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Tuesday, May 21, 2002
Titanium dioxide plus sunlight spells clean
Photocatalytic technology used to keep walls, tiles and toilets white
By ERIKO ARITA
The exterior walls of the Toto Research Institute in Chigasaki, Kanagawa Prefecture, look like patchwork; some square areas are dark with stains while others are spotless.
"The clean parts are coated with a paint using photocatalytic technology," said Yoshimitsu Saeki, general manager of the institute, which is operated by Toto Ltd.
Saeki said this process is caused by the self-cleaning effects of titanium dioxide photocatalysis -- the chemical reaction between titanium dioxide and sunlight, known to researchers as the "Light Cleaning Revolution."
Two properties of titanium dioxide in paint contrive to produce the self-cleaning process, he said.
First, the oxidizing power of the substance in the presence of sunlight decomposes organic material, including oil, germs and bacteria.
The other property, known as "superhydrophilicity," is when water expands on the surface of the painted walls in the presence of sunlight, forming a thin layer between the titanium dioxide and dust, washing it out, according to Saeki, who added that it also prevents fogging.
Saeki and other workers employed by Toto, a manufacturer of faucets, toilets and other sanitary ceramics, have developed the photocatalysis technology while working with researchers at the University of Tokyo.
Akira Fujishima, a professor in the university's applied chemistry department, noted that titanium dioxide is a metal widely used in paint and paper, as well as toothpaste and cosmetic products.
Inexpensive and known to be safe, around 2 kg of the white substance is consumed per capita in Japan on an annual basis, he said.
Applications of the new technology have been fast expanding both domestically and overseas, with an increasing number of companies launching products featuring self-cleaning and self-sterilizing properties. These include tiles, paints, glasses, walls, air-cleaner filters and deodorizers.
About 850 patent applications linked to the technology were filed in 2000 alone, while some 2,000 companies in Japan are currently working on the technology in an effort to create new products, according to Fujishima.
As examples of its use for building exteriors, Shinjuku Washington Hotel in Tokyo and part of the Sapporo Dome and Osaka Dome are coated with titanium dioxide.
Titanium dioxide-treated interior walls are also effective in breaking down formaldehyde, which causes sick building syndrome, according to Fujishima.
The technology has been applied to the medical arena as well. It has been used to kill bacteria and germs on medical equipment and to combat methicillin-resistant staphylococcus aureus, which can cause fatal infections at hospitals.
To utilize the same self-sterilizing properties, every restaurant kitchen at Universal Studios Japan in Osaka has antibacterial tiles produced by Toto, Saeki said.
Toto has been promoting research and development of photocatalytic technology in cooperation with Fujishima and other researchers at the University of Tokyo since 1991.
In 1994, the firm released self-sterilizing tiles on the market, while antifogging glass made its debut in 1997.
Toto's sales of photocatalysis-applied products rose 30 percent in fiscal 2000 from the year before.
It generated the same level of growth in fiscal 2001, according to Saeki.
"I guess the size of the market, which is now about 40 billion yen, will grow much bigger in the near future," Saeki said.
The technology has attracted significant attention from abroad.
In the United States, glass manufacturer PPG Industries, Inc. has started selling self-cleaning window glass coated with Toto-patented titanium dioxide film.
Fujishima of the University of Tokyo said, however, that 85 percent of current patents related to photocatalysis belong to Japanese researchers and companies.
Fujishima, who pioneered the technology, recalled the stroke of luck that led to its development.
Brainchild of idea
He discovered titanium dioxide photocatalytic activity in the 1960s, while he was a graduate student studying oxide semiconductors that react to light.
Fujishima placed a piece of titanium dioxide crystal and a piece of platinum into water as electrodes and exposed them to light.
He saw some bubbles rising from the surface of the electrodes.
"I found that oxygen came from the titanium dioxide piece and hydrogen came from the platinum piece," Fujishima explained.
This breaking down of water was previously thought possible only via the use of electricity.
Fujishima's discovery, however, proved that titanium dioxide exhibits a strong oxidizing power when light is shed in water, while platinum electrodes emanate hydrogen when deoxidized.
As a result, the water is split into oxygen and hydrogen.
This phenomenon is now known as the Honda-Fujishima effect, named after Fujishima and Kenichi Honda, the University of Tokyo professor under whom Fujishima was studying when he made the discovery.
The phenomenon was publicized globally when the British science journal Nature published Fujishima's paper in 1972.
Kazuhito Hashimoto, a professor at the Research Center for Advanced Science and Technology at the University of Tokyo, is another researcher who played a major role in developing photocatalytic technology.
Before moving to Fujishima's lab in 1989, Hashimoto had already succeeded in decomposing a cockroach with titanium dioxide, by placing the insect in water containing titanium dioxide powder and exposing it to light for more than a year.
The key to the next stage of development was a brainstorm borne of necessity.
Whenever Hashimoto visited Fujishima's room at the university, he had to walk by a lavatory that emitted an offensive odor.
"I thought if photocatalysis could decompose a cockroach, it should be possible to remove the dirt in the toilet," Hashimoto said, noting that dirt and cockroaches are both organic.
The chemists accordingly contacted Toto.
The scientists and Toto started working together in 1990 to develop an antibacterial tile. They and announced their success in developing photocatalysis technology in 1993.
Although Toto's success triggered a photocatalytic products boom, the technology is still under development and has its limitations.
The amount of organic material the photocatalysis can break down is limited, and it is not yet known how long the effects last, Fujishima said.
In addition, because titanium dioxide only reacts to ultraviolet rays, it is difficult to use the technology in locations that are never exposed to sunlight, he said.
Race for technology
Recently, some companies have been striving to clear these hurdles.
Last year, venture business Ecodevice Co. released titanium dioxide powder and paint that reacts to visible rays.
Shinichi Sugihara, president of Ecodevice Co., said his company succeeded in developing visible ray-responsive titanium dioxide by altering the temperature while making the metal.
"I thought that if we could make titanium dioxide that reacts to visible light, it would be much more convenient to use," Sugihara said.
The company released a new titanium dioxide product on April 17.
It reacts to visible rays and its surface is coated with apatite, which the company codeveloped with the National Institute of Advanced Industrial Science and Technology.
However, the effect of titanium dioxide products responsive to visible rays have yet to be fully verified.
In an attempt to map out an appropriate method of measuring the effectiveness of products, manufacturers in the field have established an organization.
The group, which now comprises 100 member firms, is currently working on the details of such a method.
Thus far, Japan is a leading player in the field, but an increasing number of overseas firms are getting involved.
"The United States and Europe were behind us in the past, but they are now engaging in research and development more vigorously," said Fujishima, adding that China, South Korea and Taiwan have launched similar research.
Hashimoto said he is now trying to find out if photocatalysis can purify agricultural waste water, which contains agricultural chemicals such as phosphorus and nitrogen used in fertilizer and pesticides.
"In the future, I hope to develop the technology so that it can be fully utilized in water purification," Hashimoto said.