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

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 1 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 2 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. 3 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. 4 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 5 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 6 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 7 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 8 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 HAuCl4 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 9 List of Abbreviation AAO AFM AgNO3 CNT CVD DPSSL EDX FFT GO HAuCl4 H2O2 HOPG H2SO4 IPA ITO KClO3 KMnO4 K2S2O8 NMR NPs P2O5 rGO R6G SAED SEM SERS SiC SiN SPR TEM TiO2 UV – Vis NIR XPS Anodized Alumina Atomic Force Microscopy Silver Nitrate Carbon Nanotube Chemical Vapor Deposition Diode – Pumped Solid State Lasers Dispersive X – Ray Spectroscopy Fast Fourier Transform Graphene Oxide Chloroauric Acid Hydrogen Peroxide Highly Ordered Pyrolytic Graphene Sulfuric Acid Isopropyl Alcohol Indium Tin Oxide Potassium Chlorate Potassium Permanganate Potassium Peroxydisulfate Nuclear Magnetic Resonance Nanoparticles Phosphorus Pentoxide Reduced Graphene Oxide Rhodamine 6G Selected Area Electron Diffraction Scanning Electron Microscopy Surface Enhanced Raman Scattering Silicon Carbide Silicon Nitride Surface Plasmonic Resonance Transmission Electron Microscopy Titanium Oxide Ultraviolet – Visible Near Infrared X – Ray Photoelectron Spectroscopy 10 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 11 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 electrical 2 , 3 , mechanical 4 , thermal 5 , 6 , optical 7 , 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. 12 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 graphite 13 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 edges 16 . The high solubility and hydrophilic nature of the GO is attributed to the presence of these groups 17 . 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 13 chemically reduced to form Reduced Graphene Oxide (rGO). A number of approaches to reduce GO have been studied. Examples include thermal18, chemical19 and 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 14 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 GO 24 . 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 15 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. 16 Figure 1-1: Proposed chemical structure of Graphene Oxide. Adapted from Ref[25] Hofmann and Holst proposed the first model of GO26. They suggested that oxygen was bonded to carbon on the hexagon planes by epoxy linkages, with carbon to oxygen ratio equals two. Hofmann’s model ruled out the presence of hydrogen atom in GO but this is not really the case for GO. The model was then modified by Ruess to account for the hydrogen content in GO. In Ruess’s model, he assumed hydroxyl group was bonded to the fourth vacancies of the carbon atoms, which are 17 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 18 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 one 29. 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 mechanism 30 . The most common reagent used to reduce GO is 19 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 20 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 versa 40 . 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 21 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 Oxide 44 (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. 22 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 films 46 . 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 23 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 sp3 carbons. The sp2 carbon has an energy band gap that is dependent on the size of its domain 48 , 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 reported 50 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 24 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 devices 51 . 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 25 pattern GO or graphene with ultra-high resolution 62 , this method is very time consuming and requires special instruments such as AFM. Alternatively, laser reduction of GO has shown to be a promising candidate to create micropatterns on GO/rGO, without the need of any shadow mask. Therefore, laser reduction is a non – contact method and a great potential to fabricate and integrate graphene – based microdevices. In fact, this method enables great flexibility 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. 26 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. 27 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. 28 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 29 diluting process, 30% H2O2 (50mL) was added and the solution turned bright yellow. The solution was then left overnight. The filtering setup was shown in Fig 2.1c. The supernatant was decanted and the bottom mixture was filtered with filter paper. The residue was rinsed with 3.4% HCl solution to remove residual salts. The resulting wet solid was then centrifuge at 1000rpm. The supernatant was then decanted and the remaining solid was re – dispersed in acetone. The mixture was then filtered with a PTFE membrane where additional acetone was added to rinse off residual acid. The solid GO was dried in air and detached from the membrane for storage. 