POLITECNICO DI MILANO

School of Industrial and Information Engineering

Department of Chemistry, Materials and Chemical Engineering

“Giulio Natta”

Master of Science in Materials Engineering and Nanotechnology

Ascorbic Acid GO Reduction as Alternative rGO Production

Supervisor: Prof. Luca MAGAGNIN Co-Supervisor: Ing. Lorenzo PEDRAZZETTI

Master degree thesis of:

Boquan CHEN Matr. 835889

Academic year 2016/2017

I

TABLE OF CONTENTS TABLEofCONTENTS...............................................................................................I

LISTOFFIGURESANDTABLES..............................................................................IV

ABSTRACT.............................................................................................................1

INTRODUCTION.....................................................................................................3

CHAPTER1 STATEOFTHEARTS......................................................................51.1Graphene................................................................................................................51.1.1Structuresofgraphene...........................................................................................61.1.2Propertiesofgraphene...........................................................................................81.1.2.1Electricalproperties.......................................................................................................81.1.2.2Opticalproperties...........................................................................................................91.1.2.3Thermalproperties.........................................................................................................91.1.2.4Mechanicalproperties..................................................................................................101.1.2.5Chemicalproperties.....................................................................................................10

1.2Grapheneproductionmethods.............................................................................111.2.1Exfoliation.............................................................................................................111.2.1.1Mechanicalexfoliation.................................................................................................111.2.1.2Liquid-phaseexfoliation...............................................................................................121.2.1.3Gas-phaseexfoliation...................................................................................................13

1.2.2Chemicalvapordeposition(CVD).........................................................................131.2.3SiCHeatingandEpitaxialgrowth..........................................................................151.2.3.1SiCheating....................................................................................................................151.2.3.2Epitaxialgrowth............................................................................................................16

1.2.4Electrochemicalmethod.......................................................................................171.2.5ArcDischargeMethod..........................................................................................181.2.6Organicsynthesis..................................................................................................191.2.7Carbonnanotubes(CNTs)cutting.........................................................................201.2.8Reductionofgrapheneoxidemethods.................................................................201.2.8.1Oxidationofgraphite...................................................................................................211.2.8.2Reductionofgrapheneoxide.......................................................................................23

1.3Grapheneoxide.....................................................................................................301.3.1GOproduction.......................................................................................................30

II

1.3.2GOstructureandproperties.................................................................................301.3.3GOapplications.....................................................................................................31

1.4Chemicalreductionmethod..................................................................................321.4.1Hydrazinehydrate.................................................................................................321.4.2Sodiumborohydride.............................................................................................341.4.3Ethylenediamine...................................................................................................351.4.4Sodiumcitrate.......................................................................................................351.4.5L-cysteine..............................................................................................................361.4.6Ammoniaandammoniavapor..............................................................................361.4.7Hydroiodicacid.....................................................................................................36

1.5Chemicalreducedgrapheneoxide.........................................................................371.5.1Properties..............................................................................................................371.5.2Applications...........................................................................................................381.5.2.1Energystorage..............................................................................................................381.5.2.2Transparentconductiveapplications...........................................................................401.5.2.3Graphenecomposites..................................................................................................411.5.2.4Sensor...........................................................................................................................411.5.2.5Semiconductor.............................................................................................................43

1.6Supercapacitor......................................................................................................431.6.1Supercapacitors.....................................................................................................431.6.2NiObasedsupercapacitor.....................................................................................46

CHAPTER2 MATERIALSANDCHARACTERIZATIONMETHODS.......................472.1Grapheneoxide.....................................................................................................472.2Chemicals..............................................................................................................472.2.1Ascorbicacid(AA)..................................................................................................472.2.2Ammonia...............................................................................................................482.2.3Nickelsulfamate....................................................................................................48

2.3X-rayinspection....................................................................................................482.4SEM.......................................................................................................................492.5Raman...................................................................................................................502.6Surfacechargeanalysis.........................................................................................522.7Potentiostat..........................................................................................................522.7.1Anodicoxidation...................................................................................................532.7.2Cyclicvoltammetry................................................................................................53

III

CHAPTER3 EXPERIMENTALWORKS..............................................................553.1ChemicalreductionofGOandGOHbyascorbicacid(AA)......................................553.1.1Precursorpreparation...........................................................................................553.1.2Chemicalreductionwithammonia.......................................................................553.1.3Chemicalreductionwithoutammonia.................................................................563.1.4Graphenedepositionpreparation........................................................................563.1.5Characterizationanddiscussion...........................................................................563.1.5.1Characterizationofsurfacecharge...............................................................................563.1.5.2CharacterizationofRamanspectrum...........................................................................57

3.2Electrochemicaldepositionofnickel/rGOcomposite.............................................613.2.1Electrochemicaldeposition...................................................................................613.2.2Anodeoxidationofnickellayer.............................................................................623.2.3Characterizationofthickness................................................................................623.2.4Characterizationofcyclicvoltammetry................................................................633.2.5CharacterizationofSEM........................................................................................67

CHAPTER4 CONCLUSIONS............................................................................71ACKNOWLEDGEMENT..........................................................................................73REFERENCES........................................................................................................75

IV

LIST OF FIGURES AND TABLES Figure 1 Structure of different isomer of carbon: fullerene, carbon nanotube and graphene

sheets. ........................................................................................................................... 5Figure 2 Ideal graphene structure. ...................................................................................... 6Figure 3 (a) Rippled graphene; (b) wrinkled graphene and (c) crumpled graphene. ........ 7Figure 4 Theoretical band structure of graphene in first Brillouin zone. The valence and

conduction bands meet at the Fermi level. ................................................................. 8Figure 5 Exfoliation of graphene by tape. ........................................................................... 11Figure 6 A) solvent ions intercalation, weaken the interlayer interaction; B) graphite

compound ions are substituted by solvent ions; C) Pure sonication treatment of

graphite. ...................................................................................................................... 12Figure 7 Single-crystal monolayer graphene grown on a hydrogen-terminated Ge(110)

surface. ........................................................................................................................ 14Figure 8 Possible mechanisms of CVD producing graphene. ........................................... 15Figure 9 Basics of graphene growth by thermal decomposition of SiC and structural model.

..................................................................................................................................... 16Figure 10 Simulation for the mechanism of carbon atoms emerging from metal substrate.

..................................................................................................................................... 16Figure 11 Different method of electrochemical process to exfoliate graphite sheet. ........ 17Figure 12 Organic synthesis of tri-perylene bisimides by tetrabromo-perylene bisimides

monomer. .................................................................................................................... 19Figure 13 Mechanisms for intercalation and unwrapping of multiwall nanotubes. ....... 20Figure 14 Synthetic routes of graphene oxide and graphene. ........................................... 21Figure 15 (a) Scheme of GO peparation from graphite; (b) Scheme of GO sheet............. 21Figure 16 Scheme of GO sheet and the possible oxigen-containing groups: ................... 30Figure 17 (a)Graphene without functional group; (b)graphene oxide; ........................... 38(c)reduced graphene(containing residual functional groups and defects). ..................... 38Figure 18 DNA sequence detecting by measuring (a)ionic current; (b)tunnelling current;

(c)in-plane current; .................................................................................................... 42(d)in-plane current affected by DNA physisorption. ........................................................ 42Figure 19 Charge storage mechamisn of supercapacitor. ................................................. 44Figure 20 Rayleigh scattering has emission light wavelength equals to incident light, but

Raman scattering has wavelength changes. .............................................................. 50

V

Figure 21 Raman spectrum of graphene. .......................................................................... 50Figure 22 potentiostat model 273A provided by EG&G and scheme of the circuits. ....... 53Table 1 surface charge of rGO and rGOH reduced by ascorbic acid at different

concentration. ............................................................................................................. 56Table 2 sueface charge of rGO with and without ammonia promoted dispersion. .......... 57The three peaks in the spectrums are D, G, G’(2D) peaks from left to right. ................... 58Figure 23 Raman specrum of rGO and rGOH reduced by 0.5mM ascorbic acid. ............ 58Figure 24 Raman specrum of rGO and rGOH reduced by 1mM ascorbic acid. ................ 58Figure 25 Raman specrum of rGO and rGOH reduced by 2mM ascorbic acid. ............... 59Figure 26 Raman specrum of rGO reduced by ascorbic acid at different concentrations.

.................................................................................................................................... 60Figure 27 Raman specrum of rGOH reduced by ascorbic acid at different concentrations.

.................................................................................................................................... 60Figure 28 GO was being reduced with 1mM AA at 90℃. ................................................. 61Figure 29 photo of GO/NiOOH surface(left) and rGO/NiOOH surface(right). ............... 63Figure 30 Cyclic voltammetric curve for GO and rGO with scan rate 10mV/s. ............... 64Figure 31 Cyclic voltammetric curve for GO and rGO with scan rate 20mV/s. ................ 64Figure 32 Cyclic voltammetric curve for GO and rGO with scan rate 50mV/s. ............... 65Figure 33 Cyclic voltammetric curve for GO with different scan rates. ............................ 66Figure 34 Cyclic voltammetric curve for rGO with different scan rates. .......................... 66Table 3 Integrated areas for GO and rGO with different scan rates. ................................ 67Figure 35 SEM image of GO/NiOOH composite surface 15KX magnification. ................ 67Figure 36 SEM image of GO/NiOOH composite surface 1KX magnification. ................ 68Figure 37 SEM image of rGO/NiOOH composite surface 15KX magnification. ............. 68Figure 38 SEM image of rGO/NiOOH composite surface 1KX magnification. ................ 69

1

ABSTRACT Graphene has attracted the attention of scientists and engineers recent years. The unique

structures and extraordinary properties make people to look forward next generation solution

of the nowadays industries that encounter bottlenecks. For decades hard working, the

knowledge structure has been built initially and many possible applications are surprising us.

However, the production of graphene is remaining in small scale. To enhance the production

ability, we proposed a possible method for graphene production and its application.

The work was aim at defining an environment-friendly chemical reduction method with

ascorbic acid, explore the application of the product in the enhancement of capacitance for

nickel-based supercapacitor and verifying its availability.

The first chapter is to briefly introduce the basic information about graphene and the

existing production methods, especially the chemical reduction method of graphene oxide.

The possible and existing applications were also introduced.

The second chapter is to clarify the properties of chemicals we used in the work and the

working devices for reduction and characterizing methods for the products.

The third chapter is the experimental works. The first part was to apply ascorbic acid as

reductant in reducing commercial graphene oxide and the characterization of the obtained

reduced graphene oxide (rGO) samples by surface charge analysis and Raman. We selected

the best performed sample of rGO to proceed the second part of experiments. The second part

focus on the application of rGO. The rGO was composited with nickel hydroxide to enhance

electrical performance of the existing nickel-based supercapacitor. The results of XRF, cyclic

voltammetry and SEM showed that the rGO successfully enlarged the capacitance of nickel-

based supercapacitor.

2

3

INTRODUCTION Graphene is a two-dimensional existence which break the law that matter can only exist

in three-dimensional status. Early back to 1947, graphene has been studied theoretical by

P.R.Wallace with tight-binding model1. In the past, graphene was always assumed to be a

theoretical structure that is not able to stably exist independently. By the time it was

discovered, it caused the shock of physical sciences. Graphene is a common raw material,

where the nearest appearance to our life is core of pencil. We have written with a pencil, left

graphite traces across the flat paper. In fact, what left on the paper is the multilayer graphene.

In October 2004, Geim and Novoselov2, physicists at the University of Manchester, succeeded

in isolating graphene during the process of stripping graphite from tape. This confirms that

graphene can exist independently. They were also awarded Nobel Prize in Physics "for

groundbreaking experiments regarding the two-dimensional material graphene”.

Graphene is a "super-material", is the thinnest and the hardest nanomaterials, harder

than diamond but weight almost zero. At room temperature, electrons transmission could be

faster than any known conductor. It can be applied in transparent touch panel or solar cells.

