PROCESSING OF CARBONACEOUS MATERIALS
USING DONOR-ACCEPTOR INTERACTIONS
Submitted in total fulfillment of the requirements for the
degree of
Doctor of Philosophy in Chemistry
By
Desi Hamed Gharib
May, 2018
Faculty of Science, Engineering & Technology Department of Chemistry & Biotechnology
i
Abstract
Graphene and other carbonaceous materials such as carbon nanotubes (CNTs) have
attracted immense interest due to their outstanding chemical and mechanical properties.
Their large scale production and processing has thus been of great interest to realize their
potential applications. For instance, the large scale graphene production from cheap and
readily available graphite, has been seen as the most promising route for various
applications. However, the difficulties in exfoliating graphite, as well as its intrins ic
insolubility in solvents due to presence of strong interlayer van der Waal forces, namely
interactions, still remain a key challenge. Additionally, even though much progress
has been made in the large scale synthesis of CNTs, their high natural tendency to form
bundles, ropes or aggregates as a consequence of also strong interactions still limit
their processing and development for further applications. This thesis will explore a one-
pot approach to effectively disrupt and cleave the network of interactions in both
graphite and CNTs, and further enhance the dispersion of the resultant materials in
organic solvents. This was achieved through a donor-acceptor interaction mechanism,
between the electron rich (graphite/CNTs), and specially designed electron deficient
molecules, acceptors (denoted A1 and A2). As a result, minimal energy input of manual
grinding of graphite/CNTs with acceptor induces donor-acceptor interactions which
preferentially weakens the interactions, with the ultimate result being solid
exfoliation. It is also evidenced that stable dispersions are formed immediately after
addition of the solvent to the ground graphite/CNTs-acceptor material, indicat ing
enhanced dispersion only after donor-acceptor interactions. Graphite exfoliation and CNT
dispersion is further enhanced after mild bath sonication (30 minutes) of the acceptor-
ii
graphite/CNT mixture in solvent with 13 and 200-fold increment in yield of graphene and
CNTs respectively dispersed in N-methylpyrrollidone (NMP) and A1 for instance
compared to that without the acceptor respectively. The use of electron acceptors
therefore, allows high dispersions, not only in high boiling point solvents whose surface
energy matches that of graphene/CNTs e.g. NMP and dimethylformamide (DMF), but
also in low boiling point solvent with mediocre properties, for example, chloroform in
the case of graphite. Moreover, the use of novel dihydrolevoglucosenone (DHLG) in the
liquid phase processing step as a green solvent alternative to toxic NMP is also reported.
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To my daughters, husband, mum and dad
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Acknowledgements
I would like to express my gratitude to my principal supervisor, Dr. Francois Malherbe
for his research guidance, advice and encouragement throughout the course of my PhD.
My heartfelt gratitude is also extended to my co-supervisor: Professor Simon Moulton
for his research guidance and for providing the much needed clarity for this thesis.
Professor Kazuki Sada for his helpful feedback. Dr. Shannon Notley for helpful
discussions during the initial stages of the project. I am also appreciative to Shaun
Gietman, Caitlyn Ingham, Dr. Tammana Tasnuva, Dr. Elizabeth Awuor, Safi and Arlene.
I also extend my gratitude to Dr. Mohammed Al Kobaisi and Ali Ramezan Nejad for
helping me run my TEM samples, Dr. Hayden Webb for his constant support in Raman
analysis, Dr. James Wang for SEM and XRD analysis and Dr. Deming Zhu for XPS
analysis. Many thanks to FSET support staff, Angela, Chris, Andrea and Savi. I am also
grateful to Circa group Australia PTY and especially the CEO, Dr. Tony Duncan for
providing us with dihydrolevoglucosenone. Base Titanium Company, Kenya and
especially Mr. Collin Forbes, for awarding me the travel grant to cover my family's
relocation cost to Melbourne. Miss Stephanie Twolands, Aunt Zenab, Mariam, Salma,
Amani, Binti, Shamsa, Hanifa, Khalfan, Mahmoud and my mother and father in law:
Mwanaisha Hussein and Omar Abdalla, for believing in me and helping towards my
relocation to arrive in Melbourne. I would also extend appreciation to my dearest
husband, Hussein Omar for his constant support, love and encouragement especially
throughout the many dark moments of my studies. Also to my beautiful daughters, Fatma
and Aisha for the many sacrificed play events so that I could stay indoors and do my PhD
write ups. You guys are simply the best!!! To my parents: Hamed Gharib and Fatuma
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Bwajuma for laying the foundations to dream big and for their enormous love, support,
guidance and prayers. Who I am today is a true reflection of the amount of faith and hard
work they put into me. My siblings: Mariam and Mohammed Hamed, awesome growing
up with you guys! Swinburne University of Technology, for providing me with the PhD
scholarship to ensure that my studies run smoothly. Finally, to the Almighty Allah, his
blessings still do remain immeasurable.
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Declaration
I, Desi Hamed Gharib, declare that this PhD thesis titled 'Processing of Carbonaceous
Materials using Donor-Acceptor Interactions' is no more than 100,000 words in length,
exclusive of tables, figures, appendices, references and footnotes. This thesis contains
no materials that have been previously published, in whole or in part, for the award of
any other academic degree or diploma, and has not been previously published by
another person. Except where otherwise indicated, this thesis is my own work.
Desi Hamed Gharib
2018.
vii
List of Publications
1. D. H. Gharib, S. Gietman, F. Malherbe and S. E. Moulton, High yield, solid
exfoliation and liquid dispersion of graphite driven by a donor-acceptor
interaction, Carbon, 123, 695-707, 2017
2. S.M. Notley and D.H. Gharib (Australian National University),World Intellectual
Property Organization Patent, WO2017063026, 2017
3. S. Gietman, Solution processing of graphene and hybrid materials using donor-
acceptor interactions for polyimide applications, Bachelor of Science (Honors),
thesis, Swinburne University of Agriculture and Technology, 2016.
Supervisors: D. H. Gharib, S. E. Moulton and F. Malherbe
4. D. H. Gharib and S. M. Notley (Australian National University) Australian patent,
AU2015904218, 2015
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Conference Presentation
1. Desi H. Gharib, Simon Moulton and Francois Malherbe, High Yield, Solid
Exfoliation and Liquid Dispersion of Graphite to Graphene Driven by a Donor-
Acceptor Interaction, Swinburne Celebrates Research Conference, Swinburne
University, Hawthorn Campus, Melbourne, Australia, 22-23 July, 2017
2. Desi H. Gharib, Shannon Notley, Kazuki Sada, Simon Moulton & François
Malherbe, High Yield Exfoliation and Solubility Shift of Graphite into Pristine
Graphene Driven by a Charge Transfer Interaction, 11th Annual International
Electromaterials Science Symposium, Deakin University, Burwood Campus,
Melbourne, Australia, 10 –12 February, 2016
Manuscripts in preparation
1. D. H. Gharib, S. E. Moulton and F. Malherbe, Ultrahigh dispersion and
solubility of carbon nanotubes in organic solvents using molecularly designed
electron acceptors
2. S. Gietman, D. H. Gharib, S. E. Moulton and F. Malherbe , Effects of
Carbonaceous Nanofillers on the Properties of Polyimide Composite Films (Not
included in this thesis)
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List of Abbreviations and Acronyms
oC degrees Centigrade
Absorption co-efficient
δT Solvent Hildebrand parameter
D Dispersive Hansen solubility parameter
δP Polar Hansen solubility parameter
δH Hydrogen-bonding Hansen solubility parameter
Lambda
m micrometer
L microliter
A1 Acceptor 1
A2 Acceptor 2
c Concentration
CDCl3 Deuterated chloroform
CHCl3 chloroform
CG Graphene concentration
CG+A1 Concentration of graphene exfoliated in A1
cm Centimeter
x
CNT Carbon nanotube
cP Centipoise
CVD Chemical Vapor deposition
DC Direct Current
DCM dichloromethane
DHLG dihydrolevoglucosenone
DMAc dimethylacetamide
DMF dimethylformamide
DMSO dimethyl sulfoxide
FE-TEM Field effect transmission electron microscopy
FTIR Fourier transform infrared
GO Graphene oxide
Hz Hertz
1HNMR Proton nuclear magnetic resonance
HOPG Highly oriented pyrolytic graphite
kV Kilovolt
g Gram
L Litre
A Ampere
xi
M Molar
mg milligram
min Minute
mL milliliter
mol Mol
mmol millimolar
Mw Molecular weight
MWCNT Multi-walled carbon nanotube
nm Nanometer
NMP N-methyl-pyrrolidone
p.p.m Parts per million
PNIPAAm Poly(N-isopropylacrylamide)
rGO Reduced graphene oxide
Rs Sheet resistance
r.p.m Revolution per minute
r.t Room temperature
SEM Scanning electron microscopy
SWCNT Single-walled carbon nanotube
t thickness (film)
xii
TPa Terapascals
1,2DCB 1,2 dichlorobenzene
Uv Ultraviolet
Vis Visible
w.t Weight
X.P.S Xray Photoelectron Microscopy
X.R.D X ray Diffraction
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Table of Contents
Abstract ............................................................................................................................ i
Acknowledgements ........................................................................................................ iv
Declaration ..................................................................................................................... vi
List of Publications ....................................................................................................... vii
Conference Presentation ............................................................................................. viii
Manuscripts in preparation ........................................................................................ viii
List of Abbreviations and Acronyms ........................................................................... ix
List of Figures ............................................................................................................. xvii
Chapter 1 Introduction .................................................................................................. 1
1-1 Research Background and Motivation ................................................................... 1
1-2 Previous Work and Inspiration ............................................................................... 2
1-3 Thesis Overview ..................................................................................................... 5
1-4 Thesis aims and objectives ..................................................................................... 5
Chapter 2 Literature review .......................................................................................... 8
2-1 Graphene................................................................................................................. 8
2-2 Properties ................................................................................................................ 9
2-3 Production............................................................................................................. 10
2-3-1 Bottom- up......................................................................................................... 11
2-3-1-1 Chemical vapor deposition (CVD)............................................................. 11
2-3-1-2 Epitaxial growth from SiC ......................................................................... 12
2-3-2 Top-down .......................................................................................................... 12
2-3-2-1 Chemical exfoliation: Graphene via graphite/graphene oxide ................... 13
2-3-2-2 Mechanical exfoliation ............................................................................... 15
2-3-2-2-1 Micromechanical exfoliation/cleavage ................................................... 16
xiv
2-3-2-2-2 Mechanical Milling using ball milling and manual grinding techniques 17
2-3-2-2-3 Mechano-Chemical exfoliation of graphite ............................................ 18
2-4 Liquid phase exfoliation of graphite to graphene ................................................. 20
2-4-1 Liquid phase exfoliation in organic solvents ................................................. 20
2-4-2 Liquid phase exfoliation with addition of intercalants .................................. 23
2-4-3 Mechano-chemical assisted liquid phase exfoliation .................................... 24
2-4-4 Liquid phase exfoliation via donor-acceptor interactions ............................. 25
2-4-5 Liquid phase exfoliation in green polar aprotic organic solvents .................. 26
2-5 Carbon Nanotubes ................................................................................................ 28
2-5-1 Brief Overview, Properties and Applications................................................ 28
2-5-2 CNTs production ........................................................................................... 30
2-5-3 CNT processing ............................................................................................. 31
2-5-3-1 Solid Phase Processing/ Debundling of CNTs ........................................... 32
2-5-3-2 Liquid Phase Processing/Debundling and Dispersion of CNTs ................ 33
Chapter 3 Materials and Methods .............................................................................. 36
3-1 Materials ............................................................................................................... 36
3-2 Methods ................................................................................................................ 36
3-2-1 Synthesis of acceptors ................................................................................... 36
3-2-1-1 Synthesis of A1 (N, N’-bis-(2-ethylhexyl)pyromellitic diimide)............... 37
3-2-1-2 Synthesis of A2 (N,N’-bis-(2-ethylhexyl)-1,4,5,8-naphthalene tetra carboxydiamide) ...................................................................................................... 38
3-2-2 Exfoliation of graphite ................................................................................... 40
(Starting graphite material (a), partially exfoliated graphite via donor- acceptor interaction (b), further exfoliated and dispersed graphene and graphite sheets via addition of solvent and mild bath sonication (c) and final dispersed graphene sheets recovered from the supernatant following centrifugation (d)) ................................ 41
3-2-3 Debundling and dispersion of CNTs ............................................................. 41
3-2-4 Preparation of conducting films .................................................................... 41
3-3 Characterization .................................................................................................... 42
3-3-1 1H Nuclear Magnetic Resonance (1H NMR) ................................................ 42
3-3-2 X-ray Diffraction Spectroscopy (XRD) ........................................................ 42
3-3-3 Ultraviolet-Visible (UV-Vis) Spectroscopy .................................................. 43
3-3-4 Raman Spectroscopy ..................................................................................... 44
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3-3-5 X-ray photoelectron spectroscopy (XPS) ...................................................... 45
3-3-6 Field emission scanning electron microscope (FE-SEM) ............................. 45
3-3-7 Field emission transmission electron microscope (FE-TEM) ....................... 45
3-3-8 Electrical conductivity ................................................................................... 46
Chapter 4 Solid phase exfoliation................................................................................ 47
4-1 Introduction........................................................................................................... 48
4-2 Aims and Objectives............................................................................................. 50
4-3 Experimental procedure........................................................................................ 51
4-4 Results and Discussion ......................................................................................... 51
4-4-1 Mechano-chemical Solid Exfoliation of Graphite using Donor-acceptor Interaction................................................................................................................ 51
4-4-2 Morphology of Solid State Exfoliated Graphite............................................ 55
4-4-3 Crystallinity of solid state exfoliated graphite............................................... 58
4-5 Conclusion ............................................................................................................ 62
Chapter 5 Liquid phase exfoliation ............................................................................. 63
5-1 Intoduction ............................................................................................................ 64
5-2 Aim and Objectives .............................................................................................. 65
5-3 Experimental Procedure........................................................................................ 66
5-4 Results and Discussion ......................................................................................... 66
5-4-1 Optimization of Solvents and Continued Liquid Phase Exfoliation.............. 66
5-4-2 Crystallinity of liquid phase exfoliated and dispersed graphite .................... 75
5-4-3 Effect of acceptor concentration on liquid phase exfoliation and dispersion 78
5-4-4 Morphology and Quality of liquid phase exfoliation and dispersion of graphite .................................................................................................................... 81
5-4-5 Preparation of conducting films of liquid phase acceptor exfoliated graphite and subsequent acceptor removal............................................................................ 82
5-4-6 Morphology of liquid phase exfoliated and dispersed graphite .................... 85
5-4-7 Quality of liquid phase exfoliated and dispersed graphite ............................ 87
5-4-8 Electrical properties of liquid phase exfoliated and dispersed exfoliated graphite .................................................................................................................... 91
5-5 Conclusion ............................................................................................................ 94
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Chapter 6 Processing of Carbon Nanotubes .............................................................. 95
6-1 Introduction........................................................................................................... 95
6-2 Aims and Objectives............................................................................................. 97
6-3 Experimental Procedure........................................................................................ 98
6-4 Results and Discussion ....................................................................................... 100
6-4-1 Solid Phase Processing of CNTs ................................................................. 100
6-4-2 Continued Liquid Phase Processing ............................................................ 104
6-4-3 Crystallinity of dispersed MWCNTs ........................................................... 112
6-4-4 Defect analysis of CNT dispersions ............................................................ 113
6-4-5 Morphology of CNT dispersions ................................................................. 115
6-4-6 Electrical properties of CNT dispersions..................................................... 116
6-5 Conclusion .......................................................................................................... 118
Chapter 7 General Discussions and Conclusions..................................................... 120
7-1 Research overview and Challenges .................................................................... 120
7-2 Research Objectives............................................................................................ 122
7-3 Research Outcomes ............................................................................................ 123
7-4 Future directions and applications ...................................................................... 124
References.................................................................................................................... 128
xvii
List of Figures
Figure 1-1 Cleavage of interactions in pyrene polymer gels to induce solvation and
thermoresponsivity using electron deficient acceptors via donor-acceptor interaction[3].
.......................................................................................................................................... 4
Figure 2-1 Structure of graphite and graphene[7] ........................................................... 8
Figure 2-2 Types of mechanical forces used for graphite exfoliation and the auxiliary
route for fragmentation[22]. ........................................................................................... 15
Figure 2-3 Graphene dispersibility, CG, as a function of a) solvent surface
tension(mJ/m2) b) solvent Hildebrand parameter (δT), c) dispersive Hansen solubility
parameter (δD), d) polar Hansen solubility parameter (δP), and e) hydrogen-bonding
Hansen solubility parameter (δH)[64]. ............................................................................ 22
Figure 2-4 Scheme for the production of dihydrolevoglucosenone (DHLG)[81]. ............. 28
Figure 2-5 Graphene and carbon nanotubes as (A) single wall carbon nanotube
(SWCNT) and (B) multi-wall carbon nanotube (MWCNT) structures[17]. .................. 30
Figure 3-1 Electron acceptors A1 (a) ball and stick structures, (b) simulated 3D and (c)
molecular and A2 (b) ball and stick structures, (c) and (d) simulated 3D, and, (e) and (f)
the molecular structure. .................................................................................................. 37
Figure 3-2 Synthesis of A1 (N, N’-bis-(2-ethylhexyl)pyromellitic diimide) ................ 37
Figure 3-3 1H NMR spectrum (42.5 MHz, CDCL3 r. t.) of A1 ..................................... 38
Figure 3-4 Synthesis of A2 (N,N’-bis-(2-ethylhexyl)-1,4,5,8-naphthalene tetra
carboxydiamide) ............................................................................................................. 38
Figure 3-5 1H NMR spectrum (42.5 MHz, CDCl3, r. t.) of A2...................................... 39
Figure 3-6 Schematic of the experimental process to form graphene dispersions via
donor-acceptor interactions in organic solvents. ............................................................ 40
Figure 3-7 Representative conductive film sample prepared from filtration of 6 mL
dispersion of A1 exfoliated graphite in NMP. ................................................................ 42
Figure 3-8 UV–Vis spectra of a) Acceptor 1 and b) Acceptor 2 in NMP. The spectra are
featureless above 450 nm................................................................................................ 44
Figure 4-1 Images of a) graphite, b) graphite and A1 and c) graphite and A2 before and
........................................................................................................................................ 52
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Figure 4-2 Effect of silica addition to the co-grinding of graphite and Acceptor 1 ...... 55
Figure 4-3 SEM images of (a) graphite with the inset image showing the laminar
structure of the graphite, (b) graphite ground with A1, and (c) graphite ground with A2.
........................................................................................................................................ 57
Figure 4-4 XRD diffraction patterns of ground materials, (a) graphite (d002-0.351 nm)
(b) graphite ground with A1 (d002-0.346 nm) and (c) graphite ground with A2 (d002-
0.347 nm). Inset is a zoomed region of the low intensity region. ................................... 59
Figure 5-1. Images of graphene dispersions exfoliation without acceptor (G) and with
acceptorA1 (G + A1) and A2 (G + A2) in NMP, chloroform (CHCl3), DMF and DHLG
before and after sonication and centrifuging. ................................................................. 69
Figure 5-2 Concentrations (mg/mL) of graphene dispersions in DHLG, NMP, DMF and
Chloroform, with and without acceptor, before and after sonication. ............................ 71
Figure 5-3 XRD diffraction patterns of supernatant of the exfoliated and dispersed
graphite. (a) Precursor graphite (shown for comparison), (b) graphite exfoliated with
acceptor A1, and (c) graphite exfoliated with acceptor A2. Samples were dispersed
sonicated for 30 minutes in DHLG, NMP, DMF and chloroform. ................................. 77
Figure 5-4 Effect of concentration of acceptor on the yield of exfoliated graphite in a)
A1, and b) A2. Where no results are presented for the different acceptors it indicates
that it was not possible to form stable dispersions at those acceptor concentrations. .... 79
Figure 5-5 TEM images of graphene following exfoliation of graphite with a) A1 and
b) A2 in NMP. ................................................................................................................ 81
Figure 5-6 XPS survey spectra of films of exfoliated graphite in a) A1 before washing
b) after washing with chloroform c) A2 before washing and d) after washing with
chloroform ...................................................................................................................... 84
Figure 5-7 SEM images of surface of films of graphite exfoliated (a) without acceptor,
(b) with A1, and (c) with A2. ......................................................................................... 86
Figure 5-8 Raman spectra for various graphite a) exfoliated films in A1 and b) A2 in
the solvents NMP, DHLG, DMF, chloroform. The precursor graphite is included for
comparison...................................................................................................................... 88
Figure 6-1 Schematic of the experimental process to form debundled CNTs via donor-
acceptor interactions in organic solvents. Starting CNT bundles (a), partially debundled
xix
CNT via solid phase donor- acceptor interaction (b), further debundled and dispersed
CNT via addition of solvent and mild bath sonication (c) and final dispersed debundled
CNT recovered from the supernatant following centrifugation (d) ................................ 99
Figure 6-2 Optical images of raw MWCNTs and acceptor (A1 and A2) before a) and
d), after grinding b) and e), following addition of solvent (NMP) c) and f) ................ 101
Figure 6-3 Concentrations (mg/mL) of supernatant of dispersions of MWCNTs a) and
SWCNTs b), optical images of dispersions of MWCNTs c) and SWCNTs d) in DHLG,
NMP and DMF with and without addition of acceptor (A1 and A2) after 30 minutes
sonication followed by centrifugation at 3000 rpm, for 30 minutes. ............................ 106
Figure 6-4 XRD diffraction patterns of supernatant of the acceptor (A1 and A2)
dispersed MWCNTs in NMP. The raw MWCNTs material is shown for comparison. 113
Figure 6-5 Raman spectra of MWCNTs exfoliated in (a) A1 and (b) A2 in the indicated
solvents. The raw MWCNTs is included for comparison. ........................................... 115
Figure 6-6 TEM images of MWCNT starting material (a) and following dispersion
with Acceptor 1 in DHLG (b) and (c). ......................................................................... 116
Figure 6-7 Electrical conductivity of films formed from MWCNTs dispersion with
acceptors A1 and A2 in various organic solvents ......................................................... 117
Figure 7-1 Future directions and potential applications of acceptor exfoliated materials.