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 30 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 2.2 Micro - patterning of GO In this work, mainly two methods were used to create micropatterns on GO film – Focused laser pruning and camera flash lithography. 2.2.1 Focused laser pruning technique 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 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. 32 Flash reduction was done with a single, close-up (60oC) and high laser power (>35mW), both rGO and GO became strongly adhered to the substrate and could not be removed after 90 seconds of sonication in water-acetone mixture. 44 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. 3.3.2 Feature size and resolution of GO – rGO micropatterns 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. By varying the temperature of the heating stage as well as the laser power, micro-patterns can be created. Fig 3.3 shows the development of a rectangular strip using laser power of 14.2mW at different temperature, 50oC, 80oC and 100oC, of the heating stage. Positive development occurs when the temperature of the heating stage was at 80oC and unsuccessful development of the micro patterns at 50oC and 100oC. This is in agreement with the scientist guide as shown earlier. 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. The resolution of the micropatterns was further investigated by irradiating GO with various rectangular box of different dimension - 1μm, 5μm, 10μm, 25μm and 50μm. The sample was sonicated in acetone water mixture and the result was shown in Fig 3.4(a). From Fig 3.4(a), it was evident that creating a pattern of 5 - 10μm was the resolution for this technique. Fig 3.4(b) was a checkered box pattern with spacing between each rectangle being 10μm. Fig 3.4(c) was a NUS micro pattern on GO while 47 Fig 4.4(d) shows a rectangular strip with a width of 10μm with 10μm spacing in between each strip. 3.2.3 Proposed Mechanism We propose the following mechanism for the positive and negative micropatterning processes. Controlling the temperature was crucial in enhancing the formation of bonds between GO and SiO2 substrate. When the temperature of the heating stage was below 50oC, there was insufficient formation of bonds between GO and the substrate. As a result, the GO was readily removed during sonication. At 80oC and above, the higher temperature facilitated the formation of strong adhesive force between GO and the substrate. As a result, none of the GO was removed during the short duration of sonication. Heating the sample at a temperature range of 50oC to 80oC corresponded to the intermediate state and thus only part of GO was removed during sonication. This was denoted as partial development. It should be noted that if the time of sonication was set longer than 90 seconds, all the GO would eventually be removed for the temperature range explored (22oC - 110oC) in this work. The addition of the focused laser beam provided a number of advantages. Firstly the focused laser beam served as a direct writing tool for localized micro-patterning in converting the exposed GO to reduced form of GO (rGO). Secondly the localized heating caused by the focused laser beam served as a localized “welding” tool to further pin the GO/rGO onto the substrate against the effect of sonication. Thirdly, at a lower laser power, the focused laser beam helped to create more structural defect in the sample and rendered them to be more susceptible to removal by sonication. Thus the focused laser presented an interesting and very useful option to achieve control 48 developments on the substrate. As shown in the scientist’s guide in Fig. 4.2(a), with sample heated at temperature below 50oC, normally the GO would be removed after sonication. However, after the sample was exposed to focused laser beam with a power above 30 mW, we could achieve the negative development where laser defined region remained after sonication. In this case, we utilized the “welding” attribute of the laser beam. On the other hand, heating the substrate at a higher temperature (60oC to 85oC) facilitated stronger adhesion between the GO and the substrate. But the presence of a focused laser beam at moderate power (10-20 mW) provided the destructive effect with sample exposed to the laser beam became physically damaged. And when the sample was subsequently subjected to sonication treatment, these damaged regions became more readily removed by sonication. Thus giving rise to the positive development as illustrated in Fig 3.1(c). To further elucidate the effect of laser beam on the sample, we carried out more systematic characterizations of the sample after laser irradiation. Laser irradiation can create more defects on GO83,84,85,86. Fig 3.2(b) shows the optical image of GO after irradiated with laser of power 15.6mW and 30.1mW respectively. Note that the temperature of the sample stage was maintained at 80 oC during the focused laser treatment. At 15.6mW, the optical shows many “spots” on the square box but at 30.1mW. such “spots” were significantly less visible. This