Oxygen adsorption onto pure and doped Al surfaces – the role of surface dopants

Abstract

Using density functional theory (DFT) with the PBE0 density functional we investigated the role of surface dopants in the molecular and dissociative adsorption of O₂ onto Al clusters of types Al₅₀, Al₅₀Al_ad, Al₅₀X and Al₄₉X, where X represents a dopant atom of the following elements Si, Mg, Cu, Sc, Zr, and Ti. Each dopant atom was placed on the Al(111) surface as an adatom or as a substitutional atom, in the last case replacing a surface Al atom. We found that for the same dopant geometry, the closer is the ionization energy of the dopant element to that of elemental Al, the more exothermic is the dissociative adsorption of O₂ and the stronger are the bonds between the resulting O atoms and the surface. Additionally we show that the Mulliken concept of electronegativity can be applied in the prediction of the dissociative adsorption energy of O₂ on the doped surfaces. The Mulliken modified second-stage electronegativity of the dopant atom is proportional to the exothermicity of the dissociative adsorption of O₂. For the same dopant element in an adatom position the dissociation of O₂ is more exothermic when compared to the case where the dopant occupies a substitutional position. These observations are discussed in view of the overlap population densities of states (OPDOS) computed as the overlap between the electronic states of the adsorbate O atoms and the clusters. It is shown that a more covalent character in the bonding between the Al surface and the dopant atom causes a more exothermic dissociation of O₂ and stronger bonding with the O atoms when compared to a more ionic character in the bonding between the dopant and the Al surface. The extent of the adsorption site reconstruction is dopant atom dependent and is an important parameter for determining the mode of adsorption, adsorption energy and electronic structure of the product of O₂ adsorption. The PBE0 functional could predict the existence of the O₂ molecular adsorption product for many of the cases investigated here.