Exploring the interactions of ZDDP additive with hematite surfaces: a DFT+<i>U</i> study

The class of zinc dialkyldithiophosphates (ZDDPs) has been the most widely used anti-wear additive class in the automotive industry for over 60 years, yet the pathway to the generation of the protective tribofilm remains elusive. In this context, density functional theory (DFT) can be utilized to investigate the interactions between ZDDPs and materials surfaces. We employed DFT+<i>U</i>calculations to examine the electronic structure of bulk hematite and three relevant (0001) surface terminations: Fe-O-Fe, O-Fe-Fe, and HO-Fe-Fe. Our results demonstrate that, while the Fe-O-Fe and HO-Fe-Fe slabs are insulating, the O-Fe-Fe terminated slab is metallic due to the formation of surface states from O dangling bonds. Additionally, we found that ZDDP binds more strongly on the Fe-O-Fe slab, leading to changes in ZDDP geometry and atomic charges. Minimal changes are observed when bound to the other surfaces. We have provided an in-depth study of the electronic structure of hematite and its surfaces, and their interaction with ZDDP. We include a detailed study of the first-principles Hubbard U and Hund J for Fe 3d orbitals in bulk hematite, finding a negligible self-consistency effect but a significant projector dependence. The new insights from this work provide a new path that can be used to understand the decomposition pathways of ZDDPs on metallic surfaces.