optical image suggests that rGO caused by laser reduction with power at 15.6mW was rougher compared to situation where a higher laser power of 30.1mW was used. Measurements of the roughness were carried out by Atomic Force Microscope (AFM). Fig 3.2(c) shows a plot of root mean square roughness of the laser treated sample against the power of the focused laser used during the experiment. The roughness of the sample increased 49 with the laser power used initially. This was an indication of the physical damage caused by the moderate laser beam power. As the laser power increased further, the roughness of the sample decreased. This was an indication of the annealing contribution due to the higher laser power. A rougher sample could suggest that more defective sites were present in the sample and hence the rougher portion of the sample could be preferentially removed during sonication in the event of positive development. The AFM measurement shows that there was a large increase in the roughness of rGO using laser power of 10mW to 20mW. This implies that positive development of the pattern would be more likely to occur at this laser power range (as the rougher rGO would be easier to be removed) and this is in agreement with the scientist guide shown in Fig 3.2(a). 3.2.4 Development of 3-dimensional GO-rGO layered microstructure We have demonstrated that it is possible to fabricate GO or rGO micropatterns on the substrate with controlled positive or negative development. After the fabrication of the patterned layer, it is possible to deposit additional layer on top and the same patterning process can be repeated. As a result, we could create threedimensional (3D) micropatterned multilayered structures and we could design and define a wide variety of micro-pattern in each layer en route to three-dimensional layered structures. As examples, we created layered and patterned 3D structures using the positive development technique. The temperature of the heating stage and the laser power used to create the 3D structures were 80oC and 12.6mW respectively. Fig 3.5(a) shows the schematic diagram of the processes. Firstly, a portion of GO was laser reduced to rGO. Next, the sample was sonicated for 90 seconds to remove rGO 50 (laser irradiated portion). Thirdly, GO was spin-coated on the sample. The thickness of this could be controlled by the concentration of GO and the speed at which the sample was rotated. Then, the sample was exposed to laser irradiation again and sonication followed after that. A series of laser irradiation, sonication and spincoating could be repeated to create multilayered structure with the added attribute that micropatterns could be defined and fabricated in each layer. In this way, we created 3D layered structure with micro-patterning. 51 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 measurements show that each step was about 65-70nm. (e) Taiji logo with one 52 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. Fig 3.5(b) shows the intermediate step of a checkered-box GO pattern after sonication and 3.5(c) shows the final pattern after GO was spin-coated on the sample. Yellowish portion represents a thicker GO coating and light blue portion corresponds to the thinner coating. Fig 3.5(d) shows a staircase structure with two coatings of GO. Optical image shows the color contrast of each step and the height of each step was measured by AFM. AFM characterization shows that each step height was about 6070nm. Fig 3.5(e-g) shows the process of patterning a three-dimensional micropatterned logo with 3 layers. Fig 3.5(e) shows one coating GO and Fig 3.5(f) shows the sample with two different coatings. Laser was irradiated on the two small circular spot in Fig 3.5(f) and finally removed through sonication for 90 seconds as shown in Fig 3.5(g). 3.4. Conclusion In conclusion, when laser was irradiated on GO, it can be converted to rGO. By tuning the laser power and temperature of the GO during the patterning process, rGO (positive development) or GO (negative development) can be preferentially removed by ultrasonication. Positive development of the patterns requires more careful experimental control as the conditions for this type of development works for a narrower range of laser power and temperature. After which a new layer of GO can be spin-coated and the process can be repeated en route to 3D multilayered layered structure with the pre-defined micropatterns in each layer. The end product is akin to the stacked microelectronic devices except that, instead of Si based system, we have a 53 complete GO-rGO system. Such 3D micropatterned and multilayered structure could offer more potential applications for graphene-based devices, such as all carbon electronic devices87. 54 Chapter 4 Microlandscaping on Graphene Oxide Film via Localized Decoration of Ag Nanoparticles In this chapter, the main contribution to the development of hybrid graphene materials is the new technique to selectively and locally decorate Ag nanoparticles on GO. It is a new technique to locally and selectively decorate Ag nanoparticles on GO/rGO sheets via simultaneous laser reduction GO in aqueous silver nitrate solution. This technique also allows one to tune the size, shape and density of silver nanoparticles on GO/rGO sheets. 