## 3 Results
### PM Filtration Characteristics  
The PM filtration characteristics of a PVDF NF layer based on a nylon mesh substrate were analyzed. Figure 7 shows the pressure difference, filtration efficiency, and quality  
Figure 5: High-speed image sequence of DI water droplet dripping onto the ZnO NWs-PVDF NF layer surface. Figure 6: **(a)** I-V characteristics and (b) I-t characteristics (at 1V bias) of ZnO NWs on electrospun PVDF NF under continuous light source of solar simulator (at 24 mW\(\cdot\)cm\({}^{-2}\)) and dark conditions. factor on the layers of the PVDF NF surface, with and without ZnO NWs. The pressure difference and filtration efficiency were measured in a filter test chamber on the upstream and downstream sides of the filter. The filter layer was inserted into a test chamber with a diameter of 50 mm. Intense combustion and dilution were performed at the upstream side of the filter chamber, where PM concentration was maintained to the level of 150 \(\upmu\)g\(\cdot\)m\({}^{-3}\). At the downstream side of the filter chamber, a face velocity (24.8 cm\(\cdot\)s\({}^{-1}\)) on the filter layer was controlled through vacuum pump suction for 0.5 h.  
The \(\Delta P\) of the ZnO NWs-PVDF NF layer shows 4.129 \(\pm\) 0.0175 kPa, which is 12.5 times higher than a PVDF NF layer (0.329 \(\pm\) 0.0040 kPa). However, the filtration efficiency (\(\eta\), %) of the ZnO NWs-PVDF NF layer was over 90.9%, regardless of the PM size. The filtration efficiency of 0.3 \(\upmu\)m size particle was 94.3%, which is higher than that of other particle sizes conditions. The quality factor (QF), which shows correlation between pressure difference and filtration efficiency, is expressed as Equation (4):  
\[\text{QF}=-\frac{ln(1-\eta)}{\Delta P} \tag{4}\]  
In the ZnO NWs-PVDF NF layer, QF is 0.69 kPa\({}^{-1}\) (@ 0.3 PM), which is lower than the PVDF NF (16.09 kPa\({}^{-1}\) @ 0.3 PM) layer due to the high pressure drop. The PVDF NF layer has superior QF due to high filtration efficiency (99.4% @ 0.3 PM) with low pressure difference. The ZnO NWs-PVDF NF layer is required to ensure a decreasing pressure difference of the filter for multifunctional filter applications. In summary, a high-density ZnO NW structures on a PVDF NF layer (a ZnO NWs-PVDF NF layer filter) shows high filtration efficiency (94.3%) for fine particulate matter (<0.3 \(\upmu\)m) under 24.8 cm s\({}^{-1}\) face velocity. The nylon mesh layer is used as a supporting layer for the ZnO NWs-PVDF NF filter layer, which maintains NF structure without geometric deformation due to the low-pressure differential variation (<\(\pm\)0.5%). The ZnO NW array is fabricated with high crystallinity on the NF surface without chemical reaction to PVDF via low temperature (80 \({}^{\circ}\)C) hydrothermal synthesis. The high surface-area-to-volume ratio of the ZnO NWs-PVDF NF layer has possibilities as a photoelectric generator, which has been demonstrated through the sensitivity (S) analysis of the filter under visible light radiation at ambient temperature. The filter characteristics analysis has shown that the structure of the ZnO NWs-PVDF NF layer filter has remaining limitations for immediate commercial use as a multifunctional filter because the hydrophilic filter layer, when exposed to a humid environment, is susceptible to pressure drop characteristics. However, these challenges in the filter performance can be overcome by improving the air permeability of the filter. In particular, electrospinninging conditions allow easy control of the film geometry controls, such as changes in NF diameter and film thickness, which can reduce the pressure differences. In hydrothermal synthesis, the aspect ratio of NW structures can be simply  
Figure 7: PM Filtration performance analysis of electrospun PVDF NF layer, with and without ZnO NWs, via measurement of pressure difference (\(\Delta P\)), filtration efficiency (\(\eta\)), and quality factor (QF). controlled to improve the functionality of filters with photocurrent properties.