods suited to large scale implementation are reviewed, with a specific
emphasis on liquid phase techniques. These include the exfoliation of graphene based
derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO), as well as
the exfoliation of pristine graphene in both non-aqueous and aqueous mediums. The
surfactant assisted ultrasonic exfoliation of graphite in aqueous mediums is then
31
discussed. In particular, the role of surfactant during the exfoliation process and the
properties of the graphene particles produced using this method are examined. Many of
the discussions contained in this chapter were first published in a peer reviewed article
(Reproduced from Ref. 65 with permission from The Royal Society of Chemistry).
3.2 Structure of Graphene
Graphene is defined as a single, infinite monolayer of sp2 bonded carbon atoms
arranged in a hexagonal lattice. The thickness of an individual graphene sheet is 0.35
nm, whilst the interatomic distance between adjacently bonded carbon atoms within the
lattice is 1.42 Å.19
Each of the carbon atoms within a pristine graphene lattice possess a
single s orbital and two in-plane p orbitals which hybridise to form three sp2 orbitals and
one p orbital, giving rise to the planar arrangement of graphene. These orbitals also
define the highly conjugated, aromatic structure of graphene through bonds which
exist between each carbon atom and three of its nearest neighbours in the lattice.
Additionally, each carbon atom also possesses a orbital which contributes to a system
of delocalised electrons both above and below the basal plane. Therefore, although
graphene is three dimensional, it is commonly considered a two dimensional particle
due to its ultra-high aspect ratio.
Despite the unique two dimensional structure of graphene, it also exhibits a strong
relationship with carbon materials of other dimensionalities, in particular, graphite
(Figure 3.1). Graphene sheets occur naturally in three-dimensional bulk graphite as
stacked layers held together by long-range van der Waals intermolecular forces. The
van der Waals interaction energy between adjacent graphene layers in graphite is 2
eV/nm2, while the interlayer spacing between these layers is 0.34 nm. As the number of
graphene layers in a sample increases from a single layer the mechanical, electrical and
32
thermal properties of the graphene deteriorate, so that at 10 layers of graphene, the
material exhibits an electronic structure that is indistinguishable from bulk graphite.20
However, as the screening length of pristine graphene is only 0.5 nm, equivalent to two
sheets of graphene, it is important to recognise that layered graphene sheets of as little
as five layers exhibit distinct surface and bulk properties.21
Consequently, it is
important that graphene samples are produced with as few carbon layers as possible in
order to take full advantage of the superior properties of graphene.
Figure 3.1: Graphene is a 2D building material for carbon materials of all other
dimensionalities. It can be wrapped up into 0D buckyballs, rolled into 1D nanotubes or stacked
into 3D graphite. (Reprinted by permission from Macmillan Publishers Ltd: Nature Materials,
Geim, A. K.; Novoselov, K. S., The rise of graphene. Nat. Mater. 2007, 6, (3), 183-191.,
Copyright 2007).
3.2.1 Properties of Graphene
The unique, two-dimensional structure of graphene gives rise to a material which
showcases a combination of impressive properties. Indeed, many of these properties are
33
the highest ever recorded for a material, with some even approaching their theoretical
limit. For instance, the excellent electronic properties exhibited by pristine monolayer
graphene originate from extensive delocalisation of -electrons along the graphene
basal plane. Consequently, defect-free monolayer graphene has been shown to possess
charge carrier mobilities of 2.5 × 105 cm
2 V
-1 at room temperature
22, along with current
capacities several orders of magnitude higher than copper.23
Similarly, the strength of
the C=C bonds in the graphene lattice is responsible for the superior mechanical
features displayed by graphene, with the single carbon layers having a Young’s
modulus of 1 TPa and tensile strength of 130 GPa.24
In addition to electronic and
mechanical properties, graphene also demonstrates a remarkable thermal conductivity of
5300 W m-1
K-1
, the highest of any known material.25
Due to the two dimensional
nature of graphene, the material is also predicted to possess a high specific surface area
of 2630 m2/g
26. Furthermore, several studies have shown the effective gas barrier
properties27
, high flexural strength28
and exceptional optical transparency29
characteristic to single layer graphene. Together, these features occur as a direct result
of the unique planar structure of pristine graphene.
3.2.2 Surface Charge and Intermolecular Forces Present on Graphene
The unique planar structure of graphene also dictates the surface forces demonstrated by
the graphene surface. The conjugated structure present along the graphene basal plane
gives rise to a system of delocalised -electrons which can allow dispersion forces to
occur. As result, the extended conjugation present in graphene has been shown to
facilitate intermolecular interactions with polycyclic aromatic compounds, which can
interact with the graphene surface through - stacking.30
Cationic species are also
34
able to interact with the delocalised -electron system on graphene through cation-
interactions.31
Graphene exfoliated from natural sources of graphite typically exhibits defects within
the lattice due to the formation of grain boundaries and imperfect crystal lattices. At
these boundaries and along the edges of graphene, the most common types of defect
functional groups that exist are oxygenated functional groups such as ether, carbonyl,
carboxylic acid and alcohol groups.32
As a result, hydrogen bonding interactions can
occur depending on the type of edge group and pH. A small negative charge is also
typically exhibited within the vicinity of these groups. Conversely, the presence of
defects within the lattice not only has a detrimental effect on the electronic and
mechanical properties of graphene, but also acts as a weak point in the lattice.
Consequently, defects are known to cause unzipping of the conjugated lattice
structure.30
This effect can be reduced through either synthetic, highly ordered graphitic
precursors where applicable, or carefully selected graphene production methods.
3.3 Graphene Production Methods
The relatively simple, two-dimensional structure of graphene is the primary reason that
graphene monolayers were not successfully isolated until a decade ago. Prior to 2004,
graphene was viewed as a theoretical, two dimensional macromolecule, with the
assumption that the stability of the carbon lattice was insufficient to maintain the planar
configuration of a graphene sheet.33
This understanding was transformed with the
release of Geim and Novoselov’s seminal work documenting the first successful
fabrication of graphene17
, for which they received the Nobel Prize in Physics in 2010.
The method continues to be used in preparing pristine graphene for small-scale
research, and is known as mechanical exfoliation.
35
Mechanical exfoliation is a technique that utilises the intermolecular forces acting on a
graphene sheet in order to remove graphene from bulk graphite. In particular, it
requires balancing the cohesive forces between carbon layers in graphite and the
adhesive forces used to delaminate graphitic layers. Novoselov et al. used common
adhesive tape in order to overcome the weak cohesive forces (300 nN/µm2) required to
exfoliate layers of graphene from graphite.17
Using a stick-and-peel process, the
researchers repeatedly removed graphitic multilayers from a piece of highly ordered
pyrolytic graphite (HOPG) attached to photoresist until a graphene monolayer sample
was obtained. The graphene flakes were then transferred to a Si / SiO2 substrate by
dissolving the photoresist with acetone and floating the graphene onto the substrate. A
silicon oxide layer of 300 nm provided suitable contrast to identify the graphene films
manually using polarised optical microscopy. In this manner, the mechanical
exfoliation of graphene from bulk graphite was achieved, providing a simple, effective
method of preparing defect-free graphene samples.
However, the mechanical exfoliation method used to produce graphene presents a
number of disadvantages. The mechanical exfoliation of graphene commonly produces
in graphitic films of non-uniform thickness and size. Given the irregularity of the
particles both in size and quantity, this method could also prove unfavourable when
attempting to locate the graphene films on the substrate, by contributing to low yields
and a labour intensive process overall. Furthermore, production of graphene using
mechanical exfoliation requires the sample to be floated using an organic solvent or
transferred to a substrate during the production process. The presence of a solid or
volatile transfer medium severely limits practical considerations such as storage,
handling and methods of application, and may also be incompatible with further
36
manufacturing and processing stages. Therefore, the mechanical exfoliation of
graphene is generally unsuitable for implementation in mass production processes.
3.3.1 Current Methods of Graphene Synthesis and Production
Despite the introduction of the mechanical exfoliation method over a decade ago, a
solution to preparing defect free graphene in commercial, large-scale quantities at low
cost still remains elusive. Consequently there is a great amount of interest in graphene
processing techniques within the scientific community, with a wide variety of methods
being presented in the literature. The majority of these methods can be divided into
approximately a dozen broad categories. Of these methods, three main techniques have
the potential to be suited to the fabrication and manipulation of graphene on a commercial
scale: chemical vapour deposition (CVD), growth on silicon carbide and liquid phase
exfoliation.
3.3.1.1 Chemical Vapour Deposition (CVD)
Chemical vapour deposition is a well-established, “bottom-up” technique used to
generate large, individual, high-quality graphene sheets. It involves the decomposition
and deposition of hydrocarbon precursors onto transition metal substrates in order to
synthesise graphene layers. In one of the early investigations involving this technique,
nanometre-sized graphene layers were deposited onto a Pt (1 1 1) substrate through the
decomposition of ethylene.34
Graphene has been deposited on a wide range of
transition metal substrates including Ni35
, Pd36, 37
, Ru38, 39
, Ir40
, and Cu2, 3, 41
by
decomposing hydrocarbons such as methane42-44
, benzene45
, carbon monoxide, ethanol41
and actetylene46
precursors. Although CVD typically produces graphene layers of
slightly lesser structural quality in terms of lattice structure that those produced using
the mechanical exfoliation method, it is capable of generating large areas of uniformly
37
structured, polycrystalline graphene.3 For example, large area graphene sheets up to 30
inches in size have been deposited using CVD onto thin copper substrates.47
These
layers can be precision patterned, and thus have great potential in applications requiring
large area, high quality, mass produced graphene components such as nano- and
microelectronics, transparent conductive layers, sensors and flexible opto-electronic
devices.48
However, a number of disadvantages remain with the technique, including
difficulties in controlling grain size, ripples in the graphene layers, as well as the
number of graphene layers deposited.3 The CVD graphene synthesis process is also
generally expensive due to high energy demands compared to other production
methods, and can require the removal of the substrate, typically through complex
transfer processes. Consequently, graphene produced using CVD is not appropriate for
use in all large-scale commercial applications.
3.3.1.2 Growth on Silicon Carbide
The growth of graphene on silicon carbide is another “bottom-up” method of producing
graphene with the potential to be used for large-scale fabrication and manipulation.
This process centres on the thermal decomposition of a silicon carbide substrate under
ultra-high vacuum, and involves the sublimation of silicon atoms from the substrate,
followed by reorganisation of the carbon-rich surface to produce graphene layers. The
size of the graphene sheets is limited only by the size of the supporting substrate.
Meanwhile, the thickness, charge carrier mobility and carrier density of the resulting
graphene sheets are largely influenced by the surface properties of the SiC substrate.49
Nevertheless, graphene grown on silicon carbide generally exhibits high quality lattice
structures, as evidenced by high charge carrier mobilities that are slightly lower than
that of mechanically exfoliated graphene.50
It has also been shown that the growth of
graphene layers on silicon carbide provides greater control over the number of graphene
38
layers compared to the CVD method.51
However, despite these advantages, significant
challenges remain with implementing the technique in large-scale commercial
processes. These include the high cost of the SiC substrates, as well as the extreme
temperatures necessary to produce the graphene layers. In particular, the decomposition
of silicon carbide typically requires temperatures in excess of 1000 °C, which are
generally not compatible with the use of silicon carbide as a semiconductor material in
high-power electronics. Thus, the ability to grow high quality, single layer graphene on
SiC substrates has great potential in niche electronic applications, yet is unsuitable for
industrial applications that require large-scale fabrication and manipulation of graphene.
3.3.1.3 Liquid Phase Exfoliation
Liquid phase exfoliation comprises a family of techniques with the ability to generate
large-scale quantities of mono- and few layer graphene particles in suspension.52-54
It
involves exposing bulk graphite powder to a liquid in which an increase in the total
surface area of graphite crystals is favoured. Ultrasonication is generally then applied to
encourage exfoliation of the graphitic sheets from the bulk graphite. Exfoliation of
graphitic layers can also be initiated by less common methods such as rapid
depressurisation in a supercritical fluid55
, electrochemical techniques56
, high-speed
laminar flows57
, as well as by spontaneous, self-exfoliation58
. Centrifugation is often
performed following exfoliation in order to separate the multilayer residues from the
mono-, bi and few-layer graphitic sheets. Thus, liquid phase exfoliation can result in
the production of stable suspensions enriched with pristine graphene nanosheets.
Graphene suspensions produced through liquid phase exfoliation possess a number of
important advantages and limitations. One of the most significant advantages of this
technique compared with other graphene production methods is its ability to reliably
39
manufacture monolayer graphene in scalable quantities, a feature essential for large-
scale industrial implementation. Indeed, bulk quantities of graphene generated using
liquid phase exfoliation techniques are currently available on the tonne scale
commercially.2 Graphene particles dispersed in the liquid phase also have the
advantage of being transferred to substrates conveniently, with liquid phase exfoliated
graphene being found to be compatible with a variety of industrial processes including
spin coating, spray coating59
, drop casting60
, and immersion dip coating61
. However,
liquid phase exfoliation often results in graphitic layers of varying lateral size and
thickness, producing layered materials with overall lower quality than those produced
using CVD and SiC supported growth. Consequently, this method is generally not
suited to the fabrication of high-performance electronic components or semiconductor
devices, but rather, is finding use in applications such as conductive inks62
, paints,
functional coatings63
, reinforced composites53
and hierarchically structured bulk
materials.
3.4 Liquid Phase Production Methods
There are a variety of studies in the literature, along with several recent reviews 5, 53, 63-
69, that describe the production of graphene-based materials in the liquid phase. Some
studies focus on the production of pristine, unmodified graphene, which can be
exfoliated using both aqueous and non-aqueous techniques. A large proportion focus on
wet chemical routes towards common graphene-based derivatives such as GO and rGO,
terms which are often used synonymously with graphene. While these materials exhibit
distinct properties and are generally of lesser quality than pristine graphene, they are a
popular substitute for the carbon monolayers in studies as they are easily produced
using scalable, top-down approaches. Here, the exfoliation of pristine graphene in
40
aqueous and non-aqueous media is discussed, as well as related wet chemical
techniques which are used to exfoliate graphene-based derivatives in the liquid phase.
3.4.1 Exfoliation of Graphene-Based Derivatives
Two of the most common graphene-based derivatives produced in the liquid phase are
GO and rGO.70, 71
At present, there are a variety of wet chemical techniques that can be
used to synthesise GO from graphite powder, including the Brodie72
, Staudenmaier73
,
Hofmann74
, and Hummers method75
. Various modifications to the Hummers method
have been applied in the literature, and have proved popular for producing graphene
analogues thus far, as these methods are generally faster and safer than other existing
methods.19
In the Hummers method, oxidation of graphite powder is generally
performed by treating graphite powder with concentrated sulfuric acid, sodium nitrate
and potassium permanganate in solution. This process breaks the ordered, conjugated
carbon backbone and introduces oxygen containing functional groups along the
graphene basal plane. The oxygenated moieties intercalate between the graphene
sheets, thereby increasing the interlayer distance between adjacent graphene sheets in
bulk graphite. Given the increased hydrophilicity imparted by the oxygenated surface
groups, ultrasonication can then be applied to further separate the resulting few, bilayer
or single layer GO. These GO sheets can then be used as precursors in the production
of rGO. For this conversion, a reducing agent is required to remove the oxygenated
surface groups and restore the sp2 carbon lattice structure. While hydrazine hydrate
76-79
is of the most common reducing agents employed in the production of rGO, a variety of
other organic reducing agents have also been explored including phenyl hydrazine80
,
ascorbic acid81
, hydroxylamine82
, hydroquinone83
and pyrrole84
.
41
Although the production of GO and rGO yields graphitic sheets with lattice structures
similar to that of defect-free graphene, the resulting layers are chemically distinct from
pristine graphene monolayers (Figure 3.2). GO layers possess a variety of oxygenated
moieties, including hydroxyl and epoxy groups along the basal plane, as well as
carboxyl, carbonyl and phenol groups along the edges of the sheets.85
These can assist
in maintaining particle stability, while polar oxygen-containing functionalities in
particular can improve particle dispersion in the aqueous phase.71
During the reduction
of GO, these oxygenated functional groups are removed from the carbon plane while the
sp2 planar carbon arrangements are restored. However, this process often results in
incomplete restoration of the conjugated aromatic system, with residual oxygen
functionalities along with other hetero atoms and structural defects remaining within the
rGO structure.85
Without the stabilising effects of the oxygenated functional groups
along the basal plane, the prepared graphitic layers tend to reaggregate irreversibly
through van der Waals interactions.86
Consequently, the chemical structure of rGO
particles and suspensions differ to those of GO, as well as defect-free graphene
synthesised directly from graphite in the absence of any chemical modification.
Figure 3.2: Schematic model of graphene oxide sheet (above) and highly reduced graphene
oxide sheet (below). Compton and Nguyen 201068
; Copyright Wiley-VCH Verlag GmbH &
Co. KGaA. Reproduced with permission.
42
3.5 Production of Pristine Graphene Particle Dispersions
Exfoliation of graphene from graphite in the liquid phase without significant chemical
modification can be achieved using both aqueous and non-aqueous systems. Non-
aqueous systems generally employ either organic solvents or ionic liquids, while
aqueous systems use surfactants or dispersed polymers to enable exfoliation and
stabilisation of graphene layers. The exfoliation process in both aqueous and non-
aqueous systems relies on matching the cohesive energy between graphene sheets in the
bulk, and the interfacial energy between the graphite surface and solution phase.
Practically, this can be achieved through judicious selection of the exfoliating liquid or
exfoliating additives. In particular, liquids possessing a liquid-vapour interfacial
tension of approximately 41 mJ m-2
are able to minimise the energy input required to
attain effective separation of graphene sheets beyond the range of the van der Waals
forces responsible for interlayer cohesion.
However, employing a liquid with an appropriate surface tension is often insufficient to
cause spontaneous exfoliation of graphene layers from graphite alone. Instead, the
application of shear forces is typically required in order to facilitate separation of
individual graphene sheets from bulk graphite. Shear forces are most often imparted
using tip or bath ultrasonication, which involves the application of high frequency
ultrasound to a solution. This sonication process causes cavitation, where micron-sized
bubbles spontaneously form, grow and collapse in solution. The resulting pressure
waves apply a shear force on the lamellar graphite structure87
, which causes exfoliation
of graphene sheets. At sufficient time scales, sonication not only introduces
topographical defects along the graphene surface, but also preferentially fragments
sheets where these defects are located.88
Thus, sonication is often required in order to
43
increase the concentration of individual, isolated graphene sheets within both aqueous
and non-aqueous systems89
.
3.5.1 Exfoliation of Graphene Using Non-Aqueous Systems
There are a number of studies in the literature describing the exfoliation of pristine
graphene from graphite in non-aqueous systems. Exfoliation of defect-free graphene in
the liquid phase was first demonstrated concurrently in non-aqueous conditions by
Blake et al.90
and Hernandez et al.91
The method reported by Blake and co-workers was
developed as an alternative to the reduction of graphene-oxide and was intended for use
in thin, transparent conducting films. It involved exfoliating graphene in the liquid
phase via sonication of natural graphite crystals in dimethylformamide (DMF) and
subsequent centrifugation. Atomic force microscopy (AFM) and transmission electron
microscopy (TEM) evidence indicated up to 50% of the resulting exfoliated flakes were
monolayer.
Similarly, Hernandez and colleagues prepared dispersions of graphite powder in a
number of organic solvents. N-methylpyrrolidone (NMP), N,N-dimethylacetamide
(DMA), -butyrolactone (GBL) and 1,3-dimethyl-2-imidazolidinone (DMEU) followed
by sonication and mild centrifugation. These organic solvents were selected primarily
because of their use in dispersing carbon nanotubes in previous studies. As the three
dimensional structure of nanotubes are a derivative of graphene sheets, it was reasoned
that similar interactions occurred between the solvent and nanotube sidewall, and
between the solvent and graphene. These solvents however, are often costly,
environmentally harmful and difficult to remove after exfoliation due to their high
boiling points.5 It was shown that the exfoliation of graphite using NMP resulted in a
44
wide variation in flake thicknesses, with approximately 28% of flakes appearing as
monolayer graphene and an overall graphene concentration of ~1% w/w.
Since these initial reports, a variety of other solvents have been used to demonstrate the
successful exfoliation of graphene in the liquid phase, with over 60 different solvents
studied thus far.5 For example, Hamilton et al.
92 demonstrated the use of ortho-
dichlorobenzene (ODCB) in exfoliating graphene from a range of graphitic materials.
Exfoliation of microcrystalline synthetic graphite, expanded graphite and HOPG
yielded graphitic concentrations of 0.03 mg/mL, 0.02 mg/mL and 0.02 mg/mL in
ODCB respectively. In order to overcome the difficulties associated with removing
high boiling point solvents from exfoliated graphene, O’Neill and coworkers
investigated the use of low boiling point solvents such as acetone, chloroform and
isopropanol to exfoliate pristine graphene layers.59
In those experiments, graphene
concentrations of up to 0.5 mg/mL were achieved, with the graphene particles
exhibiting a lateral size of ~ 1 m and thickness of less than 10 layers. More recently,
Liu and colleagues developed a green approach to the exfoliation of defect-free
graphene nanosheets by using a 40% isopropanol-water mixture to exfoliate graphene
from graphite.93
The resultant graphene particles exhibited an average lateral dimension
of less than 1 m, together with particle thicknesses of ~ 2 nm, while the concentration
of graphene particles was as high as 0.27 mg/mL. While NMP has remained popular in
exfoliating graphene since early studies88, 91, 94-96
, other solvents such as pyridine97
,
dimethyl sulfoxide (DMSO), formamide, ethanol, toluene, and low molecular-weight
alkanes98
have also been successfully applied to the exfoliation of graphene from
graphite.
In addition to organic solvents, ionic liquids have also had limited use in the exfoliation
of graphene.99, 100
Imidazolium-based ionic liquids have received particular attention
45
from various research groups since being successfully used to exfoliate carbon
nanotubes.65
For instance, graphene concentrations of 0.95 mg/L were achieved by
Wang et al. when using 1-butyl-3-methyl-imidazolium bis(trifluoro-methane-
sulfonyl)imide.101
Predominantly the particles were less than 5 sheets in thickness and
up to 1 m in lateral size. Yu et al. used ionic liquids containing alkyl-3-
methylimidazolium cations to intercalate and exfoliate expanded graphite, yielding
single and few layer graphene, with a lower defect density than that of rGO produced
through Hummers method.102
More recently, Elbourne and co-workers used a number
of different imidazolium-based ionic liquids to facilitate the spontaneous exfoliation of
graphene from HOPG at room temperature.103
Despite the success of ionic liquids in
exfoliating pristine graphene from graphite, the relatively high cost of these liquids is
likely to present a barrier towards scaling for industrial quantities.
3.5.2 Exfoliation of Graphene Using Aqueous Systems
The exfoliation of defect-free graphene using aqueous systems is a popular class of
methods explored in numerous studies in the literature. Exfoliating graphene layers
from bulk graphite in aqueous conditions provides significant advantages over similar
methods which use organic solvents and ionic liquids. These include much lower costs
associated with using water as the liquid medium, as well as fewer potential safety and
environmental issues. Furthermore, aqueous based exfoliation of graphene can exhibit
greater efficiency than non-aqueous techniques.5 Nevertheless, given the liquid-vapour
surface tension of pure water is 72 mJ m-2
, additives are often necessary in order to
reduce the liquid-vapour interfacial tension of an aqueous solution enough to favour
graphene exfoliation and stabilise the separated graphene layers against reaggregation.
46
Recently however, investigations have shown that graphene layers can be exfoliated in
aqueous conditions in the absence of any stabilising additives.104, 105
For instance,
Srivastava et al. showed that graphene monolayers could be produced from HOPG in
aqueous solution through the use of sonication and subsequent recycling of graphitic
residues alone.104
In this instance, judicious selection of processing parameters was
required in order to ensure a minimal net energy cost during the exfoliation process.
However, the resultant dispersions exhibited an overall poorer concentration and
stability compared to those produced using stabilisers. The prepared solutions, which
had a relatively low graphene concentration equivalent to 0.096 mg/mL were shown to
have approximately only 10% of the original graphitic concentration remaining in
suspension after seven months. Despite these examples, previous work surrounding the
exfoliation of graphene in aqueous systems has generally focused on systems requiring
the use of surfactants, polymers or other additives to facilitate exfoliation and to
stabilise the graphene sheets against reaggregation.
Graphene layers can be exfoliated and stabilised in the aqueous phase with the
assistance of a wide variety of polymers as well as biomolecules, aromatic molecules
and other additives.5 For instance, Bourlinos et al. successfully exfoliated graphene
from graphite powder in an aqueous solution of the polymer, polyvinylpyrrolidone
(PVP) under ultrasonication, with the resultant suspensions exhibiting graphene
concentrations of 0.15 - 0.20 mg/mL.89
The study also showed that solutions of
albumin, a protein, and sodium carboxymethylcellulose, a cellulose derivative, could be
used to exfoliate and stabilise graphene particles in aqueous suspensions. Cui and co-
workers used several vinylimidazole-based polymers to produce stable graphene
suspensions with concentrations of up to 1.12 mg/mL, and a production yield
approaching 3%.106
The production yield in this case was measured with respect to the
47
few-layer graphene content and compared to the initial mass of graphite employed.
These results of the study were notable, given that graphene suspensions stabilised
using polymers rarely exhibit such high production yields with graphene concentrations
above 1.0 mg/mL. Other polymers which have also been used in the literature to
facilitate the exfoliation of graphene from graphite include polyvinyl alcohol (PVA),
dextran107
, polysaccharides108
and triphenylene derivatives109
. While the addition of
polymers is one common method of effecting the exfoliation and stabilisation of
graphene layers in aqueous-based dispersions, a variety of less common additives have
also been studied for this purpose including gelatin110
, DNA111
, gum arabic112
, pyrene
derivatives113, 114
and urea115
.
3.6 Surfactant-Assisted Exfoliation of Graphene
The exfoliation and dispersion of graphene in surfactant solutions is a popular strategy
for producing graphene as it provides a number of advantages over other liquid phase
processes. For example, solvent-based methods generally require extensive
ultrasonication and result in dispersions with poorer stability compared to those which
utilise a surfactant. Furthermore, when surfactants are employed in the aqueous phase,
a higher degree of exfoliation is usually possible, which in turn, yields more highly
concentrated suspensions of graphene. Like other aqueous exfoliation methods, the
surfactant-assisted variation uses water, rather than hazardous solvents, as the
exfoliation medium. This, coupled with the option of using environmentally friendly
surfactants, allows for exfoliation processes with green chemistry. Surfactant-assisted
exfoliation has also been shown to be a facile, scalable and cost-effective way of
producing pristine graphene sheets with minimal basal plane defects.
48
The surfactant plays a critical role in promoting the exfoliation and dispersion of
graphene in aqueous surfactant solutions. In these systems, the purpose of the
surfactant is two-fold.116
Firstly, the presence of the surfactant assists in the reduction
of the liquid-vapour interfacial tension of the solution. As stated earlier, the liquid-
vapour interfacial tension of pure water is 72 mJ m-2
, while a liquid-vapour interfacial
tension of approximately 41 mJ m-2
is required to minimise the energy necessary to
delaminate and separate sheets of graphene beyond the range of the cohesive van der
Waals forces. It is possible to adjust the liquid-vapour interfacial tension of a solution
of graphite through the addition of an appropriate amount of surfactant. In this manner,
the interfacial tension of the solution may be reduced to approximately 41 mJ m-2
so
that the interfacial energy between the graphite surface and solution phase matches the
cohesive energy responsible for binding the graphene sheets together in bulk graphite.
In this case, it is neither energetically favourable nor unfavourable for the graphene
sheets to exist in bulk graphite. Thus, there is a greater probability of the graphene
sheets separating from the bulk and dispersing in solution when a shear force is applied.
Secondly, the surfactant adsorbs to the exposed lateral plane of graphene, thereby
inhibiting reaggregation of the graphene sheets through steric hindrance or electrostatic
repulsion, depending on nature of the surfactant. Indeed, Smith et al. showed that non-
ionic surfactants stabilise graphene particles via steric interactions, while the
concentration of graphene dispersions prepared using ionic surfactants are controlled by
the size of the repulsive charge barrier between particles, indicating the presence of
electrostatic stabilisation mechanisms.117
Thus, the type of surfactant is also a critical
component in determining the concentration of graphene dispersion.
The first sonication-based exfoliation of graphite in an aqueous system was performed
by Lotya et al. using the anionic surfactant, sodium dodecyl benzene sulfonate (SDBS).9
49
Although the graphene concentration was maximised for concentrations of SDBS
approaching the CMC, the concentration typically remained low after centrifugation,
reaching only a maximum of 0.05 mg/mL. TEM measurements, as shown in Figure 3.3,
indicated 45% of exfoliated graphitic material was less than 5 layers thick, with only
3% being monolayer graphene, far less than that achievable using solvent exfoliation.
Thermogravimetric analysis and high-resolution transmission electron microscopy
(HRTEM) measurements also indicated the presence of residual surfactant. This was
cited as the main reason for the relatively low DC conductivity (35 S/m) of films made
by vacuum depositing the obtained graphene.
Figure 3.3: Left: A TEM image of a monolayer graphene flake, exfoliated using SDBS. Right:
A high-resolution TEM image of a surfactant exfoliated monolayer graphene flake. Inset: Fast
Fourier transform (equivalent to an electron diffraction pattern) of the image. (Adapted with
permission from Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L.
S.; Blighe, F. M.; De, S.; Wang, Z.; McGovern, I. T.; Duesberg, G. S.; Coleman, J. N., Liquid
Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions. J. Am.
Chem. Soc. 2009, 131 (10), 3611–3620. Copyright (2009) American Chemical Society).
The ultrasonic exfoliation of graphene in the aqueous surfactant solutions has been
reported extensively in a number of studies and reviews5, 65, 66, 118
since initial
investigations by Lotya et al. §A.1 of the Appendix provides a summary of these
reports, which employ a wide variety of surfactants in generating pristine graphene
50
dispersions in the aqueous phase. Indeed, the work of Guadia et al. (Figure 3.4),
indicates practically any type of surfactant can be used to promote exfoliation. For
instance, sodium cholate has successfully been used in several studies to produce
graphene dispersions with concentrations below 0.3 mg/mL.119-122
The resultant flakes
generally exhibited a higher degree of exfoliation compared to those exfoliated using
SDBS, with as much as 80% of the total number of particles existing as monolayer
graphene. The cationic surfactant, cetyl trimethylammonium bromide (CTAB) has also
been shown to facilitate the exfoliation of graphene from both powdered graphite123, 124
and HOPG87
in the aqueous phase. It was found that graphene dispersions derived from
powdered graphite generally yield particles with lateral sizes below 200 nm, compared
to those produced from HOPG which resulted in average lateral dimensions of above
500 nm. This is most likely due to the highly ordered nature of the HOPG, which
characteristically designed to exhibit large crystal grain sizes. The concentration of the
resultant graphene dispersions produced using powdered graphite was reported to be as
high as 0.55 mg/mL.123
Non-ionic tri-block copolymers containing polyethylene oxide
(PEO) and polypropylene oxide (PPO) groups are another category of surfactants which
have proved popular in the study of aqueous-phase exfoliated graphene suspensions.
One of these surfactants, Pluronic F127, was used in a study by Seo et al. to prepare
graphene dispersions with a very low concentration of 0.078 mg/mL.125
In contrast a
similar surfactant, Pluronic F123, was employed by Guardia et al. in the preparation of
graphene suspensions with far higher concentrations of 1.5 mg/mL.126
However, it is
important to note that in all these cases, the concentration and properties of the resulting
graphene suspensions are not only determined by the type of surfactant employed, but
also various processing factors, which are unique to each study.
51
Figure 3.4: Concentration of graphene in aqueous dispersions, using a range of different
surfactants to facilitate exfoliation. The graphene concentration was estimated using UV–Vis
absorption measurements for two initial surfactant concentrations (0.5% and 1.0% wt./vol).
(Reproduced from Carbon, 49, Guardia, L.; Fernández-Merino, M.J; Paredes, J. I; Solís-
Ferández, P.; Villar-Rodil, S.; Martínez-Alonso, A. and Tasćon, J. M. D, High-throughput
production of pristine graphene in an aqueous dispersion assisted by non-ionic surfactants,
1653-1662, Copyright 2011, with permission from Elsevier).
In recent years, a series of processing modifications have been proposed so as to reduce
the limitations and improve the efficiency of the surfactant-assisted exfoliation process.
For instance, it is necessary for the surfactant to adsorb onto the graphene surface
during the exfoliation process in order to create an extra repulsive barrier against re-
aggregation. This, however, limits the efficiency of production as the adsorption
depletes surfactant from solution, raising the interfacial tension beyond the range where
exfoliation of graphite into graphene occurs. Consequently, strategies which overcome
this self-limiting process have been investigated such as the continuous addition of
surfactant (Figure 3.5).116
Through this process, graphene was successfully exfoliated
using a solution of the non-ionic tri-block copolymer surfactant, Pluronic F108, yielding
a suspension with an exceptionally high graphitic concentration of 15 mg/mL. As
52
ultrasonication is the common method of applying the shear force necessary to facilitate
graphene exfoliation, extended sonication times have also been shown to increase the
concentration of the resultant suspensions.126
Redispersion of the graphitic sediment
following centrifugation, and repetition of the sonication process has also been shown to
improve the concentration of the resultant graphene dispersions.127
Apart from
ultrasonication, other shearing techniques have also been studied for the exfoliation of
graphene in surfactant solutions. For instance, Paton and co-workers showed that
defect-free, surfactant exfoliated graphene could be produced using large-scale shear
mixers, which are more suited to the commercial production of graphene dispersion.128
Thus, a variety of individual process modifications are capable of enhancing the quality
of the dispersions produced using the basic surfactant-assisted exfoliation technique.
Figure 3.5: Schematic representation of the production of highly concentrated suspensions of
graphene through surfactant assisted exfoliation with continuous addition of polymeric
surfactant (Reprinted with permission from Notley, S. M., Highly concentrated aqueous
suspensions of graphene through ultrasonic exfoliation with continuous surfactant addition.
Langmuir 2012, 28, (40), 14110-14113. Copyright 2012 American Chemical Society.)
53
3.7 Properties and Features of Surfactant Exfoliated
Graphene
The surfactant assisted exfoliation method yields graphene layers with unique properties
and features compared to graphene derivatives such as GO or rGO produced though
common wet chemical techniques. The chemical oxidisation of graphene involves
breaking the ππ bonds existing between adjacent, planar carbon atoms. This not only
facilitates exfoliation of graphitic layers but causes defects in the lattice, converting the
sp2 hybridised carbon atoms to sp
3 tetrahedral carbon, thereby disrupting the π
conjugation. Although subsequent reduction of the graphene sheets to form rGO can be
performed, the step is typically incomplete and seldom restores the graphene sheets to
their pristine planar state. Conversely, the graphene lattice is preserved during the
liquid phase exfoliation due to the physical adsorption of the stabilisation surfactant.
The effect of chemical modification on the graphene lattice during processing and the
presence of an adsorbed surfactant is readily observed through a number of
characterisation methods.
Liquid-phase exfoliated graphene can be distinguished from other graphitic materials
using Raman scattering. Raman scattering is a non-destructive technique that reveals
the scattering behaviour of phonons with both the electronic and crystallographic
structure of graphene. Although most graphitic materials scatter phonons in the region
800-2000 cm-1
, Raman scattering is particularly effective in distinguishing ordered and
disordered carbonaceous materials based on the intensity, wavelength and breadth of
three prominent peaks: the D, G and 2D peak, subject to the excitation wavelength.129
Typical Raman spectra for graphite and exfoliated graphene are shown in Figure 3.6.
54
Figure 3.6: Raman spectra of (a) pristine natural graphite powder, (b) graphene dispersion
stabilised by the non-ionic surfactant P-123, and (c) chemically reduced graphene oxide
(Reproduced from Carbon, 49, Guardia, L.; Fernández-Merino, M.J; Paredes, J. I; Solís-
Ferández, P.; Villar-Rodil, S.; Martínez-Alonso, A. and Tasćon, J. M. High-throughput
production of pristine graphene in an aqueous dispersion assisted by non-ionic surfactants,
1653-1662, Copyright (2011), with permission from Elsevier).
The G peak at 1570 cm-1
appears as a consequence of the in-plane sp2 carbon structure,
and is evident in Raman spectra for all graphitic materials. For rGO, the peak is broader
that that observed for graphene.70
The D peak at 1360 cm-1
, however results from the
out-of-plane breathing mode of the sp2 carbon atoms and indicates the proportion of sp
3
carbon atoms in a sample. The peak occurs when defects in the graphene monolayers
are present, and when the sp3 edges of small flakes are included in the measurement.
129
In the case of liquid-phase exfoliated graphene, the intensity of this peak is significantly
greater than that of graphite powder, as well as mechanically exfoliated graphene. This
is due primarily to the lateral size of the graphene flakes, which are smaller than the
area of the laser used to perform measurements. As a result, the sp3 carbons that form
the edges of the flakes typically contribute to the measured D peak. The structure of
55
rGO and GO increases the peak absorbance70
due to the presence of oxygen containing
functional groups on the graphene surface. The peak at approximately 2700 cm-1
, is
known as the 2D peak and given the intensity, wavelength and shape of the peak,
provides an indication of the number of stacked graphene layers. In the case of
mechanical or liquid-phase exfoliated graphene, the 2D peak is a single, symmetric peak
centred on 2640 cm-1
19
with an excitation wavelength of 633 nm. Increasing the
number of stacked graphene layers increases the breadth of the peaks and shifts the peak
to between 2600 and 2750 cm-1
.
Regardless of the type of process used to exfoliate graphitic materials in the liquid
phase, the resulting flakes also demonstrate a broad variation in lateral size. The flake
size is highly dependent on the liquid phase process employed in delaminating the
individual layers from the bulk graphite. In the case of surfactant stabilised graphene,
TEM and AFM measurements show the resulting lamellar material typically have a
lateral size of between 100 – 750 nm.9, 87, 126, 130
However, these flakes can be as small
as 30 nm 123
and as large as 1 µm 121
depending somewhat on the starting graphite
material, in addition to being irregularly shaped. Similarly, rGO and GO flakes have
similar variation in shape but are typically larger in size, with flakes having an area of
between 100 and 300 µm2.131
The size of graphene-based particles has a direct influence on their properties. For
example, different sizes of GO particles have shown to exhibit different levels of
cytotoxicity.132
As a result, the inability to separate graphene materials based on size is
a clear disadvantage of the technique. However, size selection of GO dispersions has
been demonstrated by varying the sonication regime applied during the exfoliation
process133
or through size exclusion chromatography122
.
56
The presence of ionisable edge groups or defect points can lead to the development of
surface charge in solvents such as water. GO is negatively charged across a broad pH
region, with the zeta potential becoming more negative as the pH increases, from
approximately -20 mV at pH 2 to -45 mV at pH 11.123, 134
This behaviour is attributed
to the presence of carboxylic acid and phenolic hydroxyl groups on the particle surface
and edges. In the case where surfactants facilitate exfoliation of graphene, studies have
shown the surfactant is adsorbed to the graphene surface, with zeta potential plateauing
with increasing surfactant concentration.123, 135
When either an anionic9 or cationic
surfactant123, 135
is used to facilitate exfoliation, the zeta potential reflects the overall
anionic or cationic charge of the graphene particles imparted by the surfactant, in
addition to the charge imparted by the negatively charged edge groups. For example, in
the case of the anionic surfactant SDBS, the zeta potential decreases from -30 mV at pH
2, to -70 mV at pH 12, while CTAB, a cationic surfactant exhibits a decrease in zeta
potential from 5 mV at pH 4 to -30 mV at pH 10 which suggests that even after
extensive dialysis, specifically bound surfactant monomer molecules are still present on
the graphene surface.
3.8 Conclusion
Graphene has been the subject of considerable research in recent years, due to the
promising prospects posed by its unique two dimensional structure and impressive
material properties. In this chapter, a brief overview of this existing research was
presented, with a particular focus on the chemical structure, properties and production
of graphene. The two dimensional structure of graphene is closely related to both the
reactivity and material properties of the carbon layers. The material properties
exhibited by graphene also rely heavily on the number of graphene particles in a
57
sample, highlighting the importance of producing graphene with as few layers as
possible.
Of the three production methods suited to generating few-layer graphene in commercial
quantities, only those performed in the liquid phase are suitable for the large-scale
production of graphene used in industrial applications and processes. A variety of
liquid phase methods were therefore presented in this chapter including the wet
chemical techniques used to generate as GO and rGO, as well as the exfoliation of
defect-free graphene in the aqueous and non-aqueous phase. The ultrasonic surfactant-
assisted exfoliation of graphene however, exhibits a number of advantages over other
liquid phase techniques in terms of processing, as well as the concentration and quality
of the graphene particles produced. The resultant particles are chemically distinct from
those graphitic materials which undergo chemical modification during production and
can be identified and characterised through a number of different experimental
techniques.
58
CHAPTER 4
Chapter 4
Experiment Materials and Techniques
4.1 Introduction
The main goal of this thesis is to use the intermolecular interactions inherent to
surfactant exfoliated graphene to investigate the potential of these particles in a series of
applications governed by adsorption mechanisms. This is achieved through a range of
experiments, all of which involve the use of graphene suspensions prepared by the
ultrasonic surfactant-assisted exfoliation of graphite in the aqueous phase. These
suspensions, in turn, are used to prepare samples for subsequent characterisation and
analysis in each study. Often, the methods used to prepare these samples, such as the
particle stabilised foams generated in Chapter 7 or the dispersions prepared in Chapter
8, are limited to a single chapter. Moreover, they are simple and straightforward,
making detailed explanations unnecessary. Others, such as the preparation of multilayer
thin films through dip coating and the layer-by-layer technique, are common to a
number of chapters and are more complex. Similarly, certain characterisation
techniques are shared across a number of chapters. These techniques require a more
advanced understanding to not only correctly interpret their results, but to also recognise
the ability and limitations of the technique
This chapter provides an overview of the primary experimental techniques used in
subsequent chapters to prepare and characterise samples. It begins with the general
protocol used to produce surfactant exfoliated graphene in the following studies,
together with an analysis of the properties and features of the resultant particles. A
59
description of the methods used to prepare samples in this thesis is then given, with a
focus on those methods involving the adsorption of surfactant exfoliated graphene to a
solid surface. Finally, selected laboratory techniques used to characterise samples are
presented, with specific emphasis on those which can directly measure the effect of
surfactant exfoliated graphene adsorbed at an interface. Many of the results and
associated discussions contained in this chapter were first published in a peer reviewed
article136
(Reprinted with permission from Sham, A. Y. W.; Notley, S. M., Layer-by-
Layer Assembly of Thin Films Containing Exfoliated Pristine Graphene Nanosheets
and Polyethyleneimine. Langmuir 2014, 30, (9), 2410-2418. Copyright 2014 American
Chemical Society).
