Towards Graphene Based Transparentconductive Coating

8/11/2019 Towards Graphene Based Transparentconductive Coating

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Abstract

Graphene is a relatively new material which has had a lot of publicity recently as a group of scientist

was rewarded the Nobel Prize in physics for a comprehensive work about single layer graphene in

2010. After the Nobel Prize many people got up their eyes for the subject and that the numbers ofrapports have increased from around 500 in 2006 to 3000 in 2010.

An ideal sheet of graphene consists of a two-dimensional honeycomb structure of coal atoms in a

single layer, the sheet is entirely sp-2 hybridized. To be considered to be graphene the structure can

have no more than ten layers. Graphite and graphene shares many similarities between each other

and a frequent expression is that graphene is just one layer of graphite.

In this study, a new chemical process is used and robust solution process has been developed to

obtain high quality graphene sheets in aqueous solution and these sheets were characterized by

XRD, FTIR, TGA, UV-Vis, Raman, AFM, TEM and XPS.

Graphene sheets developed in this work holds a great promise in the development of carbon-based

transparent conductive coatings to replace costly indium tin oxide-based transparent conductive

coating, which is necessary for energy saving windows, solar cells, display and LED etc.



Table of contents

Introduction ........................................................................................................................................................ 5

Project description ......................................................................................................................................... 5

Purpose .......................................................................................................................................................... 5

Theory ................................................................................................................................................................. 6

Graphene is a material with a great potential. .............................................................................................. 6

Properties of graphene................................................................................................................................... 8

The synthesis of graphene.............................................................................................................................. 9

Oxidation of graphite to graphene oxide ..................................................................................................... 10

Washing of graphene oxide .......................................................... .............................................................. .. 11

Reduction of graphene oxide ....................................................................................................................... 12

Dextran ......................................................................................................................................................... 13

Dextrin .......................................................................................................................................................... 15

Method of analysis ....................................................................................................................................... 16

Experimental procedure ................................................................................................................................... 29

Oxidation of graphite ................................................................................................................................... 29

Washing of graphene oxide .......................................................... .............................................................. .. 31

Reduction of graphene oxide ....................................................................................................................... 33

Annealing of reduced GO ............................................................................................................................. 35Results & Discussion.............................................................................................................................................. 37

UV-Vis Spectrophotometry .......................................................................................................................... 39

XPS – analysis ............................................................................................................................................... 47

XRD – analysis ............................................................................................................................................... 53

TG – analysis ................................................................................................................................................. 56

AFM analysis ................................................................................................................................................. 61

TEM – analysis .............................................................................................................................................. 65

FTIR – analysis .............................................................................................................................................. 66

Conclusions ........................................................................................................................................................... 74

Oxidation of graphite ............................................................. ................................................................. .......... 74

Reduction of GO ............................................................................................................................................... 75

Summation ....................................................................................................................................................... 76

Future work .......................................................... ................................................................. ................................ 77

References ............................................................................................................................................................ 78

Quoted works and sources ................................................................................................................................... 78

Appendix ............................................................................................................................................................... 82



Theory

Graphene is a material with a great potential.

Graphene is the name given to a flat monolayer of carbon atoms tightly packed into a two-

dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all otherdimensionalities (Figure 1). (1).

Figure 1. Mother of all graphitic forms. Graphene is a 2D building materials of all other dimensionalities. It can be

wrapped up into 0D buckyballs, rolled into 1D nano tubes or stacked into graphite

Graphene is a material that has experienced a tremendous increase in interest in the last few years.

This is evident considering that the numbers of rapports have increased from around 500 in 2006 to

3000 in 2010. (2)

One of the reasons why there is such a big interest in the development of coal based semi-

conductors such as grapheme and carbon nano-tubes is the possibility to use them as a replacement

for rare earth metals.

Rare earth metals are, despite the name, moderately abundant in the earth’s core although most of

them are not concentrated enough to make mining economically viable. The current production is

also heavily centralized to China, which some countries without a domestic production see as a

drawback.



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The demand for rare earth metals is estimated at 134, 000 tons per year, and a global production of

124, 000 tons the production is covered by above-ground stocks or inventories. World-wide demand

is projected to rise to 180, 000 tons annually by 2012, while it is unlikely that new mine output will

close the gap in short term (3). By 2014 global demand for rare earth metals may exceed 200, 000

tons per year. Chinas output may reach 160, 000 tons per year (up from 130.000 tons in 2008) in

2014, an additional capacity shortfall of 40,000 tons per year may occur. New mining projects could

easily take 10 years for development. (4)

Also there is another more environmental reason why graphene attracts so much attention around

the world. While rare earth minerals have to be mined and processed from ore, graphene can be

synthesized from a variety of organic material. In an age where energy efficiency and recyclability

considerations are vital, this could be a key reason to choose graphene when possible.

As mentioned before graphene seems to have many fields of application, these ranges from

electronics to optics (5) and medical science (6). For example graphene can be used to create good

capacitor when either combined with a polymer as a composite (7) or used in its pure form (8).Graphene has also been used to construct sensitive sensors that can detect very low traces of a

specific prostrate antigen. (6)



Properties of graphene

Structure

An ideal sheet of graphene consists of a two-dimensional honeycomb structure of coal atoms in asingle layer, the sheet is entirely sp-2 hybridized. To be considered to be graphene the structure can

have no more than ten layers. (9) (10)

Graphite and graphene shares many similarities between each other and a frequent expression is

that graphene is just one layer of graphite. Actually one of the best current methods to produce high

quality graphene is through micromechanical cleavage (1). By repeatedly exposing a piece of graphite

to the mechanical stress of a piece of tape the weaker bindings between the planes are destroyed.

After creating a sufficient number of flakes the tape can then be dissolved and the graphene

collected.

However if the graphene sheets are re-stacked on top of another again the crystalline structure of

graphite should not appear.

Instead of writing graphene nano sheets repeatedly the term GN will hereafter be used.

The size of the GN sheets can have a wide range of lateral sizes depending on which method that

creates the nano sheets. The smallest sheets are defined as nano-ribbons with a lateral size of 10 nm

and the larger droplets are in the range of several hundred to 2500nm (11)

General properties

It has been reported that graphene has very high stiffness and breaking strength. The values

measured corresponds to a Young’s modulus of E = 1.0 TPa and intrinsic strength of 130 GPa, which

would suggest that graphene is the strongest material ever measured. (12) It also reported that

graphene has a very high thermal conductivity, measured between 4.84-5.30 kW m-1 K-1. (13) The

carrier mobility and electron density of graphene has also been measured to extremely high values,

200 000 cm2 V-1 s-1 and 2*1011 cm-2 respectively. (14)

Except from the other more common properties graphene also has a quantum Hall effect and an

ambipolar field effect in room temperature. (15) (16)



9

The synthesis of graphene

When producing a material, the method that is used is either top-down or bottom up. The top-down

method is when you use some kind of base material and break it down to the desired material by a

chemical or physical process. The bottom-up method is a kind of reverse method where you use thesmallest building blocks of the material and put them together to the desired material.

Figure 2 Graphite structure

Top-down methods

Many different approaches to produce graphene by top-down methods have been explored. Themost common methods of producing graphene are by using graphite as a raw material and then

separate its layers by chemical, mechanical or thermal methods (1)

Bottom-up methods

The most common type of bottom up method is by vapor deposition of graphene on some sort of

substrate, mainly a noble metal but also transition metals as cobalt can be used. (17) (18)

There has also been some success with epitaxial growth of graphene on thin SiC-wafers. The idea is

that silica has a higher sublimation rate than graphene which leaves a layer of coal atoms which

rearrange itself into a layer of graphene. (19)

It is also possible to combine the two techniques by chemical vapor deposition on silica carbide. (20)



Oxidation of graphite to graphene oxide

Graphene Oxide

In order to produce graphene from graphite the reaction has to proceed in two steps. The first step

is an oxidation from graphite to graphene-oxide. Thereafter another step follows where thegraphene oxide is reduced to graphene.

Two main reasons for using this procedure exist. The first reason is that while a graphite flake has a

very low solubility in water, the oxidized graphite is soluble to a large extent.

The second reason is that the methods that are turning graphite into graphene in one step often

exert a lot of force on the sheets and introduce more defects to the finished product. This is often

done by using extremely high temperatures or pressure. By dividing up the process in two steps less

stress is introduced in each step and the total defects introduced are often less than in a one-step

approach. (1) (2) (3) (4)

It is also worth to consider that for a coating application, a water based solution is suitable for most

types of applications.

The structure of graphene oxide is complicated and dependent of many parameters in the process

under which it is formed. In Figure 3, one possible structure of graphene oxide is shown.

Figure 3 A possible structure of graphene oxide

The most common way to oxidize graphene is using the Hummers method which was discovered as

early as 1958 (21). In short, the method is a development of Benjamin Brodies Oxidation where

sulfuric acid, sodium nitrate and potassium permanganate were used instead of potassium chlorate

and fuming nitric acid. Benjamin Brodie is credited with the first recorded chemical oxidation of

graphite.

The oxidized form of graphite will hereafter be designated of GO in this report.

During the oxidation oxygen reacts to different sites on the graphite sheet. It is believed that when

an oxygen molecule attaches there is also an opportunity for another oxygen molecule to attachitself to an adjacent carbon forming an epoxy pair on the surface. Although this reaction contributes



11

to the weakening of the van der Waals forces it only lowers the amount of fracture stress a sheet can

sustain with 16% (22) (23)

Although much research has been done to identify the chemical pathway, no consensus has been

reached so far. The reason why no significant gain has been made is that it is very difficult to

characterize the complexes formed during the reaction. This in turn is due to the very low surfacearea of graphite combined with the fact that only the edge sites form surface complexes. (24)

Somehow during that process the van der Waals forces are overcome by a force driving an expansion

of the internal distance between the layers. This force results in the increase of distance between the

layers and later also a delamination. (23) (25)

It is stated by McAllister et al (McAllister, o.a., 2007) that the exfoliation and later oxidation of the

graphene sheet is driven by gaseous products. These are derived from the exothermic decomposition

of hydroxyl and epoxy groups of the GO and not from the vaporization of intercalated species. This

proved to be an important distinction as thermal energy gained locally heats up the sample andenables faster reaction rates. This would lead to a rapid build-up in pressure which results in a

uniform exfoliation of the material. The removal of carbon atoms from graphene oxide was also

confirmed in a rapport by Paci et al. (23) (25)

Figure 3 A mix of highly dispersed GO and less dispersed GO

Washing of graphene oxide

After the oxidation of graphite there exists a highly acidic mixture of reacted and unreacted graphite

together with ions that are left from the potassium permanganate. This mixture makes it hard to

analyze.