4.1 Introduction Graphene oxide (GO) has been receiving much attention 88 in recent years. Compared to graphene, GO is significantly less conducting and this problem can be mitigated by partial reduction of its functional group. Some of these methods include thermal89,90, chemical91 and electrical92. In order to improve the performance of these graphene based materials in different applications, functionalization of graphene with metal nanoparticle is an attractive process93,94. For such integration, several approaches have been laid out in order to decorate graphene sheets with metal nanoparticles 95 , 96 . Most of these methods involve the use chemicals via redox reactions. Although these methods offer the possibility to be scaled up for mass productions, it cannot create micropatterns on the graphene films. With the advance in the miniaturization of microdevices, the ability in creating micropatterned functional components is always desirable. Hence, in order to further extend the range of potential applications of graphene based hybrid 55 materials, it would be important to develop techniques to selectively and locally decorate one or more types of metal nanoparticles on graphene or graphene oxide. Kamat has suggested decorating two different types of metal nanoparticles on a single sheet of GO to carry out selective oxidation and reduction processes at different sites97. Such design of bi-functional materials allows one to design “smart” sensors that has the capability to detect and destroy pollutants. Here, we report a direct and facile method of selectively incorporating and anchoring silver nanoparticles on graphene oxide (GO) or reduced graphene oxide (rGO) directly by focused laser beam. On top of selective anchoring of silver nanoparticles on rGO, we are able to tune the shapes and size distribution of these nanoparticles, leading to flexibility in the optical sensing of graphene functional materials. In this work, we limit our discussion using silver nanoparticles but similar technique can be employed to anchor other nanoparticles such as gold and cobalt on rGO. Hence, it is possible to selectively decorate two or more metal nanoparticles with morphology control, leading to a multi-functional material or creating a PN junction on rGO. This technique employed the laser-assisted photo-reduction of silver ions present in the aqueous solution in which GO/rGO was submerged. When the GO is irradiated by the laser beam, electron hole pairs are created locally. This “activated” GO region provides an active platform for the reduction of Ag+ in the solution. Hence, the Ag+ ions become reduced and adhere onto the surface of the GO, giving rise to formation of NPs on the surface. With precision control, a wide variety of micro-landscapes comprising of decorated array of Ag NPs onto rGO were achieved. 56 On top of that, by varying the laser power used and changing the concentration of the aqueous silver nitrate solution we could control the size and density of the decorated silver nanoparticles. Such controls allow us to locally modify the functional property of the hybrid material with some implications to applications. In addition, this technique to create decorated array of Ag NPs on rGO is friendly to different substrates such as SiO2, quartz or glass. In our work, we demonstrated that the rGOAg nanoparticles composite has shown remarkable and tunable raman enhancement. It can also be used in H2O2 sensing and the composite exhibits enhancement in photocurrent. 4.2 Experimental Methods 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 98 was then spin coated on the substrate. The schematic diagram for the scanning focused laser system used in this work is shown in Fig 4.1. GO was irradiated with the focused laser beam (wavelength=532nm, Suntech VD-IIIA DPSS Laser) with the sample placed in a substrate holder. The SiO2 substrate with the GO was immersed in 0.01M of aqueous silver nitrate during laser irradiation. The experiment was repeated using 0.05M and 0.1M of aqueous silver nitrate. At each concentration, focused laser beam with relatively high (~5.5mW), mid(~3.5mW) and low(~2mW) laser power was used to 57 scan across the sample. The scanning speed of the laser was kept constant at 50 microns per second. To prepare the sample for UV-Vis absorption, GO was first spin coated on a piece of quartz. A large area (3mm x 3mm) was scanned using focused laser beam. The substrate was placed in the UV-Vis NIR Spectrophotometer (UV3600) to obtain the absorption spectrum. The chemical composition on the surface of laser treated GO in silver nitrate solution was analyzed using Thermal Scientific Theta Probe X-ray Spectroscopy (XPS). The XPS spectral deconvolution was achieved by a curve fitting procedure using the manufacturer’s standard software. Component peak shape and full-widthsat-half-maximums (FWHMs) for a particular peak envelope were kept the same during curve fitting. To prepare the sample for XPS measurement, focused laser beam was raster through 0.7mm by 0.7mm GO previously deposited on SiO2. Silver foil was also prepared for XPS measurement. In order to image the Ag nanoparticles formed using Transmission Electron Microscope (TEM), focused photo-reduction