4.2 Preparation and Characterisation of Graphene
Suspensions
The use of surfactant exfoliated graphene particles is a common element throughout the
proceeding experimental chapters. Each experimental chapter comprises one study,
with the studies being presented in the order in which they were performed. The
graphene particles used for each study were produced using variations on a basic liquid-
phase exfoliation method, which was developed for the first study. Over time,
alterations and improvements to this protocol were applied to reflect developments in
our understanding of the surfactant exfoliation process and to accommodate individual
experiment requirements. This section of the chapter outlines the basic liquid phase
exfoliation method used to prepare the graphene particles, including background
information relevant to the specific surfactants employed during the process. It also
presents the characterization of the resultant graphene particles, which serve to provide
representative properties for the prepared material.
60
4.2.1 Exfoliating Surfactant
In this thesis, three surfactants were used to facilitate the exfoliation of graphene from
graphite in the liquid phase. These surfactants are shown in Figure 4.1 and include cetyl
trimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and Pluronic
F108. The surfactants were selected based on their chemical structure and electrostatic
charge so as to facilitate the desired type of intermolecular interaction necessary for
each study presented.
Figure 4.1: Chemical structures for surfactants used to achieve the exfoliation of graphene from
graphite in this thesis, including a) CTAB, b) SDS and c) Pluronic F108
The non-ionic surfactant, Pluronic F108 was the main surfactant used to exfoliate and
stabilise graphene particles in suspension throughout this thesis. Pluronic F108 belongs
to a class of surfactants known by the non-proprietary name, poloxamers, as well as the
trade names Pluronic®
and Synperonic®. Poloxomers are non-ionic, tri-block
copolymer surfactants that are characterised by a polyethylene oxide-polypropylene
CTAB
SDS
Pluronic F108
61
oxide-polyethylene oxide (PEO-PPO-PEO) structure. As the length of the PPO and
PEO chains are highly tailorable, a wide range of poloxomers are possible, with many
of these surfactants being used in industrial applications as antifoaming agents, wetting
agents, dispersants, thickeners, and emulsifiers depending on their degree of
hydrophilicity or hydrophobicity. As the PEO and PPO segments in poloxomers are
considered to have hydrophilic and hydrophobic character respectively, the overall
hydrophobicity or hydrophilicity of a particular poloxomer is determined by the ratio of
the length of the PPO to the PEO chains in the copolymer, known as the hydrophilic-
lipophilic balance (HLB). Pluronic F108 is very hydrophilic, with a HLB value > 24.137
Such interactions are important as they are likely to play a role in adsorption processes.
Pluronic F108 demonstrates a number of advantages over other surfactants used to
stabilise graphene particles in aqueous suspensions. Firstly, unlike ionic surfactants,
which stabilise exfoliated graphene particles through electrostatic interactions, non-
ionic surfactants such as Pluronic F108 typically stabilise particles through steric
hindrance, which is largely insensitive to high salt concentrations or changes in pH.
Furthermore, unlike monomeric surfactants such as CTAB and SDS, poloxomers like
Pluronic F108 tend to become irreversibly adsorbed along the graphene surface during
the exfoliation process.116
In this case, changes in graphene concentration are unlikely
to affect the surfactant surface excess, ensuring effective particle stabilization. Pluronic
F108 also has the advantage of yielding very highly concentrated graphene dispersions
through the ultrasonic exfoliation of graphite with continuous surfactant addition.116
62
4.2.2 Materials
For the preparation of the Pluronic F108 exfoliated graphene produced throughout this
thesis, synthetic graphite powder with a nominal particle size of less than 20 m was
used as received from Sigma Aldrich. The poloxamer, Pluronic F108 (Mn ~14.6 kDa,
HO(C2H4O)141(C3H6O)44(C2H4O)141H) was also obtained from Sigma Aldrich. Where
appropriate, NaCl and KNO3 were used as electrolytes. All solutions were prepared
using ultra-pure water with a pH of 6.8 and resistivity of 18.2 MΩ cm. Solutions were
adjusted to the appropriate pH using NaOH and HCl.
4.2.3 Methods
Stock graphene suspensions were prepared via the method of ultrasonic exfoliation of
graphite, with continuous surfactant addition.116
In a typical experiment, a 10%w/w
solution of Pluronic F108 (90 mL) was prepared at a rate of approximately one drop per
second to a 2% w/w suspension of graphite (98 mL) powder in Milli-Q water under
ultrasonication for 90 mins at 40 W. Ultrasonication was performed using a tip
sonicator (‘‘Cell Disruptor’’ W-220F, Heat Systems-Ultrasonics Inc.). The suspension
was then centrifuged at 2500 rpm for 20 minutes to sediment larger, non-exfoliated
graphite particles. The resulting suspension was dialyzed (dialysis tubing, 14 kDa
MWCO, Sigma Aldrich) against Milli-Q water for a minimum of 48 hours to remove
unadsorbed surfactant from the stock solution. Samples of the suspension were
weighed, dried and reweighed to determine the average graphitic content of the
suspensions in Chapter 5.
Exfoliated graphene particles were characterised using zeta potential measurements,
Raman spectroscopy, UV-Vis spectrophotometry, TEM and a dynamic light scattering
63
(DLS) technique. The zeta potential of the graphene particles was determined using a
Malvern Zetasizer Nano. Zeta potential measurements were performed at intervals of
0.5 pH units between pH 3.5 and 9.5. pH adjustment of the suspension was performed
manually using the appropriate amount of NaOH or HCl. Raman spectroscopy was
conducted on the graphene particles using a Horiba Jobin Yvon Raman system, with
633 nm excitation laser. Samples were prepared by direct deposition of the undiluted
graphene suspension onto silicon wafers and measured in the dry state. The UV-Vis
spectrum of a diluted graphene suspension was measured using a Cary 300 UV-Vis
spectrophotometer over the wavelength range of 200 - 800 nm. TEM was performed on
the graphene particles using a Hitachi H7100FA 125 kV system. Samples were
prepared by direct deposition of 100 L undiluted graphene suspensions onto TEM
grids (C-flatTM
, 200 mesh grid, Pro Sci Tech) and imaged after vacuum drying. Particle
sizing was performed using dynamic light scattering (DLS) with the Malvern Zetasizer
Nano.
4.2.4 Characterisation of Graphene Particle Suspensions
Dispersions of graphene particles were produced using the ultrasonic exfoliation of
graphite, with continuous surfactant addition. Visual observations indicated the
resulting graphene suspensions were stable against reaggregation for a period of several
months when stored without agitation. Such particle stability can be attributed to the
adsorption of surfactant on to the graphene surface. Given the low surface energy
inherent to the conjugated graphene lattice, the surfactant, Pluronic F108 is expected to
adsorb to the graphene surface primarily through intermolecular interactions with the
hydrophobic (i.e. PPO) portions of the surfactant. The hydrophilic portions of the
64
surfactant are expected to extend into solution in order to maximise favourable
hydrogen-bonding interactions with the surrounding water molecules.
The optical, vibrational and physical properties of the exfoliated graphene particles were
characterised prior to their use in subsequent experiments, using UV-Vis spectroscopy,
Raman spectroscopy, TEM, DLS and zeta potential measurements. The UV-Vis
spectrum of a Pluronic F108 exfoliated graphene solution (0.5107 mg/g) is shown in
Figure 4.2. The spectrum shows a single prominent peak at 259 nm, which is indicative
of extended electronic conjugation present in pristine graphene sheets.79
The sample
also exhibits a strong absorbance across all wavelengths, suggesting the spectrum is
distinct from that of rGO and GO. Furthermore, the concentration of graphene could be
compared to that determined gravimetrically, from the UV-Vis absorbance at the
wavelength 660 nm, using an extinction co-efficient, ε, of 13.90 L g-1
cm-1
based on the
recent study by Lotya et al.9 Applying the Beer-Lambert law yields a concentration
6.490 × 10-3
mg/mL. This value is significantly lower than that determined
gravimetrically, suggesting the presence of residual surfactant in the samples, which is
likely to be associated with residual water molecules given the highly hydroscopic
nature of the surfactant. Thus, after learning of this technique, UV-Vis spectroscopy
was used as the preferred method of determining graphene concentration in subsequent
studies (Chapter 6 through to Chapter 8).
65
Figure 4.2: UV-Vis spectra of Pluronic F108 exfoliated graphene
Samples of the undiluted stock suspension (3.112 mg/g) and the starting synthetic
graphite powder were also analysed using Raman spectroscopy (Figure 4.3). Both
spectra exhibit prominent peaks at approximately 1320-1330, 1570-1600 and 2665-
2675 cm-1
consistent with the D, G and 2D peaks common to all graphitic materials.6
The D peak is associated the degree of sp3 carbon present in the sample, whilst the G
peak arises from in-plane vibrations.71
In the case of the exfoliated graphene sample,
the D peak is attributed to the sp3 carbon present as edge defects on the nanoscopic
particles, whilst the G peak is associated with sp2 carbon content. Here, the ratio
between the intensity of the D and G peaks is higher for the exfoliated graphene
suspension than the synthetic graphite powder sample due to the increased proportion of
edges present, indicating successful exfoliation of graphite. The shape, spread and
position of the 2D peak in the samples is also indicative of the number of graphene
layers present. A single sharp 2D peak at 2668 cm-1
is present in the exfoliated
graphene sample, characteristic of single layer graphene. As the number of graphene
layers increases, this peak is known to shift to approximately 2700 cm-1
and transforms
into a doublet. This is indeed the case with the synthetic graphite powder tested,
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
200 300 400 500 600 700 800
Ab
sorb
ance
(a.
u.)
Wavelength (nm)
66
although the peak shifts to only 2673 cm-1
. Thus, the prepared graphene particles are
likely to exist as single or bi-layer graphene.
Figure 4.3: Raman spectra of undiluted graphene stock suspension and synthetic graphite
powder, measured with 633 nm laser.
The number of layers, as well as the lateral size of the Pluronic F108 exfoliated
graphene particles were investigated using TEM imaging. Figure 4.4 shows
representative images of the prepared particles. The graphene particles appear as highly
transparent, layers ranging in size from approximately 100 nm to 1 m. The high
degree of transparency observed indicates the particles are thin and consist of very few
layers. Visual evidence of isolated and overlapping sheets suggesting the sample
contains single, bi- and few layer graphene, which is consistent with the results obtained
through Raman spectroscopy.
1200 1700 2200 2700
Inte
nsi
ty (
a.u
.)
Raman Shift (cm-1)
Pluronic F108 Exfoliated Graphene Synthetic Graphite Powder
67
(a) (b)
Figure 4.4: TEM images of diluted graphene stock suspension yielding particles comprised of
single, bilayer and few layer graphene.
The mean lateral particle size, ⟨𝐿⟩ , of a 0.00017 mg/g solution of dispersed graphene
particles were estimated using a DLS technique described by Lotya et al.138
This
method involves determining the peak intensity of a particle size distribution (PSD)
which is then applied to estimate the mean lateral size of the dispersed graphene
particles using the empirical formula:
⟨𝐿⟩ = (0.07 ± 0.03)𝑎𝐷𝐿𝑆(1.5+0.15)
4.1
Where:
⟨𝐿⟩ = Mean lateral particle size (nm)
𝑎𝐷𝐿𝑆 = Particle size with peak intensity (nm)
A Zetasizer Nano was used to obtain the PSD of the solution, with an average 𝑎𝐷𝐿𝑆 of
307.7 nm, yielding a ⟨𝐿⟩ value of 377.82 nm. This value is consistent with visual
observations of the graphene particles obtained through TEM imaging.
68
The zeta potential of the exfoliated graphene particles in NaCl and KNO3 was measured
as a function of pH in order to determine the effective surface charge of the graphene
particles. The zeta potential measurements of the graphene particles were consistent
with literature for surfactant exfoliated graphene116
, exhibiting a slight negative charge
of between -6.38 and -18.2 mV in 10-4
M NaCl and -8.6 and -25.1 mV in 10-4
M KNO3,
becoming more negative with increasing pH. This negative charge is acquired due to
the oxygenated edge defect sites on the graphene sheets, introduced during the
sonication procedure.123, 124
Thus, as the pH decreases, protonation of the oxygen
containing surface groups occurs, resulting in a less negative zeta potential. Regardless
of the pH however, the zeta potential values still remain well within the region of
colloidal instability (±25 mV), suggesting the particles will tend towards reaggregation
on the basis of insufficient electrostatic repulsion. However, previous studies indicate
agglomeration is prevented by the irreversible adsorption of block co-polymer
surfactant molecules on the surface of the graphene sheets.116
As the stock suspension
was stable and surfactant is non-ionic, this suggests the observed colloidal stability is
imparted primarily through steric repulsion.
69
Figure 4.5: Zeta potential measurement of graphene exfoliated in surfactant solution in 10-4
M
NaCl (blue diamonds), and graphene exfoliated in surfactant solution in 10-4
M KNO3 (red
squares).
4.3 Sample Preparation Techniques Involving Adsorption
4.3.1 Layer-By-Layer Self Assembly
Layer-by-Layer (LbL) self-assembly is a well-established method of creating thin films
on substrates139
and was used in this thesis to prepare the multilayer films presented in
Chapter 5 and Chapter 6. The technique involves the build-up of consecutive layers of
two or more distinct, yet complementary chemical species to form a film, shown
schematically in Figure 4.6 for two chemical components. During this process, a
substrate is exposed to a solution containing one of the chemical components to allow
adsorption of molecules from solution onto the surface of the substrate. Any
unadsorbed molecules are removed from the surface through a subsequent rinsing step.
The remaining chemical component is then used to repeat the process, thereby forming
a bilayer. Further deposition of bilayers results in the formation of a multilayer thin
-30
-25
-20
-15
-10
-5
0
3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Zeta
Po
ten
tial
(m
V)
pH (units)
70
film. To date, the LbL technique has been successfully applied to a range of systems
involving polyelectrolyte140-142
, proteins and enzymes143, 144
, DNA145
, colloidal
particles146, 147
, various inorganic species148, 149
, nanoparticles148, 150
, dyes151
, quantum
dots152
and macromolecules153
using a variety of fabrication methods including spin
coating154
, spray coating155
and flow-based techniques156
.
Figure 4.6: Schematic of LbL thin film assembly technique.
LbL self-assembled films demonstrate a range of common features, regardless of the
chemical species employed in constructing the thin films. In particular, the resultant
multilayer films typically exhibit uniform surface coverage, in addition to highly
controllable thicknesses in the nanometre range. Furthermore, multilayer films
produced using LbL deposition can be applied to conform to nearly any shape or surface
chemistry. Another advantage of LbL deposition is the broad range of functional
molecules that can be incorporated into the resultant thin films. By controlling the
71
thickness, structure and composition of the films, this method also allows the
construction of nanocomposite films with tailored functional properties.
In order to successfully form a thin multilayer film through LbL deposition however, it
is essential that the participating chemical components exhibit attractive intermolecular
interactions between one another. A variety of different intermolecular interactions can
be utilised in order to assemble films through LbL deposition including electrostatic,
hydrophobic, hydrogen bonding, charge-transfer, host-guest and coordination chemistry
interactions.157, 158
However, LbL deposition is most widely applied to systems
involving the alternating deposition of oppositely charged chemical species onto a
charged substrate. These types of multilayers utilise charge compensation mechanisms
in order to successfully construct the multilayer. Consequently, the growth of each
layer is self-limited as the attractive interactions between the adjacent layers are
compensated by the repulsive electrostatic interactions within the layer. Similarly, layer
growth is restricted for films assembled using other types of intermolecular interactions,
when a balance between interactions both within and between layers is achieved. In this
study, investigations are limited to thin films assembled using electrostatic and
hydrogen bonding interactions.
4.3.2 Dip Coating
In this thesis, multilayer samples were prepared through layer-by-layer deposition onto
substrates using an automated dip coater. In its simplest form, dip coating can be
achieved by manually dipping and removing substrates from a vessel containing the
desired coating solution. Nevertheless, automated systems provide a number of distinct
advantages over manual dip coating. Automated dip coating systems are capable of
accurately controlling the dip and withdrawal rates of substrates from solution. These
72
systems are also capable of immersing the substrates to a desired depth and at a desired
angle, reproducibly. The systems also prove less labour intensive than manual dip
coating when a greater number of layers are required. Consequently, an automated
system was used in Chapter 6 due to the high number of dipped layers required.
The automated dip coater used in this project is shown in Figure 4.7 and consists of a
horizontal rail system with vertical dipping arm. A series of clips located at the end of
the dipping arm permit up to five sample substrates to be attached, allowing samples to
be dipped concurrently. During operation, the vertical dipping arm transverses along
the rail system a known distance and then moves vertically, dipping the substrate into
and out of the sample reservoir. This particular dip coater allows a series of dips and
withdrawals to be programmed, enabling sequential adsorption of solutions onto
substrates.
Figure 4.7: KSV NIMA dip coater multi vessel system used to assemble LbL thin films. The
dipping arm traverses horizontally along the rail system and dips the substrate in the sample
reservoir in order to form a single adsorbed layer. The process is then repeated with the other
sample reservoirs to assemble the multilayer film.
73
4.4 Characterising Samples Prepared Through Adsorption
4.4.1 Atomic Force Microscopy
AFM is an analytical technique commonly used to probe surface properties. A Bruker
Multimode VIII AFM was used in this thesis to obtain high resolution topographical
images and perform quantitative nanomechanical mapping on a variety of samples.
4.4.1.1 Apparatus
An AFM is comprised of a laser, probe, split photodiode detector, piezoelectric scanner
and feedback loop as shown in Figure 4.8.
Figure 4.8: Schematic illustration and photo of an AFM. Inset photograph shows a MultiMode
8 AFM, which was used for the majority of AFM work in this thesis.
74
The probe used in AFM imaging comprises a sharp tip positioned at the end of a
cantilever. The cantilever is usually either rectangular or V-shaped and has a length of
around 100 µm, while the tip is usually less than 10 nm in diameter at the apex. The tip
and cantilever are manufactured from Si or Si3N4 using conventional semiconductor
fabrication methods.
Analysing a sample using AFM involves positioning the cantilever tip over the sample
surface, and reflecting the laser beam off the back of the cantilever into the split
photodiode detector. As the tip approaches the surface, tip-sample interactions occur,
causing the cantilever to deflect. As the cantilever bends, the position of the reflected
beam changes on the photodiode detector. The photodiode is split into four quadrants,
which are used to determine the position of the reflected laser beam in the lateral and
normal directions relative to a reference set-point established at zero deflection. As the
path length between the cantilever and photodiode detector is significantly greater than
that of the length of the cantilever, deflection of the cantilever results in an
amplification of the change in laser position. Consequently, the system is able to
accurately detect small changes on the surface in the z-direction, down to the molecular
level.
The position of the sample surface relative to the cantilever is controlled by the
mechanical movement of the piezoelectric scanner. When a voltage is applied across
the piezoelectric stage, the material undergoes displacement proportional to the applied
voltage. This enables movement of the sample in the x-y plane and allows the tip to
raster across sections of the sample surface during imaging. The piezoelectric scanner
also allows the distance between the surface and the tip to be controlled precisely in the
z-direction using feedback from the photodiode detector. The photodiode converts a
change in the laser spot position to a voltage, which is then passed through the feedback
75
loop to the piezoelectric sensor in order to adjust the position of the sample along the z-
axis in real time. In this manner, the AFM is able to measure a quantity of interest, such
as topographical height at each point on the surface.
4.4.1.2 Basic Operational Modes
AFM are typically capable of operating in a variety of modes which differ according to
the way in which the tip contacts the sample. Contact mode and tapping mode are two
of the most basic AFM modes upon which other advanced, proprietary scanning modes
are based. In contact mode, the tip is in continuous contact with the sample surface at a
constant cantilever deflection. To achieve constant deflection of the cantilever, the
piezoelectric scanner adjusts the position of the sample along the z-axis in real time, in
response to changes in the deflection of the cantilever. As the tip is in constant contact
with the sample surface, however, this mode is not well suited to fragile surfaces. In
these cases, tapping mode provides an alternative, less destructive mode of operation,
avoiding drag and lateral forces along the sample surface. In tapping mode, the
cantilever is oscillated near its resonance frequency using a piezoelectric cantilever
holder. When the tip makes contact with the sample surface, the amplitude of
oscillation decreases. During tapping mode measurements, constant reduced amplitude
is maintained throughout the scan using the feedback loop. In this thesis, measurements
were performed using two proprietary AFM modes based on tapping mode:
ScanAsystTM
and PeakForce QNM®.
4.4.1.3 ScanAsyst Imaging
Mode
ScanAsyst is an AFM imaging mode that was used in this thesis to obtain topographical
images of LbL self-assembled thin film surfaces. In ScanAsyst mode, a system of tip-
76
sample interaction feedback and software-based algorithms are used to automatically
and continually optimise imaging parameters throughout the scan, providing enhanced
force control. As such, ScanAsyst typically enables the tip to selectively apply smaller
forces to the sample surface than those typically applied with tapping mode when
required, and is therefore ideal for imaging heterogeneous surfaces.
4.4.1.4 PeakForce QNM (Quantitative Nanomechanical Mapping) Mode
PeakForce QNM was used in this thesis to probe the mechanical properties across
hydrogen-bonded LbL thin film surfaces. PeakForce QNM is able to perform
topographical imaging while simultaneously yielding information on a number of
different mechanical surface properties. NanoScope® Analysis software is used to
quantify these properties through a variety of parameters including the modulus,
adhesion, deformation and dissipation. In order to accurately determine these material
properties, calibration of the deflection sensitivity, spring constant and tip radius of the
cantilever are required prior to measuring the sample surface. To measure mechanical
quantities using PeakForce QNM, a force curve is recorded at each point as the tip is
rastered over the sample surface. Sample properties are then extracted in-real time
based on specific regions of the AFM force curve (Figure 4.9). For example, in
NanoScope Analysis, the pull-off force experienced by the tip during retraction of the
tip from the sample surface is quantified by the adhesion parameter. The dissipation
parameter corresponds to the energy of adhesion, whilst the deformation parameter is
associated with the depth of indentation.
77
Figure 4.9: Regions of an AFM force curve with corresponding QNM parameters. The
compliance (linear) region of the retract curve fitted to the DMT model provides the Young’s
modulus, the area between the approach and retract curves is associated with dissipation, the
pull-off force is associated with adhesion while deformation corresponds with depth of
indentation.
NanoScope Analysis also provides a number of different theoretical models to calculate
the Young’s modulus, each of which is suited to a specific application. The Derjaguin-
Muller-Toporov (DMT) approach was used, as it is involves modelling the tip as a
sphere and is suitable for generic samples. The DMT modulus parameter is equivalent
to the Young’s modulus, and is calculated by first fitting the retract portion of the
compliance (linear) region to the DMT model (Equation 4.2) 159
:
𝐸𝑟 =3(𝐹𝑡𝑖𝑝 − 𝐹𝑎𝑑ℎ)
4√𝑅𝑑3
4.2
Where:
𝐸𝑟 = Reduced modulus (J)
𝐹𝑡𝑖𝑝 = Normal force on tip (N)
78
𝐹𝑎𝑑ℎ = Adhesive force between AFM tip and sample (N)
𝑑 = Deformation depth (m)
𝑅 = Tip radius (m)
The Young’s modulus of a material is then related to the reduced Young’s modulus by
Equation 4.3 :
1
𝐸𝑟=
(1 − ν𝑠2)
𝐸𝑠+
(1 − ν𝐼2)
𝐸𝐼
4.3
Where:
𝜈𝑟 = Poisson’s ratio of sample
𝐸𝑠 = Young’s modulus of sample (N/m2)
𝜈𝐼 = Poisson’s ratio of tip (J)
𝐸𝐼 = Young’s modulus of tip (N/m2)
4.4.2 Quartz Crystal Microbalance
A quartz crystal microbalance (QCM) is used throughout Chapter 5 and Chapter 6 of
this thesis in order to monitor the progression of film growth on a substrate. The QCM
is a gravimetric technique capable of monitoring the deposition of thin films with
masses in the order of 10-9
g/m2 by sensing an addition of mass to a surface. The QCM
KSV 500 quartz crystal microbalance used in this project is comprised of a closed
chamber and quartz crystal resonator as shown in Figure 4.10. Solutions are introduced
into the chamber through the top inlet valve, and adsorbates are allowed to adsorb onto
the surface of a quartz crystal resonator. After a predetermined amount of time, the
solution is extracted from the closed chamber using the outlet valve while the next
solution is introduced into the chamber.
79
(a)
(b)
Figure 4.10: (a) Schematic illustration and photograph of a QCM. Inset photograph shows a
KSV QCM-500 used for the majority of QCM work in this thesis. (b) schematic illustration of a
quartz crystal resonator showing the top (active) surface with desired surface coating, and the
bottom surface with electrode coating.
The key component of a QCM is the quartz crystal resonator, which is used to sense the
mass of the adsorbed species. Quartz crystal resonators are typically around 300 m
80
thick160
, and are sandwiched between two gold electrodes, which are used to apply an
AC voltage across the quartz crystal resonator. As quartz is piezoelectric, oscillating
deformation of the quartz crystal resonator occurs, with the piezoelectric properties and
crystalline structure of the quartz crystal determining the mode of deformation and
oscillation frequency. Overwhelmingly, quartz crystal resonators used in QCM
apparatus are manufactured from AT-cut crystals, which yield oscillating transverse
shear deformation at a frequency of 5 MHz. A standing shear wave is generated across
the surface of the quartz crystal resonator when the appropriate excitation frequency is
applied. The excitation frequency occurs when a crystal thickness is an odd multiple of
half the acoustic wavelength and includes odd numbered harmonics such as the
fundamental, third, fifth, seventh harmonic. Consequently, when a mass is adsorbed to
the resonator surface, the frequency at which the resonator oscillates decreases
proportionally to the adsorbed mass. In this manner, the adsorption of material to the
quartz crystal surface can be monitored by the change in frequency.
Although a QCM is able to obtain signals from the lowest odd harmonic frequencies, a
single harmonic frequency is typically chosen for subsequent analysis. Lower harmonic
frequencies facilitate better energy trapping of the standing wave than higher harmonic
frequencies.161
In this thesis, the third harmonic frequency was chosen for subsequent
analysis, as the fundamental (resonant) frequency is responsible for driving the
oscillation of the quartz crystal resonator. As such, the signal cannot be used for further
analysis as it is typically influenced by a combination of effects arising from the noise
inherent to the driving oscillation and the change in frequency due to adsorption.
81
4.4.2.1 Analysis of Adsorbed Mass
The addition of mass at a resonator surface can be analysed using a number of different
modelling methods.160
Two of the most common methods however, are the Voigt
model and the Sauerbrey equation. The Sauerbrey equation162
provides a simple
method with which to calculate the theoretical mass of a rigid layer adsorbed to the
surface of a QCM electrode based on the physical parameters of the quartz crystal
resonator and the change in frequency experienced when a mass is adsorbed to the
surface of the resonator. In contrast, the Voigt model assumes the oscillation of the
adsorbed film is not coupled to that of the sensor and corrects for the deviation using the
material and mechanical properties of the adsorbing species such as density, viscosity,
elastic modulus. The Voigt model163
is typically suited to systems in which viscoelastic
layers are present and generally requires computational analysis in order to model
correctly. However, using the Voigt model for even the simplest systems can be
computationally intensive, with most analyses delivering accurate results for systems up
to only a few adsorbed layers. Consequently, the use of the Sauerbrey equation is
favoured where appropriate due to its simplicity, while the Voigt model is generally
only applied when conditions do not permit the use of the Sauerbrey equation, such as
in the presence of viscous layers. For this reason, effort was directed towards the use of
the Sauerbrey equation throughout the QCM analysis sections of this thesis.
The Sauerbrey equation provides a simple method with which to calculate the
theoretical mass of a layer adsorbed to the surface of a QCM electrode. It relates the
mass deposited on the electrode with the change in oscillation of the QCM resonator.
This method assumes the adsorbed material forms a rigid layer on top of the quartz
resonator and that the energy used to produce oscillation of the resonator is transferred
through the layer without energy losses. In such a case, the system oscillates effectively
82
as a resonator of mass equivalent to that of the quartz crystal resonator and the adsorbed
film. As a result, the resonant frequency of the quartz crystal resonator is inversely
proportional to the mass of the adsorbed layer, according to the Sauerbrey equation
(Equation 4.4):
∆𝑓 =−2𝑓0
2∆𝑚
𝐴√𝜌𝑞𝜇𝑞
4.4
Where:
∆𝑓 = Measured frequency shift (Hz)
𝑓0 = Resonant frequency of quartz crystal resonator (Hz)
∆𝑚 = Mass change per unit area (g/cm2)
𝐴 = Piezoelectrically active area (i.e Area between electrodes, cm2)
𝜌𝑞 = Density of quartz, 2.648 g/cm3
𝜇𝑞 = Shear modulus of quartz, 2.947 × 1011
g/cm s2
However, in order for the Sauerbrey equation to remain valid, the system under
investigation must satisfy a series of conditions. Firstly, rigid layer deposition is
required in order to ensure the layer fully couples to the oscillation of the quartz crystal
resonator. Viscous layers do not fully couple to the oscillating QCM sensor. As a
result, points within the layer that are further from the electrode surface will not
oscillate with the same phase and frequency as that of the QCM crystal. In this case,
applying the Sauerbrey equation to this system will result in an underestimation of the
mass adsorbed. Consequently, the Sauerbrey mass is only applicable when the
dissipation is low. In order to successfully apply the Sauerbrey equation, layers with a
uniform thickness are also required. In order to ensure accurate results, the layers are
83
also required to adsorb strongly enough to the crystal surface to prevent detachment.
This again is necessary, in order to ensure complete coupling of the layer to the
oscillating surface, with slippage between the adsorbed mass and the oscillating surface
resulting in a lower sensed mass. In addition, it has been shown quantitatively that the
Sauerbrey equation remains valid when the ∆𝑓 values of the different overtones are in
agreement when scaled by the overtone, 𝑛.163 This condition was satisfied for all
solution conditions trialled in this thesis, with the maximum average difference between
overtones for each experiment being less than 12.99 Hz, approximately 1% of the
overall frequency change experienced during the experiments. The dissipation
parameter can also be used to assess whether the Sauerbrey equation may be applied to
a specific system.
4.4.2.2 Dissipation
The dissipation is a parameter that quantifies the energy dampening of the system and is
a key factor in determining which mode of analysis is appropriate for specific data sets
obtained through QCM. Dissipation is a parameter that quantifies the sum of all energy
losses in the QCM chamber per oscillation cycle and thus, provides an indication of the
rigidity of an adsorbed multilayer film. A viscous layer is characteristic of a soft film
and occurs when either the attractive intermolecular interactions between the adsorbed
species within the layer are weak, or there is a greater amount of entrained water content
within the layer. As a result, viscous layers absorb and dissipate kinetic energy from an
adjacent resonator throughout the layer, thereby giving rise to high dissipation.
Conversely, a system comprised of a rigid layer typically exhibits greater intermolecular
interactions between the particles and chemical species within the layer, allowing the
applied energy to transfer more efficiently through the layer. Consequently, dissipation
values, ∆𝐷𝑛, were used to characterise the rigidity of the prepared films in order to
84
determine the validity of the Sauerbrey equation in this thesis. Although film rigidity is
a continuum, a threshold for ∆𝐷𝑛/ (−∆𝑓𝑛
𝑛) of 4 × 10
-7 Hz
-1 160
was defined as a guide in
order to distinguish between films that were more rigid and films that exhibit more
viscoelastic behaviour.
4.4.3 Pendant Drop Technique
The pendant drop technique is a method commonly used to determine the surface
tension of a liquid in a gaseous phase. It involves suspending a droplet of the liquid
from a needle as shown in Figure 4.11 and measuring optically the contour of the
droplet at equilibrium. In this setup, the pressure inside the droplet is higher than that of
the surrounding phase due to the difference in interfacial energy between the two
phases. As a result, the droplet adopts a curved shape, which also deforms under
gravity in the z-direction due to the hydrostatic pressure acting on the droplet. Here, the
surface tension of the liquid is related to the radii of curvature of the droplet by the
Young-Laplace equation (Equation 4.5).
Figure 4.11: Screenshot of CAM 2008 software showing droplet contour and model fitting to
the Young-Laplace equation.
85
∆𝑃 = 𝛾 (1
𝑅1+
1
𝑅2) + 𝜌𝑔ℎ
4.5
Where:
∆𝑃 = Pressure difference between liquid and gaseous phase (Pa)
𝛾 = Interfacial surface tension (N m-1
)
𝑅1 = 𝑅2, Radius of curvature (m)
𝜌 = Density of liquid (g m-2
)
𝑔 = Acceleration due to gravity, 9.81 m s-2
ℎ = Height from reference plane (m)
In order to determine the surface tension, the outline of the droplet is recorded using a
camera then fit numerically to the Young-Laplace equation.
4.5 Chapter Conclusion
In this thesis, the potential for surfactant-stabilised graphene particles to be used in
various applications controlled by adsorption mechanisms will be investigated through a
series of experiments. In this chapter, the core experimental techniques used to prepare
and characterise samples in later sections were presented. These included the initial
protocol used to prepare surfactant-stabilised graphene suspensions, which forms the
basis of experimental studies described in proceeding chapters. It is important to note
that this method evolves in subsequent sections based on the specific requirements of
each study, and an improved understanding of the surfactant exfoliation process. This
chapter also described two methods, dip coating and layer-by-layer assembly, which are
used to prepare samples on solid surfaces through the adsorption of surfactant stabilised
graphene in Chapter 5 and Chapter 6. Characterisation techniques that are able to
86
directly measure the effect of surfactant exfoliated graphene adsorbed at an interface
were also discussed, with a specific emphasis on the apparatus associated with each
technique. Together, these techniques support investigations presented in following
chapters.
87
CHAPTER 5
Chapter 5
Creating Electrostatically Bonded Multilayer
Films with Surfactant Exfoliated Graphene
5.1 Introduction
Thin films with customisable properties are an emerging technology which holds great
potential in a diverse range of biomedical, energy and sensing applications. These films
can be produced using Layer-by-Layer (LbL) deposition, a well-established technique
which relies on the consecutive adsorption of complementary chemical species from
solution onto a solid substrate. To date, much of the research involving LbL deposition
has focused on the build-up of polymer thin films through electrostatic interactions.
These films are usually produced using the alternate adsorption of soluble cationic and
anionic polymeric species such as polyelectrolytes in order to yield a film with a
multilayer structure. The popularity of such films is due in part to the ease at which
electrostatic interactions are established, as well as the strength of attachment between
oppositely charged polymeric species.
More broadly however, the LbL technique itself presents a highly versatile method of
altering the bulk and surface properties of films based on film thickness, chemical
composition and deposition conditions. In particular, it is capable of creating robust,
tailorable films which can be engineered to incorporate a wide variety of functional
molecules and particles. Thus, employing innovative particles with unique material
properties offers a potential means of enhancing and extending the properties LbL films.
Graphene particles pose an attractive research opportunity in this regard, given the
88
superior electrical, mechanical and thermal properties of the material. However, the
ability to incorporate graphene particles without prior electrostatic surface modification
into LbL films has received little attention in the literature.
The experiments presented in this chapter are concerned with the assembly of
electrostatically structured LbL films containing low-charge surfactant exfoliated
graphene. Chapter 4 begins with an overview of the literature pertaining to
electrostatically assembled LbL films, including the general features of these films and
previously studied systems. The materials and methods used to assemble and analyse
the films are then described. In this study, the LbL films were assembled through
alternating deposition of a cationic polyelectrolyte, PEI and negatively charged
surfactant exfoliated graphene particle modified with Pluronic F108 surfactant onto a
negatively charged silica substrate. The remainder of the chapter describes the
experiments and subsequent analysis performed on this system in order to investigate
the effect of deposition conditions on film assembly. Many of the experiments, results
and associated discussions contained in this chapter were first published in a peer
reviewed article136
(Reprinted with permission from Sham, A. Y. W.; Notley, S. M.,
Layer-by-Layer Assembly of Thin Films Containing Exfoliated Pristine Graphene
Nanosheets and Polyethyleneimine. Langmuir 2014, 30, (9), 2410-2418. Copyright
2014 American Chemical Society).
5.2 Background
LbL self-assembly is a powerful surface coating technique which has shown great
promise in thin film applications since its introduction nearly 30 years ago. While LbL
films were proposed by Iler in 1966164
, it was only later that Decher et al. pioneered the
LbL assembly technique, forming nanostructured LbL multilayer films for the first time
89
in the early 1990’s.140, 165, 166
Since then, progress in LbL films has increased rapidly,
revealing a wide variety of potential applications. For instance, LbL films may enable
controlled targeting and release of encapsulated compounds and proteins in advanced
drug delivery systems.167-170
LbL assembled films could also be applied to objects
requiring surface modification or coatings. One example is medical implants, where
LbL films could be used to improve biocompatibility by altering the interactions
between the implant and surrounding tissue.171, 172
Recent advances in LbL thin films
also suggest a myriad of other promising applications, including conductive coatings173,
174, functional nanocomposite films
175, selective membranes
155, sensors
176-178,
micropatterning179
, artificial cell walls180
, nanobioreactors181
, microelectronic devices182
and fuel cells183, 184
.
5.2.1 Electrostatically Assembled LbL Thin Films
The majority of LbL self-assembled multilayer films encountered in the literature are
constructed using purely electrostatic intermolecular interactions between ionic
chemical species. The build-up of LbL films in this manner generally involves first
exposing a solid, charged substrate to a solution containing an oppositely charged
molecule or particle. The resultant electrostatic attraction between the surface and the
dispersed species drives irreversible adsorption at the interface. As adsorption proceeds
over time, a layer of adsorbate forms at the surface, facilitating charge compensation of
the substrate. Further deposition of the charged chemical species results in
overcompensation, leading to charge reversal of the active surface. The surplus charge
also gives rise to electrostatic repulsion within the deposited layer, which prevents
further adsorption of the species, causing self-limiting growth of the layer. After
deposition of the initial layer, the film is typically rinsed in order to remove any
90
remaining unadsorbed material. The film is then exposed to a solution containing a
chemical component of opposite charge to that in the uppermost layer. This chemical
species again adsorbs though electrostatic interactions, facilitated by the local charge
present at the exposed film surface. As the second layer is deposited, interdigitation of
the layers occur, causing charge overcompensation. Adsorption of the second layer
results in the formation of a bilayer, with further bilayer deposition resulting in the
formation of an electrostatically assembled multilayer thin film.
Figure 5.1: Schematic illustration of general electrostatically bonded LbL assembly process.
5.2.2 Benefits and Features of Electrostatic LbL Deposition
Constructing LbL multilayer thin films using electrostatic interactions provides a
number of benefits and features with respect to film assembly, structure and functional
91
properties. For instance, electrostatic LbL film deposition is not reliant on specific
chemical systems or the presence of certain functional groups, instead requiring only the
participation of soluble, multiply-charged chemical species in order to assemble stable
multilayer films. The use of multi-charged chemical species maximise the number
intermolecular contacts in the film, and results in strong, long-range attractive
interactions that are generally uninfluenced by inexact positional matching of charged
groups. Electrostatic LbL assembled films are therefore more easily established
compared to films structured through highly oriented interactions, such as hydrogen
bonding. Consequently, electrostatic LbL film deposition have been shown to be
compatible with a wide variety of charged chemical species including
polyelectrolytes140-142, 185, 186
, proteins143
, enzymes143, 144
, DNA145
, colloid particles146, 147
and inorganic nanoparticles148-150
enabling films with a variety of chemical
compositions. As a result, electrostatically assembled LbL films are also capable of
possessing a broad range of functional properties that are distinct from the underlying
substrate. These properties can include altered adhesion187
, elasticity,
biocompatibility175
, mechanical properties188
and wettability189, 190
depending on the
properties of the participating chemical species, deposition conditions and thickness of
the film.
Another advantage of electrostatically assembled LbL thin films is the ease at which
thin film nanocomposites and polymeric blends with specific nanostructures can be
produced in aqueous environments. Often, thermodynamic factors governing the
formation of mixtures act to prevent polymeric materials, molecules and particles from
forming nanoscale blends or ordered structures, due to the low entropic drive towards
mixing.191
However, electrostatically assembled LbL films provide an effective means
of controlling the morphology of thin films when used in conjunction with highly
92
charged, long-chained molecules such as strong polyelectrolytes. Polyelectrolyte
multilayer films are a widely studied class of electrostatically assembled LbL thin films
whose adsorption kinetics and internal structure vary based on a number of factors
including composition, deposition conditions and film thickness. Thus, combining
polyelectrolyte LbL thin films with novel particles using electrostatic LbL thin film
assembly provides a potential route towards highly tailorable nanoscale blended thin
films with functional properties. As such, this chapter focuses on the preparation of
electrostatically assembled LbL thin films comprised of pristine graphene particles
stabilised with a polymeric block co-polymer surfactant, and a cationic polyelectrolyte.
5.2.3 Polyelectrolyte Multilayer Film Growth
Polyelectrolyte multilayer films can generally be classified into one of two basic
categories, depending on the type of growth regime observed. Thin films which
experience linear growth in thickness and mass are generally the most common and
widely studied. They consist of highly stratified layers separated by indistinct
interfacial regions, which are caused by polyelectrolyte chains diffusing a short distance
into directly neighboring layers.141, 192
In contrast, the presence of exponential growth
at low numbers of adsorbed layers indicates greater diffusion of polymeric chains
throughout the films.193
For instance, greater chain interpenetration has been shown to
result in chain segments existing in the 3-4 layers above and below the point of
adsorption in a film142
, resulting in films with a more homogeneous, blended structure.
Thus, the degree to which at least one polymeric species is able to diffuse between
adjacent layers during the LbL deposition process is a key factor in the growth and
nanostructure of the resultant films.194
93
However, there are a number of other factors which influence the successful build-up
and morphology of polyelectrolyte based LbL thin films. For instance, the degree of
chain interpenetration in a polyelectrolyte multilayer film has been shown to be affected
by both the magnitude of electrostatic interactions between participating species in the
film and the chain conformation. Electrostatic interactions are essential to the assembly
of the film and are dependent on the charge density along the polyelectrolyte and
electrolyte concentration, and to a lesser extent on polyelectrolyte concentration,
deposition time and molecular weight.140, 185
The charge density along a polyelectrolyte
is influenced by the charge state of the polyelectrolyte, with “strong” polyelectrolytes
dissociating completely in solution to possess a fixed charge, while “weak”
polyelectrolytes exhibit only a partial, mobile charge along the molecule. The pH of a
solution controls the charge density of weak polyelectrolytes by altering the degree of
protonation along the chains, thereby dictating chain conformation.195
The chain
conformation also has a significant effect on film growth, as it controls the extent to
which charge overcompensation occurs during film deposition.196
Chain conformation
is governed by physical properties specific to the polyelectrolyte, such as the persistence
length12
and solvent quality of the molecule.185
In addition to these properties, the
polyelectrolyte chain conformation is also affected by a variety of deposition conditions
such as pH195
, temperature197
, ionic strength198
, salt type185, 199
, deposition technique193
.
