PROCESSING AND APPLICATIONS OF CARBON NANOTUBES AND GRAPHENE MEI XIAOGUANG (B.Eng., Tsinghua University, China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgements Firstly, I would like to express my earnest gratitude to my supervisor Asst. Prof. Ouyang Jianyong for his supervision and guidance during my Ph. D candidature. His invaluable advice and enthusiastic encouragement never fail to inspire me. I am indebted to him more than he knows. I am equally thankful to my group members Mr. Fan Benhu, Mr. Wu Zhonglian, Ms. Zhang Hongmei, Mr. Li Aiyuan, Ms. Zhou Dan, Ms. Xia Yijie, Mr. Sun Kuan, Ms. Cho Swee Jen and Mr. Neo Chin Yong for their devoted help and cooperation in my research work. Special thanks go to Ms. Cho Swee Jen for her contribution in the fabricating and characterization of the dye sensitized solar cells. I am grateful to the lab technicians in the department of materials science and engineering for their trainings and technical support. I am also deeply indebted to my family for their unconditional love and endless support in the past years. Finally, I want to express my thanks to the National University of Singapore and Singapore Ministry of Education for their financial support. i Table of contents Acknowledgements i Table of contents . ii Summary v List of Tables . vii List of Figures . viii List of publications . xiii Chapter 1. Introduction . 1.1. Structure of carbon nanotubes . 1.2. Properties of carbon nanotubes . 1.2.1. Electronic and electrical properties . 1.2.2. Optical properties . 1.2.3. Mechanical properties 1.2.4. Thermal properties 1.3. Dispersion of carbon nanotubes 1.4. Applications of carbon nanotubes . 10 1.5. Structure and morphology of graphene . 12 1.6. Properties of graphene 14 1.6.1. Electronic and electrical properties . 14 1.6.2. Optical properties . 15 1.6.3. Mechanical properties 16 1.6.4. Thermal properties 16 1.7. Synthesis of graphene 17 1.7.1. Mechanical exfoliation of graphite . 17 1.7.2. Direct growth 18 1.7.3. Reduction of graphene oxide 18 ii 1.8. Applications of graphene . 20 Chapter 2. Experimental 22 2.1. Materials 22 2.2. Experimental techniques 23 2.2.1. Preparation of CNT gels by mechanical grinding 23 2.2.2. Preparation of SWCNT films by gel coating and spray coating . 23 2.2.3. Preparation of CNT Counter electrode 24 2.2.4. Fabrication of DSCs . 25 2.2.5. Oxidization of graphite . 25 2.2.6. Exfoliation and reduction of GO . 26 2.2.7. Re-dispersion of rGO 27 2.2.8. Preparation of GO films by spin coating 28 2.2.9. Preparation of transparent rGO films by the reduction of GO films . 28 2.3. Characterization 29 2.3.1. Dynamic mechanical characterization . 29 2.3.2. Conductivity measurement 30 2.3.3. Morphology characterization . 31 2.3.4. Thermal analyses 31 2.3.5. Optical spectroscopy . 31 2.3.6. Photovoltaic performance characterization . 32 2.3.7. Electrochemical characterization . 33 2.3.8. X-ray photoelectron spectroscopy 34 Chapter 3. Gels of carbon nanotubes and a nonionic surfactant prepared by mechanical grinding 35 3.1. Introduction 35 3.2. Results and discussion . 37 3.2.1. Gels of MWCNTs and POETE 37 3.2.2. Gels of SWCNTs and POETE . 41 3.3. Conclusion . 44 Chapter 4. Highly conductive and transparent single-walled carbon nanotube thin films fabricated by gel coating . 45 4.1. Introduction 45 iii 4.2. Results and discussion . 46 4.2.1. Fabrication of SWCNT films by gel coating . 46 4.2.2. Optical and electrical properties of SWCNT films . 53 4.3. Conclusion . 61 Chapter 5. High-performance dye-sensitized solar cells with gel-coated binderfree carbon nanotube films as counter electrode . 63 5.1. Introduction 63 5.2. Results and discussion . 65 5.2.1. DSCs with SWCNT films as counter electrode . 65 5.2.2. DSCs with MWCMT films as counter electrode . 75 5.2.3. DSCs with SWCNT/CMC and MWCNT/CMC films as counter electrode 78 5.3. Conclusion . 81 Chapter 6. Ultrasonication-assisted ultrafast reduction of graphene oxide by zinc powder at room temperature . 82 6.1. Introduction 82 6.2. Results and discussion . 82 6.2.1. GO Reduction with Zn Powder . 82 6.2.2. Structure and Properties of Zn-rGO . 87 6.3. Conclusion . 94 Chapter 7. Metal reduced graphene thin films as transparent and flexible electrodes . 