...................................................................................................................................... 127
1
Chapter 1 Introduction
1-1 Research Background and Motivation
This thesis evolved from work done by the PhD candidate during the initial stages of the
PhD on the exfoliation of graphite using a water- surfactant system to yield graphene that
proved to have the following issues: low yield, solvent limitations and noncompetit ive
conductivity. This was attributed to the strong interlayer van der Waal interactions in
graphite, namely interactions that limit its efficient exfoliation to graphene and
subsequent dispersibility in not only aqueous solvents such as water but also in organic
solvents [1]. To overcome these interactions and exfoliate and disperse graphite in
water, additional assistants such as surfactants compounded with prolonged high energy
tip sonication had to be employed of up to 4 hours which was detrimental to the quality
of graphene produced. Furthermore, the incorporation of surfactants, which are insula t ive
in nature [2] dictated that these surfactants had to be removed following exfoliation so as
to not only improve the electrical properties but yield pristine graphene. Complete
surfactant elimination via vigorous washing was also not effective in improving the
electrical properties which meant that an additional step of high temperature annealing of
the graphene of over 250 oC in nitrogen had to be done which is not conducive for an
industrial process.The new direction is presented in this thesis to address the cleavage of
these interactions to increase the yield, dispersibility in organic solvents and
electrical properties not only in graphite but also on similar carbonaceous materials such
as CNTs.
2
This work has been developed on the basis of previous work that the PhD candidate
completed during her Master’s degree at Hokkaido University, Japan [3]. This approach
has proved to be very successful and the major outcomes will form the core of this thesis.
1-2 Previous Work and Inspiration
Previously the PhD candidate worked on the synthesis of thermoresponsive polymer gels
with a lower critical solution temperature (LCST) via a volume phase transition
specifically in organic solvents [3]. Prior to this, the synthesis of thermorespons ive
polymer gels with volume phase transition had been mainly studied on hydrogels such as
poly(N-isopropylacrylamide) (PNIPAM) gel [4], due to their ambient LCST temperatures
(~34 oC) that are attractive especially for biomedical applications such as drug delivery
[5]. Thermoresponsivity in PNIPAM, has been attributed to presence of non-covalent
inter-molecular hydrogen bonding between the hydrophilic amide groups in the polymer
chains and water at lower temperatures that cause solvation and vice versa at higher
temperatures >LCST that cause the polymer gels to collapse.
On the other hand, thermoresponsive gels with a volume phase transition in organic
solvents, which could be attractive for sensory applications where residual water is
undesirable had received less attention due to their high LCST temperatures (> 100 °C),
and how to control LCST temperature in organic solvents still remained unclear mainly
because of weak solvent-polymer interaction in organic solvents [5]. In order to induce
solvation and thermoresponsivity in polymer gels there were three important prerequisites
in the system design:
3
Synthesize a polymer gel with relatively strong intramolecular interactions such
as interactions.
Introduce external molecules such as electron deficient aromatic molecules
(acceptors) that could effectively not only interact with the pyrene groups but do
so in an easily reversible manner such as donor-acceptor interactions.
To further induce and improve solvation of the polymer gel in organic solvents
and prevent intramolecular acceptor interactions, non-polar branched alky chains
were attached to the aromatic core of the acceptors.
A ternary system was thus designed consisting of a gel bearing pyrene side groups
(P1) with relatively strong non covalent interactions. These were found to
be responsible for the poor solvation of these gels in organic solvents and thus the gel
would collapse in the solvent (Figure 1-1). However, introduction of electron
deficient acceptors A1, A2 or A3 induced swelling of the gel P1 due to donor-acceptor
interaction. Increasing the temperature of the system then resulted into breaking of
the donor-acceptor interaction forces and replacement by the initial interactions
resulting into gradual release of the solvent and shrinking of gel P1. The reverse was
also found to be true once the system is cooled resulting into thermoresponsivity.
Therefore, the electron deficient acceptors A1, A2 and A3 were found to be very
effective in cleavage of the interactions in pyrene based polymer gels and
inducing solvation of the polymer gel system in organic solvents [3].
4
Figure 1-1 Cleavage of interactions in pyrene polymer gels to induce solvation and
thermoresponsivity using electron deficient acceptors via donor-acceptor interaction [3].
As a result of the success of the electron acceptors in successfully interrupting the
interactions and inducing solvation of the pyrene gels, it was hypothesized that a similar
strategy will work on cleavage of interactions in graphite and similar carbonaceous
materials such as CNTs. The similarities of the polymer gel system with graphene/CNT
include:
Strong interactions in graphite and CNTs
Poor exfoliation yield and dispersibilility of graphene/CNTs in solvents as a
consequence of these interactions
Therefore, the key to efficient exfoliation and dispersion of graphene and CNTs is to
introduce electron deficient acceptors that will be electrostatically be attracted to the
5
electron rich surface of graphene/CNTs and non-covalently bond via a donor-acceptor
interaction mechanism.
1-3 Thesis Overview
This document will first discuss the improvements made to the exfoliation and dispersion
of graphite to yield and disperse graphene in various solvents in high yield using a
fundamental approach to cleave the interlayer graphitic interactions through donor-
acceptor interactions. The current research and industrial challenges in graphene
processing from cheap and readily available graphite, namely low yield (and to improve
yield, high energy mechanical methods are used which is detrimental to the quality of
graphene), poor graphene dispersibility especially in organic solvents will be addressed.
dihydrolevoglucosenone, (DHLG) will also be presented for the first time as novel,
renewable and green solvent for graphene processing alternative to replace the more toxic
petroleum based solvents currently used. The versatility of this processing method will
also be investigated by extending its use to process other carbon based materials: Carbon
Nanotubes (CNTs) both Single walled (SW) and Multi walled (MW) was also studied.
1-4 Thesis aims and objectives
Graphene, identified as one of the most promising materials for various applications
especially in electronic devices is difficult to produce in large scale. Its production from
graphite has been identified as a promising route due to its ubiquitous nature and low cost.
However, this approach has a significant limitation, namely the strong interlayer
interactions between the graphene sheets in graphite limiting its exfoliation and
subsequent dispersion in a wide range of solvents. Furthermore, the high energy needed
6
to break these interactions in order to increase the yield of graphene is not only
expensive for an industrial process but results in defects resulting in poor quality material.
In addition, since most of the promising graphite processing solvents are toxic there is a
need to identify nontoxic and environmental friendly alternatives. This thesis will explore
the possibility of using non-covalent donor-acceptor interaction mechanisms to exfoliate
graphite using reduced energy and expand the scope of graphene dispersion solvents. The
use of an alternative non-toxic and environmental friendly solvent for graphene
processing will also be investigated. To investigate the versatility of this approach, the
approach is extended to other carbon based materials such as CNTs. CNTs have very poor
dispersibility due to their strong tendency to agglomerate to form bundles, ropes or
aggregates as a result of their strong interactions.
1-5 Thesis Organization
Chapter 1 introduces the research topic of graphene processing including the current
challenges in its processing that limit its widespread industrial applications. It then briefly
introduces the proposed approach of using electron acceptors and why there is motiva t ion
to address these challenges. The main objectives of the study are then summarized
including a brief discussion on how the approach can also be extended to process other
carbon based materials such as CNTs. The chapter ends with a brief summary of the thesis
organization.
Chapter 2 gives an extensive literature review on graphene, its properties, applications
and its production with a main focus on top-down production methods from readily
available and cheap graphite. It then presents the limitations of current processing
methods and justifies why the research objectives are important. The strategy of using
7
donor-acceptor interactions to solve these limitations is presented. Finally a brief
introduction into CNTs and their processing limitations is discussed and how the
approach of cleavage of the interactions using donor-acceptor interactions can be
extended to CNT processing.
Chapter 3 gives a detailed description of materials and methods used and why each
method and approach were chosen and applied including the synthesis of acceptors,
graphite exfoliation to graphene, subsequent dispersion in solvents and characteriza t ion
protocols. It also describes how the graphene methodology is adopted to debundle and
disperse CNTs both SWCNTs and MWCNTs.
Chapter 4 discusses the initial step of graphite exfoliation with electron acceptors to yield
solid phase exfoliated graphene.
Chapter 5 discusses the second step of the exfoliation and dispersion process: continued
liquid phase exfoliation and dispersion in organic solvents. This chapter also discusses
DHLG as a green, renewable and environmental friendly alternative solvent to the more
toxic graphene processing solvents. Its performance in liquid phase exfoliation and
dispersion of graphite is compared to other traditional toxic petroleum-based solvents.
Chapter 6 discusses the versatility of this approach through extension of the strategy to
exfoliate graphite with electron acceptors to debundle and disperse CNTs in organic
solvents. Both SWCNTs and MWCNTs are investigated.
Chapter 7 concludes the thesis with a brief summary on research findings and how the
research outcomes meet the research objectives. It also includes recommendations for
future works, applications and future directions including prospects of the project.
8
Chapter 2 Literature review
2-1 Graphene
Graphene is a two dimensional crystalline nanomaterial consisting of a single atomic
layer of sp2 hybridized carbon atoms bonded in a hexagonal lattice similar to a
honeycomb, with an inter-atom distance of 0.142 nm [6]. Graphite is a natural minera l
that consists of many single sheets of graphene that are held together by strong van der
Waal forces, namely interactions. If one manages to overcome these interactions and
peel off a single layer from graphite then they have graphene (Figure 2-1).
Figure 2-1 Structure of graphite and graphene [7]
Even though graphene has been known to exist as the building block of graphite for years,
it was believed that its isolation from graphite to yield free standing graphene will be
impossible because of it being thermodynamically unstable at finite temperatures and thus
would decompose, crumble or collapse into other stable carbon allotropes [8].
9
However, all this changed in 2004, when two scientists: Professor Andre Geim
and Konstantin Novoselov of the University of Manchester, Britain, were able to isolate
a single layer of free standing graphene from a block of graphite by peeling off flakes
using a sticky tape until they were able to peel off a flake that was one atom thick:
graphene [9].
They then conducted a series of experiments to determine its properties that showed
immense potential to revolutionize the future of products in various applications. Andre
Geim and Konstatin Novoselev were thus awarded the Nobel prize in physics in 2010 for
their groundbreaking experiments pertinent to graphene [10][11]. Graphene’s stability
has since been attributed to the atomic scale ripples that occur on the surface which act to
minimize its surface energy [12].
2-2 Properties
Graphene has attracted significant attention due to its superior electronic, mechanical and
thermal properties. For instance, it has the highest intrinsic electron mobility known
(about 100 times that of silicon), an extremely high charge carrier mobility (both electrons
and holes are charge carriers, up to 2×105 cm2 V-1 s-1, close to the Dirac point, at an
electron density of 2×1011 cm-2, and with an ability to exhibit ballistic charge transport all
which makes graphene have extremely high electrical conductivity [13][14][8]. Graphene
is also the strongest material, with regards to its elastic properties and intrinsic breaking
strength, ever measured, 100 times stronger than steel [15]. Furthermore, exfoliated
graphite flakes have been found to have exceptional Young modulus values (>0.5–1 TPa)
with large spring constants (1–5 N m1) [16]. Graphene also has a very high specific area
(2630 m2 -1) [17] and high thermal conductivity.
10
In the case of thermal conductivity, graphene produced via mechanical exfoliation has a
measured thermal conductivity in the range of 4800–5300 Wm-1 K-1. At these values, the
thermal conductivity of graphene is more than ten times higher that of copper [18].
Graphene, being only one atom thick is also very transparent with a very high light
transmittance of ~98% and is thus the thinnest and lightest material known to man [19].
Graphene also has excellent gas permeability despite it only being a single atom thick
[20]. All these outstanding properties have led to graphene being often referred to as a
miracle material in the world of material science [18].
2-3 Production
Even though graphene has a long list of superlatives, the key challenge for its utiliza t ion
in various applications has not only been how to produce high quality graphene but also
in large scale. As a result, in recent years, much research efforts have been focused in
increasing the quality and yield of graphene to provide for large scale industr ia l
applications. These have endangered a suite of methods to produce graphene such that it
is possible to identify a well suited production approach for a particular application.
Basically there are two main distinct types of production methods: bottom-up and top
down approaches. In the bottom-up approach, generation of graphene is from suitably
designed molecular building blocks undergoing chemical reaction to form covalently
linked 2D networks using chemical vapor deposition (CVD) or epitaxial growth. The
latter approach mainly occurs via exfoliation of graphite into graphene [21][22].
However, the bottom-up approach will only be briefly discussed and the focus will be on
the top-down method from naturally occurring graphite due to low cost and ease of
availability of graphite making this process to hold high industrial potential [21].
11
2-3-1 Bottom- up
Bottom up methods involve the epitaxial growth of graphene from suitably designed
atomic and molecular scale components that act as building blocks onto a solid substrate
such as copper or nickel using chemical vapor deposition (CVD) or via reduction of
silicon carbide [23][24]. Notably, bottom-up graphene synthesis techniques are mainly
limited to growth on a solid (ideally catalytically active) surface. This is due to the fact
that since the organic synthesis starts from small molecular modules, when performed in
liquid media, are both size limited, because macromolecules become more and more
insoluble with increasing size, and suffer from the occurrence of side reactions with
increasing molecular weight [21]. This process is also expensive as it requires the use of
high temperatures, reduced pressures, expensive substrates and specialized equipment
and is therefore suited for very high end applications especially those requiring
fabrication of flexible electrodes and electronics with maximum transparency and high
electron mobility. Furthermore, even though these methods are highly advantageous due
to their ability to yield pristine graphene, these methods may not be suitable for
applications that require porous network structures for increased permeability and bulk
surface area [25].
2-3-1-1 Chemical vapor deposition (CVD)
This method involves the growth of large-area uniform polycrystalline graphene films on
a metal substrate such as copper or nickel films [26]. Even though the growth of graphene
on polycrystalline nickel films leads to a combination of single, few and multiple layered
graphene due to the grain boundaries on nickel films, the percentage of monolayer
graphene can still be increased by using single crystalline Ni (111) substrates [27].
12
On the other hand growth of graphene on copper substrates is more advantageous in that
only monolayer graphene can form. However, once the growth of graphene on the copper
substrate is complete, it then has to be transferred to the final substrate such as a dielectric
surface and silicon wafers, for fabrication of electric devices which is tedious. Overall,
the main limitations of this method include presence of defects, grain boundaries,
inclusions of thicker layers mainly in the case of nickel substrate, and, most importantly
is energy intensive [26]. Therefore, due to the overall cost of production this method is
better suited for applications such as transparent conductive coating applications such as
touch screens.
2-3-1-2 Epitaxial growth from SiC
Graphene layers can also be grown on either the silicon or carbon face of SiC wafer
through a sublimation process where the Si wafers sublime under very high temperatures
(above 1000 oC) leaving a graphitized surface [27]. This process results in very high
quality graphene with sheets approaching hundreds of micrometers in size. Apart from
the high temperature required for growth, the other issues include the growth of the
second or third layers at the edges of the surface, an increase in the size of the crystallites
and control of unintentional doping from the substrate and buffer layers [25].
2-3-2 Top-down
The top-down approach of graphene synthesis from readily available and inexpens ive
graphite is seen as the most promising route especially for applications such as conductive
inks or energy storage due to low production costs, scalability and reproducibility [25].
In this method graphene is produced from bulk graphite by peeling it off layer by layer
13
and overcoming the interactions, in between adjacent layers. However, the ease of
processing graphite to yield graphene remains a key challenge due to these
interactions (Figure 2-1). These sheets can be peeled off by applying an external force
that exceeds the strength of these interactions. Therefore, additional physical means
need to be used to provide the required energy such as ball milling, ultrasonication, shear
mixing and pressure homogenization. The discussion of graphene production from
graphite will be divided into two sections: chemical or mechanical exfoliations.
2-3-2-1 Chemical exfoliation: Graphene via graphite/graphene oxide
Graphene from graphite/graphene oxide is the most popular form of graphene available
on the market. In this process, graphene is produced from an intermediate materia l:
graphite oxide or graphene oxide which is then reduced by chemical or thermal treatments
[28][25][29]. Graphene oxide is either produced using the Brodie [30], Staudenmaier
[31], or Hummers method [32], or some variation of these methods. All three methods
involve oxidation of graphite to various levels. Brodie and Staudenmaier use a
combination of potassium chlorate (KClO3) with nitric acid (HNO3) to oxidize graphite.
The Hummers method is currently the most popular and it involves treatment of graphite
with potassium permanganate (KMnO4) and sulfuric acid (H2SO4).
The first step of this method involves the intercalation of graphite using chemical
additives: a mixture of sulphuric acid and graphite is combined with potassium
permanganate and sodium nitrate which act as catalysts. Graphite is thus oxidized and
functional oxygenated groups such as hydroxyl (-OH), or epoxide (C-O-C) in the basal
plane and carbonyl (C=O) and carboxylic (-COOH) on the edges are added on the
graphene surface in graphite. Due to the nature of these oxygenated functional groups i.e.
14
hydrophilicity, the graphitic oxide is highly dispersible and stable in water especially
under mild sonication [33]. In the second step, graphene is produced from graphene oxide
via a reduction process [28]. However, it should be noted that during the oxidation step,
defects in the form of oxygenated functional groups are introduced which can only be
partially removed by the reduction process. The reduction method can either be chemical
using reducing agents such as hydrazine hydrate and sodium borohydride [34]. Also
thermal reduction methods of thermal treatment such as heating graphene oxide at high
temperatures (>1000 oC) can be used where the oxygen functional groups are decomposed
to carbon dioxide and water [35].
Even though this approach is very popular due to high scalability, excellent yield and
ability to disperse functionalized graphene in various solvents especially water it faces
many disadvantages. These include: aggressive chemical treatment which destroys the
sp2 structure of the basal plane in graphene and introduces functional groups thus
hindering applications of this form of graphene where pristine graphene is required. Even
though these functional groups can be removed via chemical and thermal reduction,
which adds yet another processing step, the properties of this graphene are still inferior to
those of pristine graphene [36].
Furthermore, the reduction process cannot remove all structural defects introduced by the
oxidation process and thus the presence of residual oxides, i.e. epoxy bridges, hydroxyl
groups and carboxyl groups makes graphene oxide a poor electrical conductor [29]. Also
the hummers method for instance utilizes sodium nitrate, hydrazine, concentrated
sulphuric acid and potassium permanganate which are in turn very toxic and not
environmental friendly [18].
15
2-3-2-2 Mechanical exfoliation
Mechanical exfoliation methods involve the use of external forces such as shear forces,
ball milling, and sonication to peel off the graphene layers from graphite layer by layer
and overcome the interlayer van der Waal forces. The mechanical forces used to
overcome this attractive forces can be classified as either normal forces or lateral/shear
forces [22]. In the latter, the self-lubricating ability of graphite is exploited in the lateral
directions. It is also important to note that in most high yield graphite exfolia t ion
techniques these two types of forces are always used as a prerequisite of exfoliation and
almost together. However, a key disadvantage is that the mechanical forces used to exfoliate
graphite to graphene can also fragment large graphite particles or graphene layers into smaller
ones, reducing the lateral sizes of the sheets, and is thus not desirable for achieving large-area
graphene. In contrast, it can also facilitate exfoliation, because smaller graphite flakes are
easier to exfoliate than larger ones as there is smaller collective van der Waals interaction
forces between the layers in smaller graphite flakes [22].
Figure 2-2 Types of mechanical forces used for graphite exfoliation and the auxiliary
route for fragmentation [22].
16
Mechanical exfoliation methods can be divided into three categories: a) micromechanica l
exfoliation/cleavage b) mechanical milling using ball milling and manual grinding
techniques and finally c) liquid phase exfoliation (LPE) mainly via sonication. In this
thesis, micromechanical exfoliation/cleavage is discussed first followed by mechanica l
milling using ball milling and manual grinding and finally the focus is shifted to LPE and
it is then demonstrated how mechanical milling specifically ball milling or manual
grinding can be used to not only complement but improve the yield of graphene produced
by LPE.
2-3-2-2-1 Micromechanical exfoliation/cleavage
This method is also generally referred to as the scotch tape method and was mainly used
in the field of crystallography [37]. This method was used to isolate free standing
graphene leading to its discovery [26]. The main attraction of this approach lies in the fact
that no specialized equipment is required as the graphite, in the form of highly ordered
pyrolytic graphite (HOPG) or flakes, are placed between an adhesive tape such as scotch
tape and following multiple peelings of flakes stuck on the initial adhesive tape, the
graphitic layer becomes thinner and thinner and eventually one ends up with free standing
graphene [26]. To further characterize the graphene stuck to the tape it is essential to
remove it from the tape which can be done by placing the tape in an organic solvent such
as acetone followed by dipping of a silicon wafer into the graphene solution. Using this
approach large graphene sheets of up to 10 m in sizes have been isolated [20]. However
even though this method is simple, produces high quality graphene with excellent
electrical and electronic properties, scalability still remains a huge challenge as the
process is not only time consuming but it is very labor intensive and is thus only used in
17
fundamental research. There is also the challenge of inevitable contamination from the
glue tape.
2-3-2-2-2 Mechanical Milling using ball milling and manual grinding techniques
The use of mechanical milling to produce shear forces that can effectively disrupt the
interactions in graphite has been explored. Mechanical milling in a ball milling
device is a popular industrial technique that is used in the powder industry to generate
shear forces on a large scale. However, it is important to note that while shear forces are
highly desirable in lateral exfoliation of graphite to graphene, and achieving large sized
graphene flakes, there can be secondary collisions or vertical impacts applied by the balls
during rolling actions causing large scale exfoliated flakes to fragment into small ones
and sometimes this can even destroy the crystalline nature of structures to form
amorphous or non-equilibrium phases. Therefore, in order to attain high-quality and
large-sized graphene, the secondary effect should be minimized at all costs [22].
