# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 5 Titanium-Based Nanomaterials  
Among the various nanomaterials explored as photocatalysts for indoor pollutant removal, titanium-based materials, particularly titanium dioxide (TiO\({}_{2}\)), or titania, are the most widely used. TiO\({}_{2}\) is extensively researched due to its high photocatalytic efficiency, stability, low cost, low toxicity, abundance, and environmental friendliness. The morphology of the catalyst significantly impacts the photocatalytic degradation of pollutants, prompting studies on different forms of TiO\({}_{2}\) nanomaterials, such as nanoparticles [99], nanotubes [100], and nanosheets [101], to identify the most effective structure.  
During photocatalytic oxidation, TiO\({}_{2}\) absorbs radiation with energy exceeding its band gap, leading to the formation of photogenerated electrons. However, since TiO\({}_{2}\) has a wide band gap (3.2 eV), ultraviolet (UV) light is required to initiate photocatalytic reactions. In indoor environments, where visible and near-infrared (IR) light predominate, this limits TiO\({}_{2}\)'s photocatalytic applications [102]. To address this challenge, researchers have investigated various doping strategies, using both metal and non-metal materials, to narrow the band gap and enhance TiO\({}_{2}\)'s efficiency under visible light [103].  
In addition to photocatalytic oxidation, UV-irradiated TiO\({}_{2}\) exhibits another photoinduced phenomenon known as super-hydrophilicity [104], which reduces the water contact angle to nearly 0 degrees. This creates a uniform water film that covers the surface, preventing contact between the surface and volatile compounds. As a result, applying TiO\({}_{2}\) coatings on building facades can protect surfaces from volatile compounds and oxidize air pollutants through photocatalytic oxidation [105].  
As mentioned earlier, the morphology of the photocatalyst plays a crucial role in determining its properties and photocatalytic activity. TiO\({}_{2}\) nanomaterials are typically classified into four categories based on their dimensionality: nanoparticles (0D) [106], nanotubes (1D) [107], nanosheets (2D) [108], and porous structures (3D) [109]. Each of these structures has been studied to assess which form is the most efficient for photocatalysis. Comparisons have been made with the well-known TiO\({}_{2}\) P25, a standard reference photocatalyst composed of 75% anatase and 25% rutile, commonly used in photocatalytic processes [110].  
Due to the differences in lattice structures, anatase, and rutile have different structures of the electronic band and therefore different band gaps. For anatase phase TiO\({}_{2}\) the band gap between the valence band and conduction band is 3.2 eV, instead for rutile TiO\({}_{2}\) the band gap is 3.02 eV [111]. Therefore, to overcome this band gap it is necessary the adsorption of a photon with a wavelength of about 388 nm for anatase [80] and 410 nm for rutile [111]. Hence, the adsorption of a photon with \(\lambda<388\) nm is needed to activate the anatase phase TiO\({}_{2}\) semiconductor. Accordingly, TiO\({}_{2}\) does not have a high photocatalytic efficiency in indoor conditions where radiation is mostly in the visible-near IR range. However, when anatase and rutile as nanoparticles, are coupled with a heterojunction, TiO\({}_{2}\) is able to adsorb visible light [80].  
The synthesis reactions and doping processes can further influence the morphology of nanomaterials. TiO\({}_{2}\)'s photocatalytic activity is affected by several factors, including crystallite size, porosity, specific surface area, and crystalline phases. A larger surface area can enhance photocatalytic efficiency by increasing the number of adsorbed molecules. However, it can also introduce more crystal defects, which may promote the recombination of electron-hole pairs, ultimately reducing photocatalytic efficiency [112].