was carried out with GO on SiN TEM grid. GO was first drop casted on Silicon Nitride (SiN) grid. After drying under ambient conditions, silver nitrate solution of concentration 0.01M was then drop casted onto SiN. Focused laser beam was then irradiated on GO with silver nitrate solution. In this way, Ag nanoparticles formed on the GO can be imaged directly using FEI Titan 80-300S/TEM operated at 200kV in TEM mode. 58 For electrochemical studies, a three-electrode cell was assembled with the GO/ ITO, rGO/ ITO or Ag-rGO/ ITO as the working electrode, platinum rod as the counter electrode and Ag/AgCl as the reference electrode. Prior to electrochemical studies, all samples on ITO substrates were masked with Scotch tape to expose only 2 mm × 2 mm area of the respective samples. This step was carried out to avoid interference from ITO contacting with electrolyte and analyte. All potentials were referenced to the Ag/AgCl (3 M KCl) electrode. Phosphate buffer saline (0.1 M, pH 7.4) was used as the electrolyte. All measurements were carried out using an Autolab PGSTAT 30 potentiostat/galvanostat at room temperature. To prepare samples for photocurrent measurement, conventional UV photolithography was employed to fabricate patterned electrode on Si(100) with 100nm of SiO substrate. Au/Cr were then thermal evaporated as the material for the electrode. The spacing between electrodes were approximately 10µm. GO was then spin coated on the substrate for 20 seconds with a speed of 1000 revolutions per minute. The thickness of GO was about 65-70nm. 2 samples of GO were reduced via focused laser beam of wavelength 532nm under different chemical environment– one in ambient99 and another in 0.1M of AgNO3. 59 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. 4.3 Results and Discussions The irradiation of GO in aqueous silver nitrate results in the formation of silver nanoparticles (Ag, NPs) deposited only on the laser scanned region. Fig 4.2(a) shows a Scanning Electron Microscope (JEOL-JSM 7001f) image of a square decorated with Ag, NPs. The size of the square was approximately 50μm x 50μm. Fig 4.2(b) shows a zoom in view around the edge of the square (denoted by the red box). It shows that the nanoparticles only formed on the laser irradiated region. Fig 4.2(c) shows that the size of the nanoparticles formed is approximately 50-100nm using high laser power ~5.5mW. The EDX (insert) shows a strong silver peak indicating that the 60 nanoparticles observed in the SEM image could be either solid silver nitrate or silver particles. In another words, the chemical state of silver could be in ionic form or elemental form. The laser power and the concentration of AgNO3 solution used in Fig 4.2(a-c) was 5.5mW and 0.01M respectively. 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 Optical properties of the decorated film were studied with UV-Vis spectroscopy. It is widely known that silver nanoparticles exhibit surface plasmon resonance 100 . Fig 4.2(d) shows a Surface Plasmon Resonance (SPR) band at approximately 409nm when GO in silver nitrate solution was treated with laser power of 5.5mW. This indicates that silver nanoparticles were formed during the laser irradiation. The intense absorption band around 409nm is due to the collective oscillation of the free electrons in silver nanoparticles resulting from the interaction with the incoming electromagnetic radiation101. The electric field of this incoming radiation induces the formation of a dipole in the nanoparticles and a restoring force in the nanoparticles tries to compensate for this, resulting in a unique resonance wavelength. The oscillation wavelength is dependent on a number of factors such as the particle sizes and shapes102. The broad full width half maximum (FWHM) of the UV – Vis absorption peak in Fig 4.2(d) suggest that nanoparticles with a wide range of sizes were created during the irradiation process. This was in agreement with the SEM picture shown in Fig 4.2(c). When laser was irradiated on a piece of clean SiO2 substrate, there was no silver nanoparticles formed as indicated by the absence of the SPR band and SEM imaging. When the sample was irradiated using a lower laser power of 2.0mW, there is a corresponding red shift in the UV-Vis absorption spectrum, indicating that the size of Ag nanoparticles decreases. In another sample, a 50 x 50μm box was first irradiated with laser power of 3.5mW under ambient condition. The sample was then immersed in 0.01M of AgNO3 and irradiated with the same laser power. A 100 x 100μm box was drawn overlapping 62 with the first box drawn as shown in Fig 4.3(a). Fig 4.3(b) shows the SEM picture of the zoom in red color box in Fig 4.3(a). It was that the region prior treated with laser shows a significant decrease in the number of silver nanoparticles formed. Fig 4.3(c) shows the zoom in the silver nanoparticles rich region while Fig 5.3(d) shows the zoom in region of the prior laser treated region. It was known that after laser reduction, the oxygen containing functional group of GO decreased significantly, as verified by XPS in previous work. Hence, the reactive oxygen function group in GO played a significant role in the mechanism for the formation of AgNPs on GO/rGO in this case. 