Chemistry Reference
In-Depth Information
candidates in the design of high-performance composite materials. Another key
opportunity arises from the fact that CNTs can be easily internalized by cells and
therefore can act like delivery vehicles for a variety of biological cargos, ranging
from small molecules to biomacromolecules such as proteins and DNA/RNA.
Furthermore, the intrinsic physical properties of CNTs can be utilized for multi-
modal imaging and therapy [
12
,
13
].
This chapter is intended to introduce the reader to the world of CNTs, giving a
snapshot of the most salient production and characterization techniques and putting
particular emphasis on their properties and their chemical modification, which can
lead to many interesting applications.
2 Production of SWCNTs and MWCNTs
There are several methods of making CNTs. Most probably carbon nanotubes had
been around for a long time before their discovery as secondary products of various
vapor deposition or carbon combustion processes, but electron microscopy was not
advanced enough to identify them.
The first mass production of CNTs was obtained just by tuning the arc discharge
synthesis method described by Bacon and Iijima in their pioneer article of 1991.
In fact, CNTs were first produced in reasonable quantity just 1 year later by
Ebbesen and Ajayan [
15
].
Other methods successfully explored were laser ablation, whose principle is
similar to the arc discharge technique but may result in different purity of
nanotubes, together with chemical vapor deposition (CVD), which nowadays is
one of the most promising methods employed for large-scale production of carbon
fibers and fullerenes [
16
].
Whichever is the method chosen for CNT production, common prerequisites are
an active catalyst, a source of carbon, and suitable energy.
2.1 Arc Discharge
The arc discharge method provides both SWCNTs and MWCNTs on a large scale.
In a typical arc discharge process, two graphite electrodes are used to produce a
direct current electric arc discharge under inert gas atmosphere, such as Ar or He, at
pressures between 100 and 1,000 Torr. The synthesis is carried out at low voltage
(25-40 V) and high current (50-150 A) and the reaction time varies from 30 s to
10 min (Fig.
3
).
The temperature in the inter-electrode zone is so high (4,000
C) that carbon
sublimes from the positive electrode, which is consumed. The plasma formed
between the electrodes acquires the shape of carbon nanotubes and fullerenes
when it condenses at the cathode.
