by Professor Saikat Talapatra
Over the past several decades there has been an explosive growth in research
and development related to nano materials. Among these one material, carbon
Nanotubes, has led the way in terms of its fascinating structure as well as
its ability to provide function-specific applications ranging from electronics,
to energy and biotechnology1,2.
Carbon nanotubes (CNTs) can be viewed as carbon whiskers, which are tubules
of nanometer dimensions with properties close to that of an ideal graphite fiber.
Due to their distinctive structures they can be considered as matter in one-dimension
In other words, a carbon nanotube is a honeycomb lattice rolled on to itself,
with diameters of the order of nanometers and lengths of up to several micrometers.
Generally, two distinct types of CNTs exist depending whether the tubes are
made of more than one graphene sheet (multi walled carbon nanotube, MWNT) or
only one graphene sheet (single walled carbon nanotube, SWNT). For a detailed
description on CNTs please refer to the article
by Prof. M. Endo.
A Truly Multifunctional Material
Irrespective of the number of walls, CNTs are envisioned as new engineering
materials which possess unique physical properties suitable for a variety of
applications. Such properties include large mechanical strength, exotic electrical
characteristics and superb chemical and thermal stability. Specifically, the
development of techniques for growing carbon nanotubes in a very controlled
fashion (such as aligned CNT architectures on various substrates )3-7
as well as on a large scale, presents investigators all over the world with
enhanced possibilities for applying these controlled CNTs architectures to the
fields of Vacuum microelectronics, Cold-cathode flat panel displays, Field emission
devices, Vertical interconnect assemblies, Gas breakdown sensors, Bio Filtration,
On chip thermal management, etc.
Apart from their outstanding structural integrity as well as chemical stability,
the property that makes carbon nanotubes truly multifunctional in nature is
the fact that carbon nanotubes have lot to offer (literally) in terms of specific
surface area. Depending on the type of CNTs the specific surface areas may range
from 50 m2/gm to several hundreds of m2/gm and with appropriate
purification processes the specific surface areas can be increased up to ~1000
Extensive theoretical and experimental studies have shown that the presence
of large specific surface areas is accompanied by the availability of different
adsorption sites on the nanotubes8. For example,
In CNTs produced using catalyst assisted chemical vapor deposition the adsorption
occurs only on the outer surface of the curved cylindrical wall of the CNTs.
This is because the production process of the CNTs using metal catalysts usually
leads to nanotubes with closed ends, thereby restricting the access of the hollow
interior space of the tube.
However, there are simple procedures (mild chemical or thermal treatments)
which can remove the end caps of the MWNTs thereby presenting the possibility
of another adsorption site (inside the tube) in MWNTs as schematically shown
in Figure 1. Similarly, the large scale production process of SWNTs lead to
the bundling of the SWNTs. Due to this bundling effect, SWNT bundles provide
various high energy binding sites (for example grooves, Figure 1.). What this
means is then that large surfaces are available in small volume and these surfaces
can interact with other species or can be tailored and functionalized.
Possible binding sites available for adsorption on (left) MWNTs and
(right) SWNTs surfaces.
Our group's own research interests are directed into utilizing these
materials in different applications related to energy and the environment, where
their high specific surfaces areas play a crucial role. Two of such energy related
applications are discussed below:
- CNT Based Electrochemical Double Layer Capacitors
- CNT Based catalyst support
CNT Based Electrochemical Double Layer Capacitors
Electrochemical Double Layer Capacitors (EDLC's: Also referred to as
Super Capacitors and Ultra-Capacitors) are envisioned as devices that will have
the capability of providing high energy density as well as high power density9-11.
With extremely high life-span and charge-discharge cycle capabilities EDLC's
are finding versatile applications in the military, space, transportation, telecommunications
and nanoelectronics industries.
An EDLC contains two non reactive porous plates (electrodes or collectors with
extremely high specific surface area), separated by a porous membrane and immersed
in an electrolyte. Various studies have shown the suitability of CNTs as EDLC
electrodes. However, proper integration of CNTs with collector electrodes in
EDLCs are needed for minimizing the overall device resistance in order to enhance
the performance of CNT based supercapacitors. A strategy for achieving this
could be growing CNTs directly on metal surfaces and using them as EDLC electrodes12
(Figure 2). EDLC electrodes with very low equivalent series resistance (ESR)
and high power densities can be obtained by using such approaches.
Artist rendition of EDLC formed by aligned MWNT grown directly on metals
(b) An electrochemical impedance spectroscopy plot showing low ESR of
such EDLC devices and (c) very symmetric and near rectangular cyclic
voltamograms of such devices indicating impressive capacitance behavior.
CNT Based Catalyst Support
Catalysts play an important role in our existence today. Catalysts are small
particles (~ 10-9 meter, or nanometer) which due to their unique
surface properties can enhance important chemical reactions leading to useful
products. In any kind of catalytic process, the catalysts are dispersed on high
surface area materials, known as the catalyst support. The support provides
mechanical strength to the catalysts in addition to enhance the specific catalytic
surface and enhancing the reaction rates. CNTs, due to their high specific surface
areas, outstanding mechanical as well as thermal properties and chemically stability
can potentially become the material of choice for catalyst support in a variety
of catalyzed chemical reactions.
We are presently exploring the idea of using CNTs as catalyst support in the
Fischer Tropsch (FT) synthesis process13. The FT
reaction can convert a mixture of carbon monoxide and hydrogen in to a wide
range of straight chained and branched olefins and paraffins and oxygenates
(leading to the production of high quality synthetic fuels). Our preliminary
FT synthesis experiments on CNT supported FT catalysts (generally cobalt and
iron) shows that the conversion of CO and H2 obtained with FT catalyst
loaded CNTs is orders of magnitude higher than that obtained with conventional
FT catalysts (Figure 3), indicating that CNTs offer a new breed of non-oxide
based catalyst supports with superior performance for FT synthesis.
paper used as catalyst support for FT synthesis and comparison of conversion
ratio's of Co and H2
So far, CNT research has provided substantial excitement, and novel possibilities
in developing applications based on interdisciplinary nanotechnology. The area
of large scale growth of CNTs is quiet mature now and hence it could be expected
that several solid large volume applications will emerge in the near future14.
Professor Saikat Talapatra acknowledges the financial support provided by
the Office of Research and Development (ORDA) at SIUC through faculty start-up
funds and a seed grant, by the Illinois Department of Commerce and Economic
Opportunity through the Office of Coal Development and the Illinois Clean Coal
Institute and by NSF-ECCS (grant # 0925682) for carrying out some of the research
themes described in this article. ST would also like to thank his collaborators
as well as his past and present group members for actively participating in
various research efforts undertaken in his lab.
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Capacitor Electrodes using Aligned Carbon Nanotubes Grown Directly on Metals",
Nanotechnology 20, 395202 (2009).
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- Unpublished data in collaboration with Prof. K. Mondal (Dept. of Mechanical
Engineering and Energy Processes at Southern Illinois University Carbondale)
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Copyright AZoNano.com, Professor Saikat Talapatra (Southern Illinois University Carbondale)