Chairman, Kanagawa Academy of Science and Technology
Discovery and applications of photocatalysis
—Creating a comfortable future by making use of light energy—
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 30nm x 50nm 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.
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| Fig. 3 Large Image |
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| Fundamental fields in application of photocatalyst |
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| Fig. 4 Large Image |
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| Applications of photocatalyst |
