TVE 15069
Examensarbete 30 hpDecember 2015
Modelling the exfoliation of graphite for production of graphene
Mehwish Abro
Institutionen för teknikvetenskaperDepartment of Engineering Sciences
Teknisk- naturvetenskaplig fakultet UTH-enheten
Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0
Postadress: Box 536 751 21 Uppsala
Telefon: 018 – 471 30 03
Telefax: 018 – 471 30 00
Hemsida: http://www.teknat.uu.se/student
Abstract
Modelling the exfoliation of graphite for production ofgraphene
Mehwish Abro
The aim of my thesis is to make a theoretical model of data obtained from liquid-phase exfoliation of graphene. The production of graphene in the liquid phase exfoliation is a cost efficient method One part of this work is devoted to learn the method of production of graphene by the shear mixing technique from the graphite and to estimate some important parameters which are crucial for the process.
Other part of my work is based on studying the liquid-phase exfoliation mechanism of graphene through ultra-sonication technique. This method is time consuming as compared to shear-mixing.
TVE 15069 DecemberExaminator: Nora MassziÄmnesgranskare: Dr. Subimal Majee, Dr. Zhibin ZhangHandledare: Dr. Asim Aijaz
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Acknowledgements
Firstly, I would like to thank my supervisor Dr. Zhibin Zhang who gives me the opportunity to work his
group and complete my master thesis project with his research group.
I would also like to thank my co-supervisor Dr. Subimal Majee for his advisement, encouragement and
guidance during my project research as well as going out of his way to help me succeed and pursue my
goals during my time at Uppsala University. He helps me in teaching the different experimental methods
in the lab. I would also like to thank my other group members.
I would like to express my gratitude to my home country DEAN Prof. Dr. BS Chowdhry , he encouraged
and guided me throughout my university education.
A special feeling of gratitude goes to my loving parents, especially my father engr. Abdul Ghani Abro, my mother Rukhsana Abro whose prayers and words of encouragement helped me to reach this point.
I am especially thankful to my sister, brothers, my friends, and relatives who encouraged and supported
while I was working on this thesis.
I would also like thank to my best friend in Uppsala University; he encouraged me a lot throughout my
research work. I cannot conclude these acknowledgments without recognizing the lunch break time chit
chat, which also greatly supported the completion of this work.
Finally, I dedicate my work to my supervisors who have supported me throughout my research work process. I will always appreciate all they have done.
Mehwish Abro,
Uppsala University, Sweden
January, 2016
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Table of Contents:
Chapter # 1 Introduction
Introduction……………………………………..….……….………………...…... 8
Structure….…………………..….…………….…….…………………………...…..8
Properties of Graphene……………………………………………..…………….….9
Common Methods for graphene synthesis……….…………………….…............…10
Common methods, yields and their Applications….……………….……….………11
Liquid Phase Exfoliation…………….…………...………………………….…...…12
My Thesis Organization…….……….…………………..…………………………14
Chapter # 2 Shear Exfoliation of Graphite
Introduction of process…………………...…………………………………….…..15
Mechanism of process ………………………………………………………….….16
Experimental Procedure…………………………………………...………………..17
Experimental results and discussions.……………………………………..…….….17
Theatrical modelling of the experimental results.………….………………...……..19
Viscosity of Solvents………………………………………………………………...22
Conclusion of chapter 2………………………………………………………….…..23
Chapter # 3 Sonication of Graphite
Introduction of process...…………………………………………………………...…24
Experimental procedure ………………………………………………………………24
Experimental results and discussions……………...………………………………….24
Comparison of both methods…………………...……………………………………..27
Theatrical modelling of the experimental results……………………...……………....28
Conclusion of Chapter 3……………………………………………………………….29
Conclusion……………………………………………………………………………...30
Future Work……………..…………………………………….………………………..30
Bibliography…………………….………………………………………..……………..31
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List of Figures
1. Graphene.layout ….……………….…………………………………………………………..…8 2. Synthesis of graphene by two different methods in Liquid based exfoliation…………...……12 3. Shear mixing machine……………………………………….…………………………………15 4. Attractions in the Layers of graphene…………………………………………...….……….…16 5. Variation of exfoliated graphene flake thickness and lateral diameters with increasing
shear mixing speeds. The variation is for ethanol based exfoliation……...……………..........18
6. Variation of exfoliated graphene flake thickness and lateral diameters with increasing shear mixing speeds. The variation is for cyclohexanone based exfoliation…………………18
7. Variation of exfoliated graphene flake thickness and lateral diameters with increasing shear mixing speeds. The variation is for NMP based exfoliation …………………….......…19
8. Graph of force of friction and applied force……………………………………………….......20
9. Graph of Exfoliation and applied force……………………………………………………......20
10. Graph of force of friction and applied force……………………………………………….......20