5.2.4 Graphene Based Materials in Electrostatic Multilayer Films
The ability to incorporate graphene particles in to the nanostructure of polyelectrolyte
multilayer thin films is highly desirable, as it provides a potential route for producing
novel conducting films90
, or hybrid composites with enhanced mechanical properties
amongst other applications. Despite this, the majority of previous work surrounding the
94
use of graphene-based materials in electrostatic LbL self-assembled films has focused
on incorporating graphene oxide or reduced graphene oxide into multilayered thin
films.79, 200
There are however, select examples of multilayer thin films containing
pristine surfactant stabilised graphene particles constructed with the aid of
polyelectrolytes through electrostatic interactions.124, 173
For instance, multilayer thin
films were constructed from surfactant-exfoliated graphene subsequently modified with
the cationic polyelectrolyte PEI and the anionic polyelectrolyte, PAA, using the LbL
technique.124
It was shown that multilayer growth was governed by parameters such as
pH and polyelectrolyte due to the weakly ionisable nature of PEI and PAA, influencing
the overall surface charge of the graphene particles and counter polyelectrolyte.
Furthermore, a lower charge on the modified graphene particle surface was found to
increase the adsorbed amount and overall surface coverage.124
Consequently, there is a clear need to study multilayers formed with polyelectrolytes
and low surface charge graphene particles, such as those found in non-ionic surfactant
exfoliated dispersions. In order to realise the full potential of these thin films however,
it is also necessary to investigate and optimise the conditions under which these
multilayers form. Here we describe the preparation of graphene multilayer films by the
LbL approach using pristine graphene particles stabilised with polymeric block co-
polymer surfactant coupled with a cationic polyelectrolyte, PEI. The small intrinsic
charge on the graphene particles due to edge defects is sufficient to allow formation of
multilayers. By modifying the effects of solution conditions under which graphene-
containing multilayers form, the quality and thickness of the resultant multilayers can be
controlled and optimised for a given application.
95
5.3 Materials
The materials listed in §4.2.2 were again used in this study. In addition to these
materials, the cationic polyelectrolyte, PEI (30% aqueous, Mn ~ 70 kDa) was also
obtained from PolySciences Inc.
5.4 Methods
5.4.1 Preparation of Stock Graphene Suspensions
Stock graphene suspensions were prepared via the method of ultrasonic exfoliation of
graphite, with continuous surfactant addition116
using the basic protocol described in
§4.2.3.
5.4.2 Particle Characterisation
The exfoliated graphene particles were characterised using zeta potential measurements,
Raman spectroscopy, UV-Vis spectrophotometry and a DLS technique. The zeta
potential of the graphene particles was determined using a Malvern Zetasizer Nano.
Zeta potential measurements were performed at intervals of 0.5 pH units between pH
3.5 and 9.5. pH adjustment of the suspension was performed manually using the
appropriate amount of NaOH or HCl. Raman spectroscopy was conducted on the
graphene particles using a Horiba Jobin Yvon Raman system, with 633 nm excitation
laser. Samples were prepared by direct deposition of the undiluted graphene suspension
onto silicon wafers and measured in the dry state. The UV-Vis spectrum of the diluted
graphene suspension was measured using a Cary 300 UV-Vis spectrophotometer over
the wavelength range of 200 - 800 nm. Particle sizing was performed using DLS with
the Malvern Zetasizer Nano according to the study by Lotya et al.138
. The results of
96
these measurements were presented in §4.2.4 as representative properties of the
graphene suspensions.
5.4.3 Adsorption Measurements
Multilayers of graphene particles and PEI were prepared using the layer-by-layer
deposition technique on silica surfaces. In a typical experiment, adsorption of PEI to
the silica surface was followed by a rinsing step with electrolyte solution. A solution of
block co-polymer stabilised graphene was then exposed to the surface, followed by
another rinse step with electrolyte solution to produce a bilayer. The process of bilayer
formation was then repeated until the appropriate number of bilayers was deposited.
Both solutions used in the deposition process were adjusted to the appropriate pH.
In this study, the formation of multilayers was monitored using a quartz crystal
microbalance (QCM, KSV QCM-500). The QCM was used to monitor the change in
resonant frequency (∆𝑓0) and overtones (∆𝑓3, ∆𝑓5, ∆𝑓7) as the multilayer was
constructed on a QCM quartz resonator crystal. The third overtone (∆𝑓3) was selected
for subsequent analysis in preference to the resonant frequency and other overtones as it
exhibits enhanced stability and consistency over the fundamental resonant frequency,
arising from better energy trapping161. As layers are deposited and the adsorbed mass
on the surface increases, the change in resonance frequency of the crystal decreases.
The Sauerbrey relationship was used to convert this change in frequency due to
adsorption, into a sensed mass. The simplified Sauerbrey relationship (Equation 5.1)
states that for a given overtone, the total change in mass is directly proportional to the
change in frequency of the quartz resonator162
.
97
∆𝑚 = −𝐶1
𝑛∆𝑓𝑛
5.1
Where:
∆𝑚 = Mass change per unit area (g/cm2)
𝑛 = Overtone number
𝐶 = Constant based on physical properties of resonator (17.7 ng/cm2 Hz)
∆𝑓𝑛 = Frequency of quartz crystal resonator for a given overtone, 𝑛 (Hz)
The relationship is valid for uniform, rigidly adsorbed films, occurring when the ∆𝑓
values of the different overtones are similar when scaled by 𝑛.163
This condition was
satisfied for all solution conditions trialled in this chapter, with the maximum average
difference between overtones for each experiment being less than 11 Hz.
The QCM was also used to measure dissipation values of the multilayer films indirectly
via impedance analysis. Dissipation is a parameter that quantifies the sum of all energy
losses in the QCM chamber per oscillation cycle and thus, provides an indication of
adsorbed multilayer film rigidity. Consequently, the dissipation values, ∆𝐷𝑛, were used
to characterize the rigidity of the prepared films. Although film rigidity is a continuum,
in this study, we define a threshold for ∆𝐷𝑛/ (−∆𝑓𝑛
𝑛) of 4 × 10
-7 Hz
-1 as a guide in order
to distinguish between films that are more rigid and films that exhibit more viscoelastic
behaviour.160
Although most samples in this study were prepared using the QCM, a small number of
multilayer films were prepared using a manual dip coating process on silicon wafer
surfaces. The samples were prepared using the same adsorption regimes used to prepare
samples for QCM measurements.
98
5.4.4 Multilayer Characterisation
The resultant multilayers were characterised using both AFM and X-ray diffraction
(XRD) measurements in the dry state. XRD measurements were performed on
multilayer samples, using a Bruker D8 Advance X-ray powder diffraction setup with Cu
Kα X-ray source operating at a voltage of 40 kV and current of 40 mA. The samples
were continuously rotated throughout.
The surfaces of the dip coated multilayers were imaged using a Multi-Mode III AFM
(Veeco Inc., USA). Standard, non-contact Si3N4 cantilevers with a spring constant of
0.4 N/m and Si tip chemistry (Veeco, USA) were used for all measurements in this
study. The surfaces were imaged using the PeakForce Tapping® Mode. The scan rate
was typically 0.5 Hz and gains of between 15 and 20.
5.5 Results and Discussion
Adsorption measurements were performed by preparing multilayer films from
exfoliated graphene suspensions and PEI using the layer-by-layer technique in a variety
of solution conditions at 23 °C. These measurements were performed to determine the
effect of pH, graphene concentration, electrolyte, electrolyte concentration and
polyelectrolyte concentration on successful film formation, adsorbed mass and film
rigidity.
5.5.1 Film Formation for Extended Films
Multilayers comprised of an extended number of graphene/PEI bilayers were
assembled. Multilayers of graphene particles and PEI were prepared using the layer-by-
layer deposition technique on silica surfaces. In a typical experiment, adsorption of PEI
99
to the silica surface was followed by a rinsing step with electrolyte solution. A solution
of block co-polymer stabilised graphene was then exposed to the surface, followed by
another rinse step with electrolyte solution to produce a bilayer. The process of bilayer
formation was then repeated until the appropriate number of bilayers was deposited.
Both solutions used in the deposition process were adjusted to the appropriate pH.
Using the successive deposition of 100 ppm PEI and 0.5391 mg/g Pluronic F108
Exfoliated graphene suspension in 10-2
M KNO3 at pH 4, multilayers comprised of 1, 3,
5, 7 and 9 bilayers were prepared. Figure 5.2 shows the kinetics of this process for the
construction of a multilayer comprised of 9 bilayers. First, a baseline was recorded with
the pH adjusted background electrolyte in the QCM chamber. PEI was then adsorbed to
the silica crystal surface for approximately 300 s, during which a corresponding
decrease in signal was recorded indicating adsorption of the polymer. An equilibrium
adsorbed amount was attained during this step. Next, a rinsing step was performed with
electrolyte for approximately 300 s, resulting in an increase in the signal suggesting
removal of loosely bound PEI from the surface. Graphene suspension was then added
to the chamber for approximately 300 s, resulting in a decrease in frequency lower than
that observed for the PEI, followed by a rinsing step with electrolyte where the signal
increased again. The process was then repeated to create the desired number of
bilayers. The decrease in signal compared to the baseline upon rinsing regardless of
species introduced to the chamber prior to the rinse step, indicates the adsorption of
successive layers and build-up of the multilayer.
100
Figure 5.2: Change in frequency as a function of time for the 9 bilayer system formed with 100
ppm PEI and 10% Graphene suspension in 10-2
M KNO3 at pH 4.
The results obtained for the Sauerbrey mass and dissipation of the 9 bilayer film are
presented in Figure 5.3. The film exhibited linear film growth with respect to the
graphene additions, suggesting it is possible to create multilayers with an extended
number of bilayers. The only exception to this observation is the first bilayer, where a
greater mass of PEI is adsorbed in order to compensate the negative surface charge of
silica.
This behaviour is consistent with the known adsorption kinetics of polyelectrolyte
multilayers, which have been shown to increase either linearly201
or exponentially202
.
Linear film growth occurs as the result of the electrostatic attraction between the surface
and the adsorbing molecule, with further deposition resulting in charge
overcompensation at the interface. This suppresses further adsorption via electrostatic
repulsion within the layer and gives rise to distinct layers with blended interfaces. It is
-120
-100
-80
-60
-40
-20
0
20
0 2000 4000 6000 8000 10000 12000
Δf 3
(H
z)
Time (s)
Rinse
PEI
Graphene
101
currently understood that exponential film growth arises as the result of molecules
adsorbing to the film via electrostatic interactions and diffusing throughout the film
towards the substrate, forming a homogeneous film.
The linear trend in observed film growth of the multilayers may be related to the size
and planar nature of the exfoliated graphene nanoparticles. The continuous surfactant
technique used to process the graphene particles in this study typically results in
graphene particles with a lateral size of up to 100 nm 116
, which is significantly larger
than PEI molecules. Furthermore, the most kinetically favourable configuration of the
sheets maximizes contact with the active surface. As a result, the size and orientation of
the graphene sheets may hinder interdigitation of PEI molecules into the graphene
layers. This could result in distinct layers where charge overcompensation is achieved
for adjacent layers, resulting in a linear growth regime.
The films also exhibited the predicted relationship between Sauerbrey mass and
dissipation. As the mass adsorbed to the resonator increases, the dissipation increases
upon addition of both PEI and graphene. Interestingly however, the change in
dissipation and adsorbed mass decreases as successive PEI layers are added, but
remains relatively constant for graphene additions. This suggests that either less PEI is
being adsorbed but the overall rate of increase in adsorbed mass remains the same, or
entrained water is removed from the multilayer upon further additions of PEI, which is
supported by the trend in overall film rigidity. As the number of bilayers increase,
dissipation as a function of frequency decreases, indicating increased overall film
rigidity. The presence of KNO3 promotes film rigidity through the removal of entrained
water within the layers, regardless of the number of layers. Additionally, as more layers
are adsorbed, layers closer to the substrate may collapse due to further interlayer
102
interactions. This would cause the film to become more rigid as an increasing number
of layers are adsorbed.
(a)
(b)
Figure 5.3: (a) Sauerbrey Mass and (b) Dissipation as a function of time for the 9 bilayer
system formed with 100 ppm PEI and 10% graphene suspension in 10⁻² M KNO₃ at pH 4.
0
1
2
3
4
5
6
7
0 2 4 6 8 10 12 14 16 18 20
Sau
erb
rey
Mas
s o
f Fi
lm (
mg/
m2)
Number of Layers
0
1
2
3
4
5
6
7
8
9
0 2 4 6 8 10 12 14 16 18 20
Dis
sip
atio
n o
f Fi
lm (
x 1
0-6
)
Number of Layers
103
5.5.2 Effect of pH and Electrolyte on Film Formation
Multilayer films were prepared at pH 4, 7 and 9 in order to determine the effect of pH
on the formation and rigidity of the multilayers. The charge on the graphene particles
increases with increasing pH hence the adsorbed amount may vary with pH. In
addition, as the pH increases, the charge on the PEI macromolecules decreases. The
multilayers, comprised of 3 bilayers each, were constructed using the consecutive
adsorption of 100 ppm PEI and 0.5391 mg/g Pluronic F108 exfoliated graphene
suspension, in 10-2
M NaCl and 10-2
M KNO3. Two different salts were used to probe
the influence of the ionic species on film formation. The graphene is stabilised using a
PEO-PPO-PEO block-co-polymer which has some small conformational changes at
elevated ionic strength which is dependent on the specific ions in solution.
For both systems shown in Figure 5.4 and Figure 5.5, it is clear that assembly proceeds
to a greater extent at either acidic or basic conditions, but not neutral conditions. This is
attributed to orientation of the molecules, arising from the protonation and
deprotonation of amine groups on PEI. At higher pH, deprotonation occurs, lowering
charge density and causing the polyamine molecules to extend away from the active
surface as previously demonstrated from surface forces measurements.203, 204
Typically
this results in a greater adsorbed amount which is in agreement with theory and
experiments of polyelectrolyte adsorption.15
Additionally, at high pH the graphene
particles acquire a more negative charge, enabling a greater amount of PEI to be
adsorbed and involved in achieving charge compensation upon the second addition of
PEI. As a result, the thickness and density of the molecules in the PEI layer increase,
causing a higher mass to adsorb to the surface during each addition of PEI at high pH.
Conversely, the PEI molecules adopt a flat conformation against the substrate at lower
pH. The graphene particles have a very low negative charge at pH 4 hence a higher
104
adsorbed amount of particles should result. At neutral pH, both graphene particles and
PEI have significant charge density which results in relatively lower adsorbed amounts
in comparison to solutions of high or low pH. Such behaviour has previously been
demonstrated for the assembly of multilayers using polyelectrolytes with weakly
ionisable groups190, 205
.
Figure 5.4: Sauerbrey mass as a function of layer number for multilayers fabricated using PEI
at various pH in 10-2
M NaCl.
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7
Sau
erb
rey
Mas
s o
f Fi
lm (
mg/
m2)
Number of Layers
pH 4
pH 7
pH 9
105
Figure 5.5: Sauerbrey mass as a function of layer number for multilayers fabricated using PEI
at various pH in 10-2
M KNO3.
Data from this series of experiments also suggested that the overall film rigidity of the
multilayers was dependent on pH. For example, multilayers formed in 10-2
M NaCl at
pH 4 demonstrate more viscoelastic behaviour, as evidenced by the higher energy
dissipation of the film which is typically greater than 10-6
per 10 Hz (Figure 5.6). In
contrast, films formed at pH 7 and 9 in 10-2
M NaCl show an increase in film rigidity
with increased pH. This can be attributed to the orientation of the PEI in addition the
oscillation of the QCM crystal. As the QCM crystal oscillates, it generates a standing
shear wave that results in a shear force parallel to the surface.160
At lower pH values,
the PEI molecules deposit along the surface and may experience more slippage due to
the shearing action of the oscillator, increasing the energy dissipated in the layer.
Conversely, at higher pH values, the cationic polyelectrolyte extends away from the
active surface. This may promote chain entanglement, increasing layer density and thus
reducing the dissipation of the film.
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7
Sau
erb
rey
Mas
s o
f F
ilm (
mg/
m2)
Number of Layers
pH 4
pH 7
pH 9
106
(a) (b)
(c)
Figure 5.6: Dissipation as a function of the normalised frequency for systems at (a) pH 4, (b)
pH 7 and (c) pH 9 in 10-² M NaCl. The red line illustrates a dissipation of 4 × 10
-7 Hz
-1,
indicative of the threshold between rigid and viscous films. Data further below the line indicate
more rigid multilayer films, whereas data above the line indicate increased viscosity.
It was also shown that film rigidity and adsorbed mass were influenced by the
electrolyte selected. At each pH level investigated, multilayers formed in the presence
of NaCl exhibited a higher Sauerbrey mass than those of KNO3. However, films
formed using KNO3 typically exhibited more rigid film behaviour, as demonstrated by
lower dissipation values. Here, the presence of K+ ions disturb the hydration layer
surrounding PEO portions of the surfactant chain206
, dehydrating the adsorbed block co-
polymer. Consequently, a reduction in entrained water occurs, causing the films to
become more dense therefore more rigid.
-5
0
5
10
15
0 10 20 30 40Dis
sip
atio
n (
x 1
0-6
)
-Δf3/n (Hz) -5
0
5
10
15
0 10 20 30 40
Dis
sip
atio
n (
x 1
0-6
)
-Δf3/n (Hz)
-5
0
5
10
15
0 10 20 30 40
Dis
sip
atio
n (
x 1
0-6
)
-Δf3/n (Hz)
107
The trend in adsorbed mass observed for the multilayers prepared at pH 4 and pH 7 in
NaCl is also of particular interest. Here, the adsorbed mass and dissipation increases
upon an addition of graphene, then decreases slightly upon adsorption of PEI. It does
not revert to the mass measured prior to adsorption of the graphene layer, expected
when the layer is completely removed. Instead, the alternating increase and decrease in
mass upon addition of solution to the QCM chamber is characteristic of water entrained
within the adsorbed layers. At pH 4 and 7, the exfoliated graphene is expected to
undergo protonation of the oxygenated defect sites to a greater extent than at pH 9. The
expulsion of entrained water upon addition of PEI suggests hydrogen bonding occurs
between the protonated defect sites on the graphene and the trapped water molecules.
Upon addition of the PEI, electrostatic interactions and interpenetration of the layers
force the entrained water from the adsorbed graphene layer.
5.5.3 Effect of PEI Concentration on Film Formation
The effect of PEI concentration on the adsorbed layer mass was also investigated.
Multilayers were prepared using concentrations of PEI between 30 and 200 ppm in a
background electrolyte solution of either 10-2
M NaCl or 10-2
M KNO₃ at pH 4.
The results for the Sauerbrey mass of multilayers prepared in NaCl and KNO3 are
presented in Figure 5.7 and Figure 5.8. For multilayers prepared in NaCl, we observe a
peculiar, non-linear relationship between PEI content and adsorbed mass. However, the
ordering of the experiments based on PEI content is not significant. This is because of
the high variability between experimental repeats, coupled with the dissimilar behaviour
observed in experiments where KNO3 is present. The high degree of variation was also
present in the KNO3 experimental data, albeit it a lower extent, with the maximum
standard error for each repeat of the multilayers prepared in NaCl and KNO3 being 1.89.
108
The maximum standard error for each of the other experiments is 1.13. Consequently,
only a general treatment of the data is presented.
Figure 5.7: Sauerbrey mass as a function of layer number for systems with varying
concentrations of PEI in 10-2
M NaCl.
Figure 5.8: Sauerbrey mass as a function of layer number for systems with varying
concentrations of PEI in 10-2
M KNO3.
Nevertheless, the variation in data may reveal important information about the
adsorption behaviour of PEI. Low concentrations of PEI were used in the experiments
order to prevent an increase in viscosity in the prepared films, which may otherwise
0
1
2
3
4
5
6
7
0 2 4 6 8
Sau
erb
rey
Mas
s o
f Fi
lm
(mg/
m2)
Number of Layers
30ppm PEI
100ppm PEI
150ppm PEI
200ppm PEI
0
1
2
3
4
5
6
0 2 4 6 8
Sau
erb
rey
Mas
s o
f Fi
lm
(mg/
m2)
Number of Layers
30ppm PEI
100ppm PEI
150ppm PEI
200ppm PEI
109
have affected the results obtained using the QCM. Furthermore, the dissipation data
confirms that the prepared multilayers are indeed rigid, indicating the discrepancy
between the experimental repeats is not caused by viscoelastic behaviour of the PEI.
One source of variation is the competitive adsorption of PEI molecules to the active
surface. It is expected that the PEI preferentially adsorbs to graphene sheets in the
vicinity of oxygenated defect sites, where the electrostatic interaction is highest.
However at low pH, the graphene particles exhibit a small negative charge, which may
be insufficient to strongly bind and incorporate the PEI molecules into the film by
electrostatic interactions, while the highly charged PEI molecules experience
intermolecular repulsion. Consequently, the adsorbed amount and composition of the
multilayers may vary widely within the same series of solution conditions.
Despite the varied PEI concentration, the adsorbed mass appears to show a high
dependence on the electrolyte used. For instance, it was found that films prepared in the
presence of NaCl rather than KNO3 exhibited a higher Sauerbrey mass and lower
dissipation. As KNO3 causes the removal of entrained water due to dehydration of the
adsorbed Pluronic F108 surfactant, it is clear that the trend is most likely due to the
presence of water within the layers.
5.5.4 Effect of Graphene Concentration on Film Formation
Formation of multilayer films as a function of graphene concentration was also
investigated. Again, the multilayers were constructed of three bilayers, using the
successive adsorption of 100 ppm PEI and non-ionic block co-polymer stabilised
graphene suspension, in 10-2
M NaCl and 10-2
M KNO3. The measurements were
performed for graphene concentrations of 2%, 5%, 10% and 20% of a 5.142 mg/g stock
110
graphene solution at pH 4, where previous results showed enhanced multilayer
formation.
Regardless of the electrolyte used, a much higher effective mass was sensed for in
systems where the graphene concentration was greater than 2% (Figure 5.9 and Figure
5.10). For the 2% graphene systems, the adsorbed mass of consecutive PEI and
consecutive graphene additions plateaus following the first addition of graphene. This
suggests that a 2% graphene concentration is too low to enable self-limiting formation
of a uniform graphene layer, suppressing further assembly. The effect is more
pronounced when KNO3 is present as the salt acts to dehydrate the bilayers. For
concentrations greater than 2%, as the number of graphene layer additions increases the
final, effective mass for each combination of conditions trialed approaches the same
value. This results in a maximum Sauerbrey mass of 4.06 mg/m2 observed for films
formed in the presence of NaCl, with graphene concentration of 20%.
Figure 5.9: Sauerbrey mass as a function of layer number for multilayers fabricated using PEI
and various concentrations of graphene suspension in 10⁻² M NaCl
0
1
2
3
4
5
0 1 2 3 4 5 6 7
Sau
erb
rey
Mas
s (m
g/m
2)
Number of Layers
2% Graphene
5% Graphene
10% Graphene
20% Graphene
111
Figure 5.10: Sauerbrey mass as a function of layer number for multilayers fabricated using PEI
and various concentrations of graphene suspensions in 10-2
M KNO3
5.5.5 Characterisation of Films with Extended Numbers of Adsorbed
Bilayers
XRD was used to further characterize multilayers adsorbed to silica QCM resonator
surfaces. Multilayer films with 1, 3 and 5 bilayers were prepared from 100 ppm PEI
and 0.5142 mg/g Pluronic F108 exfoliated graphene suspension, in 10-2
M KNO3 at pH
4. The XRD measurements (Figure 5.11) of all three samples demonstrate a peak at
26.5°, associated with the (0 0 2) diffraction line of graphite and therefore, the 3.4 Å
spacing between graphene sheets in graphite.83
As a result, reaggregation of the
graphene particles is likely to have occurred during formation of the multilayers,
although the low peak intensity shows this is a minor effect. Furthermore, none of the
three XRD measurements indicate the presence of graphene oxide, which typically
exhibits a peak at 11.8°. This indicates the sp2 structure of the adsorbed graphene
particles is conserved upon formation of multilayers.
-1
0
1
2
3
4
0 1 2 3 4 5 6 7
Sau
erb
rey
Mas
s (m
g/m
2)
Number of Layers
2% Graphene
5% Graphene
10% Graphene
112
Figure 5.11: Thin film X-ray diffraction patterns of synthetic graphite powder and multilayer
films with various thicknesses formed from 100 ppm PEI and 10% Graphene in 10-2
M KNO3.
The relationship between surface roughness and number of adsorbed bilayers was
investigated by imaging the surfaces of the dip coated multilayer films using AFM.
Films of 1 - 9 bilayers were prepared from 100 ppm PEI and 0.00034 mg/g (determined
using UV-Vis spectroscopy) Pluronic F108 exfoliated graphene suspension, in KNO3 at
pH 4, using layer by layer deposition onto silicon wafer. For each sample, height
images of 10 representative areas were obtained and the corresponding Rq values were
determined. Figure 5.12 shows representative peak force error (deflection) height
images of the prepared samples, together with corresponding average Rq value for each
sample. The average Rq value typically increases with number of adsorbed bilayers.
This is consistent with common observations; as increasingly more material is added to
the surface, the observed underlying surface roughness is intensified. This indicates
that the films remain stable during the drying and dewetting process.
0
10000
20000
30000
40000
50000
60000
70000
80000
10 20 30 40
Inte
nsi
ty (
a.u
.)
Diffraction Angle, 2 (o)
1 bilayer film
3 bilayer film
5 bilayer film
Synthetic graphitepowder
113
(a) (b)
(c) (d)
(e) (f)
Figure 5.12: AFM peak force error (deflection) images of (a) silicon wafer (b) 1, (c) 3, (d) 5, (e)
7 and (f) 9 graphene-PEI bilayer films. The samples have Rq surface roughness values of 0.191,
0.209, 0.268, 0.368, 0.256 and 0.627 nm respectively.
114
Multilayer films constructed from surfactant exfoliated graphene and PEI using the LbL
approach show great potential in providing a means to creating thin film coatings with
tailored properties. In particular, the surface roughness, overall film thickness and
viscoelastic properties of the thin films may be altered by simply modifying the number
of adsorbed layers, pH, and electrolyte used when constructing the films. This in turn,
could provide a method for producing thin films with controlled thickness, varied
adhesion properties and elasticity. Consequently, thin films prepared using this method
could prove highly desirable in conductive films and composites. One of the challenges
in preparing composites from graphene is compatibility with matrix material. Polymer-
graphene hybrid thin films can be conveniently prepared though the methods described
here. The LbL approach has been used extensively over the past decade and has a
sufficient degree of flexibility that a range of surface functionalities can be modified. It
is anticipated that multilayers incorporating graphene will have altered mechanical
properties and possess other unique properties.
5.6 Conclusion
There is a strong demand for surface coatings involving novel materials that have the
ability to improve a range of film properties. Here, a silica surface was successfully
modified through the LbL technique using pristine graphene nanoparticles. The
particles were prepared through aqueous-phase exfoliation of graphite using a non-ionic
tri-block polymeric surfactant, Pluronic F108. This method of producing graphene is
scalable and also flexible, allowing for the stabilization of particles with various
surfactants and polymers. The graphene sheets were shown by Raman and UV-Vis
spectroscopy to have few to no defects aside from the edges, which inherently exhibit a
115
small negative charge, thereby allowing the buildup of multilayers through electrostatic
interactions.
The solution conditions for the sequential adsorption of the polycation PEI and
graphene sheets had a significant influence on the adsorbed amount of material. The
ionic species, ionic strength, and pH of the solution all affected the thickness and
viscoelastic properties of each deposited bilayer. Deposition of an extended number of
bilayers was achievable, with linear film growth observed for most of the conditions
selected. This type of film growth suggests that graphene particles may hinder diffusion
of the polyelectrolyte through the films because of their lateral size and shape, thereby
preventing exponential growth of the film. It was also shown that films formed in the
presence of KNO3 had a lower adsorbed mass but were more rigid than those prepared
in NaCl solutions. This is likely due to disruption of the hydration layer surrounding
the PEO portions of the surfactant used to stabilise the graphene particles in suspension.
Importantly, the presence of the polymeric surfactant for stabilizing the graphene sheets
did not prevent the formation of multilayer films; rather, it provides another variable
that can be used to tune the film properties.
Basic and acidic conditions were shown to favour the formation of multilayers based on
the orientation and resultant packing of the PEI molecules because the charge on the
graphene sheets hardly varies over the pH range studied. The growth rate of the
multilayers was also dependent upon the graphene concentration, with a minimum
graphene concentration required to ensure complete surface coverage and charge
compensation of the adsorbed layer. By considering the effect of each of these
parameters, the deposition and formation of multilayers containing graphene can be
easily optimised for a given thin-film application. Thus, a convenient method for the
116
modification of a surface with graphene was demonstrated, along with a description of
some design rules for optimizing or tailoring the film properties.
117
CHAPTER 6
Chapter 6
Assembling Hydrogen-bonded Multilayer
Films with Surfactant Exfoliated Graphene
6.1 Chapter Introduction
Although electrostatic interactions were traditionally used to fabricate multilayer films
through the LbL technique, other non-electrostatic forces such as hydrogen bonding can
also be employed in the construction of LbL multilayer films.207, 208
These films often
exhibit responses to environmental conditions, providing a unique opportunity to extend
the functionality of multilayer films and coatings. Consequently, the electrostatically
assembled films described in Chapter 5 were followed by a series of experiments
focusing on incorporating graphene particles into hydrogen-bonded multilayer thin
films. These experiments are described in Chapter 6.
Chapter 6 begins by outlining the relevant literature, including the features of hydrogen-
bonded multilayer films, and previously studied systems. A description of the materials
and methods required to assemble and analyse the thin films are also provided. In this
study, construction of the hydrogen-bonded multilayers required first adsorbing a
precursor layer of cationic polyelectrolyte, PEI, onto a negatively charged silica
substrate through electrostatic interactions. The bulk of the film was then assembled
through sequential alternating adsorption of the polyelectrolyte, PAA, and anionic
graphene sheets modified with Pluronic F108 surfactant. This chapter describes the
experiments and subsequent analysis performed in order to ascertain the physical,
mechanical and electrical properties of the films produced. Much of the work presented
118
in this chapter was first published in a peer reviewed article61
(Reprinted from the
Journal of Colloid and Interface Science, 456, Sham, A. Y. W.; Notley, S. M.,
Graphene–polyelectrolyte multilayer film formation driven by hydrogen bonding, 32-
41, Copyright (2015), with permission from Elsevier).
6.2 Background
Hydrogen-bonded multilayer films are a relatively recent advance in the area of layer-
by-layer thin film assembly compared to electrostatic films. As with traditional
electrostatically assembled films, they are constructed on solid substrates through the
alternating deposition of two or more soluble chemical species from solution. Often,
the first layer of adsorbate is deposited onto the substrate through electrostatic
interactions to ensure sufficient coverage for subsequent layer deposition.207
Unlike
electrostatically assembled films however, further build-up of hydrogen-bonded
multilayer films relies primarily on the presence of attractive, hydrogen bonding
interactions in order to drive adsorption and film growth. As a result, the assembly of
LbL thin films through hydrogen bonding generally requires the participation of low
charge or uncharged molecules that possess multiple moieties capable of acting as
hydrogen bonding donors and acceptors. Consequently, most hydrogen-bonded thin
films systems described in the literature are constructed using pairs of weak
polyelectrolytes, or weak polyelectrolytes and non-ionic polymeric species in aqueous
environments.209
As weak polyelectrolytes are pH sensitive, the process of assembling
hydrogen-bonded thin films usually necessitates specific pH conditions. This ensures
an appropriate degree of protonation for the participating species to yield largely
uncharged molecules, which in turn promotes film growth solely through hydrogen
bonding interactions.
119
Figure 6.1: Schematic illustration of the general hydrogen-bonded LbL assembly process.
Currently, there are a number of different hydrogen-bonded LbL thin film systems
documented in the literature. The first films produced in aqueous environments
however, were reported nearly 20 years ago by Stockton and Rubner.207
Their approach
involved the alternate deposition of polyaniline (PANI) at weakly acidic pH with a
variety of non-ionic water-soluble polymers at neutral pH values. These polymers
consisted primarily of hydrogen bonding acceptors such as polyethylene oxide (PEO),
polyvinylpyrrolidone (PVPON), PVA and polyacrylamide (PAAM). It was found that
conditions which improved the number of intermolecular contacts within the film,
namely lower pH levels and adsorbed species with higher molecular weights, increased
120
bilayer thickness. Since the initial study by Stockton and Rubner, a variety of other
hydrogen-bonded thin film systems involving weak polyelectrolytes and polymers have
been investigated including PEO and the weakly anionic polyelectrolyte, PAA206, 210
,
PEO and polymethyacrylic acid (PMAA), PVPON and PAA, PVPON and PMAA210, 211
as well as PAA and poly(N-isopropylacrylamide) (PNIPAAm)212
.
6.2.1 Advantages of Hydrogen-bonded LbL Self-Assembly
Hydrogen-bonded LbL self-assembly offers a range of advantages in comparison to
electrostatically driven LbL deposition. One key advantage of the technique when used
in conjunction with weak polyelectrolytes is that it enables the formation of films which
more readily respond to subtle changes in environmental conditions.209
For example,
hydrogen-bonded films constructed using polycarboxylic acids and neutral polymeric
species can be removed from substrates in a controlled manner, by increasing the pH of
the surrounding solution.211
Increasing the pH causes ionization of the carboxylic acid
groups, which in turn, disrupts the intermolecular hydrogen bonds responsible for the
structure of the film. Thus, as the pH is increased above a critical pH value, dissolution
of the film occurs, with weakly bound systems undergoing disintegration at lower pH
values compared to more strongly bound systems.209
In addition to pH, hydrogen-
bonded films are also able to respond to other environmental conditions such as
temperature213-215
, ionic strength210, 211
, humidity154, 206, 216
, solvent185
and applied
electric field210
. These environmental stimuli have been shown to alter a variety of film
properties including film morphology, thickness216
, conductivity206
, adhesion217
and
mechanical behaviour191, 218
. Depending on film composition, hydrogen-bonded LbL
films can also exhibit well-controlled crosslinking of polymeric components within the
film structure, as well as support for the controlled disruption of hydrogen bonds at
121
neutral pH. This unique combination of features allows the films to respond effectively
to environmental stimuli, whilst simultaneously retaining structural integrity when the
surrounding pH is varied.219
As hydrogen bonding LbL self-assembly allows a broad
range of polymeric materials to be incorporated into the resultant films, the technique
also allows inclusion of uncharged polymers with low glass transition temperatures into
the film structure.209
This enables the production of mechanically flexible, yet robust
ultrathin films, which could reveal new opportunities and applications for hydrogen-
bonded LbL thin films.
6.2.2 Applications of Hydrogen-bonded Multilayers
Recently, hydrogen-bonded LbL self-assembled films have gained increasing relevance
in a wide variety of potential thin film applications due to their extended functionality.
For instance, hydrogen-bonded thin films could be used to create capsules and coatings
that are not only compatible with biological tissues, but are also capable of controlled
loading and release of active compounds based on environmental stimuli.220, 221
Indeed,
non-crosslinked hydrogen-bonded thin films have already been identified for possible
use in both pH and temperature triggered drug delivery systems.222
In addition to pH
and temperature triggered drug delivery systems, hydrogen-bonded thin films also hold
great potential in devices such as humidity sensors, as they have been shown to
experience large changes in film hydration based on environmental humidity. Thus,
unlike their electrostatic counterparts, the sensitivity of hydrogen-bonded thin films to
environmental conditions is a key feature which enables the films to be used in a
broader range of applications.
Hydrogen-bonded LbL thin film assembly also demonstrates a wealth of opportunities
with respect to the formation of robust functional thin films with tuneable functional
122
properties, structures and thicknesses. In particular, films with specific mechanical
properties can be constructed by applying appropriate deposition conditions, or
environmental conditions post self-assembly. These factors have been shown to affect
the Young's modulus of films by altering either the strength of the intermolecular bonds,
density, or the resultant nanostructure within the films.154
The properties of hydrogen-
bonded thin films and coatings could be further customised by the inclusion of
nanoparticles and functional molecules into the film structure. This concept has already
been demonstrated using gold nanoparticles223
, layered double hydroxide
nanoparticles224
and organosilicate materials225
and has the potential to be extended to a
range of other uncharged particles.
6.2.3 Graphene Based Materials in Hydrogen-bonded Multilayer Films
One type of particle which has great potential in the production of hydrogen-bonded
thin films and coatings with enhanced properties is graphene. Graphene is particularly
suited for use in functional thin films due to its remarkable electrical conductivity,
tensile strength and gas barrier properties. For example, thin conductive multilayer
films containing graphene and polyelectrolytes have already been shown to act as
effective oxygen barrier materials.226
Electromagnetic shielding is another potential
application for graphene based coatings. Consequently, the ability to incorporate
graphene into hydrogen-bonded LbL multilayer thin films is an attractive research
opportunity, as it provides a potential route to creating responsive films for use in a
range of applications such as food packaging, solar cells, and as a coating to supplement
the existing gas barrier properties of thin polymer films.
While there are several examples where pristine graphene particles or graphene
derivatives have been used to assemble electrostatic multilayer films in the literature,
123
very few studies describe the assembly hydrogen-bonded multilayer films using weak
polyelectrolytes and dispersions of surfactant stabilised graphene. In one study,
Gokhale et al.226
used PEI coated graphene nanoparticles and PAA to form multilayer
films via the LbL technique. Hydrogen bonding was promoted by introducing the PEI
coated graphene nanoparticles into the multilayer at a pH above the pKa of PEI.
Similarly, PAA was introduced into the multilayer at a pH below its pKa. This ensured
the carboxylic acid groups of the PAA and amine groups of the PEI existed in a largely
uncharged state. However, these polyelectrolytes are known to produce electrostatically
assembled multilayers in their charged states.206
As a result, residual charges on the
species may cause electrostatic interactions within localised regions of the hydrogen-
bonded multilayer film. These electrostatic interactions are not ideal as they limit the
extra functionality that may arise due to hydrogen bonding.227
Consequently, prior to the study61
upon which this chapter is based, there was a distinct
lack of experimental data describing systems which employ purely non-electrostatic
interactions such as hydrogen bonding, in the fabrication of LbL multilayer films
comprised of surfactant stabilised graphene and polyelectrolyte. Additionally, the
growth profiles and internal structures of these films required investigation and had the
potential to provide valuable information in assessing and improving the composition
and manufacture of the films. The mechanical properties of hydrogen-bonded
multilayer films containing surfactant stabilised graphene and polyelectrolyte were also
largely unknown.
Therefore, in order to address these deficiencies, the study presents a method of
assembling thin films containing defect free, surfactant stabilised graphene particles,
using hydrogen bonding. These films were prepared via the LbL approach, using the
weak anionic polyelectrolyte PAA, and pristine graphene particles stabilised with
124
Pluronic F108. Hydrogen bonding between the PEO portion of the surfactant and PAA
is sufficient to produce stable multilayer films in the absence of electrostatic attraction
between the anionic graphene particles, adsorbed non-ionic surfactant and weak anionic
polyelectrolyte. In this project, evidence obtained from quartz crystal microbalance
measurements and Raman spectroscopic information was used to support a proposed
film growth pattern and internal film structure for the films. The ability to remove the
films from surfaces was also investigated by exposing the films to aqueous solutions
with varying pH levels, and may reveal potential for the films in controlled release
applications.
6.3 Materials
The materials listed in §4.2.2 were again used in this study. The cationic
polyelectrolyte, PEI (30% aqueous, Mn ~ 70 kDa, branched) and anionic
polyelectrolyte, PAA (25%, Mn ~ 90 kDa) were also obtained from PolySciences Inc. 5
MHz quartz crystal resonators (Electrode Materials: 10 nm Ti followed by 100 nm Au,
Coating: 100 nm SiO2) were obtained from International Crystal Manufacturing Co. Inc.
Two types of substrates were used in the study: silicon wafer and quartz microscope
slides. Silicon wafer was obtained from MEMC Electronic Materials Inc. and cleaved
into pieces approximately 50 mm × 20 mm. These substrates were subsequently
cleaned using CO2 snow cleaning and rinsed with distilled ethanol and Milli-Q grade
water. Quartz microscope slides (25 mm × 51 mm × 0.5 mm) were purchased from SPI
Supplies. The slides were soaked for 15 mins in NaOH (10%) and rinsed Milli-Q grade
water. Both substrates were then plasma cleaned for 2 mins with water vapour.
125
6.4 Methods
6.4.1 Preparation of Graphene Suspension
Graphene suspensions were prepared using the method of ultrasonic exfoliation of
graphite, with continuous surfactant addition.116
A 10% w/w solution of Pluronic F108
(180 mL) was prepared and added drop wise at a rate of one drop per second to a 2%
w/w suspension of graphite powder in Milli-Q water (196 mL) under ultrasonication for
140 mins at 40 W. The suspension was then centrifuged at 2500 rpm for 20 mins to
sediment larger, non-exfoliated graphite particles. The resulting suspension was
dialyzed (Dialysis tubing, 100 kDa MWCO, Spectrum Laboratories) against Milli-Q
water for a minimum of 48 hours to remove unadsorbed surfactant from the stock
solution.
A method based on the recent study by Lotya et al was used in order to determine the
graphene concentration of the prepared suspensions.9 Samples of the graphene
suspensions were diluted by a factor of 20 and light absorption measured using UV-Vis
spectroscopy. Applying the Beer-Lambert law to the absorption intensity of the
samples at a wavelength of 660 nm and an extinction co-efficient112
, ε, of 54.22 L g-1
cm-1
, yielded an average undiluted graphene concentration of 0.0358 mg/mL (See
Appendix, §A.2.1). This extinction coefficient differs from the one previously used in
this thesis, as it has been shown to deliver a more accurate estimate of the graphene
concentration and provide a lower estimate of the graphene concentration.
