BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 14

progress was achieved by laser ablation synthesis of bundles of aligned SWNT
with small diameter distribution. Catalytic growth of nanotubes by the chemical
vapor decomposition (CVD) method was used too [54].
ARC-DISCHARG E:
In 1991, Iijima reported the preparation of a new type of finite carbon structures
consisting of needlelike tubes. The tubes were produced using an arc discharge
evaporation method similar to that used for the fullerene synthesis. The carbon
needles, ranging from 4 to 30 nm in diameter and up to 1 mm in length, were
grown on the negative end of the carbon electrode used for the direct current (dc)
arc-discharge evaporation of carbon in an argon filled vessel (100 Torr). Iijima
used an arc-discharge chamber filled with a gas mixture of 10 Torr methane and
40 Torr argon. Two vertical thin electrodes were installed in the center of the
chamber. The lower electrode, the cathode, had a shallow dip to hold a small piece
of iron during the evaporation. The arc-discharge was generated by running a dc
current of 200 A at 20 V. Laser beam vaporizes target of a mixture of graphite
and metal catalyst (Co, Ni) in a horizontal tube in a flow of inert gas at controlled
pressure and in a tube furnace at 1200°C. The nanotubes are deposited on a water-
cooled collector outside the furnace electrodes. The use of the three components
argon, iron and methane, was critical for the synthesis of SWNT. The nanotubes
had diameters of 1 nm with a broad diameter distribution between 0.7 and 1.65
nm. In the arc-discharge synthesis of nanotubes, used as anodes thin electrodes
with bored holes, which were filled with a mixture of pure powdered metals (Fe,
Ni or Co) and graphite. The electrodes were vaporized with a current of 95-105
A in 100-500 Torr of He. Large quantities of SWNT were generated by the arc-
technique. The arc was generated between two graphite electrodes in a reactor
under helium atmosphere (660 mbar)[56].
LASER-ABLATION:
In 1996, Smalley and co-workers produced high yields (>70%) of SWNT by la-
ser ablation (vaporization) of graphite rods with small amounts of Ni and Co at
1200°C. The tube grows until too many catalyst atoms aggregate on the end of the
nanotube. The large particles either detach or become over coated with sufficient
carbon to poison the catalysis. This allows the tube to terminate with a fullerene
like tip or with a catalyst particle. Both arc-discharge and laser-ablation tech-
niques have the advantage of high (>70%) yields of SWNT and the drawback that
(1) they rely on evaporation of carbon atoms from solid targets at temperatures
>3000°C, and (2) the nanotubes are tangled which makes difficult the purification
and application of the samples [56].
CHEMICAL VAPOR DEPOSITION (CVD):
Despite the described progress of synthetic techniques for nanotubes, there still
remained two major problems in their synthesis, that is, large scale production and