To separate a desired product from undesired bi-products such as ions or acidic parts it is common to

utilize the difference in solubility or size. When centrifuging a liquid containing a mixture of

substances more dense particles settles easier in the bottom of the vial due to the greater force of

gravity. Depending if your desired substance exist in the pellet or the supernatant it is easy to

remove the undesired part.



By subsequently varying both the time of centrifugation and the solvent in which the pellet is desired

(if the desired product exists in the pellet), it I possible to obtain a product which is significantly purer

than the starting material.

Reduction of graphene oxide

Graphene in its oxidized form show two unwanted properties viz. it is not conductive and it is very

sensitive to higher temperatures. By removing the oxygen containing groups formed (reducing it) it is

possible to restore these properties. This reduced form of graphene oxide (GO) will thereafter be

called Red-GO.

Many ways exist to perform the reduction but the most common way in a chemical process is to

elevate the temperature of a GO-Water solution and add a reducing agent. To further enhance this

reaction it is possible to adjust the pH and add a promotor to the reaction.

One of the most difficult parts of finding a suitable reducing substance is to make sure that the

substance also disperses the graphene sheets in the solution. The relative surface area where the

sheets can attract each other is very large which increases the demands on the reducing agent.

During the last few years a wide variety of substances has been used to reduce graphene oxide to

graphene. Among them hydrazine proved to be comparably effective of reducing the oxidized

graphene. (26) (27) (28)

A rapport written by (Peng-Gang Ren et al) concludes that the reduction of graphene oxide by

hydrazine is more dependent on the reaction temperature than the reaction time. In any

temperatures above 60°C the epoxides bonded to the graphite was removed, although to reach ahigh C/O ratio (15.1) a temperature of 95°C had to be used. (26)

During the short time that graphene has existed as a scientific subject, a lot of potential substances

have been evaluated for reducing GO. These substances are often either efficient at reducing GO,

environmentally friendly or cheap to produce, it is not common for one substance to inherit all of

these desirable properties.

In order to be able to produce graphene in an industrial scale, it is important to find a substance that

fulfill these criterions as much as possible. Some of the more promising new substances are vitamin C

and different polysaccharides (1) (2).

As vitamin C and polysaccharides already are produced in an industrial scale, the question about bulk

cost is not a problem. Furthermore the reducing capacity of these substances have been examined

and it has been confirmed that vitamin C and most of the polysaccharides are able to reduce GO.

(Shaojun, Youxing, Shaojun, & Chengzou, 2010) (Young-Kwan. Kim, 2010) (Jian, Fang, Liu, Ning,

Zhiqiang, & Xi, 2009)



13

Dextran

Description of dextran

Dextran is an α-D-1,6-glucose-linked glucan with side-chains 1-3 linked to the backbone units of theDextran biopolymer. The degree of branching is approximately 5%. The branches are mostly 1-2

glucose units long. The molecules have a molecular weight that ranges from 1,000Da to 760,000Da.

Figure 4 Structure of fragment of dextran molecule

General properties

Dextran fractions are readily soluble in water and electrolyte solutions to form clear stable solutions.

The pH does not affect solubility significantly. Concentrated solutions (<50w/v) may be prepared.

Dextran fractions are also soluble in some other solvents, notably methyl sulfide, formamide,

ethylene glycol, and glycerol. Dextran fractions are insoluble in monohydric alcohol, for example

methanol, ethanol and isopropanol, and also most ketones, such as acetone and 2-propanone.

Biocompability of dextran

The clinical use of Dextrans over the past 50 years provides impressive proof of their safety and

quality. Most of the safety studies have been related to parenterally administered Dextran solutionsin the molecular weight range 10,000Da to 70,000Da. (3)

Dextran may be ingested orally and is well tolerated. The ingestion of Dextran is followed by a rapid

increase in blood sugar and liver glycogen and is thus digestible.

Many other applications of Dextran in medicine have appeared. Dextran is an ingredient of solutions

for ophthalmic use, for intrauterine examinations, and is also used in creams and ointments. It may

therefore be concluded that Dextran has an excellent record of biocompatibility.

Enzymes (dextranases) from molds such as Penicillium and Verticillium have been shown to degrade

Dextran. The products are essentially low molecular weight sugars, for example glucose, isomaltoseetc. (3)



Similarly many bacteria produce extracellular dextranases that split Dextran into low molecular

weight sugars. Examples of these are Lactobacillus, Cellvibrio, Cytophaga, and soil Bacillus spp.

Dextran is therefore biodegradable and the Dextran by-products are readily absorbed into the

natural environment. (3)



15

Dextrin

Description of dextrin

Dextrin is a mix of polymers of D-Glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds. The

molecules have a molecular weight that ranges from 800Da to 79,000Da.

Figure 5 Structure of fragment of dextrin molecule

General properties

In solid form dextrin is a transparent, brittle solid. It is soluble in water and in dilute alcohol, but is

insoluble in anhydrous alcohol or in ether; wood spirit dissolves it freely. Dextrin is distinguished byproducing right handed rotation upon a ray of polarized light and it derives its name from this

property. (4)

Dextrin is derived from potato starch through heating in an acidic environment. Depending on the

degree of conversion, dextrin may inhibit varying solubility in water in which it forms colloidal

systems. As a polyhydroxy compound, dextrin is able to participate in a number of chemical

reactions characteristic of alcohols (esterification or etherification through substitution on hydroxyl

groups, or chemical complex formation with the hydroxyl groups. (5)

Biocompability of dextrinIn vitro studies of dextrin nanoparticles shows that they are non-cytotoxic and do not elicit a reactive

response when in contact with macrophages. An in vivo study showed that, after intravenous

injection, a relatively fast removal of the nano particles occurs in the first 3h, then continuing slowly

up to 24h. The blood clearance profile reveal a moderately long circulation time. (6)Method of

analysis



Method of analysis

XRD analysis

About 95% of all solids can be described as crystalline. When X-rays interact with a crystallinesubstance (phase), one gets a diffraction pattern.

Every crystalline substance gives a specific pattern; the same pattern; and in a mixture of substances

each produces its pattern independently of each other.

The x-ray diffraction pattern of a pure substance is therefore, like a fingerprint of the substance. The

powder diffraction method is thus ideally suited for characterization and identification of

polycrystalline phases. (11)

XRD analysis was mainly used to investigate the changes that the crystal lattice undergoes during the

reaction steps:

During the first step, the oxidation part of the process, the graphite sheets should react with

the oxygen atoms provided by the potassium permanganate. As this happens the distance

between the layers should increase.

In the second step during the reduction of the GO the adsorbed oxygen molecules will be

released and replaced with dextran or dextrin on the surface of the graphene sheets.

The crystal structure of graphite is very simple and only one peak exists at its natural state. This peak

has an intensity maximum around 27degrees (2θ). During the oxidation process this peak disappears

with time and another peak appears which have an intensity maximum around 10degrees (2θ). (12)

This change in position represents the increase in spacing between the layers.

During the reduction of the GO the peak around 10degrees (2θ) should disappear and only a slight

“bump” around 20 degrees (2θ) should be left. (13)

XRD was used to verify these steps during the reaction and also to investigate if any other defects

would be introduced to the lattice during this process.

The measured material

Crystalline materials are characterized by the orderly periodic arrangements of atoms. If broken

down to smaller pieces the structure of a crystal can be found to consist of an identical self-repeating

structure called a unit cell.

Inside the unit cell many planes exist, these are used to define directions and distances in the unit

cell.

For example a unit cell of NaCl can have both (200) planes and (220) planes due to its structure with

chloride as a FCC structure with Sodium in the interstitial spaces between the atoms (Figure 4). (31)



17

Figure 6. Structure of NaCl

The measuring equipment

XRD (x-ray diffraction) instrument essentially consists of a monochromatic x-ray emitter, a sample

holder and a detector (Figure 5).

Figure 7. X-ray analysis of crystalline materials

As the beam sent from the emitter hits the sample the wave fields interfere with each other

constructively or destroy each other depending on the alignment of the crystallographic planes in the

sample.

By changing the angle ψ/2θ, peaks in the received reflection from the sample will appear. These

depend on the fact that randomly oriented crystals in a sufficient number produce a continuous

Debye cone (Figure 6). In a linear diffraction the detector scans through an arc that intersects each

debye cone at a single point thus giving the appearance of a discrete diffraction peak.



Figure 8 Debye cone

By sampling the signal at a number of different angles a XRD diffractogram can be made.



19

Mathematical Basis:

Braggs law is a simplistic model to understand what conditions are require for diffraction.

sin2hk l

d (1-1)

Where λ is the wavelength of the incident x-rays which is monochromatic i.e. it has a specific

wavelength.

dhkl is the distance between the planes

Sin θ is the angle between the sample and the detector.

(1-2)

Where a is the lattice spacing if a cubic crystal and h, k and l are the miller indices of the Braggs-plane



UV-Vis Analysis

Spectrophotometers are mainly used to measure transmission or absorption in liquids and

transparent or opaque solids. It does so by sending a beam of light through the sample and then

monitoring the remaining light in a detector.

In the case of a UV-Vis spectrophotometer the light is in the wavelength of 800-200nm, probing

electronic transitions in the sample, figure 9 shows some typical electronic transitions. It is hard to

reach a lower wavelength than 200nm as oxygen starts to absorb light below that wavelength.

When the light passes through the sample some of the molecules in the sample will absorb lights at

various wavelengths of this spectrum, depending on their chemical bonds and structure.

As a rule energetically favored electron promotion will be from the highest occupied molecular

orbital (HOMO) to the lowest unoccupied molecular (LUMO), and the resulting species is called an

excited state. When sample molecules are exposed to light having an energy that matches a possible

electronic transition within the molecule, some of the light energy will be absorbed as the electron is

promoted to a higher energy orbital (Figure 7).