However, mechanical exfoliation method is impossible for large scale production of graphene.

Researchers have tried to solve this problem chemically, and the more mature one is the strong

acid redox method. Oxidized graphene, as a primary derivative of graphene, has a good water

solubility, but its conductivity is strongly attenuated and insoluble in organic solvents. After

annealing or chemical reduction, due to the disappearing of the functional groups, conductive

properties greatly restored. This make possible for the applications that require the advanced

electrical properties of graphene. There are a variety of known methods for chemically

reducing graphene, these studies laid a certain foundation for the future development of

graphene.

4

In this thesis, based on the redox method of graphene oxide, we discussed the process of

preparing reduced graphene oxide(rGO) by chemical reduction and especially chemical

reduction by ascorbic acid. We studied the reduction efficiency of ascorbic acid and the

electrical properties of AA-rGO. In addition, AA-rGO was applied as the composite material

with nickel oxide, we discussed the electrical properties of the anode material, studied the

effect of graphene in enhancing the capacitor of nickel oxide based supercapacitor.

5

CHAPTER 1 STATE OF THE ARTS

1.1 Graphene Carbon is a wide spread element in the nature, and it has been used as a common material

for years. The form of carbon can be defined as crystalline and non-crystalline. For non-

crystalline carbon, there are carbon black, activated carbon, carbon fiber, coal, etc. As for

crystalline carbon, it can be hard as diamond, and soft as graphite as well. The distinct

appearances of carbon element in different forms due to their different structures show that

there is a probability to apply carbon materials to many different purposes especially combine

with nanotechnology. With the rapid development of nanomaterials, carbon as a quite

common element involved into the streams of the studies. From the discovery of fullerenes3 in

1985 to carbon nanotubes4 in 1991, the special properties with carbon nanomaterials was

confirmed and was tried to apply in different areas of engineering.

Figure1Structureofdifferentisomerofcarbon:fullerene,carbon

nanotubeandgraphenesheets.

6

As we know that the pencil is made of graphite that contains layers of sheet like graphene.

When we write with pencil, parts of the sheet are easily peeled off onto the papers due to the

relative week interaction force between the layers of graphene. That is why a pencil can leave

traces on the papers. However, the graphite traces are still in the three-dimensional structure

although the number of layer drops significantly. To the extreme situation, there is a minimum

limit of the peeling, that is monolayer with the thickness of only one carbon atom. In this

extreme situation, we can get the so-called graphene.

Graphene is named by Boehm in 1962 by combining the word graphite and suffix –ene

proposed the concept of graphene. Now it is defined as a single carbon layer of the graphite

structure, describing its nature by analogy to a polycyclic aromatic hydrocarbon of quasi

infinite size. And in the scientific research area, graphite less than 10 layers can be treated as

graphene or few-layers graphene. Because of the extraordinary performance of graphene in

optics, electronics and mechanics, it attracted researchers in many countries to study its

properties, production methods and possible applications.

1.1.1 Structures of graphene

Theoretically, the ideal two-dimensional graphene is a layer of carbon atoms combine

other three adjacent carbon atoms by sp# orbitals hybridized by one 2s and two 2p$ electron

orbitals, and by the delocalized π bond constituted by the p% orbitals vertical to the graphene

Figure2Idealgraphenestructure.

7

plane, forming the bond angle 120°, bond length 1.42Å, honeycomb shaped structure. It is the

thinnest two-dimensional material we ever found, the thickness of a monolayer graphene is

only 0.335nm.

However, the real graphene sheet cannot have such a perfect crystalline plane structure.

In 2007, J.C. Meyer reported an interesting phenomenon5. They give an incident electrons

beam in TEM with varying incident angle. When the angle deviated from the normal line, the

diffraction spot became broader. This phenomenon is most obvious in the monolayer

graphene samples, but less significant in the double layer samples, and disappear in the multi-

layer samples. It demonstrated that the graphene is never an absolute plane, but exists

wrinkles like low hills (Figure 36). It can be that the single layer graphene fold itself to reduce

the surface energy by transforming from two-dimensional form to three-dimensional form.

The numerical calculation indicated that there are 8nm height wrinkles spontaneous. And it

is the necessary condition for two-dimensional single layer graphene to exist.

With this special structure, many extraordinary properties appear.

Figure3(a)Rippledgraphene;(b)wrinkledgrapheneand(c)crumpledgraphene.

8

1.1.2 Properties of graphene 1.1.2.1 Electrical properties

Contrast to the sp& orbitals in diamond, delocalized π orbitals in a plane structure offer

continuous conjugated electron clouds in the two sides of the graphene flake where the π

electrons can move freely, bring good electronic conductivity to graphene. The band theory

study also demonstrates that the ideal single layer graphene the mean free path of electron

and hollow is so large that the movement of electron is barely effected by the phonons, hence

the mobility of electrons in graphene has few responses to the temperature variation. The

highest data of the experimental mobility reached 230,000cm# 𝑉 ⋅ 𝑠, reported by Kirill Bolotin

from Columbia University7. This mobility is 100 times more than that of silicon. Another factor

that contribute to the high conductivity is its quantum tunneling effect. The applied voltage on

graphene were supposed to be a quantum barrier for electron to overcome. However, the

actual behavior of electrons is that they all passed through the barrier by quantum tunneling.

The conductivity of graphene layer is heterogeneous along the plane and the vertical direction.

By applying the vertical voltage, graphene can be made as high performance FET due to the

zero-exchange energy between electrons and hollows. By doping with some functional groups,

graphene can act like semiconductor since the opening of the band gap.

Around the zero energy (Dirac point) of the energy band, π electrons in graphene behave

like Dirac-fermion with the moving speed 1/300 of photon (Figure 48). Hence the carrier

Figure4Theoreticalbandstructureofgraphene in firstBrillouin zone.Thevalenceand

conductionbandsmeetattheFermilevel.

9

electrons in graphene can be described by Fermi-Dirac equation. It is the exception of low-

energy condensed matter that satisfies the high-energy relativistic Fermi-Dirac equation.

Therefore, graphene possesses properties that ordinary semiconductors do not show: Klein

tunneling, PN junction tunneling, quantum spin Hall effect, anomalous quantum hall effect,

etc.

1.1.2.2 Optical properties Graphene is also a high transmittance material. The theoretical derivation indicates that

the absorption coefficient of single layer graphene is irrelevant to the incident wave length,

equals to about 6.8×102𝑚45 for single photon, and the reflectivity is no more than 1%, that is,

the absorption of incident is constantly 2.3%9. Combine with the high electrical performance,

graphene can be applied in very broad areas including solar cell, transparent conductive thin

film, photodetector, LED etc.

1.1.2.3 Thermal properties The thermal conductivity and thermal stability of graphene can be another advantage for

new application. Before the discovery of graphene, many physicists believed that the two-

dimensional crystalline structure is not allowed by the thermodynamics fluctuation under a

non-zero kelvin temperature. This was also a consensus between theorists and

experimentalists. However, the single layer of graphene was produced in the lab anyway

thanks to the nano-scaled distortion of the graphene plane we mentioned before. Theoretically,

the thermal conductivity increases with the graphene surface area, decreases with the amount

of material defects due to the phonon scattering. In room temperature, the ideal thermal

conductivity reached 6600 W 𝑚𝐾 10. Experimentally, the thermal conductivity of graphene

is significantly affected by the substrate while detecting. The best experimental data could

reach 5000 W 𝑚𝐾 11, which is 10 times more than copper under the room temperature. The

studies of thermal expansion effect indicated that the thermal expansion coefficient is negative

in room temperature. When T>300K, the thermal expansion coefficient became positive

10

gradually12. These effects should be considered especially in fabricating the delicate devices

like integrated circuits and the heat dissipation models.

1.1.2.4 Mechanical properties

The high mechanical stability is the insurance of the application. Researches shows that

when the temperature increased, the Young’s modulus, tensile strength and the stretching

limit decreased. It can be explained by the energy supplying to the graphene to break the bonds

and the increasing number of defects accompany with the higher temperature. Theoretically,

Young’s modulus of graphene is about 1TPa according to simulation, and the strength is

130~180GPa13. These values indicated that the graphene is harder than its isomer diamond,

stronger than the best steel. Hence, graphene can play an important role in the new composite

material for improving the mechanical properties. Surface modification of graphene, like

hydroxyl, carboxyl can tune the thermal and mechanical properties or to offer other different

properties.

1.1.2.5 Chemical properties The surface of graphene is similar to that of graphite, which implies that graphene is

capable to adsorb and desorb many atoms and molecules like NO2, NH3, K. These adsorbents

usually lead to change of the carrier density, which is a good indicator of the environment.

Mizuta developed a new CO# sensor detecting that the resistance varied step by step with the

absorption and desorption of CO#, acted like quantum change. This type of sensor is sensitive

enough to detect CO# of 30ppb concentration, which is much improved than the technics

before14. Now the researchers are devoting to develop higher sensitive sensor that can detect

even one single molecule.

With these special properties, many applications were realized. Nowadays, the devices

that apply the graphene technics are emerging in large number and more potential

applications is being developed. But the most difficult aspect is not the imagination of us to

11

apply it, instead, the production of graphene, especially for the high quality, large area, single

layer graphene that processes the most excellent performance, is difficult to achieve.

1.2 Graphene production methods In 2004, Geim and Novoselov prepared the single layer of graphene by tapes in

Manchester University2. It is the first time that single layer graphene was produced, and two-

dimensional material was proved to exist stably in the air condition. Since then, the production

methods of graphene have been studied extensively. Recently, there are many technics are

studied to produce graphene, including CVD, exfoliation, redox graphite, etc.

1.2.1 Exfoliation 1.2.1.1 Mechanical exfoliation

As the first successful production of graphene, the mechanical exfoliation is one of the

most direct way to prepare graphene. They etched trenches with 5µm depth on the surface of

highly oriented pyrolytic graphite(HOPG) by oxygen plasma to create many platforms with the

width of 20µm~2mm. Then the etched HOPG was pressed upon the photo resist for

transferring the graphite on the platform to the resist. After the repeated peeling by tapes, the

remaining graphite sheets on the substrate was taken to perform ultrasonic cleaning to filter

out the relatively thicker sheets. At last, what they got is few-layer graphene with the help of

AFM selecting. This is the very first success of preparing graphene, although the yield is very

Figure5Exfoliationofgraphenebytape.

12

low and difficult to pick the desire piece of graphene. Therefore, we cannot count on it for large

scaled production.

Later, Novoselov further studied exfoliation method and finally prepared monolayer

graphene and prove its stability. Based on these, Meyer placed the graphene-absorbed silicon

substrate on an etched metal frame, then etched away the silicon substrate with

tetramethylammonium hydroxide and hydrofluoric acid, obtained the graphene which is

suspended. This method can be used to obtain high quality graphene, which is widely used in

basic research to obtain the physical and chemical properties of graphene. But the mechanical

exfoliation method takes a long time to control the graphene layer number and size yield is

relatively low and single graphene will be dispersed in the multi-graphene, it is difficult to be

identified and separated, let alone to be used for large-scale production.

1.2.1.2 Liquid-phase exfoliation

Instead of mechanical exfoliation, liquid-phase and gas-phase exfoliation of graphene are

alternative methods.

Figure 6 A) solvent ions intercalation, weaken the interlayer

interaction; B) graphite compound ions are substituted by solvent

ions;C)Puresonicationtreatmentofgraphite.

13

In liquid-phase exfoliation, the graphite is dissolved into a certain organic solution or

water, with the help of ultrasound or heating, to prepare a solution with the mixture of

monolayer and multilayer graphene of certain concentrations15-16. What we need to pay

attention is the surface tension of the solvent which is supposed to match that of graphene.

