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Carbon nanofibers possess excellent mechanical properties, high electrical conductivity, electromagnetic shielding and high thermal conductivity. Carbon nanofibers have average diameters ranging from 125–150 nm depending upon the precursors and technique used to manufacture them and have lengths ranging from 50–100 μm. They can form composites with various types of polymers and can be easily functionalized due to the presence of unique surface states (Figure 1.7).

Figure 1.7 Schematic illustration showing (a) the formation of bonds of carbon atoms present at different layers of carbon fiber’s surface and missing of electrons from the top layer of carbon atoms; and (b) the effect of the missing electrons from the surface of top layer atoms, creating positive and negative charges which are responsible for making the carbon fibers reactive.

Each layer of carbon nanofiber or nanotubes, contains a set of carbon atoms arranged in one plane. In x-y direction, each carbon is attached with σ-bonds. Bottom layers of carbons (in z direction) are also arranged in the same fashion, such that each carbon shares its electron with bottom layers of carbon by either σ-bonds or π-bonds as the case may be (Figure 1.7a). However, electrons of carbon atoms present in the top layer are not satisfied with its electron due to the absence of any carbon atoms above its layer. These carbon atoms are dissatisfied with electrons. As a result, these carbon atoms would either behave as positively charged (lost its electron) or negatively charged (contains more than its normal electron). However, these distributions of charges would be such that overall the carbon tube will behave like a neutral particle. These unsatisfied carbon atoms are more reactive as compared to carbon atoms present in the layers below the top layer. These carbon atoms are said to possess active sites which are designated as a surface state. The presence of surface state depends upon the types of graphene layers present on the top layers of carbon tube. Its broken graphene layers will show a large number of surface states as well as surface areas. It is for this reason that carbon nanofibers are more reactive as compared to carbon nanotubes.