3.1.1. Characterization of ZnO Particles

The characterization data of the ZnO particles are shown in Figure 2. Specifically, Figure 2a represents the SEM images of ZnO particles. It indicates that the ZnO particles have a regular spherical shape and are uniformly dispersed without agglomeration. The DLS measurements show that average hydrodynamic diameter is about 589.3 nm (Figure 2b). Figure 2c illustrates the FTIR spectrum of the ZnO particles. The single peak at 407 cm−<sup>1</sup> can be assigned to stretching mode of the Zn-O bond [32]. To further determine the composition of the resulting particles, XRD was carried out. As shown in Figure 2d, the diffraction peaks at 2θ values of 31.8◦, 34.5◦, 36.3◦, 47.8◦, 56.3◦, 63.2◦, 68.0◦ and 69.2◦ correspond to (100), (002), (101), (102), (110), (103), (112) and (201) crystal planes of ZnO, respectively [33].

**Figure 2.** Characterization of ZnO particles (**a**) scanning electron microscopy (SEM) image, (**b**) particle size distribution by intensity, (**c**) Fourier transform infrared spectroscopy (FTIR) spectrum and (**d**) X-ray diffractometry (XRD) spectrum.
