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Prospect
Various mesoscopic systems have their own unique characteristics, some of
which are of importance due to bridging function over classical and quantum
mechanics. It is quite natural that human beings living in macroscopic world
could hardly grasp the phenomena occurring in the microscopic world in an
intuitive manner. This situation offers a vital sense in the "observation" problem
necessarily accompanied with the classical means. The fundamental core of the
argument between Einstein-Podolsky-Rosen and Bohr starting in 1935 actually
lies in this point. However, recent development of experimental techniques
finally comes to suggest the possibility to realise the "Schrodinger-cat states" in
a mesoscopic system [I ,2].
Carbon nanotubes (CNTs) as well as fullerenes are splendid gift brought to the
Earth from the red giant carbon stars in the long-distant universe through the
spectroscopy. Moreover, those belong to new carbon allotropes of the
mesoscopic scale with well-defined structures. In particular, CNTs are considered
to be the materials appropriate to realise intriguing characteristics related to the
mesoscopic system based on their size and physicochemical properties.
In a mesoscopic system in which both classical- and quantum-mechanical
pictures become compatible even for a short time is realised, its pragmatic
significance would be very large considering technical level of today. This book
is expected to offer the starting point of such new developments. In this sense, I
like to express my wholehearted admiration to the eminent work of Dr. Sumio
Iijima who first discovered CNT. The timely contents of this book are readily
conceivable by the excellent authors and I also appreciate the wisdom of my
colleague editors.
Synthesis and Purification of Multi-Walled
and Single-Walled Carbon Nanotubes
Introduction
Since the discovery of carbon nanotube (CNT) by Iijima [ 11, many researchers
have been attracted to this material and a large number of studies have been piled
up. CNT was first synthesized as a by-product in arc-discharge method in
synthesis of fullerenes and are currently being prepared by many kinds of
methods including arc-discharge [2-141, laser ablation [ 15-20] and catalytic
decomposition of hydrocarbon [2 1-27]. In addition, electrolysis [28] and solarenergy
E291 methods have also been proposed. As for the application of CNT,
there has been a remarkable progress in recent days such as that to the fieldelectron
emitter [30-341, for instance. Considering such rapid growth in many
directions, we can expect that CNT could become one of the most important
materials in the 21st century. In this chapter, keeping the application of CNT in
mind, an outline of the present situation and the future of the synthesis of this
material is described. Aspects toward large-scale synthesis is given in Chap. 12.
CNT can be classified into two types: One is multi-walled CNT (MWCNT) [1,2]
and the other single-walled CNT (SWCNT) [3]. The former had been discovered
earlier than the latter. The MWCNT is comprised of 2 to 30 concentric graphitic
layers, diameters of which range from 10 to 50 nm and length of more than 10
pm. On the other hand, SWCNT is much thinner with the diameters from 1.0
to 1.4 nm.
There have been a considerable efforts at synthesis and purification of MWCNT
for the measurements of its physical properties. The time is, however, gradually
maturing toward its industrial application. As to SWCNT, it could not be
efficiently obtained at first and, furthermore, both of its purification and physicalproperties
measurement were difficult. In 1996, it became that SWCNT could be
efficiently synthesized [ 14,163 and, since then, it has become widely studied
mainly from the scicntific viewpoints. In what follows, the synthesis and
purification of MWCNT and SWCNT are to be summarised itemisingly.
MWCNT
MWCNT was originally discovered as a by-product of synthesis of C6o as
described above. The yield of MWCNT is 30 - 50 % in the electric arc-discharge
method using pure carbon. However, from academic point of view, many
researchers currently Seem to be working at SWCNT, probably tired with tedious
purification process of MWCNT particularly synthesized in arc-discharge method.
Nonetheless, MWCNT is still attractive due to their ample ability for industrial
application utilising its high chemical stability and high mechanical strength
[35]. For instance, MWCNT has intrinsic properties suitable for field emitters in
the form of a sharp tip with nanometer-scale radius of curvature, high mechanical
stiffness, chemical inertness and high electrical conductivity. In addition to these
eminent characteristics it also has the unique coaxial shape, which will afford
good possibilities to be applied to various fields of industry (see Chaps. 13 and
14).
2. I Synthesis
2.1.1 Electric arc discharge
When the arc-discharge is carried on keeping the gap between the carbon
electrodes about 1 mm, cylindrical deposit forms on the surface of the cathode.