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 63 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. X-ray Photoelectron Spectroscopy is a surface characterization tool to probe elemental composition, chemical and electronic state of the element at the surface of a material. 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 parameter 103 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. Auger parameter is defined as the energy difference between a photoelectron and an auger electron. Fig 4.4 (a-b) shows the XPS spectra of Ag3d and auger lines for silver foil respectively. Silver foil was also scanned for O1s peak and the absence of such O1s peak suggested that the foil was not oxidized. Fig 4.4 (c-d) shows the XPS spectra of Ag3d and auger lines for rGO – Ag. The auger parameter of both rGO – Ag and Ag film were 726.1eV and 726.0eV respectively. It was widely reported that the auger parameter for metallic Ag state104 is 726.0eV and this is in agreement with our XPS results. XPS further confirmed that the nanoparticles observed in SEM for laser treated GO in silver nitrate solution was silver. 64 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 Transmission Electron Microscopy (TEM) study was used to further investigates the morphology of the silver nanoparticles. The concentration of silver nitrate was 0.01M and the laser power was low (~2.0mW). Fig 4.5(a) shows part of rGO film. Small spherical Ag nanoparticles ranging from 5 ~ 17 nm were formed during the laser reduction process as shown in Fig 4.5(b-c). Selected Area Electron Diffraction (SAED) was performed on the sample. The insert SAED shows the electron diffraction pattern obtained by directing the electron beam perpendicular to one of the nanoparticles. Calculation of the lattice d-spacing FFTs high resolution TEM images correspond to that of silver (111) crystal. 66 Figure 4-5: (a-c) TEM images of silver nanoparticles. (d) HRTEM of silver nanoparticles. Insert is the Selected Area Electron Diffraction (SAED) pattern. The reduction of silver ions in the presence of GO under laser beam irradiation was reported previously. Moussa et al. proposed the following reduction mechanism for silver ions that involves photo-generated electrons from GO 105 . The reduction mechanism was summarized as: GO + hv  GO (h+ + e-) 4h+ + 2H2O  O2 + 4H+ 67 Ag+ + GO(e-)  GO + Ag GO + 4e- + 4H+  rGO + 2H2O Focused laser treated regions are rich with electrons and holes. The holes generated were taken in by the presence of water in the solution. The electrons-rich GO then attract Ag+ and transfer its electrons to Ag+ to form Ag. On the other hand, the GO can be reduced to form reduced GO. The charge transfer strongly suggests chemical bonds formation between rGO and Ag. The laser treated site attracts decoration of Ag nanoparticles onto the focused part of the sample. With this feasibility, we can achieve micro landscaping readily. In addition, Moussa’s proposed mechanism also shows that the presence of GO was important for the reduction of Ag+. This observation was also consistently with our observation that no silver formed when laser was irradiated on a substrate with the absence of GO. The size and distribution of silver nanoparticles in laser reduction was further investigated. It was found that the sizes could be controlled by varying the laser power of the focused laser beam. Fig 4.6(a-c) shows the SEM images of rGO-Ag after laser irradiation with high (~5.5mW), mid (~3.5mW) and low (~2.0mW) laser power respectively in 0.01M of silver nitrate solution. As evident from each figure that the size of silver nanoparticles decreases with laser power of the focused laser beam. In each of these experiments, the scanning speed was kept constant at 50µm/s and hence the time taken for the growth of silver nanoparticle was approximately constant. Fig 4.6(d) shows one example of the histogram for the size distribution for Fig 4.6(c). The same experiment was then repeated using 0.05M and 0.1M of silver nitrate solution. The images of each SEM picture were processed for particle size and size 68 distribution and Fig 4.6(e) shows the box-and-whisker plot of the results. Clearly, the mean size of each Ag nanoparticles decreases with laser power, suggesting that the growth rate of Ag nanoparticles increases with the intensity of focused laser beam. A large standard deviation for silver nanoparticles formed at high laser power can be observed. This was because small silver nanoparticles were also formed together with the larger silver nanoparticles at high laser power. Such a wide variety of sizes caused the standard deviation to be large. With the range of AgNO3 concentration studied in this work, the concentration of AgNO3 does not seem to have an impact on the size distribution. The concentration of GO solution used was approximately 0.6mg/mol, which is much lower than the concentration of AgNO3 used. The limiting reagent in the reduction process is the amount of GO(e-) present. Thus, the concentration of AgNO3 (excess reagent) has negligible effect on its size distribution. The growth rate is most likely to be diffusion limited, in which the diffusion coefficient is dependent on laser power used. The number density Fig 4.6(f) generally decreases with laser power. At high laser power, the diffusion coefficient is large. This would mean that the smaller nuclei have sufficient energy to overcome the energy barrier to agglomerate, forming a larger particle and hence a corresponding decreased in the number density. 