 The way in which nanotubes are formed is not exactly known. The growth mechanism is still a subject of controversy, and more than one mechanism might be operative during the formation of CNTs. One of the mechanisms consists out of three steps. First a precursor to the formation of nanotubes and fullerenes, C2, is formed on the surface of the metal catalyst particle. From this metastable carbide particle, a rodlike carbon is formed rapidly. Secondly there is a slow graphitisation of its wall. This mechanism is based on in-situ TEM observations.

 The exact atmospheric conditions depend on the technique used, later on, these will be explained for each technique as they are specific for a technique. The actual growth of the nanotube seems to be the same for all techniques mentioned.

There are several theories on the exact growth mechanism for nanotubes. One theory 13 postulates that metal catalyst particles are floating or are supported on graphite or another substrate. It presumes that the catalyst particles are spherical or pear-shaped, in which case the deposition will take place on only one half of the surface (this is the lower curvature side for the pear shaped particles). The carbon diffuses along the concentration gradient and precipitates on the opposite half, around and below the bisecting diameter. However, it does not precipitate from the apex of the hemisphere, which accounts for the hollow core that is characteristic of these filaments. For supported metals, filaments can form either by ‘extrusion (also known as base growth)’ in which the nanotube grows upwards from the metal particles that remain attached to the substrate, or the particles detach and move at the head of the growing nanotube, labelled ‘tip-growth’. Depending on the size of the catalyst particles, SWNT or MWNT are grown. In arc discharge, if no catalyst is present in the graphite, MWNT will be grown on the C2-particles that are formed in the plasma.

It is important to understand the effect of hydrogen flow rate on the formation of carbon nano and microstructure material because hydrogen is frequently present in the hydrocarbon processing system. The effect of hydrogen can be both acceleration and suppression. The effect of hydrogen acceleration on carbon formation may be interpreted in two ways. The first interpretation suggested that, hydrogen decompose inactive metal carbides to form catalytically active metal. The other interpretation pertains to the removal, by hydrogen, of the surface carbon and precursors of carbon, which block the active site. The suppressing effect has also been reported to be due to the surface hydrogenation reactions to form methane.

Considering all these theories together with our experimental results leads us to propose the following mechanism for the deposition of graphitic carbon on the metal surface. The promotional effect of hydrogen on carbon nanotubes formation from the metal catalyzed decomposition of carbon-containing gas molecules has been attributed to its ability to convert inactive metal carbides into the catalytically active metallic state as well as to prevent the formation of graphitic overlayers on the particle surface..

 Thus the catalytic decomposition of hydrocarbon is highly sensitive to substrate catalyst, while the hydrogenation of carbon is relatively less sensitive to catalyst. For the catalyst, which is not highly active for decomposition, the hydrogenation reaction becomes important and the net carbon deposition rate is lowered by hydrogen gas

Product H2H2 H2H2 H2H2 H2H2 H2H2 H2H2 Metal Carbide (Fe 3 C) Iron Catalyst

ce h

a b c

Figure 4.24 shows a schematic representation of different types of growth that can be observed in carbon filaments (Baker, 1988). The morphology of VGCF is unique in that the graphene planes are more preferentially oriented around the axis of the fiber. Figure below are scanning electron micrograph of the broken end of a thick VGCF which suggests the fiber construction by adding successive layers of carbon, resulting after heat treatment in nested graphene planes. The figure shows that the structure of VGCFs resembles that of a tree trunk, with concentric annular rings. At the centre, along the axis of symmetry, lies the original filament.

 The results of the present investigation suggest that the observed changes in catalytic activity and selectivity accompanying an increase in temperature are probably due to major alterations in the distribution of atoms at the metal/gas interface. Thermodynamically, higher temperatures favor the surface decomposition of hydrocarbon rather than the hydrogenation reactions.

 The temperature influence on the structure of the carbon materials has been emphasized. It is generally accepted that carbon materials are formed by carbon atom dissolving, diffusing, and precipitating through the catalyst droplets in CVD process. The dissolving, diffusing and precipitating rates of the carbon atoms are affected by both the carbon atoms concentration and the temperature. The carbon precipitation region on the Fe catalyst droplets can be distinguished into two areas, surface area and internal area. At low temperature, the dissolving and diffusing rates are limited by the low concentration of carbon atoms so that carbon atoms can only precipitate on the surface area of the catalyst droplets to form completely hollow CNTs.

The diameter of CNTs gets bigger with the increase in temperature. This can probably be attributed to small catalyst droplets agglomerate at high temperature to form bigger catalyst particle which will form big CNTs. High reaction temperature will promote the decomposition of hydrocarbon to increase the concentration of carbon atoms, which will increase the growth rate of CNTs to form bigger CNTs. With the increase of the temperature, the dissolving and diffusing rates of carbon atoms will increase, and carbon atoms can get to the internal area of the catalyst droplet to form CNFs

At high temperature, the carbon concentration is high enough for the precipitating at both side areas of the catalyst droplet to form Vapor grown carbon fiber. As shown in our result, the temperature has induced the reconstruction of the metal faces of the catalyst. The change in the product from CNTs to CNFs at 900 °C was as mentioned due to the change in the catalyst faces. It shows that, at this reaction temperature the Fe catalyst reconstructs the atoms in the star shape while for CNTs the shape of the catalyst are spherical. The sizes of the CNFs catalyst are much bigger than that for CNTs as shown in figure.

CNT Catalyst CNF Catalyst Figure 4.28: Schematic representation of the change in the size and the shape of the catalyst from (a) CNT to (b) CNF.