Morphology and Dispersion of Pristine and Modified Carbon Nanofibers in Water

Abstract

Carbon nanofibers have been of great interest due to their extraordinary mechanical and electronic properties. Carbon nanofibers (CNF) are different from carbon nanotubes in that they have many more walls of crystalline carbon and usually have more structural defects than carbon nanotubes. The cost of preparing carbon nanofibers is significantly less than carbon nanotubes due to the synthesis techniques used, defects and the remaining amorphous carbon. Carbon nanofibers are suitable for a range of applications such as reinforcing fillers, field emitters and nanoelectronic devices etc. (Dresselhaus, Dresselhaus et al. 2001; Safadi, Andrews et al. 2002; Gong, Li et al. 2005; Li, Zhao et al. 2005) Unfortunately, the advantages of carbon nanofibers have not been realized because of the difficulty of obtaining fully dispersed nanofibers. Although hundreds of papers have been published describing enhanced dispersion of carbon nanofibers by surface modification, plasma treatment and functionalization of the sidewalls and fiber tips, quantitative measurement of the degree of dispersion remains challenging and the nature of the dispersed entities remains unknown. Scattering methods is an ideal tool to provide structural information about nanofiber morphology. In this chapter, we review several approaches that are used to assist dispersion, including surface modification, PEG-functionalization and plasma treatment Small angle light scattering is utilized as a primary tool to assess the morphology of the carbon nanofibers and quantify dispersion of the carbon nanofibers treated through these approaches. A simplified tube or fiber model is introduced to assist in further understanding the morphology. The chapter is divided into three sections. The first section focuses on dispersion of untreated and acid-treated carbon nanofibers suspended in water. Analysis of Light scattering data provides the first insights into the mechanism by which surface treatment promotes dispersion. Both acid-treated and untreated nanofibers exhibit hierarchical morphology consisting of small-scale aggregates (bundles) that agglomerate to form fractal clusters that eventually precipitate. Although the morphology of the aggregates and agglomerates is nearly independent of surface treatment, their time evolution is quite different. Acid oxidation has little effect on bundle morphology. Rather acid treatment slows agglomeration of the bundles. The second section discusses the morphology and dispersion of solubilized carbon nanofibers. Light scattering data indicate that PEG-functionalized nanofibers are dispersed at small rod-like bundle (side-by-side