Interestingly, even though mechanical milling has been known to generate shear forces,
it had only been applied to produce disordered graphitic sheets so as to increase the
intercalation capacities for applications in lithium ion batteries rather than to produce
graphene [38][39][40]. The graphitic material samples thus analyzed even after milling
graphite for over 10 hours in a planetary mill [40] or 60 hours in a mortar grinder [41]
still showed highly crystalline graphitic nanosheet material. A selective size reduction in
the graphitic flakes to a thickness in the order of 10 nm rather than delamination to
graphene has also been reported under mechanical milling relying on shear force [42].
These studies show that large scale shear forces on their own are not very effective in
overcoming the graphitic interactions since remnants of graphitic carbon and
18
disordered carbon are still present hence alternative complementary methods have to be
explored.
2-3-2-2-3 Mechano-Chemical exfoliation of graphite
In most cases chemical reactions are not spontaneous rather an external energy such as
thermal, electrical energy has to be supplied. Furthermore, even after the energy is
supplied, it still needs to be dispersed and mass transported throughout the reactants
which can easily be done using assistants such as solvents. However, in the case of a
solvent free reaction, in order to trigger a reaction and bring the reactants in close contact
to each other, application of mechanical energy through vigorous mixing is used. This
process of using external mechanical energy or motion to trigger chemical reactions is
referred to as mechano-chemical process [43].
Adaptation of mechanical milling to exfoliate graphite and yield graphene through
addition of chemical assistants such as solvents [44], surfactants [45], inorganic salts
[46][47], dry ice [48], gases [49], polymers [50] [43] or a combination of one or more
has been explored. For instance, Zhao et al [44] showed that mechanical milling of
graphite samples in the presence of DMF solvent for 30 hours results into graphene.
Knieke et al [45] also showed that when graphite is dispersed in an aqueous surfactant
solution (Sodium dodecyl sulfate) and stressed under milling conditions mono and
multilayered graphene sheets can be produced. Yu et al [47] used mechanical milling of
graphite in the presence of sodium chloride for 1 hour, followed by washing and drying
to further remove the salts. However, an additional step of 1 hr sonication of the resultant
material in a solvent (NMP, DMF or absolute ethanol) and subsequent removal of
unexfoliated material from the dispersions by centrifugation was further used to increase
19
the yield of graphene. Lv et al [46] also produced graphene by ball milling of graphite in
the presence of sodium sulphite for 24 hours. The resultant mixture was washed in water,
filtered and dried in the oven with the final materials highly dispersible in ethanol.
Lin et al [51] also produced graphene via a two-step mechanical milling and solvent
exfoliation technique. In the first step graphite flakes were infused with a mixture of sulphuric
and nitric acid to obtain chemically modified graphite (CMG). The CMG was then ball milled
for up to 6 hours with elemental sulphur to yield graphene-sulphur composites where the
sulphur molecules are anchored onto the graphene sheets. The resultant materials were then
dispersed in a carbon disulphide solution with stirring to remove sulfur and obtain
freestanding graphene sheets. Graphene has also been produced via high energy ball
milling of graphite with molecules such as triazine derivatives including commercia l ly
available melamine for 30 minutes [52][53] and triphenylenes for 60 minutes [54].
In all these cases, where chemical assistants have been added to the mechanical milling
process to aid graphite exfoliation, exfoliation has been attributed to molecular adsorption
of the molecules from the chemical assistants to the surface of graphene which is able to
compensate the huge attractive interactions in graphite. However, even though
mechanical milling shows a huge potential in the large-scale production of graphene as
outlined in the examples above, the main challenge that still remains is the continued use
of prolonged and high energy mechanical milling techniques which can be detrimental to
the quality of graphene. Therefore, it is imperative to identify new chemical assistants
that can efficiently interrupt and weaken the graphitic interactions and consequently
lower the energy needed to achieve exfoliations.
20
2-4 Liquid phase exfoliation of graphite to graphene
This method is the most promising due to its simplicity, effectiveness, low production
cost, overall scalability, and it is non-oxidative. Graphite is directly exfoliated and
dispersed in a suitable solvent combined with mechanical, solvothermal or sonochemica l
assistance. During this process the growth and collapse of micrometer sized bubbles or
voids in liquids due to pressure fluctuations resulting from external mechanical forces act
on the graphite and induce exfoliation into single and few layered graphene [18]. Once
the graphite is exfoliated and dispersed in a suitable solvent, the dispersion can then be
directly employed for nanocomposite applications using technologies such as 3D
printing. Apart from organic solvents, another solvent that can be used for graphite
exfoliation is water. Unfortunately water cannot be used without modifications due to its
high surface tension of around 72 mNm-1, and also its hydrophilic nature makes it
incompatible with graphene/graphite [18].
However, even though the exfoliation and dispersion in water have been adequately
solved through the addition of assistants such as surfactants and polymers, the use of
organic solvents needs to be further explored, especially for applications where residual
water is undesirable, for example in electronic devices [55]. In addition most surfactants
are insulating in nature which necessitates the need for an additional washing step after
film formation or device processing.
2-4-1 Liquid phase exfoliation in organic solvents
The direct liquid phase exfoliation of bulk graphite powder in an organic solvent is a
known scalable technique [56] that could be used for applications such as functiona l
21
coatings, conducting inks, composites, batteries, supercapacitors and top down
approaches to electronics [27][57][58][59][56][60][61]. In this approach suitable solvents
are used to exfoliate graphite via a simple sonication process. However, one major
drawback is the low yield, typically around 0.01 mg/mL [56], and, for enhanced yield,
longer sonication times are required, in the order of 460 hours for high boiling point
solvents [62] and 48 hours for low boiling point ones [63]. A significant disadvantage of
prolonged sonication is that it leads to partial destruction of the graphene sheets, and from
an energy consumption perspective is impractical for large-scale applications. Increasing
the yield of graphene, while reducing sonication times still remains a key challenge for
mass production.
Furthermore, due to the intrinsic insolubility of graphite, a consequence of the extensive
network of interlayer interactions, the exfoliation and dispersion processes are
limited to solvents whose surface tension best match the surface energy of graphene.
For graphite, the surface energy is defined as the energy per unit area required to
overcome the van der Waals forces, specifically interactions, when peeling the
two sheets apart [56]. The estimated surface energy of graphene is 70 mJ/m2, therefore a
suitable solvent should have a surface tension, close to 40 mJ/m2 (Figure 2-3 a) [56] [64].
Whilst surface energy is an important parameter for describing the solvent-graphene
interaction, it has a severe limitation in that it can only be used to describe the overall
intermolecular interaction [64]. Therefore, like in most solvent-solute systems, the
intermolecular interactions are best divided into at least three types with the simplest
formulation dividing them into dispersive (D), polar (P,) and hydrogen-bonding (H)
components. Hernandez et al [64] showed that the Hildebrand solubility parameter (δT) is
22
also useful in solvent selection for graphene dispersions, with good solvents having
values close to 23 MPa1/2(Figure 2-3 b). As both surface tension and Hildebrand
parameter are related to the overall solvent-graphene interaction, they showed that
successful solvents also have Hansen solubility parameters of Dispersive (δD) ∼ 18
MPa1/2, Polar (δP) ∼ 9.3MPa1/2, and Hydrogen bonding (δH) ∼ 7.7 MPa1/2 (Figure 2-3, c,
d and e) with the dispersibility smaller for solvents with Hansen parameters further from
these values.
Figure 2-3 Graphene dispersibility, CG, as a function of a) solvent surface tension(mJ/m2)
b) solvent Hildebrand parameter (δT), c) dispersive Hansen solubility parameter (δD), d)
polar Hansen solubility parameter (δP), and e) hydrogen-bonding Hansen solubility
parameter (δH) [64].
23
However, a further complication is the best graphite exfoliation solvents often have high
boiling points, rendering downstream processing more complex and economically non-
viable. For instance, it has been shown that it is extremely difficult to remove high boiling
point solvents when processing graphene into films or composites. Consequently, it is
also almost impossible to deposit individual flakes from solvent exfoliated graphene, as
aggregation tends to occur during the slow solvent evaporation [56].
Therefore, the prospect of extending liquid phase exfoliation to non-polar solvents, such
as chloroform, which has relatively poor matching surface energy but its low boiling point
will offer significant versatility by expanding the range of solvents available and enable
development of new applications. However, low boiling point solvents still remain
unpopular because the amount of graphene obtained in dispersion is too low [62] or they
require the transfer of graphene from a suspension to solvents such as NMP [65].
2-4-2 Liquid phase exfoliation with addition of intercalants
Many industries require that the bulk quantity of graphene be supplied as either in the
form of powder, foam, film or high concentration solution. However, the high tendency
of graphene to restack during processing means that the excellent properties of individua l
graphene sheets may not be translated into bulk graphene. With the objective of
increasing the yield and enabling the scaling up of graphene production, a number of
studies have explored liquid phase exfoliation combined with the use of intercalants
[66][67] and as the magnitude of van der Waals interactions is inversely proportional to
r6, where r is the distance between the molecules, it tends to zero for interlayer distances
greater than 0.5 nm. Intuitively, the rationale behind the addition of intercalants is that,
since the molecular attractive forces between adjacent layers of graphite are relative ly
24
weak, they are susceptible to further weakening, or can even be completely overcome, by
increasing the distance between the layers [1]. However, the reported shifts in the
interlayer distances, evidenced by XRD, were too small to conclude that intercalation had
effectively occurred thus extensive sonication was still required to achieve exfolia t ion
[63][66][68][69][70][71][72].
2-4-3 Mechano-chemical assisted liquid phase exfoliation
Non-covalent mechano-chemical activation via solid phase high energy ball milling of
graphite with molecules such as triazine derivatives and triphenylenes has been shown to
effectively produce large quantities of defect free graphene [53][52][54]. In this case,
exfoliation has been attributed to molecular adsorption of the molecules to the surface of
graphene which is able to compensate the huge attractive van der Waal forces in graphite.
However, even though graphite was successfully exfoliated after ball milling, the high
energy prolonged ball milling resulted into destruction of graphene sheets with smaller
sheets produced. In addition, not all exfoliated graphite was dispersible in organic solvent.
In the case of triazine derivatives only dimethylformamide (DMF) effectively dispersed
the exfoliated graphite [53][52]. In the triphenylene system the authors only reported
enhanced dispersion of solid phase exfoliated graphite in DMF, methanol and
tetrahydrofurane (THF) and were unable to obtain any stable dispersions in non-polar
solvent chloroform [54]. The limited scope of graphite dispersible organic solvents is due
to the polar nature of both molecular additives which explains the high attainable
dispersions in water for both cases.
25
2-4-4 Liquid phase exfoliation via donor-acceptor interactions
Donor-acceptor interactions are intermolecular interactions between -electron rich
(donor) and -electron deficient (acceptor) molecules. These interactions are very
favorable for use in graphene exfoliation due to their non-covalent nature as they leave
the graphene- conjugated system intact.
For instance, the use of solvents with strong electron withdrawing or solvents with strong
electron donating functional groups to exfoliate and stabilize graphene through donor-
acceptor interactions has been explored [73][74]. It is noteworthy that, even in these
systems, exfoliation and dispersion have been mainly ascribed to matching surface
energies of the solvents. This is probably related to the fact that exfoliation is an entropy
driven process, involving disruptions of the interactions leading to a greater degree
of disorder, and has thus energetic consequences: the overall reaction must be
endothermic to satisfy the Gibbs free energy equation (G=H-TS). In general, weak
electron acceptors or donors do not match the energetic cost associated with such
interlayer graphitic cleavage and are qualified as poor additives.
Amemori et al [75] and Gharib et al [3] reported a significant change in the solubility
behavior of selected polymeric systems bearing pyrene side groups, when branched
electron-deficient acceptors were intercalated to facilitate the cleavage of the
interactions. The authors explained the increase in solubility by the occurrence of a
mechanism involving donor-acceptor electronic coupling with the network. It was
also found that the modification of acceptors with branched alkyl chains was crucial for
the successful disruption of interactions between pyrene groups.
26
A number of studies have similarly reported the use of the archetypal -conjugated n-type
organic semiconductor molecules similar to the electron deficient acceptors discussed
above. Narayan et al [76] reported the successful exfoliation of graphene using a
tetraanionic polycyclic aromatic semiconducting compound, Perylene- 3,4,9,10-
tetracarboxylate (PTCA) as a new surfactant exfoliant-cum-dispersant for graphene in
aqueous media was reported.
However, following overnight stirring of the graphene-water-PTCA mixture, prolonged
bath sonication of up to 12 hours was still used to increase the yield which makes the
overall process less energy efficient. Similarly the yield obtained from this process was
calculated after the dispersions were left to stand overnight and it is very difficult to
evaluate how these dispersions will compare under accelerated sedimentation such as
centrifuge conditions. The authors also attributed the mechanism of exfoliation to
stacking and charge-transfer interactions between the graphene and electron acceptor
PTCA molecules. Zhang et al [77] also used a similar molecular design strategy to cleave
the interactions in graphite, using electron deficient surfactants molecules with ionic
groups attached via an alkyl spacer. However, the dispersion of graphene was only
effectively increased in water and thus improvement of graphene dispersions in organic
solvents still needs to be further explored.
2-4-5 Liquid phase exfoliation in green polar aprotic organic solvents
Even though it is imperative to produce large quantities of graphene for varied
applications, to date, the best reported solvent for the liquid phase exfoliation of graphite
is NMP and DMF [56], reproductive toxicants and chemicals that are currently on the
European Candidate List of substances of very high concerns (SVHC) due to their toxicity
27
[78][79]. SVHC is the prerequisite step to any substance becoming restricted and subject to
authorization under European REACH regulation (Regulation (EC) no. 1907/2006) before use
or import into Europe is permitted. In the USA similar concerns over NMP, and DMF, have
also been raised [80]. Therefore, these solvents are an unsustainable option for graphene
processing. Unfortunately, viable alternatives for these dipolar aprotic solvents are scarce. One
alternative solvent is 1, 2-dichlorobenzene (oDCB), as it is not currently subject to REACH
restrictions. However, it appears on the international ChemSec SIN (Substitute It Now) list and
the US EPA ‘Extremely Hazardous Substances List’ due to its high aquatic toxicity[80]. In
this respect, there is a need to explore alternative solvents that meet environmental and
safety standards without compromising on performance. Dihydrolevoglucosenone
(DHLG), a novel bio-based solvent, derived in 2 simple steps from
cellulose via levoglucosenoe (Figure 2-4), has recently been reported as a replacement solvent
for organic transformations where NMP is currently the favored solvent [81]. Furthermore,
DHLG does not contain the amide functionality which has been associated with the
reproductive toxicity in NMP and DMF. In addition, DHLG does not contain any chlorine
which can present end-of-life pollution issues or create corrosive by-products if incinerated
(e.g. oDCB). Other attractive properties of DHLG include: It is biodegradable, non-mutagenic,
and has a flash point of 108 °C thus making it safer to handle than many oxygenated solvents
[81]. It is stable to oxidation and (at end-of-life) upon incineration or biodegradation yields
only carbon dioxide and water. This is an advantage over equivalent petrochemical dipolar
aprotic solvents such as NMP which liberate NOx upon decomposition [81]. DHLG therefore,
holds potential as a sustainable polar aprotic solvent for the LPE of graphite.
28
Figure 2-4 Scheme for the production of dihydrolevoglucosenone (DHLG) [81].
2-5 Carbon Nanotubes
2-5-1 Brief Overview, Properties and Applications
Since their discovery by Ijima in 1991 [82], Carbon nanotubes (CNTs), which are
essentially graphene sheets, with sp2 hybridized carbon, rolled up to form a cylinder [83],
with either open or closed ends, still attract much attention due to some of their
outstanding physical, chemical, mechanical and electronic properties [84][85][86][87].
CNTs are electronically categorized as metallic or semiconducting depending on their
geometry, diameter and chirality [84]. Armchair aligned CNTs are metallic, whereas
zigzag and chiral geometries tend to exhibit semiconductor electrical properties [88].
Furthermore, due to the covalent sp2 bonds between individual carbon atoms, a single
CNT shows a Young’s modulus of 1.2 TPa, a tensile strength around a hundred times
higher than steel and can therefore tolerate huge strains before mechanical failure [89].
Individual SWNTs can also have a thermal conductivity of 3500 W m−1 K−1 at room
temperature, based on the wall area this exceeds the thermal conductivity of diamond
[90].
29
These unique properties have allowed their use in numerous high end applications :
conductive and high strength nanocomposites for example in automotive parts, sporting
goods and boat hulls, semiconductor devices, nanoprobes, energy conversion/ storage
devices such as rechargeable batteries, sensors, field emission displays as well as in the
engineering of novel carbon based structures [91] [92]. CNTs come in two most common
types: single-walled (SWCNT) and multi-walled (MWCNT) and, though appearing to be
structurally similar, MWCNTs are essentially an array of SWCNTs within each other,
reminiscent of a telescopic structure (Figure 2-5).
SWCNTs are treated as one dimensional material due to their high aspect ratio: with a
diameter in the range of 0.8-2 nm and a length between 0.2 and 5 µm, depending on the
synthesis method [93]. MWCNTs, on the other hand, depending on the number of layers,
have typical diameters varying from 2-100 nm [94][95]. The lengths of MWCNTs can
range from less than 100 nm to several centimeters. The interlayer distance in MWCNTS
is interestingly close to the distance between graphene layers in graphite, approximate ly
0.335 nm [96].
30
Figure 2-5 Graphene and carbon nanotubes as (A) single wall carbon nanotube (SWCNT)
and (B) multi-wall carbon nanotube (MWCNT) structures [17].
2-5-2 CNTs production
In order to meet the demand for various applications based on CNTs, controllable mass
production of CNTs with desired structure and property is essential [89]. There are 4
main methods for production of CNTs: chemical vapor deposition (CVD), arc discharge,
laser ablation/vaporization and high pressure decomposition of Co (HIPCo) [97].
Out of the four methods, CVD is currently the most widely used for large scale CNT
production. In this method, a carbon feedstock is catalytically decomposed into carbon
atoms and hydrogen with the reaction initiated on a catalyst such as a transition metal.
The carbon atoms then diffuse into the metal particles until the solution (metal-carbon)
becomes saturated; when supersaturation occurs, there induces the precipitation of
graphite carbon from the metal surface, which under the right conditions forms a cylinder
(CNT) [89][97]. The transitional metal catalysts include Fe, Co, Ni, Au, Pd, Ag, Pb, Mn,
31
Cr, Ru, Mo and Cu. Recently, metal-free catalysts such as SiO2, Si, SiC, Ge, Al2O3, ZrO2,
ZnO, C60, nanodiamand, or even CNTs have been shown to also be efficient for CNT
synthesis [97]. The CVD growth offers advantages of mild synthesis condition such as
normal pressure and low growth temperature, high yield, simple facility, and a low cost.
The wall number, diameter, length, and alignment of as-produced CNTs can also be well
controlled. However, a key disadvantage is that CVD methods yield contaminants that
can influence CNT properties and often require costly thermal annealing and/or chemical
treatment for their removal. These steps can introduce defects in CNT sidewalls and
shorten CNT length. Furthermore, because SWCNT synthesis by CVD requires much
tighter process control than MWCNT synthesis, bulk SWCNT prices are still orders of
magnitude higher than for MWCNTs. Use of MWCNTs is therefore still favored
especially for applications where CNT diameter or bandgap is not critical [97].
2-5-3 CNT processing
CNTs are produced as a solid black powder, which prior to being used in most
applications, must be exfoliated or dispersed in liquid media [98]. This is because CNTs
have a high tendency to agglomerate and form bundles, ropes, or aggregates. The resultant
CNT bundles can therefore have very complex morphologies varying from tens of
nanometers in diameter and many micrometers long. In fact, studies have shown that
individual CNTs can, not only be held within a CNT bundle, but can also be entwined,
interwoven, bent, entangled or form loops around not only other CNT bundles but also
within an existing bundle [99]. The main properties of CNT bundles are inferior to those
of isolated CNTs, and the fact that it is extremely difficult to separate CNTs from bundles
represents a serious hurdle in the way of potential applications. CNTs aggregation and
32
bundling are mainly governed by two things: nanotube morphology, a consequence of
their high molecular weight and aspect ratios, as well as attractive forces between the
CNTs due to their high surface energy and interactions derived from their extended
π-electron network [100][101][102][103][96]. All these factors make CNTs have very
poor processability and dispersiblity not only in water but also organic solvents limit ing
their practical applications.
2-5-3-1 Solid Phase Processing/ Debundling of CNTs
CNTs bundle sizes are known to be efficiently reduced by mechanical milling such as
grinding or ball milling and in some cases the mechanical milling process may introduce
cuts and bends in the CNTs resulting into simultaneous cutting of CNTs into shorter
lengths [104].
Whilst shortened CNTs are highly sought after in chemical or energy-storage
applications, long CNTs are required for their application as strong and conducting
nanocables [105]. Therefore, the aspect ratios of the final materials following CNTs
grinding could be crucial depending on the targeted end applications. Pierard et al [106]
showed that MWCNTs can be cut to lengths of < 1 um, from an initial 50 um, following
mechanical milling using an agate ball for 120 hours with no amorphous carbon observed,
indicative that no major structural defects are created during milling conditions.
Similarly, Kukovecs et al [107] showed that when a low energy ball mill is used, CNT
tube entanglement and length decreased with increasing milling time to upto 140 nm
following 200 hrs of mechanical milling. Similarly, no amorphous carbon or structural
defects was observed on the CNT walls. It has also been shown that co-grinding of CNTs
with toluene for one hour can reduce the bundle diameters and agglomerate particle sizes
33
by a factor of five [108]. SWCNTs have also been shown to be debundled and
subsequently cut simply by grinding (120 minutes and 40 minutes) with soft organic
materials such as or cyclodextrin respectively.
