## 5 Titanium Dioxide as a Photocatalyst for Water Remediation Applications
### Single Atom Doped TiO\({}_{2}\) Composites
The difference between photoactivities of the composite material and bare TiO\({}_{2}\) was attributed to higher specific surface area of C-TiO\({}_{2}\), better light harvesting ability, and effective separation of photogenerated carriers [109]. Figure 8: The applicability of single-doped titania composites as catalysts in the photodegradation studies of chemical and pharmaceutical pollutants and bacteria; d-TiO\({}_{2}\)—doped titanium(IV) oxide. [PERSON] et al. prepared amorphous, sulfur-doped TiO\({}_{2}\) photocatalysts from tetrabutyl titanate by the sol-gel method with low-temperature calcination using thiourea as a sulfur source [110]. The researchers used three different calcination temperatures (300, 350, 400 \({}^{\circ}\)C) to produce three different photocatalyst samples, which were then applied in the photocatalytic tests. Photocatalytic activity was measured by the photodegradation of 10 mg \(\times\) dm\({}^{-3}\) diclofenac (DFC) in water under irradiation from white LED lamps. Out of the three materials and bare TiO\({}_{2}\), only the sample prepared at the 300 \({}^{\circ}\)C temperature revealed notable photoactivity. S-TiO\({}_{2}\) (300) degraded 93% DFC after 4 h of irradiation with visible light. Additional tests for S-TiO\({}_{2}\) (300) were performed wherein the influence of different light intensities, photocatalyst concentrations, and pH values were evaluated. The light intensity study showed that photoactivity improved with a stronger light source. LED lights were comparable to a 15A Xe lamp, with the highest photoactivity achieved for the 25A Xe lamp. As for other tests, the photoactivity of the material increased along with higher concentration of the photocatalyst (initial--0.2 g \(\times\) dm\({}^{-3}\), up to 0.8 g \(\times\) dm\({}^{-3}\)) and decreasing pH (pH 6-11 range, the best results were noted for pH = 6) [110]. In a different study, sulfur-doped anatase TiO\({}_{2}\) was used for the degradation of rhodamine B (RhB) in an aqueous solution under a 300 W Xe lamp with a 400 nm cutoff filter that acted as a visible light source [111]. The photocatalyst was prepared by low-temperature hydrothermal oxidation of titanium disulfide in deionized water. As a consequence of sulfur doping, the composite's band gap was lowered from 3.2 eV to 2.65 eV, allowing for irradiation with visible light. For the degradation tests, a standard concentration of 10 mg \(\times\) dm\({}^{-3}\) was used for the model contaminant and 1 g \(\times\) dm\({}^{-3}\) concentration of the photocatalyst. Two samples of the photocatalyst were studied against two blank samples and pure TiO\({}_{2}\). The main difference between photocatalyst samples was the synthesis process and temperature. The first sample, ST-120-4, was prepared by a hydrothermal method carried out in 120 \({}^{\circ}\)C for 4 h. The second sample, ST-550-10, was prepared by the oxidation of TiS\({}_{2}\) in air at 550 \({}^{\circ}\)C for 10 h. According to the results, the sample ST-120-4 presented the best photodegradation rates with 80% removal of RhB in the solution after 60 min of irradiation with visible light. ST-550-10 showed a 50% removal rate after the same time in the same conditions. The authors pointed out that the difference between removal rates of both samples stemmed from different surface areas of the material, for ST-120-4, this value was notably higher [111]. Among the noble metal-doped titania, a significant amount of research refers to silver-modified TiO\({}_{2}\). Silver nanoparticles (AgNPs) present a high catalytic activity with optical properties that can be easily modified by changing their size and shape. AgNPs appear to be very promising in chemical and biosensing applications. As reported, conjugates of TiO\({}_{2}\) and Ag nanoparticles have revealed better photocatalytic activity under UV and visible light than each component separately. Conjugates of Ag with TiO\({}_{2}\) also presented better adsorption to microorganisms, which resulted in significant, up to 70-fold higher, bactericidal activity [56].