11. Ultra-sonication data for Ethanol solvent…………………………………………………..….25
12. Ultra-sonication data for Cyclohexanone solvent…………………………………………..…26
13. Ultra-sonication data for NMP solvent………………………...…………………………..…..26
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List of Tables
1. Properties of Graphene……………………………………………..……………..……….9
2. Common Methods for Graphene synthesis…………………………………….………….10
3. Common methods yield and their applications……………….………………..……….…11
4. Viscosity of Solvents……………………………………………………………..………...22
5. Comparion of both methods………………..…………………………………..…………..27
6. Time for one layer exfoliation of solvent…………………………………………………..28
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Introduction
Graphene, 1-3 is a two dimensional nanomaterial consist of single atomic layer of sp2-bonded carbon
atoms. 4,5 It is organized in a honeycomb lattice/ benzene like structure, displays amazing electronic,
electrical, mechanical, optical and thermal properties.6,7,8 In the most recent decade graphene has
developed as an energizing new material, with potential to affect numerous areas of science and
innovation.9,10 With an innovative method (Scotch-Tape exfoliation) Novoselov and Geim in 2004 have
successfully exfoliated monolayers of graphene and awarded the Nobel prize in Physics.11,12 It is
semimetal material having several outstanding applications. According to the properties for example,
electronic devices as micro- and optoelectronics13,14,15,16,17, basic nanocomposites, printed electronics,
conductive coating, biological labeling 18 and batteries and supercapacitors.19 Many other possible
technological applications of monolayer graphene, e.g. in photonics and flexible electronics, ranging from
solar cells20, photodetector21,22,23 and light emitting devices24 to touch screens25, ultrafast lasers26, spin
valves 27,28 etc., are additionally being investigated.
Structure
Graphite was derived from the Greek word ` Graphein` which means to write. The term graphene was
derived from the graphite, presented by chemists Hanns-Peter Boehm and co-worker in 1986 .29,30
Graphite is the combination of the millions of the graphene layers. Two types of bond are formed among
the graphene layers. The bond which holds together the layers of graphene by the weak force called
Vander Waal attraction. The Vander Waal bond length between the adjacent graphene layers is 3.41Å
(0.341nm) .3 Due of this weak attraction between layers, the layers slide each other and the attraction is
strong enough to do the complete exfoliation into individual layers. Another bond known as Covalent
bond which present between the carbon-carbon atoms in each layer is 1.42Å (0.142nm) considered as the
strong bond .31 Both the hypothetical and experimental research proved that the properties of graphene are
mainly dependent on their geometric structures. (See Figure 1)
Figure 1 : Graphene layout adopted
(https://www.google.se/search?q=graphene+structure&espv=2&biw=1920&bih=935&source=lnms&tbm
=isch&sa=X&ved=0CAYQ_AUoAWoVChMI1KXMptTiyAIVpJdyCh11AArw#imgrc=xMbdpfJEXG22
SM%3A)
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Properties of Graphene
Graphene has many outstanding properties It is considered as the thinnest possible material in the world
and compared to the steel it is 200 times stronger material.32,33 According to Hoorad and their colleagues
the thermal conductivity of graphene is ~33 times greater than the silicon.34 Below given are the
important properties of graphene which are collected from different literature.
Sr.
Property
Value
References
1.
Large Surface Area
~ (3000 m2g-1)
30,36
2.
Stretch elasticity
~ 20%
51
3.
Optical Transparency
~ 97.7%
37,38
4.
Tensile Strength
~ (130GPa)
30,39
5.
Thermal Conductivity
~ ( (3000-5000)WmK-1)
30,40,41,42
6.
Breaking Strength
~ ( 42Nm-1 )
10,40
7.
High Carrier Mobility
~ (10,000cm2 V-1 S-1 )
36,39,41,39,42,48
8.
Young’s Modulus
~ (1.0TPa )
30,39,41,41
9.
Large Spring Constant
~ (1-5Nm-1)
30,39,49
10.
Electrical Conductivity
~ (3000 Wm-1k-1)
50
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Common Methods for Graphene synthesis There are tremendous efforts have been done to develop synthesis methods for graphene. These different
methods are used to achieve high yield of graphene for different application. Generally the synthesis
methods can be classified as the Top-Down and Bottom-Up approaches .51 All the methods have their
own pros and cons depend on final applications. Many researchers proved that the graphene production
by bottom-up methods have high quality but unfortunately suffers from low scalability. On the other hand
the graphene produced by Top-down methods have high quantity but poor quality. The production of
graphene in the large scale and at low cost as explained by the top-down techniques.30 Precise control
over graphene synthesis is therefore required for testing their fundamental physical properties and then
introduce them in promising applications.52
Top -Down Bottom-Up
Mechanical
Exfoliation4,42,53
(1) Apply the
adhesive tape on
graphite block and
peeled back
(2) Join the two
pieces of tape
together to reduce
layers
(3) Finally press the
tape on the smooth
silicon substrate &
peel back leaving
atomic single layer
thick graphene.
