# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 5 Titanium-Based Nanomaterials
### Doping
Doping with rare earth materials can also inhibit  
Figure 8: Reaction rate constants of TiO\({}_{2}\) (a), Au–TiO\({}_{2}\) containing various amounts of Au (b–e) and P25 [86]. Figure 7: Degradation of toluene using Ag-doped TiO\({}_{2}\), pure TiO\({}_{2}\), P25, and Ag-doped P25 [142]. the anatase-to-rutile phase transition in TiO\({}_{2}\), and increase its surface area. Notably, the inhibitory effect on phase transition becomes more pronounced with the ionic radius of the RE metal, leading to distinct RE-TiO\({}_{2}\) phases [145]. For instance, Ce-doping of P25 has been shown to increase the photocatalytic oxidation rate of benzene, ethylbenzene, and m-xylene under UV radiation at specific reactor concentrations (Figure 10). In the case of benzene, the maximum conversion rate increased from 10 to 22.3 ppmv/min with Ce-doped P25. Similarly, for ethylbenzene removal, the maximum conversion rate rose from about 8 to 29 ppmv/min. Furthermore, P25/Ce demonstrated a higher conversion rate for benzene and ethylbenzene removal compared to P25/Pt and P25/Fe. This increased photocatalytic efficiency is attributed to the presence of cerium species on the photocatalyst's surface, which create vacancies, charge disparities, and unsaturated chemical bonds. These factors result in increased chemisorbed oxygen on the surface, enhancing photocatalytic oxidation. Figure 10: _Cont._  
Figure 9: Ethanol conversion using TiO\({}_{2}\) and Pt-doped TiO\({}_{2}\) composed of different amounts of Pt [144]. In another study (Figure 11) [147], the photocatalytic activity of P25 doped with Eu, La, Y, and Ce under visible light was evaluated and compared with undoped P25. The undoped photocatalyst exhibited a conversion rate of 23 ppm/h for the transformation of isopropanol to acetone. In contrast, samples doped with Eu, La, and Y showed maximum conversion rates of 27, 32, and 21 ppm/h, respectively. However, the Ce-doped samples did not show any photocatalytic activity due to low concentrations of hydroxyl groups adsorbed on the surface and reduced specific surface area caused by synthesis conditions. The enhanced photocatalytic activity in the other RE-doped samples can be explained by the increase in the specific surface area induced by the dopants. Indeed, samples with larger surface areas exhibited higher photocatalytic oxidation rates for pollutants. Additionally, RE metals like Eu, La, and Y increase charge carrier separation and reduce recombination rates, further boosting photocatalytic activity. Interestingly, the concentration of RE dopants does not affect all photocatalysts uniformly. Samples of Ti\({}_{1-x}\)RE\({}_{x}\)O\({}_{2}\) were prepared with two different dopant concentrations (x = 0.01 and 0.025 mol). For La-TiO\({}_{2}\) and Y-TiO\({}_{2}\), higher dopant concentrations resulted in increased photocatalytic activity, with rates of 32 and 21 ppm/h, respectively. However, for Eu-TiO\({}_{2}\), increasing the dopant concentration caused the photocatalytic activity to drop to 19 ppm/h. Despite the high photocatalytic efficiency of noble metal- and rare earth metal-doped photocatalysts, their high cost has led to the increased use of more affordable transition metals. Transition metals such as Mn, Cu, Co, Fe, Al, and Ni have been extensively studied as dopants for TiO\({}_{2}\) semiconductors. These dopants enhance charge carrier separation, reducing the recombination of electron-hole pairs and improving photocatalytic performance. However, if the dopant concentration exceeds an optimal threshold, it can disrupt  
Figure 11: Photocatalytic activity of P25 and TiO\({}_{2}\) doped with various rare earth metals [147]. Figure 10: Photocatalytic oxidation rates of ethylbenzene (**a**), m-xylene (**b**), and benzene (**c**) using various photocatalysts [146]. the catalyst's crystallinity and act as a recombination center, reducing overall photocatalytic efficiency. Different transition metals have unique optimal concentrations, meaning the impact on pollutant removal efficiency varies.