Development of graphene oxide based functional materials

... reduction is one of the most important reactions of GO The product of the reaction is usually name as reduced Graphene Oxide (rGO) rather than graphene This is because the structure of rGO is different... between the layers of GO flakes Thermal exfoliation of GO flakes takes place through extrusion of carbon dioxide produced By measuring the mass loss of GO and the use of equation of states, the calculated... graphene oxide film via localized decoration of Ag nanoparticles, Nanoscale, 2014, 6, 3143 – 3149 H F Teoh, Y Tao, E.S Tok, G.W Ho, C.H Sow., Direct laser-enabled graphene oxide- Reduced graphene oxide

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DEVELOPMENT OF GRAPHENE BASED

FUNCTIONAL MATERIAL

TEOH HAO FATT

(B.Eng (Hons), NUS)

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

NUS GRADUATE SCHOOL FOR INTEGRATIVE

SCIENCES AND ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2014

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Declaration

I hereby declare that the thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis

This thesis has also not been submitted for any degree in any university previously

Teoh Hao Fatt

31 March 2014

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Acknowledgments

The work of this thesis would not have been possible without the advice and assistance from many individuals First and foremost, I would like to express my gratitude to my supervisor, Professor Sow Chorng Haur His patience, support and guidance has helped me tremendously in the difficulties that I have encountered in the last four years Without him, I would never be able to complete my PhD studies Secondly, I would like to thank my co – supervisor, Professor Tok Eng Soon for his advice and efforts to establish some connections with research institutes such as Agency for Science, Technology and Research (A*Star), Temasek Laboratory and DSO National Laboratories Working with researcher from other research institutes has allowed me to broaden my perspectives in the research industry Thank you Wei Beng from Temasek Laboratory and Alvin DSO, for showing and sharing with me the

“secret recipe” to synthesize good quality Graphene Oxide Without the help from both of you, I would not be able to complete this thesis

Professor Sow has assembled a great team of aspiring scientists in the lab and I would like to thank them for their great support and patience in coaching me some of the experimental characterization tools used in this thesis They are Dr Lu Junpeng, Dr Lim Xiaodai Sharon, Zheng Minrui and Tao Ye Also, I would like to thank some of the Final Year Student undergraduate students in the lab who have made my research

in the lab a fun and fulfilling one I wish you all the greatest success in wherever your careers take you

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I would also like to thank my family members for supporting me emotionally throughout these four years of research It was very draining emotionally when I had numerous failures in my experiment

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Contents

List of Abbreviation 10

List of Publications 11

Chapter 1 : Introduction to Graphene Oxide 12

1.2 Synthesis of Graphene Oxide 14

1.2.1 Brodie Recipe for making GO solution 14

1.2.2 Hummers Method 15

1.3 Chemical Structure and Properties of GO 16

1.3.1 Chemical Structure of GO 16

1.3.2 Chemical Properties and Reactivity of GO 18

1.4 Photoreduction of Graphene Oxide 22

1.4.1: Photothermal Reduction of GO 23

1.4.2: Photochemical Reduction of GO 24

1.4.3: Laser Reduction of GO 24

1.5 Challenges faced and Motivation 25

Chapter 2 : Experimental Method – Fabrication and Characterization of GO/rGO 29

2.1 Synthesis of Graphene Oxide 29

2.2 Micro - patterning of GO 31

2.2.1 Focused laser pruning technique 31

2.2.2 Camera Flash Lithography 32

2.3 Characterization of GO film 33

Chapter 3 Direct Laser-Enabled Graphene Oxide – Reduced Graphene Oxide Layered Structures with Micropatterning 39

3.1 Introduction 39

3.2 Experimental Preparations 40

3.3 Results and Discussion 41

3.3.1 Effect of laser power and temperature of the substrate on the development GO-rGO micropatterns 41

3.3.2 Feature size and resolution of GO – rGO micropatterns 45

3.2.3 Proposed Mechanism 48

3.2.4 Development of 3-dimensional GO-rGO layered microstructure 50

3.4 Conclusion 53

Chapter 4 Microlandscaping on Graphene Oxide Film via Localized Decoration of Ag Nanoparticles 55

4.1 Introduction 55

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4.2 Experimental Methods 57

4.3 Results and Discussions 60

4.4: Applications 73

4.4.1: Surface Enhanced Raman Scattering (SERS) 73

4.4.2: Photocurrent Generation 75

4.4.3: Electrochemical Sensing of H2O2 79

4.5 Conclusion 82

Chapter 5 Spontaneous Decoration of Au Nanoparticles on Micropatterned Reduced Graphene Oxide Shaped by Focused Laser Beam 83

5.1: Introduction 83

5.2 Experimental Methods 84

6.3 Results and Discussion 87

5.4 Conclusion 100

Chapter 6 Electrical Current Mediated Interconversion Between Graphene Oxide (GO) to Reduced Grapene Oxide (rGO) 102

6.1 Introduction 102

6.2: Experimental Methods 104

5.3: Results and Discussion 105

6.4 Conclusion 114

Chapter 7 : Conclusion and Future work 115

7.1: Conclusion 115

7.2: Future Work 116

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List of Figures

Figure 1-1: Proposed chemical structure of Graphene Oxide Adapted from Ref[25] 17 Figure 1-2: Reduction Mechanism for GO using hydrazine Adapted from Ref M B Smith and J March, March’s Advanced Organic Chemistry: Reactions, Mechanisms and Structure, John Wiley & Sons, New Jersey, 6th edn, 2007 20 Figure 1-3: PT: Photo-thermal reduction, PC: Photo-chemical reduction Adopted

from Ref Y.L Zhang, L Guo, H Xia, Q.D Chen, J Feng, H.B Sun, Advanced Optical Materials, 2, 10-28 (2014) Error! Bookmark not defined