6.4.2 Adsorption Measurements – Deposition and Removal of Thin Films
Multilayer films containing graphene particles and polyelectrolytes were prepared using
the layer-by-layer deposition technique on silica surfaces. In this study, the formation
126
of multilayer films was monitored using QCM (KSV QCM-500) fitted with a 5 MHz
quartz crystal resonator. First, a baseline was established with Milli-Q water at pH 4. A
solution of 100 ppm PEI at pH 4 was added to the cell and the PEI was allowed to
adsorb onto the silica surface for 10 mins, followed by a rinsing step with Milli-Q water
at pH 2 for 2 mins. A solution of PAA (100 ppm, pH 2) was then introduced into the
chamber for 2 mins, followed by another rinse cycle with Milli-Q water at pH 2 for 2
mins. It was necessary to form a precursor layer of PEI, a cationic polyelectrolyte, on
the negatively charged silica surface first, in order to create a surface to which PAA
may adsorb electrostatically. Following the adsorption of PAA, a suspension of
graphene (concentration of 10% v/v of the stock suspension) adjusted to pH 2 was then
injected into the chamber for 2 mins. This was followed by rinsing with Milli-Q water
at pH 2 for two mins to produce a bilayer. The rinse steps and addition of PAA and
graphene were repeated until 20 layers of PAA and graphene were deposited.
During construction of the multilayer on the resonating quartz crystal surface, the
change in resonant frequency (∆𝑓0) and overtones (∆𝑓3, ∆𝑓5, ∆𝑓7) were monitored. The
third overtone (∆𝑓3) was again selected for subsequent analysis. In this study, the
change in frequency was converted to a sensed mass using the simplified Sauerbrey
equation (Equation 5.1) 162
. It is appropriate to apply the Sauerbrey relationship to this
system as the 10 PAA/Graphene bilayer system investigated in this study had a
maximum average difference between overtones less than 12.99 Hz, satisfying the
criteria for the Sauerbrey equation (See Appendix, §A.2.2)
The removal of the multilayer films with pH adjusted water was also studied using the
QCM. Following the construction of the films, two separate rinse regimes were
applied. In one series of experiments, pH 9 adjusted Milli-Q water was injected into the
chamber for 5 mins. This was followed by the introduction of pH 2 adjusted Milli-Q
127
water into the chamber for 5 mins, then pH 4 adjusted Milli-Q water for 5 mins. The
other series of experiments involved injecting unadjusted Milli-Q water into the
chamber for 5 mins, followed by pH 2 adjusted Milli-Q water for 5 mins.
6.4.3 Preparation of Dip Coated Multilayer Films
Multilayer films of graphene and PEI were constructed using a KSV NIMA dip coater
multi vessel system. Samples were prepared on each type of substrate by first dipping
the substrates in 100 ppm PEI at pH 4 for 10 mins. The substrates were then dipped in
100 ppm PAA at pH 2 for 2 mins, followed by 10% v/v graphene suspension at pH 2 for
2 mins. This was repeated until the desired number of layers was assembled. The
substrates were immersed and withdrawn from the solutions at a rate of 100 mm/min.
6.4.4 Thin Film Characterisation
Optical microscopy and surface profilometery were used in order to investigate the film
coverage of the dip-coated samples. Optical micrographs were obtained using a Leica
DM4000 M system. The surface topology of the samples was measured using a Dektak
XT (Bruker) stylus profiler complete with Vision 64 software. In all measurements, a
12.5 μm radius stylus was used to scan a 15000 μm distance from the uncoated portion
across to the dip coated portion of the sample. Each scan was performed for 400 s at a
resolution of 0.125 μm and with a stylus force of 1 mg.
The composition of the dip-coated thin films was characterised using both Raman
spectroscopy and UV-Vis spectrophotometry. Raman spectra were recorded for the
samples on silicon wafer using a Horiba Jobin Yvon Raman system, with 633 nm
excitation laser. The UV-Vis spectrum of samples prepared on quartz substrates was
128
measured using a Cary 5000 UV-Vis spectrophotometer over the wavelength range of
200 - 800 nm.
6.4.5 Imaging of Surface Structure of Multilayer Films
Topographical analysis of the samples was performed using AFM techniques. 2D
topographical images of the multilayer surfaces were obtained using a MultiMode 8
AFM (Bruker) with ScanAsyst (tapping mode). ScanAsyst Air Si3N4 cantilevers
(Bruker) with a nominal spring constant of 0.4 N/m were used. The scan rate was
typically between 0.5 and 0.250 Hz.
AFM was also used to characterize the mechanical properties of the multilayer films. A
MultiMode 8 AFM complete with NanoScope 9 software was used in PeakForceTM
QNM mode to determine the height, DMT modulus, deformation, dissipation and
adhesion at each point across 1 μm × 1 μm sized areas of the prepared samples. A scan
rate of 0.25 Hz was used for all QNM measurements. The deflection sensitivity and tip
radius were calibrated for each probe prior to use, against standard sapphire and
titanium surfaces respectively. Measurements were performed in a closed environment,
where the relative humidity was altered by the use of a saturated solution of NaCl or
LiCl to achieve an average relative humidity of 26.7 and 59.5% respectively. As the
Poisson’s ratio for the samples was unknown, the value of the Poisson’s ratio was set to
0 in the NanoScope software in order to equate the measured DMT modulus values with
the reduced Young’s modulus.
129
6.5 Results and Discussion
6.5.1 QCM Adsorption Measurements
Multilayer films consisting of a single PEI precursor layer and 20 layers of PAA and
surfactant exfoliated graphene were assembled from suspension using LbL deposition
on silica surfaces. Figure 6.2 shows the total adsorbed mass of the thin films as a
function of layer number. The thin film was constructed with an initial adsorption of
PEI at pH 4, followed by alternating adsorption of PAA and graphene at pH 2. Upon
subsequent additions of graphene and PAA, the mass of the adsorbed film increases,
indicating the successful construction of a multilayer film. Furthermore, the overall
adsorbed mass of the films is significantly higher than those achieved for
electrostatically assembled thin films that incorporate graphene shown in Chapter 5.
Figure 6.2: Evolution of the Sauerbrey mass of the film as a function of the number of adsorbed
layers formed by deposition of 100 ppm PEI at pH 4 (black square), then deposition of 100 ppm
PAA and 10% graphene suspension at pH 2. Odd numbered layers (except the first layer)
indicate the addition of graphene suspension (red diamonds), while even numbered layers are
associated with additions of PAA (blue triangles).
0
20
40
60
80
100
120
140
160
180
200
0 5 10 15 20 25
Sau
erb
rey
mas
s o
f fi
lm
(mg/
m2)
Number of Layers
PEI
PAA
Graphene
130
The evidence of film growth and the chemical nature of the particular molecular species
employed in this system suggest that assembly of the films is dominated by hydrogen
bonding interactions. PAA and Pluronic F108 were selected for use in this study to
exploit the known hydrogen bonding interaction between PAA and PEO groups at low
pH, which have previously been used to successfully construct hydrogen-bonded
multilayer films206
. When graphene is exfoliated from graphite with the use of a
surfactant, surfactant species prevent reaggregation of the nanosheets by adsorbing to
the basal plane of graphene. In the case of Pluronic F108, the surfactant adsorbs
irreversibly to the basal plane of the graphene sheets through hydrophobic interactions
with the PPO portion of the molecule. It has been shown previously that graphene
sheets exfoliated with Pluronic F108 exhibit only a slight negative charge at low pH136
.
As the adsorbing surfactant Pluronic F108 is non-ionic, this charge is assumed to arise
from oxygenated moieties occurring at the edges of the graphene sheets. Meanwhile,
the carboxylic acid groups on PAA have a pKa of 4.28 228
and therefore most of the acid
groups belonging to PAA are protonated at pH 2. As a result, PAA and the PEO groups
present in Pluronic F108 should possess minimal charge at pH 2. Furthermore, under
these solution conditions, both the graphene particles and the PAA can exhibit only
residual negative charges, that when coupled together, give rise to repulsive electrostatic
interactions that do not promote film assembly. Thus, hydrogen bonding is responsible
for assembly of the film, and predominantly occurs between the protonated carboxylic
groups of the PAA and the ether functional groups on the PEO chain.
The thin films prepared and shown in Figure 6.2 exhibit a non-linear, exponential
growth profile. Previous studies have shown that the multilayer growth regime is
related to the internal structure of the multilayer229, 230
. For thin films comprising of
several layers, the growth profile typically proceeds either linearly or exponentially197,
131
231, 232. Linear film growth occurs due to attractive intermolecular interactions occurring
between the adsorbing molecules and the exposed deposition surface, with further
deposition and build-up of the layer occurring until the intermolecular interactions of
the previously exposed surface are compensated. Further film growth is suppressed by
repulsive interactions within the adsorbing layer, resulting in a stratified internal
structure232
.
Conversely, exponential film growth is currently understood to arise from the diffusion
of adsorbed species into and out of the multilayer film. In this process, molecules first
adsorb from solution to the exposed surface of the film via attractive intermolecular
interactions, then diffuse through the bulk towards the substrate. Diffusion out towards
the surface of the film also occurs during this process194
. Consequently, films that grow
exponentially characteristically have a homogeneous internal structure. In the film
prepared here, the bulk region of the film is unlikely to be highly stratified based on the
exponential growth pattern exhibited in Figure 6.2.
Further information regarding the structural arrangement of the film is provided by the
difference in mass observed between consecutive deposition steps (Figure 6.3), coupled
with the dissipation of the film (Figure 6.4) yields. The region of the film nearest to the
substrate exhibits markedly different trends to that of the bulk film, with interactions
between the initial PEI layer and the substrate affecting the deposition of the first four
layers193
. In Figure 6.3, the graphene additions show an increasing, linear trend in the
change in the mass adsorbed after each addition of graphene suspension to the system at
higher numbers of adsorbed layers. This suggests that the film is capable of sustained
multilayer growth. In Figure 6.4, the dissipation of the film increases upon the
adsorption of stabilised graphene onto the multilayer film, consistent with the formation
of a softer, more viscoelastic film during these adsorption steps. The increase in
132
viscoelasticity is most likely due to entrained water within the graphene layer, which
arises due to the hydration layer surrounding the PEO portions of the adsorbed
surfactant molecules. PEO is known to form hydrogen bonds with 2-3 water molecules
per repeat unit.233
Therefore these chains are likely to exist in an expanded
configuration within the adsorbed graphene layer, promoting the entrainment of water
within the layer when first adsorbed.
Figure 6.3: The change in the Sauerbrey mass following the addition of a layer, as a function of
the layer adsorbed to the film. The film was formed by deposition of 100 ppm PEI at pH 4, then
alternating deposition of 100 ppm PAA and 10% graphene suspension at pH 2. Odd numbered
layers (except the first layer, not shown) correspond to the change in Sauerbrey mass following
the addition of graphene solution (red diamonds), while even numbered layers are associated
with the addition of PAA (blue triangles). Trend line is associated with data points for graphene
additions.
-10
-5
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25Sau
erb
rey
Mas
s o
f la
yer
(mg/
m2)
Number of Layers
PAA
Graphene
133
Figure 6.4: Dissipation as a function of the number of adsorbed layers formed by deposition of
100 ppm PEI at pH 4 (black square), then deposition of 100 ppm PAA and 10% graphene
suspension at pH 2. Odd numbered layers (except the first layer) indicate the addition of
graphene suspension (red diamonds), while even numbered layers are associated with additions
of PAA (blue triangles).
Meanwhile, the decrease in adsorbed mass following the addition of PAA as shown in
Figure 6.3, suggests either desorption of material from the surface of the film, or the
displacement of large masses of entrained water from the film during adsorption of
PAA. The latter is strongly supported by the trend in dissipation data, where the overall
dissipation of the film decreases following each PAA step, suggesting greater overall
film rigidity. Presumably strongly bound water molecules associated with the PEO
chains are removed through the formation of hydrogen bonds between the acid and
ether groups of the respective adsorbing species.
The decrease in mass of the resulting layer and increased rigidity of the multilayer films
upon adsorption of PAA shown in Figure 6.3 and Figure 6.4 also supports the trend in
exponential film growth. Of the species that constitute an exponentially grown
multilayer film, the more mobile molecular species generally favour diffusion
throughout the film.230, 231
Figure 6.3 and Figure 6.4 show a decrease in adsorbed mass
0
10
20
30
40
50
60
70
0 5 10 15 20 25
Dis
sip
atio
n (
x 1
0-6
)
Number of Layers
PEI
PAA
Graphene
134
and a decrease in dissipation of the film upon consecutive additions of PAA. This is
consistent with PAA diffusing and interdigitating into the bulk, increasing film stiffness
by promoting hydrogen bonding interactions with the PEO chains. Thus, an adsorption
mechanism where PAA diffuses throughout the polyelectrolyte film is suggested.
Consequently, given that PAA displaces water from the film during adsorption, the
diffusing species in the film is most likely PAA. This is reasonable due to the relative
size of the PAA molecules compared to the graphene particles, and the kinetically
favoured orientation of the graphene particles. This orientation is expected to maximize
contact of the particle surfaces with the active surface of the film by adsorbing flat
against the active surface. As the graphene particles have a lateral size of
approximately 100 – 300 nm based on previous work 116
, this is likely to prevent further
diffusion of the particles into the film. However, the diffusion of PAA throughout the
film indicates that the graphene particles are neither large enough nor packed closely
enough to form a completely impermeable layer. Interestingly, the favoured orientation
of the graphene particles also suggests that bulk materials manufactured using the LbL
technique may exhibit anisotropic behaviour, despite the homogeneous internal
structure of the film.
6.5.2 Removal of Thin Films
The ability to at least partially remove or degrade functional thin films from surfaces
using pH adjusted aqueous solutions is a desirable property, as it allows an
environmental trigger to determine the performance of the film. For instance, it could
allow the controlled release of functional particles or additives embedded in the films.
To this end, multilayer films consisting of 5 bilayers were assembled through the
alternating, LbL deposition of PAA and surfactant stabilised graphene from suspension.
135
Following the construction of the films, two separate rinse regimes were applied. In
one series of experiments, pH 9 adjusted Milli-Q water was injected into the chamber
for 5 mins. This was followed by the introduction of pH 2 adjusted Milli-Q water into
the chamber for 5 mins, then pH 4 adjusted Milli-Q water for 5 mins. The other series
of experiments involved injecting unadjusted Milli-Q water into the chamber for 5 mins,
followed by pH 2 adjusted Milli-Q water for 5 mins.
Figure 6.5 shows the adsorbed mass of a thin film rinsed with unadjusted Milli-Q grade
water, as a function of the number of solutions exposed to the active surface. In this
instance, the adsorbed mass increases during the assembly of the multilayer film then
decreases sharply when rinsed with pH unadjusted water. A slight increase in mass of
the film is subsequently observed when rinsed further with pH 2 water. Overall, with
this pH rinsing method, the film mass decreases by 14.0%. Similarly, when the
constructed film is rinsed with an aqueous solution at pH 9, then with an aqueous
solution at pH 4, the film mass decreases by 23.0% (See Appendix, §A.2.3, Figure A.5).
The permanent reduction in mass, despite restoration of the original pH used during
assembly of the film indicates the change in mass is not due to reversible collapse of the
film and expulsion of water from the bulk. Here, the PAA groups may undergo
protonation again due to the second rinse cycle with water at a pH below the pKa
required to deprotonate the PAA groups. This could facilitate hydrogen bonding
between PAA and water, causing the film to swell. This is consistent with the small
increase in film mass observed following the second rinse cycle. Thus, the reduction in
mass indicates the permanent deterioration and removal of material from the film.
The rinse treatments investigated in this study are likely to drive degradation of the
films by disrupting the hydrogen-bonded structure of the films. PAA/PEO multilayer
films have been shown to disintegrate at > pH 3.6 due to the deprotonation of the
136
carboxylic acid groups on PAA, which affect the hydrogen-bonded structure of the
film.211
Similarly, for the films presented here, at pH levels greater than pH 4.28, the
majority of carboxyl groups on the PAA are deprotonated. This causes strong charge
repulsion between the anionic graphene nanosheets and PAA. When the pH is further
increased to pH 9, stronger charge repulsions are possible as the magnitude of the
negative charge around the edge of the graphene particles increases. By combining
consecutive rinse cycles, it is possible to increase the amount of material removed from
these films (See Appendix, §A.2.3, Figure A.6).
Figure 6.5: Sauerbrey mass as a function of the number of additions of solution to the QCM
chamber. The film formation phase shows the construction of a thin film with an initial
precursor layer of PEI (black square), with alternating deposition of surfactant stabilised
graphene (red diamonds) and PAA (blue triangles), followed by partial removal of the film with
pH unadjusted water (purple cross) and final rinse with pH 2 adjusted water (green asterisk).
6.5.3 Thin Films Formed Through Dip Coating
Thin films prepared with 100, 206, 300 and 400 layers of PAA and surfactant exfoliated
graphene were constructed on a variety of surfaces using dip coating. Optical
microscopy and surface profilometery were used to confirm the deposition and film
0
1
2
3
4
5
6
7
8
0 2 4 6 8 10 12 14
Sau
erb
rey
mas
s o
f fi
lm (
mg/
m2)
Number of Layers
PEI
PAA
Graphene
pH Unadjusted Rinse
pH 2 Rinse
Film Formation Rinse
137
coverage of the dip-coated samples (See Appendix, §A.2.4). Spectroscopic
measurements were then performed on the samples in an effort to confirm that the
graphene particles were retained during the dip coating process and if multilayer thin
films were also successfully constructed. The UV-Vis spectrum of a sample comprised
of a single layer of PEI and 400 graphene/PAA layers was measured using a Cary 5000
UV-Vis spectrophotometer over the wavelength range of 200 - 800 nm (See Appendix,
§A.2.5).
The dip coated thin films were also characterised using Raman spectroscopy. Raman
spectra were recorded for the samples on silicon wafer using a Horiba Jobin Yvon
Raman system, with 633 nm excitation laser. Raman spectroscopy measurements were
performed on samples consisting of between 100 and 400 layers of PAA and graphene
adsorbed onto silicon substrates (Figure 6.6). These measurements were unable to be
performed on the thin films prepared using with QCM, due to the small number of
adsorbed layers which results in a low signal-to-noise ratio. The spectra of the samples
show three main peaks located at approximately 1332, 1579 and 2664 cm-1
, named the
D, G and 2D peaks. These peaks indicate the presence of graphitic material in the
sample and are associated with sp2 carbon breathing modes (indicative of the proportion
of sp3 carbon in the sample), in plane bond stretching between sp
2 carbon atoms
(indicative of the proportion of sp2 carbon), and the number of graphene layers in a
sample.234
The ratio of the peak intensity between the D and G peaks in each sample is
consistent with the presence of defect-free graphene sheets. Furthermore, the single 2D
peak at 2664 cm-1
is symmetrical and down shifted from 2700 cm-1
, consistent with the
presence of single and few-layer graphene6, 123
.
138
Figure 6.6: Raman spectrum of multilayer films consisting of a single layer of PEI deposited at
pH 4, followed by alternating layers of PAA and Graphene at pH 2, prepared through dip
coating. Inset shows the Raman peak intensity at 1580 cm-1
as a function of the number of
adsorbed layers.
The Raman spectra of the dip coated thin films also provides further information
regarding the growth process of the films, and therefore yields evidence of their internal
structuring. Concentrations of nanoparticles have been shown previously to be directly
related to the peak intensity of its corresponding Raman spectra.235
In particular, highly
stratified multilayer films containing 2D materials and polyelectrolyte have been shown
to exhibit a linear relationship between the peak intensity of the material and the number
of layers in the film.236
The inset in Figure 6.6 shows the relationship between the peak
intensity from Raman spectra of the samples, and the number of layers deposited on the
substrate. This relationship is linear in nature, which shows the adsorption of the PAA
and surfactant stabilised graphene forms discrete layers in the multilayer film.
The QCM measurements and Raman spectra presented in Figure 6.2 and Figure 6.6
suggest two distinct film growth profiles, despite the same chemical species used in the
-50
0
50
100
150
200
250
300
1200 1700 2200 2700
Inte
nsi
ty (
a.u
.)
Raman shift (cm-1)
0 layers (Substrate)
101 layers
207 layers
301 layers
401 layers
R² = 0.9996
-100
0
100
200
300
0 100 200 300 400
Inte
nsi
ty (
a.u
.)
Number of Layers
139
construction of the films. The difference in observed film growth between the two
characterization techniques could be the result of the different deposition methods used
to prepare the samples. Flow cell properties and shear effects may influence film
assembly to such a degree that the two multilayer systems demonstrate distinct film
growth regimes. Polyelectrolytes deposited through the LbL process have been shown
previously to reorient upon drying237
, possibly affecting the film growth profile.
Alternatively, the two distinct growth patterns observed from QCM and Raman spectra
measurements may form different parts of a single growth pattern, a phenomena known
as superlinearity. Superlinear growth profiles are characterised by a transition from
exponential to linear growth193
at large numbers of deposited layers and have been
previously observed in a number of polyelectrolyte multilayer systems193, 206, 238, 239
,
including PAA/PEO multilayer films. A model for the process was proposed by
Hübsch et al.232
and later by Salomäki et al.197
. During the initial stages of this process,
the adsorption of alternating layers of chemical species proceeds exponentially. Like
systems that demonstrate purely exponential growth, this phase requires constant
diffusion of one of the species through the multilayer towards the substrate surface,
creating a diffuse zone in which the structure of the multilayer is homogeneous. As the
number of adsorbed layers increases, eventually diffusion of the more mobile species
decreases due to rearrangement of the chemical species in order to promote the most
favourable interactions in the film. As a result, the area nearest to the substrate surface
undergoes restructuring to form distinct, impenetrable layers. When diffusion of the
species entering and leaving the diffusion zone reaches equilibrium, the diffusion zone
begins to extend away from the substrate as the number of distinct layers increases. The
growth rate of the multilayer film is dependent only on the amount of chemical species
that reach the restructured zone, causing linear growth to dominate at higher numbers of
140
adsorbed bilayers. The two growth profiles resulting from Raman and QCM data in the
current study supports this proposed method of film growth. Interestingly, if the
superlinear profile applies to the films presented here, this implies that the internal
structure of the film varies with the number of adsorbed layers.
Indeed, the superlinear growth mechanism supports details of the QCM data shown in
Figure 6.3 which are not adequately addressed by the exponential growth mechanism
alone. In particular, the superlinear growth mechanism is consistent with the linear
change in mass of the film when graphene is adsorbed (Figure 6.3), as well as the loss
of mass from the film when PAA is adsorbed during the initial stages of film formation.
Assuming a superlinear growth regime and diffusion of PAA, it is expected that fewer
hydrogen bonding interactions are available to the PAA close to the substrate upon
successive deposition steps of PAA and consequently, an increasingly higher PAA
concentration is likely in the upper most layers of the film during the initial stages of
film formation. This could be the cause of the observed linear increase in the change of
the mass of the film upon the addition of graphene, resulting from an increase in the
number of hydrogen bonds possible between the PEO and PAA at the outermost layers.
The superlinear growth mechanism also explains the increasing ability for PAA to
collapse the PEO hydration, which appears as a loss of mass from the film and
increased film rigidity (Figure 6.3 and Figure 6.4). As more graphene is adsorbed onto
the film, more entrained water is likely to be present in the graphene layers due to
hydrogen bonding with the PEO groups and therefore greater loss in mass is
experienced upon each addition of PAA. As the diffusion of PAA into the film and the
restructuring zone reaches equilibrium, the concentration of PAA in the outermost
layers of the film are expected to remain constant, resulting in a linear growth pattern
141
The switch between exponential and linear growth in a superlinear growth regime
typically occurs when only a few bilayers have been adsorbed onto the substrate. It is
currently understood to coincide with the point at which the rate of diffusion of
molecules from out of the film reaches equilibrium with the rate of diffusion of
molecules into the film. Transition between the linear and exponential regimes is
dependent on the strength of the polyelectrolytes involved, but literature values indicate
this changeover point commonly occurs when between 14 and 36 layers have been
deposited.193, 206, 239
This range agrees with growth profiles obtained from the QCM and
Raman peak intensity data presented in this study, which indicates the switch between
exponential and linear film growth took place between 20 and 100 layers of PAA and
graphene.
6.5.4 Physical and Mechanical Properties of Thin Films
AFM measurements were obtained in order to sample the surface roughness and
mechanical properties of the prepared films following Raman spectroscopic analysis.
10 μm × 10 μm areas were scanned for each of the samples comprised of 100, 206, 300
and 400 layers of PAA and F108 graphene on silica, to determine surface roughness
(Figure 6.7). The AFM measurements indicated the average Rq value of the samples
ranged between 8.622 nm and 13.782 nm. Furthermore, there was an increase in
average surface roughness with number of adsorbed bilayers. This is consistent with an
increasing number of layers being adsorbed to the substrate surface in the absence of
rinsing steps237
, and can be explained by the conformation of adsorbed species on the
surface. Unlike the more mobile PAA, the lateral size of surfactant stabilised graphene
prevents the sheets from closely following the topography of the surface. As a result,
the underlying surface morphology is amplified when each succeeding layer of material
142
is deposited randomly from solution, causing the surface roughness to increase with
number of adsorbed layers.
Figure 6.7: Surface roughness as a function of the number of adsorbed layers in films
consisting of a single PEI precursor layer and 100-400 layers of PAA and Graphene at pH 2
over a 10 μm × 10 μm area.
QNM measurements were also performed on the dip coated samples in order to
determine various material and mechanical properties for the multilayer films.
NanoScope presents each of the properties as maps of the scanned surface, where the
parameter of interest is expressed as the vertical height (See Appendix, §A.2.6, Figure
A.10). Further examination of the nanomechanical mapping scans obtained in this
experiment showed areas with two distinct combinations of mechanical and
topographical features. These areas appeared to be particle reinforcement distributed
across a matrix. Figure 6.8 shows the average values for the topographical height,
reduced Young’s modulus, adhesion, dissipation and deformation of the particles and
the matrix, measured from the lowest point on the scan. Measurements were performed
at an average relative humidity of 59.5%. Each particle data point is the average data of
10 particle areas sampled on each of the three scans. Each matrix data point is the
average of typical matrix areas sampled on each of the three scans.
0
2
4
6
8
10
12
14
16
0 100 200 300 400 500
Surf
ace
Ro
ugh
ne
ss, R
q
(nm
)
Number of layers
143
(a) (b)
(c) (d)
(e)
Figure 6.8: QNM measurements for the (a) vertical height, (b) reduced Young’s modulus, (c)
adhesion, (d) deformation and (e) dissipation of thin films comprised of a single PEI precursor
layer and 100 - 400 layers of PAA and surfactant stabilised graphene. The red squares
correspond to measurements of the matrix, while blue diamonds correspond to measurements of
particles.
0
1
2
3
4
5
0 100 200 300 400
Hei
ght
(nm
)
Number of Layers
0
50
100
150
200
0 100 200 300 400
DM
T M
od
ulu
s, R
edu
ced
Y
ou
ng'
s M
od
ulu
s (
MP
a)
Number of Layers
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 100 200 300 400
Ad
hes
ion
(n
N)
Number of Layers
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 100 200 300 400
Def
orm
atio
n (
nm
)
Number of Layers
0
20
40
60
80
100
0 100 200 300 400
Dis
sip
atio
n (
eV)
Number of Layers
144
The QNM measurements indicate a clear difference in topography between the two
different areas observed in the nanomechanical scans. Figure 6.8a shows a difference in
vertical height between the matrix and particle which varies little with the number of
layers adsorbed. The lateral size of the particles is consistent with surfactant stabilised
graphene adsorbing along its basal plane.123
Compared to the matrix, the thickness of
the particles is between 0.36 and 0.77 nm, slightly less than the reported thickness of
single graphene sheets stabilised with surfactant.125
Surfactant chains adsorbed to the
graphene surface may penetrate the underlying film, thereby reducing the thickness of
the particles compared to the matrix surface.
Figure 6.8b shows the reduced Young’s modulus for the matrix and particles areas. The
samples all possessed an average reduced Young’s modulus of below 157.41 MPa,
irrespective of the number of layers adsorbed or the scanned areas considered. In
contrast, the modulus of silicon substrates in the < 1 0 0 > direction, normal to the
deposition surface is 130 GPa.201, 240
The difference in modulus between the samples
and uncoated silica is consistent with the overall adsorption of an elastic film onto an
inelastic substrate. Furthermore, this indicates that the films cover the substrate surface
and that areas of exposed silicon wafer are not responsible for the higher DMT moduli
observed at low numbers of adsorbed layers.
It is clear that a non-linear relationship exists between the reduced Young’s modulus of
the films and the number of layers deposited. The reduced Young’s modulus of the
samples, was shown to decrease sharply from 134.82 -157.49 MPa to 6.07 - 32.3 MPa
for films containing over 203 layers of PAA and surfactant-stabilised graphene,
regardless of the area considered. The trend in reduced Young’s modulus is consistent
with soft, thin films of increasing thickness being adsorbed onto rigid substrates, and
may be caused by the thickness of the film relative to indentation of the tip. The DMT
145
modulus of the films is measured based on the portion of the force curve obtained
during retraction of tip, when the tip is in the contact regime. For thinner films, the tip
is likely to indent a shorter distance during this contact regime before experiencing
compression of the film or the effect of substrate/tip interactions. However, as
ScanAsyst control was enabled throughout the scans, the set-point is optimised to
ensure the minimum force required to image the samples is applied. The high modulus
values are therefore unlikely to arise from uncontrolled penetration of the tip through
film onto the substrate. Consequently, as an increasing number of bilayers are adsorbed
to the substrate, the effect of the stiff underlying substrate on the measured elastic
modulus diminishes, and the lower Young’s modulus of the bulk film dominates the
measurement. Therefore, the reduced Young’s Moduli of samples with 300 and 400
layers of PAA and surfactant stabilised graphene are likely to represent that of the bulk
multilayer film.
Lower moduli are observed for the matrix compared to the particles, further supporting
the notion that the patch areas are adsorbed graphene sheets. However, the reduced
Young’s moduli of the graphene areas vary significantly from the out of plane Young’s
modulus for even bilayer graphene, which is predicted to be 25 MPa.241
At higher
numbers of adsorbed layers (i.e 301 layers) this can be attributed to the presence of the
multilayer film beneath the graphene sheet, which may lower the resistance to bending
of the sheet as the AFM tip indents the surface during a scan. As the number of layers
increases further, the overall rigidity of the film increases due to the higher proportion
of graphene incorporated into the film network. Additionally, graphene sheets
embedded in the film may be screened during QNM measurements due to the overlying
polyelectrolyte layers. Consequently, the overall reduced Young’s modulus value of the
146
film is likely to differ markedly from that of single layer, pristine graphene due the
proportion of PAA present in the film.
Figure 6.8 c - d shows the adhesion, dissipation and deformation of the samples as a
function of the number of adsorbed layers. It shows that for a given parameter, the
values are independent of the number of layers adsorbed. This is expected, as the
adhesion and dissipation are derived from the pull off force that is a measure of the
attractive forces between the tip and outer most layers of the samples, which should be
identical for all the samples. The dissipation measurements were also consistent
between all four samples due to the constant applied force used for all measurements, in
addition to the same adsorbed outer layers being present in all four samples.
A clear trend between the particles and matrix are observed for the adhesion, dissipation
and deformation of the samples, regardless of the number of layers adsorbed. For a
given film thickness, the matrix demonstrates greater adhesion compared to the particle
areas. This may be attributed to the electrostatic repulsive forces arising between the
negatively charged silicon nitride AFM tip and surfactant stabilised graphene sheets.
The results indicate that distinct molecular interactions take place on the surfaces of
each of these areas, which is consistent with exposed graphene particles located on the
surface. The deformation and dissipation of the matrix is also greater than that of the
surface particles, which is probably caused by the presence of viscoelastic
polyelectrolytes.
QNM measurements were also performed on dip coated thin films with 100 layers of
PAA and surfactant exfoliated graphene at an average relative humidity of 26.7%.
From these measurements, it was shown that the mechanical properties of the films
147
were related to humidity at mid to low levels (See Appendix, §A.2.6), with an increase
in the adhesion, deformation and dissipation observed at higher humidity.
The results of this study show that hydrogen-bonded multilayer films constructed from
graphene and polyelectrolytes through the LbL process hold great promise as a means
for creating tailored, functional thin films with varying mechanical properties. The LbL
assembly approach used to construct the graphene-polyelectrolyte hydrogen-bonded
films was shown to be applicable for both large and small numbers of adsorbed layers
using both manual and automated deposition processes. Study of the films also
provides greater understanding of the mechanical properties, internal structures and
growth profiles specific to hydrogen-bonded films containing graphene and
polyelectrolyte. By simply altering the number of adsorbed layers the desired elasticity,
surface roughness and structure of the films can be achieved. Furthermore, these films
exhibit an ability to disintegrate easily when rinsed with water at neutral and basic pH.
This behaviour could prove a desirable feature in the reuse or recycling of the
substrates, or draw awareness to possible limitations of these types of films, depending
on the intended application. Other combinations of surfactant and polyelectrolyte could
provide films with additional temperature responsiveness and structural crosslinking
properties, which could further enhance functional applications of the films.
Although it is beyond the scope of this study, there is also a strong possibility that the
prepared films may prevent the diffusion of gases through the film. Recent studies have
demonstrated the reduced oxygen permeability of both hydrogen-bonded graphene-
polyelectrolyte multilayer films226
and PEO/PAA multilayer films constructed at low
pH.242
In these cases the reduction in permeability was credited to the barrier properties
of graphene and the hydrogen-bonded network occurring between PEO and PAA
groups, respectively. Both elements feature in the films presented here, and are directly
148
related to the structural arrangement of the films. The results of our study indicate
either the shear effects arising from the use of different coating techniques, or a
superlinear growth profile can cause the same film system to exhibit different internal
structures, which affects the permeability of the film.243
Consequently, different
deposition technologies could affect the gas permeability of these kinds of films.
Alternatively, if superlinearity applies to these systems, the transition between growth
modes could indicate a point along the growth profile where the deposition of
subsequent layers becomes less efficient at preventing the passage of gas molecules than
previously adsorbed layers. In either case, if the films prepared in our study exhibit gas
permeability features, it could extend the functionality of the films to pH responsive,
oxygen-barrier coatings.
6.6 Conclusion
Production of responsive surface coatings containing novel materials is highly desirable,
with the process able to enhance thin film technology in a wealth of applications. The
layer-by-layer deposition of hydrogen-bonded multilayer films is one way in which to
create thin films that respond to environmental conditions. Here, a method for
successfully incorporating defect-free graphene nanoparticles into hydrogen-bonded
multilayer thin films, using the LbL deposition of surfactant stabilised graphene and
PAA from solution is presented. The formation of the thin films was facilitated through
hydrogen bonding of the carboxylic acid groups on the PAA and the ethylene oxide
groups on the adsorbed Pluronic F108, in the absence of attractive electrostatic
interactions.
QCM measurements were used to monitor the successful deposition and removal of
multilayer films consisting of a small number of surfactant stabilised graphene and PAA
149
layers on silicon substrates. The resultant films were stable at low pH and exhibited
non-linear growth overall, indicative of a homogeneous film structure. Rinsing the
films with water at neutral or basic pH resulted in degradation of the film network.
An extended number of layers were successfully deposited on the silicon substrates
using automated dip coating. Using Raman spectroscopy, the resultant films were
shown to contain pristine graphene particles unaltered by the dip coating process.
Furthermore, the spectra also suggested linear growth of the films at high numbers of
adsorbed layers, consistent with a highly stratified internal structure. The combination
of linear and exponential growth profiles at various numbers of adsorbed layers was
attributed to superlinear growth or shear effects during film assembly.
AFM QNM measurements were used to further characterize the mechanical properties
of the dip coated films. As the number of adsorbed layers increased, the reduced
Young’s modulus of the films was shown to decrease sharply reaching a plateau for thin
films with more than 300 adsorbed layers. At lower values of adsorbed layers, the
rigidity of the films was suggested to be the result of the underlying substrate, whilst the
plateau value represents the bulk reduced Young’s modulus of the films. The adhesion,
deformation and dissipation of the films were largely independent of the number of
adsorbed layers in the film. By defining the number of adsorbed graphene/PAA layers,
responsive thin films with various mechanical properties and internal structures can be
achieved for a desired application.
150
CHAPTER 7
Chapter 7
Foam Stabilization Using Surfactant
Exfoliated Graphene
7.1 Introduction
The ability to produce highly stable air-in-water foams is essential to a range of
industrially relevant processes and is the subject of continual ongoing research and
development. One method of enhancing the stability of aqueous foams is incorporating
surface active particles into the liquid phase, which prevent destabilisation and collapse
of the foam through adsorption at the bubble air-water interface. Particle affinity for the
air-water interface is highly dependent on a number of properties including particle size,
shape and wettability. Therefore, it is critical that a range of physical factors are
considered when selecting particles for use as foam stabilisers, with novel types of
particles providing a distinct opportunity to improve the stability and performance of
these foams.
Surfactant exfoliated graphene particles demonstrate distinct physical characteristics
compared to conventional foam stabilising particles. In particular, graphene particles
possess very high-aspect ratios, approaching those of two-dimensional materials. This
extreme geometry is expected to influence the energy of attachment upon adsorption at
the air-water interface, while reducing the overall mass of particles required to stabilise
air-in-water foams. Additionally, the presence of adsorbed surfactant on the graphene
surface may enable the overall particle wettability to be modified depending on solution
conditions. Despite these features however, the effect of pristine surfactant exfoliated
151
graphene and its characteristic particle geometry on foam stability has not been well
studied.
The experiments presented in this chapter aim to investigate the use of surfactant
exfoliated graphene in stabilising air-in-liquid foams. The chapter begins with an
overview of particle stabilised foams as described in the literature, including the
mechanisms and factors affecting foam stability. A brief outline of the materials and
techniques used to generate and characterise the foams is then given. The chapter ends
with a discussion of these results indicating the suitability of surfactant exfoliated
graphene as an air-in-water foam stabiliser. Many of the experiments, results and
associated discussions contained in this chapter were first published in peer reviewed
article244
(Reprinted from the Journal of Colloid and Interface Science, 469, Sham, A.
Y. W.; Notley, S. M., Foam stabilisation using surfactant exfoliated graphene, 196-204,
Copyright (2016), with permission from Elsevier).
7.2 Background
A wide variety of industrial applications across the fields of both chemistry and
engineering rely on the production of highly stable liquid foams in order to function
effectively. For instance, foams are used to achieve uniform textile dyeing in the
textiles industry and to improve the commercial value and perceived quality of personal
care products such as shampoo.245, 246
They are also employed extensively in mineral
flotation and separation processes, water and effluent treatment, firefighting foams,
materials processing, oil recovery, and in the food and beverage industries.246
In general, foams can be described as a concentrated dispersion of air bubbles
surrounded by a continuous liquid phase. The liquid phase typically appears as thin
152
layers around the bubbles that are joined in an interconnected network through regions
known as Plateau borders (Figure 7.1).247
Thus, foams characteristically possess a high
surface area per volume with respect to the interface and are not considered
permanently stable either thermodynamically or kinetically due to the number of highly
energetic air–liquid interfaces present.248
Instead, foams are usually categorised as
either unstable with lifetimes on the order seconds, or metastable, where foams exist for
hours or days before significant collapse is observed. Throughout the following
discussions however, metastable foams with lifetimes of days will be considered stable
in order to highlight superior stability. As foams possess inherent instability,
destabilisation processes play a key role in determining the foam persistence.
Figure 7.1: Schematic illustration showing the typical microstructure of an air-in-liquid foam.
7.2.1 Destabilising Foams
Three main mechanisms contribute to the destabilization of air-in-liquid foams, namely
disproportionation, drainage and coalescence (Figure 7.2).
153
Figure 7.2: Schematic illustration of the three main destabilisation mechanisms acting on air-in-
liquid foams, (a) disproportionation, (b) drainage and (c) coalescence.
Disproportionation occurs when gas diffuses from one bubble to an adjacent bubble and
is driven by dissimilar Laplace pressures within the two bubbles. The Laplace pressure
is defined as the internal pressure in each bubble relative to the pressure outside the
bubble, and is given by the Young-Laplace equation (Equation 7.1).
Δ𝑃 =2𝛾
𝑅1
7.1
Where:
Δ𝑃 = Pressure difference between the inside (𝑃1) and outside (𝑃2) of the
bubble (Pa)
𝑅1 = Radius of curvature of bubble (m)
𝛾 = Interfacial surface tension (Nm-1
)
𝑟2
𝑟1
𝑟1′
𝑟2′
154
From Equation 7.1, bubbles with smaller radii are likely to exhibit greater Laplace
pressures compared to larger bubbles and thus, the net gas diffusion flux is greatest
from small to larger bubbles. This causes small bubbles to shrink while larger bubbles
grow in size, coarsening the foam structure. In contrast, drainage involves the removal
of liquid under gravity from the thin lamellar film that separates two adjacent bubble
interfaces. This causes the liquid layer between the two bubbles to become
progressively thinner and more susceptible to rupture. The rate at which liquid drains
from the lamellar film has shown to be dependent on a number of factors including
temperature, and viscosity of the liquid in the bulk and at the surface.248
In general
however, the effect of drainage on foam stability is greatest during the initial stages of
foam degradation, and causes spherical gas bubbles to develop into polyhedral shaped
gas cells over time.248, 249
Bubble coalescence is another destabilization mechanism
related to drainage and occurs when the thin liquid film between two adjacent bubbles
ruptures, causing the bubbles to merge. The newly formed bubble possesses the same
volume as that of the two individual bubbles from which it was formed, yet experiences
an overall reduction in surface area. This destabilisation process results in coarsening of
the foam structure, and is usually preceded by drainage of the film.250
Foams are
typically destabilised by a complex combination of these three processes, which act to
promote deterioration and collapse the foam structure over time. Thus, by preventing
these mechanisms from occurring within the foam, it is possible to reduce the inherent
instability of foams and inhibit foam deterioration.
7.2.2 Foam Stabilisation Mechanisms
One common method of reducing the inherent instability of aqueous foams is through
the adsorption of surface active species at the air-water interface. Adsorption of surface
155
active species results in highly energetic air-water interfaces being replaced with less
energetic surfaces. This in turn, causes a reduction in the overall free energy of the
system, thereby stabilizing the foam structure. Materials that stabilise the bubble
interface through adsorption include classic foaming agents such as low HLB
surfactants (HLB < 10)251
in addition to polymers and proteins, as well as cations and
anions dissociated from inorganic salts248
. More recently, it has been found that solid
particles also provide effective foam stabilization, with252, 253
and without254, 255
the aid
of a surfactant.
Importantly, the thermodynamic properties specific to particles at the air-water interface
allow particles to enhance foam stability to a greater extent than traditional foaming
agents like surfactants. At equilibrium, surfactant adsorption and desorption at the air-
water interface occurs at relatively short timescales, with the energy of attachment
usually several kT per molecule.11
In contrast, the energy required to remove a
spherical particle from the air-water interface is given by Equation 7.2:
𝐸 = 𝜋𝑟2(1 ± 𝑐𝑜𝑠𝜃) 7.2
Where:
𝐸 = Energy of attachment (J)
𝑟 = Particle radius (m)
𝛾 = Interfacial surface tension (Nm-1
)
𝜃 = Contact angle the particle makes with the interface
Given the size of conventional particle foam stabilisers, the energy required to remove a
particle from the interface is typically several orders of magnitude greater than that of a
surfactant molecule. Therefore, due to the effect of particle size on attachment energy,
156
many particles can be considered irreversibly adsorbed at the interface in comparison to
surfactant molecules.
