Figure 3.4 Different synthesis approaches available for the preparation of metal catalysts.
Figure 3.5 The process of nanoparticle formation by plant extract. (Source: Dizai et al. 2014) [24].
3.4 Role of Nanocatalyst in the Production of CNF
It is well known that the mechanical, physical, and chemical properties of nanoparticles may differ considerably from those of bulk substances. The deposition over support often endows particles with novel or improved properties. These changes are provided by the change of electronic and structural properties, strong interaction with the support, and increase of the particle/support interface length taking place with metal dispersion. These factors are closely interrelated and determine, for example, the number of catalytically active sites and adsorption energies during catalytic reactions. Thus, the dispersion of a supported metal can be extremely high, making almost every metal atom accessible to reactants. Advantages of metal dispersing over support widely used for catalytic applications are also applicable for different fields of nanotechnology.
Because of their novel properties, which can be realized in a variety of ways in numerous applications, CNFs have received a great deal of attention from both the research and industrial communities. Several academic and industrial research groups have directed their efforts toward synthesizing and optimizing the growth of these CNFs. In its simplest form, the catalytic synthesis of CNFs consists of formation of these fibers on metallic catalysts in the form of powders, foils, gauzes, or supported particles. The process consists of reducing the catalyst sample in a hydrogen-inert gas stream at a somewhat lower temperature, followed by heating the catalyst up to the reaction temperature, subsequent to which the reaction mixture, consisting of hydrocarbon, hydrogen, and inert gas, is introduced into the system [25–30]. The reaction proceeds for periods ranging from a few minutes to several hours. Several different metallic and bimetallic catalysts can be used. The most commonly used catalysts for CNF synthesis are iron, cobalt, nickel, and copper, both in bulk and in supported form. Lower hydrocarbons, such as methane, ethylene, acetylene, or benzene, and carbon monoxide are the common sources of carbon material. The most common mechanism proposed in the literature for the synthesis of CNFs consists of decomposition of the hydrocarbons on the metal surface, releasing carbon atoms. These carbon atoms then form metal carbides that dissolve and diffuse through the bulk of the metal, resulting in the deposition of CNFs at the other end of the metal particles [31–33]. Despite great advances in synthesis methods and efforts to understand the mechanism of nucleation and growth of CNFs, continuous production of these fibers has proven to be challenging.
3.5 Different Types of CNF
Carbon nanofibers of various morphologies have been synthesized (Figure 3.6). Some of them are mentioned below:
Platelet-like CNF is composed of small graphene layers perpendicular to the fiber axis and the fiber must contain a non-negligible amount of hydrogen or other heteroatoms for the stabilization of the plates. The fibrils can also be coiled, as shown in Figure 3.4a,b [34]. For the growth of platelet, CNFs from propane Ni/Al2O3 catalyst have been found to be good catalyst.Figure 3.6 CNF of different morphologies: (a) Platelets, (b) Spiral platelets, (c) Fishbone hollow core, (d) Fishbone solid, (e) Ribbon, (f) Stacked cup.
Fishbone-like CNF or Herringbone CNF is where the graphene layers are inclined with respect to the fibril axis. Consequently, hydrogen is also required to stabilize the edges. Fishbone CNFs can have either a hollow core (Figure 3.4c) or a solid core (Figure 3.4d). The herringbone CNFs are synthesized over Ni/Al2O3 catalyst using methane and propane as carbon sources.
CBF Ribbons are comprised of straight, unrolled graphene layers that are parallel to the fibril axis with noncylindrical cross sections (Figure 3.4e). Transition metal Co, originated from Co(NO3)2·6H2O doped in spinning solution, serves as both template of mesopores and catalyst for graphitic structure [35].
Stacked-Cup CNF This form of CNF is a continuous layer of rolled (spiral) graphene along the fiber axis. The spiral orientation of these nanofibers yields a truncated cone arrangement along the axis with a wide internal hollow space, as shown in Figure 3.4f. SiO2 particles are used as catalysts for stacked-cup carbon nanotube formation in a spray pyrolysis chemical deposition method from ethanol where SiO2 particles are reduced to SiC following a carbon dissolution mechanism [36].
Thickened Tubular CNFs are comprised of a base structure of one of the previously mentioned catalytic nano filaments (CNF or CNT) with a variable coating of amorphous carbon produced by CVD method using Fe/Al2O3 catalyst with ethylene as a carbon precursor by CCVD.
Figure 3.7 shows CNF grown using catalytic chemical vapor deposition (CCVD) with methane as carbon source and a hydroxyapatite-supported nickel (Ni/HAp) as catalyst. The catalyst, which contained approximately 14 wt% Ni, was prepared using the incipient wetness method with an aqueous nickel nitrate solution. Three variables were evaluated to optimize the CNF growth process, including the temperature and the time of catalyst reduction as well as the reaction time, at 650 °C. Herringbone bamboo-like CNFs were grown during methane decomposition over Ni/ HAp, which was confirmed using transmission electron microscopy [37].
The diameters of the grown CNFs were determined by TEM. As shown in Figure 3.7 the CNF diameters increase as the reduction temperature increases. At 550 °C, CNFs with a diameter of 25–45 nm were grown with a predominant contribution