Figure 2-1: (a) Pre-oxidation step to expand graphite powder (b) Oxidation of

graphite (c) GO mixture was purified using vacuum filtration 31 Figure 2-2: (a): Schematic Diagram of the laser system (b) GO sample on top of the movable stage (c) TV with real – time observation of the laser irradiation

process Error! Bookmark not defined

Figure 2-3: (a) Schematic diagram of camera flash reduction (b) Sunpak camera flash (c) GO film of thickness 10um before flash reduction (d) after flash reduction 33 Figure 2-4: (a) JEOL JSM – 6700F SEM (b) SEM image of GO film with

approximately 10μm thickness 34 Figure 2-5: (a) AFM image of a laser cut GO irradiated with a 532nm DPSS laser (b) Height profile of the line scan in (a) (c) AFM machine 35 Figure 2-6: (a) Raman Spectroscopy (b) Typical Raman shifts of HOPG, GO and rGO 36 Figure 2-7: (a) XPS setup in the surface science lab (b) Typical C1s spectra of GO before reduction (c) after reduction 37 Figure 3-1: (a) Schematic diagram showing the experimental setup (b) Taiji logo with white portion being GO and light blue being rGO cause by laser reduction (c) Positive development of 2D GO Taiji structure - Laser scanned region is being removed by 90 seconds of sonication (d) Negative development of 2D rGO Taiji structure –Laser unscanned region is being removed by 90 seconds of sonication 43 Figure 3-2: (a) Scientist guide showing the temperature and laser power favorable for positive and negative development of patterns on GO The time of sonication was kept at 90 seconds (b) Optical images of rGO with laser power (right) 15.6mW and (left) 30.1mW respectively The temperature of the stage was kept constant at 80oC (c) AFM measurements on the surface roughness of laser induced rGO with laser power 45 Figure 3-3: (Top) Showing unsuccessful development of the pattern where all GO are sonicated (Middle) Showing a successful positive development of the pattern,

(Bottom) Showing an unsuccessful development of the pattern where all GO and rGO survived 90s sonication 46 Figure 3-4: (a) Limitation of the laser development technique The smallest feature that can be wriiten was approximately 5um (b) A checker box pattern with 10um gap between each rectangle (c) NUS logo with spacing of 5um for each letter (d)

Rectangle strip with 10um spacing in between 47 Figure 3-5: (a) schematic diagram for 3D patterning of GO Positive development is used to develop the 3D patterns (b) Checkered boxes after removing the scanned region through sonication The sample was then spin coated with GO again to obtain (c) where yellowish region denotes a thicker GO region and light blue region denotes

a thinner GO region (d) “Staircase” like structure with two coatings of GO AFM

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measurements show that each step was about 65-70nm (e) Taiji logo with one

coating of GO (f) two coating of GO (g) Two circular dots inside the logo are being removed through sonication after scanning with focus laser beam 52 Figure 4-1: Schematic diagram of the focused laser setup When focused laser of wavelength 532nm is scanned across GO with silver nitrate solution, Ag+ is reduced, forming Ag nanoparticles on the laser-scanned region During the same process, the focused laser beam will also reduce the underlying GO 60 Figure 4-2: (a) SEM image of a square (50µm x 50µm) decorated with Ag, NPs (b) Magnified view of side of the square It suggests that AgNPs only formed on the laser irradiated region (c) Size of the AgNPs formed ranges from 50 – 100nm Inset is the EDX (d) Without GO, no AgNPs is formed As the laser power increases, there is a red shift in the absorption peak, indicating a larger particle size 61 Figure 4-3: (a) Two boxes of sizes 50 x 50um and 100 x 100 um The smaller box was first irradiated under ambient condition The larger box was then scanned in 0.01M of AgNO3 solution (b) Showing the zoom in of the red box in (a) and observed a

significant decrease in the region prior treated with laser (smaller box) (c) showing the zoom in of the large box (d) zoom in for the smaller box 63 Figure 4-4: (a) XPS Ag3d Spectra (b) Silver Auger lines for Silver foil (c) XPS Ag3d spectra for rGO-Ag composite (d) Silver Auger lines for rGO - Ag 65 Figure 4-5: (a-c) TEM images of silver nanoparticles (d) HRTEM of silver

nanoparticles Insert is the Selected Area Electron Diffraction (SAED) pattern 67 Figure 4-6: SEM image of Ag, NPs decorated GO when GO was irradiated with 532nm focused laser beam in the presence of 0.01M of AgNO3 solution with laser power of (a) 5.5mW (b) 3.5mw and (c) 2.0mW Scale bar for (a-c)100nm (d) Size distribution of AgNPs for (c) (e) Box-and-Whisker plot for distribution of AgNPs under different conditions (f) Number density for AgNPs at different laser powers and concentration 70 Figure 4-7: SEM Images of rGO – Ag composite with micropatterns 72 Figure 4-8: (a) Silver nanorod formation with citric acid added to stabilized Ag(111) surface (b) Without the addition of citric acid 73 Figure 4-9 (a) Raman spectra for (black) as deposited GO film, rGO film after treated with focused laser beam of (Purple) 2.0mW, (Yellow) 2.5mW, (Blue) 3.0mW and (Red) 3.5mW (b) Raman Spectra of Ag – rGO film produced with focused laser beam of power (Purple) 2.0mW, (Yellow) 2.5mW, (Blue) 3.0mW, (Red) 3.5mW (c) Red squares show the Raman enhancement for G peak and black dots show

