Chemisorption mechanism of defluorinated fluorine on bcc Fe surface during formation of PTFE transfer film

Highlights: • Defluorinated fluorine is chemisorbed on Fe surface to generate iron fluoride. • Adsorption on Fe (1 0 0) surface at bridge site exhibits preferable configuration. • Electrons are transferred from Fe surface to accumulate around the adsorbed F atom. • Only the electronic structure of the topmost Fe layer is affected by the adsorbed F. • The Fe-F bond formed on the surface reveal high degrees of ionic character. Since defluorination is most likely to occur during the formation of polytetrafluoroethylene (PTFE) transfer film, understanding the adsorption mechanism of defluorinated fluorine can help describe the interfacial tribochemical reaction. XPS analysis showed that the defluorinated fluorine was chemisorbed on the Fe surface to generate iron fluoride. Density functional theory (DFT) calculations revealed that the adsorption of fluorine on the Fe (1 0 0) surface at the bridge site with 0.25 monolayer coverage exhibited the lowest adsorption energy (−4.71 eV), indicating a preferable adsorption configuration. At an identical adsorption site, the adsorption strength decreases with increasing coverage, which is attributed to the repulsive interaction of adsorbed fluorine. The chemisorption of fluorine on the Fe surface induces a considerable redistribution of charges, i.e. electrons are transferred from the Fe surface and accumulate around the adsorbed fluorine. The adsorbed fluorine mainly interacted with the topmost Fe layer. The two Fe atoms located at the bridge sites of the topmost layer play significant roles in the formation of Fe–F bonds that have high degrees of ionic character. The electronic states of Fe–F bond are derived from the hybridisation of Fe 4s, Fe 4p, Fe 3d, F 2s, and F 2p orbitals.