Plasma Treatment of PP NWFs. Plasma treatment was performed using a desktop vacuum plasma apparatus YHS-R (Sakigake-Semiconductor, Japan). The PP NWFs (40-50 mg; 4 × 4 cm 2 ) were placed in a sample tray, and a vacuum was generated with a vacuum pump. After the inside of the device reached the vacuum conditions required, plasma discharge was performed for 1 min. The plasma-treated samples were labeled as Plasma.