The interaction between solvent and the graphene is able to balance the energy for the

exfoliation. Researches indicated that graphene is better exfoliated in the solvent with its

surface tension of 40~50 mJ/m#17. To tune this value of solvent, several additives can be used,

like N-methyl-2-pyrrolidone (NMP)18, ortho-dichlorobenzene19, dimethylformamide20. They

are called dispersion media. After the ultrasound treatment, the splitted graphene with

different size and thickness are separated by centrifugation.

1.2.1.3 Gas-phase exfoliation

Gas-phase exfoliation is achieved by the decomposition of ammonia solvent. Researches

confirmed that the ammonia gas is able to permeate into the interval of layers of graphene21.

When the gas pressure is high enough to overcome the Van der Waals force, the graphene will

be exfoliated. These two exfoliation methods have the advantages of low cost and simple

operation. But the yield of the graphene is low and cluster phenomenon is serious. Combining

the liquid-phase and mechanical exfoliations, Knieke developed an improved approach that

compensated the shortcoming of liquid-phase exfoliation and realized high yielding of 25g/L

of graphene with 50% thickness less than 3nm22.

The above methods mentioned are the physical approach to produce graphene.

Combining chemical way, more methods are studied.

1.2.2 Chemical vapor deposition (CVD)

Chemical vapor deposition is one of the most widely used methods for preparing

semiconductor thin film materials on a large scale. Because of its wide application as well as

14

the mature production process, Chemical vapor deposition provides a controllable method to

produce graphene, and it is also considered as the most promising method for large scale

production of graphene sheets.

Unlike the production of CNTs, in the production of graphene with CVD, particle catalysts

are no longer required.

Researchers usually place the plane substrates (such as metal thin film) into the

decomposable precursor atmosphere like methane and ethylene up to 1000℃ vacuum

environments, after the high temperature annealing, the carbon atoms are deposited onto the

surface of the substrates to form the graphene. The last step is the removal of metal substrates

by, for example, chemical corrosion. At this point, the monolayer graphene is obtained. With

CVD, Ruoff successfully prepared graphene on the copper foil, and most of them are

monolayer graphene23. Robertson prepared multilayer graphene with APCVD24. Whang of

Sungkyunkwan University of Korea first used silicon wafer with hydrogen-terminated

germanium Ge coating as substrate, deliver CH= and H# by CVD25. The crystalline orientation

Figure7Single-crystalmonolayergraphenegrownonahydrogen-

terminatedGe(110)surface.

15

(110) of germanium lead to the coherence of the graphene growth. Due to the weak interaction

between graphene and the hydrogen-terminated layer, the graphene can be easily take off by

thermal release tape.

The advantages of CVD to produce graphene are obvious. By choosing the type of

substrate, the growth temperature and the flux, type of precursor or other parameters, we can

modify the growth of graphene like growth rate, thickness, area and so on. The most attracting

aspect is that CVD provides us a method to prepare large area graphene, and compatible with

the existing semiconductor manufacturing technics, which enable us more readily to benefit

from the excellent electrical properties of graphene in order to make revolutionary upgrade of

semiconductor industries. For some other applications, the graphene prepared on the metal

substrate must be transferred. The difficulty of transferring and the high cost of the substrate

and the CVD equipment limit the large-scale production by CVD methods.

1.2.3 SiC Heating and Epitaxial growth 1.2.3.1 SiC heating

One of these methods is growing graphene layer SiC decomposed layer. First the

merchandise on-axis 6H-SiC substrate is etched by HF or other methods after polishing in

order to remove organic and inorganic contaminations and surface oxides26. Then the

Figure8PossiblemechanismsofCVDproducinggraphene.

16

prepared substrate is place into a furnace up to 2000℃ in ambient argon atmosphere and

keep it inside for a certain period. SiC decomposes thermally into C which is remain on the

substrate and Si away from the substrate under the high temperature, the surface is

carbonized(Figure 927), homogeneous high quality graphene is produced on (0111) surface of

SiC. After, hydrogen exposures are performed and mean while the sample is annealed at 700℃.

The graphene produced by decomposition of SiC is difficult to transfer from substrate,

and it require many high growth condition like high temperature and noble gas. Therefore, the

graphene prepared by this method is mainly used for graphene device based on SiC.

1.2.3.2 Epitaxial growth

Another epitaxial growth method gave us an alternative path to obtain graphene. The

carbon is absorbed into the surface of a transition metal, ruthenium28.

Because of the temperature-dependent solubility of interstitial carbon in this metal, the

growth of graphene on the (0001) surface of Ru is controllable. The permeation is performed

Figure9 BasicsofgraphenegrowthbythermaldecompositionofSiCandstructuralmodel.

Figure10Simulationforthemechanismofcarbonatomsemergingfrommetalsubstrate.

17

at 1150℃, then slowly cooling to 825℃.At the same time as the temperature drops, large

amount of carbon atoms is driven to the surface of substrate. First the graphene will growth

like island, when the coverage reaches 80%, the second layer begins growing. The first layer of

graphene has strong interaction with the connecting Ru layer. But the second layer has no

connection with Ru, what is left is only the weak interaction with the first layer of graphene.

This second layer of graphene has the better graphene structure. Although the graphene

produced by this method usually has no uniform thickness, the further studies had realized

the production of millimeter of graphene monocrystalline by optimizing the growth conditions.

1.2.4 Electrochemical method

Due to the strong in-layer covalent bond and the weak Van der Waals interaction of

interlayer connection, the interlayer force decreases with the increase of interlayer spacing or

the decrease of the effective contact area between layers. In the constant voltage electrolysis

process, the cation and the anion of the electrolyte move toward the cathode and the anode

Figure11Differentmethodofelectrochemicalprocesstoexfoliategraphite

sheet.

18

respectively. Driven by the electric field, the ions enter into the graphite layer, which makes

the distance between the graphite layers increase, thus weakening the interlayer force. As the

electrolysis reaction progresses, graphite on the cathode graphite electrode is continuously

etched, a large amount of colored matter is peeled off on the electrode surface, and the

exfoliated graphite layer including graphite sheet and graphene disperses the electrolytic

solution. After a certain period of electrochemical peeling, part of the ultra-thin graphite sheet

evenly dispersed in the electrolyte, while some larger particles fall in the bottom of the

container29-30.

Electrochemical method is a simple and fast approach to produce graphene without

involving any toxic reagent.

1.2.5 Arc Discharge Method

Arc discharge method provides a way to prepare N-doped or P-doped multi-layer

graphene sheets. In Li’s work31, two graphite electrodes were held in a mixture of H2 and NH3

environment in 760 Torr, where NH3 is favored for N-doped graphene. The stainless chamber

was under the temperature control of water-cooling system. Under the high DC current up to

120A, maximum open circuit voltage of 60V, the two electrodes were brought together about

1mm distance. When the discharge occurs, CNTs and other forms of carbon matter could be

collected around the cathode, and on the inner wall of the chamber, graphene was deposited,

and when the chamber came back to the ambient condition, graphene could be collected from

the inner chamber wall. The result graphene was about 2~4 times of the thickness of

monolayer graphene. Another researcher Huang performed DC arc discharge with catalyst

zinc oxide mixed in the electrode rod, finally got the gram-scale yield of graphene32.

The growth mechanism of graphene prepared by arc method is still under researching.

Because the reaction temperature during preparation of the arc method is extremely high, and

19

the researchers cannot use in situ observation. In addition, the reaction process is completed

in a very short period of time, so it is hardly possible to characterize the reaction phase analysis.

1.2.6 Organic synthesis

Early work has reported that in 1958, Clar first synthesized hexa-peri-hexabenzocoronene,

which was considered as a small piece of graphene containing 42 carbon atoms33. Mullen

prepared a graphene nanoribbons(GNRs) with the length reached 12nm34. This is another

useful type of graphene, which features an infinite ordered structure of graphene along one

direction and finite length of graphene vertical to that direction. The organic synthesis of GNRs

is reported by Qian35. They took tetrabromo-perylene bisimides as monomer, which contains

benzene structures in the middle, CuI, L-proline, and K2CO3 as the catalysts at 110℃. After

polymerization, different size of GNR was synthesized. When the size of the graphene is

increased, how to solve the solubility and edge reaction becomes a key problem in organic

synthesis.

Figure12Organicsynthesisoftri-perylenebisimidesbytetrabromo-perylenebisimidesmonomer.

20

1.2.7 Carbon nanotubes (CNTs) cutting

Columnar carbon nanotubes(CNTs) are considered as the rolled up GNRs without end

groups. Therefore, GNRs can be produced by by cutting the long CNTs, inversely. Tour

reported that, at low temperature condition, with the help of KMnO4 and H2SO4, the multi-

walled CNTs were cut lengthwise by opening the C-C bonds, the openings were connected by

oxygen groups.

Cano-Marquez36 prepared GNRs by intercalating ionic Li in liquid NH3 environment,

with the subsequent heat treatment, CNTs were cut into GNRs. Further, the transition metal

nanoparticles can also corrode the wall of CNTs, hence to open the CNTs. Jiao37 deposited the

dispersed CNTs onto the pretreated Si substrates. Then spin-coat the PMMA. After short

heating, under the low pressure, the prepared CNTs underwent the Ar plasma etching to

obtain the ribbon-like graphene. This method is capable only to produce GNRs in labs, but not

applicable for large-scale production in industry.

1.2.8 Reduction of graphene oxide methods

The preparation of graphene by oxidation-reduction method is the most widely used and

can be mass-produced graphene. The raw material is mostly the common graphite. Graphite

is constituted by layers of graphene bonded by intermolecular interaction. We must extent the

Figure 13 Mechanisms for intercalation and unwrapping of multiwall

nanotubes.

21

distances of each layer by inserting the large groups like carboxyl, hydroxyl, epoxy groups and

carbonyl in order to weaken the interaction, so that we can obtain monolayer or few-layer

graphite for further process. The most used way of doing it is to chemically oxidize the graphite.

1.2.8.1 Oxidation of graphite The oxidation process of graphite is summarized as follows: graphite is oxidized by strong

oxidant, oxygen atoms enter into graphite layer by combining with the π-electrons, the in-layer

double bonds break while the oxygen atoms bonding with dense carbon network plane in the

form of functional groups such as C=O, C-OH, -COOH. In this way, the final products are

covalent graphite intercalation compounds (Figure 1538).

There are three main methods, Brodie’s method39, Stauderunaie’s method40 and

Hummers’ method41, to prepare graphite oxide from natural graphite.

Figure14Syntheticroutesofgrapheneoxideandgraphene.

Figure15(a)SchemeofGOpeparationfromgraphite;(b)SchemeofGOsheet.

22

Date back to 1859, Brodie used strong protonic acid Fuming Nitric acid system, transform

the raw material to be graphite intercalation compounds. Then used KClO3 as oxidant, the

reaction system temperature must be maintained at 0 ℃, and then reaction 20-24 hours with

continuing stirring. The oxidized graphite obtained after washing has a low oxidation degree

and needs to be oxidized several times to improve the oxidation degree and the reaction time

is relatively long. The advantage of this method is that the oxidation degree can be controlled

by the oxidation time, and the synthesized graphite oxide structure is relatively regular.

However, the use of KClO3 as oxidant, there is a certain risk.

40 years later, Staudenmaie improved Brodie’s method of oxidizing graphite with a

mixture of concentrated sulfuric acid and fuming nitric acid, and completed the oxidation

reaction more quickly. the obtained graphite oxide had the composition ratio of C/O close to

2.

In 1958, Hummer used concentrated sulfuric acid and mix with sodium nitrate system,

KMnO4 as oxidant instead of KClO3, reduced the emission of toxic gas, hence enhanced

security. It also had faster oxidation speed and degree. The products had the higher extent of

regularity, therefore easily to swell and delaminate in the solvent.

