## 5 Titanium Dioxide as a Photocatalyst for Water Remediation Applications
### Single Atom Doped TiO\({}_{2}\) Composites
Photocatalytic activity of Ag-modified TiO\({}_{2}\) was tested by measuring the degradation rate of phenol in water solution under UV and visible light irradiation generated by a 1000 W xenon lamp. Two commercially available forms of TiO\({}_{2}\), P25, and ST01 (pure anatase) were loaded with AgNPs at 0.5, 1, and 2 wt.%. Unmodified P25 degraded 33% of phenol after 60 min of irradiation with UV light and 22% under irradiation with visible light. Better results were obtained for Ag-loaded TiO\({}_{2}\). For 2 wt.% Ag-P25, the highest degradation rate of phenol at 92% under 60 min UV irradiation was observed. Overall degradation rates in visible light were significantly lower for both materials modified and unmodified. It turns out that the 1% Ag-P25 sample achieved the best results under visible light irradiation when compared to other examined TiO\({}_{2}\) photocatalysts in the study including both unmodified and Ag-modified ST01. After 60 min of irradiation with UV light, only 23% of phenol content was degraded and 15-17% when it was irradiated with visible light at the same time. The study proved that Ag loading does not have a palpable effect on the photocatalytic activity of modified ST01 under UV light. For visible light, 2% Ag-ST01 was marginally more active than other loadings [56]. In the study presented by [PERSON] et al., a composite material of TiO\({}_{2}\) nanotube (NT) arrays and silver particles was prepared in the form of Ag/AgCl/TiO\({}_{2}\)[71]. This composite material was obtained by depositing AgCl NPs on self-organized TiO\({}_{2}\) nanotubes and partial reduction of Ag\({}^{+}\) ions from AgCl after irradiation with a xenon lamp to Ag\({}^{0}\) species. The composite, as reported by the researchers, exhibited higher photocatalytic activity when irradiated by visible light. This rise in its activity was explained by plasmon resonance and charge separation effects. Electrons from Ag were excited by plasmon resonance and transferred to TiO\({}_{2}\) conduction band and, simultaneously, compensative electrons were transferred from Cl\({}^{-}\) back to AgNPs. The material's efficiency was tested by photocatalytic degradation of methyl orange in water. The photocatalytic activity was compared with amorphous and anatase TiO\({}_{2}\) NT arrays, which did not show any photocatalytic activity under visible light. Additionally, the composite was studied against another reference photocatalyst, N-doped TiO\({}_{2}\) NTs. AgCl-TiO\({}_{2}\) presented insignificant photoactivity in visible light. Only after Ag\({}^{0}\) was deposited on the surface of the composite material could it be excited by visible light [71]. A noteworthy study where Ag-TiO\({}_{2}\) was used for the photodegradation of dexamethasone in wastewater was shown by [PERSON] et al. [112]. Ag-TiO\({}_{2}\) was prepared with the modified photodeposition method. Researchers carried out experiments using different concentrations of Ag-TiO\({}_{2}\), dexamethasone, and hydrogen peroxide. Additionally, the influences of pH and temperature were analyzed during the study. In optimal conditions, dexamethasone photodegradation levels achieved for UV and visible light were 82.3% and 71.5%, respectively. In another study by [PERSON] et al. [113], Ag-doped TiO\({}_{2}\) was used in the photodegradation of lomefloxacin under UV-light produced by a mercury lamp. Materials were synthesized by liquid impregnation method in two concentrations--1% and 2%. According to the results, 2% Ag-TiO\({}_{2}\) revealed better degradation rates of lomefloxacin than pure TiO\({}_{2}\) and 1% Ag-doped particles. The achieved degradation rates reached up to 95% after 1 h of irradiation [113]. Au-doped titania has been broadly studied as a photocatalyst for its applications in the removal of water pollutants. A review on this subject was presented by [PERSON] et al. [114]. Gold as a noble metal on its own has poor reactivity. However, this is not the case for its nanoparticles, especially when their size falls below 5 nm.