**2.4. Nano-structured TiO<sup>2</sup>**

Amongst the various strategies that have been used to enhance TiO2 photocatalytic activity, improvement of morphology, crystal structure and surface area have also been considered important and widely investigated approach to achieve better photocatalytic performance. The nanotitania crystallinity can simply be enhanced by optimizing the annealing temperature. However, the stability of the structure and geometries have to be considered when annealing [84]. For the nanotitania morphology and surface area, various ordered structures have been studied. TiO2 nanotubes [85, 86], nanowires [79], nanospheres [87], etc. Tang et al. fabricated monodisperse mesoporous anatase TiO2 nanospheres using a template material and found the resulting catalysts to show high photocatalytic degradation efficiency and selectivity towards different target dye molecules and could be readily separated from a slurry system after photocatalytic reaction [87]. Anodic TiO2 nanotubes have been reported to allow a high control over the separation of photogenerated charge carriers in photocatalytic reactions. The nanotube array has as key advantage the fact that nanotube modifications can be embedded site specifically into the tube wall or at defined locations along the tube wall. This allows for engineering of reaction sites giving rise to enhanced photocatalytic efficiencies and selectivities [88].
