## 3 Result and Discussion  
### Characterization  
#### 3.1.1 Phase Composition and Morphology  
The XRD powder patterns of the material synthesized via the acid-assisted sol-gel process at 40 \({}^{\circ}\)C, 70 \({}^{\circ}\)C, and 100 \({}^{\circ}\)C and those obtained after a subsequent thermal treatment at 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, or 600 \({}^{\circ}\)C are presented in Figure 1. The uncalcined material synthesized at 40 \({}^{\circ}\)C (Figure 1A) showed broad reflections with low intensity and 2\(\theta\) positions near the main reflections of the anatase phase. The observation of peak broadening in connection with additional scattering of amorphous material (peak at 2\(\theta\)\(\approx\) 27\({}^{\circ}\)) indicates a low crystallinity of the formed TiO\({}_{2}\). When the synthesis temperature of the sol-gel process was increased to 70 or 100 \({}^{\circ}\)C, the intensity of these reflections increased. Within the powder diffraction data (cf. Figure 1B,C), all reflections can be attributed to the anatase phase (ICDD pdf 00-064-0863). After calcination, the width of the diffraction lines decreased without any change in the phase composition, except for the sample synthesized at 40 \({}^{\circ}\)which was calcined at 600 \({}^{\circ}\)C. Here, additional peaks located at 2\(\theta\) = 27.4, 36.1, and 54.3\({}^{\circ}\) can be observed and attributed to the presence of a rutile phase (ICDD pdf 01-073-2224). The sol-gel process involves hydrolysis and condensation reactions. Hydrolysis leads to the formation of hydroxides, which can condense by releasing water, forming TiO\({}_{2}\)[24; 25]. In the acid-assisted sol-gel process, the added acetic acid promotes the formation of an anatase TiO\({}_{2}\) phase and reduces the interaction between TiO\({}_{2}\) particles through adsorbates creating a steric barrier on the surface of TiO\({}_{2}\) cores [22]. The condensation reaction is therefore slower and more ordered, which leads to the formation of crystalline TiO\({}_{2}\) domains at room temperature [22]. The primary anatase crystallite size estimated by using the Scherrer equation is summarized in Table 1. The primary crystallite size increased with calcination temperature. Moreover, crystallite size seems to also be affected by the synthesis temperature. The increase in crystallite size with increasing calcination temperature was lower for materials synthesized at 70 \({}^{\circ}\)C than for those synthesized at 40 \({}^{\circ}\)C or 100 \({}^{\circ}\)C. The reason for this is still unclear. One possible explanation might be differences in the structure of the [TiO\({}_{\rm x}\)] polyhedrons of the formed amorphous TiO\({}_{2}\) phase. These structural differences could influence  
Figure 1: XRD powder patterns of TiO\({}_{2}\) synthesized at different synthesis temperatures and after a subsequent calcination step (synthesis temperature of (**A**) 40 \({}^{\circ}\)C, (**B**) 70 \({}^{\circ}\)C, and (**C**) 100 \({}^{\circ}\)C (the calcination was carried out in static air at 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C or 600 \({}^{\circ}\)C)) and reference Bragg peak positions (ICDD pdf 00-064-0863), respectively. Low-intensity peaks assigned to rutile are labeled with arrows. the ratio between the growth of existing crystallites and the formation of small new anatase crystallites during calcination.