On the other hand, Liu et al [104]showed that when a high energy ball mill is used instead,
following 10 hours of mechanical milling, the MWCNTs are debundled and the length is
simultaneously decreased to <1um from an initial of 10–100 m. However, following
extended high energy milling of up to 90 hours , the CNTs original structure are destroyed
and amorphous carbon appears. This clearly shows that the extent of debundling,
subsequent cutting of CNTs and finally severe structural defects is dependent on both the
intensity and time of mechanical milling.
2-5-3-2 Liquid Phase Processing/Debundling and Dispersion of CNTs
The liquid phase debundling and dispersion of CNTs in a carefully selected solvent using
sonic energy is a simple and popular technique used to disaggregate, debundle and
disperse the CNT bundles into individual CNTs or reduced CNT bundle sizes
[109][110][111][112][113][114][115][116][117][118]. In this approach, solvents are
chosen due to their ability to compensate for the high CNT surface energies, a
consequence of the inter CNT interactions [118]. In general, the most effic ient
successful solvents for CNT dispersions have a surface energy very close to the surface
energy of CNTs (∼70 mJ/m2)[119]. This means that successful solvents for CNT
dispersions tend to have surface tensions of ∼40 mJ/m2 with most being polar aprotic
amides such as NMP (40.1 mJ/m2) and DMF (37.1 mJ/m2) [120][121][122]. Bergin et al
[98], showed that equally important, successful solvents have Hildebrand parameter that
match well with those of CNTs at <T> 21 MPa1/2. However, only a small fraction of
34
solvents with the correct Hildebrand parameter (T) was found to successfully disperse
CNTs due to effects of surface entropy. The Hildebrand parameter was also found not to
be specific enough to identify successful solvents. Finally, Bergin et al [98] showed that
successful solvents also occupy a well-defined range of Hansen parameter space with the
level of dispersibility being more sensitive to the dispersive Hansen parameter (Dthan
the polar(P or H-bonding(HHansen parameter. The dispersion, polar, and hydrogen
bonding Hansen parameter for the nanotubes were found to be <D> 17.8 MPa1/2, <P>7.5
MPa1/2, and<H> 7 .6 MPa1/2 with the dispersibility smaller for solvents with Hansen
parameters further from these values.
However, while the approach is technically simple with a significant potential in up
scaling, the yields obtained remain very low, typically less than 0.01 mg/mL following
centrifugation [98][123] indicating the unstable nature of such dispersions [116][98]. The
Instability of these dispersions is related to the high aspect ratios of the CNTs, therefore
even though the CNT may be stable immediately following solvent dispersion, the CNTs
sediment out at faster rates with time and especially following accelerated sedimenta t ion
even under mild centrifuge conditions [118]. Therefore, it is imperative to identify
additives that can not only assist in initial interruption of the CNT interactions but
can also prevent or reduce CNT re-aggregation or sedimentation following dispersion
especially in organic solvents. It has been widely reported that the yield and stability of
CNT dispersions can be increased via the surface modification of CNTs with additives,
such as surfactants [124] polymers [125], DNA [126] non-covalent sidewall
functionalization with aromatic compounds carrying pyrene [127], anthracene [128] and
porphyrin [129] or covalent bonding [130] .These additives have the ability to interrupt
35
the CNT interactions holding the aggregates together, thereby causing them to
debundle and form a homogenous dispersion of the CNTs. However, these methods are
very often accompanied by use of prolonged and high-energy sonication. A major
drawback of sonication being that it creates defects through fragmentation of the CNTs,
which in turn affects considerably the physical-chemical properties due to the decrease in
their aspect ratio [131][132] and alternative CNT dispersion and stabilizing agents still
need to be explored.
Moreover, the challenge of identifying organic solvents with minimal environmenta l
footprint and impact is yet to be addressed. For instance, some of the best organo-solvents
used by industry for CNT dispersions include N-methyl-2-pyrrolidone (NMP) and
dimethylformamide (DMF) which are not only fossil fuel based but are highly toxic and
are currently on the European Candidate List of substances of high concerns for their
authorization due to their toxicity in accordance with Article 59(10) of the REACH
(Registration, Evaluation, Authorisation and Restriction of Chemicals) regulation [133].
In this respect, there is a need to explore alternative solvents that meet environmental and
safety standards.
36
Chapter 3 Materials and Methods
3-1 Materials
Synthetic graphite with a nominal particle size of 5 m, 2-ethylhexylamine, pyromellit ic
dianhydride (PMDA), naphthalene-1,4,5,8- tetracarboxylic acid dianhydride and organic
solvents, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), chloroform,
dichloromethane (DCM) and hexane were used as received from Sigma Aldrich.
dihydrolevoglucosenone (DHLG) was obtained from Circa Group Pty Ltd (Australia).
MWCNTs and SWCNTs were purchased from carbon allotropes, Australia and used as
received.
3-2 Methods
3-2-1 Synthesis of acceptors
Acceptor 1 (A1) and Acceptor 2 (A2) were synthesized according to methods reported
elsewhere [75]. In the optimization experiments, it was found that the branched alkyl
chain was needed to afford solubility in a range of solvents, especially non-polar ones, as
well as to reduce intramolecular interactions in the acceptor through steric effects. The
IUPAC names of the compounds are: N,N’-bis-(2-ethylhexyl) pyromellitic diimide (A1)
and N,N’-bis-(2-ethylhexyl)-1,4,5,8-naphthalenetetracarboxydiamide (A2), with
molecular masses of 328 g/mol and 348 g/mol respectively (Figure 3-1).
37
Figure 3-1 Electron acceptors A1 (a) ball and stick structures, (b) simulated 3D and (c)
molecular and A2 (b) ball and stick structures, (c) and (d) simulated 3D, and, (e) and (f)
the molecular structure [134].
3-2-1-1 Synthesis of A1 (N, N’-bis-(2-ethylhexyl)pyromellitic diimide)
Figure 3-2 Synthesis of A1 (N, N’-bis-(2-ethylhexyl)pyromellitic diimide)
4.4 g (20 mmol) of pyromellitic dianhydride was degassed and 44 mL of dehydrated DMF
was added. 5.7 g (44 mmol) of 2-ethylhexylamine was added and the mixture heated at
150 °C under reflux under nitrogen for 24 hours (Figure 3-2). The reaction mixture was
cooled, the organic layer washed with water then extracted with dichloromethane. The
organic layer was dried using magnesium sulphate and filtered and the solvents
evaporated. The crude product was purified using column chromatography, silica gel;
38
eluent dichloromethane/hexane (2:1) to obtain A1 as an off white solid with the chemical
structure confirmed by 1H NMR spectroscopy (Figure 3-3).
Figure 3-3 1H NMR spectrum (42.5 MHz, CDCL3 r. t.) of A1 [134]
3-2-1-2 Synthesis of A2 (N,N’-bis-(2-ethylhexyl)-1,4,5,8-naphthalene tetra carboxydiamide)
Figure 3-4 Synthesis of A2 (N,N’-bis-(2-ethylhexyl)-1,4,5,8-naphthalene tetra
carboxydiamide)
39
2.7 g (10.0 mmol) of naphthalene-1,4,5,8- tetracarboxylic acid dianhydride was degassed
in a round bottomed flask and 50 mL of DMF (dehydrated) was added. 2.9 g (22.0 mmol)
of 2-ethylhexylamine was then added and heated to 150 °C under reflux under nitrogen
for 24 hours(Figure 3-4). The reaction mixture was cooled, poured into water and the
organic layer extracted with dichloromethane, and again washed with water. The organic
layer was dried using magnesium sulphate and filtered, and the solvent was evaporated
under reduced pressure. The crude product was purified using column chromatography
silica gel; eluent dichloromethane to obtain A2 as a pink solid with the chemical structure
confirmed by 1H NMR spectroscopy (Figure 3-5).
Figure 3-5 1H NMR spectrum (42.5 MHz, CDCl3, r. t.) of A2 [134]
40
3-2-2 Exfoliation of graphite
In a typical experiment, 50 mg of graphite powder was ground for 2 minutes with A1 or
A2 (10-100 mg) in a vial followed by the addition of 10 mL of solvent. Identical
suspensions in solvent only, without acceptors, were prepared as controls. All suspensions
were subjected to low energy bath sonication (Unisonics, Australia, 50 Hz) for 30 minutes
at ambient temperature, followed by centrifugation (Eba 20, Hettich, Zentrifugen) at 3000
rpm for 30 minutes to sediment unexfoliated materials, and the supernatant was carefully
removed by decantation of the top half using a pippete to give stock graphene suspensions
(Figure 3-6).
Figure 3-6 Schematic of the experimental process to form graphene dispersions via
donor-acceptor interactions in organic solvents [134].
41
(Starting graphite material (a), partially exfoliated graphite via donor- acceptor interaction
(b), further exfoliated and dispersed graphene and graphite sheets via addition of solvent
and mild bath sonication (c) and final dispersed graphene sheets recovered from the
supernatant following centrifugation (d))
3-2-3 Debundling and dispersion of CNTs
In a typical experiment, 50 mg of MWCNTs (5 mg/mL) was ground for 2 minutes with
the acceptor (0.1 g A1 and 0.075 g A2, respectively) in a vial followed by the addition of
10 mL of an organic solvent. In the case of SWCNT due to high cost of material 10 mg
of SWCNTs (1 mg/mL) was ground with A1/A2 (20 mg, 10 mg respectively) followed
by addition of 10 mL of organic solvent. Identical suspensions in solvent only, without
acceptors, were prepared as controls. All suspensions were subjected to low energy bath
sonication (Unisonics, Australia, 50 Hz) for 30 minutes at ambient temperature, followed
by centrifugation (Eba 20, Hettich, Zentrifugen) at 3000 rpm for 30 minutes. The
supernatant was collected to give CNTs dispersions, that were then used for spectroscopic
analysis and to prepare conducting films by vacuum filtration onto a porous alumina
membrane with 20 nm pore size.
3-2-4 Preparation of conducting films
In order to characterize the quality and properties of the graphene and CNT dispersions,
thick conductive films (0.5m thickness) were prepared for some techniques: Raman,
SEM, XRD, XPS and electrical conductivity measurements (Figure 3-7). 6 mL of the
graphene and CNTs dispersions were filtered onto a porous alumina membrane 20 nm
(0.02 µm) pore size using Buchner vacuum filtration and subsequently washed with
42
solvent to remove any traces of residual acceptor followed by drying overnight in an oven
at 80°C n air.
Figure 3-7 Representative conductive film sample prepared from filtration of 6 mL
dispersion of A1 exfoliated graphite in NMP.
3-3 Characterization
3-3-1 1H Nuclear Magnetic Resonance (1H NMR)
1H NMR measurements of the acceptors was recorded on Spinsolve carbon (magritek,
SPA409) bench top NMR at 42.5 MHz at room temperature to identify the chemical
structure of the acceptors. 10 mg of A1/A2 was diluted with 5 mL CDCL3 and analyzed
in solution.
3-3-2 X-ray Diffraction Spectroscopy (XRD)
XRD was used to investigate the crystallinity and extent of exfoliation of the exfoliated
and dispersed materials (Graphene/CNT) through comparison to the untreated materials.
XRD patterns of the precursor graphite, CNTs, ground graphite, ground graphite with the
43
acceptor and the solvent exfoliated and subsequently centrifuged acceptor-graphite and
CNTs dispersions were recorded on a Bruker Advanced X-Ray Solutions D8 (40 kV, 40
mA) with CuK radiation ( = 0.154 nm), at a scan rate of 2°/min in the range of 2 theta
from 10 to 70°.
3-3-3 Ultraviolet-Visible (UV-Vis) Spectroscopy
Graphene and CNTs dispersed in solvents, were characterized using UV-Vis
spectroscopy (Shimadzu UV-2600 Spectrophotometer) so as to determine the exfolia t ion
yields. The suspensions with A1/A2 were diluted prior to the measurements to obtain
meaningful absorbance readings. The solution was transferred to a UV quartz cell where
measurements were done over a range of 200-800 nm. Since the spectra obtained for A1
and A2 show that both acceptors do not absorb anywhere from 450 nm onwards (Figure
3-8), the literature molar absorptivity of solvent exfolia ted graphene obtained at 660 nm
(α660 = 2,460 mL/mg/m) [56] was used. The concentration of suspended graphene was
then obtained by applying Beer’s law: A = cl, where A is the absorbance, l [m] is the
optical path length, and [L g1 m1] is the absorption coefficient. ( is determined
experimentally by filtering a known volume of dispersion, e.g. via vacuum filtration, onto
a filter of known mass, and measuring the resulting mass using a microbalance).
44
Figure 3-8 UV–Vis spectra of a) Acceptor 1 and b) Acceptor 2 in NMP. The spectra are
featureless above 450 nm [134].
For CNTs calculations, concentration of suspended CNTs which was obtained by
measuring the absorbance at 660 nm, and applying Beer’s law using A/l = 660C, with a
literature extinction coefficient of CNTs (α660 = 3264 mL/mg/m) [98]. However, studies
on the molar extinction of MWCNTs (α660) would need to be conducted for comparative
studies with SWCNTs for various polar aprotic solvents which is beyond the scope of this
thesis.
3-3-4 Raman Spectroscopy
Raman spectroscopy was used to characterize the quality of exfoliated graphite and
dispersed CNTs. The spectra were taken on a confocal Raman microscope (WiTec Alpha
300R) employing a grating spectrometer with a Peltier-cooled CCD detector coupled to
a confocal microscope. The Raman scattering was excited with an argon ion laser (λ=
532.1 nm), focusing on the sample was done with a 10x S5 microscope objective
(NA=0.85) with the minimum laser power to obtain an optical signal over a scan range
of 1000-3000 cm-1. Samples were prepared by the direct deposition of the undiluted
45
graphene dispersion onto an alumina membrane and measured in their dried state (Film
thickness 0.5 m, Figure 3-7). For samples exfoliated with an acceptor, the graphene
films were further washed in excess exfoliation solvent to remove residual acceptor. All
spectral data was processed with Origin 3.2 software.
3-3-5 X-ray photoelectron spectroscopy (XPS)
XPS analysis was carried out on the washed exfoliated graphite film (Figure 3-7) to
ascertain whether any acceptor was still present. Residual acceptor in the films was
removed by repeated washing with the respective exfoliating solvent until the acceptor in
the wash filtrate was undetectable by UV/Vis spectroscopy. The XPS analysis was
performed with a Kratos AXIS NOVA spectrometer (Kratos Analytical, Inc., Manchester,
UK) using a monochromated Al k x-ray source operating at a power of 150 W. Survey
spectra were acquired at 160 and 20 eV pass energies, respectively. Three spots on each
surface with an elliptical area of approximately 0.3 x 0.7 mm were analyzed.
3-3-6 Field emission scanning electron microscope (FE-SEM)
The morphology and size of the particles forming the graphene films were determined
using a, ZEISS SUPRA 40VP FE-SEM at 3 kV. Samples were mounted on the SEM plate
using a conductive tape and gold coated. The film thickness (t) of all graphene and CNT
films was also obtained from the film cross section using SEM images.
3-3-7 Field emission transmission electron microscope (FE-TEM)
Graphene/MWCNT dispersions was added to an equal volume of isopropanol to dilute it
as it was found that it was too concentrated to achieve a good TEM image as well as to
46
aid evaporation of the solvent. FE-TEM samples of exfoliated graphite and MWCNTs
were made by drop casting from the diluted dispersion onto holey carbon films (copper
grids, 400 mesh size) and the images were recorded on a Jeol 1010 TEM with an operating
voltage of 100 kV with Gatan Orius SC600 CCD Camera. The images were taken at
random locations across the grids, to ensure a non-biased assessment then used to
determine the quality and degree of exfoliation.
3-3-8 Electrical conductivity
Electrical conductivity measurements were performed using a 4-point probe (Jandel,
model RM 3000) so as to evaluate the electrical properties of the films. The sheet
resistance (Rs) of all the films were measured, using the four-point conductivity probe
method, after drying the films overnight at 70 °C. The DC conductivities was then
calculated as shown below with the film thickness (t) obtained from film cross section
SEM images.
Conductivity(S/m) = (Rs)−1 / t×10−9
47
Chapter 4 Solid phase exfoliation
The majority of this chapter has been published:
D. H. Gharib, S. Gietman, F. Malherbe and S. E. Moulton, High yield, solid exfolia t ion
and liquid dispersion of graphite driven by a donor-acceptor interaction, Carbon, 123,
695-707, 2017
The PhD candidate performed all of the experiments and through the assistance of the
other article authors analyzed the results and wrote the manuscript.
48
4-1 Introduction
Graphene has attracted significant attention since its discovery due to its outstanding
mechanical and electronic properties [14][135]. Several methods have thus been explored
to prepare graphene, with each method having advantages and disadvantages as outlined
in chapter 2, Section 2-3. Of all methods discussed, micromechanical cleavage of graphite
using scotch tape [136] is a simple process that produces very high quality and pristine
graphene. However, it is too involved, lacks controllability and there is always the
possibility of inevitable contamination from the glue tape [22]. In addition, the yield is
too low to upscale. Mechanical exfoliation, relying on a similar concept such as used in
the scotch tape method, via mechanical peeling of graphene from graphite would
therefore be very promising in terms of quality of material once the yield and potential
scalability is improved.
Graphite consists of many layers of graphene that are held together by interactions.
In order to delaminate graphene from graphite, these interactions have to be overcome.
The use of mechanical milling to produce shear forces that can effectively disrupt the
interactions in graphite has been explored. Mechanical milling in a ball milling
device is a popular industrial technique that is used in the powder industry to generate
shear forces on a large scale. Interestingly, even though mechanical milling has been
known to generate shear forces, it had only been applied to produce disordered graphit ic
sheets so as to increase the intercalation capacities for applications in lithium ion batteries
rather than to produce graphene [38][39][40]. The graphitic material samples thus
analyzed even after milling graphite for over 10 hours in a planetary mill [40] or 60 hours
in a mortar grinder [41] still showed highly crystalline graphitic nanosheet material. A
49
selective size reduction in the graphitic flakes to a thickness in the order of 10 nm rather
than delamination to graphene has also been reported under mechanical milling relying
on shear force [42]. These studies show that large scale shear forces on their own are not
very effective in overcoming the graphitic interactions since remnants of graphit ic
carbon and disordered carbon are still present hence alternative complementary methods
have to be explored.
Adaptation of mechanical milling to exfoliate graphite and yield graphene through
addition of chemical assistants such as solvents [44], surfactants [45], inorganic salts
[46][47], dry ice [48], gases [49], polymers [50] [43] or a combination of one or more
has been explored. In all these cases, where chemical assistants have been added to the
mechanical milling process to aid graphite exfoliation, via a mechano-chemical process,
exfoliation has been attributed to molecular adsorption of the molecules from the
chemical assistants to the surface of graphene which is able to compensate the huge
attractive interactions in graphite. However, even though mechanical milling shows
a huge potential in the large-scale production of graphene as outlined in the examples
above, the main challenge that still remains is the continued use of prolonged and high
energy mechanical milling techniques which can be detrimental to the quality of
graphene. Therefore, it is imperative to identify new chemical assistants that can
efficiently interrupt and weaken the graphitic interactions and consequently lower
the energy needed to achieve exfoliations.
50
4-2 Aims and Objectives
In this chapter a mechano-chemical process employing both the use of mechanical forces
(grinding) and chemical assistant (acceptor) to achieve solid phase graphite exfoliation is
discussed. The solid phase exfoliation approach using chemical assistants can, not only
be used for the efficient exfoliation of graphite into graphene with high yield but,
simultaneously be used to easily introduce desired functionalities through surface
modification of graphene, and therefore efficiently improve the dispersion ability
following end applications where this is needed [43]. The rationale is that in order to
exfoliate graphite, the graphitic interactions have to be overcome. Mechanical means
such as shear forces have proven to be insufficient on their own to completely overcome
these interactions when used in large scale. Furthermore, even though graphene has been
produced when chemical assistants are used in the mechanical exfoliation, high energy
and continuous mechanical milling conditions have always been used, which is
detrimental to the quality of graphene. Therefore, there is still the need to identify
chemical assistants that can further efficiently weaken these graphitic interactions and
lower the energy and time needed to exfoliate graphite by mechanical means. Herein, the
use of electron deficient acceptors (A1 and A2) to disrupt and weaken the graphitic
interactions through donor-acceptor interactions is explored. The shear forces were
generated via 2 minutes of manual grinding and served a dual purpose in not only
activating the donor (graphene)-acceptor interactions via a mechano-chemical process but
also delaminating the weakened graphene layers following donor-acceptor interactions
from graphite.
51
4-3 Experimental procedure
Briefly, in a typical experiment, 20 mg of graphite powder was ground for 2 minutes with
acceptor A1 or A2 (50 mg) on a glass slide. The ground sample was then directly placed
on an XRD or SEM sample holder using conducting tape and analyzed without further
treatments. Identical samples, without acceptors, were also prepared as controls. In the
case of co-grinding with silica, 50 mg of graphite was ground with 75 mg of A1 in a 15
mL cylindrical glass vial. A similar protocol was followed whereas an additional 50 mg
of silica was added to the acceptor-graphite mixture prior to grinding.
4-4 Results and Discussion
4-4-1 Mechano-chemical Solid Exfoliation of Graphite using Donor-acceptor
Interaction
In order to trigger and maximize donor-acceptor interactions so as to interrupt the
graphitic interactions, a mechano-chemical approach was used whereby the graphite
was ground with an acceptor. Figure 4-1 shows the glass vials containing the various
materials before and after grinding. When graphite is ground without the presence of
acceptor there appears to be no visible changes to the material (Figure 4-1a). However,
immediately after co-grinding graphite with the acceptors to form a solid mixture, an
unusual phenomenon was observed whereby the walls of the vial became smeared with a
fine dark material (Figure 4-1b and c). Since the glass vial is made of silica, the glass
surface is therefore embedded with ethereal linkages (Si-O-Si) making it negative ly
charged and thus it is likely that the ground graphite containing an electron deficient
acceptor (Figure 3-1), due to the presence of strong electron withdrawing nitrogen and
oxygen groups that make the aromatic core positively charged, is electrostatica l ly
52
adsorbed on the surface of the glass resulting into dark smears. In the case of graphite
only, the electron cloud on the graphene surface makes it negatively charged and
therefore the graphite repels the negatively charged silica surface of glass resulting into
no dark smears on the vial wall.
