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 degradations of the gaseous pollutants. A major drawback of TiO2 is its wide energy band gaps of 3.2 and 3.0 eV for the 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 the 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 the dopants and the combined-semiconductors 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).

This work presents the development of titanium dioxide (TiO2) film immobilized on circular glass sheets for photocatalytic degradation of caffeine under ultraviolet C (UVC) irradiation. TiO2 was synthesized through the ultrasonic-assisted sol–gel method and immobilized on circular glass sheets by the doctor blade technique. Polyvinylpyrrolidone (PVP) was used to mix with the TiO2 precursor solution to enhance film adhesion on the glass surface. TiO2 film was mainly composed of anatase phase with a small amount of rutile phase. Caffeine removal was found to increase with increasing irradiation time. Caffeine (20 mg/L) in the synthetic wastewater could not be detected after 3 h of UVC irradiation. The reaction rate of caffeine degradation followed the pseudo-first-order model. The concentrated caffeine solutions required a longer irradiation time for degradation. The used TiO2-coated glass sheets could be easily separated from the treated wastewater and reusable. The caffeine removal efficiency of TiO2-coated glass sheets in each cycle maintained a high level (~100%) during fifteen consecutive cycles.

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