**2.2 Changes in the structure and surface of titanium dioxide**

Strong light absorption and suitable redox potential are prerequisites for photocatalytic reactions. Growing interest has focused on doped TiO2 catalysts (Ohno et al., 2003; Luo et al., 2004; Li et al., 2005; Labat et al., 2008; Yang et al., 2008; Long et al., 2009; Zhang et al., 2010; Zaleska et al., 2010; Iwaszuk & Nolan, 2011; Long & English, 2011; Spadavecchia et al., 2011; Kumar & Devi, 2011;), however current achievements are still far from the ideal goal.

In order to extend the photocatalytic activity in the region of visible light, and in order to achieve a better use of solar radiation, several approaches have been proposed for tuning the band gap response of titania to the visible region. Doping or incorporate trace impurities in the structure of TiO2 in order to obtain materials with photocatalytic activity maximized in the visible region are strategies widely used (Ohno et al., 2003; Li et al., 2005; Zaleska et al., 2010). These strategies include doping with transition metals (Nogueira & Jardim, 1998; Yamashita et al., 2001; Cavalheiro et al., 2008; Zaleska et al., 2010), nonmetals (Ohno et al., 2003; Li et al., 2005), and the inclusion of low-valence íons on the surface of the semiconductor (for example, Ag+, Ni3+, V3+ e Sc3+). Certain metals, when incorporated to titanium dioxide, are able to decrease the band gap, making possible in some cases its application in solar photocatalysis. Furthermore, they can contribute to minimize the electron-hole recombination, increasing the photocatalytic efficiency of the semiconductor (Zaleska et al., 2010).

Coupling of two photocatalysts has also been considered effective for improvement of photocatalytic efficiency. As example, nitrogen doped TiO2 coupled with WO3 and after loaded with noble metal, resulted in a material with improved photocatalytic efficiency (Yang et al., 2006).
