# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 5 Titanium-Based Nanomaterials
### Doping
Additionally, noble metal dopants can extend the light absorption capacity of the photocatalyst into the visible light range [141; 140]. For example, a study investigated the photocatalytic activity of Ag-doped TiO\({}_{2}\) nanotubes for toluene degradation (Figure 7). Among noble metals, silver is relatively inexpensive and has been shown to improve the photocatalytic efficiency of TiO\({}_{2}\). The Ag-doped TiO\({}_{2}\) achieved a 98% toluene reduction under UV light, outperforming both P25 and undoped TiO\({}_{2}\) nanotubes [142]. This high photocatalytic activity is attributed to the ability of Ag particles to trap photogenerated electrons, limiting the recombination of charge carriers. As a result, the electrons are more efficiently transferred to oxygen adsorbed on the TiO\({}_{2}\) nanotube surface. Moreover, the Ag-doped TiO\({}_{2}\) nanotubes had a porous structure and larger specific surface area, further enhancing their photocatalytic activity. Even under visible light, the Ag-doped TiO\({}_{2}\) nanotubes showed superior photocatalytic performance compared to P25 and undoped TiO\({}_{2}\) nanotubes [142]. In another study (Figure 8) [86], the photocatalytic efficiency of Au-TiO\({}_{2}\) nanocomposite microspheres for formaldehyde removal was examined. The presence of Au nanoparticles inhibited anatase nanocrystal growth, resulting in smaller crystallite sizes and larger surface areas. These nanocomposites displayed greater photocatalytic activity than both P25 and undoped TiO\({}_{2}\) microspheres. The mechanism involves the migration of photogenerated electrons to the Au nanoparticles, which are then transferred to oxygen molecules adsorbed on the TiO\({}_{2}\) surface. As with Ag, Au forms a Schottky barrier at the metal-semiconductor junction, effectively separating the positively charged holes in TiO\({}_{2}\)'s valence band from the electrons in the Au nanoparticles, preventing their recombination. This was confirmed through photoluminescence spectra, where the Au-TiO\({}_{2}\) nanocomposite exhibited lower intensity compared to P25, indicating higher photocatalytic efficiency. However, while an increase in the atomic percentage of Au initially boosts photocatalytic performance, beyond a certain threshold, it decreases due to the formation of Au nanoparticles that act as recombination centers for electron-hole pairs [86]. Platinum is another widely studied noble metal dopant, particularly in Pt/TiO\({}_{2}\) photocatalysts, known for its high stability, pollutant removal efficiency, and regenerability. TiO\({}_{2}\) facilitates the dispersion of Pt active sites, enhancing pollutant adsorption (Figure 9) [143]. One study on Pt nanoparticles supported on faceted TiO\({}_{2}\) nanocrystals (Pt/TiO\({}_{2}\)) for ethanol removal showed that Pt deposition increased TiO\({}_{2}\)'s adsorption capacity in the visible light range. Pt/TiO\({}_{2}\) exhibited superior photocatalytic activity, achieving a maximum ethanol conversion rate of 98%, compared to only 82% for undoped TiO\({}_{2}\). However, excessive deposition time led to larger Pt particles and the formation of aggregates due to the platinum reduction mechanism on TiO\({}_{2}\)'s surface. Pt nanoparticles nucleate on oxygen vacancies with high electron density, transferring photogenerated electrons from TiO\({}_{2}\) to the Pt nuclei, leading to an electron density increase. This nucleation process limits the formation of new nuclei, causing Pt particle growth and aggregation [144]. Recently, rare earth metals (RE) such as Nd, Ce, and Ln have been utilized as dopants to enhance photocatalytic performance. The incorporation of these metals into photocatalysts introduces intermediate energy levels within the band gap, facilitating the transfer of photogenerated electrons. This reduces charge carrier recombination and improves photocatalysis. Additionally, RE dopants can alter phase structures, increase surface area, and modify surface morphology, thereby enhancing light absorption and enabling photocatalytic reactions under visible light.