These photocatalysis sheets are perfect for shoe closets as their strong odor prevention properties can be renewed indefinitely just by exposing them in the sun. The sheets are on sale now at the Communication Center on the Hongo Campus (500 yen for a two-sheet set).
Interviewer: I’ve heard that the history of research into photocatalysis at the University of Tokyo dates back 46 years.
Hashimoto: One day in the mid-1960s, my predecessor Professor Akira Fujishima was a graduate student at the University, working under the guidance of Professor Kenichi Honda. In an experiment he found that when titanium oxide and platinum electrodes were placed in water and exposed to light, the combination caused bubbles to appear in the water. The water in the experiment was decomposed into oxygen and hydrogen. It was a reaction that normally would not have been thought to be possible in electrochemistry. Professor Fujishima wrote up the results in his doctoral dissertation, but the examination board and the academic association refused to accept his findings initially. However, in 1972 his research results were published in Nature and his results were given on a front-page in the Asahi Shimbun newspaper in 1974. Following this, his experiment was finally accepted by the academic association. Under normal circumstances water does not break down at a voltage of less than 1.23v, but this becomes possible when light energy is added. This reaction has become known as the “Honda-Fujishima effect,” named after the two people involved in the discovery.
Interviewer: Why was the mass media so excited about this new, but somewhat academic discovery?
Hashimoto: It was the time of the first Oil Shock and was an era when everyone was searching for alternative energies. As hydrogen gathered from water can be used as a fuel, this discovery was catapulted into the limelight as one potential answer to energy issues.
Research into the utilization of powdered titanium dioxide began from around 1975. This method of using powdered titanium dioxide instead of electrodes is what is known as photocatalysis. However, photocatalysis of the powdered form of titanium dioxide did not produce good results. Although it was reported that results had been achieved, overall they were not very effective.
The second Oil Shock occurred in 1979. It was at that time that Dr. Tadayoshi Sakata and Dr. Tomoji Kawai of the Institute for Molecular Science, to which I was affiliated at the time, thought of adding (organic) alcohol to the water and creating a reaction that way. By adding the alcohol, they found that the powdered form of the titanium dioxide also released hydrogen in plentiful supply. It was subsequently found that the organic substance didn’t have to be alcohol. Leaves, urine, excrement…as long as it was an organic substance, anything would work. Even cockroaches were completely decomposed and produced hydrogen. This gave a tremendous boost to photocatalysis research, as everyone thought that the solution to the world’s energy problems had been found.
However, as research progressed, it was found that only small amounts of energy could be obtained through this process. The drawback of titanium dioxide photocatalysis is that it reacts exclusively under ultraviolet (UV) light, and only a very small fraction of such rays reach the Earth.
Interviewer: And this was going on around the time that you joined the laboratory of Professor Fujishima as a young researcher.
Hashimoto: In 1989 I was called in for what was initially going to be a two-year limited period, to act as a type of “stand-in” for another researcher. My child had just been born and the task in front of me was a real challenge. I was filled with a real sense of pessimism as I set about my work, realizing that I had to achieve results within the space of two years. The ceiling of my office in Engineering Building 5 was really low and my head was always scraping the asbestos ceiling. I think this had an adverse effect on my hair (laughs).
My office was on the first floor and Professor Fujishima’s was on the fourth. Every day I would go up and down to his office using the elevator, at the side of which was a restroom that was terribly dirty and smelly. It was while I was staring into the yellowed urinal that I suddenly had a eureka moment. If the photocatalysis reaction could decompose a cockroach, surely it could easily decompose the bacteria that caused the yellow stain on the urinal? Just as I had been searching around for ways to use titanium dioxide, my overworked brain and the filthy urinal nearby came together in a fortuitous combination, creating this seemingly absurd idea.
An older acquaintance of a student was working at TOTO, so I immediately got on the phone with him. His first reaction was to say, “The University of Tokyo must be the only university left that has yellowed and reeking urinals,” but he agreed to come over straight away. That person was in actual fact Professor Toshiya Watanabe (of the UTokyo Policy Alternatives Research Institute (PARI)). It was still a time when collaborations between universities and corporations were difficult to imagine and, what’s more, the theme of our collaboration was toilets! Due to these reasons, we couldn’t really get together in an open environment, so we engaged in secret joint research on Saturdays.