Particles are not only capable of improving the thermodynamic stability of foams, but
can also reduce the dominance of destabilisation mechanisms. For instance, it is
generally understood that surface active particles along the bubble surfaces can cause
repulsive interactions between adjacent bubble surfaces, thus forming a steric barrier to
coalescence.251
Sufficiently dense layers of particles at the bubble surface have also
been shown to serve as a physical impediment to disproportionation, thereby preventing
the shrinkage of bubbles and collapse of the foam. In some cases, particles present in
the aqueous phase of the film collect in the plateau borders and slow the drainage of the
interlayer film.254
However, the ability to stabilise foams using these three mechanisms
is highly reliant on a combination of factors related to the particles themselves.
Consequently, these factors require careful attention in order to predict which particles
may be used to stabilise foams.
7.2.3 Effect of Particle Properties on Foam Stability
The ability for particles to effectively stabilise foams is highly influenced by a number
of factors. These factors are often interdependent and can broadly be divided into three
aspects: particle wettability, concentration and the physical dimensions of the particle.
These characteristics are in turn, associated with other particle properties that effect
foam stability, including surface chemistry, size, shape, surface area and surface
roughness.
157
7.2.3.1 Particle Wettability
Particle wettability is one of the main factors that affect foam stability as it dictates the
energy of attachment of particles at the air-liquid interface, as shown in Equation 7.2.
Here, the particle wettability is expressed in terms of the contact angle value, , which
controls the position of a particle within the interface256
, as shown in Figure 7.3.
Particles with partial wettability (40° < < 90°) have been shown to be most efficient at
stabilizing foams via adsorption along the air water interface. In contrast, particles with
greater hydrophobicity ( > 90°) can destabilise foams through particle bridging-
dewetting mechanisms. Hydrophilic particles ( < 40°) remain dispersed within the thin
films between bubbles and generally demonstrate little to no influence on foam stability,
with the exception of those that hinder drainage by collecting in the Plateau borders.256
Figure 7.3: Schematic illustration defining the contact angle and its effect on particle
wettability.
There are a number of particle parameters that have been shown to influence the contact
angle of a particle at the interface. One of the main factors is surface chemistry, which
dictates the specific intermolecular interactions that occur along the surface of the
> 90°
Destabilises
foams
90° > > 40°
Stabilises
foams
< 40°
Little effect on
stability
Particle
Liquid
More
Hydrophobic
More
Hydrophilic
Air
158
particle and in turn, determines particle affinity for the air-water interface. It is directly
related to the composition of the bulk material, or surface modification where
applicable. To date, various materials have been studied as a means to improve foam
stabilization, including cellulose particles257
, metal oxide nanoparticles258, 259
, surface
modified iron particles260
and polymeric particles such as polyvinyl chloride (PVC)261
,
polystyrene and latex262-264
. A variety of synthetic mineral particles have also been used
to stabilise foams including silica252, 265-268
, quartz249
, laponite269
, zirconium
phosphate270
, calcium carbonate271
and graphite249
. In many cases, the surface
chemistry of the particles is also modified through the adsorption of cationic272
,
anionic271
and non-ionic surfactants in order to alter adsorption. The adsorption of
amines, as well as the addition of salt has also been used to alter a particle’s affinity for
the interface. However, while surface chemistry has a major effect on the wettability of
the particle, the contact angle is also affected by other properties which can impede
movement of the three phase line, including both shape273-275
and surface roughness251
.
7.2.3.2 Physical Dimensions
In addition to particle wettability, particle dimensions also play a key role in
determining the lifetime of particle stabilised foams. More specifically, the physical
dimensions of a particle provide a common link between particle properties which are
directly related to foam stability including shape, aspect ratio and size. While previous
work has been concerned predominantly with stabilizing foams using spherical particles
within the micron range253, 262, 265, 266, 276
, the effects of non-radially symmetric particles
and differently sized particles on foam stability have also been investigated both
theoretically and experimentally to a smaller extent. In general, these studies indicate
that particle dimensions influence foam stability by either affecting the diffusion of
159
particles towards the air-water interface, or affecting the ability for particles to create a
steric barrier to collapse.
One way in which particle shape affects the formation of a steric barrier between
bubbles is through differences in particle wetting. For instance, an analysis by Frye et
al.277
showed that particles with different shapes altered the critical contact angle above
which the local capillary pressure near the particles acts to expel liquid from the thin
film, destabilizing the foam. More specifically, rod, disc and cone-like particles were
shown to reduce the critical contact angle compared to spherical particles, suggesting
these types of particles must exhibit greater hydrophilicity than spherical particles in
order to stabilise the air-water interface. Theoretical studies have also shown that
particles with sharp angles, such as cubic or cone-like particles, dramatically reduce the
lifetime of the foams regardless of their hydrophobicity. This behaviour has been
attributed to the sharp edges on the particles disturbing the thin film surrounding the
particles, rupturing the air-water interface.277, 278
Different particle shapes also affect whether particles will arrange to create an effective
steric barrier against coalescence and collapse. Foam stabilization is usually favoured
by particle shapes that enable efficient geometric packing along the bubble surface,
although, theoretical treatments of particles at the bubble interface are often modelled
using spherical particles for simplicity.279
However, experimental studies have also
shown that non-spherical particles in the liquid phase are capable of packing together to
resist film drainage. For example, Karakashev et al. demonstrated that fibrous sepiolite
particles imparted greater stability to foams compared to spherical silica particles with
similar size and contact angle.273
Compared to spherical particles, which adsorb at the
air-water interface, fibrous particles were shown to entangle along the interface and
160
within the thin film. As a result, the elongated shape and high aspect ratio of the fibrous
particles promotes interfacial jamming, resisting drainage of the thin films.
Particle aspect ratio is also a critical factor in determining the minimum particle loading
necessary to stabilise air-water interfaces in foams. The ratio between the physical
dimensions of particles has been studied with respect to foam stability using both low
and high aspect ratio particles. Particles with low aspect ratios include traditional foam
stabilizing species such as spherical253, 273, 280
particles, or cubic277, 281
particles. In
contrast, high aspect ratio particles encompass linear particles such as nanofibrils282
and
microrods254, 283
, as well as planar particles such as platelets269, 270
. One main advantage
of using high aspect ratio particles over particles with low aspect ratios, is the smaller
mass of particles usually required to stabilise the unit area of interface. This behaviour
is caused by the mass of the particle present either side of the interface. Particles within
a thin film will rotate in order to adopt an energetically stable orientation during
adsorption.278, 281
As a result, particles which possess high aspect ratios preferentially
align along the air-water interface to maximize the area occupied by the particle,
reducing the surface energy of the bubble.47
Low-aspect ratio particles will also be
attracted to the bubble surface, although the bulk of the particle will be located either
side of the interface (Figure 7.4). Thus, high-aspect ratio particles are generally more
efficient at stabilizing bubble surfaces284
.
Figure 7.4: Schematic illustration of the effect of particle aspect ratio on particle loading for the
stabilisation of air-in-liquid foams. Low-aspect ratio particles like spherical and cubic particles
have the bulk of material either side of the interface in contrast to high-aspect ratio particles.
Schematic illustration of the effect of particle shape on particle loading
161
Another important property related to the physical dimensions of particles that affects
foam stability, is particle size. This specific particle parameter not only influences
particle adsorption energy at the interface as indicated by Equation 7.2, but also the
maximum capillary pressure, 𝑃𝑐𝑚𝑎𝑥. The maximum capillary pressure is the maximum
pressure within the aqueous film that can be resisted by the liquid menisci formed by
particles at an interface before rupture occurs.285
It plays a key role in defining the
stability of thin liquid films and has been the subject of several theoretical works by
Denkov285
, Nushtaeva286
, and more recently Kaptay279
. A generalised, semi-empirical
expression for 𝑃𝑐𝑚𝑎𝑥 in foams was developed by Kaptay is given in Equation 7.3:
𝑃𝑐𝑚𝑎𝑥 =
±2𝑝𝛾(𝑐𝑜𝑠𝜃 ± 𝑧)
𝑟
7.3
Where:
𝑃𝑐𝑚𝑎𝑥 = Maximum capillary pressure (N/m
2)
𝑟 = Particle radius (m)
𝛾 = Interfacial surface tension (Nm-1
)
𝜃 = Contact angle (°)
𝑝, 𝑧 = Parameters related to the arrangement of particles in the thin film
As a consequence of Equation 7.3, smaller particles should stabilise the film more
effectively than larger particles due to greater maximum capillary pressure.287
However, Equation 7.2 implies smaller particles have a weaker energy of attachment at
the interface compared to bigger particles, and vice versa. As a result, these two
competing effects suggest that particles should be of an intermediate size in order to
stabilise foams effectively. Indeed, previous studies have successfully stabilised
aqueous foams with particles ranging from tens of nanometers to several microns in
162
diameter.256, 266, 272, 288
Nevertheless, despite foams being stabilised using a range of
particle sizes in the literature, contradictory evidence does exist surrounding the optimal
particle size required to achieve foam stabilization.249, 270, 289
In general however,
smaller particles favour rapid diffusion towards the air-water interface from the bulk of
the liquid film, and creation of an incompressible particle layer.252
Smaller particles
also possess a higher packing efficiency and are thus able to form a more complete
layer, and therefore a more effective barrier, against bubble coalescence and collapse.251
Conversely, larger particles have demonstrated a greater propensity to bridge and
rupture the surfaces of adjacent bubbles when the lamellar film has narrowed
sufficiently promoting coalescence of the foam.288
7.2.3.3 Particle Concentration
Particle concentration in the liquid phase is another parameter that determines foam
stability, as it affects the rate and extent to which particles adsorb at the air-water
interface. Particles are required in sufficient concentrations at the air-water interface in
order to stabilise the bubble surface through interfacial jamming. However, the particle
concentration within the liquid phase affects the rate at which particles diffuse from the
bulk lamellar film to adsorb at the air-water interface. Thus, at sufficiently low
concentrations, the rate of particle adsorption at the interface is unable to stabilise
bubble surfaces before destabilization processes begin to dominate. Alternatively, the
number of particles at the interface may be inadequate to form an effective barrier to
coalescence, causing the bubble interface to rupture. Conversely, high particle
concentrations generally decrease the amount of time required for diffusion of particles
towards the air-water interface and ensure a sufficient layer of particles to form a steric
barrier against drainage, disproportionation and coalescence252
. As a consequence of
particle adsorption at the interface, the particle concentration also influences interfacial
163
elasticity, which in turn affects bubble shrinkage during destabilization processes.276
Therefore, effectively stabilised foams usually require a considerable particle
concentration of between 0.01 – 5% w/w depending on the type of surface active
species and particles involved.251, 253, 254, 265
7.2.4 Graphene-based Materials as Foam Stabilisers
Given the relationship between particle properties and foam stability, graphene
demonstrates a series of unique physical properties well suited to improving the stability
and performance of air-in-water foams. For instance, graphene is an ideal candidate for
imparting foam stability due to its extremely high surface area to volume ratio, which
may enable effective foam stabilization at low loadings in comparison to conventional
stabilizing particles.270
This particular property is also shared by other graphene-based
materials including graphene oxide and reduced graphene oxide. Pristine graphene
however, has also been shown to exhibit effective gas barrier properties27
, which could
assist in preventing foam deterioration caused by disproportionation. Surfactant
exfoliated graphene particles offer several additional properties suited to enhancing
foam stability. In particular, the particles are inherently dispersed in the aqueous phase
as a result of the exfoliation process. Furthermore, using an appropriate surfactant
during the exfoliation process also enables particle surface interactions, and therefore
particle wettability, to be tailored to particular foaming applications.
Yet, despite the potential for pristine, high-aspect ratio graphene particles to act as
effective air-in-water foam stabilisers, little research has been performed in this area.
Instead, previous work involving the use of graphene-based particles in two-phase
systems has focused on either stabilising liquid-liquid emulsions290
or assembling solid
porous structures using materials such as graphene oxide291, 292
. Nevertheless, planar
164
particles other than graphene have been successfully used to impart stability to
surfactant foams. For example, Guevara et al. provided the first experimental evidence
of the effect of planar particle aspect ratio on particle stabilised foams using high-aspect
ratio -Zirconium phosphate (Zr(HPO4)2 H₂O, ZrP) exfoliated with propylamine.270
The ZrP particles prepared in the study were layered compounds with a well-defined
disk-like structure, with aspect ratios (diameter / thickness) of up to 530. In
comparison, surfactant exfoliated graphene particles exhibit even higher aspect ratios,
approaching that of two dimensional materials.123
Investigating foams stabilised using surfactant exfoliated graphene provides a unique
opportunity to further study the stability mechanisms employed by planar particle
geometries at low particle loadings. In this study, foams were produced using the
foaming agent Pluronic F108 and were stabilised using low concentrations of Pluronic
F108 stabilised graphene. The resultant foams were monitored over time with respect
to bubble size, foam stability and volume. The effect of alkali metal salts on foam
stability was also investigated. Due to the hydration behaviour of Pluronic F108, the
addition of salt is expected to alter foam stability by affecting particle wettability.
7.3 Materials
The materials listed in §4.2.2 were again used in this study. In addition to these
materials, LiCl, NaCl, KCl and CsCl were also obtained from Sigma Aldrich.
165
7.4 Methods
7.4.1 Preparation of Stock Graphene Suspensions
Stock graphene suspensions were prepared via the method of ultrasonic exfoliation of
graphite, with continuous surfactant addition.116
In a typical experiment, a 10 % w/w
solution of Pluronic F108 (180 mL) was added at a rate of approximately one drop per
second to a 2% w/w suspension (196 mL) of graphite powder in water under
ultrasonication for 90 mins. In total, 10 batches of the suspension were prepared. The
suspensions were then centrifuged at 2500 rpm for 20 mins to sediment larger, non-
exfoliated graphite particles. The suspensions were then combined and further
concentrated by heating the solutions at 70 °C while stirring, evaporating the
suspensions down to a total volume of 500 mL. The solutions were then sonicated for a
further 20 mins at 70 W. The resulting suspension was dialysed (Dialysis tubing, 100
kDa MWCO, Spectrum Laboratories) against water for a minimum of 48 hours to
remove unadsorbed surfactant from the stock suspension.
7.4.2 Characterisation of Graphene Suspensions and Particles
The graphene concentration of the stock suspension was obtained using the method
described by Lotya et al.9 Samples of the suspension were diluted by a factor of 100
and the visible light spectra measured using UV-Vis spectroscopy. Applying the Beer-
Lambert law to the absorption intensity of the samples at a wavelength of 660 nm and
applying an extinction co-efficient112
, ε, of 54.22 L g-1
cm-1
, yielded an average stock
graphene concentration of 0.54 mg/mL. Raman spectroscopy was performed on the
graphene particles using a Renishaw inVia Reflex spectrometer system, with 532 nm
excitation laser.
166
7.4.3 Preparation of Sample Solutions
A series of concentrated and dilute graphene suspensions were prepared for use in
subsequent experiments. Concentrated suspensions of graphene were prepared by
evaporating the stock suspension (200 mL) at a temperature of 80 °C to approximately
50% and 75% of the original volume. Using UV-Vis spectroscopy, the exact graphene
concentration of these solutions was found to be 0.716 and 0.955 mg/mL. Graphene
suspensions (100 mL) with concentrations of 0.005, 0.027, 0.054, 0.108, 0.270 and
0.405 mg/mL were prepared by diluting the stock suspension with an appropriate
amount of water.
In certain experiments, the graphene suspensions required further processing steps or
the addition of solutes. For instance, a suspension containing 0.716 mg/mL graphene
was filtered using 0.02 μm syringe filter and was used in determining the free surfactant
concentration. In the case of samples containing surfactant, Pluronic F108 was added to
the graphene suspensions at a concentration of 0.1 g/mL. Solutions containing
inorganic salts were adjusted using the appropriate amount of electrolyte.
7.4.4 Surface Tension Measurements
The surface tension of the graphene suspensions in the absence of added surfactant and
salts was measured using a KSV CAM200 Goniometer. Due to the low concentrations
of graphene studied, the density of the solutions was assumed to be equal to that of
water (0.998 g/mL). The resultant droplet profiles were fitted using the Young-Laplace
equation.
167
7.4.5 Viscosity Measurements
Viscometric measurements on graphene suspensions containing Pluronic F108, in the
absence of salt, were made using an Ubbelohde viscometer. The viscometer had a
viscosity constant of 0.01 mm2/s
2. The measurements were performed in a temperature
controlled room at 21 °C. For each concentration of graphene, the elution time was
measured five times. The average value for the elution time was then multiplied by the
viscosity constant to calculate the kinematic viscosity.
7.4.6 Bubble Size Distribution Measurements
Foams containing Pluronic F108 exfoliated graphene and surfactant were also
characterised using light microscopy. In this experiment, solutions containing 0.1 g/mL
Pluronic F108 and 0, 0.405 mg/mL, and 0.955 mg/mL of exfoliated graphene were
used. Each sample consisted of a 5 mL solution sealed in a 50 mL vial, shaken
vigorously for 10 s. The vial was then inverted and the cap containing the foam
removed. The foam was then imaged at 2x magnification and 1x zoom using a
calibrated Nikon AZ 100M complete with microscope imaging software. At 0, 1 and 2
hours after foaming, approximately 10 images were captured of different areas of the
samples within 15 minutes. In these images, 187 pixels represented 1 mm. Between 11
and 225 bubbles were analysed for each system and the area of each of the bubbles was
determined using Image J Software. Each experiment was repeated 3 times and the
bubble size distributions developed using the average distribution of each set.
7.4.7 Foam Stability Measurements
The stability imparted by graphene sheets on foams was investigated by determining the
foam lifetime and initial foam volume. 5 mL of suspension was sealed in a 50 mL
168
graduated plastic vial and shaken vigorously for 10 s at 21 °C. The foam life was then
recorded using a Logitech 270 webcam and captured using time lapse software (Video
Velocity Free) at a rate of one frame every two seconds. The initial foam volume is
defined as the volume of foam formed immediately after initial agitation of the
suspension. The foam half-life is defined as the time required for the volume of foam to
decrease by half.
7.5 Results and Discussion
In this study, foams were stabilised using suspensions of graphene prepared using the
surfactant-assisted exfoliation of graphite, with continuous surfactant addition. During
the exfoliation process, surfactant adsorbs onto the graphene basal plane, preventing
particle reaggregation and stabilizing the suspended particles. For foams containing
Pluronic F108 only, the surfactant adsorbs to the air-water interface via the PPO block,
while the PEO chains adopt an extended configuration.293
Similarly, previous studies
suggest that during graphene exfoliation, the hydrophobic PPO segments of the
surfactant adsorb to the hydrophobic graphene surface, while the hydrophilic PEO
blocks extend out into solution.249, 294
Thus, the presence of the hydrophilic surfactant
blocks leads to an overall increase in the hydrophilic character of the particles.
7.5.1 Graphene Characterisation
The optical, vibrational and physical properties of the exfoliated graphene particles were
characterised in order to determine suitability as a foam stabiliser prior to their use in
subsequent foaming experiments. The Raman and UV-Vis spectra of the prepared
graphene particles are consistent with the defect-free, single to few layer graphene
produced in previous studies123
and previous chapters (See Appendix, §A.3.1).
169
7.5.2 Surface Activity of Graphene Particles
The affinity of graphene particles for the air-water interface is an important indicator of
the foam stabilization capabilities of surfactant exfoliated graphene. Here, the affinity
of particles for the air-water interface was investigated using surface tension
measurements. Figure 7.5 shows the surface tension of a range of graphene suspensions
(0 - 0.955 mg/mL) measured in the absence of any surfactant added to facilitate
foaming. The surface tension of the solutions remains invariant for suspensions with
low graphene concentration (< 0.27 mg/mL), yet decreases as the solutions become
more concentrated. A graphene suspension was filtered to determine the amount of
unadsorbed surfactant in the solutions. The surface tension of the filtrate was 63.20
mN/m, which corresponds to a very low surfactant concentration of ~ 10-5
% w/v.295
The surface tension of the filtered solution is significantly higher than that of the
corresponding unfiltered suspension (56.23 mN/m). This confirms that the graphene
particles are indeed surface active and the reduction in the observed surface tension is
not due to free surfactant. Interestingly, this occurs despite the adsorption of a relatively
hydrophilic surfactant onto the graphene surface, which would favour immersion of the
particles in the aqueous phase. Furthermore, flat, disc-like particles typically exhibit
contact angles far lower than that of spherical particles, again promoting particle
migration to the aqueous phase.273
However, previous studies indicate that exfoliated
two-dimensional materials, like graphene are stabilised against reaggregation by less
than approximately 10% surfactant coverage.294
Thus, in order to adsorb at the
interface, the particles are likely to retain significant non-polar character that arises due
to the conjugated structure of graphene.30
170
Figure 7.5: Surface tension as a function of graphene concentration for suspension of Pluronic
F108 exfoliated graphene.
7.5.3 Bubble Size Distribution
Foams were imaged using optical microscopy in order to investigate the appearance of
the foams as well as the effect of graphene concentration on bubble coarsening. For this
series of experiments, solutions containing Pluronic F108 (0.1 g/mL) and surfactant
stabilised graphene (0 – 0.72 mg/mL) were used to generate the foams. Images of the
samples were captured immediately after foaming and then again 1 and 2 hours later. A
representative image of the foams during the ageing process is presented in Figure 7.6.
The foams exhibit dry, polyhedral type structures which are common for air-in-water
foam systems, with the bubbles compacting from ideal spheres through drainage and
thinning of the interfilm. The resultant bubbles are much larger than the stabilizing
graphene particles, ranging in lateral size from 0.02 to 8.28 mm. This suggests the
graphene may conform more efficiently to the curved air-water interface in comparison
to conventional spherical stabilizing particles, thereby increasing the attachment energy
associated with the particles.
0
10
20
30
40
50
60
70
80
0 0.2 0.4 0.6 0.8 1
Surf
ace
Ten
sio
n (
mN
/m)
Graphene Concentration (mg/mL)
171
Figure 7.6: Optical microscopy image of a foam consisting of 0.955 mg/mL graphene
immediately after foaming.
The influence of graphene concentration on foam evolution was investigated by
generating bubble size distributions from the foams through image analysis of the top
layer of bubbles. Although the bubbles in persistent foams are polyhedral, it is common
practice to approximate bubbles as spheres and assign an equivalent diameter.248
Thus,
the cross-sectional area of the bubbles was converted to the diameter of an equivalent
sphere, with the resultant distribution of bubble diameters shown in Figure 7.7.
172
(a)
(b)
Figure 7.7: Bubble size distribution of foams containing various concentrations of surfactant
exfoliated graphene (a) immediately after foaming and (b) 2 hours after foaming.
Regardless of the concentration of graphene present, the foams are initially comprised
of a high proportion of small bubbles, as demonstrated by the highly skewed bubble size
distributions. After two hours, foams generated in the absence of graphene exhibit a
broadened size distribution with an average equivalent diameter of 2.51 mm. This shift
in distribution is consistent with disproportionation, which causes smaller bubbles to
shrink and disappear at the expense of larger bubbles over time.296
In contrast, the
bubble size distribution of foams stabilised by graphene particles remains skewed
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 > 5.0
Freq
uen
cy (
%)
Equivalent Diameter (mm)
0 mg/mL Graphene
0.41 mg/mL Graphene
0.96 mg/mL Graphene
0.0
5.0
10.0
15.0
20.0
25.0
30.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 > 5.0
Freq
uen
cy (
%)
Equivalent Diameter (mm)
0 mg/mL Graphene
0.41 mg/mL Graphene
0.96 mg/mL Graphene
173
towards very small bubbles, with the proportion of bubbles below 0.5 mm increasing
with graphene concentration. However, of the two systems, foams with a lower
concentration of graphene experience a shift towards larger bubble sizes. Here, foams
generated with 0.41 mg/mL graphene have an equivalent average diameter of 2.21 mm,
compared with 1.93 mm for foams generated with 0.96 mg/mL graphene. These
changes in bubble size distribution suggest the presence of graphene in the bubble film
aids in preventing drainage or gas diffusion between bubbles.297
Furthermore, the data
also shows that higher concentrations of graphene may be more effective in creating
layers of particles at the air-water interface to hinder gas diffusion or drainage.
7.5.4 Foam Stability Measurements
Stability measurements were performed on foams generated from aqueous solutions of
surfactant and surfactant stabilised graphene using manual shaking at 21 °C. The
volume of the foams was then monitored as a function of time to determine the stability
of the foams. These measurements were undertaken in order to ascertain the suitability
of surfactant graphene particles as foam stabilisers. In this manner, the effects of
particle concentration and the addition of salts on the stability of the foams were
investigated.
Foams were prepared using various concentrations of surfactant stabilised graphene (0 -
0.95 mg/mL of graphene) in order to determine the effect of particle concentration on
foam stability. 0.1 g/mL Pluronic F108 was included as a component in the starting
suspensions to enable foaming and maintain a surfactant concentration far in excess of
the CMC. The excess free surfactant ensured that observed changes in foam stability
due to the depletion of surfactant by adsorption onto the graphene particles was
negligible.
174
The stability of the resultant foams is given in Figure 7.8 as a function of graphene
concentration. Regardless of the particle concentration, the foams are at least stable on
the order of hours. Initially the foam half-life decreases, reaching a minimum at a
graphene concentration of 0.27 mg/mL, however increases for concentrations beyond
that. Thus, foams containing the particles are destabilised when small concentrations of
graphene are present in the starting solution, whilst graphene particles impart greater
stability to the foam at higher concentrations. The non-linear trend indicates that the
presence of graphene influences the rate of foam deterioration by affecting foam
drainage, rather than disproportionation. Foam drainage is well known to be affected by
both particle interactions within the interfilm, and the rheological properties of the
liquid.298
However, viscometric measurements on the starting solutions (See Appendix,
§A.3.2) indicate the absence of a similar relationship between the graphene
concentration and bulk viscosity.
Figure 7.8: Foam half-life as a function of graphene concentration for solutions containing 0.1
g/mL Pluronic F108. Error bars indicate the standard error.
0
100
200
300
400
500
600
700
800
900
0 0.2 0.4 0.6 0.8 1
Foam
Hal
f Li
fe (
min
)
Graphene Concentration (mg/mL)
175
Thus, the trend in foam stability shown in Figure 7.8 can be attributed to particle-
interface interactions that act to either increase or prevent hydrodynamic drainage
depending on the concentration of graphene present. At zero graphene concentration,
the system represents a purely surfactant foam that exhibits intermediate stability
consistent with the Gibbs-Marangoni effect. This well-known phenomenon applies to
surfactant based foams and arises due to surface tension gradients within the bubble thin
film stabilizing the interface.248
When graphene is present in the foam, the non-linear
trend in foam half-life can be explained by two competing effects, namely the rate of
particle adsorption and the influence of immersed particles on the surfactant
transportation in the bubble film. Given the high aspect ratio of the graphene sheets
evidenced by TEM imaging (§4.2.4), the particles are most likely to adsorb at the
interface along the basal plane in order to maximize the detachment energy. An
increase in particle concentration reduces the time required for a particle to diffuse and
adsorb on the surface of the bubble.252
Thus, at low concentrations (0 - 0.27 mg/mL) it
is proposed that an incomplete layer of particles at the air-water interface prevent
effective steric stabilization of the foam, while particles migrating towards the bubble
surface inhibit the surfactant transport required to stabilise surfactant rich areas through
the Gibbs-Marangoni effect. As the graphene concentration is increased above a critical
concentration, a sufficiently dense monolayer of particles may form, stabilizing the
bubbles against collapse. It is possible that increasing the graphene concentration
further stabilises the foam through the formation of densely packed particles within the
bubble walls, thereby resisting drainage of the film. Time dependent adsorption of
graphene at the bubble surface is also consistent with observations that graphene
suspensions in the absence of surfactant do not foam. This may relate to the thin, sheet-
like nature of the particles, which would require more time to make contact with the
interface, compared to a wider particle. Consequently, the addition of excess surfactant
176
is required in order to establish a foam structure that is then stabilised by graphene
particles.
The influence of graphene on foamability is shown as a function of graphene
concentration in Figure 7.9. Here, the initial foam volume decreases with increasing
graphene content, reaching a plateau value of approximately 6 mL at 0.405 mg/mL.
This concentration roughly coincides with the transition in foam stability in Figure 7.8.
As a result, the reduction in initial foam volume may reflect a change in the surface
dilatational elasticity as surfactant is displaced from the air-water interface by
stabilizing particles when the graphene concentration is increased. This is consistent
with the change in proportion of smaller bubbles present for foams generated in the
absence of graphene and in the presence of 0.405 mg/mL graphene, as shown in Figure
7.7.
Figure 7.9: Initial foam volume as a function of graphene concentration for solutions
containing 0.1 g/mL Pluronic F108 and Pluronic F108 exfoliated graphene. Error bars indicate
the standard error.
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
0 0.2 0.4 0.6 0.8 1 1.2
Init
ial F
oam
Vo
lum
e (m
L)
Graphene Concentration (mg/mL)
177
7.5.5 Effect of Salt Concentration on Foam Stability
The ability to alter the wetting properties of particles allows an opportunity to modify
the stability of particle stabilised foams. Here, the solubility of the surfactant used to
exfoliate the graphene particles was modified in an effort to change the overall affinity
of the particles for the air-water interface. The surfactant used to exfoliate the graphene
particles, Pluronic F108, possesses polyethylene oxide chains, which are well known to
strongly associate with water molecules through hydrogen bonding.233
Previous
experiments have shown that the solubility of these chains can be modified by the
addition of inorganic salts.299, 300
Therefore, to determine the effect of cations on foam
stability, aqueous foams were prepared from solutions containing 0.1 g/mL surfactant
and Pluronic F108 stabilised graphene (0.405 g/mL) in the presence of varying
concentrations of alkali metal salts. Figure 7.10 shows the stability of the resultant
foams, where the foam half-life is shown as a function of salt concentration.
Figure 7.10: Foam half-life as a function of salt concentration for solutions containing 0.1 g/mL
Pluronic F108 and 0.405 mg/mL Pluronic F108 exfoliated graphene.
10-2 10-1 100 101
0
500
1000
1500
2000
2500
3000
Foam
Hal
f Li
fe (
min
)
Concentration of Salt (M)
NaCl
LiCl
KCl
CsCl
178
The stability of the foams is strongly influenced by the presence of salt in solution and
the specific cation. In particular, foams generated using a salt concentration above 1 M
exhibit far greater stability than those with lower salt content. This behaviour can be
attributed to the dehydration capacity of cations on PEO groups in Pluronic F108.
Previous studies indicate most cations alter water structuring surrounding PEO chains,
causing the chains to deviate from their extended, hydrated conformation301
. In the case
of exfoliated graphene, this could result in a reduction in overall particle hydrophilicity,
thereby improving adsorption of the particles at the air-water interface. However, it is
clear the cations do not affect foam stability equally, with the stability of the foams
generally decreasing with the ordering Na+ > Li
+ > K
+ > Cs
+. In contrast, the foam
stability of equivalent foams generated without graphene generally decrease according
to the sequence Li+ > Na
+ > K
+ > Cs
+ (See Appendix, §A.3.3). The latter ordering
follows the Hofmeister series and is consistent with the varying degrees to which
cations are known to penetrate the water structure around PEO chains.13, 300
The extent
to which cations affect the solubility of PEO groups is attributed to two competing
effects, namely the hydration capacity of ions in solution and ions complexing with the
ether oxygen on PEO.299
Thus, the difference in cation effect between the two foam
systems may arise due to the hydration behaviour of Li+, which unlike other cations, is
known to form complexes with basic ether groups on PEO rather than with water.302
The formation of complexes improves hydration of the PEO groups. Thus, the Li+ may
alter the hydration of the PEO chains whereby differences in orientation of the Pluronic
F108 on exfoliated graphene and at the bubble surface relative to the air-water interface
have an effect on the adsorption of the surface active species. Importantly, the
discrepancy between the two orderings suggests that the cations not only affect the
solubility of PEO chains in the free surfactant, but also in surfactant adsorbed to the
graphene sheets. Thus, the addition of cations enables the wettability characteristics of
179
the graphene particles at the air-water interface to be altered to yield enhanced foam
stability.
From Figure 7.11, it can be seen that the ability to foam the solutions is also dependent
on the type and concentration of salt present in the foams. Unlike the foam lifetime, the
initial volume of foam generally decreased with increasing salt concentration regardless
of the salt, except at 3 M concentration where the samples containing NaCl, KCl and
CsCl all experienced a significant increase in foam volume. At such high salt
concentrations, aggregation of the particles may be responsible for these changes in
initial foam volume.
Figure 7.11: Initial foam volume as a function of salt concentration for solutions containing 0.1
g/mL Pluronic F108 and 0.405 mg/mL Pluronic F108 exfoliated graphene.
The results of this study indicate particles with extreme aspect ratios, such as surfactant-
exfoliated graphene hold great potential as stabilisers in aqueous foams. A major
advantage of using high-aspect ratio particles as foam stabilisers is the lower particle
loadings required in order to resist rupture and collapse of the foams. For example,
10-2 10-1 100 101
0
1
2
3
4
5
6
7
8
9
10
Init
ial F
oam
Fo
rmed
(m
L)
Concentration of Salt (M)
NaCl
LiCl
KCl
CsCl
180
foams stabilised with conventional particles typically contain 0.5 – 2% w/w particles254,
265, 269 whereas the foams presented here are stabilised with a minimum graphene
concentration of 0.716 mg/mL, equivalent to 7.17 × 10-2
% w/w. Similar foams
stabilised with graphite particles (5 μm) and Pluronic F108 exhibited a comparable
foam half-life at a particle concentration of 33% w/w.249
Thus, the physical dimensions
of particles are a critical factor in increasing foam stability at minimal particle loadings.
The ability to alter the wettability of graphene particles through the addition of simple
inorganic salts could also prove to be a highly desirable property for this system,
allowing the stability of foams to be tailored for a particular application. The adsorbed
surfactant is another feature of surfactant exfoliated graphene that improves the
versatility of the stabilizing particles in promoting or discouraging foam stability. It has
been shown previously that the surface interactions of the particles are altered by the
type of surfactant used.66
As a result, by simply choosing a suitable surfactant to
exfoliate the graphene, interactions between the surfactant and the solvent in the bubble
interlayer may be optimised in order to inhibit or promote drainage and collapse of the
foam. This may include using surfactants with shorter hydrophilic chains or even
monomeric surfactants that are not irreversibly adsorbed.116, 117, 125
7.6 Conclusion
Particle stabilised foams are a well-known processing technology with great commercial
importance across a number of engineering and scientific fields. Here, foams stabilised
using graphene were demonstrated. Graphene particles were prepared in the presence
of the non-ionic tri-block polymeric surfactant, Pluronic F108, through aqueous phase
exfoliation of graphite. These particles, which were shown earlier to possess a high
aspect ratio through TEM imaging and are shown here to be surface active, are likely to
181
favour irreversible adsorption at the air-water interface and therefore efficient
stabilisation of bubble surfaces in the foam.
Thus, the presence of graphene particles had a significant influence on the properties of
the prepared foams. The initial volume of the foams showed a non-linear trend
consistent with changes in the surface dilatational elasticity as the graphene
concentration was increased. Foam stability was also affected by the concentration of
graphene particles in solution, with low concentrations of graphene destabilising the
foam, while sufficiently high graphene concentrations improved foam stability. This
behaviour was attributed to the rate at which graphene accumulated at the air-water
interface and within the bubble film to inhibit drainage and coalescence. The
arrangement of particles at the interface and within the bubble film was further
evidenced by the evolution of bubble size distributions with time. Foams with a higher
graphene concentration retained a higher proportion of small bubbles over time,
indicating the presence of particles improves resistance to disproportionation.
The effects of added alkali metal salts on graphene stabilised surfactant foams were also
studied. It was shown that foams stabilised using Pluronic F108 exfoliated graphene
exhibited improved foam stability subject to specific ion effects. Upon addition of salt,
the foams experienced an enhanced stability according to the series Na+ > Li
+ > K
+ >
Cs+. The ranking was indicative of the relative hydration capacity of each salt with
respect to the surfactant PEO groups. Thus, by altering solvent-particle interactions,
overall changes in particle wettability and therefore foam stability were achieved.
Together, these results illustrate the potential of surfactant exfoliated graphene as a
versatile foam stabiliser.
182
CHAPTER 8
Chapter 8
Adsorption of Organic Dyes Using Surfactant
Exfoliated Graphene
8.1 Introduction
The removal of contaminants from the aqueous phase using carbon materials is a well-
established technique which forms the basis of many modern environmental
remediation and water treatment practices. These processes are generally driven by the
adsorption of impurities from the liquid phase onto the surface of the carbon material.
Most conventional carbon adsorbents are produced by combustion of natural materials
and therefore possess high adsorption capacities due to their porous or granular
structure and heterogeneous surface chemistry. However, while surface properties are
critical in determining the adsorption capacity of carbon materials, the molecular
affinity at the solid-liquid interface is also contingent on solution conditions and the
chemical structure of the adsorbing contaminant.
Organic dyes are an important category of contaminants which present a significant
source of pollution if released into the environment. They occur in effluent from the
textile, dyeing, paper, tannery and paint industries and can induce toxic effects in
aquatic and human life. Organic dyes are also frequently used to model small, organic
molecules such as herbicides and pesticides in adsorption studies. Often, these
molecules are adsorbed onto carbon materials through a highly complex balance of
electrostatic and non-electrostatic interactions.
183
Surfactant exfoliated graphene offers distinct advantages over other carbon adsorbents
in the adsorption of organic dyes. For instance, the particles possess a large specific
surface area allowing a greater number of attractive interactions, and therefore greater
potential for adsorption of contaminants. The surface properties of surfactant exfoliated
graphene can also be altered to promote attractive electrostatic interactions while
retaining the conjugated lattice structure of the particles which support Van der Waals
based interactions. Despite these features however, the adsorption capacity of
surfactant exfoliated graphene with respect to contaminants such as organic dyes has yet
to be investigated.
The experiments presented in this chapter aim to examine the adsorption of organic
dyes from the aqueous phase using surfactant exfoliated graphene. The chapter begins
with an overview of carbon-based adsorbents used in aqueous environments, with a
particular focus on the process of adsorption and factors affecting adsorption. Common
models used to analyse and characterise adsorption are also presented, followed by a
review of the known adsorption behaviour of graphene-based materials. Next, an
outline of the materials and techniques used to investigate the adsorption behaviour of
surfactant exfoliated graphene particles is given. In this project, two organic dyes,
methylene blue and methyl red were used to study the adsorption behaviour of graphene
particles stabilised using anionic, cationic and non-ionic surfactant under a variety of
solution conditions. The remainder of the chapter describes the results of these
experiments and subsequent analysis performed.
184
8.2 Background
8.2.1 Adsorption by Carbon Based Materials in the Aqueous Phase
For thousands of years, carbonaceous materials have been used in wastewater treatment
and water purification processes as an effective means of removing contaminants from
water sources. Indeed, ancient Sanskrit texts dating from approximately 2000 BC
describe the use of charcoal to filter drinking water in India, while 1600 years later, the
Greek physician Hippocrates advised treating drinking water with wood char to
eliminate undesirable taste and odour and to prevent the transmission of waterborne
diseases.303, 304
Today, carbon-based materials are used extensively for similar
purification purposes as part of more sophisticated water remediation practices. As in
antiquity, many of these materials are produced from natural sources and
characteristically appear in the form of either highly porous granular materials or finely
divided particles. For instance, conventional, highly porous activated carbons are
particularly prevalent in large-scale municipal water purification systems and industrial
wastewater treatment operations, and are often produced through chemical or physical
activation of a carbon-rich precursor such as coal, coconut shells, lignite, peat or
wood.303
Recently, carbon allotropes with long range order and well defined chemical
composition such as fullerenes305
, carbon nanotubes305-307
and graphene-based77, 308-311
materials have also attracted attention for their ability to remove contaminants from
aqueous environments. To date, carbon materials have been studied extensively as a
means of removing a wide variety of water-soluble contaminants from the aqueous
phase including heavy metals312, 313
, phenolic compounds314
, endocrine disrupting
compounds, pharmaceutical active compounds307, 315, 316
, cyanotoxins316
, and organic
dyes313, 317-319
.
185
8.2.2 Organic Dye Contaminants
Organic dyes are a major class of industrially relevant pollutants that pose a serious risk
to both human health and the environment if released into water streams. Organic dyes
often possess complex molecular structures comprised of two main components:
chromophore groups, which are responsible for imparting colour, and auxochrome
groups, which serve multiple functions including supplementing the chromophores,
rendering the dyes water soluble and promoting dye affinity for surfaces.304
As a result,
organic dyes are widely encountered as part of colouring processes within the textile320,
321, paper
304, leather tanning
322, food
323, 324, polymer
325, cosmetics
317, 326, printing and
dye manufacturing317
industries. Since these processes consume substantial volumes of
water and organic dyes are generally resistant to degradation, the dyes often remain in
industrial wastewater which is subsequently discharged into the environment. The
presence of organic dye contaminants gives rise to water with undesirable colour, which
can affect the photosynthetic mechanisms essential to aquatic life by reducing sunlight
penetration.327
Some organic dyes have also demonstrated carcinogenic, mutagenic or
teratogenic effects in various fish species, with the presence of chemical groups such as
aromatics and amines in the dye structure being shown to induce toxic effects in aquatic
life.317
While organic dye contaminants are harmful to aquatic life, they also can have
detrimental effects on human health affecting the kidney, reproductive system, liver,
brain and central nervous system.313, 320, 326
Therefore, the removal of such dyes from
wastewater prior to their release into water streams is critical. While organic dyes can
be removed from the aqueous phase using select treatment technologies including
membrane filtration and ion-exchange, one of the most effective and efficient methods
employed by industries to remove dyes from wastewater is through adsorption onto
conventional carbon-based materials.328
186
8.2.3 Adsorption Mechanisms Associated with Porous Carbon Materials
The process of removing water-soluble contaminants like organic dyes from the
aqueous phase via adsorption onto conventional porous carbon materials can generally
be divided into four different stages (Figure 8.1). These stages include external mass
transfer of adsorbate, boundary layer diffusion, intraparticle mass transfer of adsorbate
and adsorption at active sites.77, 329
Contaminant molecules are dispersed in the bulk
solution prior to adsorption on the surface of porous carbon materials. Upon
experiencing attractive intermolecular interactions with adsorbent particles, the
molecules undergo mass transfer from the external bulk solution to the internal
boundary layer surrounding the outside surface of the particles. The adsorbate
molecules then diffuse across the liquid film boundary. Next, the molecules continue to
diffuse throughout the liquid contained within the pores of the particle, leading to
intraparticle mass transfer of the adsorbate. The final stage of the removal process
involves adsorption of contaminant molecules at active sites along the particle surface
through attractive intermolecular interactions.