A spectrophotometer records the wavelengths at which absorption occurs, together with the degree

of absorption at each wavelength. The resulting spectrum is presented as a graph of absorbance

versus wavelength.

Figure 9 Electronic transition of materials



21

Mathematical basis:

The relationship between concentration and absorbance is given by the Beer-Lamberts law:

(1-3)

A = Measured Absorbance

IO= Intensity of the incident light at a given wavelength (without sample)

I = The transmitted intensity (after sample)

L = length of the cell

ϵ = a constant known as the molar absorptivity or extinction coefficient

C= the concentration of the absorbing species



FTIR analysis - Attenuated total reflectance

An FT-IR (Fourier Transform InfraRed) Spectrometer is an instrument which acquires absorption

spectra in the infrared region. Unlike a dispersive instrument, i.e. grating monochromator or

spectrograph, and FT-IR Spectrometer collects all wavelengths simultaneously.

Figure 10 FTIR device with pressure being applied

FTIR analysis was made on the samples to provide more information about the process steps, the

oxidation of graphite and reduction of GO. This is done by showing how the functional groups of the

material changes as the process progresses.

Also it is used as a source of information that works in conjunction with other methods such as XPS

analysis. Both of them can analyze the information of the functional groups but in slightly different

approaches.

Some benefits with using the FTIR analysis method are:

FTIR is a non-destructive method, information could be gained without sacrificing a large part

of the sample.

One other advantage with this method is the high signal to noise ratio, also a benefit if a small

amount of sample available for analysis.

This method gives a high spectral resolution

A FTIR instrument probes vibrations inside molecules. For a vibration to be infrared active i.e. able to

absorb infrared radiation, the dipole moment of the bond must change during the vibration. Bonds

between oxygen and carbon atoms are an example of bonds that are relatively easy to monitor with

FTIR spectroscopy.



XPS / ESCA method

XPS / ESCA method was used to examine the outermost layer of the reduced graphene sheets.

XPS (X-ray Photoelectron Spectroscopy also known as ESCA ( Electron Spectroscopy for Chemical

Analysis), is a highly surface sensitive and powerful tool for chemical surface analysis. XPS providesquantitative chemical information – the chemical composition expressed in atomic % - for the

outermost 2-20 nm surfaces. The analysis depth depends on e.g. the material analyzed, and is about

10 nm for polymers and papers and lower for metal oxides and metals.

X-Ray photoelectron spectroscopy is based on the photoelectron effect discovered by Hertz in 1887.

In this method the surface is bombarded with mono-energetic low-energy X-ray photons, which are

less disruptive than an electron beam. The energy absorbed, resulting in a direct ejection of a core

level electron, i.e. a photoelectron. In the electron emission process, a singly charged ion, M+ is

produced. (Figure 9) By analyzing the kinetic energy of these photo electrons, their binding energy

can be calculated, thus giving their origin in relation to the element and the electron shell (Figure 10).

XPS provides quantitative data on both the elemental composition and different chemical states of

an element (different functional groups, chemical bonding, and oxidation states etc.). All elements

except hydrogen and helium are detected.

Volatile samples cannot be analyzed with XPS due to ultra-high vacuum during analysis (pressure

below 1*10^7 torr).

(AvJan C. J. Bart, Plastic Additives, Advanced Industrial Analysis, P418, IOS press.

Figure 12. X-ray photoemission from a 1s core level and subsequent relaxation process leading to X-ray fluorescence and

Auger electron emissions



25

AFM – analysis

Main task of an AFM

AFM or atomic force microscopy provides a 3D profile of the surface at nanoscale, this is done bymeasuring forces between a sharp probe (<10nm) and a surface at very short distance (0,2-10nm

probe-sample separation). The probe is supported on a flexible cantilever. The AFM tip gently

touches the surface and records the small force between the probe and the surface. (32)

The probe is placed on the end of a cantilever, one could make the assumption that the end of the

cantilever acts as a spring. The force between the probe and the sample is dependent on the spring

constant effect of the cantilever and the cantilever deflection.

(1-5)

F= force

k=spring constant

x= cantilever deflection

If the spring constant of cantilever (typically ~0,1-1 N/m)) is less than the surface, the cantilever

bends and the deflection is monitored. Typical values results in forces ranging from 10-9nN to 10-6µN

in open air. (32)

Figure 13 SEM image of triangular cantilever with probe, image from mikromasch

Instrumentation

The AFM probe has a very sharp tip, often less than 100Å in diameter, at the end of a small

cantilever. The probe is attached to a piezoelectric scanner tube, which scans the probe across a

selected area of the sample surface. Interatomic forces between the probe tip and the sample

surface cause the cantilever to deflect as the samples surface topography (or other properties)

changes. A laser light reflected from the back of the cantilever measures the deflection. This

information is fed back to a computer, which in turn generates a map of topography and/or other

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properties of interest. Areas of large as about 100µm square to less than 100nm square can be

imaged depending on interest.

Depending on the properties of the material which is scanned and the surface that it is fastened on

different resolutions can be received. Also the tip of the cantilever is very important as the ultimate

resolution of an AFM is critically defined and scaled by the radius of the AFM tip. The AFM has abetter resolution in the vertical direction than the horizontal direction. A vertical resolution of

0,001nm can be achieved with a laser sensor. (3)(33)

Figure 14 A typical setup of an AFM instrument

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27

TEM – Analysis

A Transmission Electron Microscope (TEM) is used both for recording high magnification images of

samples but also for crystallographic studies.

Theoretical basis and setup

In a conventional transmission electron microscope a thin specimen is irradiated with an electron

beam of uniform current density. The electrons are emitted in the electron gun by thermionic,

Schottky or field emission. A three- or four stage condenser-lens system permits variation of the

illuminated aperture and the area of the specimen illuminated.

The electron intensity, distribution behind the specimen is imagined with a lens system, composed of

eight to three lenses onto a fluorescent screen. The image can be recorded by direct exposure of a

photographic emulsion or an image plate inside the vacuum or digitally via a fluorescent screen

coupled by a fiber optic plate to a CCD camera. (34)

Figure 15 A typical setup of a TEM instrument

Electrons interact strongly with atoms by elastic and inelastic scattering. The specimen must

therefore be very thin, typically of the order of 5-100nm for 100keV electrons, depending on the

density and elemental composition of the object and the resolution desired. Transmission electron

microscopes offer resolutions up to 0,1nm at 300 and probe diameters up to 0,34nm. (4)



29

Experimental procedure

Oxidation of graphite

Material

Potassium Permanganate>99%; CAS Number: 7722-64-7

Graphite powder, synthetic, -20+100 mesh, 99.9%; CAS Number: 7782-42-5

Graphite flake, natural, -10 mesh, 99.9%; CAS Number: 7782-42-5

Graphite powder, 7-10 micron mesh, 99,9%; CAS Number: 7782-42-5

H3PO4 (85 w %) CAS Number: 7664-38-2

H2SO4 (95-98 w %); CAS Number: 7664-93-9

H2O2 (30 w %); CAS Number: 7722-84-1

Diethyl Ether >99%; CAS Number: 60-29-7

Experimental setup

Heating plate with automatic temperature regulation

Heating bath (CAS, Size and dimension)

Glass beaker (400ml)

Cooling bath ( beaker with ice > > 400 ml)

Amount of time needed for this step

• Approximately 65h

Measurement

Concentration of product; UV-Vis spectrophotometry; Perkinelmer Lambda 650

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Procedure

The basic reaction used to perform the first step was previously reported by Marconi et. al. (35).

First, sulfuric acid was mixed with phosphoric acid in a 9:1 ratio (90 ml H2SO4, 10 ml H3PO4), thesolution was then heated to 50 degrees Celsius. Potassium permanganate was added (4,5g) and then

the solution was left to be stirred for approximately 10 minutes, this to ensure that all of the KMnO4

was dissolved.

A sample was taken before graphite was added and about two minutes into the reaction. These

samples were to be used as zero samples when analyzing the results. Also samples were taken with a

selected interval in time to investigate the possibility of in vitro measurement of progress in the

reaction. In the beginning the samples were taken with a quite small interval (~30min) to compare

with previous work regarding the oxidation. The intervals were later increased to monitor a possible

change in concentration after the alleged reaction time had passed.

All samples except the one in the trial experiment were taken with a pipet which had a large opening

(~1mm), this to ensure that small pieces of graphite did not block the entrance of the pipet. Samples

was weighed and diluted to 1/1000 in weight percentage with double distilled water and cooled in to

4˚C to avoid further reaction. The samples were measured for any change during the time of the

experiments.

The reaction time was not limited to the interval of the previous experiment (35); instead each

experiment was to be left running until there was no change in the reaction. Change being defined as

any changes either visible by the naked eye or measured with an instrument, i.e. change in viscosity,

color or the concentration of a selected substance. As the reaction progressed the reaction mixture

turned from a brown/black color to a pale grey color. At the same time the viscosity of the solution

increased, in order to sustain the stirring the power had to be increased. The increase in viscosity and

other visual changes were observed long after 12 hours had passed.

In a failed batch the water that was used in the heating bath was boiled of. This resulted in the

mixture turning to a pitch black colour, the density and other properties being somewhat unchanged.

In the later washing step of the batch most turned out to be to large particles to pass the filter.

When the reaction was deemed to be finished the reaction was stopped by adding 0, 75 ml H202

which had been mixed with ~100ml of ice. As the reaction between the acids and water produce asignificant amount of heat the procedure was also performed in a cooling bath.



31

Washing of graphene oxide


Approximately 65h

Material

Double distilled water

Hydrochloric acid (36 w %); CAS Number: 7647-01-7

Ethanol (95 w %); CAS Number: 64-17-5

Experimental setup

Glass filter disc Schott-Duran Porosity 1

Titration burette (1l Shott-Duran)

Rotina 420 Centrifuge; Hettichlab

Filter 2µm

MeasurementConcentration of product; UV-Vis spectrophotometry; Perkinelmer Lambda 650

Crystalline structure; XRD analysis Pananlytical X’Pert PRO Alpha – 1 wavelength 1.54059 Å

The structure and conductivity of the GO- flakes, AFM, NanoScope IIIa Multimode, Digital

Instruments

The functional groups of GO by FTIR; PerkinElmer Spectrometer “Spectrum one”

Decomposition properties; Perkin Elmer Pyris 1

Functional groups on surface of GO, Confocal Raman and XPS Kratos AXIS Ultra

DLD

x-rayphotoelectron spectrometer










Procedure

After the oxidation of graphite there exists a highly acidic mixture of reacted and unreacted graphite

together with ions that are left from the potassium permanganate treatment. This mixture makes it

impossible to qualitatively analyze the desired product. Therefore a purification process is needed inorder to study the results from the oxidation step.

