Morphology evolution of the aluminum surface in a fluorine-containing environment

The interaction between aluminum (Al) and F and O atoms is essential to understand the etching process of Al and alumina (Al₂O₃) by fluorine-containing gases. In addition, it also has an influence on the corrosion behavior of Al devices, e.g., the Al collector in lithium-ion batteries operates in fluorine-containing electrolytes. However, the understanding of the structural evolution of the Al surface by fluorination at the atomistic level still remains elusive. Here, the thermodynamic and kinetic behaviors of F adatoms as well as co-adsorbed F and O adatoms on typical Al surfaces have been systematically investigated by combining density functional theory (DFT) calculations, canonical Monte Carlo (CMC) simulations and reactive molecular dynamics (RMD) simulations. The results of DFT calculations indicate that there is a repulsion (about 0.07 eV on Al(111) and Al(110), and 0.7 eV on Al(100)) between the first nearest neighboring (1NN) F adatoms, while an attraction of 0.14 eV on Al(111) exists within a 1NN F-O pair. CMC simulations reveal that the configurations of co-adsorbed F and O adatoms on the Al(111) surface at medium to low temperature (<600 K) and low total coverage (<0.2 monolayer, ML) have F adatoms dispersed in the hexagonal islands of O adatoms due to the attraction within the O-O and F-O pairs and the repulsion between F adatoms. As the coverage increases, the surface undergoes serious deformation. The average 1NN coordination numbers (1st CN) of O-to-O, F-to-O and F-to-F are six, three and zero, respectively. As the temperature increases, the interactions among adsorbates begin to be disrupted: the 1st CNs of O-to-O and F-to-O decrease, while that of F-to-F increases. The O-F hexagonal pattern remains until above the Al melting temperature (>1200 K). For F adatoms, both their migration on the surface and the penetration into the subsurface are easier than those of O adatoms, confirmed by both the DFT and RMD simulations. Our study on the co-adsorbates with opposite lateral interactions is instructive for understanding the thermal etching of Al and Al₂O₃ by fluorine-containing compounds.