Effects of Monoatomic Vacancies and Alloy Atoms on Hydrogen Diffusion at Al(111) Surfaces

Abstract

 We have investigated trapping effects of monoatomic vacancies V_mono and alloy atoms on hydrogen diffusion at Al(111) surfaces. We performed first-principles calculations based on density functional theory with generalized gradient approximation in order to obtain the hydrogen adsorption energies in the vicinity of monoatomic vacancies and alloy atoms in Al(111) subsurfaces. We considered Si, Cr, Mn, Fe, Cu, Ge, and Zn as alloy atoms. We substituted one Al atom with one monoatomic vacancy or alloy atom, which corresponds to Al_0.95X_0.05(111) (X=Si, Cr, Mn, Fe, Cu, Ge, Zn, V_mono) surfaces. We found that all of monoatomic vacancies and alloy atoms increase adsorption energies. We also clarified that hydrogen atoms make strong covalent bonds in Cr, Mn, Fe-alloyed Al(111) subsurfaces, while they make only weak ionic bonds in pure and Si, Cu, Ge, Zn-alloyed Al(111) subsurfaces.

Fig. 5

Isosurfaces of electron density difference of hydrogen atoms at the most stable adsorption sites in (a) pure and (b) Cr-alloyed Al(111) subsurfaces. Electron density increase in darker (red) regions and decrease in lighter (blue) regions. The isosurface value is 0.002e Å⁻³. Black, gray (green), and white balls are aluminum, chromium and hydrogen atoms, respectively. Black lines show the supercell which we considered.