Deformation-contributed negative triboelectric property of polytetrafluoroethylene: A density functional theory calculation

Polytetrafluoroethylene (PTFE) is a promising negative triboelectric material used in high-performance triboelectric nanogenerators (TENGs). To improve the triboelectric output of PTFE, an understanding of the triboelectric properties and electron transfer factor in PTFE is highly required. However, the triboelectric properties of PTFE have not been analyzed by considering the side effects during TENG operations (e.g., deformation). Here, we investigated the change in the triboelectric properties of PTFE due to molecular structure deformation, which is driven by contact force, using density functional theory (DFT). The deformation of the molecular structure induces modification of the electronic structure and triboelectric properties. Using the linear and deformed PTFE models (80°, 70°, 60°), we determined that the energy of the lowest unoccupied molecular orbital (LUMO) is decreased under deformation using the energy band diagram and density of states (DOS) (linear: 5.831 eV, 80°: 5.358 eV, 70°: 4.028 eV, 60°: 1.729 eV). This implies that the deformation due to the contact force enhances its negative triboelectric property (i.e., electron-accepting property). We analyzed this phenomenon because carbon in the deformation region has a strongly electron-deficient state, and the positive local dipole due to that state is enhanced. In addition, we investigated the LUMO changes, in part, from anti-bonding orbital to a bonding orbital. Because the bonding orbital has a more stable energy state than the anti-bonding orbital, the energy level of the LUMO could be lowered.