by Professor Morinobu Endo
Over the last decade, nanotechnology has received lots of attention from
within society as a potential source for novel solutions to many of the world's
existing and emerging problems. Simply put, nanotechnology could provide the
ability to better understand and design complex solutions on an atomic and
molecular scale. The most attractive nanotechnology-related nanomaterial is
considered to be one-dimensional carbon nanotubes (CNT).
Geometrically, CNT can be visualized by rolling sheets of graphene into a
long hollow tubule. The unique configuration of this material imparts excellent
physico-chemical properties1. For instance, the
Young's modulus of CNT is stiffer than any other material, while their tensile
strength is 100 times that of steel. Maximal electrical current density is 100
times greater than for copper wire and carrier mobility is ca. 105
cm2/Vs. CNTs show great promise in numerous applications in the near
future2 and the excellent properties of CNT have
already resulted in their use in commercial available products.
At present, the total amount of CNTs produced commercially from around the
world reached ca. 1,000 ton/year. In this feature article, the basic structure
of CNTs is briefly described, as well as the latest advances in the large-scale
production, existing commercial uses of nanotubes are reviewed with special
emphasis on the toxicological issue of CNTs.
What is a Carbon Nanotube?
CNT can be visualized as rolling sheets of graphene (sp2 carbon honeycomb
lattice) into a cylinder of nanometer size diameter (Fig. 1 (a)). The structure
of CNT has been explored in the early years with high-resolution transmission
electron microscopy (Fig. 1 (b))3, and the results
obtained reveal that nanotubes are seamless nanoscale tubules derived from the
honeycomb lattice representing a single atomic layer of crystalline graphite,
otherwise referred to as a graphene sheet. The curvature of the nanotubes
incorporates a small amount of sp3 bonding so that the force constant in the
circumferential direction is slightly weaker than along the nanotube axis.
Figure 1. (a) CNT
could be visualized by rolling sheets of graphene (sp2 carbon honeycomb lattice)
into a cylinder of nanometer size diameter. (b) The structure of CNT has been
explored early on by high-resolution transmission electron microscopy
Since single-walled carbon nanotube (SWNT) is only one atom thick and has a
small number of atoms around its circumference, only a few wave vectors are
needed to describe the periodicity of the nanotubes. These constraints lead to
quantum confinement of the wave functions in the radial and circumferential
directions, with plane wave motion occurring only along the nanotube axis,
corresponding to a large number or closely spaced allowed wave vectors.
Carbon nanotubes can be either metallic or semiconducting, and likewise the
individual constituents of multi-wall nanotubes or single-wall nanotube bundles
can be metallic or semiconducting4. These remarkable
electronic properties follow from the electronic structure of 2D graphite under
the constraints of quantum confinement in the circumferential direction.
In the case of multi-walled carbon nanotubes (MWNTs), which typically have a
diameter less than around 100 nm, no graphitic three-dimensional stacking is
established5, even though an individual shell of the
multi layers consists of perfect graphene sheets. Also, each tube has different
and independent chirality, which might contribute to a larger inter-shell
spacing than is found in graphite. These characteristic structures of single-
and multi-walled CNTs indicate that they are unique one-dimensional materials
with fascinating electronic, chemical, mechanical, and thermal properties.
Industrial Scale Production of Carbon Nanotubes
Up until now, various synthetic methods for producing CNTs have been reported
(e.g., arc discharge, laser vaporization and catalytic chemical vapor deposition
(CVD)). The dominant recent trend is to synthesize CNTs using CVD approach since
this technique is extremely useful for the large-scale production of both SWNTs
and MWNTs3. By simultaneously feeding hydrocarbons
and nanoscale catalytic particles in the gas phase into the reaction chamber,
CNTs have been synthesized on a large-scale6.
Growing SWNTs and MWNTs in a reactor has been proposed and this involves the
catalytic deposition of hydrocarbons over the surface of nano-sized metal
particles and a continuous output by the particle of well-organized tubule of
The strong evidence of this assumption is the presence of catalytic particles at
the ends (top or root) of the tubes (Fig. 2 (a-c)). In the case of large-scale
production of SWNTs, the development of the high-pressure carbon monoxide
process gave impetus to the scientific study and applications of SWNTs7.
