A Conversation with Akira Fujishima
A
experiments I observed those semiconductor electrodes were unstable under irradiation (dissolution happened). Then I looked for stable semiconductors. I got information that Mr. Nakazumi (President of Single Crystal Manufacturing Company) was making TiO2 rutile single crystal. I asked him directly for one, and then I got it in 1967. Soon using this crystal, I obtained oxygen evolution due to water photolysis. I reported the photoinduced oxidation of water to produce oxygen on a titanium dioxide electrode in Japanese in 1969 (Fujishima, A.; Honda, K.; Kikuchi S. Photosensitized Electrolytic Oxidation on TiO2 Semiconductor Electrode. J. Chem. Soc. Japan 1969, 72, 108, in Japanese). Without light, this reaction requires a large input energy. However, with energetic UV light shining on a TiO2 surface, the input energy decreases drastically. In 1972, I reported with Prof. Honda the complete photoinduced splitting of water into hydrogen and oxygen at a doped rutile single crystal electrode (Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37− 38). In 1975, we reported the sustained production of hydrogen gas in sunlight (Fujishima, A.; Kohayakawa, K.; Honda, K. Hydrogen Production under Sunlight with an Electrochemical Photocell. J. Electrochem. Soc. 1975, 122, 1487−1489). EL: You were the pioneer in exploring various aspects of TiO2 in photocatalysis. What led you to identif y the strength of photocatalysis for environmental remediation so early? AK: At the beginning of 1980, I recognized that it was not easy to carry out highly efficient hydrogen evolution using TiO2 electrodes. An irradiated TiO2 surface, however, has very strong oxidation power. Therefore, we coated a thin film of TiO2 on various materials like glass, tiles, and so on. We then demonstrated the effectiveness of illuminated TiO2 surfaces in removing pollutants and bacteria from air, water, and solid surfaces. In addition, we discovered photoinduced hydrophilicity. This fact is also of great practical importance because it allows surfaces to be self-cleaning with rain or gentle irrigation, and glass to be antifogging and not to form water droplets, as well as self-cleaning. EL: What were some of the major challenges that you encountered in taking the laboratory work to industry for the development of commercial products? AK: About 30 years ago, I became a professor at the University of Tokyo. At that time, some top people from research units of big Japanese companies (TOTO, Daikin, Toshiba, Hitachi,...) visited me and asked to do collaborative work on photocatalysis. After several years many products (air cleaning systems, tiles, glasses, and so on) were commercialized. EL: It has been more than four decades since your work on water splitting f irst appeared. Despite rigorous effort, we still have not
paper on semiconductor photoelectrochemistry published in Nature by Fujishima and Honda in 1972 (Electrochemical Photolysis of Water at a Semiconductor Electrode; DOI: 10.1038/238037a0) led to the birth of two major disciplines, liquid junction solar cells and semiconductor-assisted photocatalysis. The concept of exciting TiO2 semiconductors with UV-light to split water has now become the basis for modern solar fuels research. For more than 45 years Prof. Fujishima has led the efforts to investigate properties of TiO2 and other semiconductor photocatalysts. The photocatalysis concepts that he has laid down in his work continue to energize young researchers today to further advance the fields of artificial photosynthesis and environmental remediation. He has also closely worked with industries to develop photocatalysis products. Many of these TiO2-based photocatalyst products can now be seen in self-cleaning glass, antibacterial tiles, paints, and air purifiers. After his tenure at the University of Tokyo, Prof. Fujishima now serves as the President of the Tokyo University of Science. During the recent 21st Topical Meeting of the International Society for Electrochemistry, Szeged, Hungary (April 23−26, 2017) I had the opportunity to discuss his successful journey into photocatalysis research (Figure 1). The following conversation provides some insights into the visionary views of Prof. Akira Fujishima.
Figure 1. With Prof. Akira Fujishima during the 21st Topical Meeting of the International Society for Electrochemistry, Szeged, Hungary. (Photo courtesy of P. Kamat).
EL (ACS Energy Letters): How did you come up with idea to test TiO2 as a photoactive semiconductor to split water during your seminal work published in 1972? AK: When I was a graduate student of the University of Tokyo, at first I followed the previous works using Ge, ZnO, CdS electrodes in aqueous solution. Of course, from these © XXXX American Chemical Society
Received: June 5, 2017 Accepted: June 5, 2017 1586
DOI: 10.1021/acsenergylett.7b00483 ACS Energy Lett. 2017, 2, 1586−1587
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designed economically sustainable, practical devices for photocatalytic H2 production. Can you identif y major challenges in utilizing the principles of photocatalysis for solar f uels? AK: One mole of water (18 g/mol) has 6 × 1023 molecules. This number is so big compared with the number of photons absorbed by the TiO2 surface. Therefore, I think that a hybrid system of Si-solar cell and water electrolysis is one of the practical ways to address the challenges. EL: What are the new challenges in photocatalysis that young researchers could address to succeed in their career? AK: Photocatalysis of TiO2 has very nice properties, especially its very strong oxidation power and super hydrophilicity. Still, additional fundamental research work is needed to make this process effective in many less-explored areas. For example, there is plenty of scope to extend the photocatalysis to agricultural approaches or medical applications.
Prashant V. Kamat, Editor-in-Chief, ACS Energy Letters
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University of Notre Dame, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
ORCID
Prashant V. Kamat: 0000-0002-2465-6819 Notes
Views expressed in this Energy Focus are those of the author and not necessarily the views of the ACS. The author declares no competing financial interest. Biography
Professor Fujishima was born in 1942 in Tokyo. He received his Ph.D. in Applied Chemistry at the University of Tokyo in 1971. He taught at Kanagawa University for four years before moving to the University of Tokyo, where he became a Professor in 1986. In 2003, he retired from this position and took on the position of Chairman at the Kanagawa Academy of Science and Technology. On January 1, 2010 he became President of Tokyo University of Science. His main interests are in photocatalysis, photoelectrochemistry, and diamond electrochemistry.
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DOI: 10.1021/acsenergylett.7b00483 ACS Energy Lett. 2017, 2, 1586−1587
