First-principles study of the TiO2(110) surface reduction upon Na adsorption
Résumé
We present a detailed theoretical study, based on density-functional calculations, of Na adsorption on TiO2(110) for coverage equal to 1/8, 1/4, and 1/2 monolayers. Two competing threefold adsorption sites are found. In the framework of the Bader theory, we analyze the electron distribution, the interfacial electron transfer, and the screening processes as a function of the coverage and the actual adsorption geometry. At low coverage, the Na atoms bind preferentially to two bridging and one in-plane O atoms. The adsorbed sodium atoms are nearly fully ionized. The excess electron states have a nonbonding Ti character and are delocalized over a few surface and subsurface sites. Such a behavior, which is at variance with the common belief that the excess states should be essentially localized on the surface fivefold titanium, is discussed in terms of the surface electrostatic potential and the atomic relaxations. According to our calculations, the onset of covalent interactions between Na atoms occurs for coverage slightly below 1/2 of monolayer, depending on the specific adsorption configuration. It is accompanied by a reduction of the positive Bader charge of the Na atoms and by a weakening of the ionic Na-O bond strength, which results in decreasing Na adsorption energies. At 1/2 monolayer coverage, the adsorption energies are practically insensitive to the details of the Na distribution among the low-energy adsorption sites, which suggests that Na-induced surface reconstructions might be affected by the intrinsic quality of the surface and the actual deposition conditions.