Liquid Phase Exfoliation
Chemical Vapor
Deposition
(CVD)37,42,53,56,58,59,60
(1)A substrate
usually copper
(sometimes Ni) is
heated in furnace
about ~1000Ċ at low
pressure
(2) CH4 and H2
gasses are added
through furnace
(3) Carbon atoms are
deposited on copper
substrate and
continuous graphene
sheets are formed.
Epitaxial
Growth on
Silicon Carbide 42,53,59,60
(1)Small amount
of SiC is placed
into box with
small hole in it
(2)Seal the box in
non-reactive
environment and
heated at about ~
1500Ċ
(3)Because of
heating, Si
molecule from the
surface of SiC
evaporates,
leaving high
quality monolayer
graphene.
/Chemical
Exfoliation53,54,55
(1) In this process
ultrasound is used
to break graphite
into flakes into
organic solvents.
(2) After some
duration, large
quantity of flakes
are produced
(3) Centrifuge
process is helpful
for enriching the
graphene quality.
Chemical Exfoliation
Via Graphene Oxide 56,57
(1) In this process the
graphite is first
oxidized and then
exposed in chemical
exfoliation to produce
graphene oxide flakes
(2) Centrifuge process
is used for further
enriching of graphene
sheets
(3) The solution is
deposited on substrates
and reduced thermally
and chemically to get
graphene.
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Common Methods Yield and their Applications
Methods flake size
(Top view)
Cost Throughput
Flake
thickness
(cross
section
view)
Number of
layers
Applications
Mechanical process
Greater
than 1 mm
low
low
10 nm
Single &
multiple
Research purpose
Chemical Exfoliation
1 μm
low
μm to few
nm
μm to
few nm
Single and
multiple
Inkjet printer ink,
polymers fillers,
coating, paint,
composites,
transparent
electrode,
sensors, energy
storage and bio
applications
Chemical
Exfoliation Via
Graphene Oxide
μm to few
nm
low
high
nm
Single and
multiple
Inkjet printer ink,
polymers fillers,
energy storage,
Battery
electrodes,
supercapacitors
CVD
cm
high
Moderate
< 1nm
Monolayer to
multilayer
Touch screen,
small windows,
flexible LCDs &
LEDs
Epitaxial Growth on
Silicon Carbide
100 mm
high
low
< 1nm
Monolayer to
multilayer
Transistor
circuits,
interconnect,
memory
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Liquid Phase Exfoliation:
The exfoliation of graphite in the liquid environments can be effectively done by exploiting shear mixing
and ultrasound to extract individual layers. The liquid phase exfoliation process normally includes three
stages.(see figure: 2)
1. Dissolve graphite in a solvent,
2. Exfoliation, and
3. Purification.
Shear Mixing Exfoliation
Ultra-sonication Exfoliation
Figure 2: Synthesis of graphene by two different methods in Liquid based exfoliation
Liquid phase exfoliation is a method to exfoliate graphite into liquid solution. It is a feasible way to
obtain colloidal suspension of graphene layers in the solution. The quality and quantity of graphene layers
are higher than those produced from graphite oxide due to the absence of oxygen functionalities which
disturb the properties like electrical conductivity and carrier mobility of graphene layers. This method is
more efficient when the applied force to the graphene can overcome the graphene–graphene interlayer
van der Waal interaction force. According to Loh et.al, the exfoliation is better when the surface energy
of the solvent is close to the graphene.61 By the mechanical force, which is sufficiently greater than
Vander Waals force, we can separate graphene layers.
Ethanol
Cyclohexanone
N-Methyl-2-
pyrrolidone
Stablizer
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The mechanical force is achieved by either sonication or shear-mixing. The long term sonication prompts
to undesirable fragmentation into exfoliated graphene layers which brings about small size graphene
layers. Generally, sonication is a process to transfer the sound energies to fragment the particles.62 It has
been found that the surface energies play important role when graphite surface is immersed in the liquid.30
Liquid phase exfoliation is one of the effective and straightforward method to decrease the strength of the
Vander Waals attractions. 63
Meanwhile in shear mixing and ultra-sonication, the growth and the breakdown of the micrometer-sized
bubbles because of pressure fluctuations, work over the bulk material and induce exfoliation.62 After
exfoliation, the interaction of solvent–graphene needs to adjust the attractive forces in between the inter-
sheets. Ideal solvents to disperse graphene are those that actually minimize the interfacial surface tension
[mN m−1] between the solvent and graphene flakes. For example, the forces that reduce the area of the
surfaces in contact.30,64
Many groups have worked in this method and made it possible to produce large scale of graphene.