As before the washing step used by Marconi et al was used (35).

It should be noted that small changes was made, mainly in order to avoid using a metal sieve when a

chemically inert material was available. Also when diluting the GO between washing steps the

volume of dilatant was not set to a specific amount, instead the volume used was adapted to the

centrifugation equipment. In general about 300ml was used, but in extraordinary cases as for the

natural graphite the total volume in some steps exceeded 1L.

A large glass bottle was used to collect and dilute the produced GO. In general the graphite oxide

solution was diluted to three times the original amount, ~500ml.

After diluting the mixture it was sifted through a combination of a glass filter disc with pore size

between 100-160µm and glass wool. Large non-oxidized particles were thereby removed from the

mixture together as the solvability of non- and partly oxidized graphite is essentially zero.

After filtration the filtrate was centrifuged at 8000rpm for about 2hours and the supernatant

containing unwanted material such as ions and acidic solution was decanted away. The pellet was

then dissolved in another substance and in this way washed in many steps. The first solvent used was

ethanol followed by another ethanol step. After this hydrochloric acid was used, the wash was thenfinished with two water washing steps. To accelerate the process a sonication device was used to

dissolve the GO. By experience some substances were better at dissolving GO than others. While the

ethanol dissolved the GO quite quickly and in a large degree, hydrochloric acid needed more of an

effort to achieve an acceptable solvability. To overcome this obstacle a longer time for sonication

was used and if necessary a small amount of double distilled water was added to the solution. These

steps are repeated to obtain a solution with fewer ions and a more neutral PH, which in turn

promotes the analysis and reduction of the obtained GO. In the case of micron size graphite the last

part of the washed GO was also filtered through a 2µm filter.

After the washing process the remaining GO is dissolved and coagulated in ether, and later movedfrom the solution onto a PTFE membrane with a 0.45 μm pore size. The solid obtained on the filter

was vacuum-dried overnight at room temperature



33

Reduction of graphene oxide


Approximately 24h

Material needed

Graphene oxide

Ammonia 28%; CAS Number: 1336-21-6

Dextrin; CAS Number: 9004-53-9

Dextran (6000Da); CAS Number: 9004-54-0

Ammonium Citrate >98%; CAS Number: 3458-72-8

Experimental setup

Heating plate with automatic temperature regulation

Heating bath

Florence flask

Condensation column

Measurement

Concentration of product; UV-Vis spectrophotometry; Perkinelmer Lambda 650

Crystalline structure; XRD analysis Pananlytical X’Pert PRO Alpha – 1 wavelength 1.54059 Å

The structure and conductivity of the GO- flakes

AFM, NanoScope IIIa Multimode, Digital InstrumentsThe functional groups on GO by FTIR; PerkinElmer Spectrometer “Spectrum one”

Decomposition properties by TG; Perkin Elmer Pyris 1

Functional groups on surface of GO, Confocal Raman and XPS Kratos AXIS UltraDLD x-ray

photoelectron spectrometer


















Procedure

The basic reaction used to perform the first step was first reported by Young-Kwan et al (27)

The obtained graphene-oxide was once again dissolved in water. This was performed by adding GO inwater to a concentration of 1 mg/ml in a small sample vessel made out of glass, and then using a

sonication device to disperse the dried GO. As it turned out the dried GO was quite hard to dissolve

after it had been dried into solid form. By subsequently using sonication and then letting the material

dissolve for at least 10 minutes almost all of the GO could finally be solved into the water. It should

be noted that there always was some GO that didn’t dissolve into the liquid which would appear as

minute solid particles into the solution. Later on the solution was transferred to the reaction vessel

and diluted to a concentration of 0,1mg/ml.

Dextran was added to the solution so that it corresponded to 0,1w% and then the reaction vessel

was mounted in a heat bath at a temperature of 95°C. A condensing vessel was also attached to thereaction vessel to prevent the water from escaping.

The temperature of which the automatic heat plate was calibrated against was the temperature

inside the reaction vessel by using water as a substitute for the reaction. By measuring the

temperature in the water with a regular thermometer against the corresponding temperature of the

oil, a fixed point for the automatic heat plate could be obtained.

The solution was left to be re-boiled in the heat bath for 24 hours, during this time the solution

changes color from brown to black. Also there is some agglomeration of R-GO during the process.

After the 24 initial hours have passed the dextrin in the solution will have reduced the GO to some

degree depending on which GO used in the reaction.

Ammonium solution (25µ) was then added to the solution; this accelerates the reduction process and

drives it forward.

In the beginning of the experiments there was an excessive agglomeration of R-GO particles when

ammonia solution was added resulting in a loss of some GO. As there was not very much material

gained from the oxidation of natural graphite the concentration had to be lowered to conserve

material until the problem was solved. It turned out that by letting the dextran on itself reduce the

GO for 24 hours most of the aggregation could be avoided.

The reactions were carried out in an excessive amount of time compared to reactions mentioned in

the previous work which was around 3 hours after ammonia was added. In general the entire

reaction time was ~72 hours but as with the oxidation time the reactions were judged individually.



35

Annealing of reduced GO


Approximately 4h

Material needed

Reduced GO

Polished steel plate

Inert gas

Experimental setup

Oven with a maximum temperature above 400ᵒC

Flask with a possibility to hold an atmosphere

Silica grease

Measurement

The structure and conductivity of the reduced and annealed GO- flakes

AFM, NanoScope IIIa Multimode, Digital Instruments



Procedure

A small steel plate was polished with sand paper of an increasingly fine grit to create a smoothsurface, a solution containing red GO was then drop casted on the polished surface.

The metal plate was transferred to a bottle which had been additionally sealed with silica greasewhich in turn was placed in an oven which was set to 400ᵒC. After four hours had passed the bottle

was removed from the oven and cooled to room temperature (25ᵒC).

To ensure that the sample should not be contaminated with particles it was kept in the bottle rightuntil the AFM analysis.



37

Results & Discussion

Table 1 shows all of the batches of GO that was obtained from the oxidation process. Each batch was

subsequently named after their completion. Which means that GO batch 1 was named GO01 and GO

batch 2 was named GO02.

Batch 1 Batch 2 Batch 3 Batch 4

Type of graphite Synthetic 8-10 micronSynthetic

Synthetic Natural

Temperature (ᵒC) 50 50 50 50

Amounts (g)

Sulfuric acid (g) 165 165 165 165Potassium permanganate

(g)

4,5 4,5 4,5 4,5

Phosphoric acid (g) 17 17 17 17

Graphite (g) 0.75 0.75 0.75 0.75

Yield (g) 0.8 0.69 0.15 0.53

Designation GO01 GO02 GO03 GO04Table 1 Scheme over the oxidation components

Batch 2 Batch 3 Batch 4 Batch 5Type of

graphite

GO04 GO03 GO03 GO02

Designation Red GO B2 Red GO B3 Red GO B4 Red GO B5

Temperature

(ᵒC)

95 95 95 95

Amounts

Graphene oxide

mg

10 10 10 10

Water (g) 99.9 99.9 99.9 99.9Reducing agent Dextran Dextran Dextran Dextran

Promoter Ammonia Ammonia AmmoniumCitrate

Ammonia

Batch 7 Batch 9 Batch 10Type of

graphite

GO01 GO01 GO02

Designation Red GO B7 Red GO B9 Red GO B10Temperature

(ᵒC)

95 95 95

Amounts

Graphene oxide

mg

10 10 5

Water (g) 99.9 99.9 49.95Reducing agent Dextran Dextran Dextrin



39

UV-Vis Spectrophotometry


During the oxidation of graphite, oxygen attach to the graphene layers and thereby increasing the

polarity of the layers which in turn increases their solubility in water. This results in a change of color

of the solution from a neutral color to a brownish color, depending on the concentration of GO the

brown color can be of different intensities. The GO that is formed has an absorption maximum at ca

230nm for a well oxidized material. (17)

Figure 16 UV-vis spectra showing the evolution of oxidation process of graphite (GO02)

Figure 16 shows the progress of oxidation of micron size graphite (GO02) with time. Initially, samples

were taken every 30 minutes while for later samples this interval increased.

As the reaction progresses, the intensity of the absorption bands at 230nm increases indicating an

increased oxidation of the graphene, this continues until about 50 hours into the reaction. This would

be equal to the orange curve in the graph. After that stage has been reached there is some

unidentified substance which is created as can be seen by an increase in absorbance at 200-230nm.

The samples were taken from the reaction and the reaction stopped by diluting the mixture in a ratio

of 1/1000 with milli-q water. The samples were then stored in 4ᵒC.



Figure 17 Entire UV-Vis spectra of graphene oxide

Figure 17 shows the absorption spectra of GO01, GO02, GO03 and GO04 from 200nm to 500nm. The

image has been limited to enhance the desired interval. The rest of the area follows the same trends

up to 800nm but does not bring any other useful information.

While samples GO01 and GO03 have a similar absorption across the entire spectra this suggests that

for a single material the oxidation result is quite consistent with small variations in the final progress

of the oxidation.

The spectra of GO02 and GO04 have a more defined absorption peak around 230nm. This suggests

that they have been more oxidized according to figure 17. Also it is worth to note that the areas

except from the peak in these latter two are essentially the same. This suggests that a higher

concentration of GO can be reached in a material without increasing the absorption throughout the

spectrum and thereby diminishing the transparency.



41

Figure 18 Different solutions of graphene oxide 1mg/ml, dispersed in milli-q water

Figure 18 shows some samples of produced GO with a concentration of 1 mg GO / 1ml H2O.

