**2.2. Oxidation methods**

These methods involve the oxidation of titanium metal using oxidants or anodization. Anodization of titanium sheet under a voltage between 10 and 20 V in 0.5% hydrogen fluoride

**Figure 3.** Schematic representation of spray pyrolysis system: (a) vertical chambers; (b) horizontal chambers and (c) chambers for film deposition [27].

leads to the formation of aligned TiO2 nanotubes whose diameter is controlled by varying the applied voltage [31]. In another study, crystallized TiO2 nanotubes were obtained when anodized titanium plate was heat treated at 500°C for 6 h in an oxygen environment [32]. Direct oxidation of the titanium metal with hydrogen peroxide has also been found to lead to the formation of TiO2 nanorods. The TiO2 can be obtained by placing a cleaned Ti metal plate in a 50 mL solution of 30 wt% H2 O2 at 353 K for 72 h [33]. Formation of crystalline TiO2 occurs via mechanism of dissolution precipitation and this phase can be controlled by addition of NaX (X = F<sup>−</sup> , Cl<sup>−</sup> , SO4 2−) inorganic salts. Addition of Na2 SO4 and NaF results in the formation of anatase phase and when rutile phase is needed, NaCl can be added during dissolution precipitation [34].

Acetone, pure oxygen and a mixture of oxygen and argon can be used as sources of oxygen for oxidation of titanium metal. Acetone is a good source of oxygen and when used at high temperatures, it results in nanorods which are well aligned and highly dense. Use of pure oxygen or a mixture of oxygen and argon results in crystal grain films and morphology of the nanoparticles can be controlled by the diffusion competition of oxygen and titanium [8].
