## 3 Result and Discussion
### Photocatalytic Results  
The uncalcined TiO\({}_{2}\) samples showed both DCF adsorption and photocatalytic degradation (cf. Figure 5A). When the TiO\({}_{2}\) was synthesized at 40 \({}^{\circ}\)C, about 75% of DCF was adsorbed within 30 min in the dark, but DCF adsorption dropped significantly when the synthesis temperature was raised to 70 \({}^{\circ}\)C or 100 \({}^{\circ}\)C. The reasons for the higher DCF adsorption of the material synthesized at 40 \({}^{\circ}\)C might be a high content of amorphous titanium dioxide, a low crystallinity of the TiO\({}_{2}\) phase, and a high carbon content. A negative effect of the presence of an amorphous TiO\({}_{2}\) phase on the photocatalytic activity has been already reported [33]. The photocatalytic DCF degradation activity of the samples synthesized at 70 \({}^{\circ}\)C or 100 \({}^{\circ}\)C is similar to that of P25, which also shows lower adsorption of DCF in the dark. The pseudo-first-order rate constant for photocatalytic DCF degradation estimated from a plot of ln(\(c_{0}\)-\(c_{t}\)-\({}^{1}\)) versus reaction time is presented in Figure 5B. The rate constant of DCF degradation increases with increasing sol-gel synthesis temperature, and for TiO\({}_{2}\) synthesized at 70 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, the estimated degradation rate constant was slightly higher than the rate constant obtained when using P25 TiO\({}_{2}\) as the photocatalyst. The impact of annealing temperature on DCF adsorption in the dark and degradation under light irradiation is shown in Figure 6A for TiO\({}_{2}\) synthesized at 70 \({}^{\circ}\)C. After calcination at 400 \({}^{\circ}\)C, the TiO\({}_{2}\) material lost its ability to adsorb DCF on its surface. On the other hand, the degradation rate of DCF in the calcined materials was significantly faster compared to the non-calined sample. HPLC chromatograms of the reaction solution after different irradiation times using the sample calcined at 500 \({}^{\circ}\)C as the photocatalyst are shown in Figure S3. The chromatograms show a rapid decrease in the DCF signal with increasing irradiation time. During the calcination step, in addition to the thermally induced TiO\({}_{2}\) crystallization, carbon-containing groups will be removed [34]. It is assumed that with increasing crystallinity, the number of DCF adsorption sites decreases; however, the number of distorted lattice structures that act as charge trapping and/or recombination sites will also decrease [35]. In such cases, a higher number of photogenerated electrons and holes can reach the TiO\({}_{2}\) surface and react with oxygen, surface hydroxyl groups, or adsorbed water molecules to produce highly reactive oxygen species like superoxide and hydroxyl radicals. This can be seen by the increased photocurrent when the material was calcined at up to 500 \({}^{\circ}\)C (Figure 4C). The comparable first-order rate constant for TiO\({}_{2}\) calcined at 400 or 500 \({}^{\circ}\)C indicates that the photocatalytic activity of the TiO\({}_{2}\) might be influenced not only  
Figure 5: (**A**) Plot of relative DCF concentration versus time for TiO\({}_{2}\) synthesized using the sol-gel process at different temperatures (without calcination) and (**B**) plot for the estimation of the pseudo-first-order rate constant (for B: c\({}_{0}\) = DCF concentration when turning on the light). by the charge carrier generation, recombination, and migration, but also by the number of available surface sites. The larger BET surface area of the sample calcined at 400 \({}^{\circ}\)C could at least partially compensate for the faster recombination of the generated charge carriers compared to the sample calcined at 500 \({}^{\circ}\)C. The pseudo-first-order degradation rate constant of anatase TiO\({}_{2}\) calcined at 400 \({}^{\circ}\)C or 500 \({}^{\circ}\)C was 1.4- and 1.5-times higher than the degradation rate obtained in the presence of P25 TiO\({}_{2}\) photocatalyst, respectively (Figure 6B).