According to the original experiment of Hummers, first step was to prepare the graphene

intercalation compound. 100g graphite powder and 50g sodium nitrate was added into the

vigorous stirring 2.3L concentrated sulfuric acid, while the whole system was kept in the ice

bath to prevent overheating. Then 300g potassium permanganate was added stepwise in order

to keep the temperature less than 20 ℃ during the exothermic reaction. Second step was to

level up the reacting solution at 35℃ for deeper oxidation reaction. The 20th mins, solution

became pasty with only small amount of gas evolution, the mixture turned into brownish grey.

At the end of the 30mins, 4.6L water was slowly added, caused violent effervescence and the

23

temperature elevated to 98℃. So far, the diluted suspension looked brown. Final step was that

the suspension was kept at 98℃ for 15mins, then diluted with warm water to 14L. 3% H#O#

was added to reduce the residual permanganate and manganese dioxide to colorless soluble

manganese sulfate.

Among the three methods, hummers’ method is the most used for oxidizing graphite due

to its high efficiency and acceptable safty. Also, many other improved hummers’ methods were

developed to avoid the release of some hazardous gases like nitrogen oxides42-43.

1.2.8.2 Reduction of graphene oxide I. Electrochemical reduction

For now, the reported electrochemically reduction of GO is categorized into two types:

Direct electrochemical reduction and two-step electrochemical reduction.

Direct electrochemical reduction is that the GO solution is directly reduced to be graphene

onto the surface of electrode by the voltage. Usually, cyclic voltammetry, linear scan

voltammetry and constant potential are used. By the time GO is contacted with electrode, the

electrochemical reduction takes place.

By DC voltage, An44 reduced GO onto the stainless steel anode with 10V voltage, Tong45

reduced GO with the mix electrolyte of GO and KNO3 with 20V voltage. The reaction time is

10mins to 8 hours. With the increase of reduction degree, the GO solution turn black from

yellow.

Cyclic voltammetry, instead, works on the voltage of a certain window of the potential

range, which made possible to define the exact redox potential and reversibility of the reaction.

In cyclic voltammetry, the scanning rate is usually set 20~100mV/s, the potential varies from

0 to 1.5V. Guo46 used cyclic voltammetry pretreatment to get the reduction potential of GO on

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glassy carbon electrode, then fix the reduction potential as -1.3V or -1.5V for 2 hours’ reaction

in the subsequent large scale experiments. Contrast to potential -1.3V, electrochemical

reaction is more effective in potential -1.5V, but there were the hydrogen bubbles follow.

On the other hand, during the electrochemical process, due to the difference in solubility

of rGO and GO, rGO is deposited on the electrode. Therefore, electrolyte and pH value of the

solution have significant effects for the coating formation. The common electrolytes are

phosphate, sodium chloride or sodium sulfate.

In two-step electrochemical reduction, GO is firstly modified on the electrode substrate.

This prepared electrode then acts as the working electrode in the conventional three-electrode

electrolysis system with a certain electrolyte in order to achieve the reduction of GO coating.

The first step, coating, can be realized by many existing deposition methods such as dip

coating47, spin coating48, layer self-assembly coating49, electrochemical deposition50, etc. The

substrate electrode can be either conductive (ITO, glassy carbon, gold, etc.) or nonconductive

(glass, plastic). Moreover, the thickness and the uniformity can be controlled by different

deposition method, deposition time and the quantity of GO in the solution. The shape and the

size of GO coating follow that of the electrode substrate. Zhou51 applied spin coating and

obtained GO coatings with different thickness around 1~2 nm, and reduced them to rGO

coatings. Liu50 deposited GO electrochemically on ITO conductive glasses as the working

electrodes, scanned by 0~ -1.0V voltage in the 0.1mol/L KCl solution with the contour glassy

carbon electrode, obtained rGO coatings on the ITO substrate. Another way to modify

electrode with GO by electrostatic attraction was studied by Raj52. He modified the glassy

carbon electrode with HDA(1,6 - ethylenediamine) by self-assembly method. Because of the

positive charge of HDA hydrolysis, the negatively charged GO flakes were attracted and self-

assemble on the surface of the modified electrode. The final product has the structure that

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from the outside to the inner layer followed by GO, HDA, glassy carbon. With this modified

electrode, Raj applied 0~ -1.4V voltage for electrochemical reduction, obtained rGO that had

relatively high degree of reduction. Similarly, Wang modified glassy carbon electrode with

APTES, then coated GO by the interaction between GO and amino group.

Compare to the direct electrochemical reduction, the selection of electrolyte and pH value

play important part as well. The common electrolytes are potassium chloride, potassium

chloride or sodium chloride. pH value usually be tuned to acidic or neutral.

Other than cyclic voltammetry, constant potential is also available for two-step reduction.

Li53 applied different constant potentials in potassium phosphate electrolyte (pH 5.1~5.5) for

3 mins. The results showed that the reduction degree increased with the voltage increase. And

when the reduction voltage is -1.8 V, the reduced graphene film can be completely peeled off

from the electrodes due to the generation of hydrogen bubbles on the electrodes. Peng48

applied -1.1V voltage in three-electrode system with sodium nitride electrolyte for 4.5 hours,

by controlling the shape and thickness of GO deposited on electrode, got different shape and

thickness rGO coatings with high reduction degree.

For the mechanism of electrochemical reduction of GO, all the reported electrochemical

reduction methods for GO include two microscopic process: electron transportation between

GO and surface of working electrode and that between rGO on the surface of electrode and GO

part. The highly negative potential can effetely reduce the oxygen-containing groups of GO.

The exact mechanism of electrochemical reduction of GO is still beyond our knowledge. One

possible mechanism reported by Dong and Zhou51 indicated the process:

GO + aHB + b𝑒4 → rGO + cH#O

where H+ plays a key part in this process. And further research is necessary for the later

development.

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II. Thermal reduction

II.1 Solid heating reduction

Solid thermal reduction is that at high temperature, the oxygen-containing functional

groups and water molecules between the oxidized graphite layers degrade to form small

molecules such as CO# or H#O, which makes the graphite lamellae overcome the interlayer

van der Waals force, hence the oxygen content decreases, meantime, the exfoliation completes.

The key factor of thermal reduction is to react in protection atmosphere like nitrogen and

argon, or high vacuum environment to avoid the oxidation of the production by oxygen in the

air environment. The temperature must high enough to provide the energy above the

activation energy for breaking the C-O bonds.

As early as 2006, Schniepp54 reported this solid thermal reduction stripping method. A

small amount of completely dried graphite oxide powder was placed in a sealed quartz tube,

under the protection of hydrogen, products was obtained at high temperature (1050℃) after

30s. Seung treated GO powders at room temperature to 2000℃ in N# atmosphere for 1 hour.

A recent study reported by Wang55 compare the mild temperature reduction under different

ambient atmosphere for 1 hour. On the other hand, the high pressure brings by CO# and H#O

not only exfoliate rGO into layers, but also remove carbon atoms from the graphene plane,

which splits the sheets into small pieces, results in the distortion and reduction of the sheet

size, or brings the defects in the plane.

II.1 Solvothermal reduction

Rajamathi56 first reported that the liquid-phase environment, instead of solid state, was

able to reduce GO into rGO by solvothermal reaction. The GO was dispersed in the solvent by

ultrasonic, then transferred into the hydrothermal kettle and sealed. After reaction at a certain

temperature (120 ~ 2000℃) for a period, the graphene was formed. Li57 dispersed 40mg GO

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in 40mL ethylene glycol by ultrasonic, added ammonia with continuous stirring to enhance

the dispersion, then 50mL solution was transferred to the kettle, reacted at 180℃ for 12 hours.

After cooling, filtering and rinsing to neutral, rGO is obtained.

For the mechanism of solvothermal reduction, it is believed that under high temperature

and pressure condition, high concentration hydrogen ions are generated. Under the catalysis

of these hydrogen ions, hydroxyl group and other oxygen-containing groups are reacted,

generate water from the surface of GO and dissolve in water, at the same time, the reaction

products enlarge the distance between layers to enable exfoliation. However, this reaction is

reversible, so the oxygen-containing groups cannot be completely removed. In addition, the

solvothermal reduction method not only removed the oxygen-containing groups from the

surface of GO, but also repaired the defects in the graphene aroma structure caused by the

oxidation.

Solid heating reduction requires high temperature up to 1050℃, consuming high energy,

low stability and difficult to massive production, while in the low temperature condition that

lower than 200℃ requires high vacuum and longer time up to several hours. Solvothermal

reduction method also requires different solvents, high pressure and the long duration.

Recently, radiation assisted thermal reduction is studied to improve the performance of above

productions.

III. Photo-reduction

III.1 Photo-thermal reduction

radiation is another source of energy. Photo-thermal reduction is able to transfer energy

to GO in super-high vacuum, Ar, H2, NH3 environment via radiation like xenon lamp

irradiation, to produce rGO58-59.

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In solution, Zhang60 radiated GO solution in water / alcohol solvent by γ-rays in order to

produce H*(radical) and e- as reductants, reduced GO into rGO. The γ-ray features a low wave

length, high frequency radiation, carrying high energy, contrast to chemical reduction, γ-ray

produces no pollution and toxic byproduct. The flash light of camera is the common source of

xenon radiation61. It’s wave length is larger than 400nm, according to theoretical calculation,

one pulse of flash light is well enough to provide energy for 1µm thick GO for reduction. Other

light sources around or above the wave length of visible light, like Hg lamp, are also useful for

photothermal reduction. Microwave is an alternative source for heating62. Heating mechanism

of microwave is the friction caused by irregular movement of polar molecules that generating

heat, which is fast and uniform. At the beginning, microwave has no different with other

heating methods that applied in solid state thermal reduction. Later, researchers found that it

was better to heat in liquid state organic solvent because the microwave work only for polar

molecules.

III.2 Photo-chemical reduction

Early research of photochemical reduction applied TiO2 as catalyst with UV radiation.

The mechanisms’ as follows63:

TiO# + hν → TiO# 𝑒 + ℎ

TiO# 𝑒 + ℎ + C#HMOH → TiO# 𝑒 +∙ C#H=OH + HB

TiO# 𝑒 + GO → TiO# + rGO

Williams64 applied this method in the mixture of TiO2 and GO in ethanol solution,

produced graphene modified by TiO2. TiO2 here acted as both a photocatalyst and a

dispersant. The radiation time is a key factor that influence the reduction degree of GO, but a

prolonged radiation time would lead to the degradation of rGO. Besides, other catalysts are

also available for photocatalysis reduction of GO, such as ZnO and WO3.

29

Without catalyst, irradiation itself is able to reduce GO effectively. Matsumoto58 radiated

GO at room temperature in H2 or N2 environment with no catalyst by UV, he analyzed that

the sp3 structures C-O-C-OH were reduced to sp2 orbitals.

III.3 Laser reduction

Smirnov65 provided a threshold of radiation wave length, 390nm(3.2eV), to categorize the

two mechanisms mentioned above. GO undergoes photothermal reduction when the radiation

wave length is larger than 390nm, GO undergoes photochemical when that of the radiation is

smaller than 390nm. However, with the application of laser, both two mechanisms may exist

at the same time. Abdelsayed66 generated stable thermal energy by laser with 532nm wave

length, reduced GO to rGO in aqueous solution. The similar experiment is performed by

Zhang67 that 790nm laser radiated directly on the substrate containing GO, the products

showed good conductivity which indicated that the rGO formed. Laser reduction of GO without

catalyst is contributed by the electronic excitation caused by the radiation, which is able to

remove the oxygen-containing groups on the surface of GO.

IV. Chemical reduction

Chemical reduction methods of GO is various. As long as reagent has reductivity, is more

or less available for reduction of GO. Commonly used reductants are hydrazine hydrate, sulfur

compounds, sodium borohydride, vitamin C, Zinc powder, etc. The carbon content of the

products CrGO of chemical reduction is in the range of 80% to 95%, and easily to introduce

defects result from the incomplete reduction, hence parts of the superior performance of rGO

maybe loss.