187
Figure 8.1: Schematic illustration of the individual stages which contribute to the overall
adsorption of contaminants onto conventional porous carbon materials. Stage 1 involves
external mass transfer of adsorbate, stage 2 consists of boundary layer diffusion, stage 3
involves intraparticle mass transfer of adsorbates and stage 4 consists of adsorption of
contaminants at active sites
8.2.4 Conditions Influencing Dye Adsorption
The extent to which organic dye molecules adsorb onto the surface of carbon materials
is affected by a number of interdependent factors. These factors play an essential role in
determining the number, types and strength of intermolecular interactions necessary to
facilitate adsorption. As a result, they are often related to the characteristics of the
adsorbent, solution conditions as well as the characteristics of the adsorbate.
8.2.4.1 Adsorbent Surface Chemistry
The surface chemistry of the adsorbent is a particularly important consideration, as it is
responsible for the presence and strength of attractive interactions responsible for the
188
adsorption of dyes from solution. Previous work relating to adsorbent surface chemistry
has been predominantly concerned with high capacity adsorbents such as activated
carbon materials.321, 330-332
These materials are produced by activating carbon-rich
precursors such as agricultural313, 319, 333
, industrial334
and biowaste335
through either
chemical treatment, followed by carbonization at high temperatures, or carbonization in
an oxidizing atmosphere. The process gives rise to chemically heterogeneous surfaces,
with the precursor composition and processing conditions controlling the degree and
type of functionalization observed.303, 330, 336
Thus, conventional carbon adsorbent
surfaces typically consist of a complex arrangement of chemically distinct areas which
exhibit varying adsorbate affinity.
Oxygenated functional groups and carbon-based -electron systems are two types of
chemical structures typically present on the surface of carbon adsorbents that are
capable of participating in dye adsorption processes. The carbon content in these types
of adsorbents generally appears as sp2 carbon atoms arranged hexagonally along atomic
planes.337
Activated carbon surfaces can also contain a number of different types of
oxygenated functional groups including acidic carboxylic, carbonyl, phenolic, carbonyl,
ether and lactonic groups.314, 338
Such oxygen-containing surface moieties are well
known to determine the adsorption capacity of carbon adsorbents such as activated
carbons and have been shown to affect the adsorption of both negatively and positively
charged dyes.331, 339
In particular, the presence of oxygenated groups such as carboxyl
and phenolic groups on activated carbons have been shown to facilitate favourable
electrostatic interactions with positively charged dyes by imparting a negative charge to
the adsorbent surface when deprotonated.340
In contrast, the adsorption of negatively
charged dyes can be prevented by the presence of oxygen-containing surface moieties,
which can serve as electron-withdrawing groups and therefore act to localise -electrons
189
on the surfaces of the adsorbents.331
A positive surface charge can also result from
oxygenated surface groups with basic character, such as pyrenes or chromes, leading to
the adsorption of negatively charged dyes.321, 341
In the absence of oxygenated surface
groups, the delocalised -electrons along the carbon basal plane have also been shown
to facilitate adsorption through - interactions with suitable dye molecules.339
While
oxygenated functional groups and carbon-based -electron systems are two of the most
common types of actives sites encountered on the carbon adsorbent surface, it has been
suggested in the literature that adsorption sites can be more broadly defined in terms of
Lewis acid and base behaviour.321, 341
Thus, the presence, type and number of chemical
structures on the adsorbent surface plays a critical role in establishing both electrostatic
and dispersion interactions essential for adsorption.
8.2.4.2 Specific Surface Area of Adsorbent
The specific surface area of the adsorbate is another important property that influences
dye adsorption. It is directly related to the quantity of active chemical sites available for
the adsorption of dye molecules along the adsorbent surface. Carbon adsorbents such as
activated carbon characteristically possess high specific surface areas, which are often a
consequence of particle porosity or particle size. The high specific surface area of these
adsorbents is typically credited with greater adsorption capacity and forms the basis
upon which highly porous carbon adsorbents, such as carbon aerogels, have been
studied as a means to improve dye adsorption.342
However, it has been shown that high
specific surface area alone does not determine adsorption capacity for porous carbon
adsorbents, with pore size and pore volume playing a key role in the allowing adsorbate
molecules to access the adsorbent surface.303
Indeed, studies have shown that dye
adsorption on activated carbon is strongly dependent on both porosity and surface area,
190
with an increase in accessible surface area yielding greater adsorption capacity with
respect to organic dyes.343
As a consequence of this, the surface area available for
adsorption is also related to the size of the adsorbing dye molecule as well as the
adsorbent dosage.
8.2.4.3 Adsorbent Dosage
Adsorbent dosage is another parameter in addition to specific surface area, which also
influences the quantity of active chemical sites available to participate in adsorption.
Studies have shown that as the amount of carbon adsorbent increases, the percentage of
dye removed from solution increases accordingly.317, 344
This can be attributed to a rise
in the total number of active adsorption sites available to participate in adsorption
within the system.340
However, it has also been found that when the concentration of
adsorbent exceeds a critical level, physical or steric hindrance may lead to active sites
being blocked from participating in adsorption processes.345
Thus, adsorbent dosage is
a key parameter in determining the optimal amount of adsorbent required to attain
maximum adsorption of adsorbate. Adsorbent dosage also has an effect on maximum
adsorption capacity through the adsorbate-to-adsorbent ratio, which is in turn affected
by the initial dye concentration.
8.2.4.4 Initial Dye Concentration
The amount of dye that is adsorbed by a carbon adsorbent is also highly dependent on
initial dye concentration. The strong correlation between dye adsorption and initial dye
concentration is due to the direct coupling of dye molecules with available active sites
on the adsorbent surface during the adsorption process. Studies have shown that as the
initial dye concentration is raised above a critical amount, the percentage of dye
191
removed decreases.77, 327
This behaviour has been attributed to the saturation of active
sites by dye molecules at higher initial dye concentrations.317
While the percentage of
dye removed from solution decreases with increases in initial dye concentration, the
adsorption capacity demonstrates an opposite trend. Systems with a higher initial dye
concentration provide a larger driving force for the mass transfer of dye to the adsorbent
surface, resulting in an increase in adsorption capacity of the adsorbent.327
As a result,
the initial dye concentration has a significant influence on certain quantitative
parameters used to characterise the efficacy of the adsorbent.
8.2.4.5 Structural Chemistry of Dyes
The structural chemistry of an adsorbing dye also plays a critical role in facilitating
adsorption onto carbon materials. More specifically, the presence or absence of certain
structural groups within organic dyes can contribute to attractive intermolecular
interactions, thereby driving adsorption along carbon surfaces. Organic dyes can be
divided into several structural categories such as azo, anthraquinone, indigoid, nitroso,
nitro and triarylmethane dyes based on the type of chromophore present within the dye
molecule.317
These chromophores and their associated auxochromes commonly possess
an aromatic or double bond system, which can result in the presence of free electrons in
the dye molecule. Studies have shown that the free electrons on dye molecules are
capable of forming attractive dispersion interactions with the delocalised -electron
systems present on carbon surfaces.330-332
However, the distribution of electrons in
adjacent aromatic structures and in turn, the surface affinity of the dye molecules, can
be reduced by the presence of adjacent electronegative atoms in dye molecules.344
In
addition to being classified by their chemical structures, organic dyes can be further
categorised by their electrostatic charge. Organic dyes are capable of acquiring a
cationic or anionic charge upon dissolution in an aqueous medium. It has been shown
192
that both cationic and anionic dyes can adsorb through attractive electrostatic
interactions with the carbon surface, provided that the surface exhibits a sufficient
opposing charge to that of the adsorbing dye.329, 331
Consequently, the type of dye
contaminant removed from solution is highly dependent on adsorbent surface chemistry.
8.2.4.6 pH
One of the most important environmental conditions which affect the adsorption
capacity of carbon adsorbents is pH. Conventional carbon adsorbents such as activated
carbon typically exhibit amphoteric character due to the presence of oxygenated surface
moieties.339
Thus, variations in solution pH are able to control both the ionization of
dye molecules and adsorbent surface properties which in turn, govern the electrostatic
interactions responsible for adsorption. More specifically, the pH influences the surface
charge of carbon adsorbents by controlling the dissociation and protonation of
oxygenated surface groups such as OH-. The pH at which the adsorbent surface charge
reaches zero, termed the point of zero charge (pHpzc), provides an important indicator of
this behaviour and its effects on adsorption, particularly for systems involving ionic
dyes.317
For instance, pH levels above pHpzc yield overall negatively charged surfaces
and have been shown to promote the adsorption of cationic dyes whilst pH levels below
the pHpzc can favour anionic dye adsorption by yielding a positively charged carbon
surface.317
It has also been shown that for anionic dyes, adsorption capacity of dyes on
activated carbon increased with acidicity and decreased in basic solutions338
, while an
opposite trend has been demonstrated in systems involving cationic dyes317
. To date,
the effect of pH on dye adsorption has been studied with respect to a number of
different carbon-based adsorbents including graphene oxide77, 310, 346
, reduced graphene
oxide77, 347
, carbon nanotubes310, 348
and activated carbons310, 329, 338, 349
.
193
8.2.4.7 Ionic Strength
The intermolecular interactions responsible for the adsorption of organic dyes along a
carbon surface can also be influenced by the ionic strength of the system. The presence
of electrolyte aids in screening electrostatically charged species in solution, such as
ionic dyes and ionised adsorbent surfaces.350
Thus, an increase in ionic strength
typically reduces the adsorption capacity of carbon adsorbents when attractive
electrostatic interactions dominate the adsorption process, with as little as 0.001 M salt
required in order to yield an appreciable difference in adsorption capacity.351, 352
Conversely, an increase in ionic strength has been shown to yield greater adsorption
when repulsive electrostatic interactions are present between the dye and adsorbent.350
However, exceptions to this trend can occur, and have been observed in cases where the
ionic strength is sufficient to induce aggregation of dye molecules.338
8.2.4.8 Temperature
The temperature of the system is another physical parameter that affects the adsorption
of organic dyes onto adsorbents. In particular, changes in temperature affect the
adsorption capacity of the adsorbent by affecting the number and strength of adsorptive
forces between the dye molecules and carbon adsorbent. Given that adsorption is a
spontaneous process and that the irreversible adsorption of molecules onto a surface can
yield a decrease in the overall entropy of the system, it is expected that an increase in
temperature will cause a decrease in dye uptake.341
In this case, the temperature
increase reduces the intermolecular forces between the dye species and active sites on
the adsorbent, leading to a reduction in overall adsorption capacity.317
Adsorption
processes which experience a reduction in adsorption capacity with increasing
temperature are classified as exothermic. There are numerous examples of organic dyes
194
adsorbing onto carbon surfaces via exothermic processes in the literature.346, 353, 354
Despite not being predicted by thermodynamics, endothermic adsorption processes in
which the amount of dye adsorbed on the carbon surface increases with temperature, are
also encountered in the literature. In these types of processes, it has been proposed that
a rise in temperature may lead to an increase in the mobility of the dye molecules and
the number of active sites on the adsorbate to a sufficient extent so as to cause an
overall increase in the entropy of the system.317
At present, many examples of the
endothermic adsorption of organic dyes onto carbon surfaces exist in the literature.329,
338, 347, 348
8.2.5 Quantitative Modelling of Adsorption Processes
Quantitative modelling of adsorption data provides an important method of
characterizing and describing the adsorption process for a given system at various
operating conditions. Systems involving the adsorption of organic dyes onto carbon
adsorbates are frequently analyzed using kinetic models and adsorption isotherm
models in the literature.
8.2.5.1 Adsorption Kinetic Models
Kinetic modelling of adsorption processes provides a powerful method of identifying
and understanding the controlling mechanism responsible for the rate of adsorption in a
system. While there are several different kinetic models proposed in the literature355, 356
,
they can generally be divided into two categories, adsorption diffusion models and
adsorption reaction models. Adsorption diffusion models are constructed under the
assumption that adsorption onto a porous material proceeds through the four distinct
steps described earlier, namely external mass transfer of adsorbate, boundary layer
195
diffusion, intraparticle mass transfer of adsorbate and adsorption at active sites.77, 329
Conversely, adsorption reaction models are based on chemical reaction kinetics and
consider the entire adsorption process without consideration of the individual steps
required for adsorption.355
Three common models used to analyse the adsorption of
organic dyes onto carbon materials include the pseudo-first order model, pseudo-second
order model and intraparticle diffusion model.
The pseudo-first order (Lagergren) model was the first adsorption kinetic model
developed and remains one of the most popular methods used to analyse kinetic
adsorption data.357
The model relies on the assumption that adsorption is driven by the
difference between the actual and equilibrium concentration of adsorbate at the surface
and that the rate of adsorption at active sites controls the overall kinetics of the
adsorption process.357
The pseudo-first order model is given by the equation:
𝑑𝑞𝑡
𝑑𝑡= 𝑘1(𝑞𝑒 − 𝑞𝑡)
8.1
Where:
𝑞𝑡 = Specific adsorption amount at 𝑡 (mg/g)
𝑡 = Time (min)
𝑘1 = Pseudo-first order rate constant (min-1
)
𝑞𝑒 = Equilibrium specific adsorption amount (mg/g)
Although often considered as an adsorption reaction type model, the pseudo-first order
model has been shown to be the limiting form of a generic kinetic model that is capable
of describing both adsorption reaction and adsorption diffusion mechanisms. However,
it is important to note that this model is only applicable to adsorption systems that are
196
either at or near equilibrium.357
To date, the model has been used to describe the
adsorption kinetics of a range of systems including the adsorption of methylene blue on
activated carbons prepared from agricultural waste358
, Acid Blue 113 onto commercial
activated carbon354
, as well as the adsorption of malachite green on reduced graphene
and graphene oxide359
.
Another type of adsorption kinetic model frequently encountered in the literature is the
pseudo-second order (Ho) model. Similar to the pseudo-first order model, the pseudo-
second order model assumes that the difference between the actual and equilibrium
concentration of adsorbate at the adsorbent surface provides the driving force for
adsorption. It also assumes that the rate of adsorption at active sites controls the overall
kinetics of the adsorption process according to a second order reaction rate.
Consequently, the pseudo-second order model is considered an adsorption reaction type
model and is represented by the expression:
𝑑𝑞𝑡
𝑑𝑡= 𝑘2(𝑞𝑒 − 𝑞𝑡)2
8.2
Where:
𝑘2 = Pseudo-second order rate constant (min-1
)
The pseudo-second order model is the most popular kinetic model used to describe
adsorption in solid/liquid systems356
and has been successfully applied to a range of
systems, including the adsorption of methylene blue on reduced graphene oxide77, 347
and activated carbon329
, methyl violet and rhodamine B on graphene oxide77
and
reactive black 5 on various activated carbons351
.
197
The intraparticle diffusion (Weber-Morris) model is a common example of an
adsorption diffusion type model. Unlike the pseudo-first order and pseudo-second order
models, it assumes that the rate of adsorption is controlled only by intraparticle mass
transfer of adsorbate via diffusion. The intraparticle diffusion model can be described
using the simplified equation:
𝑞𝑡 = 𝑘𝑝√𝑡 8.3
Where:
𝑘𝑝 = Intraparticle diffusion rate constant (mg g-1
min-0.5
)
To date, the intraparticle diffusion model has been used in the kinetic analysis of a
number of adsorption systems.77, 342, 344, 349
8.2.5.2 Adsorption Isotherm Models
Another approach used to study the adsorption of chemical species onto carbon
materials from solution at equilibrium is the modelling of adsorption isotherms.
Adsorption isotherms are critical not only in identifying key adsorption mechanisms,
but also characterising and comparing adsorption capacity and provide a quantitative
foundation for designing effective adsorption systems.360, 361
They are derived from
experimental data and describe the relationship between the surface excess
concentration of adsorbed species at an interface and the concentration or partial
pressure of an adsorbate in the adjacent liquid or vapour phase at constant temperature.
While there are a number of theoretical models that can be used to describe observed
adsorption isotherms, five of the most common models applicable to carbon materials341
are shown in Figure 8.2.
198
Figure 8.2: Schematic of adsorption isotherms commonly observed for carbon materials
including the (a) linear, (b) Langmuir, (c) Freundlich, (d) high affinity and (e) sigmoidal
isotherms.
When the adsorption process involves the adsorption of chemical species onto a solid
surface from solution at equilibrium, the adsorption isotherm is often expressed in terms
of the concentration of adsorbate in solution and specific adsorbed amount of adsorbate,
which is equivalent to the surface excess concentration. Thus, adsorption isotherms can
be used to determine the adsorption capacity of carbon materials and predict the nature
of the interaction between the adsorbate and adsorbent material in solution.
One of the simplest types of adsorption isotherm, Figure 8.2a, involves a linear increase
in adsorption with adsorbate concentration. These isotherms represent ideal adsorption
processes at low surface excess concentrations and can be described by the Henry
adsorption isotherm equation:
(a) 𝒄
𝚪
𝒄
𝚪
𝒄
𝚪
𝒄
𝚪
𝚪
𝒄
(b) (c)
(d) (e)
199
Γ = 𝑘𝐻𝑐 8.4
Where:
Γ = Surface excess (mol m-2
)
𝑘𝐻 = Henry adsorption constant (L m-2
)
𝑐 = Concentration of adsorbate (mol L-1
)
While carbon adsorbents rarely exhibit persistent linear adsorption isotherms, linear
adsorption does occur during the initial stages of all isotherms which depict adsorption
onto homogeneous surfaces341
.
The Langmuir adsorption isotherm (Figure 8.2b) is one type of isotherm which
describes the adsorption of chemical species onto homogeneous surfaces. This model
assumes a fixed number of adsorption sites across the surface, with one molecule of
adsorbate binding per site. The model also ignores interactions between adsorbates
along the surface. Langmuir adsorption isotherms are often observed for processes
involving adsorption from solution and can be described quantitatively using the
Langmuir adsorption isotherm equation:
𝜃 =𝑘𝐿𝑐
1 + 𝑘𝐿𝑐=
Γ
Γ𝑚𝑜𝑛
8.5
Where:
𝜃 = Fraction of surface covered with adsorbate
𝑘𝐿 = Langmuir constant (L mol-1
)
Γ𝑚𝑜𝑛 = Maximum amount adsorbed on the surface (mol m-2
)
200
The Langmuir constant, 𝑘, is related to both the affinity of the binding sites and energy
of adsorption while Γ𝑚𝑜𝑛 represents the maximum amount adsorbed on the surface.
Given the assumptions of the model, this parameter corresponds to a monolayer of
adsorbate. Consequently, adsorption processes described by the Langmuir adsorption
isotherm are often characterised by a sharp increase in adsorption at low concentration
of adsorbate, followed by surface saturation at high concentrations. These isotherms
have been observed for a variety dye adsorption processes which involve carbon based
materials including graphene oxide77, 347, 362
, reduced graphene oxide77, 347
, surfactant
intercalated reduced graphene oxide353
, carbon nanotubes362
, activated carbon338, 344, 362
and chemically modified activated carbon363
.
While many adsorption processes can be fit accurately using the Langmuir adsorption
isotherm, the most commonly observed type of isotherm is the Freundlich adsorption
isotherm (Figure 8.2c). The Freundlich adsorption isotherm can be described using the
empirical equation:
Γ = 𝑘𝑓𝑐1𝑛
8.6
Where:
𝑘𝑓 = Freundlich adsorption constant (L m-2
)
𝑛 = Freundlich exponent
Unlike the Langmuir adsorption isotherm, this model generally describes isotherms
which result from adsorption onto heterogeneous surfaces. Most real surfaces are
heterogeneous and therefore consist of adsorption sites which vary in binding affinity.
During the adsorption process, regions of high affinity are usually occupied by
adsorbate molecules first. Thus, adsorption processes described by the Freundlich
201
adsorption isotherm are usually characterised by a steep rise in adsorption at low
adsorbate concentrations, caused by regions of preferred adsorption. The steep increase
can also be attributed to the presence of lateral repulsion effects between adsorbates at
the surface. The Freundlich adsorption isotherm has been used to describe the
equilibrium dye adsorption capacity of a number of carbon based materials including
graphene oxide77, 364
, reduced graphene oxide77
, reduced graphene based hydrogels365
,
carbon nanotubes348
, graphene-carbon nanotube hybrid materials366
and activated
carbon349, 367, 368
.
Conversely, sigmoidal isotherms (Figure 8.2d) are encountered with certain flat,
homogeneous surfaces and often arise as a result of lateral cooperative interactions
between molecules on the surface, while high affinity isotherms (Figure 8.2e) describe
adsorption processes governed by strong interactions between the adsorbate and
adsorbent material. These isotherms are far less common than both Langmuir and
Freundlich type isotherms with respect to the removal of organic dyes from solution by
carbon adsorbents.
8.2.6 Graphene-based Materials as Carbon Adsorbents
Surfactant exfoliated graphene offers several distinct advantages over existing carbon
adsorbents with respect to the adsorption of organic dyes. For example, graphene has
the potential to outperform conventional porous carbon adsorbents in dye adsorption
processes due to its extremely high surface area to volume ratio. This property, which
is also shared by other graphene-based materials, may increase the surface area
accessible to the dye molecules, thereby enabling a greater specific adsorption capacity
compared to other adsorbents.309
The use of surfactant exfoliated graphene also enables
adsorbent surface interactions to be tailored to the removal of dye contaminants with
202
specific structural properties by employing an appropriate surfactant during the
exfoliation process. In particular, ionic surfactants could be used to promote
electrostatic interactions with charged dye molecules along the surface of the particles,
whilst retention of the pristine carbon lattice may enable π-π interactions with aromatic
dye structures. Lastly, surfactant exfoliated graphene has the advantage of being able to
be produced in scalable quantities using a simple, inexpensive process unlike activated
carbon, whose widespread use is restricted by high production costs.328
Consequently,
surfactant exfoliated graphene particles may provide a potentially cost efficient method
of removing organic dyes from effluent.
Indeed, the potential for graphene and graphene-based materials to act as effective dye
adsorbents has already been recognised and is discussed in several studies and
reviews.308, 311, 369-371
The majority of these works describe the adsorption of organic
dyes using either graphene-based nanocomposites92, 372-375
or graphene derivatives. For
instance, graphene oxide and reduced graphene have been used to remove a tremendous
variety of ionic dyes such as methylene blue77, 346, 347, 364, 376
, methyl blue377
, methyl
violet77
, orange G77
and rhodamine B77
and malachite green359, 376
amongst many others.
As with conventional carbon materials, the main interactions influencing adsorption
between ionic dyes and these graphene derivatives have been shown to be attractive
electrostatic interactions and π-π stacking.311, 378
However, due to their high aspect
ratios and accessible surface area, graphene derivatives have been found to enable faster
adsorption and exhibit higher dye adsorption capacities than conventional carbon
adsorbents.346
While there are numerous examples of graphene derivatives being employed in dye
adsorption processes throughout the literature, the use of surfactant exfoliated graphene
in the removal of organic dyes from solution is an area that has been largely unexplored.
203
It was only recently, that adsorbent materials closely resembling that of surfactant
exfoliated graphene were presented as a means of adsorbing dyes from water. Yusuf et
al.353
used a cationic surfactant, CTAB, to assist in the intercalation of reduced graphene
oxide in solution. The resultant particles were then used to adsorb two anionic azo
dyes, acid red 265 and acid orange 7 from an aqueous medium. It was shown that
adsorption was facilitated through electrostatic interactions, arising from the sulfate
anions on the dyes and the cationic, CTAB modified adsorbent surface, as well as π-π
interactions between the aromatic portions of the dye and carbon surface. Adsorption of
the dyes was also found to be highly dependent on pH, initial dye concentration and
contact time. While this study presents significant findings on the dye adsorption
behaviour of materials similar to surfactant exfoliated graphene, the results are limited
by a number of factors. In particular, the chemical structure of reduced graphene
particles are inherently different to the pristine graphene surfaces generated using the
surfactant assisted exfoliation method. The study also excludes experiments involving
the effect of electrostatic charges on adsorption with respect to both surfactant and dye
molecules. Furthermore, the use of two relatively uncommon dyes in the study also
raises difficulties in comparing the adsorption performance of the intercalated
compounds with other adsorbents. Therefore, it is essential that further studies into the
adsorption of ionic organic dyes on surfactant exfoliated graphene are conducted in
order to investigate the potential of these particles in dye removal processes.
This chapter presents for the first time, a systematic investigation into the adsorption
behaviour of defect free, surfactant assisted exfoliated graphene in aqueous solutions.
The study focuses on the adsorption of two ionic organic dyes, namely methylene blue
and methyl red from solution using graphene exfoliated from bulk graphite using both
ionic and non-ionic surfactants. Methylene blue, a cationic dye was used in this project
204
as it is commonly employed as a standard model compound in studies relating to the
removal of organic contaminants from water.347
In contrast, methyl red, an anionic dye,
was selected due to its structural features as an Azo dye. Azo dyes are the major class
of synthetic dyes used in the textile, paper, leather, rubber, food and technology
industries, and have been linked to mutagenic and carcinogenic activity in industrial
effluent.326
During this study, various parameters such as contact time, pH, temperature
and initial dye concentration were examined to determine their effect on organic dye
uptake by anionic surfactant exfoliated graphene. Adsorption kinetic and isotherm
models were also employed to analyse and characterise the adsorption behaviour of the
system.
8.3 Materials
The materials listed in §4.2.2 were again used in this study. In addition to these
materials, the surfactants SDS (CH3(CH2)11SO4Na) and CTAB ((C16H33)N(CH3)3)Br
and were also obtained from Sigma Aldrich along with two organic dyes, methylene
blue and methyl red sodium salt with properties given in Table 8.1. Anotop inorganic
0.1 and 0.02 μm syringe filters were obtained from VWR International.
205
Table 8.1: Physicochemical properties of methylene blue and methyl red.
Dye Property Methylene Blue Methyl Red
Molecular
formula
C16H18ClN3S·3H2O C15H14N3NaO2
Chemical
Structure
Ionic charge Cationic Anionic
Molecular mass
(g/mol)
319.85 291.28
pKa 3.8 379
4.8 380
Peak absorbance
at pH 5, 𝜆𝑚𝑎𝑥
(nm)
631 570
Surface area
(nm2)
0.85 343
1.60 381
8.4 Methods
8.4.1 Preparation of Stock Graphene Suspensions
Stock graphene suspensions were prepared via the method of ultrasonic exfoliation of
graphite, with continuous surfactant addition.116
In a typical experiment, surfactant
solutions were added at a rate of approximately one drop per second to a 2% w/w
suspension of graphite powder in water over 48 h. In order to produce graphene
206
suspensions stabilised using SDS, a 0.1% w/w solution of SDS (999 mL) was typically
added to the graphite solution. Similarly, a 10% w/w solution of Pluronic F108 (900
mL) was added at a rate of approximately one drop per second to a 2% w/w suspension
(980 mL) of graphite powder in water under ultrasonication for 48 h. CTAB stabilised
graphene suspensions were produced by adding a 10% w/w solution of CTAB at a rate
of approximately one drop per second to a 2% w/w suspension (980 mL of graphite
powder in water under ultrasonication for 48 h. The suspensions were then centrifuged
at 2500 rpm for 20 min to sediment larger, non-exfoliated graphite particles. The
resulting CTAB and SDS exfoliated suspensions were dialysed using 14 kDa dialysis
tubing. Concentrated suspensions of Pluronic F108 exfoliated graphene were prepared
by evaporating the suspension at a temperature of 70 °C to approximately 5% of the
original volume before being dialysed using 100 kDa dialysis tubing. In both cases,
dialysis was performed against water for a minimum of 48 h to remove unadsorbed
surfactant from the stock suspension.
The graphene concentration of the stock suspensions was obtained using the method
described by Lotya et al.9 Samples exfoliated using SDS and CTAB were diluted by a
factor of 20, while samples stabilised by Pluronic F108 were diluted by a factor of 100
before the visible light spectra measured using UV-Vis spectroscopy. Applying the
Beer-Lambert law to the absorption intensity of the samples at a wavelength of 660 nm
and applying an extinction co-efficient 112
, ε, of 54.22 L g-1
cm-1
, yielded an average
graphene concentration of 0.107 mg/mL, 0.954 mg/mL and 0.159 mg/mL for
suspensions exfoliated using SDS, Pluronic F108 and CTAB respectively.
207
8.4.2 Particle Characterisation
The exfoliated graphene particles were characterised using zeta potential measurements,
Raman spectroscopy, UV-Vis spectrophotometry and a DLS technique. The zeta
potential of the graphene particles was determined using a Malvern Zetasizer Nano
complete with MPT-2 autotitrator. Zeta potential measurements were performed at
intervals of 0.5 pH units between pH 3.0 and 9, with pH adjustment being performed
automatically using an appropriate amount of NaOH or HCl. Raman spectroscopy was
conducted on the graphene particles using a Renishaw inVia Reflex spectrometer
system, with 532 nm excitation laser. Samples were prepared by direct deposition of
the undiluted graphene suspension onto silicon wafers and measured in the dry state.
Particle sizing was performed using DLS with the Malvern Zetasizer Nano.
8.4.3 UV-Visible Spectroscopy
The main method used to analyse the concentration of organic dyes remaining in
solution was UV-Vis spectroscopy. The UV-Vis spectrum of the solutions was
measured using a Shimadzu 1800 UV-Vis spectrophotometer over the wavelength range
of 200 - 800 nm with baseline correction.
In order to determine the concentration of organic dye solutions based on UV-Vis
spectra, calibration curves of the two organic dyes were first constructed. Calibration
curves were obtained for solutions of methylene blue at pH 3, 5, 7, and 9, and methyl
red sodium salt at pH 5. For a typical calibration curve, a 100 mL stock solution of
1000 ppm organic dye solution was prepared then adjusted to the appropriate pH. In the
case of methylene blue, 0.25 - 1.5 mL of the solution was then diluted to 100 mL with
pH adjusted water to yield organic dye solutions with a concentration of 2.5, 5, 7.5, 10,
12.5, 15 ppm. For methyl red, 0.25 - 1 mL of stock solution was diluted to 100 mL to
208
yield solutions with a dye concentration of 2.5, 5, 7 and 10 ppm. Calibration curves
were constructed for each of the dyes at each pH. It was found that a linear relationship
existed between the concentration and peak absorbance, 𝜆𝑚𝑎𝑥, for each organic dye at
each pH tested.
8.4.4 Effect of Filtering Process on Measured Dye Concentration
In order to determine the effect of filtration on removal of the organic dyes from
solution, solutions of organic dyes were prepared and filtered, and their UV-Vis spectra
obtained. Solutions containing 5 ppm methyl red at pH 5 were prepared along with
solutions of 5 ppm and 150ppm methylene blue at pH 5. UV-Vis spectra were obtained
for the solutions both before and after filtration with a 0.02 μm syringe filter. Similarly,
the UV-Vis spectra of solutions containing 5 ppm methylene blue at pH 7 were
measured both before and after filtration with a 0.1 μm syringe filter. The solutions
showed a decrease of less than 5% in concentration following filtration of the solutions
using both types of filter, which can be considered negligible, given the error associated
with the measurements.
8.4.5 Adsorption of Dyes with Surfactant Exfoliated Graphene
SDS, Pluronic F108 and CTAB exfoliated graphene suspensions with a graphene
concentration of 0.053 mg/mL were adjusted to pH 5. 22.5 mL volumes of the
suspensions were then added to 0.125 mL of 1000 ppm pH 5 methylene blue or methyl
red along with 2.375 mL of pH adjusted water, yielding 25 mL solutions with an initial
dye concentration of 5 ppm (1.56 × 10-5
M and 1.72 × 10-5
M respectively). Next, the
solutions were agitated in a shaker bath at 60 rpm with a temperature of 25 °C for 48 h
209
and filtered using a 0.02 µm pore size syringe filter. The filtrate was then analysed
using UV-Vis spectroscopy to determine the dye concentration in solution.
8.4.6 Adsorption Isotherms and Temperature Effects on Dye Adsorption
500 mL SDS exfoliated graphene suspensions were diluted to 1 L with water, and
adjusted to pH 7. 22.5 mL volumes of the diluted SDS exfoliated graphene suspensions
were then added to 50 mL conical flasks. Next, 0.125, 0.25, 0.5, 1.25 and 2.5 mL of pH
7 1000 ppm methylene blue, and 2.5 mL of pH 1500 ppm methylene blue were added to
the suspensions, along with an appropriate amount of pH 7 adjusted water in order to
give 25 mL solutions with initial dye concentrations ranging from 5 ppm to 150 ppm
(4.70 × 10-4
M). The flasks were stoppered and the solutions agitated at 60 rpm in a
shaker bath heated to 25 °C. After 48 h, the solutions were filtered using a 0.1 µm pore
size syringe filter. The filtrate was then diluted by a factor of 2 for solutions with an
initial methylene blue concentration of 20 ppm, and 10 for solutions with an initial
concentration of between 50 and 150 ppm using water adjusted to pH 7. The UV-Vis
spectrum for each sample was then obtained and the peak intensity of each spectrum
used to determine the concentration of the organic dye from interpolation of the
appropriate calibration curve. The equilibrium specific adsorption amount of methylene
blue was calculated by Equation 8.7:
𝑞𝑒 =(𝐶0 − 𝐶𝑒)𝑉
𝑊
8.7
Where:
𝑞𝑒 = Equilibrium specific adsorption amount (mg/g)
𝐶0 = Initial concentration of methylene blue in solution (mg/L)
210
𝐶𝑒 = Equilibrium concentration of methylene blue in solution (mg/L)
𝑉 = Volume of solution (L)
𝑊 = Mass of adsorbent (g)
8.4.7 Adsorption Kinetics and Effect of Contact Time on Dye Adsorption
Again, 500 mL SDS exfoliated graphene suspensions were diluted to 1 L with water and
adjusted to pH 7. 90 mL volumes of the diluted suspensions were then added to 100
mL conical flasks. Next, 1, 5, and 10 mL of pH 7 1000 ppm methylene blue were
added to the suspensions, along with an appropriate amount of pH 7 adjusted water in
order to give 100 mL solutions with initial dye concentrations of 10 , 50 and 100 ppm.
The flasks were stoppered and the solutions agitated at 60 rpm in a shaker bath with a
temperature of 25 °C and 35 °C over a period of 48 h. Aliquots of approximately 10
mL were then withdrawn at 10, 30, 60, 120 (2 h), 240 (4 h), 480 (6 h), 1440 (24 h) and
2880 (48 h) min and filtered using a 0.1 μm pore size syringe filter. The UV-Vis
spectrum of each sample was obtained in order to determine the dye concentration. The
specific adsorption amount of methylene blue for each withdrawal, q𝑡, was then
calculated in a similar manner to Equation 8.7.
The effective surface charge of the graphene particles was also measured as a function
of time. In a typical experiment, 45 mL of the diluted suspension along with 4.5 mL of
water adjusted to pH 7 was introduced into the autotitrator flow system. Next, 0.5 mL
of pH 7 1000 ppm methylene blue was added to the suspension in order to yield a 50
mL solution with initial dye concentration of 10 ppm. The zeta potential was measured
over a period of 600 min, with the system monitoring the pH of the solution and
applying intermittent agitation to the solution approximately every 2 min.
211
8.4.8 Effect of pH on Dye Adsorption
100 mL of SDS exfoliated graphene suspension were diluted to 200 mL with water and
adjusted to pH 3, 5, 7 and 9. 22.5 mL volumes of the diluted graphene suspensions
were then added to 50 mL conical flasks. Next, 1.25 mL of 1000 ppm methylene blue
with corresponding pH was added to the suspensions, along with 1.25 mL of pH
adjusted water, yielding 25 mL solutions with an initial dye concentration of 50 ppm.
The solutions were then agitated in a shaker bath at 60 rpm with a temperature of 25 °C
for 48 h and filtered using a 0.1 µm syringe filter. A 10% v/v solution of the filtrate was
then prepared and analysed using UV-Vis spectroscopy to determine the dye
concentration in solution. The equilibrium specific adsorption amount of methylene
blue was then calculated.
8.5 Results and Discussion
8.5.1 Characterisation of Graphene
The optical, vibrational and physical properties of the exfoliated graphene particles were
characterised prior to their use in subsequent adsorption experiments using UV-Vis
spectroscopy, Raman spectroscopy, DLS and zeta potential measurements. The UV-Vis
and Raman spectra, together with the DLS particle sizing results are given in §A.4.1 of
the Appendix, and are consistent with earlier analyses. The zeta potential of graphene
particles exfoliated using CTAB, Pluronic F108 and SDS were measured as a function
of pH in order to determine the effective surface charge of the particles (Figure 8.3).
These measurements are consistent with current literature pertaining to surfactant
exfoliated graphene, as well as those in §4.2.4. Regardless of the surfactant employed
during the exfoliation process, the zeta potential of the graphene particles tend to
212
become progressively more negative with increasing pH. This occurs as a result of the
oxygenated edge defect sites on the graphene sheets introduced during the sonication
procedure123, 124
, which become deprotonated at higher pH.
More generally, the zeta potential values of the particles reflect the charge imparted by
the specific surfactant adsorbed along the graphene surface during the exfoliation
process. For instance, graphene particles exfoliated using SDS, an anionic surfactant,
exhibited a significant negative charge of between -47.5 and -66.8 mV, whilst graphene
particles exfoliated using the cationic surfactant, CTAB, demonstrated a considerable
positive charge of between 50.5 and 27.4 mV. In both cases, the charge exhibited by
the graphene particles is consistent with the presence of strongly bound ionic surfactant
molecules, despite extensive dialysis. Furthermore, these zeta potential values remain
largely outside the region of colloidal instability (±30 mV), indicating the particles
possess sufficient electrostatic repulsion in order to prevent particle aggregation. In
contrast, graphene particles exfoliated using the non-ionic surfactant, Pluronic F108,
exhibit low effective surface charges of between -10.0 and -35.7 mV. Unlike the SDS
and CTAB exfoliated graphene particles, the negative charge is unlikely to arise from
the presence of the surfactant, but rather the negatively charged oxygen containing edge
functional groups. The small negative charge also suggests that electrostatic repulsion
is insufficient to resist flocculation, thereby causing the particles to favour reaggregation
over time. However, previous studies indicate that Pluronic F108 exfoliated graphene
particles derive colloidal stability primarily through steric effects arising from the
irreversible adsorption of surfactant molecules on the surface of the graphene sheets.124,
136 Together, these results support earlier work which suggested that highly stable
suspensions of graphene particles with tailorable surface charges could be produced
213
through the surfactant-assisted ultrasonic exfoliation of graphite using variously charged
ionic surfactants117
.
Figure 8.3: Zeta potential measurement of graphene exfoliated using CTAB, Pluronic F108 and
SDS in NaCl 10-4
M.
8.5.2 Adsorption of Ionic Dyes with Surfactant Exfoliated Graphene
Adsorption studies were conducted using solutions of organic dye and surfactant
stabilised graphene under a variety of different solution conditions. These
measurements were performed to determine the effect of pH, temperature, dye
concentration and contact time on the adsorption capacity of surfactant stabilised
graphene particles. Experiments were also conducted in order to examine the effect of
graphene particle surface charge and overall charge exhibited by the organic dye
molecules on dye adsorption.
-80
-60
-40
-20
0
20
40
60
2 3 4 5 6 7 8 9 10
Zeta
Po
ten
tial
(m
V)
pH (units)
CTAB
Pluronic F108
SDS
214
In this study, two ionic dyes were used to simulate the removal of organic contaminants
from water using suspensions of surfactant exfoliated graphene. The dyes, which
included methylene blue and methyl red, were selected based on a number of
considerations. Firstly, methylene blue and methyl red dissociate in aqueous
environments to form cationic and anionic aromatic species respectively, which are
active in the UV-Vis region. Thus, by using simple analytical techniques such as UV-
Vis spectroscopy, the role of electrostatic and non-electrostatic interactions in the
adsorption of dyes onto graphene particles stabilised using ionic surfactants can be
investigated. Methylene blue was specifically chosen for the majority of adsorption
measurements as it exhibits a consistent UV-Vis spectrum across moderate pH values,
having a pKa of 3.8.379
Other organic dyes including methyl red, exhibit pKa values
that are closer to neutral. The use of these dyes poses difficulties in experiments
designed to investigate the effect of pH as their acid and base forms often demonstrate
significant differences in solubility and UV-Vis spectrum absorption.
8.5.3 Effect of Surfactant on Dye Adsorption
The adsorption capacity of graphene particles exfoliated using various surfactants were
investigated in order to determine the primary interactions responsible for dye
adsorption. In this series of experiments, 25 mL solutions containing either 5 ppm
methyl red or methylene blue and 0.048 mg/mL graphene exfoliated using CTAB,
Pluronic F108 and SDS at pH 5 were used. Each sample was equilibrated at 25 °C for
48 h after initial contact with dye then filtered using a 0.02 μm syringe filter. The UV-
Vis spectra of each of the samples were then obtained in order to determine the
concentration of dye remaining in solution. The results of the experiments are shown in
Table 8.2.
215
Table 8.2: Percentage of organic dye removed from solution by graphene particles produced
using surfactants with different electronic characteristics at pH 5.
Surfactant Methyl Red Removal
Efficiency (%)
Methylene Blue
Removal Efficiency (%)
CTAB 55.0 22.1
Pluronic F108 23.5 38.9
SDS 27.9 86.5
From Table 8.2, it is clear that each type of graphene suspension is able to remove both
anionic and cationic organic dyes from solution, irrespective of surfactant used during
the sonication process. The minimum amount of dye adsorbed by each type of
graphene suspension is similar, varying between 22.1 and 27.9%. However, graphene
suspensions prepared using a particular surfactant favour the adsorption of a particular
dye. Furthermore, the type of dye favoured generally exhibits an opposite charge to that
of the exfoliated particles. For instance, CTAB exfoliated graphene particles adsorbed
the greatest amount of methyl red, with over 50% of dye removed from solution. In
contrast, SDS exfoliated graphene particles were shown to be particularly effective in
the removal of the methylene blue, with less than 13.5% of dye remaining in solution
following exposure to the graphene suspension. As a consequence of this result, the
system was chosen as the basis for all subsequent adsorption experiments. Pluronic
F108 exfoliated graphene particles were also able to remove a larger percentage of
methylene blue than methyl red, albeit to a lesser extent than the SDS exfoliated
graphene suspension. Thus, it is clear that the graphene suspensions exhibit different
maximum adsorption capacities with respect to each of the organic dyes.
216
The differences in adsorption capacity between the various graphene suspensions with
respect to the two dyes may be caused by the different types of intermolecular
interactions possible between the surface of the graphene particles and dye molecules.
