Ba adsorption on the TiO2 (110) surface. A density functional study

We have carried out a DFT study on Ba adsorption on the rutile TiO2 (110) surface by means of plane-wave, plane augmented waves potential, density-functional theory calculations. We have used a model consisting on a (4×1) unit cell and tried several potential adsorption sites on the surface. We have found that the most stable site is with the Ba atom on a position where it is bound to two bridging oxygen and an in-plane oxygen atoms forming equivalent bonds. The adsorption energy is 0.71 eV referred to the formation of Ba bulk and is about 0.3 eV more stable than other adsorption sites. The Ba-surface interaction produces some surface relaxation. We have found a strong covalent character in the nature of the bonding, which contrasts with the existence of Ba2+ species after a full two electrons transfer.