XRD measurements were performed to identify the crystalline phases synthesized by the nonhydrolytic sol–gel process at 500 °C calcination temperature. The XRD patterns of the as obtained powders, undoped and iron doped TiO2 (0.5, 1 and 2 mol% Fe), are shown in Figure 3 a and b, respectively. As is seen in both figures, the three strongest interplanar distances of anatase (TiO2) appear at 3.51; 1.89 and 1.66 Å (JCPDS 78-2486). The anatase structure is preferred over other polymorphs for photocatalytic applications because of its higher electron mobility, low dielectric constant and lower density. All commonly known polymorphs of titania consist of TiO6 octahedra, which share edges and corners in different manners. The TiO6 octahedron of anatase is slightly distorted [29]. It has to be noticed that iron was not found in the XRD patterns of the investigated samples due to its very low concentrations. The average crystallite size of as prepared undoped TiO2 and iron doped TiO2 (0.5, 1 and 2 mol% Fe) calculated from the broadening of the diffraction line using Sherrer’s equation is about 20 and 12–15 nm, respectively. As is seen from the obtained values, the crystallite size of undoped TiO2 is larger than those of Fe-doped TiO2. Obviously, the Fe-doping leads to decrease of the crystallite sizes. Our results are in good accordance to the results obtained by Yang et al. [30]. However, there are previous studies which reported controversial results concerning the Fe3+ doping effect on the crystallite sizes. For example, Wang et al. [31] claimed that Fe3+ increase the crystallite sizes