Graphene based nanostructures: towards application with carbon nanofibers

Abstract

Graphene nanostructures have got great attention of the researchers worldwide in the last two decades owing to their excellent physical, chemical, optical, and electronic properties. The continuing efforts to miniaturize the electronic devices have stirred the need for searching alternate materials with high aspect ratio and improved electrical properties. Carbon nanofibers having sub-micron diameter serve as a bridge where the fundamental graphene concepts could be tangibly applied and measured compared to carbon fibers. The thesis project stems from this concept. In this context, the present research work is focused on the realization of graphene nanostructures through carbon nanofiber, its graphitic structure and electrical transport properties. The thesis systematically discusses the basic process parameters and their impact on the graphitic structure from electrospinning to stabilization and carbonization. The graphitic structure is significant as it dictates the electrical, mechanical, and chemical properties. Particularly, two different approaches are used to modify the graphitic structure of carbon nanofibers, namely creep stress induced graphitization and nanocarbon templated graphitization. Both the independent and synergistic effects of these two approaches have been explored, first on the polyacrylonitrile graphitization and then on the electrical transport properties. We showed that creep stress during stabilization improves the degree of cyclization of PAN at lower temperatures. It was found that though the degree of cyclization may be the same, the graphitic order of carbon nanofibers after carbonization shows marked difference. The application of creep stress avoids the curving of graphene planes which takes places due to in-situ hepta- and penta-ring formation. Moreover, the nanocarbon inclusion based templated graphitization improves the PAN graphitic order by lowering of ID/IG ratio, increasing the crystallite size, and enhancing the orientation of graphitic domains. The improvement of graphitic order is attributed to the anchoring of PAN chains by nanocarbon inclusion during stabilization which prevents the loss of polymer chain alignment. Further, the nanocarbon templating effect for nucleation and growth of carbon crystal was observed. The last phase of the project explores rather the synergistic effects of these both approaches on graphitization degree and electrical transport of PAN. The sp2 fraction and ordering of graphitic domains is influenced by nanocarbon inclusion (CNTs). However, the combined effect of CNTs and creep stress does increase the sp2 fraction but deteriorates their alignment. By investigating the mutual and individual effect of above approaches, we were able to conclude that the electrical transport in sub-micron carbon nanofiber is mainly dependent on the alignment of graphitic domains (sp2 clusters). The electrical transport could be understood as the cumulative effect of band-based conductivity along sp2 units and tunneling between sp2 and sp3 clusters. These findings will pave a way for fabrication of carbon fibers with tailored graphitic structure and electrical conductivity for applications in cables, energy storage and sensing electrodes.