there is not much difference between carbon nanofiber and carbon nanotube. When a single layer of graphene sheet is rolled into a cylinder (Figure 1.5a) it takes the shape of a single-walled carbon nanotube. When a multiple number of graphene sheets separated by a distance of 0.335 nm are rolled into a cylindrical shape it takes the shape of multi-walled carbon nanotubes (Figure 1.5b). However, when broken sheets of multiple graphene sheets are rolled into a cylinder it takes the shape of carbon nanofiber (Figure 1.5c). In other words, carbon nanofibers appear as cylindrical nanostructures with graphene layers organized as packed cones, cups or plates. The surface area of CNF is much larger compared to MWCNT of the same dimension, because the former has many broken graphene sheets which are not aligned as that of MWCNT (well-defined layer, as shown in the TEM image in Figure 1.5b).
Figure 1.5 Schematic illustration of (a) how single layer of graphene folds to make a single-wall carbon nanotube; (b) how multilayers of graphene fold together to form a multi-walled carbon nanotube; and (c) sketch of how broken graphene layers fold to form a carbon nanofiber, and TEM image of CNF with broken graphene layers, and (d) TEM image of packed cones, cups.
1.2.1 History of CNF
Carbon nanofibers (CNFs) were first reported by Hughes and Chambers in a patent in 1889 [5] while carrying out the synthesis of filamentous carbon. Pyrolysis of methane/hydrogen gaseous mixture yielded carbon nanofibers in the form of a filament growth. The first electron microscopy observations of hollow carbon nanofibers of 50 nm in diameter was reported in the early 1950s by the Soviet scientists Radushkevich and Lukyanovich. Then, in 1988 Morinobu Endo [6] at Shinshu University, Japan, reported carbon nanofibers with a diameter of 1 μm and length of above 1 mm. In 1991, Yoshinori Ando [7] synthesized carbon nanomaterial by Arc process which was examined by Sumio Iijima while working at NEC as a new form of carbon nanotube. Pradhan synthesized carbon nanofibers by the pyrolysis of precursor camphor. Some of the peculiar shaped carbon nanofibers synthesized are shown in Figure 1.6.
Figure 1.6 (a) TEM image showing graphene layers of (b) entwined carbon nanofibers synthesized at 1100 °C in presence of Ni: Al catalyst and n-hexane; (c, d) TEM images of open-ended bent, V-shaped inner novel growth of carbon nanotubes at 1000 °C in presence of nickel particles; and (e) SEM micrograph image of straight VGCFs (Source: Debabrata Pradhan, 2003) [4].
1.2.2 Role of Surface States in Controlling the Properties of CNFs
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.
1.3 Synthesis of Carbon Nanofiber (CNF)
Catalytic chemical vapor deposition (CCVD) or simply CVD is the foremost commercial technique for the fabrication of CNF as well as CNT. This is a batch process. This technique also produces CMF known as vapor-grown carbon nanofiber (VGCNF). In this type of process, precursor near its boiling temperature is converted into gas-phase molecules and is transported by some carrier gas to decompose at high temperatures in the presence of a transition metal catalyst kept on a substrate. A subsequent growth of the fiber around the catalyst particles is achieved. The nanofiber diameter depends on the catalyst size. There can be different designs of a CVD unit.
1.3.1 Chemical Vapor Deposition (CVD) Method
A typical CVD setup is shown in Figure 1.8. A long quartz tube (C) is inserted into two ceramic tubes (D). Electrical heating wire is wrapped over two ceramic tubes (D). These two ceramic tubes are arranged to maintain two different temperatures (A1 and A2). In the quartz tube (C) two quartz boats (B1 and B2) are kept. In boat B1 a known amount of precursor which is to be pyrolyzed is kept. In boat B2 catalyst powder is spread over the boat. Two gas cylinders (K and H) are connected to the long quartz tube (C) via two controllers (E) and flow meter (F). The outlet of tube C is connected to two bubblers (G). Temperature of A1 is maintained