Figure 4-1 Images of a) graphite, b) graphite and A1 and c) graphite and A2 before and
after grinding [134].
53
In order to investigate what processes drives the phenomenon observed in Figure 4-1b
and c, acceptors were ground alone, and the same phenomenon was observed, namely the
powder stuck to the glass vial walls (Figure 4-2 a). This is also as a result of the electron
deficient acceptor molecules being electrostatically attracted and adsorbed to the
negatively charged silica surface of glass. To further investigate the plausibility of this
assumption, control experiments were carried out whereby a few milligrams of
amorphous silica and acceptor (A1) were ground together (Figure 4-2 b). In this case the
amount of acceptor smeared on the wall of the glass vial was significantly reduced. This
indicates that there is preferential attachment of the acceptor molecules to the negative ly
charged surface of silica which contains both the ethereal linkage (SiO2) and silanol
(SiOH) groups, from adsorbed water, resulting into less smears observed even for the
graphite-acceptor mixture on the glass vial surface. (Figure 4-2 c). The higher surface
area of the amorphous silica compared to the glass surface is also thought to play a key
role in the preferential attachment of the acceptor to the silica rather than glass. A number
of studies on the adsorption of aromatic molecules bearing side chains containing for
example oxygen, nitrogen or hydrogen, on silica have also been carried out [137]. Zhao
et al [138]showed that benzyl alcohol, benzaldehyde, benzoic acid, anisole and toluene
can be adsorbed onto the silica surface. In their work they attributed the adsorption
mechanism to a combination of not only electrostatic attractive force of silica surface and
the aromatic system but also hydrogen bonding. Adsorption isotherm analysis indicated
that in the case of hydrogen bonding, the bonds are mainly between (i) the π-electron of
a benzene ring and the hydrogen atom of the hydroxyl group of the silica or, (ii) the
oxygen atoms of aromatics and the hydrogen atoms of the silanol groups or, (iii) the
hydrogen atom attached to the oxygen atom of aromatics and the oxygen atoms of the
54
silanol groups. Another study showed that acetophenones can also be adsorbed onto the
surface of silica [138]. In this case hydrogen bonding between silanol group and carbonyl
groups was also shown to be responsible for adsorption. From the chemical structure of
the acceptor (Figure 3-1), the adsorption mechanism of the acceptor should therefore not
only be through electrostatic attractions between the electron deficient aromatic core and
the electron rich surface of glass but also through hydrogen bonding between the silanol
group of silica and carbonyl groups of the acceptor. Finally, this provides a strong
indication that mechanical grinding does induce adsorption of the acceptor on the surface
of graphite, since some of the ground graphite material coats the glass vial walls (Figure
4-1 b and c). Physical changes have also been observed in other mechano-chemica l
processes as a result of a solid-solid reaction [139][140]. For instance, when N,N,N,N'-
tetraisopropyloxamide and p-cresol was ground in an agate mortar and pestle, a 1:2
complex was formed with the mixture turning from solid to liquid on grinding. In the
work presented here the change in graphite consistency from solid to dark smears (Figure
4-1 b and c) is thus a clear indication that donor-acceptor interaction of graphite with
acceptor occurs in solid phase.
55
Figure 4-2 Effect of silica addition to the co-grinding of graphite and Acceptor 1[134].
4-4-2 Morphology of Solid State Exfoliated Graphite
In order to investigate what morphological changes are occurring during the grinding
process, the ground materials were imaged using SEM. The SEM analysis of the starting
material graphite (Figure 4-3 a) shows agglomerates with lateral dimensions in the range
of 200 to 800 m. The book like structure shown in the inset clearly evidences the
presence of large (> 200 m) unexfoliated graphite as thick (> 2 m) compact bundles of
few layers graphene. In contrast, the materials recovered from the walls of the glass vial
following co-grinding with an acceptor show significantly smaller sheets, typically less
than 100 m (Figure 4-3b and c). One notable feature of the composites is the very smooth
and clean surfaces, with a higher proportion of larger size and regularly shaped particles.
Liu et al [141] also reported a significant decrease in graphite sheet sizes after ball milling
of graphite with ammonia borane for 4 hours, attributed to mechano-chemical cracking
of large grain size of pristine graphite particles into homogenous small gran size of
graphene sheets (0.4–1 m).
56
In the materials reported here, the sheets sizes following solid exfoliation with an acceptor
are much larger as a consequence of the substantial reduction in grinding time and
intensity: 2 minutes manual grinding compared to 4 hours of high energy ball milling
used. Furthermore, the sheet reduction is also indicative of successful molecular
adsorption of the acceptor on the surface of graphite which in turn effectively weakens
interactions between graphite layers, which further facilitates exfoliation of graphene
sheets from graphite particles. Finally, no observable structural defects such as holes were
found indicating that no major structural changes occurred during the solid phase grinding
process with an acceptor.
57
Figure 4-3 SEM images of (a) graphite with the inset image showing the laminar structure
of the graphite, (b) graphite ground with A1, and (c) graphite ground with A2 [134].
58
4-4-3 Crystallinity of solid state exfoliated graphite
The crystalline nature of the materials was also investigated (Figure 4-4) so as to
determine the extent of exfoliation. The XRD patterns were recorded for the ground
graphite (diffractogram a) and the composites resulting from manual co-grinding of
graphite with acceptors A1 (diffractogram b) and A2 (diffractogram c). The samples
containing the acceptors were recovered directly from the walls of the glass vial and
analysed without further treatments (i.e., no interaction with solvent). The two intense
and sharp diffraction peaks observed in the diffractogram of the untreated graphite at
~26.4 are the typical (0 0 2) reflections of three-dimensional graphite and are indicat ive
of a highly ordered material. The interplanar d-spacing can be determined using Bragg’s
Law (equation 4-1):
2dsin = n (4-1)
here d is the distance between two individual graphene sheets, the wavelength of CuK
radiation and n is an integer representing the order of the hkl reflecting plane (nth
harmonic) used for the calculation. The d002 calculated for the untreated graphite used in
this work is 0.349 nm.
59
Figure 4-4 XRD diffraction patterns of ground materials, (a) graphite (d002-0.351 nm)
(b) graphite ground with A1 (d002-0.346 nm) and (c) graphite ground with A2 (d002-
0.347 nm). Inset is a zoomed region of the low intensity region [134].
The inset of Figure 4-4 shows in greater details the significant difference in the intensit ies
observed for the (0 0 2) reflection in all ground materials. This phenomenon is explained
by the loss of long-range order following the application of shear forces during manual
grinding by hand. In raw graphite, the three-dimensional crystalline structure is
determined by the stacking of the graphene sheets, and the results presented here suggest
that the weak van der Waal’s interactions can be partially overcome by friction forces
generated during mild grinding, most probably complemented with localized thermal
energy. It is important to note that in the starting graphite no shift is observed in the
position of the (0 0 2) diffraction peak, implying that there has been no variation in the d-
spacing. Furthermore, the lack of asymmetry in the (0 0 2) peak is a strong indication of
the absence of turbostratic disorder in the final material, i.e., there is no evidence of faults
60
in the stacking of the graphene sheets. The results are in good agreement with previous
studies that showed that mechanical milling of graphitic material samples still showed
highly crystalline graphitic nanosheet materials even after prolonged milling of graphite
for over 10 hours in a planetary mill [40] or 60 hours in a mortar grinder [41].
Alternatively, as evidenced by diffractograms (b) and (c) in Figure 4-4, after the co-
grinding of graphite with the acceptors, a rather unusual feature appears in the XRD
patterns, namely a non-negligible shift of the (0 0 2) peak to higher 2θ which corresponds
to a decrease in d-spacing. Although the results may first be counterintuitive, it should be
noted that the X-ray data actually show residual materials that have not been completely
exfoliated, as they still exhibit a three-dimensional crystalline structure. Compared to the
original untreated graphite, the main diffraction peak of the graphite treated with
acceptors exhibits a decrease from about 180,000 cps to 3500 cps with A2 and 2,000 cps
with A1. This significant decrease in intensity is generally attributed to the exfoliation of
graphite into few layered graphene, [66][67] and is evidence of enhanced solid phase
exfoliation in the presence of acceptors. The occurrence of the slightly contracted
interlayer spacing is a clear indication of interactions between the acceptors and the
graphite sheets, and it can be postulated that A1/A2 will first adsorb on the surface
through - interactions. While this favors exfoliation due to a decrease in the attraction
forces between subsequent layers within the graphite, there may exist regions where the
adsorbed acceptor molecules would interact with other graphite layers to form sandwich
structures exhibiting smaller d-spacing.
While interlayer contractions are very common in lamellar materials such as anionic [142]
and cationic [143] clays, it is not anticipated or sought after in work involving production
61
of graphene where the main objective is facilitation of delamination, i.e., induce an
infinite expansion leading to complete separation of the sheets. The occurrence of this
contraction of the interlamellar distance, often referred to as a grafting process, is driven
by ionic interactions between the charged layers and the ions present in the interlayer
domain [144]. These so-called “guest-host” interactions have been extensively exploited
to generate novel hybrid materials for various applications, like adsorption of pollutants
[145] and drug delivery [146], and are often activated by mild thermal treatments. With
the materials involved here, given the mild grinding, it is very unlikely that mechanica l
or thermal effects would be the driving forces behind the shrinkage of the interlamellar
distance.
62
4-5 Conclusion
Using a mechano-chemical process of grinding graphite with acceptors A1/A2, the
graphitic interactions were disrupted allowing efficient solid exfoliation within a
short period of time: 2 minutes of manual grinding by hand compared to prolonged
mechanical milling processes outlined in literature. As a result, few layered graphene was
produced as evidenced by SEM and XRD analysis. Minimization of mechanica l
fragmentation effects has also been achieved as evidenced by the large graphitic sheets
observed in SEM even after grinding. Solvent free solid exfoliation of graphite to few
layered graphene also eliminates the high cost of solvents and will therefore allow easy
adaptability to current end product manufacturing equipment. However, the key challenge
is on how to increase the yield of pristine graphene and hence complementary techniques
such as LPE have to be explored following mechanical milling. Separation of
unexfoliated materials is also difficult and separation techniques such as centrifuga tion
are thus essential hence there is still need to enhance the solid phase graphite exfolia t ion
efficiency.
63
Chapter 5 Liquid phase exfoliation
The majority of this chapter has been published:
D. H. Gharib, S. Gietman, F. Malherbe and S. E. Moulton, High yield, solid exfoliation
and liquid dispersion of graphite driven by a donor-acceptor interaction, Carbon, 123,
695-707, 2.
The PhD candidate performed all of the experiments and through the assistance of the
other article authors analyzed the results and wrote the manuscript.
64
5-1 Intoduction
The top down approach of graphene production from low cost and readily available
graphite has been identified as a promising route to produce large quantities of defect free
graphene [147]. However, this approach has a significant disadvantage: the strong
interlayer interactions between the graphene sheets in graphite that limits its
exfoliation and subsequent dispersion in a wide range of solvents. Direct liquid phase
exfoliation (LPE) of graphite in a well-chosen organic solvent, by exploiting ultrasounds
to produce graphene [56], is a known technique that could be used for applications such
as conducting inks and electronics [148]. In this method, exfoliation has been attributed
to strong interactions between the solvent molecules and the graphitic basal planes, which
in turn results into subsequent dispersion [149].
However, one significant limitation of LPE is that only solvents with surface tensions of
about 30–40 mJ/m2, such as NMP (40 mJ/m2, [56] ) and DMF (37.1 mJ/m2, [150] ) can
be used. In addition, even in these ideal solvents, the yield is very low, typically around
0.01 mg/mL [56], and, for enhanced yield, longer sonication times are required, in the
order of 460 hours for high boiling point solvents [62] and 48 hours for low boiling point
ones [63].
A significant disadvantage of prolonged sonication is that it leads to destruction of the
graphene sheets, and, from an energy consumption perspective, is impractical and
expensive for a large-scale industrial process. Moreover, extending LPE of graphene to
lower boiling point, non-polar solvents such as chloroform would also be advantageous
since it can be very difficult to completely remove high boiling point solvents especially
when processing graphene dispersions into films or composites [149].
65
Lastly, since most of the promising graphite processing solvents are toxic and petroleum
based, there is a need to identify non-toxic, renewable and environmental friendly
alternatives.
5-2 Aim and Objectives
In this study, non-covalent, donor-acceptor interactions are used to interrupt graphitic
interactions, and continue the exfoliation and dispersion of graphite to graphene in
organic solvents. As a result of the use of electron acceptors in weakening the
interactions, the sonic energy needed to exfoliate and disperse graphene in organic
solvents should reduce resulting into a low energy exfoliation and dispersion process.
Shorter and lower energy exfoliation and dispersion processes should translate into large
graphene sheets with high lateral dimensions which result into materials with good
electrical properties. Furthermore, since the acceptor is non-polar, the acceptor-
functionalized graphene, should easily be dispersible in organic solvents and thus result
into high yield graphene dispersions even in traditionally poor solvents such as
chloroform [62]. Finally, dihydrolevoglucosenone(DHLG) [138] is explored as an
alternative non-toxic and environmental friendly solvent for graphene processing, with
potential to replace the currently used toxic and non-renewable petroleum based solvents
such as NMP and DMF.
66
5-3 Experimental Procedure
Briefly, in the first step, 50 mg of graphite powder was ground for approximately 2
minutes with A1 or A2 (10-100 mg) in a cylindrical glass vial followed by the addition
of 10 mL of solvent. Identical suspensions in solvent only, without acceptors, were
prepared as controls. All suspensions were subjected to low energy bath sonication
(Unisonics, Australia, 50 Hz) for 30 minutes at ambient temperature, followed by
centrifugation (Eba 20, Hettich, Zentrifugen) at 3000 rpm for 30 minutes to sediment
unexfoliated materials, and the supernatant was carefully removed by decantation of the
top half using a pippete to give stock graphene suspensions. In order to characterize the
quality and properties of the graphene dispersions, thick conductive films (0.5 m
thickness, Figure 3-7) were prepared for some techniques: Raman, SEM, XRD, XPS and
electrical conductivity measurements. For the film preparation, 6 mL of the graphene
dispersions were filtered onto a porous alumina membrane 20 nm (0.02 µm) pore size
using Buchner vacuum filtration and subsequently washed with solvent to remove any
traces of residual acceptor followed by drying overnight in an oven at 70°C in air.
5-4 Results and Discussion
5-4-1 Optimization of Solvents and Continued Liquid Phase Exfoliation
The poor exfoliation and dispersions of graphite in a wide range of organic solvents has
been attributed to strong interlayer interactions between the graphene sheets in
graphite. In chapter 4, a mechano-chemical process of grinding of acceptor with graphite
was found to be efficient in interruption of the interlayer interactions in graphite
leading to solid phase exfoliations. This was as a result electrostatic adsorption of the
67
electron deficient acceptor to the electron rich graphitic surface which induced donor-
acceptor interactions and interrupted the graphitic interactions. Furthermore, these
results strongly indicated that the electron acceptor is attached to the graphitic surface
during the mechano-chemical process resulting into acceptor functionalized
graphene/graphite. From the molecular structure of the acceptor (Figure 3-1), the
aromatic core is attached to a non-polar branched alkyl chain to specifically induce
acceptor solubility in organic solvents. Therefore, the acceptor functionalized graphene
should have increased dispersibility in previously elusive non polar solvents [62].
To gain further insight into the exfoliation process and the dispersion abilities of the
graphite-acceptor mixture in solution, four solvents were chosen namely DHLG, NMP,
DMF and chloroform and added to the ground graphite-acceptor materials (Figure 4-1 b
and c). These were chosen on the basis of their reported good (NMP and DMF [56]) and
poor (chloroform [63]) performances for exfoliation and dispersion. Furthermore, DHLG
was selected as a novel environmentally friendly solvent, being a bio-based compound
derived from cellulose, and is non-toxic [81]. Immediately after addition of a solvent to
the ground graphite-acceptor mixture, dark suspensions formed even prior to sonication
(Figure 5-1) indicating the presence of exfoliated graphite. In the absence of acceptors no
such dark dispersions were formed. These results provide further evidence that donor-
acceptor interactions between graphite and the acceptor occurred in the solid phase,
causing cleavage of the interlayer interactions, and hence the dispersion of exfoliated
graphite in solvent (prior to sonication). In addition, the increased dispersibility of the
ground graphite-acceptor material in the solvents is also attributed to the successful
functionalization of the non-polar acceptor on the graphitic surface via non-covalent
68
donor-acceptor interactions during the mechano-chemical process of grinding graphite
with an acceptor. Similarly Yan et al [151], showed that when graphite flakes were ball
milled for 8 hours with KOH using a planetary mill, the exfoliation of graphite layers
could be achieved by both shear forces as well as the functionalization of graphite layer
by formation of –OH groups during the solid-state mechanochemical reaction. The
resulting G-OH, showed strong hydrophilicity with good solubility in water due to
presence of OH groups. In a control experiment, when graphite was ball milled under the
same conditions, but without the incorporation of KOH, the resultant powder had poor
dispersibility in water suggesting that the solid-phase mechanochemical reaction is essential
for the formation of water-soluble G–OH.
When the ground mixtures of graphite-acceptor were subjected to mild bath sonication
for 30 minutes in the solvent, followed by centrifugation, it was possible to obtain very
stable dispersions (Figure 5-1). It was also observed that when graphite and acceptors
were not ground but only sonicated (low energy bath sonication), no dispersions formed
after addition of solvents. This provides further evidence that the grinding process, to
induce a mechano-chemical reaction between the acceptor and graphite, is critical to
create intimate contacts between the solid particles and facilitate interactions of the
acceptors, which in turn will lead to the exfoliation of graphite. Furthermore, this clearly
shows that donor-acceptor interactions between the graphite and acceptor are not
spontaneous. Rather, like in most cases of chemical reactions, an external energy
specifically mechanical, has to be supplied in order to trigger a reaction and bring the
reactants in close contact to each other.
69
Figure 5-1. Images of graphene dispersions exfoliation without acceptor (G) and with
acceptorA1 (G + A1) and A2 (G + A2) in NMP, chloroform (CHCl3), DMF and DHLG
before and after sonication and centrifuging [134].
70
The concentrations of the dispersed and exfoliated graphite before and after sonication in
the solvents were determined by UV-Vis spectroscopy (Figure 5-2). The analyzed
samples were, (i) the dark solution prior to sonication (Stage (c) in Figure 3-2, materials
and methods, chapter 3) and (ii) the supernatant collected after sonication and
centrifugation (Stage (d) in Figure 3-2, materials and methods, chapter 3). It has been
previously suggested that the most effective solvents for the exfoliation and dispersion of
graphite are those whose surface tension is close to 40 mJ/m2 equivalent to a Hildebrand
solubility parameter of 23 MPa1/2. Hernandez et al [64] showed that equally significant,
successful solvents also have Hansen solubility parameters of Dispersive (δD) ∼ 18
MPa1/2, Polar (δP) ∼ 9.3MPa1/2, and Hydrogen bonding (δH) ∼ 7.7 MPa1/2 with the
dispersibility smaller for solvents with Hansen parameters further from these values
(Figure 2-3).
In all solvents, there was a notable increase in the yield of graphene following the addition
of acceptors, which was further improved after mild bath sonication of only 30 minutes
(Figure 5-2). This significant increase in graphene concentration on addition of the
acceptor highlights that further exfoliation can be afforded through gentle bath sonication
which is unprecedented in the literature. It is attributed to the synergistic effects of
efficient exfoliation and dispersion formation as a result of the acceptors interaction with
graphite through donor-acceptor interactions. This emphasizes the crucial role played by
the solvent in liquid phase exfoliation, as documented previously [62][63][148], with high
oiling point solvents (non-volatile – NMP and DMF in Figure 5-2) typically being the
best candidates.
71
Figure 5-2 Concentrations (mg/mL) of graphene dispersions in DHLG, NMP, DMF and
chloroform, with and without acceptor, before and after sonication [134].
Table 5-1 Boiling point (BP), Solvent Hildebrand parameter (δT), dispersive Hansen
solubility parameter (δD), polar Hansen solubility parameter (δP), and hydrogen-bond ing
Hansen solubility parameter (δH) [64] for all solvents tested, DHLG, NMP, DMF and
chloroform.
Solvent BP (oC) T(MPa)1/2 D(MPa)1/2 P(MPa)1/2 HMPa)1/2
Graphene - - 18 9.3 7.7
DHLG 203 - 18.8 10.6 6.9
NMP 202 23 18 12.3 7.2
DMF 153 24.9 17.4 13.7 11.3
chloroform 61 18.9 17.8 3.1 5.7
72
The data in Figure 5-2 also shows the startling fact that the more volatile solvent
chloroform produced dispersion containing a high graphene concentration. The graphene
concentration when sonicated in chloroform (30 minutes low power followed by
centrifugation at 3000rpm) is 0.08 mg/mL for A1 and 0.1 mg/mL for A2. These values
are similar to that reported by the Coleman group (0.07 mg/mL after 5000rpm
centrifugation [63],however their samples were sonicated at lower energy for 48hrs (96
times longer than the samples reported here). Even their samples prepared at longer
sonication times (300hrs) in chloroform resulted in a concentration of approximately 0.4
mg/mL [62] which is 4 times higher than the values reported here, while their sonication
time is 600 times longer. It has been suggested that the dispersion quality is particula r ly
sensitive to the dispersive Hansen parameter, D; successful dispersions are only
achieved for solvents in the range of 15 MPa1/2 < D < 21 MPa1/2. Furthermore, reasonable
dispersions can be achieved for a much wider range of polar, P, and H-bonding, H,
Hansen parameters (from 2-3MPa1/2 to 17-18 MPa1/2 respectively). Effective solvents for
graphite exfoliation, therefore, have a non-zero polarity (δP) and hydrogen bonding (δH) values
despite the non-polar nature of graphene, with all three Hansen solubility parameters being
essential when describing the affinity between solvent and solute. Khan et al [62] suggested
that their low concentration in chloroform is predominantly due to chloroforms polar
Hansen parameter (δP -3.1) being at the very edge of the allowable range for graphene
dispersion (Table 5-1). During the LPE of graphite in solvent, the solvent –graphene
interaction also plays a key role in that this interaction is strong enough to balance the
interlayer graphitic interactions allowing exfoliation. The solvent–graphene
interactions are known to involve only weak van der Waals type bonding, specifica l ly
73
dispersive (D), dipolar (P), and hydrogen-bonding (B) interactions. Therefore, when
formulating solubility theories for graphene, these interactions are always treated
separately. When an acceptor is used, solvent-acceptor and acceptor-solvent interactions
cannot be ruled out and will possibly contribute to the overall interactions in addition to
those discussed above, meeting the allowable threshold for further graphite exfolia t ion
and dispersion. Additionally, chloroform is a non-polar solvent and the electron acceptor
is also non-polar (by virtue of the long branched non polar alkyl chains, Figure 3-1).
