## 3 Result and Discussion
### Characterization
#### 3.1.2 Textural Properties and Light Absorption  
N\({}_{2}\)-sorption isotherms for pristine TiO\({}_{2}\) synthesized at 70 \({}^{\circ}\)C as well as for materials obtained after calcination at different temperatures are presented in Figure 3A, and their pore radius distribution is shown in Figure 3B. The N\({}_{2}\)-sorption isotherms of the uncalcined sample and that of the material calcined at temperatures up to 500 \({}^{\circ}\)C can be classified as type IV with an H\({}_{3}\) hysteresis loop, which indicates the presence of mesopores [26; 27]. The adsorption isotherm shifts to areas of higher relative pressure with increasing calcination temperature, indicating the formation of larger pores with increasing temperature [28]. On the other hand, the absence of the hysteresis loop in the adsorption isotherms of the sample calcined at 600 \({}^{\circ}\)C indicates that the TiO\({}_{2}\) has almost completely lost its mesoporousity at this temperature. The derived BET surface areas (A\({}_{\textrm{BET}}\)), pore volumes (Vp), and pore size maxima (Sp) are summarized in Table 2. A relatively high BET surface area of about 300 m\({}^{2}\cdot\)g\({}^{-1}\) was measured for the uncalcined sample [29]. As expected, BET surface area and pore volume significantly decreased when the material was calcined. Interestingly, the BET surface area of the TiO\({}_{2}\) sample calcined at 500 \({}^{\circ}\)C was still two times higher in comparison to the BET surface area of P25 [30]. During calcination, the primary crystallite size of the anatase crystallites increased significantly, regardless of the applied synthesis temperatures. It is assumed that amorphous TiO\({}_{2}\) is transformed into anatase during calcination and that this transformation increases with increasing calcination temperature. The loss of the amorphous TiO\({}_{2}\) content with increasing temperature, which is associated with a collapse of the mesopores, leads to material densification and thus to a decrease in the BET surface area and the pore volume of the TiO\({}_{2}\)[31]. During crystallization, the anatase crystallites forming the wall of mesopores undergo growth, thereby reducing the number and volume of the existing voids, which are \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Tcalc** & **A\({}_{\textrm{BET}}\)** & **Vp** & **Sp** & **C-Content** & **Eg** \\  & \({}^{\circ}\)**C** & **m\({}^{2}\) g\({}^{-1}\)** & **cm\({}^{3}\) g\({}^{-1}\)** & **nm** & **wt\%** & **eV** \\ \hline TiO\({}_{2}\) & - & 325.4 & 3.8 & 2.4 & 0.66 & 3.22 \\ TiO\({}_{2}\) & 400 & 140.7 & 2.8 & 4.0 & 0.05 & 3.16 \\ TiO\({}_{2}\) & 500 & 97.4 & 2.6 & 5.4 & 0.04 & 3.13 \\ TiO\({}_{2}\) & 600 & 17.9 & 0.9 & - & 0.04 & 3.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of N\({}_{2}\) sorption and band gap estimation for materials synthesized at 70 \({}^{\circ}\)C.  
Figure 3: (**A**) Nitrogen adsorption and desorption curves of TiO\({}_{2}\) synthesized at 70 \({}^{\circ}\)C and after calcination and (**B**) the corresponding pore size distribution. responsible for the mesoporous structure. The calcination also removes organic residues from TiO\({}_{2}\), as shown by the reduced carbon content. When the sample is calcined at 600 \({}^{\circ}\)C, the amorphous part will be almost completely transformed into crystalline anatase. In addition, the transformation of the anatase phase into the rutile phase begins at this high temperature, and sintering can also occur [31]. As a sum of these processes, the BET surface area and the pore volume of the material calcined at 600 \({}^{\circ}\)C decrease drastically. Diffuse reflectance spectra (DRS) from TiO\({}_{2}\) samples synthesized at 70 \({}^{\circ}\)C are presented in Figure 4A. As expected, TiO\({}_{2}\) absorbed light only in the UV region. The calcination temperature had only a minor influence on its light adsorption. [PERSON]'s method was used to determine the band gap of the TiO\({}_{2}\) samples (Figure 4B).