Stankovich and their followers oxidized the graphite and produced the graphite oxide layer. During the
oxidation process, the functional groups like hydroxyl and epoxide attached covalently to the graphite
oxide reducing the interlayer interactions, which causes complete exfoliation and produce the single layer
GO. The presence of oxygen functional group on the GO causes sheet hydrophilic and shows less thermal,
electrical and mechanical properties as compared to pure graphene.3,65,66,67 So reduction of GO into
graphene has become main area of research.68 Recently, some groups demonstrated the exfoliation of GO
into the organic solvent like dimethyl foraminde (DMF), N-methyl-2-pyrrolidone (NMP), ethylene glycol
and tetrahydrofuran (THF) with moderate sonication. 35,48,69,70
Hernandez and their fellows in 2008 explained the first successful exfoliation of graphite in the organic
solvent such as NMP and DMF by sonication based technique.71 After centrifugation, they confirmed
their characterization through Atomic Force Microscopy (AFM) and Transmission Electron Microscopy
(TEM), that they obtained pristine graphene which were chemically unmodified. Surprisingly they
achieved closely 100% graphene nanosheet and atomic thickness was less than 6 layers and samples was
consisting of 28% monolayers. 35,48,71,72,73,74,75,76,77 But NMP and DMF liquid have a few drawbacks e.g.
NMP is an eye irritant and may be dangerous to the reproductive organs, while DMF may have toxic
effects on various organs. 78,79 It is considered that the solvents having the surface energy close to the
graphene are appropriate for the direct exfoliation of graphene .30
Mustafa Lotya et.al in 2009 demonstrated that the mechanism of liquid phase production of graphene
depends on utilizing the specific solvents whose surface energies are around that of graphene.80 These
organic solvents require unique consideration when handling. However, it is considered that the solvents
which have low boiling point is good for exfoliation of graphene but that graphene has limit application,
while on the other hand the solvents which have high boiling point like (NMP 203 ℃, cyclohexanone 156
℃ and DMF 154 ℃ ) are suitable for the exfoliation of graphene but the high boiling points of these
solvents restrict their use for the real manipulation, specifically in organic electronics.30 Many
independent group explained unfortunately that water has a surface energy (72.7 mJm-2) too much high to
use it as for exfoliation of graphene .71, 80,81
In 2010 a group from Ireland which observed the dispersion of graphene in NMP solvent by using ultra
sonication exfoliation method at low power 23W for long times up to 460hours.82 They have achieved
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the concentration of 1mg mL−1. The size and thickness of flakes decrease with sonication time. For long
sonication times the average flakes dimension still remains above 1μm . 82
The graphene application´s market is basically driven by progress in the synthesis of graphene with
properties fitting for the specific application, what's more, this circumstance is liable to proceed for the
next decade or if nothing else until each of graphene's many potential applications meets its own
particular requirements. Currently there are most likely a dozen of techniques being used and developed
to synthesis graphene of different dimensions, quality and shapes. 83,84
My thesis organization:
In the next chapter of my thesis, one can find the exfoliation of graphene in the liquid phase exfoliation
using the shear mixing method. During my stay in the Uppsala University Sweden, I was involved in the
experiments with the liquid phase exfoliation method of graphene and modelling the production
mechanism of graphene. In this chapter I have presented the detail of the protocols involved in the
synthesis of graphene and the modelling of the surface energies of Ethanol, Cyclohexanone, NMP w.r.t to
graphene and the graphical representation of reduction of the graphene flake thickness with some shear
mixing parameters.
In the third chapter, I have explained the exfoliation of graphite by using the sound waves in the liquid
solution and finally the advantages and disadvantages of both the exfoliation methods. And I have
calculated the time for exfoliation of single layer. Finally, the conclusion is summarized for both methods
and some future outlook is presented.
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Chapter # 2
Shear exfoliation of Graphene
Introduction of the process Shear mixing is a process of exfoliation of the graphite in the liquid phase by using the shear force
mechanism85. In this process, a machine (Silverson model L5M mixer) is used to generate high shear
force. The Silverson model consists of manual control system, rotor and stator part (see figure 1) and the
purpose of this machine is to mix the solution of solvent and graphite and shear the graphite. In shear
mixing process, the graphite is mixed with the organic solvent like ethanol, Cyclohexanone and NMP.
Figure 1: Shear mixing machine. a, Image of shear mixing operator. b, Rotor. c, clear view of
functioning part. d, different shape of stator. e, combination of Rotor and stator. f, exfoliated
samples
Here, we can say that high-shear mixing is a scalable alternative to the sonication process for the peeling
of layered crystal of graphite. Shear mixing is already generally used to disperse nanoparticles in solvents.
