GROWTH MECHANISM OF CNM

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 theory13 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.
THE ROLE OF HYDROGEN FLOW RATE
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
H2
H2
H2
H2
H2
Metal Carbide (Fe3 C)
H2
Iron Catalyst
Product
Product
Product
Product
e
c
h
a
b
c
CARBON NANOFIBER
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 ROLE OF REACTION TEMPERATURE

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.