## 4 Conclusions  
With the applied acid-assisted sol-gel method, a pure anatase TiO\({}_{2}\) phase with reasonable photocatalytic properties can be prepared in a facile and reproducible manner. With the obtained materials, DCF can be almost completely mineralized within 120 min via irradiation with simulated sunlight. The photocatalytic activity of the anatase phase for the degradation and mineralization of DCF depends on the temperature of the acid-assisted sol-gel process and the calcination temperature. Both parameters affect the electronic (e.g., charge carrier separation and migration) as well as structural (e.g., crystallinity and BET surface area) properties of the formed TiO\({}_{2}\) phase. To obtain TiO\({}_{2}\) with high photocatalytic activity for the degradation of pollutants, these properties should be effectively balanced.  
Using an RSM model, the optimal synthesis and calcination temperatures for photocatalytic DCF degradation and mineralization can be predicted. Moreover, the predicted optimal values correspond well with the values that were determined experimentally. The obtained results are considered as a basis for the development of composite materials based on an anatase phase that photocatalytically degrades pollutants with higher efficiency compared to a pure anatase phase.  
Figure 11: Three-dimensional surface plots for (**A**) DCF degradation rate and (**B**) TOC removal.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15031401/s1](https://www.mdpi.com/article/10.3390/app15031401/s1)]([https://www.mdpi.com/article/10.3390/app15031401/s1](https://www.mdpi.com/article/10.3390/app15031401/s1)). Figure S1: Scheme of TiO\({}_{2}\) synthesis; Figure S2: EDX results of TiO\({}_{2}\) synthesized at 70 \({}^{\circ}\)C and calcined at 400, 500, and 600 \({}^{\circ}\)C; Figure S3: Plot of HPLC chromatograms; Figure S4: Contour of actual factor coding for the rate constant; Figure S5: Contour of actual factor coding for the TOC decomposition; Table S1: Comparison of results for the rate constant using different models; Table S2: Comparison of results for the TOC removal using different models; Table S3: ANOVA result for the rate constant; Table S4: ANOVA result for the TOC decomposition.  
**Author Contributions:** Conceptualization, D.A.A. and N.S.; methodology, D.A.A.; software, D.A.A.; validation, D.A.A. and N.S.; investigation, D.A.A., H.L. and T.T.H.D.; resources, N.S.; data curation, D.A.A.; writing--original draft preparation, D.A.A.; writing--review and editing, D.A.A., H.L. and N.S.; visualization, N.S. and D.A.A.; supervision N.S., J.S. and F.F.; project administration, N.S. and J.S.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data are available from the authors based on reasonable requirements from the reader.  
**Acknowledgments:** [PERSON] thanks the EXiST project (Ethiopia Center of Excellency in Science and Technology, funding code 51235) and the Leibniz Institute for Catalysis (LIKAT Rostock) for their financial support. The help by [PERSON] (BET), [PERSON] (XRD), [PERSON] (SEM), and [PERSON] (experimental work) is gratefully acknowledged.  
**Conflicts of Interest:** The authors declare no conflicts of interest.