It includes separating of microparticle agglomerates that are weakly attached compare with the intersheet
binding strength in graphite. The exfoliation of graphite or layered material that incorporates shear mixing
as a part of the procedure is reported in different articles 3. In shear mixing, the layered material is initially
swelled by intercalation, significantly reducing the interlayer binding strength.3,35,71 The shear mixing
exfoliated the crystal material graphite to give dispersed nanosheets. However, shear mixing method just
limiting the rate step (Time) for intercalation and exfoliation, increases the potential for scale-up. The
shear mixing process has much more ability to exfoliate the untreated graphite crystal material into liquid
solution. In addition, Shear mixing process utilizes less power densities as compared to Ultra-sonication
method.
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Mechanism of the process
The shear mixing exfoliation method works for the exfoliation of graphene in liquid phase. To understand
the mechanism of exfoliation in liquid we have to first understand some term like cohesive force, surface
energy and viscosity. The cohesive force is defined as the attraction of one molecule toward other
molecule which held together the molecules of the atom in the bulk material in the layers. Surface energy
is defined as the imbalance energies at the top surfaces of the materials such as liquid or solid. In the bulk
material,
Figure 2: Attractions in the Layers of graphene
the energies in centre remain same so every molecule get force exerted equally from surrounded
molecules. The molecules present in the top layer of material have different energies because top layer
remain in the contact of the atmosphere and experience the imbalance energies in the surface. In term of
graphite, when solid graphite mixed with the solvents, they attach into the layers of graphite to reduce the
Vander Waal force in between layers (see figure 2). Actually, the total energy (ET) required to exfoliate
the graphite in the liquid is the combination of the Vander Waal forces (Evwf) and the change of the
surface energies (Es).
ET = Evwf + ∆ E
ET = Evwf + [ Eg - Es]
Where,
ET is the total energy requires for exfoliation,
Evwf is the Vander Waal force between the layers of graphite,
E is the changes in surface energies,
Es is the surface energy for the mixing organic solvent
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The exfoliation of graphite requires to overcoming these energies. Generally, the graphene layers are
attached with each other by weak Vander Waal attraction and that attractions are strong enough to
exfoliate the graphene single layer from the graphite.
Experimental procedure
We use the shear mixing process to exfoliate the graphite for production of graphene. For this we mix the
graphite powder and organic solvent in the beaker with a specified weight ratio. We also add some
polymer stabilizer along with them, in order to keep the exfoliated graphene flakes stable inside the
dispersion. The beaker is placed at the bottom stage of the shear mixer. By controlling the setup, the rotor
(outside diameter 32 mm) is inserted into the beaker. The speed of the rotor can be varied using the setup
monitor. The maximum rotor speed that can be obtained with full load is 8000 rpm. In order to see the
changes of thickness and the lateral diameters of the exfoliated graphene flakes on the types of solvents
generally used, a wide range of solvents with different surface energies are used in our study. The speed
of the rotor is varied from 1000 rpm to 6000 rpm. The process was performed for a fixed duration of 20
min. After each shear mixing completion, the supernatant obtained from the top part of the dispersion is
spin coated on Si/SiO2 substrates and the morphology of the films are characterized through atomic force
microscopy technique using non-contact mode. The thickness and the lateral diameters of the flakes are
evaluated through this process.
Experimental results and discussions
The shear mixing is first performed in ethanol based exfoliation. Initially the thickness of graphite flakes
was 1μm. When the shear speed is increases from 0 to 1000 rpm; the thickness and the diameter of
graphene flakes get affected and the graphene flake thickness is reduced from 1000 nm to 740 nm. When
the shear speed varies from 1000 to 5000 rpm, the thickness is reduced sharply with the increasing speed
and reduced down to ~100 nm. The diameter is also reduced from 20,000 nm to 2900 nm. After that the
shear speed doesn’t affect the exfoliation process and the thickness as well as the diameters remain
constant (see figure 4).
In Cyclohexanone based exfoliation, when the shear speed is increase from 0 to 1000 rpm, the thickness
and the diameter are reduced. The thickness is reduced from 1000 nm to 500 nm. When the shear speed is
increased from 1000 to 5000 rpm, the thickness is sharply reduced down to 70 nm. The diameter is also
reduced from 20,000 nm to 1000 nm. After that the shear rate don’t not affect the thickness and diameter
(see figure 5).
Similarly, when the shear mixing process is used with the NMP, the shear speed affects the thickness and
the diameter. When shear speed increases from 0 to 1000 rpm, the thickness is reduced from 1000 nm to
400 nm. When the shear speed is increased gradually from the 1000 to 5000 rpm, the thickness and
diameter are sharply reduced down. The thickness is reduced down and the minimum reduced thickness
obtained is 10 nm for 5000 rpm speed. On the other hand the diameter is also reduced from 20,000 to 250
nm After that the shear speed doesn’t affect the exfoliation process and the thickness as well as the
diameter remains constant. The NMP is greatly affecting the shear mixing process, and the thickness and
diameter reduce more as compared to the ethanol and cyclohexanone (see figure 6).
