# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 5 Titanium-Based Nanomaterials
### Doping  
As previously mentioned, the major limitation of TiO\({}_{2}\) applications is its restricted range of radiation absorbance. With a wide band gap of 3.2 eV, TiO\({}_{2}\) requires UV light to initiate photocatalytic reactions. To overcome this, doping is employed to induce a bathochromic shift, narrowing the band gap or introducing intermediate energy levels that enhance absorption in the visible light range [126]. Additionally, TiO\({}_{2}\) photocatalysts have a high charge carrier recombination rate, which decreases the efficiency of photocatalytic oxidation. Various dopants are used to reduce this recombination rate. By introducing intermediate energy levels within the band gap, doping enables TiO\({}_{2}\) to be activated by photons in the visible light spectrum [127; 128]. These modifications also improve charge carrier separation, thereby reducing recombination and enhancing photocatalytic efficiency [129]. Modifications to TiO\({}_{2}\) can include metal doping, non-metal doping, co-doping (involving different elements), or coupling with other semiconductors [126]. Various metals such as Fe, Cu, Co, Mn, Ag, and Ni have been used as dopants to reduce the band gap of TiO\({}_{2}\). Metal dopants influence photocatalytic oxidation in several ways: they can modify the photocatalyst's light absorption capacity, affect the charge transfer rate at the interface, and improve the adsorption of pollutant molecules on the catalyst surface. When a metal cation is introduced as a dopant in TiO\({}_{2}\), it creates an intermediate energy level near the valence band, enabling electron transfer from the valence band to the conduction band with less energy than undoped TiO\({}_{2}\). This shift allows the photocatalyst to be activated by visible light. Additionally, metal doping can enhance electron trapping, reducing electron-hole pair recombination, and thus increasing photocatalytic efficiency [128]. However, excessive dopant concentrations can lead to metal aggregation, forming recombination centers that diminish photocatalytic performance [130]. Therefore, factors like dopant type, concentration, and the preparation method of the doped photocatalyst are crucial for optimizing photocatalytic oxidation [80]. Despite the advantages of metal doping, some drawbacks have been identified. Metal dopants can block the porous surface sites of TiO\({}_{2}\), promote nanocrystalline growth, and reduce the specific surface area, all of which lower photocatalytic efficiency [117]. Furthermore, metals deposited on the photocatalyst surface can block photon absorption, leading to increased charge carrier recombination [131]. TiO\({}_{2}\) photocatalysts can be doped with metal ions substitutionally [132] or interstitially [133]. Introducing dopants can enhance conductivity by partially emptying the valence band (with _p-type_ dopants) or partially filling the conduction band with electrons (with n-type dopants) [134]. P-type doping, using heterocations with lower valencies than Ti\({}^{4+}\) (such as Al\({}^{3+}\) and Cr\({}^{3+}\)), creates acceptor centers that trap photogenerated electrons, which then attract holes and form charge carrier recombination centers [135]. N-type doping, achieved with heterocations of higher valencies than Ti\({}^{4+}\) (such as Nb\({}^{5+}\) and Ta\({}^{5+}\)), acts as donor centers, increasing electron concentration in the conduction band, which also promotes charge carrier recombination. Combining _p-type_ and _n-type_ materials can lead to the recombination of electron-hole pairs at the interface [111]. The most commonly used noble metals as dopants for TiO\({}_{2}\) photocatalysts include platinum (Pt) [136], palladium (Pd) [137], silver (Ag) [138], and gold (Au) [139; 140]. These dopants enhance the separation of charge carriers and decrease the recombination rate due to the formation of a Schottky barrier at the metal-semiconductor interface. This results in an increased photocatalytic efficiency, leading to a higher pollutant degradation rate.