During the mechano-chemical process, the non-polar acceptor is adsorbed on the surface
of graphite/graphene leading to acceptor functionalized graphene. This highlights the
beneficial effect of the acceptor in exfoliating in the solid phase (Chapter 4) followed by
increased exfoliation and dispersion formation in the liquid phase. The use of an acceptor
dramatically decreases the amount of energy required to form dispersions at high
concentrations in the liquid phase making this process appealing from an industr ia l
perspective.
A number of studies on the use of non-polar organic molecules aimed at improving the
liquid phase exfoliation process have also been investigated. For instance, Ciesielski et
al [149] reported graphite exfoliation with addition of 1-phenyl octane and arachidic acid
in NMP. However, even after using bath sonication of 6 hours to increase the graphene
concentration, only a 50% increase in yield was observed i.e., 0.128 mg/mL and to 0.1
mg/mL for graphene exfoliated in the presence of arachidic acid and 1-phenyloctane,
respectively compared to samples prepared just in NMP (0.075 mg/mL) [149]. In the
work by Ciesielski et al [149] they attributed the dispersion/stabilizing effects of the
aliphatic organic molecules to their higher calculated adsorption energy on graphene
(19.1 and 28.2 kcal/ mol, 1-phenyloctane and arachidic acid respectively) being higher
74
than the adsorption of solvent molecules, NMP (8.5 kcal/ mol). In other studies, Xu et al
[66] showed that the addition of naphthalene also resulted in a significant increase in the
graphene concentration. For the solvent NMP, the graphene concentration in the absence
of naphthalene was 0.08 mg/mL after bath sonication of 90 minutes and increased to 0.15
mg/mL in the presence of naphthalene [66]. As shown in the work presented in this
chapter, when an acceptor is used a 1250 % graphene concentration increase is observed
in NMP after mild bath sonication of 30 minutes showing its effectiveness to exfoliate
graphene via a donor-acceptor interaction.
Whilst high graphene concentrations were achievable in NMP, DMF and chloroform,
these solvents are known to be toxic and pose significant risks to public health and the
environment [78][79]. Using this acceptor exfoliation approach, a new environmenta l ly
safe “green” solvent derived from cellulose, dihydrolevoglucosenone (DHLG) [81] was
investigated to determine its effectiveness in forming highly concentrated dispersions of
graphene. From Figure 5-2 it is clear that this solvent is extremely efficient at aiding in
exfoliation and dispersion formation resulting in the most concentrated dispersion out of
all of the solvents. From Table 5-1, DHLG shows similar physical properties to NMP and
DMF which explains its efficiency in exfoliation of graphite but not on its high peformance.
These results are in agreement with recent studies by Salavagione et al [80], who showed
that DHLG was also much more efficient in graphite exfoliation than NMP. However, the vast
difference in its performance, compared to NMP was attributed to DHLG higher viscocity, 14
cP, being more viscous than NMP, 2 cP. Furthermore, DHLG also presents
similar δH compatibility and the closest δP match to graphene compared to NMP and DMF
(Table 5-1).
75
5-4-2 Crystallinity of liquid phase exfoliated and dispersed graphite
In order to investigate the crystallinity and extent of exfoliation of the dispersed materials,
XRD was used. Films prepared from the supernatant of the centrifuged dispersions, and
analysed by XRD showed a dramatically diminished XRD (0 0 2) diffraction peak at
26.1° compared to the starting raw material graphite (Figure 5-3), which is indicative of
complete exfoliation of graphite into graphene. In graphite exfoliated in DHLG for
instance the intensity of this peak (I26.1) decreased to 0.6 % and 0.4 % of its initial value
in A1 and A2 respectively, which is a clear evidence of exfoliation and dispersion
resulting in the loss of a three-dimensional structure in graphite. Based on the relative
intensities, the general observation comparing the various systems is that A2-treated
graphite tends to yield lower quantities of unexfoliated materials. Analysing the
diffractograms in Figure 5-3, it is noteworthy that the intensities of the A2-treated
materials in Figure 5-3(c) are much lower, indicating higher degrees of exfoliation, when
compared to the materials obtained with A1 (Fig 5-3 (b)).
Similarly, it is observed that DHLG leaves a higher proportion of unexfoliated graphite
with both acceptors, while DMF seems to give more unexfoliated materials with A1, and
chloroform has similar effects on both A1 and A2. Given the multiple parameters
involved in the systems under investigations and the diverse nature of the solvents, the
possible reasons might include: solubility of acceptors, affinity of the acceptors to
graphite and the solvent, affinity of graphite to the solvent, dispersibility of graphene in
the solvent, properties of the acceptors, molecular structures and possible steric
considerations.
76
As detailed in the 3D-optimised structures of Figure 3-1, Chapter 3, although they bear
identical functional groups, both are diimides, A1 and A2 have significantly different
optimal structures, with A1 adopting a trans conformation, while A2 most stable structure
is the cis. However, in the course of molecular interactions, these conformations may
undergo slight variations that can also be influenced by the nature and properties of the
solvent in use, such as polar, non-polar, protic and aprotic.
77
Figure 5-3 XRD diffraction patterns of supernatant of the exfoliated and dispersed
graphite. (a) Precursor graphite (shown for comparison), (b) graphite exfoliated with
acceptor A1, and (c) graphite exfoliated with acceptor A2. Samples were dispersed
sonicated for 30 minutes in DHLG, NMP, DMF and chloroform [134].
78
5-4-3 Effect of acceptor concentration on liquid phase exfoliation and dispersion
The effect of the concentration of the acceptor used in the solid phase exfoliation was
investigated to determine the optimal graphite to acceptor ratio for efficient exfolia t ion
and formation of high concentration dispersions (Figure 5-4). In all samples, the yield in
graphene also increased with increasing acceptor concentrations, up to a critical level.
However, there was a significant difference in the critical concentration of the acceptors,
depending on the solvent used: overall, the upper limit was 0.02 M for A1 and 0.01 M for
A2. Not all acceptors resulted in dispersion formation in certain solvents as indicated by
the absence of data for the acceptor, concentration and solvent (i.e., A1 at 0.034 M in
DMF).
79
Figure 5-4 Effect of concentration of acceptor on the yield of exfoliated graphite in a)
A1, and b) A2. Where no results are presented for the different acceptors it indicates
that it was not possible to form stable dispersions at those acceptor concentrations [134].
One possible explanation for this difference in critical concentrations between A1 and A2
is related to the affinity between graphite, acceptors, solvent and particularly, the
association constant (Ka) between graphene and the acceptors. Previous work done by the
candidate reported [3] for pyrene groups in 1,2 dichloroethane, Ka to be 0.74 mol-1 for A1
and 4.23 mol-1 for A2, and concluded that a linearity of the Benesi-Hildebrand and Job’s
plots implied the formation of a donor-acceptor complex with 1:1 donor-acceptor ratio.
80
Therefore, the use of acceptor A1, with a lower Ka requires a higher concentration than
A2 to achieve the same degree of association.
Another important factor is the limit of solubility of the acceptors in some organic
solvents. For instance, graphite exfoliation using NMP indicates that acceptor solubility
may play a key role in facilitating the interaction between the acceptors and graphite.
While more rationally, high concentrations can also lead to self-aggregation of acceptor
molecules, mobilising non-negligible quantities, and leaving lesser amounts to interact
with graphite. Apart from solubility limitations at higher concentrations, during
exfoliation and subsequent dispersion formation, three types of interactions will influence
the process: (i) graphene-solvent, (ii) graphene-acceptor, and, (iii) solvent-acceptor. At
moderate concentrations graphene easily interacts with both acceptor and solvent
resulting in exfoliation and dispersion. At higher acceptor concentrations there are two
scenarios that may exist, (i) graphene-acceptor interaction increases resulting in
aggregation and (ii) solvent-acceptor interaction could be dominant to graphene-acceptor-
solvent interaction resulting in decreased exfoliation and dispersion of the graphene. Thus
it is important to ensure a shift in equilibrium to drive the graphene-acceptor interaction
at the expense of solvent-acceptor interaction through matching appropriate surface
energies.
81
5-4-4 Morphology and Quality of liquid phase exfoliation and dispersion of
graphite
Transmission electron microscopy (TEM) was used to determine the morphology, quality
and degree of exfoliation of graphite. TEM of representative acceptor-exfoliated graphite
dispersed in solvent (NMP) is shown in Figure 5-5.
Figure 5-5 TEM images of graphene following exfoliation of graphite with a) A1 and
b) A2 in NMP [134].
Both samples showed exfoliated graphene sheets with long edges with evidence of limited
defaults: the edges are perfectly straight with no curved or jagged structures, and with a
notable folding of a large sheet in Figure 5-5 (a). The results are in agreement with
literature reports [66][63], the darker contrast represents the part where the sheet folds on
itself and becomes more opaque, most likely a result of sample preparation process as the
graphene dispersion is casted and dried on a TEM grid. Overall, the flakes are relative ly
large with little evidence of defects, holes or other damage indicating high quality of the
dispersion formation process.
82
5-4-5 Preparation of conducting films of liquid phase acceptor exfoliated graphite
and subsequent acceptor removal
The acceptor exfoliated graphite dispersions were used to prepare films by vacuum
filtration onto a porous alumina membrane. Whilst the use of an acceptor in both solid
and solution phase exfoliation results in high concentration dispersions of graphene, it is
important to ensure complete elimination of the acceptor in the final products (e.g., film,
fibre or membrane) prepared from these dispersions. It is noted that the inclusion of an
electron acceptor may result in decreased performance of an electronic device that relies
upon electron transfer, therefore following film preparation by filtration, residual acceptor
molecules were removed by repeatedly washing with the same solvent used for dispersion
until no acceptor was detected in the UV-Vis analysis of the filtrate. The film was then
dried overnight in a vacuum oven at 70 oC (Figure 3-7).
XPS analysis was carried out on the washed films to ascertain whether any acceptor was
still present in the graphene film. From the molecular structure of acceptor (Figure 3-1,
chapter 3), the nitrogen peak of acceptor (N 1s) should be clearly evident in the XPS
survey spectra before washing and completely disappear after washing if acceptor is
completely eliminated. However, XPS analysis of previous studies of liquid phase
exfoliated graphite in NMP have still shown traces of nitrogen from NMP even after
annealing of films at 1000 oC [56][149]. This is because it is very difficult to completely
remove high boiling point solvents such as NMP. For this reason, it was sought to
eliminate any contribution of nitrogen traces from residual solvent by using films
prepared from graphite exfoliated in chloroform and subsequently washed with
chloroform. Additionally, the use of a low boiling point solvent such as chloroform
guarantees elimination of residual solvent in the end product, as well as it can be easily
83
separated from the acceptor in the wash filtrate enabling both acceptor and solvent to be
recycled. Figure 5-6 shows the XPS survey spectra of graphene films prepared from
exfoliated graphite dispersions in A1 and A2 prior to washing (Figure 5-6 a and b
respectively) and after repeated washing with chloroform (Figure 5-6 b and d
respectively).
From the spectra it is clear that the N 1s at 400 eV, attributed to acceptor, is present before
washing (Figure 5-6 a and c) and completely disappears after washing (Figure 5-6 b and
d). These results clearly indicate that due to the non-covalent nature of interaction of
graphite with acceptor, it is possible to remove adsorbed acceptor from the end graphene
product. Furthermore, only trace amounts of oxygen (i.e., O 1s at 527.9 eV) are present
in both samples after washing with the main peak being carbon (>96 %) due to minor
graphitic oxygenation during exfoliation which is common in most graphite exfolia t ion
methods and has been attributed to physically adsorbed water or oxygen [141] again
indicating the exfoliation method produces minimal defects.
84
Figure 5-6 XPS survey spectra of films of exfoliated graphite in a) A1 before washing b)
after washing with chloroform c) A2 before washing and d) after washing with
chloroform [134].
85
5-4-6 Morphology of liquid phase exfoliated and dispersed graphite
SEM analysis was used to determine the morphology and size of the particles forming the
graphene films formed through the filtration process. SEM showed large exfoliated
graphitic material when A1 and A2 were added (Figure 5-7 b and c) compared to without
acceptor (Figure 5-7). This increase in size may be due to the added acceptor interacting
with graphene laterally leading to weak points during the manual grinding process. Then
during bath sonication, the sonic energy may preferentially act on these lateral weak
points resulting into exfoliation and larger lateral dimensioned graphene sheets. However,
in the blank sample the sonication energy randomly cuts the graphene sheets both laterally
and vertically leading to much smaller sheets. It is well considered that large graphene
sheets are highly desirable in improving the electrical and mechanical properties of
graphene composites.
86
Figure 5-7 SEM images of surface of films of graphite exfoliated (a) without acceptor,
(b) with A1, and (c) with A2 [134].
87
5-4-7 Quality of liquid phase exfoliated and dispersed graphite
Raman spectroscopy was used to study and characterize the quality of exfoliated graphite
flakes. From the spectra summarized in Figure 5-8, three typical vibrational bands of
graphene were clearly observed. The D band at around 1350 cm-1, G band at 1600 cm-1
and 2D band at 2700 cm-1.
88
Figure 5-8 Raman spectra for various graphite a) exfoliated films in A1 and b) A2 in
the solvents NMP, DHLG, DMF, chloroform. The precursor graphite is included for
comparison [134].
Previous studies have shown that the D band is not observed in micro mechanica l
exfoliated graphene, however, it is always observed in liquid phase exfoliated graphene
and it is attributed to flake edges [56][152] which acts as defects in the Raman scattering
process [68]. The D band can also be indicative of basal plane degree of functionaliza t ion
89
[152]. In general, relatively low D-band intensities indicate that the graphene flakes
contain few defects, mainly located at the flake edges rather than the basal plane. Liu et
al [67] reported that since the Raman excitation beam has a spot size comparable to that
of the sizes of most graphene flakes, the beam can always “see” a large quantity of edges
at the same time.
Furthermore it has been reported that the D peak intensity increases with decreasing size
of the flakes [56]. As evidenced from SEM of the untreated precursor graphite (Figure 4-
3 a) large graphitic flakes of lateral dimensions in the range of 200 to 800 m exist. These
large flakes are greatly reduced after solid grinding and 30 minutes of sonication to the
range of 1.0 – 6.0 m for A1 and A2 exfoliated graphite films (Figure 5-7 b) and c). The
slight increase of the D bands in the Raman spectra of all acceptor exfoliated graphite
(Figure 5-8) compared to the precursor graphite agrees with the observations of the SEMs.
The shape, and position, of the 2D band provides information on the number of graphene
layers per flake [153]. It can be noticed that as the materials move from raw graphite to
few layers of graphene, the 2D band moves from a clearly asymmetric band, with a
maximum at ~2710 cm-1 for graphite, to around ~2690 cm-1 for the material exfoliated
with the acceptors in NMP. The slight red shift, i.e. toward lower energies/wavenumbers,
is directly proportional to the number of layers whereby with more layers higher energies
are needed for an in-plane vibration to occur, hence higher wavelengths with graphite
compare to the exfoliated materials. Using similar rationale, the skewed aspects of
unexfoliated graphite is a result of the multitude of possible vibrational energies in a
multilayer where they are of the same order of magnitude but with slight variations due
to the occurrence of multiple interactions or interferences.
90
The 2D bands of acceptor exfoliated graphite appear to be slightly angled suggesting
slight aggregation during film formation. However, compared to that of the precursor
graphite, they are more symmetrical further indicating high quality exfoliated graphite.
Due to the high concentrations of the dispersions obtained with the added acceptor films
of approximately 500nm thickness were obtained by filtration with the Raman analyses
conducted on these films, and not on sieved samples or single flakes. It has been reported
in other studies that for Raman spectra recorded for more than 5 layer graphene the 2D
band will exhibit features similar to that of graphite [153]. As seen in Figure 5-8, the
Raman spectra of materials exfoliated with acceptors bear very little resemblance to the
starting raw material. More importantly, it is clear that the Raman spectra show little or
no asymmetries in the 2D band indicating that the approach developed in this work yield
mostly few layers of graphene.
The ratio of the integral intensities of the Raman peaks D to G (ID/IG) is generally used
as a measure of the degree of structural defects in graphene [153]. The ID/IG ratio for
each sample is also reported in Figure 5-8 with a clear difference in the exfolia t ion
properties of A1 and A2 being observed. All A2 exfoliated graphite films have a higher
ID/IG ratio (Figure 5-8 b) compared to A1 exfoliated graphite films (Figure 5-8 a). This
results indicate that while A2 is efficient in formation of high concentration graphene
dispersions (Figure 5-8) it however results in more few layered graphene sheets compared
to A1 exfoliated graphene during film formation. However, for all acceptor exfoliated
graphite films the ID/IG is found to be << 1.0, which is much lower than that of
chemically or thermally reduced GO (1.2–1.5) further indicating high-quality graphene
[154][155] produced from this method.
9 1
5- 4- 8 El e ct ri c al pr o pe rti e s of li q ui d p h as e e xf oli at e d a n d dis pe rs e d e xf oli at e d
g r a p hit e
St u dies ha ve s h o w n t hat li q ui d p has e e xf oliati o n of gr a p hit e t o pr o d uc e gr a p he ne i n t he
a bs e nc e of o xi d ati o n, r et ai ns t he pristi ne gr a p hitic b as al pla ne. H e nc e, t he ele ctri c al
c o n d ucti vit y of li q ui d p has e e xf oliat e d gr a p hit e s h o ul d als o r e mai n u na ffe ct e d, wit h fil ms
f or me d fr o m s uc h dis p ersi o ns ha vi n g hi g her c o n d ucti vities t ha n fil ms c o m p os e d of
c he mic all y c o n vert e d gr a p he ne w her e t he ele ctr o nic pr o p erties ar e oft e n ne gati v e l y
a ffe ct e d b y s uc h d efe c ts [ 1].
I n or d er t o e val uat e t he ele ctric al pr o p erties of t he fil ms, t he s he et r esist a nc e ( Rs - / )
of all t he fil ms w er e me as ur e d usi n g t he f o ur- p oi nt c o n d ucti vit y pr o b e met h o d aft er
dr yi n g t he fil ms o ver ni g ht at 7 0 o C ( Fi g ur e 3- 7). T he D C c o n d ucti viti es s h o w n i n T a bl e
5 - 2 w er e c alc ulat e d usi n g e q uati o n 5- 1, w her e t he fil m t hic k ness (t) ( A 1: 5 0 0 n m a n d A 2:
6 0 0 n m) w as o bt ai ne d fr o m fil m cr oss s e cti o n S E M i ma ges.
C o n d ucti vit y ( S/ m) = ( 𝑅 𝑠 ) − 1
𝑡 𝑥 1 0 − 9 ( 5- 1)
T he d at a i n T a ble 5- 2 b el o w cle arl y s h o w t hat fil m c o n d ucti vit y is g o ver ne d b y t he s ol v e nt
a n d a c c e pt or us e d t o f or m t he dis p ersi o n f oll o wi n g s oli d p has e e xf oli ati o n wit h t he fil ms
pr e p ar e d fr o m A 2 i n c hl or of or m pr o d uci n g t he hi g hest c o n d ucti vit y of 4 4. 4 x 1 0 3 S/ m, a
val ue t hat is o nl y a p pr o xi mat el y 3 ti mes l o w er t ha n t he c o n d ucti vit y of t he st arti n g
gr a p hit e mat erial ( ~ 1. 5 × 1 0 5 S/ m). T his r es ult is si g nifi c a nt gi ve n t his fil m w as pr o d uc e d
fr o m a s ol ve nt t hat is n or mall y c o nsi d er e d a p o or s ol ve nt f or t he e xf oliati o n of gr a p hit e
a n d a gai n hi g hli g hts t he e xc e pti o nal a bilit y of t he pr o c essi n g a p pr o a c h t o pr o d uc e l o w
d efe ct lar ge s he et gr a p he ne ( Fi g ur e 5- 7 a n d 5- 8 r es p e cti vel y). T his c o n d ucti vit y als o
92
compares well with free-standing films of reduced graphene oxide reported which
displayed conductivities of up to 3.5×104 S/m (after annealing at 350 oC) [33][156].
The films formed from chloroform (boiling point, 61 oC) exhibited larger conductivit ies
compared to the other solvent acceptor systems which is attributed to the difficulty in
removing solvent from the films. Studies have shown that it is difficult to completely
remove high boiling point solvents (boiling points, DHLG: 203 oC, NMP: 202 oC and
DMF: 153 oC) from graphene films even after annealing at 1000 oC leading to lower
achievable conductivities [56].