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When the shear rate is increased gradually, the thicknesses as well as the diameters of the graphene flakes
are reduced for all the above mentioned organic solvents. There is a clear trend of sharp reduction of the
thickness and the diameters after 1000 rpm and we found a critical speed of 5000 rpm above which there
is no further exfoliation. If we compare three organic solvents, we can see through graph that the ethanol
shows the less exfoliation (see Figure 4). The cyclohexanone shows the moderate exfoliation as compared
to ethanol and NMP (see figure 5). NMP based exfoliation shows better exfoliation rate as compared to
the ethanol and cyclohexanone (see Figure 6).
1000 2000 3000 4000 5000 60000
100
200
300
400
500
600
700
800
Flake thickness Flake diameter
Shear speed (rpm)
Flak
e th
ickne
ss (n
m)
2000
4000
6000
8000
10000
12000
Fla
ke d
iam
eter
(nm
)
Figure: 4 Variation of exfoliated graphene flake thickness and lateral diameters with increasing shear
mixing speeds. The variation is for ethanol based exfoliation.
1000 2000 3000 4000 5000 60000
100
200
300
400
500 Flake thickness Flake diameter
Shear rate (r.p.m)
Flak
e th
ickne
ss (n
m)
0
2000
4000
6000
8000
Flak
e dia
met
er (n
m)
Figure: 5 Variation of exfoliated graphene flake thickness and lateral diameters with increasing shear
mixing speeds. The variation is for cyclohenanone based exfoliation.
Page | 17
1000 2000 3000 4000 5000 6000
0
100
200
300
400 Flake thickness Flake diameter
Shear speed (rpm)
Flak
e th
ickn
ess
(nm
)
0
1000
2000
3000
4000
Fla
ke d
iam
eter
(nm
)
Figure: 6 Variation of exfoliated graphene flake thickness and lateral diameters with increasing
shear mixing speeds. The variation is for NMP based exfoliation.
Theoretical modelling of the experimental results The shear mixing mechanism works on the force of friction and applied force. When we use the shear
mixing process, rotor rotates and applies force into the solution. Initially, the static friction occurs when
the applied force through the rotor exerted into liquid solution (solid graphite and organic solvent), the
friction produced into the solution. With the increase of the shear mixing speeds, the applied rotational
force faced by the flakes increases as well, this allows the sliding of the graphene layers between
themselves. As a results, the friction between the two adjacent graphene layers increases linearly as
shown in Figure 3.1, which is termed as the static friction. Due to increase of the friction with the applied
force, it is hard to separate the individual layers form each other and almost no exfoliation occurs in this
region (from F0 to F1 as shown in Figure 3.2). In our practical experiments, we did not see any
exfoliation when the shear mixing speeds are increased slowly from 0 to1000 rpm. We suppose the
frictional force is so huge in this region, therefore basically it is impossible to separate the layers of
graphene. As a result, there is no reduction of either thickness or size of the flakes as shown in figure 3.3.
When further increase of the applied force, the frictional force starts to reduce and therefore the exfoliate
occurs. We can see than exponential decay of the frictional force with the applied force in the region
between F1 and F2 (Figure 3.1). In this period the graphene layers start to slide between each other and
with the increase of the applied force they separate from each other. We can say that the exfoliation starts
from this region and from our practical measurements, it occurs after 1000 rpm. As a result, both the
thickness and the size of the flakes reduce exponentially as we have shown in Figure 4, 5 and 6.
With further increase of the applied force, the frictional force becomes constant, which means easy
exfoliation of the layers, however, there is a limit of the exfoliation of the layers which depends on the
surface energies of the solvents used, we can see a stable thickness and the size of the flakes after a
certain speed of 5000 rpm (corresponds to F2 in the Figure 3.2).
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Figure 3.1: Graph of force of friction and applied force
http://www.sciencehq.com/physics/frictional-force.html
Exfoliation
Figure 3.2: Graph of Exfoliation and applied force
Thickness
Page | 19
Figure 3.3: Graph of force of friction and applied force
The surface energies are calculated through the following calculation,
ṙmin = [√Egraphene – √Esolvent
ŋ∗𝑙]
2
ṙmin = [√Egraphene – √Esolvent
ŋ]
2
∗ 1
𝑙2
𝑙 = [√Egraphene – √Esolvent
ŋ∗𝑙]
2
∗1
ṙ𝑚𝑖𝑛
𝑙 = √𝐸graphene – √Esolvent
ŋ∗
1
√ṙ𝑚𝑖𝑛
𝑙 = √𝐸graphene – √Esolvent
ŋ∗√ṙ𝑚𝑖𝑛 -----------------(1)
Where,
ṙmin ₌ Shear rate (s-1)
Eg ₌ Surface energy of graphene, (N/m)
Es ₌ Surface energy of solvent, (N/m)
ŋ ₌ solvent viscosity, (Pa.s)
𝑙 ₌ Flake length, (m)
Page | 20
Viscosity of Solvents
The viscosity of solvent is defines as the thickness of the solvents which resist to flow. It is also
important term for the exfoliation of graphite in the liquid. The viscosity of the liquids which we are using
in our experiments is given below.
