Titanium Dioxide Photocatalysis

Chemistry
Akira Fujishima
President, Tokyo University of Science, Special University Professor Emeritus, University of Tokyo, and Supreme Advisor, Kanagawa Academy of Science and Technology

Fujishima is suggested as a possible Nobel Prize winner “for the discovery of photocatalytic properties of titanium dioxide known as the Honda-Fujishima Effect”

Although titanium dioxide (TiO2) has been known for centuries as the minerals rutile and anatase, it only became commercially important in the last century when it was found to be a replacement for the white lead used in paints. And so it remained, useful but chemically uninteresting, until a paper was published in Nature in 1972 (vol. 238, issue 5358, pp 37-38) which revealed an aspect of TiO2 that had hitherto been overlooked: it was a catalyst for the photolysis of water and released hydrogen, albeit in only small amounts (0.4% energy yield). This paper resulted from doctoral work by Fujishima, who was supervised by Kenichi Honda. It has since gathered more than 6,000 citations.

While Fujishima’s discovery did not to lead to a commercial application as such, further investigation by his group in the 1990s led to a second discovery about the photocatalytic properties of titanium dioxide: its super-hydrophilicity. In other words, its surface is permanently changed by UV light in respect of the effect it has on water. Droplets on the surface no longer maintain the spherical shape determined by surface tension, but collapse to form a uniform film, and if the surface is upright or at an incline then the water runs off. Fujishima has linked this super-hydrophilicity to the formation of oxygen bridges within the TiO2.

When a titanium dioxide surface is irradiated by light, the photocatalytic effect and hydrophilicity are activated together. Any organic chemical in contact with the surface will undergo decomposition to CO2 and H2O, a process which occurs at ambient temperatures. This discovery has led to widespread use of TiO2 as a coating for glass, making this self-cleaning, and of especial benefit for side mirrors on automobiles. There is even the possibility that it could be used to clean surfaces contaminated with disease pathogens, as demonstrated by Fujishima in 1998 with E coli. It has been suggested that TiO2-coated surfaces could assist in environmental clear-up by removing atmospheric pollution, and such coated paving is now in use in Japan.

Commentary on the Chemistry Laureates by John Emsley, Chemistry correspondent, ScienceWatch.