Judging from the graph in figure 18 samples GO02 should be the most oxidized graphite followed bysample GO04 whereas samples GO01 and GO03 are substantially less oxidized. Indeed the solutionsmost oxidized according to the UV/vis spectroscopy also are the most colored samples with a darkbrown appearance. At the same time the samples less oxidized according to the UV/Vis spectroscopyare the least colored samples.

Surprisingly there is quite a difference between GO01 and GO03, this difference is not well reflectedon the graph in figure 17. A possibility that this difference has arisen is maybe that a more dilutemixture was used for these samples and the concentration of GO has to be concentrated to a certainextent to absorb enough light to show in these measurements.



Reduction of GO

When GO is being reduced the absorption band assigned to oxidized graphene at ca 230 nm will be

shifted to a higher wavelength with increasing treatment (reduction) time. This is due to the oxygen

attached on the layers being desorbed by the reducing agent (6). As an effect of this, the solution willchange color from brown to black. Figure 20 shows the spectra and Figure 22 images (showing the

colour) of the solutions. In figure 20 the absorption increases at all wavelength as the reduction

progresses and figure 22 shows the visual result of this reduction. The high absorbance at all

wavelengths is in accordance with the visual observation that the solutions become black with time

i.e. the solution absorbs light at all wavelengths.

Figure 19 The reductuion of GO02 seqentially

Figure19 shows spectra taken during the reduction of GO02 by dextran with ammonia added as a

promoter at different reaction times. The graph shows a shift in the absorption peak with time from

the oxidized form of graphite at ca 230nm to the reduced form at ca 267nm.

The figure also shows the general increase of absorption as the solution changes color from brown to

black resulting in an increased absorption in the entire spectra. The increase of absorption in theareas close to 200nm is due to the addition of ammonia during the experiment.

The measurements were made on samples taken from the reaction as it progressed, before

measurement the concentration was diluted from 0.1mg/ml to 0.01 mg/ml. A low concentration of r-

GO was necessary because of the black color of the solution which absorbs a large amount of the

light used in the analysis.



43

Figure 20 Entire spectrum of reduced GO, all measurable species



Figure 20 shows all the UV-Vis spectra of the reduced graphene solutions.

In general most of the red GO solutions has a peak ~267nm which indicates that a substantial

reduction of GO has taken place. This is due to a change in absorption maxima from230nm to 267nm

when oxygen atoms are removed from the graphene layer. (6)

The absorption at 600nm roughly corresponds to the color of the solution with a solution that has a

dark color starting with an initially higher absorption of light in the higher areas of wavelength.

By comparing the difference between the maximal absorption at ¨267nm and the “starting”

wavelength 600nm a relative measure of total concentration of red GO VS minimum absorption has

been evaluated in table 3. It shows that the sample where the strongest tendency of the specific

wavelength ~267nm is red GO B2 followed closely by red GO B5.

Between the other measurable samples it was pretty close between Red GO B3 and Red GO B7,

which could be explained by the fact that they originates from the same type of graphite and had

been reduced under the same conditions.

When considering the batches that were reduced by dextrin and ammonium citrate, more work

remain as for these samples no absorption bands were obtained in the spectra, although visibly there

were changes in color etcetera suggesting that the samples were also reduced but in a lesser degree

than the batches shown in figure 22.

The Samples were prepared by taking a sample from the final reduction product and diluting it in a

ratio of 1/10. Also the largest particles in the solution were allowed to settle for about 5 minutes,

which was done because the instrument showed an unreasonably high absorption at a relatively low

concentration of product. Water with the corresponding amount of dextrin or dextran added wasused as a zero value. The concentration of R-GO was 0.01mg/ml.

Batch Red GO B2 Red GO B3 Red GO B5 Red GO B7 Red GO B9

Starting absorbtion

(A.u)

0,352 0,194 0,420 0,280 0,238

Absorbtion maximum

(A.u)

0,139 0,095 0,173 0,139 0,128

Ratio Am/Sa 2,532 2,042 2,428 2,014 1,857

Table 3 Comparison of starting absorption and absorption maximum of all species of reduced graphene

AM = Absorption maxiumum, Sa = starting absorption



45

Figure 21 Different solutions of reduced graphene oxide 0,1mg/ml, dispersed in milli-q water

Figure 21 shows samples of the red GO produced by the various methods of the reduction process.

The reduction process of GO is very visible with a clear change in color for successful batches. Red GO

B2 which is derived from natural graphite and red GO B5 which is derived from micron sized graphite

is the most obvious examples. Also red GO B7 and red GO B9 which are derived from GO01 showpromising visual changes. In the cases where the reaction has been less successful the material rests

on the bottom of the container instead of being dispersed in the solution.

Figure 22 Absorption spectra of dextran 6000 and dextrin

In Figure 22 the UV/vis spectra of solutions of the polysaccharides Dextrin and Dextran at

concentrations of 0, 1 w% is shown. It can be seen that Dextrin has an absorption peak at 285 nm,

which is in the vicinity of the peak of reduced GO (267 nm). This is a drawback as it will interfere

when trying to map the progress of reduction of any sort of GO. Considering that if the Dextrin is

consumed in a reaction there will automatically be a decrease in that region, that will in turn make it

harder to correctly analyze the change that the absorption peak of GO undergo as it is reduced. The

samples were made by dissolving the polysaccharides in double-distilled water and using also using

the water without GO as a zero sample.



Summary UV-Vis analysis

In summary the UV-Vis investigation shows that the oxidation of all samples of graphite has been

successful. The wavelength changed for all batches of oxidized graphite from ~267nm to the

wavelength of GO ~230nm.

Depending on what type of graphite that was used as a starting material the GO reached different

degrees of oxidation. The GO that was most highly concentrated was that of micron sized synthetic

graphite closely followed by natural graphite. Larger sized synthetic graphite did not quite reach the

same level of oxidation but still managed to show an absorption peak in the known area for oxidized

graphite.

This statement is strengthened with the results from figure 16 of, a continuous oxidation of graphite.As the oxidation of graphite continues absorption of light increases throughout the entire spectrumand a distinct peak arises around 230nm.

The reduction of graphite has a course that is similar to that of the oxidation of graphite withtransition of the absorption peak from 230nm to 267nm. At the same time there is also an additionalincrease in absorption of light as the oxygen atoms are removed from the graphene layers. Thischange is visible in figure 19, where the absorption continues to increase with respect to time and atransition of the absorption peak is clearly visible.

Figure 19 and figure 21 shows this result which is visible to the human eye. Clearly the solutionswhich have had good progress according to the theory show a solution which color is intensivelycolored. Either dark brown color has arisen in the case of GO or a black tint is prominent in these ofthe reduced GO.

The batches that were reduced either by dextrin or with ammonium citrate was not possible toanalyze as the zero sample solution was hard to adjust. Especially in the case of the batch withdextrin (red GO B10) where dextrin itself has a, absorption peak around 285nm which in unsuitedwhen investigating the potential to reduce GO.



47

XPS – analysis

The samples for the XPS analysis were prepared by first polishing pieces of steel on both sides to

remove impurities and then washing them, first with ethanol and then milli-q water.

The dissolved GO was applied with a drop casting technique by using a Pasteur-pipette, the

concentration of the solution was 1mg/ml.

Reduced GO was applied in the same way but as the concentration of the solution was lower than

that of GO (0,1mg/ml) it had to be applied for three times instead of one.

Graphite was prepared in a similar way as the other samples. The graphite was mixed with water by

stirring and then applied to the sample by drop casting. As the water evaporated graphite was

accumulated in a very small area of the sample base creating a good base for analysis.



Surface composition

Table 4 shows the relative surface composition (atomic %) of each part of the process. First the

unoxidized graphite, then the GO product of the oxidation and last the GO that has been reduced by

dextran.

Relative surface composition in atomic %.

Sample C O Si N Zn Na Ca S

Graphite micron 91.1 7.7 0.2 0.2 0.2 0.3 0.3 (<0.05)

Red GO B5 54.8 28.8 14.7 0.4 - 0.1 0.5 0.7

G002 71.7 27.4 0.1 0.4 * - 0.1 (<0.05) 0.3

Table 4 Surface composition in atomic % as determined with XPS

The micron sized graphite contains mostly coals atoms on the surface but also about 7.7 percent of

oxygen. As the measurements are done with a very low pressure atmospheric oxygen should not

contribute to this number. Two similar explanations for these oxygen atoms is suggested:

The first explanation would be that oxygen atoms are intercalated between the coal sheets of the

graphite.

The second explanation would be that oxygen atoms have been absorbed in pockets and fissures inthe molecules and that the pressure that has been applied pressure is not sufficient to remove them.

A fact that is supporting these theories is that the analysis made to investigate the functional groups

in table 4 shows no bond existing between the carbon atoms and the oxygen atoms in the graphite

sample.

The amount of oxygen increases from 7.7% in original graphite to 27.4% in oxidized graphite which

suggests that the oxidation has been successful.

Apparently a large amount of silicon is present in sample Red GO B5, most likely the silica a

contaminant from the silica grease that was used to seal the equipment needed for reducing the GO.

If this is the case, this could explain also the high oxygen concentration (about the same oxygen

concentration as the oxidized sample) of this reduced sample as silica grease.

The reduced GO of sample B5 has a ratio between coal and oxygen atoms that corresponds to the

ratio that exist in dextran. This gives an indication of a successful reduction as the dextran is

supposed to both reduce the GO and also to attach to the surface of the graphene sheets and keep

them dispersed in the solution.

The low amount of residual sulfur atoms also indicate that the wash has been successful and a very

pure solution of reduced GO has been acquired.



49

Functional groups

In high-resolution carbon XPS spectra, chemical shifts in the peak position may yield information on

the chemical state of the carbon, with the binding energy positions for each carbon peak after

adjusting C1-carbon to 285.0 eV as the reference value. The chemical shifts are due to carbons indifferent functional groups with oxygen. Values are from curve fitting of the different carbon peaks

with the total amount of carbon = 100 %.

Examples of functional groups

C1: C-C, C=C, C-H (ref: 285.0 eV)

C2: C-O, C-O-C (ref: 286.5-6 eV)

C3: O-C-O, C=O (ref: 287.9 eV)

C4: O-C=O, C (=O)OH (ref: 289.3 eV)

Table 5 shows how the carbon sheets of micron sized graphite are bonded to oxygen in three

different steps of the process.