enhancement of D peak Blue line shows the number density of silver nanoparticles 74 Figure 4-10: (a) Optical image of the fabricated photocurrent device (b) Photocurrent

of Ag-rGO composite measured under 2.31 x 10-5 Torr with laser power of ~10mW and wavelength 404nm (blue), 532nm (green) and 808nm (red) (c) Photocurrent of rGO composite measured under 2.31 x 10-5 Torr with laser power of ~10mW and wavelength 404nm (blue), 532nm (green) and 808nm (red) (d) Log-log plot of

photocurrent generated with 405nm laser of varying intensities 77 Figure 4-11: CVs of (a) Ag-rGO/ ITO and (b) rGO/ ITO in PBS solution with absence and presence of H2O2. Scan rate: 50 mV/s; (c) CVs of Ag-rGO/ ITO in N2-saturated PBS solution with different concentration of H2O2 and (d) the corresponding cathodic current at -0.75 V against concentration of H2O2 81 Figure 5-1: Schematic diagram of the focused beam laser setup The laser beam is directed by mirrors into a beam splitter in the optical microscope, followed by

focusing the laser on the sample by a lens 85

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Figure 5-2: (a) SEM image showing that Au NPs selectively decorated within the box (b) Zoom in from one of the side of the square in (a) Inset is the EDX of the red box (c) Zoom in showing the size of each Au NPs was approximately 20nm Micro – patterns of Au NPs can be created showing a (d) snow – flake (e) Radioactive Hazard logo (f) Checker box design 87 Figure 5-3: (a) TEM imaging indicates polycrystalline structure of Au NPs Inset: Lattice spacing of crystalline Au found to be 0.245nm (b) SAED pattern of Au NP shown in (a) 88 Figure 5-4: (a) Optical microscopy image of reduced GO pattern across two

conducting electrodes and an isolated reduced GO pattern on the GO film (b) SEM image of reduced GO across the two conducting electrodes showing no decoration of

Au NPs (c) SEM image of the isolated reduced GO pattern decorated with Au NPs 89 Figure 5-5: (a): XPS C1s spectrum of Carbon, (b) 4f of Gold, (c) O1s of Oxygen 91 Figure 5-6: (a-b) SEM images of Au NPs on rGO of laser powers 15mW and 40mW respectively (c) Semi-log plot indicating the relationship between number density of NPs formed and their sizes, with the laser power used for irradiation 93

Figure 5-7: (a-c) Optical images of GO reduced by laser of powers 15mW, 20mW, and 40mW respectively (d) AFM measurements of surface roughness of rGO reduced with different laser powers 94

Figure 5-8: SEM images of Au NPs of different sizes for different concentration of HAuCl4 (a)0.01M (b) 0.001M and (c) 0.00001M (d) Relationship between size of NPs and concentration of HAuCl4 96 Figure 5-9: SEM images of Au NPs on rGO for concentration (a) 0.01M, (b) 0.001M and (c) 0.00001M of HAuCl4 applied (d) Relationship between size of NPs and concentration of HAuCl4 97

Figure 5-10: Raman Spectra of R6G supported on rGO with different sizes of Au NPs achieved by applying HAuCl 4 of varying concentrations 99

Figure 6-1: (a) Schematic diagram of the setup (b) Optical image showing GO before passing 3V through electrodes (c) Region between the electrodes is fully reduced Process is no longer reversible (d) Sweeping voltages varying from 1V to 4.5V are applied GO between the electrodes are fully reduced when 4.5V is applied across the electrodes 106 Figure 6-2: Showing the I-V characteristics with sweeping voltage from -2V to 2V before complete reduction between the electrodes (Red) and after complete reduction between the electrodes (Blue) 107 Figure 6-3: Snapshots at different timings of the color change when sweeping voltage

of 3.5V is applied across the electrodes (a) 0s (b) 15s (c) 28s (d) 38s 109 Figure 6-4: XPS spectral of C1s (a) As deposited before electrical reduction (b) After electrical reduction A constant voltage of 10V is applied for 15 minutes (c) Electrical re-oxidation by reverse bias of 10V applied for 15 minutes (d) Raman Spectroscopy 110 Figure 6-5: XPS spectra of O1s (a) As deposited before electrical reduction (b) After electrical reduction by applying a constant voltage of 10V for 15minutes (c)

Electrical re-oxidation by reverse bias of 10V applied for 15minutes 112

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List of Abbreviation

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List of Publications

In this thesis

1 Y.C Wan, H.F Teoh, E.S Tok, C.H Sow., Spontaneous decoration of Au

nanoparticles on micro-patterned reduced graphene oxide shaped by focused laser beam, Journal of Appl Phys, 2015, 117

2 H.F Teoh, P Dzung, W.Q Lim, J.H Chua, K.K Lee, Z.B Hu, H.R Tan,

E.S Tok, C.H Sow., Microlandscaping on a graphene oxide film via localized decoration of Ag nanoparticles, Nanoscale, 2014, 6, 3143 – 3149

3 H F Teoh, Y Tao, E.S Tok, G.W Ho, C.H Sow., Direct laser-enabled

graphene oxide-Reduced graphene oxide layered structures with micropatterning, Journal of Appl Phys, 2012, 112, 064309

4 H.F Teoh, Y Tao, E.S Tok, G.W Ho, C.H Sow., Electrical current

mediated interconversion between graphene oxide to reduced graphene oxide, Appl Phys Lett, 2011, 98, 173105