95 7.1. Introduction 95 7.2. Results and discussion . 96 7.2.1. Reduction of spin coated GO films . 96 7.2.2. Properties of the rGO films . 99 7.3. Conclusion . 103 Bibliography 109 iv Summary Both carbon nanotubes (CNTs) and graphene are low dimensional carbon isotopes with graphitic structures. They exhibit extraordinary electrical properties arising from their quantum size effects in nanometer scale and have been considered as building blocks of the next generation electronic devices. However, their practical applications have been severely impeded by lack of effective processing technique. CNTs have a strong tendency to aggregate due to their large surface area and high aspect ratio. Although random networks or oriented arrays of individual CNTs can be directly grown by chemical vapor deposition (CVD), their fabrication costs are too high for mass production. Currently, commercially available CNTs are generally in the form of large agglomerates. Thus, the CNT aggregates must be debundled and dispersed into liquid medium for most of their applications. The conventional way is to disperse CNTs in organic solvents or water dispersed with surfactant. But the concentration of CNTs in solutions is lower than 0.1 wt%, so that copious solvents are used. A novel approach was developed to dispersing CNTs directly into surfactants without any solvent. The CNTs form gels with the surfactants by mechanical grinding or ultrasonication. The concentration of CNTs can be up to wt% in the gels. CNT films can be prepared on various substrates by coating the CNT/surfactant gels and subsequently heating. Highly conductive and transparent CNT thin films can be prepared by controlling the thickness. They can have a sheet resistance of the films of 100 Ω/□ and a transmittance of 70% at 550 nm. They have v potential application as transparent electrode of optoelectronic devices. Thick CNT films can be readily obtained from the CNT/surfactant gels. They have good adhesion to substrates, and excellent catalysis for some electrochemical reaction. They were used as the counter electrode of high-performance dye-sensitized solar cells. Like CNTs, the large scale processing techniques are also very important for graphene. Solution processing techniques have advantages over the dry processing techniques in terms of the low cost and preparation of graphene films in a large area. One key step in the solution processing techniques is the reduction of graphene oxide (GO) into graphene. During my PhD program, we developed an ultrafast green method to reduce GO with Zn powder in mild acid solutions under ultrasonication. The reduction is complete within min, and the reduced graphene can have a conductivity of 15000 S/m. The reduced graphene was processed into conductive and transparent films. These graphene films are promising candidate as the transparent electrode of optoelectronic devices as well. vi List of Tables Table 5.1. Photovoltaic parameters of DSCs with Pt and a SWCNT film of m in thickness as the counter electrode. 68 Table 5.2. Photovoltaic stability of a DSC with Pt and a SWCNT film of m in thickness as the counter electrode. 73 Table 5.3. Photovoltaic parameters of DSCs with SWCNT films of different thicknesses as the counter electrode. 74 Table 5.4. Photovoltaic stability of a DSC with a m-thick MWCNT film as the counter electrode. 77 Table 5.5. Photovoltaic parameters of DSCs with SWCNT/CMC and MWCNT/CMC films as the counter electrode. The thickness of the CNT/CMC films was 10 µm. . 80 Table 6.1. Times required for GO reduction by different methods reported in literature. . 84 vii List of Figures Figure 1.1. Schematic structures of (a) a SWCNT and (b) a MWCNT (Pictures from internet). Figure 1.2. Graphene sheet segment showing indexed lattice points and the rolling vector [3]. Figure 1.3. Density of electronic states for a semiconducting SWCNT. Solid arrows depict the optical excitation and emission transitions; dashed arrows denote nonradioactive relaxation of electrons and holes before emission [3]. . Figure 1.4. Colors of SWCNTs with different chiralities [13]. Figure 1.5. The photoluminescence emission spectrum and the optical absorption spectrum of individual SWCNTs suspended in a Sodium dodecyl sulfate (SDS) solution [14]. . Figure 1.6. Schematic presentation of a functionalization process of oxidized CNTs [25]. . 