SampleC1

285.0 eV

C2

286.9 eV

C3

287.7-288.0 eV

C4

289.2-3 eV

Graphite micron * 100 - - -

G002 ref B5 88.1 7.4 3.0 1.4

G002 59.8 - 31.1 9.1

Table 5 Chemical shift of carbon for each tested substance.

Before the oxidation when the graphite exist in its original state no oxygen atoms seems to be

bonded to the carbon molecules, as this would produce a bond with a lower chemical shift.

After the oxidation of the graphite sheets some other functional groups starts to appear. Thesegroups are double and triple bonded oxygen-carbon groups. The fact that 31% of the bonds arerelated to double bonds and 9.1% are related to single bonds suggests that the oxidation of graphiteto GO have been very successful.



51

Chemical shifts in different XPS signals.

Figure 24 Chemical shift in different XPS signals

Figure 24 shows the assignment of XPS peaks (18), the Chemical shift in different XPS Signals

including the one used to determine the type of bonding in the detailed scans of the carbon peaks.



Summary of XPS – analysis

In summary the XPS analysis has been a useful tool when investigating the surface of the graphene

layers

The oxidation of micron sized synthetic graphite was investigated by comparing the oxygen that is

attached to the graphene layers.

Virgin graphite contains mostly coal atoms which could be expected but also some oxygen atoms

exist in the molecule according to table 4. The presence of oxygen atoms is a little confusing but

when looking at table 5 it becomes clear that none of them is actually bonded to the carbon surface

and thus have to exist in an unbounded state between the graphene layers.

By comparing the virgin graphite it becomes possible to estimate how successful the oxidation has

been. The sample GO02 which is listed in table 4 contains 27.4% of oxygen atoms on the surface of

the graphene layers. Also these atoms are bonded with comparatively strong bonds according to

table 5. This suggests that the oxidation has been successful in the case of micron sized graphite.

Unfortunately no resources were available to analyze another sample of oxidized graphite and the

correlation to the UV-Vis measurements are hard to compare.

The reduced GO sample is quite hard to compare as apparently there were silica present in the

sample. Probably it is a contaminant from the silica grease used for sealing the equipment. Most

likely this would be able to remove with an additional wash but in this case it was not enough time or

resources to try this approach.



53

XRD – analysis

An important part of analysis of crystal structures is the XRD-analysis. By analyzing the various

products it is possible to determine which changes in the crystal structure the graphite undergo in

each part of the process.

In general there are two major changes of the structure.

First of all is there a change of the single peak from ca 27 degrees to a peak around 10 degrees. This

first change during the oxidation is possible to evaluate as the angle of the peak is related to the

interlayer spacing of the material. In this case a shift of the diffraction peak from 27 to 10° mean that

the spacing between the layers increases. As the oxidation is supposed to create spaces between the

layers a large distance (indicated by diffraction peaks at low angles) is a sign of the oxidation being

successful.

Secondly the peak around 10 degrees should disappear when GO is reduced and the structure

thereafter depends on which substance is used as the reducing agent. This change is also possible to

evaluate but in different terms, where the disappearance of the peak at ca 10 degrees being the

most significant. But also the introduction of possible formations could be interesting.

The samples were prepared in different ways but in common for all samples are that the

measurements were done on a dry part of each substance.

The GO was collected from the part of the samples that had been vacuum dried after the last stage

of the washing. Thereafter the sample was placed in a small holder for analysis of small quantities,

always with an as smooth surface as possible in the area of analysis. The graphite used weretransferred into a container for powder samples and pressed with a moderate pressure to create a

smooth surface. Lastly the r-GO was samples from the part of the solution that had sediment and

dried in the vacuum oven. Like the dried GO a very smooth surface was created during the drying in

the oven.



x-ray diffractograms

This following part will show some diffractograms obtained by the XRD measurements and some

information and comments regarding those figures.

Figure 25 GO03, red GO B3 and synthetic graphite

Figure 25 shows XRD diffractograms of GO03, Red GO B3 and Synthetic graphite The figure shows the

oxidation of synthetic graphite to GO and the subsequent reduction of GO03 to r-GO B3.

During the oxidation of graphite there is a disappearance of the peak at 26.45 degrees and the

appearance of two peaks one weak at 19.36 and another intense at 9.6 degrees. These changes are

expected and reflect the increased spacing between the layers in graphite upon oxidation.

Also some minor peaks at 37 degrees and 29 degrees are visible. After the reduction, the intensity

decreases dramatically, however there are several smaller peaks in the diffractogram, suggesting

that some unwanted structures in the lattice of the graphene sheets remain during the entire

process.

0

100000

200000300000

400000

500000

600000

700000

800000

900000

1000000

0

5000

10000

15000

20000

25000

30000

35000

40000

0 10 20 30 40 50

I n t e n s i t y ; S y n t h e t i c g r a p h i t e

I n t e n s i t y ;

G O 0 3 a n d R e d G O B 3

Angle (2θ)

XRD analysis - Synthetic graphite

GO03 Red GO B3 Synthetic graphite



55

Figure 26 XRD curves of micron sized graphite,GO02 and red GO B2.

The graph shows the oxidation of natural graphite to GO02 and the reduction of GO02 to r-GO B5.

During the oxidation phase, the most intense peak in the native material at ca 27° disappears. At the

same time many new peaks in the material appear where the most intense peak appear at ca 10°

indicating that for the larger part of the sample spacing between the layers increases. However, the

result is not as good as for the synthetic graphite shown in figure27. An explanation to these could

be the small difference between the filter used and the smallest size of the particles which it was

supposed to filtrate (2µm to 5µm). As the particles are oxidized they are also broken up in smaller

pieces, some of them may have been able to pass through the filter.

After GO02 have been reduced by dextran the peak at 10° disappears. This indicates that the

symmetrical distance which exists in the GO is replaced by another structure, thereby proving that

the reduction has been successful (1).

Summary of XRD – analysis

The results from the XRD analysis are in a stark agreement with each other

The first stage where the peak around 27 degrees disappears and the peak at around 10 degrees

appear (2θ) is visible for all the samples of oxidized graphite. The differences between them are so

small that it suggests that the oxidation has been successful in all cases and only small differences in

this part of the process exist.

One exception of this rule may be the batch GO02 where more impurities seem to exist in the sample

(figure 28). This may depend on the small difference between the sieve used for the wash and the

virgin graphite.

0

50000

100000

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200000

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16000

0 10 20 30 40 50 I n

t e n s i t y ; S y n t h e t i c g r a p h i t e m i c r o

n

I n t e n s i t y ; G O 0 2 , R e d G O B 5

Angle (2θ)

XRD- analysis; Synthetic graphite micron

GO02 Red GO B5 Graphite micron



TG – analysis

The TG samples were prepared in the same way as the samples that were used for the XRD-

measurements.

The TG-analysis gives a lot of information about the compound tested. By making the measurements

it is possible to understand the thermal stability and other properties correlated to the thermal

stability, such as the ability of the reducing agent to restore the crystal structure of the graphene

sheets which may be altered during the oxidation.

Graphite is a quite thermally stabile compound with a decomposition temperature of ca 600ᵒC in

comparison to GO which has a decomposition temperature of ca 200°C (Marconi, o.a., 2010). This

difference is due to the oxygen atoms that have been added to the sheets in the GO.

When GO is being reduced the oxygen is removed from the surface and the decomposition

temperature is supposed to increase. Then, depending on what substance that is used to reduce the

GO and if it acts as a dispersant, the first decomposition should approach that of the reducing agent.

There should also be a decomposition temperature in the area of 600ᵒC as the graphene should be

similar to that of graphite.

The more the decomposition temperature of r-GO resembles the one of graphite instead the one of

GO the more likely is it that the sample has underwent a complete reduction.



57

Thermal decomposition curves

Figure 27 Thermal decomposition curves of GO02, red GO B5 red GO B10 and micron sized graphite

In Figure 29, the micron sized graphite shows the classical behavior of that of graphite and starts to

decompose at ca 600ᵒC.

The GO02 has a very high reduction of weight at 214ᵒC. Previous work shows that when warming

samples faster than 1ᵒC/min produces a reaction powerful enough to remove an excess of the

sample from the pan resulting in inaccurate scans. It is also shown that at about 200ᵒC the strongest

signal in the gas stream is that of CO2. This indicates that the decomposition of oxygen containingfunctional groups, rather than the vaporization of intercalants, causes the reaction. (2)

This is probably what happened in our case. Due to a rate of heating the decomposition of functional

groups was to fast thereby removing a large amount of the available material.

Although the weight loss probably is smaller in reality, the temperature at which it sets in and the

later decomposition temperature of the sheets should still be correct.

When looking at the temperature where the remaining mass is lost (ca 550 °C) it is easy to see it is

significantly lower than that of the other types of graphite. This may be due to the fact that the

sheets are less ordered and thereby more sensitive to heating. In relation with the XRD results, it isprobable that the GO contains some structures that are somewhat ordered but not thermally stabile.

If one compares the thermal decomposition of red GO B5 which have been reduced by dextran with

the decomposition of red GO B10 which have been reduced by dextrin some interesting differences

appear. The GO that has been reduced by dextrin seem to be more thermally stable, it is only in the

end that the high temperature takes its toll and most of the red GO B10 decomposes.



Figure 28 Thermal decomposition curves of Natural graphite, GO04 and red GO B2 A large weight loss of GO sets in at 217ᵒC, which for the GO samples is the highest temperature in

which the initial weight loss starts. This would imply that the sheets are less oxidized than the other

GO samples and is in stark opposite to the UV-Vis results. The most probable explanation is that the

graphite pieces of which the GO origins from is very large, which affects the thermal stability of the

produced GO. (14)

The r-GO shows a curve that is very similar to the one of r-GO B5 in figure 27, but with a final weight

that is about 4% higher. This suggests that the flakes have a more stable structure which makes them

slightly less sensitive to heat.

Figure 29 Thermal decomposition curves of synthetic graphite, GO01 and red GO B7 and red GO B9

Figure 29 shows the thermal decomposition curves of synthetic graphite, red GO B7 and red GO B9.



59

The red GO of this measurement of the same sort graphite which makes it interesting to compare

them The only difference between them is that more ammonia has been used in batch red GO B9.