Others

1 H.W Liu, J.P Lu, H.F Teoh, D.C Li, Y.P Feng, S.H Tang, C.H Sow, X.H

Zhang., Defect Engineering in CdSxSe1-x Nanobelts: An Insight into Carrier Relaxation Dynamics via Optical Pump – Terahertz Probe Spectroscopy, Journal of Phys Chem C, 2012, 116(49), 26036 – 26042

2 A.H Zhang, H.F Teoh, Z.X Dai, Y.P Feng, C Zhang., Band gap

engineering in graphene and hexagonal BN antidot lattices: A first principles study, Appl Phys Lett, 2011, 98, 023105

3 Y.Q Cai, A.H Zhang, Y.P Feng, C Zhang, H.F Teoh, G.W Ho., Strain

effects on work functions of pristine and potassium-decorated carbon nanotubes, 2009, 131, 224701

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Chapter 1 : Introduction to Graphene Oxide

Carbon, one of the most abundant elements, is essential to sustain life on Earth since the human body is mainly made up of carbon There are many allotropes of carbon: 3 -dimensional (3D) structure such as diamond or graphite, 2D such as graphene, 1D structure such as Carbon Nanotube (CNTs) as well as 0D structure such

as fullerene (also called carbon buckyballs) Out of these allotropes of carbon, discoverer of graphene and fullerene has each received Nobel Prize in Physics and Chemistry in 2010 and 1996 respectively

Before 2004, research interest in carbon primarily focused on CNTs and Carbon buckyballs When Professor Andre Geim and Konstantin Novoselov published a paper in 2004 that reports a high-level room temperature carrier mobility1

of graphene obtained by mechanical exfoliation of Highly Oriented Pyrolytic Graphene (HOPG), research in graphene has outshined the rest of the members in the carbon family

Graphene, an atomically thin layer of sp2 hybridized carbon atoms arranged in

a honeycomb lattice, is reported to possess unique electrical2 , 3, mechanical4, thermal5 , 6, optical7 , 8 and chemical properties, These properties have positioned graphene as a promising candidate in the field of nanoelectronics As such, many researchers have attempted to use graphene in various interesting applications Graphene has been used as electrodes in solar cells9, in sensing applications and also transistors10

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In order to facilitate the use of graphene in these devices, there was a need to develop methods for large-scale synthesis since the first mechanical cleaving method adopted by Andre Geim’s group does not give a high yield of graphene As such many synthesis methods have been developed in the last decade These methods ranged from the usage of chemical vapour deposition11,12 (CVD) to solution phase methods that involve chemically exfoliating graphite13 to vacuum graphitization of silicon carbide (SiC)14 to the chemical reduction of Graphene Oxide (GO) There are advantages and disadvantages in each method but GO is seen as one of the most favorable route to graphene due to its low cost and the ease to scale up production

Research in GO can be traced back to more than one hundred years ago Benjamin C Brodie, a chemist from University of Oxford, was the first person to successfully synthesize GO solution in 1859 via treatment of graphite with fuming nitric acid and potassium chlorate15 Although the synthesis of GO has been modified

a few times over the last few years by replacing those oxidizing agents with potassium permanganate and concentrated sulfuric acid, the chemical functionality of

GO does not deviate much from those synthesized by Brodie

Many researchers have conducted experiments to investigate the chemical composition in GO In a sheet of GO, its basal plane contains both hydroxyl and epoxide groups; carboxylic, ester and carbonyl groups can be located along the edges16 The high solubility and hydrophilic nature of the GO is attributed to the presence of these groups17 However, the presence of such groups destroys the aromaticity of the graphene framework, causing the carbon atoms in that GO sheet to

be sp3 hybridized To recover the electrical conductivities of GO, it can be

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chemically reduced to form Reduced Graphene Oxide (rGO) A number of approaches to reduce GO have been studied Examples include thermal18, chemical19and electrochemical20 methods It was widely reported that the conductivity of rGO is several orders of magnitude higher than that of GO but still much lower compared to graphene21 This is because during the synthesis of GO, many defects and vacancies were created and these defects are not possible to eliminate completely during reduction16 Despite the lower conductivity of rGO compared to graphene, GO and rGO have shown many promising applications in some areas such as sensing and energy storage22,23

1.2 Synthesis of Graphene Oxide

In this section of the thesis, we will highlight some of the commonly used recipes to synthesize GO

1.2.1 Brodie Recipe for making GO solution

Oxford chemist Benjamin C Brodie was the first person to synthesize GO solution He prepared his GO solution by first adding 3 portions (by weight) of potassium chlorate (KClO3) to 1 portion of graphite followed by large amount of strong fuming nitric acid (HNO3) The solution was placed in a heat bath of 60oC for three to four days, till the yellow vapors released by the solution ceased The product mixture was then washed with plentiful amount of water to remove the acids and salts

in the mixture, dried in a water bath and put it under the oxidation environment again The appearance of the substance changed after each cycle until the fourth or fifth repetition, the color of the substance turns light yellow, which would not change with

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subsequent oxidation treatment He also observed that the light yellow product cannot

be attained with one prolong oxidative treatment The light yellow crystals when observed under the microscope, was perfectly transparent

According to his elemental analysis15, the percentage compositions (by weight) of carbon, hydrogen and oxygen of the product are 61.11%, 1.85% and 37.04% respectively The corresponding chemical formula is C11H4O5

1.2.2 Hummers Method

Approximately one century later, chemist William S Hummers and Richard

E Offeman from Mellon Institution of Industrial Research have created a new recipe for the synthesis of GO solution Instead of using fuming nitric acid, Hummers and Offeman first add water – free mixture of concentrated sulfuric acid and sodium nitrate to powdered flake graphite The solution is kept at 0oC in an ice bath Next, potassium permanganate is added to the mixture, keeping the temperature of the mixture below 20oC during the process The entire oxidation process will complete in less than two hours The final product shows a larger degree of oxidation compared to the previous method This method is faster and safer as compared to Brodie’s method

