ABSTRACT
In recent years, the ability of carbon to exist in different allotropic forms has provided, besides C60, new varieties of nanoscale structures with fascinating physicochemical properties, such as the “higher” fullerenes, endohedral fullerenes, carbon nano-onions (CNOs), graphene, and single-walled and multiwalled carbon nanotubes (CNTs), among others. A significant amount of research has been done in the field of fullerenes after their discovery in 1985 by Smalley, Curl, and Kroto (Kroto et al. 1985). Fullerenes exhibit interesting properties such as high thermal stability, large surface area, and broad absorption spectra (Wang et al. 2004; Janssen et al. 2005; Shin et al. 2006). One of the most attractive and potentially useful properties of fullerenes is their ability to reversibly accept multiple electrons (Xie et al. 1992). Recently, larger morphological variations of fullerene-like all carbon structures such as CNTs and graphene have received enormous attention. The remarkable physical and chemical properties of these structures are of considerable interest. These properties include high conductivity and strong mechanical, thermal, and environmental resistance (Novoselov et al. 2004; Zhang et al. 2005; Han et al. 2007; Bekyarova et al. 2009; Rao et al. 2009). Within this large group of allotropic carbon nanomaterials, we focus on the onion-like layered structures. Sumio Iijima discovered CNOs in 1980 while looking at a sample of carbon black using a transmission electron microscope (TEM) (Iijima 1980). CNOs are spherical structures that consist of a hollow spherical fullerene core surrounded by concentric graphene layers (larger fullerenes) with increasing diameter (Iijima 1980). That is why they are referred to as “nano-onions,” “onion-like carbon,” multilayer fullerenes, multilayered round carbon particles, or “buckyonions” (Kuznetsov et al. 1994; Bates et al. 1998; Santiago et al. 2012; Costa et al. 2014). In this chapter, we discuss the preparation and characterization of CNOs; their structural,
11.1 Introduction ......................................................................................275 11.2 CNOs: Structure, Production, and Formation Mechanisms .....276 11.3 Physical Properties of CNOs ...........................................................278