As our research progressed we found that not only did the discoloration disappear, but so did the unpleasant odors. We found that titanium dioxide forms a thin film on materials, which keeps the surface clean. As a result of our efforts, TOTO launched sales of photocatalytic antibacterial tiles in 1995.
Interviewer: So was it a good thing that the target of your research moved from energy to combating dirt and grime?
Hashimoto: Energy in small amounts is not practical or usable, but dirt and grime are a different story. For example, as dirt on buildings is at the surface level, it only takes a small amount of light to implement photocatalysis. Similarly, with regards to odors, human noses are highly sensitive; thus, working with substances that are small in volume is most effective. What started out as a project to use large quantities of light to create large amounts of energy transformed into a project where small quantities of light were used to have a small-scale effect. The result was that our eyes were opened to a totally different world of possibilities.
In the process of the joint research with TOTO we made another new discovery, known as photoinduced hydrophilicity. As titanium dioxide is exposed to light, the surface structure of the water changes. This also represented a new discovery in terms of basic scientific research, and we published a paper on what we had found in Nature in 1997. The applications for this discovery were also of course new. Until this point, it had been the case that unless consistently exposed to light, the process would not function. Following the new discovery, however, it was shown that if exposed to light once, water would come between the dirt and the titanium dioxide and cause the dirt to run off. Since this effect lasts for around 20 hours, it can be sustained constantly outside as long as there is water. The areas for application of this technology thus expanded dramatically. Starting from the end of the 1990s, the photocatalysis market experienced a significant increase in growth
Interviewer: And that was the starting point for such products as self-cleaning wall materials and non-fogging side mirrors.
Hashimoto: However, in 2006 the market reached a peak. As photocatalysis of titanium dioxide could only be implemented in an outdoor environment, it was employed almost exclusively for exterior building materials. If the same process could be used in an indoor environment, the market would expand to a much larger scale. For this to be possible, however, it would be necessary to achieve photocatalysis using room light and not just sunlight. It was with this aim in mind that we launched a project in 2008 that sought to achieve titanium dioxide-based photocatalysis while working with visible light.
The results of that project are now actually coming together. The discovery of two new concepts (multi-electron catalysis and interface electron transfer) was particularly useful in helping these results to come to fruition. We are now promoting a consortium to create indoor materials that utilize visible light and multiple manufacturers are seeking to launch production of various products.
Looking back on the history of the research and development of this process, we can discern four steps: (1) the discovery of the Honda-Fujishima effect, (2) the change in focus from energy creation to preventing dirt and odors, (3) the discovery of hydrophilicity, and (4) the discovery of photocatalysis using visible light. These advances were made possible through joint research with the industrial world from the second step onwards. It was very fortunate in this case that research and industry interaction was implemented from the very early stages of development.
Interviewer: Professor Hashimoto, you are an unusual researcher in that you have published many papers, but you also have registered many patents.
Hashimoto: Although I may give off a strong impression of being a specialist in applied science, at heart I am a basic scientific researcher coming from the Faculty of Science. My feeling is that the people who put their heart and soul into basic research achieve great success when they advance into applied science. However, concentrating exclusively on basic research alone will not always produce results that are valuable to society. Many people turn their thoughts to commercial development only after completing their research, but I think it is better to engage in dialogue with the markets from the early stages of research.
I am a member of the government’s Council for Science and Technology Policy (CSTP), which puts me in the position of seeking to advance national innovation. It is important for people who are experienced on the front lines of science to get involved in policymaking. Managing a laboratory with more than 40 members while also providing input on national policy leaves me with a hectic schedule. Nevertheless, just as dialogue between researchers and the market is essential, I feel that it is also my mission to use my experiences to promote national science and technology policy.
Professor, Department of Applied Chemistry, Graduate School of Engineering