Contact Electrification of Biological and Bio-Inspired Adhesive Materials on SiO₂ Surfaces: Perspectives from DFT Calculations

In this study, we investigate the contact electrification properties of glycine, cysteine, and dimethyl siloxane on silicon dioxide (SiO₂) surfaces using density functional theory calculations. Molecule contacting through the sulfhydryl group has stronger adhesion to the SiO₂-O and SiO₂-OH surfaces. The SiOH/SiO₂-Si system has the largest adhesion energy in all molecule/SiO₂-Si contact systems and charge transfers from the molecule to the SiO₂-O and SiO₂-Si surfaces. The molecule/SiO₂-OH systems have a reverse charge transfer direction. Molecules with their sulfhydryl and hydroxyl groups facing the SiO₂-O and SiO₂-OH surfaces have more transferred charges. The NH₂/SiO₂-Si system has a larger transferred charge than other molecule/SiO₂-Si systems. The direction of charge transfer is determined by the Bader charge of the isolated surface atoms. The respective energy difference in the lowest unoccupied occupied molecular orbitals between contacting atoms influences the charge transfer. The respective energy difference in the highest occupied molecular orbitals reflects the electron attraction and affects charge transfer. Finally, the quantitative relationship between the transferred charge and energy gaps is established to evaluate the charge transfer. The findings propose a new perspective and in-depth understanding of contact electrification and shed light on the bio-inspired adhesive materials design and fabrication for engineering applications.