Alignment Effect on the Piezoelectric Properties of Ultrathin Cellulose Nanofiber Films

This study aims at evaluating the piezoelectric property of an ultrathin cellulose nanofiber (CNF) film in a thickness direction. Cellulose is known to have piezoelectric properties. However, its measurement is not easy. By making an ultrathin CNF film and eliminating space charges, the pure piezoelectric property of CNF is intended to be measured. A 600 nm thick ultrathin CNF film was prepared using a microfabrication process. The effect of alignment methods on the piezoelectric property of the ultrathin CNF film in the thickness direction was investigated with the three alignment methods: corona poling, electrical in-plane alignment, and high magnetic field (HiMA) alignment. Piezoresponse force microscopy (PFM) was utilized to analyze the piezoelectric property. By applying AC voltages using PFM, the vertical displacement on the ultrathin CNF film surface was recorded and converted into the piezoelectric coefficient, d₃₃. The aligned ultrathin CNF films show different piezoelectric coefficients. The corona-poled ultrathin CNF film shows the largest piezoelectric coefficient among the three alignment methods. Water contact angle measurement proves that the piezoelectric constant in the thickness direction is associated with hydroxyl groups in cellulose chains appearing on the surface of the ultrathin CNF films.