Titanium Dioxide and its Modified Forms as Photocatalysts for Air Treatment

Background: TiO2 has been proved as an effective photocatalyst for air purification that can produce hydroxyl radicals and superoxide radicals by the illumination of light with suitable energy. These radicals are extremely powerful agents in the degradation of gaseous pollutants. A major drawback of TiO2 is its wide energy band gaps of 3.2 and 3.0 eV for anatase and rutile phases, respectively, which are mostly equivalent to the photon wavelength absorption in the range of UV region. Methods: The modification strategies of TiO2 as photocatalysts for air treatment, such as metal doping, non-metal doping, co-doping, and coupling with other semiconductors are discussed. The photocatalytic performance of the pristine TiO2 and modified TiO2 for degradations of gaseous pollutants are reviewed. Results: Various parameters can affect the photocatalytic removal efficiencies of gaseous pollutants, such as the initial concentration of pollutants, relative humidity, light source, irradiation time, and the preparation of TiO2 photocatalysts. The optimal content of dopants and the combinedsemiconductors should be considered for preventing the recombination of electrons and holes during irradiation. Conclusion: Doping with heteroatoms and coupling could enhance the photocatalytic activity of TiO2. The modified photocatalysts could be applied for photocatalytic degradation of gaseous pollutants, including volatile organic compounds (VOCs), nitrogen oxides (NOx), and sulfur oxides (SOx).