Adsorption of Zn²⁺ on the (110) surface of TiO₂ (Rutile): A density functional molecular dynamics study

Adsorption of Zn²⁺ at the rutile TiO₂ (110)-aqueous interface was studied with Born-Oppenheimer molecular dynamics at 300 K. Simulations were carried out using the periodically repeated slab model with vacuum gap of 15 Å filled with 72 H₂O molecules. Two possible adsorption sites, monodentate above bridging oxygen (Ti-O-Ti or O_br) and bidentate above terminal oxygens (Ti-O), were investigated. Sites with different coordination environment for adsorbed Zn²⁺ differ from each other by the position of Zn²⁺ above surface level and by characteristic Zn-O distances. Obtained results gave evidence that 4-fold coordination of adsorbed Zn²⁺ is more probable than the 6-fold coordination found for aqueous species. The hydrolysis of H₂O molecules was observed in the first coordination shell of adsorbed ion, resulting in formation of OH^- groups attached to Zn²⁺. Calculated energies favor the tetrahedral bidentate structure of hydrated Zn²⁺ on the rutile surface. The model structures are compared to observed positions of Zn²⁺ above the rutile (110) surface using X-ray scattering techniques.