**2.5 Carbon nanofilaments**

Carbon nanofilaments are nanometric filaments with diameters between 1 and 200 nm and lengths of up to several microns. These materials are composed mainly of graphite type carbon whose basic structural component is graphene [69]. Graphene can be defined as the combination of carbon atoms with sp<sup>2</sup> hybridization, where each carbon atom joins three others forming a flat hexagonal tessellation (basal plane or graphene layer) [70]. The parallel stacking of several of these layers' outcomes in graphite characterized by an elevated structural order and a distance of 0.3354 nm between the distinct graphene layers (crystalline domain or interplanar distance, d002) (**Figure 5**).

#### **Figure 5.**

*Representative scheme of crystal structures of graphene (adapted from Ref. [71]).*

On the other hand, carbon nanofilaments have a structural order inferior to that of graphite and according to the Franklin classification [72] correspond to turbostratic type materials, that is, they have crystalline domains greater than graphite and smaller than non-graphitic carbons (0.3354 < d002 < 0.344 nm).

Within carbon nanofilaments we can distinguish two types: carbon nanotubes (CNT) and carbon nanofibers (CNF). The CNT can be considered as layers of graphene rolled into hollow tubes [73]. Depending on the number of layers that make up the CNT, they are classified as single wall CNT (SWCNT), formed by a single layer, or multiple wall CNT (MWCNT), formed by 2 or more concentrically coiled layers (**Figure 6a**) [73]. On the other side, the CNF can be hollow or strong

*Catalysts for the Simultaneous Production of Syngas and Carbon Nanofilaments… DOI: http://dx.doi.org/10.5772/intechopen.101320*

**Figure 6.**

*Simplified representation of the different kinds of (a) carbon nanotubes (SWCNT and MWCNT), and (b) carbon nanofibers (platelet, tubular, fishbone) (adapted from Ref. [73]).*

and are categorized with regard to their longitudinal axis according to the angle they form graphene layers (α). The most common types of CNF are platelet, parallel (also named ribbon or tubular) and fishbone (**Figure 6b**) [73]. Platelet CNF are characterized by the fact that the graphene sheets are arranged perpendicular to the growth axis of the CNF (α ≈ 180°), while in the fishbone type the angle α is between approximately 20–160° [74].

They are also called Herringbone. Finally, the parallel types would be those in which the sheets are parallel to the longitudinal axis of the CNF (α ≈ 0°). Unlike **Figure 6b**, this sort of structure can also be tubular and therefore it is not feasible to distinguish them from MWCNT by using electronic microscopy methods. However, there is some controversy, parallel type CNF tend to present areas along their structure in which the graphene layers are not oriented in parallel (α > 0°) as well as numerous imperfections such as the union of the layers' graphene inside the nanofiber (loops). Along with these three morphologies, in the CNF world there are other types of less common structures such as bamboo CNF, which are characterized by having internal holes that occur periodically due to the movement of the catalytic particle during the growth of the CNF, or the octopus-type NFCs that are generally produced when a Ni catalyst doped with Cu [75] is employed. Although there is a bibliography related to the formation of carbon filaments since the late nineteenth century, it was the discovery of the transmission electron microscope (TEM) in 1939 that really represented a breakthrough in this field since it allowed the observation in detail the morphology of this type of structures [76]. Initially, the interest in carbon formation derived from the problems that its accumulation caused in the processes of conversion of hydrocarbons (deactivation and destruction of catalysts or plugging of reactors) and therefore, the objective was to understand how and why it was generated in order to avoid their formation [77]. However, since the discovery of CNT by Iijima [78] in the 90s and due to the properties that carbon nanofilaments present (high specific surfaces and high electrical conductivities and thermal, the approach changed and numerous studies were initiated to optimize their production [79].
