Air-purification units produced by Daikin Industries (Jiji)

Self-Cleaning Tiles for the Marunouchi Building
Fujishima Akira was a graduate student at the University of Tokyo (where he is now a professor emeritus) in 1967 when he discovered the unique photocatalytic properties of titanium oxide. He and his professor, Honda Ken'ichi (now the president of Tokyo Polytechnic University), coauthored a paper on the phenomenon, which subsequently came to be known as the "Honda-Fujishima effect." Toto Ltd., a maker of sanitary ceramics, joined with the University of Tokyo research team to develop photocatalytic tiles coated with titanium oxide, which were released to market in 1994. The tiles possessed antibacterial properties, meaning that any bacteria on the surface were eliminated by the titanium oxide, which also prevented yellowing and controlled odors. These tiles were a big hit with consumers.

The hydrophilic properties of titanium oxide were discovered in 1995, and their application in the realm of photocatalysis has grown rapidly. One of the innovations that has attracted the most attention has been the use of photocatalytic tiles on the outside of the new Marunouchi Building, which opened in September 2002 in front of Tokyo Station. With the building facing a road that has heavy traffic, normal exterior walls would tend to blacken over time from the exhaust and particulate matter released by automobiles. The outside of the building, however, is self-cleaning; its tiles use photocatalysis on sunny days to break down the grime and dust, and their hydrophilic properties allow these substances to be washed away when it rains. In addition to keeping the outside of the building clean in near perpetuity, these tiles also help rid the surrounding area of nitrogen oxides, a class of pollutants. The effects of the Marunouchi Building are equivalent to some 200 poplar trees.

Research is proceeding on the potential applications of hydrophilic properties. Last summer Nissan Shatai, a company located in Hiratsuka City, Kanagawa Prefecture, put up a large net coated with titanium oxide on the west wall of a building used as an employee changing room. The net is equipped with a mechanism supplying its surface with a small amount of water. Once a window in the room is opened, a breeze enters from outside, and occupants can feel as though they are enjoying the shade of a tall tree. When the temperature outside is 31°C (88°F), the temperature inside is 2-4 degrees cooler (4-7 degrees F). As light strikes the net, the water on its surface spreads into a thin film, absorbing heat from the surrounding area as it evaporates. The water is supplied by a high-tech sprinkler device 7 meters high and 50 meters across. The cost of the water and the electricity to run the pump for an entire summer is just ¥10,000 ($95.23 at ¥105 to the dollar), far less than the ¥400,000 ($3,809) it would cost to run an air conditioner. Another advantage over air conditioning is that this setup emits no heat, meaning that it has a low environmental load. There are hopes that the heat-island effect can be limited as this sort of application become more widespread.

Wide Variety of Uses
Photocatalysis has already found its way into a variety of products all around us, including air purifiers, deodorizers for refrigerators and other uses, and vacuum cleaners. Interestingly, there have even been suits made with a photocatalytic coating that cuts the odor of cigarette smoke by 80%-100% and of sweat by 60%-100%. The coating continues to remain effective even after the suit has been dry-cleaned. More and more products making use of hydrophilic properties are appearing in other areas, including bathroom mirrors, roadside mirrors, and automobile rear-view mirrors.

When titanium oxide is combined with silver or copper, the resulting compound is highly resistant to bacteria, so it has been used on the floors in hospitals as a way of preventing infections from spreading. This market was worth ¥8 billion ($76.2 million) in fiscal 2002 (April 2002-March 2003). In the future it is hoped that this type of material will prove useful in the development of sterilized medical instruments and even in the treatment of cancer through the destruction of cancerous cells.

It is also believed that photocatalysts will prove effective in future efforts to improve the environment, and there are already many examples that are approaching the stage of practical application. At the Kanagawa Prefectural Agricultural Institute, titanium oxide is used to purify runoff that contains agricultural chemicals. Within one week, the water becomes clean enough to raise killifish.

Japan leads the world in photocatalysis, and approximately 2,000 companies in the country are undertaking research and development in this area. As there are no standard specifications, however, it is not possible at present to compare the performance of the varying products. With companies in Europe, North America, and Asia getting into this area as well, the Ministry of Economy, Trade, and Industry has been working since 2002 to develop standard specifications and hopes to take the lead in the future development of international specifications.

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Copyright (c) 2004 Web Japan. Edited by Japan Echo Inc. based on domestic Japanese news sources. Articles presented here are offered for reference purposes and do not necessarily represent the policy or views of the Japanese Government.