# Photocatalytic Cementitious Material for Eco-Efficient Construction--A Systematic Literature Review
## 2 Principles of Photocatalysis
### Semiconductor Photocatalysis  
Figure 4 shows the primary steps in the photocatalysis mechanism. Photoinduced charge separation (step 1 in Figure 4a) followed by electron transfer process for redox  
Figure 3: Applications of light-activated TiO\({}_{2}\).  
reactions (step 3 and 4 in Figure 4b) are the key steps in the photocatalysis process. These steps will generate radicals that have the power to breakdown most of organic pollutant (step 5 in Figure 4b). In the absence of adsorbate, charge separation (step 1 in Figure 4a) will be followed by surface recombination or volume recombination (step 2 in Figure 4b). In this case, the photocatalysis process could not take place and the absorbed photon energy was released as heat.  
In semiconductor, the valence band and the conduction band are separated by a bandgap \(\Delta\)E, a region devoid of energy in a perfect crystal as shown in the enlarged section of Figure 4a. The highest energy band containing electrons is called the valence band (VB); the lowest empty band is called the conduction band (CB) [26]. The energy difference between the highest occupied band (the valence band), and the lowest unoccupied band (the conduction band) is called the bandgap, \(\Delta\)E. By absorbing a photon energy equal to or greater than the band gap, \(\Delta\)E, of the semiconductor, an electron is excited from the valence band into the conduction band, leaving behind a hole in the valence band (step 1), as shown in the enlarged section of Figure 4a [27]. The created electron-hole pair has a sufficient lifetime, in the nanosecond regime, to undergo charge transfer to adsorbed species on the semiconductor surface [28]. In the absence of the adsorbate, the excited electrons at the conduction band and holes at the valence band can recombine (step 2) and dissipate the input energy as heat.  
If suitable, electron-hole scavengers, or surface defect state, is available to trap the electron or hole, and recombination is prevented and subsequent redox transfer may occur [27]. Photogenerated holes can be utilized to oxidize substrates at the semiconductor surface, as illustrated in step 3 of Figure 4b [29]. The holes trapped at the TiO\({}_{2}\) surface can react with OH\({}^{-}\) to generate OH\({}^{\bullet}\) radicals. The generated OH\({}^{\bullet}\) radicals and holes themselves are a powerful oxidant. This powerful oxidant can degrade most of organic pollutant completely into a harmless final product (e.g., CO\({}_{2}\) and H\({}_{2}\)O), as shown in step 5. In addition, the photoexcited electrons can react with electron acceptors adsorbed on the semiconductor surface (step 4). The reduction power of electrons can induce the reduction in molecular oxygen (O\({}_{2}\)) to form the superoxide anion (O\({}_{2}\)\({}^{-}\)). It has been confirmed that the superoxide is almost effective as holes and hydroxyl radicals in the chain reactions for breaking down the organic compounds [8]. Initiating an oxidation and reduction process at semiconductor surface is the principle of photodegradation [29]. Most organic photodegradation reactions utilize the oxidizing power of the holes either directly or indirectly [27]. During these processes, semiconductor remains intact and the charge transfer to the adsorbed species is continuous [28]. However, to prevent a build-up of charge, a reducible species should be provided to react with the electrons. In this system, photoinduced molecular transformations or reactions take place at the surface of a catalyst. The relevant photocatalytic processes may occur either at the air-solid interface or at the  
Figure 4: (**a**) Photoexcitation process; (**b**) Primary steps in photocatalytic mechanism.  
liquid-solid interface. Therefore, if the species are pre-adsorbed on the surface, the electron transfer processes could be more efficient.