69 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 This patterning tool allows us to carry out microlandscaping on GO by fabricating Ag, NPs micropatterns on GO. As demonstrated, the size and distribution of Ag nanoparticles can be controlled. Fig 4.7(a) shows that we can create Ag micropatterns with different sizes on the same sample. On the left, the laser power used was low compared to the right, where high laser power was used. The contrast in the size of AgNPs on both left and right region was created by changing the laser power in a single scan. Fig 4.7(b) shows a checkered 3 by 3 boxes with Ag nanoparticles decorated on selected region. Fig 4.7(c) shows 3 lines drawn on the same sample with different sizes and density of Ag nanoparticles within each line. Fig 4.7(d) shows a regular pattern with circular patches of Ag. Fig 4.7(e) can be created by laser defocus. Fig 4.7(f) shows a pattern created by lines and dots. Fig 4.7(g) shows a small dipper pattern. Fig 4.7(h) shows a box form by raster scanning different regions with different laser power. Lastly, Fig 4.7(I) shows big and small circular patches of Ag nanoparticles at different locations. 71 Figure 4-7: SEM Images of rGO – Ag composite with micropatterns. The shape of silver nanoparticles can also be controlled during the laser irradiation process by adding surfactants or capping agents in aqueous silver nitrate. These molecules can preferentially adsorb onto specific planes of silver atoms exposed on crystal surfaces. As such, this will generate direction-dependent nano crystal growth by stabilizing a particular facet where growth rate is slow on stabilized crystal planes but fast on the plane where the molecules are weakly adsorbed 106 . When citric acid was added to aqueous silver nitrate, the nanorods of silver are formed. Fig 4.8(a) and 4.8(b) shows the formation of silver nanoparticles with and without the addition of citric acid to aqueous silver nitrate. The addition of citric acid is likely to bind to the Ag (111) surface rather than Ag (100) surface107. Ag (111) plane is stabilized and the growth along Ag (100) results silver nanorods. 72 Figure 4-8: (a) Silver nanorod formation with citric acid added to stabilized Ag(111) surface. (b) Without the addition of citric acid. 4.4: Applications In this section, we will discuss and explore some of the potential applications of Ag – rGO hybrid material. These application includes but not limited to (a) Surface Enhanced Raman Scattering (SERS), (b) Photocurrent Generation and (c) Electrochemical Sensing of Hydrogen Peroxide, H2O2. 4.4.1: Surface Enhanced Raman Scattering (SERS) Surface Enhanced Raman Scattering (SERS) has received much attention in the realm of physics, chemistry and biology108,109.The Raman property of rGO-Ag composite material was investigated. In our experiments, we used different laser power during the reduction process to vary the size of the nanoparticles. Raman 73 enhancement is defined as the ratio of Raman intensity measured on rGO-Ag compared to that of laser treated GO (rGO) in ambient condition and using the same laser power used to create rGO-Ag composite. Previous reports have shown that the enhancement factor for rGO-Ag composite can be up to one order110,111 of magnitude. It is known that Raman spectrum of GO shows G band around the region 1600cm-1 as well as the D band around 1350cm-1. The G line is due to E2g phonon of sp2 carbon atoms while the D line is a breathing mode of k-point phonons of A1g symmetry112. In our sample, as shown in Fig 4.9, Raman enhancement of the G peak intensities can be up to 16 times, with the size of the Ag NPs at 35nm and number density of 1.34 x 1010 cm-2. 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 It is known that Raman spectrum of GO shows G band around the region 1600cm-1 as well as the D band around 1350cm-1. The G line is due to E2g phonon of sp2 carbon atoms while the D line is a breathing mode of k-point phonons of A1g symmetry113. In general, there are two mechanisms contributing to the surface enhancement. They are the electromagnetic and charge transfer mechanisms. Electromagnetic mechanism arises from a large increase in the local electric field caused by surface Plasmon resonances (SPR) of metallic nanoparticles114. Electromagnetic enhancement can improve up to 6 orders of magnitude for the Raman intensity 115 . The charge transfer mechanism involves the chemisorption interaction and the metal adsorbate charge transfer. Raman enhancement due to such mechanism can be up to 2 orders of magnitude 116,117 . These two mechanisms may not be independent and they can be linked together118. One of the mechanism could dominate at a particular excitation wavelength. In our case, the mechanism for ramen enhancement was likely due to charge transfer between silver nanoparticles and rGO. 4.4.2: Photocurrent Generation We further explored the application of the composites for photocurrent generation in the visible range of