# Photocatalytic Cementitious Material for Eco-Efficient Construction--A Systematic Literature Review
## 2 Principles of Photocatalysis
### TiO\({}_{2}\) Based Photocatalyst  
TiO\({}_{2}\) naturally occurs in three different types of crystal structure: anatase, rutile, and brookite [32]. Two of them, anatase and rutile, are commonly used in photocatalysis. As shown in Figure 5, the bandgap of anatase is 3.2 eV, while the bandgap energy for rutile is 3.0 eV, which is 0.2 eV narrower than anatase. Lower bandgap materials, such as rutile, should be preferred as they could absorb photon with smaller energy for photoactivation. However, in the literature, anatase shows higher photocatalytic activity than rutile under UV light irradiation. This is due to a more negative conduction band position of anatase than that of rutile (Figure 5), resulting in more reactive species formed [21].  
Figure 5: Valence and conductance band positions for various semiconductors and relevant redox couples at pH 0.  
The scientific concept of TiO\({}_{2}\) photocatalysis was introduced by [PERSON] and [PERSON] [22]. They discovered the photocatalytic water splitting in a photo-electrochemical cell. Using TiO\({}_{2}\) electrodes in a photo-electrochemical cell, water could be dissociated into its constituent parts, hydrogen and oxygen gases, under ultraviolet (UV) illumination. This discovery triggered much research in understanding the fundamental mechanism and in improving the efficiency of TiO\({}_{2}\) photocatalyst. In recent years, one of the most active areas in the development of photocatalysis technologies is their application for environmental cleanup. However, due to wide bandgap of the TiO\({}_{2}\) material, pure TiO\({}_{2}\) photocatalyst can only be activated by UV irradiation [28; 33; 34]. While it can use natural sunlight-UV irradiation, sunlight only contains a small amount of UV photon that can be used to carry out reactions at the TiO\({}_{2}\) surface. Therefore, one of the main challenges in improving the performance of TiO\({}_{2}\) photocatalyst is to extend their optical response from UV to visible region.  
Improving the optical response and photocatalytic performance of TiO\({}_{2}\) in the visible light region can be achieved by doping or sensitization as shown in Figure 6[35]. Doping is a kind of bulk chemical modification to alter the chemical composition of TiO\({}_{2}\) but maintain the integrity of TiO\({}_{2}\) crystal structure. This technique substitutes the metal (titanium) or the non-metal (oxygen) component in order to alter the optical properties of TiO\({}_{2}\) nanomaterials. Figure 6b shows an example of metal doped, V-doped TiO\({}_{2}\)[36]. V-doped TiO\({}_{2}\) is the metal doping of TiO\({}_{2}\) with vanadium (V) to shift the absorption spectra of TiO\({}_{2}\) to a lower energy region (red shift) with the increase in the concentration of dopant. The red shift is attributed to the charge transfer transition between d-electron (t2g level) of the V dopant and the CB of TiO\({}_{2}\). The bandgap of V-doped TiO\({}_{2}\) is lower than pristine TiO\({}_{2}\). Hence, it can be photoactivated by visible light irradiation.  
An alternative technique is photosensization, a photochemical process by which a chemical reaction is induced by energy transfer from the light-absorbing sensitizer [37]. As shown in Figure 6c, charge transfer from the excited sensitizer to conduction band of TiO\({}_{2}\) is a key step in these processes. The match of energy difference between the oxidation potential of the excited sensitizer and conduction band of the TiO\({}_{2}\) acts as the driving force for the charge injection process [38]. The sensitizer for TiO\({}_{2}\) materials can be any materials with lower bandgap, including semiconductors, metals, and organic dyes.