Introduction

TiO₂ has received wide attention ever since 1972 when Fujishima and Honda discovered photocatalytic splitting of water on a TiO₂ electrode under ultraviolet (UV) light [1]. Over the past decades, TiO₂ has served as a “benchmark photocatalyst” for the degradation of a wide class of organic compounds and microorganisms in the UV range [2] [3]. TiO₂ has been the most widely studied and used in many applications because of its strong oxidizing abilities, superhydrophilicity, chemical stability, long durability, nontoxicity, biocompatibility, photocorrosion-free, low cost, and transparency to visible light [4]. In addition, it has the following advantages.

• TiO₂ can be fabricated with interesting morphologies such as spheres, nanorods, fibers, tubes and sheets. This enables them to attain unique chemical, physical, and electronic properties increasing its photocatalytic efficiency [5] [6]. 
• TiO₂ nanomaterials can be prepared in large scale at mild temperatures and conditions. [7] [8]
• TiO₂ can be supported on various substrates such as glass, fibers, stainless steel, inorganic materials, sand, and activated carbon which allows its continuous reuse [9].

Therefore, tremendous interest has been shown in studies of TiO₂ nanomaterial structures and their photocatalytic applications. 

2.1. Structure of TiO₂

TiO₂ exists in three mineral forms, viz., anatase, rutile and brookite. These polymorphic phases of TiO₂ are shown in figure 3. In all the three forms, titanium (Ti⁴⁺) atoms are coordinated to six oxygen (O^2-) atoms, forming TiO₆ octahedra. Anatase is made up of face (vertice) sharing arrangements of octahedra at (001) planes (Fig. 1 a) resulting in a tetragonal structure. In rutile, the octahedra share edges at (001) planes to give a tetragonal structure (Fig. 1 b), and in brookite both edges and corners are shared to give an orthorhombic structure (Fig. 1 c)[10]   

Amongst the various phases, TiO₂ in the brookite phase has been scarcely used as photocatalysts. Between TiO₂ in the anatase and rutile phases, the anatase TiO₂ is more active for photocatalysis applications, even though the rutile TiO₂ possesses a smaller band gap.  The reasons anatase TiO₂ is more actively used for photocatalysis are as follows [11]:

• The conduction band position of anatase TiO₂ is more negative compared to rutile TiO₂. This is responsible for the higher reducing power of anatase TiO₂ towards the adsorbed reactants and thus results in the higher photocatalytic activity of anatase TiO₂ in comparison to rutile TiO₂
• Recombination reactions adversely affect photocatalytic activity and the degree of recombination is found to be higher for rutile TiO₂ than for anatase TiO₂. The recombination reactions limit rutile TiO₂’s activity.

The photocatalytic activity of TiO₂ increases with an increase in the density of physically adsorbed OH^- groups on its surface. The surface hydroxyl groups accept  holes generated by UV and solar irradiation to form hydroxyl radicals and prevent electron-hole recombination thus increasing the photocatalytic activity of TiO₂. Generally, the surface of the anatase phase of TiO₂ is very hydroxylated; whereas, rutile TiO₂, being obtained at higher temperatures, has a very low density of physically adsorbed hydroxyl groups. This leads to lower photoactivity of rutile TiO₂ in comparison to anatase TiO₂

Figure 1. Polymorphic phases of TiO2

2.2. Drawbacks of TiO₂

Besides the aforementioned inherent advantages of TiO₂, it has following drawbacks

• • The large band gap of TiO₂ (∼ 3.2 eV for anatase and brookite, ∼3.0 eV for rutile) requires an excitation wavelength that falls in the UV region. Given that less than ∼5% of the solar flux incident at the earth’s surface lies in this spectral regime (solar light consists of ∼5% UV, ∼43% visible, and ∼52% harvesting infrared), only a very small portion of the solar light could be used by pure TiO₂ photocatalysts. 
  • Massive recombination of photogenerated charge carriers limits its overall photocatalytic efficiency

2.3 Strategies to modify TiO₂ nanomaterials

Various strategies have been adopted for improving the photocatalytic efficiency of TiO₂. Some of them are listed in Fig. 2.

Figure 2. Strategies to modify TiO2

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