Heterogeneous photocatalytic oxidation process (PCO) is a promising technology for removing indoor volatile organic compounds (VOCs) contaminants. Titanium dioxide (TiO 2 ) has been regarded as the most suitable photocatalyst for its cost effectiveness, high stability and great capability to degrade various VOCs. However, no TiO 2 -based photocatalysts completely satisfy all practical requirements given photoexcited charge carriers' short lifetime and a wide band gap requiring ultraviolet (UV) radiation. Strategies for improving TiO 2 photocatalyst activities by doping with different metal and/or non-metal ions and by coupling with other semiconductors have been examined and reported. These techniques can improve PCO performance through the following mechanisms: i) by introducing an electron capturing level in the band gap that would generate some defects in the TiO 2 lattice and help capture charge carriers; ii) by slowing down the charge carrier recombination rate and increasing VOCs degradation. This paper reports the outcomes of a comprehensive literature review of TiO 2 modification techniques that include approaches for overcoming the inherent TiO 2 limitations and improving the photocatalytic degradation of VOCs. Accordingly, it focuses on the recent development of modified-TiO 2 used for degrading gas phase pollutants in ambient conditions. Modification techniques, such as metal and non-metal doping, co-doping, and the heterojunction of TiO 2 with other semiconductors, are reviewed. A brief introduction on the basics of photocatalysis and the effects of controlling parameters is presented, followed by a discussion about TiO 2 photocatalyst modification for gas phase applications. The reported experimental results obtained with PCO for eliminating VOCs are also compiled and evaluated.

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