## 5 Titanium Dioxide as a Photocatalyst for Water Remediation Applications
### Single Atom Doped TiO\({}_{2}\) Composites  
Within single-doped titania composites, various examples of non-metal doped (nitrogen, silica, sulfur, carbon) as well as noble metals-doped materials including silver, platinum, gold, and palladium can be discussed. These materials have been studied as photocatalysts for the degradation of chemical and pharmaceutical pollutants and bacteria (Figure 8). In the study presented by [PERSON] et al., the preparation of Si-doped molecularly imprinted titania photocatalyst and its application toward antibiotic degradation in wastewater was presented [106]. It was found that Si-TiO\({}_{2}\) showed a higher degradation rate of oxytetracycline under UV light produced by a xenon lamp than bare TiO\({}_{2}\) nanoparticles. Si-TiO\({}_{2}\) material was prepared by liquid phase deposition and later appropriately characterized. Interestingly, samples with Si-doped TiO\({}_{2}\) showed an 80.79% degradation rate of oxytetracycline after 120 min of irradiation. Reusability tests showed that the material  
Figure 6: Graphical explanation of TiO\({}_{2}\) doping for water remediation. Figure 7: Possible modifications of TiO\({}_{2}\) with other atoms and substances that are decomposed due to the catalytic properties of obtained composites. could be re-used, as even after five cycles of degradation, the rates were at a relatively high level of 65.6% removal of initial oxytetracycline concentration. In another study by [PERSON] et al., N-doped TiO\({}_{2}\) photodiodes with different nitrogen content were prepared using a radio frequency magnetic sputtering process to tune their optical absorption for activity in visible light [107]. These anodes were used in photo-electrochemical degradation of chloretracycline in wastewater and allowed even for a 99.6% degradation rate after 180 min of irradiation with visible light and current intensity of 0.6 A. In a different study, N-doped TiO\({}_{2}\) photocatalysts were synthesized via the sol-gel method from titanium butoxide in the presence of guanidium chloride, which served as an N source [108]. In contrast to other studies, the prepared material was not in the form of pure anatase or P25, but as a mixture of anatase and rutile (about 40:60 anatase/rutile ratio, depending on the loading), making it a mixed phase material. The photoactivity of the photocatalyst was measured by photodegradation of 10 mg \(\times\) dm\({}^{-3}\) methylene blue (MB) water solution under visible irradiation with a fluorescent lamp (112 W and 3.2 mW \(\times\) cm\({}^{-2}\)). Photocatalysts with varying ratios of N/Ti were used. The material with 1:1 N/Ti loading proved to be the most efficient and showed up to a 97% degradation rate of initial concentration of MB after 100 min of irradiation with visible light [108]. Carbon-doped TiO\({}_{2}\) nanophotocatalysts were prepared by [PERSON] et al. from TiO\({}_{2}\), starch, and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) by facile calcination and acid etching process [109]. C-TiO\({}_{2}\) photoactivity was tested by the photodegradation of tetracycline (TCL) water solution at the concentration of 10 mg \(\times\) dm\({}^{-3}\) under visible light irradiation. Five samples of the C-TiO\({}_{2}\) photocatalyst were used during the test. The samples were differentiated by starch content pre-calcification and contained either 1, 2, 3, or 5 g of starch, which acted as the C source in the material. Sample C-TiO\({}_{2}\)-2 showed the highest photocatalytic activity, reaching 90.8% reduction in TCL in the solution after 160 min of irradiation with visible light (source of the light not described). The photoactivity of the novel material was significantly higher than that obtained for bare TiO\({}_{2}\), which reached only 30.4% reduction in TCL under the same conditions. In addition, the material showed high reusability, because after five cycles, its photocatalytic activity decreased only by 8.7% between the first and the fifth cycle.