# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 5 Titanium-Based Nanomaterials
### Doping
Additionally, even with the same dopant amount, particle size and morphology of the photocatalysts can differ. After doping, the particle size of TiO\({}_{2}\) typically increases due to metal-induced agglomeration, with noticeable differences between samples [129]. Iron is frequently used as a dopant for TiO\({}_{2}\) photocatalysts because of its ability to enhance charge carrier separation and reduce recombination rates. It also induces a bathochromic shift, extending the applications of TiO\({}_{2}\) photocatalysts into the visible light range. Due to the similar ionic radii of Fe\({}^{3+}\) and Ti\({}^{4+}\), iron cations substitute into TiO\({}_{2}\)'s lattice. Iron doping also catalyzes the anatase-to-rutile phase transition. Research on Fe-doped TiO\({}_{2}\) has demonstrated high photocatalytic efficiency for toluene degradation under visible light. The 0.7% Fe/TiO\({}_{2}\) sample exhibited a maximum degradation rate of 96.5% (Figure 12). Higher Fe concentrations resulted in increased recombination rates, lowering photocatalytic efficiency. This was confirmed by photoluminescence spectra, where the 0.7% Fe/TiO\({}_{2}\) sample showed lower intensity than other doped TiO\({}_{2}\) samples. Furthermore, Fe doping improved pore volume and specific surface area [128]. In another study [129] (Figure 13), the photocatalytic activity of TiO\({}_{2}\) doped with various transition metals (Mn, Co, Cu, Ni, and Fe) for benzene degradation was compared. Doped TiO\({}_{2}\) particles were significantly larger than undoped TiO\({}_{2}\), leading to lower specific surface areas. Metal doping likely blocked photocatalyst pores and/or catalyzed particle growth, increasing the samples' specific surface area. Additionally, the fractions of anatase and rutile in the samples revealed that metal doping accelerated the anatase-to-rutile conversion. Under vacuum ultraviolet (VUV) radiation, photocatalytic efficiency followed this order: Mn/TiO\({}_{2}\) > Co/TiO\({}_{2}\) > Ni/TiO\({}_{2}\) > P25 > Cu/TiO\({}_{2}\) > anatase TiO\({}_{2}\) > Fe/TiO\({}_{2}\). The higher efficiency of Mn/TiO\({}_{2}\) and Co/TiO\({}_{2}\) aligns with the fact that MnO\({}_{\rm x}\) and CoO\({}_{\rm x}\) oxides have a strong ability to generate reactive oxygen species (ROS), which can oxidize benzene and boost photocatalytic efficiency. Further research explored the photocatalytic degradation of acetaldehyde under visible and UV light using Mn/TiO\({}_{2}\), Co/TiO\({}_{2}\), and Mn/Co/TiO\({}_{2}\). These photocatalysts removed approximately 49%, 28%, and 74% of formaldehyde under visible light, respectively, and 57%, 71%, and 91% under UV light (Figure 14). The results, particularly for Mn/Co co-doped TiO\({}_{2}\), were significantly higher than those for undoped TiO\({}_{2}\). UV-vis adsorption spectra demonstrated that doped and co-doped TiO\({}_{2}\) had higher light absorption efficiency under visible light compared to undoped TiO\({}_{2}\), with Mn/Co/TiO\({}_{2}\) showing the highest absorption capacity. However, despite high removal rates, only Mn/TiO\({}_{2}\) exhib  
Figure 12: Mineralization rate of toluene using Fe-doped TiO\({}_{2}\) containing various amounts of Fe (a–f), TiO\({}_{2}\) (g), and no catalyst (h) under visible light and one sample of Fe-doped TiO\({}_{2}\) in dark (i) [128]. ited photocatalytic activity. Photocatalytic efficiency in metal-doped TiO\({}_{2}\) under visible light depends not only on adsorption properties but also on recombination rates. Thus, Co/TiO\({}_{2}\) and Mn/Co/TiO\({}_{2}\) showed no photocatalytic activity, even with excellent visible light absorption [88]. In addition to metal doping, the use of non-metal anions such as C, N, F, and S for doping TiO\({}_{2}\) has been investigated to enhance the photocatalytic oxidation of pollutants. Both metal and non-metal dopants modify the electronic structure of TiO\({}_{2}\) by introducing new intermediate states in its band gap.