Microwave enabled dispersion of highly conductive carbon nanomaterials and their interfacial assemblies
Creators: name: Chiu, Pui Lam
Publisher: name: No Publisher Supplied
Year: 2013

description: Due to its phenomenal mechanical characteristics and remarkable electrical properties, graphene, a perfect single-atomic thick two-dimensional lattice carbon layer, has attracted extensive attention in nanoscience and condensed matter physics. With all the similarities, it is believed that graphene can compete with or even surpass carbon nanotubes in many fields, and it is expected to replace silicon in many electronic applications and in other advanced technologies. A single layer of graphene sheet was first isolated in 2004 from highly oriented pyrolysis graphite with Scotch tape. The invention of “The Scotch-tape” method seems very simple, and it has enabled a whole new path in many graphene-based research areas. It also resulted in Andre Geim and Konstantin Novoselov’s winning the 2010 Nobel Prize in physics. This solvent-free method however suffers from low yields, low repeatability, and being extremely labor intensive. Solution-based fabrications have shown to be able to overcome these problems. However, the next challenge in the graphene research field and applications is the tedious chemical path that is required to convert oxidized graphene using toxic chemicals, such as hydrazine.  In this thesis, we first developed a novel and an unprecedentedly fast and simple approach to directly exfoliate graphite flakes with the aid of both nitronium ion and microwave irradiation with the aim of solving the main research problems in the field. To utilize the produced graphene in practical applications, our knowledge of interfacial science was exploited to controllably self-assemble these wonderful materials into desired structures. The research results combined with an introduction of the development and future aspects of these fields will be presented in the five chapters of this thesis.  Chapter 1 will include a general overview of basic but important information concerning the two main carbon-based materials, carbon nanotubes and graphene. Their structures, physical properties, methods of fabrications and applications will be discussed in depth. In addition, interfacial science for self-assembly of nanomaterials will be summarized.  In Chapter 2, an efficient, simple and promising way to prepare graphene sheets directly from graphite flakes with the aid of nitronium ions and microwave irradiation will be presented. Knowledge of the chemistries related to nitronium ions and microwave has enabled us to purposely omit strong oxidants, such as KMnO4, with an aim not to heavily oxidize the materials, as many methods are based on, thus reduction reactions can be completely avoided. Experimental results demonstrate that this non-destructive method resulted in concentrated stable dispersions of flat, high-quality, conductive graphene sheets in both aqueous and organic solvents. This mildly oxidized material was extensively characterized by atomic force microscope (AFM), Infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy, thermo-gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM).  In chapter 3, we extended the nitronium ions and microwave enabled dispersed approach to carbon nanotubes. Different sources of both single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were tested and the results showed that all the CNTs from different sources can be quickly dispersed into aqueous solutions with remarkably high concentrations compared to those of graphene solutions even though the same parameters were applied during dispersion. We found that depending on the existence of a small amount of defects from the original CNT sources, the yield, and quality of the dispersed CNTs are varied.  With a long term aim of fabricating highly transparent and conductive films to replace Indium tin oxide (ITO) in a wide variety of optoelectronic devices, in Chapter 4, a new method referred to as an interfacial self-assembly approach is developed to assemble the microwave dispersed graphene and CNTs into highly conductive films. The self-assembly behavior of graphene, CNT, and a mixture of graphene and CNT with different ratios were studied separately, and the knowledge obtained was used to fabricate graphene, CNT,  and a hybrid of graphene-CNT thin films at an oil/water interface, respectively. Compared to the generally used vacuum filtration method, this new approach does not need any membrane, thus theoretically any size film can be easily fabricated. To transfer the formed films to substrates for practical applications, a simple film-transferring method was also developed. The films fabricated with different film fabrication methods will also be compared and a systematic study on how the compositions of these two materials affect the performance of the final films will be summarized.   The dispersed graphene sheets are often composed with graphene sheets of different sizes, to separate them for different applications. In Chapter 5, interfacial self-assembly reactions were also applied to separate the graphene sheets based on their size-and electronic-dependent surface energies  Chapter 6 will then focus on fine-tuning the surface chemistry of the graphene sheets and the oil/water ratio to efficiently emulsify the graphene sheets into core-shell capsules for drug delivery applications. Poly(N-isopropylacrylamide) (PNIPAA), a thermally sensitive polymer is introduced to form a temperature-sensitive and stable oil-in-water microemulsion with the ability to release the encapsulated materials in a graphene/PNIPAA shell above its transition temperature. Experimental observations show that the emulsion with graphene has a slightly increased transitional temperature from 34 °C to 38 °C.