Viscosity of solvants (Pa.S)
Ethanol 0.00107
Cyclohexanone 0.00202
NMP 0.00167
The surface energies of the three solvents e.g. Ethanol, Cyclohexanone and NMP are very important when
we talk about the exfoliation of graphite. We have taken the model reference from the literature 85
(Scalable production of large quantities of defect-free few-layers graphene by shear exfoliation in liquids)
and calculated the surface energies of the solvents.
From equation (1) , the shear rate is the mixing rate of the solution and it is given as,
ṙmin = 2 𝜋 ND
∆R
where,
N ₌ number of revolution per min,
D ₌ 0.0320 m,
∆R ₌ gap of rotor-stator,
So,
ṙmin = 2 ∗3.14∗83.3334∗ 0.0320
0.0001
ṙmin ₌ 167467 s-1
Page | 21
this is the value of the shear rate of all solvents for 5000 rpm, Now for the calculation of surface energy
of Ethanol √E Ethonal ,by rearranging the equation (1), we get
√EEthanol = √Egraphene – √ rmin * ŋ * 𝑙
= √ 0.0763 – √ 167467 * 0.00107 * 0.000007
= 0.2645 – √ 0.001254
= 0.2645 – 0.03541
√EEthanol = 0.22909
Take the squaring on both sides,
(√EEthonal ) 2 = (0.22909) 2
EEthanol = 0.0524 N/m
EEthanol = 52mJ/m2
By using the similar calculation and procedure, the surface energies of the cyclohexanone is calculated as
62 mJ/m2 and for the NMP is 69 mJ/m2.
Conclusion of chapter 2
1. We have exfoliated graphene flakes out of graphite from a novel method which is shear exfoliation
technique. In this process, the applied shear force helps to separate the graphene layers by overcoming the
vander Waal’s force of attraction.
2. We have shown the variation of the thickness and the lateral diameters of the flakes with shear mixing
speeds.
3. We have defined a critical shear mixing speed beyond which there is no more exfoliation.
4. Using an available shear mixing mechanism, we have extracted the surface energy values of some well
known solents usually used for exfoliation of graphene.
5. We have shown a correlation between frictional force and the reduction of thickness.
Page | 22
Chapter # 3
Sonication of graphite
Introduction of the process
Sonication is another method for exfoliation of graphite layers into the solution. It is based on the sound
waves in which the solution is placed in the ultra-sonication bath. The solution kept in the beaker, and
placed in the sonication bath. The sonication bath exerted the pressure in the beaker solution and then the
solution in which the organic solvent and the graphite are present experiences the force from the
sonication bath which causes reduction of the flake thickness. It is slow process for the exfoliation. The
result from the sonication process for three organic solvents ethanol, cyclohexanone and NMP are given
below.
Experimental procedure
We use the sonication mechanism in the liquid phase to exfoliate the graphite into the solution for
production of graphene. For this process, we mix the graphite powder, organic solvent and polymer
stabilizer with specific quantity into the beaker and keep the beaker into the sonication bath. We add the
polymer stabilizer into solution because to keep the graphene flakes stable into the solution. The
sonication bath works on different frequencies. We used 20 KHz frequency to disperse and exfoliation of
the solution. The sonication bath transfers the energy into the water present into the bath then the water
exerted the pressure to the breaker. The pressure then transmits in to the solution, disperse and exfoliate
the graphite into the solution. This process is time consuming and produces the graphene into the liquid.
The process was performed at different times. After the completion of ultra-sonication, we take the small
amount of supernatant from the top part of the solution to spin coat it on the substrate of Si/ SiO2. Then
we characterize the sample through the atomic force microscopy technique.
Experimental results and discussions
The ultra-sonication technique in the liquid phase exfoliation is first accomplished in ethanol based
exfoliation. In this process, initial thickness of flakes of graphite was 1μm (1000 nm). In order to increase
the ultra-sonication time (hour), the thickness of graphite flakes is decrease. Briefly, when the ultra-
sonication time is increased from 0 to 300 hour, the thickness of the graphite flakes is reduced down from
1000 nm to 80 nm for ethanol. After that the ultra-sonication time doesn’t affect the exfoliation process
and the thickness remains constant (see Figure 1).
For the cyclohexanone, when the ultra-sonication time increases from 0 to 300 hour, the thickness of
graphite flakes is get affected through the sonication time. The thickness is reduced down from 1000 nm
to 40 nm. After this thickness, there is no further reduction of the thickness (see figure 2) .