However, chemical reduction methods are most possible to apply in large-scale

production due to its cheap cost and simple operation process. Therefore, it became the most

common method for graphene production with acceptable properties. The details of chemical

30

reduction will be discussed in later section. And what we concern now is the pristine of rGO,

graphene oxide.

1.3 Graphene oxide As we mentioned before, graphene oxide is the precursor for graphene production. The

loose graphite oxide is dispersed in an alkaline solution to form a single atom-like fragment of

graphene-like structure. Because of this nature, graphite oxide has the potential for industrial

production of graphene, which has attracted many scientists to participate in the study.

1.3.1 GO production

Graphene oxide was first prepared by the Oxford University chemist Brodie in 1859 with

potassium chlorate and concentrated nitric acid, mixed in solution to perform oxidation.

Hummer provided a safer, faster and more efficient method in 1957, that is called Hummer’s

method. Recent years, Improved Hummer’s methods was proposed to enhance the efficiency.

1.3.2 GO structure and properties

The structure and properties of GO depend on the methods of oxidation and the oxidation

degree. The graphite layer structure is remained, but the distance between layers is enlarged

Figure16SchemeofGOsheetandthepossibleoxigen-containinggroups:

(A)epoxygroup;(b)hydroxyldroup;(C)carboxylgroup.

31

as two times to that of graphite due to the intercalation of oxygen-containing groups. The

oxygen-containing groups includes epoxide groups, carbonyl, hydroxyl, phenol and some

other impurity groups depend on the choice of oxidant. These groups also appear at the edge

of each single layer.

Graphene oxide features a hydrophilic nature. It’s easy to become hydrate when it contacts

with vapor. This will result in the increase of interlayer distance. With this nature, GO is easy

to dissolve in water and other polar solvents. During the heat treatment up to 300℃, GO

exfoliates and decomposes to be amorphous carbon. By controlling the heating rate and

temperature, it is possible to reduce GO by heat treatment to be graphene.

For the electrical properties of material, GO presents a very different electronic structure

from the perfect graphene, and the GO is insulating due to the oxygen-containing groups in

the basal plane that destroy the π bonds, significantly suppress the conductive capability.

Meanwhile, on the other hand, these groups provide new features, such as dispersion,

compatibility to polymer and the hydrophilic mentioned above. From this point, we can

identify the element of samples of rGO and GO.

1.3.3 GO applications

Base on the special properties of GO like high specific surface area and various functional

groups, GO can be composited with polymer and inorganic materials to modify the bulk

properties or surface properties.

In biomedicine, the high specific surface area makes it possible for drug delivering. Study

showed that GO-PEG carried a kind of non-water-soluble anti-cancer drugs had better

dispersion68. When the concentration of GO-PEG reached 100mg/L, the carrier had no

32

significant cytotoxicity, but it appeared that this system has obvious cytotoxic effect on cancer

cells.

GO can also be used in photocatalytic oxidation69. GO has negative surface charge

attribute in the oxygen-containing groups, and they are able to absorb metal ions like TiO2

material which is a high efficient photocatalyst. This process is applied for the water and gas

purification. Another mechanism for water purification with GO is using reverse osmosis. GO

could be engineered to allow water to pass, but retain some larger ions.

Other applications such as biosensor, artificial photosynthesis, supercapacitor are also

widely studied. For sure, GO as a precursor for rGO production is one of the important

applications.

1.4 Chemical reduction method

1.4.1 Hydrazine hydrate

The most successful reductant for GO reduction is hydrazine hydrate. Hydrazine hydrate

is a type of hydrate of hydrazine (N#H= ∙ H#O ). Hydrazine hydrate has extremely strong

reductivity, it’s able to react intensively with HNO&, halogen, KMnO=, etc. Hydrazine hydrate

can be used to prepare a variety of organic compounds, which is an important raw materials

and intermediates in fine chemical products. It’s also common in synthesis of blowing agent,

pesticide, medicine, water treatment agent and other products.

The process of hydrazine hydrate is simple that GO and hydrazine hydrate are mixed in a

certain proportion, reacts at elevated temperature for a certain time. Xiao70 reduced GO by

hydrazine hydrate and obtained rGO that chemical groups were highly removed, and analyzed

the time-dependence and quantity of hydrazine hydrate influence of oxygen-containing group

removal: When the reaction time was fixed and the mass ratio of GO to hydrazine hydrate

33

range from 10:7 to 10:10, the removal rate of hydroxyl (-OH) reach maximum but that of poxy

(-C-O-C-) was in the minimum. When the mass ratio was fixed, samples from reaction time

from 80 to 100 mins had the maximum chemical group removal rate which is over 99%. Hence,

she concluded that for hydrazine hydrate reactant, the best mass ratio and reaction time was

10:7 to 10:10 and 80 to 100 mins, respectively, for GO reduction.

Ma71 prepared rGO by the same method, and analyzed the products about the structure

and thermal resistance with FT-IR, raman spectrum, XRD and thermogravimetric analysis.

The results indicated that after the reduction by hydrazine hydrate, the sp3 hybrid carbon

atoms were reduced to sp2 hybrid carbon atoms, but the crystallization strength and regularity

decreased, which means that these rGO is not able to be reproduced completely as the

structure of original graphene. For the performance of thermal stability, rGO is much better

than GO. The rGO had a strong absorption capacity for those dyes feature large polarity, such

as rhodamine B and methylene blue.

Moreover, many studies indicated that the quantity of hydrazine hydrate had significant

affection on the removal of functional group. Thus, increasing the amount of hydrazine

hydrate is easier to obtain higher degree of reduction of graphene, but other researches also

referred that too much hydrazine hydrate caused the cluster of rGO72. Hence the quantity

should be well controlled during the reduction.

Although hydrazine hydrate is the strongest reductant for GO reduction demonstrated by

excellent reduction effect, hydrazine itself is also highly toxic and tend to combine with

graphene to form C-N bonds, which is unconducive for the recovery of conjugated π bonds.

Also, considering that it’s flammable, which endanger the both the process itself and the

operator. Therefore, hydrazine hydrate is not an ideal reductant for industrial production of

rGO.

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1.4.2 Sodium borohydride

Sodium borohydride (NaBH=) is an odorless white crystalline powder features a strong

hygroscopicity that it can be decomposed while in the wet air. Sodium borohydride is a

versatile reducing agent which applied in many industries. For GO reduction, sodium

borohydride is able to reduce aldehyde group (-CHO), carbonyl group (-C=O) to hydroxyl (-

OH) selectively, reduce carboxyl group (-COOH) to aldehyde group without reacting with

double bond and triple bond between carbons.

Sodium borohydride is a common reductant for rGO production as hydrazine hydride.

Choi73 compared the rGO reduced by sodium borohydride and hydrazine hydride, the sheet

resistance analysis showed that rGO reduced by sodium borohydride had a lower resistance,

but the C: O compositions are similar. It could be that the accumulation of nitrogen atoms

from hydrazine hydride behaved as donors compensating p-type hole carriers, which reduced

the carriers, therefore, the resistance of hydrazine hydride rGO was higher. Yang74 reduced GO

by sodium borohydride, the results indicated that the specific area was low but the capacitance

was high. Also, the cyclic voltammetry test confirmed its good double-layer capacitor

performance.

However, sodium borohydride is not able to reduce hydroxy group. For further reducing

hydroxy group and enhancing the dispersion of rGO, Tien75 combined sodium borohydride

and ethylene glycol with two-step method, produced silver/rGO composite coating with much

lower sheet resistance.

35

1.4.3 Ethylenediamine

Ethylenediamine is a colorless clear viscous liquid, with strong alkaline, corrosive and

ammonia smell. It’s easily absorb CO# in the air to generate non-volatile carbonates. When it

dissolves into water, it combines with water to form hydrated ethylenediamine.

Shen76 compared the reduction performance and the dispersion influence of N-

butylamine, ethylenediamine and hydrazine hydrate after 80℃ water bath for 24 hours. The

IR, XPS and Raman analysis all showed that both hydrazine hydrate and ethylenediamine had

significant reduction effect, but N-butylamin had little effect. Thermogravimetric analysis,

XRD and conductivity tests indicated that rGO reduced by ethylenediamine is not better than

that by hydrazine hydrate, but the sample doesn’t cluster obviously due to the planarization

trend of the rGO structure, dispersion is much better than that reduced by hydrazine hydrate.

It was assumed that during the reduction by ethylenediamine, chemical surface modification

took place. It is special phenomenon that other reactants do not possess. Rather,

ethylenediamine is less toxic than hydrazine hydrate, but good protection is still necessary.

1.4.4 Sodium citrate

Sodium citrate is a white or colorless crystal, odorless and tastes cool and salty spicy. It’s

a non-toxic specie and usually added to food and beverage as acidity regulator, flavoring agent

and stabilizer. It also has widespread application in various industries. For the reduction of

GO, sodium citrate shows excellent solubility and reducibility. The solubility increases with

temperature and it deliquescence in the moist atmosphere. Wan77 used sodium citrate to

prepare rGO at 90℃ oil bath for 10 hours. The mild reaction condition and the low cost of

reactant make this reduction method easier and safer to achieve. The obtained rGO had good

electron transportation performance, indicating that the oxygen-containing groups were

effectively removed. According to his work, the byproducts of reduction are only CO# and

36

H#O, which are environmentally friendly small molecule that can be easily remove by water

rinsing.

1.4.5 L-cysteine

L-cysteine is another green reductant for rGO production. It’s white crystalline, dissoluble

in water but hardly dissolve in ethanol. L-cysteine is stable in acidic environment and easily

be oxidized to be cystine in neutral and alkalic solutions78. Li79 reduced GO hydrosol at 95℃.

After rinsing, the solid products are moved into ethanol solvent to perform ultrasonic in order

to obtain stable dispersion of rGO in ethanol. The IR analysis, XRD and TG test indicated that

most oxygen-containing groups are removed and the samples showed good thermal stability.

Compare to other reactants, L-cysteine provides rGO a better dispersion and higher

conductivity in ethanol solution.

1.4.6 Ammonia and ammonia vapor

Ammonia is aqueous solution of ammonia gas, colorless and transparent. It’s easy to

volatile and diffuses pungent smell. It’s a reductive agent which is applied in many important

industrial processes such as SCR denitrification system. The rGO reduced by ammonia

features a significant decrease of resistance and less toxic nature than hydrazine hydrate. The

ammonia hydroxide is able to increase the pH value to 9~10 where the rGO reaches the best

dispersion in aqueous solution, which is another advantage of ammonia for GO reduction.

Besides, ammonia is also used for other reduction process for only adjust the pH value in order

to obtain better dispersion80.

1.4.7 Hydroiodic acid

Hydroiodic acid is the aqueous solution of hydrogen iodide. It’s colorless or light yellow

liquid. Hydroiodic acid has strong reducibility as well as strong corrosiveness.

37

For GO which is prepared on a substrate via mechanical exfoliation, epitaxial growth and

CVD, is in the form of thin film. Strong alkali, hydrazine hydrate and borohydride is not

suitable for thin film GO reduction even they are work well for reduction in solution due to the

stiffening and disintegration during the reduction. Instead, Pei81 applied hydroiodic acid as

reductant for GO thin film reduction. The hydroiodic acid rGO film had a significant drawdown

of resistance and carbon oxygen ratio was higher than that of the other chemical methods.

What important is that the flexibility was maintained while tensile strength increased with

respect to the original GO film. The possible reducing mechanism could be ring-opening

reaction of epoxy group and the substitution reaction of a hydroxyl group by a halogen atom.

Hydroiodic acid provided a new method of GO reduction in acidic environment while other

reduction methods were all processed in alkali solutions.

Besides, other reductants such as metal powder, sodium dithionate, sodium hydroxide,

glycan, alcohols, phenolic acid and various reductants were reported and corresponding

special properties of their rGO were discovered, hence specific properties are able to be

obtained with different reductants for specific application.

