The optical properties of the NP, NT, NF, NW and Degussa P-25samples annealed at 350◦C are shown in Fig. 4. Generally, the TiO2anatase phase has a strong absorption between 380 and 459 nm,and the band gap (Eg) of this phase is 3.2 eV [14]. In comparison,the absorption edge of the NT shows a notable red shift towardsthe visible light region. This change can be due to the fact thatthe hydrothermal and annealing treatments modified the UV–visabsorption characteristics of the tubular structure in comparison