**3. How to improve the catalytic performance of titanium dioxide**

Although titanium dioxide has been used successfully as a photocatalyst for several reactions. However, its wide band gap (3.2 eV) limits its photocatalytic applications. Due to its wide band gap, titanium dioxide can be used as a photocatalyst under ultraviolet radiation only. The best photocatalyst is one that can be used under visible light as well as ultraviolet light because the sunlight is mainly composed of visible light. Being longer wavelength, the visible light can not excite electrons from the valence band to the conduction band of the titanium dioxide. The photocatalytic performance of titanium dioxide can be improved in two ways.

1.By prevention of recombination of photoinduced positive holes and electrons

2.By narrowing the band gap of the TiO2

The prevention of recombination of photoinduced positive holes and electrons will permit the occurrence of redox reactions of the positive holes and electrons. These redox reactions will generate hydroxyl radicals and ultimately the photocatalytic performance will be enhanced.

The narrowing of band gap will allow the absorption of visible light by photocatalyst, so the photocatalyst will absorb a wide range of the solar spectrum. The solar spectrum consists of 47% infrared, 46% visible, and 4–5% ultraviolet light. Hence the absorption of visible light will also improve the photocatalytic performance. It has been reported that recombination of the photoinduced positive holes and electrons has a significant contribution to the photocatalytic performance of the catalysts. Most of the researchers work to develop methods and techniques to prevent the recombination of these positive holes and electrons [31–33]. The researchers have proposed that crystal structure significantly affects the photocatalytic performance of the photocatalyst as it plays a significant role in the recombination of positive holes and electrons. Hence, attempts have been made for modifications in the crystal structure to improve the photocatalytic performance of titanium dioxide.

*Photocatalytic Applications of Titanium Dioxide (TiO2) DOI: http://dx.doi.org/10.5772/intechopen.99598*

Furthermore, the low photo conversion efficiency of the TiO2 is also a challenge for the researchers. It has been reported that loss of efficiency is associated with each photo catalytic step. Due to loss of efficiency at each step, the observed photo catalytic efficiency with TiO2 reported in literature is very low. This low photo catalytic efficiency of TiO2 is the critical drawback associated with TiO2 catalyzed treatment. Hence, the photocatalytic efficiency is expected to be low compared to other advanced oxidation processes. However, the TiO2 has the ability to absorb less energetic photons. This characteristic of TiO2 enable the researcher to develop TiO2 based photo catalysts that can absorb light in visible region. The presence of hydrogen peroxide is not needed in visible light photocatalysis. The presenece of hydrogen peroxide is needed in other advanced oxidation processes.