10 Figure 1.7. Graphene as the parent of all graphitic forms [71]. 13 Figure 1.8. The schematic illustration of corrugated graphene [75]. 13 Figure 1.9. Electronic dispersion in the honeycomb lattice [78]. The magnified the image shows the linear relationship between E and k near one Dirac point . 14 Figure 1.10. (a) A Photograph of a 50-mm aperture partially covered by graphene and its bilayer. The inset shows the sample design: A 20-mm-thick metal support structure has several apertures of 20, 30, and 50 mm in diameter with graphene crystallites placed over them [84]. (b) Excitation processes responsible for absorption of light in graphene [84]. 16 Figure 1.11. Chemical structural models of (a) pristine graphene, (b) GO and (c) rGO [104]. . 20 Figure 3.1. Photographs of a MWCNT/POETE gel. (a) Phase-separation behavior of the gel (upper phase) and excess POETE (lower phase), observed after centrifugation of a ground mixture of MWCNTs and POETE. The dotted line indicates the surface of the liquid phase. (b) Extrusion of the gel from a needle. (c) Electrical conduction of the gel with the four-point probe technique. . 38 Figure 3.2. SEM images of (a) as-received MWCNTs and (b) a MWCNT/POETE gel. . 39 viii Figure 3.3. TEM image of MWCNTs. The sample was preparedby dispersing a MWCNT/POETE gel in water. . 39 Figure 3.4. Angular frequency (ω) dependencies of dynamic storage (G’, solid symbols) and loss moduli (G”, open symbols) of a MWCNT/POE gel at (a) 25 oC and (b) 125 oC. Applied strain amplitudes (c) were 1% (circles) and 5% (triangles). 40 Figure 3.5. SEM images of (a) as-received SWCNTs and (b) a SWCNT/POETE gel. . 42 Figure 3.6. Raman spectra of (a) as-received SWCNTs and (b) a SWCNT/POETE gel. . 43 Figure 3.7. Angular frequency (ω) dependencies of dynamic storage (G’, solid symbols) and loss moduli (G”, open symbols) of a SWCNT/POETE gel at 25 oC. Applied strain amplitudes (c) were 1% (circles) and 5% (triangles). . 43 Figure 4.1. TGA curves of (a) POETE, (b) a SWCNT:POETE gel, and (c) pure SWCNTs in air 47 Figure 4.2. Photographs of (a) a SWCNT:POETE layer on a glass substrate coated by doctor blade, (b) the SWCNT film obtained from the SWCNT:POETE layer by heating at 250 oC for 15 and then at 500 oC for 20 min, (c) the SWCNT film transferred onto a flexible PET substrate. Picture (d) shows the flexibility of the SWCNT film on PET. . 49 Figure 4.3. SEM images of an SWCNT film (a) after the heat treatment and (b) after heat and HNO3 treatments. (c) EDX of the particles in image (a). . 50 Figure 4.4. UV-vis-IR transmittance spectra of a SWCNT film (a) before and (b) immediately after the HNO3 treatment. The inset shows the corresponding absorption spectra. 52 Figure 4.5. (a) Raman spectra of pristine SWCNTs (solid curve) and a SWCNT film (dashed curve). (b) Raman intensity ratios of the D band to G band (ID/IG) of (i) asreceived SWCNTs and a SWCNT:POETE gel after successively heating at (ii) 300 oC, (iii) 400 oC, (iv) 500 oC and (v) after subsequent HNO3 treatment. The heating time for each heat treatment is 20 min. . 53 Figure 4.6. Sheet resistance-transmittance curves of SWCNT films prepared from SWCNT:POETE gels which were formed by dispersing SWCNTs in POETE under ultrasonication of 15 W for different periods. 54 Figure 4.7. SEM images of SWCNTs prepared from SWCNT:POETE gels which were prepared under ultrasonication at 15 W for (a) 10 min, (b) 20 min, (c) 40 and (d) 80 min. (e) Average bundle lengths of SWCNT in the SWCNT/POETE gels after ultrasonication treatment for different periods. 55 Figure 4.8. TEM images SWCNTs after ultrasonication treatment at 15 W for (a) 10 min, (b) 20 min, (c) 40 and (d) 80 min. . 56 ix [87] Balandin, A. 