light. Fig 4.10 shows an optical microscope image of a sample used in this study. It is made by first depositing 100nm thick Au/Cr electrodes on GO film and then locally formed the rGO-Ag composite, with the help of the focused laser beam that bridged across the electrode. After which the sample was dried and ready for the photocurrent experiment. By maintaining the same applied voltage bias across the electrode, we monitored the change in the current 75 across the electrodes with periodic illumination of laser beam. In these experiments, a broad beam of laser light was used to irradiate onto the device during the experiment. Different laser sources with wavelengths of 405nm, 532nm and 808nm were utilized. We ensured that the beam power was not too strong to cause sample damage during the experiment. The photoresponse of rGO-Ag composite material was then compared with laser reduced rGO without the benefit of decorated Ag NPs. The laser power used for each of the experiments was kept constant at approximately 20mW. All the experiments (both rGO-Ag and rGO) were conducted in vacuum of 2.31 x 10-5 Torr. Fig 4.10(b) and Fig 4.10(c) show the photocurrent for devices comprising of rGO-Ag and rGO respectively. The samples were subjected to the same voltage bias of 1V during the experiments. Both samples showed positive photoresponse but clearly the hybrid rGO-Ag sample performed much better. Among the laser sources used, the one emitting laser light with a wavelength of 405nm (denoted as 405nmlaser) generated the largest response. We can quantify the photoresponse by computing the ratio of the current under the irradiation to that without irradiation (i.e. dark current). When 405nm-laser was irradiated on both rGO-Ag and rGO samples, the photocurrent was increased by 125% and 12% respectively. Thus, rGO-Ag exhibits a 10-fold enhancement of the photocurrent, suggesting the synergistic effect of both Ag nanoparticles and rGO in the composite. The absorption spectrum of rGOAg composite material exhibits a peak centered around 409nm (Fig 4.10d), this is consistent with our observation that the samples exhibit the largest photocurrent for laser light with a wavelength of 405nm. It was known that when metal nanoparticles are excited with electromagnetic radiation, it induces a strong localized electromagnetic field. This localized electromagnetic field will enhance optical 76 absorption or scattering at a particular wavelength that is dependent on the shape and size of the nanoparticle119,120. As the sample exhibit largest photocurrent for laser light with wavelength of 405nm, this indicates the strong ability of the material to absorb light in this part of visible spectrum. 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 Another notable feature of the observed photocurrents is that the rGO-Ag system possesses much shorter photoresponse time compared to just rGO. As shown in Fig 4.10(d), the response time of the hybrid to the irradiation at the onset of irradiation and when the irradiation is removed is less than 100ms and 90ms respectively. On the other hand, photoresponse time of rGO obtained from exponential fittings to the data shown in Fig 4.10(c) gave a typical response time of >150 seconds. The large difference (at least 3 orders of magnitude) in the photoresponse time for the two systems suggests marked difference in photocurrent mechanism. Laser induced thermal heating, carrier’s thermalization trapping and interaction of surface states with the laser irradiation could contribute to the measured photocurrent observed in rGO as these factors tend to introduce a slower photoresponse and hence photocurrent from these contributions is characterized by the longer response time. On the other hand, the rapid response time exhibited by rGO-Ag suggests that there is a synergistic effect of both AgNPs and rGO in the composite. rGO can accept electrons from AgNPs and facilitate electron transfer within the composite film121. The photoresponse of rGO-Ag composite is dependent on laser intensity. The photocurrent increases with the light intensity, consistent with the fact that the charge carriers’ photogeneration efficiency is proportional to the absorbed photon flux. The corresponding dependency is plotted in log-log plot as shown in Fig 4.10(c). The wavelength of laser used to generate Fig 4.10(c) was 405nm. This can be fitted to a power law, Iλ ~ Pγ, where γ determines the response of the photocurrent to the light intensity. The fitting gives a linear behavior with γ = 0.62. The non-unity (0.5[...]... 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... 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 oxide2 8 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. .. – 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,... 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 materials5 6,57, 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... 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... 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... 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...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,... 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... 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