# Photocatalytic Inactivation as a Method of Elimination of _E. coli_ from Drinking Water
## 3 Inactivation Process by Photocatalysts
### Metal Oxides as Photocatalysts for the Inactivation of E. coli  
Metal oxides such as TiO\({}_{2}\), ZnO, ZrO\({}_{2}\), CeO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), SnO\({}_{2}\) and WO\({}_{3}\) have photocatalytic properties and have been reported in different studies [8; 15; 44; 45; 46; 47; 48]. The photocatalytic properties of some of these metal oxides have been harnessed for the conversion of solar energy to chemical energy in order to oxidize or reduce materials, degrade pollutants and inactivate pathogens [13]. Nanocomide made from two oxides (zinc oxide and silver oxide) has been used to inactivate _E. coli_ under both visible and ultraviolet light radiation. Despite pure zinc oxide being reported to be effective against bacteria [49], its photocatalytic activities were lower than that of the nanocomposite made from zinc oxide and silver oxide [50]. In a bid to reducing the time needed for the inactivation of bacteria, zinc oxide nanoarrays were fabricated and used to inactivate 97.5% of _E. coli_ within 60 s under ultraviolet light. However, after several cycles of use, the time taken for inactivation to take place was 600 s under similar condition [51]. Oxides of copper nanoparticles derived from citrus has been used as light-sensitive catalysts for decontaminating wastewater containing _E. coli_[52]. Magnesium oxide nanoparticles obtained via green synthesis from plant biomass was also used to inhibit _E. coli_[53]. The synthesis of magnesium oxide from plant biomass is advantageous because it is not poisonous, is environmentally sustainable and cheaply available [54; 55]. In addition, the biomass contains stabilizing and reducing agents such as phenolic compounds, natural acids, alkaloids, and terpenoids, which aid the formation of the nanoparticles of metal oxides [54; 55]. The inactivation of 60% of _Escherichia coli_ was achieved in the presence of light by using chitosan and sodium alginate functionalized by nanoparticles of ZnO and CuO. The functionalization led to the generation of more reactive oxygen species under light [56]. Titanium oxide is one of the earliest discovered catalysts and has remained the photocatalysts of choice due to its non-toxic nature, relative cheapness, good photocatalytic performance and high chemical stability [57]. It is an important nanomaterial with optical, catalytic and dielectric properties [58]. Titanium dioxide has attracted a considerable attention because of its wide and growing industrial applications in the production of  
Figure 3: Redox potential and band gap energies of some selected semiconductor photocatalysts at pH 7. Reprinted with permission from [43], Copyright (2016) American Chemical Society. solar cells, fillers, pigments including catalyst supports and as photocatalysts [8; 58]. It has three crystal forms, namely brookite, anatase and rutile. The anatase form induces maximum speed in the formation of free radicals [8]. Several studies have shown the potential of the TiO\({}_{2}\) catalyst in the successful and complete deactivation of microbial cells including cancerous cells, bacteria, viruses, fungi and algae under the irradiation of UV light source [58]. One of the numerous investigations that utilized titanium oxide as photocatalyst for inactivating _E. coli_ was reported by [PERSON] et al. [59]. In the study, a transparent film of titanium oxide was used to inactivate _E. coli_ in the presence of light. Hydrogen peroxide was introduced to increase the amount of reactive oxygen species that is present in the photocatalytic system. Despite the wide acceptance of titanium oxide as a good photocatalyst for inactivating bacteria, the limitation of titanium oxide photocatalyst is that it is only active in the ultraviolet (UV) region of the solar spectrum due to the wide band gap energy of 3.2 eV. This UV region accounts for less than 5% of the solar spectrum, which implies that pure titanium oxide under-utilizes the solar spectrum.