8-Hydroxyquinoline complexes (Alq3) on Al(111): atomic scale structure, energetics and charge distribution

Abstract

8-Hydroxyquinoline (8Hq) is known to efficiently inhibit the corrosion of aluminium by forming metal–organic layers (8Hq forms complexes with aluminium atoms). In the present work, the atomic scale structure and the energetics of 8-hydroxyquinoline complexes (Alq₃) adsorbed on an aluminium surface are investigated by dispersion-corrected DFT computations. Two scenarios are considered: (i) an Alq₃ complex, previously formed in vacuum, is deposited on a flat Al(111) surface or (ii) three deprotonated 8Hq molecules (q) directly adsorb on a defective Al(111) surface presenting Al adatoms (Al–Al(111)). For the Alq₃ formation in vacuum, each addition of a q molecule on the Al atom stabilises the system, the oxidation state of the Al atom evolving from Al^I in Alq to Al^III in Alq₂ and Alq₃. The subsequent deposition of Alq₃ on Al(111) leads to a strong bonding between the q molecules of the complex and the Al(111) surface, with a significant electron transfer occurring from the surface to the complexes (0.73 to 1.57 e). The formation on the metal surface of Alq₃ complexes via the adsorption of q molecules on an Al adatom leads to more stable structures than the ones obtained from direct adsorption of Alq₃ on Al(111). For the most stable adsorption conformation, the three q molecules are bonded to the Al adatom but only two are bonded to the aluminium surface. In that case, the total electron transfer from the Al–Al(111) surface to the q molecules is 4.40 e and the electron transfer from the Al(111) surface to the Alq₃-like species is 2.04 e. The structure, energetics and charge distribution data demonstrate an iono-covalent bonding between the q molecules and the Al atoms, in the complex as well as on the aluminium surface.