However, scientists have discovered that the structure of the final GO contains partial oxidized graphite core surrounded by GO shells A pre-treatment of graphite aiming to increase the surface area graphite flakes can help to achieve a more oxidized GO24 At present, there are many groups making some modification to Hummer’s method Different modifications will give rise to a slightly different ratio

of carbon, oxygen and hydrogen in the final GO product All this modified Hummer’s

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method gives a better yield and degree of oxidation compared to that produced by Brodie

1.3 Chemical Structure and Properties of GO

Many researchers have conducted experiments to investigate the chemical composition in GO It is very interesting and challenging at the same time to characterize the structure of GO because the chemical functionality is very dependent

on the oxidation process In general, in a sheet of GO, its basal plane contains both hydroxyl and epoxide groups; carboxylic, ester and carbonyl groups can be located along the edges In this section, we will explore the molecular structure and chemical reactivity of GO

1.3.1 Chemical Structure of GO

In general, scientists have proposed at least six chemical structures25 for GO as shown in Fig 1.1 [Ref 25] although the exact chemical structure of GO is debatable

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arranged in 1, 3-ether cyclohexane ring In Ruess’s model, it excluded C = O functional group Clauss and Boehm supplemented the models with C = C bonds, ketone and enolic groups Clauss and Boehm have also proposed the presence of carboxylic groups around the edges, accounting for the acidic nature of GO observed Later, Scholz and Boehm made some modifications to the model, after taking into account the stereochemistry of the model Scholz and Boehm modeled GO as corrugated carbon layers in a quinoidal structure27 Nakajima – Matsuo’s model was derived from the structure of fluorinated graphite oxide28

One of the recent models that compasses most of those stated earlier was the Lerf’s model Unlike many of the models discussed, Lerf’s model rejected the periodicity of GO The structure of GO in his model consists of two regions: one region consists large number of unoxidized benzene rings and another region is mainly made up of aliphatic six – membered rings The relative concentration and size of these regions are dependent on the degree of oxidation during the synthesis process The epoxide groups and benzene rings are located in the plane of carbon while hydroxyl group attached to the carbons are in a distorted tetrahedral configuration Lerf also suggested that the carbon – carbon double bonds in GO should be aromatic or conjugated because alkenes would have been cleaved during the vigorous oxidation process of making GO Lastly, Lerf also deduced from Nuclear Magnetic Resonance (NMR) spectrum the existence of strong hydrogen bonds between GO and water

1.3.2 Chemical Properties and Reactivity of GO

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Due to the presence of many organic functional groups in GO, GO can react readily with many reagents and most of the reagents are reducing reagents GO is insulating and it’s conductivity can be partially restored via reduction Hence, reduction is one of the most important reactions of GO The product of the reaction is usually name as reduced Graphene Oxide (rGO) rather than graphene This is because the structure of rGO is different from that of pristine graphene albeit there are some similarities between rGO and pristine graphene

Although reduction is very commonly defined as the gain of electrons or decrease in oxidation number, many organic chemists defined reduction as the conversion of functional group in a molecule from one category to a lower one29 There are many ways to reduce GO to rGO Some of the methods include but not limited to use of chemical reducing agents, thermal – mediated reduction, photochemical or photothermal reduction via use of focused laser beam and electrical reduction

1.3.2.1 Chemical reductions of GO

There are many reducing agents known to convert GO to rGO It is difficult to determine exact mechanism for the reduction process and most scientists would use common reducing agents previously used for small organic compounds on GO This was a reasonable approach since the proposed structure of GO contains functional groups like hydroxyl, epoxy and carboxylic Chemical reduction of GO can be classified into two groups: Reduction with well-supported mechanism and reduction with proposed mechanism30 The most common reagent used to reduce GO is

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hydrazine monohydrate31 because hydrazine monohydrate does not react with water Thus, it makes hydrazine monohydrate a popular choice to reduce GO dispersed in water The reduction mechanism for reduction of GO using hydrazine is highlighted

in the Fig 1.2 below

Figure 1-2: Reduction Mechanism for GO using hydrazine Adapted from Ref [29]

1.3.2.2 Thermal reductions of GO

Thermal reduction32,33 of GO is another commonly used method to remove oxide functionality from its surface At a temperature of 200oC, oxygen functional group in GO starts to decompose, producing carbon dioxide (CO2) gas between the layers of GO flakes Thermal exfoliation of GO flakes takes place through extrusion

of carbon dioxide produced By measuring the mass loss of GO and the use of equation of states, the calculated pressure of CO2 produced range from 40MPa at

200oC to 130MPa at 1000oC Considering GO as multilayer system and using Lifshitz’s equation, the pressure required to overcome dispersion forces between two

GO sheets is predicted to be approximately 2.5MPa34

During the thermal exfoliation process at 1050oC for 30 seconds, approximately 30% of the mass of GO is lost, creating topological and vacancies defects in a plane of rGO sheet35 These defects increase the number of scattering sites and reduce the ballistic transport path length, therefore, affecting the electronic

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properties of rGO However, bulk conductivities of 1000 – 2300 Sm-1 were reported for thermally reduced GO This indicts an overall reduction and restoration of GO electronic structure36,37,38

