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JAPAN NANONET BULLETIN
-- 44th Issue -- May 12, 2005
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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IN THIS ISSUE
Nanonet Interview:
"Discovery and applications of photocatalysis
-- Creating a comfortable future by making use of light energy --"
Akira Fujishima, Chairman, Kanagawa Academy of Science and
Technology
Young Researchers' Introduction:
"Novel photocurrent generators constructed by supramolecular
assembly"
Atsushi IKEDA, Associate Professor, Graduate School of Materials
Science, Nara Institute of Science and Technology
-- NANO CALENDAR --
For information on nanotechnology related symposiums and conferences
held in the world,
http://www.nanonet.go.jp/english/calendar/
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NANONET INTERVIEW
Discovery and applications of photocatalysis
-- Creating a comfortable future by making use of light energy --
(Issued in Japanese: December 2, 2003)
Akira Fujishima, Chairman, Kanagawa Academy of Science and
Technology
As a graduate student in the spring of 1967, Prof. Fujishima
discovered an unpredictable phenomenon. When he exposed a titanium
oxide electrode in an aqueous solution to strong light, gas bubbles
were evolved from the surface of the electrode, though no bubbles came
out from the surface when the light was switched off. He found that
the bubbles consisted of oxygen gas. He also confirmed that hydrogen
gas was generated at the counter electrode made of platinum. Thus,
water was decomposed to hydrogen and oxygen. What happened on the
surface of the titanium oxide electrode was "photocatalysis", later
called the "Honda-Fujishima effect".
The discovery, however, was not initially accepted by electrochemists
because at that time, the idea that light could also be used as energy
source had not yet taken hold among electrochemists, who maintained
that oxygen could not be generated at such a low voltage because water
electrolysis occurs at 1.5 to 2 volts or higher. However, Prof.
Fujishima's paper, which was published in the journal Nature in 1972,
turned the tables. His work drew attention from researchers around the
world, partly due to the first oil shock taking place. After that, it
became known that a Japanese researcher had found a method to extract
hydrogen -- a clean energy source -- from water using sunlight.
To find out whether Prof. Fujishima's method could generate a
sufficient amount of hydrogen as an energy source, he covered the
rooftop of a building with titanium oxide films made by heating
titanium plates in air. His experiment was conducted on a clear summer
day but only 7 liters of hydrogen per square meter of the films were
generated. The energy conversion efficiency was only 0.3%, which
indicated that the photocatalyst was not suitable for energy
conversion.
Prof. Fujishima's research on developing commercial applications for
photocatalysis began to make progress after Dr. Kazuhito Hashimoto
(now a professor at The University of Tokyo) joined the Fujishima's
research group in 1989. They concluded that although the photocatalyst
could not be used as a sunlight energy conversion material to generate
a large amount of energy, there were no other materials with an
oxidation ability as powerful as that of the photocatalyst in sunlight.
They agreed that the photocatalyst could be used to decompose
materials, which cause trouble even in small amounts. His group
started joint research with Dr. Toshiya Watanabe (now a professor at
The University of Tokyo) of the Research Institute of Toto Ltd., who
was interested in disinfection and deodorization at the time. In their
joint research, they covered the walls and floor of a hospital
operating room with tiles coated with titanium oxide. They found that
the numbers of both the bacteria on the surface of these tiles and the
bacteria in the air of the room fell sharply. Titanium oxide is now
widely used as a material for antibacterial tiles and in air-cleaning
systems.
In 1995, a new phenomenon discovered at Toto's Research Institute
helped further expand the applications of the photocatalysis. When
glass coated with titanium oxide was exposed to light, water droplets
on its surface did not keep their spherical shape but became flat on
the surface. The surface exhibited "superhydrophilicity". Prof.
Fujishima and his group found in atomic force microscopic observations
that ultraviolet light had partially removed oxygen atoms from the
surface of the titanium oxide. The areas where oxygen atoms were
removed were hydrophilic, while the areas where no oxygen atoms were
taken away were hydrophobic. Hydrophilic areas of about 30 nm x 50 nm
and hydrophobic areas of almost the same size existed side by side on
the surface. Water droplets on the surface did not remain spherical
but became flat, thereby forming a uniform film because water spread
through the hydrophilic areas. If oil is already present on the
surface, the water falling on the coated surface penetrates under the
oil and removes it easily. These coating materials based on
superhydrophilicity with a self-cleaning function are already used for
side-view mirrors of vehicles and exterior materials of buildings.
Prof. Fujishima stresses that benefits of science and technology
should be shared by everyone. He says, "The primary objective of
science and technology is to create a society where people can have
healthy, comfortable and long lives. The crucial thing in science and
technology is to develop a new concept that can be applied to actual
products and services, and these new products and services will
eventually make people happy." By making use of his discovery of
photocatalyst, he is trying to help create such a society.
(Yu Tatsukawa, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/044a.html
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YOUNG RESEARCHERS' INTRODUCTION
Novel photocurrent generators constructed by supramolecular assembly
(Issued in Japanese: December 23, 2003)
Atsushi IKEDA, Associate Professor, Graduate School of Materials
Science; Nara Institute of Science and Technology
There has been great interest devoted to the development of
photocurrent generators consisting of organic electron-donor and
electron-acceptor couples. These electron-donors or electron-acceptors
can be deposited on the electrode surface as monolayers by means of
Langmuir-Blodgett (LB) membranes and self-assembled monolayers (SAMs).
However, it is very difficult to link covalently all of the thin-layer
-forming substituents, donor units and acceptor units into one
molecular system through synthetic methods. To find a more expeditious
and more general means of designing a multilayer photocurrent
generator system on an electrode, we have taken advantage of a method
known as alternate adsorption. A C60-porphyrin dyad system produced by
alternate adsorption showed a sensitive photoelectrochemical response
under visible light irradiation and a high quantum yield (20%). The
largest advantage of the alternate adsorption method is its easiness
as it utilizes self-assembly while maintaining a high quantum yield.
The preparation of thin films with high surface concentrations of
donor-acceptor molecules is indispensable in order to achieve high
conversion efficiency, but it inevitably induces self-aggregation of
the chromophores. After photoactivation, therefore, the aggregated
donor or acceptor molecules on the electrode will be deactivated by
self-quenching. It thus occurred to us that the self-aggregation might
be suppressed by encapsulation of the donor or acceptor molecules in
the cavity of macrocyclic host molecules. The photocurrent density and
the quantum yield in the C60-porphyrin bilayer system are remarkably
improved by the addition of cyclodextrin. The high quantum yield
arises from the isolation of the porphyrin units by cyclodextrin
through host-guest interactions.
Further applications of this new concept are currently being examined
in this laboratory.
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/044b.html
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Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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