## 5 Titanium Dioxide as a Photocatalyst for Water Remediation Applications
### Multiple Atom Doped TiO\({}_{2}\) Composites  
Multiple-doped titania materials consist of titania that has been doped, for example, with nitrogen and sulfur, vanadium and nitrogen, gold and palladium as well as silver, carbon, and sulfur. The photocatalytic properties of these materials were studied toward the degradation of pharmaceutical pollutants in water and/or bactericidal activity (Figure 9). [PERSON] et al. presented TiO\({}_{2}\) NPs co-doped with multiple atoms [123]. In the study, titanium oxide was doped with silver, carbon, and sulfur, which led to a composite Ag/(C,S)-TiO\({}_{2}\) with enhanced bactericidal and sporicidal activities. Various silver loadings were used--0, 0.5, 1, 2, 10, and 20 mol%. Carbon and sulfur co-dopants seem to further enhance photocatalytic and bactericidal properties compared to regular Ag-TiO\({}_{2}\). Properties of Ag/(C,S)-TiO\({}_{2}\) were evaluated in tests of gas-phase acetaldehyde degradation as well as _Escherichia coli_ and _Bacillus subtilis_ inactivation. These co-doped Ag/(C,S)-TiO\({}_{2}\) particlesshowed significant antimicrobial activity, even without additional irradiation, against both strains. An antibacterial test was conducted by suspending bacterial spores and cells in a vegetative state in water. According to the results, 10% Ag/(C,S)-TiO\({}_{2}\) exerted the best bactericidal effect on _E. coli_ out of all samples tested with 8.9 log reduction after 30 min of exposure. Compared to bare P25 TiO\({}_{2}\) with 3.2 log reduction, it was a notable improvement. Authors pointed out that the presence of co-dopant silver and carbon played a major role in enhancing TiO\({}_{2}\) antibacterial properties. Results of the tests on _B. subtilis_ were similar. A better bactericidal effect was achieved for doped-TiO\({}_{2}\) than bare TiO\({}_{2}\). In addition, higher loadings of Ag led to improved antibacterial activity [123]. [PERSON] et al. probed N-doped TiO\({}_{2}\) in the presence of vanadium ions, which allowed them to obtain V-N co-doped TiO\({}_{2}\)[124]. The researchers attempted photodegradation of chloramphenicol and studied the possibility of degrading both bacteria and the antibiotic in close vicinity. V-N-TiO\({}_{2}\) revealed high degradation rates of chloramphenicol as well as antibacterial activity against _E. coli_. Additionally, it was possible to reuse the photocatalyst for many cycles [124]. In another study, N,S-doped nanoparticles and nanosheets were used in the photodegradation of naproxen and ibuprofen by visible light irradiation. Nanoparticles were synthesized by the sol-gel method, whereas nanosheets by the hydrothermal method. The loading of the catalyst at 2.0 g \(\times\) dm\({}^{-3}\) and pH 6 nanoparticles degraded 85% ibuprofen content and 99.3% of naproxen. The nanosheets degraded 71.6% of ibuprofen and 99.1% naproxen in 90 min. The materials could be re-used for six cycles [125]. [PERSON] et al. showed that salicylic acid could be degraded by TiO\({}_{2}\) doped with N,S,B in pairs or individually [126]. These non-metal composites were synthesized by the solvothermal method and were characterized by X-ray diffraction and photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, UV-Vis diffuse reflectance spectra, and specific surface area. The best results were achieved for the co-doped sample of Ti:N:S = 1:1:1, which resulted in 76.25% degradation of salicylic acid after 120 min [126]. Methods of paracetamol (PCT) degradation are sought after and developed due to its increasing presence in the environment as a pollutant. For this reason [PERSON] et al. prepared multi-dopant systems of gold, palladium, and gold/palladium and TiO\({}_{2}\) for the degradation of PCT through photocatalysis and sonophoctatalysis under UV-A irradiation (254 nm) and sonication (574, 861, and 1145 kHz) [127]. Noble metal-doped photocatalysts were synthesized sonolytically from noble metal substrates and powdered P25 (1-50 mg \(\times\) dm\({}^{-3}\) concentrations).