Of all the r-GO samples measured r-GO B9 shows the strangest result. With three different phases of

weight loss and then a small weight gain in the end it is hard to say anything about the result. Red GO

B9 otherwise has a visual appearance of a very well dispersed solution and a significant change incolor, from the golden brown color which is typical for GO to a black color that seems to be inherent

of GO that has been reduced by dextran (13). Coupled with UV-Vis analysis strengthening the

conclusion that reduction was successful this is a surprising result that should be investigated further.

Sample r-GO B9 has a rather large weight loss in the area of 330ᵒC follow by an even decline of the

remaining weight. It has some of the traits of a completely reduced GO but still lacks the plateau

that should exist between 400ᵒC-600ᵒC.

Figure 30 Thermal decomposition curve of Dextran

Figure 30 shows the thermal decomposition curve of solid form dextran

This picture shows the curve as a reference, with a major weight reduction at around 334ᵒC and a

steady reduction after to finally disappear completely by 825ᵒC

0

20

40

60

80

100

25 225 425 625 825

W e i g h t ( % )

Temperature ̊ C

Dextran 6000

Dextran 6000



Summary of TG – Analysis

The results from the TG-analysis show that the GO decomposes around 220ᵒC for all the samples. As

the rate of temperature increase was probably was too high for this analysis the curves shows asteep decrease in weight for the GO, which should have been more gradual.

One interesting fact gained from the thermal decomposition curves of GO is in figure 29. As micronsized graphite is of a relatively small size the thermal stability of the formed GO suffers which showsby the non-existence of GO beyond 625 degrees C. Also the natural graphite with very large flakesshows some increased thermal properties in figure 30.

Judging from the figure 29, 30 and 31 the ability of reduced Go to withstand thermal decompositionseems to be best for red GO B5, Red GO B2 and red GO B7.



61

AFM analysis

Figure 31 Topographical picture of GO01 and a line scan

This figure shows a topographical image of sample GO01. The sample was prepared by dissolving

10mg of GO in water by sonication and then letting it sediment for approximately 30 min. The clear

part of the solution was then drop-casted on a piece of Mica.

Most of the pieces studied consisted of single layer, this is proven by the fact that the height of

delaminated GO is approximately 1nm. In some cases it seems like the sheet is 2nm or more thick.

Whether it is just single layer which have been stacked on top of each other or if it is delaminated

material is hard to say. The flakes are fairly large with a lateral size up to 3µm but the size and shapes

is varying.



The reason why just one of the batches of oxidized graphite was studied was that the resources in

the form of assistance of using the AFM was limited to a certain amount of hours. As the AFM is a

sensitive instrument which requires a lot of training and uses expensive tips for the measurement

using it alone was deemed inappropriate.

As GO01 was the first sample that completed the oxidation process it was also the sample that wasdetermined to be tested to verify if the method of producing GO was successful or not. One

perquisite was that the GO had to be single layers as it was deemed important for further

applications.

Figure 32 Topographical picture of red GO B5 before conductive sweep

Figure 32 shows a topographical mapping of sample red GO B5 made by the AFM instrument. This

figure is a topographical picture of reduced and annealed GO B5 and shows that as the material dries

on the mica some point wise agglomeration appears which created an uneven structure with some

spread peaks as evidenced by the bright fields in the image.



63

Figure 33 Layered picture of red GO B5

Figure 33 is a layered picture that shows the topography in combination with the conductivity of an

annealed R-GO sample.

The conductivity is depicted as different scales of brightness in the picture. A dark color of the picture

shows an area with low conductivity whereas a bright color shows an area with high conductivity.

It becomes apparent that areas with a high topography have a lower conductivity compared to the

areas with low topography. The reason why has not been investigated in detail as there were not

enough time to study this, however one plausible reason may be that the agglomerates either are

slightly porous decreasing the effective conductivity or that it is just an effect of the thickness.

The sample was made by drop casting an r-GO solution (0,1w % r-GO) on a piece of polished steel

and annealing it in a vessel at 450ᵒC. The vessel was filled with an inert gas in the form of nitrogen.



Figure 34 AFM topographical picture of red-GO B5

This figure 34shows the topography of red-GO-B2. Between some areas with unidentified

topography sheets of red GO appears as discs of varying sized spread out over surface. The sheetsappear to vary in size from a range of a few nanometers up to a micrometer in diameter.



65

TEM – analysis

Transmission electron microscopy (TEM) was performed using a JEOL JEM- 3010 microscope. TEM

samples were prepared by placing a drop of graphene oxide/reduced graphene oxide dispersion on a

carbon coated copper grid and subsequently evaporating the water.

Figure 35 TEM image of red GO B5

Figure 35 shows a TEM image of a few layer thick graphene oxide sheets.

The TEM image of the reduced graphene flakes are of a similar shape as in the image acquired from

the AFM measurements (picture 37). The TEM – information works in conjunction with the

information from the AFM measurements, mainly about the size and shape of the particles but also

about how many layers the particles may have.



An ocular analysis of the layers in figure 46 shows that the red GO molecule of batch 5 seems to have

at least four layers but the total amount of layers is impossible to determine.

The shape of the particles is similar to the sheets in picture36 and shows that the red GO sheets

produced from micron sized GO is disc shaped particles.

In some cases during the oxidation process the sheets may fold due to the strong acids that are used

as intercalating agents.

However graphene sheets without any folds could also be observed in the TEM pictures. Whether it

depends on an effect of the layers stabilizing the sheets or a reduction process that restores the even

layers of virgin graphite is unknown.

FTIR – analysis

The picture at figure 38 shows the experimental setup (ATR) for the FTIR measurements. The samplewas placed upon the ATR crystal and the infrared sample scanned the entire adjacent sample

surface.

Figure 36 ATR crystal and sample

FTIR has been used to analyze the samples to investigate the presence of functional groups, in

particular oxidized groups.

To make it easier to write out every bond that is discussed some abbreviations has been used.

For a single bond both types of atoms that are included in the bond are written out and

the bond is represented with a dash; for example C-O for a carbon-oxygen single bond.

For a double bond both types of atoms that are included in the bond are written out and

the bond is represented with an equal sign; for example C=O for a carbon-oxygen double

bond.

Atoms that may be represented are C (Carbon), O (Oxygen), H (Hydrogen), Si (silicon) and N

(Nitrogen).



67

Figure 37 Graphite

Figure 37 Shows a FTIR-spectrum of graphite from an article written by Knower et al. (20)

The ATR crystal used in the lab was made out of diamond. If instead a crystal made out of

Germanium had been available it would probably have been possible to analyze the graphite, and

also to get better spectrums when analyzing the red GO. This is possible to perform with a

Germanium crystal because it has a high refractive index. As a result of this property the effective

depth of penetration is very low, approximately 1 micron. For most samples this will result in a weak

spectrum being produced, however this is an advantage when analyzing highly black materials;

carbon black and graphite etc. (21)

As the device in the laboratory was not suited to analyze graphite a reference spectrum from

literature had to be used, see Figure 37. This spectrum shows the multiple vibration peaks of the C=C

bonds which lies between 1400cm-1 and 1600cm-1. Also a peak around 3500cm-1 exist which is

correlated to the stretching vibration of O-H bonds, which would be because of the water absorbed

by the graphite.

Although these bands seem to be strong in the spectrum above they are only medium-weak instrength. Therefore other functional groups that may appear due to chemical changes may hide

these peaks. (21)



69

FTIR spectra

FTIR spectra of Synthetic graphite and associated samples

Figure 39 FTIR spectra for dextran, GO01, red-GO B7 and red-GO B9.

This figure(Figure 39) shows the FTIR spectra of Dextran, GO01, Red GO B7 and Red GO B9.

As the dextran molecule have been discussed and explained underneath figure 40 most of the

arguments will be related to GO and reduced GO.

If beginning with GO01 there is first a group of 3 adjacent peaks. First there are an alkoxy peak (O-H)

at 1057cm-1 but also an epoxy peak (C-O-C) at 1225cm-1 and a carboxy(C-O) group at 1407cm-1.

Also there are a group of two peaks which are the C=C aromatic peak at 1618cm-1 and a

carbonyl/carboxyl (C=O) peak at 1725cm-1. (23)

Most of these peaks are related to the bonds that oxygen atoms form with the graphene sheet

during the oxidation process, with the exception of the aromatic peak. Generally one can say thatthe higher the peaks are compared to the aromatic peak the more oxidized is the material.

The peaks below 1000cm-1 is more or less identical for all the samples with the strong peaks of the

dextran molecule hiding all the other peaks. Also the peak around 3250cm-1 is less relevant for the

interpreting of the results than the peeks situated between 1200cm-1 and 2000cm-1. Therefore the

figures showing a more general view of the sample will only be used if the product itself differs from

the other samples in these areas.



Figure 40 Enlargement of the spectrums between 1200cm-1 and 2000cm-1 for dextran 6000, GO01, red GO B7

and red GO B9.

Figure 40 shows enlarged areas of interest from the previous figure (Figure 39)

From comparing the spectra from samples Red GO B7 and Red GO B9 it becomes evident that the

end result differs as both position and intensities of the bands is completely different for the twosamples.

The sample red GO B7 follows the desired pattern with a C=C peak becoming one of the most

prominent peaks and the peaks representing oxygen associated functional groups diminishing.

This shows that red GO B9 probably had been reduced by the dextran to some extent.

Red GO B9 shows a completely different result with the peak associated to the C=O double bond

being the most prominent peak. This would suggest that red GO B9 had been poorly reduced.

However both figure 20 and 21 suggests that the samples have been reduced to a similar extent,

with red GO B7 being slightly more successful. In retrospect it is hard to predict what went

wrong with the sample red GO B9.

From the start GO01 has a quite even ratio between the peaks representing the C-O double bond

at 1750cm-1 and the bond representing the C-C double bond at 1610cm-1, the C=C peak being

approximately 20% larger. This shows that GO01 have been quite extensively oxidized during

the first step of the oxidation process.

During the reduction of GO01 there are two very different types of results if one interprets the

FTIR spectrum of Red GO B7 and Red GO B9.

Red GO B7 seems to have been reduced quite well. The C=C/C=O ratio have increased from 1.2(GO01) to a number that approaches infinity for red GO B7.



