## 3 Results
### Effect of ZPP Morphology on UV Shielding Protection  
To evaluate the UV-blocking of ZPPs in the coatings, ZPPs were mixed with a PVA solution prior to casting on the glass slide substrates. Although all hybrid films were prepared from mixtures containing the same solid content, the amount of ZnO NPs were different. The OA-ZnO NPs-PVA film was as transparent as the pristine PVA film on a black background (Figure S6) because the particle size of OA-ZnO NPs was small (11.8 \(\pm\) 1.8 nm) and the PVA can partially form a hydrophobic interaction [43]. The hybrid films containing ZPPs were less transparent or opaque on black background and appeared transparent on white background. These results posed the influence of particle size on light scattering on different backgrounds and their visible transparency. The attenuation of light in materials is a combination of absorption and the scattering of UV and visible radiation, of which the latter causes opacity [44]. Compared to pristine films, OA-ZnO NPs-PVA film showed slightly higher absorbance over the UV-visible region, where the characteristic peak of ZnO NPs at 370 nm were observed, as presented in Figure 8A [2; 23]. All the ZPPs-based PVA films presented significant enhancement of absorption over the UV and visible region than the pristine and OA-ZnO NP-PVA films, suggesting good UV and visible shielding [45]. Besides characteristic peaks of ZnO NPs, all the absorption spectra of the ZPP-based PVA films showed the scattering patterns over UV-visible region due to their large particle sizes. Hybrid PVA films containing core-shell ZPPs exhibited the highest absorbance with \(\lambda_{\text{max}}\) at 329 nm, followed by Pickering-like and hollow ZPPs-PVA films, respectively. The collaborating influence between the particle size of ZPPs and the  
Figure 7: Scheme of hollow structure formation.  
location of ZnO NPs seemed to outweigh the amount of ZnO NPs in ZPPs. ZnO NPs in the core-shell ZPP were confined in the core area (about 100 nm) and coated with a PS shell, with a refractive index (1.5865) close to PVA (1.4770). These nano-size semiconductors are in close proximity to each other, which facilitated the motion of the electron in the conducting band (CB) and the hole in valence band (VB) between adjacent ZnO NPs and performed high UV absorption, as illustrated in Figure S7[46,47]. It might be said that absorption mode plays a key role in the UV shielding of core-shell ZPPs. Aside from particle sizes and the location of ZnO NPs laying on the surface or in the shell of ZPPs, the absorbance and absorption patterns of Pickering-like and hollow ZPPs are comparable. Multiple light scattering, between the high refractive index of ZnO NPs (1.9596) at the interface and the low refractive index of the PS core for Pickering-like ZPPs or the air void (1.0003) for hollow ZPPs, is possibly the major contributor for their shielding properties [48].  
Similar results were observed in the transmission spectra as shown in Figure 8B. Transmittance of pristine and OA-ZnO NPs-PVA films were, respectively, nearly 90% and 83%, which agreed well with the optical images in Figure S6. The transmittance of ZPPs-PVA films were lower than the pristine and OA-ZnO NPs-PVA films, in which core-shell ZPPs-PVA films presented the lowest transmittance value among ZPPs, followed by Pickering-like and hollow ZPPs-based PVA films. Despite exhibiting some degree of opacity, all ZPPs-based PVA films showed ca. 45-65% transmittance in the UV region and ca. 50-80% in the visible region. The strong UV absorption and light transmittance in the visible range suggested the advantages of the encapsulation of ZnO NPs in particulate form, making them a potential candidate for UV-shielding applications.