1.3.2.3 Electrical reductions of GO

Electric induced reduction39 of GO can be carried out by applying a potential difference across two – terminal devices containing multilayer GO in between The existence of absorbed water molecules in the GO film plays a significant role in converting GO to rGO via electric means – The presence of water molecules will favor the reduction of GO and vice versa40 It is noted that during the electrical reduction process, strong electric field causes water molecules to dissociate to form

H+ ion and OH- ions Reduction of GO will take place at cathode (negative electrode) with the proposed equation GO + H+ + e- -> rGO + H2O30

One notable feature of this reduction method is that the process is reversible

As long as the GO film between two electrodes is not totally converted to rGO, rGO can be converted back41,42 to GO when the applied potential is reversed during the process

1.3.2.4 Laser reductions of GO

The use of laser to reduce GO has a few distinct advantages over the other reduction methods It includes flexible patterning and low cost43 As the use of laser is

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the main technique to reduce GO in this thesis, this technique will be discussed in greater details in the next section

1.4 Photoreduction of Graphene Oxide

The use of photoreduction technique to convert GO to rGO has become more popular in the last few years The first few photoreduction techniques include the use

of photo-catalysts such as Titanium Oxide44 (TiO2) to cause a photochemical reduction of GO and the use of camera flash of a commercial digital camera to provide a different mechanism, photothermal process, for GO reduction

In general, there are two types of reaction mechanism for the reduction process: Photothermal and photochemical The mechanism during the reduction process is deduced by the threshold effect45, where Smirnov et al investigated the

minimum photon energy required for photochemical reduction of GO is 3.2eV Hence, if the wavelength of the laser used is smaller than 390nm, then the reduction process is dominated by photochemical process while laser with wavelength greater than 390nm is accounted by photothermal process Fig 1.3 shows an overview for the various photoreduction techniques for the reduction of GO films

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Table 1: PT: Photo-thermal reduction, PC: Photo-chemical reduction Adopted from Ref [21]

1.4.1: Photothermal Reduction of GO

Cote et al is the first to report the use of flash to reduce and create patterns on

GO films46 The wavelengths of camera flash are mainly in the visible spectrum (400nm < λ < 800nm) and the reduction process is mainly dominated by the photothermal process In general, a commercial camera flash can deliver 100 – 2000mJ / cm2 of energy per pulse Taking into account of the optical absorption of

GO film with thickness 1μm (~63%), the total amount of energy that could be deposited on the GO film is approximately 63 – 1260mJ/cm2 Based on the differential scanning calorimetry (DSC) heating curve for GO47, the amount of energy required to cause deoxygenation of GO is calculated to be 70mJ/cm2 Hence, one pulse from the camera flash has more than sufficient energy to cause photothermal reduction of GO, provided that most of the absorbed light energy is converted to heat

Photothermal reduction is not limited just to the use of commercial camera flash Visible light with sufficient photo - energy will also be able to trigger such

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photothermal reduction, as long as the conversion from light energy to heat energy on

GO is efficient

1.4.2: Photochemical Reduction of GO

Matsumoto et al employed the use of UV from a 500W high pressure

mercury lamp to convert GO to rGO During the reduction process, H2 and CO2 are liberated In a sheet of GO, it consists of two main regions: hydrophobic π – conjugated sp2 domains as well as sp3 carbons with oxygen containing groups This conducting sp2 discrete small island is surrounded by insulating network of sp3carbons The sp2 carbon has an energy band gap that is dependent on the size of its domain48 , 49 The group proposed that the sp2 semiconductor domain can act as photocatalyst when irradiated with light with energy greater than the bandgap of the domains – an electron – hole pair can be generated that caused GO to be reduced with the aid of surrounding water molecules

1.4.3: Laser Reduction of GO

Theoretically, according to Smirnov threshold effect, a laser with wavelength smaller than 390nm would undergo photochemical reduction process whereas wavelength larger than 390nm would have the reduction process be dominated by photothermal process

Zhou et al has reported50 the first paper on using focused laser beam of wavelength 663nm and laser power of 80mW to directly create micro patterns on multi-layered GO film The laser energy was converted into heat energy locally and

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the temperature of the laser irradiated spot raised to 500oC, causing oxidative decomposition of GO The conductivity of rGO created via this method is approximately 1.1 S/m and the resolution of rGO patterns was approximately 20μm

Recently, Kaner’s group has developed a flexible GO patterning method to fabricate all graphene devices51 The group has used 788nm infrared laser from Lightscribe DVD, a technique that was created by Hewlett – Packard to produce laser – etched labels as well as greyscale graphics, to reduce GO and created a greyscale rGO graphics

1.5 Challenges faced and Motivation

The use of unreduced GO directly in devices is relatively few or close to none The use of GO in application is that it provides a platform to develop rGO or rGO composite hybrid materials Some of the potential applications of rGO or rGO composite hybrid materials are: electrochemical or bio sensors52,53, photo-catalyst54,55, energy storage materials56,57, supercapacitor electrodes58, electronic transistors59 etc

In order to fabricate these graphene – based devices, there is a need to address one main problem: The design and patterning of GO or graphene are needed to be designed so that it can be integrated with some others in practical applications For example, micropatterns can be created in rGO that was prepared by chemical or thermal reductions via lithography followed by oxygen plasma etching60,61 However, this method requires shadow mask to be in contact with rGO which cause damages and contaminations to rGO Although there were also reports of using AFM tip to

in the design of pattern on GO to be processed under ambient condition, even onto different flexible substrates such as mica without the need of masks, post processing

or transferring techniques63 Although the resolution for such micropatterns (~5μm) is still smaller compared to the tip technique, it is still high enough for most electronic devices In this thesis, a novel technique to create 3D GO/rGO stacked structures with micropatterning will be presented in chapter 3 This opens up more opportunity to fabricate complex devices