The different interactions can arise as the result of three distinct areas on the graphene
particles, namely the oxygen containing edge groups, the conjugated graphene structure
and the adsorbed surfactant, as well as the charge and structure of the dye molecules.
At pH 5, the electrostatic charge of the adsorbed surfactant dominates the net effective
surface charge of the graphene particles. For instance, CTAB exfoliated graphene
particles exhibit a positive charge, the Pluronic F108 exfoliated graphene possesses a
low negative charge, whilst SDS exfoliated graphene possesses a significant negative
charge. Given the pKa of methylene blue and methyl red are 3.8 and 4.8 respectively379,
380, the dye molecules exist largely in their charged states at pH 5. As a result, specific
combinations of effective surface charge on the graphene particles and charge on the
dyes can be used to facilitate electrostatic based interactions. In contrast, electron-rich
species such as the conjugated graphene structure and aromatic systems on the organic
dye molecules may enable adsorption through non-electrostatic interactions.
The ability to remove the dyes from solution, regardless of the effective surface charge
on the graphene particles, is consistent with non-electrostatic interactions between the
graphene surface and the dye molecules. It is likely that these interactions are
comprised primarily of π-π interactions, which typically exhibit greater interaction
strengths than van-der Waals interactions. π-π interactions have been shown previously
to drive adsorption of planar aromatic compounds such as Phenanthrene and biphenyl
from solution onto graphene based materials.310
As methylene blue and methyl red are
both planar molecules with aromatic π-electron systems, they possess the required
configuration in order to align and gain close proximity to the conjugated structure of
217
the graphene sheets so as to facilitate π-π stacking. However, the positioning of dye
molecules along the graphene surface and subsequent formation of π-π interactions is
likely to be prevented in areas where the stabilising surfactant is adsorbed.
Consequently, the size and number of the surfactant molecules adsorbed to the carbon
surface may affect the adsorption capacity of the graphene particles. This may account
for the slight variation in dye adsorption observed when anionic methyl red is adsorbed
using graphene exfoliated with a non-ionic and anionic surfactant.
While the adsorption measurements indicate that dye adsorption is possible through
non-electrostatic interactions, the data also suggests strong, attractive electrostatic
interactions between the dye and graphene particles promote further removal of dye
from solution. Since the zeta potential measurements shown in Figure 8.3 indicate the
electrostatic charge imparted by the ionic surfactants is distributed across the particle, it
is likely areas close in proximity to the adsorbed surfactants will experience higher
affinity for the dye molecules when favourable electrostatic interactions are permitted.
Favourable electrostatic interactions are possible between the cationic CTAB exfoliated
graphene particles and the anionic methyl red, as well as between the anionic SDS
exfoliated graphene particles and the cationic methylene blue molecules. Pluronic F108
exfoliated particles are also capable of establishing attractive electrostatic interactions
with cationic methylene blue, although to a lesser extent than the SDS exfoliated
graphene due to the magnitude of the particle surface charge. The presence of these
interactions is consistent with the results shown in Table 8.2, which demonstrate that the
graphene particles adsorb a greater percentage of dye when the dye exhibits an opposite
charge to that of the particles. Consequently, the adsorption of organic dyes onto
surfactant exfoliated graphene particles is maximised when attractive electrostatic
interactions are permitted to occur.
218
8.5.4 Effect of Contact Time on Dye Adsorption
The effect of contact time on adsorption of methylene blue onto SDS exfoliated
graphene was investigated over a period of 48 h using samples with initial dye
concentrations of 10, 50 and 100 ppm. The results of the adsorption measurements are
shown in Figure 8.4.
Figure 8.4: The effect of contact time on the specific adsorption amount of methylene blue on
SDS exfoliated graphene particles at various dye concentrations at 25 °C.
Figure 8.4 indicates that adsorption of methylene blue by SDS exfoliated graphene
occurs regardless of initial dye concentration and contact time. A high degree of
adsorption is observed only 10 mins after contact with the dye. This was the minimum
adsorption time able to be measured due to the amount of time required to filter the
solutions. As the experiment proceeds, the specific adsorbed amount of methylene blue
dye generally remains constant within experimental error. This suggests that the system
0
100
200
300
400
500
600
700
800
0 500 1000 1500 2000 2500 3000 3500
qt (
mg/
g)
Time (min)
10 ppm
50 ppm
100 ppm
219
achieves equilibrium within the first 10 min of the experiment. The rapid rate of
adsorption can be attributed to the high surface area of the graphene particles in solution
that are available to participate in the adsorption process. Given the results presented
earlier in Table 8.2, the adsorption process is likely governed by electrostatic
interactions between the cationic methylene blue and anionic SDS exfoliated graphene
particles. Consequently, the effective surface energy of the particles may also vary with
contact time.
The zeta potential of the graphene particles was monitored during the initial stages
following dye addition in order to further examine the mechanism of adsorption as a
function of contact time. As shown in Figure 8.5, the zeta potential of the graphene
particles increases from -46.3 mV during the first 240 minutes after initial contact with
the dye to an approximate plateau value of -30 mV. This initial increase in zeta
potential over time is again consistent with the electrostatically driven adsorption of
methylene blue onto SDS exfoliated graphene particles. As the amount of cationic dye
molecules adsorbed to the graphene particles increases, the number of unbalanced
negative charges on the particle surface decreases, thereby increasing the overall
effective surface charge. Typically, as the zeta potential enters the region of colloidal
instability, ±30 mV, repulsive electrostatic interactions are insufficient to maintain
particle stability. This tendency to aggregate at low surface charges is supported by
visual observations (Figure 8.6), which show significant aggregation after 120 min and
near complete aggregation of dispersed particles after 240 min. As a result, the
aggregation of the graphene particles may prove a desirable feature in the separation of
these nanoscale adsorbents from the liquid phase following removal of the dye from
solution.
220
Figure 8.5: Zeta potential measurement of SDS exfoliated graphene suspension with 10 ppm
methylene blue as a function of time at 25 °C.
Figure 8.6: Suspensions of SDS exfoliated graphene with 10 ppm methylene blue 10, 30, 60,
120, 240, 480, 1140 and 2880 min after initial contact with dye at 25 °C.
8.5.5 Adsorption Kinetics
The mechanism controlling the adsorption of methylene blue from solution by SDS
exfoliated graphene was further investigated by modelling the adsorption kinetics of the
process. While SDS exfoliated graphene particles are expected to be non-porous, the
presence of adsorbed surfactant on the graphene surface may enable intraparticle
diffusion processes to occur. Furthermore, the variation in the experimental
measurements coupled with the evidence of equilibrium in Figure 8.4 prevents the
-60
-50
-40
-30
-20
-10
0
0 100 200 300 400 500 600
Zeta
Po
ten
tial
(m
V)
Time (min)
221
pseudo-first order kinetic model, a common method of analysing adsorption kinetics
from being applied to the data. As a result, these two factors restrict the types of kinetic
models that can be applied to analyse the results.
The data presented in Figure 8.4 was analysed using two common kinetic models, the
pseudo-second order and intra-particle diffusion kinetic model. The linearised pseudo-
second order kinetic model is given by the equation:
𝑡
𝑞𝑡=
1
𝑘2𝑞𝑒2
+1
𝑞𝑒𝑡
8.8
Where:
𝑞𝑡 = Specific adsorption amount at 𝑡 (mg/g)
𝑡 = Time (min)
𝑘2 = Pseudo-second order rate constant (min-1
)
𝑞𝑒 = Equilibrium specific adsorption amount (mg/g)
The pseudo-second order model is applicable to processes which exhibit a linear trend
with a slope and intercept of 1
𝑞𝑒 and
1
𝑘2𝑞𝑒2 when
1
𝑞𝑡 is plotted against 𝑡. The linear form of
the intra-particle diffusion model can be expressed using the equation:
𝑞𝑡 = 𝑘𝑝𝑡0.5 + 𝐶 8.9
Where:
𝑘𝑝 = Intraparticle diffusion rate constant (mg g-1
min-0.5
)
𝐶 = Constant (mg/g)
222
Adsorption processes that can be described by the intra-particle diffusion model should
demonstrate a linear trend when 𝑞𝑡 is plotted with respect to 𝑡0.5. The slope and
intercept of the graph provides the model constants 𝐶 and 𝑘𝑝.
The results of the kinetic models are shown in Figure 8.7 and Figure 8.8 with relevant
constants and model fitting parameters given in Table 8.3. The pseudo-second-order
model offers excellent agreement with the experimental data for all dye concentrations
studied, as evidenced by the very high coefficients of determination for the linear
regression. The model also predicts the equilibrium specific adsorbed amount of
methylene blue accurately, with less than 2.3% difference between the experimental and
calculated 𝑞𝑒 values. In contrast, the intra-particle diffusion model is likely to provide a
poor description of the experimental data, with 𝑅2 values less than 0.5. In the event that
the model is applicable, 𝐶 indicates whether intraparticle diffusion is the rate controlling
step in the adsorption process. Here, 𝐶 is non-zero for all dye concentrations studied,
indicating that adsorption of methylene blue dye onto SDS exfoliated graphene occurs
through a complex adsorption mechanism instead of simple intraparticle diffusion.
Figure 8.7: Pseudo-second order model adsorption kinetics for methylene blue adsorbed onto
SDS exfoliated graphene.
0
2
4
6
8
10
12
14
16
18
0 500 1000 1500 2000 2500 3000 3500
t/q
t
Time (min)
10ppm
50ppm
100ppm
223
Figure 8.8: Intra-particle diffusion model adsorption kinetics for methylene blue adsorbed onto
SDS exfoliated graphene.
Table 8.3: Kinetic parameters for pseudo-second order model and intra-particle diffusion
model.
𝑪𝟎
(ppm)
Pseudo-Second Order Intra-Particle Diffusion
𝒒𝒆,𝒆𝒙𝒑,
(mg/g)
𝒒𝒆,𝒄𝒂𝒍𝒄
(mg/g)
𝒌𝟐
(min-1
)
𝑹𝟐 𝒌𝒑
(mg g-1
min-
0.5)
𝑪
(mg/g)
𝑹𝟐
10 147.72 172.41 0.000967 0.9996 -0.0689 175.36 0.1051
50 523.00 526.31 0.000269 0.9997 -0.7409 544.93 0.2364
100 602.00 588.23 -0.00011 0.9995 -1.238 658.82 0.3026
8.5.6 Effect of Temperature on Dye Adsorption
As temperature is also a critical factor in determining the adsorption capacity of a
material, the effect of temperature on the adsorption of methylene blue was also
investigated. The adsorption of dye onto SDS exfoliated graphene particles was studied
0
100
200
300
400
500
600
700
800
0.00 10.00 20.00 30.00 40.00 50.00 60.00
qt
t0.5
10ppm
50ppm
100ppm
224
at equilibrium using initial dye concentrations of 5, 10, 20, 50 and 100 ppm at 25, 35
and 45 °C. The results of the these experiments are shown in Figure 8.9, where the
specific adsorbed amount of methylene blue, 𝑞𝑒, is presented in terms of the equilibrium
concentration of methylene blue in solution, 𝐶𝑒.
Figure 8.9: The effect of temperature on the adsorption of pH 7 methylene blue on SDS
exfoliated graphene particles.
The majority of adsorption experiments described in this study were performed at 25
°C, where SDS exfoliated graphene exhibits a maximum adsorption capacity for
methylene blue of 782.3 mg/g. This value is greater than that of nearly 200 other
adsorbents listed in the literature333
, including various natural materials, bioadsorbents,
agricultural wastes, industrial adsorbents, as well as activated carbons and coals used for
commercial and research purposes. It is also greater than those of other graphene-based
materials including reduced graphene oxide347
and graphene oxide364
. However, there
remains a small number of materials which exhibit greater maximum adsorption
capacities than SDS exfoliated graphene including commercial activated carbon (980.3
mg/g)358
, PMAA modified biomass of baker’s yeast (869.6 mg/g)382
and teak wood bark
(914.59 mg/g)383
. In general, these adsorbents require at least 30 minutes to attain
0
100
200
300
400
500
600
700
800
900
0 20 40 60 80 100 120 140
qe (
mg/
g)
Ce (mg/L)
25°C
35°C
45°C
225
maximum adsorption of dye, unlike the SDS exfoliated graphene particles, which have
been shown to reach equilibrium within 10 minutes (Figure 8.4). Thus, SDS exfoliated
graphene particles exhibit excellent adsorption characteristics with respect to methylene
blue at 25 °C.
From Figure 8.9, it is clear that an increase in temperature reduces the adsorption
capacity of SDS exfoliated graphene. As the temperature increases from 25 °C to 45
°C, the maximum adsorption capacity of SDS exfoliated graphene decreases to 701.4
mg/L. Despite a reduction in adsorption capacity, these values remain far greater than
other graphene-based materials such as reduced graphene oxide, which has maximum
adsorption capacities ranging from 153.85 to 204.08 mg/g when the temperature is
varied from 20 to 40 °C.347
Consequently, SDS exfoliated graphene remains an
effective adsorbent under different environmental temperatures. The inverse
relationship between adsorption capacity and temperature such as that shown in Figure
8.9 is indicative of an exothermic process384
, which is consistent with the standard
thermodynamics of adsorption.
8.5.7 Equilibrium Adsorption Isotherm
In order to further investigate the process governing the adsorption of methylene blue
onto SDS exfoliated graphene, the data presented in Figure 8.9 was analysed using
several adsorption isotherm models. The first model applied to the data was the linear
adsorption isotherm model, which describes the simplest case of adsorption. Figure 8.9
shows a non-linear relationship between 𝐶𝑒 and 𝑞𝑒 at 25 °C, and appears consistent with
either Freundlich or Langmuir type adsorption. As the temperature is increased to 35
°C and 45 °C however, the relationship between 𝐶𝑒 and 𝑞𝑒, tends towards a linear trend.
This is supported by the higher 𝑅2 regression model fitting parameters shown in Table
226
8.4. Overall, the linear adsorption isotherm model does not accurately describe the
experimental data.
Table 8.4: Linear adsorption isotherm model fitting parameters.
Temperature (°C) 𝒌𝑯 𝑹𝟐
25 8.139 0.5351
35 6.881 0.8073
45 6.488 0.9634
In addition to the linear adsorption isotherm model, two frequently utilised adsorption
isotherm models, the Langmuir and Freundlich adsorption isotherms were also used to
analyse the data. The linearised Langmuir adsorption isotherm equation is given by:
𝐶𝑒
𝑞𝑒=
𝐶𝑒
𝑞𝑚𝑎𝑥+
1
𝑞𝑚𝑎𝑥𝑘𝐿
8.10
Where:
𝐶𝑒 = Equilibrium dye concentration (mol m-2
)
𝑞𝑒 = Equilibrium specific adsorption amount (mg/g)
𝑞𝑚𝑎𝑥 = Maximum specific adsorption amount (mg/g)
𝑘𝐿 = Langmuir constant (L g-1
)
Thus, processes which are described by the Langmuir model of adsorption should
exhibit a linear trend with a slope and intercept of 𝑞𝑚𝑎𝑥 and 𝑘𝐿 when 𝐶𝑒
𝑞𝑒 is plotted
against 𝐶𝑒. The commonly used linear form of the Freundlich adsorption isotherm
equation is given by Equation 8.11:
227
Ln𝑞𝑒 = Ln𝑘𝑓 +1
𝑛Ln𝐶𝑒
8.11
Where:
𝑘𝑓 = Freundlich adsorption constant (L/g)
𝑛 = Freundlich exponent
Given this equation, adsorption processes that are consistent with the Freundlich
adsorption isotherm equation should demonstrate a linear trend when Ln𝑞𝑒 is plotted
against Ln𝐶𝑒. In these cases, 𝑛 and 𝑘𝑓 can then be evaluated from the slope and
intercept of the plot.
The Langmuir adsorption isotherm as applied to the experimental data is shown in
Figure 8.10, with relevant constants and model fitting parameters given in Table 8.5.
Figure 8.10 shows good agreement between the adsorption model and experimental data
at 25 °C, which is supported by the high coefficient of determination presented in Table
8.5. However, the other 𝑅2 values suggest the data becomes less consistent with the
Langmuir model of adsorption as the temperature increases. Additionally, the
theoretical 𝑞𝑚𝑎𝑥 values shown in Table 8.5 do not correspond well with experimental
results, indicating the adsorption process is not adequately described by the Langmuir
isotherm.
228
Figure 8.10: Langmuir adsorption isotherms for methylene blue on SDS exfoliated graphene
particles.
Table 8.5: Langmuir adsorption isotherm constants and model fitting parameters.
Temperature (°C) 𝒒𝒎𝒂𝒙 (mg/g) 𝒌𝑳 (L/g) 𝑹𝟐
25 833.33 0.085 0.9781
35 769.23 0.046 0.8896
45 1428.57 0.008 0.6550
The Freundlich adsorption isotherm is shown in Figure 8.11, with relevant constants
and model fitting parameters given in Table 8.6. Figure 8.11 shows reasonable
agreement between the experimental data and adsorption model at all temperatures
studied. This is supported by the high 𝑅2 values given in Table 8.6, which suggest
better overall agreement with the Freundlich model than the Langmuir adsorption
isotherm. Consequently, the data indicates that the surfaces of SDS exfoliated graphene
particles tend to behave like heterogeneous surfaces during the adsorption process.
Such a result is consistent with the known surface properties of surfactant exfoliated
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 20 40 60 80 100 120 140
Ce/q
e (
g/L)
Ce (mg/L)
25°C
35°C
45°C
229
graphene, including the presence of chemically distinct species in specific areas along
the surface and edges of the carbon lattice.
Figure 8.11: Freundlich adsorption isotherms for methylene blue on SDS exfoliated graphene
particles.
The relationship between temperature and the isotherm constants shown in Table 8.6
also suggests the experimental data is compatible with the Freundlich adsorption
isotherm. As the temperature decreases, the slope of the isotherm 1
𝑛 approaches 0,
indicating the surface becomes more heterogeneous.385
This could directly reflect
changes in the amount of adsorbed SDS present on the graphene surface with
temperature, as the quantity of adsorbed ionic surfactant at a solid surface generally
decreases at heightened temperatures due to an increase in kinetic energy of the
species.386
While 1
𝑛 is related to surface heterogeneity, 𝑛 indicates the favourability of
the adsorption process, with adsorption typically occurring for 𝑛 < 1. The values for 1
𝑛
in Table 8.6, suggests that adsorption of the dye onto the graphene particles is
0
1
2
3
4
5
6
7
-2 -1 0 1 2 3 4 5 6
Ln q
e
Ln Ce
25°C
35°C
45°C
230
favourable at all temperatures studied, yet becomes less favourable as the temperature is
increased. Given that dye adsorption does occur in this experiment and is likely to be
driven primarily by strong electrostatic interactions involving the charged surfactant,
this trend is also consistent with a reduction in the amount of surfactant adsorbed on the
graphene particles as the temperature is raised. The proposed relationship between
temperature and surface heterogeneity is also supported by the trend in 𝑘𝐹, which is
observed to decrease with increased temperature. 𝑘𝐹 is an approximate indicator of
adsorption capacity, suggesting that the adsorption capacity of surfactant exfoliated
graphene particles decreases upon raising the temperature. Thus, the values for the
isotherm constants shown in Table 8.6 suggest the experimental data is reasonably
consistent with the Freundlich adsorption isotherm model.
Table 8.6: Freundlich adsorption isotherm constants and model fitting parameters.
Temperature (°C) 𝟏𝒏⁄ 𝒌𝑭 (L/g) 𝑹𝟐
25 0.354 4.95 0.9667
35 0.307 4.85 0.8809
45 0.782 2.83 0.9767
8.5.8 Effect of pH on Adsorption
In order to determine the effect of pH on the adsorption capacity of SDS exfoliated
graphene, the adsorption of methylene blue was investigated at pH 3,5,7 and 9.
Although pH has little effect on the ionization of methylene blue, it does significantly
alter the net surface charge of SDS exfoliated graphene particles. These changes in
surface charge arise due to the protonation or dissociation of oxygen containing groups
231
on the edges of graphene layers, rather than the ionisation of SDS, which is insensitive
to pH. Since electrostatic interactions are primarily responsible for the adsorption
process, the amount of methylene blue adsorbed onto the particles may vary with pH.
The specific adsorbed amount of methylene blue at equilibrium, 𝑞𝑒, is shown as a
function of pH in Figure 8.12. The specific adsorbed amount of dye varies little with
pH and is well within experimental error for all pH conditions tested. Thus, the
attractive interactions between the oxygenated edge groups and dye caused by changes
in pH are likely to have a minor effect on the amount of methylene blue adsorbed. This
behaviour is in direct contrast with other graphene based materials such as graphene
oxide and reduced graphene oxide, which have both shown increases in methylene blue
removal efficiency with pH.
Figure 8.12: The effect of pH on the specific adsorption amount of methylene blue on SDS
exfoliated graphene particles at 25 °C. The error bars indicate one standard deviation.
300
350
400
450
500
550
600
2 3 4 5 6 7 8 9 10
qe (
mg/
g)
pH (Units)
232
8.6 Conclusions
The ability to adsorb pollutants onto carbon-based materials is a major processing
technology critical to many wastewater treatment operations and environmental
remediation activities. It is particularly efficient at removing biologically harmful
contaminants such as organic dyes which often resist degradation in aqueous mediums.
Here, surfactant exfoliated graphene particles were used to adsorb two ionic organic
dyes, methylene blue and methyl red from solution. The graphene particles were
prepared in the presence of a cationic, non-ionic and anionic surfactant through aqueous
phase exfoliation of graphite. It was shown using zeta potential measurements that the
overall effective surface charge of these particles was dominated by the electrostatic
charge on the stabilising surfactant. In the case of the non-ionic surfactant, particle
surface charge was largely influenced by the presence of negatively charged oxygenated
edge groups.
The charge on the stabilising surfactant also had a significant influence on the
adsorption of the two ionic dyes. The percentage of dye removed by each type of
surfactant stabilised graphene was maximised when a dye with opposite charge to that
of the graphene surface was adsorbed from solution. For example, graphene particles
stabilised by the cationic surfactant, CTAB adsorbed the greatest amount of methyl red,
an anionic dye, whilst graphene particles exfoliated using SDS, an anionic surfactant
removed the greatest amount of methylene blue, a cationic dye, from solution.
Graphene particles stabilised by the non-ionic surfactant, Pluronic F108 adsorbed the
least amount of dye overall. This adsorption behaviour was attributed primarily to the
strong attractive electrostatic interactions between the ionic dyes and surfactants.
Nevertheless, a degree of adsorption occurred in all cases regardless of the type of dye
233
or graphene studied, suggesting π-π interactions also played a secondary role in dye
adsorption.
The effect of adsorbent properties and solution conditions on dye adsorption was also
studied with respect to the adsorption of methylene blue on SDS exfoliated graphene
particles. The adsorption of dye was found to be exceedingly rapid, reaching
equilibrium within the first 10 mins of contact for all initial dye concentrations tested.
Corresponding zeta potential measurements and visual observations taken at various
times during the adsorption process and that the graphene particles underwent a
reduction in particle surface charge which was coupled with progressive particle
aggregation, consistent with electrostatically driven adsorption. Meanwhile, the
temperature of the solution also had a significant influence on dye removal, with lower
temperatures favouring dye adsorption. Indeed, at 25 °C, the SDS exfoliated graphene
had a maximum adsorption capacity of 782.3 mg/g , greater than that of many other
adsorbents in the literature. In contrast, adsorption capacity showed little variation with
the pH of the solution, indicating that the attractive interactions between the oxygenated
edge groups and dye had a negligible effect on the amount of methylene blue adsorbed.
The adsorption data was also analysed using a series of theoretical models in an effort to
characterise the main underlying adsorption mechanism and predict the maximum
adsorption capacity of methylene blue with respect to SDS exfoliated graphene. It was
shown that the experimental kinetic data provided excellent agreement with the
adsorption parameters predicted using the pseudo-second order kinetics model. The
results from the adsorption experiments were also consistent with the Freundlich
adsorption isotherm, suggesting adsorption of methylene blue occurs at active sites
along the chemically heterogeneous SDS exfoliated graphene surface. Consequently,
by analysing the adsorption data using theoretical models, the underlying adsorption
234
process could be accurately characterised. Furthermore, altering dye-particle
interactions through changes in processing conditions enabled overall changes in
adsorption efficiency. Thus, the results of these experiments collectively illustrate the
potential of surfactant exfoliated graphene as effective carbon-based adsorbents.
235
CHAPTER 9
Chapter 9
Conclusions and Further Work
9.1 Conclusions
Graphene is an extraordinary material that has captured the attention of both research
and industry through its potential to deliver enhanced performance to existing
applications and its prospects in future disruptive technologies. However, mass
producing pristine graphene in a form which can be easily integrated and used in
products and applications has proved one of the most challenging obstacles towards the
widespread, practical implementation of graphene. The ultrasonic exfoliation of
graphene in aqueous surfactant solutions provides an established method of producing
defect-free graphene that is suited to large scale manufacture. It also offers an efficient
route towards the transfer and integration of such particles into products and
applications through adsorption.
In this thesis, the fundamental adsorption behaviour of surfactant exfoliated graphene
was investigated through four different proof-of-concept case studies, with each study
presenting a potential application governed by adsorption mechanisms. Each case study
made use of a different kind of interface or different types of intermolecular interactions
in order to explore the range of adsorption processes available to surfactant exfoliated
graphene particles. During these experiments, the suitability of the particles in each
application was also considered by examining whether the result of the adsorption
process addressed the basic, practical requirements of the application under
236
investigation. Together, the studies conducted in this thesis revealed a number of
significant findings.
Electrostatically Assembled Multilayer Thin Films
This study demonstrated the adsorption of surfactant exfoliated graphene at a solid-
liquid interface using electrostatic interactions. Here, multilayer thin films comprised of
the cationic polyelectrolyte, PEI, and low-surface charge anionic surfactant stabilised
graphene, were successfully assembled layer-by-layer on a silicon substrate using
electrostatic interactions. The deposition and thickness of the resultant films were
shown using QCM measurements to be affected by a range of solution conditions such
as ionic species, ionic strength, pH and graphene concentration. Furthermore, the
resultant films exhibited linear film growth for most of the solution conditions trialled,
indicating that the films possessed a highly stratified internal structure. Consequently,
the results of the study showed the potential for surfactant exfoliated graphene particles
to be used in the production of surface coatings and nanocomposites with highly
tailorable film properties.
Multilayer Thin Films Assembled using Hydrogen Bonding
This study illustrated the adsorption of surfactant exfoliated graphene at a solid-liquid
interface using hydrogen bonding. The investigation involved the preparation of
multilayer thin films containing Pluronic F108 exfoliated graphene and the
polyelectrolyte, PAA at low pH using the manual deposition of layers and automated
dip coating. The bulk of the thin films were formed through hydrogen bonding
interactions between the carboxylic acid groups on the PAA and the ethylene oxide
groups on the adsorbed Pluronic F108, in the absence of electrostatic interactions.
237
QCM measurements and Raman spectroscopy indicated that the resultant films
exhibited a super-linear growth regime consistent with a homogeneous internal
structure. Rinsing the films with neutral or basic solutions enabled partial removal of
the films, which was attributed to degradation of the hydrogen bonding interactions in
the film. AFM QNM measurements were used to characterise the mechanical
properties of the dip coated films and indicated the adhesion, deformation and
dissipation of the films were largely independent of the number of adsorbed layers in
the film. Films with more than 300 layers however, exhibited a severe reduction in
Young’s modulus compared to films comprised of fewer adsorbed layers. Together,
these results reveal important information surrounding the growth mechanisms,
mechanical properties and the effect of environmental conditions on hydrogen-bonded
thin films that contain surfactant exfoliated graphene. Furthermore, the work
successfully demonstrates the capacity for surfactant exfoliated graphene to be used in
functional coatings designed to respond to changes in environmental conditions.
Graphene as an Aid to Foam Stabilisation
The adsorption of surfactant exfoliated graphene at a liquid-vapour interface formed the
basis of this investigation. Here, foams were produced using the foaming agent
Pluronic F108 and were stabilised using low concentrations of Pluronic F108 exfoliated
graphene, which adsorbed along the air-water interface. It was shown using foaming
experiments that the presence of graphene particles had a significant influence on
surfactant foam stability and volume. For instance, the presence of the graphene
particles destabilised the foams at low graphene concentrations, whilst yielding an
increase in foam stability above a critical concentration. This behaviour was attributed
to the rate at which graphene accumulated at the air-water interface and within the
bubble film to inhibit drainage and coalescence. These observations were further
238
supported by the analysis of bubble size distributions, which showed that foams retained
a higher proportion of bubbles over time when higher graphene concentrations were
employed, suggesting that in these cases the graphene particles form a physical barrier
against disproportionation. The effect of alkali metal salts on the particle stabilised
foams was also studied and showed that foams experienced enhanced stability based on
specific ion effects. Thus, the work presented in this study demonstrates for the first
time the potential for surfactant exfoliated graphene particles to be used in the
stabilisation of surfactant foams for industrial purposes.
Graphene as an Adsorbent for Organic Dyes in Water
The work presented in this study illustrated the adsorption of organic molecules along
the surface of surfactant exfoliated graphene particles suspended in the liquid phase.
Investigations consisted of adsorbing methyl red and methylene blue, an anionic and
cationic organic dye onto CTAB, SDS and Pluronic F108 exfoliated graphene particles
suspended in solution. The surface charge on the graphene particles had a significant
influence on the adsorption of the two dyes, with adsorption being driven primarily
through favourable electrostatic interactions between the dye molecules and the
graphene particle surface. The effect of solution conditions on dye adsorption was also
studied with respect to the adsorption of methylene blue on SDS exfoliated graphene
particles. The adsorption process was found to be rapid, exothermic and largely
resistant to changes in pH. Furthermore, exfoliated graphene particles were shown to
have a maximum adsorption capacity for methylene blue of 782.3 mg/g, greater than
that of many other adsorbents presented in the literature. It was also demonstrated that
the adsorption data provided excellent agreement with adsorption parameters predicted
using two common kinetic and thermodynamic models, the pseudo-second order
kinetics model and the Freundlich adsorption isotherm. Together, the results of the
239
study showed for the first time, the immense capacity for surfactant exfoliated graphene
particles to be used in the removal of organic contaminants from aqueous environments.
Thus, the potential for the adsorption behaviour of surfactant exfoliated graphene to be
used for practical purposes was demonstrated using these four case studies. Together,
the results highlight the versatility of surfactant exfoliated graphene particles, which are
derived not only from properties related to their unique planar carbon structure, but also
the presence of the surfactant. Furthermore, the work of this thesis emphasises the wide
range of interactions which can be used to successfully deploy these particles in a range
of adsorption-based applications.
9.2 Further Work
The experiments presented in this thesis were largely performed using graphene
particles stabilised by the poloxamer, Pluronic F108. As a result, the most obvious
option for further investigation is conducting the experiments with other types of
poloxamers, as well as other types of surfactants in an effort to alter the strength of the
surface interactions occurring at the graphene surface. The effect of exfoliation
processing parameters such as sonication power, solution volume, time, surfactant
concentration and temperature on graphene production could also be explored.
Alternative methods of removing the excess surfactant from the graphene suspensions
could also be investigated, such as the method employed by Wang et al.387
where
centrifugation and separation of the graphitic material is repeatedly performed before
dilution of the suspension.
Throughout this project, a number of attempts were made to characterise the specific
surface area of the exfoliated graphene particles, as well as quantify the amount of
240
surfactant adsorbed on the particles during the exfoliation. These attempts, which
primarily involved the use of Brauner-Emmett-Teller (BET) nitrogen adsorption and
proton nuclear magnetic resonance (NMR) techniques were ultimately unsuccessful in
providing correct results. This was due in part, to the difficulties associated with
particle aggregation, and in determining the relaxation parameters required for NMR
analysis, which are unique to both particle material and particle size. However, an
accurate determination of the specific surface area and amount of surfactant adsorbed
along the graphene surface would prove invaluable in further analysing the data
presented in this thesis.
Consequently, there are a number of topics that apply broadly to this thesis and which
warrant further investigation. However, there are also several other experiments which
could also complement the core work in each of the studies presented. These
experiments are outlined below.
Electrostatically Assembled Multilayer Thin Films
Investigating the use of electrostatically assembled multilayer thin films
containing surfactant exfoliated graphene in the creation of hollow capsules.
These studies would require not only the ability to deposit the films on
templating particles, but also the removal of the core particles whilst retaining
the structure of the film.
Studying the inclusion of other functional materials and molecules in the
assembly of the multilayer films to enhance film properties.
241
Multilayer Thin Films Assembled using Hydrogen Bonding
Repeating the AFM QNM experiments with an average relative humidity greater
than 59.5%. Studies have shown that exceptionally high levels of humidity
nearing 100% enable entrained water within hydrogen-bonded multilayer films
to act as a plasticiser.206
However, such high levels of humidity may prove
incompatible with the electronic systems present in the AFM QNM apparatus.
Determining whether the multilayer thin films exhibit other functional
properties, in particular, gas barrier properties.
Graphene as an Aid to Foam Stabilisation
Using different gases in the formation of foams to further investigate the density
of graphene particles located at the bubble surface. As different gases exhibit
different solubilities in the aqueous phase, the rates of disproportionation are
likely to differ with the use of different gasses. If the bubble surface is covered
by a sufficiently dense layer of graphene particles however, disproportionation
may be prevented regardless of the type of gas employed during foaming.
Using poloxamers with different PEO chain lengths such as P105, P103, P104 or
L101 in the production of the foams and exfoliated graphene to investigate the
effect of particle wettability on foam stability.
Graphene as an Adsorbent for Organic Dyes in Water
Determining the effect of ionic strength on the adsorption capacity of SDS
exfoliated graphene with respect to methylene blue.
242
Investigating the effect of dialysis time on the adsorption capacity of SDS
exfoliated graphene with respect to methylene blue. As the adsorption of
monomeric surfactants is reversible, varying the time for dialysis may affect the
amount of surfactant adsorbed. This may enable graphene particles to be
produced with the same type of surface chemistry, yet graduated surface charge.
Examining the effect of alkali metal salts on the adsorption capacity of Pluronic
F108 exfoliated graphene with respect to aromatic molecules. As indicated by
the studies performed in Chapter 7, the presence of alkali metal cations is likely
to affect the conformation of the PEO chains on the surfactant. This may allow
aromatic molecules to gain closer proximity to the graphene conjugated lattice,
which could enable adsorption through a greater number of van der Waals based
interactions.
Investigating the adsorption of inorganic molecules and ions using ionic and
non-ionic surfactant exfoliated graphene.
243
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285
APPENDIX
A.1 Literature Review
This section summarises studies in the literature that describe the method of ultrasonic
exfoliation of graphene from graphite in aqueous surfactant solutions. The studies are
ordered chronologically, beginning with the earliest studies. The experiment conditions
and results for each study are given in terms of initial and final concentrations, as well
as sonication and centrifugation parameters (Table A.1). Much of the work presented in
this section was first presented in the supplementary material associated with a peer
reviewed article (Reproduced from Ref. 65 with permission from The Royal Society of
Chemistry.)
Table A.1: Labels used to describe exfoliation conditions of studies shown in Table A.2.
Label Parameters
𝐶𝐺,𝑖 Initial graphite concentration
𝐶𝐺,𝑓 Final maximum graphene concentration reported
𝐶𝑠𝑢𝑟𝑓 Initial surfactant concentration
Sonication Sonication power, sonication duration
Centrifugation Centrifuge rotor speed, centrifuge duration
286
Tab
le A
.2:
Su
mm
ary
of
stud
ies
in t
he
lite
ratu
re d
escr
ibin
g t
he
ult
raso
nic
exfo
liat
ion o
f gra
phen
e fr
om
gra
phit
e in
aq
ueo
us
surf
acta
nt
solu
tio
ns.
Ref
.
9
87
11
9
Sel
ecte
d P
rop
erti
es a
nd
Res
ult
s
Pro
po
rtio
n o
f fl
akes
, by
num
ber
:
Few
lay
er g
raph
ene
flak
es:
45
%
Mono
layer
gra
ph
ene:
3%
Lat
eral
siz
e:
Pea
k w
idth
:15
0 n
m
Pea
k l
eng
th:
250
nm
Rat
e o
f ex
foli
atio
n p
ropo
rtio
nal
to 𝑡
0.5
Th
icknes
s o
f fl
akes
: ~
1.3
nm
Lat
eral
siz
e:
Av
erag
e le
ng
th:
700
nm
Av
erag
e w
idth
: 50
0 n
m
Pro
po
rtio
n o
f fl
akes
, by
num
ber
:
Mono
layer
gra
ph
ene:
80
%
Pa
rtic
le
Ch
ara
cter
isa
tio
n
Met
ho
ds
UV
-Vis
, H
RT
EM
, ze
ta
pote
nti
al,
SE
M,
Ram
an, F
TIR
, X
PS
,
XR
D,
AF
M,
TG
A.
UV
-Vis
, S
EM
, E
DX
,
AF
M,
HR
TE
M
UV
-Vis
, A
FM
, R
aman
𝑪𝑮
,𝒇 ,
(m
g/m
L)
0.0
5
- 0.0
9
(Fo
llo
win
g
sonic
atio
n a
nd
centr
ifu
gat
ion
)
Ex
foli
ati
on
Co
nd
itio
ns
𝐶𝐺
,𝑖:
0.1
mg
/mL
𝐶𝑆
𝑢𝑟
𝑓:
0.5
mg
/mL
So
nic
atio
n:
30
min
Cen
trif
ugat
ion:
500
rp
m, 90
min
𝐶𝐺
,𝑖:
10
mg/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.5
M
So
nic
atio
n:
4 h
Cen
trif
ugat
ion:2
00
00
rp
m, 45
min
𝐶𝐺
,𝑖:
0.8
5 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
29
mg/m
L
So
nic
atio
n:
51
- 5
2 W
, 1 h
Cen
trif
ugat
ion:
750
-150
00
rp
m, 10
-60
min
Met
ho
d
Aq
ueo
us,
su
rfac
tant
assi
sted
ex
foli
atio
n o
f
gra
phen
e w
ith b
ath
sonic
atio
n a
nd
centr
ifu
gat
ion
.
HO
PG
so
nic
ated
in
surf
acta
nt
solu
tion
in
gla
cial
ace
tic
acid
, re
flux
ed
und
er n
itro
gen
atm
osp
her
e,
then
dec
ante
d a
nd
centr
ifu
ged
. R
esult
ant
resi
du
e re
susp
ended
in
DM
F.
Aq
ueo
us,
su
rfac
tant
assi
sted
ex
foli
atio
n o
f
gra
phen
e w
ith s
onic
atio
n
and
cen
trif
ugat
ion
. S
tep
gra
die
nt
centr
ifu
gat
ion
,
foll
ow
ed b
y d
ensi
ty
gra
die
nt
ult
race
ntr
ifug
atio
n.
Su
rfa
cta
nt
Res
po
nsi
ble
fo
r
Ex
foli
ati
on
So
diu
m
do
decy
lben
zen
e-
sulf
on
ate
(S
DB
S)
Cety
ltri
meth
yl
am
mo
niu
mb
rom
ide
(CT
AB
)
So
diu
m C
hola
te (
SC
)
287
12
0
12
1
11
7
Gra
ph
itic
fla
kes
wit
h a
hig
her
deg
ree
of
exfo
liat
ion
co
mp
ared
wit
h S
DB
S.
Ab
le t
o p
rep
are
gra
ph
ene
thin
fil
ms
wit
h a
dir
ect
con
du
ctiv
ity
of
1.5
× 1
04 S
m-1
Pro
po
rtio
n o
f fl
akes
, by
nu
mb
er:
Mo
no
lay
er g
rap
hen
e: 2
0%
Lat
eral
par
ticl
e si
ze:
Av
erag
e le
ng
th:
1 µ
m
Av
erag
e w
idth
: 40
0 n
m
Mea
n f
lak
e le
ng
th i
nv
erse
ly p
ropo
rtio
nal
to
cen
trif
ug
atio
n r
ate
Lat
eral
par
ticl
e si
ze:
Mea
n l
eng
th i
n a
ll c
ases
: 0
.75 μ
m
Ion
ic s
urf
acta
nts
yie
ld c
on
centr
atio
ns
of
gra
ph
ene
dep
end
ent
on
zet
a po
ten
tial
(el
ectr
ost
atic
stab
ilis
atio
n).
N
on
-io
nic
su
rfac
tan
ts s
tab
ilis
e v
ia
ster
ic i
nte
ract
ion
s.
UV
-Vis
, T
EM
,
FT
IR,
SE
M,
Ram
an,
AF
M
UV
-Vis
, T
EM
,
Ram
an,
SE
M,
TG
A
UV
-Vis
, ze
ta
po
tenti
al,
Ram
an,
TE
M,
0.0
4
0.3
0.0
26
(Ach
iev
ed w
ith
bo
th I
GE
PA
L
CO
-89
0 a
nd
SC
)
𝐶𝐺
,𝑖:
0.7
5 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.5
mg
/mL
So
nic
atio
n:
14
0 m
in (
1st s
tag
e),
14
0 m
in (
2nd s
tag
e)
Cen
trif
ug
atio
n:
50
00
rp
m,
90
min
𝐶𝐺
,𝑖:
5 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:0
.01
- 2
0 m
g/m
L
So
nic
atio
n:
16
W , 2
4 h
Cen
trif
ug
atio
n:
50
0 -
20
0 r
pm
,
30
- 9
0 m
in
𝐶𝐺
,𝑖:
5 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.1
mg
/mL
So
nic
atio
n:
56
2.5
W,
30
min
,
Cen
trif
ug
atio
n:
15
00
rp
m,
90
min
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h b
ath
son
icat
ion
and
cen
trif
ug
atio
n.
Mu
ltip
le
son
icat
ion
sta
ges
per
form
ed b
efo
re
cen
trif
ug
atio
n.
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h b
ath
son
icat
ion
and
cen
trif
ug
atio
n.
So
nic
atio
n t
ime
and
cen
trif
ug
atio
n s
pee
d
wer
e v
arie
d f
acto
rs.
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h t
ip
son
icat
ion
and
cen
trif
ug
atio
n.
SC
SC
SD
S,
SD
BS
, li
thiu
m
do
dec
yl
sulf
ate
(LD
S),
CT
AB
,
Tw
een
® 8
0,
tetr
ad
ecy
ltri
met
hy
la
mm
on
ium
bro
mid
e
(TT
AB
), S
C,
sod
ium
deo
xy
cho
late
(D
OC
)
an
d s
od
ium
tau
rod
eox
ych
ola
te
(TD
OC
),
IGE
PA
L®
CO
-89
0, T
rito
n™
X-
10
0 a
nd
Tw
een
20
288
Tab
le A
.2:
Su
mm
ary
of
stud
ies
in t
he
lite
ratu
re d
escr
ibin
g t
he
ult
raso
nic
exfo
liat
ion o
f gra
phen
e fr
om
gra
phit
e in
aq
ueo
us
surf
acta
nt
solu
tio
ns
(co
nt.