Diameter of this cathode deposit is the same as that of the anode stick. Under the
conditions that diameter of the anode carbon is 8 mm with the arc-electric current
of 80 A (voltage is about 23.5 V) and He pressure of 300 Torr, the cathode
deposit grows at the rate of ca. 2-3 mm per min. This cylindrical cathode
deposit consists of two portions; the inside is black fragile core and the outside
hard shell. The inner core has the fabric structure growing along the length of the
cathode-deposit cylinder, the inside of which includes nanotubes and polyhedral
graphitic nanoparticles. The outer-shell part consists of the crystal of graphite.
Figure 1 shows a rotating-cathode arc-discharge method [6a] which enables longterm
operation.
MWCNT grows only inside the cathode deposit and does not exist in other
places in the reactor. Quantity of MWCNT obtained depends on the pressure of
He atmosphere in the reactor, which is the most important parameter. The
highest quantity of MWCNT is obtained when the pressure of He is ca. 500
Torr. When this value becomes below 100 Torr, almost no MWCNT grow. This
contrasts to that the highest quantity of fullerene is obtained when the pressure
becomes 100 Torr or less.
Another important parameter is the electric current for discharge. If the current
density is too high, the quantity of the hard shell increases and that of the
MWCNT decreases. To keep the arc discharge stable and the electrode cool are
effective to increase in the product quantity of MWCNT. A considerable quantity
of graphite is produced in the cathode deposit even under the most suitable
condition to the synthesis of MWCNT.
The bundle of MWCNT can be released in ultrasonic cleaner using ethanol as the
solvent.
MWCNT synthesized by catalytic decomposition of hydrocarbon does not
contain nanoparticle nor amorphous carbon and hence this method is suitable for
mass production. The shape of MWCNT thus produced, however, is not straight
more often than that synthesized by arc-discharge method. This difference could
be ascribed to the structure without pentagons nor heptagons in graphene sheet of
the MWCNT synthesized by the catalytic decomposition of hydrocarbon, which
would affect its electric conductivity and electron emission.
Crucial point in this method lies in controlled production of MWCNT with
regard to length, diameter and alignment. To overcome these problems, novel
catalyst methods have been developed. Li et a1 [25] have reported a method for
producing aligned CNT (nanotubes brushes) grown on silicates by using Fe
particle on meso-porous silica. Terrones et al. [26] have developed a controlled
production method of aligned-MWCNT bundles (see Fig. 5) by using thin film
of Co catalyst patterned on the silica substrate.
Conclusion
MWCNT was first discovered by arc-discharge method of pure carbon and
successive discovery of SWCNT was also based on the same method in which
carbon is co-evaporated with metallic element. Optimisation of such metallic
catalyst has recently been performed. Although these electric arc methods can
produce gram quantity of MWCNT and SWCNT, the raw product requires rather
tedious purification process.
The laser-ablation method can produce SWCNT under co-evaporation of metals
like in the electric arc-discharge method. As metallic catalyst Fe, Co or Ni plays
the important role and their combination or addition of the third element such as
Y produces SWCNT in an efficient manner. But it is still difficult in the laserablation
method to produce gram quantity of SWCNT. Nonetheless, remarkable
progress in the research of physical properties has been achieved in thus
synthesized SWCNT.
Fe, Co or Ni is also crucial in the catalytic decomposition of hydrocarbon. In
order to efficiently obtain CNT and to control its shape, it is necessary and
indispensable io have enough information on chemical interaction between
carbon and these metals. It is quite easy for the catalytic synthesis method to
scale up the CNT production (see Chap. 12). In this sense, this method is
considered to have the best possibility for mass production. It is important to
further improve the process of catalytic synthesis and, in order to do so,
clarification of the mechanism of CNT growth is necessary to control the
synthesis. CNT can be synthesized by the chemical reaction at relatively low temperature fortunately. There could be, in general, a lot of possibilities in the
control of chemical reaction at 1000-1500°C. It is of much interest to watch the
development of study along this line.
The study on CNT commenced in Japan and, nowadays, a large number of
investigators from all over the world participate in the research. It is considercd
that it is now high time for the turning point in the study on CNT in the sense
that the phase of research should shift from basic to applied science including
more improvement in efficiency of the synthesis, separation and purification. It
is expected that CNT will be one of the most important materials in the 21st
century and, hence, it is the most exciting thing for us to participate in science
and technology of CNT.