Dissolution, processing and fluid structure of graphene and carbon nanotube in superacids: The route toward high performance multifunctional materials.
Doctor of Philosophy
Carbon allotropes have taken central stage of nanotechnology in the last two decades. Today, fullerenes, carbon nanotubes (CNTs), and graphene are essential building blocks for nanotechnology. Their superlative electrical, thermal and mechanical properties make them desirable for a number of technological applications ranging from lightweight strong materials to electrical wires and support for catalysts. However, transferring the exceptional single molecule properties into macroscopic objects has presented major challenges. This thesis demonstrates that carbon nanotubes and graphite dissolve in superacids and these solution can processed into macroscopic objects. Chapter 2 reviews neat CNT fiber literature. Specifically, the two main processing methods —solid– state and solution spinning — are discussed. CNT aspect ratio and fibers structure are identified as the main variables affecting fiber properties. Chapter 3 shows that graphite can be exfoliated into single-layer graphene by spontaneous dissolution in chlorosulfonic acid. The dissolution is general and can be applied to various forms of graphite, including graphene nanoribbons. Dilute solutions of graphene can be used to form transparent conductive films. At high concentration, graphene and graphene nanoribbons in chlorosulfonic acid forms a liquid crystal and can be spun directly into continuous fibers. Chapter 4 describes a solution–based method to form a thin CNT network. This network is an ideal specimen support for electron microscopy. Imaging nanoparticles with atomic resolution and sample preparation from reactive fluids demonstrate the unique feature of solution–based CNT support compared to state–of–the–art TEM supports . Chapter 5 describes CNT liquid crystalline phase. Specifically, CNT nematic droplets shape and merging dynamics are analyzed. Despite nanotube liquid crystals having been reported in various CNT systems, a number of anomalies such as low order parameter and spaghetti–like, nematic droplets are reported. However, CNTs in chlorosulfonic acid show elongated, bipolar droplets typical of other rod–like molecules. Moreover, their large aspect ratio allows capturing the transition from homogeneous to bipolar transition expected from scaling arguments.The equilibrium shape and merging dynamics demonstrate the liquid nature of CNT liquid crystals. Chapter 6 describes the CNT/chlorosulfonic acid fiber spinning. The influence of starting material, spinning dope concentration, spin draw ratio and coagulation on fiber properties is discussed. The linear scaling of fiber strength with CNT aspect ratio is demonstrated experimentally, once the best properties from different batches are compared. Moreover, Successful multi–hole spinning demonstrates the intrinsic scalability of wet spinning to meet the typical production output of industrial–scale spinning. Chapter 7 compares acid–spun CNT fibers to other CNTs fibers as well as existing engineered materials. Acid–spun CNT fibers combine the typical specific strength of high–strength carbon fibers to the thermal and electrical conductivity of metals. These properties are obtained because of a highly aligned, dense structure. The combined strength and electrical conductivity allow acid-spun fibers to be used as structural as well as conducting wire while the combined electrical and thermal properties allow for exceptional field emission properties. In conclusion, we demonstrate that multifunctional properties of carbon nanotubes that have fuelled much of the research in the past 20 years, can be attained on a macroscopic level via rational design of fluid–phase processing.