Enhanced photocatalytic disinfection process
Doping Titanium Dioxide
Titanium dioxide (TiO2) is a wide bandgap semiconductor that has been extensively used in selective oxidation and reduction, condensation reactions, polymerization catalysis, substitutional perfluorination of olefins, phosphines, and phosphates, photovoltaics, and in photo-degradation of organic and inorganic compounds. The high photoactivity, thermal and chemical stability, low cost, and nontoxicity of TiO2 present the compound as a good candidate for such applications.
Upon illumination of the TiO2 with light of equal or greater energy than the band gap such as UV light, charge transfer from valence band to conduction band occurs, creating a hole and a free electron. These species can either recombine or migrate to the surface and react with surface-bound adsorbates, typically oxygen or water in most photooxidative processes. Since UV light makes up of less than 5% of the solar spectrum that reaches the earth’s surface, the commercial application of these processes has been retarded (as visible light by itself cannot produce any photoactivity in TiO2).
In order to make the application of TiO2 commercially viable a number of methods of doping TiO2 (doping refers to the intentional introduction of impurities to the material for the purpose of modifying electrical characteristics) had been investigated. These methods required either expensive ion-implantation facilities or suffered from thermal instability and low quantum yields due to the continuum of interband states found in transitional metals.
Finally in 1985 it was Shinri Sato (Sato, S. Chem. Phys. Lett., 1986, 123, 126.) who correctly identified nitrogen as the right element to dope TiO2. After calcining a commercial source of Ti(OH)4, Sato found that the resulting TiO2 displayed improved photoactivity in the visible light region (434 nm) and absorbed light at longer wavelengths than typical of TiO2. After subjecting his original sample of Ti(OH)4 to analysis, he found that it contained NH4Cl as an impurity. Sato concluded correctly that it was the existence of nitrogen species originating from the NH4Cl that imparted the difference.
Source: http://www.bama.ua.edu/~chem/seminars/student_seminars/spring07/s07-papers/hennek-sem.pdf
Enhanced photocatalytic disinfection process
Researchers at the University of Illinois have developed an enhanced photocatalytic disinfection process that uses visible light to destroy harmful bacteria and viruses, even in the dark. The disinfection process can be used to purify drinking water, sanitize surgical instruments and remove unwanted fingerprints from delicate electrical and optical components.
Jian Ku Shang, a professor of materials science and engineering at the University of Illinois and his team used titanium-oxide doped with nitrogen as a catalyst in the disinfection process. When visible light strikes this doped catalyst, electron-hole pairs are produced. The holes react with water to produce oxidizing agents, primarily hydroxyl radicals, which kill bacteria and viruses. However the electrons and holes quickly recombine, severely limiting the effectiveness of the catalyst.
To improve the efficiency of the catalyst, Shang and collaborators at the University of Illinois and at the Chinese Academy of Sciences added palladium nanoparticles to the catalyst. The palladium nanoparticles trap the electrons, allowing the holes to react with water to produce oxidizing agents that kill bacteria and viruses.
When the light is turned off, the palladium nanoparticles slowly release the trapped electrons, which can then react with water to produce additional oxidizing agents. Prof. Shang said “In a sense, the material remembers that it was radiated with light. This ‘memory effect’ can last up to 24 hours.” Although the disinfection efficiency in the dark is not as high as it is in visible light, it enables the continuous operation of a unique, robust catalytic disinfection system driven by solar or other visible light illumination.
Source: http://www.news.illinois.edu/news/10/0119photocatalyst.html
January 20, 2010