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      Chemical Vapor Deposition

Q1: Potential Applications of Carbon Nanotubes.
• Additives in ploymers
• Catalysts
     Electron field emitters for
     cathode ray lighting elements
     flat panel display
     gas-discharge tubes in telecom networks
• Electromagnetic-wave absorption and shielding
• Energy conversion
• Lithium-battery anodes
• Hydrogen storage
• Nanotube composites (by filling or coating);
• Nanoprobes for
     STM, AFM, and EFM tips
     nanolithography
     nanoelectrodes
     drug delivery
• sensors
• Reinforcements in composites
• Supercapacitor

Q2: What are Carbon Nanotubes?

Carbon nanotubes are hexagonally shaped arrangements of carbon atoms that have been rolled into tubes. These tiny straw-like cylinders of pure carbon have useful electrical properties. They have already been used to make tiny transistors and one-dimensional copper wire.
They were developed by using nanotechnology, a relatively new field that involves building electronic circuits and devices from single atoms and molecules. Nano means one thousand millionth of a unit. A nanometer is therefore one thousand millionth of a meter. The first nanofabrication experiments occurred in 1990 when individual xenon atoms were placed on a nickel substrate and used to spell out a company logo. One primary goal of nanotechnology is to build computer chips and other devices that are thousands of times smaller than they are now.
Carbon nanotubes have enormous theoretical possibilities but have not lived up to the hype surrounding their development. Researchers have continued to look for ways to use them, however, as successful applications have the potential to be highly lucrative.
Scientists have recently succeeded in altering carbon nanotubes so that they supply electrons when exposed to light.
This was done by having two flat rings of carbon molecules sandwich a ferrocene (iron) molecule. Ferrocene is known for its tendency to relinquish electrons. When exposed to visible light, the carbon atoms accepted the ferrocene molecule.
This is the first time that carbon nanotubes have been hybridized to undergo light-induced electron transfer. Researchers say that these modified carbon nanotubes are the first step in building solar cells using this technology.
The newly-discovered ability of carbon nanotubes to serve as electron sources has great potential. Carbon nanotubes may one day replace the metal filaments in X-ray machines, which tend to burn out quickly. Scientists hope to use them to develop portable X-ray machines for use in airport security, ambulances, and customs work.
Carbon nanotubes also have great significance for use in flat-panel displays, microwave generators, devices for electric surge protection, and high intensity lamps.

Chemical Vapor Deposition
"Chemical vapor deposition (CVD) of hydrocarbons over a metal catalyst is a classical method that has been used to produce various carbon materials such as carbon fibers, filaments, etc. for over twenty years. Large amounts of CNTs can be formed by catalytic CVD of acetylene over Co and Fe catalysts supported on silica or zeolite. The carbon deposition activity seems to relate to the cobalt content of the catalyst, whereas the CNTs’ selectivity seems to be a function of the pH in catalyst preparation. Fullerenes and bundles of SWNTs were also found among the MWNTs produced on the carbon/zeolite catalyst.
Some researchers are experimenting with the formation of CNTs from ethylene. Supported catalysts (Fe, Co, Ni), containing either a single metal or a mixture of metals, seem to induce the growth of isolated SWNTs or SWNT bundles in the ethylene atmosphere. The production of SWNTs, as well as double-walled CNTs (DWNTs), on Mo and Mo-Fe alloy catalysts has also been demonstrated. CVD of carbon within the pores of a thin alumina template (called a membrane) with or without a Ni catalyst has been achieved. (Reference ??) Ethylene was used with reaction temperatures of 545oC for Ni-catalyzed CVD, and 900oC for an uncatalyzed process. The resultant carbon nanostructures have open ends, with no caps.
Methane has also been used as a carbon source. In particular it has been used to obtain ‘nanotube chips’ containing isolated SWNTs at controlled locations. High yields of SWNTs have been obtained by catalytic decomposition of an H2/CH4 mixture over well-dispersed metal particles (Co, Ni, Fe) on MgO at 1000oC. It has been reported that the synthesis of composite powders containing well-dispersed CNTs can be achieved by selective reduction in an H2/CH4 atmosphere of oxide solid solutions (between a non-reducible oxide such as Al2O3 or MgAl2O4 and one or more transition metal oxides). The reduction produces very small transition metal particles at a temperature of usually >800oC. The decomposition of CH4 over the freshly formed nanoparticles prevents their further growth, and thus results in a very high proportion of SWNTs and fewer MWNTs." [Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, Chapman and Hall (2002)]