The conversion of GO to rGO is deemed as a crucial step for electronic application Photoreduction of GO provides a more refined control over the extent of reduction of GO, compared to chemical or thermal method The extent of reduction can be controlled by tuning the laser intensity, irradiation time or wavelength and the degree of reduction is often reflected in the electrical conductivity of rGO, which is dependent on C/O ratio of rGO Although photoreduced rGO does not show higher electrical conductivity compared with thermal and chemical methods, the ability to control the conductivity of rGO film over at least three orders of magnitude makes it a unique strategy

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According to the most common model of GO (Lerf – Klinowski), it contains

reactive functional groups such as hydroxyl, carbonyl, carboxylic, epoxy and ester

that could react with other chemicals to form composite compounds These composite

GO based compounds open up new possibilities and further improve the electrical

properties of the material For example, 3D GO encapsulated with gold nanoparticles

could be very effective to detect the differentiation potential64 of neural stem cells

(NSCs) from the Surface Enhanced Raman Spectroscopy However, there are many

challenges that needs to be addressed before one could synthesize such GO based

functional material for groundbreaking applications Firstly, it is not possible to just

selectively react with one of these functional group during the synthesis process Most

of the functionalization involves more than one reactive group and results in

complicated products, making purification almost impossible Secondly, the existing

methods do not allow one to selectively and locally react with GO sheets Thirdly, the

properties of the synthesized GO – based hybrid materials are not easily tunable to

accommodate the requirements for a particular application

This thesis has addressed some of the challenges stated above in fabricating

versatile GO based functional material Chapter 3 of this thesis has illustrated a novel

technique to develop 3D GO – rGO stacked structures via the use of focused laser

beam With this technique, one will be able to create GO – rGO – GO stacked

structures with micro-patterning In chapter 4 and chapter 5, the thesis has presented a

technique to selectively and locally decorate Ag and Au nanoparticles on GO In

addition, the electrical and optical properties of such hybrid structures can be easily

tuned by changing some of the experimental conditions

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In Chapter 6, the thesis will present a novel method to reversibly convert GO

to rGO via electrical method This technique will allow us to “write” and “rewrite” on

GO films This study could present and offer more possibilities in the application of

GO in memory devices since one could “erase” the reduced GO

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Chapter 2 : Experimental Method – Fabrication and Characterization of GO/rGO

2.1 Synthesis of Graphene Oxide

GO was prepared using a modified Hummer’s method from expanded graphite powders (pre-oxidation step) using HNO3, H2SO4 and KMnO4 in an ice bath as detailed below based on a recipe for 12g of GO synthesis

In the pre – oxidation step, concentrated H2SO4 (75mL) was heated to 80oC in

a 250mL round bottom flask placed in a heat bath as shown in Fig 2.1a Next, 15g of Potassium Peroxydisulfate K2S2O8 and 15g of phosphorous (V) oxide P2O5 was added

to the round bottom flask and stirred until fully dissolved Then, 20g of graphite powder was added and kept at 80oC for 4.5h After 4.5h, the mixture was cooled and transferred to a large flask to be diluted with 2 litres of DI water This mixture was filtered and the residue was further washed until the pH of the solution reach about 5.5 Finally, the residue was left to dry overnight in vacuum

The setup for the oxidation step was shown in Fig 2.1b The pre – oxidized graphite (12g) was transferred into a 2L Erlenmeyer flash with concentrated H2SO4

(460ml) and chill to 0oC Next, 60g of KMnO4 was added very slowly into the flask, keeping the temperature of the mixture below 10oC The mixture was then transferred and kept in a water bath of temperature 35oC for 2h After 2h, the flask was transferred back into the ice bath where DI water (1L) was slowly added in the flask with agitation The temperature of the mixture was kept below 55oC After the

A stock solution of GO with specific concentration (in mg/ml) can be prepared using the above solid GO dispersed in either water or organic solvent Large free standing multilayered GO sheets were fabricated by vacuum filtration of GO dispersion using anodized alumina (AAO) membranes with a nominal pore size of 0.02μm or direct evaporation of GO solution from a Teflon dish The GO sheets were

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peeled off from the AAO membranes or Teflon dish after drying

Figure 2-1: (a) Pre-oxidation step to expand graphite powder (b) Oxidation of graphite (c) GO mixture was purified using vacuum filtration

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The schematic diagram for the laser pruning setup is shown in Fig 2.2 Suntech VD-IIIA DPSS Diode laser ( λ = 532nm) was guided to the microscope using two reflecting mirrors The beam splitter then directed the laser to the 100x objective lens and the focused laser spot size was approximately 1.5μm in diameter

The sample was placed on a MICOS X-Y stage that was connected to a computer, which control the movement of the stage via Microsoft Visual Basic software The minimum step size of the stage was approximately 500nm A JVC CCD camera was connected to the microscope so that the entire laser cutting process can be monitored through a TV Through this process, a variety of patterns can be created

2.2.2 Camera Flash Lithography

The desired pattern was first printed on overhead transparency films using a commercial laser printer and to be used as a photo-mask A commercial flash camera Sunpak auto 383 (Fig 2.3) with a window size of 30mm x 50mm or UV light source was then used as the source of illumination to shine onto the GO sheets through the photo-mask thereby creating a pattern of rGO on the GO sheet as illustrated in Fig 2.3 All reduction was carried out in air using either flash camera or UV light source

Figure 2-2: (a): Schematic Diagram of the laser system (b) GO sample on top of the movable stage (c) TV with real – time observation of the laser irradiation process