The higher conductivity of films formed from Chloroform/NMP in A2 compared to
Chloroform/NMP in A1 is also in good agreement with the XRD diffractogram data in
Figure 5-3, which showed that the d002 peak intensities of the A2-treated materials are
much lower (Figure 5-3 c), indicating higher degrees of exfoliation, when compared to
the materials obtained with A1 (Fig 5-3 b). In the case of films formed in DMF A2, even
though A2 showed more exfoliated flakes than A1, A2 films had lower conductivit ies
compared to A1 which is attributed to the lower solubility of A2 in DMF compared to A1
(Figure 5-4), which even though results into more exfoliated materials, the yield of
monolayered graphene might be much less compared to A2 in chloroform/NMP.
Interestingly, even though DHLG produces the highest dispersion of all solvents it also
produces the lowest film conductivity. From Table 5-1, DHLG has the highest boiling
point of all solvents, 203 oC, almost similar to NMP's 202 oC. Therefore, like for NMP
films , the high boiling point of DHLG means that it is very difficult to completely remove
the solvent hence, even after drying the films overnight at 70 oC , there can still be traces
of residual solvents within the film that further lowers the electrical properties of the films
93
formed from such dispersions. However, the high boiling point of DHLG alone does not
explain the drastic differences with films formed from NMP which exhibited much higher
conductivity. A complementary explanation is that DHLG's much higher viscosity of 14 cP
compared to 2 cP of NMP means that there might be less efficient flake size separation during
centrifugation resulting into less sedimentation of aggregated or multilayer flakes compared
to monolayer graphene. Indeed this is true, as evidenced by the relative intensities of the d002
diffraction peak in XRD spectral analysis (Figure 5-3) which showed that DHLG leaves a
higher proportion of unexfoliated graphite with both acceptors compared to NMP, DMF
or even Chloroform. TEM studies of DHLG and comparison to NMP exfoliated and
centrifuged graphene dispersions, including statistical analysis of flake dimensions and flake
thickness will need to be done to evaluate the number of stacked monolayer per flakes using
techniques such as the edge counting method [62]. Finally, the high conductivity obtained
from these graphene films suggest that the graphene dispersions may be suitable for
application in transparent electrodes and other electronic and electrical devices.
Table 5-2 Electrical conductivity (x103 S/m) of films formed from graphite exfoliated
with acceptor A1 and A2 in solvents DHLG, DMF, NMP and CHCl3 [134].
Film Electrical conductivity (x kS/m)
DHLG NMP DMF CHCl3
A1 1.0±0.04 5.8±0.30 1.8±0.05 5.09±0.33
A2 0.8±0.02 12.5±0.28 0.85±0.01 44.41±0.65
94
5-5 Conclusion
As a result of cleavage of graphitic interactions via a donor-acceptor interaction
mechanism, graphite exfoliation was further enhanced after mild bath sonication (30
minutes) of the acceptor-graphite in solvent with 13 fold increment in yield of graphene
dispersed in N-methylpyrrollidone (NMP) and A1 for instance compared to that without
the acceptor. The use of electron acceptors therefore, allowed high dispersions, not only
in high boiling point solvents whose surface energy matches that of graphene e.g. NMP
and dimethylformamide (DMF), but also in low boiling point solvent with mediocre
properties, for example, chloroform. Moreover, the use of novel dihydrolevoglucosenone
(DHLG) in the liquid phase processing step as a high performance green solvent
alternative to toxic NMP and DMF was also reported. The use of electron acceptors in
weakening the interactions also meant that the sonic energy needed to exfoliate and
disperse graphene in organic solvents was reduced resulting into a low energy exfolia t ion
and dispersion process. Shorter and lower energy exfoliation and dispersion processes
also translated into large graphene sheets/higher aspect ratios of which resulted into
materials with good electrical properties.
95
Chapter 6 Processing of Carbon Nanotubes
6-1 Introduction
Since their discovery [82], carbon nanotubes (CNTs), which are essentially graphene
sheets, rolled up to form a cylinder [83], still attract much attention due to their
outstanding physical, chemical, mechanical and electronic properties [84][85][86][87].
These unique properties have allowed their use in numerous high end applications such
as conductive and high strength nanocomposites, semiconductor devices and in energy
conversion/storage devices [91][92]. CNTs come in two most common types: single-
walled (SW) and multi-walled (MW), and, though appearing to be structurally similar,
MWCNTs are essentially an array of SWCNTs within each other, reminiscent of a
telescopic structure [94](Figure 2-5,chapter 2).
However, CNTs are produced as a solid black powder, which prior to being used in most
applications, must be exfoliated and dispersed in liquid media [98]. This is because CNTs
have a high tendency to agglomerate and form bundles, ropes, or aggregates. The resultant
CNT bundles can therefore have very complex morphologies varying from tens of
nanometers in diameter and many micrometers long. In fact, studies have shown that
individual CNTs can, not only be held within a CNT bundle, but can also be entwined,
interwoven, bent, entangled or form loops around not only other CNT bundles but also
within an existing bundle [99]. The main properties of CNT bundles are inferior to those
of isolated CNTs, and the fact that it is extremely difficult to separate CNTs from bundles
represents a serious hurdle in the way of potential applications. CNTs aggregation and
bundling are mainly governed by two things: nanotube morphology, a consequence of
their high molecular weight and aspect ratios, as well as attractive forces between the
96
CNTs due to their high surface energy and interactions derived from their extended
π-electron network [100][101][102][103][96]. All these factors make CNTs have very
poor dispersiblity not only in water but also organic solvents limiting their practical
applications.
The liquid phase debundling and dispersion of CNTs in a carefully selected solvent using
sonic energy is a simple and popular technique used to disaggregate, debundle and
disperse the CNT bundles into individual CNTs or reduced CNT bundle sizes
[109][110][111][112][113][114][115][116][117][118]. In this approach, only solvents
with a matched surface energy to the CNT, generally high boiling point amides, are
employed [157][98]. These solvents are chosen due to their ability to compensate for the
high CNT surface energies, a consequence of the inter CNT interactions [118].
However, while the approach is technically simple with a significant potential in up
scaling, the yields obtained remain very low, typically less than 0.01 mg/mL following
centrifugation [98][123], indicating the unstable nature of such dispersions. The
instability of these dispersions is related to the high aspect ratios of the CNTs, therefore
even though the CNT may be stable immediately following solvent dispersion, the CNTs
sediment out at faster rates with time and especially following accelerated sedimenta t ion
even under mild centrifuge conditions [118]. Therefore, it is imperative to identify
additives that can not only assist in initial interruption of the CNT interactions but
can also prevent or reduce CNT re-aggregation or sedimentation following dispersion
especially in organic solvents. Moreover, the challenge of identifying organic solvents
with minimal environmental footprint and impact is yet to be addressed even for CNT
systems.
97
For instance some of the best solvents used by industry for CNT dispersions include NMP
and DMF which are not only fossil fuel based but are highly toxic and are currently on
the European Candidate List of substances of high concerns for their authorization due to
their toxicity in accordance with Article 59 of the REACH (Registration, Evaluat ion,
Authorisation and Restriction of Chemicals) regulation [133]. In this respect, like in
graphene processing, there is a need to explore alternative solvents that meet
environmental and safety standards.
6-2 Aims and Objectives
In chapter 4 and 5, the successful exfoliation of graphite in organic solvents stabilized by
non-covalent electronic interactions between an electron-rich donor (graphite) and an
electron-deficient acceptor was reported. A significant novelty was the use of low power
sonication, for relatively short intervals to achieve high yield graphite exfoliation. The
yield of graphene was considerably improved, a twentyfold increase, compared to system
without an acceptor. The addition of an electron deficient acceptor was found to be crucial
in the successful disruption of interactions in graphite leading to solid exfoliat ions
and successive dispersions in conventional amide solvents. Since CNTs can be
considered to be rolled up graphene sheets, a similar strategy making use of electro -
deficient acceptors was extended to interrupt the inter CNTs interactions, thereby
causing the agglomerate to debundle and disperse in organic solvents (Figure 6-1).
Following dispersion in solvents, the electron acceptor acts as a stabilizer by preventing
the reaggregation of debundled/dispersed CNTs through prevention/reduction of inter
CNT interactions. Furthermore, due to the non-covalent nature of donor-acceptor
interactions, the electronic properties of CNTs should be maintained, unlike the situation
98
with covalent fuctionalization methods that can alter CNT properties. DHLG was also
explored as an environmental friendly and non-toxic alternative to the traditional NMP
and DMF. In this work both SWCNTs and MWCNTs were studied. Compared to pristine
SWCNTs, MWCNTs have the advantage of low cost and hence hold high industr ia l
potential. It should also be noted that most research efforts have been focused on
dispersion of SWCNTs in organic solvents, due to their uncomplicated structure with few
research reports of pristine MWCNT dispersions.
6-3 Experimental Procedure
Briefly, 50 mg of MWCNTs (5mg/mL) was ground with A1/A2 (0.1 g, 0.075 g
respectively) for 2 minutes followed by addition of 10 mL of organic solvent. The amount
of optimum acceptor required for maximum dispersion was estimated from previous
experiments with graphene (Chapter 4 and 5). In the case of SWCNTs due to the high
cost of material, 10 mg of SWCNTs (1 mg/mL) was ground with A1/A2 (20 mg, 10 mg
respectively) followed by addition of 10 mL of organic solvent. Identical suspensions in
solvent only, without acceptors, were also prepared as controls for both MWCNTs and
SWCNTs. All suspensions were then subjected to low energy bath sonication (Unisonics,
Australia, 50 Hz) for 30 minutes at ambient temperature, followed by centrifugation (Eba
20, Hettich, Zentrifugen) at 3000 rpm, for 30 minutes to sediment larger aggregated
materials, and the supernatant was removed to give CNT dispersions. For the
characterization of the CNT dispersions, the supernatant dispersions, were diluted in their
respective solvents for UV-Vis measurements and TEM. However, for characteriza t ion
of the dispersions using XRD and Raman measurements, conducting films were prepared
by filtration of supernatant dispersion onto a porous alumina membrane, 20 nm (0.02 µm)
99
pore size, followed by washing with solvent to remove the electron acceptor and drying
the films overnight at 70 oC in an oven. It should however be noted that due to the high
cost of the SWCNTs, all characterization was only exhaustively done on MWCNTs.
Figure 6-1 Schematic of the experimental process to form debundled CNTs via donor-
acceptor interactions in organic solvents. Starting CNT bundles (a), partially debundled
CNT via solid phase donor- acceptor interaction (b), further debundled and dispersed
CNT via addition of solvent and mild bath sonication (c) and final dispersed debundled
CNT recovered from the supernatant following centrifugation (d).
100
6-4 Results and Discussion
6-4-1 Solid Phase Processing of CNTs
Studies on the exfoliation of graphite with acceptors, discussed in Chapter 4, showed that
mechano-chemical activation via solid phase manual grinding in the presence of an
acceptor was key to trigger donor-acceptor interactions and exfoliate graphite to graphene .
This strategy is also extended here to induce donor-acceptor interactions and interrupt the
inter CNT interactions, debundle and pave way for a low sonic energy and high yield
dispersion process once the solvent is added.
In the first step, the acceptors were thus ground with CNTs for 2 minutes, generating
immediately, a smooth, dark solid readily stuck on the walls of the glass vial (Figure 6-2
b and e). Similar observations were also made for the case of SWCNTs. This phenomenon
was however not observed when the CNTs were ground without an acceptor. As
discussed in chapter 4, this phenomenon is attributed to the electrophilic nature of the
acceptors, a consequence of the strong electron withdrawing oxygen and nitrogen groups
that polarize the electron density away from the aromatic core. Hence, the electron
acceptors are not only attracted to the electron rich CNT surface but also to the negative ly
charged surface of the glass which is decorated with ethereal linkages (Si-O-Si).
101
Figure 6-2 Optical images of raw MWCNTs and acceptor (A1 and A2) before a) and d),
after grinding b) and e), following addition of solvent (NMP) c) and f).
Studies of grinding of graphite with an acceptor (chapter 4) showed that it was possible
to interrupt the graphitic interactions, exfoliate graphite to graphene and few layered
graphene just by 2 minutes of mechanical grinding with an acceptor. Therefore, a similar
solid phase debundling/deaggregation of CNT to semi debundled CNT by interruption of
the inter CNT interactions in the presence of acceptors may also be occurring in this
case (Figure 6-1, Stage b). The proposed mechanism is a two-step process whereby in the
first step, the electron deficient acceptor is electrostatically attracted and adsorbed onto
the electron rich CNT surface, with vigorous mechanical grinding not only bringing the
acceptor and CNT into close contact, but also providing the necessary energy to trigger a
mechano-chemical process, resulting into a donor-acceptor interaction.
102
Once a single or multiple face of the CNTs has interacted with the acceptor, the inter CNT
interactions between adjacent CNTs are therefore further interrupted and weakened
and in most cases may lead to isolation of individual CNTs from the CNT bundles.
However, even though the CNTs bundle sizes are known to be efficiently reduced by
mechanical milling such as grinding or ball milling, in some cases the mechanical milling
process may introduce cuts and bends in the CNTs resulting into simultaneous cutting of
CNTs into shorter lengths [104]. Whilst shortened CNTs are highly sought after in
chemical or energy-storage applications, long CNTs are required for their application as
strong and conducting nanocables [105]. Therefore, the aspect ratios of the final materials
following CNTs grinding could be crucial depending on the targeted end applications.
Pierard et al [106] showed that MWCNTs can be cut to lengths of < 1 um, from an init ia l
50 um, following mechanical milling using an agate ball for 120 hours with no amorphous
carbon observed, indicative that no major structural defects are created during milling
conditions. Similarly, Kukovecs et al [107] showed that when a low energy ball mill is
used, CNT tube entanglement and length decreased with increasing milling time to up to
140 nm following 200 hrs of mechanical milling. Similarly, no amorphous carbon or
structural defects was observed on the CNT walls. It has also been shown that co-grinding
of CNTs with toluene for one hour can reduce the bundle diameters and agglomerate
particle sizes by a factor of five [108]. SWCNTs have also been shown to be debundled
and subsequently cut simply by grinding (120 minutes and 40 minutes) with soft organic
materials such as or cyclodextrin respectively.
103
On the other hand, Liu et al [104]showed that when a high energy ball mill is used instead,
following 10 hours of mechanical milling, the MWCNTs are debundled and the length is
simultaneously decreased to <1um from an initial of 10–100 m. However, following
extended high energy milling of up to 90 hours , the CNTs original structure are destroyed
and amorphous carbon appears. This clearly shows that the extent of debundling,
subsequent cutting of CNTs and finally severe structural defects is dependent on both the
intensity and time of mechanical milling. Hence it is safe to say that the relatively short 2
minutes manual grinding by hand of the CNTs with acceptors will favor mostly cleavage
of inter CNT interactions rather than shorten the CNTs or lead to structural defects.
However, for applications that require that the CNTs be significantly cut into shorter
lengths, mechanical milling of the CNTs with acceptors for longer times would need to
be explored. Of interest in such a study would be whether the cutting of CNTs to shorter
lengths would be achieved at much shorter times and using low energy. Most importantly,
a short time, solid state CNT debundling process should be highly desirable to industry
as it avoids not only sonication in strong acids and oxidants which can severely damage
the small-diameter nanotubes, but also the long time sonication in solvents, includ ing
toxic ones which could make scaling-up difficult.
However, due to the limited scope of this thesis, more experimental studies on the CNTs
size distribution through statistical analysis from high-resolution TEM studies will need
to be done to shed more light on the associated true effects of grinding of acceptors with
CNTs.
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6-4-2 Continued Liquid Phase Processing
In Chapter 5, DHLG, a bio-based, non-toxic and environmentally friendly solvent gave
excellent results in exfoliation of graphite, with great potential to replace the more toxic
NMP and DMF. As in graphite exfoliation studies, where exfoliating solvent selection is
heavily based on previous good solvents for CNT dispersion, DHLG should also hold
potential to be a good solvent for CNT dispersion. Three solvents were selected to test
the dispersibility of the CNTs in organic solvents: polar aprotic and amide based NMP
and DMF being the industry standard and polar aprotic DHLG being a renewable and
non- toxic green solvent with potential to replace the more toxic industry standards [133].
The solvents were added to the ground solid mixture of acceptor-CNTs (Figure 6-2 b)
and e)). On addition of the solvent, the dark CNT-acceptor smears on the vial wall readily
dissolved into a black dispersion, indicative of slight CNT dispersion forming even prior
to sonication. (Figure 6-c) and f)). Similar observations were made in DMF and DHLG.
This is indicative that non-covalent surface modifications of the CNTS with acceptor
occurred in the solid phase via donor-acceptor interactions. During mechano-chemica l
activation via grinding, the electron deficient acceptor adsorbs on the surface of the
electron rich CNTs via a donor-acceptor interaction mechanism weakening the inter CNT
interactions leading to debundling of the CNTs. In addition, the branched, non-polar
alkyl chains of the acceptor ensure that once it is adsorbed on the CNT surface, it can
easily interact with the non-polar solvent and further enhance the CNT dispersion leading
to higher dispersions even prior to sonication.
105
Interestingly, it has been shown that the liquid phase dispersion of CNTs in solvents is
highly unstable under accelerated sedimentation even under mild centrifuge conditions
[98][116]Error! Bookmark not defined.. In all experiments following 30 minutes of mild bath
onication, the dispersions were centrifuged at 3000 rpm for 30 minutes so as to test the
stability of dispersions especially when an acceptor was used. In addition, it has been
shown that dispersions containing mostly individual CNTs or small bundles can be
obtained after centrifugation [115] [158]. After centrifugation, the supernatant was then
carefully separated from the undispersed and large aggregate material and the UV-Vis
spectra was collected to determine the concentration of dispersed CNTs: the higher the
concentration the greater the CNT dispersibility and stability (Figure 6-3).
106
Figure 6-3 Concentrations (mg/mL) of supernatant of dispersions of MWCNTs a) and SWCNTs b), optical images of dispersions of MWCNTs c) and
SWCNTs d) in DHLG, NMP and DMF with and without addition of acceptor (A1 and A2) after 30 minutes sonication followed by centrifugation at
3000 rpm, for 30 minutes.
107
In general, the most efficient solvents for CNT dispersions have a surface energy very
close to the surface energy of CNTs (∼70 mJ/m2) [119]. This means that successful
solvents for CNT dispersions tend to have surface tensions of ∼40 mJ/m2 with most being
polar aprotic amides such as NMP (40.1 mJ/m2) and DMF (37.1 mJ/m2) [120][121][122].
Bergin et al [98], showed that equally important, successful solvents have a Hildebrand
parameter (T) that match well with that of CNTs at <T> 21 MPa1/2. However, only a
small fraction of solvents with the correct Hildebrand parameter (T) was found to
successfully disperse CNTs due to effects of surface entropy. The Hildebrand parameter
was also found not to be specific enough to identify successful solvents. Finally, Bergin
et al [98] showed that successful solvents also occupy a well-defined range of Hansen
parameter space with the level of dispersibility being more sensitive to the dispersive
Hansen parameter (Dthan the polar (P or H-bonding (HHansen parameter. The
dispersion, polar, and hydrogen bonding Hansen parameters for the CNTs were found to
be <D> 17.8 MPa1/2, <P>7.5 MPa1/2, and <H> 7 .6 MPa1/2 with the dispersibility smaller
for solvents with Hansen parameters further from these values.
In all CNT dispersions with an acceptor, higher concentration of supernatant dispersions
were obtained following centrifuge at 3000 rpm, for 30 minutes compared to that without
an acceptor (Figure 6-3) except for MWCNTs dispersed in DHLG and A2 (Figure 6-3 a).
With CNTs dispersions made with NMP in the absence of an acceptor, even though the
samples looked well dispersed and promising prior to centrifugation, after the samples
are centrifuged at 3000 rpm, for 30 minutes all CNTs sedimented out (Figure 6-3 a and
b) and no nanotubes could be detected after centrifuge (Figure 6-3), final concentration
of 0.01 mg/mL. Bergin et al [118] observed similar results in SWCNTs where the CNTs
concentration in NMP was 0.116 mg/mL before centrifuge and dropped to 0.01 mg/mL
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under mild centrifuge conditions of 5,500 rpm for 90 minutes which they attributed to
CNT instability against aggregation and under accelerated sedimentation conditions. The
instability of CNT dispersions is also related to their high aspect ratios, therefore even
though the CNT may be stable immediately following solvent dispersion, the CNTs
sediment out at faster rates with time and especially following accelerated sedimenta t ion
even under mild centrifuge conditions [118]. Sun et al [159] also observed relative ly
stable dispersions of MWCNTs in NMP with a final estimated concentration reported to
be larger than 0.1 mg/mL. However, these samples were not only sonicated for 60
minutes, twice the amount of time compared to the samples reported here, but were not
centrifuged but rather allowed to sediment under gravity over a period of one week.
