Ultraviolet water purifiers have been on the market for years and have gained popularity among those looking for a chemical-free, more natural approach to clean drinking water. Now researchers at the University of Illinois have improved on the process by using visible, rather than UV, light. What’s more, the new process works even after the light is turned off – disinfecting in the dark. Based on a new photocatalyst, the process also can be used to sanitize surgical equipment and to clean delicate electrical and optical components. “The new catalyst also has a unique catalytic memory effect that continues to kill deadly pathogens for up to 24 hours after the light is turned off,” said Jian-Ku Shang, a professor of materials science and engineering at the university. Jian-Ku Shang, a professor of materials science and engineering at the University of Illinois, holds a sample of a new photocatalytic material that uses visible light to destroy harmful bacteria and viruses, even in the dark. Courtesy of L. Brian Stauffer. In earlier research, he and his group developed a catalytic material that works with visible light by doping a titanium-oxide matrix with nitrogen, enabling the disinfecting process to be activated by sunlight or by standard indoor lighting. “When visible light strikes this catalyst, electron-hole pairs are produced in the matrix,” Shang said. “Many of these electrons and holes quickly recombine, however, severely limiting the effectiveness of the catalyst.” Damage to E. coli cells by the photocatalytic process is shown. Courtesy of Jian-Ku Shang. Alone, the nitrogen-doped titanium oxide kills bacteria, but not very efficiently. To create the catalytic memory, the researchers added nanoparticles of palladium to the surface of the fibers so that, when the light is turned off, the nanoparticles slowly release the trapped electrons, which then react with water to produce additional oxidizing agents. They tested the process using a metal halogen desk lamp and a solution of Escherichia coli. A glass filter on the lamp provided zero light intensity below 400 nm. They shone the light on the fibers for 10 hours to simulate daylight and stored them in a dark environment for various periods. “In a sense, the material remembers that it was radiated with light,” Shang said. “This ‘memory effect’ can last up to 24 hours.” Even though the disinfection efficiency in the dark is not as high as it is in visible light, it still enables continuous operation of the process, which Shang suggests could lend itself to environmental applications. Shown is a schematic of the process in which photoelectrons flow to palladium-oxide (PdO) nanoparticles on a titanium-oxide (TiON) matrix under visible light illumination and the process of discharging of PdO nanoparticles when the visible light is switched off. (Note that A refers to an electron acceptor that can accept surface photoelectrons, while D refers to an electron donor that can donate electrons and react with surface holes.) Courtesy of Qi Li, Yin Wai Li, Zhiquan Liu, Rongcai Xie and Jian-Ku Shang, J Mat Chem, 2010, Vol. 20, pp. 1068-1072, DOI: 10.1039/b917239d – Reproduced by permission of the Royal Society of Chemistry.) “In the near term, we are exploring applications of this technology in two areas – disinfecting municipal water and building point-of-use devices for purifying drinking water,” he said. “For the long term, we are interested in understanding the mechanism underlying the catalytic memory effect, designing new materials with even stronger memory effect and using the materials for control of infectious diseases.” The work was supported by the National Science Foundation through the university’s Center of Advanced Materials for the Purification of Water with Systems. Some of the work was performed at the university’s Frederick Seitz Materials Research Laboratory, which is partially supported by the US Department of Energy.