BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 15

ordered synthesis. But in 1996, a CVD method emerged as a new candidate for
nanotube synthesis. This method is capable of controlling growth direction on a
substrate and synthesizing13 large quantity of nanotubes. In this process a mixture
of hydrocarbon gas, acetylene, methane or ethylene and nitrogen was introduced
into the reaction chamber. During the reaction, nanotubes were formed on the
substrate by the decomposition of the hydrocarbon at temperatures 700-900°C
and atmospheric pressure. The process has two main advantages: the nanotubes
are obtained at much lower CVD reactor temperature, although this is at the cost
of lower quality, and the catalyst can be deposited on a substrate, which allows for
the formation of novel structures [56].
THE SUBSTRATE:
The preparation of the substrate and the use of the catalyst deserve special atten-
tion, because they determine the structure of the tubes. The substrate is usually
silicon, but also, glass and alumina are used. The catalysts are metal nanopar-
ticles, like Fe, Co and Ni, which can be deposited on silicon substrates either
from solution, electron beam evaporation or by physical sputtering. The nanotube
diameter depends on the catalyst particle size, therefore, the catalyst deposition
technique, in particular the ability to control the particle size, is critical to de-
velop nano devices. Porous silicon is an ideal substrate for growing self-oriented
nanotubes on large surfaces. It has been proven that nanotubes grow at a higher
ratio (length per minute), and they are better aligned than on plain silicon. The
nanotubes grow parallel to each other and perpendicular to the substrate surface,
because of catalyst surface interaction and the van der Waals forces developed
between the tubes [56].
THE SOL-GEL:
The sol-gel method uses a dried silicon gel, which has undergone several chemi-
cal processes, to grow highly aligned nanotubes. The substrate can be reused after
depositing new catalyst particles on the surface. The length of the nanotube arrays
increases with the growth time, and reaches about 2 mm after 48h growth [56].
GAS PHASE METAL CATALYST:
In the methods described above, the metal catalysts are deposited or embedded on
the substrate before the deposition of the carbon begins. A new method is to use a
gas phase for introducing the catalyst, in whichboth the catalyst and the hydrocar-
bon gas are fed into a furnace, followed by catalytic reaction in the gas phase. The
latter method is suitable for large-scale synthesis, because the nanotubes are free
from catalytic supports and the reaction can be operated continuously. A high-
pressure carbon monoxide (CO) reaction method, in which CO gas reacts with
iron pentacarbonyl, Fe(CO) 5 to form SWNT, has been developed. SWNT have
also been synthesized from a mixture of benzene and ferrocene, Fe(C 5 H 5 ) 2 in a