Page | 23
In NMP based exfoliation, the affect is much severe. When ultra-sonication time increases from 0 to 300
hour, the thickness is reduced down from 1000 nm to 10 nm. After this point, it doesn’t affect the
exfoliation process and the thickness remains constant. (see figure 3).
-100 0 100 200 300 400 500 600 700 80060
80
100
120
140
160
180
200
Flak
e th
ickn
ess
(nm
)
Sonication time (hour)
Figure 1: Ultra-sonication data for Ethanol solvent
Page | 24
-100 0 100 200 300 400 500 600 700 80020
40
60
80
100
120
140
160
Flak
e th
ickn
ess
(nm
)
Sonication time (hour)
Figure 2: Ultra-sonication data for Cyclohexanone solvent
-100 0 100 200 300 400 500 600 700 8000
10
20
30
40
50
60
70
80
90
Flak
e th
ickn
ess
(nm
)
Sonication time (hour)
Figure 3: Ultra-sonication data for NMP solvent
Page | 25
Comparison of the both methods
We have used to different methods in liquid phase exfoliation for production of graphene. But methods
have some advantages and disadvantages. If we compare both methods, according to some parameters,
we have some conclusion given below.
Parameter Shear Mixing Exfoliation Ultra-sonication Exfoliation
Time
Less time
Longer time
Thickness of final
product
same
same
Scalability
Yes
No
Concentration of
final product
same
same
Page | 26
Theoretical modelling of the experimental results
Here is the calculation of one layer exfoliation.The distance between the graphite layers is 0.34 nm. and
initially the thickness of grahite is 1000 nm and we notice that the linear exfoliation happens upto 300
hours and the thickness reduces at 80 nm for ethonol.
The exfoliated thickness at 300 hour is = 1000 - 80
= 920 nm
We know that the distance between the layers is 0.34 nm.
The number of layers exfoliated at 300 hour is given as
Number of layers exfoliated = 920
0.34
Number of layers exfoliated of ethanol = 2705
So,
One layers exfoliation time (min) is = 300 ∗ 60
2705
One layers exfoliation time is = 6.65 min for ethanol
Now the remaining layers at 80 nm = 80
0.34
Remaining layers = 235
So now we can know that the total number of layers (LT) at initial 1000 nm thickness is
LT = 2705 + 235
LT = 2940
On the other hand, if we calculate the exfoliation of single layer time with same method for
Cyclohexanone and NMP, it is given as,
Solvent Time(min) Reduced
thickness(nm)
# of layers
exfoliated
1 layer
exfoliated time
Remaining layers
Cyclohexanone
NMP
300
300
960
990
2823
2911
6.37
6.1
117
29
Page | 27
We can see the exfoliation time for the one layer from the graphite flakes is reduces when the surface
energy value of the corresponding solvent is close to that of the graphene. Since, from our previous
chapter, we concluded that the surface energy of NMP is much close to graphene and from this chapter
we concluded that the time required for one layer exfoliation is minimum for NMP, we can correlate
both conclusions.
Conclusion of chapter 3:
We can see the exfoliation time for the one layer from the graphite flakes is reduces when the surface
energy value of the corresponding solvent is close to that of the graphene. Since, from our previous
chapter, we concluded that the surface energy of NMP is much close to graphene and from this chapter
we concluded that the time required for one layer exfoliation is minimum for NMP, we can correlate
both conclusions.
1. We have exfoliated graphene flakes out of graphite through the ultra-sonication technique. In this
process, the applied sound energy helps to separate the graphene layers by overcoming the vander Waal’s
force of attraction.
2. We have shown the variation of the thickness with ultrasonication time
3. We have defined a critical ultrasonication time, above which there is no more exfoliation.
4. We have calculted the single layer exfoliation time using the data obtained from the measurements.
Page | 28
Conclusion
In this project, we have used to different methods for the production of graphene from graphite. The
methods are based on the liquid phase exfoliation. Through these two methods we have come to know
that the surface energy is one of the main parameters for the exfoliation. We used three different organic
solvents for the exfoliation. These were Ethanol, Cyclohexanone and NMP. The liquid which has surface
energy close to the graphene is better for the exfoliation. These two methods produced graphene in which
the shear mixing exfoliation takes less times for the production of graphene on the other hand, the ultra-
sonication consumes more time.
If we compare both methods, the shear mixing exfoliation is more scalable method because it can produce
large quantity of flakes compared to the ultra-sonication based exfoliation.
Future work
Our work gave a solid indication of the choice of solvent requires for the efficient exfoliation. In future,
efforts should be devoted to use the track presented here to exfoliate the graphene flakes efficiently with
further optimization of the methods.
Finally a way should found to produce large amount of high quality graphene through both methods in the
liquid phase exfoliation so that we can use it in our daily life application.
Page | 29
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