Figure 2. shows the
presence of catalytic particles at the ends of the
Regarding the bulk production of MWNTs for industrial applications, it is
important to mention that at the end of 1980, Showa-Denko Co. Ltd and Hyperion
Catalysis International, Inc. (Cambridge, MA) commenced production of several
tons of catalytically grown CNTs annually. At present, the total amount of the
commercially available MWNTs around the world has reached 1,000 ton/year. It is
expected that the global carbon-nanotube revenue in 2015 will reach US$500
million8. The most interesting point is that all
companies selected a catalytic CVD method for the large-scale production of
Application of Carbon Nanotubes
Due to their small dimensions and excellent physicochemical properties, CNTs
have been proposed for a wide range of applications. Some of the potential
applications of CNT include multi-functional composites, electrochemical
electrodes and/or additives, field emitters as well as nano-sized semiconductor
devices2. CNTs are also used as fillers in both
anode and cathode materials of lithium-ion secondary batteries9,10.
MWNTs can be used as scanning probe microscope tips to obtain high-resolution
images and in the near future, thin MWNTs will be used as field emission
electron sources for flat-panel displays. Chemically functionalized MWNTs also
give a high sensing ability for chemical and biological groups interacting with
In addition, CNTs are an ideal candidate for fillers in polymer composites.
The smallest working composite gear has been prepared by mixing nanotubes into
molten nylon and then injecting into the tiny mold. This piece exhibits a high
mechanical strength, high abrasion resistance and also good electrical and
thermal conductivity. Further progress has to be carried out in order to fully
utilize these nanotube/polymer composites, for example the optimization of
surface properties, the homogeneous dispersion without physical damage, the
development of an effective alignment method (also evaluation method) and
A super rubber sealant capable of withstanding high temperature and pressure
was successfully fabricated by Professor Morinobu Endo and his colleagues at the
Carbon Science & Technology. This was done by incorporating
surface-modified nanotubes into rubber11. Based on
our estimates and after surveying the depth and temperature of oil resources,
the development of a super rubber technology capable of withstanding 260°C under
239 MPa of pressure will contribute to a revolutionary enhancement in oil
recovery efficiency from the current 35 % to more than 70 % by excavating
previously inaccessible deposits.
Another potential application of CNT is in the fabrication of
super-capacitors and electrochemical actuators used in artificial muscles.
Nanotube actuators can operate at low voltages and temperatures as high as
350°C. Currently, super-capacitors are incorporated into hybrid vehicles as they
could provide rapid acceleration and store breaking energy electrically.
The possibility of using CNTs as nanowires is envisaged due to their observed
ballistic transport. For the fabrication of nanotube field effect transistors,
SWNTs were connected to metal nano-electrodes. The performance is excellent in
terms of switching speed owing to their low capacitance. An inherent problem
associated with CNT lies in the difficulty in manipulating them. From a
commercial viewpoint, further technical progress is required, such as selective
growth of nanotubes using self-assembly techniques.
Carbon Nanotube Biocompatibility
Much attention was paid on the toxicity of CNTs due to their nanoscale
dimension and their morphological features similar to that of asbestos12,13. Therefore, toxicological evidence
of CNT is strongly needed to prevent risks and occupational disorders in workers
and to promote their safe use in consumer products. Our preliminary studies on
the biological response of CNTs indicates their potential toxic nature is
significantly low14,15. However,
a more thorough and long-term study has to be conducted to determine the toxic
nature of various types of CNTs such as direct aspiration of tubes in human
These tiny, black and tubular-type nanomaterials will change the way we live,
work and communicate. A large number of CNT-derived products are already in use
and their viability strongly depends on the success of their commercialization.
Before considering the use of CNTs in commercial products as a success, at
least four obstacles have to be resolved:
1. How to obtain high purity CNTs as metallic impurities
often remain after the fabrication process which can give rise to toxic
2. How to manipulate these tiny materials.
3. How to control the chirality of
4. The most important but critical "safety" issue has
to be clarified based on long-term and systematic biological studies.
Extensive and intensive efforts in both academy and industry are looking for
a solution to these obstacles and once a solution has been reached, CNTs will
play a key important role as an innovative material of 21st century
in a number of industrial processes.
We have reached beyond the first mountain of science, the second mountain of
technology and the third mountain of economy by producing CNTs successfully on a
large-scale at reasonable cost (Fig. 3). Now we are striving to climb the
mountain of society. By sharing information on risks and benefits of CNTs with
all stakeholders, we will finally reach the top of a nanotube mountain and prove
CNT is an innovative material for the 21st century.
Figure 3. Carbon
nanotube as a leading-edge of nanotechnology must go beyond the four mountains
as an innovative and fundamental technology of 21st century.
Worldwide collaboration on science is the key issue for the success.
This work was in part supported by the CLUSTER (second stage) and MEXT grants
(No 19002007), Japan.
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