Figure 2-3: (a) Schematic diagram of camera flash reduction (b) Sunpak camera flash (c) GO film of thickness 10um before flash reduction (d) after flash reduction

2.3 Characterization of GO film

Scanning Electron Microscopy (SEM)

of these secondary electrons will allow surface imaging to be carried out All the SEM images in this report were captured by JEOL JSM-6400F FE SEM The energy of the electron beam was set at 10kV, current of 10μA and the pressure of the chamber was maintained at a pressure of 9.63 x 10-5 Pa This SEM is also equipped with Energy Dispersive X-ray Spectroscopy (EDX) that allows elemental mapping of the sample

to be carried out The resolution of SEM is about 5nm and the sample can be tilted about 30o As such, cross sectional view on GO or rGO can be obtained

Atomic Force Microscopy (AFM)

The use of AFM will allow us to image the surface topology of GO after irradiated by focused laser beam In Fig 2.5 (a), it shows a typical AFM scan of the laser cut GO The width of the laser cut was approximately 2μm and Fig 2.5(b) shows the depth of laser cut sample was approximately 75nm The laser power used was

~10mW

Raman Spectroscopy

a sample and the shift in this wavelength is captured, known as Raman Shift This shift will then be able to provide information such as vibrational and rotational modes

in the molecules The Raman spectroscopy used in this work was Reinshaw Invia System with 2400/mm grating and the wavelength of the monochromatic light was 532nm

Fig 2.6(b) shows a typical Raman Spectra for GO film, laser reduced rGO film and Highly Oriented Pyrolytic Graphite (HOPG) The raman spectra of GO or rGO includes a D (~1350cm-1) and G (~1580cm-1) peak 2D (~2700cm-1) peak is able to distinguish the number of layers in graphene by determining the shape, position and width of the peak66 D peak does not exist from HOPG

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X – Ray Photoelectron Spectroscopy (XPS)

XPS is a surface characterization tool to probe elemental composition, chemical and electronic state of the element at the surface of a material based on photoelectric effect XPS spectra are obtained when a beam of x-rays was irradiated onto the sample and measuring the kinetic energy of the electrons emitted Most of the electrons emitted without energy losses are from 3λ from the surface, where λ denotes the Inelastic Mean Free Path (IMFP) of electrons

It is known that photoelectron lines and auger lines exhibit differences in chemical shift, which are a function of chemical environment of the atom The auger parameter67 is a useful method to identify chemical states of various elements because this parameter is independent on reference energy, work function as well as charging effect The XPS spectrum presented in this work was collected using Thermal Scientific Theta Probe X-ray Spectroscopy (XPS) with a monochromatic Al Kα source The XPS spectral deconvolution was achieved by a curve fitting procedure using the manufacturer’s standard software

Figure 2-7: (a) XPS setup in the surface science lab (b) Typical C1s spectra of GO before reduction (c) after reduction.

Transmission Electron Microscopy (TEM)

Wide resolution range of the TEM (JEOL JEM-2010F) allows this technique to be highly useful in determining the size, microstructures as well as lattice arrangements

of the nanostructures On one hand, low resolution TEM image provides information

on the size, shape and morphology of the nanostructures On the other hand, high resolution TEM (HRTEM) image coupled with selected area electron diffraction (SAED) technique provides information with regards to lattice spacing and crystallization direction of the nanostructures etc

TEM imaging of the samples were normally carried out with an acceleration voltage of 200 kV in order to obtain clear atomically-resolved HRTEM images For studies of the CNTs, the samples were scratched off the substrate and dropped onto copper (Cu) grids with carbon films

3.1 Introduction

Graphene, a single atomic layer of aromatic carbon atoms, has drawn much attention due to its unique electrical properties such as massless fermions, ballistic electronic transport and ultrahigh electron mobility69 , 70 Hence, researchers have explored the applications of graphene in various interesting devices such as transistors, gas sensors and electrodes in solar cells71,72 In order to facilitate the use of graphene in these devices, many synthesis methods have been developed These methods ranged from the usage of chemical vapor deposition73,74 (CVD) to solution phase methods that involve chemically exfoliating graphite75

Previous efforts have demonstrated that graphene-based platelets can be assembled into a two-dimensional (2D) constructs such as thin films or papers The ability to assemble these platelets into three-dimensional (3D) structures could result

in carbon materials exhibiting novel physical and electronic properties for

rGO structure was found to exhibit good cyclic stability and energy storage capacities comparable to existing thin film supercapacitor81 Here, we report a simple and yet versatile method to fabricate micro-patterned multilayered 3D GO-rGO structures with hierarchical control through a series of steps involving (i) irradiation of focused laser beam (Suntech VD-IIIA DPSS Laser, λ = 532nm) at elevated temperature on

GO deposited on SiO2/Si substrate, (ii) ultrasonicating in acetone-water mixture and (iii) controlled spincoating of GO solution on the resulting patterned substrate

3.2 Experimental Preparations

To prepare the sample for laser patterning, the substrate, silicon dioxide (SiO2) was first cleaned via ultra-sonication in ethanol and subsequently isopropyl alcohol (IPA) for a period of 10 minutes Next, it was placed in oxygen plasma chamber for further treatment GO prepared via Modified Hummer’s method was then spin coated

on the substrate for 20 seconds with a speed of 1000 revolutions per minute The thickness of GO was about 65nm – 70nm as determined by Atomic Force Microscopy (AFM, Nanoman, DI300)

GO was irradiated with a focused laser beam (wavelength=532nm, Suntech IIIA DPSS Laser) with the sample placed on a heating stage at an initial temperature