1.5 Chemical reduced graphene oxide

1.5.1 Properties

The main idea of redox methods is to increase the interlayer distance by inserting the

oxygen-containg groups with graphite oxidation process in order to obtain single-layer

structure, then remove these groups by reductants to recover the original carbon plane with

purely in-plane sp2 orbitals. Since the chemical reduction process is not possible to remove all

the lateral groups away, the byproducts of reduction that may remain on the solution and

difficult to separate and the possible damage of the graphene plane during oxidation and

38

reduction, the properties of these final rGO productions deviate from the perfect graphene

sheets. Hence, chemical rGO is supposed to be discussed individually with respect to graphene.

Many properties such as dispersion, thermal properties, optical properties, especially

electrical properties maybe affected by the reduction methods and the residual groups.

Conversely, the properties unique for rGO such as porous-containing may somehow useful for

some specific applications.

1.5.2 Applications

However, rGO has most of properties that have recovered as graphene. Together with the

unique properties, many applications derive from rGO or enhancement was reported and

produced.

1.5.2.1 Energy storage Graphite is applied as anode material for lithium cell. For improving the capacity,

graphene was introduced as the anode material. The theoretical specific capacity of graphene

anode (LiC& ) is 744mAh/g82, which is almost 2 times of that of graphite anode (LiCU ).

Experimentally, Yoo83 obtained 540mAh/g capacity with pure graphene as anode material for

lithium cell. Wang84 doped graphene anode with nitrogen, greatly enhanced the capacity to

Figure17(a)Graphenewithoutfunctionalgroup;(b)grapheneoxide;

(c)reducedgraphene(containingresidualfunctionalgroupsanddefects).

39

900mAh/g. The factors that influence the storage of lithium for graphene are distance between

graphene sheets, ratio of Raman peaks IW/IX and the C: O ratio.

Transition metal oxide is another choice of anode material features a high specific capacity

of 700~1000mAh/g. However, the drawbacks are obvious. Oxides of transition metal has low

conductivity, also, the significant vary in volume during the releasing of lithium limits its

performance. Carbon based material is suitable to improve the stability of metal oxides as

anode. The composite material of graphene and transition metal oxides are widely studied.

Cobalt graphene composite has a high capacity (1018mAh/g) even after more than 500 cycles85,

the researchers attribute this superior performance to the homogeneous coverage of CoO by

graphene and the small size (5nm) of CoO particles. Iron oxide and graphene composite also

showed a high performance as a three-dimensional porous anode with a high specific surface

area of 95.22 m#/g86. Other graphene composites were also studied, such as manganese and

IV group compounds. Besides, graphene can be composited in cathode for lithium cell as well

to improve the electrical properties.

For developing energy storage systems with higher energy density, lithium-sulfur cells

with theoretical energy densities of up to 2600Wh/kg87 have received extensive attention in

recent years. However, the insulating properties of cathodic sulfur and the high solubility of

lithium sulfur compounds in liquid-phase electrodes are two of the key factors that hamper

the commercialization of lithium-sulfur cells. To overcome these two problems, a conductive

coating system is suggested for not only inhibiting the dissolution of lithium sulfide during

charging and discharging processes, but also significantly improving the conductivity. The

coating material is usually carbon material, graphene is one of them due to its superior

electrical properties, high specific surface area and mechanical properties.

40

The fuel cell is a kind of green energy storage system which converts chemical energy into

electric energy, electrode applies carbon material. The chemical reaction includes hydrogen

oxidation reaction and oxygen reduction reaction. Usually, to enhance the reaction conversion

rate, the platinum material is added on the surface of the carbon material88. The larger specific

surface area it has, the more weight of platinum catalyst per unit filled with can be, thereby

enhancing the overall energy density of the cell. Graphene is proved to be substitution of

traditional carbon materials due to the high electrical performance. Experiments

demonstrated that the N-doped graphene is better than platinum carbon black in electrical

properties89. And graphene is more effective in hydrogen storage than alloy like LaNiM.

In the outermost layer of solar cells, electrode should be transparent in order to let the

solar light pass through. The currently used material is mainly ITO, but the content of indium

in the earth is very rare that increase the price of raw material. Nowadays, ITO transparent

has been widely used in metal oxide solid-state dye-sensitized solar cells (DSSC). Graphene

has the similar work function (4.2eV) to CNTs, which is compatible to the existing components,

hence it is possible to apply in DSSC. Compare with PEDOT-PSS electrode, when

graphene/PEDOT-PSS composite worked as auxiliary electrode, the energy conversion is

much higher than that of pure PEDOT-PSS90. The high surface area and structure defects make

it a high catalytic activity. In addition, monolayer graphene absorption of visible light is only

2.3%, and its infrared light penetration is better than ITO, allowing more sunlight pass through

the solar cells to enhance the photoelectric conversion efficiency, therefore, graphite

application of transparent electrodes in solar cells has been paid much attention.

1.5.2.2 Transparent conductive applications As we mentioned before, ITO is applied in many areas such as solar cell, OLED, LCD,

touch panel, etc. They have different extent requirement for electrode resistance from few

ohms to hundred ohms. The most vital disadvantage of ITO is the brittleness, which limits the

application in soft electronic devices. Recently, soft transparent conductive materials are

41

developed such as conducting polymer91, silver nanowires92, metal mesh93 and CNTs94.

Graphene is another choice. Graphene has transmittance and surface resistance similar to ITO,

what most important is that graphene is flexible. Compare to other material mentioned above,

graphene has lower surface resistance except metal mesh.

1.5.2.3 Graphene composites Thanks to the excellent conductivity, graphene can be composited with insulate material

to provide electrical properties. The most interesting application is the conductive coating. As

many other trials like conducting polymers, our purpose is to introduce conjugate π bonds.

The coating properties can be modified by doping or quantity of graphene in order to adapt to

different applications, like anti-static coating95, conductive ink96, conductive paste97, anti-

corrosion coating, etc98.

The same idea is also suitable for thermal conductive graphene composites. The improved

thermally conductive material can be applied in many industries. For example, the package of

CPU needs thermal interlayer material to ensure the heat transportation from chip to outer

package, and another layer of thermal interlayer material between the CPU package and the

external CPU cooler.

By composite graphene, we can introduce the high mechanical strength into the materials.

This is already successfully developed in textile field to produce high toughness conforms to

fiber99, which is also able to applied in capacitor components.

1.5.2.4 Sensor

The unique two-dimensional structure of graphene makes it a bright future in the field of

sensors. The large surface area makes it very sensitive to the surrounding environment. Even

adsorption or release of a gas molecule can be detected100-102. This detection can be divided

into direct detection and indirect detection. The adsorption and desorption processes of

42

monoatoms can be directly observed by transmission electron microscopy (TEM)103. The

adsorption and release processes of single atoms can be indirectly detected by measuring the

Hall effect method104-105. When a gas molecule is adsorbed on the graphene surface, a localized

change in resistance occurs at the adsorption site. Of course, this effect also occurs in other

substances, but graphene has a high electrical conductivity and low noise of good quality, able

to detect this small resistance changes. Mizuta14 developed a new CO# sensor detecting that

the resistance varied step by step with the absorption and desorption of CO# , acted like

quantum change. This type of sensor is sensitive enough to detect CO# of 30ppb concentration.

Graphene as a new photosensitive material, is expected to utilize its special structure, so

that the photosensitive ability of the element could be better almost 1,000 times than that of

traditional CMOS or CCD, and the energy consumption reduce to 1/10106. Like many new

photographic technologies, this technology will be first applied in the field of surveillance and

satellite imagery. But the study also pointed out that this technology will eventually be applied

to the commercial digital camera.

Features of graphene are available to be modified via the modification of functional

groups, besides, it has large contact area, the thickness down to the atomic scale, the structure

Figure18DNAsequencedetectingbymeasuring(a)ioniccurrent;(b)tunnellingcurrent;(c)in-planecurrent;

(d)in-planecurrentaffectedbyDNAphysisorption.

43

of molecular gates and so on, which enable graphene to bacterial detection and diagnostic

devices. Researchers believed that graphene is a potential material for fast detection of DNA

sequence base on the different resistance when different bases (A, C, G, T) pass through the

nanoholes107(Figure 18). According to the small voltage different, we can figure out which base

is passing through.

1.5.2.5 Semiconductor

Graphene possesses desirable properties for excellent integrated circuit electronic device.

Graphene has a high carrier mobility, and low noise, allowing it to be used as a channel in a

field effect transistor(FET)108. Report indicated that for graphene epitaxial grew on SiC is

suitable for the large-scale production of IC109. IBM researchers have demonstrated a field-

effect transistor (FET) made of graphene material with a cut-off frequency of 100 GHz which

is far higher than the existing applications110. Graphene has special π orbitals in the plane, the

electrons can freely move without scattering by the lattice, presents a ballistic transport which

is able to apply in low noise amplifier. When it is used in doubler by applying the special

ambipolar properties, both electrons and holes are able to transport signals, besides, graphene

doubler needs no filter before signal output which improve the efficiency of the system111.

1.6 Supercapacitor

1.6.1 Supercapacitors

Li-ion, NiMH and other new cells can provide reliable energy storage solutions, and has

been widely used in many fields. It is well known that electrochemical reactions, which

produce Faraday charge transfer to store charge, have a short service life and are more

susceptible to temperature, which is also a problem faced by designers of lead-acid cells. In

addition, large current will directly affect the life of these cells, therefore, for some applications

44

that require long life and high reliability, these cells based on chemical reaction are facing

problems to be solved.

Supercapacitor is also called electrochemical capacitor, which is capable to be applied in

high power charge-discharge cycle and long cycle life devices for energy storage. Although

supercapacitor has energy density much higher than traditional ceramic capacitor and

electrolytic capacitor, the general performance is still inferior to traditional lithium cell and

fuel cell.

According to the storage mechanism, supercapacitor can be classified as double-layer

capacitors and pseudo-capacitor(Figure 19112).

The former one is that the cation absorption of electrode surface results in the forming of

electric double layer to contribute capacitance. The storage performance is positively related

to the specific surface area, conductivity and porosity of the active material. Usually, carbon

based materials are applied as active material, but few carbon electrode materials have

excellent performance in all these three properties. Therefore, carbon materials for double-

layer capacitor need deeper researches. The electrical properties of the latter type are reflected

Figure19Chargestoragemechamisnofsupercapacitor.

45

by the storage effect of reversible redox reaction on the surface of electrode. The energy is

stored and released during the reaction between electrode and electrolyte. Usually metal

oxides and conductive polymers, which contain redox active sites on their surface, are applied.

The material choice of supercapacitor bases on the performance as well as cost, is mainly

carbon based material.

Contrast to double-layer capacitor, pseudo-capacitor has larger capacitance, but the

structure is easy to be destroyed due to the poor conductivity of the material, so the energy

density and cycle performance are relatively poor. On the other hand, double-layer capacitor

features longer cycling time but low capacitance.

To further improve the energy density of super capacitor, a hybrid surpercapacitor, also

known as asymmetric supercapacitor, was developed in recent years. One of the electrodes

applies redox-active material to store and convert energy through electrochemical reactions,

another electrode applies carbon material to store energy through a double-layer. In this

hybrid supercapacitor, the energy storage process still occurs mainly on the surface of

electrodes. The specific capacitance, specific surface area and structural stability of the

electrode material are determinants of the energy storage and conversion performance of the

hybrid super capacitor.

Therefore, for improving the energy density and power density, no matter double-layer

supercapacitor, pseudo-capacitor, or hybrid supercapacitor, the electrode material must have

a large specific surface area, high conductivity and structural stability characteristics.

46

1.6.2 NiO based supercapacitor

For pseudo-capacitor transition metal oxides, their large capacitance due to multielectron

transfer during fast faradaic reactions play an important role. There are many metal oxides

candidate for pseudo-capacitor, such as RuO#, Co&O=, K$MnO# 113. However, the expensive

price of RuO# limited its viability. NiO could be a promising alternative candidate for

electrode material due to its low cost and high capacitance.