71

It was indicated in figure 20 that the GO01 had been reduced quite well in the Uv-Vis analysis.

This gives more credit to this conclusion. The peak that exist at 1430cm-1 is probably also a

peak related to the C=C bond, which should be more significant as oxygen is desorbed and the

sheet structure restored. (21)

The red GO B9 shows an IR-spectrum that does not make very much sense if compared to theUv-Vis measurements (figure 20) which indicate a very good reduction of GO01. The reaction

should in theory have been similar to that of Red GO B7 but in the IR-results it seems to have

gone the other way.

No explanation of this reaction pathway could be thought of.

Figure 41 Enlargement of the spectrums between 1200cm-1

and 2000 cm-1

for dextran, GO03, red GO, B3 and red GO B4

Figure 41 shows the IR spectrum of GO03, Dextran, red GO B3 and red GO B4.

First of all, there is a large similarity in the spectra of GO03 to the spectrum previously presented for

sample GO 01, indicating that the oxidation of these two samples were similar. Further, comparing

the spectrum from red GO B3 and Red GO B4 that the spectra are very similar, indicating that the

same (or close to) reduction was obtained for both samples. There is a difference in which promoter

that is used in the reduction but that is not reflected at all in the results. When compared both of the

peaks show only a minimal difference.

As the sample reduced by ammonium citrate (red GO B4) was not possible to characterize with UV-

vis spectroscopy in a very good way this is encouraging news. This could mean that although it was

not possible to distinguish in the UV-Vis it is still possible to reduce GO with ammonium citrate.



FTIR spectrum of micron sized synthetic graphite and associated samples

Figure 42 FTIR spectra for dextran, GO01, red-GO B5 and red-GO B10.

Again, the spectrum of sample GO 02 is fairly similar to the spectra of the GO presented above.

However, when examining the spectrum of sample red GO B5, the spectrum is very contradictive to

the results from the UV-Vis measurements.

The reduction of the top at 1720c m-1 compared to the top at 1650 c m-1 is not visible in figure 42.

Instead it looks like the peak at 1650c m-1 vanishes. This would show that the sample has not

been reduced, but rather that the amount of C=O double bonds groups has increased. As we

know from the XPS measurements in table 5 that this is not the case and a contradiction exists.

It is hard to say what why a sample that has been judged to be reduced by the UV-Vis analysis

and the visual appearance does not give an equally good result in the FTIR measurements.

Probably some containment or residue has affected the reduction process. Although this is not

desired in a real process it shows that this method is somewhat robust against pollution in

general.

This case has some resemblance to the case of red GO B7 VS red GO B9.

Probably more measurements have to be made to determine these results. As the opposite result

of which you would expect after a successful reduction appears in more than once case it is not

just a single incident. Also it occurs in two different types of red GO which suggests that it has

more to do with the method of reducing GO than the starting GO itself.

The red GO B10 has an intense peak around 2690-2840cm-1 suggesting that it contains a lot of C-H

groups. The source of this peak is also unknown.



73

Figure 43 Enlargement of the spectrums between 1200cm-1 and 2000cm-1 for dextran 6000, GO02, red GO B5

and red GO B10.

In the oxidized material (GO 02) the peaks representing double bonded carbon and double

bonded oxygen is quite even.

The reduction of the GO to reduced form is not easy to explain. It would be expected that red GO

B5 would have reduced the peak at 1720c m-1 compared to the top at 1650 c m-1. Instead it is the

opposite situation that is expressed by the analysis. Both by the visual appearance and the UV-

VIS measurement red GO B5 looks like a god candidate. It could be that the silica gel that

probably contaminated the solution affects the results.

Summary FTIR – Analysis

In summary the FTIR analysis produces a lot of information regarding both the oxidation of graphite

and the reduction of GO.

Most samples of GO seems to be well oxidized with bindings representing the double bond between

coal atoms and oxygen atoms in relatively large amounts. The one with the most bonds in relation

being GO02 (figure 43) followed by GO01 (figure 39).

When comparing the Red GO the samples get very hard to compare. Generally it seems that double

bonded oxygen atoms have been reduced compared to double bonded carbon which should be a

sign of a successful reduction but there are too many and prominent exceptions to state much fromthese measurements.



Conclusions


The oxidation of the graphite was continuously monitored with UV-Vis spectroscopy and shows that

all samples have reached the desired wavelength around 230nm for oxidized graphite but the type of

virgin graphite is important for the final result. Micron sized graphite shows the highest

concentration of dissolvable GO for an equal concentration followed by natural graphite and then

the larger size synthetic graphite.

Two batches with the same type of graphite were prepared and they reached a near equal UV-Vis

appearance which strengthens the argument that the starting material is a very important

parameter, more so than exact reaction time and other variables.

When it comes to the XPS analysis only one oxidized sample was tested i.e. GO02. The analysis of

GO02 shows that the oxidation of the graphene layers was successful with the amount of bonded

oxygen increasing from 0% to 27.4% in the selected sample. Also the amount of sulfur atoms of the

surface is only 0.3% which shows that the wash was highly successful in removing unwanted

substances.

The oxygen atoms in GO02 were bonded to the surface of the graphene layers in different bonds

with single carbon oxygen being more common and more complex carbon oxygen bonding being

relatively rare.

The TG samples confirmed that the thermal stability of the GO had decreased significantly in all the

samples due to the oxygen bonds. At 220ᵒC the thermal decomposition starts compared to 600ᵒC for

virgin graphite. The graphite which was most sensitive to heat was GO02, whether it was due to the

small size of the sheets making them less thermally stable or that the oxidation had been more

successful is hard to predict.

Lastly the FTIR measurement shows that the ratio between the C=O double bonds and C=C double

bonds has increased significantly. This was true for all samples with micron sized being having the

highest ratio and GO03 is having the lowest ratio.



75

Reduction of GO

The UV-Vis analysis shows that it was possible to use dextran to reduce GO this was evidenced by a

shift in the absorption band from 230nm for GO to 267nm for the reduced samples. This is

happening continuously during the reaction and for the chosen method lasts for about 75 hours.

All types of GO reached the desired wavelength but the baseline for the absorption and the degree of

how distinct the transited peak was different for the different samples.

The most successful batches of red GO according to the UV-Vis results and the visual appearance was

Red GO B2, Red GO B5 and red GO B7. These samples were also very dark indicating a high

concentration of dispersed red GO.

The reduction of GO was also investigated by XPS. As resources was limited the only sample that was

analyzed was red GO B5, GO derived from micron sized graphite reduced by dextran. As the sample

was most likely contaminated by silica grease there is not much to tell from this sample.

According to the XRD analysis there was a clear change in the crystal lattice during the reduction

process. The peak around 10ᵒ (2θ) disappears for all samples and a slight bump appears around 20ᵒ

(2θ). This is a sign that the clear crystal structure that existed in GO disappears and is replaced with a

less ordered structure. This shows that the oxygen atoms on the surface of the graphene layers are

replaced as the reaction continues and that the flakes gets dispersed or arrange them self in a less

ordered fashion than in native- or oxidized graphite. (13)

A reversed reaction also takes place when the GO is being reduced. The thermal stability is increased

to a level somewhere between that of GO and graphite as evidenced from TG measurements. This istrue for all samples of reduced GO, although slight differences exist between the different batches.

The AFM analysis shows the molecules of red GO 5 as disc shaped particles instead of the “ice shard”

appearance that the GO of red had. It is hard to estimate if this is due to the reducing process or ifsome difference between the virgin graphite shape remains. Unfortunately no more time wasavailable to investigate this.

To support the AFM measurements a TEM analysis was made on red GO B5. The size of the particlesis very similar to that of the AFM measurements which shows that the shape of the particle with allcertainty is disc shaped. Also no ridges or abnormalities are visible on the picture which suggests that

the method have been relatively gentle to the structure of the graphene layer.

The FTIR measurements are difficult to evaluate as other peaks are very prominent and hides somepeaks of interest. In general the spectra is showing a reduction of the C=O double bond compared tothat of the C=C double bond but sadly exceptions do exist on important batches such as the one ofred GO B5.

Annealing the red GO samples seem to enhance the conductivity in the graphene samples accordingto the AFM measurements.



Summation

The type of starting material has a big impact on the final result of the red GO. Although all samples

could be oxidized and then reduced according to the UV-Vis analysis the extent of these reactions

varied.

By using micron sized synthetic sized graphite instead of larger synthetic sized graphite a higher

concentration of water dispersed GO and red GO was attainable. This could suggest some size

dependency of the reactions.

One exception of this would be the natural graphite which also was able to produce solutions with

relatively high concentrations of GO and red GO. This suggests that also the structure of the graphite

is important in addition to the size of the flakes.

If choosing which type of graphite which is most suitable to produce red-GO with these chemical

methods micron sized graphite is probably the best candidate.

This conclusion is due to the fact that when oxidized and subsequently reduced, micron sized

graphite produced solutions with relatively high concentrations of GO and red GO.

This would imply that GO derived from micron sized is very well oxidized and thereby water soluble.

It also implies that the red GO also easily dispersed when reduced by dextran and theoretically that

the reduction was succesfull.



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Appendix

XRD

Figure 44 GO04, red GO B2 and Natural graphite

Figure 45 XRD curves of GO01 and synthetic graphite

0

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a t u r a l g r a p h i t e

I n t e n s i t

y ; G O 0 4 a n d R e d G O B 2

Angle (2θ)

XRD analysis - Natural graphite

GO04 Red GO B2 Natural graphite



83

Figure 46 XRD curves of micron sized graphite, GO02 and red GO B10

0

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0 10 20 30 40 50 I n t e n s i t y ; s y n t h e t i c g r a p h i t e

i c r o n

I n t e n s i t y ; G O 0 2 , a n d R e d G O B 1 0

Angle (2θ)

XRD- analysis; Synthetic graphite micron

GO02 Red GO B10 Graphite micron



TG

Figure 47 Thermal decomposition curves of GO03, red GO B3 and red GO B4

The curve of r-GO B3 looks like it has been very poorly reduced. Judging from the curve there is

weight reduction as early as 175ᵒC, which slowly transits into a larger loss at 300ᵒC. After that there

is a small weight until the end temperature.

Towards Graphene Based Transparentconductive Coating

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