).
R
ef.
125
123
Sel
ecte
d P
rop
erti
es a
nd
Res
ult
s
Gra
ph
ene
thic
kn
esse
s ra
ng
e fr
om
1 t
o 4
nm
,
con
sist
ent
wit
h 1
- 1
0 l
ayer
s o
f st
ack
ed g
raph
ene.
Lat
eral
siz
e o
f fl
akes
bet
wee
n 5
0 a
nd
sev
eral
hu
nd
red
nm
.
Ab
ilit
y t
o e
nh
ance
ex
foli
atio
n e
ffic
ien
cy b
y t
un
ing
surf
ace
inte
ract
ion
s o
f se
lect
ed P
luro
nic
an
d
Tet
ron
ic s
urf
acta
nts
.
Ap
pro
xim
atel
y 8
0%
of
par
ticl
es e
ith
er m
ono
lay
er
or
bil
ayer
gra
ph
ene.
Ty
pic
al f
lak
e d
iam
eter
: <
30 n
m
Pa
rtic
le
Ch
ara
cter
isa
tio
n
Met
ho
ds
SE
M,
Ram
an,
AF
M
AF
M,
Ram
an,
zeta
po
tenti
al,
turb
idim
eter
𝑪𝑮
,𝒇 ,
(m
g/m
L)
𝐶𝐺
,𝑖:
75 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
1%
w/v
So
nic
atio
n:
16
-
18
W,
30
min
Cen
trif
ug
atio
n:
75
0 -
15
00
0
rpm
, 10
– 6
0
min
~0
.55
Ex
foli
ati
on
Co
nd
itio
ns
𝐶𝐺
,𝑖:
75 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
1%
w/v
So
nic
atio
n:
16
- 1
8 W
, 3
0 m
in
Cen
trif
ug
atio
n:
75
0 -
15
00
0
rpm
, 10
– 6
0 m
in
𝐶𝐺
,𝑖:
1%
w/w
𝐶𝑆
𝑢𝑟
𝑓:
0.1
– 0
.9 ×
10
-3 M
So
nic
atio
n:
30
W, 5
min
Cen
trif
ug
atio
n:
150
0 g
, 5
min
Met
ho
d
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h
tem
per
atu
re c
on
troll
ed
tip
son
icat
ion
, th
en
cen
trif
ug
atio
n.
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h t
ip
son
icat
ion
. S
ub
seq
uen
t
cen
trif
ug
atio
n a
nd
dia
lysi
s w
as p
erfo
rmed
.
Su
rfa
cta
nt
Res
po
nsi
ble
fo
r
Ex
foli
ati
on
Plu
ron
ic F
127
, P
12
3,
P1
04
, P
103
, F
84
,F38
,
F6
8, F
88
, F
98
, F
108
,
F7
7, F
87
, a
nd
Tet
ron
ic® 9
08,
11
07
,
13
07
an
d 9
04
CT
AB
289
12
6
Pro
po
rtio
n o
f fl
akes
, by
nu
mb
er:
Mo
no
lay
er g
rap
hen
e: 1
0 -
15%
Lat
eral
siz
e o
f fl
akes
wit
hin
th
e 50
- 2
00
nm
ran
ge.
UV
-Vis
, A
FM
,
ST
M,
Ram
an,
XP
S,
SE
M,
TG
A
1.5
(Ach
iev
ed w
ith
P-1
23
, at
exte
nd
ed
son
icat
ion
tim
e
of
5 h
)
𝐶𝐺
,𝑖::
10
0 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.5
% a
nd
1.0
% w
/v
So
nic
atio
n:
40
kH
z, 2
-5 h
Cen
trif
ug
atio
n:
50
0 g
, 5
min
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h b
ath
son
icat
ion
and
cen
trif
ug
atio
n.
Plu
ron
ic P
-123
,
Tw
een
80
, B
rij®
70
0,
Gu
m a
rab
ic f
rom
aca
cia
tre
e, T
rito
n
X-1
00,
Tw
een
85
,
Bri
j 3
0,
Po
lyv
iny
lpy
rro
lid
on
e
(PV
P),
n-D
od
ecy
l b
-
D-m
alt
osi
de
(DB
DM
),
Po
ly(s
od
ium
4-
sty
ren
esu
lfo
na
te)
(PS
S),
3-[
(3-
Ch
ola
mid
op
rop
yl)
di
met
hy
l a
mm
on
io]-
1-
Pro
pa
nes
ulf
on
ate
(CH
AP
S),
DO
C,
SD
BS
, 1
-
Py
ren
ebu
tyri
c a
cid
(PB
A),
Sod
ium
do
dec
yl
sulp
ha
te
(SD
S),
TD
OC
,
Hex
ad
ecy
ltri
met
hy
la
mm
on
ium
bro
mid
e
(HT
AB
)
290
Tab
le A
.2:
Su
mm
ary
of
stud
ies
in t
he
lite
ratu
re d
escr
ibin
g t
he
ult
raso
nic
exfo
liat
ion o
f gra
phen
e fr
om
gra
phit
e in
aq
ueo
us
surf
acta
nt
solu
tio
ns
(co
nt.
).
Ref
.
124
38
8
Sel
ecte
d P
rop
erti
es a
nd
Res
ult
s
Ty
pic
al f
lak
e d
iam
eter
: 50
– 6
0 n
m
Max
imu
m f
lak
e d
iam
eter
: <
200
nm
Mea
n l
ater
al p
arti
cle
size
(S
tock
): 0
.58
μm
Cle
ar s
epar
atio
n o
f p
arti
cles
bas
ed o
n s
ize:
2nd f
ract
ion
, m
ean
lat
eral
siz
e:1
.17
μm
14
th f
ract
ion
, m
ean
lat
eral
siz
e: 0
.45
μm
Pa
rtic
le
Ch
ara
cter
isa
tio
n
Met
ho
ds
AF
M,
Ram
an,
Zet
a
po
tenti
al
UV
-Vis
, T
EM
,
Ram
an
𝑪𝑮
,𝒇 ,
(m
g/m
L)
0.1
-
Ex
foli
ati
on
Co
nd
itio
ns
𝐶𝐺
,𝑖:
1%
w/w
𝐶𝑆
𝑢𝑟
𝑓:
6 ×
10
-4 M
So
nic
atio
n:
30
W, 5
min
Cen
trif
ug
atio
n:1
500
g,
15
min
𝐶𝐺
,𝑖:2
0 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.3
mg
/mL
So
nic
atio
n:
8 h
Cen
trif
ug
atio
n:
50
0 r
pm
, 4
5
min
Met
ho
d
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h t
ip
son
icat
ion
. S
ub
seq
uen
t
cen
trif
ug
atio
n a
nd
dia
lysi
s w
as p
erfo
rmed
,
foll
ow
ed b
y t
reat
men
t
wit
h p
oly
elec
tro
lyte
s.
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h b
ath
son
icat
ion
and
cen
trif
ug
atio
n.
Par
tial
evap
ora
tion
of
liq
uid
in
the
dis
per
sio
n w
as
per
form
ed,
foll
ow
ed b
y
dil
uti
on
wit
h e
thy
len
e
gly
col
bef
ore
per
form
ing
size
ex
clu
sio
n
chro
mat
og
raph
y.
Su
rfa
cta
nt
Res
po
nsi
ble
fo
r
Ex
foli
ati
on
CT
AB
SC
291
38
9
39
0
11
6
Pro
po
rtio
n o
f fl
akes
, by
nu
mb
er:
Mo
no
lay
er g
rap
hen
e: 8
%
Few
lay
er g
raph
ene:
82
%
Lat
eral
siz
e an
d t
hic
kn
ess
of
gra
ph
itic
lay
ers
is
red
uce
d a
fter
a c
om
bin
atio
n o
f b
all
mil
ling
and
tip
son
icat
ion
.
SA
ED
mea
sure
men
ts s
ho
w t
he
gra
ph
itic
mat
eria
l
pro
du
ced
exh
ibit
s si
x-f
old
sy
mm
etry
ty
pic
al o
f
mo
no
lay
er g
raph
ene
Ram
an s
pec
tra
sho
w t
he
gra
phit
ic m
ater
ial
pro
du
ced
con
sist
s o
f m
ono
lay
er a
nd
few
lay
er
gra
ph
ene
Sim
ilar
lat
eral
fla
ke
size
to
rep
ort
ed b
y G
riff
ith
and
No
tley
123,
and
No
tley
124.
UV
-Vis
, Z
eta
po
tenti
al,
TG
A,
TE
M,
AF
M,
SE
M
TE
M,
AF
M,
SE
M,
Ram
an,
UV
-Vis
,
zeta
po
ten
tial
TE
M,
zeta
po
ten
tial
,
Ram
an
7.1
6 (
ach
iev
ed
wit
h r
ecy
cled
sed
imen
t
15
(A
chie
ved
wit
h P
luro
nic
F1
08
)
𝐶𝐺
,𝑖:
200
mg
/mL
𝐶𝑆
𝑢𝑟
𝑓:
3 m
g/m
L
So
nic
atio
n:
10
0 W
, 2
0 k
Hz,
24
h
Cen
trif
ug
atio
n:
50
0 –
130
00
rpm
, 90
min
𝐶𝑖:
10 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.0
5%
w/w
Mil
ling
: 1
00
rp
m,
1 -
12
h
So
nic
atio
n:
80
W, 2
h
Cen
trif
ug
atio
n:
50
00
rp
m,
20
min
𝐶𝐺
,𝑖:
5%
w/w
(b
atch
ad
dit
ion
),
7.5
% w
/w (
con
tin
uo
us
add
itio
n)
𝐶𝑆
𝑢𝑟
𝑓:
7.5
% w
/w (
con
tinu
ou
s
add
itio
n)
So
nic
atio
n:
60
W
Cen
trif
ug
atio
n:
15
00
g, 1
5 m
in
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h t
ip
son
icat
ion
and
cen
trif
ug
atio
n.
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h b
all
mil
lin
g w
ith
or
wit
hou
t
sub
seq
uen
t ti
p
son
icat
ion
, fo
llo
wed
by
cen
trif
ug
atio
n a
nd
op
tio
nal
rec
ycl
ing o
f
sed
imen
t.
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h t
ip
son
icat
ion
and
bat
ch o
r
con
tin
uo
us
add
itio
n o
f
surf
acta
nt.
S
ub
sequ
ent
cen
trif
ug
atio
n a
nd
dia
lysi
s w
as p
erfo
rmed
.
TD
OC
SD
S
Plu
ron
ic F
108
, F
12
7,
an
d C
TA
B,
TT
AB
an
d D
TA
B
292
Tab
le A
.2:
Su
mm
ary
of
stud
ies
in t
he
lite
ratu
re d
escr
ibin
g t
he
ult
raso
nic
exfo
liat
ion o
f gra
phen
e fr
om
gra
phit
e in
aq
ueo
us
surf
acta
nt
solu
tio
ns
(co
nt.
).
R
ef.
39
1
39
2
Sel
ecte
d P
rop
erti
es a
nd
Res
ult
s
Incr
ease
s in
so
nic
atio
n t
ime
aid i
n c
leav
ing
th
e
gra
ph
ene
wit
hin
th
e b
asal
pla
ne,
th
ereb
y
incr
easi
ng
th
e g
raph
ene
con
cen
trat
ion
Ph
ysi
cal
pro
per
ties
of
gra
ph
ene
pro
du
ced
usi
ng
SD
C:
Lat
eral
siz
e: <
10
0 n
m
Av
erag
e th
ickn
ess:
1.5
nm
Nu
mb
er o
f la
yer
s:1
- 5
Hig
h g
rap
hen
e co
nce
ntr
atio
n a
ttri
bu
ted t
o
spec
iall
y d
esig
ned
su
rfac
tan
t st
ruct
ure
co
nsi
stin
g
of
ion
ic g
rou
ps
atta
ched
to
ele
ctro
n-d
efic
ien
t
-
con
jug
ated
un
it v
ia f
lex
ible
sp
acer
s.
Ph
ysi
cal
pro
per
ties
of
gra
ph
ene
pro
du
ced
usi
ng
ND
I-1
:
Th
ickn
ess:
1.5
- 2
.0 n
m
Mo
no
lay
er g
rap
hen
e: 6
%
Sp
ecif
ic s
urf
ace
area
: 5
3.5
7 m
2 g
-1
Ele
ctri
cal
con
du
ctiv
ity
: 4
71
7 S
m-1
Pa
rtic
le
Ch
ara
cter
isa
tio
n
Met
ho
ds
UV
-Vis
, R
aman
,
AF
M,
SE
M,
TG
A,
Zet
a p
ote
nti
al,
FT
-
IR,
Ph
oto
lum
ines
cen
ce
spec
tro
sco
py
1H
an
d 1
3C
NM
R,
TE
M,
AF
M,
SE
M,
XP
S,
XR
D,
Ram
an,
Zet
a p
ote
nti
al,
BE
T
𝑪𝑮
,𝒇 ,
(m
g/m
L)
2.5
8 (
Ach
iev
ed
wit
h S
DC
)
5.0
(A
chie
ved
wit
h N
DI-
1)
Ex
foli
ati
on
Co
nd
itio
ns
𝐶𝐺
,𝑖:
100
– 7
00 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
100
-30
0 m
g/m
L (
SC
),
10
-180
mg
/mL
(S
DC
)
So
nic
atio
n:
12
0 W
, 3
h
Cen
trif
ug
atio
n:
10
00
0 r
pm
, 1
5
min
𝐶𝐺
,𝑖:
500
mg
/mL
𝐶𝑆
𝑢𝑟
𝑓:
1 -
10
mg
/mL
So
nic
atio
n:
20
kH
z, 1
hr
Cen
trif
ug
atio
n:
100
0
- 1
30
0
rpm
, 30
min
Met
ho
d
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h b
ath
son
icat
ion
and
cen
trif
ug
atio
n.
Su
rfac
tan
t co
nce
ntr
atio
n
and
in
itia
l g
rap
hit
e
con
cen
trat
ion
wer
e
exp
erim
enta
l v
aria
ble
s
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h
tem
per
atu
re c
on
troll
ed
tip
son
icat
ion
, th
en
cen
trif
ug
atio
n
Su
rfa
cta
nt
Res
po
nsi
ble
fo
r
Ex
foli
ati
on
SC
an
d S
od
ium
deo
xy
cho
late
(S
DC
)
N,N
0-b
is-[
2-(
eth
an
oic
aci
d s
od
ium
)]-
1,4
,5,8
-na
ph
tha
len
e
dii
mid
e(N
DI-
1)
an
d
N,N
0-b
is[2
-
(eth
an
esu
lfon
ic a
cid
sod
ium
)]-1
,4,5
,8-
na
ph
tha
len
e d
iim
ide
(ND
I-2
)
293
39
3
39
4
39
5
Ph
ysi
cal
dim
ensi
on
s o
f g
raph
ene
pro
du
ced
:
Av
erag
e la
tera
l si
ze o
f fl
akes
: 3 µ
m
Th
ickn
ess:
1 –
4 n
m
Gra
ph
ene
con
centr
atio
n i
ncr
ease
s w
ith
in
itia
l
surf
acta
nt
con
cen
trat
ion u
nti
l a
pea
k v
alu
e is
reac
hed
. W
hen
th
e su
rfac
tan
t co
nce
ntr
atio
n i
s
furt
her
in
crea
sed
, th
e g
rap
hen
e co
nce
ntr
atio
n w
ill
exp
erie
nce
a d
ecre
ase
if a
n i
on
ic s
urf
acta
nt
is
emp
loy
ed, o
r b
e m
ain
tain
ed i
f a
no
n-i
on
ic
surf
acta
nt
is e
mp
loy
ed d
uri
ng s
onic
atio
n.
Ph
ysi
cal
dim
ensi
on
s o
f g
raph
ene
pro
du
ced
usi
ng
SD
OC
:
Su
rfac
e ar
ea:
46
.83
µm
2
Th
ickn
ess:
1 t
o 3
nm
Py
ren
es s
urf
acta
nts
in
terf
ere
wit
h a
ccu
rate
gra
ph
ene
char
acte
risa
tio
n w
hen
usi
ng
Ram
an
spec
tro
sco
py
an
d X
PS
Py
ren
e-b
ased
su
rfac
tan
ts s
elf-
asse
mb
le i
nto
rib
bo
n-l
ike
stru
ctu
res
and f
lak
es w
hen
su
rfac
tan
t
is n
ot
rem
ov
ed a
fter
fil
teri
ng
.
TE
M,
SA
ED
, X
RD
,
Ram
an,
lase
r
dif
frac
tom
etry
UV
-Vis
, X
PS
,
Ram
an,
XR
D,
TE
M
SE
M,
XP
S,
Cry
o-
TE
M,
Ram
an
-
~1
.2 (
Ach
iev
ed
wit
h T
wee
n 8
0)
0.1
- 1
𝐶𝐺
,𝑖:
6 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
30 m
g/m
L
So
nic
atio
n:
20
0 W
, 2
2.5
kH
z,
10
min
– 6
h
𝐶𝐺
,𝑖:
5 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.0
25
- 2
.5 m
g/m
L
So
nic
atio
n:
10
0 W
, 8
h
Cen
trif
ug
atio
n:
50
0-
30
00
rp
m,
1 –
10 h
𝐶𝐺
,𝑖:
20 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
1 m
g/m
L
So
nic
atio
n:
15
W, 1
h
Cen
trif
ug
atio
n:
50
00
rp
m,
4 h
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h t
ip
son
icat
ion
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h b
ath
son
icat
ion
th
en
cen
trif
ug
atio
n u
sin
g
var
ied
su
rfac
tan
t
con
cen
trat
ion
s,
son
icat
ion
tim
es a
nd
cen
trif
ug
atio
n r
ates
.
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h
tem
per
atu
re c
on
troll
ed
tip
son
icat
ion
th
en
cen
trif
ug
atio
n
1H
,1H
,11
Hey
coso
flu
or-
1-d
eca
no
le
po
lyg
lyci
dy
l et
her
SD
OC
, S
DB
S,
SD
S
HT
AB
, T
wee
n 8
0,
Tri
ton
X-1
00
1-P
yre
nec
arb
ox
yli
c
aci
d (
PC
A),
1-
py
ren
ebu
tyri
c a
cid
(PB
A),
1-
py
ren
esu
lfo
nic
aci
d
sod
ium
sa
lt
(Py
SA
SS
)
294
Tab
le A
.2:
Su
mm
ary
of
stud
ies
in t
he
lite
ratu
re d
escr
ibin
g t
he
ult
raso
nic
exfo
liat
ion o
f gra
phen
e fr
om
gra
phit
e in
aq
ueo
us
surf
acta
nt
solu
tio
ns
(co
nt.
).
R
ef.
38
7
Sel
ecte
d P
rop
erti
es a
nd
Res
ult
s
Ab
le t
o p
rep
are
sam
ple
s w
hic
h e
xh
ibit
sep
arat
e
gra
ph
ene
mo
no
lay
ers
usi
ng
th
e L
ang
mu
ir-
Blo
dg
ett
met
hod
fo
r A
FM
ch
arac
teri
sati
on
.
Mea
sure
d t
hic
kn
ess
of
gra
ph
ene
mo
no
lay
ers
usi
ng
AF
M:
2 n
m
Flo
ccu
lati
on
alw
ays
ob
serv
ed f
or
gra
ph
ene
dis
per
sion
s ex
foli
ated
usi
ng
CT
AB
aft
er s
urf
acta
nt
exch
ang
e.
Pa
rtic
le
Ch
ara
cter
isa
tio
n
Met
ho
ds
AF
M,
TE
M,
Ram
an
𝑪𝑮
,𝒇 ,
(m
g/m
L)
-
Ex
foli
ati
on
Co
nd
itio
ns
𝐶𝐺
,𝑖:
5 m
g/m
L
𝐶𝑆
𝑢𝑟
𝑓:
0.5
– 1
mg
/mL
So
nic
atio
n:
40
%, 2
h
Cen
trif
ug
atio
n:
30
00
g, 3
0 m
in
(lo
w s
pee
d),
250
00
g, 2
h (
hig
h
spee
d)
Met
ho
d
Aq
ueo
us,
su
rfac
tan
t
assi
sted
ex
foli
atio
n o
f
gra
ph
ene
wit
h t
ip
son
icat
ion
, fo
llo
wed
by
low
, th
en h
igh
sp
eed
cen
trif
ug
atio
n f
oll
ow
ed
by
su
rfac
tan
t ex
chan
ge
wit
h t
he
Ben
zox
azin
e
surf
acta
nt,
BM
100
0
Su
rfa
cta
nt
Res
po
nsi
ble
fo
r
Ex
foli
ati
on
BM
100
0, P
luro
nic
F1
08
an
d L
64
,
Tw
een
20
, C
TA
B
an
d S
DS
295
A.2 Assembling Hydrogen-bonded Multilayer Films with
Surfactant Exfoliated Graphene
This section provides additional results and analyses related to Chapter 6. Much of the
results, analyses and discussions presented in this section were first presented in
supplementary material associated with a peer reviewed article61
(Reprinted from the
Journal of Colloid and Interface Science, 456, Sham, A. Y. W.; Notley, S. M.,
Graphene–polyelectrolyte multilayer film formation driven by hydrogen bonding, 32-
41, Copyright (2015), with permission from Elsevier).
A.2.1 Characterisation of Graphene Suspensions
The UV-Vis spectrum of Pluronic F108 exfoliated graphene is shown in Figure A.1.
The concentration of graphene in suspension was again determined by applying the
Beer-Lambert law to the absorption intensity of the samples at a wavelength of 660 nm.
Applying an extinction co-efficient112
, ε, of 54.22 L g-1
cm-1
and accounting for dilution
yielded an average stock graphene concentrations of 0.0358 mg/mL.
Figure A.1: UV-Vis spectra of Pluronic F108 exfoliated graphene.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
300 400 500 600 700 800
Ab
sorp
tio
n (
a.u
.)
Wavelength (nm)
296
A.2.2 QCM Multilayer Adsorption Measurements
Multilayer films consisting of a single PEI precursor layer and 10 PAA/surfactant
exfoliated graphene layer pairs, were assembled from suspension using LbL deposition.
To ensure the Sauerbrey equation remains applicable for this system, the ∆𝑓𝑛 values
measured must agree over all overtones when scaled by 𝑛. Figure A.2 shows ∆𝑓𝑛𝑛
for
each harmonic, 𝑛. It is clear that the data is consistent across all overtones, having an
average maximum difference of less than 12.99 Hz.
Figure A.2: ∆𝑓𝑛𝑛
as a function of time for the multilayer film formed by the deposition of 100
ppm PEI at pH 4, then alternating deposition of 100 ppm PAA and 10% graphene suspension at
pH 2.
Figure A.3 shows the kinetics of the process used to assemble the multilayer films.
First, a baseline was recorded with the pH 4 adjusted Milli-Q grade water in the QCM
chamber. A solution of PEI at pH 4 was then adsorbed to the silica crystal surface for
approximately 600 s, during which a corresponding decrease in signal was recorded
indicating adsorption of the polymer. Next, a rinsing step was performed with pH 2
adjusted Milli-Q water for approximately 120 s, resulting in an increase in the signal
-1400
-1200
-1000
-800
-600
-400
-200
0
200
0 2000 4000 6000
Δf n
/n (
Hz)
Time (s)
3rd Overtone, n = 3
5th Overtone, n = 5
7th Overtone, n = 7
297
suggesting removal of loosely bound PEI from the surface. A solution of PAA at pH 2
was then added to the chamber for approximately 120 s, resulting in a decrease in
frequency lower than that observed for the PEI. An equilibrium adsorbed amount was
attained during this step. Next, a rinsing step with pH 2 adjusted Milli-Q water was
performed, where the signal increased again indicating the removal of excess PAA.
Graphene suspension was then introduced into the chamber for 120 s, resulting in a
decrease in the frequency. A rinsing step with pH 2 adjusted Milli-Q water was then
performed. The last four adsorption steps were then repeated until 10 PAA/surfactant
exfoliated graphene bilayers were deposited. The corresponding dissipation data for the
process is shown in Figure A.4.
Figure A.3: Change in frequency as a function of time for the multilayer film formed by the
deposition of 100 ppm PEI at pH 4, then alternating deposition of 100 ppm PAA and 10%
graphene suspension at pH 2.
-4000
-3500
-3000
-2500
-2000
-1500
-1000
-500
0
500
0 1000 2000 3000 4000 5000 6000
Δf 3
(H
z)
Time (s)
PAA Graphene
PEI
pH 4 Rinse
298
Figure A.4: Change in dissipation as a function of time for the multilayer film formed by the
deposition of 100 ppm PEI at pH 4 (black square), then alternating deposition of 100 ppm PAA
and 10% graphene suspension at pH 2.
A.2.3 Removal of Multilayer Thin Films
The effect of weakly basic rinse solutions on the degradation of constructed multilayer
films was demonstrated using thin films constructed from a layer of PEI at pH 4,
followed by the deposition of alternating 10 layers of PAA and graphene at pH 2. The
film was then rinsed with pH 9 and pH 4 adjusted water. Figure A.5 shows the
adsorbed mass of the film throughout the formation of the film and during subsequent
rinse cycles.
-10
0
10
20
30
40
50
60
70
80
90
100
0 1000 2000 3000 4000 5000 6000
Dis
sip
atio
n (
× 1
0-6
)
Time (s)
Graphene
PEI PAA pH 4
Rinse
299
Figure A.5: Sauerbrey mass as a function of the number of additions of solution to the QCM
chamber. The film formation phase shows the construction of a thin film with an initial
precursor layer of PEI (black square), with alternating deposition of surfactant stabilised
graphene (red diamonds) and PAA (blue triangles), followed by partial removal of the film with
pH 9 adjusted water (orange plus sign) and final rinse with pH 4 adjusted water (blue cross).
To demonstrate the cumulative effect of multiple rinse cycles on the removal of
constructed multilayer films, thin films were constructed by depositing a layer of PEI at
pH 4, followed by the deposition of alternating 10 layers of PAA and graphene at pH 2.
The resulting film was then rinsed twice using unadjusted Milli-Q water, followed by
pH 9 adjusted Milli-Q water. The pH 2 Milli-Q water was introduced into the QCM
chamber after the initial rinse with unadjusted Milli-Q water, and after the rinse with pH
9 Milli-Q water. Figure A.6 shows the adsorbed mass of the film throughout the
formation of the film and during subsequent rinse cycles.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 2 4 6 8 10 12 14
Sau
erb
rey
mas
s o
f fi
lm (
mg/
m2)
Number of Layers
PEI
PAA
Graphene
pH 9 Rinse
pH 4 Rinse
Film Formation Rinse
300
Figure A.6: Sauerbrey mass as a function of the number of additions of solution to the QCM
chamber. The film formation phase shows the construction of a thin film with alternating
deposition of graphene and PAA, followed by removal of the film using unadjusted Milli-Q
water and Milli-Q water at pH 9.
The combination of rinse cycles at different pH levels resulted in the removal of 79.5%
of the total mass of the film. It is foreseeable that an even greater number of rinse
cycles extending to higher pH levels could result in complete removal of the multilayer
system from the substrate.
A.2.4 Surface Coverage of Dip-Coated Thin Films
Preliminary, qualitative observations to determine the presence of deposited films on
the silicon substrates were performed using optical microscopy (Figure A.7).
Examination of the samples indicates that material was successfully deposited onto the
silicon surfaces using the dip coating process, as evidenced by the textured areas where
dip coating was performed at both low and high magnification. However, further
examination of the edges of the substrate, and interface between the uncoated and dip
0
1
2
3
4
5
6
7
8
9
0 2 4 6 8 10 12 14 16 18
Sau
erb
rey
Mas
s o
f fi
lm (
mg/
m2)
Number of Layers
PEI
PAA
Graphene
pH UnadjustedRinse
pH 2 Rinse
pH 9 Rinse
Film Formation Rinse (Neutral pH)
Rinse (pH 9)
301
coated portions of the substrate at low magnification showed evidence of a drying front.
This dewetting behaviour could be associated with an inability to strongly adsorb layers
to the active surface, resulting in non-uniform adsorption. Dewetting of the films could
also indicate lateral shrinkage of the films, and possibly rearrangement of the internal
multilayer structure.
(a)
(b)
(c)
(d)
Figure A.7: Optical micrographs for (a) 50 bilayer sample and (b) 200 bilayer sample at 1.5x
magnification, and (c) 50 bilayer sample and (d) 200 bilayer sample at 100x magnification.
The films were further investigated using stylus profilometry in an effort to determine
the quality of film coverage. Stylus profilometry was used to measure longer cross
sections spanning the coated and uncoated portions of the samples. This enabled a
direct comparison of the surface roughness corresponding to uncoated and coated
302
portions of the substrates. The samples showed a significant increase in surface
roughness along the coated portions of the substrate (Figure A.8) compared to the
uncoated portions of the film. This again suggests the deposition of a thin film.
Furthermore, the difference in profile height between the wetted, coated portions and
uncoated portions of the substrate is relatively constant overall, indicating that coating
of the substrates is generally uniform. However, the dip coated portion of the substrate
did indeed show surface features with a height difference in the order of hundreds of
nanometres. This height difference between areas in the films could be attributed to the
presence of drying artefacts comprised of graphitic aggregates. Therefore, the
introduction of a rinsing stage following deposition of the thin films could act to reduce
the aggregation and deposition of these aggregates. Although the stylus profilometry
technique benefits from an ability to obtain longer profiles along a surface, the
technique involves the stylus being constantly engaged with the surface, without height
adjustment of the stylus as it scans along the sample surface. As a result, the technique
is primarily used for hard surfaces and is not well suited to the PAA/graphene films in
this study.
303
Figure A.8: Surface profile of 50 PAA/graphene bilayer sample obtained through stylus
profilometry. The direction of the scan was from left (bare silicon substrate) to right (multilayer
film).
A.2.5 Characterisation of Graphene in Dip-Coated Thin Films
UV-Vis measurements were performed on a quartz substrate sample with a single layer
of PEI, and 400 graphene/PAA layers. The resultant spectrum in Figure A.9 shows a
single peak at 271 nm, consistent with the signal associated with the extended
conjugation of defect-free graphene.79
This indicates that the dip coating deposition
process results in thin films which contain pristine graphene sheets.
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-1 1 3 5 7 9 11 13 15
Tota
l Pro
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(n
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Lateral Distance (mm)
Silicon Substrate Drying Front Bulk film
304
Figure A.9: UV-Vis spectra of 100 ppm PEI deposited at pH 4, followed by the deposition of
200 Bilayers of PAA and Pluronic F108 exfoliated graphene deposited at pH 2.
A.2.6 Quantitative Nanomechanical Measurements
QNM measurements were performed on samples assembled with a single layer of PEI
and between 100 and 400 layers of PAA and surfactant stabilised graphene in order to
determine various material and mechanical properties of the multilayer films.
NanoScope Analysis presents each of the properties as maps of the scanned surface,
where the parameter of interest is expressed as the vertical height. Figure A.10 shows
the results of a typical QNM scan for a film comprised of a single precursor PEI layer,
followed by 400 layers of PAA and surfactant stabilised graphene. In Figure A.11, the
Young’s modulus, adhesion, dissipation and deformation are shown for typical sample
surfaces, which include both particle and matrix areas for films comprised of a single
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0.03
0.04
0.05
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0.08
0.09
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200 300 400 500 600 700 800
Ab
sorb
ance
(a.
u.)
Wavelength (nm)
305
layer of PEI followed by 100 - 400 deposition steps of PAA and surfactant stabilised
graphene.
(a) (b)
(c) (d)
(e)
Figure A.10: QNM scan for the (a) reduced Young’s modulus, (b) adhesion, (c) deformation,
(d) dissipation and (e) topographical height of a film assembled from an initial layer of PEI
followed by 400 layers of PAA and Graphene. Scan performed at 59.5% humidity.
306
(a) (b)
(c) (d)
Figure A.11: QNM measurements for the (a) reduced Young’s modulus, (b) adhesion, (c)
deformation and (d) dissipation of thin films comprised of a single PEI precursor layer and 100-
400 layers of PAA and graphene.
The relationship between relative humidity and the mechanical properties of the
multilayer films was also studied. Humidity was selected as an environmental factor for
further investigation, as LbL multilayer films containing PAA and PEO have been
shown previously to swell with water when exposed to high humidity environments.216
In order to determine whether the water acted as a plasticizer for the films, QNM
measurements were performed on dip coated films with 100 PAA and graphene layers
at an average relative humidity of 26.7% and compared to those performed on the
sample at 59.5% relative humidity.
0
20
40
60
80
100
120
140
0 100 200 300 400
DM
T M
od
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Y
ou
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MP
a)
Number of Layers
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Ad
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Number of Layers
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Def
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Dis
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Number of Layers
307
Figure A.12 shows the effect of humidity on the mechanical properties of the multilayer
film at low to mid-levels. Regardless of the area considered the DMT modulus
decreases when the relative humidity is increased from 26.7% to 59.5%. The film loses
rigidity upon exposure to a humid environment and this is most likely caused by the
increased amount of water acting as a plasticizer within the film. Sorption of water into
the films is also responsible for the trends in deformation and dissipation when the
humidity is increased. Compared to the graphene particles, there is a greater ability for
the matrix to achieve water uptake from the atmosphere by hydration of PAA and PEO
chains on the surfactant. As a consequence, the matrix should be softer and therefore
less resistant to deformation at higher levels of humidity. This behaviour is
demonstrated in the large difference in deformation observed between the particles and
the matrix at mid-range humidity, compared to low humidity. This also accounts for the
higher dissipation of the films at a relative humidity of 59.5%, as it is expected that
softer films would exhibit improved energy dissipation compared to more rigid films.
In addition to the DMT modulus, deformation and dissipation, the surface adhesion of
the films is also dependent on humidity. A greater adhesion force was observed for
films exposed to 59.5% humidity compared to those at lower humidity levels. This
most probably arises due to the lower rigidity of the films, which allows the polymer
chains to conform under the graphene particle or across the AFM tip during a
measurement, leading to increased adhesion. From these measurements, it was shown
that the mechanical properties of the films were largely independent of humidity at mid
to low levels of humidity.
308
(a) (b)
(c) (d)
Figure A.12: QNM measurements for the (a) reduced Young’s modulus, (b) adhesion, (c)
deformation and (d) dissipation of films containing a single PEI precursor layer and 100 PAA
and surfactant stabilised graphene layers at 26.7% and 59.5% humidity. The red squares
correspond to measurements of the matrix, while blue diamonds correspond to measurements of
particles.
A.3 Foam Stabilization Using Surfactant Exfoliated
Graphene
This section provides additional results and analyses related to Chapter 7. Much of the
results, analyses and discussions presented in this section were first presented in
supplementary material associated with the peer reviewed article244
(Reprinted from the
Journal of Colloid and Interface Science, 469, Sham, A. Y. W.; Notley, S. M., Foam
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300
0 20 40 60
DM
T M
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(M
Pa)
Relative Humidity (%)
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0.1
0.2
0.3
0.4
0.5
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Ad
hes
ion
(n
N)
Relative Humidity (%)
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0 20 40 60
Def
orm
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n (
nm
)
Relative Humidity (%)
0
20
40
60
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0 20 40 60
Dis
sip
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eV)
Relative Humidity (%)
309
stabilisation using surfactant exfoliated graphene, 196-204, Copyright (2016), with
permission from Elsevier).
A.3.1 Characterisation of Graphene Suspensions
The UV Visible spectrum of Pluronic F108 exfoliated graphene is shown in Figure
A.13. The spectrum shows a single prominent peak at 269 nm, which is in good
agreement with the results shown in §4.2.4and occurs as a result of the extended
electronic conjugation found in the pristine graphene sheets. As in Chapter 4, the
concentration of graphene in the samples was also determined by applying the Beer-
Lambert law to the absorption intensity of the samples at a wavelength of 660 nm.
Applying an extinction co-efficient112
ε, of 54.22 L g-1
cm-1
and accounting for dilution
yielded an average stock graphene concentration of 0.54 mg/mL.
Figure A.13: UV-Vis spectra of Pluronic F108 exfoliated graphene.
The particles were also characterised using Raman spectroscopy (Figure A.14). The
resultant spectrum exhibits three distinctive peaks at approximately 1342, 1577 and
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0.1
0.2
0.3
0.4
0.5
0.6
200 300 400 500 600 700 800
Ab
sorb
ance
(a.
u)
Wavelength (nm)
310
2708 cm-1
consistent with the D, G and 2D peaks common to all graphitic materials.
The D peak is related to the degree of sp3 carbon present in the sample, whilst the G
peak arises from in-plane vibrations.71
In the case of surfactant exfoliated graphene, the
D peak is attributed to the sp3 carbon present as edge defects on the nanoscopic
particles, whilst the G peak is associated with sp2 carbon content. Here, the ratio
between the intensity of the D and G peaks is low, due to the large size of the graphene
particles relative to the proportion of edges present, indicating successful exfoliation of
graphite. The shape, spread and position of the 2D peak in the samples is also related to
the number of graphene layers present. A triplet 2D peak at 2708 cm-1
is apparent,
indicating the presence of single or bilayer graphene in the sample.
Figure A.14: Raman spectra of Pluronic F108 exfoliated graphene, measured with 532 nm
laser.
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20000
25000
30000
800 1300 1800 2300 2800 3300
Inte
nsi
ty (
a.u
.)
Raman Shift (nm)
311
A.3.2 Bulk Rheological Behaviour
In an effort to identify the primary stabilization mechanism behind the increase in foam
stability at high graphene concentrations, the bulk rheological properties of the graphene
solutions were studied. Figure A.15 shows the kinematic viscosity as a function of
graphene concentration for solutions containing between 0 and 0.405 mg/mL surfactant
stabilised graphene. Here, the relationship between the kinematic viscosity and
graphene concentration does not correspond with the trend between graphene
concentration and foam stability observed in Figure 7.8. This suggests the viscosity of
the solutions does not dictate drainage of the foams and therefore particle-interface
interactions are likely to determine foam stability.
Figure A.15: Kinematic viscosity as a function of graphene concentration for solutions
containing 0.1 g/mL Pluronic F108 and Pluronic F108 exfoliated graphene.
A.3.3 Salt Effects on Foam Stability
In order to compare the effect of salt on particle wettability and foam stability, the
stability of foams generated from solutions containing only surfactant and alkali metal
7.95
8
8.05
8.1
8.15
8.2
8.25
8.3
0.000 0.100 0.200 0.300 0.400 0.500
Kin
em
atic
Vis
cosi
ty (
mm
2/s
)
Graphene Concentration (mg/mL)
312
salts were studied. Figure A.16 shows the foam lifetime as a function of salt
concentration for solutions containing 0.1 mg/mL Pluronic F108 and 0.01 – 3 M alkali
chloride salts. It is clear that the foam half-life increases due to the presence of cations,
with significant improvement to the foam half-life observed for solutions containing
above 0.5 M salt. The extent of the increase in foam half-life increases according to the
series Li+ > Na
+ > K
+ > Cs
+. This particular ordering is consistent with the ordering to
which the cations alter the solubility of PEO groups in water, corresponding to the
Hofmeister series13, 300
.
Figure A.16: Foam half-life as a function of salt concentration for solutions containing 0.1
g/mL Pluronic F108 and alkali metal salts
10-2 10-1 100 101 0.00
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1500.00
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2500.00
3000.00
Foam
Hal
f Li
fe (
min
)
Concentration of Salt (M)
LiCl
NaCl
KCl
CsCl
313
A.4 Adsorption of Organic Dyes Using Surfactant Exfoliated
Graphene
This section provides additional results and analyses related to Chapter 8.
A.4.1 Characterisation of Graphene Suspensions
The UV Visible spectrum of SDS, CTAB and Pluronic F108 exfoliated graphene is
shown in Figure A.17. The spectra show a single prominent peak at 268 nm, which is
good agreement with the results shown previously and is consistent with the extended
electronic conjugation found in the pristine graphene sheets. The concentration of
graphene in the samples was again determined by applying the Beer-Lambert law to the
absorption intensity of the samples at a wavelength of 660 nm. Applying an extinction
co-efficient112
ε, of 54.22 L g-1
cm-1
and accounting for dilution yielded an average
stock graphene concentrations of 0.107, 0.159 and 0.954 mg/mL for the stock SDS,
CTAB and Pluronic F108 exfoliated graphene suspensions respectively.
Figure A.17: UV-Vis spectra of SDS, CTAB and Pluronic F108 exfoliated graphene.
0
0.2
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0.6
0.8
1
1.2
1.4
200 300 400 500 600 700 800
Ab
sorb
ance
(a.
u)
Wavelength (nm)
SDS
CTAB
Pluronic F108
314
The mean lateral size, ⟨𝐿⟩ of CTAB, SDS and Pluronic F108 exfoliated graphene
particles was estimated using the method described by Lotya et al.138
A Zetasizer Nano
was used to obtain the PSD of CTAB, SDS and Pluronic F108 exfoliated graphene
suspensions with a concentration of 0.0535 mg/mL The average 𝑎𝐷𝐿𝑆 values for each
type of suspension were then used to estimate the ⟨𝐿⟩ of the dispersed graphene particles
using Equation 4.1. The average 𝑎𝐷𝐿𝑆 values and corresponding ⟨𝐿⟩ values for each of
the suspensions are given in Table A.3.
Table A.3: Mean lateral particle sizes as determined through DLS for SDS, CTAB and Pluronic
F108 exfoliated graphene.
Surfactant Peak intensity,
𝒂𝑫𝑳𝑺 (nm)
Mean lateral particle size, ⟨𝑳⟩
(nm)
SDS 418.83 600.01
CTAB 229.17 242.84
Pluronic F108 434.57 634.14
The graphene particles were also characterised using Raman spectroscopy (Figure
A.18). The resultant spectra exhibit three distinctive peaks at approximately 1350, 1581
and 2697 cm-1
consistent with the D, G and 2D peaks common to all graphitic materials.
The ratio between the intensity of the D and G peaks is low for the SDS and Pluronic
F108 exfoliated samples, suggesting the graphene particles have a large lateral size
relative to the proportion of edges present. This is consistent with the measurements
surrounding the mean lateral size of the particles and suggests successful exfoliation. In
contrast, CTAB demonstrates a higher ratio between the intensity of the D and G peaks,
which is again consistent with mean lateral size calculations indicating the presence of
smaller particle sizes. Triplet 2D peaks at 2697 cm-1
are apparent for all three spectra,
315
with the location of the peaks indicating the presence of single or bilayer graphene in
the sample.
Figure A.18: Raman spectra of SDS, CTAB and Pluronic F108 exfoliated graphene.
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Inte
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.)
Raman Shift (cm-1)
SDS
CTAB
Pluronic F108