To further improve the capacitance of NiO electrode, graphene could be applied to make

a NiO graphene composite. Graphene as a carbon material has been reported to have high

specific capacity more than 200F/g114. Although the specific surface area of graphene is not as

high as activated carbon, graphene has excellent conductivity and mesoporous structure which

is better than microporous structure of activated carbon where the electrolyte is not able to

permeate. Also, considering the high mechanical strength, graphene can be an ideal material

for electrochemical capacitor.

47

CHAPTER 2 MATERIALS AND

CHARACTERIZATION METHODS

2.1 Graphene oxide To produce reduced graphene oxide, GO source is the fundamental material. As we

mentioned before, there are several methods to produce GO from natural graphite. But the

production of graphene oxide is quite mature today, to ensure the constant original condition

and high quality of GO source, in the experiments, GO will be provided by commercial

production. The commercial GO we used is a provided by Graphendo, given in GO fine powder

crystalline.

As we know, GO is hydrophilic and easy to cluster. For the next step reduction, we need a

finely dispersion of GO in water. To ensure the dispersion, pH control and ultrasonic are

applied.

2.2 Chemicals

2.2.1 Ascorbic acid(AA)

L-Ascorbic acid is one of the forms of vitamin C, which has a strong reductive ability and

nontoxic property, naturally exists in living body as a reducing agent, is also widely used as

food additive. The diene alcohol group in AA structure presents reductivity, is easily oxidized

into a diketone and become dehydro-AA. In fields other than food industry, AA is applied in

the preparation of nano metal powder115. For reduction of GO, AA has comparable reductive

48

ability with other reductants but very environmentally friendly, it is an ideal reductant for

production of rGO. In our works, the L-AA is provided by Sigma Aldrich.

2.2.2 Ammonia

The ammonia we mentioned is the aqueous solution of ammonia gas. Ammonia has

strong pungent odor and volatile nature, the solution colorless and weakly alkaline. In this

experiment, the main purpose of ammonia water is to adjust the pH value of the solution so as

to maximize the surface charge of GO and rGO, in order to enhance the static repulsive force

between the suspensions and achieve the effect of stable dispersion.

It should be noted that the ammonia itself also has a certain reductivity, which is able to

reduce GO. But because of the use of ammonia in the test is very little, and the reduction

process at the elevated temperature, the reduction effect can be ignored80. The ammonia in

the work is provided by Sigma Aldrich, given in aqueous solution 32%.

2.2.3 Nickel sulfamate

Nickel sulfamate features a green crystalline which is easily dissolve in water and ethanol.

It’s aqueous solution is acidic. Nickel sulfamate is a highly precise and quality surface

treatment chemical which utilized the outstanding plating characteristic. In our work, nickel

sulfamate commercial plating solution was employed to carry on the Nickel active layer

deposition.

2.3 X-ray inspection X-ray fluorescence is a fast and non-destructive measurement. The short-wavelength high

energy X-ray or γ-ray make the specimen ionize. When energy received is higher than energy

required for inner electrons to escape away, the electronic structure will be unstable and tend

49

to compensate the inner orbital by outer electrons. This process cause an emission of light,

which is corresponding to the energy difference between to orbitals. By detecting this

illumination, we can obtain the information from the specimen.

The thickness detection is processed by collecting the intensity of a specific material

response. The response of substrate varies with the thickness of coating. Because the substrate

material hide deeper cause its response weaker and coating response stronger.

2.4 SEM SEM is a microscopic inspection method between transmission electron microscopy

(TEM) and optical microscope. It can directly use the material properties of the sample surface

to make microscopic imaging. SEM generates an image of the sample by scanning the sample

with a focused electron beam. When a very fine high energy incident electron beam strikes the

surface of a sample, the excited region will produce secondary electrons, Auger electrons,

characteristic x-rays and continuum X-rays, backscattered electrons, and so on.

The imaging depends mainly on the secondary electrons to give morphology in the scale

down to nano scale. Backscattered electrons refer to a portion of the incident electrons that

are reflected by the inner bulk atoms, including elastic backscattered electrons and inelastic

back reflection electrons. The yield of backscattered electrons increases with the increase of

atomic number. Therefore, using back reflection electron as imaging signal can not only

analyze the topographic features, but also can be used to show the atomic number contrast,

qualitative analysis of components.

50

2.5 Raman Raman spectroscopy is based on the analysis of the Raman scattering effects that take

place when light passing through the sample. The different scattering spectra are generated by

different incident frequencies, sample lattice, vibration mode and other mode. For the study

of graphene and its derivatives, Raman spectroscopy is a very effective tool.

The basic principle of Raman spectroscopy: When the sample receives different incident

lights, the incident lights interact with the electron cloud and the molecular bond, results in

the electron transitions. The electron transitions to the virtual state, then the electron

Figure20Rayleighscatteringhasemission lightwavelengthequals to

incidentlight,butRamanscatteringhaswavelengthchanges.

Figure21Ramanspectrumofgraphene.

51

immediately falls back from the virtual level to low energy state with a corresponding light

emission. The emission light is a scattered light. When the final state is different from original

state, the scattering is called Stokes scattering or Anti-stokes scattering. They are both belong

to Raman scattering; the emission light wavelength will be different from the incident light.

By analyzing the frequency, intensity, peak position and half-width of the Raman

spectrum of the sample, we can get the information of the composition, dispersity, lattice

symmetry and so on. Based on the present study, we know that there are three characteristic

peaks in the Raman spectrum of graphene116-118 (Figure 21):

D peak (around 1350cm45) due to the defects of graphene which is explained by the double

resonant Raman scattering theory;

G peak (around 1580cmcm45) due to the in-plane E2g vibration of carbon atoms, is the

main characteristic peaks, representing the order degree of crystal, which is very sensitive to

stress, reflecting the layer number of graphene when combine with the intensity of 2D peak.

Increasing the number of layer, n, the position of G peak moves towards low wave number, the

displacement has the relation 1/n with layer number. In addition, G peak is easy to be affected

by doping, hence the position and half-width can be an indication of doping level.

2D peak, also called G’ peak, due to dual-phonon resonance, is easily affected by incident

light, for 514nm incident light, 2D peak locates around 2700 cm45, which is also an indication

of layer number.

Intensity ratio R=ID/IG has an experience relation119 with grain size

L] ∝ 44 (𝐼W 𝐼X) ∝ 1 𝑅

where L] represents the average size of sp# orbital area in graphene structure.

52

2.6 Surface charge analysis Surface charge is an important parameter reflecting the solid ion electrodynamics

behavior. The stable dispersion of the graphene oxide and graphene suspension depends on

the negative potential of the surface of the nanoparticle. If the surface potential of graphene

oxide and graphene cannot produce a sufficient repulsive force therebetween, the suspension

will be unstable, leading to the occurrence of clusters.

The studies show that the surface of the graphene oxide is negatively charged80, 120. As the

pH value increases, the surface negative charge of the nanoparticles increases obviously. At

pH = 10, the surface potential reached a maximum121. Compare to graphene, the absolute value

of the surface potential of the graphene oxide is remarkably improved, thereby generating a

larger electrostatic repulsive force between the particles, which contributes to the formation

of a stable colloidal suspension. According to the same study, after the absolute value of the

potential reached its maximum value, the potential decreased and the dispersion became

unstable with the increase of pH value.

2.7 Potentiostat Potentiostat is an electronic instrument that measures the current flow between working

electrode and a reference electrode while controlling the voltage difference between them.

Potentiostat works with three-electrode system including working electrode, counter electrode

and reference electrode immerse in conductive electrolyte. The reference electrode is needed

because we would like to study electrochemical reactions only in a specific electrode, the use

of reference electrode, which remains constant potential, allows us to measure the reaction

with no interference or contribution from the counter electrode.

In our work, anodic oxidation and cyclic voltammetry test were processed in assistance of

potentiostat model 273A provided by EG&G (Figure 22).

53

2.7.1 Anodic oxidation

Anodic oxidation is the electrochemical oxidation of a metal or an alloy. During this

process, the surface of the metal or alloy work as anode. The aim of this step is to thicken the

oxide/hydroxide film on top of metallic Nickel. Some oxide coatings are perfect protection of

metal substrates, preventing them from corrosion. If the oxide layer has stronger mechanical

properties, the artificial anodic oxidation is necessary too. Although the oxidation layer of

metal could form naturally in the atmosphere, but the structure and thickness is beyond

control. Positive anodic oxidation provides a method to define the desired structure and

thickness by controlling the oxidation current, duration and so on.

2.7.2 Cyclic voltammetry

Voltammetric analysis is a method of electrochemical analysis based on the voltage-

current behavior of the electrode in the solution. Different from potential analysis, the

voltammetric method measures the current of system at a certain potential, while the potential

analysis method measures the system potential at zero current. This method controls the

potential of the electrode to scan at different rates in one or more triangular waveforms over

time. The potential range includes where different reduction and oxidation reactions occur

alternately on the electrode, and the current-potential curve is recorded. According to the

shape of the curve, it is possible to judge the reversibility of the electrode reaction, the

Potentiostat

I V S

Vv

CAVi

Rm

C

C

CEWE

RE

I/E Converter

Control Amp

Electrometer

Cell Switch

Figure22potentiostatmodel273AprovidedbyEG&Gandschemeofthecircuits.

54

possibility of interphase adsorption, new phase formation, and the nature of the coupling

reaction. It is commonly used to measure electrode reaction parameters, determine its control

steps and reaction mechanism, and observe what reactions can occur within the entire

potential scan range, and its nature. For a new electrochemical system, the preferred method

of research is often cyclic voltammetry.

The pulse voltage of the isosceles triangle is applied to the working electrode, and the

resulting current-voltage curve consists of two branches. If the first part of the potential scan

is the cathode scan, electro-active substances is reduced in the electrode, resulting in reduction

wave, then the latter part of the potential scan is the anode scan, the reduction product will re-

oxidation in the electrode to produce oxidation. Therefore, a triangular wave scan to complete

a reduction and oxidation cycle, so it is called cyclic voltammetry, the current-voltage curve is

called cyclic voltammogram.

55

CHAPTER 3 EXPERIMENTAL WORKS

3.1 Chemical reduction of GO and GOH by

ascorbic acid(AA)

3.1.1 Precursor preparation

The experiments are divided into 2 groups. One is the reduction of GO, another is the

reduction of GOH. For the first group experiment, we used GO at a concentration of 0.1mg/mL

dissolve in aqueous solution. Ammonia was added in order to adjust pH value to 9~10,

promoting the colloidal stability of GO sheets. For further breaking the clustered GO,

improving the dispersion of GO sheets, ultrasonic treatment was performed for 6 to 10 minutes.

After sonication, the solution was placed in a magnetic stirrer to maintain uniformity and

dispersion of the solution.

The resulting solution were considered to be monolayer/FL GO sheets suspension which

is ready to be highly efficient reduced by the later procedures.

3.1.2 Chemical reduction with ammonia

The resulting solutions were divided into four equal parts, in each parts, the ascorbic acid

was added at a concentration of 0mM / 0.5mM / 1mM / 2mM respectively, where the first part

is the reference. The solutions with ascorbic acid at concentration of 0.5mM / 1mM / 2mM

were then heated up to 95℃ and kept in temperature not lower than 95℃ for 15 minutes to

perform the reduction, while the reference was kept in room temperature. After that, all

solutions were cooled naturally down to room temperature. The same procedures were

performed in the second group with GOH at the same concentration.

56

3.1.3 Chemical reduction without ammonia

As we mentioned, the ammonia could be an important factor for dispersion because it is

able to modify the pH value of solution, further increase the surface charge of the reduced

sheets. For comparison, the same experiments were performed except the pH adjusting. In

order to reach similar dispersion, we elongated the duration of ultrasonic to 20 minutes.

