A Case Study About The Growth Mechanism of CNT |ChemFam #08|

In my previous post, I was talking about the types and synthesis of carbon nanotubes. Today, I am going to talk about the growth mechanism that involves in the growth of nanotubes by the already discussed CVD method.

The yield and quality of the CNTs produced by the CVD method greatly depends on certain parameters. Some of the heavily influenced parameters are -

The particle size of the catalyst
The concentration of the catalyst used
Pressure
The time of growth
The temperature of the growth
The gas flow rate

A metal catalyst is essential for the growth of MWCNTs( Multi Wall Carbon Nanotubes) by CVD( Chemical Vapor Deposition) method, which was not the case for the growth of MWCNTs by the laser ablation and arc discharge method. In case of SWCNT( Single Wall Carbon Nanotubes), the use of metal is required in all three processes i.e. laser ablation, arc discharge or CVD. So, the growth mechanism of CNTs are different for different methods being used. There are two basic mechanisms for the growth of CNTs based on the substrate bound catalyst. They are called as Tip growth model and Bottom growth model.

Tip Growth Model

Tip growth model is also called as top carbon diffusion process. The decomposition of carbon source on the exposed surface area of the metal catalyst results in the formation of some carbon species. These species readily dissolves in the catalyst surface. They then diffuses through it until they are precipitated at the other end in the form of carbon nanotubes. In this model, the catalyst particle always sits on the top or surface of the growing nanotube. This is the reason why this model is called as tip growth model.

Root Growth Model
The model is also called as bottom carbon diffusion. This process functions in a way different than the tip growth model. The carbon species formed on the exposed surface of the metal catalyst dissolves and diffuses through the catalyst until they are precipitated on the top of the metal particle in the form of nanotubes. The catalyst stays on the substrate.

The formation of whether SWNTs or MWNTs is governed by the size of the catalyst particle being used. If the catalyst being used has size in few nanometers then SWNTs are formed while if it is larger then MWNTs are formed. In general, arc discharge and laser grown CNTs possesses superior crystallinity then the CVD based CNTs. However, in terms of yield and purity CVD based CNTs are way more superior than the CNTs produced by the other two methods. CVD offers harnessing of plenty of hydrocarbons in all three states of matter, thus enabling the use of various substrates. This allows growth of CNTs in various forms which can be powder, aligned or entangled, thin or thick films or could even be straight or coiled nanotubes. CVD also offers better control over the various parameters.

The growth of CNTs can further be discussed again on the basis of metal based catalyst and non metal based catalysts.

Metal based catalysts
The name itself indicates that here the use of metals as catalysts brings about the formation of nanotubes. Although, the growth mechanism of CNTs is yet to be fully developed; however the proposed general process by researchers involving the absorption and decomposition of hydrocarbons has a rigid base. The resulting carbon dissolves into particles and upon supersaturation, the carbon precipitates in tubular crystalline form. The metal catalyst have to be dispersed well on the substrate to form CNTs. Thus allowing easy formation of nanoclusters on the substrate surface. Strong interaction between the catalyst clusters and substrates leads to formation of CNTs by the already discussed root growth model. The cracking of hydrocarbons produces extremely mobile carbon atoms on the metal surface and they rapidly diffuse through or over the metal particles. The catalyst particle is present at the root and graphite precipitates around the catalyst and cylindrical structures are formed in a nested fashion. The growth of nanotubes stopped when the catalyst particle is wholly covered by layers of carbon.

When the interaction between the catalyst clusters and substrate is weak, the growth of nanotube occurs by tip growth mechanism. The catalyst particles are lifted off the substrate in aid of the cylindrical structures formed below it as a result of the weak interaction. The tip carrying the catalyst particle grows towards a region of higher feed gas concentration while the base of CNTs remain anchored to the substrate. The tip growth mechanism stops when the catalyst is completely covered by carbon layers or when the supply of feed gas is cut-off. The growth of CNTs over Fe catalyst on aluminium occurs by tip growth mechanism and finally metastable Fe₃C is formed.

Non metal based catalysts
These are also called as ceramic or semiconductor catalyst. SiC is most widely used among non metal based catalysts. Carbon nanostructures are formed over SiC by the carbothermal reduction of silica. In the initial stage SiO₂ is reduced to SiC by a carbothermal reaction. In the next step carbon nanoparticles merges and finally growth of CNTs occur on the surface of SiC through precipitation of carbon atoms or decomposition of SiC particles. The carbon generation reaction involved in the process is —

SiC(s) + CO(g) → SiO(g) + 2C(s)