Furthermore, they did not report a final concentration after sedimentation studies obtained
and calculated from UV-Vis spectra hence it is very difficult to compare the stability of
these dispersions to the samples discussed here. However, when an acceptor is used to
debundle and disperse the CNTs in NMP, the dispersions are very stable even under
centrifuge conditions, 30 minutes at 3000 rpm, for instance, a 200 fold and 20 fold
increment in A1 and A2 respectively compared to the sample without an acceptor (0.01
mg/mL). The same was also observed with SWCNTs with a concentration of 0.12 mg/mL
in A1 and 0.18 mg/mL in A2: a 12 and 18 fold improvement in yield respectively
compared to 0.01 mg/mL without an acceptor. However, it is important to note that for
SWCNTs the initial starting CNT concentration was 1 mg/mL compared to that of
MWCNT of 5 mg/mL. Hence, higher concentration dispersions of SWCNTs are possible
once the initial starting concentration is increased. Similar comparable results were
obtained with DMF, with a concentration of 1.27 mg/mL in A1 and 1.92 mg/mL in
MWCNTs dispersed in A2, a 6 fold and 9 fold increment respectively compared to 0.48
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mg/mL without an acceptor. For SWCNTs in DMF, the concentration improved to 0.12
mg/mL in A1 and 0.18 mg/mL in A2: a 12 and 18 fold improvement in yield respectively
compared to 0.01 mg/mL without an acceptor. Compared to the sample of NMP without
an acceptor, DMF showed better solvent dispersion and stabilizing properties even under
mild centrifuge conditions. This is in agreement with Inam et al [160] who showed that
solvent dispersions of MWCNTs prepared with DMF were very stable against
sedimentation with no signs of agglomeration even after a few weeks [160]. Even though
it has been demonstrated that successful solvents tend to have surface tensions close to
40 mJ/m2 [119], this seems to be a very useful guide for finding new solvents but not
perfect as is the case here when comparing NMP and DMF dispersions without an
acceptor. NMP with a perfect surface tension of 40.1 mJ/m2 should have higher
dispersions compared to DMF with a lower surface tension of 37.1 mJ/m2, yet DMF
outperformed NMP in terms of stability when the samples were exposed to accelerated
sedimentation under centrifuge conditions. Bergin et al [98] also observed significa ntly
lower dispersibilty in NMP with an almost perfect surface tension compared to
cyclohexyl-pyrrolidinone (surface tension-38.8 mJ/m2). In fact even though alternative
solvent parameters have been used to describe or predict good solvents such as the
Hildebrand parameter(T, NMP still has a much closer T at 23.0 compared to DMF at
24.5, much closer to that of CNTs at 21 (Table 6-1). Similarly, in the case of DHLG, the
concentration increased to 1.56 mg/mL when an acceptor (A1) is used compared to 0.48
mg/mL when an acceptor is not used, 3 times increment in yield. The sample of DHLG
in A2 showed a similar dispersion concentration of 0.42 mg/mL compared to that without
an acceptor. These results are however very promising since this is the first time that
DHLG has been shown to be a potential non-toxic and environmental friendly alternative
110
in CNT processing to the more toxic and fossil based NMP and DMF. Even when an
acceptor is not used, DHLG shows a concentration of 0.48 mg/mL which is 48 times
higher in yield compared to the more conventional NMP and two times that of DMF
hence holds huge industrial potential. The high concentration obtained for DHLG are
attributed to its matched solvent Hildebrand and Hansen Solubility parameters being very
close to those of CNTs as is also the case of NMP and DMF (Table 6-1). Similarly DHLG's
high viscosity of 14 cP could also play a key role as has been previously reported [80].
Table 6-1 Solvent Hildebrand parameter (δT), dispersive Hansen solubility parameter
(δD), polar Hansen solubility parameter (δP), and hydrogen-bonding Hansen solubility
parameter (δH) [64] for all solvents tested, DHLG, NMP, DMF and chloroform [98][80].
Solvent T(MPa)1/2 D(MPa)1/2 P(MPa)1/2 HMPa)1/2
CNTs[98] 21 17.8 7.5 7.6
DHLG - 18.8 10.6 6.9
NMP 23 18 12.3 7.2
DMF 24.9 17.4 13.7 11.3
Furthermore, the solvent-acceptor type-CNT combination also played a key role with A1
being a better dispersant in DHLG and NMP while A2 was better in DMF for the case of
MWCNTs. For SWCNTs a trend was observed whereas A2 was a better dispersant for
all solvents than A1. Similar trends were observed for graphite exfoliation (Chapter 5)
and this was attributed to the possible higher association constant (Ka) of 4.23 mol-1 of
A2 compared to 0.74 mol-1 of A1, calculated from previous work with pyrene gels [75].
111
Therefore, A2 interacts more efficiently and strongly with the CNTs surface during the
mechano-chemical process resulting into higher dispersions. However, there appears that
other factors could play a key role in the dispersion of MWCNTs in solvents such as the
smaller acceptor size of A1 means that it can easily penetrate into the telescopic structure
of MWCNTs more easily and efficiently compared to A2 leading to higher dispersions
obtained in two of the three solvents tested. However, more computational studies and
molecular modelling of the interaction of the acceptors with CNTs in solvents would need
to be done which is beyond the scope of this thesis. Furthermore, the results clearly show
that MWCNTs and SWCNTs interact differently with the acceptor once the solvent is
added which may lead to variations in the association constant (Ka) with acceptor. Studies
have shown that SWCNTs have larger aspect ratios and more perfect sp2 backbones than
MWCNTs making them have a more hydrophobic tubular surface [161]. Therefore, the
very exposed surface of SWCNTs means that the electrons are readily available to
interact with the acceptor molecules, resulting into higher functionalization that aid in
solvent dispersions compared to MWCNTs. Solvent effects can also not be ruled out
given that significant differences are observed in some cases, notably between DMF and
NMP. It is well documented that CNT dispersion in solvent occurs because of the strong
interaction between solvent, specifically the amide functionality, N-C=O and CNT
sidewall which means that the energetic penalty for dispersion and subsequent solvation
becomes small [98]. Several studies have also demonstrated that solvent effects and their
interactions with CNTs greatly affect the dispersion abilities. For instance, Lewis basicity,
polarity and geometry (e.g. ring structure) of organic solvents have been shown to play a
key role in CNT dispersion [109][157][121]. In these studies NMP and DMF have a lone
pair of electrons acting as a lewis base with no hydrogen donors which are essential for
112
good CNT dispersions [110]. An ideal solvent interacts less with both acceptor and CNTs,
allowing more acceptor molecules to interact with the target CNTs.
6-4-3 Crystallinity of dispersed MWCNTs
The crystalline nature and extent of debundling of the MWCNTs was also investigated
using X-ray diffraction spectroscopy (Figure 6-4). MWCNTs films prepared from the
supernatant of the centrifuged dispersions exhibited the typical peaks at 25.72° and 44.3°
in the XRD spectra, corresponding to the graphitic (002) and (100) reflections,
respectively [162][163]. The (002) peak is an indicator of 3 dimensional structure in
graphitic materials [163]. The reduction of the intensity of this peak (I25.72) in the films
prepared from the supernatant dispersion of acceptor A1 and A2, by 84 % compared to
the raw MWCNT (Figure 6-4) is a clear indication of loss of this 3 dimensional structure
upon dispersion indicative of a reduction or complete removal of aggregates as a result
of efficient debundling and dispersion of MWCNTs in solvent in the presence of
acceptors.
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Figure 6-4 XRD diffraction patterns of supernatant of the acceptor (A1 and A2)
dispersed MWCNTs in NMP. The raw MWCNTs material is shown for comparison.
6-4-4 Defect analysis of CNT dispersions
To verify whether the dispersion process with an acceptor introduces defects that may
affect the intrinsic bulk properties of the MWCNTs, Raman measurements of the films
prepared from all the acceptor-MWCNTs dispersed samples was done. The Raman
spectra were then compared to that of the raw MWCNTs starting material. Each spectrum
showed the three characteristic vibrational bands namely, the D, G and 2D bands at ~1350
cm-1, ~1600 cm-1 and ~2700 cm-1, respectively typical of graphitic materials (Figure 6-5)
[164]. It has been shown that the D band is usually present and characteristic of
amorphous graphitic impurities in CNT samples as well as defect sites on the CNTs. The
D band is seen to be present in all samples including the raw MWCNTs starting material
in agreement with literature reports [165]. In addition, the intensity of the D band of the
114
acceptor-dispersed MWCNT films increases compared to the raw MWCNT starting
material. This is attributed to a higher degree of disorder as a result of effective dispersion
and reduced bundle sizes in the configuration of the MWCNTs, compared to the raw,
bundled MWCNTs [165]. The ratio of the integral intensities of the Raman peaks, D to
G (ID/IG) is generally used as a measure of the degree of structural defects in graphit ic
materials [153]. The ID/IG ratio for each sample is also reported in Figure 6-5 with a
clear difference in the exfoliation properties of A1 and A2 being observed. All A1 films
formed from dispersions of debundled MWCNTs have a higher ID/IG ratio (Figure 6-5,
a) compared to A2 (Figure 6-5, b). This results indicate that while A1 is efficient in
formation of high concentration dispersions in some solvents, DHLG and NMP (Figure
6-3), it however results in slightly less efficient debundling compared to A2 MWCNT
dispersions. Overall, a consistent trend is observed with regard to the ID/IG ratios of
materials processed with the acceptors: MWCNTs dispersed with acceptors exhibited
ID/G ratio closer to that of the raw MWCNT starting materials. Hence it is safe to say
that the dispersion process does not generate defects in the final dispersed material. This
follows from the method of using mild and short sonication times (30 minutes).
115
Figure 6-5 Raman spectra of MWCNTs exfoliated in (a) A1 and (b) A2 in the indicated
solvents. The raw MWCNTs is included for comparison.
6-4-5 Morphology of CNT dispersions
Transmission electron microscopy (TEM) was used to determine the morphology and
extent of debundling of MWCNTs in the supernatant of dispersed MWCNTs dispersion.
The TEM of the starting raw MWCNTs was also imaged and shown for comparison
(Figure 6-6 a). The TEM of representative acceptor-dispersed MWCNTs (A1, DHLG) is
shown in figure 6-6 (b and c). Compared to the starting raw material which shows highly
aggregated and entangled MWCNTs bundles Figure 6-6 (a), the supernatant of the
acceptor dispersed MWCNT shows the presence of debundled MWCNTs (Figure 6-6 (b)
with no evidence of agglomerates like those present in the raw material. Upon a closer
look of individual tubes Figure 6-6 (c), the tube was seen to be highly transparent to the
electron beam with no observable holes or damages hence showing that the dispersion
method does not result into defects, also a consequence of reduced mild 30 minutes bath
sonication.
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Figure 6-6 TEM images of MWCNT starting material (a) and following dispersion with
Acceptor 1 in DHLG (b) and (c).
6-4-6 Electrical properties of CNT dispersions
It is widely accepted that the presence of agglomerates in MWCNTs dispersions can lead
to a decrease in electrical properties especially during formation of nanocomposites [160].
The electrical properties of the supernatant of the MWCNTs dispersions in an acceptor
were evaluated as this would directly relate to their performance in thin film deposition
or in composite formation. The sheet resistance (Rs) were measured using the four-point
conductivity probe method after drying the films overnight at 70 oC in air. The DC
conductivities reported in Figure 6-7 were calculated using Equation 7-1, where t is the
film thickness: 500 nm.
Conductivity (S/m) = (𝑅𝑠)−1
𝑡𝑥10−9 (7-1)
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Figure 6-7 Electrical conductivity of films formed from MWCNTs dispersion with
acceptors A1 and A2 in various organic solvents
Figure 6-7 clearly shows that film conductivity is dependent on the nature of the solvent
and acceptor used to form the dispersion with the films prepared from A2 having higher
conductivities than A1 in all solvents except DHLG which had almost similar values to
A1. These results are in agreement with Raman spectra which showed that A1 films
formed from dispersions of debundled MWCNTs have a higher ID/IG ratio (Figure 6-4,
a) compared to A2 (Figure 6-4, b) indicative of A1 being slightly less efficient in
debundling compared to A2. Films of A2 MWCNTs dispersed in DMF produced the
highest conductivity of 48,781 S/m. It is important to note that this film was also formed
from the highest dispersions of MWCNTs achieved in all solvents and acceptor tested
(Figure 6-3). Therefore, it is not surprising that this sample gives the highest conductivity
further reinforcing A2 is a better dispersant of MWCNTs in DMF as previously discussed
118
in section: 6-4-2 above. Even though DHLG produces relatively good dispersions, it also
produces the lowest film conductivity. From Table 5-1, DHLG has the highest boiling
point of all solvents, 203 oC, which means that it is very difficult to completely remove
the solvent hence even after drying the films overnight at 70 oC , there can be still traces
of residual solvents within the film that further lowers the electrical properties of the films
formed from such dispersions. However, the high boiling point of DHLG alone does not
explain the drastic differences with films formed from NMP which exhibited much higher
conductivity. A complementary explanation would also be that DHLG has a much higher
viscocity of 14 cP compared to 2 cP of NMP for instance which means that there might be less
efficient flake size separation during centrifugation resulting into less sedimentation of
aggregated CNTs. The relative good conductivity of all the films is attributed to the low
defect of the processing method as is also evidenced by Raman spectroscopy (Figure 6-
5) and TEM (Figure 6-6)
6-5 Conclusion
The approach of using a ternary system of CNTs, solvent and electron deficient acceptor
enabled the effective interruption of the inter CNT interactions via a donor-acceptor
interaction mechanism. As a result, the yield of debundled and dispersed CNTs in organic
solvents increased with the dispersions stable even after centrifuge. The use of the
electron acceptors was also essential in lowering the energy needed for effective
debundling and solvent dispersion of the CNTs to only short periods of 30 minutes under
mild bath sonication. More so, the solvent scope was also extended to DHLG, a non-toxic
alternative to the current industrial standards NMP and DMF. These studies show the
importance of designing additives that assist the solvent in cleavage of the inter CNT
119
interactions to enable efficient debundling, dispersion as well as reduce/eliminate the
tendancy to re-agglomerate in solvents. While the theory of matched surface energies for
efficient dispersion is important to consider, it is highlighted in this study that the ability
of specifically designed acceptors to cleave the interaction plays an equally important
role.
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Chapter 7 General Discussions and Conclusions
7-1 Research overview and Challenges
Graphene, identified as one of the most promising materials for various applications
especially in electronic devices, due to its excellent electrical and mechanical properties,
is difficult to produce in large scale. The top down approach of graphene production from
low cost and readily available graphite has been identified as a promising route to produce
large quantities of defect free graphene [147]. However, this approach has a significant
disadvantage: the strong interlayer interactions between the graphene sheets in
graphite that limits its exfoliation and subsequent dispersion in a wide range of solvents .
Micromechanical cleavage of graphite using scotch tape [136] is a simple process that
can be used to produce very high quality and pristine graphene. However, it is too
involved, lacks controllability and there is always the possibility of inevitab le
contamination from the glue tape [22]. In addition, the yield is too low to upscale.
Mechanical exfoliation using shear forces and relying on a similar concept such as used
in the scotch tape method, has been explored to increase the yield of graphene. However,
such methods have still proven to be insufficient on their own to completely overcome
these interactions when used in large scale. Furthermore, even though graphene has been
produced when chemical assistants are used in the mechanical exfolia tion, high energy
and prolonged mechanical milling conditions of are still used, which is detrimental to the
quality of graphene.
Direct liquid phase exfoliation (LPE) of graphite in a well-chosen organic solvent, by
exploiting ultrasounds to produce graphene [56], is also another known technique that
could be used for applications such as conducting inks and electronics [148].
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In this method, exfoliation has been attributed to strong interactions between the solvent
molecules and the graphitic basal planes, which in turn results into subsequent dispersion
[149]. Whilst LPE is a promising approach for industrial use, this method has a significant
limitation as has been previously highlighted in Chapter 5, section 5-1. Additionally, even
though much progress has been made in the large scale synthesis of CNTs, their high
natural tendency to form bundles, ropes or aggregates as a consequence of also strong
interactions still limit their processing and development for further applications. The
resultant CNT bundles can also have very complex morphologies varying from tens of
nanometers in diameter and many micrometers long. Studies have shown that individua l
CNTs can, not only be held within a CNT bundle, but can also be entwined, interwoven,
bent, entangled or form loops around not only other CNT bundles but also within an
existing bundle [99]. The main properties of CNT bundles are inferior to those of isolated
CNTs, and the fact that it is extremely difficult to separate CNTs from bundles represents
a serious hurdle in the way of potential applications. The liquid phase debundling and
dispersion of CNTs in a carefully selected solvent using sonic energy is a simple and
popular technique used to disaggregate, debundle and disperse the CNT bundles into
individual CNTs or reduced CNT bundle sizes. In this approach, only solvents with a
matched surface energy to the CNT, generally high boiling point amides, are employed
[157][98]. However, while the approach is technically simple with a significant potential
in up scaling, the yields obtained also remain very low, typically less than 0.01 mg/mL
following centrifugation [98][123], indicating the unstable nature of such dispersions.
The instability of these dispersions is related to the high aspect ratios of the CNTs,
therefore even though the CNT may be stable immediately following solvent dispersion,
the CNTs sediment out at faster rates with time and especially following accelerated
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sedimentation even under mild centrifuge conditions [118]. Therefore, it is imperative to
identify additives that can not only assist in initial interruption of the CNT
interactions but can also prevent or reduce CNT re-aggregation or sedimenta t ion
following dispersion especially in organic solvents.
7-2 Research Objectives
This main aim of this study was to interrupt the interlayer interactions between the
graphene sheets in graphite that limit its exfoliation and subsequent dispersion in a wide
range of solvents using donor-acceptor interactions. Once successful, a similar strategy is
extended to cleave the interlayer interactions between graphitic sheets in other
carbonaceous materials such as CNTs. In order to achieve this, several objectives were
set:
Synthesize electron deficient acceptors (A1 and A2)
Through a mechano-chemical process, induce and maximize donor-acceptor
interactions between graphite and acceptor, exfoliate graphite while
simultaneously reducing the energy needed for mechanical exfoliations
Enhance the yield and dispersibility of graphene produced from liquid phase
exfoliation of graphite
Reduce the sonic energy needed to increase the yield of graphene from liquid
phase exfoliation of graphite
Expansion of scope of graphite exfoliating solvents to include low boiling point
solvents and identify high performing, non-toxic and environmental friendly
solvent alternatives for graphite exfoliations
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Extend a similar strategy to process other carbonaceous materials such as CNTs
which also have very poor processability due to interactions between CNTs
7-3 Research Outcomes
This thesis set to explore how to effectively disrupt and simultaneously lower the energy
needed to cleave the network of interactions in both graphite and CNTs, as well as
further enhance the dispersion of the resultant materials in organic solvents. This was
successfully achieved through a donor-acceptor interaction mechanism, between the
electron rich (graphite/CNTs), and specially designed electron deficient molecules,
acceptors (A1 and A2). As a result, minimal energy input of manual grinding (2 minutes)
of graphite/CNTs with acceptor, via a mechanochemical process, induced donor-acceptor
interactions which preferentially weakened the interactions, with the ultimate result
being solid exfoliation in graphite and solid phase debundling of CNTs. This outcome of
solid phase graphite exfoliation/CNT debundling within a short period of time: 2 minutes
of manual grinding by hand compared to prolonged mechanical milling processes
outlined in literature, is significant for two major reasons a) minimization of mechanica l
fragmentation that cause defects in graphitic sheets/CNTs tube lengths which is crucial
for enhancing the electrical properties of these materials. b) Solvent free process also
eliminates the high cost of solvents and will therefore allow easy adaptability to current
end product manufacturing equipment.
It was also evidenced that stable dispersions were formed immediately after addition of
the solvent to the ground graphite/CNTs-acceptor material, indicating enhanced
dispersion only after donor-acceptor interactions. Graphite exfoliation and CNT
dispersion was further enhanced after mild bath sonication (30 minutes) of the acceptor-
124
graphite/CNT mixture in solvent with 13 and 200 fold increment in yield of graphene and
CNTs respectively dispersed in N-methylpyrrollidone (NMP) and A1 for instance
compared to that without the acceptor respectively. The use of electron acceptors
therefore, allowed high dispersions, not only in high boiling point solvents whose surface
energy matches that of graphene/CNTs e.g. NMP and dimethylformamide (DMF), but
also in low boiling point solvent with mediocre properties, for example, chloroform in
the case of graphite. Moreover, the use of novel dihydrolevoglucosenone (DHLG) in the
liquid phase processing step as a high performance green solvent alternative to toxic NMP
and DMF was also reported. The use of electron acceptors in weakening the
interactions also meant that the sonic energy needed to exfoliate and disperse
graphene/CNTs in organic solvents was reduced resulting into a low energy exfolia t ion
and dispersion process. Shorter and lower energy exfoliation and dispersion processes
also translated into large graphene sheets/higher aspect ratios of CNTs which resulted
into materials with good electrical properties.
7-4 Future directions and applications
The effective cleavage of the network of interactions in both graphite and CNTs
using donor-acceptor interactions meant that a higher yield of materials could be
produced using a very low energy process. These materials are attractive to a wide variety
of industrial applications as outlined in Figure 7-1 and as a result two patents have since
been filed:
125
S.M. Notley and D.H. Gharib (Australian National University),World Intellectual
Property Organization Patent, WO2017063026, 2017
D. H. Gharib and S. M. Notley (Australian National University) Australian patent,
AU2015904218, 2015.
Furthermore, these materials have recently been applied to improve the electrical, thermal
and mechanical properties of polyimide composites (Yet to be published work). For
instance polyimide composites containing graphene-MWCNTs composite materials at
0.56 wt % loading yielded increases of 5 % and 40% in thermal stability and Young’s
modulus in the polyimide properties respectively. Furthermore, at this loading, electrical
conductivity was improved by several orders of magnitude to 1.60 x 10-4 S/m, from an
insulating material to a semiconductor. The composites fabricated could find potential
applications in high performance polymer application in aerospace engineering and
flexible electronic technologies. This clearly shows that these novel, high quality carbon
based materials processed using this electron acceptor method hold huge industr ia l
potential.
Even though the research objectives were realized there are still key areas for future
research (Figure 7-1) these include:
Quantification of the energy needed in mechanical exfoliations using controlled
techniques such as ball milling
Preliminary studies showed that extremely non-polar and polar solvents such as
mineral oils and water can still not be used for the effective processing of both
graphite/CNTs even when an acceptor is used. There is still need to design novel
126
acceptors or modify the acceptors to further increase the scope of solvents to
extremely non-polar and polar solvents
Further characterizations of materials especially statistical analysis of the size
distributions for both graphitic sheets and CNTs bundle sizes following
dispersion in solvents using techniques such as Atomic force microscopy AFM)
and High resolution TEM with electron diffraction capability.
Application of the novel materials in energy storage, conductive inks, electronic
devices.
Explore similar strategy of using donor-acceptor interactions to exfoliate other
Van der waal bonded layered materials such as molybdenum disulphide (MoS2),
tungsten disulphide(WS2) and boron nitride (BN)
127
Figure 7-1 Future directions and potential applications of acceptor exfoliated materials.
128
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