3.1.4 Graphene deposition preparation

The substrates we used were silicon wafers of the size 1cm×1cm. After cutting, the

substrate sheets were cleaned by water and acetone in order to remove inorganic and organic

impurity. Each of the solutions was dropped onto the substrate precisely in 200 µL, then dried

naturally at room temperature. The final dried up samples were rGO coatings which were

ready to perform the analysis.

3.1.5 Characterization and discussion

For each of the samples, we performed surface charge analysis. For rGO and rGOH

samples with ammonia, Raman spectroscopy was performed.

3.1.5.1 Characterization of surface charge The results of surface charge test are listed below:

Table1surfacechargeofrGOandrGOHreducedbyascorbicacidatdifferentconcentration.

Surface charge potential

(mV)

Concentration of AA (mM)

0 (reference) 0.5 1.0 2.0

rGO -35.6 -34.0 -36.5 -36.3

rGOH -57.7 -55.2 -50.1 -57.7

57

The surface charge of rGO and rGOH are both negative, which provided repulsive forces

between the sheets. Hence the reduced suspension can have better dispersion. rGOH sheets

carried more negative charge than rGO did because of the hydrogen groups.

Table2suefacechargeofrGOwithandwithoutammoniapromoteddispersion.

Surface charge potential

(mV)

Concentration of AA (mM)

0 (reference) 1.0 2.0

rGO with ammonia -35.6 -36.5 -36.3

rGO without ammonia -13.6 -18.5 -19

The comparison of surface charge of rGO with ammonia and that of rGO without

ammonia shows that the ammonia significantly enlarged the surface charge of rGO, which is

consistent with the data provide in the essay. The study80 indicated that even with high

concentration ammonia, its reduction effect is less enough to be ignored compare with the

reduction effect of ascorbic acid. We can consider that ammonia in our experiment only works

as a modifier of pH value of solution instead of a reductant. Hence, ammonia decreased the

trend of cluster and further enhanced the electrodynamic effect in the later experiments that

utilized the negative surface charge of rGO.

3.1.5.2 Characterization of Raman spectrum For materials consist with only carbon, the chemical bonds are all homonuclear diatom

symmetry bonds. They are inactive to IR detection, which requires dipole changes. In general,

homonuclear diatom are not sensitive, but highly active to Raman. Hence it is easy to get

informations for graphene by performing Raman inspections.

For rGO/rGOH with three different concentration of reductant, the comparisions of the

characteristic peaks are as follow:

58

ThethreepeaksinthespectrumsareD,G,G’(2D)peaksfromlefttoright.

Figure23RamanspecrumofrGOandrGOHreducedby0.5mMascorbicacid.

Figure24RamanspecrumofrGOandrGOHreducedby1mMascorbicacid.

59

The characterization of rGOH is much worse than that of rGO, all the characteristic peaks

are broad and 2D peaks are even not obvious that we could hardly tell information from this

picture. The curves for rGOH is similar to that for thermal reduced graphene oxide, which

could be other reducing mechanisms worked during the reactions122. The later studies would

focus on rGO samples.

The intensity ratios 𝐼#W/𝐼X of rGO reduced by ascorbic acid are 0.38 / 0.35 /0.28 for

reductant concentration at 0.5mM / 1mM / 2mM respectively, among three spectrums, 1mM

ascorbic acid rGO has relative high G peak. These results indicated that ascorbic acid worked

in reducing GO, but not effective enough. The rGO reduced by 1mM ascorbic acid featured the

best reduction effect: The G peak is the high, indicated that the in-plane vibration of C-C bonds

were well recovered; 𝐼#W/𝐼X ratio was the relatively high, indicated that the layer number of

graphene were fewer; 𝐼W/𝐼X was the highest, which indicated that the reduction effect of the

sample could be ideal and the defects is minimal among three samples.

Figure25RamanspecrumofrGOandrGOHreducedby2mMascorbicacid.

60

Figure26RamanspecrumofrGOreducedbyascorbicacidatdifferentconcentrations.

Figure27RamanspecrumofrGOHreducedbyascorbicacidatdifferentconcentrations.

61

The result rGOH samples are not consistent with what we expected. G’ peak is weak and

broad due to the low degree of graphitization123. Hence in the later works, rGO was the only

one we used to perform the electrochemical experiments instead of rGOH.

3.2 Electrochemical deposition of nickel/rGO

composite

3.2.1 Electrochemical deposition

We chose austenite 304 stainless steel as substrate, which acted as working cathode, for

nickel deposition. Steels were cleaned by water then acetone to remove the impurity. The

substrate was seal partially with the rectangular exposure area of 4cm# by Kapton tape. The

counter anode was nickel metal mesh.

Figure28GOwasbeingreducedwith1mMAAat90℃.

62

Electrolyte was prepared by mixing 70mL commercial nickel sulfamate solution and

30mL rGO solution that reduced by ascorbic acid at concentration of 1mM. Also, for

comparison, we also prepared electrolyte mixed with 70mL nickel sulfamate solution and

30mL GO solution as reference.

To keep the current density at 10mA 𝑐𝑚#, the electrochemical deposition was performed

under a certain current of 40mA, at an elevated temperature of 50℃ for 52 minutes. After

deposition, the sample was cleaned by DI water and completely dried by nitrogen gas. The

samples were obviously coated by a layer of grey nickel metal.

3.2.2 Anode oxidation of nickel layer

In order to obtain the useful metal oxide for supercapacitor, the last step samples needed

to be oxidized to nickel oxide(NiO). Because the oxidation processes were performed in weak

alkali environment since sodium perborate (Na#B=O2) was chosen as the electrolyte, the final

oxidation products would be nickel oxide hydroxide(NiOOH).

At this stage, the nickel coated stainless steel worked as anode and titanium mesh worked

as cathode. The electrolyte concentration was 1mM (sodium perborate). We set the current

density at 150𝜇𝐴 𝑐𝑚#, the current we set for 4𝑐𝑚# anode is 600𝜇𝐴. The anodizing process

was performed at room temperature with companion of magnetic stirrer for 1 hour. After the

anodizing, the samples were cleaned by DI water and dried by nitrogen gas. The oxidized

samples appeared to be darker than they are before, indicating that the surfaces were oxidized

to be NiO and NiOOH which have deeper color nature than nickel metal.

3.2.3 Characterization of thickness

Thickness tests was performed by XRF provide by Fischer Technology, model XAN –FD

BC. The GO/NiOOH layer is 11.45μm in average and the rGO/NiOOH layer is 11.04μm in

63

average. The results indicated that nickel was successfully deposited on stainless steel

substrate with a satisfactory thickness for the next step oxidation of nickel metal.

3.2.4 Characterization of cyclic voltammetry

The cyclic voltammetry tests were performed inside 6mM KOH electrolyte to ensure

enough conductivity and Ag/AgCl reference electrode was used to ensure the accuracy of

obtained data for working electrode. The potential scans were performed in triangular

waveforms, window is from -0.4V to -0.9V, started and ended at -0.4V.

The pseudocapacitance of the composite films can be estimated from the cyclic

voltammetry diagrams by integrating the area under the current-potential curve.

Figure29photoofGO/NiOOHsurface(left)andrGO/NiOOHsurface(right).

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Figure30CyclicvoltammetriccurveforGOandrGOwithscanrate10mV/s.

Figure31CyclicvoltammetriccurveforGOandrGOwithscanrate20mV/s.

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The diagrams above are the cyclic voltammetry results for each scan rates. We can see

from each picture that the curve of GO and rGO composite samples appear

pseudocapacitances due to their composite material nickel hydroxide and the curves for rGO

are apparently show better pseudocapacitance effect than GO at each scan rate.

Figure32CyclicvoltammetriccurveforGOandrGOwithscanrate50mV/s.

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The two diagrams below are the comparison of curves for each scan rate. In both GO and

rGO composite. We can see that the faster scan rate induces the higher current.

Figure33CyclicvoltammetriccurveforGOwithdifferentscanrates.

Figure34CyclicvoltammetriccurveforrGOwithdifferentscanrates.

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Table3IntegratedareasforGOandrGOwithdifferentscanrates.

Scan rate(mV/s) 10 20 50

Integrate area

(mV·mA)

GO 0.928049 3.984490 10.210200

rGO 4.145590 8.699690 16.441200

By comparing the data of GO and rGO, we can easily identify that integrate areas of rGO

with each scan rate are apparently larger than that of GO. The addition of rGO in the composite

had significantly enlarged the capacitance of the nickel based supercapacitor.

Combine the results of curves and data in the table, we can suggest a possible way for the

improvement of capacitance for the electrochemical capacitors by using rGO/nickel composite.

3.2.5 Characterization of SEM

Figure35SEMimageofGO/NiOOHcompositesurface15KXmagnification.

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Figure36SEMimageofGO/NiOOHcompositesurface1KXmagnification.

Figure37SEMimageofrGO/NiOOHcompositesurface15KXmagnification.

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From the SEM images, we can see that the composites of NiOOH/rGO and NiOOH/GO

were both well coated on the surface of substrate respectively in 15K times magnification image.

The particle sizes ranged from 0.1µm to few µm scale. In the same area size, there were more

the surface particles in rGO/nickel hydroxide than in GO/nickel hydroxide, which means the

former composite featured finer particle sizes.

In the images of 1K times magnification, we can see that there were few large particle in

the surface of coating. They could be carbonaceous material, but further confirmation is still

needed.

Figure38SEMimageofrGO/NiOOHcompositesurface1KXmagnification.

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71

CHAPTER 4 CONCLUSIONS

In this thesis, we reduced commercial GO and GOH powders with ascorbic acid, a non-

toxic and environmentally friendly material, with a simple one-step process and studied the

compositions and properties of the reduced samples by mean of surface charge analysis and

Raman spectroscopy. The results indicated a successful reduction of GO that rGO appeared

negative surface charge for enhanced dispersion and electrodynamic process, and ideal

composition for recovered electrical properties. These fine functional powders could be useful

for making composites to improve the charge carriers transport properties of other material.

We proposed an application in the electronics field that rGO was composited with nickel

hydroxide, which is used as the electrode material for supercapacitors, to improve the

pseudocapacitance of the existing nickel application. The nickel source and graphene

suspension were mixed together to deposit a layer of NiOOH/rGO composite. According to the

results of thickness test, the composite was successfully coated on the substrate. The obtained

NiOOH/rGO sample and the reference sample were tested with cyclic voltammetry and SEM.

What clear from the electrochemical inspections is that rGO paired NiOOH layer appeared

higher pseudocapacitance than GO paired NiOOH: the capacitances of NiOOH/rGO electrode

were all higher than that of NiOOH/GO electrode at each scan rate. The improvement is clear

and signicifant.

This result made it possible to develop a high capacitance substrate with rGO

enhancement suitable for supercapacitor applications with a non-toxic and environment-

friendly reductant, ascorbic acid. Future work should be devoted to improve yield of the

reduction reaction and to optimize the quality of the final products; also, it would be useful to

try to obtain a prototype of NiOOH/rGO based supercapacitor to test its actual electronic

properties.

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73

ACKNOWLEDGEMENT

First of all, I would like to thank Professor Luca Magagnin for giving me this opportunity

to undertake this work, also for his support and guidance throughout the this work.

Sincere and special gratitude goes to my advisor Lorenzo Pedrazzetti who turned me from

a stranger of the laboratory to a skilled researcher. He provided lots of help that was

indispensable during the work. Also, I would like to thank all the other friends in the lab that

kindly helped me whenever I need.

The most remote thank goes my parents, supported me mentally and financially to study

abroad. Thank you all my new and old classmates and professors who composed a colorful life

in Italy for these two years.

I must thank my University, Politecnico di Milano, for the financial supports that

acknowledged my academic achievements and for the academic guidance that improved my

study ability.

74

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