# Photocatalytic Inactivation as a Method of Elimination of _E. coli_ from Drinking Water
## 3 Inactivation Process by Photocatalysts
### Inactivation of _E. coli_ by Heterojunction Systems  
One of the most common heterojunction systems is the type II heterojunction system, which is formed by two semiconductors (SC I and SC II) that possess wide and narrow band gaps, respectively. The valence band and the conduction band of semiconductor 1 (SC I)are more than the valence band and the conduction band of semiconductor 2 (SC II). This makes it possible for photo-induced holes to move from the valence band of SC I to the valence band of SC II, whereas the light-excited electrons will migrate in the reverse direction (SC I to SC II). The movement in different direction boosts the charge separation, which culminate in better photocatalytic performance. Another common heterojunction system is the Z-scheme heterojunction system [93]. The Z-scheme and other heterojunction schemes are illustrated in Figure 4a-f and it could be with or without semiconductor at its interface. The semiconductor acts as the electron mediator for charge relay and recombination. Without the mediator, the photo-excited charge in the valence band of SC I recombine with the low-lying conduction band of SC II at the interface of SC I and SC II [94]. The s-scheme has also been reported as another heterojunction system. In the s-scheme heterojunction system, two n-type semiconductors with different band structures are combined together. One of the photocatalyst is the oxidation photocatalyst, while the other one is the reduction photocatalyst [95]. The Schottky scheme has equally been reported as another way of improving the photocatalytic efficiency of the photocatalysts. In this heterojunction, a semiconductor makes contact with a metal to generate a potential difference known as the Schottky barrier. The generation of this potential difference is as a result of non-similar Fermi levels and work function between the semiconductor and the metal. The Schottky barrier function elongates the lifespan of the electron by preventing the recombination of the charge carriers [96; 97]. The combination of nano-Ag\({}_{2}\)ZrO\({}_{3}\) and graphitic carbon nitride has been used to fabricate a type II heterojunction system and the resultant photocatalyst was used for eliminating gram-negative _E. coli_. Exposure of the _E. coli_ to visible light in the presence of this heterojunction photocatalysts led to the reduction of the population of _E. coli_ to 3%, but longer exposure time achieved a reduction of the _E. coli_ to 1.27% [99]. The Z-scheme heterojunction system made from silver iodide and organic framework having a covalent bond was also investigated for photocatalytic inactivation of _E. coli_. The inactivation was affected under visible light irradiation by the hydroxyl radicals, holes and peroxyl radical as revealed by the scavenger experiment [100]. Complete bacteria disinfection  
Figure 4: Examples of heterojunction systems. Reproduced from [98], Copyright (2019), with permission from Elsevier. (\(1.5\times 10^{6}\) CFU mL\({}^{-1}\)) was achieved in 40 min when visible light was used, unlike in the dark where there was no inactivation of viable cells. After four cycles of using the photocatalyst, there was still complete inactivation of the bacteria, which shows that the photocatalyst was reusable [100]. Another Z-scheme heterojunction system that was used for the inactivation of _E. coli_ under visible light irradiation was made from nanocomposites of AgI/Bi\({}_{2}\)Sn\({}_{2}\)O\({}_{7}\). The investigation by electron spin resonance and radical trapping experiments showed that 7.48-log of the bacteria was inactivated via \(\cdot\)OH, \(\cdot\)O\({}_{2}\)\({}^{-}\), e\({}^{-}\) and h\({}^{+}\)[101]. The Z-scheme heterojunction made from graphitic carbon nitride and nickel phosphide was also used to inactivate _E. coli_ via the oxidation by holes in the presence of visible light. The number of _E.