## 2 ZnO-Based Nanostructures Preparation
### Sol-Gel Method  
The sol-gel method is one of the most effective chemical methods for nanocomposites preparation with desired properties and advantages, such as low cost, mild reaction, environmental friendliness, reliability, and simplicity [43]. In the preparation of photocatalytic materials, the materials synthesized by this method have better photocatalytic activity [44]. ZnO nanoparticles were successfully synthesized using the gel-sol method by [PERSON] [45], and the surface morphology was verified by Fe-SEM images (Figure 1), which was confirmed to have a rod-like structure with a dense particle structure. Varieties of ZnO nanostructures have been discovered, which are in the form of nanorods, nanotubes, nanobelts, nano springs, nano spirals, nano rings, and many more [46]. Among these structures, the rod-like structure is the best nanostructure compared to others due to their one-dimensional nanostructures (such as nanorods, nanowires, and nanotubes) that can facilitate more efficient carrier transport for the decreased grain boundaries, surface defects, disorders, and discontinuous interfaces [47; 48]. In the process of preparing ZnO-based nanomaterials by the sol-gel method, the influence of factors such as solution drop acceleration rate, reaction temperature, pH, etc., will have a significant impact on the antibacterial properties of the materials [49]. Effects of various preparation influencing factors on the antibacterial properties of ZnO were tested, and pH was proven to be the most important influencing factor [50]. The reason is suggested to be that the neutral and acidic solution environment is more suitable for Zn\({}^{2+}\) to function and achieve an antibacterial effect. As long as the optimal pH and preparation temperature are found, the sol-gel method will be one of the effective preparation methods with high efficiency and low cost. Therefore, in recent studies, the sol-gel method is used more in the synthetization of ZnO-based nanoparticles [51; 52; 53].  
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Method** & **Preparation Shape** & **Advantages** & **Disadvantages** & **References** \\ \hline \multirow{6}{*}{Sol–gel} & Nanorods; & & & \\ Nanotubes; & (1) Uniform doping; & (2) Longer reaction time; & (32–34) \\ Nano springs; & (3) Low synthesis temperature. & (3) Organics escape. & & \\ Nano rings. & & & & \\ \hline \multirow{6}{*}{Co-precipitation} & Homogeneous and spherical; & (1) Simple preparation process; & (1) Additional precipitant; & \\ Nanobelts; & (2) Low cost & (3) Short synthesis cycle. & (2) High temperature calcination; & [35; 36] \\ \cline{1-1}  & Nano springs. & & (3) Short synthesis cycle. & (3) Uneven dispersion. & \\ \hline \multirow{6}{*}{Microwaves-assisted} & Nanorods; & (1) High synthesis efficiency; & (1) Indeterminate form; & \\ Nanotubes; & (2) Energy saving; & (2) Large investment; & (3) High requirements for & [37; 38; 39] \\ \cline{1-1}  & Nano springs. & & properties. & equipment. & \\ \hline \multirow{6}{*}{ Hydrothermal} & Nanobelts; & (1) Less thermal stress; & (1) Inconvenient to observe; & \\ \cline{1-1}  & Nano springs; & (2) High particle purity; & (3) High equipment requirements; & \\ \cline{1-1}  & Nano spirals. & (4) Low cost. & (4) Technical difficulty; & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Advantages and disadvantages of different antibacterial synthesis methods.