Co-Adsorption of H2O, OH, and Cl on Aluminum and Intermetallic Surfaces and Its Effects on the Work Function Studied by DFT Calculations

The energetics of adsorption of H₂O layers and H₂O layers partially replaced with OH or Cl on an Al(111) surface and on selected surfaces of intermetallic phases, Mg₂Si and Al₂Cu, was studied by first-principle calculations using the density function theory (DFT). The results show that H₂O molecules tended to bind to all investigated surfaces with an adsorption energy in a relatively narrow range, between -0.8 eV and -0.5 eV, at increased water coverage. This can be explained by the dominant role of networks of hydrogen bonds at higher H₂O coverage. On the basis of the work function, the calculated Volta potential data suggest that both intermetallic phases became less noble than Al(111); also, the Volta potential difference was larger than 1 V when the coverage of the Cl-containing ad-layer reached one monolayer. The energetics of H₂O dissociation and substitution by Cl as well as the corresponding work function of each surface were also calculated. The increase in the work function of the Al(111) surface was attributed to the oxidation effect during H₂O adsorption, whereas the decrease of the work function for the Mg₂Si(111)-Si surface upon H₂O adsorption was explained by atomic and electronic rearrangements in the presence of H₂O and Cl.