## 3 Types of Nanomaterials for Photocatalytic Antimicrobials in Water Treatment  
A variety of nanomaterials as efficient adsorption materials and catalytic degradation and purification of wastewater will be discussed in this section. As shown in Figure 7, various microbial contaminants and their sources, as well as different nanomaterials for removal applications are revealed [76]. Limitations of single nanomaterials can be overcome  
Figure 5: Electron microscope image of ZnO nanoparticles: the concentration of precursor is (a) 5 mM, (b) 10 mM, and (c) 20 mM. Reprinted/adapted with permission from Ref. [72]. Copyright © 2022, Elsevier Ltd.  
Figure 6: XRD patterns of ZnO samples synthesized with different precursor concentrations. Reprinted/adapted with permission from Ref. [72]. Copyright © 2022, Elsevier Ltd.  
by various strategies such as polymer/metal oxide nanocomposite [77], metal oxide/metal-based nanomaterials [78,79], polymer/metal oxide nanocomposite [80,81], and polymer-structure-based materials [82,83].  
The nanocomposite is a safe, non-toxic, green and environmentally friendly nanomaterial, usually prepared with ZnO and TiO\({}_{2}\) as substrates. The addition of TiO\({}_{2}\) and ZnO NPs in some polymers such as polypropylene matrix would increase the dielectric constant of the nanocomposite, thereby enhancing the photocatalytic ability of the material [84]. In addition, the polymer boundary layer transition zone forms a crystalline structure, which increases conductivity and acts as a tuning surfactant [85], which greatly enhances the photoreactivity of the catalytic material. A variety of composite nanomaterials have been reported that can be applied to organic pollutants and microbial contamination removal from water [86]. Moreover, the addition of Ag, zinc, zeolite, and titanium is obtained having better efficiencies in pathogenic pathogens and microorganisms removal from water [87,88]. For example, silver NPs with polyurethane will flexibly remove almost 100% of _B. subtilis_ and _E. coli_ from water [89,90].  
Among the composite nanomaterial antibacterial agents, ZnO-based nanomaterials have the advantages of strong compatibility, green friendliness, lower costs, and simple preparation, so they occupy a large proportion of the related studies in this field in recent years. Studies showed that ZnO has stronger direct interactions with bacterial and pathogen cell surfaces than other semiconductor materials [91]. In addition, ZnO nanomaterials leak Zn\({}^{2+}\) in solution, which can exacerbate toxicity to bacteria, pathogens, and viruses. Compared with Cu\({}^{+}\), Fe\({}^{2+}\), and Al\({}^{3+}\), Zn\({}^{2+}\) showed a better ability to fight microbial contaminants [92]. Therefore, ZnO-based nanomaterials have a better potential to combat microbial contamination, which is of practical significance for the study of antimicrobial contamination. Among types of microbial pollution in the water environment, bacterial pollution is still the most important problem to be solved. Thus, in the next section, the antimicrobial properties and mechanisms of ZnO-based nanomaterials, as well as methods for studying antimicrobial properties are introduced, and the methods for improving their catalytic properties are summarized and discussed.