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Introduction 1.1 Structure of carbon nanotubes Carbon nanotubes (CNTs) were firstly discovered in 1991 by Sumio Iijima as a byproduct of C60 [1] As members of fullerene family, CNTs can be considered as rolling of graphite sheets into seamless cylinders (Fig 1.1) They can be classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (MWCNT)... spectra of a 30 nm GO film before and after reduction and (b) ID/IG ratios of GO films reduced by Al with different thickness 100 Figure 7.5 (a) XPS surveys of a GO film and a rGO film, and high resolution C1s peaks of (b) a GO film and (c) a rGO film 101 Figure 7.6 (a) Sheet resistance vs transmittance of rGO films with different thickness (b) Dependence of the thickness and transmittance of. .. fieldeffect mobility of graphene is one order higher than that of Si [120] By tailoring the morphology of graphene, the Ion/Ioff ratios of graphene based TFT can be increased to 107 [121] Graphene can also be used as high performance chemical sensors [122-125] or biosensors [126,127], because the conductivity of monolayer graphene has a strong dependence on surface absorption The sensibility of graphene toward... such as epoxy and other polymers, are usually used to stabilize CNT films on substrates But these inert binders can lower the catalytic and conductive capabilities of the CNT films since they reduce the contact area between CNTs and electrolyte and block the inter-nanotube charge transport [70] 1.5 Structure and morphology of graphene Graphene is composed of a single-layer of sp2 hybridized carbon atoms... 1.8 The schematic illustration of corrugated graphene [75] 13 1.6 Properties of graphene 1.6.1 Electronic and electrical properties Fig 1.9 shows the energy dispersion in the honeycomb lattice [78] The π and π* bands in graphene have asymmetric structures The two bands are connected only at the six corners of the first Brillouin zone of the honeycomb lattice Electrons and holes near these corners behave... shown in Fig 1.11c, there are 19 a lot of vacancies left on the basal plane of rGO, and some functional groups on the plane edges cannot be completely removed Figure 1.11 Chemical structural models of (a) pristine graphene, (b) GO and (c) rGO [104] 1.8 Applications of graphene Graphene has many similar applications as CNTs due to their similar properties Individual graphene monolayers can be used as TFT... the number of graphene sheet layers (a) (b) Figure 1.1 Schematic structures of (a) a SWCNT and (b) a MWCNT (Pictures from internet) A SWCNT has a cylinder shape with a typical diameter of around 1 nm and is made of a single layer of graphene sheet [2] The SWCNT structure is related to the rolling direction of the graphene sheet, which is called the chirality [3] As shown in Fig 1.2, a pair of indices... Photographs of a GO solution (A) before and (B) after reduction with Zn powder (b) Photographs of Zn-rGO dispersed in solutions of (C) Brij 30, (D) SDBS, (E) cetrimonium bromide, (F) poly(sodium 4-styrenesulfonate) acid and (G) CMC (c) Photographs of hydrazine-rGO dispersed in (H) water and (I) a SDBS solution 83 Figure 6.2 SEM images of (a) GO and (b) Zn-rGO, AFM images of (c) GO and (d)... presentation of a functionalization process of oxidized CNTs [25] 1.4 Applications of carbon nanotubes CNTs have a lot of attractive applications based on their unique structures and properties For example, the 1D structure of CNTs gives rise to applications such as AFM tips [ 38 ] and quantum wires [ 39 ], while their hollow structure leads to applications like hydrogen storage [40,41] The semiconducting... survey scans of graphite, GO and Zn-rGO (b) Deconvolution of C1s XPS of GO (c) C1s peaks of graphite (solid curve) and Zn-rGO (dash curve) 92 xi Figure 6.8 Photograph of a rGO film transferred onto a glass substrate 93 Figure 7.1 SEM images of GO nanosheets deposited on Si substrate by spin coating at 3000 rpm Concentrations of GO suspensions are (a) 0.2 mg/ml and (b) 2 mg/ml (c) SEM image of a rGO . 1.3. Dispersion of carbon nanotubes 8 1.4. Applications of carbon nanotubes 10 1.5. Structure and morphology of graphene 12 1.6. Properties of graphene 14 1.6.1. Electronic and electrical. PROCESSING AND APPLICATIONS OF CARBON NANOTUBES AND GRAPHENE MEI XIAOGUANG (B.Eng., Tsinghua University, China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (MWCNT) according to the number of graphene sheet layers. Figure